Induction of heme oxygenase-1 and heat shock protein 70 in rat hepatocytes: The role of calcium signaling

Stress response genes including heat shock proteins are induced under a variety of conditions to confer cellular protection. This study investigated the role of calcium signaling in the induction of two stress response genes, heme oxygenase-1/hsp32 and hsp70, in isolated rat hepatocytes. Both genes were induced by cellular glutathione depletion. This induction could be inhibited by BAPTA-AM. Culturing in a calcium-free medium prevented the induction of hsp70 gene expression after glutathione depletion without affecting heme oxygenase-1 gene expression. Thapsigargin increased the gene expression of heme oxygenase-1 but not that of hsp70. Thapsigargin-induced heme oxygenase-1 induction was completely inhibited by BAPTA-AM. Incubation with the Ca²⁺-ionophore A23187 augmented heme oxygenase-1 (two-fold) and hsp70 (5.2-fold) mRNA levels. Our data suggests a significant role of Ca²⁺-dependent pathways in the induction of the two stress genes. An increase in the cytoplasmic Ca²⁺ activity seems to play a key role in the cascade of signaling leading to the induction of the two genes. However, the source of Ca²⁺ that fluxes into the cytoplasm seems to be different. Our data provides evidence for a compartmentalization of calcium fluxes, i.e. the Ca²⁺ flux from intracellular stores (e.g. the endoplasmic reticulum) plays a major role in the induction of heme oxygenase-1. By contrast, Ca²⁺ flux from the extracellular medium seems to be a mechanism initiating the cellular signaling cascade leading to hsp70 gene induction.     

Molecular control of mitochondrial calcium uptake

The recently identified Mitochondrial Calcium Uniporter (MCU) is the protein of the inner mitochondrial membrane responsible for Ca²⁺ uptake into the matrix, which plays a role in the control of cellular signaling, aerobic metabolism and apoptosis. At least two properties of mitochondrial calcium signaling are well defined: (i) mitochondrial Ca²⁺ uptake varies greatly among different cells and tissues, and (ii) channel opening is strongly affected by extramitochondrial Ca²⁺ concentration, with low activity at resting and high capacity after cellular stimulation. It is now becoming clear that these features of the mitochondrial Ca²⁺ uptake machinery are not embedded in the MCU protein itself, but are rather due to the contribution of several MCU interactors. The list of the components of the MCU complex is indeed rapidly growing, thus revealing an unexpected complexity that highlights the pleiotropic role of mitochondrial calcium signaling.     

Modeling Ca2+ signaling differentiation during oocyte maturation

Ca²⁺ is a fundamental intracellular signal that mediates a variety of disparate physiological functions often in the same cell. Ca²⁺ signals span a wide range of spatial and temporal scales, which endow them with the specificity required to induce defined cellular functions. Furthermore, Ca²⁺ signaling is highly plastic as it is modulated dynamically during normal physiological development and under pathological conditions. However, the molecular mechanisms underlying Ca²⁺ signaling differentiation during cellular development remain poorly understood. Oocyte maturation in preparation for fertilization provides an exceptionally well-suited model to elucidate Ca²⁺ signaling regulation during cellular development. This is because a Ca²⁺ signal with specialized spatial and temporal dynamics is universally essential for egg activation at fertilization. Here we use mathematical modeling to define the critical determinants of Ca²⁺ signaling differentiation during oocyte maturation. We show that increasing IP₃ receptor (IP₃R) affinity replicates both elementary and global Ca²⁺ dynamics observed experimentally following oocyte maturation. Furthermore, our model reveals that because of the Ca²⁺ dependency of both SERCA and the IP₃R, increased IP₃R affinity shifts the system's equilibrium to a new steady state of high cytosolic Ca²⁺, which is essential for fertilization. Therefore our model provides unique insights into how relatively small alterations of the basic molecular mechanisms of Ca²⁺ signaling components can lead to dramatic alterations in the spatio-temporal properties of Ca²⁺ dynamics.     

