Hippocampal Dentate Granule Cells Research Articles

Methylation determines the extracellular calcium sensitivity of the leak channel NALCN in hippocampal dentate granule cells

The sodium leak channel NALCN is a key player in establishing the resting membrane potential (RMP) in neurons and transduces changes in extracellular Ca2+ concentration ([Ca2+]e) into increased neuronal excitability as the downstream effector of calcium-sensing receptor (CaSR). Gain-of-function mutations in the human NALCN gene cause encephalopathy and severe intellectual disability. Thus, understanding the regulatory mechanisms of NALCN is important for both basic and translational research. This study reveals a novel mechanism for NALCN regulation by arginine methylation. Hippocampal dentate granule cells in protein arginine methyltransferase 7 (PRMT7)-deficient mice display a depolarization of the RMP, decreased threshold currents, and increased excitability compared to wild-type neurons. Electrophysiological studies combined with molecular analysis indicate that enhanced NALCN activities contribute to hyperexcitability in PRMT7−/− neurons. PRMT7 depletion in HEK293T cells increases NALCN activity by shifting the dose-response curve of NALCN inhibition by [Ca2+]e without affecting NALCN protein levels. In vitro methylation studies show that PRMT7 methylates a highly conserved Arg1653 of the NALCN gene located in the carboxy-terminal region that is implicated in CaSR-mediated regulation. A kinase-specific phosphorylation site prediction program shows that the adjacent Ser1652 is a potential phosphorylation site. Consistently, our data from site-specific mutants and PKC inhibitors suggest that Arg1653 methylation might modulate Ser1652 phosphorylation mediated by CaSR/PKC-delta, leading to [Ca2+]e-mediated NALCN suppression. Collectively, these data suggest that PRMT7 deficiency decreases NALCN methylation at Arg1653, which, in turn, decreases CaSR/PKC-mediated Ser1652 phosphorylation, lifting NALCN inhibition, thereby enhancing neuronal excitability. Thus, PRMT7-mediated NALCN inhibition provides a potential target for the development of therapeutic tools for neurological diseases.

Adult Neurogenesis in the Development of Epilepsy

Compelling evidence indicates that hippocampal dentate granule cells are generated throughout human life and into old age. While animal studies demonstrate that these new neurons are important for memory function, animal research also implicates these cells in the pathogenesis of temporal lobe epilepsy. Several recent preclinical studies in rodents now suggest that targeting these new neurons can have disease-modifying effects in epilepsy.

Impact of mTOR hyperactive neurons on the morphology and physiology of adjacent neurons: Do PTEN KO cells make bad neighbors?

Hyperactivation of the mechanistic target of rapamycin (mTOR) pathway is associated with epilepsy, autism and brain growth abnormalities in humans. mTOR hyperactivation often results from developmental somatic mutations, producing genetic lesions and associated dysfunction in relatively restricted populations of neurons. Disrupted brain regions, such as those observed in focal cortical dysplasia, can contain a mix of normal and mutant cells. Mutant cells exhibit robust anatomical and physiological changes. Less clear, however, is whether adjacent, initially normal cells are affected by the presence of abnormal cells. To explore this question, we used a conditional, inducible mouse model approach to delete the mTOR negative regulator phosphatase and tensin homolog (PTEN) from <1% to >30% of hippocampal dentate granule cells. We then examined the morphology of PTEN-expressing granule cells located in the same dentate gyri as the knockout (KO) cells. Despite the development of spontaneous seizures in higher KO animals, and disease worsening with increasing age, the morphology and physiology of PTEN-expressing cells was only modestly affected. PTEN-expressing cells had smaller somas than cells from control animals, but other parameters were largely unchanged. These findings contrast with the behavior of PTEN KO cells, which show increasing dendritic extent with greater KO cell load. Together, the findings indicate that genetically normal neurons can exhibit relatively stable morphology and intrinsic physiology in the presence of nearby pathological neurons and systemic disease.

Exercise-induced enhancement of synaptic function triggered by the inverse BAR protein, Mtss1L.

