Matrix stiffness regulates endothelial cell proliferation through septin 9. PLoS One. 2012;7:e46889. [PMID: 23118862 PMCID: PMC3485289 DOI: 10.1371/journal.pone.0046889]

Synthesis and use of thermoplastic polymers for tissue engineering purposes

Thermoplastic polymers provide a versatile platform to mimic various aspects of physiological extracellular matrix properties such as chemical composition, stiffness, and topography for use in cell and tissue engineering applications. In this review, we provide a brief overview of the most promising thermoplastic polymers, and in particular the thermoplastic polyesters, such as poly(lactic acid), poly(glycolic acid), and polycaprolactone, and the thermoplastic elastomers, such as polyurethanes, polyhydroxyalkanoates, and poly(butyl cyanoacrylate). A particular focus has been made on the synthesis processes, the processability and the biocompatibility. We also discuss how these materials can be applied in tissue engineering, mimicking tissues' structure and function, and stimulate mesenchymal stem cells differentiation and mechanotransduction.

Paganella L, Narciso ML, Supponen O. Cyclic jetting enables microbubble-mediated drug delivery

The pursuit of targeted therapies capable of overcoming biological barriers, including the blood-brain barrier, has spurred the investigation of stimuli-responsive microagents that can improve therapeutic efficacy and reduce undesirable side effects. Intravenously administered, ultrasound-responsive microbubbles are promising agents with demonstrated potential in clinical trials, but the mechanism underlying drug absorption remains unclear. Here we show that ultrasound-driven single microbubbles puncture the cell membrane and induce drug uptake through stable cyclic microjets. Our theoretical models successfully reproduce the observed bubble and cell dynamic responses. We find that cyclic jets arise from shape instabilities, as opposed to classical inertial jets that are driven by pressure gradients, enabling microjet formation at mild ultrasound pressures below 100 kPa. We also establish a threshold for bubble radial expansion beyond which microjets form and facilitate cellular permeation and show that the stress generated by microjetting outperforms previously suggested mechanisms by at least an order of magnitude. Overall, this work elucidates the physics behind microbubble-mediated targeted drug delivery and provides the criteria for its effective and safe application.

Microvascular endothelial cells display organ-specific responses to extracellular matrix stiffness

The extracellular matrix was originally thought of as simply a cellular scaffold but is now considered a key regulator of cell function and phenotype from which cells can derive biochemical and mechanical stimuli. Age-associated changes in matrix composition drive increases in matrix stiffness. Enhanced matrix stiffness promotes the progression of numerous diseases including cardiovascular disease, musculoskeletal disease, fibrosis, and cancer. Macrovascular endothelial cells undergo endothelial dysfunction in response to enhanced matrix stiffness. However, endothelial cells are highly heterogeneous, adopting structural and gene expression profiles specific to their organ of origin. Endothelial cells isolated from different vessels (i.e. arteries, veins or capillaries) respond differently to changes in substrate stiffness. It is unknown whether microvascular endothelial cells isolated from different organs also display organ-specific responses to substrate stiffness. In this study, we compare the response of microvascular endothelial cells isolated from both the mouse lung and mammary gland to a range of physiologically relevant substrate stiffnesses. We find that endothelial origin influences microvascular endothelial cell response to substrate stiffness in terms of both proliferation and migration speed. In lung-derived endothelial cells, proliferation is bimodal, where both physiologically soft and stiff substrates drive enhanced proliferation. Conversely, in mammary gland-derived endothelial cells, proliferation increases as substrate stiffness increases. Substrate stiffness also promotes enhanced endothelial migration. Enhanced stiffness drove greater increases in migration speed in mammary gland-derived than lung-derived endothelial cells. However, stiffness-induced changes in microvascular endothelial cell morphology were consistent between both cell lines, with substrate stiffness driving an increase in endothelial volume. Our research demonstrates the importance of considering endothelial origin in experimental design, especially when investigating how age-associated changes in matrix stiffness drive endothelial dysfunction and disease progression.

Progress in Biomaterials-Enhanced Vascularization by Modulating Physical Properties

Sufficient vascular system and adequate blood perfusion is crucial for ensuring nutrient and oxygen supply within biomaterials. Actively exploring the optimal physical properties of biomaterials in various application scenarios has provided clues for enhancing vascularization within materials, leading to improved outcomes in tissue engineering and clinical translation. Here we focus on reviewing the physical properties of biomaterials, including pore structure, surface topography, and stiffness, and their effects on promoting vascularization. This angiogenic capability has the potential to provide better standardized research models and personalized treatment strategies for bone regeneration, wound healing, islet transplantation and cardiac repair.

Human Chorionic Membrane-derived Tunable Hydrogels for Vascular Tissue Engineering Strategies

One of the foremost targets in the advancement of biomaterials to engineer vascularized tissues is not only to replicate the composition of the intended tissue but also to create thicker structures incorporating a vascular network for adequate nutrients and oxygen supply. For the first time, to the best of current knowledge, a clinically relevant biomaterial is developed, demonstrating that hydrogels made from the human decellularized extracellular matrix can exhibit robust mechanical properties (in the kPa range) and angiogenic capabilities simultaneously. These properties enable the culture and organization of human umbilical vein endothelial cells into tubular structures, maintaining their integrity for 14 days in vitro without the need for additional polymers or angiogenesis-related factors. This is achieved by repurposing the placenta chorionic membrane (CM), a medical waste with an exceptional biochemical composition, into a valuable resource for bioengineering purposes. After decellularization, the CM underwent chemical modification with methacryloyl groups, giving rise to methacrylated CM (CMMA). CMMA preserved key proteins, as well as glycosaminoglycans. The resulting hydrogels rapidly photopolymerize and have enhanced strength and customizable mechanical properties. Furthermore, they demonstrate angio-vasculogenic competence in vitro and in vivo, holding significant promise as a humanized platform for the engineering of vascularized tissues.

The alarmin, interleukin-33, increases vascular tone via extracellular signal regulated kinase-mediated Ca2+ sensitization and endothelial nitric oxide synthase inhibition

Alarmins are classified by their release from damaged or ruptured cells. Many alarmins have been found to increase vascular tone and oppose endothelium-dependent dilatation (EDD). Interleukin (IL)-33 plays a prominent role in lung injury and can be released during vascular injury and in chronic studies found to be cardioprotective. Our recent work has implicated IL-33 in acute vascular dysfunction following inhalation of engineered nanomaterials (ENM). However, the mechanisms linking IL-33 to vascular tone have not been interrogated. We therefore aimed to determine whether IL-33 directly influenced microvascular tone and endothelial function. Isolated feed arteries and in vivo arterioles from male and female Sprague-Dawley rats were used to determine direct vascular actions of IL-33. Mesenteric feed arteries and arterioles demonstrated reduced intraluminal diameters when treated with increasing concentrations of recombinant IL-33. IL-33 activated extracellular signal regulated kinase (ERK)1/2 of rat aortic smooth muscle cells but not phosphorylation of myosin light chain kinase. This suggested IL-33 may sensitize arterioles to Ca2+-mediated responses. Indeed, IL-33 augmented the myogenic- and phenylephrine-induced vasoconstriction. Additionally, incubation of arterioles with 1 ng IL-33 attenuated ACh-mediated EDD. Mechanistically, in human aortic endothelial cells, we demonstrate that IL-33-mediated ERK1/2 activation leads to inhibitory phosphorylation of serine 602 on endothelial nitric oxide synthase. Finally, we demonstrate that IL-33-ERK1/2 contributes to vascular tone following two known inducers of IL-33; ENM inhalation and the rupture endothelial cells. The present study provides novel evidence that IL-33 increases vascular tone via canonical ERK1/2 activation in microvascular smooth muscle and endothelium. Altogether, it is suggested IL-33 plays a critical role in microvascular homeostasis following barrier cell injury. KEY POINTS: Interleukin (IL)-33 causes a concentration-dependent reduction in feed artery diameter. IL-33 acts on vascular smooth muscle cells to augment Ca2+-mediated processes. IL-33 causes inhibitory phosphorylation of endothelial nitric oxide synthase and opposes endothelium-dependent dilatation. Engineered nanomaterial-induced lung injury and endothelial cell rupture in part act through IL-33 to mediate increased vascular tone.

