Advances in intracellular Ca²⁺ signalling

It was over 125 years ago that Sidney Ringer, working in his laboratory at University College Hospital in London, discovered the requirement for calcium in maintaining contraction of the heart. Similar findings were subsequently made in many different physiological systems, leading Nobel Laureate Otto Loewi to exclaim in 1959: ‘Ja, Kalzium! Das ist alles’. Loewi's proclamation is not unfettered hyperbole. We now know that cytoplasmic Ca²⁺ can control myriad responses within a cell, ranging from neurotransmitter release operating over a sub‐milliseconds time scale to cell growth and proliferation, which can occur hours to days after termination of the initial Ca²⁺ signal (Clapham, 2007). Cytoplasmic Ca²⁺ can also trigger cell death through the highly orchestrated sequence of events that develops during apoptosis. Inherent to the use of a multifarious messenger like Ca²⁺ is the old chestnut of specificity: how are some responses within a cell activated and not others when intracellular Ca²⁺ rises? What has become apparent in recent years is that where Ca²⁺ goes up within the cytoplasm and for how long it stays up are key factors that help determine a selective response (Parekh, 2011). Moreover, dysregulation of the spatial or temporal profile of the Ca²⁺ signal can contribute to patho‐physiology, occasionally with devastating consequences.

Intracellular Ca²⁺ can be rapidly increased through the opening of Ca²⁺‐permeable ion channels in either intracellular stores like the endoplasmic reticulum or in the plasma membrane. The kinetics of the Ca²⁺ signal are dictated by properties of the channels themselves, local Ca²⁺ buffering and Ca²⁺ clearance mechanisms including Ca²⁺ pumps and Na⁺–Ca²⁺ exchange. Important advances in these areas, and how they impact on cell function in health and disease, were described in a Gordon Research Conference on intracellular Ca²⁺ signalling, held in 2015 at the Sunday River Resort in Newry, Maine, USA. This issue brings together five review articles from speakers at that meeting which reflect the versatility of Ca²⁺ signalling.

Ambudkar provides a comprehensive overview of Ca²⁺ signalling in exocrine glands and how the signals can be hijacked in diseases such as autoimmune primary Sjögren's syndrome (Ambudkar, 2016). A major route for Ca²⁺ influx in virtually all eukaryotic cells is through store‐operated CRAC channels in the plasma membrane (Hoth & Penner, 1992; Parekh & Penner, 1997). These channels are activated following InsP ₃‐dependent release of Ca²⁺ from the endoplasmic reticulum (ER). As store Ca²⁺ content falls, the ER Ca²⁺ sensors STIM1 and STIM2 migrate towards the plasma membrane to open Orai proteins, the pore‐forming subunits of the CRAC channels (Soboloff et al. 2012; Amcheslavsky et al. 2015). The author highlights a form of channel amplification in which Ca²⁺ entry through Orai1, which by itself is sufficient to activate NFAT transcription factors (Kar & Parekh, 2015), stimulates insertion of TRPC1/TRPC3 channels (Ambudkar, 2016). The presence of these large conductance non‐selective cation channels leads to different intracellular Ca²⁺ signatures with distinct downstream consequences.

Bogeski and colleagues discuss the emerging role of the STIM/Orai pathway in cancer focusing on melanoma (Stanisz et al. 2016), the most lethal form of skin cancer. Melanoma arises from malignant transformation of melanocytes, melanin‐pigment‐synthesizing cells that occupy the epidermal basement layer of the skin. Melanin helps protect against UV irradiation. It turns out that Orai1, gated by STIM2 in this system, controls proliferation, migration, invasion and melanin production in melanocytes and melanoma. Targeting CRAC channels could therefore be an effective strategy for reducing the risk of UV‐driven melanoma following excessive sun exposure.

To activate Orai channels, ER‐resident STIM protein multimers migrate across the ER to occupy specialized regions juxtaposed against the plasma membrane. At these specialized ER–plasma membrane junctions, STIM is close enough to bind to and open Orai channels. How the junctions form, how they are maintained and how the compartmentalized Ca²⁺ signals are decoded are described by Tepikin and colleagues (Okeke et al. 2016). What emerges is an appreciation of how versatile these regions actually are, integrating Ca²⁺, cAMP and redox signalling with lipid transport between the ER and plasma membrane.

Physiologically, CRAC channels are activated by store depletion following InsP ₃‐driven Ca²⁺ release from the ER. Prole and Taylor describe how InsP ₃ receptors form signalling hubs through interactions with proteins that regulate channel activity and which can be regulated themselves (Prole & Taylor, 2016). Additional proteins located within the hubs are activated by local Ca²⁺ flux through the InsP ₃ receptors. This latter process provides a means whereby spatially restricted Ca²⁺ signals on the ER surface can be selectively guided to downstream effectors. The number of proteins that are thought to interact with the InsP ₃ receptor is sizeable, which perhaps helps explain why the InsP ₃ receptor is so large compared with most other ion channels.

An interesting form of InsP ₃ receptor regulation, described by Yule and colleagues (Wang et al. 2016), is through cleavage by caspases at a DEVD substrate motif site between amino acids 1888 and 1891. The receptor is cut into two fragments: a larger N‐terminal component that harbours the InsP ₃ binding site and a smaller C‐terminal stump with the channel pore. Despite cleavage, the fragmented receptor migrates at the same molecular weight as the full length receptor, indicating the former preserves the same tetrameric arrangement as the normal protein. Remarkably, the cleaved receptor retains gating by InsP ₃ in response to Gq‐coupled receptor activation (Wang et al. 2016). How fragmentation alters the overall spatiotemporal profile of InsP ₃‐driven Ca²⁺ release is unclear but one could well imagine altered regulation by components within the signalling hub described by Taylor and colleagues. Ryanodine‐sensitive Ca²⁺ release channels also maintain function after proteolytic cleavage (George et al. 2004), suggesting fragmentation might be a form of long‐term modulation in Ca²⁺ release channels that helps them adapt to changes in the environment.

We have seen significant strides in our understanding of Ca²⁺ signalling over the last couple of years in all areas of Ca²⁺ homeostasis. However, these advances have spawned new questions, which are being vigorously pursued. We can look forward with great anticipation to further breakthroughs in the coming years.