Isolation of a calreticulin-like calcium binding protein from bovine brain

Endoplasmic Reticulum Stress Response of Bombyx Mori Calreticulin

We isolated a calreticulin cDNA from the silkworm, Bombyx mori. The cDNA encodes 398 amino acid residues of B. mori calreticulin, with an endoplasmic reticulum retentional HDEL motif at its C-terminus and a predicted molecular mass of 45,801 Da. The B. mori calreticulin shows high protein homology with calreticulin from G. mellonella (88%), A. aegypti (71%), D. melanogaster (69%) and H. sapiens (63%). The highest level of mRNA expression of B. mori calreticulin was exhibited in the fat body of this insect. Although expression of B. mori calreticulin was affected by disturbances in intracellular calcium levels, other ER stress conditions such as inhibition of intracellular protein transport, reduction of disulfide formation, glycosylation inhibition, heat shock and oxidative stress did not disrupt induction of B. mori calreticulin.

Increased expression of endoplasmic reticulum stress proteins following chronic valproate treatment of rat C6 glioma cells

The anticonvulsant sodium valproate has been shown to be an effective treatment for bipolar disorder, however, its precise mechanism of action has yet to be determined. It has been suggested that adaptational changes in gene expression are critical for valproate's prophylactic effects. Previous studies in our lab have shown that one gene that may be regulated by valproate is the 78-kilodalton glucose-regulated protein (GRP78). 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. All three proteins showed similar concentration-dependent increases in messenger RNA abundance. Chronic (seven days) treatment significantly increased GRP78 and GRP94 messenger RNA expression, whereas calreticulin expression increased after both acute and chronic treatment. Increases in mRNA expression corresponded to a similar increase in protein expression. The roles of GRP78, GRP94 and calreticulin as molecular chaperones and calcium binding proteins, suggest that these results might have functional relevance to the therapeutic action of valproate.

Increased calreticulin stability in differentiated NG-108-15 cells correlates with resistance to apoptosis induced by antisense treatment

Since its first identification as a high-affinity calcium-binding protein over two decades ago [T.J. Ostwald and D.H. MacLennan, Isolation of a high-affinity calcium-binding protein from sarcoplasmic reticulum, J. Biol. Chem., 249 (1974) 974-979], calreticulin has become recognized as a multifunctional protein involved in a wide variety of cellular processes. We have previously shown that it has a protective function in Ca2+-mediated cell death [N. Liu, R.E. Fine, E. Simons and R.J. Johnson, Decreasing calreticulin expression lowers the Ca2+ response to bradykinin and increases sensitivity to ionomycin in NG-108-15 cells, J. Biol. Chem. , 269 (1994) 28635-28639]. We report here that in NG-108-15 neuroblastomaxglioma hybrid cells, calreticulin protein levels increase markedly when these cells are induced to differentiate by treating them with N,N-dibutyryl cAMP (db-cAMP). We demonstrate that the reason for this increase is mostly due to a large increase in the turnover time of calreticulin in differentiated cells. We also show that a calreticulin antisense oligonucleotide, CrtAS1, previously described by Liu and co-workers [N. Liu, R.E. Fine, E. Simons and R.J. Johnson, Decreasing calreticulin expression lowers the Ca2+ response to bradykinin and increases sensitivity to ionomycin in NG-108-15 cells, J. Biol. Chem., 269 (1994) 28635-28639] causes cell death in undifferentiated NG-108-15 cells when antisense treatment is extended for more than 24 h. This effect is not seen in NG-108-15 cells that have been induced to differentiate with db-cAMP until the cells have been treated with antisense for more than 4 days, due to the increased stability of Crt in these cells. Our results indicate that the mechanism by which these cells die is likely to be apoptosis.

Calreticulin expression in spinal motoneurons of the rat

We have examined the expression of calreticulin in rat spinal motoneurons in order to reveal the occurrence and distribution of Ca2(+)-storage organelles in these neurons. Calreticulin, the non-muscle equivalent of calsequestrin, is the low-affinity, high-capacity calcium-binding protein responsible for intracompartmental Ca2(+)-storage in a number of different cell types. The results of the present immunohistochemical study show that all spinal motoneurons express calreticulin at approximately the same level; no significant differences in cytoplasmic immunostaining intensity were observed between different motoneuron pools or between small and large spinal motoneurons. Immunoelectron microscopy revealed that the intracellular localization of calreticulin within spinal motoneurons was confined to the endoplasmic reticulum and to spherical or pleiomorphic, frequently 'coated' vesicles with a diameter ranging between 120 and 150 nm. Some of these vesicles may represent the so-called calciosomes, the intracellular Ca2(+)-storage vesicles described in liver cells and in cerebellar Purkinje cells. The molecular components responsible for the uptake and release of Ca2+ from the Ca2(+)-storage organelles in spinal motoneurons still remain to be identified.

