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. Author manuscript; available in PMC: 2016 Jan 1.
Published in final edited form as: Dev Biol. 2014 Nov 4;397(1):140–149. doi: 10.1016/j.ydbio.2014.10.020

Retinoic acid regulates Kit translation during spermatogonial differentiation in the mouse

Jonathan T Busada 1, Vesna A Chappell 1, Bryan A Niedenberger 1, Evelyn P Kaye 1, Brett D Keiper 2, Cathryn A Hogarth 4, Christopher B Geyer 1,3,5
PMCID: PMC4268412  NIHMSID: NIHMS640342  PMID: 25446031

Abstract

In the testis, a subset of spermatogonia retains stem cell potential, while others differentiate to eventually become spermatozoa. This delicate balance must be maintained, as defects can result in testicular cancer or infertility. Currently, little is known about the gene products and signaling pathways directing these critical cell fate decisions. Retinoic acid (RA) is a requisite driver of spermatogonial differentiation and entry into meiosis, yet the mechanisms activated downstream are undefined. Here, we determined a requirement for RA in the expression of KIT, a receptor tyrosine kinase essential for spermatogonial differentiation. We found that RA signaling utilized the PI3K/AKT/mTOR signaling pathway to induce the efficient translation of mRNAs for Kit, which are present but not translated in undifferentiated spermatogonia. Our findings provide an important molecular link between a morphogen (RA) and the expression of KIT protein, which together direct the differentiation of spermatogonia throughout the male reproductive lifespan.

Keywords: Translation, spermatogonia, retinoic acid, testis, spermatogenesis, Kit

INTRODUCTION

Spermatogenesis begins in the neonatal mouse testis with the transition of quiescent prospermatogonia (also called gonocytes) into a heterogeneous population of undifferentiated and differentiated spermatogonia at approximately 3-4 days postpartum (dpp). A subset of undifferentiated spermatogonia retain stem cell potential, while the rest differentiate to eventually enter meiosis as leptotene spermatocytes by ~10 dpp (reviewed in (de Rooij and Russell 2000; Oatley and Brinster 2012; Yoshida 2012)). It is unclear how spermatogonia make this critical cell fate decision, but afterwards there is a clear demarcation in the expression of specific proteins in undifferentiated (As-Aal) and differentiated (A1-) spermatogonia. One critical cell surface marker that distinguishes differentiated spermatogonia is KIT (also called c-KIT). This receptor tyrosine kinase is required for fertility (Besmer et al. 1993; Loveland and Schlatt 1997; Kissel et al. 2000), and has been particularly useful in the study of male germ cells; antibodies raised against KIT enable isolation of differentiated spermatogonia, and KIT expression is used to follow germ cell fate changes in transgenic and knockout mice or following various treatments (Prabhu et al. 2006; Mithraprabhu and Loveland 2009). KIT is bound by KIT ligand (KITL), which is expressed by neighboring Sertoli cells in the seminiferous epithelium (Loveland and Schlatt 1997). This binding activates the PI3K intracellular signaling pathway in spermatogonia, which is required for entry into meiosis (Blume-Jensen et al. 2000; Kissel et al. 2000). KIT is also essential during differentiation of a number of stem cell populations (Edling and Hallberg 2007), including embryonic stem cells (ESCs) (Bashamboo et al. 2006).

There is no consensus as to when KIT is first expressed in spermatogonia in the neonatal testis, although it is clear that Kit mRNA is present in undifferentiated spermatogonia and in prospermatogonia in the neonatal testis without detectable protein (Schrans-Stassen et al. 1999; Prabhu et al. 2006; Oatley et al. 2009; Yang et al. 2013b). The initiation of KIT protein expression in differentiating spermatogonia may involve a change in the utilization of Kit mRNAs. It has been clear for a long time that mRNAs are not all translated with equivalent efficiency (Lodish 1976); those experiencing greater initiation activity recruit more ribosomes (polyribosomes, or polysomes) will be more efficiently translated, while those that initiate poorly lack ribosomes and will not produce protein. We recently assessed changes in mRNA translation efficiency during neonatal testis development using translation state array analysis (TSAA). This is a genome-wide microarray approach in which mRNAs are identified based on changes in their association with polysomes (described in (Arava 2003)). We identified ~3,000 mRNAs encoding both germ cell-specific and housekeeping proteins that become 2:2-fold enriched in polysomes in the neonatal testis from 1 to 4 dpp without a significant change in overall mRNA abundance (Chappell et al. 2013). We found that Kit mRNAs underwent significant movement (~5-fold) into heavy polysomes over that interval, and this provides a potential mechanism for the initial expression of KIT protein in spermatogonia.

The morphogen all-trans retinoic acid (ATRA, hereafter abbreviated as RA) regulates the sex-specific timing of meiotic initiation in mice, and has been implicated in spermatogonial differentiation in the testis (Koshimizu et al. 1995; Gaemers et al. 1998; Bowles et al. 2006; Koubova et al. 2006; Bowles and Koopman 2007). Evidence supporting a role for RA in regulation of KIT expression has been largely gathered from in vitro experimentation. Although RA can increase Kit mRNA levels in vitro (Pellegrini et al. 2008; Zhou et al. 2008a; Raverdeau et al. 2012), this is not likely the primary mechanism regulating KIT protein production in vivo, as non-translating mRNA is already present in both prospermatogonia and in undifferentiated spermatogonia without detectable protein (Schrans-Stassen et al. 1999; Prabhu et al. 2006; Yang et al. 2013a; Yang et al. 2013b). We recently found that injection of RA into mice at 1 dpp (prior to endogenous exposure to RA at 3-4 dpp) induced precocious KIT protein expression in spermatogonia (Busada et al. 2014).

