Conversion of mouse fibroblasts into cardiomyocyte-like cells using small molecule treatments

Enhancing Cardiomyocyte Purity through Lactate-Based Metabolic Selection

BACKGROUND

Direct reprogramming of fibroblasts into chemically induced cardiomyocyte-like cells (CiCMs) through small molecules presents a promising cell source for cardiac regeneration and therapeutic development. However, the contaminating non-cardiomyocytes, primarily unconverted fibroblasts, reduce the effectiveness of CiCMs in various applications. This study investigated a metabolic selection approach using lactate to enrich CiCMs by exploiting the unique metabolic capability of cardiomyocytes to utilize lactate as an alternative energy source.

METHODS

Primary mouse embryonic fibroblasts (pMEFs) were reprogrammed into CiCMs and subjected to a glucose-depleted, lactate-supplemented medium for 4 days. Afterward, cell viability was analyzed, and cardiomyocyte efficiency was assessed through the expression of cardiac-specific markers. Additionally, electrophysiological function was evaluated by examining drug-induced responses.

RESULTS

The lactate treatment led to a significant decrease in the viability of non-cardiomyocytes (pMEF-LAC), while CiCMs (CiCM-LAC) showed minimal cell death. Specifically, the expression of all cardiac-related markers was increased in CiCM-LAC. Metabolically purified CiCMs exhibited enhanced contractile force and increased contraction frequency compared to non-purified CiCMs, as well as an elevated responsiveness to drugs.

CONCLUSION

This study demonstrates that lactate-based metabolic selection is an effective and practical approach for enriching CiCMs, offering a cost-effective alternative to other purification methods. The application of this strategy could potentially broaden the accessibility and utility of reprogrammed cardiomyocytes in cardiac regeneration and therapeutic development.

Direct Cardiac Reprogramming: Current Status and Future Prospects

Advances in cellular reprogramming articulated the path for direct cardiac lineage conversion, bypassing the pluripotent state. Direct cardiac reprogramming attracts major attention because of the low or nil regenerative ability of cardiomyocytes, resulting in permanent cell loss in various heart diseases. In the field of cardiology, balancing this loss of cardiomyocytes was highly challenging, even in the modern medical world. Soon after the discovery of cell reprogramming, direct cardiac reprogramming also became a promising alternative for heart regeneration. This review mainly focused on the various direct cardiac reprogramming approaches (integrative and non-integrative) for the derivation of induced autologous cardiomyocytes. It also explains the advancements in cardiac reprogramming over the decade with the pros and cons of each approach. Further, the review highlights the importance of clinically relevant (non-integrative) approaches and their challenges for the prospective applications for personalized medicine. Apart from direct cardiac reprogramming, it also discusses the other strategies for generating cardiomyocytes from different sources. The understanding of these strategies could pave the way for the efficient generation of integration-free functional autologous cardiomyocytes through direct cardiac reprogramming for various biomedical applications.

Three-dimensional heart extracellular matrix enhances chemically induced direct cardiac reprogramming

Direct cardiac reprogramming has emerged as a promising therapeutic approach for cardiac regeneration. Full chemical reprogramming with small molecules to generate cardiomyocytes may be more amenable than genetic reprogramming for clinical applications as it avoids safety concerns associated with genetic manipulations. However, challenges remain regarding low conversion efficiency and incomplete cardiomyocyte maturation. Furthermore, the therapeutic potential of chemically induced cardiomyocytes (CiCMs) has not been investigated. Here, we report that a three-dimensional microenvironment reconstituted with decellularized heart extracellular matrix can enhance chemical reprogramming and cardiac maturation of fibroblasts to cardiomyocytes. The resultant CiCMs exhibit elevated cardiac marker expression, sarcomeric organization, and improved electrophysiological features and drug responses. We investigated the therapeutic potential of CiCMs reprogrammed in three-dimensional heart extracellular matrix in a rat model of myocardial infarction. Our platform can facilitate the use of CiCMs for regenerative medicine, disease modeling, and drug screening.

