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 PMP22 in myelination, a regulatory function in cell growth has been suspected. 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 observed previously in NIH3T3 fibroblasts. To prove a growth regulatory function of PMP22, we generated Schwann cell cultures by infection with retroviral PMP22 expression vectors that constitutively expressed PMP22 cDNA sequences, in either the sense or antisense orientation. Transduced cells carrying the sense construct overexpressed PMP22 mRNA and protein, whereas in cells infected with an antisense PMP22 expression vector PMP22 mRNA levels were reduced markedly. Altered levels of PMP22 significantly modulated Schwann cell proliferation, as judged by 5-bromo-2'-deoxy-uridine incorporation into replicated DNA. In asynchronously dividing cultures enhanced expression of PMP22 decreased DNA synthesis to 60% of the control level. Conversely, reduced levels of PMP22 mRNA led to enhanced DNA synthesis of similar to 150%. Further cell cycle analyses by flow cytometry revealed that overexpression of PMP22 delayed serum- and forskolin-stimulated entry of resting Schwann cells from G(0)/G(1) into the S+G(2)/M phases by similar to 8 h, whereas underexpression of PMP22 mRNA slightly increased the proportion of cells that entered the S+G(2)/M phases. Our results indicate that PMP22 acts as a negative modulator of Schwann cell growth and delays the transition from G(0)/G(1) to the S phase of the cell cycle.