Upregulation of the AMPK-FOXO1-PDK4 pathway is a primary mechanism of pyruvate dehydrogenase activity reduction in tafazzin-deficient cells.

Barth syndrome (BTHS) is a rare disorder caused by mutations in the TAFAZZIN gene. Previous studies from both patients and model systems have established metabolic dysregulation as a core component of BTHS pathology. In particular, features such as lactic acidosis, pyruvate dehydrogenase (PDH) deficiency, and aberrant fatty acid and glucose oxidation have been identified. However, the lack of a mechanistic understanding of what causes these conditions in the context of BTHS remains a significant knowledge gap, and this has hindered the development of effective therapeutic strategies for treating the associated metabolic problems. In the current study, we utilized tafazzin-knockout C2C12 mouse myoblasts (TAZ-KO) and cardiac and skeletal muscle tissue from tafazzin-knockout mice to identify an upstream mechanism underlying impaired PDH activity in BTHS. This mechanism centers around robust upregulation of pyruvate dehydrogenase kinase 4 (PDK4), resulting from hyperactivation of AMP-activated protein kinase (AMPK) and subsequent transcriptional upregulation by forkhead box protein O1 (FOXO1). Upregulation of PDK4 in tafazzin-deficient cells causes direct phospho-inhibition of PDH activity accompanied by increased glucose uptake and elevated intracellular glucose concentration. Collectively, our findings provide a novel mechanistic framework whereby impaired tafazzin function ultimately results in robust PDK4 upregulation, leading to impaired PDH activity and likely linked to dysregulated metabolic substrate utilization. This mechanism may underlie previously reported findings of BTHS-associated metabolic dysregulation.

Insights into the roles of inositol hexakisphosphate kinase 1 (IP6K1) in mammalian cellular processes.

Inositol phosphates and their metabolites play a significant role in several biochemical pathways, gene expression regulation, and phosphate homeostasis. Among the different inositol phosphates, inositol hexakisphosphate (IP6) is a substrate of inositol hexakisphosphate kinases (IP6Ks), which phosphorylate one or more of the IP6 phosphate groups. Pyrophosphorylation of IP6 leads to the formation of inositol pyrophosphates, high-energy signaling molecules that mediate physiological processes through their ability to modify target protein activities, either by directly binding to their target protein or by pyrophosphorylating protein serine residues. 5-diphosphoinositol pentakisphosphate, the most abundant inositol pyrophosphate in mammals, has been extensively studied and found to be significantly involved in a wide range of physiological processes. Three IP6K (IP6K1, IP6K2, and IP6K3) isoforms regulate IP7 synthesis in mammals. Here, we summarize our current understanding of IP6K1's roles in cytoskeletal remodeling, trafficking, cellular migration, metabolism, gene expression, DNA repair, and immunity. We also briefly discuss current gaps in knowledge, highlighting the need for further investigation.

Decreased pyruvate dehydrogenase activity in Tafazzin-deficient cells is caused by dysregulation of pyruvate dehydrogenase phosphatase 1 (PDP1).

