Hydrostatic pressure-induced internalization of flagellar axonemes, disassembly, and reutilization during flagellar regeneration in Polytomella

Pressure reduction affects growth and morphology of Chlamydomonas reinhardtii

Cellular perception of pressure is a largely unknown field in microalgae research although it should be addressed for optimization of a photobioreactor design regarding typically occurring pressure cycles. Also for the purpose of using microalgae as basic modules for material cycles in controlled ecological life support systems, the absence of pressure in outer space or the low absolute pressures on other planets is an abiotic factor that needs to be considered for design of integrated microalgae-based modules. The aim of this work is to study the effects of lowered pressure and pressure changes on photosynthesis as well as morphology. Two Chlamydomonas reinhardtii wild-type strains were exposed to controlled pressure patterns during batch cultivations. Sudden pressure changes should test for existing threshold values for cell survival to mimic such events during space missions. Algae were grown inside a 2 L photobioreactor with an integrated vacuum pump ensuring constant pressures down to 700 mbar. Cultivation samples were analyzed for OD₇₅₀, cell dry weight, and morphology via light microscope. Chlamydomonas reinhardtii CC-1690 cells showed decreased growth rates, higher carbon dioxide uptake rates, and unchanged oxygen production rates at lower pressures. For sudden pressures changes in the range of 300 mbar no fatal threshold was determined. This study shows that pressure reduction affects growth, gas exchange rates, and morphology. Within the tested pressure range no fatal threshold value was reached.

Mechanism of ciliary disassembly

As motile organelles and sensors, cilia play pivotal roles in cell physiology, development and organ homeostasis. Ciliary defects are associated with a class of cilia-related diseases or developmental disorders, termed ciliopathies. Even though the presence of cilia is required for diverse functions, cilia can be removed through ciliary shortening or resorption that necessitates disassembly of the cilium, which occurs normally during cell cycle progression, cell differentiation and in response to cellular stress. The functional significance of ciliary resorption is highlighted in controlling the G1-S transition during cell cycle progression. Internal or external cues that trigger ciliary resorption initiate signaling cascades that regulate several downstream events including depolymerization of axonemal microtubules, dynamic changes in actin and the ciliary membrane, regulation of intraflagellar transport and posttranslational modifications of ciliary proteins. To ensure ciliary resorption, both the active disassembly of the cilium and the simultaneous inhibition of ciliary assembly must be coordinately regulated.

EFFECTS OF HIGH PRESSURE IN THE ARMORED DINOFLAGELLATE SCRIPPSIELLA HEXAPRAECINGULA (PERIDINIALES, DINOPHYCEAE): CHANGES IN THECAL PLATE PATTERN AND MICROTUBULE ASSEMBLY(1)

The possible role of cortical microtubules in dinoflagellates was studied using high-pressure treatments applied to nonmotile cells (just after ecdysis) of Scrippsiella hexapraecingula T. Horig. et Chihara. Whereas considerable disorganization of cortical microtubules was observed when cells were exposed to high-pressure treatments of 98 MPa or more for 5-15 min, they were mostly intact in cells exposed to a pressure of <98 MPa for 5 min. After nonmotile cells were exposed to high-pressure treatments sufficient to disorganize the cortical microtubules, they produced new motile cells with thecal plate patterns that differed considerably from the pattern known for this species. Increasing the intensity of high pressure applied to nonmotile cells resulted in an increase in the number of cells that exhibited disorganized cortical microtubules as well as a change in their thecal plate pattern, suggesting that high pressure disorganizes cortical microtubules leading to a change in the thecal plate pattern.

The ubiquitin conjugation system is involved in the disassembly of cilia and flagella

The disassembly of cilia and flagella is linked to the cell cycle and environmental cues. We have found that ubiquitination of flagellar proteins is an integral part of flagellar disassembly. Free ubiquitin and the ubiquitin-conjugating enzyme CrUbc13 are detected in flagella, and several proteins are ubiquitinated in isolated flagella when exogenous ubiquitin and adenosine triphosphatase are added, suggesting that the ubiquitin conjugation system operates in flagella. Levels of ubiquitinated flagellar proteins increase during flagellar resorption, especially in intraflagellar transport (IFT) mutants, suggesting that disassembly products are labeled with ubiquitin and transported to the cell body by IFT. Substrates of the ubiquitin conjugation system include α-tubulin (but not β-tubulin), a dynein subunit (IC2), two signaling proteins involved in the mating process, cyclic guanosine monophosphate–dependent kinase, and the cation channel polycystic kidney disease 2. Ubiquitination of flagellar proteins is enhanced early in mating, suggesting that ubiquitination also plays an active role in regulating signaling pathways in flagella.

The dcc mutation affects ciliary length in Tetrahymena thermophila

We have characterized ciliogenesis in a mutant Tetrahymena thermophila that both fails to regain motility following deciliation and that fails to complete cytokinesis. Scanning electron microscopic (SEM) observations revealed that starved deciliated cells regenerated fewer, shorter cilia at the restrictive temperature than similarly treated cells incubated at the permissive temperature. Transmission electron microscopic evaluation of isolated, regenerated cilia revealed no structural abnormalities. Incorporation of S-35 methionine was similar during ciliary regeneration at both the restrictive and permissive temperatures, indicating the mutant phenotype was not due to a simple failure in translation or transcription. Mutant cells incubated in growth medium at the restrictive temperature arrested in cytokinesis and assembled a large number of abnormally short cilia. These cells also developed irregular surface projections that were not visible on wild-type cells. These observations suggest that ciliogenesis can be initiated in growing cells as well as in starved deciliated cells but that elongation is inhibited before cilia reach full length. The mutation was named dcc for defective in ciliogenesis and cytokinesis.

Chlamydomonas alpha-tubulin is posttranslationally modified in the flagella during flagellar assembly

The principal alpha-tubulin within Chlamydomonas reinhardtii flagellar axonemes differs from the major alpha-tubulin in the cell body. We show that these two isoelectric variants of alpha-tubulin are related to one another since posttranslational modification of the cell body precursor form converts it to the axonemal form. During flagellar assembly, precursor alpha-tubulin enters the flagella and is posttranslationally modified within the flagellar matrix fraction prior to or at the time of its addition to the growing axonemal microtubules. Experiments designed to identify the nature of this posttranslational modification have also been conducted. When flagella are induced to assemble in the absence of de novo protein synthesis, tritiated acetate can be used to posttranslationally label alpha-tubulin in vivo and, under these conditions, no other flagellar polypeptides exhibit detectable labeling.

Multiple forms of tubulin in Polytomella and Chlamydomonas: evidence for a precursor of flagellar alpha-tubulin

The quadriflagellate alga polytomella agilis contains several alpha-tubulins with distinct isoelectric points (McKeithan, T.W., and J.L. Rosenbaum, 1981, J. Cell Biol., 91:352-360). While alpha-3 is the major component in flagella, alpha-1 predominates in cytoskeletal microtubules. For determination of whether the differences in alpha- tubulins are due to distinct genes or to posttranslational modification of a common alpha-tubulin precursor, poly A+ RNA was isolated from deflagellated and control (nonregenerating) cells and translated in vitro. Approximately twice as much alpha-1 was synthesized using RNA from deflagellated as compared to control cells; however, there was no detectable synthesis in vitro of alpha-3 in either. These results suggest that alpha -3 tubulin is formed in vivo by posttranslational modification of a form co- migrating with, and possibly identical to, cytoskeletal alpha-tubulin. In the related alga chlamydomonas reinhardii deflagellation greatly stimulates synthesis of tubulin and tubulin mRNA. As in polytomella, the principal alpha-tubulin synthesized both in vivo and in vitro following deflagellation in chlamydomonas is more basic than the major alpha-tubulin and appears to correspond to alpha-1 tubulin in polytomella. The conversion of alpha-1 to alpha-3 receives additional support from in vivo labeling and pulse-chase experiments. In addition, in both polytomella and chlamydomonas some conversion of alpha-1 to alpha-3 appears to occur even when protein synthesis is inhibited.

Synthesis, transport, and utilization of specific flagellar proteins during flagellar regeneration in Chlamydomonas

We labeled gametes of Chlamydomonas with 10-min pulses of 35SO4(-2) before and at various times after deflagellation, and isolated whole cells and flagella immediately after the pulse. The labeled proteins were separated by one- or two-dimensional gel electrophoresis, and the amount of isotope incorporated into specific proteins was determined. Individual proteins were identified with particular structures by correlating missing axonemal polypeptides with ultrastructural defects in paralyzed mutants, or by polypeptide analysis of flagellar fractions. Synthesis of most flagellar proteins appeared to be coordinately induced after flagellar amputation. The rate of synthesis for most quantified proteins increased at least 4- to 10-fold after deflagellation. The kinetics of synthesis of proteins contained together within a structure (e.g., the radial spoke proteins [RSP] ) were frequently similar; however, the kinetics of synthesis of proteins contained in different structures (e.g., RSP vs. alpha- and beta-tubulins) were different. Most newly synthesized flagellar proteins were rapidly transported into the flagellum with kinetics reflecting the rate of growth of the organelle; exceptions included a central tubule complex protein (CT1) and an actinlike component, both of which appeared to be supplied almost entirely from pre-existing, unlabeled pools. Isotope dilution experiments showed that, for most quantified axonemal proteins, a minimum of 35-40% of the polypeptide chains used in assembling a new axoneme was synthesized during regeneration; these proteins appeared to have predeflagellation pools of approximately the same size relative to their stoichiometries in the axoneme. In contrast, CT1 and the actinlike protein had comparatively large pools.

Multiple forms of tubulin in the cytoskeletal and flagellar microtubules of Polytomella

The alga polytomella contains several organelles composed of microtubules, including four flagella and hundreds of cytoskeletal microtubules. Brown and co-workers have shown (1976. J. Cell Biol. 69:6-125; 1978, Exp. Cell Res. 117: 313-324) that the flagella could be removed and the cytoskeletans dissociated, and that both structures could partially regenerate in the absence of protein synthesis. Because of this, and because both the flagella and the cytoskeletons can be isolated intact, this organism is particularly suitable for studying tubulin heterogeneity and the incorporation of specific tubulins into different microtubule-containing organelles in the same cell. In order to define the different species of tubulin in polytonella cytoplasm, a (35)S- labeled cytoplasmic fraction was subjected to two cycles of assembly and disassembly in the presence of unlabeled brain tubulin. Comparison of the labeled polytomella cytoplasmic tubulin obtained by this procedure with the tubulin of isolated polytomella flagella by two-dimensional gel electrophoresis showed that, whereas the beta-tubulin from both cytoplasmic and flagellar tubulin samples comigrated, the two alpha-tubulins had distinctly different isoelectic points. As a second method of isolating tubulin from the cytoplasm, cells were gently lysed with detergent and intact cytoskeletons obtained. When these cytoskeletons were exposed to cold temperature, the proteins that were released were found to be highly enriched in tubulin; this tubulin, by itself, could be assembled into microtubules in vitro. The predominant alpha-tubulin of this in vitro- assembled cytoskeletal tubulin corresponded to the major cytoplasmic alpha-tubulin obtained by coassembly of labeled polytomella cytoplasmic extract with brain tubulin and was quite distinct from the alpha-tubulin of purified flagella. These results clearly show that two different microtubule-containing organelles from the same cell are composed of distinct tubulins.

Regeneration of cilia in starved Tetrahymena thermophila involves induced synthesis of ciliary proteins but not synthesis of membrane lipids

The synthesis of ciliary-membrane phospholipids and ciliary proteins was studied after deciliation in starving Tetrahymena thermophila cells. Deciliated cells regenerated the new ciliary membrane without any induced phospholipid synthesis. The constant cell volume found during the regrowth of the cilia suggests that renewal of ciliary membranes takes place by insertion of intracellular membrane material into the cell surface. In contrast with the absence of induced phospholipid synthesis during ciliary regeneration, the synthesis of ciliary proteins was found to be induced. This enhanced synthetic activity was made possible by an increased rate of intracellular protein degradation in regenerating cells. It was found that the extent of the induced synthesis strongly depends upon the growth conditions of the cells before starvation. Furthermore, it was shown that the degree of induced protein synthesis is greater for higher-molecular-weight ciliary proteins than for lower-molecular-weight species.

Formation and positioning of surface-related structures in protozoa

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