Evolving Evidence of Calreticulin as a Pharmacological Target in Neurological Disorders

Antibodies to calnexin and mutated calreticulin are common in human sera

PURPOSE OF THE STUDY

Calreticulin is an endoplasmic reticulum chaperone protein, which is involved in protein folding and in peptide loading of major histocompatibility complex class I molecules together with its homolog calnexin. Mutated calreticulin is associated with a group of hemopoietic disorders, especially myeloproliferative neoplasms. Currently only the cellular immune response to mutated calreticulin has been described, although preliminary findings have indicated that antibodies to mutated calreticulin are not specific for myeloproliferative disorders. These findings have prompted us to characterize the humoral immune response to mutated calreticulin and its chaperone homologue calnexin.

PATIENTS AND METHODS

We analyzed sera from myeloproliferative neoplasm patients, healthy donors and relapsing-remitting multiple sclerosis patients for the occurrence of autoantibodies to wild type and mutated calreticulin forms and to calnexin by enzyme-linked immunosorbent assay.

RESULTS

Antibodies to mutated calreticulin and calnexin were present at similar levels in serum samples of myeloproliferative neoplasm and multiple sclerosis patients as well as healthy donors. Moreover, a high correlation between antibodies to mutated calreticulin and calnexin was seen for all patient and control groups. Epitope binding studies indicated that cross-reactive antibodies bound to a three-dimensional epitope encompassing a short linear sequence in the C-terminal of mutated calreticulin and calnexin.

CONCLUSION

Collectively, these findings indicate that calreticulin mutations may be common and not necessarily lead to onset of myeloproliferative neoplasm, possibly due to elimination of cells with mutations. This, in turn, may suggest that additional molecular changes may be required for development of myeloproliferative neoplasm.

Clioquinol induces autophagy by down-regulation of calreticulin in human neurotypic SH-SY5Y cells

Clioquinol (CQ) is considered as a promising drug of neurodegenerative diseases. However, the underlying mechanism is unclear. Our previous study has proved that CQ induces S-phase cell cycle arrest through the elevation of intracellular calcium concentration ([Ca2+]i) with high levels of SERCA2. Furthermore, it could induce autophagy in an intracellular calcium independent manner in human neurotypic SH-SY5Y cells. In this study, the involvement of calreticulin (CRT) in autophagy induced by CQ was investigated. Our results illustrated the endoplasmic reticulum (ER) stress induced by CQ and DTT led to the cell death in different manners. DTT, an ER stress positive control, induced UPR accompanied with up-regulation of CRT and apoptosis, while CQ inhibited UPR accompanied with down-regulation of CRT，resulting in autophagy. Then, overexpression of CRT was shown to cause UPR and decrease [Ca2+]i, leading to cell apoptosis and inhibition of S-phase arrest induced by CQ. While the UPR was alleviated and autophagy was further enhanced in CRT deficient cells by using targeted siRNA. Meanwhile, down-regulation of CRT resulted in [Ca2+]i overload and induction of S-phase arrest. Finally, we found that the effect of CQ on the HT22 cells was similar to that on the SH-SY5Y cells. Our data showed for the first time that CQ decreased expression of CRT, leading to autophagy, an increase of [Ca2+]i, and cell S-phase arrest in the neurotypic cells. The present study describes the cellular signal pathways regulating autophagy by CQ and highlights the potential therapeutic application of CQ in neurodegenerative disorders.

Oxytocin action on components of endoplasmic reticulum in hippocampal neuronal cells

Despite the known importance of the endoplasmic reticulum (ER) in protein synthesis and vesicular transport, it is not clear whether neuropeptide and neuromodulator oxytocin can directly affect components of the ER in neuronal cells. Therefore, in the present study, we hypothesize that incubation of hippocampal neuronal cells in a presence of oxytocin 1) plays a role in the regulation of the expression of selected ER chaperone components and molecules involved in unfolded protein response pathway 2) affects distribution of the intracellular fluorescence signal highly selective for the ER. We found that oxytocin (1 μM) after 60 min significantly decreased the gene expression of oxidoreductase Ero1β, chaperone glucose-regulated proteins (Grp) 78 and Grp94. A significant decrease in GRP78 protein levels in response to oxytocin treatment occurred after 30, 60 and 120 min. We also observed a time-dependent increase in calreticulin protein levels with a statistically significant increase observed after 360 min. We found that the dynamics of the ER network changes significantly within 2 h of incubation under the influence of oxytocin. In conclusion we have shown that ER chaperones, oxidoreductases and trafficking molecules in neuronal cells are changing in response to oxytocin treatment in a short-term scenario potentially relevant for growth of dendrites and axons.

