Comparison of stem-spermatogonial renewal and mitotic activity in the gamma-irradiated mouse and rat

Radiotherapy and Testicular Function: A Comprehensive Review of the Radiation-Induced Effects with an Emphasis on Spermatogenesis

This comprehensive review explores the existing literature on the effects of radiotherapy on testicular function, focusing mainly on spermatogenic effects, but also with a brief report on endocrine abnormalities. Data from animal experiments as well as results on humans either from clinical studies or from accidental radiation exposure are included to demonstrate a complete perspective on the level of vulnerability of the testes and their various cellular components to irradiation. Even relatively low doses of radiation, produced either from direct testicular irradiation or more commonly from scattered doses, may often lead to detrimental effects on sperm count and quality. Leydig cells are more radioresistant; however, they can still be influenced by the doses used in clinical practice. The potential resultant fertility complications of cancer radiotherapy should be always discussed with the patient before treatment initiation, and all available and appropriate fertility preservation measures should be taken to ensure the future reproductive potential of the patient. The topic of potential hereditary effects of germ cell irradiation remains a controversial field with ethical implications, requiring future research.

Inhibition of p38MAPK signalling pathway alleviates radiation-induced testicular damage through improving spermatogenesis

BACKGROUND AND PURPOSE

Damage to the testis following exposure to ionizing radiation has become an urgent problem to be solved. Here we have investigated if inhibition of p38 mitogen-activated protein kinase (p38MAPK) signalling could alleviate radiation-induced testicular damage.

EXPERIMENTAL APPROACH

In mice exposed to whole body radiation (2-6 Gy), morphological changes of the epididymis and testis was measured by histochemical staining. immunohistochemical and immunofluorescence procedures and western blotting were used to monitor expression and cellular location of proteins. Expression of genes was assessed by qPCR and RNA-Seq was used to profile gene expression.

KEY RESULTS

Exposure to ionizing radiation induced dose-dependent damage to mouse testis. The sperm quality decreased at 6 and 8 weeks after 6 Gy X-ray radiation. Radiation decreased PLZF+ cells and increased SOX9+ cells, and affected the expression of 969 genes, compared with data from non-irradiated mice. Expression of genes related to p38MAPK were enriched by GO analysis and were increased in the irradiated testis, and confirmed by qPCR. Levels of phospho-p38MAPK protein increased at 28 days after irradiation. In irradiated mice, SB203580 treatment increased spermatozoa, SOX9+ cells, the area and diameter of seminiferous tubules, sperm movement rate and density. Furthermore, SB203580 treatment increased SCP3+ cells, accelerating the process of spermatogenesis.

CONCLUSION AND IMPLICATIONS

Exposure to ionizing radiation clearly changed gene expression in mouse testis, involving activation of p38MAPK signalling pathways. Inhibition of p38MAPK by SB203580 partly alleviated the testicular damage caused by radiation and accelerated the recovery of sperms through promoting spermatogenesis.

The hot-spot p53R172H mutant promotes formation of giant spermatogonia triggered by DNA damage

Overexpression of mutant p53 is a common finding in most cancers but testicular tumours accumulate wild-type p53 (wtp53). In contrast to the accepted concept that p53 homozygous mutant mice do not accumulate mutant p53 in normal cells, our study on a mutant p53 mouse model of Li-Fraumeni syndrome harbouring the hot-spot p53R172H mutation described an elevated level of mutant p53 in non-cancerous mouse tissues. Here we use detailed immunohistochemical analysis to document the expression of p53R172H in mouse testis. In developing and adult testes, p53R172H was expressed in gonocytes, type A, Int, B spermatogonia as well as in pre-Sertoli cells and Leydig cells but was undetectable in spermatocytes and spermatids. A similar staining pattern was demonstrated for wtp53. However, the intensity of wtp53 staining was generally weaker than that of p53R172H, which indicates that the expression of p53R172H can be a surrogate marker of p53 gene transcription. Comparing the responses of wtp53 and p53R172H to irradiation, we found persistent DNA double-strand breaks in p53R172H testes and the formation of giant spermatogonia (GSG) following persistent DNA damage in p53R172H and p53-null mice. Strikingly, we found that p53R172H promotes spontaneous formation of GSG in non-stressed p53R172H ageing mice. Two types of GSG: Viable and Degenerative GSG were defined. We elucidate the factors involved in the formation of GSG: the loss of p53 function is a requirement for the formation of GSG whereas DNA damage acts as a promoting trigger. The formation of GSG does not translate to higher efficacy of testicular tumorigenesis arising from mutant p53 cells, which might be due to the presence of delayed-onset of p53-independent apoptosis.

