Targeting SHIP paralogs to promote microglial effector function in the CNS

Abstract

The two SH2-containing inositol 5'-phosphatases , SHIP1 (INPP5D) and SHIP2 (INPPL1), play an essential role in modulation of cellular signaling by transforming the PI3K product PI(3,4,5)P3 into PI(3,4)P2. PI3K signaling triggers activation of downstream signaling cascades that drive survival, effector functions, differentiation, and proliferation. SHIP1 can also mask the cytoplasmic tails of key receptors or their adaptor proteins such as DAP12, thus preventing PI3K recruitment to Trem2, a critical receptor for microglial function. Several GWAS studies correlated single nucleotide polymorphisms (SNPs) in INPP5D with Alzheimer's Disease (AD). However, it remains unclear whether these SNPs are deleterious or protective in AD and how they alter SHIP1 protein expression. SHIP2 overexpression has also been correlated with AD, suggesting that both SHIP1 and SHIP2 might be therapeutic targets. To study how SHIP1 and SHIP2 modulate microglial functions we used small molecule inhibitors and agonists of these enzymes. In our initial study we found that both SHIP paralogs are expressed in murine microglia and that Pan- SHIP1/2 inhibition increases lysosomal size and enhances microglial phagocytosis of Ab1-42 fibrils and dead neurons using both flow cytometry and confocal microscopy. Our lead Pan-SHIP1/2 inhibitor, K161, showed Blood Brain Barrier penetration as detected in the cerebral cortex of treated mice with mass spectrometry. K161 treatment of WT mice showed no difference in microglial frequency or lysosomal content in vivo; however, we observed a significant 2 increase in Ab1-42 and dead neurons phagocytosis ex vivo in microglia in K161- treated mice versus controls. Subsequently, we discovered a novel and highly potent SHIP1 selective agonist (K306) via artificial intelligence guided computational screening. We found that K306 can reduce the release of inflammatory cytokines in macrophages and microglial cells stimulated with LPS or Ab1-42. Interestingly, K306 didn't alter microglial phagocytic uptake of cargo, but did promote degradation of phagocytosed lipid-laden cargo - defining a novel role of SHIP1 in degradation of lipid cargo in microglia. These results highlight the importance of SHIP1 and SHIP2 in microglial biology and their modulation as therapeutics in different stages of neurodegenerative disease where microglia play a major role, such as AD.