The contribution of the SPCA1 Ca2+ pump to the Ca2+ accumulation in the Golgi apparatus of HeLa cells assessed via RNA-mediated interference

Loss of the Atp2c1 secretory pathway Ca(2+)-ATPase (SPCA1) in mice causes Golgi stress, apoptosis, and midgestational death in homozygous embryos and squamous cell tumors in adult heterozygotes

Loss of one copy of the human ATP2C1 gene, encoding SPCA1 (secretory pathway Ca(2+)-ATPase isoform 1), causes Hailey-Hailey disease, a skin disorder. We performed targeted mutagenesis of the Atp2c1 gene in mice to analyze the functions of this Golgi membrane Ca(2+) pump. Breeding of heterozygous mutants yielded a normal Mendelian ratio among embryos on gestation day 9.5; however, null mutant (Spca1(-/-)) embryos exhibited growth retardation and did not survive beyond gestation day 10.5. Spca1(-/-) embryos had an open rostral neural tube, but hematopoiesis and cardiovascular development were ostensibly normal. Golgi membranes of Spca1(-/-) embryos were dilated, had fewer stacked leaflets, and were expanded in amount, consistent with increased Golgi biogenesis. The number of Golgi-associated vesicles was also increased, and rough endoplasmic reticulum had fewer ribosomes. Coated pits, junctional complexes, desmosomes, and basement membranes appeared normal in mutant embryos, indicating that processing and trafficking of proteins in the secretory pathway was not massively impaired. However, apoptosis was increased, possibly the result of secretory pathway stress, and a large increase in cytoplasmic lipid was observed in mutant embryos, consistent with impaired handling of lipid by the Golgi. Adult heterozygous mice appeared normal and exhibited no evidence of Hailey-Hailey disease; however, aged heterozygotes had an increased incidence of squamous cell tumors of keratinized epithelial cells of the skin and esophagus. These data show that loss of the Golgi Ca(2+) pump causes Golgi stress, expansion of the Golgi, increased apoptosis, and embryonic lethality and demonstrates that SPCA1 haploinsufficiency causes a genetic predisposition to cancer.

Calcium in the Golgi apparatus

The secretory-pathway Ca2+-ATPases (SPCAs) represent a recently recognized family of phosphorylation-type ATPases that supply the lumen of the Golgi apparatus with Ca2+ and Mn2+ needed for the normal functioning of this structure. Mutations of the human SPCA1 gene (ATP2C1) cause Hailey-Hailey disease, an autosomal dominant skin disorder in which keratinocytes in the suprabasal layer of the epidermis detach. We will first review the physiology of the SPCAs and then discuss how mutated SPCA1 proteins can lead to an epidermal disorder.

Identification and localisation of SERCA 2 isoforms in mammalian sperm

Upon binding to the egg's zona pellucida, capacitated spermatozoa will undergo a calcium-dependent exocytotic event called acrosome reaction. During this process, Ca2+ depletion from internal stores is followed by an important rise in [Ca2+]i due to a massive Ca2+ influx. Previous reports have shown that the acrosome can act as a Ca2+ store and that depletion of thapsigargin-sensitive stores induces acrosome exocytosis in capacitated spermatozoa from different mammalian species. The effect of thapsigargin, a specific inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases (SERCAs), suggests the presence and implication of SERCA in the active Ca2+ uptake during mammalian sperm capacitation. Although the presence of a thapsigargin-sensitive Ca2+-ATPase has been debated, the aim of this study was to clearly determine whether SERCAs are present in mammalian spermatozoa. Using three different anti-SERCA 2 antibodies, mono- and polyclonal, which recognised the same protein, we successfully identified and localised SERCA 2 in human, mouse and bovine sperm. Western blot analysis suggests that more than one SERCA 2 splice variant are present, one detected in the fraction containing the outer acrosomal membranes and another one present in the subcellular fraction containing the sperm midpiece. These results were confirmed by indirect immunofluorescence where SERCA 2 was observed in the acrosome and midpiece regions of human sperm. SERCA 2 immunohistochemical studies on human testis and PCR-amplification of mRNA encoding for each SERCA 2 splice variant in spermatogenic cells support the presence of this Ca2+-ATPase family in mature spermatozoa. In this paper, we clearly demonstrate, for the first time, the presence of SERCA 2 in mammalian sperm.

