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Somatic stem cells (SSCs) play a major role in tissue homeostasis and respond to a panoply of micro-environmental cues by adjusting their quiescence and activation profiles. How these cells integrate and decode multiple niche signals remains elusive. In recent years, Ca2+ signaling has emerged as one of the key intracellular pathways that allow stem cells to dynamically adjust their fate and either to remain quiescent for future needs or to become activated to generate new progeny. Interestingly, not only distinct Ca2+ signatures are associated with the quiescence and activation states of stem cells, but also various extracellular cues impinge on Ca2+ pathways to dynamically regulate the responses of stem cells to different niche signals. This Viewpoint article deals with how Ca2+ signaling may be used to decode and integrate different niche factors and how Ca2+ fluctuations of distinct amplitudes, frequencies, and overall intracellular levels may trigger the differential gene transcription program. Knowledge about mechanisms that allow SSCs to translate the complexity of extracellular niche signaling into intrinsic states of cell quiescence and activation is crucial for understanding life-long tissue homeostasis and regeneration.

Summary The neural stem cells (NSCs) residing in the olfactory epithelium (OE) regenerate damaged olfactory sensory neurons throughout adulthood. The accessibility and availability of these NSCs in living individuals, including humans, makes them a promising candidate for harvesting their potential for cell replacement therapies. However, this requires an in-depth understanding of their developmental potential after grafting. Here, we investigated the developmental potential and plasticity of mouse OE-derived NSCs after grafting into the adult subventricular zone (SVZ) neurogenic niche. Our results showed that OE-derived NSCs integrate and proliferate just like endogenous SVZ stem cells, migrate with similar dynamics as endogenous neuroblasts toward the olfactory bulb, and mature and acquire similar electrophysiological properties as endogenous adult-born bulbar interneurons. These results reveal the developmental potential and plasticity of OE-derived NSCs in vivo and show that they can respond to heterotopic neurogenic cues to adapt their phenotype and become functional neurons in ectopic brain regions., Graphical Abstract, Highlights • OE-derived NSCs integrate in the SVZ after heterotopic transplantation • OE-derived NSCs respond to SVZ niche factors and change their developmental program • The development of OE-derived and SVZ NSCs are indistinguishable • OE-derived NSCs grafted into the SVZ become functional bulbar interneurons, In this article, Saghatelyan and colleagues show that olfactory epithelium-derived neural stem cells adapt their developmental program in response to heterotopic neurogenic niche factors after grafting in vivo. They migrate and differentiate into ectopic brain region neurons that are indistinguishable in their morpho-functional properties from endogenous adult-born neurons.

The Mechanistic Target Of Rapamycin Complex 1 (mTORC1) pathway controls several aspects of neuronal development. Mutations in regulators of mTORC1, such as Tsc1 and Tsc2, lead to neurodevelopmental disorders associated with autism, intellectual disabilities and epilepsy. The correct development of inhibitory interneurons is crucial for functional circuits. In particular, the axonal arborisation and synapse density of parvalbumin (PV)-positive GABAergic interneurons change in the postnatal brain. How and whether mTORC1 signaling affects PV cell development is unknown. Here, we show that Tsc1 haploinsufficiency causes a premature increase in terminal axonal branching and bouton density formed by mutant PV cells, followed by a loss of perisomatic innervation in adult mice. PV cell-restricted Tsc1 haploinsufficient and knockout mice show deficits in social behavior. Finally, we identify a sensitive period during the third postnatal week during which treatment with the mTOR inhibitor Rapamycin rescues deficits in both PV cell innervation and social behavior in adult conditional haploinsufficient mice. Our findings reveal a role of mTORC1 signaling in the regulation of the developmental time course and maintenance of cortical PV cell connectivity and support a mechanistic basis for the targeted rescue of autism-related behaviors in disorders associated with deregulated mTORC1 signaling., Regulators of mTORC1, such as Tsc1, are involved in neurodevelopmental disorders associated with autism, intellectual disabilities and epilepsy. The authors identify a role of mTORC1 signaling in the regulation and maintenance of cortical parvalbumin cell connectivity and social behaviour.

Coronavirus disease 19 (COVID-19) is a respiratory illness caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). COVID-19 pathogenesis causes vascular-mediated neurological disorders via elusive mechanisms. SARS-CoV-2 infects host cells via the binding of viral Spike (S) protein to transmembrane receptor, angiotensin-converting enzyme 2 (ACE2). Although brain pericytes were recently shown to abundantly express ACE2 at the neurovascular interface, their response to SARS-CoV-2 S protein is still to be elucidated. Using cell-based assays, we found that ACE2 expression in human brain vascular pericytes was increased upon S protein exposure. Pericytes exposed to S protein underwent profound phenotypic changes associated with an elongated and contracted morphology accompanied with an enhanced expression of contractile and myofibrogenic proteins, such as α-smooth muscle actin (α-SMA), fibronectin, collagen I, and neurogenic locus notch homolog protein-3 (NOTCH3). On the functional level, S protein exposure promoted the acquisition of calcium (Ca2+) signature of contractile ensheathing pericytes characterized by highly regular oscillatory Ca2+ fluctuations. Furthermore, S protein induced lipid peroxidation, oxidative and nitrosative stress in pericytes as well as triggered an immune reaction translated by activation of nuclear factor-kappa-B (NF-κB) signaling pathway, which was potentiated by hypoxia, a condition associated with vascular comorbidities that exacerbate COVID-19 pathogenesis. S protein exposure combined to hypoxia enhanced the production of pro-inflammatory cytokines involved in immune cell activation and trafficking, namely macrophage migration inhibitory factor (MIF). Using transgenic mice expressing the human ACE2 that recognizes S protein, we observed that the intranasal infection with SARS-CoV-2 rapidly induced hypoxic/ischemic-like pericyte reactivity in the brain of transgenic mice, accompanied with an increased vascular expression of ACE2. Moreover, we found that SARS-CoV-2 S protein accumulated in the intranasal cavity reached the brain of mice in which the nasal mucosa is deregulated. Collectively, these findings suggest that SARS-CoV-2 S protein impairs the vascular and immune regulatory functions of brain pericytes, which may account for vascular-mediated brain damage. Our study provides a better understanding for the mechanisms underlying cerebrovascular disorders in COVID-19, paving the way to develop new therapeutic interventions.

Summary During adulthood, the activation of adult neural stem cells (NSCs) has been mostly studied ex vivo in post-mortem tissues or in vivo in anesthetized animals. This protocol presents an approach that allows for the long-term and minimally invasive investigation of adult NSC activation and physiology in freely behaving animals. By combining specific NSC labeling and mini-endoscopic microscopy, live imaging of NSC division and Ca2+ activity can be performed continuously for 2–3 days and even up to several months. For complete details on the use and execution of this protocol, please refer to Gengatharan et al. (2021)., Graphical abstract, Highlights • A protocol for mini-endoscopic imaging of adult neural stem cells (NSCs) • Live imaging of NSCs activation in freely behaving mice • In vivo mini-endoscopic imaging and analysis of Ca2+ dynamics in NSCs • A protocol for labeling adult NSCs based on coincident activity of two promoters, During adulthood, the activation of adult neural stem cells (NSCs) has been mostly studied ex vivo in post-mortem tissues or in vivo in anesthetized animals. This protocol presents an approach that allows for the long-term and minimally invasive investigation of adult NSC activation and physiology in freely behaving animals. By combining specific NSC labeling and mini-endoscopic microscopy, live imaging of NSC division and Ca2+ activity can be performed continuously for 2–3 days and even up to several months.

