SUMMARY Aberrant aggregation of RNA binding protein TDP-43 in neurons is a hallmark of frontotemporal lobar degeneration caused by progranulin haploinsufficiency 1 , 2 . However, the mechanism leading to TDP-43 proteinopathy remains unclear. Here we use single-nucleus RNA-sequencing (snRNA-seq) to show that progranulin deficiency promotes microglial transition from a homeostatic to disease-specific state that causes endolysosomal dysfunction and neurodegeneration. These defects persist even when Grn −/− microglia are cultured ex vivo . In addition, snRNA-seq reveals selective loss of excitatory neurons at disease end-stage, characterized by prominent nuclear and cytoplasmic TDP-43 granules and nuclear pore defects. Remarkably, conditioned media from Grn −/− microglia is sufficient to promote TDP-43 granule formation, nuclear pore defects and cell death in excitatory neurons via the complement activation pathway. Consistent with these results, deleting C1qa and C3 mitigates microglial toxicity, and rescues TDP-43 proteinopathy and neurodegeneration. These results uncover previously unappreciated contributions of chronic microglial toxicity to TDP-43 proteinopathy during neurodegeneration.

Triglycerides (triacylglycerols, TGs) store metabolic energy in organisms and have industrial uses for foods and fuels. Excessive accumulation of TGs in humans causes obesity and is associated with metabolic diseases 1 . TG synthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes 2 – 4 whose structures and catalytic mechanisms are unknown. Here we determined the structure of dimeric human DGAT1, a member of the membrane-bound O -acyltransferase (MBOAT) family, by cryo-electron microscopy at 3.0-Å resolution. DGAT1 forms a homodimer through N-terminal segments and a hydrophobic interface, with putative active sites within the membrane region. A structure obtained with oleoyl-CoA substrate resolved at 3.2-Å shows that the CoA moiety binds DGAT1 on the cytosolic side and the acyl group lies deep within a hydrophobic channel, positioning the acyl-CoA thioester bond near an invariant catalytic histidine residue. The reaction center is located inside a large cavity, which opens laterally to the membrane bilayer, providing lipid access to the active site. A lipid-like density, possibly an acyl-acceptor molecule, is located within the reaction center, orthogonal to acyl-CoA. Insights provided by the DGAT1 structures, together with mutagenesis and functional studies, give rise to a model of catalysis for DGAT’s generation of TGs.