Disrupted hippocampal network physiology following PTEN deletion from newborn dentate granule cells

Vector-mediated PTEN deletion in the adult dentate gyrus initiates new growth of granule cell bodies and dendrites and expansion of mossy fiber terminal fields that continues for months

Embryonic and early postnatal deletion of the gene phosphatase and tensin homolog (PTEN) results in neuronal hypertrophy, formation of aberrant neural networks and spontaneous seizures. Our previous studies document that deletion of PTEN in mature neurons also causes growth of cortical neuron cell bodies and dendrites, but it is unknown how this growth alters connectivity in mature circuits. Here, we explore consequences of deleting PTEN in a focal area of the dentate gyrus in adult male and female mice. PTEN deletion was accomplished by injecting AAV-Cre unilaterally into the dentate gyrus of double transgenic mice with lox-P sites flanking exon 5 of the PTEN gene and stop/flox tdTomato in the Rosa locus (PTEN^f/f/Rosa^tdTomato). Focal deletion led to progressive increases in the size of the dentate gyrus at the injection site, enlargement of granule cell bodies, and increases in dendritic length and caliber. Quantitative analysis of dendrites by Golgi staining revealed dramatic increases in spine numbers throughout the proximo-distal extent of the dendritic tree, suggesting that dendritic growth is sufficient to induce new synapse formation by input neurons with intact PTEN expression. Tract tracing of input pathways to the dentate gyrus from the ipsilateral entorhinal cortex and commissural/associational system revealed that laminar specificity of termination of inputs is maintained. Mossy fiber axons from PTEN-deleted granule cells expanded their terminal field in CA3 where PTEN expression was intact and supra-granular mossy fibers developed in some mice. These findings document that persistent activation of mTOR via PTEN deletion in fully mature neurons re-initiates a state of robust cell-intrinsic growth, upending connectional homeostasis in fully mature hippocampal circuits.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

Neurovascular Development in Pten and Tsc2 Mouse Mutants

Hyperactivation of the mechanistic target of rapamycin (mTOR) signaling pathway is linked to more than a dozen neurologic diseases, causing a range of pathologies, including excess neuronal growth, disrupted neuronal migration, cortical dysplasia, epilepsy and autism. The mTOR pathway also regulates angiogenesis. For the present study, therefore, we queried whether loss of Pten or Tsc2, both mTOR negative regulators, alters brain vasculature in three mouse models: one with Pten loss restricted to hippocampal dentate granule cells [DGC-Pten knock-outs (KOs)], a second with widespread Pten loss from excitatory forebrain neurons (FB-Pten KOs) and a third with focal loss of Tsc2 from cortical excitatory neurons (f-Tsc2 KOs). Total hippocampal vessel length and volume per dentate gyrus were dramatically increased in DGC-Pten knock-outs. DGC-Pten knock-outs had larger dentate gyri overall, however, and when normalized to these larger structures, vessel density was preserved. In addition, tests of blood-brain barrier integrity did not reveal increased permeability. FB-Pten KOs recapitulated the findings in the more restricted DGC-Pten KOs, with increased vessel area, but preserved vessel density. FB-Pten KOs did, however, exhibit elevated levels of the angiogenic factor VegfA. In contrast to findings with Pten, focal loss of Tsc2 from cortical excitatory neurons produced a localized increase in vessel density. Together, these studies demonstrate that hypervascularization is not a consistent feature of mTOR hyperactivation models and suggest that loss of different mTOR pathway regulatory genes exert distinct effects on angiogenesis.

mTORC2 Inhibition Improves Morphological Effects of PTEN Loss, But Does Not Correct Synaptic Dysfunction or Prevent Seizures

