Androgen receptor roles in spermatogenesis and fertility: lessons from testicular cell-specific androgen receptor knockout mice

Abstract

Androgens are critical steroid hormones that determine the expression of the male phenotype, including the outward development of secondary sex characteristics as well as the initiation and maintenance of spermatogenesis. Their actions are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily. AR functions as a ligand-dependent transcription factor, regulating expression of an array of androgen-responsive genes. Androgen and the AR play important roles in male spermatogenesis and fertility. The recent generation and characterization of male total and conditional AR knockout mice from different laboratories demonstrated the necessity of AR signaling for both external and internal male phenotype development. As expected, the male total AR knockout mice exhibited female-typical external appearance (including a vagina with a blind end and a clitoris-like phallus), the testis was located abdominally, and germ cell development was severely disrupted, which was similar to a human complete androgen insensitivity syndrome or testicular feminization mouse. However, the process of spermatogenesis is highly dependent on autocrine and paracrine communication among testicular cell types, and the disruption of AR throughout an experimental animal cannot answer the question about how AR in each type of testicular cell can play roles in the process of spermatogenesis. In this review, we provide new insights by comparing the results of cell-specific AR knockout in germ cells, peritubular myoid cells, Leydig cells, and Sertoli cells mouse models that were generated by different laboratories to see the consequent defects in spermatogenesis due to AR loss in different testicular cell types in spermatogenesis. Briefly, this review summarizes these results as follows: 1) the impact of lacking AR in Sertoli cells mainly affects Sertoli cell functions to support and nurture germ cells, leading to spermatogenesis arrest at the diplotene primary spermatocyte stage prior to the accomplishment of first meiotic division; 2) the impact of lacking AR in Leydig cells mainly affects steroidogenic functions leading to arrest of spermatogenesis at the round spermatid stage; 3) the impact of lacking AR in the smooth muscle cells and peritubular myoid cells in mice results in similar fertility despite decreased sperm output as compared to wild-type controls; and 4) the deletion of AR gene in mouse germ cells does not affect spermatogenesis and male fertility. This review tries to clarify the useful information regarding how androgen/AR functions in individual cells of the testis. The future studies of detailed molecular mechanisms in these in vivo animals with cell-specific AR knockout could possibly lead to useful insights for improvements in the treatment of male infertility, hypogonadism, and testicular dysgenesis syndrome, and in attempts to create safe as well as effective male contraceptive methods.

Figure 1

The mating strategy to generate T-AR^−/y, S-AR^−/y, L-AR^−/y, PM-AR^−/y, and G-AR^−/y mice.

Figure 2

The internal genitalia of 14-wk-old male AR^+/y (A), S-AR^−/y (B), L-AR^−/y (C), PM-AR^−/y (D), and T-AR^−/y (E). Arrows indicate the testis. [Sections of figures were reproduced from Refs. and . Copyright 2004 and 2006, respectively, Proceedings of the National Academy of Sciences.] F, Body weight, testis weight, and serum testosterone levels (mean ± sem) of 14-wk-old male AR^+/y, G-AR^−/y, PM-AR^−/y, S-AR^−/y, L-AR^−/y, and T-AR^−/y mice. ^a, Significant difference (P < 0.05; t test) compared with AR^+/y. All of these results originated from our previous publications. The other studies also showed the similar trend of serum testosterone level changes in T-AR^−/y mice and S-AR^−/y mice compared with AR^+/y controls, although this was not statistically significant due to a wide variation of serum testosterone levels between animals within the same genotype groups.

Figure 3

Analysis of germ cell DNA content of 14-wk-old male AR^+/y (A), S-AR^−/y (B), L-AR^−/y (C), PM-AR^−/y (D), and G-AR^−/y (E) mice by using flow cytometry. 1N represents haploid cells, 2N represents diploid cells, and 4N represents tetraploid cells. Compared with AR^+/y testis (A), S-AR^−/y testis showed 3-fold increase in diploid cells, 2-fold increase in tetraploid cells, and 11-fold reduced haploid cells (B); L-AR^−/y testis showed 4-fold increase in tetraploid cells and 2.8-fold reduced haploid cells (C). There were similar distributions of DNA content histogram picks between PM-AR^−/y (D), G-AR^−/y (E), and AR^+/y testis. Histology of testis by hematoxylin and eosin staining in testicular sections from 14-wk-old AR^+/y (F), S-AR^−/y (G), L-AR^−/y (H), PM-AR^−/y (I), and G-AR^−/y (J) mice. Four to six 14-wk-old mice from individual groups were killed, and testes were excised for histology section. Compared with AR^+/y testis (F), S-AR^−/y testis showed that decrease of lumen formation in seminiferous tubules as well as germ cell development stopped at diplotene primary spermatocyte (G); L-AR^−/y testis showed that decrease of lumen formation in seminiferous tubules as well as germ cell development stopped at round spermatid and no further differentiated elongated spermatid or released spermatozoa can be found (H); PM-AR^−/y (I) and G-AR^−/y (J) testis showed relatively comparable seminiferous tubule diameters and full range of germ cell development. [Sections of figures were reproduced from Refs. and . Copyright 2006, Proceedings of the National Academy of Sciences.] All of these results originated from our previous publications.

Figure 4

Diagram of germ cell progression in T-AR^−/y, S-AR^−/y, L-AR^−/y, PM-AR^−/y, G-AR^−/y, and AR^+/y testis. AR^+/y, G-AR^−/y, and PM-AR^−/y testis can achieve full germ cell progression. However, spermatogenesis in the T-AR^−/y, S-AR^−/y, and L-AR^−/y testis ceases predominately at the pachytene, diplotene, and round spermatid stages, respectively.