Elfn1 regulates target-specific release probability at CA1-interneuron synapses

ELFN1 is a new extracellular matrix (ECM)-associated protein

AIMS

The ELFN1, discovered in 2007, is a single-pass transmembrane protein. Studies conducted thus far to elucidate the function of the Elfn1 have been limited only to animal studies. These studies have reported that ELFN1 is a universal binding partner of metabotropic glutamate receptors (mGluRs) in the central nervous system and its functional deficiency has been associated with the pathogenesis of neurological and neuropsychiatric diseases. In 2021, we described the first disease-associated human ELFN1 pathogenic gene mutation. Severe joint laxity, which was the most striking finding of this new disease and was clearly seen in the patients since early infancy, showed that the ELFN1 may have a possible function in the connective tissue besides the nervous system. Here, we present the first experimental evidence of the extracellular matrix (ECM)-related function of the ELFN1.

MATERIALS AND METHODS

Primary skin fibroblasts were isolated from the skin biopsies of ELFN1 mutated patients and healthy foreskin donors. For the clinical trial in a dish, in vitro ECM and DEM (decellularized ECM) models were created from skin fibroblasts. All the in vitro models were comparatively characterized and analyzed.

KEY FINDINGS

The mutation in the ELFN1 signal peptide region of patients resulted in a severe lack of ELFN1 expression and dramatically altered the characteristic morphology and behavior (growth, proliferation, and motility) of fibroblasts.

SIGNIFICANCE

We propose that ELFN1 is involved in the cell-ECM attachment, and its deficiency is critical enough to cause a loss of cell motility and soft ECM stiffness.

Arid1b haploinsufficiency in cortical inhibitory interneurons causes cell-type-dependent changes in cellular and synaptic development

Autism spectrum disorder (ASD) presents with diverse cognitive and behavioral abnormalities beginning during early development. Although the neural circuit mechanisms remain unclear, recent work suggests pathology in cortical inhibitory interneurons (INs) plays a crucial role. However, we lack fundamental information regarding changes in the physiology of synapses to and from INs in ASD. Here, we used transgenic mice to conditionally knockout one copy of the high confidence ASD risk gene Arid1b from the progenitors of parvalbumin-expressing fast-spiking (PV-FS) INs and somatostatin-expressing non-fast-spiking (SST-NFS) INs. In brain slices, we performed paired whole-cell recordings between INs and excitatory projection neurons (PNs) to investigate changes in synaptic physiology. In neonates, we found reduced synaptic input to INs but not PNs, with a concomitant reduction in the frequency of spontaneous network events, which are driven by INs in immature circuits. In mature mice, we found a reduction in the number of PV-FS INs in cortical layers 2/3 and 5. However, changes in PV-FS IN synaptic physiology were cortical layer and PN cell-type dependent. In layer 5, synapses from PV-FS INs to subcortical-projecting PNs were weakened. In contrast, in layer 2/3, synapses to and from PV-FS INs and corticocortical-projecting PNs were strengthened, leading to enhanced feedforward inhibition of input from layer 4. Finally, we found a novel synaptic deficit among SST-NFS INs, in which excitatory synapses from layer 2/3 PNs failed to facilitate. Our data highlight that changes in unitary synaptic dynamics among INs in ASD depend on neuronal cell-type.

Novel pharmacological targets for GABAergic dysfunction in ADHD

Attention deficit/hyperactivity disorder (ADHD) is a neurodevelopment disorder that affects approximately 5% of the population. The disorder is characterized by impulsivity, hyperactivity, and deficits in attention and cognition, although symptoms vary across patients due to the heterogenous and polygenic nature of the disorder. Stimulant medications are the standard of care treatment for ADHD patients, and their effectiveness has led to the dopaminergic hypothesis of ADHD in which deficits in dopaminergic signaling, especially in cortical brain regions, mechanistically underly ADHD pathophysiology. Despite their effectiveness in many individuals, almost one-third of patients do not respond to stimulant treatments and the long-term negative side effects of these medications remain unclear. Emerging clinical evidence is beginning to highlight an important role of dysregulated excitatory/inhibitory (E/I) balance in ADHD. These deficits in E/I balance are related to functional abnormalities in glutamate and Gamma-Aminobutyric Acid (GABA) signaling in the brain, with increasing emphasis placed on GABAergic interneurons driving specific aspects of ADHD pathophysiology. Recent genome-wide association studies (GWAS) have also highlighted how genes associated with GABA function are mutated in human populations with ADHD, resulting in the generation of several new genetic mouse models of ADHD. This review will discuss how GABAergic dysfunction underlies ADHD pathophysiology, and how specific receptors/proteins related to GABAergic interneuron dysfunction may be pharmacologically targeted to treat ADHD in subpopulations with specific comorbidities and symptom domains. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Different states of synaptic vesicle priming explain target cell type-dependent differences in neurotransmitter release

Pronounced differences in neurotransmitter release from a given presynaptic neuron, depending on the synaptic target, are among the most intriguing features of cortical networks. Hippocampal pyramidal cells (PCs) release glutamate with low probability to somatostatin expressing oriens-lacunosum-moleculare (O-LM) interneurons (INs), and the postsynaptic responses show robust short-term facilitation, whereas the release from the same presynaptic axons onto fast-spiking INs (FSINs) is ~10-fold higher and the excitatory postsynaptic currents (EPSCs) display depression. The mechanisms underlying these vastly different synaptic behaviors have not been conclusively identified. Here, we applied a combined functional, pharmacological, and modeling approach to address whether the main difference lies in the action potential-evoked fusion or else in upstream priming processes of synaptic vesicles (SVs). A sequential two-step SV priming model was fitted to the peak amplitudes of unitary EPSCs recorded in response to complex trains of presynaptic stimuli in acute hippocampal slices of adult mice. At PC-FSIN connections, the fusion probability (Pfusion) of well-primed SVs is 0.6, and 44% of docked SVs are in a fusion-competent state. At PC-O-LM synapses, Pfusion is only 40% lower (0.36), whereas the fraction of well-primed SVs is 6.5-fold smaller. Pharmacological enhancement of fusion by 4-AP and priming by PDBU was recaptured by the model with a selective increase of Pfusion and the fraction of well-primed SVs, respectively. Our results demonstrate that the low fidelity of transmission at PC-O-LM synapses can be explained by a low occupancy of the release sites by well-primed SVs.

Functional specialization of hippocampal somatostatin-expressing interneurons

Hippocampal somatostatin-expressing (Sst) GABAergic interneurons (INs) exhibit considerable anatomical and functional heterogeneity. Recent single-cell transcriptome analyses have provided a comprehensive Sst-IN subpopulations census, a plausible molecular ground truth of neuronal identity whose links to specific functionality remain incomplete. Here, we designed an approach to identify and access subpopulations of Sst-INs based on transcriptomic features. Four mouse models based on single or combinatorial Cre- and Flp- expression differentiated functionally distinct subpopulations of CA1 hippocampal Sst-INs that largely tiled the morpho-functional parameter space of the Sst-INs superfamily. Notably, the Sst;;Tac1 intersection revealed a population of bistratified INs that preferentially synapsed onto fast-spiking interneurons (FS-INs) and were sufficient to interrupt their firing. In contrast, the Ndnf;;Nkx2-1 intersection identified a population of oriens lacunosum-moleculare INs that predominantly targeted CA1 pyramidal neurons, avoiding FS-INs. Overall, our results provide a framework to translate neuronal transcriptomic identity into discrete functional subtypes that capture the diverse specializations of hippocampal Sst-INs.

