Active zone structure-function relationships at the neuromuscular junction

The Mechanism of α2 adrenoreceptor-dependent Modulation of Neurotransmitter Release at the Neuromuscular Junctions

α2-Adrenoreceptors (ARs) are main Gi-protein coupled autoreceptors in sympathetic nerve terminals and targets for dexmedetomidine (DEX), a widely used sedative. We hypothesize that α2-ARs are also potent regulators of neuromuscular transmission via G protein-gated inwardly rectifying potassium (GIRK) channels. Using extracellular microelectrode recording of postsynaptic potentials, we found DEX-induced inhibition of spontaneous and evoked neurotransmitter release as well as desynchronization of evoked exocytotic events in the mouse diaphragm neuromuscular junction. These effects were suppressed by SKF-86,466, a selective α2-AR antagonist. An activator of GIRK channels ML297 had the same effects on neurotransmitter release as DEX. By contrast, inhibition of GIRK channels with tertiapin-Q prevented the action of DEX on evoked neurotransmitter release, but not on spontaneous exocytosis. The synaptic vesicle exocytosis is strongly dependent on Ca2+ influx through voltage-gated Ca2+ channels (VGCCs), which can be negatively regulated via α2-AR - GIRK channel axis. Indeed, inhibition of P/Q-, L-, N- or R-type VGCCs prevented the inhibitory action of DEX on evoked neurotransmitter release; antagonists of P/Q- and N-type channels also suppressed the DEX-mediated desynchronization of evoked exocytotic events. Furthermore, inhibition of P/Q-, L- or N-type VGCCs precluded the frequency decrease of spontaneous exocytosis upon DEX application. Thus, α2-ARs acting via GIRK channels and VGCCs (mainly, P/Q- and N-types) exert inhibitory effect on the neuromuscular communication by attenuating and desynchronizing evoked exocytosis. In addition, α2-ARs can suppress spontaneous exocytosis through GIRK channel-independent, but VGCC-dependent pathway.

Three-dimensional in vitro models of neuromuscular tissue

Skeletal muscle is a dynamic tissue in which homeostasis and function are guaranteed by a very defined three-dimensional organization of myofibers in respect to other non-muscular components, including the extracellular matrix and the nervous network. In particular, communication between myofibers and the nervous system is essential for the overall correct development and function of the skeletal muscle. A wide range of chronic, acute and genetic-based human pathologies that lead to the alteration of muscle function are associated with modified preservation of the fine interaction between motor neurons and myofibers at the neuromuscular junction. Recent advancements in the development of in vitro models for human skeletal muscle have shown that three-dimensionality and integration of multiple cell types are both key parameters required to unveil pathophysiological relevant phenotypes. Here, we describe recent achievement reached in skeletal muscle modeling which used biomaterials for the generation of three-dimensional constructs of myotubes integrated with motor neurons.

Adaptive Remodeling of the Neuromuscular Junction with Aging

Aging is associated with gradual degeneration, in mass and function, of the neuromuscular system. This process, referred to as “sarcopenia”, is considered a disease by itself, and it has been linked to a number of other serious maladies such as type II diabetes, osteoporosis, arthritis, cardiovascular disease, and even dementia. While the molecular causes of sarcopenia remain to be fully elucidated, recent findings have implicated the neuromuscular junction (NMJ) as being an important locus in the development and progression of that malady. This synapse, which connects motor neurons to the muscle fibers that they innervate, has been found to degenerate with age, contributing both to senescent-related declines in muscle mass and function. The NMJ also shows plasticity in response to a number of neuromuscular diseases such as amyotrophic lateral sclerosis (ALS) and Lambert-Eaton myasthenic syndrome (LEMS). Here, the structural and functional degradation of the NMJ associated with aging and disease is described, along with the measures that might be taken to effectively mitigate, if not fully prevent, that degeneration.

