Determining the affinity and stoichiometry of interactions between unmodified proteins in solution using Biacore.

We describe a general Biacore method for measuring equilibrium binding affinities and stoichiometries for interactions between unmodified proteins and their unmodified ligands free in solution. Mixtures of protein and ligand are preequilibrated at different ratios in solution and then analyzed by Biacore using a sensor chip surface that detects only unbound analyte. Performing the Biacore analysis under mass transport limited conditions allows the concentration of unbound analyte to be determined from the initial velocity of binding. Plots of initial velocity versus the concentration of the varied binding partner are fitted to a quadratic binding equation to give the affinity and stoichiometry of binding. We demonstrate the method using soluble Her2 extracellular domain binding to monovalent, bivalent, and trivalent forms of an anti-Her2 antibody. The affinity we measured agrees with that obtained from conventional Biacore kinetic analysis, and the stoichiometries for the resulting 1:1, 1:2, and 1:3 complexes were confirmed by gel filtration with in-line light scattering. The method is applicable over an affinity range of approximately 100 pM to 1 µM and is particularly useful when there is concern that covalently modifying one or the other binding partner might affect its binding properties or where multivalency might otherwise complicate a quantitative analysis of binding.

Tumor necrosis factor family receptors regulating bone turnover: new observations in osteoblastic and osteoclastic cell lines.

While the tumor necrosis factor (TNF) family members RANKL and TNF-alpha are critical regulators of osteoclast formation, functions of other TNFs in bone are poorly understood. Here we consider the roles in regulating bone turnover of TNF receptors (TNF-R) also expressed by osteoblasts and osteoblast precursors. TNF receptors in osteoblasts and preosteoblasts include TNFR1 (p55), DR3 (TNFR25), DR5 (TRAIL-R2) and Fas, and possibly FN14 and DR4 (TRAIL-R1). Osteoblasts also produce soluble TNF receptors, DcR2, osteoprotegerin, and sDR3; these bind the TNFs TRAIL, RANKL, TL1A, and Apo3L and block ligand effects on cell surface receptors. Activation of DR3 regulates osteoblast maturation and may control the decision to exit the pool of differentiation-competent preosteoblasts. A major natural ligand for DR3, TL1A, is produced by vascular cells adjacent to differentiating osteoblasts and possibly by Fcgamma-stimulated osteoclast precursors. The activity of DR3 is regulated by osteoblast production of its soluble DR3 splice variant. Activation of TNFR1 or DR5 by TNF-alpha or TRAIL may regulate osteoblast connectivity, which is important to bone turnover. When there is a source for Fas ligand, such as an inflammatory infiltrate, activation of Fas may lead to apoptosis of any bone cell. TNF receptors are thus implicated in multiple aspects of bone turnover.

Serum phosphate and lactate vary with FSH in an early postmenopausal population.

OBJECTIVE: We examined serum in recent postmenopausal women to determine the relationship of menopausal status, as FSH level, to serum acid-base balance. DESIGN, SETTING, AND PATIENTS: Serum electrolytes of 58 women, aged 53-58, were measured relative to serum FSH. The subjects were over one year since the last menstrual period and were from an academic practice setting. RESULTS: In women with FSH <35 IU/L (n=20, mean 16.6 IU/L, SD 6.8), phosphate and lactate were reduced relative to women with FSH >35 IU/L (n=38, mean 84.8 IU/L, SD 34.5). No other major anions showed significant differences. Both groups were analyzed by mass spectroscopy for fatty acids and anionic metabolic intermediates. Lactate was the predominant anion in the organic group but accounted for only about 10% of the FSH-responsive anion change. This change was mainly due to a 0.1-mM increase in phosphate in the high FSH group. CONCLUSIONS: There is a direct correlation of early postmenopausal FSH to increasing serum phosphate. Changes in phosphorus may reflect differences in the rate of bone loss.

CHDL: a cadherin-like domain in Proteobacteria and Cyanobacteria.

