Physiological and morphological characterization of dentate granule cells in the p35 knock-out mouse hippocampus: evidence for an epileptic circuit

Post-Stroke Environmental Enrichment Improves Neurogenesis and Cognitive Function and Reduces the Generation of Aberrant Neurons in the Mouse Hippocampus

Ischemic lesions stimulate adult neurogenesis in the dentate gyrus, however, this is not associated with better cognitive function. Furthermore, increased neurogenesis is associated with the formation of aberrant neurons. In a previous study, we showed that a running task after a stroke not only increases neurogenesis but also the number of aberrant neurons without improving general performance. Here, we determined whether stimulation in an enriched environment after a lesion could increase neurogenesis and cognitive function without enhancing the number of aberrant neurons. After an ischemic stroke induced by MCAO, animals were transferred to an enriched environment containing a running wheel, tunnels and nest materials. A GFP-retroviral vector was delivered on day 3 post-stroke and a modified water maze test was performed 6 weeks after the lesion. We found that the enriched environment significantly increased the number of new neurons compared with the unstimulated stroke group but not the number of aberrant cells after a lesion. Increased neurogenesis after environmental enrichment was associated with improved cognitive function. Our study showed that early placement in an enriched environment after a stroke lesion markedly increased neurogenesis and flexible learning but not the formation of aberrant neurons, indicating that rehabilitative training, as a combination of running wheel training and enriched environment housing, improved functional and structural outcomes after a stroke.

Granule cell dispersion in two mouse models of temporal lobe epilepsy and reeler mice is associated with changes in dendritic orientation and spine distribution

Temporal lobe epilepsy is characterized by hippocampal neuronal death in CA1 and hilus. Dentate gyrus granule cells survive but show dispersion of the compact granule cell layer. This is associated with decrease of the glycoprotein Reelin, which regulates neuron migration and dendrite outgrow. Reelin-deficient (reeler) mice show no layering, their granule cells are dispersed throughout the dentate gyrus. We studied granule cell dendritic orientation and distribution of postsynaptic spines in reeler mice and two mouse models of temporal lobe epilepsy, namely the p35 knockout mice, which show Reelin-independent neuronal migration defects, and mice with unilateral intrahippocampal kainate injection. Granule cells were Golgi-stained and analyzed, using a computerized camera lucida system. Granule cells in naive controls exhibited a vertically oriented dendritic arbor with a small bifurcation angle if positioned proximal to the hilus and a wider dendritic bifurcation angle, if positioned distally. P35 knockout- and kainate-injected mice showed a dispersed granule cell layer, granule cells showed basal dendrites with wider bifurcation angles, which lost position-specific differences. Reeler mice lacked dendritic orientation. P35 knockout- and kainate-injected mice showed increased dendritic spine density in the granule cell layer. Molecular layer dendrites showed a reduced spine density in kainate-injected mice only, whereas in p35 knockouts no reduced spine density was seen. Reeler mice showed a homogenous high spine density. We hypothesize that granule cells migrate in temporal lobe epilepsy, develop new dendrites which show a spread of the dendritic tree, create new spines in areas proximal to mossy fiber sprouting, which is present in p35 knockout- and kainate-injected mice and loose spines on distal dendrites if mossy cell death is present, as it was in kainate-injected mice only. These results are in accordance with findings in epilepsy patients.

Congenital hypothyroidism impairs spine growth of dentate granule cells by downregulation of CaMKIV

Congenital hypothyroidism (CH), a common neonatal endocrine disorder, can result in cognitive deficits if delay in diagnose and treatment. Dentate gyrus (DG) is the severely affected subregion of the hippocampus by the CH, where the dentate granule cells (DGCs) reside in. However, how CH impairs the cognitive function via affecting DGCs and the underlying mechanisms are not fully elucidated. In the present study, the CH model of rat pups was successfully established, and the aberrant dendrite growth of the DGCs and the impaired cognitive behaviors were observed in the offspring. Transcriptome analysis of hippocampal tissues following rat CH successfully identified that calcium/calmodulin-dependent protein kinase IV (CaMKIV) was the prominent regulator involved in mediating deficient growth of DGC dendrites. CaMKIV was shown to be dynamically regulated in the DG subregion of the rats following drug-induced CH. Interference of CaMKIV expression in the primary DGCs significantly reduced the spine density of dendrites, while addition of T3 to the primary DGCs isolated from CH pups could facilitate the spine growth of dendrites. Insights into relevant mechanisms revealed that CH-mediated CaMKIV deficiency resulted in the significant decrease of phosphorylated CREB in DGCs, in association with the abnormality of dendrites. Our results have provided a distinct cell type in hippocampus that is affected by CH, which would be beneficial for the treatment of CH-induced cognitive deficiency.

Cyclin-Dependent Kinases (CDK) and Their Role in Diseases Development-Review

Cyclin-dependent kinases (CDKs) are involved in many crucial processes, such as cell cycle and transcription, as well as communication, metabolism, and apoptosis. The kinases are organized in a pathway to ensure that, during cell division, each cell accurately replicates its DNA, and ensure its segregation equally between the two daughter cells. Deregulation of any of the stages of the cell cycle or transcription leads to apoptosis but, if uncorrected, can result in a series of diseases, such as cancer, neurodegenerative diseases (Alzheimer’s or Parkinson’s disease), and stroke. This review presents the current state of knowledge about the characteristics of cyclin-dependent kinases as potential pharmacological targets.

Possible interplay between the theories of pharmacoresistant epilepsy

Epilepsy is one of the oldest known neurological disorders and is characterized by recurrent seizure activity. It has a high incidence rate, affecting a broad demographic in both developed and developing countries. Comorbid conditions are frequent in patients with epilepsy and have detrimental effects on their quality of life. Current management options for epilepsy include the use of anti-epileptic drugs, surgery, or a ketogenic diet. However, more than 30% of patients diagnosed with epilepsy exhibit drug resistance to anti-epileptic drugs. Further, surgery and ketogenic diets do little to alleviate the symptoms of patients with pharmacoresistant epilepsy. Thus, there is an urgent need to understand the underlying mechanisms of pharmacoresistant epilepsy to design newer and more effective anti-epileptic drugs. Several theories of pharmacoresistant epilepsy have been suggested over the years, the most common being the gene variant hypothesis, network hypothesis, multidrug transporter hypothesis, and target hypothesis. In our review, we discuss the main theories of pharmacoresistant epilepsy and highlight a possible interconnection between their mechanisms that could lead to the development of novel therapies for pharmacoresistant epilepsy.

Hippocampal adult-born granule cells drive network activity in a mouse model of chronic temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is characterized by recurrent seizures driven by synchronous neuronal activity. The reorganization of the dentate gyrus (DG) in TLE may create pathological conduction pathways for synchronous discharges in the temporal lobe, though critical microcircuit-level detail is missing from this pathophysiological intuition. In particular, the relative contribution of adult-born (abGC) and mature (mGC) granule cells to epileptiform network events remains unknown. We assess dynamics of abGCs and mGCs during interictal epileptiform discharges (IEDs) in mice with TLE as well as sharp-wave ripples (SPW-Rs) in healthy mice, and find that abGCs and mGCs are desynchronized and differentially recruited by IEDs compared to SPW-Rs. We introduce a neural topic model to explain these observations, and find that epileptic DG networks organize into disjoint, cell-type specific pathological ensembles in which abGCs play an outsized role. Our results characterize identified GC subpopulation dynamics in TLE, and reveal a specific contribution of abGCs to IEDs.

