Osteopontin adhesion receptors on gingival fibroblasts

Methods for studying tooth root cementum by light microscopy

The tooth root cementum is a thin, mineralized tissue covering the root dentin that is present primarily as acellular cementum on the cervical root and cellular cementum covering the apical root. While cementum shares many properties in common with bone and dentin, it is a unique mineralized tissue and acellular cementum is critical for attachment of the tooth to the surrounding periodontal ligament (PDL). Resources for methodologies for hard tissues often overlook cementum and approaches that may be of value for studying this tissue. To address this issue, this report offers detailed methodology, as well as comparisons of several histological and immunohistochemical stains available for imaging the cementum–PDL complex by light microscopy. Notably, the infrequently used Alcian blue stain with nuclear fast red counterstain provided utility in imaging cementum in mouse, porcine and human teeth. While no truly unique extracellular matrix markers have been identified to differentiate cementum from the other hard tissues, immunohistochemistry for detection of bone sialoprotein (BSP), osteopontin (OPN), and dentin matrix protein 1 (DMP1) is a reliable approach for studying both acellular and cellular cementum and providing insight into developmental biology of these tissues. Histological and immunohistochemical approaches provide insight on developmental biology of cementum.

Effects of irradiation on cementum matrix cytokins function during periodontal regeneration

The influence of gamma-ray irradiation on a cementum-impregnated gelatine membrane (CGM) was analyzed with emphasis on its function during periodontal regeneration. In brief, proteins were extracted from gamma-ray irradiated cementum (gammaC). With the gammaC protein, sample cells (gingival fibroblasts, periodontal ligament cells, and alveolar bone cells) were co-cultured, and cytological parameters (cell attachment, cell differentiation and alkaline phosphatase activity) were analyzed. Additionally, kinetics of some gene expression was analysed using reverse transcript RT-PCR, which included osteoproteogerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) mRNA. BMP-2 and osteonectin were resistant to gamma-rays, and other cytokines involved in regeneration were decreased. Thus, the attachment activity of osteoblasts to gammaC protein was higher than that of non-irradiated cementum (control C). The expression of OPG/OCIF mRNA was lower in co-cultured cells with gammaC protein than those with in control C protein. Together the results imply that some cytokine in intact cementum prevents the attachment (differentiation) of bone cells onto the root surface, which may explain why the introduction of CGM following gingival flap surgery induces new cementum, new ligament and new bone formation, but CGM irradiated with gamma-rays for clinical use causes ankylosis.

Upregulation of osteopontin expression in rat spinal cord microglia after traumatic injury

Osteopontin is a noncollagenous extracellular matrix protein that is expressed in various tissues. Recent studies have shown the upregulation of osteopontin expression in the ischemic cortex after cerebral infarction. We demonstrate here the upregulation of osteopontin expression in the spinal cord after compression injury. Laboratory rats were used in a compression model of spinal cord injury (30-g load for 5 min). Northern blot analysis showed that osteopontin mRNA expression levels reached a peak 3 days after injury (sevenfold; p < 0.05). In situ hybridization demonstrated osteopontin mRNA expression in necrotic areas from 24 h, peaking 3 days after injury. Immunohistochemistry detected osteopontin protein immunoreactivity from 12 h, peaking at 3 days. The peak time and distribution of osteopontin protein expression were coincident with those of osteopontin mRNA expression. Osteopontin expression in our model preceded that shown in the previously reported cerebral infarction models. Osteopontin protein was found in the cytoplasm at 3 days and secreted into the extracellular matrix at 7 days. Triple immunolabeling showed that osteopontin was localized in activated microglia surrounded by astrocytes.

CD44 is not an adhesive receptor for osteopontin

Osteopontin is a secreted glycoprotein with adhesive and migratory functions. Cellular interactions with osteopontin are mediated through integrin receptors which recognize the RGD domain. Recently, CD44, a non-integrin, multifunctional adhesion molecule was identified as an osteopontin receptor. CD44 is a ubiquitous surface molecule that exists as a number of different isoforms, generated by alternative splicing. To analyze which forms of CD44 mediate binding to osteopontin, we used the standard form of CD44 as CD44-human immunoglobulin fusion proteins and several splice variants in enzyme-linked immunosorbant assays. Multiple preparations of osteopontin were used including native osteopontin derived from smooth muscle cells, human urinary osteopontin, full-length recombinant osteopontin, and two recombinant osteopontin fragments expected to be formed following thrombin cleavage. Our data show that although the CD44-hlg fusion proteins could interact with hyaluronic acid as expected, there was no interaction between CD44H, CD44E, CD44v3,v8-v10, or CD44v3 with osteopontin. These studies suggest that CD44-osteopontin interactions may not be common in vivo and may be limited to a specific CD44 isoform(s), and/or a particular modified form of osteopontin.

