- calcium ATPase
- calcium channels
- calcium signaling
- cellular function
- excitable cells
- reactive oxygen species
- second messengers
How does a person pen 1200 words on “calcium rules” when a search for “calcium” in PubMed produces >530 000 references, 15 000 of which are from 2016 alone? To face this challenge, I decided to recount my life’s journey unveiling some of these rules, a journey that has led me to conclude that this versatile element, so important to the central processes of life, does in fact rule the cell.
I first became acquainted with the calcium ion (Ca2+) in the mid-1960s when I studied Ca2+ efflux from the squid giant axon under the guidance of Eduardo Rojas in Montemar, Chile. In 1968, we reported that this complex process exhibits metabolism-dependent components controlled by mitochondrial function. Squid seasons were short, precluding us from testing, as planned, the effects of removing external sodium (Na+) on Ca2+ efflux. Thus, we missed the discovery of the Na+-Ca2+-exchanger reported in the late 1960s by Reuter and Seitz in cardiac muscle and in the squid axon by Peter Baker and colleagues.
What did we know then about Ca2+ rules? From the pioneering studies of Sydney Ringer, we were aware that heart contraction required Ca2+, but the underlying mechanisms emerged many years later. In the early 1960s, for instance, 3 independent research teams led by Wilhelm Hasselbach in Germany, Setsuro Ebashi in Japan, and Annemarie Weber in the United States reported Ca2+ uptake by the sarcoplasmic reticulum and discovered the sarcoplasmic reticulum calcium pump, later known as the sarco/endoplasmic reticulum calcium ATPase pump. They also showed conclusively that an increase in intracellular Ca2+ concentration promotes Ca2+ binding to troponin C, which allows the actin-myosin interaction that triggers muscle contraction (for historical aspects, see ref. 1). Later studies by many groups on the workings of the sarco/endoplasmic reticulum calcium ATPase pump, including analysis of its crystal structure and our own studies of how the physical state of the lipids surrounding the pump control its rotational motion and activity, provided key information regarding the mechanisms underlying the active transport of ions across biological membranes.
Shortly after, several groups reported that Ca2+ also governed cardiac muscle contraction. Before long, we learned that the mechanisms underlying excitation-contraction coupling exhibited similar characteristics in skeletal and cardiac muscle but also displayed some significant differences; the use of selective blockers of voltage-gated Ca2+ channels allowed significant progress in this regard. Central among these differences was the finding that Ca2+ entry by voltage-gated Ca2+ channels causes the cytoplasmic Ca2+ increase elicited by the cardiac action potential, whereas contraction of skeletal muscle fibers proceeds in the absence of Ca2+ entry. In addition, several early reports showed that the Na+-Ca2+-exchanger plays a key role in cardiac contraction, and that phospholamban, an intrinsic membrane protein absent from the sarcoplasmic reticulum of fast-twitch skeletal muscle, tightly controls the cardiac sarco/endoplasmic reticulum calcium ATPase pump.2
In the early 1960s, a few studies reported that by carrying out Ca2+ uptake and release, mitochondria played a role in cellular Ca2+ signaling.1 In subsequent years, however, we saw the fall of mitochondria—from influential subjects to humble vassals within the Ca2+ kingdom—a position from which they have reemerged more recently in full glory—as pivotal actors governing many cellular functions directed by Ca2+. The recent identification of the mitochondrial Ca2+ uniporter complex has generated strong interest in its functional connections with Ca2+ channels present in the endoplasmic reticulum and the plasma membrane.
Calcium domains expanded significantly in the 1980s and acquired new subjects with the key discovery of the second messenger role of inositol 1,4,5-trisphosphate and the identification of the endoplasmic reticulum Ca2+ release channels: the ryanodine receptor (RyR) in striated muscle and the inositol 1,4,5-trisphosphate receptor in a variety of cell types. Now we know that first messengers generate complex intracellular Ca2+ signaling patterns, which by triggering on and off cellular mechanisms leads to changes in Ca2+ signal amplitude and frequency, generating time-dependent local or global domains that are decoded by intracellular sensors and receptors to control selective biological responses. To this purpose, Ca2+ harnesses a large toolkit to reign over a wide range of mammalian cellular functions, comprising muscle contraction, secretion of neurotransmitters and hormones, gene transcription, synaptic plasticity, learning and memory, development, proliferation, and fertilization.3 Yet Ca2+ is not an entirely benign ruler; if uncontrolled, it can kill cells through apoptosis and necrosis. Moreover, mishandled Ca2+ signals in the heart lead to cardiac arrhythmia and hypertrophy, ischemic heart disease, and heart failure, whereas uncontrolled Ca2+ signals in neuronal cells can cause aged-related neurological diseases.
Cells have developed sophisticated mechanisms to keep Ca2+ signals in check, including transporters and pumps that entail a significant cost in metabolic energy. Cells also possess a variety of Ca2+-binding components that limit local increases in Ca2+ concentration and significantly hinder its diffusion through the cytoplasm. Restricted Ca2+ diffusion creates an intriguing problem for Ca2+ signal propagation along the cell interior. This issue acquires special relevance in neurons, where activity-generated postsynaptic Ca2+ signals initiate a complex cascade of events, including the generation of nuclear Ca2+ signals that promote the expression of numerous genes involved in sustained synaptic plasticity, currently considered the cellular correlate of learning and memory.
Evidence collected over the last decades has shown that protein kinases and phosphatases modulate the function of many proteins engaged in Ca2+ signaling. In addition, crosstalk between Ca2+ signaling and reactive oxygen and nitrogen species is becoming a relevant subject because of its physiological and pathological significance. Several key proteins involved in Ca2+ signaling, including RyR channels, are highly redox-sensitive.4 Although RyR channels are widely accepted as muscle Ca2+ release channels, our current focus is the study of neuronal Ca2+ signals generated by RyR-mediated Ca2+ release. We are presently testing the hypotheses that neuronal RyR channels act as postsynaptic coincidence detectors of activity-generated Ca2+ and reactive oxygen species signals, and that the ensuing RyR-mediated dendritic Ca2+ signals have key roles in synaptic plasticity, learning, and memory. Moreover, the aging process and age-related neurodegenerative diseases entail increased levels of cellular reactive oxygen species. We propose that by enhancing RyR activation by Ca2+, this reactive oxygen species increase elicits anomalous Ca2+ signaling, with consequent deleterious effects for neuronal function.
What makes Ca2+ such a special element for life? This question certainly deserves an answer because the preceding sections highlight how Ca2+ reigns over so many fundamental biological processes. As lucidly described in evolutionary terms in a recent article,5 the special role of Ca2+ may derive from its peculiar coordination chemistry, which allows Ca2+ binding to many different proteins at sites that do not accept Mg2+.
Over the past 6 decades, we have witnessed spectacular developments in our understanding of the mechanisms whereby Ca2+ rules over many aspects of cell function; yet important challenges remain. Among these challenges, we must unravel the complex and orchestrated connections that occur between Ca2+ signaling and other cellular signaling pathways. Recognition of subcellular Ca2+ microdomains is emerging with increasing importance. It is also urgent that we advance our knowledge of the cellular mechanisms that perturb Ca2+ homeostasis and signaling during aging and pathology. In the end, unveiling the scope and complexity of this beautiful biology—of Ca2+ rules—will continue to delight and yield insights of great relevance to understand fundamental aspects of cellular function.
Sources of Funding
This work was supported by FONDECYT-1140545 and BNI P-09-015F.
The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.
Circulation is available at http://circ.ahajournals.org.
- © 2017 American Heart Association, Inc.