Rat testicular myoid cells respond to endothelin: characterization of binding and signal transduction pathway

Insights on Platelet-Derived Growth Factor Receptor α-Positive Interstitial Cells in the Male Reproductive Tract

Male infertility is a significant factor in approximately half of all infertility cases and is marked by a decreased sperm count and motility. A decreased sperm count is caused by not only a decreased production of sperm but also decreased numbers successfully passing through the male reproductive tract. Smooth muscle movement may play an important role in sperm transport in the male reproductive tract; thus, understanding the mechanism of this movement is necessary to elucidate the cause of sperm transport disorder. Recent studies have highlighted the presence of platelet-derived growth factor receptor α (PDGFRα)-positive interstitial cells (PICs) in various smooth muscle organs. Although research is ongoing, PICs in the male reproductive tract may be involved in the regulation of smooth muscle movement, as they are in other smooth muscle organs. This review summarizes the findings to date on PICs in male reproductive organs. Further exploration of the structural, functional, and molecular characteristics of PICs could provide valuable insights into the pathogenesis of male infertility and potentially lead to new therapeutic approaches.

Triterpene betulin may be involved in the acute effects of pulp and paper mill effluent on testis physiology in zebrafish

Pulp and paper mill effluent can cause changes in the morphology and energy metabolism in the zebrafish (Danio rerio) testis. Betulin, a naturally occurring triterpene is commonly present in this type of effluent and is suspected of being involved in these effects. The aim of this study was to compare the effects pulp and paper mill effluent and betulin on various aspects of testicular physiology in the zebrafish. This included the in vitro effects of effluent and betulin on testicular lactate content and lactate dehydrogenase (LDH) activity. In addition, the effects of betulin on glucose uptake, glycogen, alanine aminotransferase (ALT), reactive oxygen and nitrogen species formation and oxidative damage in the testes were determined. Furthermore, we compared the effects and mechanism of action of betulin and effluent on calcium homeostasis in testes. In vitro exposure to both effluent and betulin decreased lactate and calcium influx, possibly due to the activation of the sodium‑calcium exchanger (NCX) pump. Additionally, betulin-treated testes had higher reactive oxygen species (ROS) and reduced glutathione (GSH) content, as well as increased glutathione transferase (GST) activity and a tendency towards decreased catalase (CAT) activity. Thus, this study shows that alterations in testis physiology caused by the pulp and paper mill effluent in the testis may be due in part to the actions of betulin.

ATP activation of peritubular cells drives testicular sperm transport

Spermatogenesis, the complex process of male germ cell proliferation, differentiation, and maturation, is the basis of male fertility. In the seminiferous tubules of the testes, spermatozoa are constantly generated from spermatogonial stem cells through a stereotyped sequence of mitotic and meiotic divisions. The basic physiological principles, however, that control both maturation and luminal transport of the still immotile spermatozoa within the seminiferous tubules remain poorly, if at all, defined. Here, we show that coordinated contractions of smooth muscle-like testicular peritubular cells provide the propulsive force for luminal sperm transport toward the rete testis. Using a mouse model for in vivo imaging, we describe and quantify spontaneous tubular contractions and show a causal relationship between peritubular Ca2+ waves and peristaltic transport. Moreover, we identify P2 receptor-dependent purinergic signaling pathways as physiological triggers of tubular contractions both in vitro and in vivo. When challenged with extracellular ATP, transport of luminal content inside the seminiferous tubules displays stage-dependent directionality. We thus suggest that paracrine purinergic signaling coordinates peristaltic recurrent contractions of the mouse seminiferous tubules to propel immotile spermatozoa to the rete testis.

HGF Modulates Actin Cytoskeleton Remodeling and Contraction in Testicular Myoid Cells

The presence of the HGF/Met system in the testicular myoid cells was first discovered by our group. However, the physiological role of this pathway remains poorly understood. We previously reported that HGF increases uPA secretion and TGF-β activation in cultured tubular fragments and that HGF is maximally expressed at Stages VII–VIII of the seminiferous epithelium cycle, when myoid cell contraction occurs. It is well known that the HGF/Met pathway is involved in cytoskeletal remodeling; moreover, the interaction of uPA with its receptor, uPAR, as well as the activation of TGF-β have been reported to be related to the actin cytoskeleton contractility of smooth muscle cells. Herein, we report that HGF induces actin cytoskeleton remodeling in vitro in isolated myoid cells and myoid cell contraction in cultured seminiferous tubules. To better understand these phenomena, we evaluated: (1) the regulation of the uPA machinery in isolated myoid cells after HGF administration; and (2) the effect of uPA or Met inhibition on HGF-treated tubular fragments. Because uPA activates latent TGF-β, the secretion of this factor was also evaluated. We found that both uPA and TGF-β activation increase after HGF administration. In testicular tubular fragments, HGF-induced TGF-β activation and myoid cell contraction are abrogated by uPA or Met inhibitor administration.

