Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) reveals potential lipid markers between infrapatellar fat pad biopsies of osteoarthritis and cartilage defect patients

Tissue and Extracellular Matrix Remodeling of the Subchondral Bone during Osteoarthritis of Knee Joints as revealed by Spatial Mass Spectrometry Imaging

Osteoarthritis (OA) is a degenerative condition of the skeletal extracellular matrix (ECM) marked by the loss of articular cartilage and changes to subchondral bone homeostasis. Treatments for OA beyond full joint replacement are lacking primarily due to gaps in molecular knowledge of the biological drivers of disease. Mass Spectrometry Imaging (MSI) enables molecular spatial mapping of the proteomic landscape of tissues. Histologic sections of human tibial plateaus from knees of human OA patients and cadaveric controls were treated with collagenase III to target ECM proteins prior to MS Imaging of bone and cartilage proteins using a timsTOF fleX mass spectrometer. Spatial MSI data of the knee were processed and automatically segmented identifying distinct areas of knee joint damage. ECM peptide markers were compared between i) the medial halves of OA patient joints and the medial side of non-OA (cadaveric) joints, and ii) between the same medial OA tissues and their corresponding, less OA impacted, lateral joint halves. Distinct peptide signatures distinguished OA medial tissues from the cadaveric medial and OA lateral tissues (AUROC >0.85). Overall, 31 peptide candidates from ECM proteins, including Collagen alpha-1(I), Collagen alpha-1(III), and surprisingly, Collagen alpha-1(VI) and Collagen alpha-3(VI), exhibited significantly elevated abundance in diseased tissues. Highly specific hydroxyproline-containing collagen peptides, mainly from collagen type I, dominated OA subchondral bone directly under regions of lost cartilage. The identification of specific protein markers for subchondral bone remodeling in OA advances our molecular understanding of disease progression in OA and provides potential new biomarkers for OA detection and disease grading.

Systemic and joint adipose tissue lipids and their role in osteoarthritis

Osteoarthritis (OA) is a major disease whose prevalence increases with aging, sedentary lifestyles, and obesity. The association between obesity and OA has been well documented, but the precise mechanisms underlying this heightened risk remain unclear. While obesity imposes greater forces on joints, systemic fat-derived factors such as lipids or adipokine may potentially act on the pathophysiology of OA, but the exact role of these factors in weight-bearing and non-weight-bearing joints remains elusive. Intra-articular adipose tissues (IAAT) have gained significant attention for actively participating in OA pathogenesis by interacting with various joint tissues. Lipid content has been proposed as a diagnostic target for early OA detection and a potential source of biomarkers. Moreover, targeting a specific IAAT called infrapatellar fat pad (IFP) and its lipids hold promise for attenuating OA-associated inflammation. Conversely, bone marrow adipose tissue (BMAT), which was long thought to be an inert filling tissue, is now increasingly considered a dynamic tissue whose volume and lipid content regulate bone remodeling in pathological conditions. Given OA's ability to alter adipose tissues, particularly those within the joint (IFP and BMAT), and the influence of adipose tissues on OA pathogenesis, this review examines the lipids produced by OA-associated adipose tissues, shedding light on their potential role in OA pathophysiology and highlighting them as potential therapeutic targets.

Role of joint adipose tissues in osteoarthritis

Osteoarthritis (OA) is the most common musculoskeletal disease, without any curative treatment. Obesity being the main modifiable risk factor for OA, much attention focused on the role of adipose tissues (AT). In addition to the involvement of visceral and subcutaneous AT via systemic ways, many arguments also highlight the involvement of local AT, present in joint tissues. Local AT include intra-articular AT (IAAT), which border the synovium, and bone marrow AT (BMAT) localized within marrow cavities in the bones. This review describes the known features and involvement of IAAT and BMAT in joint homeostasis and OA. Recent findings evidence that alteration in magnetic resonance imaging signal intensity of infrapatellar fat pad can be predictive of the development and progression of knee OA. IAAT and synovium are partners of the same functional unit; IAAT playing an early and pivotal role in synovial inflammation and fibrosis and OA pain. BMAT, whose functions have only recently begun to be studied, is in close functional interaction with its microenvironment. The volume and molecular profile of BMAT change according to the pathophysiological context, enabling fine regulation of haematopoiesis and bone metabolism. Although its role in OA has not yet been studied, the localization of BMAT, its functions and the importance of the bone remodelling processes that occur in OA argue in favour of a role for BMAT in OA.

Spatial analysis of the osteoarthritis microenvironment: techniques, insights, and applications

Osteoarthritis (OA) is a debilitating degenerative disease affecting multiple joint tissues, including cartilage, bone, synovium, and adipose tissues. OA presents diverse clinical phenotypes and distinct molecular endotypes, including inflammatory, metabolic, mechanical, genetic, and synovial variants. Consequently, innovative technologies are needed to support the development of effective diagnostic and precision therapeutic approaches. Traditional analysis of bulk OA tissue extracts has limitations due to technical constraints, causing challenges in the differentiation between various physiological and pathological phenotypes in joint tissues. This issue has led to standardization difficulties and hindered the success of clinical trials. Gaining insights into the spatial variations of the cellular and molecular structures in OA tissues, encompassing DNA, RNA, metabolites, and proteins, as well as their chemical properties, elemental composition, and mechanical attributes, can contribute to a more comprehensive understanding of the disease subtypes. Spatially resolved biology enables biologists to investigate cells within the context of their tissue microenvironment, providing a more holistic view of cellular function. Recent advances in innovative spatial biology techniques now allow intact tissue sections to be examined using various -omics lenses, such as genomics, transcriptomics, proteomics, and metabolomics, with spatial data. This fusion of approaches provides researchers with critical insights into the molecular composition and functions of the cells and tissues at precise spatial coordinates. Furthermore, advanced imaging techniques, including high-resolution microscopy, hyperspectral imaging, and mass spectrometry imaging, enable the visualization and analysis of the spatial distribution of biomolecules, cells, and tissues. Linking these molecular imaging outputs to conventional tissue histology can facilitate a more comprehensive characterization of disease phenotypes. This review summarizes the recent advancements in the molecular imaging modalities and methodologies for in-depth spatial analysis. It explores their applications, challenges, and potential opportunities in the field of OA. Additionally, this review provides a perspective on the potential research directions for these contemporary approaches that can meet the requirements of clinical diagnoses and the establishment of therapeutic targets for OA.