Rationale GDF11 (Growth Differentiation Factor 11) is a member of the transforming growth factor β (TGFβ) super family of secreted factors. A recent study showed that reduced GDF11 blood levels with aging was associated with pathological cardiac hypertrophy (PCH), and restoring GDF11 to normal levels in old mice rescued PCH. Objective To determine if and by what mechanism GDF11 rescues aging dependent PCH. Methods and Results 24-month-old C57BL/6 mice were given a daily injection of either recombinant (r) GDF11 at 0.1mg/kg or vehicle for 28 days. rGDF11 bioactivity was confirmed in-vitro. After treatment, rGDF11 levels were significantly increased but there was no significant effect on either heart weight (HW) or body weight (BW). HW/BW ratios of old mice were not different from 8 or 12 week-old animals, and the PCH marker ANP was not different in young versus old mice. Ejection fraction, internal ventricular dimension, and septal wall thickness were not significantly different between rGDF11 and vehicle treated animals at baseline and remained unchanged at 1, 2 and 4 weeks of treatment. There was no difference in myocyte cross-sectional area rGDF11 versus vehicle-treated old animals. In vitro studies using phenylephrine-treated neonatal rat ventricular myocytes (NRVM), to explore the putative anti-hypertrophic effects of GDF11, showed that GDF11 did not reduce NRVM hypertrophy, but instead induced hypertrophy. Conclusions Our studies show that there is no age-related PCH in disease free 24-month-old C57BL/6 mice and that restoring GDF11 in old mice has no effect on cardiac structure or function.

Rationale Autologous bone marrow- or cardiac-derived stem cell therapy for heart disease has demonstrated safety and efficacy in clinical trials but functional improvements have been limited. Finding the optimal stem cell type best suited for cardiac regeneration is key toward improving clinical outcomes. Objective To determine the mechanism by which novel bone-derived stem cells support the injured heart. Methods and Results Cortical bone stem cells (CBSCs) and cardiac-derived stem cells (CDCs) were isolated from EGFP+ transgenic mice and were shown to express c-kit and Sca-1 as well as 8 paracrine factors involved in cardioprotection, angiogenesis and stem cell function. Wild-type C57BL/6 mice underwent sham operation (n=21) or myocardial infarction (MI) with injection of CBSCs (n=67), CDCs (n=36) or saline (n=60). Cardiac function was monitored using echocardiography. Only 2/8 paracrine factors were detected in EGFP+ CBSCs in vivo (basic fibroblast growth factor and vascular endothelial growth factor) and this expression was associated with increased neovascularization of the infarct border zone. CBSC therapy improved survival, cardiac function, regional strain, attenuated remodeling, and decreased infarct size relative to CDC- or saline-treated MI controls. By 6 weeks, EGFP+ cardiomyocytes, vascular smooth muscle and endothelial cells could be identified in CBSC- but not in CDC-treated animals. EGFP+ CBSC-derived isolated myocytes were smaller and more frequently mononucleated, but were functionally indistinguishable from EGFP- myocytes. Conclusions CBSCs improve survival, cardiac function, and attenuate remodeling through two mechanisms:1) secretion of pro-angiogenic factors that stimulate endogenous neovascularization, and 2) differentiation into functional adult myocytes and vascular cells.

Rationale Catecholamines increase cardiac contractility, but exposure to high concentrations or prolonged exposures can cause cardiac injury. A recent study demonstrated that a single subcutaneous injection of isoproterenol (ISO; 200 mg/kg) in mice causes acute myocyte death (8-10%) with complete cardiac repair within a month. Cardiac regeneration was via endogenous cKit+ cardiac stem cell (CSC)-mediated new myocyte formation. Objective Our goal was to validate this simple injury/regeneration system and use it to study the biology of newly forming adult cardiac myocytes. Methods and Results C57BL/6 mice (n=173) were treated with single injections of vehicle, 200mg/kg or 300mg/kg ISO, or with two daily doses of 200mg/kg ISO for 6 days. Echocardiography revealed transiently increased systolic function and unaltered diastolic function 1 day after single ISO injection. Single ISO injections also caused membrane injury in about 10% of myocytes but few of these myocytes appeared to be necrotic. Circulating troponin I levels after ISO were elevated, further documenting myocyte damage. However, myocyte apoptosis was not increased after ISO injury. Heart weight to body weight ratio and fibrosis were also not altered 28 days after ISO injection. Single or multiple dose ISO injury was not associated with an increase in the percentage of 5-ethynyl-2’-deoxyuridine (EdU)-labeled myocytes. Furthermore, ISO injections did not increase new myocytes in cKit+/Cre × R-GFP transgenic mice. Conclusions A single dose of ISO causes injury in about 10% of the cardiomyocytes. However, most of these myocytes appear to recover and do not elicit cKit+ cardiac stem cell (CSC)-derived myocyte regeneration.

