Ca2+ signaling and calcium binding chaperones of the endoplasmic reticulum

Acidic calcium stores of Saccharomyces cerevisiae

Fungi and animals constitute sister kingdoms in the eukaryotic domain of life. The major classes of transporters, channels, sensors, and effectors that move and respond to calcium ions were already highly networked in the common ancestor of fungi and animals. Since that time, some key components of the network have been moved, altered, relocalized, lost, or duplicated in the fungal and animal lineages and at the same time some of the regulatory circuitry has been dramatically rewired. Today the calcium transport and signaling networks in fungi provide a fresh perspective on the scene that has emerged from studies of the network in animal cells. This review provides an overview of calcium signaling networks in fungi, particularly the model yeast Saccharomyces cerevisiae, with special attention to the dominant roles of acidic calcium stores in fungal cell physiology.

Ca2+ homeostasis modulation enhances the amenability of L444P glucosylcerebrosidase to proteostasis regulation in patient-derived fibroblasts

Gaucher's disease is caused by deficiency of lysosomal glucocerebrosidase (GC) activity and accumulation of GC substrate, glucosylceramide. A number of point mutations in GC encoding gene have been reported to destabilize the enzyme native structure, resulting in protein misfolding and degradation. Particularly, the L444P GC variant, often associated with neuropathic manifestations of the disease, is severely destabilized and immediately degraded, resulting in complete loss of enzymatic activity. In addition, glucosylceramide accumulation causes Ca(2+) efflux from the endoplasmic reticulum (ER) through ryanodine receptors (RyRs) in the neurons of Gaucher's disease patients. We hypothesized that excessive [Ca(2+)](ER) efflux impairs ER folding and studied how modulation of [Ca(2+)](ER) affects folding of L444P GC in patient-derived fibroblasts. We report that RyRs blockers mediated [Ca(2+)] modulation, recreating a "wild type-like" folding environment in the ER, more amenable to rescuing the folding of mutated L444P GC through proteostasis regulation. Treating patient-derived fibroblasts with a RyRs blocker and a proteostasis modulator, MG-132, results in enhanced folding, trafficking, and activity of the severely destabilized L444P GC variant. Global gene expression profiling and mechanistic studies were conducted to investigate the folding quality control expression pattern conducive to native folding of mutated L444P GC and revealed that the ER-lumenal BiP/GRP78 plays a key role in the biogenesis of this GC variant.

Antarctic krill 454 pyrosequencing reveals chaperone and stress transcriptome

BACKGROUND

The Antarctic krill Euphausia superba is a keystone species in the Antarctic food chain. Not only is it a significant grazer of phytoplankton, but it is also a major food item for charismatic megafauna such as whales and seals and an important Southern Ocean fisheries crop. Ecological data suggest that this species is being affected by climate change and this will have considerable consequences for the balance of the Southern Ocean ecosystem. Hence, understanding how this organism functions is a priority area and will provide fundamental data for life history studies, energy budget calculations and food web models.

METHODOLOGY/PRINCIPAL FINDINGS

The assembly of the 454 transcriptome of E. superba resulted in 22,177 contigs with an average size of 492bp (ranging between 137 and 8515bp). In depth analysis of the data revealed an extensive catalogue of the cellular chaperone systems and the major antioxidant proteins. Full length sequences were characterised for the chaperones HSP70, HSP90 and the super-oxide dismutase antioxidants, with the discovery of potentially novel duplications of these genes. The sequence data contained 41,470 microsatellites and 17,776 Single Nucleotide Polymorphisms (SNPs/INDELS), providing a resource for population and also gene function studies.

CONCLUSIONS

This paper details the first 454 generated data for a pelagic Antarctic species or any pelagic crustacean globally. The classical "stress proteins", such as HSP70, HSP90, ferritin and GST were all highly expressed. These genes were shown to be over expressed in the transcriptomes of Antarctic notothenioid fish and hypothesized as adaptations to living in the cold, with the associated problems of decreased protein folding efficiency and increased vulnerability to damage by reactive oxygen species. Hence, these data will provide a major resource for future physiological work on krill, but in particular a suite of "stress" genes for studies understanding marine ectotherms' capacities to cope with environmental change.

Adult onset cardiac dilatation in a transgenic mouse line with Galβ1,3GalNAc α2,3-sialyltransferase II (ST3Gal-II) transgenes: a new model for dilated cardiomyopathy

Sugar chain abnormalities in glycolipids and glycoproteins are associated with various diseases. Here, we report an adult onset cardiac dilatation in a transgenic mouse line with Galβ1,3GalNAc α2,3-sialyltransferase II (ST3Gal-II) transgenes. The transgenic hearts at the end-stage, at around 7 months old, were enlarged, with enlarged cavities and thin, low-tensile walls, typical of dilated cardiomyopathy. Although no apparent change was found in heart gangliosides, glycosylation of heart proteins was altered. Interestingly, sugar moieties not directly related to the ST3Gal-II catalytic reaction were also changed. Significant increases in calreticulin and calnexin were observed in hearts of the transgenic mice. These results suggest that expression of ST3Gal-II transgenes induces abnormal protein glycosylation, which disorganizes the endoplasmic/sarcoplasmic reticulum quality control system and elevates the calreticulin/calnexin level, resulting in suppression of cardiac function. The transgenic mice showed 100% incidence of adult onset cardiac dilatation, suggesting great potential as a new model for dilated cardiomyopathy.

PERK activation at low glucose concentration is mediated by SERCA pump inhibition and confers preemptive cytoprotection to pancreatic β-cells

A decrease in the ATP/energy status of the cell in response to a decrease in glucose concentration results in SERCA pump inhibition, the efflux of Ca²⁺ from the ER and the activation of PERK, which plays an important role in both pancreatic β-cell function and survival.

