Common molecular pathways mediate long-term potentiation of synaptic excitation and slow synaptic inhibition

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Learning-induced bidirectional enhancement of inhibitory synaptic metaplasticity

Training rodents in a particularly difficult olfactory-discrimination (OD) task results in the acquisition of the ability to perform the task well, termed 'rule learning'. In addition to enhanced intrinsic excitability and synaptic excitation in piriform cortex pyramidal neurons, rule learning results in increased synaptic inhibition across the whole cortical network to the point where it precisely maintains the balance between inhibition and excitation. The mechanism underlying such precise inhibitory enhancement remains to be explored. Here, we use brain slices from transgenic mice (VGAT-ChR2-EYFP), enabling optogenetic stimulation of single GABAergic neurons and recordings of unitary synaptic events in pyramidal neurons. Quantal analysis revealed that learning-induced enhanced inhibition is mediated by increased quantal size of the evoked inhibitory events. Next, we examined the plasticity of synaptic inhibition induced by long-lasting, intrinsically evoked spike firing in post-synaptic neurons. Repetitive depolarizing current pulses from depolarized (-70 mV) or hyperpolarized (-90 mV) membrane potentials induced long-term depression (LTD) and long-term potentiation (LTP) of synaptic inhibition, respectively. We found a profound bidirectional increase in the ability to induce both LTD, mediated by L-type calcium channels, and LTP, mediated by R-type calcium channels after rule learning. Blocking the GABAB receptor reversed the effect of intrinsic stimulation at -90 mV from LTP to LTD. We suggest that learning greatly enhances the ability to modify the strength of synaptic inhibition of principal neurons in both directions. Such plasticity of synaptic plasticity allows fine-tuning of inhibition on each particular neuron, thereby stabilizing the network while maintaining the memory of the rule. KEY POINTS: Olfactory discrimination rule learning results in long-lasting enhancement of synaptic inhibition on piriform cortex pyramidal neurons. Quantal analysis of unitary inhibitory synaptic events, evoked by optogenetic minimal stimulation, revealed that enhanced synaptic inhibition is mediated by increased quantal size. Surprisingly, metaplasticity of synaptic inhibition, induced by intrinsically evoked repetitive spike firing, is increased bidirectionally. The susceptibility to both long-term depression (LTD) and long-term potentiation (LTP) of inhibition is enhanced after learning. LTD of synaptic inhibition is mediated by L-type calcium channels and LTP by R-type calcium channels. LTP is also dependent on activation of GABAB receptors. We suggest that learning-induced changes in the metaplasticity of synaptic inhibition enable the fine-tuning of inhibition on each particular neuron, thereby stabilizing the network while maintaining the memory of the rule.

Unique transcriptomes of sensory and non-sensory neurons: insights from Splicing Regulatory States

Alternative Splicing (AS) programs serve as instructive signals of cell type specificity, particularly within the brain, which comprises dozens of molecularly and functionally distinct cell types. Among them, retinal photoreceptors stand out due to their unique transcriptome, making them a particularly well-suited system for studying how AS shapes cell type-specific molecular functions. Here, we use the Splicing Regulatory State (SRS) as a novel framework to discuss the splicing factors governing the unique AS pattern of photoreceptors, and how this pattern may aid in the specification of their highly specialized sensory cilia. In addition, we discuss how other sensory cells with ciliated structures, for which data is much scarcer, also rely on specific SRSs to implement a proteome specialized in the detection of sensory stimuli. By reviewing the general rules of cell type- and tissue-specific AS programs, firstly in the brain and subsequently in specialized sensory neurons, we propose a novel paradigm on how SRSs are established and how they can diversify. Finally, we illustrate how SRSs shape the outcome of mutations in splicing factors to produce cell type-specific phenotypes that can lead to various human diseases.

Impairments in hippocampal oscillations accompany the loss of LTP induced by GIRK activity blockade

Learning and memory occurrence requires of hippocampal long-term synaptic plasticity and precise neural activity orchestrated by brain network oscillations, both processes reciprocally influencing each other. As G-protein-gated inwardly rectifying potassium (GIRK) channels rule synaptic plasticity that supports hippocampal-dependent memory, here we assessed their unknown role in hippocampal oscillatory activity in relation to synaptic plasticity induction. In alert male mice, pharmacological GIRK modulation did not alter neural oscillations before long-term potentiation (LTP) induction. However, after an LTP generating protocol, both gain- and loss-of basal GIRK activity transformed LTP into long-term depression, but only specific suppression of constitutive GIRK activity caused a disruption of network synchronization (δ, α, γ bands), even leading to long-lasting ripples and fast ripples pathological oscillations. Together, our data showed that constitutive GIRK activity plays a key role in the tuning mechanism of hippocampal oscillatory activity during long-term synaptic plasticity processes that underlies hippocampal-dependent cognitive functions.

Cranial irradiation impairs intrinsic excitability and synaptic plasticity of hippocampal CA1 pyramidal neurons with implications for cognitive function

Radiation therapy is a standard treatment for head and neck tumors. However, patients often exhibit cognitive impairments following radiation therapy. Previous studies have revealed that hippocampal dysfunction, specifically abnormal hippocampal neurogenesis or neuroinflammation, plays a key role in radiation-induced cognitive impairment. However, the long-term effects of radiation with respect to the electrophysiological adaptation of hippocampal neurons remain poorly characterized. We found that mice exhibited cognitive impairment 3 months after undergoing 10 minutes of cranial irradiation at a dose rate of 3 Gy/min. Furthermore, we observed a remarkable reduction in spike firing and excitatory synaptic input, as well as greatly enhanced inhibitory inputs, in hippocampal CA1 pyramidal neurons. Corresponding to the electrophysiological adaptation, we found reduced expression of synaptic plasticity marker VGLUT1 and increased expression of VGAT. Furthermore, in irradiated mice, long-term potentiation in the hippocampus was weakened and GluR1 expression was inhibited. These findings suggest that radiation can impair intrinsic excitability and synaptic plasticity in hippocampal CA1 pyramidal neurons.

