Characterization of the promoter region of the murine Catsper2 gene

CATSPER2 (Cation channel sperm-associated protein 2) protein, which is part of the calcium CATSPER channel located in the membrane of the flagellar principal piece of the sperm cell, is only expressed in the testis during spermatogenesis. Deletions or mutations in the Catsper2 gene are associated with the deafness-infertility syndrome (DIS) and non-syndromic male infertility. However, the mechanisms by which Catsper2 is regulated are unknown. Here, we report the characterization of the promoter region of murine Catsper2 and the role of CTCF and CREMτ in its transcription. We report that the promoter region has transcriptional activity in both directions, as determined by observing luciferase activity in mouse Sertoli and GC-1 spg transfected cells. WGBS data analysis indicated that a CpG island identified in silico is non-methylated; Chromatin immunoprecipitation (ChIP)-seq data analysis revealed that histone marks H3K4me3 and H3K36me3 are present in the promoter and body of the Catsper2 gene respectively, indicating that Catsper2 is subject to epigenetic regulation. In addition, the murine Catsper2 core promoter was delimited to a region between -54/+189 relative to the transcription start site (TSS), where three CTCF and one CRE binding site were predicted. The functionality of these sites was determined by mutation of the CTCF sites and deletion of the CRE site. Finally, ChIP assays confirmed that CREMτ and CTCF bind to the Catsper2 minimal promoter region. This study represents the first functional analysis of the murine Catsper2 promoter region and the mechanisms that regulate its expression.

Down Regulation of Catsper1 Expression by Calmodulin Inhibitor (Calmidazolium): Possible Implications for Fertility

The CatSper channel localizes exclusively in the flagella of sperm cells. The Catsper1 protein, together with three pore units, is essential for the CatSper Channel formation, which produces flagellum hyperactivation and confers sperm fertility. Catsper1 expression is dependent on Sox transcription factors, which can recognize in vitro at least three Sox binding sites on the promoter. Sox transcription factors have calmodulin-binding domains for nuclear importation. Calmodulin (CaM) is affected by the specific inhibitor calmidazolium (CMZ), which prevents the nuclear transport of Sox factors. In this work, we assess the regulation of the Catsper1 promoter in vivo by Sox factors in the murine testis and evaluate the effects of the inhibitor calmidazolium on the expression of the Casper genes, and the motility and fertility of the sperm. Catsper1 promoter has significant transcriptional activity in vivo; on the contrary, three Sox site mutants in the Catsper1 promoter reduced transcriptional activity in the testis. CaM inhibition affects Sox factor nuclear transport and has notable implications in the expression and production of Catsper1, as well as in the motility and fertility capability of sperm. The molecular mechanism described here might conform to the basis of a male contraceptive strategy acting at the transcriptional level by affecting the production of the CatSper channel, a fundamental piece of male fertility.

Human sperm ion channel (dys)function: implications for fertilization

BACKGROUND

Intensive research on sperm ion channels has identified members of several ion channel families in both mouse and human sperm. Gene knock-out studies have unequivocally demonstrated the importance of the calcium and potassium conductances in sperm for fertility. In both species, the calcium current is carried by the highly complex cation channel of sperm (CatSper). In mouse sperm, the potassium current has been conclusively shown to be carried by a channel consisting of the pore forming subunit SLO3 and auxiliary subunit leucine-rich repeat-containing 52 (LRRC52). However, in human sperm it is controversial whether the pore forming subunit of the channel is composed of SLO3 and/or SLO1. Deciphering the role of the proton-specific Hv1 channel is more challenging as it is only expressed in human sperm. However, definitive evidence for a role in, and importance for, human fertility can only be determined through studies using clinical samples.

OBJECTIVE AND RATIONALE

This review aims to provide insight into the role of sperm ion channels in human fertilization as evidenced from recent studies of sperm from infertile men. We also summarize the key discoveries from mouse ion channel knock-out models and contrast the properties of mouse and human CatSper and potassium currents. We detail the evidence for, and consequences of, defective ion channels in human sperm and discuss hypotheses to explain how defects arise and why affected sperm have impaired fertilization potential.

SEARCH METHODS

Relevant studies were identified using PubMed and were limited to ion channels that have been characterized in mouse and human sperm. Additional notable examples from other species are included as appropriate.

OUTCOMES

There are now well-documented fundamental differences between the properties of CatSper and potassium channel currents in mouse and human sperm. However, in both species, sperm lacking either channel cannot fertilize in vivo and CatSper-null sperm also fail to fertilize at IVF. Sperm-lacking potassium currents are capable of fertilizing at IVF, albeit at a much lower rate. However, additional complex and heterogeneous ion channel dysfunction has been reported in sperm from infertile men, the causes of which are unknown. Similarly, the nature of the functional impairment of affected patient sperm remains elusive. There are no reports of studies of Hv1 in human sperm from infertile men.

