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Tafazzin: Biological Overview | References

BIOLOGICAL OVERVIEW

Barth syndrome (BTHS) is an X-linked multisystemic disorder presenting with dilated cardiomyopathy, skeletal myopathy, cyclic neutropenia, and growth retardation. The disease is caused by mutations in the tafazzin gene that encodes a putative phospholipid acyltransferase. Accordingly, BTHS patients have phospholipid abnormalities, in particular, a reduced content and an altered composition of cardiolipin. Among the many cardiolipin species that can be formed by various fatty acid combinations, BTHS specifically affects the concentration of tetralinoleoyl-cardiolipin. This species is formed by remodeling of the four acyl groups of cardiolipin, suggesting that tafazzin is involved in the acyl group exchange. BTHS has generally been considered a mitochondrial disease because pathologic mitochondria have been found in tissue biopsies from BTHS patients and because cardiolipin is a specific mitochondrial lipid. However, the presence of multiple tafazzin transcripts raises the prospect that other cellular functions are affected as well (Xu, 2006 and references therein).

Tafazzins from Drosophila and mammalian cells assemble in large protein complexes with a short half-life

Tafazzin is a transacylase that affects cardiolipin fatty acid composition and mitochondrial function. Mutations in human tafazzin cause Barth syndrome yet the enzyme has mostly been characterized in yeast. To study tafazzin in higher organisms, this study isolated mitochondria from Drosophila and mammalian cell cultures. Tafazzin was found to bind to multiple protein complexes in these organisms, and that the interactions of tafazzin lack strong specificity. Very large Tafazzin complexes are detected only in the presence of cardiolipin, but smaller complexes remain intact even upon treatment with phospholipase A2. In mammalian cells, Tafazzin has a half-life of only 3–6 h, which is much shorter than the half-life of other mitochondrial proteins. The data suggest that Tafazzin is a transient resident of multiple protein complexes (Xu, 2015).

The results confirm and expand previous observations made in yeast, in particular the association of tafazzin with heterogeneous protein complexes, and the requirement for cardiolipin to stabilize the largest of these complexes. The latter is consistent with the general role of cardiolipin in promoting supercomplex formation. Although some variations exist between yeast, flies, and mammals in terms of the assembly pattern of tafazzin, common to all is that tafazzin is recovered mostly in the 100–400 kDa zone of the Blue Native-PAGE gel while only a tiny portion migrates with supercomplexes. It was also confirmed that the smaller tafazzin complexes are not assembly intermediates of the larger ones (Xu, 2015).

Protein–protein interactions seem to be involved in the formation of tafazzin complexes in the 100–400 kDa weight range because they remain stable in the absence of cardiolipin and do not disintegrate upon treatment with phospholipase A2. However, the tafazzin–protein interactions are relatively non-specific, prompting complex formation not only in mitochondria but also in microsomes. Therefore, it could be possible that tafazzin could also interact with itself. Indeed, when tafazzin was over-expressed in insect cells, evidence for the presence of tafazzin dimers was found, but the overall distribution of the enzyme was shifted towards smaller complexes compared to wild-type Drosophila. These data suggest that the formation of tafazzin complexes in wild-type mitochondria is not the result of tafazzin oligomerization but requires the involvement of other proteins (Xu, 2015).

It was found that the half-life of tafazzin is only a few hours in H9c2 and HEK 293 cells, which is much shorter than the half-life of other mitochondrial proteins such as VDAC, ATP synthase, and the respiratory complexes. It is known that mitochondrial proteins exhibit different rates of degradation and that protein degradation can function as a control mechanism. The short half-life of tafazzin is interesting in light of a recent study showing that tafazzin nearly vanishes from yeast cells in which the MICOS complex has been altered by Aim24 deletion combined with Mic22/Mic26 mutations. Thus, the state of the MICOS complex, a large protein assembly that resides near cristae junctions and bridges the outer and the inner membrane, has an impact on the steady-state concentration of tafazzin (Xu, 2015).

