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Gene name - Mitoguardin
Synonyms - Cytological map position - 7E9-7E9 Function - Transmembrane Keywords - a mitochondrial outer-membrane protein that regulates endoplasmic reticulum-mitochondrial contact sites (ERMCSs) - required for autophagy |
Symbol - Miga
FlyBase ID: FBgn0030037 Genetic map position - chrX:8,433,923-8,436,801 Classification - Mitoguardin Cellular location - mitochondrial surface transmembrane |
Mitochondrial malfunction and autophagy defects are often concurrent phenomena associated with neurodegeneration. This study shows that Miga, a mitochondrial outer-membrane protein that regulates endoplasmic reticulum-mitochondrial contact sites (ERMCSs), is required for autophagy. Loss of Miga results in an accumulation of autophagy markers and substrates, whereas PI3P and Syx17 levels are reduced. Further experiments indicated that the fusion between autophagosomes and lysosomes is defective in Miga mutants. Miga binds to Atg14 and Uvrag; concordantly, Miga overexpression results in Atg14 and Uvrag recruitment to mitochondria. The heightened PI3K activity induced by Miga requires Uvrag, whereas Miga-mediated stabilization of Syx17 is dependent on Atg14. Miga-regulated ERMCSs are critical for PI3P formation but are not essential for the stabilization of Syx17. In summary, this study identified a mitochondrial protein that regulates autophagy by recruiting two alternative components of the PI3K complex present at the ERMCSs (Xu, 2022).
Eukaryotic cells are compartmentalized into different organelles that execute distinct functions and communicate with each other through indirect signal transduction or direct organelle-organelle contacts. Mitochondria and the adjacent endoplasmic reticulum (ER) form contacts, which are characterized by a 10-30 nm distance between the two organelles. These contacts mediate lipid exchange and calcium flux between the ER and mitochondria. It has been reported that ER-mitochondrial contact sites (ERMCSs) are important platforms for regulating macroautophagy (hereafter referred to as autophagy) and mitophagy (Xu, 2022).
Autophagosome formation at the ERMCSs in mammalian cells has been reported. Upon starvation, the ER-resident SNARE protein syntaxin 17 (STX17 in mammals; Syx17 in flies) recruits the PI3K complex subunit Atg14 to the ERMCSs and triggers autophagosome formation. However, Syx17 was not required for autophagosome formation in flies , and the major role of Syx17 in both mammals and flies is to mediate the fusion between autophagosome and lysosome. In addition, VAPB and PTPIP51, a pair of ERMCS tethers, also regulate autophagy. Increased ERMCS formation facilitated by VAPB or PTPIP51 overexpression inhibits autophagy; conversely, the weakening of contact by knockdown of these tethers stimulates autophagosome formation. Recent studies have shown that autophagy occurs at ERMCSs to supply free fatty acids for mitochondrial energy metabolism, while mitochondrial respiratory chain activity supports autophagy through the regulation of ERMCS formation. In addition to regulating autophagy at the initiation stage, in a previous study, it was determined that mitochondria play a crucial role in the late stage of autophagy. The loss of Tom40, a key subunit of the mitochondrial protein import channel, results in blockage of autophagosome and lysosome fusion. It was also found that defects in several general mitochondrial metabolic processes, such as ATP production, mitochondrial protein synthesis, or the citrate cycle, do not cause the autophagy defects observed in Tom40-depleted tissues. This implied that the autophagy defects caused by blocking mitochondrial protein import are rather specific. It is therefore hypothesized that certain mitochondrial proteins regulate autophagy directly (Xu, 2022).
In the present study, it was demonstrated that Miga, a mitochondrial outer-membrane protein, is required for autophagy. Loss of Miga led to defects in autophagosome-lysosome fusion. Miga is an evolutionarily conserved protein, with orthologs from worms to humans. In a previous study, it was found to be localized on the mitochondrial outer membrane to regulate mitochondrial fusion by stabilizing MitoPLD. Miga interacts with the ER-localized VAP protein to establish ERMCSs. The interactions between Miga and VAP proteins are regulated by the phosphorylation of the FFAT motif in Miga. A recent study also reported that MIGA2 (the human ortholog of Miga) regulates ERMCSs and contacts between mitochondria and lipid droplets (LDs) . Loss of Miga led to the degeneration of photoreceptor cells in flies. Overexpression of Miga in fly eyes resulted in increased ERMCSs and severe eye degeneration. In mice, loss of MIGA2 led to anxiety-like behavior. This study found that Miga interacts with Atg14 and Uvrag to regulate PI3K activity and Syx17 stability, thereby modulating autophagy (Xu, 2022).