WIDER IMPLICATIONS

Recent studies using sperm from infertile men have given new insight and critical evidence supporting the supposition that calcium and potassium conductances are essential for human fertility. However, it should be highlighted that many fundamental questions remain regarding the nature of molecular and functional defects in sperm with dysfunctional ion channels. The development and application of advanced technologies remains a necessity to progress basic and clinical research in this area, with the aim of providing effective screening methodologies to identify and develop treatments for affected men in order to help prevent failed ART cycles. Conversely, development of drugs that block calcium and/or potassium conductances in sperm is a plausible strategy for producing sperm-specific contraceptives.

Regulation of CATSPER1 expression by the testis-determining gene SRY

CATSPER1 gene encodes a pore-forming and pH-sensing subunit of the CatSper Ca²⁺- permeable channel, a protein in the flagellum essential for sperm hyperactivation. Previous studies have shown that the murine Catsper1 gene promoter is regulated by different Sox proteins. Likewise, it is acknowledged that the human CATSPER1 gene promoter sequence is enriched in potential interaction sites for the sex-determining region Y gene (SRY), which suggest a novel regulatory transcriptional mechanism for CatSper1 channel expression. Therefore, in this work, we sought to determine whether the human CATSPER1 gene expression is regulated by the SRY transcription factor. To this end, a series of deletions and mutations were introduced in the wild- type CATSPER1 gene promoter to eliminate the SRY sites, and the different constructs were tested for their ability to activate transcription in human embryonic kidney and murine spermatogonial germ cell lines (HEK-293 and GC1-spg, respectively) using luciferase assays. In addition, by using a strategy that combines electrophoretic mobility shift assays (EMSA) and chromatin immunoprecipitation (ChIP) we investigated whether the CATSPER1 gene expression is regulated by the SRY transcription factor both in vitro and in vivo. Our results show that the transcriptional factor SRY specifically binds to different sites in the promoter sequence and has the ability to control CATSPER1 gene transcription.

Human CATSPER1 Promoter Is Regulated by CREB1 and CREMτ Transcriptional Factors In Vitro

BACKGROUND

The CATSPER1 gene encodes a CATSPER channel protein that selectively permeates Ca²⁺ ions, and CATSPER expression in sperm is essential for flagellum hyperactivation and, thus, male fertility. Little is known regarding the transcriptional regulation of CATSPER1, but previous studies have performed in silico analyses of transcription factor binding sites, including three CRE sites designated 0-2, in which CRE0 is located near the transcription start site.

OBJETIVES

We investigate if overexpression of CREB-A and CREMτ transcription factors might regulate CATSPER1 expression.

MATERIAL AND METHODS

In this study, the transcriptional regulation of the CATSPER1 gene by CREB-A and CREMτ transcriptions factors was determined by dual-luciferase assays in HEK293 and GC1-spg cells, and important CRE sites were mutated and analyzed for transcriptional regulation.

RESULTS

The deletion of the CRE1 site dramatically increased the transcriptional activity of the CATSPER1 promoter in HEK293 and GC1-spg cells. In HEK293 cells, the CREB-A transcription factor positively regulated CATSPER1 gene expression, while the presence of CREB-A and CREMτ factors synergistically enhanced promoter activity in these cells. In contrast, deletion of CRE0 prevented any transcriptional activity of the CATSPER1 promoter in GC1-spg spermatogonial cells, but expression of either CREB-A or CREMτ restored such transcriptional activity.

CONCLUSIONS

The human CATSPER1 promoter is positively regulated in vitro by CREB-A in HEK293 and GC1-spg cells. Both lines showed differential transcriptional regulation, which was defined by the factors and coactivators present in each cell line as well as the context in which the CRE sites were found in the promoter.

Catsper1 promoter is bidirectional and regulates the expression of a novel lncRNA

The Catsper1 gene, whose expression is restricted to male germ cells, has great importance in reproductive biology because of its function in sperm motility and fertilization. We previously reported that the promoter of this gene has transcriptional activity in either direction in a heterologous system. In the present study, we found that the Catsper1 promoter has in vitro transcriptional activity in either orientation in GC-1 spg mouse spermatogonial cells. The results also showed that this promoter regulates the expression of a new divergent Catsper1 gene named Catsper1au (Catsper1 antisense upstream transcript). Catsper1au is expressed in adult male mouse testis and liver tissues but not in female mouse liver or ovary tissues. In the testis, Catsper1au is expressed in embryos at 11.5 days post-coitum and from newborns to adults. This gene is also expressed in 1- to 3-week postnatal hearts and in 1-week to adult stage livers. The analysis of the 1402 bp whole genome sequence revealed that Catsper1au is an intronless and polyadenylated lncRNA, located in the nuclei of Sertoli and spermatogenic cells from adult testis. These data indicate that Catsper1au is divergently expressed from the Catsper1 promoter and could regulate gene expression during spermatogenesis.

