Ca2+ signaling and calcium binding chaperones of the endoplasmic reticulum

Regulation of Protein Compartmentalization Expands the Diversity of Protein Function

Proteins destined for the secretory pathway are translocated into the endoplasmic reticulum (ER) by signal sequences that vary widely in their functional properties. We have investigated whether differences in signal sequence function have been exploited for cellular benefit. A cytosolic form of the ER chaperone calreticulin was found to arise by an aborted translocation mechanism dependent on its signal sequence and factors in the ER lumen and membrane. A signal sequence that functions independently of these accessory translocation factors selectively eliminated cytosolic calreticulin. In vivo replacement of endogenous calreticulin with a constitutively translocated form influenced glucocorticoid receptor-mediated gene activation without compromising chaperone activity in the ER. Thus, in addition to its well-established ER lumenal functions, calreticulin has an independent role in the cytosol that depends critically on its inefficient compartmentalization. We propose that regulation of protein translocation represents a potentially general mechanism for generating diversity of protein function.

Regulation of gene expression by the amyloid precursor protein: inhibition of the JNK/c-Jun pathway

The amyloid precursor protein (APP) has been suggested to regulate gene expression. GeneChip analysis and in vitro kinase assays revealed potent APP-dependent repression of c-Jun, its target gene SPARC and reduced basal c-Jun N-terminal kinase (JNK) activity in PC12 cells overexpressing APP. UV-induced activation of the JNK signalling pathway and subsequent apoptosis were likewise reduced by APP and this effect could be mimicked by the indirect JNK inhibitor CEP-11004. Treatment with a gamma-secretase inhibitor did not affect APP-mediated downmodulation of the JNK signalling pathway, suggesting that the effects might be mediated via alpha-secretase processing of APP. In support of these data, overexpression of the Swedish mutant of APP did not inhibit SPARC expression, UV-induced JNK activation and cell death. Our data suggest an important physiological role of APP and alpha-secretase activity in the control of JNK/c-Jun signalling, target gene expression and cell death activation in response to cytotoxic stress.

Calreticulin signals upstream of calcineurin and MEF2C in a critical Ca(2+)-dependent signaling cascade

We uncovered a new pathway of interplay between calreticulin and myocyte-enhancer factor (MEF) 2C, a cardiac-specific transcription factor. We establish that calreticulin works upstream of calcineurin and MEF2C in a Ca²⁺-dependent signal transduction cascade that links the endoplasmic reticulum and the nucleus during cardiac development. In the absence of calreticulin, translocation of MEF2C to the nucleus is compromised. This defect is reversed by calreticulin itself or by a constitutively active form of calcineurin. Furthermore, we show that expression of the calreticulin gene itself is regulated by MEF2C in vitro and in vivo and that, in turn, increased expression of calreticulin affects MEF2C transcriptional activity. The present findings provide a clear molecular explanation for the embryonic lethality observed in calreticulin-deficient mice and emphasize the importance of calreticulin in the early stages of cardiac development. Our study illustrates the existence of a positive feedback mechanism that ensures an adequate supply of releasable Ca²⁺ is maintained within the cell for activation of calcineurin and, subsequently, for proper functioning of MEF2C.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

Release of Ca2+ from the endoplasmic reticulum contributes to Ca2+ signaling in Dictyostelium discoideum

Ca2+ responses to two chemoattractants, folate and cyclic AMP (cAMP), were assayed in Dictyostelium D. discoideum mutants deficient in one or both of two abundant Ca2+-binding proteins of the endoplasmic reticulum (ER), calreticulin and calnexin. Mutants deficient in either or both proteins exhibited enhanced cytosolic Ca2+ responses to both attractants. Not only were the mutant responses greater in amplitude, but they also exhibited earlier onsets, faster rise rates, earlier peaks, and faster fall rates. Correlations among these kinetic parameters and the response amplitudes suggested that key events in the Ca2+ response are autoregulated by the magnitude of the response itself, i.e., by cytosolic Ca2+ levels. This autoregulation was sufficient to explain the altered kinetics of the mutant responses: larger responses are faster in both mutant and wild-type cells in response to both folate (vegetative cells) and cAMP (differentiated cells). Searches of the predicted D. discoideum proteome revealed three putative Ca2+ pumps and four putative Ca2+ channels. All but one contained sequence motifs for Ca2+- or calmodulin-binding sites, consistent with Ca2+ signals being autoregulatory. Although cytosolic Ca2+ responses in the calnexin and calreticulin mutants are enhanced, the influx of Ca2+ from the extracellular medium into the mutant cells was smaller. Compared to wild-type cells, Ca2+ release from the ER in the mutants thus contributes more to the total cytosolic Ca2+ response while influx from the extracellular medium contributes less. These results provide the first molecular genetic evidence that release of Ca2+ from the ER contributes to cytosolic Ca2+ responses in D. discoideum.

PICK1 is a calcium-sensor for NMDA-induced AMPA receptor trafficking

Regulation of AMPA receptor (AMPAR) trafficking results in changes in receptor number at the postsynaptic membrane, and hence modifications in synaptic strength, which are proposed to underlie learning and memory. NMDA receptor-mediated postsynaptic Ca2+ influx enhances AMPAR internalisation, but the molecular mechanisms that trigger such trafficking are not well understood. We investigated whether AMPAR-associated protein-protein interactions known to regulate receptor surface expression may be directly regulated by Ca2+. PICK1 binds the AMPAR GluR2 subunit and is involved in AMPAR internalisation and LTD. We show that PICK1 is a Ca2+-binding protein, and that PICK1-GluR2 interactions are enhanced by the presence of 15 muM Ca2+. Deletion of an N-terminal acidic domain in PICK1 reduces its ability to bind Ca2+, and renders the GluR2-PICK1 interaction insensitive to Ca2+. Overexpression of this Ca2+-insensitive mutant occludes NMDA-induced AMPAR internalisation in hippocampal neurons. This work reveals a novel postsynaptic Ca2+-binding protein that provides a direct mechanistic link between NMDAR-mediated Ca2+ influx and AMPAR endocytosis.