We identified a cadherin-like domain (CHDL) using computational analysis. The CHDL domain is mostly distributed in Proteobacteria and Cyanobacteria, although it is also found in some eukaryotic proteins. Prediction of three-dimensional protein folding indicated that the CHDL domain has an immunoglobulin beta-sandwich fold and belongs to the cadherin superfamily. The CHDL domain does not have LDRE and DxNDN motifs, which are conserved in the cadherin domain, but has three other motifs: PxAxxD, DxDxD and YT-V/I-S/T-D, which might contribute to forming a calcium-binding site. The identification of this cadherin-like domain indicates that the cadherin superfamily may exhibit wider sequence and structural diversity than previously appreciated. Domain architecture analysis revealed that the CHDL domain is also associated with other adhesion domains as well as enzyme domains. Based on computational analysis and previous experimental data, we predict that the CHDL domain has calcium-binding and also carbohydrate-binding activity.

In vitro differentiation of CD14 cells from osteopetrotic subjects: contrasting phenotypes with TCIRG1, CLCN7, and attachment defects.

UNLABELLED: We studied osteoclastic differentiation from normal and osteopetrotic human CD14 cells in vitro. Defects in acid transport, organic matrix removal, and cell fusion with deficient attachment were found. Analysis of genotypes showed that TCIRG1 anomalies correlated with acid transport defects, but surprisingly, organic matrix removal failure correlated with CLCN7 defects; an attachment defect had normal TCIRG1 and CLCN7. INTRODUCTION: Osteopetrotic subjects usually have normal macrophage activity, and despite identification of genetic defects associated with osteopetrosis, the specific developmental and biochemical defects in most cases are unclear. Indeed, patients with identical genotypes often have different clinical courses. We classified defects in osteoclast differentiation in vitro using four osteopetrotic subjects without immune or platelet defects, three of them severe infantile cases, compared with normals. MATERIALS AND METHODS: Osteoclast differentiation used isolated CD14 cells; results were correlated with independent analysis of two key genes, CLCN7 and TCIRG1. CD14 cell attachment and cell surface markers and extent of differentiation in RANKL and colony-stimulating factor (CSF)-1 were studied using acid secretion, bone pitting, enzyme, and attachment proteins assays. RESULTS AND CONCLUSIONS: CD14 cells from all subjects had similar lysosomal and nonspecific esterase activity. With the exception of cells from one osteopetrotic subject, CD14 cells from osteopetrotic and control monocytes attached similarly to bone or tissue culture substrate. Cells from one osteopetrotic subject, with normal CLCN7 and TCIRG1, did not attach to bone, did not multinucleate, and formed no podosomes or actin rings in RANKL and CSF-1. Attachment defects are described in osteopetrosis, most commonly mild osteopetrosis with Glantzman's thrombasthenia. However, this case, with abnormal integrin alphavbeta3 aggregates and no osteoclasts, seems to be unique. Two subjects were compound heterozygotes for TCIRG1 defects; both had CD14 cells that attached to bone but did not acidify attachments; cell fusion and attachment occurred, however, in RANKL and CSF-1. This is consistent with TCIRG1, essential for H+-ATPase assembly at the ruffled border. A compound heterozygote for CLCN7 defects had CD14 cells that fused in vitro, attached to bone, and secreted acid, TRACP, and cathepsin K. However, lacunae were shallow and retained demineralized matrix. This suggests that CLCN7 may not limit H+-ATPase activity as hypothesized, but may be involved in control of organic matrix degradation or removal.

Expression and function of TNF-family proteins and receptors in human osteoblasts.

