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Stentor comprises a genus of freshwater protists that has long enthralled cell and developmental biologists. These organisms are large polyploid single cells that possess highly polarised and complex structures. Normally elongated in a trumpet-like shape, Stentor cells also have the ability to contract their cell body, and do so in response to mechanical or light stimuli. Interestingly, this response is subject to habituation, meaning the cell can ‘learn’ after repeated exposure to ignore these stimuli and stay elongated. Perhaps the most remarkable characteristic of Stentor is the ability of these cells to fully regenerate after being cut in half, perfectly preserving the original cell structure. Numerous microscopic studies analysed the minute morphological details of Stentor regeneration, but for many decades, there were no tools available for molecular and genetic studies. However, recent developments should now allow researchers to probe the molecular details of regeneration in a single-celled organism. Key Concepts Stentor coeruleus is a large and highly polarised single cell with complex cell biology. Stentor cells exhibit a rapid contractile response in reaction to light and mechanical stimulation, but they can habituate to these stimuli over time. Stentor cells possess the remarkable ability to fully regenerate themselves after being cut in half. Keywords: Stentor coeruleus; ciliates; protists; regeneration; habituation

Eukaryotic organelles encapsulate defined subsets of cellular biochemical pathways. For example, beta oxidation of fatty acids occurs inside mitochondria while fatty acid chain elongation takes place on the endoplasmic reticulum membrane. Organelle membranes isolate reactions from each other and store intermediates and products, and can thus be viewed as “reaction vessels”, playing roles analogous to the reflux columns and holding tanks of a chemical factory. To develop an effective chemical manufacturing process, it is not enough to focus just on the chemistry, i.e. the reactants and solvents that directly participate in reactions. The size and design of the reaction vessels is of equal importance. Likewise, within a cell, the size of organelles will influence the rates of biochemical pathways contained within them. Organelle surface area can limit the rate of import of substrates and efflux of products, while the volume of the organelle can dictate the quantity of intermediates that can build up (Figure 1). Many key metabolic enzymes are organelle membrane proteins, and in such cases increased surface area could allow larger numbers of molecules into the membrane to increase metabolic flux. Figure 1 Organelles as reaction vessels. Substrate in cytoplasm (Sc) is imported into an organelle through its bounding membrane to provide substrate inside the organelle (So). This organellar substrate is then subjected to several enzymatic steps in the organelle ... The influence of organelle size on metabolism is indicated by the fact that in cells specialized for certain pathways, the organelles that contain these pathways are enlarged compared to other cell types. Secretory cells are an obvious example, in which the requirement for a high rate of flux of secreted proteins is met by a massive over proliferation of endoplasmic reticulum and Golgi apparatus. Other examples include enlarged lipid droplets in adipose cells, proliferation of microvilli on the surface of cells lining the intestine, increased surface area and volume of rhodopsin containing vesicles in rods versus cones, and changes in mitochondrial abundance as a function of respiratory state. If, as we hypothesize, organelle size affects metabolism and signaling, then reprogramming of organelle size could be used as a novel strategy for reprogramming cellular state and behavior, with direct applications in medicine and biotechnology. Organelle-directed medicine and biotechnology Cytopathologists diagnose cancer by visual assessment of cell geometry including organelle size. For example, enlarged nuclei in a pap test indicates early stage cervical cancer. Cytopathology texts are full of such examples, but we don’t understand why these changes occur. According to the hypothesis of this review, these changes of cell geometry in cancer arise because cells have adapted to the metabolic alterations that are a hallmark of cancer [1]. Could we attack cancer cells by reprogramming organelle size? We can distinguish two possible reasons for organelle size alteration in cancer cells, which in turn predict two possible ways that organelle targeted therapy could be useful (Figure 2). First, if organelle size is adjusted as a response to pathological alterations in cell metabolism, then if we could reprogram organelle size in a cancer cell using small molecules that target the size control pathway, the cell might die due to a mismatch between organelle size and metabolic state. Alternatively, organelle size alterations might arise from pathological alterations in signaling pathways that impinge on the size control system, and then alterations in cell metabolism or behavior would be a downstream effect of the change to organelle size. In this case, it might be possible to drive the cell back to a less malignant state by driving its organelles towards a more normal size range. Either outcome would be therapeutically useful, but so far this “organelle directed medicine” strategy has not to our knowledge been tested in any cancer model system. Figure 2 Organelle size changes in disease: two strategies for organelle directed medicine. Disease causing mutations (for example, loss of tumor suppressor genes or activation of oncogenes) cause organelle size changes that are observed by the cytopathologist. ... Reprogramming organelle size would also have applications in metabolic engineering. Increasing the size of organelles that encapsulate key steps of metabolite production, especially those involving toxic intermediates, could greatly enhance metabolite production. For example biodiesel production could be enhanced by targeting genes that control lipid droplet size [2–3] thereby enhancing the ability of the cell to store triglyceride (TG). Before we can implement or test these applications in medicine and biotechnology, we need to obtain mechanistic understanding of how organelle size is regulated.

Basal bodies and centrioles play central roles in microtubule (MT)-organizing centres within many eukaryotes. They share a barrel-shaped cylindrical structure composed of nine MT triplet blades. Here, we report the structure of the basal body triplet at 33 A resolution obtained by electron cryo-tomography and 3D subtomogram averaging. By fitting the atomic structure of tubulin into the EM density, we built a pseudo-atomic model of the tubulin protofilaments at the core of the triplet. The 3D density map reveals additional densities that represent non-tubulin proteins attached to the triplet, including a large inner circular structure in the basal body lumen, which functions as a scaffold to stabilize the entire basal body barrel. We found clear longitudinal structural variations along the basal body, suggesting a sequential and coordinated assembly mechanism. We propose a model in which δ-tubulin and other components participate in the assembly of the basal body.

Cilia are motile and sensory organelles with critical roles in physiology. Ciliary defects can cause numerous human disease symptoms including polycystic kidneys, hydrocephalus, and retinal degeneration. Despite the importance of these organelles, their assembly and function is not fully understood. The unicellular green alga Chlamydomonas reinhardtii has many advantages as a model system for studies of ciliary assembly and function. Here we describe our initial efforts to build a chemical-biology toolkit to augment the genetic tools available for studying cilia in this organism, with the goal of being able to reversibly perturb ciliary function on a rapid time-scale compared to that available with traditional genetic methods. We screened a set of 5520 compounds from which we identified four candidate compounds with reproducible effects on flagella at nontoxic doses. Three of these compounds resulted in flagellar paralysis and one induced flagellar shortening in a reversible and dose-dependent fashion, accompanied by a reduction in the speed of intraflagellar transport. This latter compound also reduced the length of cilia in mammalian cells, hence we named the compound “ciliabrevin” due to its ability to shorten cilia. This compound also robustly and reversibly inhibited microtubule movement and retrograde actin flow in Drosophila S2 cells. Ciliabrevin may prove especially useful for the study of retrograde actin flow at the leading edge of cells, as it slows the retrograde flow in a tunable dose-dependent fashion until flow completely stops at high concentrations, and these effects are quickly reversed upon washout of the drug. © 2011 Wiley-Liss, Inc.

Interaction between homologous chromosomes is a key organizing force in the interphase nucleus. Establishment of these interactions requires large-scale chromosome movement and provides a model system for studying homologue pairing in meiosis. Keywords: nuclear organization; chromatin; meiosis; transvection; Drosophila