Alk1 controls arterial endothelial cell migration in lumenized vessels

Abstract

Heterozygous loss of the arterial-specific TGFβ type I receptor, activin receptor-like kinase 1 (ALK1; ACVRL1), causes hereditary hemorrhagic telangiectasia (HHT). HHT is characterized by development of fragile, direct connections between arteries and veins, or arteriovenous malformations (AVMs). However, how decreased ALK1 signaling leads to AVMs is unknown. To understand the cellular mis-steps that cause AVMs, we assessed endothelial cell behavior in alk1-deficient zebrafish embryos, which develop cranial AVMs. Our data demonstrate that alk1 loss has no effect on arterial endothelial cell proliferation but alters arterial endothelial cell migration within lumenized vessels. In wild-type embryos, alk1-positive cranial arterial endothelial cells generally migrate towards the heart, against the direction of blood flow, with some cells incorporating into endocardium. In alk1-deficient embryos, migration against flow is dampened and migration in the direction of flow is enhanced. Altered migration results in decreased endothelial cell number in arterial segments proximal to the heart and increased endothelial cell number in arterial segments distal to the heart. We speculate that the consequent increase in distal arterial caliber and hemodynamic load precipitates the flow-dependent development of downstream AVMs.

ALK1 signaling in development and disease: new paradigms

“…As described above, arterial EC migration against the direction of blood flow is impaired in zebrafish acvrl1 mutants [183], and similar abnormal EC migratory behavior was recently described in Eng -deleted ECs in mice. In the latter model, Eng -deleted cells migrate aberrantly in the direction of flow and accumulate in arterioles, and enlarged arterioles ultimately become the afferent segments of AVMs [89].…”

“…In these mutants, AVMs invariably develop between 32 and 48 hours post-fertilization in cranial vessels that underlie the hindbrain; therefore, AVM development can be imaged in embryos expressing EC-localized fluorescent transgenes using time-lapse confocal microscopy [67, 68, 183]. These imaging studies have revealed that AVM development is a two-step process (Figure 3).…”

“…In step 1, loss of acvrl1 reduces arterial EC migration against the direction of blood flow and enhances migration in the direction of blood flow, resulting in a skewed distribution of ECs toward distal arterial segments. The consequent increase in distal arterial EC number increases distal arterial caliber [68, 183]. In step 2, directly downstream of this enlarged arterial segment, a normally transient arteriovenous shunt is aberrantly retained and enlarged in acvrl1 mutants, generating an embryonic lethal AVM.…”

“…In zebrafish, acvrl1 is expressed in arterial ECs only after the onset of blood flow, and in mice, Acvrl1 expression is enhanced by and dependent on blood flow [1, 67, 68, 184]. Furthermore, loss of blood flow phenocopies molecular and cellular defects observed in high-flow regions of the zebrafish acvrl1 mutant vasculature—including enhanced arterial EC expression of promigratory tip cell markers cxcr4a and dll4 , decreased arterial EC expression of the vasconstrictive peptide edn1 , and aberrant arterial EC migration in the direction of flow—and molecular defects caused by flow loss can be rescued by restoration of arterial EC Bmp10/Alk1 signaling [68, 136, 183]. Because ALK1 ligands BMP9 and BMP10 are secreted into the bloodstream, the role of blood flow in ALK1 signaling certainly includes distribution of these endocrine ligands to arterial ECs [124, 136].…”

“…In zebrafish embryos, acvrl1 -positive cranial arterial ECs migrate toward the heart, which is the source of the ALK1 ligand BMP10 [136, 183], suggesting that retrograde migration may be a chemotactic response to a BMP10 gradient. However, it is also possible that the direction of flow exclusively dictates the direction of migration.…”