Ligand-dependent regulation of retinoic acid receptor alpha in rat testis: in vivo response to depletion and repletion of vitamin A

FSH regulates RA signaling to commit spermatogonia into differentiation pathway and meiosis

BACKGROUND

Spermatogenesis is a complex process that is controlled by interactions between germ cells and somatic cells. The commitment of undifferentiated spermatogonia to differentiating spermatogonia and normal spermatogenesis requires the action of gonadotropins. Additionally, numerous studies revealed the role of retinoic acid signaling in induction of germ cell differentiation and meiosis entry.

MAIN TEXT

Recent studies have shown that expression of several RA signaling molecules including Rdh10, Aldh1a2, Crabp1/2 are influenced by changes in gonadotropin levels. Components of signaling pathways that are regulated by FSH signaling such as GDNF, Sohlh1/2, c-Kit, DMRT, BMP4 and NRGs along with transcription factors that are important for proliferation and differentiation of spermatogonia are also affected by retinoic acid signaling.

CONCLUSION

According to all studies that demonstrate the interface between FSH and RA signaling, we suggest that RA may trigger spermatogonia differentiation and initiation of meiosis through regulation by FSH signaling in testis. Therefore, to the best of our knowledge, this is the first time that the correlation between FSH and RA signaling in spermatogenesis is highlighted.

Nuclear import of NLS- RARα is mediated by importin α/β

The promyelocytic leukemia-retinoic acid receptor α (PML/RARα) is hypothesized to play a vital role in the pathogenesis of acute promyelocytic leukemia (APL). A previous study has demonstrated that PML/RARα is cleaved by neutrophil elastase (NE) in early myeloid cells, which leads to an increase in the nuclear localization signal (NLS) in RARα and in the incidence of APL. In this study, we explored the effects of NLS-RARα on acute myeloid leukemia (AML) cells and studied the mechanism of its localization. LV-NLS-RARα recombinant lentivirus and negative control LV-NC lentivirus were transfected into HL-60 cells and U937 cells while mutant NLS-RARα were transfected into U937 cells, and all groups were treated with 1α, 25-dihydroxyvitamin D3(1,25D3). The results showed that NLS-RARα was located mainly in the nucleus while mutant NLS-RARα was located in the cytoplasm. Overexpression of NLS-RARα downregulated the expression of CD11b, CD11c, CD14, and three forms of CEBPβ compared to the overexpression of NC and mutant NLS-RARα. It was speculated that the abnormal localization of NLS-RARα was mediated via importin-α/β in the pathogenesis of APL. By producing point mutations in the two NLSs in NLS-RARα, we showed that the nuclear import of NLS-RARα was mainly dependent on the NLS of the RARα portion. Subsequently, we found that importin-α1 (KPNA2)/importin-β1 (KPNB1) participates in the nuclear transport of NLS-RARα. Taken together, abnormal localization of NLS-RARα blocks the differentiation of APL cells, and nuclear localization of NLS-RARα depends on NLS of the RARα portion and is mediated via binding with importin-α/β.

The change in retinoic acid receptor signaling induced by prenatal marginal vitamin A deficiency and its effects on learning and memory

This study investigated the prenatal marginal vitamin A deficiency (mVAD)-related impairment in learning and memory and the interactions between RARα, Src and NR1. Learning and memory were assessed in adult rats that were exposed to prenatal mVAD with Morris water maze. The average escape time was longer in mVAD rats, and they passed the hidden platform fewer times during the memory retention test than normal vitamin A intake (VAN) rats. The mRNA and protein levels of RARα, Src and NR1 in mVAD rats were significantly lower than those in VAN rats. RARα and Src, but not NR1, were in the same protein complex. RA treatment-induced increase in RARα, Src and NR1 expressions in mVAD neurons was much lower than that in VAN neurons. Overexpression of RARα gene in VAN neurons induced an increase in RARα, Src and NR1 expressions, while silencing of RARα gene induced a decrease in expressions of RARα and Src, but not that of of NR1. In mVAD neurons, however, overexpression of RARα did not induce an increase in NR1 expression, while silencing of RARα gene had no effect on Src and NR1 expressions. Furthermore, inhibition of Src was associated with a decrease in NR1 expression but not that of RARα. Prenatal mVAD was associated with impaired learning and memory in adult rats. It is possible that mVAD-related decrease in RARα led to a decrease in Src expression, which in turn down-regulated NR1 expression and Ca²⁺ influx and eventually caused learning and memory deficits.

Expression and Subcellular Localization of Retinoic Acid Receptor-α (RARα) in Healthy and Varicocele Human Spermatozoa: Its Possible Regulatory Role in Capacitation and Survival

Varicocele, an abnormal tortuosity and dilation of veins of the pampiniform plexus, is the most common identifiable and correctable cause of male infertility. It is now becoming apparent that signaling through vitamin A metabolites, such as all-trans retinoic acid (ATRA), is indispensable for spermatogenesis and disruption of retinoic acid receptor-α (RARα) function may result in male sterility and aberrant spermatogenesis. Herein, we investigated by Western blot and immunogold electron microscopy the expression profiles and subcellular localization of RARα in healthy and varicocele human sperm; in addition, we analyzed the effects of ATRA on cholesterol efflux and sperm survival utilizing enzymatic colorimetric CHOD-PAP method and Eosin Y technique, respectively. In varicocele samples, a strong reduction of RARα expression was observed. Immunogold labeling evidenced cellular location of RARα also confirming its reduced expression in "varicocele" samples. Sperm responsiveness to ATRA treatment was reduced in varicocele sperm. Our study showed that RARα is expressed in human sperm probably with a dual role in promoting both cholesterol efflux and survival. RARα might be involved in the pathogenesis of varicocele as its expression is reduced in pathologic samples. Thus, ATRA administration in procedures for artificial insemination or dietary vitamin A supplementation might represent a promising therapeutic approach for the management of male infertility.

