Hybrid cardiomyocytes derived by cell fusion in heterotopic cardiac xenografts

Conserved pathway activation following xenogeneic, heterotypic fusion

Fusion between cells of different organisms (i.e., xenogeneic hybrids) can occur, and for humans this may occur in the course of tissue transplantation, animal handling, and food production. Previous work shows that conferred advantages are rare in xenogeneic hybrids, whereas risks of cellular dysregulation are high. Here, we explore the transcriptome of individual xenogeneic hybrids of human mesenchymal stem cells and murine cardiomyocytes soon after fusion and ask whether the process is stochastic or involves conserved pathway activation. Toward this end, single-cell RNA sequencing was used to analyze the transcriptomes of hybrid cells with respect to the human and mouse genomes. Consistent with previous work, hybrids possessed a unique transcriptome distinct from either fusion partner but were dominated by the cardiomyocyte transcriptome. New in this work is the documentation that a few genes that were latent in both fusion partners were consistently expressed in hybrids. Specifically, human growth hormone 1, murine ribosomal protein S27, and murine ATP synthase H⁺ transporting, mitochondrial Fo complex subunit C2 were expressed in nearly all hybrids. The consistent activation of latent genes between hybrids suggests conserved signaling mechanisms that either cause or are the consequence of fusion of these 2 cell types and might serve as a target for limiting unwanted xenogeneic fusion in the future.-Yuan, C., Freeman, B. T., McArdle, T. J., Jung, J. P., Ogle, B. M. Conserved pathway activation following xenogeneic, heterotypic fusion.

New Myocyte Formation in the Adult Heart: Endogenous Sources and Therapeutic Implications

Death of adult cardiac myocytes and supportive tissues resulting from cardiovascular diseases such as myocardial infarction is the proximal driver of pathological ventricular remodeling that often culminates in heart failure. Unfortunately, no currently available therapeutic barring heart transplantation can directly replenish myocytes lost from the injured heart. For decades, the field has struggled to define the intrinsic capacity and cellular sources for endogenous myocyte turnover in pursuing more innovative therapeutic strategies aimed at regenerating the injured heart. Although controversy persists to this day as to the best therapeutic regenerative strategy to use, a growing consensus has been reached that the very limited capacity for new myocyte formation in the adult mammalian heart is because of proliferation of existing cardiac myocytes but not because of the activity of an endogenous progenitor cell source of some sort. Hence, future therapeutic approaches should take into consideration the fundamental biology of myocyte renewal in designing strategies to potentially replenish these cells in the injured heart.

Heterotopic Implantation of Decellularized Pulmonary Artery Homografts In A Rodent Model: Technique Description and Preliminary Report

PURPOSE

Despite a substantial amount of literature on tissue-guided regeneration, decellularization process, repopulation time points and stem cell turnover, more in-depth study on the argument is required. Currently, there are plenty of reports involving large animals, as well as clinical studies facing cardiac repair with decellularized homografts, but no exhaustive rodent models are described. The purpose of this study was to develop such a model in rats; preliminary results are also herein reported.

MATERIAL AND METHODS

Fresh or decellularized pulmonary homografts from wild type rats were implanted in the abdominal aorta of green fluorescent protein positive rats. Three experimental groups were build up: sham, fresh homograft recipients and decellularized homograft recipients. The homograft decellularization process was performed with three cycles of detergent-enzymatic treatment protocol. Surgical technique of pulmonary homograft implantation and postoperative ultrasonographic evaluation were also reported; gross, histology and immunohistochemistry analysis on unimplanted and postoperative homografts were also carried out.

RESULTS

The median total recipient operating time was 148 minutes, with a surgical success rate of 82%. The decellularization protocol resulted effective and showed a complete decellularization with intact extracellular matrix. At 15 days from surgery, the implanted decellularized pulmonary homografts exhibited cell repopulation in the outer media wall and partial endothelial lining in absence of rejection.

CONCLUSIONS

Our technique is a feasible and reproducible model that can be fundamental for building a valid study for further exploitation on the field. Even in a short-term follow up, the decellularized pulmonary homografts showed autologous repopulation in absence of rejection.

