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

Overexpression of androgen receptors in target musculature confers androgen sensitivity to motoneuron dendrites

The dendritic arbors of spinal motoneurons are dynamically regulated by a variety of factors, and several lines of evidence indicate that trophic interactions with the target musculature are of central importance. In highly androgen-sensitive motoneuron populations, androgens are thought to regulate motoneuron dendrites through their action at the receptor-enriched target musculature. Using rats transgenically modified to overexpress androgen receptor (AR) in skeletal muscle, we directly tested the hypothesis that the enhanced expression of AR in the target musculature can underlie the androgenic regulation of motoneuron dendritic morphology. The morphology of motoneurons innervating the quadriceps muscle was examined in wild-type (WT) rats as well as in rats that had been transgenically modified to overexpress ARs in their skeletal musculature. Motoneurons innervating the vastus lateralis muscle of the quadriceps in gonadally intact male rats, and castrated males with or without androgen replacement, were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. In WT rats, quadriceps motoneuron dendrites were insensitive to hormonal manipulation. In contrast, quadriceps motoneuron dendrites in gonadally intact transgenic males were larger than those of WT males. Furthermore, overexpression of ARs in the quadriceps muscle resulted in androgen sensitivity in dendrites, with substantial reductions in dendritic length occurring after castration; this reduction was prevented with testosterone replacement. Thus, it appears that the androgen sensitivity of motoneuron dendrites is conferred indirectly via the enrichment of ARs in the musculature.

Skeletal muscle in motor neuron diseases: therapeutic target and delivery route for potential treatments

Lower motor neuron (LMN) degeneration occurs in several diseases that affect patients from neonates to elderly and can either be genetically transmitted or occur sporadically. Among diseases involving LMN degeneration, spinal muscular atrophy (SMA) and spinal bulbar muscular atrophy (Kennedy's disease, SBMA) are pure genetic diseases linked to loss of the SMN gene (SMA) or expansion of a polyglutamine tract in the androgen receptor gene (SBMA) while amyotrophic lateral sclerosis (ALS) can either be of genetic origin or occur sporadically. In this review, our aim is to put forward the hypothesis that muscle fiber atrophy and weakness might not be a simple collateral damage of LMN degeneration, but instead that muscle fibers may be the site of crucial pathogenic events in these diseases. In SMA, the SMN gene was shown to be required for muscle structure and strength as well as for neuromuscular junction formation, and a subset of SMA patients develop myopathic pathology. In SBMA, the occurrence of myopathic histopathology in patients and animal models, along with neuromuscular phenotype of animal models expressing the androgen receptor in muscle only has lead to the proposal that SBMA may indeed be a muscle disease. Lastly, in ALS, at least part of the phenotype might be explained by pathogenic events occuring in skeletal muscle. Apart from its potential pathogenic role, skeletal muscle pathophysiological events might be a target for treatments and/or be a preferential route for targeting motor neurons.

Neonatal androgen-dependent sex differences in lumbar spinal cord dopamine concentrations and the number of A11 diencephalospinal dopamine neurons

A(11) diencephalospinal dopamine (DA) neurons provide the major source of DA innervation to the spinal cord. DA in the dorsal and ventral horns modulates sensory, motor, nociceptive, and sexual functions. Previous studies from our laboratory revealed a sex difference in the density of DA innervation in the lumbar spinal cord. The purpose of this study was to determine whether sex differences in spinal cord DA are androgen dependent, influenced by adult or perinatal androgens, and whether a sex difference in the number of lumbar-projecting A(11) neurons exists. Adult male mice have significantly higher DA concentrations in the lumbar spinal cord than either females or males carrying the testicular feminization mutation (tfm) in the androgen receptor (AR) gene, suggesting an AR-dependent origin. Spinal cord DA concentrations are not changed following orchidectomy in adult male mice or testosterone administration to ovariectomized adult female mice. Administration of exogenous testosterone to postnatal day 2 female mice results in DA concentrations in the adult lumbar spinal cord comparable to those of males. Male mice display significantly more lumbar-projecting A(11) DA neurons than females, particularly in the caudal portion of the A(11) cell body region, as determined by retrograde tract tracing and immunohistochemistry directed toward tyrosine hydroxylase. These results reveal an AR-dependent sex difference in both the number of lumbar-projecting A(11) DA neurons and the lumbar spinal cord DA concentrations, organized by the presence of androgens early in life. The AR-dependent sex difference suggests that this system serves a sexually dimorphic function in the lumbar spinal cord.

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

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.