Exercise is a potent enhancer of learning and memory, yet we know little of the underlying mechanisms that likely include alterations in synaptic efficacy in the hippocampus. To address this issue, we exposed mice to a single episode of voluntary exercise, and permanently marked activated mature hippocampal dentate granule cells using conditional Fos-TRAP mice. Exercise-activated neurons (Fos-TRAPed) showed an input-selective increase in dendritic spines and excitatory postsynaptic currents at 3 days post-exercise, indicative of exercise-induced structural plasticity. Laser-capture microdissection and RNASeq of activated neurons revealed that the most highly induced transcript was Mtss1L, a little-studied I-BAR domain-containing gene, which we hypothesized could be involved in membrane curvature and dendritic spine formation. shRNA-mediated Mtss1L knockdown in vivo prevented the exercise-induced increases in spines and excitatory postsynaptic currents. Our results link short-term effects of exercise to activity-dependent expression of Mtss1L, which we propose as a novel effector of activity-dependent rearrangement of synapses.

Sub‐neurodegenerative Levels of Binge Alcohol Reduce Calcium‐independent Phospholipase A2 (iPLA2) Levels in Rat Brain: An Unrecognized Initial Sign of Neuroinflammation

Selective degeneration of dentate granule cells in hippocampus (HC), pyramidal cells in entorhinal cortex (EC), and olfactory bulb neurons occurs in adult male rats subjected to severe alcohol (ethanol) binges over 4 days that generate maximal blood alcohol levels of 350–450 mg/dl. Concurrent with brain damage specifically in these brain regions, significant alterations are elicited by alcohol in the levels of phospholipase A2 (PLA2) enzymes, i.e., (1) increases in Ca2+‐dependent cytosolic cPLA2 and secreted sPLA2, two membrane phospholipid‐metabolizing enzymes causing neuroinflammation via mobilization of omega‐6 arachidonic acid, a largely pro‐oxidative neurocellular mediator; and (2) reductions in Ca2+‐independent iPLA2, known to selectively regulate membrane release and turnover of anti‐inflammatory, anti‐oxidative omega‐3 docosahexaenoic acid (DHA). Furthermore, these neurodegenerative and neuroinflammatory changes are replicated in vitro, along with a loss in DHA content itself, in rat organotypic adult‐age HC‐EC slice cultures binge‐treated with 100 mM (~460 mg/dl) alcohol. Importantly, DHA pre‐supplementation of the slices prevents the neurodegeneration and PLA2 changes caused by alcohol (Tajuddin et al. 2014). The objective of this study was to determine if phospholipid‐mediated neuroinflammatory changes exist from alcohol in the absence of neurodegeneration. Here we report that rats subjected to less severe binges ‐ and thus lower blood alcohol levels of ~200 mg/dl, which are insufficient to cause detectable brain damage and/or neuroinflammatory cPLA2 and sPLA2 elevations ‐nevertheless cause significantly reduced iPLA2 levels that are largely reversed by DHA supplementation. The results imply that iPLA2 depletion and, we suspect, loss of cellular/membrane DHA (that iPLA2 regulates), are critical, previously unappreciated first indicators or markers of brain neuroinflammation due to binge alcohol. Similar changes in iPLA2, potentially due to its sensitivity, may have a bearing on other forms of brain insults causing neuroinflammation and ultimately neurodamage.Support or Funding InformationResearch support: NIH U01 AA018279 (MAC and HYK) and NIH T32 AA013527 (Loyola Medical Center Alcohol Research Program)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

A neuronal molecular switch through cell-cell contact that regulates quiescent neural stem cells.

The quiescence of radial neural stem cells (rNSCs) in adult brain is regulated by environmental stimuli. However, little is known about how the neurogenic niche couples the external signal to regulate activation and transition of quiescent rNSCs. Here, we reveal that long-term excitation of hippocampal dentate granule cells (GCs) upon voluntary running leads to activation of adult rNSCs in the subgranular zone and thereby generation of newborn neurons. Unexpectedly, the role of these excited GC neurons in NSCs depends on direct GC-rNSC interaction in the local niche, which is through down-regulated ephrin-B3, a GC membrane-bound ligand, and attenuated transcellular EphB2 kinase-dependent signaling in the adjacent rNSCs. Furthermore, constitutively active EphB2 kinase sustains the quiescence of rNSCs during running. These findings thus elucidate the physiological significance of GC excitability on adult rNSCs under external environments and indicate a key-lock switch regulation via cell-cell contact for functional transition of rNSCs.

