Osteoarthritis and metabolic dysregulation: insights from a preclinical model

Abstract

This thesis aims to identify the effect of metabolic factors, inflammatory processes and obesity in the pathophysiology of osteoarthritis (OA), using a high-fat diet and/or traumatic injury in a small animal model. The first part of this thesis describes, the rat Groove model of OA, using a one-time trigger, with and without induced metabolic dysregulation. In this model, the femoral articular cartilage is damaged without damaging the subchondral bone. This resulted in mild knee joint degeneration as observed by histology as well as µCT. However, synovial inflammation or osteophyte formation was not observed. To study the metabolic phenotype of OA in animals, a high-fat (HF) diet was used, inducing metabolic dysregulation and not a full metabolic syndrome. Using only a HF diet resulted in a more inflamed joint with increased synovial membrane inflammation without cartilage degeneration. HF diet feeding in addition to groove surgery resulted in an increased progression of osteoarthritic features, with inflammatory responses like increased synovitis, increased CD68 expression, and osteophyte formation. It was concluded that an additional trigger is needed to develop metabolic OA when the animal is metabolically challenged. The second part of this thesis focuses on the further validation of the Groove model. Pain was determined by the vonFrey method together with dynamic weight bearing. The Groove model induced pain in the experimental paws without clear changes in weight bearing. When the model is combined with a HF diet, similar pain was observed compared to standard diet fed rats with reduced weight bearing of the experimental paw. Also systemic alterations with macrophage infiltration in the spinal cord and astrocyte activation in the dorsal root ganglia as a result of groove surgery in metabolic challenged rats were observed. The presence of activated macrophages was studied in-vivo by SPECT/CT using a new folate conjugate with an albumin binding entity. When groove surgery was combined with HF diet feeding, increased folate receptor expressing macrophages were observed. The actual presence of activated macrophages was additionally confirmed by immunohistochemistry of CD68. This technique provides opportunities to monitor inflammation longitudinally, especially in metabolic OA. Next, potential biomarkers were studied. Local and systemic oxylipins, signalling mediators capable of modulating the inflammatory state of the joint, are responsive in early stages of OA, especially when inflammation is present. These oxylipins act different in the local and peripheral compartment. Another biomarker studied is Fibulin, capable to inhibit angiogenesis and chondrocyte differentiation. Fibulin3-3 was increased and associated with histological joint degeneration and cartilage damage. In the final part of this thesis, a newly developed drug was studied, the fusion-protein of two individual anti-inflammatory cytokines IL-4 and IL-10. This fusion-protein administered by repetitive intra-articular injections reduced pain and this transient effect was observed in all animals independent of the diet. By histology, there was a small decrease in synovitis, but no effect on cartilage degeneration. This thesis results in a better understanding on how obesity influences the pathophysiology of OA and consequently more accurate treatment strategies to prevent the occurrence and progression of metabolic related OA.