Rat models of post-irradiation recovery of spermatogenesis: interstrain differences

Recently, we reported large differences between rat strains in spermatogenesis recovery at 10 weeks after 5-Gy irradiation suggesting that there are interstrain as well as interspecies differences in testicular radiation response. To determine whether these interstrain differences in sensitivity might be a result of the particular dose and time-point chosen, we performed dose-response and time-course studies on sensitive Brown-Norway (BN) and more resistant spontaneously hypertensive rats (SHR) and Sprague-Dawley (SD) rats. Type A spermatogonia were observed in atrophic tubules at 10 weeks after irradiation in all strains indicating that tubular atrophy was caused by a block in their differentiation, but the doses to produce the block ranged from 4.0 Gy in BN to 10 Gy in SD rats. Although the numbers of type A spermatogonial were unaffected at doses below 6 Gy, higher doses reduced their number, indicating that stem cell killing also contributed to the failure of recovery. After 10 weeks, there was no further recovery and even a decline in spermatogonial differentiation in BN rats, but in SHR rats, sperm production returned to control levels by 20 weeks after 5.0 Gy and, after 7.5 Gy, differentiation resumed in 60% of tubules by 30 weeks. Suppression of testosterone and gonadotropins after irradiation restored production of differentiated cells in nearly all tubules in BN rats and in all tubules in SHR rats. Thus, the differences in recovery of spermatogenesis between strains were a result of both quantitative differences in their sensitivities to a radiation-induced, hormone-dependent block of spermatogonial differentiation and qualitative interstrain differences in the progression of post-irradiation recovery. The progression of recovery in SHR rats was similar to the prolonged delays in recovery of human spermatogenesis after cytotoxic agent exposure and thus may be a system for investigating a phenomenon also observed in men.

Differences in radiation sensitivity of recovery of spermatogenesis between rat strains

Previous studies with Lewis/Brown-Norway (BN) F1 hybrid rats indicated that spermatogenesis was much more sensitive to ionizing radiation than in the widely studied outbred Sprague Dawley stock, suggesting that there were genetically based differences; however, the relative sensitivities of various inbred strains had not been established. As a first step to defining the genes responsible for these differences, we compared the sensitivities of seven rat strains to radiation damage of spermatogenesis. Recovery of spermatogenesis was examined 10 weeks after 5-Gy irradiation of seven strains (BN, Lewis, Long-Evans, Wistar Kyoto, spontaneously hypertensive [SHR], Fischer 344, and Sprague Dawley). The percentages of tubules containing differentiated cells and testicular sperm counts showed that BN and Lewis were most sensitive to radiation (< 2% of tubules recovered, < 2 × 10(5) late spermatids per testis), Long-Evans, Wistar Kyoto, Fischer, and SHR were more resistant, and Sprague Dawley was the most resistant (98% of tubules recovered, 2 × 10(7) late spermatids per testis). Although increases in intratesticular testosterone levels and interstitial fluid volume after irradiation had been suggested as factors inhibiting recovery of spermatogenesis, neither appeared to correlate with the radiation sensitivity of spermatogenesis in these strains. In all strains, the atrophic tubules without differentiated germ cells nevertheless showed the presence of type A spermatogonia, indicating that their differentiation was blocked. Thus, we conclude that the differences in radiation sensitivity of recovery of spermatogenesis between rat strains of different genetic backgrounds can be accounted for by differences in the extent of the radiation-induced block of spermatogonial differentiation.

