Cardiomyogenic differentiation-independent improvement of cardiac function by human cardiomyocyte progenitor cell injection in ischaemic mouse hearts

Evaluation and manipulation of tissue and cellular distribution of cardiac progenitor cell-derived extracellular vesicles

Cardiac progenitor cell-derived extracellular vesicles (CPC-EVs) have been successfully applied via different delivery routes for treating post-myocardial infarction injury in several preclinical models. Hence, understanding the in vivo fate of CPC-EVs after systemic or local, i.e. myocardial, delivery is of utmost importance for the further therapeutic application of CPC-EVs in cardiac repair. Here, we studied the tissue- and cell distribution and retention of CPC-EVs after intramyocardial and intravenous injection in mice by employing different EV labeling and imaging techniques. In contrast to progenitor cells, CPC-EVs demonstrated no immediate flush-out from the heart upon intramyocardial injection and displayed limited distribution to other organs over time, as determined by near-infrared imaging in living animals. By employing CUBIC tissue clearing and light-sheet fluorescent microscopy, we observed CPC-EV migration in the interstitial space of the myocardium shortly after EV injection. Moreover, we demonstrated co-localization with cTnI and CD31-positive cells, suggesting their interaction with various cell types present in the heart. On the contrary, after intravenous injection, most EVs accumulated in the liver. To potentiate such a potential systemic cardiac delivery route, targeting the cardiac endothelium could provide openings for directed CPC-EV therapy. We therefore evaluated whether decorating EVs with targeting peptides (TPs) RGD-4C or CRPPR connected to Lamp2b could enhance EV delivery to endothelial cells. Expression of both TPs enhanced CPC-EV uptake under in vitro continuous flow, but did not affect uptake under static cell culture conditions. Together, these data demonstrate that the route of administration influences CPC-EV biodistribution pattern and suggest that specific TPs could be used to target CPC-EVs to the cardiac endothelium. These insights might lead to a better application of CPC-EV therapeutics in the heart.

Systemic Mesenchymal Stem Cell-Derived Exosomes Reduce Myocardial Infarct Size: Characterization With MRI in a Porcine Model

The observations that mesenchymal stem cells (MSCs) exert cardiac protection and repair via their secretome with the active component(s) identified as exosomes underpinned our test of the efficacy of MSC exosomes in a porcine model of myocardial infarction (MI) when administered systemically by the convenient method of intravenous (IV) bolus injection. Results show that 7 days of IV exosomes results in clear reduction (30-40%) of infarct size measured at both 7 and 28 days post-MI, despite near identical release of hs Troponin T. Together with reduced infarct size, exosome treatment reduced transmurality and lessened wall thinning in the infarct zone. Exosome treated pigs showed relative preservation of LV function with significant amelioration of falls in fractional wall thickening compared with control. However, global measures of LV function were less protected by exosome treatment. It is possible that greater preservation of global LV function may have been attenuated by increased cardiac fibrosis, as T1 values showed significant increase in the exosome pigs compared to control particularly in the infarct related segments. Taken together, these results show clear effects of IV exosomes administered over 7 days to reduce infarct size with relatively preserved cardiac function compared to control treated infarct pigs.

Human epicardium-derived cells reinforce cardiac sympathetic innervation

RATIONALE

After cardiac damage, excessive neurite outgrowth (sympathetic hyperinnervation) can occur, which is related to ventricular arrhythmias/sudden cardiac death. Post-damage reactivation of epicardium causes epicardium-derived cells (EPDCs) to acquire a mesenchymal character, contributing to cardiac regeneration. Whether EPDCs also contribute to cardiac re/hyperinnervation, is unknown.

AIM

To investigate whether mesenchymal EPDCs influence cardiac sympathetic innervation.

METHODS AND RESULTS

Sympathetic ganglia were co-cultured with mesenchymal EPDCs and/or myocardium, and neurite outgrowth and sprouting density were assessed. Results showed a significant increase in neurite density and directional (i.e. towards myocardium) outgrowth when ganglia were co-cultured with a combination of EPDCs and myocardium, as compared to cultures with EPDCs or myocardium alone. In absence of myocardium, this outgrowth was not directional. Neurite differentiation of PC12 cells in conditioned medium confirmed these results via a paracrine effect, in accordance with expression of neurotrophic factors in myocardial explants co-cultured with EPDCs. Of interest, EPDCs increased the expression of nerve growth factor (NGF) in cultured, but not in fresh myocardium, possibly due to an "ischemic state" of cultured myocardium, supported by TUNEL and Hif1α expression. Cardiac tissues after myocardial infarction showed robust NGF expression in the infarcted, but not remote area.

