Role of the oligodendrocyte-enriched Ectonucleotide pyrophosphatase-6 in energy metabolism

Abstract

Despite the continuing global rise in obesity prevalence, few effective medicines are available to ameliorate this disease. There remains a need to identify new therapeutic targets. Previously, our genome-wide association studies (GWAS) study in a population isolate identified that variants in and around the human ENPP6 gene (encoding the ectonucleotide pyrophosphatase-6 enzyme ENPP6) were associated with reduced visceral fat accumulation. Moreover, our preliminary data showed that mice lacking the Enpp6 gene (Enpp6–/–) exhibited reduced visceral adiposity and improved glucose tolerance after high-fat-diet (HFD) feeding compared to Enpp6+/+ littermates. The ENPP6 enzyme belongs to a family of ENPP(1–7) enzymes, which have recently emerged as potential mechanistically distinct therapeutic targets for metabolic and vascular diseases. Of these, ENPP6 is a lysophospholipase C type enzyme. In parallel with our work, ENPP6 deficiency was linked by others to excessive hepatic lipid accumulation as a result of local choline deficiency.

To better understand the role of ENPP6 in nutrient metabolism, we characterised the Enpp6–/– mice and tested the hypothesis that choline deficiency was the underlying mechanism for their metabolic phenotype.

We showed that HFD-fed mixed-strain B6129F1-Enpp6–/– mice exhibited reduced visceral adipocyte size and lower liver fat accumulation. Lower liver choline level was found in HFD-fed B6129F1-Enpp6–/– mice compared to the wildtype, consistent with our hypothesis. The improved metabolic phenotype of B6129F1-Enpp6–/– mice was reversed by dietary choline supplementation, but it was not further improved by short-term dietary choline deficiency, which unexpectedly caused higher liver fat accumulation in B6129F1-Enpp6–/– mice. Additionally, the metabolic benefits of Enpp6 deficiency were strain dependent.

In contrast to B6129F1-Enpp6–/– mice, congenic (5 generations back-cross to C57BL/6J strain) C57BL/6J-Enpp6–/– mice were more prone to HFD-induced obesity. These mice exhibited worsened metabolic profiles including increased whole-body weight and fat mass, impaired insulin sensitivity, and a trend for higher liver fat accumulation compared to their littermate controls. This discrepancy may be explained by the significant difference in endogenous Enpp6 expression in the different background strains. ENPP6 is highly expressed in brain newly formed oligodendrocytes (NFOs), but the biological function of ENPP6 in NFOs and whether it is involved in energy balance is unknown. To gain insight into this, this thesis also determined the impact of ENPP6 inhibition/deficiency on oligodendrocyte development by tracking oligodendrocyte lineage marker expression and assessed the metabolic consequences of oligodendrocyte-specific knockdown of Enpp6 in mouse NFOs. No evidence of altered metabolism was found in oligodendrocyte-specific Enpp6 knockdown (Pdgfrα-Cre+Enpp6fl/fl) compared to their littermates (Pdgfrα-Cre–Enpp6fl/fl) after HFD feeding.

Additionally, a potential risk of demyelination caused by ENPP6 deficiency was ruled out, since there was no difference in myelin basic protein (MBP) expression in the corpus callosum of young or adult B6129F1-Enpp6–/– mice brain. In vitro data showed that ENPP6 inhibition by the selective ENPP6 inhibitor T11, or genetic ablation of Enpp6, resulted in impaired oligodendrocyte differentiation without affecting total MBP expression. Nevertheless, Enpp6 deficiency did alter expression of key signalling pathways associated with lipid metabolites G-protein coupled receptor 17 (GPR17), suggesting an alternative yet unknown role for ENPP6 in NFO/oligodendrocyte function.

Taken together, the data suggest a strain dependent metabolic role for ENPP6, with some support for ENPP6 inhibition as a potential therapeutic strategy for visceral obesity and diabetes that may relate to individual variation in ENPP6 expression as revealed by mouse strain differences.