Possible involvement of peroxynitrite in promotion of proliferative activity in the neural stem/progenitor cells after hippocampal dentate granule cell loss

Possible involvement of peroxynitrite in promotion of proliferative activity in the neural stem/progenitor cells after hippocampal dentate granule cell loss

Chemogenetic silencing of hippocampal neurons suppresses epileptic neural circuits.

We investigated how pathological changes in newborn hippocampal dentate granule cells (DGCs) lead to epilepsy. Using a rabies virus-mediated retrograde tracing system and a designer receptors exclusively activated by designer drugs (DREADD) chemogenetic method, we demonstrated that newborn hippocampal DGCs are required for the formation of epileptic neural circuits and the induction of spontaneous recurrent seizures (SRS). A rabies virus-mediated mapping study revealed that aberrant circuit integration of hippocampal newborn DGCs formed excessive de novo excitatory connections as well as recurrent excitatory loops, allowing the hippocampus to produce, amplify, and propagate excessive recurrent excitatory signals. In epileptic mice, DREADD-mediated-specific suppression of hippocampal newborn DGCs dramatically reduced epileptic spikes and SRS in an inducible and reversible manner. Conversely, specific activation of hippocampal newborn DGCs increased both epileptic spikes and SRS. Our study reveals an essential role for hippocampal newborn DGCs in the formation and function of epileptic neural circuits, providing critical insights into DGCs as a potential therapeutic target for treating epilepsy.

Self-reinforcing effects of mTOR hyperactive neurons on dendritic growth

Loss of the mTOR pathway negative regulator PTEN from hippocampal dentate granule cells leads to neuronal hypertrophy, increased dendritic branching and aberrant basal dendrite formation in animal models. Similar changes are evident in humans with mTOR pathway mutations. These genetic conditions are associated with autism, cognitive dysfunction and epilepsy. Interestingly, humans with mTOR pathway mutations often present with mosaic disruptions of gene function, producing lesions that range from focal cortical dysplasia to hemimegalanecephaly. Whether mTOR-mediated neuronal dysmorphogenesis is impacted by the number of affected cells, however, is not known. mTOR mutations can produce secondary comorbidities, including brain hypertrophy and seizures, which could exacerbate dysmorphogenesis among mutant cells. To determine whether the percentage or “load” of PTEN knockout granule cells impacts the morphological development of these same cells, we generated two groups of PTEN knockout mice. In the first, PTEN deletion rates were held constant, at about 5%, and knockout cell growth over time was assessed. Knockout cells exhibited significant dendritic growth between 7 and 18 weeks, demonstrating that aberrant dendritic growth continues even after the cells reach maturity. In the second group of mice, PTEN was deleted from 2 to 37% of granule cells to determine whether deletion rate was a factor in driving this continued growth. Multivariate analysis revealed that both age and knockout cell load contributed to knockout cell dendritic growth. Although the mechanism remains to be determined, these findings demonstrate that large numbers of mutant neurons can produce self-reinforcing effects on their own growth.

Preferential Targeting of Lateral Entorhinal Inputs onto Newly Integrated Granule Cells.

