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Mitochondrial Ca2+ homeostasis in intact cells
R Rizzuto1, C Bastianutto, M Brini
1Department of Biomedical Sciences, University of Padova, Italy.
The Journal of Cell Biology
|September 1, 1994
Summary
Researchers developed a new method to track calcium (Ca2+) levels within mitochondria. This technique reveals how localized calcium signals activate mitochondrial metabolism, impacting cellular processes like proliferation and secretion.
Area of Science:
- Cell Biology
- Biochemistry
- Physiology
Background:
- Calcium ions (Ca2+) regulate numerous cellular processes, including cytosolic enzymes and organelle metabolic pathways.
- Monitoring Ca2+ concentration within specific cellular compartments, like mitochondria, was previously challenging.
- The development of Ca2+-sensitive photoproteins offers a novel approach to study organelle Ca2+ dynamics.
Purpose of the Study:
- To detail the use of chimeric recombinant aequorin targeted to mitochondria for monitoring mitochondrial Ca2+ dynamics.
- To investigate the spatiotemporal dynamics of mitochondrial Ca2+ concentration ([Ca2+]m) in response to cellular stimuli.
- To explore the relationship between cytosolic Ca2+ gradients and mitochondrial Ca2+ uptake, and its impact on metabolic activity.
Main Methods:
- Utilizing molecularly engineered, Ca2+-sensitive photoproteins (aequorin) targeted to mitochondria.
- Employing transient or permanent transfection in various cell models to express the aequorin construct.
- Stimulating cells with agonists that generate inositol trisphosphate (InsP3) and monitoring [Ca2+]m and NAD(P)H fluorescence.
Main Results:
- Mitochondrial Ca2+ concentration ([Ca2+]m) increases rapidly and transiently upon stimulation with InsP3-generating agonists.
- Fast mitochondrial Ca2+ accumulation is dependent on localized cytosolic Ca2+ gradients near InsP3-sensitive channels, not just general store release.
- Evidence suggests microheterogeneity in Ca2+ sensing among mitochondria within the same cell, with ~30% sensing localized cytosolic Ca2+ microdomains.
- Observed changes in [Ca2+]m are sufficient to rapidly activate mitochondrial dehydrogenases, indicated by NAD(P)H fluorescence.
- A model is proposed where receptor activation leads to increased cytosolic and mitochondrial Ca2+, activating metabolic activity.
Conclusions:
- Chimeric aequorin provides a robust method for studying mitochondrial Ca2+ dynamics in diverse cell types.
- Localized cytosolic Ca2+ signaling plays a critical role in the rapid uptake and metabolic activation of mitochondria.
- Mitochondrial Ca2+ signaling is integrated with cytosolic Ca2+ signaling to coordinate cellular energy production with cellular demands.