mTORC1 Is a Local, Postsynaptic Voltage Sensor Regulated by Positive and Negative Feedback Pathways

17β-estradiol status alters NMDAR function and antipsychotic-like activity in female rats

Low 17β-estradiol (E2) in females of reproductive age, and marked E2 decline with menopause, contributes to heightened symptom severity in schizophrenia (i.e. cognitive dysfunction) and diminished response to antipsychotic medications. However, the underlying mechanisms are unknown. N-methyl-D-aspartate receptor (NMDAR) hypofunction contributes to the pathophysiology of schizophrenia, yet impact of E2 depletion on NMDAR function is not well characterized. Quantitative electroencephalography (qEEG), specifically gamma power, is a well-established functional readout of cortical activity that is elevated in patients with schizophrenia and is sensitive to alterations in NMDAR function. Using qEEG and touchscreen cognitive assessments, present studies investigated the effects of E2 on NMDAR function by administering MK-801 (NMDAR antagonist) to ovariectomized rats with or without E2 implants (Ovx+E and Ovx, respectively). Ovx rats were more sensitive to MK-801-induced elevations in gamma power and attentional impairments compared to Ovx+E rats. Further investigation revealed these effects were mediated by reduced synaptic GluN2A expression. Consistent with clinical reports, olanzapine (second-generation antipsychotic) was less effective in mitigating MK-801-induced elevations in gamma power in Ovx rats. Lastly, we examined antipsychotic-like activity of a Group II metabotropic glutamate receptor (mGlu2/3) positive allosteric modulator (PAM), SBI-0646535, as a novel therapeutic in E2-deprived conditions. SBI-0646535 reversed MK-801-induced elevations in gamma power regardless of E2 status. Collectively, these studies established a relationship between E2 deprivation and NMDAR function that is in part GluN2A-dependent, supporting the notion that E2 deprivation increases susceptibility to NMDAR hypofunction. This highlights the need to examine age/hormone-specific factors when considering antipsychotic response and designing novel pharmacotherapies.

Hippocampal Inhibitory Interneuron-Specific DREADDs Treatment Alters mTORC1-4E-BP Signaling and Impairs Memory Formation

Control of protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) is essential for learning and memory. However, the cell-type-specific and spatiotemporal regulation of this pathway during memory formation is not well understood. In this study, we expressed artificial human muscarinic M3 [hM3D(Gq)] or M4 [hM4D(Gi)] designer receptors exclusively activated by designer drugs (DREADDs) in hippocampal CA1 excitatory or inhibitory neurons of adult mice. We studied the impact of clozapine-N-oxide (CNO), a synthetic DREADDs agonist, on the mTORC1 pathway and long-term memory. hM3D(Gq) and hM4D(Gi) activate or inactivate, respectively, mTORC1 signaling in hippocampal interneurons, as indicated by the phosphorylation of its targets, eukaryotic initiation factor 4E-binding proteins (4E-BP1/2) and ribosomal protein S6 (S6). Activation of either hM3D(Gq) or hM4D(Gi) in mice immediately after training in memory tasks impaired long-term memory formation in inhibitory, but not in excitatory neurons. The findings underscore the importance of activity-dependent mTORC1-4E-BP1/2 signaling in hippocampal inhibitory interneurons for memory formation.

17β-estradiol status alters NMDAR function and antipsychotic-like activity in female rats

Low 17β-estradiol (E2) in females of reproductive age, and marked E2 decline with menopause, contributes to heightened symptom severity in schizophrenia (i.e. cognitive dysfunction) and diminished response to antipsychotic medications. However, the underlying mechanisms are unknown. N-methyl-D-aspartate receptor (NMDAR) hypofunction contributes to the pathophysiology of schizophrenia, yet impact of E2 depletion on NMDAR function is not well characterized. Quantitative electroencephalography (qEEG), specifically gamma power, is a well-established functional readout of cortical activity that is elevated in patients with schizophrenia and is sensitive to alterations in NMDAR function. Using qEEG and touchscreen cognitive assessments, present studies investigated the effects of E2 on NMDAR function by administering MK-801 (NMDAR antagonist) to ovariectomized rats with or without E2 implants (Ovx+E and Ovx, respectively). Ovx rats were more sensitive to MK-801-induced elevations in gamma power and attentional impairments compared to Ovx+E rats. Further investigation revealed these effects were mediated by reduced synaptic GluN2A expression. Consistent with clinical reports, olanzapine (second-generation antipsychotic) was less effective in mitigating MK-801-induced elevations in gamma power in Ovx rats. Lastly, we examined antipsychotic-like activity of a Group II metabotropic glutamate receptor (mGlu2/3) positive allosteric modulator (PAM), SBI-0646535, as a novel therapeutic in E2-deprived conditions. SBI-0646535 reversed MK-801-induced elevations in gamma power equally regardless of E2 status. Collectively, these studies established a relationship between E2 deprivation and NMDAR function that is in part GluN2A-dependent, supporting the notion that E2 deprivation increases susceptibility to NMDAR hypofunction. This highlights the need to examine age/hormone-specific factors when considering antipsychotic response and designing novel pharmacotherapies.

