Nathalie Brouard

A role for pericytes as microenvironmental regulators of human skin tissue regeneration

The cellular and molecular microenvironment of epithelial stem and progenitor cells is poorly characterized despite well-documented roles in homeostatic tissue renewal, wound healing, and cancer progression. Here, we demonstrate that, in organotypic cocultures, dermal pericytes substantially enhanced the intrinsically low tissue-regenerative capacity of human epidermal cells that have committed to differentiate and that this enhancement was independent of angiogenesis. We used microarray analysis to identify genes expressed by human dermal pericytes that could potentially promote epidermal regeneration. Using this approach, we identified as a candidate the gene LAMA5, which encodes laminin alpha5, a subunit of the ECM component laminin-511/521 (LM-511/521). LAMA5 was of particular interest as we had previously shown that it promotes skin regeneration both in vitro and in vivo. Analysis using immunogold localization revealed that pericytes synthesized and secreted LAMA5 in human skin. Consistent with this observation, coculture with pericytes enhanced LM-511/521 deposition in the dermal-epidermal junction of organotypic cultures. We further showed that skin pericytes could also act as mesenchymal stem cells, exhibiting the capacity to differentiate into bone, fat, and cartilage lineages in vitro. This study suggests that pericytes represent a potent stem cell population in the skin that is capable of modifying the ECM microenvironment and promoting epidermal tissue renewal from non-stem cells, a previously unsuspected role for pericytes.

Greater Bone Formation of Y2 Knockout Mice Is Associated with Increased Osteoprogenitor Numbers and Altered Y1 Receptor Expression

Germ line or hypothalamus-specific deletion of Y2 receptors in mice results in a doubling of trabecular bone volume. However, the specific mechanism by which deletion of Y2 receptors increases bone mass has not yet been identified. Here we show that cultured adherent bone marrow stromal cells from Y2-/- mice also demonstrate increased mineralization in vitro. Isolation of two populations of progenitor cell types, an immature mesenchymal stem cell population and a more highly differentiated population of progenitor cells, revealed a greater number of the progenitor cells within the bone of Y2-/- mice. Analysis of Y receptor transcripts in cultured stromal cells from wild-type mice revealed high levels of Y1 but not Y2, Y4, Y5, or y6 receptor mRNA. Interestingly, germ line Y2 receptor deletion causes Y1 receptor down-regulation in stromal cells and bone tissue possibly due to the lack of feedback inhibition of NPY release and subsequent overstimulation of Y1 receptors. Furthermore, deletion of Y1 receptors resulted in increased bone mineral density in mice. Together, these findings indicate that the greater number of mesenchymal progenitors and the altered Y1 receptor expression within bone cells in the absence of Y2 receptors are a likely mechanism for the greater bone mineralization in vivo and in vitro, opening up potential new treatment avenues for osteoporosis.

Prospective Isolation of Mesenchymal Stem Cells from Mouse Compact Bone

Bone marrow from numerous species, including rodents and man, has been shown to contain a rare population of cells known as marrow stromal cells or mesenchymal stem cells (MSC). Given the innate ability of these cells to give rise to multiple tissue types including bone, fat and cartilage, there is considerable interest in utilizing MSC in a broad repertoire of cell-based therapies for the treatment of human disease. In order for such therapies to be realized, a preclinical animal model in which to refine strategies utilizing MSC is required.We have described methodology allowing for the prospective isolation by fluorescence activated cell sorting (FACS) of a highly purified population of MSC from murine compact bone (CB). These cells are multipotent and capable of extensive proliferation in vitro and thus represent an ideal source of cells with which to explore both the fundamental biology of MSC and their efficacy in a variety of cellular therapies.

BMP inhibition stimulates WNT-dependent generation of chondrogenic mesoderm from embryonic stem cells

WNT and bone morphogenetic protein (BMP) signaling are known to stimulate hemogenesis from pluripotent embryonic stem (ES) cells. However, osteochondrogenic mesoderm was generated effectively when BMP signaling is kept to a low level, while WNT signaling was strongly activated. When mesoderm specification from ES cells was exogenous factor dependent, WNT3a addition supported the generation of cardiomyogenic cells expressing lateral plate/extraembryonic mesoderm genes, and this process involved endogenous BMP activities. Exogenous BMP4 showed a similar effect that depended on endogenous WNT activities. However, neither factor induced robust chondrogenic activity. In support, ES cell differentiation in the presence of either WNT3a or BMP4 was associated with elevated levels of both Bmp and Wnt mRNAs, which appeared to provide sufficient levels of active BMPs and WNTs to promote the nonchondrogenic mesoderm specification. The osteochondrogenic mesoderm expressed PDGFRalpha, which also expressed genes that mark somite and rostral presomitic mesoderm. A strong WNT signaling was required for generating the mesodermal progeny, while approximately 50- to 100-fold lower concentration of WNT3a was sufficient for specifying axial mes(end)oderm. Thus, depending on the dose and cofactor (BMP), WNT signaling stimulates the generation of different biological activities and specification of different types of mesodermal progeny from ES cells.