Diseases involving the Golgi calcium pump

Secretory-pathway Ca2(+)-transport ATPases (SPCA) provide the Golgi apparatus with Ca2+ and Mn2+ needed for the normal functioning of this organelle. Loss of one functional copy of the human SPCA1 gene (ATP2C1) causes Hailey-Hailey disease, a rare skin disorder characterized by recurrent blisters and erosions in the flexural areas. Here, we will review the properties and functional role of the SPCAs. The relationship between Hailey-Hailey disease and its defective gene (ATP2C1) will be adressed as well.

Calcium gradients and the Golgi

Changes in intracellular free calcium regulate many intracellular processes. With respect to the secretory pathway and the Golgi apparatus, changes in calcium concentration occurring either in the adjacent cytosol or within the lumen of the Golgi act to regulate Golgi function. Conversely, the Golgi sequesters calcium to shape cytosolic calcium signals as well as initiate them by releasing calcium via inositol-1,4,5-triphosphate (IP(3)) receptors, located on Golgi membranes. Local calcium transients juxtaposed to the Golgi (arising from release by the Golgi or other organelles) can activate calcium dependent signalling molecules located on or around the Golgi. This review focuses on the reciprocal relationship between the cell biology of the Golgi apparatus and intracellular calcium homeostasis.

Autosomal-dominant calcium ATPase disorders

Darier disease (DD) and Hailey-Hailey disease (HHD) are the only known autosomal-dominant Ca2+ ATPase disorders. Epidermal symptoms selectively occur in the affected individuals, the precise reason for which is still not fully understood. Here, we review the clinical, epidermal, and molecular features of the two genodermatoses. It is concluded that epidermal Ca2+ regulation disturbances and epigenetic factors may play an even more prominent role in the pathogenesis of DD and HHD than earlier appreciated.

Distribution of secretory pathway Ca2+ ATPase (SPCA1) in neuronal and glial cell cultures

1. Secretory pathway Ca(2+) ATPase type 1 (SPCA1) is a newly recognized Ca(2+)/Mn(2+)-transporting pump localized in membranes of the Golgi apparatus. 2. The expression level of SPCA1 in brain tissue is relatively high in comparison with other tissues. 3. With the aim to determine the expression of SPCA1 within the different types of neural cells, we investigated the distribution of SPCA1 in neuronal, astroglial, oligodendroglial, ependymal, and microglial cell cultures derived from rat brains. 4. Western Blot analysis with rabbit anti-SPCA1 antibodies revealed the presence of SPCA1 in homogenates derived from neuronal, astroglial, ependymal, and oligodendroglial, but not from microglial cells. 5. Cell cultures that gave rise to positive signal in the immunoblot analysis were also examined immunocytochemically. 6. Immunocytochemical double-labeling experiments with anti-SPCA1 serum in combination with antibodies against cell-type specific proteins showed a localization of the SPCA1signal within cells stained positively also for GFAP, alpha-tubulin or MBP. 7. These results definitely established the expression of SPCA1 in astroglial, ependymal, and oligodendroglial cells. 8. In addition, the evaluation of neuronal cultures for the presence of SPCA1 revealed an SPCA1-specific immunofluorescence signal in cells identified as neurons.

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.

Contributions of Intracellular Compartments to Calcium Dynamics: Implicating an Acidic Store

Many cells show a plateau of elevated cytosolic Ca(2+) after a long depolarization, suggesting delayed Ca(2+) release from intracellular compartments such as mitochondria and endoplasmic reticulum (ER). Mouse pancreatic beta-cells show a thapsigargin-sensitive plateau ('hump') of Ca(2+) after a 30 s depolarization but not after a 10 s depolarization. Surprisingly, this hump depends primarily on compartments other than the mitochondria or ER. It is reduced by only 22% upon blocking mitochondrial Na(+)-Ca(2+) exchange and by only 18% upon blocking ryanodine or IP(3) receptors together. Further, the time course of ER Ca(2+) measured by a targeted cameleon does not depend on the duration of depolarizations. Instead, the hump is reduced 35% by treatments with the dipeptide glycylphenylalanine beta-napthylamide, a tool often used to lyse lysosomes. We show that this dipeptide does not disturb ER functions, but it lyses acidic compartments and releases Ca(2+) into the cytosol. Moreover, it induces leaks in and possibly lyses insulin granules and stops mobilization of secretory granules to the readily releasable pool in beta-cells. We conclude that the dipeptide compromises dense-core secretory granules and that these granules comprise an acidic calcium store in beta-cells whose loading and/or release is sensitive to thapsigargin and which releases Ca(2+) after cytosolic Ca(2+) elevation.

