Mechanoinduction of PTHrP/cAMP‐signaling governs proteoglycan production in mesenchymal stromal cell‐derived neocartilage

Abstract

AbstractAbnormal mechanical loading is one of the major risk factors for articular cartilage degeneration. Engineered mesenchymal stromal cell (MSC)‐derived cartilage holds great promise for cell‐based cartilage repair. However, physiological loading protocols were shown to reduce matrix synthesis of MSC‐derived neocartilage in vitro and the regulators of this undesired mechanoresponse remain poorly understood. Parathyroid hormone‐related protein (PTHrP) is involved in cartilage development and can affect extracellular matrix (ECM) production during MSC chondrogenesis opposingly, depending on a continuous or transient exposure. PTHrP is induced by various mechanical cues in multiple tissues and species; but whether PTHrP is regulated in response to loading of human engineered neocartilage and may affect matrix synthesis in a positive or negative manner is unknown. The aim of this study was to investigate whether dynamic loading adjusts PTHrP‐signaling in human MSC‐derived neocartilage and whether it regulates matrix synthesis and other factors involved in the MSC mechanoresponse. Interestingly, MSC‐derived chondrocytes significantly upregulated PTHrP mRNA (PTHLH) expression along with its second messenger cAMP in response to loading in our custom‐built bioreactor. Exogenous PTHrP(1‐34) induced the expression of known mechanoresponse genes (FOS, FOSB, BMP6) and significantly decreased glycosaminoglycan (GAG) and collagen synthesis similar to loading. The adenylate‐cyclase inhibitor MDL‐12,330A rescued the load‐mediated decrease in GAG synthesis, indicating a direct involvement of cAMP‐signaling in the reduction of ECM production. According to COL2A1‐corrected hypertrophy‐associated marker expression, load and PTHrP treatment shared the ability to reduce expression of MEF2C and PTH1R. In conclusion, the data demonstrate a significant mechanoinduction of PTHLH and a negative contribution of the PTHrP‐cAMP signaling axis to GAG synthesis in MSC‐derived chondrocytes after loading. To improve ECM synthesis and the mechanocompetence of load‐exposed neocartilage, inhibition of PTHrP activity should be considered for MSC‐based cartilage regeneration strategies.