Expression patterns of hedgehog signaling peptides in naturally acquired equine osteochondrosis

Etiology and Pathogenesis of Osteochondrosis

Osteochondrosis is a common and clinically important joint disorder that occurs in human beings and in multiple animal species, most commonly pigs, horses, and dogs. This disorder is defined as a focal disturbance of enchondral ossification and is regarded as having a multifactorial etiology, with no single factor accounting for all aspects of the disease. The most commonly cited etiologic factors are heredity, rapid growth, anatomic conformation, trauma, and dietary imbalances; however, only heredity and anatomic conformation are well supported by the scientific literature. The way in which the disease is initiated has been debated. Although formation of a fragile cartilage, failure of chondrocyte differentiation, subchondral bone necrosis, and failure of blood supply to the growth cartilage all have been proposed as the initial step in the pathogenesis, the recent literature strongly supports failure of blood supply to growth cartilage as being the most likely. The term osteochondrosis has been used to describe a wide range of different lesions among different species. We suggest a refinement of this terminology to include the modifiers latens (lesion confined to epiphyseal cartilage), manifesta (lesion accompanied by delay in endochondral ossification), and dissecans (cleft formation through articular cartilage). The purpose of this review is to provide an overview of the disease, focusing on the most commonly cited theories, recent research findings, and our own views regarding the etiology and pathogenesis of osteochondrosis, in order to provide a better understanding of this apparently complex disease.

Impact of feeding and housing on the development of osteochondrosis in foals-A longitudinal study

Osteochondrosis dissecans (OCD) is a developmental orthopedic disease caused by a failure of the endochondral ossification in epiphyseal plates and joint cartilage. This trouble may induce the presence of osteochondral fragments in the articulation, fissures or subchondral bone cysts in the growth cartilage. Occurrence of osteochondrosis is influenced by a complex interaction of different factors. Among these, the effect of the housing and the feeding of the foals during their first months of life, have been described as risk factors for the development of osteochondrosis. The aim of this study was to investigate the evolution of OCD lesions with a longitudinal study in 204 young foals from 6 to 18 months in comparison to the type of feeding and the type of housing conditions. These factors and OCD status were obtained by a questionnaire and radiological examination, respectively. This allowed dividing the foals into four groups according to the initial OCD status and the evolution of the condition. As a result, we found that foals fed with concentrates show a higher probability to develop OCD lesions (p=0.06), while foals not receiving concentrates, had a higher probability to heal from existing OCD lesions (p=0.001). This study supports the theory that management factors such as feeding or housing may influence the evolution of the osteochondrosis disease.

Identification of novel osteochondrosis--Associated genes

During the early stages of articular osteochondrosis, cartilage is retained in subchondral bone, but the pathophysiology of this condition of growing humans and domestic animals is poorly understood. A subtractive hybridization study was undertaken to compare gene expression between the cartilage of early experimentally induced equine osteochondrosis lesions and control cartilage. Of the many putative differentially expressed genes identified, eight were confirmed by quantitative PCR analysis as differentially expressed, in addition to those already known to be associated with early lesions. Genes encoding vacuolar H(+)-ATPase V0 subunit d2 (ATP6V0D2), cathepsin K, integrin-binding sialoprotein, integrin αV, low density lipoprotein receptor-related protein 4, lumican, osteopontin, and thymosin β4 (TMSB4) were expressed at higher levels in lesions than in control cartilage. These genes included 34 genes not previously identified in cartilage. Some genes identified as associated with early lesions are known chondrocyte hypertrophy-associated genes, and in transmission electron microscopy studies normal hypertrophic chondrocytes were observed in lesions. Differential expression of ATP6V0D2 and TMSB4 in the cartilage of early naturally occurring osteochondrosis lesions was confirmed by immunohistochemistry. These results identify novel osteochondrosis-associated genes and provide evidence that articular osteochondrosis does not necessarily result from failure of chondrocytes to undergo hypertrophy.