Defects in both mitochondria and autophagy are hallmarks of several types of neurodegenerative diseases. This study found that Miga establishes a direct link between mitochondria and autophagy to maintain cellular homeostasis (Xu, 2022).
It is striking that a mitochondrial protein directly regulates autophagy by interacting with the core components of the autophagy machinery. In the present study, it was found that the mitochondrial protein Miga forms complexes with Uvrag and Atg14 to regulate PI3P production and to stabilize Syx17 during autophagy (Xu, 2022).
Miga interacts with Vap33 to mediate formation of ERMCSs. Overexpression of wild-type Miga, but not MigaFM, led to increased PI3P levels, implying that Miga-induced ERMCSs are required for regulating PI3P formation. However, the ERMCS tether function of Miga is neither required for recruiting Uvrag nor for binding to Atg14 and Syx17 stabilization. It has been shown previously that Atg14 and other components of the PI3K complex, such as Atg16 and Vps34, are enriched in ERMCSs upon starvation. The question remains as to why the PI3K complex needs to be present. Phosphatidylinositol (PI) is a substrate required for the PI3K complex to produce PI3P. PI is synthesized on the ER, and ERMCSs are the sites for the transfer of PI between the ER and mitochondria. During autophagy, the PI3K complex promotes PI3P formation to facilitate autophagic processes, and ERMCSs represent platforms to access PI. It is believed that the enrichment of the PI3K complex at ERMCSs is needed to assess the supply of PI. The present study found that MigaFM failed to promote PI3P formation, although it was still able to recruit key PI3K components, such as Uvrag or Atg14. This implied that PI3P formation during autophagy not only requires the activity of the PI3K complex but also PI supplied from ERMCSs (Xu, 2022).
Previous studies reported that ERMCSs are required for the initiation of autophagy. In the current study, it was found that in Miga mutants, autophagic processes were blocked at the autophagosome-lysosome fusion stage, while autophagosome formation was largely unaffected. Lack of Miga led to a reduction in PI3P and Syx17 levels. Previous studies have demonstrated that PI3K is not only essential for autophagy initiation but is also recruited to the autophagosome together with the HOPS complex to facilitate autophagosome and lysosome fusion in mammalian cells. The remaining PI3P in Miga mutants is probably sufficient for autophagosome formation but not enough for the autophagosome-lysosome fusion process. This study found that the loss of Miga reduced co-localization of FYVE-GFP and Atg8a but not co-localization of FYVE-GFP and CathL. This suggests that the reduction of PI3P in autophagosomes, but not lysosomes, might contribute to fusion defects (Xu, 2022).
The fusion defects observed in Miga mutants were not identical to those found in mutants without Syx17 or HOPS components. The puncta of autophagosome markers are larger in Miga mutants than those in mutants without Syx17 or HOPS components, possibly due to the combined effects of reduction of PI3P and Syx17. In worms and mammalian cells, the lack of EPG5 prevents autophagosome maturation and induces the ectopic fusion of autophagosomes with various endocytic vesicles. The enlarged Atg8a-positive structures in Miga mutants might also be a result of the ectopic fusion of autophagosomes with other vesicles (Xu, 2022).
In mammals, both UVRAG and ATG14 are required for autophagy. In flies, Uvrag regulates PI3P formation under fed conditions, and Atg14 is required for PI3P-positive autophagosome formation. This study found that Miga overexpression induces PI3P formation; additionally, Uvrag, but not Atg14, is required during this process. It was also found that Miga overexpression leads to an upregulation of numerous autophagy markers, such as Atg9, Syx17, Atg18a, Rab7, and LAMP, among others. However, the expression levels or patterns of p62 and Atg8a did not change significantly upon Miga overexpression. This implied that Miga overexpression is not sufficient to fully activate autophagy (Xu, 2022).
