TrkB (tropomyosin-related kinase B) controls the assembly and maintenance of GABAergic synapses in the cerebellar cortex

Ablation of TrkB from Enkephalinergic Precursor-Derived Cerebellar Granule Cells Generates Ataxia

In ataxia disorders, motor incoordination (ataxia) is primarily linked to the dysfunction and degeneration of cerebellar Purkinje cells (PCs). In spinocerebellar ataxia 6 (SCA6), for example, decreased BDNF-TrkB signalling appears to contribute to PC dysfunction and ataxia. However, abnormal BDNF-TrkB signalling in granule cells (GCs) may contribute to PC dysfunction and incoordination in ataxia disorders, as TrkB receptors are also present in GCs that provide extensive input to PCs. This study investigated whether dysfunctional BDNF-TrkB signalling restricted to a specific subset of cerebellar GCs can generate ataxia in mice. To address this question, our research focused on TrkbPenk-KO mice, in which the TrkB receptor was removed from enkephalinergic precursor-derived cerebellar GCs. We found that deleting Ntrk2, encoding the TrkB receptor, eventually interfered with PC function, leading to ataxia symptoms in the TrkbPenk-KO mice without affecting their cerebellar morphology or levels of selected synaptic markers. These findings suggest that dysfunctional BDNF-TrkB signalling in a subset of cerebellar GCs alone is sufficient to trigger ataxia symptoms and may contribute to motor incoordination in disorders like SCA6.

Neuroligin-1 dependent phosphotyrosine signaling in excitatory synapse differentiation

Introduction

The synaptic adhesion molecule neuroligin-1 (NLGN1) is involved in the differentiation of excitatory synapses, but the precise underlying molecular mechanisms are still debated. Here, we explored the role of NLGN1 tyrosine phosphorylation in this process, focusing on a subset of receptor tyrosine kinases (RTKs), namely FGFR1 and Trks, that were previously described to phosphorylate NLGN1 at a unique intracellular residue (Y782).

Methods

We used pharmacological inhibitors and genetic manipulation of those RTKs in dissociated hippocampal neurons, followed by biochemical measurement of NLGN1 phosphorylation and immunocytochemical staining of excitatory synaptic scaffolds.

Results

This study shows that: (i) the accumulation of PSD-95 at de novo NLGN1 clusters induced by neurexin crosslinking is reduced by FGFR and Trk inhibitors; (ii) the increase in PSD-95 puncta caused by NLGN1 over-expression is impaired by FGFR and Trk inhibitors; (iii) TrkB activation by BDNF increases NLGN1 phosphorylation; and (iv) TrkB knock-down impairs the increase of PSD-95 puncta caused by NLGN1 over-expression, an effect which is not seen with the NLGN1 Y782A mutant.

Discussion

Together, our data identify TrkB as one of the major RTKs responsible for NLGN1 tyrosine phosphorylation, and reveal that TrkB activity is necessary for the synaptogenic effects of NLGN1.

Engineered Mesenchymal Stem Cells Over-Expressing BDNF Protect the Brain from Traumatic Brain Injury-Induced Neuronal Death, Neurological Deficits, and Cognitive Impairments

Traumatic brain injury (TBI) causes transitory or permanent neurological and cognitive impairments, which can intensify over time due to secondary neuronal death. However, no therapy currently exists that can effectively treat brain injury following TBI. Here, we evaluate the therapeutic potential of irradiated engineered human mesenchymal stem cells over-expressing brain-derived neurotrophic factor (BDNF), which we denote by BDNF-eMSCs, in protecting the brain against neuronal death, neurological deficits, and cognitive impairment in TBI rats. BDNF-eMSCs were administered directly into the left lateral ventricle of the brain in rats that received TBI damage. A single administration of BDNF-eMSCs reduced TBI-induced neuronal death and glial activation in the hippocampus, while repeated administration of BDNF-eMSCs reduced not only glial activation and delayed neuronal loss but also enhanced hippocampal neurogenesis in TBI rats. In addition, BDNF-eMSCs reduced the lesion area in the damaged brain of the rats. Behaviorally, BDNF-eMSC treatment improved the neurological and cognitive functions of the TBI rats. The results presented in this study demonstrate that BDNF-eMSCs can attenuate TBI-induced brain damage through the suppression of neuronal death and increased neurogenesis, thus enhancing functional recovery after TBI, indicating the significant therapeutic potential of BDNF-eMSCs in the treatment of TBI.

Hippocampal parvalbumin interneurons play a critical role in memory development

Episodic memories formed in early childhood rapidly decay, but their latent traces remain stored long term. These memories require the dorsal hippocampus (dHPC) and seem to undergo a developmental critical period. It remains to be determined whether the maturation of parvalbumin interneurons (PVIs), a major mechanism of critical periods, contributes to memory development. Here, we show that episodic infantile learning significantly increases the levels of parvalbumin in the dHPC 48 h after training. Chemogenetic inhibition of PVIs before learning indicated that these neurons are required for infantile memory formation. A bilateral dHPC injection of the γ-aminobutyric acid type A receptor agonist diazepam after training elicited long-term memory expression in infant rats, although direct PVI chemogenetic activation had no effect. Finally, PVI activity was required for brain-derived neurotrophic factor (BDNF)-dependent maturation of memory competence, i.e., adult-like long-term memory expression. Thus, dHPC PVIs are critical for the formation of infantile memories and for memory development.

