Role of axons in the regulation of P_O biosynthesis by Schwann cells

The role of axons in the expression of the major myelin glycoprotein, P_O, has been investigated using neuron/Schwann cell cultures. These cultures were either nonmyelinating or myelinating due to growth in defined medium or in medium containing serum and chick embryo extract, respectively. The neurons and Schwann cells used in the studies were derived from embryonic day 15 rat dorsal root ganglia (DRG), and the Schwann cells from these ganglia are shown not to synthesize appreciable levels of P_o prior to growth in culture. Myelinating cultures of Schwann cells and neurons grown together for 18–21 days synthesize P_o that is readily identified by immunoblotting. The nonmyelinating cultures, which do not assemble basal lamina, also synthesize P_o that is detectable by either [³H]mannose precursor incorporation or by immunoblotting. The steady‐state level of P_o in the nonmyelinating cultures is less than that of the myelinating cultures, and the P_o that is synthesized by the former appears to be catabolized shortly after its biosynthesis. Since nonmyelinating Schwann cells synthesize P_o when in contact with neurites in vitro, we have examined the ability of such nonmyelinating cells to express the glycoprotein in vivo. Very little steady‐state P_o is detected in immunoblots of the adult rat cervical sympathetic trunk (CST), a nerve in which approximately 99% of the axons are nonmyelinated. Similarly, the amounts of [³H]mannose and [³H]amino acids that are incorporated into newly synthesized P_o are much lower in the CST than in the adult sciatic nerve. The Schwann cells of the CST are induced to synthesize myelin after growth in serum‐containing medium with embryonic DRG neurites that are devoid of endogenous Schwann cells due to prior treatment with an antimitotic agent. Immunoblot analysis of these cultures reveals a level of P_o that is comparable to that of myelinating DRG Schwann cell/neuron cultures. These data indicate that DRG neurites supply a signal that can initiate the synthesis of P_o in the absence of basal lamina and myelination. Furthermore, this signal may be reduced or altered in the CST.