Cellular cardiomyoplasty improves survival after myocardial injury

Electrically Conductive Collagen-PEDOT:PSS Hydrogel Prevents Post-Infarct Cardiac Arrhythmia and Supports hiPSC-Cardiomyocyte Function

Myocardial infarction (MI) causes cell death, disrupts electrical activity, triggers arrhythmia, and results in heart failure, whereby 50-60% of MI-associated deaths manifest as sudden cardiac deaths (SCD). The most effective therapy for SCD prevention is implantable cardioverter defibrillators (ICDs). However, ICDs contribute to adverse remodeling and disease progression and do not prevent arrhythmia. This work develops an injectable collagen-PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) hydrogel that protects infarcted hearts against ventricular tachycardia (VT) and can be combined with human induced pluripotent stem cell (hiPSC)-cardiomyocytes to promote partial cardiac remuscularization. PEDOT:PSS improves collagen gel formation, micromorphology, and conductivity. hiPSC-cardiomyocytes in collagen-PEDOT:PSS hydrogels exhibit near-adult sarcomeric length, improved contractility, enhanced calcium handling, and conduction velocity. RNA-sequencing data indicate enhanced maturation and improved cell-matrix interactions. Injecting collagen-PEDOT:PSS hydrogels in infarcted mouse hearts decreases VT to the levels of healthy hearts. Collectively, collagen-PEDOT:PSS hydrogels offer a versatile platform for treating cardiac injuries.

Persistence of intramyocardially transplanted murine induced pluripotent stem cell-derived cardiomyocytes from different developmental stages

Background

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) are regarded as promising cell type for cardiac cell replacement therapy, but it is not known whether the developmental stage influences their persistence and functional integration in the host tissue, which are crucial for a long-term therapeutic benefit. To investigate this, we first tested the cell adhesion capability of murine iPSC-CM in vitro at three different time points during the differentiation process and then examined cell persistence and quality of electrical integration in the infarcted myocardium in vivo.

Methods

To test cell adhesion capabilities in vitro, iPSC-CM were seeded on fibronectin-coated cell culture dishes and decellularized ventricular extracellular matrix (ECM) scaffolds. After fixed periods of time, stably attached cells were quantified. For in vivo experiments, murine iPSC-CM expressing enhanced green fluorescent protein was injected into infarcted hearts of adult mice. After 6–7 days, viable ventricular tissue slices were prepared to enable action potential (AP) recordings in transplanted iPSC-CM and surrounding host cardiomyocytes. Afterwards, slices were lysed, and genomic DNA was prepared, which was then used for quantitative real-time PCR to evaluate grafted iPSC-CM count.

Results

The in vitro results indicated differences in cell adhesion capabilities between day 14, day 16, and day 18 iPSC-CM with day 14 iPSC-CM showing the largest number of attached cells on ECM scaffolds. After intramyocardial injection, day 14 iPSC-CM showed a significant higher cell count compared to day 16 iPSC-CM. AP measurements revealed no significant difference in the quality of electrical integration and only minor differences in AP properties between d14 and d16 iPSC-CM.

Conclusion

The results of the present study demonstrate that the developmental stage at the time of transplantation is crucial for the persistence of transplanted iPSC-CM. iPSC-CM at day 14 of differentiation showed the highest persistence after transplantation in vivo, which may be explained by a higher capability to adhere to the extracellular matrix.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-020-02089-5.

Treatment with mononuclear cell populations improves post-infarction cardiac function but does not reduce arrhythmia susceptibility

Background

Clinical and experimental data give evidence that transplantation of stem and progenitor cells in myocardial infarction could be beneficial, although the underlying mechanism has remained elusive. Ventricular tachyarrhythmia is the most frequent and potentially lethal complication of myocardial infarction, but the impact of mono nuclear cells on the incidence of ventricular arrhythmia is still not clear.

Objective

We aimed to characterize the influence of splenic mononuclear cell populations on ventricular arrhythmia after myocardial infarction.

Methods

We assessed electrical vulnerability in vivo in mice with left ventricular cryoinfarction 14 days after injury and intramyocardial injection of specific subpopulations of mononuclear cells (MNCs) (CD11b-positive cells, Sca-1-positive cells, early endothelial progenitor cells (eEPCs)). As positive control group we used embryonic cardiomyocytes (eCMs). Epicardial mapping was performed for analysing conduction velocities in the border zone. Left ventricular function was quantified by echocardiography and left heart catheterization.

Results

In vivo pacing protocols induced ventricular tachycardia (VT) in 30% of non-infarcted mice. In contrast, monomorphic or polymorphic VT could be evoked in 94% of infarcted and vehicle-injected mice (p<0.01). Only transplantation of eCMs prevented post-infarction VT and improved conduction velocities in the border zone in accordance to increased expression of connexin 43. Cryoinfarction resulted in a broad aggravation of left ventricular function. All transplanted cell types augmented left ventricular function to a similar extent.

Conclusions

Transplantation of different MNC populations after myocardial infarction improves left ventricular function similar to effects of eCMs. Prevention of inducible ventricular arrhythmia is only seen after transplantation of eCMs.

Regulation of cardiomyocyte maturation during critical perinatal window

A primary limitation in the use of pluripotent stem cell-derived cardiomyocytes (PSC-CMs) for both patient health and scientific investigation is the failure of these cells to achieve full functional maturity. In vivo, cardiomyocytes undergo numerous adaptive structural, functional and metabolic changes during maturation. By contrast, PSC-CMs fail to fully undergo these developmental processes, instead remaining arrested at an embryonic stage of maturation. There is thus a significant need to understand the biological processes underlying proper CM maturation in vivo. Here, we discuss what is known regarding the initiation and coordination of CM maturation. We postulate that there is a critical perinatal window, ranging from embryonic day 18.5 to postnatal day 14 in mice, in which the maturation process is exquisitely sensitive to perturbation. While the initiation mechanisms of this process are unknown, it is increasingly clear that maturation proceeds through interconnected regulatory circuits that feed into one another to coordinate concomitant structural, functional and metabolic CM maturation. We highlight PGC1α, SRF and the MEF2 family as transcription factors that may potentially mediate this cross-talk. We lastly discuss several emerging technologies that will facilitate future studies into the mechanisms of CM maturation. Further study will not only produce a better understanding of its key processes, but provide practical insights into developing a robust strategy to produce mature PSC-CMs.

Overexpression of Cx43 in cells of the myocardial scar: Correction of post-infarct arrhythmias through heterotypic cell-cell coupling

Ventricular tachycardia (VT) is the most common and potentially lethal complication following myocardial infarction (MI). Biological correction of the conduction inhomogeneity that underlies re-entry could be a major advance in infarction therapy. As minimal increases in conduction of infarcted tissue markedly influence VT susceptibility, we reasoned that enhanced propagation of the electrical signal between non-excitable cells within a resolving infarct might comprise a simple means to decrease post-infarction arrhythmia risk. We therefore tested lentivirus-mediated delivery of the gap-junction protein Connexin 43 (Cx43) into acute myocardial lesions. Cx43 was expressed in (myo)fibroblasts and CD45⁺ cells within the scar and provided prominent and long lasting arrhythmia protection in vivo. Optical mapping of Cx43 injected hearts revealed enhanced conduction velocity within the scar, indicating Cx43-mediated electrical coupling between myocytes and (myo)fibroblasts. Thus, Cx43 gene therapy, by direct in vivo transduction of non-cardiomyocytes, comprises a simple and clinically applicable biological therapy that markedly reduces post-infarction VT.