Optimized protocol for immunostaining of experimental GFP-expressing and human hearts

Morphological and histochemical analysis of the heart is fundamental for the understanding of cardiac physiology and pathology. The accurate detection of different myocardial cell populations, as well as the high-resolution imaging of protein expression and distribution, within the diverse intracellular compartments, is essential for basic research on disease mechanisms and for the translatability of the results to human pathophysiology. While enormous progress has been made on the imaging hardware and methods and on biotechnological tools [e.g., use of green fluorescent protein (GFP), viral-mediated gene transduction] to investigate heart cell structure and function, most of the protocols to prepare heart tissue samples for analysis have remained almost identical for decades. We here provide a detailed description of a novel protocol of heart processing, tailored to the simultaneous detection of tissue morphology, immunofluorescence markers and native emission of fluorescent proteins (i.e., GFP). We compared a variety of procedures of fixation, antigen unmasking and tissue permeabilization, to identify the best combination for preservation of myocardial morphology and native GFP fluorescence, while simultaneously allowing detection of antibody staining toward sarcomeric, membrane, cytosolic and nuclear markers. Furthermore, with minimal variations, we implemented such protocol for the study of human heart samples, including those already fixed and stored with conventional procedures, in tissue archives or bio-banks. In conclusion, a procedure is here presented for the laboratory investigation of the heart, in both rodents and humans, which accrues from the same tissue section information that would normally require the time-consuming and tissue-wasting observation of multiple serial sections.

Microenvironment-induced myofibroblast-like conversion of engrafted keratinocytes

Myofibroblasts, recognized classically by α-smooth muscle actin (α-SMA) expression, play a key role in the wound-healing process, promoting wound closure and matrix deposition. Although a body of evidence shows that keratinocytes explanted onto a wound bed promote closure of a skin injury, the underlying mechanisms are not well understood. The basal layer of epidermis is rich in undifferentiated keratinocytes (UKs). We showed that UKs injected into granulation tissue could switch into α-SMA positive cells, and accelerate the rate of skin wound healing. In addition, when the epidermis sheets isolated from foreskin cover up the wound bed or are induced in vitro, keratinocytes located at the basal layers or adjacent sites were observed to convert into myofibroblast-like cells. Thus, UKs have a potential for myofibroblastic transition, which provides a novel mechanism by which keratinocyte explants accelerate skin wound healing.

Identification and characterization of novel Kirrel isoform during myogenesis

Somatic cell fusion is an essential component of skeletal muscle development and growth and repair from injury. Additional cell types such as trophoblasts and osteoclasts also require somatic cell fusion events to perform their physiological functions. Currently we have rudimentary knowledge on molecular mechanisms regulating somatic cell fusion events in mammals. We therefore investigated during in vitro murine myogenesis a mammalian homolog, Kirrel, of the Drosophila Melanogaster genes Roughest (Rst) and Kin of Irre (Kirre) which regulate somatic muscle cell fusion during embryonic development. Our results demonstrate the presence of a novel murine Kirrel isoform containing a truncated cytoplasmic domain which we term Kirrel B. Protein expression levels of Kirrel B are inverse to the occurrence of cell fusion events during in vitro myogenesis which is in stark contrast to the expression profile of Rst and Kirre during myogenesis in Drosophila. Furthermore, chemical inhibition of cell fusion confirmed the inverse expression pattern of Kirrel B protein levels in relation to cell fusion events. The discovery of a novel Kirrel B protein isoform during myogenesis highlights the need for more thorough investigation of the similarities and potential differences between fly and mammals with regards to the muscle cell fusion process.

The contribution of stem cell therapy to skeletal muscle remodeling in heart failure

BACKGROUND

The aim of our study was to investigate whether stem cell (SC) therapy with human amniotic fluid stem cells (hAFS, fetal stem cells) and rat adipose tissue stromal vascular fraction cells-GFP positive cells (rSVC-GFP) was able to produce favorable effects on skeletal muscle (SM) remodeling in a well-established rat model of right heart failure (RHF).

METHODS

RHF was induced by monocrotaline (MCT) in Sprague-Dawley rats. Three weeks later, four millions of hAFS or rSVC-GFP cells were injected via tail vein. SM remodeling was assessed by Soleus muscle fiber cross sectional area (CSA), myocyte apoptosis, myosin heavy chain (MHC) composition, satellite cells pattern, and SC immunohistochemistry.

