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Ependymal cells reside in the adult spinal cord and display stem cell properties in vitro. They proliferate after spinal cord injury and produce neurons in lower vertebrates but predominantly astrocytes in mammals. The mechanisms underlying this glial-biased differentiation remain ill-defined. We addressed this issue by generating a molecular resource through RNA profiling of ependymal cells before and after injury. We found that these cells activate STAT3 and ERK/MAPK signaling post injury and downregulate cilia-associated genes and FOXJ1, a central transcription factor in ciliogenesis. Conversely, they upregulate 510 genes, seven of them more than 20-fold, namely Crym, Ecm1, Ifi202b, Nupr1, Rbp1, Thbs2 and Osmr—the receptor for oncostatin, a microglia-specific cytokine which too is strongly upregulated after injury. We studied the regulation and role of Osmr using neurospheres derived from the adult spinal cord. We found that oncostatin induced strong Osmr and p-STAT3 expression in these cells which is associated with reduction of proliferation and promotion of astrocytic versus oligodendrocytic differentiation. Microglial cells are apposed to ependymal cells in vivo and co-culture experiments showed that these cells upregulate Osmr in neurosphere cultures. Collectively, these results support the notion that microglial cells and Osmr/Oncostatin pathway may regulate the astrocytic fate of ependymal cells in spinal cord injury.

Atr is a serine/threonine kinase, known to sense single-stranded DNA breaks and activate the DNA damage checkpoint by phosphorylating Chek1, which inhibits Cdc25, causing cell cycle arrest. This pathway has not been implicated in neuroregeneration. We show that in Drosophila sensory neurons removing Atr or Chek1, or overexpressing Cdc25 promotes regeneration, whereas Atr or Chek1 overexpression, or Cdc25 knockdown impedes regeneration. Inhibiting the Atr-associated checkpoint complex in neurons promotes regeneration and improves synapse/behavioral recovery after CNS injury. Independent of DNA damage, Atr responds to the mechanical stimulus elicited during regeneration, via the mechanosensitive ion channel Piezo and its downstream NO signaling. Sensory neuron-specific knockout of Atr in adult mice, or pharmacological inhibition of Atr-Chek1 in mammalian neurons in vitro and in flies in vivo enhances regeneration. Our findings reveal the Piezo-Atr-Chek1-Cdc25 axis as an evolutionarily conserved inhibitory mechanism for regeneration, and identify potential therapeutic targets for treating nervous system trauma., The Atr-Check1 pathway is involved in cell cycle and the DNA damage response. Here, the authors show that the Atr-Check1 pathway can inhibit axon regeneration in response to Piezo-mediated mechanosensation, affecting functional recovery.

International audience; Spinal cord injuries (SCI) are disastrous neuropathologies causing permanent disabilities. The availability of different strains of mice is valuable for studying the pathophysiological mechanisms involved in SCI. However, strain differences have a profound effect on spontaneous functional recovery after SCI. CX3CR1+/eGFP and Aldh1l1-EGFP mice that express green fluorescent protein in microglia/monocytes and astrocytes, respectively, are particularly useful to study glial reactivity. Whereas CX3CR1+/eGFP mice have C57BL/6 background, Aldh1l1-EGFP are in Swiss Webster background. We first assessed spontaneous functional recovery in CX3CR1+/eGFP and Aldh1l1-EGFP mice over 6 weeks after lateral spinal cord hemisection. Second, we carried out a longitudinal follow-up of lesion evolution using in vivo T2-weighted magnetic resonance imaging (MRI). Finally, we performed in-depth analysis of the spinal cord tissue using ex vivo T2-weighted MRI as well as detailed histology. We demonstrate that CX3CR1+/eGFP mice have improved functional recovery and reduced anxiety after SCI compared with Aldh1l1-EGFP mice. We also found a strong correlation between in vivo MRI, ex vivo MRI, and histological analyses of the injured spinal cord in both strain of mice. All three modalities revealed no difference in lesion extension and volume between the two strains of mice. Importantly, histopathological analysis identified decreased gliosis and increased serotonergic axons in CX3CR1+/eGFP compared with Aldh1l1-EGFP mice following SCI. These results thus suggest that the strain-dependent improved functional recovery after SCI may be linked with reduced gliosis and increased serotonergic innervation.

