Epigenetic dysregulation of articular cartilage during progression of hip femoroacetabular impingement disease

Femoroacetabular impingement (FAI) is an important trigger of hip osteoarthritis (OA). Epigenetic changes in DNA methyltransferase 3B (DNMT3B) attenuate catabolic gene expression in cartilage hemostasis. This study aimed to examine the articular chondrocyte catabolic state and DNMT3B and 4-aminobutyrate aminotransferase promoter (ABAT) expression during OA progression in FAI. Cartilage samples were collected from the impingement zone of 12 patients with cam FAI (early-FAI) and 12 patients with advanced OA secondary to cam FAI (late-FAI-OA). Five healthy samples were procured from cadavers (ND: nondiseased). Explants were cultured under unstimulated conditions, catabolic stimulus (IL1β), or anabolic stimulus (TGFβ). Histology was performed with safranin-O/fast-green staining. Gene expression was analyzed via qPCR for GAPDH, DNMT3B, ABAT, MMP-13, COL10A1. Methylation specific PCR assessed methylation status at the ABAT promoter. Cartilage samples in early-FAI and late-FAI-OA showed a histological OA phenotype and increased catabolic marker expression (MMP13/COL10A1, ND vs. early-FAI, p = 0.004/p < 0.001, ND vs. late-FAI-OA, p < 0.001/p < 0.001). RT-PCR confirmed DNMT3B underexpression (ND vs. early-FAI, p < 0.001, early-FAI vs. late-FAI-OA, p = 0.016) and ABAT overexpression (ND vs. early-FAI, p < 0.001, early vs. late-FAI-OA, p = 0.035) with advanced disease. End-stage disease showed ABAT promoter hypomethylation. IL1β stimulus accentuated ABAT promoter hypomethylation and led to further ABAT and catabolic marker overexpression in early-FAI and late-FAI-OA while TGFβ normalized these alterations in gene expression. Catabolic and epigenetic molecule expression suggested less catabolism in early-stage disease. Sustained inflammation induced ABAT promoter hypo-methylation causing a catabolic phenotype. Suppression of ABAT by methylation control could be a new target for therapeutic intervention to prevent OA progression in hip FAI.

PROMIS Physical Function and Pain Interference Scores Correlate with the Lower Extremity Toronto Extremity Salvage Score

The Toronto Extremity Salvage Score (TESS) and the National Institutes of Health Patient-Reported Outcomes Measurement Information System (PROMIS) are both utilized to measure patient-reported outcomes in adults with musculoskeletal oncologic conditions. However, the relationship between them has not been studied. We sought to describe a link between Lower Extremity (LE) TESS and PROMIS Physical Function (PF) scores, as well as between LE TESS and Pain Interference (PI) scores, to develop a method for converting scores between TESS and PROMIS and to examine whether TESS and PROMIS captured differences in pain and function between clinically relevant subgroups in our population.

Methods

Our study population consisted of 125 adult patients who underwent surgical treatment of a lower-extremity musculoskeletal tumor at a single sarcoma center between December 2015 and October 2018. The LE TESS questionnaire was administered to patients via paper and the PROMIS PF and PI were administered via iPad at a preoperative appointment. The relationship between LE TESS and PROMIS measures was analyzed with use of generalized linear modeling. Subgroup analyses were performed with a 2-tailed t test or 1-way analysis of variance.

Results

PROMIS PF had a very strong positive correlation with LE TESS (r = 0.83) and was related through the following equation: PROMIS PF = 0.00294 × (LE TESS)² + 22.6. PROMIS PI had a strong negative correlation with LE TESS (r = -0.77) and was related through the following equation: PROMIS PI = -0.00259 × (LE TESS)² + 73.8. PROMIS PF and PI performed similarly to LE TESS across multiple patient subgroups and captured the expected differences between subgroups.

Conclusions

LE TESS and PROMIS PF appeared to measure similar information in patients with an orthopaedic oncologic condition. Moreover, PROMIS PI scores were strongly correlated with functional disability as measured with the LE TESS. Understanding the relationship between TESS and PROMIS will allow the comparison and combination of data for both clinical and research purposes.

Level of Evidence

Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Did the Physical and Mental Health of Orthopaedic Patients Change After the Onset of the COVID-19 Pandemic?

BACKGROUND

The 2019 novel coronavirus (COVID-19) pandemic has been associated with poor mental health outcomes and widened health disparities in the United States. Given the inter-relationship between psychosocial factors and functional outcomes in orthopaedic surgery, it is important that we understand whether patients presenting for musculoskeletal care during the pandemic were associated with worse physical and mental health than before the pandemic's onset.

QUESTIONS/PURPOSES

(1) Did patients seen for an initial visit by an orthopaedic provider during the COVID-19 pandemic demonstrate worse physical function, pain interference, depression, and/or anxiety than patients seen before the pandemic, as measured by the Patient-Reported Outcomes Measurement Information System (PROMIS) instrument? (2) During the COVID-19 pandemic, did patients living in areas with high levels of social deprivation demonstrate worse patterns of physical function, pain interference, depression, or anxiety on initial presentation to an orthopaedic provider than patients living in areas with low levels of social deprivation, compared with prepandemic PROMIS scores?

METHODS

This was a retrospective, comparative study of new patient evaluations that occurred in the orthopaedic department at a large, urban tertiary care academic medical center. During the study period, PROMIS computer adaptive tests were routinely administered to patients at clinical visits. Between January 1, 2019, and December 31, 2019, we identified 26,989 new patients; we excluded 4% (1038 of 26,989) for being duplicates, 4% (1034 of 26,989) for having incomplete demographic data, 44% (11,925 of 26,989) for not having a nine-digit home ZIP Code recorded, and 5% (1332 of 26,989) for not completing all four PROMIS computer adaptive tests of interest. This left us with 11,660 patients in the "before COVID-19" cohort. Between January 1, 2021 and December 31, 2021, we identified 30,414 new patients; we excluded 5% (1554 of 30,414) for being duplicates, 4% (1142 of 30,414) for having incomplete demographic data, 41% (12,347 of 30,414) for not having a nine-digit home ZIP Code recorded, and 7% (2219 of 30,414) for not completing all four PROMIS computer adaptive tests of interest. This left us with 13,152 patients in the "during COVID-19" cohort. Nine-digit home ZIP Codes were used to determine patients' Area Deprivation Indexes, a neighborhood-level composite measure of social deprivation. To ensure that patients included in the study represented our overall patient population, we performed univariate analyses on available demographic and PROMIS data between patients included in the study and those excluded from the study, which revealed no differences (results not shown). In the before COVID-19 cohort, the mean age was 57 ± 16 years, 60% (7046 of 11,660) were women, 86% (10,079 of 11,660) were White non-Hispanic, and the mean national Area Deprivation Index percentile was 47 ± 25. In the during COVID-19 cohort, the mean age was 57 ± 16 years, 61% (8051 of 13,152) were women, 86% (11,333 of 13,152) were White non-Hispanic, and the mean national Area Deprivation Index percentile was 46 ± 25. The main outcome measures in this study were the PROMIS Physical Function ([PF], version 2.0), Pain Interference ([PI], version 1.1), Depression (version 1.0), and Anxiety (version 1.0). PROMIS scores follow a normal distribution with a mean t-score of 50 and a standard deviation of 10. Higher PROMIS PF scores indicate better self-reported physical capability, whereas higher PROMIS PI, Depression, and Anxiety scores indicate more difficulty managing pain, depression, and anxiety symptoms, respectively. Clinically meaningful differences in PROMIS scores between the cohorts were based on a minimum clinically important difference (MCID) threshold of 4 points. Multivariable linear regression models were created to determine whether presentation to an orthopaedic provider during the pandemic was associated with worse PROMIS scores than for patients who presented before the pandemic. Regression coefficients (ß) represent the estimated difference in PROMIS scores that would be expected for patients who presented during the pandemic compared with patients who presented before the pandemic, after adjusting for confounding variables. Regression coefficients were evaluated in the context of clinical importance and statistical significance. Regression coefficients equal to or greater than the MCID of 4 points were considered clinically important, whereas p values < 0.05 were considered statistically significant.