In this study, we identify a novel mechanism by which RA regulates KIT protein synthesis in differentiating spermatogonia. Our results indicate that Kit mRNA translates poorly in the absence of RA, but becomes recruited to polysomes in response to RA in the neonatal testis, resulting in the first appearance of KIT protein in neonatal spermatogonia. We show that the first population of KIT+ spermatogonia is also STRA8+, supporting the concept that RA exposure is required for KIT expression. However, KIT expression is not dependent upon STRA8, which suggests that there are independent RA-directed pathways regulating KIT and STRA8 expression. Importantly, we discovered that RA-activated PI3K and AKT signaling is required for induction of KIT but not STRA8. Taken together, these results suggest a novel non-genomic pathway in which RA activates the PI3K-AKT-mTOR signaling network to regulate KIT expression and spermatogonia differentiation in the neonatal testis.

MATERIALS AND METHODS

Animal treatments and tissue collection

All animal procedures were performed in accordance with the National Research Council Guide for the Care and Use of Laboratory Animals and approved by the Animal Care and Use Committee of East Carolina University (AUP #A178) and Washington State University (ASAF #1519). All analyses were performed using CD-1 mice (Charles River Laboratories), excepting Stra8 knockout mice, which were congenic C57Bl/6 (Griswold Lab). Administration of exogenous RA was done as previously described (Busada et al. 2014). Briefly, neonatal mice received one subcutaneous injection of 100 μg all-trans RA (Cat# R2625, Sigma-Aldrich) dissolved in 10 μl dimethyl sulfoxide (DMSO) or DMSO alone at 1 dpp, and were euthanized by decapitation 24 hours after injection (see Fig. S1A). To generate RA-deficient testes, neonatal mice received daily oral treatments of 100 μg/g body weight WIN 18,446, suspended in 5 μl 1% gum tragacanth, for 4 days, beginning at birth, and were euthanized by decapitation 24 hours following the final treatment. Vehicle control animals received 5 μl 1% gum tragacanth alone.

Polysome gradient analysis

Resolution of polysomes, ribosomal subunits, and initiation complexes was performed with 15-45% sucrose gradients by centrifugation as previously described (Chappell et al. 2013). Testis lysates from 1 dpp and 4 dpp mice, and mice treated with DMSO and RA were prepared as previously described (Chappell et al. 2013).

Quantitative RT-PCR

Quantitative RT-PCR analyses were performed with RNA isolated whole testis lysate and with RNA from the heavy polysome fractions (fractions 9-14). RNA was isolated from 2 dpp prospermatogonia cultured in serum-free media for 24 hours in a previous study (Hogarth et al. 2011). Fifty ng of RNA was subjected to reverse transcription and qPCR in the same reaction tube using iScript One-Step RT-PCR kit with SYBR green (BioRad). Amplification and detection of specific gene products were performed using the iCycler IQ real-time PCR detection system. Threshold temperatures were selected automatically, and all amplifications were followed by melt-curve analysis. Primers were designed on either side of exon junctions to avoid amplification of residual genomic DNA. Sequences were as follows: Rpl19 (NM_009078, 5’-GAAATCGCCAATGCCAACTC and 5’-TCTTAGACCTGCGAGCCTCA), Stra8 (NM_009292, 5’-GAGGTCAAGGAAGAATATGC and 5’-CAGAGACAATAGGAAGTGTC), Kit (NM_001122733, 5’-CATGGCGTTCCTCGCCT and 5’-GCCCGAAATCGCAAATCTTT), and B2m (NM_009735, (5’-CCGTGATCTTTCTGGTT and 5’-CGTAGCAGTTCAGTATGTTCG The following comparative quantitation equations were used to calculate relative mRNA levels for pooled polysomal fractions: mRNA level = (2ΔCT), where ΔCt = (Ct gene of interest) − (Ct min), where Ct min = the minimum Ct value within the gene set and controls. For qRT-PCR on total RNA, Ct values were normalized to either B2m (Fig. 1 and 2) or Rpl19 (Fig. 4 and Fig. S1) and relative mRNA quantities were calculated by the 2ΔΔCt, with ΔCt = (Ct untreated gene) − (Ct treated gene).

Fig. 1. KIT protein levels increase during neonatal testis development.

Fig. 1

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(A-B) Immunolabeling for KIT on mouse testis sections at 1 dpp (A) or 4 dpp (B). Sections were stained for KIT protein (green), phalloidin-594 was added to visualize F-Actin and outline the testis cords (red), and DAPI was added to visualize nuclei (blue). White arrows indicate KIT+ interstitial cells, and yellow arrows indicate KIT+ germ cells (A-B). (C) Kit amplicons from RT-PCR using total RNA isolated from 2 dpp THY1+ prospermatogonia (Psg), and whole testis lysate of mice aged 1 dpp and 4 dpp. MW = molecular weight ladder, and NT = no template control. (D) Kit mRNA abundance was assessed by qRT-PCR using total testis RNA from 1 and 4 dpp mice. (E) Kit mRNA abundance in heavy polysome fractions. Asterisk indicates statistical significance (P≤0.01). Scale bar (in A) = 40 μm.