Mechanical strategies to promote vascularization for tissue engineering and regenerative medicine

Vascularization is a major challenge in the field of tissue engineering and regenerative medicine. Mechanical factors have been demonstrated to play a fundamental role in vasculogenesis and angiogenesis and can affect the architecture of the generated vascular network. Through the regulation of mechanical factors in engineered tissues, various mechanical strategies can be used to optimize the preformed vascular network and promote its rapid integration with host vessels. Optimization of the mechanical properties of scaffolds, including controlling scaffold stiffness, increasing surface roughness and anisotropic structure, and designing interconnected, hierarchical pore structures, is beneficial for the in vitro formation of vascular networks and the ingrowth of host blood vessels. The incorporation of hollow channels into scaffolds promotes the formation of patterned vascular networks. Dynamic stretching and perfusion can facilitate the formation and maturation of preformed vascular networks in vitro. Several indirect mechanical strategies provide sustained mechanical stimulation to engineered tissues in vivo, which further promotes the vascularization of implants within the body. Additionally, stiffness gradients, anisotropic substrates and hollow channels in scaffolds, as well as external cyclic stretch, boundary constraints and dynamic flow culture, can effectively regulate the alignment of vascular networks, thereby promoting better integration of prevascularized engineered tissues with host blood vessels. This review summarizes the influence and contribution of both scaffold-based and external stimulus-based mechanical strategies for vascularization in tissue engineering and elucidates the underlying mechanisms involved.

Mesenchymal Osr1+ cells regulate embryonic lymphatic vessel formation

The lymphatic system is formed during embryonic development by the commitment of specialized lymphatic endothelial cells (LECs) and their subsequent assembly in primary lymphatic vessels. Although lymphatic cells are in continuous contact with mesenchymal cells during development and in adult tissues, the role of mesenchymal cells in lymphatic vasculature development remains poorly characterized. Here, we show that a subpopulation of mesenchymal cells expressing the transcription factor Osr1 are in close association with migrating LECs and established lymphatic vessels in mice. Lineage tracing experiments revealed that Osr1+ cells precede LEC arrival during lymphatic vasculature assembly in the back of the embryo. Using Osr1-deficient embryos and functional in vitro assays, we show that Osr1 acts in a non-cell-autonomous manner controlling proliferation and early migration of LECs to peripheral tissues. Thereby, mesenchymal Osr1+ cells control, in a bimodal manner, the production of extracellular matrix scaffold components and signal ligands crucial for lymphatic vessel formation.

Matrix stiffness regulates the triad communication of adipocytes/macrophages/endothelial cells through CXCL13

Adipose tissue remodeling and plasticity are dynamically regulated by the coordinated functions of adipocytes, macrophages, and endothelial cells and extracellular matrix (ECM) that provides stiffness networks in adipose tissue component cells. Inflammation and fibrosis are crucial exogenous factors that dysregulate adipose tissue functions and drastically change the mechanical properties of the ECM. Therefore, communication among the ECM and adipose tissue component cells is necessary to understand the multifaceted functions of adipose tissues. To obtain in vivo stiffness, we used genipin as a crosslinker for collagen gels. Meanwhile, we isolated primary adipocytes, macrophages, and endothelial cells from C57BL/6J mice and incubated these cells in the differentiation media on temperature-responsive culture dishes. After the differentiation, these cell sheets were transferred onto genipin-crosslinked collagen gels with varying matrix stiffness. We found that inflammatory gene expressions were induced by hard matrix, whereas antiinflammatory gene expressions were promoted by soft matrix in all three types of cells. Interestingly, the coculture experiments of adipocytes, macrophages, and endothelial cells showed that the effects of soft or hard matrix stiffness stimulation on adipocytes were transmitted to the distant adipose tissue component cells, altering their gene expression profiles under normal matrix conditions. Finally, we identified that a hard matrix induces the secretion of CXCL13 from adipocytes, and CXCL13 is one of the important transmitters for stiffness communication with macrophages and endothelial cells. These findings provide insight into the mechanotransmission into distant cells and the application of stiffness control for chronic inflammation in adipose tissues with metabolic dysregulation.

Mechanical regulation of lymphocyte activation and function

Mechanosensing, or how cells sense and respond to the physical environment, is crucial for many aspects of biological function, ranging from cell movement during development to cancer metastasis, the immune response and gene expression driving cell fate determination. Relevant physical stimuli include the stiffness of the extracellular matrix, contractile forces, shear flows in blood vessels, complex topography of the cellular microenvironment and membrane protein mobility. Although mechanosensing has been more widely studied in non-immune cells, it has become increasingly clear that physical cues profoundly affect the signaling function of cells of the immune system. In this Review, we summarize recent studies on mechanical regulation of immune cells, specifically lymphocytes, and explore how the force-generating cytoskeletal machinery might mediate mechanosensing. We discuss general principles governing mechanical regulation of lymphocyte function, spanning from the molecular scale of receptor activation to cellular responses to mechanical stimuli.

The concerted action of SEPT9 and EPLIN modulates the adhesion and migration of human fibroblasts

Septins are cytoskeletal proteins that participate in cell adhesion, migration, and polarity establishment. The septin subunit SEPT9 directly interacts with the single LIM domain of epithelial protein lost in neoplasm (EPLIN), an actin-bundling protein. Using a human SEPT9 KO fibroblast cell line, we show that cell adhesion and migration are regulated by the interplay between both proteins. The low motility of SEPT9-depleted cells could be partly rescued by increased levels of EPLIN. The normal organization of actin-related filopodia and stress fibers was directly dependent on the expression level of SEPT9 and EPLIN. Increased levels of SEPT9 and EPLIN enhanced the size of focal adhesions in cell protrusions, correlating with stabilization of actin bundles. Conversely, decreased levels had the opposite effect. Our work thus establishes the interaction between SEPT9 and EPLIN as an important link between the septin and the actin cytoskeleton, influencing cell adhesion, motility, and migration.

Phototuning of Hyaluronic-Acid-Based Hydrogel Properties to Control Network Formation in Human Vascular Endothelial Cells

In vitro network formation by endothelial cells serves as a fundamental model for studies aimed at understanding angiogenesis. The morphogenesis of these cells to form a network is intricately regulated by the mechanical and biochemical properties of the extracellular matrix. Here the effects of modulating these properties in hydrogels derived from phenolated hyaluronic acid (HA-Ph) and phenolated gelatin (Gelatin-Ph) are presented. Visible-light irradiation in the presence of tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate and sodium persulfate induces the crosslinking of these polymers, thereby forming a hydrogel and degrading HA-Ph. Human vascular endothelial cells form networks on the hydrogel prepared by visible-light irradiation for 45 min (42 W cm-2 at 450 nm) but not on the hydrogels prepared by irradiation for 15, 30, or 60 min. The irradiation time-dependent degradation of HA-Ph and the changes in the mechanical stiffness of the hydrogels, coupled with the expressions of RhoA and β-actin genes and CD44 receptors in the cells, reveal that the network formation is synergistically influenced by the hydrogel stiffness and HA-Ph degradation. These findings highlight the potential of tailoring HA-based hydrogel properties to modulate human vascular endothelial cell responses, which is critical for advancing their application in vascular tissue engineering.