Protein kinase R-like ER kinase (PERK) is activated at physiologically low glucose concentrations in pancreatic β-cells. However, the molecular mechanisms by which PERK is activated under these conditions and its role in β-cell function are poorly understood. In this report, we investigated, in dispersed rat islets of Langerhans and mouse insulinoma-6 (MIN6) cells, the relationship between extracellular glucose concentration, the free endoplasmic reticulum (ER) calcium concentration ([Ca²⁺]_ER) measured directly using an ER targeted fluorescence resonance energy transfer-based calcium sensor, and the activation of PERK. We found that a decrease in glucose concentration leads to a concentration-dependent reduction in [Ca²⁺]_ER that parallels the activation of PERK and the phosphorylation of its substrate eukaryotic initiation factor-2α. We provide evidence that this decrease in [Ca²⁺]_ER is caused by a decrease in sarcoplasmic/ER Ca²⁺-ATPase pump activity mediated by a reduction in the energy status of the cell. Importantly, we also report that PERK-dependent eukaryotic initiation factor-2α phosphorylation at low glucose concentration plays a significant role in 1) the regulation of both proinsulin and global protein synthesis, 2) cell viability, and 3) conferring preemptive cytoprotection against ER stress. Taken together, these results provide evidence that a decrease in the ATP/energy status of the cell in response to a decrease in glucose concentration results in sarcoplasmic/ER Ca²⁺-ATPase pump inhibition, the efflux of Ca²⁺ from the ER, and the activation of PERK, which plays an important role in both pancreatic β-cell function and survival.

Prion protein misfolding affects calcium homeostasis and sensitizes cells to endoplasmic reticulum stress

Prion-related disorders (PrDs) are fatal neurodegenerative disorders characterized by progressive neuronal impairment as well as the accumulation of an abnormally folded and protease resistant form of the cellular prion protein, termed PrP^RES. Altered endoplasmic reticulum (ER) homeostasis is associated with the occurrence of neurodegeneration in sporadic, infectious and familial forms of PrDs. The ER operates as a major intracellular calcium store, playing a crucial role in pathological events related to neuronal dysfunction and death. Here we investigated the possible impact of PrP misfolding on ER calcium homeostasis in infectious and familial models of PrDs. Neuro2A cells chronically infected with scrapie prions showed decreased ER-calcium content that correlated with a stronger upregulation of UPR-inducible chaperones, and a higher sensitivity to ER stress-induced cell death. Overexpression of the calcium pump SERCA stimulated calcium release and increased the neurotoxicity observed after exposure of cells to brain-derived infectious PrP^RES. Furthermore, expression of PrP mutants that cause hereditary Creutzfeldt-Jakob disease or fatal familial insomnia led to accumulation of PrP^RES and their partial retention at the ER, associated with a drastic decrease of ER calcium content and higher susceptibility to ER stress. Finally, similar results were observed when a transmembrane form of PrP was expressed, which is proposed as a neurotoxic intermediate. Our results suggest that alterations in calcium homeostasis and increased susceptibility to ER stress are common pathological features of both infectious and familial PrD models.

Cellular stress responses, the hormesis paradigm, and vitagenes: novel targets for therapeutic intervention in neurodegenerative disorders

Despite the capacity of chaperones and other homeostatic components to restore folding equilibrium, cells appear poorly adapted for chronic oxidative stress that increases in cancer and in metabolic and neurodegenerative diseases. Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This article introduces the concept of hormesis and its applications to the field of neuroprotection. It is argued that the hormetic dose response provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose-response relationships, their mechanistic foundations, and their relationship to the concept of biological plasticity, as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This article describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways, including sirtuin and Nrf2 and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. Particular attention is given to the emerging role of nitric oxide, carbon monoxide, and hydrogen sulfide gases in hormetic-based neuroprotection and their relationship to membrane radical dynamics and mitochondrial redox signaling.

Ca(2+) sources for the exocytotic release of glutamate from astrocytes

Astrocytes can exocytotically release the gliotransmitter glutamate from vesicular compartments. Increased cytosolic Ca(2+) concentration is necessary and sufficient for this process. The predominant source of Ca(2+) for exocytosis in astrocytes resides within the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate and ryanodine receptors of the ER provide a conduit for the release of Ca(2+) to the cytosol. The ER store is (re)filled by the store-specific Ca(2+)-ATPase. Ultimately, the depleted ER is replenished by Ca(2+) which enters from the extracellular space to the cytosol via store-operated Ca(2+) entry; the TRPC1 protein has been implicated in this part of the astrocytic exocytotic process. Voltage-gated Ca(2+) channels and plasma membrane Na(+)/Ca(2+) exchangers are additional means for cytosolic Ca(2+) entry. Cytosolic Ca(2+) levels can be modulated by mitochondria, which can take up cytosolic Ca(2+) via the Ca(2+) uniporter and release Ca(2+) into cytosol via the mitochondrial Na(+)/Ca(2+) exchanger, as well as by the formation of the mitochondrial permeability transition pore. The interplay between various Ca(2+) sources generates cytosolic Ca(2+) dynamics that can drive Ca(2+)-dependent exocytotic release of glutamate from astrocytes. An understanding of this process in vivo will reveal some of the astrocytic functions in health and disease of the brain. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Decoded calreticulin-deficient embryonic stem cell transcriptome resolves latent cardiophenotype

Genomic perturbations that challenge normal signaling at the pluripotent stage may trigger unforeseen ontogenic aberrancies. Anticipatory systems biology identification of transcriptome landscapes that underlie latent phenotypes would offer molecular diagnosis before the onset of symptoms. The purpose of this study was to assess the impact of calreticulin-deficient embryonic stem cell transcriptomes on molecular functions and physiological systems. Bioinformatic surveillance of calreticulin-null stem cells, a monogenic insult model, diagnosed a disruption in transcriptome dynamics, which re-prioritized essential cellular functions. Calreticulin-calibrated signaling axes were uncovered, and network-wide cartography of undifferentiated stem cell transcripts suggested cardiac manifestations. Calreticulin-deficient stem cell-derived cardiac cells verified disorganized sarcomerogenesis, mitochondrial paucity, and cytoarchitectural aberrations to validate calreticulin-dependent network forecasts. Furthermore, magnetic resonance imaging and histopathology detected a ventricular septal defect, revealing organogenic manifestation of calreticulin deletion. Thus, bioinformatic deciphering of a primordial calreticulin-deficient transcriptome decoded at the pluripotent stem cell stage a reconfigured multifunctional molecular registry to anticipate predifferentiation susceptibility toward abnormal cardiophenotype.