CONCLUSION

Neurite outgrowth and density increases significantly in the presence of EPDCs by a paracrine effect, indicating a new role for EPDCs in the occurrence of sympathetic re/hyperinnervation after cardiac damage.

Human Cardiac Progenitor Cells Enhance Exosome Release and Promote Angiogenesis Under Physoxia

Studies on cardiac progenitor cells (CPCs) and their derived exosomes therapeutic potential have demonstrated only modest improvements in cardiac function. Therefore, there is an unmet need to improve the therapeutic efficacy of CPCs and their exosomes to attain clinically relevant improvement in cardiac function. The hypothesis of this project is to assess the therapeutic potential of exosomes derived from human CPCs (hCPCs) cultured under normoxia (21% O₂), physoxia (5% O₂) and hypoxia (1% O₂) conditions. hCPCs were characterized by immunostaining of CPC-specific markers (NKX-2.5, GATA-4, and c-kit). Cell proliferation and cell death assay was not altered under physoxia. A gene expression qPCR array (84 genes) was performed to assess the modulation of hypoxic genes under three different oxygen conditions as mentioned above. Our results demonstrated that very few hypoxia-related genes were modulated under physoxia (5 genes upregulated, 4 genes down regulated). However, several genes were modulated under hypoxia (23 genes upregulated, 9 genes downregulated). Furthermore, nanoparticle tracking analysis of the exosomes isolated from hCPCs under physoxia had a 1.6-fold increase in exosome yield when compared to normoxia and hypoxia conditions. Furthermore, tube formation assay for angiogenesis indicated that exosomes derived from hCPCs cultured under physoxia significantly increased tube formation as compared to no-exosome control, 21% O₂, and 1% O₂ groups. Overall, our study demonstrated the therapeutic potential of physoxic oxygen microenvironment cultured hCPCs and their derived exosomes for myocardial repair.

Therapeutic Application of Mesenchymal Stem Cells Derived Extracellular Vesicles for Immunomodulation

The immunosuppressive potential of mesenchymal stem cells has been extensively investigated in many studies in vivo and in vitro. In recent years, a variety preclinical and clinical studies have demonstrated that mesenchymal stem cells ameliorate immune-mediated disorders, including autoimmune diseases. However, to date mesenchymal stem cells have not become a widely used therapeutic agent due to safety challenges, high cost and difficulties in providing long term production. A key mechanism underpinning the immunomodulatory effect of MSCs is the production of paracrine factors including growth factors, cytokines, chemokines, and extracellular vesicles (EVs). MSCs derived EVs have become an attractive therapeutic agent for immunomodulation and treatment of immune-mediated disorders. In addition to many preclinical studies of MSCs derived EVs, their beneficial effects have been observed in patients with both acute graft-vs.-host disease and chronic kidney disease. In this review, we discuss the current findings in the field of MSCs derived EVs-based therapies in immune-mediated disorders and approaches to scale EV production for clinical use.

Exosomes in ischemic heart disease: novel carriers for bioinformation

The occurrence of ischemic heart disease(IHD) is a multi-step chain process from potential risk factors to overt clinical diseases. Vascular cells, blood cells, cardiomyocytes and stem cells are all involved in the pathophysiological links via continual and polynary crosstalk. Exosomes,as powerful vectors for intercellular communication,have been a hotspot for basic and clinical research. Plenty of evidence has shown that exosomes largely participate in the evolution of IHD, including endothelial dysfunction, lipid deposition, atheromatous plaque formation and rupture, myocardial ischemia-reperfusion(I/R) injury,and heart failure (HF), while the rules for detailed communication in the different stages of this continuous disease are still poorly understood. This review will systematically describe characteristics of exosomal crosstalk between different cells in the diverse periods, and also cast light on the potential and challenges for exosome application as therapeutic targets, hoping to offer supporting background for the following research.

