Asymmetrically Positioned Flagellar Control Units Regulate Human Sperm Rotation

Abstract

The ability of sperm to fertilize an egg is controlled by ion channels, one of which is the pHdependent calcium channel of sperm CatSper. For CatSper to be fully activated, the cytoplasmic pH must be alkaline, which is accomplished by either proton transporters, or a faster mechanism, such as the voltage-gated proton channel Hv1. To ensure effective regulation, these channels and regulatory proteins must be tightly compartmentalized. Here, we characterize human sperm nanodomains that are comprised of Hv1, CatSper and regulatory protein ABHD2. Superresolution microscopy revealed that Hv1 forms asymmetrically positioned bilaterally distributed longitudinal lines that span the entire length of the sperm tail. Such a distribution provides a direct structural basis for the selective activation of CatSper, and subsequent flagellar rotation along the long axis that, together with hyperactivated motility, enhances sperm fertility. Indeed, Hv1 inhibition leads to a decrease in sperm rotation. Thus, sperm ion channels are organized in distinct regulatory nanodomains that control hyperactivated motility and rotation.

Rotational motion and rheotaxis of human sperm do not require functional CatSper channels and transmembrane Ca 2+ signaling

“…Semi‐automated analysis of blinking events revealed the rotation frequency of each sperm cell in the field of view. In non‐capacitated and capacitated control sperm from healthy donors, the rotation frequency was normally distributed (Fig D) with a mean value of 4.8 ± 1.5 Hz ( n = 1,455) and 7.0 ± 2.2 Hz ( n = 1,097), respectively (Fig E) (Rigler & Thyberg, ; Aitken et al , ; Miller et al , ). Bicarbonate (25 mM) used for capacitation stimulates cAMP synthesis (Carlson et al , ; Tresguerres et al , ; Brenker et al , ) and, thereby, accelerates the flagellar beat (Esposito et al , ; Xie et al , ) and rotation frequency (Miki & Clapham, ).…”

“…The Ca 2+ channel CatSper has been implicated in rolling and rheotactic steering of mammalian sperm (Miki & Clapham, ; Miller et al , ). We show that, in fact, rolling and rheotaxis of both human and mouse sperm do not require Ca 2+ influx via CatSper.…”

“…In Catsper1 −/− mouse sperm, longitudinal rolling and rheotaxis were abolished (Miki & Clapham, ), suggesting that control of [Ca 2+ ] i by CatSper is required for rolling and rheotaxis of mammalian sperm. For human sperm, it was specifically proposed that rolling is created by asymmetrical Ca 2+ influx via CatSper channels, stimulated by local pH i signaling (Miller et al , ). The H + channel Hv1 is organized along the flagellum of human sperm in two threads near two of the four CatSper threads (Miller et al , ).…”

“…For human sperm, it was specifically proposed that rolling is created by asymmetrical Ca 2+ influx via CatSper channels, stimulated by local pH i signaling (Miller et al , ). The H + channel Hv1 is organized along the flagellum of human sperm in two threads near two of the four CatSper threads (Miller et al , ). It was proposed that H + efflux via Hv1 organizes localized Ca 2+ signaling that, ultimately, creates an asymmetry in calcium‐dependent inhibition of dynein‐powered microtubule sliding (Miller et al , ).…”

“…The H + channel Hv1 is organized along the flagellum of human sperm in two threads near two of the four CatSper threads (Miller et al , ). It was proposed that H + efflux via Hv1 organizes localized Ca 2+ signaling that, ultimately, creates an asymmetry in calcium‐dependent inhibition of dynein‐powered microtubule sliding (Miller et al , ). However, the concept that rolling and rheotaxis are enabled by Ca 2+ influx cannot be reconciled with the finding that rolling of mouse sperm does not require extracellular Ca 2+ (Babcock et al , ; Muschol et al , ), and that exposure of human sperm to gradients of flow velocities does not evoke measurable changes in [Ca 2+ ] i (Zhang et al , ).…”