Cellular Respiration: Mapping mitochondrial aging
Measuring mitochondrial respiration in frozen tissue samples provides the first comprehensive atlas of how aging affects mitochondrial function in mice.
- Department of Laboratory Medicine and Pathology, University of Washington Medical Center, United States
There are multiple theories as to why animals grow old and die. One of the first theories of aging blames free radicals for our eventual demise (Harman, 2003). Free radicals are unstable molecules that can damage cellular components, and they are thought to originate from mitochondria – the organelles responsible for harnessing energy from nutrients.
Mitochondria use a metabolic process called respiration to store energy from nutrients in the form of a molecule known as ATP, which powers many essential cellular processes. This mechanism mostly takes place in the mitochondrial inner membrane, which houses protein complexes that make up an electron transport chain – the final stage of respiration responsible for driving the bulk of ATP production. It is generally accepted that mitochondria become less active in aging animals and that their dysfunction is a key contributor to the aging process (López-Otín et al., 2023).
Markers of mitochondrial dysfunction have been seen in a range of aging model organisms, from yeast to nematode worms (Hughes and Gottschling, 2012; Berry et al., 2023), as well as in aged human cells in culture (Lerner et al., 2013). In mice, inducing mitochondrial dysfunction causes premature aging (Trifunovic et al., 2004; Kujoth et al., 2005), and interventions that protect or restore mitochondrial function have successfully delayed aging in several models (Schriner et al., 2005; Berry et al., 2023). However, the extent and magnitude of mitochondrial decline with age and its impact on aging more generally is not fully understood.
A device called a ‘respirometer’ can be used to measure mitochondrial activity by detecting how much oxygen these organelles are consuming. However, until recently, this approach could only be applied to freshly isolated mitochondria obtained from mammalian tissues through a long and laborious process, making them difficult to study in large numbers. This limitation has prevented comprehensive analyses of mitochondrial respiration in mammalian tissues. Now, in eLife, William Wong and colleagues from Johns Hopkins University – including Dylan Sarver as first author – report the first large-scale analysis of mammalian mitochondrial respiration and how this process changes with aging (Sarver et al., 2024).
Using a recently developed protocol for respiratory analysis of frozen tissue samples (Acin-Perez et al., 2020), Sarver et al. measured a proxy of mitochondrial respiration in over 1000 samples from a large cohort of young and old mice of both sexes. This included tissues with reportedly high mitochondrial activity, such as certain brain regions, several skeletal muscles, the heart, and the kidneys. The samples also included metabolic tissues like the liver or pancreas, as well as sections of the gastrointestinal tract, the skin and the eyes.
Due to the process of freezing and thawing, the mitochondria in the samples were not intact and therefore could not be isolated. As a result, Sarver et al. measured mitochondrial respiration at three different sites on the electron transport chain in cellular extracts enriched with mitochondrial membranes. The proteins making up this chain are likely to remain relatively stable in mitochondria whose membrane integrity has been lost, which allows measurements that indicate the maximum capacity of the mitochondria to produce ATP to be taken.
Analyzing the differences between old and young animals revealed a net decline in mitochondrial activity in most tissues with age, most notably in samples from the brain and metabolic tissues. These results are consistent with our current understanding of the energetic demands of various tissues and how they decline over time. They also support previous findings which demonstrated age-related changes in mitochondrial gene expression and electric activity (Schaum et al., 2020).
Intriguingly, in older animals, respiration increased in some tissues with high-energy demand, such as the heart and skeletal muscles, which is potentially at odds with the observation that these organs perform less well with age. Analyzing differences between samples from males and females also revealed that age has a much larger effect on mitochondrial activity across all tissues than sex.
Although the findings do not reveal the mechanisms behind the observed changes, Sarver et al. offer a roadmap for future studies aimed at better understanding the role of mitochondrial respiration in aging. Notably, as the team acknowledged, the techniques used to generate this ‘respiration atlas’ cannot recapitulate the full extent of mitochondrial physiology. Their method does not account for important physiological variables that might impact respiration, such as nutrient and oxygen availability, transport of reagents and products across mitochondrial membranes, and how proteins in the electron transport chain are organized. At the moment, these can only be assessed in intact mitochondria or tissue preparations.
