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RESEARCH REPORT |
N402, Dows Institute for Dental Research and the Department of Prosthodontics, University of Iowa College of Dentistry, Iowa City, IA 52242;
* corresponding author, galen-schneider{at}uiowa.edu
| ABSTRACT |
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KEY WORDS: Cbfa1 osseointegration dental implants osteoblasts mineralization
| INTRODUCTION |
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During bone formation, the osteogenic marker Cbfa1 has been shown to be critical in the regulatory processes of bone differentiation (Ducy et al., 1997). Cbfa1 belongs to the Runt family of transcription factors (Xiao et al., 1998), and regulates osteoblast differentiation and expression of key osteoblast genes necessary for the development of a mineralized phenotype (Xiao et al., 1998; Ducy et al., 1999). However, the effects of different implant surface microtopographies on gene expression of key osteogenic factors are not fully understood.
The hypothesis of the current study was that roughened implant surface microtopographies differentially affect Cbfa1 and BSPII gene expression, as well as mineralization. We utilized a previously characterized, rapidly mineralizing UMR-106-01 BSP osteoblast model (Stanford et al., 1995), and a non-transformed primary rat calvarial osteoblast (RCOB) model for comparison (Bowers et al., 1992). In the current study, rapidly mineralizing UMR-BSP osteoblasts were grown on either rough or grooved implant surfaces. Enhanced mineralization, as well as increased gene expression of Cbfa1 and BSPII, was noted on rougher surfaces as compared with grooved, relative to tissue culture plastic. However, in the RCOB cultures, less BSPII gene expression was seen on rough surfaces relative to grooved. These results suggest that osteoblast gene expression and subsequent mineralization are affected by roughened implant surface microtopographies during the osseointegration of dental implants.
| MATERIALS & METHODS |
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Alizarin Red S Staining for Mineralization
UMR osteoblasts were plated in triplicate at a density of 50,000 cells/10-µL droplet cultures onto prepared titanium discs or onto tissue culture plastic and placed into a 37-degree incubator. After 1 hr of attachment, 1.0 mL of medium was added to each culture. Following an incubation period of 48 hrs, ß-glycerophosphate (5 mM final concentration) was added, and the cultures were incubated for an additional 24 hrs. The medium was then removed, and the culture was gently rinsed with phosphate-buffered saline (PBS) and fixed for 10 min in 10% formalin. After fixation, the cultures were rinsed with Nanopure water (no calcium ion) and stained for calcium with 2% Alizarin Red-S (AR-S) for 10 min at room temperature. Wells were washed 4x with Nanopure water and evaluated for intensity of AR-S staining. Triplicate cultures were scanned for quantitation by means of the NIH Scion imaging analysis program. Each culture was then scanned 3x and averaged. The averaged scan value of the non-mineralizing control (UI cells) was subtracted from all BSP scan values. The experimental BSP cultures were expressed as a percentage of the BSP on the rough-surface group. Statistical analysis (N = 3) was performed by a one-way analysis of variance (ANOVA) with Tukeys Multiple Comparison Test to a significance level of P < 0.05.
Cell Proliferation and Attachment Assay
Microdots of UI cells and BSP cells were plated in triplicate (50,000 cells/10-µL dot) on tissue culture plastic (TCP) and on cpTi discs polished to either a rough (sandblasted) or grooved (600 grit) finish. Cultures were allowed to attach for 1 hr before being flooded with 1 mL of medium. At 48 hrs, 5 mM ß-glycerophosphate was added to all cultures. At 72 hrs, the medium was removed from the wells, unattached cells were quantitated with the model ZM Coulter counter, and triplicate cultures were averaged. Statistical analysis (N = 5) was performed by one-way analysis of variance (ANOVA) with Tukeys Multiple Comparison Test to a significance level of P < 0.05.
Real Time RT-PCR
Real Time PCR primers and probes were designed (Fig. 1
) with Primer Express software (Perkin Elmer, Boston, MA, USA) from the known rat sequence that corresponds to exons 1 and 2 (Isoform 1), and 3 and 4 (Isoform 3, Runt-domain) of the Cbfa1 gene (Xiao et al., 1998), as well as exons 6 and 7 of BSPII (Oldberg et al., 1988). The probe was designed to overlie the exon junction, so that it was unlikely to hybridize to genomic DNA that may have been present. This ensured that if genomic DNA contaminants were present, their PCR amplification was non-contributory to the Real Time analysis. We used the TaqMan Ribosomal RNA Control Reagents Kit (Perkin Elmer) to detect 18s ribosomal RNA as an endogenous control.
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Ct was performed by a one-way analysis of variance (ANOVA) with Tukeys Multiple Comparison Test or a paired one-tailed t test to a confidence level of P < 0.05. | RESULTS |
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Cbfa1 and BSPII Gene Expression are Affected by Implant Surface Microtopographies
Differences in levels of mineralization were noted in osteoblasts cultured on rough vs. grooved implant surfaces. We used Real Time RT-PCR strategies to determine if differences in the levels of Cbfa1 and BSPII gene expression would also differ (Fig. 4
). Quantitative Real Time PCR analysis detected Cbfa1 and BSPII in the mineralizing osteoblast cultures grown on both rough and grooved implant surfaces, relative to tissue culture plastic. The steady-state mRNA levels of Cbfa1 were significantly (P < 0.03) greater, nearly four-fold, in the UMR osteoblast cultures grown on the roughened implant surface as compared with grooved (Fig. 4A
). This result was also found in the non-transformed RCOB cultures, where, again, a significant (P < 0.001), three-fold increase in Cbfa1 gene expression was noted on the rough surface (Fig. 4B
). In addition, the expression of the extracellular matrix protein specific to mineralizing tissues, BSPII, whose gene expression is thought to be regulated by the Cbfa1 transcription factor (Ducy et al., 1997), was slightly increased (P < 0.01) in UMR osteoblasts grown on the roughened surface as well (Fig. 4C
). However, when analyzed in the RCOB cultures, there was six-fold less (P < 0.001) BSPII gene expression on rough surface microtopographies relative to grooved (Fig. 4D
).
