Volume 52(12): 1543–1559, 2004
Journal of Histochemistry & Cytochemistry
http://www.jhc.org
ARTICLE
Immunolocalization of Actin in Paramecium Cells
Roland Kissmehl, Ivonne M. Sehring, Erika Wagner, and Helmut Plattner
The Journal of Histochemistry & Cytochemistry
Department of Biology, University of Konstanz, Konstanz, Germany
S U M M A R Y We have selected a conserved immunogenic region from several actin genes
of Paramecium, recently cloned in our laboratory, to prepare antibodies for Western blots
and immunolocalization. According to cell fractionation analysis, most actin is structurebound. Immunofluorescence shows signal enriched in the cell cortex, notably around ciliary
basal bodies (identified by anti-centrin antibodies), as well as around the oral cavity, at the
cytoproct and in association with vacuoles (phagosomes) up to several m in size. Subtle
strands run throughout the cell body. Postembedding immunogold labeling/EM analysis
shows that actin in the cell cortex emanates, together with the infraciliary lattice, from
basal bodies to around trichocyst tips. Label was also enriched around vacuoles and vesicles
of different size including “discoidal” vesicles that serve the formation of new phagosomes. By all methods used, we show actin in cilia. Although none of the structurally welldefined filament systems in Paramecium are exclusively formed by actin, actin does display
some ordered, though not very conspicuous, arrays throughout the cell. F-actin may somehow serve vesicle trafficking and as a cytoplasmic scaffold. This is particularly supported by
the postembedding/EM labeling analysis we used, which would hardly allow for any largescale redistribution during preparation. (J Histochem Cytochem 52:1543–1559, 2004)
Actin is a highly flexible cytoskeletal component
that participates in many static and dynamic functions
in eukaryotic cells (Pollard et al. 2000). This includes
reversible self-assembly of monomeric G-actin to F-actin
filaments. Also generally known is that these filaments
may be more or less bundled and can serve different
functions, such as structural enforcement and restructuring of the cell cortex, rearrangement of cortical
components during intracellular signaling, organelle
dynamics and transport, etc. The latter includes wellestablished functions such as phagosome formation
and plasma streaming, i.e., cyclosis (Shimmen and
Yokota 2004). However, quite recent results highlight
a much broader functional spectrum of F-actin than
previously assumed. This applies to early steps of exocytosis, including dense core vesicle docking (Morales
et al. 2000; Pendleton and Koffer 2001; Manneville et
al. 2003; Gasman et al. 2004), late steps of endocytosis (Engqvist-Goldstein and Drubin 2003; Guilherme
Correspondence to: H. Plattner, Department of Biology, University of Konstanz, P.O. Box 5560, 78457 Konstanz, Germany. E-mail:
helmut.plattner@uni-konstanz.de
Received for publication May 10, 2004; accepted August 13,
2004 [DOI: 10.1369/jhc.4A6379.2004].
© The Histochemical Society, Inc.
0022-1554/04/$3.30
KEY WORDS
actin
cilia
immunolocalization
microfilaments
vesicle traffic
Paramecium
et al. 2004), exo-endocytosis coupling (Valentijn et
al. 1999), endo-phagosome interaction (Kjeken et al.
2004), delivery of endocytosed receptors to lysosomes
for degradation (Stoorvogel et al. 2004), vacuole fusion in yeast (Merz and Wickner 2004), and positioning of the nucleus (Starr and Han 2003). Some aspects
are still poorly understood, particularly, e.g., the role
of actin in flagella of algae (Mitchell 2000; Hayashi et
al. 2001; Hirono et al. 2003), whereas its occurrence
in cilia has remained a matter of debate. Another line
of experiments concerns the potential role of actin in
mediating the connection between cortical Ca2-stores
and the plasma membrane (Patterson et al. 1999; Rosado and Sage 2000; Kunzelmann-Marche et al. 2001;
Wang et al. 2002).