Developmental Fate Determination and Marker Discovery in Hematopoietic Stem Cell Biology Using Proteomic Fingerprinting

In hematopoiesis, co-expression of Sca-1 and c-Kit defines cells (LS+K) with long term reconstituting potential. In contrast, poorly characterized LS−K cells fail to reconstitute lethally irradiated recipients. Relative quantification mass spectrometry and transcriptional profiling were used to characterize LS+K and LS−K cells. This approach yielded data on >1200 proteins. Only 32% of protein changes correlated to mRNA modulation demonstrating post-translational protein regulation in early hematopoietic development. LS+K cells had lower expression of protein synthesis proteins but did express proteins associated with mature cell function. Major increases in erythroid development proteins were observed in LS−K cells; based on this assessment of erythroid potential we showed them to be principally erythroid progenitors, demonstrating effective use of discovery proteomics for definition of primitive cells.

Isolation of the stromal-vascular fraction of mouse bone marrow markedly enhances the yield of clonogenic stromal progenitors

The low incidence of CFU-F significantly complicates the isolation of homogeneous populations of mouse bone marrow stromal cells (BMSCs), a common problem being contamination with hematopoietic cells. Taking advantage of burgeoning evidence demonstrating the perivascular location of stromal cell stem/progenitors, we hypothesized that a potential reason for the low yield of mouse BMSCs is the flushing of the marrow used to remove single-cell suspensions and the consequent destruction of the marrow vasculature, which may adversely affect recovery of BMSCs physically associated with the abluminal surface of blood vessels. Herein, we describe a simple methodology based on preparation and enzymatic disaggregation of intact marrow plugs, which yields distinct populations of both stromal and endothelial cells. The recovery of CFU-F obtained by pooling the product of each digestion (1631.8 + 199) reproducibly exceeds that obtained using the standard BM flushing technique (14.32 + 1.9) by at least 2 orders of magnitude (P < .001; N = 8) with an accompanying 113.95-fold enrichment of CFU-F frequency when plated at low oxygen (5%). Purified BMSC populations devoid of hematopoietic contamination are readily obtained by FACS at P0 and from freshly prepared single-cell suspensions. Furthermore, this population demonstrates robust multilineage differentiation using standard in vivo and in vitro bioassays.

Aryl hydrocarbon receptor–dependent enrichment of a megakaryocytic precursor with a high potential to produce proplatelets

The mechanisms regulating megakaryopoiesis and platelet production (thrombopoiesis) are still incompletely understood. Identification of a progenitor with enhanced thrombopoietic capacity would be useful to decipher these mechanisms and to improve our capacity to produce platelets in vitro. Differentiation of peripheral blood CD34(+) cells in the presence of bone marrow-human mesenchymal stromal cells (MSCs) enhanced the production of proplatelet-bearing megakaryocytes (MKs) and platelet-like elements. This was accompanied by enrichment in a MK precursor population exhibiting an intermediate level of CD41 positivity while maintaining its expression of CD34. Following sorting and subculture with MSCs, this CD34(+)CD41(low) population was able to efficiently generate proplatelet-bearing MKs and platelet-like particles. Similarly, StemRegenin 1 (SR1), an antagonist of the aryl hydrocarbon receptor (AhR) transcription factor known to maintain CD34 expression of progenitor cells, led to an enriched CD34(+)CD41(low) fraction and to an increased capacity to generate proplatelet-producing MKs and platelet-like elements ultrastructurally and functionally similar to circulating platelets. The effect of MSCs, like that of SR1, appeared to be mediated by an AhR-dependent mechanism because both culture conditions resulted in repression of its downstream effector CYP1B1. This newly described isolation of a precursor exhibiting strong MK potential could be exploited to study normal and abnormal thrombopoiesis and for in vitro platelet production.