The loss of sarco/endoplasmic reticulum calcium transport ATPase 3 expression is an early event during the multistep process of colon carcinogenesis

Calcium accumulation in the endoplasmic reticulum is accomplished by sarco/endoplasmic reticulum calcium transport ATPases (SERCA enzymes). To better characterize the role of SERCA3 in colon carcinogenesis, its expression has been investigated in colonic epithelium, benign lesions, adenomas, and adenocarcinomas. In addition, the regulation of SERCA3 expression was analyzed in the context of the adenomatous polyposis coli/beta-catenin/T-cell factor 4 (TCF4) pathway and of specificity protein 1 (Sp1)-like factor-dependent transcription. We report that SERCA3 expression increased along the crypts as cells differentiated in normal colonic mucosa and in hyperplastic polyps, was moderately and heterogeneously expressed in colonic adenomas with expression levels inversely correlated with the degree of dysplasia, was barely detectable in well and moderately differentiated adenocarcinomas, and was absent in poorly differentiated tumors. Inhibition of Sp1-like factor-dependent transcription blocked SERCA3 expression during cell differentiation, and SERCA3 expression was induced by the expression of dominant-negative TCF4 in colon cancer cells. These data link SERCA3 expression to the state of differentiation of colonic epithelial cells, and relate SERCA3 expression, already decreased in adenomas, to enhanced adenomatous polyposis coli/beta-catenin/TCF4-dependent signaling and deficient Sp1-like factor-dependent transcription. In conclusion, intracellular calcium homeostasis becomes progressively anomalous during colon carcinogenesis as reflected by deficient SERCA3 expression.

Enamel matrix protein interactions

The recognized structural proteins of the enamel matrix are amelogenin, ameloblastin, and enamelin. While a large volume of data exists showing that amelogenin self-assembles into multimeric units referred to as nanospheres, other reports of enamel matrix protein-protein interactions are scant. We believe that each of these enamel matrix proteins must interact with other organic components of ameloblasts and the enamel matrix. Likely protein partners would include integral membrane proteins and additional secreted proteins.

INTRODUCTION

The purpose of this study was to identify and catalog additional proteins that play a significant role in enamel formation.

MATERIALS AND METHODS

We used the yeast two-hybrid assay to identify protein partners for amelogenin, ameloblastin, and enamelin. Once identified, RT-PCR was used to assess gene transcription of these newly identified and potential "enamel" proteins in ameloblast-like LS8 cells.

RESULTS

In the context of this yeast assay, we identified a number of secreted proteins and integral membrane proteins that interact with amelogenin, ameloblastin, and enamelin. Additionally, proteins whose functions range from the inhibition of soft tissue mineralization, calcium ion transport, and phosphorylation events have been identified as protein partners to these enamel matrix proteins. For each protein identified using this screening strategy, future studies are planned to confirm this physiological relationship to biomineralization in vivo.

CONCLUSION

Identifying integral membrane proteins of the secretory surface of ameloblast cells (Tomes' processes) and additional enamel matrix proteins, based on their abilities to interact with the most abundant enamel matrix proteins, will better define the molecular mechanisms of enamel formation at its most rudimentary level.

Purification and biochemical characterization of native ERp29 from rat liver

ERp29 is a recently characterized resident of the ER (endoplasmic reticulum) lumen that has broad biological significance, being expressed ubiquitously and abundantly in animal cells. As an apparent housekeeper, ERp29 is thought to be a general folding assistant for secretory proteins and to probably function as a PDI (protein disulphide isomerase)-like molecular chaperone. In the present paper, we report the first purification to homogeneity and direct functional analysis of native ERp29, which has led to the unexpected finding that ERp29 lacks PDI-like folding activities. ERp29 was purified 4800-fold in non-denaturing conditions exploiting an unusual affinity for heparin. Two additional biochemical hallmarks that will assist the classification of ERp29 homologues were identified, namely the idiosyncratic behaviours of ERp29 on size-exclusion chromatography (M(r)<globular homodimer) and SDS/PAGE (M(r)>monomeric mass). In contrast with PDI and parallel-purified co-residents (calreticulin, ERp60), native ERp29 lacked classical chaperone, disulphide reductase and isomerase, and calcium-binding activities. In the chaperone assays, ERp29 neither protected substrate proteins against thermal aggregation nor interacted stably with chemically denatured proteins as detected by cross-linking. ERp29 also did not exhibit helper activity toward calreticulin (chaperone) or PDI and ERp60 (disulphide reductase). By refuting long-standing predictions about chaperone activity, these results expose ERp29 as a functionally distinct member of the ER machinery and prompt a revised hypothesis that ERp29 acts as a non-classical folding assistant. The native preparation and biochemical hallmarks established here provide a useful foundation for ongoing efforts to resolve the functional orphan status of ERp29.

Overexpression of calreticulin sensitizes SERCA2a to oxidative stress

Calreticulin (CRT), a Ca(2+)-binding molecular chaperone in the endoplasmic reticulum, plays a vital role in cardiac physiology and pathology. Oxidative stress is a main cause of myocardiac disorder in the ischemic heart, but the function of CRT under oxidative stress is not fully understood. In this study, the effect of overexpression of CRT on sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA) 2a under oxidative stress was examined using myocardiac H9c2 cells transfected with the CRT gene. The in vitro activity of SERCA2a and uptake of (45)Ca(2+) into isolated microsomes were suppressed by H(2)O(2) in CRT-overexpressing cells compared with controls. Moreover, SERCA2a protein was degraded via a proteasome-dependent pathway following the formation of a complex with CRT under the stress with H(2)O(2). Thus, we conclude that overexpression of CRT enhances the inactivation and degradation of SERCA2a in the cells under oxidative stress, suggesting some pathophysiological functions of CRT in Ca(2+) homeostasis of myocardiac disease.

