Abstract
Osteoblasts support hematopoietic cell development, including B lymphopoiesis. We have previously shown that the nuclear factor of activated T cells (NFAT) negatively regulates osteoblast differentiation and bone formation. Interestingly, in smooth muscle, NFAT has been shown to regulate the expression of vascular cellular adhesion molecule-1 (VCAM-1), a mediator of cell adhesion and signaling during leukocyte development. To examine whether NFAT signaling in osteoblasts regulates hematopoietic development in vivo, we generated a mouse model expressing dominant-negative NFAT driven by the 2.3 kb fragment of the collagen-αI promoter to disrupt NFAT activity in osteoblasts (dnNFATOB). Bone histomorphometry showed that dnNFATOB mice have significant increases in bone volume (44%) and mineral apposition rate (131%) and decreased trabecular thickness (18%). In the bone microenvironment, dnNFATOB mice displayed a significant increase (87%) in Lineage−cKit+Sca-1+ (LSK) cells and significant decreases in B220+CD19−IgM− pre-pro-B cells (41%) and B220+CD19+IgM+ immature B cells (40%). Concurrent with these findings, LSK cell differentiation into B220+ cells was inhibited when cocultured on differentiated primary osteoblasts harvested from dnNFATOB mice. Gene expression and protein levels of VCAM-1 in osteoblasts decreased in dnNFATOB mice compared to controls. These data suggest that osteoblast-specific NFAT activity mediates early B lymphopoiesis, possibly by regulating VCAM-1 expression on osteoblasts.
Highlights
The family of nuclear factor of activated T-cell (NFAT) transcription factors is composed of five proteins (NFATc1– c4, NFAT-5) and is known for its role in T-cell development and differentiation [1, 2]
We demonstrate that the inhibition of NFAT activity in osteoblasts increases the number of hematopoietic stem/progenitor cells (HSPCs) and decreases the production of Blineage cells in dnNFATOB mice when compared to control mice
To determine whether NFAT signaling in osteoblasts regulates hematopoiesis in the bone marrow microenvironment, we generated collagen αI promoter (ColI)-dominant-negative form of NFAT (dnNFAT) transgenic mice
Summary
The family of nuclear factor of activated T-cell (NFAT) transcription factors is composed of five proteins (NFATc1– c4, NFAT-5) and is known for its role in T-cell development and differentiation [1, 2]. Previous studies have shown that Cn/NFAT signaling positively regulates osteoblast differentiation and bone formation, but these reports were conducted using animal models with global overexpression or knockout of calcineurin Aalpha and NFATc1 [5,6,7]. This model does not sufficiently represent the effects of Cn/NFAT signaling in osteoblasts because it is not an osteoblast-specific model. VCAM-1 gene expression and protein levels were reduced when NFAT activation was inhibited These results suggest that NFAT expression in osteoblasts regulates hematopoiesis in the bone marrow microenvironment, possibly by transcriptional regulation of VCAM-1
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