Mitochondria and aging in older individuals: an analysis of DNA methylation age metrics, leukocyte telomere length, and mitochondrial DNA copy number in the VA normative aging study

Long-term demyelination and aging-associated changes in mice corpus callosum; evidence for the role of accelerated aging in remyelination failure in a mouse model of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disorder affecting the central nervous system. Evidence suggests that age-related neurodegeneration contributes to disability progression during the chronic stages of MS. Aging is characterized by decreased regeneration potential and impaired myelin repair in the brain. It is hypothesized that accelerated cellular aging contributes to the functional decline associated with neurodegenerative diseases. We assessed the impact of aging on myelin content in the corpus callosum (CC) and compared aging with the long-term demyelination (LTD) consequents induced by 12 weeks of feeding with a cuprizone (CPZ) diet. Initially, evaluating myelin content in 2-, 6-, and 18-month-old mice revealed a reduction in myelin content, particularly at 18 months. Myelin thickness was decreased and the g-ratio increased in aged mice. Although a lower myelin content and higher g-ratio were observed in LTD model mice, compared to the normally aged mice, both aging and LTD exhibited relatively similar myelin ultrastructure. Our findings provide evidence that LTD exhibits the hallmarks of aging such as elevated expression of senescence-associated genes, mitochondrial dysfunction, and high level of oxidative stress as observed following normal aging. We also investigated the senescence-associated β-galactosidase activity in O4+ late oligodendrocyte progenitor cells (OPCs). The senescent O4+/β-galactosidase+ cells were elevated in the CPZ diet. Our data showed that the myelin degeneration in CC occurs throughout the lifespan, and LTD induced by CPZ accelerates the aging process which may explain the impairment of myelin repair in patients with progressive MS.

Biomarkers of Aging and Relevant Evaluation Techniques: A Comprehensive Review

The risk of developing chronic illnesses and disabilities is increasing with age. To predict and prevent aging, biomarkers relevant to the aging process must be identified. This paper reviews the known molecular, cellular, and physiological biomarkers of aging. Moreover, we discuss the currently available technologies for identifying these biomarkers, and their applications and potential in aging research. We hope that this review will stimulate further research and innovation in this emerging and fast-growing field.

Prenatal air pollution exposure in relation to the telomere-mitochondrial axis of aging at birth: A systematic review

BACKGROUND

Telomere length (TL) and mitochondrial DNA (mtDNA) are central markers of vital biological mechanisms, including cellular aging. Prenatal air pollution exposure may impact molecular markers of aging leading to adverse health effects.

OBJECTIVE

To perform a systematic review on human population-based studies investigating the association between prenatal air pollution exposure and TL or mtDNA content at birth.

METHODOLOGY

Searches were undertaken on PubMed and Web of Science until July 2023. The framework of the review was based on the PRISMA-P guidelines.

RESULTS

Nineteen studies studied prenatal air pollution and TL or mtDNA content at birth. Studies investigating TL or mtDNA content measured at any other time or did not evaluate prenatal air pollution were excluded. Twelve studies (including 4381 participants with study sample range: 97 to 743 participants) investigated newborn TL and eight studies (including 3081 participants with study sample range: 120 to 743 participants) investigated mtDNA content at birth. Seven studies focused on particulate matter (PM2.5) exposure and newborn TL of which all, except two, showed an inverse association in at least one of the gestational trimesters. Of the eight studies on mtDNA content, four focused on PM2.5 air pollution with two of them reporting an inverse association. For PM2.5 exposure, observations on trimester-specific effects were inconsistent. Current literature showing associations with other prenatal air pollutants (including nitrogen oxides, sulfur dioxide, carbon monoxide and ozone) is inconsistent.

CONCLUSION

This review provides initial evidence that prenatal PM2.5 exposure impacts the telomere-mitochondrial axis of aging at birth. The current evidence did not reveal harmonious observations for trimester-specific associations nor showed consistent effects of other air pollutants. Future studies should elucidate the specific contribution of prenatal exposure to pollutants other than PM in relation to TL and mtDNA content at birth, and the potential later life health consequences.

