Interaction of RAFT1 with gephyrin required for rapamycin-sensitive signaling

Gephyrin phosphorylation facilitates sexually dimorphic development and function of parvalbumin interneurons in the mouse hippocampus

The precise function of specialized GABAergic interneuron subtypes is required to provide appropriate synaptic inhibition for regulating principal neuron excitability and synchronization within brain circuits. Of these, parvalbumin-type (PV neuron) dysfunction is a feature of several sex-biased psychiatric and brain disorders, although, the underlying developmental mechanisms are unclear. While the transcriptional action of sex hormones generates sexual dimorphism during brain development, whether kinase signaling contributes to sex differences in PV neuron function remains unexplored. In the hippocampus, we report that gephyrin, the main inhibitory post-synaptic scaffolding protein, is phosphorylated at serine S268 and S270 in a developmentally-dependent manner in both males and females. When examining GphnS268A/S270A mice in which site-specific phosphorylation is constitutively blocked, we found that sex differences in PV neuron density in the hippocampal CA1 present in WT mice were abolished, coincident with a female-specific increase in PV neuron-derived terminals and increased inhibitory input onto principal cells. Electrophysiological analysis of CA1 PV neurons indicated that gephyrin phosphorylation is required for sexually dimorphic function. Moreover, while male and female WT mice showed no difference in hippocampus-dependent memory tasks, GphnS268A/S270A mice exhibited sex- and task-specific deficits, indicating that gephyrin phosphorylation is differentially required by males and females for convergent cognitive function. In fate mapping experiments, we uncovered that gephyrin phosphorylation at S268 and S270 establishes sex differences in putative PV neuron density during early postnatal development. Furthermore, patch-sequencing of putative PV neurons at postnatal day 4 revealed that gephyrin phosphorylation contributes to sex differences in the transcriptomic profile of developing interneurons. Therefore, these early shifts in male-female interneuron development may drive adult sex differences in PV neuron function and connectivity. Our results identify gephyrin phosphorylation as a new substrate organizing PV neuron development at the anatomical, functional, and transcriptional levels in a sex-dependent manner, thus implicating kinase signaling disruption as a new mechanism contributing to the sex-dependent etiology of brain disorders.

Intracellular signaling mechanisms that shape postsynaptic GABAergic synapses

Postsynaptic GABAergic receptors interact with various membrane and intracellular proteins to mediate inhibitory synaptic transmission. They form structural and/or signaling synaptic protein complexes that perform a variety of postsynaptic functions. In particular, the key GABAergic synaptic scaffold, gephyrin, and its interacting partners govern downstream signaling pathways that are essential for GABAergic synapse development, transmission, and plasticity. In this review, we discuss recent researches on GABAergic synaptic signaling pathways. We also outline the main outstanding issues that need to be addressed in this field and highlight the association of dysregulated GABAergic synaptic signaling with the onset of various brain disorders.

Engineered Extracellular Vesicles with Compound-Induced Cargo Delivery to Solid Tumors

Efficient delivery of functional factors into target cells remains challenging. Although extracellular vesicles (EVs) are considered to be potential therapeutic delivery vehicles, a variety of efficient therapeutic delivery tools are still needed for cancer cells. Herein, we demonstrated a promising method to deliver EVs to refractory cancer cells via a small molecule-induced trafficking system. We generated an inducible interaction system between the FKBP12-rapamycin-binding protein (FRB) domain and FK506 binding protein (FKBP) to deliver specific cargo to EVs. CD9, an abundant protein in EVs, was fused to the FRB domain, and the specific cargo to be delivered was linked to FKBP. Rapamycin recruited validated cargo to EVs through protein-protein interactions (PPIs), such as the FKBP-FRB interaction system. The released EVs were functionally delivered to refractory cancer cells, triple negative breast cancer cells, non-small cell lung cancer cells, and pancreatic cancer cells. Therefore, the functional delivery system driven by reversible PPIs may provide new possibilities for a therapeutic cure against refractory cancers.

Age and diet shape the genetic architecture of body weight in diversity outbred mice

Understanding how genetic variation shapes a complex trait relies on accurately quantifying both the additive genetic and genotype–environment interaction effects in an age-dependent manner. We used a linear mixed model to quantify diet-dependent genetic contributions to body weight measured through adulthood in diversity outbred female mice under five diets. We observed that heritability of body weight declined with age under all diets, except the 40% calorie restriction diet. We identified 14 loci with age-dependent associations and 19 loci with age- and diet-dependent associations, with many diet-dependent loci previously linked to neurological function and behavior in mice or humans. We found their allelic effects to be dynamic with respect to genomic background, age, and diet, identifying several loci where distinct alleles affect body weight at different ages. These results enable us to more fully understand and predict the effectiveness of dietary intervention on overall health throughout age in distinct genetic backgrounds.

Body weight is one trait influenced by genes, age and environmental factors. Both internal and external environmental pressures are known to affect genetic variation over time. However, it is largely unknown how all factors – including age – interact to shape metabolism and bodyweight.

Wright et al. set out to quantify the interactions between genes and diet in ageing mice and found that the effect of genetics on mouse body weight changes with age. In the experiments, Wright et al. weighed 960 female mice with diverse genetic backgrounds, starting at two months of age into adulthood. The animals were randomized to different diets at six months of age. Some mice had unlimited food access, others received 20% or 40% less calories than a typical mouse diet, and some fasted one or two days per week.

Variations in their genetic background explained about 80% of differences in mice’s weight, but the influence of genetics relative to non-genetic factors decreased as they aged. Mice on the 40% calorie restriction diet were an exception to this rule and genetics accounted for 80% of their weight throughout adulthood, likely due to reduced influence from diet and reduced interactions between diet and genes. Several genes involved in metabolism, neurological function, or behavior, were associated with mouse weight.

The experiments highlight the importance of considering interactions between genetics, environment, and age in determining complex traits like body weight. The results and the approaches used by Wright et al. may help other scientists learn more about how the genetic predisposition to disease changes with environmental stimuli and age.

