The Na,K-ATPase alpha4 gene (Atp1a4) encodes a ouabain-resistant alpha subunit and is tightly linked to the alpha2 gene (Atp1a2) on mouse chromosome 1

Clinical significance of P-class pumps in cancer

P-class pumps are specific ion transporters involved in maintaining intracellular/extracellular ion homeostasis, gene transcription, and cell proliferation and migration in all eukaryotic cells. The present review aimed to evaluate the role of P-type pumps [Na⁺/K⁺ ATPase (NKA), H⁺/K⁺ ATPase (HKA) and Ca2⁺-ATPase] in cancer cells across three fronts, namely structure, function and genetic expression. It has been shown that administration of specific P-class pumps inhibitors can have different effects by: i) Altering pump function; ii) inhibiting cell proliferation; iii) inducing apoptosis; iv) modifying metabolic pathways; and v) induce sensitivity to chemotherapy and lead to antitumor effects. For example, the NKA β2 subunit can be downregulated by gemcitabine, resulting in increased apoptosis of cancer cells. The sarcoendoplasmic reticulum calcium ATPase can be inhibited by thapsigargin resulting in decreased prostate tumor volume, whereas the HKA α subunit can be affected by proton pump inhibitors in gastric cancer cell lines, inducing apoptosis. In conclusion, the present review highlighted the central role of P-class pumps and their possible use and role as anticancer cellular targets for novel therapeutic chemical agents.

The Na+ and K+ transport system of sperm (ATP1A4) is essential for male fertility and an attractive target for male contraception†

One of the mechanisms that cells have developed to fulfil their specialized tasks is to express different molecular variants of a particular protein that has unique functional properties. Na,K-ATPase (NKA), the ion transport mechanism that maintains the transmembrane Na+ and K+ concentrations across the plasma membrane of cells, is one of such protein systems that shows high molecular and functional heterogeneity. Four different isoforms of the NKA catalytic subunit are expressed in mammalian cells (NKAα1, NKAα2, NKAα3, and NKAα4). NKAα4 (ATP1A4) is the isoform with the most restricted pattern of expression, being solely produced in male germ cells of the testis. NKAα4 is abundant in spermatozoa, where it is required for sperm motility and hyperactivation. This review discusses the expression, functional properties, mechanism of action of NKAα4 in sperm physiology, and its role in male fertility. In addition, we describe the use of NKAα4 as a target for male contraception and a potential approach to pharmacologically block its ion transport function to interfere with male fertility.

Molecular cloning and characterization of porcine Na⁺/K⁺-ATPase isoform α4

Na⁺/K⁺-ATPase is responsible for maintaining electrochemical gradients of Na⁺ and K⁺, which is essential for a variety of cellular functions including neuronal activity. The α-subunit of the Na⁺/K⁺-ATPase is composed of four different polypeptides (α1-α4) encoded by different genes. Na,K-ATPase α4, encoded by the ATP1A4 gene, is expressed in testis and in male germ cells of humans, rats and mice. The α4 polypeptide has an important role in sperm motility, and is essential for male fertility. Here we present the RT-PCR cloning and characterization of the porcine ATP1A4 cDNA coding for Na⁺/K⁺-ATPase polypeptide α4. The Na⁺/K⁺-ATPase polypeptide α4, consisting of 1030 amino acids, displays a high homology with its human counterpart (86%). Phylogenetic analysis demonstrated that porcine Na⁺/K⁺-ATPase polypeptide α4 is closely related to other mammalian counterparts. In addition, the genomic structure of the porcine ATP1A4 gene was determined, and the intron-exon organization was found to be similar to that of the human ATP1A4 gene. The promoter sequence for the porcine ATP1A4 gene was also identified. Investigation of the genetic variation in the porcine ATP1A4 gene revealed a missense A/G SNP in exon 18. This A/G polymorphism results in a substitution of a methionine to a glycine residue (M888G). A very high overall DNA methylation rate of the ATP1A4 gene, 70-80%, was observed in both brain and liver. Expression analysis demonstrated that the porcine ATP1A4 gene is predominantly expressed in testis. The sequence of the porcine ATP1A4 cDNA encoding the Na⁺/K⁺-ATPase α4 protein has been submitted to GenBank under the accession number GenBank Accession No. MG587082.