Wnt/β-catenin signaling of cartilage canal and osteochondral junction chondrocytes and full thickness cartilage in early equine osteochondrosis

The objective of this study was to elucidate gene and protein expression of Wnt signaling molecules in chondrocytes of foals having early osteochondrosis (OC) versus normal controls. The hypothesis was that increased expression of components of Wnt signaling pathway in osteochondral junction (OCJ) and cartilage canal (CC) chondrocytes would be found in early OC when compared to controls. Paraffin-embedded osteochondral samples (7 OC, 8 normal) and cDNA from whole cartilage (7 OC, 10 normal) and chondrocytes surrounding cartilage canals and osteochondral junctions captured with laser capture microdissection (4 OC, 6 normal) were obtained from femoropatellar joints of 17 immature horses. Equine-specific Wnt signaling molecule mRNA expression levels were evaluated by two-step real-time qPCR. Spatial tissue protein expression of β-catenin, Wnt-11, Wnt-4, and Dkk-1 was determined by immunohistochemistry. There was significantly decreased Wnt-11 and increased β-catenin, Wnt-5b, Dkk-1, Lrp6, Wif-1, Axin1, and SC-PEP gene expression in early OC cartilage canal chondrocytes compared to controls. There was also significantly increased β-catenin gene expression in early OC osteochondral junction chondrocytes compared to controls. Based on this study, abundant gene expression differences in OC chondrocytes surrounding cartilage canals suggest pathways associated with catabolism and inhibition of chondrocyte maturation are targeted in early OC pathogenesis.

Age-dependent expression of osteochondrosis-related genes in equine leukocytes

INTRODUCTION

Osteochondrosis (OC) is a developmental disease in horses which has a significant impact on the horse's welfare and performance. The early disturbance in the process of endochondral ossification progresses to inflammatory and repair processes in older horses. Previously, differentially expressed genes in leukocytes of OC-affected horses have been identified. The aim of the present study is to detect age-related changes in these differentially expressed genes.

MATERIALS AND METHODS

The expression of OC-related genes was analysed by real-time PCR and subsequent statistical analysis (ΔΔCT) in the leukocytes of 135 Belgian Warmblood horses divided into three different age groups: <12 months (n=47), 18-24 months (n=50) >30 months (n=38).

RESULTS

Relative expression of genes of horses less than 12 months of age showed significant induction of the genes MGAT4A, PRKCG, MHCI, ApoB, ApoB3G, B4GALT6 and a significantly lower expression of the genes OAS3. Horses of 18-24 months of age, showed a significantly higher expression of the genes TBC1D9, MGAT4A, IFIH1, MHCIIa and MMP1. Horses of more than 30 months of age showed a significantly higher expression of the genes MGAT4A, HP, SECTM1 compared with their age-matched control groups.

CONCLUSIONS

The study demonstrates that OC-related genes are differentially expressed in horses of different ages compared with their age-matched controls. Some of the genes may be implicated in cell signalling and differentiation as well as carbohydrate and lipid metabolism and inflammation. However, the causal relationship between the differentially expressed genes and the development and progression of the OC lesions needs to be determined.

Alterations in sclerostin protein in lesions of equine osteochondrosis

Osteochondrosis (OC) is a common and clinically important joint disease that occurs in many species, including humans, pigs, chickens and horses. It has been described as a focal failure of endochondral ossification (EO), but no cellular/molecular mechanisms are fully described that explain the cause of this condition. Recently a Wnt signalling inhibitor, sclerostin, has been described in osteoarthritic cartilage, where it has been proposed to protect damaged cartilage from degradation. Cartilage degradation is a key event in EO, thus, abnormalities of sclerostin in growth cartilage could, potentially, lead to a failure of EO and, thus, OC. The aim of this study was to describe the distribution of sclerostin protein in normal and OC growth cartilage.