STX17, the mammalian ortholog of Syx17, is an autophagosome-localized Q-SNARE that mediates autophagosome and lysosome fusion through interactions with SNAP29 and VAMP8/Vamp7. STX17 contains two tandem transmembrane domains that have low hydrophobicity but are required for autophagosome localization. In fed mammalian cells, STX17 reportedly localizes to the ER, mitochondria, and cytosol. STX17 was enriched in ERMCSs upon autophagy stimulation and was present on completely closed autophagosomes. The detailed translocation mechanism remains unclear. In flies, Syx17 shows diffusely dispersed patterns, and there is no mitochondria-specific localization under normal fed conditions. Syx17 forms puncta and co-localizes with Atg8-positive autophagosomes upon starvation. It was found that Miga is required for the stabilization of Syx17. Miga does not bind to Syx17 but stabilizes it through Atg14. It is puzzling why a mitochondrial protein would be required for the stabilization of a protein that functions in autophagosome maturation. It has been reported that there are three-way contacts among the ER, mitochondria, and late endosomes. It is possible that Miga, Vap33, and Atg14 mediate the contact between the ER, mitochondria, and autophagosomes. Autophagosome-associated Atg14 further stabilizes Syx17 to mediate the fusion between autophagosomes and lysosomes. GFP-Atg14 formed large puncta instead of decreasing in the Miga mutant clones. One possible explanation for this is that the overexpression of GFP-Atg14 overrides the requirement of Miga to stabilize it, but the overexpression of GFP-Atg14 per se is not sufficient to fully rescue the autophagy defects in the Miga mutant. Therefore, similar to other autophagy markers, GFP-Atg14 puncta accumulated in the Miga mutant clones (Xu, 2022).
In summary, this study identified a mitochondrial protein, Miga, that regulates autophagic processes by interacting with Atg14 and Uvrag. This delineates a link between mitochondria and macroautophagy. However, this study did not solve how Miga stabilizes Atg14 and Syx17. It is possible that Miga mediates the three-way contact between the ER, mitochondria, and autophagosomes. Miga interacts with Atg14 and stabilizes Atg14. Furthermore, Atg14 interacts with Syx17 to stabilize it. It is not clear how the relay is carried out during autophagy (Xu, 2022).
Both Atg14 and Uvrag interact with MigaN (1-252 aa), but there is no evident competition between Atg14 and Uvrag. The exact regions of Miga that bind to each protein were not identified in this study (Xu, 2022).
Miga-mediated endoplasmic reticulum-mitochondria contact sites regulate neuronal homeostasis
Endoplasmic reticulum (ER)-mitochondria contact sites (ERMCSs) are crucial for multiple cellular processes such as calcium signaling, lipid transport, and mitochondrial dynamics. However, the molecular organization, functions, regulation of ERMCS, and the physiological roles of altered ERMCSs are not fully understood in higher eukaryotes. This study found that Miga, a mitochondrion located protein, markedly increases ERMCSs and causes severe neurodegeneration upon overexpression in fly eyes. Miga interacts with an ER protein Vap33 through its FFAT-like motif and an amyotrophic lateral sclerosis (ALS) disease related Vap33 mutation considerably reduces its interaction with Miga. Multiple serine residues inside and near the Miga FFAT motif were phosphorylated, which is required for its interaction with Vap33 and Miga-mediated ERMCS formation. The interaction between Vap33 and Miga promoted further phosphorylation of upstream serine/threonine clusters in the Miga protein that fine-tuned Miga activity. Protein kinases CKI and CaMKII contribute to Miga hyperphosphorylation. MIGA2, encoded by the miga mammalian ortholog, has conserved functions in mammalian cells. A model is proposed that shows Miga interacts with Vap33 to mediate ERMCSs and excessive ERMCSs lead to neurodegeneration (Xu, 2020).
ERMCS mediates the crosstalk between ER and mitochondria. VAP proteins are ER membrane proteins that often serve as an 'entry point' to tether different organelles to ER and mediate the communications between ER and the tethered organelles. VAP proteins contains the MSP domain that binds to proteins with FFAT motifs. In ALS patients, a point mutation in the VAPB MSP domain was identified. The resultant mutant VAPB has reduced affinity to the FFAT motif containing proteins. Indeed, the disease mutation mimic Vap33 had less affinity to Miga than its wild type form. The reduction of organelle contacts including ERMCSs might contribute to the disease conditions in the ALS patients. This study found that increasing ERMCSs by enhancing the interaction between Miga and VAP proteins also led to neurodegeneration, suggesting that the proper amount of contacts and the right distance between ER and mitochondria are critical for neuronal homeostasis (Xu, 2020).
