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.
•Mitochondria and ER are tethered.•Tethering controls multiple mitochondrial and ER functions.•Molecules involved in tethering are being discovered and characterized.
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.
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 Ca2+‐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.
The authors report the identification of the putative tumor suppressor trichoplein/mitostatin as a novel component of the interface between mitochondria and endoplasmic reticulum. Here it interacts with the mitofusin 2 to negatively regulate the tethering between these organelles.
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.
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 VO₂max 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 γ coactivator (PGC)-1α/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 α-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 α-subunit of ATP synthase in muscle from control subjects. However, Mfn2 was unchanged after chronic exercise in individuals with diabetes, whereas porin and α-subunit of ATP synthase were increased. Acute exercise caused a fourfold increase in PGC-1α expression in muscle from control subjects but not in subjects with diabetes. CONCLUSIONS: Our results demonstrate alterations in the regulatory pathway that controls PGC-1α 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-1α and Mfn2.
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.
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
splice variant ERMIT2 as the ER tethering partner of MFN2. Splicing of
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
knockout mice. Thus, ER-specific
variants display entirely extramitochondrial MFN2 functions involved in interorganellar tethering and liver metabolic activities.
REPLY TO FILADI ET AL Naon, Deborah; Zaninello, Marta; Giacomello, Marta ...
Proceedings of the National Academy of Sciences,
03/2017, Letnik:
114, Številka:
12
Journal Article
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 α and ...Interleukin-6
Daniel Bach 1 ,
Deborah Naon 1 ,
Sara Pich 1 ,
Francesc X. Soriano 1 ,
Nathalie Vega 2 ,
Jennifer Rieusset 2 ,
Martine Laville 2 ,
Christelle Guillet 3 ,
Yves Boirie 3 ,
Harriet Wallberg-Henriksson 4 ,
Melania Manco 5 ,
Menotti Calvani 5 ,
Marco Castagneto 5 ,
Manuel Palacín 1 ,
Geltrude Mingrone 5 ,
Juleen R. Zierath 4 ,
Hubert Vidal 2 and
Antonio Zorzano 1
1 Departament de Bioquímica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, and IRB-PCB, Parc Científic
de Barcelona, Barcelona, Spain
2 Institut National de la Santé et de la Recherche Médicale Unité-449 and Centre de Recherche en Nutrition Humaine de Lyon,
Faculté de Médecine R.T.H., Laennec Lyon, France
3 Unite du Metabolisme Proteino-Energetique, UMR Universite d’Auvergne/INRA, CRNH, Centre Hospitalier Universitaire, Clermont-Ferrand,
France
4 Department of Surgical Sciences, Section for Integrative Physiology, Karolinska Institutet, Stockholm, Sweden
5 Istituto di Medicina Interna, Facoltà di Medicina e Chirurgia, Università Cattolica, Roma, Italy
Address correspondence and reprint requests to Dr. Antonio Zorzano, Universitat de Barcelona, Departament de Bioquímica i
Biologia Molecular, Facultat de Biologia, Avda. Diagonal 645, Barcelona, Spain, 08071. E-mail: azorzano{at}pcb.ub.es
Abstract
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)α 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 TNFα 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, TNFα
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.
BPD, bilio-pancreatic diversion
Mfn2, mitofusin-2
TNF, tumor necrosis factor
Footnotes
Accepted June 1, 2005.
Received January 7, 2005.
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