Soluble forms of NCAM and F3 neuronal cell adhesion molecules promote Schwann cell migration: identification of protein tyrosine phosphatases zeta/beta as the putative F3 receptors on Schwann cells.

Neural cell adhesion molecule (NCAM) and F3 are both axonal adhesion molecules which display homophilic (NCAM) or heterophilic (NCAM, F3) binding activities and participate in bidirectional exchange of information between neurones and glial cells. Engineered Fc chimeric molecules are fusion proteins that contain the extracellular part of NCAM or F3 and the Fc region of human IgG1. Here, we investigated the effect of NCAM-Fc and F3-Fc chimeras on Schwann cell (SC) migration. Binding sites were identified at the surface of cultured SCs by chimera coated fluorospheres. The functional effect of NCAM-Fc and F3-Fc binding was studied in two different SC migration models. In the first, migration is monitored at specific time intervals inside a 1-mm gap produced in a monolayer culture of SCs. In the second, SCs from a dorsal root ganglion explant migrate on a sciatic nerve cryosection. In both systems addition of the chimeras significantly increased the extent of SC migration and this effect could be prevented by the corresponding anti-NCAM or anti-F3 blocking antibodies. Furthermore, antiproteoglycan-type protein tyrosine phosphatase zeta/beta (RPTPzeta/beta) antibodies identified the presence of RPTPzeta/beta on SCs and prevented the enhancing effect of soluble F3 on SC motility by 95%. The F3-Fc coated Sepharose beads precipitated RPTPzeta/beta from SC lysates. Altogether these data point to RPTPzeta/beta is the putative F3 receptor on SCs. These results identify F3 and NCAM receptors on SC as potential mediators of signalling occurring between axons and glial cells during peripheral nerve development and regeneration.

The neuron-glia signal beta-neuregulin promotes Schwann cell motility via the MAPK pathway.

Neuregulins constitute a family of related growth factors that play important roles in Schwann cell development and maturation. We investigated the involvement of beta-neuregulin in Schwann cell migration, using a simple in vitro bioassay. Pure Schwann cells were prepared from the sciatic nerves of 5-day-old rats and were grown in defined medium, with or without serum, until a monolayer of confluent cells was formed. A cell-free area was then generated by inflicting a scratch resulting in a 1-mm-wide gap. Schwann cell migration within the gap was monitored microscopically at given time intervals and was quantified using an image analysis system. The extent of cell proliferation was estimated by BrdU incorporation, and cell migration was quantified both in the absence and presence of cytosine arabinoside. We found that, in the absence of serum, beta-neuregulin at a dose submaximal for proliferation increased the rate of Schwann cell migration by 84%. A more moderate effect was observed when beta-neuregulin was applied in the presence of serum which, however, is by itself responsible for increased Schwann cell motility. To assess the signal transduction pathways involved in this procedure we used one inhibitor of MAPK, PD098059, two inhibitors of PI-3-kinase, wortmannin, and LY0294002, and three different PKC inhibitors. Of these PD098059 inhibited the neuregulin-induced enhancement in Schwann cell migration by 40%, the two PI-3-kinase inhibitors yielded an approximately 20% inhibition while the PKC inhibitors were ineffective. Our data indicate that the action of beta-neuregulin on Schwann cell motility is primarily mediated via the MAPK pathway.