Sialosylcholesterol induces reorganization of astrocyte filament network

Sialosylcholesterol induces the differentiation of astrocytes with respect to their morphological appearance (Kato et al., Brain Res. 438 (1988) 277-285; Ito et al., 481 (1989) 335-343), while in a cell-free condition it depolymerizes the astrocyte cellular filaments, the glia filaments and microfilaments (Ito et al., J. Neurochem. 61 (1993) 80-84). To solve this paradox, we examined hetero-interaction between the glia filaments and microfilaments in the presence of sialosylcholesterol. Each filament was prepared in a depolymerized form in low ionic strength, and was adjusted to physiological ionic strength to prevent from repolymerization by sialosylcholesterol. When the two filament preparations in this form were mixed, repolymerization took place in spite of the presence of sialosylcholesterol. The filament formed in the mixture was found almost exclusively composed of vimentin and actin, the major component of the glia filaments and microfilaments preparation, respectively. An excess amount of vimentin over actin in the precipitate implicated that the main mechanism for the hetero-polymerization was the enhancement of vimentin polymerization by actin. To support this view, pre-polymerization of the microfilaments before mixing with the depolymerized glia filaments resulted in a marked decrease in polymerization of the glia filaments. A similar hetero-interaction was found between the purified vimentin and actin. When polymerized vimentin and actin were directly depolymerized by sialosylcholesterol and mixed, polymer formation was demonstrated between these two proteins. Electronmicroscopy indicated direct interaction of the actin filament with the vimentin filament. The results indicate that sialosylcholesterol induces reorganization of the cellular filament network, such as disorganization of vimentin and actin filaments, and provokes their hetero-interaction to form the hetero-filament. Hence, this may be one of the key mechanisms for the induction of cellular differentiation by sialocylcholesterol.