IHH and binding partners regulating progression of joint formation

Abstract

Functional synovial joints require proper joint development. Phalangeal joint forms by segmentation of cartilage template. This process starts with de-differentiation of cells at the future joint site (interzone), that progresses to cavitation. Indian hedgehog (IHH), an important morphogen in developmental patterning, is necessary for joint formation as mutations in this gene give rise to a rare congenital disease, Brachydactyly type A1 (BDA1) where joint formation is affected. A common molecular outcome of IHH mutations causing BDA1 is reduced binding affinity with its receptors (PTCH1) and regulators (HIP1), leading to reducing signaling capacity but increase in the signaling range resulting in more IHH in the developing interzone. However, the molecular and cellular impact for this increased concentration of IHH leading to abnormal joint formation in not known. Single cell transcriptomics analysis of WT and BDA1 developing mouse phalangeal joint interzones at E14.5 showed cellular apoptosis was affected, and identified two differentially expressed genes, Cdon and Gli3 to be associated with BDA1. Interestingly, Cdon, has been demonstrated to have dependence receptor function linked to apoptosis. Previously, we showed that apoptosis is required for phalangeal joint interzone cavitation. Indeed, inactivation of Cdon in mice and in a mouse model of BDA1 previously generated can modify apoptosis in phalangeal joint formation and progression to cavitation. Importantly, the ability for CDON to induce apoptosis is suppressed when IHH is in excess. Further preliminary findings of apoptosis from overexpression studies in ATDC5 cells and assessments using the developing chick neural tube as a model system, support the proposed the apoptosis relationship between IHH and CDON, and a potential interplay between CDON and GAS1 in regulating apoptosis. Thus, we proposed a model by which a precise level of IHH regulated apoptosis that is needed for interzone cavitation in phalangeal joint formation, and this is achieved through the interaction with CDON and/or GAS1. While further investigations are needed, our findings have provided further novel insights into the role of the hedgehog morphogenic gradient in regulating organogenesis in developmental process.