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a Departments of Cell Biology and
b Radiotherapy, University Medical Center Utrecht, 3584 CX, Utrecht, The Netherlands
ABSTRACT
Using immunohistochemistry, the expression of the D-type cyclin proteins was studied in the developing and adult mouse testis. Both during testicular development and in adult testis, cyclin D1 is expressed only in proliferating gonocytes and spermatogonia, indicating a role for cyclin D1 in spermatogonial proliferation, in particular during the G1/S phase transition. Cyclin D2 is first expressed at the start of spermatogenesis when gonocytes produce A1 spermatogonia. In the adult testis, cyclin D2 is expressed in spermatogonia around stage VIII of the seminiferous epithelium when Aal spermatogonia differentiate into A1 spermatogonia and also in spermatocytes and spermatids. To further elucidate the role of cyclin D2 during spermatogenesis, cyclin D2 expression was studied in vitamin A-deficient testis. Cyclin D2 was not expressed in the undifferentiated A spermatogonia in vitamin A-deficient testis but was strongly induced in these cells after the induction of differentiation of most of these cells into A1 spermatogonia by administration of retinoic acid. Overall, cyclin D2 seems to play a role at the crucial differentiation step of undifferentiated spermatogonia into A1 spermatogonia. Cyclin D3 is expressed in both proliferating and quiescent gonocytes during testis development. Cyclin D3 expression was found in terminally differentiated Sertoli cells, in Leydig cells, and in spermatogonia in adult testis. Hence, although cyclin D3 may control G1/S transition in spermatogonia, it probably has a different role in Sertoli and Leydig cells. In conclusion, the three D-type cyclins are differentially expressed during spermatogenesis. In spermatogonia, cyclins D1 and D3 seem to be involved in cell cycle regulation, whereas cyclin D2 likely has a role in spermatogonial differentiation.
gene regulation, Sertoli cells, signal transducers, spermatogenesis, testis
INTRODUCTION
In mammals, three D-type cyclins have been identified: D1, D2, and D3. Most cells express D3 and either D1 or D2. In vitro, all three D-type cyclins are able to regulate the G1/S transition, as can be deduced from studies using antisense mRNA and antibody microinjection, and from overexpression experiments in cell lines [1–5]. Cyclin D1 was the first D-type cyclin found in mammals [6, 7]. It is intriguing that increased cyclin D1 levels have been found in two opposing cell cycle situations: cell proliferation [8] and cell cycle arrest [9]. Also, cyclin D1 protein levels were found to be up-regulated after apoptosis induction (e.g., by kainic acid treatment [10] or nerve growth factor deprivation [11]). Several factors that are involved in G1/S cell cycle control, including the retinoblastoma protein [12], p53 [13], and p21Cip1/WAF1 [14] are able to regulate cyclin D1 synthesis, indicating that the different G1/S regulators are functionally linked. Recently, a new role for cyclin D1 has been described as a ligand for the estrogen receptor (ER); cyclin D1 competes with estrogen for ER binding and is able to activate the ER [15].
Cyclins D2 and D3 have been implicated in proliferation and differentiation of several cell types, including granulosa cells, granulocytes, and megakaryocytes [5, 16, 17]. Although cyclin D2-deficient mice develop normally, cyclin D2-deficient female mice are sterile due to an inability of the granulosa cells to respond to FSH, whereas male mutants show testis hypoplasia [18]. Cyclin D3 is strongly up-regulated in leukemic cells induced to undergo differentiation and is highly expressed in quiescent, differentiating, and terminally differentiated cells. This suggests that cyclin D3 has a role in promotion and, possibly, maintenance of the terminally differentiated state of some cell types [5].
Primordial germ cells (PGCs) are the first known cells in the germ cell lineage and, upon arrival in the genital ridges, become gonocytes when they are enclosed in the seminiferous cords at Fetal Day (E) 12.5 [19, 20]. The gonocytes further proliferate until they are arrested in the G0/G1 phase of the cell cycle at E16 [21]. In mice, gonocytes resume proliferation within a few days after birth and give rise to adult-type spermatogonia [20]. In the adult mouse testis, the undifferentiated spermatogonia are at the beginning of the spermatogenic lineage. These cells are on the basal membrane of the seminiferous tubules and can be subdivided according to their topographical arrangement into A single (As), A paired (Apr), or A aligned (Aal) spermatogonia [22, 23]. The undifferentiated spermatogonia proliferate during part of the cycle of the seminiferous epithelium (stages X–II) and then become quiescent (stages III–VII) until most of the Aal cells differentiate into differentiating type A1 spermatogonia in stage VIII. After a series of divisions (A2 to B), differentiating spermatogonia finally divide into spermatocytes that move toward a more adluminal position of the seminiferous tubules. After meiosis, the spermatocytes become round spermatids, which develop into elongating spermatids without further divisions.
