Huntingtin‐associated protein‐1 is a synapsin I‐binding protein regulating synaptic vesicle exocytosis and synapsin I trafficking

Abstract

Huntingtin-associated protein-1 (HAP1) is involved in intracellular trafficking, vesicle transport, and membrane receptor endocytosis. However, despite such diverse functions, the role of HAP1 in the synaptic vesicle (SV) cycle in nerve terminals remains unclear. Here, we report that HAP1 functions in SV exocytosis, controls total SV turnover and the speed of vesicle fusion in nerve terminals and regulates glutamate release in cortical brain slices. We found that HAP1 interacts with synapsin I, an abundant neuronal phosphoprotein that associates with SVs during neurotransmitter release and regulates synaptic plasticity and neuronal development. The interaction between HAP1 with synapsin I was confirmed by reciprocal co-immunoprecipitation of the endogenous proteins. Furthermore, HAP1 co-localizes with synapsin I in cortical neurons as discrete puncta. Interestingly, we find that synapsin I localization is specifically altered in Hap1 À/À cortical neurons without an effect on the localization of other SV proteins. This effect on synapsin I localization was not because of changes in the levels of synapsin I or its phosphorylation status in Hap1 À/À brains.Furthermore, fluorescence recovery after photobleaching in transfected neurons expressing enhanced green fluorescent protein-synapsin Ia demonstrates that loss of HAP1 protein inhibits synapsin I transport. Thus, we demonstrate that HAP1 regulates SV exocytosis and may do so through binding to synapsin I.

Transcriptional control of synaptic components by the clock machinery

“…In addition, NSF (N‐ethylmaleimide‐sensitive fusion protein) is an ATPase that regulates the decoupling of vesicle complexes (Südhof, b). Synaptic vesicles are also regulated by proteins ensuring interaction with the cytoskeleton and intracellular trafficking, such as huntingtin‐associated protein 1 (HAP1), synapsin1 (SYN1) (Mackenzie et al., , ; Twelvetrees et al., ), and 14‐3‐3 proteins (Broadie, Rushton, Skoulakis, & Davis, ; Geiger et al., ). Several of these synaptic components show different levels of support for transcriptional regulation by the clock machinery (Table ).…”

Molecular insights into cortico-striatal miscommunications in Huntington's disease

“…Htt may also directly affect presynaptic vesicular function, including neurotransmitter release. Mutant Htt binds to synaptic vesicles through interaction with Hap1 [22], which is involved in synaptic vesicle exocytosis by interacting with synapsin-1 [23]. Consistent with this mechanism, targeted expression of mHtt-exon1 to the presynaptic terminal interferes with presynaptic glutamate release in the striatum, and produces disease phenotypes in mice [24].…”

A Poly-Glutamine Region in theDrosophilaVAChT Dictates Fill-Level of Cholinergic Synaptic Vesicles

“…120Q mutants were also shown to reduce glutamate release suggesting a direct interaction between extended polyQ domains and synaptic release (Li et al, 2003). HAP1 has also been shown to bind synapsin 1 (Mackenzie et al, 2016) which is critical for SV pool mobilization and formation (Rosahl et al, 1995; Akbergenova and Bykhovskaia, 2010). We therefore theorize that the polyQ region in VAChT may play a similar role in trafficking the transporter to the SV, plasma membrane and/or SV pool formation.…”