Mature dentate granule cells in the hippocampus receive input from the entorhinal cortex via the perforant path in precisely arranged lamina, with medial entorhinal axons innervating the middle molecular layer and lateral entorhinal cortex axons innervating the outer molecular layer. Although vastly outnumbered by mature granule cells, adult-generated newborn granule cells play a unique role in hippocampal function, which has largely been attributed to their enhanced excitability and plasticity (Schmidt-Hieber et al., 2004; Ge et al., 2007). Inputs from the medial and lateral entorhinal cortex carry different informational content. Thus, the distribution of inputs onto newly integrated granule cells will affect their function in the circuit. Using retroviral labeling in combination with selective optogenetic activation of medial or lateral entorhinal inputs, we examined the functional innervation and synaptic maturation of newly generated dentate granule cells in the mouse hippocampus. Our results indicate that lateral entorhinal inputs provide the majority of functional innervation of newly integrated granule cells at 21 d postmitosis. Despite preferential functional targeting, the dendritic spine density of immature granule cells was similar in the outer and middle molecular layers, which we speculate could reflect an unequal distribution of shaft synapses. However, chronic blockade of neurotransmitter release of medial entorhinal axons with tetanus toxin disrupted normal synapse development of both medial and lateral entorhinal inputs. Our results support a role for preferential lateral perforant path input onto newly generated neurons in mediating pattern separation, but also indicate that medial perforant path input is necessary for normal synaptic development.SIGNIFICANCE STATEMENT The formation of episodic memories involves the integration of contextual and spatial information. Newly integrated neurons in the dentate gyrus of the hippocampus play a critical role in this process, despite constituting only a minor fraction of the total number of granule cells. Here we demonstrate that these neurons preferentially receive information thought to convey the context of an experience. Each newly integrated granule cell plays this unique role for ∼1 month before reaching maturity.

Signaling Pathways and Cellular Mechanisms Regulating Mossy Fiber Sprouting in the Development of Epilepsy.

The sprouting of hippocampal dentate granule cell axons, termed mossy fibers, into the dentate inner molecular layer is one of the most consistent findings in tissue from patients with mesial temporal lobe epilepsy. Decades of research in animal models have revealed that mossy fiber sprouting creates de novo recurrent excitatory connections in the hippocampus, fueling speculation that the pathology may drive temporal lobe epileptogenesis. Conducting definitive experiments to test this hypothesis, however, has been challenging due to the difficulty of dissociating this sprouting from the many other changes occurring during epileptogenesis. The field has been largely driven, therefore, by correlative data. Recently, the development of powerful transgenic mouse technologies and the discovery of novel drug targets has provided new tools to assess the role of mossy fiber sprouting in epilepsy. We can now selectively manipulate hippocampal granule cells in rodent epilepsy models, providing new insights into the granule cell subpopulations that participate in mossy fiber sprouting. The cellular pathways regulating this sprouting are also coming to light, providing new targets for pharmacological intervention. Surprisingly, many investigators have found that blocking mossy fiber sprouting has no effect on seizure occurrence, while seizure frequency can be reduced by treatments that have no effect on this sprouting. These results raise new questions about the role of mossy fiber sprouting in epilepsy. Here, we will review these findings with particular regard to the contributions of new granule cells to mossy fiber sprouting and the regulation of this sprouting by the mTOR signaling pathway.

Stochastic Modeling of Radiation-induced Dendritic Damage on in silico Mouse Hippocampal Neurons

Cognitive dysfunction associated with radiotherapy for cancer treatment has been correlated to several factors, one of which is changes to the dendritic morphology of neuronal cells. Alterations in dendritic geometry and branching patterns are often accompanied by deficits that impact learning and memory. The purpose of this study is to develop a novel predictive model of neuronal dendritic damages caused by exposure to low linear energy transfer (LET) radiation, such as X-rays, γ-rays and high-energy protons. We established in silico representations of mouse hippocampal dentate granule cell layer (GCL) and CA1 pyramidal neurons, which are frequently examined in radiation-induced cognitive decrements. The in silico representations are used in a stochastic model that describes time dependent dendritic damage induced by exposure to low LET radiation. Changes in morphometric parameters, such as total dendritic length, number of branch points and branch number, including the Sholl analysis for single neurons are described by the model. Our model based predictions for different patterns of morphological changes based on energy deposition in dendritic segments (EDDS) will serve as a useful basis to compare specific patterns of morphological alterations caused by EDDS mechanisms.

Triheptanoin protects against status epilepticus-induced hippocampal mitochondrial dysfunctions, oxidative stress and neuronal degeneration.