Identification and structural analysis of a selective tropomyosin receptor kinase C (TRKC) inhibitor

Tropomyosin receptor kinases (TRKs) are a family of TRKA, TRKB and TRKC isoforms. It has been widely reported that TRKs are implicated in a variety of tumors with several Pan-TRK inhibitors currently being used or evaluated in clinical treatment. However, off-target adverse events frequently occur in the clinical use of Pan-TRK inhibitors, which result in poor patient compliance, even drug discontinuation. Although a subtype-selectivity TRK inhibitor may avert the potential off-target adverse events and can act as a more powerful tool compound in the biochemical studies on TRKs, the high sequence similarities of TRKs hinder the development of subtype-selectivity TRK inhibitors. For example, no selective TRKC inhibitor has been reported. Herein, a selective TRKC inhibitor (L13) was disclosed, with potent TRKC inhibitory activity and 107.5-/34.9-fold selectivity over TRKA/B (IC₅₀ TRKA/B/C = 1400 nM, 454 nM, 13 nM, respectively). Extensive molecular dynamics simulations illustrated that key interactions of L13 with the residues and diversely conserved water molecules in the ribose regions of different TRKs may be the structural basis of selectivity. This will provide inspiring insights into the development of subtype-selectivity TRK inhibitors. Moreover, L13 could serve as a tool compound to investigate the distinct biological functions of TRKC and a starting point for further research on drugs specifically targeting TRKC.

GABAergic System Dysfunction in Autism Spectrum Disorders

Autism spectrum disorder (ASD) refers to a series of neurodevelopmental diseases characterized by two hallmark symptoms, social communication deficits and repetitive behaviors. Gamma-aminobutyric acid (GABA) is one of the most important inhibitory neurotransmitters in the central nervous system (CNS). GABAergic inhibitory neurotransmission is critical for the regulation of brain rhythm and spontaneous neuronal activities during neurodevelopment. Genetic evidence has identified some variations of genes associated with the GABA system, indicating an abnormal excitatory/inhibitory (E/I) neurotransmission ratio implicated in the pathogenesis of ASD. However, the specific molecular mechanism by which GABA and GABAergic synaptic transmission affect ASD remains unclear. Transgenic technology enables translating genetic variations into rodent models to further investigate the structural and functional synaptic dysregulation related to ASD. In this review, we summarized evidence from human neuroimaging, postmortem, and genetic and pharmacological studies, and put emphasis on the GABAergic synaptic dysregulation and consequent E/I imbalance. We attempt to illuminate the pathophysiological role of structural and functional synaptic dysregulation in ASD and provide insights for future investigation.

Pleiotropic effects of BDNF on the cerebellum and hippocampus: Implications for neurodevelopmental disorders

Brain-derived neurotrophic factor (BDNF) is one of the most studied neurotrophins in the mammalian brain, essential not only to the development of the central nervous system but also to synaptic plasticity. BDNF is present in various brain areas, but highest levels of expression are seen in the cerebellum and hippocampus. After birth, BDNF acts in the cerebellum as a mitogenic and chemotactic factor, stimulating the cerebellar granule cell precursors to proliferate, migrate and maturate, while in the hippocampus BDNF plays a fundamental role in synaptic transmission and plasticity, representing a key regulator for the long-term potentiation, learning and memory. Furthermore, the expression of BDNF is highly regulated and changes of its expression are associated with both physiological and pathological conditions. The purpose of this review is to provide an overview of the current state of knowledge on the BDNF biology and its neurotrophic role in the proper development and functioning of neurons and synapses in two important brain areas of postnatal neurogenesis, the cerebellum and hippocampus. Dysregulation of BDNF expression and signaling, resulting in alterations in neuronal maturation and plasticity in both systems, is a common hallmark of several neurodevelopmental diseases, such as autism spectrum disorder, suggesting that neuronal malfunction present in these disorders is the result of excessive or reduced of BDNF support. We believe that the more the relevance of the pathophysiological actions of BDNF, and its downstream signals, in early postnatal development will be highlighted, the more likely it is that new neuroprotective therapeutic strategies will be identified in the treatment of various neurodevelopmental disorders.

Deletion of TrkB in parvalbumin interneurons alters cortical neural dynamics

Signaling by neurotrophins such as the brain‐derived neurotrophic factor (BDNF) is known to modulate development of interneurons, but the circuit effects of this modulation remain unclear. Here, we examined the impact of deleting TrkB, a BDNF receptor, in parvalbumin‐expressing (PV) interneurons on the balance of excitation and inhibition (E‐I) in cortical circuits. In the mouse olfactory cortex, TrkB deletion impairs multiple aspects of PV neuronal function including synaptic excitation, intrinsic excitability, and the innervation pattern of principal neurons. Impaired PV cell function resulted in aberrant spiking patterns in principal neurons in response to stimulation of sensory inputs. Surprisingly, dampened PV neuronal function leads to a paradoxical decrease in overall excitability in cortical circuits. Our study demonstrates that, by modulating PV circuit plasticity and development, TrkB plays a critical role in shaping the evoked pattern of activity in a cortical network.

Assessment of the toxicity and toxicokinetics of the novel potent tropomyosin receptor kinase (Trk) inhibitor LPM4870108 in rhesus monkeys

LPM4870108 is a tropomyosin receptor kinase (Trk) inhibitor that is currently under consideration for human clinical trials. We characterized the toxicity and toxicokinetic properties of LPM4870108 following its oral administration to rhesus monkeys (5, 10, or 20 mg/kg/day for 4 weeks with a 4-week recovery period). No evidence of LPM4870108 toxicity was observed over this study as reflected by an absence of difference in body weight, ophthalmoscopy, urinalysis, gross, or histopathology findings. No significant differences in toxicity-related outcomes were detected when comparing LPM4870108 and control groups, and no significant treatment-related changes in food consumption, electrocardiogram results, blood pressure, hematological parameters, biochemical values, organ weight, or bone marrow parameters were observed. Treatment caused dose-dependent effects of gait disturbance, impaired balance, poor coordination, and decreased grip strength in all LPM4870108-treated animals, with these effects being attributable to excessive on-target Trk receptor inhibition. After the 4-week recovery period, all these abnormal treatment-related findings had fully or partially resolved. The toxicokinetic study of monkeys revealed that the LPM4870108 exposure increased with dose. Overall, LPM4870108 exhibited a safety profile in treated monkeys, indicating that the Highest Non-Severely Toxic Dose (HNSTD) for LPM4870108 in monkeys was 20 mg/kg/day.

