calcium channel blockers The mammalian myocardial response t

The mammalian myocardial response to increased workload and to injury is conditioned by the fact that, shortly after the post-natal period, CMs permanently withdraw from the cell cycle (Nadal-Ginard., 1978). The molecular mechanism of terminal differentiation in CMs has not yet been completely defined. Different experimental models indicate that calcium channel blockers lineage determination genes directly interact with the retinoblastoma (Rb) pocket proteins to produce and maintain the terminally differentiated state (Tam et al., 1995; MacLellan et al., 2005). After more than 3 decades of research, there are literally hundreds of papers in reputed journals documenting that mature myocytes in and from the adult myocardium do not re-enter the cell cycle and in the very rare occasions they do, do not undergo mitosis but apoptose, at least in part, due to a deficit in centriole formation (Schneider et al., 1994). This block of cell division (which does not necessarily blocks bi-nucleation and DNA endo-reduplication) has been shown both in vivo and in vitro in different mammalian species from mouse to human. This behaviour is in contraposition to the replication capacity of foetal and neonatal mammalian myocytes, which do not yet express pRB, and in the mouse can extend up to 7–8days post-natally (Porrello et al., 2011) as well as those of adult invertebrates, fishes and reptiles (Kikuchi et al., 2012). Furthermore, adult mammalian myocytes can also be co-axed to re-enter the cell cycle at a significant rate through genetic manipulations at the expense of the stability of their differentiated function (MacLellan et al., 2005). Notwithstanding the elegant studies documenting that adult cardiomyocytes divide in species other than mammals (Zhang et al., 2013), these findings cannot and should not be extrapolated to the adult mammalian heart. Despite the overwhelming observational and experimental evidence summarized above, a provocative recent publication by Senyo et al., (2013) claimed that adult myocyte renewal is mostly accomplished by division of pre-existing cardiomyocytes, which was estimated to be a more relevant source than resident (or circulating) stem/progenitor cells. Indeed, the authors stated that the birth of cardiomyocytes from pre-existing cardiomyocytes has a projected rate of roughly 0.76% per year in the young adult mouse under normal homeostatic conditions, a rate that increases after myocardial injury in the border region. Also the authors interpreted their findings as a demonstration that cardiac progenitors do not have a significant function in myocardial homeostasis in mammals and thus their role after injury is also limited (Senyo et al., 2013). This publication has been taken by some as a demonstration of the negligible or secondary role of resident CSCs in adult mammalian myocardium homeostasis and regeneration. Interestingly, two previous publications from the same laboratory on the same topic and using the same experimental animal models (Hsieh et al., 2007; Loffredo et al., 2011), reached exactly the opposite conclusions; that is, new CMs originated not from pre-existing CMs but from a stem cell population which they did not identify. Additionally, Marban\'s group using the same genetic cell fate tracking mouse model, concluded that CM renewal after AMI is accomplished by both pre-existing CM division and resident stem cell differentiation, with a higher contribution by the latter which is further amplified by cardiosphere-derived cell therapy (Malliaras et al., 2013). In contrast, we have reported that the resident stem cells are not only necessary but sufficient for full myocardial regeneration after diffuse and segmental damage and we have also ruled out any meaningful contribution of mature CM division (Ellison et al., 2013). The contradictory results pointed out above are, at least in part, due to a misconception of the strength of the transgenic mouse models used for the test. The so-called Z/EG mouse is a double-reporter mouse line that switches from expressing lacZ to enhanced green fluorescent protein (eGFP) in the targeted cells upon Cre-mediated excision (Hsieh et al., 2007) (Fig. 2). Although powerful and useful, this mouse is not fully effective when applied to the genetic labelling of adult CMs by cross-breeding Z/EG with α-MHC-MerCreMer mice where Cre is expressed under the control of MYH6 (α-MHC) promoter. Its main drawback is that when the MerCreMer construct is induced by Tamoxifen, only ~80% of the cardiomyocytes recombine the floxed transgene and turn on the eGFP marker (Hsieh et al., 2007). Therefore, one of every 5 cells continues to express the constitutive transgene, β-galactosidase, and their origin cannot be ascertained (Hsieh et al., 2007). For this reason, new CM formation has to be quantified indirectly by the “dilution” of the eGFP positive myocytes by the new myocytes, which, because they did not exist at the time of tamoxifen induction, should express the constitutive β-galactosidase reporter. Because all numbers are relative, the formation of new β-galactosidase-expressing myocytes by necessity leads to an increase of the % of β-galactosidase (β-Gal)-expressing cells and the corresponding decrease in the % of the eGFP-expressing cohort. This indirect measurement reduces the sensitivity and accuracy of the test, particularly if the number of newly formed myocytes is small.