A rat H1t‐GFP transgene recapitulates endogenous H1t expression pattern in mouse

H1 histones bind to linker DNA. H1t (H1f6), a testis‐specific linker histone variant, is present in pachytene spermatocytes and spermatids. The expression of H1t histone coincides with the acquisition of metaphase I competence in pachytene spermatocytes. Here we report the generation of H1t‐GFP transgenic mice. The H1t‐GFP (H1 histone testis‐green fluorescence protein) fusion protein expression recapitulates the endogenous H1t expression pattern. This protein appears first in mid pachytene spermatocytes in stage V seminiferous tubules, persists in round spermatids and elongating spermatids, but is absent in elongated spermatids. The strong green fluorescence signal, due to the high abundance of H1t‐GFP, is maintained in spermatocytes after induction towards metaphase I through treatment with okadaic acid. Therefore, H1t‐GFP can be used as a visual marker for monitoring the progression of meiosis in vitro and in vivo, as well as fluorescence‐activated cell sorting (FACS) sorting of germ cells.     

Aging results in differential regulation of DNA repair pathways in pachytene spermatocytes in the Brown Norway rat

The present trend of increasing paternal age is accompanied by concerns for the development of complex multigene diseases (e.g., autism and schizophrenia) in progeny. Recent studies have established strong correlations between male age, increased oxidative stress, decreased sperm quality, and structural aberrations of chromatin and DNA in spermatozoa. We tested the hypothesis that increasing age would result in altered gene expression relating to oxidative stress and DNA damage/repair in germ cells. To test this hypothesis, pachytene spermatocytes and round spermatids were isolated from Brown Norway (BN) rats at 4 (young) and 18 (aged) mo of age. Microarray analysis was used to compare gene expression between the groups. The probe sets with significantly altered expression were linked to DNA damage/repair and oxidative stress in pachytene spermatocytes but not in round spermatids. Further analysis of pachytene spermatocytes demonstrated that genes involved in the base excision repair (BER) and nucleotide excision repair (NER) pathways were specifically altered. Quantitative RT-PCR confirmed that NER genes were upregulated (>1.5-fold), whereas BER genes were downregulated (>1.5-fold). At the protein level the members of the BER pathway were also altered by up to 2.3-fold; levels of NER proteins remained unchanged. Furthermore, there was an increase in 8-oxo-2'-deoxyguanosine (8-oxodG) immunoreactivity in testes from aged males and in the number of spermatozoa positive for 8-oxodG. In conclusion, aging is associated with differential regulation of DNA repair pathways with a decrease in the BER pathway leading to deficient repair of 8-oxo-dG lesions in germ cells and spermatozoa.     

Effects of acute and chronic cyclophosphamide treatment on meiotic progression and the induction of DNA double-strand breaks in rat spermatocytes

Male rats treated with cyclophosphamide, an alkylating agent commonly used clinically in both acute and chronic regimens, present with damaged male germ cells and abnormal progeny outcome. The extent and type of damage induced by cyclophosphamide largely depend on the germ cell type exposed to the drug and its ability to respond to insult. In the present study, the response of pachytene spermatocytes to damage was evaluated by assessing their ability to undergo meiotic G2/MI transition following exposure to acute or chronic cyclophosphamide. Male rats were given an acute high dose (70 mg/kg, once) or chronic low doses (6 mg/kg, daily for 5-6 wk) of cyclophosphamide. Pachytene spermatocytes were isolated, cultured, and induced to undergo G2/MI transition with okadaic acid. To determine the effect of DNA damage on meiotic progression, induction of DNA double-strand breaks was detected after each treatment regimen by the formation of foci of phosphorylated histone H2AX. The transition from G2 to MI was impaired after acute cyclophosphamide treatment; this impairment in the progression of pachytene spermatocytes was correlated with extensive DNA double-strand breaks. In contrast, despite the presence of significant levels of DNA damage, meiotic progression was not impaired in spermatocytes after chronic cyclophosphamide exposure. We suggest that the cell cycle impairment induced after acute cyclophosphamide treatment could be mediated by a G2/M checkpoint activated in response to DNA damage. The absence of impairment after chronic treatment raises concern about the functionality of defense mechanisms in male germ cells after repeated exposure to low doses of genotoxic agents.     