Molecular characterization of the endoplasmic reticulum: insights from proteomic studies

The endoplasmic reticulum (ER) is a multifunctional intracellular organelle responsible for the synthesis, processing and trafficking of a wide variety of proteins essential for cell growth and survival. Therefore, comprehensive characterization of the ER proteome is of great importance to the understanding of its functions and has been actively pursued in the past decade by scientists in the proteomics field. This review summarizes major proteomic studies published in the past decade that focused on the ER proteome. We evaluate the data sets obtained from two different organs, liver and pancreas each of which contains a primary cell type (hepatocyte and acinar cell) with specialized functions. We also discuss how the nature of the proteins uncovered is related to the methods of organelle purification, organelle purity and the techniques used for protein separation prior to MS. In addition, this review also puts emphasis on the biological insights gained from these studies regarding the molecular functions of the ER including protein synthesis and translocation, protein folding and quality control, ER-associated degradation and ER stress, ER export and membrane trafficking, calcium homeostasis and detoxification and drug metabolism.

Rab32 modulates apoptosis onset and mitochondria-associated membrane (MAM) properties

The mitochondria-associated membrane (MAM) has emerged as an endoplasmic reticulum (ER) signaling hub that accommodates ER chaperones, including the lectin calnexin. At the MAM, these chaperones control ER homeostasis but also play a role in the onset of ER stress-mediated apoptosis, likely through the modulation of ER calcium signaling. These opposing roles of MAM-localized chaperones suggest the existence of mechanisms that regulate the composition and the properties of ER membrane domains. Our results now show that the GTPase Rab32 localizes to the ER and mitochondria, and we identify this protein as a regulator of MAM properties. Consistent with such a role, Rab32 modulates ER calcium handling and disrupts the specific enrichment of calnexin on the MAM, while not affecting the ER distribution of protein-disulfide isomerase and mitofusin-2. Furthermore, Rab32 determines the targeting of PKA to mitochondrial and ER membranes and through its overexpression or inactivation increases the phosphorylation of Bad and of Drp1. Through a combination of its functions as a PKA-anchoring protein and a regulator of MAM properties, the activity and expression level of Rab32 determine the speed of apoptosis onset.

Gene expression in primary cultured astrocytes affected by aluminum: alteration of chaperons involved in protein folding

OBJECTIVES

Aluminum is notorious as a neurotoxic metal. The aim of our study was to determine whether endoplasmic reticulum (ER) stress is involved in aluminum-induced apoptosis in astrocytes.

METHODS

Mitochondrial RNA (mRNA) was analyzed by reverse transcription (RT)-PCR following pulse exposure of aluminum glycinate to primary cultured astrocytes. Tunicamycin was used as a positive control.

RESULTS

Gene expression analysis revealed that Ire1β was up-regulated in astrocytes exposed to aluminum while Ire1α was up-regulated by tunicamycin. Exposure to aluminum glycinate, in contrast to tunicamycin, seemed to down-regulate mRNA expression of many genes, including the ER resident molecular chaperone BiP/Grp78 and Ca(2+)-binding chaperones (calnexin and calreticulin), as well as stanniocalcin 2 and OASIS. The down-regulation or non-activation of the molecular chaperons, whose expressions are known to be protective by increasing protein folding, may spell doom for the adaptive response. Exposure to aluminum did not have any significant effects on the expression of Bax and Bcl2 in astrocytes.

CONCLUSIONS

The results of this study demonstrate that aluminum may induce apoptosis in astrocytes via ER stress by impairing the protein-folding machinery.

Endoplasmic reticulum Ca2+ increases enhance mutant glucocerebrosidase proteostasis

Altering intracellular calcium levels is known to partially restore mutant enzyme homeostasis in several lysosomal storage diseases, but why? We hypothesize that endoplasmic reticulum (ER) calcium level increases enhance the folding, trafficking and function of these mutant misfolding/degradation-prone lysosomal enzymes by increasing chaperone function. Herein, we report that increasing ER calcium levels by reducing ER calcium efflux through the ryanodine receptor (antagonists or RNAi) or by promoting ER calcium influx by SERCA2b overexpression enhances mutant glucocerebrosidase (GC) homeostasis in Gaucher’s disease patient-derived cells. Post-translational regulation of the calnexin folding pathway by increasing the ER calcium concentration appears to enhance the capacity of this chaperone system to fold mutant misfolding-prone enzymes, increasing the folded mutant GC population that can engage the trafficking receptor at the expense of ER-associated degradation, increasing the lysosomal GC concentration.