Tissue selectivity and potential clinical applications of trenbolone (17beta-hydroxyestra-4,9,11-trien-3-one): A potent anabolic steroid with reduced androgenic and estrogenic activity

Recently, the development of selective androgen receptor modulators (SARMs) has been suggested as a means of combating the deleterious catabolic effects of hypogonadism, especially in skeletal muscle and bone, without inducing the undesirable androgenic effects (e.g., prostate enlargement and polycythemia) associated with testosterone administration. 17beta-Hydroxyestra-4,9,11-trien-3-one (trenbolone; 17beta-TBOH), a synthetic analog of testosterone, may be capable of inducing SARM-like effects as it binds to androgen receptors (ARs) with approximately three times the affinity of testosterone and has been shown to augment skeletal muscle mass and bone growth and reduce adiposity in a variety of mammalian species. In addition to its direct actions through ARs, 17beta-TBOH may also exert anabolic effects by altering the action of endogenous growth factors or inhibiting the action of glucocorticoids. Compared to testosterone, 17beta-TBOH appears to induce less growth in androgen-sensitive organs which highly express the 5alpha reductase enzyme (e.g., prostate tissue and accessory sex organs). The reduced androgenic effects result from the fact that 17beta-TBOH is metabolized to less potent androgens in vivo; while testosterone undergoes tissue-specific biotransformation to more potent steroids, dihydrotestosterone and 17beta-estradiol, via the 5alpha-reductase and aromatase enzymes, respectively. Thus the metabolism of 17beta-TBOH provides a basis for future research evaluating its safety and efficacy as a means of combating muscle and bone wasting conditions, obesity, and/or androgen insensitivity syndromes in humans, similar to that of other SARMs which are currently in development.

Testosterone metabolites differentially maintain adult morphology in a sexually dimorphic neuromuscular system

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). Androgens are necessary for the development of the SNB neuromuscular system, and in adulthood, continue to influence the morphology and function of the motoneurons and their target musculature. However, estrogens are also involved in the development of the SNB system, and are capable of maintaining function in adulthood. In this experiment, we assessed the ability of testosterone metabolites, estrogens and nonaromatizable androgens, to maintain neuromuscular morphology in adulthood. Motoneuron and muscle morphology was assessed in adult normal males, sham-castrated males, castrated males treated with testosterone, dihydrotestosterone, estradiol, or left untreated, and gonadally intact males treated with the 5alpha-reductase inhibitor finasteride or the aromatase inhibitor fadrozole. After 6 weeks of treatment, SNB motoneurons were retrogradely labeled with cholera toxin-HRP and reconstructed in three dimensions. Castration resulted in reductions in SNB target muscle size, soma size, and dendritic morphology. Testosterone treatment after castration maintained SNB soma size, dendritic morphology, and elevated target muscle size; dihydrotestosterone treatment also maintained SNB dendritic length, but was less effective than testosterone in maintaining both SNB soma size and target muscle weight. Treatment of intact males with finasteride or fadrozole did not alter the morphology of SNB motoneurons or their target muscles. In contrast, estradiol treatment was completely ineffective in preventing castration-induced atrophy of the SNB neuromuscular system. Together, these results suggest that the maintenance of adult motoneuron or muscle morphology is strictly mediated by androgens.

Testosterone represses ubiquitin ligases atrogin-1 and Murf-1 expression in an androgen-sensitive rat skeletal muscle in vivo

Skeletal muscle atrophy induced by denervation and metabolic diseases has been associated with increased ubiquitin ligase expression. In the present study, we evaluate the influence of androgens on muscle ubiquitin ligases atrogin-1/MAFbx/FBXO32 and Murf-1/Trim63 expression and its correlation with maintenance of muscle mass by using the testosterone-dependent fast-twitch levator ani muscle (LA) from normal or castrated adult male Wistar rats. Gene expression was determined by qRT-PCR and/or immunoblotting. Castration induced progressive loss of LA mass (30% of control, 90 days) and an exponential decrease of LA cytoplasm-to-nucleus ratio (nuclear domain; 22% of control after 60 days). Testosterone deprivation induced a 31-fold increase in LA atrogin-1 mRNA and an 18-fold increase in Murf-1 mRNA detected after 2 and 7 days of castration, respectively. Acute (24 h) testosterone administration fully repressed atrogin-1 and Murf-1 mRNA expression to control levels. Atrogin-1 protein was also increased by castration up to 170% after 30 days. Testosterone administration for 7 days restored atrogin-1 protein to control levels. In addition to the well known stimulus of protein synthesis, our results show that testosterone maintains muscle mass by repressing ubiquitin ligases, indicating that inhibition of ubiquitin-proteasome catabolic system is critical for trophic action of androgens in skeletal muscle. Besides, since neither castration nor androgen treatment had any effect on weight or ubiquitin ligases mRNA levels of extensor digitorum longus muscle, a fast-twitch muscle with low androgen sensitivity, our study shows that perineal muscle LA is a suitable in vivo model to evaluate regulation of muscle proteolysis, closely resembling human muscle responsiveness to androgens.

