Cardiovascular consequences of sickle cell disease.

Sickle cell disease (SCD) is an inherited blood disorder caused by a single point mutation within the beta globin gene. As a result of this mutation, hemoglobin polymerizes under low oxygen conditions causing red blood cells to deform, become more adhesive, and increase in rigidity, which affects blood flow dynamics. This process leads to enhanced red blood cell interactions with the endothelium and contributes to vaso-occlusion formation. Although traditionally defined as a red blood cell disorder, individuals with SCD are affected by numerous clinical consequences including stroke, painful crisis episodes, bone infarctions, and several organ-specific complications. Elevated cardiac output, endothelium activation along with the sickling process, and the vaso-occlusion events pose strains on the cardiovascular system. We will present a review of the cardiovascular consequences of sickle cell disease and show connections with the vasculopathy related to SCD. We will also highlight biophysical properties and engineering tools that have been used to characterize the disease. Finally, we will discuss therapies for SCD and potential implications on SCD cardiomyopathy.

Biomimetic microsystems for cardiovascular studies.

Traditional tissue culture platforms have been around for several decades and have enabled key findings in the cardiovascular field. However, these platforms failed to recreate the mechanical and dynamic features found within the body. Organs-on-chips (OOCs) are cellularized microfluidic-based devices that can mimic the basic structure, function, and responses of organs. These systems have been successfully utilized in disease, development, and drug studies. OOCs are designed to recapitulate the mechanical, electrical, chemical, and structural features of the in vivo microenvironment. Here, we review cardiovascular-themed OOC studies, design considerations, and techniques used to generate these cellularized devices. Furthermore, we will highlight the advantages of OOC models over traditional cell culture vessels, discuss implementation challenges, and provide perspectives on the state of the field.

A microfluidic computational fluid dynamics model for cellular interaction studies of sickle cell disease vaso-occlusions.

Individuals with sickle cell disease are plagued with vaso-occlusions, chronic blockages within the vasculature. Several factors including stiffer sickle red blood cells and increased cell aggregation contribute to vaso-occlusion formation; however much remains to be understood. We present a computational fluid dynamics blood flow simulation within a microfluidic platform using the Carreau model and Murray's law. Vaso-occlusions form preferentially near bifurcations within 60 s in the sickle cell disease simulation. Velocity profiles and shear rates align with clinical and experimental reports. We assert that results from this study can be utilized to inform experimental investigations and microfluidic system design decisions.

Microfluidics for investigating vaso-occlusions in sickle cell disease.

SCD stems from amutation in the beta globin gene. Upon deoxygenation, hemoglobin polymerizes and triggers RBC remodeling. This phenomenon is central to SCD pathogenesis as individuals suffering from the disease are plagued by painful vaso-occlusive crises episodes. These episodes are the result of a combination of processes including inflammation, thrombosis, and blood cell adhesion to the vascular wall which leads to blockages within the vasculature termed vaso-occlusions. Vaso-occlusive episodes deprive tissues of oxygen and are a major contributor to SCD-related complications; unfortunately, the complex mechanisms that contribute to vaso-occlusions are not well understood. Vaso-occlusions can occur in post-capillary venules; hence, the microvasculature is a prime target for SCD therapies. Traditional in vitro systems poorly recapitulate architectural and dynamic flow properties of in vivo systems. However, microfluidic devices can capture features of the native vasculature such as cellular composition, flow, geometry, and ECM presentation. This review, although not comprehensive, highlights microfluidic approaches that aim to improve our current understanding of the pathophysiological mechanisms surrounding SCD. Microfluidic platforms can aid in identifying factors that may contribute to disease severity and can serve as suitable test beds for novel treatment strategies which may improve patient outcomes.

Angiotensin II Induced Cardiac Dysfunction on a Chip.

