What do synaptic endings store

Fragments of other tRNA Glu species accounted for the fifth and sixth most abundant sequences. The fifth sequence does not map perfectly to any known mouse genomic sequence for a tRNA, perhaps as a result of a splice variation or mouse genomic strain difference and the sixth most abundant sequence mapped to a fragment of tRNA Glu TTC. The northerns were conducted three times, each with RNA isolated from a different preparation of mouse SVs. Unlike the T. To test whether AGO2 resided within the SVs, or was associated with the exterior of the SVs, we treated the sample with the protease trypsin.

To verify that we were not inducing the degradation of the vesicles with the application of trypsin we also tested for the degradation of synaptophysin.

Synaptophysin can be cleaved by trypsin, but the trypsin cleavage sites reside within the lumen of the SVs. Synaptophysin was not degraded in the trypsin treated samples Fig. Surprisingly, the stem and loop structure determined in this manner closely resembled known crystal structures of the tRNA anticodon stem and loop, particularly at the anticodon region Fig.

The sequence of this pseudo-anticodon is GGU and thus would recognize threonine codons, a codon not present in vertebrate genomes. Base-pair probabilities are shown in a heat map from 0—1; for non-paired nucleotides, heat map indicates probability of being unpaired. The heat map shows the positional entropy from 0—1.

The tertiary structure was aligned in pymol to a known crystal structure of a tRNA, shown in blue yeast tRNA Phe 1ehz The stem and loop structure determined in this manner closely resembled known crystal structures of the tRNA anticodon stem and loop, particularly at the anticodon region orange.

The sequence of this pseudo-anticodon is GGU. We immunopurified T. We extended our findings to heterogeneous SVs isolated from the mouse brain. One potential interpretation for the presence of the precursors in association with the SVs is that the presynaptic terminal is a site of cytoplasmic maturation and cleavage of SV sRNAs. The smaller size of the mouse SVs may make them less soluble in detergent, or the sRNAs are in a trypsin resistant complex in the mouse SVs.

The most abundant sRNA present in T. Two pathways are known to produce these fragments in vertebrates. The most abundant miRNA and the third most abundant class of sequences was miR, a miRNA important for neuronal development, synaptogenesis and post-mitotic neuronal functioning Members of the miR family miR99a and miR, the fourth and seventh most abundant mouse sRNAs are miRNAs that have been shown to co-enrich with polyribosomes in mammalian neurons and regulate the mammalian target of rapamycin mTOR pathway The mechanism by which miRNAs are taken in at the synapse is not known.

However, a previous study has demonstrated that miR99a is released from synaptosomes in an activity and calcium dependent manner, consistent with the release from SVs and synthetic miRNAs were taken up by synaptosomes via an unspecified endocytic pathway Long-term changes in synaptic plasticity require protein synthesis, the local dendritic regulation of which is still an active area of research 6 , 7 , 8 , 9.

One common and critical component of every model of local protein synthesis is the role activity plays in the up-regulation and down-regulation of translation. Activity at a local synapse is driven by the presynaptic fusion of synaptic vesicles with the plasma membrane and the subsequent release of the contents residing within the SV lumen.

Currently most of our understanding of SV content release has focused on small molecule neurotransmitters that rapidly bind metabotropic and ionotropic receptors leading to near instantaneous changes in target membrane excitability. It provides the potential for a direct means of controlling translation locally, which may prove at least as important as the proposed calcium induced mechanisms for controlling translation.

Methods were adapted from Ohsawa Louis, MO. Separate bead columns were prepared for electric organ and mouse brains to ensure no contamination. Negative stain of isolated synaptic vesicles of T. SVs were isolated and concentrated as described above. For each assay, 1 preparation of vesicles was utilized and split into 5 equal samples. Isolated SVs were passed through a 0.

