Constitutive cardiomyocyte proliferation in the leopard gecko (Eublepharis macularius)

Abstract

Although the contractile function of the heart is universally conserved, the organ itself varies in structure across species. This variation includes the number of ventricular chambers (one, two, or an incompletely divided chamber), the structure of the myocardial wall (compact or trabeculated), and the proliferative capacity of the resident cardiomyocytes. Whereas zebrafish are capable of comparatively high rates of constitutive cardiomyocyte proliferation, humans and rodents are not. However, for most species, the capacity to generate new cardiomyocytes under homeostatic conditions remains unclear. Here, we investigate cardiomyocyte proliferation in the lizard Eublepharis macularius, the leopard gecko. As for other lizards, the leopard gecko heart has a partially septated ventricular lumen with a trabeculated myocardial wall. To test our hypothesis that leopard gecko cardiomyocytes routinely proliferate, we performed 5-bromo-2'-deoxyuridine incorporation and immunostained for the mitotic marker phosphorylated histone H3 (pHH3) and the DNA synthesis phase (S phase) marker proliferating cell nuclear antigen (PCNA). Using double immunofluorescence, we co-localized pHH3 or PCNA with the cardiomyocyte marker myosin heavy chain (MHC). We found that ~0.5% of cardiomyocytes were mitotically active (pHH3+/MHC+), while ~10% were in S phase (PCNA+/MHC+). We also determined that cell cycling by gecko cardiomyocytes is not impacted by caudal autotomy (tail loss), a dramatic form of self-amputation. Finally, we show that populations of cardiac cells are slow cycling. Overall, our findings provide predictive evidence that geckos may be capable of spontaneous cardiac self-repair and regeneration following a direct injury.

Cutting the molecular brakes to achieve cardiac regeneration

“…Better understanding of the natural course balancing hemodynamic load with metabolic demand that nature designed and refined to grow a heart to maturity could reveal the "holy-grail" approach to MI therapy. Interestingly, in some species (amphibians and fishes) [8] safe cell-cycle re-entry occurs naturally in a functionally meaningful way to elicit cardiac regeneration-so why don't adult mammals do this?…”

Reptiles as a Model System to Study Heart Development

“…For example, there are two instances where septation has evolved to such an extent that the ventricle is functionally separated into a low-pressure right side and a high-pressure left side, namely, in monitor lizards and python snakes (Burggren and Johansen 1982;Wang et al 2003). In these animals, there is a small luminal continuity between the left and right side (which closes in systole) and some mixing of bloodstreams, or shunting, is inevitable (Ishimatsu et al 1988;Jensen et al 2010). Although the variation in ventricular septation makes the hearts of reptiles particularly interesting, reptilian heart development offers insight into the evolution of a number of important processes.…”

“…Cardiomyocyte proliferation underlies the formation of chambers from the embryonic heart tube (Sedmera et al 2003). Cell-cycle activity and possibly proliferation of heart cells has been assessed by the detection of BrdU of pulselabeled nuclei, or by the use of antibodies against PCNA and pHH3 located in the nucleus (Riquelme et al 2011;Jensen et al 2013a;Jacyniak and Vickaryous 2018). That said, it is difficult to assign correctly a cell cycle-marker-positive nucleus to its native cytoplasm whereby cardiomyocyte identity can be established by the detection of sarcomeric proteins (Ang et al 2010;Leone et al 2015).…”

Intrinsic Heart Regeneration in Adult Vertebrates May be Strictly Limited to Low‐Metabolic Ectotherms

“…Time calibrated (million years ago, MYA) vertebrate phylogeny at the level of orders with indications of currently known regenerative potential in species members of different orders. Polypteriformes: juvenile Polypterus senegalus; [22] Cypriniformes: adult Danio rerio, [7] adult Devario aequipinnatus, [8] adult Carassius auratus; [9] Characiformes: adult surface and Pachón cave ecotypes of Astyanax mexicanus; [10] Beloniformes: adult Oryzias latipes; [23] Anura: adult Xenopus laevis; [24] Caudata: adult Ambystoma mexicanum, [12] adult Notophthalmus viridescens; [3,11] Rodentia: neonatal Mus musculus, [14] neonatal Rattus norvegicus; [15] Lagomorpha: embryonic Oryctolagus cuniculus; [15,17] Primates: embryonic Homo sapiens; [19] Artiodactyla: embryonic Ovis aries; [18] Squamata: adult Eublepharis macularius; [6] Crocodylia: juvenile Alligator mississippiensis; [5] Galliformes: embryonic Gallus gallus. [13] the other characteristics of regenerative hearts, it is tempting to speculate that there is a tight link between cardiac metabolism and regenerative capability.…”

“…[ 3 ] Contrary to appendage regeneration, which can be documented as far back as the Carboniferous in the fossil record, [ 4 ] our knowledge of the ancestry of the capacity to restore soft tissues like the heart is very limited, since soft tissue rarely leaves fossil traces. Today, we know of no adult mammal nor bird that is capable of intrinsic heart regeneration, and there are only a few indicative but inconclusive reports of cardiac regenerative capacity in reptiles [ 5,6 ] ( Figure 1 ). On the other hand, heart regenerative ability has been documented in a few species of adult fish and amphibians: zebrafish, [ 7 ] giant danio, [ 8 ] goldfish, [ 9 ] Mexican tetra, [ 10 ] Eastern newt, [ 3,11 ] and axolotl [ 12 ] as well as some embryonic and/or neonatal birds and mammals: embryonic chicken, [ 13 ] neonatal mouse, [ 14 ] neonatal rat, [ 15 ] embryonic rabbit, [ 16,17 ] embryonic sheep, [ 18 ] and possibly embryonic human [ 19 ] (Figure 1).…”