Purification and characterization of a soluble calnexin from human placenta

Calnexin Is Involved in Forskolin-induced Syncytialization in Cytotrophoblast Model BeWo Cells

Calnexin (CNX), a membrane-bound molecular chaperone, is involved in protein folding and quality control of nascent glycoproteins in the endoplasmic reticulum. We previously suggested critical roles of calreticulin, a functional paralogue of CNX, in placentation, including invasion of extravillous trophoblasts and syncytialization of cytotrophoblasts. However, the roles of CNX in placentation are unclear. In human choriocarcinoma BeWo cells, which serve as an experimental model of syncytialization, CNX knockdown suppressed forskolin-induced cell fusion and β-human chorionic gonadotropin (β-hCG) induction. Cell-surface luteinizing hormone/chorionic gonadotropin receptor, a β-hCG receptor, was significantly down-regulated in CNX-knockdown cells, which suggested the presence of a dysfunctional autocrine loop of β-hCG up-regulation. In this study, we also found abundant CNX expression in normal human placentas. Collectively, our results revealed the critical role of CNX in the syncytialization-related signaling in a villous trophoblast model and suggest a link between CNX expression and placenta development.

Cancer anti-angiogenesis vaccines: Is the tumor vasculature antigenically unique?

Angiogenesis is essential for the growth and metastasis of solid tumors. The tumor endothelium exists in a state of chronic activation and proliferation, fueled by the tumor milieu where angiogenic mediators are aberrantly over-expressed. Uncontrolled tumor growth, immune evasion, and therapeutic resistance are all driven by the dysregulated and constitutive angiogenesis occurring in the vasculature. Accordingly, great efforts have been dedicated toward identifying molecular signatures of this pathological angiogenesis in order to devise selective tumor endothelium targeting therapies while minimizing potential autoimmunity against physiologically normal endothelium. Vaccination with angiogenic antigens to generate cellular and/or humoral immunity against the tumor endothelium has proven to be a promising strategy for inhibiting or normalizing tumor angiogenesis and reducing cancer growth. Here we review tumor endothelium vaccines developed to date including active immunization strategies using specific tumor endothelium-associated antigens and whole endothelial cell-based vaccines designed to elicit immune responses against diverse target antigens. Among the novel therapeutic options, we describe a placenta-derived endothelial cell vaccine, ValloVax™, a polyvalent vaccine that is antigenically similar to proliferating tumor endothelium and is supported by pre-clinical studies to be safe and efficacious against several tumor types.

Purification and characterization of calreticulin: a Ca²⁺-binding chaperone from sheep kidney

Calreticulin (CRT) is a molecular chaperone with a molecular mass of 46 kDa present in the endoplasmic reticulum (ER). This protein is primarily involved in the regulation of intracellular Ca(2+) homeostasis and Ca(2+) storage in the ER. CRT also plays a significant role in autoimmunity and cancer. This protein contains three distinct structural domains with specialized functions. Here, we are reporting a simple procedure for the purification of CRT from mammalian kidney. To isolate CRT, sheep kidney was crushed and kept for 12 h in the extraction buffer. The lysate was centrifuged, and supernatant was precipitated by ammonium sulphate. The precipitate of 90 % ammonium sulphate was extensively dialyzed and loaded on DEAE-Hi-Trap FF and Mono Q chromatography columns. The purity of CRT was confirmed by SDS-PAGE. Finally, the protein was identified by matrix-assisted laser desorption/ionization time of flight. The purified protein was further characterized for secondary structural elements using the far-UV circular dichroism measurements. Our purification procedure is fast and simple with high yield.

Probing the structure of human protein disulfide isomerase by chemical cross-linking combined with mass spectrometry

Protein disulfide-isomerase (PDI) is a four-domain flexible protein that catalyzes the formation of disulfide bonds in the endoplasmic reticulum. Here we have analyzed native PDI purified from human placenta by chemical cross-linking followed by mass spectrometry (CXMS). In addition to PDI the sample contained soluble calnexin and ERp72. Extensive cross-linking was observed within the PDI molecule, both intra- and inter-domain, as well as between the different components in the mixture. The high sensitivity of the analysis in the current experiments, combined with a likely promiscuous interaction pattern of the involved proteins, revealed relatively densely populated cross-link heat maps. The established X-ray structure of the monomeric PDI could be confirmed; however, the dimer as presented in the existing models does not seem to be prevalent in solution as modeling on the observed cross-links revealed new models of dimeric PDI. The observed inter-protein cross-links confirmed the existence of a peptide binding area on calnexin that binds strongly both PDI and ERp72. On the other hand, interaction sites on PDI and ERp72 could not be uniquely identified, indicating a more non-specific interaction pattern.

BIOLOGICAL SIGNIFICANCE

The present work demonstrates the use of chemical cross-linking and mass spectrometry (CXMS) for the determination of a solution structure of natural human PDI and its interaction with the chaperones ERp72 and calnexin. The data shows that the dimeric structure of PDI may be more diverse than indicated by present models. We further observe that the temperature influences the cross-linking pattern of PDI, but this does not influence the overall folding pattern of the molecule.