Improved heart repair upon myocardial infarction: Combination of magnetic nanoparticles and tailored magnets strongly increases engraftment of myocytes

Cell replacement in the heart is considered a promising strategy for the treatment of post-infarct heart failure. Direct intramyocardial injection of cells proved to be the most effective application route, however, engraftment rates are very low (<5%) strongly hampering its efficacy. Herein we combine magnetic nanoparticle (MNP) loading of EGFP labeled embryonic cardiomyocytes (eCM) and embryonic stem cell-derived cardiomyocytes (ES-CM) with application of custom designed magnets to enhance their short and long-term engraftment. To optimize cellular MNP uptake and magnetic force within the infarct area, first numerical simulations and experiments were performed in vitro. All tested cell types could be loaded efficiently with SOMag5-MNP (200 pg/cell) without toxic side effects. Application of a 1.3 T magnet at 5 mm distance from the heart for 10 min enhanced engraftment of both eCM and ES-CM by approximately 7 fold at 2 weeks and 3.4 fold (eCM) at 8 weeks after treatment respectively and also strongly improved left ventricular function at all time points. As underlying mechanisms we found that application of the magnetic field prevented the initial dramatic loss of cells via the injection channel. In addition, grafted eCM displayed higher proliferation and lower apoptosis rates. Electron microscopy revealed better differentiation of engrafted eCM, formation of cell to cell contacts and more physiological matrix formation in magnet-treated grafts. These results were corroborated by gene expression data. Thus, combination of MNP-loaded cells and magnet-application strongly increases long-term engraftment of cells addressing a major shortcoming of cardiomyoplasty.

Long-Term Cell Survival and Hemodynamic Improvements after Neonatal Cardiomyocyte and Satellite Cell Transplantation into Healed Myocardial Cryoinfarcted Lesions in Rats

Cell engraftment is a new strategy for the repair of ischemic myocardial lesions. The hemodynamic effectiveness of this strategy, however, is not completely elucidated yet. In a rat model of cryothermia-induced myocardial dysfunction, we investigated whether syngeneic transplantation of neonatal cardiomyocytes or satellite cells is able to improve left ventricular performance. Myocardial infarction was induced in female Lewis rats by a standardized cryolesion to the obtuse margin of the left ventricle. After 4 weeks, 5 × 106 genetically male neonatal cardiomyocytes (n= 16) or satellite cells (n = 16) were engrafted into the myocardial scar. Sham-transplanted animals (n = 15) received injections with cell-free medium. Sham-operated animals (n = 15) served as controls. Left ventricular performance was analyzed 4 months after cell engraftment. Chimerism after this sex-mismatched transplantation was evaluated by detection of PCR-amplified DNA of the Y chromosome. The average heart weight of the infarcted animals significantly exceeded that of controls (p < 0.05). In sham-transplanted animals, mean aortic pressure, left ventricular systolic pressure, aortic flow (indicator of cardiac output), and left ventricular systolic reserve were significantly lower (p < 0.05) compared with sham-operated controls. This was associated with deterioration of ventricular diastolic function (maximal negative dP/dt, time constants of isovolumic relaxation; p < 0.05). Transplantation of satellite cells was found more effective than transplantation of neonatal cardiomyocytes, resulting in i) normalization of mean aortic pressure compared with sham-operated controls, and ii) significantly improved left ventricular systolic pressure and aortic flow (p < 0.05) compared with sham-transplanted animals. Left ventricular systolic reserve and diastolic function, however, were improved by neither satellite cell nor neonatal cardiomyocyte transplantation. Analysis of male genomic DNA revealed 3.98 ± 2.70 ng in hearts after neonatal cardiomyocyte engraftment and 6.16 ± 4.05 ng in hearts after satellite cell engraftment, representing approximately 103 viable engrafted cells per heart. Our study demonstrates i) long-term survival of both neonatal cardiomyocytes and satellite cells after transplantation into cryoinfarcted rat hearts, ii) slight superiority of satellite cells over neonatal cardiomyocytes in improving global left ventricular pump performance, and iii) no effect of both transplant procedures on diastolic dysfunction.

Intramyocardially Transplanted Neonatal Cardiomyocytes (NCMs) Show Structural and Electrophysiological Maturation and Integration and Dose-Dependently Stabilize Function of Infarcted Rat Hearts

Cardiac cell replacement therapy is a promising therapy to improve cardiac function in heart failure. Persistence, structural and functional maturation, and integration of transplanted cardiomyocytes into recipients' hearts are crucial for a safe and efficient replacement of lost cells. We studied histology, electrophysiology, and quantity of intramyocardially transplanted rat neonatal cardiomyocytes (NCMs) and performed a detailed functional study with repeated invasive (pressure-volume catheter) and noninvasive (echocardiography) analyses of infarcted female rat hearts including pharmacological stress before and 3 weeks after intramyocardial injection of 5 × 106 (low NCM) or 25 × 106 (high NCM) syngeneic male NCMs or medium as placebo (Ctrl). Quantitative real-time polymerase chain reaction (PCR) for Y-chromosome confirmed a fivefold higher persisting male cell number in high NCM versus low NCM after 3 weeks. Sharp electrode measurements within viable slices of recipient hearts demonstrated that transplanted NCMs integrate into host myocardium and mature to an almost adult phenotype, which might be facilitated through gap junctions between host myocardium and transplanted NCMs as indicated by connexin43 in histology. Ejection fraction of recipient hearts was severely impaired after ligation of left anterior descending (LAD; pressure-volume catheter: 39.2 ± 3.6%, echocardiography: 39.9 ± 1.4%). Repeated analyses revealed a significant further decline within 3 weeks in Ctrl and a dose-dependent stabilization in cell-treated groups. Consistently, stabilized cardiac function/morphology in cell-treated groups was seen in stroke volume, cardiac output, ventricle length, and wall thickness. Our findings confirm that cardiac cell replacement is a promising therapy for ischemic heart disease since immature cardiomyocytes persist, integrate, and mature after intramyocardial transplantation, and they dose-dependently stabilize cardiac function after myocardial infarction.

HIF2α induces cardiomyogenesis via Wnt/β-catenin signaling in mouse embryonic stem cells

Background

Embryonic stem cells (ESCs) are pluripotent stem cells and can differentiate into cardiomyocytes when cultured in appropriate conditions. The function of hypoxia-inducible factors (HIFs) has been identified in directing the formation of cardiac lineages. The purpose of this study was to investigate the ability of HIF2α to induce differentiation of ESCs into cardiomyocytes and to explore the potential underlying molecular mechanisms.

Methods

Cardiac differentiation from mouse ESCs was analyzed using the “hanging drop” method, and success was determined by assaying the numbers of beating embryoid bodies and the expression level of cardiac markers. The expression of HIF2α was then manipulated during cardiac differentiation with piggyBac transposon and the lentivirus system. The underlying mechanism was finally examined via administering selective inhibitors of the Wnt/β-catenin signaling pathway.

