Regulation of cardiolipin synthase levels in Saccharomyces cerevisiae

Cardiolipin function in the yeast S. cerevisiae and the lessons learned for Barth syndrome

Cardiolipin (CL) is the signature phospholipid (PL) of mitochondria and plays a pivotal role in mitochondrial and cellular function. Disruption of the CL remodeling gene tafazzin (TAZ) causes the severe genetic disorder Barth syndrome (BTHS). Our current understanding of the function of CL and the mechanism underlying the disease has greatly benefited from studies utilizing the powerful yeast model Saccharomyces cerevisiae. In this review, we discuss important findings on the function of CL and its remodeling from yeast studies and the implications of these findings for BTHS, highlighting the potential physiological modifiers that may contribute to the disparities in clinical presentation among BTHS patients.

E3 ligase subunit Fbxo15 and PINK1 kinase regulate cardiolipin synthase 1 stability and mitochondrial function in pneumonia

Acute lung injury (ALI) is linked to mitochondrial injury, resulting in impaired cellular oxygen utilization; however, it is unknown how these events are linked on the molecular level. Cardiolipin, a mitochondrial-specific lipid, is generated by cardiolipin synthase (CLS1). Here, we show that S. aureus activates a ubiquitin E3 ligase component, Fbxo15, that is sufficient to mediate proteasomal degradation of CLS1 in epithelia, resulting in decreased cardiolipin availability and disrupted mitochondrial function. CLS1 is destabilized by the phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (PINK1), which binds CLS1 to phosphorylate and regulates CLS1 disposal. Like Fbxo15, PINK1 interacts with and regulates levels of CLS1 through a mechanism dependent upon Thr219. S. aureus infection upregulates this Fbxo15-PINK1 pathway to impair mitochondrial integrity, and Pink1 knockout mice are less prone to S. aureus-induced ALI. Thus, ALI-associated disruption of cellular bioenergetics involves bioeffectors that utilize a phosphodegron to elicit ubiquitin-mediated disposal of a key mitochondrial enzyme.

The topology and regulation of cardiolipin biosynthesis and remodeling in yeast

The signature mitochondrial phospholipid cardiolipin plays an important role in mitochondrial function, and alterations in cardiolipin metabolism are associated with human disease. Topologically, cardiolipin biosynthesis and remodeling are complex. Precursor phospholipids must be transported from the ER, across the mitochondrial outer membrane to the matrix-facing leaflet of the inner membrane, where cardiolipin biosynthesis commences. Post-synthesis, cardiolipin undergoes acyl chain remodeling, requiring additional trafficking steps, before it achieves its final distribution within both mitochondrial membranes. This process is regulated at several points via multiple independent mechanisms. Here, we review the regulation and topology of cardiolipin biosynthesis and remodeling in the yeast Saccharomyces cerevisiae. Although cardiolipin metabolism is more complicated in mammals, yeast have been an invaluable model for dissecting the steps required for this process.

Deacylation on the matrix side of the mitochondrial inner membrane regulates cardiolipin remodeling

Our understanding of the clinically relevant tafazzin-mediated cardiolipin (CL) remodeling pathway is incomplete. In this study, a new trafficking step required for CL remodeling has been identified. Further, it is demonstrated that flux through this CL remodeling pathway is controlled by the strength of the mitochondrial electrochemical gradient.

The mitochondrial-specific lipid cardiolipin (CL) is required for numerous processes therein. After its synthesis on the matrix-facing leaflet of the inner membrane (IM), CL undergoes acyl chain remodeling to achieve its final form. In yeast, this process is completed by the transacylase tafazzin, which associates with intermembrane space (IMS)-facing membrane leaflets. Mutations in TAZ1 result in the X-linked cardiomyopathy Barth syndrome. Amazingly, despite this clear pathophysiological association, the physiological importance of CL remodeling is unresolved. In this paper, we show that the lipase initiating CL remodeling, Cld1p, is associated with the matrix-facing leaflet of the mitochondrial IM. Thus monolysocardiolipin generated by Cld1p must be transported to IMS-facing membrane leaflets to gain access to tafazzin, identifying a previously unknown step required for CL remodeling. Additionally, we show that Cld1p is the major site of regulation in CL remodeling; and that, like CL biosynthesis, CL remodeling is augmented in growth conditions requiring mitochondrially produced energy. However, unlike CL biosynthesis, dissipation of the mitochondrial membrane potential stimulates CL remodeling, identifying a novel feedback mechanism linking CL remodeling to oxidative phosphorylation capacity.