Extracellular matrix stiffness mediates insulin secretion in pancreatic islets via mechanosensitive Piezo1 channel regulated Ca2+ dynamics

The pancreatic islet is surrounded by ECM that provides both biochemical and mechanical cues to the islet β-cell to regulate cell survival and insulin secretion. Changes in ECM composition and mechanical properties drive β-cell dysfunction in many pancreatic diseases. While several studies have characterized changes in islet insulin secretion with changes in substrate stiffness, little is known about the mechanotransduction signaling driving altered islet function in response to mechanical cues. We hypothesized that increasing matrix stiffness will lead to insulin secretion dysfunction by opening the mechanosensitive ion channel Piezo1 and disrupting intracellular Ca2+ dynamics in mouse and human islets. To test our hypothesis, mouse and human cadaveric islets were encapsulated in a biomimetic reverse thermal gel (RTG) scaffold with tailorable stiffness that allows formation of islet focal adhesions with the scaffold and activation of Piezo1 in 3D. Our results indicate that increased scaffold stiffness causes insulin secretion dysfunction mediated by increases in Ca2+ influx and altered Ca2+ dynamics via opening of the mechanosensitive Piezo1 channel. Additionally, inhibition of Piezo1 rescued glucose-stimulated insulin secretion (GSIS) in islets in stiff scaffolds. Overall, our results emphasize the role mechanical properties of the islet microenvironment plays in regulating function. It also supports further investigation into the modulation of Piezo1 channel activity to restore islet function in diseases like type 2 diabetes (T2D) and pancreatic cancer where fibrosis of the peri-islet ECM leads to increased tissue stiffness and islet dysfunction.

Modular Multiwell Viscoelastic Hydrogel Platform for Two- and Three-Dimensional Cell Culture Applications

Hydrogels have gained significant popularity as model platforms to study reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within the same 96-well plate for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and is used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for two-dimensional (2D) cell culture applications was demonstrated by measuring both population-level and single-cell-level metrics via microplate reader and high-content imaging. Finally, a 96-well hydrogel array was utilized for three-dimensional (3D) cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adaptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

Fundamental considerations for designing endothelialized in vitro models of thrombosis

Endothelialized in vitro models for cardiovascular disease have contributed greatly to our current understanding of the complex molecular mechanisms underlying thrombosis. To further elucidate these mechanisms, it is important to consider which fundamental aspects to incorporate into an in vitro model. In this review, we will focus on the design of in vitro endothelialized models of thrombosis. Expanding our understanding of the relation and interplay between the different pathways involved will rely in part on complex models that incorporate endothelial cells, blood, the extracellular matrix, and flow. Importantly, the use of tissue-specific endothelial cells will help in understanding the heterogeneity in thrombotic responses between different vascular beds. The dynamic and complex responses of endothelial cells to different shear rates underlines the importance of incorporating appropriate shear in in vitro models. Alterations in vascular extracellular matrix composition, availability of bioactive molecules, and gradients in concentration and composition of these molecules can all regulate the function of both endothelial cells and perivascular cells. Factors modulating these elements in in vitro models should therefore be considered carefully depending on the research question at hand. As the complexity of in vitro models increases, so can the variability. A bottom-up approach to designing such models will remain an important tool for researchers studying thrombosis. As new techniques are continuously being developed and new pathways are brought to light, research question-dependent considerations will have to be made regarding what aspects of thrombosis to include in in vitro models.

The role of cell-matrix adhesion and cell migration in breast tumor growth and progression

During breast cancer progression, there is typically increased collagen deposition resulting in elevated extracellular matrix rigidity. This results in changes to cell-matrix adhesion and cell migration, impacting processes such as the epithelial-mesenchymal transition (EMT) and metastasis. We aim to investigate the roles of cell-matrix adhesion and cell migration on breast tumor growth and progression by studying the impacts of different types of extracellular matrices and their rigidities. We embedded MCF7 spheroids within three-dimensional (3D) collagen matrices and agarose matrices. MCF7 cells adhere to collagen but not agarose. Contrasting the results between these two matrices allows us to infer the role of cell-matrix adhesion. We found that MCF7 spheroids exhibited the fastest growth rate when embedded in a collagen matrix with a rigidity of 5.1 kPa (0.5 mg/mL collagen), whereas, for the agarose matrix, the rigidity for the fastest growth rate is 15 kPa (1.0% agarose) instead. This discrepancy is attributable to the presence of cell adhesion molecules in the collagen matrix, which initiates collagen matrix remodeling and facilitates cell migration from the tumor through the EMT. As breast tumors do not adhere to agarose matrices, it is suitable to simulate the cell-cell interactions during the early stage of breast tumor growth. We conducted further analysis to characterize the stresses exerted by the expanding spheroid on the agarose matrix. We identified two distinct MCF7 cell populations, namely, those that are non-dividing and those that are dividing, which exerted low and high expansion stresses on the agarose matrix, respectively. We confirmed this using Western blot which showed the upregulation of proliferating cell nuclear antigen, a proliferation marker, in spheroids grown in the 1.0% agarose (≈13 kPa). By treating the embedded MCF7 spheroids with an inhibitor or activator of myosin contractility, we showed that the optimum spheroids' growth can be increased or decreased, respectively. This finding suggests that tumor growth in the early stage, where cell-cell interaction is more prominent, is determined by actomyosin tension, which alters cell rounding pressure during cell division. However, when breast tumors begin generating collagen into the surrounding matrix, collagen remodeling triggers EMT to promote cell migration and invasion, ultimately leading to metastasis.

Extracellular matrix stiffness controls cardiac fibroblast proliferation via the nuclear factor-Y (NF-Y) transcription factor

The proliferative expansion of cardiac fibroblasts (CF) contributes towards cardiac fibrosis, which results in myocardial stiffening, cardiac dysfunction, and heart failure. CF sense and respond to increased stiffness of their local extracellular matrix, modulating their phenotype towards increased collagen synthesis and higher proliferation, leading potentially to a vicious circle of positive feedback. Here we describe a novel mechanism that mediates increased CF proliferation in response to a pathologically stiff Exteracellular matrix (ECM). The mechanism we describe is independent of the well-characterised mechano-sensitive transcript factors, YAP-TEAD and MKL1-SRF, which our data indicate are only responsible for part of the genes induced by stiffened ECM. Instead, our data identify Nuclear Factor-Y (NF-Y) as a novel mechanosensitive transcription factor, which mediates enhanced CF proliferation in response to a stiff ECM. We show that levels of NF-YA protein, the major regulatory subunit of NF-Y, and NF-Y transcriptional activity, are increased by a stiff ECM. Indeed, NF-Y activity drives the expression of multiple cell-cycle genes. Furthermore, NF-YA protein levels are dependent on FAK signalling suggesting a mechanistic link to ECM composition. Consistent with its role as a mechano-sensor, inhibition of NF-Y using siRNA or dominant negative mutant blocks CF proliferation on plastic in vitro, which models a stiff ECM, whereas ectopic expression of NF-YA increases the proliferation of cells interacting under conditions that model a physiologically soft ECM. In summary, our data demonstrate that NF-Y is a biomechanically sensitive transcription factor that promotes CF proliferation in a model of pathologically stiffened ECM.

Septin9 DNA methylation is associated with breast cancer recurrence or metastasis

OBJECTIVE

We aimed to explore the prognostic value of Septin9 DNA methylation in breast cancer.

METHODS

Breast cancer patients with and without recurrence or metastasis and matched non-breast cancer patients were screened retrospectively from 2014 to 2016. Bisulfite conversion and fluorescence quantitative methylation-specific polymerase chain reaction were used to detect the Septin9 methylation status and distribution levels in patient breast tissues.

RESULTS

Septin9 DNA methylation was more frequent in breast cancer tissues than in non-breast cancer tissues, but was not significantly correlated with any relevant breast cancer patient clinicopathological characteristic. Septin9 methylation rates were higher in patients with recurrence or metastasis. Septin9 methylation, tumor size, lymph node status, and progesterone receptor (PR) expression could influence prognosis. Septin9 methylation was significantly associated with worse disease-free survival in breast cancer patients, with receiver operating characteristic curve analysis indicating that it had good prognostic ability, with an area under the curve (AUC) value of 0.719. The AUC values increased when Septin9 methylation was combined with tumor size, lymph node status, and PR to predict prognosis.

CONCLUSIONS

Septin9 DNA methylation was an independent predictors of breast cancer prognostic risk. This could possibly help improve comprehensive prognosis prediction methods when combined with other risk factors.