Cardiolipin (CL), the signature lipid of the mitochondrial inner membrane, is critical for maintaining optimal mitochondrial function and bioenergetics. Disruption of CL metabolism, caused by mutations in the CL remodeling enzyme TAFAZZIN, results in the life-threatening disorder Barth syndrome (BTHS). While the clinical manifestations of BTHS, such as dilated cardiomyopathy and skeletal myopathy, point to defects in mitochondrial bioenergetics, the disorder is also characterized by broad metabolic dysregulation, including abnormal levels of metabolites associated with the tricarboxylic acid (TCA) cycle. Recent studies have identified the inhibition of pyruvate dehydrogenase (PDH), the gatekeeper enzyme for TCA cycle carbon influx, as a key deficiency in various BTHS model systems. However, the molecular mechanisms linking aberrant CL remodeling, particularly the primary, direct consequence of reduced tetralinoleoyl-CL (TLCL) levels, to PDH activity deficiency are not yet understood. In the current study, we found that remodeled TLCL promotes PDH function by directly binding to and enhancing the activity of PDH phosphatase 1 (PDP1). This is supported by our findings that TLCL uniquely activates PDH in a dose-dependent manner, TLCL binds to PDP1 in vitro, TLCL-mediated PDH activation is attenuated in the presence of phosphatase inhibitor, and PDP1 activity is decreased in Tafazzin-knockout (TAZ-KO) C2C12 myoblasts. Additionally, we observed decreased mitochondrial calcium levels in TAZ-KO cells and treating TAZ-KO cells with calcium lactate (CaLac) increases mitochondrial calcium and restores PDH activity and mitochondrial oxygen consumption rate. Based on our findings, we conclude that reduced mitochondrial calcium levels and decreased binding of PDP1 to TLCL contribute to decreased PDP1 activity in TAZ-KO cells.

Upregulation of the AMPK-FOXO1-PDK4 pathway is a primary mechanism of pyruvate dehydrogenase activity reduction and leads to increased glucose uptake in tafazzin-deficient cells.

Barth syndrome (BTHS) is a rare disorder caused by mutations in the TAFAZZIN gene. Previous studies from both patients and model systems have established metabolic dysregulation as a core component of BTHS pathology. In particular, features such as lactic acidosis, pyruvate dehydrogenase (PDH) deficiency, and aberrant fatty acid and glucose oxidation have been identified. However, the lack of a mechanistic understanding of what causes these conditions in the context of BTHS remains a significant knowledge gap, and this has hindered the development of effective therapeutic strategies for treating the associated metabolic problems. In the current study, we utilized tafazzin-knockout C2C12 mouse myoblasts (TAZ-KO) and cardiac and skeletal muscle tissue from tafazzin-knockout mice to identify an upstream mechanism underlying impaired PDH activity in BTHS. This mechanism centers around robust upregulation of pyruvate dehydrogenase kinase 4 (PDK4), resulting from hyperactivation of AMP-activated protein kinase (AMPK) and subsequent transcriptional upregulation by forkhead box protein O1 (FOXO1). Upregulation of PDK4 in tafazzin-deficient cells causes direct phospho-inhibition of PDH activity accompanied by increased glucose uptake and elevated intracellular glucose concentration. Collectively, our findings provide a novel mechanistic framework whereby impaired tafazzin function ultimately results in robust PDK4 upregulation, leading to impaired PDH activity and likely linked to dysregulated metabolic substrate utilization. This mechanism may underlie previously reported findings of BTHS-associated metabolic dysregulation.

Anomalous peroxidase activity of cytochrome c is the primary pathogenic target in Barth syndrome.

Barth syndrome (BTHS) is a life-threatening genetic disorder with unknown pathogenicity caused by mutations in TAFAZZIN (TAZ) that affect remodeling of mitochondrial cardiolipin (CL). TAZ deficiency leads to accumulation of mono-lyso-CL (MLCL), which forms a peroxidase complex with cytochrome c (cyt c) capable of oxidizing polyunsaturated fatty acid-containing lipids. We hypothesized that accumulation of MLCL facilitates formation of anomalous MLCL-cyt c peroxidase complexes and peroxidation of polyunsaturated fatty acid phospholipids as the primary BTHS pathogenic mechanism. Using genetic, biochemical/biophysical, redox lipidomic and computational approaches, we reveal mechanisms of peroxidase-competent MLCL-cyt c complexation and increased phospholipid peroxidation in different TAZ-deficient cells and animal models and in pre-transplant biopsies from hearts of patients with BTHS. A specific mitochondria-targeted anti-peroxidase agent inhibited MLCL-cyt c peroxidase activity, prevented phospholipid peroxidation, improved mitochondrial respiration of TAZ-deficient C2C12 myoblasts and restored exercise endurance in a BTHS Drosophila model. Targeting MLCL-cyt c peroxidase offers therapeutic approaches to BTHS treatment.