Tafazzin degrades rapidly, regardless of the protein complex it is associated with, which indicates it is highly susceptible to proteases. It has been shown that the degradation of mutated tafazzins is catalyzed by iAAA, but it remains unclear at this point which degradation system is involved in maintaining the steady-state level of wild-type tafazzin. Continuous clearance of tafazzin from the mitochondria ensures that its steady-state concentration can be altered rapidly (Xu, 2015).

In summary, data from this study suggest that tafazzin is incorporated into multiple protein complexes but resides there only for a relatively short period of time. During its lifetime, tafazzin exchanges acyl groups between membrane phospholipids, a reaction that is thought to support membrane dynamics or crista assembly by lowering the energy of curvature formation. Recent evidence has implicated a protein complex formed by prohibitins and DNAJC19 in cardiolipin remodeling, which suggests that tafazzin may operate in the vicinity of this complex. The short-lived nature of tafazzin leads to the speculation that tafazzin participates in transient protein assemblies that are formed as intermediates in the biogenesis of mitochondrial membranes (Xu, 2015).

Tafazzin senses curvature

The lipid composition of mitochondrial inner membranes has attracted attention because of its high content of phosphatidylethanolamines and cardiolipins, lipid species that are both known to form nonlamellar lipid phases with negatively curved lipid monolayers. Mitochondrial lipids of mammals are particularly rich in 1',3'-bis[1,2-dilinoleoyl-sn-glycero-3-phospho]-sn-glycerol, a cardiolipin with four diunsaturated linoleic acid chains. The concentration of this lipid in the inner membranes of mitochondria is determined to a considerable extent by the phospholipid-lysophospholipid transacylase tafazzin. Disturbances in the cardiolipin content of the inner mitochondrial membrane, including shifts in the composition of cardiolipin fatty acids, have been related to Barth syndrome, an X chromosome–linked genetic disorder that is caused by mutations to tafazzin. However, previous studies have indicated that tafazzin has little selectivity among possible lipid substrates, raising the question as to why tafazzin mutations would cause such a specific outcome. Schlame (2013) shed light on this question, reporting model membrane studies suggesting that selective enrichment of linoleic acid in cardiolipins is related to a strong preference of tafazzin to transacylate phospholipids in negatively curved lipid monolayers as found in the cristae of mitochondrial inner membranes (Gawrisch, 2012).

Tafazzin is expressed in high amounts in the mitochondria of cardiac and skeletal muscles. The enzyme is located in the intermembrane space between inner and outer membranes of mitochondria, where it associates peripherally with membrane leaflets. The amino acid sequence of the protein predicted that tafazzin is a phospholipid acyltransferase, with recombinant expression of tafazzin in Sf9 cells demonstrating unambiguously that it functions as a phospholipid-lysophospholipid transacylase. In Barth syndrome patients, tafazzin is mutated and has impaired activity, resulting in a deficiency of tetralinoleoyl-cardiolipin in mitochondria. The genetic disorder has been associated with weakening and enlargement of the heart (dilated cardiomyopathy); low counts of neutrophils, an abundant type of white blood cell (neutropenia); and weakness of skeletal muscles (skeletal myopathy), all of which are related to ultrastructural abnormalities in mitochondria of the corresponding tissues (Gawrisch, 2012 and references therein).

In the previous characterization of tafazzin, the reasons for specific enrichment of mitochondria with tetralinoleoyl-cardiolipin remained mysterious: contrary to expectations the extracted protein was shown to transacylate all phospholipids and lysolipids, regardless of their polar head groups or hydrophobic hydrocarbon chains. Tafazzin even transfers acyl chains to the sn-1 and sn-2 positions of glycerol in phospholipids with equal probabilit9. What is then the mechanism responsible for the specific enrichment? (Gawrisch, 2012).