Na+, K+-ATPase β1 subunit associates with α1 subunit modulating a "higher-NKA-in-hyposmotic media" response in gills of euryhaline milkfish, Chanos chanos

The euryhaline milkfish (Chanos chanos) is a popular aquaculture species that can be cultured in fresh water, brackish water, or seawater in Southeast Asia. In gills of the milkfish, Na⁺, K⁺-ATPase (i.e., NKA; sodium pump) responds to salinity challenges including changes in mRNA abundance, protein amount, and activity. The functional pump is composed of a heterodimeric protein complex composed of α- and β-subunits. Among the NKA genes, α1-β1 isozyme comprises the major form of NKA subunits in mammalian osmoregulatory organs; however, most studies on fish gills have focused on the α1 subunit and did not verify the α1-β1 isozyme. Based on the sequenced milkfish transcriptome, an NKA β1 subunit gene was identified that had the highest amino acid homology to β233, a NKA β1 subunit paralog originally identified in the eel. Despite this high level of homology to β233, phylogenetic analysis and the fact that only a single NKA β1 subunit gene exists in the milkfish suggest that the milkfish gene should be referred to as the NKA β1 subunit gene. The results of accurate domain prediction of the β1 subunit, co-localization of α1 and β1 subunits in epithelial ionocytes, and co-immunoprecipitation of α1 and β1 subunits, indicated the formation of a α1-β1 complex in milkfish gills. Moreover, when transferred to hyposmotic media (fresh water) from seawater, parallel increases in branchial mRNA and protein expression of NKA α1 and β1 subunits suggested their roles in hypo-osmoregulation of euryhaline milkfish. This study molecularly characterized the NKA β1 subunit and provided the first evidence for an NKA α1-β1 association in gill ionocytes of euryhaline teleosts.

Green fluorescence protein driven by the Na,K-ATPase α4 isoform promoter is expressed only in male germ cells of mouse testis

PURPOSE

Expression of the Na,K-ATPase α4 isoform is required for sperm motility and fertility and is controlled by the Atp1a4 promoter. Here, we have investigated the specific tissue, cell type and developmental regulation of expression mediated by the Atp1a4 promoter.

METHODS

We have inserted the green fluorescent protein (GFP), downstream of the endogenous Atp1a4 promoter, in place of the Na,K-ATPase α4 gene, and used it as a marker for α4 expression in mice (Atp1a4 ( null(GFP) ) mice).

RESULTS

Replacement of α4 by GFP completely disrupted α4 expression and activity, produced sperm morphological and functional abnormalities, and caused infertility of Atp1a4 ( null(GFP) ) male mice. Immunoblot analysis of Atp1a4 ( null(GFP) ) mouse tissues showed GFP expression in testis. This particular expression pattern was found in adult, but not in mouse embryos or in 7, 18 day old mice. In agreement with expression of GFP, adult Atp1a4 ( null(GFP) ) mouse testis displayed the typical fluorescence of GFP. Immunocytochemistry of testis identified GFP in more differentiated male germ cells, but not in spermatogonia, Leydig or Sertoli cells. Further analysis, using immunoblot of fluorescently sorted testis cells with cell specific markers, detected GFP only in spermatocytes, spermatids and spermatozoa. While epididymis showed GFP expression, this was confined to the spermatozoa within the epididymal tubules.

CONCLUSIONS

Our results show that the Atp1a4 promoter drives GFP expression exclusively in male germ cells of the testis, where it restricts it to post-meiotic stages of spermatogenesis. These findings highlight the exquisite spatial and temporal control of expression exerted by the Atp1a4 promoter on Na,K-ATPase α4, which is particularly well suited to fulfill the special functions of spermatozoa.

The pumps that fuel a sperm's journey

The sole purpose of a sperm cell is to carry genetic information from a male to a female egg. In order to accomplish this quest, the sperm cell must travel a long distance through a constantly changing environment. The success of this journey depends on membrane proteins that are uniquely expressed in sperm cells. One of these proteins is the α4 isoform of the sodium pump. This pump is optimized to cope with the ionic environment characteristic of the female reproductive tract, and its activity may be tightly coupled with secondary transporters that maintain cytoplasmic pH. Pharmacological inhibition of α4 is sufficient to inhibit sperm motility, and significant differences around the inhibitor-binding site compared with the ubiquitous α1 isoform, make α4 a feasible target in rational drug development.

