Title:
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Trafficking of VEGFR2 in angiogenesis : the role of myosin Vb
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Angiogenesis is the formation of new blood vessels from existing vasculature. It is a process
fundamental to normal development and tissue repair, and is implicated in many pathological
conditions. The major pro-angiogenic factor is VEGF, for which the major receptor is
VEGFR2. Blood vessels are lined with endothelial cells that express VEGFR2 to detect
VEGF in surrounding tissue. This detection mediates cell responses to initiate fOlmation of
angiogenic sprouts.
VEGFR2 belongs to the family of receptor tyrosine kinases. Ligand binding to the
extracellular domain results in receptor dimerisation and autophosphorylation of the
intracellular kinase domain. This activates multiple downstream signalling cascades, which in
the case of VEGFR2 have four main outcomes: proliferation, migration, permeability and
survival. Receptor activation is often followed by internalisation and degradation to downregulate
signalling pathways. VEGFR2 has unusual trafficking kinetics for a receptor tyrosine
kinase because it constitutively internalises and recycles back to the cell surface in the
absence of VEGF. It is distributed such that a significant proportion is localised to an
intracellular endosomal storage pool. Moreover, the behaviour of VEGFR2 during
angiogenesis depends on the location of the endothelial cell within the growing sprout. The
importance of this unusual trafficking in relation to signal transduction is poorly understood.
My aim was to elucidate how VEGFR2 trafficking controls angiogenic signalling.
I worked to identify sorting proteins required for VEGFR2 trafficking. I investigated proteins
that are known to mediate the trafficking of other cargoes. Using an ELISA-based screen, I
discovered several potential regulators of VEGFR2, including the motor protein myosin Vb.
Myosin Vb traffics organelles along actin filaments and has been described in the recycling of
many receptors. I used biochemical methods to establish an essential role for myosin Vb in
VEGFR2 recycling in unstimulated conditions. I show that myosin Vb depletion impairs
vessel formation in an organotypic angiogenesis assay and disrupts phosphorylation of several
kinases activated upon VEGF stimulation in endothelial cells. Furthermore, I show that
myosin Vb is required for the polarised distribution of VEGFR2 in endothelial cells to enable
chemotactic migration towards VEGF. My data suggest that myosin Vb-dependent
constitutive trafficking of inactive VEGFR2 is necessary for angiogenesis.
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