Immunolocalization of retinoic acid receptor-alpha, -beta, and -gamma, in bovine and canine sperm

Retinoic acid is an important regulator of cellular proliferation and differentiation. The action of retinoic acid is mediated by retinoic acid receptors (RARs) and the retinoid X receptors. The objective was to elucidate the protein localization and expression of RARα, RARβ, and RARγ in bull and dog sperm. Bull and dog sperm were subjected to an immunostaining procedure to determine presence of RARα, RARβ, and RARγ. We concluded that all three receptors were present in different regions of bull and dog sperm at varying levels. Protein expression in bull and dog sperm lysates was investigated using protein dot-blot analyses. The protein levels of RARα and RARγ were higher than the protein level of RARβ in bull and dog sperm. Protein sequences of RARα, RARβ, and RARγ for bull and dog were 98%, 89%, and 98%, respectively, on similarity alignment. In conclusion, the presence of RARα, RARβ, and RARγ receptors supported their role in sperm structure and function.

A novel cytoplasmic adaptor for retinoic acid receptor (RAR) and thyroid receptor functions as a Derepressor of RAR in the absence of retinoic acid

In most mammalian cells, the retinoic acid receptor (RAR) is nuclear rather than cytoplasmic, regardless of its cognate ligand, retinoic acid (RA). In testis Sertoli cells, however, RAR is retained in the cytoplasm and moves to the nucleus only when RA is supplied. This led us to identify a protein that regulates the translocation of RAR. From yeast two-hybrid screening, we identified a novel RAR-interacting protein called CART1 (cytoplasmic adaptor for RAR and TR). Systematic interaction assays using deletion mutants showed that the C-terminal CoRNR box of CART1 was responsible for the interaction with the NCoR binding region of RAR and TR. Such interaction was impaired in the presence of ligand RA, as further determined by GST pulldown assays in vitro and immunoprecipitation assays in vivo. Fluorescence microscopy showed that unliganded RAR was captured by CART1 in the cytoplasm, whereas liganded RAR was liberated and moved to the nucleus. Overexpression of CART1 blocked the transcriptional repressing activity of unliganded apoRAR, mediated by corepressor NCoR in the nucleus. CART1 siRNA treatment in a mouse Sertoli cell line, TM4, allowed RAR to move to the nucleus and blocked the derepressing function of CART1, suggesting that CART1 might be a cytoplasmic, testis-specific derepressor of RAR.

Activity of retinoic acid receptor-alpha is directly regulated at its protein kinase A sites in response to follicle-stimulating hormone signaling

Retinoic acid receptor-alpha (RARA) is crucial for germ cell development in the testis, as shown by the degenerated testis in Rara gene knockout mice, which are sterile. Similarly, FSH is known to regulate Sertoli cell proliferation and differentiation, indirectly controlling the quantity of the spermatogenic output. Interestingly, FSH inhibited, via activation of FSH receptor, cAMP, and protein kinase A (PKA), the nuclear localization and transcriptional activity of RARA. Given that retinoic acid, the ligand for RARA, is known to regulate cell proliferation and differentiation, we investigated whether FSH regulates RARA by a direct posttranslational phosphorylation mechanism. Mutagenesis of serine 219 (S219) and S369 at the PKA sites on RARA to either double alanines or double glutamic acids showed that both PKA sites are important for RARA activity. The negative charges at the PKA sites, whether they are from glutamic acids or phosphorylation of serines, decreased the nuclear localization of RARA, heterodimerization with retinoid X receptor-alpha, and the transcriptional activity of the receptor. On the other hand, the double-alanine mutant that cannot be phosphorylated at the 219 and 369 amino acid positions did not respond to cAMP and PKA activation. Wild-type and double-mutant RARA interacted with PKA, but only in the presence of cAMP or FSH. These results together suggest that FSH may regulate cell proliferation and differentiation of Sertoli cells, at least partially, by directly affecting the PKA sites of RARA and controlling the transcriptional function of the receptor.

Temporal profiling of rat transcriptomes in retinol-replenished vitamin A-deficient testis