Immunohistochemistry (IHC) was used to localise sclerostin protein in normal and OC growth cartilage. Growth cartilage was harvested from the distal femur of horses aged between 6 and 18 months. Cartilage was classified as normal or having lesions consistent with a diagnosis of early OC. IHC was used to identify sclerostin protein in cartilage sections. Sclerostin protein distribution was semiquantified using a grading system and shown to be upregulated throughout all three zones of cartilage in lesions of OC (IHC score 8.1 compared to IHC score of 0.88). These results indicate that sclerostin may be contributing to the development of OC lesions by inhibiting extracellular matrix remodelling or may reflect the response of damaged cartilage. Clearly, further work is required to fully characterise this observation but, with antisclerostin antibodies used to treat human osteoporosis, the possibility of development of a systemic treatment of OC remains a potential goal.

IHH gene polymorphism among three horse breeds and its application for association test in horses with osteochondrosis

Genetic polymorphism of IHH gene were investigated in Angloarabian, Polish Coldblood and Polish Halfbred horses with the inclusion of a group of Polish Halfbreds affected by osteochondrosis. IHH is a good candidate gene for association study of developmental disorders mainly affecting skeleton development. DNA sequence spanning IHH gene annotated in the horse genome and its putative promoter were investigated using SANGER sequencing. Analysis of genetic variability at polymorphic sites in the IHH gene body and the promoter region confirmed genetic differences between warmblood and coldblood horse breeds. A test for allelic and genotypic association at particular SNP sites revealed no association with osteochondrosis in investigated group of Polish Halfbreds. It was concluded that participation of different warmblood breeds in pedigrees of Polish Halfbreds make it difficult to search for genetic variants being associated with this complex disorder in this breed. IHH gene polymorphism investigated among three different horse populations would be valuable for further studies on equine bone developmental disorders.

Chondrogenic effects of exogenous retinoic acid or a retinoic acid receptor antagonist (LE135) on equine chondrocytes and bone marrow-derived mesenchymal stem cells in monolayer culture

OBJECTIVE

To determine effects of various concentrations of retinoic acid (RA) or a synthetic RA receptor antagonist (LE135) on equine chondrocytes or bone marrow-derived equine mesenchymal stem cells (BMDMSCs) in monolayer cultures.

SAMPLE

Articular cartilage and BMDMSCs from 5 clinically normal horses.

PROCEDURES

Monolayers of chondrocytes cultured in standard media and of BMDMSCs cultured in chondrogenic media were treated with RA at concentrations of 0, 0.1, 1, or 10 μM or LE135 at concentrations of 0, 0.1, 1, or 10 μM on day 0. On days 7 and 14, samples were analyzed for DNA concentration, chondrocyte morphology or features consistent with chondrogenesis (ie, chondral morphology [scored from 0 to 4]), and gene expression of collagen type Ia (CI), collagen type II (CII), and aggrecan.

RESULTS

Chondrocytes treated with RA had more mature chondral morphology (range of median scores, 3.0 to 4.0) than did untreated controls (range of median scores, 0.5 to 0.5). Chondrocytes treated with LE135 did not sustain chondrocyte morphology. All BMDMSCs had evidence of chondral morphology or high CII:CI ratio. Retinoic acid (1 or 10 μM) or LE135 (10 μM) treatment decreased DNA content of BMDMSC cultures. At 0.1 and 1 μM concentrations, LE135 weakly but significantly increased chondral morphology scores, compared with untreated controls, but lack of aggrecan expression and lack of increased CII:CI ratio, compared with that of controls, did not affect chondrogenesis.

CONCLUSIONS AND CLINICAL RELEVANCE

RA promoted maturation and hypertrophy in chondrocytes but not BMDMSCs in monolayer cultures. Deficiency or blockade of RA may prevent hypertrophy and maturation of differentiated chondrocytes.

Developmental mechanisms in articular cartilage degradation in osteoarthritis

Osteoarthritis is the most common arthritic condition, which involves progressive degeneration of articular cartilage. The most recent accomplishments have significantly advanced our understanding on the mechanisms of the disease development and progression. The most intriguing is the growing evidence indicating that extracellular matrix destruction in osteoarthritic articular cartilage resembles that in the hypertrophic zone of fetal growth plate during endochondral ossification. This suggests common regulatory mechanisms of matrix degradation in OA and in the development and can provide new approaches for the treatment of the disease by targeting reparation of chondrocyte phenotype.