RESULTS

We found no clinically important differences in baseline physical and mental health PROMIS scores between new patients who presented to an orthopaedic provider before the COVID-19 pandemic and those who presented during the COVID-19 pandemic (PROMIS PF: ß -0.2 [95% confidence interval -0.43 to 0.03]; p = 0.09; PROMIS PI: ß 0.06 [95% CI -0.13 to 0.25]; p = 0.57; PROMIS Depression: ß 0.09 [95% CI -0.14 to 0.33]; p = 0.44; PROMIS Anxiety: ß 0.58 [95% CI 0.33 to 0.84]; p < 0.001). Although patients from areas with high levels of social deprivation had worse PROMIS scores than patients from areas with low levels of social deprivation, patients from areas with high levels of social deprivation demonstrated no clinically important differences in PROMIS scores when groups before and during the pandemic were compared (PROMIS PF: ß -0.23 [95% CI -0.80 to 0.33]; p = 0.42; PROMIS PI: ß 0.18 [95% CI -0.31 to 0.67]; p = 0.47; PROMIS Depression: ß 0.42 [95% CI -0.26 to 1.09]; p = 0.23; PROMIS Anxiety: ß 0.84 [95% CI 0.16 to 1.52]; p = 0.02).

CONCLUSION

Contrary to studies describing worse physical and mental health since the onset of the COVID-19 pandemic, we found no changes in the health status of orthopaedic patients on initial presentation to their provider. Although large-scale action to mitigate the effects of worsening physical or mental health of orthopaedic patients may not be needed at this time, orthopaedic providers should remain aware of the psychosocial needs of their patients and advocate on behalf of those who may benefit from intervention. Our study is limited in part to patients who had the self-agency to access specialty orthopaedic care, and therefore may underestimate the true changes in the physical or mental health status of all patients with musculoskeletal conditions. Future longitudinal studies evaluating the impact of specific COVID-19-related factors (for example, delays in medical care, social isolation, or financial loss) on orthopaedic outcomes may be helpful to prepare for future pandemics or natural disasters.

LEVEL OF EVIDENCE

Level II, prognostic study.

A Novel Model of Hip Femoroacetabular Impingement in Immature Rabbits Reproduces the Distinctive Head-Neck Cam Deformity

BACKGROUND

Femoroacetabular impingement (FAI) is a leading cause of hip pain in young adults and often leads to degenerative osteoarthritis (OA). A small animal model of hip deformities is crucial for unraveling the pathophysiology of hip OA secondary to FAI.

PURPOSES

To (1) characterize a new minimally invasive surgical technique to create a proximal femoral head-neck deformity in a skeletally immature rabbit model and (2) document the effect of an injury to the medial proximal femoral epiphysis on head-neck morphology at 28 days after the injury.

STUDY DESIGN

Controlled laboratory study.

METHODS

Six-week-old New Zealand White rabbits (n = 10) were subjected to right hip surgery, with the left hip used as a control. An epiphyseal injury in the medial femoral head was created using a 1.6-mm drill. Hips were harvested bilaterally at 28 days after surgery. Alpha and epiphyseal shaft angles were measured on radiographs. Alpha angles at the 1- and 3-o'clock positions were measured on the oblique axial plane of micro-computed tomography images. Bone bar formation secondary to growth plate injuries was confirmed using alcian blue hematoxylin staining.

RESULTS

All hips in the study group showed a varus-type head-neck deformity, with lower epiphyseal shaft angles on anteroposterior radiographs versus those in the control group (133°± 8° vs 142°± 5°, respectively; P = .022) and higher epiphyseal shaft angles on lateral radiographs (27°± 12° vs 10°± 7°, respectively; P < .001). The mean alpha angles in the study group were higher at both the 1- (103°± 14° vs 46°± 7°, respectively; P < .002) and 3-o'clock (99°± 18° vs 35°± 11°, respectively; P < .002) positions than those in the control group. Alcian blue hematoxylin staining of all hips in the study group indicated that the injured physis developed a bony bar, leading to growth plate arrest on the medial femoral head.

CONCLUSION

The proposed model led to growth arrest at the proximal femoral physis, resulting in a femoral head-neck deformity similar to human FAI.

CLINICAL RELEVANCE

Our novel small animal model of a femoral head-neck deformity is a potential platform for research into the basic mechanisms of FAI disease progression and the development of disease-modifying therapies.

Distinct Pattern of Inflammation of Articular Cartilage and the Synovium in Early and Late Hip Femoroacetabular Impingement

BACKGROUND

The molecular mechanism of how femoroacetabular impingement (FAI) morphology leads to hip osteoarthritis (OA) is yet to be determined. The expression and location of inflammation-related molecules during early- and late-stage FAI have not been previously described. Moreover, the characterization of intra-articular inflammation away from the cam deformity as well as the nature of adjacent synovial tissue have also not been extensively reported.

HYPOTHESIS

Early-stage FAI has a similar expression of inflammation-related markers in the head-neck and acetabular cartilage but less synovitis than late-stage FAI.

STUDY DESIGN

Controlled laboratory study.

METHODS

Head-neck cartilage, acetabular cartilage, and synovial samples were obtained from patients undergoing hip preservation surgery for the treatment of symptomatic cam FAI (early FAI group; n = 15) and advanced OA secondary to cam FAI (late FAI group; n = 15). Samples procured from healthy young adult donors served as the control group (n = 7). Cartilage degeneration was assessed by histology, and the expression of inflammation-related proteins (interleukin-1 beta [IL-1β], matrix metalloproteinase-13 [MMP-13], a disintegrin and metalloproteinase with thrombospondin motifs-4 [ADAMTS-4], type II collagen [COL2], and aggrecan neoepitope [NITEGE]) was measured by immunostaining. Synovial samples in the early and late FAI groups were examined for synovitis and the expression of IL-1β.

RESULTS

Head-neck cartilage in the early FAI group showed significantly more degeneration than the control group and an increased expression of inflammation-related proteins (IL-1β: 69.7% ± 18.1% vs 20.2% ± 4.9%, respectively; MMP-13: 79.6% ± 12.6% vs 25.3% ± 9.5%; ADAMTS-4: 83.9% ± 12.2% vs 24.3% ± 11.1%; NITEGE: 89.7% ± 7.7% vs 39.8% ± 20.5%) (P < .001). Head-neck and acetabular cartilage in the early and late FAI groups showed a similar degree of degeneration. Moreover, a similar expression of inflammation-related proteins was observed between the early and late FAI groups for head-neck cartilage (IL-1β: 69.7% ± 18.1% vs 72.5% ± 13.2%; MMP-13: 79.6% ± 12.6% vs 71.4% ± 18.8%; ADAMTS-4: 83.9% ± 12.2% vs 82.6% ± 12.5%; COL2: 93.6% ± 3.9% vs 92.5% ± 5.8%; NITEGE: 89.7% ± 7.7% vs 95.7% ± 4.7%) and acetabular cartilage (IL-1β: 83.3% ± 24.8% vs 80.7% ± 15.6%; MMP-13: 94.3% ± 9.7% vs 85.2% ± 12.3%; ADAMTS-4: 98.5% ± 2.3% vs 98.4% ± 3.4%; COL2: 99.8% ± 0.7% vs 99.7% ± 1.1%; NITEGE: 96.7% ± 6.7% vs 99.2% ± 2.2%). In contrast, synovitis was minimal with a low expression of IL-1β in the early FAI group compared with the late FAI group.

