Microglial\ufeff signalling pathway deficits associated with the patient derived R47H TREM2 variants linked to AD indicate inability to activate inflammasome

Katharina Cosker,Stephen J Neame,Thomas M Piers,James Staddon,Anna Mallach,Jennifer M Pocock,Janhavi Limaye,John Hardy

doi:10.1038/s41598-021-91207-1

Abstract

The R47H variant of the microglial membrane receptor TREM2 is linked to increased risk of late onset Alzheimer’s disease. Human induced pluripotent stem cell derived microglia (iPS-Mg) from patient iPSC lines expressing the AD-linked R47Hhet TREM2 variant, common variant (Cv) or an R47Hhom CRISPR edited line and its isogeneic control, demonstrated that R47H-expressing iPS-Mg expressed a deficit in signal transduction in response to the TREM2 endogenous ligand phosphatidylserine with reduced pSYK-pERK1/2 signalling and a reduced NLRP3 inflammasome response, (including ASC speck formation, Caspase-1 activation and IL-1beta secretion). Apoptotic cell phagocytosis and soluble TREM2 shedding were unaltered, suggesting a disjoint between these pathways and the signalling cascades downstream of TREM2 in R47H-expressing iPS-Mg, whilst metabolic deficits in glycolytic capacity and maximum respiration were reversed when R47H expressing iPS-Mg were exposed to PS+ expressing cells. These findings suggest that R47H-expressing microglia are unable to respond fully to cell damage signals such as phosphatidylserine, which may contribute to the progression of neurodegeneration in late-onset AD.

Highlights

The R47H variant of the microglial membrane receptor triggering receptor on myeloid cells 2 (TREM2) is linked to increased risk of late onset Alzheimer’s disease
In TREM2 common variant (Cv, control) induced pluripotent stem cell derived microglia (iPS-Mg) lines, activation of TREM2 by 2 min of antibody crosslinking with F(ab’)[2] fragments resulted in increased pSYK signalling compared with IgG2B isotype control
We have previously shown that iPS-Mg harbouring the NHD variant T66M showed a significant deficit in phagocytosis of dead cells[10] and we were able to repeat this under our current conditions (Supplementary Fig. 1iii)

Summary

Introduction

The R47H variant of the microglial membrane receptor TREM2 is linked to increased risk of late onset Alzheimer’s disease. Apoptotic cell phagocytosis and soluble TREM2 shedding were unaltered, suggesting a disjoint between these pathways and the signalling cascades downstream of TREM2 in R47H-expressing iPS-Mg, whilst metabolic deficits in glycolytic capacity and maximum respiration were reversed when R47H expressing iPS-Mg were exposed to PS+ expressing cells. These findings suggest that R47H-expressing microglia are unable to respond fully to cell damage signals such as phosphatidylserine, which may contribute to the progression of neurodegeneration in late-onset AD. The NLRP3 inflammasome is regulated by phosphorylation[21,22] via pSyk[23,24,25] and pERK1/2 s ignalling[26,27], it is not known whether these signalling pathways downstream of TREM2 are linked to inflammasome activation

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Results

Conclusion