Involvement of p32 and microtubules in alteration of mitochondrial functions by rubella virus

The interaction of the rubella virus (RV) capsid (C) protein and the mitochondrial p32 protein is believed to participate in virus replication. In this study, the physiological significance of the association of RV with mitochondria was investigated by silencing p32 through RNA interference. It was demonstrated that downregulation of p32 interferes with microtubule-directed redistribution of mitochondria in RV-infected cells. However, the association of the viral C protein with mitochondria was not affected. When cell lines either pretreated with respiratory chain inhibitors or cultivated under (mild) hypoxic conditions were infected with RV, viral replication was reduced in a time-dependent fashion. Additionally, RV infection induces increased activity of mitochondrial electron transport chain complex III, which was associated with an increase in the mitochondrial membrane potential. These effects are outstanding among the examples of mitochondrial alterations caused by viruses. In contrast to the preferential localization of p32 to the mitochondrial matrix in most cell lines, RV-permissive cell lines were characterized by an almost exclusive membrane association of p32. Conceivably, this contributes to p32 function(s) during RV replication. The data presented suggest that p32 fulfills an essential function for RV replication in directing trafficking of mitochondria near sites of viral replication to meet the energy demands of the virus.

A novel function of the human CLS1 in phosphatidylglycerol synthesis and remodeling

Phosphatidylglycerol (PG) is a precursor for the biosynthesis of cardiolipin and a signaling molecule required for various cellular functions. PG is subjected to remodeling subsequent to its de novo biosynthesis in mitochondria to incorporate appropriate acyl content for its biological functions and to prevent the harmful effect of lysophosphatidylglycerol (LPG) accumulation. Yet, a gene encoding a mitochondrial LPG acyltransferase has not been identified. In this report, we identified a novel function of the human cardiolipin synthase (hCLS1) in regulating PG remodeling. In addition to the reported cardiolipin synthase activity, the recombinant hCLS1 protein expressed in COS-7 cells and Sf-9 insect cells exhibited a strong acyl-CoA-dependent LPG acyltransferase activity, which was further confirmed by purified hCLS1 protein overexpressed in Sf-9 cells. The recombinant hCLS1 displayed an acyl selectivity profile in the order of in the order of C18:1>C18:2>C18:0>C16:0, which is similar to that of hCLS1 toward PGs in cardiolipin synthesis, suggesting that the PG remodeling by hCLS1 is an intrinsic property of the enzyme. In contrast, no significant acyltransferase activity was detected from the recombinant hCLS1 enzyme toward lysocardiolipin which shares a similar structure with LPG. In support of a key function of hCLS1 in PG remodeling, overexpression of hCLS1 in COS-7 cells significantly increased PG biosynthesis concurrent with elevated levels of cardiolipin without any significant effects on the biosynthesis of other phospholipids. These results demonstrate for the first time that hCLS1 catalyzes two consecutive steps in cardiolipin biosynthesis by acylating LPG to PG and then converting PG to cardiolipin.

Ups1p and Ups2p antagonistically regulate cardiolipin metabolism in mitochondria

Cardiolipin, a unique phospholipid composed of four fatty acid chains, is located mainly in the mitochondrial inner membrane (IM). Cardiolipin is required for the integrity of several protein complexes in the IM, including the TIM23 translocase, a dynamic complex which mediates protein import into the mitochondria through interactions with the import motor presequence translocase–associated motor (PAM). In this study, we report that two homologous intermembrane space proteins, Ups1p and Ups2p, control cardiolipin metabolism and affect the assembly state of TIM23 and its association with PAM in an opposing manner. In ups1Δ mitochondria, cardiolipin levels were decreased, and the TIM23 translocase showed altered conformation and decreased association with PAM, leading to defects in mitochondrial protein import. Strikingly, loss of Ups2p restored normal cardiolipin levels and rescued TIM23 defects in ups1Δ mitochondria. Furthermore, we observed synthetic growth defects in ups mutants in combination with loss of Pam17p, which controls the integrity of PAM. Our findings provide a novel molecular mechanism for the regulation of cardiolipin metabolism.

