Functional robustness of adult spermatogonial stem cells after induction of hyperactive Hras

SPRY4-dependent ERK negative feedback demarcates functional adult stem cells in the male mouse germline†

Niche-derived growth factors support self-renewal of mouse spermatogonial stem and progenitor cells through ERK MAPK signaling and other pathways. At the same time, dysregulated growth factor-dependent signaling has been associated with loss of stem cell activity and aberrant differentiation. We hypothesized that growth factor signaling through the ERK MAPK pathway in spermatogonial stem cells is tightly regulated within a narrow range through distinct intracellular negative feedback regulators. Evaluation of candidate extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK)-responsive genes known to dampen downstream signaling revealed robust induction of specific negative feedback regulators, including Spry4, in cultured mouse spermatogonial stem cells in response to glial cell line-derived neurotrophic factor or fibroblast growth factor 2. Undifferentiated spermatogonia in vivo exhibited high levels of Spry4 mRNA. Quantitative single-cell analysis of ERK MAPK signaling in spermatogonial stem cell cultures revealed both dynamic signaling patterns in response to growth factors and disruption of such effects when Spry4 was ablated, due to dysregulation of ERK MAPK downstream of RAS. Whereas negative feedback regulator expression decreased during differentiation, loss of Spry4 shifted cell fate toward early differentiation with concomitant loss of stem cell activity. Finally, a mouse Spry4 reporter line revealed that the adult spermatogonial stem cell population in vivo is demarcated by strong Spry4 promoter activity. Collectively, our data suggest that negative feedback-dependent regulation of ERK MAPK is critical for preservation of spermatogonial stem cell fate within the mammalian testis.

A method for multiple sampling mouse sperm in vivo†

Mice are the most widely used animal model to study human diseases. However, the difficulty of in vivo recovery of mouse sperm has posed a limitation with its use in reproductive biology research. Several published techniques for obtaining sperm samples in vivo have been described, but most of them have several caveats. Critical limitations include poor reliability and significant mortality (Electroejaculation and drug-induced ejaculation), or the need for a large number of animals, careful programming, and laborious work (directed mating). Here, we describe a new approach for in vivo collection of sperm in the mouse via direct puncture of the epididymis to address these limitations. In addition, the technique is easy, safe, and reliable, allowing the animal to recover and maintain its fertility. In this way, punctual experiments could be carried out, or even more so, serial sampling of the same animal over time. Therefore, our approach allows for long-term and time-course experiments to study sperm characteristics under different treatments or conditions while maintaining the spermatogenic niche in vivo. In summary, we present our original approach as a powerful research tool to facilitate the study of spermatozoa relevant to various areas of biomedical research.

Defining Gene Function in Spermatogonial Stem Cells Through Conditional Knockout Approaches

Mammalian male fertility is maintained throughout life by a population of self-renewing mitotic germ cells known as spermatogonial stem cells (SSCs). Much of our current understanding regarding the molecular mechanisms underlying SSC activity is derived from studies using conditional knockout mouse models. Here, we provide a guide for the selection and use of mouse strains to develop conditional knockout models for the study of SSCs, as well as their precursors and differentiation-committed progeny. We describe Cre recombinase-expressing strains, breeding strategies to generate experimental groups, and treatment regimens for inducible knockout models and provide advice for verifying and improving conditional knockout efficiency. This resource can be beneficial to those aiming to develop conditional knockout models for the study of SSC development and postnatal function.

Loss of tyrosine kinase receptor Ephb2 impairs proliferation and stem cell activity of spermatogonia in culture†

Germline stem and progenitor cells can be extracted from the adult mouse testis and maintained long-term in vitro. Yet, the optimal culture conditions for preserving stem cell activity are unknown. Recently, multiple members of the Eph receptor family were detected in murine spermatogonia, but their roles remain obscure. One such gene, Ephb2, is crucial for maintenance of somatic stem cells and was previously found enriched at the level of mRNA in murine spermatogonia. We detected Ephb2 mRNA and protein in primary adult spermatogonial cultures and hypothesized that Ephb2 plays a role in maintenance of stem cells in vitro. We employed CRISPR-Cas9 targeting and generated stable mutant SSC lines with complete loss of Ephb2. The characteristics of Ephb2-KO cells were interrogated using phenotypic and functional assays. Ephb2-KO SSCs exhibited reduced proliferation compared to wild-type cells, while apoptosis was unaffected. Therefore, we examined whether Ephb2 loss correlates with activity of canonical pathways involved in stem cell self-renewal and proliferation. Ephb2-KO cells had reduced ERK MAPK signaling. Using a lentiviral transgene, Ephb2 expression was rescued in Ephb2-KO cells, which partially restored signaling and proliferation. Transplantation analysis revealed that Ephb2-KO SSCs cultures formed significantly fewer colonies than WT, indicating a role for Ephb2 in preserving stem cell activity of cultured cells. Transcriptome analysis of wild-type and Ephb2-KO SSCs identified Dppa4 and Bnc1 as differentially expressed, Ephb2-dependent genes that are potentially involved in stem cell function. These data uncover for the first time a crucial role for Ephb2 signaling in cultured SSCs.

Spermatogonial stem cells are a promising and pluripotent cell source for regenerative medicine

Regenerative medicine has been shown to hold enormous potential to treat traumatic and degenerative diseases, and substantial advancements have been made in the recent decades. In particular, different cell types were evaluated in basic research and preclinical studies on cell-based therapy applications. Despite the extraordinary achievements made in experimental studies and clinical practice, a considerable number of obstacles, such as the cellular source, ethical and safety issues, hinder further clinical applications. Spermatogonial stem cells (SSCs) are gradually becoming the research focus of cell-based regenerative medicine owing to their unique merits over other types of stem cells, particularly the lack of ethical concerns and lower immunogenicity. In addition, SSCs have been successfully induced to differentiate into other cell types under different appropriate conditions in compelling studies. Based on these properties, we systemically reviewed the development of SSCs as an attractive cell source for cell-based regenerative medicine.