mPFC GABAergic transmission mediated the role of BDNF signaling in cognitive impairment but not anxiety induced by adolescent social stress

Depression with comorbid anxiety or cognitive symptoms can vary in terms of symptoms, pathophysiology and antidepressant efficacy, but the underlying neurobiological mechanisms remain to be elucidated. Previous studies from our group and others have shown that as a classic animal model of depression, adolescent social stress (ASS) could stably induce a variety of emotional and cognitive alterations in adult animals, and accompanied by transcriptional decrease in brain-derived neurotrophic factor (BDNF) total and promoter IV levels in the medial prefrontal cortex (mPFC). The present study further identified the GABAergic synaptic and molecular changes downstream of BDNF signaling impairment in the mPFC and roles in various behavioral phenotypes induced by ASS. We found that ASS induced a set of emotional and cognitive symptoms, including decreased social interest, impaired cognitive function, and increased anxiety-like behavior, as well as decreased GABAergic transmission in the mPFC. The specific deletion of BDNF promoter IV directly caused impairments in social interest, cognitive function, and inhibition of GABAergic transmission, but no changes in anxiety-like behavior. Acute microinjections of tropomyosin-related kinase B (TrkB) agonists into the mPFC and chronic antidepressant treatment ameliorated the changes in social behavior and cognition, as well as the reduction in GABAergic synaptic transmission in the mPFC, but not anxiety in previously stressed adult mice. These results suggest that the downstream GABAergic transmission of BDNF signaling in the mPFC involved in depression with comorbid cognitive dysfunction induced by ASS and can be used as a therapeutic target for the treatment of cognitive dysfunction in depression. This article is part of the special issue on Stress, Addiction and Plasticity.

Brain-Derived Neurotrophic Factor Val66Met polymorphism interacts with adolescent stress to alter hippocampal interneuron density and dendritic morphology in mice

Brain-derived neurotrophic factor (BDNF) plays essential roles in GABAergic interneuron development. The common BDNF val66met polymorphism, leads to decreased activity-dependent release of BDNF. The current study used a humanized mouse model of the BDNF val66met polymorphism to determine how reduced activity-dependent release of BDNF, both on its own, and in combination with chronic adolescent stress hormone, impact hippocampal GABAergic interneuron cell density and dendrite morphology. Male and female Val/Val and Met/Met mice were exposed to corticosterone (CORT) or placebo in their drinking water from weeks 6-8, before brains were perfuse-fixed at 15 weeks. Cell density and dendrite morphology of immunofluorescent labelled inhibitory interneurons; somatostatin, parvalbumin and calretinin in the CA1, and 3 and dentate gyrus (DG) across the dorsal (DHP) and ventral hippocampus (VHP) were assessed by confocal z-stack imaging, and IMARIS dendritic mapping software. Mice with the Met/Met genotype showed significantly lower somatostatin cell density compared to Val/Val controls in the DHP, and altered somatostatin interneuron dendrite morphology including branch depth, and spine density. Parvalbumin-positive interneurons were unchanged between genotype groups, however BDNF val66met genotype influenced the dendritic volume, branch level and spine density of parvalbumin interneurons differentially across hippocampal subregions. Contrary to this, no such effects were observed for calretinin-positive interneurons. Adolescent exposure to CORT treatment also significantly altered somatostatin and parvalbumin dendrite branch level and the combined effect of Met/Met genotype and CORT treatment significantly reduced somatostatin and parvalbumin dendrite spine density. In sum, the BDNF^Val66Met polymorphism significantly alters somatostatin and parvalbumin-positive interneuron cell development and dendrite morphology. Additionally, we also report a compounding effect of the Met/Met genotype and chronic adolescent CORT treatment on dendrite spine density, indicating that adolescence is a sensitive period of risk for Val66Met polymorphism carriers.

Characterization of on-target adverse events caused by TRK inhibitor therapy

BACKGROUND

The tropomyosin receptor kinase (TRK) pathway controls appetite, balance, and pain sensitivity. While these functions are reflected in the on-target adverse events (AEs) observed with TRK inhibition, these AEs remain under-recognized, and pain upon drug withdrawal has not previously been reported. As TRK inhibitors are approved by multiple regulatory agencies for TRK or ROS1 fusion-positive cancers, characterizing these AEs and corresponding management strategies is crucial.

PATIENTS AND METHODS

Patients with advanced or unresectable solid tumors treated with a TRK inhibitor were retrospectively identified in a search of clinical databases. Among these patients, the frequency, severity, duration, and management outcomes of AEs including weight gain, dizziness or ataxia, and withdrawal pain were characterized.

RESULTS

Ninety-six patients with 15 unique cancer histologies treated with a TRK inhibitor were identified. Weight gain was observed in 53% [95% confidence interval (CI), 43%-62%] of patients and increased with time on TRK inhibition. Pharmacologic intervention, most commonly with glucagon-like peptide 1 analogs or metformin, appeared to result in stabilization or loss of weight. Dizziness, with or without ataxia, was observed in 41% (95% CI, 31%-51%) of patients with a median time to onset of 2 weeks (range, 3 days to 16 months). TRK inhibitor dose reduction was the most effective intervention for dizziness. Pain upon temporary or permanent TRK inhibitor discontinuation was observed in 35% (95% CI, 24%-46%) of patients; this was more common with longer TRK inhibitor use. TRK inhibitor reinitiation was the most effective intervention for withdrawal pain.

CONCLUSIONS

TRK inhibition-related AEs including weight gain, dizziness, and withdrawal pain occur in a substantial proportion of patients receiving TRK inhibitors. This safety profile is unique relative to other anticancer therapies and warrants careful monitoring. These on-target toxicities are manageable with pharmacologic intervention and dose modification.