A somatic mutation in MEN1 gene detected in periventricular nodular heterotopia tissue obtained from depth electrodes

Periventricular nodular heterotopia (PNH) is a common structural malformation of cortical development. Mutations in the filamin A gene are frequent in familial cases with X‐linked PNH. However, many cases with sporadic PNH remain genetically unexplained. Although medically refractory epilepsy often brings attention to the underlying PNH, patients are often not candidates for surgical resection. This limits access to neuronal tissue harboring causal mutations. We evaluated a patient with PNH and medically refractory focal epilepsy who underwent a presurgical evaluation with stereotactically placed electroencephalographic (SEEG) depth electrodes. Following SEEG explantation, we collected trace tissue adherent to the electrodes and extracted the DNA. Whole‐exome sequencing performed in a Clinical Laboratory Improvement Amendments–approved genetic diagnostic laboratory uncovered a de novo heterozygous pathogenic variant in novel candidate PNH gene MEN1 (multiple endocrine neoplasia type 1; c.1546dupC, p.R516PfsX15). The variant was absent in an earlier exome profiling of the venous blood–derived DNA. The MEN1 gene encodes the ubiquitously expressed, nuclear scaffold protein menin, a known tumor suppressor gene with an established role in the regulation of transcription, proliferation, differentiation, and genomic integrity. Our study contributes a novel candidate gene in PNH generation and a novel practical approach that integrates electrophysiological and genetic explorations of epilepsy.

Restrained Dendritic Growth of Adult-Born Granule Cells Innervated by Transplanted Fetal GABAergic Interneurons in Mice with Temporal Lobe Epilepsy

The dentate gyrus (DG) is a region of the adult rodent brain that undergoes continuous neurogenesis. Seizures and loss or dysfunction of GABAergic synapses onto adult-born dentate granule cells (GCs) alter their dendritic growth and migration, resulting in dysmorphic and hyperexcitable GCs. Additionally, transplants of fetal GABAergic interneurons in the DG of mice with temporal lobe epilepsy (TLE) result in seizure suppression, but it is unknown whether increasing interneurons with these transplants restores GABAergic innervation to adult-born GCs. Here, we address this question by birth-dating GCs with retrovirus at different times up to 12 weeks after pilocarpine-induced TLE in adult mice. Channelrhodopsin 2 (ChR2)-enhanced yellow fluorescent protein (EYFP)-expressing medial-ganglionic eminence (MGE)-derived GABAergic interneurons from embryonic day (E)13.5 mouse embryos were transplanted into the DG of the TLE mice and GCs with transplant-derived inhibitory post-synaptic currents (IPSCs) were identified by patch-clamp electrophysiology and optogenetic interrogation. Putative synaptic sites between GCs and GABAergic transplants were also confirmed by intracellular biocytin staining, immunohistochemistry, and confocal imaging. 3D reconstructions of dendritic arbors and quantitative morphometric analyses were carried out in >150 adult-born GCs. GABAergic inputs from transplanted interneurons correlated with markedly shorter GC dendrites, compared to GCs that were not innervated by the transplants. Moreover, these effects were confined to distal dendritic branches and a short time window of six to eight weeks. The effects were independent of seizures as they were also observed in naïve mice with MGE transplants. These findings are consistent with the hypothesis that increased inhibitory currents over a smaller dendritic arbor in adult-born GCs may reduce their excitability and lead to seizure suppression.

Multifaceted Regulation of ALDH1A1 by Cdk5 in Alzheimer's Disease Pathogenesis

This study revealed multifaceted regulation of ALDH1A1 by Cdk5 in Alzheimer's disease (AD) pathogenesis. ALDH1A1 is a multifunctional enzyme with dehydrogenase, esterase, and anti-oxidant activities. ALDH1A1 is also a major regulator of retinoic acid (RA) signaling, which is critical for normal brain homeostasis. We identified ALDH1A1 as both physiological and pathological target of Cdk5. First, under neurotoxic conditions, Cdk5-induced oxidative stress upregulates ALDH1A1 transcription. Second, Cdk5 increases ALDH1A1 levels by preventing its ubiquitylation via direct phosphorylation. Third, ALDH1A1 phosphorylation increases its dehydrogenase activity by altering its tetrameric state to a highly active monomeric state. Fourth, persistent oxidative stress triggered by deregulated Cdk5 inactivates ALDH1A1. Thus, initially, the good Cdk5 attempts to mitigate ensuing oxidative stress by upregulating ALDH1A1 via phosphorylation and paradoxically by increasing oxidative stress. Later, sustained oxidative stress generated by Cdk5 inhibits ALDH1A1 activity, leading to neurotoxicity. ALDH1A1 upregulation is highly neuroprotective. In human AD tissues, ALDH1A1 levels increase with disease severity. However, ALDH1A1 activity was highest at mild and moderate stages, but declines significantly at severe stage. These findings confirm that during the initial stages, neurons attempt to upregulate and activate ALDH1A1 to protect from accruing oxidative stress-induced damage; however, persistently deleterious conditions inactivate ALDH1A1, further contributing to neurotoxicity. This study thus revealed two faces of Cdk5, good and bad in neuronal function and survival, with a single substrate, ALDH1A1. The bad Cdk5 prevails in the end, overriding the good Cdk5 act, suggesting that Cdk5 is an effective therapeutic target for AD.

The Expression Alteration of BC1 RNA and its Interaction with Eukaryotic Translation Initiation Factor eIF4A Post-Status Epilepticus

Abnormal dendritic sprouting and synaptic remodelling are important pathological features of temporal lobe epilepsy. BC1 RNA is a translation repressor involved in the regulation of the dendritic protein synthesis and mRNA transport, which is essential for dendritic development and plasticity. The expression alteration of BC1 RNA in the pilocarpine induced epilepsy model remains unknown. It is unclear if the interactions between BC1 RNA and eukaryotic initiation factor 4A (eIF4A) exists in this model. The purpose of this study was to investigate the expression changes of BC1 RNA and its interactions with eIF4A post-status epilepticus (SE). Chloride lithium and pilocarpine were used to induce the SE rat model. Either a whole brain or hippocampus tissues were collected at different time points after SE. The expression patterns of BC1 was detected by qPCR and in situ hybridization. The levels of eIF4AI/II protein expression were analyzed via western blotting and immunohistochemistry. The BC1 RNA-eIF4AI/II interaction was determined by electrophoretic mobility shift assay (EMSA). We found that the BC1 RNA levels decreased in hippocampus 3d, 1w and 2w post-SE before the levels recovered. The eIF4AI/II began to rise 3d post-SE and reached the maximum level 1w post-SE. After 1w post-SE the levels decreased in the hippocampal CA1, CA3 and DG subregions. EMSA analysis showed that BC1 RNA specifically interacted with the eIF4AI/II. The BC1 RNA-eIF4AI/II complex reduced to the lowest level 1w post-SE. Our results suggested that BC1 has a negative regulatory correlation with eIF4AI/II, where BC1 RNA could be involved in epileptogenesis by regulating dendritic protein synthesis.

Intrauterine inflammation reduces postnatal neurogenesis in the hippocampal subgranular zone and leads to accumulation of hilar ectopic granule cells

Prenatal inflammation is associated with poor neurobehavioral outcomes in exposed offspring. A common route of exposure for the fetus is intrauterine infection, which is often associated with preterm birth. Hippocampal development may be particularly vulnerable to an inflammatory insult during pregnancy as this region remains highly neurogenic both prenatally and postnatally. These studies sought to determine if intrauterine inflammation specifically altered hippocampal neurogenesis and migration of newly produced granule neurons during the early postnatal period. Microglial and astroglial cell populations known to play a role in the regulation of postnatal neurogenesis were also examined. We show that intrauterine inflammation significantly reduced hippocampal neurogenesis between postnatal days 7 (P7) and P14 as well as decreased granule cell density at P28. Ectopic migration of granule cells was observed in LPS-exposed mice at P14, but not at P28. Intrauterine inflammation had no effect on hippocampal astrocyte or microglia density or on apoptosis rate at the postnatal time points examined. Thus, exposure to intrauterine inflammation disrupts early postnatal neurogenesis and leads to aberrant migration of newly born granule cells.

