INTRODUCTION

Luteinizing hormone (LH) receptor is expressed in the Leydig cells in the testis and mediates the effects of LH, such as stimulation of steroidogenesis. In 1989, the cDNA for this receptor molecule was cloned from the rat luteal cDNA library (McFarland et al., 1989), and thereafter the existence of multiple species of LH receptor mRNA was revealed by means of cloning of cDNA molecules, Northern hybridization and poly-merase chain reaction (Aatsinki et al., 1992; Koo et al., 1994; Wang et al., 1991). Previous studies indicated that the expression patterns of the mRNA transcripts depend on sex, age or experimental conditions (Iizuka et al., 1996; LaPolt et al., 1991; Tena-Sempere et al., 1994; Veldhuizen-Tsoerkan et al., 1994; Vihko et al., 1992; Zhang et al., 1994). Ovary and fetal testis express major transcripts of 6.5-7.0 kb and 2.5-2.7 kb, whereas in adult testis an additional 1.8 kb transcript is detected. LaPolt et al., (1991) showed that the expression of the 1.8 kb transcript detected in adult testis was not decreased after a hCG injection, while other transcripts were down-regulated. Furthermore, 1.8 kb transcript was expressed persistently after the destruction of mature Leydig cells by an ethyl-ene dimethane sulfonate (EDS) injection (Tena-Sempere et al., 1994; Veldhuizen-Tsoerkan et al., 1994). However, the regulatory mechanism for the expression patterns have yet to be clarified. Recently we have studied the effects of unilateral cryptorchidism (UCD) on the expression of mRNAs for FSH and LH receptor and found that the expression pattern of LH receptor mRNA transcripts was markedly altered in the abdominal testis 4 weeks after the operation (Iizuka et al., 1996) It is possible that an alteration in the paracrine environment of the Leydig cells occurs along with the degeneration of seminiferous epithelium in abdominal testis, leading to changes in the expression pattern of LH receptor mRNA transcripts. However, we could not rule out the possibility that exposure of Leydig cells to abdominal high temperature affected the expression pattern of LH receptor mRNA directly, since the whole testis was exposed to unusual higher temperature in abdomen in cryptorchidism. If high temperature around the Leydig cells is responsible for the changes in the expression pattern, degeneration of seminiferous epithelium without an elevation of temperature would not induce such alterations. It is reported that ligation of efferent ducts induces pressure atrophy of the seminiferous epithelium (Smith, 1962). In the present study, therefore, we examined whether efferent duct ligation (EDL) affects the expression pattern of LH receptor mRNA transcripts. In addition, we examined the sequential changes in the expression pattern of the receptor mRNA in the UCD rats during degeneration of the seminiferous epithelium to discuss the relationship between the mRNA expression and the loss of germ cells.

MATERIALS AND METHODS

RESULTS

Effects of efferent duct ligation

Fourteen days after unilateral ligation of efferent ducts, the weight of efferent duct-ligated testis was significantly decreased as compared to that of contralateral intact testis in the same rat (Fig. 1). As shown in Fig. 2, few spermatozoa and spermatids were observed in the ligated testis. Some tubules were lined only by Sertoli cells and spermatogonia while other tubules contained thin layers of spermatocytes. The recovery of poly (A)⁺ RNA exraction per tissue weight was lower in the ligated than intact testis (data not shown). Figure 3 shows the result of Northern hybridization. In control testis, a major transcript of 1.8 kb and other transcripts of 6.5, 2.6 and 1.4 kb were detected. The expression of LH receptor mRNA transcripts was markedly altered 14 days after EDL operation: the intensity of the 1.8 kb transcript was relatively decreased below the other four species. This band pattern was similar to that seen in abdominal testis in the cryptorchid rats in our previous study (Iizuka et al., 1996). In contralateral intact testis, there were no changes in either histology or expression of mRNA relative to control testis.

Fig. 1

Testicular weight of control and unilateral EDL rats. Values are given as the means ± SEM and the numbers of animals are shown in parentheses. cont: testis of control rat, lig: testis with efferent duct ligation, and int: contralateral intact testis of the EDL rat. *P < 0.01 compared to intact testes of EDL rat (Student's t-test).

Fig. 2

Histology of control testis (A), and efferent duct-ligated testis (B) and contralateral intact testis (C) of EDL rat. The control testis and intact testis of EDL rat showed active spermatogenic features. On the other hand, spermatozoa and spermatids were rare, and thin layers of spermatocytes, spermatogonia and Sertoli cells were observed in seminiferous epithelia in the efferent duct-ligated testis. Bar = 100 μm.

