Neutral Sphingomyelinase-2 Deficiency Ameliorates Alzheimer's Disease Pathology and Improves Cognition in the 5XFAD Mouse

Recent evidence implicates exosomes in the aggregation of A beta and spreading of tau in Alzheimer's disease. In neural cells, exosome formation can be blocked by inhibition or silencing of neutral sphingomyelinase-2 (nSMase2). We generated genetically nSMase2-deficient 5XFAD mice (fro; 5XFAD) to assess AD-related pathology in a mouse model with consistently reduced ceramide generation. We conducted in vitro assays to assess A beta(42) aggregation and glial clearance with and without exosomes isolated by ultracentrifugation and determined exosome-induced amyloid aggregation by particle counting. We analyzed brain exosome content, amyloid plaque formation, neuronal degeneration, sphingolipid, A beta(42) and phospho-tau levels, and memory-related behaviors in 5XFAD versus fro; 5XFAD mice using contextual and cued fear conditioning. Astrocyte-derived exosomes accelerated aggregation of A beta(42) and blocked glial clearance of A beta(42) in vitro. A beta(42) aggregates were colocalized with extracellular ceramide in vitro using a bifunctional ceramide analog preloaded into exosomes and in vivo using anticeramide IgG, implicating ceramide-enriched exosomes in plaque formation. Compared with 5XFAD mice, the fro; 5XFAD mice had reduced brain exosomes, ceramide levels, serum anticeramide IgG, glial activation, total A beta(42) and plaque burden, tau phosphorylation, and improved cognition in a fear-conditioned learning task. Ceramide-enriched exosomes appear to exacerbate AD-related brain pathology by promoting the aggregation of A beta. Reduction of exosome secretion by nSMase2 loss of function improves pathology and cognition in the 5XFAD mouse model.