Mechanisms of adipocyte loss in mouse models of familial partial lipodystrophy 2

PROJECT SUMMARY/ABSTRACT Lipodystrophies are rare diseases, characterized by a striking loss, redistribution, and dysfunction of adipose tissue, and accompanied by metabolic dysfunction. Familial partial lipodystrophy 2 (FPLD2) is the most common type of lipodystrophy, caused by mutations in LMNA, which encodes the nuclear lamina proteins lamin A/C. Lamin A/C is crucial for nuclear function and controls gene expression, but its role in adipocyte maintenance and function is incompletely understood. Our lab created a constitutive adipocyte specific Lmna knockout mouse model (LmnaADKO) to better characterize the roles of Lmna in adipocytes. LmnaADKO mice develop adipose tissues that are subsequently lost starting at 4 weeks of age; LmnaADKO mice have hepatic steatosis and reduced circulating leptin concentrations, closely recapitulating FPLD2. Analyses of LmnaADKO mice suggest that lamin A/C is required for adipocyte maintenance, substantially advancing our mechanistic understanding, since previous studies did not reveal how loss of Lmna function leads to lipodystrophy. To study mechanisms of how knockout of Lmna in adipocytes leads to subsequent loss of adipocytes, we generated mice in which Lmna can be inducibly knocked out with administration of tamoxifen (LmnaiADKO). This approach allows us to use adult mice, which have more adipose tissue, and to temporally synchronize adipocyte loss. In preliminary experiments, we have tested multiple mechanisms in vivo that could contribute to loss of adipocytes in LmnaiADKO mice. Altered adipogenesis seems unlikely, considering that functional white adipose tissue (WAT) develops in LmnaADKO mice and patients with FPLD2. We also did not observe signs of increased lipolytic capacity or increased acute cell death in WAT lacking Lmna in adipocytes. However, we did observe that adipocytes lacking Lmna are visibly smaller and misshapen compared to controls, suggesting adipocytes are losing their lipid stores, and are unable to maintain viability. Bulk RNA-seq and proteomics from LmnaiADKO WAT revealed downregulation of lipogenic pathways, and upregulation of inflammatory genes. We hypothesize that lamin A/C is required to maintain mature adipocyte characteristics, and that absence of functional lamin A/C leads to adipocyte loss through reduced lipogenic gene expression, driven by altered interactions between Lmna and chromatin. My project will test this hypothesis by (1) evaluating changes in lipogenic and mitochondrial genes in Lmna knockout adipocytes and performing DamID and FISH to study changes in chromatin-lamina interactions that may underlie changes in gene expression and (2) assessing decreased de novo lipogenesis in LmnaiADKO mice, in addition to restoring lipogenic gene expression to Lmna KO adipocytes to prevent WAT loss. Ultimately, these studies will reveal underlying molecular mechanisms of FPLD2, uncovering novel therapeutic targets for lipodystrophy patients while bolstering our understanding of fundamental adipocyte biology.