Splice variants of mitofusin 2 shape the endoplasmic reticulum and tether it to mitochondria.

In eukaryotic cells, different organelles interact at membrane contact sites stabilized by tethers. Mitochondrial mitofusin 2 (MFN2) acts as a membrane tether that interacts with an unknown partner on the endoplasmic reticulum (ER). In this work, we identified the MFN2 splice variant ERMIT2 as the ER tethering partner of MFN2. Splicing of MFN2 produced ERMIT2 and ERMIN2, two ER-specific variants. ERMIN2 regulated ER morphology, whereas ERMIT2 localized at the ER-mitochondria interface and interacted with mitochondrial mitofusins to tether ER and mitochondria. This tethering allowed efficient mitochondrial calcium ion uptake and phospholipid transfer. Expression of ERMIT2 ameliorated the ER stress, inflammation, and fibrosis typical of liver-specific Mfn2 knockout mice. Thus, ER-specific MFN2 variants display entirely extramitochondrial MFN2 functions involved in interorganellar tethering and liver metabolic activities.

Inhibition of autophagy curtails visual loss in a model of autosomal dominant optic atrophy.

In autosomal dominant optic atrophy (ADOA), caused by mutations in the mitochondrial cristae biogenesis and fusion protein optic atrophy 1 (Opa1), retinal ganglion cell (RGC) dysfunction and visual loss occur by unknown mechanisms. Here, we show a role for autophagy in ADOA pathogenesis. In RGCs expressing mutated Opa1, active 5' AMP-activated protein kinase (AMPK) and its autophagy effector ULK1 accumulate at axonal hillocks. This AMPK activation triggers localized hillock autophagosome accumulation and mitophagy, ultimately resulting in reduced axonal mitochondrial content that is restored by genetic inhibition of AMPK and autophagy. In C. elegans, deletion of AMPK or of key autophagy and mitophagy genes normalizes the axonal mitochondrial content that is reduced upon mitochondrial dysfunction. In conditional, RGC specific Opa1-deficient mice, depletion of the essential autophagy gene Atg7 normalizes the excess autophagy and corrects the visual defects caused by Opa1 ablation. Thus, our data identify AMPK and autophagy as targetable components of ADOA pathogenesis.

Cytoskeletal transgelin 2 contributes to gender-dependent adipose tissue expandability and immune function.

During adipogenesis, preadipocytes' cytoskeleton reorganizes in parallel with lipid accumulation. Failure to do so may impact the ability of adipose tissue (AT) to shift between lipid storage and mobilization. Here, we identify cytoskeletal transgelin 2 (TAGLN2) as a protein expressed in AT and associated with obesity and inflammation, being normalized upon weight loss. TAGLN2 was primarily found in the adipose stromovascular cell fraction, but inflammation, TGF-ß, and estradiol also prompted increased expression in human adipocytes. Tagln2 knockdown revealed a key functional role, being required for proliferation and differentiation of fat cells, whereas transgenic mice overexpressing Tagln2 using the adipocyte protein 2 promoter disclosed remarkable sex-dependent variations, in which females displayed "healthy" obesity and hypertrophied adipocytes but preserved insulin sensitivity, and males exhibited physiologic changes suggestive of defective AT expandability, including increased number of small adipocytes, activation of immune cells, mitochondrial dysfunction, and impaired metabolism together with decreased insulin sensitivity. The metabolic relevance and sexual dimorphism of TAGLN2 was also outlined by genetic variants that may modulate its expression and are associated with obesity and the risk of ischemic heart disease in men. Collectively, current findings highlight the contribution of cytoskeletal TAGLN2 to the obese phenotype in a gender-dependent manner.-Ortega, F. J., Moreno-Navarrete, J. M., Mercader, J. M., Gómez-Serrano, M., García-Santos, E., Latorre, J., Lluch, A., Sabater, M., Caballano-Infantes, E., Guzmán, R., Macías-González, M., Buxo, M., Gironés, J., Vilallonga, R., Naon, D., Botas, P., Delgado, E., Corella, D., Burcelin, R., Frühbeck, G., Ricart, W., Simó, R., Castrillon-Rodríguez, I., Tinahones, F. J., Bosch, F., Vidal-Puig, A., Malagón, M. M., Peral, B., Zorzano, A., Fernández-Real, J. M. Cytoskeletal transgelin 2 contributes to gender-dependent adipose tissue expandability and immune function.

Critical reappraisal confirms that Mitofusin 2 is an endoplasmic reticulum-mitochondria tether.

