Abstract only
Resident stem/progenitor cell populations have been identified within many vertebrate tissues, including the hearts of teleost fish, urodeles, and mammals. Although often rare, these ...populations are considered crucial for normal physiological repair, and may also play a role in tissue regeneration. Here we provide preliminary evidence of the presence of resident cardiac stem/progenitor cells within the lizard
Eublepharis macularius
(the leopard gecko). Using Proliferating Cell Nuclear Antigen (PCNA) immunostaining, we determined that gecko cardiomyocytes continue to proliferate into adulthood. Next, we performed a 5‐bromo‐2'‐deoxyuridine (BrdU) pulse‐chase experiment to identify slow cycling cells within the normal (uninjured) heart. We found BrdU label‐retaining cells within cardiac tissues chased for 4, 6 and 20 weeks within the epicardium and myocardium of the ventricular wall and trabeculae, indicating that quiescent or slow‐cycling cells are present. Finally, we observed that select BrdU label‐retaining cells at 20 weeks were also immunopositive for the cardiac progenitor marker c‐kit. Based on these findings, we predict that putative cardiac stem/progenitor cells are present in the gecko heart.
The role of erythropoietin (EPO) has extended beyond hematopoiesis to include cytoprotection, inotropy, and neurogenesis. Extra-renal EPO has been reported for multiple tissue/cell types, but the ...physiological relevance remains unknown. Although the EPO receptor is expressed by multiple cardiac cell types and human recombinant EPO increases contractility and confers cytoprotection against injury, whether the heart produces physiologically meaningful amounts of EPO in vivo is unclear. We show a distinct circadian rhythm of cardiac EPO mRNA expression in adult mice and increased mRNA expression during embryogenesis, suggesting physiological relevance to cardiac EPO production throughout life. We then generated constitutive, cardiomyocyte-specific EPO knockout mice driven by the Mlc2v promoter (EPOfl/fl:Mlc2v-cre+/−; EPOΔ/Δ-CM). During cardiogenesis, cardiac EPO mRNA expression and cellular proliferation were reduced in EPOΔ/Δ-CM hearts. However, in adult EPOΔ/Δ- CM mice, total heart weight was preserved through increased cardiomyocyte cross-sectional area, indicating the reduced cellular proliferation was compensated for by cellular hypertrophy. Echocardiography revealed no changes in cardiac dimensions, with modest reductions in ejection fraction, stroke volume, and tachycardia, whereas invasive hemodynamics showed increased cardiac contractility and lusitropy. Paradoxically, EPO mRNA expression in the heart was elevated in adult EPOΔ/Δ-CM, along with increased serum EPO protein content and hematocrit. Using RNA fluorescent in situ hybridization, we found that Epo RNA colocalized with endothelial cells in the hearts of adult EPOΔ/Δ-CM mice, identifying the endothelial cells as a cell responsible for the EPO hyper-expression. Collectively, these data identify the first physiological roles for cardiomyocyte-derived EPO. We have established cardiac EPO mRNA expression is a complex interplay of multiple cell types, where loss of embryonic cardiomyocyte EPO production results in hyper-expression from other cells within the adult heart.
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•Cardiac EPO expression exhibits circadian rhythm in adult mice.•EPO plays a relevant role in cardiogenesis.•There is a compensatory hyper-expression of EPO in EPOΔ/Δ-CM mice.•Endothelial cells are a source of EPO hyper-expression in adult mouse LV.
Abstract only
Erythropoietin (EPO) is widely recognized as the principle regulator of erythropoiesis, however, extra‐erythropoietic functions have been identified including cytoprotection, cardiac ...inotropy, cellular proliferation, and embryonic development. Whole body deletion of either EPO or the EPO receptor is embryonic lethal with impaired cardiogenesis leading to ventricular hypoplasia. While multiple extra‐renal tissue and cell types produce EPO, whether the heart is a direct source remains unclear. Human recombinant EPO increases myocardial contractility and confers cytoprotection against cardiac injury, which suggests a role for EPO signalling in the heart. Our objectives were to (1) confirm whether the heart produces EPO and (2) determine if there is a role for paracrine EPO signalling during cardiogenesis. We generated constitutive, cardiomyocyte‐specific EPO knockout mice driven by the Mlc2v promoter (EPO
fl/fl
:Mlc2v‐cre
+/−
; EPO
Δ/Δ‐CM
). We confirmed that the heart is a source of EPO expression with a distinct circadian rhythm in adult hearts and increased expression during embryonic development. During cardiogenesis, cardiac EPO expression was reduced, but not eliminated, in EPO
Δ/Δ‐CM
hearts with decreased cardiac cell proliferation. This suggests EPO signalling is partially compensated by an alternate cardiac cell type during cardiac development. In adult EPO
Δ/Δ‐CM
mice, global cardiac mass was preserved while cardiomyocyte cross‐sectional area was increased. Taken together, cellular cardiomyocyte hypertrophy in the absence of gross organ hypertrophy, and the observed reduction in cardiac cell proliferation during cardiogenesis, points towards a reduction in the overall number of cardiomyocytes in EPO
Δ/Δ‐CM
mice. Collectively, these data identify the first physiological roles of extra‐renal EPO by confirming that the heart is a source of EPO and that paracrine expression is required for cardiogenesis. Further, cardiac EPO expression is a complex interplay of multiple cell types where loss of cardiomyocyte production results in compensation from other cardiac cell lineages.
Support or Funding Information
This work was funded in part by the Canadian Institutes of Health Research (JAS), the Natural Sciences and Engineering Research Council of Canada (KRB, MKV, and JAS), the Canadian Glycomics Network (JAS), and the Heart and Stroke Foundation of Canada (KRB and JAS). JAS is also a new investigator with the Heart and Stroke Foundation of Ontario.
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