Cell line dependent RNA expression profiles of prion-infected mouse neuronal cells

The overall impact of prion disease on gene expression is not well characterized. We have carried out a large-scale expression analysis of specific cell types commonly employed in studies of prion disease. Neuroblastoma cells (N2a) and hypothalamic neuronal cells (GT1) can be persistently infected with mouse-adapted scrapie prions, the latter demonstrating cytopathologic effects associated with prion neuropathology. Exploiting a mouse DNA microarray containing approximately 21,000 spotted cDNAs, we have identified several hundred differentially expressed sequences in the two cell lines when infected with prion strain RML. ScN2a and ScGT1 cells demonstrate unique changes in RNA profiles and both differ from the reported changes in human microglia and prion-infected brain studies albeit with some overlap. In addition, several of the identified changes are shared in common with other neurodegenerative diseases such as Alzheimer's disease. The results illustrate that prion infection differs in effect depending on cell type, which could be exploited for diagnostic or therapeutic intervention.

Surface calreticulin mediates muramyl dipeptide-induced apoptosis in RK13 cells

Calreticulin (CRT) is a binding protein for apoptotic N-acetylmuramyl-L-alanyl-D-isoglutamine (L,D-MDP) or peptidoglycan in RK(13) cells. CRT on RK(13) cell surface (srCRT) forms complex(es) with tumor necrosis factor receptor 1 (TNFR1) and TNFR-associated death domain (TRADD) protein of the cell membrane. CRT polyclonal or monoclonal antibody binding to RK(13) srCRT dose-dependently inhibited L,D-MDP-induced apoptosis. In RK(13) cells, L,D-MDP up-regulated the TNFR1.TRADD complex of the plasma membrane and subsequently induced cytosolic TRADD-Fas-associated death domain protein complex. Biotinylated srCRT was capable of calcium-dependent binding of Sepharose-immobilized L,D-MDP or peptidoglycan. However, Toll-like receptors TLR-2 and TLR-4, Nod2, and CD14 of RK(13) cells did not specifically bind Sepharose-immobilized L,D-MDP. High concentrations (5-40 mm) of EGTA dose-dependently inhibited free L,D-MDP binding to purified RK(13) cell CRT and promoted free L,D-MDP dissociation from RK(13) cell CRT.MDP complex. Different concentrations of EGTA (0-40 mm) added to Dulbecco's modified essential medium with 1.8 mm calcium or phosphate-buffered saline with 0.18 mm calcium have different effects on medium free calcium concentrations but have identical inhibiting effects on L,D-MDP-induced apoptosis. More inhibition of the L,D-MDP-induced apoptotic DNA ladders and caspase-3 activity in RK(13) cells was obtained with EGTA pretreatment (83%) than just EGTA + L,D-MDP (47%). The knocking down of srCRT by antisense oligonucleotide CRTAS121 (250 nmol/ml) and stealth small interfering RNA CRT_siR479 (150 pm/ml) for 2 days (44 and 66%, respectively), resulted in the inhibition of L,D-MDP-induced caspase-3 activity (47 and 65%, respectively). The results suggest that (a) the binding of L,D-MDP to srCRT is calcium-dependent, i.e. on srCRT-bound calcium, and (b) it is srCRT, not TLR-2, TLR-4, Nod2 or CD14, that mediates L,D-MDP-induced RK(13) cell apoptosis through activating the TNFR1. TRADD-Fas-associated death domain protein apoptotic pathway.

Calreticulin, a Ca2+-binding chaperone of the endoplasmic reticulum

Calreticulin is a 46-kDa Ca2+-binding chaperone found across a diverse range of species. The protein is involved in the regulation of intracellular Ca2+ homeostasis and endoplasmic reticulum (ER) Ca2+ storage capacity. Calreticulin is also an important molecular chaperone involved in "quality control" within secretory pathways. The protein contains structurally and functionally unique domains with specialized functions. Studies on calreticulin knockout mice indicate that the protein is essential in early cardiac development. The protein also plays an important role in autoimmunity and cancer.

Cellular Functions of Endoplasmic Reticulum Chaperones Calreticulin, Calnexin, and ERp57

Glycosylated proteins destined for the cell surface or to be secreted from the cell are trafficked through the endoplasmic reticulum during synthesis and folding. Correct folding is determined in large part by the sequence of the protein, but it is also assisted by interaction with enzymes and chaperones of the endoplasmic reticulum. Calreticulin, calnexin, and ERp57 are among the endoplasmic chaperones that interact with partially folded glycoproteins and determine if the proteins are to be released from the endoplasmic reticulum to be expressed, or alternatively, if they are to be sent to the proteosome for degradation. Studies on the effect of alterations in the expression and function of these proteins are providing information about the importance of this quality control system, as well as uncovering other important functions these proteins play outside of the endoplasmic reticulum.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

cDNA microarray analysis identifies genes induced in common by peptide growth factors and androgen in human prostate epithelial cells