At least in mammals, retinoic acid is a pivotal factor in maintaining the functionality of the testis, in particular, for the progression of germ cells from mitosis to meiosis. Removal of dietary vitamin A or a targeted deletion of retinoic acid receptor alpha gene (Rara), the receptor for retinoic acid, in mice, led to testicular degeneration by a dramatic loss of germ cells and a loss of control of the spermatogenic cycle. The germ cells that remained in the vitamin A deficient (VAD) rat testis were spermatogonia and a few preleptotene spermatocytes. Spermatogenesis can be reinitiated by injection of VAD rats with retinol, the metabolic precursor of retinoic acid, but to date, the functions of retinoic acid in the testis remain elusive. We have applied DNA microarray technology to investigate the time-dependent transcriptome changes that occur 4 to 24 h after retinol replenishment in the VAD rat testis. The retinol-regulated gene expression occurred both in germ cells and Sertoli cells. Bioinformatic analyses revealed time-dependent clusters of genes and canonical pathways that may have critical functions for proper progression through spermatogenesis. In particular, gene clusters that emerged dealt with: (1) cholesterol and oxysterol homeostasis, * (2) the regulation of steroidogenesis, (3) glycerophospholipid metabolism, (4) the regulation of acute inflammation, (5) the regulation of the cell cycle including ubiquitin-mediated degradation of cell cycle proteins and control of centrosome and genome integrity, and (6) the control of membrane scaffolding proteins that can integrate multiple small GTPase signals within a cell. These results provide insights into the potential role of retinoic acid in the testis.

Small ubiquitin-like modifier-2 modification of retinoic acid receptor-alpha regulates its subcellular localization and transcriptional activity

The retinoic acid receptor-alpha (Rara) gene is critical for germ cell development in the testis, as demonstrated by infertile Rara knockout male mice. The encoded protein for Rara (RARA) is expressed in both Sertoli cells and germ cells, but it is not always in the nucleus. Previously, all-trans retinoic acid (ATRA) was shown to increase the nuclear localization and transcriptional activity of RARA in Sertoli cells. Here, we identified a small ubiquitin-like modifier-2 (SUMO-2) modification as a novel posttranslational regulatory mechanism controlling the ATRA-dependent RARA subcellular localization and transcription. ATRA increased the SUMO-2 modification of RARA. In the presence of ATRA, lysine 166 (K166) and K171 of RARA were modified at a physiological concentration of SUMO-2, whereas in the absence of ATRA, K399 was the only site that was modified, but at a higher SUMO-2 concentration. However, K399 was critical for ATRA-controlled nuclear trafficking of RARA. In the presence of ATRA, a K399 mutation to arginine resulted in the cytoplasmic localization of K399R mutant, indicating that K166 and K171 sumoylations were inhibitory to nuclear localization. This may be due to SUMO/sentrin-specific peptidase 6 (SENP6) not being able to bind K399R mutant to desumoylate K166 and K171 in Sertoli cells, whereas it can bind RARA with intact K399. On the other hand, functional K166 and K171 sites for sumoylation were required for a full transcriptional activity, when K399 was intact. These results together suggest that both K166 and K171 sumoylation and desumoylation are critical for optimal RARA function.

Expression of retinoic acid receptor alpha in the germline is essential for proper cellular association and spermiogenesis during spermatogenesis

Signaling through vitamin A metabolites is indispensable for spermatogenesis, and disruption of retinoic acid receptor alpha (RARalpha) function resulted in male sterility and aberrant spermatogenesis, which resembled vitamin A deficiency. Here we investigated the lineage- and cell-specific role of RARalpha-mediated signaling during spermatogenesis using germ-cell transplantation and genetically manipulated mouse models. We demonstrated that RARalpha-deficient germ-cell stem cells were able to repopulate germ-cell-depleted wild-type testes and initiate spermatogenesis; however, improper cellular associations and abnormal sperm formation were observed. We further generated RARalpha-deficient mice that expressed RARalpha-EGFP fusion protein uniquely in haploid germ cells. Strikingly, spermatid orientation, alignment and release, as well as sperm morphology, were normal and there was a partial rescue of sterility. These data provide the first direct evidence for a distinct requirement of RARalpha-mediated retinoid signaling specifically in germ cells.

Changes in the expression and subcellular localization of RARalpha in the rat hippocampus during postnatal development

Retinoic acid receptors (RARs) are reported to mediate the effects of retinoid acid and participate in the maintenance of normal hippocampal function during embryonic and postnatal stages. RARalpha is the only one that has been reported to be continuously expressed among RARs in the CA1-CA3 areas of the hippocampus, at both the mRNA and the protein level. Here, we show the expression and subcellular localization of RARalpha in granule and pyramidal cells in various regions of the hippocampus during postnatal development of rats. We discovered that the expression level of RARalpha in postnatal hippocampal tissue gradually decreased over time with increasing developmental maturity of the nervous system. Moreover, the subcellular localization of RARalpha expression showed a phenomenon of intracellular translocation during the postnatal development period. This new discovery is inconsistent with a traditional viewpoint according to which RARalpha, as a nuclear transcription factor, is mainly expressed inside nucleus. This phenomenon suggests that RARalpha may have different actions during each stage of hippocampal development.