Stress-induced Cdk5 activity enhances cytoprotective basal autophagy in Drosophila melanogaster by phosphorylating acinus at serine437

Cdk5 is a post-mitotic kinase with complex roles in maintaining neuronal health. The various mechanisms by which Cdk5 inhibits and promotes neurodegeneration are still poorly understood. Here, we show that in Drosophila melanogaster Cdk5 regulates basal autophagy, a key mechanism suppressing neurodegeneration. In a targeted screen, Cdk5 genetically interacted with Acinus (Acn), a primarily nuclear protein, which promotes starvation-independent, basal autophagy. Loss of Cdk5, or its required cofactor p35, reduces S437-Acn phosphorylation, whereas Cdk5 gain-of-function increases pS437-Acn levels. The phospho-mimetic S437D mutation stabilizes Acn and promotes basal autophagy. In p35 mutants, basal autophagy and lifespan are reduced, but restored to near wild-type levels in the presence of stabilized Acn^S437D. Expression of aggregation-prone polyQ-containing proteins or the Amyloid-β42 peptide, but not alpha-Synuclein, enhances Cdk5-dependent phosphorylation of S437-Acn. Our data indicate that Cdk5 is required to maintain the protective role of basal autophagy in the initial responses to a subset of neurodegenerative challenges.

Inhibition of Cdk5 rejuvenates inhibitory circuits and restores experience-dependent plasticity in adult visual cortex

Cyclin-dependent kinase 5 (Cdk5) acts as an essential modulator for neural development and neurological disorders. Here we show that Cdk5 plays a pivotal role in modulating GABAergic signaling and the maturation of visual system. In adult mouse primary visual cortex, Cdk5 formed complex with the GABA synthetic enzyme glutamate decarboxylase GAD67, but not with GAD65. In addition to enhancement in the surface level of NR2B-containing NMDA receptors, inhibition of Cdk5 reduced the protein levels of GADs and Otx2, while leaving intact the expression of vesicular GABA transporter and subunits of GABA_A or AMPA receptors. Whole-cell patch-clamp recording in layer II/III pyramidal neurons revealed a decrease in the frequency of miniature inhibitory postsynaptic current (mIPSC). Consequently, pharmacological inhibition and genetic knockdown of Cdk5 in adult mice led to a restoration of juvenile-like ocular dominance plasticity in vivo and long-term synaptic potential in layer II/III induced by white matter stimulation in vitro. Interestingly, we did not observe an alteration of perineuronal nets of extracellular matrix, but a reinstatement of the capability to evoke long-term depression at inhibitory synapses (iLTD), which depended on presynaptic endocannabinoid receptors and was a sign of the rejuvenated GABAergic synapses. Enhancement of GABA signaling by diazepam impeded ocular dominance plasticity rescued by Cdk5 inhibition. These results thus suggest that a physiological role of Cdk5 in visual cortex is to consolidate and stabilize neural circuits through controlling GABAergic signaling.

Betaine in the Brain: Characterization of Betaine Uptake, its Influence on Other Osmolytes and its Potential Role in Neuroprotection from Osmotic Stress

Betaine (N-trimethylglycine), a common osmolyte, has received attention because of the number of clinical reports associating betaine supplementation with improved cognition, neuroprotection and exercise physiology. However, tissue analyses report little accumulation of betaine in brain tissue despite the presence of betaine/GABA transporters (BGT1) at the blood brain barrier and in nervous tissue, calling into question whether betaine influences neuronal function directly or indirectly. Therefore, the focus of this study was to determine what capacity nervous tissue has to accumulate betaine, specifically in the hippocampus, a region of the brain associated with learning and memory and one that is particularly susceptible to damage (e.g., seizure activity). Here we report that hippocampal slices actively accumulate betaine in a time, dose and osmolality dependent manner, resulting in peak intracellular concentrations four times extracellular concentrations within 8 h. Our data also indicate that betaine uptake differentially influences the accumulation of other osmolytes. Under isosmotic conditions, betaine uptake minimally impacted some osmolytes (e.g., glycerylphosphorylcholine and glutamate) while significantly reducing others (taurine, creatine, and myo-inositol). Under osmotic stress (hyperosmotic) conditions, we observed dramatic changes in osmolytes like glycine and glutamine-key players in inhibitory neurotransmission-and little change in osmolytes such as taurine, creatine and myo-inositol when betaine was available. These data suggest that betaine may influence pathways of inhibitory neurotransmitter production/recycling in addition to serving as an osmolyte and metabolic intermediate. In sum, our data provide detailed characterization of betaine uptake in the hippocampus that implicates betaine in the modulation of hippocampal neurophysiology and neuroprotection.

Adult hippocampal neurogenesis poststroke: More new granule cells but aberrant morphology and impaired spatial memory

Stroke significantly stimulates neurogenesis in the adult dentate gyrus, though the functional role of this postlesional response is mostly unclear. Recent findings suggest that newborn neurons generated in the context of stroke may fail to correctly integrate into pre-existing networks. We hypothesized that increased neurogenesis in the dentate gyrus following stroke is associated with aberrant neurogenesis and impairment of hippocampus-dependent memory. To address these questions we used the middle cerebral artery occlusion model (MCAO) in mice. Animals were housed either under standard conditions or with free access to running wheels. Newborn granule cells were labelled with the thymidine analoque EdU and retroviral vectors. To assess memory performance, we employed a modified version of the Morris water maze (MWM) allowing differentiation between hippocampus dependent and independent learning strategies. Newborn neurons were morphologically analyzed using confocal microscopy and Neurolucida system at 7 weeks. We found that neurogenesis was significantly increased following MCAO. Animals with MCAO needed more time to localize the platform and employed less hippocampus-dependent search strategies in MWM versus controls. Confocal studies revealed an aberrant cell morphology with basal dendrites and an ectopic location (e.g. hilus) of new granule cells born in the ischemic brain. Running increased the number of new neurons but also enhanced aberrant neurogenesis. Running, did not improve the general performance in the MWM but slightly promoted the application of precise spatial search strategies. In conclusion, ischemic insults cause hippocampal-dependent memory deficits which are associated with aberrant neurogenesis in the dentate gyrus indicating ischemia-induced maladaptive plasticity in the hippocampus.

Biological functions of CDK5 and potential CDK5 targeted clinical treatments

Cyclin dependent kinases are proline-directed serine/threonine protein kinases that are traditionally activated upon association with a regulatory subunit. For most CDKs, activation by a cyclin occurs through association and phosphorylation of the CDK’s T-loop. CDK5 is unusual because it is not typically activated upon binding with a cyclin and does not require T-loop phosphorylation for activation, even though it has high amino acid sequence homology with other CDKs. While it was previously thought that CDK5 only interacted with p35 or p39 and their cleaved counterparts, Recent evidence suggests that CDK5 can interact with certain cylins, amongst other proteins, which modulate CDK5 activity levels. This review discusses recent findings of molecular interactions that regulate CDK5 activity and CDK5 associated pathways that are implicated in various diseases. Also covered herein is the growing body of evidence for CDK5 in contributing to the onset and progression of tumorigenesis.

