PMR1/SPCA Ca2+ pumps and the role of the Golgi apparatus as a Ca2+ store

The MLCK-mediated alpha1-adrenergic inotropic effect in atrial myocardium is negatively modulated by PKCepsilon signaling

The present study examined the role of myosin light chain kinase (MLCK), PKC isozymes, and inositol 1,4,5-trisphosphate (IP(3)) receptor in the positive inotropic effect of alpha(1)-adrenergic stimulation in atrial myocardium. We measured inotropic effects of phenylephrine (0.3-300 microM) in isolated left atrial preparations (1 Hz, 37 degrees C, 1.8 mM Ca(2+), 0.3 microM nadolol) from male 8-week FVB mice (n=200). Phenylephrine concentration-dependently increased force of contraction from 1.5+/-0.1 to 2.8+/-0.1 mN (mean+/-s.e.m., n=42), which was associated with increased MLC-2a phosphorylation at serine 21 and 22 by 67% and translocation of PKCepsilon but not PKCalpha to membrane (+30%) and myofilament (+50%) fractions.MLCK inhibition using ML-7 or wortmannin right-shifted the concentration-response curve of phenylephrine, reducing its inotropic effect at 10 microM by 73% and 81%, respectively. The compound KIE1-1 (500 nM), an intracellularly acting PKCepsilon translocation inhibitor peptide, prevented PKCepsilon translocation and augmented the maximal inotropic effect of phenylephrine by 40%. In contrast, inhibition of Ca(2+)-dependent PKC translocation (KIC1-1, 500 nM) had no effect. Chelerythrine, a PKC inhibitor, decreased basal force without changing the inotropic effect of phenylephrine. The IP(3) receptor blocker 2-APB (2 and 20 microM) concentration-dependently decreased basal force, but did not affect the concentration-response curve of phenylephrine. These results indicate that activation of MLCK is required for the positive inotropic effect of alpha(1)-adrenergic stimulation, that the Ca(2+)-independent PKCepsilon negatively modulates this effect, and that PKCalpha and IP(3) receptor activation is not involved.

Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+and Mn2+homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene ( SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol ( 1 , 4 , 5 )-trisphosphate-mediated Ca2+signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+“spherites” and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+in transporting epithelia: to transport or store Ca2+in bulk without compromising its use as a signal.

Identification of new Golgi complex specific proteins by direct organelle proteomic analysis

The Golgi complex is in the crossroad of the endocytic and secretory pathways. Its function is to post-translationally modify and sort proteins and lipids, and regulate the membrane balance in the cell. To understand the structure-function relationship of the Golgi complex the Golgi proteome has to be identified first. We have used a direct organelle proteomic analysis to identify new Golgi complex proteins. Enriched stacked Golgi membrane fractions from rat livers were isolated, and the proteins from these membranes were subsequently digested into peptides. The peptides were fractionated by cation-exchange chromatography followed by protein identification by automated capillary-LC/ESI-MS/MS analysis and database searches. Two different search programs, ProID and MASCOT were used. This resulted in a total of 1125 protein identifications in two experiments. In addition to the known Golgi resident proteins, a significant number of unknown proteins were identified. Some of these were further characterized in silico using different programs to provide insight into their structure, intracellular localization and biological functions. The Golgi localization of two of these newly identified proteins was also confirmed by indirect immunofluorescence.

Atp2b2, encoding plasma membrane Ca2+-ATPase type 2, (PMCA2) exhibits tissue-specific first exon usage in hair cells, neurons, and mammary glands of mice

