Analysis in vivo using a new method, ARGO (Analysis of Red Green Offset), reveals complexity and cell-type specificity in presynaptic turnover of synaptic vesicle protein Synaptogyrin/SNG-1

In long-lived cells such as neurons, proteostasis involves the regulated degradation and replacement of proteins to ensure their quality and appropriate abundance. Synaptic vesicle (SV) protein turnover in neurons is important for controlling the SV pool size to maintain appropriate levels of neurotransmission; yet, it is incompletely understood, partly due to limited tools for quantifying protein turnover in vivo. We present ARGO (Analysis of Red-Green Offset), a fully genetically encoded, ratiometric fluorescence imaging method that visualizes and quantifies protein turnover with subcellular resolution in vivo. ARGO is inexpensive, modular, and scalable for use in genetically tractable experimental organisms. Using ARGO, we examine the turnover of Synaptogyrin/SNG-1, an evolutionarily conserved, integral SV protein, in C. elegans neurons. We show that the SNG-1 turnover rate is consistent across presynapses within a single neuron but varies between neuron classes. Notably, we find SNG-1 and can exist in two distinct, non-intermixing populations within each presynapse. Further, we present an initial mutant analysis of uba-1, the sole E1 ubiquitin ligase in C. elegans, showing that we can detect slowed SNG-1 turnover even though steady-state SNG-1 abundance is not increased compared to wild-type. These results provide new hints for the regulation of SV pool size. Significance Statement- In long-lived cells, a proteins rates of synthesis and degradation together determine its abundance, yet regulation of protein turnover is largely unknown due to the lack of simple methods for in vivo quantification. - Using C. elegans, the authors develop ARGO, a genetically encoded, microscopy approach that quantifies a proteins turnover. Results suggest that synaptic vesicle protein Synaptogyrin/SNG-1 can partition into two presynaptic pools with distinct half-lives. - This provides a powerful tool to study protein turnover, reveals an unexpected complexity in SNG-1 turnover, and lays the groundwork for future investigations of synaptic vesicle protein compartmentalization, sorting, and degradation.