ATP2C1 is specifically localized in the basal layer of normal epidermis and its depletion triggers keratinocyte differentiation

BACKGROUND

ATP2C1 is a calcium/manganese-ATPase localized in the Golgi apparatus and known as responsible gene for Hailey-Hailey disease. But its localization and roles in the epidermis are not fully elucidated.

OBJECTIVE

To explore the localization and biological role of ATP2C1 in normal epidermis in terms of differentiation states.

METHODS

We examined the immunohistochemical distribution of ATP2C1 in normal epidermis and measured the expression of ATP2C1 in cultured keratinocytes following forced detachment from culture dish or following treatment with high concentrations of calcium. Furthermore, we knockdown ATP2C1 expression in cultured keratinocytes by using RNA interference procedure to abrogate cation accumulation in cell organelles.

RESULTS

ATP2C1 is specifically localized at the basal cell layer in normal epidermis. Neither detachment of keratinocyte from culture dish nor treatment with high concentrations of calcium suppressed ATP2C1 expression, while both procedures induced differentiation markers, K10 keratin and involucrin. In contrast, knockdown of ATP2C1 induced these differentiation markers of cultured keratinocytes. Furthermore, treatment of keratinocytes with a calcium ionophore, A23187, did not up-regulate differentiation markers of keratinocytes, while a more manganese selective ionophore Br-A23187 up-regulated these differentiation markers.

CONCLUSION

Our results suggest that ATP2C1 plays an essential role for basal keratinocytes to keep in the undifferentiated state and that its reduction evokes differentiation and up-localization to suprabasal layers most likely via the manganese starvation in the Golgi apparatus of keratinocytes.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Subcellular mobility of the calpain/calpastatin network: an organelle transient

Calpain (Cp) is a calcium (Ca(2+))-dependent cysteine protease. Activation of the major isoforms of Cp, CpI and CpII, are required for a number of important cellular processes including adherence, shape change and migration. The current concept that cytoplasmic Cp locates and associates with its regulatory subunit (Rs) and substrates as well as translocates throughout the cell via random diffusion is not compatible with the spatial and temporal constraints of cellular metabolism. The novel finding that Cp and Rs function relies upon tenacious hydrophobic interactions with organelle membranes offers a unifying explanation for the paradoxical and puzzling features of Cp activation and regulation such as how nM concentrations of intracellular Ca(2+) can activate Cp molecules requiring muM to mM concentrations of Ca(2+) for in vitro activation, and how this protease can spatially and temporally locate specific substrates and translocate throughout the cell. We hypothesize that Cp and its regulatory moieties associate with organelles to facilitate the activation of this protease resulting in the cleavage of substrates and aid in its translocation throughout the cell.

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.

Functional comparison between secretory pathway Ca2+/Mn2+-ATPase (SPCA) 1 and sarcoplasmic reticulum Ca2+-ATPase (SERCA) 1 isoforms by steady-state and transient kinetic analyses

Steady-state and transient kinetic studies were performed to functionally analyze the overall and partial reactions of the Ca(2+) transport cycle of the human secretory pathway Ca(2+)/Mn(2+)-ATPase 1 (SPCA1) isoforms: SPCA1a, SPCA1b, SPCA1c, and SPCA1d (encoded by ATP2C1, the gene defective in Hailey-Hailey disease) upon heterologous expression in mammalian cells. The expression levels of SPCA1 isoforms were 200-350-fold higher than in control cells except for SPCA1c, whose low expression level appears to be the effect of rapid degradation because of protein misfolding. Relative to SERCA1a, the active SPCA1a, SPCA1b, and SPCA1d enzymes displayed extremely high apparent affinities for cytosolic Ca(2+) in activation of the overall ATPase and phosphorylation activities. The maximal turnover rates of the ATPase activity for SPCA1 isoforms were 4.7-6.4-fold lower than that of SERCA1a (lowest for the shortest SPCA1a isoform). The kinetic analysis traced these differences to a decreased rate of the E(1) approximately P(Ca) to E(2)-P transition. The apparent affinity for inorganic phosphate was reduced in the SPCA1 enzymes. This could be accounted for by an enhanced rate of the E(2)-P hydrolysis, which showed constitutive activation, lacking the SERCA1a-specific dependence on pH and K(+).

