Adenophostin A and ribophostin, but not inositol 1,4,5-trisphosphate or manno-adenophostin, activate the Ca2+ release-activated Ca2+ current, I(CRAC), in weak intracellular Ca2+ buffer

Regulation of store-operated Ca2+ entry by IP3 receptors independent of their ability to release Ca2

Loss of endoplasmic reticular (ER) Ca2+ activates store-operated Ca2+ entry (SOCE) by causing the ER localized Ca2+ sensor STIM to unfurl domains that activate Orai channels in the plasma membrane at membrane contact sites (MCS). Here, we demonstrate a novel mechanism by which the inositol 1,4,5 trisphosphate receptor (IP3R), an ER-localized IP3-gated Ca2+ channel, regulates neuronal SOCE. In human neurons, SOCE evoked by pharmacological depletion of ER-Ca2+ is attenuated by loss of IP3Rs, and restored by expression of IP3Rs even when they cannot release Ca2+, but only if the IP3Rs can bind IP3. Imaging studies demonstrate that IP3Rs enhance association of STIM1 with Orai1 in neuronal cells with empty stores; this requires an IP3-binding site, but not a pore. Convergent regulation by IP3Rs, may tune neuronal SOCE to respond selectively to receptors that generate IP3.

d-chiro-Inositol Ribophostin: A Highly Potent Agonist of d-myo-Inositol 1,4,5-Trisphosphate Receptors: Synthesis and Biological Activities

Analogues of the Ca²⁺-releasing intracellular messenger d-myo-inositol 1,4,5-trisphosphate [1, Ins(1,4,5)P₃] are important synthetic targets. Replacement of the α-glucopyranosyl motif in the natural product mimic adenophostin 2 by d-chiro-inositol in d-chiro-inositol adenophostin 4 increased the potency. Similar modification of the non-nucleotide Ins(1,4,5)P₃ mimic ribophostin 6 may increase the activity. d-chiro-Inositol ribophostin 10 was synthesized by coupling as building blocks suitably protected ribose 12 with l-(+)-3-O-trifluoromethylsulfonyl-6-O-p-methoxybenzyl-1,2:4,5-di-O-isopropylidene-myo-inositol 11. Separable diastereoisomeric 3-O-camphanate esters of (±)-6-O-p-methoxy-benzyl-1,2:4,5-di-O-isopropylidene-myo-inositol allowed the preparation of 11. Selective trans-isopropylidene deprotection in coupled 13, then monobenzylation gave separable regioisomers 15 and 16. p-Methoxybenzyl group deprotection of 16, phosphitylation/oxidation, then deprotection afforded 10, which was a full agonist in Ca²⁺-release assays; its potency and binding affinity for Ins(1,4,5)P₃R were similar to those of adenophostin. Both 4 and 10 elicited a store-operated Ca²⁺ current I_CRAC in patch-clamped cells, unlike Ins(1,4,5)P₃ consistent with resistance to metabolism. d-chiro-Inositol ribophostin is the most potent small-molecule Ins(1,4,5)P₃ receptor agonist without a nucleobase yet synthesized.

Stimulation of inositol 1,4,5-trisphosphate (IP3) receptor subtypes by adenophostin A and its analogues

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca(2+) channels. Most animal cells express mixtures of the three IP3R subtypes encoded by vertebrate genomes. Adenophostin A (AdA) is the most potent naturally occurring agonist of IP3R and it shares with IP3 the essential features of all IP3R agonists, namely structures equivalent to the 4,5-bisphosphate and 6-hydroxyl of IP3. The two essential phosphate groups contribute to closure of the clam-like IP3-binding core (IBC), and thereby IP3R activation, by binding to each of its sides (the α- and β-domains). Regulation of the three subtypes of IP3R by AdA and its analogues has not been examined in cells expressing defined homogenous populations of IP3R. We measured Ca(2+) release evoked by synthetic adenophostin A (AdA) and its analogues in permeabilized DT40 cells devoid of native IP3R and stably expressing single subtypes of mammalian IP3R. The determinants of high-affinity binding of AdA and its analogues were indistinguishable for each IP3R subtype. The results are consistent with a cation-π interaction between the adenine of AdA and a conserved arginine within the IBC α-domain contributing to closure of the IBC. The two complementary contacts between AdA and the α-domain (cation-π interaction and 3″-phosphate) allow activation of IP3R by an analogue of AdA (3″-dephospho-AdA) that lacks a phosphate group equivalent to the essential 5-phosphate of IP3. These data provide the first structure-activity analyses of key AdA analogues using homogenous populations of all mammalian IP3R subtypes. They demonstrate that differences in the Ca(2+) signals evoked by AdA analogues are unlikely to be due to selective regulation of IP3R subtypes.

