Utilization of Human Samples for Assessment of Mitochondrial Bioenergetics: Gold Standards, Limitations, and Future Perspectives

Mitochondrial bioenergetic function is a central component of cellular metabolism in health and disease. Mitochondrial oxidative phosphorylation is critical for maintaining energetic homeostasis, and impairment of mitochondrial function underlies the development and progression of metabolic diseases and aging. However, measurement of mitochondrial bioenergetic function can be challenging in human samples due to limitations in the size of the collected sample. Furthermore, the collection of samples from human cohorts is often spread over multiple days and locations, which makes immediate sample processing and bioenergetics analysis challenging. Therefore, sample selection and choice of tests should be carefully considered. Basic research, clinical trials, and mitochondrial disease diagnosis rely primarily on skeletal muscle samples. However, obtaining skeletal muscle biopsies requires an appropriate clinical setting and specialized personnel, making skeletal muscle a less suitable tissue for certain research studies. Circulating white blood cells and platelets offer a promising primary tissue alternative to biopsies for the study of mitochondrial bioenergetics. Recent advances in frozen respirometry protocols combined with the utilization of minimally invasive and non-invasive samples may provide promise for future mitochondrial research studies in humans. Here we review the human samples commonly used for the measurement of mitochondrial bioenergetics with a focus on the advantages and limitations of each sample.

Targeted metabolomics connects thioredoxin-interacting protein (TXNIP) to mitochondrial fuel selection and regulation of specific oxidoreductase enzymes in skeletal muscle

Thioredoxin-interacting protein (TXNIP) is an α-arrestin family member involved in redox sensing and metabolic control. Growing evidence links TXNIP to mitochondrial function, but the molecular nature of this relationship has remained poorly defined. Herein, we employed targeted metabolomics and comprehensive bioenergetic analyses to evaluate oxidative metabolism and respiratory kinetics in mouse models of total body (TKO) and skeletal muscle-specific (TXNIP(SKM-/-)) Txnip deficiency. Compared with littermate controls, both TKO and TXNIP(SKM-/-) mice had reduced exercise tolerance in association with muscle-specific impairments in substrate oxidation. Oxidative insufficiencies in TXNIP null muscles were not due to perturbations in mitochondrial mass, the electron transport chain, or emission of reactive oxygen species. Instead, metabolic profiling analyses led to the discovery that TXNIP deficiency causes marked deficits in enzymes required for catabolism of branched chain amino acids, ketones, and lactate, along with more modest reductions in enzymes of β-oxidation and the tricarboxylic acid cycle. The decrements in enzyme activity were accompanied by comparable deficits in protein abundance without changes in mRNA expression, implying dysregulation of protein synthesis or stability. Considering that TXNIP expression increases in response to starvation, diabetes, and exercise, these findings point to a novel role for TXNIP in coordinating mitochondrial fuel switching in response to nutrient availability.

Inborn errors of mitochondrial fatty acid oxidation

Inborn errors of the mitochondrial beta-oxidation of long-chain fatty acids represent an evolving field of inherited metabolic disease. Fatty acid oxidation defects demonstrate an abnormal response to the process of fasting adaptation and affect those tissues that utilize fatty acids as an energy source. These tissues include cardiac and skeletal muscle and liver. Muscle directly uses fatty acids as an energy source whilst hepatic metabolism of fatty acids is mostly directed toward the synthesis of ketone bodies for energy utilization by tissues such as brain. The clinical phenotypes of fatty acid oxidation disorders include disease of one or more of these fatty acid-metabolizing tissues. In this review, we provide an overview of the pathway, discuss the disorders that are well established, and describe recent advances in the field. Currently available diagnostic procedures are critically evaluated.

Fatal hepatic short-chain L-3-hydroxyacyl-coenzyme A dehydrogenase deficiency: clinical, biochemical, and pathological studies on three subjects with this recently identified disorder of mitochondrial beta-oxidation

This report describes the clinical, biochemical, and pathological findings in three infants with hepatic short-chain L-3-hydroxyacyl-coenzyme A dehydrogenase (SCHAD) deficiency, a recently recognized disorder of the mitochondrial oxidation of straight-chain fatty acids. Candidate subjects were identified from an ongoing study of infant deaths. SCHAD analysis was performed on previously frozen liver and skeletal muscle on subjects with a characteristic urine organic acid profile. Autopsy findings were correlated with the biochemical abnormalities. Enzyme analysis in liver revealed marked deficiency in SCHAD with residual activities of 3-11%. All subjects had normal activity in skeletal muscle. However, Western blot analysis of SCHAD revealed an identical truncated protein in both liver and muscle from one patient, suggesting that SCHAD is similar in liver and muscle and that the normal activity in muscle may be due to other enzymes with C4 activity. Autopsy findings revealed marked steatosis and a muscle pattern consistent with spinal muscular atrophy in one patient. Lipid storage was less pronounced in one patient and not detected in the third patient who had a well-documented history of recurrent hypoglycemia. This is the initial pathological characterization of this enzyme defect, and our observations suggest that SCHAD deficiency is a very severe disorder contributing to early infant death.

Sudden and unexpected neonatal death: a protocol for the postmortem diagnosis of fatty acid oxidation disorders

Fatty acid oxidation (FAO) disorders are frequently reported as the cause of sudden and unexpected death, but their postmortem identification remains difficult. Over a period of 5 years, the authors have identified 44 cases representing five FAO disorders and 19 additional cases without a diagnosis of a specific defect. Among the two groups, 13 patients died in the neonatal period, 10 in the FAO group, and three from the undetermined defect group. This outcome was consistently associated with exclusive breast feeding and presumably poor caloric intake. The diagnosis of FAO disorder in these cases was based on the analysis of postmortem liver and bile. In postmortem liver, informative findings are microvesicular steatosis, elevated fatty acid concentrations, glucose depletion, and low carnitine concentration. Bile carnitine analysis and acylcarnitine profiling have expanded significantly the effectiveness of the initial protocol and could lead, based on preliminary observations, to better identification of patients who may have been missed or left undetermined by the analysis of liver only. If an autopsy is not performed, informative findings can still be obtained by analysis of blood spots collected for newborn screenings and by biochemical testing of parents and asymptomatic siblings.

Retrospective biochemical screening of fatty acid oxidation disorders in postmortem livers of 418 cases of sudden death in the first year of life

OBJECTIVE

Fatty acid oxidation (FAO) disorders are frequently reported as the cause of sudden and unexpected death, but their postmortem recognition remains difficult. We have devised a biochemical protocol in which informative findings in liver tissue are microvesicular steatosis, elevated concentrations of C8-C16 fatty acids, glucose depletion, and low carnitine concentration.

STUDY DESIGN

We analyzed 27 cases representing five FAO disorders and compared the results with those obtained in a retrospective blinded analysis of 418 cases of sudden infant death (313 SIDS, 45 infections, and 34 accidents and abuse).

RESULTS

All cases of accidents and abuse correctly tested negative. Among the others, 25 (6%) showed at least two abnormal findings. Of these, 14 closely matched the biochemical profiles seen in specific FAO disorders. These included 2 cases with medium-chain acyl-CoA dehydrogenase deficiency, 4 cases consistent with glutaric acidemia type 2, 4 cases with either very long-chain acylcoenzyme A dehydrogenase deficiency or long-chain 3-hydroxy-acyl-coenzyme A dehydrogenase deficiency, and 4 cases predicted to be affected with carnitine uptake defect.

CONCLUSION

The results of this study support the view that approximately 5% of all cases of sudden infant death are likely caused by an FAO disorder.