Endothelial cells regulate alveolar morphogenesis by constructing basement membranes acting as a scaffold for myofibroblasts.

Alveologenesis is a spatially coordinated morphogenetic event, during which alveolar myofibroblasts surround the terminal sacs constructed by epithelial cells and endothelial cells (ECs), then contract to form secondary septa to generate alveoli in the lungs. Recent studies have demonstrated the important role of alveolar ECs in this morphogenetic event. However, the mechanisms underlying EC-mediated alveologenesis remain unknown. Herein, we show that ECs regulate alveologenesis by constructing basement membranes (BMs) acting as a scaffold for myofibroblasts to induce septa formation through activating mechanical signaling. Rap1, a small GTPase of the Ras superfamily, is known to stimulate integrin-mediated cell adhesions. EC-specific Rap1-deficient (Rap1iECKO) mice exhibit impaired septa formation and hypo-alveolarization due to the decreased mechanical signaling in myofibroblasts. In Rap1iECKO mice, ECs fail to stimulate integrin ß1 to recruit Collagen type IV (Col-4) into BMs required for myofibroblast-mediated septa formation. Consistently, EC-specific integrin ß1-deficient mice show hypo-alveolarization, defective mechanical signaling in myofibroblasts, and disorganized BMs. These data demonstrate that alveolar ECs promote integrin ß1-mediated Col-4 recruitment in a Rap1-dependent manner, thereby constructing BMs acting as a scaffold for myofibroblasts to induce mechanical signal-mediated alveologenesis. Thus, this study unveils a mechanism of organ morphogenesis mediated by ECs through intrinsic functions.

Rap1 small GTPase is essential for maintaining pulmonary endothelial barrier function in mice.

Vascular permeability is dynamically but tightly controlled by vascular endothelial (VE)-cadherin-mediated endothelial cell-cell junctions to maintain homeostasis. Thus, impairments of VE-cadherin-mediated cell adhesions lead to hyperpermeability, promoting the development and progression of various disease processes. Notably, the lungs are a highly vulnerable organ wherein pulmonary inflammation and infection result in vascular leakage. Herein, we showed that Rap1, a small GTPase, plays an essential role for maintaining pulmonary endothelial barrier function in mice. Endothelial cell-specific Rap1a/Rap1b double knockout mice exhibited severe pulmonary edema. They also showed vascular leakage in the hearts, but not in the brains. En face analyses of the pulmonary arteries and 3D-immunofluorescence analyses of the lungs revealed that Rap1 potentiates VE-cadherin-mediated endothelial cell-cell junctions through dynamic actin cytoskeleton reorganization. Rap1 inhibits formation of cytoplasmic actin bundles perpendicularly binding VE-cadherin adhesions through inhibition of a Rho-ROCK pathway-induced activation of cytoplasmic nonmuscle myosin II (NM-II). Simultaneously, Rap1 induces junctional NM-II activation to create circumferential actin bundles, which anchor and stabilize VE-cadherin at cell-cell junctions. We also showed that the mice carrying only one allele of either Rap1a or Rap1b out of the two Rap1 genes are more vulnerable to lipopolysaccharide (LPS)-induced pulmonary vascular leakage than wild-type mice, while activation of Rap1 by administration of 007, an activator for Epac, attenuates LPS-induced increase in pulmonary endothelial permeability in wild-type mice. Thus, we demonstrate that Rap1 plays an essential role for maintaining pulmonary endothelial barrier functions under physiological conditions and provides protection against inflammation-induced pulmonary vascular leakage.

Mechanical loading of intraluminal pressure mediates wound angiogenesis by regulating the TOCA family of F-BAR proteins.

