Human VCP mutant ALS/FTD microglia display immune and lysosomal phenotypes independently of GPNMB

Background: Microglia play crucial roles in mediating neuronal homeostasis but have been implicated in contributing to amyotrophic lateral sclerosis (ALS). However, the role of microglia in ALS remains incompletely understood. Methods: Here, we generated highly enriched cultures of VCP mutant microglia derived from human induced pluripotent stem cells (hiPSCs) to investigate their cell autonomous and non-cell autonomous roles in ALS pathogenesis. We used RNA-sequencing, proteomics and functional assays to study hiPSC derived VCP mutant microglia and their effects on hiPSC derived motor neurons and astrocytes. Results: Transcriptomic, proteomic and functional analyses revealed immune and lysosomal dysfunction in VCP mutant microglia. Stimulating healthy microglia with inflammatory inducer lipopolysaccharide (LPS) showed partial overlap with VCP mutant microglia in their reactive transformation. LPS-stimulated VCP mutant microglia displayed differential activation of inflammatory pathways compared with LPS-stimulated healthy microglia. Conserved gene expression changes were identified between VCP mutant microglia, SOD1 mutant mice microglia, and postmortem ALS spinal cord microglial signatures, including increased expression of the transmembrane glycoprotein GPNMB. While knockdown of GPNMB affected inflammatory and phagocytosis processes in microglia, this was not sufficient to ameliorate cell autonomous phenotypes in VCP mutant microglia. Secreted factors from VCP mutant microglia were sufficient to activate the JAK-STAT pathway in hiPSC derived motor neurons and astrocytes. Conclusions: VCP mutant microglia undergo cell autonomous reactive transformation involving immune and lysosomal dysfunction that partially recapitulate key phenotypes of microglia from ALS models and post mortem tissue and are independent of GPNMB. VCP mutant microglia elicit non cell autonomous responses in motor neurons and astrocytes involving the JAK-STAT pathway. Overall design: To study differences in ALS or LPS treated microglia and their effects on other CNS cell types we differentiated microglia from human iPSC lines from VCP mutant ALS or healthy control backgrounds and conducted RNA sequencing of microglia monocultures or astrocytes or motor neurons treated with microglia conditioned media. To investigate the role of GPNMB in microglia we conducted RNA sequencing of samples treated with either an siRNA targeting GPNMB or a scrambled control siRNA.