Taking Heart Development to the Next Level

Current in vitro systems are powerful tools for studying early heart specification but lack the ability to model morphological events. Reporting in this issue of Cell Stem Cell, Rossi et al., 2021Rossi G. Broguiere N. Miyamoto M. Boni A. Guiet R. Girgin M. Kelly R.G. Kwon C. Lutolf M.P. Capturing Cardiogenesis in Gastruloids.Cell Stem Cell. 2021; 28 (Published online November 10, 2020)https://doi.org/10.1016/j.stem.2020.10.013Abstract Full Text Full Text PDF Scopus (39) Google Scholar present a patterned embryonic organoid model (gastruloid) that mimics aspects of early cardiogenesis. Current in vitro systems are powerful tools for studying early heart specification but lack the ability to model morphological events. Reporting in this issue of Cell Stem Cell, Rossi et al., 2021Rossi G. Broguiere N. Miyamoto M. Boni A. Guiet R. Girgin M. Kelly R.G. Kwon C. Lutolf M.P. Capturing Cardiogenesis in Gastruloids.Cell Stem Cell. 2021; 28 (Published online November 10, 2020)https://doi.org/10.1016/j.stem.2020.10.013Abstract Full Text Full Text PDF Scopus (39) Google Scholar present a patterned embryonic organoid model (gastruloid) that mimics aspects of early cardiogenesis. The development of our organs is initiated by dynamic and complex interactions between early embryonic progenitors during gastrulation. Among these, the heart is the first functional organ to emerge but the early stages of cardiogenesis are still poorly understood. Consequently, how early embryonic interactions specify, position, and shape the heart, and how they fail in cardiac developmental disorders, is still unclear. One bottleneck has been the lack of developmental in vitro models allowing rapid testing of hypotheses based on in vivo findings. Early cardiac specification has been successfully modeled using 2D monolayer and 3D embryonic body differentiation approaches (Protze et al., 2019Protze S.I. Lee J.H. Keller G.M. Human Pluripotent Stem Cell-Derived Cardiovascular Cells: From Developmental Biology to Therapeutic Applications.Cell Stem Cell. 2019; 25: 311-327https://doi.org/10.1016/j.stem.2019.07.010Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar). PSC-derived cardiac cell types, aggregates, and engineered constructs have been generated and used to study mechanisms of specification, improve cardiomyocyte maturation, analyze contractions, model disease, and test drugs (Karbassi et al., 2020Karbassi E. Fenix A. Marchiano S. Muraoka N. Nakamura K. Yang X. Murry C.E. Cardiomyocyte maturation: advances in knowledge and implications for regenerative medicine.Nat. Rev. Cardiol. 2020; 17 (10.1038/s41569-019-0331-x): 341-359Crossref PubMed Scopus (123) Google Scholar; Stein et al., 2020Stein J.M. Mummery C.L. Bellin M. Engineered models of the human heart: directions and challenges.Stem Cell Reports. 2020; (S2213-6711(20)30461-6)https://doi.org/10.1016/j.stemcr.2020.11.013Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar). In contrast to specification, development of self-organized organoid-based models of the heart have lagged behind. Key processes of early cardiogenesis such as migration of cardiac progenitors, positioning of the cardiac crescent, heart tube formation, looping, growing of multiple chambers, septation, etc. have not been observed in vitro. In this respect, the cardiac field is trailing behind morphogenetic modeling of other major organs using principles of self-organization (Lancaster and Huch, 2019Lancaster M.A. Huch M. Disease modelling in human organoids.Dis. Model. Mech. 2019; 12 (dmm039347)https://doi.org/10.1242/dmm.039347Crossref PubMed Scopus (124) Google Scholar; Schutgens and Clevers, 2019Schutgens F. Clevers H. Human Organoids: Tools for Understanding Biology and Treating Diseases.Annu. Rev. Pathol. 2019; 15 (2129253558)https://doi.org/10.1146/annurev-pathmechdis-012419-032611Crossref PubMed Scopus (105) Google Scholar). In the current issue of Cell Stem Cell, Rossi et al., 2021Rossi G. Broguiere N. Miyamoto M. Boni A. Guiet R. Girgin M. Kelly R.G. Kwon C. Lutolf M.P. Capturing Cardiogenesis in Gastruloids.Cell Stem Cell. 2021; 28 (Published online November 10, 2020)https://doi.org/10.1016/j.stem.2020.10.013Abstract Full Text Full Text PDF Scopus (39) Google Scholar use self-organizing mouse gastruloids as a model to study the earliest stages of heart development before heart tube formation. The authors present a polarized embryonic organoid system with an anterior beating domain. Single-cell RNA sequencing (scRNA-seq) of a time-course between 96 h and168 h reveals an embryo-like cell type composition, which recapitulates the expected relationships between different lineages. Moreover, they show that the anterior cardiac domain closely recapitulates the spatio-temporal changes in gene expression known to be essential in the course of cardiogenesis ranging from progenitor (Mesp1) over differentiation markers (Nkx2-5) to the eventual expression of functional cardiomyocyte genes (Actn2 and Ryr2). Organ systems usually develop in complex environments strongly defined by crosstalk of neighboring tissues. Due to the embryo-like organization of gastruloids the authors were able to further look into certain aspects of tissue-tissue interactions. Within the observed anterior cardiac domain, the authors identify Flk1+ cells, which later give rise to a vascular-like CD31+ compartment suggesting a co-development of cardiovascular lineages. scRNA-seq data of later stage gastruloids showed expression of a genetic signature indicating endothelial maturation. Early cardiogenesis is characterized by the migration of two cell populations that form the first (FHF) and second (SHF) heart fields, which eventually give rise to the main structures of the heart. FHF progenitors initially migrate anteriorly to form the cardiac crescent, while SHF progenitors contribute to this heart-forming region in a delayed