Valproate regulates inositol synthesis by reducing expression of myo-inositol-3-phosphate synthase.

Inositol depletion is a hypothesized mechanism of action of mood stabilization drugs used in the treatment of bipolar disorder. It was previously reported that the mood stabilizer valproate (VPA) increased phosphorylation of myo-inositol-3-phosphate synthases (MIPS), the rate limiting enzyme of inositol synthesis. Phosphosites were identified and examination of site-directed mutants suggested that phosphorylation leads to decreased enzymatic activity. In this study, we examined the extent of MIPS phosphorylation in response to VPA and used two interaction screens to identify protein kinases that interact with MIPS. Using an epitope tagged MIPS construct, we determined the fraction of phosphorylated MIPS to be very low (less than 2% of total), and we could not detect phosphorylation of untagged MIPS in response to VPA. In vitro analyses of phosphorylation revealed that putative protein kinases, PKC and CKII, have low specificity toward MIPS. These findings suggest that VPA likely depletes inositol via a mechanism other than MIPS phosphorylation. Consistent with this, mRNA levels of the MIPS-encoding gene INO1 and MIPS protein levels were significantly reduced during the mid-log growth phase in response to VPA treatment. These findings suggest that the mechanism whereby VPA causes inositol depletion is by reducing expression of the rate-limiting enzyme MIPS.

mRNA decapping activators Pat1 and Dhh1 regulate transcript abundance and translation to tune cellular responses to nutrient availability.

We have examined the roles of yeast mRNA decapping-activators Pat1 and Dhh1 in repressing the translation and abundance of specific mRNAs in nutrient-replete cells using ribosome profiling, RNA-Seq, CAGE analysis of capped mRNAs, RNA Polymerase II ChIP-Seq, and TMT-mass spectrometry of mutants lacking one or both factors. Although the Environmental Stress Response (ESR) is activated in dhh1Δ and pat1Δ mutants, hundreds of non-ESR transcripts are elevated in a manner indicating cumulative repression by Pat1 and Dhh1 in wild-type cells. These mRNAs show both reduced decapping and diminished transcription in the mutants, indicating that impaired mRNA turnover drives transcript derepression in cells lacking Dhh1 or Pat1. mRNA degradation stimulated by Dhh1/Pat1 is not dictated by poor translation nor enrichment for suboptimal codons. Pat1 and Dhh1 also collaborate to reduce translation and protein production from many mRNAs. Transcripts showing concerted translational repression by Pat1/Dhh1 include mRNAs involved in cell adhesion or utilization of the poor nitrogen source allantoin. Pat1/Dhh1 also repress numerous transcripts involved in respiration, catabolism of non-preferred carbon or nitrogen sources, or autophagy; and we obtained evidence for elevated respiration and autophagy in the mutants. Thus, Pat1 and Dhh1 function as post-transcriptional repressors of multiple pathways normally activated only during nutrient limitation.

Decapping factor Dcp2 controls mRNA abundance and translation to adjust metabolism and filamentation to nutrient availability.

Degradation of most yeast mRNAs involves decapping by Dcp1/Dcp2. DEAD-box protein Dhh1 has been implicated as an activator of decapping, in coupling codon non-optimality to enhanced degradation, and as a translational repressor, but its functions in cells are incompletely understood. RNA-Seq analyses coupled with CAGE sequencing of all capped mRNAs revealed increased abundance of hundreds of mRNAs in dcp2Δ cells that appears to result directly from impaired decapping rather than elevated transcription. Interestingly, only a subset of mRNAs requires Dhh1 for targeting by Dcp2, and also generally requires the other decapping activators Pat1, Edc3, or Scd6; whereas most of the remaining transcripts utilize nonsense-mediated mRNA decay factors for Dcp2-mediated turnover. Neither inefficient translation initiation nor stalled elongation appears to be a major driver of Dhh1-enhanced mRNA degradation. Surprisingly, ribosome profiling revealed that dcp2Δ confers widespread changes in relative translational efficiencies (TEs) that generally favor well-translated mRNAs. Because ribosome biogenesis is reduced while capped mRNA abundance is increased by dcp2Δ, we propose that an increased ratio of mRNA to ribosomes increases competition among mRNAs for limiting ribosomes to favor efficiently translated mRNAs in dcp2Δ cells. Interestingly, genes involved in respiration or utilization of alternative carbon or nitrogen sources are upregulated, and both mitochondrial function and cell filamentation are elevated in dcp2Δ cells, suggesting that decapping sculpts gene expression post-transcriptionally to fine-tune metabolic pathways and morphological transitions according to nutrient availability.

