Oct4-GFP expression during transformation of gonocytes into spermatogonial stem cells in the perinatal mouse testis

Comparison of POU5F1 gene expression and protein localization in two differentiated and undifferentiated spermatogonial stem cells

The POU domain, class 5, transcription factor 1 (POU5F1), plays a vital role in creating pluripotency and maintaining self-renewal of the spermatogonial stem cells (SSCs). In this experimental research, the gene and protein expression of POU5F1 in two populations of differentiated and undifferentiated spermatogonia were examined, by immunohistochemistry (IMH), immunocytochemistry (ICC) and Fluidigm real-time RT-PCR. Our study was extended with online databases and the creation of PPI networks. The results indicated that the POU5F1 protein was localized in the basal compartment of seminiferous tubules. Under in vitro conditions, isolated SSC colonies were ICC-positive for the POU5F1, but the protein expression level of POU5F1 in the undifferentiated populations was higher than that in differentiated. A significant POU5F1 mRNA expression was seen in passage 4 compared to passage 0 for both populations. POU5F1 has a significantly higher mRNA expression in undifferentiated SSCs than that in differentiated SSCs, also in mESCs than in SSC-like cells. Bioinformatic analysis on POU5F1 shows its impressive connection with other genes involved in spermatogonia differentiation. These results support the advanced investigations of spermatogonia differentiation, both in vitro and in vivo. A better understanding of the POU5F1 gene and its function during differentiation will give the scientific community an open perspective for the development of direct differentiation of SSC to other male germline cells which is very important in infertility treatment.

POU5F1 Protein and Gene Expression Analysis in Neonate and Adult Mouse Testicular Germ Cells by Immunohistochemistry and Immunocytochemistry

POU5F1 (POU class 5 homeobox 1) is a transcription factor that is critically involved in the self-renewal of undifferentiated embryonic stem cells. In this present study, we have developed our study to analyze the expression of the POU5F1 in the neonatal and adult mice testis section and isolated spermatogonial stem cells (SSCs). We also examine POU5F1 protein localization by three various kinds of antibodies. In this experimental research, to enhance our understanding of the POU5F1 expression levels, protein localization, and function in testicular germ cell, we used immunohistochemistry, immunocytochemistry, and Fluidigm real-time polymerase chain reaction (RT-PCR) analysis in the mouse testis section and neonatal and adult SSCs, and also we used protein-protein network analysis and gene enrichment analysis for genes involved in testicular development. Counting POU5F1-positive cells represented significantly higher expression (p < 0.05) of POU5F1 in the adult testis in comparison to the neonate. Finally, Fluidigm RT-PCR showed a significant expression (p < 0.05) level of germ cells gene POU5F1 in neonate SSCs (1-2 week) than 16-24 week SSCs. The illustrated results identify POU5F1 as a necessary transcription factor of testicular germ cells and can be supportive for the investigation of the development and differentiation of SSCs.

Postnatal germ cell development in cryptorchid boys

Cryptorchidism is associated with infertility in adulthood. Early orchiopexy is suggested to reduce the risk. Information is lacking on the potential link between infant germ cell maturation and the risk of future infertility. The objective of the study was to evaluate age-related germ cell development in cryptorchidism. Immunostaining for markers of germ cell development (octamer-binding transcription factor 3/4 [OCT3/4], placental alkaline phosphatase [PLAP], KIT proto-oncogene [C-KIT], podoplanin [D2-40], Lin-28 homolog A [LIN28], and G antigen 7 [GAGE-7]) was performed in testicular biopsies from 40 cryptorchid boys aged 4-35 months. Germ cell numbers and distributions were evaluated in cross sections of seminiferous tubules, with and without immunostaining. OCT3/4, D2-40, and LIN28 were generally expressed in the early stages of germ cell development, as shown by positive expression in germ cells in the central region of seminiferous tubules. In contrast, PLAP and GAGE-7 were expressed in both central and peripheral parts of the tubules in the early stages of development and expressed mainly in a peripheral position with advancing age. Germ cell maturation was delayed in this study population as compared with that observed in our previous study on germ cell markers in a healthy population. The number of GAGE-7-positive germ cells per tubular cross section obtained by immunostaining was significantly higher than that obtained by standard hematoxylin and eosin staining. Double immunostaining revealed heterogeneity in germ cell development in cryptorchid testes. These results shed light on the pathophysiology of germ cell development in boys with cryptorchidism.

