Predominant integrin ligands expressed by osteoblasts show preferential regulation in response to both cell adhesion and mechanical perturbation

Osteogenic Differentiation of Human Mesenchymal Stromal Cells on Surface-Modified Titanium Alloys for Orthopedic and Dental Implants

Purpose

Surface properties of titanium alloys, used for orthopedic and dental applications, are known to affect implant interactions with host tissues. Osteointegration, bone growth and remodeling in the area surrounding the implants can be implemented by specific biomimetic treatments; these allow the preparation of micro/nanostructured titanium surfaces with a thickened oxide layer, doped with calcium and phosphorus ions. We have challenged these experimental titanium alloys with primary human bone marrow stromal cells to compare the osteogenic differentiation outcomes of the cells once they are seeded onto the modified surfaces, thus simulating a prosthetic device-biological interface of clinical relevance.

Methods

A specific anodic spark discharge was the biomimetic treatment of choice, providing experimental titanium disks treated with different alkali etching approaches. The disks, checked by electron microscopy and spectroscopy, were subsequently used as substrates for the proliferation and osteogenic differentiation of human cells. Expression of markers of the osteogenic lineage was assessed by means of qualitative and quantitative PCR, by cytochemistry, immunohistochemistry Western blot and matrix metalloprotease activity analyses.

Results

Metal surfaces were initially less permissive for cell growth. Untreated control substrates were less efficient in sustaining mineralized matrix deposition upon osteogenic induction of the cells. Interestingly, bone sialo protein and matrix metalloprotease 2 levels were enhanced on experimental metals compared to control surfaces, particularly for titanium oxide coatings etched with KOH.

Discussion

As a whole, the KOH-modification of titanium surfaces seems to allow the best osteogenic differentiation of human mesenchymal stromal cells, representing a possible plus for future clinical prosthetic applications.

Rheological properties, biocompatibility and in vivo performance of new hydrogel-based bone fillers

Three different heterologous substitutes for bone regeneration, manufactured with equine-derived cortical powder, cancellous chips and demineralized bone matrix granules, were compared in vitro and in vivo.

Differentiating zones at periodontal ligament-bone and periodontal ligament-cementum entheses

BACKGROUND AND OBJECTIVE

The structural and functional integrity of bone-periodontal ligament (PDL)-cementum complex stems from the load-bearing attachment sites (entheses) between soft (PDL) and hard (bone, cementum) tissues. These attachment sites are responsible for the maintenance of a bone-PDL-cementum complex biomechanical function. The objective was to investigate changes in spatiotemporal expression of key biomolecules in developing and functionally active entheses.

MATERIAL AND METHODS

Multilabeling technique was performed on hemimandibles of 3 wk and 3 mo-old scleraxis-GFP transgenic mice for CD146, CD31, NG2, osterix and bone sialoprotein. Regions of dominant stretch within the PDL were evaluated by identifying directionality of collagen fibrils, PDL fibroblasts and PDL cell cytoskeleton.

RESULTS

CD146+ cells adjacent to CD31+ vasculature were identified at PDL-bone enthesis. NG2+ cells were located at coronal bone-PDL and apical cementum-PDL entheses in the 3-wk-old group, but at 3 mo, NG2 was positive at the entheses of the apical region and alveolar crest. NG2 and osterix were colocalized at the osteoid and cementoid regions of the PDL-bone and PDL-cementum entheses. Bone sialoprotein was prominent at the apical region of 3-wk-old mice. The directionality of collagen fibers, fibroblasts and their cytoskeleton overlapped, except in the apical region of 3 wk.

CONCLUSION

Colocalization of biomolecules at zones of the PDL adjacent to attachment sites may be essential for the formation of precementum and osteoid interfaces at a load-bearing bone-PDL-tooth fibrous joint. Biophysical cues resulting from development and function can regulate recruitment and differentiation of stem cells potentially from a vascular origin toward osteo- and cemento-blastic lineages at the PDL-bone and PDL-cementum entheses. Investigating the coupled effect of biophysical and biochemical stimuli leading to cell differentiation at the functional attachment sites is critical for developing regeneration strategies to enable functional reconstruction of the periodontal complex.

