Routes of Ca2+ Shuttling during Ca2+ Oscillations: FOCUS ON THE ROLE OF MITOCHONDRIAL Ca2+ HANDLING AND CYTOSOLIC Ca2+ BUFFERS

Our plain-English summary of Routes of Ca2+ Shuttling during Ca2+ Oscillations: FOCUS ON THE ROLE OF MITOCHONDRIAL Ca2+ HANDLING AND CYTOSOLIC Ca2+ BUFFERS, by L. Pecze, W. Blum, B. Schwaller (The Journal of Biological Chemistry, 2015).

The study

• Authors: L. Pecze, W. Blum, B. Schwaller
• Journal: The Journal of Biological Chemistry
• Published: 2015;290(47):28214-28230
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

In some cell types, Ca(2+) oscillations are strictly dependent on Ca(2+) influx across the plasma membrane, whereas in others, oscillations also persist in the absence of Ca(2+) influx. We observed that, in primary mesothelial cells, the plasmalemmal Ca(2+) influx played a pivotal role. However, when the Ca(2+) transport across the plasma membrane by the "lanthanum insulation method" was blocked prior to the induction of the serum-induced Ca(2+) oscillations, mitochondrial Ca(2+) transport was found to be able to substitute for the plasmalemmal Ca(2+) exchange function, thus rendering the oscillations independent of extracellular Ca(2+). However, in a physiological situation, the Ca(2+)-buffering capacity of mitochondria was found not to be essential for Ca(2+) oscillations. Moreover, brief spontaneous Ca(2+) changes were observed in the mitochondrial Ca(2+) concentration without apparent changes in the cytosolic Ca(2+) concentration, indicating the presence of a mitochondrial autonomous Ca(2+) signaling mechanism. In the presence of calretinin, a Ca(2+)-buffering protein, the amplitude of cytosolic spikes during oscillations was decreased, and the amount of Ca(2+) ions taken up by mitochondria was reduced. Thus, the increased calretinin expression observed in mesothelioma cells and in certain colon cancer might be correlated to the increased resistance of these tumor cells to proapoptotic/pronecrotic signals. We identified and characterized (experimentally and by modeling) three Ca(2+) shuttling pathways in primary mesothelial cells during Ca(2+) oscillations: Ca(2+) shuttled between (i) the endoplasmic reticulum (ER) and mitochondria, (ii) the ER and the extracellular space, and (iii) the ER and cytoplasmic Ca(2+) buffers.

© 2015 by The American Society for Biochemistry and Molecular Biology, Inc.

In plain English

This laboratory study did not examine asbestos. The authors studied rises and falls (oscillations) of calcium in mesothelial cells from mice, using fluorescent sensors and a mathematical model. After 24 hours without serum, about 70% of cells showed oscillations when serum was added back. Removing outside calcium stopped the oscillations. When lanthanum blocked calcium movement across the cell membrane before serum was added, serum still triggered slow oscillations, each with a matching mitochondrial calcium spike. Lanthanum added after oscillations began stopped them. Lowering mitochondrial calcium uptake reduced oscillation frequency. The authors also made cells produce calretinin, a calcium-binding protein they note is found in human mesothelioma. Only about 10 to 20% of these cells oscillated, versus 60 to 70% without it. In those that did, the first calcium spike was on average half the size, and mitochondrial calcium uptake was reduced. The authors suggested that higher calretinin in mesothelioma cells might be correlated or causally linked to these tumor cells' resistance to cell-death signals. This was not tested here. They noted their simplified model could not reproduce random or spatial features.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) study of cultured cells combined with mathematical modeling
• Who was studied: Primary mouse mesothelial cells isolated from 4 to 6 month old C57Bl/6J mice, studied at the University of Fribourg, Switzerland; no human subjects
• Exposure: No asbestos or other fiber exposure was studied. The cells received laboratory treatments: serum (1% FCS) added after 24 hours without serum, removal of outside calcium with EGTA, lanthanum to block calcium movement across the cell membrane, the drugs CCCP, CGP-37157 and Ru-360, shRNA to reduce the mitochondrial calcium uniporter (MCU), and overexpression of calretinin
• Main results:
  • About 70% of primary mouse mesothelial cells responded to serum readministration with calcium oscillations
  • Without serum, a small fraction of cells (2 to 3%) showed isolated rises in mitochondrial calcium with no detectable change in cytosolic calcium
  • Removing outside calcium (adding 10 mM EGTA) during ongoing oscillations stopped them immediately
  • When lanthanum blocked calcium transport across the cell membrane before serum was added, serum induced long-lasting but slow cytosolic oscillations, each accompanied by a mitochondrial calcium spike
  • Lanthanum added while serum-induced oscillations were already under way blocked the oscillations completely
  • Pretreatment with Ru-360 reduced average oscillation frequency by approximately 30% in each time segment
  • Reducing MCU messenger RNA by 60 to 90% decreased initial mitochondrial calcium uptake by 40 to 60%. Oscillation frequency was lower in all time windows (50 oscillating cells per condition)
  • Among cells made to express EBFP-calretinin, only about 10 to 20% showed oscillations, compared with about 60 to 70% of non-infected cells. Oscillations occurred only in cells with low EBFP-calretinin expression
  • In the EBFP-calretinin-expressing cells that oscillated, the amplitude of the first calcium spike after serum was on average half that of cells without calretinin, and the rise in mitochondrial calcium was clearly diminished
  • The model predicted that more calretinin would increase oscillation frequency; the experiments did not support this prediction
  • Calretinin protein in the unmodified primary mouse mesothelial cells was below the detection limit of the Western blot
• The authors' conclusion: The authors concluded that three calcium shuttling pathways operate during oscillations in primary mesothelial cells: between the endoplasmic reticulum and mitochondria, between the endoplasmic reticulum and the space outside the cell, and between the endoplasmic reticulum and cytoplasmic calcium buffers. They suggested that increased calretinin in mesothelioma cells and certain colon cancers might be correlated or causally linked to these tumor cells' resistance to cell-death signals.
• Limitations the authors state:
  • The mathematical model is a minimal deterministic point model and cannot reproduce the random or spatial features of calcium oscillations
  • It is unclear whether Ru-360 acts in intact cells only by blocking mitochondrial calcium uptake or also by blocking calcium entry from outside the cell
  • Experimental testing of the model's predictions was hampered by a lack of drugs that reach the mitochondrial inner membrane immediately
  • The model did not consider calcium microdomains, and calretinin's calcium-binding properties inside these cells may differ from those measured in vitro

Topics

Medical terms in this study, explained in our Lexicon: Mesothelial Cells, Calretinin, Mesothelioma, Colorectal Cancer.

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