The beads were pelleted and the supernatant collected for imaging. Three separate images from each field were taken using laser lines and filter cubes paired to eliminate fluorescent cross talk between the dyes: laser line with filter cube 73 HE was used for Pacific Blue, laser line with filter cube 38 HE was used for SYTO 12 and laser line with filter cube 74 HE was used for FM 4— Suitable spots detected in the FM4—64 channel were marked.

The other channels were then quantified for label. Electrophoresis and blotting are previously described Northern blots were conducted in triplicate, in order to reduce bias from any single preparation, each northern was conducted on RNA isolated from a single fish. Prehybridization buffer: 0. SVs from T. As a further enrichment, SVs from T. For in situ hybridization ISH , frozen T. Probes were hybridized to sections as previously described 54 , with minor modifications in amplification strategy.

Sequences used for probe generation are listed below. One preparation of vesicles was split into 10 equal samples. Westerns were conducted using antibodies against synaptophysin or AGO2. RNA was extracted after the treatments and quantified as described above. How to cite this article : Li, H. Dale, H. The action of certain esters and ethers of choline and their relation to muscarine.

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The Third Thudichum Lecture. Perreault, J. Ro-associated Y RNAs in metazoans: evolution and diversification. The isolation of pure cholinergic synaptic vesicles from the electric organs of elasmobranch fish of the family Torpedinidae. Hell, J. Uptake of GABA by rat brain synaptic vesicles isolated by a new procedure. EMBO J. Lamparski, H. Production and characterization of clinical grade exosomes derived from dendritic cells.

Although a great deal of variation exists in the size and shape of boutons of individual neurons, synapses can be identified by the presence of the following:.

View an EM of a nerve ending with flat vesicles. Numerous variations of the "model" neuron described above exist. An important modification, which occurs especially in receptor neurons, involves the designation of a neuronal process as a dendrite or as an axon. Classically, the axon has been identified as the myelinated or unmyelinated process that transmits signals away from the cell body.

The classical view of the dendrite is that of an unmyelinated tube of cytoplasm which carries information toward the cell body. However, this distinction does not hold for ALL neurons. Some cells have a myelinated process that transmits signals toward the cell body. Morphologically the "dendrite" and the "axon" may, therefore, be indistinguishable. Neither the position of the cell body nor the presence or absence of myelin is always a useful criterion for understanding the orientation of the neuron.

The region of impulse initiation is more reliable guide to understanding the functional focal point of the cell. This region is analogous to the initial segment of the model neuron, discussed above. Routinely the fiber or process, which contains the initial segment or trigger zone, is referred to as an axon. Note, as shown in Figure 8. A number of conventions have evolved to classify and name neurons.

Through this approach cells are classified as unipolar, bipolar and multipolar neurons as shown in Figure 8. Unipolar cells have only one cell process, and are primarily found in invertebrates. However, vertebrate sensory neurons are another form of this type of cell. Because these cells start out developmentally as bipolar neurons and then become unipolar as they mature, they are called pseudo-unipolar cells.

Bipolar cells are present in the retina and the olfactory bulb. Multipolar cells make up the remainder of neuronal types and are, consequently, the most numerous type. These have been further sub-categorized into Golgi type II cells that are small neurons, usually interneurons, and Golgi type I cells that are large multipolar neurons. Cells are also named for their shape e.

More recently, cells have been named for their function or the neurotransmitter they contain e. This description is possible because of the development of histochemical and immunocytochemical methods to specifically identify the neurotransmitter type used by neurons. Two variations in cell morphology.

On the left is the pyramidal cell named for its characteristic pyramid shape. This cell is prominent in the cerebral cortex. On the right is the cell soma and dendrites of the Purkinje cell found in the cerebellum and named for the scientist, Purkinje. Axolemma is the plasmalemma of the axon.

Endoplasmic reticulum is a labyrinthine, membrane bounded compartment in the cytoplasm where lipids are synthesized and membrane bound proteins are made. In some regions of the neuron ER is devoid of ribosomes and is termed smooth ER.

