The identity of the calcium-storing, inositol 1,4,5—trisphosphate-sensitive organelle in non-muscle cells: calciosome, endoplasmic reticulum … or both?

223, 124-137 21 Vogel, K. S. and Davies, A. M. (1989) Soc. Neurosci. Abstr. 14, 536.1 22 Ehrensberger, U. and Rohrer, H. (1988) Dev. Biol. 126, 420-432 23 Martin, P., Khan, A. and Lewis, J. (1989) Development 106, 335-346 24 Pittman, R. N., Patterson, W. and Vos, P, (1990) Soc. Neurosci. Abstr. 16, 68.11 25 Diamond, J., Coughlin, M., Macintyre, L., Holmes, M. and Visheau, B. (1987) Proc. NatlAcad. Sci. USA 84, 6596-6600 26 Owen, D. J., Logan, A. and Robinson, P. P. (1989) Brain Res. 476, 248-255 27 Ebendal, T. and Jacobson, C. O. (1977) Exp. Cell. Res. 105, 379-387 28 Lumsden, A. G. S. and Davies, A. M. (1983) Nature 306, 786-788 29 Lumsden, A. G. S. and Davies, A. M. (1986) Nature 323, 538-539 30 Tessier-Lavigne, M., Placzek, M., Lumsden, A. G. S., Dodd, J. and Jessell, T. M. (1988) Nature 336, 775-778 31 Placzek, M., Tessier-Lavigne, M., Jessell, T. and Docld, J. (1990) Development 110, 19-30 32 Placzek, M., Tessier-Lavigne, M., Yamada, T., Jessell, T. and Dodd, J. Cold Spring Harbor Symp. Quant. Biol. 55 (in press) 33 Weber, A. (1938) Biomorphosis 1, 30-35 34 O'Leary, D. D. M. and Takahashi, T. (1988) Neuron 1, 901-910 35 Heffner, C. D., Lumsden, A. G. S. and O'Leary, D. D. M. (1990) Science 247, 217-220

36 O'Leary, D. D. M. et al. (_-old Spring Harbor Symp. Quant. BioL 55 (in press) 37 Bolz, J., Novak, N. and G6tz, M. (1990) Soc. Neurosci. Abstr. 16, 464.6 38 Bolz, J., Novak, N., Gbtz, M. and Bonhoeffer, T. (1990) Nature 346, 359-362 39 Kurotani, T., Yamamoto, N. and Toyama, K. (1990) 5oc. Neurosci. Abstr. 16, 409.3 40 Davies, A. M. and Lumsden, A. G. S. (1986) J. Comp. Neurol. 253, 13-24 41 Riggott, M. J. and Moody, S. A. (1987) J. Comp. Neurol. 258, 580-596 42 Phelan, K. A. and Hollyday, M. (1990) J. Neurosci. 10, 2699-2716 43 Lewis, J., Chevalliar, A., Kieny, M. and Wolper~, L. (1981) J. Embryo/. Exp. Morphol. 64, 211-232 44 Tosney, K. W. (1987) Dev. Biol. 122,540-558 45 Okamoto, H. and Kuwada, J. Y. (1990) Soc. Neurosci. Abstr. 16, 262.3 46 McCaig, C. D. (1986) J. Physiol. 375, 39-54 47 Harris, W. A. (1989) Nature 339, 218-221 48 Zigrnond, S. A. (1977) J, Cell Biol. 75, 606-616 49 Tranquillo, R. T., Lauffenburger, D. A. and Zigmond, S. H. (1988) J. Cell. Biol. 106, 303-309 50 Berg, H. C. and Purcell, E. M. (1977) Biophys. J. 20, 193-219 51 Tranquillo, R. T. in Motility and Taxis (Lackie, J. and Armitage, J., eds), Cambridge University Press (in press) 52 Sullivan, S. J. and Zigmond, S. H. (1980) J. Cell Biol. 85, 703-711 53 Sutter, A., Riopelle, R. J., Harris-Warrick, R. M. and Shooter, E. M. (1979) J. Biol. Chem. 254, 5872-5982