Delineating Calcium Signaling Machinery in Plants: Tapping the Potential through Functional Genomics

Plants have developed calcium (Ca²⁺) signaling as an important mechanism of  regulation of  stress perception,  developmental cues, and  responsive gene  expression. The  post-genomic era has witnessed the successful unravelling of the functional characterization of genes and the creation of large datasets of molecular information. The major elements of Ca²⁺ signaling machinery include Ca²⁺ sensors and responders such as Calmodulins (CaMs), Calmodulin-like proteins (CMLs), Ca²⁺/CaM-dependent protein kinases (CCaMKs), Ca²⁺-dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) as well as transporters, such as Cyclic nucleotide-gated channels (CNGCs), Glutamate-like receptors (GLRs), Ca²⁺-ATPases, Ca²⁺/H⁺ exchangers (CAXs) and mechanosensitive channels. These elements play an important role in the regulation of physiological processes and plant responses to various stresses. Detailed genomic analysis can help us in the identification of potential molecular targets that can be exploited towards the development of stress-tolerant crops. The information sourced from model systems through omics approaches helps in the prediction and simulation of regulatory networks involved in responses to different stimuli at the molecular and cellular levels. The molecular delineation of Ca²⁺ signaling pathways could be a stepping stone for engineering climate-resilient crop plants. Here, we review the recent developments in Ca²⁺ signaling in the context of transport, responses, and adaptations significant for crop improvement through functional genomics approaches.     

Mitochondria and calcium signaling in embryonic development

Calcium (Ca²⁺) is a simple but critical signal for controlling various cellular processes and is especially important in fertilization and embryonic development. The dynamic change of cellular Ca²⁺ concentration and homeostasis are tightly regulated. Cellular Ca²⁺ increases by way of Ca²⁺ influx from extracellular medium and Ca²⁺ release from cellular stores of the endoplasmic reticulum (ER) and sarcoplasmic reticulum (SR). The elevated Ca²⁺ is subsequently sequestered by expelling it out of the cell or by pumping back to the ER/SR. Mitochondria function as a power house for energy production via oxidative phosphorylation in most eukaryotes. In addition to this well-known function, mitochondria are also recognized to regulate Ca²⁺ homeostasis through different mechanisms. Although critical roles of Ca²⁺ signaling in fertilization and embryonic development are known, the involvement of mitochondria in these processes are not fully understood. This review is focused on the role of mitochondrial respiratory chain complex I in the regulation of Ca²⁺ signaling pathway and gene expression in embryonic development, especially on the new findings in the cardiac development of Xenopus embryos. The data demonstrate that mitochondria modulate Ca²⁺ signaling and the Ca²⁺-dependent NFAT pathway and its target gene which are essential for embryonic heart development.     

Imaging of mitochondrial Ca2+ dynamics in astrocytes using cell-specific mitochondria-targeted GCaMP5G/6s: Mitochondrial Ca2+ uptake and cytosolic Ca2+ availability via the endoplasmic reticulum store

Mitochondrial Ca²⁺ plays a critical physiological role in cellular energy metabolism and signaling, and its overload contributes to various pathological conditions including neuronal apoptotic death in neurological diseases. Live cell mitochondrial Ca²⁺ imaging is an important approach to understand mitochondrial Ca²⁺ dynamics. Recently developed GCaMP genetically-encoded Ca²⁺ indicators provide unique opportunity for high sensitivity/resolution and cell type-specific mitochondrial Ca²⁺ imaging. In the current study, we implemented cell-specific mitochondrial targeting of GCaMP5G/6s (mito-GCaMP5G/6s) and used two-photon microscopy to image astrocytic and neuronal mitochondrial Ca²⁺ dynamics in culture, revealing Ca²⁺ uptake mechanism by these organelles in response to cell stimulation. Using these mitochondrial Ca²⁺ indicators, our results show that mitochondrial Ca²⁺ uptake in individual mitochondria in cultured astrocytes and neurons can be seen after stimulations by ATP and glutamate, respectively. We further studied the dependence of mitochondrial Ca²⁺ dynamics on cytosolic Ca²⁺ changes following ATP stimulation in cultured astrocytes by simultaneously imaging mitochondrial and cytosolic Ca²⁺ increase using mito-GCaMP5G and a synthetic organic Ca²⁺ indicator, x-Rhod-1, respectively. Combined with molecular intervention in Ca²⁺ signaling pathway, our results demonstrated that the mitochondrial Ca²⁺ uptake is tightly coupled with inositol 1,4,5-trisphosphate receptor-mediated Ca²⁺ release from the endoplasmic reticulum and the activation of G protein-coupled receptors. The current study provides a novel approach to image mitochondrial Ca²⁺ dynamics as well as Ca²⁺ interplay between the endoplasmic reticulum and mitochondria, which is relevant for neuronal and astrocytic functions in health and disease.     

