Scalable Cardiac Differentiation of Pluripotent Stem Cells Using Specific Growth Factors and Small Molecules

Heart in a dish - choosing the right in vitro model

The heart is the first functional organ established during embryogenesis. Investigating heart development and disease is a fascinating and crucial field of research because cardiovascular diseases remain the leading cause of morbidity and mortality worldwide. Therefore, there is great interest in establishing in vitro models for recapitulating both physiological and pathological aspects of human heart development, tissue function and malfunction. Derived from pluripotent stem cells, a large variety of three-dimensional cardiac in vitro models have been introduced in recent years. In this At a Glance article, we discuss the available methods to generate such models, grouped according to the following classification: cardiac organoids, cardiac microtissues and engineered cardiac tissues. For these models, we provide a systematic overview of their applications for disease modeling and therapeutic development, as well as their advantages and limitations to assist scientists in choosing the most suitable model for their research purpose.

Novel method of differentiating human induced pluripotent stem cells to mature cardiomyocytes via Sfrp2

Current methods to generate cardiomyocytes from induced pluripotent stem cells (iPSc) utilize broad-spectrum pharmacological inhibitors. These methods give rise to cardiomyocytes which are typically immature. Since we have recently demonstrated that cardiomyogenesis in vitro and in vivo requires Sfrp2, we asked if Sfrp2 would drive differentiation of human iPSc into cardiomyocytes. Indeed, we found that Sfrp2 induced robust cardiac differentiation. Importantly, replacement of broad spectrum pharmacological inhibitors with Sfrp2 gave rise to mature cardiomyocytes as evidenced by their sarcomere structure, electrophysiological profiles, and ability to form gap junctions.

Temporal Control of the WNT Signaling Pathway During Cardiac Differentiation Impacts Upon the Maturation State of Human Pluripotent Stem Cell Derived Cardiomyocytes

Inefficient differentiation and insufficient maturation are barriers to the application of human pluripotent stem cell (hPSC)-derived cardiomyocytes (CMs) for research and therapy. Great strides have been made to the former, and multiple groups have reported cardiac differentiation protocol that can generate hPSC-CMs at high efficiency. Although many such protocols are based on the modulation of the WNT signaling pathway, they differ in their timing and in the WNT inhibitors used. Little is currently known about whether and how conditions of differentiation affect cardiac maturation. Here we adapted multiple cardiac differentiation protocols to improve cost-effectiveness and consistency, and compared the properties of the hPSC-CMs generated. Our results showed that the schedule of differentiation, but not the choice of WNT inhibitors, was a critical determinant not only of differentiation efficiency, which was expected, but also CM maturation. Among cultures with comparable purity, hPSC-CMs generated with different differentiation schedules vary in the expression of genes which are important for developmental maturation, and in their structural, metabolic, calcium transient and proliferative properties. In summary, we demonstrated that simple changes in the schedule of cardiac differentiation could promote maturation. To this end, we have optimized a cardiac differentiation protocol that can simultaneously achieve high differentiation efficiency and enhanced developmental maturation. Our findings would advance the production of hPSC-CMs for research and therapy.

Continuous human iPSC-macrophage mass production by suspension culture in stirred tank bioreactors

Macrophages derived from human induced pluripotent stem cells (iPSCs) have the potential to enable the development of cell-based therapies for numerous disease conditions. We here provide a detailed protocol for the mass production of iPSC-derived macrophages (iPSC-Mac) in scalable suspension culture on an orbital shaker or in stirred-tank bioreactors (STBRs). This strategy is straightforward, robust and characterized by the differentiation of primed iPSC aggregates into 'myeloid-cell-forming-complex' intermediates by means of a minimal cytokine cocktail. In contrast to the 'batch-like differentiation approaches' established for other iPSC-derived lineages, myeloid-cell-forming-complex-intermediates are stably maintained in suspension culture and continuously generate functional and highly pure iPSC-Mac. Employing a culture volume of 120 ml in the STBR platform, ~1-4 × 107 iPSC-Mac can be harvested at weekly intervals for several months. The STBR technology allows for real-time monitoring of crucial process parameters such as biomass, pH, dissolved oxygen, and nutrition levels; the system also promotes systematic process development, optimization and linear upscaling. The process duration, from the expansion of iPSC until the first iPSC-Mac harvest, is 28 d. Successful application of the protocol requires expertise in pluripotent stem cell culture, differentiation and analytical methods, such as flow cytometry. Fundamental know-how in biotechnology is also advantageous to run the process in the STBR platform. The continuous, scalable production of well-defined iPSC-Mac populations is highly relevant to various fields, ranging from developmental biology, immunology and cell therapies to industrial applications for drug safety and discovery.