The Cdk5-Mcl-1 axis promotes mitochondrial dysfunction and neurodegeneration in a model of Alzheimer's disease

Cdk5 deregulation is highly neurotoxic in Alzheimer's disease (AD). We identified Mcl-1 as a direct Cdk5 substrate using an innovative chemical screen in mouse brain lysates. Our data demonstrate that Mcl-1 levels determine the threshold for cellular damage in response to neurotoxic insults. Mcl-1 is a disease-specific target of Cdk5, which associates with Cdk5 under basal conditions, but is not regulated by it. Neurotoxic insults hyperactivate Cdk5 causing Mcl-1 phosphorylation at T92. This phosphorylation event triggers Mcl-1 ubiquitylation, which directly correlates with mitochondrial dysfunction. Consequently, ectopic expression of phosphorylation-dead T92A-Mcl-1 fully prevents mitochondrial damage and subsequent cell death triggered by neurotoxic treatments in neuronal cells and primary cortical neurons. Notably, enhancing Mcl-1 levels offers comparable neuroprotection to that observed upon Cdk5 depletion, suggesting that Mcl-1 degradation by direct phosphorylation is a key mechanism by which Cdk5 promotes neurotoxicity in AD. The clinical significance of the Mcl-1-Cdk5 axis was investigated in human AD clinical specimens, revealing an inverse correlation between Mcl-1 levels and disease severity. These results emphasize the potential of Mcl-1 upregulation as an attractive therapeutic strategy for delaying or preventing neurodegeneration in AD.

p39 Is Responsible for Increasing Cdk5 Activity during Postnatal Neuron Differentiation and Governs Neuronal Network Formation and Epileptic Responses

Two distinct protein cofactors, p35 and p39, independently activate Cyclin-dependent kinase 5 (Cdk5), which plays diverse roles in normal brain function and the pathogenesis of many neurological diseases. The initial discovery that loss of p35 impairs neuronal migration in the embryonic brain prompted intensive research exploring the function of p35-dependent Cdk5 activity. In contrast, p39 expression is restricted to the postnatal brain and its function remains poorly understood. Despite the robustly increased Cdk5 activity during neuronal differentiation, which activator is responsible for enhancing Cdk5 activation and how the two distinct activators direct Cdk5 signaling to govern neuronal network formation and function still remains elusive. Here we report that p39, but not p35, is selectively upregulated by histone acetylation-mediated transcription, which underlies the robust increase of Cdk5 activity during rat and mouse neuronal differentiation. The loss of p39 attenuates overall Cdk5 activity in neurons and preferentially affects phosphorylation of specific Cdk5 targets, leading to aberrant axonal growth and impaired dendritic spine and synapse formation. In adult mouse brains, p39 deficiency results in dysregulation of p35 and Cdk5 targets in synapses. Moreover, in contrast to the proepileptic phenotype caused by the lack of p35, p39 loss leads to deficits in maintaining seizure activity and induction of immediate early genes that control hippocampal excitability. Together, our studies demonstrate essential roles of p39 in neuronal network development and function. Furthermore, our data support a model in which Cdk5 activators play nonoverlapping and even opposing roles to govern balanced Cdk5 signaling in the postnatal brain.

SIGNIFICANCE STATEMENT

Neuronal network development requires tightly regulated activation of Cyclin-dependent kinase 5 (Cdk5) by two distinct cofactors, p35 and p39. Despite the well-known p35-dependent Cdk5 function, why postnatal neurons express abundant p39 in addition to p35 remained unknown for decades. In this study, we discovered that selective upregulation of p39 is the underlying mechanism that accommodates the increased functional requirement of Cdk5 activation during neuronal differentiation. In addition, we demonstrated that p39 selectively directs Cdk5 to phosphorylate protein substrates essential for axonal development, dendritic spine formation, and synaptogenesis. Moreover, our studies suggest opposing roles of p39 and p35 in synaptic Cdk5 function and epileptic responses, arguing that cooperation between Cdk5 activators maintains balanced Cdk5 signing, which is crucial for postnatal brain function.

A Tale of the Good and Bad: Remodeling of the Microtubule Network in the Brain by Cdk5

Cdk5, a cyclin-dependent kinase family member, is a global orchestrator of neuronal cytoskeletal dynamics. During embryogenesis, Cdk5 is indispensable for brain development. In adults, it is essential for numerous neuronal processes, including higher cognitive functions such as learning and memory formation, drug addiction, pain signaling, and long-term behavior changes through long-term potentiation and long-term depression, all of which rely on rapid alterations in the cytoskeleton. Cdk5 activity becomes deregulated in various brain disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, attention-deficit hyperactivity disorder, epilepsy, schizophrenia, and ischemic stroke; these all result in profound remodeling of the neuronal cytoskeleton. This Commentary specifically focuses on the pleiotropic contribution of Cdk5 in regulating neuronal microtubule remodeling. Because the vast majority of the physiological substrates of Cdk5 are associated with the neuronal cytoskeleton, our emphasis is on the Cdk5 substrates, such as CRMP2, stathmin, drebrin, dixdc1, axin, MAP2, MAP1B, doublecortin, kinesin-5, and tau, that have allowed to unravel the molecular mechanisms through which Cdk5 exerts its divergent roles in regulating neuronal microtubule dynamics, both in healthy and disease states.

The Ever-Changing Morphology of Hippocampal Granule Neurons in Physiology and Pathology

Newborn neurons are continuously added to the hippocampal dentate gyrus throughout adulthood. In this review, we analyze the maturational stages that newborn granule neurons go through, with a focus on their unique morphological features during each stage under both physiological and pathological circumstances. In addition, the influence of deleterious (such as schizophrenia, stress, Alzheimer's disease, seizures, stroke, inflammation, dietary deficiencies, or the consumption of drugs of abuse or toxic substances) and neuroprotective (physical exercise and environmental enrichment) stimuli on the maturation of these cells will be examined. Finally, the regulation of this process by proteins involved in neurodegenerative and neurological disorders such as Glycogen synthase kinase 3β, Disrupted in Schizophrenia 1 (DISC-1), Glucocorticoid receptor, pro-inflammatory mediators, Presenilin-1, Amyloid precursor protein, Cyclin-dependent kinase 5 (CDK5), among others, will be evaluated. Given the recently acquired relevance of the dendritic branch as a functional synaptic unit required for memory storage, a full understanding of the morphological alterations observed in newborn neurons may have important consequences for the prevention and treatment of the cognitive and affective alterations that evolve in conjunction with impaired adult hippocampal neurogenesis.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Differentiation of apical and basal dendrites in pyramidal cells and granule cells in dissociated hippocampal cultures

Hippocampal pyramidal cells and dentate granule cells develop morphologically distinct dendritic arbors, yet also share some common features. Both cell types form a long apical dendrite which extends from the apex of the cell soma, while short basal dendrites are developed only in pyramidal cells. Using quantitative morphometric analyses of mouse hippocampal cultures, we evaluated the differences in dendritic arborization patterns between pyramidal and granule cells. Furthermore, we observed and described the final apical dendrite determination during dendritic polarization by time-lapse imaging. Pyramidal and granule cells in culture exhibited similar dendritic patterns with a single principal dendrite and several minor dendrites so that the cell types were not readily distinguished by appearance. While basal dendrites in granule cells are normally degraded by adulthood in vivo, cultured granule cells retained their minor dendrites. Asymmetric growth of a single principal dendrite harboring the Golgi was observed in both cell types soon after the onset of dendritic growth. Time-lapse imaging revealed that up until the second week in culture, final principal dendrite designation was not stabilized, but was frequently replaced by other minor dendrites. Before dendritic polarity was stabilized, the Golgi moved dynamically within the soma and was repeatedly repositioned at newly emerging principal dendrites. Our results suggest that polarized growth of the apical dendrite is regulated by cell intrinsic programs, while regression of basal dendrites requires cue(s) from the extracellular environment in the dentate gyrus. The apical dendrite designation is determined from among multiple growing dendrites of young developing neurons.