CONCLUSION

Hip cartilage exhibited an OA phenotype in patients with early-stage FAI, similar to what was observed in hip OA secondary to FAI. Severe synovitis was only evident with late-stage FAI.

CLINICAL RELEVANCE

This study supports the concept that early hip impingement is associated with cartilage degeneration and catabolism.

Inflammatory Response of Articular Cartilage to Femoroacetabular Impingement in the Hip

BACKGROUND

Femoroacetabular impingement (FAI) has been proposed as an etiologic factor in up to 50% of hips with osteoarthritis (OA). Inflammation is thought to be one of the main initiators of OA, yet little is known about the origin of intra-articular inflammation in FAI hips.

HYPOTHESIS

Articular cartilage from the impingement zone of patients with FAI has high levels of inflammation, reflecting initial inflammatory process in the hip.

STUDY DESIGN

Controlled laboratory study.

METHODS

Head-neck cartilage samples were obtained from patients with cam FAI (cam FAI, early FAI; n = 15), advanced OA secondary to cam FAI (FAI OA, late FAI; n = 15), and advanced OA secondary to developmental dysplasia of the hip (DDH OA, no impingement; n = 15). Cartilage procured from young adult donors (n = 7) served as control. Safranin O-stained sections were assessed for cartilage abnormality. Tissue viability was detected by TUNEL assay. Immunostaining of interleukin 1β (IL-1β), catabolic markers (matrix metalloproteinase 13 [MMP-13], a disintegrin and metalloproteinase with thrombospondin motif 4 [ADAMTS-4], aggrecan antibody to C-terminal neoepitope [NITEGE]), and an anabolic marker (type II collagen [COL2]) was performed to evaluate molecular inflammation and metabolic activity. The average percentage of immunopositive cells from the total cell count was calculated. Kruskal-Wallis test followed by Steel-Dwass post hoc test was used for multiple comparisons.

RESULTS

Microscopic osteoarthritic changes were more prevalent in cartilage of cam FAI and FAI OA groups compared with DDH OA and control groups. Cartilage in cam FAI and FAI OA groups, versus the DDH group, had higher expression of inflammatory molecules IL-1β (69.7% ± 18.1% and 72.5% ± 13.2% vs 32.7% ± 14.4%, respectively), MMP-13 (79.6% ± 12.6% and 71.4% ± 18.8% vs 38. 5% ± 13.3%), ADAMTS-4 (83.9% ± 12.2% and 82.6% ± 12.5% vs 45.7% ± 15.5%), and COL2 (93.6% ± 3.9% and 92.5% ± 5.8% vs 53.3% ± 21.0%) (P < .001). Expression of NITEGE was similar among groups (cam FAI, 89.7% ± 7.7%; FAI OA, 95.7% ± 4.7%; DDH OA, 93.9% ± 5.2%; P = .0742). The control group had minimal expression of inflammatory markers. Inflammatory markers were expressed in all cartilage zones of early and late FAI but only in the superficial zone of the no impingement group.

CONCLUSION

Cartilage from the impingement zone in FAI is associated with a high expression of inflammatory markers, extending throughout all cartilage zones.

CLINICAL RELEVANCE

Inflammation associated with FAI likely has a deleterious effect on joint homeostasis. Further clinical and translational studies are warranted to assess whether and how surgical treatment of FAI reduces molecular inflammation.

Inhibition of 4-aminobutyrate aminotransferase protects against injury-induced osteoarthritis in mice

Recently we demonstrated that ablation of the DNA methyltransferase enzyme, Dnmt3b, resulted in catabolism and progression of osteoarthritis (OA) in murine articular cartilage through a mechanism involving increased mitochondrial respiration. In this study, we identify 4-aminobutyrate aminotransferase (Abat) as a downstream target of Dnmt3b. Abat is an enzyme that metabolizes γ-aminobutyric acid to succinate, a key intermediate in the tricarboxylic acid cycle. We show that Dnmt3b binds to the Abat promoter, increases methylation of a conserved CpG sequence just upstream of the transcriptional start site, and inhibits Abat expression. Dnmt3b deletion in articular chondrocytes results in reduced methylation of the CpG sequence in the Abat promoter, which subsequently increases expression of Abat. Increased Abat expression in chondrocytes leads to enhanced mitochondrial respiration and elevated expression of catabolic genes. Overexpression of Abat in murine knee joints via lentiviral injection results in accelerated cartilage degradation following surgical induction of OA. In contrast, lentiviral-based knockdown of Abat attenuates the expression of IL-1β-induced catabolic genes in primary murine articular chondrocytes in vitro and also protects against murine articular cartilage degradation in vivo. Strikingly, treatment with the FDA-approved small-molecule Abat inhibitor, vigabatrin, significantly prevents the development of injury-induced OA in mice. In summary, these studies establish Abat as an important new target for therapies to prevent OA.

Variation in the Delivery of Inpatient Orthopaedic Care to Medicaid Beneficiaries within a Single Metropolitan Region

BACKGROUND

There is variability in access to and utilization of orthopaedic care, particularly for those with Medicaid insurance. One potential contributor is perceived unwillingness of surgeons and hospitals to accept underinsured patients. We used administrative data to examine the payer mix for select inpatient orthopaedic surgical procedures at all hospitals within a single region, hypothesizing that the delivery of orthopaedic surgery to Medicaid beneficiaries varies highly at the hospital level.

METHODS

Using administrative data, we analyzed inpatient hospitalizations for elective cases (total knee or hip arthroplasty; spinal decompression or fusion) and trauma cases (hip hemiarthroplasty; femoral or tibial and fibular fracture repair) among 22 hospitals in a single region from 2011 to 2016 for patients who were 18 to 64 years of age. The primary outcome was the percentage of each hospital's caseload with Medicaid listed as the primary payer. The secondary outcome measured each hospital's Medicaid percentage against the percentage of Medicaid-insured individuals within 10 miles of the hospital (Medicaid share ratio), using a ratio of 1 as a benchmark. To quantify variation, we calculated a weighted coefficient of variation of the Medicaid share ratio for all cases combined, elective cases only, and trauma cases only.

RESULTS

For all cases (n = 19,204), the mean percentage of Medicaid-funded surgical procedures was 7.6% (range, 0.2% to 57.3%). The mean Medicaid share ratio was 1.0 (range, 0.05 to 4.20). Across 22 hospitals, the weighted coefficient of variation for Medicaid share was 69, indicating very high variation. For elective cases alone, the mean percentage of Medicaid-funded surgical procedures was 5.5% (range, 0.2% to 64.6%). The mean Medicaid share ratio was 0.71 (range, 0.05 to 4.73), and the weighted coefficient of variation was 93. For trauma cases alone, Medicaid-funded surgical procedures were 14.7% (range, 0.0% to 35.7%). The mean Medicaid share ratio was 2.0 (range, 0 to 3.93), and the weighted coefficient of variation was 34.