Fig. 2. RA stimulates KIT expression in spermatogonia by increasing mRNA utilization.

Fig. 2

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(A-B) Immunolabeling for KIT on mouse testis sections treated at 1 dpp with DMSO (vehicle (A)) or RA (B) and euthanized 24 hours later. Sections were labeled with anti-KIT (green), phalloidin-594 was added to visualize testis cords (red), and DAPI stained nuclei (blue). (C) Kit mRNA levels were assessed by qRT-PCR from DMSO- and RA-treated mice. (D) Kit mRNA abundance in heavy polysomes was assessed by qRT-PCR using total testis RNA from DMSO and RA dpp mice. Scale bar (in A) = 40 μm. Asterisks indicate statistical significance with P≤0.01.

Fig. 4. KIT expression is not dependent upon STRA8.

Fig. 4

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Stra8+/− and Stra8−/− mice were injected with DMSO (A, C, E, G) or RA (B, D, F, H) at 1 dpp and then euthanized 24 hours later. (A-H) Immunolabeling was performed to detect STRA8 (green in A-B and E-F) or KIT (green in C-D and G-H). Phalloidin-594 was used in each section to visualize outline of the testis cords (red). (A-B and E-F) As expected, STRA8 was undetectable in spermatogonia in both Stra8+/− (C) and Stra8−/− (E) testes from DMSO-treated mice. However, RA induced STRA8 in spermatogonia from Stra8+/− (B) but not Stra8−/− (F) mice. (C-D and G-H) KIT protein was undetectable in spermatogonia in both Stra8+/− (C) and Stra8−/− (G) testes from DMSO-treated mice. In contrast, RA induced KIT expression in both Stra8+/− (D) and Stra8−/− (H) testes. Scale bar (in A) = 30 μm.

Immunoblotting

Immunoblotting was performed using conventional methods. Blocking and antibody incubations were done in 1X PBST + 3% BSA. Primary antibodies used were against phospho-mTOR (1:1,000, #5536, Cell Signaling Technology), phospho-EIF4EBP1 (1:1,000, #2855, Cell Signaling Technology), and RPS6 (1:2,000, #2217S, Cell Signaling Technology), and were incubated overnight at 4°C. Secondary anti-rabbit-HRP (1:3000, #7074S, Cell Signaling Technology) was incubated for 1 hour at room temperature. Blots were developed using LumiGlo detection reagent (Cell Signaling Technology).

Indirect Immunofluorescence

Sections from at least 4 testes from different mice were analyzed, and experiments performed at least thrice. Immunolabeling was done using conventional methods. Briefly, testes were fixed for 2 hours in 4% PFA and then washed and incubated overnight in 30% sucrose prior to freezing in O.C.T. Five μm sections were blocked for 30 minutes at room temperature and then incubated in primary antibody for 1 hour at room temperature or overnight at 4°C. Primary antibodies used were against the following: KIT (1:1000, #3074, rabbit monoclonal, Cell Signaling Technology) KIT (1:400, #1494, goat polyclonal, Santa Cruz Biotechnology), STRA8 (1:3000, #49602, Abcam), ZBTB16/PLZF (1:500, #22839, Santa Cruz Biotechnology), FOXO1 (1:100, #2880, Cell Signaling Technology), and phospho-mTOR (1:50, #2976, Cell Signaling Technology). Primary antibody was omitted as a negative control. Following stringency washes, sections were incubated in secondary antibody (1:500, Alexa Fluor donkey anti-rabbit-488 or anti-goat-594, Invitrogen) with or without phalloidin-635 or -594 (both at 1:500, Invitrogen) for 1 hour at room temperature. Blocking and antibody incubations were done in 1X PBS containing 3% BSA + 0.1% Triton X-100, and stringency washes were done with 1X PBS +0.1% Triton X-100. Coverslips were mounted with Vectastain containing DAPI (Vector Laboratories), and images obtained using a Fluoview FV1000 confocal laser-scanning microscope (Olympus America).

Testis explant cultures

Testes isolated from 1 dpp neonatal mice were detunicated, cut in thirds, and washed in Krebs-Ringer Bicarbonate Buffer (Sigma-Aldrich). Testis pieces were cultured in hanging drops with 30 μl of DMEM/F12 (Life Technology) containing 1X penicillin-streptomycin in a humidified incubator with 5% CO2 at 37°C. To inhibit PI3K, PDK1, or AKT, explants were pretreated for 2 hours with 20 μM Ly294002 (Sigma-Aldrich), for 1 hour with 15 μM GSK2334470 (Tocris Bioscience), or for 2 hours with 200 μM AKT1/2 inhibitor (Abcam, 142088) followed by culture for 6 hours in DMEM/F12 plus DMSO and inhibitor or 10 μM RA and inhibitor. Controls were pretreated for 2 hours with DMSO and then cultured for 6 hours in DMSO or RA. Explants were then rinsed in 1X PBS and prepared for immunolabeling as described above.

Statistics

Statistical analyses of the qRT-PCR results were performed using the Student’s t-test, and the level of significance was set at P≤0.01.