Oxidative protein folding in the endoplasmic reticulum: tight links to the mitochondria-associated membrane (MAM)

The production of secretory proteins at the ER (endoplasmic reticulum) depends on a ready supply of energy and metabolites as well as the close monitoring of the chemical conditions that favor oxidative protein folding. ER oxidoreductases and chaperones fold nascent proteins into their export-competent three-dimensional structure. Interference with these protein folding enzymes leads to the accumulation of unfolded proteins within the ER lumen, causing an acute organellar stress that triggers the UPR (unfolded protein response). The UPR increases the transcription of ER chaperones commensurate with the load of newly synthesized proteins and can protect the cell from ER stress. Persistant stress, however, can force the UPR to commit cells to undergo apoptotic cell death, which requires the emptying of ER calcium stores. Conversely, a continuous ebb and flow of calcium occurs between the ER and mitochondria during resting conditions on a domain of the ER that forms close contacts with mitochondria, the MAM (mitochondria-associated membrane). On the MAM, ER folding chaperones such as calnexin and calreticulin and oxidoreductases such as ERp44, ERp57 and Ero1alpha regulate calcium flux from the ER through reversible, calcium and redox-dependent interactions with IP3Rs (inositol 1,4,5-trisphophate receptors) and with SERCAs (sarcoplasmic/endoplasmic reticulum calcium ATPases). During apoptosis progression and depending on the identity of the ER chaperone and oxidoreductase, these interactions increase or decrease, suggesting that the extent of MAM targeting of ER chaperones and oxidoreductases could shift the readout of ER-mitochondria calcium exchange from housekeeping to apoptotic. However, little is known about the cytosolic factors that mediate the on/off interactions between ER chaperones and oxidoreductases with ER calcium channels and pumps. One candidate regulator is the multi-functional molecule PACS-2 (phosphofurin acidic cluster sorting protein-2). Recent studies suggest that PACS-2 mediates localization of a mobile pool of calnexin to the MAM in addition to regulating homeostatic ER calcium signaling as well as MAM integrity. Together, these findings suggest that cytosolic, membrane and lumenal proteins combine to form a two-way switch that determines the rate of protein secretion by providing ions and metabolites and that appears to participate in the pro-apoptotic ER-mitochondria calcium transfer.

Store-operated CRAC channels: function in health and disease

Elevation of cytosolic Ca(2+) levels through the activation of store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels is involved in mediating a disparate array of cellular responses. These include secretion, metabolism and gene expression, as well as cell growth and proliferation. Moreover, emerging evidence points to the involvement of aberrant CRAC channel activity in human diseases, such as certain types of immunodeficiency and autoimmunity disorders, allergy, and inflammatory bowel disease. This article summarizes recent advances in understanding the gating and function of CRAC channels, their links to human disease and key issues for the development of channel blockers.

Mutational analysis of calnexin

Calnexin is a type I endoplasmic reticulum lectin-like chaperone protein. In this study, we have used site-specific mutagenesis to investigate the functional importance of glutamate E351 found at the tip of the P-domain of calnexin, and tryptophan W428 found in the carbohydrate binding region of the globular domain of the protein. The E351 and W428 calnexin mutants lost the ability to inhibit aggregation of IgY (glycosylated substrate). The E351 mutation led to slightly enhanced ERp57 binding to calnexin, whereas W428 greatly enhanced binding of ERp57 to calnexin. These findings indicate that modification of a residue(s) in the carbohydrate binding region may have a profound effect on the structural and functional properties of the P-domain and consequently on association of calnexin with the folding enzyme ERp57.

The asthma-associated ORMDL3 gene product regulates endoplasmic reticulum-mediated calcium signaling and cellular stress

Alterations of protein folding or Ca(2+) levels within the endoplasmic reticulum (ER) result in the unfolded-protein response (UPR), a process considered as an endogenous inducer of inflammation. Thereby, understanding how genetic factors modify UPR is particularly relevant in chronic inflammatory diseases such as asthma. Here we identified that ORMDL3, the only genetic risk factor recently associated to asthma in a genome wide study, alters ER-mediated Ca(2+) homeostasis and facilitates the UPR. Heterologous expression of human ER-resident transmembrane ORMDL3 protein increased resting cytosolic Ca(2+) levels and reduced ER-mediated Ca(2+) signaling, an effect reverted by co-expression with the sarco-endoplasmic reticulum Ca(2+) pump (SERCA). Increased ORMDL3 expression also promoted stronger activation of UPR transducing molecules and target genes while siRNA-mediated knock-down of endogenous ORMDL3 potentiated ER Ca(2+) release and attenuated the UPR. In conclusion, our findings are consistent with a model in which ORMDL3 binds and inhibits SERCA resulting in a reduced ER Ca(2+) concentration and increased UPR. Thus, we provide a first insight into the molecular mechanism explaining the association of ORMDL3 with proinflammatory diseases.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Molecular cloning of calnexin and calreticulin in the Pacific oyster Crassostrea gigas and its expression in response to air exposure

Calnexin (CNX) and calreticulin (CRT) are endoplasmic reticulum (ER) chaperones. CNX is a type I transmembrane protein and CRT is a soluble CNX homologue. In the ER, CNX and CRT are important for Ca(2+) homeostasis and protein maturation. Here, we describe the full-length cDNA of the first mollusk CNX (cgCNX) and a second mollusk CRT (cgCRT) from the oyster Crassostrea gigas. CgCNX, containing 3255bp, was composed of a 1764bp open reading frame (ORF) that encodes a 588-amino acid protein. CgCRT, containing 1727bp, was composed of a 1242bp ORF that encodes a 414-amino acid protein. CgCNX and cgCRT contains an N-terminal 21- and 16-amino acid sequence, respectively, which is characteristic of a signal sequence. At the C-terminus, cgCRT also contains the KDEL (-Lys-Asp-Glu-Leu) peptide motif suggesting that cgCRT localizes in the ER. Northern blot analysis showed that both cgCNX and cgCRT mRNAs are induced by air exposure. The expression patterns of cgCNX mRNA differed from those of cgCRT during air exposure. This suggests that these two molecular chaperones have different roles in the response to air exposure.

