Contemporary perspective on endogenous myocardial regeneration

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

The concept of tissue engineering evolved long before the phrase was forged, driven by the thromboembolic complications associated with the early total artificial heart programs of the 1960s. Yet more than half a century of dedicated research has not fulfilled the promise of successful broad clinical implementation. A historical account outlines reasons for this scientific impasse. For one, there was a disconnect between distinct eras each characterized by different clinical needs and different advocates. Initiated by the pioneers of cardiac surgery attempting to create neointimas on total artificial hearts, tissue engineering became fashionable when vascular surgeons pursued the endothelialisation of vascular grafts in the late 1970s. A decade later, it were cardiac surgeons again who strived to improve the longevity of tissue heart valves, and lastly, cardiologists entered the fray pursuing myocardial regeneration. Each of these disciplines and eras started with immense enthusiasm but were only remotely aware of the preceding efforts. Over the decades, the growing complexity of cellular and molecular biology as well as polymer sciences have led to surgeons gradually being replaced by scientists as the champions of tissue engineering. Together with a widening chasm between clinical purpose, human pathobiology and laboratory-based solutions, clinical implementation increasingly faded away as the singular endpoint of all strategies. Moreover, a loss of insight into the healing of cardiovascular prostheses in humans resulted in the acceptance of misleading animal models compromising the translation from laboratory to clinical reality. This was most evident in vascular graft healing, where the two main impediments to the in-situ generation of functional tissue in humans remained unheeded–the trans-anastomotic outgrowth stoppage of endothelium and the build-up of an impenetrable surface thrombus. To overcome this dead-lock, research focus needs to shift from a biologically possible tissue regeneration response to one that is feasible at the intended site and in the intended host environment of patients. Equipped with an impressive toolbox of modern biomaterials and deep insight into cues for facilitated healing, reconnecting to the “user needs” of patients would bring one of the most exciting concepts of cardiovascular medicine closer to clinical reality.

Circular RNA (circRNA) CDYL Induces Myocardial Regeneration by ceRNA After Myocardial Infarction

Background

The aim of the study was to assess the effect of circRNA CDYL on myocardial angiogenesis after acute myocardial infarction (AMI).

Material/Methods

We compared changes in circRNA CDYL and myocardial angiogenesis in myocardial infarction tissue and normal heart tissue by establishing a myocardial infarction mouse model to clarify the relationship between circRNA CDYL and changes in myocardial infarction and myocardial angiogenesis. Secondly, we used the RegRNA website to predict downstream miRNA, and we performed gain-of-function and loss-of-function experiments.

Results

CircCDYL was downregulated in myocardial tissues and hypoxia myocardial cells, and overexpression and downregulation of circCDYL improved and aggravated, respectively, heart function after AMI. CircCDYL overexpression and downregulation can promote and inhibit, respectively, proliferation of cardiomyocytes in vitro. Finally, we found that circCDYL can sponge miR-4793-5p and regulate its expression, and then miR-4793-5p regulates APP expression.

Conclusions

CircCDYL can promote the proliferation of cardiomyocytes through the miR-4793-5p/APP pathway.

Biomimetic Model of Contractile Cardiac Tissue with Endothelial Networks Stabilized by Adipose-Derived Stromal/Stem Cells

Cardiac tissue engineering strategies have the potential to regenerate functional myocardium following myocardial infarction. In this study, we utilized novel electrospun fibrin microfiber sheets of different stiffnesses (50.0 ± 11.2 kPa and 90.0 ± 16.4 kPa) to engineer biomimetic models of vascularized cardiac tissues. We characterized tissue assembly, electrophysiology, and contractility of neonatal rat ventricular cardiomyocytes (NRVCMs) cultured on these sheets. NRVCMs cultured on the softer substrates displayed higher conduction velocities (CVs) and improved electrophysiological properties. Human umbilical vein endothelial cells (HUVECs) formed dense networks on the sheets when co-cultured with human adipose-derived stem/stromal cells (hASCs). To achieve vascularized cardiac tissues, we tested various tri-culture protocols of NRVCM:hASC:HUVEC and found that a ratio of 1,500,000:37,500:150,000 cells/cm2 enabled the formation of robust endothelial networks while retaining statistically identical electrophysiological characteristics to NRVCM-only cultures. Tri-cultures at this ratio on 90 kPa substrates exhibited average CVs of 14 ± 0.6 cm/s, Action Potential Duration (APD)80 and APD30 of 152 ± 11 ms and 71 ± 6 ms, respectively, and maximum capture rate (MCR) of 3.9 ± 0.7 Hz. These data indicate the significant potential of generating densely packed endothelial networks together with electrically integrated cardiac cells in vitro as a physiologic 3D cardiac model.