The identity of the caldum-storing, inositol 1, 4, 5tn'sphosphate-sensitive organelle in non-musde cells: calciosome, endoplasmk reticulum . . . or both? M i c h e l F. Rossier a n d James W . P u t n e y , Jr Michd F. Rossierand James W. Putney, # are at the Laboratory of Cellular and Molecular Pharmacology, National Institute of Environmental Health Sciences, NIH, Research Triangle Park, NC 27709, USA.

Although the initial phase of receptor-mediated Ca 2+ signaling, involving Ca z+ release from intracellular stores by inositol 1,4,5-trisphosphate, is relatively well characterized, the nature of the organdie releasing Ca 2+ is a controversial subject. At issue is the question of whether Ca 2+ is released from the endoplasmic reticulum, or from a more specialized organelle called the 'calciosome'. In this review, we attempt to analyse the arguments for and against these two views, and attempt to reconcile some of the apparently conflicting findings by proposing a hypothetical model of the inositol 1, 4,5-trisphosphate-sensitive Ca e+ pool. In 1983, inositol 1, 4, 5-trisphosphate (IP3) was shown to release Ca 2+ from intracellular organelles of permeabilized pancreatic acinar cells 1. Since that important observation, the release of Ca 2+ by IP3 from specific intracellular stores has been extensively investigated, with the use of either permeabilized cells or preparations of subcellular fractions (for a review, see Ref. 2). IP3 is believed to act through a specific receptor, which functions as an intracellular, ligand-gated Ca 2+ channel. The IP3 receptor, which is particularly abundant in cerebellum, has been recently purified from this tissue and functionally reconstituted in liposomes 3'4. This glycoprotein of 260 kDa appeared to be structurally similar to (but functionally distinct from) the ryanodine receptor in skeletal muscle 5. However, despite these significant advances in under-

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standing the receptor at the molecular level, the nature of the intracellular organelle involved in the IP 3 response is still a controversial issue. This might be partly because of limitations in the experimental approaches available to investigate the characteristics of this organelle. The basic controversy centers around the issue of whether the IP3-sensitive organelle is in fact the endoplasmic reticulum, as originally envisaged or, alternatively, a unique and specialized Ca2+-storin~ and -releasing organelle, called the calciosome '°. The purpose of this review is to discuss, and to a limited extent evaluate, some of the arguments that have arisen in this debate. The endoplasmic reticulum as the location of the IPa-sensitive Ca 2+ pool In the first reports of biological activity of IP3, it was demonstrated in permeabilized cells that the IP3sensitive pool was capable of accumulating and releasing its Ca 2+ in the presence of various inhibitors of mitochondrial metabolism. This indicated that the organelle of interest was non-mitochondrial. In nonmuscle cells, it was believed that the only ubiquitous intracellular structures capable of sequestering significant amounts of Ca 2+ were the mitochondria and the endoplasmic reticulum. Thus, the latter was quickly adopted as the site of action of IP3. Investigators next tried to confirm this assignment by subcellular fractionation, using the Ca2+-mobilizing TINS, Vol. 14, No. 7, 1991

action of IP3 as a specific marker for this intracellular organelle. This task was not as simple as it first appeared, largely because of the generally diminished response to IP3 in cellular homogenates. Nonetheless, the earliest subcellular fractionation studies did in fact localize the IP3-sensitive Ca2+ pool in the microsomal fraction 7'8, which is also the fraction most enriched in endoplasmic reticulum. Upon further fractionation of the microsomes, a positive correlation was found between marker enzymes for the endoplasmic reticulum and ATP-dependent Ca2+ uptake 9-n or IP3-induced Ca 2+ release 12. This latter report rapidly reinforced the consensus that IP3 acts on the endoplasmic reticulum, although this correlation could only reflect a co-purification of the endoplasmic reticulum with the IP3-sensitive organelle. This view was further strengthened, albeit indirectly, by the observation that in liver the presence of glucose-6phosphate increases Ca2+ uptake into the IPasensitive pool. The mechanism proposed for this effect is that release of phosphate by glucose-6phosphatase, an enzyme specifically located in the lumen of the endoplasmic reticulum, increases the Ca2+-buffering capacity of this organelle 13. Similarly, the uptake of Ca2+ into the IP3-sensitive pool is augmented in the presence of oxalate 14'15, another property generally attributed to endoplasmic

