Sustained signalling by PTH modulates IP3 accumulation and IP3 receptors through cyclic AMP junctions

Structural Mechanisms of Store-Operated and Mitochondrial Calcium Regulation: Initiation Points for Drug Discovery

Calcium (Ca2+) is a universal signaling ion that is essential for the life and death processes of all eukaryotes. In humans, numerous cell stimulation pathways lead to the mobilization of sarco/endoplasmic reticulum (S/ER) stored Ca2+, resulting in the propagation of Ca2+ signals through the activation of processes, such as store-operated Ca2+ entry (SOCE). SOCE provides a sustained Ca2+ entry into the cytosol; moreover, the uptake of SOCE-mediated Ca2+ by mitochondria can shape cytosolic Ca2+ signals, function as a feedback signal for the SOCE molecular machinery, and drive numerous mitochondrial processes, including adenosine triphosphate (ATP) production and distinct cell death pathways. In recent years, tremendous progress has been made in identifying the proteins mediating these signaling pathways and elucidating molecular structures, invaluable for understanding the underlying mechanisms of function. Nevertheless, there remains a disconnect between using this accumulating protein structural knowledge and the design of new research tools and therapies. In this review, we provide an overview of the Ca2+ signaling pathways that are involved in mediating S/ER stored Ca2+ release, SOCE, and mitochondrial Ca2+ uptake, as well as pinpoint multiple levels of crosstalk between these pathways. Further, we highlight the significant protein structures elucidated in recent years controlling these Ca2+ signaling pathways. Finally, we describe a simple strategy that aimed at applying the protein structural data to initiating drug design.

New Insights in the IP3 Receptor and Its Regulation

The inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) is a Ca²⁺-release channel mainly located in the endoplasmic reticulum (ER). Three IP₃R isoforms are responsible for the generation of intracellular Ca²⁺ signals that may spread across the entire cell or occur locally in so-called microdomains. Because of their ubiquitous expression, these channels are involved in the regulation of a plethora of cellular processes, including cell survival and cell death. To exert their proper function a fine regulation of their activity is of paramount importance. In this review, we will highlight the recent advances in the structural analysis of the IP₃R and try to link these data with the newest information concerning IP₃R activation and regulation. A special focus of this review will be directed towards the regulation of the IP₃R by protein-protein interaction. Especially the protein family formed by calmodulin and related Ca²⁺-binding proteins and the pro- and anti-apoptotic/autophagic Bcl-2-family members will be highlighted. Finally, recently identified and novel IP₃R regulatory proteins will be discussed. A number of these interactions are involved in cancer development, illustrating the potential importance of modulating IP₃R-mediated Ca²⁺ signaling in cancer treatment.

Selective inhibition of histamine-evoked Ca2+ signals by compartmentalized cAMP in human bronchial airway smooth muscle cells

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β₂-adrenoceptors, via cAMP and PKA, inhibit histamine-evoked Ca²⁺ signals in human bronchial airway smooth muscle cells.

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Responses to other Ca²⁺-mobilizing receptors are unaffected or minimally affected by cAMP.

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There is no consistent relationship between the amounts of cAMP produced by different stimuli and inhibition of histamine-evoked Ca²⁺ release.

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Local delivery of cAMP within hyperactive signaling junctions stimulates PKA.

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PKA inhibits an early step in the signaling pathway activated by H₁ histamine receptors.

Intracellular Ca²⁺ and cAMP typically cause opposing effects on airway smooth muscle contraction. Receptors that stimulate these pathways are therapeutic targets in asthma and chronic obstructive pulmonary disease. However, the interactions between different G protein-coupled receptors (GPCRs) that evoke cAMP and Ca²⁺ signals in human bronchial airway smooth muscle cells (hBASMCs) are poorly understood. We measured Ca²⁺ signals in cultures of fluo-4-loaded hBASMCs alongside measurements of intracellular cAMP using mass spectrometry or [³H]-adenine labeling. Interactions between the signaling pathways were examined using selective ligands of GPCRs, and inhibitors of Ca²⁺ and cAMP signaling pathways. Histamine stimulated Ca²⁺ release through inositol 1,4,5-trisphosphate (IP₃) receptors in hBASMCs. β₂-adrenoceptors, through cAMP and protein kinase A (PKA), substantially inhibited histamine-evoked Ca²⁺ signals. Responses to other Ca²⁺-mobilizing stimuli were unaffected by cAMP (carbachol and bradykinin) or minimally affected (lysophosphatidic acid). Prostaglandin E₂ (PGE₂), through EP₂ and EP₄ receptors, stimulated formation of cAMP and inhibited histamine-evoked Ca²⁺ signals. There was no consistent relationship between the inhibition of Ca²⁺ signals and the amounts of intracellular cAMP produced by different stimuli. We conclude that β-adrenoceptors, EP₂ and EP₄ receptors, through cAMP and PKA, selectively inhibit Ca²⁺ signals evoked by histamine in hBASMCs, suggesting that PKA inhibits an early step in H₁ receptor signaling. Local delivery of cAMP within hyperactive signaling junctions mediates the inhibition.