Prostate cancer cells initially require androgen for continued proliferation, but invariably become androgen independent or unresponsive and recur after treatment by androgen ablation. Exploitation of common signaling components downstream of their specific receptors (i.e., androgen receptor (AR), insulin-like growth factor 1 (IGF-1) receptor, and epidermal growth factor (EGF) receptor) could provide a mechanism by which androgen independent cells survive and proliferate. Our objective was to design and implement prostate enriched cDNA microarrays to identify genes induced in prostate epithelial cells in a similar temporal pattern by both androgen and IGF or EGF. AR positive and AR negative human prostate epithelial cells of the M12 line were exposed in parallel to DHT, EGF, or IGF for 0, 6, or 24 h. RNA extracted from each of these groups was analyzed by cDNA microarrays composed of a unique set of 6373 prostate-derived cDNA clones from the Prostate Expression Database (PEDB). We observed statistically significant changes in 20 genes induced in common after 6 and 24 h exposure to androgen or these growth factors, and validated the microarray results by RT-PCR for three or four of these genes: v-myc, isocitrate dehydrogenase, and calnexin. Androgen response element binding motifs were identified in the upstream sequence in 16 of these 20 genes. These results provide comprehensive and unique insights into potential mechanisms by which peptide growth factors provide alternate pathways to control prostate epithelial cell proliferation in malignant states.

Ca2+ regulation and gene expression in normal brain aging

Understanding the cellular mechanisms that characterize the functional changes of the aged brain is an ongoing and formidable challenge for the neuroscience community. Evidence now links changes in Ca(2+) influx and homeostasis with perturbations induced by the aging process in the function of the main intracellular organelles involved in Ca(2+) regulation: the endoplasmic reticulum and mitochondria. New perspectives are also offered by recent gene microarray studies, illustrating the multifactorial nature of the aging process.

Biophysical characterization of ERp29. Evidence for a key structural role of cysteine 125

ERp29 is a major resident of the endoplasmic reticulum (ER) that seemingly plays an important role in most animal cells. Although a protein-folding association is widely supported, ERp29's specific molecular function remains unknown. A chaperone activity was postulated from evidence that ERp29 forms multimers like the classical ER chaperones, but conflicting results have emerged from our recent studies. Here a biophysical approach was used to clarify this issue and also reveal a key structural role for ERp29's characteristic cysteine, Cys-125. Applying hydrodynamic parameters derived from sedimentation and dynamic light-scattering analyses, a model of ERp29's quaternary structure was assembled from existing tertiary substructures. Comparison with Windbeutel, an ERp29-like protein from fruit fly with specialized chaperone activity, revealed similar tri-lobar gross structures but some finer differences consistent with functional divergence. Solubility and hydrophobic probe assays revealed moderate surface hydrophobicity, which was reduced in mutant ERp29 in which serine replaced Cys-125. This mutant was also relatively labile to proteolytic degradation, providing two reasons for the strict conservation of Cys-125. No multimerization was observed with untagged ERp29, which existed as tight homodimers (K(d) < 50 nm), whereas His-tagged ERp29 artifactually formed 670-kDa oligomers. These findings distinguish ERp29 biophysically from its peers in the ER including Windbeutel, endorsing our postulate that ERp29 adds a distinct type of folding activity to the ER machinery. By invoking novel functional associations for Cys-125 and the adjoining linker, new clues about how ERp29 might work have also arisen.

Dietary magnesium depletion does not promote oxidative stress but targets apical cells within the mouse caput epididymidis

It is well documented that a dietary deficiency in magnesium can induce oxidative stress and an inflammatory response in animal models. In our study, we have investigated these responses in the mouse epididymis after mice had been fed a magnesium-deficient diet for a 2-week duration. The extracellular and intracellular concentrations of magnesium where shown to be depleted on this diet. This was followed, however, only in the liver of the Mg-deficient animals, by an increase in both alpha 2-macroglobulin (alpha-2m), an acute phase marker, and interleukin-6 transcripts suggesting that an inflammatory response had been initiated. These changes were correlated with a decrease in circulating neutrophils. To address the question of whether or not peroxidation was induced in mouse epididymis following hypomagnesia, we have monitored the level of endogenous peroxidation, their ability to respond to induced peroxidation as well as the expression and activity of the enzymatic glutathione peroxidase (GPX) antioxidant family. To evaluate if the epididymis had evolved specific protections against peroxidation, other organs such as the liver and the kidney were monitored in parallel. We detected no evidence for increased peroxidation in any of the mouse organs tested. However, GPX activity was found to be significantly lower in the liver and the kidney of Mg-deficient animals while it was unchanged in the epididymides of the same animals during the deficiency. Histological analysis of the epididymis showed no major difference in the overall cytological aspect of the organ. Segment 2 of the caput, however presented a significant increase in the number of apically located cells or blebbing cells. Immunohistochemical analysis proved that these cells were epididymal apical cells and not infiltrated leukocytes. These observations suggested that the mouse caput epididymidis segment 2 specifically responded to Mg deficiency via the apical cells. Finally, a comparative analysis of stress response genes was conducted in control and magnesium-deficient caput epididymidis samples. It brought forward some genes that might be involved in the peculiar response of the caput epithelium following hypomagnesia.

Darier’s disease: from dyskeratosis to endoplasmic reticulum calcium ATPase deficiency

The skin is the body's largest organ and has an essential barrier protective function against physical, chemical, and pathogen aggressions and prevents fluid loss. The outer layer of the skin, known as the epidermis, plays a key role in this protection, through a tightly regulated differentiation programme from basal keratinocytes to the stratum corneum at the skin surface. During this process, keratinocytes from the base of the epidermis undergo major morphological and functional changes during their migration through the spinous and granular layers, to become terminally differentiated corneocytes which will be shed from the skin's surface. The role of extracellular Ca2+ in cell-to-cell adhesion and in epidermal differentiation was known to be important, but the identification of the sarco/endoplasmic reticulum Ca2+ transport ATPase (ATP2A2) as the defective gene in a rare genetic skin disease known as Darier's disease, came as a surprise and shed light on the key role of Ca2+ signaling in the homeostasis of the epidermis.

