Combination of reverse and chemical genetic screens reveals angiogenesis inhibitors and targets

Abstract

We combined reverse and chemical genetics to identify targets and compounds modulating blood vessel development. Through transcript profiling in mice, we identified 150 potentially druggable microvessel-enriched gene products. Orthologs of 50 of these were knocked down in a reverse genetic screen in zebrafish, demonstrating that 16 were necessary for developmental angiogenesis. In parallel, 1280 pharmacologically active compounds were screened in a human cell-based assay, identifying 28 compounds selectively inhibiting endothelial sprouting. Several links were revealed between the results of the reverse and chemical genetic screens, including the serine/threonine (S/T) phosphatases ppp1ca, ppp1cc, and ppp4c and an inhibitor of this gene family; Endothall. Our results suggest that the combination of reverse and chemical genetic screens, in vertebrates, is an efficient strategy for the identification of drug targets and compounds that modulate complex biological systems, such as angiogenesis.

Figure 1. Experimental Overview of the Parallel Reverse and Chemical Genetic Screens

(A) The reverse genetic (RG) screen comprised 5856 genes derived from cDNA libraries, constructed from pooled mouse vascular fragments isolated in vivo. Vascular and corresponding nonvascular samples from various tissues and stages were isolated and underwent RNA extraction, amplification, and labeling. After mRNA expression profiling, 150 mouse genes were selected for functional validation. Fifty mouse genes had suitable zebrafish orthologs, and their functional importance in angiogenesis was evaluated by morpholino-mediated knockdown in zebrafish embryos. Sixteen of the 50 knockdowns resulted in vascular defects. (B) In a separate chemical genetic (CG) screen, 1280 compounds were screened in a human cellular angiogenesis assay. Ninety compounds inhibited in vitro angiogenesis, and 28 of these, targeting 69 proteins, did not affect fibroblast scattering. At the intersection of the two screens, three genes encoding members of the PPP1 and PPP2 families of S/T phophatases were identified as angiogenesis targets.

Figure 2. Validation of Vascular Expression of Gene Products Identified in the Reverse Genetic Screen

(A–G) Gene expression determined by nonradioactive mRNA in situ hybridization (blue) on mouse tissues. (A) sat1 expression was restricted to the microvasculature in the heart at embryonic day (E) 17.5. (B) Expression of sat1 in the vasculature of kidney glomeruli (G) at E17.5. Arrows indicates afferent and efferent arterioles. (C and D) At E17.5, ppap2a was mainly expressed in large vessels, here exemplified by two vessels in skeletal muscle (skeletal m.) and a vein (vein). (E–G) The S/T protein phosphatases ppp1ca, ppp1cc, and ppp4c were all expressed in the vasculature as well as in other cell types. (E) Vascular ppp1ca staining in the brain at E17.5 (arrowheads). (F) Vascular expression of ppp1cc in the brain at E14.5 (arrowheads). (G) Vascular expression of ppp4c in skeletal muscle at E17.5 (arrowheads). Scale bars represent 100 μm.

Figure 3. Twenty-Eight Compounds Inhibit VEGF-A-Induced HUVEC Sprouting Angiogenesis

VEGF-A-driven HUVEC angiogenesis was quantified as the average cumulative sprout length (black bars). The VEGF-A-driven sprouting was set to 100%, and the basal sprouting in the absence of VEGF-A was set to 0%. The red line is a reference line for the basal HUVEC sprouting. Scattering of clustered NHDFs in collagen gels in the presence of basal medium was set to 100% (gray bars). Twenty-eight compounds from the LOPAC library inhibited sprouting at > 50% (black bars) and NHDF scattering at < 50% (gray bars).

Figure 4. Knockdown of ppp1ca, ppp1cc, and ppp4c Results in Defects in Endothelial Path Finding and Tubulogenesis

(A–L) Images are all lateral views of the trunk vasculature at 48–50 hpf. (A–C) Control embryo injected with a mixed-base morpholino. (D–L) Embryos injected with morpholinos against (D–F) ppp1ca, (G–I) ppp1cc, and (J–L) ppp4c. Endothelial cells (shown with green fluorescence in [A], [D], [G], and [J]) were identified by using the Tg(fli1:egfp)^y1 line. Dorsal aortas are marked with arrowheads, examples of perfused ISVs are marked with arrows, and examples of nonperfused ISVs are labeled with asterisks. (D and J) The ppp1ca and ppp4c knockdowns resulted in enlarged ISVs, whereas (G) knockdown of ppp1cc resulted in excessive branching of the ISVs. (B) At 48–50 hpf, circulation as observed by microangiography was observed in control injected embryos (mixed-base MPO) in the dorsal aorta, cardinal vein, and ISVs. (E and K) Rhodamine-dextran dye (red) often entered the ventral aspect of the ISVs in the ppp1ca and ppp4c knockdowns (a more severely affected embryo is shown for ppp4c), but a circulatory loop was not established. (H) The ppp1cc knockdown embryos showed either an absence of circulation or thin vessels with reduced circulation. (C, F, I, and L) Merged images of the embryos shown in the previous two panels.

Figure 5. Treatment of Embryos with Endothall Results in Defects in Tubulogenesis

(A, D, and G) Endothelial cells in the trunk were analyzed for defects in migration in the Tgfli1: (egfp)^y1 line (green fluorescence) at 48–50 hpf. (B, E, and H) Analysis of circulation defects by using microangiography (rhodamine-dextran imaging, red) at 48–50 hpf. (A–I) are lateral views of the trunk. (A–C) Treatment with either carrier (DMSO control) or (D–I) Endothall did not alter endothelial migration (see Tgfli1:egfp)^y1 in [A], [D], and [G]). (B, E, and H) Microangiography revealed defects is circulation consistent with defects in tubulogenesis. Two classes of affected embryos are shown. In weakly affected embryos (low effect), some of the ISVs would fail to circulate rhodamine-dextran. In more severely affected embryos (high effect), most of the ISVs failed to transfer dye. (C, F, and I) Merged images of the previous two panels. Dorsal aortas are marked with arrowheads, examples of perfused ISVs are marked with arrows, and examples of non-perfused ISVs are labeled with asterisks. (J) Dose response to bathing embryos in Endothall. Embryos were scored based on circulation in the axial vessels and the ISVs as in Table 2. Vascular collapse was observed with the highest concentration of Endothall.