Cyclin D mRNA has been found and localized in the mouse testis in various studies. Cyclin D1 mRNA was found in Sertoli cells [24]. Cyclin D2 mRNA was found in a few spermatogonia in the adult mouse, whereas cyclin D2 was also present in Sertoli cells during testicular development [25]. Other studies, however, indicate low, if any, expression of cyclin D2 in the mouse [24] or human [26] testis. Finally, cyclin D3 mRNA was clearly observed in round spermatids and in Sertoli cells. Correspondingly, cyclin D3 protein was found in round spermatids, in addition to spermatogonia and spermatocytes in the mouse testis [27]. In the present study, the role of the D-type cyclins was further investigated through a detailed immunohistochemical analysis of the protein expression patterns in the developing, normal adult, and vitamin A-deficient (VAD) adult mouse testis.
MATERIALS AND METHODS
Animals and Fixation
A series of pregnant FvB/NiCO mice, E14 through E19, and a series of newborn mice (Day 1 through Day 4) were obtained from Broekman Instituut B.V. (Someren, The Netherlands) and were reared at the Central Laboratory Animal Institute, Utrecht, The Netherlands. Both fetal and newborn mice were decapitated, after which the testes were dissected.
For one experiment, VAD mice were obtained as described before [28]. Briefly, breeding pairs of Nc/Cpb-U mice (Central Laboratory Animal Institute) were fed a VAD diet (Teklad Trucking, Madison, WI) for at least 4 wk. Male weanlings received the same diet until they became VAD. Their body weights slightly decreased at the age of 14–16 wk. Some were given an i.p. injection of 1 mg all-trans retinoic acid in 16% dimethyl sulfoxide (DMSO; Sigma, St. Louis, MO). Twenty-four h after treatment, animals were killed by cervical dislocation, and testes were dissected.
Animals were housed in standard temperature-controlled conditions in a photoperiod of 14L:10D with food and water available ad libitum. The animal experimentation was approved by the Ethical Committee for Animal Experimentation of the Medical School, Utrecht University.
For histology and immunohistochemistry, testes were fixed in 10% neutral buffered formalin for 4 h and postfixed in a diluted Bouins solution (71% picric acid [0.9%], 24% formaldehyde [37%], 5% acetic acid) for 16 h at 4°C. Tissues were dehydrated and washed in 70% ethyl alcohol prior to embedding in paraffin (Stemcowax, Adamas Instruments, Amerongen, The Netherlands).
Immunohistochemistry
Paraffin sections of testes, 5 µm thick, were mounted together on a silane-coated slide. At least three separate series of animals were used. Unmasking of cyclin D1 was established by incubation of the sections for 15 min in 5 mM trypsin at 37°C. Unmasking of cyclin D2 and cyclin D3 was established by boiling the sections for 10 min in 0.01 M sodium citrate using a microwave oven (H2500, Bio-Rad, Veenendaal, The Netherlands). Endogenous peroxidase was blocked by incubation with 0.35% H2O2 in PBS for 10 min. The slides were washed in PBS and then incubated with 10% normal horse serum for cyclin D1 or with normal goat serum for cyclins D2 and D3 to block nonspecific binding sites of the antibodies. Subsequently, the slides were incubated with primary antibodies against cyclin D1 (NCLcyclinD1, Novo Castra Laboratories Ltd., Newcastle Upon Tyne, United Kingdom), cyclin D2 (C-17, SC-181, Santa Cruz Biotechnologies Inc., Santa Cruz, CA) and cyclin D3 (C-16, SC-182, Santa Cruz Biotechnologies), diluted 1:20, 1:200, and 1:200, respectively in PBS including 5% normal horse serum for cyclin D1 and normal rabbit serum for cyclin D2 and D3 in a humidified chamber overnight at 4°C. After extensive washing steps in PBS, slides were incubated for 60 min with a biotinylated horse anti-mouse immunoglobulin G (Elite ABC-peroxidase staining kit, Vector Laboratories Inc., Burlingame, CA) for cyclin D1, or a biotinylated goat anti-rabbit (Santa Cruz Biotechnologies) for cyclins D2 and D3, diluted 1:200 in PBS including 5% normal horse serum for cyclin D1 or normal rabbit serum for cyclin D2 and D3, in a humidified chamber. The avidin-biotin complex reaction was performed according to the manufacturer's protocol. To visualize bound antibodies, sections were washed in PBS and covered with 0.3 µg/µl 3, 3' diaminodibenzene (DAB, Sigma) in PBS, to which 0.03% H2O2 was added. Sections were counterstained with Mayers hematoxylin.