Gentamicin binds to the lectin site of calreticulin and inhibits its chaperone activity

Recently, it became clear that aminoglycoside antibiotics affect protein-protein interactions involving protein disulfide isomerase as well as protein synthesis in the endoplasmic reticulum. In this study, we used affinity column chromatography to screen gentamicin-binding proteins in microsomes derived from bovine kidney in order to learn about the possible mechanisms of gentamicin-associated nephrotoxicity. One of the gentamicin-binding proteins was identified as calreticulin (CRT) by N-terminal amino acid sequence analysis. Interestingly, gentamicin inhibited the chaperone and oxidative refolding activities of CRT when N-glycosylated substrates such as alpha1-antitrypsin and alpha-mannosidase were used as substrates, but it did not inhibit the chaperone activity of CRT when unglycosylated citrate synthase was used. Moreover, CRT suppressed the aggregation of deglycosylated and denatured alpha-mannosidase, but gentamicin did not inhibit its chaperone activity. Experiments with domain mutants suggest that the lectin site of CRT is the main target for gentamicin binding and that binding of gentamicin to this site inhibits the chaperone activity of CRT.

S100A6 (Calcyclin) is a prostate basal cell marker absent in prostate cancer and its precursors

S100A6 (Calcyclin) is a calcium-binding protein that has been implicated in a variety of biological functions as well as tumorigenesis. The aim of our study was to investigate the involvement of S100A6 during prostate cancer development and progression. Using immunohistochemistry, the expression of S100A6 was examined in benign (n=66), premalignant (n=10), malignant (n=66) and metastatic prostate (n=5) tissues arranged in a tissue-microarray or whole sections as well as in prostate cancer cell lines. The S100A6 immunostaining pattern in tissues was compared with that of cytokeratin 5 (a basal cell marker) and 18 (a benign luminal cell marker). In all cases of benign epithelium, intense S100A6 expression was seen in the basal cell layer with absent staining in luminal cells. In all cases of prostatic adenocarcinoma (matched), metastatic lesions and 3/10 high-grade prostatic intraepithelial neoplasia lesions, an absence of S100A6 was seen. Western blotting and RT–PCR analysis of cell lines showed S100A6 expression to be absent in LNCaP, LNCaP-LN3 and LNCaP-Pro5 but present in Du145, PC3, PC-3M and PC-3M-LN4. LNCaP cells treated with 5-Azacytidine, caused re-expression of S100A6 mRNA. Sequencing of bisulphite modified DNA showed CpG methylation within the S100A6 promoter region and exon 1 of LNCaP, LNCaP-LN3 and LNCaP-Pro5 cell lines but not in Du145 cells. Our data suggest that loss of S100A6 protein expression is common in prostate cancer development and may occur at an early stage. The mechanism of loss of expression may involve hypermethylation of CpG sites. The finding of intense S100A6 expression in the basal cells of benign glands but loss of expression in cancer could be useful as a novel diagnostic marker for prostate cancer.

Bcl2-low-expressing MCF7 cells undergo necrosis rather than apoptosis upon staurosporine treatment

We present a ribozyme-based strategy for studying the effects of Bcl2 down-regulation. The anti-bcl2 hammerhead ribozyme Rz-bcl2 was stably transfected into MCF7 cancer cells and the cleavage of Bcl2 mRNA was demonstrated using a new assay for cleavage product detection, while Western blot analysis showed a concomitant depletion of Bcl2 protein. Rz-bcl2-expressing cells were more sensitive to staurosporine than control cells. Moreover, both molecular and cellular read-outs indicated that staurosporine-induced cell death was necrosis rather than apoptosis in these cells. The study of the effects of Bcl2 down-regulation was extended to the global MCF7 protein expression profile, exploiting a proteomic approach. Two reference electro-pherograms of Rz-bcl2-transfected cells, one with the ribozyme in a catalytically active form and the other with the ribozyme in a catalytically inactive form, were obtained. When comparing the two-dimensional maps, 53 differentially expressed spots were found, four of which were identified by MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS as calreticulin, nucleophosmin, phosphoglycerate kinase and pyruvate kinase. How the up-regulation of these proteins might help to explain the modification of Bcl2 activity is discussed.

The Ca2+ Homeostasis Defects in a pgm2Δ Strain of Saccharomyces cerevisiae Are Caused by Excessive Vacuolar Ca2+ Uptake Mediated by the Ca2+-ATPase Pmc1p

Loss of the major isoform of phosphoglucomutase (PGM) causes an accumulation of glucose 1-phosphate when yeast cells are grown with galactose as the carbon and energy source. Remarkably, the pgm2Delta strain also exhibits a severe imbalance in intracellular Ca(2+) homeostasis when grown under these conditions. In the present study, we examined how the pgm2Delta mutation alters yeast Ca(2+) homeostasis in greater detail. We found that a shift from glucose to galactose as the carbon source resulted in a 2-fold increase in the rate of cellular Ca(2+) uptake in wild-type cells, whereas Ca(2+) uptake increased 8-fold in the pgm2Delta mutant. Disruption of the PMC1 gene, which encodes the vacuolar Ca(2+)-ATPase Pmc1p, suppressed the Ca(2+)-related phenotypes observed in the pgm2Delta strain. This suggests that excessive vacuolar Ca(2+) uptake is tightly coupled to these defects in Ca(2+) homeostasis. An in vitro assay designed to measure Ca(2+) sequestration into intracellular compartments confirmed that the pgm2Delta mutant contained a higher level of Pmc1p-dependent Ca(2+) transport activity than the wild-type strain. We found that this increased rate of vacuolar Ca(2+) uptake also coincided with a large induction of the unfolded protein response in the pgm2Delta mutant, suggesting that Ca(2+) uptake into the endoplasmic reticulum compartment was reduced. These results indicate that the excessive Ca(2+) uptake and accumulation previously shown to be associated with the pgm2Delta mutation are due to a severe imbalance in the distribution of cellular Ca(2+) into different intracellular compartments.