COVID-19 is a respiratory disease caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). COVID-19 pathogenesis causes vascular-mediated neurological disorders via still elusive mechanisms. SARS-CoV-2 infects host cells by binding to angiotensin-converting enzyme 2 (ACE2), a transmembrane receptor that recognizes the viral spike (S) protein. Brain pericytes were recently shown to express ACE2 at the neurovascular interface, outlining their possible implication in microvasculature injury in COVID-19. Yet, pericyte responses to SARS-CoV-2 is still to be fully elucidated. Using cell-based assays, we report that ACE2 expression in human brain vascular pericytes is highly dynamic and is increased upon S protein stimulation. Pericytes exposed to S protein underwent profound phenotypic changes translated by increased expression of contractile and myofibrogenic proteins, namely α-smooth muscle actin (α-SMA), fibronectin, collagen I, and neurogenic locus notch homolog protein-3 (NOTCH3). These changes were associated to an altered intracellular calcium (Ca2+) dynamic. Furthermore, S protein induced lipid peroxidation, oxidative and nitrosative stress in pericytes as well as triggered an immune reaction translated by activation of nuclear factor-kappa-B (NF-κB) signalling pathway, which was potentiated by hypoxia, a condition associated to vascular comorbidities, which exacerbate COVID-19 pathogenesis. S protein exposure combined to hypoxia enhanced the production of pro-inflammatory cytokines involved in immune cell activation and trafficking, namely interleukin-8 (IL-8), IL-18, macrophage migration inhibitory factor (MIF), and stromal cell-derived factor-1 (SDF-1). Finally, we found that S protein could reach the mouse brain via the intranasal route and that reactive ACE2-expressing pericytes are recruited to the damaged tissue undergoing fibrotic scarring in a mouse model of cerebral multifocal micro-occlusions, a main reported vascular-mediated neurological condition associated to COVID-19. Our data demonstrate that the released S protein is sufficient to mediate pericyte immunoreactivity, which may contribute to microvasculature injury in absence of a productive viral infection. Our study provides a better understanding for the possible mechanisms underlying cerebrovascular disorders in COVID-19, paving the way to develop new therapeutic interventions.

Several native and engineered heat‐stable DNA polymerases from a variety of sources are used as powerful tools in different molecular techniques, including polymerase chain reaction, medical diagnostics, DNA sequencing, biological diversity assessments, and in vitro mutagenesis. The DNA polymerase from the extreme thermophile, Thermus scotoductus strain K1, (TsK1) was expressed in Escherichia coli, purified, and characterized. This enzyme belongs to a distinct phylogenetic clade, different from the commonly used DNA polymerase I enzymes, including those from Thermus aquaticus and Thermus thermophilus. The enzyme demonstrated an optimal temperature and pH value of 72–74°C and 9.0, respectively, and could efficiently amplify 2.5 kb DNA products. TsK1 DNA polymerase did not require additional K+ ions but it did need Mg2+ at 3–5 mM for optimal activity. It was stable for at least 1 h at 80°C, and its half‐life at 88 and 95°C was 30 and 15 min, respectively. Analysis of the mutation frequency in the amplified products demonstrated that the base insertion fidelity for this enzyme was significantly better than that of Taq DNA polymerase. These results suggest that TsK1 DNA polymerase could be useful in various molecular applications, including high‐temperature DNA polymerization., The DNA polymerase from the extreme thermophile Thermus scotoductus strain K1 (TsK1) was expressed in Escherichia coli, purified, and characterized. The enzyme demonstrated an optimal temperature and pH value of 72–74°C and 9.0, respectively, and could efficiently amplify 2.5 kb DNA products. TsK1 DNA polymerase did not require additional K+ ions but it did need Mg2+ at 3–5 mM for optimal activity. The base insertion fidelity for this enzyme was significantly better than that of Taq DNA polymerase.

Neuronal migration is a fundamental brain development process that allows cells to move from their birthplaces to their sites of integration. Although neuronal migration largely ceases during embryonic and early postnatal development, neuroblasts continue to be produced and to migrate to a few regions of the adult brain such as the dentate gyrus and the subventricular zone (SVZ). In the SVZ, a large number of neuroblasts migrate into the olfactory bulb (OB) along the rostral migratory stream (RMS). Neuroblasts migrate in chains in a tightly organized micro-environment composed of astrocytes that ensheath the chains of neuroblasts and regulate their migration; the blood vessels that are used by neuroblasts as a physical scaffold and a source of molecular factors; and axons that modulate neuronal migration. In addition to diverse sets of extrinsic micro-environmental cues, long-distance neuronal migration involves a number of intrinsic mechanisms, including membrane and cytoskeleton remodeling, Ca2+signaling, mitochondria dynamics, energy consumption, and autophagy. All these mechanisms are required to cope with the different micro-environment signals and maintain cellular homeostasis in order to sustain the proper dynamics of migrating neuroblasts and their faithful arrival in the target regions. Neuroblasts in the postnatal brain not only migrate into the OB but may also deviate from their normal path to migrate to a site of injury induced by a stroke or by certain neurodegenerative disorders. In this review, we will focus on the intrinsic mechanisms that regulate long-distance neuroblast migration in the adult brain and on how these pathways may be modulated to control the recruitment of neuroblasts to damaged/diseased brain areas.

Neural stem cells (NSCs) in the adult brain transit from the quiescent state to proliferation to produce new neurons. The mechanisms regulating this transition in freely behaving animals are, however, poorly understood. We customized invivo imaging protocols to follow NSCs for several days up to months, observing their activation kinetics in freely behaving mice. Strikingly, NSC division is more frequent during daylight and is inhibited by darkness-induced melatonin signaling. The inhibition of melatonin receptors affected intracellular Ca2+ dynamics and promoted NSC activation. We further discovered a Ca2+ signature of quiescent versus activated NSCs and showed that several microenvironmental signals converge on intracellular Ca2+ pathways to regulate NSC quiescence and activation. Invivo NSC-specific optogenetic modulation of Ca2+ fluxes to mimic quiescent-state-like Ca2+ dynamics in freely behaving mice blocked NSC activation and maintained their quiescence, pointing to the regulatory mechanisms mediating NSC activation in freely behaving animals.

