Dynamic Compression and Aggrecan Modulate Pericellular Matrix Development in Growth Plate Chondrocytes

Abstract

Cartilage tissue in the long bones is subject to frequent changes in mechanical stress that are transduced through the extracellular matrix to chondrocytes. A specialized matrix called the pericellular matrix (PCM) surrounds chondrocytes and is known to provide biomechanical and biochemical cues to the cells. The PCM is composed of proteoglycans (e.g. aggrecan), glycosaminoglycans, and is the primary localization of type VI collagen (ColVI), which acts as an anchor to hold the chondrocytes to the PCM via β‐integrin transmembrane receptors. Although biomechanical signaling from the matrix to chondrocytes has been extensively studied, detailed studies of effects of mechanotransduction on PCM ultrastructure have not been performed. As a starting point, we developed two culture systems to study matrix formation by single chondrocytes. First, we used a 3D alginate hydrogel bead culture model to assess PCM development in murine hindlimb growth plate chondrocytes in static non‐compressed conditions. Second, we designed and used a microfluidic cell compression device stimulated by pressurized air to examine PCM development under one hour of dynamic compression (1 Hz) per day for four days. The device allows five different magnitudes (3.5–22%) of compressive strain to be tested simultaneously. We compared PCM development in wild type (WT) and Aggrecan (Acan) mutant chondrocytes that lack the proteoglycan component of the PCM. PCM development was assessed using immunofluorescence staining for ColVI, imaged with confocal microscopy, and measured using a MATLAB image processing program. Results show that under non‐compressed conditions, WT chondrocytes form a ring of ColVI at the cell surface within 1 day of culture and this ring expanded radially away from the membrane and increased in area over 21‐days of culturing. By contrast, null Acan chondrocytes did not show detectable ColVI deposition until day 4, and only after 7 days of culture did Acan mutant ColVI deposition equal that of day 1 WT growth plate chondrocyte (distance = 1.12±0.79 vs. 1.17±1.34 μm, respectively). The distance and area of ColVI matrix deposition was directly proportional to the magnitude of dynamic compression in the WT growth plate chondrocytes and similar studies are currently underway with Acan mutant chondrocytes. Our preliminary studies have demonstrated complementary culture systems that can be used to quantitatively analyze effects of compressive strain and mutant phenotypes on chondrocyte matrix development. Our continuing studies will use these tools to determine how chondrocytes build strong matrix and how intracellular and extracellular factors influence matrix structure and integrity. These studies of matrix structure are intended to support tissue engineering of cartilage, which despite decades of research has had little success in recapitulating the biomechanical functions of native cartilage.Support or Funding InformationGrant AR070242 from NIH/NAIMS, Holland Regenerative Medicine Program