The reactivation of demembranated human spermatozoa lacking outer dynein arms is independent of pH

Ambroxol-Enhanced Frequency and Amplitude of Beating Cilia Controlled by a Voltage-Gated Ca2+ Channel, Cav1.2, via pHi Increase and [Cl-]i Decrease in the Lung Airway Epithelial Cells of Mice

Ambroxol (ABX), a frequently prescribed secretolytic agent which enhances the ciliary beat frequency (CBF) and ciliary bend angle (CBA, an index of amplitude) by 30%, activates a voltage-dependent Ca2+ channel (CaV1.2) and a small transient Ca2+ release in the ciliated lung airway epithelial cells (c-LAECs) of mice. The activation of CaV1.2 alone enhanced the CBF and CBA by 20%, mediated by a pHi increasei and a [Cl-]i decrease in the c-LAECs. The increase in pHi, which was induced by the activation of the Na+-HCO3- cotransporter (NBC), enhanced the CBF (by 30%) and CBA (by 15-20%), and a decrease in [Cl-]i, which was induced by the Cl- release via anoctamine 1 (ANO1), enhanced the CBA (by 10-15%). While a Ca2+-free solution or nifedipine (an inhibitor of CaV1.2) inhibited 70% of the CBF and CBA enhancement using ABX, CaV1.2 enhanced most of the CBF and CBA increases using ABX. The activation of the CaV1.2 existing in the cilia stimulates the NBC to increase pHi and ANO1 to decrease the [Cl-]i in the c-LAECs. In conclusion, the pHi increase and the [Cl-]i decrease enhanced the CBF and CBA in the ABX-stimulated c-LAECs.

Na+/H+ Exchangers (NHEs) in Mammalian Sperm: Essential Contributors to Male Fertility

Na+/H+ exchangers (NHEs) are known to be important regulators of pH in multiple intracellular compartments of eukaryotic cells. Sperm function is especially dependent on changes in pH and thus it has been postulated that NHEs play important roles in regulating the intracellular pH of these cells. For example, in order to achieve fertilization, mature sperm must maintain a basal pH in the male reproductive tract and then alkalize in response to specific signals in the female reproductive tract during the capacitation process. Eight NHE isoforms are expressed in mammalian testis/sperm: NHE1, NHE3, NHE5, NHE8, NHA1, NHA2, NHE10, and NHE11. These NHE isoforms are expressed at varying times during spermatogenesis and localize to different subcellular structures in developing and mature sperm where they contribute to multiple aspects of sperm physiology and male fertility including proper sperm development/morphogenesis, motility, capacitation, and the acrosome reaction. Previous work has provided evidence for NHE3, NHE8, NHA1, NHA2, and NHE10 being critical for male fertility in mice and NHE10 has recently been shown to be essential for male fertility in humans. In this article we review what is known about each NHE isoform expressed in mammalian sperm and discuss the physiological significance of each NHE isoform with respect to male fertility.

Carbocisteine stimulated an increase in ciliary bend angle via a decrease in [Cl-]i in mouse airway cilia

Carbocisteine (CCis), a mucoactive agent, is widely used to improve respiratory diseases. This study demonstrated that CCis increases ciliary bend angle (CBA) by 30% and ciliary beat frequency (CBF) by 10% in mouse airway ciliary cells. These increases were induced by an elevation in intracellular pH (pH_i; the pH_i pathway) and a decrease in the intracellular Cl^- concentration ([Cl^-]_i; the Cl^- pathway) stimulated by CCis. The Cl^- pathway, which is independent of CO₂/HCO₃^-, increased CBA by 20%. This pathway activated Cl^- release via activation of Cl^- channels, leading to a decrease in [Cl^-]_i, and was inhibited by Cl^- channel blockers (5-nitro-2-(3-phenylpropylamino) benzoic acid and CFTR(inh)-172). Under the CO₂/HCO₃^--free condition, the CBA increase stimulated by CCis was mimicked by the Cl^--free NO₃^- solution. The pH_i pathway, which depends on CO₂/HCO₃^-, increased CBF and CBA by 10%. This pathway activated HCO₃^- entry via Na⁺/HCO₃^- cotransport (NBC), leading to a pH_i elevation, and was inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid. The effects of CCis were not affected by a protein kinase A inhibitor (1 μM PKI-A) or Ca²⁺-free solution. Thus, CCis decreased [Cl^-]_i via activation of Cl^- channels including CFTR, increasing CBA by 20%, and elevated pH_i via NBC activation, increasing CBF and CBA by 10%.

