Origin of quantal size variation and high-frequency miniature postsynaptic currents at the Caenorhabditis elegans neuromuscular junction

Rebalancing the motor circuit restores movement in a Caenorhabditis elegans model for TDP-43 toxicity

Amyotrophic lateral sclerosis can be caused by abnormal accumulation of TAR DNA-binding protein 43 (TDP-43) in the cytoplasm of neurons. Here, we use a C. elegans model for TDP-43-induced toxicity to identify the biological mechanisms that lead to disease-related phenotypes. By applying deep behavioral phenotyping and subsequent dissection of the neuromuscular circuit, we show that TDP-43 worms have profound defects in GABA neurons. Moreover, acetylcholine neurons appear functionally silenced. Enhancing functional output of repressed acetylcholine neurons at the level of, among others, G-protein-coupled receptors restores neurotransmission, but inefficiently rescues locomotion. Rebalancing the excitatory-to-inhibitory ratio in the neuromuscular system by simultaneous stimulation of the affected GABA- and acetylcholine neurons, however, not only synergizes the effects of boosting individual neurotransmitter systems, but instantaneously improves movement. Our results suggest that interventions accounting for the altered connectome may be more efficient in restoring motor function than those solely focusing on diseased neuron populations.

Presynaptic Congenital Myasthenic Syndromes: Understanding Clinical Phenotypes through In vivo Models

Presynaptic congenital myasthenic syndromes (CMS) are a group of genetic disorders affecting the presynaptic side of the neuromuscular junctions (NMJ). They can result from a dysfunction in acetylcholine (ACh) synthesis or recycling, in its packaging into synaptic vesicles, or its subsequent release into the synaptic cleft. Other proteins involved in presynaptic endplate development and maintenance can also be impaired.Presynaptic CMS usually presents during the prenatal or neonatal period, with a severe phenotype including congenital arthrogryposis, developmental delay, and apnoeic crisis. However, milder phenotypes with proximal muscle weakness and good response to treatment have been described. Finally, many presynaptic genes are expressed in the brain, justifying the presence of additional central nervous system symptoms.Several animal models have been developed to study CMS, providing the opportunity to identify disease mechanisms and test treatment options. In this review, we describe presynaptic CMS phenotypes with a focus on in vivo models, to better understand CMS pathophysiology and define new causative genes.

Complexins: small but capable

Despite intensive research, it is still unclear how an immediate and profound acceleration of exocytosis is triggered by appropriate Ca(2+)-stimuli in presynaptic terminals. This is due to the fact that the molecular mechanisms of "docking" and "priming" reactions, which set up secretory vesicles to fuse at millisecond time scale, are extremely hard to study. Yet, driven by a fruitful combination of in vitro and in vivo analyses, our mechanistic understanding of Ca(2+)-triggered vesicle fusion has certainly advanced in the past few years. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. In particular, we will focus on the role of the small regulatory factor complexin whose function in Ca(2+)-dependent exocytosis has been controversially discussed for more than a decade. Special emphasis will also be laid on the functional relationship of complexin and synaptotagmin, as both proteins possibly act as allies and/or antagonists to govern SNARE-mediated exocytosis.

Postsynaptic current bursts instruct action potential firing at a graded synapse

Nematode neurons generally produce graded potentials instead of action potentials (APs). It is unclear how the graded potentials control postsynaptic cells under physiological conditions. Here we show that postsynaptic currents (PSCs) frequently occur in bursts at the neuromuscular junction of C. elegans. Cholinergic bursts concur with facilitated AP firing, elevated cytosolic [Ca²⁺], and contraction of the muscle whereas GABA ergic bursts suppress AP firing. The bursts, distinct from artificially evoked responses, are characterized by a persistent current (the primary component of burst-associated charge transfer)and increased frequency and mean amplitude of PSC events. The persistent current of cholinergic PSC bursts is mostly mediated by levamisole-sensitive acetylcholine receptors, which correlates well with locomotory phenotypes of receptor mutants. Eliminating command interneurons abolishes the bursts whereas mutating SLO-1 K⁺ channel, a potent presynaptic inhibitor of exocytosis, greatly increases the mean burst duration. These observations suggest that motoneurons control muscle by producing PSC bursts.