fashion. In their scRNA-seq dataset the authors identify cell populations with transcriptional signatures reminiscent of FHF and SHF. To confirm the presence of these cardiac progenitor populations in the gastruloids, the authors isolate FHF and SHF progenitors based on the expression of the cell surface marker Cxcr4 and show that Gata6+/Cxcr4+ cells express higher levels of SHF markers (Tbx1, Fgf10, and Isl1). In addition, RNAscope analysis reveals that FHF and SHF markers are expressed in mutual exclusive domains. Eventually, using a dual FHF/SHF reporter/tracer mESC line, the authors underline the presence of FHF/SHF progenitors in the anterior domain. According to the authors, the spatial distribution of these cell populations suggests an in-vivo-like fashion. In the developing embryo, the process of cardiogenesis does not occur in isolation, but is instead tightly linked to, for instance, the morphogenetic movements related to the closure of the foregut (Miquerol and Kelly, 2013Miquerol L. Kelly R.G. Organogenesis of the vertebrate heart.Wiley Interdiscip. Rev. Dev. Biol. 2013; 2 (10.1002/wdev.68): 17-29Crossref PubMed Scopus (61) Google Scholar). Modeling processes that depend on close interaction of multiple organ systems is a challenging task. The authors make a first attempt in this regard by investigating potential interactions between cardiomyocytes, anterior endoderm, and endothelial cells in the gastruloid system. This is an important first step and it also corroborates the necessity to control these processes with high reproducibility, especially as we are learning that transcriptional states of cardiac progenitors are more dynamic than previously thought (Tyser et al., 2021Tyser R.C.V. Ibarra-Soria X. McDole K.A. Jayaram S. Godwin J. van den Brand T.A.H. Miranda A.M.A. Scialdone A. Keller P.J. Marioni J.C. Srinivas S. Characterization of a common progenitor pool of the epicardium and myocardium.Science. 2021; 43 (eabb2986)https://doi.org/10.1126/science.abb2986Crossref Scopus (20) Google Scholar). Taken together, Rossi et al. have further developed the gastruloid platform to model the earliest stages of cardiac specification and some aspects of morphogenesis. A big advantage of this system is the possibility to study the dynamic interactions of endodermal and ectodermal tissues with cardiac progenitors as they specify and morph (Moris et al., 2020Moris N. Martinez Arias A. Steventon B. Experimental embryology of gastrulation: pluripotent stem cells as a new model system.Curr. Opin. Genet. Dev. 2020; 64: 78-83Crossref PubMed Scopus (11) Google Scholar). This can be done at a much higher throughput, including the introduction of genetic modifications, and then compared to developing mouse embryos. As with all in vitro models, gastruloids are therefore a strong complement to, but not a substitute for, in vivo studies. Although gastruloids are simpler than embryos, they are still highly complex structures containing all three germ layers and >30 different cell types. They are therefore more suitable for studying the embryo-like context of organogenesis and less so for organ- and cell-type-specific questions and applications. Looking forward, we can expect that also human gastruloids (Moris et al., 2020Moris N. Martinez Arias A. Steventon B. Experimental embryology of gastrulation: pluripotent stem cells as a new model system.Curr. Opin. Genet. Dev. 2020; 64: 78-83Crossref PubMed Scopus (11) Google Scholar) can also be optimized to mimic the earliest stages of heart development and thereby aid in the assessment of associated developmental defects. A more faithful recapitulation of early cardiac structures in gastruloids would be highly complementary to established systems (Protze et al., 2019Protze S.I. Lee J.H. Keller G.M. Human Pluripotent Stem Cell-Derived Cardiovascular Cells: From Developmental Biology to Therapeutic Applications.Cell Stem Cell. 2019; 25: 311-327https://doi.org/10.1016/j.stem.2019.07.010Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, Stein et al., 2020Stein J.M. Mummery C.L. Bellin M. Engineered models of the human heart: directions and challenges.Stem Cell Reports. 2020; (S2213-6711(20)30461-6)https://doi.org/10.1016/j.stemcr.2020.11.013Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar) and emerging human cardiac-specific self-organizing “cardioids” (Hofbauer et al., 2020Hofbauer P. Jahnel S. Papai N. Giesshammer M. Penc M. Tavernini K. Grdseloff N. Meledeth C. Deyett A. Schmidt C. et al.Cardioids reveal self-organizing principles of human cardiogenesis.bioRxiv. 2020; 146 (dev.174961–50)https://doi.org/10.1101/2020.07.06.189431Crossref Google Scholar). Overall, the cardiac field will therefore benefit from a wider arsenal of in vitro diferentiation platforms. Depending on the question and suitability, these can then be specifically deployed together with model organisms. A greater diversity of well-established models in combination with cutting edge technologies will allow us to address the outstanding mysteries of developmental biology and face the challenge of growing medical needs due to cardiovascular disease. The Institute for Molecular Biotechnology (IMBA) filed a patent application ( EP20164637.9 ) with P. Hofbauer, S. Jahnel, N. Papai, and S. Mendjan named as inventors. The application covers the methods for generation of different types of cardiac organoids (cardioids) from hPSCs. Capturing Cardiogenesis in GastruloidsRossi et al.Cell Stem CellNovember 10, 2020In BriefRossi et al. describe an embryonic organoid model that mimics the early development of the heart, from the generation of cardiovascular precursor cells to the specification of the first and second heart fields. These axially patterned organoids support the formation of cardiac crescent and early cardiac tube-like structures while reproducing the cell diversity and tissue-tissue interactions typical of embryos. Full-Text PDF