Quantifying Microvascular Structure in Healthy and Infarcted Rat Hearts Using Optical Coherence Tomography Angiography.

Myocardial infarction (MI) is a life-threatening medical emergency resulting in coronary microvascular dysregulation and heart muscle damage. One of the primary characteristics of MI is capillary loss, which plays a significant role in the progression of this cardiovascular condition. In this study, we utilized optical coherence tomography angiography (OCTA) to image coronary microcirculation in fixed rat hearts, aiming to analyze coronary microvascular impairment post-infarction. Various angiographic metrics are presented to quantify vascular features, including the vessel area density, vessel complexity index, vessel tortuosity index, and flow impairment. Pathological differences identified from OCTA analysis are corroborated with histological analysis. The quantitative assessments reveal a significant decrease in microvascular density in the capillary-sized vessels and an enlargement for the arteriole/venule-sized vessels. Further, microvascular tortuosity and complexity exhibit an increase after myocardial infarction. The results underscore the feasibility of using OCTA to offer qualitative microvascular details and quantitative metrics, providing insights into coronary vascular network remodeling during disease progression and response to therapy.

Genetically Encoded XTEN-based Hydrogels with Tunable Viscoelasticity and Biodegradability for Injectable Cell Therapies.

While direct cell transplantation holds great promise in treating many debilitating diseases, poor cell survival and engraftment following injection have limited effective clinical translation. Though injectable biomaterials offer protection against membrane-damaging extensional flow and supply a supportive 3D environment in vivo that ultimately improves cell retention and therapeutic costs, most are created from synthetic or naturally harvested polymers that are immunogenic and/or chemically ill-defined. This work presents a shear-thinning and self-healing telechelic recombinant protein-based hydrogel designed around XTEN - a well-expressible, non-immunogenic, and intrinsically disordered polypeptide previously evolved as a genetically encoded alternative to PEGylation to "eXTENd" the in vivo half-life of fused protein therapeutics. By flanking XTEN with self-associating coil domains derived from cartilage oligomeric matrix protein, single-component physically crosslinked hydrogels exhibiting rapid shear thinning and self-healing through homopentameric coiled-coil bundling are formed. Individual and combined point mutations that variably stabilize coil association enables a straightforward method to genetically program material viscoelasticity and biodegradability. Finally, these materials protect and sustain viability of encapsulated human fibroblasts, hepatocytes, embryonic kidney (HEK), and embryonic stem-cell-derived cardiomyocytes (hESC-CMs) through culture, injection, and transcutaneous implantation in mice. These injectable XTEN-based hydrogels show promise for both in vitro cell culture and in vivo cell transplantation applications.

Multiplexed Functional Assessments of MYH7 Variants in Human Cardiomyocytes.

BACKGROUND: Pathogenic autosomal-dominant missense variants in MYH7 (myosin heavy chain 7), which encodes the sarcomeric protein (ß-MHC [beta myosin heavy chain]) expressed in cardiac and skeletal myocytes, are a leading cause of hypertrophic cardiomyopathy and are clinically actionable. However, ≈75% of MYH7 missense variants are of unknown significance. While human-induced pluripotent stem cells (hiPSCs) can be differentiated into cardiomyocytes to enable the interrogation of MYH7 variant effect in a disease-relevant context, deep mutational scanning has not been executed using diploid hiPSC derivates due to low hiPSC gene-editing efficiency. Moreover, multiplexable phenotypes enabling deep mutational scanning of MYH7 variant hiPSC-derived cardiomyocytes are unknown. METHODS: To overcome these obstacles, we used CRISPRa On-Target Editing Retrieval enrichment to generate an hiPSC library containing 113 MYH7 codon variants suitable for deep mutational scanning. We first established that ß-MHC protein loss occurs in a hypertrophic cardiomyopathy human heart with a pathogenic MYH7 variant. We then differentiated the MYH7 missense variant hiPSC library to cardiomyocytes for multiplexed assessment of ß-MHC variant abundance by massively parallel sequencing and hiPSC-derived cardiomyocyte survival. RESULTS: Both the multiplexed assessment of ß-MHC abundance and hiPSC-derived cardiomyocyte survival accurately segregated all known pathogenic variants from synonymous variants. Functional data were generated for 4 variants of unknown significance and 58 additional MYH7 missense variants not yet detected in patients. CONCLUSIONS: This study leveraged hiPSC differentiation into disease-relevant cardiomyocytes to enable multiplexed assessments of MYH7 missense variants for the first time. Phenotyping strategies used here enable the application of deep mutational scanning to clinically actionable genes, which should reduce the burden of variants of unknown significance on patients and clinicians.