Different actin isoforms occurring in many organisms may serve specific functions in the respective cells
(Pollard et al. 2000; Wagner et al. 2002). For localization, antibodies (ABs) may be used at the light microscope (LM) and electron microscope (EM) levels, as
well as for Western blots. Bicyclic peptide toxins, phalloidin or jasplakinolide, can bind rather specifically to
F-actin, thus allowing fluorescence labeling (Wieland
and Faulstich 1978; Bubb et al. 2000). This or the alternative approach, F-actin disruption by toxins of the
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1544
type cytochalasin B and D or latrunculin A, is also
widely used for functional analyses also with ciliates
(see below).
In previous times, mainly before molecular biology
approaches could be undertaken, biochemical, functional, and immunolocalization studies were tried to
probe the potential function of F-actin in ciliates such
as Paramecium (Tiggemann and Plattner 1981; Cohen
et al. 1984; Fok et al. 1985; Kersken et al. 1986a,b),
Tetrahymena (Mitchell and Zimmerman 1985; Hirono
et al. 1987b,1989; Hoey and Gavin 1992), Pseudomicrothorax (Hauser et al. 1980), Histriculus (PérezRomero et al. 1999), Climacostomum (Fahrni 1992),
and Spirostomum (Zackroff and Hufnagel 1998). However, with ciliates, F-actin–disrupting drugs frequently
had to be used in conspicuously high concentrations
to abolish, e.g., phagocytosis (Fok et al. 1987; Zackroff and Hufnagel 1998,2002). With a variety of protozoa of the phylum Alveolata, actin genes or partial
sequences of it have been cloned. This holds in particular for ciliates, such as Tetrahymena (Zimmerman et
al. 1983; Cupples and Pearlman 1986; Hirono et al.
1987a) and Paramecium (Díaz-Ramos et al. 1998), but
also for their pathogenic relatives of the group of Apicomplexa such as Toxoplasma (Delbac et al. 2001).
Our present analysis also addresses some special
subcellular structures in Paramecium cells that contain
multiple filament systems (Allen 1971; Cohen et al.
1984,1987; Cohen and Beisson 1988; Keryer et al.
1990a,b; Allen et al. 1998; Beisson et al. 2001; Clérot
et al. 2001). We focus on regions with dense-core secretory vesicles (“trichocysts”), cortical filament bundles
(“infraciliary lattice,” cf. Allen 1971,1988), the narrow space between the plasma membrane and tightly
attached cortical Ca2-stores (“alveolar sacs,” see
Plattner and Klauke 2001), in addition to abundant
vesicles of the phago-/lysosomal and recycling system
(Fok and Allen 1990; Allen and Fok 2000). Recent
cloning of several actin genes of Paramecium tetraurelia in our laboratory opened up a new way to structural localization with this cell, whose regular “design”
facilitates such studies. So far, studies on actin in Paramecium have not addressed all relevant aspects, and
many aspects have remained controversial.
Materials and Methods
Stocks and Cultures
The wild-type strain of P. tetraurelia used was stock 7S.
Cells were cultivated in a decoction of dried lettuce monoxenically inoculated with Enterobacter aerogenes as a food organism, supplemented with 0.4 g·ml1 -sitosterol (Sonneborn 1970). For subcellular fractionation, we used axenic
cultures (Kaneshiro et al. 1979). Cells were grown at 25C to
early stationary phase as previously described (Kissmehl et
al. 1996).
Kissmehl, Sehring, Wagner, Plattner
Expression of Paramecium Actin-specific Peptides
in Escherichia coli
For heterologous expression of actin-specific peptides we selected the amino acid sequence of actin1-1 (accession number AJ537442). After changing all deviant Paramecium glutamine codons (TAA and TAG) into universal glutamine
codons (CAA and CAG) by PCR methods, the coding regions of either E57-P243 (N-terminal region) or L251-G366
(C-terminal region) of Paramecium actin1-1 were cloned
into the XhoI/BamH1 restriction sites of pET 16b expression vector of the pET System from Novagen (Madison, WI)
which employs a His10 tag for purification of the recombinant peptides.