Rationale Sorafenib is an effective treatment for renal cell carcinoma, but recent clinical reports have documented its cardiotoxicity through an unknown mechanism. Objective Determining the mechanism of sorafenib-mediated cardiotoxicity. Methods and Results Mice treated with sorafenib or vehicle for 3 weeks underwent induced myocardial infarction (MI) after 1 week of treatment. Sorafenib markedly decreased 2-week survival relative to vehicle-treated controls but echocardiography at 1 and 2 weeks post-MI detected no differences in cardiac function. Sorafenib-treated hearts had significantly smaller diastolic and systolic volumes and reduced heart weights. High doses of sorafenib induced necrotic death of isolated myocytes in vitro, but lower doses did not induce myocyte death or affect inotropy. Histological analysis documented increased myocyte cross-sectional area despite smaller heart sizes following sorafenib treatment, further suggesting myocyte loss. Sorafenib caused apoptotic cell death of cardiac- and bone-derived c-kit+ stem cells in vitro and decreased the number of BrdU+ myocytes detected at the infarct border zone in fixed tissues. Sorafenib had no effect on infarct size, fibrosis or post-MI neovascularization. When sorafenib-treated animals received metoprolol treatment post-MI, the sorafenib-induced increase in post MI mortality was eliminated, cardiac function was improved, and myocyte loss was ameliorated. Conclusions Sorafenib cardiotoxicity results from myocyte necrosis rather than from any direct effect on myocyte function. Surviving myocytes undergo pathological hypertrophy. Inhibition of c-kit+ stem cell proliferation by inducing apoptosis exacerbates damage by decreasing endogenous cardiac repair. In the setting of MI, which also causes large-scale cell loss, sorafenib cardiotoxicity dramatically increases mortality.

Rationale Cortical bone stem cells (CBSCs) have been shown to reduce ventricular remodeling and improve cardiac function in a murine myocardial infarction (MI) model. These effects were superior to other stem cell types that have been used in recent early stage clinical trials. However, CBSC efficacy has not been tested in a preclinical large animal model using approaches that could be applied to patients. Objective To determine if post–MI transendocardial injection of allogeneic CBSCs reduces pathological structural and functional remodeling and prevents the development of heart failure in a swine MI model. Methods and Results Female Göttingen swine underwent left anterior descending coronary artery occlusion, followed by reperfusion (ischemia–reperfusion MI). Animals received, in a randomized, blinded manner, 1:1 ratio, CBSCs (n = 9) (2×107 cells total) or placebo (vehicle; VEH, n = 9) through NOGA® guided transendocardial injections. 5–ethynyl–2’deoxyuridine (EdU), a thymidine analog, containing minipumps were inserted at the time of MI induction. At 72hrs (n=8) initial injury and cell retention were assessed. At 3 Months post–MI, cardiac structure and function was evaluated by serial echocardiography, and terminal invasive hemodynamics. CBSCs were present in the MI border zone and proliferating at 72hrs post–MI but had no effect on initial cardiac injury or structure. At 3 months, CBSC–treated hearts had significantly reduced scar size, smaller myocytes and increased myocyte nuclear density. Noninvasive echocardiographic measurements showed that left ventricular (LV) volumes and ejection fraction were significantly more preserved in CBSC–treated hearts and invasive hemodynamic measurements documented improved cardiac structure and functional reserve. The number of EdU+ cardiac myocytes was increased in CBSC– vs. VEH– treated animals. Conclusions CBSC administration into the MI border zone reduces pathological cardiac structural and functional remodeling and improves LV functional reserve. These effects reduce those processes that can lead to heart failure with reduced ejection fraction (HFrEF).

Rationale: Adoptive transfer of multiple stem cell types has only had modest effects on the structure and function of failing human hearts. Despite increasing the use of stem cell therapies, consensus on the optimal stem cell type is not adequately defined. The modest cardiac repair and functional improvement in patients with cardiac disease warrants identification of a novel stem cell population that possesses properties that induce a more substantial improvement in patients with heart failure. Objective: To characterize and compare surface marker expression, proliferation, survival, migration, and differentiation capacity of cortical bone stem cells (CBSCs) relative to mesenchymal stem cells (MSCs) and cardiac-derived stem cells (CDCs), which have already been tested in early stage clinical trials. Methods and Results: CBSCs, MSCs, and CDCs were isolated from Gottingen miniswine or transgenic C57/BL6 mice expressing enhanced green fluorescent protein and were expanded in vitro. CBSCs possess a unique surface marker profile, including high expression of CD61 and integrin β4 versus CDCs and MSCs. In addition, CBSCs were morphologically distinct and showed enhanced proliferation capacity versus CDCs and MSCs. CBSCs had significantly better survival after exposure to an apoptotic stimuli when compared with MSCs. ATP and histamine induced a transient increase of intracellular Ca 2+ concentration in CBSCs versus CDCs and MSCs, which either respond to ATP or histamine only further documenting the differences between the 3 cell types. Conclusions: CBSCs are unique from CDCs and MSCs and possess enhanced proliferative, survival, and lineage commitment capacity that could account for the enhanced protective effects after cardiac injury.

Visual Abstract

Determination of fundamental mechanisms of disease often hinges on histopathology visualization and quantitative image analysis. Currently, the analysis of multi-channel fluorescence tissue images is primarily achieved by manual measurements of tissue cellular content and sub-cellular compartments. Since the current manual methodology for image analysis is a tedious and subjective approach, there is clearly a need for an automated analytical technique to process large-scale image datasets. Here, we introduce Nuquantus (Nuclei quantification utility software) - a novel machine learning-based analytical method, which identifies, quantifies and classifies nuclei based on cells of interest in composite fluorescent tissue images, in which cell borders are not visible. Nuquantus is an adaptive framework that learns the morphological attributes of intact tissue in the presence of anatomical variability and pathological processes. Nuquantus allowed us to robustly perform quantitative image analysis on remodeling cardiac tissue after myocardial infarction. Nuquantus reliably classifies cardiomyocyte versus non-cardiomyocyte nuclei and detects cell proliferation, as well as cell death in different cell classes. Broadly, Nuquantus provides innovative computerized methodology to analyze complex tissue images that significantly facilitates image analysis and minimizes human bias.