In negative control sections, the cyclin D2 and cyclin D3 antibodies were incubated with fivefold excess of cyclin D2 and cyclin D3 protein, respectively (Santa Cruz Biotechnologies), prior to use in the immunohistochemical analysis. Negative control sections for cyclin D1 were treated as described earlier except that the primary antibody was omitted during the procedure and was replaced by normal mouse serum.
In adult testes immunohistochemically stained for cyclin D2, in random tubular cross-sections in stages VII and VIII of the cycle of the seminiferous epithelium, a total of 50 A spermatogonia were studied in each of three mice to establish the percentage of cyclin D2-positive A spermatogonia. In testes of fetal and newborn mice, immunohistochemically stained for cyclin D1, D2, and D3, in each of three mice, 100 gonocytes were scored at random to establish the percentage of positive staining gonocytes.
RESULTS
Localization of Cyclins D1, D2, and D3 in the Adult Testis
Immunohistochemistry was performed to localize the different D-type cyclins in spermatogenic cell types. Staining for cyclin D1 was seen in nuclei of spermatogonia in all stages of the cycle of the seminiferous epithelium (Fig. 1, A and B; Table 1). In addition, Leydig cells showed a light nuclear cyclin D1 staining. When the primary antibody was omitted and replaced by normal mouse serum, no staining was observed (Fig. 1G).
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Cyclin D2 staining was observed in nuclei of A-spermatogonia present in stages VII and VIII of the cycle of the seminiferous epithelium (Fig. 1C). In these stages, about 39% ± 2% of the A-spermatogonia were stained. In other stages of the cycle of the seminiferous epithelium only a very weak staining in a few spermatogonia could be detected. Furthermore, cyclin D2 was clearly present in nuclei of pachytene spermatocytes from stage IV onward and, although very weakly, in nuclei of round spermatids until stage V of the cycle of the seminiferous epithelium (Fig. 1C, Table 1). In addition, a weak cyclin D2 staining was present in the cytoplasm of these cells. In Sertoli and Leydig cells, no staining for cyclin D2 was observed. Incubation of the primary antibody with a fivefold excess of cyclin D2 protein resulted in a complete loss of staining (Fig. 1H).
Immunohistochemical staining for cyclin D3 was seen in the nuclei of some of the spermatogonia present in all stages of the cycle of the seminiferous epithelium (Fig. 1F; Table 1). Cyclin D3 staining was also detected in the sex vesicle of pachytene and diplotene spermatocytes from stage VII until stage XI of the cycle of the seminiferous epithelium (Fig. 1F). In addition, a clear cyclin D3 staining was seen in nuclei of all Sertoli cells (Fig. 1F). Finally, about 70% of the Leydig cells stained for cyclin D3. When the primary antibody was incubated with a fivefold excess of cyclin D3 protein, no staining was observed (Fig. 1I).
Cyclin D2 Expression in Vitamin A-Deficient Testes
The immunohistochemical results suggested a role for cyclin D2 in the differentiation of undifferentiated spermatogonia. To investigate this in further detail, the vitamin A deficiency model was used. Vitamin A deficiency in the mouse results in a spermatogenic block at the moment at which the undifferentiated spermatogonia start to differentiate into A1 spermatogonia [28–30]. After all-trans retinoic acid treatment, spermatogenesis resumes, resulting in the formation of A1 spermatogonia within 24 h [31]. To investigate the expression of cyclin D2 before and during the time when undifferentiated spermatogonia differentiate into A1 spermatogonia, immunohistochemistry was performed on the VAD testis and 24 h after retinoic acid injection. In the VAD testis, cyclin D2 staining was present only in the cytoplasm of Sertoli cells (Fig. 1D). No nuclear cyclin D2 staining was present in either somatic or germ cells. However, 24 h after retinoic acid injection, nuclear cyclin D2 staining was clearly present in about 30% of the spermatogonia (Fig. 1E). Cyclin D2 staining in the cytoplasm of Sertoli cells remained but seemed weaker.