Differential nanoscale organization of excitatory synapses onto excitatory vs. inhibitory neurons

A key feature of excitatory synapses is the existence of subsynaptic protein nanoclusters (NCs) whose precise alignment across the cleft in a transsynaptic nanocolumn influences the strength of synaptic transmission. However, whether nanocolumn properties vary between excitatory synapses functioning in different cellular contexts is unknown. We used a combination of confocal and DNA-PAINT super-resolution microscopy to directly compare the organization of shared scaffold proteins at two important excitatory synapses-those forming onto excitatory principal neurons (Ex→Ex synapses) and those forming onto parvalbumin-expressing interneurons (Ex→PV synapses). As in Ex→Ex synapses, we find that in Ex→PV synapses, presynaptic Munc13-1 and postsynaptic PSD-95 both form NCs that demonstrate alignment, underscoring synaptic nanostructure and the transsynaptic nanocolumn as conserved organizational principles of excitatory synapses. Despite the general conservation of these features, we observed specific differences in the characteristics of pre- and postsynaptic Ex→PV nanostructure. Ex→PV synapses contained larger PSDs with fewer PSD-95 NCs when accounting for size than Ex→Ex synapses. Furthermore, the PSD-95 NCs were larger and denser. The identity of the postsynaptic cell was also represented in Munc13-1 organization, as Ex→PV synapses hosted larger Munc13-1 puncta that contained less dense but larger and more numerous Munc13-1 NCs. Moreover, we measured the spatial variability of transsynaptic alignment in these synapse types, revealing protein alignment in Ex→PV synapses over a distinct range of distances compared to Ex→Ex synapses. We conclude that while general principles of nanostructure and alignment are shared, cell-specific elements of nanodomain organization likely contribute to functional diversity of excitatory synapses.

Complex N-glycosylation of mGluR6 is required for trans-synaptic interaction with ELFN adhesion proteins

Synaptic transmission from retinal photoreceptors to downstream ON-type bipolar cells (BCs) depends on the postsynaptic metabotropic glutamate receptor mGluR6, located at the BC dendritic tips. Glutamate binding to mGluR6 initiates G-protein signaling that ultimately leads to BC depolarization in response to light. The mGluR6 receptor also engages in trans-synaptic interactions with presynaptic ELFN adhesion proteins. The roles of post-translational modifications in mGluR6 trafficking and function are unknown. Treatment with glycosidase enzymes PNGase F and Endo H demonstrated that both endogenous and heterologously expressed mGluR6 contain complex N-glycosylation acquired in the Golgi. Pull-down experiments with ELFN1 and ELFN2 extracellular domains revealed that these proteins interact exclusively with the complex glycosylated form of mGluR6. Mutation of the four predicted N-glycosylation sites, either singly or in combination, revealed that all four sites are glycosylated. Single mutations partially reduced, but did not abolish, surface expression in heterologous cells, while triple mutants had little or no surface expression, indicating that no single glycosylation site is necessary or sufficient for plasma membrane trafficking. Mutation at N445 severely impaired both ELFN1 and ELFN2 binding. All single mutants exhibited dendritic tip enrichment in rod BCs, as did the triple mutant with N445 as the sole N-glycosylation site, demonstrating that glycosylation at N445 is sufficient but not necessary for dendritic tip localization. The quadruple mutant was completely mislocalized. These results reveal a key role for complex N-glycosylation in regulating mGluR6 trafficking and ELFN binding, and by extension, function of the photoreceptor synapses.

Genomic analysis identifies risk factors in restless legs syndrome

Restless legs syndrome (RLS) is a neurological condition that causes uncomfortable sensations in the legs and an irresistible urge to move them, typically during periods of rest. The genetic basis and pathophysiology of RLS are incompletely understood. Here, we present a whole-genome sequencing and genome-wide association meta-analysis of RLS cases (n = 9,851) and controls (n = 38,957) in three population-based biobanks (All of Us, Canadian Longitudinal Study on Aging, and CARTaGENE). Genome-wide association analysis identified nine independent risk loci, of which eight had been previously reported, and one was a novel risk locus (LMX1B, rs35196838, OR = 1.14, 95% CI = 1.09-1.19, p-value = 2.2 × 10-9). A genome-wide, gene-based common variant analysis identified GLO1 as an additional risk gene (p-value = 8.45 × 10-7). Furthermore, a transcriptome-wide association study also identified GLO1 and a previously unreported gene, ELFN1. A genetic correlation analysis revealed significant common variant overlaps between RLS and neuroticism (rg = 0.40, se = 0.08, p-value = 5.4 × 10-7), depression (rg = 0.35, se = 0.06, p-value = 2.17 × 10-8), and intelligence (rg = -0.20, se = 0.06, p-value = 4.0 × 10-4). Our study expands the understanding of the genetic architecture of RLS and highlights the contributions of common variants to this prevalent neurological disorder.

Silences, spikes and bursts: Three-part knot of the neural code

When a neuron breaks silence, it can emit action potentials in a number of patterns. Some responses are so sudden and intense that electrophysiologists felt the need to single them out, labelling action potentials emitted at a particularly high frequency with a metonym - bursts. Is there more to bursts than a figure of speech? After all, sudden bouts of high-frequency firing are expected to occur whenever inputs surge. The burst coding hypothesis advances that the neural code has three syllables: silences, spikes and bursts. We review evidence supporting this ternary code in terms of devoted mechanisms for burst generation, synaptic transmission and synaptic plasticity. We also review the learning and attention theories for which such a triad is beneficial.

Post-synaptic GABAA receptors potentiate transmission by recruiting CaV2 channels to their inputs

We describe a retrograde synaptic signal at the C. elegans GABAergic neuromuscular junction. At this synapse, GABA release is controlled by two voltage-activated calcium channels (UNC-2/CaV2 and EGL-19/CaV1), and muscle responses are mediated by a single GABA receptor (UNC-49/GABAA). Mutations inactivating UNC-49 or those preventing UNC-49 synaptic clustering cause retrograde defects in GABAergic motor neurons, whereby UNC-2/CaV2 levels at active zones, UNC-2 current, and pre-synaptic GABA release are decreased. Inactivating post-synaptic GABAA receptors has no effect on GABA neuron EGL-19/CaV1 levels nor on several other pre-synaptic markers. The effect of GABAA receptors on pre-synaptic strength is not a consequence of decreased GABA transmission and is input selective. Finally, pre-synaptic UNC-2/CaV2 levels are increased when post-synaptic GABAA receptors are increased but are unaffected by increased extra-synaptic receptors. Collectively, these results suggest that clustered post-synaptic GABAA receptors adjust the strength of their inputs by recruiting CaV2 to contacting active zones.

Presynaptic Kainate Receptors onto Somatostatin Interneurons Are Recruited by Activity throughout Development and Contribute to Cortical Sensory Adaptation

Somatostatin (SST) interneurons produce delayed inhibition because of the short-term facilitation of their excitatory inputs created by the expression of metabotropic glutamate receptor 7 (mGluR7) and presynaptic GluK2-containing kainate receptors (GluK2-KARs). Using mice of both sexes, we find that as synaptic facilitation at layer (L)2/3 SST cell inputs increases during the first few postnatal weeks, so does GluK2-KAR expression. Removal of sensory input by whisker trimming does not affect mGluR7 but prevents the emergence of presynaptic GluK2-KARs, which can be restored by allowing whisker regrowth or by acute calmodulin activation. Conversely, late trimming or acute inhibition of Ca2+/calmodulin-dependent protein kinase II is sufficient to reduce GluK2-KAR activity. This developmental and activity-dependent regulation also produces a specific reduction of L4 GluK2-KARs that advances in parallel with the maturation of sensory processing in L2/3. Finally, we find that removal of both GluK2-KARs and mGluR7 from the synapse eliminates short-term facilitation and reduces sensory adaptation to repetitive stimuli, first in L4 of somatosensory cortex, then later in development in L2/3. The dynamic regulation of presynaptic GluK2-KARs potentially allows for flexible scaling of late inhibition and sensory adaptation.SIGNIFICANCE STATEMENT Excitatory synapses onto somatostatin (SST) interneurons express presynaptic, calcium-permeable kainate receptors containing the GluK2 subunit (GluK2-KARs), activated by high-frequency activity. In this study we find that their presence on L2/3 SST synapses in the barrel cortex is not based on a hardwired genetic program but instead is regulated by sensory activity, in contrast to that of mGluR7. Thus, in addition to standard synaptic potentiation and depression mechanisms, excitatory synapses onto SST neurons undergo an activity-dependent presynaptic modulation that uses GluK2-KARs. Further, we present evidence that loss of the frequency-dependent synaptic components (both GluK2-KARs and mGluR7 via Elfn1 deletion) contributes to a decrease in the sensory adaptation commonly seen on repetitive stimulus presentation.