Cardiosphere-Derived Cells and Ischemic Heart Failure

After a myocardial infarction, heart tissue becomes irreversibly damaged, leading to scar formation and inevitably ischemic heart failure. Of the many available interventions after a myocardial infarction, such as percutaneous intervention or pharmacological optimization, none can reverse the ischemic insult on the heart and restore cardiac function. Thus, the only available cure for patients with scarred myocardium is allogeneic heart transplantation, which comes with extensive costs, risks, and complications. However, multiple studies have shown that the heart is, in fact, not an end-stage organ and that there are endogenous mechanisms in place that have the potential to spark regeneration. Stem cell therapy has emerged as a potential tool to tap into and activate this endogenous framework. Particularly promising are stem cells derived from cardiac tissue itself, referred to as cardiosphere-derived cells (CDCs). CDCs can be extracted and isolated from the patient's myocardium and then administered by intramyocardial injection or intracoronary infusion. After early success in the animal model, multiple clinical trials have demonstrated the safety and efficacy of autologous CDC therapy in humans. Clinical trials with allogeneic CDCs showed early promising results and pose a potential "off-the-shelf" therapy for patients in the acute setting after a myocardial infarction. The mechanism responsible for CDC-induced cardiac regeneration seems to be a combination of triggering native cardiomyocyte proliferation and recruitment of endogenous progenitor cells, which most prominently occurs via paracrine effects. A further understanding of the mediators involved in paracrine signaling can help with the development of a stem cell-free therapy, with all the benefits and none of the associated complications.

From state-of-the-art cell therapy to endogenous cardiac repair

Clinical heart failure prevention and contemporary therapy often involve breaking the vicious cycle of global haemodynamic consequences of myocardial decay. The lack of effective regenerative therapies results in a primary focus on preventing further deterioration of cardiac performance. The cellular transplantation hypothesis has been evaluated in many different preclinical models and a handful of important clinical trials. The primary expectation that cellular transplants will be embedded into failing myocardium and fuse with existing functioning cells appears unlikely. A multitude of cellular formulas, access routes and clinical surrogate endpoints for evaluation add to the complexity of cellular therapies. Several recent large clinical trials have provided insights into both the regenerative potential and clinical improvement from non-regenerative mechanisms. Initiating endogenous repair seems to be another meaningful alternative to recover structural integrity in myocardial injury. This option may be achieved using a transcoronary sinus catheter intervention, implying the understanding of basic principles in biology. With intermittent reduction of outflow in cardiac veins (PICSO), vascular cells appear to be activated and restart a programme similar to pathways in the developing heart. Structural regeneration may be possible without requiring exogenous agents, or a combination of both approaches may become clinical reality in the next decade.

Current Progress in the Rejuvenation of Aging Stem/Progenitor Cells for Improving the Therapeutic Effectiveness of Myocardial Repair

Ischemic heart disease affects a majority of people, especially elderly patients. Recent studies have utilized autologous adult stem/progenitor cells as a treatment option to heal cardiac tissue after myocardial infarction. However, donor cells from aging patients are more likely to be in a senescent stage. Rejuvenation is required to reverse the damage levied by aging and promote a youthful phenotype. This review aims to discuss current strategies that are effective in rejuvenating aging cardiac stem cells and represent novel therapeutic methods to treat the aging heart. Recent literature mainly focuses on three approaches that aim to reverse cardiac aging: genetic modification, pharmaceutical administration, and optimization of extracellular factors. In vitro genetic modification can be used to overexpress or knock down certain genes and allow for reversal of the aging phenotype. Pharmaceutical administration is another approach that allows for manipulation of signaling pathways related to cell proliferation and cell senescence. Since the stem cell niche can contribute to the age-related decline in stem cell function, rejuvenation strategies also include optimization of extracellular factors. Overall, improving the intrinsic properties of aging stem cells as well as the surrounding environment allows these cells to adopt a phenotype similar to their younger counterparts.

Possible Muscle Repair in the Human Cardiovascular System

The regenerative potential of tissues and organs could promote survival, extended lifespan and healthy life in multicellular organisms. Niches of adult stemness are widely distributed and lead to the anatomical and functional regeneration of the damaged organ. Conversely, muscular regeneration in mammals, and humans in particular, is very limited and not a single piece of muscle can fully regrow after a severe injury. Therefore, muscle repair after myocardial infarction is still a chimera. Recently, it has been recognized that epigenetics could play a role in tissue regrowth since it guarantees the maintenance of cellular identity in differentiated cells and, therefore, the stability of organs and tissues. The removal of these locks can shift a specific cell identity back to the stem-like one. Given the gradual loss of tissue renewal potential in the course of evolution, in the last few years many different attempts to retrieve such potential by means of cell therapy approaches have been performed in experimental models. Here we review pathways and mechanisms involved in the in vivo repair of cardiovascular muscle tissues in humans. Moreover, we address the ongoing research on mammalian cardiac muscle repair based on adult stem cell transplantation and pro-regenerative factor delivery. This latter issue, involving genetic manipulations of adult cells, paves the way for developing possible therapeutic strategies in the field of cardiovascular muscle repair.

Ablation of periostin inhibits post-infarction myocardial regeneration in neonatal mice mediated by the phosphatidylinositol 3 kinase/glycogen synthase kinase 3β/cyclin D1 signalling pathway

AIMS

To resolve the controversy as to whether periostin plays a role in myocardial regeneration after myocardial infarction (MI), we created a neonatal mouse model of MI to investigate the influence of periostin ablation on myocardial regeneration and clarify the underlying mechanisms.