Although Miga overexpression greatly increased ERMCSs in both fly eyes and fat body tissues, the loss of Miga did not have obvious effects on the ERMCSs in fly fat body tissues. The ERMCSs (10-30 nm), especially the tight contacts (about 10 nm), are rare in the wild type fly fat body. It might be the reason for the difficulty to detect ERMCS reduction in this tissue. A Miga genomic rescue fragment with point mutations in the FFAT motif was introduced in Miga mutant animals in this study. Although it rescued the fatality of Miga mutant, the rescued animals were short lived and had eye degeneration. It suggested that the interaction between Miga and Vap33 had physiological importance. It is interesting that the rescued animals always losing R7/R8 upon aging but not other photoreceptor cells, while the loss of photoreceptor cells in Miga mutant animals during aging did not have preference to any particular photoreceptor cells. Drosophila has compound eyes composed with repeat units call ommatidia. Each ommatidia consists of eight photoreceptors: the outer photoreceptor cells R1-R6 and the inner photoreceptor cells R7 and R8. The R1-R6 cells express a single opsin Rh1 and are involved in image formation and motion detection. The R7 cells express one of two opsins Rh3 and Rh4. The R8 cells are localized beneath the R7 and express one of the three opsins: Rh3, Rh5, and Rh6. R7 and R8 cells are involved in color vision and polarized light detection. However, no obvious difference of ERMCSs between the outer and inner photoreceptor cells was observed. Further investigation is required to understand why the FFAT motif in Miga is particularly important for the inner photoreceptor cells. MigaFM rescued the lethality of Miga mutants, suggesting that at least some functions of Miga do not need its FFAT motif. The study in mammalian cells indicated that MIGA2 links mitochondria to lipid droplets (LD) through its C-terminal region and is required for adipocyte differentiation. The LD interaction region of MIGA2 is conserved in fly Miga protein. It needs further investigation whether Miga plays a role in lipid metabolism in Drosophila (Xu, 2020).
It has been reported that the ERMCSs marked the mitochondrial fission positions. However, presenting ERMCS is not sufficient to induce mitochondrial fission. It has been observed that both mitochondrial fission and fusion could happen at ERMCSs. When Miga is overexpressed, increased number was observed of mitochondria with ERMCSs, increased ERMCS length per mitochondrion, and decreased distance between ER and mitochondria at the ERMCSs. Instead of promoting mitochondrial fission, enhanced mitochondrial fusion and dramatically increased mitochondrial length were observed when Miga was overexpressed. Interestingly, when MigaFM was overexpressed, there is no increase of ERMCSs and the mitochondria length was also significantly shorter than that in the wildtype Miga overexpressed tissues. It has been suggested that Miga function in mitochondrial fusion might be coupled with its function in mediating ERMCSs (Xu, 2020).
This study found that Miga was hyperphosphorylated at multiple sites. The phosphorylation on the cluster V was essential for the interaction between Miga and VAP protein and the interaction mediated ERMCS establishment. Upon cell stress stimuli, such as starvation, the phosphorylation was increased and so was the ERMCSs. Therefore, the phosphorylation in the cluster V provides a switch to modulate ERMCSs and the subsequent communications between ER and mitochondria. In addition to the cluster V, Miga was also phosphorylated at multiple sites in the clusters I, II, and III (see Miga was phosphorylated at multiple clusters
This study identified that CKI and CaMKII were required for Miga phosphorylation. ERMCSs play a key role in calcium flux between ER and mitochondria. The activity of CaMKII is regulated by Ca²+/calmodulin. Therefore, the local concentration of calcium might function as a trigger to modulate CaMKII activity and further modify the ERMCSs through the phosphorylation of Miga. We found that the reduction of mobility shift of Miga upon the RNAi of CaMKII was much smaller than that in the cluster V mutants, suggesting that other kinases are involved in the phosphorylation of the cluster V in Miga. Experiments were not performed to test whether CKI or CamKII could modify Miga overexpression phenotypes in fly eyes because of the shutdown of facilities during COVID-19 pandemic. It would be interesting to know whether these kinases are required for Miga overexpression mediated eye degeneration (Xu, 2020).
