SHIP1 regulates TREM2 signalling and macrophage functions in a hiPSC-derived model

Genetic and functional studies strongly implicate microglia in the pathology of Alzheimer's disease. The Triggering Receptor Expressed on Myeloid cells 2 (TREM2) pathway is an important functional regulator of microglia, promoting phagocytosis of apoptotic cells, debris and pathogenic proteins. Genome-wide association studies have identified risk-bearing polymorphisms in several members of the pathway including INPP5D, encoding the inositol phosphatase SHIP1. Recent studies utilising in vitro and in vivo disease-relevant models have identified a role for SHIP1 in modulating TREM2 signalling and neurotoxic microglial activation. In this study, we characterised the role of SHIP1 in regulating TREM2 signalling and myeloid functions using human-induced pluripotent stem cell (hiPSC)-derived macrophages. Characterisation of isogenic SHIP1 knockout (SHIP1 KO) macrophages showed decreased expression of the lipopolysaccharide (LPS) coreceptor CD14, which translated to lower secretion of pro-inflammatory cytokines. Additionally, loss of SHIP1 in this macrophage model attenuated TREM2 signalling, which was further reflected in a reduced level of phagocytosis of apoptotic neurons compared to the parental line. The observed effects on TREM2 signalling did not correspond to changes in total protein levels or shedding. Further examination of macrophage scavenger receptor expression revealed reduced cell surface levels of CD163 and CD206 involved in resolution of inflammation, but increased levels of the phagocytic receptor MerTK. Morphological analysis revealed a less amoeboid-activated phenotype and increased adhesion in the SHIP1 KO macrophages. Taken together, our data indicate that SHIP1 plays a key role in modulating TREM2 signalling, inflammation and phagocytosis.