Written By: Kathleen Cunningham
Original Article: Pires & Early eLIFE 2018.
The Gist of It
Strokes are serious events resulting from reduced or blocked blood flow in the brain. During a stroke, the reduced blood flow causes neurons in the brain to die from lack of oxygen. In the United States, stroke is the fifth leading cause of death and the number one cause of disability (National Stroke Association). Although great strides have been made in surgical intervention and treatment to help survival during a stroke, much is still unknown about what interventions we can do to prevent or reduce neuron death. Researchers from Reno School of Medicine investigated one mechanism by looking at calcium channels on the lining of blood vessels. Channels are like a tunnel in your cell membrane that allows the passage of specific types of ions and small molecules. In this case, the researchers took arteries from the brain of mice and showed that the calcium channels in the mouse arteries opened with low oxygen conditions (like during a stroke!), allowing more calcium to flow into the cell. The channels opened due to the production of a specific type of molecule called a free radical during stroke-like conditions. This inflow of calcium stimulated a signaling pathway in the cell that allows the arteries to dilate and increased the blood flow. When the researchers induced a stroke in mice that lacked the calcium channel on the blood vessel lining, the blood vessels dilated less and more neurons died from lack of oxygen. Excitingly, the researchers could treat with drugs that opened the channel in normal mice while they were having a stroke– and those mice had less neuron death.
The Nitty Gritty
Pires and Earley from the University of Nevada in this study used ex vivo arteries from mice expressing the fluorescent calcium indicator Gcamp6 in the arterial endothelium to investigate the role of TRPA1 calcium channels in response to hypoxia. First, the researchers measured TRPA1 Gcamp sparklets of calcium activity in response to the peroxidated lipid 4-HNE, which has previously been shown to activate the TRPA1 channel during hypoxic conditions. 4-HNE increased the frequency and the number of sparklet sites in the endothelium, which were ablated in the presence of a TRPA1 channel antagonist. The researchers then carefully held other conditions constant while exposing the arteries to hypoxic conditions, which caused the formation of endogenous 4-HNE and activated TRPA1-mediated calcium influx. The generation of 4-HNE required the production of intracellular reactive oxygen species (ROS) and was blocked by intracellular superoxide dismutase (SOD) or by the mitochondrial-targetted antioxident mitoTEMPO. Vasodilation of the pial arteries and of penetrating arterioles in response to hypoxia was blocked by treatment by a TRPA1 inhibitor or in arteries from TRPA1 ecKO mice compared to wildtype littermates. The researchers finally demonstrated that the dilation of arteries and arterioles in response to TRPA1 channel activity was protective against ischemic strokes induced in mice using a middle cerebral artery occlusion (MCAO) model. Wildtype TRPA1 mice had considerably less ischemic damage after MCAO stroke than ecKO littermates. Further, wildtype C57/bl6 micce were more resistant to damage from ischemic stroke when treated with the TRPA1-activating compound cinnamaldehyde. The authors identify this pathway as an adaptive response to lower oxygen levels and that TRPA1 activation may be neuroprotective during ischemic stroke.