GRAMD1B is a novel regulator of lipid homeostasis, autophagic flux, and phosphorylated tau.

Lipid dyshomeostasis and tau pathology are found in frontotemporal lobar degeneration (FTLD) and Alzheimer’s disease (AD). However, the relationship between lipid dyshomeostasis and tau pathology and molecular mechanisms underlying this relationship remain unclear. Using single-cell RNA-sequencing, we report that GRAM Domain Containing 1B (GRAMD1B), a nonvesicular cholesterol transporter, is increased in excitatory neurons of human neural organoids from patient-derived induced pluripotent stem cells with the MAPT R406W mutation compared to isogenic controls. Mutant neural organoids also exhibit altered lipid species, increased phosphorylated tau, and decreased neuronal activity. Increased GRAMD1B is also found in human FTLD and AD cases and PS19 tau mice. Phosphorylated tau, free cholesterol, and lipid droplets are also increased in human FTLD and AD cases. Furthermore, GRAMD1B overexpression increases pathological tau and lipid dyshomeostasis, which may be due to blocking of autophagic flux. Our findings suggest GRAMD1B may be a new player in the regulation of autophagic flux, cholesterol transport, and tau pathology in FTLD and AD. Human iPSC lines were requested from the Tau consortium including iPSC F11362.11C11(WT: CRISPR-Cas9 edited isogenic control), iPSC F11362.11F10 (HET: MAPT R406W/R406R heterozygous), and iPSC GIH-143 (HOM: MAPT R406W/R406W homozygous). Human neural organoids (HNOs) were formed according to previously published protocols. On Day 70 and Day 120, ten healthy HNOs of similar size were selected from each line. Five HNOs were combined to form a technical replicate. Each replicate was gently dissociated following the Single Cell Preparation Guide established by 10x genomics.