Neurexins, neuroligins and LRRTMs: synaptic adhesion getting fishy

SGLT2 inhibition, plasma proteins, and heart failure: a proteome-wide Mendelian Randomization and colocalization study

Objective

To investigate the causal contributions of Sodium-glucose cotransporter 2 (SGLT2) inhibition on Heart Failure (HF) and identify the circulating proteins that mediate SGLT2 inhibition's effects on HF.

Methods

Applying a two-sample, two-step Mendelian Randomization (MR) analysis, we aimed to estimate: (1) the causal impact of SGLT2 inhibition on HF; (2) the causal correlation of SGLT2 inhibition on 4,907 circulating proteins; (3) the causal association of SGLT2 inhibition-driven plasma proteins on HF. Genetic variants linked to SGLT2 inhibition derived from the previous studies. The 4,907 circulating proteins were derived from the deCODE study. Genetic links to HF were obtained through the Heart Failure Molecular Epidemiology for Therapeutic Targets (HERMES) consortium.

Results

SGLT2 inhibition demonstrated a lower risk of HF (odds ratio [OR] = 0.44, 95% CI [0.26, 0.76], P = 0.003). Among 4,907 circulating proteins, we identified leucine rich repeat transmembrane protein 2 (LRRTM2), which was related to both SGLT2 inhibition and HF. Mediation analysis revealed that the impact of SGLT2 inhibition on HF operates indirectly through LRRTM2 [β = -0.20, 95% CI (-0.39, -0.06), P = 0.02] with a mediation proportion of 24.6%. Colocalization analysis provided support for the connections between LRRTM2 and HF.

Conclusion

The study indicated a causative link between SGLT2 inhibition and HF, with plasma LRRTM2 potentially serving as a mediator.

Subsynaptic positioning of AMPARs by LRRTM2 controls synaptic strength

Recent evidence suggests that nano-organization of proteins within synapses may control the strength of communication between neurons in the brain. The unique subsynaptic distribution of glutamate receptors, which cluster in nanoalignment with presynaptic sites of glutamate release, supports this hypothesis. However, testing it has been difficult because mechanisms controlling subsynaptic organization remain unknown. Reasoning that transcellular interactions could position AMPA receptors (AMPARs), we targeted a key transsynaptic adhesion molecule implicated in controlling AMPAR number, LRRTM2, using engineered, rapid proteolysis. Severing the LRRTM2 extracellular domain led quickly to nanoscale declustering of AMPARs away from release sites, not prompting their escape from synapses until much later. This rapid remodeling of AMPAR position produced significant deficits in evoked, but not spontaneous, postsynaptic receptor activation. These results dissociate receptor numbers from their nanopositioning in determination of synaptic function and support the novel concept that adhesion molecules acutely position receptors to dynamically control synaptic strength.

Developmental exposure to mepanipyrim induces locomotor hyperactivity in zebrafish (Danio rerio) larvae

Mepanipyrim is a widely used fungicide, and residues of mepanipyrim are frequently detected in commodities. However, the neurotoxicity and underlying mechanisms of mepanipyrim are still insufficiently understood. In this study, zebrafish embryos at 0.5-1.0 post-fertilization hours (hpf) were exposed to 0.1, 1, 10 and 100 μg/L mepanipyrim for 7 days. Our results showed that mepanipyrim could cause the locomotor hyperactivity and increase the concentration of γ-amino butyric acid (GABA) and the Na⁺/K⁺- and Ca²⁺-ATPase activities in zebrafish larvae. We have conducted the RNA-sequence and RT-qPCR to analyze the gene expressions. The mRNA expression levels of calcium/sodium ion conduction associated genes were observably up-regulated, demonstrating that mepanipyrim could enhance the cell energy metabolism, the synaptic transmission and skeletal muscle contraction, which were consistent with the locomotor hyperactivity. Meanwhile, exposure to mepanipyrim could significantly change the gene expression levels of gad1, bdnf, nlgn1, and type A and B GABA receptors in zebrafish larvae. This is the first study focusing on the underlying mechanisms of the neurotoxic effects that are induced by mepanipyrim.

