Exploring the role of protein arginine modifying enzymes in the oligodendrocyte lineage

The oligodendrocyte lineage is critical for proper functioning of the central nervous system (CNS). Previous research has highlighted the importance of epigenetic regulation in oligodendrocyte development. However, the regulation of arginine residues on histones still remains poorly understood. This work aims to provide novel insights into the molecular and functional aspects of two arginine-modifying enzymes: PRMT5 and PADI2. Using an integrated biochemical, molecular and genetic approach, we identify the arginine methyltransferase PRMT5 is a key regulator of oligodendrocyte progenitor (OPC) proliferation and survival. These effects were likely mediated by a p53-dependent increase in the expression of pro-apoptotic (i.e., Perp) and anti-proliferative (i.e. Cdkn1a) genes, and accompanied by the down-regulation of oligodendrocyte lineage specific genes. In addition, we unveiled an interesting cross-talk between repressive symmetric di-methylation of the R3 residue (catalyzed by PRMT5) and acetylation of the K5 residue on histone H4. To assess the functional relevance of these findings, we ablated the Prmt5 gene in OPC by crossing Prmt5-flox/flox mice with Olig1-cre mice. The mutants displayed hypomyelination and a premature death phenotype, indicating an essential role of PRMT5 during developmental myelination. In addition, we identified non-histone substrates of PRMT5 in OPC using an unbiased approach based on quantitative mass spectrometry and found that HNRNPA1 (a well-established splicing factor in the oligodendrocyte lineage) was methylated by PRMT5 at its RNA binding region, which may underlie the role of PRMT5 in regulating alternative splicing during developmental myelination. Because PRMT5 was necessary for OPC proliferation and survival, we further studied its role in glioma cells derived from OPC. Interestingly, PRMT5 formed distinct protein complexes in normal OPC, and in glioma cells. Knock-down of PRMT5 in tumor cells resulted in a more modest effect on growth arrest and survival, which was inversely correlated with the level of malignancy. Finally, we also evaluated the potential role of the citrullination enzyme PADI2 in myelination. Mass-spectrometry analysis of purified myelin fractions from Padi2-knockout and control mice, revealed a gender-specific altered abundance of several myelin proteins in the knockout mice. Together these studies reveal a multifaceted functional role of arginine-modifying enzymes in the oligodendrocyte lineage, which encompasses histones, non-histone substrates, and myelin protein components, that requires further investigation.