METHODS AND RESULTS

Neonatal periostin-knockout mice and their wildtype littermates were subjected to MI or sham surgery. In the wildtype mice after MI, fibrosis was detectable at 3 days and fibrotic tissue was completely replaced by regenerated myocardium at 21 days. In contrast, in the knockout mice, significant fibrosis in the infarcted area was present at even 3 weeks after MI. Levels of phosphorylated-histone 3 and aurora B in the myocardium, detected by immunofluorescence and western blotting, were significantly lower in knockout than in wildtype mice at 7 days after MI. Similarly, angiogenesis was decreased in the knockout mice after MI. Expression of both the endothelial marker CD-31 and α-smooth muscle actin was markedly lower in the knockout than in wildtype mice at 7 days after MI. The knockout MI group had elevated levels of glycogen synthase kinase (GSK) 3β and decreased phosphatidylinositol 3-kinase (PI3K), phosphorylated serine/threonine protein kinase B (p-Akt), and cyclin D1, compared with the wildtype MI group. Similar effects were observed in experiments using cultured cardiomyocytes from neonatal wildtype or periostin knockout mice. Administration of SB216763, a GSK3β inhibitor, to knockout neonatal mice decreased myocardial fibrosis and increased angiogenesis in the infarcted area after MI.

CONCLUSION

Ablation of periostin suppresses post-infarction myocardial regeneration by inhibiting the PI3K/GSK3β/cyclin D1 signalling pathway, indicating that periostin is essential for myocardial regeneration.

OX40 regulates pressure overload-induced cardiac hypertrophy and remodelling via CD4+ T-cells

OX40, which belongs to the tumour necrosis factor (TNF)-receptor family, is a costimulatory receptor that can potentiate T-cell receptor signalling on the surface of T-lymphocytes. The role of OX40 in non-immune systems, particularly the cardiovascular system, has not been defined. In the present study, we observed a noticeable increase in OX40 expression during cardiac remodelling in rodent heart. In the present study, cardiac hypertrophy was induced by aortic banding (AB) in OX40 knockout (KO) mice and wild-type (WT) mice. After 8 weeks, the OX40 KO mice showed significantly attenuated cardiac hypertrophy, fibrosis and inflammation as well as preserved cardiac function compared with the WT mice. Follow-up in vitro studies suggested that CD4⁺ T-lymphocyte proliferation and pro-inflammatory cytokine release were significantly decreased, whereas anti-inflammatory cytokine release was considerably increased in OX40 KO mice compared with WT mice as assessed by Cell Counting Kit-8 (CCK-8) assay and ELISA. Co-culturing neonatal rat cardiomyocytes with the activated supernatant of CD4⁺ T-lymphocytes from OX40 KO mice reduced the hypertrophy response. Interestingly, OX40 KO mice with reconstituted CD4⁺ T-lymphocytes presented deteriorated cardiac remodelling. Collectively, our data indicate that OX40 regulates cardiac remodelling via the modulation of CD4⁺ T-lymphocytes.

Sphingosine 1-phosphate elicits RhoA-dependent proliferation and MRTF-A mediated gene induction in CPCs

Although c-kit(+) cardiac progenitor cells (CPCs) are currently used in clinical trials there remain considerable gaps in our understanding of the molecular mechanisms underlying their proliferation and differentiation. G-protein coupled receptors (GPCRs) play an important role in regulating these processes in mammalian cell types thus we assessed GPCR mRNA expression in c-kit(+) cells isolated from adult mouse hearts. Our data provide the first comprehensive overview of the distribution of this fundamental class of cardiac receptors in CPCs and reveal notable distinctions from that of adult cardiomyocytes. We focused on GPCRs that couple to RhoA activation in particular those for sphingosine-1-phosphate (S1P). The S1P2 and S1P3 receptors are the most abundant S1P receptor subtypes in mouse and human CPCs while cardiomyocytes express predominantly S1P1 receptors. Treatment of CPCs with S1P, as with thrombin and serum, increased proliferation through a pathway requiring RhoA signaling, as evidenced by significant attenuation when Rho was inhibited by treatment with C3 toxin. Further analysis demonstrated that both S1P- and serum-induced proliferation are regulated through the S1P2 and S1P3 receptor subtypes which couple to Gα12/13 to elicit RhoA activation. The transcriptional co-activator MRTF-A was activated by S1P as assessed by its nuclear accumulation and induction of a RhoA/MRTF-A luciferase reporter. In addition S1P treatment increased expression of cardiac lineage markers Mef2C and GATA4 and the smooth muscle marker GATA6 through activation of MRTF-A. In conclusion, we delineate an S1P-regulated signaling pathway in CPCs that introduces the possibility of targeting S1P2/3 receptors, Gα12/13 or RhoA to influence the proliferation and commitment of c-kit(+) CPCs and improve the response of the myocardium following injury.