Ceramide Metabolism and Transport: Implications on the Initiation of Apoptosis

Apoptosis is a process by which unwanted cells are eliminated in a controlled manner. Early in apoptosis, ceramide levels rise and the mitochondrial outer membrane becomes permeable to proteins. The permeability of the outer membrane is attributed to the self-assembly of ceramide in form of channels. In the only direct structural study, to date, ceramide channels were visualized in liposomes using transmission electron microscopy. Those channels were of various sizes, averaging 10 nm in diameter. In concert, using electrophysiological techniques, the estimated diameter of ceramide channels was also around 10 nm. These channels are large enough to release all the pro-apoptotic intermembrane space proteins to initiate apoptosis. Dihydroceramide desaturase converts the inactive precursor, dihydroceramide to ceramide. Both long and short chain dihydroceramides inhibit ceramide channel formation in mitochondria. The inhibition is strong as one tenth as much dihydroceramide inhibited the outer membrane permeabilization by 95% (C2) and 51% (C16). Other mitochondrial components are not required for such inhibition as comparable amounts prevented the permeabilization of liposomes. Hence, the apoptogenic activity of ceramide may depend on the ceramide to dihydroceramide ratio perhaps resulting in a more abrupt transition from the normal to the apoptotic state. The location of the desaturase is the endoplasmic reticulum (ER). Only minimal activity was measured in mitochondria. However, newly synthesized ceramide from 14C-C8-dihydroceramide or 3H-sphingosine (in the ER) can transfer rapidly to mitochondria (40 % in 10 min) and permeabilize them to cytochrome c and adenylate kinase. The transfer of sphingolipids is bidirectional and non-specific. The transfer mechanism is consistent with direct membrane contact, since reducing the organellar concentrations by half resulted in a four-fold reduction of the transfer rate. Thus this ceramide exchange obviates the need for a complete ceramide de novo pathway in mitochondria in order for cells to use ceramide to activate mitochondria-mediated apoptosis. These results demonstrate the ability of ceramide to form large channels capable of releasing proteins from mitochondria. Ceramide can rapidly reach mitochondria and there are mechanisms to control the propensity for ceramide channel formation. Clearly ceramide channels play a central role in the decision to undergo apoptosis

Mysterious sphingolipids: metabolic interrelationships at the center of pathophysiology

Metabolic pathways are complex and intertwined. Deficiencies in one or more enzymes in a given pathway are directly linked with genetic diseases, most of them having devastating manifestations. The metabolic pathways undertaken by sphingolipids are diverse and elaborate with ceramide species serving as the hubs of sphingolipid intermediary metabolism and function. Sphingolipids are bioactive lipids that serve a multitude of cellular functions. Being pleiotropic in function, deficiency or overproduction of certain sphingolipids is associated with many genetic and chronic diseases. In this up-to-date review article, we strive to gather recent scientific evidence about sphingolipid metabolism, its enzymes, and regulation. We shed light on the importance of sphingolipid metabolism in a variety of genetic diseases and in nervous and immune system ailments. This is a comprehensive review of the state of the field of sphingolipid biochemistry

Mitochondrial physiology

As the knowledge base and importance of mitochondrial physiology to evolution, health and disease expands, the necessity for harmonizing the terminology concerning mitochondrial respiratory states and rates has become increasingly apparent. The chemiosmotic theory establishes the mechanism of energy transformation and coupling in oxidative phosphorylation. The unifying concept of the protonmotive force provides the framework for developing a consistent theoretical foundation of mitochondrial physiology and bioenergetics. We follow the latest SI guidelines and those of the International Union of Pure and Applied Chemistry (IUPAC) on terminology in physical chemistry, extended by considerations of open systems and thermodynamics of irreversible processes. The concept-driven constructive terminology incorporates the meaning of each quantity and aligns concepts and symbols with the nomenclature of classical bioenergetics. We endeavour to provide a balanced view of mitochondrial respiratory control and a critical discussion on reporting data of mitochondrial respiration in terms of metabolic flows and fluxes. Uniform standards for evaluation of respiratory states and rates will ultimately contribute to reproducibility between laboratories and thus support the development of data repositories of mitochondrial respiratory function in species, tissues, and cells. Clarity of concept and consistency of nomenclature facilitate effective transdisciplinary communication, education, and ultimately further discovery

