Ceramide-mediated clustering is required for CD95-DISC formation

Abstract

Early events required for induction of apoptosis by CD95 are preassociation of CD95, the formation of the deathinducing signaling complex (DISC) and clustering of CD95 in distinct membrane domains. Here, we identify the molecular ordering of these events and show that the acid sphingomyelinase (ASM) functions upstream of the DISC to mediate CD95 clustering in ceramide-enriched membrane platforms, an event that is required for DISC formation. Experiments in ASM-deficient cells revealed that CD95 ligation, in the absence of ceramide generation, triggers o1% of full caspase 8 activation at the receptor. This event, however, is both necessary and sufficient to trigger translocation of ASM onto the outer leaflet of the plasma membrane, ASM activation and ceramide release, but insufficient for apoptosis induction. Ceramidemediated CD95 clustering then amplifies the primary CD95 signaling and drives the second step of CD95 signaling, that is, formation of the DISC yielding 100% caspase activity and apoptosis. These studies suggest that the most parsimonious interpretation of the molecular ordering of the earliest events in CD95 signaling, at least in some cells, is: CD95 ligation-1% of maximum caspase 8 activation-ASM translocation-ceramide generation-CD95clustering-DISC formation-100% of maximum caspase 8 activation-apoptosis.

New insights into apoptosis signaling by Apo2L/TRAIL

“…Instead, receptor O-glycosylation appeared to be required for efficient ligand-induced clustering of DR4 and DR5, which triggers DISC assembly and caspase-8 activation. Clustering of the Fas receptor has been shown to depend on the generation of ceramide-enriched membrane microdomains by acid sphyngomyelinase (Grassme et al, 2001(Grassme et al, , 2003. Similarly, several studies have suggested a role of ceramide-rich platforms in signaling Apoptosis signaling by Apo2L/TRAIL F Gonzalvez and A Ashkenazi by DR4 and DR5 (Martin et al, 2005;Dumitru et al, 2007).…”

The Role of Sphingolipids and Ceramide in Pulmonary Inflammation in Cystic Fibrosis

“…We suggested that ceramide-enriched membrane domains serve to re-organize receptor molecules and intracellular signaling molecules in the cell membane [51], which permits the specific receptor to transmit its signal into the cell. Ceramide-enriched membrane domains exhibit very different biophysical properties than other parts of the plasma membrane resulting in preferential trapping of activated receptor molecules and/or signaling molecules in these domains.…”

On the trail of cell death pathways in prostate cancer

“…Autocatalytic activation of pro-caspase 8 can lead directly to the proteolytic activation of pro-caspase 3, and the rapid initiation of DNA fragmentation (the so called extrinsic pathway), or it can lead to the initiation of mitochondrial membrane transition through the insertion of t-Bax into the mitochondrial membrane (the intrinsic pathway), formation of the apoptosome and slower activation of pro-caspase 3 and DNA fragmentation. Although it is not yet established what leads to the choice of which pathway is selected after assembly of the DISC, it has been suggested that it may be related to the amount of pro-caspase 8 recruited to the DISC which in turn may be dictated by other components required for the assembly of the DISC such as acid sphingomyelinase, 21 selective recruitment of the inhibitor cFLIP to the DISC, 22,23 or more fundamental changes in the biosynthesis and surface expression of specific death receptors. 24 Activation of caspase 8 is necessary but not sufficient for the induction of cell death, inhibition of the receptor tyrosine kinase mediated cell survival pathways is also necessary to ensure that cells complete the apoptotic process.…”