Physiological mechanisms and therapeutic potential of bone mechanosensing

Skeletal loading is an important physiological regulator of bone mass. Theoretically, mechanical forces or administration of drugs that activate bone mechanosensors would be a novel treatment for osteoporotic disorders, particularly age-related osteoporosis and other bone loss caused by skeletal unloading. Uncertainty regarding the identity of the molecular targets that sense and transduce mechanical forces in bone, however, has limited the therapeutic exploitation of mechanosesning pathways to control bone mass. Recently, two evolutionally conserved mechanosensing pathways have been shown to function as "physical environment" sensors in cells of the osteoblasts lineage. Indeed, polycystin-1 (Pkd1, or PC1) and polycystin-2 (Pkd2, or PC2' or TRPP2), which form a flow sensing receptor channel complex, and TAZ (transcriptional coactivator with PDZ-binding motif, or WWTR1), which responds to the extracellular matrix microenvironment act in concert to reciprocally regulate osteoblastogenesis and adipogenesis through co-activating Runx2 and a co-repressing PPARγ activities. Interactions of polycystins and TAZ with other putative mechanosensing mechanism, such as primary cilia, integrins and hemichannels, may create multifaceted mechanosensing networks in bone. Moreover, modulation of polycystins and TAZ interactions identify novel molecular targets to develop small molecules that mimic the effects of mechanical loading on bone.

A Comparative Evaluation between New Ternary Zirconium Alloys as Alternative Metals for Orthopedic and Dental Prosthetic Devices

Purpose

We assessed in vitro the corrosion behavior and biocompatibility of four Zr-based alloys (Zr97.5 Nb1.5 VM1.0 ; VM, valve metal: Ti, Mo, W, Ta; at%) to be used as implant materials, comparing the results with grade-2 titanium, a biocompatible metal standard.

Methods

Corrosion resistance was investigated by open circuit potential and electrochemical impedance spectroscopy measurements as a function of exposure time to an artificial physiological environment (Ringer's solution). Human bone marrow stromal cells were used to evaluate biocompatibility of the alloys and their influence on growth kinetics and cell osteogenic differentiation through histochemical and gene expression analyses.

Results

Open circuit potential values indicated that Zr-based alloys and grade-2 Ti undergo spontaneous passivation in the simulated aggressive environment. High impedance values for all samples demonstrated improved corrosion resistance of the oxide film, with the best protection characteristics displayed by Zr97.5 Nb1.5 Ta1.0. Cells seeded on all surfaces showed the same growth kinetics, although matrix mineralization and alkaline phosphatase activity were maximal on Zr97.5 Nb1.5 Mo1.0 and Zr97.5 Nb1.5 Ta1.0. Markers of ongoing proliferation, however, such as podocalyxin and CD49f, were still overexpressed on Zr97.5 Nb1.5 Mo1.0 even upon osteoinduction. No relevant effects were noted for the CD146-expressing population of bone progenitors. Nonetheless, the presence of a more differentiated cell population on Zr97.5 Nb1.5 Ta1.0 samples was inferable by comparing mineralization data and transcript levels of osteogenic markers (osteocalcin, osteopontin, bone sialoprotein, and RUNX2).

Conclusions

The combination of passivation, corrosion resistance and satisfactory biotolerance to bone progenitors make the Zr-based alloys promising implant materials. Among those we tested, Zr97.5 Nb1.5 Ta1.0 seems to be the most appealing.

Targeting integrins to promote bone formation and repair

The ageing skeleton experiences a progressive decline in the rate of bone formation, which can eventually result in osteoporosis--a common disease characterized by reduced bone mass and altered bone microarchitecture which can result in fractures. One emerging therapy involves the identification of molecules that target bone-marrow mesenchymal stromal cells (MSCs) and promote their differentiation into osteoblasts, thereby counteracting bone loss. This Review highlights the discovery that some integrins, a family of heterodimeric transmembrane proteins that can interact with matrix proteins and generate intracellular signals, can be targeted to promote homing of MSCs to bone, osteogenic differentiation and bone formation. Specifically, priming of the α(5)β(1) integrin, which is required for osteoblastic differentiation of MSCs, leads to increased bone formation and improved bone repair in mice. Additionally, treatment with a peptidomimetic ligand of the α(4)β(1) integrin coupled to an agent with a high affinity for bone improves the homing of MSCs to bone and promotes osteoblast differentiation and bone formation, leading to increased bone mass in osteopenic mice. Strategies that target key integrins expressed by MSCs might, therefore, translate into improved therapies for age-related bone loss and possibly other disorders.