Axons in the mature central nervous system (CNS) fail to regenerate after axotomy, partly due to the inhibitory environment constituted by reactive glial cells producing astrocytic scars, chondroitin sulfate proteoglycans, and myelin debris. We investigated this inhibitory milieu, showing that it is reversible and depends on glial metabolic status. We show that glia can be reprogrammed to promote morphological and functional regeneration after CNS injury in Drosophila via increased glycolysis. This enhancement is mediated by the glia derived metabolites: L-lactate and L-2-hydroxyglutarate (L-2HG). Genetically/pharmacologically increasing or reducing their bioactivity promoted or impeded CNS axon regeneration. L-lactate and L-2HG from glia acted on neuronal metabotropic GABAB receptors to boost cAMP signaling. Local application of L-lactate to injured spinal cord promoted corticospinal tract axon regeneration, leading to behavioral recovery in adult mice. Our findings revealed a metabolic switch to circumvent the inhibition of glia while amplifying their beneficial effects for treating CNS injuries.

Spinal cord injury (SCI) is a tragic event causing irreversible losses of sensory, motor, and autonomic functions, that may also be associated with chronic neuropathic pain. Serotonin (5-HT) neurotransmission in the spinal cord is critical for modulating sensory, motor, and autonomic functions. Following SCI, 5-HT axons caudal to the lesion site degenerate, and the degree of axonal degeneration positively correlates with lesion severity. Rostral to the lesion, 5-HT axons sprout, irrespective of the severity of the injury. Unlike callosal fibers and cholinergic projections, 5-HT axons are more resistant to an inhibitory milieu and undergo active sprouting and regeneration after central nervous system (CNS) traumatism. Numerous studies suggest that a chronic increase in serotonergic neurotransmission promotes 5-HT axon sprouting in the intact CNS. Moreover, recent studies in invertebrates suggest that 5-HT has a pro-regenerative role in injured axons. Here we present a brief description of 5-HT discovery, 5-HT innervation of the CNS, and physiological functions of 5-HT in the spinal cord, including its role in controlling bladder function. We then present a comprehensive overview of changes in serotonergic axons after CNS damage, and discuss their plasticity upon altered 5-HT neurotransmitter levels. Subsequently, we provide an in-depth review of therapeutic approaches targeting 5-HT neurotransmission, as well as other pre-clinical strategies to promote an increase in re-growth of 5-HT axons, and their functional consequences in SCI animal models. Finally, we highlight recent findings signifying the direct role of 5-HT in axon regeneration and suggest strategies to further promote robust long-distance re-growth of 5-HT axons across the lesion site and eventually achieve functional recovery following SCI.

Nuclear magnetic resonance imaging (NMRI) is a powerful tool for biological investigations. Nevertheless, the imaging resolution performance results in the combination of the magnetic field (B0) and the antenna efficiency. This latter one results in a compromise between the size of the sample, the location of the region of interest and the homogeneity requirement. In the context of spinal cord imaging on mice, a ribbon solenoid coil is used to enhance the efficiency of the MRI experiment. This paper details the calculation of the local magnetization contribution to the induced voltage of MRI coils. The modeling is illustrated on ribbon solenoid antennas used in emitter–receiver mode for the study. The analytical model, which takes into account the emitting mode, the receiving step and the imaging sequence, is compared to the measurement performed on a 9.4 T VARIAN MRI apparatus. The efficiency of the antenna, in terms of signal to noise ratio, is significantly enhanced with respect to a commercial quadrature volumic antenna, given a significant advantage for the study of spinal cord injuries.