Cardiolipin metabolism regulates expression of muscle transcription factor MyoD1 and muscle development.

The mitochondrial phospholipid cardiolipin (CL) is critical for numerous essential biological processes, including mitochondrial dynamics and energy metabolism. Mutations in the CL remodeling enzyme TAFAZZIN cause Barth syndrome, a life-threatening genetic disorder that results in severe physiological defects, including cardiomyopathy, skeletal myopathy, and neutropenia. To study the molecular mechanisms whereby CL deficiency leads to skeletal myopathy, we carried out transcriptomic analysis of the TAFAZZIN-knockout (TAZ-KO) mouse myoblast C2C12 cell line. Our data indicated that cardiac and muscle development pathways are highly decreased in TAZ-KO cells, consistent with a previous report of defective myogenesis in this cell line. Interestingly, the muscle transcription factor myoblast determination protein 1 (MyoD1) is significantly repressed in TAZ-KO cells and TAZ-KO mouse hearts. Exogenous expression of MyoD1 rescued the myogenesis defects previously observed in TAZ-KO cells. Our data suggest that MyoD1 repression is caused by upregulation of the MyoD1 negative regulator, homeobox protein Mohawk, and decreased Wnt signaling. Our findings reveal, for the first time, that CL metabolism regulates muscle differentiation through MyoD1 and identify the mechanism whereby MyoD1 is repressed in CL-deficient cells.

Decapping factor Dcp2 controls mRNA abundance and translation to adjust metabolism and filamentation to nutrient availability.

Degradation of most yeast mRNAs involves decapping by Dcp1/Dcp2. DEAD-box protein Dhh1 has been implicated as an activator of decapping, in coupling codon non-optimality to enhanced degradation, and as a translational repressor, but its functions in cells are incompletely understood. RNA-Seq analyses coupled with CAGE sequencing of all capped mRNAs revealed increased abundance of hundreds of mRNAs in dcp2 Δ cells that appears to result directly from impaired decapping rather than elevated transcription, which was confirmed by ChIP-Seq analysis of RNA Polymerase II occupancies genome-wide. Interestingly, only a subset of mRNAs requires Dhh1 for targeting by Dcp2, and also generally requires the other decapping activators Pat1, Lsm2, Edc3 or Scd6; whereas most of the remaining transcripts utilize NMD factors for Dcp2-mediated turnover. Neither inefficient translation initiation nor stalled elongation appears to be a major driver of Dhh1-enhanced mRNA degradation. Surprisingly, ribosome profiling revealed that dcp2 Δ confers widespread changes in relative TEs that generally favor well-translated mRNAs. Because ribosome biogenesis is reduced while capped mRNA abundance is increased by dcp2 Δ, we propose that an increased ratio of mRNA to ribosomes increases competition among mRNAs for limiting ribosomes to favor efficiently translated mRNAs in dcp2 Δ cells. Interestingly, genes involved in respiration or utilization of alternative carbon or nitrogen sources are derepressed, and both mitochondrial function and cell filamentation (a strategy for nutrient foraging) are elevated by dcp2 Δ, suggesting that mRNA decapping sculpts gene expression post-transcriptionally to fine-tune metabolic pathways and morphological transitions according to nutrient availability.