Study of the in vitro corrosion behavior and biocompatibility of Zr-2.5Nb and Zr-1.5Nb-1Ta (at%) crystalline alloys

The in vitro corrosion behavior and biocompatibility of two Zr alloys, Zr-2.5Nb, employed for the manufacture of CANDU reactor pressure tubes, and Zr-1.5Nb-1Ta (at%), for use as implant materials have been assessed and compared with those of Grade 2 Ti, which is known to be a highly compatible metallic biomaterial. The in vitro corrosion resistance was investigated by open circuit potential and electrochemical impedance spectroscopy (EIS) measurements, as a function of exposure time to an artificial physiological environment (Ringer's solution). Open circuit potential values indicated that both the Zr alloys and Grade 2 Ti undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface, in the aggressive environment. It also indicated that the tendency for the formation of a spontaneous oxide is greater for the Zr-1.5Nb-1Ta alloy and that this oxide has better corrosion protection characteristics than the ones formed on Grade 2 Ti or on the Zr-2.5Nb alloy. EIS study showed high impedance values for all samples, increasing with exposure time, indicating an improvement in corrosion resistance of the spontaneous oxide film. The fit obtained suggests a single passive film presents on the metals surface, improving their resistance with exposure time, presenting the highest values to the Zr-1.5Nb-1Ta alloy. For the biocompatibility analysis human osteosarcoma cell line (Saos-2) and human primary bone marrow stromal cells (BMSC) were used. Biocompatibility tests showed that Saos-2 cells grow rapidly, independently of the surface, due to reduced dependency from matrix deposition and microenvironment recognition. BMSC instead display a reduced proliferation, possibly caused by a reduced crosstalk with the metal surface microenvironment. However, once the substrate has been colonized, BMSC seem to respond properly to osteoinduction stimuli, thus supporting a substantial equivalence in the biocompatibility among the Zr alloys and Grade 2 titanium. In summary, high in vitro corrosion resistance together with satisfactory biocompatibility make the Zr-2.5Nb and Zr-1.5Nb-1Ta crystalline alloys promising biomaterials for surgical implants.

Osteoblast physiology in normal and pathological conditions

Osteoblasts are mononucleated cells that are derived from mesenchymal stem cells and that are responsible for the synthesis and mineralization of bone during initial bone formation and later bone remodelling. Osteoblasts also have a role in the regulation of osteoclast activity through the receptor activator of nuclear factor κ-B ligand and osteoprotegerin. Abnormalities in osteoblast differentiation and activity occur in some common human diseases such as osteoporosis and osteoarthritis. Recent studies also suggest that osteoblast functions are compromised at sites of focal bone erosion in rheumatoid arthritis.

Bone tissue engineering: a review in bone biomimetics and drug delivery strategies

Critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of a tissue-engineered scaffold is to use engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. A synthetic bone scaffold must be biocompatible, biodegradable to allow native tissue integration, and mimic the multidimensional hierarchical structure of native bone. In addition to being physically and chemically biomimetic, an ideal scaffold is capable of eluting bioactive molecules (e.g., BMPs, TGF-betas, etc., to accelerate extracellular matrix production and tissue integration) or drugs (e.g., antibiotics, cisplatin, etc., to prevent undesired biological response such as sepsis or cancer recurrence) in a temporally and spatially controlled manner. Various biomaterials including ceramics, metals, polymers, and composites have been investigated for their potential as bone scaffold materials. However, due to their tunable physiochemical properties, biocompatibility, and controllable biodegradability, polymers have emerged as the principal material in bone tissue engineering. This article briefly reviews the physiological and anatomical characteristics of native bone, describes key technologies in mimicking the physical and chemical environment of bone using synthetic materials, and provides an overview of local drug delivery as it pertains to bone tissue engineering is included.

Cyclo-DfKRG peptide modulates in vitro and in vivo behavior of human osteoprogenitor cells on titanium alloys

The first aim of the present study was to investigate the capacity of a cyclo-DfKRG-coated hydroxyapatite-titanium alloy (Ti-HA-RGD) to activate in vitro human osteoprogenitor cells adhesion and differentiation. The second purpose was to examine in vivo the role of a autologous cell seeding on cyclo-DfKRG-functionalized materials to provide bone repair after implantation in femoral condyle of rabbits. Our in vitro results have demonstrated that both titanium alloy functionalized with hydroxyapatite (Ti-HA-RGD and Ti-HA) contributed to higher cell adhesion than titanium alloy alone respectively 85 and 55% vs 15% compared to tissue culture polystyrene after one hour of cell seeding. As for differentiation, after 3 days of culture, Ti-HA presented the highest increase of ALP mRNA of all surfaces studied. Ti-HA-RGD showed an intermediate value about half as high as Ti-HA. Moreover after 3 days, both Ti-HA and Ti-HA-RGD surfaces showed the highest increase of cbfa1 mRNA expression. Two weeks following implantation, in vivo findings revealed that percentage of lacunae contact observed with pre-cellularized Ti-HA-RGD samples remains significantly lower than with Ti-HA group (10.5+/-9.6 % vs 33.7+/-11.5 %, P<0.03). Meanwhile, RGD peptide coating had no significant additional effect on the bone implant contact and area. Moreover, histomorphometry analysis revealed that implantation of pre-cellularized RGD coated materials with ROP cells increased significantly peri-implant fibrous area (24+/-11.6% vs 3+/-1.7% for Ti-HA-RGD, P<0.02). RGD coatings demonstrated osteoblastic adhesion, differentiation and in vivo bone regeneration at most equivalent to HA coatings.

