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Query: UMLS:C0038454 (stroke)
 147,016 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A mutation at the PF10 locus of the unicellular green alga Chlamydomonas reinhardtii leads to abnormal cell motility. The asymmetric form of the ciliary beat stroke characteristic of wild-type flagella is modified by this mutation to a nearly symmetric beat. We report here that this abnormal motility is a conditional phenotype that depends on light intensity. In the absence of light or under low light intensities, the motility is more severely impaired than at higher light intensities. By UV mutagenesis we obtained 11 intragenic and 70 extragenic strains that show reversion of the pf10 motility phenotype observed in low light. The intragenic events reverted the motility phenotype of the pf10 mutation completely. The extragenic events define at least seven suppressor loci; these map to linkage groups IV, VII, IX, XI, XII and XVII. Suppressor mutations at two of the seven loci (LIS1 and LIS2) require light for their suppressor activity. Forty-eight of the 70 extragenic suppressors were examined in heterozygous diploid cells; 47 of these mutants were recessive to the wild-type allele and one mutant (bop5-1) was dominant to the wild-type allele. Complementation analysis of the 47 recessive mutants showed unusual patterns. Most mutants within a recombinationally defined group failed to complement one another, although there were pairs that showed intra-allelic complementation. Additionally, some of the mutants at each recombinationally defined locus failed to complement mutants at other loci. They define dominant enhancers of one another.
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PMID:A genetic analysis of suppressors of the PF10 mutation in Chlamydomonas reinhardtii. 322 13

The biflagellate alga Chlamydomonas reinhardi was studied with the light and electron microscopes to determine the behavior of flagella in the living cell and the structure of the basal apparatus of the flagella. During normal forward swimming the flagella beat synchronously in the same plane, as in the human swimmer's breast stroke. The form of beat is like that of cilia. Occasionally cells swim backward with the flagella undulating and trailing the cell. Thus the same flagellar apparatus produces two types of motion. The central pair of fibers of both flagella appear to lie in the same plane, which coincides with the plane of beat. The two basal bodies lie in a V configuration and are joined at the top by a striated fiber and at the bottom by two smaller fibers. From the area between the basal bodies four bands of microtubules, each containing four tubules, radiate in an X-shaped pattern, diverge, and pass under the cell membrane. Details of the complex arrangement of tubules near the basal bodies are described. It seems probable that the connecting fibers and the microtubules play structural roles and thereby maintain the alignment of the flagellar apparatus. The relation of striated fibers and microtubules to cilia and flagella is reviewed, particularly in phytoflagellates and protozoa. Structures observed in the transitional region between the basal body and flagellar shaft are described and their occurrence is reviewed. Details of structure of the flagellar shaft and flagellar tip are described, and the latter is reviewed in detail.
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PMID:Flagellar motion and fine structure of the flagellar apparatus in Chlamydomonas. 534 Oct 20

Analysis of serial cross-sections of the Chlamydomonas flagellum reveals several structural asymmetries in the axoneme. One doublet lacks the outer dynein arm, has a beak-like projection in its B-tubule, and bears a two-part bridge that extends from the A-tubule of this doublet to the B-tubule of the adjacent doublet. The two doublets directly opposite the doublet lacking the arm have beak-like projections in their B-tubules. These asymmetries always occur in the same doublets from section to section, indicating that certain doublets have consistent morphological specializations. These unique doublets give the axoneme an inherent structural polarity. All three specializations are present in the proximal portion of the axoneme; based on their frequency in random cross-sections of isolated axonemes, the two-part bridge and the beak-like projections are present in the proximal one quarter and one half of the axoneme, respectively, and the outer arm is absent from the one doublet greater than 90% of the axoneme's length. The outer arm-less doublet of each flagellum faces the other flagellum, indicating that each axoneme has the same rotational orientation relative to the direction of its effective stroke. This strongly suggests that the direction of the effective stroke is controlled by a structural component within the axoneme. The striated fibers are associated with specific triplets in a manner suggesting that they play a role in setting up or maintaining the 180 degrees rotational symmetry of the two flagella.
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PMID:Outer doublet heterogeneity reveals structural polarity related to beat direction in Chlamydomonas flagella. 622 2

