Exercise Protein CTRP9 Fights Muscle Loss by Clearing Inflammatory Triggers

Sarcopenia affects an estimated 10–27% of older adults and is a major driver of disability, falls, fractures, and loss of independence. Despite the clinical burden, disease-modifying therapies remain elusive. This study, published in Cell Death & Disease, identifies a previously unrecognized axis — CTRP9–LAMP2A–NLRP3 — as a central regulator of skeletal muscle aging, providing a compelling new therapeutic target.

The research used a replicative senescence model in C2C12 myoblasts, distinguishing young cells (below 14 mean population doublings, MPD) from senescent cells (above 21 MPD). In senescent cells, CTRP9 mRNA and protein expression were significantly reduced compared to young cells, accompanied by a parallel decline in LAMP2A protein levels — the rate-limiting receptor for chaperone-mediated autophagy (CMA). Concurrently, NLRP3 protein accumulated and IL-1β secretion increased substantially, consistent with a pro-inflammatory, pro-atrophic cellular state. β-galactosidase staining and p21/p53 expression confirmed genuine replicative senescence.

To validate these findings in vivo, the team studied CTRP9 knockout (KO) mice versus wild-type (WT) controls at both young (3 months) and old (23 months) ages (n = 10 per group, all male C57BL/6J). Old KO mice showed markedly worse muscle phenotypes than old WT mice: lower grip strength, reduced suspension impulse in the hanging grid test, shortened exhaustive running distance, and significantly reduced lean mass on DEXA imaging. Histologically, gastrocnemius cross-sectional area was decreased and Fbxo32 (atrogin-1, a key atrophy E3 ubiquitin ligase) expression was elevated in KO mice. LAMP2A protein was further suppressed in KO muscle, while NLRP3 and cleaved caspase-1 were elevated, confirming NLRP3 inflammasome activation in the absence of CTRP9.

Treatment with the biologically active globular domain of CTRP9 (gCTRP9) restored LAMP2A expression and CMA flux in senescent C2C12 cells, as demonstrated by KFERQ-Dendra reporter assays (a validated live-cell CMA activity readout). Selective degradation of NLRP3 through the CMA-lysosomal pathway was confirmed by co-immunoprecipitation of NLRP3 with Hsc70 (the CMA chaperone) and by showing that LAMP2A knockdown (siRNA) abolished gCTRP9's ability to reduce NLRP3 levels. gCTRP9 treatment also rescued myogenic differentiation — increasing MYOD1 expression and fusion index — while reducing Fbxo32 and improving myotube diameter and integrity in senescent cells. Critically, the same benefits were replicated in primary human myoblasts isolated from elderly donors (≥60 years), which showed significantly lower CTRP9 and LAMP2A and higher NLRP3 compared to young donors (18–34 years). gCTRP9 partially reversed this molecular signature in human elderly myoblasts, strengthening translational relevance.

Serum CTRP9 levels measured across 86 human participants (stratified into young, middle-aged, and elderly groups) also declined progressively with age, and correlated positively with grip strength metrics — paralleling the animal and cell data. Together, these findings establish CTRP9 as a bona fide autocrine myokine whose age-related decline drives CMA dysfunction, NLRP3 accumulation, chronic inflammaging, and ultimately sarcopenic muscle loss. The identification of a druggable axis — gCTRP9 supplementation or LAMP2A upregulation to degrade NLRP3 — represents a mechanistically grounded therapeutic strategy for preserving muscle mass and function in aging.