Effects of vitamin A deficiency on mitochondrial function in rat liver and heart

The aim of this study was to investigate comparative effects of vitamin A deficiency on respiratory activity and structural integrity in liver and heart mitochondria. Male rats were fed a liquid control diet (control rats) or a liquid vitamin A-deficient diet (vitamin A-deficient rats) for 50 days. One group of vitamin-A deficient rats was refed a control diet for 15 days (vitamin A-recovered rats). To assess the respiratory function of mitochondria the contents of coenzyme Q (ubiquinone, CoQ), cytochrome c and the activities of the whole electron transport chain and of each of its respiratory complexes were evaluated. Chronic vitamin A deficiency promoted a significant increase in the endogenous coenzyme Q content in liver and heart mitochondria when compared with control values. Vitamin A deficiency induced a decrease in the activity of complex I (NADH–CoQ reductase) and complex II (succinate–CoQ reductase) and in the levels of complex I and cytochrome c in heart mitochondria. However, NADH and succinate oxidation rates were maintained at the control levels due to an increase in the CoQ content in accordance with the kinetic behaviour of CoQ as an homogeneous pool. On the contrary, the high CoQ content did not affect the electron-transfer rate in liver mitochondria, whose integrity was preserved from the deleterious effects of the vitamin A deficiency. Ultrastuctural assessment of liver and heart showed that vitamin A deficiency did not induce appreciable alterations in the morphology of their mitochondria. After refeeding the control diet, serum retinol, liver and heart CoQ content and the activity of complex I and complex II in heart mitochondria returned to normality. However, the activities of both whole electron transfer chain and complex I in liver were increased over the control values. The interrelationships between physiological antioxidants in biological membranes and the beneficial effects of their administration in mitochondrial diseases are discussed.

Male sterility in mice lacking retinoic acid receptor alpha involves specific abnormalities in spermiogenesis

The severe degeneration of the germinal epithelium and subsequent male sterility observed in mice null for the retinoic acid receptor alpha (RARalpha) gene suggested its critical role in spermatogenesis, although the etiology and progression of these abnormalities remain to be determined. Previous studies have revealed that elongated spermatids in RARalpha(-/-) testes were improperly aligned at the tubular lumen and did not undergo spermiation at stage VIII(*). We now report a distinctive failure of step 8-9 spermatids to orient properly with regard to the basal aspect of Sertoli cells, resulting in stage VIII(*)-IX(*) tubules with randomly oriented spermatids. By in situ terminal deoxynucleotidyltransferase-mediated deoxy-UTP nick end labeling (TUNEL), we noted that elongating spermatids frequently underwent apoptosis. Immunohistochemical analysis revealed that while activated caspase-3, the primary effector caspase in the apoptotic cell death machinery, was detected in the nuclei of primary spermatocytes in the first wave of spermatogenesis and occasionally in spermatogonia of both normal and mutant testes, it was not involved in the death of elongating spermatids in RARalpha(-/-) testes. Thus, sterility in RARalpha(-/-) males was associated with specific defects in spermiogenesis, which may correlate with a failure in both spermatid release and spermatid orientation to the basal aspect of Sertoli cells at stage VIII(*) in young adult RARalpha(-/-) testis. Further, the resulting apoptosis in elongating spermatids appears to involve pathways other than that mediated by activated caspase-3.

Differential Effects of Phthalates on the Testis and the Liver1

Phthalates have been shown to elicit contrasting effects on the testis and the liver, causing testicular degeneration and promoting abnormal hepatocyte proliferation and carcinogenesis. In the present study, we compared the effects of phthalates on testicular and liver cells to better understand the mechanisms by which phthalates cause testicular degeneration. In vivo treatment of rats with di-(2-ethylhexyl) phthalate (DEHP) caused a threefold increase of germ cell apoptosis in the testis, whereas apoptosis was not changed significantly in livers from the same animals. Western blot analyses revealed that peroxisome proliferator-activated receptor (PPAR) alpha is equally abundant in the liver and the testis, whereas PPAR gamma and retinoic acid receptor (RAR) alpha are expressed more in the testis. To determine whether the principal metabolite of DEHP, mono-(2-ethylhexyl) phthalate (MEHP), or a strong peroxisome proliferator, 4-chloro-6(2,3-xylindino)-2-pyrimidinylthioacetic acid (Wy-14,643), have a differential effect in Sertoli and liver cells by altering the function of RAR alpha and PPARs, their nuclear trafficking patterns were compared in Sertoli and liver cells after treatment. Both MEHP and Wy-14,643 increased the nuclear localization of PPAR alpha and PPAR gamma in Sertoli cells, but they decreased the nuclear localization of RAR alpha, as previously shown. Both PPAR alpha and PPAR gamma were in the nucleus and cytoplasm of liver cells, but RAR alpha was predominant in the cytoplasm, regardless of the treatment. At the molecular level, MEHP and Wy-14,643 reduced the amount of phosphorylated mitogen-activated protein kinase (activated MAPK) in Sertoli cells. In comparison, both MEHP and Wy-14,643 increased phosphorylated MAPK in liver cells. These results suggest that phthalates may cause contrasting effects on the testis and the liver by differential activation of the MAPK pathway, RAR alpha, PPAR alpha, and PPAR gamma in these organs.

Different Testicular Gene Expression Patterns in the First Spermatogenic Cycle of Postnatal and Vitamin A-Deficient Rat Testis1

Spermatogenesis is a complicated process of germ cell differentiation, involving programmatic expression of diverse cell type- and developmental stage-specific genes. To date, the vitamin-A-deficiency (VAD) rats and postnatal rats are two models commonly used to study spermatogenesis. In the present study, we studied the expression of 1185 known genes in the vitamin-A-deficient and retinol-reinitiated spermatogenesis of rat testis using Clontech Atlas rat cDNA expression arrays. The mRNA expression patterns of post-vitamin-A (PVA) testis on Days 15 and 35 were compared with those of the spermatogenic arrested rat testis on Day 0. About 9% (110/1185) of the genes studied were highly expressed. When compared with VAD rat testis on Day 0, 20 and 31 genes were differentially expressed by a factor of twofold or greater on Days 15 and 35, respectively. Four genes (cytochrome P450 17, sulfated glycoprotein 2, protein kinase inhibitor, and cathepsin L) that play important roles in spermatogenesis were selected and their gene expression patterns were confirmed by semiquantitative reverse transcription-polymerase chain reaction. Comparison of the expression patterns of these genes between PVA-VAD and postnatal rat testis in developmentally matched stages revealed substantial differences during the early stages of spermatogenesis. This discrepancy could be caused by either the presence of arrested but mature somatic cells in the PVA-VAD testis that may contribute to a unique gene expression pattern in this model or the direct effect of retinol on spermatogenesis. Therefore, caution is needed in interpreting the gene expression data using the PVA-VAD and postnatal rat models in studying spermatogenesis in rat testes.