Role and regulation of EGFR in actin remodeling in sperm capacitation and the acrosome reaction

To bind and fertilize the egg, the spermatozoon should undergo few biochemical and motility changes in the female reproductive tract collectively called capacitation. The capacitated spermatozoon binds to the egg zona pellucida, and then undergoes the acrosome reaction (AR), which allows its penetration into the egg. The mechanisms regulating sperm capacitation and the AR are not completely understood. In the present review, we summarize some data regarding the role and regulation of the epidermal growth factor receptor (EGFR) in these processes. In the capacitation process, the EGFR is partially activated by protein kinase A (PKA), resulting in phospholipase D (PLD) activation and actin polymerization. Protein kinase C alpha (PKCα), which is already activated at the beginning of the capacitation, also participates in PLD activation. Further activation of the EGFR at the end of the capacitation enhances intracellular Ca(2+) concentration leading to F-actin breakdown and allows the AR to take place. Under in vivo conditions, the EGFR can be directly activated by its known ligand epidermal growth factor (EGF), and indirectly by activating PKA or by transactivation mediated by G protein-coupled receptors (GPCRs) activation or by ouabain. Under physiological conditions, sperm PKA is activated mainly by bicarbonate, which activates the soluble adenylyl cyclase to produce cyclic adenosine monophosphate (cAMP), the activator of PKA. The GPCR activators angiotensin II or lysophosphatidic acid, as well as ouabain and EGF are physiological components present in the female reproductive tract.

Eya4 regulation of Na+/K+-ATPase is required for sensory system development in zebrafish

To investigate the mechanisms by which mutations in the human transcriptional co-activator EYA4 gene cause sensorineural hearing loss that can occur in association with dilated cardiomyopathy, we studied eya4 expression during zebrafish development and characterized eya4 deficiency. eya4 morphant fish embryos had reduced numbers of hair cells in the otic vesicle and lateral line neuromasts with impaired sensory responses. Analyses of candidate genes that are known to be expressed in a temporal and spatial pattern comparable to eya4 focused our analyses on atp1b2b, which encodes the beta2b subunit of the zebrafish Na+/K+-ATPase. We demonstrate atp1b2b levels are reduced in eya4 morphant fish and that morpholino oligonucleotides targeting the atp1b2b gene recapitulated the eya4 deficiency phenotypes, including heart failure, decreased sensory hair cell numbers in the otic vesicle and neuromasts, and abnormal sensory responses. Furthermore, atp1b2b overexpression rescued these phenotypes in eya4 morphant fish. We conclude that eya4 regulation of Na+/K+-ATPase is crucial for the development of mechanosensory cells and the maintenance of cardiac function in zebrafish.

Analysis of a quantitative trait locus for seizure susceptibility in mice using bacterial artificial chromosome-mediated gene transfer

PURPOSE

Previous quantitative trait loci (QTL) mapping studies from our laboratory identified a 6.6 Mb segment of distal chromosome 1 that contains a gene (or genes) having a strong influence on the difference in seizure susceptibility between C57BL/6 (B6) and DBA/2 (D2) mice. A gene transfer strategy involving a bacterial artificial chromosome (BAC) DNA construct that contains several candidate genes from the critical interval was used to test the hypothesis that a strain-specific variation in one (or more) of the genes is responsible for the QTL effect.

METHODS

Fertilized oocytes from a seizure-sensitive congenic strain (B6.D2-Mtv7a/Ty-27d) were injected with BAC DNA and three independent founder lines of BAC-transgenic mice were generated. Seizure susceptibility was quantified by measuring maximal electroshock seizure threshold (MEST) in transgenic mice and nontransgenic littermates.

RESULTS

Seizure testing documented significant MEST elevation in all three transgenic lines compared to littermate controls. Allele-specific RT-PCR analysis confirmed gene transcription from genome-integrated BAC DNA and copy-number-dependent phenotypic effects were observed.

CONCLUSIONS

Results of this study suggest that the gene(s) responsible for the major chromosome 1 seizure QTL is found on BAC RPCI23-157J4 and demonstrate the utility of in vivo gene transfer for studying quantitative trait genes in mice. Further characterization of this transgenic model will provide new insight into mechanisms of seizure susceptibility.