Potential roles of voltage-gated ion channel disruption in Tuberous Sclerosis Complex

Tuberous Sclerosis Complex (TSC) is a lynchpin disorder, as it results in overactive mammalian target of rapamycin (mTOR) signaling, which has been implicated in a multitude of disease states. TSC is an autosomal dominant disease where 90% of affected individuals develop epilepsy. Epilepsy results from aberrant neuronal excitability that leads to recurring seizures. Under neurotypical conditions, the coordinated activity of voltage-gated ion channels keep neurons operating in an optimal range, thus providing network stability. Interestingly, loss or gain of function mutations in voltage-gated potassium, sodium, or calcium channels leads to altered excitability and seizures. To date, little is known about voltage-gated ion channel expression and function in TSC. However, data is beginning to emerge on how mTOR signaling regulates voltage-gated ion channel expression in neurons. Herein, we provide a comprehensive review of the literature describing common seizure types in patients with TSC, and suggest possible parallels between acquired epilepsies with known voltage-gated ion channel dysfunction. Furthermore, we discuss possible links toward mTOR regulation of voltage-gated ion channels expression and channel kinetics and the underlying epileptic manifestations in patients with TSC.

In vitro modulation of mTOR and mGlur5 influence α-synuclein accumulation

One of the main hallmarks of Parkinson's disease (PD) is abnormal alpha-synuclein (α-syn) aggregation which forms the main component of intracellular Lewy body inclusions. This short report used preformed α-syn fibrils, as well as an A53T mutant α-syn adenovirus to mimic conditions of pathological protein aggregation in dopaminergic human derived SH-SY5Y neural cells. Since there is evidence that the mTOR pathway and glutamatergic signaling each influence protein aggregation, we also assessed the impact of the mTOR inhibitor, rapamycin and the mGluR5 allosteric modulator, CTEP. We found that both rapamycin and CTEP induced a significant reduction of α-syn fibrils in SH-SY5Y cells and this effect was associated with a reduction in mTOR signaling and enhancement in autophagic pathway factors. These data support the possibility that CTEP (or rapamycin) might be a useful pharmacological approach to target abnormal α-syn accumulation by promoting intracellular degradation or enhanced clearance.

Aberrant DJ-1 expression underlies L-type calcium channel hypoactivity in dendrites in tuberous sclerosis complex and Alzheimer's disease

L-type voltage-gated calcium (Ca2+) channels (L-VGCC) dysfunction is implicated in several neurological and psychiatric diseases. While a popular therapeutic target, it is unknown whether molecular mechanisms leading to disrupted L-VGCC across neurodegenerative disorders are conserved. Importantly, L-VGCC integrate synaptic signals to facilitate a plethora of cellular mechanisms; however, mechanisms that regulate L-VGCC channel density and subcellular compartmentalization are understudied. Herein, we report that in disease models with overactive mammalian target of rapamycin complex 1 (mTORC1) signaling (or mTORopathies), deficits in dendritic L-VGCC activity are associated with increased expression of the RNA-binding protein (RBP) Parkinsonism-associated deglycase (DJ-1). DJ-1 binds the mRNA coding for the alpha and auxiliary Ca2+ channel subunits CaV1.2 and α2δ2, and represses their mRNA translation, only in the disease states, specifically preclinical models of tuberous sclerosis complex (TSC) and Alzheimer's disease (AD). In agreement, DJ-1-mediated repression of CaV1.2/α2δ2 protein synthesis in dendrites is exaggerated in mouse models of AD and TSC, resulting in deficits in dendritic L-VGCC calcium activity. Finding of DJ-1-regulated L-VGCC activity in dendrites in TSC and AD provides a unique signaling pathway that can be targeted in clinical mTORopathies.

