Age-dependent modifications of axons, mitochondrial dynamics, and Ca2+ homeostasis underlie the vulnerability of aging white matter to ischemia

Aging white matter (WM) is increasingly susceptible to neurodegenerative diseases and stroke. Among others, changes in ATP production, mitochondrial dynamics, and Ca2+ homeostasis contribute to increased susceptibility of aging WM to stroke. We utilized a pure white matter tract, mouse optic nerve (MON), obtained from 2and 12-month old mice to quantify axon structure and function using electrophysiology, 3D-EM, immunoblots, and ATP assays. Functionally, aging axons did not recover as well after ischemia compared to young axons. Structurally, aging axons became thicker with lower G ratios, indicating increased myelin thickness, as well as increased internodal distances and nodal lengths. Aging axonal mitochondria were larger and thicker with lower incidence of smooth endoplasmic reticulum (SER) association. An increase of mitochondrial fusion proteins with aging resulted in aggregation of mitochondria and together with lower ATP levels suggested mitochondrial dysfunction. Our results suggest that aging alters axonal and mitochondrial structure and function resulting from reduced ATP production that may disrupt Ca2+ homeostasis to underlie the vulnerability of aging WM to ischemia and neurodegenerative diseases. Introduction Stroke is the fourth leading cause of death in the United States, on average killing one person every four minutes (Go et al., 2014). About 87% of all strokes are ischemic strokes, in which blood flow to the brain is blocked, depriving the brain of oxygen and glucose (Go et al., 2014). Aging is a significant risk factor for stroke. It has been previously thought that age-related cardiovascular changes, such as atherosclerosis, would account for the increased risk of stroke with age. However, intrinsic changes to the central nervous system itself contribute to this increased risk (Baltan et al., 2008). White matter is injured in most strokes (Baltan et al., 2008). The white matter contains no neural soma and is composed of myelinated axons, oligodendrocytes, and