The transcription factor CREMtau and cAMP regulate promoter activity of the Na,K-ATPase alpha4 isoform

The Na,K-ATPase is an essential enzyme of the plasma membrane that plays a key role in numerous cell processes that depend on the transcellular gradients of Na(+) and K(+). Among the various isoforms of the catalytic subunit of the Na,K-ATPase, alpha4 exhibits the most limited pattern of expression, being restricted to male germ cells. Activity of alpha4 is essential for sperm function, and alpha4 is upregulated during spermatogenesis. The present study addressed the transcriptional control of the human Na,K-ATPase alpha4 gene, ATP1A4. We describe that a 5' untranslated region of the ATP1A4 gene (designated -339/+480 based on the ATP1A4 transcription initiation site) has promoter activity in luciferase reporter assays. Computer analysis of this promoter region revealed consensus sites (CRE) for the cyclic AMP (cAMP) response element modulator (CREM). Accordingly, dibutyryl cAMP (db-cAMP) and ectopic expression of CREMtau, a testis specific splice variant of CREM were able to activate the ATP1A4 promoter driven expression of luciferase in HEK 293 T, JEG-3 and GC-1 cells. Further characterization of the effect of db-cAMP and CREMtau on deleted constructs of the ATP1A4 promoter (-339/+80, and +25/+480), and on the -339/+480 region carrying mutations in the CRE sites showed that db-cAMP and CREMtau effect required the CRE motif located 263 bp upstream the transcription initiation site. EMSA experiments confirmed the CRE sequence as a bonafide CREMtau binding site. These results constitute the first demonstration of the transcriptional control of ATP1A4 gene expression by cAMP and by CREMtau, a transcription factor essential for male germ cell gene expression.

The Na,K-ATPase alpha4 isoform from humans has distinct enzymatic properties and is important for sperm motility

In the rat, the Na,K-ATPase alpha4 isoform exhibits unique enzymatic characteristics and is important for sperm motility. In this work, we studied expression, localization and function of alpha4 in human spermatozoa. We show two catalytically active Na,K-ATPase alpha polypeptides with different ouabain affinity and identified expression of alpha1, alpha4, beta1 and beta3 isoforms in the gametes. In addition, human sperm presented two Na,K-ATPases composed of alpha4, alpha4beta1 and alpha4beta3. Kinetic analysis of these isozymes produced in insect cells showed that, compared with human alpha1beta1, alpha4beta1 and alpha4beta3 exhibit higher Na(+) and lower K(+) affinity and higher sensitivity to ouabain. These particular enzymatic properties suggested a role for alpha4 in sperm function. Using computer-assisted sperm analysis (CASA), we found that ouabain inhibition of alpha4 significantly decreased percentage sperm motility. In contrast, ouabain did not affect linearity of forward progression, amplitude of lateral head displacement, beat cross frequency and sperm straight-line, curvilinear or average path velocities. This suggests a primary role of alpha4 in flagellar motility. Accordingly, we found alpha4 in the sperm tail, predominating in the mid-piece of the flagellum. Therefore, similar to the rat ortholog, human Na,K-ATPase alpha4 isoform has a distinct activity that is essential for sperm function.

Different expression and activity of the alpha1 and alpha4 isoforms of the Na,K-ATPase during rat male germ cell ontogeny

Two catalytic isoforms of the Na,K-ATPase, alpha1 and alpha4, are present in testis. While alpha1 is ubiquitously expressed in tissues, alpha4 predominates in male germ cells. Each isoform has distinct enzymatic properties and appears to play specific roles. To gain insight into the relevance of the Na,K-ATPase alpha isoforms in male germ cell biology, we have studied the expression and activity of alpha1 and alpha4 during spermatogenesis and epididymal maturation. This was explored in rat testes at different ages, in isolated spermatogenic cells and in spermatozoa from the caput and caudal regions of the epididymis. Our results show that alpha1 and alpha4 undergo differential regulation during development. Whereas alpha1 exhibits only modest changes, alpha4 increases with gamete differentiation. The most drastic changes for alpha4 take place in spermatocytes at the mRNA level, and with the transition of round spermatids into spermatozoa for expression and activity of the protein. No further changes are detected during transit of spermatozoa through the epididymis. In addition, the cellular distribution of alpha4 is modified with development, being diffusely expressed at the plasma membrane and intracellular compartments of immature cells, finally to localize to the midregion of the spermatozoon flagellum. In contrast, the alpha1 isoform is evenly present along the plasma membrane of the developing and mature gametes. In conclusion, the Na,K-ATPase alpha1 and alpha4 isoforms are functional in diploid, meiotic and haploid male germ cells, alpha4 being significantly upregulated during spermatogenesis. These results support the importance of alpha4 in male gamete differentiation and function.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