It has been reported that presenilins and γ-secretase activity are concentrated in the ERMCSs. Increased ERMCSs have also been observed in the fibroblasts from patients with sporadic Alzheimer's disease. However, whether ERMCS change is the cause or the consequence of the diseases was not known. These data provided evidence that enhanced ERMCSs by Miga overexpression or other means cause severe degeneration in neurons and muscles. It provided a direct evidence that enhanced ERMCSs is devastating to the cellular homeostasis and leads to neurodegeneration (Xu, 2020).
Mitochondria undergo frequent morphological changes through fission and fusion. Mutations in core members of the mitochondrial fission/fusion machinery are responsible for severe neurodegenerative diseases. However, the mitochondrial fission/fusion mechanisms are poorly understood. The loss of a mitochondrial protein encoding gene, mitoguardin (miga), leads to mitochondrial defects and neurodegeneration in fly eyes. Mammals express two orthologs of miga: Miga1 and Miga2. Both MIGA1 and MIGA2 form homotypic and heterotypic complexes on the outer membrane of the mitochondria. Loss of MIGA results in fragmented mitochondria, whereas overexpression of MIGA leads to clustering and fusion of mitochondria in both fly and mammalian cells. MIGA proteins function downstream of mitofusin and interact with MitoPLD to stabilize MitoPLD and facilitate MitoPLD dimer formation. Therefore, it is proposed that MIGA proteins promote mitochondrial fusion by regulating mitochondrial phospholipid metabolism via MitoPLD (Zhang, 2016).
Mitochondria have been identified to be involved in oxidative phosphorylation, lipid metabolism, cell death, and cell proliferation. Previous studies have demonstrated that mitoguardin (Miga), a mitochondrial protein that governs mitochondrial fusion, mitochondria-endoplasmic reticulum (ER) contacts, lipid formation, and autophagy, is crucial for ovarian endocrine and follicular development. Nevertheless, whether mammalian MIGA1 or MIGA2 (MIGA1,-2) regulates ovarian granulosa cell proliferation remains unclear. This study revealed that mammalian MIGA1,-2 promotes cell proliferation and regulates the phosphorylation and localization of Yes-associated protein 1 (YAP1) in ovarian granulosa cells. MIGA2 upregulation resulted in reduced YAP1 activity, while MIGA2 removal led to increased YAP1 activity. Further analysis indicated that MIGA1,-2 regulated YAP1 via the Hippo signaling pathway and regulated protein kinase B (AKT) activity in collaboration with YAP1. In addition, lysophosphatidic acid (LPA) regulated MIGA2 expression and AKT activity by activating YAP1. Briefly, it was demonstrated that the mitochondrial MIGA1 and MIGA2, especially MIGA2, promoted cellular proliferation by activating AKT and regulating the Hippo/YAP1 signaling pathway in ovarian granulosa cells, which may contribute to the molecular pathogenesis of reproductive endocrine diseases, such as polycystic ovary syndrome (PCOS) (Yan, 2023).
Lipid transport proteins at membrane contacts, where organelles are closely apposed, are critical in redistributing lipids from the endoplasmic reticulum (ER), where they are made, to other cellular membranes. Such protein-mediated transfer is especially important for maintaining organelles disconnected from secretory pathways, like mitochondria. This study identified mitoguardin-2, a mitochondrial protein at contacts with the ER and/or lipid droplets (LDs), as a lipid transporter. An x-ray structure shows that the C-terminal domain of mitoguardin-2 has a hydrophobic cavity that binds lipids. Mass spectrometry analysis reveals that both glycerophospholipids and free-fatty acids co-purify with mitoguardin-2 from cells, and that each mitoguardin-2 can accommodate up to two lipids. Mitoguardin-2 transfers glycerophospholipids between membranes in vitro, and this transport ability is required for roles both in mitochondrial and LD biology. While it is not established that protein-mediated transfer at contacts plays a role in LD metabolism, these findings raise the possibility that mitoguardin-2 functions in transporting fatty acids and glycerophospholipids at mitochondria-LD contacts (Hong, 2022).
