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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., (Copyright © 2011 Wiley Periodicals, Inc.)

Alkaptonuria (AKU) results from defective homogentisate1,2-dioxygenase (HGD), causing degenerative arthropathy. The deposition of ochronotic pigment in joints is so far attributed to homogentisic acid produced by the liver, circulating in the blood and accumulating locally. Human normal and AKU osteoarticular cells were tested for HGD gene expression by RT-PCR, mono- and 2D-Western blotting. HGD gene expression was revealed in chondrocytes, synoviocytes, osteoblasts. Furthermore, HGD expression was confirmed by Western blotting, that also revealed the presence of five enzymatic molecular species. Our findings indicate that AKU osteoarticular cells produce the ochronotic pigment in loco and this may strongly contribute to induction of ochronotic arthropathy., (Copyright © 2011 Wiley Periodicals, Inc.)

Objectives: Alkaptonuria (AKU) is an orphan disease that has an estimated prevalence of 0.3/100,000. The disease is caused by the lack of activity of homogentisic acid oxidase (HGO), an enzyme involved in tyrosine and phenylalanine metabolism. To date, there is only one drug, the nitisinone, with orphan designation authorised by both Food and Drug Administration (FDA) and European Medical Agency (EMA) for AKU. A clinical trial on AKU patients using nitisinone has recently been completed but it needs further investigation for long-term therapy. In recent years our group has developed a series of AKU in vitro models using cell lines, primary chondrocytes and human plasma in order to test the efficacy of new substances, mainly antioxidant compounds, for AKU therapy. Herein, we report the optimisation of an ex vivo reproducible culture method exploiting cartilage slices in order to investigate the deposition of ochronotic pigment in this kind of connective tissue., Methods: Human normal cartilage slices, obtained after surgery for prosthesis replacement, were cultured for several days in the presence of a sublethal concentration of homogentisic acid (HGA)., Results: After two months of incubation with HGA, the peculiar melanin-like ochronotic pigmentation can be observed into the cartilage tissue., Conclusions: This novel organo-typic ex vivo model could be extremely useful to investigate the efficacy of substances able to ameliorate the conditions of AKU patients. Moreover, it could be used for genetic and proteomic investigations to better define AKU pathophysiology.

Objectives: Alkaptonuria (AKU) is a genetic disorder caused by lack of the enzyme responsible for breaking down homogentisic acid (HGA), an intermediate in tyrosine metabolism. HGA is deposited as a polymer, termed ochronotic pigment, in collagenous tissues. Pigmentation is progressive over many years, leading to CTDs including severe arthropathies. To investigate the mechanism of pigmentation and to determine how it leads to arthropathy, we aimed to develop an in vitro model of ochronosis., Methods: Osteosarcoma cell lines MG63, SaOS-2 and TE85 were cultured in medium containing HGA from 0.1 μM to 1 mM. Cultures were examined by light microscopy and transmission electron microscopy, and Schmorl's stain was used to detect pigment deposits in vitro, following the observation that this stain identifies ochronotic pigment in AKU tissues. The effects of HGA on cell growth and collagen synthesis were also determined., Results: There was a dose-related deposition of pigment in cells and associated matrix from 33 μM to 0.33 mM HGA. Pigmentation in vitro was much more rapid than in vivo, indicating that protective mechanisms exist in tissues in situ. Pigment deposition was dependent on the presence of cells and was observed at HGA concentrations that were not toxic. There was an inhibition of cell growth and a stimulation of type I collagen synthesis up to 0.33 mM HGA, but severe cell toxicity at 1 mM HGA., Conclusion: We have developed an in vitro model of ochronosis that should contribute to understanding joint destruction in AKU and to the aetiology of OA.

Alkaptonuria (AKU) is a rare autosomal recessive disease, associated with deficiency of homogentisate 1,2-dioxygenase activity in the liver. This leads to an accumulation of homogentisic acid (HGA) and its oxidized derivatives in polymerized form in connective tissues especially in joints. Currently, AKU lacks an appropriate therapy. Hence, we propose a new treatment for AKU using the antioxidant N-acetylcysteine (NAC) administered in combinations with ascorbic acid (ASC) since it has been proven that NAC counteracts the side-effects of ASC. We established an in vitro cell model using human articular primary chondrocytes challenged with an excess of HGA (0.33 mM). We used this experimental model to undertake pre-clinical testing of potential antioxidative therapies for AKU, evaluating apoptosis, viability, proliferation, and metabolism of chondrocytes exposed to HGA and treated with NAC and ASC administered alone or in combination addition of both. NAC decreased apoptosis induced in chondrocytes by HGA, increased chondrocyte growth reduced by HGA, and partially restored proteoglycan release inhibited by HGA. A significantly improvement in efficacy was found with combined addition of the two antioxidants in comparison with NAC and ASC alone. Our novel in vitro AKU model allowed us to demonstrate the efficacy of the co-administration of NAC and ASC to counteract the negative effects of HGA for the treatment of ochronotic arthropathy., ((c) 2010 Wiley-Liss, Inc.)