The Biology of Cellular Aging

The hallmarks framework

The influential hallmarks-of-ageing framework groups the process into interlinked categories: genomic instability, telomere attrition, epigenetic drift, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion and altered intercellular communication.

The value of the framework is that it treats ageing as several connected processes rather than one clock, which is also why interventions targeting a single hallmark tend to produce partial effects.

Senescence and the SASP

Senescent cells stop dividing but stay metabolically active and secrete a mix of inflammatory cytokines, proteases and growth factors known as the senescence-associated secretory phenotype. That secretion can push neighbouring cells toward senescence as well.

Senescence is protective in the short term — it prevents damaged cells from replicating — and harmful when senescent cells accumulate faster than the immune system clears them.

NAD+ and mitochondria

NAD+ is the central redox cofactor of metabolism and the required substrate for sirtuins and PARP enzymes. Tissue NAD+ levels decline with age in multiple species, and both reduced synthesis and increased consumption have been proposed as causes.

Because sirtuins regulate mitochondrial biogenesis, falling NAD+ links nutrient sensing directly to mitochondrial capacity — one of the tighter causal threads in the field.

Inflammaging

Chronic low-grade inflammation rises with age even without infection, driven by senescent cell secretion, mitochondrial DNA leakage into the cytosol and reduced barrier integrity. It appears both as a consequence of the other hallmarks and as a driver of them.

What this means for peptide research

Mitochondrial-derived peptides such as MOTS-c and humanin attract attention because they sit at the intersection of energy sensing, stress response and inter-organelle signalling. The evidence remains preclinical, and that distinction should be stated plainly rather than implied away.