Application of Small Molecules in the Central Nervous System Direct Neuronal Reprogramming

The lack of regenerative capacity of neurons leads to poor prognoses for some neurological disorders. The use of small molecules to directly reprogram somatic cells into neurons provides a new therapeutic strategy for neurological diseases. In this review, the mechanisms of action of different small molecules, the approaches to screening small molecule cocktails, and the methods employed to detect their reprogramming efficiency are discussed, and the studies, focusing on neuronal reprogramming using small molecules in neurological disease models, are collected. Future research efforts are needed to investigate the in vivo mechanisms of small molecule-mediated neuronal reprogramming under pathophysiological states, optimize screening cocktails and dosing regimens, and identify safe and effective delivery routes to promote neural regeneration in different neurological diseases.

Uncovering the Underlying Mechanisms of Ketamine as a Novel Antidepressant

Major depressive disorder (MDD) is a devastating psychiatric disorder which exacts enormous personal and social-economic burdens. Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, has been discovered to exert rapid and sustained antidepressant-like actions on MDD patients and animal models. However, the dissociation and psychotomimetic propensities of ketamine have limited its use for psychiatric indications. Here, we review recently proposed mechanistic hypotheses regarding how ketamine exerts antidepressant-like actions. Ketamine may potentiate α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR)-mediated transmission in pyramidal neurons by disinhibition and/or blockade of spontaneous NMDAR-mediated neurotransmission. Ketamine may also activate neuroplasticity- and synaptogenesis-relevant signaling pathways, which may converge on key components like brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB) and mechanistic target of rapamycin (mTOR). These processes may subsequently rebalance the excitatory/inhibitory transmission and restore neural network integrity that is compromised in depression. Understanding the mechanisms underpinning ketamine’s antidepressant-like actions at cellular and neural circuit level will drive the development of safe and effective pharmacological interventions for the treatment of MDD.

Complex regulation of Gephyrin splicing is a determinant of inhibitory postsynaptic diversity

Gephyrin (GPHN) regulates the clustering of postsynaptic components at inhibitory synapses and is involved in pathophysiology of neuropsychiatric disorders. Here, we uncover an extensive diversity of GPHN transcripts that are tightly controlled by splicing during mouse and human brain development. Proteomic analysis reveals at least a hundred isoforms of GPHN incorporated at inhibitory Glycine and gamma-aminobutyric acid A receptors containing synapses. They exhibit different localization and postsynaptic clustering properties, and altering the expression level of one isoform is sufficient to affect the number, size, and density of inhibitory synapses in cerebellar Purkinje cells. Furthermore, we discovered that splicing defects reported in neuropsychiatric disorders are carried by multiple alternative GPHN transcripts, demonstrating the need for a thorough analysis of the GPHN transcriptome in patients. Overall, we show that alternative splicing of GPHN is an important genetic variation to consider in neurological diseases and a determinant of the diversity of postsynaptic inhibitory synapses.

The protein gephyrin is involved in organizing synapses. Here, the authors show how different transcripts of gephyrin form and regulate inhibitory synapses.

IQSEC3 Deletion Impairs Fear Memory Through Upregulation of Ribosomal S6K1 Signaling in the Hippocampus

BACKGROUND

IQSEC3, a gephyrin-binding GABAergic (gamma-aminobutyric acidergic) synapse-specific guanine nucleotide exchange factor, was recently reported to regulate activity-dependent GABAergic synapse maturation, but the underlying signaling mechanisms remain incompletely understood.

METHODS

We generated mice with conditional knockout (cKO) of Iqsec3 to examine whether altered synaptic inhibition influences hippocampus-dependent fear memory formation. In addition, electrophysiological recordings, immunohistochemistry, and behavioral assays were used to address our question.

RESULTS

We found that Iqsec3-cKO induces a specific reduction in GABAergic synapse density, GABAergic synaptic transmission, and maintenance of long-term potentiation in the hippocampal CA1 region. In addition, Iqsec3-cKO mice exhibited impaired fear memory formation. Strikingly, Iqsec3-cKO caused abnormally enhanced activation of ribosomal P70-S6K1-mediated signaling in the hippocampus but not in the cortex. Furthermore, inhibiting upregulated S6K1 signaling by expressing dominant-negative S6K1 in the hippocampal CA1 of Iqsec3-cKO mice completely rescued impaired fear learning and inhibitory synapse density but not deficits in long-term potentiation maintenance. Finally, upregulated S6K1 signaling was rescued by IQSEC3 wild-type, but not by an ARF-GEF (adenosine diphosphate ribosylation factor-guanine nucleotide exchange factor) inactive IQSEC3 mutant.

CONCLUSIONS

Our results suggest that IQSEC3-mediated balanced synaptic inhibition in hippocampal CA1 is critical for the proper formation of hippocampus-dependent fear memory.

New insights into neuropeptides regulation of immune system and hemopoiesis: effects on hematologic malignancies

Several neurotransmitters and neuropeptides were reported to join to or to cooperate with different cells of the immune system, bone marrow, and peripheral cells and numerous data support that neuroactive molecules might control immune system activity and hemopoiesis operating on lymphoid organs, and the primary hematopoietic unit, the hematopoietic niche. Furthermore, many compounds seem to be able to take part to the leukemogenesis and lymphomagenesis process, and in the onset of multiple myeloma. In this review, we will assess the possibility that neurotransmitters and neuropeptides may have a role in the onset of haematological neoplasms, may affect the response to treatment or may represent a useful starting point for a new therapeutic approach. More in vivo investigations are needed to evaluate neuropeptide's role in haematological malignancies and the possible utilization as an antitumor therapeutic target. Comprehending the effect of the pharmacological administration of neuropeptide modulators on hematologic malignancies opens up new possibilities in curing clonal hematologic diseases to achieve more satisfactory outcomes.

Short-term dietary restriction maintains synaptic plasticity whereas short-term overfeeding alters cellular dynamics in the aged brain: evidence from the zebrafish model organism

Increased caloric intake (OF) impairs quality of life causing comorbidities with other diseases and cognitive deficits, whereas dietary restriction (DR) increases healthspan by preventing age-related deteriorations. To understand the effects of these opposing dietary regimens on the cellular and synaptic dynamics during brain aging, the zebrafish model, which shows gradual aging like mammals, was utilized. Global changes in cellular and synaptic markers with respect to age and a 12 week dietary regimen of OF and DR demonstrated that aging reduces the levels of the glutamate receptor subunits, GLUR2/3, inhibitory synaptic clustering protein, GEP, synaptic vesicle protein, SYP, and early-differentiated neuronal marker, HuC. DR significantly elevates levels of glutamate receptor subunits, GLUR2/3, and NMDA clustering protein, PSD95, levels, while OF subtly increases the level of the neuronal protein, DCAMKL1. These data suggest that decreased caloric intake within the context of aging has more robust effects on synapses than cellular proteins, whereas OF alters cellular dynamics. Thus, patterns like these should be taken into account for possible translation to human subjects.

