No abstract

Stress conditions affecting the functions of the endoplasmic reticulum (ER) cause the accumulation of unfolded proteins. ER stress is counteracted by the unfolded-protein response (UPR). However, under prolonged stress the UPR initiates a proapoptotic response. Mounting evidence indicate that the ER chaperone calnexin is involved in apoptosis caused by ER stress. Here, we report that overexpression of calnexin in Schizosaccharomyces pombe induces cell death with apoptosis markers. Cell death was partially dependent on the Ire1p ER-stress transducer. Apoptotic death caused by calnexin overexpression required its transmembrane domain (TM), and involved sequences on either side of the ER membrane. Apoptotic death caused by tunicamycin was dramatically reduced in a strain expressing endogenous levels of calnexin lacking its TM and cytosolic tail. This demonstrates the involvement of calnexin in apoptosis triggered by ER stress. A genetic screen identified the S. pombe homologue of the human antiapoptotic protein HMGB1 as a suppressor of apoptotic death due to calnexin overexpression. Remarkably, overexpression of human calnexin in S. pombe also provoked apoptotic death. Our results argue for the conservation of the role of calnexin in apoptosis triggered by ER stress, and validate S. pombe as a model to elucidate the mechanisms of calnexin-mediated cell death. INTRODUCTIONThe endoplasmic reticulum (ER) is a specialized organelle playing essential and central roles in the biology of the cell. The ER is the site of synthesis and folding of secreted, membrane-bound and some organelle-targeted proteins (Bukau et al., 2000;Fewell et al., 2001). Protein folding in the ER is assisted by a battery of molecular chaperones and foldases (Bukau et al., 2000;Fewell et al., 2001;Trombetta and Parodi, 2003;Helenius and Aebi, 2004). In addition, the ER contains several factors required for optimum protein folding, including, ATP, Ca 2ϩ , and an oxidizing environment to allow disulphide-bond formation. Proper ER function is critical for numerous aspects of cell physiology, including vesicle trafficking and lipid and membrane biogenesis, as well as protein targeting and secretion (Lai et al., 2007).The ER is highly sensitive to stresses perturbing the cellular energy levels and ER lipid or glycolipid imbalances or changes in the redox state or Ca 2ϩ concentration (Breckenridge et al., 2003;Boyce and Yuan, 2006;Szegezdi et al., 2006). Such stresses reduce the protein folding capacity of the ER, which results in the accumulation and aggregation of unfolded proteins, a condition referred to as ER stress. When the capacity of the ER to fold proteins properly is compromised or overwhelmed, a highly conserved unfolded-protein response (UPR) signal-transduction pathway is activated. The ER response to stress is basically conserved from yeast to mammalian cells (Patil and Walter, 2001;Ron and Walter, 2007). To counter ER stress, the UPR halts general protein synthesis and up-regulates the transcription of genes encoding ER resident chaperone...

The placenta produces numerous miRNAs with some of them being released in the maternal circulation. These miRNA genes are encoded into specific clusters and expressed preferentially by placental cells, in a time-dependent manner. They were shown to be dysregulated in plasma and placenta from women suffering from GDM and associated with pregnancy and birth-related outcomes. The discovery of pregnancy-related miRNAs and their respective characterization will provide us with important information as to their function in maternal and placental metabolic regulation. More studies are needed to determine whether they will be useful for early screening of GDM.

Folding of newly synthesized proteins within the ER (endoplasmic reticulum) is a rate-limiting step in protein secretion. Thus ER molecular chaperones and foldases have a major impact in determining the rate and yield of these crucial cellular processes. Calnexin is a key ER chaperone implicated in the folding, retention and targeting for degradation of proteins that go through the secretory pathway. Calnexin molecules contain a highly conserved central domain (hcd) that has been proposed to be involved in the interaction with folding substrates and other chaperones. To gain a better understanding of the roles played by calnexin in the secretory pathway, we examined the efficiency of fission yeast (Schizosaccharomyces pombe) strains expressing calnexin mutants to secrete different model proteins. Remarkably, calnexin hcd-deletion mutants, although devoid of detectable chaperone activity in vitro, confer viability and cause a considerable increase in the secretion of heterologous cellulase. Surprisingly the quality-control efficiency, measured as the activity/amount ratio of secreted model protein, was not severely reduced in these calnexin hcd-deletion mutant strains. Our results indicate that the essential function of calnexin does not reside in its role in the folding or in the retention of misfolded proteins. These observations suggest the existence of a highly stringent quality control mechanism in the ER of S. pombe that might reduce the secretion efficiency of endogenous proteins.