Detecting respiratory chain deficiency in osteoblasts of older patients

Mitochondria contain their own genome which encodes 13 essential mitochondrial proteins and accumulates somatic variants at up to 10 times the rate of the nuclear genome. These mitochondrial genome variants lead to respiratory chain deficiency and cellular dysfunction. Work with the PolgAmut/PolgAmut mouse model, which has a high mitochondrial DNA mutation rate, showed enhanced levels of age related osteoporosis in affected mice along with respiratory chain deficiency in osteoblasts. To explore whether respiratory chain deficiency is also seen in human osteoblasts with age, we developed a protocol and analysis framework for imaging mass cytometry (IMC) in bone tissue sections to analyse osteoblasts in situ. We have demonstrated significant increases in complex I deficiency with age in human osteoblasts. This work is consistent with findings from the PolgAmut/PolgAmut mouse model and suggests that respiratory chain deficiency, as a consequence of the accumulation of age related mitochondrial DNA mutations, may have a significant role to play in the pathogenesis of human age related osteoporosis.

The Spectrum of Maculopathy in Mitochondrial DNA A3243G Mutation: A Case Series of Six Patients

Background The mitochondrial DNA (mtDNA) A3243G point mutation encompasses a heterogenous group of disorders including mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), maternally inherited diabetes and deafness (MIDD), and, rarely, chronic progressive external ophthalmoplegia (CPEO). Regardless of the clinical phenotype, a specific retinopathy has been associated with the presence of this mitochondrial DNA mutation. We present six female patients exhibiting retinopathy of the A3243G point mutation at various stages. History and Signs Six female patients (37 – 70 years old) with the A3243G point mutation (four MELAS, one MIDD, and one CPEO) exhibited a maculopathy. Visual acuity ranged from 1/60 to 10/10. Visual field abnormalities varied from minimal decreased sensitivity to absolute central scotomas. They all exhibited, at various degrees, a characteristic pattern of perimacular and peripapillary retinal pigment epithelium (RPE) alterations, with mottled dys-autofluorescence and RPE atrophy and deposits on OCT. Therapy and Outcome The level of visual impairment depended on the foveal involvement and the extension of RPE atrophy. The severity of the maculopathy was not related to age. In the only long-term follow-up (15 years), evolution was slowly progressive. Conclusions A single mtDNA point mutation at locus 3243 can result in a variety of clinical presentations (MELAS, MIDD, or CPEO). Ocular involvement may manifest as a perimacular/peripapillary RPE atrophy/deposit, which can variably impact central visual function (from asymptomatic to legal blindness). The discovery of such a maculopathy should prompt the ophthalmologist to complete the personal and family history, namely, asking for the presence of diabetes mellitus and/or deafness.

Bi-phasic dynamics of the mitochondrial DNA mutation m.3243A>G in blood: An unbiased, mutation level-dependent model implies positive selection in the germline.

The A-to-G point mutation at position 3243 in the human mitochondrial genome (m.3243A>G) is the most common pathogenic mtDNA variant responsible for disease in humans. It is widely accepted that m.3243A>G levels decrease in blood with age, and correction representing ~2% annual decline is often applied to account for this change in mutation level. Here we report that recent data indicate the dynamics of m.3243A>G are far more complex and depend on the blood mutation level in a bi-phasic way. As a consequence, the traditional 2% correction, which is adequate on 'average', creates opposite predictive biases at high and low mutation levels. Thus, overall accuracy of traditional correction depends on the proportion of individuals with high and low mutant levels in the dataset. Unbiased age correction is needed to circumvent these drawbacks of the standard model. We propose to abolish both biases by using an approach where correction depends on mutation level in biphasic way, to account for the biphasic dynamics of m.3243A>G in blood. The significance of removing bias was further tested using germline selection as a model, in which we detected mutation patterns consistent with the possibility of positive selection for m.3243A>G. We conclude that use of bi-phasic approach will greatly improve the predictive accuracy of modeling data for changes in mtDNA mutations in the germline and in somatic cells during aging.

