Apotransferrin and the cytoskeleton of oligodendroglial cells

Apotransferrin (aTf), has been shown to accelerate the differentiation of oligodendroglial cells (OLGcs) in primary cultures and to increase the expression of different components of the myelin cytoskeleton (CSK). We examined the incorporation and distribution of human aTf (aTfh) exogenously added to OLGcs cultures and its effects on the CSK of the OLGcs. When OLGcs treated with aTfh were extracted with a CSK-stabilizing buffer containing detergent, aTfh was found in the soluble fraction. In vitro experiments showed that purified tubulin was not altered by the addition of aTfh. In OLGc primary cultures treated with aTfh, this glycoprotein showed a punctate distribution pattern along the OLGc processes. Treatment of the cultures with colchicine, cytochalasin, or taxol induced a displacement of the immunoreactivity of aTfh toward the OLGc soma. Analysis of the effects of aTfh on the cell distribution of tyrosinated and detyrosinated tubulin and STOP (stable tubule only polypeptide), showed that aTfh added to OLGc cultures promoted changes suggesting a stabilizing effect on the microtubules (MT) at the tip of the processes. Kinesin and dynein were found to colocalize with the aTfh, indicating that these motors participate in the transport of the added glycoprotein. Moreover, after treatment with aTfh, clathrin immunoreactivity was displaced from the OLGc body toward the cell processes. These results indicate that although aTfh added to OLGcs does not interact directly with CSK components, it seems to be transported in clathrin coated vesicles from the cell body to the tips of the OLGc processes where it promotes their stabilization. This mechanism may be of importance in the increased formation of the myelin membrane induced by aTf.