PTPσ functions as a presynaptic receptor for the glypican-4/LRRTM4 complex and is essential for excitatory synaptic transmission

Leukocyte common antigen-related receptor protein tyrosine phosphatases-comprising LAR, PTP delta, and PTP sigma-are synaptic adhesion molecules that organize synapse development. Here, we identify glypican 4 (GPC-4) as a ligand for PTP sigma. GPC-4 showed strong (nanomolar) affinity and heparan sulfate (HS)-dependent interaction with the Ig domains of PTP sigma. PTP sigma bound only to proteolytically cleaved GPC-4 and formed additional complex with leucine-rich repeat transmembrane protein 4 (LRRTM4) in rat brains. Moreover, single knockdown (KD) of PTP sigma, but not LAR, in cultured neurons significantly reduced the synaptogenic activity of LRRTM4, a postsynaptic ligand of GPC-4, in heterologous synapse-formation assays. Finally, PTP sigma KD dramatically decreased both the frequency and amplitude of excitatory synaptic transmission. This effect was reversed by wild-type PTP sigma, but not by a HS-binding-defective PTP sigma mutant. Our results collectively suggest that presynaptic PTP sigma, together with GPC-4, acts in a HS-dependent manner to maintain excitatory synapse development and function.