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.     

Reduction in neuronal L-type calcium channel activity in a double knock-in mouse model of Alzheimer's disease

Increased function of neuronal L-type voltage-sensitive Ca^2 + channels (L-VSCCs) is strongly linked to impaired memory and altered hippocampal synaptic plasticity in aged rats. However, no studies have directly assessed L-VSCC function in any of the common mouse models of Alzheimer's disease where neurologic deficits are typically more robust. Here, we used cell-attached patch-clamp recording techniques to measure L-VSCC activity in CA1 pyramidal neurons of partially dissociated hippocampal “zipper” slices prepared from 14-month-old wild-type mice and memory-impaired APP/PS1 double knock-in mice. Surprisingly, the functional channel density of L-VSCCs was significantly reduced in the APP/PS1 group. No differences in voltage dependency and unitary conductance of L-VSCCs were observed. The results suggest that mechanisms for Ca^2 + dysregulation can differ substantially between animal models of normal aging and models of pathological aging.     

Distribution of Preproatrial Natriuretic Peptide mRNA in Rat Brain Detected by In Situ Hybridization of DNA Oligonucleotides: Enrichment in Hypothalamic and Limbic Regions

The expression and distribution of mRNA encoding preproatrial natriuretic peptide (ppANP) in rat brain has been investigated by in situ hybridization of two 35S-labeled synthetic DNA oligonucleotides, based on a cDNA clone sequence that encodes rat ppANP. The highest relative concentrations of ppANP mRNA were detected in the medial preoptic hypothalamic nucleus ("anteroventral/third ventricle region") and the medial habenula. Moderate concentrations of ppANP mRNA were observed in the CA1 pyramidal cells of the hippocampus, the endopiriform nucleus, the arcuate nucleus, the zona incerta, and cells of the pontine tegmental and peduculopontine nuclei. Several of these regions, including the habenula and the hypothalamic areas, have previously been reported to contain atrial natriuretic peptide (ANP)-like immunoreactivity, but the expression of ppANP mRNA in CA1 pyramidal cells suggests the occurrence of differential translation of ppANP mRNA into protein product in different brain regions, or the existence of different immunological forms of the peptide. The abundance of ppANP mRNA in brain was relatively low in comparison with that previously reported for many other mRNA species encoding other brain neuropeptides. These results demonstrate that ANP gene expression occurs in discrete neuronal populations of the CNS and that studies of the regulation of this expression should now be possible using quantitative in situ hybridization.     

Mitochondrial potassium channels and uncoupling proteins in synaptic plasticity and neuronal cell death

The function of the nervous system relies upon synaptic transmission, a process in which a neurotransmitter released from pre-synaptic terminals of one neuron (in response to membrane depolarization and calcium influx) activates post-synaptic receptors on dendrites of another neuron. Synapses are subjected to repeated bouts of oxidative and metabolic stress as the result of changing ion gradients and ATP usage. Mitochondria play central roles in meeting the demands of synapses for ATP and in regulating calcium homeostasis, and mitochondrial dysfunction can cause dysfunction and degeneration of synapses, and can trigger cell death. We have identified two types of mitochondrial proteins that serve the function of protecting synapses and neurons against dysfunction and death. Mitochondrial ATP-sensitive potassium (MitoKATP) channels modulate inner membrane potential and oxyradical production; mitochondrial potassium fluxes can affect cytochrome c release and caspase activation and may determine whether neurons live or die in experimental models of stroke and Alzheimer's disease. Uncoupling proteins (UCPs) are a family of mitochondrial membrane proteins that uncouple electron transport from ATP production by transporting protons across the inner membrane. Neurons express at least three UCPs including the widely expressed UCP-2 and the neuron-specific UCP-4 and UCP-5 (BMCP-1). We have found that UCP-4 protects neurons against apoptosis by a mechanism involving suppression of oxyradical production and stabilization of cellular calcium homeostasis. The expression of UCP-4 is itself regulated by changes in energy metabolism. In addition to their roles in neuronal cell survival and death, MitoKATP channels and UCPs may play roles in regulating neuronal differentiation during development and synaptic plasticity in the adult.     

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.     

