Turnover of Tubulin in Ciliary Outer Doublet Microtubules

History of research concerning the ependyma: a view from inside the human brain

The history of research concerning ependymal cells is reviewed. Cilia were identified along the surface of the cerebral ventricles c1835. Numerous anatomical and histopathological studies in the late 1800's showed irregularities in the ependymal surface that were thought to be indicative of specific pathologies such as syphilis; this was subsequently disproven. The evolution of thoughts about functions of cilia, the possible role of ependyma in the brain-cerebrospinal fluid barrier, and the relationship of ependyma to the subventricular zone germinal cells is discussed. How advances in light and electron microscopy and cell culture contributed to our understanding of the ependyma is described. Discoveries of the supraependymal serotoninergic axon network and supraependymal macrophages are recounted. Finally, the consequences of loss of ependymal cells from different regions of the central nervous system are considered.

In-cell structural insight into the stability of sperm microtubule doublet

The propulsion for mammalian sperm swimming is generated by flagella beating. Microtubule doublets (DMTs) along with microtubule inner proteins (MIPs) are essential structural blocks of flagella. However, the intricate molecular architecture of intact sperm DMT remains elusive. Here, by in situ cryo-electron tomography, we solved the in-cell structure of mouse sperm DMT at 4.5-7.5 Å resolutions, and built its model with 36 kinds of MIPs in 48 nm periodicity. We identified multiple copies of Tektin5 that reinforce Tektin bundle, and multiple MIPs with different periodicities that anchor the Tektin bundle to tubulin wall. This architecture contributes to a superior stability of A-tubule than B-tubule of DMT, which was revealed by structural comparison of DMTs from the intact and deformed axonemes. Our work provides an overall molecular picture of intact sperm DMT in 48 nm periodicity that is essential to understand the molecular mechanism of sperm motility as well as the related ciliopathies.

Cilia locally synthesize proteins to sustain their ultrastructure and functions

Cilia are microtubule-based hair-like organelles propelling locomotion and extracellular liquid flow or sensing environmental stimuli. As cilia are diffusion barrier-gated subcellular compartments, their protein components are thought to come from the cell body through intraflagellar transport or diffusion. Here we show that cilia locally synthesize proteins to maintain their structure and functions. Multicilia of mouse ependymal cells are abundant in ribosomal proteins, translation initiation factors, and RNA, including 18 S rRNA and tubulin mRNA. The cilia actively generate nascent peptides, including those of tubulin. mRNA-binding protein Fmrp localizes in ciliary central lumen and appears to function in mRNA delivery into the cilia. Its depletion by RNAi impairs ciliary local translation and induces multicilia degeneration. Expression of exogenous Fmrp, but not an isoform tethered to mitochondria, rescues the degeneration defects. Therefore, local translation defects in cilia might contribute to the pathology of ciliopathies and other diseases such as Fragile X syndrome.

Testing the time-of-flight model for flagellar length sensing

A combination of quantitative imaging, modeling, and genetics has been used to test a proposed mechanism for measuring the size of an organelle. One way to measure distance is to send a clock out on a train and measure the elapsed time when the train returns. We tested a molecular version of this model as a possible regulator of intraflagellar transport by altering the return speed of the transport machinery and probing the effect on a known length-dependent process.

Cilia and flagella are microtubule-based organelles that protrude from the surface of most cells, are important to the sensing of extracellular signals, and make a driving force for fluid flow. Maintenance of flagellar length requires an active transport process known as intraflagellar transport (IFT). Recent studies reveal that the amount of IFT injection negatively correlates with the length of flagella. These observations suggest that a length-dependent feedback regulates IFT. However, it is unknown how cells recognize the length of flagella and control IFT. Several theoretical models try to explain this feedback system. We focused on one of the models, the “time-of-flight” model, which measures the length of flagella on the basis of the travel time of IFT protein in the flagellar compartment. We tested the time-of-flight model using Chlamydomonas dynein mutant cells, which show slower retrograde transport speed. The amount of IFT injection in dynein mutant cells was higher than that in control cells. This observation does not support the prediction of the time-of-flight model and suggests that Chlamydomonas uses another length-control feedback system rather than that described by the time-of-flight model.

Centriole/basal body morphogenesis and migration during ciliogenesis in animal cells

Cilia, either motile or immotile, exist on most cells in the human body. There are several different mechanisms of ciliogenesis, which enable the production of many kinds of cilia and flagella: motile and immotile, transient and long-lived. These can be linked to the cell cycle or associated with differentiation. A primary cilium is extended from a basal body analogous to the mitotic centrioles, whereas the several hundred centrioles needed to form the cilia of a multi-ciliated cell can be generated by centriolar or acentriolar pathways. Little is known about the molecular control of these pathways and most of our knowledge comes from ultrastructural studies. The increasing number of genetic diseases linked to dysfunctional cilia and basal bodies has renewed interest in this area, and recent proteomic and cell biological studies in model organisms have helped to shed light on the molecular components of these enigmatic organelles.

