関: pulsating vacuole

WordNet

capable of contracting or being contracted; "the contractile wings of an insect"
a tiny cavity filled with fluid in the cytoplasm of a cell

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収縮性の
空胞,液胞(細胞内にあり分泌液を含む小腔)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/10/13 08:53:52」(JST)

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Protist Paramecium aurelia with contractile vacuoles

A contractile vacuole (abbreviation: CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole.

Overview[edit]

The CV pumps excess water out of the cell. In freshwater environments the concentration of solutes inside the cell is higher than outside the cell (i.e., the environment is hypotonic or hypoosmotic). Under these conditions water flows from the environment into the cell by osmosis. The CV serves as a protective mechanism that prevents the cell from absorbing too much water and possibly exploding. The contractile vacuole should not be confused with the vacuole, a different organelle much more common than the CV.

The CV, as its name suggests, expels water out of the cell by contracting. The growth (water gathering) and contraction (water expulsion) of the CV are periodical. One cycle takes several seconds, depending on the species and the environment's osmolarity. The stage in which water flows into the CV is called diastole. The contraction of the CV and the expulsion of water out of the cell is called systole.

Water always flows first from outside the cell into the cytoplasm, and only then from the cytoplasm into the CV. Species that possess a CV always use it, even at very hypertonic (high concentration of solutes) environments, since the cell tends to adjust its cytoplasm to become even more hyperosmotic than the environment. The amount of water expelled from the cell and the rate of contraction are related to the osmolarity of the environment. In hyperosmotic environments less water will be expelled and the contraction cycle will be longer.

The most researched CVs belong to the protists Paramecium, Amoeba, Dictyostelium and Trypanosoma, and to a lesser extent the green alga Chlamydomonas. Not all species that possess a CV are freshwater organisms; some marine and even soil microorganisms also have a CV. The CV is predominant in species that do not have a cell wall, but there are exceptions (notably Chlamydomonas). Evolutionarily, the CV was mostly eliminated in multicellular organisms, but it still exists in the unicellular stage of several multicellular fungi, as well as in several types of cells in sponges (amoebocytes, pinacocytes, and choanocytes).^[1]

The number of CVs per cell varies, depending on the species. Amoeba have one, Dictyostelium discoideum, Paramecium aurelia and Chlamydomonas reinhardtii have two, and giant amoeba, such as Chaos carolinensis, have many. The number of CVs in each species is mostly constant, and is therefore used for species characterization in systematics. The CV has several structures attached to it in most cells, such as membrane folds, tubules, water tracts and small vesicles. These structures are commonly known as the spongiome; the CV together with the spongiome is sometimes called the contractile vacuole complex (CVC). The spongiome serves several functions in water transport into the CV and in localization and docking of the CV within the cell.

Paramecium and Amoeba possess large CVs (average diameter of 13 and 45 µm, respectively), which are relatively comfortable to isolate, manipulate and assay. The smallest known CVs belong to Chlamydomonas, with a diameter of 1.5 µm. In Paramecium, which presumably has the most complex and highly evolved CV, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm which can be opened and closed.^[2] Other protists, such as Amoeba, have CVs that move to the surface of the cell when full and undergo exocytosis. In Amoeba contractile vacuoles collect excretory waste, such as ammonia, from the intracellular fluid by both diffusion and active transport.

Water flow into the CV[edit]

A Dictyostelium discoideum (slime mold) cell exhibiting a prominent contractile vacuole on its left side

The way in which water enters the CV had been a mystery for many years, but several discoveries since the 1990s have improved understanding of this issue. Water could theoretically cross the CV membrane by osmosis, but only if the inside of the CV is hyperosmotic (higher solute concentration) to the cytoplasm. The discovery of proton pumps in the CV membrane^[3] and the direct measurement of ion concentrations inside the CV using microelectrodes^[4] led to the following model: the pumping of protons either into or out of the CV causes different ions to enter the CV. For example, some proton pumps work as cation exchangers, whereby a proton is pumped out of the CV and a cation is pumped at the same time into the CV. In other cases, protons pumped into the CV drag anions with them (carbonate, for example), to balance the pH. This ion flux into the CV causes an increase in CV osmolarity and as a result water enters the CV by osmosis. Water has been shown in at least some species to enter the CV through aquaporins.^[5]

Acidocalcisomes have been implied to work alongside the contractile vacuole in responding to osmotic stress. They were detected in the vicinity of the vacuole in Trypanosoma cruzi and were shown to fuse with the vacuole when the cells were exposed to osmotic stress. Presumably the acidocalcisomes empty their ion contents into the contractile vacuole, thereby increasing the vacuole's osmolarity.^[6]

Unresolved issues[edit]

The CV indeed does not exist in higher organisms, but some of its unique characteristics are used by the former in their own osmoregulatory mechanisms. Research on the CV can therefore help us understand how osmoregulation works in all species. Many issues regarding the CV remain, as of 2010, unsolved:

Contraction. It is not completely known what causes the CV to contract, and whether it is an active process which costs energy or a passive collapse of the CV membrane. Evidence for involvement of actin and myosin, prominent contractile proteins which are found in many cells, are ambiguous.
Membrane composition. Although it is known that several proteins decorate the CV membrane (V-H+-ATPases, aquaporins), a complete list is missing. The composition of the membrane itself and its similarities to and differences from other cellular membranes are also not clear.
Contents of the CV. Several studies have shown the ion concentrations inside some of the largest CVs but not in the smallest ones (such as in the important model organism Chlamydomonas rheinhardii). The reasons and mechanisms for ion exchange between the CV and cytoplasm are not entirely clear.