Extracellular Vesicles Released by Human Induced-Pluripotent Stem Cell-Derived Cardiomyocytes Promote Angiogenesis

Although cell survival post-transplantation is very low, emerging evidence using stem cell therapy for myocardial repair points toward a primary role of paracrine signaling mechanisms as the basis for improved cardiac function, decreased fibrosis, and increased angiogenesis. Recent studies have demonstrated that extracellular vesicles (EVs) such as exosomes secreted by stem cells stimulate angiogenesis, provide cytoprotection, and modulate apoptosis. However, the angiogenic potential of EVs secreted from human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM), a terminally differentiated cell type, has not been elucidated yet. Therefore, the main objective of this study is to isolate, characterize, and evaluate the in vitro angiogenic potential of EVs collected from hiPSC-CM conditioned media. The hiPSC-CM were cultured for 2 weeks and EVs were isolated from cell culture medium. Isolated EVs were characterized by transmission electron microscopy (TEM), nanoparticle tracking analysis, and immunoblotting. Furthermore, the angiogenic potential of these EVs was evaluated by tube formation, wound-healing, and cell-proliferation assays in bovine aortic endothelial cells (BAEC). In addition, gene expression levels of growth factors was evaluated in hiPSC-derived endothelial cells (hiPSC-EC) treated with hiPSC-CM-derived EV (CM-EVs) to assess their role in promoting angiogenesis. TEM imaging of CM-EVs showed a presence of a double-membrane bound structure, which is a characteristic of EV. Nanoparticle tracking analysis further confirmed the size and shape of the secreted particles to be consistent with EVs. Furthermore, EV-specific markers (CD63 and HSP70) were enriched in these particles as illustrated by immunoblotting. Most importantly, BAEC treated with 100 μg/ml of CM-EVs showed significant increases in tube formation, wound closure, and cell proliferation as compared to control (no-EVs). Finally, treatment of hiPSC-EC with CM-EVs induced increased expression of pro-angiogenic growth factors by the endothelial cells. Overall, our results demonstrated that EVs isolated from hiPSC-CM enhance angiogenesis in endothelial cells. This acellular/cell-free approach constitutes a potential translational therapeutic to induce angiogenesis in patients with myocardial infarction.

Cardiac Progenitor Cell-Derived Extracellular Vesicles Reduce Infarct Size and Associate with Increased Cardiovascular Cell Proliferation

Cell transplantation studies have shown that injection of progenitor cells can improve cardiac function after myocardial infarction (MI). Transplantation of human cardiac progenitor cells (hCPCs) results in an increased ejection fraction, but survival and integration are low. Therefore, paracrine factors including extracellular vesicles (EVs) are likely to contribute to the beneficial effects. We investigated the contribution of EVs by transplanting hCPCs with reduced EV secretion. Interestingly, these hCPCs were unable to reduce infarct size post-MI. Moreover, injection of hCPC-EVs did significantly reduce infarct size. Analysis of EV uptake showed cardiomyocytes and endothelial cells primarily positive and a higher Ki67 expression in these cell types. Yes-associated protein (YAP), a proliferation marker associated with Ki67, was also increased in the entire infarcted area. In summary, our data suggest that EV secretion is the driving force behind the short-term beneficial effect of hCPC transplantation on cardiac recovery after MI.

Triggering Endogenous Cardiac Repair and Regeneration via Extracellular Vesicle-Mediated Communication

A variety of paracrine signals create networks within the myocardium and mediate intercellular communications. Indeed, paracrine stimulation of the endogenous regenerative program of the heart, mainly based on resident cardiac progenitor cell (CPC) activation together with cardiomyocyte proliferation, has become increasingly relevant for future cardiac medicine. In the last years, it has been shown that extracellular vesicles (EV), including exosomes (Ex), are powerful conveyors of relevant biological effects. EV have been proposed not only as promising therapeutic tool for triggering cardiac regeneration and improving repair, but also as means of better understanding the physiological and pathological relationships between specific cardiac components, including cardiomyocytes and fibroblasts. Actually, EV from different kinds of exogenous stem cells have been shown to mediate beneficial effects on the injured myocardium. Moreover, endogenous cells, like CPC can instruct cardiovascular cell types, including cardiomyocytes, while cardiac stromal cells, especially fibroblasts, secrete EV that modulate relevant aspects of cardiomyocyte biology, such as hypertrophy and electrophysiological properties. Finally, cardiomyocytes too may release EV influencing the function of other cardiac cell types. Therefore, EV-based crosstalk is thought to be important in both physiology and pathology, being involved in the responses of the heart to noxious stimuli. In this review we will discuss the role of EV in both regulating cardiac homeostasis and driving heart regeneration. In particular, we will address their role in: (i) providing cardio-protection and enhancing cardiac repair mechanisms; (ii) CPC biology; and (iii) influencing adult cardiomyocyte behavior.