ATPases, ion exchangers and human sperm motility

Human sperm has several mechanisms to control its ionic milieu, such as the Na,K-ATPase (NKA), the Ca-ATPase of the plasma membrane (PMCA), the Na+/Ca2+-exchanger (NCX) and the Na+/H+-exchanger (NHE). On the other hand, the dynein-ATPase is the intracellular motor for sperm motility. In this work, we evaluated NKA, PMCA, NHE, NCX and dynein-ATPase activities in human sperm and investigated their correlation with sperm motility. Sperm motility was measured by Computer Assisted Semen Analysis. It was found that the NKA activity is inhibited by ouabain with twoKi(7.9×10−9and 9.8×10−5 M), which is consistent with the presence of two isoforms of α subunit of the NKA in the sperm plasma membranes (α1 and α4), being α4 more sensitive to ouabain. The decrease in NKA activity is associated with a reduction in sperm motility. In addition, sperm motility was evaluated in the presence of known inhibitors of NHE, PMCA and NCX, such as amiloride, eosin, and KB-R7943, respectively, as well as in the presence of nigericin after incubation with ouabain. Amiloride, eosin and KB-R7943 significantly reduced sperm motility. Nigericin reversed the effect of ouabain and amiloride on sperm motility. Dynein-ATPase activity was inhibited by acidic pH and micromolar concentrations of Ca2+. We explain our results in terms of inhibition of the dynein-ATPase in the presence of higher cytosolic H+and Ca2+, and therefore inhibition of sperm motility.

Regulation of axonemal motility in demembranated equine sperm

Equine in vitro fertilization is not yet successful because equine sperm do not effectively capacitate in vitro. Results of previous studies suggest that this may be due to failure of induction of hyperactivated motility in equine sperm under standard capacitating conditions. To evaluate factors directly affecting axonemal motility in equine sperm, we developed a demembranated sperm model and analyzed motility parameters in this model under different conditions using computer-assisted sperm analysis. Treatment of ejaculated equine sperm with 0.02% Triton X-100 for 30 sec maximized both permeabilization and total motility after reactivation. The presence of ATP was required for motility of demembranated sperm after reactivation, but cAMP was not. The calculated intracellular pH of intact equine sperm was 7.14 ± 0.07. Demembranated sperm showed maximal total motility at pH 7. Neither increasing pH nor increasing calcium levels, nor any interaction of the two, induced hyperactivated motility in demembranated equine sperm. Motility of demembranated sperm was maintained at free calcium concentrations as low as 27 pM, and calcium arrested sperm motility at much lower concentrations than those reported in other species. Calcium arrest of sperm motility was not accompanied by flagellar curvature, suggesting a failure of calcium to induce the tonic bend seen in other species and thought to support hyperactivated motility. This indicated an absence, or difference in calcium sensitivity, of the related asymmetric doublet-sliding proteins. These studies show a difference in response to calcium of the equine sperm axoneme to that reported in other species that may be related to the failure of equine sperm to penetrate oocytes in vitro under standard capacitating conditions. Further work is needed to determine the factors that stimulate hyperactivated motility at the axonemal level in equine sperm.

Motility of fish spermatozoa: from external signaling to flagella response

For successful fertilization, spermatozoa must access, bind, and penetrate an egg, processes for which activation of spermatozoa motility is a prerequisite. Fish spermatozoa are stored in seminal plasma where they are immotile during transit through the genital tract of most externally fertilizing teleosts and chondrosteans. Under natural conditions, motility is induced immediately following release of spermatozoa from the male genital tract into the aqueous environment. The nature of an external trigger for the initiation of motility is highly dependent on the aquatic environment (fresh or salt water) and the species' reproductive behavior. Triggering signals include osmotic pressure, ionic and gaseous components of external media and, in some cases, egg-derived substances. Extensive study of environmental factors influencing fish spermatozoa motility has led to the proposal of several mechanisms of activation in freshwater and marine fish. However, the signal transduction pathways initiated by these mechanisms remain clear. This review presents the current knowledge with respect to (1) membrane reception of the activation signal and its transduction through the spermatozoa plasma membrane via the external membrane components, ion channels, and aquaporins; (2) cytoplasmic trafficking of the activation signal; (3) final steps of the signaling, including signal transduction to the axonemal machinery, and activation of axonemal dyneins and regulation of their activity; and (4) pathways supplying energy for flagellar motility.