CONCLUSIONS

Delivery of care was highly variable when benchmarking against the insurance composition of each hospital's surrounding community. Although generalizability to other regions is limited, our findings support previously asserted notions that delivery of orthopaedic care may differ on the basis of socioeconomic markers (such as insurance status). If not addressed, these inequities may exacerbate existing racially and socioeconomically based disparities in care.

Epigenetic and therapeutic implications of dnmt3b in temporomandibular joint osteoarthritis

Temporomandibular joint (TMJ) arthritis causes severe debilitation and has few treatment options. Here, we found a small molecule, DNA methyltransferase 3B (Dnmt3b), as a putative therapeutic target, partially rescued osteoarthritic phenotype. Dnmt3b was detected differentially expressed in cell zones of mandibular condylar cartilage and the expression of Dnmt3b decreased in the progression of TMJ osteoarthritis. Dnmt3b deficiency using conditional knockout mice led to the onset of osteoarthritis-like conditions including cartilage clefts, cartilage matrix loss and premature chondrocyte hypertrophy, which suggested that Dnmt3b functioned as a osteoarthritis suppressor. Dnmt3b gain-of-function in TMJ stem/progenitor cells showed increases in collagen type II but decreases in collagen type X, whereas Dnmt3b knockdown had opposite effects with attenuated collagen type II but increased collagen type X. Dnmt3b acted via Wnt/β-catenin signaling and Dnmt3b regulated TMJ stem/progenitor cells differentiation by inducing their premature progression towards hypertrophic chondrocytes through β-catenin transnucleation and activation. Finally, local Dnmt3b delivery partially rescued cartilage degradation in experimentally induced osteoarthritis. Thus, novel molecules in articular cartilage, such as Dnmt3b, may have therapeutic effects for TMJ osteoarthritis.

Cyclooxygenase-2 deficiency causes delayed ossification of lumbar vertebral endplates

Based on previous findings that cyclooxygenase-2 (COX-2) is a critical molecule in chondrocyte differentiation and skeletal repair, we hypothesized that COX-2 deficiency or inhibition affects the ossification of vertebral endplates (VEP) and degeneration of intervertebral discs (IVD) and thus is involved in the pathogenesis of low back pain (LBP). We aimed to delineate the COX-2 working mechanism and its interacting molecules, and to explore the effect of NSAIDs and selective COX-2 inhibitor on degenerative spinal diseases. Here, lumbar spinal samples harvested from Cox-2 mutant (Cox-2-/-) and wild type (WT) mice were used for histological examinations. Nucleus pulposus (NP) cells isolated from rat were treated with PGE-2. Mouse endplate chondrocytes (mEC) isolated from mice were treated with a recombinant sonic hedgehog (Shh) protein. A mouse IVD organ culture system was established and treated COX-2 inhibitor Celecoxib. Human lumbar endplate chondrocytes were cultured and treated with Celecoxib. Immunohistochemical (IHC) studies were done in the human and mouse VEP samples. Radiographic and histological examinations revealed delayed VEP ossification in Cox-2-/- mice compared to WT ones. Decreased PGE-2 expression was found to promote Shh expression in rat NP cells, while Shh increased noggin expression in mEC. IHC showed that noggin expression was increased while pSmad1 expression decreased in the VEP of Cox-2-/- mice. Human VEP samples from patients with severe IVD degeneration showed decreased expression of Shh and noggin and increased expression of COX-2 and pSmad1 compared with milder cases. In cultured mouse IVDs and human endplate chondrocytes, Celecoxib enhanced expression of Shh and noggin and decreased Smad1 phosphorylation. In conclusion, COX-2/PGE-2 axis plays an important role in VEP ossification and IVD degeneration through crosstalk with Shh and BMP signaling pathways. These findings may facilitate clinical use of COX-2 inhibitor to prevent LBP progression.

Systemic EP4 Inhibition Increases Adhesion Formation in a Murine Model of Flexor Tendon Repair

Flexor tendon injuries are a common clinical problem, and repairs are frequently complicated by post-operative adhesions forming between the tendon and surrounding soft tissue. Prostaglandin E2 and the EP4 receptor have been implicated in this process following tendon injury; thus, we hypothesized that inhibiting EP4 after tendon injury would attenuate adhesion formation. A model of flexor tendon laceration and repair was utilized in C57BL/6J female mice to evaluate the effects of EP4 inhibition on adhesion formation and matrix deposition during flexor tendon repair. Systemic EP4 antagonist or vehicle control was given by intraperitoneal injection during the late proliferative phase of healing, and outcomes were analyzed for range of motion, biomechanics, histology, and genetic changes. Repairs treated with an EP4 antagonist demonstrated significant decreases in range of motion with increased resistance to gliding within the first three weeks after injury, suggesting greater adhesion formation. Histologic analysis of the repair site revealed a more robust granulation zone in the EP4 antagonist treated repairs, with early polarization for type III collagen by picrosirius red staining, findings consistent with functional outcomes. RT-PCR analysis demonstrated accelerated peaks in F4/80 and type III collagen (Col3a1) expression in the antagonist group, along with decreases in type I collagen (Col1a1). Mmp9 expression was significantly increased after discontinuing the antagonist, consistent with its role in mediating adhesion formation. Mmp2, which contributes to repair site remodeling, increases steadily between 10 and 28 days post-repair in the EP4 antagonist group, consistent with the increased matrix and granulation zones requiring remodeling in these repairs. These findings suggest that systemic EP4 antagonism leads to increased adhesion formation and matrix deposition during flexor tendon healing. Counter to our hypothesis that EP4 antagonism would improve the healing phenotype, these results highlight the complex role of EP4 signaling during tendon repair.

Development of antisense oligonucleotide (ASO) technology against Tgf-β signaling to prevent scarring during flexor tendon repair

Flexor tendons (FT) in the hand provide near frictionless gliding to facilitate hand function. Upon injury and surgical repair, satisfactory healing is hampered by fibrous adhesions between the tendon and synovial sheath. In the present study we used antisense oligonucleotides (ASOs), specifically targeted to components of Tgf-β signaling, including Tgf-β1, Smad3 and Ctgf, to test the hypothesis that local delivery of ASOs and suppression of Tgf-β1 signaling would enhance murine FT healing by suppressing adhesion formation while maintaining strength. ASOs were injected in to the FT repair site at 2, 6 and 12 days post-surgery. ASO treatment suppressed target gene expression through 21 days. Treatment with Tgf-β1, Smad3 or Ctgf ASOs resulted in significant improvement in tendon gliding function at 14 and 21 days, relative to control. Consistent with a decrease in adhesions, Col3a1 expression was significantly decreased in Tgf-β1, Smad3 and Ctgf ASO treated tendons relative to control. Smad3 ASO treatment enhanced the maximum load at failure of healing tendons at 14 days, relative to control. Taken together, these data support the use of ASO treatment to improve FT repair, and suggest that modulation of the Tgf-β1 signaling pathway can reduce adhesions while maintaining the strength of the repair.

BMP-2 induces ATF4 phosphorylation in chondrocytes through a COX-2/PGE2 dependent signaling pathway

OBJECTIVE

Bone morphogenic protein (BMP)-2 is approved for fracture non-union and spine fusion. We aimed to further dissect its downstream signaling events in chondrocytes with the ultimate goal to develop novel therapeutics that can mimic BMP-2 effect but have less complications.