The human Na,K-ATPase alpha4 isoform is a ouabain-sensitive alpha isoform that is expressed in sperm

The Na,K-ATPase generates electrochemical gradients across the plasma membrane that are responsible for numerous cellular and physiological processes. The active Na,K-ATPase is minimally composed of an alpha and a beta subunit and families of isoforms for both subunits exist. Recent studies have identified a physiological role for the rat Na,K-ATPase alpha4 isoform in sperm motility. However, very little is known about the human Na,K-ATPase alpha4 isoform other than its genomic sequence and structure and its mRNA expression pattern. Here, the human alpha4 isoform of the Na,K-ATPase is cloned, expressed, and characterized. Full length cDNAs encoding the putative human alpha4 isoform of the Na,K-ATPase were identified from a number of ESTs and a protein product corresponding to this isoform was shown to be expressed from these cDNAs. The human Na,K-ATPase alpha4 isoform protein was found to be expressed in mature sperm in human testes sections and it is localized specifically to the principle piece of human sperm. In addition, the presence of the Na,K-ATPase alpha4 isoform is absent in immature testes however its expression appears coincident with sexual maturity. And finally, the human Na,K-ATPase alpha4 isoform was shown to be as sensitive to cardiac glycoside inhibition as the human Na,K-ATPase alpha1 isoform. Considering the important role of the rat Na,K-ATPase alpha4 isoform in rat sperm motility, the demonstration that the human alpha4 isoform is a sperm-specific protein localized to the flagellum suggests a role for the human Na,K-ATPase alpha4 isoform in human sperm physiology.

K+-induced hyperpolarization in rat mesenteric artery: identification, localization and role of Na+/K+-ATPases

1. Mechanisms underlying K(+)-induced hyperpolarizations in the presence and absence of phenylephrine were investigated in endothelium-denuded rat mesenteric arteries (for all mean values, n=4). 2. Myocyte resting membrane potential (m.p.) was -58.8+/-0.8 mV. Application of 5 mM KCl produced similar hyperpolarizations in the absence (17.6+/-0.7 mV) or presence (15.8+/-1.0 mV) of 500 nM ouabain. In the presence of ouabain +30 microM barium, hyperpolarization to 5 mM KCl was essentially abolished. 3. In the presence of 10 microM phenylephrine (m.p. -33.7+/-3 mV), repolarization to 5 mM KCl did not occur in the presence or absence of 4-aminopyridine but was restored (-26.9+/-1.8 mV) on addition of iberiotoxin (100 nM). Under these conditions the K+-induced repolarization was insensitive to barium (30 microM) but abolished by 500 nM ouabain alone. 4. In the presence of phenylephrine + iberiotoxin the hyperpolarization to 5 mM K(+) was inhibited in the additional presence of 300 nM levcromakalim, an action which was reversed by 10 microM glibenclamide. 5. RT-PCR, Western blotting and immunohistochemical techniques collectively showed the presence of alpha(1)-, alpha(2)- and alpha(3)-subunits of Na(+)/K(+)-ATPase in the myocytes. 6. In K(+)-free solution, re-introduction of K(+) (to 4.6 mM) hyperpolarized myocytes by 20.9+/-0.5 mV, an effect unchanged by 500 nM ouabain but abolished by 500 microM ouabain. 7. We conclude that under basal conditions, Na(+)/K(+)-ATPases containing alpha(2)- and/or alpha(3)-subunits are partially responsible for the observed K(+)-induced effects. The opening of myocyte K(+) channels (by levcromakalim or phenylephrine) creates a 'K(+) cloud' around the cells which fully activates Na(+)/K(+)-ATPase and thereby abolishes further responses to [K(+)](o) elevation.