PI3K/AKT/mTOR signalling inhibitor chrysophanol ameliorates neurobehavioural and neurochemical defects in propionic acid-induced experimental model of autism in adult rats

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder marked by social and communication deficits as well as repetitive behaviour. Several studies have found that overactivation of the PI3K/AKT/mTOR signalling pathways during brain development plays a significant role in autism pathogenesis. Overexpression of the PI3K/AKT/mTOR signalling pathway causes neurological disorders by increasing cell death, neuroinflammation, and oxidative stress. Chrysophanol, also known as chrysophanic acid, is a naturally occurring chemical obtained from the plant Rheum palmatum. This study aimed to examine the neuroprotective effect of CPH on neurobehavioral, molecular, neurochemical, and gross pathological alterations in ICV-PPA induced experimental model of autism in adult rats. The effects of ICV-PPA on PI3K/AKT/mTOR downregulation in the brain were studied in autism-like rats. Furthermore, we investigated how CPH affected myelin basic protein (MBP) levels in rat brain homogenate and apoptotic biomarkers such as caspase-3, Bax, and Bcl-2 levels in rat brain homogenate and blood plasma samples. Rats were tested for behavioural abnormalities such as neuromuscular dysfunction using an actophotometer, motor coordination using a beam crossing task (BCT), depressive behaviour using a forced swim test (FST), cognitive deficiency, and memory consolidation using a Morris water maze (MWM) task. In PPA-treated rats, prolonged oral CPH administration from day 12 to day 44 of the experimental schedule reduces autistic-like symptoms. Furthermore, in rat brain homogenates, blood plasma, and CSF samples, cellular, molecular, and cell death markers, neuroinflammatory cytokines, neurotransmitter levels, and oxidative stress indicators were investigated. The recent findings imply that CPH also restores abnormal neurochemical levels and may prevent autism-like gross pathological alterations, such as demyelination volume, in the rat brain.

mTORC1 function in hippocampal parvalbumin interneurons: regulation of firing and long-term potentiation of intrinsic excitability but not long-term contextual fear memory and context discrimination

Hippocampal CA1 parvalbumin-expressing interneurons (PV INs) play a central role in controlling principal cell activity and orchestrating network oscillations. PV INs receive excitatory inputs from CA3 Schaffer collaterals and local CA1 pyramidal cells, and they provide perisomatic inhibition. Schaffer collateral excitatory synapses onto PV INs express Hebbian and anti-Hebbian types of long-term potentiation (LTP), as well as elicit LTP of intrinsic excitability (LTP_IE). LTP_IE requires the activation of type 5 metabotropic glutamate receptors (mGluR5) and is mediated by downregulation of potassium channels Kv1.1. It is sensitive to rapamycin and thus may involve activation of the mammalian target of rapamycin complex 1 (mTORC1). LTP_IE facilitates PV INs recruitment in CA1 and maintains an excitatory-inhibitory balance. Impaired CA1 PV INs activity or LTP affects network oscillations and memory. However, whether LTP_IE in PV INs plays a role in hippocampus-dependent memory remains unknown. Here, we used conditional deletion of the obligatory component of mTORC1, the Regulatory-Associated Protein of mTOR (Raptor), to directly manipulate mTORC1 in PV INs. We found that homozygous, but not heterozygous, conditional knock-out of Rptor resulted in a decrease in CA1 PV INs of mTORC1 signaling via its downstream effector S6 phosphorylation assessed by immunofluorescence. In whole-cell recordings from hippocampal slices, repetitive firing of CA1 PV INs was impaired in mice with either homozygous or heterozygous conditional knock-out of Rptor. High frequency stimulation of Schaffer collateral inputs that induce LTP_IE in PV INs of control mice failed to do so in mice with either heterozygous or homozygous conditional knock-out of Rptor in PV INs. At the behavioral level, mice with homozygous or heterozygous conditional knock-out of Rptor showed similar long-term contextual fear memory or contextual fear memory discrimination relative to control mice. Thus, mTORC1 activity in CA1 PV INs regulates repetitive firing and LTP_IE but not consolidation of long-term contextual fear memory and context discrimination. Our results indicate that mTORC1 plays cell-specific roles in synaptic plasticity of hippocampal inhibitory interneurons that are differentially involved in hippocampus-dependent learning and memory.

