Endothelial-to-mesenchymal transition is a dynamic process in which endothelial cells suppress constituent endothelial properties and take on mesenchymal cell behaviors. To begin the process, ...endothelial cells loosen their cell-cell junctions, degrade the basement membrane, and migrate out into the perivascular surroundings. These initial endothelial behaviors reflect a transient modulation of cellular phenotype, that is, a phenotypic modulation, that is sometimes referred to as partial endothelial-to-mesenchymal transition. Loosening of endothelial junctions and migration are also seen in inflammatory and angiogenic settings such that endothelial cells initiating endothelial-to-mesenchymal transition have overlapping behaviors and gene expression with endothelial cells responding to inflammatory signals or sprouting to form new blood vessels. Reduced endothelial junctions increase permeability, which facilitates leukocyte trafficking, whereas endothelial migration precedes angiogenic sprouting and neovascularization; both endothelial barriers and quiescence are restored as inflammatory and angiogenic stimuli subside. Complete endothelial-to-mesenchymal transition proceeds beyond phenotypic modulation such that mesenchymal characteristics become prominent and endothelial functions diminish. In proadaptive, regenerative settings the new mesenchymal cells produce extracellular matrix and contribute to tissue integrity whereas in maladaptive, pathologic settings the new mesenchymal cells become fibrotic, overproducing matrix to cause tissue stiffness, which eventually impacts function. Here we will review what is known about how TGF (transforming growth factor) β influences this continuum from junctional loosening to cellular migration and its relevance to cardiovascular diseases.
EndMT is an intricate cellular differentiation process whereby endothelial cells detach and migrate away from the endothelium and, to varying extents, decrease endothelial properties and acquire ...mesenchymal features. First described in developing heart valves as an epithelial mesenchymal transformation, EndMT begins in response to an external signal, often transforming growth factor-β (TGFβ). The endothelial cells lose luminal-abluminal polarity, extend filopodia and migrate into extravascular space where they take up residence, in the case of heart valves, as valve interstitial cells. Properly controlled EndMT is essential for heart valve development: too little and valves fail to form; too much and the valves thicken and cannot close properly. EndMT appears to persist past embryonic development as endothelial cells expressing EndMT markers can be detected
in vivo
in adult ovine and human valves.
In vitro
, valve endothelial cells treated with TGFβ undergo robust EndMT; hence valve endothelial cells can serve as a prototype for revealing regulators of EndMT.
Reactivation of EndMT in post-natal settings has emerged as a potential mechanism for adaptation to new physiologic settings and at the same time for maladaptive responses to disease
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. This is exemplified by EndMT in cardiac valves, which are lined with a specialized endothelium that originates from FLK1+ (a.k.a. VEGFR2+) progenitor cells in cardiac mesoderm, distinct from vascular endothelium
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. Although endocardial and vascular endothelium are molecularly similar
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, endocardial endothelial cells exhibit a distinct plasticity, which may provide valve endothelial cells with unique capabilities for adaptation and function over a lifetime.
The mitral valve, lined with a unique endothelium, provides an exemplary setting to unravel the causes and nuances of endothelial-to-mesenchymal transition (EndMT), and may provide guideposts for understanding EndMT more broadly as its reactivation in post-natal settings has emerged as a potential mechanism for adaptation to both physiologic and pathologic settings.
PURPOSE OF REVIEWCapillary malformations, the most common type of vascular malformation, are caused by a somatic mosaic mutation in GNAQ, which encodes the Gαq subunit of heterotrimeric G-proteins. ...How the single amino acid change – predicted to activate Gαq – causes capillary malformations is not known but recent advances are helping to unravel the mechanisms.
RECENT FINDINGSThe GNAQ R183Q mutation is present not only in endothelial cells isolated from skin and brain capillary malformations but also in brain tissue underlying the capillary malformation, raising questions about the origin of capillary malformation-causing cells. Insights from computational analyses shed light on the mechanisms of constitutive activation and new basic science shows Gαq plays roles in sensing shear stress and in regulating cerebral blood flow.
SUMMARYSeveral studies confirm the GNAQ R183Q mutation in 90% of nonsyndromic and Sturge–Weber syndrome (SWS) capillary malformations. The mutation is enriched in endothelial cells and blood vessels isolated from skin, brain, and choroidal capillary malformations, but whether the mutation resides in other cell types must be determined. Further, the mechanisms by which the R183Q mutation alters microvascular architecture and blood flow must be uncovered to develop new treatment strategies for SWS in particular, a devastating disease for which there is no cure.
The mechanism of benefit of corticosteroid therapy in infantile hemangioma is unknown. In this study, hemangioma-derived stem cells showed vasculogenic activity in vivo when implanted into nude mice. ...Systemic treatment with dexamethasone or pretreatment of the stem cells with dexamethasone inhibited vasculogenesis. Dexamethasone suppressed the production of vascular endothelial growth factor A (VEGF-A). Silencing of VEGF-A with short hairpin RNA also inhibited vasculogenesis.
In a murine model, dexamethasone inhibited the vasculogenic potential of stem cells derived from human infantile hemangioma. The corticosteroid also inhibited the expression of VEGF-A by hemangioma-derived stem cells, and silencing of VEGF-A expression in these cells inhibited vasculogenesis in vivo.
Infantile hemangioma is the most common tumor of infancy, affecting about 10% of infants of mixed European descent by the age of 1 year.
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Infantile hemangioma occurs more frequently in female infants and in premature and low-birth-weight newborns.
