Morphology and inter-vesicle distances in condensates of synaptic vesicles and synapsin

Synaptic vesicle clusters or pools are functionally important constituents of chemical synapses. In the so-called reserve and the active pools, neurotransmitter-loaded synaptic vesicles (SVs) are stored and conditioned for fusion with the synaptic membrane and subsequent neurotransmitter release during synaptic activity. Vesicle clusters can be considered as so-called membrane-less compartments, which form by liquid-liquid phase separation (LLPS). Synapsin as one of the most abundant synaptic proteins has been identified as a major driver of pool formation. It has been shown to induce LLPS and form condensates on its own in solution, but also has been shown to integrate vesicles into condensates in vitro. In this process, the intrinsically disordered region of synapsin is believed to play a critical role. Here we first investigate the solution structure of synapsin and SVs separately by small-angle X-ray scattering (SAXS). In the limit of low momentum transfer q, the scattering curve for synapsin gives clear indication for supra-molecular aggregation (condensation). We then study mixtures of SVs and synapsin forming condensates, aiming at the morphology and inter-vesicle distances, i.e the structure of the condensates in solution. To obtain the structure factor S(q) quantifying inter-vesicle correlation, we divide the scattering curve of condensates by that of pure SV suspensions. Analysis of S(q) in combination with numerical simulations of cluster aggregation indicates a non-compact fractal-like vesicular fluid with rather short inter-vesicle distances at the contact sites.