Delayed cytoprotection induced by hypoxic preconditioning in cultured neonatal rat cardiomyocytes: role of GRP78

Hypoxic preconditioning (HPC) has been well demonstrated to have potent protective effects in many cell types; however, the mechanisms responsible for this phenomenon are not fully understood. Recently, glucose-regulated protein 78 (GRP78), an inducible molecular chaperon, was indicated to be associated with ischemic preconditioning. We hypothesized that HPC protects cardiomyocytes against hypoxia by inducing GRP78 in cultured neonatal rat cardiomyocytes. HPC was induced by exposing cardiomyocytes to brief hypoxia (1% O(2), 30 min) followed by reoxygenation. GRP78 was expressed constitutively in cultured cardiomyocytes and its expression was enhanced at 12 h, peaked at 24 h (207.3+/-23.6% of the baseline), and was sustained for up to 72 h after HPC. Twenty-four hours after HPC, the myocytes were subjected to prolonged hypoxia (1% O(2), 12 h). The lactic dehydrogenase (LDH) release and malondialdehyde (MDA) content were reduced, while cell viability and superoxide dismutase (SOD) activity were increased in the preconditioned cells compared with the non-HPC cells. The GRP78 protein level was higher in cells exposed to both HPC and hypoxia than in the cells exposed to HPC alone or hypoxia alone. Heat shock protein 70 (HSP70) was induced in parallel by late HPC. Transfection of GRP78 antisense oligonucleotides blocked GRP78 expression but not HSP70, resulting in attenuated cardioprotection afforded by late HPC. Furthermore, inducing GRP78 by gene transfer protected cardiomyocytes from hypoxic injury. These findings demonstrate that the induction of GRP78 partially mediates the late HPC, suggesting that GRP78 is a novel mechanism responsible for the late cytoprotection of HPC.     

A systems-theoretical framework for health and disease: inflammation and preconditioning from an abstract modeling point of view

Modern advances in molecular biology have produced enormous amounts of data characterizing physiological and disease states in cells and organisms. While bioinformatics has facilitated the organizing and mining of these data, it is the task of systems biology to merge the available information into dynamic, explanatory and predictive models. This article takes a step into this direction. It proposes a conceptual approach toward formalizing health and disease and illustrates it in the context of inflammation and preconditioning. Instead of defining health and disease states, the emphasis is on simplexes in a high-dimensional biomarker space. These simplexes are bounded by physiological constraints and permit the quantitative characterization of personalized health trajectories, health risk profiles that change with age, and the efficacy of different treatment options. The article mainly focuses on concepts but also briefly describes how the proposed concepts might be formulated rigorously within a mathematical framework.     

Junctophilin type 2 is associated with caveolin-3 and is down-regulated in the hypertrophic and dilated cardiomyopathies

Functional coupling between the sarcolemmal membrane and the sarcoplasmic reticulum is based on distinct structures called junctional membrane complexes (JMCs). Recently, junctophilins are found to be responsible for normal formation of JMCs. In the present study, we found that junctophilin type 2 (JP-2), a unique isoform in the heart, was localized in caveolin-rich membranes, and that the expression of JP-2 was up-regulated during normal development and down-regulated in a hypertrophic or a dilated cardiomyopathic mouse model. The expression levels of JP-2 may be associated with the development of T-tubules and impaired Ca(2+)-induced Ca(2+) release in the heart.     

A novel neural-specific BMP antagonist, Brorin-like, of the Chordin family

We identified a gene encoding a novel secreted protein in mice, humans, and zebrafish. As the protein of 222 amino acids is similar to Brorin, a secreted BMP antagonist, which is a member of the Chordin family, we named it Brorin-like. Recombinant Brorin-like protein weakly but significantly inhibited the activity of BMP in mouse preosteoblastic cells and promoted neurogenesis in mouse neural precursor cells. Brorin-like was predominantly expressed in the adult brain and embryonic neural tissues. The inhibition of Brorin-like functions in zebrafish resulted in the impairment of neural development. Brorin-like potentially plays roles in neural development and functions.     