Acquired cilia dysfunction in chronic rhinosinusitis

BACKGROUND

Cilia are complex and powerful cellular structures of the respiratory mucosa that play a critical role in airway defense. Respiratory epithelium is lined with cilia that perform an integrated and coordinated mechanism called mucociliary clearance. Mucociliary clearance is the process by which cilia transport the mucus blanket overlying respiratory mucosa to the gastrointestinal tract for ingestion. It is the primary means by which the airway clears pathogens, allergens, debris, and toxins. The complex structure and regulatory mechanisms that dictate the form and function of normal cilia are not entirely understood, but it is clear that ciliary dysfunction results in impaired respiratory defense.

METHODS

A literature review of the current knowledge of cilia dysfunction in chronic rhinosinsusitis was conducted.

RESULTS

Ciliary dysfunction may be primary, the result of genetic mutations resulting in abnormal cilia structure, or, more commonly, secondary, the result of environmental, infectious, or inflammatory stimuli that disrupt normal motility or coordination. Patients with chronic rhinosinusitis (CRS) have been found to have impaired mucociliary clearance. Many biochemical, environmental, and mechanical stimuli have been shown to influence ciliary beat frequency, and common microbial pathogens of respiratory mucosa such as Pseudomonas aeruginosa and Haemophilus influenzae have developed toxins that appear to interrupt normal mucociliary function. Furthermore, inflammatory mediators known to be present in patients with CRS appear to impair secondarily mucociliary clearance.

CONCLUSION

The goal of this article is to summarize the recent developments in the understanding of cilia dysfunction and mucociliary clearance in CRS.

Mechanisms of mammalian ciliary motility: Insights from primary ciliary dyskinesia genetics

Motile cilia and flagella are organelles that, historically, have been poorly understood and inadequately investigated. However, cilia play critical roles in fluid clearance in the respiratory system and the brain, and flagella are required for sperm motility. Genetic studies involving human patients and mouse models of primary ciliary dyskinesia over the last decade have uncovered a number of important ciliary proteins and have begun to elucidate the mechanisms underlying ciliary motility. When combined with genetic, biochemical, and cell biological studies in Chlamydomonas reinhardtii, these mammalian genetic analyses begin to reveal the mechanisms by which ciliary motility is regulated.

Cilia dysfunction

Cilia are complex and powerful cellular structures that serve a multitude of functions across many types of organisms. In humans, one of the most critical roles of cilia is defense of the airway. The respiratory epithelium is lined with cilia that normally carry out an integrated and coordinated mechanism called mucociliary clearance. Mucociliary clearance, the process by which cilia transport the viscous mucus blanket of the upper airway to the gastrointestinal tract, is the primary means by which the upper airway clears itself of pathogens, allergens, debris, and toxins. The complex structure and regulatory mechanisms that dictate the form and function of normal cilia are not entirely understood, but it is clear that ciliary dysfunction results in impaired respiratory defense. Ciliary dysfunction may be primary, the result of genetic mutations resulting in abnormal cilia structure, or secondary, the result of environmental, infectious or inflammatory stimuli that disrupt normal motility or coordination.

Epithelium, cilia, and mucus: their importance in chronic rhinosinusitis

Chronic rhinosinusitis is a common disease resulting from inflammation of the sinonasal mucosa. It has long been recognized that patients with chronic rhinosinusitis have impaired capacity to clear sinonasal secretions. However, the cause of this pathologic process is not well understood. In this article the components of mucociliary clearance, including cilia, mucus production, and cilia beat frequency, are reviewed and alterations of the system discussed regarding contribution to the disease process.

