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Dystroglycan (Dag1) is a transmembrane glycoprotein that links the extracellular matrix to the actin cytoskeleton. Mutations inDag1or the genes required for its glycosylation result in dystroglycanopathy, a type of congenital muscular dystrophy characterized by a wide range of phenotypes including muscle weakness, brain defects, and cognitive impairment. We investigated interneuron (IN) development, synaptic function, and associated seizure susceptibility in multiple mouse models that reflect the wide phenotypic range of dystroglycanopathy neuropathology. Mice that model severe dystroglycanopathy due to forebrain deletion ofDag1orPOMT2, which is required for Dystroglycan glycosylation, show significant impairment of CCK+/CB1R+IN development. CCK+/CB1R+IN axons failed to properly target the somatodendritic compartment of pyramidal neurons in the hippocampus, resulting in synaptic defects and increased seizure susceptibility. Mice lacking the intracellular domain of Dystroglycan have milder defects in CCK+/CB1R+IN axon targeting, but exhibit dramatic changes in inhibitory synaptic function, indicating a critical postsynaptic role of this domain. In contrast, CCK+/CB1R+IN synaptic function and seizure susceptibility was normal in mice that model mild dystroglycanopathy due to partially reduced Dystroglycan glycosylation. Collectively, these data show that inhibitory synaptic defects and elevated seizure susceptibility are hallmarks of severe dystroglycanopathy, and show that Dystroglycan plays an important role in organizing functional inhibitory synapse assembly.

Hilar mossy cells regulate network function in the hippocampus through both direct excitation and di-synaptic inhibition of dentate granule cells (DGCs). Substantial mossy cell loss accompanies hippocampal circuit changes in epilepsy. We examined the contribution of surviving mossy cells to network activity in the reorganized dentate gyrus after pilocarpine-induced status epilepticus (SE). To examine functional circuit changes, we optogenetically stimulated mossy cells in acute hippocampal slices from male mice. In control mice, activation of mossy cells produced monosynaptic excitatory and di-synaptic GABAergic currents in DGCs. In pilocarpine-treated mice, mossy cell density and excitation of DGCs were reduced in parallel, with only a minimal reduction in feedforward inhibition, enhancing the inhibition/excitation ratio. Surprisingly, mossy cell-driven excitation of parvalbumin-positive (PV+) basket cells, primary mediators of feed-forward inhibition, was maintained. Our results suggest that mossy cell outputs reorganize following seizures, increasing their net inhibitory effect in the hippocampus.SIGNIFICANCE STATEMENTHilar mossy cell loss in epilepsy is associated with hippocampal hyperexcitability, potentially as a result of disrupted dentate microcircuit function. We used transgenic mice, translational mouse modeling, viral vectors, and optogenetics to selectively examine functional changes to mossy cell outputs following status epilepticus (SE). Interestingly, the outputs of surviving mossy cells exhibited adaptive plasticity onto target parvalbumin-positive (PV+) interneurons, resulting in a relative increase in their inhibitory control of dentate granule cells (DGCs). Our findings suggest that residual mossy cell outputs can reorganize in a homeostatic manner, which may provide clues for therapeutic targeting of this microcircuit.

Protein biomarkers are often measured at hospital presentation to diagnose traumatic brain injury (TBI) and predict patient outcomes. However, a biomarker measurement at this single time point is no more accurate at predicting patient outcomes than less invasive and more cost-effective methods. Here, we review evidence that TBI biomarkers provide greater prognostic value when measured repeatedly over time, such that a trajectory of biomarker concentrations can be evaluated. PubMed, Google Scholar, and Cochrane Central Register were searched to identify studies from the last decade in which established TBI biomarkers had been measured at more than one time point following acute TBI, and which related their findings to patient outcomes. Twenty-two studies were identified, 18 of which focused on adults and 4 of which focused on children. Three general biomarker trajectories were identified: persistently high, persistently low, and reversal of decreasing concentrations. Downtrend reversal was highly specific to predicting poor patient outcomes. Four studies demonstrated that biomarker trajectories can be affected by therapeutic interventions. Additional studies demonstrated that biomarkers measured at a later time point offered superior prognostic value than a single measurement obtained at initial hospital presentation. Among other details, longitudinal biomarker trajectory assessments may identify ongoing injury and predict patient deterioration before clinical symptoms develop and thus help guide therapeutic interventions.

In temporal lobe epilepsy, sprouting of hippocampal mossy fiber axons onto dentate granule cell dendrites creates a recurrent excitatory network. However, unlike mossy fibers projecting to CA3, sprouted mossy fiber synapses depress upon repetitive activation. Thus, despite their proximal location, relatively large presynaptic terminals, and ability to excite target neurons, the impact of sprouted mossy fiber synapses on hippocampal hyperexcitability is unclear. We find that despite their short-term depression, single episodes of sprouted mossy fiber activation in hippocampal slices initiated bursts of recurrent polysynaptic excitation. Consistent with a contribution to network hyperexcitability, optogenetic activation of sprouted mossy fibers reliably triggered action potential firing in postsynaptic dentate granule cells after single light pulses. This pattern resulted in a shift in network recruitment dynamics to an “early detonation” mode and an increased probability of release compared with mossy fiber synapses in CA3. A lack of tonic adenosine-mediated inhibition contributed to the higher probability of glutamate release, thus facilitating reverberant circuit activity.

