The Mother Centriole Plays an Instructive Role in Defining Cell Geometry

Citation: Feldman JL, Geimer S, Marshall WF ( The Mother Centriole Plays an Instructive Role in Defining Cell Geometry Jessica L. Feldman 0 1 Stefan Geimer 0 1 Wallace F. Marshall 0 1 0 Academic Editor: Hiroshi Hamada, Osaka University , Japan 1 1 Department of Biochemistry and Biophysics, University of California San Francisco , San Francisco , California, United States of America, 2 Biologie/Elektronenmikroskopie NW I/B 1, Universita t Bayreuth , Bayreuth , Germany Centriole positioning is a key step in establishment and propagation of cell geometry, but the mechanism of this positioning is unknown. The ability of pre-existing centrioles to induce formation of new centrioles at a defined angle relative to themselves suggests they may have the capacity to transmit spatial information to their daughters. Using three-dimensional computer-aided analysis of cell morphology in Chlamydomonas, we identify six genes required for centriole positioning relative to overall cell polarity, four of which have known sequences. We show that the distal portion of the centriole is critical for positioning, and that the centriole positions the nucleus rather than vice versa. We obtain evidence that the daughter centriole is unable to respond to normal positioning cues and relies on the mother for positional information. Our results represent a clear example of ''cytotaxis'' as defined by Sonneborn, and suggest that centrioles can play a key function in propagation of cellular geometry from one generation to the next. The genes documented here that are required for proper centriole positioning may represent a new class of ciliary disease genes, defects in which would be expected to cause disorganized ciliary position and impaired function. - A fundamental question in cell biology is how cell geometry is established and maintained [14]. Cell geometry refers to the characteristic positioning of organelles within the cell body in order for a cell to be able to carry out its specified function. Despite the importance of cell geometry in tissue organization and cell function, the mechanistic origins of cell geometry remain a mystery. Further compounding the mystery is the fact that, as demonstrated by the classic experiments of Beisson and Sonneborn [5], cell organization can be propagated through cell division, alleviating the need for cells to re-establish their infrastructure after each round of mitosis, and potentially allowing a coherent organization to be maintained across developing tissue during proliferative growth. Many organelles take part in this elaborate cellular patterning. One organelle that is often found in specific subcellular locations is the centriole. Centrioles are nonmembrane-bound organelles composed of nine triplet microtubule blades arranged around a central cartwheel structure. Centrioles are found as a pair, composed of a mother and a daughter, which is duplicated during each cell cycle. Mother centrioles are so-called because they were assembled in a previous cell cycle to the daughter centriole. Mother centrioles have unique ultrastructural modifications [6] and are decorated with a number of molecules not found on daughter centrioles. Centrioles have two main functions in the cell. First, centrioles together with pericentriolar material comprise the centrosome, the major microtubule-organizing center of the cell. Indeed, centrioles are the highly stable, core nucleating centers for the centrosome, providing it with persisting structural integrity [7] and attaching it to cytoplasmic microtubules during G1 [8]. Second, centrioles serve as basal bodies to nucleate the assembly of cilia. In order to carry out these functions in the cell, centrioles often need to be specifically localized. Although originally named for their centralized location, centrioles are repositioned to more peripheral sites during cell-state transitions such as wound healing, cell migration, and cell growth [911]. The importance of centriole positioning for development and physiology is perhaps most clearly illustrated in situations involving cilia, which are assembled from centrioles. The problem of ciliary positioning is 2-fold. First, centrioles must migrate to the proper region on the cell surface where they will dock and assemble cilia. Second, once centrioles reach the cell surface, they must become properly oriented so as to create a proper directional stroke in the case of motile cilia, or so they are oriented to participate in signaling as in the case of a primary cilium. Perturbation in either step of ciliary positioning has severely deleterious effects in humans [12]. For example, inability of centrioles to properly migrate prior to ciliary assembly has recently been linked to Meckel-Gruber syndrome [13]. Additionally, proper orientation of cilia via centriole positioning towards the posterior of embryonic node cells is critical for establishing leftright asymmetry during mammalian development [14]. Centrioles must also be properly positioned when they serve as basal bodies in multiciliated cells such as in the tracheal epithelium. Centriole orientation, and the resulting proper Cells are not just homogenous bags of enzymes, but instead have a precise and complex internal architecture. However, the mechanisms that define this architecture remain unclear. How do different organelles find their proper location within the cell? We have begun to address this question for one particular organelle, the centriole, using a genetic approach. Our approach relies on the fact that centrioles are required for the assembly of cilia and flagella, which are used for swimming. We studied the unicellular green alga Chlamydomonas, which use flagella to swim towards a light source. We screened for mutants that could not swim towards light, and found a set of mutants in which the centrioles and flagella are displaced from their normal location within the cell. Using these mutants, we have obtained evidence that centrioles play a role in positioning other structures within the cell, such as the nucleus. We also found that in these cells, which contain two centrioles differing in age, the older centriole plays a role in positioning the newer centriole, suggesting that cells may have a way to propagate spatial patterns from one generation to the next. alignment of respiratory cilia, is required for effective mucus clearing in the airway [15]. In all cases in which cilia act either to drive fluid flow or act as sensors, it is important that they be placed on the appropriate region of the cell surface; for example, in cells lining a duct, the cilia would have to face the lumen of the duct, which requires specific positioning of centrioles on a limited patch of cell surface. It is clear that centriole positioning is critical in many aspects of cell behavior, especially in placing a cilium that will interact with the extracellular environment. Centriole position may also serve a function in intr (...truncated)