Mitochondria are organelles found within most eukaryotic cell types that are primarily responsible for bulk ATP production that is required for cell survival. Additionally, mitochondria sequester pro-apoptotic factors and release them upon initiation of the apoptotic process. Based on these two fundamental roles, it is easy to understand how mitochondria are essential in determining the health of the cell. In fact, damage to mitochondria has been shown to be the principal insult during many forms of ischemia/reperfusion injury, such as a heart attack or stroke.
During an ischemic event, an area of tissue is prevented from receiving oxygen and substrate rich blood by the blockage of an artery. The area of tissue becomes hypoxic and mitochondria are unable to perform the processes required to generate ATP for the cell. Conditions which accompany ischemia can cause the opening of an indiscriminate mitochondrial inner membrane pore and allow the equilibration of solutes less than 1500 Daltons during a process known as the mitochondrial permeability transition [MPT]. After the mitochondria have undergone the MPT cells will progress through apoptosis or necrosis leading to an area infarct observed after heart attack or stroke. Currently, the pore that opens to cause the MPT is not known, however previous work by our group and others have shown that the activity of a mitochondrial calcium- independent phospholipase A2 [iPLA2] accompanies MPT and sensitizes the pore opening.
We first identified the iPLA2 activity in isolated mitochondria upon uncoupling of the electron transport chain from ATP synthesis, opening of the pore responsible for the MPT, and by forming an artificial pore in the mitochondrial membrane using alamethicin. Additionally, we showed that there was no discernable iPLA2 activity in energized mitochondria. With this in mind, we hypothesized that the iPLA2 was specifically regulated by the loss of membrane potential that accompanied all of the previously identified activators. Utilizing isolated mitochondria, we have developed a model system in which we can selectively modulate the generation of membrane potential via the inhibition of specific components of the electron transport chain. Using this system we can assess the ability of the iPLA2 to become active upon loss of membrane potential. We have also used this system to examine the role of the iPLA2 in other possible physiological processes.
Here we show that the mitochondrial iPLA2 is primarily regulated by small decreases in membrane potential and is reversible upon reenergization. Additionally, we propose a new potential physiological function for mitochondrial stress induced iPLA2 activity. As mentioned above, iPLA2 activity sensitizes the pore responsible for the MPT to opening. Because iPLA2 activity occurs upstream of the MPT, we propose that the iPLA2 could serve as a pharmacological target to prevent the damage associated with ischemia/reperfusion injury.