Knee adipose tissue: from its implication in osteoarthritis to its supposed role in tissue engineering

Knee osteoarthritis (OA) has long been considered as a cartilage disease. However, it is now well established that cartilage defects are the ultimate OA feature only. Several studies investigating the mechanisms of OA onset/progression have recognized the infrapatellar fat pad as an active OA-player but also as a possible source of stem cells with regenerative potential for cartilage. Is it really like that? a cautious approach should be adopted.

To date, the knee is broadly recognized as a complex and dynamic organ, where the balanced/imbalanced dialog among its constituent structures (i.e., cartilage, bone, synovium, meniscus, fat pads) plays a fundamental role in maintaining joint health or driving osteoarthritis (OA) progression, respectively¹. Therefore, cartilage damage, associated with eventual formation of osteophytes, is only the final characterizing feature of OA, which leads to pain, disability and compromised quality of life². Age, gender, obesity and previous joint injuries are risk factors for knee OA, whose pathogenesis remains poorly understood, despite being the prevalent disease in elderly population^2,3. The incidence of OA increases with the rise of these predisposing factors; one in three people over age 65, particularly women, are affected by OA⁴. Moreover, obesity increases the risk of OA in both weight-bearing and non-weight-bearing joints, doubling the lifetime risk of symptomatic OA compared to individuals with a Body Mass Index (BMI) of less than 25⁵.

According to epidemiological studies, the link between obesity and OA cannot be explained solely by the mechanical overload associated with excess weight⁶. Obesity, compounded by aging, drastically increases chronic low-grade inflammation (inflammaging) that, in turn, contributes to OA development^7,8. Currently, there is growing attention on the endocrine activity of adipose tissue in relation to OA onset/progression^6,9. Moreover, in addition to the systemic role of visceral and subcutaneous adipose tissues, the pro-inflammatory contribution of local adipose tissues in OA cannot be excluded^6,10,11.

Several adipose tissues are recognized within the knee joint, and they are known to interact with neighbouring tissues, possibly influencing joint homeostasis. These include the infrapatellar fat pad (IFP), the suprapatellar fat pad (SFP), and other small fat pads like the posterior knee fat pad and posterior suprapatellar fat pad. In efforts to identify OA-linked factors, IFP tissue has been recognized as the most closely related to knee disease. It is not merely a passive bystander with a cushion-like function, but rather an active player capable of influencing joint homoeostasis¹².

Structurally, the IFP distinguishes for lobules of white fibrous adipose tissue, with a mean diameter of 1.15 ± 0.11 mm, which are separated by thin connective septa (0.22 ± 0.034 mm)^13,14. Topographically, it is related to the inferior pole of the patella superiorly, the patellar tendon anteriorly, the anterior tibia and the anterior horns of the menisci posteroinferiorly, and the femoral condyles and intercondylar notch posteriorly¹⁵. On its posterior aspect, the IFP is bordered by the synovial membrane (SM); this closeness is so intimate that the tissues cannot be easily separated, suggesting anatomical continuity and leading to the concept of an anatomo-functional unit (AFU). Imaging, histopathology and molecular biology evidence corroborate the existence of the AFU¹⁶. Interestingly, despite the pathophysiological changes associated with aging, the AFU plays a role in both initiating and perpetuating the onset and progression of osteoarthritis (OA), as well as in OA-associated knee joint pain, making it a target of the disease itself^17,18,19.