Non-viral approaches for somatic cell reprogramming into cardiomyocytes

Heart disease is the leading cause of human deaths worldwide. Due to lacking cardiomyocytes with replicative capacity and cardiac progenitor cells with differentiation potential in adult hearts, massive loss of cardiomyocytes after ischemic events produces permanent damage, ultimately leading to heart failure. Cellular reprogramming is a promising strategy to regenerate heart by induction of cardiomyocytes from other cell types, such as cardiac fibroblasts. In contrast to conventional virus-based cardiac reprogramming, non-viral approaches greatly reduce the potential risk that includes disruption of genome integrity by integration of foreign DNAs, expression of exogenous genes with oncogenic potential, and appearance of partially reprogrammed cells harmful for the physiological functions of tissues/organs, which impedes their in-vivo applications. Here, we review the recent progress in development of non-viral approaches to directly reprogram somatic cells towards cardiomyocytes and their therapeutic application for heart regeneration.

Direct cell reprogramming: approaches, mechanisms and progress

The reprogramming of somatic cells with defined factors, which converts cells from one lineage into cells of another, has greatly reshaped our traditional views on cell identity and cell fate determination. Direct reprogramming (also known as transdifferentiation) refers to cell fate conversion without transitioning through an intermediary pluripotent state. Given that the number of cell types that can be generated by direct reprogramming is rapidly increasing, it has become a promising strategy to produce functional cells for therapeutic purposes. This Review discusses the evolution of direct reprogramming from a transcription factor-based method to a small-molecule-driven approach, the recent progress in enhancing reprogrammed cell maturation, and the challenges associated with in vivo direct reprogramming for translational applications. It also describes our current understanding of the molecular mechanisms underlying direct reprogramming, including the role of transcription factors, epigenetic modifications, non-coding RNAs, and the function of metabolic reprogramming, and highlights novel insights gained from single-cell omics studies.

Biphasic Electrical Pulse by a Micropillar Electrode Array Enhances Maturation and Drug Response of Reprogrammed Cardiac Spheroids

Direct reprogramming is an efficient strategy to produce cardiac lineage cells necessary for cardiac tissue engineering and drug testing for cardiac toxicity. However, functional maturation of reprogrammed cardiomyocytes, which is of great importance for their regenerative potential and drug response, still remains challenging. In this study, we propose a novel electrode platform to promote direct cardiac reprogramming and improve the functionality of reprogrammed cardiac cells. Nonviral cardiac reprogramming was improved via a three-dimensional spheroid culture of chemically induced cardiomyocytes exposed to a small-molecule cocktail. A micropillar electrode array providing biphasic electrical pulses mimicking the heartbeat further enhanced maturation and electrophysiological properties of reprogrammed cardiac spheroids, leading to proper responses and increased sensitivity to drugs. On the basis of our results, we conclude that our device may have a wider application in the generation of functional cardiac cells for regenerative medicine and screening of novel drugs.

Impelling force and current challenges by chemicals in somatic cell reprogramming and expansion beyond hepatocytes

In the field of regenerative medicine, generating numerous transplantable functional cells in the laboratory setting on a large scale is a major challenge. However, the in vitro maintenance and expansion of terminally differentiated cells are challenging because of the lack of specific environmental and intercellular signal stimulations, markedly hindering their therapeutic application. Remarkably, the generation of stem/progenitor cells or functional cells with effective proliferative potential is markedly in demand for disease modeling, cell-based transplantation, and drug discovery. Despite the potent genetic manipulation of transcription factors, integration-free chemically defined approaches for the conversion of somatic cell fate have garnered considerable attention in recent years. This review aims to summarize the progress thus far and discuss the advantages, limitations, and challenges of the impact of full chemicals on the stepwise reprogramming of pluripotency, direct lineage conversion, and direct lineage expansion on somatic cells. Owing to the current chemical-mediated induction, reprogrammed pluripotent stem cells with reproducibility difficulties, and direct lineage converted cells with marked functional deficiency, it is imperative to generate the desired cell types directly by chemically inducing their potent proliferation ability through a lineage-committed progenitor state, while upholding the maturation and engraftment capacity posttransplantation in vivo. Together with the comprehensive understanding of the mechanism of chemical drives, as well as the elucidation of specificity and commonalities, the precise manipulation of the expansion for diverse functional cell types could broaden the available cell sources and enhance the cellular function for clinical application in future.

mRNA-Driven Generation of Transgene-Free Neural Stem Cells from Human Urine-Derived Cells