Cyclic AMP Recruits a Discrete Intracellular Ca2+ Store by Unmasking Hypersensitive IP3 Receptors

Inositol 1,4,5-trisphosphate (IP₃) stimulates Ca²⁺ release from the endoplasmic reticulum (ER), and the response is potentiated by 3′,5′-cyclic AMP (cAMP). We investigated this interaction in HEK293 cells using carbachol and parathyroid hormone (PTH) to stimulate formation of IP₃ and cAMP, respectively. PTH alone had no effect on the cytosolic Ca²⁺ concentration, but it potentiated the Ca²⁺ signals evoked by carbachol. Surprisingly, however, the intracellular Ca²⁺ stores that respond to carbachol alone could be both emptied and refilled without affecting the subsequent response to PTH. We provide evidence that PTH unmasks high-affinity IP₃ receptors within a discrete Ca²⁺ store. We conclude that Ca²⁺ stores within the ER that dynamically exchange Ca²⁺ with the cytosol maintain a functional independence that allows one store to be released by carbachol and another to be released by carbachol with PTH. Compartmentalization of ER Ca²⁺ stores adds versatility to IP₃-evoked Ca²⁺ signals.

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Cyclic AMP directly potentiates IP₃-evoked Ca²⁺ release

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The Ca²⁺ stores released by IP₃ alone or IP₃ with cAMP are functionally independent

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Cyclic AMP unmasks high-affinity IP₃ receptors in a discrete ER Ca²⁺ store

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Independent regulation of discrete Ca²⁺ stores increases signaling versatility

Prostaglandin E2 Inhibits Histamine-Evoked Ca2+ Release in Human Aortic Smooth Muscle Cells through Hyperactive cAMP Signaling Junctions and Protein Kinase A

In human aortic smooth muscle cells, prostaglandin E₂ (PGE₂) stimulates adenylyl cyclase (AC) and attenuates the increase in intracellular free Ca²⁺ concentration evoked by activation of histamine H₁ receptors. The mechanisms are not resolved. We show that cAMP mediates inhibition of histamine-evoked Ca²⁺ signals by PGE₂. Exchange proteins activated by cAMP were not required, but the effects were attenuated by inhibition of cAMP-dependent protein kinase (PKA). PGE₂ had no effect on the Ca²⁺ signals evoked by protease-activated receptors, heterologously expressed muscarinic M3 receptors, or by direct activation of inositol 1,4,5-trisphosphate (IP₃) receptors by photolysis of caged IP₃. The rate of Ca²⁺ removal from the cytosol was unaffected by PGE₂, but PGE₂ attenuated histamine-evoked IP₃ accumulation. Substantial inhibition of AC had no effect on the concentration-dependent inhibition of Ca²⁺ signals by PGE₂ or butaprost (to activate EP₂ receptors selectively), but it modestly attenuated responses to EP₄ receptors, activation of which generated less cAMP than EP₂ receptors. We conclude that inhibition of histamine-evoked Ca²⁺ signals by PGE₂ occurs through “hyperactive signaling junctions,” wherein cAMP is locally delivered to PKA at supersaturating concentrations to cause uncoupling of H₁ receptors from phospholipase C. This sequence allows digital signaling from PGE₂ receptors, through cAMP and PKA, to histamine-evoked Ca²⁺ signals.

Regulation of IP3 receptors by cyclic AMP

Ca²⁺ and cAMP are ubiquitous intracellular messengers and interactions between them are commonplace. Here the effects of cAMP on inositol 1,4,5-trisphosphate receptors (IP₃Rs) are briefly reviewed. All three subtypes of IP₃R are phosphorylated by cAMP-dependent protein kinase (PKA). This potentiates IP₃-evoked Ca²⁺ release through IP₃R1 and IP₃R2, but probably has little effect on IP₃R3. In addition, cAMP can directly sensitize all three IP₃R subtypes to IP₃. The high concentrations of cAMP required for this PKA-independent modulation of IP₃Rs is delivered to them within signalling junctions that include type 6 adenylyl cyclase and IP₃R2.