The alpha2 nicotinic acetylcholine receptor, a subunit with unique and selective expression in inhibitory interneurons associated with principal cells

Nicotinic acetylcholine receptors (nAChRs) play crucial roles in various human disorders, with the α7, α4, α6, and α3-containing nAChR subtypes extensively studied in relation to conditions such as Alzheimer's disease, Parkinson's disease, nicotine dependence, mood disorders, and stress disorders. In contrast, the α2-nAChR subunit has received less attention due to its more restricted expression and the scarcity of specific agonists and antagonists for studying its function. Nevertheless, recent research has shed light on the unique expression pattern of the Chrna2 gene, which encodes the α2-nAChR subunit, and its involvement in distinct populations of inhibitory interneurons. This review highlights the structure, pharmacology, localization, function, and disease associations of α2-containing nAChRs and points to the unique expression pattern of the Chrna2 gene and its role in different inhibitory interneuron populations. These populations, including the oriens lacunosum moleculare (OLM) cells in the hippocampus, Martinotti cells in the neocortex, and Renshaw cells in the spinal cord, share common features and contribute to recurrent inhibitory microcircuits. Thus, the α2-nAChR subunit's unique expression pattern in specific interneuron populations and its role in recurrent inhibitory microcircuits highlight its importance in various physiological processes. Further research is necessary to uncover the comprehensive functionality of α2-containing nAChRs, delineate their specific contributions to neuronal circuits, and investigate their potential as therapeutic targets for related disorders.

Differential nanoscale organization of excitatory synapses onto excitatory vs inhibitory neurons

A key feature of excitatory synapses is the existence of subsynaptic protein nanoclusters whose precise alignment across the cleft in a trans-synaptic nanocolumn influences the strength of synaptic transmission. However, whether nanocolumn properties vary between excitatory synapses functioning in different cellular contexts is unknown. We used a combination of confocal and DNA-PAINT super-resolution microscopy to directly compare the organization of shared scaffold proteins at two important excitatory synapses - those forming onto excitatory principal neurons (Ex→Ex synapses) and those forming onto parvalbumin-expressing interneurons (Ex→PV synapses). As in Ex→Ex synapses, we find that in Ex→PV synapses presynaptic Munc13-1 and postsynaptic PSD-95 both form nanoclusters that demonstrate alignment, underscoring synaptic nanostructure and the trans-synaptic nanocolumn as conserved organizational principles of excitatory synapses. Despite the general conservation of these features, we observed specific differences in the characteristics of pre- and postsynaptic Ex→PV nanostructure. Ex→PV synapses contained larger PSDs with fewer PSD-95 NCs when accounting for size than Ex→Ex synapses. Furthermore, the PSD-95 NCs were larger and denser. The identity of the postsynaptic cell also had a retrograde impact on Munc13-1 organization, as Ex→PV synapses hosted larger Munc13-1 puncta that contained less dense but larger and more numerous Munc13-1 NCs. Moreover, we measured the spatial variability of transsynaptic alignment in these synapse types, revealing protein alignment in Ex→PV synapses over a distinct range of distances compared to Ex→Ex synapses. We conclude that while general principles of nanostructure and alignment are shared, cell-specific elements of nanodomain organization likely contribute to functional diversity of excitatory synapses. Understanding the rules of synapse nanodomain assembly, which themselves are cell-type specific, will be essential for illuminating brain network dynamics.

GABAA receptors as targets for treating affective and cognitive symptoms of depression

In the past 20 years, our understanding of the pathophysiology of depression has evolved from a focus on an imbalance of monoaminergic neurotransmitters to a multifactorial picture including an improved understanding of the role of glutamatergic excitatory and GABAergic inhibitory neurotransmission. FDA-approved treatments targeting the glutamatergic [esketamine for major depressive disorder (MDD)] and GABAergic (brexanolone for peripartum depression) systems have become available. This review focuses on the GABAA receptor (GABAAR) system as a target for novel antidepressants and discusses the mechanisms by which modulation of δ-containing GABAARs with neuroactive steroids (NASs) or of α5-containing GABAARs results in antidepressant or antidepressant-like actions and discusses clinical data on NASs. Moreover, a potential mechanism by which α5-GABAAR-positive allosteric modulators (PAMs) may improve cognitive deficits in depression is presented.

Target cell-specific synaptic dynamics of excitatory to inhibitory neuron connections in supragranular layers of human neocortex

Rodent studies have demonstrated that synaptic dynamics from excitatory to inhibitory neuron types are often dependent on the target cell type. However, these target cell-specific properties have not been well investigated in human cortex, where there are major technical challenges in reliably obtaining healthy tissue, conducting multiple patch-clamp recordings on inhibitory cell types, and identifying those cell types. Here, we take advantage of newly developed methods for human neurosurgical tissue analysis with multiple patch-clamp recordings, post-hoc fluorescent in situ hybridization (FISH), machine learning-based cell type classification and prospective GABAergic AAV-based labeling to investigate synaptic properties between pyramidal neurons and PVALB- vs. SST-positive interneurons. We find that there are robust molecular differences in synapse-associated genes between these neuron types, and that individual presynaptic pyramidal neurons evoke postsynaptic responses with heterogeneous synaptic dynamics in different postsynaptic cell types. Using molecular identification with FISH and classifiers based on transcriptomically identified PVALB neurons analyzed by Patch-seq, we find that PVALB neurons typically show depressing synaptic characteristics, whereas other interneuron types including SST-positive neurons show facilitating characteristics. Together, these data support the existence of target cell-specific synaptic properties in human cortex that are similar to rodent, thereby indicating evolutionary conservation of local circuit connectivity motifs from excitatory to inhibitory neurons and their synaptic dynamics.

Cortical interneurons: fit for function and fit to function? Evidence from development and evolution

Cortical inhibitory interneurons form a broad spectrum of subtypes. This diversity suggests a division of labor, in which each cell type supports a distinct function. In the present era of optimisation-based algorithms, it is tempting to speculate that these functions were the evolutionary or developmental driving force for the spectrum of interneurons we see in the mature mammalian brain. In this study, we evaluated this hypothesis using the two most common interneuron types, parvalbumin (PV) and somatostatin (SST) expressing cells, as examples. PV and SST interneurons control the activity in the cell bodies and the apical dendrites of excitatory pyramidal cells, respectively, due to a combination of anatomical and synaptic properties. But was this compartment-specific inhibition indeed the function for which PV and SST cells originally evolved? Does the compartmental structure of pyramidal cells shape the diversification of PV and SST interneurons over development? To address these questions, we reviewed and reanalyzed publicly available data on the development and evolution of PV and SST interneurons on one hand, and pyramidal cell morphology on the other. These data speak against the idea that the compartment structure of pyramidal cells drove the diversification into PV and SST interneurons. In particular, pyramidal cells mature late, while interneurons are likely committed to a particular fate (PV vs. SST) during early development. Moreover, comparative anatomy and single cell RNA-sequencing data indicate that PV and SST cells, but not the compartment structure of pyramidal cells, existed in the last common ancestor of mammals and reptiles. Specifically, turtle and songbird SST cells also express the Elfn1 and Cbln4 genes that are thought to play a role in compartment-specific inhibition in mammals. PV and SST cells therefore evolved and developed the properties that allow them to provide compartment-specific inhibition before there was selective pressure for this function. This suggest that interneuron diversity originally resulted from a different evolutionary driving force and was only later co-opted for the compartment-specific inhibition it seems to serve in mammals today. Future experiments could further test this idea using our computational reconstruction of ancestral Elfn1 protein sequences.

