Androgen Action via the Androgen Receptor in Neurons Within the Brain Positively Regulates Muscle Mass in Male Mice

Abstract

Although it is well established that exogenous androgens have anabolic effects on skeletal muscle mass in humans and mice, data from muscle-specific androgen receptor (AR) knockout (ARKO) mice indicate that myocytic expression of the AR is dispensable for hind-limb muscle mass accrual in males. To identify possible indirect actions of androgens via the AR in neurons to regulate muscle, we generated neuron-ARKO mice in which the dominant DNA binding-dependent actions of the AR are deleted in neurons of the cortex, forebrain, hypothalamus, and olfactory bulb. Serum testosterone and luteinizing hormone levels were elevated twofold in neuron-ARKO males compared with wild-type littermates due to disruption of negative feedback to the hypothalamic-pituitary-gonadal axis. Despite this increase in serum testosterone levels, which was expected to increase muscle mass, the mass of the mixed-fiber gastrocnemius (Gast) and the fast-twitch fiber extensor digitorum longus hind-limb muscles was decreased by 10% in neuron-ARKOs at 12 weeks of age, whereas muscle strength and fatigue of the Gast were unaffected. The mass of the soleus muscle, however, which consists of a high proportion of slow-twitch fibers, was unaffected in neuron-ARKOs, demonstrating a stimulatory action of androgens via the AR in neurons to increase the mass of fast-twitch hind-limb muscles. Furthermore, neuron-ARKOs displayed reductions in voluntary and involuntary physical activity by up to 60%. These data provide evidence for a role of androgens via the AR in neurons to positively regulate fast-twitch hind-limb muscle mass and physical activity in male mice.

The Role of Central Androgen Receptor Actions in Regulating the Hypothalamic-Pituitary-Ovarian Axis

“…Hence, the observed effects in this NeurARKO model are due to the selective loss of AR function specifically in the brain and pituitary. Furthermore, in support of our findings, using quantitative real-time PCR analysis in the same mouse model, Davey et al[41] observed a > 99% reduction in AR exon 3 mRNA in the cortex, hypothalamus, forebrain, and olfactory bulb, a 77% decrease in the cerebellum, and a 35% reduction in the pituitary.…”

“…Interestingly, our NeurARKO (which exhibits a 35% reduction of pituitary AR expression) did not present with reduced LH in the afternoon of proestrus. This inconsistency may be explained by the presence of more functional AR mRNA in the pituitary in our model [41], possibly due to an earlier time of collection prior to the onset of the LH surge or potential hypothyroidism in the PitARKO model due to this model being created by the use of gonadotropin alpha subunit (αGSU)-Cre that is expressed in not only pituitary gonadotropes but also the thyrotropes. E2-treated NeurARKO females lack an LH surge, and while these mice appeared to cycle at a normal frequency, the cycles appear more irregular than those of WT females.…”

Androgen Receptor in Neurons Slows Age-Related Cortical Thinning in Male Mice

“…In agreement, wheel running activity, which we have previously established as a sensitive marker of androgen activity, remained unaltered after AR inactivation in neurons. However, seminal vesicle weight, a frequently used readout for androgen levels, was increased in N‐ARKO mice, despite less than in previous N‐ARKO mouse models with a complete neuronal AR inactivation in the hypothalamus . Because seminal vesicle weight might be more sensitive than serum T reflecting changes in androgen activity in male rodents, we determined the bone phenotype of N‐ARKO mice after ORX and hormone replacement in order to rule out potential confounding from subtle alterations in sex steroid levels.…”

“…Here, inactivation of AR in neurons did not recapitulate the detrimental effects of androgen deficiency on hindlimb muscle mass, grip strength, and wheel running activity, thus excluding the possibility that the bone phenotype of N‐ARKO mice was secondary to a loss of muscle function. Our results differ from those obtained by Davey and colleagues showing that AR signaling in brain, including hypothalamic regions, positively regulates hindlimb muscle mass. Whether differences between studies reflect a role for hypothalamic AR remains to be determined.…”

“…However, seminal vesicle weight, a frequently used readout for androgen levels, (24) was increased in N-ARKO mice, despite less than in previous N-ARKO mouse models with a complete neuronal AR inactivation in the hypothalamus. (41) Because seminal vesicle weight might be more sensitive than serum T reflecting changes in androgen activity in male rodents, (44) we determined the bone phenotype of N-ARKO mice after ORX and hormone replacement in order to rule out potential confounding from subtle alterations in sex steroid levels. N-ARKO mice receiving a fixed amount of T displayed a similar accelerated cortical thinning to that observed in gonadally intact animals, demonstrating that a disturbed HPG axis was not interfering with our observations.…”