Selective determinants of inositol 1,4,5-trisphosphate and adenophostin A interactions with type 1 inositol 1,4,5-trisphosphate receptors

BACKGROUND AND PURPOSE

Adenophostin A (AdA) is a potent agonist of inositol 1,4,5-trisphosphate receptors (IP(3) R). AdA shares with IP(3) the essential features of all IP(3) R agonists, namely structures equivalent to the 4,5-bisphosphate and 6-hydroxyl of IP(3) , but the basis of its increased affinity is unclear. Hitherto, the 2'-phosphate of AdA has been thought to provide a supra-optimal mimic of the 1-phosphate of IP(3) .

EXPERIMENTAL APPROACH

We examined the structural determinants of AdA binding to type 1 IP(3) R (IP(3) R1). Chemical synthesis and mutational analysis of IP(3) R1 were combined with (3) H-IP(3) binding to full-length IP(3) R1 and its N-terminal fragments, and Ca(2+) release assays from recombinant IP(3) R1 expressed in DT40 cells.

KEY RESULTS

Adenophostin A is at least 12-fold more potent than IP(3) in functional assays, and the IP(3) -binding core (IBC, residues 224-604 of IP(3) R1) is sufficient for this high-affinity binding of AdA. Removal of the 2'-phosphate from AdA (to give 2'-dephospho-AdA) had significantly lesser effects on its affinity for the IBC than did removal of the 1-phosphate from IP(3) (to give inositol 4,5-bisphosphate). Mutation of the only residue (R568) that interacts directly with the 1-phosphate of IP(3) decreased similarly (by ~30-fold) the affinity for IP(3) and AdA, but mutating R504, which has been proposed to form a cation-π interaction with the adenine of AdA, more profoundly reduced the affinity of IP(3) R for AdA (353-fold) than for IP(3) (13-fold).

CONCLUSIONS AND IMPLICATIONS

The 2'-phosphate of AdA is not a major determinant of its high affinity. R504 in the receptor, most likely via a cation-π interaction, contributes specifically to AdA binding.

Regulation of Ca2+ signaling with particular focus on mast cells

Calcium signals mediate diverse cellular functions in immunological cells. Early studies with mast cells, then a preeminent model for studying Ca2+-dependent exocytosis, revealed several basic features of calcium signaling in non-electrically excitable cells. Subsequent studies in these and other cells further defined the basic processes such as inositol 1,4,5-trisphosphate-mediated release of Ca2+ from Ca2+ stores in the endoplasmic reticulum (ER); coupling of ER store depletion to influx of external Ca2+ through a calcium-release activated calcium (CRAC) channel now attributed to the interaction of the ER Ca2+ sensor, stromal interacting molecule-1 (STIM1), with a unique Ca2+-channel protein, Orai1/CRACM1, and subsequent uptake of excess Ca2+ into ER and mitochondria through ATP-dependent Ca2+ pumps. In addition, transient receptor potential channels and ion exchangers also contribute to the generation of calcium signals that may be global or have dynamic (e.g., waves and oscillations) and spatial resolution for specific functional readouts. This review discusses past and recent developments in this field of research, the pharmacologic agents that have assisted in these endeavors, and the mast cell as an exemplar for sorting out how calcium signals may regulate multiple outputs in a single cell.

Inositol trisphosphate analogues selective for types I and II inositol trisphosphate receptors exert differential effects on vasopressin-stimulated Ca2+ inflow and Ca2+ release from intracellular stores in rat hepatocytes

