Mutations in Atlastin-3: implications for ER membrane fusion and crosstalk with mitochondria
13 June 2017
UAntwerp, Campus Drie Eiken, Building Q, Promotiezaal - Universiteitsplein 1 - 2610 Wilrijk (Antwerp) (route: UAntwerpen, Campus Drie Eiken
4:00 PM - 6:00 PM
Vincent Timmerman, Sophie Janssens
PhD defence Michiel Krols - Department of Biomedical Sciences
The endoplasmic reticulum (ER) is a complex network of sheets and tubules that are continuously through the concerted actions of membrane shaping proteins. The relevance of this remodeling process is underscored by the fact that mutations in atlastins (ATL), the proteins mediating ER fusion in mammalia, lead to neurodegenerative disease characterized by length-dependent degeneration of axons in the peripheral and central nervous system. While the mechanism of ATL-mediated membrane fusion is partially understood, how defects in this process lead to neurodegeneration is unknown.
Here we show that the sensory neuropathy-causing ATL3 variants retain their dimerization-dependent GTPase activity, but are unable to form the crossover dimer necessary for membrane fusion. Instead they adopt a conformation that promotes aberrant membrane tethering. This leads to a collapse of the ER network characterized by bundles of ER tubules running in parallel, as observed by volume electron microscopy and TEM in patient derived fibroblasts. Given the redundancy of the three ATL proteins in membrane fusion, this enhanced ER membrane tethering likely represents a toxic gain-of-function that has major implications on normal ER dynamics and its interaction with other organelles.
Given its central and widespread position within a cell, the ER is a pivotal player in inter-organelle communication and several lines of evidence imply an important role for ER-mitochondria contact sites in neurodegeneration. Here we demonstrate that in cells expressing disease-causing mutations in the ER membrane fusion protein ATL3, mitochondrial trafficking is altered. As a consequence, mitochondrial distribution towards neurites is impaired when mutant ATL3 is expressed in primary cortical neurons. Given that ATL3 is an ER protein and that these mutations affect ER dynamics, we hypothesized that these defects could be due to altered signaling at ER-mitochondria contact sites.
Indeed, we found that the number of ER-mitochondria contact sites is increased in HeLa cells expressing mutant ATL3. As a result, ER-mitochondria crosstalk is augmented, resulting in upregulated lipid levels, autophagy induction and ER-mitochondria calcium transfer. These findings show that disruption of ER dynamics has repercussions also for mitochondrial trafficking and homeostasis, which likely has adverse effects on neuronal survival that may underlie the neurodegenerative phenotype caused by mutations in ATL3.