Defective cholesterol clearance limits remyelination in the aged central nervous system

Keeping cholesterol at bay A decline in tissue repair is a universal hallmark of aging. The failure to regenerate myelin sheaths in multiple sclerosis lesions contributes to chronic progressive disease and disability. Understanding the cause and preventing this failure is a key goal in regenerative medicine. Cantuti-Castelvetri et al. report that the self-limiting inflammatory response, which is necessary for remyelination to occur, is maladaptive in the central nervous system (CNS) of old mice (see the Perspective by Chen and Popko). Cholesterol-rich myelin debris overwhelmed the efflux capacity of phagocytes, resulting in a transition of free cholesterol into crystals, thereby inducing lysosomal rupture and inflammasome stimulation. Thus, drugs being developed to promote cholesterol clearance in human atherosclerosis lesions may also be good candidates for regenerative medicine in the CNS. Science, this issue p. 684; see also p. 635 Overloading phagocytes with cholesterol drives inflammation and limits tissue regeneration in brains from older mice. Age-associated decline in regeneration capacity limits the restoration of nervous system functionality after injury. In a model for demyelination, we found that old mice fail to resolve the inflammatory response initiated after myelin damage. Aged phagocytes accumulated excessive amounts of myelin debris, which triggered cholesterol crystal formation and phagolysosomal membrane rupture and stimulated inflammasomes. Myelin debris clearance required cholesterol transporters, including apolipoprotein E. Stimulation of reverse cholesterol transport was sufficient to restore the capacity of old mice to remyelinate lesioned tissue. Thus, cholesterol-rich myelin debris can overwhelm the efflux capacity of phagocytes, resulting in a phase transition of cholesterol into crystals and thereby inducing a maladaptive immune response that impedes tissue regeneration.