Ribosomal proteins in bone development – RiBone

Bone is a complex and dynamic tissue constituted by a collagen matrix filled in with crystal of hydroxyapatite. Chondrocytes, the cells of cartilage, are involved in bone biology and more precisely during the bone growth. In the endochondral ossification, the skeletal cartilage templates are replaced by bone. After birth, the epiphyseal plate, located between the epiphysis and the diaphysis, is responsible for growth in length of the bone. In this stratified cartilage structure, chondrocytes are proliferating with high rate and then become hypertrophic and secrete a high amount of collagen matrix. To achieve all this processes, cells require high translational capacity, to meet the demands of proliferation, collagen matrix production, within the stratified growth plate. Our team described recently a severe form of spondyloepimetaphyseal dysplasia in two patients (SEMD) that differs from SEMD caused by matrilin 3 or collagen type II mutations. Exome sequencing in patients and their parents reveal a de novo variant located in the same gene, RPL13, in the two patients. Both de novo variants were predicted to abolish the natural donor splice site of intron 5 of the RPL13 gene, therefore coding for a 18 amino acids longer protein (+2kDa).
It is the first time that a mutation in RPL13 gene is described causing a defect in skeletal development, suggesting that RPL13 is mandatory for a well-balanced bone growth. As RPL13 is a ribosomal protein belonging to the large 60S subunit, this new form of SEMD refer to a novel ribosomopathy in which ribosome biogenesis or an alteration of ribosomal structural protein could lead to pathologies. There are a group of human disorders that often affect a specific tissue such as blood compartment in Diamond Blackfan Anemia. Numerous of these ribosomopathies can result in skeletal anomalies (hypoplasia of craniofacial bones, short stature or anomalies in digits) indicating that ribosome formation and composition play critical role in bone growth.
Our preliminary data show that the ribosome is fully build in patient mesenchymal stem cells. Thus, we hypothesized that mesenchymal stem cells with RPL13-mutated ribosomes are not able to well differentiate into chondrocytes to achieve long bone growth. This fact could be due to a defect in translation of key factors that are involved in chondrocyte differentiation process which explain the selective tissue affected by RPL13 deleterious variant. This hypothesis could be extended to the concept of “specializes ribosomes”. We hypothesize that, along chondrogenic differentiation, cells arbor a various compostion in ribosomal proteins leading to a selection in mRNAs actively translated.
To our knowledge, no study has ever investigated the control performed by ribosomes at a post transcriptonal level in the differenciation of chondrocytes. We will deeply analyse the physiopathological role of RPL13 variant at a cellular level and in a Crispr/Cas9 generated strain mice. Ribone project will also use innovative techniques to reveal the actively translated mRNAs associated to the heterogeneity of ribosomes in chondrocytes.
The Overall objective of this project is to investigate the role of ribosomes into the control of proliferation and differentiation of chondrocytes. This project will contribute to a better understanding of bone growth through the study of pathophysiological process of a severe dysplasia due to a deleterious variant in a ribosomal protein. It will gain insight into post-transcriptionnal regulation given by ribosomes that occur in bone development and would provide new targets for the treatment of cartilage diseases.