RESULTS

KIT protein is induced in spermatogonia in the neonatal testis

Cell surface expression of KIT protein is a defining feature of differentiating spermatogonia in the neonatal, pre-pubertal, and adult testis (reviewed in (Mithraprabhu and Loveland 2009)). However, there have been differing reports regarding when KIT is first detectable in neonatal spermatogonia (Yoshinaga et al. 1991; Prabhu et al. 2006; Pellegrini et al. 2008). Clarification of the onset of KIT expression will help define the originating population of differentiating spermatogonia during the first wave of spermatogenesis. We detected KIT in the neonatal testis using immunolabeling on cryosections. As shown previously, KIT protein was detectable in a subset of interstitial Leydig cells and peritubular myoid cells (Rothschild et al. 2003) as well as faintly in a small subset (6.0 ± 2.8%) of prospermatogonia in the newborn testis (Fig. 1A). By 4 dpp, prospermatogonia have transitioned to spermatogonia, and there was a dramatic increase in both the number of germ cells expressing KIT protein (38.6 ± 8.3%) and the intensity of the signal in a subset of spermatogonia (Fig. 1B).

Kit mRNA becomes more efficiently translated during spermatogonial development

Previous studies reported the presence of Kit mRNA in undifferentiated spermatogonia without detectable KIT protein (Prabhu et al. 2006; Yang et al. 2013b). We confirmed these results and found abundant Kit mRNA in isolated 2 dpp THY1+ cell fraction enriched for prospermatogonia, as well as in whole testis lysates of both 1 dpp and 4 dpp neonatal mice (Fig. 1C). We determined that there was a ~1.8-fold increase in Kit mRNA abundance in the testis from 1 to 4 dpp (Fig. 1D). This small increase in Kit mRNA was consistent with previous microarray results (Shima et al. 2004), and accompanied an approximate doubling of the germ cell population over the same interval (Chappell et al. 2013). Using qRT-PCR, we measured a 10.0-fold increase in Kit mRNA association with heavy polysomes at 4 dpp compared to 1 dpp (Fig. 1E). This robust increase in Kit mRNA translational efficiency accompanied the dramatic increase of KIT protein in spermatogonia at 4 dpp, but was only accompanied by a small increase in total Kit mRNA. Taken together, these findings indicate that Kit mRNAs underwent significant recruitment into polysomes to be translated during spermatogonia development.

RA increases KIT protein expression in spermatogonia

The dramatic increase in KIT protein at 4 dpp coincided with the onset of endogenous RA signaling (Zhou et al. 2008b; Snyder et al. 2010), and RA treatment of spermatogonial cell cultures increased KIT protein (Yang et al. 2013b). This suggested that RA might regulate KIT protein synthesis in a similar fashion in vivo. Therefore, we tested the hypothesis that RA stimulated Kit translation in vivo by injecting mice with either vehicle or 100 μg RA at 1 dpp, approximately 2 days before endogenous RA signaling (Fig. S1A). This in vivo model of RA-activated spermatogonial differentiation induced near-physiologic levels of Stra8 mRNA (Fig. S1B) (Busada et al. 2014) and induced STRA8 protein (Fig. S1C-D). In addition, exposure to exogenous RA for 24 hours increased the germ cell population by ~60% (Fig. S1E) (Busada et al. 2014).

In comparison to vehicle-treated control, there was a dramatic increase in KIT protein in response to RA (Fig. 2A-B). We next examined mRNA levels in vehicle- and RA-treated testes, and found a small 1.6-fold increase in Kit mRNA levels in response to RA (Fig. 2C). We next assessed Kit message utilization in RA-treated testes by quantifying heavy polysome-bound Kit mRNA by qRT-PCR. RA stimulated 25-fold recruitment of Kit mRNA to heavy polysomes in comparison to vehicle controls (Fig. 2D). The precocious stimulation of Kit mRNA translation in response to exogenous RA resembled that affecting Kit mRNA from 1 to 4 dpp (Fig. 1F). These results suggest a novel role for RA in stimulating translation in the neonatal testis.

KIT and STRA8 colocalize in spermatogonia in the neonatal testis

In response to RA, the population of STRA8+ spermatogonia increased dramatically from 1 to 4 dpp (Zhou et al. 2008b; Snyder et al. 2011; Busada et al. 2014). Both STRA8 and KIT are markers of differentiating/differentiated spermatogonia, and if both were upregulated in response to RA, then they should colocalize in neonatal spermatogonia. To test this, we performed co-antibody labeling against STRA8 and KIT on testis sections from mice aged 1 and 4 dpp. As shown above in Fig. 1A, KIT was faintly detectable in a very small subset of prospermatogonia at 1 dpp (Fig. 3A), similar to results for STRA8 (Fig. 3B). A subset of spermatogonia became STRA8+ at 4 dpp (Fig. 3E), and we found that 72% of these were also KIT+ (Fig. 3D and Fig. 3F, quantitated in Fig. S2). Therefore, initial expression of KIT occurred predominantly in the subset of STRA8+ germ cells, suggesting a shared relationship to RA signaling. To further test the association of RA signaling and KIT protein expression, we performed co-immunolabeling for STRA8 and KIT on testis sections from 1 dpp mice treated for 24 h with RA. Testes treated with vehicle contained few STRA8+/KIT+ spermatogonia (Fig. 3G-I). In contrast, treatment with RA resulted in a dramatic increase in both KIT+ (Fig. 3J) and STRA8+ germ cells (Fig. 3K). Of these, 80% were STRA8+/KIT+ positive (Fig. 3L and Fig. S2). The remaining 20% of labeled cells were positive for one marker only, with 7% single KIT+ and 13% single STRA8+.