Atp2b2 encodes the plasma membrane Ca(2+)-ATPase type 2 (PMCA2) expressed in various tissues, including stereocilia of cochlear and vestibular hair cells, cerebellar Purkinje cells, and lactating mammary epithelia. Mutations of the gene lead to deafness, ataxia, and reduced Ca(2+) levels in milk. Heterozygous mutants also have abnormal hearing, suggesting that precise regulation of Atp2b2 is required for normal function. In this study, we describe Atp2b2 5'-untranslated region genomic structure and transcript usage in mice. Using 5'-rapid amplification of cDNA ends, we observed four transcripts: types alpha, beta, mu and delta, each splicing into a common ATG-containing exon. Types alpha and beta correspond to previously published mammalian cDNA sequences. Types mu and delta constitute novel 5'-untranslated region sequences, and were observed at high levels only in lactating mammary gland. Using real-time reverse transcriptase polymerase chain reaction, we quantified relative transcript usage across several tissues. We show that alpha and beta are abundant throughout the CNS, as well as the cochlea. When we microdissected the cochlea into hair cell and spiral ganglion containing fractions, we found that cochlear hair cell expression is mediated through the type alpha transcript. In situ hybridization studies in cerebellum using exon-specific probes revealed that alpha dominates in Purkinje neurons, while beta is enriched in cerebellar granule neurons. We compared 5'-untranslated region sequence across multiple species, and found high conservation around the first exons for alpha and beta in mammals, but not other species. The regions around the mu and delta first exons are highly conserved between rat and mouse, but less so with other species. Our results show that expression of Atp2b2 is highly regulated, using four different transcriptional start regions, two of which are differentially expressed in neuronal tissue. This suggests that unique regulatory mechanisms are used to control Atp2b2 expression in different types of cells.

Calcium transport and signaling in the mammary gland: targets for breast cancer

The mammary gland is subjected to extensive calcium loads during lactation to support the requirements of milk calcium enrichment. Despite the indispensable nature of calcium homeostasis and signaling in regulating numerous biological functions, the mechanisms by which systemic calcium is transported into milk by the mammary gland are far from completely understood. Furthermore, the implications of calcium signaling in terms of regulating proliferation, differentiation and apoptosis in the breast are currently uncertain. Deregulation of calcium homeostasis and signaling is associated with mammary gland pathophysiology and as such, calcium transporters, channels and binding proteins represent potential drug targets for the treatment of breast cancer.

Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca(2+) is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca(2+) concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca(2+) concentration and generation of complex Ca(2+) signals. In this review, we consider the various components of the Ca(2+)-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca(2+) stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca(2+) signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.

Cytosolic Ca2+ signals depending on the functional state of the Golgi in HeLa cells

The Golgi apparatus is, like the endoplasmic reticulum, an inositol-1,4,5-trisphosphate-sensitive Ca2+ store, but its role in setting up Ca2+ signals is not well understood. We have now measured histamine-induced Ca2+ signals in HeLa cells pretreated with brefeldin A, a fungal metabolite that leads to the fragmentation and subsequent disappearance of the Golgi apparatus by its reabsorption within the endoplasmic reticulum. Ca2+ responses in which the free cytoplasmic Ca2+ concentration returned to resting levels during the histamine stimulation (mainly baseline Ca2+ oscillations or a single Ca2+ peak) occurred more often in brefeldin A pretreated cells, resulting in a lower Ca2+ plateau in population measurements. The latencies before the onset of the Ca2+ signals were longer after brefeldin A pretreatment. These results suggest that the integrity of the Golgi apparatus contributes to the shaping of intracellular Ca2+ signals.

Keratinocytes cultured from patients with Hailey-Hailey disease and Darier disease display distinct patterns of calcium regulation

BACKGROUND

Hailey-Hailey disease (HHD) (OMIM 16960) and Darier disease (DD) (OMIM 124200) are dominantly inherited acantholytic skin diseases, respectively, caused by mutations in the genes encoding the Golgi secretory pathway Ca2+-ATPase (SPCA1, ATP2C1) and the sarco/endoplasmic reticulum Ca2+-ATPase type 2 (SERCA2, ATP2A2) genes.

OBJECTIVES

To investigate calcium regulation in keratinocytes cultured from patients with HHD and DD by measuring intracellular calcium resting levels and the cellular responses to ATP and thapsigargin.

METHODS

The study was carried out using keratinocyte cultures established from four patients with HHD and four with DD. Calcium concentrations were measured with fluorescence ratio imaging using fura-2 loading.

RESULTS

Control and HHD keratinocytes displayed approximately the same Ca2+ levels in resting phase, while DD keratinocytes showed elevated Ca2+ levels. Application of ATP caused less pronounced elevation of intracellular calcium concentration ([Ca2+]i) in both HHD and DD keratinocytes than in control cells. HHD keratinocytes did not lower their [Ca2+]i as efficiently as control keratinocytes after treatment with thapsigargin. In addition, DD keratinocytes were practically incapable of lowering their [Ca2+]i after treatment with thapsigargin.

CONCLUSIONS

The results demonstrate that the defects in SPCA1 and SERCA2 calcium ATPases result in distinct patterns of calcium metabolism. This is also supported by the different clinical features of the diseases.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Hailey-Hailey disease as an orthodisease of PMR1 deficiency in Saccharomyces cerevisiae

The term orthodisease has recently been introduced to define human disorders in which the pathogenic gene has orthologs in model organism genomes. Here, we describe Hailey-Hailey disease (HHD), a blistering skin disorder caused by haploinsufficiency of ATP2C1 as an orthodisease from a Saccharomyces cerevisiae perspective. ATP2C1 encodes the human secretory pathway Ca(2+)/Mn(2+) ATPase hSPCA1 and is orthologous to the PMR1 gene in S. cerevisiae. hSPCA1 fully complements PMR1 deficiency in yeast and pmr1DeltaS. cerevisiae has proved to be a valuable tool to screen ATP2C1 mutations and address potential pathogenic/pharmacologic mechanisms in HHD. Consequently, this human skin disorder is an ideal example of an orthodisease.

Secretory pathway Ca2+-ATPase (SPCA1) Ca2+ pumps, not SERCAs, regulate complex [Ca2+]i signals in human spermatozoa

The sarcoplasmic-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitors thapsigargin (0.1-1 μM) and cyclopiazonic acid (10 μM), failed to affect resting [Ca2+] in human spermatozoa. Slow progesterone-induced [Ca2+ i]i oscillations in human spermatozoa, which involve cyclic emptying-refilling of an intracellular Ca2+ store were also insensitive to these inhibitors. Non-selective doses of thapsigargin (5-30 μM, 50-300 times the saturating dose for SERCA inhibition), caused elevation of resting [Ca2+]i and partial, dose-dependent disruption of oscillations. A 10-40 μM concentration of bis(2-hydroxy-3-tert-butyl-5-methyl-phenyl)methane (bis-phenol), which inhibits both thapsigargin-sensitive and -insensitive microsomal Ca2+ ATPases, caused elevation of resting [Ca2+]i and inhibition of [Ca2+]i oscillations at doses consistent with inhibition of thapsigargin-resistant, microsomal ATPase and liberation of stored Ca2+. Low doses of bis-phenol had marked effects on [Ca2+]i oscillation kinetics. Application of the drug to cells previously stimulated with progesterone had effects very similar to those observed when it was applied to unstimulated cells, suggesting that the sustained Ca2+ influx induced by progesterone is not mediated via mobilisation of Ca2+ stores. Western blotting for human sperm proteins showed expression of secretory pathway Ca2+ ATPase (SPCA1). Immunolocalisation studies revealed expression of SPCA1 in all cells in an area behind the nucleus, extending into the midpiece. Staining for SERCA, carried out in parallel, detected no expression with either technique. We conclude that: (1) intracellular Ca2+ store(s) and store-dependent [Ca2+]i oscillations in human spermatozoa rely primarily on a thapsigargin/cyclopiazonic acid-insensitive Ca2+ pump, which is not a SERCA as characterised in somatic cells; (2) effects of high-dose thapsigargin on spermatozoa primarily reflect non-specific actions on non-SERCAs and; (3) secretory pathway Ca2+ ATPases contribute at least part of this non-SERCA Ca2+ pump activity.

