Androgen receptor immunoreactivity in skeletal muscle: Enrichment at the neuromuscular junction

Abstract

Potential cellular targets of androgen action within skeletal muscle of the rat were determined by comparing the cellular distribution of androgen receptor (AR)-positive nuclei in the highly androgen-responsive levator ani (LA) muscle with that of the relatively androgen-unresponsive extensor digitorum longus (EDL) muscle. We found that androgen responsiveness correlates with AR expression in muscle fibers and not in fibroblasts. Results indicate that a much higher percentage of myonuclei in the LA are AR(+) than in the EDL (74% vs. 7%), correlating with differences in androgen responsiveness. Both muscles contain an equivalent proportion of AR(+) fibroblasts (approximately 62%). AR(+) nuclei were not observed in terminal Schwann cells in either muscle. These results suggest that ARs within LA muscle fibers mediate the androgen-dependent survival and growth of the LA muscle and its motoneurons. We also observed an unexpected enrichment of AR(+) myonuclei and fibroblasts proximate to neuromuscular junctions, suggesting that ARs at muscle synapses may selectively regulate synapse-specific genes important for the survival and growth of motoneurons. Although castration reduced the proportion of AR(+) fibroblasts in both muscles, the proportion of AR(+) myonuclei was reduced only in the LA. As expected, testosterone treatment prevented these effects of castration but, unexpectedly, increased the proportion of AR(+) myonuclei in the EDL to above normal. These results suggest that how AR expression in skeletal muscle is influenced by androgens depends not only on the particular muscle but on the particular cell type within that muscle.

The spinal nucleus of the bulbocavernosus: Firsts in androgen-dependent neural sex differences

“…In particular, it was established only quite recently which cells in muscle tissue express AR and are therefore potential direct sites of androgen action. A study of the distribution of receptor expression shows that about 75% of nuclei in muscle fibers of the LA are AR-positive (AR+), compared to 5-10% for a relatively nonandrogen sensitive limb muscle ( Figure 5; Monks et al, 2004). The LA and limb muscle contained equivalent proportions of AR+ fibroblasts, and terminal Schwann cells were not AR + in either muscle (Monks et al, 2004).…”

“…A study of the distribution of receptor expression shows that about 75% of nuclei in muscle fibers of the LA are AR-positive (AR+), compared to 5-10% for a relatively nonandrogen sensitive limb muscle ( Figure 5; Monks et al, 2004). The LA and limb muscle contained equivalent proportions of AR+ fibroblasts, and terminal Schwann cells were not AR + in either muscle (Monks et al, 2004). Thus, both muscle fibers and fibroblasts are potential direct sites of androgen action in striated muscle, but the effect of androgens on muscle size correlates with AR expression by myofiber nuclei.…”

Effect of Gonadal Hormones on the Cross‐Sectional Area of Pubococcygeus Muscle Fibers in Male Rat

“…Likewise, treatment with testosterone reaching at supraphysiological plasmatic levels increases both measures consistent with the large force exerted by this muscle (Tucek et al, 1976;Gutmann and Carlson, 1979;Godinho et al, 1987;Monks et al, 2004;Axell et al, 2006). Now we have found that Pcm in males is also sensitive to castration and testosterone replacement.…”

“…Thus, it is possible that gonadal hormones could be affecting this process. In this regard, it is known that, whereas castration reduces the weight (Antonio et al, 1999), fiber size (Nnodim, 1999;Monks et al, 2004), and synthesis of contractile proteins (Ferry et al, 1999) of the bulbocavernosus/levator ani muscle complex, the administration of androgens reverses or prevents the castration effects (Balice-Gordon et al, 1990;Venable, 1966;Antonio et al, 1999;Monks et al, 2004), recovering the intracellular physiology (Gori et al, 1969) to promote muscle protein synthesis and the incorporation of satellite cells into the muscle fibers (Nnodim, 2001) through the activation of receptors found in muscle fibers, fibroblasts close to neuromuscular junctions (Monks et al, 2004) and in muscle satellite cells (Sinha-Hikim et al, 2003. In addition, estrogen receptors are also present in the bulbocavernosus/levator ani muscle complex (Dubé et al, 1976;Dionne et al, 1979), and participate in the metabolism of this muscle Knudsen and Max, 1980).…”

Androgen regulation of axon growth and neurite extension in motoneurons

“…In vivo studies have shown that androgens are also responsible for process outgrowth in motoneurons, for example maintaining appropriate adult dendritic length (Kurz et al, 1986;Sasaki and Arnold, 1991;Rand andBreedlove, 1995, Fargo andSengelaub, 2007a,b) and enhancing axon regeneration after peripheral nerve axotomy (Kujawa et al, 1991). Additionally, AR is present in both motoneurons and muscles in vivo (Matsumoto, 1997;Yu and McGinnis, 2001;Tetzlaff et al, 2007a;Monks et al, 2004), and blocking androgen receptors with flutamide or hydroxyflutamide abolishes the ability of androgens to maintain dendritic length (Rand and Breedlove, 1995) and enhance axon regeneration (Yu and Cao, 1991;Kujawa et al, 1995). However, one possible difference between in vitro and in vivo studies of androgen regulation of process outgrowth in motoneurons relates to the question of whether androgens act directly on motoneurons to affect changes in cellular morphology.…”