RETROVIRAL-MEDIATED GENE-TRANSFER OF THE PERIPHERAL MYELIN PROTEIN PMP22 IN SCHWANN-CELLS - MODULATION OF CELL-GROWTH

The peripheral myelin gene PMP22 is the rat and human homologue of the murine growth arrest-specific gene gas3. Besides a putative role of P MP22 in myelination, a regulatory function in cell growth has been sus pected. Here we have investigated both the expression of PMP22 during cell cycle progression of cultured rat Schwann cells and the influence of altered levels of PMP22 on Schwann cell growth. When resting cells were stimulated to begin the cell cycle, the regulation of PMP22 mRNA resembled the growth arrest-specific pattern of gas3 expression obser ved previously in NIH3T3 fibroblasts. To prove a growth regulatory fun ction of PMP22, we generated Schwann cell cultures by infection with r etroviral PMP22 expression vectors that constitutively expressed PMP22 cDNA sequences, in either the sense or antisense orientation. Transdu ced cells carrying the sense construct overexpressed PMP22 mRNA and pr otein, whereas in cells infected with an antisense PMP22 expression ve ctor PMP22 mRNA levels were reduced markedly. Altered levels of PMP22 significantly modulated Schwann cell proliferation, as judged by 5-bro mo-2'-deoxy-uridine incorporation into replicated DNA. In asynchronous ly dividing cultures enhanced expression of PMP22 decreased DNA synthe sis to 60% of the control level. Conversely, reduced levels of PMP22 m RNA led to enhanced DNA synthesis of similar to 150%. Further cell cyc le analyses by flow cytometry revealed that overexpression of PMP22 de layed serum- and forskolin-stimulated entry of resting Schwann cells f rom G(0)/G(1) into the S+G(2)/M phases by similar to 8 h, whereas unde rexpression of PMP22 mRNA slightly increased the proportion of cells t hat entered the S+G(2)/M phases. Our results indicate that PMP22 acts as a negative modulator of Schwann cell growth and delays the transiti on from G(0)/G(1) to the S phase of the cell cycle.