Results

Overexpressing HIF2α can significantly drive mouse ESCs to form cardiomyocytes. Contrarily, knockdown of HIF2α inhibits the emergence of cardiac cells. In addition, the cardiomyogenesis-promoting effect of HIF2α occurred by increasing the protein level of β-catenin, an effector that contributes to cardiac differentiation at an early stage of ESC differentiation.

Conclusion

HIF2α has a cardiomyogenesis-promoting effect in ESCs via enhancing the activation of the Wnt/β-catenin signaling pathway. Our results may be beneficial for generating and applying cardiomyocytes from ESCs safely and effectively in the future.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0447-7) contains supplementary material, which is available to authorized users.

Tissue engineering: construction of a multicellular 3D scaffold for the delivery of layered cell sheets

Many tissues, such as the adult human hearts, are unable to adequately regenerate after damage.(2,3) Strategies in tissue engineering propose innovations to assist the body in recovery and repair. For example, TE approaches may be able to attenuate heart remodeling after myocardial infarction (MI) and possibly increase total heart function to a near normal pre-MI level.(4) As with any functional tissue, successful regeneration of cardiac tissue involves the proper delivery of multiple cell types with environmental cues favoring integration and survival of the implanted cell/tissue graft. Engineered tissues should address multiple parameters including: soluble signals, cell-to-cell interactions, and matrix materials evaluated as delivery vehicles, their effects on cell survival, material strength, and facilitation of cell-to-tissue organization. Studies employing the direct injection of graft cells only ignore these essential elements.(2,5,6) A tissue design combining these ingredients has yet to be developed. Here, we present an example of integrated designs using layering of patterned cell sheets with two distinct types of biological-derived materials containing the target organ cell type and endothelial cells for enhancing new vessels formation in the "tissue". Although these studies focus on the generation of heart-like tissue, this tissue design can be applied to many organs other than heart with minimal design and material changes, and is meant to be an off-the-shelf product for regenerative therapies. The protocol contains five detailed steps. A temperature sensitive Poly(N-isopropylacrylamide) (pNIPAAM) is used to coat tissue culture dishes. Then, tissue specific cells are cultured on the surface of the coated plates/micropattern surfaces to form cell sheets with strong lateral adhesions. Thirdly, a base matrix is created for the tissue by combining porous matrix with neovascular permissive hydrogels and endothelial cells. Finally, the cell sheets are lifted from the pNIPAAM coated dishes and transferred to the base element, making the complete construct.

Clinical application of adult stem cells for therapy for cardiac disease

INTRODUCTION

Cardiovascular disease is a major cause of death worldwide. Different medical and surgical therapeutic options are well established, but a significant number of patients are not amenable to standard therapeutic options. Cell-based therapies after clinical application have shown different results in recent years. Here, we are giving a comprehensive overview on major available clinical data regarding cell therapy.

BACKGROUND

Cell-based therapies and tissue engineering provide new promising platforms to develop upcoming therapeutic options. Initial clinical trials were able to generate promising results. A variety of different stem cell types have been used for the clinical application. Different adult cardiac stem cells and progenitor cells, including mesenchymal, CD34(+) and CD133(+) autologous human bone marrow-derived stem cells (BMCs), human myoblasts, and peripheral blood-derived stem and progenitor cells (PBSCs) have been used for the therapy for end-stage heart failure. Future experiments will show the importance of novel cell populations and clarify the mechanism causing cell therapy-mediated observed effects.

CONCLUSION

Several clinical trials have reported on sole therapy, as well as combined application of autologous adult stem cells with conventional revascularization. The reported promising findings encourage further research in the field of the translational research.

Impact of ischemia-reperfusion on extracellular matrix processing and structure of the basement membrane of the heart

PURPOSE

Acute ischemic injury is a strong inductor of cardiac remodelling, resulting in structural changes of the extracellular matrix (ECM) and basement membrane (BM). In a large animal model of ischemia-reperfusion (I/R) we investigated the post-ischemic liberation of the collagen-IV-fragments Tumstatin (TUM; 28 kDa-fragment of collagen-IV-alpha-3), Arresten (ARR; 26 kDa-fragment of collagen-IV-alpha-1) and Endorepellin (LG3, 85 kDa-fragment of perlecan) which are biologically active in angiogenesis and vascularization in the post-ischemic myocardium.

METHODS AND RESULTS

In this blinded study, 30 pigs were randomized to 60 min of global I/R at either 4°C or 32°C or served as control. Three transmyocardial tissue samples were collected prior to ischemia and within 30 min and 150 min of reperfusion. Tissue content of TUM, ARR and LG3 was analyzed by western blotting and immunostaining. Within 150 min of mild hypothermic I/R a significantly increased tissue content of ARR (0.17±0.14 vs. 0.56±0.56; p = 0.001) and LG3 (1.13±0.34 vs. 2.51±1.71, p<0.001) was observed. In contrast, deep hypothermic I/R was not associated with a significant release of cleavage products. Cleavage of TUM remained unchanged irrespective of temperature. Increased matrix processing following mild hypothermia I/R is further supported by a >11fold elevation of creatine kinase (2075±2595 U/l vs. 23248±6551 U/l; p<0.001) in the coronary sinus plasma samples. Immunostaining demonstrated no changes for ARR and LG3 presentation irrespective of temperature. In contrast, TUM significantly decreased in the BM surrounding cardiomyocytes and capillaries after mild and deep hypothermic I/R, thus representing structural alterations of the BM in these groups.

CONCLUSION

The study demonstrates an early temperature-dependent processing of Col-IV as major component of the BM of cardiomyocytes and vascular endothelium. These observations support the protective effects of deep hypothermia during I/R. Furthermore, the results suggest an increased structural remodelling of the myocardial basement membrane with potential functional impairment during mild hypothermic I/R which may contribute to the progression to post-ischemic heart failure.

Expression of CD80 on cultured neonatal mice cardiomyocytes and attenuation of cytotoxic T lymphocyte-mediated lysis

Transplantation of cultured heterogeneous proliferating cardiomyocytes is a promising therapeutic approach for the treatment of the damage cardiac area resulting from myocardial infarction. However, the chances of recipient rejection are high. How to reduce the immunogenicity of heterogeneous cardiomyocytes and attenuate immune rejection is one of the key stumbling blocks in the application of these cells. In this study, we determined that cultured neonatal cardiomyocytes from mice can express CD80 after culture. CD80 is one of the key costimulatory molecules. Most scholars believe that the main function of CD80 is to activate and boost immune rejection. However, recent studies have shown that CD80 may primarily bind with CTLA-4 and inhibit the immune response. To further study how CD80 worked on these cells, a cytotoxic T-lymphocyte (CTL) assay was performed. The results showed that activated allogenic CTLs lysed cultured cardiomyocytes lacking CD80 expression, but they did not efficiently lyse cardiomyocytes expressing CD80. If we blocked the CD80 with anti-CD80 monoclonal antibody (mAb), the percentages of cardiomyocytes lysis were significantly increased. CD80 can bind CD28, CTLA-4, PD-L1, and even B7-H1, but the main ligands are CD28 and CTLA-4. Thus, we blocked the two ligands separately. When anti-CTLA-4 mAb was applied, the percentages of cardiomyocytes lysis were significantly increased, but when anti-CD28 mAb was applied, the percentages of cell lysis were the same as the intact control. The results indicated that CD80 and CTLA-4 played an important role on the attenuation of CTL-mediated lysis. To our knowledge, this study, for the first time, proves that cardiomyocytes can express CD80 and this expression pattern can resist CTL-mediated lysis through CTLA-4 pathway. The results could have implications in efforts to improve therapeutic strategies for cardiomyocyte transplantation.

