Mikko Lammi

The human musculoskeletal system is repeatedly subjected to physical loading. In order toadapt to the prevailing loading regimer, the cells of the skeletal tissues, such as bone and cartilage, must detect loading forces and express the correct genes to produce appropriate proteins for tissue remodeling.

   Loading of the musculoskeletal system subjects the chondrocyte, the main cell type present in articular cartilage, to physical loading factors, such as hydrostatic pressure and mechanical stretching. The transcriptional responses of chondrocytes exposed to these physical loading factors are not well understood. In the first two parts of this thesis, the cDNA array technique was used to study gene expression profiles of chondrocytic cells exposed to hydrostatic pressure of cyclic m echanical stretching. Several genes were found to be regulated by pressure or stretching. The results suggetsed that high static 30 MPa (300 bar) hydrostatic pressure caused a heat shock protein response, growth arrest and adaptation or differentiation of chondrocytic cells. Lower 5 MPa (50 bar) hydrostatic pressure of both the cyclic and continuous caused changes in the activity of several seemingly unrelated genes. Cyclic elongation of 8% caused more gene regulation events than 5 MPa hydrostatic pressure. Both 5 MPa pressure exposure modes, as weel as 8% cyclic stretching affected the transcription of different sets of genes. Transcription regulation events triggered by cyclic stretching were evidence that changes taking place in cell cycle regulation and in the metabolism of the extracellular matrix.

   Low intensity pulsed ultrasound (LIPUS) therapy is approved for the treatment of human bone fractures. Despite numerous previous studies, the effectiveness of the treatment and mechanism of action on osteoblast have not been definitively demonstrated. In the third part of the thesis, the transcriptional response of osteoblastic cells exposed to LIPUS was recorded at the level of the whole human genome. The in vitro experimental setup was optimised in order to expose cells to a defined dose of ultrasound thus avoiding thermal effects and unwanted reflections. The analysis revealed that LIPUS induced changes in the regulation of the genes encoding plasma membrane proteins and transporters, as well as transcription factors. However, LIPUS did not promote the appearance of an osteoblastic phenotype in the cells.

   Mechanical loading of articular cartilage also exposes chondrocytes to osmotic stress. It has been clained that chondrocytes use a small amino acid-like molecule, taurine, to protect themselves against a decrease of osmotic pressure. However, very little is known about the role of taurine in chondrocyte adaptation to hypertonic challenge. In the fourth part of this thesis, the roles of taurine and its plasma membrane transporter (TauT) were studied in articular chondrocytes and chondrocytic cells. It was demonstrated that TauT expression was regulated in anisotonic conditions. The localization of TauT on plasma membrane was confirmed. The beneficial effects of taurine on osteogenesis or chondrogenesis of mesenchymal stem cells could not be confirmed in this study.