In elegant new experiments, Schlame (2012) combined radiolabeling, MS and 31P-NMR of various lipid compositions before and after treatment with tafazzin to investigate a 'thermodynamic remodeling' hypothesis-the idea that not only chemical identity but also the physical properties of the lipid domain affect tafazzin function. In particular, phospholipids whose polar head group has a relatively small volume, such as phosphatidylethanolamine and cardiolipin, and unsaturated hydrocarbon chains, such as linoleic acid, have a shape asymmetry that leads to their accumulation in negatively curved lipid monolayers. Indeed, the formation of cardiolipin microdomains in negatively curved regions of Escherichia coli membranes was confirmed experimentally (Gawrisch, 2012).

This enrichment may even trigger a transition to an inverse hexagonal lipid phase (HII) consisting of inverted cylindrical micelles that have a water-filled, polar core. The micelles are packed into a hexagonal lattice, which gives the phase its name. For cardiolipin, HII phase formation is triggered by addition of divalent cations and can easily be detected by solid-state 31P-NMR. Schlame (2012) observed that partial conversion of cardiolipin-containing lipid-water dispersions to an HII phase, as monitored by NMR, correlate with tafazzin-induced enrichment of cardiolipin with linoleoyl hydrocarbon chains, as detected by MS. Further work will be needed to determine whether the HII phase itself is the site of tafazzin action or whether the enzyme acts at the transition structures between lamellar and HII phases that also have negatively curved lipid monolayers and may be more easily accessible for the peripheral protein. The selective enrichment in tetralinoleoyl-cardiolipin by tafazzin suggests that lipids with the diunsaturated linoeoyl chains enrich specifically in those highly curved membrane regions (Gawrisch, 2012).

The tafazzin story is an excellent example of how the search for a locus of a genetic disorder stimulated biochemical and biophysical investigations that linked the disease to the consequences of malfunction of a phospholipid-lysophospholipid transacylase, the lack of tetralinoleoyl-cardiolipin in mitochondrial inner membranes and the impairment of mitochondrial function. This remarkable depth of understanding of the molecular mechanisms of Barth syndrome pathogenesis has already resulted in easier diagnosis and earlier treatment of the disorder and also points at opportunities for basic research to develop new treatments. Tafazzin joins the ranks of proteins, such as protein kinase C, whose membrane association and function are influenced by lipids that prefer to be located in monolayers with negative curvature. Negative curvature packing stress was also identified as major cofactor in the formation of metarhodopsin-II, the photointermediate of the G protein-coupled membrane receptor rhodopsin, which activates the G protein transducin. Considering that phosphatidylethanolamines are very common phospholipids, it is almost certain that more proteins will be found whose function is regulated by negative curvature in lipid monolayers (Gawrisch, 2012).

Role of calcium-independent phospholipase A2 in the pathogenesis of Barth syndrome

Quantitative and qualitative alterations of mitochondrial cardiolipin have been implicated in the pathogenesis of Barth syndrome, an X-linked cardioskeletal myopathy caused by a deficiency in tafazzin, an enzyme in the cardiolipin remodeling pathway. A tafazzin-deficient Drosophila model of Barth syndrome that is characterized by low cardiolipin (CL) concentration, abnormal cardiolipin fatty acyl composition, abnormal mitochondria, and poor motor function has been generated earlier. This study shows that tafazzin deficiency in Drosophila disrupts the final stage of spermatogenesis, spermatid individualization, and causes male sterility. This phenotype can be genetically suppressed by inactivation of the gene encoding a calcium-independent phospholipase A2, iPLA2-VIA, which also prevents cardiolipin depletion/monolysocardiolipin accumulation, although in wild-type flies inactivation of the iPLA2-VIA does not affect the molecular composition of cardiolipin. Furthermore, it was shown that treatment of Barth syndrome patients' lymphoblasts in tissue culture with the iPLA2 inhibitor, bromoenol lactone, partially restores their cardiolipin homeostasis. Taken together, these findings establish a causal role of cardiolipin deficiency in the pathogenesis of Barth syndrome and identify iPLA2-VIA as an important enzyme in cardiolipin deacylation, and as a potential target for therapeutic intervention (Malhotra, 2009).