Young developmental age cardiac extracellular matrix promotes the expansion of neonatal cardiomyocytes in vitro

A major limitation to cardiac tissue engineering and regenerative medicine strategies is the lack of proliferation of postnatal cardiomyocytes. The extracellular matrix (ECM) is altered during heart development, and studies suggest that it plays an important role in regulating myocyte proliferation. Here, the effects of fetal, neonatal and adult cardiac ECM on the expansion of neonatal rat ventricular cells in vitro are studied. At 24h, overall cell attachment was lowest on fetal ECM; however, ~80% of the cells were cardiomyocytes, while many non-myocytes attached to older ECM and poly-l-lysine controls. After 5 days, the cardiomyocyte population remained highest on fetal ECM, with a 4-fold increase in number. Significantly more cardiomyocytes stained positively for the mitotic marker phospho-histone H3 on fetal ECM compared with other substrates at 5 days, suggesting that proliferation may be a major mechanism of cardiomyocyte expansion on young ECM. Further study of the beneficial properties of early developmental aged cardiac ECM could advance the design of novel biomaterials aimed at promoting cardiac regeneration.

Electrophysiological integration and action potential properties of transplanted cardiomyocytes derived from induced pluripotent stem cells

AIMS

Induced pluripotent stem cell-derived cardiomyocytes (iPSCM) are regarded as promising cell type for cardiac cell replacement therapy. We investigated long-term electrophysiological integration and maturation of transplanted iPSCM, which are essential for therapeutic benefit.

METHODS AND RESULTS

Murine iPSCM expressing enhanced green fluorescent protein and a puromycin resistance under control of the α-myosin heavy chain promoter were purified by antibiotic selection and injected into adult mouse hearts. After 6-12 days, 3-6 weeks, or 6-8 months, viable slices of recipient hearts were prepared. Slices were focally stimulated by a unipolar electrode placed in host tissue, and intracellular action potentials (APs) were recorded with glass microelectrodes in transplanted cells and neighbouring host tissue within the slices. Persistence and electrical integration of transplanted iPSCM into recipient hearts could be demonstrated at all time points. Quality of coupling improved, as indicated by a maximal stimulation frequency without conduction blocks of 5.77 ± 0.54 Hz at 6-12 days, 8.98 ± 0.38 Hz at 3-6 weeks and 10.82 ± 1.07 Hz at 6-8 months after transplantation. AP properties of iPSCM became more mature from 6-12 days to 6-8 months after transplantation, but still differed significantly from those of host APs.

CONCLUSION

Transplanted iPSCM can persist in the long term and integrate electrically into host tissue, supporting their potential for cell replacement therapy. Quality of electrical integration improves between 6-12 days and 6-8 months after transplantation, and there are signs of an electrophysiological maturation. However, even after 6-8 months, AP properties of transplanted iPSCM differ from those of recipient cardiomyocytes.

Identification and characterization of CW108F, a novel β-carboline compound that promotes cardiomyogenesis of stem cells

AIMS

The aim of this study was to identify new compounds that induce cardiomyocyte differentiation of stem cells through cell-based screening and investigate lineage specificity and mechanisms in vitro.

MAIN METHODS

Embryoid bodies (EBs) formed from TC-1/KH2 mouse embryonic stem cells (ESCs) carrying the gene for enhanced green fluorescent protein (EGFP) under the control of the α-myosin heavy chain (MHC) promoter were treated with test compounds. The number of cardiomyocyte-like (EGFP-expressing) cells in EBs was determined by fluorescence-activated cell sorting. Cardiomyocyte differentiation was further confirmed using lineage-specific biochemical assays and by investigating the expression of cardiomyocyte-specific and "stemness"-associated genes. Nuclear factor-kappaB (NF-κB) signaling activity was measured in A549 cells using a reporter-gene assay.

KEY FINDINGS

A β-carboline compound, designated CW108F, increased the number of mouse ESCs expressing α-MHC promoter-driven EGFP and the proportion of beating EBs. CW108F also increased expression of MHC in P19 stem cells, but did not induce osteogenesis of MC3T3-E1 cells, suggesting lineage-specific activity toward cardiomyocytes. CW108F upregulated expression of cardiac-specific GATA-4 and atrial natriuretic factor (ANF) genes in TC-1/KH2 cells, but downregulated expression of the stemness genes, Oct-4 and brachyury. CW108F inhibited NF-κB transcriptional activity, an effect that might contribute to its cardiomyogenesis-promoting activity.

SIGNIFICANCE

The results of this study suggest that the novel β-carboline, CW108F, promotes the differentiation of ESCs into cardiomyocytes and may be useful for investigating molecular pathways of cardiomyogenesis and generating cardiomyocytes from ESCs.

Embryonic cardiomyocyte, but not autologous stem cell transplantation, restricts infarct expansion, enhances ventricular function, and improves long-term survival

AIMS

Controversy exists in regard to the beneficial effects of transplanting cardiac or somatic progenitor cells upon myocardial injury. We have therefore investigated the functional short- and long-term consequences after intramyocardial transplantation of these cell types in a murine lesion model.

METHODS AND RESULTS

Myocardial infarction (MI) was induced in mice (n = 75), followed by the intramyocardial injection of 1-2×10(5) luciferase- and GFP-expressing embryonic cardiomyocytes (eCMs), skeletal myoblasts (SMs), mesenchymal stem cells (MSCs) or medium into the infarct. Non-treated healthy mice (n = 6) served as controls. Bioluminescence and fluorescence imaging confirmed the engraftment and survival of the cells up to seven weeks postoperatively. After two weeks MRI was performed, which showed that infarct volume was significantly decreased by eCMs only (14.8±2.2% MI+eCM vs. 26.7±1.6% MI). Left ventricular dilation was significantly decreased by transplantation of any cell type, but most efficiently by eCMs. Moreover, eCM treatment increased the ejection fraction and cardiac output significantly to 33.4±2.2% and 22.3±1.2 ml/min. In addition, this cell type exclusively and significantly increased the end-systolic wall thickness in the infarct center and borders and raised the wall thickening in the infarct borders. Repetitive echocardiography examinations at later time points confirmed that these beneficial effects were accompanied by better survival rates.

CONCLUSION

Cellular cardiomyoplasty employing contractile and electrically coupling embryonic cardiomyocytes (eCMs) into ischemic myocardium provoked significantly smaller infarcts with less adverse remodeling and improved cardiac function and long-term survival compared to transplantation of somatic cells (SMs and MSCs), thereby proving that a cardiomyocyte phenotype is important to restore myocardial function.