Dopamine D1 and D5 receptors modulate spike timing-dependent plasticity at medial perforant path to dentate granule cell synapses

Although evidence suggests that DA modulates hippocampal function, the mechanisms underlying that dopaminergic modulation are largely unknown. Using perforated-patch electrophysiological techniques to maintain the intracellular milieu, we investigated how the activation of D1-type DA receptors regulates spike timing-dependent plasticity (STDP) of the medial perforant path (mPP) synapse onto dentate granule cells. When D1-type receptors were inhibited, a relatively mild STDP protocol induced LTP only within a very narrow timing window between presynaptic stimulation and postsynaptic response. The stimulus protocol produced timing-dependent LTP (tLTP) only when the presynaptic stimulation was followed 30 ms later by depolarization-induced postsynaptic action potentials. That is, the time between presynaptic stimulation and postsynaptic response was 30 ms (Δt = +30 ms). When D1-type receptors were activated, however, the same mild STDP protocol induced tLTP over a much broader timing window: tLTP was induced when -30 ms ≤ Δt ≤ +30 ms. The result indicated that D1-type receptor activation enabled synaptic potentiation even when postsynaptic activity preceded presynaptic stimulation within this Δt range. Results with null mice lacking the Kv4.2 potassium channel and with the potassium channel inhibitor, 4-aminopyridine, suggested that D1-type receptors enhanced tLTP induction by suppressing the transient IA-type K(+) current. Results obtained with antagonists and DA receptor knock-out mice indicated that endogenous activity of both D1 and D5 receptors modulated plasticity in the mPP. The DA D5 receptors appeared particularly important in regulating plasticity of the mPP onto the dentate granule cells.

An evo-devo approach to thyroid hormones in cerebral and cerebellar cortical development: etiological implications for autism

The morphological alterations of cortical lamination observed in mouse models of developmental hypothyroidism prompted the recognition that these experimental changes resembled the brain lesions of children with autism; this led to recent studies showing that maternal thyroid hormone deficiency increases fourfold the risk of autism spectrum disorders (ASD), offering for the first time the possibility of prevention of some forms of ASD. For ethical reasons, the role of thyroid hormones on brain development is currently studied using animal models, usually mice and rats. Although mammals have in common many basic developmental principles regulating brain development, as well as fundamental basic mechanisms that are controlled by similar metabolic pathway activated genes, there are also important differences. For instance, the rodent cerebral cortex is basically a primary cortex, whereas the primary sensory areas in humans account for a very small surface in the cerebral cortex when compared to the associative and frontal areas that are more extensive. Associative and frontal areas in humans are involved in many neurological disorders, including ASD, attention deficit-hyperactive disorder, and dyslexia, among others. Therefore, an evo-devo approach to neocortical evolution among species is fundamental to understand not only the role of thyroid hormones and environmental thyroid disruptors on evolution, development, and organization of the cerebral cortex in mammals but also their role in neurological diseases associated to thyroid dysfunction.

LIS1 deficiency promotes dysfunctional synaptic integration of granule cells generated in the developing and adult dentate gyrus

Type I lissencephaly, a neuronal migration disorder characterized by cognitive disability and refractory epilepsy, is often caused by heterozygous mutations in the LIS1 gene. Histopathologies of malformation-associated epilepsies have been well described, but it remains unclear whether hyperexcitability is attributable to disruptions in neuronal organization or abnormal circuit function. Here, we examined the effect of LIS1 deficiency on excitatory synaptic function in the dentate gyrus of hippocampus, a region believed to serve critical roles in seizure generation and learning and memory. Mice with heterozygous deletion of LIS1 exhibited robust granule cell layer dispersion, and adult-born granule cells labeled with enhanced green fluorescent protein were abnormally positioned in the molecular layer, hilus, and granule cell layer. In whole-cell patch-clamp recordings, reduced LIS1 function was associated with greater excitatory synaptic input to mature granule cells that was consistent with enhanced release probability at glutamatergic synapses. Adult-born granule cells that were ectopically positioned in the molecular layer displayed a more rapid functional maturation and integration into the synaptic network compared with newborn granule cells located in the hilus or granule cell layer or in wild-type controls. In a conditional knock-out mouse, induced LIS1 deficiency in adulthood also enhanced the excitatory input to granule cells in the absence of neuronal disorganization. These findings indicate that disruption of LIS1 has direct effects on excitatory synaptic transmission independent of laminar disorganization, and the ectopic position of adult-born granule cells within a malformed dentate gyrus critically influences their functional maturation and integration.

Synaptic connections of hilar basal dendrites of dentate granule cells in a neonatal hypoxia model of epilepsy

Numerous animal models of epileptogenesis demonstrate neuroplastic changes in the hippocampus. These changes occur not only for the mature neurons and glia, but also for the newly generated granule cells in the dentate gyrus. One of these changes, the sprouting of mossy fiber axons, is derived predominantly from newborn granule cells in adult rats with pilocarpine-induced temporal lobe epilepsy. Newborn granule cells also mainly contribute to another neuroplastic change, hilar basal dendrites (HBDs), which are synaptically targeted by mossy fibers in the hilus. Both sprouted mossy fibers and HBDs contribute to recurrent excitatory circuitry that is hypothesized to be involved in increased seizure susceptibility and the development of spontaneous recurrent seizures (SRS) that occur following the initial pilocarpine-induced status epilepticus. Considering the putative role of these neuroplastic changes in epileptogenesis, a critical question is whether similar anatomic phenomena occur after epileptogenic insults to the immature brain, where the proportion of recently born granule cells is higher due to ongoing maturation. The current study aimed to determine if such neuroplastic changes could be observed in a standardized model of neonatal seizure-inducing hypoxia that results in development of SRS. We used immunoelectron microscopy for the immature neuronal marker doublecortin to label newborn neurons and their HBDs following neonatal hypoxia. Our goal was to determine whether synapses form on HBDs from neurons born after neonatal hypoxia. Our results show a robust synapse formation on HBDs from animals that experienced neonatal hypoxia, regardless of whether the animals experienced tonic-clonic seizures during the hypoxic event. In both cases, the axon terminals that synapse onto HBDs were identified as mossy fiber terminals, based on the appearance of dense core vesicles. No such synapses were observed on HBDs from newborn granule cells obtained from sham animals analyzed at the same time points. This aberrant circuit formation may provide an anatomic substrate for increased seizure susceptibility and the development of epilepsy.

Inhibition and interneuron distribution in the dentate gyrus of p35 knockout mice

The p35 knockout (p35-/-) mouse is an animal model of temporal lobe epilepsy that recapitulates key neuroanatomic abnormalities-granule cell dispersion and mossy fiber sprouting-observed in the hippocampal formation of humans, as well as spontaneous seizure activity. It is a useful model in which to study the relationship between the abnormal neuronal structure and seizure activity to further our understanding of cortical dysplasia in epileptogenesis. Our previous work using this mouse model characterized the anatomic features of the dentate granule cells and the functional implications of these abnormalities on increased recurrent excitation. These data also suggested that there might be compromised inhibition in this animal model. We pursued this possibility, focusing our investigation on inhibitory circuitry. In preliminary investigations using neuroanatomic tools (immunocytochemistry, camera lucida reconstructions of individually labeled interneurons, and electron microscopy) combined with intracellular electrophysiology, we observed no significant reduction in the number of symmetric versus asymmetric synaptic contacts on dentate granule cell somata, and no statistically significant changes in evoked early or late inhibition. Although there were some abnormalities in the morphology/distribution of inhibitory interneurons (as well as a larger population of dentate granule cells) of the dentate gyrus, overall inhibition in the p35 knockout mouse appeared to be largely intact.