Expression of mRNA Encoding Mcu and Other Mitochondrial Calcium Regulatory Genes Depends on Cell Type,Neuronal Subtype,and Ca2+ Signaling

Uptake of Ca²⁺ into the mitochondrial matrix controls cellular metabolism and survival-death pathways. Several genes are implicated in controlling mitochondrial Ca²⁺ uptake (mitochondrial calcium regulatory genes, MCRGs), however, less is known about the factors which influence their expression level. Here we have compared MCRG mRNA expression, in neural cells of differing type (cortical neurons vs. astrocytes), differing neuronal subtype (CA3 vs. CA1 hippocampus) and in response to Ca²⁺ influx, using a combination of qPCR and RNA-seq analysis. Of note, we find that the Mcu-regulating Micu gene family profile differs substantially between neurons and astrocytes, while expression of Mcu itself is markedly different between CA3 and CA1 regions in the adult hippocampus. Moreover, dynamic control of MCRG mRNA expression in response to membrane depolarization-induced Ca²⁺ influx is also apparent, resulting in repression of Letm1, as well as Mcu. Thus, the mRNA expression profile of MCRGs is not fixed, which may cause differences in the coupling between cytoplasmic and mitochondrial Ca²⁺, as well as diversity of mitochondrial Ca²⁺ uptake mechanisms.     

Calcium signaling and epigenetics: A key point to understand carcinogenesis

Calcium (Ca²⁺) signaling controls a wide range of cellular processes, including the hallmarks of cancer. The Ca²⁺ signaling system encompasses several types of proteins, such as receptors, channels, pumps, exchangers, buffers, and sensors, of which several are mutated or with altered expression in cancer cells. Since epigenetic mechanisms are disrupted in all stages of carcinogenesis, and reversibly regulate gene expression, they have been studied by different research groups to understand their role in Ca²⁺ signaling remodeling in cancer cells and the carcinogenic process. In this review, we link Ca²⁺ signaling, cancer, and epigenetics fields to generate a comprehensive landscape of this complex group of diseases.     

Calcium decoding mechanisms in plants

Ca²⁺ is a crucial second messenger that is involved in mediating responses to various biotic and abiotic environmental cues and in the regulation of many developmental processes in plants. Intracellular Ca²⁺ signals are realized by spatially and temporally defined changes in Ca²⁺ concentration that represent stimulus-specific Ca²⁺ signatures. These Ca²⁺ signatures are sensed, decoded and transmitted to downstream responses by a complex tool kit of Ca²⁺ binding proteins that function as Ca²⁺ sensors. Plants possess an extensive and complex array of such Ca²⁺ sensors that convey the information presented in the Ca²⁺ signatures into phosphorylation events, changes in protein-protein interactions or regulation of gene expression. Prominent Ca²⁺ sensors like, Calmodulins (CaM), Calmodulin-like proteins (CMLs), calcium dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs) exist in complex gene families and form intricate signaling networks in plants that are capable of robust and flexible information processing. In this review we reflect on the recently gained knowledge about the mechanistic principles of these Ca²⁺ sensors, their biochemical properties, physiological functions and newly identified targets proteins. These aspects will be discussed in the context of emerging functional principles that govern the information processing via these signaling modules.