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TISSUE-SPECIFIC STEM CELLS

A Road Map Toward Defining the Role of Smad Signaling in Hematopoietic Stem Cells

^a Molecular Medicine and Gene Therapy, Institute of Laboratory Medicine and The Lund Strategic Research Center for Stem Cell Biology and Cell Therapy, Lund University Hospital, Lund, Sweden;
^b Umeå Center for Molecular Medicine, Umeå University, Umeå, Sweden

Key Words. Hematopoietic stem cells • Smad signaling • Transforming growth factor-ß • Bone morphogenic proteins • Activin

Correspondence: Stefan Karlsson, M.D., Ph.D., Molecular Medicine and Gene Therapy, Institute of Laboratory Medicine and The Lund Strategic Research Center for Stem Cell Biology and Cell Therapy, Lund University Hospital, BMC A12, 221 84, Lund, Sweden. Telephone: 46-46-222-0577, Fax: 46-46-222-0568; e-mail: Stefan.Karlsson{at}molmed.lu.se

Received on June 14, 2005; accepted for publication on December 9, 2005.

The transforming growth factor-ß (TGF-ß) superfamily encompasses the ligands and receptors for TGF-ß, bone morphogenic proteins (BMPs), and Activins. Cellular response to ligand is context-dependent and may be controlled by specificity and/or redundancy of expression of these superfamily members. Several pathways within this family have been implicated in the proliferation, differentiation, and renewal of hematopoietic stem cells (HSCs); however, their roles and redundancies at the molecular level are poorly understood in the rare HSC. Here we have characterized the expression of TGF-ß superfamily ligands, receptors, and Smads in murine HSCs and in the Lhx2-hematopoietic progenitor cell (Lhx2-HPC) line. We demonstrate a remarkable likeness between these two cell types with regard to expression of the majority of receptors and Smads necessary for the transduction of signals from TGF-ß, BMP, and Activin. We have also evaluated the response of these two cell types to various ligands in proliferation assays. In this regard, primary cells and the Lhx2-HPC line behave similarly, revealing a suppressive effect of Activin-A that is similar to that of TGF-ß in bulk cultures and no effect of BMP-4 on proliferation. Signaling studies that verify the phosphorylation of Smad2 (Activin and TGF-ß) and Smad1/5 (BMP) confirm cytosolic responses to these ligands. In addition to providing a thorough characterization of TGF-ß superfamily expression in HSCs, our results define the Lhx2-HPC line as an appropriate model for molecular characterization of Smad signaling.

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