Endoplasmic reticulum resident, immunoglobulin joining chain, can be secreted by perturbation of the calcium concentration in the endoplasmic reticulum

We established a transient human joining (J)-chain gene expression system in the baby hamster kidney (BHK) cell. The J-chain was detected as a 29-kDa single band on Western blotting. Immunofluorescent staining of the transfectant revealed an exclusive localization of the J-chain in the endoplasmic reticulum (ER). Intracellular transport experiment revealed that incubating conditions favorable for vesicular stomatitis virus glycoprotein (VSV-G) transport did not allow the J-chain to exit from the ER. Analysis of glycosylation status of the J-chain in the transfectant was examined by tunicamycin treatment, endoglycosidase H digestion, and also by treatment with brefeldin A. It was found that an N-glycosylation consensus site of the J-chain was functional, and intracellular J-chain was endoglycosidase H sensitive. These results indicate that, in the absence of any immunoglobulin molecules, J-chain localizes exclusively in the ER. We also tested whether the J-chain could be exported from the ER by perturbing the Ca2+ concentration in the ER. Cultivation of the J-chain transfectant in the presence of ionomycin resulted in the time-dependent secretion of the J-chain. The secreted J-chain was modified by the Golgi resident glycosylation enzymes, indicating that the secreted J-chain passed through the normal exocytic pathway.

Toward a consensus on the operation of receptor-induced calcium entry signals

Receptor-induced Ca2+ signals involve both Ca2+ release from intracellular stores and extracellular Ca2+ entry across the plasma membrane. The channels mediating Ca2+ entry and the mechanisms controlling their function remain largely a mystery. Here we critically assess current views on the Ca2+ entry process and consider certain modifications to the widely held hypothesis that Ca2+ store emptying is the fundamental trigger for receptor-induced Ca2+ entry channels. Under physiological conditions, receptor-induced store depletion may be quite limited. A number of distinct channel activities appear to mediate receptor-induced Ca2+ entry, and their activation is observed to occur through quite diverse coupling processes.

The calcium pump of the endoplasmic reticulum plays a role in midline signaling during early zebrafish development

During early vertebrate development, a signaling network is activated along the midline of the embryo. This signaling network induces the neural tube floor plate and ventral brain regions. In turn, induction of the ventral brain region is important for bilateral division of the forebrain and bilateral separation of the eyes. The present study provides direct evidence for a role of the endoplasmic reticulum Ca(2+) pump in zebrafish midline signaling. The endoplasmic reticulum Ca(2+) pump was inhibited in zebrafish embryos using thapsigargin or cyclopiazonic acid. Inhibition of the endoplasmic reticulum Ca(2+) pump during early gastrulation induces cyclopia, mimicking defects observed in cyclops, squint, one-eyed pinhead, and silberblick mutant embryos. In contrast, inhibition of the endoplasmic reticulum Ca(2+) pump during mid-gastrulation does not induce cyclopia, but does induce tail defects, mimicking defects observed in no-tail mutant embryos. This study is the first to relate thapsigargin and cyclopiazonic acid with induction of cyclopia. In addition, obtained results provide new information on the roles of Ca(2+) in embryonic development and may lead to new insights on the mechanisms underlying holoprosencephaly, a relatively common brain defect in human development.

Inverse gene expression patterns for macrophage activating hepatotoxicants and peroxisome proliferators in rat liver

Macrophage activation contributes to adverse effects produced by a number of hepatotoxic compounds. Transcriptional profiles elicited by two macrophage activators, LPS and zymosan A, were compared to those produced by 100 paradigm compounds (mostly hepatotoxicants) using cDNA microarrays. Several hepatotoxicants previously reported to activate liver macrophages produced transcriptional responses similar to LPS and zymosan, and these were used to construct a gene signature profile for macrophage activators in the liver. Measurement of cytokine mRNAs in the same liver samples by RT-PCR independently confirmed that these compounds are associated with macrophage activation. In addition to expected effects on acute phase proteins and metabolic pathways that are regulated by LPS and inflammation, a strong induction was observed for many endoplasmic reticulum-associated stress/chaperone proteins. Additionally, many genes in our macrophage activator signature profile were well-characterized PPARalpha-induced genes which were repressed by macrophage activators. A shared gene signature profile for peroxisome proliferators was determined using a training set of clofibrate, WY 14643, diethylhexylphthalate, diisononylphthalate, perfluorodecanoic acid, perfluoroheptanoic acid, and perfluorooctanoic acid. The signature profile included macrophage activator-induced genes that were repressed by peroxisome proliferators. NSAIDs comprised an interesting pharmacological class in that some compounds, notably diflunisal, co-clustered with peroxisome proliferators whereas several others co-clustered with macrophage activators, possibly due to endotoxin exposure secondary to their adverse effects on the gastrointestinal system. While much of these data confirmed findings from the literature, the transcriptional patterns detected using this toxicogenomics approach showed relationships between genes and biological pathways requiring complex analysis to be discerned.

Endoplasmic reticulum Ca(2+) homeostasis and neuronal death

The endoplasmic reticulum (ER) is a universal signalling organelle, which regulates a wide range of neuronal functional responses. Calcium release from the ER underlies various forms of intracellular Ca(2+) signalling by either amplifying Ca(2+) entry through voltage-gated Ca(2+) channels by Ca(2+)-induced Ca(2+) release (CICR) or by producing local or global cytosolic calcium fluctuations following stimulation of metabotropic receptors through inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The ER Ca(2+) store emerges as a single interconnected pool, thus allowing for a long-range Ca(2+) signalling via intra-ER tunnels. The fluctuations of intra-ER free Ca(2+) concentration regulate the activity of numerous ER resident proteins responsible for post-translational protein folding and modification. Disruption of ER Ca(2+) homeostasis results in the developing of ER stress response, which in turn controls neuronal survival. Altered ER Ca(2+) handling may be involved in pathogenesis of various neurodegenerative diseases including brain ischemia and Alzheimer dementia.

