Search

SummaryThe Tricarboxylic Acid Cycle (TCA) cycle is arguably the most critical metabolic cycle in physiology and exists as an essential interface coordinating cellular metabolism, bioenergetics, and redox homeostasis. Despite decades of research, a comprehensive investigation into the consequences of TCA cycle dysfunction remains elusive. Here, we targeted two TCA cycle enzymes, fumarate hydratase (FH) and succinate dehydrogenase (SDH), and combined metabolomics, transcriptomics, and proteomics analyses to fully appraise the consequences of TCA cycle inhibition (TCAi) in kidney epithelial cells. Our comparative approach shows that TCAi elicits a convergent rewiring of redox and amino acid metabolism dependent on the activation of ATF4 and the integrated stress response (ISR). Furthermore, we also uncover a divergent metabolic response, whereby acute FHi, but not SDHi, can maintain asparagine levels via reductive carboxylation and maintenance of cytosolic aspartate synthesis. Our work highlights an important interplay between the TCA cycle, redox biology and amino acid homeostasis.HighlightsTCA cycle inhibition promotes GSH synthesis and impairs de novo aspartate and proline synthesisDisruption of mitochondrial thiol redox homeostasis phenocopies TCA cycle inhibition by promoting GSH synthesis and impairing proline and aspartate synthesisAcute FHi, but not SDHi, can maintain asparagine levels via reductive carboxylation and maintenance of cytosolic aspartate synthesisTCA cycle inhibition mimics an amino acid deprivation-type response and activates ATF4 via the integrated stress response to maintain redox and amino acid homeostasis

Mfn2 is a mitochondrial fusion protein with bioenergetic functions implicated in the pathophysiology of neuronal and metabolic disorders. Understanding the bioenergetic mechanism of Mfn2 may aid in designing therapeutic approaches for these disorders. Here we show using endoplasmic reticulum (ER) or mitochondria‐targeted Mfn2 that Mfn2 stimulation of the mitochondrial metabolism requires its localization in the ER, which is independent of its fusion function. ER‐located Mfn2 interacts with mitochondrial Mfn1/2 to tether the ER and mitochondria together, allowing Ca2+ transfer from the ER to mitochondria to enhance mitochondrial bioenergetics. The physiological relevance of these findings is shown during neurite outgrowth, when there is an increase in Mfn2‐dependent ER‐mitochondria contact that is necessary for correct neuronal arbor growth. Reduced neuritic growth in Mfn2 KO neurons is recovered by the expression of ER‐targeted Mfn2 or an artificial ER‐mitochondria tether, indicating that manipulation of ER‐mitochondria contacts could be used to treat pathologic conditions involving Mfn2., The bioenergetic role of Mfn2 relies on its localization in the ER. Mfn2 localization to mitochondria is dispensable in the presence of mitochondrial Mfn1, which tethers ER and mitochondria, and facilitates Ca2+ transfer.

The epicardium plays a crucial role in embryonic heart development and adult heart repair; however, the molecular events underlying its maturation remain unknown. Wt1, one of the main markers of the embryonic epicardium, is essential for epicardial development and function. Here, we analyse the transcriptomic profile of epicardial-enriched cells at different stages of development and from control and epicardial-specific Wt1 knockout (Wt1KO) mice. Transcriptomic and cell morphology analyses of epicardial cells from epicardial-specific Wt1KO mice revealed a defect in the maturation process of the mutant epicardium, including sustained upregulation of Bmp4 expression and the inability of mutant epicardial cells to transition into a mature squamous phenotype. We identified Bmp4 as a transcriptional target of Wt1, thus providing a molecular basis for the retention of the cuboidal cell shape observed in the Wt1KO epicardium. Accordingly, inhibition of the Bmp4 signalling pathway both ex vivo and in vivo rescued the cuboidal phenotype of the mutant epicardium. Our findings indicate the importance of the cuboidal-to-squamous transition in epicardial maturation, a process regulated by Wt1.

The epicardium plays a crucial role in embryonic heart development and adult heart repair; however, the molecular events underlying its maturation remain unknown.

The formation of neurites is an important process affecting the cognitive abilities of an organism. Neurite growth requires the addition of new membranes, but the metabolic remodeling necessary to supply lipids for membrane expansion is poorly understood. Here, we show that synaptic activity, one of the most important inducers of neurite growth, transcriptionally regulates the expression of neuronal glucose transporter Glut3 and rate-limiting enzymes of glycolysis, resulting in enhanced glucose uptake and metabolism that is partly used for lipid synthesis. Mechanistically, CREB regulates the expression of Glut3 and Siah2, the latter and LDH activity promoting the normoxic stabilization of HIF-1 alpha that regulates the expression of rate-limiting genes of glycolysis. The expression of dominant-negative HIF-1 alpha or Glut3 knockdown blocks activity-dependent neurite growth in vitro while pharmacological inhibition of the glycolysis and specific ablation of HIF-1 alpha in early postnatal mice impairs the neurite architecture. These results suggest that the manipulation of neuronal glucose metabolism could be used to treat some brain developmental disorders.

Mitochondria morphology constantly changes through fission and fusion processes that regulate mitochondrial function, and it therefore plays a prominent role in cellular homeostasis. Cell death progression is associated with mitochondrial fission. Fission is mediated by the mainly cytoplasmic Drp1, which is activated by different post-translational modifications and recruited to mitochondria to perform its function. Our research and other studies have shown that in the early moments of excitotoxic insult Drp1 must be nitrosylated to mediate mitochondrial fragmentation in neurons. Nonetheless, mitochondrial fission is a multistep process in which filamentous actin assembly/disassembly and myosin-mediated mitochondrial constriction play prominent roles. Here we establish that in addition to nitric oxide production, excitotoxicity-induced mitochondrial fragmentation also requires activation of the actomyosin regulator ROCK. Although ROCK1 has been shown to phosphorylate and activate Drp1, experiments using phosphor-mutant forms of Drp1 in primary cortical neurons indicate that in excitotoxic conditions, ROCK does not act directly on Drp1 to mediate fission, but may act on the actomyosin complex. Thus, these data indicate that a wider range of signaling pathways than those that target Drp1 are amenable to be inhibited to prevent mitochondrial fragmentation as therapeutic option.

Mitochondrial fusion and fission is a dynamic process critical for the maintenance of mitochondrial function and cell viability. During excitotoxicity neuronal mitochondria are fragmented, but the mechanism underlying this process is poorly understood. Here, we show that Mfn2 is the only member of the mitochondrial fusion/fission machinery whose expression is reduced in in vitro and in vivo models of excitotoxicity. Whereas in cortical primary cultures, Drp1 recruitment to mitochondria plays a primordial role in mitochondrial fragmentation in an early phase that can be reversed once the insult has ceased, Mfn2 downregulation intervenes in a delayed mitochondrial fragmentation phase that progresses even when the insult has ceased. Downregulation of Mfn2 causes mitochondrial dysfunction, altered calcium homeostasis, and enhanced Bax translocation to mitochondria, resulting in delayed neuronal death. We found that transcription factor MEF2 regulates basal Mfn2 expression in neurons and that excitotoxicity-dependent degradation of MEF2 causes Mfn2 downregulation. Thus, Mfn2 reduction is a late event in excitotoxicity and its targeting may help to reduce excitotoxic damage and increase the currently short therapeutic window in stroke.