Hypothesis Relating the Structure, Biochemistry and Function of Active Zone Material Macromolecules at a Neuromuscular Junction

This report integrates knowledge of in situ macromolecular structures and synaptic protein biochemistry to propose a unified hypothesis for the regulation of certain vesicle trafficking events (i.e., docking, priming, Ca²⁺-triggering, and membrane fusion) that lead to neurotransmitter secretion from specialized “active zones” of presynaptic axon terminals. Advancements in electron tomography, to image tissue sections in 3D at nanometer scale resolution, have led to structural characterizations of a network of different classes of macromolecules at the active zone, called “Active Zone Material’. At frog neuromuscular junctions, the classes of Active Zone Material macromolecules “top-masts”, “booms”, “spars”, “ribs” and “pins” direct synaptic vesicle docking while “pins”, “ribs” and “pegs” regulate priming to influence Ca²⁺-triggering and membrane fusion. Other classes, “beams”, “steps”, “masts”, and “synaptic vesicle luminal filaments’ likely help organize and maintain the structural integrity of active zones. Extensive studies on the biochemistry that regulates secretion have led to comprehensive characterizations of the many conserved proteins universally involved in these trafficking events. Here, a hypothesis including a partial proteomic atlas of Active Zone Material is presented which considers the common roles, binding partners, physical features/structure, and relative positioning in the axon terminal of both the proteins and classes of macromolecules involved in the vesicle trafficking events. The hypothesis designates voltage-gated Ca²⁺ channels and Ca²⁺-gated K⁺ channels to ribs and pegs that are connected to macromolecules that span the presynaptic membrane at the active zone. SNARE proteins (Syntaxin, SNAP25, and Synaptobrevin), SNARE-interacting proteins Synaptotagmin, Munc13, Munc18, Complexin, and NSF are designated to ribs and/or pins. Rab3A and Rabphillin-3A are designated to top-masts and/or booms and/or spars. RIM, Bassoon, and Piccolo are designated to beams, steps, masts, ribs, spars, booms, and top-masts. Spectrin is designated to beams. Lastly, the luminal portions of SV2 are thought to form the bulk of the observed synaptic vesicle luminal filaments. The goal here is to help direct future studies that aim to bridge Active Zone Material structure, biochemistry, and function to ultimately determine how it regulates the trafficking events in vivo that lead to neurotransmitter secretion.

Homeostatic plasticity induced by increased acetylcholine release at the mouse neuromuscular junction

At the neuromuscular junction (NMJ), changes to the size of the postsynaptic potential induce homeostatic compensation. At the Drosophila NMJ, increased glutamate release causes a compensatory decrease in quantal content, but it is unknown if this mechanism operates at the cholinergic mammalian NMJ. We addressed this question by recording endplate potentials (EPP) and muscle contraction in 3-month and 24-month ChAT-ChR2-EYFP mice that overexpress vesicular acetylcholine transporter and release more acetylcholine per vesicle. At 3 months, the quantal content of EPPs from ChAT-ChR2-EYFP mice were not different from WT controls, however tetanic depression was greater, and quantal size during high-frequency stimulation and the size of the readily releasable pool (RRP) were decreased. At 24 months of age, quantal content was reduced in ChAT-ChR2-EYFP mice, which normalized synaptic depression despite smaller RRP. The effect of pancuronium on indirect evoked muscle twitch was not different between groups. These results indicate that an increase in the amount of acetylcholine per vesicle induces two distinct age-dependent homeostatic mechanisms compensating excessive acetylcholine release.

Super-resolution microscopy for analyzing neuromuscular junctions and synapses

Super-resolution microscopy techniques offer subdiffraction limited resolution that is two- to ten-fold improved compared to that offered by conventional confocal microscopy. This breakthrough in resolution for light microscopy has contributed to new findings in neuroscience and synapse biology. This review will focus on the Structured Illumination Microscopy (SIM), Stimulated emission depletion (STED) microscopy, and Stochastic optical reconstruction microscopy (STORM) / Single molecule localization microscopy (SMLM) techniques and compare them for the better understanding of their differences and their suitability for the analysis of synapse biology. In addition, we will discuss a few practical aspects of these microscopic techniques, including resolution, image acquisition speed, multicolor capability, and other advantages and disadvantages. Tips for the improvement of microscopy will be introduced; for example, information resources for recommended dyes, the limitations of multicolor analysis, and capabilities for live imaging. In addition, we will summarize how super-resolution microscopy has been used for analyses of neuromuscular junctions and synapses.