Immunity, Ion Channels and Epilepsy

Epilepsy is a common chronic neurological disorder in modern society. One of the major unmet challenges is that current antiseizure medications are basically not disease-modifying. Among the multifaceted etiologies of epilepsy, the role of the immune system has attracted considerable attention in recent years. It is known that both innate and adaptive immunity can be activated in response to insults to the central nervous system, leading to seizures. Moreover, the interaction between ion channels, which have a well-established role in epileptogenesis and epilepsy, and the immune system is complex and is being actively investigated. Some examples, including the interaction between ion channels and mTOR pathways, will be discussed in this paper. Furthermore, there has been substantial progress in our understanding of the pathophysiology of epilepsy associated with autoimmune encephalitis, and numerous neural-specific autoantibodies have been found and documented. Early recognition of immune-mediated epilepsy is important, especially in cases of pharmacoresistant epilepsy and in the presence of signs of autoimmune encephalitis, as early intervention with immunotherapy shows promise.

Modeling Epilepsy Using Human Induced Pluripotent Stem Cells-Derived Neuronal Cultures Carrying Mutations in Ion Channels and the Mechanistic Target of Rapamycin Pathway

Epilepsy is a neurological disorder that affects over 65 million people globally. It is characterized by periods of seizure activity of the brain as a result of excitation and inhibition (E/I) imbalance, which is regarded as the core underpinning of epileptic activity. Both gain- and loss-of-function (GOF and LOF) mutations of ion channels, synaptic proteins and signaling molecules along the mechanistic target of rapamycin (mTOR) pathway have been linked to this imbalance. The pathogenesis of epilepsy often has its roots in the early stage of brain development. It remains a major challenge to extrapolate the findings from many animal models carrying these GOF or LOF mutations to the understanding of disease mechanisms in the developing human brain. Recent advent of the human pluripotent stem cells (hPSCs) technology opens up a new avenue to recapitulate patient conditions and to identify druggable molecular targets. In the following review, we discuss the progress, challenges and prospects of employing hPSCs-derived neural cultures to study epilepsy. We propose a tentative working model to conceptualize the possible impact of these GOF and LOF mutations in ion channels and mTOR signaling molecules on the morphological and functional remodeling of intrinsic excitability, synaptic transmission and circuits, ultimately E/I imbalance and behavioral phenotypes in epilepsy.

Fragile X Syndrome: From Molecular Aspect to Clinical Treatment

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by the full mutation as well as highly localized methylation of the fragile X mental retardation 1 (FMR1) gene on the long arm of the X chromosome. Children with FXS are commonly co-diagnosed with Autism Spectrum Disorder, attention and learning problems, anxiety, aggressive behavior and sleep disorder, and early interventions have improved many behavior symptoms associated with FXS. In this review, we performed a literature search of original and review articles data of clinical trials and book chapters using MEDLINE (1990-2021) and ClinicalTrials.gov. While we have reviewed the biological importance of the fragile X mental retardation protein (FMRP), the FXS phenotype, and current diagnosis techniques, the emphasis of this review is on clinical interventions. Early non-pharmacological interventions in combination with pharmacotherapy and targeted treatments aiming to reverse dysregulated brain pathways are the mainstream of treatment in FXS. Overall, early diagnosis and interventions are fundamental to achieve optimal clinical outcomes in FXS.

AMPA Receptor Expression Requirement During Long-Term Memory Retrieval and Its Association with mTORC1 Signaling