Mitochondrial physiology

As the knowledge base and importance of mitochondrial physiology to evolution, health and disease expands, the necessity for harmonizing the terminology concerning mitochondrial respiratory states and rates has become increasingly apparent. The chemiosmotic theory establishes the mechanism of energy transformation and coupling in oxidative phosphorylation. The unifying concept of the protonmotive force provides the framework for developing a consistent theoretical foundation of mitochondrial physiology and bioenergetics. We follow the latest SI guidelines and those of the International Union of Pure and Applied Chemistry (IUPAC) on terminology in physical chemistry, extended by considerations of open systems and thermodynamics of irreversible processes. The concept-driven constructive terminology incorporates the meaning of each quantity and aligns concepts and symbols with the nomenclature of classical bioenergetics. We endeavour to provide a balanced view of mitochondrial respiratory control and a critical discussion on reporting data of mitochondrial respiration in terms of metabolic flows and fluxes. Uniform standards for evaluation of respiratory states and rates will ultimately contribute to reproducibility between laboratories and thus support the development of data repositories of mitochondrial respiratory function in species, tissues, and cells. Clarity of concept and consistency of nomenclature facilitate effective transdisciplinary communication, education, and ultimately further discovery

Very long chain ceramides interfere with C16-ceramide-induced channel formation: A plausible mechanism for regulating the initiation of intrinsic apoptosis

AbstractMitochondria mediate both cell survival and death. The intrinsic apoptotic pathway is initiated by the permeabilization of the mitochondrial outer membrane to pro-apoptotic inter-membrane space (IMS) proteins. Many pathways cause the egress of IMS proteins. Of particular interest is the ability of ceramide to self-assemble into dynamic water-filled channels. The formation of ceramide channels is regulated extensively by Bcl-2 family proteins and dihydroceramide. Here, we show that the chain length of biologically active ceramides serves as an important regulatory factor. Ceramides are synthesized by a family of six mammalian ceramide synthases (CerS) each of which produces a subset of ceramides that differ in their fatty acyl chain length. Various ceramides permeabilize mitochondria differentially. Interestingly, the presence of very long chain ceramides reduces the potency of C16-mediated mitochondrial permeabilization indicating that the intercalation of the lipids in the dynamic channel has a destabilizing effect, reminiscent of dihydroceramide inhibition of ceramide channel formation (Stiban et al., 2006). Moreover, mitochondria isolated from cells overexpressing the ceramide synthase responsible for the production of C16-ceramide (CerS5) are permeabilized faster upon the exogenous addition of C16-ceramide whereas they are resistant to permeabilization with added C24-ceramide. On the other hand mitochondria isolated from CerS2-overexpressing cells show the opposite pattern, indicating that the product of CerS2 inhibits C16-channel formation ex vivo and vice versa. This interplay between different ceramide metabolic enzymes and their products adds a new dimension to the complexity of mitochondrial-mediated apoptosis, and emphasizes its role as a key regulatory step that commits cells to life or death

Regulation of ceramide channel formation and disassembly: Insights on the initiation of apoptosis

AbstractSphingolipid research has surged in the past two decades and has produced a wide variety of evidence supporting the role of this class of molecules in mediating cellular growth, differentiation, senescence, and apoptosis. Ceramides are a subgroup of sphingolipids (SLs) that are directly involved in the process of initiation of apoptosis. We, and others, have recently shown that ceramides are capable of the formation of protein-permeable channels in mitochondrial outer membranes under physiological conditions. These pores are indeed good candidates for the pathway of release of pro-apoptotic proteins from the mitochondrial intermembrane space (IMS) into the cytosol to initiate intrinsic apoptosis. Here, we review recent findings on the regulation of ceramide channel formation and disassembly, highlighting possible implications on the initiation of the intrinsic apoptotic pathway