Targeted CRISPR activation is functional in engineered human pluripotent stem cells but undergoes silencing after differentiation into cardiomyocytes and endothelium.

Human induced pluripotent stem cells (hiPSCs) offer opportunities to study human biology where primary cell types are limited. CRISPR technology allows forward genetic screens using engineered Cas9-expressing cells. Here, we sought to generate a CRISPR activation (CRISPRa) hiPSC line to activate endogenous genes during pluripotency and differentiation. We first targeted catalytically inactive Cas9 fused to VP64, p65 and Rta activators (dCas9-VPR) regulated by the constitutive CAG promoter to the AAVS1 safe harbor site. These CRISPRa hiPSC lines effectively activate target genes in pluripotency, however the dCas9-VPR transgene expression is silenced after differentiation into cardiomyocytes and endothelial cells. To understand this silencing, we systematically tested different safe harbor sites and different promoters. Targeting to safe harbor sites hROSA26 and CLYBL loci also yielded hiPSCs that expressed dCas9-VPR in pluripotency but silenced during differentiation. Muscle-specific regulatory cassettes, derived from cardiac troponin T or muscle creatine kinase promoters, were also silent after differentiation when dCas9-VPR was introduced. In contrast, in cell lines where the dCas9-VPR sequence was replaced with cDNAs encoding fluorescent proteins, expression persisted during differentiation in all loci and with all promoters. Promoter DNA was hypermethylated in CRISPRa-engineered lines, and demethylation with 5-azacytidine enhanced dCas9-VPR gene expression. In summary, the dCas9-VPR cDNA is readily expressed from multiple loci during pluripotency but induces silencing in a locus- and promoter-independent manner during differentiation to mesoderm derivatives. Researchers intending to use this CRISPRa strategy during stem cell differentiation should pilot their system to ensure it remains active in their population of interest.

Stacking thick perfusable human microvascular grafts enables dense vascularity and rapid integration into infarcted rat hearts.

Fabrication of large-scale engineered tissues requires extensive vascularization to support tissue survival and function. Here, we report a modular fabrication approach, by stacking of patterned collagen membranes, to generate thick (2 mm and beyond), large, three-dimensional, perfusable networks of endothelialized vasculature. In vitro, these perfusable vascular networks exhibit remodeling and evenly distributed perfusion among layers, while maintaining their patterned, open-lumen architecture. Compared to non-perfusable, self-assembled vasculature, constructs with perfusable vasculature demonstrated increased gene expression indicative of vascular development and angiogenesis. Upon implantation onto infarcted rat hearts, perfusable vascular networks attain greater host vascular integration than self-assembled controls, indicated by 2.5-fold greater perfused vascular density measured by histological analysis and 5-fold greater perfusion rate measured by optical microangiography. Together, the success of fabricating thick, perfusable tissues with dense vascularity and rapid anastomoses represents an important step forward for vascular bioengineering, and paves the way towards more complex, large scale, highly metabolic engineered tissues.

Cell based dATP delivery as a therapy for chronic heart failure.