METHODS

BMP-2 effect on cyclooxygenase (COX)-2 expression was examined using Real time quantitative PCR (RT-PCR) and Western blot analysis. Genetic approach was used to identify the signaling pathway mediating the BMP-2 effect. Similarly, the pathway transducing the PGE2 effect on ATF4 was investigated. Immunoprecipitation (IP) was performed to assess the complex formation after PGE2 binding.

RESULTS

BMP-2 increased COX-2 expression in primary mouse costosternal chondrocytes (PMCSC). The results from the C9 Tet-off system demonstrated that endogenous BMP-2 also upregulated COX-2 expression. Genetic approaches using PMCSC from ALK2(fx/fx), ALK3(fx/fx), ALK6(-/-), and Smad1(fx/fx) mice established that BMP-2 regulated COX-2 through activation of ALK3-Smad1 signaling. PGE-2 EIA showed that BMP-2 increased PGE2 production in PMCSC. ATF4 is a transcription factor that regulates bone formation. While PGE2 did not have significant effect on ATF4 expression, it induced ATF4 phosphorylation. In addition to stimulating COX-2 expression, BMP-2 also induced phosphorylation of ATF4. Using COX-2 deficient chondrocytes, we demonstrated that the BMP-2 effect on ATF4 was COX-2-dependent. Tibial fracture samples from COX-2(-/-) mice showed reduced phospho-ATF4 immunoreactivity compared to wild type (WT) ones. PGE2 mediated ATF4 phosphorylation involved signaling primarily through the EP2 and EP4 receptors and PGE2 induced an EP4-ERK1/2-RSK2 complex formation.

CONCLUSIONS

BMP-2 regulates COX-2 expression through ALK3-Smad1 signaling, and PGE2 induces ATF4 phosphorylation via EP4-ERK1/2-RSK2 axis.

Cellular and molecular factors in flexor tendon repair and adhesions: a histological and gene expression analysis

Flexor tendon healing is mediated by cell proliferation, migration, and extracellular matrix synthesis that contribute to the formation of scar tissue and adhesion. The biological mechanisms of flexor tendon adhesion formation have been linked to transforming growth factor β (TGF-β). To elucidate the cellular and molecular events in this pathology, we implanted live flexor digitorum longus grafts from the reporter mouse Rosa26(LacZ/+) in wild-type recipients, and used histological β-galactosidase (β-gal) staining to evaluate the intrinsic versus extrinsic cellular origins of scar, and reverse transcription-polymerase chain reaction to measure gene expression of TGF-β and its receptors, extracellular matrix proteins, and matrix metalloproteinases (MMPs) and their regulators. Over the course of healing, graft cellularity and β-gal activity progressively increased, and β-gal-positive cells migrated out of the Rosa26(LacZ/+) graft. In addition, there was an evidence of influx of host cells (β-gal-negative) into the gliding space and the graft, suggesting that both graft and host cells contribute to adhesions. Interestingly, we observed a biphasic pattern in which Tgfb1 expression was the highest in the early phases of healing and gradually decreased thereafter, whereas Tgfb3 increased and remained upregulated later. The expression of TGF-β receptors was also upregulated throughout the healing phases. In addition, type III collagen and fibronectin were upregulated during the proliferative phase of healing, confirming that murine flexor tendon heals by scar tissue. Furthermore, gene expression of MMPs showed a differential pattern in which inflammatory MMPs were the highest early and matrix MMPs increased over time. These findings offer important insights into the complex cellular and molecular factors during flexor tendon healing.

Gene expression analysis of the pleiotropic effects of TGF-β1 in an in vitro model of flexor tendon healing

Flexor tendon injuries are among the most challenging problems for hand surgeons and tissue engineers alike. Not only do flexor tendon injuries heal with poor mechanical strength, they can also form debilitating adhesions that may permanently impair hand function. While TGF-β1 is a necessary factor for regaining tendon strength, it is associated with scar and adhesion formation in the flexor tendons and other tissues as well as fibrotic diseases. The pleiotropic effects of TGF-β1 on tendon cells and tissue have not been characterized in detail. The goal of the present study was to identify the targets through which the effects of TGF-β1 on tendon healing could be altered. To accomplish this, we treated flexor tendon tenocytes cultured in pinned collagen gels with 1, 10 or 100 ng/mL of TGF-β1 and measured gel contraction and gene expression using RT-PCR up to 48 hours after treatment. Specifically, we studied the effects of TGF-β1 on the expression of collagens, fibronectin, proteoglycans, MMPs, MMP inhibitors, and the neotendon transcription factors, Scleraxis and Mohawk. Area contraction of the gels was not dose-dependent with the TGF-β1 concentrations tested. We observed dose-dependent downregulation of MMP-16 (MT3-MMP) and decorin, and upregulation of biglycan, collagen V, collagen XII, PAI-1, Scleraxis, and Mohawk by TGF-β1. Inter-gene analyses were also performed to further characterize the expression of ECM and MMP genes in the tenocyte-seeded collagen gels. These analyses illustrate that TGF-β1 tilts the balance of gene expression in favor of ECM synthesis rather than the matrix-remodeling MMPs, a possible means by which TGF-β1 promotes adhesion formation.

Bone marrow-derived matrix metalloproteinase-9 is associated with fibrous adhesion formation after murine flexor tendon injury

The pathogenesis of adhesions following primary tendon repair is poorly understood, but is thought to involve dysregulation of matrix metalloproteinases (Mmps). We have previously demonstrated that Mmp9 gene expression is increased during the inflammatory phase following murine flexor digitorum (FDL) tendon repair in association with increased adhesions. To further investigate the role of Mmp9, the cellular, molecular, and biomechanical features of healing were examined in WT and Mmp9^−/− mice using the FDL tendon repair model. Adhesions persisted in WT, but were reduced in Mmp9^−/− mice by 21 days without any decrease in strength. Deletion of Mmp9 resulted in accelerated expression of neo-tendon associated genes, Gdf5 and Smad8, and delayed expression of collagen I and collagen III. Furthermore, WT bone marrow cells (GFP⁺) migrated specifically to the tendon repair site. Transplanting myeloablated Mmp9^−/− mice with WT marrow cells resulted in greater adhesions than observed in Mmp9^−/− mice and similar to those seen in WT mice. These studies show that Mmp9 is primarily derived from bone marrow cells that migrate to the repair site, and mediates adhesion formation in injured tendons. Mmp9 is a potential target to limit adhesion formation in tendon healing.

Unique angiogenic and vasculogenic properties of renal cell carcinoma in a xenograft model of bone metastasis are associated with high levels of vegf-a and decreased ang-1 expression

Management of various tumor metastases to bone has dramatically improved, but this is not so for renal cell carcinoma (RCC), which is a difficult surgical problem due to its great vascularity. Furthermore, the unique mechanisms that mediate RCC vasculogenesis in bone remain unknown. To understand this process we developed a xenograft model that recapitulates highly vascular RCC versus less vascular tumors that metastasize to bone. Human tumor cell lines of RCC (786-O), prostate cancer (PC3), lung cancer (A549), breast cancer (MDA-MB231), and melanoma (A375) were transduced with firefly luciferase (Luc), injected into the tibiae of nude mice, and differences in growth, osteolysis, and vascularity were assessed by longitudinal bioluminescent imaging, micro-CT for measurement of calcified tissues and vascularity and histology. The results showed that while RCC-Luc has reduced growth and osteolytic potential versus the other tumor lines, it displayed a significant increase in vascular volume (p < 0.05). This expansion was due to 3- and 5-fold increases in small and large vessel numbers respectively. In vitro gene expression profiling revealed that RCC-Luc expresses significantly (p < 0.05) more vegf-a (10-fold) and 20- to 30-fold less ang-1 versus the other lines. These data demonstrate the utility of this model to study the unique vasculogenic properties of RCC bone metastases.