Hyperactivation of PI3K/PTEN-mTOR signaling during neural development is associated with focal cortical dysplasia (FCD), autism, and epilepsy. mTOR can signal through two major hubs, mTORC1 and mTORC2, both of which are hyperactive following PTEN loss of function (LOF). Here, we tested the hypothesis that genetic inactivation of the mTORC2 complex via deletion of Rictor is sufficient to rescue morphologic and electrophysiological abnormalities in the dentate gyrus caused by PTEN loss, as well as generalized seizures. An established, early postnatal mouse model of PTEN loss in male and female mice showed spontaneous seizures that were not prevented by mTORC2 inactivation. This lack of rescue occurred despite the normalization or amelioration of many morphologic and electrophysiological phenotypes. However, increased excitatory connectivity proximal to dentate gyrus granule neuron somas was not normalized by mTORC2 inactivation. Further studies demonstrated that, although mTORC2 inactivation largely rescued the dendritic arbor overgrowth caused by PTEN LOF, it increased synaptic strength and caused additional impairments of presynaptic function. These results suggest that a constrained increase in excitatory connectivity and co-occurring synaptic dysfunction is sufficient to generate seizures downstream of PTEN LOF, even in the absence of characteristic changes in morphologic properties.SIGNIFICANCE STATEMENT Homozygous deletion of the Pten gene in neuronal subpopulations in the mouse serves as a valuable model of epilepsy caused by mTOR hyperactivation. To better understand the physiological mechanisms downstream of Pten loss that cause epilepsy, as well as the therapeutic potential of targeted gene therapies, we tested whether genetic inactivation of the mTORC2 complex could improve the cellular, synaptic, and in vivo effects of Pten loss in the dentate gyrus. We found that mTORC2 inhibition improved or rescued all morphologic effects of Pten loss in the dentate gyrus, but synaptic changes and seizures persisted. These data suggest that synaptic dysfunction can drive epilepsy caused by hyperactivation of PI3K/PTEN-mTOR, and that future therapies should focus on this mechanistic link.

OUP accepted manuscript

Epilepsy is one of the most frequent neurological diseases, with focal epilepsy accounting for the largest number of cases. The genetic alterations involved in focal epilepsy are far from being fully elucidated.

Here, we show that defective lipid signalling caused by heterozygous ultra-rare variants in PIK3C2B, encoding for the class II phosphatidylinositol 3-kinase PI3K-C2β, underlie focal epilepsy in humans. We demonstrate that patients’ variants act as loss-of-function alleles, leading to impaired synthesis of the rare signalling lipid phosphatidylinositol 3,4-bisphosphate, resulting in mTORC1 hyperactivation. In vivo, mutant Pik3c2b alleles caused dose-dependent neuronal hyperexcitability and increased seizure susceptibility, indicating haploinsufficiency as a key driver of disease. Moreover, acute mTORC1 inhibition in mutant mice prevented experimentally induced seizures, providing a potential therapeutic option for a selective group of patients with focal epilepsy.

Our findings reveal an unexpected role for class II PI3K-mediated lipid signalling in regulating mTORC1-dependent neuronal excitability in mice and humans.

Life and death in the hippocampus: What's bad?

The hippocampal formation is crucial for the generation and regulation of several brain functions, including memory and learning processes; however, it is vulnerable to neurological disorders, such as epilepsy. Temporal lobe epilepsy (TLE), the most common type of epilepsy, changes the hippocampal circuitry and excitability, under the contribution of both neuronal degeneration and abnormal neurogenesis. Classically, neurodegeneration affects sensitive areas of the hippocampus, such as dentate gyrus (DG) hilus, as well as specific fields of the Ammon's horn, CA3, and CA1. In addition, the proliferation, migration, and abnormal integration of newly generated hippocampal granular cells (GCs) into the brain characterize TLE neurogenesis. Robust studies over the years have intensely discussed the effects of death and life in the hippocampus, though there are still questions to be answered about their possible benefits and risks. Here, we review the impacts of death and life in the hippocampus, discussing its influence on TLE, providing new perspectives or insights for the implementation of new possible therapeutic targets. This article is part of the Special Issue "NEWroscience 2018".

