Anti-RANKL (receptor activator of nuclear factor kappa-B ligand) agents function by blocking the differentiation of osteoclasts, thereby proving useful in the clinical management of postmenopausal osteoporosis. The effects of such agents on osseointegration is less well understood. The purpose of the current study was to investigate whether osteoprotegerin (OPG), an osteoclast inhibitor, enhances the known anabolic effects of mechanical loading (VEH) and intermittent PTH (iPTH) using a well-established rabbit model of osseointegration. In the first set of experiments, OPG was administered either alone or combined with iPTH to study its effects on measured bone mass. The second set of experiments was conducted using a higher dosage of OPG (10 mg/kg) to explore its early impact at the cellular and molecular levels. All subjects had mechanical load applied to the implant on one extremity, and no load applied on the contralateral side. In the first set of experiments, OPG alone decreased peri-implant bone mass compared to the mechanical loading group, whereas OPG + iPTH increased peri-implant bone mass compared to the OPG group. In the second set of experiments, high-dose OPG significantly decreased osteoclast number (-74.3%) at 1 week. However, this effect was not sustained as osteoclast number returned to baseline by 2 weeks. These results suggest that systemic administration of OPG does not enhance osseointegration, but rather has a detrimental effect.

Figure 2 and its caption were inadvertently omitted from the original article. The figure and caption are given below.

Background: Periprosthetic joint infection (PJI) remains a devastating complication following total joint arthroplasty. Current animal models of PJI do not effectively recreate the clinical condition and thus provide limited help in understanding why treatments fail. We developed a mouse model of the first-stage surgery of a 2-stage revision for PJI involving a 3-dimensionally printed Ti-6Al-4V implant and a mouse-sized cement spacer that elutes vancomycin. Methods: Vancomycin was mixed with polymethylmethacrylate (PMMA) cement and inserted into custom-made mouse-sized spacer molds. Twenty C57BL/6 mice received a proximal tibial implant and an intra-articular injection of 3 × 10 5 colony-forming units of Staphylococcus aureus Xen36. At 2 weeks, 9 mice underwent irrigation and debridement of the leg with revision of the implant to an articulating vancomycin-loaded PMMA spacer. Postoperatively, mice underwent radiography and serum inflammatory-marker measurements. Following euthanasia of the mice at 6 weeks, bone and soft tissues were homogenized to quantify bacteria within periprosthetic tissues. Implants and articulating spacers were either sonicated to quantify adherent bacteria or examined under scanning electron microscopy (SEM) to characterize the biofilm. Results: Vancomycin-loaded PMMA spacers eluted vancomycin for ≤144 hours and retained antimicrobial activity. Control mice had elevated levels of inflammatory markers, radiographic evidence of septic loosening of the implant, and osseous destruction. Mice treated with a vancomycin-loaded PMMA spacer had significantly lower levels of inflammatory markers (p < 0.01), preserved tibial bone, and no intra-articular purulence. Retrieved vancomycin-loaded spacers exhibited significantly lower bacterial counts compared with implants (p < 0.001). However, bacterial counts in periprosthetic tissue did not significantly differ between the groups. SEM identified S. aureus encased within biofilm on control implants, while vancomycin-loaded spacers contained no bacteria. Conclusions: This animal model is a clinically representative model of PJI treatment. The results suggest that the antimicrobial effects of PMMA spacers are tightly confined to the articular space and must be utilized in conjunction with thorough tissue debridement and systemic antibiotics. Clinical Relevance: These data provide what we believe to be the first insight into the effect of antibiotic-loaded cement spacers in a clinically relevant animal model and justify the adjunctive use of intravenous antibiotics when performing a 2-stage revision for PJI.