The cardiolipin metabolism defect associated with Barth syndrome is manifested by the triad of CL depletion, monolyso-CL accumulation, and CL species diversification, i.e., the generation of CL molecules with different fatty acyl compositions. It is not clear whether the abnormal CL homeostasis actually plays a role in the pathogenesis of Barth syndrome, and if so, which aspect is the key factor. This study addresses this issue in a Drosophila model of Barth syndrome (Malhotra, 2009).

It was found that tafazzin deficiency in Drosophila, which alters CL homeostasis and reduces CL levels, also disrupts spermatid individualization during spermatogenesis, resulting in male sterility, and that this male-sterile phenotype can be suppressed by inactivation of the CL-degrading enzyme iPLA2-VIA, which partially restores CL homeostasis in double-mutant flies. These observations suggest that CL content, or at least the MLCL/CL ratio, plays a critical role in Drosophila spermatid individualization. It has been recently shown that the final stage of spermatid differentiation in Drosophila involves an apoptosis-like mechanism, in which the cytochrome c-dependent caspase activation is required for the elimination of excess cytoplasm. Cardiolipin has been shown to play important roles in mitochondria-dependent apoptosis and a recent report demonstrats that CL deficiency increases cells' resistance to apoptosis. Therefore, CL deficiency in Drosophila testes may prevent the syncytial spermatids from initiating the apoptosis-like mechanism required for normal spermatid individualization. (Malhotra, 2009).

The cardinal characteristics of Barth syndrome are cardioskeletal myopathy, exercise intolerance, neutropenia, abnormal mitochondria, and altered CL metabolism. Because in eukaryotes CL is localized exclusively in mitochondria and is required for optimal mitochondrial function, it has been generally assumed that the defective CL metabolism causes the pathophysiology of Barth syndrome. This study tested this hypothesis by genetically manipulating CL metabolism in the Drosophila model. It was found that partial restoration of CL homeostasis through genetic inactivation of iPLA2-VIA suppresses the male-sterile phenotype of tafazzin-deficient flies; this provides the direct evidence that altered CL metabolism is a major contributing factor in Barth syndrome (Malhotra, 2009).

The most abundant CL molecular species from various organisms and tissues contain only 1 or 2 types of fatty acids. In many mammalian tissues, the predominant fatty acyl moiety in CL is linoleic acid (C18:2). For example, 80% of CL molecules in heart and skeletal muscle are tetralinoleoyl CL. However, the role of CL molecular species in vivo remains speculative. The characteristic fatty acyl composition of CL in vivo is achieved through tafazzin-dependent remodeling of nascent CL. However, tafazzin deficiency, such as in Barth syndrome, results not only in abnormal CL acyl composition, but also in CL depletion and monolyso-CL accumulation. Thus, it is unclear which aspect of the CL metabolic disorder contributes to the pathogenesis of Barth syndrome. The finding in this study that the male-sterile phenotype of tafazzin-deficient flies can be suppressed by genetic inactivation of iPLA2-VIA, which prevents CL depletion and monolyso-CL accumulation without correcting the abnormal CL acyl composition, suggests that the abnormal levels of CL and/or monolyso-CL are important pathogenetic factors. Because a cardiolipin synthase mutant of yeast exhibits abnormal mitochondrial function, it is likely that the low CL content is critical in the molecular mechanism of Barth syndrome. Nevertheless, because Barth syndrome is a multisystem disorder, involvement of monolyso-CL accumulation and abnormal CL acyl composition may also play a role in certain tissues and organs (Malhotra, 2009).