Asymmetric distribution of UCH-L1 in spermatogonia is associated with maintenance and differentiation of spermatogonial stem cells

Asymmetric division of germline stem cells in vertebrates was proposed a century ago; however, direct evidence for asymmetric division of mammalian spermatogonial stem cells (SSCs) has been scarce. Here, we report that ubiquitin carboxy-terminal hydrolase 1 (UCH-L1) is expressed in type A (A(s), A(pr), and A(al)) spermatogonia located at the basement membrane (BM) of seminiferous tubules at high and low levels, but not in differentiated germ cells distant from the BM. Asymmetric segregation of UCH-L1 was associated with self-renewal versus differentiation divisions of SSCs as defined by co-localization of UCH-L1(high) and PLZF, a known determinant of undifferentiated SSCs, versus co-localization of UCH-L1(low/-) with proteins expressed during SSC differentiation (DAZL, DDX4, c-KIT). In vitro, gonocytes/spermatogonia frequently underwent asymmetric divisions characterized by unequal segregation of UCH-L1 and PLZF. Importantly, we could also demonstrate asymmetric segregation of UCH-L1 and PLZF in situ in seminiferous tubules. Expression level of UCH-L1 in the immature testis where spermatogenesis was not complete was not affected by the location of germ cells relative to the BM, whereas UCH-L1-positive spermatogonia were exclusively located at the BM in the adult testis. Asymmetric division of SSCs appeared to be affected by interaction with supporting somatic cells and extracelluar matrix. These findings for the first time provide direct evidence for existence of asymmetric division during SSCs self-renewal and differentiation in mammalian spermatogenesis.

Enhancement of A spermatogonial proliferation and differentiation in irradiated rats by gonadotropin-releasing hormone antagonist administration

The initial changes in the numbers, proliferation, and differentiation of A spermatogonia in irradiated rats after the administration of a GnRH antagonist, which is known to induce differentiation in this system, were investigated. LBNF1 rats were given 6 Gy of gamma-irradiation; some were treated with the GnRH antagonist Cetrorelix beginning 15 weeks after irradiation. Although the spermatogonia in the irradiated rats without hormone treatment continue to proliferate (labeling and mitotic indexes of 24% and 18%, respectively), they underwent apoptosis (apoptotic indexes of 21% by the terminal transferase-mediated end labeling assay and 9% by nuclear morphology), resulting in a constant number of A spermatogonia. Whole mount analysis of clones ofA spermatogonia revealed that larger clones were more likely to undergo apoptosis than mitosis. Hormone administration decreased the intratesticular testosterone concentration to 6% of the level in irradiated rats within 1 week. Concomitantly, there was a decrease in spermatogonial apoptotic indexes to 43% of levels in irradiated-only rats, leading to an increases in their numbers by 150%, their diameters by 11%, and their labeling indexes by 31%. The sizes of the mitotic clones gradually increased, and clones of more than eight cells appeared at week 3 of hormone treatment. A spermatogonial differentiation began at week 4, and by week 6.6, differentiation occurred in 30% of the tubules. Thus, suppression of intratesticular testosterone by the GnRH antagonist may be responsible for the immediate changes in spermatogonial numbers and kinetics, but several additional steps are required before differentiation begins, which did not occur until week 4.

The tao of stem cells in the germline

Germline stem cells (GSCs) are the self-renewing population of germ cells that serve as the source for gametogenesis. GSCs exist in diverse forms, from those that undergo strict self-renewing asymmetric divisions in Drosophila to those that maintain their population by balancing between mitosis and differentiation in Caenorhabditis elegans. Most vertebrate spermatogonial GSCs appear to adopt an intermediate strategy. In most animals, GSCs are established during preadult gonadogenesis following the proliferation and migration of embryonic primordial germ cells. GSCs produce numerous gametes throughout the sexually active period of adult life. The establishment and self-renewing division of GSCs are controlled by extracellular signals such as hormones from the hypothalamic-pituitary axis and local interactions between GSCs and their neighboring cells. These extracellular signals may then influence differential gene expression, cell cycle machinery, and cytoskeletal organization of GSCs for their formation and/or divisional asymmetry. In addition, the GSC mechanism is related to that for germline and sex determination. Current knowledge has provided a solid framework for further study of GSCs and stem cells in general.