Currently, there are no medications available to treat aseptic loosening of orthopedic implants. Using osteoprotegerin fusion protein (OPG‐Fc), we previously blocked instability‐induced osteoclast differentiation and peri‐prosthetic osteolysis. Wnt/β‐catenin signaling, which regulates OPG secretion from osteoblasts, also modulates the bone tissue response to mechanical loading. We hypothesized that activating Wnt/β‐catenin signaling by inhibiting glycogen synthase kinase‐3β (GSK‐3β) would reduce instability‐induced bone loss through regulation of both osteoblast and osteoclast differentiation. We examined effects of GSK‐3β inhibition on regulation of RANKL and OPG in a rat model of mechanical instability‐induced peri‐implant osteolysis. The rats were treated daily with a GSK‐3β inhibitor, AR28 (20 mg/kg bw), for up to 5 days. Bone tissue and blood serum were assessed by qRT‐PCR, immunohistochemistry, and ELISA on days 3 and 5, and by micro‐CT on day 5. After 3 days of treatment with AR28, mRNA levels of β‐catenin, Runx2, Osterix, Col1α1, and ALP were increased leading to higher osteoblast numbers compared to vehicle‐treated animals. BMP‐2 and Wnt16 mRNA levels were downregulated by mechanical instability and this was rescued by GSK‐3β inhibition. Osteoclast numbers were decreased significantly after 3 days of GSK‐3β inhibition, which correlated with enhanced OPG mRNA expression. This was accompanied by decreased serum levels of TRAP5b on days 3 and 5. Treatment with AR28 upregulated osteoblast differentiation, while osteoclastogenesis was blunted, leading to increased bone mass by day 5. These data suggest that GSK‐3β inactivation suppresses osteolysis through regulating both osteoblast and osteoclast differentiation in a rat model of instability‐induced osteolysis.

Background: Periprosthetic joint infection (PJI) is a devastating complication following total joint arthroplasty. Current animal models of PJI are limited because of a lack of quantitative methods and failure to effectively recreate the periprosthetic space. We therefore developed a murine PJI model involving a 3-dimensionally printed Ti-6Al-4V implant capable of bearing weight and permitting quantitative analysis of periprosthetic bacterial load and evaluation of biofilm. Methods: Twenty-five 12-week-old C57BL/6 mice received a unilateral proximal tibial implant and intra-articular injection of either 3 × 10 5 colony forming units (CFUs) of Staphylococcus aureus Xen 36 or saline solution. Postoperatively, mice underwent gait analysis, knee radiographs, and serum inflammatory marker measurements. Following euthanasia at 2 or 6 weeks, bone and soft tissues were homogenized to quantify bacteria within periprosthetic tissues. Implants were either sonicated to quantify adherent bacteria or examined under scanning electron microscopy (SEM) to characterize biofilm. Results: All mice survived surgery and were not systemically septic. The control mice immediately tolerated weight-bearing and had normal inflammatory markers and radiographic signs of osseointegration. Infected mice had difficulty walking over time, exhibited radiographic findings of septic implant loosening, and had significantly elevated inflammatory markers. Periprosthetic tissues of the infected animals displayed a mean of 4.46 × 10 6 CFUs of S. aureus at 2 weeks and 2.53 × 10 5 CFUs at 6 weeks. Viable S. aureus was quantified on retrieved implant surfaces. SEM demonstrated S. aureus cocci in clusters encased within biofilm. Conclusions: This animal model is, to our knowledge, the most clinically representative PJI replication to date. It is the first that we know of to produce infection through the same method hypothesized to occur clinically, utilize a weight-bearing implant that can osseointegrate, and provide quantitative data on 8 aspects of PJI, including radiographic features, inflammatory markers, and bacterial loads. Clinical Relevance: This novel animal model is, to our knowledge, the first to provide a load-bearing translational representation of clinical PJI that effectively recreates the periprosthetic space.

➤The poor treatment outcomes for periprosthetic joint infection (PJI) reflect the limited understanding that currently exists regarding the pathogenesis of this devastating clinical problem.➤Current animal models of PJI are limited in their translational nature primarily because of their inability to recreate the periprosthetic environment.➤A greater mechanistic understanding of the musculoskeletal and immune systems of small animals, such as mice and rats, provides a more robust platform for modeling and examining the pathogenesis of PJI.➤A clinically representative PJI model must involve an implant that recreates the periprosthetic space and be amenable to methodologies that identify implant biofilm as well as quantify the peri-implant bacterial load.

