Document Type

Dissertation

Date of Degree

Spring 2015

Degree Name

PhD (Doctor of Philosophy)

Degree In

Biomedical Engineering

First Advisor

Jose G. Assouline

Abstract

One of the leading causes of death and hospital stays in the United States, myocardial infarction (MI) occurs when coronary blockages lead to downstream ischemia in the myocardium. Following the MI, the heart activates a number of pathways to repair or remodel the infarcted zone. Endothelial cells respond to ischemia by de-differentiating to form neovasculature and myofibroblasts. The resident cardiac differentiable stem cells (CDCs) are recruited via local cytokines and chemokines to the infarct zone where they too differentiate into myofibroblasts. Mesenchymal stem cells (MSCs) of the bone marrow respond to circulating factors by immobilizing to the heart and differentiating down cardiac lineages. In regenerative medicine approaches, these processes are exploited to augment the resident supply of reparative cells.

Clinical trials to transplant cardiac stem cells into MI zones have been met with mixed results. When CDCs are harvested from autologous or type-matched donors, the cells are prepared with a minimum of manipulations, but the yield is quite small. Conversely, MSCs from bone marrow are highly proliferative, but the manipulations in culture required to trigger cardiac differentiation have been found to transform the cell into a more immunogenic phenotype. In addition, there is a dearth of in vivo evidence for the fate of transplanted cells. Currently, intracardiac echocardiographs are used to assess the infarcted area and to guide delivery of stem cell transplants. However, this modality is invasive, short-term, and does not image the transplanted cells directly.

In this project, I addressed these shortcomings with a regenerative medicine and bioimaging approach. Our lab has developed multimodal nanoparticles based on a core of mesoporous silica, functionalized with fluorescein or tetramethylrhodamine isothiocyanate for visibility in fluorescent microscopy, Gd2O3 for magnetic resonance imaging (MRI), and trifluoropropyl moieties for ultrasound applications. After establishing in vitro models of cardiac stem cells using CDCs and MSCs, the particles were implemented and characterized in vitro. At a concentration of 125 μg/mL in culture, the particles are highly biocompatible, and labeled cells were found to be fluorescent, echogenic, and detectable with MRI in prepared agar phantoms. Ex vivo mouse hearts, first mounted in agar phantoms, then left in situ, were implemented as a model for guided delivery using ultrasound and follow-up cell tracking with MRI.

These results in this project demonstrate the feasibility of this highly novel and practical approach. Additional studies will be carried out to evaluate the biocompatibility and retention versus clearance in live animal models, prior to the carrying out of true pre-clinical models for myocardial infarction.

Public Abstract

Every year in the U.S. an estimated 1.5 million individuals suffer a first, recurrent, or “silent” heart attack, causing a total financial burden of $11.5 billion. After a heart attack, several processes activate the body’s native stem cell population to repair the heart, using resident stem cells from the heart as well as mobilized stem cells from the bone marrow. Over the past 2 decades, clinical trials have performed stem cell transplants to provide an additional supply of cells to the heart for it to repair itself. Results have been mixed, and one factor is the requirement for precise placement of the transplanted cells. Current approaches are invasive, short-lived, and only track the damaged tissue, not the transplanted cells themselves.

In order to address these shortcomings, I have evaluated a novel approach for direct, non-invasive tracking of stem cells transplanted following heart attack. Our lab has developed nanoparticles that are engulfed by the stem cells. Labeled cells are tracked using ultrasound during the transplantation procedure, and follow-up ultrasound and MRI scans can be used to verify that the transplanted cells remain in their desired location. This novel technology represents a new approach to tracking stem cells in clinical applications as well as gathering new data about heart repair in animal models.

Keywords

publicabstract, Magnetic Resonance Imaging, Mesenchymal Stem Cells, Mesoporous Silica Nanoparticles, Myocardial Infarction, Ultrasound

Pages

xiv, 135 pages

Bibliography

Includes bibliographical references (pages 119-135).

Copyright

Copyright 2015 Sean K. Sweeney

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