CYTOSKELETAL STRUCTURES MAY LINK CHOLESTEROL SYNTHESIS TO GLIAL DIFFERENTIATION

Abstract

Cholesterol synthesis is a highly critical process in developing brain, particularly in glia, because cholesterol is a major constituent of all cellular membranes, including myelin. We have shown recently that the rate limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase (HMG-R), is actively regulated in cultured C-6 glial cells, a model of the differentiating glial cell, and that this regulation involves microtubules. This study elucidates further the role of cytoskeletal structures. Microfilaments were disrupted by exposure of the cells to cytochalasin D (cD). Within 1 h of the addition of 1 μM cD, there resulted a 50% reduction in HMG-R activity, and after 3 h an 80% reduction. A decrease of 50% occurred with just 0.2 μM cD (3 h). The effects were fully reversible. The specificity of the effect was indicated by the findings of no change in the activities of five other critical glial enzymes or in the rate of total protein synthesis. That the decrease in HMG-R activity caused by cD is related to a decrease in enzyme content was suggested by the observation that cycloheximide blocked the recovery of activity after removal of cD. A relation of the effect of cD on reductase to disruption of microfilaments was suggested by striking concomitant morphological changes caused by the drug. Coupled with our previous findings, the observations suggest a link between a microfilament-microtubular system and both glial differentiation and cholesterol biosynthesis. Such a link would be of major neurobiological importance because it would provide a mechanism for coupling membrane biogenesis to cellular differentiation. (Supported by NIH grant R01 HD-07464).