Color-coded fluorescent protein imaging of angiogenesis: the AngioMouse models

Abstract

We have utilized multicolored fluorescent proteins to develop three imaging models of tumor angiogenesis. In one model, the nonluminous induced capillaries are clearly visible by contrast against the very bright tumor green fluorescent protein (GFP) fluorescence examined either intravitally or by whole-body imaging in real time. Intravital images of an orthotopic model of human pancreatic tumors expressing GFP visualized angiogenic capillaries at both primary and metastatic sites. Whole-body optical imaging showed that blood vessel density increased linearly over a 20-week period in an orthotopic model of human breast cancer expressing GFP. Opening a reversible skin-flap in the light path markedly reduces signal attenuation, increasing detection sensitivity many-fold and enables vessels to be externally visualized in GFP-expressing tumors growing on internal organs. In another model, dual-color fluorescence imaging was effected by using red fluorescent protein (RFP)-expressing tumors growing in GFP-expressing transgenic mice that express GFP in all cells. This dual-color model visualizes with great clarity the details of the tumor-stroma interaction, especially tumor-induced angiogenesis. The GFP-expressing tumor vasculature, both nascent and mature, are readily distinguished interacting with the RFP-expressing tumor cells. The third model involves a transgenic mouse in which the regulatory elements of the stem cell marker nestin drive GFP (ND-GFP). The ND-GFP mouse expresses GFP in nascent blood vessels. RFP-expressing tumors transplanted to nestin-GFP mice enable specific visualization of nascent vessels. The ND-GFP mouse was utilized to develop a rapid in vivo/ex vivo fluorescent angiogenesis assay by implanting Gelfoam which was vascularized by fluorescent nascent blood vessels. This process could be markedly stimulated or inhibited by specific compounds. We also observed, using ND-GFP mice, that the hair follicle is angiogenic and that the hair-follicle vascular network is a prime target for chemotherapy drugs which cause hair loss (chemotherapy-induced alopecia). These fluorescent models, generally termed AngioMouse, can quantitatively determine efficacy of antiangiogenesis compounds.