DNA polymerase-beta and poly(ADP)ribose polymerase mRNAs are differentially expressed during the development of male germinal cells.

We have examined the steady-state mRNA levels in spermatogenic cells of two nuclear enzymes that appear to be involved in DNA repair, DNA polymerase-beta (pol-beta) and poly(ADP)ribose polymerase (PADPRP). Two pol-beta mRNAs of 1.3 kb and 1.4 kb were detected in extracts from mouse testes. In leptotene/zygotene spermatocytes a low level of the 1.4-kb mRNA was observed. Both pol-beta mRNAs were found in meiotic pachytene spermatocytes, with the 1.3-kb form being more abundant. In contrast, the 1.4-kb form was more abundant in haploid round spermatids. Polysome gradient analyses indicated that the two pol-beta mRNAs were predominantly present in the nonpolysomal fractions of spermatocytes. In round spermatids, a larger fraction of the 1.4-kb pol-beta mRNA was associated with polysomes, correlating well with the higher levels of pol-beta enzyme detected during spermiogenesis. The pattern of PADPRP mRNA expression differed from the expression of pol-beta mRNA. The two PADPRP mRNAs of 3.7 and 3.8 kb were present in type A and type B spermatogonia, reached their highest levels in pachytene spermatocytes, and were greatly reduced in haploid round and elongating spermatids. Most of the pachytene spermatocyte PADPRP and mRNAs were present in polysomes, whereas a greater percentage of PADPRP mRNAs in round spermatids were detected in the nonpolysomal fractions. This finding correlates with the immunocytochemical nuclear localization of this enzyme in pachytene spermatocytes. These data demonstrate that different developmental patterns of mRNA expression and translational regulation exist for the pol-beta and PADPRP mRNAs during differentiation of male germinal cells.     

Human spermatogenic cell marker proteins detected by two-dimensional electrophoresis

Highly homogeneous populations of human pachytene spetmatocytes and round spermatids have been obtained from normal adult testis using unit gravity (STA-PUT) sedimentation. Contaminating Leydig cells have been removed by density centrifugation in discontinuous Percoll gradients to yield resultant germ cell purities of 90–95% for pachytene spermatocytes and 89–96% for round spermatids. The total cellular polypeptide composition of separated human germ cells has been analyzed by two-dimensional polyacrylamide gel electrophoresis to compare 1) human and mouse pachytene spermatocytes (species specificity), 2) samples of human spermatocytes obtained from different individuals (allo specificity), and 3) pachytene spermatocytes and round spermatids from the same patients (stage specificity). Mouse and human germ cells have been found to exhibit extensive homology, but identified marker proteins limited to human spermatocytes include a group of polypeptides at p45/5.9 as well as a protein at p67/5.2. Proteins unique to mouse germ cells include component p65/5.5. Comparisons between different preparations of human pachytene spermatocytes have revealed about 90% electrophoretic homology, but some striking allotypic variations have been noted including the proteins at p45/5.9. Finally, presumptive stage-specific spermatogenic cell markers have been identified including the p45/5.9 polypeptides that are present only in human spermatocytes. Although the physiological roles of particular marker proteins have not yet been determined, the present findings indicate that purified spermatogenic cell populations may be analyzed biochemically to identify constituents important in the regulation of sperm development in man.     

