Development-associated myristoylated alanine-rich C kinase substrate phosphorylation in rat brain

In neuronal cells, myristoylated alanine-rich C kinase substrate (MARCKS), localized to particular areas of the synaptic membrane, is active during brain development. The destination of phosphorylated MARCKS is thought to be the cytoplasm where it is probably inactive. We compared MARCKS phosphorylation in the brains of embryonic, perinatal, and adult rats to determine its possible involvement in neurogenesis.

We prepared crude and partially purified extracts from various brain regions of rats aged between embryonic day 14 (E14) and 7 weeks after birth and assayed them for MARCKS phosphorylation by immunochemical methods. The isotypes of protein kinase C (PKC) were immunochemically identified in crude brain extracts from embryonic and postnatal rats. Despite negligible MARCKS phosphorylation, E16 brain extracts contained both MARCKS and PKCgamma, delta, epsilon, and lambda. MARCKS and polypeptides were clearly phosphorylated (49 and 45 kDa, respectively) in brain extracts purified on a DE52 column. Embryonic brain extracts manifested a high-molecular-weight activity capable of suppressing polypeptide phosphorylation. This activity was markedly decreased on the day of birth and almost undetectable in the brains of 9-day-old rats.

The embryonic rat brain appears to contain a protein(s) that suppresses the phosphorylation of other proteins including MARCKS. We posit that this inhibitory activity represents a factor(s) that plays a role in the regulation of neurogenesis beginning on the day on which MARCKS appears in the embryonic brain.