Modulation of the Ca2 permeable cation channel TRPV4 by cytochrome P450 epoxygenases in vascular endothelium

TRPV4 is a broadly expressed Ca2+-permeable cation channel in the vanilloid subfamily of transient receptor potential channels. TRPV4 gates in response to a large variety of stimuli, including cell swelling, warm temperatures, the synthetic phorbol ester 4alpha-phorbol 12,13-didecanoate (4alpha-PDD), and the endogenous lipid arachidonic acid (AA). Activation by cell swelling and AA requires cytochrome P450 (CYP) epoxygenase activity to convert AA to epoxyeicosatrienoic acids (EETs) such as 5,6-EET, 8,9-EET, which both act as direct TRPV4 agonists. To evaluate the role of TRPV4 and its modulation by the CYP pathway in vascular endothelial cells, we performed Ca2+ imaging and patch-clamp measurements on mouse aortic endothelial cells (MAECs) isolated from wild-type and TRPV4(-/-) mice. All TRPV4-activating stimuli induced robust Ca2+ responses in wild-type MAECs but not in MAECs isolated from TRPV4(-/-) mice. Upregulation of CYP2C expression by preincubation with nifedipine enhanced the responses to AA and cell swelling in wild-type MAECs, whereas responses to other stimuli remained unaffected. Conversely, inhibition of CYP2C9 activity with sulfaphenazole abolished the responses to AA and hypotonic solution (HTS). Moreover, suppression of EET hydrolysis using 1-adamantyl-3-cyclo-hexylurea or indomethacin, inhibitors of soluble epoxide hydrolases (sEHs), and cyclooxygenases, respectively, enhanced the TRPV4-dependent responses to AA, HTS, and EETs but not those to 4alpha-PDD or heat. Together, our data establish that CYP-derived EETs modulate the activity of TRPV4 channels in endothelial cells and shows the unraveling of novel modulatory pathways via CYP2C modulation and sEH inhibition.

The Secretory Pathway Ca2+/Mn2+-ATPase 2 Is a Golgi-localized Pump with High Affinity for Ca2+ Ions

Accumulation of Ca(2+) into the Golgi apparatus is mediated by sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs) and by secretory pathway Ca(2+)-ATPases (SPCAs). Mammals and birds express in addition to the housekeeping SPCA1 (human gene name ATP2C1, cytogenetic position 3q22.1) a homologous SPCA2 isoform (human gene name ATP2C2, cytogenetic position 16q24.1). We show here that both genes present an identical exon/intron layout. We confirmed that hSPCA2 has the ability to transport Ca(2+), demonstrated its Mn(2+)-transporting activity, showed its Ca(2+)- and Mn(2+)-dependent phosphoprotein intermediate formation, and documented the insensitivity of these functional activities to thapsigargin inhibition. The mRNA encoding hSPCA2 showed a limited tissue expression pattern mainly confined to the gastrointestinal and respiratory tract, prostate, thyroid, salivary, and mammary glands. Immunocytochemical localization in human colon sections presented a typical apical juxtanuclear Golgi-like staining. The expression in COS-1 cells allowed the direct demonstration of (45)Ca(2+) (K(0.5) = 0.27 microm) or (54)Mn(2+) transport into an A23187-releasable compartment.

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.

A plethora of interacting organellar Ca2+ stores

The endoplasmic reticulum is not the only major agonist-releasable Ca2+ store within cells; it is now clear that virtually all organelles so far studied have the ability to act as mobilizable Ca2+ stores. From recent findings with regard to Ca2+ transportation and Ca2+ homeostasis within a variety of cell organelles such as the mitochondria, nucleus, Golgi and lysosomes, it emerges that many of these organellar Ca2+ stores appear to interact with each other, adding a further level of complexity to Ca2+ signalling events.

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.

Deficiency of ATP2C1, a Golgi ion pump, induces secretory pathway defects in endoplasmic reticulum (ER)-associated degradation and sensitivity to ER stress

Relatively few clues have been uncovered to elucidate the cell biological role(s) of mammalian ATP2C1 encoding an inwardly directed secretory pathway Ca2+/Mn2+ pump that is ubiquitously expressed. Deficiency of ATP2C1 results in a human disease (Hailey-Hailey), which primarily affects keratinocytes. ATP2C1-encoded protein is detected in the Golgi complex in a calcium-dependent manner. A small interfering RNA causes knockdown of ATP2C1 expression, resulting in defects in both post-translational processing of wild-type thyroglobulin (a secretory glycoprotein) as well as endoplasmic reticulum-associated protein degradation of mutant thyroglobulin, whereas degradation of a nonglycosylated misfolded secretory protein substrate appears unaffected. Knockdown of ATP2C1 is not associated with elevated steady state levels of ER chaperone proteins, nor does it block cellular activation of either the PERK, ATF6, or Ire1/XBP1 portions of the ER stress response. However, deficiency of ATP2C1 renders cells hypersensitive to ER stress. These data point to the important contributions of the Golgi-localized ATP2C1 protein in homeostatic maintenance throughout the secretory pathway.

