Changes in myelin and axonal protein organization in the aged rhesus monkey brain

The steady impairment of normal cognitive performance is a common feature of aging in both humans and various animal models. An age-related decline in brain white matter volume and changes in myelin ultrastructure and biochemistry are proposed to underlie age-related cognitive decline, presumably by dampening axonal conduction. Age-dependent changes in the expression of 2' , 3'-cyclic nucleotide phosphodiesterase (CNP), a protein crucial to myelin and axonal maintenance, and its proteolysis by calpain-1 are central events in the aging brain and may mediate the effects of age on molecular organization in the axon. Using the aged rhesus monkey as a model for understanding age-related changes in myelin and white matter, an age-dependent accumulation of CNP, independent of increases in CNP gene expression, is observed throughout the brain in detergent-soluble homogenates. With age, CNP also accumulates in detergent-insoluble fractions of myelin, showing altered partitioning in fractions isolating with lipid rafts and those not. Ubiquitinated CNP can be immunoprecipitated from raft-associated fractions and high molecular weight aggregates are detectable. In MO3.13 cells overexpressing increasing amounts of EGFP-CNP, ubiquitin and CNP-positive aggregates appear. In myelin from aged animals, CNP undergoes proteolysis, evidenced by the appearance of low molecular weight fragments throughout the brain, some of which can be generated in vitro by calpain-1 and partially prevented ex vivo by inhibiting calpain-1. CNP proteolysis with age is associated with equally widespread activation of the protease, calpain-1, largely in white matter microglia. The molecular organization of the axon in the region of the node of Ranvier is partly determined and maintained by myelin. Thus, in the presence of age-related changes in myelin, in both the rat and monkey optic nerve, voltage-gated potassium channels (Kv) are mislocalized into the paranodal region. This mislocalization is accompanied by a breakdown in the normal molecular compartmentalization of the paranode, indicated by increasingly irregular contactin-associated protein (caspr)-positive profiles. Hence, in the aged monkey, the accumulation and proteolysis of CNP is a result of both ineffective and inappropriate proteolysis and it is proposed that changes in CNP lead to variation in axonal molecular organization and perhaps to cognitive decline in the rhesus monkey.