Oligodendrocyte maturation and progenitor cell proliferation are independently regulated by thyroid hormone

The development of oligodendrocyte progenitor cells is regulated by epigenetic factors which control their proliferation and differentiation. When oligodendrocyte progenitor cells, purified on a Percoll centrifugation gradient from neonate rat brain, are cultured in serum-free medium in the presence of platelet-derived-growth factor (PDGF), they divide and their differentiation is delayed. Triiodothyronine (T3) treatment of progenitor cells blocks their proliferation and induces their differentiation into oligodendrocytes. T3 also induces morphological differentiation of oligodendrocytes as indicated by the marked increase in the length of oligodendrocyte processes. To determine whether the effects of T3 on progenitor cell proliferation and oligodendrocyte maturation are causally related, or instead, are independent, we examined the influence of T3 on secondary cultures of postmitotic oligodendrocytes. We show that T3 increases morphological and functional maturation of postmitotic oligodendrocytes as indicated by a well developed network of branched processes and by the expression of myelin/oligodendrocyte glycoprotein (MOG) and glutamine synthetase (GS). T3 increases glutamine synthetase activity and its message level after a lag period of 24-48 h, and these levels increase through a posttranscriptional event. In contrast, no effect of T3 was observed on myelin basic protein (MBP) gene expression as determined by Northern blot analysis. Our results indicate that thyroid hormones participate in the control of the progenitor cell proliferation and differentiation as well as in oligodendrocyte maturation and that these two T3-regulated events are independent.