Britta Trappmann

Vascular endothelial growth factor (VEGF), a major regulator of blood vessel formation, is naturally bound to heparan sulfate proteoglycans in the extracellular matrix (ECM). Yet, how the physical presentation of VEGF by the matrix impacts its signaling potential remains fully unknown. To address this question, we have developed a tunable heparin-containing hydrogel model that recapitulates natural VEGF binding modes with full and independent control over physical properties. Using this model, we show that the degree of heparin flexibility in the hydrogel network determines the mobility of bound VEGF and, in turn, its ability to interact with VEGF receptor 2. We demonstrate that VEGF mobility is driven by a relay mechanism, in which VEGF molecules directly switch from one heparin chain to another. When strong electrostatic interactions between heparin and other hydrogel constituents immobilize the sugar backbone, this relay mechanism is impeded, in turn reducing VEGF’s ability to reach its target receptor at the cell membrane, thereby reducing its bioactivity. This work identifies heparin flexibility within the ECM as a previously unknown regulator of the microenvironment, which will not only contribute to a better mechanistic understanding of how the ECM regulates growth factor bioactivity, but it will also provide an important design criterion for the development of tissue-engineered biomaterials that require vascularization.

We develop a biomimetic model to uncover the synergistic effects of luminal flow and chemokine gradients on angiogenic sprouting.

SUMMARY: Mechanical stimuli, particularly laminar blood flow, play a crucial role in shaping the vascular system. Changes in the rate of blood flow manifest in altered shear stress, which activates signaling cascades that drive vascular remodeling. Consistently, dysregulation of the endothelial response to fluid shear forces and aberrant flow patterns both lead to pathological conditions, including impaired blood vessel development and atherosclerosis. Despite its importance, the mechanisms driving the coordinated cell behavior underlying vascular remodeling are not fully understood. Combining classical cell biological approaches with advanced image analysis, mathematical modeling, biomimetic strategies, and in vivo studies, we identify the planar cell polarity (PCP) protein Vangl1 as an enforcer of flow-dependent cell dynamics in the vascular system. We demonstrate that shear stress triggers the relocation of Vangl1 from an internal reservoir to the plasma membrane at the initiation of cell remodeling. Membrane enrichment of Vangl1 is mediated by a Coronin1C-dependent shift in the equilibrium between endo- and exocytosis and results in the spatial reorganization of another essential PCP protein, Frizzled6 (Fzd6). The resulting mutual exclusion of the core PCP proteins Fzd6 and Vangl1 augments differential junctional and cytoskeletal dynamics along the flow axis. Loss of Vangl1 limits the ability of endothelial cells to respond to shear forces in a coordinated fashion, resulting in irregular cell alignment along the flow direction and erroneous vessel sprouting. Together, these studies introduce core PCP signaling as a determinant of collective cell dynamics and organization of the vascular system.

Tissue homeostasis and disease states rely on the formation of new blood vessels through angiogenic sprouting, which is tightly regulated by the properties of the surrounding extracellular matrix. While physical cues, such as matrix stiffness or degradability, have evolved as major regulators of cell function in tissue microenvironments, it remains unknown whether and how physical cues regulate endothelial cell migration during angiogenesis. To investigate this, a biomimetic model of angiogenic sprouting inside a tunable synthetic hydrogel is created. It is shown that endothelial cells sense the resistance of the surrounding matrix toward proteolytic cleavage and respond by adjusting their migration phenotype. The resistance cells encounter is impacted by the number of covalent matrix crosslinks, crosslink degradability, and the proteolytic activity of cells. When matrix resistance is high, cells switch from a collective to an actomyosin contractility‐dependent single cellular migration mode. This switch in collectivity is accompanied by a major reorganization of the actin cytoskeleton, where stress fibers are no longer visible, and F‐actin aggregates in large punctate clusters. Matrix resistance is identified as a previously unknown regulator of angiogenic sprouting and, thus, provides a mechanism by which the physical properties of the matrix impact cell migration modes through cytoskeletal remodeling.

In both physiological processes and disease contexts, migrating cells have the ability to adapt to conditions in their environment. As an in vivo model for this process, we use zebrafish primordial germ cells that migrate throughout the developing embryo. When migrating within an ectodermal environment, the germ cells form fewer and smaller blebs when compared with their behavior within mesodermal environment. We find that cortical tension of neighboring cells is a parameter that affects blebbing frequency. Interestingly, the change in blebbing activity is accompanied by the formation of more actin-rich protrusions. These alterations in cell behavior that correlate with changes in RhoA activity could allow the cells to maintain dynamic motility parameters, such as migration speed and track straightness, in different settings. In addition, we find that the polarity of the cells can be affected by stiff structures positioned in their migration path This article has an associated ‘The people behind the papers’ interview.

While matrix stiffness regulates cell behavior on 2D substrates, recent studies using synthetic hydrogels have suggested that in 3D environments, cell behavior is primarily impacted by matrix degradability, independent of stiffness. However, these studies did not consider the potential impact of other confounding matrix parameters that typically covary with changes in stiffness, particularly, hydrogel swelling and hydrolytic stability, which may explain the previously observed distinctions in cell response in 2D versus 3D settings. To investigate how cells sense matrix stiffness in 3D environments, a nonswelling, hydrolytically stable, linearly elastic synthetic hydrogel model is developed in which matrix stiffness and degradability can be tuned independently. It is found that matrix degradability regulates cell spreading kinetics, while matrix stiffness dictates the final spread area once cells achieve equilibrium spreading. Importantly, the differentiation of human mesenchymal stromal cells toward adipocytes or osteoblasts is regulated by the spread state of progenitor cells upon initiating differentiation. These studies uncover matrix stiffness as a major regulator of cell function not just in 2D, but also in 3D environments, and identify matrix degradability as a critical microenvironmental feature in 3D that in conjunction with matrix stiffness dictates cell spreading, cytoskeletal state, and stem cell differentiation outcomes.

A major deficit in tissue engineering strategies is the lack of materials that promote angiogenesis, wherein endothelial cells from the host vasculature invade the implanted matrix to form new blood vessels. To determine the material properties that regulate angiogenesis, we have developed a microfluidic in vitro model in which chemokine-guided endothelial cell sprouting into a tunable hydrogel is followed by the formation of perfusable lumens. We show that long, perfusable tubes only develop if hydrogel adhesiveness and degradability are fine-tuned to support the initial collective invasion of endothelial cells and, at the same time, allow for matrix remodeling to permit the opening of lumens. These studies provide a better understanding of how cell-matrix interactions regulate angiogenesis and, therefore, constitute an important step towards optimal design criteria for tissue-engineered materials that require vascularization.