Subcellular distribution of the calcium-storing inositol 1,4,5-trisphosphate-sensitive organelle in rat liver. Possible linkage to the plasma membrane through the actin microfilaments

Calcium binding proteins in the sarcoplasmic/endoplasmic reticulum of muscle and nonmuscle cells

In this paper we review some of the large quantities of information currently available concerning the identification, structure and function of Ca(2+)-binding proteins of endoplasmic and sarcoplasmic reticulum membranes. The review places particular emphasis on identification and discussion of Ca2+ 'storage' proteins in these membranes. We believe that the evidence reviewed here supports the contention that the Ca(2+)-binding capacity of both calsequestrin and calreticulin favor their contribution as the major Ca(2+)-binding proteins of muscle and nonmuscle cells, respectively. Other Ca(2+)-binding proteins discovered in both endoplasmic reticulum and sarcoplasmic reticulum membranes probably contribute to the overall Ca2+ storage capacity of these membrane organelles, and they also play other important functional role such as posttranslational modification of newly synthesized proteins, a cytoskeletal (structural) function, or movement of Ca2+ within the lumen of the sarcoplasmic/endoplasmic reticulum towards the storage sites.

Do submaximal InsP3 concentrations only induce the partial discharge of permeabilized hepatocyte calcium pools because of the concomitant reduction of intraluminal Ca2+ concentration?

In several types of cells whose cytoplasmic Ca2+ is regulated by inositol phosphate derivatives, low concentrations of InsP3 added to permeabilized cell suspensions induce the rapid discharge of part of the InsPs-sensitive Ca2+ pool instead of slow monophasic release of Ca2+ from the entire pool. As a tentative explanation for this puzzling observation, sometimes called 'quantal release', it was suggested that the reduced intraluminal Ca2+ concentration remaining in the Ca2+ pool after a certain amount of Ca2+ had been released might allosterically reduce the channels' affinity for InsP3 and the corresponding InsP3-dependent Ca2+ efflux, and thus result in partial pool discharge (Irvine, R.F. (1990) FEBS Lett. 263, 5-9). We have tested this hypothesis by manipulating the Ca2+ pool contents with ionophore, and found that the rate of InsP3-dependent Ca2+ efflux after ionophore-induced partial discharge of the Ca2+ pools was much faster than what was predicted on the basis of this hypothesis. Heterogeneity of the Ca2+ pools appears to be a more likely reason for the 'quantal release' behavior.

Intracellular calcium: molecules and pools

The complex nature of intracellular calcium storage pools has been examined at many levels in the past year. Additional molecules associated with calcium stores have been identified and their localization examined. The convergence of molecular biology, cell biology and biochemistry has now allowed the details of calcium signalling to be meaningfully explored.

Localisation of the [32P]IP3 binding site on human platelet intracellular membranes isolated by high-voltage free-flow electrophoresis

This study reports the localisation of the [32P]IP3 binding site on highly purified membrane fractions prepared using high-voltage free-flow electrophoresis. Binding studies on mixed membranes, carried out at 4 degrees C, revealed a binding site with a Kd = 86 nM and beta max = 5.3 pmol/mg protein. The binding was potently inhibited by heparin. High-voltage free-flow electrophoresis was used to further purify surface and intracellular membranes. The intracellular membranes showed a 5-fold enrichment of binding sites with respect to the parent mixed membranes with the same Kd (80 nM), but the surface membranes showed an absence of binding activity. The results indicate the localisation of the IP3 receptor on highly purified intracellular membranes.

Inositol 1,4,5-trisphosphate compartmentalization in tracheal smooth muscle

Pool sizes of inositol phosphate species in myo-[3H]inositol-labeled porcine tracheal smooth muscle were determined under three conditions: (a) unstimulated; (b) stimulated with carbachol; (c) atropine-relaxed from a carbachol contraction. In unstimulated muscle, the inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) content was 14 pmol/100 nmol lipid P1. This is equivalent to a mean [Ins(1,4,5)P3] of about 3 microM (in total cellular water), a level about 30-fold in excess of that required for Ca2+ release from Ins(1,4,5)P3-sensitive sarcoplasmic reticulum (SR). Pool sizes of breakdown products of Ins(1,4,5)P3 were relatively small or absent in unstimulated muscle, suggesting that, under this condition, Ins(1,4,5)P3 was sequestered and had limited access to Ins(1,4,5)P3 5-phosphatase and/or 3-kinase. During carbachol stimulation, the Ins(1,4,5)P3 pool did not increase while those of other mono-, di-, and trisphosphate isomers increased over 10-fold. Subsequent atropine-induced relaxation resulted in a partial depletion (40%) of total tissue Ins(1,4,5)P3. Decreases in Ins(1,4,5)P3 were paralleled by decreases in Ins(1,4)P2 and Ins(1,3,4)P3. During contraction a portion of total tissue Ins(1,4,5)P3 has access to Ins(1,4,5)P3 3-kinase and 5-phosphatase and to Ins(1,4,5)P3-sensitive SR, though during antagonist-induced relaxation access to Ins(1,4,5)P3-sensitive SR for Ca2+ release is restricted. Data are consistent with a mechanism by which a large pool of Ins(1,4,5)P3 present in the unstimulated state in a sequestered compartment can contribute in activated muscle to increases in [Ins(1,4,5)P3] in a nonsequestered compartment, controlling SR Ca2+ release.