Phosphatidic acid inhibits inositol synthesis by inducing nuclear translocation of kinase IP6K1 and repression of myo-inositol-3-P synthase.

Inositol is an essential metabolite that serves as a precursor for structural and signaling molecules. Although perturbation of inositol homeostasis has been implicated in numerous human disorders, surprisingly little is known about how inositol levels are regulated in mammalian cells. A recent study in mouse embryonic fibroblasts demonstrated that nuclear translocation of inositol hexakisphosphate kinase 1 (IP6K1) mediates repression of myo-inositol-3-P synthase (MIPS), the rate-limiting inositol biosynthetic enzyme. Binding of IP6K1 to phosphatidic acid (PA) is required for this repression. Here, we elucidate the role of PA in IP6K1 repression. Our results indicate that increasing PA levels through pharmacological stimulation of phospholipase D (PLD) or direct supplementation of 18:1 PA induces nuclear translocation of IP6K1 and represses expression of the MIPS protein. We found that this effect was specific to PA synthesized in the plasma membrane, as endoplasmic reticulum-derived PA did not induce IP6K1 translocation. Furthermore, we determined that PLD-mediated PA synthesis can be stimulated by the master metabolic regulator 5' AMP-activated protein kinase (AMPK). We show that activation of AMPK by glucose deprivation or by treatment with the mood-stabilizing drugs valproate or lithium recapitulated IP6K1 nuclear translocation and decreased MIPS expression. This study demonstrates for the first time that modulation of PA levels through the AMPK-PLD pathway regulates IP6K1-mediated repression of MIPS.

Isoforms of the transcriptional cofactor SIN3 differentially regulate genes necessary for energy metabolism and cell survival.

The SIN3 scaffolding protein is a conserved transcriptional regulator known to fine-tune gene expression. In Drosophila, there are two major isoforms of SIN3, SIN3 220 and SIN3 187, which each assemble into multi-subunit histone modifying complexes. The isoforms have distinct developmental expression patterns and non-redundant functions. Gene regulatory network analyses indicate that both isoforms affect genes encoding proteins in pathways such as the cell cycle and cell morphogenesis. Interestingly, the SIN3 187 isoform uniquely regulates a subset of pathways including post-embryonic development, phosphate metabolism and apoptosis. Target genes in the phosphate metabolism pathway include nuclear-encoded mitochondrial genes coding for proteins responsible for oxidative phosphorylation. Here, we investigate the physiological effects of SIN3 isoforms on energy metabolism and cell survival. We find that ectopic expression of SIN3 187 represses expression of several nuclear-encoded mitochondrial genes affecting production of ATP and generation of reactive oxygen species (ROS). Forced expression of SIN3 187 also activates several pro-apoptotic and represses a few anti-apoptotic genes. In the SIN3 187 expressing cells, these gene expression patterns are accompanied with an increased sensitivity to paraquat-mediated oxidative stress. These findings indicate that SIN3 187 influences the regulation of mitochondrial function, apoptosis and oxidative stress response in ways that are dissimilar from SIN3 220. The data suggest that the distinct SIN3 histone modifying complexes are deployed in different cellular contexts to maintain cellular homeostasis.

Current Knowledge on the Role of Cardiolipin Remodeling in the Context of Lipid Oxidation and Barth Syndrome.

