Semaphorin 6A regulates angiogenesis by modulating VEGF signaling

Abstract

Formation of new vessels during development and in the mature mammal generally proceeds through angiogenesis. Although a variety of molecules and signaling pathways are known to underlie endothelial cell sprouting and remodeling during angiogenesis, many aspects of this complex process remain unexplained. Here we show that the transmembrane semaphorin6A (Sema6A) is expressed in endothelial cells, and regulates endothelial cell survival and growth by modulating the expression and signaling of VEGFR2, which is known to maintain endothelial cell viability by autocrine VEGFR signaling. The silencing of Sema6A in primary endothelial cells promotes cell death that is not rescued by exogenous VEGF-A or FGF2, attributable to the loss of prosurvival signaling from endogenous VEGF. Analyses of mouse tissues demonstrate that Sema6A is expressed in angiogenic and remodeling vessels. Mice with null mutations of Sema6A exhibit significant defects in hyaloid vessels complexity associated with increased endothelial cell death, and in retinal vessels development that is abnormally reduced. Adult Sema6A-null mice exhibit reduced tumor, matrigel, and choroidal angiogenesis compared with controls. Sema6A plays important roles in development of the nervous system. Here we show that it also regulates vascular development and adult angiogenesis. IntroductionAngiogenesis during development and in adult mammals proceeds largely through endothelial cell sprouting and sprouts remodeling to generate a network of vessels where blood can flow. VEGF-A, its receptor VEGFR2, and coreceptor neuropilin-1, Notch ligands, and Notch receptors, B-ephrin ligands, and EphB receptors, and other molecules are critical mediators of angiogenesis, but aspects of this process are not explained and other molecules probably contribute.The semaphorins comprise a family of membrane-bound and secreted proteins implicated in the development of the neural system, and in modulating immune responses and tumor growth in the adult. [1][2][3] Semaphorins are characterized by the presence of a Sema domain, a 500 residue N-terminal domain, structurally similar to the extracellular domain of ␣-integrins, and a determinant of receptor binding specificity. 4 There are 8 classes of semaphorins, including classes 1 and 2 found mostly in invertebrates, classes 3 to 7 found in vertebrates, and the viral (V) class encoded by viruses. Semaphorins signal through their plexin receptors, but whereas membrane-bound semaphorins bind directly to their cognate plexin receptors, the secreted class-3 semaphorins require neuropilin-1 or 2 as coreceptors. 5,6 Plexin receptors can elicit multiple signaling pathways in response to semaphorin activation, as they associate with various signaling protein partners. 6 Many secreted class-3 semaphorins have been reported to inhibit angiogenesis. 2 Semaphorin-3A (Sema-3A), which binds neuropilin-1 and signals through plexinA1, A2, or A4, inhibits endothelial cell responses to VEGF 165 in vitro. [7][8][9][10][11][12] However, it is not clear whether ...

Endothelial RAF1/ERK activation regulates arterial morphogenesis

“…Although semaphorins were originally thought to mediate axon guidance during the development of central nervous system, 42 a number of the family members such as SEMA3A, SEMA3E, SEMA3F, SEMA4D, and SEMA6A are expressed in EC and play various roles in endothelium. [43][44][45] In particular, SEMA3E 30,43 and SEMA6A 45 have been implicated in regulating vascular patterning and EC repulsion, and this is supported by our data.…”

Nrf2 in ischemic neurons promotes retinal vascular regeneration through regulation of semaphorin 6A

“…Two studies investigating Sema6A in the context of endothelial cells demonstrated contrasting effects of this molecule on angiogenesis. One study demonstrated a proangiogenic effect of Sema6A in the setting of retinal vascular development and found that endothelial cell expression of Sema6A promoted EC survival, VEGFR2 expression, and responsiveness to VEGF (38). A second study of endothelial cells demonstrated that Sema6A can exert an antiangiogenic effect.…”

The role of the semaphorins in cancer

“…Such semaphorins are therefore considered as targets for the development of novel cancer therapeutics. Interestingly, some semaphorins such as sema3C, sema6A and sema3E display dual activities and have been characterized in some publications as inducers of tumor progression 108,[111][112][113][114] and in other publications as inhibitors of tumor progression. 61,92,115,116 The mechanisms responsible for this duality are not yet completely clear and are likely the result of post translational processing and the formation of complex associations between semaphorin receptors and other types of membrane bound receptors such as various tyrosinekinase receptors and adhesion receptors.…”

“…255 However, sema6A may also be required for the survival of endothelial cells. Silencing its expression in endothelial cells leads to apoptosis and in-vivo to reduced tumor angiogenesis and tumor development due to inhibition of VEGF induced VEGFR-2 mediated signal transduction which can be rescued by the addition of the exogenous soluble extracellular domain of sema6A, 108 suggesting that it can affect tumor angiogenesis and tumor progression utilizing diverse mechanisms of action. In addition, inhibition of sema6A expression as well as MICAL-1 expression in melanoma cells containing mutated B-RAF inhibited tumor progression while overexpression of sema6A in these cells promoted tumor progression, 256 suggesting that the effects of sema6A on tumor progression may vary depending on tumor type.…”