How Localized Z-Disc Damage Affects Force Generation and Gene Expression in Cardiomyocytes

The proper function of cardiomyocytes (CMs) is highly related to the Z-disc, which has a pivotal role in orchestrating the sarcomeric cytoskeletal function. To better understand Z-disc related cardiomyopathies, novel models of Z-disc damage have to be developed. Human pluripotent stem cell (hPSC)-derived CMs can serve as an in vitro model to better understand the sarcomeric cytoskeleton. A femtosecond laser system can be applied for localized and defined damage application within cells as single Z-discs can be removed. We have investigated the changes in force generation via traction force microscopy, and in gene expression after Z-disc manipulation in hPSC-derived CMs. We observed a significant weakening of force generation after removal of a Z-disc. However, no significant changes of the number of contractions after manipulation were detected. The stress related gene NF-kB was significantly upregulated. Additionally, α-actinin (ACTN2) and filamin-C (FLNc) were upregulated, pointing to remodeling of the Z-disc and the sarcomeric cytoskeleton. Ultimately, cardiac troponin I (TNNI3) and cardiac muscle troponin T (TNNT2) were significantly downregulated. Our results allow a better understanding of transcriptional coupling of Z-disc damage and the relation of damage to force generation and can therefore finally pave the way to novel therapies of sarcomeric disorders.

Cardiac Remodeling and Repair: Recent Approaches, Advancements, and Future Perspective

The limited ability of mammalian adult cardiomyocytes to proliferate following an injury to the heart, such as myocardial infarction, is a major factor that results in adverse fibrotic and myocardial remodeling that ultimately leads to heart failure. The continued high degree of heart failure-associated morbidity and lethality requires the special attention of researchers worldwide to develop efficient therapeutics for cardiac repair. Recently, various strategies and approaches have been developed and tested to extrinsically induce regeneration and restoration of the myocardium after cardiac injury have yielded encouraging results. Nevertheless, these interventions still lack adequate success to be used for clinical interventions. This review highlights and discusses both cell-based and cell-free therapeutic approaches as well as current advancements, major limitations, and future perspectives towards developing an efficient therapeutic method for cardiac repair.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

A method for differentiating human induced pluripotent stem cells toward functional cardiomyocytes in 96-well microplates

The capacity of pluripotent stem cells both for self-renewal and to differentiate into any cell type have made them a powerful tool for studying human disease. Protocols for efficient differentiation towards cardiomyocytes using defined, serum-free culture medium combined with small molecules have been developed, but thus far, limited to larger formats. We adapted protocols for differentiating human pluripotent stem cells to functional human cardiomyocytes in a 96-well microplate format. The resulting cardiomyocytes expressed cardiac specific markers at the transcriptional and protein levels and had the electrophysiological properties that confirmed the presence of functional cardiomyocytes. We suggest that this protocol provides an incremental improvement and one that reduces the impact of heterogeneity by increasing inter-experimental replicates. We believe that this technique will improve the applicability of these cells for use in developmental biology and mechanistic studies of disease.

Prediction of Human Induced Pluripotent Stem Cell Cardiac Differentiation Outcome by Multifactorial Process Modeling