Probability of self-renewing divisions of spermatogonial stem cells in colonies, formed after fission neutron irradiation

Repopulating spermatogenic colonies, found in the seminiferous epithelium after irradiation with fast-fission neutrons, were studied to determine the chance that a stem cell Asingle (As) spermatogonium would complete a self-renewing division (P). Mathematical formulas originally derived for such studies in haemopoietic colonies were employed, and a method specifically aimed at spermatogenic colonies was developed. The results showed that during the first division after irradiation, P is close to 1.0. P decreases in later generations, but remains 0.7 or higher up to the 4th or 5th divisions. The mean value for P was over 0.8, which is higher than the value of 0.6-0.7 found for stem cells in haemopoietic colonies.

Regulation of spermatogonial proliferation

In nonprimates the spermatogonial compartment can be subdivided into (morphologically) undifferentiated spermatogonia and differentiating spermatogonia. Each cycle of the seminiferous epithelium the proliferative activity of the undifferentiated spermatogonia is stimulated, probably by factors secreted by Sertoli cells. Subsequently, during a period of active proliferation many Aal spermatogonia are formed. In the normal situation around epithelial stage III, proliferation is inhibited by the differentiating spermatogonia by way of a negative feedback system, probably involving a spermatogonial chalone. Then most of the Aal spermatogonia formed differentiate into Al spermatogonia. For this differentiation vitamin A, or factors secreted by Sertoli cells under the influence of vitamin A, is/are necessary. In the normal situation there is no regulation of the density of the undifferentiated spermatogonia. Different tubular areas can contain widely varying numbers of stem cells and other undifferentiated spermatogonia and consequently can produce widely varying numbers of Al spermatogonia. Only in extreme circumstances, such as after irradiation, the stem cells change the ratio between self-renewing and differentiating divisions in favor of self-renewal. Furthermore, in this situation the proliferation of the undifferentiated spermatogonia is not inhibited at epithelial stage III because of the lack of differentiating spermatogonia. Density regulation does take place during the development of the differentiating spermatogonia. In the chinese hamster it appeared that despite the variation in the numbers of Al spermatogonia produced in different areas, the density of the preleptotene spermatocytes was very much the same. It was found that the even distribution of spermatocytes in the epithelium was achieved by a density-dependent degeneration of differentiating spermatogonia in such a way that many of the latter cells degenerated in high density areas and only few or none in low density areas. In primates the undifferentiated spermatogonia can be subdivided into Ap and Ad spermatogonia. Both Ap and Ad spermatogonia can be seen to be topographically arranged in clones of 1 or 2n cells in situations in which their density is low. The Ad spermatogonia do not proliferate, but after cell loss these cells were found to transform into Ap spermatogonia that start to proliferate. The Ap spermatogonia only divide once every epithelial cycle, renewing themselves and giving rise to B spermatogonia. In the monkey the number of Ap spermatogonia could be increased by FSH treatment. Hence, there may be a correlation between FSH levels and the numbers of Ap spermatogonia. Fu

Feedback regulation of the proliferation of the undifferentiated spermatogonia in the Chinese hamster by the differentiating spermatogonia

In the seminiferous epithelium the differentiating spermatogonia proliferate following a very strict synchronous pattern, and undergo the S phase during parts of particular epithelial stages. The undifferentiated spermatogonia do not divide synchronously and display maximum proliferative activity in stages XI-III. Hence the S-phase-specific cytotoxic agent Ara-C kills different proportions of these two cell types dependent on the epithelial stage. We have studied the effect of several combinations of degrees of cell loss to both compartments on proliferation of the undifferentiated spermatogonia. It was found that when the differentiating spermatogonia are removed, the proliferation of the undifferentiated spermatogonia is not inhibited at epithelial stage III, as seen in controls. However, when the undifferentiated spermatogonia were already arrested in G1, removal of the differentiating spermatogonia did not evoke proliferation again. When the population of undifferentiated spermatogonia was reduced in an area where the differentiating spermatogonia were left intact, the inhibition of the proliferation of undifferentiated spermatogonia took place around stage III as usual. It is concluded that in the normal adult seminiferous epithelium, the length of the period of active proliferation of the undifferentiated spermatogonia is regulated by negative feedback from the differentiating spermatogonia.

