The temporal basis of angiogenesis

Abstract

The process of new blood vessel growth (angiogenesis) is highly dynamic, involving complex coordination of multiple cell types. Though the process must carefully unfold over time to generate functional, well-adapted branching networks, we seldom hear about the time-based properties of angiogenesis, despite timing being central to other areas of biology. Here, we present a novel, time-based formulation of endothelial cell behaviour during angiogenesis and discuss a flurry of our recent, integrated in silico/in vivo studies, put in context to the wider literature, which demonstrate that tissue conditions can locally adapt the timing of collective cell behaviours/decisions to grow different vascular network architectures. A growing array of seemingly unrelated 'temporal regulators' have recently been uncovered, including tissue derived factors (e.g. semaphorins or the high levels of VEGF found in cancer) and cellular processes (e.g. asymmetric cell division or filopodia extension) that act to alter the speed of cellular decisions to migrate. We will argue that 'temporal adaptation' provides a novel account of organ/disease-specific vascular morphology and reveals 'timing' as a new target for therapeutics. We therefore propose and explain a conceptual shift towards a 'temporal adaptation' perspective in vascular biology, and indeed other areas of biology where timing remains elusive.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Keywords: active perception; adaptation; dynamics; morphogenesis; time; vascular.

Figure 1.

Angiogenesis and the ‘central pattern generator’ (CPG): key concepts. (a) New blood vessels sprout from pre-existing ones (active migratory cells, white; inhibited stalk cells, dark grey), tip cells leading sprouts fuse to form vessel loops that can support blood flow; reproduced with permission from [1]. This repeats to build the vascular network. (b) The CPG. VEGF/Notch signalling selects a heterogeneous pattern of active migratory cells and inhibited stalk cells from homogeneous (light grey) ECs. (c) Changing the speed that the CPG takes to select the active/inhibited (white/dark grey) phenotypes can alter vascular network structure as many more cells remain homogenous while they decide (light grey); adapted with permission from [1].

Figure 2.

Time-ordering events and active perception. (a) Traditional feed-forward schematic view of the regulation of cell behaviour. (b) Schematic ordered chronologically reverses the flow. (c) The CPG spatio-temporally reveals active perception mechanisms: green dots indicate that sensors (VEGFR receptors) reside on the cell membrane, which is deformed and moved as actin polymerizes to form filopodia creating a sensorimotor loop (active perception).

Figure 3.

Cell rearrangement during sprouting. (a) (Top) The terms ‘tip’ and ‘stalk’ are positional, relating to regions of the sprout. (Middle and lower) Active migrating cells from within the stalk region rearrange positions at a rate inversely proportional to their Notch receptor activity. Reproduced with permission from [51]. (b) The interplay of the CPG and differential adhesion driven cell rearrangement making the timing explicit. This reveals that the length of time the CPG takes to re-establish differential states will be critical to determining the resulting network structure.

Figure 4.

Sema3E-PlexinD1 speeds up the CPG increasing branching density (a) pathway schematic showing crosstalk with the CPG. (b) MSM simulations predict the sparser branching seen in vivo due to longer time spent collectively deciding states via increased lateral inhibition strength. Reproduced with permission from [1].

Figure 5.

Pathologically high VEGF synchronized the CPG and promoted vessel expansion. (a) MSM simulation: t1 (time point 1) has a normal VEGF-A (linear gradient) extending above the sprout driving the CPG to rapidly select tip/stalk cells and promotes branching; high uniform VEGF-A levels are simulated from t2 through t6 mimicking VEGF-A intraocular injection in the mouse retina, switching the vessel to expand, not branch, with synchronizing oscillations in Dll4 (high Dll4, green; low Dll4, purple). (b) Images of the sprouting front of WT P5 mouse retinas not injected (wt) and injected with mVegf165 (high VEGF injected) labelled with Dll4 protein (red) and endothelial cell nuclei (ERG; blue). (c) High magnification of (b). Reproduced with permission from [35].

Figure 6.

Temporal regulation of vascular patterning overview. Reproduced with permission from [35]. (Online version in colour.)