Human neural stem cells (NSCs) hold enormous promise for neurological disorders, typically requiring their expandable and differentiable properties for regeneration of damaged neural tissues. Despite the therapeutic potential of induced NSCs (iNSCs), a major challenge for clinical feasibility is the presence of integrated transgenes in the host genome, contributing to the risk for undesired genotoxicity and tumorigenesis. Here, we describe the advanced transgene-free generation of iNSCs from human urine-derived cells (HUCs) by combining a cocktail of defined small molecules with self-replicable mRNA delivery. The established iNSCs were completely transgene-free in their cytosol and genome and further resembled human embryonic stem cell-derived NSCs in the morphology, biological characteristics, global gene expression, and potential to differentiate into functional neurons, astrocytes, and oligodendrocytes. Moreover, iNSC colonies were observed within eight days under optimized conditions, and no teratomas formed in vivo, implying the absence of pluripotent cells. This study proposes an approach to generate transplantable iNSCs that can be broadly applied for neurological disorders in a safe, efficient, and patient-specific manner.

Partial reprogramming as a therapeutic approach for heart disease: A state-of-the-art review

Heart disease such as myocardial infarction is the first cause of mortality in all countries. Today, cardiac cell-based therapy using de novo produced cardiac cells is considered as a novel approach for cardiac regenerative medicine. Recently, an alchemy-like approach, known as direct reprogramming or direct conversion, has been developed to directly convert somatic cells to cardiac cells in vitro and in vivo. This cellular alchemy is a short-cut and safe strategy for generating autologous cardiac cells, and it can be accomplished through activating cardiogenesis- or pluripotency-related factors in noncardiac cells. Importantly, pluripotency factors-based direct cardiac conversion, known as partial reprogramming, is shorter and more efficient for cardiomyocyte generation in vitro. Today, this strategy is achievable for direct conversion of mouse and human somatic cells to cardiac lineage cells (cardiomyocytes and cardiac progenitor cells), using transgene free, chemical-based approaches. Although, heart-specific partial reprogramming seems to be challenging for in vivo conversion of cardiac fibroblasts to cardiac cells, but whole organism-based in vivo partial reprogramming ameliorates cellular and physiological hallmarks of aging and prolongs lifespan in mouse. Notably, cardiac cells produced using partial reprogramming strategy can be a useful platform for disease modeling, drug screening and cardiac cell-based therapy, once the safety issues are overcome. Herein, we discuss about all progresses in de novo production of cardiac cells using partial reprogramming-based direct conversion, as well as give an overview about the potential applications of this strategy in vivo and in vitro.

Pluripotent stem cells: induction and self-renewal

Pluripotent stem cells (PSCs) lie at the heart of modern regenerative medicine due to their properties of unlimited self-renewal in vitro and their ability to differentiate into cell types representative of the three embryonic germ layers-mesoderm, ectoderm and endoderm. The derivation of induced PSCs bypasses ethical concerns associated with the use of human embryonic stem cells and also enables personalized cell-based therapies. To exploit their regenerative potential, it is essential to have a firm understanding of the molecular processes associated with their induction from somatic cells. This understanding serves two purposes: first, to enable efficient, reliable and cost-effective production of excellent quality induced PSCs and, second, to enable the derivation of safe, good manufacturing practice-grade transplantable donor cells. Here, we review the reprogramming process of somatic cells into induced PSCs and associated mechanisms with emphasis on self-renewal, epigenetic control, mitochondrial bioenergetics, sub-states of pluripotency, naive ground state, naive and primed. A meta-analysis identified genes expressed exclusively in the inner cell mass and in the naive but not in the primed pluripotent state. We propose these as additional biomarkers defining naive PSCs.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Specific Cell (Re-)Programming: Approaches and Perspectives

Many disorders are manifested by dysfunction of key cell types or their disturbed integration in complex organs. Thereby, adult organ systems often bear restricted self-renewal potential and are incapable of achieving functional regeneration. This underlies the need for novel strategies in the field of cell (re-)programming-based regenerative medicine as well as for drug development in vitro. The regenerative field has been hampered by restricted availability of adult stem cells and the potentially hazardous features of pluripotent embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). Moreover, ethical concerns and legal restrictions regarding the generation and use of ESCs still exist. The establishment of direct reprogramming protocols for various therapeutically valuable somatic cell types has overcome some of these limitations. Meanwhile, new perspectives for safe and efficient generation of different specified somatic cell types have emerged from numerous approaches relying on exogenous expression of lineage-specific transcription factors, coding and noncoding RNAs, and chemical compounds.It should be of highest priority to develop protocols for the production of mature and physiologically functional cells with properties ideally matching those of their endogenous counterparts. Their availability can bring together basic research, drug screening, safety testing, and ultimately clinical trials. Here, we highlight the remarkable successes in cellular (re-)programming, which have greatly advanced the field of regenerative medicine in recent years. In particular, we review recent progress on the generation of cardiomyocyte subtypes, with a focus on cardiac pacemaker cells. Graphical Abstract.