Immediately following embryonic cleavage, the cells of Astrephomene have four equal-sized basal bodies, two of which are connected by a striated distal fibre and two striated proximal fibres. The four microtubular rootlets, which alternate between having 3/1 and 2 members, are arranged cruciately. The two basal bodies that are connected by the striated fibres then extend into flagella, while the two accessory basal bodies are now markedly shorter. At this stage the flagellar apparatus has 180 degrees rotational symmetry and is very similar to the flagellar apparatus of the unicellular Chlamydomonas and related algae. Development proceeds with a number of concurrent events. The basal bodies begin to separate at their proximal ends and become nearly parallel. Each striated proximal fibre detaches at one end from one of the basal bodies. Each half of the flagellar apparatus, which consists of a flagellum and attached basal body, an accessory basal body, two rootlets and a striated fibre (formerly one of the proximal striated fibres), rotates about 90 degrees, the two halves rotating in opposite directions. An electron-dense strut forms near one two-membered rootlet and grows past both basal bodies. During this time a fine, fibrous component appears between newly developed spade-like structures and associated amorphous material connected to each basal body. The basal bodies continue to separate as the distal fibre stretches and finally detaches from one of them. These processes result in the loss of the 180 degree rotational symmetry present in previous stages. Although the flagella continue to separate, there is no further reorganization of the components of the flagellar apparatus. In the mature cell of Astrephomene, the two flagella are inserted separately and are parallel. The four microtubular rootlets are no longer arranged cruciately. Three of the rootlets are nearly parallel, while the fourth is approximately perpendicular to the other three. A straited fibre connects each basal body to the underside of the strut. These fibres run in the direction of the effective stroke of the flagella and might be important either in anchoring the basal bodies or in the initiation of flagellar motion. Unlike the case in the unicellular Chlamydomonas, the two flagella beat in the same direction and in parallel planes. The flagella of a given cell may or may not beat in synchrony. The combination of this type of flagellar motion and the parallel, separate flagella appears to be suited to the motion of this colonial organism.
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PMID:Ultrastructure and development of the flagellar apparatus and flagellar motion in the colonial graeen alga Astrephomene gubernaculifera. 663 Mar 10

Dynein ATPases are microtubule motors that are critical to diverse processes such as vesicle transport and the beating of sperm tails; however, their mechanism of force generation is unknown. Each dynein comprises a head, from which a stalk and a stem emerge. Here we use electron microscopy and image processing to reveal new structural details of dynein c, an isoform from Chlamydomonas reinhardtii flagella, at the start and end of its power stroke. Both stem and stalk are flexible, and the stem connects to the head by means of a linker approximately 10 nm long that we propose lies across the head. With both ADP and vanadate bound, the stem and stalk emerge from the head 10 nm apart. However, without nucleotide they emerge much closer together owing to a change in linker orientation, and the coiled-coil stalk becomes stiffer. The net result is a shortening of the molecule coupled to an approximately 15-nm displacement of the tip of the stalk. These changes indicate a mechanism for the dynein power stroke.
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PMID:Dynein structure and power stroke. 1261 Jun 3

Thin section electron micrographs of rapidly fixed Chlamydomonas cells were used to establish a relationship between flagellar bends and orientation of the central pair microtubule complex. Using conditions that preserve flagellar waveforms during both forward swimming (asymmetric bends) and backward swimming (symmetric bends), we found that central pair orientation differs in bent regions and straight regions. During forward swimming, a plane through the two central pair microtubules is parallel to the bend plane throughout principal bends, in both effective stroke and recovery stroke phases of the beat cycle. In these curved segments, the C1 microtubule always faces the outer edge of the curve. This parallel orientation twists in straight regions both proximal and distal to bends. During backward swimming episodes induced by photoshock, when Chlamydomonas flagella beat with principal and reverse bends of similar magnitude, the central pair twists by 180 degrees between successive bends. These observations support a model in which central pair orientation in Chlamydomonas is linked to doublet-specific dynein activation, and bend propagation is linked to rotation of the central pair complex.
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PMID:Orientation of the central pair complex during flagellar bend formation in Chlamydomonas. 1450 9

The unicellular green alga Chlamydomonas reinhardtii steers through water with a pair of cilia (eukaryotic flagella). Long-term observation of the beating of its cilia with controlled stimulation is improving our understanding of how a cell responds to sensory inputs. Here we describe how to record ciliary motion continuously for long periods. We also report experiments on the network of intracellular signaling that connects the environment inputs with response outputs. Local spatial changes in ciliary response on the time scale of the underlying biochemical dynamics are observed. Near-infrared light monitors the cells held by a micropipette. This condition is tolerated well for hours, not interfering with ciliary beating or sensory transduction. A computer integrates the light stimulation of the eye of Chlamydomonas with the ciliary motion making possible long-term correlations. Measures of ciliary responses include the beating frequency, stroke velocity, and stroke duration of each cilium, and the relative phase of the cis and trans cilia. The stationarity and dependence of the system on light intensity was investigated. About 150,000,000 total beat cycles and up to 8 h on one cell have been recorded. Each beat cycle is resolved so that each asynchronous beat is detected. Responses extend only a few hundred milliseconds, but there is a persistence of momentary changes that last much longer. Interestingly, we see a response that is linear with absolute light intensity as well as different kinds of response that are clearly nonlinear, implying two signaling pathways from the cell body to the cilia.
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PMID:An electro-optic monitor of the behavior of Chlamydomonas reinhardtii cilia. 1583 39

