RI COBRE: REGULATION OF GROWTH PLATE DEVELOPMENT BYNUCLEAR/CYTOPLASMIC FACTORS

Rhode Island Hospital, Providence RI

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our long-term goal is to understand the molecular mechanisms regulating growth plate development. The last step in maturation of the growth plate is the differentiation of proliferative chondrocytes into hypertrophic chondrocytes, which subsequently undergoes endochondral ossification. Runx 2/Cbfa1 is a transcription factor necessary for chondrocyte differentiation and hypertrophy. HDAC4 functions as a negative regulator of chondrocyte hypertrophy by binding and inhibiting Runx 2/Cbfa1 expression in the nucleus. Our recent findings indicate that HDAC4 nuclear-cytoplasm shuttling and degradation occurs in chondrocytes, allowing chondrocyte differentiation and further hypertrophy. However, the mechanisms underlying HDAC4 shuttling and degradation are unclear. The overall hypothesis includes two parts: Hypothesis 1: HDAC4 nuclear-cytoplasmic shuttling controls chondrocyte differentiation and is dependent on the Ca2+/calmodulin signaling pathway. Specific Aims 1: To determine whether activation of the Ca2+/calmodulin signaling pathway prevents nuclear entry of HDAC4 and enhances the binding of HDAC4 to the cytoplasmic binding protein 14-3-3. This may impair HDAC4-mediated inhibition of chondrocyte differentiation in the nucleus. Hypothesis 2: P38 MAPK activity controls chondrocyte hypertrophy by increasing caspase-regulated degradation of HDAC4, which releases Runx2 from a repressive influence of HDAC4. Specific Aims 2: To determine if caspases induce the degradation of HDAC4 is controlled by p38 MAPK by using constitutively active MKK6 to elevate p38 and dominant negative p38 MAPK to repress p38 in the presence or absence of caspase inhibitors, which in turn increases Runx2 activity. New Experiment " Inducible deletion of Ihh prevents cartilage degeneration in mouse OA model" has been approved by COBRE External Advisory Committee. Abstract Recently, we performed a miRNA expression profile using Microarray analysis in OA cartilage. We found the expression of miRNA-1 is undetectable while miRNA-31 is overexpression in the OA cartilage (154 fold increase) in comparison with the adjacent relative normal cartilage. Overexpression of miRNA-31 increases the expression of Indian Hedgehog (Ihh) and MMP-1,-3, and 13. Our further pilot study let us believe that Ihh plays a critical role in OA development. Our rationale for this focus is based on our and others'findings. Collective evidence include: a) Ihh is a key regulator of chondrocyte hypertrophy and endochondral bone formation [1] [2];b) Ihh is mainly expressed in the developmental growth plate, and it is almost undetectable in normal adult articular cartilage;c) excessive amounts of Ihh are synthesized by chondrocytes in OA patients;d) Ihh promotes chondrocyte hypertrophy and increases MMP production, which subsequently induces cartilage degeneration;e) Knockdown Ihh in cell culture results in suppression of MMP release. Our comparative study of normal and OA patients indicates that OA cartilage degeneration is accompanied by a chondrocyte response to this damage which involves enhanced Ihh synthesis. The increase of Ihh in OA may involve not only accelerated processes but also the initiation of events that are not ordinarily encountered in healthy cartilage. These findings support the notion that elevated Ihh signaling in the joint may contribute significantly to cartilage matrix degeneration in OA. However, direct genetic evidence for Ihh in OA has not been reported because tissue-specific activation of the Ihh gene (targeted by Col2a1-Cre) died shortly after birth. In this study, we will specifically delete the Ihh gene in chondrocytes in adult mice by generating Ihh conditional activated mice through Col2a1-CreERT2;Ihhfl / Ihhfl (provide by Dr. Beate Lanske, Harvard School of Dental Medicine) to confirm and extend these findings. Hypothesis: Inducible deletion of Ihh prevents cartilage degeneration in OA mouse model Specific Aim: We will determine whether disrupting Ihh signaling pathway in vivo will attenuate OA progression in Col2a1-CreERT2;Ihhfl / Ihhfl mouse OA model induced by surgery. Tamoxifen (TM) will be delivered intraperitoneally for 5 consecutive days to remove Ihh. The resulting changes in OA cartilage will be evaluated by X-ray and histologically by Safranin-O staining, and quantified using the Modified Mankin score. Expression of type II, IX and X collagens and matrix metalloproteinase (MMP), -3, -9, and -13 will be further examined by immunohistochemistry and in situ hybridization (ISH). The change of cartilage degeneration will be monitored using MMPSense probe in vivo by fluorescence-based quantitative tomography, a non-invasive in vivo imaging technique (VisEn Medical). Summary Ihh expression is markedly elevated in OA cartilage. We further demonstrate that Ihh promotes chondrocyte hypertrophy and induces the release of matrix metalloproteinase in vitro. Thus, Ihh may activate cartilage catabolism during arthritis. In this application, we propose to evaluate novel strategies for prevention of Ihh induced cartilage degeneration in arthritis by the direct reduction of Ihh in vivo.