Toward defining the role of the synovium in mitigating normal articular cartilage wear and tear

Abstract

Cartilage repair has been studied extensively in the context of injury and disease, but the joint's management of regular sub-injurious damage to cartilage, or 'wear and tear,' which occurs due to normal activity, is poorly understood. We hypothesize that this cartilage maintenance is mediated in part by cells derived from the synovium that migrate to the worn articular surface. Here, we demonstrate in vitro that the early steps required for such a process can occur. First, we show that under physiologic mechanical loads, chondrocyte death occurs in the cartilage superficial zone along with changes to the cartilage surface topography. Second, we show that synoviocytes are released from the synovial lining under physiologic loads and attach to worn cartilage. Third, we show that synoviocytes parachuted onto a simulated or native cartilage surface will modify their behavior. Specifically, we show that synoviocyte interactions with chondrocytes lead to changes in synoviocyte mechanosensitivity, and we demonstrate that cartilage-attached synoviocytes can express COL2A1, a hallmark of the chondrogenic phenotype. Our findings suggest that synoviocyte-mediated repair of cartilage 'wear and tear' as a component of joint homeostasis is feasible and is deserving of future study.

Keywords: Cartilage; Mechanobiology; Stem cell; Synovium.

Fig. 1.

A) Reciprocal friction shear loading of articular cartilage. B) Representative viability staining of B) unloaded and C) loaded cartilage with corresponding red and green fluorescence channel intensity, SZ on right. Bar: 50 μm. D) AFM Imaging of Cartilage Surface. Topography, Roughness (RQ, scale ± 2 μm), and Hertz elastic modulus plot of cartilage surface (scale: 0–400 kPa) of control and after 2 h of physiologic friction loading. E) H&E and PRG4 staining (brown) of juvenile bovine cartilage, control (2 images at left) and after friction loading (2 images at right) at 409 kPa for 2 h, showing decreased superficial lubricin after shear and possible loss of surface lining chondrocytes. Scale bar: 100 μm.

Fig. 2.

A) Reciprocal shear loading of synovium on glass. B) Quantification of release of synovial cells, living (calcein) and dead (ethidium homodimer), into a bath of media:synovial fluid (1:1). FLS (green) attachment to C) control and D) friction shear-loaded articular cartilage plugs (chondrocyte nuclei labeled blue). N = 4 cartilage plugs for each condition. Scale bar: 200 μm.

Fig. 3.

A) FLS parachuted (Fura-red dye- red) on articular chondrocyte (AC) monolayer. B) Fraction of responding cells. FLS parachuted on an AC monolayer showed a significant decrease in response rate for young pool cells (n = 5 slides each, p = 0.026). C) Peak latencies of responding cells. All comparisons not marked “ns” are significant, with p < 0.001. D) Dye transfer from FLS to articular chondrocyte monolayer quantified with flow cytometry. Cells in the upper-left quadrant were DiI+/Calcein−, indicating no dye transfer has occurred, and cells in the upper-right quadrant were DiI+/Calcein+, indicating that dye transfer has occurred. E) AFM imaging of articular cartilage monolayer revealed a non-uniform substrate both in terms of topography and elastic modulus F) COL2A1 reporter human cells parachuted on engineered cartilage. Cellular morphology was non-uniform, and COL2A1 expression levels (red fluorescence) varied. Below image, pixel intensity for GFP (green, successfully transduced cells), tdTomato (red, cells expressing COL2A1), and the red:green ratio. Scale bar: 100 μm. G) COL2A1 reporter bovine cells on cartilage explant. Cells localized to a suspected defect site where they expressed COL2A1. Scale bar: 100 μm.