Barth syndrome (BTHS, OMIM 302060) is a genetic disorder caused by variants of the TAFAZZIN gene (G 4.5, OMIM 300394). This debilitating disorder is characterized by cardio- and skeletal myopathy, exercise intolerance, and neutropenia. TAFAZZIN is a transacylase that catalyzes the second step in the cardiolipin (CL) remodeling pathway, preferentially converting saturated CL species into unsaturated CLs that are susceptible to oxidation. As a hallmark mitochondrial membrane lipid, CL has been shown to be essential in a myriad of pathways, including oxidative phosphorylation, the electron transport chain, intermediary metabolism, and intrinsic apoptosis. The pathological severity of BTHS varies substantially from one patient to another, even in individuals bearing the same TAFAZZIN variant. The physiological modifier(s) leading to this disparity, along with the exact molecular mechanism linking CL to the various pathologies, remain largely unknown. Elevated levels of reactive oxygen species (ROS) have been identified in numerous BTHS models, ranging from yeast to human cell lines, suggesting that cellular ROS accumulation may participate in the pathogenesis of BTHS. Although the exact mechanism of how oxidative stress leads to pathogenesis is unknown, it is likely that CL oxidation plays an important role. In this review, we outline what is known about CL oxidation and provide a new perspective linking the functional relevance of CL remodeling and oxidation to ROS mitigation in the context of BTHS.

The paradoxical role of inositol in cancer: a consequence of the metabolic state of a tumor.

Inositol is an essential nutrient, obtained either by uptake from the environment or by de novo synthesis from glucose. Inositol and its derivatives exhibit tumor-suppressive effects, potentially mediated by inhibition of the ERK-MAPK or PI3K-Akt pathways. Accordingly, many cancers have been documented to silence expression of the ISYNA1 gene, which encodes the rate-limiting enzyme of inositol synthesis. Paradoxically, recent studies have also reported upregulation of ISYNA1 in some cancers. Upregulation may reflect a compensatory response brought about by defective inositol uptake or oncogenic mutations that preclude its tumor-suppressive effects. In these scenarios, de novo synthesis of inositol may be upregulated to promote cell proliferation. The role of inositol in cancer is further complicated by its ability to inhibit the master metabolic regulator AMPK, which upon activation can either decrease cell proliferation and metastasis or promote cell survival. Due to its potential dual role in cancer, inositol homeostasis must be tightly regulated in tumor cells. Thus, whether inositol acts to suppress or promote tumor progression is determined by the metabolic profile and oncogenic background of the cancer.

Inositol depletion regulates phospholipid metabolism and activates stress signaling in HEK293T cells.

Inositol plays a significant role in cellular function and signaling. Studies in yeast have demonstrated an "inositol-less death" phenotype, suggesting that inositol is an essential metabolite. In yeast, inositol synthesis is highly regulated, and inositol levels have been shown to be a major metabolic regulator, with its abundance affecting the expression of hundreds of genes. Abnormalities in inositol metabolism have been associated with several human disorders. Despite its importance, very little is known about the regulation of inositol synthesis and the pathways regulated by inositol in human cells. The current study aimed to address this knowledge gap. Knockout of ISYNA1 (encoding myo-inositol-3-P synthase 1) in HEK293T cells generated a human cell line that is deficient in de novo inositol synthesis. ISYNA1-KO cells exhibited inositol-less death when deprived of inositol. Lipidomic analysis identified inositol deprivation as a global regulator of phospholipid levels in human cells, including downregulation of phosphatidylinositol (PI) and upregulation of the phosphatidylglycerol (PG)/cardiolipin (CL) branch of phospholipid metabolism. RNA-Seq analysis revealed that inositol deprivation induced substantial changes in the expression of genes involved in cell signaling, including extracellular signal-regulated kinase (ERK), and genes controlling amino acid transport and protein processing in the endoplasmic reticulum (ER). This study provides the first in-depth characterization of the effects of inositol deprivation on phospholipid metabolism and gene expression in human cells, establishing an essential role for inositol in maintaining cell viability and regulating cell signaling and metabolism.

NAD supplementation improves mitochondrial performance of cardiolipin mutants.