Recognition Memory Induces Natural LTP-like Hippocampal Synaptic Excitation and Inhibition

Synaptic plasticity is a cellular process involved in learning and memory by which specific patterns of neural activity adapt the synaptic strength and efficacy of the synaptic transmission. Its induction is governed by fine tuning between excitatory/inhibitory synaptic transmission. In experimental conditions, synaptic plasticity can be artificially evoked at hippocampal CA1 pyramidal neurons by repeated stimulation of Schaffer collaterals. However, long-lasting synaptic modifications studies during memory formation in physiological conditions in freely moving animals are very scarce. Here, to study synaptic plasticity phenomena during recognition memory in the dorsal hippocampus, field postsynaptic potentials (fPSPs) evoked at the CA3–CA1 synapse were recorded in freely moving mice during object-recognition task performance. Paired pulse stimuli were applied to Schaffer collaterals at the moment that the animal explored a new or a familiar object along different phases of the test. Stimulation evoked a complex synaptic response composed of an ionotropic excitatory glutamatergic fEPSP, followed by two inhibitory responses, an ionotropic, GABA_A-mediated fIPSP and a metabotropic, G-protein-gated inwardly rectifying potassium (GirK) channel-mediated fIPSP. Our data showed the induction of LTP-like enhancements for both the glutamatergic and GirK-dependent components of the dorsal hippocampal CA3–CA1 synapse during the exploration of novel but not familiar objects. These results support the contention that synaptic plasticity processes that underlie hippocampal-dependent memory are sustained by fine tuning mechanisms that control excitatory and inhibitory neurotransmission balance.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

De novo truncating NOVA2 variants affect alternative splicing and lead to heterogeneous neurodevelopmental phenotypes

Alternative splicing (AS) is crucial for cell-type specific gene transcription and plays a critical role in neuronal differentiation and synaptic plasticity. De novo frameshift variants in NOVA2, encoding a neuron-specific key splicing factor, have been recently associated with a new neurodevelopmental disorder (NDD) with hypotonia, neurological features, and brain abnormalities. We investigated eight unrelated individuals by exome sequencing (ES) and identified seven novel pathogenic NOVA2 variants, including two with a novel localization at the KH1 and KH3 domains. In addition to a severe NDD phenotype, novel clinical features included psychomotor regression, attention deficit-hyperactivity disorder (ADHD), dyspraxia, and urogenital and endocrinological manifestations. To test the effect of the variants on splicing regulation, we transfected HeLa cells with wildtype and mutant NOVA2 cDNA. The novel variants NM_002516.4: c.754_756delCTGinsTT p.(Leu252Phefs*144) and c.1329dup p.(Lys444Glnfs*82) all negatively affected AS events. The distal p.(Lys444Glnfs*82) variant, causing a partial removal of the KH3 domain, had a milder functional effect leading to an intermediate phenotype. Our findings expand the molecular and phenotypic spectrum of NOVA2-related NDD, supporting the pathogenic role of AS disruption by truncating variants and suggesting that this is a heterogeneous condition with variable clinical course. This article is protected by copyright. All rights reserved.

G-Protein-Gated Inwardly Rectifying Potassium (Kir3/GIRK) Channels Govern Synaptic Plasticity That Supports Hippocampal-Dependent Cognitive Functions in Male Mice

The G-protein-gated inwardly rectifying potassium (Kir3/GIRK) channel is the effector of many G-protein-coupled receptors (GPCRs). Its dysfunction has been linked to the pathophysiology of Down syndrome, Alzheimer's and Parkinson's diseases, psychiatric disorders, epilepsy, drug addiction, or alcoholism. In the hippocampus, GIRK channels decrease excitability of the cells and contribute to resting membrane potential and inhibitory neurotransmission. Here, to elucidate the role of GIRK channels activity in the maintenance of hippocampal-dependent cognitive functions, their involvement in controlling neuronal excitability at different levels of complexity was examined in C57BL/6 male mice. For that purpose, GIRK activity in the dorsal hippocampus CA3-CA1 synapse was pharmacologically modulated by two drugs: ML297, a GIRK channel opener, and Tertiapin-Q (TQ), a GIRK channel blocker. Ex vivo, using dorsal hippocampal slices, we studied the effect of pharmacological GIRK modulation on synaptic plasticity processes induced in CA1 by Schaffer collateral stimulation. In vivo, we performed acute intracerebroventricular (i.c.v.) injections of the two GIRK modulators to study their contribution to electrophysiological properties and synaptic plasticity of dorsal hippocampal CA3-CA1 synapse, and to learning and memory capabilities during hippocampal-dependent tasks. We found that pharmacological disruption of GIRK channel activity by i.c.v. injections, causing either function gain or function loss, induced learning and memory deficits by a mechanism involving neural excitability impairments and alterations in the induction and maintenance of long-term synaptic plasticity processes. These results support the contention that an accurate control of GIRK activity must take place in the hippocampus to sustain cognitive functions.SIGNIFICANCE STATEMENT Cognitive processes of learning and memory that rely on hippocampal synaptic plasticity processes are critically ruled by a finely tuned neural excitability. G-protein-gated inwardly rectifying K+ (GIRK) channels play a key role in maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Here, we demonstrate that modulation of GIRK channels activity, causing either function gain or function loss, transforms high-frequency stimulation (HFS)-induced long-term potentiation (LTP) into long-term depression (LTD), inducing deficits in hippocampal-dependent learning and memory. Together, our data show a crucial GIRK-activity-mediated mechanism that governs synaptic plasticity direction and modulates subsequent hippocampal-dependent cognitive functions.

5-HT6R null mutatrion induces synaptic and cognitive defects

Serotonin 6 receptor (5-HT6R) is a promising target for a variety of human diseases, such as Alzheimer's disease (AD) and schizophrenia. However, the detailed mechanism underlying 5-HT6R activity in the central nervous system (CNS) is not fully understood. In the present study, 5-HT6R null mutant (5-HT6R^-/- ) mice were found to exhibit cognitive deficiencies and abnormal anxiety levels. 5-HT6R is considered to be specifically localized on the primary cilia. We found that the loss of 5-HT6R affected the Sonic Hedgehog signaling pathway in the primary cilia. 5-HT6R^-/- mice showed remarkable alterations in neuronal morphology, including dendrite complexity and axon initial segment morphology. Neurons lacking 5-HT6R exhibited increased neuronal excitability. Our findings highlight the complexity of 5-HT6R functions in the primary ciliary and neuronal physiology, supporting the theory that this receptor modulates neuronal morphology and transmission, and contributes to cognitive deficits in a variety of human diseases, such as AD, schizophrenia, and ciliopathies.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

New insights on epigenetic mechanisms supporting axonal development: histone marks and miRNAs

Mechanisms supporting axon growth and the establishment of neuronal polarity have remained largely disconnected from their genetic and epigenetic fundamentals. Recently, post-transcriptional modifications of histones involved in chromatin folding and transcription, and microRNAs controlling translation have emerged as regulators of axonal specification, growth, and guidance. In this article, we review novel evidence supporting the concept that epigenetic mechanisms work at both transcriptional and post-transcriptional levels to shape axons. We also discuss the role of splicing on axonal growth, as one of the most (if not the most) powerful post-transcriptional mechanism to diversify genetic information. Overall, we think exploring the gap between epigenetics and axonal growth raises new questions and perspectives to the development of axons in physiological and pathological contexts.