Extracellular vesicles and cardiovascular disease therapy

Cardiovascular disease (CVD) constitutes one of the leading causes of mortality worldwide, therefore representing a major public health concern. Despite recent advances in the treatment of patients with acute myocardial infarction (AMI), such as bypass surgery or percutaneous coronary intervention, pathological cardiac remodeling often predisposes survivors to fatal heart failure. In this context, the proven efficacy of stem cell-regenerative therapies constitutes a promising therapeutic perspective with is nevertheless slow down by safety and ethical concerns. Recent studies have underscored the capacity of stem cell-derived extracellular vesicles (EV) to recapitulate the regenerative properties of their parental cells therefore offering a therapeutic alternative to cell therapy in cardiovascular regenerative medicine. In this article, we review the functional relevance of using stem cell-derived EV as therapeutically agents and detail the identified molecular pathways that they used to exert their effects. We also discuss the advantages of using such an acellular regenerative therapy, in regard with parental stem cells, and address the limitations, which would need to be resolved, before their clinical translation.

Myocardial Regeneration via Progenitor Cell-Derived Exosomes

In the past 20 years, a variety of cell products has been evaluated in terms of their capacity to treat patients with acute myocardial infarction and chronic heart failure. Despite initial enthusiasm, therapeutic efficacy has overall been disappointing, and clinical application is costly and complex. Recently, a subset of small extracellular vesicles (EVs), commonly referred to as "exosomes," was shown to confer cardioprotective and regenerative signals at a magnitude similar to that of their donor cells. The conceptual advantage is that they may be produced in industrial quantities and stored at the point-of-care for off-the-shelf application, ideally without eliciting a relevant recipient immune response or other adverse effects associated with viable cells. The body of evidence on beneficial exosome-mediated effects in animal models of heart diseases is rapidly growing. However, there is significant heterogeneity in terms of exosome source cells, isolation process, therapeutic dosage, and delivery mode. This review summarizes the current state of research on exosomes as experimental therapy of heart diseases and seeks to identify roadblocks that need to be overcome prior to clinical application.

Potential Role of Exosomes in Mending a Broken Heart: Nanoshuttles Propelling Future Clinical Therapeutics Forward

Stem cell transplantation therapy is a promising adjunct for regenerating damaged heart tissue; however, only modest improvements in cardiac function have been observed due to poor survival of transplanted cells in the ischemic heart. Therefore, there remains an unmet need for therapies that can aid in attenuating cardiac damage. Recent studies have demonstrated that exosomes released by stem cells could serve as a potential cell-free therapeutic for cardiac repair. These exosomes/nanoshuttles, once thought to be merely a method of waste disposal, have been shown to play a crucial role in physiological functions including short- and long-distance intercellular communication. In this review, we have summarized studies demonstrating the potential role of exosomes in improving cardiac function, attenuating cardiac fibrosis, stimulating angiogenesis, and modulating miRNA expression. Furthermore, exosomes carry an important cargo of miRNAs and proteins that could play an important role as a diagnostic marker for cardiovascular disease post-myocardial infarction. Although there is promising evidence from preclinical studies that exosomes released by stem cells could serve as a potential cell-free therapeutic for myocardial repair, there are several challenges that need to be addressed before exosomes could be fully utilized as off-the-shelf therapeutics for cardiac repair.

Cardiac Progenitor Cells and the Interplay with Their Microenvironment

The microenvironment plays a crucial role in the behavior of stem and progenitor cells. In the heart, cardiac progenitor cells (CPCs) reside in specific niches, characterized by key components that are altered in response to a myocardial infarction. To date, there is a lack of knowledge on these niches and on the CPC interplay with the niche components. Insight into these complex interactions and into the influence of microenvironmental factors on CPCs can be used to promote the regenerative potential of these cells. In this review, we discuss cardiac resident progenitor cells and their regenerative potential and provide an overview of the interactions of CPCs with the key elements of their niche. We focus on the interaction between CPCs and supporting cells, extracellular matrix, mechanical stimuli, and soluble factors. Finally, we describe novel approaches to modulate the CPC niche that can represent the next step in recreating an optimal CPC microenvironment and thereby improve their regeneration capacity.

Cytochalasin B-induced membrane vesicles convey angiogenic activity of parental cells

Naturally occurring extracellular vesicles (EVs) play essential roles in intracellular communication and delivery of bioactive molecules. Therefore it has been suggested that EVs could be used for delivery of therapeutics. However, to date the therapeutic application of EVs has been limited by number of factors, including limited yield and full understanding of their biological activities. To address these issues, we analyzed the morphology, molecular composition, fusion capacity and biological activity of Cytochalasin B-induced membrane vesicles (CIMVs). The size of these vesicles was comparable to that of naturally occurring EVs. In addition, we have shown that CIMVs from human SH-SY5Y cells contain elevated levels of VEGF as compared to the parental cells, and stimulate angiogenesis in vitro and in vivo.

