Expression of the enkephalin precursor gene in rat Sertoli cells. Regulation by follicle-stimulating hormone

The opioid peptide leucine enkephalin modulates hypothalamic-hypophysial axis in the cichlid fish Oreochromis mossambicus

In vertebrates, opioid peptides are thought to be involved in the regulation of reproduction; however, the significance of enkephalins in testicular function remains unclear. We examined the influence of δ-opioid receptor agonist leucine enkephalin (L-ENK) on the hypophysial-testicular axis of the cichlid fish Oreochromis mossambicus. Treatment with a low dose of L-ENK (60 µg) caused a significant increase in the numbers of primary and secondary spermatocytes and early and late spermatids, concomitant with intense immunolabelling of testicular androgen receptors, but did not significantly alter serum luteinizing hormone (LH) and 11-ketotestosterone (11-KT) levels compared to those of controls. Nevertheless, treatment with a high dose of L-ENK (200 µg) caused a significant reduction in the numbers of secondary spermatocytes as well as late spermatids associated with marginal immunolabelling of androgen receptors and significantly lower concentrations of serum 11-KT and LH compared to controls. In addition, the serum cortisol level was not affected in low-dose L-ENK-treated fish, but its level was significantly increased in the high-dose L-ENK-treated group. Together, these findings indicate that a low dose of L-ENK stimulates the germ cells at the meiosis stage and promotes further stages of spermatogenesis, whereas a high concentration of L-ENK inhibits spermatogenesis at the advanced stages. This effect appears to be mediated through the suppression of testicular steroidogenesis and the reduction of LH release in the pituitary gland of tilapia. The findings also suggest that elevated L-ENK levels in teleosts may exert their inhibitory influence on the hypophysial-testicular axis via glucocorticoids.

Testosterone activates mitogen-activated protein kinase and the cAMP response element binding protein transcription factor in Sertoli cells

The androgen testosterone is essential for the Sertoli cell to support the maturation of male germ cells and the production of spermatozoa (spermatogenesis). In the classical view of androgen action, binding of androgen to the intracellular androgen receptor (AR) produces a conformational change in AR such that the receptor-steroid complex has high affinity for specific DNA regulatory elements and is able to stimulate gene transcription. Here, we demonstrate that testosterone can act by means of an alternative, rapid, and sustainable mechanism in Sertoli cells that is independent of AR-DNA interactions. Specifically, the addition of physiological levels of testosterone to Sertoli cells stimulates the mitogen-activated protein kinase signaling pathway and causes phosphorylation of the cAMP response element binding protein transcription factor on serine 133, a modification known to be required for Sertoli cells to support spermatogenesis. Androgen-mediated activation of mitogen-activated protein kinase and cAMP response element binding protein occurs within 1 min, extends for at least 12 h and requires AR. Furthermore, androgen induces endogenous cAMP response element binding protein-mediated transcription in Sertoli cells. These newly identified mechanisms of androgen action in Sertoli cells suggest new targets for developing male contraceptive agents.

Inducible cAMP early repressor ICER down-regulation of CREB gene expression in Sertoli cells

The cAMP response element binding protein (CREB) and the cAMP-responsive element modulator (CREM) are cyclically expressed in the seminiferous tubules during spermatogenesis. In the somatic Sertoli cells, which are the major supporters of germ cell development in the seminiferous tubules, the expression of CREB is cyclical and appears to be regulated by the levels of cAMP produced in response to the pituitary derived follicle-stimulating hormone FSH. Cyclic AMP response elements (CREs) located in the promoter of the CREB gene were shown earlier to be implicated in an autopositive feedback loop that up-regulates the expression of CREB. Here we show that in Sertoli cells FSH-mediated induction of the CREM repressor isoform, ICER (inducible cAMP early repressor) is correlated with the inhibition and delay of CREB gene expression in the seminiferous tubules. ICER binds to the two CREs located in the promoter of the CREB gene and in transient transfection assays of Sertoli cells, ICER expression vectors down-regulate transcription of a reporter gene driven by the CREB gene promoter. In addition, analyses of ICER and CREB gene expression in isolated segments of rat seminiferous tubules reveals stage-specific and cycle-dependent expression of ICER. The periods of enhanced expression of ICER correspond to the stages of spermatogenesis with the lowest levels of CREB expression. We suggest that the expression of ICER in Sertoli cells may contribute to the periodic repression of CREB gene expression during the repeated 12-day cycles of spermatogenesis, and may be required to reset the levels of activator CREB prior to the initiation of each new cycle of spermatogenesis.

