The complexity of cardiolipin in health and disease

Abstract

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

Figure 1. Cardiolipin, the center of mitochondrial physiology

The importance of cardiolipin (CL) to normal mitochondrial function is now appreciated to be multifaceted [–4, 8]. (i) Although not absolutely required, cardiolipin significantly improves the efficiency and adaptability of the OXPHOS machinery by at least 2 distinct mechanisms. First, cardiolipin stabilizes higher order assemblies of respiratory complexes, called supercomplexes, and is required for the association of the major ADP/ATP carrier with respiratory supercomplexes [2, 7, 8]. This is predicted to increase both the efficiency of electron flow and ADP/ATP exchange. Second, cardiolipin functions as a proton trap that restricts the flow of protons [75]. (ii) One consequence of reduced functionality of the electron transport chain in the absence of cardiolipin is the generation of a weaker electrochemical gradient (ΔΨ). This negatively impacts the biogenesis of proteins destined for the mitochondrial matrix and/or IM via translocases in the IM (TIM22 and TIM23) [2]. Interestingly, cardiolipin is also required for the biogenesis of proteins destined for the outer membrane (OM; import of OM proteins is membrane potential-independent) presumably by stabilizing the OM translocases, TOM and SAM (sorting and assembly machinery) [1]. (iii) Mitochondria are highly dynamic organelles with their shape and cellular numbers dictated by balanced fission and fusion events. IM fusion, performed by dynamin-related GTPases, Mgm1p in yeast and OPA1 in mammals, requires a balance of long and short isoforms. Cardiolipin restricts the short isoforms to cardiolipin-enriched membranes and promotes sMgm1p GTPase activity, which is required for IM fusion [76, 77]. Mitochondrial division is mediated by the dynamin-related GTPase, DRP1 (Dnm1p in yeast). Overexpression of a DRP1 variant defective in cardiolipin binding alters mitochondrial morphology consistent with it functioning as a dominant-negative allele [78]. (iv) The importance of cardiolipin and perhaps mature cardiolipin to cristae morphology is highlighted by the repeated observation in multiple cardiolipin-deficient models of mitochondria with abnormal IM ultrastructure [, , –56]. (v) The presence of cardiolipin in membranes identifies those membranes as being mitochondrial. Thus, not surprisingly, the ability to recognize and/or bind cardiolipin-containing membranes is one method through which diverse cellular activities impinge at the level of the mitochondrion. For instance, cardiolipin promotes apoptosis by serving as a recruitment platform for caspase 8 downstream of Fas receptor signaling [79].

Figure 2. Overview of cardiolipin biosynthesis and remodeling

The biosynthesis of cardiolipin (CL) occurs in the mitochondrion. Phosphatidylglycerolphosphate synthase (Pgs1p) catalyzes the first and committed step in cardiolipin biosynthesis producing the short-lived phosphatidylglycerolphosphate (PGP) from the condensation of cytidine 5′-diphosphate-diacylglycerol (CDP-DAG) and glycerol-3-phosphate (G3P). PGP is dephosphorylated to phosphatidylglycerol (PG) by a phosphatase, identified recently in yeast as Gep4p and even more recently in mammals as the phylogenetically unrelated PTPMT1 [15, 16]. When cardiolipin synthase, Crd1p, forms CL from PG and another molecule of CDP-DAG, the result is immature cardiolipin characterized by a random assortment of attached acyl chains that are saturated and variable in length. Acyl chain remodeling is responsible for the final molecular composition of mature cardiolipin which is typically defined by the symmetric incorporation of unsaturated longer fatty acyl chains [–12]. The remodeling process is initiated by a phospholipase, in yeast the cardiolipin deacylase Cld1p [21] and in mammals the calcium-independent iPLA_2γ [–26], which remove an acyl chain from cardiolipin generating the remodeling intermediate, monolysocardiolipin (MLCL). The regeneration of cardiolipin from monolysocardiolipin is accomplished by up to three distinct proteins, tafazzin (Taz1p), monolysocardiolipin acyltransferase 1 (MLCLAT1), and acyl-CoA:lysocardiolipin acyltransferase-1 (ALCAT1). Whereas MLCLAT1 and ALCAT1 utilize acyl-CoA as the acyl chain donor for the reacylation of MLCL [33, 38, 39], Taz1p is a transacylase that takes an acyl chain from another phospholipid, preferentially phosphatidylcholine (PC) or phosphatidylethanolamine, and adds it to monolysocardiolipin [28]. Also notable is that the acyl chain specificity of these three enzymes is not the same. Tafazzin lacks acyl chain specificity [29], MLCLAT1 prefers 18:2 linoleoyl-CoA [33, 34], and ALCAT1 utilizes acyl-CoA loaded with diverse long-chain, unsaturated acyl chains [38, 39]. Black boxes highlight currently undefined processes/information. Amazingly, each cardiolipin remodeling enzyme resides in a distinct subcellular compartment suggestive of distinct functional outcomes. Whereas Taz1p and MLCLAT1 remodeling generates, and perhaps maintains, the tissue-specific, homogenous mature cardiolipin, ALCAT1 remodeling produces “bad” cardiolipin that is associated with pathologic processes.