CDDO-Me Attenuates Astroglial Autophagy via Nrf2-, ERK1/2-SP1- and Src-CK2-PTEN-PI3K/AKT-Mediated Signaling Pathways in the Hippocampus of Chronic Epilepsy Rats

Clasmatodendrosis is an autophagic astroglial death showing extensive swollen cell bodies with vacuoles and disintegrated/beaded processes. This astroglial degeneration is closely relevant to the synchronous epileptiform discharges. However, the underlying molecular mechanisms and the roles of clasmatodendrosis in spontaneous seizure activity are still unknown. The 2-cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me; RTA 402) is one of the activators for nuclear factor-erythroid 2-related factor 2 (Nrf2) that is a redox-sensitive transcription factor. In the present study, we explored the effects of CDDO-Me on clasmatodendrosis in chronic epilepsy rats, which could prevent epilepsy-related complications. In the present study, clasmatodendritic astrocytes showed reduced Nrf2 expression and its nuclear accumulation, which were restored by CDDO-Me. CDDO-Me also abrogated heat shock protein 25 (HSP25) upregulation in clasmatodendritic astrocytes by regulating extracellular signal-related kinases 1/2 (ERK1/2)-specificity protein 1 (SP1)- and Src-casein kinase 2 (CK2)-phosphatase and tensin homolog deleted on chromosome 10 (PTEN)-phosphatidylinositol-3-kinase (PI3K)-AKT-glycogen synthase kinase 3β (GSK3β)-bax-interacting factor 1 (Bif-1)-mediated signaling pathways in chronic epilepsy rats. In addition, CDDO-Me ameliorated spontaneous seizure duration, but not seizure frequency and behavioral seizure severity. Therefore, our findings suggest that clasmatodendrosis may affect seizure duration in chronic epilepsy rats, and that CDDO-Me may attenuate autophagic astroglial degeneration by regulating various signaling pathways.

mTOR-driven neural circuit changes initiate an epileptogenic cascade

Mutations in genes regulating mTOR pathway signaling are now recognized as a significant cause of epilepsy. Interestingly, these mTORopathies are often caused by somatic mutations, affecting variable numbers of neurons. To better understand how this variability affects disease phenotype, we developed a mouse model in which the mTOR pathway inhibitor Pten can be deleted from 0 to 40 % of hippocampal granule cells. In vivo, low numbers of knockout cells caused focal seizures, while higher numbers led to generalized seizures. Generalized seizures coincided with the loss of local circuit interneurons. In hippocampal slices, low knockout cell loads produced abrupt reductions in population spike threshold, while spontaneous excitatory postsynaptic currents and circuit level recurrent activity increased gradually with rising knockout cell load. Findings demonstrate that knockout cells load is a critical variable regulating disease phenotype, progressing from subclinical circuit abnormalities to electrobehavioral seizures with secondary involvement of downstream neuronal populations.

Dual-specificity phosphatases in mental and neurological disorders

The dual-specificity phosphatase (DUSP) family includes a heterogeneous group of protein phosphatases that dephosphorylate both phospho-tyrosine and phospho-serine/phospho-threonine residues within a single substrate. These protein phosphatases have many substrates and modulate diverse neural functions, such as neurogenesis, differentiation, and apoptosis. DUSP genes have furthermore been associated with mental disorders such as depression and neurological disorders such as Alzheimer's disease. Herein, we review the current literature on the DUSP family of genes concerning mental and neurological disorders. This review i) outlines the structure and general functions of DUSP genes, and ii) overviews the literature on DUSP genes concerning mental and neurological disorders, including model systems, while furthermore providing perspectives for future research.