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These tumors can be solitary or multiple; they appear early in postnatal life, grow rapidly during infancy, and involute spontaneously in early childhood. Infantile hemangioma is usually harmless; however, about 10% of hemangiomas are destructive, disfiguring, and even vision- or life-threatening. Corticosteroids given orally or by intralesional injection have been the first-line treatment for problematic hemangiomas since the 1960s. Nevertheless, . . .
Vascularization of tissues is a major challenge of tissue engineering (TE). We hypothesize that blood-derived endothelial progenitor cells (EPCs) have the required proliferative and vasculogenic ...activity to create vascular networks in vivo. To test this, EPCs isolated from human umbilical cord blood or from adult peripheral blood, and human saphenous vein smooth muscle cells (HSVSMCs) as a source of perivascular cells, were combined in Matrigel and implanted subcutaneously into immunodeficient mice. Evaluation of implants at one week revealed an extensive network of human-specific lumenal structures containing erythrocytes, indicating formation of functional anastomoses with the host vasculature. Quantitative analyses showed the microvessel density was significantly superior to that generated by human dermal microvascular endothelial cells (HDMECs) but similar to that generated by human umbilical vein endothelial cells (HUVECs). We also found that as EPCs were expanded in culture, their morphology, growth kinetics, and proliferative responses toward angiogenic factors progressively resembled those of HDMECs, indicating a process of in vitro maturation. This maturation correlated with a decrease in the degree of vascularization in vivo, which could be compensated for by increasing the number of EPCs seeded into the implants. Our findings strongly support the use of human EPCs to form vascular networks in engineered organs and tissues.
Propranolol and atenolol, current therapies for problematic infantile hemangioma (IH), are composed of R(+) and S(-) enantiomers: the R(+) enantiomer is largely devoid of beta blocker activity. We ...investigated the effect of R(+) enantiomers of propranolol and atenolol on the formation of IH-like blood vessels from hemangioma stem cells (HemSCs) in a murine xenograft model. Both R(+) enantiomers inhibited HemSC vessel formation in vivo. In vitro, similar to R(+) propranolol, both atenolol and its R(+) enantiomer inhibited HemSC to endothelial cell differentiation. As our previous work implicated the transcription factor sex-determining region Y (SRY) box transcription factor 18 (SOX18) in propranolol-mediated inhibition of HemSC to endothelial differentiation, we tested in parallel a known SOX18 small-molecule inhibitor (Sm4) and show that this compound inhibited HemSC vessel formation in vivo with efficacy similar to that seen with the R(+) enantiomers. We next examined how R(+) propranolol alters SOX18 transcriptional activity. Using a suite of biochemical, biophysical, and quantitative molecular imaging assays, we show that R(+) propranolol directly interfered with SOX18 target gene trans-activation, disrupted SOX18-chromatin binding dynamics, and reduced SOX18 dimer formation. We propose that the R(+) enantiomers of widely used beta blockers could be repurposed to increase the efficiency of current IH treatment and lower adverse associated side effects.
Vascular endothelial growth factor receptor–1 (VEGFR-1) is a member of the VEGFR family, and binds to VEGF-A, VEGF-B, and placental growth factor. VEGFR-1 contributes to tumor growth and metastasis, ...but its role in the pathological formation of blood vessels is still poorly understood. Herein, we used infantile hemangioma (IH), the most common tumor of infancy, as a means to study VEGFR-1 activation in pathological vasculogenesis. IH arises from stem cells (HemSCs) that can form the three most prominent cell types in the tumor: endothelial cells, pericytes, and adipocytes. HemSCs can recapitulate the IH life cycle when injected in immuncompromised mice, and are targeted by corticosteroids, the traditional treatment for IH. We report here that VEGF-A or VEGF-B induces VEGFR-1–mediated ERK1/2 phosphorylation in HemSCs and promotes differentiation of HemSCs to endothelial cells. Studies of VEGFR-2 phosphorylation status and down-regulation of neuropilin-1 in the HemSCs demonstrate that VEGFR-2 and NRP1 are not needed for VEGF-A– or VEGF-B–induced ERK1/2 activation. U0216-mediated blockade of ERK1/2 phosphorylation or shRNA-mediated suppression of VEGFR-1 prevents HemSC-to-EC differentiation. Furthermore, the down-regulation of VEGFR-1 in the HemSCs results in decreased formation of blood vessels in vivo and reduced ERK1/2 activation. Thus, our study reveals a critical role for VEGFR-1 in the HemSC-to-EC differentiation that underpins pathological vasculogenesis in IH.
Here we investigated whether endothelial colony forming cells (ECFC) and mesenchymal progenitor cells (MPC) form vascular networks and restore blood flow in ischemic skeletal muscle, and whether host ...myeloid cells play a role. ECFC + MPC, ECFC alone, MPC alone, or vehicle alone were injected into the hind limb ischemic muscle one day after ligation of femoral artery and vein. At day 5, hind limbs injected with ECFC + MPC showed greater blood flow recovery compared with ECFC, MPC, or vehicle. Tail vein injection of human endothelial specific Ulex europaeus agglutinin-I demonstrated an increased number of perfused human vessels in ECFC + MPC compared with ECFC. In vivo bioluminescence imaging showed ECFC persisted for 14 days in ECFC + MPC-injected hind limbs. Flow cytometric analysis of ischemic muscles at day 2 revealed increased myeloid lineage cells in ECFC + MPC-injected muscles compared to vehicle-injected muscles. Neutrophils declined by day 7, while the number of myeloid cells, macrophages, and monocytes did not. Systemic myeloid cell depletion with anti-Gr-1 antibody blocked the improved blood flow observed with ECFC + MPC and reduced ECFC and MPC retention. Our data suggest that ECFC + MPC delivery could be used to reestablish blood flow in ischemic tissues, and this may be enhanced by coordinated recruitment of host myeloid cells.
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