Osteopontin is a hematopoietic stem cell niche component that negatively regulates stem cell pool size

Stem cells reside in a specialized niche that regulates their abundance and fate. Components of the niche have generally been defined in terms of cells and signaling pathways. We define a role for a matrix glycoprotein, osteopontin (OPN), as a constraining factor on hematopoietic stem cells within the bone marrow microenvironment. Osteoblasts that participate in the niche produce varying amounts of OPN in response to stimulation. Using studies that combine OPN-deficient mice and exogenous OPN, we demonstrate that OPN modifies primitive hematopoietic cell number and function in a stem cell–nonautonomous manner. The OPN-null microenvironment was sufficient to increase the number of stem cells associated with increased stromal Jagged1 and Angiopoietin-1 expression and reduced primitive hematopoietic cell apoptosis. The activation of the stem cell microenvironment with parathyroid hormone induced a superphysiologic increase in stem cells in the absence of OPN. Therefore, OPN is a negative regulatory element of the stem cell niche that limits the size of the stem cell pool and may provide a mechanism for restricting excess stem cell expansion under conditions of niche stimulation

Osteopontin as a Mediator of NKT Cell Function in T Cell-Mediated Liver Diseases

AbstractBoth osteopontin (OPN) and natural killer T (NKT) cells play a role in the development of immunological disorders. We examined a functional link between OPN and NKT cells. Concanavalin A (Con A)-induced hepatitis is a well-characterized murine model of T cell-mediated liver diseases. Here, we show that NKT cells secrete OPN, which augments NKT cell activation and triggers neutrophil infiltration and activation. Thus, OPN- and NKT cell-deficient mice were refractory to Con A-induced hepatitis. In addition, a neutralizing antibody specific for a cryptic epitope of OPN, exposed by thrombin cleavage, ameliorated hepatitis. These findings identify NKT cell-derived OPN as a novel target for the treatment of inflammatory liver diseases

The role of osteopontin in tendon tissue remodeling after denervation-induced mechanical stress deprivation

It has been shown that musculoskeletal tissues undergo dynamic tissue remodeling by a process that is quite sensitive to the mechanical environment. However, the detailed molecular mechanism underlying this process remains unclear. We demonstrate here that after denervation-induced mechanical stress deprivation, tendons undergo dynamic tissue remodeling as evidenced by a significant reduction of the collagen fibril diameter. Importantly, the transient up-regulation of osteopontin (OPN) expression was characteristic during the early phase of tendon tissue remodeling. Following this dynamic change of OPN expression, matrix metalloproteinase (MMP)-13 expression was induced, which presumably accounts for the morphological changes of tendon by degrading tendon collagen fibrils. The modulation of MMP-13 expression by OPN was specific, since the expression of MMP-2, which is also known to be involved in tissue remodeling, did not alter in the tendons under the absence or presence of OPN. We also demonstrate that the modulation of MMP-13 expression by OPN is due to the signaling through cell surface receptors for OPN. Thus, we conclude that OPN plays a crucial role in conveying the effect of denervation-induced mechanical stress deprivation to the tendon fibroblasts to degrade the extracellular matrices by regulating MMP-13 expression in tendon fibroblasts

Override of the Osteoclast Defect in Osteopontin-Deficient Mice by Metastatic Tumor Growth in the Bone

Osteopontin (OPN) is a major noncollagenous protein of bone that is frequently up-regulated in tumors, where it enhances tumor growth. OPN-deficient mice are resistant to stimulated bone resorption, including that occurring after ovariectomy. Using a new syngeneic model of bone metastasis (r3T), we examined whether OPN-deficient mice are similarly resistant to bone loss resulting from osteolytic tumor growth. Transformed mammary epithelial cells, r3T, which express parathyroid hormone-related protein but not receptor activator of nuclear factor-κB ligand, were injected via the intracardiac route into both wild-type and OPN(−/−) mice. We measured tumor burden in the bone by quantitative polymerase chain reaction assay and evaluated bone loss by X-ray and microCT. Unexpectedly, bone loss was similar in OPN(−/−) and wild-type mice bearing similar-sized tumors. Osteoclast number was comparable in both genotypes, and the expression of bone sialoprotein was similar in tumor-bearing bones of both genotypes, excluding two potential mechanisms of overriding the defect. Taken together, these results indicate that in the absence of OPN, the bone loss associated with tumor growth at the bone site proceeds rapidly despite the osteoclast defects documented in OPN(−/−) mice, suggesting that the mechanism of bone loss due to tumor growth differs from that occurring in other pathologies

KC is degraded by proteases in the EP infected chambers.

<p>(A) Protease activity in chamber fluid from EP or S. mitis infected chambers 24 h after infection. Increased fluorescence results from cleavage of the substrate Val-Leu-Lys-AMC. Inh - reaction performed in the presence of protease inhibitor cocktail. (B) Protease activity (slope of the linear portion of the curve; change in fluorescence/minute) in EP and S. mitis chamber fluid, n = 3 individual mice. (C) Western blot of KC levels after incubation in chamber fluid. 25 ng of KC was added to 10 μl of chamber fluid and incubated for 14 h in the presence or absence of protease inhibitor cocktail (Inh), then separated on a 4–20% SDS Page gel. KC runs at about 15 kD; the lower band (*) is likely a degradation product. Representative results of three independent determinations. Average values (% KC detected without vs with protease inhibitor) were 34.50 ± 6.054 for EP and 119.0 ± 25.95 for S. mitis, p<0.05.</p

Summary of expression of all cytokines measured.

<p>^a) P value (EP vs <i>S</i>. <i>mitis</i>) determined by unpaired t-test.</p><p>^b) Acute responder cytokines are defined as those expressed significantly (p<0.001) higher at 2 h than at 12 h after infection. EP- endodontic pathogens, Sm–<i>S</i>. <i>mitis</i>.</p><p>Summary of expression of all cytokines measured.</p