Complexin: Does it deserve its name?

Knockout and other perturbations of complexins have provided important insights and elicited controversies about their role in neurotransmitter release. New work by Yang et al. in this issue of Neuron adds important detail and complexity to existing concepts—particularly on the nature of a Ca2+-dependent complexin-synaptotagmin switch for the triggering of exocytosis. But it also provokes thoughts about alternative interpretations, which might result in a simpler model of complexin function

Extensions and block decompositions for finite-dimensional representations of equivariant map algebras

Suppose a finite group acts on a scheme $X$ and a finite-dimensional Lie algebra $\mathfrak{g}$. The associated equivariant map algebra is the Lie algebra of equivariant regular maps from $X$ to $\mathfrak{g}$. The irreducible finite-dimensional representations of these algebras were classified in previous work with P. Senesi, where it was shown that they are all tensor products of evaluation representations and one-dimensional representations. In the current paper, we describe the extensions between irreducible finite-dimensional representations of an equivariant map algebra in the case that $X$ is an affine scheme of finite type and $\mathfrak{g}$ is reductive. This allows us to also describe explicitly the blocks of the category of finite-dimensional representations in terms of spectral characters, whose definition we extend to this general setting. Applying our results to the case of generalized current algebras (the case where the group acting is trivial), we recover known results but with very different proofs. For (twisted) loop algebras, we recover known results on block decompositions (again with very different proofs) and new explicit formulas for extensions. Finally, specializing our results to the case of (twisted) multiloop algebras and generalized Onsager algebras yields previously unknown results on both extensions and block decompositions.Comment: 41 pages; v2: minor corrections, formatting changed to match published versio

The patch-clamp technique in the study of secretion.

One of the basic cellular functions of virtually every cell type is the exocytotic release of molecules synthesized, stored and packaged into intracellular vesicles or granules. Over decades much effort has been concentrated on elucidating the chain of events leading to exocytosis. Unfortunately, the nature of the process that ultimately induces membrane fusion is not known, nor has it been established definitively whether or not the final steps in the secretory cascade are identical in different cells. Although the fusion between vesicle and plasma membrane has been neatly documented by electron micrographs, it was only recently that the technique of time-resolved membrane capacitance measurement has provided a more detailed insight into mechanistic aspects of exocytosis, both in terms of the fusion event and the steps involved in stimulus-secretion coupling

Non-negative Matrix Factorization as a Tool to Distinguish Between Synaptic Vesicles in Different Functional States

Synaptic vesicles (SVs) undergo multiple steps of functional maturation (priming) before being fusion competent. We present an analysis technique, which decomposes the time course of quantal release during repetitive stimulation as a sum of contributions of SVs, which existed in distinct functional states prior to stimulation. Such states may represent different degrees of maturation in priming or relate to different molecular composition of the release apparatus. We apply the method to rat calyx of Held synapses. These synapses display a high degree of variability, both with respect to synaptic strength and short-term plasticity during high-frequency stimulus trains. The method successfully describes time courses of quantal release at individual synapses as linear combinations of three components, representing contributions from functionally distinct SV subpools, with variability among synapses largely covered by differences in subpool sizes. Assuming that SVs transit in sequence through at least two priming steps before being released by an action potential (AP) we interpret the components as representing SVs which had been ‘fully primed’, ‘incompletely primed’ or undocked prior to stimulation. Given these assumptions, the analysis reports an initial release probability of 0.43 for SVs that were fully primed prior to stimulation. Release probability of that component was found to increase during high-frequency stimulation, leading to rapid depletion of that subpool. SVs that were incompletely primed at rest rapidly obtain fusion-competence during repetitive stimulation and contribute the majority of release after 3–5 stimuli

Calmodulin mediates rapid recruitment of fast-releasing synaptic vesicles at a calyx-type synapse.

In many synapses, depletion and recruitment of releasable synaptic vesicles contribute to use-dependent synaptic depression and recovery. Recently it has been shown that high- frequency presynaptic stimulation enhances recovery from depression, which may be mediated by Ca2+. We addressed this issue by measuring quantal release rates at the calyx of Held synapse and found that transmission is mediated by a heterogeneous population of vesicles, with one subset releasing rapidly and recovering slowly and another one releasing reluctantly and recovering rapidly. Ca2+ promotes refilling of the rapidly releasing synaptic vesicle pool and calmodulin inhibitors block this effect. We propose that calmodulin- dependent refilling supports recovery from synaptic depression during high-frequency trains in concert with rapid recovery of the slowly releasing vesicles

Synaptotagmin has an essential function in synaptic vesicle positioning for synchronous release in addition to its role as a calcium sensor.

SummaryA multitude of synaptic proteins interact at the active zones of nerve terminals to achieve the high temporal precision of neurotransmitter release in synchrony with action potentials. Though synaptotagmin has been recognized as the Ca2+ sensor for synchronous release, it may have additional roles of action. We address this question at the calyx of Held, a giant presynaptic terminal, that allows biophysical dissection of multiple roles of molecules in synaptic transmission. Using high-level expression recombinant adenoviruses, in conjunction with a stereotactic surgery in postnatal day 1 rats, we overcame the previous inability to moleculary perturb the calyx by overexpression of a mutated synaptotagmin. We report that this mutation leaves intrinsic Ca2+ sensitivity of vesicles intact while it destabilizes the readily releasable pool of vesicles and loosens the tight coupling between Ca2+ influx and release, most likely by interfering with the correct positioning of vesicles with respect to Ca2+ channels

Multiple roles of calcium ions in the regulation of neurotransmitter release.

The intracellular calcium concentration ([Ca2+]) has important roles in the triggering of neurotransmitter release and the regulation of short-term plasticity (STP). Transmitter release is initiated by quite high concentrations within microdomains, while short-term facilitation is strongly influenced by the global buildup of “residual calcium.” A global rise in [Ca2+] also accelerates the recruitment of release-ready vesicles, thereby controlling the degree of short-term depression (STD) during sustained activity, as well as the recovery of the vesicle pool in periods of rest. We survey data that lead us to propose two distinct roles of [Ca2+] in vesicle recruitment: one accelerating “molecular priming” (vesicle docking and the buildup of a release machinery), the other promoting the tight coupling between releasable vesicles and Ca2+ channels. Such coupling is essential for rendering vesicles sensitive to short [Ca2+] transients, generated during action potentials