Reduced activity of GIRK1-containing heterotetramers is sufficient to affect neuronal functions, including synaptic plasticity and spatial learning and memory

KEY POINTS

G-protein inwardly rectifying K⁺ (GIRK) channels consist of four homologous subunits (GIRK1-4) and are essential regulators of electrical excitability in the nervous system. GIRK2-null mice have been widely investigated for their distinct behaviour and altered depotentiation following long-term potentiation (LTP), whereas GIRK1 mice are less well characterized. Here we utilize a novel knockin mouse strain in which the GIRK1 subunit is fluorescently tagged with yellow fluorescent protein (YFP-GIRK1) and the GIRK1-null mouse line to investigate the role of GIRK1 in neuronal processes such as spatial learning and memory, locomotion and depotentiation following LTP. Neurons dissected from YFP-GIRK1 mice had significantly reduced potassium currents and this mouse line phenotypically resembled GIRK1-null mice, making it a 'functional knockdown' model of GIRK1-containing channels. YFP-GIRK1 and GIRK1-null mice had increased locomotion, reduced spatial learning and memory and blunted depotentiation following LTP.

ABSTRACT

GIRK channels are essential for the slow inhibition of electrical activity in the nervous system and heart rate regulation via the parasympathetic system. The implications of individual GIRK isoforms in specific physiological activities are based primarily on studies conducted with GIRK-null mouse lines. Here we utilize a novel knockin mouse line in which YFP was fused in-frame to the N-terminus of GIRK1 (YFP-GIRK1) to correlate GIRK1 spatial distribution with physiological activities. These mice, however, displayed spontaneous seizure-like activity and thus were investigated for the origin of such activity. We show that GIRK tetramers containing YFP-GIRK1 are correctly assembled and trafficked to the plasma membrane, but are functionally impaired. A battery of behavioural assays conducted on YFP-GIRK1 and GIRK1-null (GIRK1^-/- ) mice revealed similar phenotypes, including impaired nociception, reduced anxiety and hyperactivity in an unfamiliar environment. However, YFP-GIRK1 mice exhibited increased home-cage locomotion while GIRK1^-/- mice did not. In addition, we show that the GIRK1 subunit is essential for intact spatial learning and memory and synaptic plasticity in hippocampal brain slices. This study expands our knowledge regarding the role of GIRK1 in neuronal processes and underlines the importance of GIRK1-containing heterotetramers.

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.

Ih, GIRK, and KCNQ/Kv7 channels differently modulate sharp wave - ripples in the dorsal and ventral hippocampus

Sharp waves and ripples (SPW-Rs) are endogenous transient patterns of hippocampus local network activity implicated in several functions including memory consolidation, and they are diversified between the dorsal and the ventral hippocampus. Ion channels in the neuronal membrane play important roles in cell and local network function. In this study, using transverse slices and field potential recordings from the CA1 field of rat hippocampus we show that GIRK and KCNQ2/3 potassium channels play a higher role in modulating SPW-Rs in the dorsal hippocampus, while I_h and other KCNQ (presumably KCNQ5) channels, contribute to shaping SPW-R activity more in the ventral than in dorsal hippocampus. Specifically, blockade of I_h channels by ZD 7288 reduced the rate of occurrence of SPW-Rs and increased the generation of SPW-Rs in the form of clusters in both hippocampal segments, while enhanced the amplitude of SPW-Rs only in the ventral hippocampus. Most effects of ZD 7288 appeared to be independent of NMDA receptors' activity. However, the effects of blockade of NMDA receptors depended on the functional state of I_h channels in both hippocampal segments. Blockade of GIRK channels by Tertiapin-Q increased the rate of occurrence of SPW-Rs only in the dorsal hippocampus and the probability of clusters in both segments of the hippocampus. Blockade of KCNQ2/3 channels by XE 991 increased the rate of occurrence of SPW-Rs and the probability of clusters in the dorsal hippocampus, and only reduced the clustered generation of SPW-Rs in the ventral hippocampus. The blocker of KCNQ1/2 channels, that also enhances KCNQ5 channels, UCL 2077, increased the probability of clusters and the power of the ripple oscillation in the ventral hippocampus only. These results suggest that GIRK, KCNQ and I_h channels represent a key mechanism for modulation of SPW-R activity which act differently in the dorsal and ventral hippocampus, fundamentally supporting functional diversification along the dorsal-ventral axis of the hippocampus.

Impairments of Synaptic Plasticity Induction Threshold and Network Oscillatory Activity in the Hippocampus Underlie Memory Deficits in a Non-Transgenic Mouse Model of Amyloidosis

In early Alzheimer disease (AD) models synaptic failures and upstreaming aberrant patterns of network synchronous activity result in hippocampal-dependent memory deficits. In such initial stage, soluble forms of Amyloid-β (Aβ) peptides have been shown to play a causal role. Among different Aβ species, Aβ_25-35 has been identified as the biologically active fragment, as induces major neuropathological signs related to early AD stages. Consequently, it has been extensively used to acutely explore the pathophysiological events related with neuronal dysfunction induced by soluble Aβ forms. However, the synaptic mechanisms underlying its toxic effects on hippocampal-dependent memory remain unresolved. Here, in an in vivo model of amyloidosis generated by intracerebroventricular injections of Aβ_25-35 we studied the synaptic dysfunction mechanisms underlying hippocampal cognitive deficits. At the synaptic level, long-term potentiation (LTP) of synaptic excitation and inhibition was induced in CA1 region by high frequency simulation (HFS) applied to Schaffer collaterals. Aβ_25-35 was found to alter metaplastic mechanisms of plasticity, facilitating long-term depression (LTD) of both types of LTP. In addition, aberrant synchronization of hippocampal network activity was found while at the behavioral level, deficits in hippocampal-dependent habituation and recognition memories emerged. Together, our results provide a substrate for synaptic disruption mechanism underlying hippocampal cognitive deficits present in Aβ_25-35 amyloidosis model.