Different localization of inositol 1,4,5-trisphosphate and ryanodine binding sites in rat liver

The distribution of inositol 1,4,5-trisphosphate and ryanodine binding sites between plasma membrane, microsomal, and mitochondrial fractions of rat liver were compared. IP3 bound mostly to the plasma membrane fraction (Kd = 6 nM; Bmax = 802 fmol/mg protein). Some IP3 binding sites were also present in the microsomal and mitochondrial fractions (Kd = 2.5 and 2.9 nM; Bmax = 35 and 23 fmol/mg protein respectively). The possibility that these binding sites are due to contamination of the fractions with plasma membrane cannot be excluded. Binding of IP3 to the plasma membrane was inhibited by heparin but not by either caffeine or tetracaine. High-affinity ryanodine binding sites were present mostly in the microsomal fraction (Kd = 13 nM; Bmax = 301 fmol/mg protein). Lower affinity binding sites were also found to be present in the mitochondrial and plasma membrane fractions. Binding of ryanodine to the microsomal fraction was inhibited by both caffeine and tetracaine but not by heparin. These data demonstrate that IP3 and ryanodine binding sites are present in different cellular compartments in the liver. These differences in the localization of the binding sites might be indicative of their functional differences.

Purification of an inositol 1,4,5-trisphosphate-binding calreticulin-containing intracellular compartment of HL-60 cells

To investigate the identity of Ins(1,4,5)P3-sensitive intracellular Ca2+ stores in myeloid cells, we have developed a method that yields subcellular fractions highly enriched in Ins(1,4,5)P3 binding. HL-60 cells were disrupted by nitrogen cavitation, and subcellular fractions were obtained by differential centrifugation, followed by Percoll- and sucrose-density-gradient separations. A subcellular fraction enriched 26-fold in Ins(1,4,5)P3-binding sites was obtained. This fraction showed no enrichment in plasma-membrane markers and only a comparatively moderate enrichment (7-fold) in endoplasmic-reticulum markers. The ratio between specific enrichment of Ins(1,4,5)P3 binding and endoplasmic-reticulum markers in the different fractions varied over 50-fold, from less than 0.1 to greater than 5. The purified Ins(1,4,5)P3-binding fraction was enriched to a similar extent (27-fold) in the putative intravesicular Ca(2+)-storage protein calreticulin. Our results favour the concept of a distinct Ins(1,4,5)P3-binding, calreticulin-containing compartment (i.e. the calciosome) in HL-60 cells.

Calcium signaling mechanisms in the gastric parietal cell

Gastric hydrochloric acid (HCl) secretion is stimulated in vivo by histamine, acetylcholine, and gastrin. In vitro studies have shown that histamine acts mainly via a cAMP-dependent pathway, and acetylcholine acts via a calcium-dependent pathway. Histamine also elevates intracellular calcium ([Ca2+]i) in parietal cells. Both gastrin and acetylcholine release histamine from histamine-containing cells. In humans, rats, and rabbits, there is considerable controversy as to whether or not gastrin receptors are also present on the parietal cell. We utilized digitized video image analysis techniques in this study to demonstrate gastrin-induced changes in intracellular calcium in single parietal cells from rabbit in primary culture. Gastrin also stimulated a small increase in [14C]-aminopyrine (AP) accumulation, an index of acid secretory responsiveness in cultured parietal cells. In contrast to histamine and the cholinergic agonist, carbachol, stimulation of parietal cells with gastrin led to rapid loss of the calcium signaling response, an event that is presumed to be closely related to gastrin receptor activation. Moreover, different calcium signaling patterns were observed for histamine, carbachol, and gastrin, Previous observations coupled with present studies using manganese, caffeine, and ryanodine suggest that agonist-stimulated increases in calcium influx into parietal cells do not occur via voltage-sensitive calcium channels or nonspecific divalent cation channels. It also appears to be unlikely that release of intracellular calcium is mediated by a muscle or neuronal-type ryanodine receptor. We hypothesize that calcium influx may be mediated by either a calcium exchange mechanism or by an unidentified calcium channel subtype that possesses different molecular characteristics as compared to muscle, nerve, and certain secretory cell types such as, for example, the adrenal chromaffin cell. Release of intracellular calcium may be mediated via both InsP3-sensitive and -insensitive mechanisms. The InsP3-insensitive calcium pools, if present, do not appear, however, to possess ryanodine receptors capable of modulating calcium efflux from these storage sites.