Obtaining control of cell surface functionalizations via Pre-targeting and Supramolecular host guest interactions

The use of mammalian cells for therapeutic applications is finding its way into modern medicine. However, modification or "training" of cells to make them suitable for a specific application remains complex. By envisioning a chemical toolbox that enables specific, but straight-forward and generic cellular functionalization, we investigated how membrane-receptor (pre)targeting could be combined with supramolecular host-guest interactions based on β-cyclodextrin (CD) and adamantane (Ad). The feasibility of this approach was studied in cells with membranous overexpression of the chemokine receptor 4 (CXCR4). By combining specific targeting of CXCR4, using an adamantane (Ad)-functionalized Ac-TZ14011 peptide (guest; KD = 56 nM), with multivalent host molecules that entailed fluorescent β-CD-Poly(isobutylene-alt-maleic-anhydride)-polymers with different fluorescent colors and number of functionalities, host-guest cell-surface modifications could be studied in detail. A second set of Ad-functionalized entities enabled introduction of additional surface functionalities. In addition, the attraction between CD and Ad could be used to drive cell-cell interactions. Combined we have shown that supramolecular interactions, that are based on specific targeting of an overexpressed membrane-receptor, allow specific and stable, yet reversible, surface functionalization of viable cells and how this approach can be used to influence the interaction between cells and their surroundings.

Exosomes from Cardiomyocyte Progenitor Cells and Mesenchymal Stem Cells Stimulate Angiogenesis Via EMMPRIN

To date, cellular transplantation therapy has not yet fulfilled its high expectations for cardiac repair. A major limiting factor is lack of long-term engraftment of the transplanted cells. Interestingly, transplanted cells can positively affect their environment via secreted paracrine factors, among which are extracellular vesicles, including exosomes: small bi-lipid-layered vesicles containing proteins, mRNAs, and miRNAs. An exosome-based therapy will therefore relay a plethora of effects, without some of the limiting factors of cell therapy. Since cardiomyocyte progenitor cells (CMPC) and mesenchymal stem cells (MSC) induce vessel formation and are frequently investigated for cardiac-related therapies, the pro-angiogenic properties of CMPC and MSC-derived exosome-like vesicles are investigated. Both cell types secrete exosome-like vesicles, which are efficiently taken up by endothelial cells. Endothelial cell migration and vessel formation are stimulated by these exosomes in in vitro models, mediated via ERK/Akt-signaling. Additionally, these exosomes stimulated blood vessel formation into matrigel plugs. Analysis of pro-angiogenic factors revealed high levels of extracellular matrix metalloproteinase inducer (EMMPRIN). Knockdown of EMMPRIN on CMPCs leads to a diminished pro-angiogenic effect, both in vitro and in vivo. Therefore, CMPC and MSC exosomes have powerful pro-angiogenic effects, and this effect is largely mediated via the presence of EMMPRIN on exosomes.

Stem cells for the treatment of heart failure

Stem cell-based therapy is currently tested in several trials of chronic heart failure. The main question is to determine how its implementation could be extended to standard clinical practice. To answer this question, it is helpful to capitalize on the three main lessons drawn from the accumulated experience, both in the laboratory and in the clinics. Regarding the cell type, the best outcomes seem to be achieved by cells the phenotype of which closely matches that of the target tissue. This argues in favor of the use of cardiac-committed cells among which the pluripotent stem cell-derived cardiac progeny is particularly attractive. Regarding the mechanism of action, there has been a major paradigm shift whereby cells are no longer expected to structurally integrate within the recipient myocardium but rather to release biomolecules that foster endogenous repair processes. This implies to focus on early cell retention, rather than on sustained cell survival, so that the cells reside in the target tissue long enough and in sufficient amounts to deliver the factors underpinning their action. Biomaterials are here critical adjuncts to optimize this residency time. Furthermore, the paracrine hypothesis gives more flexibility for using allogeneic cells in that targeting an only transient engraftment requires to delay, and no longer to avoid, rejection, which, in turn, should simplify immunomodulation regimens. Regarding manufacturing, a broad dissemination of cardiac cell therapy requires the development of automated systems allowing to yield highly reproducible cell products. This further emphasizes the interest of allogeneic cells because of their suitability for industrially-relevant and cost-effective scale-up and quality control procedures. At the end, definite confirmation that the effects of cells can be recapitulated by the factors they secrete could lead to acellular therapies whereby factors alone (possibly clustered in extracellular vesicles) would be delivered to the patient. The production process of these cell-derived biologics would then be closer to that of a pharmaceutical compound, which could streamline the manufacturing and regulatory paths and thereby facilitate an expended clinical use.

