Elsevier

Pharmacological Research

Volume 127, January 2018, Pages 116-128
Pharmacological Research

Review
The use and abuse of Cre/Lox recombination to identify adult cardiomyocyte renewal rate and origin

https://doi.org/10.1016/j.phrs.2017.06.012Get rights and content

Abstract

The adult mammalian heart, including the human, is unable to regenerate segmental losses after myocardial infarction. This evidence has been widely and repeatedly used up-to-today to suggest that the myocardium, contrary to most adult tissues, lacks an endogenous stem cell population or more specifically a bona-fide cardiomyocyte-generating progenitor cell of biological significance. In the last 15 years, however, the field has slowly evolved from the dogma that no new cardiomyocytes were produced from shortly after birth to the present consensus that new cardiomyocytes are formed throughout lifespan. This endogenous regenerative potential increases after various forms of injury. Nevertheless, the degree/significance and more importantly the origin of adult new cardiomyocytes remains strongly disputed. Evidence from independent laboratories has shown that the adult myocardium harbours bona-fide tissue-specific cardiac stem cells (CSCs). Their transplantation and in situ activation have demonstrated the CSCs regenerative potential and have been used to develop regeneration protocols which in pre-clinical tests have shown to be effective in the prevention and treatment of heart failure. Recent reports purportedly tracking the c-kit+CSC’s fate using Cre/lox recombination in the mouse have challenged the existence and regenerative potential of the CSCs and have raised scepticism about their role in myocardial homeostasis and regeneration. The validity of these reports, however, is controversial because they failed to show that the experimental approach used is capable to both identify and tract the fate of the CSCs. Despite these serious shortcomings, in contraposition to the CSCs, these publications have proposed the proliferation of existing adult fully-matured cardiomyocytes as the relevant mechanism to explain cardiomyocyte renewal in the adult. This review critically ponders the available evidence showing that the adult mammalian heart possesses a definable myocyte-generating progenitor cell of biological significance. This endogenous regenerative potential is expected to provide the bases for novel approaches of myocardial repair in the near future.

Graphical abstract

Cardiomyocyte (CM) renewal in adult myocardial tissue in response to CM loss depends on resident endogenous cardiac stem/progenitor cells (eCSCs) activation and ensuing differentiation in new fully-functional CMs.

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Section snippets

Cardiomyocytes renewal in adulthood: in search of their origin

The adult mammalian heart has been considered for a long time as a post-mitotic organ lacking any muscle regenerative capacity [1]. Indeed, cardiomyocytes (CMs), the cells that define heart histology and function, have been commonly thought to be all formed during embryonic life or shortly thereafter in the neonatal period [2], [3]. Afterwards CMs become fully mature adult cells undergoing a process known as terminal differentiation mainly described as a permanent withdrawal from the cell cycle

Genetic cell-fate mapping strategy using Cre/Lox recombination

To correctly discuss the relevant knowledge addressing the origin of new CMs during adulthood and after injury, it is imperative to consider the advantages/limitations of the technologies used to generate the relevant data. Because genetic cell-fate mapping is the technique used to support the currently most controversial view about adult cardiomyogenesis in homeostasis and repair, despite excellent reviews have been published on this topic [30], here we summarize the main features of this

Adult cardiomyocytes are terminally differentiated cells with no replicative capacity

As above stated, shortly after the post-natal period, CMs permanently withdraw from the cell cycle becoming terminally-differentiated cells [2]. Mature CMs indeed do not re-enter the cell cycle and in the very rare occasions they do, do not undergo mitosis but die by apoptosis [58]. CM permanent withdrawal from the cell cycle has been shown both in vivo and in vitro in different mammalian species from mouse to human. Furthermore, experimental evidences show that if adult mammalian CMs are

The CSCs are necessary and sufficient for adult myocardium cellular homeostasis as well as functional and anatomical repair after major damage

From 2003 on, we and others have shown that the heart, like most other adult mammalian tissues, contains a pool of resident tissue-specific CSCs [21], [22], [69], [70], [71], [72], [73], [74], [75], [76], [77]. These cells have all the characteristics expected from a tissue stem cell: multipotency, clonogenicity, self-renewing and capacity to differentiate in vivo and in vitro into the four main myocardial cell types: CMs, endothelial and smooth vascular and connective tissue cells (Fig. 3) [21]

Controversy over the role and myogenic properties of the eCSCs

From the results summarized above, it was reasonable to expect that identification of the CSCs and characterization of their properties in vitro and in vivo would have put to rest most outstanding questions about the intrinsic regenerative capacity of the adult myocardium and about the origin of the CMs born in adulthood. Despite this solid and reproducible data, however, considerable confusion has arisen recently about the nature, physiological role and regenerative capacity of what has come

Conclusions and future perspective

Based on data accumulated over the past decade, it is evident that the vast majority of adult mammalian cardiomyocytes are terminally differentiated cells, which are not a source of cardiomyocyte renewal in the adult mammalian heart. Indeed, even though it is possible to identify rare cardiomyocytes undergoing mitotic division, they seem to be recently born and still immature myocytes which have not yet reached terminal differentiation. Furthermore, it is a fact that all the experimental

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