Total amount of acetylated tubulin in sea urchin cilia of varying lengths.

Introduction: Cilia are easily observable membrane-bound microtubule projections from a cell. The point from which a cilium projects away from the cell is referred to as the basal body, a structure of triplet microtubules that anchor the cilium (Hammond, Cai, and Verhey, 2008). The building blocks used to construct the extending cilium are transported from the base toward the protruding (+) end in a process called intraflagellar transport (Eggenschwiler and Anderson, 2007). The constructed microtubules are arranged in two patterns, either 9+2 meaning nine outer doublet microtubules surrounding two central microtubules or 9+0 meaning nine outer doublet microtubules organized in the same manner but without the central pair (Satir, 2005). The microtubules in cilia are known to be more stable—resistant to depolymerization, the breaking down of microtubules—than those of the cellular cytoskeleton due to post-translational modifications (Hammond et al., 2008). Post-translational modifications generally occur in polymerized tubulin, tubulin already assembled in a microtubule (Hammond et al., 2008). One post-translational modification that stabilizes microtubules is acetylation the binding of acetyl groups to α-tubulin (Westermann and Weber, 2003). Studies have not found that acetylation of α-tubulin is necessary for cell survival but allows for many specialized functions (Westermann and Weber, 2003). It was believed that 9+2 cilia had significant function in organisms and 9+0 (primary) cilia were vestigial because they generally lacked the dynein motors necessary for cilia movement (Satir and Christensen, 2007). Recent studies have identified the prevalence of primary cilia throughout the body and the ability of some primary cilia to be motile (Satir and Christensen, 2007). This knowledge instigated investigations of the purpose of primary cilia. Data, found when studying the membrane of cilia, supports that cilia can act as receptors for factors influencing cell growth, cell division, and maintenance of a differentiated state (Satir and Christensen, 2007). There is also evidence that Hedgehog and Wnt signaling pathways during development act through cilia (Satir and Christensen, 2007). Much of this evidence was acquired by identifying abnormal Hedgehog and Wnt signaling in individuals with conditions known to be caused by cilia defects such as polycystic kidney disease, certain cancers, and retinal degeneration (Satir and Christensen, 2007). Therefore better understanding of the structure and receptive functions of cilia could lead to breakthrough treatments of these disorders and pathologies. Defects causing a cilium to be unstable, more likely to depolymerize, would inhibit that cilia’s ability to move or to act as a receptor. A cilium needs enough acetylation to stabilize its structure allowing it to properly function. It is hypothesized that the total amount of acetylated tubulin within a cilium increases with increased length of the cilium. Since sea urchin development is easily-observable and there are cilia present on the cells it can be used as a model organism to study the structure of cilia. In this study, cilia on sea urchin embryos were immunofluorescently labeled to detect the amount of acetylated tubulin in each cilium.