Alternative Splicing by NOVA Factors: From Gene Expression to Cell Physiology and Pathology

NOVA1 and NOVA2, the two members of the NOVA family of alternative splicing factors, bind YCAY clusters of pre-mRNAs and assemble spliceosomes to induce the maintenance/removal of introns and exons, thus governing the development of mRNAs. Members of other splicing families operate analogously. Activity of NOVAs accounts for up to 700 alternative splicing events per cell, taking place both in the nucleus (co-transcription of mRNAs) and in the cytoplasm. Brain neurons express high levels of NOVAs, with NOVA1 predominant in cerebellum and spinal cord, NOVA2 in the cortex. Among brain physiological processes NOVAs play critical roles in axon pathfinding and spreading, structure and function of synapses, as well as the regulation of surface receptors and voltage-gated channels. In pathology, NOVAs contribute to neurodegenerative diseases and epilepsy. In vessel endothelial cells, NOVA2 is essential for angiogenesis, while in adipocytes, NOVA1 contributes to regulation of thermogenesis and obesity. In many cancers NOVA1 and also NOVA2, by interacting with specific miRNAs and by additional mechanisms, activate oncogenic roles promoting cell proliferation, colony formation, migration, and invasion. In conclusion, NOVAs regulate cell functions of physiological and pathological nature. Single cell identification and distinction, and new therapies addressed to NOVA targets might be developed in the near future.

Sevoflurane inhibits neuronal migration and axon growth in the developing mouse cerebral cortex

The highly organized laminar structure of the mammalian brain is dependent on successful neuronal migration, and migration deficits can cause lissencephaly and behavioral and cognitive defects. Here, we investigated the contribution of neuronal migration dysregulation to anesthesia-induced neurotoxicity in the fetal brain. Pregnant C57BL/6 mice at embryonic day 14.5 received 2.5% sevoflurane daily for two days. Cortical neuron migration and axon lengths were evaluated using GFP immunostaining. Morris water maze tests were performed to assess the effects of sevoflurane exposure on spatial memory in offspring. We found that sevoflurane exposure decreased axon length and caused cognitive defects in young mice. RNA sequencing revealed that these defects were associated with reduced neuro-oncological ventral antigen 2 (Nova2) expression. In utero electroporation experiments using Nova2 shRNA recapitulated this finding. Nova2 shRNA inhibited neuronal migration and decreased axon lengths. Finally, we found that Netrin-1/Deleted in Colorectal Cancer (Dcc) proteins acted downstream of Nova2 to suppresses neuronal migration. These findings describe a novel mechanism by which prenatal anesthesia exposure affects embryonic neural development and postnatal behavior.

Muscarinic Receptors, from Synaptic Plasticity to its Role in Network Activity

Acetylcholine acting via metabotropic receptors plays a key role in learning and memory by regulating synaptic plasticity and circuit activity. However, a recent overall view of the effects of muscarinic acetylcholine receptors (mAChRs) on excitatory and inhibitory long-term synaptic plasticity and on circuit activity is lacking. This review focusses on specific aspects of the regulation of synaptic plasticity and circuit activity by mAChRs in the hippocampus and cortex. Acetylcholine increases the excitability of pyramidal neurons, facilitating the generation of dendritic Ca²⁺-spikes, NMDA-spikes and action potential bursts which provide the main source of Ca²⁺ influx necessary to induce synaptic plasticity. The activation of mAChRs induced Ca²⁺ release from intracellular IP₃-sensitive stores is a major player in the induction of a NMDA independent long-term potentiation (LTP) caused by an increased expression of AMPA receptors in hippocampal pyramidal neuron dendritic spines. In the neocortex, activation of mAChRs also induces a long-term enhancement of excitatory postsynaptic currents. In addition to effects on excitatory synapses, a single brief activation of mAChRs together with short repeated membrane depolarization can induce a long-term enhancement of GABA A type (GABA_A) inhibition through an increased expression of GABA_A receptors in hippocampal pyramidal neurons. By contrast, a long term depression of GABA_A inhibition (iLTD) is induced by muscarinic receptor activation in the absence of postsynaptic depolarizations. This iLTD is caused by an endocannabinoid-mediated presynaptic inhibition that reduces the GABA release probability at the terminals of inhibitory interneurons. This bidirectional long-term plasticity of inhibition may dynamically regulate the excitatory/inhibitory balance depending on the quiescent or active state of the postsynaptic pyramidal neurons. Therefore, acetylcholine can induce varied effects on neuronal activity and circuit behavior that can enhance sensory detection and processing through the modification of circuit activity leading to learning, memory and behavior.

Heterosynaptic Plasticity Determines the Set Point for Cortical Excitatory-Inhibitory Balance

Excitation in neural circuits must be carefully controlled by inhibition to regulate information processing and network excitability. During development, cortical inhibitory and excitatory inputs are initially mismatched but become co-tuned or balanced with experience. However, little is known about how excitatory-inhibitory balance is defined at most synapses or about the mechanisms for establishing or maintaining this balance at specific set points. Here we show how coordinated long-term plasticity calibrates populations of excitatory-inhibitory inputs onto mouse auditory cortical pyramidal neurons. Pairing pre- and postsynaptic activity induced plasticity at paired inputs and different forms of heterosynaptic plasticity at the strongest unpaired synapses, which required minutes of activity and dendritic Ca²⁺ signaling to be computed. Theoretical analyses demonstrated how the relative rate of heterosynaptic plasticity could normalize and stabilize synaptic strengths to achieve any possible excitatory-inhibitory correlation. Thus, excitatory-inhibitory balance is dynamic and cell specific, determined by distinct plasticity rules across multiple excitatory and inhibitory synapses.

