Vascular mural cells (vMCs) are essential components of the vertebrate vascular system, controlling blood vessel maturation and homeostasis. Discrete molecular mechanisms have been associated with vMC development and differentiation. The function of hemodynamic forces in controlling vMC recruitment is unclear. Using transgenic lines marking developing vMCs in zebrafish embryos, we find that vMCs are recruited by arterial-fated vessels and that the process is flow dependent. We take advantage of tissue-specific CRISPR gene targeting to demonstrate that hemodynamic-dependent Notch activation and the ensuing arterial genetic program is driven by endothelial primary cilia. We also identify zebrafish foxc1b as a cilia-dependent Notch-specific target that is required within endothelial cells to drive vMC recruitment. In summary, we have identified a hemodynamic-dependent mechanism in the developing vasculature that controls vMC recruitment.
Dynamic modulation of endothelial cell-to-cell and cell–to–extracellular matrix (ECM) adhesion is essential for blood vessel patterning and functioning. Yet the molecular mechanisms involved in this process have not been completely deciphered. We identify the adhesion G protein–coupled receptor (ADGR) Latrophilin 2 (LPHN2) as a novel determinant of endothelial cell (EC) adhesion and barrier function. In cultured ECs, endogenous LPHN2 localizes at ECM contacts, signals through cAMP/Rap1, and inhibits focal adhesion (FA) formation and nuclear localization of YAP/TAZ transcriptional regulators, while promoting tight junction (TJ) assembly. ECs also express an endogenous LPHN2 ligand, fibronectin leucine-rich transmembrane 2 (FLRT2), that prevents ECM-elicited EC behaviors in an LPHN2-dependent manner. Vascular ECs of lphn2a knock-out zebrafish embryos become abnormally stretched, display a hyperactive YAP/TAZ pathway, and lack proper intercellular TJs. Consistently, blood vessels are hyperpermeable, and intravascularly injected cancer cells extravasate more easily in lphn2a null animals. Thus, LPHN2 ligands, such as FLRT2, may be therapeutically exploited to interfere with cancer metastatic dissemination.
Intestinal smooth muscle cells (iSMCs) are a crucial component of the adult gastrointestinal tract and support intestinal differentiation, peristalsis and epithelial homeostasis during development. Despite these crucial roles, the origin of iSMCs and the mechanisms responsible for their differentiation and function remain largely unknown in vertebrates. Here, we demonstrate that iSMCs arise from the lateral plate mesoderm (LPM) in a stepwise process. Combining pharmacological and genetic approaches, we show that TGFβ/Alk5 signaling drives the LPM ventral migration and commitment to an iSMC fate. The Alk5-dependent induction of zeb1a and foxo1a is required for this morphogenetic process: zeb1a is responsible for driving LPM migration around the gut, whereas foxo1a regulates LPM predisposition to iSMC differentiation. We further show that TGFβ, zeb1a and foxo1a are tightly linked together by miR-145 In iSMC-committed cells, TGFβ induces the expression of miR-145, which in turn is able to downregulate zeb1a and foxo1a The absence of miR-145 results in only a slight reduction in the number of iSMCs, which still express mesenchymal genes but fail to contract. Together, our data uncover a cascade of molecular events that govern distinct morphogenetic steps during the emergence and differentiation of vertebrate iSMCs.
Modulation of endothelial cell (EC) adhesion to extracellular matrix (ECM) in response to mechanostimuli is essential for blood vessel patterning and functioning, yet the underpinning molecular mechanisms are deciphered only in part. We identify the adhesion G protein-coupled receptor Latrophilin 2 (LPHN2) as a novel determinant of vascular morphogenesis and endothelial barrier function. In cultured ECs, endogenous LPHN2 localizes at ECM adhesions, signals through cAMP/Rap1, and negatively regulates ECM-elicited YAP/TAZ mechanosignaling and haptotaxis. ECs also express endogenous fibronectin-leucine-rich transmembrane 2 (FLRT2) ligand that promotes cAMP/Rap1 signaling and hinders haptotaxis via LPHN2. To validate these findings in vivo, we generated lphn2a mutant zebrafish embryos in which vascular ECs are abnormally stretched, display YAP/TAZ hyperactivation, and lack proper intercellular junctions. Consistently, intravascularly injected cancer cells extravasate more easily in lphn2a null embryos compared to control animals. Overall, we unveil in vascular ECs a novel crosstalk between LPHN2 and the Hippo pathway, that may be therapeutically exploited to interfere with cancer metastatic dissemination.