Genome-wide DNA-binding profile of SRY-box transcription factor 3 (SOX3) in mouse testes

Spermatogenesis is the male version of gametogenesis, where germ cells are transformed into haploid spermatozoa through a tightly controlled series of mitosis, meiosis and differentiation. This process is reliant on precisely timed changes in gene expression controlled by several different hormonal and transcriptional mechanisms. One important transcription factor is SRY-box transcription factor 3 (SOX3), which is transiently expressed within the uncommitted spermatogonial stem cell population. Sox3-null mouse testes exhibit a block in spermatogenesis, leading to infertility or subfertility. However, the molecular role of SOX3 during spermatogonial differentiation remains poorly understood because the genomic regions targeted by this transcription factor have not been identified. In this study we used chromatin immunoprecipitation sequencing to identify and characterise the endogenous genome-wide binding profile of SOX3 in mouse testes at Postnatal Day 7. We show that neurogenin3 (Neurog3 or Ngn3) is directly targeted by SOX3 in spermatogonial stem cells via a novel testes-specific binding site. We also implicate SOX3, for the first time, in direct regulation of histone gene expression and demonstrate that this function is shared by both neural progenitors and testes, and with another important transcription factor required for spermatogenesis, namely promyelocytic leukaemia zinc-finger (PLZF). Together, these data provide new insights into the function of SOX3 in different stem cell contexts.

The role of gonadotrophins in gonocyte transformation during minipuberty

PURPOSE

Postnatal surge of gonadotrophins, Luteinizing hormone (LH) and Follicle-Stimulating hormone (FSH) known as minipuberty, is critical for gonocyte maturation into spermatogonial stem cells (SSC) in the testis. Gonadotrophins are essential for optimum fertility in men, but very little is known how they regulate germ cells during minipuberty. This study examined whether gonadotrophins play a role on gonocyte transformation in vivo.

METHODS

Testes from hypogonadal (hpg) mice and their wild type (WT) littermates (n = 6/group) were weighed, and processed in paraffin at postnatal days (D) 0, 3, 6 and 9. Mouse VASA homologue (germ cell marker), anti-Müllerian hormone (Sertoli cell marker) antibodies and DAPI (nuclei marker) were used for immunofluorescence followed by confocal imaging. Germ cells on or off basement membrane (BM) and Sertoli cells/tubule were counted using Image J and analyzed with GraphPad.

RESULTS

Comparing to WT littermates, there were significantly fewer germ cells on BM/tubule (p < 0.05) in D9 hpg mice, whereas there was no significant difference for germ cells off BM/tubule and Sertoli cells/tubule between littermates. However, testicular weight was significantly reduced in D3-D9 hpg mice comparing to WT littermates.

CONCLUSION

Gonadotrophin deficiency reduced D9 germ cells on BM indicating impaired gonocyte transformation into SSC. This suggests that gonadotrophins may mediate gonocyte transformation during minipuberty.

Does the apoptosis pathway play a critical role in gonocyte transformation?

PURPOSE

Undescended Testes (UDT) are prevalent in 2%-5% of male infants and cause malignancy and infertility. During germ cell development, abnormal gonocytes usually undergo apoptosis. This process is believed to involve BAX (Bcl-2 Associated X) protein in clearing abnormal gonocytes which may fail in UDT, resulting in persisting gonocytes causing seminomas later in life. We aim to investigate the role of BAX in gonocyte apoptosis.

MATERIALS AND METHODS

BAXKO (BAX-knockout) mice were back-crossed to OG2 mice (Oct4-promoter driving enhanced green fluorescent protein-eGFP) to produce BAXOG2 mice. Testes (wildtype-BAX^+/+, heterozygous-BAX^+/- and homozygous-BAX^-/- mice, n = 6/group) on postnatal days 1, 3, 6, 9 were fixed and embedded in OCT for frozen sectioning. Sections were labeled with Anti-Müllerian Hormone (Sertoli cell marker), Mouse Vasa Homolog (germ cell marker) and DAPI (nucleus marker) and imaged using confocal microscopy. Oct4-GFP+ve germ cells, germ cells on/off the basement membrane and Sertoli cells were counted using ImageJ followed by data analysis with GraphPad.