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

Regulation of Presynaptic Release Machinery by Cell Adhesion Molecules

The synapse is a highly specialized asymmetric structure that transmits and stores information in the brain. The size of pre- and postsynaptic structures and function is well coordinated at the individual synapse level. For example, large postsynaptic dendritic spines have a larger postsynaptic density with higher α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) number on their surface, while juxtaposing presynaptic terminals have a larger active zone and higher release probability. This indicates that pre- and postsynaptic domains bidirectionally communicate to coordinate assembly of specific molecules on both sides of the synaptic cleft. Cell adhesion molecules (CAMs) that localize at synapses form transsynaptic protein interactions across the synaptic cleft and play important roles in synapse formation and regulation. The extracellular domain of CAMs is essential for specific synapse formation and function. In contrast, the intracellular domain is necessary for binding with synaptic molecules and signal transduction. Therefore, CAMs play an essential role on synapse function and structure. In fact, ample evidence indicates that transsynaptic CAMs instruct and modulate functions at presynaptic sites. This chapter focuses on transsynaptic protein interactions that regulate presynaptic functions emphasizing the role of neuronal CAMs and the intracellular mechanism of their regulation.

Circuit-selective cell-autonomous regulation of inhibition in pyramidal neurons by Ste20-like kinase

Maintaining an appropriate balance between excitation and inhibition is critical for neuronal information processing. Cortical neurons can cell-autonomously adjust the inhibition they receive to individual levels of excitatory input, but the underlying mechanisms are unclear. We describe that Ste20-like kinase (SLK) mediates cell-autonomous regulation of excitation-inhibition balance in the thalamocortical feedforward circuit, but not in the feedback circuit. This effect is due to regulation of inhibition originating from parvalbumin-expressing interneurons, while inhibition via somatostatin-expressing interneurons is unaffected. Computational modeling shows that this mechanism promotes stable excitatory-inhibitory ratios across pyramidal cells and ensures robust and sparse coding. Patch-clamp RNA sequencing yields genes differentially regulated by SLK knockdown, as well as genes associated with excitation-inhibition balance participating in transsynaptic communication and cytoskeletal dynamics. These data identify a mechanism for cell-autonomous regulation of a specific inhibitory circuit that is critical to ensure that a majority of cortical pyramidal cells participate in information coding.

Theta patterns of stimulation induce synaptic and intrinsic potentiation in O-LM interneurons

Brain oscillations have long-lasting effects on synaptic and cellular properties. For instance, synaptic stimulation at theta (θ) frequency induces persistent depression of both excitatory synaptic transmission and intrinsic excitability in CA1 principal neurons. However, the incidence of θ activity on synaptic transmission and intrinsic excitability in hippocampal GABAergic interneurons is unclear. We report here the induction of both synaptic and intrinsic potentiation in oriens-lacunosum moleculare (O-LM) interneurons following stimulation of afferent glutamatergic inputs in the θ frequency range (∼5 Hz). Long-term synaptic potentiation (LTP) is induced by synaptic activation of calcium-permeable AMPA receptors (CP-AMPAR), whereas long-term potentiation of intrinsic excitability (LTP-IE) results from the mGluR1-dependent down-regulation of Kv7 voltage-dependent potassium channel and hyperpolarization activated and cyclic nucleotide-gated (HCN) channel through the depletion of phosphatidylinositol-4,5-biphosphate (PIP2). LTP and LTP-IE are reversible, demonstrating that both synaptic and intrinsic changes are bidirectional in O-LM cells. We conclude that synaptic activity at θ frequency induces both synaptic and intrinsic potentiation in O-LM interneurons, i.e., the opposite of what is typically seen in glutamatergic neurons.

Targeting prefrontal cortex GABAergic microcircuits for the treatment of alcohol use disorder

Developing novel treatments for alcohol use disorders (AUDs) is of paramount importance for improving patient outcomes and alleviating the suffering related to the disease. A better understanding of the molecular and neurocircuit mechanisms through which alcohol alters brain function will be instrumental in the rational development of new efficacious treatments. Clinical studies have consistently associated the prefrontal cortex (PFC) function with symptoms of AUDs. Population-level analyses have linked the PFC structure and function with heavy drinking and/or AUD diagnosis. Thus, targeting specific PFC cell types and neural circuits holds promise for the development of new treatments. Here, we overview the tremendous diversity in the form and function of inhibitory neuron subtypes within PFC and describe their therapeutic potential. We then summarize AUD population genetics studies, clinical neurophysiology findings, and translational neuroscience discoveries. This study collectively suggests that changes in fast transmission through PFC inhibitory microcircuits are a central component of the neurobiological effects of ethanol and the core symptoms of AUDs. Finally, we submit that there is a significant and timely need to examine sex as a biological variable and human postmortem brain tissue to maximize the efforts in translating findings to new clinical treatments.

Subsynaptic mobility of presynaptic mGluR types is differentially regulated by intra- and extracellular interactions

Presynaptic metabotropic glutamate receptors (mGluRs) are essential for the control of synaptic transmission. However, how the subsynaptic dynamics of these receptors is controlled and contributes to synaptic signaling remain poorly understood quantitatively. Particularly, since the affinity of individual mGluR subtypes for glutamate differs considerably, the activation of mGluR subtypes critically depends on their precise subsynaptic distribution. Here, using superresolution microscopy and single-molecule tracking, we unravel novel molecular mechanisms that control the nanoscale distribution and mobility of presynaptic mGluRs in hippocampal neurons. We demonstrate that the high-affinity group II receptor mGluR2 localizes diffusely along the axon, and is highly mobile, while the low-affinity group III receptor mGluR7 is stably anchored at the active zone. We demonstrate that intracellular interactions modulate surface diffusion of mGluR2, while immobilization of mGluR7 at the active zone relies on its extracellular domain. Receptor activation or increases in synaptic activity do not alter the surface mobility of presynaptic mGluRs. Finally, computational modeling of presynaptic mGluR activity revealed that this particular nanoscale arrangement directly impacts their ability to modulate neurotransmitter release. Altogether, this study demonstrates that distinct mechanisms control surface mobility of presynaptic mGluRs to contribute differentially to glutamatergic synaptic transmission.

Genomic Correlates of Outcome in Tumor-Infiltrating Lymphocyte Therapy for Metastatic Melanoma

PURPOSE

Adoptive cell therapy (ACT) of tumor-infiltrating lymphocytes (TIL) historically yields a 40%-50% response rate in metastatic melanoma. However, the determinants of outcome are largely unknown.

EXPERIMENTAL DESIGN

We investigated tumor-based genomic correlates of overall survival (OS), progression-free survival (PFS), and response to therapy by interrogating tumor samples initially collected to generate TIL infusion products.

RESULTS

Whole-exome sequencing (WES) data from 64 samples indicated a positive correlation between neoantigen load and OS, but not PFS or response to therapy. RNA sequencing analysis of 34 samples showed that expression of PDE1C, RTKN2, and NGFR was enriched in responders who had improved PFS and OS. In contrast, the expression of ELFN1 was enriched in patients with unfavorable response, poor PFS and OS, whereas enhanced methylation of ELFN1 was observed in patients with favorable outcomes. Expression of ELFN1, NGFR, and PDE1C was mainly found in cancer-associated fibroblasts and endothelial cells in tumor tissues across different cancer types in publicly available single-cell RNA sequencing datasets, suggesting a role for elements of the tumor microenvironment in defining the outcome of TIL therapy.

CONCLUSIONS

Our findings suggest that transcriptional features of melanomas correlate with outcomes after TIL therapy and may provide candidates to guide patient selection.

Optimizing interneuron circuits for compartment-specific feedback inhibition

Cortical circuits process information by rich recurrent interactions between excitatory neurons and inhibitory interneurons. One of the prime functions of interneurons is to stabilize the circuit by feedback inhibition, but the level of specificity on which inhibitory feedback operates is not fully resolved. We hypothesized that inhibitory circuits could enable separate feedback control loops for different synaptic input streams, by means of specific feedback inhibition to different neuronal compartments. To investigate this hypothesis, we adopted an optimization approach. Leveraging recent advances in training spiking network models, we optimized the connectivity and short-term plasticity of interneuron circuits for compartment-specific feedback inhibition onto pyramidal neurons. Over the course of the optimization, the interneurons diversified into two classes that resembled parvalbumin (PV) and somatostatin (SST) expressing interneurons. Using simulations and mathematical analyses, we show that the resulting circuit can be understood as a neural decoder that inverts the nonlinear biophysical computations performed within the pyramidal cells. Our model provides a proof of concept for studying structure-function relations in cortical circuits by a combination of gradient-based optimization and biologically plausible phenomenological models.

