Concentration dependent effects of fibroblast-like synoviocytes on collagen gel multiscale biomechanics & neuronal signaling: Implications for modeling human ligamentous tissues.

Abnormal loading of a joint's ligamentous capsule activates nociceptive afferent fibers, which reside in the capsule's collagenous matrix alongside fibroblast-like synoviocytes (FLS) and transmit pain to the DRG. This study integrated FLS into a DRG-collagen gel model to mimic the anatomy and physiology of human joint capsules; using this new model, the effect of FLS on multiscale biomechanics and cell physiology under load were investigated. Primary FLS cells were co-cultured with DRGs at low or high concentrations, to simulate variable anatomical FLS densities, and failed in tension. Given their roles in collagen degradation and nociception, MMP-1 and neuronal expression of the neurotransmitter substance P were probed after gel failure. The amount of FLS did not alter (p>0.3) the failure force, displacement, or stiffness. FLS doubled regional strains at both low (p<0.01) and high (p=0.01) concentrations. For high FLS, the collagen network showed more reorganization at failure (p<0.01). Although total MMP-1 and neuronal substance P were the same regardless of FLS concentration before loading, expression of both increased after failure, but only with low FLS (p=0.02). The concentration-dependent effect of FLS on microstructure and cellular responses implies capsule regions with different FLS densities experience variable microenvironments. This study presents a novel DRG-FLS co-culture collagen gel system that provides a platform for investigating the complex biomechanics and physiology of human joint capsules, and is the first relating DRG and FLS interactions.