The importance of immunohistochemical analyses in evaluating the phenotype of Kv channel knockout mice

To gain insights into the phenotype of voltage-gated potassium (Kv)1.1 and Kv4.2 knockout mice, we used immunohistochemistry to analyze the expression of component principal or α subunits and auxiliary subunits of neuronal Kv channels in knockout mouse brains. Genetic ablation of the Kv1.1 α subunit did not result in compensatory changes in the expression levels or subcellular distribution of related ion channel subunits in hippocampal medial perforant path and mossy fiber nerve terminals, where high levels of Kv1.1 are normally expressed. Genetic ablation of the Kv4.2 α subunit did not result in altered neuronal cytoarchitecture of the hippocampus. Although Kv4.2 knockout mice did not exhibit compensatory changes in the expression levels or subcellular distribution of the related Kv4.3 α subunit, we found dramatic decreases in the cellular and subcellular expression of specific Kv channel interacting proteins (KChIPs) that reflected their degree of association and colocalization with Kv4.2 in wild-type mouse and rat brains. These studies highlight the insights that can be gained by performing detailed immunohistochemical analyses of Kv channel knockout mouse brains.

Are developmental dysplastic lesions epileptogenic?

Cortical dysplasia of various types, reflecting abnormalities of brain development, have been closely associated with epileptic activities. Yet, there remains considerable discussion about if/how these structural lesions give rise to seizure phenomenology. Animal models have been used to investigate the cause-effect relationships between aberrant cortical structure and epilepsy. In this article, we discuss three such models: (1) the Eker rat model of tuberous sclerosis, in which a gene mutation gives rise to cortical disorganization and cytologically abnormal cellular elements; (2) the p35 knockout mouse, in which the genetic dysfunction gives rise to compromised cortical organization and lamination, but in which the cellular elements appear normal; and (3) the methylazoxymethanol-exposed rat, in which time-specific chemical DNA disruption leads to abnormal patterns of cell formation and migration, resulting in heterotopic neuronal clusters. Integrating data from studies of these animal models with related clinical observations, we propose that the neuropathologic features of these cortical dysplastic lesions are insufficient to determine the seizure-initiating process. Rather, it is their interaction with a more subtly disrupted cortical "surround" that constitutes the circuitry underlying epileptiform activities as well as seizure propensity and ictogenesis.

Mossy cell dendritic structure quantified and compared with other hippocampal neurons labeled in rats in vivo

Mossy cells are likely to contribute to normal hippocampal function and to the pathogenesis of neurologic disorders that involve the hippocampus, including epilepsy. Mossy cells are the least well-characterized excitatory neurons in the hippocampus. Their somatic and dendritic morphology has been described qualitatively but not quantitatively. In the present study rat mossy cells were labeled intracellularly with biocytin in vivo. Somatic and dendritic structure was reconstructed three-dimensionally. For comparison, granule cells, CA3 pyramidal cells, and CA1 pyramidal cells were labeled and analyzed using the same approach. Among the four types of hippocampal neurons, granule cells had the smallest somata, fewest primary dendrites and dendritic branches, and shortest total dendritic length. CA1 pyramidal cells had the most dendritic branches and longest total dendritic length. Mossy cells and CA3 pyramidal cells both had large somata and similar total dendritic lengths. However, mossy cell dendrites branched less than CA3 pyramidal cells, especially close to the soma. These findings suggest that mossy cells have dendritic features that are not identical to any other type of hippocampal neuron. Therefore, electrotonic properties that depend on soma-dendritic structure are likely to be distinct in mossy cells compared to other neurons.

Mutation of Semaphorin-6A disrupts limbic and cortical connectivity and models neurodevelopmental psychopathology

Psychiatric disorders such as schizophrenia and autism are characterised by cellular disorganisation and dysconnectivity across the brain and can be caused by mutations in genes that control neurodevelopmental processes. To examine how neurodevelopmental defects can affect brain function and behaviour, we have comprehensively investigated the consequences of mutation of one such gene, Semaphorin-6A, on cellular organisation, axonal projection patterns, behaviour and physiology in mice. These analyses reveal a spectrum of widespread but subtle anatomical defects in Sema6A mutants, notably in limbic and cortical cellular organisation, lamination and connectivity. These mutants display concomitant alterations in the electroencephalogram and hyper-exploratory behaviour, which are characteristic of models of psychosis and reversible by the antipsychotic clozapine. They also show altered social interaction and deficits in object recognition and working memory. Mice with mutations in Sema6A or the interacting genes may thus represent a highly informative model for how neurodevelopmental defects can lead to anatomical dysconnectivity, resulting, either directly or through reactive mechanisms, in dysfunction at the level of neuronal networks with associated behavioural phenotypes of relevance to psychiatric disorders. The biological data presented here also make these genes plausible candidates to explain human linkage findings for schizophrenia and autism.

Stereologic estimation of hippocampal GluR2/3- and calretinin-immunoreactive hilar neurons (presumptive mossy cells) in two mouse models of temporal lobe epilepsy

PURPOSE

Hippocampal mossy cells receive dense innervation from dentate granule cells and, in turn, mossy cells innervate both granule cells and interneurons. Mossy cell loss is thought to trigger granule cell mossy fiber sprouting, which may affect granule cell excitability. The aim of this study was to quantify mossy cell loss in two animal models of temporal lobe epilepsy, and determine whether there exists a relationship between mossy cell loss, mossy fiber sprouting, and granule cell dispersion.

METHODS

Representative hippocampal sections from p35 knockout mice and mice with unilateral intrahippocampal kainate injection were immunolabeled for GluR2/3, two subunits of the amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor and calretinin to identify mossy cells. Mossy fibers were immunostained against synaptoporin.

KEY FINDINGS

p35 Knockout mice showed no hilar cell death, but moderate mossy fiber sprouting and granule cell dispersion. In the kainate-injected hippocampus, there was an 80% and 85% reduction of GluR2/3- and GluR2/3/calretinin-positive hilar neurons, respectively, and dense mossy fiber sprouting and significant granule cell dispersion. In the contralateral hippocampus there was a 52% loss of GluR2/3-, but only a 20% loss of GluR2/3-calretinin-immunoreactive presumptive mossy cells, and granule cell dispersion; no mossy fiber sprouting was observed.

SIGNIFICANCE

These results indicate a probable lack of causality between mossy cell death and mossy fiber sprouting.

A new hypothesis of drug refractory epilepsy: neural network hypothesis

Drug refractory is an important clinical problem in epilepsy, affecting a substantial number of patients globally. Mechanisms underlying drug refractory need to be understood to develop rational therapies. Current two prevailing theories on drug refractory epilepsy (DRE) include the target hypothesis and the transporter hypothesis. However, those hypotheses could not be adequate to explain the mechanisms of all the DRE. Thus, we propose another possible mechanism of DRE, which is neural network hypothesis. It is hypothesized that seizure-induced alterations of brain plasticity including axonal sprouting, synaptic reorganization, neurogenesis and gliosis could contribute to the formation of abnormal neural network, which has not only avoided the inhibitory effect of endogenous antiepileptic system but also prevented the traditional antiepileptic drugs from entering their targets, eventually leading to DRE. We will illustrate this hypothesis at molecular and structural level based on our recent studies and other related researches.