Discrepant Results of Experimental Human Mesenchymal Stromal Cell Therapy after Myocardial Infarction: Are Animal Models Robust Enough?

Background

Human mesenchymal stromal cells (MSCs) have been reported to preserve cardiac function in myocardial infarction (MI) models. Previously, we found a beneficial effect of intramyocardial injection of unstimulated human MSCs (uMSCs) on cardiac function after permanent coronary artery ligation. In the present study we aimed to extend this research by investigating the effect of intramyocardial injection of human MSCs pre-stimulated with the pro-inflammatory cytokine interferon-gamma (iMSCs), since pro-inflammatory priming has shown additional salutary effects in multiple experimental disease models.

Methods

MI was induced in NOD/Scid mice by permanent ligation of the left anterior descending coronary artery. Animals received intramyocardial injection of uMSCs, iMSCs or PBS. Sham-operated animals were used to determine baseline characteristics. Cardiac performance was assessed after 2 and 14 days using 7-Tesla magnetic resonance imaging and pressure-volume loop measurements. Histology and q-PCR were used to confirm MSC engraftment in the heart.

Results

Both uMSC and iMSC therapy had no significant beneficial effect on cardiac function or remodelling in contrast to our previous studies.

Conclusions

Animal models for cardiac MSC therapy appear less robust than initially envisioned.

Cellular and molecular characterization of human cardiac stem cells reveals key features essential for their function and safety

Cell therapy of heart diseases is emerging as one of the most promising known treatments in recent years. Transplantation of cardiac stem cells (CSCs) may be one of the best strategies to cure adult or pediatric heart diseases. As these patient-derived stem cells need to be isolated from small heart biopsies, it is important to select the best isolation method and CSC subpopulation with the best cardiogenic functionality. We employed three different protocols including c-KIT(+) cell sorting, clonogenic expansion, and explants culture to isolate c-KIT(+) cells, clonogenic expansion-derived cells (CEDCs), and cardiosphere-derived cells (CDCs), respectively. Evaluation of isolated CSC characteristics in vitro and after rat myocardial infarction (MI) model transplantation revealed that although c-KIT(+) and CDCs had higher MI regenerative potential, CEDCs had more commitment into cardiomyocytes and needed lower passages that were essential to reach a definite cell count. Furthermore, genome-wide expression analysis showed that subsequent passages caused changes in characteristics of cells, downregulation of cell cycle-related genes, and upregulation of differentiation and carcinogenic genes, which might lead to senescence, commitment, and possible tumorigenicity of the cells. Because of different properties of CSC subpopulations, we suggest that appropriate CSCs subpopulation should be chosen based on their experimental or clinical use.

iPSC-derived human cardiac progenitor cells improve ventricular remodelling via angiogenesis and interstitial networking of infarcted myocardium

We investigate the effects of myocardial transplantation of human induced pluripotent stem cell (iPSC)‐derived progenitors and cardiomyocytes into acutely infarcted myocardium in severe combined immune deficiency mice. A total of 2 × 10⁵ progenitors, cardiomyocytes or cell‐free saline were injected into peri‐infarcted anterior free wall. Sham‐operated animals received no injection. Myocardial function was assessed at 2‐week and 4‐week post‐infarction by using echocardiography and pressure‐volume catheterization. Early myocardial remodelling was observed at 2‐week with echocardiography derived stroke volume (SV) in saline (20.45 ± 7.36 μl, P < 0.05) and cardiomyocyte (19.52 ± 3.97 μl, P < 0.05) groups, but not in progenitor group (25.65 ± 3.61 μl), significantly deteriorated as compared to sham control group (28.41 ± 4.41 μl). Consistently, pressure–volume haemodynamic measurements showed worsening chamber dilation in saline (EDV: 23.24 ± 5.01 μl, P < 0.05; ESV: 17.08 ± 5.82 μl, P < 0.05) and cardiomyocyte (EDV: 26.45 ± 5.69 μl, P < 0.05; ESV: 18.03 ± 6.58 μl, P < 0.05) groups by 4‐week post‐infarction as compared to control (EDV: 15.26 ± 2.96 μl; ESV: 8.41 ± 2.94 μl). In contrast, cardiac progenitors (EDV: 20.09 ± 7.76 μl; ESV: 13.98 ± 6.74 μl) persistently protected chamber geometry against negative cardiac remodelling. Similarly, as compared to sham control (54.64 ± 11.37%), LV ejection fraction was preserved in progenitor group from 2‐(38.68 ± 7.34%) to 4‐week (39.56 ± 13.26%) while cardiomyocyte (36.52 ± 11.39%, P < 0.05) and saline (35.34 ± 11.86%, P < 0.05) groups deteriorated early at 2‐week. Improvements of myocardial function in the progenitor group corresponded to increased vascularization (16.12 ± 1.49/mm² to 25.48 ± 2.08/mm² myocardial tissue, P < 0.05) and coincided with augmented networking of cardiac telocytes in the interstitial space of infarcted zone.