Mechanical tensile stress effects on the expression of bone sialoprotein in bovine cementoblasts

OBJECTIVE

To develop a new cementoblast culture method and to detect bone sialoprotein (BSP) expression in response to high and low mechanical tensile stress in cementoblast in vitro.

MATERIALS AND METHODS

Cementoblasts were collected from the roots of newborn bovine teeth and were identified with cementum-derived attachment protein (CAP) antibody 3G9. Cell proliferation was evaluated by MTT [3-(4,5-dimethylthazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay, and mineralization was confirmed by von Kossa staining. Mechanical tensile stress was applied in vitro to the cementoblast with the use of a uniaxial four-point bending system with 2000 or 4000 microstrains, at a frequency of 0.5 Hz for 3, 6, 12, 24, or 36 hours. BSP mRNA level was quantified by real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR).

RESULTS

A large amount of cementoblast was observed to be expressing CAP. Cementoblasts had a proliferation tendency similar to that of osteoblasts but different from that of periodontal ligament (PDL) cells. Cementoblasts had the ability to become mineralized between osteoblasts and PDL cells. The mechanical tensile stress significantly up-regulated BSP mRNA expression, which reached a peak at 24 hours in both 2000 and 4000 microstrain groups (P < .01) and was tenfold and sixfold higher than that of controls, respectively. BSP expression dropped toward baseline levels at 36 hours in both groups.

CONCLUSIONS

Mechanical tensile stress up-regulated the expression of BSP. Low mechanical tensile stress induced earlier and more intensive up-regulation of BSP mRNA; this might represent the optimal stimuli for cementoblast activity.

Stress fracture and military medical readiness: bridging basic and applied research

PURPOSE

Military recruits and distance runners share a special risk of stress fracture injury. Recent efforts by US and Israeli military-sponsored researchers have uncovered important mechanisms and practical low-cost interventions. This article summarizes key findings relevant to prevention of stress fracture, including simple strategies to identify and to mitigate risk.

METHODS

Published research supported through the Bone Health and Military Medical Readiness research program and related military bone research was analyzed for contributions to preventing stress fracture in military recruits and optimizing bone health.

RESULTS

Thousands of military recruits helped test hypotheses about predictors of risk, safer exercise regimens, and rest, nutrition, gait training, and technology interventions to reduce stress fracture risk. Concurrent cellular, animal, and human laboratory studies were used to systematically investigate mechanisms of mechanical forces acting on bone and interactions through muscle, hormonal and genetic influences, and metabolism. The iterative and sometimes simultaneous process of basic discovery and field testing produced new knowledge that will provide safer science-based physical training.

DISCUSSION

Human training studies evaluating effects on bone require special commitment from investigators and funders due to volunteer compliance and attrition challenges. The findings from multiple studies indicate that measures of bone elasticity, fragility, and geometry are as important as bone mineral density in predicting fracture risk, with applications for new measurement technologies. Risk may be reduced by high intakes of calcium, vitamin D, and possibly protein (e.g., milk products). Prostaglandin E2, insulin-like growth factor 1, and estrogens are important mediators of osteogenesis, indicating reasons to limit the use of certain drugs (e.g., ibuprofen), to avoid excessive food restriction, and to treat hypogonadism. Abnormal gait may be a correctable risk factor. Brief daily vibration may stimulate bone mineral accretion similar to weight-bearing exercise. Genetic factors contribute importantly to bone quality, affecting fracture susceptibility and providing new insights into fracture healing and tissue reengineering.

Response of cementoblast-like cells to mechanical tensile or compressive stress at physiological levels in vitro

To clarify the role of cementoblast in orthodontic-related root resorption, this study was attempted to examine whether murine cementoblast-like cells are responsive to mechanical stress, and how mechanical forces regulate bone sialoprotein (BSP) and osteopontin (OPN) gene expression in these cells in vitro. In this force-loading model, defined and reproducible mechanical loadings of different magnitudes and types were applied up to 24 h. Besides a transitory and reversible change in cell proliferation, remarkable alterations in gene transcription of BSP and OPN were found. BSP mRNA was suppressed by the stresses. Three and six hours-loadings at 2,000 microstrain up-regulated the expression of OPN mRNA, while the other loadings inhibited it. The study also concluded that 4,000 microstrain was likely to exert more influence on cementoblast-like cells than 2,000 microstrain. Furthermore, no obvious evidence indicated the difference between tension and compression. These results suggested that cementoblast-like cells are sensitive to mechanical stress, and may play a role in regulating orthodontic-related root resorption/repair.

