title: Embryonic stem cell-derived cardiomyogenesis: a novel role for calreticulin as a regulator.
abstract: A role for calreticulin, an endoplasmic reticulum (ER)-resident, Ca(2+)-binding chaperone, has recently emerged in the context of cardiomyogenesis. We previously proposed calreticulin to be a novel cardiac fetal gene, because calreticulin knockout causes embryonic lethality in mice as a result of cardiac defects, it is transiently activated during heart development, and heart-targeted overexpression of constitutively active calcineurin in calreticulin-null mice rescues the lethal phenotype. Calreticulin affects Ca(2+) homeostasis and expression of adhesion-related genes. Using cardiomyocytes derived from both calreticulin-null and wild-type embryonic stem (ES) cells, we show here that cardiomyogenesis from calreticulin-null ES cells is accelerated but deregulated, such that the myofibrils of calreticulin-null cardiomyocytes become disorganized and disintegrate with time in culture. We have previously shown that the disorganization of the actin cytoskeleton in calreticulin-null cells may be explained, at least in part, by the downregulation of adhesion proteins, implying that calreticulin ablation causes adhesion-related defects. Here, upon examination of adhesion proteins, we found that vinculin is downregulated in calreticulin-null cardiomyocytes. We also found c-Src activity to be higher in calreticulin-null cardiomyocytes than in wild-type cardiomyocytes, and c-Src activity is affected by both calreticulin and [Ca(2+)]. Finally, we show that calreticulin and calsequestrin, the major Ca(2+) storage proteins of the ER and sarcoplasmic reticulum, respectively, exhibit alternate distributions. This suggests that calreticulin may have a housekeeping role to play in mature cardiomyocytes as well as during cardiomyogenesis. We propose here that calreticulin, an ER Ca(2+) storage protein, is a crucial regulator of cardiomyogenesis whose presence is required for controlled cardiomyocyte development from ES cells.
authors: Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada.
authors: Sylvia
authors: S
authors: Papp
authors: Ewa
authors: E
authors: Dziak
authors: Michal
authors: M
authors: Opas
mesh_terms: D000818:Animals; D015153:Blotting, Western; D037282:Calreticulin / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002454:Cell Differentiation / Q000187:drug effects; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D053595:Embryonic Stem Cells / Q000166:cytology* / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism / Q000648:ultrastructure; D015759:Ionomycin / Q000494:pharmacology; D051379:Mice; D008853:Microscopy; D046529:Microscopy, Electron, Transmission; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000378:metabolism / Q000648:ultrastructure; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000648:ultrastructure
chemical_list: D037282:Calreticulin; D004533:Egtazic Acid; D015759:Ionomycin; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid