Peptidylarginine deiminase 2 (PAD2) overexpression in transgenic mice leads to myelin loss in the central nervous system

Abstract

Demyelination in the central nervous system is the hallmark feature in multiple sclerosis (MS). The mechanism resulting in destabilization of myelin is a complex multi-faceted process, part of which involves deimination of myelin basic protein (MBP). Deimination, the conversion of protein-bound arginine to citrulline, is mediated by the peptidylarginine deiminase (PAD) family of enzymes, of which the PAD2 and PAD4 isoforms are present in myelin. To test the hypothesis that PAD contributes to destabilization of myelin in MS, we developed a transgenic mouse line (PD2) containing multiple copies of the cDNA encoding PAD2, under the control of the MBP promoter. Using previously established criteria, clinical signs were more severe in PD2 mice than in their normal littermates. The increase in PAD2 expression and activity in white matter was demonstrated by immunohistochemistry, reverse transcriptase-PCR, enzyme activity assays, and increased deimination of MBP. Light and electron microscopy revealed more severe focal demyelination and thinner myelin in the PD2 homozygous mice compared with heterozygous PD2 mice. Quantitation of the disease-associated molecules GFAP and CD68, as measured by immunoslot blots, were indicative of astrocytosis and macrophage activation. Concurrently, elevated levels of the pro-inflammatory cytokine TNF-alpha and nuclear histone deimination support initiation of demyelination by increased PAD activity. These data support the hypothesis that elevated PAD levels in white matter represents an early change that precedes demyelination.