Loss of TrkB Signaling in Parvalbumin-Expressing Basket Cells Results in Network Activity Disruption and Abnormal Behavior

The GABAergic system is regulated by the brain-derived neurotrophic factor (BDNF)/Tropomyosin-related kinase B (TrkB) pathway, but the cell-intrinsic role of TrkB signaling in parvalbumin cortical interneuron development and function is unclear. We performed conditional ablation of the TrkB receptor in parvalbumin-expressing (PV) interneurons to study whether postnatal loss of TrkB in parvalbumin cells affects their survival, connectivity, spontaneous and evoked neuronal activity and behavior. Using in vivo recordings of local field potentials, we found reduced gamma oscillations in the sensory cortex of PVcre+; TrkBF/F conditional knockout mice (TrkB cKO), along with increased firing of putative excitatory neurons. There was a significant downregulation in parvalbumin neuron number in cerebral and cerebellar cortices of TrkB cKO mice. In addition, inhibitory synaptic connections between basket cells and pyramidal neurons were profoundly reduced in the neocortex of TrkB cKO mice and there was a loss of cortical volume. TrkB cKO mice also showed profound hyperactivity, stereotypies, motor deficits and learning/memory defects. Our findings demonstrate that the targeting and/or synapse formation of PV-expressing basket cells with principal excitatory neurons require TrkB signaling in parvalbumin cells. Disruption of this signaling has major consequences for parvalbumin interneuron connectivity, network dynamics, cognitive and motor behavior.

Trkb-IP3 Pathway Mediating Neuroprotection in Rat Hippocampal Neuronal Cell Culture Following Induction of Kainic Acid

BACKGROUND

Following brain injury, development of hippocampal sclerosis often led to the temporal lobe epilepsy which is sometimes resistant to common anti-epileptic drugs. Cellular and molecular changes underlying epileptogenesis in animal models were studied, however, the underlying mechanisms of kainic acid (KA) mediated neuronal damage in rat hippocampal neuron cell culture alone has not been elucidated yet.

METHODS

Embryonic day 18 (E-18) rat hippocampus neurons were cultured with poly-L-lysine coated glass coverslips. Following optimisation, KA (0.5 μM), a chemoconvulsant agent, was administered at three different time-points (30, 60 and 90 min) to induce seizure in rat hippocampal neuronal cell culture. We examined cell viability, neurite outgrowth density and immunoreactivity of the hippocampus neuron culture by measuring brain derived neurotrophic factor (BDNF), γ-amino butyric acid A (GABAA) subunit α-1 (GABRA1), tyrosine receptor kinase B (TrkB), and inositol trisphosphate receptor (IP3R/IP3) levels.

RESULTS

The results revealed significantly decreased and increased immunoreactivity changes in TrkB (a BDNF receptor) and IP3R, respectively, at 60 min time point.

CONCLUSION

The current findings suggest that TrkB and IP3 could have a neuroprotective role which could be a potential pharmacological target for anti-epilepsy drugs.

Pathobiological expression of the brain-derived neurotrophic factor (BDNF) in cerebellar cortex of sudden fetal and infant death victims

Brain-derived neurotrophic factor (BDNF), a neurotrophin of the central nervous system, is able to regulate neuronal differentiation and modulate synaptic plasticity, being particularly involved in the development of the cerebellar cortical structure. The main aim of this study was to delineate, by immunohistochemistry, the BDNF expression in human cerebellar cortex of victims of fetal and infant death. The study was performed on a total of 45 cases, aged between 25 gestational weeks and 6 postnatal months, including 29 victims of sudden fetal and infant death and 16 age-matched subjects who died of known causes (Controls). We observed, in sudden death groups compared with Controls, a significantly higher incidence of defective BDNF expression in granule layers of the cerebellar cortex, which was particularly evident in the posterior lobule, a region that participates in respiratory control. These results were related to maternal smoking, allowing to speculate that nicotine, in addition to the well-known damages, can exert adverse effects during cerebellar cortex development, in particular in hindering the BDNF expression in the posterior lobule. This implies modifications of synaptic transmission in the respiratory circuits, with obvious deleterious consequences on survival.

Comparative Response of Brain to Chronic Hypoxia and Hyperoxia

Two antithetic terms, hypoxia and hyperoxia, i.e., insufficient and excess oxygen availability with respect to needs, are thought to trigger opposite responses in cells and tissues. This review aims at summarizing the molecular and cellular mechanisms underlying hypoxia and hyperoxia in brain and cerebral tissue, a context that may prove to be useful for characterizing not only several clinically relevant aspects, but also aspects related to the evolution of oxygen transport and use by the tissues. While the response to acute hypoxia/hyperoxia presumably recruits only a minor portion of the potentially involved cell machinery, focusing into chronic conditions, instead, enables to take into consideration a wider range of potential responses to oxygen-linked stress, spanning from metabolic to genic. We will examine how various brain subsystems, including energetic metabolism, oxygen sensing, recruitment of pro-survival pathways as protein kinase B (Akt), mitogen-activated protein kinases (MAPK), neurotrophins (BDNF), erythropoietin (Epo) and its receptors (EpoR), neuroglobin (Ngb), nitric oxide (NO), carbon monoxide (CO), deal with chronic hypoxia and hyperoxia to end-up with the final outcomes, oxidative stress and brain damage. A more complex than expected pattern results, which emphasizes the delicate balance between the severity of the stress imposed by hypoxia and hyperoxia and the recruitment of molecular and cellular defense patterns. While for certain functions the expectation that hypoxia and hyperoxia should cause opposite responses is actually met, for others it is not, and both emerge as dangerous treatments.