Inhibition of human sperm motility by contraceptive anti-eppin antibodies from infertile male monkeys: effect on cyclic adenosine monophosphate

Epididymal protease inhibitor (eppin [official symbol, SPINLW1]) is of interest as a male contraceptive target because of its specificity and location on the human sperm surface. We have examined the effect of anti-eppin antibodies from infertile male monkeys and the effect of recombinant human semenogelin on human sperm motility. Anti-eppin antibodies significantly decreased the progressive motility of human spermatozoa as measured by decreased total distance traveled, decreased straight-line distance, and decreased velocity. Anti-eppin treatment of spermatozoa significantly increased the amount of cAMP present in nonprogressive spermatozoa; however, approximately 25% of antibody-treated spermatozoa could be rescued by the addition of cAMP-acetoxymethyl ester, indicating that anti-eppin-treated spermatozoa have a compromised ability to utilize cAMP. Addition of recombinant human semenogelin has a concentration-dependent inhibitory effect on progressive motility (increased tortuosity and decreased velocity). We tested the hypothesis that anti-eppin antibodies bound to eppin would subsequently block semenogelin binding to eppin. Anti-eppin antibodies from infertile monkeys inhibited eppin from binding to semenogelin. Addition of affinity-purified antibodies made to the dominant C-terminal epitope of eppin had an inhibitory effect on progressive motility (increased tortuosity, decreased velocity, and straight distance). Our results suggest that the eppin-semenogelin binding site is critical for the removal of semenogelin in vivo during semen liquefaction and for the initiation of progressive motility. We conclude that the eppin-semenogelin binding site on the surface of human spermatozoa is an ideal target for a nonsteroidal male contraceptive.

Insect sperm motility

The flagellosperm of insects, although following a general ground plan, exhibit considerable variation in morphology and ultrastructure across taxa, consistent with a history of rapid and divergent evolution. Sperm competition, which occurs when sperm of two or more males compete for the fertilization of a female's ova, has been recognized as a significant driving force in the evolution of insect sperm structure. Despite a considerable volume of data on sperm morphology, little is known about the motility of insect sperm. Understanding insect sperm motility would help to refine models of sexual selection on insect sperm, and would throw light on the selective mechanisms that shape insect sperm structure and function. This review updates our present knowledge of the proximate and ultimate aspects of insect sperm motility.

Regulation of mammalian ciliary beating

Recent advances in our understanding of the structure-function relationship of motile cilia with the 9 + 2 microtubular arrangement have helped explain some of the mechanisms of ciliary beat regulation by intracellular second messengers. These second messengers include cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) as well as calcium and pH. cAMP activates protein kinase A (PKA), which is localized to the axoneme. The cAMP-dependent phosphorylation of PKA's main target, originally described as p29 in Paramecium, seems to increase ciliary beat frequency (CBF) directly. The mechanism by which cGMP increases CBF is less well defined but involves protein kinase G and possibly PKA. Protein kinase C inhibits ciliary beating. The regulation mechanisms of CBF by calcium remain somewhat controversial, favoring an immediate, direct action of calcium on ciliary beating and a second cyclic nucleotide-dependent phase. Finally, intracellular pH likely affects CBF through direct influences on dynein arms.

Increase in intracellular pH induces phosphorylation of axonemal proteins for activation of flagellar motility in starfish sperm

Increased intracellular pH ([pH]i) activates dynein in sea urchin and mammalian sperm and induces activation of flagellar motility. It is thought that cAMP-dependent protein phosphorylation is associated with motility activation through increasing [pH]i, but little attention has been given to the cAMP-independent phosphorylation also induced by the[pH]i increase. The present study demonstrates that the increase in[pH]i in starfish sperm induces the phosphorylation of axonemal proteins and activation of flagellar motility independently of cAMP. Flagellar motility of intact sperm was activated when the [pH]i was raised by addition of NH4Cl. Histidine, which is known to activate motility of starfish sperm, also raised the [pH]i during the motility activation. In addition, motility of demembranated sperm flagella was activated in a pH-dependent manner without cAMP. These results indicate that in starfish sperm it is the increase in [pH]i that induces activation of flagellar motility. Moreover, phosphorylation of axonemal proteins (of molecular mass 25, 32 and 45 kDa) was observed during the pH-dependent and cAMP-independent motility activation of demembranated sperm. This suggests that the increase in [pH]i regulates flagellar motility via cAMP-independent phosphorylation of axonemal proteins.

