Decorin Deficiency Promotes D-Galactose-Induced Skeletal Muscle Atrophy and Fibrosis by Regulating ITGB1/Akt/mTOR Signalling Pathway.

<p><b>BACKGROUND</b></p><p>Primary sarcopenia is an age-associated disorder with progressive and generalised loss of skeletal muscle strength and mass. Skeletal muscle fibrosis is one of the significant pathological manifestations of age-associated sarcopenia.

Decorin, a small dermatan-sulfate proteoglycan, participates in extracellular matrix assembly. In numerous studies, the involvement of decorin is not restricted to matrix structural proteins, and it also affects a diverse variety of biological functions like cell growth, adhesion, migration, proliferation and differentiation.

Additionally, it modulates the process of inflammation and fibrillogenesis. Based on these preclinical evidences, we hypothesised that decorin may potentially play an important role in skeletal muscles during ageing.</p><p><b>METHODS</b></p><p>Natural ageing mice (Dcn +/+), D-galactose (D-gal)-induced Dcn +/+, Dcn -/- mice and NOR-10 cell models were established.

Grip strength and exercise capacity were evaluated, after the mice were sacrificed to collect their gastrocnemius muscles for assessment of atrophy and fibrosis by haematoxylin and eosin staining, qRT-PCR and Western blotting. Co-immunoprecipitation was used to identify the interaction between decorin and integrin β1 (ITGB1).</p><p><b>RESULTS</b></p><p>The expression levels of decorin were reduced at both mRNA and protein levels in muscle during natural ageing in mice (-75.3% of mRNA level; -29.5% of protein level) and NOR-10 cells (-78% of protein level).

si-Dcn promoted the expression of α-SMA (+37.2%) and fibronectin (+53.1%), which are related to muscle fibrosis in D-gal-induced NOR-10 cells. In addition, in Dcn -/–D-gal mice, which exhibited more aggravated muscle atrophy including smaller grip strength (-21.7%, p < 0.001), downregulation of the ratio of gastrocnemius weight (-7.3%), fibre size (Gast: -15.3%) and increased levels of α-SMA (+70.2%), MuRF-1 (+30.2%), NLRP3(+19.4%) and p21(+27.2%) proteins compared with Dcn +/+-D-gal mice.

In terms of the underlying mechanisms, the Akt/mTOR signalling pathway was downregulated in Dcn -/–D-gal mice compared with aged Dcn +/+-D-gal mice (p-S473-Akt/Akt: -38.1%; p-Ser2448-mTOR/mTOR: -28.8%; p-p70S6K/p70: -40.3%; p-4E-BP1: -42.3%, p < 0.05). Decorin activation enhanced ITGB1 expression (+46.7% vs.

NC, p < 0.01). si-ITGB1 suppressed the expression of p-S473-Akt and p-Ser2448-mTOR in NOR-10 cells with Dcn overexpression.

The interaction between decorin and ITGB1 was found in skeletal muscle, suggesting that the regulation of decorin on Akt/mTOR might depend on ITGB1. Notably, decorin deficiency increased the accumulation of p62 (+47.2%, p < 0.05) and LC3b (+50.9%, p < 0.01).</p><p><b>CONCLUSIONS</b></p><p>The comprehensive results show that decorin plays a sarcoprotective role by activating the ITGB1/Akt/mTOR pathway and may serve as a potential therapeutic reagent in age-associated sarcopenia.</p>