Germ cell removal after induction of cryptorchidism in adult rats

Cryptorchidism is associated with male infertility due to germ cell loss in response to elevated temperature. However, there is a great deal of contradictory information prevalent on the status of germ cells and their process of removal in the cryptorchid testis. In the present study, we investigate the cell removal from cryptorchid rat testis by the methods of morphology and stereology. The testis weight is reduced according to previous reports after surgical induction of cryptorchidism. Interestingly, the epididymal weight is significantly increased in 7 days after surgery, and the caput epididymis tubules show filling with countless round germ cells. We found that the elongating spermatids (steps 10-13), newborn spermatids (step 1) and the dividing spermatocytes are the most susceptible cells to elevated temperature, and are the first disappeared cells from the seminiferous tubules after surgery. Germ cell removal followed the order, starting first with elongating spermatids and newborn spermatids, followed by round spermatids and elongated spermatids and later extending to spermatocytes.     

Determination of daily sperm production in stallion testes by enumeration of germ cells in homogenates

Thirty adult stallion testes were selected with high (n = 15) and low (n = 15) Daily Sperm Production (DSP)/testis. Parenchymal samples were prepared for morphometric analysis, and the numbers of germ cells and Sertoli cells were determined. Testicular samples were homogenized, and germ cells and Sertoli cells were enumerated using phase contrast microscopy. Numbers of germ cells and Sertoli cells and potential DSP during spermatogenesis were determined. Significant correlations existed between morphometric and homogenate determinations of number per testis of preleptotene, leptotene plus zygotene primary spermatocytes (r = 0.58; P < 0.001), pachytene plus diplotene primary spermatocytes (r = 0.67; P < 0.0001), all primary spermatocytes (r = 0.67; P < 0.0001), round spermatids (r = 0.72; P < 0.0001), and Sertoli cells (r = 0.70; P < 0.0001). Significant correlations (P < 0.0001) existed between morphometric and homogenate determination of DSP/testis based on preleptotene, leptotene plus zygotene primary spermatocytes (r = 0.78), pachytene plus diplotene primary spermatocytes (r = 0.88), and round spermatids (r = 0.85). Using morphometric determination as the standard, the sensitivity (i.e., ability to detect low DSP/testis) and specificity (i.e., ability to detect high DSP/testis) by homogenate enumeration of germ cells was 81 and 93% for round spermatids, 100 and 24% for pachytene plus diplotene primary spermatocytes, and 67 and 87% for preleptotene, leptotene plus zygotene primary spermatocytes, respectively. Enumeration of primary spermatocytes in homogenates was less accurate than enumeration of round or elongated spermatids. Enumeration of round and elongated spermatids in homogenates was a rapid and useful method for determining DSP in horses, and it may prove to be a useful technique for quantitating potential DSP from testicular biopsies.     

Stage- and cell-specific expression of cyclic adenosine 3',5'-monophosphate-dependent protein kinases in rat seminiferous epithelium.

Expression of mRNAs in the rat testis encoding cyclic AMP (cAMP)-dependent protein kinases (PKAs) was studied. A microdissection method was used to isolate 10 pools of seminiferous tubules representing various stages of the cycle of the seminiferous epithelium in combination with Northern blots and in situ hybridization. The results showed a differential expression of the four isoforms of the regulatory subunits (PKA-R) at various stages of the cycle. RI alpha mRNA was detected at approximately the same levels at all stages while expression of RI beta mRNA was low at stages XIII-III, started to increase at stages IV-V, and reached a maximum at stages VIII-XI. The level of RII alpha mRNA was low at stages II-VI, increased markedly at stage VIIa,b, and reached maximal levels at stages VIIc,d and VIII, followed by a reduced expression at later stages, RII beta mRNA levels increased significantly at stage VI with maximal levels at stages VII and VIII. In situ hybridization of sections from the adult rat testis revealed RI alpha mRNA in the layers of pachytene spermatocytes and round spermatids of all stages. RI beta mRNA was detected over late pachytene spermatocytes and round spermatids of stages VII-XIII. RII alpha mRNA was seen in the layers of round spermatids of stages VII-VIII and elongating spermatids of later stages while RII beta mRNA was detected only in the round spermatid region of stages VII-VIII and in some tubules of stages I-VI. These data show that mRNAs encoding PKA-R are expressed in a stage-specific manner in differentiating male germ cells with different patterns of expression for each subunit; this suggests specific roles for these protein kinases at different times of spermatogenesis.     