Integrins, insulin like growth factors, and the skeletal response to load

Bone loss during skeletal unloading, whether due to neurotrauma resulting in paralysis or prolonged immobilization due to a variety of medical illnesses, accelerates bone loss. In this review the evidence that skeletal unloading leads to bone loss, at least in part, due to disrupted insulin like growth factor (IGF) signaling, resulting in reduced osteoblast proliferation and differentiation, will be examined. The mechanism underlying this disruption in IGF signaling appears to involve integrins, the expression of which is reduced during skeletal unloading. Integrins play an important, albeit not well defined, role in facilitating signaling not only by IGF but also by other growth factors. However, the interaction between selected integrins such as alphaupsilonbeta3 and beta1 integrins and the IGF receptor are of especial importance with respect to the ability of bone to respond to mechanical load. Disruption of this interaction blocks IGF signaling and results in bone loss.

Bone sialoprotein plays a functional role in bone formation and osteoclastogenesis

Bone sialoprotein (BSP) and osteopontin (OPN) are both highly expressed in bone, but their functional specificities are unknown. OPN knockout (^−/−) mice do not lose bone in a model of hindlimb disuse (tail suspension), showing the importance of OPN in bone remodeling. We report that BSP^−/− mice are viable and breed normally, but their weight and size are lower than wild-type (WT) mice. Bone is undermineralized in fetuses and young adults, but not in older (≥12 mo) BSP^−/− mice. At 4 mo, BSP^−/− mice display thinner cortical bones than WT, but greater trabecular bone volume with very low bone formation rate, which indicates reduced resorption, as confirmed by lower osteoclast surfaces. Although the frequency of total colonies and committed osteoblast colonies is the same, fewer mineralized colonies expressing decreased levels of osteoblast markers form in BSP^−/− versus WT bone marrow stromal cultures. BSP^−/− hematopoietic progenitors form fewer osteoclasts, but their resorptive activity on dentin is normal. Tail-suspended BSP^−/− mice lose bone in hindlimbs, as expected. In conclusion, BSP deficiency impairs bone growth and mineralization, concomitant with dramatically reduced bone formation. It does not, however, prevent the bone loss resulting from loss of mechanical stimulation, a phenotype that is clearly different from OPN^−/− mice.

Osteoblast cytoskeletal modulation in response to compressive stress at physiological levels

Biomechanical force is one of the major epigenetic factors that determine the form and differentiation of skeletal tissues. In this study, osteoblastic cells UMR-106 were exposed to compressive forces at 1000 mustrain and 4000 mustrain via a four-point bending system, and analyzed by MTT and LSCM techniques. Cell proliferation activity decreased shortly after loading but recovered to normal levels within 24 h. And the cytoskeleton depolymerized at first, but then gradually repolymerized. To find out the role of cytoskeleton in mechanotransduction, we examined the relationship between cytoskeleton construction and c-fos expression. A transient stress-induced upregulation in c-fos mRNA and c-Fos protein was discovered when cells were exposed to physiological forces. And the upregulation in c-fos expression was blocked by cytochalasin D (Depolymerizing agent of microfilament). It gave clues that the organization of cytoskeleton was an important link in transcriptional control in response to low-mechanical stimulation.

In VitroMicrodistraction of Preosteoblasts: Distraction Promotes Proliferation and Oscillation Promotes Differentiation

Osteoblast biology is influenced in vivo by a 3-dimensional (3D) extracellular matrix that mediates their adhesion and interaction and by a constant state of compressive and tensile forces. To study the role of mechanical stress on osteoblasts in vitro, these parameters must be addressed. Therefore, this study describes the use of a novel, in vitro system that subjects cells to distractive and compressive forces in a 3D environment. This system, termed a microdistractor system, was used to apply linear forces to 3D collagen type I gels containing preosteoblasts. Gels were induced for up to 16 days in osteogenic medium and subjected to either constant linear distraction (distraction gels) or to repeating cycles of distraction and compression (oscillation gels). The effect of these stresses was evaluated over time by measuring proliferation rates, protein synthesis (i.e., cellular activity), and osteogenic differentiation levels. While linear forces in general appeared to increase protein synthesis, force-specific effects on proliferation and differentiation were observed. Specifically, distraction forces appeared to enhance MC3T3 proliferation while distraction/compressive forces appeared to accelerate their osteogenic differentiation program. Therefore, these results suggest that the microdistraction system may be an appropriate in vitro system for the study of mechanobiology in osteoblast phenotype.