Structural requirements for alpha 9 beta 1-mediated adhesion and migration to thrombin-cleaved osteopontin

Osteopontin is an RGD-containing glycoprotein that mediates adhesion and migration of a variety of different cell types. We recently showed that a recombinant osteopontin fragment that was expected to be formed following thrombin cleavage was not only biologically active, but also could support alpha 9 beta 1-mediated adhesion, an activity not found in the full-length molecule. In this study we defined binding sites on the N-terminal osteopontin fragment important for alpha 9 beta 1-mediated cell interactions. In addition to adhesion, we showed that alpha 9 beta 1 could mediate cell migration, a function not previously identified for this integrin. Using site-directed mutagenesis, we made specific mutations in the RGD region of the N-terminal osteopontin fragment. Mutation of RGD to RGE resulted in a 50% decrease in cell adhesion. The residual binding to the RGE mutant could be blocked by alpha 9 and beta 1 antibodies. Adhesion to the RGE mutant was due to residual recognition of the RGE site by alpha 9 beta 1 since mutants containing more drastic mutations in the RGD domain achieved by mutating RGD to RAA or by eliminating the RGD completely failed to support cell adhesion and migration. In contrast, alpha 9 beta 1-mediated adhesion to tenascin was RGD independent. These data demonstrate that alpha 9 beta 1 is one of the few integrin receptors that can interact with two distinct RGD-containing ligands through different adhesive domains.

Coordinate expression of OPN and associated receptors during monocyte/macrophage differentiation of HL-60 cells

Promyelocytic leukemia HL-60 cells promoted by PMA to differentiate along the monocyte pathway adhere to tissue culture plates. To explore the regulation of adhesion molecules in cells promoted to differentiate, the expression and secretion of osteopontin (OPN) and expression of associated cell surface receptors, CD44 and integrin subunits alpha(v), beta3, beta1, were examined. Results were as follows: 1) PMA induced OPN mRNA and OPN secretion into media; 2) untreated cells expressed beta1 and CD44 mRNA, and PMA induced alpha(v), and beta3 mRNA and increased beta1 and CD44 mRNA expression; 3) PMA increased levels of alpha(v), beta3, beta1 and CD44 protein on the cell surface; and 4) retinoic acid, which promotes granulocytic differentiation of HL-60 cells, did not affect OPN, alpha(v), beta3, beta1, or CD44 mRNA or protein expression. These data suggest that induction of OPN and associated receptors may play a role during monocytic differentiation of HL-60 cells.

Mechanical stimulation of osteopontin mRNA expression and synthesis in bone cell cultures

We have shown earlier that mechanical stimulation by intermittent hydrostatic compression (IHC) promotes alkaline phosphatase and procollagen type I gene expression in calvarial bone cells. The bone matrix glycoprotein osteopontin (OPN) is considered to be important in bone matrix metabolism and cell-matrix interactions, but its role is unknown. Here we examined the effects of IHC (13 kPa) on OPN mRNA expression and synthesis in primary calvarial cell cultures and the osteoblast-like cell line MC3T3-E1. OPN mRNA expression declined during control culture of primary calvarial cells, but not MC3T3-E1 cells. IHC upregulated OPN mRNA expression in late released osteoblastic cell cultures, but not in early released osteoprogenitor-like cells. Also, in both proliferating and differentiating MC3T3-E1 cells, OPN mRNA expression and synthesis were enhanced by IHC, differentiating cells being more responsive than proliferating cells. These results suggest a role for OPN in the reaction of bone cells to mechanical stimuli. The severe loss of OPN expression in primary bone cells cultured without mechanical stimulation suggests that disuse conditions down-regulate the differentiated osteoblastic phenotype.