Ceramide forms channels in mitochondrial outer membranes at physiologically relevant concentrations

Abstract

Recent evidence suggests that the ability of ceramides to induce apoptosis is due to a direct action on mitochondria. Mitochondria are known to contain enzymes responsible for ceramide synthesis and hydrolysis and mitochondrial ceramide levels have been shown to be elevated prior to the mitochondrial phase of apoptosis. Ceramides have been reported to induce the release of intermembrane space proteins from mitochondria, which has been linked to their ability to form large channels in membranes. The aim of this study was to determine if the membrane concentration of ceramide required for the formation of protein permeable channels is within the range that is present in mitochondria during the induction phase of apoptosis. Only a very small percentage of the ceramide actually inserts into the mitochondrial membranes. The permeability of the mitochondrial outer membrane correlates directly with the level of ceramide in the membrane. Importantly, the concentration of ceramide at which significant channel formation occurs is consistent with the level of mitochondrial ceramide that occurs during the induction phase of apoptosis (4 pmol ceramide/ nanomole phospholipid). Similar results were obtained with short-and long-chain ceramide. Ceramide channel formation is specific to mitochondrial membranes in that no channel formation occurs in the plasma membranes of erythrocytes even at concentrations 20 times higher than those required for channel formation in mitochondrial outer membranes. Thus, ceramide channels are good candidates for the pathway by which proapoptotic proteins are released from mitochondria during the induction phase of apoptosis.

Anti-apoptotic Bcl-2 Family Proteins Disassemble Ceramide Channels

“…Ceramide-induced permeabilization of the MOM shows the characteristics of an organized channel (23,24). As expected for a channel, ceramide allows the bi-directional flux of cytochrome c across the MOM and not just its release (23).…”

“…Previous studies show that ceramide possesses the ability to form large protein-permeable channels in planar phospholipid as well as MOMs of isolated mitochondria (6,(22)(23)(24)(25). Although channel formation by ceramide occurs in the MOM, it does not occur in the plasma membrane and thus is somehow influenced by the membrane environment (24).…”

“…Although channel formation by ceramide occurs in the MOM, it does not occur in the plasma membrane and thus is somehow influenced by the membrane environment (24). Remarkably, channel formation depends on the presence of the 4 -5-trans double bond of the sphingoid base backbone of ceramide.…”

Ceramide Synthase-dependent Ceramide Generation and Programmed Cell Death

“…Cer channels forming in the plasma membrane of apoptotic cells might directly or indirectly promote permeability to vital dyes and LDH. However, no channels were observed when Cer was added to the plasma membranes of erythrocytes, suggesting that the channel-forming ability of Cer is location-specific (69,70). Whether Cer channels actually form in mitochondria in vivo or whether they can form in the plasma membrane or other organelles during programmed cell death remains to be demonstrated.…”

The deficit in long-term potentiation induced by chronic administration of amyloid-β is attenuated by treatment of rats with a novel phospholipid-based drug formulation, VP025

“…Among the factors which contribute to loss of mitochondrial membrane patency and ultimately, activation of caspase 3, is ceramide, which is generated when sphingomyelinase hydrolyses the sphingophospholipid, sphingomyelin. It has been suggested that caspase 8 can act on membrane-associated sphingomyelinase in endosomes or lysosomes, and in this way lead to the generation of ceramide (Smyth et al, 1996) which can increase mitochondrial membrane permeability and trigger cytochrome c release (Roberts, 2005;Siskind et al, 2006).…”