Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Biomedical Engineering

First Advisor

Dr. Hainsworth Y. Shin

Second Advisor

Dr. Abhijit R. Patwardhan

Abstract

Hypercholesterolemia is a significant risk factor in the development of cardiovascular disease and is associated with chronic leukocyte adhesion in the microvasculature. While the underlying mechanisms behind this have yet to be determined, it may be possible that hypercholesterolemia impairs the fluid shear stress (FSS) inactivation of neutrophils through the rigidifying effect of cholesterol on membrane fluidity. FSS restricts surface expression of CD18 integrins through cathepsin B (ctsB) proteolysis, which minimizes neutrophil adhesivity. If hypercholesterolemia blocks FSS mechanotransduction, then the inhibition of CD18 cleavage may link pathologic blood cholesterol elevations with dysregulated neutrophil adhesion. We hypothesized that elevated cholesterol contributes to dysregulated neutrophil adhesion by impairing ctsB FSS-induced CD18 cleavage through membrane fluidity changes.

In the first part of this study, we demonstrated that FSS-induced CD18 cleavage is a robust response of neutrophils and involves selective cleavage of macrophage 1-antigen (Mac1) through ctsB proteolysis. The second part of this study confirmed that ctsB regulates neutrophil adhesion through its proteolytic actions on Mac1, an important integrin involved in adhesion and chemotaxis. Specifically, ctsB accelerated neutrophil motility through an effect on Mac1 integrins during pseudopod retraction. Furthermore, by using a flow-based assay to quantify the mechanoregulation of neutrophil adhesivity, we demonstrated that FSS-induced ctsB release promoted neutrophil detachment from platelet-coated substrates and unstimulated endothelium. For the third part of this study, we linked cholesterol-related membrane fluidity changes with the ability of FSS to restrict neutrophil adhesion through Mac1. We also determined that pathologic cholesterol elevations associated with hypercholesterolemia could block FSS-induced Mac1 cleavage and were linked to disrupted tissue blood flow. This was accomplished using low-density lipoprotein receptor deficient (LDLR-/-) mice fed a high-fat diet.

Ultimately, the results provided in the present study confirmed that cholesterol-related changes in membrane fluidity blocked the ability of ctsB to regulate neutrophil adhesion through FSS-induced Mac1 cleavage. This implicates an impaired neutrophil FSS mechanotransduction response in the dysregulation of neutrophil adhesion associated with hypercholesterolemia. Since dysregulated adhesion may be one of the earliest upstream features of cardiovascular disease associated with hypercholesterolemia, the present study provides a foundation for identifying a new mechanobiological factor in the pathobiology of microcirculatory dysfunction.

Digital Object Identifier (DOI)

http://dx.doi.org/10.13023/ETD.2016.287

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