title: Enhanced proliferation of monolayer cultures of embryonic stem (ES) cell-derived cardiomyocytes following acute loss of retinoblastoma.
abstract: BACKGROUND
  Cardiomyocyte (CM) cell cycle analysis has been impeded because of a reliance on primary neonatal cultures of poorly proliferating cells or chronic transgenic animal models with innate compensatory mechanisms.
  
  
  RESULTS
  We describe an in vitro model consisting of monolayer cultures of highly proliferative embryonic stem (ES) cell-derived CM. Following induction with ascorbate and selection with puromycin, early CM cultures are >98% pure, and at least 85% of the cells actively proliferate. During the proliferative stage, cells express high levels of E2F3a, B-Myb and phosphorylated forms of retinoblastoma (Rb), but with continued cultivation, cells stop dividing and mature functionally. This developmental transition is characterized by a switch from slow skeletal to cardiac TnI, an increase in binucleation, cardiac calsequestrin and hypophosphorylated Rb, a decrease in E2F3, B-Myb and atrial natriuretic factor, and the establishment of a more negative resting membrane potential. Although previous publications suggested that Rb was not necessary for cell cycle control in heart, we find following acute knockdown of Rb that this factor actively regulates progression through the G1 checkpoint and that its loss promotes proliferation at the expense of CM maturation.
  
  
  CONCLUSIONS
  We have established a unique model system for studying cardiac cell cycle progression, and show in contrast to previous reports that Rb actively regulates both cell cycle progression through the G1 checkpoint and maturation of heart cells. We conclude that this in vitro model will facilitate the analysis of cell cycle control mechanisms of CMs.
authors.affiliation: Laboratory of Cardiovascular Science, National Institute on Aging, Baltimore, Maryland, USA.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Yamanaka
authors.forename: Ihor
authors.initials: I
authors.lastname: Zahanich
authors.forename: Robert P
authors.initials: RP
authors.lastname: Wersto
authors.forename: Kenneth R
authors.initials: KR
authors.lastname: Boheler
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D002460:Cell Line; D049109:Cell Proliferation / Q000187:drug effects; D002469:Cell Separation; D004352:Drug Resistance, Microbial; D055724:Electrophysiological Phenomena; D053595:Embryonic Stem Cells / Q000166:cytology* / Q000187:drug effects; D016193:G1 Phase / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000187:drug effects; D011691:Puromycin / Q000494:pharmacology; D051381:Rats; D016160:Retinoblastoma Protein / Q000172:deficiency* / Q000378:metabolism; D016196:S Phase / Q000187:drug effects; D019831:Sodium-Calcium Exchanger / Q000378:metabolism
chemical_list: D016160:Retinoblastoma Protein; D019831:Sodium-Calcium Exchanger; C110374:sodium-calcium exchanger 1; D011691:Puromycin