PTEN Is Required for The Anti-Epileptic Effects of AMPA Receptor Antagonists in Chronic Epileptic Rats

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is one of the ligand-gated ion channels for glutamate, which is an important player in the generation and spread of seizures. The efficacy of AMPAR functionality is regulated by the trafficking, synaptic targeting, and phosphorylation. Paradoxically, AMPAR expression and its phosphorylation level are decreased in the epileptic hippocampus. Therefore, the roles of AMPAR in seizure onset and neuronal hyperexcitability in ictogenesis remain to be elucidated. In the present study, we found that AMPAR antagonists (perampanel and GYKI 52466) decreased glutamate ionotropic receptor AMPA type subunit 1 (GRIA1) surface expression in the epileptic rat hippocampus. They also upregulated phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expression and restored to basal levels the upregulated phosphoinositide 3-kinase (PI3K)/AKT1 phosphorylations. Dipotassium bisperoxovanadium(pic) dihydrate (BpV(pic), a PTEN inhibitor) co-treatment abolished the anti-epileptic effects of perampanel and GYKI 52466. Therefore, our findings suggest that PTEN may be required for the anti-epileptic effects of AMPAR antagonists.

LncRNA CASC2 inhibits astrocytic activation and adenosine metabolism by regulating PTEN in pentylenetetrazol-induced epilepsy model

Growing evidence has indicated that long noncoding RNAs (lncRNAs) are closely implicated in the progress of epilepsy. However, the expression profile and potential function of long noncoding RNAs cancer susceptibility candidate 2 (lncRNA CASC2) in epilepsy are poorly studied. The aim of this study was to testify the influence of lncRNA CASC2 on epilepsy in rat and cell models of epileptic seizure. We adopted qRT-PCR on the hippocampus of rats following pentylenetetrazol (PTZ)-stimulated epilepsy. To further examine the correlation between lncRNA CASC2 and Phosphatase and tensin homolog (PTEN), we detected the effects of lncRNA CASC2 on PTEN expression. We found that lncRNA CASC2 and PTEN expression were positively correlated in PTZ-induced epileptic rat. Overexpression of lncRNA CASC2 prolonged the latency and reduced the frequency of epileptic seizure, suppressed the activation of astrocytes and the release of adenosine in epileptic rat, whereas downregulation of lncRNA CASC2 exhibited the opposite effects. Meanwhile, lncRNA CASC2 decreased the adenosine metabolism related proteins expression of p38, Equilibrative nucleoside transporter 1 (ENT1) and Adenosine Kinase (ADK). In PTZ-treated astrocytes, PTEN was found to be a direct target of lncRNA CASC2. Additionally, downregulation of PTEN attenuated the protective effect of lncRNA CASC2 overexpression in epileptic seizure. Our findings manifested the key role of lncRNA CASC2 in the occurrence of epilepsy by targeting PTEN, which provided a novel target for epilepsy therapy.

AAVshRNA-mediated PTEN knockdown in adult neurons attenuates activity-dependent immediate early gene induction

Genetic deletion or knockdown of PTEN enables regeneration of CNS axons, enhances sprouting of intact axons after injury, and induces de novo growth of uninjured adult neurons. It is unknown, however how PTEN deletion in mature neurons alters neuronal physiology. As a first step to address this question, we used immunocytochemistry for activity-dependent markers to assess consequences of PTEN knockdown in cortical neurons and granule cells of the dentate gyrus. In adult rats that received unilateral intra-cortical injections of AAV expressing shRNA against PTEN, immunostaining for c-fos under resting conditions (home cage, HC) and after 1 h of exploration of a novel enriched environment (EE) revealed no hot spots of c-fos expression that would suggest abnormal activity. Counts revealed similar numbers of c-fos positive neurons in the area of PTEN deletion vs. homologous areas in the contralateral cortex in the HC and similar induction of c-fos with EE. However, IEG induction in response to high frequency stimulation (HFS) of the cortex was attenuated in areas of PTEN deletion. In rats with AAVshRNA-mediated PTEN deletion in the dentate gyrus, induction of the IEGs c-fos and Arc with HFS of the perforant path was abrogated in areas of PTEN deletion. Immunostaining using phosphospecific antibodies for phospho-S6 (a downstream marker for mTOR activation) and phospho-ERK1/2 revealed abrogation of S6 phosphorylation in PTEN-deleted areas but preserved activation of phosphorylation of ERK1/2. SIGNIFICANCE STATEMENT: Deletion or knockdown of the tumor suppressor gene PTEN enables regenerative growth of adult CNS axons after injury, which is accompanied by enhanced recovery of function. Consequently, PTEN represents a potential target for therapeutic interventions to enhance recovery after CNS injury. Here we show that activity-dependent IEG induction is attenuated in PTEN-depleted neurons. These findings raise the intriguing possibility that functional recovery due to regenerative growth may be limited by the disruption of plasticity-related signaling pathways, and that recovery might be enhanced by restoring PTEN expression after regenerative growth has been achieved.