The mature acyl chain composition of CL is achieved through a remodeling process, which requires the action of tafazzin. It has been previously shown that tafazzin catalyzes phospholipid-lysophospholipid transacylation that involves both deacylation of a phospholipid such as CL and reacylation of a monolyso-phospholipid, such as monolyso-CL. Unlike the CoA-dependent deacylation-reacylation cycle (Lands cycle), in which a nascent phospholipid is deacylated by a phospholipase A to yield a free fatty acid and a lysophospholipid that is then reacylated by an acyl-CoA-dependent acyltransferase, transacylation does not require acyl-CoA, and proceeds directly by transferring a fatty acyl chain from a phospholipid to a lysophospholipid; no phospholipase is involved and no free fatty acid is generated in the process. It was found that although the calcium-independent phospholipase A2, iPLA2-VIA, is not required for CL remodeling, in the absence of tafazzin, i.e., in the Barth syndrome model, the enzyme plays a major role in the depletion of CL and the accumulation of monolyso-CL (Malhotra, 2009).

The finding that the phenotypic features of tafazzin deficiency can be suppressed by inhibiting iPLA2-VIA activity identifies this enzyme as a potential target for therapeutic intervention in Barth syndrome. Indeed, it was found that treatment of cultured lymphoblasts from Barth patients with the iPLA2 inhibitor BEL partially restores CL homeostasis. The calcium-independent iPLA2-VIA has been implicated in a variety of biological processes, including phospholipid remodeling, arachidonic acid release, apoptosis, and store-operated calcium entry. In addition, iPLA2-VIA knockout mice develop age-dependent neurological impairment and mutations in the iPLA2-VIA gene have been identified in patients with infantile neuroaxonal dystrophy and neurodegeneration with iron accumulation in the brain. Therefore, a therapeutic approach to Barth syndrome based on the inhibition of iPLA2-VIA is likely to require either careful titration of the phospholipase inhibitor, or even its tissue-specific targeting (Malhotra, 2009).

A Drosophila model of Barth syndrome

Barth syndrome is an X-linked disease presenting with cardiomyopathy and skeletal muscle weakness. It is caused by mutations in tafazzin (taz), a putative acyl transferase that has been associated with altered metabolism of the mitochondrial phospholipid cardiolipin. To investigate the molecular basis of Barth syndrome, this study created Drosophila mutants, resulting from imprecise excision of a P element inserted upstream of the coding region of the tafazzin gene. Homozygous flies for that mutation are unable to express the full-length isoform of tafazzin, as documented by RNA and Western blot analysis, but two shorter tafazzin transcripts are still present, although the expression levels of their encoded proteins are too low to be detectable by Western blotting. The tafazzin mutation causes an 80% reduction of cardiolipin and a diversification of its molecular composition, similar to the changes seen in Barth patients. Other phospholipids, like phosphatidylcholine and phosphatidylethanolamine, are not affected. Flies with the tafazzin mutation show reduced locomotor activity, measured in flying and climbing assays, and their indirect flight muscles display frequent mitochondrial abnormalities, mostly in the cristae membranes. Thus, tafazzin mutations in Drosophila generate a Barth-related phenotype, with the triad of abnormal cardiolipin, pathologic mitochondria, and motor weakness, suggesting causal links between these findings. The study concludes that a lack of full-length tafazzin is responsible for the cardiolipin deficiency, which is integral to the disease mechanism, leading to mitochondrial myopathy (Xu, 2006).

Experimental models of BTHS have been developed in yeast, cultured fibroblasts, and lymphoblasts. These models have provided important insights into the underlying molecular defect but have not been useful to study the organ-specific aspects of the disease. This work with Drosophila establishes a BTHS model in an organism with differentiated tissues, among them the highly developed flight muscle apparatus. These mutant flies are unable to produce the full-length isoform of tafazzin, and exhibit cardiolipin deficiency, abnormal muscle mitochondria, and poor motor performance. Replication in Drosophila of the specific pathologies of human BTHS strongly suggests a causal link between these features. The study proposes that aberrant cardiolipin induces mitochondrial malformations, which in turn causes the myopathy. The data show that full-length tafazzin is essential for the formation of normal cardiolipin and that the shorter tafazzin species are unable to compensate for the lack of full-length tafazzin. It is yet to be demonstrated whether or not short tafazzins are actually expressed as proteins from the corresponding transcripts. Given the low expression level of tafazzin, this endeavor will require careful analysis of concentrated subcellular fractions from Drosophila at different developmental stages (Xu, 2006).