Long-term survival of neonatal porcine Sertoli cells in non-immunosuppressed rats

Sertoli cells from the testis contain immunoprotective properties which allow them to survive as allografts and also to protect islets and adrenal chromafin cells from immune rejection without the use of immunosuppressive drugs. Experiments were designed to determine whether xenogeneic neonatal porcine Sertoli cells (NPSCs) survive transplantation in rats without the use of immunosuppression. NPSCs (92.2 +/- 5.1%) were isolated, cultured and then transplanted under the kidney capsule of non-immunosuppressed Lewis rats. To assess survival, grafts were removed after 4, 20, 30, 40, 60, and 90 days post-transplant and immunostained for the Sertoli cell marker vimentin. Survival was confirmed by polymerase chain reaction (PCR) for the porcine mitochondrial cytochrome oxidase II (COII) subunit gene, a marker for porcine tissue. In both methods, NPSCs were detected in the grafts for at least 90 days. Histologically, NPSCs were clustered in small aggregates or organized in tubule-like structures. When stained for the presence of proliferating cell nuclear antigen (PCNA), many Sertoli cells stained positive at 20 days post-transplant, indicating not only cell survival but also Sertoli cell proliferation. The number of PCNA positive cells decreased somewhat by 40 days with almost no positive Sertoli cells at 60 and 90 days. These data demonstrate that NPSCs survive long-term following xenotransplantation in rats, which to our knowledge is the first report of a discordant xenograft surviving without immunosuppression in a non-immunoprivileged site. Further study of the mechanism of NPSC xenograft survival may provide clues for promoting a local tolerogenic environment.

Peroxisome proliferators disrupt retinoic acid receptor alpha signaling in the testis

Peroxisome proliferators include a diverse group of chemicals, some of which have been demonstrated to be testicular toxicants. However, the mechanism by which peroxisome proliferators, such as phthalates, cause testicular damage is not clear. It is known that retinoic acid receptor alpha (RARalpha) and its retinoic acid ligand, the acid form of vitamin A, are required for spermatogenesis. It has been demonstrated that the absence of RARalpha gene or vitamin A in the animal leads to testis degeneration and sterility. Therefore, any compound that disrupts the action of vitamin A in the testis could potentially be damaging to male fertility. The current investigation examined a novel hypothesis that a mechanism of degeneration by peroxisome proliferators in the testis is due, in part, to disruption of the critical RARalpha signaling pathway. We show that peroxisome proliferators were able to disrupt the retinoic acid-induced nuclear localization of RARalpha and the retinoic acid-stimulated increase in transcriptional activity of a retinoic acid-responsive reporter gene in Sertoli cells. Concomitantly, peroxisome proliferators increased the nuclear localization of PPARalpha and the transcriptional activity of a peroxisome proliferator-responsive reporter gene in these cells. These results indicate that peroxisome proliferators can indeed shift the balance of nuclear localization for RARalpha and PPARalpha, resulting in deactivation of the critical RARalpha transcriptional activity in Sertoli cells.

Positive regulation of retinoic acid receptor alpha by protein kinase C and mitogen-activated protein kinase in sertoli cells

Retinoic acid receptor alpha (RARalpha) is required for normal testis function. Similar to other steroid hormone receptors, RARalpha appears to undergo an activation process by which it translocates from the cytoplasm to the nucleus where it acts as a transcription factor. In this report, we demonstrate that RARalpha nuclear trafficking in Sertoli cells is positively regulated by phorbol-12-myristate-13-acetate-activated protein kinase C without the requirement of ligand, retinoic acid. Protein kinase C then stimulates the downstream mitogen-activated protein kinase, and the nuclear localization of RARalpha is dependent on activation of both kinases. The increase in RARalpha nuclear translocation is also coupled with enhanced transcriptional activity of RARalpha. This mechanism of RARalpha positive regulation is unique, different from that of its negative regulation, that has previously been shown to be dependent on cAMP-dependent protein kinase A and more importantly, dependent on its ligand. However, the mechanism by which retinoic acid positively influences the nuclear localization of RARalpha is not due to retinoic acid directly increasing protein kinase C or mitogen-activated protein kinase activities. Nonetheless, the positive influence of retinoic acid is also dependent on these two kinases as determined by inhibitor studies. These results suggest two mechanisms for RARalpha activation in Sertoli cells: one involving only the two kinases, the other involving both the ligand and the two kinases. These regulatory mechanisms for RARalpha activation, both positive and negative, may be critical for the proper function of RARalpha in the testis.