Simultaneous assessment of mitochondrial DNA copy number and nuclear epigenetic age towards predictive models of development and aging

OBJECTIVE

Mitochondrial dysfunction and nuclear epigenetic alterations, two hallmarks of aging, are associated with aberrant development and complex disease risk. Here, we report a method for the simultaneous assessment of mitochondrial DNA copy number (mtDNA-CN) and DNA methylation age (DNAm age) from the same DNA extraction using quantitative polymerase chain reaction (qPCR) and array data, respectively.

RESULT

We present methods for the concurrent estimation of mtDNA-CN and DNAm age from the same DNA samples. This includes qPCR to estimate mtDNA-CN, representing the number of circular mitochondrial genomes in a cell, and DNA methylation microarray data to estimate the epigenetic age of an individual. Further, we provide a method for the combination of these metrics into a shared metric termed 'mtEpiAge'. This approach provides a valuable tool for exploring the interplay between mitochondrial dysfunction and nuclear epigenetic alterations, and their associations with disease and aging.

Stress, epigenetics, and aging: Unraveling the intricate crosstalk

Aging, as a complex process involving multiple cellular and molecular pathways, is known to be exacerbated by various stresses. Because responses to these stresses, such as oxidative stress and genotoxic stress, are known to interplay with the epigenome and thereby contribute to the development of age-related diseases, investigations into how such epigenetic mechanisms alter gene expression and maintenance of cellular homeostasis is an active research area. In this review, we highlight recent studies investigating the intricate relationship between stress and aging, including its underlying epigenetic basis; describe different types of stresses that originate from both internal and external stimuli; and discuss potential interventions aimed at alleviating stress and restoring epigenetic patterns to combat aging or age-related diseases. Additionally, we address the challenges currently limiting advancement in this burgeoning field.

Low mitochondrial DNA copy number in peripheral blood mononuclear cells is associated with future mortality risk: a long-term follow-up study from Japan

OBJECTIVES

The mitochondrial DNA (mtDNA) is unique and circular with multiple copies of the genome. The lower mtDNA copy number (mtDNA-CN) in leukocytes is associated with the risk of all-cause mortality. However, its long-term association is unknown. Thus, the study examined the association between mtDNA-CN and the risk of all-cause mortality in a long-term follow-up study in the Japanese population.

DESIGN

This longitudinal study included the study cohort from an annual, population-based health checkup in the town of Yakumo, Hokkaido, Japan.

SETTING AND PARTICIPANTS

814 participants (baseline age range: 38-80 years, mean: 56.3 years) were included in this study in 1990. They were followed-up regarding mortality for about 30 years (median: 28.1 years) till 2019.

MEASURES

The genomic DNA was extracted from peripheral blood mononuclear cells and the mtDNA-CN was measured using real-time polymerase chain reaction. The level of the mtDNA-CN was divided into tertiles (low, middle, and high). The participants were categorized based on their age into middle-aged (<60 years old) or old-aged (≥60 years old). Survival analysis was performed for tertile of mtDNA-CN and compared using the log-rank test. Univariate and multivariable Cox proportional hazard regression analyses were performed to assess the association between mtDNA-CN and all-cause mortality. The model adjusted with age, sex, body mass index, systolic blood pressure, smoking habit, alcohol consumption, exercise habit, and education level.

RESULTS

The low levels of mtDNA-CN resulted in a significant decrease in cumulative survival rate (P <  0.05). The risk of mortality was significantly higher in the middle-aged cohort when mtDNA-CN levels were low (hazard ratios [95% confidence intervals]: 1.98 [1.10-3.56]).

CONCLUSION

This study demonstrated that leukocyte mtDNA-CN is associated with future mortality risk. Our study findings may lead to further research on the early prediction of mortality and its underlying mechanisms.

Analysis of mitochondrial DNA copy number variation in Brazilian farmers occupationally exposed to pesticides

The use of pesticide use has been linked to the higher production of reactive oxygen species, resulting in oxidative stress, which in turn can cause genomic instability. A marker for instability is the copy number variation of the mitochondrial genome (mtDNAcn), which has been found to be altered in diverse human diseases, including tumors. This research aimed to examine the variation of mtDNAcn in individuals occupationally exposed to pesticides. Real-time PCR assays were conducted on 154 individuals (78 exposed and 76 non-exposed). Pesticide-exposed ndividuals exhibited a significant reduction in mtDNAcn (1.11 ± 0.37mtDNAcn/genome) compared to non-exposed individuals (1.30 ± 0.33mtDNAcn/genome; p = 0.001). The multivariate analysis indicated that individuals who reported using haloxyfop and copper sulfate demonstrated an increase (β = 0.200, p = 0.053) and a decrease (β=-0.2, p = 0.021), respectively, in mtDNAcn. In conclusion, our findings suggest that chronic exposure to pesticides results in changes in mtDNAcn.