Regulation of human airway ciliary beat frequency by intracellular pH

pHi affects a number of cellular functions, but the influence of pHi on mammalian ciliary beat frequency (CBF) is not known. CBF and pHi of single human tracheobronchial epithelial cells in submerged culture were measured simultaneously using video microscopy (for CBF) and epifluorescence microscopy with the pH-sensitive dye BCECF. Baseline CBF and pHi values in bicarbonate-free medium were 7.2 +/- 0.2 Hz and 7.49 +/- 0.02, respectively (n = 63). Alkalization by ammonium pre-pulse to pHi 7.78 +/- 0.02 resulted in a 2.2 +/- 0.1 Hz CBF increase (P < 0.05). Following removal of NH4Cl, pHi decreased to 7.24 +/- 0.02 and CBF to 5.8 +/- 0.1 Hz (P < 0.05). Removal of extracellular CO2 to change pHi resulted in similar CBF changes. Pre-activation of cAMP-dependent protein kinase (10 microM forskolin), broad inhibition of protein kinases (100 microM H-7), inhibition of PKA (10 microM H-89), nor inhibition of phosphatases (10 microM cyclosporin + 1.5 microM okadaic acid) changed pHi-mediated changes in CBF, nor were they due to [Ca2+]i changes. CBF of basolaterally permeabilized human tracheobronchial cells, re-differentiated at the air-liquid interface, was 3.9 +/- 0.3, 5.7 +/- 0.4, 7.0 +/- 0.3 and 7.3 +/- 0.3 Hz at basolateral i.e., intracellular pH of 6.8, 7.2, 7.6 and 8.0, respectively (n = 18). Thus, intracellular alkalization stimulates, while intracellular acidification attenuates human airway CBF. Since phosphorylation and [Ca2+]i changes did not seem to mediate pHi-induced CBF changes, pHi may directly act on the ciliary motile machinery.

Effects of Ca-ATPase inhibitors on the intracellular calcium activity and motility of human spermatozoa

Although evidence suggests that high intracellular calcium activity ([Ca2+]i) inhibits sperm motility, data concerning [Ca2+]i within, or slightly above, the physiological range are sparse, particularly in mammalian sperm. We investigated inhibitors of the sarcoplasmic/endoplasmic reticulum Ca-ATPase (SERCA) and the plasma membrane Ca-ATPase with the objective of increasing the intracellular calcium ion activity in human spermatozoa to study its effect on motility and other functions. Thapsigargin (20 micromol/L) increased [Ca2+]i from 140 +/- 7 nmol/L over an approximately 2-min period to reach a plateau of 530 +/- 84 nmol/L (mean +/- SEM, n = 3, p < 0.05). In sperm suspended in calcium-free medium thapsigargin increased [Ca2+]i from 13 +/- 3.3 to 35 +/- 7.5 nmol/L (p < 0.01), consistent with the release of calcium from intracellular stores. Cyclopiazonic acid (60 micromol/L) caused a transient decrease in [Ca2+]i. Quercetin, (200 micromol/L) caused a rapid increase in [Ca2+]i to 1280 +/- 90 nmol/L, after which [Ca2+]i fell quickly at first but then more slowly. Thapsigargin (20 micromol/L) caused approximately 70% of sperm to acrosome react in < or = 5 min, but once acrosome reacted, many sperm died over the next 30 min. Lower concentrations of thapsigargin caused fewer acrosome reactions but were less toxic. Both thapsigargin and quercetin caused rapid dose-dependent decreases in sperm motility. The results are consistent with high [Ca2+]i in the range observed in caput epididymal or cryopreserved spermatozoa inhibiting motility, but might be confounded by other events following the acrosome reaction.

Hyperactivated motility of bull sperm is triggered at the axoneme by Ca2+ and not cAMP

Hyperactivated motility, a swimming pattern of mammalian sperm in the oviduct, is essential for fertilization in vivo. It is characterized by high-amplitude flagellar waves and, usually, highly asymmetrical flagellar beating. It had been suggested, but not tested, that Ca2+ and cAMP switch on hyperactivation by directly affecting the flagellar axoneme. In this study, the direct affects of these agents on the axoneme were tested by using detergent-demembranated bull sperm. As confirmed by TEM, treatment of sperm with 0.2% Triton X-100 disrupted the plasma, acrosomal, and inner mitochondrial membranes, leaving axonemes intact. In the presence of 2 mM ATP, the percentage of reactivated sperm that were hyperactivated increased to 80% when free Ca2+ was increased from 50 to 400 nM. The effect of the Ca2+ in this range was to increase beat asymmetry by increasing the curvature of the principal bend. No additional increases were observed above 400 nM free Ca2+, but motility was suppressed at 1 mM. The ability of Ca2+ to produce hyperactivation depended on ATP availability, such that more ATP was required to produce the high amplitude flagellar bends characteristic of hyperactivated motility than to produce activated motility. Cyclic AMP was not required for reactivation, nor for hyperactivation. Production of hyperactivated motility also required an alkaline environment (pH 7.9-8.5). These results suggest that, provided sufficient ATP is present and pH is sufficiently alkaline, Ca2+ switches on hyperactivation by enabling curvature of the principal bends to increase.