NAADP-induced Ca(2+ signaling in response to endothelin is via the receptor subtype B and requires the integrity of lipid rafts/caveolae

We have investigated the role of NAADP-mediated Ca(2+) mobilization in endothelin (ET) signaling via endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) in rat peritubular smooth muscle cells. Microinjection and extracellular application of NAADP were both able to elicit Ca(2+) release which was blocked by inhibitory concentrations of NAADP, by impairing Ca(2+) uptake in acidic stores with bafilomycin, and by thapsigargin. Ca(2+) release in response to selective ETB stimulation was abolished by inhibition of NAADP signaling through the same strategies, while these treatments only partially impaired ETA-dependent Ca(2+) signaling, showing that transduction of the ETB signal is dependent on NAADP. In addition, we show that lipid rafts/caveolae contain ETA, ETB, and NAADP/cADPR generating enzyme CD38 and that stimulation of ETB receptors results in increased CD38 activity; interestingly, ETB- (but not ETA-) mediated Ca(2+) responses were antagonized by disruption of lipid rafts/caveolae with methyl-beta-cyclodextrin. These data demonstrate a primary role of NAADP in ETB-mediated Ca(2+) signaling and strongly suggest a novel role of lipid rafts/caveolae in triggering ET-induced NAADP signaling.

Oligozoospermia with normal fertility in male mice lacking the androgen receptor in testis peritubular myoid cells

Androgens and the androgen receptor (AR) play important roles in the testes. Previously we have shown that male total AR knockout (T-AR-/y) mice revealed incomplete germ cell development and lowered serum testosterone levels, which resulted in azoospermia and infertility. However, the consequences of AR loss in particular types of testicular cells remain unclear. Using a Cre-loxP conditional knockout strategy, we generated a tissue-selective knockout mouse with the AR gene deleted in testis peritubular myoid cells (PM-AR-/y). Phenotype analyses showed that PM-AR-/y mice were indistinguishable from WT AR (AR+/y) mice with the exception of smaller testes size. PM-AR-/y mice have serum testosterone concentrations comparable with AR+/y mice. PM-AR-/y mice have oligozoospermia in the epididymis; however, fertility was normal. Although normal germ cell distribution ratio was found, total germ cell number decreased in PM-AR-/y mice. Further mechanistic studies demonstrated that PM-AR-/y mice have defects in the expression of Sertoli cells' functional marker genes such as tranferrin, epidermal fatty acid-binding protein, androgen-binding protein, and other junction genes including occludin, testin, nectin, zyxin, vinculin, laminingamma3, gelsolin, connection43, and N-cadherin. Furthermore, there were defects in peritubular myoid cell contractility-related genes such as endothelin-1, endothelin receptor A and B, adrenomedullin, adrenomedullin receptor, and vasopressin receptor 1a. Together, our PM-AR-/y mice provide in vivo evidence for the requirement of functional AR in peritubular myoid cells to maintain normal Sertoli cells function and peritubular myoid cell contractility, thus ensuring normal spermatogenesis and sperm output.

Platelet-derived growth factor-BB-induced hypertrophy of peritubular smooth muscle cells is mediated by activation of p38 MAP-kinase and of Rho-kinase

