EPHB4-RASA1-Mediated Negative Regulation of Ras-MAPK Signaling in the Vasculature: Implications for the Treatment of EPHB4- and RASA1-Related Vascular Anomalies in Humans

EPHB4-RASA1 Inhibition of PIEZO1 Ras Activation Drives Lymphatic Valvulogenesis

BACKGROUND

EPHB4 (ephrin receptor B4) and the RASA1 (p120 Ras GTPase-activating protein) are necessary for the development of lymphatic vessel (LV) valves. However, precisely how EPHB4 and RASA1 regulate LV valve development is unknown. In this study, we examine the mechanisms by which EPHB4 and RASA1 regulate the development of LV valves.

METHODS

We used LV-specific inducible EPHB4-deficient mice and EPHB4 knockin mice that express a form of EPHB4 that is unable to bind RASA1 yet retains protein tyrosine kinase activity (EPHB4 2YP) to study the role of EPHB4 and RASA1 in LV valve development in the embryo and LV valve maintenance in adults. We also used human dermal lymphatic endothelial cells in vitro to study the role of EPHB4 and RASA1 as regulators of LV valve specification induced by oscillatory shear stress, considered the trigger for LV valve specification in vivo.

RESULTS

LV valve specification, continued valve development postspecification, and LV valve maintenance were blocked upon induced loss of EPHB4 in LV. LV valve specification and maintenance were also impaired in EPHB4 2YP mice. Defects in LV valve development were reversed by inhibition of the Ras-MAPK (mitogen-activated protein kinase) signaling pathway. In human dermal lymphatic endothelial cells, loss of expression of EPHB4 or its ephrin b2 ligand, loss of expression of RASA1, and inhibition of physical interaction between EPHB4 and RASA1 resulted in dysregulated oscillatory shear stress-induced Ras-MAPK activation and impaired expression of LV specification markers that could be rescued by Ras-MAPK pathway inhibition. The same results were observed when human dermal lymphatic endothelial cells were stimulated with the Yoda1 agonist of the PIEZO1 oscillatory shear stress sensor. Although Yoda1 increased the number of LV valves when administered to wild-type embryos, it did not increase LV valve number when administered to EPHB4 2YP embryos.

CONCLUSIONS

EPHB4 is necessary for LV valve specification, continued valve development postspecification, and valve maintenance. LV valve specification requires physical interaction between EPHB4 and RASA1 to limit activation of the Ras-MAPK pathway in lymphatic endothelial cells. Specifically, EPHB4-RASA1 physical interaction is necessary to dampen Ras-MAPK activation induced through the PIEZO1 oscillatory shear stress sensor. These findings reveal the mechanism by which EPHB4 and RASA1 regulate the development of LV valves.

Novel postzygotic RASA1 mutation in a patient with Parkes Weber syndrome: A case report and literature review

Key Clinical Message

Not only germline but also postzygotic mutations in the RASA1 or EPHB4 genes can lead to capillary malformation-arteriovenous malformation (CM-AVM) syndrome. As it is not always possible to clinically distinguish between constitutional variants and postzygotic mosaicism, a sufficiently high sequencing depth must be used in genetic diagnostics to detect both.

Abstract

Capillary malformation-arteriovenous malformation (CM-AVM) syndrome, with or without Parkes Weber syndrome, is a rare autosomal dominant disease caused by pathogenic RASA1 or EPHB4 variants. Up to 80% of CM-AVM cases have an affected parent. Gene panel sequencing was performed for a 4-year-old girl with multiple CMs, two capillary stains on the left leg, and associated overgrowth of the second toe. We also reviewed published cases with mosaic RASA1 and EPHB4 mutations. A mosaic RASA1 loss-of-function mutation was detected with a variant allele frequency (VAF) of 20% in the blood and oral epithelial cells of the index patient. The literature review illustrates that the severity of the clinical phenotype does not correlate with the VAF. We also identified a germline nonsense variant in the patient's TEK gene. However, inactivating TEK variants do not cause a vascular phenotype but can confer an increased risk for primary congenital glaucoma with variable expressivity. The case presented here illustrates that the choice of the sequencing depth of a diagnostic next-generation sequencing test for CM-AVM patients should always take mosaicism into account and that a good knowledge of the sequenced genes and associated disease mechanisms is necessary for adequate genetic counseling.

Another face of RASA1: Report of familial germline variant in RASA1 with dysmorphic features

RASopathies encompass a diverse set of disorders affecting genes that encode proteins within the RAS-MAPK pathway. RASA1 mutations are the cause of an autosomal dominant disorder called capillary malformation-arteriovenous malformation type 1 (CM-AVM1). Unlike other RASopathies, facial dysmorphism has not been described in these patients. We phenotypically delineated a large family of individuals with multifocal fast-flow capillary malformations, severe lymphatic anomalies of perinatal onset, and dysmorphic features not previously described. Sequencing studies were performed on probands and related family members, confirming the segregation of dysmorphic features in affected members of a novel heterozygous variant in RASA1 (NM_002890.3:c.2366G>A, p.(Arg789Gln)). In this work, we broaden the phenotypic spectrum of CM-AVM type 1 and propose a new RASA1 variant as likely pathogenic.

