Every day, the human body’s pool of adult stem cells, termed hematopoietic stem cells (HSCs), develop into nearly one trillion mature blood cells, including red blood cells, white blood cells, platelets and immune cells. These blood cell-forming HSCs primarily reside in trabecular bone marrow (TBM), which is the tissue in the porous end of long bones such as femurs. HSC transplantation is the most successful stem cell therapy, but its clinical impact has been limited by the availability of HSCs. HSC’s are difficult to grow outside the body where they tend to change (differentiate) and lose their ability to develop into any blood cell type. To address the need for a scalable method to grow HSCs without allowing differentiation, the goal of this CAREER project is to develop biomaterial models that mimic the microenvironments of TBM for HSC growth and maintenance. Once developed, the biomaterial models will be combined and integrated into a TBM model in bioreactor designed to growth HSCs. Successful TBM models will advance understanding of bone remodeling associated with aging and the progression of diseases such as osteoporosis and bone cancers, and they may also serve as an alternative to animals during preclinical drug testing. Furthermore, the project will be used to recruit and educate students with diverse backgrounds to face emerging challenges at the intersection between engineering and medicine. Activities include an integrated lecture and lab curriculum for a tissue engineering course, an interdisciplinary team-based (engineering and biology) capstone projects for undergraduate students, and a summer research program for high school girls aligned with the existing Engineering the Cell program.
The investigator’s long-term research goals are to deliver translational bioengineered solutions that can advance our understanding of the trabecular bone marrow (TBM) in health and disease and harness the regenerative potential of the TBM. Toward this goal, the aim of this CAREER project is to elucidate the dynamic structure-function relationship that maintains hematopoietic activity in the TBM and to apply this knowledge to design a scalable bioreactor for expanding hematopoietic stem cells (HSCs). Two observations, (1) that blood-forming HSCs primarily reside and function in cavities that maintain comparable bone thickness and cavity diameters while undergoing constant bone remodeling and (2) that aging-associated decrease in trabecular bone thickness and increase in cavity diameters are related to a decrease in hematopoietic activity, suggest a possible relationship between anatomy and function that maintains hematopoietic activity in the TBM. Thus, the project’s central hypothesis is that there exist optimal dimensions and spatial arrangement that effectively coordinate bidirectional crosstalk between the endosteal and vascular niches, which are the two important anatomical and functional niches in the TBM. The project builds on the investigator’s development of (1) inverted colloidal crystal (ICC) hydrogel scaffolds that closely emulate anatomical and physical features of the vascular niche and (2) demineralized bone paper that preserves intact biochemical and structural aspects of the endosteal niche and exhibits biological significance in reproducing the surface and subsurface bone tissue complexity of bone, including osteocytes. The Research Plan is organized under four Objectives: (1) To develop an endosteal niche model that recapitulates bone remodeling-related HSC biology by refining an endosteal niche model in which osteocytes buried in layers of demineralized bone paper under mechanoculture re-create physiological aspects of bone surface and subsurface; (2) To develop a vascular niche model that emulates vascular perfusion-related HSC biology by refining the model of the vascular niche to mimic molecular gradients under perfusion within a microfluidic hydrogel scaffold that supports culture of bone marrow stromal cells (BMSCs); (3) To combine the endosteal and vascular niche models into an integrated TBM model by integrating individually optimized models into a single platform to re-create the differentiation-suppressing quality of the endosteal niche and the proliferation-stimulating quality of the vascular niche and (4) To assemble multiple TBM models into a scalable bioreactor that will maintain a stable level of molecular and mechanical interactions between the endosteal and vascular niches to support the expansion phase of HSCs. To determine the competitive advantages and weaknesses of the ex vivo HSC expansion bioreactor compared to other solutions, a limiting dilution assay will be conducted to determine the in vivo repopulating potential of the culture-expanded HSCs by intravenous injection into sublethally irradiated mice.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
