Tipping the Balance: Robustness of Tip Cell Selection, Migration and Fusion in Angiogenesis

Abstract

Vascular abnormalities contribute to many diseases such as cancer and diabetic retinopathy. In angiogenesis new blood vessels, headed by a migrating tip cell, sprout from pre-existing vessels in response to signals, e.g., vascular endothelial growth factor (VEGF). Tip cells meet and fuse (anastomosis) to form blood-flow supporting loops. Tip cell selection is achieved by Dll4-Notch mediated lateral inhibition resulting, under normal conditions, in an interleaved arrangement of tip and non-migrating stalk cells. Previously, we showed that the increased VEGF levels found in many diseases can cause the delayed negative feedback of lateral inhibition to produce abnormal oscillations of tip/stalk cell fates. Here we describe the development and implementation of a novel physics-based hierarchical agent model, tightly coupled to in vivo data, to explore the system dynamics as perpetual lateral inhibition combines with tip cell migration and fusion. We explore the tipping point between normal and abnormal sprouting as VEGF increases. A novel filopodia-adhesion driven migration mechanism is presented and validated against in vivo data. Due to the unique feature of ongoing lateral inhibition, ‘stabilised’ tip/stalk cell patterns show sensitivity to the formation of new cell-cell junctions during fusion: we predict cell fates can reverse. The fusing tip cells become inhibited and neighbouring stalk cells flip fate, recursively providing new tip cells. Junction size emerges as a key factor in establishing a stable tip/stalk pattern. Cell-cell junctions elongate as tip cells migrate, which is shown to provide positive feedback to lateral inhibition, causing it to be more susceptible to pathological oscillations. Importantly, down-regulation of the migratory pathway alone is shown to be sufficient to rescue the sprouting system from oscillation and restore stability. Thus we suggest the use of migration inhibitors as therapeutic agents for vascular normalisation in cancer.

A truncation allele in vascular endothelial growth factor c reveals distinct modes of signaling during lymphatic and vascular development

“…The memAgent-Spring model (MSM) (Bentley et al, 2008;Bentley et al, 2009) was used to investigate four possible autocrine effects of Vegfc/Flt4 signaling to identify the best match to experimental data. The MSM has previously been calibrated to match wild-type sprouting data; therefore, Vegfc/Flt4 signaling is encompassed in existing parameters.…”

SRF selectively controls tip cell invasive behavior in angiogenesis

“…Our results instead provide the first experimental evidence for a crucial role of filopodia formation in the process of tip cell competition. Using computational modeling of tip cell selection, Bentley and colleagues previously predicted that filopodia act as amplifiers of DLL4/Notch-mediated lateral inhibition, as a cell that extends more and longer filopodia will be able to reach more VEGF and thus receive more VEGFR2 (KDR) signaling input driving DLL4 expression (Bentley et al, 2008;Bentley et al, 2009).…”

Computational Modeling of Angiogenesis: Towards a Multi-Scale Understanding of Cell–Cell and Cell–Matrix Interactions

“…The model is used to study the interaction between Dll4-Notch1-signaling with VEGF-induces tip cell activation [14]. The robustness of tip cell selection is investigated by applying a VEGF gradient perpendicular to the vessel [7] ; each cell senses the same level of VEGF, which combined with a Dll4-Notch1-based lateral inhibition mechanism produces a pattern of alternating tip and stalk cells. The tip cells grow long filopodia that may meet up to form a connected vessel; anastomosis.…”