PTEN in Autism and Neurodevelopmental Disorders

Phosphatase and tensin homolog (PTEN) is a classical tumor suppressor that antagonizes phosphatidylinositol 3-phosphate kinase (PI3K)/AKT signaling. Although there is a strong association of PTEN germline mutations with cancer syndromes, they have also been described in a subset of patients with autism spectrum disorders with macrocephaly characterized by impairments in social interactions and communication, repetitive behavior and, occasionally, epilepsy. To investigate PTEN's role during neurodevelopment and its implication for autism, several conditional Pten knockout mouse models have been generated. These models are valuable tools to understand PTEN's spatiotemporal roles during neurodevelopment. In this review, we will highlight the anatomical and phenotypic results from animal studies and link them to cellular and molecular findings.

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.

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.

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.

Contributions of Adult-Generated Granule Cells to Hippocampal Pathology in Temporal Lobe Epilepsy: A Neuronal Bestiary

Hippocampal neurogenesis continues throughout life in mammals – including humans. During the development of temporal lobe epilepsy, newly-generated hippocampal granule cells integrate abnormally into the brain. Abnormalities include ectopic localization of newborn cells, de novo formation of abnormal basal dendrites, and disruptions of the apical dendritic tree. Changes in granule cell position and dendritic structure fundamentally alter the types of inputs these cells are able to receive, as well as the relative proportions of remaining inputs. Dendritic abnormalities also create new pathways for recurrent excitation in the hippocampus. These abnormalities are hypothesized to contribute to the development of epilepsy, and may underlie cognitive disorders associated with the disease as well. To test this hypothesis, investigators have used pharmacological and genetic strategies in animal models to alter neurogenesis rates, or ablate the newborn cells outright. While findings are mixed and many unanswered questions remain, numerous studies now demonstrate that ablating newborn granule cells can have disease modifying effects in epilepsy. Taken together, findings provide a strong rationale for continued work to elucidate the role of newborn granule cells in epilepsy: both to understand basic mechanisms underlying the disease, and as a potential novel therapy for epilepsy.

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.

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 STATEMENT

Hyperactivation 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.

PTEN Loss Increases the Connectivity of Fast Synaptic Motifs and Functional Connectivity in a Developing Hippocampal Network