Fig. 3. KIT and STRA8 colocalize in neonatal germ cells.

Fig. 3

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(A-I) Immunolabeling of mouse testis sections from mice aged 1 dpp (A-C) and 4 dpp (D-F) as well as on those from mice treated at 1 dpp with the vehicle (G-I) or RA (J-L) and euthanized 24 hours later. Antibodies were used to detect KIT (green) and STRA8 (red), and F-actin was labeled with phalloidin (blue). Red arrows indicate representative KIT+ germ cells. Green arrows indicate representative STRA8+ germ cells, and yellow arrows indicate double KIT+/STRA8+ germ cells. Scale bar (in A) = 50 μm.

KIT expression is not dependent upon STRA8

STRA8 is induced in differentiating germ cells in response to RA and is required for fertility in both sexes (Baltus et al. 2006; Koubova et al. 2006; Mark et al. 2008). However, despite years of study, the function of STRA8 in germ cell development remains unknown. Since we found KIT expression was induced by RA in STRA8+ germ cells, we asked whether stimulation of Kit mRNA translation was dependent upon STRA8 function. To address this, we injected Stra8+/− and Stra8−/− male pups (Anderson et al. 2008) at 1 dpp with RA and assessed KIT protein expression 24 hours later. As expected, RA injection induced STRA8+ spermatogonia in Stra8+/− but not Stra8-/− testes (Fig. 4A-D). In contrast, there were similar numbers of KIT+ spermatogonia in testes from both Stra8+/− and Stra8 −/− pups (Fig. 4E-H), demonstrating that RA-stimulated translation of Kit mRNAs was independent of STRA8 function.

KIT is not expressed in the absence of RA signaling

Next we tested the requirement for RA in KIT protein expression by blocking RA synthesis in mice at 0 dpp with WIN 18,446, an inhibitor of aldehyde dehydrogenase activity (Amory et al. 2011; Hogarth et al. 2013). Mice were then euthanized at 4 dpp, and we observed a 33% decrease in the germ cell population in response to WIN 18,446 (Fig. 5E-G). As expected, STRA8 protein and mRNA were present in spermatogonia from 4 dpp mice treated with vehicle, but absent in spermatogonia from mice treated with WIN 18,446 (Fig. 5A-B and 5H). Similar numbers of KIT+ Leydig and peritubular myoid cells were observed in both vehicle- and WIN 18,446-treated testes (Fig. 5C-D), indicating that RA signaling did not affect KIT expression in somatic cells. However, compared to controls, WIN 18,446 treatment blocked expression of KIT protein in spermatogonia (Fig. 5C-D), confirming the requirement for RA signaling in germ cell KIT expression. Furthermore, treatment with WIN 18,446 caused a 1.6 fold reduction in Kit mRNA compared to vehicle treated testes (Fig. 5H). This reduction in Kit mRNA is consistent with the small increase in Kit mRNA observed from 1-4 dpp (Fig. 1D) and in response to RA treatment (Fig. 2C). Therefore, we found that inhibition of RA signaling resulted in loss of KIT protein expression in spermatogonia and was accompanied by only a small decrease in mRNA levels and did affect its expression in neighboring somatic cells.

Fig. 5. Loss of RA signaling prevents both STRA8 and KIT expression.

Fig. 5

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(A-F) Immunolabeling of mouse testis sections from 4 dpp mice treated with vehicle (A, C, E) or WIN 18,446 (B, D, F). Sections were incubated with antibodies for STRA8 (green in A and B), KIT (green in C and D), or ZBTB16 (green in E and F). Phalloidin-594 was added to visualize testis cords (in red), and nuclei were stained with DAPI (blue). Yellow arrows indicate KIT+ germ cells (C) and white arrows indicate KIT+ interstitial cells (B and D). (G) ZBTB16+ cells quantitated from E-F. (H) Message levels for Stra8 and Kit were determined by qRT-PCR of RNA isolated from whole testis lysate of vehicle- or WIN 18,446-treated mice. Relative mRNA quantities were compared to WIN 18,446-treated samples, which were set at 1. Asterisks indicate statistical significance at P<0.01. Scale bar (in A) = 30 μm.

RA activates the PI3K-AKT-mTOR signaling pathway in spermatogonia

Next, we aimed to determine the mechanism by which RA stimulated the translation of Kit mRNAs during spermatogonial differentiation. PI3K-AKT-mTOR signaling is a common way by which cells regulate translation (Shimobayashi and Hall 2014), and its activation is required for spermatogonial differentiation and entry into meiosis (Goertz et al. 2011). We adapted neonatal testis explant cultures for this purpose (Fig. 6A-B), and we can recapitulate robust STRA8 and KIT expression by 6 hours (Fig. 6C-D). We tested whether RA stimulated PI3K/AKT signaling by treating explants with RA and assessing the localization of FOXO1, which becomes phosphorylated following AKT activation and relocates from the nucleus to the cytoplasm in spermatogonia (Brunet et al. 1999; Goertz et al. 2011). We found that FOXO1 was predominantly nuclear in vehicle-treated explants (Fig. 6E), but relocated to the cytoplasm in response to RA treatment (Fig. 6F). We confirmed this activation following in vivo RA treatment, and found increased levels of phospho-mTOR (Fig. S3A). Immunofluorescence labeling revealed that these changes occurred in the cytoplasm of germ cells, while the signal was not detectable in Sertoli or Leydig cells (Fig. S3B-C).

