Tissue specific adult stem cells can be isolated from various tissues and show great capacity of self-renewal and multipotency, thus making it a valuable cell source for regenerative medicine. However, despite its function in homeostasis and tissue repair, it is not clear whether normal adult stem cells could be involved in diseases, like cancer stem cells. Therefore, we investigated this issue in the cardiovascular system, wound healing model and knee joint system. Cardiovascular disease is the number one killer worldwide, which is responsible for approximately 40% of annual deaths in United States. Although various vascular stem cells and progenitor cells were isolated from blood vessel wall, it is not clear whether and how these stem cells contribute to diseases. In the general wound healing, although bone marrow cells and hematopoietic stem cells were identified as the sources of myofibroblasts, which are generally believed to be the culprit of scar tissue formation, it is not clear whether local tissue specific stem cells could be another source of myofibroblasts. In knee joint, osteoarthritis represents structural breakdown of the synovial joint, affecting 70 million people in the United States. However previous identified synovial stem cells are only characterized by non-specific surface markers and their response to biomaterials are not well understood. Therefore, in this dissertation work, we will address these fundamental issues to elucidate the role of adult stem cells in tissue remodeling and diseases.
It is generally accepted that the de-differentiation of smooth muscle cells (SMCs) from contractile to proliferative or synthetic phenotype plays an important role during vascular remodeling and diseases. In the first 3 chapters, we provide evidence that challenges this dogma. We identify a new type of multipotent vascular stem cell (MVSC) in blood vessel wall. MVSCs express markers including Sox17, Sox10 and S100β, are cloneable, have telomerase activity, and can differentiate into neural cells and mesenchymal stem cell (MSC)-like cells that subsequently differentiate into SMCs. On the other hand, we use lineage tracing with smooth muscle myosin heavy chain (SM-MHC) as a marker to show that MVSCs and proliferative or synthetic SMCs do not arise from the de-differentiation of mature SMCs. Upon vascular injuries, MVSCs, instead of SMCs, become proliferative, and MVSCs can differentiate into SMCs and chondrogenic cells, thus contributing to vascular remodeling and neointimal hyperplasia. Moreover, we also isolated MVSCs from neointima and plaque from patients with atherosclerosis. These findings support a new hypothesis that the differentiation of MVSCs rather than the de-differentiation of SMCs contributes to vascular remodeling and diseases. The MVSCs instead of SMCs should be treated as the targets for drug screening and development.
In the second part of the work, we investigated the general wound healing process and remodeling of implanted artificial biomaterials. It is generally accepted that myofibroblasts play a retractile role in wound contraction and are involved in the synthesis of extracellular matrix components to form the scar tissues. We performed detailed characterization and found a novel type of stem cells showing similar feature with MVSCs, which express markers including Sox10, Sox17 and S100β, can spontaneously differentiate into myofibroblasts and eventually SMCs. In vivo studies also identified Sox10+ cells at early stage of wound healing, remodeling of vascular grafts and implanted biomaterials. This work identified a new precursor of myofibroblasts and provided new tools for further research.
In the last part, we identified and characterized a new type of stem cells from the synovial membrane of knee joint, named neural crest cell-like synovial stem cells (NCCL-SSCs). NCCL-SSCs showed the characteristics of neural crest stem cells: they expressed markers such as Sox10, Sox17 and S100, were clonable, and could differentiate into neural lineages as well as mesenchymal lineages. However, lineage tracing with Wnt-1 as marker showed that NCCL-SSCs were not derived from neural crest. When treated with transforming growth factor 1 (TGF-1), NCCL-SSCs differentiated into MSCs, lost the expression of Sox17, and lost the differentiation potential into neural lineages, but retained the potential of differentiating into mesenchymal lineages. To determine the responses of NCCL-SSCs to microfibrous scaffolds for tissue engineering, electrospun composite scaffolds with various porosities were fabricated by co-electrospinning of structural and sacrificial microfibers. Interestingly, microfibrous scaffolds with higher porosity increased the expression of chondrogenic and osteogenic genes but suppressed smooth muscle and adipogenic genes. These results suggest that NCCL-SSCs have tremendous potential for tissue engineering and their differentiation can be controlled by both soluble chemical factors and the biophysical factors such as the porosity of the scaffold.
In summary, this dissertation work identified new tissue specific adult stem cells and performed detailed characterization. This work demonstrated the role of adult stem cells in vascular diseases and scar formation for the first time, indicating that many diseases might be stem cell diseases like cancer. Stem cells instead of somatic cells might be the target for the design of novel therapeutic strategy. Furthermore, we also provide new insight into the response of stem cells on physical properties of biomaterials, thus providing guidance for the design of suitable scaffolds for tissue engineering. We hope this dissertation work could promote the research in investigating the function of stem cells in not only regenerative medicine, but also diseases, thus contributing to our healthcare system.