The fountain of youth of mitochondrial research

Abstract

A new wave of studies is untangling the connection between primary genetic mitochondrial diseases and the role of mitochondria in aging: what are the implications for longevity?

Mitochondria in our cells take components from the food we eat and turn them into energy. This essential role in energy metabolism is fundamental to the way cells work and the existence of complex life. Mitochondria are not only essential cellular energy factories; they also have critical functions that extend to cell signaling and are essential for everything from muscle and neuronal function to responses to viral infections. However, as they play such a pivotal role in health, defects in mitochondrial functions can also be responsible for a wide range of diseases. One group includes primary mitochondrial diseases, a range of genetic disorders that can result in heart failure, blindness, muscle weakness, and death. Another is secondary mitochondrial diseases: neurodegenerative disorders, cancer, diabetes, and obesity that have a component of mitochondrial dysfunction. Critically, defects in mitochondria, as well as primary and secondary mitochondrial diseases, have also been linked with aging‐associated processes.

One of the major theories of biological aging is based on the gradual loss of mitochondrial integrity. “With aging, mitochondrial function deteriorates due to multiple intertwined mechanisms including the accumulation of mitochondrial DNA (mtDNA) mutations, deficient proteostasis leading to the destabilization of respiratory chain complexes, reduced turnover of the organelle, and changes in mitochondrial dynamics,” recently remarked Guido Kroemer and colleagues in a review on mitochondrial dysfunction and aging (López‐Otín et al, ²⁰²³). As a consequence of this progressive deterioration, the authors noted, mitochondria's contribution to energy metabolism declines while the production of reactive oxygen species (ROS) increases, which in turn damages mitochondrial membranes and leads to inflammation and cell death (Fig 1).

Figure 1. Mitochondria run parallel in mature cardiomyocytes.

Adult mouse primary cardiomyocytes stained with TMRE (mitochondria, orange) and NucBlue (nuclei, blue). Winner of the Best Mitochondria Image 2022 award of the World Mitochondria Society (https://wms‐site.com/alert‐on‐mitochondria/1121‐winner‐of‐the‐best‐mitochondria‐image‐2022‐erminia‐donnarumma‐institut‐pasteur). Credit: Erminia Donnarumma, T. Wai’s Lab, Institut Pasteur, France. Reproduced with permission.

Understanding the links between mitochondrial defects, cellular damage, and eventually aging, is not straightforward. However, although the underlying mechanisms remain largely unknown, a flurry of recent studies has detailed how the decline of major mitochondrial stress pathways such as those regulating the mitochondrial unfolded protein response, mitochondrial membrane dynamics, and mitophagy (autophagy of the mitochondria) impacts cellular homeostasis and leads to systemic deterioration (Fig 2; Lima et al, ²⁰²²).

Figure 2. Pleiotropic effects of mitochondria in aging.

Mitochondrial alterations in aging initiate with a decline in mitochondria stress‐response (MSR) pathways leading to the accumulation of mtDNA mutations, release of damaged toxic mitochondrial material (for example, intracellular molecules from senescent or dying cells, termed damage‐associated molecular patterns or DAMPs), generation of mitochondrial ROS, proteotoxicity, and deregulated metabolites (TCA intermediates, NAD⁺). These alterations have a broad detrimental effect on cellular homeostasis and, through a complex signaling mechanism (involving mitokines, metabolites, and more), contribute to systemic organismal decline and the onset of several age‐related diseases. Pharmacological modulation of the MSR, such as through the use of NAD⁺ enhancers or mitophagy inducers, can be effective strategies to prevent aging‐related cellular and organismal decline. In the figure, IR injury stands for ischemia‐reperfusion injury; NFLD for nonalcoholic fatty liver disease; and NASH for nonalcoholic steatohepatitis. Reproduced from Lima et al (2022) with permission.

Understanding the links between mitochondrial defects, cellular damage, and eventually aging, is not straightforward.

“There is a whole range of data pointing to an involvement of mitochondria in aging, but we seem to be far from a consensus,” said Mike Murphy, an expert in mitochondrial redox biology at the University of Cambridge, UK. “It used to be simple, with the idea that mitochondrial ROS caused mtDNA damage that thereby increased ROS and set in train a vicious cycle. But [as more recent studies have shown] that's clearly not the case. So, how mitochondria contribute to aging is now murky.”