Fig. 6. RA signals through PI3K-AKT to stimulate Kit translation.

Fig. 6

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(A) Model of ex vivo testis cultures. Testes are removed from 1 dpp mice cut into pieces and cultured in hanging drops in the presence of the vehicle or RA. (B) Ex vivo testis cultures are pre-treated for 2 hours with inhibitors for either PI3K or AKT followed by treatment with vehicle or RA (+) the inhibitor for 6 hours. (C-F) Immunolabeling was performed on testes cultured with the vehicle or RA for 6 hours. Sections were stained for KIT (Green, C and D) and STRA8 (Red, C-D) or for FOXO1 (Green, E-F). Phalloidin was added to visualize testis cords (Blue in C-D, and Red in E-F). (G-J) Testis cultures were pretreated with inhibitors of PI3K (G,I) or AKT (H,J) for two hours followed by co-culture with the vehicle or RA and inhibitor for 6 hours. Immunolabeling was performed on testis sections stained for FOXO1 (Green G-H) or KIT (Green, I-J) and STRA8 (Red, I-J). Phalloidin was added to visualize testis cords (Blue in G-H, and Red in I-J). Scale bar (in C) = 40 μm.

To test the requirement for the PI3K-AKT signaling pathway in stimulation of KIT protein expression, we pharmacologically inhibited PI3K, PDK1 (a kinase that acts downstream of PI3K to activate AKT and other substrates), and AKT and then treated with RA for 6 h (Fig. 6B). The exclusive nuclear localization of FOXO1 following treatment with the inhibitors verified successful inhibition (Fig. 6G-H and Fig. S3D). The inhibition of PI3K, PDK1 and AKT followed by treatment with RA did not affect induction of STRA8 (Fig. 6I-J and Fig. S3E) but blocked induction of KIT (Fig. 6I-J and Fig. S3E). Taken together, these results demonstrate that RA-induced activation of the PI3K-AKT-mTOR signaling pathway is required for expression of KIT.

DISCUSSION

The newborn mouse testis contains an apparently homogenous population of quiescent prospermatogonia, which transition into type A spermatogonia after birth in the mouse. These type A spermatogonia either remain as SSCs or differentiate during the first wave of spermatogenesis. Currently, little is known about the mechanisms that control these germ cell fate decisions. However, it is known that RA is required for the differentiation of spermatogonia and their entry into meiosis (Bowles et al. 2006; Koubova et al. 2006; Bowles and Koopman 2007; MacLean et al. 2007; de Rooij and Griswold 2012). In this study, we examined the relationship between RA signaling and expression of the receptor tyrosine kinase KIT in differentiating spermatogonia in the neonatal testis. We found that RA induced the recruitment of Kit mRNAs into polysomes for efficient translation through activation of the PI3K-AKT-mTOR signaling pathway in differentiating spermatogonia. The results from this study provide a critical link between RA and KIT, two key determinants of spermatogonial differentiation, and demonstrate a novel mechanism by which RA regulates gene expression through translational control.

There is currently a great deal of interest in determining the molecular mechanisms by which the mammalian SSC population is established after the initiation of spermatogenesis at 3-4 dpp. It is known that the undifferentiated state is maintained by signals such as glial cell-derived neurotrophic factor (GDNF), which is received from Sertoli cells by the subset of spermatogonia expressing the co-receptors GFRA1 and RET (Meng et al. 2000; Kubota et al. 2004; Naughton et al. 2006). Equally important, however, may be the avoidance of differentiation signals. The only known ligand-receptor interaction required for spermatogonial differentiation is KIT ligand (KITL) and KIT. Sertoli cells provide KITL, which is received by the subset of spermatogonia that express KIT. In this study, we show how RA directs expression of KIT protein, which provides a direct link between RA signaling and spermatogonial cell fate determination. After 3-4 dpp, testis cords become differentially exposed to RA (Snyder et al. 2010), as shown in Fig. 7 by testis cord-specific STRA8 and KIT expression. It is not clear how specific cords avoid RA exposure, although it may be through continuous expression of the CYP26 enzymes (predominantly CYP26A1 and CYP26B1), which protect prospermatogonia from RA in the fetal testis (Bowles et al. 2006; Koubova et al. 2006; MacLean et al. 2007). The presence of an RA-free environment is likely to be a critical facilitator or determinant of SSC development.

Fig. 7. RA may signal spermatogonia differentiation by activating both genomic and non-genomic pathways.

Fig. 7

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(A) Co-immunolabeling was performed for STRA8 (red) and KIT (green) in 4 dpp testes, and phalloidin was added to visualize testis cords (blue). Cords exposed to RA (as indicated by STRA8+/KIT+ spermatogonia) are circled in yellow, and those not exposed are encircled in white. (B) A testis cord, such as the ones shown in A, is depicted in longitudinal form. Regional exposure to RA would result in the differentiation of subpopulations of spermatogonia (yellow bands), while the other areas of the cord are unexposed so that those spermatogonia remain undifferentiated (white band). (C) Diagram of the canonical genomic RA signaling pathway, in which RA regulates transcription by binding to retinoic acid receptors (RAR) at retinoic acid response elements (RARE) in the promoter of target genes such as Stra8. (D) In the proposed alternative non-genomic pathway, RA regulates translation by binding to RARs, which activate the PI3K-AKT-mTOR signaling cascade to stimulate translation of mRNAs such as Kit.

