Generation of Ppp4c conditional knockout (Ppp4cflox) mouse strain
Using CRISPR/Cas9 technology, a Ppp4c allele in which exon 3 is flanked by loxP sites (Fig. 1A-D) was introduced into the mouse germ line. Expression of Cre recombinase results in deletion of exon 3 and frame-shift mutation (Fig. 1E) and generates an allele (Ppp4cΔ) leading to inactivation of PPP4C protein. Germ-line generation of the Ppp4cΔ allele was achieved by using the oocyte-specific Zp3-Cre transgene. We found that Ppp4c+/Δanimals from Ppp4c+/Δ × Ppp4c+/Δ crosses were viable and phenotypically similarly normal, however, Ppp4cΔ/Δ embryos died during the embryo stage (data not shown), indicating that PPP4 may play an essential role in embryogenesis.
Conditional knockout of Ppp4c in Sertoli cells results in impaired testis development and infertility in mice
To address the role of PPP4 in Sertoli cells and male reproduction, we introduced the Sertoli cell-expressed Amh-Cre transgene into the Ppp4cflox strain to obtain Ppp4cflox/flox,Amh-Cre males (Ppp4c Amh-cKO). A previous study reported that Cre recombinase was specifically expressed in Sertoli cells in Amh-Cre mice at E14.5 (ref. 38).
We found that Ppp4cflox/flox,Amh-Cre animals were fully viable. No gross abnormalities of external genitalia were observed in 2-month-old Ppp4cflox/flox,Amh-Cre males (Fig. 2A). However, the testes size from the Ppp4cflox/flox,Amh-Cre was dramatically smaller (Fig. 2B, C), and testes weight was only ≈4% of that of control littermates (Fig. 2D). The rest of the reproductive tract including vas deferens, epididymis, and seminal vesicles was observed in the mutants (Fig. 2B). Thus, the regression of the Müllerian duct system and Wolffian duct differentiation were largely normal. Although the body weight was not changed between mutant and control mice (Fig. 2E), the ratio of testes weight/body weight was significantly reduced in Ppp4cflox/flox,Amh-Cre males (Fig. 2F). Then, we evaluated the impact of Ppp4c-deficiency in Sertoli cells on male fertility using a successive breeding assay. Adult Ppp4cflox/flox,Amh-Cre or littermate control male mice were mated with wild-type female mice. The probability of observing a vaginal plug was not obviously different between the two groups of mice (data not shown). However, after a total of 87 matings, control mice sired 12 pups per litter, whereas the Ppp4cflox/flox,Amh-Cre male mice did not produce any progeny (Fig. 2G and H). These results indicated that Ppp4c-deficiency in Sertoli cells could injure testes development and induce male infertility in mice.
Aberrant histology of testes and non-obstructive azoospermia (NOA) of Ppp4cflox/flox,Amh-Cre males
To investigate the functions of PPP4 in spermatogenesis, the histology of testes from control and Ppp4cflox/flox,Amh-Cre mice was examined. Histologically, Ppp4cflox/flox,Amh-Cre testes from 2-month-old animals bore no resemblance to age-matched control testes. Mutant testes completely lacked the normal tubular architecture observed in control testes (Fig. 3A, Band C) and consisted primarily of scattered cells and only some tubular architecture (Fig. 3D, E andF). In addition, mature spermatozoa with crescent-shaped heads were identified in the control epididymis (Fig. 3G). In contrast, no spermatozoa were found in the Ppp4cflox/flox,Amh-Cre epididymis (Fig. 3H). This aberrant histology exhibited in Ppp4cflox/flox,Amh-Cre males was very similar to human NOA, which is one of the major causes of male infertility in humans.
Ectopic expression of FOXL2 protein in the testes of Ppp4cflox/flox,Amh-Cre mice
To examine the cell types of aberrant Ppp4cflox/flox,Amh-Cre testes, the expression of several germ and somatic cell marker proteins was examined by immunostaining and western blotting. Germ cell-specific marker MVH was detected in the germ cells of the control testes (Fig. 4A, arrow), whereas no MVH signal was detected in the Ppp4cflox/flox,Amh-Cre testes (Fig. 4B). This result indicated that the germ cell was completely lost in the PPP4C-deficient testes. In normal testes, steroidogenic enzyme 3β-HSD was specifically expressed in Leydig cells with a strong signal (Fig. 4C, arrow). However, in Ppp4cflox/flox,Amh-Cre testes, partial cells showed strong signals of 3β-HSD (Fig. 4D, arrow), but more disorganized cells showed weak signals (Fig. 4D, arrowhead). Normally, SOX9 (Fig. 4E, arrow) and WT1 (Fig. 4G, arrow) proteins are specifically expressed in Sertoli cells of seminiferous tubules. However, almost no SOX9 signals were detected in the Ppp4cflox/flox, Amh-Cre testes except for some remnant tubular architecture (Fig. 4F). In comparison, more weak signals of WT1 were observed in the mutant testes (Fig. 4H, arrow).
