Direct monitoring of the free Ca2+ concentration in the lumen of the endoplasmic reticulum (ER) is an important but still unsolved experimental problem. We have shown that a Ca(2+)‐sensitive ...photoprotein, aequorin, can be addressed to defined subcellular compartments by adding the appropriate targeting sequences. By engineering a new aequorin chimera with reduced Ca2+ affinity, retained in the ER lumen via interaction of its N‐terminus with the endogenous resident protein BiP, we show here that, after emptying the ER, Ca2+ is rapidly re‐accumulated up to concentrations of > 100 microM, thus consuming most of the reporter photoprotein. An estimate of the steady‐state Ca2+ concentration was obtained using Sr2+, a well‐known Ca2+ surrogate which elicits a significantly slower rate of aequorin consumption. Under conditions in which the rate and extent of Sr2+ accumulation in the ER closely mimick those of Ca2+, the steady‐state mean lumenal Sr2+ concentration (Sr2+er) was approximately 2 mM. Receptor stimulation causes, in a few seconds, a 3‐fold decrease of the Sr2+er, whereas specific inhibition of the ER Ca2+ ATPase leads to an approximately 10‐fold drop in a few minutes.
Luminous proteins include primary light producers, such as aequorin, and secondary photoproteins that in some organisms red-shift light emission for better penetration in space. When expressed in ...heterologous systems, both types of proteins may act as versatile reporters capable of monitoring phenomena as diverse as calcium homoeostasis, protein sorting, gene expression, and so on. The Ca(2+)-sensitive photoprotein aequorin was targeted to defined intracellular locations (organelles, such as mitochondria, endoplasmic reticulum, sarcoplasmic reticulum, Golgi apparatus and nucleus, and cytoplasmic regions, such as the bulk cytosol and the subplasmalemmal rim), and was used to analyse Ca(2+) homoeostasis at the subcellular level. We will discuss this application, reviewing its advantages and disadvantages and the experimental procedure. The applications of green fluorescent protein (GFP) are even broader. Indeed, the ability to molecularly engineer and recombinantly express a strongly fluorescent probe has provided a powerful tool for investigating a wide variety of biological events in live cells (e.g. tracking of endogenous proteins, labelling of intracellular structures, analysing promoter activity etc.). More recently, the demonstration that, using appropriate mutants and/or fusion proteins, GFP fluorescence can become sensitive to physiological parameters or activities (ion concentration, protease activity, etc.) has further expanded its applications and made GFP the favourite probe of cell biologists. We will here present two applications in the field of cell signalling, i.e. the use of GFP chimaeras for studying the recruitment of protein kinase C isoforms and the activity of intracellular proteases.
With this overview of the role of mitochondria in the realm of calcium signalling we have tried to provide a chronological perspective, from the very early days to the present. We have briefly ...sketched a timeline of the research on calcium and mitochond ria during the course of the century. Particular attention is paid to recent developments which have contributed to a renewed interest in calcium handling by this organelle.
The notion that the agonist-dependent increases in intracellular Ca2+concentration, on ubiquitous signalling mechanism, occur with a tightly regulated spatio-temporal pattern has become an ...established concept in modern cell biology. As a consequence, the concept is emerging that the recruitment of specific intracellular targets and effector system mechanisms depends on exposure to local Ca2+ that differs substantially from the mean Ca2+. A striking example is provided by mitochondria, intracellular organelles that have been overlooked for a long time in the field of calcium signalling due to the low affinity of their Ca2+- uptake pathways. We will summarize here some of the evidence indicating that these organelles actively participate in Ca2+homeostasis in physiological conditions (with consequences not only for the control of their function, but also for the modulation of the complexity of calcium signals) because they have the capability to respond to microdomains of high Ca2+ transiently generated in their proximity by the opening of Ca2+channels.
In 1995, IBM Global Services began implementing a business model that included support for the growth and development of communities of practice focused on the competencies of the organization. This ...paper describes the experience working with these communities over a five-year period, concentrating specifically on how the communities evolved. An evolution model based on observing over 60 communities is presented, and the evolution is discussed in terms of people and organization behavior, supporting processes, and enabling technology factors. Also described are specific scenarios of communities within IBM Global Services at various stages of evolution.
Ca2+ is a key regulator not only of multiple cytosolic enzymes, but also of a variety of metabolic pathways occuring within the lumen of intracellular organelles. Until recently, no technique to ...selectively monitor the Ca2+ concentration within defined cellular compartments was available. We have recently proposed the use of molecularly engineered Ca2+-sensitive photoproteins to obtain such a result and demonstrated the application of this methodology to the study of mitochondrial and nuclear Ca2+ dynamics. We here describe in more detail the use of chimeric recombinant aequorin targeted to the mitochondria. The technique can be applied with equivalent results to different cell models, transiently or permanently transfected. In all the cell types we analyzed, mitochondrial Ca2+ concentration ( Ca2+ m) increases rapidly and transiently upon stimulation with agonists coupled to InsP3 generation. We confirm that the high speed of mitochondrial Ca2+ accumulation with this type of stimuli depends on the generation of local gradients of Ca2+ in the cytosol, close to the channels sensitive to InsP3. In fact, only activation of these channels, but not the simple release from internal stores, as that elicited by blocking the intracellular Ca2+ ATPases, results in a fast mitochondrial Ca2+ accumulation. We also provide evidence in favor of a microheterogeneity among mitochondria of the same cells, about 30% of them apparently sensing the microdomains of high cytosolic Ca2+ concentration ( Ca2+ c). The changes in ( Ca2+ m) appear sufficiently large to induce a rapid activation of mitochondrial dehydrogenases, which can be followed by monitoring the level of NAD(P)H fluorescence. A general scheme can thus be envisaged by which the triggering of a plasma membrane receptor coupled to InsP3 generation raises the Ca2+ concentration both in the cytoplasm (thereby triggering energy-consuming processes, such as cell proliferation, motility, secretion, etc.) and in the mitochondria, where it activates the metabolic activity according to the increased cell needs.
Studies on pancreatic acinar cells provided the original evidence for the Ca
2+ releasing action of inositol 1,4,5-trisphosphate (IP
3). Ironically, this system has presented problems for the general ...theory that IP
3 acts primarily on the endoplasmic reticulum (ER), because the IP
3-elicited Ca
2+ release occurs in the apical pole, which is dominated by zymogen granules (ZGs) and apparently contains very little ER. Using confocal and two-photon microscopy and a number of different ER-specific fluorescent probes, we have now investigated in detail the distribution of the ER in living pancreatic acinar cells. It turns out that although the bulk of the ER, as expected, is clearly located in the baso-lateral part of the cell, there is significant invasion of ER into the granular pole and each ZG is in fact surrounded by strands of ER. This structural evidence from living cells, in conjunction with recent functional studies demonstrating the high Ca
2+ mobility in the ER lumen, provides the framework for a coherent and internally consistent theory for cytosolic Ca
2+ signal generation in the apical secretory pole, in which the primary Ca
2+ release occurs from ER extensions in the granular pole supplied with Ca
2+ from the main store at the base of the cell by the tunnel function of the ER.
Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is ...generally referred to as 'accidental cell death' (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. 'Regulated cell death' (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death.
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