Purification of Recombinant Actin1-1 Peptides
Recombinant actin1-1 peptides, actin1-1E57-P243 and actin11L251-G366 were purified by affinity chromatography on Ni2nitrilotriacetate agarose under native conditions, as recommended by the manufacturer (Novagen). The recombinant
peptides were eluted with a step gradient, 100 to 1000 mM
imidazole in 50 mM sodium phosphate (pH 6.0) with 300
mM NaCl added. The fractions collected were analyzed on
SDS polyacrylamide gels, and those containing the recombinant peptides were pooled and dialyzed in phosphate-buffered saline (PBS).
Antibodies Used
ABs against the two recombinant actin peptides, actin1-1E57-P243
and actin1-1L251-G366, were raised either in rabbits or mice.
After several boosts, positive sera were taken at day 60 and
purified by two subsequent chromatography steps, a first
step on a His-tag peptide column (24-amino acid peptide, to
remove His tag-specific ABs), followed by an affinity step on
the corresponding actin1-1 peptide. One of these ABs recognizes the N-terminal and the other the C-terminal region of
actin1-1, yet results achieved in this study were indistinguishable with either type of ABs. Therefore, no further distinction is made, unless indicated. We used the sequence of
Paramecium actin 1-1 because it is rather similar for numerous isoforms that we have cloned (R. Kissmehl, J. Mansfeld,
E. Wagner, I. Sehring, H. Plattner, unpublished data) and
thus should allow us to establish an overall distribution of
actin, notably of F-actin, in Paramecium.
Mouse polyclonal ABs against Paramecium actin1-1 were
selectively used for the colocalization at the LM level, in conjunction with an anti-centrin (Dictyostelium discoideum)
polyclonal AB produced in rabbits (designation HisDdCentrin2 from R. Gräf, University of Munich) used to identify ciliary basal bodies (Daunderer et al. 2001).
Cell Fractionation
Cells were deciliated by a Mn2-shock (for details, see below) and cilia were purified by differential centrifugation
(Nelson 1995). Whole-cell homogenates were prepared in
phase buffer (20 mM Tris-maleate, 20 mM NaOH, 20 mM
NaCl, 250 mM sucrose, pH 7.0) by 100 hand strokes in a
glass homogenizer equipped with a Teflon pestle. Soluble
and particulate fractions were separated by centrifugation at
100,000 g for 60 min at 4C. Cell surface complexes (“cortices”) were prepared according to Lumpert et al. (1990),
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The Journal of Histochemistry & Cytochemistry
Actin in Paramecium
and trichocysts were isolated by the method of Glas-Albrecht
and Plattner (1990). A protease-inhibitor cocktail containing 15 M pepstatin A, 100 mU/ml aprotinin, 100 M leupeptin, 0.26 mM TAME, 28 M E64, and 0.2 mM Pefabloc
SC was used throughout.
diluted 1:50, were applied for 90 min, followed by 4 15
min washes in PBS. A second labeling with anti-centrin ABs
from rabbits was performed as described above, using Texas
Red–conjugated anti-rabbit ABs. Anti-rabbit and anti-mouse
fluorescent AB conjugates were from Sigma-Aldrich (St Louis,
MO) and Serva (Heidelberg), respectively.
Electrophoretic Techniques and Western Blot Analysis
Light Microscopy. Cells were mounted with Mowiol sup-
Protein samples were denatured by boiling for 3 min in sample buffer (0.4 M Tris–HCl, 1% SDS, 0.5% DTT, 20% glycerol, pH 8.0) and subjected to electrophoresis on linear
gradient (5–20%) SDS polyacrylamide gels using the discontinuous buffer system of Laemmli (1970). Before electrophoresis, samples were alkylated for 30 min at 20C by 2%
iodoacetamide. Protein standards were used in accordance
with manufacturer directions. Gels were either stained with
Coomassie blue R250 or prepared for electrophoretic protein transfer onto nitrocellulose membranes. Protein blotting
was performed at 2 mA/cm2 for 1 hr according to the technique of Kyhse-Andersen (1989) using the semidry blotter
from BioRad (Munich, Germany). ABs were diluted 1:1000
in 0.25% (w/v) non-fat dry milk and Tris-buffered saline,
pH 7.5, and applied overnight at 4C. AB binding was visualized by a second AB coupled to alkaline phosphatase (Sigma:
Taufkirchen, Germany) using 5-bromo-4-chloro-3-indolyl
phosphate and Nitro Blue tetrazolium as substrates.