Physical mapping and characterization of the human Na,K-ATPase isoform, ATP1A4

Four isoforms of the catalytic alpha subunit of the Na,K-ATPase have been previously identified. We characterized and mapped a genomic copy of the human ATP1A4 isoform between D1S2707 and WI-9524, telomeric to a nearby isoform ATP1A2, and within a candidate region at 1q23 for familial hemiplegic migraine (FHM). Human ATP1A4 gene shares 84% identity with the mouse Atp1a4 gene, and both consist of 22 exons and 21 introns. The predicted polypeptide is 1029 amino acids and shares 82 and 79.8% identity, respectively, with human ATP1A2 and ATP1A1. ATP1A4 is larger than other isoforms and most divergent at the N-terminus. ATP1A4 and ATP1A2 are paralogous genes with the same number and organization of putative H-transmembrane domains, conserved exon-intron boundaries, and are found approximately 8.5 kb apart. Expression analysis of the ATP1A4 gene revealed a new major approximately 7.5 kb transcript in human skeletal muscle, with expression also shown in mouse muscle. Predictive analysis of promoter regions identified muscle specific regulatory elements for ATP1A4 and Atp1a4. Mutation analysis among eight affected individuals from a single large, highly penetrant FHM family was negative in ATP1A4 and ATP1A2 although multiple polymorphisms were identified.

Two Na,K-ATPase beta 2 subunit isoforms are differentially expressed within the central nervous system and sensory organs during zebrafish embryogenesis

We have identified cDNAs encoding a second zebrafish ortholog of the human Na,K-ATPase beta 2 subunit. The beta 2b cDNA encodes a 292 amino acid-long polypeptide with 74% identity to the previously characterized zebrafish beta 2a subunit. By using a zebrafish meiotic mapping panel, we determined that the beta 2b gene (atp1b2b) was tightly linked to markers on linkage group 5, whereas the beta 2a gene was located on linkage group 23. In situ hybridization analysis shows that in developing zebrafish embryos, atp1b2a and atp1b2b are predominantly expressed in the nervous system. beta 2a transcripts were abundantly expressed throughout brain as well as spinal cord neurons and lateral line ganglia. In contrast, beta 2b mRNA expression was primarily detected in sensory organs, including retina, otic vesicles, and lateral line neuromast cells. These results suggest that the beta 2a and beta 2b genes play distinct roles in developing brain and sensory organs, and raise the possibility that the functions encoded by the single mammalian beta 2 gene may be partitioned between the two zebrafish beta 2 orthologs.

Structure, promoter analysis, and chromosomal localization of the murine H(+)/K(+)-ATPase alpha 2 subunit gene

The H(+)/K(+)-ATPase alpha2 subunit (HK alpha 2) of distal colon and renal collecting ducts plays a critical role in potassium and acid-base homeostasis. The isolation and complete sequence of the murine HK alpha 2 gene are reported. The HK alpha 2 gene contains 23 exons and spans 23.5 kb of genomic DNA. The exon/intron organization is comparable to that of the human ATP1AL1 gene. Primer extension and 5'-rapid amplification of cDNA ends of distal colon RNA were used to map the transcription initiation site. Fluorescence in situ hybridization analysis localized the HK alpha 2 gene to murine chromosome 14C3. Sequence analysis of 7.2 kb of the 5'-flanking region revealed numerous consensus sites for transcription factors, including two potential glucocorticoid response elements. Transient transfection of promoter-luciferase constructs demonstrated strong basal HK alpha 2 promoter activity in renal collecting duct cells but not in fibroblasts or in a medullary thick ascending limb of Henle's loop cell line. Deletion analysis revealed that the proximal 0.2 kb of the promoter was sufficient to confer activity in collecting duct cells. These data should prove important in elucidation of the mechanisms controlling the differential, tissue-specific expression of the HK alpha 2 gene.

The reproductive importance of P-type ATPases

P-type ATPases are integral membrane proteins that use the free energy of ATP hydrolysis to generate transmembrane electrochemical ion gradients to support a variety of cellular processes. They have eight signature motifs, eight or ten transmembrane domains, highly conserved phosphorylation and ATP-binding sites, and similar hydropathic profiles. This review summarizes recent insights in the relationship of P-type ATPases to successful reproduction, and the hormone dependence of some family members. Because protein topology is central to understanding the pump action of this family of enzymes, this review also describes the dramatic change in the primary structure of one family member that may mediate transcription in the uterus.