Inhibitory Plasticity: From Molecules to Computation and Beyond

Synaptic plasticity is the cellular and molecular counterpart of learning and memory and, since its first discovery, the analysis of the mechanisms underlying long-term changes of synaptic strength has been almost exclusively focused on excitatory connections. Conversely, inhibition was considered as a fixed controller of circuit excitability. Only recently, inhibitory networks were shown to be finely regulated by a wide number of mechanisms residing in their synaptic connections. Here, we review recent findings on the forms of inhibitory plasticity (IP) that have been discovered and characterized in different brain areas. In particular, we focus our attention on the molecular pathways involved in the induction and expression mechanisms leading to changes in synaptic efficacy, and we discuss, from the computational perspective, how IP can contribute to the emergence of functional properties of brain circuits.

Metabotropic Modulation of Potassium Channels During Synaptic Plasticity

Besides their primary function mediating the repolarization phase of action potentials, potassium channels exquisitely and ubiquitously regulate the resting membrane potential of neurons and therefore have a key role establishing their intrinsic excitability. This group of proteins is composed of a very diverse collection of voltage-dependent and -independent ion channels, whose specific distribution is finely tuned at the level of the synapse. Both at the presynaptic and postsynaptic membranes, different types of potassium channels are subjected to modulation by second messenger signaling cascades triggered by metabotropic receptors, which in this way serve as a link between neurotransmitter actions and changes in the neuron membrane excitability. On the one hand, by regulating the resting membrane potential of the postsynaptic membrane, potassium channels appear to be critical towards setting the threshold for the induction of long-term potentiation and depression. On the other hand, these channels maintain the presynaptic membrane potential under control, therefore influencing the probability of neurotransmitter release underlying different forms of short-term plasticity. In the present review, we examine in detail the role of metabotropic receptors translating their activation by different neurotransmitters into a final effect modulating several types of potassium channels. Furthermore, we evaluate the consequences that this interplay has on the induction and maintenance of different forms of synaptic plasticity.

Hippocampal long-term synaptic depression and memory deficits induced in early amyloidopathy are prevented by enhancing G-protein-gated inwardly rectifying potassium channel activity

Hippocampal synaptic plasticity disruption by amyloid‐β (Aβ) peptides + thought to be responsible for learning and memory impairments in Alzheimer's disease (AD) early stage. Failures in neuronal excitability maintenance seems to be an underlying mechanism. G‐protein‐gated inwardly rectifying potassium (GirK) channels control neural excitability by hyperpolarization in response to many G‐protein‐coupled receptors activation. Here, in early in vitro and in vivo amyloidosis mouse models, we study whether GirK channels take part of the hippocampal synaptic plasticity impairments generated by Aβ_1–42. In vitro electrophysiological recordings from slices showed that Aβ_1–42 alters synaptic plasticity by switching high‐frequency stimulation (HFS) induced long‐term potentiation (LTP) to long‐term depression (LTD), which led to in vivo hippocampal‐dependent memory deficits. Remarkably, selective pharmacological activation of GirK channels with ML297 rescued both HFS‐induced LTP and habituation memory from Aβ_1–42 action. Moreover, when GirK channels were specifically blocked by Tertiapin‐Q, their activation with ML297 failed to rescue LTP from the HFS‐dependent LTD induced by Aβ_1–42. On the other hand, the molecular analysis of the recorded slices by western blot showed that the expression of GIRK1/2 subunits, which form the prototypical GirK channel in the hippocampus, was not significantly regulated by Aβ_1–42. However, immunohistochemical examination of our in vivo amyloidosis model showed Aβ_1–42 to down‐regulate hippocampal GIRK1 subunit expression. Together, our results describe an Aβ‐mediated deleterious synaptic mechanism that modifies the induction threshold for hippocampal LTP/LTD and underlies memory alterations observed in amyloidosis models. In this scenario, GirK activation assures memory formation by preventing the transformation of HFS‐induced LTP into LTD.

Alternative splicing programming of axon formation

Alternative pre-mRNA splicing generates multiple mRNA isoforms of different structures and functions from a single gene. While the prevalence of alternative splicing control is widely recognized, and the underlying regulatory mechanisms have long been studied, the physiological relevance and biological necessity for alternative splicing are only slowly being revealed. Significant inroads have been made in the brain, where alternative splicing regulation is particularly pervasive and conserved. Various aspects of brain development and function (from neurogenesis, neuronal migration, synaptogenesis, to the homeostasis of neuronal activity) involve alternative splicing regulation. Recent studies have begun to interrogate the possible role of alternative splicing in axon formation, a neuron-exclusive morphological and functional characteristic. We discuss how alternative splicing plays an instructive role in each step of axon formation. Converging genetic, molecular, and cellular evidence from studies of multiple alternative splicing regulators in different systems shows that a biological process as complicated and unique as axon formation requires highly coordinated and specific alternative splicing events. This article is categorized under: RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Development.

Neuromodulators and Long-Term Synaptic Plasticity in Learning and Memory: A Steered-Glutamatergic Perspective

The molecular pathways underlying the induction and maintenance of long-term synaptic plasticity have been extensively investigated revealing various mechanisms by which neurons control their synaptic strength. The dynamic nature of neuronal connections combined with plasticity-mediated long-lasting structural and functional alterations provide valuable insights into neuronal encoding processes as molecular substrates of not only learning and memory but potentially other sensory, motor and behavioural functions that reflect previous experience. However, one key element receiving little attention in the study of synaptic plasticity is the role of neuromodulators, which are known to orchestrate neuronal activity on brain-wide, network and synaptic scales. We aim to review current evidence on the mechanisms by which certain modulators, namely dopamine, acetylcholine, noradrenaline and serotonin, control synaptic plasticity induction through corresponding metabotropic receptors in a pathway-specific manner. Lastly, we propose that neuromodulators control plasticity outcomes through steering glutamatergic transmission, thereby gating its induction and maintenance.

Degeneracy in hippocampal physiology and plasticity

Degeneracy, defined as the ability of structurally disparate elements to perform analogous function, has largely been assessed from the perspective of maintaining robustness of physiology or plasticity. How does the framework of degeneracy assimilate into an encoding system where the ability to change is an essential ingredient for storing new incoming information? Could degeneracy maintain the balance between the apparently contradictory goals of the need to change for encoding and the need to resist change towards maintaining homeostasis? In this review, we explore these fundamental questions with the mammalian hippocampus as an example encoding system. We systematically catalog lines of evidence, spanning multiple scales of analysis that point to the expression of degeneracy in hippocampal physiology and plasticity. We assess the potential of degeneracy as a framework to achieve the conjoint goals of encoding and homeostasis without cross-interferences. We postulate that biological complexity, involving interactions among the numerous parameters spanning different scales of analysis, could establish disparate routes towards accomplishing these conjoint goals. These disparate routes then provide several degrees of freedom to the encoding-homeostasis system in accomplishing its tasks in an input- and state-dependent manner. Finally, the expression of degeneracy spanning multiple scales offers an ideal reconciliation to several outstanding controversies, through the recognition that the seemingly contradictory disparate observations are merely alternate routes that the system might recruit towards accomplishment of its goals.

