Flow-mediated mechanotransduction in mouse proximal tubule

The most important unresolved mystery in kidney proximal tubule is that the fractional reabsorption of salt and water remains constant despite the large variations in glomerular filtering load, i.e. glomerular tubular balance (GTB). In the proximal tubule of rat kidneys, the Na+ and HCO 3- reabsorption vary proportionally with changes in axial flow rate, a behavior termed "perfusion-absorption balance". This feature is a critical component of GTB, but the basic mechanism by which the tubule epithelial cells sense axial flow rate and convert the mechanical signal to physiological responses remains unexplained. The purpose of this thesis is to investigate flow-mediated mechanotransduction in mouse proximal tubule cells. We begin by demonstrating that "perfusion-absorption balance" is also present in isolated mouse proximal tubule. A global mathematical model was utilized to calculate the force and torque distribution on microvilli. In vitro microperfusion studies were performed on mouse proximal tubule, in which volume and HCO3- reabsorption were recorded and used as an input to the model. The model shows a perfectly linear relationship between hydrodynamic torque and Na + and HCO3- reabsorption, indicating that the torque signals transporter activities in response to flow. In addition, this flow-dependent Na+ and HCO3- reabsorption was greatly reduced by cytochalasin D, an actin disrupting drug, suggesting that the actin cytoskeleton plays an important role in regulating this flow-dependent behavior. Next, we selected a mouse proximal tubule cell line to examine the structural and functional responses of these cells to fluid shear. Cytoskeleton reorganization was dramatic. We witnessed flow-induced disappearance of stress fibers from cell base, formation of tight and adherens junctions, and accumulation of focal adhesions. Upregulation and trafficking of the transporters were seen on both luminal and peritubular sides of the cell membrane. An intact cytoskeleton network is necessary to facilitate the flow-mediated transporter activities, but not all proteins are dependent on the actin cytoskeleton. These studies investigate the mechanotransduction cascade in mouse proximal tubule cells, and will enhance on understanding of the mechanism of flow-dependent proximal tubule transport.