Transplanted human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) improve ventricular performance when delivered acutely post-myocardial infarction but are ineffective in chronic myocardial infarction/heart failure. 2'-deoxy-ATP (dATP) activates cardiac myosin and potently increases contractility. Here we engineered hPSC-CMs to overexpress ribonucleotide reductase, the enzyme controlling dATP production. In vivo, dATP-producing CMs formed new myocardium that transferred dATP to host cardiomyocytes via gap junctions, increasing their dATP levels. Strikingly, when transplanted into chronically infarcted hearts, dATP-producing grafts increased left ventricular function, whereas heart failure worsened with wild-type grafts or vehicle injections. dATP-donor cells recipients had greater voluntary exercise, improved cardiac metabolism, reduced pulmonary congestion and pathological cardiac hypertrophy, and improved survival. This combination of remuscularization plus enhanced host contractility offers a novel approach to treating the chronically failing heart.

Gene editing to prevent ventricular arrhythmias associated with cardiomyocyte cell therapy.

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) offer a promising cell-based therapy for myocardial infarction. However, the presence of transitory ventricular arrhythmias, termed engraftment arrhythmias (EAs), hampers clinical applications. We hypothesized that EA results from pacemaker-like activity of hPSC-CMs associated with their developmental immaturity. We characterized ion channel expression patterns during maturation of transplanted hPSC-CMs and used pharmacology and genome editing to identify those responsible for automaticity in vitro. Multiple engineered cell lines were then transplanted in vivo into uninjured porcine hearts. Abolishing depolarization-associated genes HCN4, CACNA1H, and SLC8A1, along with overexpressing hyperpolarization-associated KCNJ2, creates hPSC-CMs that lack automaticity but contract when externally stimulated. When transplanted in vivo, these cells engrafted and coupled electromechanically with host cardiomyocytes without causing sustained EAs. This study supports the hypothesis that the immature electrophysiological prolife of hPSC-CMs mechanistically underlies EA. Thus, targeting automaticity should improve the safety profile of hPSC-CMs for cardiac remuscularization.

Graft-host coupling changes can lead to engraftment arrhythmia: a computational study.

After myocardial infarction (MI), a significant portion of heart muscle is replaced with scar tissue, progressively leading to heart failure. Human pluripotent stem cell-derived cardiomyocytes (hPSC-CM) offer a promising option for improving cardiac function after MI. However, hPSC-CM transplantation can lead to engraftment arrhythmia (EA). EA is a transient phenomenon arising shortly after transplantation then spontaneously resolving after a few weeks. The underlying mechanism of EA is unknown. We hypothesize that EA may be explained partially by time-varying, spatially heterogeneous, graft-host electrical coupling. Here, we created computational slice models derived from histological images that reflect different configuration of grafts in the infarcted ventricle. We ran simulations with varying degrees of connection imposed upon the graft-host perimeter to assess how heterogeneous electrical coupling affected EA with non-conductive scar, slow-conducting scar and scar replaced by host myocardium. We also quantified the effect of variation in intrinsic graft conductivity. Susceptibility to EA initially increased and subsequently decreased with increasing graft-host coupling, suggesting the waxing and waning of EA is regulated by progressive increases in graft-host coupling. Different spatial distributions of graft, host and scar yielded markedly different susceptibility curves. Computationally replacing non-conductive scar with host myocardium or slow-conducting scar, and increasing intrinsic graft conductivity both demonstrated potential means to blunt EA vulnerability. These data show how graft location, especially relative to scar, along with its dynamic electrical coupling to host, can influence EA burden; moreover, they offer a rational base for further studies aimed to define the optimal delivery of hPSC-CM injection. KEY POINTS: Human pluripotent stem cell-derived cardiomyocytes (hPSC-CM) hold great cardiac regenerative potential but can also cause engraftment arrhythmias (EA). Spatiotemporal evolution in the pattern of electrical coupling between injected hPSC-CMs and surrounding host myocardium may explain the dynamics of EA observed in large animal models. We conducted simulations in histology-derived 2D slice computational models to assess the effects of heterogeneous graft-host electrical coupling on EA propensity, with or without scar tissue. Our findings suggest spatiotemporally heterogeneous graft-host coupling can create an electrophysiological milieu that favours graft-initiated host excitation, a surrogate metric of EA susceptibility. Removing scar from our models reduced but did not abolish the propensity for this phenomenon. Conversely, reduced intra-graft electrical connectedness increased the incidence of graft-initiated host excitation. The computational framework created for this study can be used to generate new hypotheses, targeted delivery of hPSC-CMs.