Proximity proteomics of C9orf72 dipeptide repeat proteins identifies molecular chaperones as modifiers of poly-GA aggregation

The most common inherited cause of two genetically and clinico-pathologically overlapping neurodegenerative diseases, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), is the presence of expanded GGGGCC intronic hexanucleotide repeats in the C9orf72 gene. Aside from haploinsufficiency and toxic RNA foci, another non-exclusive disease mechanism is the non-canonical translation of the repeat RNA into five different dipeptide repeat proteins (DPRs), which form neuronal inclusions in affected patient brains. While evidence from cellular and animal models supports a toxic gain-of-function of pathologic poly-GA, poly-GR, and poly-PR aggregates in promoting deposition of TDP-43 pathology and neurodegeneration in affected brain areas, the relative contribution of DPRs to the disease process in c9FTD/ALS patients remains unclear. Here we have used the proximity-dependent biotin identification (BioID) proximity proteomics approach to investigate the formation and collective composition of DPR aggregates using cellular models. While interactomes of arginine rich poly-GR and poly-PR aggregates overlapped and were enriched for nucleolar and ribosomal proteins, poly-GA aggregates demonstrated a distinct association with proteasomal components, molecular chaperones (HSPA1A/HSP70, HSPA8/HSC70, VCP/p97), co-chaperones (BAG3, DNAJA1A) and other factors that regulate protein folding and degradation (SQSTM1/p62, CALR, CHIP/STUB1). Experiments in cellular models of poly-GA pathology show that molecular chaperones and co-chaperones are sequestered to the periphery of dense cytoplasmic aggregates, causing depletion from their typical cellular localization. Their involvement in the pathologic process is confirmed in autopsy brain tissue, where HSPA8, BAG3, VCP, and its adapter protein UBXN6 show a close association with poly-GA aggregates in the frontal cortex, temporal cortex, and hippocampus of c9FTLD and c9ALS cases. The association of heat shock proteins and co-chaperones with poly-GA led us to investigate their potential role in reducing its aggregation. We identified HSP40 co-chaperones of the DNAJB family as potent modifiers that increased the solubility of poly-GA, highlighting a possible novel therapeutic avenue and a central role of molecular chaperones in the pathogenesis of human C9orf72-linked diseases.

Impact of Calreticulin and Its Mutants on Endoplasmic Reticulum Function in Health and Disease

The endoplasmic reticulum (ER) performs key cellular functions including protein synthesis, lipid metabolism and signaling. While these functions are spatially isolated in structurally distinct regions of the ER, there is cross-talk between the pathways. One vital player that is involved in ER function is the ER-resident protein calreticulin (CALR). It is a calcium ion-dependent lectin chaperone that primarily assists in glycoprotein synthesis in the ER as part of the protein quality control machinery. CALR also buffers calcium ion release and mediates other glycan-independent protein interactions. Mutations in CALR have been reported in a subset of chronic blood tumors called myeloproliferative neoplasms. The mutations consist of insertions or deletions in the CALR gene that all cause a + 1 bp shift in the reading frame and lead to a dramatic alteration of the amino acid sequence of the C-terminal domain of CALR. This alters CALR function and affects cell homeostasis. This chapter will discuss how CALR and mutant CALR affect ER health and disease.

The Role of Calcium Signaling in Regulation of Epithelial-Mesenchymal Transition

Despite substantial advances in the field of cancer therapeutics, metastasis is a significant challenge for a favorable clinical outcome. Epithelial to mesenchymal transition (EMT) is a process of acquiring increased motility, invasiveness, and therapeutic resistance by cancer cells for their sustained growth and survival. A plethora of intrinsic mechanisms and extrinsic microenvironmental factors drive the process of cancer metastasis. Calcium (Ca2+) signaling plays a critical role in dictating the adaptive metastatic cell behavior comprising of cell migration, invasion, angiogenesis, and intravasation. By modulating EMT, Ca2+ signaling can regulate the complexity and dynamics of events leading to metastasis. This review summarizes the role of Ca2+ signal remodeling in the regulation of EMT and metastasis in cancer.

Calreticulin regulated intrinsic apoptosis through mitochondria-dependent and independent pathways mediated by ER stress in arsenite exposed HT-22 cells

Arsenic is a naturally occurring environmental toxicant. Chronic exposure to arsenic is linked with neurological damage. Although the mechanisms remain to be elucidated, it is currently believed that neural cell apoptosis is one of the underlying mechanisms of arsenic-induced neurotoxicity. Calreticulin (CRT) is a quality control chaperone located in the lumen of the endoplasmic reticulum (ER), which participates in many signaling pathways including apoptosis. However, the role of CRT in apoptosis is controversial. Whether CRT plays a role in arsenite-induced apoptosis and the relationship between CRT and ER stress-mediated apoptosis have not been mentioned before. In this study, we found that CRT expression as well as the cell apoptosis levels increased in a dose dependent manner upon arsenite exposure in HT-22 cells, a mouse hippocampal neural cell line. In addition, arsenite exposure resulted in the up-regulation of ER stress indicator GRP78 and ER stress-related proteins including p-PERK, ATF4, CHOP, calpain2 and cleaved caspases-12, accompanied by the down-regulation of Bcl-2 and up-regulation of Bax and cleaved caspase-3. Silence of CRT remarkably alleviated arsenite-induced apoptosis and reversed the expression of the proteins above. Our findings confirmed the role of CRT in the induction of apoptosis upon arsenite exposure and suggested that CRT mediated the intrinsic apoptotic cell death including both mitochondria-dependent (PERK/ATF4/CHOP/Bcl-2) and independent (calpain2/caspases-12) pathways initiated by ER stress, which we believed to be a previously undocumented property of arsenite-induced apoptosis.