Pulling on my heartstrings: mechanotransduction in cardiac development and function

3D printing of thick myocardial tissue constructs with anisotropic myofibers and perfusable vascular channels

Engineering of myocardial tissues has become a promising therapeutic strategy for treating myocardial infarction (MI). However, a significant challenge remains in generating clinically relevant myocardial tissues that possess native microstructural characteristics and fulfill the requirements for implantation within the human body. In this study, a thick 3D myocardial construct with anisotropic myofibers and perfusable branched vascular channels is created with clinically relevant dimensions using a customized beam-scanning stereolithography printing technique. To obtain tissue-specific matrix niches, a decellularized extracellular matrix microfiber-reinforced gelatin-based bioink is developed. The bioink plays a crucial role in facilitating the precise manufacturing of a hierarchical microstructure, enabling us to better replicate the physiological characteristics of the native myocardial tissue matrix in terms of structure, biomechanics, and bioactivity. Through the integration of the tailored bioink with our printing method, we demonstrate a biomimetic architecture, appropriate biomechanical properties, vascularization, and improved functionality of induced pluripotent stem cell-derived cardiomyocytes in the thick tissue construct in vitro. This work not only offers a novel and effective means to generate biomimetic heart tissue in vitro for the treatment of MI, but also introduces a potential methodology for creating clinically relevant tissue products to aid in other complex tissue/organ regeneration and disease model applications.

Functional microvascularization of human myocardium in vitro

In this study, we report static and perfused models of human myocardial-microvascular interaction. In static culture, we observe distinct regulation of electrophysiology of human induced pluripotent stem cell derived-cardiomyocytes (hiPSC-CMs) in co-culture with human cardiac microvascular endothelial cells (hCMVECs) and human left ventricular fibroblasts (hLVFBs), including modification of beating rate, action potential, calcium handling, and pro-arrhythmic substrate. Within a heart-on-a-chip model, we subject this three-dimensional (3D) co-culture to microfluidic perfusion and vasculogenic growth factors to induce spontaneous assembly of perfusable myocardial microvasculature. Live imaging of red blood cells within myocardial microvasculature reveals pulsatile flow generated by beating hiPSC-CMs. This study therefore demonstrates a functionally vascularized in vitro model of human myocardium with widespread potential applications in basic and translational research.

Effect of Blood Flow on Cardiac Morphogenesis and Formation of Congenital Heart Defects

Congenital heart disease (CHD) affects about 1 in 100 newborns and its causes are multifactorial. In the embryo, blood flow within the heart and vasculature is essential for proper heart development, with abnormal blood flow leading to CHD. Here, we discuss how blood flow (hemodynamics) affects heart development from embryonic to fetal stages, and how abnormal blood flow solely can lead to CHD. We emphasize studies performed using avian models of heart development, because those models allow for hemodynamic interventions, in vivo imaging, and follow up, while they closely recapitulate heart defects observed in humans. We conclude with recommendations on investigations that must be performed to bridge the gaps in understanding how blood flow alone, or together with other factors, contributes to CHD.

Cardiac forces regulate zebrafish heart valve delamination by modulating Nfat signaling

In the clinic, most cases of congenital heart valve defects are thought to arise through errors that occur after the endothelial–mesenchymal transition (EndoMT) stage of valve development. Although mechanical forces caused by heartbeat are essential modulators of cardiovascular development, their role in these later developmental events is poorly understood. To address this question, we used the zebrafish superior atrioventricular valve (AV) as a model. We found that cellularized cushions of the superior atrioventricular canal (AVC) morph into valve leaflets via mesenchymal–endothelial transition (MEndoT) and tissue sheet delamination. Defects in delamination result in thickened, hyperplastic valves, and reduced heart function. Mechanical, chemical, and genetic perturbation of cardiac forces showed that mechanical stimuli are important regulators of valve delamination. Mechanistically, we show that forces modulate Nfatc activity to control delamination. Together, our results establish the cellular and molecular signature of cardiac valve delamination in vivo and demonstrate the continuous regulatory role of mechanical forces and blood flow during valve formation.