The microbial diversity of high-altitude geothermal springs has been recently assessed to explore their biotechnological potential. However, little is known regarding the microbiota of similar ecosystems located on the Armenian Highland. This review summarizes the known information on the microbiota of nine high-altitude mineralized geothermal springs (temperature range 25.8–70 °C and pH range 6.0–7.5) in Armenia and Nagorno-Karabakh. All these geothermal springs are at altitudes ranging from 960–2090 m above sea level and are located on the Alpide (Alpine–Himalayan) orogenic belt, a seismically active region. A mixed-cation mixed-anion composition, with total mineralization of 0.5 mg/L, has been identified for these thermal springs. The taxonomic diversity of hot spring microbiomes has been examined using culture-independent approaches, including denaturing gradient gel electrophoresis (DGGE), 16S rRNA gene library construction, 454 pyrosequencing, and Illumina HiSeq. The bacterial phyla Proteobacteria, Bacteroidetes, Cyanobacteria, and Firmicutes are the predominant life forms in the studied springs. Archaea mainly include the phyla Euryarchaeota, Crenarchaeota, and Thaumarchaeota, and comprise less than 1% of the prokaryotic community. Comparison of microbial diversity in springs from Karvachar with that described for other terrestrial hot springs revealed that Proteobacteria, Bacteroidetes, Actinobacteria, and Deinococcus–Thermus are the common bacterial groups in terrestrial hot springs. Contemporaneously, specific bacterial and archaeal taxa were observed in different springs. Evaluation of the carbon, sulfur, and nitrogen metabolism in these hot spring communities has revealed diversity in terms of metabolic activity. Temperature seems to be an important factor in shaping the microbial communities of these springs. Overall, the diversity and richness of the microbiota are negatively affected by increasing temperature. Other abiotic factors, including pH, mineralization, and geological history, also impact the structure and function of the microbial community. More than 130 bacterial and archaeal strains (Bacillus, Geobacillus, Parageobacillus, Anoxybacillus, Paenibacillus, Brevibacillus Aeribacillus, Ureibacillus, Thermoactinomyces, Sporosarcina, Thermus, Rhodobacter, Thiospirillum, Thiocapsa, Rhodopseudomonas, Methylocaldum, Desulfomicrobium, Desulfovibrio, Treponema, Arcobacter, Nitropspira, and Methanoculleus) have been reported, some of which may be representative of novel species (sharing 91–97% sequence identity with their closest matches in GenBank) and producers of thermozymes and biomolecules with potential biotechnological applications. Whole-genome shotgun sequencing of T. scotoductus K1, as well as of the potentially new Treponema sp. J25 and Anoxybacillus sp. K1, were performed. Most of the phyla identified by 16S rRNA were also identified using metagenomic approaches. Detailed characterization of thermophilic isolates indicate the potential of the studied springs as a source of biotechnologically valuable microbes and biomolecules.

The genus Thermus represents a group of archetypal extreme thermophiles found in both natural and man-made thermal environments. Thermozymes mined from these bacteria have found their use in a broad range of biotechnological and industrial applications, including feed and food production, pharmaceutical and paper-pulp industries, organic synthesis, starch-processing, etc. Moreover, several molecular biological tools and host-vector systems for heterologous expression of extremozymes of thermophiles and hyperthermophiles were developed on the basis of representatives of this genus. Several strains of the genus Thermus are also well known as metal reducers with high potency in use for bioremediation. This chapter summarizes the diversity, distribution, main physiometabolic characteristics of members of the genus Thermus, as well as their biotechnological capacity and applications.

Cell migration is a dynamic process that entails extensive protein synthesis and recycling, structural remodeling, and considerable bioenergetic demand. Autophagy is one of the pathways that maintain cellular homeostasis. Time-lapse imaging of autophagosomes and ATP/ADP levels in migrating cells in the rostral migratory stream of mouse revealed that decreases in ATP levels force cells into the stationary phase and induce autophagy. Pharmacological or genetic impairments of autophagy in neuroblasts using either bafilomycin, inducible conditional mice, or CRISPR/Cas9 gene editing decreased cell migration due to the longer duration of the stationary phase. Autophagy is modulated in response to migration-promoting and inhibiting molecular cues and is required for the recycling of focal adhesions. Our results show that autophagy and energy consumption act in concert in migrating cells to dynamically regulate the pace and periodicity of the migratory and stationary phases to sustain neuronal migration.

Mesodiencephalic dopamine (mDA) neurons play a wide range of brain functions. Distinct subtypes of mDA neurons regulate these functions but the molecular mechanisms that drive the mDA circuit formation are largely unknown. Here we show that autophagy, the main recycling cellular pathway, is present in the growth cones of developing mDA neurons and its level changes dynamically in response to guidance cues. To characterize the role of autophagy in mDA axon growth/guidance, we knocked-out (KO) essential autophagy genes (Atg12, Atg5) in mice mDA neurons. Autophagy deficient mDA axons exhibit axonal swellings and decreased branching both in vitro and in vivo, likely due to aberrant microtubule looping. Strikingly, deletion of autophagy-related genes, blunted completely the response of mDA neurons to chemo-repulsive and chemo-attractive guidance cues. Our data demonstrate that autophagy plays a central role in regulating mDA neurons development, orchestrating axonal growth and guidance.

Here, we ask how neural stem cells (NSCs) transition in the developing neocortex from a rapidly to a slowly proliferating state, a process required to maintain lifelong stem cell pools. We identify LRIG1, known to regulate receptor tyrosine kinase signaling in other cell types, as a negative regulator of cortical NSC proliferation. LRIG1 is expressed in murine cortical NSCs as they start to proliferate more slowly during embryogenesis and then peaks postnatally when they transition to give rise to a portion of adult NSCs. Constitutive or acute loss of Lrig1 in NSCs over this developmental time frame causes stem cell expansion due to increased proliferation. LRIG1 controls NSC proliferation by associating with and negatively regulating the epidermal growth factor receptor (EGFR). These data support a model in which LRIG1 dampens the stem cell response to EGFR ligands within the cortical environment to slow their proliferation as they transition to postnatal adult NSCs.

Mutations in regulators of the Mechanistic Target Of Rapamycin Complex 1 (mTORC1), such as Tsc1/2, lead to neurodevelopmental disorders associated with autism, intellectual disabilities and epilepsy. Whereas the effects of mTORC1 signaling dysfunction within diverse cell types are likely critical for the onset of the diverse neurological symptoms associated with mutations in mTORC1 regulators, they are not well understood. In particular, the effects of mTORC1 dys-regulation in specific types of inhibitory interneurons are unclear.Here, we showed that Tsc1 haploinsufficiency in parvalbumin (PV)-positive GABAergic interneurons either in cortical organotypic cultures or in vivo caused a premature increase in their perisomatic innervations, followed by a striking loss in adult mice. This effects were accompanied by alterations of AMPK-dependent autophagy in pre-adolescent but not adult mice. PV cell-restricted Tsc1 mutant mice showed deficits in social behavior. Treatment with the mTOR inhibitor Rapamycin restricted to the third postnatal week was sufficient to permanently rescue deficits in both PV cell innervation and social behavior in adult conditional haploinsufficient mice. All together, these findings identify a novel role of Tsc1-mTORC1 signaling in the regulation of the developmental time course and maintenance of cortical PV cell connectivity and provide a mechanistic basis for the targeted rescue of autism-related behaviors in disorders associated with deregulated mTORC1 signaling.