Role of GirK Channels in Long-Term Potentiation of Synaptic Inhibition in an In Vivo Mouse Model of Early Amyloid-β Pathology

Imbalances of excitatory/inhibitory synaptic transmission occur early in the pathogenesis of Alzheimer’s disease (AD), leading to hippocampal hyperexcitability and causing synaptic, network, and cognitive dysfunctions. G-protein-gated potassium (GirK) channels play a key role in the control of neuronal excitability, contributing to inhibitory signaling. Here, we evaluate the relationship between GirK channel activity and inhibitory hippocampal functionality in vivo. In a non-transgenic mouse model of AD, field postsynaptic potentials (fPSPs) from the CA3–CA1 synapse in the dorsal hippocampus were recorded in freely moving mice. Intracerebroventricular (ICV) injections of amyloid-β (Aβ) or GirK channel modulators impaired ionotropic (GABA_A-mediated fPSPs) and metabotropic (GirK-mediated fPSPs) inhibitory signaling and disrupted the potentiation of synaptic inhibition. However, the activation of GirK channels prevented Aβ-induced changes in GABA_A components. Our data shows, for the first time, the presence of long-term potentiation (LTP) for both the GABA_A and GirK-mediated inhibitory postsynaptic responses in vivo. In addition, our results support the importance of an accurate level of GirK-dependent signaling for dorsal hippocampal performance in early amyloid pathology models by controlling the excess of excitation that disrupts synaptic plasticity processes.

Differential NOVA2-Mediated Splicing in Excitatory and Inhibitory Neurons Regulates Cortical Development and Cerebellar Function

RNA-binding proteins (RBPs) regulate genetic diversity, but the degree to which they do so in individual cell types in vivo is unknown. We developed NOVA2 cTag-crosslinking and immunoprecipitation (CLIP) to generate functional RBP-RNA maps from different neuronal populations in the mouse brain. Combining cell type datasets from Nova2-cTag and Nova2 conditional knockout mice revealed differential NOVA2 regulatory actions on alternative splicing (AS) on the same transcripts expressed in different neurons. This includes functional differences in transcripts expressed in cortical and cerebellar excitatory versus inhibitory neurons, where we find NOVA2 is required for, respectively, development of laminar structure, motor coordination, and synapse formation. We also find that NOVA2-regulated AS is coupled to NOVA2 regulation of intron retention in hundreds of transcripts, which can sequester the trans-acting splicing factor PTBP2. In summary, cTag-CLIP complements single-cell RNA sequencing (RNA-seq) studies by providing a means for understanding RNA regulation of functional cell diversity.

Mapping of In Vivo RNA-Binding Sites by Ultraviolet (UV)-Cross-Linking Immunoprecipitation (CLIP)

RNA "CLIP" (cross-linking immunoprecipitation), the method by which RNA-protein complexes are covalently cross-linked and purified and the RNA sequenced, has attracted attention as a powerful means of developing genome-wide maps of direct, functional RNA-protein interaction sites. These maps have been used to identify points of regulation, and they hold promise for understanding the dynamics of RNA regulation in normal cell function and its dysregulation in disease.

Cell type-specific CLIP reveals that NOVA regulates cytoskeleton interactions in motoneurons

Background

Alternative RNA processing plays an essential role in shaping cell identity and connectivity in the central nervous system. This is believed to involve differential regulation of RNA processing in various cell types. However, in vivo study of cell type-specific post-transcriptional regulation has been a challenge. Here, we describe a sensitive and stringent method combining genetics and CLIP (crosslinking and immunoprecipitation) to globally identify regulatory interactions between NOVA and RNA in the mouse spinal cord motoneurons.

Results

We developed a means of undertaking motoneuron-specific CLIP to explore motoneuron-specific protein–RNA interactions relative to studies of the whole spinal cord in mouse. This allowed us to pinpoint differential RNA regulation specific to motoneurons, revealing a major role for NOVA in regulating cytoskeleton interactions in motoneurons. In particular, NOVA specifically promotes the palmitoylated isoform of the cytoskeleton protein Septin 8 in motoneurons, which enhances dendritic arborization.

Conclusions

Our study demonstrates that cell type-specific RNA regulation is important for fine tuning motoneuron physiology and highlights the value of defining RNA processing regulation at single cell type resolution.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1493-2) contains supplementary material, which is available to authorized users.

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

Engaging homeostatic plasticity to treat depression

Major depressive disorder (MDD) is a complex and heterogeneous mood disorder, making it difficult to develop a generalized, pharmacological therapy that is effective for all who suffer from MDD. Through the fortuitous discovery of N-methyl-D-aspartate receptor (NMDAR) antagonists as effective antidepressants, we have gained key insights into how antidepressant effects can be produced at the circuit and molecular levels. NMDAR antagonists act as rapid-acting antidepressants such that relief from depressive symptoms occurs within hours of a single injection. The mode of action of NMDAR antagonists seemingly relies on their ability to activate protein-synthesis-dependent homeostatic mechanisms that restore top-down excitatory connections. Recent evidence suggests that NMDAR antagonists relieve depressive symptoms by forming new synapses resulting in increased excitatory drive. This event requires the mammalian target of rapamycin complex 1 (mTORC1), a signaling pathway that regulates synaptic protein synthesis. Herein, we review critical studies that shed light on the action of NMDAR antagonists as rapid-acting antidepressants and how they engage a neuron's or neural network's homeostatic mechanisms to self-correct. Recent studies notably demonstrate that a shift in γ-amino-butyric acid receptor B (GABA_BR) function, from inhibitory to excitatory, is required for mTORC1-dependent translation with NMDAR antagonists. Finally, we discuss how GABA_BR activation of mTORC1 helps resolve key discrepancies between rapid-acting antidepressants and local homeostatic mechanisms.