CRaTER enrichment for on-target gene editing enables generation of variant libraries in hiPSCs.

Standard transgenic cell line generation requires screening 100-1000s of colonies to isolate correctly edited cells. We describe CRISPRa On-Target Editing Retrieval (CRaTER) which enriches for cells with on-target knock-in of a cDNA-fluorescent reporter transgene by transient activation of the targeted locus followed by flow sorting to recover edited cells. We show CRaTER recovers rare cells with heterozygous, biallelic-editing of the transcriptionally-inactive MYH7 locus in human induced pluripotent stem cells (hiPSCs), enriching on average 25-fold compared to standard antibiotic selection. We leveraged CRaTER to enrich for heterozygous knock-in of a library of variants in MYH7, a gene in which missense mutations cause cardiomyopathies, and recovered hiPSCs with 113 different variants. We differentiated these hiPSCs to cardiomyocytes and show MHC-ß fusion proteins can localize as expected. Additionally, single-cell contractility analyses revealed cardiomyocytes with a pathogenic, hypertrophic cardiomyopathy-associated MYH7 variant exhibit salient HCM physiology relative to isogenic controls. Thus, CRaTER substantially reduces screening required for isolation of gene-edited cells, enabling generation of functional transgenic cell lines at unprecedented scale.

Epicardially secreted fibronectin drives cardiomyocyte maturation in 3D-engineered heart tissues.

Ischemic heart failure is due to irreversible loss of cardiomyocytes. Preclinical studies showed that human pluripotent stem cell (hPSC)-derived cardiomyocytes could remuscularize infarcted hearts and improve cardiac function. However, these cardiomyocytes remained immature. Incorporating hPSC-derived epicardial cells has been shown to improve cardiomyocyte maturation, but the exact mechanisms are unknown. We posited epicardial fibronectin (FN1) as a mediator of epicardial-cardiomyocyte crosstalk and assessed its role in driving hPSC-derived cardiomyocyte maturation in 3D-engineered heart tissues (3D-EHTs). We found that the loss of FN1 with peptide inhibition F(pUR4), CRISPR-Cas9-mediated FN1 knockout, or tetracycline-inducible FN1 knockdown in 3D-EHTs resulted in immature cardiomyocytes with decreased contractile function, and inefficient Ca2+ handling. Conversely, when we supplemented 3D-EHTs with recombinant human FN1, we could recover hPSC-derived cardiomyocyte maturation. Finally, our RNA-sequencing analyses found FN1 within a wider paracrine network of epicardial-cardiomyocyte crosstalk, thus solidifying FN1 as a key driver of hPSC-derived cardiomyocyte maturation in 3D-EHTs.

Cardiomyocyte Apoptosis Is Associated with Contractile Dysfunction in Stem Cell Model of MYH7 E848G Hypertrophic Cardiomyopathy.

Missense mutations in myosin heavy chain 7 (MYH7) are a common cause of hypertrophic cardiomyopathy (HCM), but the molecular mechanisms underlying MYH7-based HCM remain unclear. In this work, we generated cardiomyocytes derived from isogenic human induced pluripotent stem cells to model the heterozygous pathogenic MYH7 missense variant, E848G, which is associated with left ventricular hypertrophy and adult-onset systolic dysfunction. MYH7E848G/+ increased cardiomyocyte size and reduced the maximum twitch forces of engineered heart tissue, consistent with the systolic dysfunction in MYH7E848G/+ HCM patients. Interestingly, MYH7E848G/+ cardiomyocytes more frequently underwent apoptosis that was associated with increased p53 activity relative to controls. However, genetic ablation of TP53 did not rescue cardiomyocyte survival or restore engineered heart tissue twitch force, indicating MYH7E848G/+ cardiomyocyte apoptosis and contractile dysfunction are p53-independent. Overall, our findings suggest that cardiomyocyte apoptosis is associated with the MYH7E848G/+ HCM phenotype in vitro and that future efforts to target p53-independent cell death pathways may be beneficial for the treatment of HCM patients with systolic dysfunction.