α-synuclein (α-syn) aggregation into insoluble deposits, referred to as Lewy bodies (LBs) is the paramount pathological hallmark of Parkinson’s disease and related α-synucleinopathies. However, how these aggregates affect neuronal homeostasis leading to neurodegeneration remains elusive. This gap in knowledge is mainly due to the lack of proper cellular and animal models to undertake such investigations. We have addressed this by developing a light-inducible protein aggregation (LIPA) system that allows for real-time induction of α-syn inclusions formation with remarkable spatial and temporal resolution in both cell culture and in vivo paradigms. We report that LIPA-induced aggregates auto-perpetuate for several days after transient light induction, and faithfully mimic several authentic features of LBs in vivo and in cell culture. Moreover, optogenetically induced α-syn aggregation in mouse midbrains compromised the nigrostriatal transmission and induced a significant dopaminergic neuronal loss and behavioural impairment. Our system provides a novel, dependable and invaluable tool by which to generate, visualize and dissect the role of protein aggregates in neurodegeneration.

The adult mouse olfactory bulb is continuously supplied with new neurons that mostly differentiate into granule cells (GCs). Different subtypes of adult-born GCs have been identified, but their maturational profiles and their roles in bulbar network functioning and odor behavior remain elusive. It is also not known whether the same subpopulations of GCs born during early postnatal life (early-born) or during adulthood (adult-born) differ in their morpho-functional properties. Here, we show that adult-born calretinin-expressing (CR+) and non-expressing (CR−) GCs, as well as early-born CR+ GCs, display distinct inhibitory inputs but indistinguishable excitatory inputs and similar morphological characteristics. The frequencies of inhibitory post-synaptic currents were lower in early-born and adult-born CR+ GCs than in adult-born CR− neurons. These findings were corroborated by the reduced density of gephyrin+ puncta on CR+ GCs. CR+ GCs displayed a higher level of activation following olfactory tasks based on odor discrimination, as determined by an immediate early gene expression analysis. Pharmacogenetic inhibition of CR+ GCs diminished the ability of the mice to discriminate complex odor mixtures. Altogether, our results indicate that distinct inhibitory inputs are received by adult-born CR+ and CR− GCs, that early- and adult-born CR+ neurons have similar morpho-functional properties, and that CR+ GCs are involved in complex odor discrimination tasks.

The adult olfactory bulb is continuously supplied with neuronal precursors that differentiate into granule and periglomerular cells. Little is known about the structural dynamic of adult-born granule cells (GCs) at their different maturational stages, the mechanisms controlling the integration of new neurons into the pre-existing neuronal circuitry, or the role of principal cell activity in these processes. We used two-photon time-lapse imaging to reveal a high level of filopodia formation and retraction on the distal dendrites of adult-born GCs at their early maturational stages. This dynamic decreased as the adult-born interneurons matured. Filopodia formation/retraction on the dendrites of adult-born GCs at the early maturational stages depended on the activation of NMDA receptors (NMDARs). The stimulation of mitral cells using a pattern that mimics activity of these principal neurons to odor presentation promotes the NMDAR-dependent filopodia dynamic of adult-born GCs during their early but not late maturational stages. Moreover, NMDA iontophoresis was sufficient to induce the formation of new filopodia on the distal dendrites of immature adult-born GCs. The maturation of adult-born interneurons was accompanied by a progressive hyperpolarization of the membrane potential and an increased Mg2+block of NMDARs. Decreasing the extracellular Mg2+concentration led to filopodia formation on the dendrites of mature adult-born GCs following NMDA iontophoresis. Our findings reveal an increased structural dynamic of adult-born GCs during the early stages of their integration into the mouse bulbar circuitry and highlight a critical period during which the principal cells' activity influences filopodia formation/retraction on the dendrites of interneurons.

Neuronal precursors produced in the subventricular zone throughout an animal's life migrate tangentially along the rostral migratory stream and, once in the olfactory bulb (OB), turn to migrate radially to the bulbar layers, where they differentiate into interneurons. Despite extensive investigations, it has remained largely unknown whether the same molecular mechanisms control OB neurogenesis during early postnatal development and in adulthood. In this study, we show that the extracellular matrix glycoprotein tenascin-R (TNR) is produced in the granule cell layer of the OB and that its expression increases during postnatal development. Time-lapse video imaging and morphological analyses revealed that a lack of TNR decreases the radial migration of neuronal precursors in the adult, but not in the developing OB. A lack of TNR also reduces spine development of newborn neurons in adult mice. To understand the functional consequences of a lack of TNR, we performed electrophysiological and behavioral studies on young and adult mice. Electrophysiological recordings showed that mitral cells, the target cells of newly generated interneurons, receive reduced spontaneous and evoked inhibitory activity in adult, but not young, TNR knock-out mice. Moreover, the synchronized activity of mitral cells was decreased in the OB of adult TNR knock-out mice. Behavioral studies revealed that the lower numbers of newborn interneurons in the adult OB induce alterations in short-term odor memory. Our results indicate that TNR modulates adult but not developmental neurogenesis in the OB and also highlight that the regulation of OB neurogenesis can vary during an animal's lifetime.

Granule cells (GCs) in the olfactory bulb (OB) play an important role in odor information processing. Although they have been classified into various neurochemical subtypes, the functional roles of these subtypes remain unknown. We used in vivo two-photon Ca2+ imaging combined with cell-type-specific identification of GCs in the mouse OB to examine whether functionally distinct GC subtypes exist in the bulbar network. We showed that half of GCs express Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα+) and that these neurons are preferentially activated by olfactory stimulation. The higher activity of CaMKIIα+ neurons is due to the weaker inhibitory input that they receive compared to their CaMKIIα-immunonegative (CaMKIIα-) counterparts. In line with these functional data, immunohistochemical analyses showed that 75%-90% of GCs expressing the immediate early gene cFos are CaMKIIα+ in naive animals and in mice that have been exposed to a novel odor and go/no-go operant conditioning, or that have been subjected to long-term associative memory and spontaneous habituation/dishabituation odor discrimination tasks. On the other hand, a perceptual learning task resulted in increased activation of CaMKIIα- cells. Pharmacogenetic inhibition of CaMKIIα+ GCs revealed that this subtype is involved in habituation/dishabituation and go/no-go odor discrimination, but not in perceptual learning. In contrast, pharmacogenetic inhibition of GCs in a subtype-independent manner affected perceptual learning. Our results indicate that functionally distinct populations of GCs exist in the OB and that they play distinct roles during different odor tasks.

In mammals, new neurons in the adult olfactory bulb originate from a pool of neural stem cells in the subventricular zone of the lateral ventricles. Adult-born cells play an important role in odor information processing by adjusting the neuronal network to changing environmental conditions. Olfactory bulb neurogenesis is supported by several non-neuronal cells. In this review, we focus on the role of astroglial cells in the generation, migration, integration, and survival of new neurons in the adult forebrain. In the subventricular zone, neural stem cells with astrocytic properties display regional and temporal specificity when generating different neuronal subtypes. Non-neurogenic astrocytes contribute to the establishment and maintenance of the neurogenic niche. Neuroblast chains migrate through the rostral migratory stream ensheathed by astrocytic processes. Astrocytes play an important regulatory role in neuroblast migration and also assist in the development of a vasculature scaffold in the migratory stream that is essential for neuroblast migration in the postnatal brain. In the olfactory bulb, astrocytes help to modulate the network through a complex release of cytokines, regulate blood flow, and provide metabolic support, which may promote the integration and survival of new neurons. Astrocytes thus play a pivotal role in various processes of adult olfactory bulb neurogenesis, and it is likely that many other functions of these glial cells will emerge in the near future.

