Cardiac and lung endothelial cells in response to fluid shear stress on physiological matrix stiffness and composition

Integration of Extracellular Matrices into Organ-on-Chip Systems

The extracellular matrix (ECM) is a complex, dynamic network present within all tissues and organs that not only acts as a mechanical support and anchorage point but can also direct fundamental cell behavior, function, and characteristics. Although the importance of the ECM is well established, the integration of well-controlled ECMs into Organ-on-Chip (OoC) platforms remains challenging and the methods to modulate and assess ECM properties on OoCs remain underdeveloped. In this review, current state-of-the-art design and assessment of in vitro ECM environments is discussed with a focus on their integration into OoCs. Among other things, synthetic and natural hydrogels, as well as polydimethylsiloxane (PDMS) used as substrates, coatings, or cell culture membranes are reviewed in terms of their ability to mimic the native ECM and their accessibility for characterization. The intricate interplay among materials, OoC architecture, and ECM characterization is critically discussed as it significantly complicates the design of ECM-related studies, comparability between works, and reproducibility that can be achieved across research laboratories. Improving the biomimetic nature of OoCs by integrating properly considered ECMs would contribute to their further adoption as replacements for animal models, and precisely tailored ECM properties would promote the use of OoCs in mechanobiology.

The Development of Small-Caliber Vascular Grafts Using Human Umbilical Artery: An Evaluation of Methods

Due to the prevalence of cardiovascular disease in the United States, small-caliber vascular grafts for coronary bypass surgery continue to be in high demand. Human umbilical arteries, an underutilized resource, were decellularized using zwitterionic (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate [CHAPS]) and ionic (sodium dodecyl sulfate [SDS]) detergents and evaluated as potential vascular grafts. Vessels were tested for decellularization efficacy, mechanical integrity, and recellularization potential. Hematoxylin and eosin staining and DNA quantification revealed moderate to successful removal of cells in both conditions. While CHAPS-decellularized vessels displayed collagen structure most similar to intact tissue, both CHAPS- and SDS-decellularized vessels demonstrated burst pressures lower than that of intact tissue. Alcian Blue staining and sulfated glycosaminoglycan (sGAG) quantification indicated the preservation of sGAG content after both decellularization pathways. Both conditions were also capable of recellularization with human umbilical vein endothelial cells, and the use of a basic fibroblast growth factor treatment did not have a significant effect on the density of adhered cells after 5 days. Whole CHAPS-decellularized vessels were successfully recellularized. Additionally, an evaluation of the effects of freeze-thaw cycles was performed. In summary, human umbilical arteries present a promising alternative for small-caliber vascular grafts due to their high availability and ability to be decellularized and recellularized for safe and successful implantation. Impact Statement Coronary heart disease accounts for one of nine deaths in the United States each year. Bypass surgery has been shown to decrease the risk of heart attack; however, many patients do not have a suitable saphenous vein, which is required to redirect blood flow around their blocked arteries. In this study, we evaluate decellularized umbilical artery as a potential small-diameter vascular graft based on its mechanical properties and its recellularization potential.

Revealing anisotropic elasticity of endothelium under fluid shear stress

Endothelium lining interior surface of blood vessels experiences various physical stimulations in vivo. Its physical properties, especially elasticity, play important roles in regulating the physiological functions of vascular systems. In this paper, an integrated approach is developed to characterize the anisotropic elasticity of the endothelium under physiological-level fluid shear stress. A pressure sensor-embedded microfluidic device is developed to provide fluid shear stress on the perfusion-cultured endothelium and to measure transverse in-plane elasticities in the directions parallel and perpendicular to the flow direction. Biological atomic force microscopy (Bio-AFM) is further exploited to measure the vertical elasticity of the endothelium in its out-of-plane direction. The results show that the transverse elasticity of the endothelium in the direction parallel to the perfusion culture flow direction is about 70% higher than that in the direction perpendicular to the flow direction. Moreover, the transverse elasticities of the endothelium are estimated to be approximately 120 times larger than the vertical one. The results indicate the effects of fluid shear stress on the transverse elasticity anisotropy of the endothelium, and the difference between the elasticities in transverse and vertical directions. The quantitative measurement of the endothelium anisotropic elasticity in different directions at the tissue level under the fluid shear stress provides biologists insightful information for the advanced vascular system studies from biophysical and biomaterial viewpoints. STATEMENT OF SIGNIFICANCE: In this paper, we take advantage an integrated approach combining microfluidic devices and biological atomic force microscopy (Bio-AFM) to characterize anisotropic elasticities of endothelia with and without fluidic shear stress application. The microfluidic devices are exploited to conduct perfusion cell culture of the endothelial cells, and to estimate the in-plane elasticities of the endothelium in the direction parallel and perpendicular to the shear stress. In addition, the Bio-AFM is utilized for characterization of the endothelium morphology and vertical elasticity. The measurement results demonstrate the very first anisotropic elasticity quantification of the endothelia. Furthermore, the study provides insightful information bridging the microscopic sing cell and macroscopic organ level studies, which can greatly help to advance vascular system research from material perspective.