Spinal cord injuries (SCI) lead to major disabilities affecting > 2.5 million people worldwide. Major shortcomings in clinical translation result from multiple factors, including species differences, development of moderately predictive animal models, and differences in methodologies between preclinical and clinical studies. To overcome these obstacles, we first conducted a comparative neuroanatomical analysis of the spinal cord between mice, Microcebus murinus (a nonhuman primate), and humans. Next, we developed and characterized a new model of lateral spinal cord hemisection in M. murinus. Over a 3-month period after SCI, we carried out a detailed, longitudinal, behavioral follow-up associated with in vivo magnetic resonance imaging ((1)H-MRI) monitoring. Then, we compared lesion extension and tissue alteration using 3 methods: in vivo (1)H-MRI, ex vivo (1)H-MRI, and classical histology. The general organization and glial cell distribution/morphology in the spinal cord of M. murinus closely resembles that of humans. Animals assessed at different stages following lateral hemisection of the spinal cord presented specific motor deficits and spinal cord tissue alterations. We also found a close correlation between (1)H-MRI signal and microglia reactivity and/or associated post-trauma phenomena. Spinal cord hemisection in M. murinus provides a reliable new nonhuman primate model that can be used to promote translational research on SCI and represents a novel and more affordable alternative to larger primates. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s13311-017-0589-9) contains supplementary material, which is available to authorized users.

Alzheimer's disease (AD) is one of the major neurodegenerative diseases that deteriorates cognitive functions and primarily affects associated brain regions involved in learning and memory, such as the neocortex and the hippocampus. Following the discovery and establishment of its role as a neurotransmitter, serotonin (5-HT), was found to be involved in a multitude of neurophysiological processes including mnesic function, through its dedicated pathways and interaction with cholinergic, glutamatergic, GABAergic and dopaminergic transmission systems. Abnormal 5-HT neurotransmission contributes to the deterioration of cognitive processes in ageing, AD and other neuropathologies, including schizophrenia, stress, mood disorders and depression. Numerous studies have confirmed the pathophysiological role of the 5-HT system in AD and that several drugs enhancing 5-HT neurotransmission are effective in treating the AD-related cognitive and behavioural deficits. Here we present a comprehensive overview of the role of serotonergic neurotransmission in brain development, maturation and ageing, discuss its role in higher brain function and provide an in depth account of pathological modifications of serotonergic transmission in neurological diseases and AD.

Alzheimer's disease (AD) is a progressive neurodegenerative disease that impairs mnesic functions. The histopathology of the disease is manifested by the accumulation of intracellular β-amyloid (Aβ) and subsequent formation of neuritic plaques as well as the presence neurofibrillary tangles in specific brain regions associated with learning and memory including the hippocampus. Here, we analysed the effect of chronic (1 month) food diets containing low (LTrP), normal (NTrP) and high tryptophan (HTrP), 0.04, 0.20 and 0.40 g/100 g, respectively, on CA1 serotonin transporter (SERT) fibre density, intraneuronal Aβ deposition and total number of serotonergic (5-HT) neurons in an AD triple transgenic (3xTg-AD) mouse model. Nontransgenic (non-Tg) animals fed with HTrP displayed increased SERT fibre density in CA1 (35%) and in stratum lacunosum moleculare (S.Mol) (48%) compared to LTrP diet. Transgenic animals showed increased SERT fibre density in CA1 S.Mol compared to diet-matched non-Tg irrespective of dietary tryptophan content (104% for LTrP, 74% for NTrP and 35% for HTrP); no differences were observed in the total number of 5-HT neurons neither in the dorsal nor in the median raphe nuclei. However, and more relevant to AD, HTrP diet reduced intraneuronal Aβ density (by a 17%) in transgenic animals compared to transgenic animals fed with NTrP diet. Our results show that increased dietary TrP intake reduces intraneuronal Aβ load in the 3xTg-AD mouse model of AD, suggesting that enhanced TrP intake and in consequence a potential increase in 5-HT neurotransmission may be effective in reducing plaque pathology in AD.