SummaryHilar mossy cells control network function in the hippocampus through both direct excitation and di-synaptic inhibition of dentate granule cells (DGCs). Substantial mossy cell loss occurs after epileptic seizures; however the contribution of surviving mossy cells to network activity in the reorganized dentate gyrus is unknown. To examine functional circuit changes after pilocarpine-induced status epilepticus, we optogenetically stimulated mossy cells in acute hippocampal slices. In control mice, activation of mossy cells produced monosynaptic excitatory and di-synaptic GABAergic currents in DGCs. In pilocarpine-treated mice, mossy cell density and excitation of DGCs were reduced in parallel, with only a minimal reduction in feedforward inhibition, enhancing the inhibition:excitation ratio. Surprisingly, mossy cell-driven excitation of parvalbumin-positive basket cells, the primary mediators of feed-forward inhibition, was maintained, indicating increased connectivity between surviving mossy cells and these targets. Our results suggest that mossy cell outputs reorganize following seizures, increasing their net inhibitory effect in the hippocampus.

α2δ proteins (CACNA2D1-4) are required for normal neurological function and contribute to membrane trafficking of voltage-gated calcium channels, through which calcium entry initiates numerous physiological processes. However, it remains unclear how α2δ proteins influence calcium-mediated signalling to control neuronal output. Using whole-cell recordings of mouse Purkinje cells, we show that α2δ-2 is required for functional coupling of postsynaptic voltage-dependent calcium entry with calcium-dependent effector mechanisms controlling two different outputs, depolarization-induced suppression of excitation and spike afterhyperpolarization. Our findings indicate an important role for α2δ-2 proteins in regulating functional postsynaptic calcium channel coupling in neurons, providing new context for understanding the effects of α2δ mutations on neuronal circuit function and presenting additional potential avenues to manipulate α2δ-mediated signalling for therapeutic gain. KEY POINTS: Calcium influx, via voltage-dependent calcium channels, drives numerous neuronal signalling processes with precision achieved in part by tight coupling between calcium entry and calcium-dependent effectors. α2δ proteins are important for neurological function and contribute to calcium channel membrane trafficking, although how α2δ proteins influence postsynaptic calcium-dependent signalling is largely unexplored. Here it is shown that loss of α2δ-2 proteins disrupts functional calcium coupling to two different postsynaptic calcium-dependent signals in mouse Purkinje cell neurons, retrograde endocannabinoid signalling and the action potential afterhyperpolarization. The findings provide new insights into the control of calcium coupling as well as new roles for α2δ-2 proteins in neurons.

In the last 12,000 years, human societies have scaled up from small bands to large states of millions and even billions. Many modern societies and even groups of societies cooperate on large-scale projects with relatively low levels of conflict, but the scale and intensity of cooperation varies dramatically between societies. Here we attempt to formalize dynamics that may be driving this rapid increase in cooperation and the differences we see between societies. Our model extends an N-person stag hunt to include population growth dynamics, “stags” with different sized payoffs, and competition for these stags. An increasing number of cooperators is required to access larger stags. The payoff from these stags in turn increases carrying capacity, which increases competition for the stag. As population size increases, new cooperative thresholds are attainable, and as population size shrinks, previously attainable thresholds fall out of reach. Among other predictions, we show that when a new threshold is accessible to a population, the level of cooperation will increase to reach this threshold. However, when the next threshold is out of reach, cooperation decreases as individuals refrain from costly cooperation, preferring a smaller stag. This model offers a framework for understanding the rapid increase in the scale of human cooperation and decline of violence, differences between societies, and challenges to future cooperation.

The Coronavirus Disease 2019 (COVID-19) pandemic has impacted many aspects of neuroscience research. At the 2020 Society of Neuroscience in Anesthesiology and Critical Care (SNACC) Annual Meeting, the SNACC Research Committee met virtually to discuss research challenges encountered during the COVID-19 pandemic along with possible strategies for facilitating research activities. These challenges and recommendations are included in this consensus statement. The objectives are to: (1) provide an overview of the disruptions and challenges to neuroscience research caused by the COVID-19 pandemic, and; (2) put forth a set of consensus recommendations for strengthening research sustainability during and beyond the current pandemic. Specific recommendations are highlighted for adapting laboratory and human subject study operations to optimize safety. Complementary research activities are also outlined, for both laboratory and clinical researchers, if specific investigations are impossible due to regulatory or societal changes. The role of virtual platforms is also discussed with respect to fostering new collaborations, scheduling research meetings, and holding conferences such that scientific collaboration and exchange of ideas can continue. Our hope is for these recommendations to serve as a valuable resource for investigators in the neurosciences and other research disciplines for current and future research disruptions.

Coronavirus disease (COVID-19), the disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus, is responsible for a global pandemic characterized by high transmissibility and morbidity. Healthcare workers (HCWs) are at risk of contracting COVID-19, but this risk has been mitigated through the use of personal protective equipment such as N95 Filtering Facepiece Respirators (FFRs). At times the high demand for FFRs has exceeded supply, placing HCWs at increased exposure risk. Effective FFR decontamination of many FFR models using ultraviolet-C germicidal irradiation (UVGI) has been well-described, and could maintain respiratory protection for HCWs in the face of supply line shortages. Here, we detail the construction of an ultraviolet-C germicidal irradiation (UVGI) device using previously existing components available at our institution. We provide data on UV-C dosage delivered with our version of this device, provide information on how users can validate the UV-C dose delivered in similarly constructed systems, and describe a simple, novel methodology to test its germicidal effectiveness using in-house reagents and equipment. As similar components are readily available in many hospitals and industrial facilities, we provide recommendations on the local construction of these systems, as well as guidance and strategies towards successful institutional implementation of FFR decontamination.