Cardiolipin (CL) deficiency causes mitochondrial dysfunction and aberrant metabolism that are associated in humans with the severe disease Barth syndrome (BTHS). Several metabolic abnormalities are observed in BTHS patients and model systems, including decreased oxidative phosphorylation, reduced tricarboxylic acid (TCA) cycle flux, and accumulated lactate and D-ß-hydroxybutyrate, which strongly suggests that nicotinamide adenine dinucleotide (NAD) redox metabolism may be altered in CL-deficient cells. In this study, we identified abnormal NAD+ metabolism in multiple BTHS model systems and demonstrate that supplementation of NAD+ precursors such as nicotinamide mononucleotide (NMN) improves mitochondrial function. Improved mitochondrial function in the Drosophila model was associated with restored exercise endurance, which suggests a potential therapeutic benefit of NAD+ precursor supplementation in the management of BTHS patients.

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.

Studying Lipid-Related Pathophysiology Using the Yeast Model.

Saccharomyces cerevisiae, commonly known as baker's yeast, is one of the most comprehensively studied model organisms in science. Yeast has been used to study a wide variety of human diseases, and the yeast model system has proved to be an especially amenable tool for the study of lipids and lipid-related pathophysiologies, a topic that has gained considerable attention in recent years. This review focuses on how yeast has contributed to our understanding of the mitochondrial phospholipid cardiolipin (CL) and its role in Barth syndrome (BTHS), a genetic disorder characterized by partial or complete loss of function of the CL remodeling enzyme tafazzin. Defective tafazzin causes perturbation of CL metabolism, resulting in many downstream cellular consequences and clinical pathologies that are discussed herein. The influence of yeast research in the lipid-related pathophysiologies of Alzheimer's and Parkinson's diseases is also summarized.

The Role of the UPR Pathway in the Pathophysiology and Treatment of Bipolar Disorder.

Bipolar disorder (BD) is a mood disorder that affects millions worldwide and is associated with severe mood swings between mania and depression. The mood stabilizers valproate (VPA) and lithium (Li) are among the main drugs that are used to treat BD patients. However, these drugs are not effective for all patients and cause serious side effects. Therefore, better drugs are needed to treat BD patients. The main barrier to developing new drugs is the lack of knowledge about the therapeutic mechanism of currently available drugs. Several hypotheses have been proposed for the mechanism of action of mood stabilizers. However, it is still not known how they act to alleviate both mania and depression. The pathology of BD is characterized by mitochondrial dysfunction, oxidative stress, and abnormalities in calcium signaling. A deficiency in the unfolded protein response (UPR) pathway may be a shared mechanism that leads to these cellular dysfunctions. This is supported by reported abnormalities in the UPR pathway in lymphoblasts from BD patients. Additionally, studies have demonstrated that mood stabilizers alter the expression of several UPR target genes in mouse and human neuronal cells. In this review, we outline a new perspective wherein mood stabilizers exert their therapeutic mechanism by activating the UPR. Furthermore, we discuss UPR abnormalities in BD patients and suggest future research directions to resolve discrepancies in the literature.

Valproate activates the Snf1 kinase in Saccharomyces cerevisiae by decreasing the cytosolic pH.

Valproate (VPA) is a widely used mood stabilizer, but its therapeutic mechanism of action is not understood. This knowledge gap hinders the development of more effective drugs with fewer side effects. Using the yeast model to elucidate the effects of VPA on cellular metabolism, we determined that the drug upregulated expression of genes normally repressed during logarithmic growth on glucose medium and increased levels of activated (phosphorylated) Snf1 kinase, the major metabolic regulator of these genes. VPA also decreased the cytosolic pH (pHc) and reduced glycolytic production of 2/3-phosphoglycerate. ATP levels and mitochondrial membrane potential were increased, and glucose-mediated extracellular acidification decreased in the presence of the drug, as indicated by a smaller glucose-induced shift in pH, suggesting that the major P-type proton pump Pma1 was inhibited. Interestingly, decreasing the pHc by omeprazole-mediated inhibition of Pma1 led to Snf1 activation. We propose a model whereby VPA lowers the pHc causing a decrease in glycolytic flux. In response, Pma1 is inhibited and Snf1 is activated, resulting in increased expression of normally repressed metabolic genes. These findings suggest a central role for pHc in regulating the metabolic program of yeast cells.