Recently, it was reported that mechanistic/mammalian target of rapamycin complex 1 (mTORC1) activity during memory retrieval is required for normal expression of aversive and non-aversive long-term memories. Here we used inhibitory-avoidance task to evaluate the potential mechanisms by which mTORC1 signaling pathway participates in memory retrieval. First, we studied the role of GluA-subunit trafficking during memory recall and its relationship with mTORC1 pathway. We found that pretest intrahippocampal infusion of GluR23ɣ, a peptide that selectively blocks GluA2-containing AMPA receptor (AMPAR) endocytosis, prevented the amnesia induced by the inhibition of mTORC1 during retrieval. Additionally, we found that GluA1 levels decreased and GluA2 levels increased at the hippocampal postsynaptic density subcellular fraction of rapamycin-infused animals during memory retrieval. GluA2 levels remained intact while GluA1 decreased at the synaptic plasma membrane fraction. Then, we evaluated the requirement of AMPAR subunit expression during memory retrieval. Intrahippocampal infusion of GluA1 or GluA2 antisense oligonucleotides (ASO) 3 h before testing impaired memory retention. The memory impairment induced by GluA2 ASO before retrieval was reverted by GluA23ɣ infusion 1 h before testing. However, AMPAR endocytosis blockade was not sufficient to compensate GluA1 synthesis inhibition. Our work indicates that de novo GluA1 and GluA2 AMPAR subunit expression is required for memory retrieval with potential different roles for each subunit and suggests that mTORC1 might regulate AMPAR trafficking during retrieval. Our present results highlight the role of mTORC1 as a key determinant of memory retrieval that impacts the recruitment of different AMPAR subunits.

Mammalian Target of Rapamycin-RhoA Signaling Impairments in Direct Striatal Projection Neurons Induce Altered Behaviors and Striatal Physiology in Mice

BACKGROUND

As an integrator of molecular pathways, mTOR (mammalian target of rapamycin) has been associated with diseases including neurodevelopmental, psychiatric, and neurodegenerative disorders such as autism spectrum disorder, schizophrenia, and Huntington's disease. An important brain area involved in all these diseases is the striatum. However, the mechanisms behind how mTOR is involved in striatal physiology and its relative role in distinct neuronal populations in these striatal-related diseases still remain to be clarified.

METHODS

Using Drd1-Cre mTOR-conditional knockout male mice, we combined behavioral, biochemical, electrophysiological, and morphological analysis aiming to untangle the role of mTOR in direct pathway striatal projection neurons and how this would impact on striatal physiology.

RESULTS

Our results indicate deep behavioral changes in absence of mTOR in Drd1-expressing neurons such as decreased spontaneous locomotion, impaired social interaction, and decreased marble-burying behavior. These alterations were accompanied by a Kv1.1-induced increase in the fast phase of afterhyperpolarization and coincident decreased distal spine density in striatal direct pathway striatal projection neurons. The physiological changes were mechanistically independent of protein synthesis but sensitive to pharmacological blockade of transforming protein RhoA activity.

CONCLUSIONS

These results identify mTOR signaling as an important regulator of striatal functions through an intricate mechanism involving RhoA and culminating in Kv1.1 overfunction, which could be targeted to treat striatal-related monogenic disorders associated with the mTOR signaling pathway.

Control of Slc7a5 sensitivity by the voltage-sensing domain of Kv1 channels

Many voltage-dependent ion channels are regulated by accessory proteins. We recently reported powerful regulation of Kv1.2 potassium channels by the amino acid transporter Slc7a5. In this study, we report that Kv1.1 channels are also regulated by Slc7a5, albeit with different functional outcomes. In heterologous expression systems, Kv1.1 exhibits prominent current enhancement ('disinhibition') with holding potentials more negative than −120 mV. Knockdown of endogenous Slc7a5 leads to larger Kv1.1 currents and strongly attenuates the disinhibition effect, suggesting that Slc7a5 regulation of Kv1.1 involves channel inhibition that can be reversed by supraphysiological hyperpolarizing voltages. We investigated chimeric combinations of Kv1.1 and Kv1.2, demonstrating that exchange of the voltage-sensing domain controls the sensitivity and response to Slc7a5, and localize a specific position in S1 with prominent effects on Slc7a5 sensitivity. Overall, our study highlights multiple Slc7a5-sensitive Kv1 subunits, and identifies the voltage-sensing domain as a determinant of Slc7a5 modulation of Kv1 channels.

