Mechanisms to separately regulate synaptic vesicle release and recycling


Date:
July 19, 2021
Source:
Johannes Gutenberg Universitaet Mainz
Summary:
Interactions of two voltage-gated calcium channels and a pump enable
separate control of exocytosis and endocytosis at chemical synapses.



FULL STORY ========================================================================== Chemical synapses transmit information within the nervous system. When
a presynaptic cell is electrically excited, synaptic vesicles fuse
with the presynaptic membrane causing messenger substances within the
vesicles to be released into the synaptic cleft. These then bind to
receptors in the postsynaptic cell where they trigger an electrical
signal once again. The temporal and spatial sequence of the incoming
signals determines how information is processed and transmitted in the
brain. In order to sustain their function in the long term, chemical
synapses need to recycle synaptic vesicles to make them available for
renewed signal transmission.


========================================================================== Professor Carsten Duch and Professor Martin Heine and their respective
research groups at Johannes Gutenberg University Mainz (JGU) are
investigating how the release and recycling of synaptic vesicles are coordinated. "Exocytosis and endocytosis rates at chemical synapses need
to be coordinated to achieve reliable signal transmission in the brain,"
the biologists explained. Together with Dr. Ulrich Thomas, group leader
at the Leibniz Institute for Neurobiology in Magdeburg, Duch and Heine
have revealed in a PNAS paper how spatiotemporally separated presynaptic calcium signals independently regulate exocytosis and endocytosis of
synaptic vesicles, i.e., their release and recycling.

Co-existence of different types of voltage-gated calcium channels at
the presynapse At chemical synapses, incoming electrical impulses are
converted into chemical signals and relayed on to the next cell. The
process entails calcium ions first flowing through voltage-dependent
membrane channels into the presynapse, i.e., the upstream nerve cell
that transmits the signal to the postsynaptic cell.

This calcium influx is tightly constrained in both time and space and
results in exocytosis of synaptic vesicles from a specialized vesicle reservoir.

Presynaptic calcium signals also regulate synaptic vesicle recycling,
but here the temporal and spatial requirements are different. One
unresolved question is how presynaptic electrical activity can lead
to calcium signals with different temporal and spatial profiles in the presynaptic terminal.

By combining genetic modifications with electrophysiological and optophysiological measurements at the neuromuscular synapse of the
Drosophila melanogaster genetic model organism, the research team was
able to demonstrate that the presynapse harbors two different types
of voltage-gated calcium channels, Cav2 and Cav1. These, however, were
found to be spatially segregated.

Both types of channels open when electrical signals arrive, but only Cav2 channels, which reside in active zones of the presynapse, are required for exocytosis of synaptic vesicles. Cav1 channels are situated outside active zones and augment endocytosis of synaptic vesicles via activity-dependent calcium influx. Thus, knockdown of Cav2 by means of genetic manipulation prevents synaptic transmission, whereas knockdown of Cav1 decreases
the rate of synaptic vesicle endocytosis, thereby enhancing synaptic
depression during sustained activity. This is how calcium signals mediated
by two different populations of largely independent voltage-gated calcium channels regulate two essential functions of the presynapse in response to neuronal activity, namely the release and recycling of synaptic vesicles.

Functional separation of Cav1 and Cav2 by means of a calcium pump A
key question was how calcium signals through different channels could be functionally separated at the nanometer scale of the presynaptic terminal, because calcium after all is a highly diffusible intracellular messenger.

According to the researchers, different vital functions of calcium signals through Cav1 and Cav2 channels are separated by a membrane-anchored
calcium buffer. Cav2 channels are found within presynaptic active zones
at distances of 70 to 140 nanometers from readily releasable synaptic
vesicles. This distinct localization of Cav2 results in the emergence
of temporally and spatially tightly regulated calcium signals within
so-called nano-domains during presynaptic electrical activity, and these
are essential for temporally precise synaptic transmission. Cav1 localizes around active zones, in theory allowing calcium influx simultaneously
through both types of channels to result in mixed signals with no
measurable delay. However, mixed signals of this type are prevented by the plasma membrane calcium pump (PMCA). PMCA is located outside active zones
and isolates them from the dynamic regulation of endocytosis achieved
by Cav1-mediated calcium influx. Because Cav1, Cav2, and PMCA have
been identified also at central synapses in the brains of mammals, these proteins may represent a conserved functional triad for separate activity- dependent regulation of exocytosis and endocytosis of synaptic vesicles.

Calcium channels and the regulation of essential synaptic functions In the future, Duch's and Heine's research groups will continue to explore the interactions of calcium channels and their associated molecules at the presynapse. Calcium signals in the presynaptic terminal regulate other essential synaptic functions beyond exocytosis and endocytosis. These
include the regulation of synaptic vesicle movements between distinct specialized reservoirs as well as the control of fixed synaptic
transmission strengths, which are restored by compensatory mechanisms
after perturbation. This homeostatic synaptic plasticity is essential for reliably processing information in the brain. As part of a project within Collaborative Research Center 1080 on Molecular and Cellular Mechanisms
in Neural Homeostasis, Duch's and Heine's groups are investigating how spatiotemporally separated presynaptic calcium signals independently
control exocytosis and endocytosis, the transport of vesicles between
different reservoirs, and synaptic homeostasis. "Calcium signals are
extremely well suited to precisely adapt a variety of vital synaptic
functions to differing neuronal activities, but we are only just starting
to work out the mechanisms that independently regulate these functions,"
Duch and Heine commented on their neurobiology research.

========================================================================== Story Source: Materials provided by
Johannes_Gutenberg_Universitaet_Mainz. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Niklas Krick, Stefanie Ryglewski, Aylin Pichler, Arthur Bikbaev,
Torsten
Go"tz, Oliver Kobler, Martin Heine, Ulrich Thomas, Carsten Duch.

Separation of presynaptic Cav2 and Cav1 channel function in
synaptic vesicle exo- and endocytosis by the membrane anchored Ca2+
pump PMCA.

Proceedings of the National Academy of Sciences, 2021; 118 (28):
e2106621118 DOI: 10.1073/pnas.2106621118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210719103100.htm

--- up 10 weeks, 3 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)