title: Astrocytes secrete exosomes enriched with proapoptotic ceramide and prostate apoptosis response 4 (PAR-4): potential mechanism of apoptosis induction in Alzheimer disease (AD).
abstract: Amyloid protein is well known to induce neuronal cell death, whereas only little is known about its effect on astrocytes. We found that amyloid peptides activated caspase 3 and induced apoptosis in primary cultured astrocytes, which was prevented by caspase 3 inhibition. Apoptosis was also prevented by shRNA-mediated down-regulation of PAR-4, a protein sensitizing cells to the sphingolipid ceramide. Consistent with a potentially proapoptotic effect of PAR-4 and ceramide, astrocytes surrounding amyloid plaques in brain sections of the 5xFAD mouse (and Alzheimer disease patient brain) showed caspase 3 activation and were apoptotic when co-expressing PAR-4 and ceramide. Apoptosis was not observed in astrocytes with deficient neutral sphingomyelinase 2 (nSMase2), indicating that ceramide generated by nSMase2 is critical for amyloid-induced apoptosis. Antibodies against PAR-4 and ceramide prevented amyloid-induced apoptosis in vitro and in vivo, suggesting that apoptosis was mediated by exogenous PAR-4 and ceramide, potentially associated with secreted lipid vesicles. This was confirmed by the analysis of lipid vesicles from conditioned medium showing that amyloid peptide induced the secretion of PAR-4 and C18 ceramide-enriched exosomes. Exosomes were not secreted by nSMase2-deficient astrocytes, indicating that ceramide generated by nSMase2 is critical for exosome secretion. Consistent with the ceramide composition in amyloid-induced exosomes, exogenously added C18 ceramide restored PAR-4-containing exosome secretion in nSMase2-deficient astrocytes. Moreover, isolated PAR-4/ceramide-enriched exosomes were taken up by astrocytes and induced apoptosis in the absence of amyloid peptide. Taken together, we report a novel mechanism of apoptosis induction by PAR-4/ceramide-enriched exosomes, which may critically contribute to Alzheimer disease.
authors.affiliation: Program in Developmental Neurobiology, Institute of Molecular Medicine and Genetics, Medical College of Georgia, Georgia Health Sciences University, Augusta, Georgia 30912, USA.
authors.forename: Guanghu
authors.initials: G
authors.lastname: Wang
authors.forename: Michael
authors.initials: M
authors.lastname: Dinkins
authors.forename: Qian
authors.initials: Q
authors.lastname: He
authors.forename: Gu
authors.initials: G
authors.lastname: Zhu
authors.forename: Christophe
authors.initials: C
authors.lastname: Poirier
authors.forename: Andrew
authors.initials: A
authors.lastname: Campbell
authors.forename: Margot
authors.initials: M
authors.lastname: Mayer-Proschel
authors.forename: Erhard
authors.initials: E
authors.lastname: Bieberich
mesh_terms: D000544:Alzheimer Disease / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D000682:Amyloid / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D001253:Astrocytes / Q000378:metabolism* / Q000473:pathology; D053148:Caspase 3 / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D002518:Ceramides / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000235:genetics; D055354:Exosomes / Q000235:genetics / Q000378:metabolism*; D005260:Female; D006801:Humans; D008297:Male; D051379:Mice; D013108:Sphingomyelin Phosphodiesterase / Q000235:genetics / Q000378:metabolism
chemical_list: D000682:Amyloid; D051017:Apoptosis Regulatory Proteins; D002518:Ceramides; C101296:prostate apoptosis response-4 protein; C491484:SMPD3 protein, human; C491483:Smpd3 protein, mouse; D013108:Sphingomyelin Phosphodiesterase; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse; D053148:Caspase 3