Endosome is a membrane-bounded organelle that carries materials ingested by endocytosis and passes them to lysosomes and peroxisomes for degradation. It also functions in the nerve ending to recycle synaptic vesicles. Golgi apparatus is a collection of stacked, smooth-surfaced membrane bound organelles where proteins and lipids made in the endoplasmic reticulum are modified and sorted. Lysosomes contain enzymes that digest compounds that originate inside or outside the cells.

They are involved in converting proteins to amino acids and glycogen to glucose, the basic nutrient of neurons. Their enzymes act at an acidic pH. As will be described later, they also serve as vesicles for reverse transport from axon terminals to the soma.

Many lysosomes become degraded to lipofuscin granules, which accumulate as the organism ages and are regarded as neuronal refuse. Lysosomes form from the budding off the Golgi apparatus.

They have a variety of membrane-bound shapes and sizes, ranging in size between and nm in diameter. Microfilaments are 7 nm in diameter filaments arranged as a paired helix of two strands of globular actin.

Microfilaments are especially prominent in synaptic terminals, in dendritic spines, and in association with the axolemma. Microtubules are 20 to 25 nm diameter tubular structures that run in loose bundles around the nucleus and funnel into the base of the axonal and dendritic processes where they form parallel arrays distributed longitudinally.

The MAPS regulate the polymerization of tubulin subunits to form the microtubules. In addition, microtubules are not continuous, and each microtubule is composed of numerous nm units.

Microtubules are involved in axoplasmic transport see below. Mitochondria are distributed ubiquitously throughout the cytoplasm of the entire nerve cell and are especially plentiful at presynaptic specializations. Neurofilaments are a type of intermediate filament found in nerve cells. Neurofilaments are involved in the maintenance of the neuron's shape and mechanical strength. Although neuronal neurofilaments are classified as intermediate filaments, their composition in neurons is different than that found in other cells.

They are composed of three subunits that are arranged to form a nm diameter tubule. It is the neurofilament that stains with heavy metal to permit the visualization of neuronal shape. Neurofilaments run in loose bundles around the cell nucleus and other organelles and funnel into the base of the axonal and dendritic processes where they form parallel arrays distributed longitudinally.

Neurofilaments are more abundant than microtubules in axons, whereas microtubules are more abundant than neurofilaments in dendrites. It is the neurofilaments that undergo modification in the Alzheimer's disease to form neurofibrillary tangles. Nucleolus is in the center of the nuclei of all neurons. It is a prominent, deeply stained spherical inclusion about one-third the size of the nucleus. The nucleolus synthesizes ribosomal RNA, which has a major role in protein synthesis.

Nucleus of the neuron is large and round and is usually centrally located. In some cells, masses of deeply staining chromatin are visible in the nucleus. The nuclear membrane of neurons is like that of other cells - a double membrane punctuated by pores nuclear pores which are involved in nuclear-cytoplasmic interactions.

The nucleus in neurons is spherical and ranges in diameter from 3 to 18 micrometers depending on the size of the neuron. Neurons with long axons have a larger cell body and nucleus.

As in other cells, the principal component of the nucleus is deoxyribonucleic acid DNA , the substance of the chromosomes and genes. Peroxisomes are small membrane bounded organelles that use molecular oxygen to oxidize organic molecules. They contain some enzymes that either produce or degrade hydrogen peroxide. Plasmalemma of the neuron appears in the electron microscope as a typical bi-layered cellular membrane, approximately 10 nm thick.

Postsynaptic density is darkly staining material of postsynaptic cell adjacent to the synapse. Receptors, ion channels, and other signaling molecules are likely bound to this material. Presynaptic density is the region of darkly staining material of the presynaptic membrane where synaptic vesicles are hypothesized to dock prior to fusion with the presynaptic membrane.