The Evolution of the Stem Cell Theory for Heart Failure

Various stem cell-based approaches for cardiac repair have achieved encouraging results in animal experiments, often leading to their rapid proceeding to clinical testing. However, freewheeling evolutionary developments of the stem cell theory might lead to dystopian scenarios where heterogeneous sources of therapeutic cells could promote mixed clinical outcomes in un-stratified patient populations. This review focuses on the lessons that should be learnt from the first generation of stem cell-based strategies and emphasizes the absolute requirement to better understand the basic mechanisms of stem cell biology and cardiogenesis. We will also discuss about the unexpected “big bang” in the stem cell theory, “blasting” the therapeutic cells to their unchallenged ability to release paracrine factors such as extracellular membrane vesicles. Paradoxically, the natural evolution of the stem cell theory for cardiac regeneration may end with the development of cell-free strategies with multiple cellular targets including cardiomyocytes but also other infiltrating or resident cardiac cells.

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Varied sources of therapeutic cells and low repair ability of the failing heart contribute to mixed results in clinical trials.

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Consensus is still lacking concerning the appropriate type of therapeutic stem cells.

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A clear understanding of cardiac development and adult cardiogenesis might increase the efficiency of regenerative therapies.

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Delivery of stem cell-derived paracrine factor alone to the damaged heart may be sufficient to activate repair mechanisms.

Myocardial infarction models in NOD/Scid mice for cell therapy research: permanent ischemia vs ischemia-reperfusion

Myocardial infarction animal studies are used to study disease mechanisms and new treatment options. Typically, myocardial infarction (MI) is induced by permanent occlusion of the left anterior descending artery. Since in MI patients coronary blood flow is often restored new experimental models better reflecting clinical practice are needed. Here, permanent ischemia MI (PI group) was compared with transient ischemia (45 min) (IR group) in immunodeficient NOD/Scid mice. Cardiac function, infarct size, wall thickness and total collagen deposition were significantly reduced only in PI mice. Cardiac inflammatory cells and serum cytokine levels were less dynamic in IR animals compared to PI. So although IR better reflects clinical practice, it is secondary to PI for investigating cell therapy, since it induces too little damage to provide a measurable therapeutic window. MI did result in significant changes in the inflammatory state, indicating this immunodeficient mouse strain is valuable to study human cell therapy.

mAb C19 targets a novel surface marker for the isolation of human cardiac progenitor cells from human heart tissue and differentiated hESCs

AIMS

Cardiac progenitor cells (CPCs) have been isolated from adult and developing hearts using an anti-mouse Sca-1 antibody. However, the absence of a human Sca-1 homologue has hampered the clinical application of the CPCs. Therefore, we generated novel monoclonal antibodies (mAbs) specifically raised against surface markers expressed by resident human CPCs. Here, we explored the suitability of one of these mAbs, mAb C19, for the identification, isolation and characterization of CPCs from fetal heart tissue and differentiating cultures of human embryonic stem cells (hESCs).

METHODS & RESULTS

Using whole-cell immunization, mAbs were raised against Sca-1+ CPCs and screened for reactivity to various CPC lines by flow cytometry. mAb C19 was found to be specific for Sca-1+ CPCs, with high cell surface binding capabilities. mAb C19 stained small stem-like cells in cardiac tissue sections. Moreover, during differentiation of hESCs towards cardiomyocytes, a transient population of cells with mAb C19 reactivity was identified and isolated using magnetic-activated cell sorting. Their cell fate was tracked and found to improve cardiomyocyte purity from hESC-derived cultures. mAb C19+ CPCs, from both hESC differentiation and fetal heart tissues, were maintained and expanded in culture, while retaining their CPC-like characteristics and their ability to further differentiate into cardiomyocytes by stimulation with TGFβ1. Finally, gene expression profiling of these mAb C19+ CPCs suggested a highly angiogenic nature, which was further validated by cell-based angiogenesis assays.

CONCLUSION

mAb C19 is a new surface marker for the isolation of multipotent CPCs from both human heart tissues and differentiating hESCs.