Adenylate kinase in sea urchin embryonic cilia

Sea urchin embryos swim by ciliary movement. Hypertonic shock causes deciliation and loss of motility. Within 2-4 h, cilia regenerate and the embryos swim again. Regeneration of cilia occurs multiple times. The adenylate kinase (AK) activity of isolated cilia was studied. A 130-kDa Sp-AK isozyme, present in sperm flagella, is also present in embryonic cilia. AK activity is responsible for approximately 93% of nonmitochondrial ATP regeneration from ADP in embryonic cilia. This is unlike sea urchin sperm flagella, where approximately 31% of the nonmitochondrial ATP regeneration is from the 130-kDa Sp-AK isozyme and approximately 69% from the flagellar creatine kinase (Sp-CK). Embryos were deciliated 1-3 times and after a 2-h period of regeneration the major ciliary axonemal proteins such as the tubulins appeared constant in amount. However, a moderate decrease in ATPase activity, and a large decrease of total AK activity, were measured. The decrease in AK activity paralleled the decrease in embryo swimming velocity. Embryos were deciliated once and cilia regeneration followed for 4 h. ATPase activity recovered to control levels by 3 h, but AK activity and swimming velocity remained lower than in controls. Detergent solubility data and kinetic experiments indicate that, in addition to the 130-kDa Sp-AK, there is at least one additional AK isozyme in embryonic cilia. Analysis of the S. purpuratus genome indicates five AK isozymes in addition to the 130-kDa Sp-AK isozyme. Decreased swimming velocity of embryos with regenerated cilia suggests that regenerated cilia are not as functionally perfect as naturally grown cilia.

Flagellar length control in chlamydomonas--paradigm for organelle size regulation

A fundamental unsolved question in cell biology is how the cell controls the size of its organelles. Cilia and flagella are an ideal test case to study the mechanism of organelle size control, because their size is easily measured and can be represented by a single number-length. Moreover, the involvement of cilia in many developmental and physiological processes suggests an understanding of their size control system is critical for understanding ciliary diseases, many of which (e.g., autosomal recessive polycystic kidney disease) are known to involve abnormally short cilia. The flagella of the model organism Chlamydomonas reinhardtii provide the best genetic and cell-biological system to study length control of cilia. Studies in this organism using genetics, biochemistry, imaging, and mathematical modeling have revealed many genes involved in length control of cilia and flagella, and have suggested several testable models for length regulation.

Tektin interactions and a model for molecular functions

Tektins from echinoderm flagella were analyzed for microheterogeneity, self-associations and association with tubulin, resulting in a general model of tektin filament structure and function applicable to most eukaryotic cilia and flagella. Using a new antibody to tektin consensus peptide RPNVELCRD, well-characterized chain-specific antibodies and quantitative gel densitometry, tektins A, B and C were found to be present in equimolar amounts in Sarkosyl-urea-stable filaments. In addition, two isoforms of tektin A are present in half-molar ratios to tektins B and C. Cross-linking of AB filaments indicates in situ nearest neighbor associations of tektin A1B and A2B heterodimers, -trimers, -tetramers and higher oligomers. Soluble purified tektin C is cross-linked as homodimers, trimers and tetramers, but not higher oligomers. Tektin filaments associate with both loosely bound and tightly bound tubulin, and with the latter in a 1:1 molar ratio, implying a specific, periodic association of tightly bound tubulin along the tektin axis. Similarly, in tektin-containing Sarkosyl-stable protofilament ribbons, two polypeptides ( approximately 67/73 kDa, homologues of rib72, efhc1 and efhc2) are present in equimolar ratios to each other and to individual tektins, co-fractionating with loosely bound tubulin. These results suggest a super-coiled arrangement of tektin filaments, the organization of which has important implications for the evolution, assembly and functions of cilia and flagella.

Preferential incorporation of tubulin into the junctional region of ciliary outer doublet microtubules: a model for treadmilling by lattice dislocation

Even in the presence of colchicine or Taxol(R), sea urchin embryonic cilia undergo substantial steady-state turnover, with a rate of tubulin incorporation approaching half that seen in full regeneration [Stephens: Mol Biol Cell 8:2187-2198, 1997]. Preliminary experiments suggest that tubulin incorporates differentially into the most stable portion of the outer doublet, the junctional protofilaments [Stephens: Cell Struct Funct 24:413-418, 1999]. To explore this possibility further, embryos of the sea urchin Tripneustes gratilla, a ciliary length inducible system [Stephens: J Exp Zool 269:106-115, 1994a], were pulse labeled with (3)H leucine during steady-state turnover or induced elongation, followed by regeneration in the presence of unlabeled leucine. Cilia were isolated by hypertonic shock and fractionated into detergent-soluble membrane plus matrix, thermally-solubilized microtubule walls, and insoluble 9-fold symmetric remnants of A-B junctional protofilaments plus associated architectural elements. The fractions were resolved by SDS-PAGE and the specific activity of alpha-tubulin was determined. In cilia undergoing turnover or elongation during an isotope pulse, the specific activity of tubulin in the junctional region approximated that of precursor membrane plus matrix tubulin but surpassed that of the tubule wall by a factor of approximately 1.5. In cilia regenerated during an isotope chase, the specific activity of junctional tubulin exceeded that of both the membrane plus matrix and the tubule wall by a similar factor. These data indicate that tubulin is preferentially incorporated into junctional protofilaments during steady-state turnover, induced elongation and regeneration. A model for directional incorporation based on surface lattice discontinuities in the outer doublet is proposed.

Cellular Samurai: katanin and the severing of microtubules

Recent biochemical studies of the AAA ATPase, katanin, provide a foundation for understanding how microtubules might be severed along their length. These in vitro studies are complemented by a series of recent reports of direct in vivo observation of microtubule breakage, which indicate that the in vitro phenomenon of catalysed microtubule severing is likely to be physiological. There is also new evidence that microtubule severing by katanin is important for the production of non-centrosomal microtubules in cells such as neurons and epithelial cells. Although it has been difficult to establish the role of katanin in mitosis, new genetic evidence indicates that a katanin-like protein, MEI-1, plays an essential role in meiosis in C. elegans. Finally, new proteins involved in the severing of axonemal microtubules have been discovered in the deflagellation system of Chlamydomonas.