Effect of high intensity aerobic exercise and mesterolone on remodeling of Achilles tendon of C57BL/6 transgenic mice

The effect of mesterolone and intensive treadmill training (6 weeks, 5 days/week, means: 15.82 m/min and 45.8 min/day) in Achilles tendon remodeling was evaluated. Sedentary mice treated with mesterolone (Sed-M) or vehicle (Sed-C, placebo/control) and corresponding exercised (Ex-M and Ex-C) were examined. SDS-polyacrylamide gel electrophoresis was used for determining collagen bands and hydroxyproline concentration. Collagen fibril diameter, the area and number of fibrils contained in an area probe, and the ultrastructure of fibroblasts (tenocytes) were determined. The presence of collagen was notable in the tendons of all groups. Collagen alpha(1/)alpha(2) bands in Sed-M, Ex-C, and Ex-M were higher than in Sed-C, as shown by hydroxyproline content, but collagen beta-chain appeared only in Ex-C. Noticeable bands of non-collagenous proteins were found in Sed-M and Ex-M. The number of fibrils in the area probe increased markedly in Sed-M and Ex-C (12-fold), but their diameter and area were unchanged compared with Sed-C. In Ex-M, the fibril number decreased by three-fold to 3.5-fold compared with Sed-M and Ex-C, whereas diameter and area increased. Sed-C tenocytes appeared quiescent, whereas those in the other groups seemed to be engaged in protein synthesis. The density of tenocytes was smaller in Sed-C than in Ex-C, Sed-M, and Ex-M. Thus, mechanical stimuli and mesterolone alter the morphology of tenocytes and the composition of the tendon, probably through fibrillogenesis and/or increased intermolecular cross-links. The ergogenic effect is evidenced by the activation of collagenous and non-collagenous protein synthesis and the increase in the diameter and area of collagen fibrils. This study might be relevant to clinical sports medicine.

Androgen signaling in myocytes contributes to the maintenance of muscle mass and fiber type regulation but not to muscle strength or fatigue

Muscle frailty is considered a major cause of disability in the elderly and chronically ill. However, the exact role of androgen receptor (AR) signaling in muscle remains unclear. Therefore, a postmitotic myocyte-specific AR knockout (mARKO) mouse model was created and investigated together with a mouse model with ubiquitous AR deletion. Muscles from mARKO mice displayed a marked reduction in AR protein (60-88%). Interestingly, body weights and lean body mass were lower in mARKO vs. control mice (-8%). The weight of the highly androgen-sensitive musculus levator ani was significantly reduced (-46%), whereas the weights of other peripheral skeletal muscles were not or only slightly reduced. mARKO mice had lower intra-abdominal fat but did not demonstrate a cortical or trabecular bone phenotype, indicating that selective ablation of the AR in myocytes affected male body composition but not skeletal homeostasis. Furthermore, muscle contractile performance in mARKO mice did not differ from their controls. Myocyte-specific AR ablation resulted in a conversion of fast toward slow fibers, without affecting muscle strength or fatigue. Similar results were obtained in ubiquitous AR deletion, showing lower body weight, whereas some but not all muscle weights were reduced. The percent slow fibers was increased, but no changes in muscle strength or fatigue could be detected. Together, our findings show that myocyte AR signaling contributes to the maintenance of muscle mass and fiber type regulation but not to muscle strength or fatigue. The levator ani weight remains the most sensitive and specific marker of AR-mediated anabolic action on muscle.

The organizational hypothesis and final common pathways: Sexual differentiation of the spinal cord and peripheral nervous system

In honor of the 50th anniversary of the "organizational hypothesis," this paper reviews work on sexual differentiation of the spinal cord and peripheral nervous system. Topics considered include the spinal nucleus of the bulbocavernosus, the ejaculation center, the cremaster nucleus, sensory and autonomic neurons, and pain. These relatively simple neural systems offer ample confirmation that early exposure to testicular hormones masculinizes the nervous system, including final common pathways. However, I also discuss findings that challenge, or at least stretch, the organizational hypothesis, with important implications for understanding sex differences throughout the nervous system.

Regulation of myogenic differentiation by androgens: cross talk between androgen receptor/ beta-catenin and follistatin/transforming growth factor-beta signaling pathways

Androgens are important regulators of body composition and promote myogenic differentiation and inhibit adipogenesis of mesenchymal, multipotent cells. Here, we investigated the mechanisms by which androgens induce myogenic differentiation of mesenchymal multipotent cells. Incubation of mesenchymal multipotent C3H 10T1/2 cells with testosterone and dihydrotestosterone promoted nuclear translocation of androgen receptor (AR)/beta-catenin complex and physical interaction of AR, beta-catenin, and T-cell factor-4 (TCF-4). Inhibition of beta-catenin by small inhibitory RNAs significantly decreased testosterone-induced stimulation of myogenic differentiation. Overexpression of TCF-4, a molecule downstream of beta-catenin in Wnt signaling cascade, in C3H 10T1/2 cells significantly up-regulated expression of myoD and myosin heavy chain II proteins and of follistatin (Fst), which binds and antagonizes native ligands of the TGF-beta/Smad pathway. Gene array analysis of C3H 10T1/2 cells treated with testosterone revealed that testosterone up-regulated the expression of Fst and modified the expression of several signaling molecules involved in the TGF-beta/Smad pathway, including Smad7. Lowering of testosterone levels in mice by orchidectomy led to a significant decrease in Fst and Smad7 expression; conversely, testosterone supplementation in castrated mice up-regulated Fst and Smad7 mRNA expression in androgen-responsive levator ani muscle. Testosterone-induced up-regulation of MyoD and myosin heavy chain II proteins in C3H 10T1/2 cells was abolished in cells simultaneously treated with anti-Fst antibody, suggesting an essential role of Fst during testosterone regulation of myogenic differentiation. In conclusion, our data suggest the involvement of AR, beta-catenin, and TCF-4 pathway during androgen action to activate a number of Wnt target genes, including Fst, and cross communication with the Smad signaling pathway.