KIT is essential for the spermatogonia transition from mitosis to meiosis (Kissel et al. 2000), and KIT expression provides a hallmark to identify differentiated (KIT+) spermatogonia. However, it is currently unclear what regulatory steps direct the expression of this critical differentiation signal. It has been established that Kit mRNA is present in the neonatal testis and in undifferentiated spermatogonia without detectable protein (Schrans-Stassen et al. 1999; Koubova et al. 2006; Oatley and Brinster 2006; Prabhu et al. 2006; Grasso et al. 2012; Yang et al. 2013b), and our results here support those findings. It is curious that prospermatogonia and undifferentiated spermatogonia transcribe the Kit gene, only to prevent translation of resulting mRNAs. Importantly, KIT protein regulates survival, proliferation, and migration (reviewed in (Mauduit et al. 1999)) of male germ cells before (primordial germ cells, or PGCs) and after (differentiated spermatogonia) those in which it is repressed (prospermatogonia and undifferentiated spermatogonia) (Mithraprabhu and Loveland 2009). It is possible that germ cells lack the requisite controls to alternatively activate Kit transcription in PGCs, repress it for ~1 week in prospermatogonia and undifferentiated spermatogonia, and then reactivate it in differentiating spermatogonia. A mechanism has been proposed for the repression of Kit transcription in spermatogonia from 7 dpp mice through an interaction of ZBTB16/PLZF with the Kit promoter (Filipponi et al. 2007; Hobbs et al. 2010). However, this is unlikely to repress Kit in the neonatal testis in vivo, as both molecules are co-expressed in spermatogonia through at least 6 dpp ((Busada et al. 2014), Niedenberger et al., in preparation, Hermann et al., in preparation).

In the adult testis, it was recently shown that, in seminiferous tubule sections cultured in the presence of GDNF, Kit mRNA levels increased during specific stages II-V but did not result in KIT protein (Grasso et al. 2012). Interestingly, it is inferred from mRNA localization studies that these stages have low levels of endogenous RA exposure (Vernet et al. 2006; Sugimoto et al. 2012). KIT protein is then detectable in stages VII-VIII as spermatogonia differentiate from Aal-A1 (Oakberg 1971; Schrans-Stassen et al. 1999). Given the results from this current study, it seems likely that the same mechanism of RA-induced translational control is at work in the adult testis in a stage-specific manner, coinciding with hypothesized cyclical changes in RA levels (Hogarth and Griswold 2013).

RA-stimulated KIT protein expression in a subset of cultured undifferentiated spermatogonia was recently shown to involve the microRNAs miR-221/222 (Yang et al. 2013b). We saw a much more robust response in vivo, which may be due in part to the addition of growth factors to the culture medium such as GDNF and bFGF in that study; both factors maintain spermatogonial stem cells (SSCs) in an undifferentiated state (Kubota et al. 2004), and may oppose endogenous mechanisms regulating normal KIT expression, including the upregulation of miR221/222 (Yang et al. 2013b). While it is likely that repression of Kit translation is regulated in part by miRNAs in vivo, they are unlikely to be the primary mechanism, as evidenced by the observation that spermatogonial stem cell renewal and mitotic proliferation appear unaffected in the miRNA-lacking Dicer1-null mice (Romero et al. 2011), which would not be expected if KIT expression was largely misregulated.

It has been known for a long time that mRNAs are not all translated with equivalent efficiency (Lodish 1976); those that recruit more ribosomes (polyribosomes, or polysomes) will be more efficiently translated, while those that lack ribosomes will not produce protein. A recent study reported that mRNA translational efficiency is likely a more accurate predictor of the proteome than mRNA levels (Schwanhausser et al. 2011). In accordance with this, we recently identified a large number of mRNAs that undergo recruitment to polysomes without significant changes in abundance during neonatal germ cell development (Chappell et al. 2013). This suggests that the initial differentiation of neonatal mammalian male germ cells may be regulated in large part by translational control.

The most well studied action of RA is genomic, in which transcription is directed through RA receptors (RARs) bound to RA response elements (RAREs) present in the promoters of target genes such as Stra8 ((Bowles and Koopman 2007) and Fig. 7B). However, relatively modest changes in the transcriptome after RA exposure (Shima et al. 2004; Zhou et al. 2008a) suggests other avenues of RA-based regulation may be utilized in the neonatal testis. It has become clear that there are also non-genomic consequences of signaling downstream of RA (reviewed in (Al Tanoury et al. 2013)), as with other hormone-receptor interactions, examples of which involve signaling through the estrogen receptor (ER) (Levin 2014), androgen receptor (AR) (Walker 2003), and glucocorticoid receptor (GR) (Revollo and Cidlowski 2009). RAR alpha (RARA) was shown in several cell types (SH-SY5Y, NIH3T3, and MEFs) to be bound to the regulatory subunit (p85) of PI3K; addition of RA recruited the catalytic subunit (p110) to induce rapid phosphorylation of ERK and AKT (Lopez-Carballo et al. 2002; Masia et al. 2007). We provide compelling evidence here that RA-induced activation of PI3K/AKT signaling is required for translational control leading to production of KIT protein (Fig. 7D). The PI3K/AKT signaling network is complex, and has been shown to be involved in both self-renewal of SSCs downstream of GDNF (Lee et al. 2007) as well as in spermatogonial differentiation downstream of KIT (Kissel et al. 2000). PI3K signaling is also required to maintain mouse embryonic stem cells (mESCs) in an undifferentiated state. However, our data suggest that RA activates the mTOR signaling axis, which regulates translation and was not shown to regulate pluripotency in mESCs (Watanabe et al. 2006). Together, this reveals that the PI3K-AKT signaling network interprets a variety of signals to direct multiple different cell fate decisions.

