Chronic pharmacologic manipulation of dopamine transmission ameliorates metabolic disturbance in a syndrome caused by mutated trappc9

Loss-of-function mutations of the gene encoding the trafficking protein particle complex subunit 9 (trappc9) cause autosomal recessive intellectual disability and obesity by unknown mechanisms. Genome-wide analysis links trappc9 to non-alcoholic fatty liver disease (NAFLD). Trappc9-deficient mice have been shown to appear overweight shortly after weaning. Here, we analyzed serum biochemistry and histology of adipose and liver tissues to determine the incidence of obesity and NAFLD in trappc9-deficient mice and combined transcriptomic and proteomic analyses, pharmacological studies, and biochemical and histological examinations of postmortem mouse brains to unveil mechanisms involved. We found that trappc9-deficient mice presented with systemic glucose homeostatic disturbance, obesity and NAFLD, which were relieved upon chronic treatment combining dopamine receptor D2 (DRD2) agonist quinpirole and DRD1 antagonist SCH23390. Blood glucose homeostasis in trappc9-deficient mice was restored upon administrating quinpirole alone. RNA-sequencing analysis of DRD2- containing neurons and proteomic study of brain synaptosomes revealed signs of impaired neurotransmitter secretion in trappc9-deficient mice. Biochemical and histological studies of mouse brains showed that trappc9-deficient mice synthesized dopamine normally, but their dopamine-secreting neurons had a lower abundance of structures for releasing dopamine in the striatum. Our study suggests that trappc9 loss- of-function causes obesity and NAFLD by constraining dopamine synapse formation. Overall design: Abdominal fat pads were collected aseptically from three WT and three trappc9 KO mice aged 3-4 weeks. Tissues were washed three times in Hank's balanced salt solutions, minced into small pieces, and then treated with 1 mg mL-1 type-I collagenase at 37 oC with gentle mixing at 120 rpm. Enzymatic digestions were terminated by adding 10 mL of a-MEM media containing 10% FBS, 1x L-glutamine, and 1x penicillin/ streptomycin. The digestion mixtures were filtered through a 70-µm cell strainer (ThermoFisher), and the filtrates were centrifuged at 1200 rpm for 5 min. The cell pellets were washed once in phosphate saline and resuspended in 1 mL TRIzol reagents (Invitrogen) for RNA sequencing (RNAseq) analysis.