Prefrontal Cortex GABAergic Deficits and Circuit Dysfunction in the Pathophysiology and Treatment of Chronic Stress and Depression

Psychiatric diseases, notably major depression, are associated with imbalance of excitatory and inhibitory neurotransmission within the prefrontal cortex (PFC) and related limbic brain circuitry. In many cases these illnesses are precipitated or exacerbated by chronic stress, which also alters excitatory and inhibitory neurotransmitter systems. Notably, exposure to repeated uncontrollable stress causes persistent changes in the synaptic integrity and function of the principal glutamatergic excitatory neurons in the PFC, characterized by neuronal atrophy and loss of synaptic connections. This can lead to dysfunction of the PFC circuitry that is necessary for execution of adaptive behavioral responses. In addition, an emerging literature shows that chronic stress also causes extensive alteration of GABAergic inhibitory circuits in the PFC, leading to the hypothesis that inhibitory neurotransmitter deficits contribute to changes in PFC neuronal excitability and cognitive impairments. Here we review evidence in rodents and human, which point to the mechanisms underlying stress-induced alterations of GABA transmission in the PFC, and its relevance to circuit dysfunction in mood and stress related disorders. These findings suggest that alterations of GABA interneurons and inhibitory neurotransmission play a causal role in the development of stress-related neurobiological illness, and could identify a new line of GABA related therapeutic targets.

Severe Intellectual Disability and Enhanced Gamma-Aminobutyric Acidergic Synaptogenesis in a Novel Model of Rare RASopathies

BACKGROUND

Dysregulation of Ras-extracellular signal-related kinase (ERK) signaling gives rise to RASopathies, a class of neurodevelopmental syndromes associated with intellectual disability. Recently, much attention has been directed at models bearing mild forms of RASopathies whose behavioral impairments can be attenuated by inhibiting the Ras-ERK cascade in the adult. Little is known about the brain mechanisms in severe forms of these disorders.

METHODS

We performed an extensive characterization of a new brain-specific model of severe forms of RASopathies, the KRAS^12V mutant mouse.

RESULTS

The KRAS^12V mutation results in a severe form of intellectual disability, which parallels mental deficits found in patients bearing mutations in this gene. KRAS^12V mice show a severe impairment of both short- and long-term memory in a number of behavioral tasks. At the cellular level, an upregulation of ERK signaling during early phases of postnatal development, but not in the adult state, results in a selective enhancement of synaptogenesis in gamma-aminobutyric acidergic interneurons. The enhancement of ERK activity in interneurons at this critical postnatal time leads to a permanent increase in the inhibitory tone throughout the brain, manifesting in reduced synaptic transmission and long-term plasticity in the hippocampus. In the adult, the behavioral and electrophysiological phenotypes in KRAS^12V mice can be temporarily reverted by inhibiting gamma-aminobutyric acid signaling but not by a Ras-ERK blockade. Importantly, the synaptogenesis phenotype can be rescued by a treatment at the developmental stage with Ras-ERK inhibitors.

CONCLUSIONS

These data demonstrate a novel mechanism underlying inhibitory synaptogenesis and provide new insights in understanding mental dysfunctions associated to RASopathies.

Gephyrin and the regulation of synaptic strength and dynamics at glycinergic inhibitory synapses

Glycinergic synapses predominate in brainstem and spinal cord where they modulate motor and sensory processing. Their postsynaptic mechanisms have been considered rather simple because they lack a large variety of glycine receptor isoforms and have relatively simple postsynaptic densities at the ultrastructural level. However, this simplicity is misleading being their postsynaptic regions regulated by a variety of complex mechanisms controlling the efficacy of synaptic inhibition. Early studies suggested that glycinergic inhibitory strength and dynamics depend largely on structural features rather than on molecular complexity. These include regulation of the number of postsynaptic glycine receptors, their localization and the amount of co-localized GABA_A receptors and GABA-glycine co-transmission. These properties we now know are under the control of gephyrin. Gephyrin is the first postsynaptic scaffolding protein ever discovered and it was recently found to display a large degree of variation and regulation by splice variants, posttranslational modifications, intracellular trafficking and interactions with the underlying cytoskeleton. Many of these mechanisms are governed by converging excitatory activity and regulate gephyrin oligomerization and receptor binding, the architecture of the postsynaptic density (and by extension the whole synaptic complex), receptor retention and stability. These newly uncovered molecular mechanisms define the size and number of gephyrin postsynaptic regions and the numbers and proportions of glycine and GABA_A receptors contained within. All together, they control the emergence of glycinergic synapses of different strength and temporal properties to best match the excitatory drive received by each individual neuron or local dendritic compartment.

Prenatal Cocaine Exposure Upregulates BDNF-TrkB Signaling

Prenatal cocaine exposure causes profound changes in neurobehavior as well as synaptic function and structure with compromised glutamatergic transmission. Since synaptic health and glutamatergic activity are tightly regulated by brain-derived neurotrophic factor (BDNF) signaling through its cognate tyrosine receptor kinase B (TrkB), we hypothesized that prenatal cocaine exposure alters BDNF-TrkB signaling during brain development. Here we show prenatal cocaine exposure enhances BDNF-TrkB signaling in hippocampus and prefrontal cortex (PFCX) of 21-day-old rats without affecting the expression levels of TrkB, P75^NTR, signaling molecules, NMDA receptor—NR1 subunit as well as proBDNF and BDNF. Prenatal cocaine exposure reduces activity-dependent proBDNF and BDNF release and elevates BDNF affinity for TrkB leading to increased tyrosine-phosphorylated TrkB, heightened Phospholipase C-γ1 and N-Shc/Shc recruitment and higher downstream PI3K and ERK activation in response to ex vivo BDNF. The augmented BDNF-TrkB signaling is accompanied by increases in association between activated TrkB and NMDARs. These data suggest that cocaine exposure during gestation upregulates BDNF-TrkB signaling and its interaction with NMDARs by increasing BDNF affinity, perhaps in an attempt to restore the diminished excitatory neurotransmission.