Endoplasmic reticulum proteins in cardiac development and dysfunction

An understanding of cardiac pathologies and the molecular mechanisms thereof is essential for the development of therapies for cardiovascular disease, a common cause of death in Western societies. Investigations into heart diseases have shown that the endoplasmic reticulum and its diverse functions may lie at the center of many cardiac pathologies. Animal models have demonstrated that in numerous cases, faulty endoplasmic reticulum activity is manifested in defective cardiogenesis or impaired heart function. These findings suggest that the endoplasmic and sarcoplasmic reticulum membranes may represent functionally independent organelles responsible for specialized functions in the heart. This review addresses the molecular pathways linking endoplasmic reticulum function and malfunction with impaired cardiac phenotypes. The endoplasmic reticulum affects cardiac development and function through Ca2+-dependent pathways, its catalytic role in the proper folding and targeting of membrane-bound and secretory proteins, and its response to cellular stress events, particularly hypoxic conditions. These pathways present potential novel targets for treatment of cardiac disease.

Mitochondrial Ca2+ channels: Great unknowns with important functions

Mitochondria process local and global Ca(2+) signals. Thereby the spatiotemporal patterns of mitochondrial Ca(2+) signals determine whether the metabolism of these organelles is adjusted or cell death is executed. Mitochondrial Ca(2+) channels of the inner mitochondrial membrane (IMM) actually implement mitochondrial uptake from cytosolic Ca(2+) rises. Despite great efforts in the past, the identity of mitochondrial Ca(2+) channels is still elusive. Numerous studies aimed to characterize mitochondrial Ca(2+) uniport channels and provided a detailed profile of these great unknowns with important functions. This mini-review revisits previous research on the mechanisms of mitochondrial Ca(2+) uptake and aligns them with most recent findings.

Abnormal calcium homeostasis in peripheral neuropathies

Abnormal neuronal calcium (Ca2+) homeostasis has been implicated in numerous diseases of the nervous system. The pathogenesis of two increasingly common disorders of the peripheral nervous system, namely neuropathic pain and diabetic polyneuropathy, has been associated with aberrant Ca2+ channel expression and function. Here we review the current state of knowledge regarding the role of Ca2+ dyshomeostasis and associated mitochondrial dysfunction in painful and diabetic neuropathies. The central impact of both alterations of Ca2+ signalling at the plasma membrane and also intracellular Ca2+ handling on sensory neurone function is discussed and related to abnormal endoplasmic reticulum performance. We also present new data highlighting sub-optimal axonal Ca2+ signalling in diabetic neuropathy and discuss the putative role for this abnormality in the induction of axonal degeneration in peripheral neuropathies. The accumulating evidence implicating Ca2+ dysregulation in both painful and degenerative neuropathies, along with recent advances in understanding of regional variations in Ca2+ channel and pump structures, makes modulation of neuronal Ca2+ handling an increasingly viable approach for therapeutic interventions against the painful and degenerative aspects of many peripheral neuropathies.

Calreticulin and focal-contact-dependent adhesion

Cell adhesion is regulated by a variety of Ca2+-regulated pathways that depend on Ca2+-binding proteins. One such protein is calreticulin, an ER-resident protein. Calreticulin signalling from within the ER can affect processes outside the ER, such as expression of several adhesion-related genes, most notably vinculin and fibronectin. In addition, changes in the expression level of calreticulin strongly affect tyrosine phosphorylation of cellular proteins, which is known to affect many adhesion-related functions. While calreticulin has been localized to cellular compartments other than the ER, it appears that only the ER-resident calreticulin affects focal-contact-dependent adhesion. In contrast, calreticulin residing outside the ER may be involved in contact disassembly and other adhesion phenomena. Here, we review the role of calreticulin in focal contact initiation, stabilization, and turnover. We propose that calreticulin may regulate cell-substratum adhesion by participating in an "ER-to-nucleus" signalling and in parallel "ER-to-cell surface" signalling based on posttranslational events.

Reticulon-4A (Nogo-A) redistributes protein disulfide isomerase to protect mice from SOD1-dependent amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease inherited in a small subset of patients. The SOD1(G93A) transgenic mouse models this subset of patients, and studies of this strain have suggested that endoplasmic reticulum (ER) stress and deficits in ER chaperone function are contributors to ALS pathophysiology. Here, we demonstrate that the reticulon family of proteins is a novel regulator of the ER chaperone protein disulfide isomerase (PDI), and that through PDI, reticulon-4A (Nogo-A) can protect mice against the neurodegeneration that characterizes ALS. We show that overexpressing reticulon protein induces a punctate redistribution of PDI intracellularly, both in vitro and in vivo. Conversely, reduction of endogenous NogoA expression causes a more homogeneous expression pattern in vivo. These effects occur without induction of the unfolded protein response. To examine the effect of PDI redistribution on ALS disease progression, we conducted survival and behavior studies of SOD1(G93A) mice. Deletion of a single copy of the NogoA,B gene accelerates disease onset and progression, while deletion of both copies further worsens disease. We conclude that NogoA contributes to the proper function of the ER resident chaperone PDI, and is protective against ALS-like neurodegeneration. Our results provide a novel intracellular role for reticulon proteins and support the hypothesis that modulation of PDI function is a potential therapeutic approach to ALS.

