Caveolin-1 impairs PKA-DRP1-mediated remodelling of ER–mitochondria communication during the early phase of ER stress

Cell Death and Differentiation, Sep 2018

Close contacts between endoplasmic reticulum and mitochondria enable reciprocal Ca2+ exchange, a key mechanism in the regulation of mitochondrial bioenergetics. During the early phase of endoplasmic reticulum stress, this inter-organellar communication increases as an adaptive mechanism to ensure cell survival. The signalling pathways governing this response, however, have not been characterized. Here we show that caveolin-1 localizes to the endoplasmic reticulum–mitochondria interface, where it impairs the remodelling of endoplasmic reticulum–mitochondria contacts, quenching Ca2+ transfer and rendering mitochondrial bioenergetics unresponsive to endoplasmic reticulum stress. Protein kinase A, in contrast, promotes endoplasmic reticulum and mitochondria remodelling and communication during endoplasmic reticulum stress to promote organelle dynamics and Ca2+ transfer as well as enhance mitochondrial bioenergetics during the adaptive response. Importantly, caveolin-1 expression reduces protein kinase A signalling, as evidenced by impaired phosphorylation and alterations in organelle distribution of the GTPase dynamin-related protein 1, thereby enhancing cell death in response to endoplasmic reticulum stress. In conclusion, caveolin-1 precludes stress-induced protein kinase A-dependent remodelling of endoplasmic reticulum–mitochondria communication.

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Caveolin-1 impairs PKA-DRP1-mediated remodelling of ER–mitochondria communication during the early phase of ER stress

Abstract Close contacts between endoplasmic reticulum and mitochondria enable reciprocal Ca2+ exchange, a key mechanism in the regulation of mitochondrial bioenergetics. During the early phase of endoplasmic reticulum stress, this inter-organellar communication increases as an adaptive mechanism to ensure cell survival. The signalling pathways governing this response, however, have not been characterized. Here we show that caveolin-1 localizes to the endoplasmic reticulum–mitochondria interface, where it impairs the remodelling of endoplasmic reticulum–mitochondria contacts, quenching Ca2+ transfer and rendering mitochondrial bioenergetics unresponsive to endoplasmic reticulum stress. Protein kinase A, in contrast, promotes endoplasmic reticulum and mitochondria remodelling and communication during endoplasmic reticulum stress to promote organelle dynamics and Ca2+ transfer as well as enhance mitochondrial bioenergetics during the adaptive response. Importantly, caveolin-1 expression reduces protein kinase A signalling, as evidenced by impaired phosphorylation and alterations in organelle distribution of the GTPase dynamin-related protein 1, thereby enhancing cell death in response to endoplasmic reticulum stress. In conclusion, caveolin-1 precludes stress-induced protein kinase A-dependent remodelling of endoplasmic reticulum–mitochondria communication. Introduction Communication between the endoplasmic reticulum (ER) and mitochondria is essential to coordinate cellular responses [1, 2]. Both organelles form contact points via cholesterol-rich microdomains, termed mitochondria-associated ER membranes (MAM) [3], which allow for efficient Ca2+ transfer from ER to mitochondria [4] and either stimulate mitochondrial bioenergetics [5] or initiate apoptosis [6]. Previously, we showed that disruption of the ER protein folding capacity, termed ER stress, during its early stage increases the ER–mitochondria contacts, thus leading to an adaptive increase in mitochondrial ATP production [7]. This appears to be a generic response to acute stress, as we also observed such changes upon inhibition of the nutrient-sensing kinase mammalian target of rapamycin complex 1 (mTORC1) [8]. On the other hand, alterations in ER–mitochondria contacts have also been reported in various models of chronic disease [9,10,11,12]. Among the regulators of the ER–mitochondria interface, Calnexin is an ER-resident chaperone that regulates the Ca2+-handling machinery at MAM. Upon ER stress, Calnexin translocates from MAM to the ER, fulfilling a dual objective: (a) to reinforce protein folding at the ER, and (b) to enhance ER–mitochondria Ca2+ transfer [13]. Therefore, MAM composition is dynamic and requires appropriate membrane organization. Caveolin-1 (CAV1) is a scaffolding protein that controls intracellular cholesterol transport [14] and numerous processes related to cell death and survival at the plasma membrane [15]. A recent report showed that CAV1 is enriched at MAM, and its ablation greatly reduces ER–mitochondria contact sites while increasing inter-organelle cholesterol transfer [16]. These observations agree with previous studies showing a requirement for CAV1 presence in lipid raft-like domains of the ER [17] to protect against mitochondrial dysfunction induced by cholesterol overload [18]. Nonetheless, it remains unclear how CAV1 impacts on signalling cascades that regulate organelle communication. Regarding the potential pathways governing ER–mitochondria interaction, ER stress and mTORC1 inhibition share in common the activation of cAMP-dependent protein kinase (PKA). mTORC1 inhibition leads to PKA-mediated inhibitory phosphorylation of Dynamin-related protein-1 (DRP1) on Ser637 [19]. DRP1 is a GTPase that orchestrates mitochondrial fragmentation by forming a constrictive ring around mitochondria, and thus its inhibition by PKA promotes mitochondrial elongation. Of note, DRP1-mediated fragmentation occurs at the ER–mitochondria interface [20] and is regulated by ER-localized proteins [21, 22]. Similarly, ER stress also leads to PKA activation as a protective mechanism [23], which is partially due to DRP1 phosphorylation [24]. Interestingly, DRP1 phosphorylation at Ser637 upon ER stress has been associated with its translocation to the ER, where it participates in ER expansion triggered to cope with protein misfolding [25]. Therefore, PKA regulates the dynamics of both organelles by determining DRP1 distribution and function [26]. Interestingly, CAV1 and CAV3 reportedly serve as PKA-anchoring proteins on the surface of lipid droplets in adipocytes [27] and in T-tubules in cardiomyocytes [28], respectively. However, whether CAV1 modulates the PKA-DRP1 axis required for the regulation of ER–mitochondria communication remains unexplored. In light of these observations, we hypothesized that CAV1 regulates ER–mitochondria interaction during early ER stress by modulating PKA-mediated DRP1 phosphorylation at MAM. Consistent (...truncated)


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Roberto Bravo-Sagua, Valentina Parra, Carolina Ortiz-Sandoval, Mario Navarro-Marquez, Andrea E. Rodríguez, Natalia Diaz-Valdivia, Carlos Sanhueza, Camila Lopez-Crisosto, Nasser Tahbaz, Beverly A. Rothermel, Joseph A. Hill, Mariana Cifuentes, Thomas Simmen, Andrew F. G. Quest, Sergio Lavandero. Caveolin-1 impairs PKA-DRP1-mediated remodelling of ER–mitochondria communication during the early phase of ER stress, Cell Death and Differentiation, 2018, DOI: 10.1038/s41418-018-0197-1