Motor development in individuals with congenital adrenal hyperplasia: strength, targeting, and fine motor skill

This study investigated early androgen influence on the development of human motor and visuomotor characteristics. Participants, ages 12-45 years, were individuals with congenital adrenal hyperplasia (CAH), a disorder causing increased adrenal androgen production before birth (40 females, 29 males) and their unaffected relatives (29 females, 30 males). We investigated grip strength and visuomotor targeting tasks on which males generally outperform females, and fine motor pegboard tasks on which females generally outperform males. Physical characteristics (height and weight) were measured to explore whether body parameters could explain differences in motor skills. Females with CAH were stronger and showed better targeting than unaffected females and showed reduced fine visuomotor skill on one pegboard measure, with no difference on the other. Males with CAH were weaker than unaffected males in grip strength but did not differ on the targeting or pegboard measures. Correction for body size could not explain the findings for females, but suggests that the reduced strength of males with CAH may relate to their smaller stature. Further, the targeting advantage in females with CAH persisted following adjustment for their greater strength. Results in females support the hypothesis that androgen may masculinize, or promote, certain motor characteristics at which males excel, and contribute to defeminization of certain fine motor characteristics at which females excel. Thus, these data suggest that organizational effects of androgens on behavior during prenatal life may extend to motor characteristics and may contribute to general sex differences in motor-related behaviors; however, alternative explanations based on activational influences of androgen or altered experiential factors cannot be excluded without further study.

Androgen receptor and Kennedy disease/spinal bulbar muscular atrophy

Kennedy Disease/Spinal Bulbar Muscular Atrophy (KD/SBMA) is a progressive neurodegenerative disease caused by genetic polyglutamine expansion of the androgen receptor. We have recently found that overexpression of wildtype androgen receptor in skeletal muscle of transgenic mice results in a KD/SBMA phenotype. This surprising result challenges the orthodox view that KD/SBMA requires expression of polyglutamine expanded androgen receptor within motoneurons. Theories relating to the etiology of this disease drawn from studies of human patients, cellular and mouse models are considered with a special emphasis on potential myogenic contributions to as well as the molecular etiology of KD/SBMA.

Androgen regulation of axon growth and neurite extension in motoneurons

Androgens act on the CNS to affect motor function through interaction with a widespread distribution of intracellular androgen receptors (AR). This review highlights our work on androgens and process outgrowth in motoneurons, both in vitro and in vivo. The actions of androgens on motoneurons involve the generation of novel neuronal interactions that are mediated by the induction of androgen-dependent neurite or axonal outgrowth. Here, we summarize the experimental evidence for the androgenic regulation of the extension and regeneration of motoneuron neurites in vitro using cultured immortalized motoneurons, and axons in vivo using the hamster facial nerve crush paradigm. We place particular emphasis on the relevance of these effects to SBMA and peripheral nerve injuries.

Androgen receptors and testosterone in men--effects of protein ingestion, resistance exercise and fiber type

The purpose of this study was to examine the impact of protein ingestion on circulating testosterone and muscle androgen receptor (AR) as well as on insulin-like growth factor-I (MGF and IGF-IEa) responses to a resistance exercise (RE) bout in (57-72 year) men. Protein (15 g whey) (n=9) or placebo (n=9) was consumed before and after a RE bout (5 sets of 10 repetition maximums), and vastus lateralis muscle biopsies were taken pre, 1 and 48 h post-RE. The protein ingestion blunted the RE-induced increase in serum free and total testosterone while the RE bout significantly increased muscle AR mRNA levels in older men (P<0.05). However, protein ingestion did not significantly affect AR mRNA or protein expression, or MGF and IGF-IEa mRNA expression at 1 and 48 h post-RE. Immunohistochemical staining of muscle cross-sections was done with antibodies specific to AR and MyHC I and II and showed that there seems to be within or near the type-I muscle fibers a greater staining of ARs than within or near the type-II fibres. In conclusion, the protein ingestion hinders RE-induced increase in serum testosterone in older men but may not significantly affect muscle AR, MGF or IGF-IEa gene expression. Furthermore, the present study shows that even older men are able to increase muscle AR mRNA expression in response to a RE bout.

