Michael J. Berridge

The versatility and universality of calcium signalling

The universality of calcium as an intracellular messenger depends on its enormous versatility. Cells have a calcium signalling toolkit with many components that can be mixed and matched to create a wide range of spatial and temporal signals. This versatility is exploited to control processes as diverse as fertilization, proliferation, development, learning and memory, contraction and secretion, and must be accomplished within the context of calcium being highly toxic. Exceeding its normal spatial and temporal boundaries can result in cell death through both necrosis and apoptosis.

Calcium signalling: dynamics, homeostasis and remodelling

Ca2+ is a highly versatile intracellular signal that operates over a wide temporal range to regulate many different cellular processes. An extensive Ca2+-signalling toolkit is used to assemble signalling systems with very different spatial and temporal dynamics. Rapid highly localized Ca2+ spikes regulate fast responses, whereas slower responses are controlled by repetitive global Ca2+ transients or intracellular Ca2+ waves. Ca2+ has a direct role in controlling the expression patterns of its signalling systems that are constantly being remodelled in both health and disease.

Neuronal Calcium Signaling

I would like to thank Martin Bootman for preparing the figures. This work was supported by a grant from the European Commission BIOMED2 (BMH4-CT96-0656).

Calcium - a life and death signal

One of the most versatile and universal signalling agents in the human body is the calcium ion, Ca2+. How does this simple ion act during cell birth, life and death, and how does it regulate so many different cellular processes?

Neural and developmental actions of lithium: A unifying hypothesis

AFRC Unit of Insect Neurophysiology and Pharmacology Department of Zoology Cambridge CB2 3EJ England tSmith Kline & French Research Ltd. The Frythe, Welwyn Hertfordshire AL6 9AR England *MRC Molecular Neurobiology Unit University of Cambridge Medical School Cambridge CB2 2QH England Lithium, with an atomic weight of 6.9, is the smallest of the alkali metals, yet this simple ion can exert a profound ef- fect on both human behavior and early embryonic devel- opment. Manic-depressive psychosis, characterized by dramatic swings in mood, can be effectively controlled by maintaining a serum level of Li+ of ~1 mM. Despite its therapeutic success, little is known about the way Li+ can modify neurotransmission within the central nervous sys- tem (CNS). Many of the proposed mechanisms have sug- gested an inhibitory effect on components of various neu- rotransmitter signaling pathways, such as cyclic AMP formation, cyclic GMP formation, G proteins, or inositol phosphate metabolism (Hallcher and Sherman, 1960; Berridge et al., 1962). Only the latter provides a plausible explanation of the Li+ conundrum, i.e., the reason this ion

The endoplasmic reticulum: a multifunctional signaling organelle.

The endoplasmic reticulum (ER) is a multifunctional signaling organelle that controls a wide range of cellular processes such as the entry and release of Ca(2+), sterol biosynthesis, apoptosis and the release of arachidonic acid (AA). One of its primary functions is as a source of the Ca(2+) signals that are released through either inositol 1,4,5-trisphosphate (InsP(3)) or ryanodine receptors (RYRs). Since these receptors are Ca(2+)-sensitive, the ER functions as an excitable system capable of spreading signals throughout the cell through a process of Ca(2+)-induced Ca(2+) release (CICR). This regenerative capacity is particularly important in the control of muscle cells and neurons. Its role as an internal reservoir of Ca(2+) must be accommodated with its other major role in protein synthesis where a constant luminal level of Ca(2+) is essential for protein folding. The ER has a number of stress signaling pathways that activate various transcriptional cascades that regulate the luminal content of the Ca(2+)-dependent chaperones responsible for the folding and packaging of secretory proteins.Another emerging function of the ER is to regulate apoptosis by operating in tandem with mitochondria. Anti-apoptotic regulators of apoptosis such as Bcl-2 may act by reducing the ebb and flow of Ca(2+) through the ER/mitochondrial couple. Conversely, the presenilins that appear to increase the Ca(2+) content of the ER lumen make cells more susceptible to apoptosis.

