Schwann cells stimulated by axolemma-enriched fractions express cyclic AMP responsive element binding protein

Both axolemma-enriched fractions (AEF) and cyclic AMP have been shown to regulate the proliferation and differentiation of cultured primary Schwann cells (SC). We have evaluated the role of CREB, a transcription factor that binds to the cAMP-responsive element, in mediating the AEF-stimulated SC proliferation and differentiation. We detected CREB in nuclear extracts derived from SC stimulated with 40 micrograms/ml of AEF for 16, 24, 48, 72, and 96 hr, using a DNA-electrophoretic mobility shift assay. Unstimulated quiescent SC contained low levels of CREB which increased to a maximal level after 48 hr of AEF treatment. Using anti-CREB antibodies and Western blot analysis, after 24 hr of AEF treatment we first detected CREB as a 45 kDa protein which reached a maximal level of expression after 72 hr. Double labeled immunocytochemistry using anti-CREB and anti-5-bromo-2'-deoxy-uridine antibodies demonstrated maximal CREB expression after 72 hr of AEF treatment, closely coinciding with the temporal expression of SC proliferation. At all times examined, all AEF-treated SC labeled by anti-CREB antibodies were also labeled with anti-BrdU antibodies. These observations are consistent with the view that CREB could play an important role in the induction of SC proliferation by AEF.