Engineered tissue vascularization and engraftment depends on host model.

Developing vascular networks that integrate with the host circulation and support cells engrafted within engineered tissues remains a key challenge in tissue engineering. Most previous work in this field has focused on developing new methods to build human vascular networks within engineered tissues prior to their implant in vivo, with substantively less attention paid to the role of the host in tissue vascularization and engraftment. Here, we assessed the role that different host animal models and anatomic implant locations play in vascularization and cardiomyocyte survival within engineered tissues. We found major differences in the formation of graft-derived blood vessels and survival of cardiomyocytes after implantation of identical tissues in immunodeficient athymic nude mice versus rats. Athymic mice supported robust guided vascularization of human microvessels carrying host blood but relatively sparse cardiac grafts within engineered tissues, regardless of implant site. Conversely, athymic rats produced substantive inflammatory changes that degraded grafts (abdomen) or disrupted vascular patterning (heart). Despite disrupted vascular patterning, athymic rats supported > 3-fold larger human cardiomyocyte grafts compared to athymic mice. This work demonstrates the critical importance of the host for vascularization and engraftment of engineered tissues, which has broad translational implications across regenerative medicine.

CRaTER enrichment for on-target gene-editing enables generation of variant libraries in hiPSCs.

Standard transgenic cell line generation requires screening 100-1000s of colonies to isolate correctly edited cells. We describe CR ISPR a On- T arget E diting R etrieval (CRaTER) which enriches for cells with on-target knock-in of a cDNA-fluorescent reporter transgene by transient activation of the targeted locus followed by flow sorting to recover edited cells. We show CRaTER recovers rare cells with heterozygous, biallelic-editing of the transcriptionally-inactive MYH7 locus in human induced pluripotent stem cells (hiPSCs), enriching on average 25-fold compared to standard antibiotic selection. We leveraged CRaTER to enrich for heterozygous knock-in of a library of single nucleotide variants (SNVs) in MYH7 , a gene in which missense mutations cause cardiomyopathies, and recovered hiPSCs with 113 different MYH7 SNVs. We differentiated these hiPSCs to cardiomyocytes and show MYH7 fusion proteins can localize as expected. Thus, CRaTER substantially reduces screening required for isolation of gene-edited cells, enabling generation of transgenic cell lines at unprecedented scale.

Cardiomyocyte apoptosis contributes to contractile dysfunction in stem cell model of MYH7 E848G hypertrophic cardiomyopathy.

Missense mutations in myosin heavy chain 7 ( MYH7 ) are a common cause of hyper-trophic cardiomyopathy (HCM), but the molecular mechanisms underlying MYH7 -based HCM remain unclear. In this work, we generated cardiomyocytes derived from isogenic human induced pluripotent stem cells to model the heterozygous pathogenic MYH7 missense variant, E848G, which is associated with left ventricular hypertrophy and adultonset systolic dysfunction. MYH7 E848G/+ increased cardiomyocyte size and reduced the maximum twitch forces of engineered heart tissue, consistent with the systolic dysfunction in MYH7 E848G HCM patients. Interestingly, MYH7 E848G/+ cardiomyocytes more frequently underwent apoptosis that was associated with increased p53 activity relative to controls. However, genetic ablation of TP53 did not rescue cardiomyocyte survival or restore engineered heart tissue twitch force, indicating MYH7 E848G/+ cardiomyocyte apoptosis and contractile dysfunction are p53-independent. Overall, our findings suggest that cardiomyocyte apoptosis plays an important role in the MYH7 E848G/+ HCM phenotype in vitro and that future efforts to target p53-independent cell death pathways may be beneficial for the treatment of HCM patients with systolic dysfunction.