Extracellular vesicles: potential roles in regenerative medicine

Extracellular vesicles (EV) consist of exosomes, which are released upon fusion of the multivesicular body with the cell membrane, and microvesicles, which are released directly from the cell membrane. EV can mediate cell–cell communication and are involved in many processes, including immune signaling, angiogenesis, stress response, senescence, proliferation, and cell differentiation. The vast amount of processes that EV are involved in and the versatility of manner in which they can influence the behavior of recipient cells make EV an interesting source for both therapeutic and diagnostic applications. Successes in the fields of tumor biology and immunology sparked the exploration of the potential of EV in the field of regenerative medicine. Indeed, EV are involved in restoring tissue and organ damage, and may partially explain the paracrine effects observed in stem cell-based therapeutic approaches. The function and content of EV may also harbor information that can be used in tissue engineering, in which paracrine signaling is employed to modulate cell recruitment, differentiation, and proliferation. In this review, we discuss the function and role of EV in regenerative medicine and elaborate on potential applications in tissue engineering.

Myoblasts and embryonic stem cells differentially engraft in a mouse model of genetic dilated cardiomyopathy

The functional and architectural benefits of embryonic stem cells (ESC) and myoblasts (Mb) transplantations into infarcted myocardium have been investigated extensively. Whereas ESC repopulated fibrotic areas and contributed to myocardial regeneration, Mb exerted their effects through paracrine secretions and scar remodeling. This therapeutic perspective, however, has been less explored in the setting of nonischemic dilated cardiomyopathies (DCMs). Our aim was to compare the integration and functional efficacy of ESC committed to cardiac fate by bone morphogenic protein 2 (BMP-2) pretreatment and Mb used as gold standard following their transplantation into the myocardium of a mouse model of laminopathy exhibiting a progressive and lethal DCM. After 4 and 8 weeks of transplantation, stabilization was observed in Mb-transplanted mice (P = 0.008) but not in groups of ESC-transplanted or medium-injected animals, where the left ventricular fractional shortening (LVFS) decreased by 32 ± 8% and 41 ± 8% respectively. Engrafted differentiated cells were consistently detected in myocardia of mice receiving Mb, whereas few or no cells were detected in the hearts of mice receiving ESC, except in two cases where teratomas were formed. These data suggest that committed ESC fail to integrate in DCM where scar tissue is absent to provide the appropriate niche, whereas the functional benefits of Mb transplantation might extend to nonischemic cardiomyopathy.

Substrate stiffness modulates gene expression and phenotype in neonatal cardiomyocytes in vitro

Biomaterials to be used as cell delivery systems for cardiac tissue engineering should be able to comply with cardiac muscle contractile activity, while favoring cell survival and neo-angiogenesis in a hostile environment. Biocompatible synthetic materials can be tailored to mimic cardiac tissue three-dimensional organization in the micro- and nanoscales. Nonetheless, they usually display mechanical properties that are far from those of the native myocardium and thus could affect host cell survival and activity. In the present investigation, inert poly-ε-caprolactone planar layers were manufactured to change the surface stiffness (with Young's modulus ranging from 1 to 133 MPa) without changing matrix chemistry. These substrates were challenged with neonatal murine cardiomyocytes to study the possible effect of substrate stiffness on such cell behavior without changing biological cues. Interestingly, softer substrates (0.91±0.08 and 1.53±0.16 MPa) were found to harbor mostly mature cardiomyocytes having assembled sarcomeres, as shown by the expression of alpha actinin and myosin heavy chain in typical striations and the upregulation of sarcomeric actin mRNA. On the other hand, a preferential expression of immature cardiac cell genes (Nkx-2.5) and proteins (GATA-4) in cardiac cells grown onto stiffer materials (49.67±2.56 and 133.23±8.67 MPa) was detected. This result could not be ascribed to significant differences in cell adhesion or proliferation induced by the substrates, but to the stabilization of cardiomyocyte differentiated phenotype induced by softer layers. In fact, cardiac cell electromechanical coupling was shown to be more organized on softer surfaces, as highlighted by connexin 43 distribution. Moreover, a differential regulation of genes involved in extracellular matrix remodeling was detected on soft films (0.91±0.08 MPa) as compared with the stiffest (133.23±8.67 MPa). Finally, the upregulation of a number of genes involved in inflammatory processes was detected when the stiffest polymer is used. These events highlight the differences in cell mechanosensitivity in a heterogeneous cell preparation and are likely to contribute to the differences encountered in cardiac cell phenotype induced by substrate stiffness.