Molecular regulation of osteoblasts for tissue engineered bone repair

Role of integrin subunits in mesenchymal stem cell differentiation and osteoblast maturation on graphitic carbon-coated microstructured surfaces

Surface roughness, topography, chemistry, and energy promote osteoblast differentiation and increase osteogenic local factor production in vitro and bone-to-implant contact in vivo, but the mechanisms involved are not well understood. Knockdown of integrin heterodimer alpha2beta1 (α2β1) blocks the osteogenic effects of the surface, suggesting signaling by this integrin homodimer is required. The purpose of the present study was to separate effects of surface chemistry and surface structure on integrin expression by coating smooth or rough titanium (Ti) substrates with graphitic carbon, retaining surface morphology but altering surface chemistry. Ti surfaces (smooth [Ra < 0.4 μm], rough [Ra ≥ 3.4 μm]) were sputter-coated using a magnetron sputtering system with an ultrapure graphite target, producing a graphitic carbon thin film. Human mesenchymal stem cells and MG63 osteoblast-like cells had higher mRNA for integrin subunits α1, α2, αv, and β1 on rough surfaces in comparison to smooth, and integrin αv on graphitic-carbon-coated rough surfaces in comparison to Ti. Osteogenic differentiation was greater on rough surfaces in comparison to smooth, regardless of chemistry. Silencing integrins β1, α1, or α2 decreased osteoblast maturation on rough surfaces independent of surface chemistry. Silencing integrin αv decreased maturation only on graphitic carbon-coated surfaces, not on Ti. These results suggest a major role of the integrin β1 subunit in roughness recognition, and that integrin alpha subunits play a major role in surface chemistry recognition.

Short-term administration of small molecule phenamil induced a protracted osteogenic effect on osteoblast-like MC3T3-E1 cells

Sustained administration (21-day treatment) of the small molecule phenamil has been proposed as an alternative osteogenic factor when used in conjunction with a biodegradable scaffold for in vitro osteogenesis. While promising, the major issue associated with small molecules is non-specific cytotoxicity. The aim of this study was to minimize the side-effects from small-molecule drugs by reducing the frequency of administration. Toward this goal, we investigated whether a shorter phenamil treatment is sufficient to induce in vitro osteogenesis. We compared the effects of short-term (12 h) and continuous treatments of phenamil on osteoblastic differentiation and mineralization. Alkaline phosphatase (ALP) and osteopontin (OPN) activity were used as markers for osteoblastic differentiation. Measurement of the calcium content of the extracellular matrix was used as the hallmark for in vitro bone formation after 21 days of culture. Our findings revealed that both short and continuous phenamil treatment triggers osteoblastic differentiation and mineralization of MC3T3-E1 cells on a biodegradable polymeric scaffold composed of polylactic-co-glycolic acid (PLAGA) at the same time points. In addition, in order to fabricate a phenamil-loaded PLAGA scaffold, the small molecule phenamil was physically absorbed onto the surface of scaffolds and the bioactivity of the loaded scaffolds was evaluated. Furthermore, biochemical analysis indicated that short phenamil treatment of cells was accompanied by upregulation in protein expression of integrin α5, p125(FAK) and phosphorylation of CREB. These effects may contribute to the downstream signalling cascade necessary for osteogenesis, and such responses may account for our observed protracted osteogenic differentiation in vitro. Copyright © 2013 John Wiley & Sons, Ltd.

Activation of cyclic amp/protein kinase: a signaling pathway enhances osteoblast cell adhesion on biomaterials for regenerative engineering

Osteoblast cell adhesion on biomaterials is an important goal for implants to be useful in bone regeneration technologies. The adhesion of osteoblastic cells to biomaterials has been investigated in the field of bone regenerative engineering. Previous work from our group demonstrated that osteoblastic cells adhering to biodegradable biomaterials require the expression of integrins on the cell surface. However, the underlying molecular signaling mechanism is still not fully clear. We report here that cyclic adenosine monophosphate (cAMP), a small signaling molecule, regulates osteoblast cell adhesion to biomaterial surfaces. We used an in vitro cell adhesion assay to demonstrate that at 0.1 mM, 8-Br-cAMP, a cell-permeable cAMP analog, significantly enhances osteoblast-like cells' (MC3T3-E1) adherence to biomaterials. Moreover, we demonstrate that a commonly used cAMP-elevating agent, forskolin, promotes cell adhesion similar to that of the cell permeable cAMP analog. By using different target-specific cAMP analogs: 8-CPT-2Me-cAMP which specifically activates exchange protein activated by cAMP (Epac), and 6-Bnz-cAMP which specifically activates protein kinase A (PKA), we observed that the PKA signaling pathway plays a dominant role in this process. Thus, this report suggests a new method to enhance osteoblast cell adhesion on biodegradable biomaterials for bone regenerative engineering applications.