Assessment and management of the orthopedic and other complications of Proteus syndrome

PURPOSE

A multidisciplinary workshop was convened at the National Institutes of Health (NIH) to discuss the management of the orthopedic and other complications of Proteus syndrome (PS), a progressive, disproportionate overgrowth disorder. While PS poses many complex challenges, the focus of the workshop was the management of the asymmetric and disorganized skeletal overgrowth that characterizes this multisystem disorder.

METHODS

Workshop participants developed recommendations for clinical research and patient management and surveillance to maximize the benefits and reduce the risks of surgical and other interventions.

RESULTS

Recommendations for clinical care and management included assessments of skeletal overgrowth and its progression with modalities such as X-ray, magnetic resonance imaging (MRI), dual-energy X-ray absorptiometry, and computerized tomography (CT) imaging. The recommendations also cover the assessment of non-orthopedic complications of PS that significantly impact surgical risk, such as pulmonary embolism and lung bullae. Surgical considerations in PS include assessment of the contribution of contractures to deformities and prophylactic soft-tissue release, aggressive and early use of epiphysiodesis and epiphysiostasis, amputation, and spinal bracing.

CONCLUSION

Decisions on the timing of orthopedic procedures in children with PS are challenging because they entail balancing the risks of intervention in this high-risk and complex population against the increasing morbidity that patients experience with progressive bony overgrowth. If surgery is delayed too long, the condition may become inoperable. We hope that these recommendations will help clinicians gather appropriate data and assist their patients in making timely treatment decisions.

Establishment of an index with increased sensitivity for assessing murine arthritis

The goals of our study were to establish quantitative outcomes for assessing murine knee arthritis and develop an Arthritis Index that incorporates multiple outcomes into a single calculation that provides enhanced sensitivity. Using an accepted model of meniscal/ligamentous injury (MLI)-induced osteoarthritis (OA), we assessed mouse knee arthritis using several approaches. Histology-based methods were performed to visualize joint tissues including articular cartilage and subchondral bone. Accepted histologic scoring methods and histomorphometry were performed to grade cartilage degeneration and determine articular cartilage area, respectively. MicroCT was used to visualize and quantify the bony structures of the joint including osteophytes and joint bone volume. A statistical algorithm was then developed that combined histologic scores and cartilage areas into a single Arthritis Index. MLI induced progressive, OA-like articular cartilage degeneration characterized by increasing (worsening) histologic score and decreasing cartilage area. MicroCT revealed osteophytes and increased joint bone volume between the femoral and tibial physes following MLI. Lastly, an Arthritis Index calculation was established, which incorporated histologic scoring and cartilage area. The Arthritis Index provided enhanced quantitative sensitivity in assessing the level of joint degeneration compared to either histologic scoring or cartilage area determination alone; when using the Index, between 29% and 43% fewer samples are needed to establish statistical significance in studies of murine arthritis. Arthritis in the mouse knee can be quantitatively assessed by histologic scoring, measuring cartilage area, and determining joint bone volume. Enhanced sensitivity can be achieved by performing the Arthritis Index calculation, a novel method for quantitatively assessing mouse knee arthritis.

Impact of Smad3 loss of function on scarring and adhesion formation during tendon healing

Studies were performed evaluating the role of Smad3, a transcription factor mediating canonical TGF-β signaling, on scarring and adhesion formation using an established flexor digitorum longus (FDL) tendon repair model. In unoperated animals the metatarsophalangeal (MTP) range of motion (ROM) was similar in Smad3(-/-) and wild-type (WT) mice while the basal tensile strength of Smad3(-/-) tendons was significantly (39%) lower than in WT controls. At 14 and 21 days following repair Smad3(-/-) MTP ROM reached approximately 50% of the basal level and was twice that observed in WT tendon repairs, consistent with reduced adhesion formation. Smad3(-/-) and WT maximal tensile repair strength on post-operative day 14 was similar. However, Smad3(-/-) tendon repairs maximal tensile strength on day 21 was 42% lower than observed in matched WT mice, mimicking the relative decrease in strength observed in Smad3(-/-) FDL tendons under basal conditions. Histology showed reduced "healing callus" in Smad3(-/-) tendons while quantitative PCR, in situ hybridization, and immunohistochemistry showed decreased col3a1 and col1a1 and increased MMP9 gene and protein expression in repaired Smad3(-/-) tendons. Thus, Smad3(-/-) mice have reduced collagen and increased MMP9 gene and protein expression and decreased scarring following tendon FDL tendon repair.

Teriparatide therapy enhances devitalized femoral allograft osseointegration and biomechanics in a murine model

Despite the remarkable healing potential of long bone fractures, traumatic injuries that result in critical defects require challenging reconstructive limb sparing surgery. While devitalized allografts are the gold standard for these procedures, they are prone to failure due to their limited osseointegration with the host. Thus, the quest for adjuvants to enhance allograft healing remains a priority for this unmet clinical need. To address this, we investigated the effects of daily systemic injections of 40 μg/kg teriparatide (recombinant human parathyroid hormone) on the healing of devitalized allografts used to reconstruct critical femoral defects (4mm) in C57Bl/6 mice. The femurs were evaluated at 4 and 6 weeks using micro CT, histology, and torsion testing. Our findings demonstrated that teriparatide induced prolonged cartilage formation at the graft-host junction at 4 weeks, which led to enhanced trabeculated bone callus formation and remarkable graft-host integration at 6-weeks. Moreover, we observed a significant 2-fold increase in normalized callus volume (1.04 ± 0.3 vs. 0.54 ± 0.14 mm³/mm; p < 0.005), and Union Ratio (0.28 ± 0.07 vs. 0.13 ± 0.09; p < 0.005), compared to saline treated controls at 6-weeks. Teriparatide treatment significantly increased the torsional rigidity (1175 ± 311 versus 585 ± 408 N.mm²) and yield torque (10.5 ± 4.2 versus 6.8 ± 5.5 N.mm) compared to controls. Interestingly, the Union Ratio correlated significantly with the yield torque and torsional rigidity (R²=0.59 and R²=0.77, p < 0.001, respectively). These results illustrate the remarkable potential of teriparatide as an adjuvant therapy for allograft repair in a mouse model of massive femoral defect reconstruction, and warrant further investigation in a larger animal model at longer time intervals to justify future clinical trials for PTH therapy in limb sparing reconstructive procedures.

Axin2 regulates chondrocyte maturation and axial skeletal development

Axis inhibition proteins 1 and 2 (Axin1 and Axin2) are scaffolding proteins that modulate at least two signaling pathways that are crucial in skeletogenesis: the Wnt/beta-catenin and TGF-beta signaling pathways. To determine whether Axin2 is important in skeletogenesis, we examined the skeletal phenotype of Axin2-null mice in a wild-type or Axin1(+/-) background. Animals with disrupted Axin2 expression displayed a runt phenotype when compared to heterozygous littermates. Whole-mount and tissue beta-galactosidase staining of Axin2(LacZ/LacZ) mice revealed that Axin2 is expressed in cartilage tissue, and histological sections from knockout animals showed shorter hypertrophic zones in the growth plate. Primary chondrocytes were isolated from Axin2-null and wild-type mice, cultured, and assayed for type X collagen gene expression. While type II collagen levels were depressed in cells from Axin2-deficient animals, type X collagen gene expression was enhanced. There was no difference in BrdU incorporation between null and heterozygous mice, suggesting that loss of Axin2 does not alter chondrocyte proliferation. Taken together, these findings reveal that disruption of Axin2 expression results in accelerated chondrocyte maturation. In the presence of a heterozygous deficiency of Axin1, Axin2 was also shown to play a critical role in craniofacial and axial skeleton development.

