Thursday, April 9, 2015

#expbio ASPET Blogging: RGS6 protects the heart from ischemic injury

Ischemic heart disease, also known as coronary heart disease, is the leading cause of death in the United States. Ischemia is when blood flow to the heart is decreased or blocked because of atherosclerotic plaque build-up in blood vessels. When blood flow is blocked, cell death can occur and the damaged area is called an infarct. Dr. Boyd Rorabaugh, at Ohio Northern University, studies ischemic heart disease and found that proteins called Regulators of G-protein signaling (RGS) can protect the heart from damage following ischemia.

Figure 1: RGS proteins inactivate G proteins, leading to 
changes in cell signaling. (Source: Dr. Rorabaugh's poster)

As mentioned in a recent post, RGS proteins work by “turning off” a G-protein responsible for conducting a signal from a receptor to proteins within the cell. Dr. Rorabaugh found that when RGS proteins are not able to turn off the Gαi2 protein in the heart, the heart is protected from damage following ischemia (Waterson et al., 2011, free full text). In his study presented at Experimental Biology, he used mice with specific RGS proteins knocked out (KO) to find out which one is responsible for the ischemic protection (RGS1, 2, 4, 2/4, or 5-KO). He used the mouse hearts in a common experimental model, called the Langendorff isolated heart model. In this experiment, the heart is isolated from the body and kept pumping using a solution with nutrients and oxygen, sort of like fake blood. Check out this cool video of a Langendorff heart experiment to see how it looks!

To mimic ischemia as it would occur in an animal, the solution was blocked from entering the heart for either 15 or 30 minutes and then started again to allow the heart to recover. This would be similar to a heart attack in which the person’s blood flow was restarted using CPR or by fibrinolytic drugs used in the emergency room. Measurement of the damaged area, the infarct, after the procedure allowed Dr. Rorabaugh to figure out which missing RGS protein was protecting the heart.

Figure 2: Basic heart anatomy. Arrows indicate blood flow. 
De-oxygenated blood flows into the right atrium, through the 
right ventricle, and then to the lungs where it is re-oxygenated. 
The oxygenated blood flows into the left atrium, through the 
left ventricle, and out to the body. (Source)
The infarct size in RGS1-, RGS2-, RGS4-, RGS2/4-, and RGS5-KO mice was not different from the control heart’s infarct size after 30 minutes of ischemia. However, in the RGS6-KO mice, the infarct size was increased when compared with control hearts that express RGS6. This is opposite of what he expected based on his previous research showing disruption of the interactions of RGS proteins with Gαi2 were cardioprotective. RGS6 deletion also increased the rate and degree of cardiac contracture. The increase in contracture is associated with the onset of irreversible cellular damage, the infarct. This may seem contradictory, heart contracture sounds like a good thing, but the intensity of contracture actually stiffens the muscle so that pumping is less efficient.

In other measures of post-ischemic recovery of contractile function there were no differences between RGS6-KO hearts and control hearts. Dr. Rorabaugh thought that maybe if less tissue was damaged there might be a difference in contractile recovery so he repeated the experiment using only 15 minutes of ischemia. There were no differences in infarct size or recovery. This suggests that RGS6 expression protects the heart from ischemia-induced cell death but not myocardial stunning, which is when tissue does not die after recovery but the heart just does not contract properly.

The hunt continues for figuring out which RGS protein in his earlier study was responsible for cardioprotection. There are over 20 known RGS proteins but only some of those mediate the Gαi2 protein so he can narrow it down somewhat. Knowing which RGS protein is protecting the heart from ischemic damage is exciting. It could lead to more specific and improved treatments that would reduce the amount of damage caused by decreased blood flow to the heart. Dr. Rorabaugh will continue to study cardiac ischemic injury and the role of RGS proteins in the heart.

No comments:

Post a Comment