Disease relevance of Calr

• Structural homology between the rat calreticulin gene product and the Onchocerca volvulus antigen Ral-1 [1].
• Calreticulin inhibits the MEK1,2-ERK1,2 pathway in alpha 1-adrenergic receptor/Gh-stimulated hypertrophy of neonatal rat cardiomyocytes [2].
• We have reconstituted the binding of recombinant calreticulin to two glycoprotein substrates, vesicular stomatitis virus G protein and influenza hemagglutinin, in vitro [3].
• This is the first report of calreticulin inhibiting a down-regulatory function of a sterol hormone and may help explain the refractoriness of the secondary hyperparathyroidism of many chronic renal failure patients to 1,25-(OH)2D3 [4].
• To determine whether intra-uterine growth retardation or programmed hypertension was associated with altered calreticulin or calsequestrin expression, effects of prenatal glucocorticoid overexposure (maternal dexamethasone treatment on days 15-21 of pregnancy) were examined during fetal life and postnatal development until adulthood (24 weeks) [5].

High impact information on Calr

• Autocrine motility factor receptor (AMF-R) is a marker for a smooth subdomain of the ER, shown here by confocal microscopy to be distinct from, yet closely associated with the calnexin- or calreticulin-labeled ER [6].
• The structure type observed for CRT(189-288) provides an additional basis for functional studies of the abundant endoplasmic reticulum chaperone calreticulin [7].
• In this system UDP-Glc:glycoprotein glucosyltransferase (GT) serves as a glycoprotein specific folding sensor by specifically glucosylating N-linked glycans in misfolded glycoproteins thus retaining them in the calnexin/calreticulin chaperone cycle [8].
• While the mRNA levels for N-CAM decreased after such treatment, the levels of mRNAs for glutamine synthetase and calreticulin increased [9].
• A dynamic multimolecular assembly termed the peptide-loading complex (PLC) participates in this process and is composed of MHC class I molecules, calreticulin, ERp57, and tapasin bound to the transporter associated with antigen processing (TAP) peptide transporter [10].

Chemical compound and disease context of Calr

• To elucidate whether G(h) mediates norepinephrine-stimulated intracellular signal transductions leading to activation of extracellular signal-regulated kinases (ERKs) and neonatal rat cardiomyocyte hypertrophy, we examined the effects of G(h) on the activation of ERKs and inhibitory effects of CRT on alpha(1)-adrenoceptor/G(h) signaling [2].
• We report that treatment of rat C6 glioma cells with valproate can also increase the expression of additional endoplasmic reticulum stress proteins, GRP94 and calreticulin [11].

Biological context of Calr

• Upon culture in a differentiation medium containing fetal calf serum (1%) and retinoic acid (10 nm), cells transfected with the calreticulin gene were highly susceptible to apoptosis compared with controls [12].
• Calreticulin has been shown to be essential for cardiac and neural development in mice, but the mechanism by which it functions in cell differentiation is not fully understood [12].
• The complete amino acid sequence of calreticulin, deduced from the cDNA sequence, shares a high degree of identity with that of the analogous mouse protein [13].
• These results suggest that calreticulin, which is not specific to testis, is closely associated with spermatogenesis of rats [13].
• The latter two findings were paralleled in activated lymphocytes by an increase of expression of ER markers, involved (calreticulin; Ca2+ ATPase) or not (protein disulfide isomerase) in the regulation of Ca2+ homeostasis [14].

Anatomical context of Calr

• Calreticulin (CRT) is an abundant, soluble molecular chaperone of the endoplasmic reticulum [15].
• Immunogold labelling established the ER localization of calreticulin in transfected and nontransfected COS cells [16].
• An endoplasmic reticulum protein, calreticulin, is transported into the acrosome of rat sperm [13].
• Immunohistochemical studies revealed that calreticulin was present in the acrosome of both round spermatids and mature sperm [13].
• A monoclonal antibody, C6, raised against skeletal-muscle calsequestrin cross-reacted with calreticulin in SDS/PAGE immunoblots, but polyclonal antibodies reacted with native calreticulin only weakly, or not at all, after SDS denaturation [17].