Mechanisms and implications of reactive oxygen species generation during the unfolded protein response: roles of endoplasmic reticulum oxidoreductases, mitochondrial electron transport, and NADPH oxidase

Cellular mechanisms governing redox homeostasis likely involve their integration with other stresses. Endoplasmic reticulum (ER) stress triggers complex adaptive or proapoptotic signaling defined as the unfolded protein response (UPR), involved in several pathophysiological processes. Since protein folding is highly redox-dependent, convergence between ER stress and oxidative stress has attracted interest. Evidence suggests that ROS production and oxidative stress are not only coincidental to ER stress, but are integral UPR components, being triggered by distinct types of ER stressors and contributing to support proapoptotic, as well as proadaptive UPR signaling. Thus, ROS generation can be upstream or downstream UPR targets and may display a UPR-specific plus a nonspecific component. Enzymatic mechanisms of ROS generation during UPR include: (a) Multiple thiol-disulfide exchanges involving ER oxidoreductases including flavooxidase Ero1 and protein disulfide isomerase (PDI); (b) Mitochondrial electron transport; (c) Nox4 NADPH oxidase complex, particularly Nox4. Understanding the roles of such mechanisms and how they interconnect with the UPR requires more investigation. Integration among such ROS sources may depend on Ca(2+) levels, ROS themselves, and PDI, which associates with NADPH oxidase and regulates its function. Oxidative stress may frequently integrate with a background of ER stress/UPR in several diseases; here we discuss a focus in the vascular system.

Axotomy depletes intracellular calcium stores in primary sensory neurons

BACKGROUND

The cellular mechanisms of neuropathic pain are inadequately understood. Previous investigations have revealed disrupted Ca signaling in primary sensory neurons after injury. The authors examined the effect of injury on intracellular Ca stores of the endoplasmic reticulum, which critically regulate the Ca signal and neuronal function.

METHODS

Intracellular Ca levels were measured with Fura-2 or mag-Fura-2 microfluorometry in axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent L4 neurons isolated from hyperalgesic rats after L5 spinal nerve ligation, compared to neurons from control animals.

RESULTS

Endoplasmic reticulum Ca stores released by the ryanodine-receptor agonist caffeine decreased by 46% in axotomized small neurons. This effect persisted in Ca-free bath solution, which removes the contribution of store-operated membrane Ca channels, and after blockade of the mitochondrial, sarco-endoplasmic Ca-ATPase and the plasma membrane Ca ATPase pathways. Ca released by the sarco-endoplasmic Ca-ATPase blocker thapsigargin and by the Ca-ionophore ionomycin was also diminished by 25% and 41%, respectively. In contrast to control neurons, Ca stores in axotomized neurons were not expanded by neuronal activation by K depolarization, and the proportionate rate of refilling by sarco-endoplasmic Ca-ATPase was normal. Luminal Ca concentration was also reduced by 38% in axotomized neurons in permeabilized neurons. The adjacent neurons of the L4 dorsal root ganglia showed modest and inconsistent changes after L5 spinal nerve ligation.

CONCLUSIONS

Painful nerve injury leads to diminished releasable endoplasmic reticulum Ca stores and a reduced luminal Ca concentration. Depletion of Ca stores may contribute to the pathogenesis of neuropathic pain.

Tubeimoside-1 exerts cytotoxicity in HeLa cells through mitochondrial dysfunction and endoplasmic reticulum stress pathways

Traditional Chinese herbal medicines are a great source of cancer chemotherapeutic agents. Tubeimoside-1 (TBMS1) is a triterpenoid saponin extracted from Bolbostemma paniculatum (Maxim.) Franquet (Cucurbitaceae), a Chinese herb with anticancer potential named as "Tu Bei Mu". In the present study, we used proteomics to examine the cytotoxic effects of TBMS1 on HeLa cells. Protein profiling of TBMS1-treated HeLa cells revealed profound protein alterations related to energy metabolism and protein synthesis and folding, suggesting that mitochondria and endoplasmic reticulum (ER) play a role in TBMS1-initiated apoptosis. TBMS1 induced the depletion of mitochondrial transmembrane potential (DeltaPsi(m)), leading to the activation of caspase-dependent apoptotic cell death. Unfolded Protein Response (UPR) signaling pathways are also activated after TBMS1 treatment and these changes were accompanied by increased expression of GADD153/CHOP, a transcription factor associated with growth arrest and apoptosis in the event of prolonged ER stress. Salubrinal (Sal), a selective inhibitor for ER stress, partially abrogated the TBMS1-related cell death. These results suggest that TBMS1 exerts cytotoxicity in HeLa cells through both mitochondrial dysfunction and ER stress cell death pathways.

Absence of high‐affinity calreticulin autoantibodies in patients with systemic rheumatic diseases and coeliac disease

Calreticulin has been reported to be an autoantigen in various autoimmune connective tissue diseases and in coeliac disease. Previous studies have used incubation buffers with low salt and low detergent concentrations (low stringency conditions) with serum albumin or other proteins as a blocking agent. Using these conditions we found a relatively high level of non-specific binding in many sera. Antibodies to proteins that are used as blocking reagents in ELISA (bovine serum albumin (BSA), ovalbumin, skimmed milk powder) are frequently present in sera, and these may cause false-positive results. Moreover, the low isoelectric point of calreticulin and its chaperone properties may give rise to false-positive results under low stringency conditions. We report that the use of a simple buffer without protein (50 mM Tris, pH 7.5, 1% Tween 20, 0.3 M NaCl) removes most of the problems with unwanted binding (high stringency conditions). Using the high stringency conditions, we screened sera from 107 patients with systemic lupus erythematosus, sera from patients with other systemic autoimmune diseases and from children with coeliac disease for the presence of high-affinity calreticulin autoantibodies by immunoblotting and ELISA. None of the sera contained high-affinity calreticulin antibodies. It is concluded that calreticulin is not a common autoantigen in patients with autoimmune connective tissue diseases or coeliac disease.

Neuroserpin polymers activate NF-kappaB by a calcium signaling pathway that is independent of the unfolded protein response

The autosomal dominant dementia familial encephalopathy with neuroserpin inclusion bodies is characterized by the accumulation of ordered polymers of mutant neuroserpin within the endoplasmic reticulum of neurones. We show here that intracellular neuroserpin polymers activate NF-kappaB by a pathway that is independent of the IRE1, ATF6, and PERK limbs of the canonical unfolded protein response but is dependent on intracellular calcium. This pathway provides a mechanism for cells to sense and react to the accumulation of folded structures of mutant serpins within the endoplasmic reticulum. Our results provide strong support for the endoplasmic reticulum overload response being independent of the unfolded protein response.

