Role of Energy Metabolism and Mitochondrial Function in Inflammatory Bowel Disease.

Inflammatory bowel disease (IBD) is a chronic recurring inflammation of the intestine which can be debilitating for those with intractable disease. However, the etiopathogenesis of inflammatory bowel disorders remains to be solved. The hypothesis that mitochondrial dysfunction is a crucial factor in the disease process is being validated by an increasing number of recent studies. Thus mitochondrial alteration in conjunction with previously identified genetic predisposition, changes in the immune response, altered gut microbiota, and environmental factors (eg, diet, smoking, and lifestyle) are all posited to contribute to IBD. The implicated factors seem to affect mitochondrial function or are influenced by mitochondrial dysfunction, which explains many of the hallmarks of the disease. This review summarizes the results of studies reporting links between mitochondria and IBD that were available on PubMed through March 2021. The aim of this review is to give an overview of the current understanding of the role of mitochondria in the pathogenesis of IBD.

We address the effect of energy metabolism and mitochondrial function on the pathogenesis of inflammatory bowel disease. Because many studies on this topic have been published recently, it is important to give an overview of the results of that work.

Expression of Oxidative Phosphorylation Complexes and Mitochondrial Mass in Pediatric and Adult Inflammatory Bowel Disease.

INTRODUCTION: Inflammatory bowel disease (IBD), which includes Crohn's disease (CD) and ulcerative colitis (UC), is a multifactorial intestinal disorder but its precise etiology remains elusive. As the cells of the intestinal mucosa have high energy demands, mitochondria may play a role in IBD pathogenesis. The present study is aimed at evaluating the expression levels of mitochondrial oxidative phosphorylation (OXPHOS) complexes in IBD. Material and Methods. 286 intestinal biopsy samples from the terminal ileum, ascending colon, and rectum from 124 probands (34 CD, 33 UC, and 57 controls) were stained immunohistochemically for all five OXPHOS complexes and the voltage-dependent anion-selective channel 1 protein (VDAC1 or porin). Expression levels were compared in multivariate models including disease stage (CD and UC compared to controls) and age (pediatric/adult). RESULTS: Analysis of the terminal ileum of CD patients revealed a significant reduction of complex II compared to controls, and a trend to lower levels was evident for VDAC1 and the other OXPHOS complexes except complex III. A similar pattern was found in the rectum of UC patients: VDAC1, complex I, complex II, and complex IV were all significantly reduced, and complex III and V showed a trend to lower levels. Reductions were more prominent in older patients compared to pediatric patients and more marked in UC than CD. CONCLUSION: A reduced mitochondrial mass is present in UC and CD compared to controls. This is potentially a result of alterations of mitochondrial biogenesis or mitophagy. Reductions were more pronounced in older patients compared to pediatric patients, and more prominent in UC than CD. Complex I and II are more severely compromised than the other OXPHOS complexes. This has potential therapeutic implications, since treatments boosting biogenesis or influencing mitophagy could be beneficial for IBD treatment. Additionally, substances specifically stimulating complex I activity should be tested in IBD treatment.

Age-Related Deterioration of Mitochondrial Function in the Intestine.

Aging is an important and inevitable biological process in human life, associated with the onset of chronic disease and death. The mechanisms behind aging remain unclear. However, changes in mitochondrial function and structure, including reduced activity of the mitochondrial respiratory chain and increased production of reactive oxygen species-thus oxidative damage-are believed to play a major role. Mitochondria are the main source of cellular energy, producing adenosine triphosphate (ATP) via oxidative phosphorylation. Accumulation of damaged cellular components reduces a body's capacity to preserve tissue homeostasis and affects biological aging and all age-related chronic conditions. This includes the onset and progression of classic degenerative diseases such as cardiovascular disease, kidney failure, neurodegenerative diseases, and cancer. Clinical manifestations of intestinal disorders, such as mucosal barrier dysfunction, intestinal dysmotility, and chronic obstipation, are highly prevalent in the elderly population and have been shown to be associated with an age-dependent decline of mitochondrial function. This review summarizes our current understanding of the role of mitochondrial dysfunction in intestinal aging.

Alterations of Oxidative Phosphorylation Complexes in Papillary Thyroid Carcinoma.