The endoplasmic reticulum (ER)-mitochondria contact site (ERMCS) is crucial for exchanging biological molecules such as phospholipids and Ca(2+) ions between these organelles. Mitoguardin-2 (MIGA2), a mitochondrial outer membrane protein, forms the ERMCS in higher eukaryotic cells. This study reports the crystal structures of the MIGA2 Lipid Droplet (LD) targeting domain and the ER membrane protein VAPB bound to the phosphorylated FFAT motif of MIGA2. These structures reveal that the MIGA2 LD targeting domain has a large internal hydrophobic pocket that accommodates phospholipids and that two phosphorylations of the FFAT motif are required for tight interaction of MIGA2 with VAPB, which enhances the rate of lipid transport. Further biochemical studies show that MIGA2 transports phospholipids between membranes with a strong preference for binding and trafficking phosphatidylserine (PS). These results provide a structural and molecular basis for understanding how MIGA2 mediates the formation of ERMCS and facilitates lipid trafficking at the ERMCS (Kim, 2022).
Physical contact between organelles is vital to the function of eukaryotic cells. Lipid droplets (LDs) are dynamic organelles specialized in lipid storage that interact physically with mitochondria in several cell types. The mechanisms coupling these organelles are, however, poorly understood, and the cell-biological function of their interaction remains largely unknown.This study discovered in adipocytes that the outer mitochondrial membrane protein MIGA2 links mitochondria to LDs. An amphipathic LD-targeting motif was identified, and it was revealed that MIGA2 binds to the membrane proteins VAP-A or VAP-B in the endoplasmic reticulum (ER). In adipocytes MIGA2 is involved in promoting triglyceride (TAG) synthesis from non-lipid precursors. These data indicate that MIGA2 links reactions of de novo lipogenesis in mitochondria to TAG production in the ER, thereby facilitating efficient lipid storage in LDs. Based on its presence in many tissues, MIGA2 is likely critical for lipid and energy homeostasis in a wide spectrum of cell types (Freyre, 2019).
Search PubMed for articles about Drosophila Miga
Freyre C. A. C., Rauher, P. C., Ejsing, C. S., Klemm, R. W. (2019). MIGA2 Links Mitochondria, the ER, and Lipid Droplets and Promotes De Novo Lipogenesis in Adipocytes. Mol Cell76(5):811-825 e814. PubMed ID: 31628041
Hong Z., Adlakha, J., Wan, N., Guinn, E., Giska, F., Gupta, K., Melia, T. J., Reinisch, K. M. (2022). Mitoguardin-2-mediated lipid transfer preserves mitochondrial morphology and lipid droplet formation. J Cell Biol 221(12). PubMed ID: 36282247
Kim H., Lee, S., Jun, Y., Lee, C. (2022). Structural basis for mitoguardin-2 mediated lipid transport at ER-mitochondrial membrane contact sites. Nat Commun13(1):3702. PubMed ID: 35764626
Xu, L., Wang, X., Zhou, J., Qiu, Y., Shang, W., Liu, J. P., Wang, L. and Tong, C. (2020). Miga-mediated endoplasmic reticulum-mitochondria contact sites regulate neuronal homeostasis. Elife 9. PubMed ID: 32648543
Xu, L., Qiu, Y., Wang, X., Shang, W., Bai, J., Shi, K., Liu, H., Liu, J. P., Wang, L. and Tong, C. (2022). ER-mitochondrial contact protein Miga regulates autophagy through Atg14 and Uvrag. Cell Rep 41(5): 111583. PubMed ID: 36323251
Yan, M. Q., Zhu, B. H., Liu, X. H., Yang, Y. M., Duan, X. Y., Wang, Y., Sun, H., Feng, M., Li, T., Liu, X. M. (2023). Mitoguardin 1 and 2 promote granulosa cell proliferation by activating AKT and regulating the Hippo-YAP1 signaling pathway. Cell Death Dis14(11):779. PubMed ID: 38012141
Zhang, Y., Liu, X., Bai, J., Tian, X., Zhao, X., Liu, W., Duan, X., Shang, W., Fan, H. Y., Tong, C. (2016). Mitoguardin Regulates Mitochondrial Fusion through MitoPLD and Is Required for Neuronal Homeostasis. Mol Cell 61(1):111-124. PubMed ID: 26711011
date revised: 26 November, 2023
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