The influence of microenvironment stiffness on endothelial cell fate: Implication for occurrence and progression of atherosclerosis

Atherosclerosis, the primary cause of cardiovascular diseases (CVDs), is characterized by phenotypic changes in fibrous proliferation, chronic inflammation and lipid accumulation mediated by vascular endothelial cells (ECs) and vascular smooth muscle cells (SMCs) which are correlated with the stiffening and ectopic remodeling of local extracellular matrix (ECM). The native residents, ECs and SMCs, are not only affected by various chemical factors including inflammatory mediators and chemokines, but also by a range of physical stimuli, such as shear stress and ECM stiffness, presented in the microenvironmental niche. Especially, ECs, as a semi-selective barrier, can sense mechanical forces, respond quickly to changes in mechanical loading and provide context-specific adaptive responses to restore homeostasis. However, blood arteries undergo stiffening and lose their elasticity with age. Reports have shown that the ECM stiffening could influence EC fate by changing the cell adhesion, spreading, proliferation, cell to cell contact, migration and even communication with SMCs. The cell behaviour changes mediated by ECM stiffening are dependent on the activation of a signaling cascade of mechanoperception and mechanotransduction. Although the substantial evidence directly indicates the importance of ECM stiffening on the native ECs, the understanding about this complex interplay is still largely limited. In this review, we systematically summarize the roles of ECM stiffening on the behaviours of endothelial cells and elucidate the underlying details in biological mechanism, aiming to provide the process of how ECs integrate ECM mechanics and the highlights for bioaffinity of tissue-specific engineered scaffolds.

Endothelial glycocalyx sensitivity to chemical and mechanical sub-endothelial substrate properties

Glycocalyx (GCX) is a carbohydrate-rich structure that coats the surface of endothelial cells (ECs) and lines the blood vessel lumen. Mechanical perturbations in the vascular environment, such as blood vessel stiffness, can be transduced and sent to ECs through mechanosensors such as GCX. Adverse stiffness alters GCX-mediated mechanotransduction and leads to EC dysfunction and eventually atherosclerotic cardiovascular diseases. To understand GCX-regulated mechanotransduction events, an in vitro model emulating in vivo vessel conditions is needed. To this end, we investigated the impact of matrix chemical and mechanical properties on GCX expression via fabricating a tunable non-swelling matrix based on the collagen-derived polypeptide, gelatin. To study the effect of matrix composition, we conducted a comparative analysis of GCX expression using different concentrations (60-25,000 μg/mL) of gelatin and gelatin methacrylate (GelMA) in comparison to fibronectin (60 μg/mL), a standard coating material for GCX-related studies. Using immunocytochemistry analysis, we showed for the first time that different substrate compositions and concentrations altered the overall GCX expression on human umbilical vein ECs (HUVECs). Subsequently, GelMA hydrogels were fabricated with stiffnesses of 2.5 and 5 kPa, representing healthy vessel tissues, and 10 kPa, corresponding to diseased vessel tissues. Immunocytochemistry analysis showed that on hydrogels with different levels of stiffness, the GCX expression in HUVECs remained unchanged, while its major polysaccharide components exhibited dysregulation in distinct patterns. For example, there was a significant decrease in heparan sulfate expression on pathological substrates (10 kPa), while sialic acid expression increased with increased matrix stiffness. This study suggests the specific mechanisms through which GCX may influence ECs in modulating barrier function, immune cell adhesion, and mechanotransduction function under distinct chemical and mechanical conditions of both healthy and diseased substrates.

Modular multiwell viscoelastic hydrogel platform for two- and three-dimensional cell culture applications

Hydrogels have gained significant popularity as model platforms to study the reciprocal interactions between cells and their microenvironment. While hydrogel tools to probe many characteristics of the extracellular space have been developed, fabrication approaches remain challenging and time-consuming, limiting multiplexing or widespread adoption. Thus, we have developed a modular fabrication approach to generate distinct hydrogel microenvironments within 96-well plates for increased throughput of fabrication as well as integration with existing high-throughput assay technologies. This approach enables in situ hydrogel mechanical characterization and was used to generate both elastic and viscoelastic hydrogels across a range of stiffnesses. Additionally, this fabrication method enabled a 3-fold reduction in polymer and up to an 8-fold reduction in fabrication time required per hydrogel replicate. The feasibility of this platform for cell culture applications was demonstrated by measuring both population-level and single cell-level metrics via microplate reader and high-content imaging. Finally, the 96-well hydrogel array was utilized for 3D cell culture, demonstrating the ability to support high cell viability. Together, this work demonstrates a versatile and easily adoptable fabrication approach that can support the ever-expanding tool kit of hydrogel technologies for cell culture applications.

3D gradient printing based on digital light processing

3D gradient printing is a type of fabrication technique that builds three-dimensional objects with gradually changing properties. Gradient digital light processing based 3D printing has garnered considerable attention in recent years. This function-oriented technology precisely manipulates the performance of different positions of materials and prints them as a monolithic structure to realize specific functions. This review presents a conceptual understanding of gradient properties, covering an overview of current techniques and materials that can produce gradient structures, as well as their limitations and challenges. The principle of digital light processing (DLP) technology and feasible strategies for 3D gradient printing to overcome any barriers are also presented. Additionally, this review discusses the promising future of 4D bioprinting systems based on DLP printing.

Understanding Arrhythmogenic Cardiomyopathy: Advances through the Use of Human Pluripotent Stem Cell Models

Cardiomyopathies (CMPs) represent a significant healthcare burden and are a major cause of heart failure leading to premature death. Several CMPs are now recognized to have a strong genetic basis, including arrhythmogenic cardiomyopathy (ACM), which predisposes patients to arrhythmic episodes. Variants in one of the five genes (PKP2, JUP, DSC2, DSG2, and DSP) encoding proteins of the desmosome are known to cause a subset of ACM, which we classify as desmosome-related ACM (dACM). Phenotypically, this disease may lead to sudden cardiac death in young athletes and, during late stages, is often accompanied by myocardial fibrofatty infiltrates. While the pathogenicity of the desmosome genes has been well established through animal studies and limited supplies of primary human cells, these systems have drawbacks that limit their utility and relevance to understanding human disease. Human induced pluripotent stem cells (hiPSCs) have emerged as a powerful tool for modeling ACM in vitro that can overcome these challenges, as they represent a reproducible and scalable source of cardiomyocytes (CMs) that recapitulate patient phenotypes. In this review, we provide an overview of dACM, summarize findings in other model systems linking desmosome proteins with this disease, and provide an up-to-date summary of the work that has been conducted in hiPSC-cardiomyocyte (hiPSC-CM) models of dACM. In the context of the hiPSC-CM model system, we highlight novel findings that have contributed to our understanding of disease and enumerate the limitations, prospects, and directions for research to consider towards future progress.

Control of blood capillary networks and holes in blood-brain barrier models by regulating elastic modulus of scaffolds

The blood-brain barrier (BBB) is a type of capillary network characterized by a highly selective barrier, which restricts the transport of substances between the blood and nervous system. Numerous in vitro models of the BBB have been developed for drug testing, but a BBB model with controllable capillary structures remains a major challenge. In this study, we report for the first time a unique method of controlling the blood capillary networks and characteristic holes formation in a BBB model by varying the elastic modulus of a three-dimensional scaffold. The characteristic hole structures are formed by the migration of endothelial cells from the model surface to the interior, which have functions of connecting the model interior to the external environment. The hole depth increased, as the elastic modulus of the fibrin gel scaffold increased, and the internal capillary network length increased with decreasing elastic modulus. Besides, internal astrocytes and pericytes were also found to be important for inducing hole formation from the model surface. Furthermore, RNA sequencing indicated up-regulated genes related to matrix metalloproteinases and angiogenesis, suggesting a relationship between enzymatic degradation of the scaffolds and hole formation. The findings of this study introduce a new method of fabricating complex BBB models for drug assessment.