that this IP3-binding protein, which is identical to the Purkinje cell marker P400 protein 21, is unusually abundant in these cells and has not been detected in other cells of the cerebellar cortex 2°. The reason for this over-expression in Purkinje cells is still unknown, but the atypically high level of the receptor at this site suggests that caution must be used in extrapolating experimental findings to other systems in the CNS and periphery.

Arguments against the endoplasmic reticulum

One piece of rather circumstantial evidence against the involvement of the endoplasmic reticulum in IP3induced Ca2+ release is the lack of correlation between the amount of endoplasmic reticulum present in the cytosol of a given cell type and the sensitivity of this cell to a challenge with IP3. For example, platelets or differentiated neutrophils appear to be almost completely devoid of endoplasmic reticulum in their mature form but they release as much Ca 2+ in response to IP3 as do hepatocytes or exocrine pancreatic cells, in which stacked endoplasmic reticulum cisternae represent approximately half of the cellular membranes 22. In addition, the endoplasmic reticulum content of a cell can change dramatically during its life span. These variations, observed, for example, in adrenal cortical cells upon chronic stimulation or in reticulum16,17. hepatocytes in response to chronic treatment with More recently, the distribution of two distinct Ca2+ cytochrome P450 substrates, reflect the complex and ATPase-like proteins in adrenal chromaffin cells and varying activities of this organelle in different cell their relationship to the agonist-sensitive Ca2+ store types. Therefore, it could he argued that a specialized have been reported 18. The presence of a 140 kDa and independent cellular organelle might be required protein, presumably associated with the IP3-sensitive for an efficient control of the cytosolic free Ca2+ pool, was detected close to the nucleus - a region concentration. enriched in endoplasmic reticulum. In other studies, In most cell types, it appears that only a fraction of antibodies raised against the IP3 receptor isolated the non-mitochondfial Ca2+ stores are responsive to from cerebellum have been used to determine the the actions of IP3, while there is substantial evidence cellular distribution of this protein in Purkinje cells 5' 19. that additional pools are involved in other aspects of The immunoreactivity detected by the peroxidase intracellular Ca2+ signaling. One example is the technique was localized in the soma and dendrites of caffeine-sensitive pool, which has been proposed to Purkinje cells, not only to the smooth and rough play a role in the generation of intracellular Ca2+ endoplasmic reticulum, but also to other structures, oscillations 23. Since these pools are also clearly nonsuch as the nuclear membrane, the cis cisternae of mitochondrial, they are therefore also assigned (in a Golgi, and the sub-plasmalemmal cisternae. No label- similar way to the IP3-sensitive pool) to the endoing was detected in the mitochondria or the plasma plasmic reticulum. Thus, if the IP3-sensitive pool is membrane. identical to the whole of, or a portion of, the endoBy the use of gold-immunolabeled ultrathin plasmic reticulum, then the existence of these other cryosections, which give a higher spatial resolution, a non-mitochondrial pools require at the very least quantitative study of the distribution of the IP3 some sub-specialization of Ca2+-metabolizing endoreceptor in these cells was then undertaken 2°. This plasmic reticulum to account for these apparently revealed that the receptor is mainly concentrated in different signaling functions. cisternal stacks, in cisternal singlets or doublets, and There is considerable evidence that at least a in other smooth-surfaced vesicular and tubular pro- portion of the IP3-sensitive pool might he localized files. In contrast, rough endoplasmic reticulum was near the plasma membrane, where one would not labeled much less, although it was often in direct expect to find traditionally defined endoplasmic reticuluminal continuity with heavily labeled smooth el- lure. However, such a location is consistent with some ements. Hypolemmal cisternae were labeled to an current models relating intracellular Ca2+ release intermediate degree and smooth cisternae of the to plasma membrane permeability 15'24-26 and is dendritic spine apparatus were always markedly supported by several experimental observations. labeled. The Purkinje cell dendritic spines are (1) Serial injections of IP3 into immature Xenopus enriched in quisqualate-preferring glutamate recep- ooeytes27 or Limlglus photoreceptor cells 28 produce tors, and the presence of Ca2+ in the spinal apparatus different responses depending on the position and the has been demonstrated by electron probe X-ray depth of the micropipette within the cell. (2) Muscarmicroanalysis, suggesting a physiological role for the inic stimulation of parotid cells 29 or cholinergic stimuIP3 receptor in this region of the cell. However, the lation of exocrine pancreatic cells 3° induces the functional relevance of the presence of this protein in opening of Ca2+-sensitive ionic channels in the plasma other structures is less certain. It is also noteworthy membrane before the Ca2+ concentration, detectable

