Regulation of Macrophage- and Microglia-mediated STING Signaling in Glioblastoma

Glioblastoma is the most common primary malignant brain tumor in adults with a median survival of less than 15 months despite aggressive standard of care including surgery, radiation and chemotherapy. Novel therapies are therefore in critical need. Tumor-derived 2′3′-Cyclic GMP-AMP (cGAMP) induced by chemotherapy and radiation activates STING in myeloid cells as an immunotransmitter to elicit anti-tumor response. Macrophages and microglia are both potent responder to cGAMP in non-tumor conditions, however, in GBM, macrophages and microglia, which are one of the most abundant cells in the tumor microenvironment, are profoundly immunosuppressive. It is unclear through what molecular mechanisms and fundamental biology that macrophages and microglia minimize STING activation in GBM. While STING signaling is activated by a series of well-characterized protein phosphorylation, the mechanisms of inactivation/dephosphorylation of STING signaling is unclear. Protein phosphatase 2A (PP2A) is a ubiquitous serine/threonine phosphatase comprised of a catalytic (C), regulatory (B) and scaffolding (A) subunit and accounts for 50-70% of the total serine/threonine phosphatase activity. The specificity of PP2A is determined by its regulatory B subunit. We have found that inhibition of PP2A catalytic subunit (PP2Ac) in both macrophages and microglia enhances STING signaling. Mice with macrophages/microglia specific deletion of PP2Ac have increased tumor T cells infiltration and reduced tumor growth. Moreover, from unbiased screening of all known regulatory B subunits, we find that striatin4, a specific B subunit of PP2A, has a similar role as PP2Ac in suppressing STING signaling in macrophages/microglia but not in glioma cells. In this project, we propose to: Aim 1) Determine the impact of PP2Ac inhibition in macrophages/microglia on STING pathway in glioma microenvironment in vivo. Aim 2) Elucidate the molecular mechanisms underlying STING inhibition by striatin4-PP2Ac complex in macrophages/microglia. We will also use clinically annotated GBM samples to verify the clinical relevance of striatin4-PP2A. These findings will firmly establish the role of the specific striatin4-PP2A complex in regulating macrophage/microglia functions and also provide the mechanistic foundation to target this specific PP2A complex leading to precise targeting of PP2A as a novel therapy for glioma. This proposed study will also address the fundamental biology about how glioma cells and macrophages/microglia communicate through cGAMP and how macrophage/microglia turn off STING activation through PP2A. We appreciate that PP2A complexes play significant roles not only in macrophages/microglia, but also in glioma cells and other cell types. Our long-term goal is to identify the specific PP2A complexes in glioma cells, immune cells, astrocytes and other cell types within the central neural system relevant to the modulation of immune response in order to develop novel therapeutics for GBM. We believe this study fits to the mission of NINDS to seek fundamental knowledge about the brain and nervous system and to use that knowledge to reduce the burden of neurological disease such as brain tumor.