RESULTS

hAFS and rSVC-GFP injection produced significant SC homing in Soleus (0.68 ± 1.0 and 0.67 ± 0.75% respectively), with a 50% differentiation toward smooth muscle and endothelial cells. Pro-inflammatory cytokines were down regulated to levels similar to those of controls. SC-treated (SCT) rats showed increased CSA (p<0.004 vs MCT) similarly to controls with a reshift toward the slow MHC1 isoform. Apoptosis was significantly decreased (11.12.± 8.8 cells/mm(3) hAFS and 13.1+7.6 rSVC-GFP) (p<0.001 vs MCT) and similar to controls (5.38 ± 3.0 cells/mm(3)). RHF rats showed a dramatic reduction of satellite cells(MCT 0.2 ± 0.06% Pax7 native vs controls 2.60 ± 2.46%, p<0.001), while SCT induced a repopulation of both native and SC derived satellite cells (p<0.005).

CONCLUSIONS

SC treatment led to SM remodeling with satellite cell repopulation, decreased atrophy and apoptosis. Modulation of the cytokine milieu might play a crucial pathophysiological role with a possible scenario for autologous transplantation of SC in pts with CHF myopathy.

Stem-cell therapy in an experimental model of pulmonary hypertension and right heart failure: role of paracrine and neurohormonal milieu in the remodeling process

BACKGROUND

In this study we investigated the effect of human amniotic fluid stem (hAFS) cells and rat adipose tissue stromal vascular fraction GFP-positive cell (rSVC-GFP) therapy and the contribution of the paracrine and neurohormonal milieu to cardiac and pulmonary vascular remodeling in a rat model of pulmonary hypertension (PH) and right heart failure (RHF).

METHODS

Sprague-Dawley rats were injected with monocrotaline (MCT). Four million hAFS or rSVC-GFP cells were injected via the tail vein 3 weeks after MCT. RHF was confirmed by RV hypertrophy/dilation and by brain natriuretic peptide (BNP) level. Cytokine profile was assessed by Multiplex array. Stem cell (SC) differentiation was studied by immunofluorescence.

RESULTS

MCT rats showed eccentric RV hypertrophy with increased RV dilation (measured as right ventricular mass/right ventricular volume [RVM/RVV]: MCT, 1.46 ± 0.12; control, 2.33 ± 0.24; p = 0.01), and increased RV hypertrophy (measured as LVM/RVM: MCT, 1.58 ± 0.06; control, 2.83 ± 0.1; p < 0.00001), increased BNP (MCT, 5.2 ± 1.2; control, 1.5 ± 0.1; p < 0.001) and both pro- and anti-inflammatory cytokines. SC produced a fall of BNP (hAFS, 2.1 ± 0.7; rSVC-GFP, 1.98 ± 1.3; p < 0.001) and pro-inflammatory cytokines. Positive RV remodeling with decreased RV dilation (RVM/RVV: hAFS, 1.87 ± 0.44; rSVC-GFP, 2.12 ± 0.24; p < 0.03 and p < 0.05 vs MCT) and regression of RV hypertrophy (LVM/RVM: hAFS, 2.06 ± 0.08; rSVC-GFP, 2.16 ± 0.08; p < 0.00001 vs MCT) was seen together with a decrease in medial wall thickness of pulmonary arterioles (hAFS, 35.33 ± 2.78%; rSVC-GFP, 37.15 ± 2.92%; p = 0.0001 vs MCT).

CONCLUSIONS

SC engrafted in the lung, heart and skeletal muscle modulated the pro- and anti-inflammatory cytokine milieu, and produced a positive neurohormonal response. This was accompanied by positive cardiac and pulmonary vascular remodeling, with formation mainly of new vascular cells.

Human mesenchymal stem cells reprogram adult cardiomyocytes toward a progenitor-like state through partial cell fusion and mitochondria transfer

Because stem cells are often found to improve repair tissue including heart without evidence of engraftment or differentiation, mechanisms underlying wound healing are still elusive. Several studies have reported that stem cells can fuse with cardiomyocytes either by permanent or partial cell fusion processes. However, the respective physiological impact of these two processes remains unknown in part because of the lack of knowledge of the resulting hybrid cells. To further characterize cell fusion, we cocultured mouse fully differentiated cardiomyocytes with human multipotent adipose-derived stem (hMADS) cells as a model of adult stem cells. We found that heterologous cell fusion promoted cardiomyocyte reprogramming back to a progenitor-like state. The resulting hybrid cells expressed early cardiac commitment and proliferation markers such as GATA-4, myocyte enhancer factor 2C, Nkx2.5, and Ki67 and exhibited a mouse genotype. Interestingly, human bone marrow-derived stem cells shared similar reprogramming properties than hMADS cells but not human fibroblasts, which suggests that these features might be common to multipotent cells. Furthermore, cardiac hybrid cells were preferentially generated by partial rather than permanent cell fusion and that intercellular structures composed of f-actin and microtubule filaments were involved in the process. Finally, we showed that stem cell mitochondria were transferred into cardiomyocytes, persisted in hybrids and were required for somatic cell reprogramming. In conclusion, by providing new insights into previously reported cell fusion processes, our data might contribute to a better understanding of stem cell-mediated regenerative mechanisms and thus, the development of more efficient stem cell-based heart therapies.

