Recreating the Cardiac Microenvironment in Pluripotent Stem Cell Models of Human Physiology and Disease

Abstract

The advent of human pluripotent stem cell (hPSC) biology has opened unprecedented opportunities for the use of tissue engineering to generate human cardiac tissue for in vitro study. Engineering cardiac constructs that recapitulate human development and disease requires faithful recreation of the cardiac niche in vitro. Here we discuss recent progress in translating the in vivo cardiac microenvironment into PSC models of the human heart. We review three key physiologic features required to recreate the cardiac niche and facilitate normal cardiac differentiation and maturation: the biochemical, biophysical, and bioelectrical signaling cues. Finally, we discuss key barriers that must be overcome to fulfill the promise of stem cell biology in preclinical applications and ultimately in clinical practice.

Keywords: cardiac myocytes; cardiac niche; cellular microenvironment; disease modeling; pluripotent stem cells; tissue engineering.

Figure 1, Key Figure. Recreating the Cardiac Niche In Vitro

The cardiac niche constitutes a cardiogenic microenvironment that controls cardiac development, function and disease. It harbors extrinsic cues that exhibit an interdependence between bioelectrical, biochemical and biophysical signals. These microenvironmental cues control cardiac myocyte biology and are interconnected by cell-cell and cell-ECM interactions. Examples of approaches to recreate this complex network of signals in vitro in for recapitulating the cardiac niche into cellular models of human heart development and disease.are shown within colored circles.

Figure 2. Cell-Cell Interactions in the Myocardium

Cardiac myocytes directly interact with each other and with adjacent cell populations to receive signaling cues critical for normal heart development. Two important cell populations are epicardial (depicted in blue) and endocardial (depicted in red) cells. Fibroblasts and vascular cells have been omitted for visual clarity. Biochemical signaling between cells centers on transmembrane ligand-receptor interactions as well as diffusible growth factors that regulate intracellular signaling pathways. Intercalated discs at the longitudinal borders of cardiac myocytes connect the myocardium into a functional syncytium. Biophysical signaling relies on fascia adherens junctions and desmosomes to sense and transmit mechanical forces in a bi-directional and longitudinal way between neighboring cells. Fascia adherens junctions consist of cadherins that link intracellular actin filaments, while desmosomes link desmosomal cadherins to intracellular intermediate filaments. Both anchors form attachment sites between adjacent cells that allow cytoskeletal remodeling in response to intercellular mechanical stress. Bioelectrical signaling between cardiac myocytes is mediated by gap junctions that transmit electrical impulses between cells.

Figure 3. Cell-ECM Interactions in the Myocardium

Cardiac cells are surrounded by the extracellular matrix (ECM). It is the anchoring source that provides structural as well as molecular support to cardiac cells. The ECM is composed of a wide variety of micro-and macromolecules including fibrous proteins, proteoglycans and incorporates growth factors, cytokines and chemokines. As such, the ECM provides important biochemical and biophysical signaling cues to cardiac myocytes. The balance between synthesis and degradation of the ECM is maintained by matrix metalloproteinases. Cardiac myocytes connect to the ECM at costamere complexes. Costameres are mechanotransducing complexes that mainly link Z-discs laterally to the ECM and consist of integrin and dystrophin-glycoprotein complexes. Integrins play a critical role in forming focal adhesions in vitro that couple actin filaments through linker proteins to the ECM. At costameres the incoming mechanical load regulates biochemical signaling pathways. Cytoskeletal organization and cell geometry are significantly influenced by how the ECM is presented to the cell. As a result dysregulation of the ECM may lead to cytoskeletal disarray and aberrant cell shape. The viscoelastic properties of the ECM also influence the contractile properties of the myocardium. In addition, the intracavitary pressure of the native heart is transmitted through the ECM and acts as a major determinant of the biomechanical forces exerted on myocardial cells. Aspects of cell-cell interactions have been deemphasized to reduce visual clutter.