Calcium and cell function

From the birth of cells to their death, elevations in intracellular Ca²⁺ concentrations mediate diverse biological functions from gene expression and neurotransmitter release to cell proliferation and apoptosis (Clapham, 2007). Ca²⁺ signals drive these functions by binding to numerous Ca²⁺-sensitive effector proteins which translate the signals into specific cellular responses depending on the spatiotemporal dynamics of the Ca²⁺ signals. Further upstream, the Ca²⁺ signals themselves are generated by a large repertoire of ion channels, both plasma membrane and intracellular, and sculpted by a variety of pumps, transporters, and Ca²⁺ buffers. With so many different players involved, it is only natural that dysfunction of Ca²⁺ signalling pathways is implicated in numerous human diseases and syndromes ranging from immune deficiencies to cancer, and autism to neurodegenerative diseases (Feske, 2009; Supnet & Bezprozvanny, 2010; Surmeier et al. 2010; Berridge, 2012, 2017). These and related topics were the focus of the FASEB conference on calcium and cell function that was sponsored by The Journal of Physiology and held in beautiful Lake Tahoe, California, in 2018. The collection of reviews and articles in this symposium issue of the Journal illustrates a sampling of these advances.

Nowhere is the role of cellular Ca²⁺ elevations better exemplified than in the brain where Ca²⁺ signals drive myriad effector functions from fast neurotransmitter release and excitation–transcription coupling in neurons (Dolmetsch, 2003; Neher & Sakaba, 2008; Ma et al. 2014) to gliotransmitter release and cytokine generation in astrocytes (Santello & Volterra, 2009; Sofroniew, 2014; Toth et al. 2019). How so many different effector functions are regulated by the same common messenger is a topic that has fascinated – and confounded – calcium biologists for decades. Exemplifying the compartmentalized nature of Ca²⁺ signalling pathways and high specificity seen for coupling between specific pathways and effector functions, Ege Kavalali describes one manifestation of this specificity – in the Ca²⁺ regulation of spontaneous neurotransmitter release which appears to occur by mechanisms distinct from those involved in regulated evoked transmission (Kavalali, 2020). And in astrocytes, the most prevalent glial cell type in the brain, a rapidly growing literature implicates these cells in critical brain functions and pathologies from learning and memory to modulation of pain sensation. The review by Alexej Verkhratsky addresses the repertoire of ion channels contributing to astrocyte Ca²⁺ signals and how these may impact astrocyte physiology (Verkhratsky et al. 2019). In a complementary review, Masamitsu Iino discusses the invention of intriguing new tools to monitor astrocyte Ca²⁺ signals including a new generation of organelle-targeted genetically encoded indicators that offer the promise to revolutionize measurements of Ca²⁺ signals in hard to reach intracellular organelles (Okubo & Iino, 2020). Novel genetically encoded tools are also the topic of Henry Colecraft’s review on voltage-gated Ca²⁺ (Cav) channels (Colecraft, 2020). He describes an intriguing form of Cav channel inhibition mediated by the Ras-like GTPase protein which holds promise to be harnessed for designer genetically voltage-dependent inhibitors of Cav1.2 channels.

In addition to these players, two players that sit squarely at the nexus of the calcium signalling machinery in most cells and which cooperate with each other to regulate local and global calcium signals are store-operated Orai channels and the endoplasmic reticulum. This Ca²⁺ signalling pathway, mediated by opening of Orai channels via interactions with the ER-resident STIM proteins is a ubiquitous mechanism in most eukaryotic cells (Prakriya & Lewis, 2015). Several reviews in this issue examine the mechanisms of STIM1 activation, Orai1 gating, and regulation of the pathway by mitochondria. In particular, Hogan and colleagues describe elegant work on the molecular and structural underpinnings of how STIM1 is kept quiescent at rest and unfolds in response to store depletion to reveal the catalytic domain that binds and activates Orai channels (Gudlur et al. 2020). Their work reveals a unique molecular mechanism by which the catalytic domain of STIM1 is maintained in a hidden/inactive state via an intramolecular autoinhibitory clamp, which is released upon depletion of ER Ca²⁺ stores. On the channel side, Yeung and colleagues describe the molecular basis of the allosteric process that culminates in the opening of the Orai1 channel pore following STIM1 binding (Yeung et al. 2019). This work reveals a critical role for transmission of the gating signal through the Orai1 transmembrane domains to open a hydrophobic gate. And Parekh and colleagues expound on an intriguing mode of the regulation of the Orai signalling pathway by Ca²⁺ uptake into the mitochondria through a mechanism that appears to be primarily regulated by the mitochondrial uniporter (Samanta et al. 2020).

A challenge in studying the cell biology of store-operated calcium signalling is the paucity of tools to effectively probe the local contact sites between the ER and the plasma membrane where the Ca²⁺ entry occurs. Yubin Zhou and colleagues describe fascinating new approaches and a general guide for selecting methods and tools for probing interorganellar membrane contact sites (Jing et al. 2020). Such tools could aid the visualization and interrogation of these structures in both fixed and living cells. In addition to the ER, another fascinating organelle that exemplifies local signalling in distinct compartments is the signalling machinery in cilia. Remarkable advances have occurred in understanding cilia physiology in recent years in large part due to selective targeting of genetically encoded dyes into the cilium compartment and heroic electro-physiological recordings of ionic currents in the cilia membranes (Delling et al. 2013, 2016). Markus Delling describes these advances in a fascinating piece on ion signalling in the cilia of the embryonic node (Tajhya & Delling, 2020). Finally, Andrea Meredith and colleagues describe intriguing daytime/night-time differences in the regulation of voltage-gated Ca²⁺ channels in the suprachiasmatic nucleus, and the possible consequences of this regulation for SCN neuron firing (McNally et al. 2020), highlighting a major role for L-type Cav channels in regulating circadian rhythms.

Together, these articles demonstrate the remarkable progress and exciting developments that have occurred in this evolving field. Still, most calcium physiologists would concur that what we know likely represents just the tip of a large iceberg and there is still much be learnt on how Ca²⁺ signals are generated, regulated, and transduced into physiological responses and modified in disease states. As exemplified by the discoveries described in this collection of reviews and papers from this symposium, we can look forward with great anticipation to further breakthroughs in the coming years.