Autism Spectrum Disorder Risk Factor Met Regulates the Organization of Inhibitory Synapses

A common hypothesis explains autism spectrum disorder (ASD) as a neurodevelopmental disorder linked to excitatory/inhibitory (E/I) imbalance in neuronal network connectivity. Mutation of genes including Met and downstream signaling components, e.g., PTEN, Tsc2 and, Rheb are involved in the control of synapse formation and stabilization and were all considered as risk genes for ASD. While the impact of Met on glutamatergic synapses was widely appreciated, its contribution to the stability of inhibitory, GABAergic synapses is poorly understood. The stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin. Here, we show in vivo and in vitro that Met is necessary and sufficient for the stabilization of GABAergic synapses via induction of gephyrin clustering. Likewise, we provide evidence for Met-dependent gephyrin clustering via activation of mTOR. Our results support the notion that deficient GABAergic signaling represents a pathomechanism for ASD.

RNA-binding proteins balance brain function in health and disease

Posttranscriptional gene expression including splicing, RNA transport, translation, and RNA decay provides an important regulatory layer in many if not all molecular pathways. Research in the last decades has positioned RNA-binding proteins (RBPs) right in the center of posttranscriptional gene regulation. Here, we propose interdependent networks of RBPs to regulate complex pathways within the central nervous system (CNS). These are involved in multiple aspects of neuronal development and functioning, including higher cognition. Therefore, it is not sufficient to unravel the individual contribution of a single RBP and its consequences but rather to study and understand the tight interplay between different RBPs. In this review, we summarize recent findings in the field of RBP biology and discuss the complex interplay between different RBPs. Second, we emphasize the underlying dynamics within an RBP network and how this might regulate key processes such as neurogenesis, synaptic transmission, and synaptic plasticity. Importantly, we envision that dysfunction of specific RBPs could lead to perturbation within the RBP network. This would have direct and indirect (compensatory) effects in mRNA binding and translational control leading to global changes in cellular expression programs in general and in synaptic plasticity in particular. Therefore, we focus on RBP dysfunction and how this might cause neuropsychiatric and neurodegenerative disorders. Based on recent findings, we propose that alterations in the entire regulatory RBP network might account for phenotypic dysfunctions observed in complex diseases including neurodegeneration, epilepsy, and autism spectrum disorders.

Tuning GABAergic Inhibition: Gephyrin Molecular Organization and Functions

To be highly reliable, synaptic transmission needs postsynaptic receptors (Rs) in precise apposition to the presynaptic release sites. At inhibitory synapses, the postsynaptic protein gephyrin self-assembles to form a scaffold that anchors glycine and GABAARs to the cytoskeleton, thus ensuring the accurate accumulation of postsynaptic receptors at the right place. This protein undergoes several post-translational modifications which control protein-protein interaction and downstream signaling pathways. In addition, through the constant exchange of scaffolding elements and receptors in and out of synapses, gephyrin dynamically regulates synaptic strength and plasticity. The aim of the present review is to highlight recent findings on the functional role of gephyrin at GABAergic inhibitory synapses. We will discuss different approaches used to interfere with gephyrin in order to unveil its function. In addition, we will focus on the impact of gephyrin structure and distribution at the nanoscale level on the functional properties of inhibitory synapses as well as the implications of this scaffold protein in synaptic plasticity processes. Finally, we will emphasize how gephyrin genetic mutations or alterations in protein expression levels are implicated in several neuropathological disorders, including autism spectrum disorders, schizophrenia, temporal lobe epilepsy and Alzheimer's disease, all associated with severe deficits of GABAergic signaling. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Glycine Protects Muscle Cells From Wasting in vitro via mTORC1 Signaling

Glycine supplementation can protect skeletal muscles of mice from cancer-induced wasting, but the mechanisms underlying this protection are not well-understood. The aim of this study was to determine whether exogenous glycine directly protects skeletal muscle cells from wasting. C2C12 muscle cells were exposed to non-inflammatory catabolic stimuli via two models: serum withdrawal (SF) for 48 h; or incubation in HEPES buffered saline (HBS) for up to 5 h. Cells were supplemented with glycine or equimolar concentrations of L-alanine. SF- and HBS-treated myotubes (with or without L-alanine) were ~20% and ~30% smaller than control myotubes. Glycine-treated myotubes were up to 20% larger (P < 0.01) compared to cells treated with L-alanine in both models of muscle cell atrophy. The mTORC1 inhibitor rapamycin prevented the glycine-stimulated protection of myotube diameter, and glycine-stimulated S6 phosphorylation, suggesting that mTORC1 signaling may be necessary for glycine's protective effects in vitro. Increasing glycine availability may be beneficial for muscle wasting conditions associated with inadequate nutrient intake.

Regulation of nuclear-cytoplasmic partitioning by the lin-28-lin-46 pathway reinforces microRNA repression of HBL-1 to confer robust cell-fate progression in C. elegans

MicroRNAs target complementary mRNAs for degradation or translational repression, reducing or preventing protein synthesis. In Caenorhabditis elegans, the transcription factor HBL-1 (Hunchback-like 1) promotes early larval (L2)-stage cell fates, and the let-7 family microRNAs temporally downregulate HBL-1 to enable the L2-to-L3 cell-fate progression. In parallel to let-7-family microRNAs, the conserved RNA-binding protein LIN-28 and its downstream gene lin-46 also act upstream of HBL-1 in regulating the L2-to-L3 cell-fate progression. The molecular function of LIN-46, and how the lin-28-lin-46 pathway regulates HBL-1, are not understood. Here, we report that the regulation of HBL-1 by the lin-28-lin-46 pathway is independent of the let-7/lin-4 microRNA complementary sites (LCSs) in the hbl-1 3'UTR, and involves stage-specific post-translational regulation of HBL-1 nuclear accumulation. We find that LIN-46 is necessary and sufficient to prevent nuclear accumulation of HBL-1. Our results illuminate that robust progression from L2 to L3 cell fates depends on the combination of two distinct modes of HBL-1 downregulation: decreased synthesis of HBL-1 via let-7-family microRNA activity, and decreased nuclear accumulation of HBL-1 via action of the lin-28-lin-46 pathway.