Abstract Haematopoietic stem cells (HSCs) are the self-renewing progenitors that continuously populate the haemato-immune cell lineages throughout life, and constitute the therapeutic component of bone marrow transplants. A major biomedical goal has been to understand the native specification of HSCs during embryonic development as a means to inform in vitro directed differentiation of pluripotent stem cells. Across vertebrate phyla, HSCs derive from haemogenic endothelium in the ventral floor of the primitive dorsal aorta (DA), also known as the descending aorta in mammals. Competent HSC-fated cells in the endothelium likely receive instructive signaling from neighbouring cells that constitute a “specification niche.” We previously showed that experimental manipulations leading to defects in the most ventral compartment of the somite, the sclerotome, are correlated with HSC defects, raising the possibility that sclerotome patterning is required for HSC specification. Here we show that in zebrafish, specific sclerotome-derived cells contact the DA immediately prior to the emergence of HSCs. These cells subsequently give rise to vascular smooth muscle cells (VSMCs). When sclerotome patterning is disrupted, VSMCs are diminished, and HSC specification fails. We conclude that sclerotome-derived VSMC progenitors contribute to the embryonic HSC specification niche, most likely by providing unknown HSC inductive signals.
Feline leukemia virus subgroup C receptor 1 (Flvcr1) encodes two heme exporters: FLVCR1a, which localizes to the plasma membrane, and FLVCR1b, which localizes to mitochondria. Here, we investigated the role of the two Flvcr1 isoforms during erythropoiesis. We showed that, in mice and zebrafish, Flvcr1a is required for the expansion of committed erythroid progenitors but cannot drive their terminal differentiation, while Flvcr1b contributes to the expansion phase and is required for differentiation. FLVCR1a-down-regulated K562 cells have defective proliferation, enhanced differentiation, and heme loading in the cytosol, while FLVCR1a/1b-deficient K562 cells show impairment in both proliferation and differentiation, and accumulate heme in mitochondria. These data support a model in which the coordinated expression of Flvcr1a and Flvcr1b contributes to control the size of the cytosolic heme pool required to sustain metabolic activity during the expansion of erythroid progenitors and to allow hemoglobinization during their terminal maturation. Consistently, reduction or increase of the cytosolic heme rescued the erythroid defects in zebrafish deficient in Flvcr1a or Flvcr1b, respectively. Thus, heme export represents a tightly regulated process that controls erythropoiesis.
Dynamic modulation of endothelial cell (EC) adhesion to extracellular matrix (ECM) in response to mechanostimuli is essential for blood vessel patterning and functioning. Yet, the molecular mechanisms involved in this biological process are far to be completely deciphered. Here, we identify the adhesion G protein-coupled receptor (ADGR) Latrophilin 2 (LPHN2) as a novel determinant of vascular morphogenesis and endothelial barrier function. In cultured ECs, endogenous LPHN2 localizes at ECM adhesions, signals through cAMP/Rap1, and, via its fibronectin-leucine-rich transmembrane (FLRT)-binding domain, negatively regulates ECM-elicited haptotaxis. ECs also express endogenous FLRT2 ligand that promotes cAMP/Rap1 signaling and hinders haptotaxis in a LPHN2-dependent manner. Vascular ECs of lphn2a knock-out zebrafish embryos become abnormally stretched, display a hyperactive Hippo mechanosensing pathway, and lack proper intercellular junctions. Indeed, intravascularly injected cancer cells extravasate more easily in lphn2a null animals. Thus, LPHN2 ligands, such as FLRT2, may be therapeutically exploited to interfere with cancer metastatic dissemination.
During neuronal development and maturation, microRNAs (miRs) play diverse functions ranging from early patterning, proliferation and commitment to differentiation, survival, homeostasis, activity and plasticity of more mature and adult neurons. The role of miRs in the differentiation of olfactory receptor neurons (ORNs) is emerging from the conditional inactivation of Dicer in immature ORN, and the depletion of all mature miRs in this system. Here, we identify specific miRs involved in olfactory development, by focusing on mice null for Dlx5, a homeogene essential for both ORN differentiation and axon guidance and connectivity. Analysis of miR expression in Dlx5(-/-) olfactory epithelium pointed to reduced levels of miR-9, miR-376a and four miRs of the -200 class in the absence of Dlx5. To functionally examine the role of these miRs, we depleted miR-9 and miR-200 class in reporter zebrafish embryos and observed delayed ORN differentiation, altered axonal trajectory/targeting, and altered genesis and position of olfactory-associated GnRH neurons, i.e. a phenotype known as Kallmann syndrome in humans. miR-9 and miR-200-class negatively control Foxg1 mRNA, a fork-head transcription factor essential for development of the olfactory epithelium and of the forebrain, known to maintain progenitors in a stem state. Increased levels of z-foxg1 mRNA resulted in delayed ORN differentiation and altered axon trajectory, in zebrafish embryos. This work describes for the first time the role of specific miR (-9 and -200) in olfactory/GnRH development, and uncovers a Dlx5-Foxg1 regulation whose alteration affects receptor neuron differentiation, axonal targeting, GnRH neuron development, the hallmarks of the Kallmann syndrome.
