Developmental expression of protein kinase C isozymes in oligodendrocytes and their differentail modulation by 4?-phorbol-12, 13-dibutyrate

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Abstract
Myelin gene expression in normal oligodendrocytes (OLG) depends on developmentally regulated protein kinase C (PKC) enzyme activity (Asotra and Macklin: J Neurosci Res 34:571–588, 1993). We studied the developmental expression of the Ca++-dependent PKC-α, -βI, -βII and -γ isozymes, and the Ca++-independent PKC-δ, -ϵ, -ζ and -η isozymes in enriched rat brain OLG cultures. In A2B5+ O-2A progenitors, only PKC-δ, PKC-ϵ and PKC-ζ were detected immunocytochemically. In 04+ proligondendrocytes, PKC-βI, -δ and -ζ were expressed moderately and low levels of PKC-α and -ϵ were detected. GD3+ OLG, GC+ OLG and MBP+ OLG showed increased levels of PKC-α, -βI, -δ and -ζ isozymes. PKC-βII, -γ and -η were poorly expressed in OLG. On immunoblots, PKC-α was present early and increased continually up to 18 days but PKC-βI increased until 12 days in cultured OLG. High levels of PKC-δ, PKC-ϵ and PKC-η, the most abundant PKC isozymes in OLG, were maintained up to 12 days and were then slightly reduced. Interestingly, relatively high levels of PKC-α, PKC-βI, PKC-βII, PKC-γ and PKC-ϵ isozymes were detected in purified myelin membrane although greater levels of PKC-δ were found in OLG than in purified myelin. Thus, most of the PKC isozymes found in cultured OLG were also present in myelin, although at different levels. Treatment with 50 nM 4β-phorbol-12,13-dibutyrate (PDB) caused a delayed downregulation of PKC-δ levels after 8 hr without modulating the expression of other PKC isozymes in 1-day OLG; in the 3-day-old and 6-day-old OLG, PDB downmodulated PKC-βI, -δ and ϵ isozymes with only a minor effect on PKC-α and no reduction in PKC-ζ. Induction or downmodulation of individual PKC isozymes by phorbol esters appears to depend on the differentiation state of OLG. These data suggest that PKC-βI, -δ and -ϵ isozymes have an important function in different cellular events of OLG differentiation. We conclude that the PKC-dependent modulation of myelin gene expression in OLG results predominantly from the Ca++-dependent PKC-βI isozyme activity and the Ca++-independent PKC-δ and PKC-ϵ activities in a cell differentiation state-dependent manner. Because of continuous presence of PKC-ζ isozyme in all the OLG phenotypes and its regulation by a mechanism unlike that for the regulation of other PKC isozymes, PKC-ζ may have a unique role in OLG function.

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