Gene expression profiling from leukocytes of horses affected by osteochondrosis

Osteochondrosis (OC) is a developmental disease that affects growing horses and that severely affects their ability to perform. The genetic basis of its pathogenesis is poorly understood. The aim of the study was to analyze the transcript profile of leukocytes from horses affected with OC. Two transcriptome libraries were constructed from leukocytes of OC-affected and non-OC-affected horses using digital gene expression analysis (DGE) and real-time PCR. Statistical analysis allowed selection of 1,008 tags upregulated in the non-OC-affected group and 1,545 tags upregulated in the OC-affected group. Among these genes, 16 regulated genes and 5 housekeeping genes were selected. Metabolic pathways analysis showed an obvious dysregulation of several signaling pathways related to cartilage formation or cartilage repair, including Wnt, Indian hedgehog, and TGF-beta signaling. Other genes, including ISG, ApoB, MGAT4, and TBC1D9, showed a significantly different expression between groups. These genes may play a role in high carbohydrate diet, abnormal insulin metabolism, or inflammation, mechanisms suspected to be involved in OC. This DGE analysis of the transcript profile of leukocytes from OC-affected horses demonstrated significant differences in comparison to the control library. These results open new perspectives for the understanding of equine OC.

Altered gene expression in early osteochondrosis lesions

Osteochondrosis is a condition involving defective endochondral ossification and retention of cartilage in subchondral bone. The pathophysiology of this condition is poorly characterized, but it has been proposed that the fundamental defect is failure of chondrocyte hypertrophy. The aim of the current study was to characterize phenotypic changes in chondrocytes associated with the initiation of osteochondrosis. Early lesions were induced in an equine model of osteochondrosis by feeding foals a high energy diet for 8 or 15 weeks. Lesions in articular-epiphyseal growth cartilage were examined histologically and by quantitative PCR analysis of expression of a number of genes representative of pathways that regulate chondrocyte behavior during endochondral ossification. There were more cells present in clusters in the lesions compared to normal articular cartilage. Expression of matrix metalloproteinase-13, type I collagen, type X collagen, and Runx2 mRNA was significantly greater in the lesions compared to normal cartilage from the same joint. Expression of vascular endothelial growth factor, type II collagen, connective tissue growth factor, aggrecan, Sox9, and fibroblast growth factor receptor 3 mRNA was not significantly different in lesions than in control cartilage. These observations suggest that osteochondrosis does not result from failure of chondrocytes to undergo hypertrophy.

Chondrocytes harvested from osteochondritis dissecans cartilage are able to undergo limited in vitro chondrogenesis despite having perturbations of cell phenotype in vivo

Our objective was to characterize the variation in gene expression for key genes associated with chondrogenic phenotype of osteochondrosis (OC)-affected and normal chondrocytes, and to identify whether OC chondrocytes can redifferentiate and regain a phenotype similar to normal chondrocytes if appropriate chondrogenic signals are given. Equine articular cartilage removed at surgery to treat clinically significant OC lesions was collected (n = 10), and the gene expression evaluated and compared to aged-matched normal samples (n = 10). Cartilage was harvested from normal (n = 4) and OC (n = 3) joints from horses at necropsy. Chondrogenic pellet cultures were established following monolayer proliferation. After 14 days in culture, the pellets were assessed by histochemical and pellet weight analysis, assay of glycosaminoglycan (GAG) content, and gene expression. Chondrocytes from OC cartilage expressed significantly more Coll-I, -II, -III, and -X than chondrocytes from normal cartilage (all p < 0.0001). Furthermore, OC chondrocytes expressed significantly more MMP-13, ADAMTS-4 (both p < 0.0001), and TIMP-1 (p < 0.001) and significantly less TIMP-2 and TIMP-3. Pellets created from OC chondrocytes contained significantly less GAG (p = 0.0069) and expressed significantly less Sox9 and significantly more superficial zone protein (SZP) (p = 0.0105) than pellets created from normal cartilage. The results suggest that chondrocytes from OC cartilage at the time of surgical treatment have perturbations in phenotype compared to cells from normal cartilage. Despite these differences, following monolayer expansion and pellet culture under chondrogenic conditions, chondrocytes derived from OC cartilage retain some ability to undergo chondrogenic differentiation and synthesize an appropriate cartilage-like matrix. However, this chondrogenic differentiation potential is inferior to that seen in aged-matched normal chondrocytes.