Telomere Dynamics in Livestock

Telomeres are repeated sequences of nucleotides at the end of chromosomes. They deteriorate across mitotic divisions of a cell. In Homo sapiens this process of lifetime reduction has been shown to correspond with aspects of organismal aging and exposure to stress or other insults. The early impetus to characterize telomere dynamics in livestock related to the concern that aged donor DNA would result in earlier cell senescence and overall aging in cloned animals. Telomere length investigations in dairy cows included breed effects, estimates of additive genetic control (heritability 0.12 to 0.46), and effects of external stressors on telomere degradation across animal life. Evaluation of telomeres with respect to aging has also been conducted in pigs and horses, and there are fewer reports of telomere biology in beef cattle, sheep, and goats. There were minimal associations of telomere length with animal productivity measures. Most, but not all, work in livestock has documented an inverse relationship between peripheral blood cell telomere length and age; that is, a longer telomere length was associated with younger age. Because livestock longevity affects productivity and profitability, the role of tissue-specific telomere attrition in aging may present alternative improvement strategies for genetic improvement while also providing translational biomedical knowledge.

Quantifying human genome parameters in aging

Healthy human longevity is a global goal of the world health system. Determining the causes and processes influencing human longevity is the primary fundamental goal facing the scientific community. Currently, the main efforts of the scientific community are aimed at identifying the qualitative characteristics of the genome that determine the trait. At the same time, when evaluating qualitative characteristics, there are many challenges that make it difficult to establish associations. Quantitative traits are burdened with such problems to a lesser extent, but they are largely overlooked in current genomic studies of aging and longevity. Although there is a wide repertoire of quantitative trait analyses based on genomic data, most opportunities are ignored by authors, which, along with the inaccessibility of published data, leads to the loss of this important information. This review focuses on describing quantitative traits important for understanding aging and necessary for analysis in further genomic studies, and recommends the inclusion of the described traits in the analysis. The review considers the relationship between quantitative characteristics of the mitochondrial genome and aging, longevity, and age-related neurodegenerative diseases, such as the frequency of extensive mitochondrial DNA (mtDNA) deletions, mtDNA half-life, the frequency of A>G replacements in the mtDNA heavy chain, the number of mtDNA copies; special attention is paid to the mtDNA methylation sign. A separate section of this review is devoted to the correlation of telomere length parameters with age, as well as the association of telomere length with the amount of mitochondrial DNA. In addition, we consider such a quantitative feature as the rate of accumulation of somatic mutations with aging in relation to the lifespan of living organisms. In general, it may be noted that there are quite serious reasons to suppose that various quantitative characteristics of the genome may be directly or indirectly associated with certain aspects of aging and longevity. At the same time, the available data are clearly insufficient for definitive conclusions and the determination of causal relationships.

The prenatal environment and its influence on maternal and child mitochondrial DNA copy number and methylation: A review of the literature

Mitochondrial DNA (mtDNA) is sensitive to environmental stressors and associated with human health. We reviewed epidemiological literature examining associations between prenatal environmental, dietary, and social exposures and alterations in maternal/child mtDNA copy number (mtDNAcn) and mtDNA methylation. Evidence exists that prenatal maternal exposures are associated with alterations in mtDNAcn for air pollution, chemicals (e.g. metals), cigarette smoke, human immunodeficiency virus (HIV) infection and treatment. Evidence for their associations with mtDNA methylation was limited. Given its potential implications as a disease pathway biomarker, studies with sufficient biological specificity should examine the long-term implications of prenatal and early-life mtDNA alterations in response to prenatal exposures.

Biomarkers of aging through the life course: A Recent Literature Update

Purpose of review

The development of biomarkers of aging has greatly advanced epidemiological studies of aging processes. However, much debate remains on the timing of aging onset and the causal relevance of these biomarkers. In this review, we discuss the most recent biomarkers of aging that have been applied across the life course.

