Direct Recording and Molecular Identification of the Calcium Channel of Primary Cilia

Abstract

Primary cilia are solitary, tiny organelles that project from the membrane of most mammalian cells. Housing components of the cell division apparatus between cell divisions, they also serve as specialized compartments for calcium signaling and Hedgehog signaling pathways. The ionic permeability of the primary cilia membrane, the ionic concentrations within the cilia, and the diffusion barrier created by the cilia neck are unknown. Here, we report the first direct measurements and identification of ion channels from intact primary cilia. We demonstrate that a channel composed of two TRP channel proteins, PKD1-L1 and PKD2-L1. This channel richly populates cilia at a density that rival those of voltage-gated ion channels in nerve and muscle. This heteromeric channel conducts a moderately calcium-selective current that is regulated by second messengers of the Gαq/11 pathway. We developed a transgenic mouse in which only cilia express a fluorophore and employ it to measure currents in the cilia of diverse tissues. In conjunction with cilia-specific ratiometric Ca-sensor, we show that the PKD1-L1/PKD2-L1 heteromeric channel establishes the cilia as a unique calcium compartment within cells that regulates established Hedgehog pathways.