What human muscle biopsies reveal about aging-related cell signals and strength

Since the previous research note · Earlier expectation not tested

Track continuity

Since the previous research note: Old mouse muscle shows distinct signs of senescence in support cells and fibers

Previous expectation: Safely targeting harmful cellular states in aging muscle might help older adults retain strength and independence.

What this study adds: The July 2023 study demonstrated that several senescence-associated markers could be characterised in human muscle and compared with morphology and physical function across middle and older ages. Its inconsistent exploratory associations did not test whether targeting those states improves function.

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What changed in this study

Researchers are interested in cellular senescence, a stress-related cellular state that can accompany aging, because it might help explain why muscle becomes less strong or less capable over time. But finding senescence-associated signals in human muscle is not the same as showing that those signals cause weakness.

A study published on July 11, 2023, tested an important part of that problem. It asked whether researchers could characterise several senescence-associated markers directly in human skeletal muscle and compare them with muscle structure and physical function. The answer was yes. The study also showed why the next step is interpretation, not a simple senescence score for aging muscle.

A feasibility study in living human muscle

The research used muscle biopsies and detailed physical assessments from 40 participants in the MASS_Lifecourse Study. The group included 24 men and 16 women, aged approximately 47 to 84. Tissue was taken from the vastus lateralis, a large muscle in the thigh, under local anaesthesia.

The participants were not a cross-section of every older adult. They were community-dwelling white British men and women from a deeply assessed research cohort, and the group was generally healthy and physically active. Only four participants met the study’s definition of probable sarcopenia, based on low grip strength. That makes the sample useful for asking whether these measurements can be made in relatively healthy human muscle, but less representative of people with advanced muscle weakness or disability.

Researchers compared people at different ages, rather than following each person repeatedly through the aging process. The results therefore show how measurements differed between participants, not how an individual’s muscle changed year by year.

The laboratory work combined spatially resolved methods, including immunohistochemistry, immunofluorescence, RNA in situ hybridisation and fluorescence in situ hybridisation. In practical terms, the researchers could examine where particular signals appeared in the muscle tissue instead of relying only on a blended measurement from the whole sample.

Four markers captured different kinds of cellular evidence

No single marker can establish that a cell is senescent. The study therefore used several measurements that capture different features associated with the state.

p16 was measured through the presence of p16 messenger RNA signals. This is a gene-expression measurement, showing where the relevant transcript could be detected. Telomere-associated DNA-damage foci, or TAF, indicated damage signals at telomeres, the protective structures at chromosome ends. HMGB1 and Lamin B1 were nuclear-protein measurements. In this study, changes involving these proteins were interpreted as signals that can accompany senescence-associated changes in the nucleus.

The direction of a result mattered. More p16 or more telomere-associated damage was not interpreted in the same way as a loss of HMGB1 or Lamin B1 staining. A positive signal also did not amount to an unquestionable census of senescent cells. It was one piece of evidence about a cellular state, interpreted alongside other markers and the location of the signal within the tissue.

The location mattered because skeletal muscle contains more than contractile muscle fibres. The imaging and staining approaches helped distinguish nuclei and tissue regions, allowing the researchers to examine signals in relation to muscle fibres and surrounding structures. This offered a closer view of the muscle niche than a blood measurement or a bulk tissue average alone.

The age patterns did not form one senescence score

The study did not find a compelling overall relationship between the measured markers and age. Some patterns appeared stronger in women than in men, but they were exploratory, inconsistent and not statistically conclusive.

One result illustrates why a single direction would have been misleading. In women, the percentage of nuclei with p16 signals showed a negative correlation with age. That is not the simple pattern someone might expect if p16-positive measurements always rose steadily as muscle aged. The researchers also found that men had a higher percentage of fibres positive for a particular p16 measurement than women, while other p16 measurements did not show the same difference.

These findings do not establish that p16-related activity declines in every woman’s muscle with age, nor do they show a biological difference between women and men in how senescence contributes to muscle aging. The sample was small, and several related measurements were examined. A result that points in an unexpected direction can be useful for refining the question, but it needs confirmation in a larger group.

The same caution applies to the other markers. The measurements were feasible, yet feasibility did not produce a unified biological pattern that could be used to rank participants by muscle senescence or predict who would lose function.

Muscle structure and function pointed in different directions

The researchers then compared the tissue measurements with physical function, body composition and muscle morphology. In women, HMGB1 positivity showed a positive correlation with grip strength. TAF showed different relationships with body mass, muscle mass and the study’s gait measure. p16 was also related to performance on the five-chair-stand test in an exploratory analysis.

These examples are useful because they did not all point the same way, and they did not create a consistent pathway from one marker to one functional outcome. The relationships were correlations between measurements taken from different people. They do not show that changing a marker would change strength, walking or the ability to stand from a chair.

The anatomical comparisons supplied another layer of information. Larger muscle fibres were generally associated with greater muscle mass, while fibrotic area showed an opposing relationship with muscle mass in the age-related comparisons. Fibre size had a stronger relationship with grip strength in women than in men, although the relationship remained modest. These morphology findings were distinct from the marker findings: the size and condition of the tissue did not simply mirror one molecular measurement.

That separation is important for understanding what a biopsy can add. A tissue sample can connect cellular signals with the physical architecture of muscle, while strength, walking and chair-stand tests show what the person can do. The two kinds of evidence may eventually help explain one another, but this study did not establish which one comes first or whether either directly drives the other.

Why the findings remain exploratory

The researchers used Spearman correlation coefficients to describe how pairs of measurements changed together. Because the study included fewer than 50 people and examined many possible relationships, the authors focused on the direction and size of the correlations rather than presenting them as definitive statistically significant findings. Larger-looking effects in women were not consistent across markers and outcomes.

Several measurement boundaries also matter. Attempts to characterise the senescence-associated markers p21 and SA-β-Gal were unsuccessful. The study did not measure the senescence-associated secretory phenotype, or SASP, and it did not analyse muscle fibre types. Grip strength was used as a surrogate rather than a direct measurement of quadriceps strength. The tissue measurements also covered only a limited set of markers, so they could not capture every feature that might be relevant to senescence.

The healthy composition of the cohort further narrows the interpretation. People with more severe muscle impairment may show different relationships, while the small number of participants with probable sarcopenia makes it impossible to draw firm conclusions about that condition. The study’s cross-sectional comparisons also cannot reveal whether a marker rises, falls or changes in a particular person before their function changes.

For these reasons, the paper’s main advance was methodological and observational. It demonstrated that researchers can study several senescence-associated signals inside human skeletal muscle and relate them to morphology and function. It did not validate a diagnostic marker, define a clinical threshold, predict an individual’s future decline or test a treatment.

What this could mean for preserving muscle strength

Being able to examine cellular states inside living human muscle gives researchers a way to ask more precise questions about why strength and mobility change with age. If larger studies establish which combinations of tissue signals consistently accompany functional decline, and if experiments then show that those states contribute to dysfunction, the measurements could help guide treatments intended to preserve muscle strength, everyday mobility and independence.

That possibility remains conditional. A future intervention would need to change the relevant cellular state safely and demonstrate better human function, not simply alter a biopsy marker. The MASS_Lifecourse findings provide an enabling measurement approach for that longer-term work: they show how tissue characterisation, muscle structure and physical performance can be studied together without claiming that a detected signal is already a cause or a cure.

Key Sources

• Primary source: Determining the feasibility of characterising cellular senescence in human skeletal muscle and exploring associations with muscle morphology and physical function at different ages: findings from the MASS_Lifecourse Study.
  DOI or Journal Page · Peer-Reviewed Paper · 2023-07-11