Proteomics of the human meniscus in health and osteoarthritis

Abstract: Introduction: Osteoarthritis (OA) is one of the most common causes of knee pain, and the most common form ofarthritis. It causes pain and disability, and there is currently no cure. Recent research in knee OA highlights therole of the meniscus in OA pathology. In this thesis, the overall aim was to study the proteome and microstructureof the human meniscus, both in healthy subjects and OA patients, in order to increase our knowledge about thehuman meniscus and the processes that occur during degeneration and OA.Design: In the first part of this thesis, posterior horns of medial and lateral menisci from deceased donors and OApatients were analysed with histology and micro-computed tomography (μCT), in order to compare themicrostructure of healthy and OA menisci (paper I). In the second, and major part of this thesis, human menisciwere analysed with mass spectrometry (MS)-based proteomics. First, meniscus body samples and articularcartilage samples from deceased donors were compared using two different MS-based methods – dataindependentacquisition (DIA) and data-dependent acquisition (DDA) (paper II). Next, three zones (peripheral,middle and inner zone) of the body of the medial meniscus from deceased donors were compared using DIA(paper III). Lastly, meniscal plugs from the same meniscal posterior horns that had been used in paper I, wereanalysed with DIA.Results: In paper I, we found that higher histopathological scores were associated with more degeneration, andthat medial menisci from OA patients had the highest degree of degeneration. We also observed that the degreeof degeneration varied between the different sections of the meniscus. In paper II, we showed that the majority ofthe proteins were similar between articular cartilage and meniscus, but that there were nonetheless someproteomic differences. Furthermore, we found that DIA identified more differentially expressed proteins, with lessmissing values. In paper III, we observed that many proteins were common to all zones of the meniscus, and thatthe majority of meniscal proteins were associated with ECM organisation. The largest zonal differences werebetween the peripheral and inner zones. In paper IV, where we compared menisci from OA patients and deceaseddonors, we showed that the largest differences could be seen between the medial menisci from the two groups,and that there was an increase of proteoglycans and proteins such as MMP3 and TIMP1.Conclusions: Due to the many advantages of DIA, we considered it superior to DDA and used it in subsequentMS analyses. Although articular cartilage and meniscus had similar proteomic compositions, we detected severaldifferences by MS. We also detected large differences between the peripheral and inner zones of the healthymeniscus body, probably related to the higher degree of cellularity and vascularisation in the peripheral zone. Onthe other hand, the middle and inner zones of the meniscus contained more extracellular proteins and thereforeappeared to be more similar to articular cartilage. Comparison of healthy and OA menisci revealed increaseddegeneration of the OA menisci, with more disorganised collagen networks and the presence of cysts andcalcifications. The OA menisci also had an increase of proteins such as MMP3 and TIMP1, which suggestssimultaneous activation of both catabolic and anabolic processes. The increased degeneration in OA meniscisuggests a strong association between OA and meniscal degeneration.