Chemical modulation of cell fates: in situ regeneration

Chemical modulation of cell fates has been widely used to promote tissue and organ regeneration. Small molecules can target the self-renewal, expansion, differentiation, and survival of endogenous stem cells for enhancing their regenerative power or induce dedifferentiation or transdifferentiation of mature cells into proliferative progenitors or specialized cell types needed for regeneration. Here, we discuss current progress and potential using small molecules to promote in vivo regenerative processes by regulating the cell fate. Current studies of small molecules in regeneration will provide insights into developing safe and efficient chemical approaches for in situ tissue repair and regeneration.

Epigenetic Control of Reprogramming and Transdifferentiation by Histone Modifications

Somatic cells can be reprogrammed to pluripotent stem cells or transdifferentiate to another lineage cell type. Much efforts have been made to unravel the epigenetic mechanisms underlying the cell fate conversion. Histone modifications as the major epigenetic regulator are implicated in various aspects of reprogramming and transdifferentiation. Here, we discuss the roles of histone modifications on reprogramming and transdifferentiation and hopefully provide new insights into induction and promotion of the cell fate conversion by modulating histone modifications.

Aging adult porcine fibroblasts can support nuclear transfer and transcription factor-mediated reprogramming

Somatic cell nuclear transfer (SCNT) and induced pluripotent stem cells (iPSCs) technology are two classical reprogramming methods. Donor cell types can affect the reprogramming results in the above two methods. We here used porcine embryonic fibroblasts (PEFs) and adult porcine ear skin fibroblasts (APEFs) and adipose-derived stem cells (ADSCs) as donor cells for SCNT and source cells for iPSCs to study their in vitro developmental capability and colony-formation efficiency, respectively. For SCNT, fusion and cleavage rate has no significant difference among PEFs, ADSCs and APEFs. The rate and total cell number of blastocysts in the APEF group were significant lower than that in PEFs and ADSCs. For transcription factor-mediated reprogramming, the reprogramming efficiency of ADSCs were significantly higher than PEFs and APEFs and there is no significant difference between PEFs and APEFs. Furthermore, PEFs, APEFs and ADSCs can be used to generate iPSCs. Fianlly, somatic cloned pigs could still be successfully generated from APEFs, suggesting terminally differentiated aging adult somatic cells could be reprogrammed into a totipotent state. Considering the easy availability of animal tissue and the costs of establishing cell lines, aging porcine ear fibroblasts can support nuclear transfer-mediated and transcription factor-based reprogramming.

Small molecules for reprogramming and transdifferentiation

Pluripotency reprogramming and transdifferentiation induced by transcription factors can generate induced pluripotent stem cells, adult stem cells or specialized cells. However, the induction efficiency and the reintroduction of exogenous genes limit their translation into clinical applications. Small molecules that target signaling pathways, epigenetic modifications, or metabolic processes can regulate cell development, cell fate, and function. In the recent decade, small molecules have been widely used in reprogramming and transdifferentiation fields, which can promote the induction efficiency, replace exogenous genes, or even induce cell fate conversion alone. Small molecules are expected as novel approaches to generate new cells from somatic cells in vitro and in vivo. Here, we will discuss the recent progress, new insights, and future challenges about the use of small molecules in cell fate conversion.

Demethylation of H3K27 Is Essential for the Induction of Direct Cardiac Reprogramming by miR Combo

RATIONALE

Direct reprogramming of cardiac fibroblasts to cardiomyocytes has recently emerged as a novel and promising approach to regenerate the injured myocardium. We have previously demonstrated the feasibility of this approach in vitro and in vivo using a combination of 4 microRNAs (miR-1, miR-133, miR-208, and miR-499) that we named miR combo. However, the mechanism of miR combo mediated direct cardiac reprogramming is currently unknown.

OBJECTIVE

Here, we investigated the possibility that miR combo initiated direct cardiac reprogramming through an epigenetic mechanism.