“It used to be simple, with the idea that mitochondrial ROS caused mtDNA damage that thereby increased ROS and set in train a vicious cycle. But […] that's clearly not the case” Mike Murphy

Murphy explained that mutations in nutrient‐sensing pathways that affect lifespan may also be connected to mitochondrial failures. “If I had to bet on a link between mitochondria and aging, I would suggest cell fate – such as stem cell survival and/or transition to senescence – possibly with a link to inflammatory signaling. However, these are educated guesses at best.”

Despite, or perhaps because of, the lack of a consolidated picture of the role of mitochondria in aging, news of advances in the field is released almost daily, both in scientific papers and general media, and it can be difficult to keep up with the pace of progress. For instance, a team from the United States and Germany led by Shahaf Peleg and Andrew P. Wojtovich has recently applied a new approach to harnessing the role of mitochondria in the aging process that they say could lead to new treatments for age‐related diseases (Berry et al, ²⁰²³).

Mitochondria are electrically charged and often described as the “power‐packs” within cells. The researchers started from the notion that the voltage across the inner membrane of mitochondria decreases with age. This membrane potential is directly linked to basic mitochondrial functions that have cell‐wide consequences, like the production of ATP. Using optogenetics, the scientists expressed a light‐activated proton pump related to rhodopsin in the inner membrane of mitochondria of the nematode Caenorhabditis elegans during adulthood. When exposed to light, the extra proton pump increased mitochondrial membrane potential, thus charging the mitochondria, resulting in a significant improvement in the apparent health of the genetically engineered nematodes and an extension of their lifespan with respect to controls.

Specific proteins are among the potential targets in the search for therapies for mitochondria‐based, age‐related diseases. One example is voltage‐dependent anion channels (VDAC), which are located in the outer mitochondrial membrane and control the flux of ions and metabolites, thus mediating the organelle's metabolic cross‐talk with the rest of the cell. “We demonstrated that apoptosis induction leads to VDAC1 [one of three VDAC isoforms found in mammals] oligomerization regardless of the cell type or apoptosis inducer used,” said Varda Shoshan‐Barmatz, a leading expert in VDAC and its network of protein interactions, based at Ben‐Gurion University of the Negev in Beer‐Sheva, Israel (Fig 3).

Figure 3. Proposed mechanism for activators of VDAC1 promoter leading to VDAC1 overexpression, oligomerization, and activation of apoptosis and/or inflammation.

(A) Apoptosis stimuli, stress, and pathological conditions activating VDAC1 promoter via transcription factors and Ca²⁺, thereby, inducing VDAC1 transcription, which leads to enhanced VDAC1 expression. (B) Overexpressed VDAC1 shifts the equilibrium to the VDAC1 oligomeric state with a large channel, mediating the release of apoptogenic proteins, leading to apoptosis and/or to the release of mtDNA or/and oxidized mtDNA fragments (fr‐mtDNA) leading to inflammation. VDAC1‐interacting molecules such as DIDS, VBIT‐4 and VBIT‐12, inhibit VDAC1 oligomerization and, thereby, the formation of the large channel and release of pro‐apoptotic proteins, subsequently, inhibiting apoptosis. (C) Summary of sequence of events induced by apoptosis stimuli, stress, and disease conditions leading to apoptosis and/or inflammation. Modified from Shoshan‐Barmatz et al (2020) with permission.

The theory Shoshan‐Barmatz et al (2020) are proposing thus hinges around the role of VDAC1 as the link between mitochondria dysfunction and a wide range of diseases, a crossroad between metabolism, cell survival, and cell death. “Our results support the tight coupling between VDAC1 overexpression, VDAC1 oligomerization, apoptosis induction, and pathological states like diabetes, neurodegenerative, cardiac, and autoimmune diseases,” Shoshan‐Barmatz explained. “As such, up‐regulation of the expression of VDAC1 may represent a new common mode of action of apoptosis induction, thus inhibiting VDAC1 overexpression and/or its oligomerization, as an early stage of apoptosis, might represent an effective target to treat these diseases.”

Presenting more evidence to support this hypothesis, Shoshan‐Barmatz and colleagues have also shown that amyloid‐beta induced VDAC1 overexpression and apoptotic cell death in a mouse model of Alzheimer's disease, and that VBIT‐4—a newly developed inhibitor of VDAC1—prevented neuronal loss, inflammation signaling, and improved cognitive function, raising hopes that it could become a promising candidate for treatment (Verma et al, ²⁰²²).

