Deficient Surveillance and Phagocytic Activity of Myeloid Cells Within Demyelinated Lesions in Aging Mice Visualized by Ex Vivo Live Multiphoton Imaging

Aging impairs regenerative processes including remyelination, the synthesis of a new myelin sheath. Microglia and other infiltrating myeloid cells such as macrophages are essential for remyelination through mechanisms that include the clearance of inhibitory molecules within the lesion. Prior studies have shown that the quantity of myeloid cells and the clearance of inhibitory myelin debris are deficient in aging, contributing to the decline in remyelination efficiency with senescence. It is unknown, however, whether the impaired clearance of debris is simply the result of the reduced number of phagocytes or if the dynamic activity of myeloid cells within the demyelinating plaque also declines with aging and this question is relevant to the proper design of therapeutics to mobilize myeloid cells for repair. Herein, we describe a high-resolution multiphoton ex vivo live imaging protocol that visualizes individual myelinated/demyelinated axons and lipid-containing myeloid cells to investigate the demyelinated lesion of aging female mice. Wefound that aging lesions have fewer myeloid cells and that these have reduced phagocytosis of myelin. Although the myeloid cells are actively migratory within the lesion of young mice and have protrusions that seem to survey the environment, this motility and surveillance is significantly reduced in aging mice. Our results emphasize the necessity of not only increasing the number of phagocytes, but also enhancing their activity once they are within demyelinated lesions. The high-resolution live imaging of demyelinated lesions can serve as a platform with which to discover pharmacological agents that rejuvenate intralesional remodeling that promotes the repair of plaques.