We studied how tumor necrosis-factor (TNF)-family proteins interact with osteoblasts to resolve several controversial points. We measured expression of TNFs, TNF-receptors, and nonsignaling (decoy) TNF receptors in human osteoblasts derived from mesenchymal stem cells and in MG63 human osteosarcoma cells using unamplified mRNA screening, with secondary Western or PCR analysis where indicated, and studied the effects of TNFs on osteoblasts in cell culture. Expression of TNFs and receptors was similar in MG63 cells and osteoblasts. TNF-R1 (p55), TRAIL receptor 1 and 2 (DR4 and 5), and Fas were expressed; RANK was undetectable. TNF-family ligands RANKL, TRAIL, and TNFalpha were expressed, but mRNAs were typically at low levels relative to receptors, suggesting that osteoblastic TNF signals, including RANKL, require specific stimuli. Flow cytometry of MG63 cells confirmed TNFalpha receptors and identified subpopulations with high surface-bound TNFalpha. Decoy receptors expressed included a novel soluble form of TNFRSF25 (formerly DR3 or Apo3), implicated in rheumatoid-arthritis linkage studies, as well as osteoprotegerin, a well-characterized osteoblast protein that binds TRAIL and RANKL, and DcR2, which binds TRAIL. Osteoblast apoptosis was studied using terminal deoxynucleotidyl transferase labeling and annexin V binding. MG63 cells were resistant to apoptosis by exogenous TNFalpha except when grown in media promoting osteoblast-like growth or matrix nodules. However, in media supporting osteoblast-like phenotype, apoptosis was induced by anti-Fas or TNF, in contrast to other studies with human osteoblasts. TRAIL caused cell retraction, supporting functional TRAIL response in cell differentiation, but did not cause apoptosis. We conclude that human osteoblasts have functional receptors for FasL, TNFalpha, TRAIL, but not RANKL, and that osteoblasts are protected by multiple nonsignaling TNF receptors against destruction by TNF-family proteins under conditions favoring cell growth.

Tenascin cytotactin epidermal growth factor-like repeat binds epidermal growth factor receptor with low affinity.

Select epidermal growth factor (EGF)-like (EGFL) repeats of human tenascin cytotactin (tenascin C) can stimulate EGF receptor (EGFR) signaling, but activation requires micromolar concentrations of soluble EGFL repeats in contrast to subnanomolar concentrations of classical growth factors such as EGF. Using in silico homology modeling techniques, we generated a structure for one such repeat, the 14th EGFL repeat (Ten14). Ten14 assumes a tight EGF-like fold with truncated loops, consistent with circular dichroism studies. We generated bound structures for Ten14 with EGFR using two different approaches, resulting in two distinctly different conformations. Normal mode analysis of both structures indicated that the binding pocket of EGFR exhibits a significantly higher mobility in Ten14-EGFR complex compared to that of the EGF-EGFR complex; we hypothesized this may be attributed to loss of key high-affinity interactions within the Ten14-EGFR complex. We proved the efficacy of our in silico models by in vitro experiments. Surface plasmon resonance measurements yielded equilibrium constant K(D) of 74 microM for Ten14, approximately three orders of magnitude weaker than that of EGF. In accordance with our predicted bound models, Ten14 in monomeric form does not bind EGFR with sufficient stability so as to induce degradation of receptor, or undergo EGFR-mediated internalization over either the short (20 min) or long (48 h) term. This transient interaction with the receptor on the cell surface is in marked contrast to other EGFR ligands which cause EGFR transit through, and signaling from intracellular locales in addition to cell surface signaling.

Death receptor-3 mediates apoptosis in human osteoblasts under narrowly regulated conditions.

