Pharmacological Therapy in the Heart as an Alternative to Cellular Therapy: A Place for the Brain Natriuretic Peptide?

Abstract

The discovery that stem cells isolated from different organs have the ability to differentiate into mature beating cardiomyocytes has fostered considerable interest in developing cellular regenerative therapies to treat cardiac diseases associated with the loss of viable myocardium. Clinical studies evaluating the potential of stem cells (from heart, blood, bone marrow, skeletal muscle, and fat) to regenerate the myocardium and improve its functional status indicated that although the method appeared generally safe, its overall efficacy has remained modest. Several issues raised by these studies were notably related to the nature and number of injected cells, as well as the route and timing of their administration, to cite only a few. Besides the direct administration of cardiac precursor cells, a distinct approach to cardiac regeneration could be based upon the stimulation of the heart's natural ability to regenerate, using pharmacological approaches. Indeed, differentiation and/or proliferation of cardiac precursor cells is controlled by various endogenous mediators, such as growth factors and cytokines, which could thus be used as pharmacological agents to promote regeneration. To illustrate such approach, we present recent results showing that the exogenous administration of the natriuretic peptide BNP triggers "endogenous" cardiac regeneration, following experimental myocardial infarction.

Figure 1

Cardiovascular cell lineage. Schematic representation depicting the origin of cardiomyocytes and endothelial and smooth muscle cells, as well as the conduction system. Several proteins are associated with the different stage of differentiation of the cardiac cells: islet-1, Flk-1, and c-kit are expressed at an undifferentiated stage, whereas the expression of Nkx2.5 and Sca-1 identifies more differentiated cardiac precursor cells.

Figure 2

Current state of stem cell therapy for acute myocardial infarction in clinical trials. The optimal cell population for cardiac regenerative cell therapies requires autologous or allogeneic origin, rapid ex vivo expansion, and cardiac commitment including differentiation to cardiomyocytes. Numerous clinical trials have been undertaken with moderate results (for reviews see [, , –174]). ADRC: adipose-derived stem cells; BMC: bone marrow cells; CDC: cardiosphere-derived cells; CM: cardiomyocytes; EPC: endothelial progenitor cells; HSC: hematopoietic stem cells; LVEF: left ventricular ejection fraction; MSC: mesenchymal stem cells; SC: stem cells.

Figure 3

Heart expression of BNP is age-dependent and cell specific. (a) Representative microscopy pictures of neonatal and adult hearts stained for BNP (in red) and DAPI (nuclei in blue). High magnification of positive cells in top right inserts. The scale bars represent 100 μm. (b) mRNA expression of BNP using quantitative PCR, in non-myocyte cells (NMCs) (blue) and cardiomyocytes (red). Results expressed as fold-increase above the levels in NMCs. n = 7 cardiomyocyte samples compared to 9 NMC samples. Data are means ± SEM, ^∗ P < 0.05. (c) Determination of BNP protein levels in neonatal and adult hearts by western blot analysis. BNP protein expression with representative western blot and quantification relative to tubulin, expressed as fold changes relative to the average level of neonatal hearts. Data are means ± SEM, ^∗ P < 0.05. a.u.: arbitrary unit.

Figure 4

Cardiac precursor cells express BNP in neonatal and adult murine hearts. Cardiac precursor cells were defined as small Nkx2.5⁺ cells or c-kit⁺/Nkx2.5⁺ cells or Sca-1⁺/Nkx2.5⁺cells. Photomicrographs of neonatal or adult heart sections stained for BNP and DAPI (Nuclei) associated with staining for either Nkx2.5 and laminin (a), Nkx2.5 and c-kit (b), or Nkx2.5 and Sca-1 (c). Scale bars represent 80 μm for the pictures in (a) and 10 μm for the pictures in (b) and (c). Yellow arrows depict cells which are considered as being CPCs expressing BNP.

Figure 5

Survival curves of NPR-A KO and NPR-B deficient pups. The results are represented as percentages of the total number of pups (n = 66 for NPR-A KO and 96 for NPR-B deficient mice) obtained in heterozygous breeding (+/−  × +/−) and compared to the expected percentages (25%).

Figure 6

BNP modulation of cardiac precursor cell (CPC) proliferation and differentiation. BNP is secreted during physiological growth by cardiomyocytes, fibroblasts, and endothelial cells. CPCs can also secrete BNP. BNP stimulates via NPR-A CPC proliferation and via NPR-B CPC differentiation into cardiomyocytes. In pathophysiological conditions, it seems that the secreted BNP is devoid of biological activity, suggesting that BNP can be injected to stimulate the CPC proliferation and differentiation. This representation is based on the results previously published [117].