Changes in synaptic strength and connectivity are thought to be a major mechanism through which many gene variants cause neurological disease. Hyperactivation of the PI3K-mTOR signaling network, via loss of function of repressors such as PTEN, causes epilepsy in humans and animal models, and altered mTOR signaling may contribute to a broad range of neurological diseases. Changes in synaptic transmission have been reported in animal models of PTEN loss; however, the full extent of these changes, and their effect on network function, is still unknown. To better understand the scope of these changes, we recorded from pairs of mouse hippocampal neurons cultured in a two-neuron microcircuit configuration that allowed us to characterize all four major connection types within the hippocampus. Loss of PTEN caused changes in excitatory and inhibitory connectivity, and these changes were postsynaptic, presynaptic, and transynaptic, suggesting that disruption of PTEN has the potential to affect most connection types in the hippocampal circuit. Given the complexity of the changes at the synaptic level, we measured changes in network behavior after deleting Pten from neurons in an organotypic hippocampal slice network. Slices containing Pten-deleted neurons showed increased recruitment of neurons into network bursts. Importantly, these changes were not confined to Pten-deleted neurons, but involved the entire network, suggesting that the extensive changes in synaptic connectivity rewire the entire network in such a way that promotes a widespread increase in functional connectivity.SIGNIFICANCE STATEMENT Homozygous deletion of the Pten gene in neuronal subpopulations in the mouse serves as a valuable model of epilepsy caused by mTOR hyperactivation. To better understand how gene deletions lead to altered neuronal activity, we investigated the synaptic and network effects that occur 1 week after Pten deletion. PTEN loss increased the connectivity of all four types of hippocampal synaptic connections, including two forms of increased inhibition of inhibition, and increased network functional connectivity. These data suggest that single gene mutations that cause neurological diseases such as epilepsy may affect a surprising range of connection types. Moreover, given the robustness of homeostatic plasticity, these diverse effects on connection types may be necessary to cause network phenotypes such as increased synchrony.

Apoptosis induced by the methanol extract of Salvia miltiorrhiza Bunge in non-small cell lung cancer through PTEN-mediated inhibition of PI3K/Akt pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Salvia miltiorrhiza Bunge, a well-known traditional Chinese medicinal (TCM) plant, has been used to treat cardiovascular diseases since thousands of years. Many studies reported that the active component tanshinones displayed a variety of biological activities: anti-thrombous, anti-allergic, anti-inflammatory, antioxidant and anti-tumor promoting. But the mechanism of how the active components working still need to be clarified. The anti-tumor effect of compounds of tanshinone (CTN), the methanol extract of Salvia miltiorrhiza Bunge roots, was investigated. The aim of this study was to investigate the effects of CTN on the growth inhibition, apoptosis and molecular targets of human non-small cell lung cancer (NSCLC).

MATERIALS AND METHODS

CTN-induced cytotoxicity was determined by MTT assay. The cell survival was evaluated using clonogenic survival assay. The morphology of Glc-82 cells after treatment with CTN was determined by fluorescence microscopy. Cell cycle distribution was revealed by flow cytometry. The apoptotic cells were quantified with annexin V-FITC/PI staining and flow cytometry, and observed using Hoechst 33258 staining and TUNEL assays. The expression levels of proteins were analyzed using western blot. Tumor growth was assessed by subcutaneous inoculation of cells into BALB/c nude mice.

RESULTS

CTN inhibited the proliferation of NSCLC in a dose-dependent manner and induced both early and late apoptosis. Treatment of Glc-82 cells with CTN (5-80μg/ml) significantly (p<0.05) suppressed the cell proliferation in a concentration and time-dependent manner. CTN induced significant (p<0.05) and dose-dependent apoptosis of Glc-82 cells. Cell cycle assay showed that CTN induced a G₂/M phase arrest, and significantly (p<0.05) increased expression of p53 and p21, actived caspase-3/9 and PARP1, which suggest the involvement of the mitochondria in the apoptotic signals. In addition, CTN decreased expression of the anti-apoptotic protein Bcl-2, Bcl-xl and increased expression of the pro-apoptotic protein Bax. Result also showed that CTN could increase expression levels of PTEN, and reduce the phosphorylated levels of Akt (protein kinase B) on Thr 308 and Ser 473 domain. In vivo assay showed that the antitumor effect of CTN was significantly augmented without increasing toxicity in nude mice bearing Glc-82 xenograft.

CONCLUSION

The PTEN/Akt signaling axis is defined as a critical pathway regulated by PTEN in NSCLC. CTN, the methanol extract of Salvia miltiorrhiza Bunge, are the active compounds as shown by their ability to induce apoptosis through the mitochondrial pathway of apoptosis and PTEN-mediated inhibition of PI3K/Akt pathway. CTN could inhibit tumor growth more efficiently, which supports the ethno-medicinal use of this herb as an alternative or complementary therapy for NSCLC.