Researchers identified a pre-synaptic core-module that coordinates synaptic organization in the mammalian brain, according to a study published in Nature Communications. The tripartite complex assembles the primary synaptic cell adhesion molecules Neurexin 1 to 3 and LAR-type receptor protein-tyrosine-phosphatases alongside the previously uncharacterized tetraspanins T178A and T178B.
The team used multi-epitope affinity purifications paired with quantitative mass spectrometry and immuno-electron microscopy to map synaptic protein networks in adult mouse brains. Analysis via native gel electrophoresis revealed that Neurexins, LAR-PTPRs, and T178B co-migrate at an apparent molecular mass of roughly 400 kilodaltons, indicating an equimolar one-to-one-to-one stoichiometry. The complexes assemble inside the endoplasmic reticulum during biogenesis and embed preferentially into the synaptic plasma membrane.
Biochemical assays showed that the ternary complex relies directly on the transmembrane domains of the proteins. Experiments using a GPI-anchored variant of Neurexin-3 lacking a transmembrane domain failed to bind T178B or PTPRs, though binding to extracellular ligands remained intact. In reconstitution assays using cultured tsA cells, the presence of T178B increased the interaction between Neurexins and PTPRs tenfold. Structural modeling using the AlphaFold3 algorithm showed the transmembrane domains of Neurexin and PTPR juxtaposed and enclosed by the transmembrane segments of T178B.
Viral knockdown of T178B destabilized the entire core-module in cultured neurons and hippocampal CA1 pyramidal cells. The loss caused a strong reduction in LAR-PTPRs, redistributed synaptic Neurexins, and altered pre-synaptic neurotransmitter release, shifting paired-pulse ratios at stimulation frequencies of 10 and 50 hertz. Enzymatic removal of heparan sulfate chains also dissociated post-synaptic AMPA receptors and leucine-rich repeat proteins without breaking the core Neurexin-T178-PTPR assembly.