Why do developing zebrafish atrioventricular heart valves become hyperplastic under certain hemodynamic conditions? This study suggests that part of the answer lies in how the mechanosensitive Nfat pathway regulates the valve mesenchymal-to-endothelial transition.

Patient-Specific 3D Bioprinted Models of Developing Human Heart

The heart is the first organ to develop in the human embryo through a series of complex chronological processes, many of which critically rely on the interplay between cells and the dynamic microenvironment. Tight spatiotemporal regulation of these interactions is key in heart development and diseases. Due to suboptimal experimental models, however, little is known about the role of microenvironmental cues in the heart development. This study investigates the use of 3D bioprinting and perfusion bioreactor technologies to create bioartificial constructs that can serve as high-fidelity models of the developing human heart. Bioprinted hydrogel-based, anatomically accurate models of the human embryonic heart tube (e-HT, day 22) and fetal left ventricle (f-LV, week 33) are perfused and analyzed both computationally and experimentally using ultrasound and magnetic resonance imaging. Results demonstrate comparable flow hemodynamic patterns within the 3D space. We demonstrate endothelial cell growth and function within the bioprinted e-HT and f-LV constructs, which varied significantly in varying cardiac geometries and flow. This study introduces the first generation of anatomically accurate, 3D functional models of developing human heart. This platform enables precise tuning of microenvironmental factors, such as flow and geometry, thus allowing the study of normal developmental processes and underlying diseases.

Contractility of Induced Pluripotent Stem Cell-Cardiomyocytes With an MYH6 Head Domain Variant Associated With Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a clinically and anatomically severe form of congenital heart disease; however, its etiology remains largely unknown. We previously demonstrated that genetic variants in the MYH6 gene are significantly associated with HLHS. Additionally, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from an HLHS-affected family trio (affected parent, unaffected parent, affected proband) carrying an MYH6-R443P head domain variant demonstrated dysmorphic sarcomere structure and increased compensatory MYH7 expression. Analysis of iPSC-CMs derived from the HLHS trio revealed that only beta myosin heavy chain expression was observed in CMs carrying the MYH6-R443P variant after differentiation day 15 (D15). Functional assessments performed between D20-D23 revealed that MYH6-R443P variant CMs contracted more slowly (40 ± 2 vs. 47 ± 2 contractions/min, P < 0.05), shortened less (5.6 ± 0.5 vs. 8.1 ± 0.7% of cell length, P < 0.05), and exhibited slower shortening rates (19.9 ± 1.7 vs. 28.1 ± 2.5 μm/s, P < 0.05) and relaxation rates (11.0 ± 0.9 vs. 19.7 ± 2.0 μm/s, P < 0.05). Treatment with isoproterenol had no effect on iPSC-CM mechanics. Using CRISPR/Cas9 gene editing technology, introduction of the R443P variant into the unaffected parent's iPSCs recapitulated the phenotype of the proband's iPSC-CMs, and conversely, correction of the R443P variant in the proband's iPSCs rescued the cardiomyogenic differentiation, sarcomere organization, slower contraction (P < 0.05) and decreased velocity phenotypes (P < 0.0001). This is the first report to identify that cardiac tissues from HLHS patients with MYH6 variants can exhibit sarcomere disorganization in atrial but not ventricular tissues. This new discovery was not unexpected, since MYH6 is expressed predominantly in the postnatal atria in humans. These findings demonstrate the feasibility of employing patient-derived iPSC-CMs, in combination with patient cardiac tissues, to gain mechanistic insight into how genetic variants can lead to HLHS. Results from this study suggest that decreased contractility of CMs due to sarcomere disorganization in the atria may effect hemodynamic changes preventing development of a normal left ventricle.