Time-course of the electrophysiological maturation and integration of transplanted cardiomyocytes

Electrophysiological maturation and integration of transplanted cardiomyocytes are essential to enhance safety and efficiency of cell replacement therapy. Yet, little is known about these important processes. The aim of our study was to perform a detailed analysis of electrophysiological maturation and integration of transplanted cardiomyocytes. Fetal cardiomyocytes expressing enhanced green fluorescent protein were transplanted into cryoinjured mouse hearts. At 6, 9 and 12 days after transplantation, viable slices of recipient hearts were prepared and action potentials of transplanted and host cardiomyocytes within the slices were recorded by microelectrodes. In transplanted cells embedded in healthy host myocardium, action potential duration at 50% repolarization (APD50) decreased from 32.2 ± 3.3 ms at day 6 to 27.9 ± 2.6 ms at day 9 and 19.6 ± 1.6 ms at day 12. The latter value matched the APD50 of host cells (20.5 ± 3.2 ms, P=0.78). Integration improved in the course of time: 26% of cells at day 6 and 53% at day 12 revealed no conduction blocks up to a stimulation frequency of 10 Hz. APD50 was inversely correlated to the quality of electrical integration. In transplanted cells embedded into the cryoinjury, which showed no electrical integration, APD50 was 49.2 ± 4.3 ms at day 12. Fetal cardiomyocytes transplanted into healthy myocardium integrate electrically and mature after transplantation, their action potential properties after 12 days are comparable to those of host cardiomyocytes. Quality of electrical integration improves over time, but conduction blocks still occur at day 12 after transplantation. The pace of maturation correlates with the quality of electrical integration. Transplanted cells embedded in cryoinjured tissue still possess immature electrophysiological properties after 12 days.

Live monitoring of small vessels during development and disease using the flt-1 promoter element

Vessel formation is of critical importance for organ function in the normal and diseased state. In particular, the labeling and quantitation of small vessels prove to be technically challenging using current approaches. We have, therefore, established a transgenic embryonic stem (ES) cell line and a transgenic mouse model where the vascular endothelial growth factor receptor VEGFR-1 (flt-1) promoter drives the expression of the live reporter eGFP. Fluorescence microscopy and immunostainings revealed endothelial-specific eGFP labeling of vascular networks. The expression pattern recapitulates that of the endogenous flt-1 gene, because small and large vessels are labeled by eGFP during embryonic development; after birth, the expression becomes more restricted to small vessels. We have explored this in the cardiovascular system more in detail and found that all small vessels and capillaries within the heart are strongly eGFP+. In addition, myocardial injuries have been induced in transgenic mice and prominent vascular remodeling, and an increase in endothelial cell area within the peri-infarct area could be observed underscoring the utility of this mouse model. Thus, the transgenic flt-1/eGFP models are powerful tools to investigate and quantify vascularization in vivo and to probe the effect of different compounds on vessel formation in vitro.

Cell lineages, growth and repair of the mouse heart

The formation of the heart involves diversification of lineages which differentiate into distinct cardiac cell types or contribute to different regions such as the four cardiac chambers. The heart is the first organ to form in the embryo. However, in parallel with the growth of the organism, before or after birth, the heart has to adapt its size to maintain pumping efficiency. The adult heart has only a mild regeneration potential; thus, strategies to repair the heart after injury are based on the mobilisation of resident cardiac stem cells or the transplantation of external sources of stem cells. We discuss current knowledge on these aspects and raise questions for future research.

Cardiomyoplasty Improves Contractile Reserve after Myocardial Injury in Mice: Functional and Morphological Investigations with Reconstructive Three-Dimensional Echocardiography

Cellular cardiomyoplasty (CMP) is a novel therapeutic approach to myocardial injury (MI). Post-MI remodeling of the left ventricle (LV) comprises dilatation and impairment of systolic function and gives rise to progressive hemodynamic deterioration. We aimed to investigate: a) the impact of CMP on global and regional parameters of LV remodeling (LVR) as well as contractile reserve and b) the suitability and validity of different echocardiographic methods in this scenario. Murine ventricular cardiomyocytes (E13.5–E16.5) were transplanted into cryolesioned hearts of male HIM-OF1 mice. Echocardiography was performed at rest 4 and 14 days postoperatively. For quantification of akinetic myocardial mass and contractile reserve 2 weeks postoperatively additionally low-dose dobutamine stress echocardiography was conducted. Reconstructive 3D-echocardiography (r3D-echo) was compared to “plain” echocardiographic investigations and was compared to invasive measurements with conduction catheter. CMP significantly attenuated LV dilatation and reduced LV function decline on day 14, as obtained with all echocardiographic modalities and confirmed with conduction catheter measurements. In contrast to plain echocardiography and invasive testing, r3D-echo allowed noninvasive quantification of scar size and assessment of regional contractile reserve. Cell transplanted hearts demonstrated a significant decrease of akinetic myocardial mass (-CMP: 13 ± 2%; +CMP 7 ± 1%; p < 0.001) and increased regional contractile reserve, an indirect sign of myocardial viability. The present study demonstrates beneficial effects of CMP on global and regional parameters of LVR and contractile reserve after MI. In contrast to “simple” 2D echocardiography, r3D-echo allowed the assessment of regional contractile reserve and quantification of akinetic myocardial mass as additive functional and morphological measures of LVR.

Derivation of functional ventricular cardiomyocytes using endogenous promoter sequence from murine embryonic stem cells

The purpose of this study is to establish a murine embryonic stem cell (mESC) line for isolation of functional ventricular cardiomyocytes (VCMs) and then to characterize the derived VCMs. By crossing the myosin light chain 2v (Mlc2v)-Cre mouse line with the reporter strain Rosa26-yellow fluorescent protein (YFP), we generated mESC lines from these double transgenic mice, in which Cre-mediated removal of a stop sequence results in the expression of YFP under the control of the ubiquitously active Rosa26 promoter specifically in the VCM. After induction of differentiation via embryoid body (EB) formation, contracting YFP(+) cells were detected within EBs and isolated by fluorescence-activated cell sorting. N-cadherin, the cadherin expressed in cardiomyocytes, and the major cardiac connexin (Cx) isoform, Cx43, were detected in the respective adherens and gap junctions in these VCMs. Using current clamp recordings we demonstrated that mESC-derived VCMs exhibited action potential characteristics comparable to those of neonatal mouse VCMs. Real-time intracellular calcium [Ca(2+)](i) imaging showed rhythmic intracellular calcium transients in these VCMs. The amplitude and frequency of calcium transients were increased by isoproterenol stimulation, suggesting the existence of functional β-adrenergic signaling. Moreover, [Ca(2+)](i) oscillations responded to increasing frequencies of external electrical stimulation, indicating that VCMs have functional excitation-contraction coupling, a key factor for the ultimate cardiac contractile performance. The present study makes possible the production of homogeneous and functional VCMs for basic research as well as for cardiac repair and regeneration.