Slitrk1 is localized to excitatory synapses and promotes their development

Following the migration of the axonal growth cone to its target area, the initial axo-dendritic contact needs to be transformed into a functional synapse. This multi-step process relies on overlapping but distinct combinations of molecules that confer synaptic identity. Slitrk molecules are transmembrane proteins that are highly expressed in the central nervous system. We found that two members of the Slitrk family, Slitrk1 and Slitrk2, can regulate synapse formation between hippocampal neurons. Slitrk1 is enriched in postsynaptic fractions and is localized to excitatory synapses. Overexpression of Slitrk1 and Slitrk2 in hippocampal neurons increased the number of synaptic contacts on these neurons. Furthermore, decreased expression of Slitrk1 in hippocampal neurons led to a reduction in the number of excitatory, but not inhibitory, synapses formed in hippocampal neuron cultures. In addition, we demonstrate that different leucine rich repeat domains of the extracellular region of Slitrk1 are necessary to mediate interactions with Slitrk binding partners of the LAR receptor protein tyrosine phosphatase family, and to promote dimerization of Slitrk1. Altogether, our results demonstrate that Slitrk family proteins regulate synapse formation.

SALM5 trans-synaptically interacts with LAR-RPTPs in a splicing-dependent manner to regulate synapse development

Synaptogenic adhesion molecules play critical roles in synapse formation. SALM5/Lrfn5, a SALM/Lrfn family adhesion molecule implicated in autism spectrum disorders (ASDs) and schizophrenia, induces presynaptic differentiation in contacting axons, but its presynaptic ligand remains unknown. We found that SALM5 interacts with the Ig domains of LAR family receptor protein tyrosine phosphatases (LAR-RPTPs; LAR, PTPδ, and PTPσ). These interactions are strongly inhibited by the splice insert B in the Ig domain region of LAR-RPTPs, and mediate SALM5-dependent presynaptic differentiation in contacting axons. In addition, SALM5 regulates AMPA receptor-mediated synaptic transmission through mechanisms involving the interaction of postsynaptic SALM5 with presynaptic LAR-RPTPs. These results suggest that postsynaptic SALM5 promotes synapse development by trans-synaptically interacting with presynaptic LAR-RPTPs and is important for the regulation of excitatory synaptic strength.

Mice lacking the synaptic adhesion molecule Neph2/Kirrel3 display moderate hyperactivity and defective novel object preference

Synaptic adhesion molecules regulate diverse aspects of neuronal synapse development, including synapse specificity, formation, and maturation. Neph2, also known as Kirrel3, is an immunoglobulin superfamily adhesion molecule implicated in intellectual disability, neurocognitive delay associated with Jacobsen syndrome, and autism spectrum disorders. We here report mice lacking Neph2 (Neph2(-/-) mice) display moderate hyperactivity in a familiar, but not novel, environment and defective novel object recognition with normal performances in Morris water maze spatial learning and memory, contextual fear conditioning and extinction, and pattern separation tests. These mice also show normal levels of anxiety-like behaviors, social interaction, and repetitive behaviors. At the synapse level, Neph2(-/-) dentate gyrus granule cells exhibit unaltered dendritic spine density and spontaneous excitatory synaptic transmission. These results suggest that Neph2 is important for normal locomotor activity and object recognition memory.

Synapse assembly and neurodevelopmental disorders

In this review we examine the current understanding of how genetic deficits associated with neurodevelopmental disorders may impact synapse assembly. We then go on to discuss how the critical periods for these genetic deficits will shape the nature of future clinical interventions.

Towards a molecular characterization of autism spectrum disorders: an exome sequencing and systems approach

The hypothetical 'AXAS' gene network model that profiles functional patterns of heterogeneous DNA variants overrepresented in autism spectrum disorder (ASD), X-linked intellectual disability, attention deficit and hyperactivity disorder and schizophrenia was used in this current study to analyze whole exome sequencing data from an Australian ASD cohort. An optimized DNA variant filtering pipeline was used to identify loss-of-function DNA variations. Inherited variants from parents with a broader autism phenotype and de novo variants were found to be significantly associated with ASD. Gene ontology analysis revealed that putative rare causal variants cluster in key neurobiological processes and are overrepresented in functions involving neuronal development, signal transduction and synapse development including the neurexin trans-synaptic complex. We also show how a complex gene network model can be used to fine map combinations of inherited and de novo variations in families with ASD that converge in the L1CAM pathway. Our results provide an important step forward in the molecular characterization of ASD with potential for developing a tool to analyze the pathogenesis of individual affected families.