The Catsper channel and its roles in male fertility: a systematic review

The Catsper channel is a sperm-specific, Ca²⁺-permeable, pH-dependent, and low voltage-dependent channel that is essential for the hyperactivity of sperm flagellum, chemotaxis towards the egg, capacitation and acrosome reaction. All of these physiological events require calcium entry into sperm cells. Remarkably, Catsper genes are exclusively expressed in the testis during spermatogenesis, and are sensitive to ion channel-induced pH change, such as NHEs, Ca²⁺ATPase, K⁺ channel, Hv1 channel and HCO₃^- transporters. Furthermore, the Catsper channel is regulated by some physiological stimulants, such as progesterone, cyclic nucleotides (e.g., cAMP, cGMP), zona pellucida (ZP) glycoproteins and bovine serum albumin (BSA). All of these factors normally stimulate Ca²⁺ entry into sperm through the Catsper channel. In addition, the Catsper channel may be a potential target for male infertility treatment or contraception. This review will focus on the structure, functions, regulation mechanisms and medicinal targets of the Catsper channel.

Quantification of CatSper1 expression in human spermatozoa and relation to functional parameters

STUDY QUESTION

Is CatSper1 expression in human spermatozoa related to semen parameter values and sperm functions?

SUMMARY ANSWER

CatSper1 expression is positively related to progressive and hyperactivated (HA) motility, [Ca(2+)]i responsiveness to progesterone but not the acrosome reaction (AR).

WHAT IS KNOWN ALREADY

The role of cationic channel of sperm (CatSper) in sperm functions is clear in animal models but less defined in human sperm cells. Current knowledge is mostly based on low specificity CatSper inhibitors showing agonistic and toxic effects on human spermatozoa and is thus of little help in clarifying the role of the CatSper channel in human sperm functions.

STUDY DESIGN, SIZE, DURATION

CatSper1 protein expression was evaluated in 115 men undergoing semen analysis for couple infertility. CatSper1 expression was related to routine semen parameters, motility kinematic parameters and basal and progesterone-stimulated [Ca(2+)]i and the AR.

PARTICIPANTS/MATERIALS, SETTING, METHODS

CatSper1 expression was evaluated (n = 85 normozoospermic, n = 30 asthenozoospermic patients) by immunofluorescence coupled to flow cytometry leading to quantitative measurement of the percentage of ejaculated sperm cells expressing the protein. Semen analysis was evaluated according to World Health Organization guidelines. Kinematic parameters were evaluated by a computer-aided sperm analysis system. [Ca(2+)]i was measured by a spectrofluorimetric method in fura-2-loaded spermatozoa. The AR was evaluated in live sperm cells by fluorescent-labeled lectin.

MAIN RESULTS AND THE ROLE OF CHANCE

CatSper1 protein expression in spermatozoa was reduced in asthenozoospermic men (mean ± SD: 53.0 ± 15.5%, n = 30 versus 67.9 ± 17.1% in normozoospermic, n = 85, P < 0.01) and was significantly correlated with progressive (r = 0.36, P < 0.001), total (r = 0.35, P < 0.001) and HA (r = 0.41, P < 0.005) motility. In addition to a higher percentage of spermatozoa not expressing CatSper1, asthenozoospermic men showed a large number of spermatozoa with immunofluorescent signal localized outside the principal piece compared with those in normozoospermia. A significant positive correlation was found between CatSper1 protein expression and the increase of [Ca(2+)]i in response to progesterone (r = 0.36, P < 0.05, n = 40) but not with basal [Ca(2+)]i. No correlation was found with the AR, either basal or in response to progesterone.

LIMITATIONS, REASONS FOR CAUTION

The study is partly descriptive. Furthermore, we cannot rule out the possibility that some round cells remain after a single round of 40% density gradient centrifugation or that this step may have removed some defective or slow swimming sperm, and therefore this preparation may not be representative of the entire sperm sample. Although our data suggest that CatSper1 may be a useful marker for infertility, and a possible contraceptive target, any clinical application is limited without further research.

WIDER IMPLICATIONS OF THE FINDINGS

Our results demonstrate an association of CatSper1 expression with human sperm progressive and HA motility and provide preliminary evidence that lack of expression or mislocalization of CatSper1 in spermatozoa may be involved in the pathogenesis of asthenozoospermia. However, mechanistic studies are needed to confirm that the correlations between CatSper1 expression and sperm functions are causative.

STUDY FUNDING/COMPETING INTERESTS

Supported by grants from Ministry of University and Scientific Research (PRIN project to E.B. and FIRB project to S.M.) and by Regione Toscana (to G.F.). L.T. was recipient of a grant from Accademia dei Lincei (Rome, Italy). The authors have no conflicts of interest to declare.