Fibroblasts facilitate the engraftment of embryonic stem cell-derived cardiomyocytes on three-dimensional collagen matrices and aggregation in hanging drops

There is growing interest in the use of cardiomyocytes purified from embryonic stem (ES) cells for tissue engineering and cardiomyoplasty. However, most transplanted cells are lost shortly after transplantation due to the lack of integration into the host tissue and subsequent apoptosis. Here we examine whether murine embryonic fibroblasts (MEFs) can support the integration of purified murine ES cell-derived cardiomyocytes in a 3-dimensional tissue culture model based on a freezed-dryed collagen matrix with tubular structure. Collagen matrix was seeded either with cardiomyocytes alone or in combination with MEFs. The collagen sponges that were transplanted with cardiomyocytes alone showed neither morphological nor functional integration of viable cells. Cardiomyocytes also did not appear to be capable of attaching quantitatively to any of 16 different 2-dimensional biomaterials. However, cardiomyocytes co-cultured with MEFs formed fiber-like structures of rod-shaped cells with organized sarcomeric structure that contracted spontaneously. Electrical coupling between cardiomyocytes was suggested by strong expression of connexin 43. In addition, MEFs as well as cardiac fibroblasts supported re-aggregation of dissociated cardiomyocytes in hanging drops in the absence of collagen matrix. We conclude that fibroblasts promote cardiomyocyte engraftment and formation of functional 3-dimensional tissue in vitro. Elucidation of the mechanism of this phenomenon may help improve the integration of cardiomyocytes in vivo.

Engineered fetal cardiac graft preserves its cardiomyocyte proliferation within postinfarcted myocardium and sustains cardiac function

The goal of cellular cardiomyoplasty is to replace damaged myocardium by healthy myocardium achieved by host myocardial regeneration and/or transplantation of donor cardiomyocytes (CMs). In the case of CM transplantation, studies suggest that immature CMs may be the optimal cell type to survive and functionally integrate into damaged myocardium. In the present study, we tested the hypothesis that active proliferation of immature CMs contributes graft survival and functional recovery of recipient myocardium. We constructed engineered cardiac tissue from gestational day 14 rat fetal cardiac cells (EFCT) or day 3 neonatal cardiac cells (ENCT). Culture day 7 EFCTs or ENCTs were implanted onto the postinfarct adult left ventricle (LV). CM proliferation rate of EFCT was significantly higher than that of ENCT at 3 days and 8 weeks after the graft implantation, whereas CM apoptosis rate remained the same in both groups. Echocardiogram showed that ENCT implantation sustained LV contraction, whereas EFCT implantation significantly increased the LV contraction at 8 weeks versus sham group (p < 0.05, analysis of variance). These results suggest that active CM proliferation may play a critical role in immature donor CM survival and the functional recovery of damaged recipient myocardium.

Comparison of myocardial remodeling between cryoinfarction and reperfused infarction in mice

Myocardial infarction is associated with inflammatory reaction leading to tissue remodeling. We compared tissue remodeling between cryoinfarction (cMI) and reperfused myocardial infarction (MI) in order to better understand the local environment where we apply cell therapies. Models of closed-chest one-hour ischemia/reperfusion MI and cMI were used in C57/Bl6-mice. The reperfused MI showed rapid development of granulation tissue and compacted scar formation after 7 days. In contrast, cMI hearts showed persistent cardiomyocyte debris and cellular infiltration after 7 days and partially compacted scar formation accompanied by persistent macrophages and myofibroblasts after 14 days. The mRNA of proinflammatory mediators was transiently induced in MI and persistently upregulated in cMI. Tenascin C and osteopontin-1 showed delayed induction in cMI. In conclusion, the cryoinfarction was associated with prolonged inflammation and active myocardial remodeling when compared to the reperfused MI. These substantial differences in remodeling may influence cellular engraftment and should be considered in cell therapy studies.

Cardiac cell therapies: the next generation

Although significant advances have been made in terms of pharmacological, catheter-based, and surgical palliation, heart failure remains a fatal disease. As a curative concept, regenerative medicine aims at the restoration of the physiologic cellular composition of diseased organs. So far, clinical cardiac regeneration attempts have only been moderately successful, but a better understanding of myocardial cell homeostasis and somatic as well as embryonic stem cell biology has opened the door for the development of more potent therapeutic cardiac regeneration strategies. Accumulating evidence indicates that the postnatal mammalian heart retains a pool of tissue-specific progenitor cells and is also repopulated by cells from extracardiac sources. However, this intrinsic myocardial regeneration potential clearly needs to be augmented by either manipulation of the cell cycle of differentiated cells, activation of resident cardiac progenitor cells, and/or the transplantation of exogenous cells. This review summarizes the recent developments in cardiac regenerative medicine, many of which may find their way into the clinical setting in the foreseeable future.

A three-dimensional gel bioreactor for assessment of cardiomyocyte induction in skeletal muscle-derived stem cells

Skeletal muscle-derived stem cells (MDSCs) are able to differentiate into cardiomyocytes (CMs). However, it remains to be investigated whether differentiated CMs contract similar to native CMs. Here, we developed a three-dimensional collagen gel bioreactor (3DGB) that induces a working CM phenotype from MDSCs, and the contractile properties are directly measured as an engineered cardiac tissue. Neonate rat MDSCs were isolated from hind-leg muscles via the preplate technique. Isolated MDSCs were approximately 60% positive to Sca-1 and negative to CD34, CD45, or c-kit antigens. We sorted Sca-1(-) MDSCs and constructed MDSC-3DGBs by mixing MDSCs with acid soluble rat tail collagen type-I and matrix factors. MDSC-3DGB exhibited spontaneous cyclic contraction by culture day 7. MDSC-3DGB expressed cardiac-specific genes and proteins. Histological assessment revealed that cardiac-specific troponin-T and -I expressed in a typical striation pattern and connexin-43 was expressed similar to the native fetal ventricular papillary muscle. beta-Adrenergic stimulation increased MDSC-3DGB spontaneous beat frequency. MDSC-3DGB generated contractile force and intracellular calcium ion transients similar to engineered cardiac tissue from native cardiac cells. Results suggest that MDSC-3DGB induces a working CM phenotype in MDSCs and is a useful 3D culture system to directly assess the contractile properties of differentiated CMs in vitro.

Clinical application of stem cells in the cardiovascular system

Regenerative medicine encompasses "tissue engineering" - the in vitro fabrication of tissues and/or organs using scaffold material and viable cells - and "cell therapy" - the transplantation or manipulation of cells in diseased tissue in vivo. In the cardiovascular system, tissue engineering strategies are being pursued for the development of viable replacement blood vessels, heart valves, patch material, cardiac pacemakers and contractile myocardium. Anecdotal clinical applications of such vessels, valves and patches have been described, but information on systematic studies of the performance of such implants is not available, yet. Cell therapy for cardiovascular regeneration, however, has been performed in large series of patients, and numerous clinical studies have produced sometimes conflicting results. The purpose of this chapter is to summarize the clinical experience with cell therapy for diseases of the cardiovascular system, and to analyse possible factors that may influence its outcome.