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by fura-2, begins to rise in the cytosol. (3) Following lysis and sonication of pancreatic islet cells attached to positively charged Sephadex beads, a significant portion of the IP3-sensitive Ca2+ pool remains associated with the beads 31. (4) In rat hepatocytes, the subcellular fraction enriched in plasma membrane shows the highest IP3-binding capacity32'33. (5) Immunocytochemical localization of the IP3 receptor in Purkinje cells has revealed the presence of at least some of this protein in structures just beneath the plasma membrane 19. A sub-plasmalemmal position for the IP3-sensitive pool is reminiscent of the organization of the junctional sarcoplasmic reticulum in skeletal and smooth muscle cells - systems in which the distinction between the organelles involved in Ca2+ signaling and generic endoplasmic reticulum is quite well established. Finally, recent subcellular fractionation studies of various tissues have given results that differ significantly from the earlier fractionation data obtained from exocrine pancreas. Indeed, clear separations have been obtained between the various enzyme markers for the endoplasmic reticulum and for either Ca2+uptake, IP3 binding or IP3-induced Ca2+ release in neutrophils a4, HL60 cells 6, parotid acinar cells a5, and in adrenal cortex a6, brain a7 and liver cells aa.

The sarcoplasmic reticulum: a paradigm for calciosomes The various discrepancies between the properties of endoplasmic reticulum and the characteristics of the IP3-sensitive Ca2+ pool led to the proposal of the existence in non-muscle cells of a distinct and specialized organelle, called the 'calciosome', that is devoted to intracellular Ca 2+ homeostasis 6. Potential similarities between this organelle and the sarcoplasmic reticulum of muscle cells were therefore investigated 22"38. A landmark in the conceptualization and characterization of calciosomes was the discovery that various non-muscle cell types express a Ca2+-binding protein that cross-reacts with antibodies raised against calsequestrin, an acidic protein specifically located in terminal cisternae of the sarcoplasmic reticulum 6'39'4°. This protein is believed to permit the concentrated sequestration of large quantities of rapidly available Ca2+ by preventing the precipitation of Ca2+ with anions like phosphate in the lumen of the sarcoplasmic reticulum, while at the same time not interfering with the rapid release of Ca2+ upon stimulation. Thus, the calsequestrin-like protein in nonmuscle cells was proposed to be involved in Ca2+ buffering the lumen of calciosomes, and was suggested as a marker for this organdie. Immunocytological studies in hepatocytes and pancreatic acinar cells revealed a distribution of this protein in vesicles throughout the cytoplasm; these vesicles did not appear to be continuous (although they were often in close apposition) with typical elements of the endoplasmic reticulum or Golgi complex, and they were completely distinct from the mitochondria, lysosomes and secretory granules. In double-labeling experiments, the calsequestrin-positive vesicles often appeared to be endowed with a Ca2+ pump similar to the Ca 2+ ATPase of the sarcoplasmic reticulum, but never with markers of the endoplasmic reticulum 4a. In addition, a partial segregation of the calsequestrin-like protein and the Caz+ pump within the calciosome, as 312