Human cardiomyocytes (CMs) have potential for use in therapeutic cell therapy and high-throughput drug screening. Because of the inability to expand adult CMs, their large-scale production from human pluripotent stem cells (hPSC) has been suggested. Significant improvements have been made in understanding directed differentiation processes of CMs from hPSCs and their suspension culture-based production at chemically defined conditions. However, optimization experiments are costly, time-consuming, and highly variable, leading to challenges in developing reliable and consistent protocols for the generation of large CM numbers at high purity. This study examined the ability of data-driven modeling with machine learning for identifying key experimental conditions and predicting final CM content using data collected during hPSC-cardiac differentiation in advanced stirred tank bioreactors (STBRs). Through feature selection, we identified process conditions, features, and patterns that are the most influential on and predictive of the CM content at the process endpoint, on differentiation day 10 (dd10). Process-related features were extracted from experimental data collected from 58 differentiation experiments by feature engineering. These features included data continuously collected online by the bioreactor system, such as dissolved oxygen concentration and pH patterns, as well as offline determined data, including the cell density, cell aggregate size, and nutrient concentrations. The selected features were used as inputs to construct models to classify the resulting CM content as being "sufficient" or "insufficient" regarding pre-defined thresholds. The models built using random forests and Gaussian process modeling predicted insufficient CM content for a differentiation process with 90% accuracy and precision on dd7 of the protocol and with 85% accuracy and 82% precision at a substantially earlier stage: dd5. These models provide insight into potential key factors affecting hPSC cardiac differentiation to aid in selecting future experimental conditions and can predict the final CM content at earlier process timepoints, providing cost and time savings. This study suggests that data-driven models and machine learning techniques can be employed using existing data for understanding and improving production of a specific cell type, which is potentially applicable to other lineages and critical for realization of their therapeutic applications.

Fluorescence-based actin turnover dynamics of stem cells as a profiling method for stem cell functional evolution, heterogeneity and phenotypic lineage parsing

Stem cells are widely explored in regenerative medicine as a source to produce diverse cell types. Despite the wide usage of stem cells like mesenchymal stem cells (MSCs) and induced pluripotent stem cells (iPSCs), there is a lack of robust methods to rapidly discern the phenotypic and functional heterogeneity of stem cells. The organization of actin cytoskeleton has been previously used to discern divergent stem cell differentiation pathways. In this paper, we highlight the versatility of a cell profiling method for actin turnover dynamics. Actin filaments in live stem cells are labeled using SiR-actin, a cell permeable fluorogenic probe, to determine the endogenous actin turnover. Live MSC imaging after days of induction successfully demonstrated lineage specific change in actin turnover. Next, we highlighted the differences in the cellular heterogeneity of actin dynamics during adipogenic or osteogenic MSC differentiation. Next, we applied the method to differentiating iPSCs in culture, and detected a progressive slowdown in actin turnover during differentiation upon stimulation with neural or cardiac media. Finally, as a proof of concept, the actin dynamic profiling was used to isolate MSCs via flow cytometry prior to sorting into three distinct sub-populations with low, intermediate or high actin dynamics. A greater fraction of MSCs with more rapid actin dynamics demonstrated increased inclination for adipogenesis, whereas, slower actin dynamics correlated with increased osteogenesis. Together, these results show that actin turnover can serve as a versatile biomarker to not only track cellular phenotypic heterogeneity but also harvest live cells with potential for differential phenotypic fates.

Selection of human induced pluripotent stem cells lines optimization of cardiomyocytes differentiation in an integrated suspension microcarrier bioreactor

Background

The production of large quantities of cardiomyocyte is essential for the needs of cellular therapies. This study describes the selection of a human-induced pluripotent cell (hiPSC) line suitable for production of cardiomyocytes in a fully integrated bioprocess of stem cell expansion and differentiation in microcarrier stirred tank reactor.

Methods

Five hiPSC lines were evaluated first for their cardiac differentiation efficiency in monolayer cultures followed by their expansion and differentiation compatibility in microcarrier (MC) cultures under continuous stirring conditions.

Results

Three cell lines were highly cardiogenic but only one (FR202) of them was successfully expanded on continuous stirring MC cultures. FR202 was thus selected for cardiac differentiation in a 22-day integrated bioprocess under continuous stirring in a stirred tank bioreactor. In summary, we integrated a MC-based hiPSC expansion (phase 1), CHIR99021-induced cardiomyocyte differentiation step (phase 2), purification using the lactate-based treatment (phase 3) and cell recovery step (phase 4) into one process in one bioreactor, under restricted oxygen control (< 30% DO) and continuous stirring with periodic batch-type media exchanges. High density of undifferentiated hiPSC (2 ± 0.4 × 10⁶ cells/mL) was achieved in the expansion phase. By controlling the stirring speed and DO levels in the bioreactor cultures, 7.36 ± 1.2 × 10⁶ cells/mL cardiomyocytes with > 80% Troponin T were generated in the CHIR99021-induced differentiation phase. By adding lactate in glucose-free purification media, the purity of cardiomyocytes was enhanced (> 90% Troponin T), with minor cell loss as indicated by the increase in sub-G1 phase and the decrease of aggregate sizes. Lastly, we found that the recovery period is important for generating purer and functional cardiomyocytes (> 96% Troponin T). Three independent runs in a 300-ml working volume confirmed the robustness of this process.

