Impaired Myelination in the Aging Primate Brain: Quantification using a Combination of Novel Label‐Free Spectral Confocal Reflectance (SCoRe) Microscopy and Fluorescent Immunohistochemistry (IHC)
Authors: Trecartin Katelyn , Tyler William , Haydar Tarik , Rosene Douglas 
Published: June 2021

IntroductionNormal aging in the rhesus monkey is characterized by cognitive decline in learning, memory, and executive function. Monkeys are free of frank neurodegenerative diseases like Alzheimer's, and cortical neurons do not degenerate. Instead, age‐related myelin pathology accumulates and correlates with cognitive decline. Hence, quantifying myelination and assessing individual myelin sheaths is paramount to the study of normal aging. One of the challenges in understanding white matter pathology is the absence of suitable methods to measure myelin dynamics in the primate brain. While electron microscopy has identified ballooning degeneration and loss of myelin sheaths at the ultrastructural level, it has limited applicability to examination of large brain regions and requires unique fixation procedures that make other analyses difficult. Similarly, traditional myelin stains are very difficult to quantify and regular immunohistochemical markers fail to show the pathology directly. As an alternative, we investigated using the novel, label‐free microscopy method SCoRe (spectral confocal reflectance microscopy) in combination with IHC markers of normal myelin to measure myelin integrity in the aging primate brain.MethodsUsing SCoRe and IHC in brain tissue of 10 behaviorally tested adult rhesus monkeys (5 young and 5 old), we examined myelination in the cingulum bundle and cingulate gyrus of the frontal lobe, two regions previously identified for their involvement in cognitive functions compromised in aging. SCoRe was used to visualize individual myelinated axons in combination with a paranodal marker (anti‐Caspr) and axonal marker (anti‐neurofilament H—NFH). Specific measurements were made of the internode length of myelinated axons (anti‐Caspr + SCoRe) and percent of myelinated fibers (anti‐NFH + SCoRe). We also assessed remyelination efforts using a novel, selective antibody (BCAS1) marking actively myelinating oligodendrocytes (anti‐BCAS + anti‐CC1).ResultsWe found that the percentage of myelinated fibers decreases in the aging brain, which is consistent with previous findings in mouse, non‐human primate, and human. We also report an age‐related decrease in internode length and an increase in paranodes that support previous electron microscopy studies in the non‐human primate brain. Additionally, there is a decrease in the number of actively myelinating oligodendrocytes in the aging brain, corroborating previous data showing impaired remyelination with age.ConclusionsThese results demonstrate that we can use label‐free confocal microscopy techniques combined with IHC to quantify age‐related changes in myelin in the brain. Furthermore, these results suggest that myelin disturbances may be exacerbated (or caused) by a decrease in actively myelinating oligodendrocytes and provide an area for further research into the mechanisms of white matter pathology. This combination of techniques should be widely applicable in the study of white matter pathology that develops in health and disease.Support or Funding Information[Supported by NIH Grants R01‐AG04364 and R21‐061678]This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.