The Adult Human Brain Harbors Multipotent Perivascular Mesenchymal Stem Cells

Blood vessels and adjacent cells form perivascular stem cell niches in adult tissues. In this perivascular niche, a stem cell with mesenchymal characteristics was recently identified in some adult somatic tissues. These cells are pericytes that line the microvasculature, express mesenchymal markers and differentiate into mesodermal lineages but might even have the capacity to generate tissue-specific cell types. Here, we isolated, purified and characterized a previously unrecognized progenitor population from two different regions in the adult human brain, the ventricular wall and the neocortex. We show that these cells co-express markers for mesenchymal stem cells and pericytes in vivo and in vitro, but do not express glial, neuronal progenitor, hematopoietic, endothelial or microglial markers in their native state. Furthermore, we demonstrate at a clonal level that these progenitors have true multilineage potential towards both, the mesodermal and neuroectodermal phenotype. They can be epigenetically induced in vitro into adipocytes, chondroblasts and osteoblasts but also into glial cells and immature neurons. This progenitor population exhibits long-term proliferation, karyotype stability and retention of phenotype and multipotency following extensive propagation. Thus, we provide evidence that the vascular niche in the adult human brain harbors a novel progenitor with multilineage capacity that appears to represent mesenchymal stem cells and is different from any previously described human neural stem cell. Future studies will elucidate whether these cells may play a role for disease or may represent a reservoir that can be exploited in efforts to repair the diseased human brain

Studies on myeloid differentiation: cytokine influence and identification of a novel marker gene

Hematopoiesis is a highly complex process by which a range of specialized blood cells are generated from a small pool of multipotent stem cells in the bone marrow. The survival, proliferation, and differentiation of hematopoietic stem cells are tightly regulated by both soluble and membrane-bound cytokines produced within the bone marrow microenvironment. Kit Ligand (KL) and Flt3 Ligand (FL) are two important hematopoietic cytokines, and signal via related tyrosine kinase receptors; c-kit and flt3. The first part of this thesis is focused on the influence of KL and FL on differentiation of a stem and progenitor cell-enriched cell population (c-kit+Lin- cells), isolated from mouse bone marrow. We found that KL and FL have different effects, and favor development of granulocytic and monocytic cells, respectively. A clear discrepancy was also seen on the expansion of multilineage progenitors (pre-CFCmulti) and granulocyte/macrophage colony-forming progenitors (CFC-G/M) which was strongly favored by KL. Furthermore, FL induced development of a biphenotypic cell population coexpressing monocytic and B-cell characteristics, restricted to the macrophage lineage. The second part of this thesis, describes the identification and characterization of a novel myeloid-associated differentiation marker gene (MYADM) which is preferentially expressed in myeloid cells, but absent in B and T lymphocytes. The predicted 32-kDa protein contains eight transmembrane domains and localizes to intracellular membranes. Although, the function of MYADM is unknown, antisense oligonucleotides inhibit colony formation of c-kit+Lin- cells, suggesting an important role for MYADM in myeloid differentiation. To gain further insight into the transcriptional control of the MYADM gene, we have analyzed a 1.5-kb sequence upstream the transcription start site for promoter activity in myeloid and lymphoid cells. We conclude that the sequence analyzed in this study does not confer the promoter tissue-specificity, thus additional regulatory elements may be located outside the region upon which this study has concentrated