N-cadherin is an in vivo substrate for protein tyrosine phosphatase sigma (PTP sigma) and participates in PTP sigma-mediated inhibition of axon growth

Protein tyrosine phosphatase sigma (PTP sigma) belongs to the LAR family of receptor tyrosine phosphatases and was previously shown to negatively regulate axon growth. The substrate for PTP sigma and the effector(s) mediating this inhibitory effect were unknown. Here we report the identification of N-cadherin as an in vivo substrate for PTP sigma. Using brain lysates from PTP sigma knockout mice, in combination with substrate trapping, we identified a hyper-tyrosine-phosphorylated protein of similar to 120 kDa in the knockout animals (relative to sibling controls), which was identified by mass spectrometry and immunoblotting as N-cadherin. beta-Catenin also precipitated in the complex and was also a substrate for PTP sigma. Dorsal root ganglion (DRG) neurons, which highly express endogenous N-cadherin and PTP sigma, exhibited a faster growth rate in the knockout mice than in the sibling controls when grown on laminin or N-cadherin substrata. However, when N-cadherin function was disrupted by an inhibitory peptide or lowering calcium concentrations, the differential growth rate between the knockout and sibling control mice was greatly diminished. These results suggest that the elevated tyrosine phosphorylation of N-cadherin in the PTP sigma(-/-) mice likely disrupted N-cadherin function, resulting in accelerated DRG nerve growth. We conclude that N-cadherin is a physiological substrate for PTP sigma and that N-cadherin (and likely beta-catenin) participates in PTP sigma-mediated inhibition of axon growth.