plemented with n-propylgallate to reduce fading. To analyze
fluorescence staining, we used a conventional LM, type Axiovert 100TV (Zeiss; Oberkochen, Germany), or a confocal
laser scanning microsope (CLSM) type LSM 510 (Zeiss)
equipped with a Plan-Apochromat 63 oil immersion objective (numeric aperture 1.4). Images acquired with the LSM
510 software were processed with Photoshop software (Adobe
Systems, San Jose, CA).
Immunofluorescence Labeling
Basic Procedure. Cells were washed twice in 5 mM Pipes
buffer, pH 7.0, containing 1 mM KCl and 1 mM CaCl2.
Cells were fixed in 4% (w/v) freshly depolymerized formaldehyde for 20 min at room temperature. Cells were then
permeabilized and fixed in a mixture of 0.5% digitonin and
4% formaldehyde, dissolved in 5 mM Pipes buffer, pH 7.0,
for 30 min. Cells were washed twice in PBS, 2 10 min in
PBS with 50 mM glycine added and 30 min in this solution
with 1% bovine serum albumin (BSA) added. The rabbit
anti-actin AB was applied in a dilution of 1:50 in PBS (1%
BSA) for 90 min at room temperature. After 4 15 min
washes in the same solvent, FITC-conjugated anti-rabbit
ABs, diluted 1:50, were applied for 90 min, followed by 4
15 min washes in PBS. Samples were shaken gently during
all incubation and washing steps.
Deciliated Cells. Cells were washed twice in 5 mM Pipes
buffer, pH 7.0, each containing 1 mM KCl and CaCl2, at
room temperature and suspended in 50 mM MnCl2 solution
in 10 mM Tris-HCl, pH 7.2. After 2 min at 4C, cells were
removed by centrifugation and resuspended in the same solution. After 10 min of gentle shaking, 90–95% of cells were
deciliated. Deciliated cells were removed by centrifugation
and washed twice in Pipes buffer before further use.
Deciliated cells were fixed in 8% (w/v) freshly depolymerized formaldehyde with 0.5% digitonin, 1 mM ATP, 10
mM MgCl2, and 10 mM KCl added, for 20 min on ice in
Pipes buffer, pH 7.0. After fixation, cells were washed twice
in PBS, 2 10 min in PBS with 50 mM glycine added and
30 min in this solution with 1% BSA added. The mouse
anti-actin AB was applied in a dilution of 1:50 in PBS (1%
BSA) for 90 min at room temperature. After 4 15 min
washes in the same solvent, FITC-conjugated anti-mouse ABs,
Fixation and Embedding for Postembedding
EM Analysis
Using a quenched-flow apparatus (Knoll et al. 1991), Paramecium cells were rapidly injected into 8% formaldehyde
plus 0.1% glutaraldehyde dissolved in Pipes buffer, pH 7.2
(0C), with 1 mM KCl and CaCl2 each added, further fixed
for 60 min at 4C, washed in PBS (pH 7.4) 50 mM glycine
(2 10 min), dehydrated by increasing ethanol concentrations (30%, 50%, 70%, 90%, 96%, 2 15 min each, and
2 100%, 30 min each), and impregnated with LR Gold
resin (London Resin, London, UK) at 0C, with two changes
in 2-hr intervals each and then overnight, followed by UVlight polymerization at 35C for 72 hr.
Immunogold Labeling and EM Analysis
Postembedding Method. Ultrathin sections mounted on
formvar-coated Ni grids were pretreated (2 10 min) with 20
l of PBS, then for 10 min with PBS with 50 mM glycine
added, and finally immersed in PBS supplemented with 0.5%
BSA and 0.5% goat serum (2 10 min, room temperature), to
eliminate nonspecific gold adsorption. Grids were then incubated with rabbit AB, diluted 1:20 in PBS supplemented with
0.3% BSA-c (BioTrend, Köln, Germany), pH 7.4, 1 hr at room
temperature. BSA-c as an acetylated form reduces nonspecific
adsorption of gold conjugates due to increased net charge.
