Myelinated, but not unmyelinated axons, reversibly down-regulate N-CAM in Schwann cells

Summary

There is evidence from chicks and mice that N-CAM expression in Schwann cells is subject to significant regulation during development and following injury. In the present work, rat sciatic nerve and immunohistochemical methods have been used to study developmental and injury-related modulation of N-CAM in Schwann cells, using cell type specific markers to identify different Schwann cell populations, and cell counting to quantify their size. The study has sought to determine unambiguously whether immature Schwann cells in developing nerves and denervated Schwann cells in injured adult nerves express surface N-CAM, and has investigated the temporal relationship between the gradual loss of surface N-CAM and the differentiation of myelin-forming Schwann cells, monitored by the sequential appearance of the glycolipid galactocerebroside and the myelin-specific protein P₀. Further points examined are whether this down-regulation of N-CAM is rapidly reversible following loss of axonal contact, and whether N-CAM reappearance in Schwann cells depends on protein synthesis.

In nerves from 17- to 18-day embryos, 90% of the Schwann cells, identified with Ran-1 antibodies, expressed surface N-CAM. In nerves from newborn rats many cells are in the early stage of myelin synthesis and therefore express galactocerebroside, although they have not yet acquired P₀. Suspension staining of dissociated cells from this nerve showed that 92% of the galactocerebroside-positive cells were also N-CAM positive. In suspension staining of nerves from 5-day, 10-day and adult rats, P₀-positive cells were essentially N-CAM negative. If, however, cells from 10-day nerves were placed in culture and immunostained after, 3, 6, 9 and 24 h, N-CAM appeared in the P₀-positive cells that had formed myelinin vivo. The percentage of P₀-positive cells that also expressed N-CAM at these time points was 10, 28, 56 and 92%, respectively. Cell division was not a prerequisite for N-CAM reappearance which was, however, blocked by cycloheximide, an inhibitor of protein synthesis. Schwann cells in transected adult sciatic nerves in which regeneration was prevented, expressed surface N-CAM 2 months after injury, indicating that in the absence of axonal contact Schwann cells express N-CAM indefinitely.

The results indicate that in myelin-forming Schwann cells N-CAM synthesis is continuously suppressed by ongoing axonal signals. Thus axon-Schwann cell signals are involved not only in up-regulation but also in reversible down-regulation of Schwann cell molecules.