Immunohistochemical detection of the retinoid acid receptors (RXR-alpha, -beta, -gamma) and Farnesoid X-activated receptor (FXR) in the marbled newt along the annual cycle

Retinoid acid receptors (RXR-alpha, -beta, -gamma) and Farnesoid X-activated receptor (FXR) expression in the testis of the marbled newt were investigated with special attention to the changes during the annual testicular cycle, using light microscopy immunohistochemistry and Western blot analysis. The annual testicular cycle of the marbled newt (Triturus marmoratus marmoratus) comprises three periods: (a) proliferative period (germ cell proliferation from primordial germ cells to round spermatids, April-June); (b) spermiogenesis period (July-September); and (c) quiescence period (interstitial and follicular cells form the glandular tissue, October-April). In the proliferative period, primordial germ cells and primary spermatogonia immunostained intensely to the three types of RXRs and also to FXR. In the other periods, immunostaining to these antibodies was weak or absent. Secondary spermatogonia stained weakly to the four antibodies in the proliferative period, and only to FXR, also weakly, in the spermiogenesis period. Immunoreactive primary spermatocytes were weakly labeled with the RXR antibodies in the proliferative period. Spermatids and spermatozoa did not stain to any antibody in any period. Follicular cells only immunostained to RXR-gamma and only in the quiescence period when they are forming the glandular tissue, together with the interstitial cells. As follicular cells, interstitial cells only immunostained in the quiescence period; however, they immunoreacted to the three types of RXRs. These findings suggest that in the newt, RXRs and FXR are involved in spermatogenesis control by regulating the proliferation of primordial germ cells and spermatogonia. In addition, RXR-gamma seems to be also involved in the development of the glandular (steroidogenic) tissue.

Follicle-stimulating hormone and leukemia inhibitory factor regulate Sertoli cell retinol metabolism

Sertoli cells, the somatic epithelial cells of the seminiferous tubules, provide both structural and biochemical support for developing male germ cells. The Sertoli cells are targets of retinoid action in the testis. We have found that FSH, (Bu)(2)cAMP, and leukemia inhibitory factor elicit substantial changes in the metabolism of [(3)H]retinol (vitamin A) in primary cultures of purified rat Sertoli cells. Addition of (Bu)(2)cAMP for 2 h or FSH for 6 h results in a 3-fold increase in the metabolism of [(3)H]retinol to [(3)H]retinoic acid ([(3)H]RA); the esterification of [(3)H]retinol to [(3)H]retinyl esters, especially [(3)H]retinyl palmitate, is also increased by approximately 5-fold. The addition of 1 microM all-trans-RA also elicits changes in [(3)H]retinol metabolism, but in this case the metabolism of [(3)H]retinol to [(3)H]RA is inhibited, whereas the metabolism of [(3)H]retinol to [(3)H]retinyl esters is increased by over 50-fold. Leukemia inhibitory factor increases the esterification of [(3)H]retinol by 2- to 3-fold. FSH leads to a reduction in the level of cellular retinol binding protein I transcripts, whereas RA increases the cellular retinol binding protein I messenger RNA level by about 2-fold at approximately 24 h. Levels of AHD-2 (aldehyde dehydrogenase-2) and RALDH-2 (retinaldehyde dehydrogenase-2) messenger RNAs, which encode enzymes that convert [(3)H]retinaldehyde to [(3)H]RA, are increased by about 2-fold by FSH, whereas no change in CYP26 (RA hydroxylase) expression is seen. Our results suggest that one function of FSH (and/or (Bu)(2)cAMP) in Sertoli cells is to increase the metabolism of retinol to the biologically active metabolite RA and to retinyl esters.

Cellular expression of retinal dehydrogenase types 1 and 2: effects of vitamin A status on testis mRNA

We examined expression of retinal dehydrogenase (RALDH) types 1 and 2 in liver and lung, and the effect of vitamin A status on testis expression by in situ hybridization. Liver expressed RALDH1 and RALDH2 only in stellate cells and hepatocytes, respectively. Lung expressed RALDH1 and RALDH2 throughout the epithelia of the airways, from the principal bronchi to the respiratory bronchiole. Vitamin A-sufficient rats expressed RALDH1 in spermatocytes, with less intense expression in spermatogonia and spermatids, and expressed RALDH2 in interstitial cells, spermatogonia, and spermatocytes. Neither Sertoli nor peritubular cells showed detectable RALDH1 or RALDH2 mRNA. Vitamin A deficiency produced a sevenfold increase in RALDH1 and a 70-fold decrease in RALDH2 mRNA in testis. In each case, the net change reflected extensive loss of germ cells, increased intensity of expression in residual germ cells, and expression in Sertoli and peritubular cells. Low-dose RA relatively early during vitamin A depletion supported spermatogenesis and affected expression of both RALDHs, but did not reinstate "vitamin A normal" expression patterns. These results show that: RALDH1 and RALDH2 have distinct mRNA expression patterns in multiple cell types in three vitamin A target tissues; RALDH expression occurs in cell types that express cellular retinol-binding protein and retinol dehydrogenase isozymes (except stellate cells, for which retinol dehydrogenase expression remains unknown); vitamin A deficiency and RA supplementation affects the loci and intensity of RALDH mRNAs in testis; and low-dose RA does not substitute completely for retinol. Overall, these data provide insight into the unique functions of RALDH1 and RALDH2 in retinoid metabolism.

