Myocytic androgen receptor controls the strength but not the mass of limb muscles

Abstract

The anabolic effects of androgens on skeletal muscles are thought to be mediated predominantly through the androgen receptor (AR), a member of the ligand-dependent nuclear receptor superfamily. However, despite numerous studies performed in men and in rodents, these effects remain poorly understood. To characterize androgen signaling in skeletal muscles, we generated mice in which the AR is selectively ablated in myofibers. We show that myocytic AR controls androgen-induced insulin-like growth factor IEa (IGF-IEa) expression in the highly androgen-sensitive perineal muscles and that it mediates androgen-stimulated postnatal hypertrophy of these muscles. In contrast, androgen-dependent postnatal hypertrophy of limb muscle fibers is independent of myocytic AR. Thus, androgens control perineal and limb muscle mass in male mice through myocytic AR-dependent and -independent pathways, respectively. Importantly, we also show that AR deficiency in limb myocytes impairs myofibrillar organization of sarcomeres and decreases muscle strength, thus demonstrating that myocytic AR controls key pathways required for maximum force production. These distinct androgen signaling pathways in perineal and limb muscles may allow the design of screens to identify selective androgen modulators of muscle strength.

Fig. 1.

Characterization of BC/LA muscles in AR^skm-/y and AR^(i)skm-/y mice. (A) BC/LA weight from 6- and 13-wk-old AR^L2/y and AR^skm-/y mice (n = 7). (B) Gross morphology of BC/LA (Upper) and histological analysis of H&E-stained BC muscle (Lower) of 13-wk-old AR^L2/y and AR^skm-/y mice. (Scale bar, 20 μm.) (C) Mean fiber CSA of BC muscle from 13-wk-old AR^L2/y and AR^skm-/y mice. (D and E) BC/LA weight from 8-wk-old (D) and 25-wk-old (E) AR^L2/y and AR^(i)skm-/y mice (n = 7). (F) BC/LA weight from 25-wk-old sham-operated (sham) and orchidectomized (Orx) AR^L2/y mice, and from 25-wk-old AR^skm-/y mice (n = 7). (G) IGF-IEa mRNA levels from 13-wk-old AR^L2/y and AR^skm-/y mice (n = 8). In A and C–G, error bars indicate SEM; *, P < 0.05.

Fig. 2.

Characterization of hindlimb skeletal muscles in AR^skm-/y mice and AR^(i)skm-/y mice. (A) Gastrocnemius weight from 6- and 13-wk-old AR^L2/y and AR^skm-/y mice (n = 7). (B) Gross morphology of gastrocnemius (G) and soleus (S) muscles (Upper) and histological analysis of H&E-stained gastrocnemius muscle (Lower) from 13-wk-old AR^L2/y and AR^skm-/y mice. (Scale bar, 20 μm.) (C) Mean fiber CSA of gastrocnemius muscle from 13-wk-old AR^L2/y and AR^skm-/y mice. (D) Gastrocnemius weight from 12- and 32-wk-old AR^L2/y and AR^(i)skm-/y mice (n = 7). Gastrocnemius weight (E), mean fiber CSA (F), and IGF-IEa mRNA levels (G) from 25-wk-old sham-operated (sham) and orchidectomized (Orx) AR^L2/y and AR^skm-/y mice (n = 6). In A and C–G, error bars indicate SEM. In A, F, and G, *P < 0.05. In G, ^#P ≥ 0.05.

Fig. 3.

Characterization of muscle contractile functions in AR^skm-/y mice. (A) Maximal grip strength of AR^L2/y and AR^skm-/y mice between the ages of 6 and 65 wk (n = 8–10). (B) In situ absolute maximal isometric tetanic force of tibialis anterior muscle from 20-wk-old AR^L2/y and AR^skm-/y mice (n = 7–8). (C) In vitro absolute maximal isometric tetanic force of EDL and of soleus muscles from 20-wk-old AR^L2/y and AR^skm-/y mice (n = 7–8). In A–C, error bars indicate SEM; *P < 0.05. In C, ^#P = 0.84. (D) Ultrastructure of EDL muscle from 15-wk-old AR^L2/y and AR^skm-/y mice. Black arrowheads indicate Z line disruptions; white arrowheads indicate loss of myofilaments; white arrows indicate sarcoplasm. A, A band; I, I band; H. H band; M, M line; Mt, mitochondria; Tt, T-tubules; Z, Z line.

Fig. 4.

Schematic model of androgen signaling in perineal and limb skeletal muscles.