Efficacy of colistin-impregnated beads to prevent multidrug-resistant A. baumannii implant-associated osteomyelitis

Osteomyelitis (OM) from multidrug-resistant (MDR) Acinetobacter has emerged in >30% of combat-related injuries in Iraq and Afghanistan. While most of these strains are sensitive to colistin, the drug is not available in bone void fillers for local high-dose delivery. To address this, we developed a mouse model with MDR strains isolated from wounded military personnel. In contrast to S. aureus OM, which is osteolytic and characterized by biofilm in necrotic bone, A. baumannii OM results in blastic lesions that do not contain apparent biofilm. We also found that mice mount a specific IgG response against three proteins (40, 47, and 56 kDa) regardless of the strain used, suggesting that these may be immuno-dominant antigens. PCR for the A. baumannii-specific parC gene confirmed a 100% infection rate with 75% of the MDR strains, and in vitro testing confirmed that all strains were sensitive to colistin. We also developed a real-time quantitative PCR (RTQ-PCR) assay that could detect as few as 10 copies of parC in a sample. To demonstrate the efficacy of colistin prophylaxis in this model, mice were treated with either parenteral colistin (0.2 mg colistinmethate i.m. for 7 days), local colistin (PMMA bead impregnated with 1.0 mg colistin sulfate), or an unloaded PMMA bead control. While the parenteral colistin failed to demonstrate any significant effects versus the placebo, the colistin PMMA bead significantly reduced the infection rate such that only 29.2% of the mice had detectable levels of parC at 19 days (p < 0.05 vs. i.m. colistin and placebo).

Remodeling of murine intrasynovial tendon adhesions following injury: MMP and neotendon gene expression

Tendon injury frequently results in the formation of adhesions that reduce joint range of motion. To study the cellular, molecular, and biomechanical events involved in intrasynovial tendon healing and adhesion formation, we developed a murine flexor tendon healing model in which the flexor digitorum longus (FDL) tendon of C57BL/6 mice was transected and repaired using suture. This model was used to test the hypothesis that murine flexor tendons heal with differential expression of matrix metalloproteases (MMPs), resulting in the formation of scar tissue as well as the subsequent remodeling of scar and adhesions. Healing tendons were evaluated by histology, gene expression via real-time RT-PCR, and in situ hybridization, as well as biomechanical testing to assess the metatarsophalangeal (MTP) joint flexion range of motion (ROM) and the tensile failure properties. Tendons healed with a highly disorganized fibroblastic tissue response that was progressively remodeled through day 35 resulting in a more organized pattern of collagen fibers. Initial repair involved elevated levels of Mmp-9 at day 7, which is associated with catabolism of damaged collagen fibers. High levels of Col3 are consistent with scar tissue, and gradually transition to the expression of Col1. Scleraxis expression peaked at day 7, but the expression was limited to the original tendon adjacent to the injury site, and no expression was present in granulation tissue involved in the repair response. The MTP joint ROM with standardized force on the tendon was decreased on days 14 and 21 compared to day 0, indicating the presence of adhesions. Peak expressions of Mmp-2 and Mmp-14 were observed at day 21, associated with tendon remodeling. At day 28, two genes associated with neotendon formation, Smad8 and Gdf-5, were elevated and an improvement in MTP ROM occurred. Tensile strength of the tendon progressively increased, but by 63 days the repaired tendons had not reached the tensile strength of normal tendon. The murine model of primary tendon repair, described here, provides a novel mechanism to study the tendon healing process, and further enhances the understanding of this process at the molecular, cellular, and biomechanical level.

Adhesions in a murine flexor tendon graft model: autograft versus allograft reconstruction

Reconstruction of flexor tendons often results in adhesions that compromise joint flexion. Little is known about the factors involved in the formation of flexor tendon graft adhesions. In this study, we developed and characterized a novel mouse model of flexor digitorum longus (FDL) tendon reconstruction with live autografts or reconstituted freeze-dried allografts. Grafted tendons were evaluated at multiple time points up to 84 days post-reconstruction. To assess the flexion range of the metatarsophalangeal joint, we developed a quantitative outcome measure proportional to the resistance to tendon gliding due to adhesions, which we termed the Gliding Coefficient. At 14 days post-grafting, the Gliding Coefficient was 29- and 26-fold greater than normal FDL tendon for both autografts and allografts, respectively (p < 0.001), and subsequently doubled for 28-day autografts. Interestingly, there were no significant differences in maximum tensile force or stiffness between live autograft and freeze-dried allograft repairs over time. Histologically, autograft healing was characterized by extensive remodeling and exuberant scarring around both the ends and the body of the graft, whereas allograft scarring was abundant only near the graft-host junctions. Gene expression of GDF-5 and VEGF were significantly increased in 28-day autografts compared to allografts and to normal tendons. These results suggest that the biomechanical advantages for tendon reconstruction using live autografts over devitalized allografts are minimal. This mouse model can be useful in elucidating the molecular mechanisms in tendon repair and can aid in preliminary screening of molecular treatments of flexor tendon adhesions.

Tamoxifen-inducible CreER-mediated gene targeting in periosteum via bone-graft transplantation

Periosteum plays a key role in bone repair through activation of residing stem and/or progenitor cells. The molecular signals regulating differentiation and expansion of periosteal stem cells during early repair are poorly understood. Understanding the molecular basis for initiation and completion of bone healing is vital for the success of bone-tissue engineering and regeneration therapy for impaired bone healing. We established a live-bone-graft transplantation model that allows us to quantitatively evaluate the fate of the periosteal cells and cell-initiated endochondral bone healing with use of a transgenic and knockout mouse model. By combining this live-bone-graft transplantation method with a tamoxifen-inducible CreER-mediated gene recombination model (R26CreER), we developed a novel approach to efficiently delete genes in periosteal cells during the initiation of skeletal repair. This approach allows us to use floxed mice to examine the function of genes whose germline deletion results in lethality during development. Successful bone repair and regeneration therapies require a deeper understanding of the signals and signaling pathways that are critical for the morphogenesis of the repair tissues. Early lethality in genetically manipulated mice prohibits an understanding of the function of genes in the adult repair process. Our current approach overcomes this encumbrance and enables examination of gene function in a time-dependent and repair-tissue-specific manner.

