VITA Peter M. Govey
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The Pennsylvania State University The Graduate School Department of Bioengineering MULTISCALE ANABOLIC BONE RESPONSES TO FLUID FLOW AND POST-IRRADITION LOADING A Dissertation in Bioengineering by Peter M. Govey © 2015 Peter M. Govey Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2015 The dissertation of Peter M. Govey was reviewed and approved* by the following: Henry J. Donahue Michael and Myrtle Baker Professor of Orthopaedics and Rehabilitation, Cellular and Molecular Physiology, and Bioengineering Dissertation Advisor Chair of Committee Justin Brown Assistant Professor of Bioengineering Gregory Lewis Assistant Professor of Orthopaedics and Rehabilitation Christopher Niyibizi Associate Professor of Orthopaedics and Rehabilitation Christopher Siedlecki Professor of Surgery and Bioengineering Cheng Dong Distinguished Professor of Bioengineering Head of the Department of Bioengineering *Signatures are on file in the Graduate School. ii ABSTRACT Bone responds to its mechanical environment, strengthening in response to daily use and wasting during disuse. Examples of disuse include bed rest, immobility, and microgravity experienced in space. Bone resorption may be exacerbated by aging, radiation exposure, sex hormone reduction, and various other pathologies, leading to osteoporosis. Resulting insufficiency fractures represent a tremendous toll on our population’s health, and current treatments such as anti-resorptive bisphosphonates are stop-gap measures with accompanying complications. Improved treatment and preventative strategies require an understanding of the body’s innate anabolic response to loading. This work examines bone’s anabolic response on multiple scales—first examining the cellular signaling response to mechanical stimulation, and extending this to an animal model of loading. A literature review reveals that bone adaptation to its mechanical environment, from embryonic through adult life, is thought to be the product of increased osteoblastic differentiation from putative mesenchymal stem cells (MSCs). These heterogeneous populations of multipotent stem cells are subject to a variety of biophysical cues within their native microenvironments. Bone marrow-derived MSCs—the most broadly studied source of osteoblastic progenitors—undergo osteoblastic differentiation in vitro in response to biophysical signals, including hydrostatic pressure, fluid flow and accompanying shear stress, substrate strain and stiffness, substrate topography, and electromagnetic fields. Additionally, stem cells may be subject to indirect regulation by mechano-sensing osteocytes positioned to more readily detect these same loading- iii induced signals within the bone matrix. Such paracrine and juxtacrine regulation of differentiation by osteocytes occurs in vitro. However, the osteocytic signaling molecules responsible for mediating this response remain unknown. We addressed this gap in knowledge by mapping a time course of broad in vitro signaling responses in fluid flowed osteocytic MLO-Y4 cells to physiological magnitude fluid shear stress. We hypothesized that high-throughput analyses of gene transcripts and proteins using DNA microarrays and mass-spectrometry, respectively, would reveal mechano-sensitive shifts in response to fluid flow. Osteocytic MLO-Y4 cells were exposed to 2 hours of 10 dyn/cm2 oscillating fluid shear stress and post-incubated for 0, 2, 8, and 24 hours. Fluid flow-regulated gene transcripts included known and unknown mechano-sensitive genes. qPCR analysis confirmed novel up-regulation of two of the most highly-upregulated gene transcripts by microarray: Cxcl1 and Cxcl2. Proteomic analysis identified notable up-regulation of ATP-producing enzyme NDK, calcium- binding Calcyclin, and G protein-coupled receptor kinase 6. Integrative analysis of transcriptomic and proteomic data revealed additional signaling nodes, including transcription factors c-Myc, c-Jun, and RelA/NF-κB. Finally, gene ontology analysis identified associations with various signaling pathways and functions, including inflammatory pathways, immune cell trafficking, and hematological system function. We followed up with a second analysis of these fluid flowed MLO-Y4 RNA samples using next-generation RNA-Seq. We sought to confirm previous results from DNA microarray, compare these analytical techniques, and identify additional novel signaling transcripts, particularly small RNA. Though a similar number of gene transcripts were significantly regulated, many more of these transcripts met a >1.5 fold- iv change cutoff, and the majority were newly identified. Once again, gene ontology analysis associated an inflammatory response, immune cell trafficking, and hematological system