Dietary and Pharmacological Interventions That Inhibit Mammalian Target of Rapamycin Activity Alter the Brain Expression Levels of Neurogenic and Glial Markers in an Age-and Treatment-Dependent Manner

Intermittent fasting (IF) and its mimetic, rapamycin extend lifespan and healthspan through mechanisms that are not fully understood. We investigated different short-term durations of IF and rapamycin on cellular and molecular changes in the brains of young (6-10 months) and old (26-31 months) zebrafish. Interestingly, our results showed that IF significantly lowered glucose levels while increasing DCAMKL1 in both young and old animals. This proliferative effect of IF was supported by the upregulation of foxm1 transcript in old animals. Rapamycin did not change glucose levels in young and old animals but had differential effects depending on age. In young zebrafish, proliferating cell nuclear antigen and the LC3-II/LC3-I ratio was decreased, whereas glial fibrillary acidic protein and gephyrin were decreased in old animals. The changes in proliferative markers and a marker of autophagic flux suggest an age-dependent interplay between autophagy and cell proliferation. Additionally, changes in glia and inhibitory tone suggest a suppressive effect on neuroinflammation but may push the brain toward a more excitable state. Mammalian target of rapamycin (mTOR) activity in the brain following the IF and rapamycin treatment was differentially regulated by age. Interestingly, rapamycin inhibited mTOR more potently in young animals than IF. Principal component analysis supported our conclusion that the regulatory effects of IF and rapamycin were age-specific, since we observed different patterns in the expression levels and clustering of young and old animals. Taken together, our results suggest that even a short-term duration of IF and rapamycin have significant effects in the brain at young and old ages, and that these are age and treatment dependent.

Treatment with Mesenchymal-Derived Extracellular Vesicles Reduces Injury-Related Pathology in Pyramidal Neurons of Monkey Perilesional Ventral Premotor Cortex

Functional recovery after cortical injury, such as stroke, is associated with neural circuit reorganization, but the underlying mechanisms and efficacy of therapeutic interventions promoting neural plasticity in primates are not well understood. Bone marrow mesenchymal stem cell-derived extracellular vesicles (MSC-EVs), which mediate cell-to-cell inflammatory and trophic signaling, are thought be viable therapeutic targets. We recently showed, in aged female rhesus monkeys, that systemic administration of MSC-EVs enhances recovery of function after injury of the primary motor cortex, likely through enhancing plasticity in perilesional motor and premotor cortices. Here, using in vitro whole-cell patch-clamp recording and intracellular filling in acute slices of ventral premotor cortex (vPMC) from rhesus monkeys (Macaca mulatta) of either sex, we demonstrate that MSC-EVs reduce injury-related physiological and morphologic changes in perilesional layer 3 pyramidal neurons. At 14-16 weeks after injury, vPMC neurons from both vehicle- and EV-treated lesioned monkeys exhibited significant hyperexcitability and predominance of inhibitory synaptic currents, compared with neurons from nonlesioned control brains. However, compared with vehicle-treated monkeys, neurons from EV-treated monkeys showed lower firing rates, greater spike frequency adaptation, and excitatory:inhibitory ratio. Further, EV treatment was associated with greater apical dendritic branching complexity, spine density, and inhibition, indicative of enhanced dendritic plasticity and filtering of signals integrated at the soma. Importantly, the degree of EV-mediated reduction of injury-related pathology in vPMC was significantly correlated with measures of behavioral recovery. These data show that EV treatment dampens injury-related hyperexcitability and restores excitatory:inhibitory balance in vPMC, thereby normalizing activity within cortical networks for motor function.SIGNIFICANCE STATEMENT Neuronal plasticity can facilitate recovery of function after cortical injury, but the underlying mechanisms and efficacy of therapeutic interventions promoting this plasticity in primates are not well understood. Our recent work has shown that intravenous infusions of mesenchymal-derived extracellular vesicles (EVs) that are involved in cell-to-cell inflammatory and trophic signaling can enhance recovery of motor function after injury in monkey primary motor cortex. This study shows that this EV-mediated enhancement of recovery is associated with amelioration of injury-related hyperexcitability and restoration of excitatory-inhibitory balance in perilesional ventral premotor cortex. These findings demonstrate the efficacy of mesenchymal EVs as a therapeutic to reduce injury-related pathologic changes in the physiology and structure of premotor pyramidal neurons and support recovery of function.