Recent findings

The most recently developed aging biomarkers that have been applied across the life course can be designated into three categories: epigenetic clocks, epigenetic markers of chronic inflammation, and mitochondrial DNA copy number. While these have been applied at different life stages, the development, validation, and application of these markers has been largely centered on populations of older adults. Few studies have examined trajectories of aging biomarkers across the life course. As the wealth of molecular and biochemical data increases, emerging biomarkers may be able to capture complex and system-specific aging processes. Recently developed biomarkers include novel epigenetic clocks; clocks based on ribosomal DNA, transcriptomic profiles, proteomics, metabolomics, and inflammatory markers; clonal hematopoiesis of indeterminate potential gene mutations; and multi-omics approaches.

Summary

Attention should be placed on aging at early and middle life stages to better understand trajectories of aging biomarkers across the life course. Additionally, novel biomarkers will provide greater insight into aging processes. The specific mechanisms of aging reflected by these biomarkers should be considered when interpreting results.

Association of telomere length and mitochondrial DNA copy number, two biomarkers of biological aging, with the risk of venous thromboembolism

BACKGROUND

Venous thromboembolism (VTE) is the third most common cardiovascular disease and occurs in all age groups, albeit the risk increases considerably with age. Previous research indicates mitochondrial dysfunction and telomere shortening in cardiovascular aging. However, in the context of VTE this has not been investigated in detail.

AIM

We aimed to explore biomarkers reflecting biological aging (i.e. human mitochondrial DNA copy number (mtDNA) and telomere length) and their association with VTE.

METHODS

mtDNA and telomere length were measured in a case-control study of 116 patients with a history of VTE and 128 age- and sex-matched healthy individuals from isolated blood using a qPCR-based assay kit. Cases had at least one unprovoked VTE event and were enrolled no earlier than 3 months after the last VTE event.

RESULTS

The mtDNA copy number was significantly lower in VTE cases compared to controls (median [IQR]: 663 per diploid cells [78.75-2204.5] vs. 2832 per diploid cells [724-4350]; p < 0.001). After adjustment for age, sex, BMI, and smoking, mtDNA copy number was independently associated with VTE risk (odds ratio per increase in 400 mtDNA per diploid cell: 0.889, 95%CI 0.834-0.947). mtDNA copy numbers were significantly different between women and men (2375 [455-3737] women vs. 893 [152-3154] men; p < 0.001). The analysis of telomere length showed no significant difference between patients and healthy controls.

CONCLUSION

Lower mtDNA levels were found in patients with VTE compared to controls, indicating an association of biological aging with risk of VTE.

Interrelationships and determinants of aging biomarkers in cord blood

BACKGROUND

Increasing evidence supports the concept of prenatal programming as an early factor in the aging process. DNA methylation age (DNAm age), global genome-wide DNA methylation (global methylation), telomere length (TL), and mitochondrial DNA content (mtDNA content) have independently been shown to be markers of aging, but their interrelationship and determinants at birth remain uncertain.

METHODS

We assessed the inter-correlation between the aging biomarkers DNAm age, global methylation, TL and mtDNA content using Pearson's correlation in 190 cord blood samples of the ENVIRONAGE birth cohort. TL and mtDNA content was measured via qPCR, while the DNA methylome was determined using the human 450K methylation Illumina microarray. Subsequently, DNAm age was calculated according to Horvath's epigenetic clock, and mean global, promoter, gene-body, and intergenic DNA methylation were determined. Path analysis, a form of structural equation modeling, was performed to disentangle the complex causal relationships among the aging biomarkers and their potential determinants.

RESULTS

DNAm age was inversely correlated with global methylation (r = -0.64, p < 0.001) and mtDNA content (r = - 0.16, p = 0.027). Cord blood TL was correlated with mtDNA content (r = 0.26, p < 0.001) but not with global methylation or DNAm age. Path analysis showed the strongest effect for global methylation on DNAm age with a decrease of 0.64 standard deviations (SD) in DNAm age for each SD (0.01%) increase in global methylation (p < 0.001). Among the applied covariates, newborn sex and season of delivery were the strongest determinants of aging biomarkers.