The brain contains billions of nerve cells—neurons—that can be classified into different types depending on their shape, connectivity and activity. A particularly diverse group of neurons is that of inhibitory neurons, named after their suppressive effect on neural activity. Presumably, their diverse properties allow inhibitory neurons to fulfil different functions, but what these functions are is currently unclear. In this paper, we investigated if a particular function can explain the existence and properties of the two most common inhibitory cell classes: The need to regulate activity in different physical parts (compartments) of the neurons they target. We investigated this function in a computer model, using optimization to find the neuron properties best-suited for compartment-specific inhibition. Our key result is that after the optimization, model neurons largely fell into two classes that resembled the two types of biological neurons. In particular, the optimized neurons were connected to only one compartment of other neurons. This suggests that the diversity of inhibitory neurons is well suited for compartment-specific inhibition. In the future, our approach of optimizing neural properties might be used to investigate other functions (or dysfunctions) of neuron diversity.

Selective Enrichment of Munc13-2 in Presynaptic Active Zones of Hippocampal Pyramidal Cells That Innervate mGluR1α Expressing Interneurons

Selective distribution of proteins in presynaptic active zones (AZs) is a prerequisite for generating postsynaptic target cell type-specific differences in presynaptic vesicle release probability (P_v) and short-term plasticity, a characteristic feature of cortical pyramidal cells (PCs). In the hippocampus of rodents, somatostatin and mGluR1α expressing interneurons (mGluR1α+ INs) receive small, facilitating excitatory postsynaptic currents (EPSCs) from PCs and express Elfn1 that trans-synaptically recruits mGluR7 into the presynaptic AZ of PC axons. Here we show that Elfn1 also has a role in the selective recruitment of Munc13-2, a synaptic vesicle priming and docking protein, to PC AZs that innervate mGluR1α+ INs. In Elfn1 knock-out mice, unitary EPSCs (uEPSCs) in mGluR1α+ INs have threefold larger amplitudes with less pronounced short-term facilitation, which might be the consequence of the loss of either mGluR7 or Munc13-2 or both. Conditional genetic deletion of Munc13-2 from CA1 PCs results in the loss of Munc13-2, but not mGluR7 from the AZs, and has no effect on the amplitude of uEPSCs and leaves the characteristic short-term facilitation intact at PC to mGluR1α+ IN connection. Our results demonstrate that Munc13-1 alone is capable of imposing low P_v at PC to mGluR1α+ IN synapses and Munc13-2 has yet an unknown role in this synapse.

Membrane trafficking and positioning of mGluRs at presynaptic and postsynaptic sites of excitatory synapses

The plethora of functions of glutamate in the brain are mediated by the complementary actions of ionotropic and metabotropic glutamate receptors (mGluRs). The ionotropic glutamate receptors carry most of the fast excitatory transmission, while mGluRs modulate transmission on longer timescales by triggering multiple intracellular signaling pathways. As such, mGluRs mediate critical aspects of synaptic transmission and plasticity. Interestingly, at synapses, mGluRs operate at both sides of the cleft, and thus bidirectionally exert the effects of glutamate. At postsynaptic sites, group I mGluRs act to modulate excitability and plasticity. At presynaptic sites, group II and III mGluRs act as auto-receptors, modulating release properties in an activity-dependent manner. Thus, synaptic mGluRs are essential signal integrators that functionally couple presynaptic and postsynaptic mechanisms of transmission and plasticity. Understanding how these receptors reach the membrane and are positioned relative to the presynaptic glutamate release site are therefore important aspects of synapse biology. In this review, we will discuss the currently known mechanisms underlying the trafficking and positioning of mGluRs at and around synapses, and how these mechanisms contribute to synaptic functioning. We will highlight outstanding questions and present an outlook on how recent technological developments will move this exciting research field forward.

Synaptic Dynamics Convey Differential Sensitivity to Input Pattern Changes in Two Muscles Innervated by the Same Motor Neurons

Postsynaptic responses depend on input patterns as well as short-term synaptic plasticity, summation, and postsynaptic membrane properties, but the interactions of those dynamics with realistic input patterns are not well understood. We recorded the responses of the two pyloric dilator (PD) muscles, cpv2a and cpv2b, that are innervated by and receive identical periodic bursting input from the same two motor neurons in the lobster Homarus americanus. Cpv2a and cpv2b showed quantitative differences in membrane nonlinearities and synaptic summation. At a short timescale, responses in both muscles were dominated by facilitation, albeit with different frequency and time dependence. Realistic burst stimulations revealed more substantial differences. Across bursts, cpv2a showed transient depression, whereas cpv2b showed transient facilitation. Steady-state responses to bursting input also differed substantially. Neither muscle had a monotonic dependence on frequency, but cpv2b showed particularly pronounced bandpass filtering. Cpv2a was sensitive to changes in both burst frequency and intra-burst spike frequency, whereas, despite its much slower responses, cpv2b was largely insensitive to changes in burst frequency. Cpv2a was sensitive to both burst duration and number of spikes per burst, whereas cpv2b was sensitive only to the former parameter. Neither muscle showed consistent sensitivity to changes in the overall spike interval structure, but cpv2b was surprisingly sensitive to changes in the first intervals in each burst, a parameter known to be regulated by dopamine (DA) modulation of spike propagation of the presynaptic axon. These findings highlight how seemingly minor circuit output changes mediated by neuromodulation could be read out differentially at the two synapses.

Astrocytes in Neural Circuits: Key Factors in Synaptic Regulation and Potential Targets for Neurodevelopmental Disorders

Astrocytes are the major glial cells in the brain, which play a supporting role in the energy and nutritional supply of neurons. They were initially regarded as passive space-filling cells, but the latest progress in the study of the development and function of astrocytes highlights their active roles in regulating synaptic transmission, formation, and plasticity. In the concept of "tripartite synapse," the bidirectional influence between astrocytes and neurons, in addition to their steady-state and supporting function, suggests that any negative changes in the structure or function of astrocytes will affect the activity of neurons, leading to neurodevelopmental disorders. The role of astrocytes in the pathophysiology of various neurological and psychiatric disorders caused by synaptic defects is increasingly appreciated. Understanding the roles of astrocytes in regulating synaptic development and the plasticity of neural circuits could help provide new treatments for these diseases.

Mechanisms Underlying Target Selectivity for Cell Types and Subcellular Domains in Developing Neocortical Circuits

The cerebral cortex contains numerous neuronal cell types, distinguished by their molecular identity as well as their electrophysiological and morphological properties. Cortical function is reliant on stereotyped patterns of synaptic connectivity and synaptic function among these neuron types, but how these patterns are established during development remains poorly understood. Selective targeting not only of different cell types but also of distinct postsynaptic neuronal domains occurs in many brain circuits and is directed by multiple mechanisms. These mechanisms include the regulation of axonal and dendritic guidance and fine-scale morphogenesis of pre- and postsynaptic processes, lineage relationships, activity dependent mechanisms and intercellular molecular determinants such as transmembrane and secreted molecules, many of which have also been implicated in neurodevelopmental disorders. However, many studies of synaptic targeting have focused on circuits in which neuronal processes target different lamina, such that cell-type-biased connectivity may be confounded with mechanisms of laminar specificity. In the cerebral cortex, each cortical layer contains cell bodies and processes from intermingled neuronal cell types, an arrangement that presents a challenge for the development of target-selective synapse formation. Here, we address progress and future directions in the study of cell-type-biased synaptic targeting in the cerebral cortex. We highlight challenges to identifying developmental mechanisms generating stereotyped patterns of intracortical connectivity, recent developments in uncovering the determinants of synaptic target selection during cortical synapse formation, and current gaps in the understanding of cortical synapse specificity.

