Evidence for the colocalization of the axonal mitogen for Schwann cells and oligodendrocytes

Axons that normally will encounter either CNS or PNS glia have been shown to contain a powerful mitogen for both Schwann cells and oligodendrocytes. The normally nonmyelinated, nonglial ensheathed cerebellar granule cells have been shown to possess a proliferative signal for Schwann cells, suggesting that a glial mitogen is common to all axons. To determine if a glial mitogen capable of stimulating both Schwann cells and oligodendrocytes is colocalized on all types of axons we have (1) cocultured granule cells with oligodendrocytes, (2) incubated oligodendrocytes with granule cell membranes, and (3) evaluated the ability of heparin extracts of granule cell membranes, splenic nerve microsomes, and axolemma-enriched fractions isolated from rat and bovine CNS to stimulate mitosis of cultured oligodendrocytes. Neither the intact granule cells nor the granule cell membrane fraction stimulated cultured oligodendrocytes to divide. However, heparin extracts of the granule cell membranes were significantly mitogenic to the cultured oligodendrocytes. Heparin extracts of splenic nerve microsomes were more mitogenic than the comparable extract obtained from bovine CNS axolemma-enriched fractions. These results suggest that the neuronal mitogen for oligodendroglia is colocalized with the neuronal mitogen for Schwann cells.