Exploring the central modulation hypothesis: do ancient memory mechanisms underlie the pathophysiology of trigger points?

A myofascial trigger point (TrP) is a point of focal tenderness, associated with a taut band of muscle fibers, that can develop in any skeletal muscle. TrPs are a common source of pain and motor dysfunction in humans and other vertebrates. There is no universally accepted pathophysiology to explain the etiology, symptomatology and treatment of TrPs. This article reviews and extends the author's previously published hypothesis for the pathophysiology of TrPs, "Trigger Points and Central Modulation-A New Hypothesis." The author proposes that central nervous system-maintained global changes in α-motoneuron function, resulting from sustained plateau depolarization, rather than a local dysfunction of the motor endplate, underlie the pathogenesis of TrPs.

High resolution electrophysiological techniques for the study of calcium-activated exocytosis

BACKGROUND

Neurotransmitters, neuropeptides and hormones are released from secretory vesicles of nerve terminals and neuroendocrine cells by calcium-activated exocytosis. A key step in this process is the formation of a fusion pore between the vesicle membrane and the plasma membrane. Exocytotic fusion leads to an increase in plasma membrane area that can be measured as a proportional increase in plasma membrane capacitance.

SCOPE OF REVIEW

High resolution capacitance measurements in single cells, nerve terminals and small membrane patches have become possible with the development of the patch clamp technique. This review discusses the methods of whole cell patch clamp capacitance measurements and their use in conjunction with voltage clamp pulse stimulation and with stimulation by photorelease of caged calcium. It also discusses patch capacitance measurements for the study of single exocytotic events and fusion pore properties in neuroendocrine cells and nerve terminals.

MAJOR CONCLUSIONS

Capacitance measurements provide high resolution information on the extent and time course of fusion for the characterization of vesicle pools and the kinetics of exocytosis. They allow the characterization of the mode of fusion including distinction of single vesicle full fusion, transient kiss-and-run fusion or multivesicular compound exocytosis. Furthermore, measurement of fusion pore conductances and their dynamic behavior has enabled the characterization of fusion pore properties in a way that resembles the characterization of ion channel function through single channel recordings.

GENERAL SIGNIFICANCE

The combination of patch clamp capacitance measurements with pharmacological and molecular manipulations of exocytosis is emerging as a powerful approach to investigate the molecular mechanisms of calcium-activated exocytotic fusion pore formation. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Genetic dissection of ion currents underlying all-or-none action potentials in C. elegans body-wall muscle cells

Although the neuromuscular system of C. elegans has been studied intensively, little is known about the properties of muscle action potentials (APs). By combining mutant analyses with in vivo electrophysiological recording techniques and Ca2+ imaging, we have established the fundamental properties and molecular determinants of body-wall muscle APs. We show that, unlike mammalian skeletal muscle APs, C. elegans muscle APs occur in spontaneous trains, do not require the function of postsynaptic receptors, and are all-or-none overshooting events, rather than graded potentials as has been previously reported. Furthermore, we show that muscle APs depend on Ca2+ entry through the L-type Ca2+ channel EGL-19 with a contribution from the T-type Ca2+ channel CCA-1. Both the Shaker K+ channel SHK-1 and the Ca2+/Cl−-gated K+ channel SLO-2 play important roles in controlling the speed of membrane repolarization, the amplitude of afterhyperpolarization (AHP) and the pattern of AP firing; SLO-2 is also important in setting the resting membrane potential. Finally, AP-elicited elevations of [Ca2+]i require both EGL-19 and the ryanodine receptor UNC-68. Thus, like mammalian skeletal muscle, C. elegans body-wall myocytes generate all-or-none APs, which evoke Ca2+ release from the sarcoplasmic reticulum (SR), although the specific ion channels used for AP upstroke and repolarization differ.