Biallelic mutations in ELFN1 gene associated with developmental and epileptic encephalopathy and joint laxity

ELFN1, a transmembrane leucine rich repeat protein, is involved in signal transduction in both neural cells and ROD ON-bipolar synaptogenesis. We present three siblings with developmental and epileptic encephalopathy and co-morbidities due to ELFN1 gene mutation; this is the first report in literature defining the human phenotype of ELFN1 gene mutation. Clinical, electrophysiological, and radiological findings along with comprehensive genetic studies of the patients and their family members are presented. Developmental and epileptic encephalopathy, autistic features, pyramidal signs, joint laxity, and dysmorphic features are the characteristic findings of this new clinical entity, involving mainly nervous system and possibly connective tissue. Whole exome sequence analysis followed by Sanger sequencing in all family members revealed disease-causing 8 bp frameshift mutation depicted as NM_001128636.2: c.42_49delGGCCGCCA; p. (Ala15Profs*241) in ELFN1. The variant, located in the signal peptide domain in the ELFN1 gene, was found to be homozygous in three patients, and heterozygous in the parents and three healthy siblings. Segregation analysis in family members together with pathogenicity assessment tools strongly supported the damaging effect of the frameshift variant on the function of the ELFN1 protein. Mutations in ELFN1 gene may be considered in patients with neonatal and infantile-onset epileptic encephalopathy before the full clinical picture is apparent.

Regulation and dysregulation of neuronal circuits by KARs

Kainate receptors (KARs) constitute a family of ionotropic glutamate receptors (iGluRs) with distinct physiological roles in synapses and neuronal circuits. Despite structural and biophysical commonalities with the other iGluRs, AMPA receptors and NMDA receptors, their role as post-synaptic receptors involved in shaping EPSCs to transmit signals across synapses is limited to a small number of synapses. On the other hand KARs regulate presynaptic release mechanisms and control ion channels and signaling pathways through non-canonical metabotropic actions. We review how these different KAR-dependent mechanisms concur to regulate the activity and plasticity of neuronal circuits in physiological conditions of activation of KARs by endogenous glutamate (as opposed to pharmacological activation by exogenous agonists). KARs have been implicated in neurological disorders, based on genetic association and on physiopathological studies. A well described example relates to temporal lobe epilepsy for which the aberrant recruitment of KARs at recurrent mossy fiber synapses takes part in epileptogenic neuronal activity. In conclusion, KARs certainly represent an underestimated actor in the regulation of neuronal circuits, and a potential therapeutic target awaiting more selective and efficient genetic tools and/or ligands.

Hippocampal Somatostatin Interneurons, Long-Term Synaptic Plasticity and Memory

A distinctive feature of the hippocampal structure is the diversity of inhibitory interneurons. These complex inhibitory interconnections largely contribute to the tight modulation of hippocampal circuitry, as well as to the formation and coordination of neuronal assemblies underlying learning and memory. Inhibitory interneurons provide more than a simple transitory inhibition of hippocampal principal cells (PCs). The synaptic plasticity of inhibitory neurons provides long-lasting changes in the hippocampal network and is a key component of memory formation. The dendrite targeting interneurons expressing the peptide somatostatin (SOM) are particularly interesting in this regard because they display unique long-lasting synaptic changes leading to metaplastic regulation of hippocampal networks. In this article, we examine the actions of the neuropeptide SOM on hippocampal cells, synaptic plasticity, learning, and memory. We address the different subtypes of hippocampal SOM interneurons. We describe the long-term synaptic plasticity that takes place at the excitatory synapses of SOM interneurons, its singular induction and expression mechanisms, as well as the consequences of these changes on the hippocampal network, learning, and memory. We also review evidence that astrocytes provide cell-specific dynamic regulation of inhibition of PC dendrites by SOM interneurons. Finally, we cover how, in mouse models of Alzheimer’s disease (AD), dysfunction of plasticity of SOM interneuron excitatory synapses may also contribute to cognitive impairments in brain disorders.

A High-Resolution Method for Quantitative Molecular Analysis of Functionally Characterized Individual Synapses

Elucidating the molecular mechanisms underlying the functional diversity of synapses requires a high-resolution, sensitive, diffusion-free, quantitative localization method that allows the determination of many proteins in functionally characterized individual synapses. Array tomography permits the quantitative analysis of single synapses but has limited sensitivity, and its application to functionally characterized synapses is challenging. Here, we aim to overcome these limitations by searching the parameter space of different fixation, resin, embedding, etching, retrieval, and elution conditions. Our optimizations reveal that etching epoxy-resin-embedded ultrathin sections with Na-ethanolate and treating them with SDS dramatically increase the labeling efficiency of synaptic proteins. We also demonstrate that this method is ideal for the molecular characterization of individual synapses following paired recordings, two-photon [Ca²⁺] or glutamate-sensor (iGluSnFR) imaging. This method fills a missing gap in the toolbox of molecular and cellular neuroscience, helping us to reveal how molecular heterogeneity leads to diversity in function.

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Etching and antigen retrieval enhance immunoreactions in epoxy-resin-embedded tissue

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Biocytin-filled nerve cells can be visualized in epoxy-resin-embedded tissue

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Molecular composition of functionally characterized individual synapses is revealed

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Multiplexed, postembedding reactions are compatible with STED imaging

Variability in the Munc13-1 content of excitatory release sites

The molecular mechanisms underlying the diversity of cortical glutamatergic synapses are still incompletely understood. Here, we tested the hypothesis that presynaptic active zones (AZs) are constructed from molecularly uniform, independent release sites (RSs), the number of which scales linearly with the AZ size. Paired recordings between hippocampal CA1 pyramidal cells and fast-spiking interneurons in acute slices from adult mice followed by quantal analysis demonstrate large variability in the number of RSs (N) at these connections. High-resolution molecular analysis of functionally characterized synapses reveals variability in the content of one of the key vesicle priming factors – Munc13-1 – in AZs that possess the same N. Replica immunolabeling also shows a threefold variability in the total Munc13-1 content of AZs of identical size and a fourfold variability in the size and density of Munc13-1 clusters within the AZs. Our results provide evidence for quantitative molecular heterogeneity of RSs and support a model in which the AZ is built up from variable numbers of molecularly heterogeneous, but independent RSs.

Endocannabinoid system in the neurodevelopment of GABAergic interneurons: implications for neurological and psychiatric disorders

In mature mammalian brains, the endocannabinoid system (ECS) plays an important role in the regulation of synaptic plasticity and the functioning of neural networks. Besides, the ECS also contributes to the neurodevelopment of the central nervous system. Due to the increase in the medical and recreational use of cannabis, it is inevitable and essential to elaborate the roles of the ECS on neurodevelopment. GABAergic interneurons represent a group of inhibitory neurons that are vital in controlling neural network activity. However, the role of the ECS in the neurodevelopment of GABAergic interneurons remains to be fully elucidated. In this review, we provide a brief introduction of the ECS and interneuron diversity. We focus on the process of interneuron development and the role of ECS in the modulation of interneuron development, from the expansion of the neural stem/progenitor cells to the migration, specification and maturation of interneurons. We further discuss the potential implications of the ECS and interneurons in the pathogenesis of neurological and psychiatric disorders, including epilepsy, schizophrenia, major depressive disorder and autism spectrum disorder.