Fig. 3

Northern hybridization of poly (A)+ RNA (4 μg / lane) isolated from testis of a control rat, and efferent duct-ligated testis and contralateral intact testis of an EDL rat. Samples were run on a 1 % denaturing agarose gel, transferred to nylon membrane, and hybridized to the cDNA probe corresponding to the extracellular domain of the LH receptor. Hybridizations were performed on individual samples prepared from three (control) or four (EDL) rats. Representative results are shown in A. The membrane was exposed to X-ray film for 6 days. Band intensities of all samples were optically measured by NIH image, and the mean profile of each experimental group is indicated in B. ADU, arbitrary densitometric units. The intensities of major signals subtracted with background levels are indicated in table C.

Effects of unilateral cryptorchidism

Figure 4 summarizes the changes in testicular weight after the UCD operation. Weight of abdominal testis was decreased, while that of scrotal testis showed no changes. Figure 5 shows the histological features after UCD operation. Scrotal testis did not show any changes (Fig. 5A-D), while abdominal testis showed remarkable changes. Three days after the operation, the seminiferous epithelium was disorganized and multinucleated cells containing spermatids were present in some sections of seminiferous tubules (Fig. 5E). A small number of spermatozoa and spermatids was observed at day 7 (Fig. 5F), and only a few spermatocytes were observed at day 14 (Fig. 5G). There were no significant changes in histology between day 14 and 28 (Fig. 5H). The recovery of poly (A)⁺ RNA extraction was always lower in abdominal testis than in scrotal or control testis (data not shown). The band patterns of LH receptor mRNA transcripts in abdominal testes gradually changed, while the patterns in scrotal testes did not show any changes (Fig. 6A). Densitometric analyses of the band patterns showed that 1.8 kb transcript was the most abundant and shorter transcripts exceeded longer ones in expression amount before the operation (Fig. 6B). The first change after the operation was a relative decrease of intensity of 1.8 kb transcript, which occurred within 3 days. In addition, the intensity of other transcripts, in particular those of 6.5 kb and 2.6 kb, increased gradually for 14 days.

Fig. 4

Testicular weight of UCD rats. Stippled bars represent the scrotal testes and solid bars the abdominal testes. Values are given as the means ± SEM and the numbers in the column represent rats examined. *P< 0.001 compared to scrotal testes (Student's t-test).

Fig. 5

Histological changes after the UCD operation. A-D show scrotal and E-H show contralateral abdominal testes of UCD rats 3 (A, E), 7 (B, F), 14 (C, G) and 28 (D, H) days after the operation. In abdominal testis at day 3 (E), multinucleate cells containing spermatids were observed (arrow). Staining of the multinucleate cells were faint at day 7 (F). Most seminiferous tubules contain only Sertoli cells and spermatogonia at day 14 (G) and 28 (H). Scrotal testes did not show any histological changes. Bar =100 μm.

Fig. 6

Northern hybridization of poly (A)+ RNA samples extracted from the testes of control rats (0 day) and abdominal or scrotal testes at different times following the induction of UCD. Samples were run on a 1% denaturing agarose gel, transferred to nylon membrane, and hybridized to the LH receptor cDNA probe. Sample preparation and hybridization were repeated three times, and similar results obtained each time. A representative result is shown in A. Six micrograms poly (A)+ RNA was loaded per well. The exposure time was 10 days (abdominal testes) or 4 weeks (scrotal testes). Intensities of signals from abdominal testes are indicated in B. The intensities of major signals subtracted with background levels are indicated in table C.

DISCUSSION

Recently we found that UCD operation induced marked alterations in the expression pattern of mRNA transcripts for LH receptor (Iizuka et al., 1996). To understand the biological meaning of the heterogeneity of LH receptor mRNA, we aimed to clarify the cause for the alterations in the expression pattern. We hypothesized that changes in the expression pattern of the receptor mRNA were related to degeneration of germ cells. However, we could not rule out the possibility that the exposure of Leydig cells to unusual high temperature influenced the mRNA expression. In the present study, therefore, we examined the effects of EDL on the mRNA expression, in order to clarify whether the expression pattern might be altered when germ cells were degenerated in ordinary scrotal temperature. The treatment resulted in a degeneration of seminiferous epithelium, which is in agreement with a previous study by Smith (1962). Northern hybridization revealed increased expression of 6.5 and 2.6 kb transcripts and relatively poor expression of 1.8 kb transcript which is the most abundant in intact testis. In these changes, increases of 6.5 and 2.6 kb transcripts may be explained as previously reported (Iizuka et al., 1996). Namely, Leydig cell component is relatively increased in the efferent duct-ligated testis because of loss of germ cells, which results in increase of LH receptor mRNA amount per poly (A)⁺ RNA weight. Therefore, the changes in expression pattern of LH receptor mRNA in the ligated testis are similar to those observed in the abdominal testis of UCD rats. This result demonstrates that an elevation of temperature was not necessary to the changes in the expression pattern of LH receptor mRNA, but a treatment which degenerates seminiferous epithelium could induce the expression changes.