Coronavirus disease (COVID-19), the disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus, is responsible for a global pandemic characterized by high transmissibility and morbidity. Healthcare workers (HCWs) are at risk of contracting COVID-19, but this risk has been mitigated through the use of personal protective equipment such as N95 Filtering Facepiece Respirators (FFRs). At times the high demand for FFRs has exceeded supply, placing HCWs at increased exposure risk. Effective FFR decontamination of many FFR models using ultraviolet-C germicidal irradiation (UVGI) has been well-described, and could maintain respiratory protection for HCWs in the face of supply line shortages. Here, we detail the construction of an ultraviolet-C germicidal irradiation (UVGI) device using previously existing components available at our institution. We provide data on UV-C dosage delivered with our version of this device, provide information on how users can validate the UV-C dose delivered in similarly constructed systems, and describe a simple, novel methodology to test its germicidal effectiveness using in-house reagents and equipment. As similar components are readily available in many hospitals and industrial facilities, we provide recommendations on the local construction of these systems, as well as guidance and strategies towards successful institutional implementation of FFR decontamination.

Epileptic seizures potently modulate hippocampal adult neurogenesis, and adult-born dentate granule cells contribute to the pathologic retrograde sprouting of mossy fiber axons, both hallmarks of temporal lobe epilepsy. The characteristics of these sprouted synapses, however, have been largely unexplored, and the specific contribution of adult-born granule cells to functional mossy fiber sprouting is unknown, primarily due to technical barriers in isolating sprouted mossy fiber synapses for analysis. Here, we used DcxCreERT2 transgenic mice to permanently pulse-label age-defined cohorts of granule cells born either before or after pilocarpine-induced status epilepticus (SE). Using optogenetics, we demonstrate that adult-born granule cells born before SE form functional recurrent monosynaptic excitatory connections with other granule cells. Surprisingly, however, although healthy mossy fiber synapses in CA3 are well characterized “detonator” synapses that potently drive postsynaptic cell firing through their profound frequency-dependent facilitation, sprouted mossy fiber synapses from adult-born cells exhibited profound frequency-dependent depression, despite possessing some of the morphological hallmarks of mossy fiber terminals. Mature granule cells also contributed to functional mossy fiber sprouting, but exhibited less synaptic depression. Interestingly, granule cells born shortly after SE did not form functional excitatory synapses, despite robust sprouting. Our results suggest that, although sprouted mossy fibers form recurrent excitatory circuits with some of the morphological characteristics of typical mossy fiber terminals, the functional characteristics of sprouted synapses would limit the contribution of adult-born granule cells to hippocampal hyperexcitability in the epileptic hippocampus. SIGNIFICANCE STATEMENT In the hippocampal dentate gyrus, seizures drive retrograde sprouting of granule cell mossy fiber axons. We directly activated sprouted mossy fiber synapses from adult-born granule cells to study their synaptic properties. We reveal that sprouted synapses from adult-born granule cells have a diminished ability to sustain recurrent excitation in the epileptic hippocampus, which raises questions about the role of sprouting and adult neurogenesis in sustaining seizure-like activity.

α2δ proteins (Cacna2d1-4) are auxiliary subunits of voltage-dependent calcium channels that also drive synapse formation and maturation. Because cerebellar Purkinje cells (PCs) only express one isoform of this family, α2δ-2 (Cacna2d2), we used PCs as a model system to examine roles of α2δ in excitatory synaptic function in a Cacna2d2 knockout mouse. Whole-cell recordings of PCs from acute cerebellar slices revealed altered climbing fiber (CF)-evoked complex spike generation, as well as increased amplitude and faster decay of CF-evoked excitatory postsynaptic currents (EPSCs). CF terminals in the KO were localized more proximally on PC dendrites, as indicated by VGLUT2+ immunoreactive puncta, and computational modeling demonstrated that the increased EPSC amplitude can be partly attributed to the more proximal location of CF terminals. In addition, CFs in KO mice exhibited increased multivesicular transmission, corresponding to greater sustained responses during repetitive stimulation, despite a reduction in the measured probability of release. Electron microscopy demonstrated that mutant CF terminals had twice as many vesicle release sites, providing a morphologic explanation for the enhanced glutamate release. Though KO CFs evoked larger amplitude EPSCs, the charge transfer was the same as wildtype as a result of increased glutamate re-uptake, producing faster decay kinetics. Together, the larger, faster EPSCs in the KO explain the altered complex spike responses, which degrade information transfer from PCs and likely contribute to ataxia in Cacna2d2 KO mice. Our results also illustrate the multidimensional synaptic roles of α2δ proteins.Significance Statementα2δ proteins (Cacna2d1-4) regulate synaptic transmission and synaptogenesis, but co-expression of multiple α2δ isoforms has obscured a clear understanding of how various α2δ proteins control synaptic function. We focused on roles of the α2δ-2 protein (Cacna2d2), whose deletion causes cerebellar ataxia and epilepsy in mice and humans. Because cerebellar Purkinje cells only expresses this single isoform, we studied excitatory climbing fiber synaptic function onto Purkinje cells in Cacna2d2 knockout mice. Using optical and electrophysiological analysis, we provide a detailed description of the changes in Purkinje cells lacking α2δ-2, and provide a comprehensive mechanistic explanation for how functional synaptic phenotypes contribute to the altered cerebellar output.

