Bypassing heterogeneity: the road to embryonic stem cell-derived cardiomyocyte specification

A realistic appraisal of the use of embryonic stem cell-based therapies for cardiac repair

Despite the well-documented capacity of embryonic stem cells (ESCs) to differentiate into cardiomyocytes, transplantation of ESCs or ESC-derived cells is plagued by several formidable problems, including graft rejection, arrhythmias, and potential risk of teratomas. Life-long immunosuppression is a disease in itself. Transplantation of human ESC-derived cells in primates causes life-threatening arrhythmias, and the doses used to show efficacy are not clinically relevant. In contemporary clinical research, the margin of tolerance for such catastrophic effects as malignancies is zero, and although the probability of tumours can be reduced by ESC differentiation, it is unlikely to be completely eliminated, particularly when billions of cells are injected. Although ESCs and ESC-derived cells were touted as capable of long-term regeneration, these cells disappear rapidly after transplantation and there is no evidence of long-term engraftment, let alone regeneration. There is, however, mounting evidence that they act via paracrine mechanisms-just like adult cells. To date, no controlled clinical trial of ESC-derived cells in cardiovascular disease has been conducted or even initiated. In contrast, adult cells have been used in thousands of patients with heart disease, with no significant adverse effects and with results that were sufficiently encouraging to warrant Phase II and III trials. Furthermore, induced pluripotent stem cells offer pluripotency similar to ESCs without the need for lifelong immunosuppression. After two decades, the promise that ESC-derived cells would regenerate dead myocardium has not been fulfilled. The most reasonable interpretation of current data is that ESC-based therapies are not likely to have clinical application for heart disease.

Directed Differentiation of Embryonic Stem Cells Into Cardiomyocytes by Bacterial Injection of Defined Transcription Factors

Forced expression of defined transcriptional factors has been well documented as an effective method for cellular reprogramming or directed differentiation. However, transgene expression is not amenable for therapeutic application due to potential insertional mutagenesis. Here, we have developed a bacterial type III secretion system (T3SS)-based protein delivery tool and shown its application in directing pluripotent stem cell differentiation by a controlled delivery of transcription factors relevant to early heart development. By fusing to an N-terminal secretion sequence for T3SS-dependent injection, three transcriptional factors, namely Gata4, Mef2c, and Tbx5 (abbreviated as GMT), were translocated into murine embryonic stem cells (ESCs), where the proteins are effectively targeted to the nucleus with an average intracellular half-life of 5.5 hours. Exogenous GMT protein injection activated the cardiac program, and multiple rounds of GMT protein delivery significantly improved the efficiency of ESC differentiation into cardiomyocytes. Combination of T3SS-mediated GMT delivery and Activin A treatment showed an additive effect, resulting in on average 60% of the ESCs differentiated into cardiomyocytes. ESC derived cardiomyocytes displayed spontaneous rhythmic contractile movement as well as normal hormonal responses. This work serves as a foundation for the bacterial delivery of multiple transcription factors to direct cell fate without jeopardizing genomic integrity.

Hold or fold--proteins in advanced heart failure and myocardial recovery

Advanced heart failure (AHF) describes the subset of heart failure patients refractory to conventional medical therapy. For some AHF patients, the use of mechanical circulatory support (MCS) provides an intermediary "bridge" step for transplant-eligible patients or an alternative therapy for transplant-ineligible patients. Over the past 20 years, clinical observations have revealed that approximately 1% of patients with MCS undergo significant reverse remodeling to the point where the device can be explanted. Unfortunately, it is unclear why some patients experience durable, sustained myocardial remission, while others redevelop heart failure (i.e. which hearts "hold" and which hearts "fold"). In this review, we outline unmet clinical needs related to treating patients with MCS, provide an overview of protein dynamics in the reverse-remodeling process, and propose specific areas where we expect MS and proteomic analyses will have significant impact on our understanding of disease progression, molecular mechanisms of recovery, and provide new markers with prognostic value that can positively impact patient care. Complimentary perspectives are provided with the goal of making this important topic accessible and relevant to both a clinical and basic science audience, as the intersection of these disciplines is required to advance the field.

