Manipulation of primary cilia using optical tweezers

Nonmotile primary cilia are expressed in most renal epithelial cells. The primary cilia of renal epithelia bend with the urinary flow within the renal tubules. It has been postulated that the cilia are a mechano-sensor of urinary flow. However, the mechanical characteristics of an individual primary cilium are not fully understood. In the present study, we successfully manipulated micron-sized primary cilia of the Madin-Darby canine kidney (MDCK) cell lines derived from the collecting duct, by using optical tweezers. The primary cilia of MDCK cells, cultured on flexible plastic films, could be clearly visualized from the side by video microscopy. Optical tweezers utilize the radiation pressure from the concentrated laser beam to trap and manipulate a small microparticle, and forces on the objects in the trap can be evaluated. We showed that we could apply forces in the piconewton range to a polystyrene bead flixed on the tip of the primary cilium by using optical tweezers; by that means, the primary cilium could be bent and stretched. The elasticity of each primary cilium was analyzed by measuring the displacement of the bead.