Quantitative mouse model of implant-associated osteomyelitis and the kinetics of microbial growth, osteolysis, and humoral immunity

Although osteomyelitis (OM) remains a serious problem in orthopedics, progress has been limited by the absence of an in vivo model that can quantify the bacterial load, metabolic activity of the bacteria over time, immunity, and osteolysis. To overcome these obstacles, we developed a murine model of implant-associated OM in which a stainless steel pin is coated with Staphylococcus aureus and implanted transcortically through the tibial metaphysis. X-ray and micro-CT demonstrated concomitant osteolysis and reactive bone formation, which was evident by day 7. Histology confirmed all the hallmarks of implant-associated OM, namely: osteolysis, sequestrum formation, and involucrum of Gram-positive bacteria inside a biofilm within necrotic bone. Serology revealed that mice mount a protective humoral response that commences with an IgM response after 1 week, and converts to a specific IgG2b response against specific S. aureus proteins by day 11 postinfection. Real-time quantitative PCR (RTQ-PCR) for the S. aureus specific nuc gene determined that the peak bacterial load occurs 11 days postinfection. This coincidence of decreasing bacterial load with the generation of specific antibodies is suggestive of protective humoral immunity. Longitudinal in vivo bioluminescent imaging (BLI) of luxA-E transformed S. aureus (Xen29) combined with nuc RTQ-PCR demonstrated the exponential growth phase of the bacteria immediately following infection that peaks on day 4, and is followed by the biofilm growth phase at a significantly lower metabolic rate (p < 0.05). Collectively, these studies demonstrate the first quantitative model of implant-associated OM that defines the kinetics of microbial growth, osteolysis, and humoral immunity following infection.

Lead induces chondrogenesis and alters transforming growth factor-beta and bone morphogenetic protein signaling in mesenchymal cell populations

BACKGROUND

It has been established that skeletal growth is stunted in lead-exposed children. Because chondrogenesis is a seminal step during skeletal development, elucidating the impact of Pb on this process is the first step toward understanding the mechanism of Pb toxicity in the skeleton.

OBJECTIVES

The aim of this study was to test the hypothesis that Pb alters chondrogenic commitment of mesenchymal cells and to assess the effects of Pb on various signaling pathways.

METHODS

We assessed the influence of Pb on chondrogenesis in murine limb bud mesenchymal cells (MSCs) using nodule formation assays and gene analyses. The effects of Pb on transforming growth factor-beta (TGF-beta) and bone morphogenetic protein (BMP) signaling was studied using luciferase-based reporters and Western analyses, and luciferase-based assays were used to study cyclic adenosine monophosphate response element binding protein (CREB), beta-catenin, AP-1, and nuclear factor-kappa B (NF-kappaB) signaling. We also used an ectopic bone formation assay to determine how Pb affects chondrogenesis in vivo.

RESULTS

Pb-exposed MSCs showed enhanced basal and TGF-beta/BMP induction of chondrogenesis, evidenced by enhanced nodule formation and up-regulation of Sox-9, type 2 collagen, and aggrecan, all key markers of chondrogenesis. We observed enhanced chondrogenesis during ectopic bone formation in mice preexposed to Pb via drinking water. In MSCs, Pb enhanced TGF-beta but inhibited BMP-2 signaling, as measured by luciferase reporter assays and Western analyses of Smad phosphorylation. Although Pb had no effect on basal CREB or Wnt/beta-catenin pathway activity, it induced NFkappaB signaling and inhibited AP-1 signaling.

CONCLUSIONS

The in vitro and in vivo induction of chondrogenesis by Pb likely involves modulation and integration of multiple signaling pathways including TGF-beta, BMP, AP-1, and NFkappaB.

Bone morphogenetic protein 2 activates Smad6 gene transcription through bone-specific transcription factor Runx2

BMP-2 plays an essential role in osteoblast and chondrocyte differentiation, but its signaling mechanism has not been fully defined. In the present studies, we investigated the mechanism through which BMP-2 activates the Smad6 gene. A -2006/+45 Smad6 promoter-luciferase construct was generated along with deletions and Runx2 binding site mutations to examine the role of Smad1 and Runx2 signaling following BMP-2 stimulation in osteoblasts. Transfection of Runx2 or treatment with BMP-2-stimulated promoter activity of the -2006/+45 and -1191/+45 reporters but not the -829/+45 and -374/+45 reporters. No Smad1/5 binding site is present in the -1191/-829 region of the Smad6 promoter. Mutation of the OSE2-a site (-1036/-1031) completely abolished the stimulatory effect of Runx2 as well as BMP-2 on the -2006/+45 and -1191/+45 Smad6 reporters. Gel shift and chromatin immunoprecipitation (ChIP) assays showed that Runx2 binds the OSE2-a element. ChIP assays demonstrated that Smad1 also interacts with the OSE2-a site at the Smad6 promoter through Runx2. The protein degradation of Runx2 is mediated by the E3 ubiquitin ligase Smurf1. In the present studies, we found that Smurf1 binds the OSE2-a site through Runx2 and inhibits Smad6 gene transcription. Treatment with BMP-2 and transfection of Smad1 abolished Smurf1 binding to the OSE2 site. These results show that Smad1 binding excludes Smurf1 interaction with the OSE2 site and promotes Smad6 gene transcription.

Alteration of femoral bone morphology and density in COX-2-/- mice

A role of COX-2 in pathological bone destruction and fracture repair has been established; however, few studies have been conducted to examine the involvement of COX-2 in maintaining bone mineral density and bone micro-architecture. In this study, we examined bone morphology in multiple trabecular and cortical regions within the distal and diaphyseal femur of 4-month-old wild-type and COX-2-/- mice using micro-computed tomography. Our results demonstrated that while COX-2-/- female mice had normal bone geometry and trabecular microarchitecture at 4 months of age, the male knockout mice displayed reduced bone volume fraction within the distal femoral metaphysis. Furthermore, male COX-2-/- mice had a significant reduction in cortical bone mineral density within the central cortical diaphysis and distal epiphysis and metaphysis. Consistent with the observed reduction in cortical mineral density, biomechanical testing via 4-point-bending showed that male COX-2-/- mice had a significant increase in postyield deformation, indicating a ductile bone phenotype in male COX-2-/- mice. In conclusion, our study suggests that genetic ablation of COX-2 may have a sex-related effect on cortical bone homeostasis and COX-2 plays a role in maintaining normal bone micro-architecture and density in mice.

Wnt-mediated regulation of chondrocyte maturation: modulation by TGF-beta

Wnt proteins are expressed during limb morphogenesis, yet their role and mechanism of action remains unclear during long bone growth. Wnt expression, effects and modulation of signaling events by BMP and transforming growth factor-beta (TGF-beta) were evaluated in chick embryonic chondrocytes. Chondrocyte cell cultures underwent spontaneous maturation with increased expression of colX and this was associated with an increase in the expression of multiple Wnts, including Wnt 4, 5a, 8c, and 9a. Both parathyroid hormone related peptide (PTHrP) and TGF-beta inhibited colX, but had disparate effects on Wnt expression. While TGF-beta strongly inhibited all Wnts, PTHrP did not inhibit either Wnt8c or Wnt9a and had lesser effects on the expression of the other Wnts. BMP-2 induced colX expression, and also markedly increased Wnt8c expression. Overexpression of beta-catenin and/or T cell factor (TCF)-4 also induced the type X collagen promoter. Overexpression of Wnt8c induced maturation, as did overexpression of beta-catenin. The Wnt8c/beta-catenin maturational effects were enhanced by BMP-2 and inhibited by TGF-beta. TGF-beta also inhibited activation of the Topflash reporter by beta-catenin, suggesting a direct inhibitory effect since the Topflash reporter contains only beta-catenin binding sequences. In turn beta-catenin inhibited activation of the p3TP-Luc reporter by TGF-beta, although the effect was partial. Thus, Wnt/beta-catenin signaling is a critical regulator of the rate of chondrocyte differentiation. Moreover, this pathway is modulated by members of the TGF-beta family and demonstrates the highly integrated nature of signals controlling endochondral ossification.