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| DISCUSSION |
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Osteoblast differentiation and responses during osseointegration vary and are affected by the implant surface microtopography, associated extracellular matrix proteins, and their respective integrin receptors (Schneider and Burridge, 1994; Okamoto et al., 1998; Cooper et al., 1999; Sykaras et al., 2000; Schneider et al., 2001). A direct association of osteoblasts with their associated bone matrices defines osseointegration with the implant surface in the absence of soft or fibrous tissue (Albrektsson et al., 1981). Many studies examining cell adhesion and morphology, DNA synthesis, integrin and extracellular matrix expression, and enzyme activity have been done to elucidate osteoblastic response to titanium alloys (Puleo et al., 1991; Vrouwenvelder et al., 1993; Schneider and Burridge, 1994; Gronowicz and McCarthy, 1996; Sinha and Tuan, 1996). However, many of the molecular and genotypic events taking place at the osteoblast cell level during osseointegration are still largely unknown.
The purpose of our study was to begin to address these molecular events with respect to Cbfa1 and BSPII gene expression in relation to differences in mineralization associated with roughened implant surface microtopographies. Data from this study provide novel cellular and molecular insight as to the effects of implant surface microtopographies, in association with the surrounding matrix, on molecular signaling mechanisms that regulate osteoblast mineralization during osseointegration.
Following the preparation of two different implant surface conditions, rough and grooved, we utilized the UMR-BSP-106-01 osteoblast cell model (Stanford et al., 1995; Schneider et al., 2001) and the non-transformed primary RCOB model to assess differences in levels of mineralization when osteoblasts were cultured onto these prepared implant surfaces. Osteoblasts grown on roughened surfaces, as previously reported (Bowers et al., 1992), revealed more mineralization as compared with those on grooved surfaces.
As a direct result of these differences in mineralization, and because of the known function of Cbfa1 in the regulation of osteoblast differentiation and subsequent mineralization, we wanted to determine if differences in levels of Cbfa1 expression would also be seen under in vitro conditions simulating osseointegration. To analyze and quantify the results, we used Real Time RT-PCR strategies that can determine absolute mRNA copy number (Aarskog and Vedeler, 2000; Bustin, 2000). Cbfa1 gene expression was enhanced in the UMR and rat calvarial osteoblasts grown on the rough implant surfaces relative to grooved. Analysis of the suggested Cbfa1 transcriptionally regulated BSPII gene (Ducy et al., 1997) revealed slight enhancement of expression in UMR osteoblasts, but reduced expression in RCOBs on the rough surfaces relative to grooved. However, expression of both genes was higher on both types of rough implant surfaces relative to tissue culture plastic. Differences in BSPII gene expression between the UMR and rat calvarial osteoblasts could be for several reasons. The expression of Cbfa1 might be higher in the RCOBs cultured on the rough surface microtopographies, as compared with the grooved, but the permissive activity of this Cbfa1 transcription factor might be impeded as a result of an unknown effect of the rough, but not the grooved, implant surface microtopography on this intracellular signaling cascade. This impediment in Cbfa1 function, but not expression, may lead to a reduction of BSPII gene expression as seen in the RCOB cultures on the rough relative to the grooved surfaces. The impediment of Cbfa1 transcription factor with a BSPII gene could also be associated with interference of Cbfa1 with the recently described osterix gene (Nakashima et al., 2002).
Additionally, on the grooved surface, Cbfa1 was expressed at a level similar to that of BSPII gene expression on the same surface in the RCOBs, where, as in the UMR model, a slight increase in BSPII gene expression relative to Cbfa1 was noted. The modest increase in BSPII gene expression relative to Cbfa1 in the UMR model, vs. the reduction of BSPII relative to Cbfa1 on rough surfaces in the RCOB cells, may be attributed to the transformed state of the UMR model, where overexpression of BSPII has been reported, and thus may be an artifact of the culture model (Stanford et al., 1995). Thus, the UMR cells may be able to override the lack of Cbfa1 permissive activity seen in the RCOB model on rough microtopographies by the ability of the UMR model to overexpress that BSPII gene as a function of its transformed state. Differences in gene expression on rough and grooved microtopographies in both the UMR and RCOB cells were not attributed to variations in rates of cell proliferation or number, since no significant differences in these variables were noted.
We show here that different implant surface microtopographies (rough vs. grooved) can alter the expression of key osteogenic regulatory genes such as Cbfa1 and BSPII. This suggests that the interaction of the osteoblasts with the extracellular matrix components on the different implant surface microtopographies can influence gene expression. Perhaps this occurs as a result of differences in cell adhesion and shape, as a result of integrin-mediated adhesion and regulation of downstream signaling cascades as previously reported (Schneider et al., 2001). It could also be a result of extracellular matrix spatial and temporal expression profile changes that would be under the control of the transcription factor Cbfa1, such as bone sialoprotein (BSPII) (Ducy et al., 1997; Schneider et al., 1999).
Thus, roughened implant surface microtopographies may contribute to the regulation of osteoblast differentiation by influencing the level of gene expression of key osteogenic factors. A better understanding of these molecular processes will lead to the development of more advanced therapeutic prosthetic interventions associated with dental implant therapy and tissue-engineering biological applications.
| ACKNOWLEDGMENTS |
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Received January 15, 2002; Last revision February 11, 2003; Accepted February 11, 2003
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