Growth of mitochondria biotech

Being one of the “hallmarks of aging,” targeting mitochondrial dysfunction to increase longevity and tackle age‐related diseases has become an attractive strategy for a rapidly growing number of biotech companies. “Mitochondria companies make up one of the biggest subcategories in longevity biotech,” wrote Nathan Cheng, a Toronto‐based entrepreneur and founder of Longevity Marketcap (https://longevitymarketcap.com/) in his online overview of the sector. “This makes sense as mitochondria are an extremely critical component of our cells. There are ~20+ longevity mitochondria companies [globally] currently, ranging from early stage‐startups to Nasdaq‐listed public companies. From small molecule drugs to gene therapies and mitochondrial transfusions,” (https://sub.longevitymarketcap.com/p/022‐mitochondria‐longevity‐companies).

Biotechnology companies engaged in mitochondria research generally focus on age‐related diseases such as neurodegenerative disorders, rare genetic diseases caused by mitochondrial DNA mutation, metabolic disorders, or on the effects of oxidative damage.

Transplantation of healthy mitochondria into tissue or organs to fix mitochondrial dysfunction and restore cellular health, has been long been proposed as a “revolutionary” therapeutic approach, and has received extensive media coverage. Despite some concern about its real efficacy and safety (Lightowlers et al, ²⁰²⁰; see also Box 1), several companies are working on this type of treatment for specific illnesses and possibly to extend lifespan. One example is Mitrix Bio, a preclinical biotech startup developing mitochondrial transplantation therapeutics, based in Pleasanton, California. In 2022, Mitrix announced early results of an 18‐month project involving mitochondrial transplants in brain, eye, liver, immune system, and skin. According to the company, these tests—among the first to apply mitochondrial transplantation to address adult age‐related diseases‐showed apparent age reversal in multiple endpoints in both human cells and animal disease models, suggesting therapeutic potential for diseases such as Alzheimer's, macular degeneration, cardiovascular disease, frailty, and immune senescence.

Box 1. The ambiguous allure of mitochondrial medicine.

Mitochondrial science is an exciting and rapidly evolving field. This wave of interest, however, has caused ripples that extend far beyond the limits of scientific research and its translation into well‐tested therapies.

Mitochondria—often used as a branding term—are seemingly everywhere, from dietary supplements to skin products, a trendy focus for anti‐aging strategies in personal care. A host of private longevity clinics in various parts of the world, offer “mitochondrial repair treatments,” or tailored recommendations “to boost your mitochondria function.”

Mitochondrial transfer has become available for infertility treatment at many centers worldwide, despite the fact that its efficacy remains unproven and safety uncertain (Chinnery, 2020). The technique varies depending whether the scope is preventing the transmission of maternally inherited mitochondrial DNA diseases or treating infertility. In the latter, the cytoplasm from a healthy young donor is transferred to oocytes of a prospective mother with infertility. Alternatively, spindle transfer can be used to transfer the nuclear DNA from a mother's oocyte to an enucleated donor oocyte before in vitro fertilization (see Figure B1).

Earlier this year, The Guardian reported the news of the birth of the first “three‐parent baby” in the United Kingdom, where mitochondrial replacement therapy is strictly regulated (https://www.theguardian.com/science/2023/may/09/first‐uk‐baby‐with‐dna‐from‐three‐people‐born‐after‐new‐ivf‐procedure). In the absence of mitochondrial disorders, the main concern in using mitochondrial replacement therapy to address infertility hinges around the lack of data on the long‐term effects of such an approach.

In Greece, a recent pilot trial of 25 couples with fertility problems found that one of the six children born through maternal spindle transfer (see Figure B1) had some 30–60% of the total mitochondrial DNA inherited from the mother at birth, despite the fact that mitochondrial DNA carryover was < 1% at the blastocyst stage (https://www.fertstert.org/article/S0015‐0282(23)00136‐X/fulltext). The reasons why this increase in mitochondrial DNA from the mother—a phenomenon called reversion—occurs are not clear, but experts have raised concerns that this could increase risks of future health issues for the newborn due to the mismatch between nuclear and mitochondrial genomes, as observed in mice (Chinnery, 2020).

While the future of mitochondrial medicine and its multiple applications has substantial promise, some specialists urge more caution when dealing with the implications of basic research for the public sphere.