The repertoire of Na,K-ATPase alpha and beta subunit genes expressed in the zebrafish, Danio rerio

We have identified a cohort of zebrafish expressed sequence tags encoding eight Na,K-ATPase alpha subunits and five beta subunits. Sequence comparisons and phylogenetic analysis indicate that five of the zebrafish alpha subunit genes comprise an alpha1-like gene subfamily and two are orthologs of the mammalian alpha3 subunit gene. The remaining alpha subunit clone is most similar to the mammalian alpha2 subunit. Among the five beta subunit genes, two are orthologs of the mammalian beta1 isoform, one represents a beta2 ortholog, and two are orthologous to the mammalian beta3 subunit. Using zebrafish radiation hybrid and meiotic mapping panels, we determined linkage assignments for each alpha and beta subunit gene. Na,K-ATPase genes are dispersed in the zebrafish genome with the exception of four of the alpha1-like genes, which are tightly clustered on linkage group 1. Comparative mapping studies indicate that most of the zebrafish Na,K-ATPase genes localize to regions of conserved synteny between zebrafish and humans. The expression patterns of Na,K-ATPase alpha and beta subunit genes in zebrafish are quite distinctive. No two alpha or beta subunit genes exhibit the same expression profile. Together, our data imply a very high degree of Na,K-ATPase isoenzyme heterogeneity in zebrafish, with the potential for 40 structurally distinct alpha/beta subunit combinations. Differences in expression patterns of alpha and beta subunits suggest that many of the isoenzymes are also likely to exhibit differences in functional properties within specific cell and tissue types. Our studies form a framework for analyzing structure function relationships for sodium pump isoforms using reverse genetic approaches.

Comparative physical and transcript maps of approximately 1 Mb around loop-tail, a gene for severe neural tube defects on distal mouse chromosome 1 and human chromosome 1q22-q23

The homozygous loop-tail (Lp) mouse has a severe neural tube closure defect, analogous to the craniorachischisis phenotype seen in humans. Linkage analysis and physical mapping have previously localized the Lp locus to a region on mouse chromosome 1 defined by the markers D1Mit113-Tagln2. Here we report the construction of sequence-ready bacterial clone contigs encompassing the Lp critical region in both mouse and the orthologous human region (1q22-q23). Twenty-two genes, one EST, and one pseudogene have been identified using a combination of EST database screening, exon amplification, and genomic sequence analysis. The preliminary gene map is Cen-Estm33-AA693056-Ly9-Cd48-Slam-Cd84-Kiaa1215-Nhlh1-Kiaa0253-Copa-Pxf-H326-Pea15-Casq1-Atp1a4-Atp1a2-Estm34-Kcnj9-Kcnj10-Kiaa1355-Tagln2-Nesg1-Crp-Tel. The genes between Slam and Kiaa1355 are positional candidates for Lp. The comparative gene content and order are identical between mouse and human, indicating a high degree of conservation between the two species in this region. Together, the physical and transcript maps described here serve as resources for the identification of the Lp mutation and further define the conservation of this genomic region between mouse and human.

The alpha4 isoform of the Na,K-ATPase is expressed in the germ cells of the testes

In addition to the three isoforms of the catalytic subunit of the Na, K-ATPase originally identified (alpha1, alpha2, and alpha3), a fourth alpha polypeptide (alpha4) has recently been found in mammalian cells. This novel alpha-subunit of the Na,K-ATPase is selectively expressed in male gonadal tissues. In the testes, alpha4 is functionally active and comprises approximately half of the Na, K-ATPase activity of the organ. At present, the pattern of expression of the alpha4 polypeptide within the cells of the male gonad is unknown. By in situ hybridization, immunocytochemistry, and the ouabain inhibition profile of Na,K-ATPase activity, we show that the alpha4-subunit is expressed in the germ cells of rat testes. The highest amounts of the isoform are found in spermatozoa, where it constitutes two thirds of the Na,K-ATPase activity of the gametes. The other Na pump present in the cells is the ubiquitously expressed alpha1 polypeptide. The characteristic localization of alpha4 in the gonad is further supported by the drastic reduction of the polypeptide in mice that are infertile as a consequence of arrest in maturation of the germ cells. In addition, GC-1spg cells, a murine cell line derived from testis spermatogonia, also contain the Na, K-ATPase alpha4 polypeptide. However, the level of expression of the isoform in these cells is much lower than in the spermatozoa, a fact that may depend on the limited ability of the GC-1spg cells to differentiate in vitro. The particular expression of the Na,K-ATPase alpha4 isoform we encounter and the specific enzymatic properties of the polypeptide suggests its importance for ionic homeostasis of the germ cells of the testes.

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.