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Objective- The Wnt/β-catenin pathway orchestrates development of the blood-brain barrier, but the downstream mechanisms involved at different developmental windows and in different central nervous system (CNS) tissues have remained elusive. Approach and Results- Here, we create a new mouse model allowing spatiotemporal investigations of Wnt/β-catenin signaling by induced overexpression of Axin1, an inhibitor of β-catenin signaling, specifically in endothelial cells ( Axin1 iEC- OE). AOE (Axin1 overexpression) in Axin1 iEC- OE mice at stages following the initial vascular invasion of the CNS did not impair angiogenesis but led to premature vascular regression followed by progressive dilation and inhibition of vascular maturation resulting in forebrain-specific hemorrhage 4 days post-AOE. Analysis of the temporal Wnt/β-catenin driven CNS vascular development in zebrafish also suggested that Axin1 iEC- OE led to CNS vascular regression and impaired maturation but not inhibition of ongoing angiogenesis within the CNS. Transcriptomic profiling of isolated, β-catenin signaling-deficient endothelial cells during early blood-brain barrier-development (E11.5) revealed ECM (extracellular matrix) proteins as one of the most severely deregulated clusters. Among the 20 genes constituting the forebrain endothelial cell-specific response signature, 8 ( Adamtsl2, Apod, Ctsw, Htra3, Pglyrp1, Spock2, Ttyh2, and Wfdc1) encoded bona fide ECM proteins. This specific β-catenin-responsive ECM signature was also repressed in Axin1 iEC- OE and endothelial cell-specific β-catenin-knockout mice ( Ctnnb1-KOiEC) during initial blood-brain barrier maturation (E14.5), consistent with an important role of Wnt/β-catenin signaling in orchestrating the development of the forebrain vascular ECM. Conclusions- These results suggest a novel mechanism of establishing a CNS endothelium-specific ECM signature downstream of Wnt-β-catenin that impact spatiotemporally on blood-brain barrier differentiation during forebrain vessel development. Visual Overview- An online visual overview is available for this article., SCOPUS: ar.j, info:eu-repo/semantics/published

The circumventricular organs (CVOs) in the central nervous system (CNS) lack a vascular blood-brain barrier (BBB), creating communication sites for sensory or secretory neurons, involved in body homeostasis. Wnt/β-catenin signaling is essential for BBB development and maintenance in endothelial cells (ECs) in most CNS vessels. Here we show that in mouse development, as well as in adult mouse and zebrafish, CVO ECs rendered Wnt-reporter negative, suggesting low level pathway activity. Characterization of the subfornical organ (SFO) vasculature revealed heterogenous claudin-5 (Cldn5) and Plvap/Meca32 expression indicative for tight and leaky vessels, respectively. Dominant, EC-specific β-catenin transcription in mice, converted phenotypically leaky into BBB-like vessels, by augmenting Cldn5+vessels, stabilizing junctions and by reducing Plvap/Meca32+ and fenestrated vessels, resulting in decreased tracer permeability. Endothelial tightening augmented neuronal activity in the SFO of water restricted mice. Hence, regulating the SFO vessel barrier may influence neuronal function in the context of water homeostasis., SCOPUS: ar.j, info:eu-repo/semantics/published

The ultimate formation of a four-chambered heart allowing the separation of the pulmonary and systemic circuits was key for the evolutionary success of tetrapods. Complex processes of cell diversification and tissue morphogenesis allow the left and right cardiac compartments to become distinct but remain poorly understood. Here, we describe an unexpected laterality in the single zebrafish atrium analogous to that of the two atria in amniotes, including mammals. This laterality appears to derive from an embryonic antero-posterior asymmetry revealed by the expression of the transcription factor gene meis2b. In adult zebrafish hearts, meis2b expression is restricted to the left side of the atrium where it controls the expression of pitx2c, a regulator of left atrial identity in mammals. Altogether, our studies suggest that the multi-chambered atrium in amniotes arose from a molecular blueprint present before the evolutionary emergence of cardiac septation and provide insights into the establishment of atrial asymmetry., SCOPUS: ar.j, info:eu-repo/semantics/published