CONCLUSIONS

We provide insight into molecular aging signatures at the start of life, including their interrelations and determinants, showing that cord blood DNAm age is inversely associated with global methylation and mtDNA content but not with newborn telomere length. Our findings demonstrate that cord blood TL and DNAm age relate to different pathways/mechanisms of biological aging and can be influenced by environmental factors already at the start of life. These findings are relevant for understanding fetal programming and for the early prevention of noncommunicable diseases.

Prenatal lead exposure, telomere length in cord blood, and DNA methylation age in the PROGRESS prenatal cohort

BACKGROUND

Lead is a ubiquitous pollutant with deleterious effects on human health and remains a major current public health concern in developing countries. This heavy metal may interfere with nucleic acids via oxidative stress or epigenetic changes that affect biological markers of aging, e.g., telomere length and DNA methylation (DNAm). Telomere shortening associates with biological age in newborns, and DNA methylation at specific CpG sites can be used to calculate "epigenetic clocks".

OBJECTIVE

The aim of this study was to examine the associations of prenatal lead exposures with telomere length and DNA-methylation-based predictors of age in cord blood.

DESIGN

The study included 507 mother-child pairs from the Programming Research in Obesity, Growth, Environment and Social Stressors (PROGRESS) study, a birth cohort in Mexico City. Maternal blood (second trimester, third trimester and at delivery) and bone lead levels (one month postpartum) were measured using inductively coupled plasma-mass spectrometry and X-ray fluorescence, respectively. Cord blood leukocyte telomere length was measured using quantitative PCR and apparent age by DNA methylation biomarkers, i.e., Horvath's DNA methylation age and the Knight's predictor of gestational age.

RESULTS

Average maternal age was 28.5 ± 5.5 years, and 51.5% reported low socioeconomic status. Children's mean telomere length was 1.2 ± 1.3 relative units, and mean DNA methylation ages using the Horvath's and Knight's clocks were -2.6 ± 0.1 years and 37.9 ± 1.4 weeks (mean ± SD), respectively. No significant associations were found between maternal blood and bone lead concentrations with telomere length and DNAm age in newborns.

CONCLUSION

We found no associations of prenatal lead exposure with telomere length and DNA methylation age biomarkers.

Integrating DNA Methylation Measures of Biological Aging into Social Determinants of Health Research

PURPOSE OF REVIEW

Acceleration of biological processes of aging is hypothesized to drive excess morbidity and mortality in socially disadvantaged populations. DNA methylation measures of biological aging provide tools for testing this hypothesis.

RECENT FINDINGS

Next-generation DNA methylation measures of biological aging developed to predict mortality risk and physiological decline are more predictive of morbidity and mortality than the original epigenetic clocks developed to predict chronological age. These new measures show consistent evidence of more advanced and faster biological aging in people exposed to socioeconomic disadvantage and may be able to record the emergence of socially determined health inequalities as early as childhood. Next-generation DNA methylation measures of biological aging also indicate race/ethnic disparities in biological aging. More research is needed on these measures in samples of non-Western and non-White populations. New DNA methylation measures of biological aging open opportunities for refining inference about the causes of social disparities in health and devising policies to eliminate them. Further refining measures of biological aging by including more diversity in samples used for measurement development is a critical priority for the field.

Telomere length and mitochondrial DNA copy number in multidrug-resistant tuberculosis

Multidrug resistant tuberculosis (MDR-TB) is a severe disease that requires prolonged chemotherapy and is associated with an increased probability of treatment failure and death. MDR-TB is a state of heightened oxidative stress and inflammation, which could be related to the aging-related processes and immunosenescence. We, therefore, tested the hypothesis that MDR-TB is associated with alterations in aging biomarkers in peripheral blood cells. We investigated 51 MDR-TB patients and 57 healthy individuals and carried out an analysis of covariance to assess the possible impact of different variables on biomarker perturbations. The results showed that MDR-TB patients had significantly reduced telomere length (TL) and increased mitochondrial DNA copy number (mtDNA CN) (P < 0.05) in comparison to the controls, and MDR-TB infection was the main influencing factor. Male sex and extrapulmonary TB strongly influenced mtDNA CN increment, and MDR-TB patients with normal weight had longer telomeres than those who were underweight (P < 0.05). In conclusion, the evidence for shorter telomeres and higher mtDNA CN in the peripheral blood cells of MDR-TB patients was obtained indicating the connection between MDR-TB and aging biomarkers. The observed associations highlight a complicated interplay between MDR-TB and immunosenescence, thus further studies are required to achieve full understanding.