Decreased Hippocampal Expression, but Not Functionality, of GABAB Receptors After Transient Cerebral Ischemia in Rats

Abstract : We have examined the effects of transient global ischemia on both the gene expression levels and the functionality of GABA_B receptors in rat brain, using antisense in situ hybridization and electrophysiological evaluations. At the level of gene expression, no significant change in GABA_B receptor expression was observed in any hippocampal subfield at either 6 or 12 h after challenge. At 24 h postchallenge, however, a significant decrease in GABA_B receptor expression was observed in both the CA1 and CA3 subfields, whereas no change was observed in the dentate granule cell layer. Although expression in both the vulnerable CA1 and less vulnerable CA3 subfields was diminished at this time postchallenge, there was no significant difference in the degree of the diminished expression between these subfields. At the functional level, the dose-dependent ability of baclofen (1-100 μ M ) to inhibit an evoked excitatory postsynaptic potential (f-EPSP) in the CA1 subfield was evaluated at 24 h postischemia, in comparison with the dose-response observed in sham-operated subjects. No significant differences were observed in the efficacy of GABA_B receptor-mediated inhibition of the elicited f-EPSP at any of the baclofen concentrations examined. These data demonstrate that although the mRNA expression levels for the GABA_B receptor are diminished in both vulnerable and less vulnerable neurons of Ammon's horn at 24 h following transient global ischemia, the functionality of the GABA_B receptor system is maintained at this time postchallenge.     

A Myxococcus xanthus cell density-sensing system required for multicellular development

Abstract Progression through early Myxococcus xanthus multicellular fruiting body development requires the generation of and response to extracellular A signal. Extracellular A signal is a specific set of amino acids at an extracellular concentration greater than 10 μM. It functions as a cell density signal during starvation that allows the cells to sense that a minimal cell density has been reached and development can proceed. The generation of extracellular A signal requires the products of three asg genes. They have recently been identified as AsgA, a fused two-component histidine protein kinase and response regulator; AsgB, a putative DNA-binding protein; and AsgC, the M. xanthus major sigma factor. Other elements of the A signaling pathway map to the sasB locus and appear to be A signal transducers. These elements are regulators of the earliest A signal-dependent gene, whose promoter is a member of the sigma-54 family. Continued study of the A signaling pathway is expected to identify additional components of this network required for the complex behavioural response of fruiting body formation.     

Reciprocal altruism in birds: A critical review

Many proposed examples of reciprocal altruism are either misidentified or involve questionable assumptions concerning the costs and benefits accruing to the interactors. Waltz's (Am. Nat. 118: 588–592, 1981) definition of reciprocal altruism as an interaction in which “one individual aids another in anticipation that the recipient will return the favor benefiting the actor in the future” is not sufficiently restrictive: there must also be a direct fitness cost to the individual performing the original beneficent act that is less than the fitness benefit received when the act is reciprocated (again at a cost) by the second individual.Several recurring problems in identifying potential examples of reciprocal altruism are discussed, including the assumption that restraint is an act of altruism and the misclassification of “generational mutualisms,” in which individuals helping to raise young are “repaid” one generation later by the offspring they assisted in raising. No definite case of reciprocal altruism is currently known in birds, but examples in which this phenomenon may be involved include helping behavior in a few cooperative breeders and communal feeding in several taxa including gulls, jays, and juncos.     

GABA neurons in seizure disorders: A review of immunocytochemical studies

Immunocytochemical studies have identified alterations in GABA neurons in several models of seizure disorders. However, the changes have varied among different epilepsy models, and these variations presumably reflect the diversity of mechanisms that can lead to seizure disorders. In models of cortical focal epilepsy, there is strong evidence fordecreases in the number of GABAergic elements, and the changes closely parallel the time course of seizure development. By contrast, in some genetic models of epilepsy,increases in the number of immunocytochemically-detectable neurons have been observed in selected brain regions. In several models of temporal lobe epilepsy, there presently is little immunocytochemical evidence for alterations of GABA neurons within the hippocampal formation despite physiological demonstrations of decreased GABA-mediated inhibition in this region. However, it remains possible that certain types of GABA neurons could be differentially affected in some seizure disorders while other types are preserved. Thus, distinguishing between different classes of GABA neurons and determining their functional roles represent major challenges for future studies of GABA neurons in seizure disorders.Special issue dedicated to Dr. Eugene Roberts.