Homogentisic acid induces morphological and mechanical aberration of ochronotic cartilage in alkaptonuria

Abstract

Alkaptonuria (AKU) is a disease caused by a deficient homogentisate 1,2-dioxygenase activity leading to systemic accumulation of homogentisic acid (HGA), that forms a melanin-like polymer that progressively deposits onto connective tissues causing a pigmentation called "ochronosis" and tissue degeneration. The effects of AKU and ochronotic pigment on the biomechanical properties of articular cartilage need further investigation. To this aim, AKU cartilage was studied using thermal (thermogravimetry and differential scanning calorimetry) and rheological analysis.We found that AKU cartilage had a doubled mesopore radius compared to healthy cartilage. Since the mesoporous structure is the main responsible for maintaining a correct hydrostatic pressure and tissue homoeostasis, drastic changes of thermal and rheological parameters were found in AKU. In particular, AKU tissue lost its capability to enhance chondrocytes metabolism (decreased heat capacity) and hence the production of proteoglycans. A drastic increase in stiffness and decrease in dissipative and lubricant role ensued in AKU cartilage. Multiphoton and scanning electron microscopies revealed destruction of cell-matrix microstructure and disruption of the superficial layer. Such observations on AKU specimens were confirmed in HGAtreated healthy cartilage, indicating that HGA is the toxic responsible of morphological and mechanical alterations of cartilage in AKU. K E Y W O R D Salkaptonuria, glycosaminoglycan, heat capacity, mesoporosity, ochronosis, rheology

<p>Alkaptonuria: Current Perspectives</p>

“…Characteristics of AKU cartilage were recently studied using thermal (thermogravimetry and differential scanning calorimetry) and rheological analysis. 51 Drastic changes in thermal and rheological parameters were found, indicating that AKU tissue is not able to enhance chondrocyte metabolism and produce proteoglycans. Consequently, its stiffness drastically rises while dissipative and lubricant roles diminish, manifested also as destruction of the cell-matrix microstructure and disruption of the superficial layer.…”

“…HGA-treated healthy cartilage manifested the same features, indicating that HGA is actually the toxic agent responsible for alterations in cartilage in AKU. 51 There have been some specific features of AKU cartilage described recently. In normal cartilage, collagen fibers were cross-linked covalently to other-type collagen molecules, especially to homotypic polymers of type II collagen, whereas in AKU cartilage high amounts of type III collagen were present.…”

Homogentisic acid-derived pigment as a biocompatible label for optoacoustic imaging of macrophages

“…Decades of continuous HGA buildup in patients’ serum, urine, and interstitial fluids are required for alkaptonuria to manifest and result in the characteristic ochronotic pigment deposits primarily found in joints. Cartilage degeneration 33 and SAA amyloid–pigment coaggregations have further been reported 34 .…”

Ochronotic pigmentation is caused by homogentisic acid and is the key event in alkaptonuria leading to the destructive consequences of the disease—A review

“…The amount of OP formed is proportionate to the concentrations of HGA in the medium . ex vivo “organotypic” approaches have modeled AKU by studying the effect of HGA on human cartilage explants . In these studies, pericellular pigmentation of cartilage explants is visible after 2 months of incubation with HGA…”

“…Development of these model systems has not only been useful in studying the development of OP under controlled conditions, but also a number of other pathophysiological events closely associated with ochronosis including secondary amyloidosis, perturbed redox homeostasis, and disorder of the cartilage extracellular matrix, including proteoglycan loss and collagen fibril rearrangement . Oxidative stress is a frequent and major consequence of HGA exposure in these studies, as indicated by direct oxidative modification to proteins and depletion of endogenous antioxidants such as glutathione .…”