BACKGROUND Long-term fixation of uncemented joint implants requires early mechanical stability and implant osseointegration. To date, osseointegration has been unreliable and remains a major challenge in cementless total knee arthroplasty. We developed a murine model in which an intra-articular proximal tibial titanium implant with a roughened stem can be loaded through the knee joint. Using this model, we tested the hypothesis that intermittent injection of parathyroid hormone (iPTH) would increase proximal tibial cancellous osseointegration. METHODS Ten-week-old female C57BL/6 mice received a subcutaneous injection of PTH (40 μg/kg/day) or a vehicle (n = 45 per treatment group) five days per week for six weeks, at which time the baseline group was killed (n = 6 per treatment group) and an implant was inserted into the proximal part of the tibiae of the remaining mice. Injections were continued until the animals were killed at one week (n = 7 per treatment group), two weeks (n = 14 per treatment group), or four weeks (n = 17 per treatment group) after implantation. Outcomes included peri-implant bone morphology as analyzed with micro-computed tomography (microCT), osseointegration percentage and bone area fraction as shown with backscattered electron microscopy, cellular composition as demonstrated by immunohistochemical analysis, and pullout strength as measured with mechanical testing. RESULTS Preimplantation iPTH increased the epiphyseal bone volume fraction by 31.6%. When the data at post-implantation weeks 1, 2, and 4 were averaged for the iPTH-treated mice, the bone volume fraction was 74.5% higher in the peri-implant region and 168% higher distal to the implant compared with the bone volume fractions in the same regions in the vehicle-treated mice. Additionally, the trabecular number was 84.8% greater in the peri-implant region and 74.3% greater distal to the implant. Metaphyseal osseointegration and bone area fraction were 28.1% and 70.1% higher, respectively, in the iPTH-treated mice than in the vehicle-treated mice, and the maximum implant pullout strength was 30.9% greater. iPTH also increased osteoblast and osteoclast density by 65.2% and 47.0%, respectively, relative to the values in the vehicle group, when the data at post-implantation weeks 1 and 2 were averaged. CONCLUSIONS iPTH increased osseointegration, cancellous mass, and the strength of the bone-implant interface. CLINICAL RELEVANCE Our murine model is an excellent platform on which to study biological enhancement of cancellous osseointegration.