Triheptanoin, the triglyceride of heptanoate, is anaplerotic (refills deficient tricarboxylic acid cycle intermediates) via the propionyl-CoA carboxylase pathway. It has been shown to be neuroprotective and anticonvulsant in several models of neurological disorders. Here, we investigated the effects of triheptanoin against changes of hippocampal mitochondrial functions, oxidative stress and cell death induced by pilocarpine-induced status epilepticus (SE) in mice. Ten days of triheptanoin pre-treatment did not protect against SE, but it preserved hippocampal mitochondrial functions including state 2, state 3 ADP, state 3 uncoupled respiration, respiration linked to ATP synthesis along with the activities of pyruvate dehydrogenase complex and oxoglutarate dehydrogenase complex 24h post-SE. Triheptanoin prevented the SE-induced reductions of hippocampal mitochondrial superoxide dismutase activity and plasma antioxidant status as well as lipid peroxidation. It also reduced neuronal degeneration in hippocampal CA1 and CA3 regions 3days after SE. In addition, heptanoate significantly reduced hydrogen peroxide-induced cell death in cultured neurons. In situ hybridization localized the enzymes of the propionyl-CoA carboxylase pathway, specifically Pccα, Pccβ and methylmalonyl-CoA mutase to adult mouse hippocampal pyramidal neurons and dentate granule cells, indicating that anaplerosis may occur in neurons. In conclusion, triheptanoin appears to have anaplerotic and antioxidant effects which contribute to its neuroprotective properties.

Prenatal Alcohol Exposure Leads to Enhanced Serine 9 Phosphorylation of Glycogen Synthase Kinase-3β (GSK-3β) in the Hippocampal Dentate Gyrus of Adult Mouse.

The goal of this study was to evaluate the expression and serine 9 phosphorylation of glycogen synthase kinase (GSK-3β) within the adult hippocampal dentate gyrus (DG) in a preclinical mouse model of fetal alcohol spectrum disorders. GSK-3β is a multifunctional kinase that modulates many hippocampal processes affected by gestational alcohol, including synaptic plasticity and adult neurogenesis. GSK-3β is a constitutively active kinase that is negatively regulated by phosphorylation at the serine 9 residue. We utilized a well-characterized limited access "drinking-in-the-dark" paradigm of prenatal alcohol exposure (PAE) and measured p(Ser9)GSK-3β and total GSK-3β within adult DG by Western blot analysis. In addition, we evaluated the expression pattern of both p(Ser9)GSK-3β and total GSK-3β within the adult hippocampal dentate of PAE and control mice using high-resolution confocal microscopy. Our findings demonstrate a marked 2.0-fold elevation of p(Ser9)GSK-3β in PAE mice, concomitant with a more moderate 36% increase in total GSK-3β. This resulted in an approximate 63% increase in the p(Ser9)GSK-3β/GSK-3β ratio. Immunostaining revealed robust GSK-3β expression within Cornu Ammonis (CA) pyramidal neurons, hilar mossy cells, and a subset of GABAergic interneurons, with low levels of expression within hippocampal progenitors and dentate granule cells. These findings suggest that PAE may lead to a long-term disruption of GSK-3β signaling within the DG, and implicate mossy cells, GABAergic interneurons, and CA primary neurons as major targets of this dysregulation.

PTEN deletion increases hippocampal granule cell excitability in male and female mice

Deletion of the mTOR pathway inhibitor PTEN from postnatally-generated hippocampal dentate granule cells causes epilepsy. Here, we conducted field potential, whole cell recording and single cell morphology studies to begin to elucidate the mechanisms by which granule cell-specific PTEN-loss produces disease. Cells from both male and female mice were recorded to identify sex-specific effects. PTEN knockout granule cells showed altered intrinsic excitability, evident as a tendency to fire in bursts. PTEN knockout granule cells also exhibited increased frequency of spontaneous excitatory synaptic currents (sEPSCs) and decreased frequency of inhibitory currents (sIPSCs), further indicative of a shift towards hyperexcitability. Morphological studies of PTEN knockout granule cells revealed larger dendritic trees, more dendritic branches and an impairment of dendrite self-avoidance. Finally, cells from both female control and female knockout mice received more sEPSCs and more sIPSCs than corresponding male cells. Despite the difference, the net effect produced statistically equivalent EPSC/IPSC ratios. Consistent with this latter observation, extracellularly evoked responses in hippocampal slices were similar between male and female knockouts. Both groups of knockouts were abnormal relative to controls. Together, these studies reveal a host of physiological and morphological changes among PTEN knockout cells likely to underlie epileptogenic activity. Significance statementHyperactivation of the mTOR pathway is associated with numerous neurological diseases, including autism and epilepsy. Here, we demonstrate that deletion of the mTOR negative regulator, PTEN, from a subset of hippocampal dentate granule impairs dendritic patterning, increases excitatory input and decreases inhibitory input. We further demonstrate that while granule cells from female mice receive more excitatory and inhibitory input than males, PTEN deletion produces mostly similar changes in both sexes. Together, these studies provide new insights into how the relatively small number (≈200,000) of PTEN knockout granule cells instigates the development of the profound epilepsy syndrome evident in both male and female animals in this model.

