<p>Roundabout (Robo) receptors were first identified in neurons as guidance molecules,
however growing evidence suggests that they also play a role in other cells. The aim of
this thesis was to characterise the expression and function of a novel endothelial specific
member of this family, Robo4. This study revealed that Robo4 is expressed primarily in
vessels but also differentially expressed in tumour vessels. Interestingly Robo4 was
primarily located within cytoplasmic vesicles coated with clathrin, suggesting that the
presence of Robo4 on the cell surface is being tightly regulated. Overexpression of
Robo4 induced filopodia and pseudopodia formation and actin re-organisation into stress
fibres. It co-localised with actin and tubulin suggesting an important interaction between
Robo4 and the cytoskeleton. Robo4's function in endothelial cells was directly
investigated using two approaches, overexpression using adenovirus and knockdown
using small interfering RNA. Functional cell-based assays revealed that disrupting
Robo4's level of expression negatively affects endothelial cell functions that are required
during angiogenesis, such as proliferation, migration and tubulogenesis. Overexpression
of a truncated version of Robo4, which lacks the C-terminus, provided clues regarding
Robo4's function. The intracellular domain is critical for Robo4's localisation and its
association with the cytoskeleton. It is also required for pseudopodia formation. Other
findings include possible cleavage of Robo4 and Robo4 homodimerisation and
heterodimerisation with Robo1. Taken together, the findings presented in this study
strongly suggest a role for Robo4 in endothelial cell guidance. Cell guidance during
angiogenesis is poorly understood therefore the identification of a new molecule
potentially involved in this mechanism will hopefully help elucidate the process.</p