Addictive nicotine alters local circuit inhibition during the induction of in vivo hippocampal synaptic potentiation

The drug addiction process shares many commonalities with normal learning and memory. Addictive drugs subvert normal synaptic plasticity mechanisms, and the consequent synaptic changes underlie long-lasting modifications in behavior that accrue during the progression from drug use to addiction. Supporting this hypothesis, it was recently shown that nicotine administered to freely moving mice induces long-term synaptic potentiation of the perforant path connection to granule cells of the dentate gyrus. The perforant path carries place and spatial information that links the environment to drug taking. An example of that association is the nicotine-induced synaptic potentiation of the perforant path that was found to underlie nicotine-conditioned place preference. The present study examines the influence of nicotine over local GABAergic inhibition within the dentate gyrus during the drug-induced synaptic potentiation. In vivo recordings from freely moving mice suggested that both feedforward and feedback inhibition onto granules cells were diminished by nicotine during the induction of synaptic potentiation. In vitro brain slice studies indicated that nicotine altered local circuit inhibition within the dentate gyrus leading to disinhibition of granule cells. These changes in local inhibition contributed to nicotine-induced in vivo synaptic potentiation, thus, likely contributed to drug-associated memories. Through this learning process, environmental features become cues that motivate conditioned drug-seeking and drug-taking behaviors.

Conditional deletion of neuronal cyclin-dependent kinase 5 in developing forebrain results in microglial activation and neurodegeneration

Neuronal migration disorders are often identified in patients with epilepsy refractory to medical treatment. The prolonged or repeated seizures are known to cause neuronal death; however, the mechanism underlying seizure-induced neuronal death remains to be elucidated. An essential role of cyclin-dependent kinase 5 (Cdk5) in brain development has been demonstrated in Cdk5(-/-) mice, which show neuronal migration defects and perinatal lethality. Here, we show the consequences of Cdk5 deficiency in the postnatal brain by generating Cdk5 conditional knockout mice, in which Cdk5is selectively eliminated from neurons in the developing forebrain. The conditional mutant mice were viable, but exhibited complex neurological deficits including seizures, tremors, and growth retardation. The forebrain not only showed disruption of layering, but also neurodegenerative changes accompanied by neuronal loss and microglial activation. The neurodegenerative changes progressed with age and were accompanied by up-regulation of the neuronal tissue-type plasminogen activator, a serine protease known to mediate microglial activation. Thus age-dependent neurodegeneration in the Cdk5 conditional knockout mouse brain invoked a massive inflammatory reaction. These findings indicate an important role of Cdk5 in inflammation, and also provide a mouse model to examine the possible involvement of inflammation in the pathogenesis of progressive cognitive decline in patients with neuronal migration disorders.

Pharmacogenetic analysis reveals a post-developmental role for Rac GTPases in Caenorhabditis elegans GABAergic neurotransmission

The nerve-cell cytoskeleton is essential for the regulation of intrinsic neuronal activity. For example, neuronal migration defects are associated with microtubule regulators, such as LIS1 and dynein, as well as with actin regulators, including Rac GTPases and integrins, and have been thought to underlie epileptic seizures in patients with cortical malformations. However, it is plausible that post-developmental functions of specific cytoskeletal regulators contribute to the more transient nature of aberrant neuronal activity and could be masked by developmental anomalies. Accordingly, our previous results have illuminated functional roles, distinct from developmental contributions, for Caenorhabditis elegans orthologs of LIS1 and dynein in GABAergic synaptic vesicle transport. Here, we report that C. elegans with function-altering mutations in canonical Rac GTPase-signaling-pathway members demonstrated a robust behavioral response to a GABA(A) receptor antagonist, pentylenetetrazole. Rac mutants also exhibited hypersensitivity to an acetylcholinesterase inhibitor, aldicarb, uncovering deficiencies in inhibitory neurotransmission. RNA interference targeting Rac hypomorphs revealed synergistic interactions between the dynein motor complex and some, but not all, members of Rac-signaling pathways. These genetic interactions are consistent with putative Rac-dependent regulation of actin and microtubule networks and suggest that some cytoskeletal regulators cooperate to uniquely govern neuronal synchrony through dynein-mediated GABAergic vesicle transport in C. elegans.

Arrested maturation of excitatory synapses in autosomal dominant lateral temporal lobe epilepsy

A subset of central glutamatergic synapses are coordinatelypruned and matured by unresolved mechanisms during early postnatal life. We report that human epilepsy gene LGI1, mutated in autosomal dominant lateral temporal lobe epilepsy (ADLTE), mediates this process in hippocampus. We introduced full-length genes encoding (1) ADLTE truncated mutant LGI1 (835delC) and (2) excess wild-type LGI1 proteins into transgenic mice. We discovered that the normal postnatal Kv1 channel-dependent down-regulation of presynaptic release probability and Src kinase-related decrease of postsynaptic NR2B/NR2A ratio were arrested by ADLTE mutant LGI1, and contrastingly, were magnified by excess wild-type LGI1. Concurrently, mutant LGI1 inhibited dendritic pruning and increased the spine density to markedly increase excitatory transmission. Inhibitory transmission, by contrast, was unaffected. Furthermore, mutant LGI1 promoted epileptiform discharge in vitro and kindling epileptogenesis in vivo with partial GABA_A receptor blockade. Thus, LGI1 represents the first human gene mutated to promote epilepsy through impaired glutamatergic circuit maturation.

Inhibitory inputs to hippocampal interneurons are reorganized in Lis1 mutant mice

Epilepsy and brain malformation are commonly associated with excessive synaptic excitation and decreased synaptic inhibition of principal neurons. However, few studies have examined the state of synaptic inhibition of interneurons in an epileptic, malformed brain. We analyzed inhibitory inputs, mediated by gamma-aminobutyric acid (GABA), to hippocampal interneurons in a mouse model of type 1 lissencephaly, a neurological disorder linked with severe seizures and brain malformation. In the disorganized hippocampal area CA1 of Lis1(+/-) mice, we initially observed a selective displacement of fast-spiking, parvalbumin-positive basket-type interneurons from stratum oriens (SO) locations to s. radiatum and s. lacunosum-moleculare (R/LM). Next, we recorded spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) onto visually identified interneurons located in SO or R/LM of Lis1(+/-) mice and age-matched littermate controls. We observed significant, layer-specific reorganizations in GABAergic inhibition of interneurons in Lis1 mutant mice. Spontaneous IPSC frequency onto SO interneurons was significantly increased in hippocampal slices from Lis1(+/-) mice, whereas mIPSC mean amplitude onto these interneurons was significantly decreased. In addition, the weighted decay times of sIPSCs and mIPSCs were significantly increased in R/LM interneurons. Taken together, these findings illustrate the extensive redistribution and reorganization of inhibitory connections between interneurons that can take place in a malformed brain.

Regulation of hippocampal and behavioral excitability by cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that has been implicated in learning, synaptic plasticity, neurotransmission, and numerous neurological disorders. We previously showed that conditional loss of Cdk5 in adult mice enhanced hippocampal learning and plasticity via modulation of calpain-mediated N-methyl-D-aspartic acid receptor (NMDAR) degradation. In the present study, we characterize the enhanced synaptic plasticity and examine the effects of long-term Cdk5 loss on hippocampal excitability in adult mice. Field excitatory post-synaptic potentials (fEPSPs) from the Schaffer collateral CA1 subregion of the hippocampus (SC/CA1) reveal that loss of Cdk5 altered theta burst topography and enhanced post-tetanic potentiation. Since Cdk5 governs NMDAR NR2B subunit levels, we investigated the effects of long-term Cdk5 knockout on hippocampal neuronal excitability by measuring NMDAR-mediated fEPSP magnitudes and population-spike thresholds. Long-term loss of Cdk5 led to increased Mg²⁺-sensitive potentials and a lower threshold for epileptiform activity and seizures. Biochemical analyses were performed to better understand the role of Cdk5 in seizures. Induced-seizures in wild-type animals led to elevated amounts of p25, the Cdk5-activating cofactor. Long-term, but not acute, loss of Cdk5 led to decreased p25 levels, suggesting that Cdk5/p25 may be activated as a homeostatic mechanism to attenuate epileptiform activity. These findings indicate that Cdk5 regulates synaptic plasticity, controls neuronal and behavioral stimulus-induced excitability and may be a novel pharmacological target for cognitive and anticonvulsant therapies.