The papillary thyroid carcinoma (PTC) is the most common malignant tumor of the thyroid gland, with disruptive mutations in mitochondrial complex I subunits reported at very low frequency. Furthermore, metabolic diversity of PTC has been postulated owing to variable messenger RNA (mRNA) expression of genes encoding subunits of the oxidative phosphorylation (OXHPOS) complexes. The aim of the present study was to evaluate the metabolic diversity of the OXPHOS system at the protein level by using immunohistochemical staining. Analysis of 18 human PTCs revealed elevated mitochondrial biogenesis but significantly lower levels of OXPHOS complex I in the tumor tissue (p < 0.0001) compared to the adjacent normal tissue. In contrast, OXPHOS complexes II⁻V were increased in the majority of PTCs. In three PTCs, we found pathologic mutations within mitochondrially encoded complex I subunits. Our data indicate that PTCs are characterized by an oncocytic metabolic signature that is in low complex I is combined with elevated mitochondrial mass and high complex II⁻V levels, which might be an important factor for tumor formation.

Oxidative Phosphorylation System in Gastric Carcinomas and Gastritis.

Switching of cellular energy production from oxidative phosphorylation (OXPHOS) by mitochondria to aerobic glycolysis occurs in many types of tumors. However, the significance of this switching for the development of gastric carcinoma and what connection it may have to Helicobacter pylori infection of the gut, a primary cause of gastric cancer, are poorly understood. Therefore, we investigated the expression of OXPHOS complexes in two types of human gastric carcinomas ("intestinal" and "diffuse"), bacterial gastritis with and without metaplasia, and chemically induced gastritis by using immunohistochemistry. Furthermore, we analyzed the effect of HP infection on several key mitochondrial proteins. Complex I expression was significantly reduced in intestinal type (but not diffuse) gastric carcinomas compared to adjacent control tissue, and the reduction was independent of HP infection. Significantly, higher complex I and complex II expression was present in large tumors. Furthermore, higher complex II and complex III protein levels were also obvious in grade 3 versus grade 2. No differences of OXPHOS complexes and markers of mitochondrial biogenesis were found between bacterially caused and chemically induced gastritis. Thus, intestinal gastric carcinomas, but not precancerous stages, are frequently characterized by loss of complex I, and this pathophysiology occurs independently of HP infection.

Alterations of oxidative phosphorylation in meningiomas and peripheral nerve sheath tumors.

BACKGROUND: Changes in the mode of aerobic energy production are observed in many solid tumors, though the kinds of changes differ among tumor types. We investigated mitochondrial energy metabolism in meningiomas and peripheral nerve sheath tumors, taking into consideration the histologic heterogeneity of these tumors. METHODS: Oxidative phosphorylation (OXPHOS) complexes and porin (a marker for mitochondrial mass) were analyzed by immunohistochemical staining of meningiomas (n = 76) and peripheral nerve sheath tumors (schwannomas: n = 10; neurofibromas: n = 4). The enzymatic activities of OXPHOS complexes and citrate synthase were determined by spectrophotometric measurement. Western blot analysis of OXPHOS complexes, porin, and mitochondrial transcription factor A was performed. Furthermore, mitochondrial DNA copy number was determined. RESULTS: The tumors differed with regard to mitochondrial energy metabolism. Low levels of a subset of OXPHOS complexes were frequently observed in World Health Organization grade I meningiomas (percent of cases with a reduction; complex I: 63%; complex II: 67%; complex IV: 56%) and schwannomas (complex III: 40%, complex IV: 100%), whereas in neurofibromas a general reduction of all complexes was observed. In contrast, expression of complexes III and V was similar to that in normal brain tissue in the majority of tumors. Mitochondrial mass was comparable or higher in all tumors compared with normal brain tissue, whereas mitochondrial DNA copy number was reduced. CONCLUSIONS: The reduction of OXPHOS complexes in meningiomas and peripheral nerve sheath tumors has potential therapeutic implications, since respiratory chain-deficient tumor cells might be selectively starved by inhibitors of glycolysis or by ketogenic diet.

From ventriculomegaly to severe muscular atrophy: expansion of the clinical spectrum related to mutations in AIFM1.

