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Androgens have well-established anabolic actions on skeletal muscle, although the direct effects of the androgen receptor (AR) in muscle remain unclear. We generated satellite cell-specific AR-knockout (satARKO) mice in which the AR is selectively ablated in satellite cells, the muscle precursor cells. Total-limb maximal grip strength is decreased by 7% in satARKO mice, with soleus muscles containing ∼10% more type I fibers and 10% less type IIa fibers than the corresponding control littermates. The weight of the perineal levator ani muscle is markedly reduced (-52%). Thus, muscle AR is involved in fiber-type distribution and force production of the limb muscles, while it is a major determinant of the perineal muscle mass. Surprisingly, myostatin (Mstn), a strong inhibitor of skeletal muscle growth, is one of the most androgen-responsive genes (6-fold reduction in satARKO) through direct transcription activation by the AR. Consequently, muscle hypertrophy in response to androgens is augmented in Mstn-knockout mice. Our finding that androgens induce Mstn signaling to restrain their own anabolic actions has implications for the treatment of muscle wasting disorders.-Dubois, V., Laurent, M. R., Sinnesael, M., Cielen, N., Helsen, C., Clinckemalie, L., Spans, L., Gayan-Ramirez, G., Deldicque, L., Hespel, P., Carmeliet, G., Vanderschueren, D., and Claessens, F. A satellite cell-specific knockout of the androgen receptor reveals myostatin as a direct androgen target in skeletal muscle., (© FASEB.)

The androgen receptor (AR) recognizes two types of DNA elements that are dimers of 5'-AGAACA-3'-like hexamers, either organized as inverted or direct repeats. We developed a mouse model [(specificity affecting AR knock-in (SPARKI)] in which the AR DNA-binding domain was mutated such that it lost binding to direct repeats but not to inverted elements. The impaired fertility of the male SPARKI mice correlates with the reduced motility of the spermatozoa, a characteristic that is developed during transit through the epididymis. Comparative transcriptome analyses revealed that the expression of 39 genes is changed in SPARKI epididymis. Remarkably, the expression of the steroid 5α-reductase type II (Srd5α2) gene, which metabolizes testosterone into the more potent dihydrotestosterone, is reduced 4-fold in SPARKI vs. wild type. The comparison of the SPARKI phenotype with that of Srd5α2-knockout mice shows, however, that the reduced Srd5α2 expression cannot explain all defects of the SPARKI epididymis. Moreover, we describe three new selective androgen response elements (AREs), which control the androgen responsiveness of the Srd5α2 gene. We conclude that the SPARKI model can be considered a knockout model for AR functioning via selective AREs and that this has a dramatic effect on sperm maturation in the epididymis.