References[edit]

^ Brauer EB and McKanna JA (1978). "Contractile vacuoles in cells of a freshwater sponge, Spongilla Lacustris". Cell Tissue Res 192 (2): 309–317. PMID 699019.
^ Allen RD (2000). "The contractile vacuole and its membrane dynamics". BioEssays 22 (11): 1035–1042. doi:10.1002/1521-1878(200011)22:11<1035::AID-BIES10>3.0.CO;2-A. PMID 11056480.
^ Heuser J, Zhu Q, Clarke M (1993). "Proton pumps populate the contractile vacuoles of Dictyostelium amoebae". J Cell Biol 121 (6): 1311–1327. doi:10.1083/jcb.121.6.1311. PMC 2119701. PMID 8509452.
^ Stock C, Gronlien HK, Allen RD, Naitoh Y (2002). "Osmoregulation in Paramecium: in situ ion gradients permit water to cascade through the cytosol to the contractile vacuole". J Cell Sci 115 (Pt 11): 2339–2348. PMID 12006618.
^ Nishihara E, Yokota E, Tazaki A, Orii H, Katsuhara M, Kataoka K, Igarashi H, Moriyama Y, Shimmen T, Sonobe S (2008). "Presence of aquaporin and V-ATPase on the contractile vacuole of Amoeba proteus". Biol Cell 100 (3): 179–188. doi:10.1042/BC20070091. PMID 18004980.
^ Rohloff P, Montalvetti A, Docampo R (2004). "Acidocalcisomes and the Contractile Vacuole Complex Are Involved in Osmoregulation in Trypanosoma cruzi". J Biol Chem 279 (50): 52270–52281. doi:10.1074/jbc.M410372200. PMID 15466463.

Wikisource has the text of the 1911 Encyclopædia Britannica article Contractile Vacuole.

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Dynamics of clathrin-mediated endocytosis and its requirement for organelle biogenesis in Dictyostelium.

Macro L, Jaiswal JK, Simon SM.AbstractThe protein clathrin mediates one of the major pathways of endocytosis from the extracellular milieu and plasma membrane. In single cell eukaryotes, such as S.cerevisiae, clathrin is not an essential gene raising the question whether clathrin conveys specific advantages for multicellularity. Furthermore, in contrast to mammalian cells, endocytosis in S.cerevisiae is not dependent on clathrin or the endocytic adaptor molecule AP2. We therefore sought to study the requirement for components of clathrin-mediated endocytosis (CME) in another unicellular system, the organism Dictyostelium. We have identified a heterotetrameric AP2 complex in Dictyostelium similar to that in higher eukaryotes. By simultaneously imaging fluorescently tagged clathrin and AP2 we find that, similar to higher eukaryotes, these proteins co-localize to membrane puncta that move into the cell together. We show that the contractile vacuole marker protein, dajumin-GFP, is trafficked via the cell membrane and identify it as a cargo that is internalized by CME in a clathrin-dependent, AP2 independent mechanism. This pathway is distinct from other endocytic mechanisms in Dictyostelium. Our finding that CME is required for the internalization of contractile vacuole proteins from the cell membrane explains the contractile vacuole biogenesis defect in Dictyostelium cells lacking clathrin. Our results lead to the implication that the machinery for CME and its role in organelle maintenance appeared early in eukaryotic evolution. We suggest that dependence of endocytosis on specific components of the CME pathway may have evolved later as demonstrated by internalization independent of AP2 function.
Journal of cell science.J Cell Sci.2012 Sep 19. [Epub ahead of print]
The protein clathrin mediates one of the major pathways of endocytosis from the extracellular milieu and plasma membrane. In single cell eukaryotes, such as S.cerevisiae, clathrin is not an essential gene raising the question whether clathrin conveys specific advantages for multicellularity. Further
PMID 22992464

The Nramp (Slc11) proteins regulate development, resistance to pathogenic bacteria and iron homeostasis in Dictyostelium discoideum.

Peracino B, Buracco S, Bozzaro S.AbstractThe Dictyostelium discoideum genome harbours two genes encoding members of the Nramp superfamily, which is conserved from bacteria (MntH proteins) to humans (Slc11 proteins). Nramps are proton-driven metal ion transporters with a preference for iron and manganese. Acquisition of these metal cations is vital for all cells, as they act as redox cofactors and regulate key cellular processes, such as DNA synthesis, electron transport, energy metabolism and oxidative stress. Dictyostelium Nramp1 (Slc11a1), like its mammalian ortholog, mediates resistance to infection by invasive bacteria. We have extended the analysis to the nramp2 gene, by generating single and double nramp1/nramp2 knockout mutants and cells expressing GFP fusion proteins. In contrast to Nramp1, which is recruited to phagosomes and macropinosomes, the Nramp2 protein is localized exclusively in the membrane of the contractile vacuole, a vesicular-tubular network regulating cellular osmolarity. Both proteins colocalize with the V-H(+) ATPase, which can provide the electrogenic force for vectorial transport. Like nramp1, nramp2 gene disruption affects resistance to Legionella pneumophila. Disrupting both genes additionally leads to defects in development, with strong delay in cell aggregation, formation of large streams and multi-tipped aggregates. Single and double mutants display differential sensitivity to cell growth under conditions of iron overload or depletion. The data favour the hypothesis that Nramp1 and Nramp2, under control of the V-H(+) ATPase, synergistically regulate iron homeostasis, with the contractile vacuole possibly acting as a store for metal cations.
Journal of cell science.J Cell Sci.2012 Sep 19. [Epub ahead of print]
The Dictyostelium discoideum genome harbours two genes encoding members of the Nramp superfamily, which is conserved from bacteria (MntH proteins) to humans (Slc11 proteins). Nramps are proton-driven metal ion transporters with a preference for iron and manganese. Acquisition of these metal cations
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Japanese Journal

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喜多村美香,石田正樹
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NAID 120006337815

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NAID 10031138556

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