PLEXIN-B2 FACILITATES DIFFUSE GLIOMA INVASION BY REGULATING CELL ADHESION AND ACTO-MYOSIN DYNAMICS

Abstract Diffuse invasion of glioblastoma (GBM) cells into brain tissue is a key factor for its high lethality. GBM cell migration is affected by functions of plexins, which are transmembrane receptors of semaphorins that regulate cell adhesion and cytoskeletal dynamics. Expression of Plexin-B2 is upregulated in GBM and correlates with malignancy. We show here that Plexin-B2 activity regulates biomechanical properties of GBM cells, promoting invasive growth. Plexin-B2 activity increased the capacity of GBM to invade as dispersed single cells by reducing the cell-cell adhesiveness between GBM cells, indicating that a major function of Plexin-B2 activity is to downregulate cell-cell adhesion systems. RNA-Seq analyses also revealed that GBM stem cells (GSC) with deletion of Plexin-B2 altered expression of genes related to cell adhesion and the matrisome, indicating compensatory mechanisms in cellular dynamics. Interestingly, in vivo intracranial transplant studies demonstrated that growth and invasion of Plexin-B2 mutant GSC was impaired, with mutant cells invading shorter distances and migrating mainly as groups of cells forming chains. Plexin-B2 mutant cells also were more likely to adhere to the vasculature, rather than to fiber tracts, suggesting altered biomechanical properties. This shift may be related to high stiffness of basal lamina of the vasculature, as Plexin-B2 KO cells have a preference for migration on stiff substrate in vitro. Intriguingly, the loss in Plexin-B2 expression also changed the distribution of the mechanosensor transction factor YAP, with high expression of Plexin-B2 correlating with increased nuclear YAP. Structure-function analyses revealed that the Ras-GAP domain as main signaling output of Plexin-B2. The Rap proteins are pleiotropic regulators of cell adhesion and actomysosin contractility. Our data also showed that overexpression of Plexin-B2 can lead to decreased levels of Rap1/Rap2. Thus, Plexin-B2 acts as a key regulator of the adhesion and contractility of GBM cells, thereby facilitating their diffuse invasion.