Mitochondrial respiratory chain dysfunction disrupts intracellular cholesterol homeostasis

Mitochondrial diseases are rare and severe conditions with debilitating symptoms. Biochemical defects in mitochondria however are common. The difference between these two frequencies is suspected to lie in the capability of the cells to adapt to the homeostatic disbalance. Cells activate retrograde signalling pathways in response to mitochondrial dysfunction, activating cellular transcriptional programs to correct for the disbalance. The full extent of the retrograde signals that are induced by mitochondrial dysfunctions still remains unknown. Studying these retrograde signalling pathways can lead to insights how to make the difference between restoring the homeostasis and a detrimental disease. Investigating the transcriptional responses to artificially induced mitochondrial dysfunction has proven to be lucrative in unveiling novel responses to mitochondrial defects. In this work we studied the transcriptional responses to three chemical models of mitochondrial respiratory chain dysfunction. In line with recently published work, we found a commonly upregulated stress response in all models. Interestingly, shared between all the models was a downregulation of genes enriched in cholesterol biosynthesis pathways. Upon closer examination this encompassed, almost without exception, all transcripts of the mevalonate pathway. This coordinated downregulation of transcripts had functional consequences on the output of the pathway, lowering the abundance of cholesterol precursor metabolites. Other metabolites in more upstream regions of the pathway were changed differently depending on the complex affected, suggesting other regulatory influences involved. Upon closer mechanistic investigation to the cause of the transcriptional suppression we found the main transcription factor responsible for mevalonate pathway gene transcription, the Sterol Response Element Binding Protein 2 (SREBP2), to be inhibited during complex I impairment. This was most evident in sterol depleted conditions where SREBP2 did not activate to normal levels. A decreased activation of SREBP2 is signature for an elevated concentration of ER-cholesterol. Indeed, other ER-bound cholesterol sensitive proteins confirmed this, showing a similar decreased abundance following complex I impairment in both normal and low lipid conditions. To investigate the source of this cholesterol we measured various cholesterol levels. No changes in the levels of total cholesterol (free cholesterol plus esterified cholesterol) were detected. Also, the fraction of free cholesterol was not significantly affected by complex I inhibition. Using the fluorescent cholesterol stain filipin, we were able to measure a modest but reproducible increase in intracellular free cholesterol. This provided novel mechanistic insight into the inhibition of ER sterol sensors and mevalonate pathway activity by mitochondrial dysfunction. Several well-known retrograde signalling pathways AMPK, ISR and mTOR did not explain the changes observed. The exact mechanism through which mitochondria incite an increase in intracellular free cholesterol remains unknown. Taken together this work can provide a mechanistic link between mitochondrial function and intracellular cholesterol homeostasis and could possibly link mitochondrial function to circulating lipoprotein homeostasis. Further work should focus on elucidating the possible mechanism, where differences in complex functions could provide a hint in the right direction.

Nuclear receptors and the control of metabolism

Johan Auwerx

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

2003

The genetic ablation of SRC-3 protects against obesity and improves insulin sensitivity by reducing the acetylation of PGC-1{alpha}

Johan Auwerx, Kristina Schoonjans

Transcriptional control of metabolic circuits requires coordination between specific transcription factors and coregulators and is often deregulated in metabolic diseases. We characterized here the mechanisms through which the coactivator SRC-3 controls energy homeostasis. SRC-3 knock-out mice present a more favorable metabolic profile relative to their wild-type littermates. This metabolic improvement in SRC-3(-/-) mice is caused by an increase in mitochondrial function and in energy expenditure as a consequence of activation of PGC-1alpha. By controlling the expression of the only characterized PGC-1alpha acetyltransferase GCN5, SRC-3 induces PGC-1alpha acetylation and consequently inhibits its activity. Interestingly, SRC-3 expression is induced by caloric excess, resulting in the inhibition of PGC-1alpha activity and energy expenditure, whereas caloric restriction reduces SRC-3 levels leading to enhanced PGC-1alpha activity and energy expenditure. Collectively, these data suggest that SRC-3 is a critical link in a cofactor network that uses PGC-1alpha as an effector to control mitochondrial function and energy homeostasis.

2008

Mitochondrial gene signature in the prefrontal cortex for differential susceptibility to chronic stress

Daniel Alpern, Antoine Timothée Cherix, Bart Deplancke, Sriparna Ghosal, Jocelin Grosse, Rolf Gruetter, Julie Marie Russeil, Maria del Carmen Sandi Perez, Meltem Weger

Mitochondrial dysfunction was highlighted as a crucial vulnerability factor for the development of depression. However, systemic studies assessing stress-induced changes in mitochondria-associated genes in brain regions relevant to depression symptomatology remain scarce. Here, we performed a genome-wide transcriptomic study to examine mitochondrial gene expression in the prefrontal cortex (PFC) and nucleus accumbens (NAc) of mice exposed to multimodal chronic restraint stress. We identified mitochondria-associated gene pathways as most prominently affected in the PFC and with lesser significance in the NAc. A more detailed mitochondrial gene expression analysis revealed that in particular mitochondrial DNA-encoded subunits of the oxidative phosphorylation complexes were altered in the PFC. The comparison of our data with a reanalyzed transcriptome data set of chronic variable stress mice and major depression disorder subjects showed that the changes in mitochondrial DNA-encoded genes are a feature generalizing to other chronic stress-protocols as well and might have translational relevance. Finally, we provide evidence for changes in mitochondrial outputs in the PFC following chronic stress that are indicative of mitochondrial dysfunction. Collectively, our work reinforces the idea that changes in mitochondrial gene expression are key players in the prefrontal adaptations observed in individuals with high behavioral susceptibility and resilience to chronic stress.

2020