Mouse models of mesial temporal lobe epilepsy recapitulate aspects of human epilepsy, which is characterized by neuronal network remodeling in the hippocampal dentate gyrus. Observational studies suggest that this remodeling is associated with altered Wnt pathway signaling, although this has not been experimentally examined. We used the well-characterized mouse intrahippocampal kainate model of temporal lobe epilepsy to examine associations between post-seizure hippocampal neurogenesis and altered Wnt signaling. Tissue was analyzed using immunohistochemistry and confocal microscopy, and transcriptome analysis was performed on RNA extracted from anatomically micro-dissected dentate gyri. Seizures increased neurogenesis and dendritic arborization of newborn hippocampal dentate granule cells in peri-ictal regions, and decreased neurogenesis in the ictal zone, 2-weeks after kainate injection. Interestingly, administration of the novel canonical Wnt pathway inhibitor XAV939 daily for 2-weeks after kainate injection further increased dendritic arborization in peri-ictal regions after seizure, without an effect on baseline neurogenesis in control animals. Transcriptome analysis of dentate gyri demonstrated significant canonical Wnt gene dysregulation in kainate-injected mice across all regions for Wnt3, 5a and 9a. Intriguingly, certain Wnt genes demonstrated differential patterns of dysregulation between the ictal and peri-ictal zones, most notably Wnt5B, 7B and DKK-1. Together, these results demonstrate regional variation in Wnt pathway dysregulation in the early post-ictal period, and surprisingly, suggest that some Wnt-mediated effects might actually temper aberrant neurogenesis after seizures. The Wnt pathway may therefore provide suitable targets for novel therapies that prevent network remodeling and the development of epileptic foci in high-risk patients.

Traumatic brain injury (TBI) triggers a robust increase in neurogenesis within the dentate gyrus of the hippocampus, but these new neurons undergo aberrant maturation and dendritic outgrowth. Because gamma-aminobutyric acid (GABA)(A) receptors (GABA(A)Rs) modulate dendritic outgrowth during constitutive neurogenesis and GABA(A)R-modulating sedatives are often administered to human patients after TBI, we investigated whether the benzodiazepine, diazepam (DZP), alters post-injury hippocampal neurogenesis. We used a controlled cortical impact (CCI) model of TBI in adult mice, and administered DZP or vehicle continuously for 1 week after injury via osmotic pump. Although DZP did not affect the neurogenesis rate in control mice, it almost completely prevented the TBI-induced increase in hippocampal neurogenesis as well as the aberrant dendritic growth of neurons born after TBI. DZP did not reduce cortical injury, reactive gliosis, or cell proliferation early after injury, but decreased c-Fos activation in the dentate gyrus at both early and late time-points after TBI, suggesting an association between neuronal activity and post-injury neurogenesis. Because DZP blocks post-injury neurogenesis, further studies are warranted to assess whether benzodiazepines alter cognitive recovery or the development of complications after TBI.

In temporal lobe epilepsy, sprouting of hippocampal mossy fiber axons onto dentate granule cell dendrites creates a recurrent excitatory network. However, unlike mossy fibers projecting to CA3, sprouted mossy fiber synapses depress upon repetitive activation. Thus, despite their proximal location, large presynaptic terminals, and ability to excite target neurons, the impact of sprouted mossy fiber synapses on hippocampal hyperexcitability is unclear. We find that despite their short-term depression, single episodes of sprouted mossy fiber activation in hippocampal slices initiated bursts of recurrent polysynaptic excitation. Consistent with a contribution to network hyperexcitability, optogenetic activation of sprouted mossy fibers reliably triggered action potential firing in postsynaptic dentate granule cells after single light pulses. This pattern resulted in a shift in network recruitment dynamics to an “early detonation” mode and an increased probability of release compared to mossy fiber synapses in CA3. A lack of tonic adenosine-mediated inhibition contributed to the higher probability of glutamate release thus facilitating reverberant circuit activity.Significance StatementSprouted mossy fibers are one of the hallmark histopathological findings in temporal lobe epilepsy. These fibers form recurrent excitatory synapses onto other dentate granule cells that display profound short-term depression. Here, however, we show that although these sprouted mossy fibers weaken substantially during repetitive activation, their initial high probability of glutamate release can activate reverberant network activity. Furthermore, we find that a lack of tonic adenosine inhibition enables this high probability of release and, consequently, recurrent network activity.

What We Already Know about This Topic What This Article Tells Us That Is New Background Traumatic brain injury induces cellular proliferation in the hippocampus, which generates new neurons and glial cells during recovery. This process is regulated by N-methyl-d-aspartate–type glutamate receptors, which are inhibited by ketamine. The authors hypothesized that ketamine treatment after traumatic brain injury would reduce hippocampal cell proliferation, leading to worse behavioral outcomes in mice. Methods Traumatic brain injury was induced in mice using a controlled cortical impact injury, after which mice (N = 118) received either ketamine or vehicle systemically for 1 week. The authors utilized immunohistochemical assays to evaluate neuronal, astroglial, and microglial cell proliferation and survival 3 days, 2 weeks, and 6 weeks postintervention. The Morris water maze reversal task was used to assess cognitive recovery. Results Ketamine dramatically increased microglial proliferation in the granule cell layer of the hippocampus 3 days after injury (injury + vehicle, 2,800 ± 2,700 cells/mm3, n = 4; injury + ketamine, 11,200 ± 6,600 cells/mm3, n = 6; P = 0.012). Ketamine treatment also prevented the production of astrocytes 2 weeks after injury (sham + vehicle, 2,400 ± 3,200 cells/mm3, n = 13; injury + vehicle, 10,500 ± 11,300 cells/mm3, n = 12; P = 0.013 vs. sham + vehicle; sham + ketamine, 3,500 ± 4,900 cells/mm3, n = 14; injury + ketamine, 4,800 ± 3,000 cells/mm3, n = 13; P = 0.955 vs. sham + ketamine). Independent of injury, ketamine temporarily reduced neurogenesis (vehicle-exposed, 105,100 ± 66,700, cells/mm3, n = 25; ketamine-exposed, 74,300 ± 29,200 cells/mm3, n = 27; P = 0.031). Ketamine administration improved performance in the Morris water maze reversal test after injury, but had no effect on performance in sham-treated mice. Conclusions Ketamine alters hippocampal cell proliferation after traumatic brain injury. Surprisingly, these changes were associated with improvement in a neurogenesis-related behavioral recall task, suggesting a possible benefit from ketamine administration after traumatic brain injury in mice. Future studies are needed to determine generalizability and mechanism.

