|Ph.D Student||Sabag Adi|
|Subject||The Role of Neuripilins and Plexins as Mediators of|
Semaphorins Activity in the Control of Tumor
Progression and Angiogenesis
|Department||Department of Medicine||Supervisor||Professor Emeritus Gera Neufeld|
|Full Thesis text|
Plexins are receptors for semaphorins, a large family of secreted and membrane-bound proteins that were first described as axon guidance factors in the nervous system, but are known today to participate in a variety of biological processes. Signals of class-3 semaphorins are mediated by binding to neuropilin co-receptors and it is believed that neuropilins determine binding specificity of these semaphorins while plexins function primarily as the signal transduction elements. Expression of semaphorin-receptors is not restricted to neuronal cells, and recent research has been focused on the roles of plexin-semaphorin signalling outside the nervous system. We aimed to determine whether plexin-A2 participates in determining binding specificity for semaphorins, in endothelial cell and tumor cells, and to learn about the role of plexin-A2 in development of tumors and blood vessels. We observed that signal transduction of semaphorin3B and semaphorin6A is plexin-A2 dependent. In human umbilical vain endothelial cells (HUVEC) and in U87MG glioblastoma cells, semaphorin6A signals are transduced by plexin-A2 and are not dependent on other plexin-A receptors. When plexin-A2 is silenced in U87MG cells, loss of semaphorin6A signals can be compensated for by plexin-A4 over expression. Sema3B signals require the expression of plexin-A2 and plexin-A4, as well as neuropilin-1 or neuropilin-2. Plexin-A2 expression is obligatory for sema3B signal transduction. It can compensate for the loss of Plexin-A4 but cannot be compensated for by plexin-A4 expression. Unexpectedly, inhibition of plexin-A2 resulted in inhibition of cell proliferation in vitro and promoted inhibition of tumor formation in vivo.
Some of the class-3 semaphorins have been characterized as suppressors of tumor progression. To determine if class-3 semaphorins can be used to inhibit development of glioblastoma-multiforme tumors, we expressed six recombinant semaphorins in U87MG cells. With the exception of sema3G, expression of these semaphorins in U87MG cells strongly inhibited tumor development from subcutaneously-implanted cells and from cells implanted in the cortex of mouse brains. Sema3D and sema3E displayed the strongest inhibitory effects and their expression in U373MG or in U87MG cells implanted in the brains of mice prolonged the survival of mice by more than two folds. Most of the semaphorins that we have used here with the exception of sema3D were previously characterized as angiogenesis-inhibitors. We found that sema3D also functions as an inhibitor of angiogenesis, suggesting that the anti-tumorigenic effects may be attributed to inhibition of tumor angiogenesis. These results indicate that class-3 semaphorins such as sema3D and sema3E could perhaps be used in the future to treat glioblastoma patients.