Strategies for in situ tissue engineering of vascularized bone regeneration (Review)

Numerous physiological processes occur following bone fracture, including inflammatory cell recruitment, vascularization, and callus formation and remodeling. In particular circumstances, such as critical bone defects or osteonecrosis, the regenerative microenvironment is compromised, rendering endogenous stem/progenitor cells incapable of fully manifesting their reparative potential. Consequently, external interventions, such as grafting or augmentation, are frequently necessary. In situ bone tissue engineering (iBTE) employs cell-free scaffolds that possess microenvironmental cues, which, upon implantation, redirect the behavior of endogenous stem/progenitor cells towards a pro-regenerative inflammatory response and reestablish angiogenesis-osteogenesis coupling. This process ultimately results in vascularized bone regeneration (VBR). In this context, a comprehensive review of the current techniques and modalities in VBR-targeted iBTE technology is provided.

Matrix stiffness enhances cancer-macrophage interactions and M2-like macrophage accumulation in the breast tumor microenvironment

The role of intratumor heterogeneity is becoming increasingly apparent in part due to expansion in single cell technologies. Clinically, tumor heterogeneity poses several obstacles to effective cancer therapy dealing with biomarker variability and treatment responses. Matrix stiffening is known to occur during tumor progression and contribute to pathogenesis in several cancer hallmarks, including tumor angiogenesis and metastasis. However, the effects of matrix stiffening on intratumor heterogeneity have not been thoroughly studied. In this study, we applied single-cell RNA sequencing to investigate the differences in the transcriptional landscapes between stiff and compliant MMTV-PyMT mouse mammary tumors. We found similar compositions of cancer and stromal subpopulations in compliant and stiff tumors but differential intercellular communication and a significantly higher concentration of tumor-promoting, M2-like macrophages in the stiffer tumor microenvironments. Interestingly, we found that cancer cells seeded on stiffer substrates recruited more macrophages. Furthermore, elevated matrix stiffness increased Colony Stimulating Factor 1 (CSF-1) expression in breast cancer cells and reduction of CSF-1 expression on stiffer substrates reduced macrophage recruitment. Thus, our results demonstrate that tissue phenotypes were conserved between stiff and compliant tumors but matrix stiffening altered cell-cell interactions which may be responsible for shifting the phenotypic balance of macrophages residing in the tumor microenvironment towards a pro-tumor progression M2 phenotype. STATEMENT OF SIGNIFICANCE: Cells within tumors are highly heterogeneous, posing challenges with treatment and recurrence. While increased tissue stiffness can promote several hallmarks of cancer, its effects on tumor heterogeneity are unclear. We used single-cell RNA sequencing to investigate the differences in the transcriptional landscapes between stiff and compliant MMTV-PyMT mouse mammary tumors. We found similar compositions of cancer and stromal subpopulations in compliant and stiff tumors but differential intercellular communication and a significantly higher concentration of tumor-promoting, M2-like macrophages in the stiffer tumor microenvironments. Using a biomaterial-based platform, we found that cancer cells seeded on stiffer substrates recruited more macrophages, supporting our in vivo findings. Together, our results demonstrate a key role of matrix stiffness in affecting cell-cell communication and macrophage recruitment.

Endothelium and Subendothelial Matrix Mechanics Modulate Cancer Cell Transendothelial Migration

Cancer cell extravasation, a key step in the metastatic cascade, involves cancer cell arrest on the endothelium, transendothelial migration (TEM), followed by the invasion into the subendothelial extracellular matrix (ECM) of distant tissues. While cancer research has mostly focused on the biomechanical interactions between tumor cells (TCs) and ECM, particularly at the primary tumor site, very little is known about the mechanical properties of endothelial cells and the subendothelial ECM and how they contribute to the extravasation process. Here, an integrated experimental and theoretical framework is developed to investigate the mechanical crosstalk between TCs, endothelium and subendothelial ECM during in vitro cancer cell extravasation. It is found that cancer cell actin-rich protrusions generate complex push-pull forces to initiate and drive TEM, while transmigration success also relies on the forces generated by the endothelium. Consequently, mechanical properties of the subendothelial ECM and endothelial actomyosin contractility that mediate the endothelial forces also impact the endothelium's resistance to cancer cell transmigration. These results indicate that mechanical features of distant tissues, including force interactions between the endothelium and the subendothelial ECM, are key determinants of metastatic organotropism.

Cell response to mechanical microenvironment cues via Rho signaling: From mechanobiology to mechanomedicine

Mechanical cues in the cell microenvironment such as those from extracellular matrix properties, stretching, compression and shear stress, play a critical role in maintaining homeostasis. Upon sensing mechanical stimuli, cells can translate these external forces into intracellular biochemical signals to regulate their cellular behaviors, but the specific mechanisms of mechanotransduction at the molecular level remain elusive. As a subfamily of the Ras superfamily, Rho GTPases have been recognized as key intracellular mechanotransduction mediators that can regulate multiple cell activities such as proliferation, migration and differentiation as well as biological processes such as cytoskeletal dynamics, metabolism, and organ development. However, the upstream mechanosensors for Rho proteins and downstream effectors that respond to Rho signal activation have not been well illustrated. Moreover, Rho-mediated mechanical signals in previous studies are highly context-dependent. In this review, we systematically summarize the types of mechanical cues in the cell microenvironment and provide recent advances on the roles of the Rho-based mechanotransduction in various cell activities, physiological processes and diseases. Comprehensive insights into the mechanical roles of Rho GTPase partners would open a new paradigm of mechanomedicine for a variety of diseases. STATEMENT OF SIGNIFICANCE: In this review, we highlight the critical role of Rho GTPases as signal mediators to respond to physical cues in microenvironment. This article will add a distinct contribution to this set of knowledge by intensively addressing the relationship between Rho signaling and mechanobiology/mechanotransduction/mechanomedcine. This topic has not been discussed by the journal, nor has it yet been developed by the field. The comprehensive picture that will develop, from molecular mechanisms and engineering methods to disease treatment strategies, represents an important and distinct contribution to the field. We hope that this review would help researchers in various fields, especially clinicians, oncologists and bioengineers, who study Rho signal pathway and mechanobiology/mechanotransduction, understand the critical role of Rho GTPase in mechanotransduction.

Effects of Elasticity on Cell Proliferation in a Tissue-Engineering Scaffold Pore

Scaffolds engineered for in vitro tissue engineering consist of multiple pores where cells can migrate along with nutrient-rich culture medium. The presence of the nutrient medium throughout the scaffold pores promotes cell proliferation, and this process depends on several factors such as scaffold geometry, nutrient medium flow rate, shear stress, cell-scaffold focal adhesions and elastic properties of the scaffold material. While numerous studies have addressed the first four factors, the mathematical approach described herein focuses on cell proliferation rate in elastic scaffolds, under constant flux of nutrients. As cells proliferate, the scaffold pores radius shrinks and thus, in order to sustain the nutrient flux, the inlet applied pressure on the upstream side of the scaffold pore must be increased. This results in expansion of the elastic scaffold pore, which in turn further increases the rate of cell proliferation. Considering the elasticity of the scaffold, the pore deformation allows further cellular growth beyond that of inelastic conditions. In this paper, our objectives are as follows: (i) Develop a mathematical model for describing fluid dynamics, scaffold elasticity and cell proliferation for scaffolds consist of identical nearly cylindrical pores; (ii) Solve the models and then simulate cellular proliferation within an elastic pore. The simulation can emulate real life tissue growth in a scaffold and offer a solution which reduces the numerical burdens. Lastly, our results demonstrated are in qualitative agreement with experimental observations reported in the literature.