Prospective Associations of Early Pregnancy Metal Mixtures with Mitochondria DNA Copy Number and Telomere Length in Maternal and Cord Blood

BACKGROUND

Metal exposure during pregnancy influences maternal and child health. Oxidative stress and inflammation may mediate adverse effects of heavy metals, whereas essential metals may act as antioxidants. Mitochondrial DNA is a prime target for metal-induced oxidative damage. Telomere dysfunction is attributed to imbalances between reactive oxidant species and antioxidants.

OBJECTIVES

We evaluated individual and joint associations of prenatal metals with mitochondrial DNA copy number (mtDNAcn) and telomere length (TL) in maternal and cord blood as biomarkers of inflammation and oxidative stress.

METHODS

We measured six nonessential metals (arsenic, barium, cadmium, cesium, lead, mercury) and four essential metals (magnesium, manganese, selenium, zinc) in first-trimester maternal red blood cells in Project Viva, a U.S. prebirth cohort. We measured relative mtDNAcn (n=898) and TL (n=893) in second-trimester maternal blood and mtDNAcn (n=419) and TL (n=408) in cord blood. We used multivariable linear regression and quantile g-computation to estimate associations between prenatal metals and the biomarkers. We used generalized additive models and Bayesian kernel machine regression to examine nonlinearity and interactions.

RESULTS

A 2-fold increase in maternal magnesium was associated with lower maternal [β=-0.07, 95% confidence interval (CI): -0.10, -0.01] and cord blood (β=-0.08, 95% CI: -0.20, -0.01) mtDNAcn. Lead was associated with higher maternal mtDNAcn (β=0.04, 95% CI: 0.01, 0.06). Selenium was associated with longer cord blood TL (β=0.30, 95% CI: 0.01 0.50). An association was observed between the nonessential metal mixture and higher maternal mtDNAcn (β=0.04, 95% CI: 0.01, 0.07). There was a nonlinear relationship between cord blood mtDNAcn and magnesium; maternal mtDNAcn and barium, lead, and mercury; and maternal TL and barium.

DISCUSSION

Maternal exposure to metals such as lead, magnesium, and selenium was associated with mtDNAcn and TL in maternal second trimester and cord blood. Future work will evaluate whether these biomarkers are associated with child health. https://doi.org/10.1289/EHP9294.

Aging-Induced Impairment of Vascular Function: Mitochondrial Redox Contributions and Physiological/Clinical Implications

Significance: The vasculature responds to the respiratory needs of tissue by modulating luminal diameter through smooth muscle constriction or relaxation. Coronary perfusion, diastolic function, and coronary flow reserve are drastically reduced with aging. This loss of blood flow contributes to and exacerbates pathological processes such as angina pectoris, atherosclerosis, and coronary artery and microvascular disease. Recent Advances: Increased attention has recently been given to defining mechanisms behind aging-mediated loss of vascular function and development of therapeutic strategies to restore youthful vascular responsiveness. The ultimate goal aims at providing new avenues for symptom management, reversal of tissue damage, and preventing or delaying of aging-induced vascular damage and dysfunction in the first place. Critical Issues: Our major objective is to describe how aging-associated mitochondrial dysfunction contributes to endothelial and smooth muscle dysfunction via dysregulated reactive oxygen species production, the clinical impact of this phenomenon, and to discuss emerging therapeutic strategies. Pathological changes in regulation of mitochondrial oxidative and nitrosative balance (Section 1) and mitochondrial dynamics of fission/fusion (Section 2) have widespread effects on the mechanisms underlying the ability of the vasculature to relax, leading to hyperconstriction with aging. We will focus on flow-mediated dilation, endothelial hyperpolarizing factors (Sections 3 and 4), and adrenergic receptors (Section 5), as outlined in Figure 1. The clinical implications of these changes on major adverse cardiac events and mortality are described (Section 6). Future Directions: We discuss antioxidative therapeutic strategies currently in development to restore mitochondrial redox homeostasis and subsequently vascular function and evaluate their potential clinical impact (Section 7). Antioxid. Redox Signal. 35, 974-1015.