The MGF expression of osteoblasts in response to mechanical overload

Cell proliferation and mRNA expression of insulin-like growth factor (IGF-I) and "mechanogrowth factor" (MGF) were studied in osteoblasts in response to overload. Static and cyclic-stretching were used to apply superphysiological strains to cells. Overload was found to increase cell growth. IGF-I and its splicing variant, MGF, were measured using reverse transcriptase-polymerase chain reaction method and were found to be regulated differentially by mechanical signals at the mRNA level. Cyclic-stretching had a more significant effect on cell proliferation and mRNA expression levels of IGF-I and MGF, while unstrained cells did not express MGF at the mRNA level. These results demonstrated that gene expression is regulated by mechanical stimulation. MGF expression in osteoblasts in response to strain may be related to an autocrine mechanism.

Integrins β1, α6, and α3contribute to mechanical strain-induced differentiation of fetal lung type II epithelial cells via distinct mechanisms

Mechanical forces regulate lung maturation in the fetus by promoting type II epithelial differentiation. However, the cell surface receptors that transduce these mechanical cues into cellular responses remain largely unknown. When distal lung type II epithelial cells isolated from embryonic day 19 rat fetuses were cultured on flexible plates coated with laminin, fibronectin, vitronectin, collagen, or elastin and exposed to a level of mechanical strain (5%) similar to that observed in utero, transmembrane signaling responses were induced under all conditions, as measured by ERK activation. However, mechanical stress maximally increased expression of the type II cell differentiation marker surfactant protein C when cells were cultured on laminin substrates. Strain-induced alveolar epithelial differentiation was inhibited by interfering with cell binding to laminin using soluble laminin peptides (IKVIV or YIGSR) or blocking antibodies against integrin β1, α3, or α6. Additional studies were carried out with substrates coated directly with different nonactivating anti-integrin antibodies. Blocking integrin β1and α6binding sites inhibited both cell adhesion and differentiation, whereas inhibition of α3prevented differentiation without altering cell attachment. These data demonstrate that various integrins contribute to mechanical control of type II lung epithelial cell differentiation on laminin substrates. However, they may act via distinct mechanisms, including some that are independent of their cell anchoring role.

Modulation of the responses of human osteoblast-like cells to physiologic mechanical strains by biomaterial surfaces

In a previous study we demonstrated that MG-63 cells cultured on Ti-6Al-4V discs covered by alumina ceramic and submitted to intermittent mechanical strain (IMS) presented morphological alteration associated with enhanced differentiation. Here we examine how the mechanical response of osteoblasts can be modulated by the nature of the substrate. MG-63 cells were cultured on four materials: polystyrene and Ti-6Al-4V (average roughness = 0.48 microm) as smooth substrates; Ti-6Al-4V (average roughness = 5.76 microm) and Ti-6Al-4V covered with alumina (average roughness = 5.21 microm) as rough substrates. Mechanical strains were applied for 15 min, three times a day for 1-5 days with a 600 microstrains magnitude and a 0.25 Hz frequency. IMS stimulated alkaline phosphatase activity by 25-35% on all substrates and had no effect on cell growth on either substrate. Fibronectin (FN) was chosen as representative of cell-matrix interaction. FN production was increased by 60% after 1 day of stretching only on alumina-coated discs. FN organization examined on smooth substrates was affected by 5 days of IMS, showing a thickening of the fibres. The same modifications induced by IMS were previously observed on alumina-covered discs. Vinculin expression was not affected by IMS whatever the substrate. Cell-cell interactions were determined by N-cadherin immunoblotting. N-cadherin expression was increased by IMS specifically on rough substrates. Our results suggest that the nature of the surface did not influence the up-regulation of alkaline phosphatase activity induced by IMS, but modulates specifically cell-substrate as well as cell-cell interactions in response to IMS.

Model surfaces engineered with nanoscale roughness and RGD tripeptides promote osteoblast activity

Cell adhesion to biomaterials is a prerequisite for tissue integration with the implant surface. Herein, we show that we can generate a model silica surface that contains a minimal-length arginine-glycine-aspartic acid (RGD) peptide that maintains its biological activity. In the first part of this study, attachment of MC3T3-E1 osteoblast-like cells was investigated on silicon oxide, amine terminated substrates [i.e., 3-aminopropyl triethoxysilane (APTS)], grafted RGD, and physisorbed RGD control. The APTS layer exhibited nanoscale roughness and presented amine functional groups for grafting a minimal RGD tripeptide devoid of any flanking groups or spacers. Contact angle measurements indicated that the hydrophobicity of the APTS surface was significantly lower than that of the surface with grafted RGD (RGD-APTS). Atomic force microscopy showed that surfaces covered with RGD-APTS were smoother (Ra = 0.71 nm) than those covered with APTS alone (Ra = 1.59 nm). Focusing mainly on cell morphology, experiments showed that the RGD-APTS hybrid provided an optimum surface for cell adhesion, spreading, and cytoskeletal organization. Discrete focal adhesion plaques were also observed consistent with successful cell signaling events. In a second set of experiments, smooth, monolayers of APTS (Ra = 0.1 nm) were used to prepare arginine-glycine-aspartic acid-serine (RGDS)-APTS and arginine-glycine-glutamic acid-serine (RGES)-APTS (control) substrates. Focusing mainly on cell function, integrin and gene expression were all enhanced for rate osteosarcoma cells on surfaces containing grafted RGDS. Both sets of studies demonstrated that grafted molecules of RGD(S) enhance both osteoblast-like cell adhesion and function.