Genetic inactivation of mTORC1 or mTORC2 in neurons reveals distinct functions in glutamatergic synaptic transmission

Although mTOR signaling is known as a broad regulator of cell growth and proliferation, in neurons it regulates synaptic transmission, which is thought to be a major mechanism through which altered mTOR signaling leads to neurological disease. Although previous studies have delineated postsynaptic roles for mTOR, whether it regulates presynaptic function is largely unknown. Moreover, the mTOR kinase operates in two complexes, mTORC1 and mTORC2, suggesting that mTOR's role in synaptic transmission may be complex-specific. To better understand their roles in synaptic transmission, we genetically inactivated mTORC1 or mTORC2 in cultured mouse glutamatergic hippocampal neurons. Inactivation of either complex reduced neuron growth and evoked EPSCs (eEPSCs), however, the effects of mTORC1 on eEPSCs were postsynaptic and the effects of mTORC2 were presynaptic. Despite postsynaptic inhibition of evoked release, mTORC1 inactivation enhanced spontaneous vesicle fusion and replenishment, suggesting that mTORC1 and mTORC2 differentially modulate postsynaptic responsiveness and presynaptic release to optimize glutamatergic synaptic transmission.

Transcranial focused ultrasound pulsation suppresses pentylenetetrazol induced epilepsy in vivo

BACKGROUND

Epilepsy is a neurological disorder characterized by abnormal neuron discharge, and one-third of epilepsy patients suffer from drug-resistant epilepsy (DRE). The current management for DRE includes epileptogenic lesion resection, disconnection, and neuromodulation. Neuromodulation is achieved through invasive electrical stimulus including deep brain stimulation, vagus nerve stimulation, or responsive neurostimulation (RNS). As an alternative therapy, transcranial focused ultrasound (FUS) can transcranially and non-invasively modulate neuron activity.

OBJECTIVE

This study seeks to verify the use of FUS pulsations to suppress spikes in an acute epileptic small-animal model, and to investigate possible biological mechanisms by which FUS pulsations interfere with epileptic neuronal activity.

METHODS

The study used a total of 76 Sprague-Dawley rats. For the epilepsy model, rats were administered pentylenetetrazol (PTZ) to induce acute epileptic-like abnormal neuron discharges, followed by FUS exposure. Various ultrasound parameters were set to test the epilepsy-suppressing effect, while concurrently monitoring and analyzing electroencephalogram (EEG) signals. Animal behavior was monitored and histological examinations were conducted to evaluate the hazard posed by ultrasound exposure and the expression of neuronal activity markers. Western blotting was used to evaluate the correlation between FUS-induced epileptic suppression and the PI3K-mTOR signaling pathway.

RESULTS

We observed that FUS pulsations effectively suppressed epileptic activity and observed EEG spectrum oscillations; the spike-suppressing effect depended on the selection of ultrasound parameters and highly correlated with FUS exposure level. Expression level changes of c-Fos and GAD65 were confirmed in the cortex and hippocampus, indicating that FUS pulsations deactivated excitatory cells and activated GABAergic terminals. No tissue damage, inflammatory response, or behavioral abnormalities were observed in rats treated with FUS under these exposure parameters. We also found that the FUS pulsations down-regulated the S6 phosphorylation and decreased pAKT expression.

CONCLUSION

Our results suggest that pulsed FUS exposure effectively suppresses epileptic spikes in an acute epilepsy animal model, and finds that ultrasound pulsation interferes with neuronal activity and affects the PTZ-induced PI3K-Akt-mTOR pathway, which might help explain the mechanism underlying ultrasound-related epileptic spike control.

Ubiquitin C-terminal hydrolase L1 (UCH-L1) loss causes neurodegeneration by altering protein turnover in the first postnatal weeks

Ubiquitin C-terminal hydrolase L1 (UCH-L1) is one of the most abundant and enigmatic enzymes of the CNS. Based on existing UCH-L1 knockout models, UCH-L1 is thought to be required for the maintenance of axonal integrity, but not for neuronal development despite its high expression in neurons. Several lines of evidence suggest a role for UCH-L1 in mUB homeostasis, although the specific in vivo substrate remains elusive. Since the precise mechanisms underlying UCH-L1-deficient neurodegeneration remain unclear, we generated a transgenic mouse model of UCH-L1 deficiency. By performing biochemical and behavioral analyses we can show that UCH-L1 deficiency causes an acceleration of sensorimotor reflex development in the first postnatal week followed by a degeneration of motor function starting at periadolescence in the setting of normal cerebral mUB levels. In the first postnatal weeks, neuronal protein synthesis and proteasomal protein degradation are enhanced, with endoplasmic reticulum stress, and energy depletion, leading to proteasomal impairment and an accumulation of nondegraded ubiquitinated protein. Increased protein turnover is associated with enhanced mTORC1 activity restricted to the postnatal period in UCH-L1-deficient brains. Inhibition of mTORC1 with rapamycin decreases protein synthesis and ubiquitin accumulation in UCH-L1-deficient neurons. Strikingly, rapamycin treatment in the first 8 postnatal days ameliorates the neurological phenotype of UCH-L1-deficient mice up to 16 weeks, suggesting that early control of protein homeostasis is imperative for long-term neuronal survival. In summary, we identified a critical presymptomatic period during which UCH-L1-dependent enhanced protein synthesis results in neuronal strain and progressive loss of neuronal function.