Glycine metabolism in skeletal muscle: implications for metabolic homeostasis

PURPOSE OF REVIEW

The review summarizes the recent literature on the role of glycine in skeletal muscle during times of stress.

RECENT FINDINGS

Supplemental glycine protects muscle mass and function under pathological conditions. In addition, mitochondrial dysfunction in skeletal muscle leads to increased cellular serine and glycine production and activation of NADPH-generating pathways and glutathione metabolism. These studies highlight how glycine availability modulates cellular homeostasis and redox status.

SUMMARY

Recent studies demonstrate that supplemental glycine effectively protects muscles in a variety of wasting models, including cancer cachexia, sepsis, and reduced caloric intake. The underlying mechanisms responsible for the effects of glycine remain unclear but likely involve receptor-mediated responses and modulation of intracellular metabolism. Future research to understand these mechanisms will provide insight into glycine's therapeutic potential. Our view is that glycine holds considerable promise for improving health by protecting muscles during different wasting conditions.

Potential of GPCRs to modulate MAPK and mTOR pathways in Alzheimer's disease

Despite efforts to understand the mechanism of neuronal cell death, finding effective therapies for neurodegenerative diseases is still a challenge. Cognitive deficits are often associated with neurodegenerative diseases. Remarkably, in the absence of consensus biomarkers, diagnosis of diseases such as Alzheimer's still relies on cognitive tests. Unfortunately, all efforts to translate findings in animal models to the patients have been unsuccessful. Alzheimer's disease may be addressed from two different points of view, neuroprotection or cognitive enhancement. Based on recent data, the mammalian target of rapamycin (mTOR) pathway arises as a versatile player whose modulation may impact on mechanisms of both neuroprotection and cognition. Whereas direct targeting of mTOR does not seem to constitute a convenient approach in drug discovery, its indirect modulation by other signaling pathways seems promising. In fact, G-protein-coupled receptors (GPCRs) remain the most common 'druggable' targets and as such pharmacological manipulation of GPCRs with selective ligands may modulate phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), mitogen-activated protein (MAP) kinase and mTOR signaling pathways. Thus, GPCRs become important targets for potential drug treatments in different neurodegenerative disorders including, but not limited to, Alzheimer's disease. GPCR-mediated modulation of mTOR may take advantage of different GPCRs coupled to different G-dependent and G-independent signal transduction routes, of functional selectivity and/or of biased agonism. Signals mediated by GPCRs may act as coincidence detectors to achieve different benefits in different stages of the neurodegenerative disease.

Artemisinins Target GABAA Receptor Signaling and Impair α Cell Identity

Type 1 diabetes is characterized by the destruction of pancreatic β cells, and generating new insulin-producing cells from other cell types is a major aim of regenerative medicine. One promising approach is transdifferentiation of developmentally related pancreatic cell types, including glucagon-producing α cells. In a genetic model, loss of the master regulatory transcription factor Arx is sufficient to induce the conversion of α cells to functional β-like cells. Here, we identify artemisinins as small molecules that functionally repress Arx by causing its translocation to the cytoplasm. We show that the protein gephyrin is the mammalian target of these antimalarial drugs and that the mechanism of action of these molecules depends on the enhancement of GABA_A receptor signaling. Our results in zebrafish, rodents, and primary human pancreatic islets identify gephyrin as a druggable target for the regeneration of pancreatic β cell mass from α cells.

•

Artemisinins inhibit ARX function and impair α cell identity

•

Compounds act by stabilizing gephyrin, thus enhancing GABA_A receptor signaling

•

Artemisinins increase β cell mass in zebrafish and rodent models

•

Functional and transcriptional data indicate a conserved phenotype in human islets

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.

Receptor tyrosine kinase EphA7 is required for interneuron connectivity at specific subcellular compartments of granule cells

Neuronal transmission is regulated by the local circuitry which is composed of principal neurons targeted at different subcellular compartments by a variety of interneurons. However, mechanisms that contribute to the subcellular localisation and maintenance of GABAergic interneuron terminals are poorly understood. Stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin and its interaction with the guanine nucleotide exchange factor collybistin. Lentiviral knockdown experiments in adult rats indicated that the receptor tyrosine kinase EphA7 is required for the stabilisation of basket cell terminals on proximal dendritic and somatic compartments of granular cells of the dentate gyrus. EphA7 deficiency and concomitant destabilisation of GABAergic synapses correlated with impaired long-term potentiation and reduced hippocampal learning. Reduced GABAergic innervation may be explained by an impact of EphA7 on gephyrin clustering. Overexpression or ephrin stimulation of EphA7 induced gephyrin clustering dependent on the mechanistic target of rapamycin (mTOR) which is an interaction partner of gephyrin. Gephyrin interactions with mTOR become released after mTOR activation while enhanced interaction with the guanine nucleotide exchange factor collybistin was observed in parallel. In conclusion, EphA7 regulates gephyrin clustering and the maintenance of inhibitory synaptic connectivity via mTOR signalling.

Mechanical Signaling in the Pathophysiology of Critical Illness Myopathy

The complete loss of mechanical stimuli of skeletal muscles, i.e., the loss of external strain, related to weight bearing, and internal strain, related to the contraction of muscle cells, is uniquely observed in pharmacologically paralyzed or deeply sedated mechanically ventilated intensive care unit (ICU) patients. The preferential loss of myosin and myosin associated proteins in limb and trunk muscles is a significant characteristic of critical illness myopathy (CIM) which separates CIM from other types of acquired muscle weaknesses in ICU patients. Mechanical silencing is an important factor triggering CIM. Microgravity or ground based microgravity models form the basis of research on the effect of muscle unloading-reloading, but the mechanisms and effects may differ from the ICU conditions. In order to understand how mechanical tension regulates muscle mass, it is critical to know how muscles sense mechanical information and convert stimulus to intracellular biochemical actions and changes in gene expression, a process called cellular mechanotransduction. In adult skeletal muscles and muscle fibers, this process may differ, the same stimulus can cause divergent response and the same fiber type may undergo opposite changes in different muscles. Skeletal muscle contains multiple types of mechano-sensors and numerous structures that can be affected differently and hence respond differently in distinct muscles.