Substrate Stiffness Regulates the Proliferation and Apoptosis of Periodontal Ligament Cells through Integrin-Linked Kinase ILK

The hallmark of orthodontic tooth movement (OTM) is time-consuming during clinical treatments. The acceleration of OTM through modulating proliferation and apoptosis of periodontal ligament cells (PDLCs) possesses the potential application in clinical treatments. Here, we established an in vitro model with a graded increase in substrate stiffness to investigate the underlying mechanism of proliferation and apoptosis of PDLCs. The role of the integrin-linked kinase (ILK) in response to substrate stiffness was investigated by the depletion model of PDLCs. We found that the proliferation and apoptosis of PDLCs show a stiffness-dependent property with stiffer substrates favoring increased bias at the transcript level. Depleting integrin-linked kinase diluted the correlation between PDLCs behaviors and substrate stiffness. Our results suggest that ILK plays a significant role in modulating PDLC proliferation and apoptosis and can serve as a potential target for accelerating OTM.

Atherosclerosis and endothelial mechanotransduction: current knowledge and models for future research

Endothelium health is essential to the regulation of physiological vascular functions. Because of the critical capability of endothelial cells (ECs) to sense and transduce chemical and mechanical signals in the local vascular environment, their dysfunction is associated with a vast variety of vascular diseases and injuries, especially atherosclerosis and subsequent cardiovascular diseases. This review describes the mechanotransduction events that are mediated through ECs, the EC subcellular components involved, and the pathways reported to be potentially involved. Up-to-date research efforts involving in vivo animal models and in vitro biomimetic models are also discussed, including their advantages and drawbacks, with recommendations on future modeling approaches to aid the development of novel therapies targeting atherosclerosis and related cardiovascular diseases.

Mechanical regulation of the early stages of angiogenesis

Favouring or thwarting the development of a vascular network is essential in fields as diverse as oncology, cardiovascular disease or tissue engineering. As a result, understanding and controlling angiogenesis has become a major scientific challenge. Mechanical factors play a fundamental role in angiogenesis and can potentially be exploited for optimizing the architecture of the resulting vascular network. Largely focusing on in vitro systems but also supported by some in vivo evidence, the aim of this Highlight Review is dual. First, we describe the current knowledge with particular focus on the effects of fluid and solid mechanical stimuli on the early stages of the angiogenic process, most notably the destabilization of existing vessels and the initiation and elongation of new vessels. Second, we explore inherent difficulties in the field and propose future perspectives on the use of in vitro and physics-based modelling to overcome these difficulties.

Recent advancements and future requirements in vascularization of cortical organoids

The fields of tissue engineering and disease modeling have become increasingly cognizant of the need to create complex and mature structures in vitro to adequately mimic the in vivo niche. Specifically for neural applications, human brain cortical organoids (COs) require highly stratified neurons and glial cells to generate synaptic functions, and to date, most efforts achieve only fetal functionality at best. Moreover, COs are usually avascular, inducing the development of necrotic cores, which can limit growth, development, and maturation. Recent efforts have attempted to vascularize cortical and other organoid types. In this review, we will outline the components of a fully vascularized CO as they relate to neocortical development in vivo. These components address challenges in recapitulating neurovascular tissue patterning, biomechanical properties, and functionality with the goal of mirroring the quality of organoid vascularization only achieved with an in vivo host. We will provide a comprehensive summary of the current progress made in each one of these categories, highlighting advances in vascularization technologies and areas still under investigation.

Combined role for YAP-TEAD and YAP-RUNX2 signalling in substrate-stiffness regulation of cardiac fibroblast proliferation

Cardiac fibrosis is associated with increased stiffness of the myocardial extracellular matrix (ECM) in part mediated by increased cardiac fibroblast proliferation However, our understanding of the mechanisms regulating cardiac fibroblast proliferation are incomplete. Here we characterise a novel mechanism involving a combined activation of Yes-associated protein (YAP) targets RUNX Family Transcription Factor 2 (RUNX2) and TEA Domain Transcription Factor (TEAD). We demonstrate that cardiac fibroblast proliferation is enhanced by interaction with a stiff ECM compared to a soft ECM. This is associated with activation of the transcriptional co-factor, YAP. We demonstrate that this stiffness induced activation of YAP enhances the transcriptional activity of both TEAD and RUNX2 transcription factors. Inhibition of either TEAD or RUNX2, using gene silencing, expression of dominant-negative mutants or pharmacological inhibition, reduces cardiac fibroblast proliferation. Using mutants of YAP, defective in TEAD or RUNX2 activation ability, we demonstrate a dual role of YAP-mediated activation of TEAD and RUNX2 for substrate stiffness induced cardiac fibroblast proliferation. Our data highlights a previously unrecognised role of YAP mediated RUNX2 activation for cardiac fibroblast proliferation in response to increased ECM stiffness.

SEPT9 Upregulation in Satellite Glial Cells Associated with Diabetic Polyneuropathy in a Type 2 Diabetes-like Rat Model

Despite the worldwide prevalence and severe complications of type 2 diabetes mellitus (T2DM), the pathophysiological mechanisms underlying the development of diabetic polyneuropathy (DPN) are poorly understood. Beyond strict control of glucose levels, clinical trials for reversing DPN have largely failed. Therefore, understanding the pathophysiological and molecular mechanisms underlying DPN is crucial. Accordingly, this study explored biochemical and neuropathological deficits in a rat model of T2DM induced through high-fat diet (HFD) feeding along with two low-dose streptozotocin (STZ) injections; the deficits were explored through serum lipid, neurobehavioral, neurophysiology, neuropathology, and immunohistochemistry examinations. Our HFD/STZ protocol induced (1) mechanical hyperalgesia and depression-like behaviors, (2) loss of intraepidermal nerve fibers (IENFs) and reduced axonal diameters in sural nerves, and (3) decreased compound muscle action potential. In addition to hyperglycemia, which was correlated with the degree of mechanical hyperalgesia and loss of IENFs, we observed that hypertriglyceridemia was the most dominant deficit in the lipid profiles of the diabetic rats. In particular, SEPT9, the fourth component of the cytoskeleton, increased in the satellite glial cells (SGCs) of the dorsal root ganglia (DRG) in the T2DM-like rats. The number of SEPT9(+) SGCs/DRG was correlated with serum glucose levels and mechanical thresholds. Our findings indicate the putative molecular mechanism underlying DPN, which presumably involves the interaction of SGCs and DRG neurons; nevertheless, further functional research is warranted to clarify the role of SEPT9 in DPN.

Extracellular Matrix Stiffness in Lung Health and Disease

The extracellular matrix (ECM) provides structural support and imparts a wide variety of environmental cues to cells. In the past decade, a growing body of work revealed that the mechanical properties of the ECM, commonly known as matrix stiffness, regulate the fundamental cellular processes of the lung. There is growing appreciation that mechanical interplays between cells and associated ECM are essential to maintain lung homeostasis. Dysregulation of ECM-derived mechanical signaling via altered mechanosensing and mechanotransduction pathways is associated with many common lung diseases. Matrix stiffening is a hallmark of lung fibrosis. The stiffened ECM is not merely a sequelae of lung fibrosis but can actively drive the progression of fibrotic lung disease. In this article, we provide a comprehensive view on the role of matrix stiffness in lung health and disease. We begin by summarizing the effects of matrix stiffness on the function and behavior of various lung cell types and on regulation of biomolecule activity and key physiological processes, including host immune response and cellular metabolism. We discuss the potential mechanisms by which cells probe matrix stiffness and convert mechanical signals to regulate gene expression. We highlight the factors that govern matrix stiffness and outline the role of matrix stiffness in lung development and the pathogenesis of pulmonary fibrosis, pulmonary hypertension, asthma, chronic obstructive pulmonary disease (COPD), and lung cancer. We envision targeting of deleterious matrix mechanical cues for treatment of fibrotic lung disease. Advances in technologies for matrix stiffness measurements and design of stiffness-tunable matrix substrates are also explored. © 2022 American Physiological Society. Compr Physiol 12:3523-3558, 2022.

