αSynuclein (αSyn) plays an important role at the synapse, to maintain neurotransmitter release via clustering synaptic vesicles (SV) and chaperoning SNARE-complex assembly. Aggregation of αSyn is a key pathological feature in multiple age-driven neurodegenerative diseases such as Parkinson’s disease (PD) and Alzheimer’s disease related dementias (ADRD) such as Lewy body dementia. Despite the involvement of βSyn and γSyn in synucleinopathies including Lewy body dementia, Gaucher’s disease, and PD, virtually nothing is known about their physiological functions in the brain. Understanding their function is the first step to finding out how their dysfunction causes PD and Alzheimer’s disease related dementias including Lewy body dementia, and how these diseases can be prevented or delayed. The objective here is to determine the effects of βSyn and γSyn on αSyn’s synaptic function. The central hypothesis is that interaction of βSyn and γSyn with αSyn causes a reduction in αSyn’s activity, leading to reduced SV clusters and SNARE-complex assembly, and altered neu- ronal activity. This hypothesis will be tested in 3 specific aims: 1) Assess the effect of βSyn and γSyn on SNARE-complex assembly; 2) Assess the effect of βSyn and γSyn on SV clustering; and 3) Determine the im- plications of synuclein interactions on SV cycling. Under aim 1, SNARE-complex assembly will be quantified in select brain areas and neurons from mice lacking βSyn and/or γSyn, in heterologous cells and using recombi- nant proteins. Under aim 2, synaptobrevin-2 binding and multimerization of αSyn, and SV pools will be quanti- fied in mice lacking βSyn and/or γSyn and using recombinant proteins. Under aim 3, SV exocytosis and cycling will be quantified in hippocampal brain slices and in neurons from mice lacking βSyn and/or γSyn with or with- out lentivirally increasing βSyn or γSyn levels. This research is innovative because it (1) tests the novel hypoth- esis that βSyn and γSyn affect the synaptic function of αSyn, (2) uses a multidisciplinary approach combining biophysical, biochemical, electrophysiological and whole animal approaches, and (3) analyzes new mouse models lacking βSyn and/or γSyn that were generated from αβγSyn triple knockout mice, that not only enable a direct comparison of the synucleins but may serve as models for synucleinopathies and Alzheimer’s disease related dementias including Lewy body dementia. Our work is significant because it (1) will clarify the im- portance of βSyn and γSyn for neuronal function, (2) will provide new insights into the molecular mechanism underlying SV binding of αSyn, (3) may uncover the contributions of βSyn and γSyn to synucleinopathies and Alzheimer’s disease related dementias including Lewy body dementia, and (4) has translational importance for the targeted development of new treatment strategies for the above-mentioned age-driven dementias aimed at targeting all synucleins instead of focusing solely on αSyn. With synuclein patho...