Mice with a targeted mutation of patched2 are viable but develop alopecia and epidermal hyperplasia

Hedgehog (Hh) signaling plays pivotal roles in tissue patterning and development in Drosophila melanogaster and vertebrates. The Patched1 (Ptc1) gene, encoding the Hh receptor, is mutated in nevoid basal cell carcinoma syndrome, a human genetic disorder associated with developmental abnormalities and increased incidences of basal cell carcinoma (BCC) and medulloblastoma (MB). Ptc1 mutations also occur in sporadic forms of BCC and MB. Mutational studies with mice have verified that Ptc1 is a tumor suppressor. We previously identified a second mammalian Patched gene, Ptc2, and demonstrated its distinct expression pattern during embryogenesis, suggesting a unique role in development. Most notably, Ptc2 is expressed in an overlapping pattern with Shh in the epidermal compartment of developing hair follicles and is highly expressed in the developing limb bud, cerebellum, and testis. Here, we describe the generation and phenotypic analysis of Ptc2(tm1/tm1) mice. Our molecular analysis suggests that Ptc2(tm1) likely represents a hypomorphic allele. Despite the dynamic expression of Ptc2 during embryogenesis, Ptc2(tm1/tm1) mice are viable, fertile, and apparently normal. Interestingly, adult Ptc2(tm1/tm1) male animals develop skin lesions consisting of alopecia, ulceration, and epidermal hyperplasia. While functional compensation by Ptc1 might account for the lack of a strong mutant phenotype in Ptc2-deficient mice, our results suggest that normal Ptc2 function is required for adult skin homeostasis.

Age-related expression of molecular regulators of hypertrophy and maturation in articular cartilage

The purpose of this study was to determine changes in the expression of regulatory molecules in normal equine articular cartilage throughout development up to 18 months of age. The hypothesis was that expression of these regulatory molecules would decrease from birth to postpubescence. Cartilage was harvested from normal femoropatellar or scapulohumeral joints from 34 fresh horse cadavers. Horses were placed in four age groups [prenatal (n = 5); prepubertal, 0-6 months (n = 11); pubertal, 7-14 months (n = 13); and postpubertal, 15-18 months (n = 5)]. Indian hedgehog (Ihh), Gli1, Gli3, Patched1 (Ptc1), Smoothened (Smo), Noggin, bone morphogenetic protein-6 (BMP-6), BMP-2, parathyroid hormone-related peptide (PTHrP), and PTH/PTHrP receptor mRNA expression levels were evaluated by real-time quantitative PCR. Spatial tissue mRNA and protein expression was determined by in situ hybridization and immunohistochemistry. The expression of PTHrP decreased (p = 0.002) in the pubertal group, while PTH/PTHrP receptor expression significantly increased (p = 0.001). No significant difference was found between groups for Ihh (p = 0.6) or Smo (p = 0.3) expression. In contrast, there was significantly increased expression of Ptc1 (p = 0.006), Gli1 (p = 0.04), and Gli3 (p = 0.007) in the pubertal group, and Gli3 (p = 0.007) remained elevated in the postpubertal group. The expression of BMP-6 significantly increased from prenatal to postnatal groups (p = 0.03) while BMP-2 expression increased during puberty and postpuberty (p = 0.03). The changes in expression of hedgehog and BMP signaling molecules in articular cartilage during postnatal development have not been shown previously. The increased expression of hedgehog receptor and transcription factors during puberty may indicate maturation of the deep articular layer during this time period.