Figure B1. Principles of mitochondrial transfer.

A prospective mother carrying a pathogenic mitochondrial DNA mutation can be offered mitochondrial transfer to reduce the risk of having a child affected by mitochondrial DNA disease. Two approaches have been used: pronuclear transfer (Panel A) and spindle transfer (Panel B), which allow the nuclear genome to be transferred to a donor zygote or oocyte containing mitochondria without the mitochondrial DNA mutation. A similar approach has been offered to women with infertility in the hope of improving their chances of having a child through mitochondrial transfer to unfertilized oocytes. Early work in this field involved the transfer of cytoplasm from a healthy young donor to oocytes that had been obtained from a prospective mother with age‐related infertility. More recently, spindle transfer has been used to transfer the nuclear DNA from an older mother's oocyte to an enucleated donor oocyte before in vitro fertilization (Panel C). Modified from Chinnery (2020) with permission.

“These are still early results, and more work remains to be done in human tests, but we think the data points to a potential breakthrough in longevity…in a shorter timeframe compared to other longevity treatments,” said Mitrix CEO Tom Benson in a related press release (https://www.mitrix.bio/uploads/1/0/6/3/10638122/mitrix_press_release_july_19_2022.pdf). “There are plenty of commonsense ways to protect mitochondrial function, like exercise and good nutrition, but none of those are likely to stop mitochondrial decline at 80 years old,” Benson added, “but with mitochondrial transplantation, we're not just tinkering around the edges, this replaces mitochondria entirely, like replacing the engine in your car to get another 100,000 miles.”

Although some enthusiasm is justifiable given the potential importance of reported results, the technology Mitrix is testing is still in development, and many outstanding questions regarding the effects of mitochondrial transplantation remain.

Mitochondrial diseases in the spotlight

Mitochondrial dysfunction can contribute to the aging process through multiple mechanisms. As mentioned above, one major hypothesis is the accumulation of mitochondrial DNA mutations over time. Function can be impaired as mutations accumulate in the mitochondria's DNA, leading to decreased energy production and increased production of ROS. These ROS can damage cellular components, with tissues and organs experiencing diminished function, thus contributing to the aging process.

However, some primary mitochondrial diseases also involve genetic mutations affecting mitochondrial function. Unlike many genetic and nongenetic disorders that involve mitochondrial mechanisms as a secondary feature, primary mitochondrial diseases are a group of genetic disorders arising by defects of the mitochondrial respiratory chain or in oxidative phosphorylation. These mutations can lead to impaired energy production and increased ROS production, similar to the effects of aging. As a result, individuals with primary mitochondrial diseases may experience accelerated aging‐like symptoms, including muscle weakness, cognitive decline, and organ dysfunction. Leigh syndrome, Alpers‐Huttenlocher syndrome, Pearson syndrome, and Leber hereditary optic neuropathy are among the most common primary mitochondrial diseases.

Caused by mutations in genes in the nuclear DNA and mitochondrial DNA, primary mitochondrial diseases are clinically heterogeneous, making diagnosis and management extremely difficult. More than 1 in 5000 people are born with a primary mitochondrial condition, making this one of the most common groups of genetic diseases and inherited neurological disorders. These defects can affect various organs and tissues in the body, leading to a wide range of symptoms, commonly including optic atrophy, chronic progressive external ophthalmoplegia, and neurodevelopmental regression. While primary mitochondrial diseases can manifest at any age, including childhood, some of these disorders may have a progressive nature, and their symptoms may worsen over time.

Specific proteins are among the potential targets in the search for therapies for mitochondria‐based, age‐related diseases.

Since there are significant similarities in the etiology, pathogenesis, and pathology of aging‐related mitochondrial dysfunction and genetic mitochondrial diseases, some specialists believe that a better understanding of the roles of mitochondria in aging processes could also help to understand mitochondrial diseases, and vice versa. “While it may seem thought provoking at first, the fact that mitochondrial diseases may be a proxy for aging is extremely interesting and it is feasible that aging and mitochondrial disease share the same cellular pathways, although likely at different levels of affectation,” remarked Albert Quintana, Universitat Autònoma de Barcelona, Spain, whose lab investigates the molecular mechanisms defining why some neuronal populations are particularly affected by mitochondrial disease.