Intersection of the unfolded protein response and hepatic lipid metabolism

The liver plays a central role in whole-body lipid metabolism by governing the synthesis, oxidization, transport and excretion of lipids. The unfolded protein response (UPR) was identified as a signal transduction system that is activated by ER stress. Recent studies revealed a critical role of the UPR in hepatic lipid metabolism. The IRE1/XBP1 branch of the UPR is activated by high dietary carbohydrates and controls the expression of genes involved in fatty acid and cholesterol biosynthesis. PERK mediated eIF2alpha phosphorylation is also required for the expression of lipogenic genes and the development of hepatic steatosis, likely by activating C/EBP and PPARgamma transcription factors. Further studies to define the molecular pathways that lead to the activation of the UPR by nutritional cues in the liver, and their contribution to human metabolic disorders such as hepatic steatosis, atherosclerosis and type 2 diabetes that are associated with dysregulation of lipid homeostasis, are warranted.

Bcl-2 family on guard at the ER

The endoplasmic reticulum (ER) is the main site for protein folding, lipid biosynthesis, and calcium storage in the cell. Disturbances of these critical cellular functions lead to ER stress. The ER responds to disturbances in its homeostasis by launching an adaptive signal transduction pathway, known as the unfolded protein response (UPR). The UPR strives to maintain ER function during stress; however, if the stress is not resolved, apoptotic responses are activated that involve cross talk between the ER and mitochondria. In addition, ER stress is also known to induce autophagy to counteract XBP-1-mediated ER expansion and assist in the degradation of unfolded proteins. One family of proteins involved in the regulation of apoptosis is that of B-cell lymphoma protein 2 (Bcl-2). Complex interactions among the three subgroups within the Bcl-2 family [the antiapoptotic, the multidomain proapoptotic, and the Bcl-2 homology domain 3 (BH3)-only members] control the signaling events of apoptosis upstream of mitochondrial outer membrane permeabilization. These proteins were found to have diverse subcellular locations to aid in the response to varied intrinsic and extrinsic stimuli. Of recent interest is the presence of the Bcl-2 family at the ER. Here, we review the involvement of proteins from each of the three Bcl-2 family subgroups in the maintenance of ER homeostasis and their participation in ER stress signal transduction pathways.

Endoplasmic reticulum: nutrient sensor in physiology and pathology

The endoplasmic reticulum (ER) is a metabolic organelle and an ideal nutrient sensor. In response to hypoglycemia, hyperglycemia or fatty acid overload, the ER triggers the unfolded protein response, which represses protein synthesis, alters insulin responsiveness and favors apoptosis. In addition, the ER affects steroid hormone activation and autophagy. The primary aim of these responses is to adjust the metabolism to environmental changes. Failure of the ER to adapt to changes in nutrient availability can result in a pathological transition in ER functions, as observed in cases of obesity-related diseases. This review highlights the recent evidence that the ER has a prominent role in cellular adaptation, as well as in the pathomechanism of type 2 diabetes.

[Molecular architecture of the sarcoplasmic reticulum and its role in the ECC]

The sarcoplasmic reticulum (SR) plays a fundamental role in excitation-contraction coupling, which propagates the electric signal conversion along the muscle fiber's plasmic membrane to a mechanical event manifested as a muscle contraction. It plays a crucial role in calcium homeostasis and intracellular calcium storage control (storage, liberation and uptake) necessary for fiber muscle contraction and then relaxation. These functions take place at the triad, made up of individualized SR subdomains where the protein-specific organization provides efficient and fast coupling. Ryanodine receptors (RyR) and dihydropyridine receptors (DHPR) mainly act in calcium exchanges in the SR. This particular structural and molecular architecture must be correlated to its functional specificity.

Roles of IP3R and RyR Ca2+ channels in endoplasmic reticulum stress and beta-cell death

OBJECTIVE

Endoplasmic reticulum (ER) stress has been implicated in the pathogenesis of diabetes, but the roles of specific ER Ca(2+) release channels in the ER stress-associated apoptosis pathway remain unknown. Here, we examined the effects of stimulating or inhibiting the ER-resident inositol trisphosphate receptors (IP(3)Rs) and the ryanodine receptors (RyRs) on the induction of beta-cell ER stress and apoptosis.

RESEARCH DESIGN AND METHODS

Kinetics of beta-cell death were tracked by imaging propidium iodide incorporation and caspase-3 activity in real time. ER stress and apoptosis were assessed by Western blot. Mitochondrial membrane potential was monitored by flow cytometry. Cytosolic Ca(2+) was imaged using fura-2, and genetically encoded fluorescence resonance energy transfer (FRET)-based probes were used to measure Ca(2+) in ER and mitochondria.

RESULTS

Neither RyR nor IP(3)R inhibition, alone or in combination, caused robust death within 24 h. In contrast, blocking sarco/endoplasmic reticulum ATPase (SERCA) pumps depleted ER Ca(2+) and induced marked phosphorylation of PKR-like ER kinase (PERK) and eukaryotic initiation factor-2alpha (eIF2alpha), C/EBP homologous protein (CHOP)-associated ER stress, caspase-3 activation, and death. Notably, ER stress following SERCA inhibition was attenuated by blocking IP(3)Rs and RyRs. Conversely, stimulation of ER Ca(2+) release channels accelerated thapsigargin-induced ER depletion and apoptosis. SERCA block also activated caspase-9 and induced perturbations of the mitochondrial membrane potential, resulting eventually in the loss of mitochondrial polarization.