Associations of Calr with chemical compounds

• This enzyme catalyzes the formation of disulfide bonds in CNX and CRT-bound glycoprotein substrates [15].
• In contrast a calreticulin mutant, which lacked the Ca(2+)-binding domain but included the KDEL sequence, could escape from the ER to a much higher extent [16].
• Immuno-gold decoration of liver ultrathin cryosections with affinity-purified antibodies against liver calreticulin revealed luminal labelling of vacuolar profiles indistinguishable from calciosomes, the subcellular structures previously identified by the use of anti-calsequestrin antibodies [17].
• Glucose trimming from newly synthesized glycoproteins regulates their interaction with the calnexin/calreticulin chaperone system [18].
• Co-overexpression of G(h) and CRT abolished norepinephrine-induced ERKs activation [2].
• In vitro and in NIH 3T3 cells in culture, the level of arginylated CRT increased with the addition of a Ca2+ chelator to the medium, whereas a decreased arginine incorporation into CRT was found in the presence of Ca2+ [19].

Physical interactions of Calr

• These results show that calreticulin prevents the binding of vitamin D receptor-retinoid X receptor-beta to the PTH VDRE in gel retardation assays and prevents the transcriptional effect of 1,25-(OH)2D3 on the PTH gene [4].
• Taken together, our results indicate that PKC interacts in vivo with calreticulin and suggest that they can operate in common signalling pathways [20].

Regulatory relationships of Calr

• Recently, it has been shown that the calreticulin (CRT) down-regulates both GTP binding and transglutaminase activities of TGII [2].
• Calreticulin inhibits vitamin D's action on the PTH gene in vitro and may prevent vitamin D's effect in vivo in hypocalcemic rats [4].
• Studies in vitro with purified calreticulin with the use of an overlay assay approach demonstrated that it binds to activated PKC isoenzymes expressed in rat hepatocytes [20].
• Calreticulin inhibits glucocorticoid- but not cAMP-sensitive expression of tyrosine aminotransferase gene in cultured McA-RH7777 hepatocytes [21].
• Heat shock-regulated expression of calreticulin in retinal pigment epithelium [22].

Other interactions of Calr

• We further demonstrated that the residues 225-251 at the tip of the CRT P-domain are involved in direct contacts with ERp57 [15].
• In the present study, we have investigated the interactions of endogenous Tg with calreticulin and with several other ER chaperones [23].
• Previously, we solved the NMR structure of the central proline-rich P-domain of CRT comprising residues 189-288 [15].
• Two-dimensional immunoblot analysis of the antigenic proteins revealed that the 56kDa and 55kDa proteins showed 4.2 and 4.0 pI values and were estimated to be protein disulfide isomerase (PDI) and calreticulin, respectively, from NH2-terminal amino acid sequence analysis [24].
• The results indicate that calreticulin is present in the acrosome of spermatids of rat testes [25].

Analytical, diagnostic and therapeutic context of Calr

• Isothermal titration microcalorimetry (ITC) now showed affinity of this fragment for ERp57 similar to that of the intact P-domain, demonstrating that CRT(221-256) may be used as a low molecular mass mimic of CRT for further investigations of the interaction with ERp57 [15].
• Northern blot analysis of RNAs from purified populations of rat spermatogenic cells indicated that the calreticulin mRNA was present in both pre- and postmeiotic cells [13].
• CRT was detected as a 59-kDa protein in rat pulp cell culture medium and dentin extracellular matrix extract by Western blotting [26].
• The expression of CRT was studied by immunohistochemistry, ultrastructural immunocytochemistry and in situ hybridization in developing rat teeth [26].
• N-Terminal sequence analysis showed that the 60 kDa protein shares a high homology with rat calreticulin, whereas the 74 kDa protein shows no homology with any known protein [27].

References