OA directly influences IFP morpho-structural characteristics. As we demonstrated for the first time¹⁷, histological, pathological, and immunohistochemical data, including quantitative reverse transcription–polymerase chain reaction (qRT-PCR), reveal distinct features for the OA-IFP compared to the non-OA-IFP (control). Typically, OA-IFP exhibits significantly higher vascularization, thicker interlobular septa, lymphocyte infiltration (absent in controls), smaller adipose lobules, and a significantly higher percentage of interleukin (IL)-6 and Monocyte Chemoattractant Protein-1 (MCP-1)-positive roundish cells and/or crown-like structures. Additionally, an increase in nerve fibers is observed in OA patients. These structural alterations in the IFP are associated with modifications in tissue biomechanics, which further perpetuate joint damage and inflammation^20,21,22,23. Regarding the SM, OA is responsible of both subintimal and synovial lining cells layer thickening. As shown by Emmi et al.¹⁹, the non-OA SM distinguishes for the typical lining layer appearance (one to three layers of lining cells), while in OA it is hyperplastic with multiple cell layers (OA intima thickness, 33.60 ± 3.8 µm). Focusing on SM compartment, CD68⁺ macrophages are moderately present in the vascular cuffing of both the IFP and the SM but are predominant throughout the sub-lining layer and within the lining layer of the SM, marking this as a distinct OA-AFU feature. In accordance with Li et al.¹, the SM and IFP, integrated in the AFU, undergo coordinated changes during OA progression.

Piezo channels (Piezo1 and Piezo2, respectively) are mechanically activated proteins that can sense compression and tensile load, in turn conveying mechanical signals, including harmful mechanical stimuli^24,25. Interestingly, Piezo channels mediate high-strain hyperphysiologic loading, representing a OA high risk factor²⁶; moreover, their expression increases and is potentiated in inflammation²⁷. Comparing the OA-AFU with the non-OA-AFU, Emmi et al.¹⁹ demonstrated, for the first time, their pattern distribution within the AFU: moderate expression of Piezo1 is detected in both the OA-IFP vessels (only slightly present in non-OA AFUs) and OA-SM vessels (not detected in non-OA AFUs). Considering Piezo1 sensitivity to flow-induced shear stress, it may be supposed its role in vascular remodelling, in turn indicating AFU plasticity; moreover, Piezo1 high-/overexpression is related to microenvironment stiffness, suggesting a correlation with OA-related fibrosis²⁸. Regarding Piezo2, the expression is moderate in the OA-SM vessels (not detected in non-OA AFUs), and strong in the OA-IFP-compartment (slight in non-OA AFUs). Piezo2 have a role in mediating mechanotransduction in the somatosensation of touch, proprioception, and pain; moreover, they have a mechanotransducer-role in the endothelial cells, contributing to stimulus dependent hyperalgesia in endothelium-related pain^29,30. Certainly, more consciousness on Piezo1/2 receptors presence/distribution within the diseased or healthy AFU may be fundamental in developing effective treatments for OA pain-management.

To date, if conservative strategies are not successful in managing knee OA, partial or total removal of the IFP and SM may be proposed for pain relief and reduction of pro-inflammatory markers (e.g., MCP1) in synovial fluid, in accordance with evidence from preclinical studies³¹. Within this context, the possibility to isolate the cell population of interest (i.e., adipose tissue-derived stem cells) from a waste tissue, to derive a therapeutic advantage following their reimplantation in a diseased target tissue (i.e., cartilage), would be highly rewarding^32,33,34,35. In addition, due to their proximity to the knee joint, IFP-stem cells may show a better potential in cartilage regeneration compared to others mesenchymal stem cells³⁶ (Table 1). However, our experience suggests that caution is necessary in considering the IFP from OA patients as a high-quality source of stem cells for regenerative purposes³⁷.

Table 1 Representative studies considering IFP-stem cells for cartilage regeneration

