Abstract
Remyelination following CNS demyelination restores rapid signal propagation and protects axons; however, its efficiency declines with increasing age. Both intrinsic changes in the oligodendrocyte progenitor cell population and extrinsic factors in the lesion microenvironment of older subjects contribute to this decline. Microglia and monocyte-derived macrophages are critical for successful remyelination, releasing growth factors and clearing inhibitory myelin debris. Several studies have implicated delayed recruitment of macrophages/microglia into lesions as a key contributor to the decline in remyelination observed in older subjects. Here we show that the decreased expression of the scavenger receptor CD36 of aging mouse microglia and human microglia in culture underlies their reduced phagocytic activity. Overexpression of CD36 in cultured microglia rescues the deficit in phagocytosis of myelin debris. By screening for clinically approved agents that stimulate macrophages/microglia, we have found that niacin (vitamin B3) upregulates CD36 expression and enhances myelin phagocytosis by microglia in culture. This increase in myelin phagocytosis is mediated through the niacin receptor (hydroxycarboxylic acid receptor 2). Genetic fate mapping and multiphoton live imaging show that systemic treatment of 9–12-month-old demyelinated mice with therapeutically relevant doses of niacin promotes myelin debris clearance in lesions by both peripherally derived macrophages and microglia. This is accompanied by enhancement of oligodendrocyte progenitor cell numbers and by improved remyelination in the treated mice. Niacin represents a safe and translationally amenable regenerative therapy for chronic demyelinating diseases such as multiple sclerosis.
Highlights
The loss of the myelin sheath has significant consequences for axonal health and conduction velocity [17, 29, 42]
As phagocytosis declines with age in macrophages [34, 39], we sought to corroborate this finding in microglia from aged (> 15 month) mice
Microglia were exposed to myelin debris and staining for intracellular iNOS demonstrate that 80% of cells were stimulated after 2 h (Fig. 1e, f)
Summary
The loss of the myelin sheath has significant consequences for axonal health and conduction velocity [17, 29, 42]. Remyelination is an endogenous process that restores rapid signal propagation and protects axons, but the process is usually incomplete and becomes inefficient with advanced age in rodents [47] Both intrinsic changes in the oligodendrocyte progenitor cell (OPC) population as well as the extrinsic lesion microenvironment are thought to underlie the decreased remyelination efficiency in older animals after demyelination, likely accelerating their rate of axonal loss [5, 12, 35, 41, 45, 46]. These results of parabiotic young macrophages altering the inhibitory lesion microenvironment of older mice suggest that pharmacological stimulation is a practical means to rejuvenate deficient macrophages/microglia to promote remyelination in older individuals
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