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

Cell number in the spinal nucleus of the bulbocavernosus (SNB) of rats was the first neural sex difference shown to differentiate under the control of androgens, acting via classical intracellular androgen receptors. SNB motoneurons reside in the lumbar spinal cord and innervate striated muscles involved in copulation, including the bulbocavernosus (BC) and levator ani (LA). SNB cells are much larger and more numerous in males than in females, and the BC/LA target muscles are reduced or absent in females. The relative simplicity of this neuromuscular system has allowed for considerable progress in pinpointing sites of hormone action, and identifying the cellular bases for androgenic effects. It is now clear that androgens act at virtually every level of the SNB system, in development and throughout adult life. In this review we focus on effects of androgens on developmental cell death of SNB motoneurons and BC/LA muscles; the establishment and maintenance of SNB motoneuron soma size and dendritic length; BC/LA muscle morphology and physiology; and behaviors controlled by the SNB system. We also describe new data on neurotherapeutic effects of androgens on SNB motoneurons after injury in adulthood.

Enhanced muscle fatigue occurs in male but not female ASIC3-/- mice

Muscle fatigue is associated with a number of clinical diseases, including chronic pain conditions. Decreases in extracellular pH activates acid-sensing ion channel 3 (ASIC3), depolarizes muscle, protects against fatigue, and produces pain. We examined whether ASIC3-/- mice were more fatigable than ASIC3+/+ mice in a task-dependent manner. We developed two exercise protocols to measure exercise-induced muscle fatigue: (fatigue task 1, three 1-h runs; fatigue task 2, three 30-min runs). In fatigue task 1, male ASIC3+/+ mice muscle showed less fatigue than male ASIC3-/- mice and female ASIC3+/+ mice. No differences in fatigue were observed in fatigue task 2. We then tested whether the development of muscle fatigue was dependent on sex and modulated by testosterone. Female ASIC3+/+ mice that were ovariectomized and administered testosterone developed less muscle fatigue than female ASIC3+/+ mice and behaved similarly to male ASIC3+/+ mice. However, testosterone was unable to rescue the muscle fatigue responses in ovariectomized ASIC3-/- mice. Plasma levels of testosterone from male ASIC3-/- mice were significantly lower than in male ASIC3+/+ mice and were similar to female ASIC3+/+ mice. Muscle fiber types, measured by counting ATPase-stained whole muscle sections, were similar in calf muscles from male and female ASIC3+/+ mice. These data suggest that both ASIC3 and testosterone are necessary to protect against muscle fatigue in a task-dependent manner. Also, differences in expression of ASIC3 and the development of exercise-induced fatigue could explain the female predominance in clinical syndromes of pain that include muscle fatigue.

Overexpression of wild-type androgen receptor in muscle recapitulates polyglutamine disease

We created transgenic mice that overexpress WT androgen receptor (AR) exclusively in their skeletal muscle fibers. Unexpectedly, these mice display androgen-dependent muscle weakness and early death, show changes in muscle morphology and gene expression consistent with neurogenic atrophy, and exhibit a loss of motor axons. These features reproduce those seen in models of Kennedy disease, a polyglutamine expansion disorder caused by a CAG repeat expansion in the AR gene. These findings demonstrate that toxicity in skeletal muscles is sufficient to cause motoneuron disease and indicate that overexpression of the WT AR can exert toxicity comparable with the polyglutamine expanded protein. This model has two clear implications for Kennedy disease: (i) mechanisms affecting AR gene expression may cause neuromuscular symptoms similar to those of Kennedy disease and (ii) therapeutic approaches targeting skeletal muscle may provide effective treatments for this disease.

Androgen receptor expression in the levator ani muscle of male mice

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic group of motoneurones that innervates the bulbocavernosus (BC) and levator ani (LA), skeletal muscles that attach to the base of the penis. In many species, including mice, rats and hamsters, the LA and BC have been found to be highly responsive to androgen and, in rats, these muscles mediate several effects of androgen on the SNB system. However, characterising the SNB system in mice is important because of the availability of genetic models in this species. In the present study, we examined AR expression in skeletal muscles of C57/BlJ6 adult male mice using immunoblotting and immunocytochemistry, comparing the BC/LA to the androgen-unresponsive extensor digitorum longus (EDL). We found similar differences in AR expression for these muscles in the mouse as previously reported for rats. In mice, the BC/LA contains more AR protein than does the EDL. At the cellular level, the LA contains a higher percentage of AR positive myonuclei and fibroblasts than does the EDL. Finally, AR expression is enriched at the neuromuscular junction of mouse LA fibres. The increased expression of AR in the LA compared to the EDL in both muscle fibres and fibroblasts indicates that each cell type may critically mediate androgen action on the SNB system in mice.