We previously reported that a soluble form of the TNF-family receptor death receptor-3 (DR3) is expressed in osteoblasts. DR3 regulates death or differentiation in other tissues, and DR3 ligands occur in bone, but the function of DR3 in the osteoblast was unknown. We studied the expression of DR3 and the effects crosslinking antibodies to DR3 or of natural DR3 ligands in human osteoblasts. Western analysis showed that nontransformed osteoblasts and the MG63 osteosarcoma cell line produce both soluble decoy receptor and transmembrane isoforms of DR3. Cell surface labeling showed that low and high DR3-expressing osteoblast populations occur. Verification of by cloning showed a point mutation in DR3 from MG63 cells. Activation of DR3 by antibody crosslinking or with DR3 ligands caused apoptosis in osteoblasts and in MG63 cells, but only in low-density cell cultures. In dense cultures apoptosis did not occur, but nuclear factor-kappaB nuclear translocation was observed under some conditions. Crosslinking of DR3 in high-density MG63 cultures blocked expression of bone matrix elements. DR3 activation in high-density nontransformed osteoblasts had only minor effects on cell maturation. We conclude that DR3 activation can mediate apoptosis in osteoblasts. Its activity is, however, highly restricted by its soluble ligand-binding isoform and possibly also by alternate survival signals. In the presence of survival signals, DR3 may affect cell maturation although effects on differentiation were clearly seen only in the MG63 transformed cell line.

Comparative modeling of TNFRSF25 (DR3) predicts receptor destabilization by a mutation linked to rheumatoid arthritis.

We constructed a three-dimensional model of TNFRSF25 (death receptor-3; DR3), a tumor necrosis-receptor family member that is expressed on immune cells and on osteoblasts, to determine whether mutations that are linked to rheumatoid arthritis are likely to have effects on receptor function. Since the crystal structure of DR3 is not known, comparative modeling was used, aligning structural elements of the primary sequences of DR3 with TNFs which have been determined by crystallography, substituting the amino acids of the target protein for those in the known structure, introducing necessary deletions or insertions, followed by energy minimization to yield a putative structure. This approach has been validated by studies of other TNF-family receptors. The results show that the DR3 extracellular domain is comprised of four homologous cysteine-rich domains (CRDs), and that a mutation linked to rheumatoid arthritis is in a region critical for structural integrity of ligand-receptor complexes at the end of CRD3. Specifically, the D158G mutation eliminates two hydrogen bonds normally present in a N/D-T-V/D-C consensus motif typically found flanking the last cysteine of each CRD. This may cause aberrations in either T cell function or in response of bone cells to DR3 ligands, which may contribute to pathology in rheumatoid arthritis. Comparison of RA mutants to mutants in other TNFRSF receptors shows that these occur in homologous positions in CRDs, so that this site is proposed to be a 'hot spot' for mutations in TNFRSF family proteins.

Negative regulation of RANKL-induced osteoclastic differentiation in RAW264.7 Cells by estrogen and phytoestrogens.

We studied estrogen effects on osteoclastic differentiation using RAW264.7, a murine monocytic cell line. Differentiation, in response to RANKL and colony-stimulating factor 1, was evaluated while varying estrogen receptor (ER) stimulation by estradiol or nonsteroidal ER agonists was performed. The RAW264.7 cells were found to express ERalpha but not ERbeta. In contrast to RANKL, which decreased ERalpha expression and induced osteoclast differentiation, 10 nm estradiol, 3 microm genistein, or 3 microm daidzein all increased ERalpha expression, stimulated cell proliferation, and decreased multinucleation, with the effects of estrogen > or = daidzein > genistein. However, no estrogen agonist reduced RANKL stimulation of osteoclast differentiation markers or its down-regulation of ERalpha expression by more than approximately 50%. Genistein is also an Src kinase antagonist in vitro, but it did not decrease Src phosphorylation in RAW264.7 cells relative to other estrogen agonists. However, both phytoestrogens and estrogen inhibited RANKL-induced IkappaB degradation and NF-kappaB nuclear localization with the same relative potency as seen in proliferation and differentiation assays. This study demonstrates, for the first time, the direct effects of estrogen on osteoclast precursor differentiation and shows that, in addition to effecting osteoblasts, estrogen may protect bone by reducing osteoclast production. Genistein, which activates ERs selectively, inhibited osteoclastogenesis less effectively than the nonselective phytoestrogen daidzein, which effectively reproduced effects of estrogen.