Caenorhabditis elegans PMR1, a P-type calcium ATPase, is important for calcium/manganese homeostasis and oxidative stress response

The Caenorhabditis elegans PMR1, a P-type Ca2+/Mn2+ ATPase, is expressed in hypodermal seam cells, intestinal cells and spermatheca; localized in Golgi complex. Knock down of pmr-1 as well as overexpression of truncated Caenorhabditis elegans PMR1, which mimics dominant mutations observed in human Hailey-Hailey disease, renders the worm highly sensitive to EGTA and Mn2+. Interestingly, pmr-1 knock down not only causes animals to become resistant to oxidative stress but also suppresses high reactive oxygen species sensitivity of smf-3 RNA-mediated interference and daf-16 worms. These findings suggest that C. elegans PMR1 has important roles in Ca2+ and Mn2+ homeostasis and oxidative stress response.

Calcium secretion into milk

Ionized calcium ([Ca(2+)]) is present in milk at concentrations around 3 mM, a concentration that drives the formation of complexes with citrate, phosphate, and casein, thereby generating compounds that carry the major portion of calcium in milk. In humans and cows, where it has been studied, changes in milk calcium appear to be regulated by the amount of citrate and casein in milk rather than changes in [Ca(2+)]. Most or all of the calcium in milk is likely derived through exocytosis of secretory vesicles derived from the Golgi compartment where a calcium ATPase mediates transport from the cytoplasm. The identity of the transporters is not yet certain but gene expression for the plasma membrane calcium ATPase, PMCA2bw, and the secretory pathway calcium ATPase, SPCA, is highly upregulated during lactation. Currently nothing appears to be known about the mechanisms that mediate transport of calcium across the basolateral membrane of the alveolar cell.

Calcium pump disorders of the skin

The causes of Darier disease (DD) and Hailey-Hailey disease (HHD) have eluded clinicians and scientists for more than 60 years. DD is characterized by loss of adhesion between suprabasal epidermal cells associated with abnormal keratinization, while loss of epidermal cell-to-cell adhesion is predominant in HHD. The genes for both conditions have recently been identified using candidate positional cloning approaches. The gene for DD (ATP2A2) encodes a calcium transport ATPase of the sarco (endo)plasmic reticulum (SERCA2) Verboomen et al. [1992: Biochem J 286(Pt 2):591-595], while the gene for HHD (ATP2C1) codes for a secretory pathway for calcium and manganese transport ATPase of the Golgi apparatus (SPCA1) Hu et al. [2000: Nat Genet 24:61-65]. These results have provided completely new insights into the role of calcium and/or manganese in maintaining skin integrity. Although the precise disease mechanisms remain to be understood, these discoveries open a new field in research for the understanding and the treatment of these distressing disorders.

A Novel Isoform of the Secretory Pathway Ca2+,Mn2+-ATPase, hSPCA2, Has Unusual Properties and Is Expressed in the Brain

Unlike lower eukaryotes, mammalian genomes have a second gene, ATP2C2, encoding a putative member of the family of secretory pathway Ca2+,Mn(2+)-ATPases, SPCA2. Human SPCA2 shares 64% amino acid identity with the protein defective in Hailey Hailey disease, hSPCA1. We show that human SPCA2 (hSPCA2) has a more limited tissue distribution than hSPCA1, with prominent protein expression in brain and testis. In primary neuronal cells, endogenous SPCA2 has a highly punctate distribution that overlaps with vesicles derived from the trans-Golgi network and is thus different from the compact perinuclear distribution of hSPCA1 seen in keratinocytes and nonpolarized cells. Heterologous expression in a yeast strain lacking endogenous Ca2+ pumps reveals further functional differences from hSPCA1. Although the Mn(2+)-specific phenotype of hSPCA2 is similar to that of hSPCA1, Ca2+ ions are transported with much poorer affinity, resulting in only weak complementation of Ca(2+)-specific yeast phenotypes. These observations suggest that SPCA2 may have a more specialized role in mammalian cells, possibly in cellular detoxification of Mn2+ ions, similar to that in yeast. We point to the close links between manganese neurotoxicity and Parkinsonism that would predict an important physiological role for SPCA2 in the brain.