Adult-born neurons adjust olfactory bulb (OB) network functioning in response to changing environmental conditions by the formation, retraction and/or stabilization of new synaptic contacts. While some changes in the odour environment are rapid, the synaptogenesis of adult-born neurons occurs over a longer time scale. It remains unknown how the bulbar network functions when rapid and persistent changes in environmental conditions occur but when new synapses have not been formed. Here we reveal a new form of structural remodelling where mature spines of adult-born but not early-born neurons relocate in an activity-dependent manner. Principal cell activity induces directional growth of spine head filopodia (SHF) followed by spine relocation. Principal cell-derived glutamate and BDNF regulate SHF motility and directional spine relocation, respectively; and spines with SHF are selectively preserved following sensory deprivation. Our three-dimensional model suggests that spine relocation allows fast reorganization of OB network with functional consequences for odour information processing., The mechanism by which adult-born neurons quickly adjust olfactory bulb network functioning is not understood. Here the authors describe a novel form of structural plasticity in which mature spines relocate toward active mitral cell dendrite along spine head filopodia via AMPA and BDNF mediated signalling.

The 2,379,636-bp draft genome sequence of Thermus scotoductus strain K1, isolated from geothermal spring outlet located in the Karvachar region in Nagorno Karabakh is presented. Strain K1 shares about 80% genome sequence similarity with T. scotoductus strain SA-01, recovered from a deep gold mine in South Africa.

The adult mammalian brain is remarkably plastic and constantly undergoes structurofunctional modifications in response to environmental stimuli. In many regions plasticity is manifested by modifications in the efficacy of existing synaptic connections or synapse formation and elimination. In a few regions, however, plasticity is brought by the addition of new neurons that integrate into established neuronal networks. This type of neuronal plasticity is particularly prominent in the olfactory bulb (OB) where thousands of neuronal progenitors are produced on a daily basis in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) towards the OB. In the OB, these neuronal precursors differentiate into local interneurons, mature, and functionally integrate into the bulbar network by establishing output synapses with principal neurons. Despite continuous progress, it is still not well understood how normal functioning of the OB is preserved in the constantly remodelling bulbar network and what role adult-born neurons play in odor behaviour. In this review we will discuss different levels of morphofunctional plasticity effected by adult-born neurons and their functional role in the adult OB and also highlight the possibility that different subpopulations of adult-born cells may fulfill distinct functions in the OB neuronal network and odor behaviour.

Neuronal migration is one of the fundamental processes underlying the proper assembly and function of neural circuitry. The majority of neuronal precursors are generated far away from their sites of integration and need to migrate substantial distances to reach their final destination. Neuronal migration occurs not only in the embryonic brain but also in a few regions of the adult brain such as the olfactory bulb (OB). The mechanisms orchestrating cell migration in the adult brain are, however, poorly understood, despite their clinical relevance. Here we describe a method for time-lapse imaging of cell migration in acute brain slices. This method, combined with genetic and/or pharmacological manipulations of different molecular pathways, makes it possible to determine the dynamics and molecular mechanisms of cell migration in the adult brain. In addition, time-lapse imaging in acute brain slices makes it possible to monitor cell movement in a microenvironment that closely resembles in vivo conditions and to study neuroblast displacement along with other cellular elements such as astrocytes and blood vessels.

International audience; The olfactory bulb is one of the few regions in the adult mammalian forebrain in which neurons are constitutively replaced throughout life. New neurons generated in the subventricular zone migrate long distances along the rostral migratory stream to the olfactory bulb where they differentiate into interneurons. Neuronal precursor generation, migration and incorporation into the bulbar network occur in an environment rich in extracellular matrix molecules. We investigated the potential role of one of the constituents of the extracellular matrix, tenascin-C (TNC), in bulbar neurogenesis and olfactory performance using TNC-deficient mice. We found that TNC deficiency resulted in a delayed onset of olfactory responses in neonatal animals. This delay normalized at around postnatal day 10. Interestingly, this delay in early olfactory performance was not due to impaired bulbar neurogenesis as proliferation, migration, incorporation and fate determination of newborn bulbar interneurons were normal in TNC-deficient animals. Thus, we conclude that a constitutive lack of TNC does not affect bulbar neurogenesis, but instead leads to ontogenetically early impairments in olfactory detection.

International audience; In mammals, the olfactory bulb (OB) constitutes one of two regions of the postnatal brain with continuous neurogenesis throughout life. Despite intense explorations of neuronal replacement in the adult OB, little is known about the mechanisms that operate at earlier postnatal stages. This question is particularly pertinent, because the majority of local interneurons are born in the neonate, when olfaction controls vital functions. Here, we analyzed the recruitment of newborn cells to the granule cell (GC) layer (GCL) and found that the postnatal mouse OB is supplied with two spatiotemporally distinct populations of newborn interneurons. Early born [postnatal day 3 (P3) to P7] GCs constitute a threefold larger population compared with those generated later (P14-P60), and some of them are produced locally within the OB itself. Newborn interneurons generated at P3-P7 were predominantly targeted to the external edge of the GCL, whereas newly generated cells were positioned deeper in older mice. Additionally, although approximately 50% of adult newborn cells were eliminated within a few weeks of reaching the OB, almost the entire population of early born GCs survived until adulthood. Importantly, early olfactory experience specifically modifies the number of newborn GCs in neonates but leaves unaltered the amount of neurons generated during adulthood. Together, these results demonstrate that early postnatal neurogenesis endows the neonate bulbar circuit with newborn GCs that differ morphologically and functionally from those produced in the adult.

Extracellular matrix molecules are important cues in the shaping of nervous system structure and function. Here, we describe a novel mechanism by which the HNK-1 carbohydrate carried by recognition molecules regulates perisomatic inhibition in the hippocampus. Neutralization of HNK-1 activity by an HNK-1 antibody results in GABA(B) receptor-mediated activation of K(+) currents in CA1 pyramidal cells, which elevates extracellular K(+) concentration and reduces evoked GABA release in perisomatic inhibitory synapses. This mechanism is supported by pharmacological analysis in hippocampal slices and data showing that the HNK-1 carbohydrate binds to GABA(B) receptors and inhibits GABA(B) receptor-activated K(+) currents in a heterologous expression system. We suggest that the HNK-1 carbohydrate is involved in homeostatic regulation of GABA(A) receptor-mediated perisomatic inhibition by suppression of postsynaptic GABA(B) receptor activity.