Localization of Cyclin D1, D2, and D3 During Testicular Development
During testicular development, very weak if any cyclin D1 staining was observed in nuclei of proliferating and quiescent gonocytes from E14 until Day 3 postpartum (pp; Fig. 2A). At Day 4 pp, cyclin D1 staining was present in the nucleus of 56% ± 2.4% of the gonocytes (Fig. 2B). However, the nuclei of proliferating Sertoli cells and Leydig cells did not stain for cyclin D1, although the cytoplasm of both these cell types was lightly stained.
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Cyclin D2 staining also was not present in the nuclei of gonocytes from E14 until Day 3 pp (Fig. 2D), however, some gonocytes showed a cytoplasmic cyclin D2 staining at E17. In Leydig cells too, cyclin D2 staining was present in the cytoplasm. From Day 3 pp onward, cyclin D2 staining was observed in the nuclei of 39% ± 7.5% of the gonocytes (Fig. 2E). At this time, Sertoli cells and Leydig cells also showed a weak nuclear staining for cyclin D2.
Cyclin D3 staining was present in nuclei of germ cells, pre-Sertoli cells, and pre-Leydig cells from E14 onward until at least Day 4 pp. However, whereas before birth all gonocytes were stained for cyclin D3 (Fig. 2G), at Days 3 and 4 only 75% ± 6.7% of the gonocytes were stained (Fig. 2H). In addition, large numbers of Leydig cells and Sertoli cells were stained for cyclin D3 during testicular development.
No staining was observed in the control sections for cyclins D1, D2, and D3 (Fig. 1, C, F, and I). In control sections for cyclin D1 staining, the primary antibody was omitted and replaced by normal mouse serum. In control sections for cyclins D2 and D3 staining, the primary antibody was incubated with a fivefold excess of cyclin D2 or cyclin D3 protein, respectively.
DISCUSSION
Using immunohistochemical analysis we were able to detect the three D-type cyclins in the testis. It is interesting that the expression patterns of cyclins D1, D2, and D3 were only partly overlapping, suggesting different roles for these proteins during spermatogenesis.
During testicular development, both proliferating (E14.5, E15.5) and quiescent (E16.5 until after birth) gonocytes express only cyclin D3. When the gonocytes start to proliferate again after birth and form A1 spermatogonia, a clear nuclear expression of cyclins D1, D2, and D3 in gonocytes, or newly formed A1 spermatogonia, or both, was observed.
In adult testis, cyclins D1 and D3 are expressed in some of the spermatogonia throughout the stages of the cycle of the seminiferous epithelium, suggesting that these cyclins are involved in the regulation of the proliferation of spermatogonia. However, cyclin D2 is expressed only in spermatogonia in epithelial stages VII and VIII, when undifferentiated spermatogonia differentiate into differentiating spermatogonia [20], suggesting that cyclin D2 is involved in this differentiation step. To further investigate this possibility, VAD mice were used. Vitamin A deficiency results in a block in spermatogenesis just at the time when undifferentiated spermatogonia differentiate into A1 spermatogonia [28–30]. After administration of retinoic acid there is a massive differentiation of undifferentiated spermatogonia into A1 spermatogonia [31]. It is interesting that the undifferentiated spermatogonia in VAD mice did not show cyclin D2 staining, whereas 24 h after retinoic acid injection, a clear staining in some of the spermatogonia was observed, again showing that cyclin D2 is specifically expressed at the time that A1 spermatogonia are formed. Hence, the expression patterns of cyclin D2 at the start of spermatogenesis, in epithelial stages VII/VIII in the adult testis, and in the VAD/vitamin A replacement model all indicate that cyclin D2 is involved in the process of differentiation of undifferentiated spermatogonia into A1 spermatogonia. The formation of A1 spermatogonia is an important event during spermatogenesis because the timed formation of these cells is the basis for the cycle of the seminiferous epithelium [20]. Another instance in which cyclin D2 seems to be involved in a differentiation process is the cerebellum [32], showing that cyclin D2 is also involved in the induction of differentiation in other cell types.
Spermatocytes and spermatids expressed both cyclins D2 and D3. Cyclin D2 is expressed in nuclei of pachytene spermatocytes, which at that time, are in the prophase of the first meiotic division. Hence, cyclins D2 and D3, as many other cell cycle proteins (e.g., p21Cip1/WAF1 [33] and cyclin B2 [34]), may be necessary during the meiotic process.