Developing zebrafish models of autism spectrum disorder (ASD)

Autism spectrum disorder (ASD) is a serious neurodevelopmental disorder with complex symptoms and unclear, multi-factorial pathogenesis. Animal (rodent) models of ASD-like behavior are extensively used to study genetics, circuitry and molecular mechanisms of ASD. The evolutionarily conserved nature of social behavior and its molecular pathways suggests that alternative experimental models can be developed to complement and enhance the existing rodent ASD paradigms. The zebrafish (Danio rerio) is rapidly becoming a popular model organism in neuroscience and biological psychiatry to study brain function, model human brain disorders and explore their genetic or pharmacological modulation. Representing highly social animals, zebrafish emerge as a strong potential model organism to study normal and pathological social phenotypes, as well as several other ASD-like symptoms. Here, we discuss the developing utility of zebrafish in modeling ASD as a new emerging field in translational neuroscience and drug discovery.

Balance within the Neurexin Trans-Synaptic Connexus Stabilizes Behavioral Control

Autism spectrum disorder (ASD) is characterized by a broad spectrum of behavioral deficits of unknown etiology. ASD associated mutations implicate numerous neurological pathways including a common association with the neurexin trans-synaptic connexus (NTSC) which regulates neuronal cell-adhesion, neuronal circuitry, and neurotransmission. Comparable DNA lesions affecting the NTSC, however, associate with a diversity of behavioral deficits within and without the autism spectrum including a very strong association with Tourette syndrome. The NTSC is comprised of numerous post-synaptic ligands competing for trans-synaptic connection with one of the many different neurexin receptors yet no apparent association exists between specific NTSC molecules/complexes and specific behavioral deficits. Together these findings indicate a fundamental role for NTSC-balance in stabilizing pre-behavioral control. Further molecular and clinical characterization and stratification of ASD and TS on the basis of NTSC status will help elucidate the molecular basis of behavior – and define how the NTSC functions in combination with other molecular determinates to strengthen behavioral control and specify behavioral deficits.

Neural cell adhesion molecules belonging to the family of leucine-rich repeat proteins

Leucine-rich repeats (LRRs) are motifs that form protein-ligand interaction domains. There are approximately 140 human genes encoding proteins with extracellular LRRs. These encode cell adhesion molecules (CAMs), proteoglycans, G-protein-coupled receptors, and other types of receptors. Here we give a brief description of 36 proteins with extracellular LRRs that all can be characterized as CAMs or putative CAMs expressed in the nervous system. The proteins are involved in multiple biological processes in the nervous system including the proliferation and survival of cells, neuritogenesis, axon guidance, fasciculation, myelination, and the formation and maintenance of synapses. Moreover, the proteins are functionally implicated in multiple diseases including cancer, hearing impairment, glaucoma, Alzheimer's disease, multiple sclerosis, Parkinson's disease, autism spectrum disorders, schizophrenia, and obsessive-compulsive disorders. Thus, LRR-containing CAMs constitute a large group of proteins of pivotal importance for the development, maintenance, and regeneration of the nervous system.

Synaptic laminae in the visual system: molecular mechanisms forming layers of perception

Synaptic connections between neurons form the basis for perception and behavior. Synapses are often clustered in space, forming stereotyped layers. In the retina and optic tectum, multiple such synaptic laminae are stacked on top of each other, giving rise to stratified neuropil regions in which each layer combines synapses responsive to a particular sensory feature. Recently, several cellular and molecular mechanisms that underlie the development of multilaminar arrays of synapses have been discovered. These mechanisms include neurite guidance and cell-cell recognition. Molecules of the Slit, Semaphorin, Netrin, and Hedgehog families, binding to their matching receptors, bring axons and dendrites into spatial register. These guidance cues may diffuse over short distances or bind to sheets of extracellular matrix, thus conditioning the local extracellular milieu, or are presented on the surface of cells bordering the future neuropil. In addition, mutual recognition of axons and dendrites through adhesion molecules with immunoglobulin domains ensures cell type-specific connections within a given layer. Thus, an elaborate genetic program assembles the parallel processing channels that underlie visual perception.