observed in sarcoplasmic reticulum, was suggested in this latter study. However, despite the apparent localization of a protein with immunological properties similar to calsequestrin in hepatocytes, it now seems clear that calsequestrin itself is not present in these cells 42'43. In addition, the purification of the amino terminus of the calsequestrin-like protein from HL60 cells, and liver and brain cells has resulted in its identification as calreticulin 44'45. Calreticulin is another sarcoplasmic reticulum Ca2+-binding protein, with one high affinity (Kd = 3 ~M) and 25 low affinity (Kd = 5 raM) Ca2+binding sites 46. This protein probably corresponds in liver to a protein previously referred to as calregulin 47'48, the intracellular localization and possible function of which remains largely unknown 42. Most significantly, in HL60 cells, calreticulin co-purifies with the IP3-binding sites and the Ca 2÷ ATPase (Lew, D. P. and van Delden, C., pers. commun.). The identification of calreticulin raised significant questions about calciosome ontogenesis. Muscle calreticulin is known to possess a Lys-Asp-Glu-Leu sequence at its carboxy terminus, which has been proposed to be a signal that retains the protein in the endoplasmic reticulum 46, whereas liver calregulin galactosylation suggests that it is processed through the trans-Golgi compartment 42. Control of the intracellular localization of protein in various cell types by alternative splicing of the retention signal is of course conceivable. It is necessary to sequence the carboxy terminus, as well as analyse the postranscriptional modifications of the protein, in order to determine the biogenic pathways of calreticulin in non-muscle cells. More importantly, the precise relationship between calciosomes and the IP3-sensitive Ca2+ pool is not yet defined and the identity of both organelles will not be definitively proven until it is possible to demonstrate morphologically the presence of calreticulin and the IP3 receptor in the same organelle.

Hypothetical models of the IP3-sensitive Ca 2+ pool The question then arises: how contradictory are the available data concerning the identity of the agonist-mobilizable Ca 2+ pool, and how can these apparent contradictions be resolved? To address this question we pose a second: what functional feature(s) of muscle sarcoplasmic reticulum can we expect to be shared with non-muscle calciosomes? One striking characteristic of the sarcoplasmic reticulum organization is the separation between Ca2+ uptake and Cae+ release sites, the latter being confined to the junctional region of the terminal cisternae. Such a segregation of IP3 receptors in strategic regions of the cell, mainly near the plasma membrane where IP3 is released, would considerably increase the efficiency of the response to an extracellular stimulus. It is noteworthy that the presence of a low affinity, high capacity Ca2+-binding protein concentrated in these specific regions would help to accumulate rapidly available Cae+ near the release sites, even if Cae÷ pumping occurs in a much broader region (see livertype calciosomes in Fig. 1). Such an organization of the Caz+ pools in hepatocytes could explain the loss of Ca2+ release activity and the lack of correlation between the IP3-binding capacity and the amount of IPa-induced Caz÷ release observed after tissue TINS, VoL 14, No. 7, 1991