The stress reaction and its molecular events: splicing variants

The growth of cells and tissues is regulated by stress. When body is injured, it manifests a large spectrum of metabolic, endocrine, and immune alterations, which is named stress reaction. Among them, the production of growth factors may play a critical role. For osteoblasts and myoblasts, IGF-I has been shown to be involved in the process of cells in response to overloads. There are two splicing forms, one is IGF-Ea, the other is the IGF-IEb in the rodents and corresponds to IGF-IEc in humans. The latter is markedly up-regulated in response to overloads. Therefore, it has been named mechanogrowth factor. The link between the mechanical stimulus and the gene expression represents a new and important area in cell science. Understanding the process of splicing in IGF-I helps one to investigate the mechanotransduction of cells in response to mechanical stimulation at molecular level.

Mechanical regulation of HB-GAM expression in bone cells

Bone adaption upon mechanical stimulation is accompanied by changes in gene expression. In this context we investigated the influence of mechanical loading on heparin binding growth associated molecule (HB-GAM) expression, an extracellular matrix molecule which in cell culture has been shown to stimulate the differentiation of osteoblasts. We obtained information on the participating signal transduction pathways using a mitogenic loading regimen. Specific inhibitors of various signal transduction pathways were added to loaded cells and to unloaded controls. By semi-quantitative PCR studies we demonstrated a rapid decrease of HB-GAM expression in primary osteoblasts and SaOs-2 cells by 20-30% upon mechanical loading within 30min. We showed that the RGD-integrin interaction is involved in the regulation of HB-GAM expression. Furthermore, integrity of the cytoskeleton, stretch-activated, and voltage-sensitive Ca(2+) channels as well as gap junctional communication are necessary for the downregulation of HB-GAM expression by mechanical loading.

Mechanical stimulation promotes osteogenic differentiation of human bone marrow stromal cells on 3-D partially demineralized bone scaffolds in vitro

Bone is a dynamic tissue that is able to sense and adapt to mechanical stimuli by modulating its mass, geometry, and structure. Bone marrow stromal cells (BMSCs) are known to play an integral part in bone formation by providing an osteoprogenitor cell source capable of differentiating into mature osteoblasts in response to mechanical stresses. Characteristics of the in vivo bone environment including the three dimensional (3-D) lacunocanalicular structure and extracellular matrix composition have previously been shown to play major roles in influencing mechanotransduction processes within bone cells. To more accurately model this phenomenon in vitro, we cultured human BMSCs on 3-D, partially demineralized bone scaffolds in the presence of four-point bending loads within a novel bioreactor. The effect of mechanical loading and dexamethasone concentration on BMSC osteogenic differentiation and mineralized matrix production was studied for 8 and 16 days of culture. Mechanical stimulation after 16 days with 10 nM dexamethasone promoted osteogenic differentiation of BMSCs by significantly elevating alkaline phosphatase activity as well as alkaline phosphatase and osteopontin transcript levels over static controls. Mineralized matrix production also increased under these culture conditions. Dexamethasone concentration had a dramatic effect on the ability of mechanical stimulation to modulate these phenotypic and genotypic responses. These results provide increased insight into the role of mechanical stimulation on osteogenic differentiation of human BMSCs in vitro and may lead to improved strategies in bone tissue engineering.

Soluble phosphate glasses: in vitro studies using human cells of hard and soft tissue origin

This report describes the short-term response of two typical cellular components of a hard/soft tissue interface such as the periodontal ligament/mandible and patellar tendon/tibia. Tissue engineering of such interfaces requires a contiguous scaffold system with at least two cell types associated with it. Human oral osteoblasts, oral fibroblasts and hand flexor tendon fibroblasts were seeded on phosphate-based glasses of different dissolution rates. Quantitative and morphological assessment of cell adhesion and proliferation for all cell types was assessed, after first elucidating an experimental composition range using MG63 cells. In addition, immunolabelling of bone-specific non-collagenous proteins bone sialoprotein, osteonectin and osteopontin was performed to determine osteoblast phenotype. Fibroblast phenotype was established by immunolabelling for prolyl-4-hydroxylase, an enzyme vital for collagen biosynthesis. Results indicated that both cell types maintained their respective phenotypes over time in culture on glass discs of generic composition (CaO)x-(Na2O)(0.5-x)-(P2O5)0.5, remained attached and proliferated dependent on glass composition and cell type. Glasses containing at least 46 mol% CaO, produced no adverse cell reaction suggesting that these compositions that support both osteoblasts and fibroblasts would be ideal as a scaffold material for engineering the hard/soft tissue interface.