Reproductive and genetic effects of continuous prenatal irradiation in the pig

The stem germ cells of the prenatal pig are highly vulnerable to the cytotoxic effects of ionizing irradiation. This study was conducted to determine whether sensitivity to killing was also marked by a sensitivity to mutation and how prenatal depletion of the germ-cell population affects reproductive performance. Germ-cell populations were reduced by continuously irradiating sows at dose rates of either 0.25 or 1.0 rad/day for the first 108 days of gestation. The prenatally irradiated boars were tested for sperm-producing ability, sperm abnormalities, dominant lethality, reciprocal translocations, and fertility. Prenatally irradiated females were allowed to bear and nurture one litter, then tested for dominant lethality in a second litter; germ cell survival and follicular development were assessed in their serially sectioned ovaries. Sperm production was not significantly affected in the 0.25-rad boars, but boars irradiated with 1.0 rad per day produced sperm at only 17% of the control level. Incidence of defective sperm was 4.9% and 11.1% in the 0.25 and 1.0 groups, respectively. Four of the 1.0-rad boars were infertile, but prenatal irradiation apparently caused neither dominant lethality nor reciprocal translocations in fertile males. Number of oocytes was reduced to 66 +/- 7% of control in the 0.25-rad gilts, but reproductive performance was unaffected and no dominant lethality was observed. Only 7 +/- 1% of the oocytes survived in the 1.0-rad group. Reproductive performance was normal for the first litter, but four of the 23 sows tested were infertile at the second litter and a significant incidence of dominant lethality was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the re-establishment of a heterogeneity in radiosensitivity among spermatogonial stem cells repopulating the mouse testis following depletion by X-rays

Earlier studies have shown that the spermatogonial stem cells of the mouse testis recovering from previous radiation or chemical mutagen exposure give subnormal yields of genetic damage with subsequent X-irradiation. This response has been investigated further: (a) with a high, 9-Gy X-ray dose given 4, 12 or 21 days after a 1-Gy conditioning dose (Expt. 1), and (b) with a 1 + 7-Gy, 24-h fractionation regime given 4 or 14 days after a 1-Gy conditioning dose (Expt. 2). In Expt. 1 the 1 + 9-Gy, 4-day interval regime gave a very low response, lower than obtained previously with an equivalent 1 + 5-Gy treatment. This suggests that a heterogeneity in radiosensitivity, such as exists in unirradiated stem cell populations and absent 24-48 h after radiation depletion, is quickly re-established among the stem cells repopulating the testis. By contrast, the 1 + 7-Gy, 24-h fractionation when given 4 days after the 1-Gy conditioning dose (Expt. 2) gave a very high yield of genetic damage, almost as high as that given by the fractionated (1 + 7 Gy) dose applied to previously unirradiated stem cells. This suggests that the newly established heterogeneity is removed by the second 1-Gy conditioning dose. With longer intervals between treatments, genetic yields consistent with additivity were obtained in Expt. 1; less clear results were obtained Expt. 2. Comparison with earlier data generally suggested that the duration of the repopulating period is dose-dependent. In a third experiment evidence was obtained that genetic damage induced by X-irradiation can be reduced by a subsequent treatment with triethylenemelamine (TEM) during the repopulating phase. This confirmed an earlier finding. Such an interaction could not be demonstrated with two X-ray treatments. An explanation for the X-ray/TEM interaction is offered.

Stem-spermatogonial survival and incidence of reciprocal translocations in the gamma-irradiated boar

To assess the effects of gamma-radiation on stem-cell survival and incidence of reciprocal translocations, boar testes were irradiated with 100, 200, or 400 rad. Stem-cell survival was markedly affected by 100 rad (51% of control) and reduced to 34% of control by 400 rad. Production of differentiating spermatogonia was all but completely interrupted by 200 rad and spermatogonial renewal was incomplete at 12 weeks. From the state of the seminiferous epithelium at 12 weeks, estimates of the percentage of permanent impairment of sperm-producing capacity ranged from 20 +/- 6 (100 rad) to 67 +/- 10 (400 rad). Incidence of translocations peaked at 200 rad and the number occurring at 100 and 400 rad was similar. Kinetics of porcine spermatogonial renewal differs considerably from those of the rodent and, relative to the rodent, this may account for the boar's higher sensitivity to stem-cell killing and lower sensitivity to translocations.