homogenization 33'49. The lost activity has been shown to be partially recovered after fusion of the vesicles through a process at least partly dependent on physiological concentrations of GTP (Ref. 49). In such a situation, the calciosome would largely be continuous with the endoplasmic reticulum, and would thus be considered as a specialized region or compartment of that organelle. One additional possibility we might consider is that calciosomes, in some cells, bud off from a specialized part of the endoplasmic reticulum to form independent organelles. (In this situation, the calciosome would truly represent a distinct organelle, although it is clear that this distinction is, from a functional standpoint, somewhat semantic.) Such a process would allow proteins containing an endoplasmic reficulum retention signal to leave the reticulum without passing through the Golgi. Mthough this pathway is at present only hypothetical, an attractive idea is that the formation of calciosomes, or their connection with other Caz+ pools, could be regulated by some of the small molecular weight GTP-binding proteins involved in the control of vesicular traffic 5°,51. Finally, interaction of the calciosomes with the cytoskeleton is probably required for transporting or maintaining these organelles at an appropriate location in the cytosol. Such an interaction has been recently suggested by the observation that treatment of a liver homogenate with cytochalasin B, a drug which disrupts actin microfilaments, significantly decreased the correlation between the amount of plasma membrane and IP3-binding sites recovered in subcellular fractions 33. It is possible that the biogenesis of the IP3-sensitive organelle will vary from one ~ell type to the other. We can therefore hypothesize that in cells endowed with a large amount of endoplasmic reticulum and that a r e highly polarized, like epithelial cells, the relevant Ca2+ stores might be rigidly maintained close to sites of IP3 fortnation, whereas in circulating, chemotactic cells, like neutrophils, rapid recruitment or re-distribution of the Ca2+ pools on one side of the cell could be necessary for the chemotactic function 38. In neuronal cells, such plasticity might allow for transportation of these pools in specific regions, for example, where synaptic contacts are established. Some of these possible configurations are illustrated in Fig. 1. Given the distinct and characteristic morphological arrangement of sarcoplasmic reticulum in muscle cells, so well suited to the specific function of these

c

SO,he

e r

- ,

oeurons Fig. 1. Hypothetical models of calciosomes (the IP3sensitive Ca2+ stores) in different cell types, illustrating possible variations of structural and functional organization. (A) In fiver, recovery of the IP3-bh~ding sites with the plasma membrane during subcellular fractionation suggests the presence of a link between the calciosomes calciosome ~ C a 2+ uptake and the cellular membrane, and there is also evidence for cytoskeletal a segregation of the Ca2+ release sites in highly specialized Ca2+ release ~ element regions of the endoplasmic reticulum. (B) In neutrophils, where the endoplasmic reticulum is much less abundant than in liver, the calciosomes might bud off from this or&anelle and might then be distributed throughout the cytosoL A rapid re-distribution of these organelles inside the cell could be a requirement for the chemotactic function of neutrophils and could involve interaction with cytoskeleton elements. (C) In neurons, the calciosomes could be transported by cytoskeleton elements and be maintained in strategic loci, such as spines of synaptic connections. A feature common to these different cells seems to be the presence of at least a fraction of the calciosomes in relatively close proximity to the plasma membrane. TINS, Vol. 14, No. 7, 1991

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Acknowledgements We are verygrateful to Drs K-H. Krause, D. Lew, J. Meldolesiand 5. 5nyder for providing us with preprints of their recent publications. We also thank Drs F. S. Menniti, A. g. Hughes, G SU. Bird and L. Turin for their comments concerning this manuscript.

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cells, it follows that similar structural diversity might occur among various non-muscle cells, such that the relationship between the endoplasmic reticulum and the IP3-sensitive Ca 2+ pools will vary significantly among cell types. The biogenesis and the location of these pools might be regulated by diverse processes such as alternative splicing of the signal peptide of their proteins, membrane traffic control by low molecular weight G proteins, or interaction of these organelles with cytoskeletal elements.

