Suppression of Osteoarthritis progression by post-natal Induction of Nkx3.2

Osteoarthritis (OA) is an incurable joint disease affecting 240 million elderly population, and major unmet medical needs exist for better therapeutic options for OA. During skeletal development, Nkx3.2 has been shown to promote chondrocyte differentiation and survival, but to suppress cartilage hypertrophy and blood vessel invasion. Here we show that Nkx3.2 plays a key role in osteoarthritis (OA) pathogenesis. Marked reduction of Nkx3.2 expression was observed in three different murine OA models. Consistent with these findings, analyses of surgery-induced and age-driven OA models revealed that cartilage-specific post-natal induction of Nkx3.2 can suppress OA progression in mice. These results suggest that Nkx3.2 may serve as a promising target for OA drug development.

Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton

The regulated expansion of chondrocytes within growth plates and joints ensures proper skeletal development through adulthood. Mutations in the transcription factor NKX3.2 underlie spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD), which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates and supernumerary distal limb joints. Whereas nkx3.2 knockdown zebrafish and mouse Nkx3.2 mutants display embryonic lethal jaw joint fusions and skeletal reductions, respectively, they lack the skeletal overgrowth seen in SMMD patients. Here, we report adult viable nkx3.2 mutant zebrafish displaying cartilage overgrowth in place of a missing jaw joint, as well as severe dysmorphologies of the facial skeleton, skullcap and spine. In contrast, cartilage overgrowth and scoliosis are absent in rare viable nkx3.2 knockdown animals that lack jaw joints, supporting post-embryonic roles for Nkx3.2. Single-cell RNA-sequencing and in vivo validation reveal increased proliferation and upregulation of stress-induced pathways, including prostaglandin synthases, in mutant chondrocytes. By generating a zebrafish model for the skeletal overgrowth defects of SMMD, we reveal post-embryonic roles for Nkx3.2 in dampening proliferation and buffering the stress response in joint-associated chondrocytes.

The role of Nkx3.2 in chondrogenesis

Transcription factor, Nkx3.2, is a member of the NK family of developmental genes and is expressed during embryogenesis in a variety of mammalian model organisms, including chicken and mouse. It was first identified in Drosophila as the Bagpipe (bap) gene, where it has been demonstrated to be essential during formation of the midgut musculature. However, mammalian homolog Nkx3.2 has been shown to play a significant role in axial and limb skeletogenesis; in particular, the human skeletal disease, spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD), is associated with mutations of the Nkx3.2 gene. In this review, we highlight the role of Nkx3.2 during musculoskeletal development, with an emphasis on the factor's role in determining chondrogenic cell fate and its subsequent role in endochondral ossification and chondrocyte survival.

Pax1 acts as a negative regulator of chondrocyte maturation

Paired box gene 1 (Pax1) indirectly promotes the early stages of chondrogenic differentiation through induction and transactivation of Nk3 homeobox 2 (Nkx3.2), a transcriptional repressor. Later in chondrogenic differentiation, Nkx3.2 blocks chondrocyte hypertrophy by repressing Runt-related transcription factor 2 (Runx2). Here we report the inhibitory action of Pax1 on chondrocyte maturation, independently of Nkx3.2. Upon cartilage formation, Pax1 expression in the ventral sclerotome was gradually decreased except for the perichondrial region of the vertebral bodies and the intervertebral region, both of which express SRY-box containing gene 9 (Sox9). Forced expression of Pax1 in the chick forelimb resulted in the formation of shortened skeletal elements with a significant reduction of proteoglycans (PGs) accumulation in cartilage as well as a lack of the cortical bone formation and vascular invasion into the primary ossification center. Pax1-misexpressing chondrocytes exhibited aberrant cell morphology with a marked downregulation of Aggrecan (Agc1). Pax1-misexpressing cultured chondrocytes failed to accumulate cartilaginous PGs and became fibroblastic, in association with downregulation of the expression of Sox9, Nkx3.2, Indian hedgehog (Ihh), type II collagen (Col2a1), Chondromodulin-1 (Chm1), and Agc1. Accumulation of cartilaginous PGs in chondrocytes was also reduced by forced expression of Pax1 and Sox9. Thus, chondrocyte maturation driven by Sox9 is antagonized by Pax1 that is downregulated during chondrogenic differentiation.