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.

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.

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.

Inositol trisphosphate producing agonists do not mobilize the thapsigargin-insensitive part of the endoplasmic-reticulum and Golgi Ca2+ store

Non-mitochondrial intracellular Ca2+ stores contain both thapsigargin-sensitive sarco(endo)plasmic-reticulum Ca2+-ATPases (SERCA) and thapsigargin-insensitive secretory-pathway Ca2+-ATPases (SPCA1). We now have studied the Ca2+-release properties of the compartments associated with these pumps in intact, i.e. non-permeabilized, cells of different origin (HeLa, keratinocytes, 16HBE14o-, COS-1, A7r5) and with different approaches (45Ca2+ fluxes, Ca2+ imaging and measurements of the free luminal [Ca2+] in the endoplasmic-reticulum and the Golgi apparatus using targeted aequorin). Application of an extracellular agonist in the absence of thapsigargin induced in all cells a Ca2+ release from both the endoplasmic-reticulum and the Golgi apparatus. The agonists were not able to release Ca2+ in the presence of 10 microM thapsigargin, except in COS-1 cells overexpressing SPCA1, where this pump not only appeared in the Golgi compartment but also overflowed into the agonist-sensitive part of the endoplasmic-reticulum. We conclude that the subcompartments of the endoplasmic-reticulum and of the Golgi complex that endogenously express SPCA1 are insensitive to agonist stimulation.

Similar Ca(2+)-signaling properties in keratinocytes and in COS-1 cells overexpressing the secretory-pathway Ca(2+)-ATPase SPCA1

Mutations in the ubiquitously expressed secretory-pathway Ca(2+)-ATPase (SPCA1) Ca(2+) pump result in Hailey-Hailey disease, which almost exclusively affects the epidermal part of the skin. We have studied Ca(2+) signaling in human keratinocytes by measuring the free Ca(2+) concentration in the cytoplasm and in the lumen of both the Golgi apparatus and the endoplasmic reticulum. These signals were compared with those recorded in SPCA1-overexpressing and control COS-1 cells. Both the sarco(endo)plasmic-reticulum Ca(2+)-ATPase (SERCA) and SPCA1 can mediate Ca(2+) uptake into the Golgi stacks. Our results indicate that keratinocytes mainly used the SPCA1 Ca(2+) pump to load the Golgi complex with Ca(2+) whereas the SERCA Ca(2+) pump was mainly used in control COS-1 cells. Cytosolic Ca(2+) signals in keratinocytes induced by extracellular ATP or capacitative Ca(2+) entry were characterized by an unusually long latency reflecting extra Ca(2+) buffering by an SPCA1-containing Ca(2+) store, similarly as in SPCA1-overexpressing COS-1 cells. Removal of extracellular Ca(2+) elicited spontaneous cytosolic Ca(2+) transients in keratinocytes, similarly as in SPCA1-overexpressing COS-1 cells. With respect to Ca(2+) signaling keratinocytes and SPCA1-overexpressing COS-1 cells therefore behaved similarly but differed from control COS-1 cells. The relatively large contribution of the SPCA1 pumps for loading the Golgi stores with Ca(2+) in keratinocytes may, at least partially, explain why mutations in the SPCA1 gene preferentially affect the skin in Hailey-Hailey patients.

Baseline cytosolic Ca2+ oscillations derived from a non-endoplasmic reticulum Ca2+ store

Cytosolic Ca(2+) oscillations can be due to cycles of release and re-uptake of internally stored Ca(2+). To investigate the nature of these Ca(2+) stores, we expressed the Pmr1 Ca(2+) pump of Caenorhabditis elegans in COS-1 cells and pretreated the cells with thapsigargin to prevent Ca(2+) uptake by the sarco(endo)plasmic reticulum Ca(2+)-ATPase. Pmr1 co-localized with the Golgi-specific 58K protein and was targeted to a Ca(2+) store that was less leaky for Ca(2+) than the endoplasmic reticulum and whose inositol trisphosphate receptors were less sensitive to inositol trisphosphate and ATP than those in the endoplasmic reticulum. ATP-stimulated Pmr1-overexpressing cells responded after a latency to extracellular Ca(2+) with a regenerative Ca(2+) signal, which could be prevented by caffeine. They also produced very stable ilimaquinone-sensitive baseline Ca(2+) spikes, even in the presence of thapsigargin. Such responses never occurred in non-transfected cells or in cells that overexpressed the type-1 sarco(endo)plasmic reticulum Ca(2+)-ATPase. Abortive Ca(2+) spikes also occurred in histamine-stimulated untransfected HeLa cells pretreated with thapsigargin, and they too were inhibited by ilimaquinone. We conclude that the Pmr1-induced Ca(2+) store, which probably corresponds to the Golgi compartment, can play a crucial role in setting up baseline Ca(2+) spiking.

The Golgi PMR1 P-type ATPase of Caenorhabditis elegans. Identification of the gene and demonstration of calcium and manganese transport

In recent years, it has been well established that the Ca(2+) concentration in the lumen of intracellular organelles is a key determinant of cell function. Despite the fact that essential functions of the Golgi apparatus depend on the Ca(2+) and Mn(2+) concentration in its lumen, little is known on the transport system responsible for ion accumulation. The Golgi ion pump PMR1 has been functionally studied only in yeast. In humans, mutations in the orthologous gene ATP2C1 cause Hailey-Hailey disease. We report here the identification of the PMR1 homologue in the model organism Caenorhabditis elegans and after ectopic expression the direct study of its ion transport in permeabilized COS-1 cells. The C. elegans genome is predicted to contain a single PMR1 orthologue on chromosome I. We found evidence for alternative splicing in the 5'-untranslated region, but no indication for the generation of different protein isoforms. C. elegans PMR1 overexpressed in COS-1 cells transports Ca(2+) and Mn(2+) with high affinity into the Golgi apparatus in a thapsigargin-insensitive manner. Part of the accumulated Ca(2+) can be released by inositol 1,4,5-trisphosphate, in agreement with the idea that the Golgi apparatus is an inositol 1,4,5-trisphosphate-sensitive Ca(2+) store.