“Mitochondria companies make up one of the biggest subcategories in longevity biotech” Nathan Cheng

Clinically and genetically, Leigh syndrome is a highly heterogeneous neurological disease resulting from mutations in the protein complexes that compose the oxidative phosphorylation system. The disease typically manifests in infancy or early childhood, and its prevalent clinical features affect the brain, muscles, and eyes. Leigh syndrome is currently incurable (Fig 4).

Figure 4. Clinical features of Leigh syndrome (LS).

The most prevalent clinical features affect the brain, muscles, and eyes. Other clinical findings include dysfunctions in the cardiovascular, gastrointestinal, renal, auditory, and hematological systems. LS can present as early onset or late‐onset with abnormalities in at least three of the organ systems highlighted. LS results from pathogenic mutations in either the nuclear DNA or mitochondrial DNA that cause abnormalities in the OXPHOS capacities of the mitochondria. Hence, biochemical findings reflect these defects. Reproduced from Bakare et al (2021) with permission.

Several studies of Leigh syndrome in mice models missing the Complex I subunit NADH:Ubiquinone Oxidoreductase Fe‐S Protein 4 (NDUFS4) have suggested that selected interventions shown to delay the onset of age‐related diseases and extend lifespan may also prolong survival and prevent disease progression. NDUFS4 is an important component of oxidative phosphorylation, a metabolic pathway that cells use to oxidize nutrients, thereby releasing energy in the form of ATP. Studies have included the use of the drug rapamycin, the NAD⁺ boosting compound nicotinamide mononucleotide, genetic inhibition of ribosomal protein S6 kinase beta‐1 (S6k1), and reducing tissue oxygen delivery by maintaining animals in a hypoxic environment (Grillo et al, ²⁰²¹).

“These interventions have identified some of the most promising treatments for mitochondrial disease,” said Quintana. “However, the underlying mechanisms are not completely understood for all treatments, and there is still a lack of clinical support for their translational potential, which need to be addressed to fully assess how closely related aging and mitochondrial disease are.”

Getting deeper into mitochondria and aging

Nevertheless, Alessandro Bitto of the University of Washington Medical Center in Seattle argued that having multiple independent studies in common mice models presents significant promise. “At a minimum, this evidence points to common underlying mechanisms between biological aging and Complex I dysfunction,” he said. “We can use geroscience to gather new insight into Complex I driven mitochondrial disorders and my hope and focus in the lab is to use the NDUFS4 model to gain mechanistic understanding of what longevity interventions are doing at a molecular level in an animal.”

…primary mitochondrial diseases are a group of genetic disorders arising from defects of the mitochondrial respiratory chain or in oxidative phosphorylation.

A recent study from Bitto's lab has shown that acarbose, a drug that extends lifespan and delays aging in mice, also suppresses symptoms of disease and improves the survival of NDUFS4 knockout mice (Bitto et al, 2023). Acarbose seems to work by remodeling the intestinal microbiome and altering the production of short‐chain fatty acids, suggesting that the gut microbiome plays a significant role in mitochondrial disease. The finding “provides further support for the model that biological aging and severe mitochondrial disorders share underlying common mechanisms,” the authors concluded (Bitto et al, ²⁰²³).

Bitto believes that clarifying the overall picture will require more than just connecting ROS, mitochondrial DNA mutations, pathogenesis of primary mitochondrial diseases, and aging. “We know mitochondrial DNA mutations accumulate with age,” he commented. “What is surprising is that the rate of accumulation varies from organ to organ and the nature of the mutations is not necessarily a footprint of ROS damage. Rather, I think that the role mitochondria cover in nutrient metabolism is an overlooked mechanism linking aging and disease.”

A word of warning also came from Quintana. “It is becoming clear that while a certain level of mitochondrial impairment may not be harmful or even have beneficial effects for cellular fitness, the cellular affectation in primary mitochondrial disease patients is commonly way beyond this threshold,” he said. “This may lead to critical differences with normative aging processes, which prompts the need for further studies to validate how applicable the knowledge gathered in primary mitochondrial diseases for our understanding of aging could be.”

…some specialists believe that a better understanding of the roles of mitochondria in aging processes could also help to understand mitochondrial diseases, and vice versa.

Although our understanding of the role of mitochondria in human health and aging is far from complete, mitochondrial science is advancing at a rapid pace. One day, specialists are confident this research will eventually translate into new therapies for combating both age‐related disfunctions and mitochondrial diseases. One day.

EMBO reports (2023) 24: e58118