Autism spectrum disorder-associated genes and the development of dentate granule cells.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by severe clinical symptoms such as the deficiency of the social communication, repetitive and stereotyped behaviors, and restricted interests. Although complex genetic and environmental factors are thought to contribute to the development of ASD, the precise etiologies are largely unknown. Neuroanatomical observations have been made of developmental abnormalities in different brain regions, including dentate gyrus of hippocampus, which is widely accepted as the center for learning and memory. However, little is known about what roles ASD-associated genes play in the development of hippocampal dentate granule cells. In this article, we summarized functions and pathophysiological significance of 6 representative ASD-associated genes, SEMA5A, PTEN, NLGN, EN-2, FMR1, and MECP2, by focusing on the development of dentate gyrus. We then introduced a recently developed gene transfer method directed to neonatal dentate granule cells. This new method will be useful for elucidating physiological as well as pathophysiological significance of ASD-associated genes in the development of hippocampal formation.

Transcriptional profile of hippocampal dentate granule cells in four rat epilepsy models

Global expression profiling of neurologic or psychiatric disorders has been confounded by variability among laboratories, animal models, tissues sampled, and experimental platforms, with the result being that few genes demonstrate consistent expression changes. We attempted to minimize these confounds by pooling dentate granule cell transcriptional profiles from 164 rats in seven laboratories, using three status epilepticus (SE) epilepsy models (pilocarpine, kainate, self-sustained SE), plus amygdala kindling. In each epilepsy model, RNA was harvested from laser-captured dentate granule cells from six rats at four time points early in the process of developing epilepsy, and data were collected from two independent laboratories in each rodent model except SSSE. Hierarchical clustering of differentially-expressed transcripts in the three SE models revealed complete separation between controls and SE rats isolated 1 day after SE. However, concordance of gene expression changes in the SE models was only 26–38% between laboratories, and 4.5% among models, validating the consortium approach. Transcripts with unusually highly variable control expression across laboratories provide a ‘red herring’ list for low-powered studies.

Loss of protohaem IX farnesyltransferase in mature dentate granule cells impairs short-term facilitation at mossy fibre to CA3 pyramidal cell synapses.

Key points Neurodegenerative disorders can exhibit dysfunctional mitochondrial respiratory chain complex IV activity.Conditional deletion of cytochrome c oxidase, the terminal enzyme in the respiratory electron transport chain of mitochondria, from hippocampal dentate granule cells in mice does not affect low‐frequency dentate to CA3 glutamatergic synaptic transmission.High‐frequency dentate to CA3 glutamatergic synaptic transmission and feedforward inhibition are significantly attenuated in cytochrome c oxidase‐deficient mice.Intact presynaptic mitochondrial function is critical for the short‐term dynamics of mossy fibre to CA3 synaptic function. Neurodegenerative disorders are characterized by peripheral and central symptoms including cognitive impairments which have been associated with reduced mitochondrial function, in particular mitochondrial respiratory chain complex IV or cytochrome c oxidase activity. In the present study we conditionally removed a key component of complex IV, protohaem IX farnesyltransferase encoded by the COX10 gene, in granule cells of the adult dentate gyrus. Utilizing whole‐cell patch‐clamp recordings from morphologically identified CA3 pyramidal cells from control and complex IV‐deficient mice, we found that reduced mitochondrial function did not result in overt deficits in basal glutamatergic synaptic transmission at the mossy‐fibre synapse because the amplitude, input–output relationship and 50 ms paired‐pulse facilitation were unchanged following COX10 removal from dentate granule cells. However, trains of stimuli given at high frequency (> 20 Hz) resulted in dramatic reductions in short‐term facilitation and, at the highest frequencies (> 50 Hz), also reduced paired‐pulse facilitation, suggesting a requirement for adequate mitochondrial function to maintain glutamate release during physiologically relevant activity patterns. Interestingly, local inhibition was reduced, suggesting the effect observed was not restricted to synapses with CA3 pyramidal cells via large mossy‐fibre boutons, but rather to all synapses formed by dentate granule cells. Therefore, presynaptic mitochondrial function is critical for the short‐term dynamics of synapse function, which may contribute to the cognitive deficits observed in pathological mitochondrial dysfunction.