To correlate the degeneration of seminiferous epithelium with the expression pattern of LH receptor mRNA, changes in both the histology and the mRNA expression were examined 3, 7, 14, and 28 days after UCD operation. In abdominal testis, spermatids were severely degenerated within 3 days and subsequently spermatocytes disappeared, which is consistent with previous results (Clegg, 1963; Iizuka et al., 1996). The changes in composition of the transcripts in abdominal testis started by day 3, proceeded throughout the experimental period, and resulted in a general shift from the short to the long form of the transcripts by day 28. Although thin layers of spermatocytes were often retained in the seminiferous tubules 14 days after EDL operation, the tubules containing spermatocytes were much fewer in the abdominal testis 14 days after UCD operation. In EDL testis, the signal of the 2.6 kb transcript predominated over the 6.5,1.8 and 1.4 kb signals bearing lower intensity. This band pattern seems to be intermediate between those of day 7 and 14 of abdominal testes in UCD rats, suggesting that the expression pattern of LH receptor mRNA is closely related to the process of germ cell degeneration. Since the expression pattern was not altered in the contralateral intact testis in EDL or UCD rats, the effects of the operation on the expression seems to occur within a testis and is independent of the circulating factors. Thus, these changes seems to be induced by some paracrine factor(s) secreted from seminiferous tubule to interstitium. Although there are many reports that paracrine factors affect the proliferation and functions of Leydig cells in vitro (for reviews see Saez, 1994; Skinner, 1991), it is unclear whether these factors have significant roles in vivo. The present results suggest that paracrine factor(s) involved in the loss of germ cells may participate in the regulation of LH receptor mRNA expression in vivo. Further analyses are needed to identify these paracrine factor(s) and to determine their source.

The present results on the expression of multiple species of LH receptor mRNA in the intact testis agree with previous studies (Iizuka et al., 1996; LaPolt et al., 1991; Wang et al., 1991). The abundant expression of 1.8 kb transcript is the most prominent feature of the adult testis, as this transcript is a minor species in ovary and fetal testis (LaPolt et al., 1991; Tena-Sempere et al., 1994; Segaloff et al., 1990; Wang et al., 1991; Zhang et al., 1994). The 1.8 kb transcript is not thought to code for the membrane-bound receptor protein, since LaPolt et al. (1991) have revealed by Northern hybridization that the 1.8 kb transcript does not hybridize to cRNA probe corresponding to the transmembrane region and Koo et al. (1994) have detected transcripts of 1.8 kb probably polyadenylated in introns 3, 4, and 10 of LH receptor gene. In testis which did not contain mature Leydig cells after EDS treatment, expression of 1.8 kb transcript was maintained without any decrease while expression of the other transcripts was severely suppressed (Tena-Sempere et al., 1994; Veldhuizen-Tsoerkan et al., 1994). The expression of longer transcripts which are thought to code for the full length of the receptor protein was restored from 20 days after the EDS treatment when mature Leydig cells were observed. Thus, it was concluded that the Leydig cell precursors express only the 1.8 kb truncated form of LH receptor mRNA, and the longer, functional receptor mRNA appears as Leydig cells mature. Recent report that transla-tional efficiency of the 1.8 kb transcript is lower than that of 6.5 kb supports this concept (Tena-Sempere et al., 1997). Kerr and Donachie (1986) studied the effects of UCD operation after EDS treatment and found that the regeneration of a new population of Leydig cells occurred more rapidly in the abdominal testis than in the scrotal testis of the same animal. Considering these previous data, our results on the expression pattern of LH receptor mRNA transcripts can be explained as follows. In intact adult testis, there were a number of Leydig cell precursors as well as mature Leydig cells, the former expressing the 1.8 kb transcript for LH receptor abundantly. When germ cells were damaged, paracrine environment of the Leydig cell precursors changed, so that the precursors ceased expressing 1.8 kb transcript and started to differentiate to mature cells which could express the longer transcripts. Consequently, the amount of 6.5 and 2.6 kb transcripts increased relative to that of 1.8 kb transcript. To evaluate this hypothesis, it is necessary to determine the source of each species of LH receptor mRNA in the testis. In addition, quantitative analyses on the expression changes of LH receptor mRNA per testis are needed to consider the effect of these changes to testicular functions, as the present study focuses the changes per poly (A)⁺ RNA weight.

In summary, the treatments which degenerate the seminiferous epithelium resulted in remarkable changes in the composition of LH receptor mRNA. These changes parallel the degeneration of germ cells, suggesting the significant role of a local communication mechanism between seminiferous tubule and interstitium, which controls the expression of LH receptor in Leydig cells.