Regulation of the Target of Rapamycin and Other Phosphatidylinositol 3-Kinase-Related Kinases by Membrane Targeting

Phosphatidylinositol 3-kinase-related kinases (PIKKs) play vital roles in the regulation of cell growth, proliferation, survival, and consequently metabolism, as well as in the cellular response to stresses such as ionizing radiation or redox changes. In humans six family members are known to date, namely mammalian/mechanistic target of rapamycin (mTOR), ataxia-telangiectasia mutated (ATM), ataxia- and Rad3-related (ATR), DNA-dependent protein kinase catalytic subunit (DNA-PKcs), suppressor of morphogenesis in genitalia-1 (SMG-1), and transformation/transcription domain-associated protein (TRRAP). All fulfill rather diverse functions and most of them have been detected in different cellular compartments including various cellular membranes. It has been suggested that the regulation of the localization of signaling proteins allows for generating a locally specific output. Moreover, spatial partitioning is expected to improve the reliability of biochemical signaling. Since these assumptions may also be true for the regulation of PIKK function, the current knowledge about the regulation of the localization of PIKKs at different cellular (membrane) compartments by a network of interactions is reviewed. Membrane targeting can involve direct lipid-/membrane interactions as well as interactions with membrane-anchored regulatory proteins, such as, for example, small GTPases, or a combination of both.

Regulation of mTORC1 by PI3K signaling

The class I phosphoinositide 3-kinase (PI3K)-mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) signaling network directs cellular metabolism and growth. Activation of mTORC1 [composed of mTOR, regulatory-associated protein of mTOR (Raptor), mammalian lethal with SEC13 protein 8(mLST8), 40-kDa proline-rich Akt substrate (PRAS40), and DEP domain-containing mTOR-interacting protein (DEPTOR)] depends on the Ras-related GTPases (Rags) and Ras homolog enriched in brain (Rheb) GTPase and requires signals from amino acids, glucose, oxygen, energy (ATP), and growth factors (including cytokines and hormones such as insulin). Here we discuss the signal transduction mechanisms through which growth factor-responsive PI3K signaling activates mTORC1. We focus on how PI3K-dependent activation of Akt and spatial regulation of the tuberous sclerosis complex (TSC) complex (TSC complex) [composed of TSC1, TSC2, and Tre2-Bub2-Cdc16-1 domain family member 7 (TBC1D7)] switches on Rheb at the lysosome, where mTORC1 is activated. Integration of PI3K- and amino acid-dependent signals upstream of mTORC1 at the lysosome is detailed in a working model. A coherent understanding of the PI3K-mTORC1 network is imperative as its dysregulation has been implicated in diverse pathologies including cancer, diabetes, autism, and aging.

Effect of Metformin, Rapamycin, and Their Combination on Growth and Progression of Prostate Tumors in HiMyc Mice

In this study, we compared the effect of oral administration of metformin (MET) and rapamycin (RAPA) alone or in combination on prostate cancer development and progression in HiMyc mice. MET (250 mg/kg body weight in the drinking water), RAPA (2.24 mg/kg body weight microencapsulated in the diet), and the combination inhibited progression of prostatic intraepithelial neoplasia lesions to adenocarcinomas in the ventral prostate (VP). RAPA and the combination were more effective than MET at the doses used. Inhibition of prostate cancer progression in HiMyc mice by RAPA was associated with a significant reduction in mTORC1 signaling that was further potentiated by the combination of MET and RAPA. In contrast, treatment with MET alone enhanced AMPK activation, but had little or no effect on mTORC1 signaling pathways in the VP of HiMyc mice. Further analyses revealed a significant effect of all treatments on prostate tissue inflammation as assessed by analysis of the expression of cytokines, the presence of inflammatory cells and NFκB signaling. MET at the dose used appeared to reduce prostate cancer progression primarily by reducing tissue inflammation whereas RAPA and the combination appeared to inhibit prostate cancer progression in this mouse model via the combined effects on both mTORC1 signaling as well as on tissue inflammation. Overall, these data support the hypothesis that blocking mTORC1 signaling and/or tissue inflammation can effectively inhibit prostate cancer progression in a relevant mouse model of human prostate cancer. Furthermore, combinatorial approaches that target both pathways may be highly effective for prevention of prostate cancer progression in men.

Collybistin binds and inhibits mTORC1 signaling: a potential novel mechanism contributing to intellectual disability and autism

Protein synthesis regulation via mammalian target of rapamycin complex 1 (mTORC1) signaling pathway has key roles in neural development and function, and its dysregulation is involved in neurodevelopmental disorders associated with autism and intellectual disability. mTOR regulates assembly of the translation initiation machinery by interacting with the eukaryotic initiation factor eIF3 complex and by controlling phosphorylation of key translational regulators. Collybistin (CB), a neuron-specific Rho-GEF responsible for X-linked intellectual disability with epilepsy, also interacts with eIF3, and its binding partner gephyrin associates with mTOR. Therefore, we hypothesized that CB also binds mTOR and affects mTORC1 signaling activity in neuronal cells. Here, by using induced pluripotent stem cell-derived neural progenitor cells from a male patient with a deletion of entire CB gene and from control individuals, as well as a heterologous expression system, we describe that CB physically interacts with mTOR and inhibits mTORC1 signaling pathway and protein synthesis. These findings suggest that disinhibited mTORC1 signaling may also contribute to the pathological process in patients with loss-of-function variants in CB.

Gephyrin: a central GABAergic synapse organizer

Gephyrin is a central element that anchors, clusters and stabilizes glycine and γ-aminobutyric acid type A receptors at inhibitory synapses of the mammalian brain. It self-assembles into a hexagonal lattice and interacts with various inhibitory synaptic proteins. Intriguingly, the clustering of gephyrin, which is regulated by multiple posttranslational modifications, is critical for inhibitory synapse formation and function. In this review, we summarize the basic properties of gephyrin and describe recent findings regarding its roles in inhibitory synapse formation, function and plasticity. We will also discuss the implications for the pathophysiology of brain disorders and raise the remaining open questions in this field.