Massively augmented hippocampal dentate granule cell activation accompanies epilepsy development

In a mouse model of temporal lobe epilepsy, multicellular calcium imaging revealed that disease emergence was accompanied by massive amplification in the normally sparse, afferent stimulation-induced activation of hippocampal dentate granule cells. Patch recordings demonstrated reductions in local inhibitory function within the dentate gyrus at time points where sparse activation was compromised. Mimicking changes in inhibitory synaptic function and transmembrane chloride regulation was sufficient to elicit the dentate gyrus circuit collapse evident during epilepsy development. Pharmacological blockade of outward chloride transport had no effect during epilepsy development, and significantly increased granule cell activation in both control and chronically epileptic animals. This apparent occlusion effect implicates reduction in chloride extrusion as a mechanism contributing to granule cell hyperactivation specifically during early epilepsy development. Glutamine plays a significant role in local synthesis of GABA in synapses. In epileptic mice, sparse granule cell activation could be restored by glutamine application, implicating compromised GABA synthesis. Glutamine had no effect on granule cell activation earlier, during epilepsy development. We conclude that compromised feedforward inhibition within the local circuit generates the massive dentate gyrus circuit hyperactivation evident in animals during and following epilepsy development. However, the mechanisms underlying this disinhibition diverge significantly as epilepsy progresses.

Long-term Fate Mapping to Assess the Impact of Postnatal Isoflurane Exposure on Hippocampal Progenitor Cell Productivity.

Exposure to isoflurane increases apoptosis among postnatally generated hippocampal dentate granule cells. These neurons play important roles in cognition and behavior, so their permanent loss could explain deficits after surgical procedures. To determine whether developmental anesthesia exposure leads to persistent deficits in granule cell numbers, a genetic fate-mapping approach to label a cohort of postnatally generated granule cells in Gli1-CreER::GFP bitransgenic mice was utilized. Green fluorescent protein (GFP) expression was induced on postnatal day 7 (P7) to fate map progenitor cells, and mice were exposed to 6 h of 1.5% isoflurane or room air 2 weeks later (P21). Brain structure was assessed immediately after anesthesia exposure (n = 7 controls and 8 anesthesia-treated mice) or after a 60-day recovery (n = 8 controls and 8 anesthesia-treated mice). A final group of C57BL/6 mice was exposed to isoflurane at P21 and examined using neurogenesis and cell death markers after a 14-day recovery (n = 10 controls and 16 anesthesia-treated mice). Isoflurane significantly increased apoptosis immediately after exposure, leading to cell death among 11% of GFP-labeled cells. Sixty days after isoflurane exposure, the number of GFP-expressing granule cells in treated animals was indistinguishable from control animals. Rates of neurogenesis were equivalent among groups at both 2 weeks and 2 months after treatment. These findings suggest that the dentate gyrus can restore normal neuron numbers after a single, developmental exposure to isoflurane. The authors' results do not preclude the possibility that the affected population may exhibit more subtle structural or functional deficits. Nonetheless, the dentate appears to exhibit greater resiliency relative to nonneurogenic brain regions, which exhibit permanent neuron loss after isoflurane exposure.