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Calcium regulates the association between mitochondria and a smooth subdomain of the endoplasmic reticulum
1Department of Pathology and Cell Biology, University of Montreal, Montreal, Quebec, Canada H3C 3J7.
Abstract:
Association between the ER and mitochondria has long been observed, and the formation of close contacts between ER and mitochondria is necessary for the ER-mediated sequestration of cytosolic calcium by mitochondria. Autocrine motility factor receptor (AMF-R) is a marker for a smooth subdomain of the ER, shown here by confocal microscopy to be distinct from, yet closely associated with the calnexin- or calreticulin-labeled ER. By EM, smooth ER AMF-R tubules exhibit direct interactions with mitochondria, identifying them as a mitochondria-associated smooth ER subdomain. In digitonin-permeabilized MDCK cells, the addition of rat liver cytosol stimulates the dissociation of smooth ER and mitochondria under conditions of low calcium. Using BAPTA chelators of various affinities and CaEGTA buffers of defined free Ca(2+) concentrations and quantitative confocal microscopy, we show that free calcium concentrations <100 nM favor dissociation, whereas those >1 microM favor close association between these two organelles. Therefore, we describe a cellular mechanism that facilitates the close association of this smooth ER subdomain and mitochondria when cytosolic free calcium rises above physiological levels.
Insights
Mitochondria and the endoplasmic reticulum (ER) associate closely. This study reveals that elevated cytosolic calcium levels trigger the association between a specific ER subdomain, marked by autocrine motility factor receptor (AMF-R), and mitochondria.
Area of Science:
- Cell Biology
- Mitochondrial Biology
- Endoplasmic Reticulum Biology
Background:
- The endoplasmic reticulum (ER) and mitochondria maintain close physical and functional associations.
- These contacts are crucial for calcium homeostasis, with mitochondria sequestering ER-released calcium.
- Autocrine motility factor receptor (AMF-R) marks a distinct smooth ER subdomain associated with mitochondria.
Purpose of the Study:
- To investigate the role of cytosolic calcium in regulating the association between the AMF-R-positive smooth ER subdomain and mitochondria.
- To identify the specific calcium concentrations that promote or inhibit ER-mitochondria contact.
Main Methods:
- Confocal and electron microscopy (EM) to visualize ER-mitochondria contacts.
- Digitonin-permeabilized cell assays with rat liver cytosol.
- Quantitative confocal microscopy using BAPTA chelators and CaEGTA buffers to precisely control free calcium concentrations.
Main Results:
- The AMF-R-labeled smooth ER subdomain directly interacts with mitochondria.
- Low cytosolic free calcium (<100 nM) promotes the dissociation of ER and mitochondria.
- High cytosolic free calcium (>1 microM) promotes close association between the ER subdomain and mitochondria.
Conclusions:
- Cytosolic calcium levels act as a critical regulator of ER-mitochondria association.
- A cellular mechanism exists where rising cytosolic calcium drives the close apposition of AMF-R-positive smooth ER and mitochondria.
- This calcium-dependent association is vital for cellular calcium signaling and organelle function.