development. Cxcl1, Cxcl2, Cxcl5, which encode proteins known to stimulate migration of bone marrow MSCs and hematopoietic stem cells (HSCs), were once again highly up-regulated. We proposed that inflammatory chemokine up-regulation might be a novel paracrine means by which osteocytic cells exposed to fluid flow increase local bone formation via osteoblastic recruitment of progenitor cells. We next explored this in mice, hypothesizing that mechanical loading on a whole tissue scale up-regulates engraftment and bone formation by osteoprogenitor cells. We first utilized in vivo cantilever loading, hypothesizing that adaptation to a maximum 1,800 μϵ for 100 cycles, 3 times/week for 3 weeks could be quantified by microCT. Previous analyses had used histomorphometry alone, which lacks the detailed trabecular characterization of microCT. Though mice all lost some cortical bone mass, we identified a statistically significant preservation of cortical minimum moment of inertia, and non- significant trends towards preserved cortical thickness and polar moment of inertia. However, there was a significant reduction in relative trabecular thickness, the first microCT evidence that cantilever loading is not well-suited for trabecular bone formation. We progressed to a model of bone formation known to induce marked trabecular and cortical adaptation: compression loading. Immediately prior to 3 weeks of loading, we conditioned these mice to engraft syngeneic donor bone marrow stem cells via lethal irradiation (600+475 cGy) and intravenous transplantation of 20 million eGFP+ whole bone marrow cells. Irradiation primed bones for cellular engraftment and prevented v immune rejection of trackable donor cells. This was also the first study of mechanical loading in irradiated or bone marrow transplanted animals, both clinically relevant scenarios. Irradiation is known to rapidly induce bone loss and increase fracture risk, in part due to depletion of bone marrow cells. Right limbs were cyclically loaded to physiological strains of ~1,640 μϵ (10 N) 5 days per week for 3 weeks. Examining donor cell presence in flushed marrows by flow cytometry and in bone by relative DNA, we observed no clear loading-mediated differences in donor cell fractions in trabecular and cortical regions. However, there were significantly fewer cells of both donor and host origin in trabecular marrow regions of loaded bones. We associate this with robust attenuation of trabecular bone loss in loaded limbs compared to non-loaded internal and sham controls (-16% BV/TV relative to baseline in loaded vs. -48% BV/TV in non- loaded, p<0.001). Loss of trabecular thickness was significantly reversed (p<0.0001), increasing 26% above baseline. Loss of cortical area was attenuated, and cortical total area increased with loading. Mineral density of both trabecular and cortical bone increased. Cxcl1 and Cxcl2 were not detected in loaded or control tibias by qPCR using current protocols. Importantly, this is the first evidence that mechanical loading attenuates bone loss in irradiated subjects. This may be informative to the care of patients receiving focal or whole body irradiation for treatment of malignancies or hematological disorders. Moreover, new bone formation points to osteoblastogenesis from either conserved endogenous cells or donor cells. Future work will examine donor cell dependency and relative contribution vs. host cells. These studies add to our knowledge of the signaling mechanisms and contexts within which bone adapts to mechanical loads. vi TABLE OF CONTENTS LIST OF FIGURES ............................................................................................................ x LIST OF TABLES ........................................................................................................... xiii ABBREVIATIONS ......................................................................................................... xiv PREFACE ........................................................................................................................ xvi ACKNOWLEDGEMENTS ............................................................................................ xvii Chapter 1. BIOPHYSICAL MECHANISMS OF STEM CELL DIFFERENTIATION 1 1.1 Mechanical loading as a modulator of bone homeostasis ....................................... 1 1.2 Direct regulation of stem cells ................................................................................ 3 1.2.1 Mechano-transduction....................................................................................... 3 1.2.2 Hydrostatic pressure and fluid flow .................................................................. 4 1.2.3 Substrate strain and stiffness ............................................................................