Ribosomes are particles composed of ribosomal RNA and ribosomal protein which associate with mRNA and catalyze the synthesis of proteins. When ribosomes are attached to the outer membranes of the ER, the organelle is termed rough ER. The rough ER, in laminae with interspersed ribosomes, is visible with the light microscope as Nissl substance. In light microscopic preparations, the appearance of Nissl substance varies in different types of neurons. It may appear as densely stained ovoids or as finely dispersed particles or aggregations of granules.

Synapse is the junction that allows signals to pass from a nerve cell to another cell or from one nerve cell to a muscle cell. The synaptic cleft is the gap between the membrane of the pre- and postsynaptic cell. In a chemical synapse the signal is carried by a diffusable neurotransmitter. This asymmetric compartmentalization of Connexins suggests that molecular rules must exist to guide specific Connexin types to particular sub-neuronal regions.

Connexin proteins are four-pass transmembrane domain proteins with N- and C-termini located intracellularly Figure 1D. Postsynaptic Cx If we look to the chemical synapse for clues, we find that the trafficking and stabilization of postsynaptic AMPA neurotransmitter receptor subtypes are regulated through interactions between its C-terminal domain and intracellular scaffolding proteins, which connects them to the cytoskeleton and other signaling molecules reviewed in Anggono and Huganir, But how do neurons target Connexins to these different neuronal compartments?

To traffic along axons and dendrites, Connexins first need to be packaged into vesicles which sort them into neuronal compartments according to the proteins on the vesicle surface. Identifying the types of vesicles in which Connexins transit would help us to understand their trafficking pathway, but these vesicles are yet to be identified.

The vesicles must next engage with the intrinsic neuronal polarity mechanisms that define dendrites and axons, particularly the motor proteins that direct traffic along microtubules to these specific regions.

These compartmental motors are distinctly organized: guidance to the presynapse along the axon requires kinesin motor proteins, and guidance to the postsynapse along the dendrite requires tethering to both kinesins and dyneins, with short-range, synaptic delivery in each compartment guided by actin-trafficked myosin motor proteins for a detailed analysis of axon and dendrite polarity differences see Rolls and Jegla, Both tubulin Brown et al.

Yet we still do not know the types of motor proteins Connexins or other electrical synapse components use to direct electrical synapse protein trafficking. However, recently some clues have started to point the field in the right direction.

Connexins likely rely on adaptor proteins to regulate their transport to the synapse. In a forward genetic screen using zebrafish, the epilepsy- and autism-associated gene Neurobeachin was identified as necessary for both electrical and chemical synapse formation Iossifov et al.

Neurobeachin is localized on vesicles which are found at the trans side of the Golgi, along dendrites, and also at chemical postsynapses Wang et al. Its localization at electrical synapses is currently unknown. Past studies show Neurobeachin regulates membrane protein trafficking of chemical synapse scaffolds including PSD95 and SAP which in turn control the trafficking of neurotransmitter receptors Medrihan et al.

In zebrafish Mauthner neurons, Neurobeachin loss results in the failure of Connexin and electrical synapse scaffold ZO1 localization. Intriguingly, Neurobeachin is both necessary and sufficient postsynaptically for electrical synapse formation in this circuit Miller et al.

This supports a model wherein Neurobeachin controls the polarized trafficking of electrical components within the postsynaptic dendrite, although the molecular mechanism remains unknown. It is attractive to speculate that perhaps Neurobeachin acts to define dendritically targeted vesicles carrying electrical synapse cargo and that it may bridge them to the motor proteins required for postsynaptic delivery.

Future experiments are required to identify how Neurobeachin functions in the dendrite to control synapse formation. The coordination of electrical and chemical synapses through a master synapse regulator such as Neurobeachin has critical implications for understanding the etiology of neurodevelopmental disorders further discussed at the end of this review. Once arriving at the synapse, Connexin vesicles must undergo exocytosis to become inserted into the membrane, allowing them to find their partner hemichannels in the neighboring neuron.