Emerging methods for and novel insights gained by absolute quantification of mitochondrial DNA copy number and its clinical applications

The past thirty years have seen a surge in interest in pathophysiological roles of mitochondria, and the accurate quantification of mitochondrial DNA copy number (mCN) in cells and tissue samples is a fundamental aspect of assessing changes in mitochondrial health and biogenesis. Quantification of mCN between studies is surprisingly variable due to a combination of physiological variability and diverse protocols being used to measure this endpoint. The advent of novel methods to quantify nucleic acids like digital polymerase chain reaction (dPCR) and high throughput sequencing offer the ability to measure absolute values of mCN. We conducted an in-depth survey of articles published between 1969 -- 2020 to create an overview of mCN values, to assess consensus values of tissue-specific mCN, and to evaluate consistency between methods of assessing mCN. We identify best practices for methods used to assess mCN, and we address the impact of using specific loci on the mitochondrial genome to determine mCN. Current data suggest that clinical measurement of mCN can provide diagnostic and prognostic value in a range of diseases and health conditions, with emphasis on cancer and cardiovascular disease, and the advent of means to measure absolute mCN should improve future clinical applications of mCN measurements.

Individual physiological and mitochondrial responses during 12 weeks of intensified exercise

AIM

Observed effects of exercise are highly variable between individuals, and subject-by-training interaction (i.e., individual response variability) is often not estimated. Here, we measured mitochondrial (citrate synthetase, cytochrome-c oxidase, succinate dehydrogenase, and mitochondrial copy-number), performance markers (Wpeak , lactate threshold [LT], and VO2peak ), and fiber type proportions/expression (type I, type IIa, and type IIx) in multiple time points during 12-week of high-intensity interval training (HIIT) to investigate effects of exercise at the individual level.

METHODS

Sixteen young (age: 33.1 ± 9.0 years), healthy men (VO2peak 35-60 ml/min/kg and BMI: 26.4 ± 4.2) from the Gene SMART study completed 12-week of progressive HIIT. Performance markers and muscle biopsies were collected every 4 weeks. We used mixed-models and bivariate growth models to quantify individual response and to estimate correlations between variables.

RESULTS

All performance markers exhibited significant (Wpeak 0.56 ± 0.33 p = 0.003, LT 0.37 ± 0.35 p = 0.007, VO2peak 3.81 ± 6.13 p = 0.02) increases overtime, with subject-by-training interaction being present (95% CI: Wpeak 0.09-0.24, LT 0.06-0.18, VO2peak 0.27-2.32). All other measurements did not exhibit significant changes. Fiber type IIa proportions at baseline was significantly associated with all physiological variables (p < 0.05), and citrate synthetase and cytochrome-c oxidase levels at baseline and overtime (i.e., intercept and slope) presented significant covariance (p < 0.05). Finally, low correlations between performance and mitochondrial markers were observed.

CONCLUSION

We identified a significant subject-by-training interaction for the performance markers. While for all other measures within-subject variability was too large and interindividual differences in training efficacy could not be verified. Changes in measurements in response to exercise were not correlated, and such disconnection should be further investigated by future studies.

Alterations of cellular aging markers in obsessive- compulsive disorder: mitochondrial DNA copy number and telomere length

BACKGROUND

The present study examined whether mitochondrial DNA copy number (mtDNAcn) and telomere length - key markers of cellular aging - were altered in male and female participants with obsessive-compulsive disorder (OCD) compared to healthy controls. We also tested for associations between these alterations and OCD-related clinical features and inflammatory index.

METHODS

A total of 235 patients with OCD (38.7% female) and 234 healthy controls (41.5% female) were included. We quantified whole-blood mtDNAcn and leukocyte telomere length using quantitative polymerase chain reaction. We also calculated the neutrophil-to-lymphocyte ratio from complete blood cell counts.

RESULTS

Multivariate analysis of covariance showed that OCD status had a significant overall effect on cellular aging markers in men (Wilks λ = 0.889, F2,275 = 17.13, p < 0.001) and women (Wilks λ = 0.742, F2,182 = 31.61, p < 0.001) after controlling for age, body mass index and childhood trauma. In post-hoc comparisons, men with OCD had lower mtDNAcn than controls (p < 0.001), but we found no between-group difference for telomere length (p = 0.55). Women with OCD had a significantly lower mtDNAcn (p < 0.001) and shortened telomere length (p = 0.023) compared to controls. Moreover, the lower mtDNAcn shown in the OCD group was significantly correlated with an increase in systemic inflammation for both sexes, as measured by neutrophil-to-lymphocyte ratio.