Cloning of complementary deoxyribonucleic acids encoding quail (Coturnix coturnix japonica) retinoic acid receptor ss isoforms and changes in their gene expression during gonadotropic growth

Retinoids have important effects on the development of the reproductive system, where they act via their specific nuclear receptors: retinoic acid receptors (RARalpha, ss, gamma) and retinoid X receptors (RXRalpha, ss, gamma). The research reported here was conducted in an effort to clone quail RARbeta+ cDNA (qRARbeta) and to evaluate the expression of qRARbeta+ mRNAs in different tissues and during the development of gonadotropic organs. Two complete cDNAs of qRARbeta1 and qRARbeta2 were isolated by a combination of reverse transcription-polymerase chain reaction and 5'- and 3'-rapid amplification of cDNA ends techniques. An RNase protection assay revealed the widespread expression of qRARbeta1 and beta2 with large tissue-specific variations. The qRARbeta1 isoform was predominant in the testis, whereas qRARbeta2 was dominant in the other tissues examined with the exception of the brain, where both isoforms were almost equally expressed. In the developing testes, the qRARbeta1 mRNA level was high between 30 and 40 days of age, the period during which the testes grew rapidly. The level declined thereafter to its initial level. In contrast, qRARbeta2 mRNA did not exhibit obvious changes. In the developing oviducts, both qRARbeta1 and beta2 mRNAs reached their peak levels by 30 days of age, just before the rapid development of the oviduct occurred, and then decreased to almost undetectable levels when the oviduct developed to the laying stage (over 2.88 g in weight). Similar expression patterns of qRARbeta1 and beta2 were also observed in the developing follicles from the prehierarchical (<2-mm diameter) to the largest preovulatory follicle. In contrast, neither qRARbeta1 nor beta2 mRNA exhibited developmental changes in the brain. These results suggest that RARbeta+ may play an important role in the development of the reproductive systems of birds.

Retinoic acid accelerates the development of reproductive organs and egg production in Japanese quail (Coturnix coturnix japonica)

The effects of retinoic acid on the development of reproductive organs and egg production in female Japanese quail (Coturnix coturnix japonica) were investigated. Female quail were fed a diet containing retinoic acid at 4 mg/kg (RA) or two diets containing retinyl acetate at 5000 IU/kg (VA1) or 14 000 IU/kg (VA2) after being fed a vitamin A-free diet for 2 wk (experiment 1). The oviduct and ovary grew more rapidly (P < 0.05) in RA-treated quail than in VA-treated quail at 5 wk of age. In addition, the body weight of RA-fed quail was also greater (P: < 0.05) than that of VA-fed quail at 5 wk. The RA-treated quail laid their first eggs approximately 5 days earlier (P < 0.05) than the VA-treated quail. Furthermore, these RA-fed quail laid more eggs (P < 0.05) than those VA-fed quail during the experimental period. To confirm the results of experiment 1, a similar experiment was conducted to record the first egg and total eggs laid by quail fed VA2 or RA (experiment 2). The early onset of oviposition was again observed in the RA-treated group (P < 0.01). These results suggest that retinoic acid has a stimulating effect on the reproductive system of female Japanese quail, as has been previously shown in the reproductive system of male Japanese quail.

Regulation of cell fate decision of undifferentiated spermatogonia by GDNF

The molecular control of self-renewal and differentiation of stem cells has remained enigmatic. Transgenic loss-of-function and overexpression models now show that the dosage of glial cell line-derived neurotrophic factor (GDNF), produced by Sertoli cells, regulates cell fate decisions of undifferentiated spermatogonial cells that include the stem cells for spermatogenesis. Gene-targeted mice with one GDNF-null allele show depletion of stem cell reserves, whereas mice overexpressing GDNF show accumulation of undifferentiated spermatogonia. They are unable to respond properly to differentiation signals and undergo apoptosis upon retinoic acid treatment. Nonmetastatic testicular tumors are regularly formed in older GDNF-overexpressing mice. Thus, GDNF contributes to paracrine regulation of spermatogonial self-renewal and differentiation.

Follicle-stimulating hormone inhibits all-trans-retinoic acid-induced retinoic acid receptor alpha nuclear localization and transcriptional activation in mouse Sertoli cell lines

The regulation of retinoic acid receptor alpha (RARalpha) signal transduction has not been well characterized. In this study, we determined whether all-trans-retinoic acid (tRA) and follicle-stimulating hormone (FSH) modulate RARalpha receptor subcellular localization, leading to changes in its transcriptional activity and protein expression in mouse Sertoli cell lines. We found that tRA induced the nuclear localization of RARalpha within 30 min and that longer term exposure increased the receptor transcriptional activity and RARalpha protein expression. Conversely, FSH suppressed the tRA-induced nuclear localization, transcriptional transactivation, and protein expression of RARalpha. Treatment with two different protein kinase A-selective antagonists reversed the inhibitory actions of FSH on tRA-dependent RARalpha nuclear localization and transcriptional activity. These results are consistent with the involvement of protein kinase A in mediating the inhibitory effects of FSH. For the first time, we demonstrate a unique signaling convergence between the RARalpha and the FSH-mediated signaling pathways, which may have significant implications in the testis because both are critical regulators of testis physiology.

