Chondrocyte mechanotransduction is the process by which cartilage cells transduce mechanical loads into biochemical and biological signals. Previous studies have identified several pathways by which chondrocytes transduce mechanical loads, yet a general understanding of which signals are activated and in what order remains elusive. This study was performed to identify candidate mediators of chondrocyte mechanotransduction using SW1353 chondrocytes embedded in physiologically stiff agarose. Dynamic compression was applied to cell-seeded constructs for 0–30 minutes, followed immediately by whole-cell metabolite extraction. Metabolites were detected via LC-MS, and compounds of interest were identified via database searches. We found several metabolites which were statistically different between the experimental groups, and we report the detection of 5 molecules which are not found in metabolite databases of known compounds indicating potential novel molecules. Targeted studies to quantify the response of central energy metabolites to compression found a transient increase in the ratio of NADP+ to NADPH and a continual decrease in the ratio of GDP to GTP, suggesting a flux of energy into the TCA cycle. These data are consistent with the remodeling of cytoskeletal components by mechanically induced signaling, and add substantial new data to a complex picture of how chondrocytes transduce mechanical loads.

Chondrocytes are the sole cell type found in articular cartilage and are repeatedly subjected to mechanical loading in vivo. We hypothesized that physiological dynamic compression results in changes in energy metabolism to produce proteins for maintenance of the pericellular and extracellular matrices. The objective of this study was to develop an in-depth understanding for the short term (<30 min.) chondrocyte response to sub-injurious, physiological compression by analyzing metabolomic profiles for human chondrocytes harvested from femoral heads of osteoarthritic donors. Cell-seeded agarose constructs were randomly assigned to experimental groups, and dynamic compression was applied for 0, 15, or 30 minutes. Following dynamic compression, metabolites were extracted and detected by HPLC-MS. Untargeted analyses examined changes in global metabolomics profiles and targeted analysis examined the expression of specific metabolites related to central energy metabolism. We identified hundreds of metabolites that were regulated by applied compression, and we report the detection of 16 molecules not found in existing metabolite databases. We observed patient-specific mechanotransduction with aging dependence. Targeted studies found a transient increase in the ratio of NADP+ to NADPH and an initial decrease in the ratio of GDP to GTP, suggesting a flux of energy into the TCA cycle. By characterizing metabolomics profiles of primary chondrocytes in response to applied dynamic compression, this study provides insight into how OA chondrocytes respond to mechanical load. These results are consistent with increases in glycolytic energy utilization by mechanically induced signaling, and add substantial new data to a complex picture of how chondrocytes transduce mechanical loads.

In articular cartilage, chondrocytes reside within a gel-like pericellular matrix (PCM). This matrix provides a mechanical link through which joint loads are transmitted to chondrocytes. The stiffness of the PCM decreases in the most common degenerative joint disease, osteoarthritis. To develop a system for modeling the stiffness of both the healthy and osteoarthritic PCM, we determined the concentration-stiffness relationships for agarose. We extended these results to encapsulate chondrocytes in agarose of physiological stiffness. Finally, we assessed the relevance of stiffness for chondrocyte mechanotransduction by examining the biological response to mechanical loading for cells encapsulated in low- and high-stiffness gels. We achieved agarose equilibrium stiffness values as large as 51.3 kPa. At 4.0% agarose, we found equilibrium moduli of 34.3 ± 1.65 kPa, and at 4.5% agarose, we found equilibrium moduli of 35.7 ± 0.95 kPa. Cyclical tests found complex moduli of ~100-300 kPa. Viability was >96% for all studies. We observed distinct metabolomic responses in >500 functional small molecules describing changes in cell physiology, between primary human chondrocytes encapsulated in 2.0 and 4.5% agarose indicating that the gel stiffness affects cellular mechanotransduction. These data demonstrate both the feasibility of modeling the chondrocyte pericellular matrix stiffness and the importance of the physiological pericellular stiffness for understanding chondrocyte mechanotransduction.

Cartilage and chondrocytes experience loading that causes alterations in chondrocyte biological activity. In vivo chondrocytes are surrounded by a pericellular matrix with a stiffness of ∼25-200 kPa. Understanding the mechanical loading environment of the chondrocyte is of substantial interest for understanding chondrocyte mechanotransduction. The first objective of this study was to analyze the spatial variability of applied mechanical deformations in physiologically stiff agarose on cellular and sub-cellular length scales. Fluorescent microspheres were embedded in physiologically stiff agarose hydrogels. Microsphere positions were measured via confocal microscopy and used to calculate displacement and strain fields as a function of spatial position. The second objective was to assess the feasibility of encapsulating primary human chondrocytes in physiologically stiff agarose. The third objective was to determine if primary human chondrocytes could deform in high-stiffness agarose gels. Primary human chondrocyte viability was assessed using live-dead imaging following 24 and 72 hours in tissue culture. Chondrocyte shape was measured before and after application of 10% compression. These data indicate that (1) displacement and strain precision are ∼1% and 6.5% respectively, (2) high-stiffness agarose gels can maintain primary human chondrocyte viability of >95%, and (3) compression of chondrocytes in 4.5% agarose can induce shape changes indicative of cellular compression. Overall, these results demonstrate the feasibility of using high-concentration agarose for applying in vitro compression to chondrocytes as a model for understanding how chondrocytes respond to in vivo loading.

Cells sense and respond to mechanical loads in a process called mechanotransduction. These processes are disrupted in the chondrocytes of cartilage during joint disease. A key driver of cellular mechanotransduction is the stiffness of the surrounding matrix. Many cells are surrounded by extracellular matrix that allows for tissue mechanical function. Although prior studies demonstrate that extracellular stiffness is important in cell differentiation, morphology and phenotype, it remains largely unknown how a cell’s biological response to cyclical loading varies with changes in surrounding substrate stiffness. Understanding these processes is important for understanding cells that are cyclically loaded during daily in vivo activities (e.g. chondrocytes and walking). This study uses high-performance liquid chromatography - mass spectrometry to identify metabolomic changes in primary chondrocytes under cyclical compression for 0–30 minutes in low- and high- stiffness environments. Metabolomic analysis reveals metabolites and pathways that are sensitive to substrate stiffness, duration of cyclical compression, and a combination of both suggesting changes in extracellular stiffness in vivo alter mechanosensitive signaling. Our results further suggest that cyclical loading minimizes matrix deterioration and increases matrix production in chondrocytes. This study shows the importance of modeling in vivo stiffness with in vitro models to understand cellular mechanotransduction.