Mixed electrical-chemical synapses at single synaptic termini represent another fascinating synaptic organization, and each component appears to be separately organized Pereda, ; Nagy et al. Intra-dendritic application of these SNAP peptides reduced both the electrical and the glutamatergic component of synaptic transmission suggesting the SNARE complex may function in Connexin insertion at the membrane Flores et al.

But again, the composition of Connexin-containing vesicles and its protein constituents remain unknown. Insight into the molecular control of Connexin vesicle trafficking and membrane insertion in neurons will be critical to understanding electrical synapse formation and plasticity.

Further insights into the cell biological framework of electrical synapses will require an identification of the type of vesicles that contain Connexins; the motor, adaptor, and vesicle fusion proteins required for their transport and membrane fusion; and to determine if these features change between electrical synapse formation and plasticity.

The elucidation of the cell biological pathways regulating electrical synapse protein trafficking will reveal whether they are the same or distinct from those of chemical synapses. The fact that electrical and chemical synapses have known distinct protein constituents suggests that at least some components will be unique, but the involvement of both Neurobeachin and SNAP suggests some molecular overlap is also present.

Besides, several trafficking conundrums remain. If Neurobeachin manages the postsynaptic trafficking of Connexins, what guides Connexin to the axon and the presynapse? And, in mammals, given that Cx36 is used within both the axon and the dendrite, how does a neuron resolve specific trafficking to these compartments?

One possibility is that Connexin trafficking depends upon posttranslational modifications to the protein, such as phosphorylation Li et al. Or instead, Neurobeachin and other adaptor proteins may bind a scaffold protein which traffics with Connexin, as is observed with chemical synapse components Tao-Cheng, ; Vukoja et al. Thus, cell-type-specific expression of these scaffolds and adaptors could result in different trafficking patterns and thus different cell biological construction of electrical synapses.

This leads us to our next question: how do electrical synapse scaffolds control electrical synapse development?

To fully appreciate electrical synapse cell biology, we must understand that each electrical synapse is composed of plaques of tens to thousands of gap junction channels Flores et al. These plaques of gap junction channels can take on many different conformations such as wide or thin ribbons and large circular regions of channels, either densely collected or with lace-like holes Nagy et al. Connexins arrive at the synapse as hemichannels that are inserted at the boundaries of existing gap junction plaques where they then find a partner hemichannel in the adjoining neuron.

Over time, the channels migrate towards the center of the plaque where they are endocytosed and sent to the lysosome for degradation Lauf et al. The half-life of Cx36 is estimated to be between 1 and 3 h in vivo , so to maintain the electrical synapse, Cx36 must continuously be made and trafficked to the correct location Flores et al. The known organizational principles of the plaque, and the turnover demand of Connexins, requires complex and ongoing molecular machinery to ensure appropriate development and homeostasis.

But what ensures that the components of the electrical synapse, including Connexins, unite at the same place over time? The gene tjp1 encodes the ZO1 protein, a membrane-associated guanylate kinase MAGUK historically known for its necessity at tight junctions Umeda et al.

Recent work in zebrafish shows that ZO1 is required for electrical synapse formation Marsh et al. For example, PSD95, SAP, and PSD93 are all postsynaptic MAGUK proteins that localize at glutamatergic chemical synapses, make up a majority of proteins in the postsynaptic density, and interact either directly or indirectly with glutamatergic neurotransmitter receptors.

Simultaneous knock-down of these three scaffolds results in smaller postsynaptic densities and a substantial reduction in chemical synapse transmission Chen et al. The unique features that facilitate their shared function at different cell-cell adhesions are exhaustively reviewed elsewhere e.

These domains interact with short ligand sequences, called PDZ binding motifs PBMs , usually found at the C-terminus of the interacting protein. At cell-cell junctions, MAGUK PDZ domains bring together the C-termini of transmembrane or auxiliary proteins to create a carefully organized hub of molecular interactions reviewed in Lee and Zheng, Additionally, these specific interactions can be regulated by posttranslational modifications to either the PDZ or the ligand motif.