Androgenic, but not estrogenic, protection of motoneurons from somal and dendritic atrophy induced by the death of neighboring motoneurons

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders or even death. We have been investigating the effects of motoneuron loss on surviving motoneurons in a lumbar motor nucleus, the spinal nucleus of the bulbocavernosus (SNB). SNB motoneurons undergo marked dendritic and somal atrophy following the experimentally induced death of other nearby SNB motoneurons. However, treatment with testosterone at the time of lesioning attenuates this atrophy. Because testosterone can be metabolized into the estrogen estradiol (as well as other physiologically active steroid hormones), it was unknown whether the protective effect of testosterone was an androgen effect, an estrogen effect, or both. In the present experiment, we used a retrogradely transported neurotoxin to kill the majority of SNB motoneurons on one side of the spinal cord only in adult male rats. Some animals were also treated with either testosterone, the androgen dihydrotestosterone (which cannot be converted into estradiol), or the estrogen estradiol. As seen previously, partial motoneuron loss led to reductions in soma area and in dendritic length and extent in surviving motoneurons. Testosterone and dihydrotestosterone attenuated these reductions, but estradiol had no protective effect. These results indicate that the neuroprotective effect of testosterone on the morphology of SNB motoneurons following partial motoneuron depletion is an androgen effect rather than an estrogen effect.

Androgens negatively regulate myostatin expression in an androgen-dependent skeletal muscle

Myostatin is an important negative regulator of skeletal muscle growth, while androgens are strong positive effectors. In order to investigate the possible interaction between myostatin and androgen pathways, we followed myostatin expression in the androgen-dependent levator ani (LA) muscle of the rat as a function of androgen status. By testosterone deprivation (castration), we induced LA growth arrest in young male rats, whilst atrophy in adult ones, however, both processes could be reversed by testosterone supplementation. After castration, a significant up-regulation of active myostatin protein (and its propeptide) was found, whereas the subsequent testosterone treatment reduced myostatin protein levels to normal values in both young and adult rats. Similarly, a testosterone-induced suppression of myostatin mRNA levels was observed in castrated adult but not in young animals. Altogether, androgens seem to have strong negative impact on myostatin expression, which might be a key factor in the weight regulation of LA muscle.

Androgen-sensitivity of somata and dendrites of spinal nucleus of the bulbocavernosus (SNB) motoneurons in male C57BL6J mice

In rats, androgens in adulthood regulate the morphology of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), including the size of their somata and the length of their dendrites. There are conflicting reports about whether androgens exert similar influences on SNB motoneurons in mice. We castrated or sham-operated C57BL6J mice at 90 days of age and, thirty days later, injected cholera toxin conjugated horseradish peroxidase into the bulbocavernosus muscle (to label SNB motoneurons) on one side, and into intrinsic foot muscles contralaterally (to label motoneurons of the retrodorsolateral nucleus (RDLN)). Castrated mice had significantly smaller SNB somas compared to sham-operated mice while there were no differences in soma size of RDLN motoneurons. Dendritic length in C57BL6J mice, estimated in 3-dimensions, also decreased significantly after adult castration. In rats, androgens act directly through androgen receptors (AR) in SNB motoneurons to control soma size and nearly all SNB motoneurons contain AR. Since SNB somata in C57BL6J mice shrank after adult castration, we used immunocytochemistry to characterize AR expression in SNB cells as well as motoneurons in the RDLN and dorsolateral nucleus (DLN). A pattern of labeling matched that seen previously in rats: the highest percentage of AR-immunoreactive motoneurons are in the SNB (98%), the lowest in the RDLN (25%) and an intermediate number in the DLN (78%). This pattern of AR labeling is consistent with the possibility that androgens also act directly on SNB motoneurons in mice to regulate soma size in mice.

Androgen-dependent pathology demonstrates myopathic contribution to the Kennedy disease phenotype in a mouse knock-in model

Kennedy disease, a degenerative disorder characterized by androgen-dependent neuromuscular weakness, is caused by a CAG/glutamine tract expansion in the androgen receptor (Ar) gene. We developed a mouse model of Kennedy disease, using gene targeting to convert mouse androgen receptor (AR) to human sequence while introducing 113 glutamines. AR113Q mice developed hormone and glutamine length-dependent neuromuscular weakness characterized by the early occurrence of myopathic and neurogenic skeletal muscle pathology and by the late development of neuronal intranuclear inclusions in spinal neurons. AR113Q males unexpectedly died at 2-4 months. We show that this androgen-dependent death reflects decreased expression of skeletal muscle chloride channel 1 (CLCN1) and the skeletal muscle sodium channel alpha-subunit, resulting in myotonic discharges in skeletal muscle of the lower urinary tract. AR113Q limb muscles show similar myopathic features and express decreased levels of mRNAs encoding neurotrophin-4 and glial cell line-derived neurotrophic factor. These data define an important myopathic contribution to the Kennedy disease phenotype and suggest a role for muscle in non-cell autonomous toxicity of lower motor neurons.

