An Emerging Role for the EphrinB EphB RASA Signaling

An Emerging Role for the EphrinB2-EphB4-RASA1 Signaling Axis in Human Congenital Cerebrovascular Disorders Normal vascular development includes de novo blood vessel formation from endothelial cells (ECs) (vasculogenesis) 1, 2, expansion of the network through vessel sprouting, splitting, and remodeling (angiogenesis) [3], and differentiation of these vessels into arteries, veins, and intervening tnf alpha inhibitor (arteriovenous specification) [4]. Studies in animal models have elucidated that these separate but overlapping mechanisms of vascular development are driven by a highly regulated genetic program that involves the expression and function of multiple key signaling pathways (e.g., ephrin-Eph, Hedgehog, VEGF, TGF-β, Wnt, and Notch) and transcription factors (e.g., HEY and HES, SOX, and Forkhead factors) [4]. Among these genetic factors, animal models have demonstrated a critical role for ephrin-Eph signaling 2, 5, 6, 7, 8. Eph receptor tyrosine kinases (RTKs) (EphA1–EphA8, EphA10, EphB1–EphB4, and EphB6) and their membrane-tethered ligands, the ephrins (ephrinA1–ephrinA5 and ephrinB1–ephrinB3), play an important role in many developmental processes during embryogenesis, including axon guidance, tissue boundary formation, bone development, and vascular and lymphatic development 9, 10. Ephrin-Eph signaling has a dizzying array of signaling modes and regulatory mechanisms (reviewed in 10, 11), including ephrin-Eph forward, reverse, parallel, and antiparallel signaling (see Glossary). Additionally, the strength of ephrin-Eph signaling through any one particular mode can be tuned by the abundance of the specific ephrin ligand cluster involved 12, 13, allowing for the dynamic regulation of multiple distinct molecular and cellular processes with high precision. Despite the complexity of ephrin-Eph signaling, the development of the vasculature appears to be highly dependent on a specific ephrin ligand–receptor pair: ephrinB2 and EphB4 [10]. While mouse and zebrafish models have provided invaluable insights into the role of these ephrin-Eph signaling molecules, recent work in human patients employing whole exome sequencing (WES) techniques and unbiased genomic analytical strategies has corroborated and extended these findings, providing novel insights. Collectively, these efforts have identified mutations in EFNB2, EPHB4, and RASA1 in both sporadic and Mendelian congenital cerebrovascular disorders (Box 1), including capillary malformation-arteriovenous malformation (CM-AVM) and Vein of Galen malformation (VOGM), indicating a conserved role of the signaling axis through vertebrates. Importantly, CM-AVM and VOGM have traditionally been considered two distinct diseases with overlapped phenotypes. However, genetic findings in human patients suggest a shared genetic mechanism underlying these two disorders, even though more comprehensive phenotyping is required to ellucidate their relationship and association. Findings in human genetics have also provided candidates for genetic testing, potential targetable nodes for therapeutic intervention, and insights into the molecular mechanisms of other more common forms of cerebrovascular disorders (see Clinician’s Corner). In this article, we will review the role of ephrinB2-EphB4-RASA1 signaling in vascular development (Figure 1, Key Figure), using data gleaned from animal models (Table 1) and, more recently, WES studies in human patients with congenital cerebrovascular disease (Table 2).
EphrinB2
EphB4
RASA1
Concluding Remarks and Future Perspectives The ephrinB2-EphB4-RASA1 signaling axis is essential for development of the vascular system. Recent successful application of WES and studies of large cohorts have associated mutations in EFNB2, EPHB4, and RASA1 with multiple cerebrovascular disorders, corroborating the findings in model organisms. However, many key questions of their roles in human diseases remain unanswered (see Outstanding Questions).