Trans-Synaptic Regulation of Metabotropic Glutamate Receptors by Elfn Proteins in Health and Disease

Trans-regulation of G protein-coupled receptors (GPCRs) by leucine-rich repeat (LRR) transmembrane proteins has emerged as a novel type of synaptic molecular interaction in the last decade. Several studies on LRR–GPCR interactions have revealed their critical role in synapse formation and in establishing synaptic properties. Among them, LRR–GPCR interactions between extracellular LRR fibronectin domain-containing family proteins (Elfn1 and Elfn2) and metabotropic glutamate receptors (mGluRs) are particularly interesting as they can affect a broad range of synapses through the modulation of signaling by glutamate, the principal excitatory transmitter in the mammalian central nervous system (CNS). Elfn–mGluR interactions have been investigated in hippocampal, cortical, and retinal synapses. Postsynaptic Elfn1 in the hippocampus and cerebral cortex mediates the tonic regulation of excitatory input onto somatostatin-positive interneurons (INs) through recruitment of presynaptic mGluR7. In the retina, presynaptic Elfn1 binds to mGluR6 and is necessary for synapse formation between rod photoreceptor cells and rod-bipolar cells. The repertoire of binding partners for Elfn1 and Elfn2 includes all group III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8), and both Elfn1 and Elfn2 can alter mGluR-mediated signaling through trans-interaction. Importantly, both preclinical and clinical studies have provided support for the involvement of the Elfn1–mGluR7 interaction in attention-deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), and epilepsy. In fact, Elfn1–mGluR7-associated disorders may reflect the altered function of somatostatin-positive interneuron inhibitory neural circuits, the mesolimbic and nigrostriatal dopaminergic pathway, and habenular circuits, highlighting the need for further investigation into this interaction.

Linear-nonlinear cascades capture synaptic dynamics

Short-term synaptic dynamics differ markedly across connections and strongly regulate how action potentials communicate information. To model the range of synaptic dynamics observed in experiments, we have developed a flexible mathematical framework based on a linear-nonlinear operation. This model can capture various experimentally observed features of synaptic dynamics and different types of heteroskedasticity. Despite its conceptual simplicity, we show that it is more adaptable than previous models. Combined with a standard maximum likelihood approach, synaptic dynamics can be accurately and efficiently characterized using naturalistic stimulation patterns. These results make explicit that synaptic processing bears algorithmic similarities with information processing in convolutional neural networks.

Neurexins: molecular codes for shaping neuronal synapses

The function of neuronal circuits relies on the properties of individual neuronal cells and their synapses. We propose that a substantial degree of synapse formation and function is instructed by molecular codes resulting from transcriptional programmes. Recent studies on the Neurexin protein family and its ligands provide fundamental insight into how synapses are assembled and remodelled, how synaptic properties are specified and how single gene mutations associated with neurodevelopmental and psychiatric disorders might modify the operation of neuronal circuits and behaviour. In this Review, we first summarize insights into Neurexin function obtained from various model organisms. We then discuss the mechanisms and logic of the cell type-specific regulation of Neurexin isoforms, in particular at the level of alternative mRNA splicing. Finally, we propose a conceptual framework for how combinations of synaptic protein isoforms act as 'senders' and 'readers' to instruct synapse formation and the acquisition of cell type-specific and synapse-specific functional properties.

Cell-type-specific recruitment of GABAergic interneurons in the primary somatosensory cortex by long-range inputs

Extensive hierarchical yet highly reciprocal interactions among cortical areas are fundamental for information processing. However, connectivity rules governing the specificity of such corticocortical connections, and top-down feedback projections in particular, are poorly understood. We analyze synaptic strength from functionally relevant brain areas to diverse neuronal types in the primary somatosensory cortex (S1). Long-range projections from different areas preferentially engage specific sets of GABAergic neurons in S1. Projections from other somatosensory cortices strongly recruit parvalbumin (PV)-positive GABAergic neurons and lead to PV neuron-mediated feedforward inhibition of pyramidal neurons in S1. In contrast, inputs from whisker-related primary motor cortex are biased to vasoactive intestinal peptide (VIP)-positive GABAergic neurons and potentially result in VIP neuron-mediated disinhibition. Regardless of the input areas, somatostatin-positive neurons receive relatively weak long-range inputs. Computational analyses suggest that a characteristic combination of synaptic inputs to different GABAergic IN types in S1 represents a specific long-range input area.

Naskar et al. show how functionally relevant brain areas interact with neurons in the primary somatosensory cortex, demonstrating that long-range projections from diverse brain areas differentially recruit specific subtypes of GABAergic neurons in S1, and each distinct subtype of GABAergic neurons differentially affects local network activity in S1.

Temporal Proteomic Profiling of SH-SY5Y Differentiation with Retinoic Acid Using FAIMS and Real-Time Searching

The SH-SY5Y cell line is often used as a surrogate for neurons in cell-based studies. This cell line is frequently differentiated with all-trans retinoic acid (ATRA) over a 7-day period, which confers neuron-like properties to the cells. However, no analysis of proteome remodeling has followed the progress of this transition. Here, we quantitatively profiled over 9400 proteins across a 7-day treatment with retinoic acid using state-of-the-art mass spectrometry-based proteomics technologies, including FAIMS, real-time database searching, and TMTpro16 sample multiplexing. Gene ontology analysis revealed that categories with the highest increases in protein abundance were related to the plasma membrane/extracellular space. To showcase our data set, we surveyed the protein abundance profiles linked to neurofilament bundle assembly, neuron projections, and neuronal cell body formation. These proteins exhibited increases in abundance level, yet we observed multiple patterns among the queried proteins. The data presented represent a rich resource for investigating temporal protein abundance changes in SH-SY5Y cells differentiated with retinoic acid. Moreover, the sample preparation and data acquisition strategies used here can be readily applied to any analogous cell line differentiation analysis.

Interplay between cell-adhesion molecules governs synaptic wiring of cone photoreceptors

Establishment of functional synaptic connections in a selective manner is essential for nervous system operation. In mammalian retinas, rod and cone photoreceptors form selective synaptic connections with different classes of bipolar cells (BCs) to propagate light signals. While there has been progress in elucidating rod wiring, molecular mechanisms used by cones to establish functional synapses with BCs have remained unknown. Using an unbiased proteomic strategy in cone-dominant species, we identified the cell-adhesion molecule ELFN2 to be pivotal for the functional wiring of cones with the ON type of BC. It is selectively expressed in cones and transsynaptically recruits the key neurotransmitter receptor mGluR6 in ON-BCs to enable synaptic transmission. Remarkably, ELFN2 in cone terminals functions in synergy with a related adhesion molecule, ELFN1, and their concerted interplay during development specifies selective wiring and transmission of cone signals. These findings identify a synaptic connectivity mechanism of cones and illustrate how interplay between adhesion molecules and postsynaptic transmitter receptors orchestrates functional synaptic specification in a neural circuit.

LRRTM4 is a member of the transsynaptic complex between rod photoreceptors and bipolar cells

Leucine rich repeat transmembrane (LRRTM) proteins are synaptic adhesion molecules with roles in synapse formation and signaling. LRRTM4 transcripts were previously shown to be enriched in rod bipolar cells (BCs), secondary neurons of the retina that form synapses with rod photoreceptors. Using two different antibodies, LRRTM4 was found to reside primarily at rod BC dendritic tips, where it colocalized with the transduction channel protein, TRPM1. LRRTM4 was not detected at dendritic tips of ON-cone BCs. Following somatic knockout of LRRTM4 in BCs by subretinal injection and electroporation of CRISPR/Cas9, LRRTM4 was abolished or reduced in the dendritic tips of transfected cells. Knockout cells had a normal complement of TRPM1 at their dendritic tips, while GPR179 accumulation was partially reduced. In experiments with heterologously expressed protein, the extracellular domain of LRRTM4 was found to engage in heparan-sulfate dependent binding with pikachurin. These results implicate LRRTM4 in the GPR179-pikachurin-dystroglycan transsynaptic complex at rod synapses.

Phenotypic profiling of mGlu7 knockout mice reveals new implications for neurodevelopmental disorders

Neurodevelopmental disorders are characterized by deficits in communication, cognition, attention, social behavior and/or motor control. Previous studies have pointed to the involvement of genes that regulate synaptic structure and function in the pathogenesis of these disorders. One such gene, GRM7, encodes the metabotropic glutamate receptor 7 (mGlu₇ ), a G protein-coupled receptor that regulates presynaptic neurotransmitter release. Mutations and polymorphisms in GRM7 have been associated with neurodevelopmental disorders in clinical populations; however, limited preclinical studies have evaluated mGlu₇ in the context of this specific disease class. Here, we show that the absence of mGlu₇ in mice is sufficient to alter phenotypes within the domains of social behavior, associative learning, motor function, epilepsy and sleep. Moreover, Grm7 knockout mice exhibit an attenuated response to amphetamine. These findings provide rationale for further investigation of mGlu₇ as a potential therapeutic target for neurodevelopmental disorders such as idiopathic autism, attention deficit hyperactivity disorder and Rett syndrome.

