Synaptojanin

Synaptojanin is a family of inositol phosphatases that regulate phospholipid signaling at neuronal synapses, playing a central role in clathrin-mediated endocytosis and synaptic vesicle recycling. In mammals, the best characterized members are synaptojanin-1 (SYNJ1) and synaptojanin-2 (SYNJ2), produced as multiple isoforms by alternative splicing. The enzymes share a two-domain catalytic core: an N-terminal Sac1 phosphatase domain and a central 5-phosphatase domain. The Sac1 domain dephosphorylates phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 3-phosphate (PI3P), while the 5-phosphatase domain removes the 5-phosphate from PI(4,5)P2 to generate PI4P. A C-terminal proline-rich region binds SH3-domain containing endocytic proteins such as dynamin, amphiphysin, and endophilin, helping recruit synaptojanin to endocytic sites.

Functionally, synaptojanin coordinates lipid signaling with coat disassembly. During synaptic vesicle recycling, hydrolysis of PI(4,5)P2 and PI4P by synaptojanin facilitates clathrin coat uncoating and vesicle reformation, enabling efficient reuse of synaptic vesicles. The activity of synaptojanin is regulated by interactions with adaptor and accessory proteins at endocytic zones, and by tissue-specific expression of isoforms.

Genetic and experimental evidence link synaptojanin to neurological function. In humans, variants in SYNJ1 have been associated with neurological disorders, including forms of early-onset parkinsonism with seizures in some cases. In model systems, loss of synaptojanin impairs synaptic vesicle endocytosis and can cause neurodegenerative phenotypes, underscoring its conserved role in synaptic maintenance.