Human gephyrin is encompassed within giant functional noncoding yin-yang sequences

Gephyrin is a highly-conserved gene that is vital for the organization of proteins at inhibitory receptors, molybdenum cofactor biosynthesis, and other diverse functions. Its specific function is intricately regulated and its aberrant activities have been observed for a number of human diseases. Here we report a remarkable yin-yang haplotype pattern encompassing gephyrin. Yin-yang haplotypes arise when a stretch of DNA evolves to present two disparate forms that bear differing states for nucleotide variations along their lengths. The gephyrin yin-yang pair consists of 284 divergent nucleotide states and both variants vary drastically from their mutual ancestral haplotype, suggesting rapid evolution. Several independent lines of evidence indicate strong positive selection on the region and suggest these high-frequency haplotypes represent two distinct functional mechanisms. This discovery holds potential to deepen our understanding of variable human-specific regulation of gephyrin while providing clues for rapid evolutionary events and allelic migrations buried within human history.

The neurology of mTOR

The mechanistic target of rapamycin (mTOR) signaling pathway is a crucial cellular signaling hub that, like the nervous system itself, integrates internal and external cues to elicit critical outputs including growth control, protein synthesis, gene expression, and metabolic balance. The importance of mTOR signaling to brain function is underscored by the myriad disorders in which mTOR pathway dysfunction is implicated, such as autism, epilepsy, and neurodegenerative disorders. Pharmacological manipulation of mTOR signaling holds therapeutic promise and has entered clinical trials for several disorders. Here, we review the functions of mTOR signaling in the normal and pathological brain, highlighting ongoing efforts to translate our understanding of cellular physiology into direct medical benefit for neurological disorders.

Gephyrin phosphorylation in the functional organization and plasticity of GABAergic synapses

Gephyrin is a multifunctional scaffold protein essential for accumulation of inhibitory glycine and GABA_A receptors at post-synaptic sites. The molecular events involved in gephyrin-dependent GABA_A receptor clustering are still unclear. Evidence has been recently provided that gephyrin phosphorylation plays a key role in these processes. Gephyrin post-translational modifications have been shown to influence the structural remodeling of GABAergic synapses and synaptic plasticity by acting on post-synaptic scaffolding properties as well as stability. In addition, gephyrin phosphorylation and the subsequent phosphorylation-dependent recruitment of the chaperone molecule Pin1 provide a mechanism for the regulation of GABAergic signaling. Extensively characterized as pivotal enzyme controlling cell proliferation and differentiation, the prolyl-isomerase activity of Pin1 has been shown to regulate protein synthesis necessary to sustain the late phase of long-term potentiation at excitatory synapses, which suggests its involvement at synaptic sites. In this review we summarize the current state of knowledge of the signaling pathways responsible for gephyrin post-translational modifications. We will also outline future lines of research that might contribute to a better understanding of molecular mechanisms by which gephyrin regulates synaptic plasticity at GABAergic synapses.

Exploring the RNA world in hematopoietic cells through the lens of RNA-binding proteins

The discovery of microRNAs has renewed interest in posttranscriptional modes of regulation, fueling an emerging view of a rich RNA world within our cells that deserves further exploration. Much work has gone into elucidating genetic regulatory networks that orchestrate gene expression programs and direct cell fate decisions in the hematopoietic system. However, the focus has been to elucidate signaling pathways and transcriptional programs. To bring us one step closer to reverse engineering the molecular logic of cellular differentiation, it will be necessary to map posttranscriptional circuits as well and integrate them in the context of existing network models. In this regard, RNA-binding proteins (RBPs) may rival transcription factors as important regulators of cell fates and represent a tractable opportunity to connect the RNA world to the proteome. ChIP-seq has greatly facilitated genome-wide localization of DNA-binding proteins, helping us to understand genomic regulation at a systems level. Similarly, technological advances such as CLIP-seq allow transcriptome-wide mapping of RBP binding sites, aiding us to unravel posttranscriptional networks. Here, we review RBP-mediated posttranscriptional regulation, paying special attention to findings relevant to the immune system. As a prime example, we highlight the RBP Lin28B, which acts as a heterochronic switch between fetal and adult lymphopoiesis.

Up-regulation of cyclin-E(1) via proline-mTOR pathway is responsible for HGF-mediated G(1)/S progression in the primary culture of rat hepatocytes

Hepatocyte growth factor (HGF) is a key ligand that elicits G1/S progression of epithelial cells, including hepatocytes. Proline is also required for DNA synthesis that is induced by growth factors in primary culture of hepatocytes. However, it remains unknown how proline contributes to the G1/S progression of hepatocytes. The primary culture of rat hepatocytes using HGF plus proline can be a conceptual model for elucidating the molecular linkage of amino acids and growth factors during G1/S progression. Using this in vitro model, we provide evidence that not only induction of cyclin-D1 by HGF but also up-regulation of cyclin-E1 by proline is required for hepatocytes to enter the S-phase. Proline-enhanced cyclin-E1 induction, without changing its mRNA level, is associated with the activation of mammalian target of rapamycin (mTOR)-dependent pathways. Indeed, proline enhanced the ribosomal protein S6 phosphorylations (i.e., mTOR target), concomitantly with an increase in cyclin-E1. Inversely, mTOR-inhibitor, rapamycin suppressed the proline-mediated induction of cyclin-E1. As a result, DNA synthesis of hepatocytes, which was induced by HGF in the presence of proline, was largely abolished by mTOR-inhibitor treatment. Such a co-mitogenic effect of proline was also dependent on collagen synthesis: collagen synthesis inhibitors, such as cis-OH-proline, diminished the proline-induced cyclin-E1, and then the G1/S progression of hepatocytes was also suppressed. Overall, proline-mediated mTOR activation and collagen synthesis were found critical for HGF-induced DNA synthesis, partly via the sufficient accumulation of cyclin-E1. This is the first report to demonstrate the molecular bridge between amino acids and growth factors that drive mitogenic outcomes.

