ER Folding Assistants Regulate ER-Mitochondria Contacts and Ca2+ Flux

• Author / Creator

  Gutiérrez, Tomás

• Eukaryotic cells are characterized by the presence of membrane-bound organelles. Close proximity between organelles has been observed for decades, but only recently we have really started to understand the importance of these contacts for cellular function. These contacts consist of areas of close apposition between the membranes of two organelles, and are the platform for the exchange of metabolites and signals which are essential for the normal function of the organelles. One of the most prominent inter-organelle contact is the one established between the endoplasmic reticulum (ER) and mitochondria. ER-mitochondria contacts are formed in a specialized region in the ER membranes known as Mitochondria-Associated Membranes (MAM), which mediates the exchange of lipids and Ca(2+) between the two organelles. Mitochondrial Ca(2+) activates several metabolic enzymes in the mitochondrial matrix, therefore the flux of Ca(2+) from ER to mitochondria is essential to maintain a normal mitochondrial metabolism and oxidative phosphorylation. Reducing Ca(2+) flux to mitochondria by increasing ER-mitochondria distance or blocking Ca(2+) channels at the MAM compromises cellular metabolism due to reduced mitochondrial energy production. However, if mitochondria are overloaded with Ca(2+), cell death pathways are activated. To regulate this exchange, the MAM is populated by proteins that control its structure and function. Some of them are tethers that form physical proteinaceous bridges between ER and mitochondria membranes; others participate in the transfer of lipids or Ca(2+), like Ca(2+) channels. One notable group of MAM regulatory proteins are ER chaperones and oxidoreductases, the proteins responsible for one of the main functions of the ER: the folding and maturation of newly synthesized proteins. Chaperones like BiP and Calnexin bind to recently synthesized peptides imported to the ER to assist on the folding and assembly of mature proteins, and prevent the aggregation of misfolded proteins. Oxidoreductases like ERp57 and TMX1 assist chaperones by catalyzing the formation, reduction and isomerization of disulfide bonds by oxidizing and reducing cysteines in the target protein. These chaperones and oxidoreductases regulate MAM function by binding and/or chemically modifying tethers and Ca(2+) channels, regulating the extent and distance of ER-mitochondria contacts, and the intensity of the release of Ca(2+) from the ER.

  In this work, we describe the role of the chaperone Calnexin and the oxidoreductase TMX1 in the regulation of MAM function. We found that Calnexin and TMX1 regulate the ER Ca(2+) pump SERCA, responsible for the loading of Ca(2+) in the ER, therefore controlling the amount of Ca(2+) available for Ca(2+) flux at the MAM. In addition, they regulate MAM formation, controlling the extent and distance of ER-mitochondria contacts. The effect of Calnexin and TMX1 in ER Ca(2+) loading and ER-mitochondria distance results in the regulation of Ca(2+) flux to mitochondria, mitochondrial Ca(2+) content, and mitochondrial metabolism.

  We also explored the role of the ER-mitochondria tether Mitofusin-2 in the regulation of MAM function. Mitofusin-2 connects ER and mitochondria together, but the loss of Mitofusin-2 activates an adaptive rescue mechanism that results in increased ER-mitochondria contacts and enhanced mitochondrial metabolism. These adaptations are characterized by the activation of ER stress signalling pathways and rearrangement of MAM protein composition.

  Taken together, this doctoral thesis provides new knowledge about the role of Calnexin, TMX1 and Mitofusin-2 in the regulation of MAM tethers and Ca(2+) channels, their impact on ER-mitochondria contact formation and Ca(2+) flux to mitochondria, and ultimately their role in the regulation of mitochondrial function. These results provide new links between ER protein folding and energy production in the cell.

• Subjects / Keywords

  • SERCA
  • Calcium
  • Mitofusin-2
  • Mitochondria-Associated Membranes
  • Endoplasmic Reticulum
  • TMX1
  • Calnexin
  • Mitochondria
  • ER stress
  • Metabolism

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-wgkc-5423

• License

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