Engineered early embryonic cardiac tissue increases cardiomyocyte proliferation by cyclic mechanical stretch via p38-MAP kinase phosphorylation

Cardiomyocyte (CM) transplantation is one therapeutic option for cardiac repair. Studies suggest that fetal CMs display the best cell type for cardiac repair, which can finitely proliferate, integrate with injured host myocardium, and restore cardiac function. We have recently developed an engineered early embryonic cardiac tissue (EEECT) using embryonic cardiac cells and have shown that EEECT contractile properties and cellular proliferative response to cyclic mechanical stretch stimulation mimic developing fetal myocardium. However, it remains unknown whether cyclic mechanical stretch-mediated high cellular proliferation activity within EEECT reflects CM or non-CM population. Studies have shown that p38-mitogen-activated protein kinase (p38MAPK) plays an important role in both cyclic mechanical stretch stimulation and cellular proliferation. Therefore, in the present study, we tested the hypothesis that cyclic mechanical stretch (0.5 Hz, 5% strain for 48 h) specifically increases EEECT CM proliferation mediated by p38MAPK activity. Cyclic mechanical stretch increased CM, but not non-CM, proliferation and increased p38MAPK phosphorylation. Treatment of EEECT with the p38MAPK inhibitor, SB202190, reduced CM proliferation. The negative CM proliferation effects of SB202190 were not reversed by concurrent stretch stimulation. Results suggest that immature CM proliferation within EEECT can be positively regulated by mechanical stretch and negatively regulated by p38MAPK inhibition.

Genomic profiling of developing cardiomyocytes from recombinant murine embryonic stem cells reveals regulation of transcription factor clusters

Cardiomyocytes derived from pluripotent embryonic stem cells (ESC) have the advantage of providing a source for standardized cell cultures. However, little is known on the regulation of the genome during differentiation of ESC to cardiomyocytes. Here, we characterize the transcriptome of the mouse ESC line CM7/1 during differentiation into beating cardiomyocytes and compare the gene expression profiles with those from primary adult murine cardiomyocytes and left ventricular myocardium. We observe that the cardiac gene expression pattern of fully differentiated CM7/1-ESC is highly similar to adult primary cardiomyocytes and murine myocardium, respectively. This finding is underlined by demonstrating pharmacological effects of catecholamines and endothelin-1 on ESC-derived cardiomyocytes. Furthermore, we monitor the temporal changes in gene expression pattern during ESC differentiation with a special focus on transcription factors involved in cardiomyocyte differentiation. Thus, CM7/1-ESC-derived cardiomyocytes are a promising new tool for functional studies of cardiomyocytes in vitro and for the analysis of the transcription factor network regulating pluripotency and differentiation to cardiomyocytes.

Paracrine effects of cell transplantation: strategies to augment the efficacy of cell therapies

Within the last few years, it has become evident that the beneficial effect of cell transplantation on ventricular function and myocardial perfusion is in large part mediated through paracrine effects on the host myocardium. Studies in which medium conditioned by cultured cells, usually mesenchymal stem cells, were injected into infarcted animal hearts have provided definitive evidence of this mechanism of action. Paracrine effects of the donor cells include but are not limited to angiogenesis, mobilization of both circulating and bone-marrow-derived stem cells, activation of cardiac-resident stem cells (CRSCs), and stabilization of the extracellular matrix (ECM). These paracrine effects can be augmented by transplantation of cells modified to express therapeutically useful transgenes, or by preconditioning through hypoxic or pharmacologic means. Strategies to enhance the paracrine effects of cell transplantation may thus be employed in the next generation of cell therapies, with greater functional benefit.

Perlecan is critical for heart stability

AIMS

Perlecan is a heparansulfate proteoglycan found in basement membranes, cartilage, and several mesenchymal tissues that form during development, tumour growth, and tissue repair. Loss-of-function mutations in the perlecan gene in mice are associated with embryonic lethality caused primarily by cardiac abnormalities probably due to hemopericards. The aim of the present study was to investigate the mechanism underlying the early embryonic lethality and the pathophysiological relevance of perlecan for heart function.

METHODS AND RESULTS

Perlecan-deficient murine embryonic stem cells were used to investigate the myofibrillar network and the electrophysiological properties of single cardiomyocytes. The mechanical stability of the developing perlecan-deficient mouse hearts was analysed by microinjecting fluorescent-labelled dextran. Maturation and formation of basement membranes and cell-cell contacts were investigated by electron microscopy, immunohistochemistry, and western blotting. Sarcomere formation and cellular functional properties were unaffected in perlecan-deficient cardiomyocytes. However, the intraventricular dye injection experiments revealed mechanical instability of the early embryonic mouse heart muscle wall before embryonic day 10.5 (E10.5). Accordingly, perlecan-null embryonic hearts contained lower amounts of the critical basement membrane components, collagen IV and laminins. Furthermore, basement membranes were absent in perlecan-null cardiomoycytes whereas adherens junctions formed and matured around E9.5. Infarcted hearts from perlecan heterozygous mice displayed reduced heart function when compared with wild-type hearts.

CONCLUSION

We propose that perlecan plays an important role in maintaining the integrity during cardiac development and is important for heart function in the adult heart after injury.

Characterisation of a soft elastomer poly(glycerol sebacate) designed to match the mechanical properties of myocardial tissue

The myocardial tissue lacks significant intrinsic regenerative capability to replace the lost cells. Therefore, the heart is a major target of research within the field of tissue engineering, which aims to replace infarcted myocardium and enhance cardiac function. The primary objective of this work was to develop a biocompatible, degradable and superelastic heart patch from poly(glycerol sebacate) (PGS). PGS was synthesised at 110, 120 and 130 degrees C by polycondensation of glycerol and sebacic acid with a mole ratio of 1:1. The investigation was focused on the mechanical and biodegrading behaviours of the developed PGS. PGS materials synthesised at 110, 120 and 130 degrees C have Young's moduli of 0.056, 0.22 and 1.2 MPa, respectively, which satisfy the mechanical requirements on the materials applied for the heart patch and 3D myocardial tissue engineering construction. Degradation assessment in phosphate buffered saline and Knockout DMEM culture medium has demonstrated that the PGS has a wide range of degradability, from being degradable in a couple of weeks to being nearly inert. The matching of physical characteristics to those of the heart, the ability to fine tune degradation rates in biologically relevant media and initial data showing biocompatibility indicate that this material has promise for cardiac tissue engineering applications.

Assessment of the effect of cardiomyocyte transplantation on left ventricular remodeling and function in post-infarction Wister rats by using high-frequency ultrasound

The effects of cardiomyocyte grafting on left ventricular (LV) remodeling and function in rats with chronic myocardial infarction were evaluated using high-frequency ultrasound. Chronic myocardial infarction was induced in 50 Wister rats by ligating the left anterior descending artery. They were randomized into two groups: a trial group that received neonatal rat cardiomyocyte transplantation (n=25) and a control group which were given intramyocardial injection of culture medium (n=25). The left ventricular (LV) geometry and function were evaluated by high-frequency ultrasound before and 4 weeks after the cell transplantation. After the final evaluation, all rats were sacrificed for histological study. The results showed that 4 weeks after the cell transplantation, as compared with the control group, the LV end-systolic dimension, end-diastolic dimension, end-systolic volume and end-diastolic volume were significantly decreased and the LV anterior wall end-diastolic thickness, LV ejection fraction and fractional shortening were significantly increased in the trial group (P<0.01). Histological study showed that transplanted neonatal rat cardiomyocytes were found in all host hearts and identified by Brdu staining. It was suggested that transplantation of neonatal rat cardiomyocytes can reverse cardiac remodeling and improve heart function in chronic myocardial infarction rats. High-frequency ultrasound can be used as a reliable technique for the non-invasive evaluation of the effect of cardiomyocyte transplantation.

