Acute complexin knockout abates spontaneous and evoked transmitter release

SNARE-mediated synaptic vesicle (SV) fusion is controlled by multiple regulatory proteins that determine neurotransmitter release efficiency. Complexins are essential SNARE regulators whose mode of action is unclear, as available evidence indicates positive SV fusion facilitation and negative 'fusion clamp'-like activities, with the latter occurring only in certain contexts. Because these contradictory findings likely originate in part from different experimental perturbation strategies, we attempted to resolve them by examining a conditional complexin-knockout mouse line as the most stringent genetic perturbation model available. We found that acute complexin loss after synaptogenesis in autaptic and mass-cultured hippocampal neurons reduces SV fusion probability and thus abates the rates of spontaneous, synchronous, asynchronous, and delayed transmitter release but does not affect SV priming or cause 'unclamping' of spontaneous SV fusion. Thus, complexins act as facilitators of SV fusion but are dispensable for 'fusion clamping' in mammalian forebrain neurons

Munc13-1 is a Ca2+-phospholipid-dependent vesicle priming hub that shapes synaptic short-term plasticity and enables sustained neurotransmission

During ongoing presynaptic action potential (AP) firing, transmitter release is limited by the availability of release-ready synaptic vesicles (SVs). The rate of SV recruitment (SVR) to release sites is strongly upregulated at high AP frequencies to balance SV consumption. We show that Munc13-1-an essential SV priming protein-regulates SVR via a Ca2+-phospholipid-dependent mechanism. Using knockin mouse lines with point mutations in the Ca2+-phospholipid-binding C2B domain of Munc13-1, we demonstrate that abolishing Ca2+-phospholipid binding increases synaptic depression, slows recovery of synaptic strength after SV pool depletion, and reduces temporal fidelity of synaptic transmission, while increased Ca2+-phospholipid binding has the opposite effects. Thus, Ca2+-phospholipid binding to the Munc13-1-C2B domain accelerates SVR, reduces short-term synaptic depression, and increases the endurance and temporal fidelity of neurotransmission, demonstrating that Munc13-1 is a core vesicle priming hub that adjusts SV re-supply to demand

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required

Acute Complexin Knockout Abates Spontaneous and Evoked Transmitter Release

Summary: SNARE-mediated synaptic vesicle (SV) fusion is controlled by multiple regulatory proteins that determine neurotransmitter release efficiency. Complexins are essential SNARE regulators whose mode of action is unclear, as available evidence indicates positive SV fusion facilitation and negative “fusion clamp”-like activities, with the latter occurring only in certain contexts. Because these contradictory findings likely originate in part from different experimental perturbation strategies, we attempted to resolve them by examining a conditional complexin-knockout mouse line as the most stringent genetic perturbation model available. We found that acute complexin loss after synaptogenesis in autaptic and mass-cultured hippocampal neurons reduces SV fusion probability and thus abates the rates of spontaneous, synchronous, asynchronous, and delayed transmitter release but does not affect SV priming or cause “unclamping” of spontaneous SV fusion. Thus, complexins act as facilitators of SV fusion but are dispensable for “fusion clamping” in mammalian forebrain neurons. : Complexins are thought to either promote synaptic vesicle fusion or act as “fusion clamps.” López-Murcia et al. show that acute genetic complexin deletion reduces the rates of all forms of transmitter release in forebrain neurons without affecting vesicle priming. Thus, complexins are facilitators of vesicle fusion and dispensable for “fusion clamping.” Keywords: complexin, synaptic vesicle, fusion, hippocampal neurons, synaptic transmissio

Optimizing Synaptic Architecture and Efficiency for High-Frequency Transmission

AbstractBursts of neuronal activity are transmitted more effectively as synapses mature. However, the mechanisms that control synaptic efficiency during development are poorly understood. Here, we study postnatal changes in synaptic ultrastructure and exocytosis in a calyx-type nerve terminal. Vesicle pool size, exocytotic efficiency (amount of exocytosis per Ca influx), Ca current facilitation, and the number of active zones (AZs) increased with age, whereas AZ area, number of docked vesicles per AZ, and release probability decreased with age. These changes led to AZs that are less prone to multivesicular release, resulting in reduced AMPA receptor saturation and desensitization. A greater multiplicity of small AZs with few docked vesicles, a larger pool of releasable vesicles, and a higher efficiency of release thus promote prolonged high-frequency firing in mature synapses

Non-monotonic response in a Fitzhugh-Nagumo neuron receiving periodic input via a static synapse.

<p>(a) Input-output response exhibits dominant <i>n:1</i>-locking interrupted by broad transition regions (b), magnified from (a). Several locking ratios <i>n:m</i> are indicated. In the transition regions, periodic, <i>n:m</i>-locked as well as nonperiodic, irregular dynamics arise. (c,d) Membrane potential dynamics (c) in the <i>4:1</i>-locking region and (d) in the irregular regime. The model parameters were <i>a</i> = 0.139, <i>b</i> = 2.54, <i>c</i> = 0.5, <i>μ</i>′ = 125 and <i>K</i>(<i>t</i>) = 2(exp(−t)−exp(−2<i>t</i>)).</p

Non-monotonic response to regular input spike sequences: increasing the input spike frequency may increase but also decrease the output spike frequency.

<p>Bottom panel: input spike frequency that slowly increases ten-fold. Top three panels: output spike responses (LIF: leaky integrate-and-fire neuron with depressive synapse, FHN: Fitzhugh-Nagumo and HH: Hodgkin-Huxley neuron, both with static synapses). Time is rescaled so that all three data sets fit in this Figure. For details of models see equations (<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004002#pcbi.1004002.e001" target="_blank">1</a>)–(<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004002#pcbi.1004002.e002" target="_blank">2</a>) for LIF, equations (<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004002#pcbi.1004002.e012" target="_blank">12</a>)–(<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004002#pcbi.1004002.e013" target="_blank">13</a>) for FHN and equations (<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004002#pcbi.1004002.e014" target="_blank">14</a>)–(<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004002#pcbi.1004002.e017" target="_blank">17</a>) for HH.</p