Abstract Abstract 3261 Osteoporosis affects up to 75% of thalassemia patients. The th3/+ mouse has hematological and bone abnormalities similar to humans with β thalassemia intermedia. In this mouse, we documented decreased bone formation as a cause of bone loss. We used the th3/+ mouse to determine mechanisms by which abnormal erythropoiesis affects osteoblast function and leads to bone loss in thalassemia. Methods: Mesenchymal stem cells (MSCs) from wild type (wt) and th3/+ mice were differentiated into osteoblasts in osteogenic media. MSC co-cultures with various hematopoietic progenitors from wt and th3/+ animals were differentiated to osteoblasts similarly. Osteogenesis was determined by Colony Forming Units for Osteoblasts (CFU-O) numbers and qPCR for osteogenesis-related genes (genes that encode for Runx2, bone sialoprotein, osterix, osteocalcin expressed through osteoblast matuaration), with four experiments performed in triplicates. Results: CFU-O numbers were decreased by 40% in MSC cultures from th3/+ mice compared to wt, while gene expression was 40 to 60% lower, indicating alterations in MSC fate that led to decreased osteogenesis. Treatment of MSC cultures with erythropoietin (EPO) antibody (EPO Ab) or soluble EPO receptor (s EPOR) reversed the decreased CFU-O numbers and expression of osteogenesis-related genes in th3/+ animals, indicating that EPO signaling guides MSC fate in thalassemia. Consistently, qPCR analysis of MSCs and osteoblasts showed increased EPO and EPOR expression in th3/+ versus wt controls. To determine the role of hematopoietic progenitors on MSC fate, we isolated various progenitors and co-cultured them with MSC from wt animals. Lin-Sca+ ckit+ (LSK) precursors from th3/+ mice yielded 50% fewer CFU-O and decreased expression of osteogenic genes by 70%. In contrast, thalassemic erythroid precursors (CD71+Ter119- cells) and mature erythroid cells (Ter119+) had little to no effect on osteogenesis respectively. LSK-MSC co-cultures from wt and th3/+ mice were then treated with EPO Ab to determine the role of EPO on LSK-MSC interactions. Indeed, a reversal of the suppressed osteogenesis (determined by CFU-O numbers and qPCR results) was seen in th3/+ with such treatment, indicating a role of EPO signaling in LSK-MSK interactions. In support of these findings, EPOR and EPO expression in thalassemia LSK was increased two fold as compared to wt. Since Bone Morphogenic Proteins (BMPs) potentiate both erythroid expansion and osteogenesis, we examined BMP-2,-4,-6 expression in LSK from wt and th3/+ mice by qPCR. Expression of all three cytokines was decreased by 60–70% in th3/+, and reversed when thalassemic LSK cells were treated with EPO-Ab, suggesting that EPO affects LSK-MSC interaction by regulating BMP expression in LSK. In summary, we observed decreased osteogenesis as result of altered MSC differentiation in the th3/+ thalassemia mouse. MSC fate is regulated by both hematopoietic progenitors and EPO. Specifically, LSK but not mature erythroid cells interact with MSCs in th3/+ to decrease osteogenesis. Our data support a dual role of EPO on osteogenesis in thalassemia. First, EPO produced by thalassemic MSCs alters osteogenesis via direct autocrine EPO signaling. Second, EPO also guides MSC-LSK interactions by regulating expression of BMPs -2,-4 and 6 in hematopoietic progenitors. Therefore, we conclude that EPO through an extra-hematopoietic action leads to bone loss in thalassemia. Disclosures: Rivella: Novartis Pharmaceuticals: Consultancy; Biomarin: Consultancy; Merganser Biotech: Consultancy, Equity Ownership, Research Funding; Isis Pharma: Consultancy, Research Funding.

Glucocorticoids (GCs) inhibit the resorptive capacity of the osteoclast by disrupting its cytoskeleton. We find that calpain-6 (Capn6), a unique, nonproteolytic member of its family, is suppressed 12-fold by dexamethasone (DEX) in the bone-degrading cell. While Capn6 abundance parallels commitment of naive bone marrow macrophages (BMMs) to the osteoclast phenotype, its excess or deletion does not affect the cell's differentiation. On the other hand, Capn6 localizes to the sealing zone, and its overexpression promotes osteoclast spreading and large actin ring formation, eventuating in stimulated bone degradation. Conversely, Capn6 knockdown impairs cytoskeletal organization and the cell's resorptive capacity. Capn6 complexes with tubulin, and its absence inhibits microtubule acetylation and stability in the osteoclast. Knockdown of Capn6 also reduces beta(3)-integrin subunit protein, another essential regulator of osteoclast cytoskeletal function. Reflecting Capn6 as a target molecule of GCs, microtubule stability and acetylation, as well as the expression of beta(3)-integrin protein, are similarly suppressed in DEX-treated osteoclasts. Moreover, overexpression of Capn6 rescues GC-mediated disruption of osteoclast cytoskeleton. Thus Capn6 promotes cytoskeletal organization and microtubule stability in osteoclasts, and its inhibition may mediate the resorption-arresting properties of GCs. (c) 2011 American Society for Bone and Mineral Research,