Second, in addition to transmembrane proteins, MAGUKs also interact with other scaffolds, regulatory proteins, signaling proteins, the cytoskeleton, and even in some cases the plasma membrane.

This array of interactions allows MAGUKs to aggregate the pieces necessary to create, maintain, and regulate a functional junction. ZO1 is found in complex with numerous proteins found at the electrical synapse including neuronal Connexins Li et al. Thus, ZO1 appears poised to act as the central hub for electrical synapse protein organization and to act as a direct link to the cytoskeleton, yet the details of how it achieves this molecular coordination remain unknown.

Finally, recent studies have shown that many MAGUK proteins are capable of phase separating, creating dynamic and selective non-membrane bound organelles. At chemical synapses, phase separation within the presynaptic active zone clusters synaptic vesicle fusion proteins while at the postsynaptic density phase separation concentrates neurotransmitter receptors reviewed in Chen et al. Thus, it is attractive to propose a model of electrical synapse formation led by ZO1 phase separation which provides a local, specialized domain to capture Connexins and other molecular machinery through both direct and indirect interactions.

This presents an exciting new avenue for future exploration. As Connexins are rapidly turned over throughout the life of the electrical synapse, ZO1 stabilizes them, aggregates necessary regulatory proteins such as kinases, and links the structure to the cytoskeleton.

The emerging evidence suggests ZO1 acts as a master organizer of electrical synapses once it is recruited to the site of the future electrical synapse. This, however, leads us to the question: what tells ZO1 where the electrical synapse should be? Although it is possible that site specification initially occurs via extracellularly secreted signals, we know that synaptic initiation and maintenance requires cell adhesion molecules CAMs. These membrane-spanning proteins have extracellular domains allowing for intercellular interactions with CAMs on an opposing cell.

Additionally, they have intracellular domains that interact with the cytoskeleton, scaffolds, and other proteins that can trigger signaling cascades and the recruitment of other molecules.

Thus, it is highly likely that neurons use CAMs to choose the right place and the right time to create an electrical synapse. Could the Connexin proteins act as the CAM for electrical synaptogenesis?

Connexins are indeed CAMs, and, in certain circumstances such as radial migration of neurons in the mouse cortex, the adhesive properties appear to be more important than the channel itself Elias et al. So it is tempting to question if Connexins coordinate the recruitment of ZO1 and other required proteins to the electrical synapse. The gap junction channel as director of synapse formation appears to be the case in the leech, where the diversity of gap junction forming Innexin proteins drives the site-specific formation of electrical synapses Baker and Macagno, However, in vertebrates, which use Connexins for their gap junctions, this may not be the case.

In Cx36 mutant mice that lack many neuronal gap junctions, electron microscopic analysis of the stereotyped dendro-dendritic electrical connections between olivary neurons found recognizable intercellular junctions still formed, but they lacked the classic electron-dense, gap junction morphology De Zeeuw et al.

A similar conclusion was found using immunohistochemistry at the MesV nucleus in Cx36 null mice, where the stereotyped electrical synapse lacked neuronal Connexin staining, yet ZO1 was still localized to the putative electrical synaptic sites Nagy and Lynn, Taken together, these results suggest that electrical synapses are specified by mechanisms other than Connexins, yet the nature of the signal remains unknown.

So what are the CAMs that specify electrical synapse sites? Vertebrate genomes contain thousands of genes that encode CAMs Zhong et al. Yet particular CAMs, such as the Nectins, may be the key as they play a critical role in establishing initial cell-cell adhesions and are known for their instructive role in adherens junction and tight junction formation in epithelia.

At these locations, they precede the cadherin-based or claudin-based adhesions that are recruited later to these sites. Nectins build up a macromolecular complex by interacting with Afadin, an intracellular scaffold that directly interfaces with the actin cytoskeleton and other important scaffolds, such as alpha-catenin and ZO1, required for adherens junction and tight junction formation respectively Yamada et al.