Neuron-specific alternative splicing of transcriptional machineries: Implications for neurodevelopmental disorders

The brain has long been known to display the most complex pattern of alternative splicing, thereby producing diverse protein isoforms compared to other tissues. Recent evidence indicates that many alternative exons are neuron-specific, evolutionarily conserved, and found in regulators of transcription including DNA-binding protein and histone modifying enzymes. This raises a possibility that neurons adopt unique mechanisms of transcription. Given that transcriptional machineries are frequently mutated in neurodevelopmental disorders with cognitive dysfunction, it is important to understand how neuron-specific alternative splicing contributes to proper transcriptional regulation in the brain. In this review, we summarize current knowledge regarding how neuron-specific splicing events alter the function of transcriptional regulators and shape unique gene expression patterns in the brain and the implications of neuronal splicing to the pathophysiology of neurodevelopmental disorders.

mTOR referees memory and disease through mRNA repression and competition

Mammalian target of rapamycin (mTOR) activity is required for memory and is dysregulated in disease. Activation of mTOR promotes protein synthesis; however, new studies are demonstrating that mTOR activity also represses the translation of mRNAs. Almost three decades ago, Kandel and colleagues hypothesised that memory was due to the induction of positive regulators and removal of negative constraints. Are these negative constraints repressed mRNAs that code for proteins that block memory formation? Herein, we will discuss the mRNAs coded by putative memory suppressors, how activation/inactivation of mTOR repress protein expression at the synapse, how mTOR activity regulates RNA binding proteins, mRNA stability, and translation, and what the possible implications of mRNA repression are to memory and neurodegenerative disorders.

GIRK2 splice variants and neuronal G protein-gated K+ channels: implications for channel function and behavior

Many neurotransmitters directly inhibit neurons by activating G protein-gated inwardly rectifying K⁺ (GIRK) channels, thereby moderating the influence of excitatory input on neuronal excitability. While most neuronal GIRK channels are formed by GIRK1 and GIRK2 subunits, distinct GIRK2 isoforms generated by alternative splicing have been identified. Here, we compared the trafficking and function of two isoforms (GIRK2a and GIRK2c) expressed individually in hippocampal pyramidal neurons lacking GIRK2. GIRK2a and GIRK2c supported comparable somato-dendritic GIRK currents in Girk2^−/− pyramidal neurons, although GIRK2c achieved a more uniform subcellular distribution in pyramidal neurons and supported inhibitory postsynaptic currents in distal dendrites better than GIRK2a. While over-expression of either isoform in dorsal CA1 pyramidal neurons restored contextual fear learning in a conditional Girk2^−/− mouse line, GIRK2a also enhanced cue fear learning. Collectively, these data indicate that GIRK2 isoform balance within a neuron can impact the processing of afferent inhibitory input and associated behavior.

G Protein-Gated Potassium Channels: A Link to Drug Addiction

G protein-gated inwardly rectifying potassium (GIRK) channels are regulators of neuronal excitability in the brain. Knockout mice lacking GIRK channels display altered behavioral responses to multiple addictive drugs, implicating GIRK channels in addictive behaviors. Here, we review the effects of GIRK subunit deletions on the behavioral response to psychostimulants, such as cocaine and methamphetamine. Additionally, exposure of mice to psychostimulants produces alterations in the surface expression of GIRK channels in multiple types of neurons within the reward system of the brain. Thus, we compare the subcellular mechanisms by which drug exposure appears to alter GIRK expression in multiple cell types and provide an outlook on future studies examining the role of GIRK channels in addiction. A greater understanding of how GIRK channels are regulated by addictive drugs may enable the development of therapies to prevent or treat drug abuse.

KCTD Hetero-oligomers Confer Unique Kinetic Properties on Hippocampal GABAB Receptor-Induced K+ Currents

GABA_B receptors are the G-protein coupled receptors for the main inhibitory neurotransmitter in the brain, GABA. GABA_B receptors were shown to associate with homo-oligomers of auxiliary KCTD8, KCTD12, KCTD12b, and KCTD16 subunits (named after their T1 K⁺-channel tetramerization domain) that regulate G-protein signaling of the receptor. Here we provide evidence that GABA_B receptors also associate with hetero-oligomers of KCTD subunits. Coimmunoprecipitation experiments indicate that two-thirds of the KCTD16 proteins in the hippocampus of adult mice associate with KCTD12. We show that the KCTD proteins hetero-oligomerize through self-interacting T1 and H1 homology domains. Bioluminescence resonance energy transfer measurements in live cells reveal that KCTD12/KCTD16 hetero-oligomers associate with both the receptor and the G-protein. Electrophysiological experiments demonstrate that KCTD12/KCTD16 hetero-oligomers impart unique kinetic properties on G-protein-activated Kir3 currents. During prolonged receptor activation (one min) KCTD12/KCTD16 hetero-oligomers produce moderately desensitizing fast deactivating K⁺ currents, whereas KCTD12 and KCTD16 homo-oligomers produce strongly desensitizing fast deactivating currents and nondesensitizing slowly deactivating currents, respectively. During short activation (2 s) KCTD12/KCTD16 hetero-oligomers produce nondesensitizing slowly deactivating currents. Electrophysiological recordings from hippocampal neurons of KCTD knock-out mice are consistent with these findings and indicate that KCTD12/KCTD16 hetero-oligomers increase the duration of slow IPSCs. In summary, our data demonstrate that simultaneous assembly of distinct KCTDs at the receptor increases the molecular and functional repertoire of native GABA_B receptors and modulates physiologically induced K⁺ current responses in the hippocampus.

SIGNIFICANCE STATEMENT

The KCTD proteins 8, 12, and 16 are auxiliary subunits of GABA_B receptors that differentially regulate G-protein signaling of the receptor. The KCTD proteins are generally assumed to function as homo-oligomers. Here we show that the KCTD proteins also assemble hetero-oligomers in all possible dual combinations. Experiments in live cells demonstrate that KCTD hetero-oligomers form at least tetramers and that these tetramers directly interact with the receptor and the G-protein. KCTD12/KCTD16 hetero-oligomers impart unique kinetic properties to GABA_B receptor-induced Kir3 currents in heterologous cells. KCTD12/KCTD16 hetero-oligomers are abundant in the hippocampus, where they prolong the duration of slow IPSCs in pyramidal cells. Our data therefore support that KCTD hetero-oligomers modulate physiologically induced K⁺ current responses in the brain.