Retinoic acid receptors and retinoid X receptors in the rat testis during fetal and postnatal development: immunolocalization and implication in the control of the number of gonocytes

Retinoids have pleiotropic effects on embryonic development and are essential for spermatogenesis in the adult, where they act via nuclear retinoid receptors: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). We used immunohistochemistry to examine the cellular localization of RARs and RXRs in the rat testis from Day 13.5 postconception (13.5 dpc) until Day 8 postpartum (8 dpp), and these findings were compared with those for immature and adult testes. RARalpha and RARbeta were detected in the interstitial tissue from 14.5 dpc, with intense staining in the gonocytes from 20. 5 dpc to 8 dpp. The nuclei of all cell types stained faintly for RARgamma from 8 dpp. Immunoreactivity for RXRalpha was intense in the gonocytes from 13.5 dpc and in the Leydig cells from 16.5 dpc, and persisted throughout the period studied. RXRbeta was always detected in the Leydig cells and during a short neonatal period in the gonocytes. RXRgamma gave a faint reaction in the nuclei of all cell types from 20.5 dpc. Unexpectedly, immunostaining for all the receptors tested, except RARgamma and RXRgamma, was detected in the cytoplasmic compartment of the cells of fetal and neonatal testes, while it was found in the nuclei in immature and adult testes. In cultures of dispersed testicular cells from 3 dpp pups, retinoic acid had a dose-dependent deleterious effect on the survival of the gonocytes and, to a lesser extent, of the somatic cells. These results suggest that retinoids act on the testicular development, especially on germ cells, via RARs and/or RXRs.

Cellular and subcellular localization of six retinoid receptors in rat testis during postnatal development: identification of potential heterodimeric receptors

Vitamin A is required in the testis for germ cell development. It acts through two families of retinoid receptors, retinoic acid receptors (RAR) and retinoid X receptors (RXR), each with three subtypes alpha, beta, and gamma. These receptors are postulated to dimerize and regulate the transcription of retinoid-responsive genes that are crucial for germ cell development. In this study, we determined the cellular and subcellular localization of six retinoid receptors in the developing rat testis to identify the specific cellular sites and times of receptor expression. Immunohistochemical results revealed the expression of RARalpha, RARbeta, RXRalpha, and RXRgamma proteins in somatic and germ cells throughout postnatal development. In contrast, the expression of RARgamma and RXRbeta did not increase until 30-35 days of age in somatic cells from the testis. Interestingly, RARalpha and RXRalpha had a similar subcellular localization pattern in Sertoli cells throughout postnatal testis development, while RARalpha and RXRgamma were both present in the nucleus of spermatocytes and elongating spermatids. These results suggest that RARalpha may potentially dimerize with RXRalpha in Sertoli cells and with RXRgamma in germ cells. In addition, we demonstrate that the only RAR in the nucleus of early meiotic germ cells is RARalpha.

Segregation of retinoic acid effects on fetal ovarian germ cell mitosis versus apoptosis by requirement for new macromolecular synthesis

Retinoic acid (RA), a naturally occurring metabolite of vitamin A, plays an essential role in regulating cellular growth, differentiation, and death in a variety of tissues, particularly during fetal development. However, essentially nothing is known of the effects of RA on fetal gametogenesis. Using a recently validated system of culturing murine fetal ovaries, herein we sought to characterize the actions of RA on female germ cell proliferation and apoptosis during oogenesis. In the absence of trophic hormone support, approximately 90% of the oogonia and oocytes present in fetal ovaries at the start of culture underwent apoptosis over a 72 h culture period (P < 0.05), whereas provision of 0.01-1 microM RA dose dependently maintained germ cell numbers. In fact, ovaries cultured with 0.1 microM RA for 72 h possessed approximately 30% more oogonia and oocytes as compared with the preculture mean number (P < 0.05). Additional experiments, using in situ DNA 3'-end-labeling and cellular morphology to assess apoptosis coupled with 5-bromo-2'-deoxyuridine incorporation to assess proliferation, revealed that RA acts as both a mitogen and a survival factor for female germ cells. Furthermore, the ability of RA to stimulate germ cell proliferation in cultured fetal ovaries was completely suppressed (P < 0.05) by cotreatment with inhibitors of transcription (alpha-amanitin, 0.1 microg/ml) or protein synthesis (cycloheximide, 1.0 microg/ml), whereas RA-mediated suppression of germ cell apoptosis was not affected by cotreatment with either macromolecular synthesis inhibitor (P > 0.05). Moreover, cotreatment of fetal ovaries with 5 microM LY294002, an inhibitor of phosphatidylinositol 3'-kinase, had no effect on RA-promoted germ cell maintenance (P > 0.05). By comparison, the antiapoptotic effects of insulin-like growth factor I on germ cells in cultured fetal ovaries were significantly attenuated by cotreating ovaries with LY294002 (P < 0.05) but not with alpha-amanitin or cycloheximide (P > 0.05). Importantly, the effect of RA on the female germ line was also observed in vivo because a single oral administration of 100 mg/kg RA to timed-pregnant female mice resulted in a significantly (P < 0.05) larger endowment of primordial oocytes in female offspring. That these actions were mediated, at least in part, by specific retinoid receptors was demonstrated by the finding of retinoic acid receptor protein in fetal female gonocytes, as assessed by immunohistochemical localization experiments. Collectively, these data indicate that RA can function, in vitro and in vivo, as a potent germ cell survival factor and mitogen during fetal oogenesis in the mouse.