A Mouse Systems Genetics Approach Reveals Common and Uncommon Genetic Modifiers of Hepatic Lysosomal Enzyme Activities and Glycosphingolipids.

Identification of genetic modulators of lysosomal enzyme activities and glycosphingolipids (GSLs) may facilitate the development of therapeutics for diseases in which they participate, including Lysosomal Storage Disorders (LSDs). To this end, we used a systems genetics approach: we measured 11 hepatic lysosomal enzymes and many of their natural substrates (GSLs), followed by modifier gene mapping by GWAS and transcriptomics associations in a panel of inbred strains. Unexpectedly, most GSLs showed no association between their levels and the enzyme activity that catabolizes them. Genomic mapping identified 30 shared predicted modifier genes between the enzymes and GSLs, which are clustered in three pathways and are associated with other diseases. Surprisingly, they are regulated by ten common transcription factors, and their majority by miRNA-340p. In conclusion, we have identified novel regulators of GSL metabolism, which may serve as therapeutic targets for LSDs and may suggest the involvement of GSL metabolism in other pathologies.

Identification of genetic modifiers of murine hepatic ß-glucocerebrosidase activity.

The acid ß-glucocerebrosidase (GCase) enzyme cleaves glucosylceramide into glucose and ceramide. Loss of function variants in the gene encoding for GCase can lead to Gaucher disease and Parkinson's disease. Therapeutic strategies aimed at increasing GCase activity by targeting a modulating factor are attractive and poorly explored. To identify genetic modifiers, we measured hepatic GCase activity in 27 inbred mouse strains. A genome-wide association study (GWAS) using GCase activity as a trait identified several candidate modifier genes, including Dmrtc2 and Arhgef1 (p=2.1x10-7), and Grik5 (p=2.1x10-7). Bayesian integration of the gene mapping with transcriptomics was used to build integrative networks. The analysis uncovered additional candidate GCase regulators, highlighting modules of the acute phase response (p=1.01x10-8), acute inflammatory response (p=1.01x10-8), fatty acid beta-oxidation (p=7.43x10-5), among others. Our study revealed previously unknown candidate modulators of GCase activity, which may facilitate the design of therapies for diseases with GCase dysfunction.

c-Abl Inhibition Activates TFEB and Promotes Cellular Clearance in a Lysosomal Disorder.

The transcription factor EB (TFEB) has emerged as a master regulator of lysosomal biogenesis, exocytosis, and autophagy, promoting the clearance of substrates stored in cells. c-Abl is a tyrosine kinase that participates in cellular signaling in physiological and pathophysiological conditions. In this study, we explored the connection between c-Abl and TFEB. Here, we show that under pharmacological and genetic c-Abl inhibition, TFEB translocates into the nucleus promoting the expression of its target genes independently of its well-known regulator, mammalian target of rapamycin complex 1. Active c-Abl induces TFEB phosphorylation on tyrosine and the inhibition of this kinase promotes lysosomal biogenesis, autophagy, and exocytosis. c-Abl inhibition in Niemann-Pick type C (NPC) models, a neurodegenerative disease characterized by cholesterol accumulation in lysosomes, promotes a cholesterol-lowering effect in a TFEB-dependent manner. Thus, c-Abl is a TFEB regulator that mediates its tyrosine phosphorylation, and the inhibition of c-Abl activates TFEB promoting cholesterol clearance in NPC models.

Hepatic metabolic response to restricted copper intake in a Niemann-Pick C murine model.

Niemann-Pick C disease (NPC) is a vesicular trafficking disorder primarily caused by mutations in the Npc1 gene and characterized by liver dysfunction and neuropathology. Altered hepatic copper metabolism has recently been reported in NPC disease. Therefore, we aimed to analyze the effects of a copper deficient diet and copper chelation using d-penicillamine on copper homeostasis in the liver of Npc1(-/-) mice of different ages. We examined liver metal ion content by AAS, and copper and iron metabolism gene expression in the liver using qPCR in Npc1(+/+) and Npc1(-/-) mice. We found higher copper and lower iron content in the liver of Npc1(-/-) mice of different ages, compared to controls; these changes in copper and iron content were correlated with increased ceruloplasmin, metallothionein 1, and transferrin receptor gene expression and decreased gene expression of Commd1, ferritin-light chain and ferroportin in the liver of Npc1(-/-) mice of different ages. Npc1(-/-) mice responded to a copper-deficient diet with a decrease in copper content in the liver, bile and heart. These results correlated with a reduction in the hepatic expression of ceruloplasmin and metallothionein 1 during the first week of treatment. d-penicillamine revealed hepatic adaptive response and an improvement in hepatic function in Npc1(-/-) mice without any effect on neurological functions. Our results confirm that the NPC1 protein is required for copper and iron homeostasis. To our knowledge, this is the first report documenting the hepatic adaptive response to low-copper intake in a Npc1(-/-) mouse model.