In the mammalian hippocampus, neurogenesis persists into adulthood, and increased generation of newborn neurons could be of clinical benefit following concussive head injuries. Post-traumatic neurogenesis has been well documented using “open” traumatic brain injury (TBI) models in rodents; however, human TBI most commonly involves closed head injury. Here we used a closed head injury (CHI) model to examine post-traumatic hippocampal neurogenesis in mice. All mice were subjected to the same CHI protocol, and a gross-motor based injury severity score was used to characterize neurologic impairment 1 h after the injury. When analyzed 2 weeks later, post-traumatic neurogenesis was significantly increased only in mice with a high degree of transient neurologic impairment immediately after injury. This increase was associated with an early increase in c-fos activity, and subsequent reactive astrocytosis and microglial activation in the dentate gyrus. Our results demonstrate that the initial degree of neurologic impairment after closed head injury predicts the induction of secondary physiologic and pathophysiologic processes, and that animals with severe neurologic impairment early after injury manifest an increase in post-traumatic neurogenesis in the absence of gross anatomic pathology.

Neuroligins, a family of postsynaptic adhesion molecules, are important in synaptogenesis through a well-characterized trans-synaptic interaction with neurexin. In addition, neuroligins are thought to drive postsynaptic assembly through binding of their intracellular domain to PSD-95. However, there is little direct evidence to support the functional necessity of the neuroligin intracellular domain in postsynaptic development. We found that presence of endogenous neuroligin obscured the study of exogenous mutated neuroligin. We therefore used chained microRNAs in rat organotypic hippocampal slices to generate a reduced background of endogenous neuroligin. On this reduced background, we found that neuroligin function was critically dependent on the cytoplasmic tail. However, this function required neither the PDZ ligand nor any other previously described cytoplasmic binding domain, but rather required a previously unknown conserved region. Mutation of a single critical residue in this region inhibited neuroligin-mediated excitatory synaptic potentiation. Finally, we found a functional distinction between neuroligins 1 and 3.

As the food industry moves toward sustainability, it is encountering, not surprisingly, a few obstacles, from labeling (after all, what is sustainable food?) to difficulties with large-scale transitioning. Editorial Board Member Tom Kimmerer recently led a dialogue with leaders in the food industry about challenges they've faced, innovations they've developed, successes, and the future.

The majority of adult hippocampal newborn cells die during early differentiation from intermediate progenitors (IPCs) to immature neurons. Neural stem cells in vivo are located in a relative hypoxic environment, and hypoxia enhances their survival, proliferation and stemness in vitro. Thus, we hypothesized that migration of IPCs away from hypoxic zones within the SGZ might result in oxidative damage, thus triggering cell death. Hypoxic niches were observed along the SGZ, composed of adult NSCs and early IPCs, and oxidative byproducts were present in adjacent late IPCs and neuroblasts. Stabilizing hypoxia inducible factor-1α with dimethyloxallyl glycine increased early survival, but not proliferation or differentiation, in neurospheres in vitro and in newly born SGZ cells in vivo. Rescue experiments in Baxfl/flmutants supported these results. We propose that localized hypoxia within the SGZ contributes to the neurogenic microenvironment and determines the early, activity-independent survival of adult hippocampal newborn cells. DOI: http://dx.doi.org/10.7554/eLife.08722.001, eLife digest The hippocampus is a region in the mammalian brain that has been implicated in the formation of new memories. This process involves the birth of new neurons, which are created at a rate of ∼4000 a day in the hippocampus of a young adult mouse. Yet only a fraction of these cells survive to form mature neurons. These cells die in two main waves – the first occurs days after they form, and the second several weeks later when as immature neurons they integrate into the brain. During this later wave, new neurons become active and survive if they connect with other nerve cells and die if they don’t. But little is known about what causes the earlier wave of cell death. The tissues that contain the precursors of new neurons often have lower oxygen levels compared to other tissues. This means that when these cells start to become neurons and leave these sites, they have to face higher levels of oxygen and may undergo “oxidative” damage. This led Chatzi et al. to ask whether such oxidative damage might cause the early loss of new neurons in the hippocampus. First, the part of the hippocampus that contains the precursor cells (called the subgranular zone or SGZ) was found to have patchy areas of low oxygen. Further experiments then revealed that chemicals that may cause oxidative damage were present in the nearby cells that had already started on the path to become new neurons. Chatzi et al. then tested whether chemically stabilizing a protein called Hypoxia Inducible Factor-1α (or HIF1α for short), which naturally helps cells to adapt to low oxygen environments, might increase the survival of the cells in the SGZ. Higher levels of HIF1α did indeed increase the survival of these cells. These findings suggest that newborn cells in the SGZ walk a tightrope between a low oxygen environment that supports the early precursors and the surrounding higher oxygen levels that can be toxic to those cells that start to become neurons. Further studies of the proteins and molecules that act downstream of HIF1α could shed light on ways to enhance the survival of these newly-generated neurons. DOI: http://dx.doi.org/10.7554/eLife.08722.002