beta-Nerve growth factor participates in an auto/paracrine pathway of regulation of the meiotic differentiation of rat spermatocytes

NGF appears to be involved in spermatogenesis. However, mice lacking NGF or TrkA genes do not survive more than a few days whereas p75(NTR) knockout mice are viable and fertile. Therefore, we addressed the effect of betaNGF on spermatogenesis by using the systems of rat germ cell culture we established previously. betaNGF did not modify the number of Sertoli cells, pachytene spermatocytes, secondary spermatocytes nor the half-life of round spermatids, but increased the number of secondary meiotic metaphases and decreased the number of round spermatids formed in vitro. These effects of betaNGF were reversible and maximal at about 4 x 10(-11) M. Conversely, K252a, a Trk-specific kinase inhibitor, enhanced the number of round spermatids above that of control cultures. The presence of betaNGF and its receptors TrkA and p75(NTR) was investigated in testis sections, in Sertoli cell and germ cell fractions, and in germ cell and Sertoli cell co-cultures. betaNGF was detected only in germ cells from pachytene spermatocytes of stages VII up to spermatids of stages IX-X. TrkA and p75(NTR) were detected in Sertoli cells and in these germ cells. Taken together, these results indicate that betaNGF should participate in an auto/paracrine pathway of regulation of the second meiotic division of rat spermatocytes in vivo.     

RNF17, a component of the mammalian germ cell nuage, is essential for spermiogenesis

Nuages are found in the germ cells of diverse organisms. However, nuages in postnatal male germ cells of mice are poorly studied. Previously, we cloned a germ cell-specific gene named Rnf17, which encodes a protein containing both a RING finger and tudor domains. Here, we report that RNF17 is a component of a novel nuage in male germ cells--the RNF17 granule, which is an electron-dense non-membrane bound spherical organelle with a diameter of 0.5 mum. RNF17 granules are prominent in late pachytene and diplotene spermatocytes, and in elongating spermatids. RNF17 granules are distinguishable from other known nuages, such as chromatoid bodies. RNF17 is able to form dimers or polymers both in vitro and in vivo, indicating that it may play a role in the assembly of RNF17 granules. Rnf17-deficient male mice were sterile and exhibited a complete arrest in round spermatids, demonstrating that Rnf17 encodes a novel key regulator of spermiogenesis. Rnf17-null round spermatids advanced to step 4 but failed to produce sperm. These results have shown that RNF17 is a component of a novel germ cell nuage and is required for differentiation of male germ cells.     

Developmental changes in cyclin B1 and cyclin-dependent kinase 1 (CDK1) levels in the different populations of spermatogenic cells of the post-natal rat testis

Spermatogenesis is a highly ordered process which requires mitotic and meiotic divisions. In this work, we studied the relative changes in the levels of the two components of the M-phase promoting factor (MPF): the regulatory subunit cyclin B1 (CycB1) and its catalytic subunit cdk1, in spermatogenic cells of rats between 16 and 90 days of life. A multivariate flow cytometry analysis of forward scatter (FSC), side scatter (SSC) and DNA content was used to identify six populations of rat germ cells: spermatogonia with preleptotene spermatocytes, young pachytene spermatocytes, middle to late pachytene spermatocytes, secondary spermatocytes with doublets of round spermatids, round spermatids, and elongated spermatids. For any population studied no significant difference in the relative cellular content of CycB1 or cdk1 proteins between animals of different ages was observed. By contrast, CycB1 and cdk1 levels were different between the different populations of germ cells. CycB1 and cdk1 were rather high in young pachytene spermatocytes and culminated in late spermatocytes, i.e. just before the first meiotic division. The relative levels of the two proteins remained high in secondary spermatocytes then decreased in round spermatids at the exit of meiosis. Similar results were obtained by Western-blot analysis of total proteins obtained from lysates of elutriated fractions of spermatocytes and spermatids. MPF activity was assessed in lysates of germ cells from 32-day-old rats or adult animals using p13suc1 agarose and histone H1 as an exogenous substrate. H1 kinase activity was higher in pachytene spermatocytes than in round spermatid fractions from both adult and young rats. These results indicate that the meiotic G2/M transition is associated to high levels of CycB1 and cdk1 leading to high MPF activity irrespective of the age of the animals.     

