SYN2

Synapsin II (SYN2) is a neuron-specific phosphoprotein of small synaptic vesicles that regulates neurotransmitter release and synaptogenesis (PMID:23956174, PMID:7587399). Genetic rescue experiments in SYN2 knockout neurons establish that the intact protein is required for normal synaptic vesicle cycling and axon outgrowth: wild-type human Synapsin II fully restores these phenotypes, whereas an ASD-associated nonsense allele (A94fs199X) is not expressed and missense alleles (Y236S, G464R) fail to rescue (PMID:23956174). The SYN2 gene maps to chromosome 3p, an autosomal location distinct from the X-linked SYN1 (PMID:7587399). Beyond these genetic and functional rescue findings, no further molecular mechanism — binding partners, vesicle-association determinants, or phosphorylation-dependent regulation — has been characterized in the available corpus.

1. 1995 Medium

   Before its chromosomal assignment, it was unknown where SYN2 resided in the genome and whether it was distinct from the X-linked SYN1; mapping placed SYN2 on autosomal chromosome 3p, defining it as a separate neuron-specific synaptic vesicle phosphoprotein gene.

   Evidence PCR from somatic hybrid cell lines, single-strand conformation analysis, FISH, and backcross genotyping in human and mouse

   PMID:7587399

   Open questions at the time

   • Mapping does not address the protein's biochemical activity or binding partners
   • No functional consequence of the locus established at this stage
2. 2013 Medium

   It was unknown whether SYN2 missense variants found in disease were functionally damaging; rescue assays in SYN2 knockout neurons showed that wild-type but not mutant Synapsin II restores synaptic vesicle cycling and axon outgrowth, establishing that the intact protein drives neurotransmitter release and synaptogenesis.

   Evidence Expression of wild-type and mutant human SYN2 in SYN2 knockout neurons with vesicle cycling and axon outgrowth readouts

   PMID:23956174

   Open questions at the time

   • Single-lab study; phenotypes not independently replicated
   • Molecular mechanism by which missense mutations disrupt function not resolved (no binding, localization, or phosphorylation analysis)
   • Direct physical partners and vesicle-association determinants not identified

• The molecular basis of Synapsin II function — its direct interaction partners, vesicle-tethering mechanism, and phosphoregulation — remains uncharacterized in the available corpus.
• No identified binding partners or substrates
• No structural model of the protein
• Mechanism linking the protein to vesicle cycling and axon outgrowth not defined