In vitro disease models offer the ability to study specific systemic features in isolation to better understand underlying mechanisms that lead to dysfunction. Here, we present a cardiac dysfunction model using angiotensin II (ANG II) to elicit pathological responses in a heart-on-a-chip platform that recapitulates native laminar cardiac tissue structure. Our platform, composed of arrays of muscular thin films (MTF), allows for functional comparisons of healthy and diseased tissues by tracking film deflections resulting from contracting tissues. To test our model, we measured gene expression profiles, morphological remodeling, calcium transients, and contractile stress generation in response to ANG II exposure and compared against previous experimental and clinical results. We found that ANG II induced pathological gene expression profiles including over-expression of natriuretic peptide B, Rho GTPase 1, and T-type calcium channels. ANG II exposure also increased proarrhythmic early after depolarization events and significantly reduced peak systolic stresses. Although ANG II has been shown to induce structural remodeling, we control tissue architecture via microcontact printing, and show pathological genetic profiles and functional impairment precede significant morphological changes. We assert that our in vitro model is a useful tool for evaluating tissue health and can serve as a platform for studying disease mechanisms and identifying novel therapeutics.

Mechanotransduction and Metabolism in Cardiomyocyte Microdomains.

Efficient contractions of the left ventricle are ensured by the continuous transfer of adenosine triphosphate (ATP) from energy production sites, the mitochondria, to energy utilization sites, such as ionic pumps and the force-generating sarcomeres. To minimize the impact of intracellular ATP trafficking, sarcomeres and mitochondria are closely packed together and in proximity with other ultrastructures involved in excitation-contraction coupling, such as t-tubules and sarcoplasmic reticulum junctions. This complex microdomain has been referred to as the intracellular energetic unit. Here, we review the literature in support of the notion that cardiac homeostasis and disease are emergent properties of the hierarchical organization of these units. Specifically, we will focus on pathological alterations of this microdomain that result in cardiac diseases through energy imbalance and posttranslational modifications of the cytoskeletal proteins involved in mechanosensing and transduction.

Synergistic effects of hypoxia and extracellular matrix cues in cardiomyogenesis.

Limited characterization of how the stem cell niche evolves has hindered our ability to mimic the physiological environment. In this paper, we hypothesized that hypoxia-induced extracellular matrix (ECM) cues may facilitate cardiomyogenesis. We evaluated the expression of four ECM proteins - fibronectin, collagen I, collagen IV, and laminin - over a period of 20 days in H1 and H9 human embryonic stem cell-derived embryoid bodies (EBs) under hypoxic (5% oxygen) and normoxic (21% oxygen) conditions. Hypoxic EBs exhibited increased collagen I, collagen IV and fibronectin expression relative to normoxic EBs between days 9-13, which coincided with increased expression of mesoderm genes. The effect of ECM cues was confirmed by plating day 9 EBs on collagen IV, gelatin, and fibronectin-rich substrates for 11 days. Hypoxia/gelatin cultures synergistically increased the cardiomyocyte yield by 1.7 and 5.5 fold relative to normoxia/gelatin and normoxia/collagen IV cultures, respectively. Current differentiation protocols may underestimate the contribution of hypoxia and ECM cues that evolve during EB maturation.

Engineering microenvironments for embryonic stem cell differentiation to cardiomyocytes.

Embryonic stem cells and induced pluripotent stem cells have the potential to be a renewable source of cardiomyocytes for use in myocardial cell replacement strategies. Although progress has been made towards differentiating stem cells to specific cell lineages, the efficiency is often poor and the number of cells generated is not suitable for therapeutic usage. Recent studies demonstrated that controlling the stem cell microenvironment can influence differentiation. Components of the extracellular matrix are important physiological regulators and can provide mechanical cues, direct differentiation and improve cell engraftment into damaged tissue. Bioreactors are used to control the microenvironment and produce large numbers of desired cells. This article describes recent methods to achieve cardiomyocyte differentiation by engineering the stem cell microenvironment. Successful translation of stem cell research to therapeutic applications will need to address large-scale cardiomyocyte differentiation and purification, assessment of cardiac function and synchronization, and safety concerns.