The guidance receptor plexin D1 is a mechanosensor in endothelial cells

Shear stress on arteries produced by blood flow is important for vascular development and homeostasis but can also initiate atherosclerosis1. Endothelial cells that line the vasculature use molecular mechanosensors to directly detect shear stress profiles that will ultimately lead to atheroprotective or atherogenic responses2. Plexins are key cell-surface receptors of the semaphorin family of cell-guidance signalling proteins and can regulate cellular patterning by modulating the cytoskeleton and focal adhesion structures3-5. However, a role for plexin proteins in mechanotransduction has not been examined. Here we show that plexin D1 (PLXND1) has a role in mechanosensation and mechanically induced disease pathogenesis. PLXND1 is required for the response of endothelial cells to shear stress in vitro and in vivo and regulates the site-specific distribution of atherosclerotic lesions. In endothelial cells, PLXND1 is a direct force sensor and forms a mechanocomplex with neuropilin-1 and VEGFR2 that is necessary and sufficient for conferring mechanosensitivity upstream of the junctional complex and integrins. PLXND1 achieves its binary functions as either a ligand or a force receptor by adopting two distinct molecular conformations. Our results establish a previously undescribed mechanosensor in endothelial cells that regulates cardiovascular pathophysiology, and provide a mechanism by which a single receptor can exhibit a binary biochemical nature.

Mechanotransduction and Uterine Blood Flow in Preeclampsia: The Role of Mechanosensing Piezo 1 Ion Channels

There is a large increase in uterine arterial blood flow during normal pregnancy. Structural and cellular adjustments occur in the uterine vasculature during pregnancy to accommodate this increased blood flow through a complex adaptive process that is dependent on multiple coordinated and interactive influences and this process is known as "vascular remodeling." The etiology of preeclampsia involves aberrant placentation and vascular remodeling leading to reduced uteroplacental perfusion. The placental ischemia leads to development of hypertension and proteinuria in the mother, intrauterine growth restriction, and perinatal death in the fetus. However, the underlying source of the deficient vascular remodeling and the subsequent development of preeclampsia remain to be fully understood. Mechanoreceptors in the vascular system convert mechanical force (shear stress) to biochemical signals and feedback mechanisms. This review focuses on the Piezo 1 channel, a mechanosensitive channel that is sensitive to shear stress in the endothelium; it induces Ca2+ entry which is linked to endothelial nitric oxide synthase (eNOS) activation as the mechanoreceptor responsible for uterine vascular dilatation during pregnancy. Here we describe the downstream signaling pathways involved in this process and the possibility of a deficiency in expression of Piezo 1 in preeclampsia leading to the abnormal vascular dysfunction responsible for the pathophysiology of the disease. The Piezo 1 ion channel is expressed in the endothelium and vascular smooth muscle cells (VSMCs) of small-diameter arteries. It plays a role in the structural remodeling of arteries and is involved in mechanotransduction of hemodynamic shear stress by endothelial cells (ECs).

Non-syndromic Mitral Valve Dysplasia Mutation Changes the Force Resilience and Interaction of Human Filamin A

Filamin A (FLNa), expressed in endocardial endothelia during fetal valve morphogenesis, is key in cardiac development. Missense mutations in FLNa cause non-syndromic mitral valve dysplasia (FLNA-MVD). Here, we aimed to reveal the currently unknown underlying molecular mechanism behind FLNA-MVD caused by the FLNa P637Q mutation. The solved crystal structure of the FLNa3-5 P637Q revealed that this mutation causes only minor structural changes close to mutation site. These changes were observed to significantly affect FLNa's ability to transmit cellular force and to interact with its binding partner. The performed steered molecular dynamics simulations showed that significantly lower forces are needed to split domains 4 and 5 in FLNA-MVD than with wild-type FLNa. The P637Q mutation was also observed to interfere with FLNa's interactions with the protein tyrosine phosphatase PTPN12. Our results provide a crucial step toward understanding the molecular bases behind FLNA-MVD, which is critical for the development of drug-based therapeutics.