During testicular development, somatic cells such as Sertoli cells and Leydig cells clearly expressed cyclin D3 in their nuclei, while shortly after birth cyclin D2 was also present. In the adult testis, the terminally differentiated Sertoli cells did not show expression of cyclin D1 or D2 protein. Lack of cyclin D1 expression was surprising because results of Ravnik and colleagues [24] showed that Sertoli cells in the adult mouse testis express cyclin D1 mRNA, as determined by in situ hybridization and Northern blot analysis. It is possible that the expression level of cyclin D1 protein in Sertoli cells is too low to detect using immunohistochemistry because of low translational activity or a high turnover of the cyclin D1 protein in these cells. In contrast, cyclin D3 was clearly expressed in differentiated Sertoli cells and in Leydig cells, which do not or rarely proliferate, respectively. So, as in quiescent gonocytes, these results also suggest that cyclin D3 can have a role not related to cell proliferation. Indeed, cyclin D3 expression has also been observed in other quiescent cell types and in terminally differentiated cells in the gastrointestinal tract [5].
Overall, the expression patterns of cyclin D2 protein in both adult and developing testes are comparable with the cyclin D2 mRNA localization studies of Nakayama et al. [25]. While in these studies a high cyclin D2 mRNA expression was found in spermatogonia, cyclin D2 levels below the in situ hybridization detection level may be present in pachytene spermatocytes. Ravnik et al. [24] previously found cyclin D3 mRNA in round spermatids and Zhang and colleagues [27] found cyclin D3 protein in some spermatogonia both during testicular development and in the adult testis, spermatocytes, spermatids, and Sertoli cells. These results in the adult testis but not in the developing testis were comparable to ours and indeed they also noted that cyclin D3 may have dual functions during spermatogenesis: regulation of proliferation in spermatogonia and a role in non-cell cycle functions such as specialized morphogenetic differentiation, chromatin remodeling, or both.
It is important to note that the function of D-type cyclins may depend on the levels of other proteins. Robker et al. [35] found that the levels of cyclin D2 relative to those of p27Kip1 are important for granulosa cell development. Granulosa cell proliferation is induced by increasing levels of cyclin D2 relative to p27Kip1 and follicular growth is terminated by down-regulation of cyclin D2 and up-regulation of p21Cip1/WAF1 and p27Kip1. In addition, p27Kip1 levels are also important for the onset of differentiation or cell cycle arrest in several cell types, including oligodendrocytes [36], while cyclin D2 overexpression in fibroblasts can shorten the G1 phase of the cell cycle [3]. In adult testis, cyclin D2 is colocalized with p21Cip1/WAF1 in pachytene spermatocytes and cyclin D3 with p27Kip1 in Sertoli cells, suggesting that the balance between a D-type cyclin and a Cip/Kip family member may also be important in the behavior of these cell types. Similar colocalizations were observed during testicular development in gonocytes when they start to proliferate after birth [21], correlating with cyclin D2 induction and loss of p27Kip1 [37].
In conclusion, testicular expression of cyclin D1 and also partially of cyclin D3 correlates with G1/S progression of gonocytes and spermatogonia. However, cyclins D2 and D3 also likely have other functions in the testis. Cyclin D2 likely plays a specific role in the differentiation program of undifferentiated spermatogonia into A1 spermatogonia and may have a role during the meiotic prophase, in particular in pachytene spermatocytes. The role of cyclin D3 in nonproliferating cells has, as yet, not been elucidated, but possibly relates to maintenance of the terminal state of cells. This clearly needs further study.
ACKNOWLEDGMENTS
The authors thank Mr. R.M.C. Scriwanek and Mr. A.N. van Rijn for assistance with photography.
FOOTNOTES
1 This work was supported by the J.A Cohen Institute for Radiopathology and Radiation Protection, Leiden, The Netherlands. 
2 Correspondence: Dirk G. de Rooij, Department of Cell Biology, University Medical Center Utrecht, AZU, RM G02.525, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands. FAX: 31 0 30 2541797; d.g.derooij{at}med.uu.nl
Accepted: August 1, 2000.
Received: April 17, 2000.
REFERENCES
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|
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 |
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|
|
|
|
 |
|
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|
|
|
|
|
|
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|
|
|
|
 |
|
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|
|
|
|
 |
|
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|
|
|
|
|
|
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|
|
|
|
 |
|
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|
|
|
|
 |
|
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|
|
|
|
 |
|
 F. D. Cunto, S. Imarisio, P. Camera, C. Boitani, F. Altruda, and L. Silengo Essential role of citron kinase in cytokinesis of spermatogenic precursors J. Cell Sci., March 14, 2003; 115(24): 4819 - 4826. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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