With an instrument that can record the motion of both cilia of the unicellular alga Chlamydomonas reinhardtii for many hours, the behavioral differences of its two cilia have been studied to determine their specific role in phototaxis. The organism was held on a fixed micropipette with the plane of ciliary beating rotated into the imaging plane of a quadrant photodetector. The responses to square-wave light patterns of a wide range of temporal frequencies were used to characterize the responses of each cilium. Eighty-one cells were examined showing an unexpectedly diverse range of responses. Plausible common signals for the linear and nonlinear signals from the cell body are suggested. Three independent ciliary measures--the beat frequency, stroke velocity, and phasing of the two cilia--have been identified. The cell body communicates to the cilia the direction of phototaxis the cell desires to go, the absolute light intensity, and the appropriate graded transient response for tracking the light source. The complexity revealed by each measure of the ciliary response indicates many independent variables are involved in the net phototactic response. In spite of their morphological similarity, the two cilia of Chlamydomonas respond uniquely. Probably the signals from the cell body fan out to independent pathways in the cilia. Each cilium modifies the input in its own way. The change in the pattern of the effective and recovery strokes of each cilium associated with negative phototaxis has been demonstrated and its involvement in phototactic turning is described.
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PMID:Ciliary behavior of a negatively phototactic Chlamydomonas reinhardtii. 1584 14

In response to light stimulation Chlamydomonas reinhardtii changes the beating frequency, beating pattern, and beating synchrony of the trans and cis cilia to steer the freely-swimming cell relative to light sources. To understand the cell steering behavior the impulse responses of the beating frequency and stroke velocity of each cilium have been obtained with high temporal resolution on cells held with a micropipette. Interestingly the response of each cilium is quite different. The trans cilium responds with less delay than the cis cilium for both beating frequency and stroke velocity. For light stimulation at 2 Hz, the critical cell-rotation frequency, both responses of the trans and cis cilia are about 180 degrees out of phase. The trans-cilium beating frequency response peaks at a stimulus frequency of 5-6 Hz, higher than the cis at 1-2 Hz. The stroke velocities of the trans and cis cilia have the same stimulus-frequency response (2 Hz), but the trans cilium has a shorter delay than the cis. The times to maximum response are much shorter than the time for a rotation of the cell. The use of two different approaches that enable the trans cilium to respond ahead of the cis for both the beating frequency and stroke velocity responses suggests the importance of both responses to phototaxis. Internal cell processing responsible for the time course of the responses is proposed.
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PMID:Linear systems analysis of the ciliary steering behavior associated with negative-phototaxis in Chlamydomonas reinhardtii. 1698 40

Outer dynein arms, the force generators for axonemal motion, form arrays on microtubule doublets in situ, although they are bouquet-like complexes with separated heads of multiple heavy chains when isolated in vitro. To understand how the three heavy chains are folded in the array, we reconstructed the detailed 3D structure of outer dynein arms of Chlamydomonas flagella in situ by electron cryo-tomography and single-particle averaging. The outer dynein arm binds to the A-microtubule through three interfaces on two adjacent protofilaments, two of which probably represent the docking complex. The three AAA rings of heavy chains, seen as stacked plates, are connected in a striking manner on microtubule doublets. The tail of the alpha-heavy chain, identified by analyzing the oda11 mutant, which lacks alpha-heavy chain, extends from the AAA ring tilted toward the tip of the axoneme and towards the inside of the axoneme at 50 degrees , suggesting a three-dimensional power stroke. The neighboring outer dynein arms are connected through two filamentous structures: one at the exterior of the axoneme and the other through the alpha-tail. Although the beta-tail seems to merge with the alpha-tail at the internal side of the axoneme, the gamma-tail is likely to extend at the exterior of the axoneme and join the AAA ring. This suggests that the fold and function of gamma-heavy chain are different from those of alpha and beta-chains.
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PMID:The architecture of outer dynein arms in situ. 1739 98


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