Furthermore, because the technique measured the maximum capacity of the electron transport chain to produce energy in compromised mitochondria, the values may not correspond to the actual level of respiration in intact mitochondria or tissue slices, nor necessarily reflect how much of this respiration actually produces ATP. Regardless, the convenience of the method and the ability to analyze samples on a much larger scale will be a driving force behind studying the broader impact of various interventions on respiration.
Geroscience, which aims to understand how the aging drives disease and develop strategies to slow it (Kennedy et al., 2014), has identified several genetic, pharmacological, and environmental interventions that can increase lifespan and reduce, if not reverse, the pace of aging in model organisms. Some, such as compounds called rapamycin or urolithin A, are known to affect mitochondrial function in certain contexts, while others have no clear effect on mitochondria. Regardless, a full-scale picture of how such treatments influence respiration systemically is still missing. Given that samples from treated animals are widely available to the scientific community through tissue repositories, applying the methods outlined by Sarver et al. offers an ideal starting point for studying the true impact of these longevity strategies on mitochondrial function. Such studies are likely to generate new and surprising hypotheses in geroscience research and contribute to our understanding of how critical mitochondrial function really is for our overall health and well-being.
References
-
The free radical theory of agingAntioxidants & Redox Signaling 5:557–561.https://doi.org/10.1089/152308603770310202
Article and author information
Author details
Publication history
Copyright
© 2024, Bitto
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 547
- views
-
- 74
- downloads
-
- 0
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Cellular Respiration: Mapping mitochondrial aging
eLife 13:e105191.
https://doi.org/10.7554/eLife.105191
Further reading
-
- Cancer Biology
- Computational and Systems Biology
Effects from aging in single cells are heterogenous, whereas at the organ- and tissue-levels aging phenotypes tend to appear as stereotypical changes. The mammary epithelium is a bilayer of two major phenotypically and functionally distinct cell lineages: luminal epithelial and myoepithelial cells. Mammary luminal epithelia exhibit substantial stereotypical changes with age that merit attention because these cells are the putative cells-of-origin for breast cancers. We hypothesize that effects from aging that impinge upon maintenance of lineage fidelity increase susceptibility to cancer initiation. We generated and analyzed transcriptomes from primary luminal epithelial and myoepithelial cells from younger <30 (y)ears old and older >55 y women. In addition to age-dependent directional changes in gene expression, we observed increased transcriptional variance with age that contributed to genome-wide loss of lineage fidelity. Age-dependent variant responses were common to both lineages, whereas directional changes were almost exclusively detected in luminal epithelia and involved altered regulation of chromatin and genome organizers such as SATB1. Epithelial expression variance of gap junction protein GJB6 increased with age, and modulation of GJB6 expression in heterochronous co-cultures revealed that it provided a communication conduit from myoepithelial cells that drove directional change in luminal cells. Age-dependent luminal transcriptomes comprised a prominent signal that could be detected in bulk tissue during aging and transition into cancers. A machine learning classifier based on luminal-specific aging distinguished normal from cancer tissue and was highly predictive of breast cancer subtype. We speculate that luminal epithelia are the ultimate site of integration of the variant responses to aging in their surrounding tissue, and that their emergent phenotype both endows cells with the ability to become cancer-cells-of-origin and represents a biosensor that presages cancer susceptibility.
-
- Computational and Systems Biology
Degree distributions in protein-protein interaction (PPI) networks are believed to follow a power law (PL). However, technical and study biases affect the experimental procedures for detecting PPIs. For instance, cancer-associated proteins have received disproportional attention. Moreover, bait proteins in large-scale experiments tend to have many false-positive interaction partners. Studying the degree distributions of thousands of PPI networks of controlled provenance, we address the question if PL distributions in observed PPI networks could be explained by these biases alone. Our findings are supported by mathematical models and extensive simulations, and indicate that study bias and technical bias suffice to produce the observed PL distribution. It is, hence, problematic to derive hypotheses about the topology of the true biological interactome from the PL distributions in observed PPI networks. Our study casts doubt on the use of the PL property of biological networks as a modeling assumption or quality criterion in network biology.
