Endothelial cilia dysfunction in pathogenesis of hereditary hemorrhagic telangiectasia

Abstract

Hereditary hemorrhagic telangiectasia (HHT) is associated with defective capillary network, leading to dilated superficial vessels and arteriovenous malformations (AVMs) in which arteries connect directly to the veins. Loss or haploinsufficiency of components of TGF-β signaling, ALK1, ENG, SMAD4, and BMP9, have been implicated in the pathogenesis AVMs. Emerging evidence suggests that the inability of endothelial cells to detect, transduce and respond to blood flow, during early development, is an underpinning of AVM pathogenesis. Therefore, components of endothelial flow detection may be instrumental in potentiating TGF-β signaling in perfused blood vessels. Here, we argue that endothelial cilium, a microtubule-based and flow-sensitive organelle, serves as a signaling hub by coupling early flow detection with potentiation of the canonical TGF-β signaling in nascent endothelial cells. Emerging evidence from animal models suggest a role for primary cilia in mediating vascular development. We reason, on recent observations, that endothelial cilia are crucial for vascular development and that embryonic loss of endothelial cilia will curtail TGF-β signaling, leading to associated defects in arteriovenous development and impaired vascular stability. Loss or dysfunction of endothelial primary cilia may be implicated in the genesis of AVMs due, in part, to inhibition of ALK1/SMAD4 signaling. We speculate that AVMs constitute part of the increasing spectrum of ciliopathy-associated vascular defects.

Keywords: BMP signaling; Endothelial cilia; TGF-β; Vascular disease; Zebrafish.

FIGURE 1

Proposed model for endothelial cilia mediated BMP/ALK1/SMAD signaling. (A) Schematic depiction of endothelial primary cilium, showing microtubule-based axoneme (yellow rods), basal body (purple cone), transition fibers (green lines) and cilia membrane (labeled). In the proposed model, ALK1 is enriched both at the plasma membrane and at the ciliary membrane. BMP9 is available under low shear stress conditions through circulation and binds to ALK1 in the EC membrane and at the ciliary membrane. Under low shear stress, the binding of BMP9 to ALK1 will result in the phosphorylation of SMAD1/5/8 and translocation of SMAD4 + phosphorylated SMAD1/5/8 complex to the nucleus, where this complex will act as a transcription factor to drive the expression of genes required for pericyte recruitment and vessel stabilization, giving rise to stable arteriovenous connections. (B) Exposure to high shear stress would result in cilia disassembly or dysfunction, thereby curtailing cilia mediated ALK1 signaling and downstream processes.

FIGURE 2

The zebrafish caudal vein plexus (CVP) is formed via BMP mediated angiogenesis. (A) Representative photomicrograph of a 28-h post fertilization (hpf) zebrafish embryo. Lateral view is shown. (B) Schematic depiction of the two conserved axial vessels in the blue boxed region in A, depicting the dorsal artery (DA) and posterior caudal vein (PCV). The arterial-fated intersegmental vessels (ISVs) emanate from the dorsal surfaces of DA and migrate dorsally in response to Vegf gradients. The PCV gives rise to venous sprouts that migrate ventrally in response to Bmp gradients to form the caudal vein plexus (CVP). (C–E) Representative maximum projections of the CVP during successive stages of sprouting, anastomosis and remodeling. The yellow arrows in (C) and (D) point to putative sites of tip cell fusion or anastomosis (F) Higher magnification of the purple boxed region in C, showing fusion of ipsilateral tip cells (G) Digital zoom of the white boxed region in F, showing progressive lumenization of site of anastomosis.

FIGURE 3

Involvement of primary cilia in the emergence of the caudal vein plexus (CVP). (A–C) Representative single stack confocal images of the CVP region in the Tg(kdrl:mCherry-CAAX) ^y171 ;(bactin::Arl13b:GFP) double transgenic embryo at approximately 28 h post fertilization (hpf). ECs express mCherry and primary cilia are labeled by GFP expression. (D–F) Higher magnification of the white boxed region in C. White arrows point to primary cilia. (G–I) Representative maximum projection confocal images of a venous sprout in the CVP region at approximately 28 hpf in the Tg(kdrl:mCherry-CAAX) ^y171 ; (BRE:eGFP) double transgenic embryom showing endothelial cells (mCherry) and BMP-responsive (BRE+) cells (eGFP) at approximately 28 hpf. White arrow points to a tip cell (J) Representative photomicrograph of wild-type embryos at approximately 28 hpf. (K) Representative photomicrographs of igu ^fo10/fo10 embryos which have a loss-of-function mutation in the ciliary basal body protein, Dzip1. Lateral images are shown. (L) Representative maximum projection image of the developing caudal vein plexus (CVP) in a wild-type embryo at 32 hpf (M) Representative maximum projection image of the developing CVP region in an igu ^fo10/fo10 embryo at 32 hpf. (N) Representative maximum projection image of the CVP in a wild-type embryo at 52 hpf (O) Representative maxmum projection image of the CVP in igu ^fo10/fo10 embryo, showing malformed CVP, evidenced by lack of CVP loops and an overall dilated morphology. This is depicted with a red asterisk. All images are lateral.