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Hematopoietic stem cells (HSCs) respond to injury by rapidly proliferating and regenerating the hematopoietic system. Little is known about the intracellular programs that are activated within HSCs during this regenerative process and how this response may be influenced by alterations in signals from the injured microenvironment. Here we have examined the regenerating microenvironment and find that following injury it has an enhanced ability to support HSCs. During this regenerative phase, both hematopoietic and stromal cell elements within the bone marrow microenvironment show increased expression of Wnt10b, which can function to enhance growth of hematopoietic precursors. In addition, regenerating HSCs show increased activation of Wnt signaling, suggesting that microenvironmental changes in Wnt expression after injury may be integrated with the responses of the hematopoietic progenitors. Cumulatively, our data reveal that growth signals in the hematopoietic system are re-activated during injury, and provide novel insight into the influence of the microenvironment during regeneration. Disclosure of potential conflicts of interest is found at the end of this article.

The hemopoietic microenvironment consists of a diverse repertoire of cells capable of providing signals that influence hemopoietic stem cell function. Although the role of osteoblasts and vascular endothelial cells has recently been characterized, the function of the most abundant cell type in the bone marrow, the adipocyte, is less defined. Given the emergence of a growing number of adipokines, it is possible that these factors may also play a role in regulating hematopoiesis. Here, we investigated the role of adiponectin, a secreted molecule derived from adipocytes, in hemopoietic stem cell (HSC) function. We show that adiponectin is expressed by components of the HSC niche and its’ receptors AdipoR1 and AdipoR2 are expressed by HSCs. At a functional level, adiponectin influences HSCs by increasing their proliferation, while retaining the cells in a functionally immature state as determined by in vitro and in vivo assays. We also demonstrate that adiponectin signaling is required for optimal HSC proliferation both in vitro and in long term hemopoietic reconstitution in vivo. Finally we show that adiponectin stimulation activates p38 MAPK, and that inhibition of this pathway abrogates adiponectin’s proliferative effect on HSCs. These studies collectively identify adiponectin as a novel regulator of HSC function and suggest that it acts through a p38 dependent pathway.

In an effort to identify the molecular changes during hematopoietic stem cell (HSC) mobilization we have analyzed genome wide changes in gene expression in HSCs in response to the chemotherapeutic agent cyclophosphamide (Cy) and the growth factor granulocyte colony stimulating factor (G). Cy/G treatment leads to an initial loss of proliferating precursors, followed by a rapid expansion of stem and progenitor cells and their migration to the peripheral blood. While this approach has been capitalized on for harvesting hematopoietic stem cells in clinical therapy, the molecular regulation of this process remains less well understood. To analyze changes in gene expression that occur in vivo as HSCs proliferate and prepare to migrate out of the bone marrow, we enriched for HSC fractions by isolating cells that express c-kit (K), Sca-1(S) and low levels of Thy1.1(T) but do not express lineage (L) markers (KTLS) from untreated mice and mice treated with Cy, G or Cy/G. The gene expression profile of these cells was examined by microarray analysis, relative to HSCs isolated from untreated mice. Interestingly, among the genes upregulated by Cy/G, a large majority (>60%) were a consequence of synergistic activity between Cy and G, while the rest of the upregulated genes could be attributed to activation by Cy or G alone. To test whether this screen allowed identification of novel candidates that regulate HSC function we analyzed the role of transforming growth factor beta induced gene (Big-h3), a gene highly upregulated after Cy/G treatment. Big-h3 is an extracellular matrix protein that mediates the adhesion and migration of various cell types, however its role in HSCs is unknown. Our experiments indicate that overexpression of Big-h3 in HSCs leads to enhanced adhesion to fibronectin as well as upregulation of specific integrins. These experiments suggest that changes in adhesion are an integral component of HSC mobilization, and that Big-h3 is one regulator of such processes. Further functional analysis of other candidates will likely lead to identification of other novel regulators of HSC development and mobilization.