Cdc42 mediates bone resorption principally by stimulating osteoclastogenesis. Whether its sister GTPase, Rac, meaningfully impacts upon the osteoclast and, if so, by what means, is unclear. We find that whereas deletion of Rac1 or Rac2 alone has no effect, variable reduction of Rac1 in osteoclastic cells of Rac2(-/-) mice causes severe osteopetrosis. Osteoclasts lacking Rac1 and Rac2 in combination (Rac double-knockout, RacDKO), fail to effectively resorb bone. By contrast, osteoclasts are abundant in RacDKO osteopetrotic mice and, unlike those deficient in Cdc42, express the maturation markers of the cells normally. Hence, the osteopetrotic lesion of RacDKO mice largely reflects impaired function, and not arrested differentiation, of the resorptive polykaryon. The dysfunction of RacDKO osteoclasts represents failed cytoskeleton organization as evidenced by reduced motility of the cells and their inability to spread or generate the key resorptive organelles (i.e. actin rings and ruffled borders), which is accompanied by abnormal Arp3 distribution. The cytoskeleton-organizing capacity of Rac1 is mediated through its 20-amino-acid effector domain. Thus, Rac1 and Rac2 are mutually compensatory. Unlike Cdc42 deficiency, their combined absence does not impact upon differentiation but promotes severe osteopetrosis by dysregulating the osteoclast cytoskeleton.

The capacity of the osteoclast (OC) to resorb bone is dictated by cytoskeletal organization, which in turn emanates from signals derived from the alpha(v)beta(3) integrin and c-Fms. Syk is key to these signals and, in other cells, this tyrosine kinase exerts its effects via intermediaries including the SLIP adaptors, SLP-76 and BLNK (B cell linker). Thus, we asked whether these two SLP proteins regulate OC function. We find BLNK-deficient OCs are normal, whereas cytoskeletal organization of those lacking SLP-76 is delayed, thus modestly reducing bone resorption in vitro. Cytoskeletal organization and bone resorption are more profoundly arrested in cultured OCs deficient in BLNK and SLP-76 double knockout (DKO) phenotypes. In contrast, stimulated bone resorption in vivo is inhibited similar to 40% in either SLP-76(-/-) or DKO mice. This observation, taken with the fact that DKO OCs are rescued by retroviral transduction of only SLP-76, indicates that SLP-76 is the dominant SLP family member in the resorptive process. We also find SLP-76 is phosphorylated in a Syk-dependent manner. Furthermore, in the absence of the adaptor protein, integrin-mediated phosphorylation of Vav3, the OC cytoskeleton-organizing guanine nucleotide exchange factor, is abrogated. In keeping with a central role of SLP-76/Vav3 association in osteoclastic resorption, retroviral transduction of SLP-76, in which the Vav binding site is disrupted (3YF), fails to normalize the cytoskeleton of DKO OCs and the resorptive capacity of the cells. Finally, c-Fms-activated Syk also exerts its OC cytoskeleton-organizing effect in a SLP-76/Vav3-dependent manner. The Journal of Immunology, 2009, 183: 1804-1812.

We examined the mechanism by which M-CSF regulates the cytoskeleton and function of the osteoclast, the exclusive bone resorptive cell. We show that binding of M-CSF to its receptor c-Fms generates a signaling complex comprising phosphorylated DAP12, an adaptor containing an immunoreceptor tyrosine-based activation motif (ITAM) and the nonreceptor tyrosine kinase Syk. c-Fms tyrosine 559, the exclusive binding site of c-Src, is necessary for regulation of DAP12/Syk signaling. Deletion of either of these molecules yields osteoclasts that fail to reorganize their cytoskeleton. Retroviral transduction of null precursors with wild-type or mutant DAP12 or Syk reveals that the SH2 domain of Syk and the ITAM tyrosine residues and transmembrane domain of DAP12 mediate M-CSF signaling. Our data provide genetic and biochemical evidence that uncovers an epistatic signaling pathway linking the receptor tyrosine kinase c-Fms to the immune adaptor DAP12 and the cytoskeleton.