The Ca2+/Mn2+ pumps in the Golgi apparatus

Recent evidence highlights the functional importance of the Golgi apparatus as an agonist-sensitive intracellular Ca(2+) store. Besides Ca(2+)-release channels and Ca(2+)-binding proteins, the Golgi complex contains Ca(2+)-uptake mechanisms consisting of the well-known sarco/endoplasmic reticulum Ca(2+)-transport ATPases (SERCA) and the much less characterized secretory-pathway Ca(2+)-transport ATPases (SPCA). SPCA supplies the Golgi compartments and, possibly, the more distal compartments of the secretory pathway with both Ca(2+) and Mn(2+) and, therefore, plays an important role in the cytosolic and intra-Golgi Ca(2+) and Mn(2+) homeostasis. Mutations in the human gene encoding the SPCA1 pump (ATP2C1) resulting in Hailey-Hailey disease, an autosomal dominant skin disorder, are discussed.

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.

SPCA1 pumps and Hailey-Hailey disease

Both the endoplasmic reticulum and the Golgi apparatus are agonist-sensitive intracellular Ca2+ stores. The Golgi apparatus has Ca2+-release channels and a Ca2+-uptake mechanism consisting of sarco(endo)plasmic-reticulum Ca2+-ATPases (SERCA) and secretory-pathway Ca2+-ATPases (SPCA). SPCA1 has been shown to transport both Ca2+ and Mn2+ in the Golgi lumen and therefore plays an important role in the cytosolic and intra-Golgi Ca2+ and Mn2+ homeostasis. Human genetic studies have provided new information on the physiological role of SPCA1. Loss of one functional copy of the SPCA1 (ATP2C1) gene causes Hailey-Hailey disease, a skin disorder arising in the adult age with recurrent vesicles and erosions in the flexural areas. Here, we review recent experimental evidence showing that the Golgi apparatus plays a much more important role in intracellular ion homeostasis than previously anticipated.

The expression, activity and localisation of the secretory pathway Ca2+ -ATPase (SPCA1) in different mammalian tissues

The distribution of the secretory pathway Ca2+ -ATPase (SPCA1) was investigated at both the mRNA and protein level in a variety of tissues. The mRNA and the protein for SPCA1 were relatively abundant in rat brain, testis and testicular derived cells (myoid cells, germ cells, primary Sertoli cells and TM4 cells; a mouse Sertoli cell line) and epididymal fat pads. Lower levels were found in aorta (rat and porcine), heart, liver, lung and kidney. SPCA activities from a number of tissues were measured and shown to be particularly high in brain, aorta, heart, fat pads and testis. As the proportion of SPCA activity compared to total Ca2+ ATPase activity in brain, aorta, fat pads and testis were relatively high, this suggests that SPCA1 plays a major role in Ca2+ storage within these tissues. The subcellular localisation of SPCA1 was shown to be predominantly around the Golgi in both human aortic smooth muscle cells and TM4 cells.

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.

Ca2+ and N-Ethylmaleimide-sensitive Factor Differentially Regulate Disassembly of SNARE Complexes on Early Endosomes

The endosome-associated protein Hrs inhibits the homotypic fusion of early endosomes. A helical region of Hrs containing a Q-SNARE motif mediates this effect as well as its endosomal membrane association via SNAP-25, an endosomal receptor for Hrs. Hrs inhibits formation of an early endosomal SNARE complex by displacing VAMP-2 from the complex, suggesting a mechanism by which Hrs inhibits early endosome fusion. We examined the regulation of endosomal SNARE complexes to probe how Hrs may function as a negative regulator. We show that although NSF dissociates the VAMP-2.SNAP-25.syntaxin 13 complex, it has no effect on the Hrs-containing complex. Whereas Ca(2+) dissociates the Hrs-containing complex but not the VAMP-2-containing SNARE complex. This is the first demonstration of differential regulation of R/Q-SNARE and all Q-SNARE-containing SNARE complexes. Ca(2+) also reverses the Hrs-induced inhibition of early endosome fusion in a tetanus toxin-sensitive manner and removes Hrs from early endosomal membranes. Moreover, Hrs inhibition of endosome fusion and its endosomal localization are sensitive to bafilomycin, implying a role for luminal Ca(2+). Thus, Hrs may bind a SNARE protein on early endosomal membranes negatively regulating trans-SNARE pairing and endosomal fusion. The release of Ca(2+) from the endosome lumen dissociates Hrs, allowing a VAMP-2-containing complex to form enabling fusion.