Physiological strains remodel extracellular matrix and cell-cell adhesion in osteoblastic cells cultured on alumina-coated titanium alloy

The effects of mechanical strains on cellular activities were assessed in an in vitro model using human osteoblastic MG-63 cells grown on titanium alloy discs coated with porous alumina and exposed to chronic intermittent loading. Strain was applied with a Dynacell device for three 15-min sequences per day for several days with a magnitude of 600 microepsilon strain and a frequency of 0.25 Hz. We have previously demonstrated that this regimen increased alkaline phosphatase activity in confluent cultures on ceramic coated titanium (alumina and hydroxyapatite) (Biomaterials 24 (2003) 3139). In this study, we analysed the production of bone matrix proteins. Osteocalcin secretion quantified by ELISA between day 5 and 11 was not affected by mechanical strain. Strain had even no quantifiable effect on collagen production from day 1 to 5 as measured by carboxy terminal collagen type I propeptide release. On the other hand, stress stimulation resulted in increased expression of fibronectin (FN) measured by Western blot after 1 day stretching. This upregulation of FN production was followed by reorganisation of the FN network after 5 days stretching observed by immunostaining. The receptors for collagen and FN, alpha2beta1, alpha5beta1 and beta1 integrins were not quantitatively affected by the strains as measured by flow cytometry. A modification of cell morphology was seen after 5 days of loading that appeared to increase cell spreading, implying consequences on intercellular contacts. For this reason, N, C11 and E-adherins were examined. We noted a selective effect characterised by increased expression of N-cadherin using both RT-PCR and Western blot analyses. We concluded that reinforcement of cell-cell adhesion and remodelling of the FN network are important adaptive responses to physiological strains for human osteoblasts grown on alumina-coated biomaterials.

Fluid flow shear stress stimulates human osteoblast proliferation and differentiation through multiple interacting and competing signal transduction pathways

This study sought to assess the role of several signaling pathways in the fluid flow shear stress-induced proliferation and differentiation of normal human osteoblasts. We evaluated the effects of an effective dose of selective inhibitors of the extracellular signal-regulated kinases (ERK) pathway (PD98059 and U0126), the nitric oxide synthase pathway (N(omega)-nitro-L-arginine methyl ester), the cyclo-oxygenase pathway (indomethacin), or the Gi/o pathway (pertussis toxin [PTX]) on the flow-mediated effects. A 30-min steady flow shear stress at 20 dynes/cm(2) increased significantly [(3)H]thymidine incorporation (an indicator of proliferation), alkaline phosphatase activity (an index of osteoblast differentiation), phosphorylation of ERK, and expression of integrin beta1. PD98059, U0126, and N(omega)-nitro-L-arginine methyl ester completely blocked the shear stress-induced increases in ERK phosphorylation, [(3)H]thymidine incorporation, and alkaline phosphatase, but without an effect on integrin beta1 expression, indicating that the ERK and nitric oxide synthase pathways are essential for the shear stress-induced proliferation and differentiation of normal human osteoblasts and that each involves ERK activation but not integrin beta1 upregulation. Indomethacin blocked the shear stress-induced osteoblast proliferation and differentiation and integrin beta1 upregulation but not ERK activation, suggesting that the cyclo-oxygenase pathway (i.e., prostacyclin and/or prostaglandin E(2)) mediates the shear stress-induced osteoblast proliferation in an ERK-independent manner. In contrast, PTX completely blocked the flow-induced increase in integrin beta1 expression but had no effect on the increase in the ERK phosphorylation or [(3)H]thymidine incorporation. PTX not only did not inhibit but also significantly enhanced the stimulatory effect of shear stress on alkaline phosphatase activity, suggesting that a PTX-sensitive signaling pathway may have an inhibitory role in osteoblast differentiation. In summary, this study shows, for the first time, that the signal transduction mechanism of shear stress in osteoblasts is complex and involves multiple ERK-dependent and independent pathways, and provides circumstantial evidence that there may be a PTX-sensitive pathway that has completing effects with an unknown pathway on the differentiation of normal human osteoblasts.