Conditions that predispose to OA and co-exist with it (e.g., age, obesity, hypertension, type 2 diabetes) build-up a chronic-stress clinical setting, characterized by oxidative stress mediators increase (i.e., reactive oxygen species)³⁸. In the knee joint, this environment affects IFP tissue with an organ dysfunction known as adiposopathy³⁹ leading to distress for residents’ cellular elements, including OA-IFP stem cells. As described in Stocco et al.⁴⁰, IFP stem cells isolated from OA-patients (n = 9; median age = 74 years; median body mass index = 29.4 Kg/m²) undergoing total knee arthroplasty show peculiar characteristics, suggesting they are primed by the inflammatory environment. Together with a distinct immunophenotype (CD73⁺/CD39⁺/CD90⁺/CD105⁺/CD44^–/+/CD45^–) associated with a high stemness grade (STAT3, NOTCH1, c-Myc, OCT-4, KLF4, and NANOG)/self-renewal potential, the OA-IFP stem cells are responsive to the microenvironmental stimuli, as suggested by detection of CD44, CD105, VEGFR2, FGFR2, IL1R, and IL6R⁴⁰. Moreover, presence of HLA-DR/CD34/Fas/FasL suggest a phenotypic reprograming ascribable to inflammation, with the ability to switch to adaptive fibrogenic reprograming in case of unbalanced mechanical stimuli/overloading (see high expression level of COL1A1 gene)^40,41,42,43. Furtherly, the significant expression of cortactin/CTTN gene advocated that the differentiation of OA-IFP stem cells might be regulated by mechano-transduction. In contrast, CD38/NADase low expression, generally upregulated in OA⁴⁴, implied OA-IFP stem cells inability to counteract NAD⁺-mediated OA inflammation leading to cellular stress (high calreticulin gene expression). Even though the OA-IFP stem cells show an anti-inflammatory signature (i.e., CD39⁺/CD73⁺⁴⁵), in vitro studies demonstrated their scarce ability in exerting a control over immune response to OA. Overall, expression of CD34, suggests a derivation of OA-IFP stem cells from adipose tissue⁴⁶ or vascular endothelial compartment⁴⁷; whereas absence of the pericyte marker PDGFRb excluded a derivation from perivascular niche.

Intense efforts are dedicated towards the identification of effective therapeutic options for OA-related cartilage lesions. However, the use of cells derived from IFP requires careful attention to avoid the regeneration of poor-quality tissue that is functionally ineffective compared to native the hyaline cartilage. Understanding the cross-talk between various cells in the knee environment, including chondrocytes, synovial cells, fibroblasts, fibroblast-like synoviocytes, osteoblasts, osteoclasts, adipocytes and immune cells, is crucial for optimizing treatment strategies^48,49,50. A particularly intriguing aspect of this dialogue is the role of small extracellular vesicles (sEVs), also known as exosomes. Exosomes serve as a novel means of cell-to-cell communication by delivering bioactive molecules that influence the local microenvironment and modulate cartilage behavior. Specifically, the cargos of exosomes, including noncoding RNAs and proteins, play a fundamental role in the progression of OA^51,52. Exosomes released by inflammatory cells, such as macrophages, carry pro-inflammatory cytokines that can alter the behavior of chondrocytes and synovial cells, contributing to cartilage degradation and pain⁵³. Furthermore, chondrocytes themselves can release exosomes containing matrix metalloproteinases (MMPs) or other inflammatory mediators, which promote the breakdown of the extracellular matrix and accelerate cartilage degeneration⁵⁴. Exosomes derived from synovial cells can also influence chondrocyte activity, either promoting or inhibiting cartilage repair depending on their molecular content^55,56. In addition, exosomes impact osteoblasts and osteoclasts, affecting subchondral bone remodeling by carrying signals that promote bone sclerosis or resorption which are two key features in the progression of OA^57,58,59. Finally, in accordance with functional studies, IFP-derived exosomes significantly promote ECM catabolism in chondrocytes also triggering senescence and exacerbating the progression of experimental OA in mice⁶⁰ (Fig. 1).

Fig. 1

Exosomes cargos from different knee cells contribute to the remodeling of the inflammatory microenvironment also driving the catabolism of the cartilage ECM and the bone with associated pain. The Figure was partly created with BioRender.com.

Exosomes are actively involved in multiple aspects of OA pathology, including inflammation, cartilage degeneration, cell communication within the joint, tissue repair, and bone remodeling⁵³. Experimental evidence highlights the potential of exosomes as both biomarkers for monitoring OA and targets for therapeutic intervention, offering a promising avenue for the development of more effective treatments for this debilitating disease⁵⁶.