Surface investigation on biomimetic materials to control cell adhesion: the case of RGD conjugation on PCL

The cell recognition of bioactive ligands immobilized on polymeric surfaces is strongly dependent on ligand presentation at the cell/material interface. While small peptide sequences such as Arg-Gly-Asp (RGD) are being widely used to obtain biomimetic interfaces, surface characteristics after immobilization as well as presentation of such ligands to cell receptors deserve more detailed investigation. Here, we immobilized an RGD-based sequence on poly(epsilon-caprolactone) (PCL), a largely widespread polymeric material used in biomedical applications, after polymer aminolysis. The surface characteristics along with the efficacy of the functionalization was monitored by surface analysis (FTIR-ATR, contact angle measurements, surface free energy determination) and spectrophotometric assays specially adapted for the analytical quantification of functional groups and/or peptides at the interface. Particular attention was paid to the evaluation of a number, morphology, and penetration depth of immobilized functional groups and/or peptides engrafted on polymeric substrates. In particular, a typical morphology in peptide distribution was evidenced on the surface raised from polymer crystallites, while a significant penetration depth of the engrafted molecules was revealed. NIH3T3 fibroblast adhesion studies verified the correct presentation of the ligand with enhanced cell attachment after peptide conjugation. Such work proposes a morphological and analytical approach in surface characterization to study the surface treatment and the distribution of ligands immobilized on polymeric substrates.

Human osteoblast cells: Isolation, characterization, and growth on polymers for musculoskeletal tissue engineering

We performed a detailed examination of the isolation, characterization, and growth of human osteoblast cells derived from trabecular bone. We further examined the morphology, phenotypic gene expression, mineralization,and growth of these human osteoblasts on polyester polymers used for musculoskeletal tissue engineering. Polylactic-co-glycolic acid [PLAGA (85:15, 50:50, 75:25)], and poly-lactic acid (L-PLA, D,L-PLA) were examined. The osteoblastic expression of key phenotypic markers osteocalcin, alkaline phosphatase, collagen, and bone sialoprotein at 4 and 8 weeks was examined. Reverse transcription-polymerase chain reaction studies revealed that trabecular-derived osteoblasts were positive for all markers evaluated with higher levels expressed over long-term culture. These cells also revealed mineralization and maturation as evidenced by energy dispersive X-ray analysis and scanning electron microscopy. Growth studies on PLAGA at 50:50,75:25, and 85:15 ratios and PLA in the L and DL isoforms revealed that human osteoblasts actively grew, with significantly higher cell numbers attached to scaffolds composed of PLAGA 50:50 in the short term and PLAGA 85:15 in the long term compared with PLA (p < 0.05). We believe human cell adhesion among these polymeric materials may be dependent on differences in cellular integrin expression and extracellular matrix protein elaboration.

Comparison of Carbonate Apatite and .BETA.-tricalcium Phosphate (Resorbable Calcium Phosphates) Implanted Subcutaneously into the Back of Rats

Bioresorption and biocompatibility of carbonate apatites, both sintered and non-sintered (S-CAP and N-CAP), and of sintered beta-tricalcium phosphate (beta-TCP) were compared by implanting particles of these materials into the back of adult rats. Bioresorption--when evaluated non-destructively with non-decalcified tissues using microfocus X-ray tomography--was essentially the same for N-CAP and beta-TCP, while S-CAP exhibited statistically lower bioresorption at 2, 4, and 12 weeks postoperatively. Biocompatibility--when evaluated by ED1 immunostaining--was in the order of beta-TCP > N-CAP > S-CAP. The intensity of ED1 immunostaining decreased with time, but persisted longer in beta-TCP than in S-CAP and N-CAP, indicating that beta-TCP produced the strongest and most enduring stimulation of macrophages. Although no statistical differences were found in tartrate-resistant acid phosphatase (TRAP) staining among the materials at each implantation period, the degree of TRAP staining for S-CAP was statistically greater at 12 weeks than at 2 and 4 weeks, indicating that osteoclast-like cells were in part responsible for the resorption of the carbonate apatite.