Metenkephalin in seminal plasma of infertile men

BACKGROUND

Enkephalin is one of the opioids, which is expressed widely in reproductive organs. However, the function of enkephalin in male reproduction is not completely understood. The effect of metenkephalin on sperm motility remains especially controversial. In this study we examined the level of metenkephalin in seminal plasma from men with normal sperm production and patients with asthenospermia, oligospermia, and azoospermia to investigate the role of metenkephalin in seminal plasma on sperm function. We also investigated the effect of metenkephalin on sperm motility in vitro.

METHODS

Sixty nine infertile patients (31 oligospermic, 21 asthenospermic, and 17 azoospermic) were included in this study. The level of metenkephalin in seminal plasma of these men was measured and the effect of the peptide on the motility of human sperm was examined in vitro. Seventeen men with normal seminograms were a control group.

RESULTS

The level of metenkephalin in the seminal plasma of semen from asthenospermic men was significantly lower than that from the controls (P < 0.05). No significant correlations between the level of metenkephalin and the mean pathing or progressive velocity of sperm, or serum hormone levels were observed. In the in vitro study, which used semen from the controls, treatment of sperm with metenkephalin (50-200 pg/mL) maintained sperm motility for 4 hours. On the other hand, motility of sperm incubated without metenkephalin began to decrease at 3 hours. Metenkephalin levels of 50 pg/ mL in seminal plasma is considered to be necessary for maintaining sperm motility.

CONCLUSION

These results suggest that metenkephalin in seminal plasma is an important clue in the investigation of decreased sperm motility.

Opioids in neural and nonneural tissues

The endogenous opioid peptides (EOP) are grouped in three families, each deriving from the posttranslational processing of a distinct precursor molecule and exhibiting high affinity for a specific opioid receptor. The genes of EOPs are expressed in a wide variety of sites, including many nerve, neurosecretory, and endocrine cells. In reviewing the vast literature on this subject, a few patterns begin to emerge. First, the distribution of EOPs in tissues appears to be a distinct characteristic of each family of opioids. Second, the EOP producing cells can be grouped into two broad categories: those expressing only one and those expressing multiple EOP genes. Most EOP-producing nerve and neurosecretory cells fall into the first category, that is, they express one EOP gene, whereas most nonneural cells fall into the second category, that is, they express multiple EOP genes. Third, it appears that there is a relationship between opioids, proliferation rate, and state of differentiation of cells, since it has been shown that (a) mitogenic factors may change the EOP profile of a cell, and that (b) opioids may inhibit the proliferation rate of normal or neoplastic cells. The physiologic implication of these observations is briefly discussed.

A germ cell factor(s) modulates preproenkephalin gene expression in rat Sertoli cells