METHODS AND RESULTS

Using a quantitative polymerase chain reaction array, we found that histone methyltransferases and demethylases that regulate the trimethylation of H3K27 (H3K27me3), an epigenetic modification that marks transcriptional repression, were changed in miR combo-treated fibroblasts. Accordingly, global H3K27me3 levels were downregulated by miR combo treatment. In particular, the promoter region of cardiac transcription factors showed decreased H3K27me3 as revealed by chromatin immunoprecipitation coupled with quantitative polymerase chain reaction. Inhibition of H3K27 methyltransferases or of the PRC2 (Polycomb Repressive Complex 2) by pharmaceutical inhibition or siRNA reduced the levels of H3K27me3 and induced cardiogenic markers at the RNA and protein level, similarly to miR combo treatment. In contrast, knockdown of the H3K27 demethylases Kdm6A and Kdm6B restored the levels of H3K27me3 and blocked the induction of cardiac gene expression in miR combo-treated fibroblasts.

CONCLUSIONS

In summary, we demonstrated that removal of the repressive mark H3K27me3 is essential for the induction of cardiac reprogramming by miR combo. Our data not only highlight the importance of regulating the epigenetic landscape during cell fate conversion but also provide a framework to improve this technique.

Neural patterning of human induced pluripotent stem cells in 3-D cultures for studying biomolecule-directed differential cellular responses

INTRODUCTION

Appropriate neural patterning of human induced pluripotent stem cells (hiPSCs) is critical to generate specific neural cells/tissues and even mini-brains that are physiologically relevant to model neurological diseases. However, the capacity of signaling factors that regulate 3-D neural tissue patterning in vitro and differential responses of the resulting neural populations to various biomolecules have not yet been fully understood.

METHODS

By tuning neural patterning of hiPSCs with small molecules targeting sonic hedgehog (SHH) signaling, this study generated different 3-D neuronal cultures that were mainly comprised of either cortical glutamatergic neurons or motor neurons.

RESULTS

Abundant glutamatergic neurons were observed following the treatment with an antagonist of SHH signaling, cyclopamine, while Islet-1 and HB9-expressing motor neurons were enriched by an SHH agonist, purmorphamine. In neurons derived with different neural patterning factors, whole-cell patch clamp recordings showed similar voltage-gated Na(+)/K(+) currents, depolarization-evoked action potentials and spontaneous excitatory post-synaptic currents. Moreover, these different neuronal populations exhibited differential responses to three classes of biomolecules, including (1) matrix metalloproteinase inhibitors that affect extracellular matrix remodeling; (2) N-methyl-d-aspartate that induces general neurotoxicity; and (3) amyloid β (1-42) oligomers that cause neuronal subtype-specific neurotoxicity.

CONCLUSIONS

This study should advance our understanding of hiPSC self-organization and neural tissue development and provide a transformative approach to establish 3-D models for neurological disease modeling and drug discovery.

STATEMENT OF SIGNIFICANCE

Appropriate neural patterning of human induced pluripotent stem cells (hiPSCs) is critical to generate specific neural cells, tissues and even mini-brains that are physiologically relevant to model neurological diseases. However, the capability of sonic hedgehog-related small molecules to tune different neuronal subtypes in 3-D differentiation from hiPSCs and the differential cellular responses of region-specific neuronal subtypes to various biomolecules have not been fully investigated. By tuning neural patterning of hiPSCs with small molecules targeting sonic hedgehog signaling, this study provides knowledge on the differential susceptibility of region-specific neuronal subtypes derived from hiPSCs to different biomolecules in extracellular matrix remodeling and neurotoxicity. The findings are significant for understanding 3-D neural patterning of hiPSCs for the applications in brain organoid formation, neurological disease modeling, and drug discovery.

Chemical transdifferentiation: closer to regenerative medicine

Cell transdifferentiation, which directly switches one type of differentiated cells into another cell type, is more advantageous than cell reprogramming to generate pluripotent cells and differentiate them into functional cells. This process is crucial in regenerative medicine. However, the cell-converting strategies, which mainly depend on the virus-mediated expression of exogenous genes, have clinical safety concerns. Small molecules with compelling advantages are a potential alternative in manipulating cell fate conversion. In this review, we briefly retrospect the nature of cell transdifferentiation and summarize the current developments in the research of small molecules in promoting cell conversion. Particularly, we focus on the complete chemical compound-induced cell transdifferentiation, which is closer to the clinical translation in cell therapy. Despite these achievements, the mechanisms underpinning chemical transdifferentiation remain largely unknown. More importantly, identifying drugs that induce resident cell conversion in vivo to repair damaged tissue remains to be the end-goal in current regenerative medicine.