MicroRNA-mediated integration of haemodynamics and Vegf signalling during angiogenesis

Abstract

Within the circulatory system, blood flow regulates vascular remodelling, stimulates blood stem cell formation, and has a role in the pathology of vascular disease. During vertebrate embryogenesis, vascular patterning is initially guided by conserved genetic pathways that act before circulation. Subsequently, endothelial cells must incorporate the mechanosensory stimulus of blood flow with these early signals to shape the embryonic vascular system. However, few details are known about how these signals are integrated during development. To investigate this process, we focused on the aortic arch (AA) blood vessels, which are known to remodel in response to blood flow. By using two-photon imaging of live zebrafish embryos, we observe that flow is essential for angiogenesis during AA development. We further find that angiogenic sprouting of AA vessels requires a flow-induced genetic pathway in which the mechano-sensitive zinc finger transcription factor klf2a induces expression of an endothelial-specific microRNA, mir-126, to activate Vegf signalling. Taken together, our work describes a novel genetic mechanism in which a microRNA facilitates integration of a physiological stimulus with growth factor signalling in endothelial cells to guide angiogenesis.

Figure 1. AA5x angiogenesis requires flow and Vegf signaling

a-k, Tg(kdrl:egfp)^la116 embryos at 72 hpf (a-c), 60 hpf (d-f) or 65 hpf (g-k). a-c, g-k Embryos subjected to microangiography. Endothelial cells are green, flow is red. a, b, Lateral views, anterior to left (a), or right (b), dorsal is up. c-k, dorsal view, anterior is up. a, b, aortic arches (AA, numbered, indicated by arrows) after severing right AA5 and 6 from ventral aorta; opa – opercular artery. c, dorsal view of embryo in a, b. d, e, Stills from Supplementary Movies 5 and 6. Embryos treated beginning at 46 hpf with BDM (d), Tricaine (e), or injected with gata1 MO (f). g, kdrl^y17 mutant embryo at 65 hpf. h-k, Embryos treated with 2.5 μM SU5416 (h) or 0.1% DMSO (i) beginning at 46 hpf or injected with 3 ng of scrambled MO (j) or 3 ng of Vegfa MO (k). c-k, Arrows: lateral dorsal aortae (LDA), arrowheads: AA5x.

Figure 2. AA5x angiogenesis requires klf2a

a, Embryos subjected to whole mount in situ hybridization at 65 hpf using indicated riboprobes (klf2a, cdh5). Lateral view, anterior to the left, dorsal is up. Embryos were injected with 2 ng of scrambled control (left) or tnnt2 (right) MO. Aortic arch region is denoted by black boxes; aortic arches 4-6 are indicated by numbered arrows in cases where staining is present. b, Microangiogram of 72 hpf Tg(kdrl:egfp)^la116 embryo injected with 11 ng-embryo scrambled control MO (top) or klf2a ATG MO (bottom); arrowheads indicate position of normal AA5x formation, arrows denote lateral dorsal aortae. Dorsal views, anterior is up.

Figure 3. miR-126 acts downstream of klf2a during AA5x development

a-f, Ventral view, anterior is up. Bracket: AA3-6. Expression of miR-126 (a-c) or cdh5 (d-f). Embryos injected with 11 ng control MO (a, d), 2 ng tnnt2 MO (b, e), 11 ng klf2a ATG MO (c, f). g-j, Tg(kdrl:egfp)^la116 embryos at 65 hpf, dorsal view, anterior is up. Endothelial cells are green, blood flow is red. Embryos injected with 20 ng of control or miR-126 MO (g), 2 ng klf2a ATG MO (h), 7 ng miR-126 MO (i), or co-injected with 2 ng klf2a ATG MO and 7 ng miR-126 MO (j). k-m, Tg(kdrl:egfp)^la116 embryos co-injected with 11 ng klf2a ATG MO and pTol-fli1ep:miR-126/mcherry (k, l) or 11 ng klf2a ATG MO and pTol-fli1ep:mcherry (m). k-m, Yellow indicates egfp and mcherry co-expression; dorsal views, anterior is up. g-m, Arrows: lateral dorsal aortae (LDA), arrowheads: AA5x.

Figure 4. Flow-mediated repression of Spred1 is required for AA5x angiogenesis

a-h, Cranial vessel expression of a miR-126 sensor at 65 hpf; lateral views, anterior to the left, dorsal is up. a, c, e, g, Expression of Egfp fused to control 3’UTR (green) and mCherry-spred1-3’UTR (red), coexpression is yellow. b, d, f, h, Expression of mCherry-spred1-3’UTR (red). Embryos co-injected with miR-126 sensor construct and 20 ng control MO (a, b), 20 ng miR-126 MO (c, d), 2 ng tnnt2 MO (e, f) , or 11 ng klf2a ATG MO (g, h). i-l, Tg(kdrl:egfp)^la116 embryos at 65 hpf, dorsal view, anterior is up. Endothelial cells in green, circulation in red. Arrow - Lateral dorsal aortae; AA5x - arrowheads. Embryos left uninjected (i), injected with 100 pg spred1 mRNA (j), 20 ng control MO (k), or 1 ng spred1 MO and 20 ng miR-126 MO (l). m, Pathway responsible for flow-stimulated angiogenesis.