Traumatic brain injury (TBI) increases hippocampal neurogenesis, which may contribute to cognitive recovery after injury. However, it is unknown whether TBI-induced adult-born neurons mature normally and functionally integrate into the hippocampal network. We assessed the generation, morphology, and synaptic integration of new hippocampal neurons after a controlled cortical impact (CCI) injury model of TBI. To label TBI-induced newborn neurons, we used 2-month-old POMC-EGFP mice, which transiently and specifically express EGFP in immature hippocampal neurons, and doublecortin-CreERT2transgenic mice crossed with Rosa26-CAG-tdTomato reporter mice, to permanently pulse-label a cohort of adult-born hippocampal neurons. TBI increased the generation, outward migration, and dendritic complexity of neurons born during post-traumatic neurogenesis. Cells born after TBI had profound alterations in their dendritic structure, with increased dendritic branching proximal to the soma and widely splayed dendritic branches. These changes were apparent during early dendritic outgrowth and persisted as these cells matured. Whole-cell recordings from neurons generated during post-traumatic neurogenesis demonstrate that they are excitable and functionally integrate into the hippocampal circuit. However, despite their dramatic morphologic abnormalities, we found no differences in the rate of their electrophysiological maturation, or their overall degree of synaptic integration when compared to age-matched adult-born cells from sham mice. Our results suggest that cells born after TBI participate in information processing, and receive an apparently normal balance of excitatory and inhibitory inputs. However, TBI-induced changes in their anatomic localization and dendritic projection patterns could result in maladaptive network properties.

Postsynaptic targeting of the Drosophila tumour suppressor discs-large (Dlg) critically depends on its SH3 and GK domains. Here, we asked whether these domains are also involved in subcellular targeting of the mammalian Dlg homolog SAP97 and its interacting partners in CNS cortical neurons by analysing a recently described mouse mutant lacking the SH3 and GK domains of SAP97. Both wildtype and truncated SAP97 were predominantly expressed in perinuclear regions, in a pattern suggesting association with the endoplasmic reticulum. Weaker immunoreactivity was found in neurites colocalizing with both dendritic and axonal markers. As SAP97 has been implicated in the early intracellular processing of the glutamate receptor GluR1, we studied biochemical maturation and subcellular localization of GluR1 in the mutants. Both the glycosylation pattern and synaptic clustering of GluR1 were indistinguishable from wildtype mice. Synaptic clustering of the guanylate kinase domain interacting protein GKAP was also intact. Our data demonstrate that truncation of the SH3 and GK domains of SAP97 in mice does neither change its subcellular distribution nor does it disrupt synaptic structure or protein clustering, as opposed to severe missorting of the respective mutant Dlg protein in Drosophila.

Dynamic regulation of AMPA-type glutamate receptors represents a primary mechanism for controlling synaptic strength, though mechanisms for this process are poorly understood. The palmitoylated postsynaptic density protein, PSD-95, regulates synaptic plasticity and associates with the AMPA receptor trafficking protein, stargazin. Here, we identify palmitate cycling on PSD-95 at the synapse and find that palmitate turnover on PSD-95 is regulated by glutamate receptor activity. Acutely blocking palmitoylation disperses synaptic clusters of PSD-95 and causes a selective loss of synaptic AMPA receptors. We also find that rapid glutamate-mediated AMPA receptor internalization requires depalmitoylation of PSD-95. In a nonneuronal model system, clustering of PSD-95, stargazin, and AMPA receptors is also regulated by ongoing palmitoylation of PSD-95 at the plasma membrane. These studies suggest that palmitate cycling on PSD-95 can regulate synaptic strength and regulates aspects of activity-dependent plasticity.

Survival of adult-born hippocampal granule cells is modulated by neural activity, and thought to be enhanced by excitatory synaptic signaling. Here, we report that a reduction in the synaptogenic protein neuroligin-1 in adult-born neurons in vivo decreased their survival, but surprisingly, this effect was independent of changes in excitatory synaptic function. Instead, the decreased survival was associated with unexpected changes in dendrite and spine morphology during granule cell maturation, suggesting a link between cell growth and survival.

Recent studies have identified myosin Va as an organelle motor that may have important functions in neurons. Abundantly expressed at the hippocampal postsynaptic density, it interacts with protein complexes involved in synaptic plasticity. It is also located in presynaptic terminals and may function to recruit vesicles in the reserve pool to the active zone. Dilute-lethal mice are spontaneous myosin Va mutants and have severe neurological symptoms. We studied hippocampal physiology at CA3-CA1 excitatory synapses in dilute-lethal mutant mice to test the hypothesis that myosin Va plays a role in pre- or postsynaptic elements of synaptic transmission. In all assays performed, the mutant synapses appeared to be functioning normally, both pre- and postsynaptically. These data suggest that myosin Va is not essential for the synaptic release machinery, postsynaptic receptor composition, or plasticity at this synapse, but does not exclude significant roles for myosin Va in other cell types nor potential compensation by other myosin V isoforms.

PSD-95 is a neuronal PDZ protein that associates with receptors and cytoskeletal elements at synapses, but whose function is uncertain. We found that overexpression of PSD-95 in hippocampal neurons can drive maturation of glutamatergic synapses. PSD-95 expression enhanced postsynaptic clustering and activity of glutamate receptors. Postsynaptic expression of PSD-95 also enhanced maturation of the presynaptic terminal. These effects required synaptic clustering of PSD-95 but did not rely on its guanylate kinase domain. PSD-95 expression also increased the number and size of dendritic spines. These results demonstrate that PSD-95 can orchestrate synaptic development and are suggestive of roles for PSD-95 in synapse stabilization and plasticity.