Regulation of plasma membrane permeability to calcium in primary cultures of rat hepatocytes

Experiments were designed to characterize the hormone sensitive transport of Ca2+ from the external media into rat hepatocytes maintained in culture. In the absence of added vasopressin, hepatocytes were nearly impermeable to Ca2+, whereas a significant and rapid influx of Ca2+ could be detected when external Ca2+ was added to hepatocytes after the addition of 20 nM vasopressin. The transport was measured as the initial rate of increase of free intracellular Ca2+ [( Ca2+]i) after Ca2+ addition to the external media. Most data were obtained from the majority of cells on a coverslip immersed in a spectrophotometric cuvette, but selected data were obtained by measuring Ca2+ changes in single cells. Ca2+ influx measured using a large number of cells was similar to data obtained using single cells. The Vmax of Ca2+ influx was 140 nM/s. Ca2+ transport was competitive with H+ so that the Km was 17.4 mM at pH 6.8, 3.7 mM at pH 7.4 and 1.8 mM at pH 7.8. Ca2+ influx was insensitive to external K+ (1 to 70 mM) and to the presence of 5 nM valinomycin, suggesting that it was independent of the electrical potential gradient across the plasma membrane. Transport also appeared to be insensitive to the activity of protein kinase C, which was varied by addition of the activator, 12-myristate 13-acetate phorbol ester, and by addition of the kinase inhibitor, staurosporine. Stimulation of transport following vasopressin addition exhibited a delay with a t1/2 of approximately 30 s. A vasopressin antagonist blocked the activation of transport, if added prior to vasopressin. However, experiments designed to determine the effect of hormone occupancy per se on transport activity indicated that continued hormone occupancy was not required. When the external medium was nominally Ca2+ free and an antagonist was added 1 min after vasopressin, Ca2+ entry, even 8 min after antagonist addition, was rapid. Conversely, preincubation with vasopressin antagonist in medium containing 0.5 mM Ca2+ dramatically lowered plasma membrane Ca2+ permeability. The ER Ca2+ pool emptied by vasopressin was refilled in the presence of vasopressin antagonist plus 0.5 mM Ca2+, but did not refill when the medium contained no added Ca2+. Under the conditions of these experiments, the Ca2+ levels of the ER hormone-sensitive Ca2+ pool were estimated as well as intracellular concentrations of inositol-1,4,5-trisphosphate. The Ca2+ levels of the endoplasmic reticulum correlated inversely with plasma membrane Ca2+ permeability, whereas cellular concentrations of inositol-1,4,5-trisphosphate did not correlate with Ca2+ permeability.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Although the initial phase of receptor-mediated Ca2+ signaling, involving Ca2+ release from intracellular stores by inositol 1,4,5-trisphosphate, is relatively well characterized, the nature of the organelle releasing Ca2+ is a controversial subject. At issue is the question of whether Ca2+ 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 Ca2+ pool.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Solubilization of rat liver inositol 1,4,5-trisphosphate receptor

High affinity Ins(1,4,5)P3-binding sites of permeabilized hepatocytes are probably the ligand recognition sites of the receptors that mediate the effects of Ins(1,4,5)P3 on intracellular Ca2+ mobilization. We have now solubilized these sites from rat liver membranes in the zwitterionic detergent, CHAPS, and shown that the solubilized sites bind Ins(1,4,5)P3 with an affinity (Kd = 7.26 +/- 0.52 nM, Hill coefficient h = 1.05 +/- 0.06) similar to that of the sites in native membranes (Kd = 6.02 +/- 1.57 nM, h = 0.99 +/- 0.02). ATP and a range of inositol phosphates (Ins(2,4,5)P3 Ins(4,5)P2, and inositol 1,4,5-trisphosphorothioate) also bound with similar affinities to the native and solubilized sites. Solubilization of the liver InsP3 receptor will allow its further characterization, purification, and comparison of its properties with those of InsP3 receptors already purified from cerebellum and smooth muscle.