Peritubular smooth muscle cells (PSMC) from rat testis in primary serum-free cultures unexpectedly undergo contraction and subsequent cell hypertrophy in response to the growth factor PDGF-BB, remaining stationary. The present study investigates the transduction pathways involved in the observed paradoxical upregulation of the differentiated phenotype and induction of hypertrophy in PSMC. PI3K, ERK, JNK, and p38 kinases, known to mediate PDGF-BB signaling in the canonic dedifferentiative and proliferative response of smooth muscle cells (SMC) were rapidly activated by PDGF-BB but only p38 remained activated after 2-day stimulation. Immunofluorescence and immunoblotting experiments showed that in 4-day treatment: (i) continuous inhibition of PI3K, of ERK, of JNK, failed to inhibit either cell enlargement and formation of prominent alpha-SM actin containing stress fibers or the typical increase in alpha-SM actin; (ii) when stimulated in the presence of the p38 inhibitor SB203580 both responses were significantly inhibited and cytofluorimetric analysis of cell size showed a remarkable reduction of the hypertrophic response. PDGF-BB was also found to activate the small GTPase RhoA and inhibition of Rho-dependent kinase ROCK by Y27632 counteracted the effects of PDGF-BB similarly to SB203580. Both the transcription factor ATF2 and the nucleosomal kinase MSK1, downstream targets of p38, were activated by PDGF-BB, but p38 inhibitor SB203580 inhibited only the phosphorylation of MSK1 which appeared unaffected by ROCK inhibitor Y27632. In concluding, p38 and the Rho-ROCK system were found to play prominent, probably independent roles in the upregulation of PSMC differentiated phenotype and induction of hypertrophy by PDGF-BB.

Immunohistochemical localisation of PDE5 in Leydig and myoid cells of prepuberal and adult rat testis

Expression of phosphodiesterase 5 (PDE5) in the rat testis at several pre and postnatal developmental stages was investigated by immunohistochemistry. The enzyme was localised in vascular smooth muscle cells, as well as in Leydig and peritubular cells. The latter were identified as myoid, based on their immunoreactivity to desmin and alpha-smooth muscle actin. The presence of PDE5 in myoid cells was confirmed by Western blot analysis and immunohistochemistry performed on highly purified cell fractions, obtained from 16-day-old rats. The expression of PDE5 in these somatic cells of rat testis is discussed in view of the roles played by cGMP signal transduction pathways in the mammalian male reproductive function.

Regulatory effect of castration on endothelins, their receptors and endothelin-converting enzyme in rat seminal vesicle

OBJECTIVE

To investigate the effects of castration on the expression of endothelins (ETs), ET receptors and ET converting enzyme-1 (ECE-1) in the rat seminal vesicle (RSV).

MATERIALS AND METHODS

Sprague-Dawley rats (3 months old) were surgically castrated or sham-operated, and then killed 7 days after surgery. Biochemical and pharmacological properties and the location of ET receptors in the RSV were determined by a series of binding experiments with [125I]ET-1, using membrane particulates and slide-mounted frozen sections of RSV. Expression levels of ETA and ETB receptor subtypes, ET-1, ET-3 and ECE-1 mRNAs were assessed by relative multiplex reverse-transcription polymerase chain reaction (RT-PCR).

RESULTS

The density of total ET receptors increased significantly in the seminal vesicle of the castrated rat. The predominance of the ETA receptor subtype in the RSV did not change with castration. Autoradiographic studies showed the presence of ET receptors on the smooth muscle and epithelium of the RSV. In addition, RT-PCR showed an up-regulation in the expression of ETA and ETB receptor subtypes, ET-1 and ECE-1 mRNAs in the seminal vesicle of the castrated rat. However, castration caused no significant change in the expression levels of ET-3 mRNA.

CONCLUSION

These findings suggest a regulatory role for testosterone in the expression of the ET receptor system in the RSV.

Expression and regulation of the prointerleukin-1alpha processing enzymes calpain I and II in the rat testis

Interleukin-1alpha (IL-1alpha) is constitutively expressed in an age- and stage-dependent manner by rat Sertoli cells. However, the mechanism of regulation of IL-1alpha is unclear in testis. We studied this regulation at the level of the enzyme calpain, a potential regulator that cleaves 32 kDa proIL-1alpha to produce mature 17 kDa IL-1alpha. Both calpain I and II were found to cleave recombinant rat testis 32proIL-1alpha in vitro. A temporary age-related increase in messenger RNA (mRNA) levels of calpain I was found in testis of 20- and 25-day-old rats, coinciding with important events of spermatogenesis and a gradual increase in IL-1alpha, while calpain II expression was constant. In response to lipopolysaccharide (LPS), calpain I protein levels were down-regulated in the seminiferous tubules, while calpain II was less affected. By contrast, the liver after LPS treatment showed up-regulated calpain I and II immunoreactive protein and reverse transcriptase chain reaction (RT-PCR) signal. Depleting Leydig cells by ethane 1,2-dimethane sulphonate treatment resulted in down-regulated calpain I mRNA and protein expression, whereas calpain II remained unchanged. In summary, there is a differential expression of calpain I and II under pathological conditions induced either by endotoxin stimuli or Leydig cell depletion, which may produce a differential effect on IL-1alpha processing.