Homeostatic control of Drosophila neuromuscular junction function

The ability to adapt to changing internal and external conditions is a key feature of biological systems. Homeostasis refers to a regulatory process that stabilizes dynamic systems to counteract perturbations. In the nervous system, homeostatic mechanisms control neuronal excitability, neurotransmitter release, neurotransmitter receptors, and neural circuit function. The neuromuscular junction (NMJ) of Drosophila melanogaster has provided a wealth of molecular information about how synapses implement homeostatic forms of synaptic plasticity, with a focus on the transsynaptic, homeostatic modulation of neurotransmitter release. This review examines some of the recent findings from the Drosophila NMJ and highlights questions the field will ponder in coming years.

Zebrafish neuromuscular junction: The power of N

In the early 1950s, Katz and his colleagues capitalized on the newly developed intracellular microelectrode recording technique to investigate synaptic transmission. For study they chose frog neuromuscular junction (NMJ), which was ideally suited due to the accessibility and large size of the muscle cells. Paradoxically, the large size precluded the use of next generation patch clamp technology. Consequently, electrophysiological study of synaptic function shifted to small central synapses made amenable by patch clamp. Recently, however, the unique features offered by zebrafish have rekindled interest in the NMJ as a model for electrophysiological study of synaptic transmission. The small muscle size and synaptic simplicity provide the singular opportunity to perform in vivo spinal motoneuron-target muscle patch clamp recordings. Additional incentive is provided by zebrafish lines harboring mutations in key synaptic proteins, many of which are embryonic lethal in mammals, but all of which are able to survive well past synapse maturation in zebrafish. This mini-review will highlight features that set zebrafish NMJs apart from traditional NMJs. We also draw into focus findings that offer the promise of identifying features that define release sites, which serve to set the upper limit of transmitter release. Since its conception several candidates representing release sites have been proposed, most of which are based on distinctions among vesicle pools in their state of readiness for release. However, models based on distinctions among vesicles have become enormously complicated and none adequately account for setting an upper limit for exocytosis in response to an action potential (AP). Specifically, findings from zebrafish NMJ point to an alternative model, positing that elements other than vesicles per se set the upper limits of release.

Lambert-Eaton Myasthenic syndrome: early diagnosis is key

Lambert-Eaton myasthenic syndrome (LEMS) is an uncommon disorder of neuromuscular transmission with distinctive pathophysiological, clinical, electrophysiological and laboratory features. There are two forms of LEMS. The paraneoplastic (P-LEMS) form is associated with a malignant tumor that is most frequently a small cell lung carcinoma (SCLC), and the autoimmune (A-LEMS) form is often related to other dysimmune diseases. Approximately 90% of LEMS patients present antibodies against presynaptic membrane P/Q-type voltage-gated calcium channels (VGCC). These antibodies are directly implicated in the pathophysiology of the disorder, provoke reduced acetylcholine (ACh) at the nerve terminal and consequently lead to muscle weakness. LEMS is clinically characterized by proximal muscle weakness, autonomic dysfunction and areflexia. In clinically suspected cases, diagnoses are confirmed by serological and electrodiagnostic tests. The detection of P/Q-type VGCC antibodies is supportive when there is clinical suspicion but should be carefully interpreted in the absence of characteristic clinical or electrodiagnostic features. Typical electrodiagnostic findings (ie, reduced compound motor action potentials (CMAPs), significant decrements in the responses to low frequency stimulation and incremental responses after brief exercise or high-frequency stimulation) reflect the existence of a presynaptic transmission defect and are key confirmatory criteria. Diagnosis requires a high level of awareness and necessitates the initiation of a prompt screening and surveillance process to detect and treat malignant tumors. In clinically affected patients without cancer and after cancer treatment, symptomatic treatment with 3,4-diaminopyridine or immunosuppressive agents can significantly improve neurologic symptoms and the quality of life. We present a detailed review of LEMS with special emphasis on the pathophysiological mechanisms, clinical manifestation and diagnostic procedure.