Target-dependent retrograde signaling mediates synaptic plasticity at the Drosophila neuromuscular junction

Neurons that innervate multiple targets often establish synapses with target-specific strengths, and local forms of synaptic plasticity. We have examined the molecular-genetic mechanisms that allow a single Drosophila motoneuron, the ventral Common Exciter (vCE), to establish connections with target-specific properties at its various synaptic partners. By driving transgenes in a subset of vCE's targets, we found that individual target cells are able to independently control the properties of vCE's innervating branch and synapses. This is achieved by means of a trans-synaptic growth factor secreted by the target cell. At the larval neuromuscular junction, postsynaptic glutamate receptor activity stimulates the release of the BMP4/5/6 homolog Glass bottom boat (Gbb). As larvae mature and motoneuron terminals grow, Gbb activates the R-Smad transcriptional regulator phosphorylated Mad (pMad) to facilitate presynaptic development. We found that manipulations affecting glutamate receptors or Gbb within subsets of target muscles led to local effects either specific to the manipulated muscle or by a limited gradient within the presynaptic branches. While presynaptic development depends on pMad transcriptional activity within the motoneuron nucleus, we find that the Gbb growth factor may also act locally within presynaptic terminals. Local Gbb signaling and presynaptic pMad accumulation within boutons may therefore participate in a "synaptic tagging" mechanism, to influence synaptic growth and plasticity in Drosophila.

Broad domains of histone H3 lysine 4 trimethylation in transcriptional regulation and disease

Histone modifications affect transcription by changing the chromatin structure. In particular, histone H3 lysine 4 trimethylation (H3K4me3) is one of the most recognized epigenetic marks of active transcription. While many studies have provided evidence of the correlation between H3K4me3 and active transcription, details regarding the mechanism involved remain unclear. The first study on the broad H3K4me3 domain was reported in 2014; subsequently, the function of this domain has been studied in various cell types. In this review, we summarized the recent studies on the role of the broad H3K4me3 domain in transcription, development, memory formation, and several diseases, including cancer and autoimmune diseases. The broadest H3K4me3 domains are associated with increased transcriptional precision of cell-type-specific genes related to cell identity and other essential functions. The broad H3K4me3 domain regulates maternal zygotic activation in early mammalian development. In systemic autoimmune diseases, high expression of immune-responsive genes requires the presence of the broad H3K4me3 domain in the promoter-proximal regions. Transcriptional repression of tumor-suppressor genes is associated with the shortening of the broad H3K4me3 domains in cancer cells. Additionally, the broad H3K4me3 domain interacts with the super-enhancer to regulate cancer-associated genes. During memory formation, H3K4me3 breadth is regulated in the hippocampus CA1 neurons. Taken together, these findings indicate that H3K4me3 breadth is essential for the regulation of the transcriptional output across multiple cell types.

Kainate Receptors, Homeostatic Gatekeepers of Synaptic Plasticity

Extensive research over the past decades has characterized multiple forms of synaptic plasticity, identifying them as key processes that allow the brain to operate in a dynamic manner. Within the wide variety of synaptic plasticity modulators, kainate receptors are receiving increasing attention, given their diversity of signaling mechanisms and cellular expression profile. Here, we summarize the experimental evidence about the involvement of kainate receptor signaling in the regulation of short- and long-term plasticity, from the perspective of the regulation of neurotransmitter release. In light of this evidence, we propose that kainate receptors may be considered homeostatic modulators of neurotransmitter release, able to bidirectionally regulate plasticity depending on the functional history of the synapse.

Beyond the Ligand: Extracellular and Transcellular G Protein-Coupled Receptor Complexes in Physiology and Pharmacology

G protein-coupled receptors (GPCRs) remain one of the most successful targets of U.S. Food and Drug Administration-approved drugs. GPCR research has predominantly focused on the characterization of the intracellular interactome's contribution to GPCR function and pharmacology. However, emerging evidence uncovers a new dimension in the biology of GPCRs involving their extracellular and transcellular interactions that critically impact GPCR function and pharmacology. The seminal examples include a variety of adhesion GPCRs, such as ADGRLs/latrophilins, ADGRBs/brain angiogenesis inhibitors, ADGRG1/GPR56, ADGRG6/GPR126, ADGRE5/CD97, and ADGRC3/CELSR3. However, recent advances have indicated that class C GPCRs that contain large extracellular domains, including group III metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7, mGluR8), γ-aminobutyric acid receptors, and orphans GPR158 and GPR179, can also participate in this form of transcellular regulation. In this review, we will focus on a variety of identified extracellular and transcellular GPCR-interacting partners, including teneurins, neurexins, integrins, fibronectin leucine-rich transmembranes, contactin-6, neuroligin, laminins, collagens, major prion protein, amyloid precursor protein, complement C1q-likes, stabilin-2, pikachurin, dystroglycan, complement decay-accelerating factor CD55, cluster of differentiation CD36 and CD90, extracellular leucine-rich repeat and fibronectin type III domain containing 1, and leucine-rich repeat, immunoglobulin-like domain and transmembrane domains. We provide an account on the diversity of extracellular and transcellular GPCR complexes and their contribution to key cellular and physiologic processes, including cell migration, axon guidance, cellular and synaptic adhesion, and synaptogenesis. Furthermore, we discuss models and mechanisms by which extracellular GPCR assemblies may regulate communication at cellular junctions. SIGNIFICANCE STATEMENT: G protein-coupled receptors (GPCRs) continue to be the prominent focus of pharmacological intervention for a variety of human pathologies. Although the majority of GPCR research has focused on the intracellular interactome, recent advancements have identified an extracellular dimension of GPCR modulation that alters accepted pharmacological principles of GPCRs. Herein, we describe known endogenous allosteric modulators acting on GPCRs both in cis and in trans.

Systematic analysis of expression signatures of neuronal subpopulations in the VTA

Gene expression profiling across various brain areas at the single-cell resolution enables the identification of molecular markers of neuronal subpopulations and comprehensive characterization of their functional roles. Despite the scientific importance and experimental versatility, systematic methods to analyze such data have not been established yet. To this end, we developed a statistical approach based on in situ hybridization data in the Allen Brain Atlas and thereby identified specific genes for each type of neuron in the ventral tegmental area (VTA). This approach also allowed us to demarcate subregions within the VTA comprising specific neuronal subpopulations. We further identified WW domain-containing oxidoreductase as a molecular marker of a population of VTA neurons that co-express tyrosine hydroxylase and vesicular glutamate transporter 2, and confirmed their region-specific distribution by immunohistochemistry. The results demonstrate the utility of our analytical approach for uncovering expression signatures representing specific cell types and neuronal subpopulations enriched in a given brain area.

ELFN2 is a postsynaptic cell adhesion molecule with essential roles in controlling group III mGluRs in the brain and neuropsychiatric behavior

The functional characterization of the GPCR interactome has predominantly focused on intracellular binding partners; however, the recent emergence of transsynaptic GPCR complexes represents an additional dimension to GPCR function that has previously been unaccounted for in drug discovery. Here, we characterize ELFN2 as a novel postsynaptic adhesion molecule with a distinct expression pattern throughout the brain and a selective binding with group III metabotropic glutamate receptors (mGluRs) in trans. Using a transcellular GPCR signaling platform, we report that ELFN2 critically alters group III mGluR secondary messenger signaling by directly altering G protein coupling kinetics and efficacy. Loss of ELFN2 in mice results in the selective downregulation of group III mGluRs and dysregulated glutamatergic synaptic transmission. Elfn2 knockout (Elfn2 KO) mice also feature a range of neuropsychiatric manifestations including seizure susceptibility, hyperactivity, and anxiety/compulsivity, which can be rescued by pharmacological augmentation of group III mGluRs. Thus, we conclude that extracellular transsynaptic scaffolding by ELFN2 in the brain is a cardinal organizational feature of group III mGluRs essential for their signaling properties and brain function.