Density dependent regulation of human Schwann cell proliferation

Cessation of division is prerequisite for Schwann cell differentiation but regulation of this critical function is poorly understood. Heregulin/forsko lin-induced growth of human Schwann cells (HSCs) in vitro was found to be s trongly regulated by cell density and thus could model some aspects of nega tive growth-regulation in vivo. To better understand this phenomenon, the p roduction of an autocrine growth-inhibitor and the role of contact-inhibiti on were investigated. The possible involvement of two membrane proteins, co ntactinhibin (CI) and peripheral myelin protein 22 (PMP22) in regulating gr owth was studied. Thymidine-labeling of HSCs on collagen-coated dishes was inhibited at cell densities less than one tenth of the density at maximal g rowth-inhibition. Medium from high density cultures did not inhibit the thy midine-labeling of HSCs at low density, a result that argues against the pr oduction of a soluble inhibitor. The expression of CI and PMP22 in nerve an d HSCs, and the effect of a function-blocking antibody to CI on HSC growth, were determined. CI was detected in fresh nerve by western blotting, and c ould easily be detected by immunocytochemistry in cultured HSCs by five day s and for several weeks thereafter. Twenty-four hour treatment with anti-CI antibody did not increase the thymidine-labeling of HSCs at any density bu t a significant increase in HSC number was observed in cultures treated wit h anti-CI for 20 days. This increase was not related to decreased cell deat h. PMP22, unlike other myelin proteins, was not down-regulated after nerve dissociation and by seven days nearly all HSCs were PMP22 positive. These r esults provide evidence for a contact-mediated mechanism of growth-regulati on in HSCs and suggest that CI is involved in this mechanism. GLIA 30:165-1 77, 2000. (C) 2000 Wiley-Liss, Inc.