Roles of the Na,K-ATPase alpha4 isoform and the Na+/H+ exchanger in sperm motility

The Na,K-ATPase generates electrochemical gradients that are used to drive the coupled transport of many ions and nutrients across the plasma membrane. The functional enzyme is comprised of an alpha and beta subunit and families of isoforms for both subunits exist. Recent studies in this laboratory have identified a biological role for the Na,K-ATPase alpha4 isoform in sperm motility. Here we further investigate the role of the Na,K-ATPase carrying the alpha4 isoform, showing again that ouabain eliminates sperm motility, and in addition, that nigericin, a H+/K+ ionophore, and monensin, a H+/Na+ ionophore, reinitiate motility. These data, along with the observation that the K+ ionophore valinomycin has no effect on the motility of ouabain-inhibited sperm, suggest that ouabain may change intracellular H+ levels in a manner that is incompatible with sperm motility. We have also localized NHE1 and NHE5, known regulators of intracellular H+ content, to the same region of the sperm as the Na,K-ATPase alpha4 isoform. These data highlight the important role of the Na,K-ATPase alpha4 isoform in regulating intracellular H(+) levels, and provide evidence suggesting the involvement of the Na+/H+ exchanger, which is critical for maintaining normal sperm motility.

The motility of demembranated human spermatozoa is inhibited by free calcium ion activities of 500 nmol/L or more

A number of studies have demonstrated that high calcium ion activities inhibit sperm motility, but little is known about the effect of different calcium activities close to the physiological range. Therefore, we investigated whether raising calcium activities within the submicromolar range would inhibit the motility of demembranated human spermatozoa. Spermatozoa were demembranated with Triton X-100 and motility was measured objectively by computer assisted semen analysis. Motility, reactivated by 1 mol adenosine 5'-triphosphate (AlphaTauP)/L, was short lived, with maximum activity only sustained for about 1 min. Reactivated motility was not affected by 50 micromol cAMP/L. The amplitude of lateral head displacement was significantly greater at room temperature than at 37 degrees C, but there were no significant differences between the percentage of sperm motile or their velocity at the two temperatures. The calcium buffer 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) at 1 mmol/L was included in the demembranation-reactivation medium, and free calcium ion activities were calibrated using the fluorescent calcium probe Fura-2. Calcium ion activities of > or =500 nmol/L significantly inhibited the percentage of demembranated-reactivated spermatozoa that were motile, and the velocity and lateral head displacement of these cells. The range of intracellular calcium activities in spermatozoa from 24 cryopreserved ejaculates was 110-534 nmol/L; roughly twice the value in fresh spermatozoa. Therefore, calcium ion activities in the range observed in cryopreserved spermatozoa can inhibit the activity of demembranated human spermatozoa.

Sperm motility is dependent on a unique isoform of the Na,K-ATPase

The Na,K-ATPase, a member of the P-type ATPases, is composed of two subunits, alpha and beta, and is responsible for translocating Na(+) out of the cell and K(+) into the cell using the energy of hydrolysis of one molecule of ATP. The electrochemical gradient it generates is necessary for many cellular functions, including establishment of the plasma membrane potential and transport of sugars and ions in and out of the cell. Families of isoforms for both the alpha and beta subunits have been identified, and specific functional roles for individual isoforms are just beginning to emerge. The alpha4 isoform is the most recently identified Na, K-ATPase alpha isoform, and its expression has been found only in testis. Here we show that expression of the alpha4 isoform in testis is localized to spermatozoa and that inhibition of this isoform alone eliminates sperm motility. These data describe for the first time a biological function for the alpha4 isoform of the Na,K-ATPase, revealing a critical role for this isoform in sperm motility.