Characteristics of Prolonged Ca 2+ Release Events Associated With the Nuclei in Adult Cardiac Myocytes

Confocal microscopy was used to study the properties of nuclear Ca 2+ regulation in adult ventricular myocytes. Prolonged nuclear Ca 2+ release (PNCR) events were identified in both intact and permeabilized rat myocytes. PNCR occurred spontaneously and was restricted to localized regions at the ends of the elongated nuclei. Typically, PNCR took the form of a rapid rise in [Ca 2+ ] followed by a maintained plateau. The mean duration of PNCR (1.78±0.19 seconds) was markedly greater than the half decay time for cytosolic Ca 2+ sparks (31.2±0.56 ms) obtained under the same conditions. The PNCR width at half maximum amplitude (5.0±0.2 μm) was also significantly greater than that of cytosolic Ca 2+ sparks (2.6±0.05 μm) obtained under the same conditions. Experiments involving the use of syto-11 to accurately locate the nuclei demonstrated that PNCR originates from the nuclear envelope or a closely associated structure. The spatial spread of PNCR was asymmetrical, with greater diffusion of Ca 2+ toward the center of the nucleus than the cytosol. Both PNCR and Ca 2+ sparks were abolished by interventions that deplete SR Ca 2+ stores or inhibit RYR activation. Experiments on intact, electrically stimulated cells revealed that diffusion of Ca 2+ from the ends of the nucleus toward the center is a prominent feature of the nucleoplasmic Ca 2+ transient. The possibility that recruitment of Ca 2+ release sites involved in PNCR might influence the temporal and spatial characteristics of the nucleoplasmic [Ca 2+ ] transient is considered.

A PMR1-like calcium ATPase ofAspergillus fumigatus: cloning, identification and functional expression inS. cerevisiae

The understanding of the controlling factors of calcium homeostasis in Aspergillus fumigatus is very poor, although this ion is involved in several important events of these particular cells. We have cloned, identified and expressed for functional complementation a PMR1-like Ca(2+)-ATPase gene from A. fumigatus. The Afpmr1 gene encodes a protein of 1061 deduced amino acids, containing all the conserved subdomains found in other P-type ATPases: the phosphatase region, phosphorylation site, FITC labelling site, ATP binding domain; E(386), N871, D875 amino acid residues for calcium ion interaction and Q880, a residue that alters ion selectivity in PMR1. The expressed AfPMR1 in S. cerevisiae K616 strain functionally complemented the deficient growth in EGTA (5-20 mM)- and MnCl2 (4 mM)-containing medium. These results demonstrate the first evidence of a Ca(2+)-ATPase in A. fumigatus and strongly suggest a role for this enzyme in calcium and manganese homeostasis.

Calcium release from the Golgi apparatus and the endoplasmic reticulum in HeLa cells stably expressing targeted aequorin to these compartments

Extracellular agonists mobilize Ca2+ from SERCA-comprising intracellular Ca2+ stores located in both the Golgi apparatus and the endoplasmic reticulum. Ca2+ release from both these compartments was studied in HeLa cells stably expressing the luminescent Ca2+ indicator aequorin specifically targeted to these compartments. Changes in lumenal [Ca2+] as detected by the aequorin measurements were correlated with parallel changes in total Ca2+ content of the stores. The latencies and initial rates of Ca2+ release from the Golgi apparatus and the endoplasmic reticulum were quite similar. However, maximal Ca2+ release measured with Golgi-targeted aequorin terminated faster than that from the endoplasmic reticulum. The rate and extent of Ca2+ depletion from both compartments correlated well with the peak amplitude of the cytosolic [Ca2+] rise. Time-course experiments further revealed that the peak of the cytosolic Ca2+ response occurred before the lumenal [Ca2+] reached its lowest level. We conclude that both the Golgi apparatus and the endoplasmic reticulum contribute to the rise in cytosolic [Ca2+] upon agonist stimulation, but the kinetics of the Ca2+ release are different.