13 Benedetti, A., Fulceri, R., Ferro, M. and ComporU, M. (1986) Trends Biochem. 5ci. 11,284-285 14 Leslie, B. A., Burgess, G. M. and Putney, J. W., Jr (1988) Cell Calcium 9, 9-16 15 Mullaney, J. M., Chueh, S-H., Ghosh, T. K. and Gill, D. L. (1987) J. Biol. Chem. 262, 13865-13872 16 Makinose, M. and Hasselbach, W. (1965) Biochem. Z. 343, 360-382 17 McGraw, C. F., Somlyo, A. V. and Blaustein, M. P. (1980) J. Cell BioL 85,228-241 18 Burgoyne, R. D. eta/. (1989) Nature 342, 72-74 19 Ross, C. A. etal. (1989) Nature 339, 468-470 20 Satoh, T. eta/. (1990)J. CellBiol. 111, 615-624 21 Furuichi, T. etaL (1989) Nature 342, 32-38 Concluding remarks 22 Pozzan, T. et al. (1988) J. Cardiovasc. PharmacoL 12, The controversy, perhaps more semantic than 80-84 functional, between defenders of calciosomes and 23 Berridge, M. J. and Irvine, R. F. (1989) Nature341, 197-205 endoplasmic reticulum seems to result not only from 24 Putney, J. W., Jr (1986) Cell Calcium 7, 1-12 Takemura, H., Hughes, A. R., Thastrup, O. and Putney, J. W., the ambiguous relationship of the calciosomes with 25 Jr (1989)J. BioL Chem. 264, 12266-12271 the IPa-regulated Ca 2÷ pool, but also from ambiguities 26 Irvine, R. F. (1990) FEB5 Lett. 263, 5-9 among the biochemical, functional and morphological 27 Busa, W. B., Fergusson, J. E., Joseph, S. K., Williamson, J. R. and Nuccitelli, R. (1985) J. Cell BioL 101,677-682 definitions of the endoplasmic reticulum itself. It now seems quite likely that the calciosome (if such a 28 Payne, R. and Fein, A. (1987) J. Cell Biol. 104, 933-937 29 Foskett, J. K., Gunter-Smith, P. J., Melvin, J. E. and Turner, distinct organelle truly exists) is derived from the R. J. (1989) Proc. NatlAcad. 5ci. USA 86, 167-171 endoplasmic reticulum (as are most other cellular 30 Osipchuk, Y. V., Wakui, M., Yule, D. I., Gallacher, D. V. and Petersen, O. H. (1990) EMBO J. 9, 697-704 organelles), and that the specific protein components required for its function in Ca 2÷ signaling are 31 Dunlop, M. E. and Larkins, R. G. (1988) Biochem. J. 253, synthesized in the endoplasmic reticulum. Therefore, 32 67-72 Guillemette, G., Balla, T., Baukal, A. J. and Cart, K. J. (1988) it is not difficult to imagine that the extent to which J. Biol. Chem. 263, 4541-4548 calciosomes have differentiated from endoplasmic 33 Rossier, M. F., Bird, G. St J. and Putney, J. W., Jr (1991) Biochem. J. 274, 643-650 reticulum may vary from one cell type to the other, the skeletal muscle cells certainly possessing the 34 Krause, K-H and Lew, D. P. (1987) J. Clin. Invest. 80, 107-116 most spectacularly specialized variation on this 35 Henne, V., Piiper, A. and Soling, H-D. (1987) FEBSLett. 