International audience; Spinal cord injury (SCI) triggers pronounced astrocyte reactivity (astrogliosis) including astroglial proliferation and migration toward the injury site participating to the formation of a glial scar. Since the mid-20 th century, SCI-induced astrogliosis was mainly regarded as detrimental for successful axonal regeneration. However, more recent studies have shown astrogliosis as a multifactorial phenomenon involving specific morphological, molecular and functional alterations in astrocytes that can also exert beneficial effects [1, 2]. It was suggested, although not proven, that SCI-induced astrogliosis depends on multiple factors such as time after lesion, injury severity and distance to the lesion site. In a recent study we had attempted to uncover the molecular involvement of astrocytes after SCI by studying their transcriptomic alterations at different stages after moderate and severe lesions [3]. Aldehyde dehydrogenase 1 family member L1 (Aldh1l1) is a pan-astrocytic marker, hence using the Aldh1l1-EGFP transgenic mice, combined with fluorescence-activated cell sorting (FACS), we isolated pure astrocyte population at different stages following SCI. Choosing lateral hemisection and complete section of the spinal cord, as moderate and severe injury models, we investigated astrocytic response at 1 and 2 weeks after lesion. We subsequently carried out astrocyte-specific RNA-sequencing and pathway analyses to unveil the molecular signature of injuries-induced astrogliosis. Our transcriptomic analyses demonstrated a dual astrocytic response depending on time post-injury and lesion severity. Following moderate SCI, astrocytes displayed a protective role and showed no changes (1 week) and even down-regulated (2 weeks) expression of transcripts involved in immune response. On the other hand, astrocytes response after severe SCI seems to be detrimental by an upsurge expression of inflammatory genes (1 week) and prevention of extracellular re-modeling (2 weeks) (3). These are the first concrete evidence of a heterogeneous astrocytic response that is driven not only by lesion severity but also time after injury (Figure 1). In parallel, using pathway analyses, we also identified in astrocytes the induction of the neural stem cell lineage and the over-expression of the neuronal progenitor gene βIII-tubulin (Tubb3, also known as TUJ1). We confirmed βIII-tubulin protein expression at tissue level using immunohistochemistry and at single cell level using FACS analyses. The sub-population of astrocytes that express βIII-tubulin was only found within 750µm distance to the lesion epicenter. Astrocytes co-expressing βIII-tubulin, also displayed alterations in their morphology from typical stellate shape to classical neuronal progenitor cells with bipolar or multipolar processes. Given that SCI induces astrocytic proliferation, we injected BrdU in Aldh1l1-EGFP mice after injury to determine the origin of eGFP/βIII-tubulin co-expressing cells. BrdU incorporation was observed into newly formed astrocytes but not in eGFP/βIII-tubulin-expressing astrocytes. This suggests that it is the resident mature astrocytes, rather than newly formed astrocytes, that undergo transdifferentiation towards neuronal lineage (Figure 1). Time-dependent analyses revealed that astrocytic conversion towards neuronal lineage starts as early as 72 hours, peaking between 1-2 weeks and continues to a lower degree up to 6 weeks after both moderate and severe SCI. Further immunostaining, using mature neuronal markers, showed that transdifferentiating astrocytes eventually express GABAergic, but not glutamatergic, markers. Moreover, we identified the fibroblast growth factor receptor 4 (Fgfr4) as a potential player responsible for SCI-induced astrocytic transdifferentiation towards neuronal lineage. Fgfr4 indeed promotes embryonic stem cell differentiation towards neuronal lineage [4] and showed pronounced over-expression from 72 hours following lesion at both RNA and protein level. Although other recent studies had shown limited astrocytes conversion towards neuronal lineage upon enforced expression of neurogenic factors, none had Editorial Figure 1: Schematic cartoon displaying summary of astrocytic responses following SCI

Neural induction is the first step in the formation of the vertebrate central nervous system. The emerging consensus of the mechanisms underling neural induction is the combined influences from inhibiting bone morphogenetic protein (BMP) signaling and activating fibroblast growth factor (FGF)/Erk signaling, which act extrinsically via either autocrine or paracrine fashions. However, do intrinsic forces (cues) exist and do they play decisive roles in neural induction? These questions remain to be answered. Here, we have identified a novel neural initiator, neuronatin (Nnat), which acts as an intrinsic factor to promote neural fate in mammals and Xenopus. ESCs lacking this intrinsic factor fail to undergo neural induction despite the inhibition of the BMP pathway. We show that Nnat initiates neural induction in ESCs through increasing intracellular Ca2+ ([Ca2+]i) by antagonizing Ca2+-ATPase isoform 2 (sarco/endoplasmic reticulum Ca2+-ATPase isoform 2) in the endoplasmic reticulum, which in turn increases the phosphorylation of Erk1/2 and inhibits the BMP4 pathway and leads to neural induction in conjunction with FGF/Erk pathway.