The apoptosis-inducing factor (AIF) functions as a FAD-dependent NADH oxidase in mitochondria. Upon apoptotic stimulation it is released from mitochondria and migrates to the nucleus where it induces chromatin condensation and DNA fragmentation. So far mutations in AIFM1, a X-chromosomal gene coding for AIF, have been described in three families with 11 affected males. We report here on a further patient thereby expanding the clinical and mutation spectrum. In addition, we review the known phenotypes related to AIFM1 mutations. The clinical course in the male patient described here was characterized by phases with rapid deterioration and long phases without obvious progression of disease. At age 2.5 years he developed hearing loss and severe ataxia and at age 10 years muscle wasting, swallowing difficulties, respiratory insufficiency and external opthamoplegia. By next generation sequencing of whole exome we identified a hemizygous missense mutation in the AIFM1 gene, c.727G>T (p.Val243Leu) affecting a highly conserved residue in the FAD-binding domain. Summarizing what is known today, mutations in AIFM1 are associated with a progressive disorder with myopathy, ataxia and neuropathy. Severity varies greatly even within one family with onset of symptoms between birth and adolescence. 3 of 12 patients died before age 5 years while others were still able to walk during young adulthood. Less frequent symptoms were hearing loss, seizures and psychomotor regression. Results from clinical chemistry, brain imaging and muscle biopsy were unspecific and inconsistent.

ELAC2 mutations cause a mitochondrial RNA processing defect associated with hypertrophic cardiomyopathy.

The human mitochondrial genome encodes RNA components of its own translational machinery to produce the 13 mitochondrial-encoded subunits of the respiratory chain. Nuclear-encoded gene products are essential for all processes within the organelle, including RNA processing. Transcription of the mitochondrial genome generates large polycistronic transcripts punctuated by the 22 mitochondrial (mt) tRNAs that are conventionally cleaved by the RNase P-complex and the RNase Z activity of ELAC2 at 5' and 3' ends, respectively. We report the identification of mutations in ELAC2 in five individuals with infantile hypertrophic cardiomyopathy and complex I deficiency. We observed accumulated mtRNA precursors in affected individuals muscle and fibroblasts. Although mature mt-tRNA, mt-mRNA, and mt-rRNA levels were not decreased in fibroblasts, the processing defect was associated with impaired mitochondrial translation. Complementation experiments in mutant cell lines restored RNA processing and a yeast model provided additional evidence for the disease-causal role of defective ELAC2, thereby linking mtRNA processing to human disease.

Deficiency of the mitochondrial phosphate carrier presenting as myopathy and cardiomyopathy in a family with three affected children.

In a family three children presented with severe neonatal lactic acidosis, hypertrophic cardiomyopathy and generalised muscular hypotonia. One child died in infancy, two survived a clinically severe neonatal period. At an age of 9 and 17years, respectively, they present with exercise intolerance, proximal muscle weakness, non-progressive hypertrophic cardiomyopathy and normal mental development. In a muscle biopsy normal activity of respiratory chain enzymes was found; however the amount of the mitochondrial phosphate carrier was decreased. This protein is expressed in two tissue-specific isoforms generated by mutually exclusive alternative splicing of the SLC25A3 gene transcript. We identified a homozygous mutation c.158-9A>G located in the 5'-intron next to exon 3A specific for heart and skeletal muscle. This creates a novel splice site resulting in a more than 95% decrease of the wild type allele.

Heterozygous mutation in the X chromosomal NDUFA1 gene in a girl with complex I deficiency.

Respiratory chain enzymes consist of multiple subunits encoded either by the mitochondrial or by the nuclear genome. Recently the first X-chromosomal mutations in complex I deficient males have been described. Heterozygous female carriers did not seem to be affected. Here, we describe a girl initially presenting with mild muscular hypotonia, a moderate lactic acidosis and an increased beta-hydroxybutyrate/acetoacetate ratio. Biochemical investigations of a muscle biopsy revealed a deficiency in the amount and activity of complex I. Mutation screening of all structural subunits of complex I identified a heterozygous mutation c.94G>C, p.Gly32Arg in the X-chromosomal NDUFA1 gene. Analysis of the cDNA showed that 72% of the expressed mRNA was mutated in the muscle biopsy sample. Investigation of the X-inactivation pattern demonstrated that 74% of the paternally inherited allele was active in the muscle. This is the first report of an X-chromosomally inherited respiratory chain defect in a heterozygous female.

Loss of mitochondria in ganglioneuromas.

