PTPσ is a receptor tyrosine phosphatase that is expressed widely in the developing nervous system and that controls the growth and retinotopic mapping of retinal axons. PTPσ is also expressed in motor neurons where its function is unclear. Given that invertebrate relatives of PTPσ can control motor axon guidance, target contact, and synaptogenesis, we have asked if extracellular ligands exist for cPTPσ, the avian PTPσ orthologue, in the neuromuscular system. Of the two major isoforms cPTPσ1 and cPTPσ2, only the shorter cPTPσ1 isoform is expressed in developing spinal motor neurons and their axons. We show that ectodomains of cPTPσ1, but not of cPTPσ2, bind specifically to developing skeletal myotubes. The putative myotube ligand is not related to the previously described binding of cPTPσ to heparan sulfates within the proteoglycans agrin and collagen XVIII, since heparinase treatment of myotubes does not alter cPTPσ1 binding and since most mutations that abolish binding of cPTPσ1 to heparin do not affect myotube binding. The expression of cPTPσ1 in motor axons and its direct binding to target myotubes suggest an isoform-specific role for axonally expressed cPTPσ1 during establishment or maintenance of neuromuscular contacts.
ASJC Scopus subject areas
- Molecular Biology
- Cellular and Molecular Neuroscience
- Developmental Neuroscience