Platelet-derived growth factor-BB stimulates hypertrophy of peritubular smooth muscle cells from rat testis in primary cultures

The tunica propria of seminiferous tubules contains a particular type of smooth muscle cell (myoid cells) arranged in a contractile epithelioid layer that is responsible for sperm and tubular fluid flow. Unlike other types of smooth muscle (SM) cells, highly purified populations of peritubular smooth muscle cells (PSMC) survive and maintain their contractile phenotype in primary cultures in controlled conditions. We used this culture model to investigate the response of the SM contractile phenotype to prolonged exposure to platelet-derived growth factor (PDGF), one of the main factors involved in vascular SM pathologies. We observed that 4-day continuous exposure of PSMC to PDGF-BB at nanomolar concentrations in plain medium enhances contractile phenotype traits and induces cell hypertrophy without inducing proliferation. In Northern and Western blotting experiments, SM-alpha-actin transcript and protein were found to be markedly increased in the PDGF-BB-treated samples, which is in line with the formation of conspicuous SM-alpha-actin-containing stress fibers. Moreover, binding sites for endothelin-1 were increased, and the calcium response to the contractile agonist, determined in single fura-2-loaded cells, was enhanced. In response to PDGF-BB, the cells underwent immediate, transient contraction, as seen in a scanning electron microscope, followed by a gradual increase in size, as evaluated by cytofluorometry, and enhancement of protein synthesis. The observed pattern of response to PDGF-BB was not accompanied by cell proliferation, as assessed by [3H]thymidine incorporation and direct cell counts. Unlike other SM cell types, in which proliferation and loss of contractile traits are induced by PDGF, chronic treatment of PSMC with this growth factor results in hypertrophy rather than hyperplasia.

Cyclic expression of endothelin-converting enzyme-1 mediates the functional regulation of seminiferous tubule contraction

The potent smooth muscle agonist endothelin-1 (ET-1) is involved in the local control of seminiferous tubule contractility, which results in the forward propulsion of tubular fluid and spermatozoa, through its action on peritubular myoid cells. ET-1, known to be produced in the seminiferous epithelium by Sertoli cells, is derived from the inactive intermediate big endothelin-1 (big ET-1) through a specific cleavage operated by the endothelin-converting enzyme (ECE), a membrane-bound metalloprotease with ectoenzymatic activity. The data presented suggest that the timing of seminiferous tubule contractility is controlled locally by the cyclic interplay between different cell types. We have studied the expression of ECE by Sertoli cells and used myoid cell cultures and seminiferous tubule explants to monitor the biological activity of the enzymatic reaction product. Northern blot analysis showed that ECE-1 (and not ECE-2) is specifically expressed in Sertoli cells; competitive enzyme immunoassay of ET production showed that Sertoli cell monolayers are capable of cleaving big ET-1, an activity inhibited by the ECE inhibitor phosphoramidon. Microfluorimetric analysis of intracellular calcium mobilization in single cells showed that myoid cells do not respond to big endothelin, nor to Sertoli cell plain medium, but to the medium conditioned by Sertoli cells in the presence of big ET-1, resulting in cell contraction and desensitization to further ET-1 stimulation; in situ hybridization analysis shows regional differences in ECE expression, suggesting that pulsatile production of endothelin by Sertoli cells (at specific "stages" of the seminiferous epithelium) may regulate the cyclicity of tubular contraction; when viewed in a scanning electron microscope, segments of seminiferous tubules containing the specific stages characterized by high expression of ECE were observed to contract in response to big ET-1, whereas stages with low ECE expression remained virtually unaffected. These data indicate that endothelin-mediated spatiotemporal control of rhythmic tubular contractility might be operated by Sertoli cells through the cyclic expression of ECE-1, which is, in turn, dependent upon the timing of spermatogenesis.

Vitamin A modulation of basement membrane production by purified testicular myoid cells

Purified myoid cells, isolated from prepubertal rat testes, cultured in a chemically defined medium for up to 1 week do not change their metabolic activities, evaluated as protein synthesis and secretion, during the culture time. We report that fibronectin, collagen IV, and laminin are synthesized and secreted by myoid cells as demonstrated by immunocytochemical and biochemical methods. The deposition of all three proteins was spotty, with different regional localizations. The effect of vitamin A on the synthesis and the secretion of the basement membrane components was also evaluated. Retinol supplementation induces a higher synthesis of fibronectin and laminin, whereas it does not change collagen IV synthesis and secretion. The secretion of the other two molecules is differentially regulated by retinol; in fact fibronectin secretion is increased, whereas laminin secretion is reduced. Similar results were obtained utilizing retinoic acid. The data we report in this paper show, for the first time, that purified testicular myoid cells synthesize and secrete fibronectin, collagen IV, and laminin and that synthesis and secretion of these components of the basement membrane are regulated by retinol. These findings reveal a new effect of vitamin A in the regulation of mammalian spermatogenesis.

