Synergetic Support of Cartilage Homeostasis via Coupled Thermal-Hydrostatic Pressure Stimuli

Cartilage homeostasis, crucial for musculoskeletal function, is orchestrated by interconnected biophysical cues. In healthy cartilage, repetitive compressive loading not only elicits a range of mechanical stimuli but also induces a gradual transient temperature increase, coined as "cartilage self-heating". Among these different stimuli, the natural coupling of hydrostatic pressure (HP) and the loading-induced temperature increase (T) stands out. Our interest in the potential significance of HP and T in regulating cartilage homeostasis arises not only from their roles as the primary external stimuli chondrocytes encounter from both mechanical and thermal perspectives, but also from their notable absence in degenerated cartilage. Moreover, prior research underscored the individual favorable influence of HP and T on chondrocytes. Inspired by all the aforementioned observations, this thesis aims to explore the collaborative regulatory impact of HP-T stimulation on cartilage homeostasis. The proposed research is structured in three steps. Each step progressively deepens our understanding on how the cooperative HP-T stimulation contributes to the maintenance of cartilage homeostasis.The first step of the thesis focused on crafting a specialized bioreactor designed to simulate the physiologically relevant dynamic hydrostatic pressure and gradual temperature increase observed within the human knee during daily activities such as walking or jogging. Through detailed analyses of 3D-cultured chondrocytes submitted to either individual or coupled HP-T stimuli, foundational insights emerged: chondroinduction and chondroprotection were maximized under the combined influence of HP and T. This suggests an intrinsic mechanism that upholds cartilage homeostasis during repetitive loading, and which absence might be related to the initiation of the vicious cycle of cartilage degeneration.Transitioning to the second step, a detailed transcriptomic analysis was undertaken on human cartilage explants subjected to various HP and/or T stimulation conditions using bulk RNA-sequencing. The results underscored the predominant role of the thermal stimulus in the HP-T regulation of the chondrocyte transcriptional profile, essentially influencing which genes are activated within these cells. Additionally, its collaboration with HP enhanced chondroinduction and chondroprotection capabilities of chondrocytes. The synergy of HP and T stimuli was further demonstrated to be accompanied by enhanced biological processes of protein modulation, such as protein translation and processing.The third step of the research centered on HSP70, a chaperon protein crucial for cellular stress responses, protein folding stabilization, and is directly related to chondrocyte function. In earlier steps of the thesis, this protein demonstrated a significant synergistic response to HP-T stimulation, reaching its peak expression when both stimuli were combined. To delve deeper into how HSP70 influences the chondrocyte response to HP-T stimulation, engineered human cartilage constructs were subjected to various stimulations under HSP70 normal or inhibited conditions. Through both gene expression and proteomics analyses, a clear pattern emerged: without sufficient HSP70 activity, the typically beneficial HP-T stimulation started to detrimentally influence chondrocyte homeostasis. ... (not able to fit the rest due to character limit ..............)