Continuous testosterone administration prevents skeletal muscle atrophy and enhances resistance to fatigue in orchidectomized male mice

Androgens promote anabolism in skeletal muscle; however, effects on subsequent muscle function are less well defined because of a lack of reliable experimental models. We established a rigorous model of androgen withdrawal and administration in male mice and assessed androgen regulation of muscle mass, structure, and function. Adult C57Bl/6J male mice were orchidectomized (Orx) or sham-operated (Sham) and received 10 wk of continuous testosterone (T) or control treatment (C) via intraperitoneal implants. Mass, fiber cross-sectional area (CSA), and in vitro contractile function were assessed for fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles. After 10 wk, Orx+C mice had reduced body weight gain (P < 0.05), seminal vesicle mass (P < 0.01), and levator ani muscle mass (P < 0.001) compared with Sham+C mice, and these effects were prevented with testosterone treatment. Orx+T mice had greater EDL (P < 0.01) and SOL (P < 0.01) muscle mass compared with Orx+C mice; however, median fiber CSA was not significantly altered in these muscles. EDL and SOL muscle force was greater in Sham+T compared with Orx+C mice (P < 0.05) in proportion to muscle mass. Unexpectedly, Orx+T mice had increased fatigue resistance of SOL muscle compared with Orx+C mice (P < 0.001). We used a rigorous model of androgen withdrawal and administration in male mice to demonstrate an essential role of androgens in the maintenance of muscle mass and force. In addition, we showed that testosterone treatment increases resistance to fatigue of slow- but not fast-twitch muscle.

Anabolic responsiveness of skeletal muscles correlates with androgen receptor protein but not mRNA

Anabolic effects of androgens on skeletal muscle are well documented, but the physiological and biochemical bases of these effects are poorly understood. Skeletal muscles that differ in their androgen responsiveness can be used to examine these mechanisms. We compared androgen receptor mRNA and protein levels of the rat levator ani, a perineal skeletal muscle that depends on androgens for its normal maintenance and function with that of the rat extensor digitorum longus, a limb muscle that does not require androgens. Western immunoblotting indicated that androgen receptor protein is significantly elevated in the levator ani relative to the extensor digitorum longus. Surprisingly, steady state androgen receptor mRNA levels were equivalent in these muscles, as determined by Northern blot analysis and quantitative RT-PCR. These results suggest that androgen responsiveness of skeletal muscles is determined by the level of androgen receptor protein in a particular muscle and that androgen receptor protein content is regulated by translational or post-translational mechanisms.

Selective androgen receptor modulator treatment improves muscle strength and body composition and prevents bone loss in orchidectomized rats

The partial agonist activity of a selective androgen receptor modulator (SARM) in the prostate was demonstrated in orchidectomized rats. In the current study, we characterized the full agonist activity of S-3-(4-acetylamino-phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamide (a structurally related SARM referred to in other publications and hereafter as S-4) in skeletal muscle, bone, and pituitary of castrated male rats. Twelve weeks after castration, animals were treated with S-4 (3 or 10 mg/kg), dihydrotestosterone (DHT) (3 mg/kg), or vehicle for 8 wk. S-4 (3 and 10 mg/kg) restored soleus muscle mass and strength and levator ani muscle mass to that seen in intact animals. Similar changes were also observed in DHT-treated (3 mg/kg) animals. Compared with the anabolic effects observed in muscle, DHT (3 mg/kg) stimulated prostate and seminal vesicle weights more than 2-fold greater than that observed in intact controls, whereas S-4 (3 mg/kg) returned these androgenic organs to only 16 and 17%, respectively, of the control levels. S-4 (3 and 10 mg/kg) and DHT (3 mg/kg) restored castration-induced loss in lean body mass. Furthermore, S-4 treatment caused a significantly larger increase in total body bone mineral density than DHT. S-4 (3 and 10 mg/kg) also demonstrated agonist activity in the pituitary and significantly decreased plasma LH and FSH levels in castrated animals in a dose-dependent manner. In summary, the strong anabolic effects of S-4 in skeletal muscle, bone, and pituitary were achieved with minimal pharmacologic effect in the prostate. The tissue-selective pharmacologic activity of SARMs provides obvious advantages over steroidal androgen therapy and demonstrates the promising therapeutic utility that this new class of drugs may hold.