In vitrostudies suggest that the intracellular C terminus of Neuroligin1 (NL1) could play a central role in the maturation of excitatory synapses. However, it is unknown how this activity affects synapsesin vivo, and whether it may impact the development of complex behaviors. To determine how NL1 influences the state of glutamatergic synapsesin vivo, we compared the synaptic and behavioral phenotypes of mice overexpressing a full-length version of NL1 (NL1FL) with mice overexpressing a version missing part of the intracellular domain (NL1ΔC). We show that overexpression of full-length NL1 yielded an increase in the proportion of synapses with mature characteristics and impaired learning and flexibility. In contrast, the overexpression of NL1ΔC increased the number of excitatory postsynaptic structures and led to enhanced flexibility in mnemonic and social behaviors. Transient overexpression of NL1FL revealed that elevated levels are not necessary to maintain synaptic and behavioral states altered earlier in development. In contrast, overexpression of NL1FL in the fully mature adult was able to impair normal learning behavior after 1 month of expression. These results provide the first evidence that NL1 significantly impacts key developmental processes that permanently shape circuit function and behavior, as well as the function of fully developed neural circuits. Overall, these manipulations of NL1 function illuminate the significance of NL1 intracellular signalingin vivo, and enhance our understanding of the factors that gate the maturation of glutamatergic synapses and complex behavior. This has significant implications for our ability to address disorders such as autism spectrum disorders.

Neural plasticity following brain injury illustrates the potential for regeneration in the central nervous system. Lesioning of the perforant path, which innervates the outer two-thirds of the molecular layer of the dentate gyrus, was one of the first models to demonstrate structural plasticity of mature granule cells (Parnavelas et al., 1974; Caceres and Steward, 1983; Diekmann et al., 1996). The dentate gyrus also harbors a continuously proliferating population of neuronal precursors that can integrate into functional circuits and show enhanced short-term plasticity (Schmidt-Hieber et al., 2004; Abrous et al., 2005). To examine the response of adult-generated granule cells to unilateral complete transection of the perforant pathin vivo, we tracked these cells using transgenic POMC-EGFP mice or by retroviral expression of GFP. Lesioning triggered a marked proliferation of newborn neurons. Subsequently, the dendrites of newborn neurons showed reduced complexity within the denervated zone, but dendritic spines still formed in the absence of glutamatergic nerve terminals. Electron micrographs confirmed the lack of intact presynaptic terminals apposing spines on mature cells and on newborn neurons. Newborn neurons, but not mature granule cells, had a higher density of dendritic spines in the inner molecular layer postlesion accompanied by an increase in miniature EPSC amplitudes and rise times. Our results indicate that injury causes an increase in newborn neurons and lamina-specific synaptic reorganization indicative of enhanced plasticity. The presence ofde novodendritic spines in the denervated zone suggests that the postlesion environment provides the necessary signals for spine formation.

Hippocampal region CA3 contains strong recurrent excitation mediated by synapses of the longitudinal association fibers. These recurrent excitatory connections may play a dominant role in determining the information processing characteristics of this region. However, they result in feedback dynamics that may cause both runaway excitatory activity and runaway synaptic modification. Previous models of recurrent excitation have prevented unbounded activity using biologically unrealistic techniques. Here, the activation of feedback inhibition is shown to prevent unbounded activity, allowing stable activity states during recall and learning. In the model, cholinergic suppression of synaptic transmission at excitatory feedback synapses is shown to determine the extent to which activity depends upon new features of the afferent input versus components of previously stored representations. Experimental work in brain slice preparations of region CA3 demonstrates the cholinergic suppression of synaptic transmission in stratum radiatum, which contains synapses of the longitudinal association fibers.

Adult-born dentate granule cells integrate into the hippocampal network, extend neurites and form synapses in otherwise mature tissue. Excitatory and inhibitory inputs innervate these new granule cells in a stereotyped, temporally segregated manner, which presents a unique opportunity to study synapse development in the adult brain. To examine the role of neuroligins as synapse-inducing molecules in vivo, we infected dividing neural precursors in adult mice with a retroviral construct that increased neuroligin-1 levels during granule cell differentiation. By 21 days post-mitosis, exogenous neuroligin-1 was expressed at the tips of dendritic spines and increased the number of dendritic spines. Neuroligin-1-overexpressing cells showed a selective increase in functional excitatory synapses and connection multiplicity by single afferent fibers, as well as an increase in the synaptic AMPA/NMDA receptor ratio. In contrast to its synapse-inducing ability in vitro, neuroligin-1 overexpression did not induce precocious synapse formation in adult-born neurons. However, the dendrites of neuroligin-1-overexpressing cells did have more thin protrusions during an early period of dendritic outgrowth, suggesting enhanced filopodium formation or stabilization. Our results indicate that neuroligin-1 expression selectively increases the degree, but not the onset, of excitatory synapse formation in adult-born neurons.

