Histochemical markers of myelin damage and impaired remyelination in the aging rhesus monkey brain: relationship to cognitive performance
Estrada, Larissa Isabel
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Myelin damage is known to increase in the normal aging brain and to correlate with age-related cognitive decline. While the causes of increased myelin damage are unknown, here we consider whether the brain’s innate capacity for remyelination diminishes with age and hence could contribute to myelin damage through slow accumulation of myelin defects. Maintenance and repair of myelin depends upon oligodendroglia precursor cells (OPCs), which must differentiate into a sufficient number of healthy mature oligodendroglia (oligos), the myelinating cell of the brain. The extracellular matrix molecule hyaluronic acid (HA) has been shown to inhibit maturation of OPCs into mature myelinating oligos. The present study examined aging changes in myelination using four markers: the damaged myelin basic protein (dMBP) antibody, a histochemical reaction to stain HA, and immunohistochemistry for OPCs and mature oligos. These markers were quantified using cell density (oligos and OPCs), percent area stained (HA and dMBP), and fluorescence intensity (HA and dMBP). Relationships between these markers, age, and behavioral measures of cognitive function were investigated using single and multiple regression analyses. Results showed that in the corpus callosum and cingulum bundle of the rhesus monkey, staining for dMBP as a marker of myelin damage strongly correlated with increases in HA. The increase in HA in the cingulum bundle correlated positively with age. OPC density increased with age in both the cingulum bundle and corpus callosum. Mature oligo density did not change significantly with age, but approached a significant increase in the cingulum and approached a significant decrease in the corpus callosum. The increase in OPC density correlated positively with both HA and dMBP in the cingulum bundle. These data are consistent with the hypothesis that HA accumulation contributes to myelin damage by inhibiting the differentiation of OPCs into mature oligodendrocytes, diminishing the brain’s innate capacity for remyelination with age.