Dynamic chromatin organization and regulatory interactions in human endothelial cell differentiation.

Vascular endothelial cells are a mesoderm-derived lineage with many essential functions, including angiogenesis and coagulation. The gene-regulatory mechanisms underpinning endothelial specialization are largely unknown, as are the roles of chromatin organization in regulating endothelial cell transcription. To investigate the relationships between chromatin organization and gene expression, we induced endothelial cell differentiation from human pluripotent stem cells and performed Hi-C and RNA-sequencing assays at specific time points. Long-range intrachromosomal contacts increase over the course of differentiation, accompanied by widespread heteroeuchromatic compartment transitions that are tightly associated with transcription. Dynamic topologically associating domain boundaries strengthen and converge on an endothelial cell state, and function to regulate gene expression. Chromatin pairwise point interactions (DNA loops) increase in frequency during differentiation and are linked to the expression of genes essential to vascular biology. Chromatin dynamics guide transcription in endothelial cell development and promote the divergence of endothelial cells from cardiomyocytes.

dATP elevation induces myocardial metabolic remodeling to support improved cardiac function.

Hallmark features of systolic heart failure are reduced contractility and impaired metabolic flexibility of the myocardium. Cardiomyocytes (CMs) with elevated deoxy ATP (dATP) via overexpression of ribonucleotide reductase (RNR) enzyme robustly improve contractility. However, the effect of dATP elevation on cardiac metabolism is unknown. Here, we developed proteolysis-resistant versions of RNR and demonstrate that elevation of dATP/ATP to ∼1% in CMs in a transgenic mouse (TgRRB) resulted in robust improvement of cardiac function. Pharmacological approaches showed that CMs with elevated dATP have greater basal respiratory rates by shifting myosin states to more active forms, independent of its isoform, in relaxed CMs. Targeted metabolomic profiling revealed a significant reprogramming towards oxidative phosphorylation in TgRRB-CMs. Higher cristae density and activity in the mitochondria of TgRRB-CMs improved respiratory capacity. Our results revealed a critical property of dATP to modulate myosin states to enhance contractility and induce metabolic flexibility to support improved function in CMs.

Lessons learnt, and still to learn, in first in human stem cell trials.

Developing cellular therapies is not straightforward. This Perspective summarizes the experience of a group of academic stem cell investigators working in different clinical areas and aims to share insight into what we wished we knew before starting. These include (1) choosing the stem cell line and assessing the genome of both the starting and final product, (2) familiarity with GMP manufacturing, reagent validation, and supply chain management, (3) product delivery issues and the additional regulatory challenges, (4) the relationship between clinical trial design and preclinical studies, and (5) the market approval requirements, pathways, and partnerships needed.

Amino acid primed mTOR activity is essential for heart regeneration.

Heart disease is the leading cause of death with no method to repair damaged myocardium due to the limited proliferative capacity of adult cardiomyocytes. Curiously, mouse neonates and zebrafish can regenerate their hearts via cardiomyocyte de-differentiation and proliferation. However, a molecular mechanism of why these cardiomyocytes can re-enter cell cycle is poorly understood. Here, we identify a unique metabolic state that primes adult zebrafish and neonatal mouse ventricular cardiomyocytes to proliferate. Zebrafish and neonatal mouse hearts display elevated glutamine levels, predisposing them to amino-acid-driven activation of TOR, and that TOR activation is required for zebrafish cardiomyocyte regeneration in vivo. Through a multi-omics approach with cellular validation we identify metabolic and mitochondrial changes during the first week of regeneration. These data suggest that regeneration of zebrafish myocardium is driven by metabolic remodeling and reveals a unique metabolic regulator, TOR-primed state, in which zebrafish and mammalian cardiomyocytes are regeneration competent.