Alzheimer’s disease (AD) is an untreatable neurodegenerative disease that deteriorates memory. Increased physical/cognitive activity reduces dementia risk by promoting neuronal and glial response. Although few studies have investigated microglial response in wild-type rodents following exposure to physical/cognitive stimulation, environmental-induced changes of microglia response to AD have been neglected. We investigated effects of running (RUN) and enriched (ENR) environments on numerical density (N v, #/mm3) and morphology of microglia in a triple transgenic (3×Tg-AD) mouse model of AD that closely mimics AD pathology in humans. We used immunohistochemical approach to characterise microglial domain by measuring their overall cell surface, volume and somata volume. 3×Tg-AD mice housed in standard control (STD) environment showed significant increase in microglial N v (11.7 %) in CA1 stratum lacunosum moleculare (S.Mol) of the hippocampus at 12 months compared to non-transgenic (non-Tg) animals. Exposure to combined RUN and ENR environments prevented an increase in microglial N v in 3×Tg-AD and reduced microglial numbers to non-Tg control levels. Interestingly, 3×Tg-AD mice housed solely in ENR environment displayed significant decrease in microglial N v in CA1 subfield (9.3 % decrease), stratum oriens (11.5 % decrease) and S.Mol (7.6 % decrease) of the hippocampus compared to 3×Tg-AD mice housed in STD environment. Morphological analysis revealed microglial hypertrophy due to pronounced increase in microglia surface, volume and somata volume (61, 78 and 41 %) in 3×Tg-AD mice housed in RUN (but not in ENR) compared to STD environment. These results indicate that exposure to RUN and ENR environments have differential effects on microglial density and activation-associated changes in microglial morphology.

Alzheimer's disease (AD) is an irreversible neurodegenerative disease that is characterised by the presence of β-amyloid (Aβ) plaques, neurofibrillary tangles (NFTs) and synaptic loss specifically in brain regions involved in learning and memory such as the neocortex and the hippocampus. Aβ depositions in the form of neuritic plaques trigger activation of microglia that is believed to be a common neuropathological feature of AD brains. As an integral part of the hippocampus, the dentate gyrus (DG) plays an important role in cognitive function. Although post-mortem studies suggest later involvement of the DG into the AD progression, changes in microglia have not been studied in this subfield of the hippocampus. In the present study the numerical density (Nv, #/mm(3)) of both resting (identified by tomato lectin staining) and activated (identified by Mac-1 immunoreactivity) microglia was analysed in the molecular layer (ML) of the DG in the triple transgenic (3xTg-AD) mouse model of AD at different ages (9, 12 and 18 months). The 3xTg-AD mouse model of AD showed a significant increase in the Nv of resting (by 75%) and activated (by 67%) at 18 months of age compared to non-Tg controls. These results indicate a complex microglial remodelling during AD progression.

Alzheimer's disease (AD) is a neurodegenerative pathology that deteriorates mnesic functions and associated brain regions including the hippocampus. Serotonin (5-HT) has an important role in cognition. We recently demonstrated an increase in 5-HT transporter (SERT) fibre density in the hippocampal CA1 in an AD triple transgenic mouse model (3xTg-AD). Here, we analyse the ultrastructural localisation, distribution and numerical density (N(v)) of hippocampal SERT axons (SERT-Ax) and terminals (SERT-Te) and their relationship with SERT fibre sprouting and altered synaptic N(v) in 3xTg-AD compared with non-transgenic control mice. 3xTg-AD animals showed a significant increase in SERT-Te N(v) in CA1 at both, 3 (95%) and 18 months of age (144%), being restricted to the CA1 stratum moleculare (S. Mol; 227% at 3 and 180% at 18 months). 3xTg-AD animals also exhibit reduced N(v) of perforated axospinous synapses (PS) in CA1 S. Mol (56% at 3 and 52% at 18 months). No changes were observed in the N(v) of symmetric and asymmetrical synapses or SERT-Ax. Our results suggest that concomitant SERT-Te N(v) increase and PS reduction in 3xTg-AD mice may act as a compensatory mechanism maintaining synaptic efficacy as a response to the AD cognitive impairment.