Proteomic profiling of H-Ras-G12V induced hypertrophic cardiomyopathy in transgenic mice using comparative LC-MS analysis of thin fresh-frozen tissue sections

Determination of disease-relevant proteomic profiles from limited tissue specimens, such as pathological biopsies and tissues from small model organisms, remains an analytical challenge and a much needed clinical goal. In this study, a transgenic mouse disease model of cardiac-specific H-Ras-G12V induced hypertrophic cardiomyopathy provided a system to explore the potential of using mass spectrometry (MS)-based proteomics to obtain a disease-relevant molecular profile from amount-limited specimens that are routinely used in pathological diagnosis. Our method employs a two-stage methanol-assisted solubilization to digest lysates prepared from 8-μm-thick fresh-frozen histological tissue sections of diseased/experimental and normal/control hearts. Coupling this approach with a nanoflow reversed-phase liquid chromatography (LC) and a hybrid linear ion trap/Fourier transform-ion cyclotron resonance MS resulted in the identification of 704 and 752 proteins in hypertrophic and wild-type (control) myocardium, respectively. The disease driving H-Ras protein along with vimentin were unambiguously identified by LC-MS in hypertrophic myocardium and cross-validated by immunohistochemistry and western blotting. The pathway analysis involving proteins identified by MS showed strong association of proteomic data with cardiovascular disease. More importantly, the MS identification and subsequent cross-validation of Wnt3a and β-catenin, in conjunction with IHC identification of phosphorylated GSK-3β and nuclear localization of β-catenin, provided evidence of Wnt/β-catenin canonical pathway activation secondary to Ras activation in the course of pathogenic myocardial hypertrophic transformation. Our method yields results indicating that the described proteomic approach permits molecular discovery and assessment of differentially expressed proteins regulating H-Ras induced hypertrophic cardiomyopathy. Selected proteins and pathways can be further investigated using immunohistochemical techniques applied to serial tissue sections of similar or different origin.

Regulation of embryonic stem cell self-renewal and differentiation by TGF-beta family signaling

Embryonic stem (ES) cells are characterized by their ability to indefinitely self-renew and potential to differentiate into all the cell lineages of the body. ES cells are considered to have potential applications in regenerative medicine. In particular, the emergence of an ES cell analogue - induced pluripotent stem (iPS) cells via somatic cell reprogramming by co-expressing a limited number of critical stemness-related transcriptional factors has solved the problem of obtaining patient-specific pluripotent cells, encouraging researchers to develop more specific and functional cell lineages from ES or iPS cells for broad therapeutic applications. ES cell fate choice is delicately controlled by a core transcriptional network, epigenetic modification profiles and complex signaling cascades both intrinsically and extrinsically. Of these signals, transforming growth factor beta (TGF-beta) family members, including TGF-beta, bone morphogenetic protein (BMP), Activin and Nodal, have been reported to influence cell self-renewal and a broad spectrum of lineage differentiation in ES cells, in accordance with the key roles of TGF-beta family signaling in early embryo development. In this review, the roles of TGF-beta family signals in coordinating ES cell fate determination are summarized.

Heterogeneous differentiation of human mesenchymal stem cells in response to extended culture in extracellular matrices

Extracellular matrix proteins (ECMs) guide differentiation of adult stem cells, but the temporal distribution of differentiation (i.e., heterogeneity) in a given population has not been investigated. We tested the effect of individual ECM proteins on lineage commitment of human bone marrow-derived mesenchymal stem cells (MSCs) over time. We exposed stem cell populations to ECM proteins representing the primary tissue structures of the body (i.e., collagens type I, III, IV; laminin; and fibronectin) and determined the lineage commitment of the stem cells at 1, 7, and 14 days. We found that collagens that can participate in the formation of fibrils guide differentiation of cardiomyocytes, adipocytes, and osteoblasts. ECMs of the basement membrane initiate differentiation of cardiomyocytes and osteoblasts but not adipocytes, and small facilitator ECMs (e.g., fibronectin) do not significantly affect stem cell differentiation. Differentiation was ECM-dependent because culture on tissue culture polystyrene, with consistent cell morphology, proliferation, and death, initiated differentiation of osteoblasts only. Thus, we show that ECMs independently trigger differentiation of human adult MSCs and that differentiation in this context can be guided down multiple lineages using the same ECM stimulus. This work highlights the importance of more clearly defining progenitor populations, especially those cultured in the presence of ECMs before transplantation.