A fitness-based interferential genetics approach using hypertoxic/inactive gene alleles as references

Genetics, genomics, and biochemistry have all been of immense help in characterizing macromolecular cell entities and their interactions. Still, obtaining an overall picture of the functioning of even a simple unicellular species has remained a challenging task. One possible way to obtain a comprehensive picture has been described: by capitalizing on the observation that the overexpression on a multicopy plasmid of apparently any wild-type gene in yeast can lead to some negative effect on cell fitness (referring to the concept of "gene toxicity"), the FIG (fitness-based interferential genetics) approach was devised for selecting normal genes that are in antagonistic (and potentially also agonistic) relationship with a particular gene used as a reference. Herein, we take a complementary approach to FIG, by first selecting a "hypertoxic" allele of the reference gene--which easily provides the general possibility of obtaining gene products with the remarkable property of being inactive without altering their macromolecular interactivity--and then looking for the genes that interact functionally with this reference. Thus, FIG and the present approach (Trap-FIG), both taking advantage of the negative effects on cell fitness induced by various quantitative modulations in cellular networks, could potentially pave the way for the emergence of efficient in situ biochemistry.

Yeast Pgc1p (YPL206c) controls the amount of phosphatidylglycerol via a phospholipase C-type degradation mechanism

The product of the open reading frame YPL206c, Pgc1p, of the yeast Saccharomyces cerevisiae displays homology to bacterial and mammalian glycerophosphodiester phosphodiesterases. Deletion of PGC1 causes an accumulation of the anionic phospholipid, phosphatidylglycerol (PG), especially under conditions of inositol limitation. This PG accumulation was not caused by increased production of phosphatidyl-glycerol phosphate or by decreased consumption of PG in the formation of cardiolipin, the end product of the pathway. PG accumulation in the pgc1Delta strain was caused rather by inactivation of the PG degradation pathway. Our data demonstrate an existence of a novel regulatory mechanism in the cardiolipin biosynthetic pathway in which Pgc1p is required for the removal of excess PG via a phospholipase C-type degradation mechanism.

New insights into the regulation of cardiolipin biosynthesis in yeast: implications for Barth syndrome

Recent studies have revealed an array of novel regulatory mechanisms involved in the biosynthesis and metabolism of the phospholipid cardiolipin (CL), the signature lipid of mitochondria. CL plays an important role in cellular and mitochondrial function due in part to its association with a large number of mitochondrial proteins, including many which are unable to function optimally in the absence of CL. New insights into the complexity of regulation of CL provide further evidence of its importance in mitochondrial and cellular function. The biosynthesis of CL in yeast occurs via three enzymatic steps localized in the mitochondrial inner membrane. Regulation of this process by general phospholipid cross-pathway control and factors affecting mitochondrial development has been previously established. In this review, novel regulatory mechanisms that control CL biosynthesis are discussed. A unique form of inositol-mediated regulation has been identified in the CL biosynthetic pathway, independent of the INO2-INO4-OPI1 regulatory circuit that controls general phospholipid biosynthesis. Inositol leads to decreased activity of phosphatidylglycerolphosphate (PGP) synthase, which catalyzes the committed step of CL synthesis. Reduced enzymatic activity does not result from alteration of expression of the structural gene, but is instead due to increased phosphorylation of the enzyme. This is the first demonstration of phosphorylation in response to inositol and may have significant implications in understanding the role of inositol in other cellular regulatory pathways. Additionally, synthesis of CL has been shown to be dependent on mitochondrial pH, coordinately controlled with synthesis of mitochondrial phosphatidylethanolamine (PE), and may be regulated by mitochondrial DNA absence sensitive factor (MIDAS). Further characterization of these regulatory mechanisms holds great potential for the identification of novel functions of CL in mitochondrial and cellular processes.