In mammalian females, 3β-HSD shows a low level in granulosa cells and oocytes of fetal mouse ovaries, but a higher level after theca cell recruitment and formation of the first antral follicles (ref. 12). Additionally, WT1 is initially expressed in somatic cells of bi-potential gonads. And in adults, its expression is maintained in ovarian granulosa cells and testicular Sertoli cells. However, SOX9 is exclusively expressed in Sertoli cells of testes, not in somatic cells of the ovary. Considering the above facts, we surmised that the WT1-positive or 3β-HSD-positive cells may be granulosa-like cells or theca-like cells.
To test these hypotheses, we first examined gonads of Ppp4cflox/flox,Amh-Cre males for the presence of FOXL2, a female-specific transcription factor expressed in the nuclei of granulosa cells and theca cells(ref. 11, 39, 40), two somatic cell types of the ovarian follicle (Fig. 4K). No FOXL2 signals were found in control testes (Fig. 4I). However, abundant cytoplasmic FOXL2-positive cells were present within mutant testes (Fig. 4G, arrow). Additionally, western blot results also showed FOXL2 expression in Ppp4cflox/flox,Amh-Cre testes with a larger size compared to that of the normal ovary (Fig. S1). Previous studies reported that post-translational regulation (including phosphorylation) of FOXL2 could change its subcellular localization from normal nuclear distribution to cytoplasmic mislocalization (ref. 41-44). So cytoplasmic location and larger amount of FOXL2 protein may be a result of its phosphorylation. Besides, western blot results showed that aromatase protein CYP11A1 was strongly expressed in mutant gonads, which was robustly expressed in granulosa cells of the ovary (Fig. S1). These results indicated that loss of the PPP4C in mouse Sertoli cells activated FOXL2 and reprogrammed testicular somatic cells into granulosa-like cells and theca-like cells.
PPP4C maintains SOX9 and suppresses FOXL2 expression in postnatal Sertoli cells
Next, we examined the time course of this aberrant testis development and the timing of FOXL2 induction. At postnatal day 7 (P7), although seminiferous tubules were present in Ppp4cflox/flox,Amh-Cre testes, partial tubules showed damaged architecture (Fig. S2). PPP4C and SOX9 double staining experiments showed that all SOX9-positive Sertoli cells in Ppp4cflox/flox,Amh-Cre testes were PPP4C-negtive, indicating specific deletion of PPP4C in Sertoli cells of mutant mice (Fig. S3). In control testes at P7, Sertoli cells strongly expressed SOX9, whereas FOXL2 was undetectable (Fig.5A-D, arrow). Surprisingly, in Ppp4cflox/flox,Amh-Cre testes at P7, we found some intratubular cells with typical Sertoli cell features including tripartite nucleoli, co-expressed SOX9, and weak FOXL2 (Fig.5 E-H, arrow), or lacked SOX9 and weakly expressed FOXL2 (Fig.5 E-H, arrowhead). Besides, FOXL2 expression was also observed in interstitial cells between seminiferous tubules, but not in Leydig cells of control testis. Meanwhile, granulosa cells and theca cells in the control ovary expressed FOXL2 but lacked SOX9 (Fig. 5I-L, arrowhead).
By 1-3months Ppp4cflox/flox,Amh-Cre testes increasingly lost normal tubular architecture and few SOX9-positive cells remained and most remnant cells strongly expressed FOXL2 (Fig. S4). Histological analysis of mutant gonads is shown in Fig. S2. These results showed that foetal loss of PPP4C caused postnatal Sertoli cells to lose the male-promoting SOX9 and instead express the female-promoting FOXL2.