A shift in cellular energy production from oxidative phosphorylation (OXPHOS) to glycolysis, even under aerobic conditions, is called the Warburg effect. To elucidate changes of the mitochondrial energy metabolism in ganglioneuroma (GN) individual OXPHOS enzymes were analyzed by activity assays and by immunohistochemical staining methods. GN (n=7) showed a significant reduction in the activity and content of OXPHOS enzymes. Citrate synthase activity was also severely diminished in GN compared to normal cortical kidney (p=0.0002) and adrenal (p=0.0024) tissues. Furthermore, the mitochondrial membrane protein porin was undetectable or significantly reduced. Accordingly, a reduction of the copy number of mitochondrial DNA was observed in GN compared to cortical kidney tissue. The striking decline of mitochondrial mass is specific for GN but not for neuroblastoma, in which a reduction of the OXPHOS complexes without reduction of mitochondrial mass was reported. Knowledge of the mechanism by which tumor cells achieve the Warburg effect will provide a starting point for functional studies aimed at restoring aerobic energy metabolism as a potential new therapeutic strategy to treat malignancies.

Heterogeneity of mitochondrial energy metabolism in classical triphasic Wilms' tumor.

Metabolic changes are observed in a variety of tumors. The nature of the changes in aerobic energy metabolism differs between tumor types. Therefore, immunohistochemical staining, enzymatic measurements and immunoblot analysis were used to determine alterations of oxidative phosphorylation (OXPHOS) in classic triphasic Wilms' Tumor (WT). Our studies revealed that the epithelial, stromal and blastemal elements of this tumor differ in their energy metabolism. Compared to unaffected kidney tissue, normal mitochondrial mass was observed in the epithelial and blastemal regions of WT, whereas the stroma showed a massive down-regulation of mitochondria, as indicated by low porin content, low citrate synthase activity, and reduced mtDNA copy number. All OXPHOS enzyme activities were reduced in all WT samples, with the exception of two epithelial-dominant cases, which showed up-regulation of complex III activity compared to control kidney tissues. Interestingly, our studies show that, even within a specific tumor entity, cell-type-specific alterations of aerobic energy metabolism can occur, although all cell types showed a clear tendency toward a reduced aerobic energy metabolism.

Alterations of respiratory chain complexes in sporadic pheochromocytoma.

Succinate dehydrogenase (SDH) has been associated with carcinogenesis in hereditary pheochromocytoma (PC) and paraganglioma. We investigated if a similar association applies to sporadic pheochromocytoma. No genetic alteration was found in the SDHB, SDHC or SDHD genes of sporadic PC. However, in eight of nine sporadic PCs the SDH activity was, on average, reduced by 40%; moreover, the activities of the other oxidative phosphorylation (OXPHOS) complexes and citrate synthase were significantly lower compared to normal kidney tissue. Furthermore, immunohistochemical staining revealed a significant down-regulation of respiratory chain complexes. Since no pathogenic mutations were detected in the von Hippel-Lindau (VHL) gene, we can rule out that VHL deficiency is causing the general reduction of OXPHOS enzymes observed in the PCs investigated. In contrast to the single enzyme defects found in a subset hereditary PCs, a more generalized reduction of mitochondrial respiration seems to be present in most sporadic PCs. Strikingly, one of the nine PCs showed specific loss of complex I and a compensatory up-regulation of complexes II-V, which is a phenotype usually characteristic of oncocytic tumors.

Respiratory chain complex I is a mitochondrial tumor suppressor of oncocytic tumors.

Oncocytic tumors, also called oxyphilic tumors, are characterized by hyperproliferation of mitochondria, which histologically presents as a fine granular eosinophilic cytoplasm. In accordance with the high mitochondrial density in oncocytomas, transcript levels of subunits of complexes of the oxidative phosphorylation (OXPHOS) system are increased. Hence, for a long time oncocytomas were presumed to have a highly active aerobic mitochondrial energy metabolism. Recently, detailed analysis of all OXPHOS complexes in a variety of oncocytomas revealed loss of complex I and compensatory up-regulation of the other complexes. In half of the oncocytoma cases examined the absence of complex I is caused by disruptive mutations in mitochondrial DNA encoding complex I subunits. The new data presented here on rare oncocytomas and the accompanying review of the literature clearly indicate that complex I deficiency in combination with up-regulation of mitochondria can be regarded as a hallmark of oncocytic tumor cells. Therefore, complex I of the respiratory chain has to be added to the growing list of mitochondrial tumor suppressors.