Regulation of GABAergic synapse development by postsynaptic membrane proteins

In the adult mammalian brain, GABAergic neurotransmission provides the majority of synaptic inhibition that balances glutamatergic excitatory drive and thereby controls neuronal output. It is generally accepted that synaptogenesis is initiated through highly specific protein-protein interactions mediated by membrane proteins expressed in developing presynaptic terminals and postsynaptic membranes. Accumulating studies have uncovered a number of membrane proteins that regulate different aspects of GABAergic synapse development. In this review, we summarize recent advances in understanding of GABAergic synapse development with a focus on postsynaptic membrane molecules, including receptors, synaptogenic cell adhesion molecules and immunoglobulin superfamily proteins.

Selective Requirement for Maintenance of Synaptic Contacts onto Motoneurons by Target-Derived trkB Receptors

Synaptic contacts onto motoneurons were studied in mice in which the gene for the trkB neurotrophin receptor was knocked out selectively in a subset of spinal motoneurons. The extent of contacts by structures immunoreactive for either of two different vesicular glutamate transporters (VGLUT1 and VGLUT2), the vesicular GABA transporter, or glutamic acid decarboxylase 67 (GAD67) with the somata of motoneurons, was studied in wild type and trkB knockout cells in tamoxifen treated male and female SLICK-trkB^−/− mice. Selective knockout of the trkB gene resulted in a marked reduction in contacts made by VGLUT2- and GAD67-immunoreactive structures in both sexes and a significant reduction in contacts containing only glycine in male mice. No reduction was found for glycinergic contacts in female mice or for VGLUT1 immunoreactive contacts in either sex. Signaling through postsynaptic trkB receptors is considered to be an essential part of a cellular mechanism for maintaining the contacts of some, but not all, synaptic contacts onto motoneurons.

Glutamatergic axon-derived BDNF controls GABAergic synaptic differentiation in the cerebellum

To study mechanisms that regulate the construction of inhibitory circuits, we examined the role of brain-derived neurotrophic factor (BDNF) in the assembly of GABAergic inhibitory synapses in the mouse cerebellar cortex. We show that within the cerebellum, BDNF-expressing cells are restricted to the internal granular layer (IGL), but that the BDNF protein is present within mossy fibers which originate from cells located outside of the cerebellum. In contrast to deletion of TrkB, the cognate receptor for BDNF, deletion of Bdnf from cerebellar cell bodies alone did not perturb the localization of pre- or postsynaptic constituents at the GABAergic synapses formed by Golgi cell axons on granule cell dendrites within the IGL. Instead, we found that BDNF derived from excitatory mossy fiber endings controls their differentiation. Our findings thus indicate that cerebellar BDNF is derived primarily from excitatory neurons--precerebellar nuclei/spinal cord neurons that give rise to mossy fibers--and promotes GABAergic synapse formation as a result of release from axons. Thus, within the cerebellum the preferential localization of BDNF to axons enhances the specificity through which BDNF promotes GABAergic synaptic differentiation.

The balancing act of GABAergic synapse organizers

GABA (γ-aminobutyric acid) is the main neurotransmitter at inhibitory synapses in the mammalian brain. It is essential for maintaining the excitation and inhibition (E/I) ratio, whose imbalance underlies various brain diseases. Emerging information about inhibitory synapse organizers provides a novel molecular framework for understanding E/I balance at the synapse, circuit, and systems levels. This review highlights recent advances in deciphering these components of the inhibitory synapse and their roles in the development, transmission, and circuit properties of inhibitory synapses. We also discuss how their dysfunction may lead to a variety of brain disorders, suggesting new therapeutic strategies based on balancing the E/I ratio.

Contactin 1 as a potential biomarker promotes cell proliferation and invasion in thyroid cancer

Contactin 1 (CNTN1) as a member of the immunoglobulin superfamily plays important role in the development of nervous system. Recent studies find that elevated CNTN1 can promote the metastasis of cancer. However, the expression and function of CNTN1 in thyroid cancer are still unknown. Here, we firstly find CNTN1 is a new gene which can be regulated by RET/PTC3 (Ret proto-oncogene and Ret-activating protein ELE1) rearrangement gene and the protein level of CNTN1 is increasing in thyroid cancer. Besides this change is positively associated with the TNM stage and tumor size. Moreover, we confirm that knockdown of CNTN1 significantly inhibits the tumor proliferation, invasiveness and represses the expression of cyclin D1 (CCND1). In conclusion, CNTN1 will be a potential diagnosis biomarker and therapy target for thyroid cancer.

Postnatal reduction of BDNF regulates the developmental remodeling of taste bud innervation

The refinement of innervation is a common developmental mechanism that serves to increase the specificity of connections following initial innervation. In the peripheral gustatory system, the extent to which innervation is refined and how refinement might be regulated is unclear. The initial innervation of taste buds is controlled by brain-derived neurotrophic factor (BDNF). Following initial innervation, taste receptor cells are added and become newly innervated. The connections between the taste receptor cells and nerve fibers are likely to be specific in order to retain peripheral coding mechanisms. Here, we explored the possibility that the down-regulation of BDNF regulates the refinement of taste bud innervation during postnatal development. An analysis of BDNF expression in Bdnf(lacZ/+) mice and real-time reverse transcription polymerase chain reaction (RT-PCR) revealed that BDNF was down-regulated between postnatal day (P) 5 and P10. This reduction in BDNF expression was due to a loss of precursor/progenitor cells that express BDNF, while the expression of BDNF in the subpopulations of taste receptor cells did not change. Gustatory innervation, which was identified by P2X3 immunohistochemistry, was lost around the perimeter where most progenitor/precursor cells are located. In addition, the density of innervation in the taste bud was reduced between P5 and P10, because taste buds increase in size without increasing innervation. This reduction of innervation density was blocked by the overexpression of BDNF in the precursor/progenitor population of taste bud cells. Together these findings indicate that the process of BDNF restriction to a subpopulation of taste receptor cells between P5 and P10, results in a refinement of gustatory innervation. We speculate that this refinement results in an increased specificity of connections between neurons and taste receptor cells during development.

