Mitochondria are dynamic organelles that play a key role in energy conversion. Optimal mitochondrial function is ensured by a quality-control system tightly coupled to fusion and fission. In this connection, mitofusin 2 (Mfn2) participates in mitochondrial fusion and undergoes repression in muscle from obese or type 2 diabetic patients. Here, we provide in vivo evidence that Mfn2 plays an essential role in metabolic homeostasis. Liver-specific ablation of Mfn2 in mice led to numerous metabolic abnormalities, characterized by glucose intolerance and enhanced hepatic gluconeogenesis. Mfn2 deficiency impaired insulin signaling in liver and muscle. Furthermore, Mfn2 deficiency was associated with endoplasmic reticulum stress, enhanced hydrogen peroxide concentration, altered reactive oxygen species handling, and active JNK. Chemical chaperones or the antioxidant N -acetylcysteine ameliorated glucose tolerance and insulin signaling in liver-specific Mfn2 KO mice. This study provides an important description of a unique unexpected role of Mfn2 coordinating mitochondria and endoplasmic reticulum function, leading to modulation of insulin signaling and glucose homeostasis in vivo.

Significance: Organelles engage in heterotypic interactions crucial for metabolic and signaling cascades. The best-studied case of this heterotypic interaction is that between the mitochondria and endoplasmic reticulum (ER), crucial for transfer of lipids and especially Ca ²⁺ between the two organelles. The original discovery that the mitochondria-shaping protein Mitofusin 2 (Mfn2) physically tethers the ER to mitochondria was recently challenged. Here, electron microscopy and fluorescent probes of organelle proximity provide definitive evidence that constitutive or acute Mfn2 ablation increases the distance between the ER and mitochondria. Functionally, this process reduces mitochondrial Ca ²⁺ uptake without altering the mitochondrial Ca ²⁺ uniporter complex in multiple tissues. Thus, the discoveries of the role of ER–mitochondria juxtaposition in cell biology based on Mfn2 as a tool remain unchallenged.

Mitofusin 2 (Mfn2) is a key protein in mitochondrial fusion and it participates in the bridging of mitochondria to the endoplasmic reticulum (ER). Recent data indicate that Mfn2 ablation leads to ER stress. Here we report on the mechanisms by which Mfn2 modulates cellular responses to ER stress. Induction of ER stress in Mfn2‐deficient cells caused massive ER expansion and excessive activation of all three Unfolded Protein Response (UPR) branches (PERK, XBP‐1, and ATF6). In spite of an enhanced UPR, these cells showed reduced activation of apoptosis and autophagy during ER stress. Silencing of PERK increased the apoptosis of Mfn2‐ablated cells in response to ER stress. XBP‐1 loss‐of‐function ameliorated autophagic activity of these cells upon ER stress. Mfn2 physically interacts with PERK, and Mfn2‐ablated cells showed sustained activation of this protein kinase under basal conditions. Unexpectedly, PERK silencing in these cells reduced ROS production, normalized mitochondrial calcium, and improved mitochondrial morphology. In summary, our data indicate that Mfn2 is an upstream modulator of PERK. Furthermore, Mfn2 loss‐of‐function reveals that PERK is a key regulator of mitochondrial morphology and function.