Conclusion

A streamlined and controllable platform for large quantity manufacturing of pure functional atrial, ventricular and nodal cardiomyocytes on MCs in conventional-type stirred tank bioreactors was established, which can be further scaled up and translated to a good manufacturing practice-compliant production process, to fulfill the quantity requirements of the cellular therapeutic industry.

Supplementary information

The online version of this article (10.1186/s13287-020-01618-6) contains supplementary material, which is available to authorized users.

Stem cell therapy of myocardial infarction: a promising opportunity in bioengineering

Myocardial infarction (MI) is a life-threatening disease resulting from irreversible death of cardiomyocytes (CMs) and weakening of the heart blood-pumping function. Stem cell-based therapies have been studied for MI treatment over the last two decades with promising outcome. In this review, we critically summarize the past work in this field to elucidate the advantages and disadvantages of treating MI using pluripotent stem cells (PSCs) including both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), adult stem cells, and cardiac progenitor cells. The main advantage of the latter is their cytokine production capability to modulate immune responses and control the progression of healing. However, human adult stem cells have very limited (if not 'no') capacity to differentiate into functional CMs in vitro or in vivo. In contrast, PSCs can be differentiated into functional CMs although the protocols for the cardiac differentiation of PSCs are mainly for adherent cells under 2D culture. Derivation of PSC-CMs in 3D, allowing for large-scale production of CMs via modulation of the Wnt/β-catenin signal pathway with defined chemicals and medium, may be desired for clinical translation. Furthermore, the technology of purification and maturation of the PSC-CMs may need further improvements to eliminate teratoma formation after in vivo implantation of the PSC-CMs for treating MI. In addition, in vitro derived PSC-CMs may have mechanical and electrical mismatch with the patient's cardiac tissue, which causes arrhythmia. This supports the use of PSC-derived cells committed to cardiac lineage without beating for implantation to treat MI. In this case, the PSC derived cells may utilize the mechanical, electrical, and chemical cues in the heart to further differentiate into mature/functional CMs in situ. Another major challenge facing stem cell therapy of MI is the low retention/survival of stem cells or their derivatives (e.g., PSC-CMs) in the heart for MI treatment after injection in vivo. This may be resolved by using biomaterials to engineer stem cells for reduced immunogenicity, immobilization of the cells in the heart, and increased integration with the host cardiac tissue. Biomaterials have also been applied in the derivation of CMs in vitro to increase the efficiency and maturation of differentiation. Collectively, a lot has been learned from the past failure of simply injecting intact stem cells or their derivatives in vivo for treating MI, and bioengineering stem cells with biomaterials is expected to be a valuable strategy for advancing stem cell therapy towards its widespread application for treating MI in the clinic.

Cryopreservation of human pluripotent stem cell-derived cardiomyocytes is not detrimental to their molecular and functional properties

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have emerged as a powerful platform for in vitro modelling of cardiac diseases, safety pharmacology and drug screening. All these applications require large quantities of well-characterised and standardised batches of hiPSC-CMs. Cryopreservation of hiPSC-CMs without affecting their biochemical or biophysical phenotype is essential for facilitating this, but ideally requires the cells being unchanged by the freeze-thaw procedure. We therefore compared the in vitro functional and molecular characteristics of fresh and cryopreserved hiPSC-CMs generated from multiple independent hiPSC lines. While the frozen hiPSC-CMs exhibited poorer replating than their freshly-derived counterparts, there was no difference in the proportion of cardiomyocytes retrieved from the mixed population when this was factored in, although for several lines a higher percentage of ventricular-like hiPSC-CMs were recovered following cryopreservation. Furthermore, cryopreserved hiPSC-CMs from one line exhibited longer action potential durations. These results provide evidence that cryopreservation does not compromise the in vitro molecular, physiological and mechanical properties of hiPSC-CMs, though can lead to an enrichment in ventricular myocytes. It also validates this procedure for storing hiPSC-CMs, thereby allowing the same batch of hiPSC-CMs to be used for multiple applications and evaluations.