A 3D multi-agent-based model for lumen morphogenesis: the role of the biophysical properties of the extracellular matrix

The correct function of many organs depends on proper lumen morphogenesis, which requires the orchestration of both biological and mechanical aspects. However, how these factors coordinate is not yet fully understood. Here, we focus on the development of a mechanistic model for computationally simulating lumen morphogenesis. In particular, we consider the hydrostatic pressure generated by the cells' fluid secretion as the driving force and the density of the extracellular matrix as regulators of the process. For this purpose, we develop a 3D agent-based-model for lumen morphogenesis that includes cells' fluid secretion and the density of the extracellular matrix. Moreover, this computer-based model considers the variation in the biological behavior of cells in response to the mechanical forces that they sense. Then, we study the formation of the lumen under different-mechanical scenarios and conclude that an increase in the matrix density reduces the lumen volume and hinders lumen morphogenesis. Finally, we show that the model successfully predicts normal lumen morphogenesis when the matrix density is physiological and aberrant multilumen formation when the matrix density is excessive.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00366-022-01654-1.

Force Sensing on Cells and Tissues by Atomic Force Microscopy

Biosensors are aimed at detecting tiny physical and chemical stimuli in biological systems. Physical forces are ubiquitous, being implied in all cellular processes, including cell adhesion, migration, and differentiation. Given the strong interplay between cells and their microenvironment, the extracellular matrix (ECM) and the structural and mechanical properties of the ECM play an important role in the transmission of external stimuli to single cells within the tissue. Vice versa, cells themselves also use self-generated forces to probe the biophysical properties of the ECM. ECM mechanics influence cell fate, regulate tissue development, and show peculiar features in health and disease conditions of living organisms. Force sensing in biological systems is therefore crucial to dissecting and understanding complex biological processes, such as mechanotransduction. Atomic Force Microscopy (AFM), which can both sense and apply forces at the nanoscale, with sub-nanonewton sensitivity, represents an enabling technology and a crucial experimental tool in biophysics and mechanobiology. In this work, we report on the application of AFM to the study of biomechanical fingerprints of different components of biological systems, such as the ECM, the whole cell, and cellular components, such as the nucleus, lamellipodia and the glycocalyx. We show that physical observables such as the (spatially resolved) Young's Modulus (YM) of elasticity of ECMs or cells, and the effective thickness and stiffness of the glycocalyx, can be quantitatively characterized by AFM. Their modification can be correlated to changes in the microenvironment, physio-pathological conditions, or gene regulation.

Modulation of Cell-Cycle Progression by Hydrogen Peroxide-Mediated Cross-Linking and Degradation of Cell-Adhesive Hydrogels

The cell cycle is known to be regulated by features such as the mechanical properties of the surrounding environment and interaction of cells with the adhering substrates. Here, we investigated the possibility of regulating cell-cycle progression of the cells on gelatin/hyaluronic acid composite hydrogels obtained through hydrogen peroxide (H₂O₂)-mediated cross-linking and degradation of the polymers by varying the exposure time to H₂O₂ contained in the air. The stiffness of the hydrogel varied with the exposure time. Human cervical cancer cells (HeLa) and mouse mammary gland epithelial cells (NMuMG) expressing cell-cycle reporter Fucci2 showed the exposure-time-dependent different cell-cycle progressions on the hydrogels. Although HeLa/Fucci2 cells cultured on the soft hydrogel (Young’s modulus: 0.20 and 0.40 kPa) obtained through 15 min and 120 min of the H₂O₂ exposure showed a G2/M-phase arrest, NMuMG cells showed a G1-phase arrest. Additionally, the cell-cycle progression of NMuMG cells was not only governed by the hydrogel stiffness, but also by the low-molecular-weight HA resulting from H₂O₂-mediated degradation. These results indicate that H₂O₂-mediated cross-linking and degradation of gelatin/hyaluronic acid composite hydrogel could be used to control the cell adhesion and cell-cycle progression.

Substrate stiffening promotes VEGF-A functions via the PI3K/Akt/mTOR pathway

While it is now well-established that substrate stiffness regulates vascular endothelial growth factor-A (VEGF-A) mediated signaling and functions, causal mechanisms remain poorly understood. Here, we report an underlying role for the PI3K/Akt/mTOR signaling pathway. This pathway is activated on stiffer substrates, is amplified by VEGF-A stimulation, and correlates with enhanced endothelial cell (EC) proliferation, contraction, pro-angiogenic secretion, and capillary-like tube formation. In the settings of advanced age-related macular degeneration, characterized by EC and retinal pigment epithelial (RPE)-mediated angiogenesis, these data implicate substrate stiffness as a novel causative mechanism and Akt/mTOR inhibition as a novel therapeutic pathway.

Gellan gum-gelatin viscoelastic hydrogels as scaffolds to promote fibroblast differentiation

Fabricating hydrogel scaffolds that are both bioreactive toward fibroblasts while still mechanically compatible with surrounding tissue is a major challenge in tissue engineering. This is because the outcome of scaffold implantation is largely determined by fibroblasts differentiating toward myofibroblasts, which is characterized by the expression of α-smooth muscle actin (α-SMA). Previous studies promoted fibroblasts differentiation by increasing scaffold substrate stiffness. However, the stiffness of scaffold has to be compatible with surrounding tissue, as mismatched stiffness may cause initial hyperplasia and inappropriate endothelial layer development. Therefore, we adjusted the hydrogel chemical component, and thus viscoelasticity to affect the mechano-signaling of fibroblasts and promote fibroblasts differentiation. Elastic gellan gum and viscoelastic gelatin were hybridized at different ratios to fabricate hydrogel scaffold with varied stress-relaxation. Vitronectin (VN) was used to further regulate the interaction between fibroblasts and the substrate. Fibroblast differentiation, characterized by α-SMA area per cell, increased from~3000-4000 μm²/cell on less viscoelastic gels to ~5000 μm²/cell on the most viscoelastic gel. Fibroblasts seeded on hydrogels had a slower migration rate on more viscoelastic hydrogels (slowest at 38 ± 14 μm/h) compared to the migration speed on less viscoelastic hydrogels (74 ± 20 μm/h). VN slowed the migration speed on all hydrogels. The organization of collagen deposited by fibroblasts cultured on the hydrogels was characterized by second harmonic generation (SHG), which showed that collagen was more organized (parallel) on more viscoelastic hydrogels. In summary, we provided a novel strategy to fabricate hydrogel scaffolds that can promote fibroblasts differentiation while keeping the stiffness compatible with blood vessels. The most viscoelastic hydrogel studied here meets these requirements best.

Sex-Specific Response to Combinations of Shear Stress and Substrate Stiffness by Endothelial Cells In Vitro

By using a full factorial design of experiment, the combinatorial effects of biological sex, shear stress, and substrate stiffness on human umbilical vein endothelial cell (HUVEC) spreading and Yes-associated protein 1 (YAP1) activity are able to be efficiently evaluated. Within the range of shear stress (0.5-1.5 Pa) and substrate stiffness (10-100 kPa), male HUVECs are smaller than female HUVECs. Only with sufficient mechanical stimulation do they spread to a similar size. More importantly, YAP1 nuclear localization in female HUVECs is invariant to mechanical stimulation within the range of tested conditions whereas for male HUVECs it increases nonlinearly with increasing shear stress and substrate stiffness. The sex-specific response of HUVECs to combinations of shear stress and substrate stiffness reinforces the need to include sex as a biological variable and multiple mechanical stimuli in experiments, informs the design of precision biomaterials, and offers insight for understanding cardiovascular disease sexual dimorphisms. Moreover, here it is illustrated that different complex mechanical microenvironments can lead to sex-specific phenotypes and sex invariant phenotypes in cultured endothelial cells.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

The glycocalyx core protein Glypican 1 protects vessel wall endothelial cells from stiffness-mediated dysfunction and disease

AIMS

Arterial stiffness is an underlying risk factor and a hallmark of cardiovascular diseases. The endothelial cell (EC) glycocalyx is a glycan rich surface layer that plays a key role in protecting against EC dysfunction and vascular disease. However, the mechanisms by which arterial stiffness promotes EC dysfunction and vascular disease are not fully understood, and whether the mechanism involves the protective endothelial glycocalyx is yet to be determined. We hypothesized that endothelial glycocalyx protects the endothelial cells lining the vascular wall from dysfunction and disease in response to arterial stiffness.