Ppp4c deletion in Sertoli cells results in damaged formation of BTB and functional change of Sertoli cells
The above experiments found partially damaged architecture in Ppp4cflox/flox,Amh-Cre testes at P7. One of the major functions of Sertoli cells is to form the structure of the BTB, which, when disrupted, results in germ cell death and spermatogenic defects. To test whether the formation of BTB was damaged in Ppp4c-deficient testes, we detected the expression of gap junction protein Connexin 43 (CX43) which is a BTB-constituted protein that not only modulates the BTB integrity (ref. 45), but also maintains the homeostasis of the BTB via its effects on tight junction reassembly (ref. 16). Here, in the seminiferous epithelium of control males, CX43 was found to be immunolocalized between Sertoli cells and spermatogonia/primary spermatocytes (Fig. 6A-C, G-I, arrow). In contrast, no immunostaining at all was detected in seminiferous tubules from Ppp4cflox/flox, Amh-Cre mice at P7, only few interstitial cells displaying weak CX43 signals (Fig. 6D-F, arrowhead), indicating that neither Sertoli cells nor spermatogonia were able to synthesize CX43 protein. Besides, at 2-month-mutant testes, although CX43 signals were found in remnant cells, they displayed several non-continuous punctate dots signals like that in the control (Fig. 6J-L, arrow). Western blot results also showed that expression of CX43 was decreased in mutant testes compared with the control at 2 months (Fig. S1). In addition, we also found that stem cell factor (SCF), a paracrine growth factor normally produced by Sertoli cells, showed aberrant increased secretion in intratubular cells and interstitial cells of mutant testes compared with the control (Fig. S5). These results indicated that damaged formation of BTB and functional change of Sertoli cells emerged in Ppp4cflox/flox,Amh-Cre testes after PP4 inactivation.
Ppp4c deletion in Sertoli cells results in partial Sertoli cell apoptosis and further massive germ cell death
We considered that integrity disruption of the BTB would result in germ cell death and spermatogenic defects, then, cellular apoptosis in the testes was analyzed with TUNEL staining. The apoptotic cells were increased significantly in the testes of Ppp4cflox/flox,Amh-Cre testes at P7 (Fig. 7D, arrow), compared with the control (Fig. 7A, arrow). However, no obvious apoptotic difference was observed between the testes of Ppp4c mutant (Fig. 7E and F) and control (Fig. 7B and C) at 1-2 months. Furthermore, we also detected another apoptosis marker protein cleavage of caspase 3 (Fig. S6). We found that apoptotic signals in mutant testes were dramatically increased relative to the control at P7. Next, we analyzed the cell types of abundant apoptotic cells existing in Ppp4cflox/flox,Amh-Cre testes at P7. Co-staining experiments showed that a few TUNEL-positive cells expressed Sertoli cell-specific protein SOX9 (Fig. S7), and most TUNEL-positive cells expressed germ cell marker protein MVH compared with the control (Fig. S8). These results suggested that Ppp4c deletion in Sertoli cells at an early stage of development would result in partial Sertoli cell apoptosis, further massive germ cell death and spermatogenic defects.
CTNNB1 is not involved in Foxl2 expression in Ppp4cflox/flox,Amh-Cre testes
Our previous study showed that, by constitutive activation of Ctnnb1 in Sertoli cells, accumulation of CTNNB1 protein in the nuclei of Sertoli cells led to the transformation of testis Sertoli cells to ovarian granulosa-like cells by inducing Foxl2 expression (ref. 8). Here we found that CTNNB1 protein was localized at the plasma membrane, not the cell nucleus, of Sertoli cells and germ cells in Ppp4cflox/flox,Amh-Cre testes (Fig. 8D-F, arrow) at P7, similar to control testes (Fig. 8A-C, arrow). At 2 months, although CTNNB1 protein was dramatically decreased in Ppp4c-mutant testes (Fig. 8J-L, arrow) compared with the control (Fig. 8G-I, arrow), only weak signals were found at the plasma membrane of scattered cells, suggesting that CTNNB1 protein may play a cell adherent role in Ppp4c-mutant testes. These findings indicated that the transformation of testis Sertoli cells to ovarian granulosa-like cells by inducing Foxl2 expression in Ppp4cflox/flox,Amh-Cre testes is not a result of the dysregulation of CTNNB1 expression.
Phosphorylation modification of histone protein is altered in Ppp4cflox/flox,Amh-Cre testes
PPP4 belongs to the phosphoprotein phosphatases (PPPs) superfamily and evidence indicates that PPP4 may play a role in the regulation of chromatin activities. Here, western blot results showed that the histone H3 and H2A protein levels were decreased in the Ppp4cflox/flox,Amh-Cre testes compared with the control testes (Fig. S1). Surprisingly, the expression of pH3 and γH2AX was completely absent in the mutant testes, indicating that protein phosphorylation modifications of histones were changed. These results suggested that PPP4 may be involved in the aberrant testis development and the transformation of testis Sertoli cells to granulosa-like cells through regulation of chromatin activities.