Cannon, Barbara, and Jan Nedergaard. Brown Adipose Tissue: Function and Physiological Significance. Physiol Rev 84: 277–359, 2004; 10.1152/physrev.00015.2003.—The function of brown adipose tissue i...

Brown Adipose Tissue: Function and Physiological Significance

The universality of calcium as an intracellular messenger depends on its enormous versatility. Cells have a calcium signalling toolkit with many components that can be mixed and matched to create a wide range of spatial and temporal signals. This versatility is exploited to control processes as diverse as fertilization, proliferation, development, learning and memory, contraction and secretion, and must be accomplished within the context of calcium being highly toxic. Exceeding its normal spatial and temporal boundaries can result in cell death through both necrosis and apoptosis.

The versatility and universality of calcium signalling

The reduction of oxygen to water proceeds via one electron at a time. In the mitochondrial respiratory chain, Complex IV (cytochrome oxidase) retains all partially reduced intermediates until full reduction is achieved. Other redox centres in the electron transport chain, however, may leak electrons to oxygen, partially reducing this molecule to superoxide anion (O2−•). Even though O2−• is not a strong oxidant, it is a precursor of most other reactive oxygen species, and it also becomes involved in the propagation of oxidative chain reactions. Despite the presence of various antioxidant defences, the mitochondrion appears to be the main intracellular source of these oxidants. This review describes the main mitochondrial sources of reactive species and the antioxidant defences that evolved to prevent oxidative damage in all the mitochondrial compartments. We also discuss various physiological and pathological scenarios resulting from an increased steady state concentration of mitochondrial oxidants.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7793.2003.00335.x

Mitochondrial formation of reactive oxygen species.

https://mootha.med.harvard.edu/PubPDFs/Wu1999.pdf

Mechanisms Controlling Mitochondrial Biogenesis and Respiration through the Thermogenic Coactivator PGC-1

A cold-inducible coactivator of nuclear receptors linked to adaptive thermogenesis.

Assessing mitochondrial dysfunction requires definition of the dysfunction to be investigated. Usually, it is the ability of the mitochondria to make ATP appropriately in response to energy demands. Where other functions are of interest, tailored solutions are required. Dysfunction can be assessed in isolated mitochondria, in cells or in vivo, with different balances between precise experimental control and physiological relevance. There are many methods to measure mitochondrial function and dysfunction in these systems. Generally, measurements of fluxes give more information about the ability to make ATP than do measurements of intermediates and potentials. For isolated mitochondria, the best assay is mitochondrial respiratory control: the increase in respiration rate in response to ADP. For intact cells, the best assay is the equivalent measurement of cell respiratory control, which reports the rate of ATP production, the proton leak rate, the coupling efficiency, the maximum respiratory rate, the respiratory control ratio and the spare respiratory capacity. Measurements of membrane potential provide useful additional information. Measurement of both respiration and potential during appropriate titrations enables the identification of the primary sites of effectors and the distribution of control, allowing deeper quantitative analyses. Many other measurements in current use can be more problematic, as discussed in the present review.

/pdf/assessing-mitochondrial-dysfunction-in-cells-3j64pz5jhm.pdf

Assessing mitochondrial dysfunction in cells

Thermogenic mechanisms in brown fat.

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca2+, generate reactive oxygen species, and undergo Ca2+-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.1.315

Mitochondria and Neuronal Survival

The release and uptake of excitatory amino acids.

1
A technique is described, based on the distribution of rubidium, acetate and methylammonium ions, for the simultaneous estimation of membrane potential and pH gradient across the inner membrane of mitochondria The technique requires less than 05 mg mitochondrial protein and is independent of many factors which interfere with electrode determinations of protonmotive force (Δp)

2
With a limiting matrix volume of 04 μl/mg mitochondrial protein, the indicated value of Δp for rat liver mitochondria is 228 mV in state 4, 170 mV in state 3, and –06 mV in the presence of rotenone and uncoupler The relative contributions of the pH gradient and membrane potential are dependent on the availability of electrophoretically and electroneutrally translocatable species in the incubation medium

3
In a sucrose-based medium containing 05 mM KCl, rotenone and uncoupler, the technique indicated a membrane potential of + 85 mV and a pH gradient of + 146 (acidic in the matrix compartment) In state 4, under no conditions examined did the pH gradient contribute more than 50% of the total protonmotive force

4
The hydrolysis of ATP generates an optimal Δp of 220 mV

5
The proton conductance of the inner membrane is potential dependent, increasing when Δp is greater than 200 mV

6
The extra-mitochondrial phosphate potential sustainable by respiration was found to change in parallel to Δp, but to exceed the latter parameter when based upon a stoichiometry of two protons translocated per ATP synthesised

https://febs.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1432-1033.1974.tb03899.x

The Influence of Respiration and ATP Hydrolysis on the Proton‐Electrochemical Gradient across the Inner Membrane of Rat‐Liver Mitochondria as Determined by Ion Distribution

David G. Nicholls

Papers

Assessing mitochondrial dysfunction in cells

Thermogenic mechanisms in brown fat.

Mitochondria and Neuronal Survival

The release and uptake of excitatory amino acids.

The Influence of Respiration and ATP Hydrolysis on the Proton‐Electrochemical Gradient across the Inner Membrane of Rat‐Liver Mitochondria as Determined by Ion Distribution