The neurogenetics of alternative splicing

Alternative precursor-mRNA splicing is a key mechanism for regulating gene expression in mammals and is controlled by specialized RNA-binding proteins. The misregulation of splicing is implicated in multiple neurological disorders. We describe recent mouse genetic studies of alternative splicing that reveal its critical role in both neuronal development and the function of mature neurons. We discuss the challenges in understanding the extensive genetic programmes controlled by proteins that regulate splicing, both during development and in the adult brain.

Astrocytes Assemble Thalamocortical Synapses by Bridging NRX1α and NL1 via Hevin

Proper establishment of synapses is critical for constructing functional circuits. Interactions between presynaptic neurexins and postsynaptic neuroligins coordinate the formation of synaptic adhesions. An isoform code determines the direct interactions of neurexins and neuroligins across the synapse. However, whether extracellular linker proteins can expand such a code is unknown. Using a combination of in vitro and in vivo approaches, we found that hevin, an astrocyte-secreted synaptogenic protein, assembles glutamatergic synapses by bridging neurexin-1alpha and neuroligin-1B, two isoforms that do not interact with each other. Bridging of neurexin-1alpha and neuroligin-1B via hevin is critical for the formation and plasticity of thalamocortical connections in the developing visual cortex. These results show that astrocytes promote the formation of synapses by modulating neurexin/neuroligin adhesions through hevin secretion. Our findings also provide an important mechanistic insight into how mutations in these genes may lead to circuit dysfunction in diseases such as autism.

NOVA regulates Dcc alternative splicing during neuronal migration and axon guidance in the spinal cord

RNA-binding proteins (RBPs) control multiple aspects of post-transcriptional gene regulation and function during various biological processes in the nervous system. To further reveal the functional significance of RBPs during neural development, we carried out an in vivo RNAi screen in the dorsal spinal cord interneurons, including the commissural neurons. We found that the NOVA family of RBPs play a key role in neuronal migration, axon outgrowth, and axon guidance. Interestingly, Nova mutants display similar defects as the knockout of the Dcc transmembrane receptor. We show here that Nova deficiency disrupts the alternative splicing of Dcc, and that restoring Dcc splicing in Nova knockouts is able to rescue the defects. Together, our results demonstrate that the production of DCC splice variants controlled by NOVA has a crucial function during many stages of commissural neuron development.

NOVA2-mediated RNA regulation is required for axonal pathfinding during development

The neuron specific RNA-binding proteins NOVA1 and NOVA2 are highly homologous alternative splicing regulators. NOVA proteins regulate at least 700 alternative splicing events in vivo, yet relatively little is known about the biologic consequences of NOVA action and in particular about functional differences between NOVA1 and NOVA2. Transcriptome-wide searches for isoform-specific functions, using NOVA1 and NOVA2 specific HITS-CLIP and RNA-seq data from mouse cortex lacking either NOVA isoform, reveals that NOVA2 uniquely regulates alternative splicing events of a series of axon guidance related genes during cortical development. Corresponding axonal pathfinding defects were specific to NOVA2 deficiency: Nova2-/- but not Nova1-/- mice had agenesis of the corpus callosum, and axonal outgrowth defects specific to ventral motoneuron axons and efferent innervation of the cochlea. Thus we have discovered that NOVA2 uniquely regulates alternative splicing of a coordinate set of transcripts encoding key components in cortical, brainstem and spinal axon guidance/outgrowth pathways during neural differentiation, with severe functional consequences in vivo.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Inhibitory and excitatory spike-timing-dependent plasticity in the auditory cortex

Synapses are plastic and can be modified by changes in spike timing. Whereas most studies of long-term synaptic plasticity focus on excitation, inhibitory plasticity may be critical for controlling information processing, memory storage, and overall excitability in neural circuits. Here we examine spike-timing-dependent plasticity (STDP) of inhibitory synapses onto layer 5 neurons in slices of mouse auditory cortex, together with concomitant STDP of excitatory synapses. Pairing pre- and postsynaptic spikes potentiated inhibitory inputs irrespective of precise temporal order within ∼10 ms. This was in contrast to excitatory inputs, which displayed an asymmetrical STDP time window. These combined synaptic modifications both required NMDA receptor activation and adjusted the excitatory-inhibitory ratio of events paired with postsynaptic spiking. Finally, subthreshold events became suprathreshold, and the time window between excitation and inhibition became more precise. These findings demonstrate that cortical inhibitory plasticity requires interactions with co-activated excitatory synapses to properly regulate excitatory-inhibitory balance.

Localization and Targeting of GIRK Channels in Mammalian Central Neurons

G protein-gated inwardly rectifying K(+) (GIRK/K(ir)3) channels are critical to brain function. They hyperpolarize neurons in response to activation of different G protein-coupled receptors, reducing cell excitability. Molecular cloning has revealed four distinct mammalian genes (GIRK1-4), which, with the exception of GIRK4, are broadly expressed in the central nervous system (CNS) and have been implicated in a variety of neurological disorders. Although the molecular structure and composition of GIRK channels are key determinants of their biophysical properties, their cellular and subcellular localization patterns and densities on the neuronal surface are just as important to nerve function. Current data obtained with high-resolution quantitative localization techniques reveal complex, subcellular compartment-specific distribution patterns of GIRK channel subunits. Recent efforts have focused on determining the associated proteins that form macromolecular complexes with GIRK channels. Demonstration of the precise subcellular compartmentalization of GIRK channels and their associated proteins represents a crucial step in understanding the contribution of these channels to specific aspects of neuronal function under both physiological and pathological conditions. Here, we present an overview of studies aimed at determining the cellular and subcellular localization of GIRK channel subunits in mammalian brain neurons and discuss implications for neuronal physiology.

GIRK Channels: A Potential Link Between Learning and Addiction

The ability of drug-associated cues to reinitiate drug craving and seeking, even after long periods of abstinence, has led to the hypothesis that addiction represents a form of pathological learning, in which drugs of abuse hijack normal learning and memory processes to support long-term addictive behaviors. In this chapter, we review evidence suggesting that G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels are one mechanism through which numerous drugs of abuse can modulate learning and memory processes. We will examine the role of GIRK channels in two forms of experience-dependent long-term changes in neuronal function: homeostatic plasticity and synaptic plasticity. We will also discuss how drug-induced changes in GIRK-mediated signaling can lead to changes that support the development and maintenance of addiction.