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 role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

Gephyrin, the enigmatic organizer at GABAergic synapses

GABA_A receptors are clustered at synaptic sites to achieve a high density of postsynaptic receptors opposite the input axonal terminals. This allows for an efficient propagation of GABA mediated signals, which mostly result in neuronal inhibition. A key organizer for inhibitory synaptic receptors is the 93 kDa protein gephyrin that forms oligomeric superstructures beneath the synaptic area. Gephyrin has long been known to be directly associated with glycine receptor β subunits that mediate synaptic inhibition in the spinal cord. Recently, synaptic GABA_A receptors have also been shown to directly interact with gephyrin and interaction sites have been identified and mapped within the intracellular loops of the GABA_A receptor α1, α2, and α3 subunits. Gephyrin-binding to GABA_A receptors seems to be at least one order of magnitude weaker than to glycine receptors (GlyRs) and most probably is regulated by phosphorylation. Gephyrin not only has a structural function at synaptic sites, but also plays a crucial role in synaptic dynamics and is a platform for multiple protein-protein interactions, bringing receptors, cytoskeletal proteins and downstream signaling proteins into close spatial proximity.

Expression and subcellular distribution of gephyrin in non-neuronal tissues and cells

Gephyrin is a scaffolding protein required for the accumulation of inhibitory neurotransmitter receptors at neuronal postsynaptic membranes. In non-neuronal tissues, gephyrin is indispensible for the biosynthesis of molybdenum cofactor, the prosthetic group of oxidoreductases including sulfite oxidase and xanthine oxidase. However, the molecular and cellular basis of gephyrin's non-neuronal function is poorly understood; in particular, the roles of its splice variants remain enigmatic. Here, we used cDNA screening as well as Northern and immunoblot analyses to show that mammalian liver contains only a limited number of gephyrin splice variants, with the C3-containing variant being the predominant isoform. Using new and established anti-gephyrin antibodies in immunofluorescence and subcellular fractionation studies, we report that gephyrin localizes to the cytoplasm of both tissue hepatocytes and cultured immortalized cells. These findings were corroborated by RNA interference studies in which the cytosolic distribution was found to be abolished. Finally, by blue-native PAGE we show that cytoplasmic gephyrin is part of a ~600 kDa protein complex of yet unknown composition. Our data suggest that the expression pattern of non-neuronal gephyrin is simpler than indicated by previous evidence. In addition, gephyrin's presence in a cytosolic 600 kDa protein complex suggests that its metabolic and/or other non-neuronal functions are exerted in the cytoplasm and are not confined to a particular subcellular compartment.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

Phosphoproteomic profiling of in vivo signaling in liver by the mammalian target of rapamycin complex 1 (mTORC1)

BACKGROUND

Our understanding of signal transduction networks in the physiological context of an organism remains limited, partly due to the technical challenge of identifying serine/threonine phosphorylated peptides from complex tissue samples. In the present study, we focused on signaling through the mammalian target of rapamycin (mTOR) complex 1 (mTORC1), which is at the center of a nutrient- and growth factor-responsive cell signaling network. Though studied extensively, the mechanisms involved in many mTORC1 biological functions remain poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We developed a phosphoproteomic strategy to purify, enrich and identify phosphopeptides from rat liver homogenates. Using the anticancer drug rapamycin, the only known target of which is mTORC1, we characterized signaling in liver from rats in which the complex was maximally activated by refeeding following 48 hr of starvation. Using protein and peptide fractionation methods, TiO(2) affinity purification of phosphopeptides and mass spectrometry, we reproducibly identified and quantified over four thousand phosphopeptides. Along with 5 known rapamycin-sensitive phosphorylation events, we identified 62 new rapamycin-responsive candidate phosphorylation sites. Among these were PRAS40, gephyrin, and AMP kinase 2. We observed similar proportions of increased and reduced phosphorylation in response to rapamycin. Gene ontology analysis revealed over-representation of mTOR pathway components among rapamycin-sensitive phosphopeptide candidates.

CONCLUSIONS/SIGNIFICANCE

In addition to identifying potential new mTORC1-mediated phosphorylation events, and providing information relevant to the biology of this signaling network, our experimental and analytical approaches indicate the feasibility of large-scale phosphoproteomic profiling of tissue samples to study physiological signaling events in vivo.

The biological role of the glycinergic synapse in early zebrafish motility

Glycine mediates fast inhibitory neurotransmission in the spinal cord, brainstem and retina. Loss of synaptic glycinergic transmission in vertebrates leads to a severe locomotion defect characterized by an exaggerated startle response accompanied by transient muscle rigidity in response to sudden acoustic or tactile stimuli. Several molecular components of the glycinergic synapse have been characterized as an outcome of genetic and physiological analyses of synaptogenesis in mammals. Recently, the glycinergic synapse has been studied using a forward genetic approach in zebrafish. This review aims to discuss molecular components of the glycinergic synapse, such as glycine receptor subunits, gephyrin, gephyrin-binding proteins and glycine transporters, as well as recent studies relevant to the genetic analysis of the glycinergic synapse in zebrafish.

Heat shock cognate protein 70 regulates gephyrin clustering

Formation and stabilization of postsynaptic glycine receptor (GlyR) clusters result from their association with the polymerized scaffold protein gephyrin. At the cell surface, lateral diffusion and local trapping of GlyR by synaptic gephyrin clusters is one of the main factors controlling their number. However, the mechanisms regulating gephyrin/GlyR cluster sizes are not fully understood. To identify molecular binding partners able to control gephyrin cluster stability, we performed pull-down assays with full-length or truncated gephyrin forms incubated in a rat spinal cord extract, combined with mass spectrometric analysis. We found that heat shock cognate protein 70 (Hsc70), a constitutive member of the heat shock protein 70 (Hsp70) family, selectively binds to the gephyrin G-domain. Immunoelectron microscopy of mouse spinal cord sections showed that Hsc70 could be colocalized with gephyrin at inhibitory synapses. Furthermore, ternary Hsc70-gephyrin-GlyR coclusters were formed following transfection of COS-7 cells. Upon overexpression of Hsc70 in mouse spinal cord neurons, synaptic accumulation of gephyrin was significantly decreased, but GlyR amounts were unaffected. In the same way, Hsc70 inhibition increased gephyrin accumulation at inhibitory synapses without modifying GlyR clustering. Single particle tracking experiments revealed that the increase of gephyrin molecules reduced GlyR diffusion rates without altering GlyR residency at synapses. Our findings demonstrate that Hsc70 regulates gephyrin polymerization independently of its interaction with GlyR. Therefore, gephyrin polymerization and synaptic clustering of GlyR are uncoupled events.