The discovery of the multiple roles of mitochondria-endoplasmic reticulum (ER) juxtaposition in cell biology often relied upon the exploitation of Mitofusin (Mfn) 2 as an ER-mitochondria tether. However, this established Mfn2 function was recently questioned, calling for a critical re-evaluation of Mfn2's role in ER-mitochondria cross-talk. Electron microscopy and fluorescence-based probes of organelle proximity confirmed that ER-mitochondria juxtaposition was reduced by constitutive or acute Mfn2 deletion. Functionally, mitochondrial uptake of Ca2+ released from the ER was reduced following acute Mfn2 ablation, as well as in Mfn2-/- cells overexpressing the mitochondrial calcium uniporter. Mitochondrial Ca2+ uptake rate and extent were normal in isolated Mfn2-/- liver mitochondria, consistent with the finding that acute or chronic Mfn2 ablation or overexpression did not alter mitochondrial calcium uniporter complex component levels. Hence, Mfn2 stands as a bona fide ER-mitochondria tether whose ablation decreases interorganellar juxtaposition and communication.

At the right distance: ER-mitochondria juxtaposition in cell life and death.

The interface between mitochondria and the endoplasmic reticulum is emerging as a crucial hub for calcium signalling, apoptosis, autophagy and lipid biosynthesis, with far reaching implications in cell life and death and in the regulation of mitochondrial and endoplasmic reticulum function. Here we review our current knowledge on the structural and functional aspects of this interorganellar juxtaposition. This article is part of a Special Issue entitled: Calcium Signaling In Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Targeting the association of calgranulin B (S100A9) with insulin resistance and type 2 diabetes.

Calgranulin B (S100A9) was recognized as a candidate type 2 diabetes (T2D) gene in the genomic profiling of muscle from a rodent model of T2D and identifying the human orthologs of genes localized in T2D susceptibility regions. Circulating and S100A9 expressions in muscle and adipose tissue, isolated fat cells, and mouse models were evaluated. A common 5'-upstream single-nucleotide polymorphism (SNP; rs3014866) for S100A9 was analyzed, as well as the effects of weight loss and treatments in vitro with recombinant S100A9. S100a9 expression was increased in muscle of diabetic mice (1.6-fold, p = 0.002), and in muscle from subjects with impaired glucose tolerance (∼4-fold, p = 0.028; n = 34). The rs3014866 SNP was associated with circulating S100A9 and the risk of T2D, having TT carriers at 28 % (p = 0.03) lower risk (n = 1,450). Indeed, increased circulating S100A9 (∼4-fold, p = 0.03; n = 206) and subcutaneous (2-fold, p = 0.01) and omental (1.4-fold, p = 0.04) S100A9 gene expressions (n = 83) in TT carriers run in parallel to decreased fasting glucose and glycated hemoglobin. Accordingly, metformin led to increased S100A9 mRNA in ex vivo-treated adipose tissue explants (n = 5/treatment). Otherwise, obese subjects showed a compensatory increase in circulating and S100A9 expressions in adipose (n = 126), as further demonstrated by decreased levels after diet- (-34 %, p = 0.002; n = 20) and surgery-induced (-58 %, p = 0.02; n = 8) weight loss. Lipopolysaccharide led to increased S100A9 in adipose from mice (n = 5/treatment) while recombinant S100A9 downregulated inflammation in adipocytes (n = 3/treatment). Current findings support the strategy of testing differentially expressed genes in mice and human orthologs associated with T2D. The increased S100A9 reported for obesity and insulin resistance may be envisioned as a compensatory mechanism for inflammation.

The promoter activity of human Mfn2 depends on Sp1 in vascular smooth muscle cells.

AIMS: Mitofusin-2 (Mfn2) expression is dysregulated in vascular proliferative disorders and its overexpression attenuates the proliferation of vascular smooth muscle cells (VSMCs) and neointimal lesion development after balloon angioplasty. We sought to gain insight into the mechanisms that control Mfn2 expression in VSMCs. METHODS AND RESULTS: We cloned and characterized 2 kb of the 5'-flanking region of the human Mfn2 gene. Its TATA-less promoter contains a CpG island. In keeping with this, 5'-rapid amplification of cDNA ends revealed six transcriptional start sites (TSSs), of which TSS2 and TSS5 were the most frequently used. The strong CpG island was found to be non-methylated under conditions characterized by large differences in Mfn2 gene expression. The proximal Mfn2 promoter contains six putative Sp1 motifs. Sp1 binds to the Mfn2 promoter and its overexpression activates the Mfn2 promoter in VSMCs. Chemical inhibition of Sp1 reduced Mfn2 expression, and Sp1 silencing reduced transcriptional activity of the Mfn2 promoter. In keeping with this view, Sp1 and Mfn2 mRNA levels were down-regulated in the aorta early after an atherogenic diet in apolipoprotein E-knockout mice or in VSMCs cultured in the presence of low serum. CONCLUSION: Sp1 is a key factor in maintaining basal Mfn2 transcription in VSMCs. Given the anti-proliferative actions of Mfn2, Sp1-induced Mfn2 transcription may represent a mechanism for prevention of VSMC proliferation and neointimal lesion and development.

Trichoplein/mitostatin regulates endoplasmic reticulum-mitochondria juxtaposition.