The stem cell movement

Stem or progenitor cell-based strategies to combat ischemic heart disease and myocardial infarction, whether autologous transplantation or stimulation of resident populations, not only require detailed insight into transdifferentiation potential and functional coupling, but the efficacy of this approach is underpinned by the need to induce appropriate migration and homing to the site of injury. This review focuses on existing insights into the trafficking of stem cells in the context of cardiac regenerative therapy, with particular focus on the wide variety of potential sources of cells, critical factors that may regulate their migration, and how extrapolating from embryonic stem/progenitor cell behavior during cardiogenesis may reveal pathways implicit in the adult heart postinjury.

Molecular signature of cardiomyocyte clusters derived from human embryonic stem cells

Human embryonic stem cells (hESCs) can differentiate in vitro into spontaneously contracting cardiomyocytes (CMs). These cells may prove extremely useful for various applications in basic research, drug discovery, and regenerative medicine. To fully use the potential of the cells, they need to be extensively characterized, and the regulatory mechanisms that control hESC differentiation toward the cardiac lineage need to be better defined. In this study, we used microarrays to analyze, for the first time, the global gene expression profile of isolated hESC-derived CM clusters. By comparing the clusters with undifferentiated hESCs and using stringent selection criteria, we identified 530 upregulated and 40 downregulated genes in the contracting clusters. To further characterize the family of upregulated genes in the hESC-derived CM clusters, the genes were classified according to their Gene Ontology annotation. The results indicate that the hESC-derived CM clusters display high similarities, on a molecular level, to human heart tissue. Moreover, using the family of upregulated genes, we created protein interaction maps that revealed topological characteristics. We also searched for cellular pathways among the upregulated genes in the hESC-derived CM clusters and identified eight significantly upregulated pathways. Real-time quantitative polymerase chain reaction and immunohistochemical analysis confirmed the expression of a subset of the genes identified by the microarrays. Taken together, the results presented here provide a molecular signature of hESC-derived CM clusters and further our understanding of the biological processes that are active in these cells.

Albumin-associated lipids regulate human embryonic stem cell self-renewal

BACKGROUND

Although human embryonic stem cells (hESCs) hold great promise as a source of differentiated cells to treat several human diseases, many obstacles still need to be surmounted before this can become a reality. First among these, a robust chemically-defined system to expand hESCs in culture is still unavailable despite recent advances in the understanding of factors controlling hESC self-renewal.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we attempted to find new molecules that stimulate long term hESC self-renewal. In order to do this, we started from the observation that a commercially available serum replacement product has a strong positive effect on the expansion of undifferentiated hESCs when added to a previously reported chemically-defined medium. Subsequent experiments demonstrated that the active ingredient within the serum replacement is lipid-rich albumin. Furthermore, we show that this activity is trypsin-resistant, strongly suggesting that lipids and not albumin are responsible for the effect. Consistent with this, lipid-poor albumin shows no detectable activity. Finally, we identified the major lipids bound to the lipid-rich albumin and tested several lipid candidates for the effect.

CONCLUSIONS/SIGNIFICANCE

Our discovery of the role played by albumin-associated lipids in stimulating hESC self-renewal constitutes a significant advance in the knowledge of how hESC pluripotency is maintained by extracellular factors and has important applications in the development of increasingly chemically defined hESC culture systems.

A Heterogeneous Expression Pattern for Nanog in Embryonic Stem Cells

Nanog is a critical homeodomain factor responsible for maintaining embryonic stem (ES) cell self-renewal and pluripotency. Of interest, Nanog expression is not homogeneous in the conventional culture of murine ES cells. A Nanog-high population expresses markers for pluripotent ES cells, whereas a Nanog-low population expresses markers for primitive endoderm, such as Gata6. Since the inner cell mass of early blastocysts has recently been reported to be heterogeneous in terms of Nanog and Gata6 expression, ES cells appear to closely resemble the developing stage from which they originate. We further demonstrate that Nanog can directly repress Gata6 expression through its binding to the proximal promoter region of the Gata6 gene and that overexpression of Nanog reduces heterogeneity during ES cell maintenance. Interestingly, Nanog heterogeneity does not correlate with the heterogeneous expression of stage-specific embryonic antigen-1, suggesting that multiple but overlapping levels of heterogeneity may exist in ES cells. These findings provide insight into the factors that control ES cell self-renewal and the earliest lineage commitment to primitive endoderm while also suggesting methods to promote homogeneity during ES cell maintenance. Disclosure of potential conflicts of interest is found at the end of this article.