Angiogenesis is regulated in coordinated fashion by chemical and mechanical cues acting on endothelial cells (ECs). However, the mechanobiological mechanisms of angiogenesis remain unknown. Herein, we demonstrate a crucial role of blood flow-driven intraluminal pressure (IP) in regulating wound angiogenesis. During wound angiogenesis, blood flow-driven IP loading inhibits elongation of injured blood vessels located at sites upstream from blood flow, while downstream injured vessels actively elongate. In downstream injured vessels, F-BAR proteins, TOCA1 and CIP4, localize at leading edge of ECs to promote N-WASP-dependent Arp2/3 complex-mediated actin polymerization and front-rear polarization for vessel elongation. In contrast, IP loading expands upstream injured vessels and stretches ECs, preventing leading edge localization of TOCA1 and CIP4 to inhibit directed EC migration and vessel elongation. These data indicate that the TOCA family of F-BAR proteins are key actin regulatory proteins required for directed EC migration and sense mechanical cell stretching to regulate wound angiogenesis.

Lysine demethylase 2B regulates angiogenesis via Jumonji C dependent suppression of angiogenic transcription factors.

Vascular endothelial growth factor (VEGF) signaling plays a central role in vascular development and maintenance of vascular homeostasis. In endothelial cells (ECs), VEGF activates the gene expression of angiogenic transcription factors (TFs), followed by induction of downstream angiogenic responsive genes. Recent findings support that histone modification dynamics contribute to the transcriptional control of genes that are important for EC functions. Lysine demethylase 2B (KDM2B) demethylates histone H3K4me3 and H3K36me2/3 and mediates the monoubiquitination of histone H2AK119. KDM2B functions as a transcriptional repressor in somatic cell reprogramming and tumor development. However, the role of KDM2B in VEGF signaling remains to be elucidated. Here, we show that KDM2B knockdown enhances VEGF-induced angiogenesis in cultured human ECs via increased migration and proliferation. In contrast, ectopic expression of KDM2B inhibits angiogenesis. The function of KDM2B may depend on its catalytic Jumonji C domain. Genome-wide analysis further reveals that KDM2B selectively controls the transcription of VEGF-induced angiogenic TFs that are associated with increased H3K4me3/H3K36me3 and decreased H2AK119ub. These findings suggest an essential role of KDM2B in VEGF signaling in ECs. As dysregulation of VEGF signaling in ECs is involved in various diseases, including cancer, KDM2B may be a potential therapeutic target in VEGF-mediated vasculopathic diseases.

Protocol for analysis of integrin-mediated cell adhesion of lateral plate mesoderm cells isolated from zebrafish embryos.

Lateral plate mesoderm (LPM) cells differentiate into various cell types including endothelial and hematopoietic cells. In zebrafish embryos, LPM cells migrate toward the midline along the ventral surfaces of somites during which their cell fate specification depends upon efficient integrin-mediated cell adhesion and migration. Herein, we present a protocol for analysis of integrin-mediated cell adhesion of LPM cells isolated from zebrafish embryos. This allows the study of the molecular mechanisms underlying integrin activation required for LPM cell fate specification. For complete details on the use and execution of this protocol, please refer to Rho et al. (2019).

Rap1b Promotes Notch-Signal-Mediated Hematopoietic Stem Cell Development by Enhancing Integrin-Mediated Cell Adhesion.

Hematopoietic stem cells (HSCs) emerge from hemogenic endothelium (HE) within the ventral portion of the dorsal aorta during vertebrate development. In zebrafish, Notch signaling induces HE specification from posterior lateral plate mesoderm (PLPM) cells as they migrate over the ventral surface of the somite. During migration, PLPM cells make close contact with Notch-ligand-expressing somitic cells to acquire HE identity. Herein, we show in zebrafish that the small GTPase Rap1b regulates HSC development by potentiating Notch-mediated HE specification. PLPM cells migrate toward the midline along the somite boundary where fibronectin accumulates. Rap1b stimulates integrin ß1 to enhance PLPM cell adhesion to fibronectin localized at the somite boundary. Rap1b-induced integrin-ß1-mediated adhesion to fibronectin leads to the spreading of PLPM cells to facilitate their physical contact with the Notch-ligand-expressing somitic cells, thereby promoting Notch-mediated HE specification. Thus, we have revealed an unexpected role of Rap1-induced integrin-mediated cell adhesion in HSC development.