The sodium/proton exchanger SLC9C1 (sNHE) is essential for human sperm motility and fertility

Abstract

Asthenozoospermia, defined by the absence or reduction of sperm motility, constitutes the most frequent cause of human male infertility. This pathological condition is caused by morphological and/or functional defects of the sperm flagellum, which preclude proper sperm progression. While in the last decade many causal genes were identified for asthenozoospermia associated with severe sperm flagellar defects, the causes of purely functional asthenozoospermia are still poorly defined. We describe here the case of an infertile man, displaying asthenozoospermia without major morphological flagellar anomalies and carrying a homozygous splicing mutation in SLC9C1 (sNHE), which we identified by whole‐exome sequencing. SLC9C1 encodes a sperm‐specific sodium/proton exchanger, which in mouse regulates pH homeostasis and interacts with the soluble adenylyl cyclase (sAC), a key regulator of the signalling pathways involved in sperm motility and capacitation. We demonstrate by means of RT‐PCR, immunodetection and immunofluorescence assays on patient's semen samples that the homozygous splicing mutation (c.2748 + 2 T > C) leads to in‐frame exon skipping resulting in a deletion in the cyclic nucleotide‐binding domain of the protein. Our work shows that in human, similar to mouse, SLC9C1 is required for sperm motility. Overall, we establish a homozygous truncating mutation in SLC9C1 as a novel cause of human asthenozoospermia and infertility.

Sperm Ion Transporters and Channels in Human Asthenozoospermia: Genetic Etiology, Lessons from Animal Models, and Clinical Perspectives

“…Very recently, the importance of SLC9C1 in human functional asthenozoospermia was confirmed by the finding of a homozygous truncating mutation in an infertile patient [ 44 ]. Before this discovery, the importance of SLC9C1 in sperm function had only been explored in model organisms.…”

“…Consistent with these results, SLC9C1 ( sNHE ) and soluble adenylate cyclase (sAC) have been reported to show co-dependent expression and to be mutually modulated by cAMP and pH/bicarbonate [ 78 ]. Interestingly the c.2748 + 2T > C splicing mutation identified in humans induces an in-frame deletion within the CNBD, which is likely to impair protein functionality [ 44 ]. The sperm of the patient carrying the SLC9C1 mutation showed reduced total and progressive motility, as well as a number of morphological defects, such as disorganization of the midpiece and an increased fraction of isolated heads and break points after sperm selection, suggesting global fragility of the sperm structure [ 44 ].…”

Blocking NHE Channels Reduces the Ability of In Vitro Capacitated Mammalian Sperm to Respond to Progesterone Stimulus

“…Moreover, whilst NHE1 channels are localized in the mid-piece of mouse sperm [ 20 ], they are present in the equatorial region, mid-piece, and proximal third of the principal piece of sheep sperm [ 16 ]. Such a species-specific distribution throughout the sperm cell is not a characteristic of NHE1, but it is also observed in sNHE [ 11 , 17 , 21 ] and other ion channels, including SLO3 and HVCN1 (reviewed in [ 2 , 22 ]). Indeed, sNHE channels in mouse sperm have an average molecular weight of 120 kDa and localize in the principal piece [ 11 ], whereas in humans they have a molecular weight from 120 to 135 kDa and are present throughout the mid- and principal pieces [ 17 , 21 ].…”

“…Such a species-specific distribution throughout the sperm cell is not a characteristic of NHE1, but it is also observed in sNHE [ 11 , 17 , 21 ] and other ion channels, including SLO3 and HVCN1 (reviewed in [ 2 , 22 ]). Indeed, sNHE channels in mouse sperm have an average molecular weight of 120 kDa and localize in the principal piece [ 11 ], whereas in humans they have a molecular weight from 120 to 135 kDa and are present throughout the mid- and principal pieces [ 17 , 21 ]. NHE5 has only been identified in the mid-piece of mouse sperm [ 20 ].…”

A Review on the Role of Bicarbonate and Proton Transporters during Sperm Capacitation in Mammals

“…Yet, it should be borne in mind that intracellular HCO 3 − levels also rely upon different carbonic anhydrase isoforms present in both plasma membrane and cytoplasm [ 20 ]. On the other hand, not only does alkalinization of sperm cytosol depend on bicarbonate transport but also on the involvement of different proton (H + ) exchangers and transporters, mainly voltage-gated proton channels, such as HVCN1 [ 3 , 21 , 22 ], monocarboxylate transporters (MCTs) [ 19 ], and Na + -H + exchangers (NHEs) [ 8 , 23 , 24 , 25 ]. In fact, the increase of pH i is known to be essential for the activation of several pH-dependent channels that play a vital function for plasma membrane hyperpolarization and acrosomal reactions [ 1 ].…”