The reticular nucleus (RT) of the thalamus, a thin sheet of GABAer- gic neurons located between the external medullary lamina and the internal capsule of the thalamus, has functionally distinct afferent and efferent connections with tha- lamic nuclei, the neocortex, the basal forebrain and the brainstem. RT is critically positioned to rhythmically pace thalamocortical networks leading to the generation of spindle activity during the early phases of sleep and during absence (spike-wave) seiz- ures. Serotonin, acting on 5-HT1A receptors on parvalbumin-containing cells of RT, has been implicated in this rhythmicity. However, the precise source(s) of 5-HT affer- ents to the RT remains to be determined. In the present study, we injected the retro- grade tracer, Fluorogold, into dorsal and ventral regions of RT to determine the ori- gins of raphe input to RT. We further characterized the distribution of 5-HT fibers to RT by using immunohistochemistry for 5-HT and for the 5HT transporter (SERT) detection. Finally, we described the presence of the two major postsynaptic 5-HT receptors in RT, 5-HT1A and 5-HT2A receptors. Our results show that the dorsal raphe nucleus and the supralemniscal nucleus (B9) of the midbrain are the principal sources of raphe projections to RT. In addition, serotonergic fibers (5-HT and SERT positive) were richly distributed throughout RT, and 5-HT1A and 5-HT2A receptors were highly expressed on RT neurons and dendrites. These findings suggest a significant 5-HT modulatory influence on GABAergic neurons of RT in the control of rhythmical (or spindle) activity in thalamocortical systems directly associated with sleep and possibly with absence seizures. Synapse 00:000-000, 2011. V C 2011 Wiley-Liss, Inc.

Alzheimer's disease (AD) affects memory and neurogenesis. Adult neurogenesis plays an important role in memory function and impaired neurogenesis contributes to cognitive deficits associated with AD. Increased physical/ cognitive activity is associated with both reduced risk of dementia and increased neurogenesis. Previous attempts to restore hippocampal neurogenesis in transgenic mice by voluntary running (RUN) and environmental enrichment (ENR) provided controversial results due to lack of non-transgenic (non-Tg) control and inclusion of social isolation as "standard" housing environment. Here, we determine the effect of RUN and ENR upon hippocampal neurogenesis in a triple transgenic (3xTg-AD) mouse model of AD, which mimics AD pathology in humans. We used single and double immunohistochemistry to determine the area density of hippocampal proliferating cells, measured by the presence of phosphorylated Histone H3 (HH3), and their potential neuronal and glial phenotype by co-localizing the proliferating cells with the immature neuronal marker doublecortin (DCX), mature neuronal marker (NeuN) and specific astroglial marker (GFAP). Our results show that 3xTg-AD mice in control environment exhibit impaired hippocampal neurogenesis compared to non-Tg animals at 9 months of age. Exposure to RUN and ENR housing restores hippocampal neurogenesis in 3xTg-AD animals to non-Tg control levels. Differentiation into neurones and glial cells is affected neither by transgenic status nor by housing environment. These results suggest that hippocampus of 3xTg-AD animals maintains the potential for cellular plasticity. Increase in physical activity and/or cognitive experience enhances neurogenesis and provides a potential for stimulation of cognitive function in AD.

The circuitry of the human brain is formed by neuronal networks embedded into astroglial syncytia. The astrocytes perform numerous functions, providing for the overall brain homeostasis, assisting in neurogenesis, determining the micro-architecture of the grey matter, and defending the brain through evolutionary conserved astrogliosis programs. Astroglial cells are engaged in neurological diseases by determining the progression and outcome of neuropathological process. Astrocytes are specifically involved in various neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and various forms of dementia. Recent evidence suggest that early stages of neurodegenerative processes are associated with atrophy of astroglia, which causes disruptions in synaptic connectivity, disbalance in neurotransmitter homeostasis, and neuronal death through increased excitotoxicity. At the later stages, astrocytes become activated and contribute to the neuroinflammatory component of neurodegeneration.