Converging Pathways in Autism Spectrum Disorders: Interplay between Synaptic Dysfunction and Immune Responses

Autism spectrum disorders (ASD) are highly heritable, yet genetically heterogeneous neurodevelopmental conditions. Recent genome-wide association and gene expression studies have provided evidence supporting the notion that the large number of genetic variants associated with ASD converge toward a core set of dysregulated biological processes. Here we review recent data demonstrating the involvement of synaptic dysfunction and abnormal immune responses in ASD, and discuss the functional interplay between the two phenomena.

Pathogenetic model for Tourette syndrome delineates overlap with related neurodevelopmental disorders including Autism

Tourette syndrome (TS) is a highly heritable neuropsychiatric disorder characterised by motor and vocal tics. Despite decades of research, the aetiology of TS has remained elusive. Recent successes in gene discovery backed by rapidly advancing genomic technologies have given us new insights into the genetic basis of the disorder, but the growing collection of rare and disparate findings have added confusion and complexity to the attempts to translate these findings into neurobiological mechanisms resulting in symptom genesis. In this review, we explore a previously unrecognised genetic link between TS and a competing series of trans-synaptic complexes (neurexins (NRXNs), neuroligins (NLGNs), leucine-rich repeat transmembrane proteins (LRRTMs), leucine rich repeat neuronals (LRRNs) and cerebellin precursor 2 (CBLN2)) that links it with autism spectrum disorder through neurodevelopmental pathways. The emergent neuropathogenetic model integrates all five genes so far found to be uniquely disrupted in TS into a single pathogenetic chain of events described in context with clinical and research implications.

Visual social preferences of lone zebrafish in a novel environment: strain and anxiolytic effects

Zebrafish (Danio rerio) have an innate tendency to join shoals. Based on this, we refined visual choice tests to focus on social interaction and novelty preference. Our design follows mouse three-chamber sociability protocols, except testing is conducted under 940 Lux fluorescent lighting. Initially, we compared performance among zebrafish strains: inbred (AB) or wild-crossbred (WIK) from Zebrafish International Resource Center, to golden and short-fin from Petco stores. AB fish exhibited a preference for shoaling; they dwelled longest near transparent boxes containing zebrafish, while short fin favored blue boxes without fish. AB and golden exhibited a strong preference for social novelty, not evident in short-fin or WIK fish. Serotonin and cannabinoids shape mammalian social behavior, and equivalents of both receptor types are expressed in the zebrafish brain. We examined the effects of the cannabinoid receptor agonist WIN 55,212-2 (1 mg/l), or serotonin 5-HT(1A) receptor agonist buspirone (10 mg/l) on Petco short-fin social choice. Fish were bath exposed to test compounds for 10 min, under these conditions [(3) H]CP55,940 (4 nm) bound to brain with a concentration of 1.9-6.4 fmol/mg 5-30 min afterward. Social approach was measured 20 min after acclimation to the test arena. WIN 55,212-2 and buspirone increased dwelling near boxed zebrafish. In zebrafish whole-brain homogenates, buspirone displaced [(3) H] 8-hydroxy-N,N-dipropylaminotetralin (dissociation constant, K(D) = 16 ± 1.2 nm) with an inhibition constant (K(i) ) of 1.8 ± 1.0 nm lower than that of WAY 100,635 (K(i) ∼1000 nm). These fish social choice tests may enhance social behavior research, and are useful for studying the effects of genetic manipulations, pharmaceuticals or environmental toxins.

Understanding the molecular diversity of GABAergic synapses

GABAergic synapses exhibit a high degree of subcellular and molecular specialization, which contrasts with their apparent simplicity in ultrastructural appearance. Indeed, when observed in the electron microscope, GABAergic synapses fit in the symmetric, or Gray’s type II category, being characterized by a relatively simple postsynaptic specialization. The inhibitory postsynaptic density cannot be readily isolated, and progress in understanding its molecular composition has lagged behind that of excitatory synapses. However, recent studies have brought significant progress in the identification of new synaptic proteins, revealing an unexpected complexity in the molecular machinery that regulates GABAergic synaptogenesis. In this article, we provide an overview of the molecular diversity of GABAergic synapses, and we consider how synapse specificity may be encoded by selective trans-synaptic interactions between pre- and postsynaptic adhesion molecules and secreted factors that reside in the synaptic cleft. We also discuss the importance of developing cataloguing tools that could be used to decipher the molecular diversity of synapses and to predict alterations of inhibitory transmission in the course of neurological diseases.