Fellowship for Chau: Microenvironmental Homing of Glioblastoma Stem Cells in the Brain

Glioblastoma (GBM) is the most aggressive of the primary brain tumors and carries a poor prognosis due to its highly infiltrative nature and tendency to progress despite surgery, radiation, and chemotherapy treatments. There is no curative treatment for GBM, however there are potential therapeutic targets. Much of the aggressive behavior of the GBM tumor is attributed to a specialized tumor cell subpopulation known as GBM stem cells that is highly migratory and self-renewing, but little is known about their homing behavior in the brain. Most GBM cells migrate outward from the tumor hypoxia/necrosis into the brain tissue making GBM difficult to completely surgically remove. However, a subpopulation of cells, specifically GBM stem cells are thought move towards hypoxic regions within the tumor called pseudopalisades which are distinct hypercellular borders surrounding hypoxic/necrotic regions. Paradoxically, these oxygen-poor microenvironments within the tumor are quite supportive of GBM stem cells and are not just areas of dying cells as previously thought. It is well established that hypoxic stress can support stem cells by increasing adaptive signaling such as self-renewal, growth, and chemoattraction through the expression of hypoxia inducible factor-1a (HIF-1a), the cell’s master oxygen sensor. HIF-1a expression is high in pseudopalisades as a response to hypoxia and GBM stem cells have been found to reside in the pseudopalisades. GBM stem cells have distinct homing patterns displaying a greater tendency for homing to specific biological niches compared to non-stem cells. Determining the homing preferences of GBM stem cells relative to the hypoxic microenvironments such as pseudopalisades has implications for targeting highly migratory tumorigenic populations for treatment. Our lab has shown that the hypercellularity of pseudopalisades is not a result of proliferation, and that there are actively migrating cells residing in pseudopalisades. This suggests that the cell accumulation and GBM stem cells found at pseudopalisades could be due to GBM stem cells migrating towards hypoxia; however little is known about the source of these cells and the mechanisms that drive homing. The overarching hypothesis for these proposed aims is that a subpopulation of GBM stem cells home towards hypoxia in vivo (Aim 1), and that there are differences in hypoxia-induced mechanisms between GBM stem and non-stem cells that drive homing in vivo (Aim 2). We propose the following 2 aims: Aim 1- To differentiate the homing preferences of GBM stem and non-stem cells towards hypoxic microenvironments in vivo Hypothesis: GBM stem cells will preferentially home to hypoxia exhibiting more self-renewal and migratory signaling than non-stem cells in the brain tissue 1.1 To determine homing preferences of GBM stem and non-stem cells towards hypoxic microenvironments 1.2 To measure the differences in self-renewal and migratory signaling of cells homed to hypoxia vs. normoxia Aim 2- To identify differences in the hypoxia-induced homing responses of GBM stem vs. non-stem cells in vivo To approach this in a comprehensive manner, we will conduct a screen of several HIF-1a-responsive migratory pathways that show differences in stem vs. non-stem cells and select the most robust candidate to knockdown. Hypothesis: There will be differences in HIF-1a-responsive migration signaling between GBM stem and non- stem cells in vivo. Knockdown of a robust HIF-1a-responsive migratory factor in GBM stem and non-stem cells will attenuate homing in vivo. 2.1 To identify robust HIF-1a-responsive migratory/homing factors in GBM stem and non-stem cells 2.2 To determine the effect of HIF-1a-responsive factor knockdown on homing capabilities of GBM stem cells and non-stem cells This study will be the first to investigate the homing capabilities of GBM stem vs. non-stem cells in hypoxic microenvironments using novel in vivo approaches. Differentiating the homing preferences of tumorigenic cell populations and HIF-1a-driven mechanisms will allow us to uncover possible GBM therapeutic targets.