Perisomatic inhibition of pyramidal cells regulates efferent signalling from the hippocampus. The striking presence of HNK-1, a carbohydrate expressed by neural adhesion molecules, on perisomatic interneurons and around somata of CA1 pyramidal neurons led us to apply monoclonal HNK-1 antibodies to acute murine hippocampal slices. Injection of these antibodies decreased GABAA receptor-mediated perisomatic inhibitory postsynaptic currents (pIPSCs) but did not affect dendritic IPSCs or excitatory postsynaptic currents. The decrease in the mean amplitude of evoked pIPSCs by HNK-1 antibodies was accompanied by an increase in the coefficient of variation of pIPSC amplitude, number of failures and changes in frequency but not amplitude of miniature IPSCs, suggesting that HNK-1 antibodies reduced efficacy of evoked GABA release. HNK-1 antibodies did not affect pIPSCs in knock-out mice deficient in the extracellular matrix molecule tenascin-R which carries the HNK-1 carbohydrate as analysed by immunoblotting in synaptosomal fractions prepared from the CA1 region of the hippocampus. For control, HNK-1 antibody was applied to acute sections of mice deficient in the neural cell adhesion molecule NCAM, another potential carrier of HNK-1, and resulted in decrease of pIPSCs as observed in wild-type mice. Reduction in perisomatic inhibition is expected to promote induction of long-term potentiation (LTP) by increasing the level of depolarization during theta-burst stimulation. Indeed, LTP was increased by HNK-1 antibody applied before stimulation. Moreover, LTP was reduced by an HNK-1 peptide mimic, but not control peptide. These results provide first evidence that tenascin-R and its associated HNK-1 carbohydrate modulate perisomatic inhibition and synaptic plasticity in the hippocampus.

The adult olfactory bulb (OB) continuously receives new interneurons that integrate into the functional neuronal network and that play an important role in odor information processing and olfactory behavior. Adult neuronal progenitors are derived from neural stem cells in the subventricular zone (SVZ) bordering the lateral ventricle. They migrate long distances along the rostral migratory stream (RMS) toward the OB where they differentiate into interneurons, mature, and establish synapses with tufted or mitral cells (MC), the principal neurons in the OB. The plasticity provided by both adult-born and pre-existing early-born neurons depends on the formation and pruning of new synaptic contacts that adapt the functioning of the bulbar network to changing environmental conditions. However, the formation of new synapses occurs over a long time scale (hours-days), whereas some changes in environmental conditions can occur more rapidly, requiring a much faster adjustment of neuronal networks. A new form of structural remodeling of adult-born, but not early-born, neurons was recently brought to light. This plasticity, which is based on the activity-dependent relocation of mature spines of GCs toward the dendrites of active principal cells, may allow a more rapid adjustment of the neuronal network in response to quick and persistent changes in sensory inputs. In this mini-review we discuss the different forms of structural plasticity displayed by adult-born and early-born neurons and the possibility that these different forms of structural remodeling may fulfill distinct roles in odor information processing.

Stroke induces the recruitment of neuronal precursors from the subventricular zone (SVZ) into the ischemic striatum. In injured areas, de-routed neuroblasts use blood vessels as a physical scaffold to their migration, in a process that resembles the constitutive migration seen in the rostral migratory stream (RMS). The molecular mechanism underlying injury-induced vasculature-mediated migration of neuroblasts in the post-stroke striatum remains, however, elusive. Using adult mice we now demonstrate that endothelial cells in the ischemic striatum produce brain-derived neurotrophic factor (BDNF), a neurotrophin that promotes the vasculature-mediated migration of neuronal precursors in the RMS, and that recruited neuroblasts maintain expression of p75NTR, a low-affinity receptor for BDNF. Reactive astrocytes, which are widespread throughout the damaged area, ensheath blood vessels and express TrkB, a high-affinity receptor for BDNF. Despite the absence of BDNF mRNA, we observed strong BDNF immunolabeling in astrocytes, suggesting that these glial cells trap extracellular BDNF. Importantly, this pattern of expression is reminiscent of the adult RMS, where TrkB-expressing astrocytes bind and sequester vasculature-derived BDNF, leading to the entry of migrating cells into the stationary phase. Real-time imaging of cell migration in acute brain slices revealed a direct role for BDNF in promoting the migration of neuroblasts to ischemic areas. We also demonstrated that cells migrating in the ischemic striatum display higher exploratory behavior and longer stationary periods than cells migrating in the RMS. Our findings suggest that the mechanisms involved in the injury-induced vasculature-mediated migration of neuroblasts recapitulate, at least partially, those observed during constitutive migration in the RMS.

As a result of brain injury, astrocytes become activated and start to proliferate in the vicinity of the injury site. Recently, we had demonstrated that these reactive astrocytes, or glia, can form self-renewing and multipotent neurospheres in vitro. In the present study, we demonstrate that it is only invasive injury, such as stab wounding or cerebral ischemia, and not noninvasive injury conditions, such as chronic amyloidosis or induced neuronal death, that can elicit this increase in plasticity. Furthermore, we find that Sonic hedgehog (SHH) is the signal that acts directly on the astrocytes and is necessary and sufficient toelicit the stem cell response both in vitro and invivo. These findings provide a molecular basis for how cells with neural stem cell lineage emerge at sites of brain injury and imply that the high levels of SHH known to enter the brain from extraneural sources after invasive injury can trigger this response.

There is a substantial body of evidence indicating that new functional neurons are constitutively generated from an endogenous pool of neural stem cells in restricted areas of the adult mammalian brain. Newborn neuroblasts from the subventricular zone (SVZ) migrate along the rostral migratory stream (RMS) to their final destination in the olfactory bulb (OB). In the RMS, neuroblasts migrate tangentially in chains ensheathed by astrocytic processes using blood vessels as a structural support and a source of molecular factors required for migration. In the OB, neuroblasts detach from the chains and migrate radially into the different bulbar layers where they differentiate into interneurons and integrate into the existing network. In this manuscript we describe the procedure for monitoring cell migration in acute slices of the rodent brain. The use of acute slices allows the assessment of cell migration in the microenvironment that closely resembling to in vivo conditions and in brain regions that are difficult to access for in vivo imaging. In addition, it avoids long culturing condition as in the case of organotypic and cell cultures that may eventually alter the migration properties of the cells. Neuronal precursors in acute slices can be visualized using DIC optics or fluorescent proteins. Viral labeling of neuronal precursors in the SVZ, grafting neuroblasts from reporter mice into the SVZ of wild-type mice, and using transgenic mice that express fluorescent protein in neuroblasts are all suitable methods for visualizing neuroblasts and following their migration. The later method, however, does not allow individual cells to be tracked for long periods of time because of the high density of labeled cells. We used a wide-field fluorescent upright microscope equipped with a CCD camera to achieve a relatively rapid acquisition interval (one image every 15 or 30 sec) to reliably identify the stationary and migratory phases. A precise identification of the duration of the stationary and migratory phases is crucial for the unambiguous interpretation of results. We also performed multiple z-step acquisitions to monitor neuroblasts migration in 3D. Wide-field fluorescent imaging has been used extensively to visualize neuronal migration. Here, we describe detailed protocol for labeling neuroblasts, performing real-time video-imaging of neuroblast migration in acute slices of the adult mouse forebrain, and analyzing cell migration. While the described protocol exemplified the migration of neuroblasts in the adult RMS, it can also be used to follow cell migration in embryonic and early postnatal brains.