Trichoplein/mitostatin (TpMs) is a keratin-binding protein that partly colocalizes with mitochondria and is often downregulated in epithelial cancers, but its function remains unclear. In this study, we report that TpMs regulates the tethering between mitochondria and endoplasmic reticulum (ER) in a Mitofusin 2 (Mfn2)-dependent manner. Subcellular fractionation and immunostaining show that TpMs is present at the interface between mitochondria and ER. The expression of TpMs leads to mitochondrial fragmentation and loosens tethering with ER, whereas its silencing has opposite effects. Functionally, the reduced tethering by TpMs inhibits apoptosis by Ca(2+)-dependent stimuli that require ER-mitochondria juxtaposition. Biochemical and genetic evidence support a model in which TpMs requires Mfn2 to modulate mitochondrial shape and tethering. Thus, TpMs is a new regulator of mitochondria-ER juxtaposition.

Subjects with early-onset type 2 diabetes show defective activation of the skeletal muscle PGC-1{alpha}/Mitofusin-2 regulatory pathway in response to physical activity.

OBJECTIVE Type 2 diabetes is associated with insulin resistance and skeletal muscle mitochondrial dysfunction. We have found that subjects with early-onset type 2 diabetes show incapacity to increase Vo(2max) in response to chronic exercise. This suggests a defect in muscle mitochondrial response to exercise. Here, we have explored the nature of the mechanisms involved. RESEARCH DESIGN AND METHODS Muscle biopsies were collected from young type 2 diabetic subjects and obese control subjects before and after acute or chronic exercise protocols, and the expression of genes and/or proteins relevant to mitochondrial function was measured. In particular, the regulatory pathway peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha/mitofusin-2 (Mfn2) was analyzed. RESULTS At baseline, subjects with diabetes showed reduced expression (by 26%) of the mitochondrial fusion protein Mfn2 and a 39% reduction of the alpha-subunit of ATP synthase. Porin expression was unchanged, consistent with normal mitochondrial mass. Chronic exercise led to a 2.8-fold increase in Mfn2, as well as increases in porin, and the alpha-subunit of ATP synthase in muscle from control subjects. However, Mfn2 was unchanged after chronic exercise in individuals with diabetes, whereas porin and alpha-subunit of ATP synthase were increased. Acute exercise caused a fourfold increase in PGC-1alpha expression in muscle from control subjects but not in subjects with diabetes. CONCLUSIONS Our results demonstrate alterations in the regulatory pathway that controls PGC-1alpha expression and induction of Mfn2 in muscle from patients with early-onset type 2 diabetes. Patients with early-onset type 2 diabetes display abnormalities in the exercise-dependent pathway that regulates the expression of PGC-1alpha and Mfn2.

Expression of Mfn2, the Charcot-Marie-Tooth neuropathy type 2A gene, in human skeletal muscle: effects of type 2 diabetes, obesity, weight loss, and the regulatory role of tumor necrosis factor alpha and interleukin-6.

The primary gene mutated in Charcot-Marie-Tooth type 2A is mitofusin-2 (Mfn2). Mfn2 encodes a mitochondrial protein that participates in the maintenance of the mitochondrial network and that regulates mitochondrial metabolism and intracellular signaling. The potential for regulation of human Mfn2 gene expression in vivo is largely unknown. Based on the presence of mitochondrial dysfunction in insulin-resistant conditions, we have examined whether Mfn2 expression is dysregulated in skeletal muscle from obese or nonobese type 2 diabetic subjects, whether muscle Mfn2 expression is regulated by body weight loss, and the potential regulatory role of tumor necrosis factor (TNF)alpha or interleukin-6. We show that mRNA concentration of Mfn2 is decreased in skeletal muscle from both male and female obese subjects. Muscle Mfn2 expression was also reduced in lean or in obese type 2 diabetic patients. There was a strong negative correlation between the Mfn2 expression and the BMI in nondiabetic and type 2 diabetic subjects. A positive correlation between the Mfn2 expression and the insulin sensitivity was also detected in nondiabetic and type 2 diabetic subjects. To determine the effect of weight loss on Mfn2 mRNA expression, six morbidly obese subjects were subjected to weight loss by bilio-pancreatic diversion. Mean expression of muscle Mfn2 mRNA increased threefold after reduction in body weight, and a positive correlation between muscle Mfn2 expression and insulin sensitivity was again detected. In vitro experiments revealed an inhibitory effect of TNFalpha or interleukin-6 on Mfn2 expression in cultured cells. We conclude that body weight loss upregulates the expression of Mfn2 mRNA in skeletal muscle of obese humans, type 2 diabetes downregulates the expression of Mfn2 mRNA in skeletal muscle, Mfn2 expression in skeletal muscle is directly proportional to insulin sensitivity and is inversely proportional to the BMI, TNFalpha and interleukin-6 downregulate Mfn2 expression and may participate in the dysregulation of Mfn2 expression in obesity or type 2 diabetes, and the in vivo modulation of Mfn2 mRNA levels is an additional level of regulation for the control of muscle metabolism and could provide a molecular mechanism for alterations in mitochondrial function in obesity or type 2 diabetes.