Testosterone regulates androgen receptor immunoreactivity in the copulatory, but not courtship, neuromuscular system in adult male green anoles

Androgens regulate the expression of male reproductive behaviour in diverse vertebrate species, often acting on androgen receptors (AR) to induce structural or functional changes in the nervous system and periphery. Male green anoles possess two sexually dimorphic neuromuscular systems, one controlling throat fan (dewlap) extension, which occurs during courtship, and the other mediating copulatory organ function. Although androgens are required for behavioural activation in both systems, testosterone has differential effects on the neuromuscular morphology. It increases the size of copulatory muscle fibres during the breeding season, but significant effects on dewlap muscle fibre size and motoneurone soma size in either system have not been detected. Corresponding to the lack of testosterone-induced morphological effects in the courtship system, relatively low levels of AR are expressed in the associated motoneurones. The present experiment had two goals, aiming to determine whether: (i) the other courtship and copulatory neuromuscular tissues express AR and (ii) testosterone and/or seasonal environmental changes regulate AR expression. The percentage of AR+ nuclei was evaluated in both the breeding and nonbreeding seasons in gonadally intact adult males (Experiment 1) and in castrated males treated with either testosterone or vehicle (Experiment 2). AR was extensively expressed in the dewlap and copulatory muscles, and in a high percentage of the copulatory motoneurones, but immunoreactivity did not vary across season. Testosterone increased the percentage of AR+ nuclei in the copulatory muscles of both breeding and nonbreeding males but not in the dewlap muscle or copulatory motoneurones. Finally, the target structures for both systems (cartilages and hemipenes) expressed AR in all animals. Therefore, the effects of testosterone on AR immunoreactivity suggest that up-regulation of the receptors may be important for morphological change. However, because all structures investigated in the present experiment expressed AR, the data also indicate that the receptors are involved with other functions.

Biogenic amine and neuropeptide inputs to identified pelvic floor motoneurons that also express SRC-1

In the rat, the neurochemical phenotypes of neurons that are presynaptic to motoneurons innervating the levator ani are poorly defined. In this study, motoneurons within the spinal nucleus of the bulbospongiosus (SNB) were revealed, using retrograde labelling, following injection of cholera toxin B subunit into the levator ani muscle. Different classes of neuron making substantial inputs onto these labelled neurons were revealed by using immunocytochemistry for dopamine beta hydroxylase, serotonin and substance P. Appositions (sites of presumptive synapses) between immunoreactive terminals and both the somata and dendrites of labelled SNB motoneurons were commonly seen suggesting that substance P, noradrenaline and serotonin are likely to exert a significant influence on the activity of perineal motoneurons and thus on sexual reflexes. Additionally, steroid receptor coactivator-1 was found to be present in the nuclei of 96% of SNB neurons retrogradely labelled from the levator ani. This suggests that practically all of the neurons that innervate the levator ani are likely to be modulated by circulating steroid hormones.

Steroid hormone masculinization of neural structure in rats: a tale of two nuclei

We review the mechanisms by which steroid hormones masculinize two different regions of the central nervous system (CNS) in rats. Although in both cases, androgens induce a male phenotype, the detailed mechanisms are remarkably different in the two models. In the spinal nucleus of the bulbocavernosus (SNB), testosterone must be present during the perinatal period to spare motoneurons and their target muscles from cell death. This masculinization of the SNB system is through activation of androgen receptors, because XY rats with a defective gene for the androgen receptor fail to develop a masculine SNB system. Interestingly, the motoneurons are spared by androgen, even though they themselves do not possess androgen receptors during the critical period for their survival. Thus, steroids can act on one part of the body to secondarily masculinize the CNS. In the posterodorsal aspect of the medial amygdala (MePD), testosterone can induce masculine development even in adulthood, indicating that there is no critical period for steroids to affect sexual differentiation of this system. In the case of the MePD, both estrogen receptors and androgen receptors appear to mediate testosterone's masculinizing influence on neural structure. The extended neural plasticity of the MePD may reflect annual "reorganization" of the brain in the seasonally breeding ancestors of laboratory rats.

Anabolic effects of androgens on muscles of female pelvic floor and lower urinary tract

PURPOSE OF REVIEW

This review discusses recently published data concerning the anabolic effects of androgens on muscle and the mechanism by which testosterone regulates body composition with special emphasis on the anabolic effects of androgens on the muscles of the pelvic floor and lower urinary tract.

RECENT FINDINGS

Androgens have direct anabolic effects on skeletal muscle. Testosterone increases lean body mass and decreases fat mass in a dose- and concentration-dependent fashion. The action of testosterone on muscle involves multiple mechanisms, including its effects on inducing protein synthesis, recruiting satellite cells, and modulating the commitment of pluripotent mesenchymal cells to myogenic lineage. Levator ani and other muscles of the pelvic floor and lower urinary tract are sensitive to the anabolic effects of testosterone. Androgen receptors are also expressed in the pelvic floor and lower urinary tract of both animals and humans. Anabolic effects of androgens may play an important role in the female pelvic-floor and lower-urinary-tract disorders. Furthermore, the interactions between androgen and nitric oxide synthase and arginase have been demonstrated, suggesting that androgens may also participate in modulating the physiological functions of lower urinary tract through nitric oxide.

SUMMARY

Androgens induce muscle hypertrophy and reduce fat mass. The action of androgens in the lower urinary tract and pelvic floor is complex and may depend on their anabolic effects, hormonal modulation, receptor expression, interaction with nitric oxide synthase, or a combination of these effects. Further studies are needed to determine the precise role of androgens in women with urinary incontinence and pelvic organ prolapse.