CONCLUSIONS

This study demonstrates that the activity of ER Ca(2+) channels regulates the susceptibility of beta-cells to ER stress resulting from impaired SERCA function. Our results also suggest the involvement of mitochondria in beta-cell apoptosis associated with dysfunctional beta-cell ER Ca(2+) homeostasis and ER stress.

Calreticulin, a multi-process calcium-buffering chaperone of the endoplasmic reticulum

Calreticulin is an ER (endoplasmic reticulum) luminal Ca2+-buffering chaperone. The protein is involved in regulation of intracellular Ca2+ homoeostasis and ER Ca2+ capacity. The protein impacts on store-operated Ca2+ influx and influences Ca2+-dependent transcriptional pathways during embryonic development. Calreticulin is also involved in the folding of newly synthesized proteins and glycoproteins and, together with calnexin (an integral ER membrane chaperone similar to calreticulin) and ERp57 [ER protein of 57 kDa; a PDI (protein disulfide-isomerase)-like ER-resident protein], constitutes the ‘calreticulin/calnexin cycle’ that is responsible for folding and quality control of newly synthesized glycoproteins. In recent years, calreticulin has been implicated to play a role in many biological systems, including functions inside and outside the ER, indicating that the protein is a multi-process molecule. Regulation of Ca2+ homoeostasis and ER Ca2+ buffering by calreticulin might be the key to explain its multi-process property.

Calcium and ATP handling in human NADH:ubiquinone oxidoreductase deficiency

Proper cell functioning requires precise coordination between mitochondrial ATP production and local energy demand. Ionic calcium (Ca(2+)) plays a central role in this coupling because it activates mitochondrial oxidative phosphorylation (OXPHOS) during hormonal and electrical cell stimulation. To determine how mitochondrial dysfunction affects cytosolic and mitochondrial Ca(2+)/ATP handling, we performed life-cell quantification of these parameters in fibroblast cell lines derived from healthy subjects and patients with isolated deficiency of the first OXPHOS complex (CI). In resting patient cells, CI deficiency was associated with a normal mitochondrial ([ATP](m)) and cytosolic ([ATP](c)) ATP concentration, a normal cytosolic Ca(2+) concentration ([Ca(2+)](c)), but a reduced Ca(2+) content of the endoplasmic reticulum (ER). Furthermore, cellular NAD(P)H levels were increased, mitochondrial membrane potential was slightly depolarized, reactive oxygen species (ROS) levels were elevated and mitochondrial shape was altered. Upon stimulation with bradykinin (Bk), the peak increases in [Ca(2+)](c), mitochondrial Ca(2+) concentration ([Ca(2+)](m)), [ATP](c) and [ATP](m) were reduced in patient cells. In agreement with these results, ATP-dependent Ca(2+) removal from the cytosol was slower. Here, we review the interconnection between cytosolic, endoplasmic reticular and mitochondrial Ca(2+) and ATP handling, and summarize our findings in patient fibroblasts in an integrative model.

Calnexin overexpression sensitizes recombinant CHO cells to apoptosis induced by sodium butyrate treatment

Sodium butyrate (NaBu) can enhance the expression of foreign genes in recombinant Chinese hamster ovary (rCHO) cells, but it can also inhibit cell growth and induce cellular apoptosis. In this study, the potential role of calnexin (Cnx) expression in rCHO cells treated with 5 mM NaBu was investigated for rCHO cells producing tumor necrosis factor receptor FC. To regulate the Cnx expression level, a tetracycline-inducible system was used. Clones with different Cnx expression levels were selected and investigated. With regard to productivity per cell (q (p)), NaBu enhanced the q (p) by over twofold. Under NaBu treatment, Cnx overexpression further enhanced the q (p) by about 1.7-fold. However, under NaBu stress, the cells overexpressing Cnx showed a poorer viability profile with a consistent difference of over 25% in the viability when compared to the Cnx-repressed condition. This drop in the viability was attributed to increased apoptosis seen in these cells as evidenced by enhanced poly (ADP-ribose) polymerase cleavage and cytochrome C release. Ca(2+) localization staining and subsequent confocal imaging revealed elevated cytosolic Ca(2+) ([Ca(2+)](c)) in the Cnx-overexpressing cells when compared to the Cnx-repressed condition, thus endorsing the increased apoptosis observed in these cells. Taken together, Cnx overexpression not only improved the q (p) of cells treated with NaBu, but it also sensitized cells to apoptosis.

ERP57 membrane translocation dictates the immunogenicity of tumor cell death by controlling the membrane translocation of calreticulin

Several pieces of experimental evidence indicate the following: 1) the most efficient antitumor treatments (this principle applies on both chemotherapy and radiotherapy) are those that induce immunogenic cell death and are able to trigger a specific antitumor immune response; and 2) the immunogenicity of cell death depends very closely on the plasma membrane quantity of calreticulin (CRT), an endoplasmic reticulum (ER) stress protein exposed to the cell membrane after immunogenic treatment. Nevertheless, the mechanisms implicated in CRT translocation are unknown. CRT is known to interact in the ER with ERP57, another ER stress protein. I sought to determine whether ERP57 would have any role in tumor immunogenicity. In this article I report that CRT exposure is controlled by ERP57 exposure. CRT and ERP57 are translocated together in the same molecular complex. ERP57 knockdown suppressed CRT exposure as well as phagocytosis by dendritic cells and abolished the immunogenicity in vivo. Knockdown or the absence of CRT abolishes ERP57 exposure. Administration of recombinant ERP57, unlike the administration of recombinant CRT, did not restore the immunogenicity of CRT or ERP57 small interfering RNA-transfected tumor cells. Together, these studies identify ERP57 as a key protein that controls immunogenicity by controlling CRT exposure and illustrate the ability of ERP57 to serve as a new molecular marker of immunogenicity.