2-Aminoethoxydiphenyl borate (2-APB) is a reliable blocker of store-operated Ca2+ entry but an inconsistent inhibitor of InsP3-induced Ca2+ release

Since its introduction to Ca2+ signaling in 1997, 2‐aminoethoxydiphenyl borate (2‐APB) has been used in many studies to probe for the involvement of inositol 1,4,5‐trisphosphate receptors in the generation of Ca2+ signals. Due to reports of some nonspecific actions of 2‐APB, and the fact that its principal antagonistic effect is on Ca2+ entry rather than Ca2+ release, this compound may not have the utility first suggested. However, 2‐APB has thrown up some interesting results, particularly with respect to store‐operated Ca2+ entry in nonexcitable cells. These data indicate that although it must be used with caution, 2‐APB can be useful in probing certain aspects of Ca2+ signaling.—Bootman, M. D., Collins, T. J., Mackenzie, L., Roderick, H. L., Berridge, M. J., Peppiatt, C. M. 2‐Aminoethoxydiphenyl borate (2‐APB) is a reliable blocker of store‐operated Ca2+ entry but an inconsistent inhibitor of InsP3‐induced Ca2+ release. FASEB J. 16, 1145–1150 (2002)

Cytosolic calcium oscillators.

Many cells display oscillations in intracellular calcium resulting from the periodic release of calcium from intracellular reservoirs. Frequencies are varied, but most oscillations have periods ranging from 5 to 60 s. For any given cell, frequency can vary depending on external conditions, particularly the concentration of natural stimuli or calcium. This cytosolic calcium oscillator is particularly sensitive to those stimuli (neurotransmitters, hormones, growth factors) that hydrolyze phosphoinositides to give diacylglycerol and inositol 1,4,5‐trisphosphate (Ins1,4,5P3). The ability of Ins1,4,5P3 to mobilize intracellular calcium is a significant feature of many of the proposed models that are used to explain oscillatory activity. Receptor‐controlled oscillator models propose that there are complex feedback mechanisms that generate oscillations in the level of Ins1,4,5P3. Second messenger‐controlled oscillator models demonstrate that the oscillator is a component of the calcium reservoir, which is induced to release calcium by a constant input of either Ins1,4,5P3 or calcium itself. In the latter case, the process of calcium‐induced calcium release might be the basis of oscillatory activity in many cell types. The function of calcium oscillations is still unknown. Because oscillator frequency can vary with agonist concentration, calcium transients might be part of a frequency‐encoded signaling system. When an external stimulus arrives at the cell surface the information is translated into a train of calcium spikes, i.e., the signal is digitized. Certain cells may then convey information by varying the frequency of this digital signal.—Berridge, M. J.; Galione, A. Cytosolic calcium oscillators. FASEB J. 2: 3074‐3082; 1988.

Inositol trisphosphate and calcium signalling mechanisms

Studies on control of fluid secretion by an insect salivary gland led to the discovery of inositol trisphosphate (IP3) and its role in calcium signalling. Many cell stimuli act on receptors that are coupled to phospholipase C that hydrolyses phosphatidylinosol 4,5-bisphosphate (PIP2) to release IP3 to the cytosol. IP3 receptors located on the endoplasmic reticulum respond to this elevation of IP3 by releasing Ca2+, which is often organized into characteristic spatial (elementary events and waves) and temporal (Ca2+ oscillations) patterns. This IP3/Ca2+ pathway is a remarkably versatile signalling system that has been adapted to control processes as diverse as fertilization, proliferation, contraction, cell metabolism, vesicle and fluid secretion and information processing in neuronal cells.

Mitochondria are morphologically and functionally heterogeneous within cells

We investigated whether mitochondria represent morphologically continuous and functionally homogenous entities within single intact cells. Physical continuity of mitochondria was determined by three‐dimensional reconstruction of fluorescence from mitochondrially targeted DsRed1 or calcein. The mitochondria of HeLa, PAEC, COS‐7, HUVEC, hepatocytes, cortical astrocytes and neuronal cells all displayed heterogeneous distributions and were of varying sizes. There was a denser aggregation of mitochondria in perinuclear positions than in the cell periphery, where individual isolated mitochondria could be seen clearly. Using fluorescence‐recovery after photobleaching, we observed that DsRed1 and calcein were highly mobile within the matrix of individual mitochondria, and that mitochondria within a cell were not lumenally continuous. Mitochondria were not electrically coupled, since only individual mitochondria were observed to depolarize following irradiation of TMRE‐loaded cells. Functional heterogeneity of mitochondria in single cells was observed with respect to membrane potential, sequestration of hormonally evoked cytosolic calcium signals and timing of permeability transition pore opening in response to tert‐butyl hydroperoxide. Our data indicate that mitochondria within individual cells are morphologically heterogeneous and unconnected, allowing them to have distinct functional properties.