Sex-specific protective effects of angiotensin II type 1 receptor knockdown on disuse muscle atrophy in the rat soleus muscle

Skeletal muscle disuse leads to atrophy. Accumulating evidence suggests that angiotensin II type 1 receptor (AT1R) contributes to sex-dependent catabolic signaling. However, the direct role of AT1R in skeletal muscle is unclear. This study investigated whether selective suppression of AT1R expression in the rat soleus muscle via adeno-associated virus serotype 9 (AAV9)-mediated short hairpin RNA (shRNA) delivery could mitigate hindlimb unloading (HU)-induced muscle atrophy, and whether this effect differed between male and female rats. Male and female rats received intramuscular injections of AAV9 vectors encoding AT1R-targeted shRNA into the soleus muscle. After 7 days of HU, <i>AT1R</i> gene copy number (digital PCR), receptor abundance (ligand-binding assay), muscle fiber cross-sectional area (CSA), and downstream molecular markers (including phosphorylated Smad2/3 and Smad1/5/8, HDAC4 expression, and the atrogenes, <i>Fbxo32</i> and <i>Trim63</i>) were assessed. Female rats exhibited higher <i>AT1R</i> gene copy numbers, which were selectively reduced by AAV9-shRNA. Ligand binding confirmed reduced receptor abundance in both sexes. CSA loss attenuation was observed exclusively in females, particularly in type I fibers. In female animals, AT1R knockdown substantially increased Smad1/5/8 phosphorylation and decreased HDAC4 expression. The AT1R copy number was positively correlated with <i>Fbxo32</i> and <i>Trim63</i> expression, independent of AAV dose. AT1R plays a sex-specific role in disuse-induced muscle atrophy, as muscle-specific AT1R knockdown conferred selective protection in female rats. The effect appears to be mediated via Smad1/5/8-HDAC4 signaling rather than oxidative stress or autophagy. This study provides mechanistic support for sex-informed therapeutic strategies targeting the muscle-localized renin-angiotensin system.<b>NEW & NOTEWORTHY</b> In this study, we demonstrate that muscle-specific knockdown of angiotensin II type 1 receptor (AT1R) via AAV9-shRNA selectively protects female rats against disuse-induced soleus muscle atrophy. The protective effect was restricted to type I fibers and associated with enhanced Smad1/5/8 signaling and reduced HDAC4 and atrogene expression. These findings reveal a sex-specific mechanism in local AT1R signaling and identify a potential therapeutic target for disuse atrophy with consideration of biological sex.