New neurons are constantly being generated in the postnatal subventricular zone. They have to migrate long distances via the rostral migratory stream (RMS) to reach their final destination in the olfactory bulb (OB). In adults, these neuronal precursors migrate in chains, ensheathed by astrocytic processes, and travel toward the OB along blood vessels (BVs) that topographically outline the RMS. The molecular and cellular mechanisms leading to the development of the RMS and the formation of the migration-promoting vasculature scaffold in the adult mice remain unclear. We now reveal that astrocytes orchestrate the formation and structural reorganization of the vasculature scaffold in the RMS and, during early developmental stages, the RMS contains only a few BVs oriented randomly with respect to the migrating neuroblasts. The first parallel BVs appeared at the outer border of the RMS, where vascular endothelial growth factor (VEGF)-expressing astrocytes are located. Gain-of-function and loss-of-function experiments revealed that astrocyte-derived VEGF plays a crucial role in the formation and growth of new BVs. Real-time videoimaging also showed that the migration of neuronal precursors in the developing RMS differs substantially from neuronal displacement in the adult migratory stream partially because of not yet fully developed vasculature scaffold. The downregulation of VEGFin vivo, specifically in the astrocytes of the developing RMS, affected the development of the vasculature scaffold and led to alterations in neuroblast migration. Altogether, our results demonstrate that astrocytes orchestrate the formation and growth of parallel BVs, crucial migration-promoting scaffolds in the adult migratory stream, via VEGF signaling.

The olfactory bulb (OB) retains a remarkable capacity to renew its interneuronal populations throughout the lifespan of animals. Neuronal precursors giving rise to the bulbar interneurons are generated in the subventricular zone and have to migrate long distances before reaching the OB. In the adult OB these neuronal precursors differentiate into distinct neuronal types, including GABAergic cells located in the granule cell layer and a diverse set of neurons in the glomerular layer comprising GABAergic and dopaminergic interneurons, as well as other neuronal subtypes expressing calretinin and calbindin. While the role of sensory activity in the integration and/or survival of newly generated cells in the olfactory system is well established, very little is known about how odorant-induced activity affects fate specification of newborn cells as well as survival and fate maintenance of preexisting neuronal populations generated in adulthood. The present study demonstrates that sensory deprivation diminishes not only the number of newborn cells in the OB, but also reduces the density of granule and periglomerular cells generated before nostril occlusion. It also shows that sensory activity has an important influence on the development and expression of dopaminergic, but not GABAergic, calretinin or calbindin phenotypes. Our data reveal that odorant-induced activity is important for the survival of both newborn and preexisting OB interneurons generated at adulthood and suggests that these chemospecific populations are differentially affected by sensory deprivation.

Neurogenesis in the adult brain occurs predominantly in the two regions, the subventricular zone (SVZ) bordering the lateral ventricle and subgranular zone (SGZ) of the hippocampus. The neuronal precursors are produced in the specialized microenvironment called neurovasculature niche. Recent evidences indicate that in addition to neurogenesis promoting environment, vasculature also serves as a substrate for migration for these newly generated cells. Importantly, under some pathological condition, including stroke, neurogenesis is enhanced in the adult brain. Newly generated neuronal precursors migrate to the sites of injury along the blood vessels and try to integrate to the damaged brain circuitry. This self-healing capacity of the adult brain is, however, insufficient to produce a noticeable amelioration in the affected neuronal network since only a tiny proportion of cells succeed to integrate and survive. Here we review the mechanisms of neuronal recruitment into the post-stroke regions with particular attention to the guidance of neuronal precursors along the blood vessels. We also outline some of the molecular factors that have been used or have a potential to be employed to improve the cell recruitment into the sites of injury.

Adult neuronal precursors retain the remarkable capacity to migrate long distances from the posterior (subventricular zone) to the most anterior [olfactory bulb (OB)] parts of the brain. The knowledge about the mechanisms that keep neuronal precursors in the migratory stream and organize this long-distance migration is incomplete. Here we show that blood vessels precisely outline the migratory stream for new neurons in the adult mammalian forebrain. Real-time video imaging of cell migration in the acute slices demonstrate that neuronal precursors are retained in the migratory stream and guided into the OB by blood vessels that serve as a physical substrate for migrating neuroblasts. Our data suggest that endothelial cells of blood vessels synthesize brain-derived neurotrophic factor (BDNF) that fosters neuronal migration via p75NTR expressed on neuroblasts. Interestingly, GABA released from neuroblasts induces Ca2+-dependent insertion of high-affinity TrkB receptors on the plasma membrane of astrocytes that trap extracellular BDNF. We hypothesize that this renders BDNF unavailable for p75NTR-expressing migrating cells and leads to their entrance into the stationary period. Our findings provide new insights into the functional organization of substrates that facilitate the long-distance journey of adult neuronal precursors.

The proper development and functioning of the vertebrate brain depends on the correct positioning of neuronal precursors which is achieved by the widespread and far-ranging migration of cells from their birthplaces. The vast majority of neuronal precursors use cellular substrates for their migration. Until very recently, it was assumed that these cellular substrates were either glial (glia-mediated or gliophilic migration) or neuronal (neuron-mediated or neurophilic migration) in nature. The widely studied examples of gliophilic and neurophilic migrations in the developing brain are displacement of neuronal precursors along the processes of radial glia in the developing cortex and migration of neurons expressing gonadotropin-releasing hormone (GnRH) along the vomeronasal axons, respectively. Recent data indicate, however, that neuronal precursors might also use blood vessels as a physical substrate for their migration. The vasculature-guided (vasophilic) migration of neuronal precursors has been observed not only under normal conditions, in the healthy brain, but also following strokes. The purpose of this review is to highlight emerging principles and delineate putative mechanisms of vasculature-guided neuronal migration under both normal and pathological conditions.

Distinct olfactory bulb (OB) interneurons are thought to become specified depending on from which of the different subregions lining the lateral ventricle wall they originate, but the role of region-specific transcription factors (TFs) in the generation of OB interneurons diversity is still poorly understood. Despite the crucial roles of the Dlx family of TFs for patterning and neurogenesis in the ventral telencephalon during embryonic development, their role in adult neurogenesis has not yet been addressed. Here we show that in the adult brain, Dlx 1 and Dlx2 are expressed in progenitors of the lateral but not the dorsal subependymal zone (SEZ), thus exhibiting a striking regional specificity. Using retroviral vectors to examine the function of Dlx2 in a cell-autonomous manner, we demonstrate that this TF is necessary for neurogenesis of virtually all OB interneurons arising from the lateral SEZ. Beyond its function in generic neurogenesis, Dlx2 also plays a crucial role in neuronal subtype specification in the OB, promoting specification of adult-born periglomerular neurons (PGNs) toward a dopaminergic fate. Strikingly, Dlx2 requires interaction with Pax6, because Pax6 deletion blocks Dlx2-mediated PGN specification. Thus, Dlx2 wields a dual function by first instructing generic neurogenesis from adult precursors and subsequently specifying PGN subtypes in conjunction with Pax6.

