Amyloid in the menisci of the knee joint is one of the most common forms of localized amyloidosis and is especially prevalent in the elderly (1–7). The amyloid is found principally within the deep central portions and on the surface of meniscal stroma. In a study of arthroscopy-derived meniscus fragments from 316 patients between 20 and 80 years of age, congophilic deposits were detected in 70% of the cases (100% in those older than age 50), with both male and female subjects equally affected (6). Notably, the presence of amyloid was not necessarily associated with degenerative or inflammatory disease, i.e., osteoarthritis, rheumatoid arthritis, or trauma (4–6), and on this basis, it was assumed that amyloid in the knee joint menisci represented a type of localized senile amyloidosis (6).
Although it was speculated that the amyloidogenic component in knee menisci was derived from extrasynovial tissue (6), neither the chemical nature of the amyloid nor its source had heretofore been established. To investigate this, we used microanalytic techniques developed in our laboratory (8, 9) to characterize the amyloid protein extracted from synovial tissue obtained at the time of knee replacement surgery in 3 patients. Amino acid sequence and mass spectrometric (MS) analyses revealed that, in all instances, the fibrils were composed of polypeptide fragments identical in sequence to the N-terminal portion of apolipoprotein A-I (Apo A-I). The Apo A-I nature of the amyloid was confirmed when the green birefringent congophilic deposits were immunostained with an antiserum specific for this molecule. The meniscal cell origin of this high-density lipoprotein was demonstrated by reverse transcriptase–polymerase chain reaction (RT-PCR), and its formation by chondrocytes was evidenced by in situ hybridization, as well as immunohistochemically. The results of our research have provided unequivocal evidence that amyloid of the knee joint menisci is derived from Apo A-I; further, we posit that this localized form of amyloidosis results from age-associated factors that render this protein amyloidogenic, and that amyloid of the knee joint menisci may have pathophysiologic consequences.
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- PATIENTS AND METHODS
The results of our studies on amyloid extracted from the menisci of 3 patients who underwent knee joint replacement surgery have provided definitive evidence of the Apo A-I derivation of this material. Direct sequence and MS/MS analyses revealed, in all cases, that the fibrils were composed of the first ∼77 amino acids of the 244-residue parent molecule. The fragmentary nature of the Apo A-I amyloid was not unexpected since deposits formed from other amyloidogenic precursor proteins (e.g., serum amyloid A or immunoglobulin light chains) typically are composed of N-terminal proteolytic products of the native molecules (13). Further proof that the meniscus-associated amyloid was Apo A-I–related was obtained from immunohistochemical studies, in which a polyclonal anti-human Apo A-I antiserum specifically immunostained the green birefringent congophilic extracellular deposits in samples from these 3 patients, as well as those from an additional 4 individuals.
In contrast to heritable cases of Apo A-I amyloidosis (14), no mutations were detected in the amyloid protein extracted from menisci and further, given the common occurrence of such deposits, the amyloid undoubtedly was the product of nonmutated human Apo A-I genes (15). Notably, amyloid consisting of wild-type Apo A-I has been found in the aortic intima of elderly individuals (16), as well as the pulmonary vasculature of elderly dogs (17). Other types of amyloid associated with the aging process also are composed of nonmutated proteins, e.g., transthyretin (18), prolactin (19), lactadherin (20), and Apo A-II (21).
Although Apo A-I is synthesized in mammals predominantly by the liver and intestines (and, to a much lesser extent, by other tissues) (22, 23), our studies indicate that this protein additionally is produced within meniscal tissue, specifically by chondrocytes, as evidenced by the results of Southern blot and in situ hybridization/immunohistochemical analyses. The synthesis of Apo A-I by these cells has been demonstrated by other investigators, who have shown that expression of this molecule occurs during chondrocyte differentiation and is markedly enhanced by small molecule ligands that activate the liver and retinal X receptors (24).
The fact that Apo A-I is produced by chondrocytes may have functional significance, since this molecule can inhibit the synthesis of proinflammatory cytokines by monocytes and macrophages (25, 26). Because Apo A-I is the major protein constituent of high-density lipoprotein and its stability is affected by interaction with this lipid complex, quantitative or qualitative alterations in knee joint cholesterol or phospholipids, as well as extracellular factors (pH, glycosaminoglycans), could lead to destabilization and self-aggregation, with eventual fibril formation (27–31). Thus, we posit that the changes in tissue structure or chemical composition of articular cartilage caused by aging are primarily responsible for the development of amyloid in the knee joint menisci.
Clinically, the occurrence of amyloid in the menisci represents a localized rather than a systemic process, and results from the in situ production of the amyloidogenic precursor Apo A-I protein. Interstitial deposition of amyloid can have pathophysiologic consequences, manifested by eventual organ failure due to replacement of normal tissue elements by insoluble fibrillar proteins or peptides, and/or cytotoxicity induced by these components (13). Conceivably, amyloid deposition in the meniscus may be a contributing, or possibly an initiating, factor in a process that results in progressive impairment of knee function. In this regard, the ability to inhibit fibrillogenesis or facilitate amyloidolysis therapeutically (32, 33) may provide a means to slow or reverse the pathologic process and, thus, the need for surgical joint replacement.