Phenotypes of SERCA and PMCA knockout mice

P-type Ca2+-ATPases of the sarco(endo)plasmic reticulum (SERCAs) and plasma membrane (PMCAs) are responsible for maintaining the Ca2+ gradients across cellular membranes that are required for regulation of Ca2+-mediated signaling and other biological processes. Gene-targeting studies of SERCA isoforms 1, 2, and 3 and PMCA isoforms 1, 2, and 4 have confirmed some of the general functions proposed for these pumps, such as a major role in excitation-contraction coupling for SERCA1 and SERCA2 and housekeeping functions for PMCA1 and SERCA2, but have also revealed some unexpected phenotypes. These include squamous cell cancer and plasticity in the regulation of Ca2+-mediated exocytosis in SERCA2 heterozygous mutant mice, modulation of Ca2+ signaling in SERCA3-deficient mice, deafness and balance disorders in PMCA2 null mice, and male infertility in PMCA4 null mice. These unique phenotypes provide new information about the cellular functions of these pumps, the requirement of their activities for higher order physiological processes, and the pathophysiological consequences of pump dysfunction.

Null Mutation in the Gene Encoding Plasma Membrane Ca2+-ATPase Isoform 2 Impairs Calcium Transport into Milk

The means by which calcium is transported into the milk produced by mammary glands is a poorly understood process. One hypothesis is that it occurs during exocytosis of secretory products via the Golgi pathway, consistent with the observation that the SPCA1 Ca2+-ATPase, which is expressed in the Golgi, is induced in lactating mammary tissue. However, massive up-regulation of the PMCA2bw plasma membrane Ca2+-ATPase also occurs during lactation and is more strongly correlated with increases in milk calcium, suggesting that calcium may be secreted directly via this pump. To examine the physiological role of PMCA2bw in lactation we compared lactating PMCA2-null mice to heterozygous and wild-type mice. Relative expression levels of individual milk proteins were unaffected by genotype. However, milk from PMCA2-null mice had 60% less calcium than milk from heterozygous and wild-type mice, the total milk protein concentration was lower, and an indirect measure of milk production (litter weights) suggested that the PMCA2-null mice produce significantly less milk. In contrast, lactose was higher in milk from PMCA2-null mice during early lactation, but by day 12 of lactation there were no differences in milk lactose between the three genotypes. These data demonstrate that the activity of PMCA2bw is required for secretion of much of the calcium in milk. This major secretory function represents a novel biological role for the plasma membrane Ca2+-ATPases, which are generally regarded as premier regulators of intracellular Ca2+.

Stimulation of Human Spermatozoa with Progesterone Gradients to Simulate Approach to the Oocyte

Progesterone is present at micromolar concentrations in the cumulus matrix, which surrounds mammalian oocytes. Exposure of human spermatozoa to a concentration gradient of progesterone (0-3 microM) to simulate approach to the oocyte induced a slowly developing increase in [Ca(2+)](i) upon which, in many cells, slow oscillations were superimposed. [Ca(2+)](i) oscillations often started at very low progesterone (<10 nm), and their frequency did not change during the subsequent rise in concentration. Oscillations also occurred, but in a much smaller proportion of cells, in response to stepped application of progesterone (3 microM). When progesterone was removed, [Ca(2+)](i) oscillations often persisted or quickly resumed. Superfusion with low-Ca(2+) bathing medium (no added Ca(2+)) did not prevent [Ca(2+)](i) oscillations, but they could be abolished by addition of EGTA or La(3+). Inhibitors of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases or inositol trisphosphate signaling had no effect on [Ca(2+)](i) oscillations, but pharmacological manipulation of ryanodine receptors affected both their frequency and amplitude. Staining of live spermatozoa with BODIPY FL-X ryanodine showed localization of ryanodine binding primarily to the caudal part of the head and mid-piece. [Ca(2+)](i) oscillations did not induce acrosome reaction, but in cells generating oscillations, the flagellar beat mode alternated in synchrony with the oscillation cycle. Flagellar bending and lateral movement of the sperm head during [Ca(2+)](i) peaks were markedly increased compared with during [Ca(2+)](i) troughs. This alternating pattern of activity is likely to facilitate zona penetration. These observations show that progesterone initiates unusual and complex store-mediated [Ca(2+)](i) signaling in human spermatozoa and identify a previously unrecognized effect of progesterone in regulating sperm "behavior" during fertilization.