Like BTHS patients, tafazzin-deficient Drosophila mutants are unable to form the dominant molecular species of cardiolipin. However, the dominant molecular species are not identical in humans and in Drosophila, suggesting that tafazzin plays a general role in cardiolipin remodeling, rather than a specific role in the formation of any particular cardiolipin species. In both humans and Drosophila, tafazzin deficiency leads to (i) a diversification of the cardiolipin pattern, i.e., formation of multiple species at the expense of the dominant species, and (ii) a decline of the total cardiolipin content (Xu, 2006).

This specific form of cardiolipin deficiency is associated with abnormalities of the cristae membranes, which bear some resemblance to abnormalities caused by age, hyperoxia, or certain mutations. In all of these examples, an atypical pattern of cristae architecture emerges, albeit with variations in shape and form. For instance, in tafazzin-deficient flight muscle, characteristic stacks of hyperdense, closely apposed membranes, were found which were deposited inside the mitochondria. A very similar morphology has been observed in heart biopsies from BTHS patients. According to electron tomographic studies, these hyperdense structures may represent collapsed cristae with obliterated intracrista space (Xu, 2006).

Physiologic consequences of tafazzin deficiency are only beginning to emerge from clinical and experimental studies. In cell cultures, poor coupling between respiration and phosphorylation has been observed, but a general breakdown of energy metabolism is unlikely to be the pathogenetic mechanism of BTHS, because the symptoms are often subtle and confined to selected organ systems. Deletion of full-length tafazzin from Drosophila does not alter lifespan or heart rate, and the mutants appear to be in good overall health. However, the tafazzin mutants may become vulnerable once they are removed from the optimal culture environment and exposed to wildlife conditions. This proposition may be tested by lifespan studies under conditions of stress (Xu, 2006).

Tafazzin deletion causes motor weakness, strongly supporting the specific role of this protein in muscle function. Muscle mitochondria may be susceptible to tafazzin deletion because they require a high concentration of cardiolipin and because they may not tolerate deviations from the normal cardiolipin composition. For instance, mammalian skeletal muscle contains cardiolipin with a very specific molecular composition, mainly consisting of tetralinoleoyl species. The requirement for specific cardiolipins may be related to the structural organization, high density, or tight association with myofibrils, which is characteristic of muscle mitochondria. The selective effect on muscle function makes Drosophila an appropriate animal model of BTHS, the further exploration of which should yield insights into the mechanism by which tafazzin mutations cause myopathy (Xu, 2006).

Search PubMed for articles about Drosophila Tafazzin

Gawrisch, K. (2012). Tafazzin senses curvature. Nat Chem Biol 8: 811-812. PubMed ID: 22987008

Malhotra, A., Edelman-Novemsky, I., Xu, Y., Plesken, H., Ma, J., Schlame, M. and Ren, M. (2009). Role of calcium-independent phospholipase A2 in the pathogenesis of Barth syndrome. Proc Natl Acad Sci U S A 106: 2337-2341. PubMed ID: 19164547

Schlame, M., Acehan, D., Berno, B., Xu, Y., Valvo, S., Ren, M., Stokes, D. L. and Epand, R. M. (2012). The physical state of lipid substrates provides transacylation specificity for tafazzin. Nat Chem Biol 8: 862-869. PubMed ID: 22941046

Xu, Y., Malhotra, A., Claypool, S.M., Ren, M. and Schlame, M. (2015). Tafazzins from Drosophila and mammalian cells assemble in large protein complexes with a short half-life. Mitochondrion 21: 27-32. PubMed ID: 25598000

Xu, Y., Condell, M., Plesken, H., Edelman-Novemsky, I., Ma, J., Ren, M. and Schlame, M. (2006). A Drosophila model of Barth syndrome. Proc Natl Acad Sci U S A 103: 11584-11588. PubMed ID: 16855048

date revised: 12 December 2022

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