The Roles of mitochondrial dysfunction and Reactive Oxygen Species in Aging and Senescence

: The aging process deteriorates organs' function at different levels, causing its progressive decline to resist stress, damage, and disease. In addition to alterations in metabolic control and gene expression, the rate of aging has been connected with the generation of high amounts of Reactive Oxygen Species (ROS). The essential perspective in free radical biology is that reactive oxygen species (ROS) and free radicals are toxic, mostly cause direct biological damage to targets, and are thus a major cause of oxidative stress. Different enzymatic and non-enzymatic compounds in the cells have roles in neutralizing this toxicity. Oxidative damage in aging is mostly high in particular molecular targets, such as mitochondrial DNA and aconitase, and oxidative stress in mitochondria can cause tissue aging across intrinsic apoptosis. Mitochondria's function and morphology are impaired through aging, following a decrease in the membrane potential by an increase in peroxide generation and size of the organelles. Telomeres may be the significant trigger of replicative senescence. Oxidative stress accelerates telomere loss, whereas antioxidants slow it down. Oxidative stress is a crucial modulator of telomere shortening, and that telomere-driven replicative senescence is mainly a stress response. The age-linked mitochondrial DNA mutation and protein dysfunction aggregate in some organs like the brain and skeletal muscle, thus contributing considerably to these post-mitotic tissues' aging. The aging process is mostly due to accumulated damage done by harmful species in some macromolecules such proteins, DNA, and lipids. The degradation of non-functional, oxidized proteins is a crucial part of the antioxidant defenses of cells, in which the clearance of these proteins occurs through autophagy in the cells, which is known as mitophagy for mitochondria.

The T414G mitochondrial DNA mutation: a biomarker of aging in human eye

The mitochondrial mutation T414G (mtDNA T414G) has been shown to accumulate in aged and sun-exposed skin. The human eye is also exposed to solar harmful rays. More precisely, the anterior structures of the eye (cornea, iris) filter UV rays and the posterior portion of the eye (retina) is exposed to visible light. These rays can catalyze mutations in mitochondrial DNA such as the mtDNA T414G, but the latter has never been investigated in the human ocular structures. In this study, we have developed a technique to precisely assess the occurrence of mtDNA T414G. Using this technique, we have quantified mtDNA T414G in different human ocular structures. We found an age-dependent accumulation of mtDNA T414G in the corneal stroma, the cellular layer conferring transparency and rigidity to the human cornea, and in the iris. Since cornea and iris are 2 anterior ocular structures exposed to solar UV rays, this suggests that the mtDNA T414G mutation is resulting from cumulative solar exposure and this could make the mtDNA T414G a good marker of solar exposure. We have previously shown that the mtDNA CD4977 and mtDNA 3895 deletions accumulate over time in photo-exposed ocular structures. With the addition of mtDNA T414G mutation, it becomes feasible to combine the levels of these different mtDNA mutations to obtain an accurate assessment of the solar exposure that an individual has accumulated during his/her lifetime.

Senescent Cell Depletion Through Targeting BCL-Family Proteins and Mitochondria

Senescent cells with replicative arrest can be generated during genotoxic, oxidative, and oncogenic stress. Long-term retention of senescent cells in the body, which is attributed to highly expressed BCL-family proteins, chronically damages tissues mainly through a senescence-associated secretory phenotype (SASP). It has been documented that accumulation of senescent cells contributes to chronic diseases and aging-related diseases. Despite the fact that no unique marker is available to identify senescent cells, increased p16INK4a expression has long been used as an in vitro and in vivo marker of senescent cells. We reviewed five existing p16INK4a reporter mouse models to detect, isolate, and deplete senescent cells. Senescent cells express high levels of anti-apoptotic and pro-apoptotic genes compared to normal cells. Thus, disrupting the balance between anti-apoptotic and pro-apoptotic gene expression, such as ABT-263 and ABT-737, can activate the apoptotic signaling pathway and remove senescent cells. Mitochondrial abnormalities in senescent cells were also discussed, for example mitochondrial DNA mutation accumulation, dysfunctional mitophagy, and mitochondrial unfolded protein response (mtUPR). The mitochondrial-targeted tamoxifen, MitoTam, can efficiently remove senescent cells due to its inhibition of respiratory complex I and low expression of adenine nucleotide translocase-2 (ANT2) in senescent cells. Therefore, senescent cells can be removed by various strategies, which delays chronic and aging-related diseases and enhances lifespan and healthy conditions in the body.