Alzheimer's disease (AD) is a neurodegenerative disease that deteriorates cognitive functions and associated brain regions such as the hippocampus, being the primary cause of dementia. Serotonin (5-HT) is widely present in the hippocampus, being an important neurotransmitter involved in learning and memory. Although recent evidence suggests alterations in 5-HT neurotransmission in AD, it is not clear how hippocampal 5-HT innervation is modified. Here, we studied hippocampal 5-HT innervation by analysing: (i) the expression, density and distribution of 5-HT transporter (SERT)-immunoreactive fibres; (ii) the specific morphological characteristics of serotonergic fibres and their relation to amyloid plaques; and (iii) the total number of 5-HT neurons within the raphe nuclei in triple transgenic mouse model of AD. We used quantitative light microscopy immunohistochemistry comparing transgenic and non-transgenic animals of different ages (3, 6, 9, 12 and 18 months). The transgenic animals showed a significant increase in SERT fibres in the hippocampus in a subfield-, strata- and age-specific manner. The increase in SERT fibres was specific to the CA1 stratum lacunosum-moleculare. An increase in SERT fibres in transgenic animals was observed at 3 months (by 61%) and at 18 months (by 74%). No changes, however, were found in the total number of raphe 5-HT neurons at any age. Our results indicate that triple transgenic mice display changes in the expression of SERT and increased SERT fibres sprouting, which may account for imbalanced serotonergic neurotransmission associated with (or linked to) AD cognitive impairment.

Astrocytes are fundamental for brain homeostasis and are at the fulcrum of neurological diseases including Alzheimer's disease (AD). Here, we monitored changes in astroglia morphology throughout the age-dependent progression of AD. We used an immunohistochemical approach that allows us to determine the domain of glial cytoskeleton, by measuring the surface, volume, and the relationship between astrocytes and neuritic plaques. We investigated astroglia in the hippocampus of a triple transgenic mouse model of AD (3xTg-AD) that mimics the progression of the human disease. The numerical density of astrocytes is affected neither by AD nor by age. We found reduction of surface and volume of GFAP profiles from early ages (6 months; 43.84 and 52.76%, respectively), persisting at 12 (40.73 and 45.39%) and 18 months (64.80 and 71.95%) in the dentate gyrus (DG) of 3xTg-AD, whereas in CA1 it appears at 18 months (29.42 and 32.74%). This cytoskeleton atrophy is accompanied by a significant reduction of glial somata volume in DG at 12 and 18 months (40.46 and 75.55%, respectively), whereas in CA1 it is significant at 18 months (42.81%). However, while astroglial atrophy appears as a generalized process, astrocytes surrounding plaques are clearly hypertrophic as revealed by increased surface (48.06%; 66.66%), and volume (57.10%; 71.06%) of GFAP profiles in DG and CA1, respectively, at 18 months. We suggest differential effects of AD on astroglial populations depending on their association with plaques accounting for the progressive disruption of neural networks connectivity and neurotransmitters imbalance which underlie mnesic and cognitive impairments observed in AD.

The formation of cerebral senile plaques composed of amyloid beta peptide (A beta) is a fundamental feature of Alzheimer's disease (AD). Glial cells and more specifically microglia become reactive in the presence of A beta. In a triple transgenic model of AD (3 x Tg-AD), we found a significant increase in activated microglia at 12 (by 111%) and 18 (by 88%) months of age when compared with non-transgenic (non-Tg) controls. This microglial activation correlated with A beta plaque formation, and the activation in microglia was closely associated with A beta plaques and smaller A beta deposits. We also found a significant increase in the area density of resting microglia in 3 x Tg-AD animals both at plaque-free stage (at 9 months by 105%) and after the development of A plaques (at 12 months by 54% and at 18 months by 131%). Our results show for the first time that the increase in the density of resting microglia precedes both plaque formation and activation of microglia by extracellular A beta accumulation. We suggest that AD pathology triggers a complex microglial reaction: at the initial stages of the disease the number of resting microglia increases, as if in preparation for the ensuing activation in an attempt to fight the extracellular A beta load that is characteristic of the terminal stages of the disease. Cell Death and Disease (2010) 1, e1; doi:10.1038/cddis.2009.2; published online 14 January 2010 Supported by the Alzheimer's Research Trust (UK) Programme Grant ART/PG2004A/1 to AV and JJR; and the Grant Agency of the Czech Republic GACR 309/09/1696 to JJR and GACR 305/08/1384 to AV.