Our results reveal an important link between a nearly ubiquitous morphogen (RA) and a molecule (KIT) that is essential for directing spermatogonial differentiation. We identify a novel mechanism by which RA regulates gene expression in the neonatal testis, and this discovery significantly advances our understanding of the molecular mechanisms controlling spermatogonia differentiation.

Supplementary Material

1

Fig. S1. RA injection induces Stra8 expression and increases the germ cell population. (A) Mice were injected at 1 dpp and euthanized 24 hours later, which is 1-2 days prior to endogenous RA signaling. (B) Stra8 mRNA levels were assessed by qRT-PCR in vehicle- and RA-treated testes and compared to 1 and 4 dpp testes. (C-D) STRA8 protein was detected (green) in testis sections from mice treated with the vehicle (C) or RA (D). Phalloidin-594 was added to visualize testis cords (red). (E) ZBTB16+ germ cells were quantitated in testis sections from mice treated at 1 dpp with the vehicle or RA. Asterisks indicate statistical significance at P≤0.01. Scale bar (in C) = 40 μm.

2

Fig. S2. Extent of STRA8 and KIT colocalization in the developing testis and in response to precocious RA. STRA8+ and KIT+ germ cells were quantified in testis sections from 4 dpp and 1 dpp mice treated for 24 h with RA. STRA8+, KIT+, and STRA8+/KIT+ germ cells were counted and presented as a percentage of labeled germ cells.

3

Fig. S3. RA treatment stimulates activation of mTOR. (A) Immunoblots of whole testis lysate from 1 dpp testes treated with the vehicle or RA for 24 hours. Blots were probed for phospho-mTOR, phospho-EIF4EBP, and RPS6. (B-C) Immunolabeling of 1 dpp testes cultured in vehicle (B) or RA treated (C) hanging drops for six hours. Sections were probed for phospho-mTOR (green) and phalloidin was added to visualize testis cords (red). (D-E) Testes from 1 dpp mice were cultured in hanging drop pretreated with an PDK1 inhibitor followed by culture for 6 h in the presence of RA + inhibitor (D-E). Testis sections were probed with antibodies against FOXO1 (green, D) or co-immunolabeled for KIT (green) and STRA8 (red, E). Scale bar = 50 μm.

HIGHLIGHTS.

  • Retinoic acid (RA) is required for KIT activation in neonatal spermatogonia in vivo

  • RA induces efficient translation of Kit mRNAs independent of STRA8 function

  • RA-induced KIT activation requires signaling through the PI3K/AKT pathway

  • RA increases phosphorylation of mTOR and EIF4EBP1

ACKNOWLEDGEMENTS

The authors thank Joani Zary-Oswald for technical assistance. The Stra8−/− mouse line was kindly provided by Dr. David Page (Whitehead Institute, MIT). This work was supported by a grant from the National Institutes of Health (HD072552) to C.B.G.

Footnotes

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Associated Data

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Supplementary Materials

1

Fig. S1. RA injection induces Stra8 expression and increases the germ cell population. (A) Mice were injected at 1 dpp and euthanized 24 hours later, which is 1-2 days prior to endogenous RA signaling. (B) Stra8 mRNA levels were assessed by qRT-PCR in vehicle- and RA-treated testes and compared to 1 and 4 dpp testes. (C-D) STRA8 protein was detected (green) in testis sections from mice treated with the vehicle (C) or RA (D). Phalloidin-594 was added to visualize testis cords (red). (E) ZBTB16+ germ cells were quantitated in testis sections from mice treated at 1 dpp with the vehicle or RA. Asterisks indicate statistical significance at P≤0.01. Scale bar (in C) = 40 μm.

NIHMS640342-supplement-1.tif (3.1MB, tif)
2

Fig. S2. Extent of STRA8 and KIT colocalization in the developing testis and in response to precocious RA. STRA8+ and KIT+ germ cells were quantified in testis sections from 4 dpp and 1 dpp mice treated for 24 h with RA. STRA8+, KIT+, and STRA8+/KIT+ germ cells were counted and presented as a percentage of labeled germ cells.

NIHMS640342-supplement-2.tif (839.6KB, tif)
3

Fig. S3. RA treatment stimulates activation of mTOR. (A) Immunoblots of whole testis lysate from 1 dpp testes treated with the vehicle or RA for 24 hours. Blots were probed for phospho-mTOR, phospho-EIF4EBP, and RPS6. (B-C) Immunolabeling of 1 dpp testes cultured in vehicle (B) or RA treated (C) hanging drops for six hours. Sections were probed for phospho-mTOR (green) and phalloidin was added to visualize testis cords (red). (D-E) Testes from 1 dpp mice were cultured in hanging drop pretreated with an PDK1 inhibitor followed by culture for 6 h in the presence of RA + inhibitor (D-E). Testis sections were probed with antibodies against FOXO1 (green, D) or co-immunolabeled for KIT (green) and STRA8 (red, E). Scale bar = 50 μm.

NIHMS640342-supplement-3.tif (13MB, tif)

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