Ral and Rab3a are major GTP-binding proteins of axonal rapid transport and synaptic vesicles and do not redistribute following depolarization stimulated synaptosomal exocytosis

We have employed high-resolution SDS polyacrylamide gels to demonstrate that there are two major low-molecular-weight GTP-binding proteins associated with axonal membranes including synaptic vesicles, rapid transported membranes and clathrin-coated vesicles. We demonstrate that one of the major proteins is Ral and that the other is Rab3A. Following the depolarization of synaptosomes resulting in increased neurotransmitter release, we see no significant dissociation of either Ral or Rab3a from synaptic vesicle derived membranes in contrast to results reported previously.

Comparison of cDNAs from bovine brain coding for two isoforms of calreticulin

Calreticulin is a major calcium-binding protein of the endoplasmic reticulum of non-muscle cells. In addition to a 1.9-kb calreticulin mRNA, some evidence has suggested the existence of another transcript of 3.75 kb, which is very similar to calreticulin. We report here the isolation and sequencing of cDNA clones from a bovine brain lambda gt11 cDNA library, two of which appear to code for calreticulin and a third for a novel isoform of calreticulin. The deduced amino-acid sequence of the novel clone shares high similarity with mouse calreticulin in the C-terminal 318 amino acids. However, its N-terminal sequence is completely divergent. Northern blot analysis of bovine cerebral cortex RNA indicates that the conserved region of the clone hybridizes to two messages of 1.9 kb and 3.75 kb. The divergent region of this clone hybridizes to the 3.75-kb message, but not to the 1.9-kb message. We believe that this novel clone corresponds to an alternate form of calreticulin which is identical to calreticulin toward the C-terminus, but completely different at the N-terminal region, and that this isoform is encoded by a much larger message.

Differences in the subcellular localization of calreticulin and organellar Ca(2+)-ATPase in neurons

It has become clear that calcium is an important mediator in the transduction of signals due to ligand binding to cell surface receptors. Cytosolic calcium is typically maintained at low levels in both muscle and non-muscle cells and intracellular sequestering of calcium appears to be important in this process. The identification of intracellular calcium pools has been the subject of much recent study, and it has been proposed that such pools would contain three components: a calcium-activated pump or Ca(2+)-ATPase, a calcium channel such as the inositol trisphosphate receptor or ryanodine receptor, and a high-capacity calcium-binding protein such as calsequestrin or calreticulin. We report here on the localization of two components, the organellar Ca(2+)-ATPase (SERCA) and calreticulin, in neuronal tissues. Using immunofluorescence and subcellular fractionation, we have found that for the most part, these two proteins do not co-localize in neuron cell bodies, dendrites, or axons; but may co-localize at the axon terminal.

Localization of gene expression of calreticulin in the brain of adult mouse

The localization of gene expression of calreticulin, a calcium-binding protein in the endoplasmic reticulum, was examined throughout the entire brain of adult mice by in situ hybridization. Calreticulin mRNA is expressed widely and heterogeneously in discrete neurons throughout the brain, but the white matters expressed it weekly or faintly. In the olfactory bulb, the mRNA is expresses moderately in the mitral cells, but weakly in the periglomerular cells and internal granule cells. In the cerebrum, the gene is expressed intensely in the piriform cortex, but weakly in neocortex, the entorhinal cortex and the amygdaloid nuclei. In the hippocampal formation, calreticulin mRNA is expressed intensely in the CA1-CA3 regions but less intensely in the granule cells of the dentate gyrus. The caudate-putamen, thalamic and hypothalamic nuclei, and mammillary nuclei express the mRNA weakly or faintly. In the mesencephalon, pons and medulla, moderate expression of the mRNA is detected in the pontine nuclei and the locus ceruleus. Weak expression of the mRNA is detected in several discrete nuclei and zones such as the substantia nigra, the superior colliculus and the central gray. Expression signals of calreticulin mRNA are faint in the inferior olive. In the cerebellum, calreticulin mRNA is expressed moderately in the Purkinje cells whereas no significant expression is detected in the granule cells. The plexus choroideus of the lateral, third and fourth ventriculi express calreticulin mRNA intensely although no distinct expression of the mRNA is discerned in the ependyma.