Characterization of different cell populations isolated from rat testis peritubular cells

Peritubular cells, a mixed population of myoid and non-myoid cells obtained by enzymatic treatments of rat seminiferous tubules, are currently utilized as a myoid cell population. We have morphologically and biochemically compared peritubular cells and purified myoid cells isolated from prepubertal rats. The two cell populations appear morphologically similar when cultured in the presence of serum: both appear to be composed of elongated cells when observed by phase-contrast microscopy. However, the two populations differ in the percentage of alkaline phosphatase positive cell since only a minority of the cells present in the peritubular cell population are positive for alkaline phosphatase expression. Secretory activity of myoid cells is higher when compared to the peritubular cell secretion. To the contrary, fibronectin is highly synthesized and secreted by the peritubular cells, suggesting that the non-myoid cells synthesize fibronectin at a high level. Myoid cells have also been cultured in a chemically defined medium without any serum addition. In this experimental condition the cells show a polygonal shape, which remains constant during culture time. Secretory activity and fibronectin synthesis and secretion of the cells cultured without serum are lower when compared to the values obtained in the presence of serum at the beginning of the culture. Moreover, the distribution of the cell-associated fibronectin is modified by the culture conditions: spotted on cells cultured in the absence of serum and fibrillar on cells cultured with serum. We therefore conclude that peritubular cells and myoid cells are significantly different cell populations and that serum addition to the culture medium dramatically influences the morphology and the metabolic activities of the myoid cells.

Inositol lipid-mediated signalling in response to endothelin and ATP in the mammalian testis

The testis is a complex organ in which local control is achieved by signalling between its constituent cells. Herein we describe the responses of cultured rat testicular cells and a mouse Sertoli cell-line to stimulation by endothelin and ATP, and elsewhere we have shown that rat peritubular myoid cells possess phosphoinositidase C-coupled V1a-vasopressin receptors identical to those of liver (Howl, J. et al, 1995, Endocrinology 136: 2206-2213). 1. Peritubular myoid cells from pre-pubertal rats responded through ETA receptors with PtdIns(4,5)P2 hydrolysis [EC50 for endothelin-1 (ET-1) approximately 0.4 nM], elevation of intracellular [Ca2+], and tyrosine phosphorylation of a variety of cellular proteins. They also showed enhanced adenylate cyclase activity, with an EC50 for ET-1 of approximately 3 nM, also through ETA receptors. Pharmacological elevation of [cAMP] did not immediately change the ET-1-stimulated formation of inositol phosphates, but attenuated the response after several hours. 2. Pre-pubertal rat Sertoli cells showed no detectable responses to ET-1, but responded to FSH with elevated [cAMP] and to ATP with PtdIns(4,5)P2 hydrolysis. PtdIns(4,5)P2 hydrolysis was equally responsive to ATP and UTP, and so appears to be activated by P2U-purinergic receptors. This response was enhanced by protein kinase C inhibition and attenuated by PKC activation. 3. Despite its lack of effect on rat Sertoli cells in primary culture, ET-1 provoked PtdIns(4,5)P2 hydrolysis in the TM4 murine Sertoli cell line (EC50 approximately 0.6 nM), and this response was negatively regulated by protein kinase C activation. 5. No receptor-stimulated activation of phosphoinositase C was detected in 'germ cell' populations, but the non-specific G protein activator A1F4-provoked inositol phosphate accumulation in these cells, so demonstrating their potential to respond through yet to be identified G protein-coupled receptors with phosphoinositidase C activation. 6. Immunoblotting studies showed the presence in rat testis of phosphoinositidase C-beta 1 and the alpha-subunits(s) of the G-protein(s) Gq and/or G11. These studies show that testicular myoid and Sertoli cells use at least three G protein-coupled receptors (V1a-vasopressins, ETA-endothelin and P2U-purinergic) to signal through phosphoinositidase C activation, that ET-1 can activate multiple signalling pathways in myoid cells, and that the ET-1-stimulated phosphoinositidase C responses of myoid and Sertoli cells have different regulatory characteristics.