RESULTS

BAX^-/-OG2 mice had significantly higher number of germ cells/tubule comparing to BAX^+/+OG2 on day 9. There were Oct4-GFP+ve gonocyte-like germ cells that persisted in the center of the tubules in BAX^-/-OG2 even after the completion of gonocyte transformation. This suggests that abnormal gonocytes in BAX^-/-OG2 mice failed to undergo apoptosis and are allowed to persist.

CONCLUSION

This study demonstrated that apoptosis is important in regulating germ cell migration and differentiation during gonocyte transformation in neonatal mice. In addition, inhibition of apoptosis results in persisting neonatal gonocytes which might become seminomas in patients with UDT.

Cryptorchidism, gonocyte development, and the risks of germ cell malignancy and infertility: A systematic review

BACKGROUND/AIM

Cryptorchidism, or undescended testis (UDT) occurs in 1%-4% of newborn males and leads to a risk of infertility and testicular malignancy. Recent research suggests that infertility and malignancy in UDT may be caused by abnormal development of the neonatal germ cells, or gonocytes, which normally transform into spermatogonial stem cells (SSC) or undergo apoptosis during minipuberty at 2-6 months in humans (2-6 days in mice). We aimed to identify the current knowledge on how UDT is linked to infertility and malignancy.

METHODS

Here we review the literature from 1995 to the present to assess the possible causes of infertility and malignancy in UDT, from both human studies and animal models.

RESULTS

Both the morphological steps and many of the genes involved in germ cell development are now characterized, but the factors involved in gonocyte transformation and apoptosis in both normal and cryptorchid testes are not fully identified. During minipuberty there is evidence for the hypothalamic-pituitary axis stimulating gonocyte transformation, but without known direct control by LH and androgen, although FSH may have a role. An arrested gonocyte maybe the origin of later malignancy at least in syndromic cryptorchid testes in humans, which is consistent with the recent finding that gonocytes are normally absent in a rodent model of congenital cryptorchidism, where malignancy has not been reported.

CONCLUSION

The results of this review strengthen the view that malignancy and infertility in men with previous UDT may be caused by abnormalities in germ cell development during minipuberty.

TYPE OF STUDY

Systematic review (secondary, filtered) LEVEL OF EVIDENCE: Level I.

Postnatal germ cell development during first 18 months of life in testes from boys with non-syndromic cryptorchidism and complete or partial androgen insensitivity syndrome

INTRODUCTION

Neonatal testicular germ cells/gonocytes, transform into stem cells for spermatogenesis during 'minipuberty', driving change in timing of surgery. This study examined gonocyte transformation in cryptorchid testes in children ≤18 months of age with unilateral, bilateral undescended testes (UDT), complete or partial androgen insensitivity syndrome (CAIS, PAIS) [3,4].

MATERIAL AND METHODS

Testicular biopsies were taken from patients with unilateral or bilateral UDT, PAIS or CAIS, aged 10 days-18 months. These testicular sections underwent immunohistochemistry with antibodies (Oct4, Ki67, C-Kit, Sox9) followed by confocal imaging, cell counting and statistical analysis.

RESULTS

Both Sertoli cells/tubule and germ cells (GC)/tubule decreased with age, and % empty tubules (no GC) increased with age but with no significant differences between patient groups. Oct4⁺ germ cells/tubule decreased with age. There are some GCs and Sertoli cells proliferating during the first year and most proliferating Oct4⁺ germ cells (Oct4⁺/Ki67⁺) were located off tubular basement membrane.

CONCLUSION

Our study showed that Oct4 expression gradually decreased after minipuberty and transformation into spermatogonia. Germ cells and Sertoli cells undergo mitosis during the first 12 months although not abundantly. We propose that Oct4⁺ gonocyte transformation into spermatogonia via proliferation and migration to the basement membrane may be delayed in UDT.