Methylome analysis of endothelial cells suggests new insights on sporadic brain arteriovenous malformation

Arteriovenous malformation of the brain (bAVM) is a vascular phenotype related to brain defective angiogenesis. Involved vessels show impaired expression of vascular differentiation markers resulting in the arteriolar to venule direct shunt. In order to clarify aberrant gene expression occurring in bAVM, here we describe results obtained by methylome analysis performed on endothelial cells (ECs) isolated from bAVM specimens, compared to human cerebral microvascular ECs. Results were validated by quantitative methylation-specific PCR and quantitative realtime-PCR. Differential methylation events occur in genes already linked to bAVM onset, as RBPJ and KRAS. However, among differentially methylated genes, we identified EPHB1 and several other loci involved in EC adhesion as well as in EC/vascular smooth muscle cell (VSMC) crosstalk, suggesting that only endothelial dysfunction might not be sufficient to trigger the bAVM phenotype. Moreover, aberrant methylation pattern was reported for many lncRNA genes targeting transcription factors expressed during neurovascular development. Among these, the YBX1 that was recently shown to target the arteridin coding gene. Finally, in addition to the conventional CpG methylation, we further considered the role of impaired CHG methylation, mainly occurring in brain at embryo stage. We showed as differentially CHG methylated genes are clustered in pathways related to EC homeostasis, as well as to VSMC-EC crosstalk, suggesting as impairment of this interaction plays a prominent role in loss of vascular differentiation, in bAVM phenotype.

A temporally-restricted pattern of endothelial cell collagen 4 alpha 1 expression during embryonic development determined with a novel knockin Col4a1-P2A-eGFP mouse line

Classical collagen type IV comprising of a heterotrimer of two collagen IV alpha 1 chains and one collagen IV alpha 2 chain is the principal type of collagen synthesized by endothelial cells (EC) and is a major constituent of vascular basement membranes. In mouse and man, mutations in genes that encode collagen IV alpha 1 and alpha 2 result in vascular dysfunction. In addition, mutations in genes that encode the Ephrin receptor B4 (EPHB4) and the p120 Ras GTPase-activating protein (RASA1) that cause increased activation of the Ras mitogen-activated protein kinase (MAPK) signaling pathway in EC result in vascular dysfunction as a consequence of impaired export of collagen IV. To understand the pathogenesis of collagen IV-related vascular diseases and phenotypes it is necessary to identify at which times collagen IV is actively synthesized by EC. For this purpose, we used CRISPR/Cas9 targeting in mice to include immediately after the terminal Col4a1 codon a sequence that specifies a P2A peptide followed by enhanced green fluorescent protein (eGFP). Analysis of eGFP expression in Col4a1-P2A-eGFP mice revealed active embryonic EC synthesis of collagen IV alpha 1 through mid to late gestation followed by a sharp decline before birth. These results provide a contextual framework for understanding the basis for the varied vascular abnormalities resulting from perturbation of EC expression and export of functional collagen IV.

Transluminal Pillars-Their Origin and Role in the Remodelling of the Zebrafish Caudal Vein Plexus

Intussusceptive pillars, regarded as a hallmark of intussusceptive angiogenesis, have been described in developing vasculature of many organs and organisms. The aim of this study was to resolve the question about pillar formation and their further maturation employing zebrafish caudal vein plexus (CVP). The CVP development was monitored by in vivo confocal microscopy in high spatio-temporal resolution using the transgenic zebrafish model Fli1a:eGPF//Gata1:dsRed. We tracked back the formation of pillars (diameter ≤ 4 µm) and intercapillary meshes (diameter > 4 µm) and analysed their morphology and behaviour. Transluminal pillars in the CVP arose via a combination of sprouting, lumen expansion, and/or the creation of intraluminal folds, and those mechanisms were not associated directly with blood flow. The follow-up of pillars indicated that one-third of them disappeared between 28 and 48 h post fertilisation (hpf), and of the remaining ones, only 1/17 changed their cross-section area by >50%. The majority of the bigger meshes (39/62) increased their cross-section area by >50%. Plexus simplification and the establishment of hierarchy were dominated by the dynamics of intercapillary meshes, which formed mainly via sprouting angiogenesis. These meshes were observed to grow, reshape, and merge with each other. Our observations suggested an alternative view on intussusceptive angiogenesis in the CVP.

An EPHB4-RASA1 signaling complex inhibits shear stress-induced Ras-MAPK activation in lymphatic endothelial cells to promote the development of lymphatic vessel valves

EPHB4 is a receptor protein tyrosine kinase that is required for the development of lymphatic vessel (LV) valves. We show here that EPHB4 is necessary for the specification of LV valves, their continued development after specification, and the maintenance of LV valves in adult mice. EPHB4 promotes LV valve development by inhibiting the activation of the Ras-MAPK pathway in LV endothelial cells (LEC). For LV specification, this role for EPHB4 depends on its ability to interact physically with the p120 Ras-GTPase-activating protein (RASA1) that acts as a negative regulator of Ras. Through physical interaction, EPHB4 and RASA1 dampen oscillatory shear stress (OSS)-induced Ras-MAPK activation in LEC, which is required for LV specification. We identify the Piezo1 OSS sensor as a focus of EPHB4-RASA1 regulation of OSS-induced Ras-MAPK signaling mediated through physical interaction. These findings contribute to an understanding of the mechanism by which EPHB4, RASA1 and Ras regulate lymphatic valvulogenesis.