Neuronal hyperactivity sustains the basal dendrites of immature dentate granule cells: time-lapse confocal analysis using hippocampal slice cultures

Dendritic morphogenesis is an essential process for the establishment of proper neural circuitry. In the epileptic hippocampus, mature dentate granule cells (GCs) possess basal dendrites (BDs), which is abnormal and is assumed to contribute to seizure progression. However, there is a lack of direct time-lapse evidence showing that neuronal hyperactivity regulates the dendritic development of GCs. In the present study, we carried out time-lapse confocal analysis of the dendritic morphogenesis of GCs in hippocampal slice cultures that were prepared from postnatal 6-day-old (P6) rats. By electroporating membrane-targeted green fluorescent protein at 5 days in vitro (DIV), we found that most of the Prox1-positive and calbindin-negative immature GCs possessed several BDs and filopodia-rich apical dendrites at 7 DIV. BDs were gradually eliminated from 7 to 9 DIV, and they completely vanished at 14 DIV in all the GCs examined. However, most BDs failed to retract from 7 to 9 DIV, when the GABA(A) receptor antagonist picrotoxin was chronically applied to induce epileptic conditions in the cultures. These effects were blocked by coapplying tetrodotoxin, a sodium channel blocker, thus convincing us that neuronal hyperactivity contributes to the maintenance of BDs. Further, in the picrotoxin-treated cultures, most of the GCs persistently exhibited several BDs even after 14 DIV. In contrast, neither the progressive pruning of the filopodia nor the branch dynamics of the apical dendrites during the culture periods was affected by picrotoxin. These results, for the first time, provide us with direct evidence that neuronal hyperactivity contributes to the stability of pre-existing BDs.

Mice lacking doublecortin and doublecortin-like kinase 2 display altered hippocampal neuronal maturation and spontaneous seizures

Mutations in doublecortin (DCX) are associated with intractable epilepsy in humans, due to a severe disorganization of the neocortex and hippocampus known as classical lissencephaly. However, the basis of the epilepsy in lissencephaly remains unclear. To address potential functional redundancy with murin Dcx, we targeted one of the closest homologues, doublecortin-like kinase 2 (Dclk2). Here, we report that Dcx; Dclk2-null mice display frequent spontaneous seizures that originate in the hippocampus, with most animals dying in the first few months of life. Elevated hippocampal expression of c-fos and loss of somatostatin-positive interneurons were identified, both known to correlate with epilepsy. Dcx and Dclk2 are coexpressed in developing hippocampus, and, in their absence, there is dosage-dependent disrupted hippocampal lamination associated with a cell-autonomous simplification of pyramidal dendritic arborizations leading to reduced inhibitory synaptic tone. These data suggest that hippocampal dysmaturation and insufficient receptive field for inhibitory input may underlie the epilepsy in lissencephaly, and suggest potential therapeutic strategies for controlling epilepsy in these patients.

Zebrafish Rohon-Beard neuron development: cdk5 in the midst

Cyclin-dependent kinase 5 (cdk5) is a proline-directed serine/threonine kinase that is activated mostly by association with its activators, p35 and p39. Initially projected as a neuron-specific kinase, cdk5 is expressed ubiquitously and its kinase activity solely depends on the presence of its activators, which are also found in some non-neuronal tissues. As a multifunctional protein, cdk5 has been linked to axonogenesis, cell migration, exocytosis, neuronal differentiation and apoptosis. Cdk5 plays a critical role in functions other than normal physiology, especially in neurodegeneration. Its contribution to both normal physiological as well as pathological processes is mediated by its specific substrates. Cdk5-null mice are embryonically lethal, therefore making it difficult to study precisely what cdk5 does to the nervous system at early stages of development, be it neuron development or programmed cell death. Zebrafish model system bypasses the impediment, as it is amenable to reverse genetics studies. One of the functions that we have followed for the cdk5 ortholog in zebrafish in vivo is its effect on the Rohon-Beard (RB) neurons. RB neurons are the primary sensory spinal neurons that die during the first two days of zebrafish development eventually to be replaced by the dorsal root ganglia (DRG). Based on ours studies and others', here we discuss possible mechanisms that may be involved in cdk5's role in RB neuron development and survival.

Unravelling the complex role of Cdk5 in the developing cerebral cortex

The normal development of the mammalian CNS is entirely dependent on the coordinated behavior of its cellular components. Particular importance is attributed to the correct morphology, migration and communication of neurons. Recent years have seen the identification of many extracellular, cell surface and intracellular signaling molecules that are important for normal CNS development, consequently triggering huge progress in our understanding of the complex processes involved. A key molecule to emerge is Cdk5. To date, Cdk5 has been functionally linked with controlled neuronal morphology, migration, synaptic function, cognition, drug addiction, neuronal death and neurodegeneration. The complexity of its function has been confirmed by the ever increasing number of diverse upstream regulators, protein substrates and biological consequences of altered catalytic function. The aim of this review is to consolidate recent findings concerning the role of Cdk5 in the developing nervous system, particularly the cerebral cortex, where its importance is most clearly evidenced.

Synaptic input to dentate granule cell basal dendrites in a rat model of temporal lobe epilepsy

In patients with temporal lobe epilepsy some dentate granule cells develop basal dendrites. The extent of excitatory synaptic input to basal dendrites is unclear, nor is it known whether basal dendrites receive inhibitory synapses. We used biocytin to intracellularly label individual granule cells with basal dendrites in epileptic pilocarpine-treated rats. An average basal dendrite had 3.9 branches, was 612 microm long, and accounted for 16% of a cell's total dendritic length. In vivo intracellular labeling and postembedding GABA-immunocytochemistry were used to evaluate synapses with basal dendrites reconstructed from serial electron micrographs. An average of 7% of 1,802 putative synapses were formed by GABA-positive axon terminals, indicating synaptogenesis by interneurons. Ninety-three percent of the identified synapses were GABA-negative. Most GABA-negative synapses were with spines, but at least 10% were with dendritic shafts. Multiplying basal dendrite length/cell and synapse density yielded an estimate of 180 inhibitory and 2,140 excitatory synapses per granule cell basal dendrite. Based on previous estimates of synaptic input to granule cells in control rats, these findings suggest an average basal dendrite receives approximately 14% of the total inhibitory and 19% of excitatory synapses of a cell. These findings reveal that basal dendrites are a novel source of inhibitory input, but they primarily receive excitatory synapses.

Involvement of septal Cdk5 in the emergence of excessive anxiety induced by stress

The aim of the present study was to evaluate whether the activation of Cdk5, a protein that has been suggested to participate in higher cognitive functions, is required for the onset of a sensitized anxiety-related behavior induced by stress. The exposure to restraint enhanced both Cdk5 expression in certain subareas of the septohippocampal system, principally in the lateral septum (LS) and septal Cdk5 kinase activity in rats. Behaviorally, restrained wild type mice showed a behavior indicative of enhanced anxiety in the elevated plus maze (EPM). In contrast, unstressed mice and stressed knockout mice, which lacked the p35 protein, the natural activator of Cdk5, displayed similar anxiety-like behavior in the EPM. Finally, the intra-LS infusion of olomoucine - a Cdk5 inhibitor - blocked the enhanced anxiety in the EPM induced by prior stress in rats. All these data provide evidence that septal Cdk5 is required in the emergence of a sensitized emotional process induced by stress.