Indirect Sertoli cell-mediated ablation of germ cells in mice expressing the inhibin-alpha promoter/herpes simplex virus thymidine kinase transgene

In the present study, we describe a novel mouse model for inducible germ cell ablation. The mice express herpes simplex virus thymidine kinase (HSV-TK) under the inhibin-alpha subunit promoter (Inhalpha). When adult transgenic (TG) mice were treated with famciclovir (FCV) for 4 wk, their spermatogenesis was totally abolished, with only Sertoli cells and few spermatids remaining in the seminiferous tubules. However, testicular steroidogenesis was not affected. Shorter treatment periods allowed us to follow up the progression of germ cell death: After 3 days, spermatogonia and preleptotene spermatocytes were no longer present. After a 1-wk treatment, spermatogonia, preleptotene, and zygotene spermatocytes were missing and the amount of pachytene spermatocytes was decreased. After a 2-wk treatment, round and elongating spermatids were present. During the third week, round spermatids were lost and, finally, after a 4-wk treatment, only Sertoli cells and few spermatids were present. Interestingly, the transgene is detected in Leydig and Sertoli cells but not in spermatogonia. This suggests that FCV is phosphorylated in Sertoli cells, and thereafter, leaks to neighboring spermatogonia, apparently through cell-cell junctions present, enabling trafficking of phosphorylated FCV. Because of the many mitotic divisions they pass through, the spermatogonia are very sensitive to toxins interfering with DNA replication, while nondividing Sertoli cells are protected. Using transillumination-assisted microdissection of the seminiferous tubules, the gene-expression patterns analyzed corresponded closely to the histologically observed progression of cell death. Thus, the model offers a new tool for studies on germ cell-Sertoli cell interactions by accurate alteration of the germ cell composition in seminiferous tubules.     

Telomere length in male germ cells is inversely correlated with telomerase activity

Telomeres, the noncoding sequences at the ends of chromosomes, progressively shorten with each cellular division. Spermatozoa have very long telomeres but they lack telomerase enzymatic activity that is necessary for de novo synthesis and addition of telomeres. We performed a telomere restriction fragment analysis to compare the telomere lengths in immature rat testis (containing type A spermatogonia) with adult rat testis (containing more differentiated germ cells). Mean telomere length in the immature testis was significantly shorter in comparison to adult testis, suggesting that type A spermatogonia probably have shorter telomeres than more differentiated germ cells. Then, we isolated type A spermatogonia from immature testis, and pachytene spermatocytes and round spermatids from adult testis. Pachytene spermatocytes exhibited longer telomeres compared to type A spermatogonia. Surprisingly, although statistically not significant, round spermatids showed a decrease in telomere length. Epididymal spermatozoa exhibited the longest mean telomere length. In marked contrast, telomerase activity, measured by the telomeric repeat amplification protocol was very high in type A spermatogonia, decreased in pachytene spermatocytes and round spermatids, and was totally absent in epididymal spermatozoa. In summary, these results indicate that telomere length increases during the development of male germ cells from spermatogonia to spermatozoa and is inversely correlated with the expression of telomerase activity.