Within the seminiferous tubule, both Sertoli and specific germ cells express opioid genes. Little is known about the paracrine regulation or role of opioid gene expression in the tubule. The present study shows that interactions among cells within the tubule may play a role in regulating preproenkephalin (PPenk) gene expression. Rat pachytene spermatocytes (PS) and round spermatids (RSd) were purified by centrifugal elutriation and established as primary cultures or co-cultured with Sertoli cells. The effects of germ cells or germ cell-conditioned media were studied to determine the expression of one of the opioid precursor genes in rat Sertoli cells, the PPenk gene. Following a 24 h co-culture with either PS or RSd, the expression of PPenk gene in Sertoli cells was increased 6.4- and 1.9-fold, respectively. Conditioned media obtained from either PS or RSd cultured for 20 h stimulated PPenk mRNA levels in Sertoli cells from as early as 2 h after exposure; maximum increases of 3.5- and 7.6-fold were observed at 12 h, respectively. The molecular weight of the germ cell factor(s) is greater than 30 kDa. 2 h after the addition of either PS- or RSd-conditioned media to Sertoli cells, small (2- to 2.6-fold, respectively) but significant (p less than 0.02) increases in extracellular cAMP levels were observed. Although both FSH and forskolin activated c-fos and PPenk gene expression in Sertoli cells, the germ cell factor(s) that stimulated PPenk mRNA levels did not affect the expression of this oncogene. These results indicate that germ cells interact with Sertoli cells, possibly by a protein(s) that acts as a short-loop paracrine factor, which regulates the expression of PPenk gene in Sertoli cells. These data suggest that stage-specific regulation of PPenk levels in Sertoli cells may occur in vivo.

Proenkephalin products are stored in the sperm acrosome and may function in fertilization

Previous studies have shown that spermatogenic cells are a major source of testicular RNA encoding the opioid peptide precursor proenkephalin, suggesting that proenkephalin-derived peptides may function as intratesticular paracrine factors produced by male germ cells. However, direct evidence for the production of proenkephalin by spermatogenic cells has been lacking. In this report, we have used polysome profile analysis, peptide quantitation, and immunocytochemistry to show that proenkephalin products are synthesized during spermatogenesis and are retained within spermatozoa of humans, hamsters, rats, and sheep. We further show that these peptides are stored in the sperm acrosome and are depleted from sperm following the acrosome reaction, an exocytotic event required for fertilization. Proenkephalin products thus may serve a dual function as sperm acrosomal factors released during the fertilization process as well as intratesticular regulators secreted by spermatogenic cells.

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).

Phorbol ester regulates the abundance of enkephalin precursor mRNA but not of amyloid beta-protein precursor mRNA in rat testicular peritubular cells

Cultured peritubular cells prepared from the testes of 20-day-old rats contained both preproenkephalin (A) mRNA (1.5 kb) and amyloid beta-protein precursor mRNA (3.6 and 2.8 kb). The phorbol ester TPA and forskolin (an adenylate cyclase activator) increased the preproenkephalin mRNA abundance to 9.0 and 5.8 times the control, respectively. TPA alone had no effect on the intracellular cAMP level. A combination of TPA and forskolin elicited a synergistic increase in the ppEnk mRNA abundance over 30-fold. Dexamethasone potentiated the effect of forskolin but not of TPA. These results suggest that TPA regulates the preproenkephalin mRNA abundance through a cAMP-independent pathway. In contrast, TPA, forskolin, and dexamethasone showed little or no effect on the abundance of amyloid beta-protein precursor mRNA.

A new species of enkephalin precursor mRNA with a distinct 5'-untranslated region in haploid germ cells

To elucidate the primary structure of preproenkephalin (A) mRNA expressed by haploid germ cells (round spermatids) in rat testis, we have screened a lambda gt11 cDNA library for preproenkephalin cDNA inserts. The largest cDNA insert contained a protein-coding sequence encoding 269 amino acid residues as well as 327 and 309 bases of the 5'- and 3'-untranslated regions, respectively. The protein-coding region plus 3'-untranslated region of the mRNA was over 99% homologous to that of brain preproenkephalin mRNA, whereas the 5'-untranslated region contained a distinct sequence including a partial sequence of intron A of the preproenkephalin gene [(1984) J. Biol. Chem. 259, 14301-14308; (1984) J. Biol. Chem. 259, 14309-14313]. Northern blot analysis using a 5'-end-specific probe showed that this type of preproenkephalin mRNA exists exclusively in the germ cells.