Identification of chromosomes associated with dental caries susceptibility using quantitative trait locus analysis in mice

Dental caries is a multifactorial, infectious disease with little known about the host genetic factors influencing susceptibility. This study aimed to identify the major candidate chromosomes for dental caries susceptibility and to detect the relevant regions within these. Quantitative trait locus (QTL) analysis was performed on genetic crosses of C3H/HeJ (caries-resistant) and C57BL/6J (caries-susceptible) mice inoculated with Streptococcus mutans serotype C. In a genomewide scan, three suggestive QTLs were detected on chromosomes 1, 2, and 7, one significant QTL was found on chromosome 2, and one highly significant QTL was detected on chromosome 8. The likelihood ratio statistic (LRS) was raised around the marker D1Mit21 in the middle region of chromosome 1, between D2Mit255 and D2Mit311 in the distal region of chromosome 2, and the region distal to D7Mit31 on chromosome 7. A significant QTL was located between the markers D2Mit237 and D2Mit101 on chromosome 2. The LRS was highly significantly raised between markers D8Mit208 and D8Mit280 on chromosome 8, and exceeded a highly significant level between markers D8Mit211 and D8Mit280. These results suggest that major gene(s) responsible for dental caries susceptibility or resistance are located in one or more of these regions.

TRAM2 protein interacts with endoplasmic reticulum Ca2+ pump Serca2b and is necessary for collagen type I synthesis

Cotranslational insertion of type I collagen chains into the lumen of the endoplasmic reticulum (ER) and their subsequent folding into a heterotrimeric helix is a complex process which requires coordinated action of the translation machinery, components of translocons, molecular chaperones, and modifying enzymes. Here we describe a role for the protein TRAM2 in collagen type I expression in hepatic stellate cells (HSCs) and fibroblasts. Activated HSCs are collagen-producing cells in the fibrotic liver. Quiescent HSCs produce trace amounts of type I collagen, while upon activation collagen synthesis increases 50- to 70-fold. Likewise, expression of TRAM2 dramatically increases in activated HSCs. TRAM2 shares 53% amino acid identity with the protein TRAM, which is a component of the translocon. However, TRAM2 has a C terminus with only a 15% identity. The C-terminal part of TRAM2 interacts with the Ca(2+) pump of the ER, SERCA2b, as demonstrated in a Saccharomyces cerevisiae two-hybrid screen and by immunoprecipitations in human cells. TRAM2 also coprecipitates with anticollagen antibody, suggesting that these two proteins interact. Deletion of the C-terminal part of TRAM2 inhibits type I collagen synthesis during activation of HSCs. The pharmacological inhibitor of SERCA2b, thapsigargin, has a similar effect. Depletion of ER Ca(2+) with thapsigargin results in inhibition of triple helical collagen folding and increased intracellular degradation. We propose that TRAM2, as a part of the translocon, is required for the biosynthesis of type I collagen by coupling the activity of SERCA2b with the activity of the translocon. This coupling may increase the local Ca(2+) concentration at the site of collagen synthesis, and a high Ca(2+) concentration may be necessary for the function of molecular chaperones involved in collagen folding.

Underglycosylation of ATF6 as a novel sensing mechanism for activation of the unfolded protein response

ATF6 is a key transcriptional activator of the unfolded protein response (UPR), which allows mammalian cells to maintain cellular homeostasis when they are subjected to a variety of environmental and physiological stresses that target the endoplasmic reticulum (ER). ATF6, a 90-kDa ER transmembrane protein, contains three evolutionarily conserved N-linked glycosylation sites within its carboxyl luminal domain. Although it is well established that p90ATF6 activation requires transit from the ER to the Golgi, where it is cleaved by the S1P/S2P protease system to generate a nuclear form p60ATF6 that acts as a transcriptional activator, the functional significance of p90ATF6 N-linked glycosylation is unknown. Here we show that ER Ca(2+) depletion stress, a triggering mechanism for the UPR, induces the formation of ATF6(f), which represents de novo partial glycosylation of newly synthesized p90ATF6. By mutating a single amino acid within the N-linked glycosylation site closest to the carboxyl terminus of p90ATF6, we recreated ATF6(f). This mutation sharply reduces p90ATF6 association with calreticulin, a major Ca(2+)-binding chaperone for N-glycoprotein. We further determined that ATF6(f) exhibits a faster rate of constitutive transport to the Golgi, resulting in a higher level of p60ATF6 in the nucleus and stronger transactivating activity in the absence of ER stress. Additional analysis of p90ATF6 mutants targeting single or multiple N-glycosylation sites also showed higher constitutive transactivating activity than wild type ATF6. Because accumulation of underglycosylated proteins in the ER is a potent inducer for the UPR, these studies uncover a novel mechanism whereby the glycosylation status of p90ATF6 can serve as a sensor for ER homeostasis, resulting in ATF6 activation to trigger the UPR.