218, theme. In non-muscle cells, the IP3 receptor, the 153-158 Ca 2+ pump and one or more specific Ca2+-binding 36 Rossier, M. F., Capponi, A. M. and Vallotton, M. B. (1989) J. Biol. Chem. 264, 14078-14084 proteins might be confined in specialized regions of 37 Alderson, B. H. and Volpe, P. (1989)Arch. Biochem. Biophys. the reticulum; these regions might bud off from it or 272, 162-174 traverse the Golgi, and might then be transported in 38 Krause, K-H. etaL (1989) Cell Calcium 10, 351-361 the cytosol to specific sites. However, it seems to be 39 Damiani, E,, Spamer, C., Heilmann, C., Salvatori, S. and Margreth, A. (1988) ,I. BioL Chem. 263,340-343 convenient at present to use the term 'calciosome' for 40 Oberdorf, J. A., Lebeche, D., Head, J. F. and Kaminer, B. the organelle functionally defined as the physiologi(1988) J. BioL Chem. 263, 6806-6809 cally significant lP3-regulated Ca 2+ store in non- 41 Hashimoto, S. etal. (1988) J. Cell Biol. 107, 2523-2531 muscle cells, even if, in some cells, this organelle 42 NguyenVan, P., Peter, F. and S61ing, H-D. (1989) J. Biol. Chem. 264, 17494-17501 coincides or is continuous with a part of the endoplasmic reticulum. Hopefully, it will soon be possible to 43 Damiani, E., Heilmann, C., Salvatori, S. and Margreth, A. (1990) Biochem. Biophys. Res. Commun. 165, 973-980 establish more definitively the identity of the calcio- 44 Krause, K-H., Simmerman, H. K. B., Jones, L. R. and some as the IP3-sensitive Ca 2+ pool and to underCampbell, K. P. (1990) Biochem. J. 270, 545-548 stand better the ontogenesis of an organelle so 45 Treves, S. eta/. (1990) Biochem. J. 271,473-480 46 Fliegel, L., Burns, K., MacLennan, D. H., Reithmeier, R. A. F. important for cell function. and Michalak, M. (1989) J. BioL Chem. 264, 21522-21528 47 Waisman, D. M., Salimath, B. P. and Anderson, M. J. (1985) J. Biol. Chem. 260, 1652-1660 Selected references 1 Streb, H., Irvine, R. F., Berridge, M. J. and Schulz, I. (1983) 48 Smith, M. J. and Koch, G. L. E. (1989) EMBO J. 8, 3581-3586 Nature 306, 67-69 49 Dawson, A. P and Comerford, J. G. (1989) Cell Calcium 10, 2 Berridge, M. J. (1987) Annu. Rev. Biochem. 56, 159-193 343-350 3 Supattapone, S., Worley, P. F., Baraban, J. M. and Snyder, 50 Wilschut, J. (1989) Curr. Opin. Cell Biol. 1,639-647 S. H. (1988)J. Biol. Chem. 263, 1530-1534 4 Ferris, C. D., Huganir, R. L., Supattapone, S. and Snyder, S. H. 51 Tooze, S. A., Weiss, U. and Huttner, W. B. (1990) Nature 347, 207-208 (1989) Nature 342, 87-89 5 Mignery, G. A., S0dhof, T. C., Takei, K. and De Camilli, P. (1989) Nature 342, 192-195 Letters to the Editor 6 Volpe, P. et al. (1988) Proc. Natl Acad. Sci. USA 85, 1091-1095 Letters to the editor of Trends in Neurosciences concerning 7 Prentki, M. eta/. (1984) Nature 309, 562-564 recently published articles in the journal are welcomed. 8 Joseph, S, K., Williams, R. J., Corkey, B. E., Matschinsky, F. M. Please mark clearly whether they are intended for and Williamson, J. R. (1984) J. Biol. Chem. 259, publication. 12952-12955 9 Preissler, M. and Williams, J. A, (1983) J. Membr. Biol. 73, Pleaseaddress letters to: 137-144 10 Prentki, M., Janjic, D., Biden, T. J., Blondel, B. and Wollheim, Editor, C. B. (1984)J. BioL Chem. 259, 10118-10123 Trends in Neurosciences, 11 Bayerd6rffer, E., Streb, H., Eckhardt, L., Haase, W. and Elsevier Trends Journals, Schulz, I. (1984) J. Membr. Biol. 81, 69-82 68 Hills Road, 12 Streb, H., Bayerd6rffer, E., Haase, W., Irvine, R. F. and Schulz, Cambridge CB2 1LA, UK. I. (1984) J. Membr. Biol. 81,241-253 TINS, Vol. 14, No. 7, 1991