Monogenic mitochondrial disease is known to cause tissue-selective manifestations, typically affecting the nervous system, liver, and muscle. However, until recently, there was relatively little human genetic evidence for a role of mitochondria in disorders of energy balance (i.e. weight) or insulin action. Several groups have identified that biallelic p.Arg707Trp mutations in MFN2 (MFN2R707W) cause a striking leptin-deficient partial lipodystrophy syndrome. It is not known why this mutation affects adipose tissue when other mutations in MFN2 cause a peripheral neuropathy. More generally, it is not clear how mitochondrial dynamics affect adipogenesis and adipokine secretion. Therefore, we aimed to broaden and deepen our understanding of the roles of the mitofusins (MFN1 and MFN2) in adipose tissue by: (i) assessing the role of the mitofusins on murine in vitro models of adipogenesis, and (ii) examining the phenotype of a Mfn2R707W knock-in mouse model.

Wild-type (WT), Mfn1-/-, and Mfn2-/-, mouse embryonic fibroblasts (MEFs) were differentiated into mature adipocytes. Despite the disruption of their mitochondrial morphology, Mfn1-/- MEFs manifested a marked increase in adipogenesis relative to WT or Mfn2-/- MEFs, as reflected by increased lipid accumulation, expression of mature adipocyte markers (Plin1, Glut4, Adipoq), and insulin-stimulated glucose uptake. In contrast, though Mfn2-/- MEFs accumulated similar amounts of lipid to wild-type cells, they showed reduced PPARG and GLUT4 expression with minimal insulin-stimulated glucose uptake. RNA sequencing revealed a pro-adipogenic transcriptional profile in Mfn1-/- MEFs including upregulation of PPARG. These findings were replicated using siRNA knock-down in 3T3-L1s, where knock-down of Mfn1 was associated with a marked increase in lipid accumulation and knock-down of Mfn2 caused a reduction in expression of Pparg, Glut4, and Plin1.

Homozygous Mfn2 p.Arg707Trp knock-in (KI) mice were generated through CRISPR-Cas9 mutagenesis. Male mice were fed either a chow diet (CD) or a 45% high-fat diet (HFD) for up to 50 weeks. Mfn2 was expressed at similar levels in wild-type (WT) and KI animals. Mitochondria were assessed morphologically using transition electron microscopy (TEM), which showed that mitochondria from brown adipose tissue (BAT) and white adipose tissue (WAT) were more circular with no morphological change in non-adipose tissues. There was no difference in body weight, fat mass, or glucose tolerance in KI but there was evidence of activation of the integrated stress response in WAT and BAT only. KI had lower leptin and adiponectin in their serum and at mRNA level, which was replicated in adipose explants.

In conclusion, loss of Mfn1 but not Mfn2, results in enhanced adipogenesis due to a combination of increased Pparg expression and a pro-adipogenic transcriptional profile. Mice homozygous for Mfn2 p.Arg707Trp show an adipose-specific mitochondrial stress response with low leptin, which is partially transcriptionally mediated. These findings underscore the importance of mitochondrial dynamics in adipose tissue function.