Biomimetic approach to cardiac tissue engineering

Here, we review an approach to tissue engineering of functional myocardium that is biomimetic in nature, as it involves the use of culture systems designed to recapitulate some aspects of the actual in vivo environment. To mimic the capillary network, subpopulations of neonatal rat heart cells were cultured on a highly porous elastomer scaffold with a parallel array of channels perfused with culture medium. To mimic oxygen supply by haemoglobin, the culture medium was supplemented with a perfluorocarbon (PFC) emulsion. Constructs cultivated in the presence of PFC contained higher amounts of DNA and cardiac markers and had significantly better contractile properties than control constructs cultured without PFC. To induce synchronous contractions of cultured constructs, electrical signals mimicking those in native heart were applied. Over only 8 days of cultivation, electrical stimulation induced cell alignment and coupling, markedly increased the amplitude of synchronous construct contractions and resulted in a remarkable level of ultrastructural organization. The biomimetic approach is discussed in the overall context of cardiac tissue engineering, and the possibility to engineer functional human cardiac grafts based on human stem cells.

Potential risks of bone marrow cell transplantation into infarcted hearts

Cellular replacement therapy has emerged as a novel strategy for the treatment of heart failure. The aim of our study was to determine the fate of injected mesenchymal stem cells (MSCs) and whole bone marrow (BM) cells in the infarcted heart. MSCs were purified from BM of transgenic mice and characterized using flow cytometry and in vitro differentiation assays. Myocardial infarctions were generated in mice and different cell populations including transgenic MSCs, unfractionated BM cells, or purified hematopoietic progenitors were injected. Encapsulated structures were found in the infarcted areas of a large fraction of hearts after injecting MSCs (22 of 43, 51.2%) and unfractionated BM cells (6 of 46, 13.0%). These formations contained calcifications and/or ossifications. In contrast, no pathological abnormalities were found after injection of purified hematopoietic progenitors (0 of 5, 0.0%), fibroblasts (0 of 5, 0.0%), vehicle only (0 of 30, 0.0%), or cytokine-induced mobilization of BM cells (0 of 35, 0.0%). We conclude that the developmental fate of BM-derived cells is not restricted by the surrounding tissue after myocardial infarction and that the MSC fraction underlies the extended bone formation in the infarcted myocardium. These findings seriously question the biologic basis and clinical safety of using whole BM and in particular MSCs to treat nonhematopoietic disorders.

Electrophysiological maturation and integration of murine fetal cardiomyocytes after transplantation

In the present study, we investigated the electrophysiological maturation and integration of immature cardiomyocytes after transplantation; maturation and integration are essential to achieve the cardiac regeneration. Murine fetal cardiomyocytes (FCMs) (d12.5-d15.5) expressing enhanced green fluorescent protein under the control of the alpha-actin promoter were injected into cryoinjured areas and adjacent myocardium of cryoinjured mouse ventricles. Viable short axis tissue slices (thickness, 150 microm) of the ventricles were prepared 5 to 6 days after transplantation. Glass microelectrodes were used for measurements of action potentials in transplanted FCMs and host cardiomyocytes within the slices. Stimulation at frequencies of up to 10 Hz was performed via a unipolar electrode placed in viable host tissue. Transplanted FCMs could be distinguished clearly from host tissue by their green fluorescence and their electrophysiological properties: maximal upstroke velocity (V(max)) was significantly lower and action potential duration at 50% repolarization (APD(50)) was significantly longer compared with values of adult cardiomyocytes. Transplanted FCMs surrounded by cryoinjured tissue showed spontaneous electrical and contractile activity, which was in no case synchronous with host tissue. V(max) and APD(50) of these nonintegrated cells matched values of cultivated dissociated FCMs. In contrast, 82% of transplanted FCMs surrounded by viable host tissue were electrically integrated; ie, electrical and contractile activity was synchronous with host tissue and these cells had more mature action potential parameters (significantly higher V(max) and shorter APD(50)) compared with nonintegrated FCMs. In conclusion, electrophysiological maturation and integration of transplanted FCMs depend on an embedment in viable host myocardium. FCMs surrounded by cryoinjured tissue maintain physiological but immature AP properties.

Toll-like receptor 4 deficiency: smaller infarcts, but no gain in function

Backgound

It has been reported that Toll-like receptor 4 (TLR4) deficiency reduces infarct size after myocardial ischemia/reperfusion (MI/R). However, measurement of MI/R injury was limited and did not include cardiac function. In a chronic closed-chest model we assessed whether cardiac function is preserved in TLR4-deficient mice (C3H/HeJ) following MI/R, and whether myocardial and systemic cytokine expression differed compared to wild type (WT).

Results

Infarct size (IS) in C3H/HeJ assessed by TTC staining after 60 min ischemia and 24h reperfusion was significantly smaller than in WT. Despite a smaller infarct size, echocardiography showed no functional difference between C3H/HeJ and WT. Left-ventricular developed pressure measured with a left-ventricular catheter was lower in C3H/HeJ (63.0 ± 4.2 mmHg vs. 77.9 ± 1.7 mmHg in WT, p < 0.05). Serum cytokine levels and myocardial IL-6 were higher in WT than in C3H/HeJ (p < 0.05). C3H/HeJ MI/R showed increased myocardial IL-1β and IL-6 expression compared to their respective shams (p < 0.05), indicating TLR4-independent cytokine activation due to MI/R.

Conclusion

These results demonstrate that, although a mutant TLR4 signaling cascade reduces myocardial IS and serum cytokine levels, it does not preserve myocardial function. The change in inflammatory response, secondary to a non-functional TLR-4 receptor, may contribute to the observed dichotomy between infarct size and function in the TLR-4 mutant mouse.

Echocardiographic assessment of left ventricular mass in neonatal and adult mice: accuracy of different echocardiographic methods

Echocardiography is an established method to estimate left-ventricular mass (LVM) in mice. Accuracy is determined by cardiac size and morphology and influenced by mathematical models. We investigated accuracy of three common algorithms in three early developmental stages. High-resolution echocardiography was performed in 35 C57/BL6-mice. Therefore, two-dimensional-guided M-mode echocardiography and parasternal short- and long-axis views in B-mode were obtained. LVM was assessed in vivo applying Penn (P), Area Length (AL), and Truncated Ellipsoid (TE) algorithms and validated with histomorphometry. Regression analysis of all mice showed fair estimation of LVM assessed with M-mode-based Penn algorithm (y = 0.6*x - 0.12, r: 0.71). In contrast two-dimensional assessment of LVM revealed close linear relationship with histomorphometry (y(AL)= 1.21*x - 12.1, r: 0.88, y(TE)= 1.38*x - 2.88, r: 0.86). Bias was lowest for LVM-AL at diastole underestimating 3.2%. In concordance with the summarized data, LVM-P revealed lower regression coefficients and significant underestimation in all three subgroups. Small hearts (<50 mg, n = 12) correlated best with LVM-AL at systole. Hearts of adolescent (50-75 mg, n = 13) and adult (75-100 mg, n = 10) mice revealed close linear relationship with LVM-AL and LVM-TE at diastole. Echocardiographic assessment of LVM is feasible in hearts weighting less than 50 mg and can be estimated best in systole. Hearts weighting more than 50 mg are estimated most accurately by means of LVM-AL at diastole.