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.

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Despite the high incidence of male infertility, only 30% of infertile men receive a causative diagnosis. To explore the regulatory mechanisms governing human germ cell function in normal and impaired spermatogenesis (crypto), we performed single-cell RNA sequencing (>30,000 cells). We find major alterations in the crypto spermatogonial compartment with increased numbers of the most undifferentiated spermatogonia (PIWIL4⁺). We also observe a transcriptional switch within the spermatogonial compartment driven by increased and prolonged expression of the transcription factor EGR4. Intriguingly, the EGR4-regulated chromatin-associated transcriptional repressor UTF1 is downregulated at transcriptional and protein levels. This is associated with changes in spermatogonial chromatin structure and fewer A_dark spermatogonia, characterized by tightly compacted chromatin and serving as reserve stem cells. These findings suggest that crypto patients are disadvantaged, as fewer cells safeguard their germline's genetic integrity. These identified spermatogonial regulators will be highly interesting targets to uncover genetic causes of male infertility.

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The hypothalamic-pituitary-gonadal (HPG) axis, which is essential for reproduction, is driven by GnRH. GnRH induces the synthesis and secretion of the two gonadotrophins LH and FSH which are fundamental for normal androgenization and spermatogenesis. Impaired pituitary or hypothalamus function can lead to hypogonadotropic hypogonadism (HH) and a GnRH stimulation-test is usually applied to diagnose malfunctioning HPG-axis. In recent years, genetic variants, including single nucleotide polymorphisms (SNPs), which might interfere with the endocrine regulation of spermatogenesis were investigated. Especially the FSHB-211 G > T SNP has been shown to impair induction of FSHB transcription which is important for FSH release.

We therefore perform stimulation tests by administering an i.v. bolus of 100 mg of a synthetic LHRH-analogue and then measure FSH and LH levels at different time points. An retrospective analysis using genetic and clinical data from our database enables us to investigate any potential FSHB-211 SNPs affect on FSH levels following GnRH stimulation.

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Life‐long sperm production leads to the assumption that male fecundity remains unchanged throughout life. However, recently it was shown that paternal age has profound consequences for male fertility and offspring health. Paternal age effects are caused by an accumulation of germ cell mutations over time. However, molecular patterns of ageing in germ cells and their impact on DNA integrity have not been studied in detail. To assess the effects of ‘pure’ ageing on male reproductive health and germ cell quality, we assembled a cohort of 198 healthy men (18–84 years) and evaluated a variety of end points. While sperm production and hormonal profiles were maintained at physiological levels over a period of six decades, we identified a steady increase of telomere length in sperm, a sharp increase in sperm DNA instability and sperm DNA methylation changes in 236 regions. In conclusion, human male germ cells present a unique germline‐specific ageing process, which likely results in diminished fecundity in elderly men and poorer health prognosis for their offspring.

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Link to Jörg Gromoll's group

Our group is working on the impact  of c.-211G>T FSHB single nucleotide polymorphism (SNP) on spermatogenesis. We and others have recently shown that this SNP is strongly associated with lowered testicular volume, reduced sperm counts and decreased FSH levels in patients carrying one or two T-alleles. However it was  not clear to which extent Sertoli cell (SC) number, Sertoli cell workload (SCWL) and thereby spermatogenic potential is affected. In the current study we demonstrated that neither SC number nor SCWL is significantly different among different genotypes of the -211G>T FSHB SNP (see figure). Thus the spermatogenic potential is maintained independent of the SNP genotype.  The previously observed clinical phenotype might be caused by a hypo-stimulated spermatogenesis and not due to a decreased SC number. Our findings support the concept for a putative treatment of infertile men with FSH to stimulate spermatogenesis further.

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Pediatric gonadotoxic treatments like cancer treatment can cause subfertility or even permanent infertility. While adult men can cryopreserve sperm as fertility reserve, this strategy cannot be applied for prepubertal patients. In this article Nina Neuhaus and Stefan Schlatt give a perspective on stem cell-based options to preserve male fertility.

Science  22 Mar 2019: Vol. 363, Issue 6433, pp. 1283-1284; DOI:10.1126/science.aaw6927
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Link to Stefan Schlatt's group

Link to Nina Neuhaus group

"RHOX transcription factor genes are mutated in infertile men"

Infertility affects 10-15% of couples with the wish for parenthood, and nearly half of these cases can be attributed to a male factor. That means around 7 percent of all males of reproductive age have fertility problems. In a collaborative study, recently published by Human Molecular Genetics, the researchers provide evidence that mutations in the reproductive homeobox (RHOX) family of transcription factors are linked to infertile men with severely low sperm count.

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Link to Jörg Gromoll's group

How do sperm find their way to the egg? It has been proposed that chemotaxis, hyperactivation and acrosomal exocytosis of sperm is controlled by a Ca2+ signal. The underlying signalling mechanism is investigated in this study. It is shown that progesterone activates the sperm specific, pH-sensitive CatSper Ca2+ channel which in turn leads to rapid Ca2+ influx.

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Link to Timo Strünker's group