The Electron Transport Chain (ETC)
The architecture of mitochondria is complex and imbues this organelle with a diverse array of signalling capabilities. Of particular interest is the electron transport chain (ETC), which is comprised of a series of protein complexes located on the inner mitochondrial membrane and is the site of oxidative phosphorylation.
There is a large voltage gradient across the inner membrane, with the matrix being ~ -180 mV with regard to the outside of the cell. This voltage gradient is due primarily to the proton (H+) gradient, which results from the reduction of oxygen by the ETC. The impermeability of the inner mitochondrial membrane to H+ allows the energy of H+ traveling back along their concentration gradient via specialized channels and pumps to be harnessed to power the movement of other ions across the membrane against their own respective concentration gradients. The combination of the ion concentration and electrical gradients results in a proton-motive force, which energizes the phosphorylation of ADP to ATP via the ATP synthetase. Thus the H+ gradient is tightly coupled to ATP production and any perturbation that dissipates the gradient for physiological purposes other than ATP production is termed an uncoupling mechanism.
Mitochondria are a hub for cellular signalling
ATP production, mitochondrial calcium buffering, and reactive oxygen species (ROS) generation are examples of mitochondrial functions that are important cellular and intracellular messengers. For example,
1) ATP and its metabolites (ADP, AMP and adenosine) directly modulate purinergic receptors and AMP kinase, which is a master regulator of cellular energetics.
2) By buffering calcium, mitochondria indirectly modulate calcium-mediated cellular signaling pathways, while calcium influx into the mitochondria activates matrix dehydrogenases and thereby accelerates electron flow through the ETC and ATP synthesis.
3) ROS directly act to modulate protein function, including stabilization of hypoxia-inducible factor, modification of cytosolic proteins and enzymes, and direct modification of ion channels.
Each of these signalling functions are critical to a variety of adaptive cellular responses to hypoxia in endogenously hypoxia-tolerant species, and also to induced protection in hypoxia-intolerant species; making the mitochondria a pivotal player in signaling mechanisms during periods of low oxygen stress.
Current Projects
We are interested in the roles played by mitochondria in protecting the cells of hypoxia-tolerant species from low oxygen stress. Using a combination of high-resolution respirometry, fluorescence microscopy, and molecular biology, we are exploring: 1) how mitochondria function as oxygen sensors in hypoxia, and 2) how they coordinate and manage beneficial cellular responses to hypoxia, while 3) avoiding the activation of cell death pathways. We are particularly interested in putative adaptations found in the mitochondria of hypoxia-tolerant species that may facilitate improved energy production efficiency in the ETC and prevent deleterious damage from ROS and calcium accumulation, which are hallmarks of cell death cascades induced by hypoxia in the brains of intolerant species.
Below, data from the doctoral thesis of Hang Cheng demonstrates that naked mole-rat brain cells are able to take up and buffer considerably more Ca2+ than mouse brain cells before activation of the mitochondrial permeability transition pore (MPTP, indicated by an increasing signal following a bolus of exogenous Ca2+ addition in panel A and the associated inset). This enhanced cellular buffering is likely due to mitochondrial uptake via the mitochondrial Ca2+-uniporter (MCU) because an antagonist of the MCU (RU360) entirely prevented Ca2+ uptake by naked mole-rat brain cells (panel C). Dr. Cheng went on to show that naked mole-rat brain cells are also better able to preserve mitochondrial membrane potential and oxidative metabolic capacity following bolus Ca2+ additions than are mouse brain cells. We are currently working to better understand how Ca2+ handling and signalling is altered by hypoxia in naked mole-rat brain.
Below, additional data from the doctoral thesis of Hang Cheng showing thermal images (FLIR imaging) of a naked mole-rat in normoxia and hypoxia, demonstrating that naked mole-rats are heterothermic but cease thermogenesis in acute hypoxia. The cause of this shift is a remarkably rapid down regulation of uncoupling protein 1 (UCP1) in naked mole-rat brown adipose tissue. By shutting down thermogenesis in hypoxia, naked mole-rats, which live in a relatively warm and thermally stable environment, likely gain considerable energy savings to support hypoxic hypometabolism. We are currently working to understand how this switch is regulated.
Other Pamenter Lab research interests: Hypoxia, Control of Breathing, Neurobiology.