Development-related aberrations in Kv1.1 α-subunit exert disruptive effects on bioelectrical activities of neurons in a mouse model of fragile X syndrome

Kv1.1, a Shaker homologue potassium channel, plays a critical role in homeostatic regulation of neuronal excitability. Aberrations in the functional properties of Kv1.1 have been implicated in several neurological disorders featured by neuronal hyperexcitability. Fragile X syndrome (FXS), the most common form of inherited mental retardation, is characterized by hyperexcitability in neural network and intrinsic membrane properties. The Kv1.1 channel provides an intriguing mechanistic candidate for FXS. We investigated the development-related expression pattern of the Kv1.1 α-subunit by using a Fmr1 knockout (KO) mouse model of FXS. Markedly decreased protein expression of Kv1.1 was found in neonatal and adult stages when compared to age-matched wild-type (WT) mice. Immunohistochemical investigations supported the delayed development-related increases in Kv1.1 expression, especially in CA3 pyramidal neurons. By applying a Kv1.1-specific blocker, dendrotoxin-κ (DTX-κ), we isolated the Kv1.1-mediated currents in the CA3 pyramidal neurons. The isolated DTX-κ-sensitive current of neurons from KO mice exhibited decreased amplitude, lower threshold of activation, and faster recovery from inactivation. The equivalent reduction in potassium current in the WT neurons following application of the appropriate amount of DTX-κ reproduced the enhanced firing abilities of KO neurons, suggesting the Kv1.1 channel as a critical contributor to the hyperexcitability of KO neurons. The role of Kv1.1 in controlling neuronal discharges was further supported by the parallel developmental trajectories of Kv1.1 expression, current amplitude, and discharge impacts, with a significant correlation between the amplitude of Kv1.1-mediated currents and Kv1.1-blocking-induced firing enhancement. These data suggest that the expression of the Kv1.1 α-subunit has a profound pathological relevance to hyperexcitability in FXS, as well as implications for normal development, maintenance, and control of neuronal activities.

mTOR signaling in VIP neurons regulates circadian clock synchrony and olfaction

The mammalian/mechanistic target of rapamycin (mTOR) kinase resides at the crux of an intracellular signaling network that controls fundamental biological processes. Dysregulation of mTOR signaling is linked to neurological and psychiatric diseases. However, the physiological functions of mTOR signaling in the adult brain are not fully understood. In the current study, we discovered that mTOR in vasoactive intestinal peptide (VIP) neurons plays a key role in regulating neurophysiology in the brain circadian clock and the olfactory system. The conditional mTOR knockout mouse will be a useful model for future investigations of mTOR and/or VIP.

Mammalian/mechanistic target of rapamycin (mTOR) signaling controls cell growth, proliferation, and metabolism in dividing cells. Less is known regarding its function in postmitotic neurons in the adult brain. Here we created a conditional mTOR knockout mouse model to address this question. Using the Cre-LoxP system, the mTOR gene was specifically knocked out in cells expressing Vip (vasoactive intestinal peptide), which represent a major population of interneurons widely distributed in the neocortex, suprachiasmatic nucleus (SCN), olfactory bulb (OB), and other brain regions. Using a combination of biochemical, behavioral, and imaging approaches, we found that mice lacking mTOR in VIP neurons displayed erratic circadian behavior and weakened synchronization among cells in the SCN, the master circadian pacemaker in mammals. Furthermore, we have discovered a critical role for mTOR signaling in mediating olfaction. Odor stimulated mTOR activation in the OB, anterior olfactory nucleus, as well as piriform cortex. Odor-evoked c-Fos responses along the olfactory pathway were abolished in mice lacking mTOR in VIP neurons, which is consistent with reduced olfactory sensitivity in these animals. Together, these results demonstrate that mTOR is a key regulator of SCN circadian clock synchrony and olfaction.