Human fertilization in vivo and in vitro requires the CatSper channel to initiate sperm hyperactivation.

The infertility of many couples rests on an enigmatic dysfunction of the man's sperm. To gain insight into the underlying pathomechanisms, we assessed the function of the sperm-specific multisubunit CatSper-channel complex in the sperm of almost 2,300 men undergoing a fertility workup, using a simple motility-based test. We identified a group of men with normal semen parameters but defective CatSper function. These men or couples failed to conceive naturally and upon medically assisted reproduction via intrauterine insemination and in vitro fertilization. Intracytoplasmic sperm injection (ICSI) was, ultimately, required to conceive a child. We revealed that the defective CatSper function was caused by variations in CATSPER genes. Moreover, we unveiled that CatSper-deficient human sperm were unable to undergo hyperactive motility and, therefore, failed to penetrate the egg coat. Thus, our study provides the experimental evidence that sperm hyperactivation is required for human fertilization, explaining the infertility of CatSper-deficient men and the need of ICSI for medically assisted reproduction. Finally, our study also revealed that defective CatSper function and ensuing failure to hyperactivate represents the most common cause of unexplained male infertility known thus far and that this sperm channelopathy can readily be diagnosed, enabling future evidence-based treatment of affected couples.

GM-CSF perturbs cell identity in mouse pre-implantation embryos.

Growth factors became attractive candidates for medium supplementation to further improve the quality of embryo culture and to mimic in vivo nutrition. Granulocyte macrophage colony-stimulating factor (GM-CSF) is a cytokine influencing the maternal-fetal interface and supporting placental development in mouse and human. It is expressed in epithelial cells of the endometrium under the regulation of estrogens. The factor is already in clinical use and a large clinical trial showed that, if supplemented to an embryo culture medium, it leads to increased survival of embryos, especially in women with previous miscarriages. Animal and cell culture studies on isolated trophectoderm cells support an effect mainly on cellular expansion. Aim of this study was to investigate, if the supplementation of GM-CSF either in a human ART medium or in a mouse optimized medium, leads to a change in cell number and cell lineages in the early pre-implantation mouse embryo. Our data shows that mouse GM-CSF increased total cell numbers with increasing concentrations. This increase of cell number has not been found in embryos cultured in ART media with or without human GM-CSF (hGM-CSF) or in a mouse medium supplemented with different concentrations of hGM-CSF. The changes were caused by a marked difference in TE and primitive endoderm cell numbers but not due to a change in epiblast cell numbers. Additionally, results show an ectopic expression of NANOG among trophectoderm cells in both, human ART media (with and without GM-CSF) and at increasing concentrations in the mouse and the human GM-CSF supplemented media. In conclusion, we could show that GM-CSF has an effect on cell identity in mice, which might probably also occur in the human. Therefore, we would like to rare awareness that the use of supplements without proper research could bare risks for the embryo itself and probably also in the post-implantation phase.

Spermatogonial stem cells: updates from specification to clinical relevance.

Human spermatogonia are target for exploration of adult stem cell characteristics and potential source for the development of therapeutic applications. Almost 50 years ago, Yves Clermont stated with regard to the nature of the true stem cells: 'there is the possibility that other classes of spermatogonia exist beside the three classes (Adark, Apale and type B)…; …we still know too little about the human spermatogonial stem cells'… This review seeks to provide current knowledge, focusing on different aspects of human spermatogonia, and novel information based on species comparisons with regard to the adaptation of their proliferative potential. Moreover, the objective is to provide an update on the state of the art concerning the potential use of human spermatogonia for clinical applications. Germ cell specification mechanisms and epigenetic as well as transcriptional features of primordial germ cells (PGC) and adult spermatogonia at the single-cell level are reviewed. Studies on single-cell analyses have been included as they provide hitherto unequaled resolution of the transcriptional profiles of unselected human testicular cells and, thereby, new insights into the molecular aspects of germ cell differentiation. Datasets on models of spermatogonial expansion were identified and spermatogonial turnover and lifetime sperm production rates in various species were calculated, based exclusively on studies employing the optical dissector approach. Finally, the state of the art concerning causes of impaired spermatogonial function and fertility preservation were comprehensively reviewed. RNA sequencing data from PGC and spermatogonia indicate that transcriptional heterogeneity is a feature of germ cells prior to differentiation. Based on these data as well as lineage-tracing studies it is now debated whether spermatogonia are a rather plastic population of undifferentiated germ cells with the stem cell niche being the regulatory unit for cell fate decisions. Based on our novel calculations we suggest that spermatogonia are adapted to the individual reproductive lifespan and that the life-long sperm output from a spermatogonium is balanced against the duration of a generation. Thereby, the risk of jeopardizing genome integrity is balanced against a maximized sperm output. With reference to Yves Clermont's statement, and based on recent datasets, we suggest that the question that needs to be answered is: 'Is there a true stem cell?' or better 'Is there a population of various cells with distinct features serving as a stem cell pool?'. This review provides an update including novel views on various aspects of spermatogonial biology (from embryonic to adult stages). We consider this review relevant for all research scientists and clinicians dealing with fertility, spermatogenesis and fertility preservation.

The C-X-C signalling system in the rodent vs primate testis: impact on germ cell niche interaction.

In zebrafish, action of the chemokine Cxcl12 is mediated through its G-protein-coupled seven-transmembrane domain receptor Cxcr4 and the atypical receptor Cxcr7. Employing this animal model, it was revealed that this Cxcl12 signalling system plays a crucial role for directed migration of primordial germ cells (PGC) during early testicular development. Importantly, subsequent studies indicated that this regulatory mechanism is evolutionarily conserved also in mice. What is more, the functional role of the CXCL12 system does not seem to be limited to early phases of testicular development. Data from mouse studies rather demonstrate that CXCL12 and its receptors are also involved in the homing process of gonocytes into their niches at the basal membrane of the seminiferous tubules. Intriguingly, even the spermatogonial stem cells (SSCs) present in the adult mouse testis appear to maintain the ability to migrate towards a CXCL12 gradient as demonstrated by functional in vitro migration assays and in vivo germ cell transplantation assays. These findings not only indicate a role of the CXCL12 system throughout male germ cell development in mice but also suggest that this system may be evolutionarily conserved. In this review, we take into account the available literature focusing on the localization patterns of the CXCL12 system not only in rodents but also in primates, including the human. Based on these data, we discuss whether the CXCL12 system is also conserved between rodents and primates and discuss the known and potential functional consequences.