Postinfarction healing dynamics in the mechanically unloaded rat left ventricle

The healing process is a key determinant for postinfarction left ventricular (LV) remodeling and the development of heart failure, which could be influenced by mechanical (pressure and/or volume) load. So far, limited information exists regarding an indepth characterization of the postinfarct healing process in the mechanically unloaded state. In the present work, we performed isogenic Lewis-to-Lewis rat abdominal heterotopic heart transplantation, which is characterized by hemodynamic unloading in the left ventricle, and simultaneously ligated the left anterior descending coronary artery (T-infarct group). Pathological evolution was dynamically compared with that of in situ infarcted Lewis hearts (I-infarct group) on days 3, 7, 14, and 35. There was a remarkable myocardial salvage in the unloaded heart, as shown by the improvement in infarct size (T-infarct group: 25.47% ± 4.31% vs. I-infarct group: 38.46% ± 4.82%, P < 0.01) and the smaller fraction of fibrosis in infarct segments (T-infarct group: 42.12% ± 8.40% vs. I-infarct group: 75.65% ± 10.51%, P < 0.01). In addition, there was a progressive disorganization of the two-dimensional collagen fiber alignment as well as retarded collagen fiber maturation in the T-infarct group. We also observed enhanced angiogenesis, lymphangiogenesis, and inflammatory cell retention in the infarct region during mechanical unloading. Moreover, capillary density and collagen deposition were significantly increased in the noninfarcted area of the unloaded heart compared with the same region in the in situ infarcted heart. In conclusion, ischemic insult in the mechanically unloaded heart elicits an altered inflammatory and healing response, which is characterized by myocardial salvage, delayed resolution of inflammation, and disorganization of the collagen orientation in the infarcted region. These findings could provide novel insights into the contribution of hemodynamic load in the postinfarction healing process. Further studies are warranted to elucidate its potential mechanism.

Cardiac interstitial cells express GATA4 and control dedifferentiation and cell cycle re-entry of adult cardiomyocytes

Interstitial cells of the adult rat heart were characterized with respect to i) expression of cardiac markers of commitment and differentiation, ii) myogenic potential in vitro and iii) ability to modulate cardiomyocyte differentiation state. We demonstrate for the first time that fibroblasts and a proportion of pericytes in the adult rat heart express the transcription factor GATA4. This appears to be a peculiar property of the heart. Fibroblasts that are also derived from the splanchnopleuric mesoderm, such as those of the gut, or fibroblasts of different embryological origin, such as those of skin and skeletal muscle, lack this property. Of note, a nestin+/GATA4+ putative stem cell population is also detected in the adult heart. GATA4+ cardiac interstitial cells do not display myogenic potential in vitro. However, cardiac fibroblasts, but not skin fibroblasts, stimulate dedifferentiation of adult cardiomyocytes and their re-entry into the cell cycle in vitro, as demonstrated by the high number of cardiomyocytes expressing Ki67, phosphorylated histone H3 (H3P) and incorporating 5-bromodeoxiuridine (BrdU) in the co-cultures. In conclusion, cardiac fibroblasts have peculiar expression of myogenic transcription factors, a property that may have an impact for reprogramming these cells to the myogenic differentiation. In addition, they are able to modulate the behavior of adult cardiomyocytes, a property that may be used to promote dedifferentiation and proliferation of cardiac cells in the damaged myocardium.

Cardiac stem cells: paradigm shift or broken promise? A view from developmental biology

For several decades, it has been known that many tissues of the human body replenish themselves with the help of specialized stem cells. Although the role of stem cells for organs with a rapid cellular turnover is well established, other organs have seemed to be exempt from stem cell-based repair. Recent studies have suggested that the heart has an inherent ability to replace its parenchymal cells continuously either by resident stem cells or by other cells that are recruited into the heart. The evidence for this acclaimed paradigm shift, however, is limited. The basis of the acclaimed beneficial effects of stem cell therapies must be investigated carefully and the fates of potential cardiac stem cells need to be studied by established cell tracing techniques.