Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) resemble fetal rather than adult myocardium, limiting their use in disease modeling, drug testing, and cardiac regenerative therapy. Varzideh et al. (Biomaterials, 2019) showed that heterotopic transplantation of human embryonic stem cell-derived cardiac organoids into immunodeficient mice induces dense host-derived neovascularization and marked transcriptional and ultrastructural maturation of graft cardiomyocytes within four weeks, but did not measure graft force or in vivo electrophysiology and did not identify the responsible vascular cues. Subsequent literature shows that in vitro organoid work has identified endothelial-derived mechanisms of maturation (laminin α5, PDGFRβ signaling), mouse developmental studies have identified angiocrines that regulate cardiomyocyte maturation, and transplant studies have demonstrated that grafted cardiomyocytes actively contribute force and that pre-vascularized grafts function better in vivo — yet no study has quantified adult-equivalent contractile force or long-term electrical stability of transplanted organoid cardiomyocytes, nor causally established which host-vessel-derived angiocrine factors and cellular interactions are necessary and sufficient for graft maturation. The problem remains open.

Background

Cardiomyocytes (CMs) derived from human pluripotent stem cells (hPSCs, either embryonic or induced pluripotent) hold promise for cardiac disease modeling, drug screening, and regenerative therapy, but they consistently emerge immature: compared with adult ventricular myocardium they show disorganized sarcomeres (the contractile units of striated muscle), incomplete T-tubules (membrane invaginations that synchronize calcium release across the cell), immature excitation-contraction coupling, fetal-like ion-channel expression, and low contractile force. Because this immaturity is a core obstacle to any application in which grafted cells must supply meaningful mechanical work, many maturation strategies have been explored — prolonged culture, electrical and mechanical stimulation, biochemical cues, and three-dimensional engineered tissues — with only partial success.

Cardiac organoids are self-organizing three-dimensional aggregates of hPSC-derived cardiac cells (with or without supporting cell types) that partially recapitulate embryonic heart microarchitecture. A long-standing hypothesis in the field is that providing a vascular niche — either vascular cells in vitro or host vessel ingrowth after transplantation — pushes hPSC-CMs toward maturity. Varzideh et al. (2019) tested the in vivo version of this hypothesis by transplanting human embryonic stem cell-derived cardiac organoids heterotopically (to a non-cardiac site, the peritoneal cavity) of immunodeficient nude mice. Four weeks after implantation, the grafts were densely infiltrated by host-derived blood vessels (about 139 vessels/mm² versus about 16 in grafts of dissociated cardiomyocytes), and the graft cardiomyocytes showed large transcriptional upregulation of contractile proteins (e.g. beta-myosin heavy chain, myosin light chain 2v), calcium-handling proteins (RYR2, SERCA2a), and ion-channel genes, several reaching adult-heart reference levels, together with ultrastructural maturation. Crucially, the study did not measure force generation of the grafted cells and did not record graft electrophysiology in vivo; its own discussion lists these as unresolved issues.

Adjacent results frame what is at stake. Engineered heart tissues and 'Cardiopatches' (hPSC-CMs cast in hydrogel strips rather than self-organized organoids) can reach near-adult contractile stress in vitro, engraft on rat hearts, and maintain electrical function without increasing arrhythmias (Shadrin et al., 2017). Co-culture with vascular cells improves human cardiac organoid maturation and contractile force in vitro, with endothelial-derived laminin alpha 5 (LAMA5) and platelet-derived growth factor receptor beta (PDGFRβ) paracrine signaling identified as mediating mechanisms (Voges et al., 2023). In mouse development, endocardial and endothelial cells secrete 'angiocrines' — paracrine factors such as Col15a1, Tgfbi, Igfbp3, Isg15, and Adm — that regulate cardiomyocyte proliferation and maturation (Rhee et al., 2021). What is missing is the causal, quantitative link in the transplant setting itself: whether host-derived vascularization and its paracrine cues bring grafted human organoid cardiomyocytes to adult-equivalent contractile force and stable electrical function, and through which specific factors and cellular interactions this occurs.

Problem Statement

Determine whether host-derived neovascularization drives maturation of transplanted human cardiac organoid cardiomyocytes to adult-equivalent function, and by what mechanism, in the transplant setting established by Varzideh et al. (2019). Two jointly linked endpoints define one objective — establishing the vascular mechanism and the functional consequence of in vivo maturation of grafted human cardiac organoids:

  1. Functional endpoint: quantify the contractile force (or stress) and the electrical behavior of grafted organoid cardiomyocytes in vivo — or ex vivo on explanted grafts — and compare them with adult myocardium, with observation extending beyond the four-week endpoint of the source study so that long-term electrical stability (stable conduction and absence of arrhythmogenic activity of the graft) can be judged.

  2. Mechanistic endpoint: identify which angiocrine factors and cellular interactions from host-recruited vessels (and other host tissues) are necessary and sufficient for the transcriptional, structural, and functional maturation programs of graft cardiomyocytes — for example by comparing vascularized with non-vascularized grafts, blocking or genetically removing candidate factors in vivo, or rescuing maturation with factor add-back in otherwise identical grafts.

References

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