METHODS AND RESULTS

Cells cultured on polyacrylamide (PA) gels of substrate stiffness 10 kPa (mimicking the subendothelial stiffness of aged, unhealthy arteries) showed a significant inhibition of glycocalyx expression compared to cells cultured on softer PA gels (2.5 kPa, mimicking the subendothelial stiffness of young, healthy arteries). Specifically, gene and protein analyses revealed that a glycocalyx core protein Glypican 1 was inhibited in cells cultured on stiff PA gels. These cells had enhanced endothelial cell dysfunction as determined by enhanced cell inflammation (enhanced inflammatory gene expression, monocyte adhesion, and inhibited nitric oxide expression), proliferation, and EndMT. Removal of Glypican 1 using gene-specific silencing with siRNA or gene overexpression using a plasmid revealed that Glypican 1 is required to protect against stiffness-mediated endothelial cell dysfunction. Consistent with this, using a model of age-mediated stiffness, older mice exhibited a reduced expression of Glypican 1 and enhanced endothelial cell dysfunction compared to young mice. Glypican 1 gene deletion in knockout mice (GPC1-/-) exacerbated endothelial dysfunction in young mice, which normally had high endothelial expression, but not in old mice that normally expressed low levels. Endothelial cell dysfunction was exacerbated in young, but not aged, Glypican 1 knockout mice (GPC1-/-).

CONCLUSION

Arterial stiffness promotes EC dysfunction and vascular disease at least partly through the suppression of the glycocalyx protein Glypican 1. Glypican 1 contributes to the protection against endothelial cell dysfunction and vascular disease in endothelial cells.

Influence of Hydrogen Peroxide-Mediated Cross-Linking and Degradation on Cell-Adhesive Gelatin Hydrogels

Hydrogen peroxide (H₂O₂) is widely used for the gelation of aqueous solutions of gelatin derivatives with phenolic hydroxyl groups (Gelatin-Ph) catalyzed by horseradish peroxidase (HRP). Apart from this, H₂O₂ is known to cause degradation/depolymerization of various polymers. Here, we prepared Gelatin-Ph hydrogels from solutions containing Gelatin-Ph and HRP by continuously supplying H₂O₂ from the gas phase and investigated the mechanical properties of resultant hydrogels and the behaviors of rat fibroblast and human adipose-derived stem cells on them. Young's modulus of the hydrogel obtained from 5 w/v % Gelatin-Ph and 1 and 5 U/mL HRP increased when the exposure time to air containing H₂O₂ (16 ppm) was extended from 15 to 30 min. However, further prolonging the exposure time to 60 min reduced Young's modulus to the same magnitude as for the hydrogels exposed to air containing H₂O₂ for 15 min. Interestingly, the cell length and aspect ratio of the cells continued to increase, as the exposure time was extended, without reflecting the decrease in Young's modulus. These results indicate that when preparing Gelatin-Ph hydrogels through HRP/H₂O₂-mediated gelation, it is necessary to consider the effect of the degradation of Gelatin-Ph caused by H₂O₂ on the mechanical properties of the resultant hydrogels and the behaviors of cells on them.

Protein Phosphatase 2A Mediates YAP Activation in Endothelial Cells Upon VEGF Stimulation and Matrix Stiffness

Angiogenesis is an essential process during development. Abnormal angiogenesis also contributes to many disease conditions such as tumor and retinal diseases. Previous studies have established the Hippo signaling pathway effector Yes-associated protein (YAP) as a crucial regulator of angiogenesis. In ECs, activated YAP promotes endothelial cell proliferation, migration and sprouting. YAP activity is regulated by vascular endothelial growth factor (VEGF) and mechanical cues such as extracellular matrix (ECM) stiffness. However, it is unclear how VEGF or ECM stiffness signal to YAP, especially how dephosphorylation of YAP occurs in response to VEGF stimulus or ECM stiffening. Here, we show that protein phosphatase 2A (PP2A) is required for this process. Blocking PP2A activity abolishes VEGF or ECM stiffening mediated YAP activation. Systemic administration of a PP2A inhibitor suppresses YAP activity in blood vessels in developmental and pathological angiogenesis mouse models. Consistently, PP2A inhibitor also inhibits sprouting angiogenesis. Mechanistically, PP2A directly interacts with YAP, and this interaction requires proper cytoskeleton dynamics. These findings identify PP2A as a crucial mediator of YAP activation in ECs and hence as an important regulator of angiogenesis.

Cellular functions of actin- and microtubule-associated septins

Septins are an integral component of the cytoskeleton, assembling into higher-order oligomers and filamentous polymers that associate with actin filaments, microtubules and membranes. Here, we review septin interactions with actin and microtubules, and septin-mediated regulation of the organization and dynamics of these cytoskeletal networks, which is critical for cellular morphogenesis. We discuss how actomyosin-associated septins function in cytokinesis, cell migration and host defense against pathogens. We highlight newly emerged roles of septins at the interface of microtubules and membranes with molecular motors, which point to a 'septin code' for the regulation of membrane traffic. Additionally, we revisit the functions of microtubule-associated septins in mitosis and meiosis. In sum, septins comprise a unique module of cytoskeletal regulators that are spatially and functionally specialized and have properties of bona fide actin-binding and microtubule-associated proteins. With many questions still outstanding, the study of septins will continue to provide new insights into fundamental problems of cytoskeletal organization and function.

A dual-ink 3D printing strategy to engineer pre-vascularized bone scaffolds in-vitro

A functional vascular supply is a key component of any large-scale tissue, providing support for the metabolic needs of tissue-remodeling cells. Although well-studied strategies exist to fabricate biomimetic scaffolds for bone regeneration, success rates for regeneration in larger defects can be improved by engineering microvascular capillaries within the scaffolds to enhance oxygen and nutrient supply to the core of the engineered tissue as it grows. Even though the role of calcium and phosphate has been well understood to enhance osteogenesis, it remains unclear whether calcium and phosphate may have a detrimental effect on the vasculogenic and angiogenic potential of endothelial cells cultured on 3D printed bone scaffolds. In this study, we presented a novel dual-ink bioprinting method to create vasculature interwoven inside CaP bone constructs. In this method, strands of a CaP ink and a sacrificial template material was used to form scaffolds containing CaP fibers and microchannels seeded with vascular endothelial and mesenchymal stem cells (MSCs) within a photo-crosslinkable gelatin methacryloyl (GelMA) hydrogel material. Our results show similar morphology of growing vessels in the presence of CaP bioink, and no significant difference in endothelial cell sprouting was found. Furthermore, our initial results showed the differentiation of hMSCs into pericytes in the presence of CaP ink. These results indicate the feasibility of creating vascularized bone scaffolds, which can be used for enhancing vascular formation in the core of bone scaffolds.

Matrix Stiffness Modulates Patient-Derived Glioblastoma Cell Fates in Three-Dimensional Hydrogels

Cancer progression is known to be accompanied by changes in tissue stiffness. Previous studies have primarily employed immortalized cell lines and 2D hydrogel substrates, which do not recapitulate the 3D tumor niche. How matrix stiffness affects patient-derived cancer cell fate in 3D remains unclear. In this study, we report a matrix metalloproteinase-degradable poly(ethylene-glycol)-based hydrogel platform with brain-mimicking biochemical cues and tunable stiffness (40-26,600 Pa) for 3D culture of patient-derived glioblastoma xenograft (PDTX GBM) cells. Our results demonstrate that decreasing hydrogel stiffness enhanced PDTX GBM cell proliferation, and hydrogels with stiffness 240 Pa and below supported robust PDTX GBM cell spreading in 3D. PDTX GBM cells encapsulated in hydrogels demonstrated higher drug resistance than 2D control, and increasing hydrogel stiffness further enhanced drug resistance. Such 3D hydrogel platforms may provide a valuable tool for mechanistic studies of the role of niche cues in modulating cancer progression for different cancer types.