The effects on electrical synapses have not been assessed. The relationship between Nectins and Afadins is likely cell type-specific, but these results support that, much like at tight junctions, these complexes form initial adhesions that lay a foundation for cadherin recruitment to the synaptic site.

But are Nectins responsible for specifying the locations of electrical synapses? Moreover, Cx36 co-immunoprecipitates with Afadin in both whole-brain and retinal homogenates Li et al. Adjacent to electrical synapses, Afadin is also present at adherens junctions where it colocalizes with Nectin and N-cadherin Li et al.

How specification proceeds to differentiate between these future structures to guide specific molecular complex formation or whether these are causally required for formation remains unclear. Alternatively, electrical synapses may use different complements of CAMs in their formation and maintenance, and to potentiate their functional plasticity. Chemical synapses use a multitude of synaptic CAMs not only to specify separate synaptic types e.

However, attempting to elucidate the requirement of these CAMs in vivo is difficult due to the pleiotropic nature of these proteins and their use at many cellular junctions.

So how can the electrical synapse CAMs be identified and studied? For the field, identifying the CAMs that specify the temporal and spatial electrical synapse dynamics is an essential hurdle that needs to be overcome to move forward in understanding the cell biology of the electrical synapse. Here we have explored several critical open questions surrounding the cell biology of the electrical synapse. Filling these gaps in knowledge will greatly impact our understanding of the development and homeostasis of electrical synapses and will provide new frontiers in regards to the etiology of neurological disorders.

Numerous human disorders are characterized by the loss of gap junction channels, and they span tissues including the skin, heart, joints, teeth, and immune system, to name just a few Jongsma and Wilders, ; van Steensel, ; Kleopa and Scherer, ; Laird, , ; Wong et al.

Indeed, the leading cause of deafness is due to the loss of Connexins expressed in the ear, which is currently, and extremely controversially, earmarked for a possible human CRISPR trial Batissoco et al. These pathologies seemingly emerge from the disruption of wide-ranging gap junction roles within cell proliferation and differentiation, morphogenesis, cell migration, growth control, and many other cell biological processes McGonnell et al. Similar disruptions are mirrored in zebrafish, where elimination of Cx36 homologs results in delayed responses to threatening stimuli and motor coordination defects Miller et al.

These behavioral defects in animal models lacking a broad class of electrical synapses are exactly what the field of neurodevelopment would expect for genes linked to disease phenotypes Mas et al. Namely, that many disorders of neurodevelopment result not in large effects with gross dysfunction, but instead are comprised of subtle molecular differences that slightly shift the functional outcomes.

Indeed, electrical synapse disruptions are proposed to contribute to the etiology of disorders such as autism Welsh et al. However, Connexin loss is not yet a well-appreciated contributor to such disorders.

We think it is likely that the growing awareness and attention electrical synapses are receiving in neural circuit formation, function, and behavior will bring to light their links to a large set of neurodevelopmental disorders.

In this review, we have made the case that Connexins are not the full story in considering the form and function of the electrical synapse. Indeed, our work on Neurobeachin, which itself is linked with both autism and epilepsy in human patients, suggests that as we begin to understand the totality of electrical synapse formation, how these structures are related to disorders of neural function will become ever more apparent.

Therefore, we fundamentally need to expand our understanding of the cell biological mechanisms that develop, maintain, and regulate electrical synapses. And we need to improve our knowledge of the mechanistic relationship between electrical and chemical synapse formation to clarify the contributions of each synapse type to development and adult neural circuit function.

In conclusion, we predict that the continuing studies of electrical synapse structure and function will provide a new framework for understanding fundamental mechanisms of brain structure and function as well as the etiology of the disease.

All authors contributed to manuscript revision, read and approved the submitted version. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abascal, F. Evolutionary analyses of gap junction protein families.

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