Synaptotagmin in asynchronous neurotransmitter release and synaptic disease
Date
2018
Authors
Shields, Mallory Catherine, author
Reist, Noreen, advisor
Garrity, Deborah, committee member
Tamkun, Michael, committee member
Tsunoda, Susan, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The majority of cell-to-cell communication relies on the stimulated release of neurotransmitter. Two forms of Ca2+-dependent stimulated release, synchronous and asynchronous, have been identified. Synchronous release is the initial release that occurs within milliseconds of stimulation. Critical for efficient synaptic communication, synchronous release is the dominant form of release at most synapses. Alternatively, asynchronous release occurs over longer time periods, with implications in synaptic plasticity and development. However, its mechanisms are poorly understood. Both synchronous and asynchronous release rely on Ca2+ sensors to confer their distinct characteristics. Synaptotagmin 1 is widely accepted as the Ca2+ sensor for fast, synchronous release, but its role in asynchronous release is unclear. Previous studies have led to the hypothesis that synaptotagmin 1, particularly Ca2+ binding by its C2A domain, is needed to inhibit aberrant asynchronous fusion events. However, recent studies have raised questions regarding the interpretation of the results that led to this conclusion. In chapter 2, I have directly tested the effect of Ca2+ binding by synaptotagmin 1's C2A domain on asynchronous release utilizing an alternant Ca2+-binding mutant. This novel mutation was designed to block Ca2+ binding without introducing the artifacts of the original Ca2+-binding mutation. By investigating asynchronous events in vivo at the Drosophila neuromuscular junction, I found no significant effect on asynchronous release when C2A Ca2+ binding was blocked. Thus, I conclude that Ca2+ binding by synaptotagmin's C2A domain is not needed for regulation of asynchronous release, in contrast to the previous study that inadvertently introduced an artifact described below. To prevent Ca2+ binding, the original aspartate to asparagine mutations (sytD-N) removed some of the negatively-charged residues that coordinate Ca2+. This simultaneously introduced aberrant fusion events, because it also interrupted the electrostatic repulsion between synaptotagmin's negatively-charged C2A Ca2+-binding pocket and the negatively-charged presynaptic membrane which is required to clamp constitutive SNARE-mediated fusion. Previous Reist lab results demonstrate that the sytD-N mutations in the C2A domain are likely behaving as ostensibly constitutively bound Ca2+. Indeed, I report that the sytD-N mutation displays slower release kinetics. To directly test if this mutation is the cause of the increase in asynchronous events, I generated additional mutations that prevent interactions with the presynaptic membrane coupled to the originally published sytD-N mutations. In chapter 3 of this dissertation, I investigated these novel mutations at the Drosophila neuromuscular junction. I reported no increase in asynchronous release relative to control, providing evidence that the increased asynchronous events in sytD-N mutants are a result of the original mutation acting as an asynchronous sensor. Together, my results contradict the current hypothesis in the field and provide the likely mechanism for the increased asynchronous release observed in the original study. This dissertation also investigated the relatively new role for synaptotagmin mutations in the etiology of neuromuscular disease. With increased availability of high-throughput sequencing, over 20 candidate genes have been implicated in different forms of congential myasthenic syndromes. These inherited disorders are caused by mutations in genes needed for effective neuromuscular signaling. Two families, presenting with similar myasthenic syndromes, carry point mutations in the C2B Ca2+ binding pocket of synaptotagmin, expressed as an autosomal dominant disorder. One of theses families contains a proline to leucine substitution (sytP-L) a residue that had not been previously investigated for synaptotagmin function. In chapter 4, I investigated the functional importance of this mutation and created a disease model for this familial condition by driving the expression of a homolous proline-leucine synaptotagmin substitution in the central nervous system of Drosophila. I demonstrated that the proline residue plays a functional role in efficient transmitter release by testing its function in an otherwise synaptotagmin null genetic background. Additionally, this mutation displayed characteristics similar to the human disorder when expressed in a heterozygous synaptotagmin background, similar to the familial expression. Namely, the sytP-L mutants exhibited a decreased release probability, which resulted in decreased evoked responses that facilitate upon high frequency stimulation, a rightward shift in Ca2+ sensitivity, and behavioral deficits, including decreased motor output and increased fatigability. Thus, these studies establish the causative nature of the sytP-L mutation in this rare form of congenital myasthenic syndrome and highlight the utility of the Drosophila system for disease modeling.
Description
Rights Access
Subject
calcium sensor
Drosophila
disease modeling
synaptic transmission
congenital myasthenia
asynchronous release