CRYP-2/cPTPRO is a neurite inhibitory repulsive guidance cue for retinal neurons in vitro

Receptor protein tyrosine phosphatases (RPTPs) are implicated as regulators of axon growth and guidance. Genetic deletions in the fly have shown that type III RPTPs are important in axon pathfinding, but nothing is known abou t their function on a cellular level. Previous experiments in our lab have identified a type III RPTP, CRYP-2/cPTPRO, specifically expressed during th e period of axon outgrowth in the chick brain; cPTPRO is expressed in the a xons and growth cones of retinal and tectal projection neurons. We construc ted a fusion protein containing the extracellular domain of cPTPRO fused to the Fc portion of mouse immunoglobulin G-1, and used it to perform in vitr o functional assays. We found that the extracellular domain of cPTPRO is an antiadhesive, neurite inhibitory molecule for retinal neurons. In addition , cPTPRO had potent growth cone collapsing activity in vitro, and locally a pplied gradients of cPTPRO repelled growing retinal axons. This chemorepuls ive effect could be regulated by the level of cGMP in the growth cone. Immu nohistochemical examination of the retina indicated that cPTPRO has at leas t one heterophilic binding partner in the retina. Taken together, our resul ts indicate that cPTPRO may act as a guidance cue for retinal ganglion cell s during vertebrate development.