International audience; Identifying and defining the characteristic features of the inhibitory neurons in the nervous system has become essential for achieving a cellular understanding of complex brain activities. For this, the olfactory bulb is ideally suited because it is readily accessible, it is a laminated structure where local interneurons can be easily distinguished from projecting neurons, and, more important, GABAergic interneurons are continuously replaced. How the newly generated neurons integrate into a preexisting neural network and how basic network functions are maintained when a large percentage of neurons are subjected to continuous renewal are important questions that have recently received new insights. Here, it is seen that the production of bulbar interneurons is specifically adapted to experience-dependent regulation of adult neural networks. In particular, the authors report the degree of sensitivity of the bulbar neurogenesis to the activity level of sensory inputs and, in turn, how the adult neurogenesis adjusts the neural network functioning to optimize information processing. By maintaining a constitutive neurogenesis sensitive to environmental cues, this neuronal recruitment leads to improving sensory abilities. This review brings together recently described properties and emerging principles of interneuron functions that may convey, into bulbar neuronal networks, a degree of circuit adaptation unmatched by synaptic plasticity alone.

International audience; The first-order sensory relay for olfactory processing, the main olfactory bulb (MOB), retains the ability to acquire new interneurons throughout life. It is therefore a particularly appropriate region for studying the role of experience in sculpting neuronal networks. We found that nostril closure decreased the number of newborn granule cells in the MOB, the complexity of their dendritic arborization, and their spine density, without affecting the preexisting population of granule cells. Accordingly, the frequency of miniature synaptic inhibitory events received by mitral cells was reduced. However, due to a compensatory increase in newborn granule cell excitability, action potential-dependent GABA release was dramatically enhanced, thus counteracting the reduction in spine density and leading to an unaltered synchronization of mitral cell firing activity. Together, this study reveals a unique form of adaptive response brought about exclusively by the cohort of newborn cells and used to maintain normal functioning of the MOB.

Cholinergic axons and nicotinic receptors are abundant in all layers of the olfactory bulb (OB), the main region of newborn neuron integration in the adult brain. Here, we report that the OB granule cell layer in mice lacking the predominant form of brain high-affinity nicotinic acetylcholine receptors (β 2 -/- mice) displayed nearly 50% more newborn neurons and significantly fewer apoptotic cells than did β 2 +/+ mice. Conversely, in vivo chronic nicotine exposure significantly decreased the number of newborn granule cells in β 2 +/+ but not β 2 -/- adult mice, confirming that the survival of newborn neurons can be controlled by the activation of β 2 -containing nicotinic acetylcholine receptors. Unexpectedly, investigating the behavioral consequence of an increased number of granule cells in β 2 -/- mice revealed that these animals have a less robust short-term olfactory memory than their wild-type counterparts. Taken together, these results provide evidence that high-affinity nicotinic receptors are involved in the maturation of adult OB local circuits. They also indicate that an increase in the number of granule cells does not necessarily correlate with better olfactory performance and further highlight the importance of cholinergic afferents for olfactory processing.

International audience; Neuroblasts arising in the adult forebrain that travel to the olfactory bulb use two modes of migration: tangentially, along the rostral migratory stream, and radially, in the core of the olfactory bulb where they start to ascend to the outer layers. Although the mechanisms of tangential migration have been extensively studied, the factors controlling radial migration remain unexplored. Here we report that the extracellular matrix glycoprotein tenascin-R, expressed in the adult mouse olfactory bulb, initiates both the detachment of neuroblasts from chains and their radial migration. Expression of tenascin-R is activity dependent, as it is markedly reduced by odor deprivation. Furthermore, grafting of tenascin-R-transfected cells into non-neurogenic regions reroutes migrating neuroblasts toward these regions. The identification of an extracellular microenvironment capable of directing migrating neuroblasts provides insights into the mechanisms regulating radial migration in the adult olfactory bulb and offers promising therapeutic venues for brain repair.

Over the past few decades, research exploring how the brain perceives, discriminates, and recognizes odorant molecules has received a growing interest. Today, olfaction is no longer considered a matter of poetry. Chemical senses entered the biological era when an increasing number of scientists started to elucidate the early stages of the olfactory pathway. A combination of genetic, biochemical, cellular, electrophysiological and behavioral methods has provided a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. Our group is exploring the physiology of the main olfactory bulb, the first processing relay in the mammalian brain. From different electrophysiological approaches, we are attempting to understand the cellular rules that contribute to the synaptic transmission and plasticity at this central relay. How olfactory sensory inputs, originating from the olfactory epithelium located in the nasal cavity, are encoded in the main olfactory bulb remains a crucial question for understanding odor processing. More importantly, the persistence of a high level of neurogenesis continuously supplying the adult olfactory bulb with newborn local neurons provides an attractive model to investigate how basic olfactory functions are maintained when a large proportion of local neurons are continuously renewed. For this purpose, we summarize the current ideas concerning the molecular mechanisms and organizational strategies used by the olfactory system to encode and process information in the main olfactory bulb. We discuss the degree of sensitivity of the bulbar neuronal network activity to the persistence of this high level of neurogenesis that is modulated by sensory experience. Finally, it is worth mentioning that analyzing the molecular mechanisms and organizational strategies used by the olfactory system to transduce, encode, and process odorant information in the olfactory bulb should aid in understanding the general neural mechanisms involved in both sensory perception and memory. Due to space constraints, this review focuses exclusively on the olfactory systems of vertebrates and primarily those of mammals.

Cell adhesion molecules have been implicated in neural development and hippocampal synaptic plasticity. Here, we investigated the role of the neural cell adhesion molecule L1 in regulation of basal synaptic transmission and plasticity in the CA1 area of the hippocampus of juvenile mice. We show that theta-burst stimulation (TBS) and pairing of low-frequency presynaptic stimulation with depolarization of postsynaptic CA1 pyramidal cells induced similar levels of LTP in L1-deficient and wild-type mice. The basal excitatory synaptic transmission and density of asymmetric excitatory synapses in the stratum radiatum were also normal in L1-deficient mice. Since L1 is expressed not only by principal cells but also by inhibitory interneurons, we recorded inhibitory postsynaptic currents (IPSCs) evoked in CA1 pyramidal cells by minimal stimulation of perisomatic interneurons. L1-deficient mice showed a reduction in the mean amplitude of putative unitary IPSCs, higher values of the coefficient of amplitude variation, higher number of failures in transmitter release, and a reduction in frequency but not amplitude of miniature IPSCs. The use-dependent modulation of inhibitory transmission by paired-pulse or short tetanic stimulation was, however, normal in L1-deficient mice. The physiological abnormalities correlated with a strong reduction in the density of inhibitory active zones, indicating that L1 is involved in establishing inhibitory perisomatic synapses in the hippocampus.

The role of the extracellular matrix molecule tenascin-R (TN-R) in regulation of synaptic transmission and plasticity in the CA1 region of the hippocampus was studied using mice deficient in expression of this molecule. The mutant mice showed normal NMDA-receptor-mediated currents but an impaired NMDA-receptor-dependent form of long-term potentiation (LTP) as compared to wild-type littermates. Reduced LTP in mutants was accompanied by increased basal excitatory synaptic transmission in synapses formed on CA1 pyramidal neurons. A possible mechanism for increased excitatory synaptic transmission in mutants could involve modulation of inhibition, since TN-R and its associated carbohydrate HNK-1 decorate perisomatic interneurons. Indeed, the amplitudes of unitary perisomatic inhibitory currents were smaller in mutants compared to wild-type mice. Thus, our data show that a deficit in TN-R results in reduction of perisomatic inhibition and, as a consequence, in an increase of excitatory synaptic transmission in CA1 to the levels close to saturation, impeding further expression of LTP.