Previous studies have shown that adenophostin A is a potent initiator of the activation of SOCs (store-operated Ca2+ channels) in rat hepatocytes, and have suggested that, of the two subtypes of Ins(1,4,5)P3 receptor predominantly present in rat hepatocytes [Ins(1,4,5)P3R1 (type I receptor) and Ins(1,4,5)P3R2 (type II receptor)], Ins(1,4,5)P3R1s are required for SOC activation. We compared the abilities of Ins(1,4,6)P3 [with higher apparent affinity for Ins(1,4,5)P3R1] and Ins(1,3,6)P3 and Ins(1,2,4,5)P4 [with higher apparent affinities for Ins(1,4,5)P3R2] to activate SOCs. The Ins(1,4,5)P3 analogues were microinjected into single cells together with fura 2, and dose-response curves for the activation of Ca2+ inflow and Ca2+ release from intracellular stores obtained for each analogue. The concentration of Ins(1,4,6)P3 which gave half-maximal stimulation of Ca2+ inflow was substantially lower than that which gave half-maximal stimulation of Ca2+ release. By contrast, for Ins(1,3,6)P3 and Ins(1,2,4,5)P3, the concentration which gave half-maximal stimulation of Ca2+ inflow was substantially higher than that which gave half-maximal stimulation of Ca2+ release. The distribution of Ins(1,4,5)P3R1 and Ins(1,4,5)P3R2 in rat hepatocytes cultured under the same conditions as those employed for the measurement of Ca2+ inflow and release was determined by immunofluorescence. Ins(1,4,5)-P3R1s were found predominantly at the cell periphery, whereas Ins(1,4,5)P3R2s were found at the cell periphery, the cell interior and nucleus. It is concluded that the idea that a small region of the endoplasmic reticulum enriched in Ins(1,4,5)P3R1 is required for the activation of SOCs is consistent with the present results for hepatocytes.

Store-operated Ca2+ entry depends on mitochondrial Ca2+ uptake

Store-operated Ca(2+) channels, which are activated by the emptying of intracellular Ca(2+) stores, provide one major route for Ca(2+) influx. Under physiological conditions of weak intracellular Ca(2+) buffering, the ubiquitous Ca(2+) releasing messenger InsP(3) usually fails to activate any store-operated Ca(2+) entry unless mitochondria are maintained in an energized state. Mitochondria rapidly take up Ca(2+) that has been released by InsP(3), enabling stores to empty sufficiently for store-operated channels to activate. Here, we report a novel role for mitochondria in regulating store-operated channels under physiological conditions. Mitochondrial depolarization suppresses store-operated Ca(2+) influx independently of how stores are depleted. This role for mitochondria is unrelated to their actions on promoting InsP(3)-sensitive store depletion, can be distinguished from Ca(2+)-dependent inactivation of the store-operated channels and does not involve changes in intracellular ATP, oxidants, cytosolic acidification, nitric oxide or the permeability transition pore, but is suppressed when mitochondrial Ca(2+) uptake is impaired. Our results suggest that mitochondria may have a more fundamental role in regulating store-operated influx and raise the possibility of bidirectional Ca(2+)-dependent crosstalk between mitochondria and store-operated Ca(2+) channels.

Determinants of adenophostin A binding to inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (IP(3)) receptors from cerebellum and recombinant type 1 IP(3) receptors expressed in Sf9 cells had indistinguishable affinities for IP(3) ( K (d)=6.40+/-0.48 nM) and adenophostin A ( K (d)=0.89+/-0.05 nM). In cytosol-like medium, each of the three mammalian IP(3) receptor subtypes when expressed in Sf9 cells bound adenophostin A with greater affinity than IP(3). It has been suggested that adenophostin A binds with high affinity only in the presence of ATP, but we found that adenophostin A similarly displaced [(3)H]IP(3) from type 1 IP(3) receptors whatever the ATP concentration. N-terminal fragments of the type 1 receptor were expressed with and without the S1 splice site; its removal had no effect on [(3)H]IP(3) binding to the 1-604 protein, but abolished binding to the 224-604 protein. The 1-604 fragment and full-length receptor bound adenophostin A with the same affinity, but the fragment had 3-fold greater affinity for IP(3), suggesting that C-terminal residues selectively inhibit IP(3) binding. The 224-604S1(+) fragment bound IP(3) and adenophostin A with increased affinity, but as with the 1-604 fragment it bound adenophostin A with only 2-fold greater affinity than IP(3). High-affinity binding of adenophostin A may be partially determined by its 2'-phosphate interacting more effectively than the 1-phosphate of IP(3) with residues within the IP(3)-binding core. This may account for the 2-fold greater affinity of adenophostin A relative to IP(3) for the minimal IP(3)-binding domain. In addition we suggest that C-terminal residues, which impede access of IP(3), may selectively interact with adenophostin A to allow it unhindered access to the IP(3)-binding domain.