Novel Insights into the Development and Function of Cilia Using the Advantages of the Paramecium Cell and Its Many Cilia

Abstract

Paramecium species, especially P. tetraurelia and caudatum, are model organisms for modern research into the form and function of cilia. In this review, we focus on the ciliary ion channels and other transmembrane proteins that control the beat frequency and wave form of the cilium by controlling the signaling within the cilium. We put these discussions in the context of the advantages that Paramecium brings to the understanding of ciliary motility: mutants for genetic dissections of swimming behavior, electrophysiology, structural analysis, abundant cilia for biochemistry and modern proteomics, genomics and molecular biology. We review the connection between behavior and physiology, which allows the cells to broadcast the function of their ciliary channels in real time. We build a case for the important insights and advantages that this model organism continues to bring to the study of cilia.

Keywords: Paramecium; calcium; cilia; electrophysiology; proteomics; signal transduction; swimming behavior.

Figure 1

Paramecium tetraurelia showing the waves of cilia beating toward the posterior of the cell. From Grass Calendar, 1985.

Figure 2

A. These images are to demonstrate that the intracellular membrane potential of Paramecium is negative (about −25 to −40 mV); the corresponding ciliary beat is toward the posterior of the cell and the cell swims forward. B. In depolarizing solutions, like high K⁺ or Ba²⁺, the cell’s membrane potential depolarizes and reaches threshold for the action potential. During the action potential calcium enters the cilia through voltage gated channels, the high levels of Ca²⁺ change the power stroke of the cilia, which now beat most strongly toward the anterior and move the cell backward. The action potential is quickly over and the calcium is removed from or sequestered in the cilia, allowing the ciliary beat and swimming to return to normal. With permission from Science [3].

Figure 3

Diagram of a cross section of a cilium. Outer arm dyneins in red; inner arm dyneins in green; doublets of microtubules in blue; radial spokes connecting central pair to outer doublets. Adapted from [30], with permission.

Figure 4

Tomogram from a Tetrahymena cilium. Isosurface renderings show the averaged 96-nm axonemal repeats of the Radial Spoke proteins. Radial spoke proteins 1, 2, and 3 are shown in green, blue, and orange, respectively. The ODA are shaded in purple, IDA are shaded in light pink, and yellow shading indicates the Nexin-Dynein Regulatory Complex. The red arrowheads indicate the connections between the radial spokes. (A) is a longitudinal-front view, (B) longitudinal back view, and (C) a bottom view. Adapted from [28], with permission.

Figure 5

Cells were stained using anti-centrin to visualize the basal bodies. Images represent stacks of Z sections, approximately 10 µm thick. The images shown are the dorsal surface of the anterior end of the cell. Basal bodies should be arranged in organized rows, as seen in the control and IFT88 depleted cells. These latter cells are controls for the effects of shortened and lost cilia because they are depleted in a protein that is important for the import of proteins into the cilia. The MKS3 depleted cell (center panel) shows the basal bodies not aligned and no longer in straight rows (white arrows) at the midline of the cell. Scale bars: 10 µm. With permission from Cilia [9].

Figure 6

Frequency of proteins organized by function from the 300 most abundant proteins from each preparation: non-fractionated cilia (C), ciliary membrane (CM), and the detergent phase of Triton X-114 phase separation (DP). With permission from Elsevier [58].