LIMITATIONS

The present cross-sectional design did not allow us to infer a causal relationship between OCD disease status and cellular aging markers.

CONCLUSION

The present study is, to our knowledge, the first to demonstrate alterations in mtDNAcn and telomere shortening in OCD. These results suggest that aging-associated molecular mechanisms may be important in the pathophysiology of OCD.

SPERMIDINE MAINTAINS TELOMERE LENGTH AND DELAYS AGING

Spermidine, a natural polyamine, has been noticed for its anti-aging properties. Supplementation of this drug prolongs lifespan and diminishes the incidence of age-related pathology. In the human population, spermidine levels decrease as aging progresses, and a potential link between diminished endogenous spermidine levels and age-related declination has been studied. At the cellular level, autophagy is the prime mode of action of spermidine known to decline with the progress of aging, similarly contributing to the accretion of impaired macromolecules and organelles through aging. Epidemiological statistics support the concept, suggesting that elevated uptake of polyamine delays aging. Here, we overview the effect of autophagy on cellular processes and age-associated diseases, emphasizing the importance of these events to the hallmarks of aging. There are numerous factors like shortening telomere, oxidative stress, mitochondrial damage, and impaired intracellular calcium signaling, which are influenced by the aging process. We hypothesize that spermidine supplements in the diet increase the telomere length. The proposed hypothesis also brings to light the differentially regulated genes involved in telomere maintenance and aging after spermidine treatment. Knowing the role of spermidine in telomere maintenance would help us understand the molecular mechanism of spermidine's effect on aging.

PCR-Based Determination of Mitochondrial DNA Copy Number in Multiple Species

Mitochondrial DNA (mtDNA) copy number is a critical component of overall mitochondrial health. In this chapter, we describe methods for simultaneous isolation of mtDNA and nuclear DNA (nucDNA), and measurement of their respective copy numbers using quantitative PCR. Methods differ depending on the species and cell type of the starting material, and availability of specific PCR reagents. We also briefly describe factors that affect mtDNA copy number and discuss caveats to its use as a biomarker.

SARS-CoV-2 and mitochondrial health: implications of lifestyle and ageing

Infection with SARs-COV-2 displays increasing fatality with age and underlying co-morbidity, in particular, with markers of the metabolic syndrome and diabetes, which seems to be associated with a "cytokine storm" and an altered immune response. This suggests that a key contributory factor could be immunosenescence that is both age-related and lifestyle-induced. As the immune system itself is heavily reliant on mitochondrial function, then maintaining a healthy mitochondrial system may play a key role in resisting the virus, both directly, and indirectly by ensuring a good vaccine response. Furthermore, as viruses in general, and quite possibly this new virus, have also evolved to modulate immunometabolism and thus mitochondrial function to ensure their replication, this could further stress cellular bioenergetics. Unlike most sedentary modern humans, one of the natural hosts for the virus, the bat, has to "exercise" regularly to find food, which continually provides a powerful adaptive stimulus to maintain functional muscle and mitochondria. In effect the bat is exposed to regular hormetic stimuli, which could provide clues on how to resist this virus. In this paper we review the data that might support the idea that mitochondrial health, induced by a healthy lifestyle, could be a key factor in resisting the virus, and for those people who are perhaps not in optimal health, treatments that could support mitochondrial function might be pivotal to their long-term recovery.

Methods for simultaneous and quantitative isolation of mitochondrial DNA, nuclear DNA and RNA from mammalian cells

The aim of this study was to assess two protocols for their capacities to simultaneously isolate RNA, mtDNA and ncDNA from mammalian cells. We compared the Invitrogen TRIzol-based method and Qiagen DNeasy columns, using the HepG2 cell line and human primary glioblastoma stem cells. Both methods allowed the isolation of all three types of nucleic acids and provided similar yields in mtDNA. However, the yield in ncDNA was more than tenfold higher on columns, as observed for both cell types. Conversely, the TRIzol method proved more reproducible and was the method of choice for isolating RNA from glioblastoma cells, as demonstrated for the housekeeping genes RPLP0 and RPS9.