Myelin-deficient (md) rats and their unaffected littermates were injected at postnatal day 4 either with a single dose of transferrin (Tf) or insulin-like growth factor one (IGF-1) singly or combined. Two weeks later, their brains were perfused and coronal sections were analyzed for MBP by in situ hybridization and for transferrin and myelin basic protein (Tf and MBP) by double immunofluorescence. Each of the factors separately had an effect on mutant animals as seen by both increased OL maturation, and MBP mRNA and protein synthesis. The combination of factors resulted in a profound enhancement of the myelinogenic properties of oligodendrocytes (OL) with a consequent increase in the number of MBP-labeled fibers. The brains of unaffected littermates also responded to growth factor(s) injection either by increasing myelination in some brain areas or by regulating the synthesis of MBP in OL. Using rat OL cultures we studied the site of transferrin action for the expression of MBP gene. We found by run off transcription that the MBP mRNA was significantly increased at the nuclear level but the PLP message was unaffected. Thus, transferrin selectively regulates MBP at the transcriptional level and together with IGF-1 synergizes to increase both the maturation and myelinogenic properties of md and normal OL.

Neurotrophin‐3 (NT‐3) and its receptor TrkC are known to be important for neuronal survival. More recently, NT‐3 has been implicated as playing a role in oligodendrocyte (OL) proliferation and survival in vitro. Examination of NT‐3 and TrkC knockout mice revealed a reduction in NT‐3‐dependent neurons. To date, no study has examined alterations in glial cell populations in these knockout mice. In this report, we demonstrate a decline in OL progenitor cell numbers within the CNS of NT‐3 and TrkC knockout mice. We also observed that immature and mature OL‐specific markers were attenuated in the NT‐3 and TrkC knockout animals. Deficiencies in other CNS glial cells, including astrocytes and ameboid microglia, were also observed. The subventricular zone (SVZ), a highly proliferative region for progenitor glial cells, was reduced in size. Furthermore, a nuclear‐specific stain revealed a decline in the numbers of pyknotic nuclei in and around the SVZ of the knockout mice. These data will support an in vivo NT‐3‐dependent mechanism for the normal development of CNS glial cells. GLIA 26:153–165, 1999. © 1999 Wiley‐Liss, Inc.

It has been established that oligodendrocytes, the myelin forming cells, participate in iron homeostasis through the synthesis and secretion of transferrin. Here we investigated whether a correlation exists between myelination, the commonly studied function of oligodendrocytes, and that of transferrin synthesis and secretion. We used a proteolipid protein mutant, the myelin deficient rat, whose condition is characterized by severe hypomyelination. We compared the ontogenic profile for transferrin gene expression in mutants with that of unaffected rat pups through northern blot analysis and in situ hybridization. Surprisingly, transferrin synthesis was null in mutant oligodendrocytes. Next, we demonstrated that a single apo-transferrin intraparenchymal injection administered to P5 rat pups enabled mutant oligodendrocytes to synthesize myelin basic protein and to myelinate axons, indicating that transferrin effects mutant oligodendrocyte maturation regardless of its source. Thus, transferrin availability is essential for oligodendrocyte maturation and function, and oligodendrocytes are most vulnerable to transferrin deficiency during the premyelinating stage.

Transplantation of oligodendrocyte (Ol) progenitor cells into the central nervous system is a promisingapproach for the treatment of myelin disorders. This approach requires providing adequate numbers of healthy cells with myelinating potential. We recently showed the successful transplantation of Ol progenitors into the myelin-deficient (md) rat brain. In the present work, CG4 cells, a cell line with properties of Ol progenitors, were labeled with fast blue and grafted into P3-P5 pups born to carrier mothers. Examination of host brains 2 weeks posttransplant indicated that CG4 cells display a much more extensive migration capacity than their wild-type counterparts. These cells synthesized myelin components. In addition, ultrastructural analysis showed myelin formation along axons of md hosts in various brain regions, including corpus callosum, cerebellum, and brainstem. Furthermore, in situ hybridization studies performed on sagittal sections revealed extensive expression of transferrin-mRNA within the md host parenchyma. The high survival and functional features displayed by CG4 cells after transplantation, together with their striking wide distribution within the host parenchyma, as assessed by the presence of myelinated fibers in mutant hosts, emphasizes the importance of using highly motile and proliferative Ol progenitor cells. Strategies to improve the condition and life span of md rat pups are currently under investigation.

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