MiP2005: Session 3

Mitochondrial Physiology Network 10.9: 38 (2005) - download pdf

 

Subplasmalemmal mitochondria modulate the activity of plasma membrane Ca2+ extrusion machinery.

Maud Frieden1, S Arnaudeau2, C Castelbou1, N Demaurex1

1Dept. Cellular Physiology Metabolism; 2Bioimaging Core Facility, University of Geneva Medical Center, 1 rue Michel-Servet, 1211 Geneva 4, Switzerland. - maud.frieden@medecine.unige.ch

    Mitochondria actively participate to the cellular calcium homeostasis and modulate the pattern of agonist-induced calcium signals by their ability to sequester and release calcium. In particular during agonist stimulation, the Ca2+ released from the ER is taken up by mitochondria and subsequently returned back to the ER, thus preventing its depletion [1]. As well, the Ca2+ that enters the cell transit through mitochondria toward the ER, short-circuiting the cytosol [2]. The calcium fluxes between mitochondria and the plasma membrane or the ER are difficult to study because, in most mammalian cells, mitochondria assemble into a dynamic network constantly remodeled by fusion and fission reactions. To study this dynamic Ca2+ connexion, we overexpressed a protein of the mitochondrial fission machinery, hFis1, and measured Ca2+ within organelles with GFP-based Ca2+ sensors. hFis1 overexpression induces a rapid fragmentation of mitochondria which then cluster around the nucleus, leaving subplasmalemmal ER regions devoid of mitochondria. We previously showed that this remodeling did not impair the ability of mitochondria to accumulate the calcium released from the ER in HeLa cells [3]. The cytosolic Ca2+ signals elicited by histamine were unaltered as long as Ca2+ was present in the extracellular medium, but were significantly blunted when Ca2+ was removed from the medium. Upon Ca2+ withdrawal, the free ER Ca2+ concentration, [Ca2+]ER, decreased rapidly and cells with fragmented mitochondria were unable to respond to repetitive stimulations. The loss of stored Ca2+ was due to an increased activity of the extrusion machinery (Ca2+-ATPase and/or Na+/Ca2+ exchanger) and was associated with an increased influx of Ca2+ and Mn2+ across Ca2+ entry channels. The increased Ca2+ influx compensated the loss of stored Ca2+, and brief Ca2+ additions between successive agonist stimulations fully corrected subsequent histamine responses. We proposed that the lack of subplasmalemmal mitochondria disrupts the transfer of Ca2+ from plasma membrane channels to the ER, and that the resulting increase in subplasmalemmal [Ca2+] enhances Ca2+ extrusion mechanisms.

1.  Arnaudeau S, Frieden M, Nakamura K, Castelbou C, Michalak M, Demaurex N (2003) Calreticulin differentially modulates calcium uptake and release in the endoplasmic reticulum and mitochondria. J. Biol. Chem. 48: 46696-46705.

2.  Malli R, Frieden M, Osibow K, Zoratti C, Mayer M, Demaurex N, Graier WF (2003) Sustained Ca2+ transfer across mitochondria is essential for mitochondrial Ca2+ buffering, store-operated Ca2+ entry, and Ca2+ store refilling. J. Biol. Chem. 278: 44769-44779.

3.  Frieden M, James D, Castelbou C, Danckaert A, Martinou J-C, Demaurex N (2004) Calcium homeostasis during mitochondrial fragmentation and perinuclear clustering induced by hFis1. J. Biol. Chem. 279: 22704-22714.


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Mitochondrial Physiology