The inflammatory process is an essential mechanism by which the body fights infection through innate immunity and is essential for initiating the healing process in response to injury to the body. Many human diseases are considered to be inflammatory diseases, such as atherosclerosis (heart disease), asthma, arthritis, and irritable bowel syndrome (IBS). The current belief is that the cause of these diseases is uncontrolled inflammation, leading to physiological changes in the body.
I am particularly interested in inflammation in the development of asthma and atherosclerosis. In asthma, the chronic inflammation leads to bronchoconstriction (tightening of the airways) causing a person to not be able to expel all of the air in the lungs, which is often described as not being able to catch one’s breath. The case for atherosclerosis is similar. In atherosclerosis, an injury to the vascular wall initiates an inflammatory response, which left unchecked leads to plaque buildup which can occlude an artery preventing blood flow. The plaque may eventually become unstable and rupture, leading to heart attack or stroke.
During my Master’s degree, I worked under Kenneth W. Rundell investigating exercise-induced asthma and the relationship between inhalation of ultra-fine particles from automobile emissions and the development of asthma. Currently, as a Ph.D. candidate, I am interested in the signaling events leading up to the development of atherosclerosis and potential mechanisms to attenuate or eliminate the excess inflammation.
Current projects in which I am involved include:
- Establishing a role for cofilin in FSS-induced actin realignment (published)
- Analyzing heparin-induced gene expression changes in vascular endothelial and smooth muscle cells
- Investigating the anti-inflammatory properties of heparin
- Identification and characterization of the heparin receptor
The relationship between fluid shear stress and atherosclerosis
It has been shown in the literature that the frictional force of blood flow (fluid shear stress - FSS) through an artery is essential for vascular homeostasis. Changes in FSS have been linked to the development of atherosclerosis. Regions of high FSS (≥15 dynes/cm2) are atheroprotective, meaning that atherosclerosis does not typically develop in these areas. On the other hand, regions of low FSS (≤4 dynes/cm2) are atheroprone, meaning that atherosclerosis can develop in these regions in predisposed individuals. These atheroprone regions are found in bifurcations and curvatures of major arteries (Figure 1).
Looking specifically at the atheroprotective areas, I am interested in how endothelial cells lining blood vessels sense and respond to high levels of FSS. Endothelial cell responses to high FSS have been previously described to include the elongation of the whole cell, the nucleus, and realignment of the actin microfilaments in the direction of the shear stress (Mengistu M, Slee JB, Lowe-Krentz LJ. 2012:177-205).
In order for the cell to realign actin microfilaments, actin binding proteins must be activated and/or inactivated during this process. In response to high FSS, phospho-cofilin (one of the major actin regulatory proteins) localization changes (Figure 2). High FSS induces decreases in cytoplasmic phospho-cofilin and increases in nuclear phospho-cofilin up to 60 mins of FSS (Slee JB and Lowe-Krentz LJ. in preparation).
This pattern of increased nuclear accumulation of cofilin after cell stress has been established in the literature for other cell stressors, such as ATP depletion. Given the evidence in the literature which suggests only unphosphorylated cofilin enters the nucleus, it was concluded that in this system, phospho-cofilin was likely not entering nucleus, but rather it is phosphorylated once it arrived. Concurrent with FSS-induced increases in phospho-cofilin, pLIMK1/2 also increases in the nucleus out to 60 mins of high FSS, suggesting cofilin is phosphorylated in the nucleus (Slee JB and Lowe-Krentz LJ. in preparation). Once cofilin is in the nucleus, it is hyopthesized that it would remodel nuclear actin to facilitate gene expression changes to facilitate responses to FSS. The current thought in the literature is that cofilin is the main shuttle for actin into the nucleus (Mengistu M, Slee JB, Lowe-Krentz LJ. 2012:177-205). Actin itself lacks a nuclear localization sequence, but has been shown to associate with cofilin to gain access to the nucleus, which is presumably the case in our system as well.
Given that cofilin is primarily regulated via phosphorylation and that its localization changes in response to FSS, the next step was to determine whether phosphorylated or unphosphorylated cofilin was necessary for this process. The results from this work suggest that it is not that cofilin exists in one form or another, but rather that the cell can control its phosphorylation state in a very time-specific manner. As shown in Figure 3, both cofilin mutants disrupt elongation in the direction of FSS compared to GFP-Vinc control (Slee JB and Lowe-Krentz LJ. in preparation). Note: S3A cofilin is constitutively active and S3D cofilin is constitutively inactive. This work suggests that cofilin is highly responsive to FSS and mediates actin realignment. The cofilin project is published in the Journal of Cellular Biochemistry (see publications below for citation).