ACh may set the dynamics of cortical function to those appropriate for learning new information. In models of the putative associative memory function of piriform cortex, selective suppression of intrinsic but not afferent fiber synaptic transmission by ACh prevents recall of previous input from interfering with the learning of new input (Hasselmo, 1993). Selective cholinergic suppression may play a similar role in the hippocampal formation, where Schaffer collateral synapses in stratum radiatum (s. rad) may store associations between activity in region CA3 and the entorhinal cortex input to region CA1 terminating in stratum lacunosum-moleculare (s. l-m). A computational model of region CA1 predicts that for effective associative memory function of the Schaffer collaterals, cholinergic suppression of synaptic transmission should be stronger in s. rad than in s. l-m. In the hippocampal slice preparation, we tested the effect of the cholinergic agonist carbachol (0.01–500 microM) on synaptic transmission in s. rad and s. l-m. Stimulating and recording electrodes were simultaneously placed in both layers, allowing analysis of the effect of carbachol on synaptic potentials in both layers during the same perfusion in each slice. Carbachol produced a significantly stronger suppression of stimulus- evoked EPSPs in s. rad than in s. l-m at all concentrations greater than 1 microM. At 100 microM, EPSP initial slopes were suppressed by 89.1 +/- 3.0% in s. rad, but only by 40.1 +/- 4.1% in s. l-m. The muscarinic antagonist atropine (1 microM) blocked cholinergic suppression in both layers. These data support the hypothesis that synaptic modification of the Schaffer collaterals may store associations between activity in region CA3 and the afferent input to region CA1 from the entorhinal cortex. In simulations, feedback regulation of cholinergic modulation based on activity in region CA1 sets the appropriate dynamics of learning for novel associations, and recall for familiar associations.

Some cases of autism spectrum disorder have mutations in the lipid phosphatase, phosphatase and tensin homolog on chromosome 10 (Pten). Tissue specific deletion of Pten in the hippocampus and cortex of mice causes anatomical and behavioral abnormalities similar to human autism. However, the impact of reductions in Pten on synaptic and circuit function remains unexplored. We usedin vivostereotaxic injections of lentivirus expressing a short hairpin RNA to knock down Pten in mouse neonatal and young adult dentate granule cells. We then assessed the morphology and synaptic physiology between 2 weeks and 4 months later. Confocal imaging of the hippocampus revealed a marked increase in granule cell size and an increase in dendritic spine density. The onset of morphological changes occurred earlier in neonatal mice than in young adults. We used whole-cell recordings from granule cells in acute slices to assess synaptic function after Pten knockdown. Consistent with the increase in dendritic spines, the frequency of excitatory miniature and spontaneous postsynaptic currents increased. However, there was little or no effect on IPSCs. Thus, Pten knockdown results in an imbalance between excitatory and inhibitory synaptic activity. Because reductions in Pten affected mature granule cells as well as developing granule cells, we suggest that the disruption of circuit function by Pten hypofunction may be ongoing well beyond early development.

Postsynaptic targeting of the Drosophila tumour suppressor discs-large (Dlg) critically depends on its SH3 and GK domains. Here, we asked whether these domains are also involved in subcellular targeting of the mammalian Dlg homolog SAP97 and its interacting partners in CNS cortical neurons by analysing a recently described mouse mutant lacking the SH3 and GK domains of SAP97. Both wildtype and truncated SAP97 were predominantly expressed in perinuclear regions, in a pattern suggesting association with the endoplasmic reticulum. Weaker immunoreactivity was found in neurites colocalizing with both dendritic and axonal markers. As SAP97 has been implicated in the early intracellular processing of the glutamate receptor GluR1, we studied biochemical maturation and subcellular localization of GluR1 in the mutants. Both the glycosylation pattern and synaptic clustering of GluR1 were indistinguishable from wildtype mice. Synaptic clustering of the guanylate kinase domain interacting protein GKAP was also intact. Our data demonstrate that truncation of the SH3 and GK domains of SAP97 in mice does neither change its subcellular distribution nor does it disrupt synaptic structure or protein clustering, as opposed to severe missorting of the respective mutant Dlg protein in Drosophila.

Excitatory synapses in the brain exhibit a remarkable degree of functional plasticity, which largely reflects changes in the number of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). However, mechanisms involved in recruiting AMPARs to synapses are unknown. Here we use hippocampal slice cultures and biolistic gene transfections to study the targeting of AMPARs to synapses. We show that AMPARs are localized to synapses through direct binding of the first two PDZ domains of synaptic PSD-95 (postsynaptic density protein of 95 kDa) to the AMPAR-associated protein, stargazin. Increasing the level of synaptic PSD-95 recruits new AMPARs to synapses without changing the number of surface AMPARs. At the same time, we show that stargazin overexpression drastically increases the number of extra-synaptic AMPARs, but fails to alter synaptic currents if synaptic PSD-95 levels are kept constant. Finally, we make compensatory mutations to both PSD-95 and stargazin to demonstrate the central role of direct interactions between them in determining the number of synaptic AMPARs.

The neurotrophin BDNF has been shown to modulate long-term potentiation (LTP) at Schaffer collateral-CA1 hippocampal synapses. Mutants in the BDNF receptor gene trkB and antibodies to its second receptor p75NTR have been used to determine the receptors and cells involved in this response. Inhibition of p75NTR does not detectably reduce LTP or affect presynaptic function, but analyses of newly generated trkB mutants implicate TrkB. One mutant has reduced expression in a normal pattern of TrkB throughout the brain. The second mutant was created by cre-loxP-mediated removal of TrkB in CA1 pyramidal neurons of this mouse. Neither mutant detectably impacts survival or morphology of hippocampal neurons. TrkB reduction, however, affects presynaptic function and reduces the ability of tetanic stimulation to induce LTP. Postsynaptic glutamate receptors are not affected by TrkB reduction, indicating that BDNF does not modulate plasticity through postsynaptic TrkB. Consistent with this, elimination of TrkB in postsynaptic neurons does not affect LTP. Moreover, normal LTP is generated in the mutant with reduced TrkB by a depolarization–low-frequency stimulation pairing protocol that puts minimal demands on presynaptic terminal function. Thus, BDNF appears to act through TrkB presynaptically, but not postsynaptically, to modulate LTP.