Munc13 and Associated Molecules

Introduction

Great efforts have gone into discovering and understanding the molecules that control and execute neurotransmitter release. Neurons in the brain communicate with each other by release of neurotransmitters. The process is mediated by membrane-anchored SNARE proteins and various regulatory proteins. UNC-13 is a conserved protein and homologs have been identified in nearly every organism examined that contains a nervous system. Munc13-1 is the most abundant isoform in the mammalian brain.

Mechanism of Action of Munc13

Munc13-1 is a large multifunctional protein essential for synaptic vesicle fusion and neurotransmitter release. Munc13-1 contains a variable N-terminal region with a C2A domain, a calmodulin-binding region, and a conserved C-terminal region that includes the C1, C2B, MUN and C2C domains. Munc13-1 MUN domain facilitates the opening of syntaxin-1 and SNARE complex formation

Fig.1 Domains of Munc13-1 and syntaxin 1. (Engel, 2016)

Munc13-1 and its homologues are primarily brain-specific, cytoplasmic proteins in the presynaptic terminal that are involved in synaptic vesicle priming and short-term synaptic plasticity. Munc13-1 has been proved to promote membrane fusion by 2 means:

• Tethers synaptic vesicles to the plasma membrane through its N- and C-terminal C2 and C1 domains
• Directly enhances SNARE assembly through the MUN domain

Evidence suggests that the MUN domain of Munc13-1 collaborates with Munc18-1 to initiate SNARE assembly, thereby priming vesicles for fast calcium-triggered vesicle fusion. Recent study reported a new function of Munc13: independent of Munc18, Munc13 promotes the proper syntaxin/synaptobrevin subconfiguration during assembly of the ternary SNARE complex.

Major Regulation of Munc13

Though the basic function of Munc13 in priming has been mapped to its conserved C-terminal domain, domains within the N-terminus of Munc13 appear to regulate the location and degree of priming and may potentiate release via additional mechanisms in response to various cues.

• Rab3/RIM Interaction

Munc13 interacts directly with the active zone protein Rim through its N-terminal RID. Besides, Rim interacts with the synaptic vesicle-associated guanosine triphosphatase Rab3. Munc13, Rab3, and Rim interact to form a tripartite complex.

• C1-Diacylglycerol Regulation

Munc13 is a well-established phorbol ester receptor in which the N-terminal C1 domain is the binding target. Binding of diacylglycerol/phorbol ester to the C1 domain induces a conformational change in Munc13 by disrupting C1 intramolecular binding and driving C1-plasma membrane interactions, thereby unleashing an undefined catalytic post priming action of Munc13 that increases vesicle release probability.

• Calmodulin-Binding Domain
• Abundance

Munc13 activity is also regulated by its abundance at the synapse. Two mechanisms have been implicated in this mode of regulation: proteasome-mediated degradation and diffusion/translocation.

• Double C2-Containing Protein Family
• ……

Fig.2 Overview of Munc13 and its associated molecules. (Das, 2013)

Future Research Directions

• Munc13 is a Molecular Target of Bryostatin 1

Munc13-1 is a C1 domain-containing protein that shares common endogenous and exogenous activators with novel and conventional PKC subtypes. Scientists explored the potential interaction between bryostatin 1 and Munc13-1. The results indicate that in vitro bryostatin 1 bind to both the isolated C1 domain of Munc13-1 and the full-length Munc13-1 protein. Together, this study characterizes Munc13-1 as a molecular target of bryostatin 1.

• Disease Target

Dysfunction of Munc13-1 has been linked to different kinds of diseases such as microcephaly, cortical hyperexcitability, and fatal myasthenia. Munc13-1 deficiency in the pancreatic islets as occurs in diabetes can reduce insulin secretion sufficient to cause abnormal glucose homeostasis. Thus, Munc13-1 tends to be a promising target for drug discovery.

Common Analysis Methods

• Single-molecule force spectroscopy
• Confocal microscopy and immunoblot analysis
• Model development
• ……

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References

1. Engel, A. G.; et al. Loss of MUNC13-1 function causes microcephaly, cortical hyperexcitability, and fatal myasthenia. Neurology Genetics. 2016, 2(5).
2. Das, J.; et al. The pre-synaptic Munc13-1 binds alcohol and modulates alcohol self-administration in Drosophila. Journal of neurochemistry. 2013, 126(6): 715-726.