Increased cortical synaptic activation of TrkB and downstream signaling markers in a mouse model of Down Syndrome

Down Syndrome (DS), trisomy 21, is characterized by synaptic abnormalities and cognitive deficits throughout the lifespan and with development of Alzheimer's disease (AD) neuropathology and progressive cognitive decline in adults. Synaptic abnormalities are also present in the Ts65Dn mouse model of DS, but which synapses are affected and the mechanisms underlying synaptic dysfunction are unknown. Here we show marked increases in the levels and activation status of TrkB and associated signaling proteins in cortical synapses in Ts65Dn mice. Proteomic analysis at the single synapse level of resolution using array tomography (AT) uncovered increased colocalization of activated TrkB with signaling endosome related proteins, and demonstrated increased TrkB signaling. The extent of increases in TrkB signaling differed in each of the cortical layers examined and with respect to the type of synapse, with the most marked increases seen in inhibitory synapses. These findings are evidence of markedly abnormal TrkB-mediated signaling in synapses. They raise the possibility that dysregulated TrkB signaling contributes to synaptic dysfunction and cognitive deficits in DS.

GABAergic control of depression-related brain states

The GABAergic deficit hypothesis of major depressive disorders (MDDs) posits that reduced γ-aminobutyric acid (GABA) concentration in brain, impaired function of GABAergic interneurons, altered expression and function of GABA(A) receptors, and changes in GABAergic transmission dictated by altered chloride homeostasis can contribute to the etiology of MDD. Conversely, the hypothesis posits that the efficacy of currently used antidepressants is determined by their ability to enhance GABAergic neurotransmission. We here provide an update for corresponding evidence from studies of patients and preclinical animal models of depression. In addition, we propose an explanation for the continued lack of genetic evidence that explains the considerable heritability of MDD. Lastly, we discuss how alterations in GABAergic transmission are integral to other hypotheses of MDD that emphasize (i) the role of monoaminergic deficits, (ii) stress-based etiologies, (iii) neurotrophic deficits, and (iv) the neurotoxic and neural circuit-impairing consequences of chronic excesses of glutamate. We propose that altered GABAergic transmission serves as a common denominator of MDD that can account for all these other hypotheses and that plays a causal and common role in diverse mechanistic etiologies of depressive brain states and in the mechanism of action of current antidepressant drug therapies.

Defects in dendrite and spine maturation and synaptogenesis associated with an anxious-depressive-like phenotype of GABAA receptor-deficient mice

Mice that were rendered heterozygous for the γ2 subunit of GABAA receptors (γ2(+/-) mice) have been characterized extensively as a model for major depressive disorder. The phenotype of these mice includes behavior indicative of heightened anxiety, despair, and anhedonia, as well as defects in hippocampus-dependent pattern separation, HPA axis hyperactivity and increased responsiveness to antidepressant drugs. The γ2(+/-) model thereby provides strong support for the GABAergic deficit hypothesis of major depressive disorder. Here we show that γ2(+/-) mice additionally exhibit specific defects in late stage survival of adult-born hippocampal granule cells, including reduced complexity of dendritic arbors and impaired maturation of synaptic spines. Moreover, cortical γ2(+/-) neurons cultured in vitro show marked deficits in GABAergic innervation selectively when grown under competitive conditions that may mimic the environment of adult-born hippocampal granule cells. Finally, brain extracts of γ2(+/-) mice show a numerical but insignificant trend (p = 0.06) for transiently reduced expression of brain derived neurotrophic factor (BDNF) at three weeks of age, which might contribute to the previously reported developmental origin of the behavioral phenotype of γ2(+/-) mice. The data indicate increasing congruence of the GABAergic, glutamatergic, stress-based and neurotrophic deficit hypotheses of major depressive disorder.

Gephyrin: a master regulator of neuronal function?

The neurotransmitters GABA and glycine mediate fast synaptic inhibition by activating ligand-gated chloride channels--namely, type A GABA (GABA(A)) and glycine receptors. Both types of receptors are anchored postsynaptically by gephyrin, which self-assembles into a scaffold and interacts with the cytoskeleton. Current research indicates that postsynaptic gephyrin clusters are dynamic assemblies that are held together and regulated by multiple protein-protein interactions. Moreover, post-translational modifications of gephyrin regulate the formation and plasticity of GABAergic synapses by altering the clustering properties of postsynaptic scaffolds and thereby the availability and function of receptors and other signalling molecules. Here, we discuss the formation and regulation of the gephyrin scaffold, its role in GABAergic and glycinergic synaptic function and the implications for the pathophysiology of brain disorders caused by abnormal inhibitory neurotransmission.

Brain-derived neurotrophic factor promotes gephyrin protein expression and GABAA receptor clustering in immature cultured hippocampal cells

Fast synaptic inhibition in the adult brain is largely mediated by GABAA receptors (GABAAR). GABAAR are anchored to synaptic sites by gephyrin, a scaffolding protein that appears to be assembled as a hexagonal lattice beneath the plasma membrane. Brain derived neurotrophic factor (BDNF) alters the clustering and synaptic distribution of GABAAR but mechanisms behind this regulation are just starting to emerge. The current study was aimed to examine if BDNF alters the protein levels and/or clustering of gephyrin and to investigate whether the modulation of gephyrin is accompanied by changes in the distribution and/or clustering of GABAAR. Exogenous application of BDNF to immature neuronal cultures from rat hippocampus increased the protein levels and clustering of gephyrin. BDNF also augmented the association of gephyrin with GABAAR and promoted the formation of GABAAR clusters. Together, these observations indicate that BDNF might regulate the assembly of GABAergic synapses by promoting the association of GABAAR with gephyrin.