Functional expression of heterologous proteins in yeast: insights into Ca2+signaling and Ca2+-transporting ATPases

The baker's yeast Saccharomyces cerevisiae is a well-developed, versatile, and widely used model organism. It offers a compact and fully sequenced genome, tractable genetics, simple and inexpensive culturing conditions, and, importantly, a conservation of basic cellular machinery and signal transducing pathways with higher eukaryotes. In this review, we describe recent technical advances in the heterologous expression of proteins in yeast and illustrate their application to the study of the Ca2+homeostasis machinery, with particular emphasis on Ca2+-transporting ATPases. Putative Ca2+-ATPases in the newly sequenced genomes of organisms such as parasites, plants, and vertebrates have been investigated by functional complementation of an engineered yeast strain lacking endogenous Ca2+pumps. High-throughput screens of mutant phenotypes to identify side chains critical for ion transport and selectivity have facilitated structure-function analysis, and genomewide approaches may be used to dissect cellular pathways involved in Ca2+transport and trafficking. The utility of the yeast system is demonstrated by rapid advances in the study of the emerging family of Golgi/secretory pathway Ca2+,Mn2+-ATPases (SPCA). Functional expression of human SPCA1 in yeast has provided insight into the physiology, novel biochemical characteristics, and subcellular localization of this pump. Haploinsufficiency of SPCA1 leads to Hailey-Hailey disease (HDD), a debilitating blistering disorder of the skin. Missense mutations, identified in patients with HHD, may be conveniently assessed in yeast for loss-of-function phenotypes associated with the disease.

Calcium pumps and keratinocytes: lessons from Darier's disease and Hailey-Hailey disease

Darier's disease and Hailey-Hailey disease are autosomal dominantly inherited skin disorders in which desmosomal adhesion between keratinocytes is abnormal. ATP2A2 and ATP2C1 have been identified as the causative genes for Darier's disease and Hailey-Hailey disease, respectively. ATP2A2 encodes the sarco(endo)plasmic reticulum Ca(2+)-ATPase isoform 2 (SERCA2) pump, while ATP2C1 encodes a secretory pathway Ca(2+)/Mn(2+)-ATPase (SPCA1) found in the Golgi apparatus. We review recent work into the function of these pumps in human keratinocytes and discuss how mutations in these genes might cause these diseases by altering the formation or stability of desmosomes.

Packing interactions between transmembrane helices alter ion selectivity of the yeast Golgi Ca2+/Mn2+-ATPase PMR1

PMR1 is the yeast secretory pathway pump responsible for high affinity transport of Mn2+ and Ca2+ into the Golgi, where these ions are sequestered and effectively removed from the cytoplasm. Phenotypic growth assays allow for convenient screening of side chains important for Ca2+ and Mn2+ transport. Earlier we demonstrated that mutant Q783A at the cytoplasmic interface of M6 could transport Ca2+, but not Mn2+. Scanning mutagenesis of side chains proximal to residue Gln-783 in membrane helices M2, M4, M5, and M6 revealed additional residues near the cytoplasmic interface, notably Leu-341 (M5), Phe-738 (M5), and Leu-785 (M6) that are sensitive to substitution. Importantly, we obtained evidence for a packing interaction between Val-335 in M4 and Gln-783 in M6 that is critical for Mn2+ transport. Thus, mutant V335G mimics the Mn2+ transport defect of Q783A and mutant V335I can effectively suppress the Mn2+-defective phenotype of Q783A. These changes in ion selectivity were confirmed by cation-dependent ATP hydrolysis using purified enzyme. Other substitutions at these sites are tolerated individually, but not in combination. Exchange of side chains at 335 and 783 also results in ion selectivity defects, suggesting that the packing interaction may be conformation-sensitive. Homology models of M4, M5, and M6 of PMR1 have been generated, based on the structures of the sarcoplasmic reticulum Ca2+-ATPase. The models are supported by data from mutagenesis and reveal that Gln-783 and Val-335 show conformation-sensitive packing at the cytoplasmic interface. We suggest that this region may constitute a gate for access of Mn2+ ions.