Alterations in Both Caliber and Myelination of Callosal Axons Elicited by Ubiquitous Genetic Ablation of c‐Jun Amino‐Terminal Kinase 3 (JNK3)

Myelination is a fundamental process supporting appropriate motor, sensory, and cognitive functions. During development, oligodendrocyte progenitor cells (OPCs) proliferate, migrate, and gradually differentiate into mature oligodendrocytes, which produce and assemble myelin sheets that insulate axons in the mammalian central nervous system. Recent evidence suggested a regulatory role of the protein kinase JNK1, one of three mammalian JNK isoforms, on the proliferation and differentiation of OPCs, but whether other JNK isoforms modulate these and other cellular events contributing to myelination has not yet been explored. Building on results from an unbiased proteomic analysis, our studies here revealed increased numbers of OPCs, but not mature oligodendrocytes, in the corpus callosum of mice featuring germline ablation of the JNK3 isoform. Ultrastructural analyses further showed an increased proportion of small caliber callosal axons in these mice, as well as thinning of their myelin sheaths. These alterations were accompanied by reduced phosphorylation of heavy chain subunits of neurofilaments (NFs), major cytoskeletal elements linking myelin to the regulation of axonal caliber. Collectively, our findings reveal previously unrecognized effects of JNK3 deletion on OPC proliferation, NF phosphorylation, callosal axon caliber, and myelin thickness in vivo, suggesting a potential involvement of this kinase on myelinogenesis and/or myelin maintenance.