The class 4 semaphorin Sema4D promotes the rapid assembly of GABAergic synapses in rodent hippocampus

Proper circuit function in the mammalian nervous system depends on the precise assembly and development of excitatory and inhibitory synaptic connections between neurons. Through a loss-of-function genetic screen in cultured hippocampal neurons, we previously identified the class 4 Semaphorin Sema4D as being required for proper GABAergic synapse development. Here we demonstrate that Sema4D is sufficient to promote GABAergic synapse formation in rodent hippocampus and investigate the kinetics of this activity. We find that Sema4D treatment of rat hippocampal neurons increases the density of GABAergic synapses as detected by immunocytochemistry within 30 min, much more rapidly than has been previously described for a prosynaptogenic molecule, and show that this effect is dependent on the Sema4D receptor PlexinB1 using PlxnB1(-/-) mice. Live imaging studies reveal that Sema4D elicits a rapid enhancement (within 10 min) in the rate of addition of synaptic proteins. Therefore, we demonstrate that Sema4D, via PlexinB1, acts to initiate synapse formation by recruiting molecules to both the presynaptic and the postsynaptic terminals; these nascent synapses subsequently become fully functional by 2 h after Sema4D treatment. In addition, acute treatment of an organotypic hippocampal slice epilepsy model with Sema4D reveals that Sema4D rapidly and dramatically alters epileptiform activity, which is consistent with a Sema4D-mediated shift in the balance of excitation and inhibition within the circuit. These data demonstrate an ability to quickly assemble GABAergic synapses in response to an appropriate signal and suggest a potential area of exploration for the development of novel antiepileptic drugs.

Integrin α3 is required for late postnatal stability of dendrite arbors, dendritic spines and synapses, and mouse behavior

Most dendrite branches and a large fraction of dendritic spines in the adult rodent forebrain are stable for extended periods of time. Destabilization of these structures compromises brain function and is a major contributing factor to psychiatric and neurodegenerative diseases. Integrins are a class of transmembrane extracellular matrix receptors that function as αβ heterodimers and activate signaling cascades regulating the actin cytoskeleton. Here we identify integrin α3 as a key mediator of neuronal stability. Dendrites, dendritic spines, and synapses develop normally in mice with selective loss of integrin α3 in excitatory forebrain neurons, reaching their mature sizes and densities through postnatal day 21 (P21). However, by P42, integrin α3 mutant mice exhibit significant reductions in hippocampal dendrite arbor size and complexity, loss of dendritic spine and synapse densities, and impairments in hippocampal-dependent behavior. Furthermore, gene-dosage experiments demonstrate that integrin α3 interacts functionally with the Arg nonreceptor tyrosine kinase to activate p190RhoGAP, which inhibits RhoA GTPase and regulates hippocampal dendrite and synapse stability and mouse behavior. Together, our data support a fundamental role for integrin α3 in regulating dendrite arbor stability, synapse maintenance, and proper hippocampal function. In addition, these results provide a biochemical and structural explanation for the defects in long-term potentiation, learning, and memory reported previously in mice lacking integrin α3.

A comprehensive small interfering RNA screen identifies signaling pathways required for gephyrin clustering

The postsynaptic scaffold protein gephyrin is clustered at inhibitory synapses and serves for the stabilization of GABA(A) receptors. Here, a comprehensive kinome-wide siRNA screen in a human HeLa cell-based model for gephyrin clustering was used to identify candidate protein kinases implicated in the stabilization of gephyrin clusters. As a result, 12 hits were identified including FGFR1 (FGF receptor 1), TrkB, and TrkC as well as components of the MAPK and mammalian target of rapamycin (mTOR) pathways. For confirmation, the impact of these hits on gephyrin clustering was analyzed in rat primary hippocampal neurons. We found that brain-derived neurotrophic factor (BDNF) acts on gephyrin clustering through MAPK signaling, and this process may be controlled by the MAPK signaling antagonist sprouty2. BDNF signaling through phosphatidylinositol 3-kinase (PI3K)-Akt also activates mTOR and represses GSK3β, which was previously shown to reduce gephyrin clustering. Gephyrin is associated with inactive mTOR and becomes released upon BDNF-dependent mTOR activation. In primary neurons, a reduction in the number of gephyrin clusters due to manipulation of the BDNF-mTOR signaling is associated with reduced GABA(A) receptor clustering, suggesting functional impairment of GABA signaling. Accordingly, application of the mTOR antagonist rapamycin leads to disinhibition of neuronal networks as measured on microelectrode arrays. In conclusion, we provide evidence that BDNF regulates gephyrin clustering via MAPK as well as PI3K-Akt-mTOR signaling.

The neuroprotective roles of BDNF in hypoxic ischemic brain injury

Hypoxia-ischemia (H/I) brain injury results in various degrees of damage to the body, and the immature brain is particularly fragile to oxygen deprivation. Hypothermia and erythropoietin (EPO) have long been known to be neuroprotective in ischemic brain injury. Brain-derived neurotrophic factor (BDNF) has recently been recognized as a potent modulator capable of regulating a wide repertoire of neuronal functions. This review was based on studies concerning the involvement of BDNF in the protection of H/I brain injury following a search in PubMed between 1995 and December, 2011. We initially examined the background of BDNF, and then focused on its neuroprotective mechanisms against ischemic brain injury, including its involvement in promoting neural regeneration/cognition/memory rehabilitation, angiogenesis within ischemic penumbra and the inhibition of the inflammatory process, neurotoxicity, epilepsy and apoptosis. We also provided a literature overview of experimental studies, discussing the safety and the potential clinical application of BDNF as a neuroprotective agent in the ischemic brain injury.

Emerging themes in GABAergic synapse development

Glutamatergic synapse development has been rigorously investigated for the past two decades at both the molecular and cell biological level yet a comparable intensity of investigation into the cellular and molecular mechanisms of GABAergic synapse development has been lacking until relatively recently. This review will provide a detailed overview of the current understanding of GABAergic synapse development with a particular emphasis on assembly of synaptic components, molecular mechanisms of synaptic development, and a subset of human disorders which manifest when GABAergic synapse development is disrupted. An unexpected and emerging theme from these studies is that glutamatergic and GABAergic synapse development share a number of overlapping molecular and cell biological mechanisms that will be emphasized in this review.