mTORC1 signaling: what we still don't know

The mammalian target of rapamycin (mTOR) is a protein kinase that plays key roles in cellular regulation. It forms complexes with additional proteins. The best-understood one is mTOR complex 1 (mTORC1). The regulation and cellular functions of mTORC1 have been the subjects of intense study; despite this, many questions remain to be answered. They include questions about the actual mechanisms by which mTORC1 signaling is stimulated by hormones and growth factors, which involves the small GTPase Rheb, and by amino acids, which involves other GTPase proteins. The control of Rheb and the mechanism by which it activates mTORC1 remain incompletely understood. Although it has been known for many years that rapamycin interferes with some functions of mTORC1, it is not known how it does this, or why only some functions of mTORC1 are affected. mTORC1 regulates diverse cellular functions. Several mTORC1 substrates are now known, although in several cases their physiological roles are poorly or incompletely understood. In the case of several processes, although it is clear that they are regulated by mTORC1, it is not known how mTORC1 does this. Lastly, mTORC1 is implicated in ageing, but again it is unclear what mechanisms account for this. Given the importance of mTORC1 signaling both for cellular functions and in human disease, it is a high priority to gain further insights into the control of mTORC1 signaling and the mechanisms by which it controls cellular functions and animal physiology.

Amino acids activate mammalian target of rapamycin complex 2 (mTORC2) via PI3K/Akt signaling

The activity of mammalian target of rapamycin (mTOR) complexes regulates essential cellular processes, such as growth, proliferation, or survival. Nutrients such as amino acids are important regulators of mTOR complex 1 (mTORC1) activation, thus affecting cell growth, protein synthesis, and autophagy. Here, we show that amino acids may also activate mTOR complex 2 (mTORC2). This activation is mediated by the activity of class I PI3K and of Akt. Amino acids induced a rapid phosphorylation of Akt at Thr-308 and Ser-473. Whereas both phosphorylations were dependent on the presence of mTOR, only Akt phosphorylation at Ser-473 was dependent on the presence of rictor, a specific component of mTORC2. Kinase assays confirmed mTORC2 activation by amino acids. This signaling was functional, as demonstrated by the phosphorylation of Akt substrate FOXO3a. Interestingly, using different starvation conditions, amino acids can selectively activate mTORC1 or mTORC2. These findings identify a new signaling pathway used by amino acids underscoring the crucial importance of these nutrients in cell metabolism and offering new mechanistic insights.

Collybistin and gephyrin are novel components of the eukaryotic translation initiation factor 3 complex

Background

Collybistin (CB), a neuron-specific guanine nucleotide exchange factor, has been implicated in targeting gephyrin-GABA_A receptors clusters to inhibitory postsynaptic sites. However, little is known about additional CB partners and functions.

Findings

Here, we identified the p40 subunit of the eukaryotic translation initiation factor 3 (eIF3H) as a novel binding partner of CB, documenting the interaction in yeast, non-neuronal cell lines, and the brain. In addition, we demonstrated that gephyrin also interacts with eIF3H in non-neuronal cells and forms a complex with eIF3 in the brain.

Conclusions

Together, our results suggest, for the first time, that CB and gephyrin associate with the translation initiation machinery, and lend further support to the previous evidence that gephyrin may act as a regulator of synaptic protein synthesis.

Cellular transport and membrane dynamics of the glycine receptor

Regulation of synaptic transmission is essential to tune individual-to-network neuronal activity. One way to modulate synaptic strength is to regulate neurotransmitter receptor numbers at postsynaptic sites. This can be achieved either through plasma membrane insertion of receptors derived from intracellular vesicle pools, a process depending on active cytoskeleton transport, or through surface membrane removal via endocytosis. In parallel, lateral diffusion events along the plasma membrane allow the exchange of receptor molecules between synaptic and extrasynaptic compartments, contributing to synaptic strength regulation. In recent years, results obtained from several groups studying glycine receptor (GlyR) trafficking and dynamics shed light on the regulation of synaptic GlyR density. Here, we review (i) proteins and mechanisms involved in GlyR cytoskeletal transport, (ii) the diffusion dynamics of GlyR and of its scaffolding protein gephyrin that control receptor numbers, and its relationship with synaptic plasticity, and (iii) adaptative changes in GlyR diffusion in response to global activity modifications, as a homeostatic mechanism.

Inhibition of the mammalian target of rapamycin signaling pathway suppresses dentate granule cell axon sprouting in a rodent model of temporal lobe epilepsy

Dentate granule cell axon (mossy fiber) sprouting is a common abnormality in patients with temporal lobe epilepsy. Mossy fiber sprouting creates an aberrant positive-feedback network among granule cells that does not normally exist. Its role in epileptogenesis is unclear and controversial. If it were possible to block mossy fiber sprouting from developing after epileptogenic treatments, its potential role in the pathogenesis of epilepsy could be tested. Previous attempts to block mossy fiber sprouting have been unsuccessful. The present study targeted the mammalian target of rapamycin (mTOR) signaling pathway, which regulates cell growth and is blocked by rapamycin. Rapamycin was focally, continuously, and unilaterally infused into the dorsal hippocampus for prolonged periods beginning within hours after rats sustained pilocarpine-induced status epilepticus. Infusion for 1 month reduced aberrant Timm staining (a marker of mossy fibers) in the granule cell layer and molecular layer. Infusion for 2 months inhibited mossy fiber sprouting more. However, after rapamycin infusion ceased, aberrant Timm staining developed and approached untreated levels. When onset of infusion began after mossy fiber sprouting had developed for 2 months, rapamycin did not reverse aberrant Timm staining. These findings suggest that inhibition of the mTOR signaling pathway suppressed development of mossy fiber sprouting. However, suppression required continual treatment, and rapamycin treatment did not reverse already established axon reorganization.