Identification of an N-domain Histidine Essential for Chaperone Function in Calreticulin

Calreticulin is an endoplasmic reticulum (ER) luminal Ca(2+)-binding chaperone involved in folding of newly synthesized glycoproteins via the "calreticulin-calnexin cycle." We reconstituted ER of calreticulin-deficient cells with N-terminal histidine (His25, His82, His128, and His153) calreticulin mutants and carried out a functional analysis. In crt(-/-) cells bradykinin-dependent Ca2+ release is altered, and the reestablishment of bradykinin-dependent Ca2+ release was used as a marker for calreticulin function. Bradykinin-dependent Ca2+ release from the ER was rescued by wild type calreticulin and by the His25, His82, or His128 mutant but not by the His153 mutant. Wild type calreticulin and the His25, His82, and His128 mutants all prevented in vitro thermal aggregation of malate dehydrogenase and IgY, whereas the His153 mutant did not, indicating that His153 chaperone function was impaired. Biophysical analysis of His153 mutant revealed that conformation changes in calreticulin mutant may be responsible for the loss of its chaperone activity. We conclude that mutation of a single amino acid residue in calreticulin has devastating consequences for its chaperone function, indicating that mutations in chaperones may play a significant role in protein folding disorders.

Calreticulin is an upstream regulator of calcineurin

Ca(2+) is a signalling molecule involved in virtually every aspect of cell function. The endoplasmic reticulum (ER) is an important and dynamic organelle responsible for storage of the majority of intracellular Ca(2+). Within the ER lumen are proteins that function as Ca(2+) buffers and/or molecular chaperones including calreticulin, a multifunctional Ca(2+)-binding protein. Calreticulin-deficiency is lethal in utero due to impaired cardiac development. In the absence of calreticulin Ca(2+) storage capacity in the ER and InsP(3) receptor mediated Ca(2+) release from ER are compromised. Remarkably, over-expression of constitutively active calcineurin in the hearts of calreticulin deficient mice rescues them from embryonic lethality and produces live calreticulin deficient animals. These observations provide first evidence that calreticulin is a key upstream regulator of calcineurin in the Ca(2+)-signalling cascade and they highlight the importance of ER during early stages of cellular commitment and tissue development during organogenesis.

Neuronal ageing from an intraneuronal perspective: roles of endoplasmic reticulum and mitochondria

The nature of brain ageing and the age-dependent decline in cognitive functions remains poorly understood. Physiological brain ageing is characterised by mild mental dysfunctions, whereas age-dependent neurodegeneration, as illustrated by Alzheimer disease (AD), results rapidly in severe dementia. These two states of the aged brain, the physiological and the pathological, are fundamentally different as the latter stems from significant neuronal loss, whereas the former develops without significant neuronal demise. In this paper, we review the changes in neuronal Ca(2+) homeostasis that occur during brain ageing, and conclude that normal, physiological ageing is characterised mainly by a decrease of neuronal homeostatic reserve, defined as the capacity to respond effectively to functional and metabolic stressors, but does not reach the trigger required to induce neuronal death. In contrast, during neurodegenerative states, Ca(2+) homeostasis is affected early during the pathological process and result in significant neuronal demise. We also review recent evidence suggesting that the endoplasmic reticulum (ER) might play an important role in controlling the balance between healthy and pathological neuronal ageing.

Death from within: apoptosis and the secretory pathway

Recent studies have highlighted the importance of the secretory pathway in stress-induced apoptotic signaling. Sensing stress at the endoplasmic reticulum and Golgi might first trigger recovery mechanisms, followed by apoptosis if repair is unsuccessful. Cleavage of endoplasmic-reticulum- or Golgi-resident proteins can signal repair or apoptosis and promote organelle disassembly during apoptosis. Initiation of apoptosis from the secretory pathway requires components of the death machinery localized to these membranes. Extensive trafficking between compartments of the secretory pathway might allow the cell to integrate signals and to determine the proper response to a particular stress.

Neuronal endoplasmic reticulum acts as a single functional Ca2+ store shared by ryanodine and inositol-1,4,5-trisphosphate receptors as revealed by intra-ER [Ca2+] recordings in single rat sensory neurones

We addressed the fundamentally important question of functional continuity of endoplasmic reticulum (ER) Ca(2+) store in nerve cells. In cultured rat dorsal root ganglion neurones we measured dynamic changes in free Ca(2+) concentration within the ER lumen ([Ca(2+)](L)) in response to activation of inositol-1,4,5-trisphosphate receptors (InsP(3)Rs) and ryanodine receptors (RyRs). We found that both receptors co-exist in these neurones and their activation results in Ca(2+) release from the ER as judged by a decrease in [Ca(2+)](L). Depletion of Ca(2+) stores following an inhibition of sarco(endoplasmic)reticulum Ca(2+)-ATPase by thapsigargin or cyclopiazonic acid completely eliminated Ca(2+) release via both InsP(3)Rs and RyRs. Similarly, when the store was depleted by continuous activation of InsP(3)Rs, activation of RyRs (by caffeine or 0.5 microM ryanodine) failed to produce Ca(2+) release, and vice versa, when the stores were depleted by activators of RyRs, the InsP(3)-induced Ca(2+) release disappeared. We conclude that in mammalian neurones InsP(3)Rs and RyRs share the common continuous Ca(2+) pool associated with ER.

Localization and regulation of Ca2+ entry and exit pathways in exocrine gland cells

Studies of Ca2+ transport pathways in exocrine gland cells have been useful, chiefly because of the polarized nature of the secretory epithelial cells. In pancreatic acinar cells, for example, Ca2+ reloading of empty intracellular stores can occur solely via Ca2+ entry through the basal part of the plasma membrane. On the other hand, the principal site for intracellular Ca2+ release-with the highest concentration of inositol 1,4,5-trisphosphate (IP(3)) receptors-is in the apical secretory pole close to the apical plasma membrane. This apical part of the plasma membrane contains the highest density of Ca2+ pumps and is therefore the principal site for Ca2+ extrusion. On the basis of the known properties of Ca2+ entry and exit pathways in exocrine gland cells, the mechanisms controlling Ca2+ exit and entry are discussed in relation to recent direct information about Ca2+ transport into and out of the endoplasmic reticulum (ER) and the mitochondria in these cells.