Mechanotransduction in Blood and Lymphatic Vascular Development and Disease

The blood and lymphatic vasculatures are hierarchical networks of vessels, which constantly transport fluids and, therefore, are exposed to a variety of mechanical forces. Considering the role of mechanotransduction is key for fully understanding how these vascular systems develop, function, and how vascular pathologies evolve. During embryonic development, for example, initiation of blood flow is essential for early vascular remodeling, and increased interstitial fluid pressure as well as initiation of lymph flow is needed for proper development and maturation of the lymphatic vasculature. In this review, we introduce specific mechanical forces that affect both the blood and lymphatic vasculatures, including longitudinal and circumferential stretch, as well as shear stress. In addition, we provide an overview of the role of mechanotransduction during atherosclerosis and secondary lymphedema, which both trigger tissue fibrosis.

Transcriptome Profiling Identifies Ribosome Biogenesis as a Target of Alcohol Teratogenicity and Vulnerability during Early Embryogenesis

Fetal alcohol spectrum disorder (FASD) is a leading cause of neurodevelopmental disability. Individuals with FASD may exhibit a characteristic facial appearance that has diagnostic utility. The mechanism by which alcohol disrupts craniofacial development is incompletely understood, as are the genetic factors that can modify individual alcohol vulnerability. Using an established avian model, we characterized the cranial transcriptome in response to alcohol to inform the mechanism underlying these cells’ vulnerability. Gallus gallus embryos having 3–6 somites were exposed to 52 mM alcohol and the cranial transcriptomes were sequenced thereafter. A total of 3422 genes had significantly differential expression. The KEGG pathways with the greatest enrichment of differentially expressed gene clusters were Ribosome (P = 1.2 x 10⁻¹⁷, 67 genes), Oxidative Phosphorylation (P = 4.8 x 10⁻¹², 60 genes), RNA Polymerase (P = 2.2 x 10⁻³, 15 genes) and Spliceosome (P = 2.6 x 10⁻², 39 genes). The preponderance of transcripts in these pathways were repressed in response to alcohol. These same gene clusters also had the greatest altered representation in our previous comparison of neural crest populations having differential vulnerability to alcohol-induced apoptosis. Comparison of differentially expressed genes in alcohol-exposed (3422) and untreated, alcohol-vulnerable (1201) transcriptomes identified 525 overlapping genes of which 257 have the same direction of transcriptional change. These included 36 ribosomal, 25 oxidative phosphorylation and 7 spliceosome genes. Using a functional approach in zebrafish, partial knockdown of ribosomal proteins zrpl11, zrpl5a, and zrps3a individually heightened vulnerability to alcohol-induced craniofacial deficits and increased apoptosis. In humans, haploinsufficiency of several of the identified ribosomal proteins are causative in craniofacial dysmorphologies such as Treacher Collins Syndrome and Diamond-Blackfan Anemia. This work suggests ribosome biogenesis may be a novel target mediating alcohol’s damage to developing neural crest. Our findings are consistent with observations that gene-environment interactions contribute to vulnerability in FASD.

Fluid mechanics as a driver of tissue-scale mechanical signaling in organogenesis

PURPOSE OF REVIEW

Organogenesis is the process during development by which cells self-assemble into complex, multi-scale tissues. Whereas significant focus and research effort has demonstrated the importance of solid mechanics in organogenesis, less attention has been given to the fluid forces that provide mechanical cues over tissue length scales.

RECENT FINDINGS

Fluid motion and pressure is capable of creating spatial gradients of forces acting on cells, thus eliciting distinct and localized signaling patterns essential for proper organ formation. Understanding the multi-scale nature of the mechanics is critically important to decipher how mechanical signals sculpt developing organs.

SUMMARY

This review outlines various mechanisms by which tissues generate, regulate, and sense fluid forces and highlights the impact of these forces and mechanisms in case studies of normal and pathological development.