Mouse minipuberty coincides with gonocyte transformation into spermatogonial stem cells: a model for human minipuberty

As the transient postnatal hormone surge in humans, known as 'minipuberty', occurs simultaneously with key steps in germ-cell development, we investigated whether similar changes occur in the hypothalamic-pituitary-testicular axis of neonatal mice at a time that would coincide with gonocyte transformation into spermatogonial stem cells (SSC). Serum and testes were collected from C57Bl/6 mice at embryonic Day 17 (E17), birth (postnatal Day 0; P0) and daily until P10. Serum FSH and testosterone levels in both serum and testes were analysed and gene expression of FSH receptor (Fshr), luteinising hormone receptor (Lhr), anti-Müllerian hormone (Amh), octamer-binding transcription factor 4 (Oct-4), membrane type 1 metalloprotease (Mt1-mmp), proto-oncogene C-kit and promyelocytic leukaemia zinc finger (Plzf ) was quantified by real-time polymerase chain reaction. We found a transient surge of serum and testicular testosterone levels between P1 and P3 and a gradual increase in FSH from P1 to P10. Testis Lhr expression remained low from P0 until P10 but Fshr expression peaked between P3 and P6 (P<0.01). The same was found for Oct-4 expression (a gonocyte marker), which surged between P3 and P6 (P<0.01). Mt1-mmp expression peaked at P3 (P<0.05). The expression pattern of both C-kit and Plzf (SSC markers) was similar with a steady increase from P1 to P10. These results show a transient activation of the hypothalamic-pituitary-testicular axis postnatally with increases in serum and testicular testosterone at P1-P3 and testicular Fshr (but not Lhr) at P3-P6. These changes coincide with increases in gene expression of Oct4, Mt1-mmp, Plzf and C-kit, reflecting gonocyte activation, migration and transformation into SSC. In conclusion, these findings suggest that 'minipuberty' does occur in mice and that gonocyte transformation may be driven by a transient FSH signalling pathway.

Functional assessment of spermatogonial stem cell purity in experimental cell populations

Historically, research in spermatogonial biology has been hindered by a lack of validated approaches to identify and isolate pure populations of the various spermatogonial subsets for in-depth analysis. In particular, although a number of markers of the undifferentiated spermatogonial population have now been characterized, standardized methodology for assessing their specificity to the spermatogonial stem cell (SSC) and transit amplifying progenitor pools has been lacking. To date, SSC content within an undefined population of spermatogonia has been inferred using either lineage tracing or spermatogonial transplantation analyses which generate qualitative and quantitative data, respectively. Therefore, these techniques are not directly comparable, and are subject to variable interpretations as to a readout that is representative of a 'pure' SSC population. We propose standardization across the field for determining the SSC purity of a population via use of a limiting dilution transplantation assay that would eliminate subjectivity and help to minimize the generation of inconsistent data on 'SSC' populations. In the limiting dilution transplantation assay, a population of LacZ-expressing spermatogonia are selected based on a putative SSC marker, and a small, defined number of cells (i.e. 10 cells) are microinjected into the testis of a germ cell-deficient recipient mouse. Using colony counts and an estimated colonization efficiency of 5%; a quantitative value can be calculated that represents SSC purity in the starting population. The utilization of this technique would not only be useful to link functional relevance to novel markers that will be identified in the future, but also for providing validation of purity for marker-selected populations of spermatogonia that are commonly considered to be SSCs by many researchers in the field of spermatogenesis and stem cell biology.

Progress and future prospect of in vitro spermatogenesis

Infertility has become a major health issue in the world. It affects the social life of couples and of all infertility cases; approximately 40–50% is due to “male factor” infertility. Male infertility could be due to genetic factors, environment or due to gonadotoxic treatment. Developments in reproductive biotechnology have made it possible to rescue fertility and uphold biological fatherhood. In vitro production of haploid male germ cell is a powerful tool, not only for the treatment of infertility including oligozoospermic or azoospermic patient, but also for the fertility preservation in pre-pubertal boys whose gonadal function is threatened by gonadotoxic therapies. Genomic editing of in-vitro cultured germ cells could also potentially cure flaws in spermatogenesis due to genomic mutation. Furthermore, this ex-vivo maturation technique with genomic editing may be used to prevent paternal transmission of genomic diseases. Here, we summarize the historical progress of in vitro spermatogenesis research by using organ and cell culture techniques and the future clinical application of in vitro spermatogenesis.