Reduced primary cilia length and altered Arl13b expression are associated with deregulated chondrocyte Hedgehog signaling in alkaptonuria

This work was supported by the Medical Research Council (MR/L002876/1), the Royal College of Surgeons of England, Fondazione Telethon Italy (GGP10058), and Associazione Italiana Malati di Alcaptonuria (AimAKU, ORPHA263402

Biochemical and proteomic characterization of alkaptonuric chondrocytes

Alkaptonuria (AKU) is a rare genetic disease associated with the accumulation of homogentisic acid (HGA) and its oxidized/polymerized products which leads to the deposition of melanin-like pigments (ochronosis) in connective tissues. Although numerous case reports have described ochronosis in joints, little is known on the molecular mechanisms leading to such a phenomenon. For this reason, we characterized biochemically chondrocytes isolated from the ochronotic cartilage of AKU patients. Based on the macroscopic appearance of the ochronotic cartilage, two sub-populations were identified: cells coming from the black portion of the cartilage were referred to as ‘black’ AKU chondrocytes, while those coming from the white portion were referred to as ‘white’ AKU chondrocytes. Notably, both AKU chondrocytic types were characterized by increased apoptosis, NO release and levels of pro-inflammatory cytokines. Transmission electron microscopy also revealed that intracellular ochronotic pigment deposition was common to both ‘white’ and ‘black’ AKU cells. We then undertook a proteomic and redox-proteomic analysis of AKU chondrocytes which revealed profound alterations in the levels of proteins involved in cell defence, protein folding and cell organization. An increased post-translational oxidation of proteins, which also involved high molecular weight protein aggregates, was found to be particularly relevant in ‘black’ AKU chondrocytes

Cytoskeleton Aberrations in Alkaptonuric Chondrocytes

Alkaptonuria (AKU) is an ultra-rare autosomal genetic disorder caused by a defect in the activity of the enzyme homogentisate 1,2-dioxygenase (HGD) that leads to the accumulation of homogentisic acid (HGA) and its oxidized product, benzoquinone acetic acid (BQA), in the connective tissues causing a pigmentation called "ochronosis". The consequent progressive formation of ochronotic aggregates generate a severe condition of oxidative stress and inflammation in all the affected areas. Experimental evidences have also proved the presence of serum amyloid A (SAA) in several AKU tissues and it allowed classifying AKU as a secondary amyloidosis. Although AKU is a multisystemic disease, the most affected system is the osteoarticular one and articular cartilage is the most damaged tissue. In this work, we have analyzed for the first time the cytoskeleton of AKU chondrocytes by means of immunofluorescence staining. We have shown the presence of SAA within AKU chondrocytes and finally we have demonstrated the co-localization of SAA with three cytoskeletal proteins: actin, vimentin and β-tubulin. Furthermore, in order to observe the ultrastructural features of AKU chondrocytes we have performed TEM analysis, focusing on the Golgi apparatus structure and, to demonstrate that pigmented areas in AKU cartilage are correspondent to areas of oxidation, 4-HNE presence has been evaluated by means of immunofluorescence. This article is protected by copyright. All rights reserved