Developmental Progenitor Cells of Articular Cartilage

Rationale: Pain, loss of mobility, and osteoarthritis resulting from both developmental and acquired articular cartilage lesions are major limiting variables in the athletic careers of many horses. In addition to the high morbidity, synovial joint injuries have substantial economic and emotional consequences for horse owners. Unfortunately, the intrinsic repair capacity of mammalian articular cartilage is very limited, and as a result, the treatment of joint surface lesions is challenging and the prognosis guarded. In two published papers, we have determined that the femorotibial joints of axolotl salamanders contain a tissue called the 'interzone' that enables a remarkable ability to fully repair even large joint surface lesions. In addition, transplanted interzone tissue in this amphibian model can generate an entirely new diarthrodial joint de novo. A mammalian homologue of this tissue does exist, but is present for only a short period of time during early limb development prior to joint cavitation. We believe that equine interzone cells may represent a far superior cell type to focus on for optimizing cell-based therapies to repair articular cartilage defects in the horse. Hypothesis/Objectives: The hypothesis to be tested is that equine interzone tissue has an enhanced biological potential relative to other cell types to differentiate into true articular chondrocytes. Study Design: The chondrogenic differentiation potential of equine interzone will be analyzed both in vitro and in vivo. In culture, cellular differentiation of interzone cells will be compared to mesenchymal 'stem' cells isolated from bone marrow and adipose tissue. For in vivo experiments, equine interzone samples will be studied as xenografts transplanted into immunologically compromised mice. Sample assessment will include mRNA, protein, cell morphology, and tissue structure analyses. Preliminary Data: Primary equine cell lines have been established that retain the expression of GDF5 and other interzone biomarkers. RNA-seq data from interzone and several different cartilaginous tissues have been generated to enable the qualitative evaluation of chondrogenic differentiation at a transcriptome and individual gene level. Expected Results: We believe the data from the proposed experiments will demonstrate that equine interzone tissue and primary interzone cells have enhanced chondrogenic differentiation potential relative to bone marrow- and adipose-derived mesenchymal stem cells. Further, we think that "chondrocytes" generated from interzone tissue will have an articular cartilage phenotype. Budget and Timeline: The project is budgeted over three years for a total cost of $121,547 distributed as follows: Year 1 ($31,045), Year 2 ($43,631), and Year 3 ($46,871). Potential Impact for Animal Health: New cell-based therapies for equine orthopaedic injuries are generating a high level of interest. The interzone is the biological progenitor of articular cartilage during normal fetal development. Studying chondrogenic differentiation of interzone cells should provide important new knowledge highly relevant to clinical efforts to create true articular chondrocytes during joint lesion repair.