Organ-Specific Endothelial Cell Differentiation and Impact of Microenvironmental Cues on Endothelial Heterogeneity

Endothelial cells throughout the body are heterogeneous, and this is tightly linked to the specific functions of organs and tissues. Heterogeneity is already determined from development onwards and ranges from arterial/venous specification to microvascular fate determination in organ-specific differentiation. Acknowledging the different phenotypes of endothelial cells and the implications of this diversity is key for the development of more specialized tissue engineering and vascular repair approaches. However, although novel technologies in transcriptomics and proteomics are facilitating the unraveling of vascular bed-specific endothelial cell signatures, still much research is based on the use of insufficiently specialized endothelial cells. Endothelial cells are not only heterogeneous, but their specialized phenotypes are also dynamic and adapt to changes in their microenvironment. During the last decades, strong collaborations between molecular biology, mechanobiology, and computational disciplines have led to a better understanding of how endothelial cells are modulated by their mechanical and biochemical contexts. Yet, because of the use of insufficiently specialized endothelial cells, there is still a huge lack of knowledge in how tissue-specific biomechanical factors determine organ-specific phenotypes. With this review, we want to put the focus on how organ-specific endothelial cell signatures are determined from development onwards and conditioned by their microenvironments during adulthood. We discuss the latest research performed on endothelial cells, pointing out the important implications of mimicking tissue-specific biomechanical cues in culture.

Mechanism of cell death of endothelial cells regulated by mechanical forces

Cell death of endothelial cells (ECs) is a common devastating consequence of various vascular-related diseases. Atherosclerosis, hypertension, sepsis, diabetes, cerebral ischemia and cardiac ischemia/reperfusion injury, and chronic kidney disease remain major causes of morbidity and mortality worldwide, in which ECs are constantly subjected to a great amount of dynamic changed mechanical forces including shear stress, extracellular matrix stiffness, mechanical stretch and microgravity. A thorough understanding of the regulatory mechanisms by which the mechanical forces controlled the cell deaths including apoptosis, autophagy, and pyroptosis is crucial for the development of new therapeutic strategies. In the present review, experimental and clinical data highlight that nutrient depletion, oxidative stress, tumor necrosis factor-α, high glucose, lipopolysaccharide, and homocysteine possess cytotoxic effects in many tissues and induce apoptosis of ECs, and that sphingosine-1-phosphate protects ECs. Nevertheless, EC apoptosis in the context of those artificial microenvironments could be enhanced, reduced or even reversed along with the alteration of patterns of shear stress. An appropriate level of autophagy diminishes EC apoptosis to some extent, in addition to supporting cell survival upon microenvironment challenges. The intervention of pyroptosis showed a profound effect on atherosclerosis. Further cell and animal studies are required to ascertain whether the alterations in the levels of cell deaths and their associated regulatory mechanisms happen at local lesion sites with considerable mechanical force changes, for preventing senescence and cell deaths in the vascular-related diseases.

In Vitro Measurements of Cellular Forces and their Importance in the Lung-From the Sub- to the Multicellular Scale

Throughout life, the body is subjected to various mechanical forces on the organ, tissue, and cellular level. Mechanical stimuli are essential for organ development and function. One organ whose function depends on the tightly connected interplay between mechanical cell properties, biochemical signaling, and external forces is the lung. However, altered mechanical properties or excessive mechanical forces can also drive the onset and progression of severe pulmonary diseases. Characterizing the mechanical properties and forces that affect cell and tissue function is therefore necessary for understanding physiological and pathophysiological mechanisms. In recent years, multiple methods have been developed for cellular force measurements at multiple length scales, from subcellular forces to measuring the collective behavior of heterogeneous cellular networks. In this short review, we give a brief overview of the mechanical forces at play on the cellular level in the lung. We then focus on the technological aspects of measuring cellular forces at many length scales. We describe tools with a subcellular resolution and elaborate measurement techniques for collective multicellular units. Many of the technologies described are by no means restricted to lung research and have already been applied successfully to cells from various other tissues. However, integrating the knowledge gained from these multi-scale measurements in a unifying framework is still a major future challenge.