Samples were washed in PBS/BSA-c (0.3%) three times,
10 min each, and treated for 1 hr with gold conjugates. We
used goat anti-rabbit IgGs coupled to gold of 5 nm (Au5)
provided by Sigma, diluted 1:30.
Preembedding Labeling. Without exception, cells were fixed
with 8% formaldehyde 0.1% glutaraldehyde and simultaneously treated with digitonin (Sigma) and the other additives, as described above for LM analysis of deciliated cells,
incubated with primary rabbit ABs against Paramecium
actin1-1, followed by Au5-conjugated second ABs, with the
aim to make the narrow subplasmalemmal space accessible.
After embedding in LR Gold (London Resin), sections were
additionally subjected to the postembedding labeling procedure with the same primary and secondary ABs, respectively.
Specificity of Immunogold Labeling and Further Processing. This was verified by the significant reduction of the
number of Au5 particles on sections incubated with ABs that
had been preadsorbed with the original antigen.
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Further Processing and Quantitative Evaluation. After
labeling, sections were rinsed with distilled water, fixed for 5
min with 2% glutaraldehyde, and routinely stained for 3
min with 2% aqueous uranyl acetate (unbuffered, pH 4.5).
EM micrographs were taken at defined magnifications and
enlarged to 77,000. Au5 grains were counted and referred
to area size determined by superposition of square lattices
with 5, 10.0 and 20.0 mm spacing, respectively, depending
on the size of the structure to be analyzed (Plattner and
Zingsheim 1983). The actual area sizes to which the numbers of gold grains were referred were determined from the
number of hit points.
The Journal of Histochemistry & Cytochemistry
Results
Actin-specific ABs, Cell Fractionation, and Western
Blot Analysis
Molecular cloning from a pilot sequencing project (Dessen et al. 2001; Sperling et al. 2002) as well as from
the ongoing Paramecium genome project of the Groupement de Recherche Européen at the Genoscope (Evry,
France) revealed that P. tetraurelia contains an actin
multigene family with at least 30 members, all encoding actin and actin-related proteins with calculated
molecular masses ranging between 38 and 45 kD (R.
Kissmehl, J. Mansfeld, I. Sehring, E. Wagner, H. Plattner, unpublished data). One of them, actin 1-1 (accession
number AJ537442), a member of the actin-1 subfamily with rather conserved immunogenic regions (Figure
1), was chosen for heterologous expression in E. coli
Kissmehl, Sehring, Wagner, Plattner
(after changing all deviant Paramecium glutamine codons
into universal glutamine codons) and subsequent production of polyclonal ABs. Various polyclonal ABs
were raised against the N-terminal (E57-P243) or C-terminal region (L251-G366, Figure 1), all readily recognizing the recombinant peptides used for immunization when tested in slot blots and Western blots (data
not shown). After affinity purification, the actin-specific
ABs were further characterized in ELISA and Western
blots. Results obtained were similar, whether ABs were
used against the N-terminal or the carboxy-terminal
region of actin 1-1, confirming their high specificity
against actin or actin-specific peptides (data not shown).
The following analyses, including Western blots, and
LM and EM analyses, have been performed predominantly with ABs against the C-terminal region of Paramecium actin1-1 (Figure 1).
Western blots from homogenates display a strong
band of 43 kD and a weak one of 40 kD (Figure 2).
Such bands also occur in the 100,000 g pellet, while
the 43 kD band is much weaker in the 100,000 g
supernatant. The 43 kD band is typical of actin, while
the 40 kD band may represent one of the shorter isoforms of the actin or actin-related gene products of
Paramecium (Kissmehl et al., unpublished data). A
43-kD band also clearly occurs in cilia and in cortices,
while it is hardly discernible in the trichocyst fraction.
Both the 100,000 g supernatant and pellet also display some very weak bands of lower size, possibly
Figure 1 Multiple alignment of the C-terminal region of Paramecium actin1-1. Actin-specific sequences from Paramecium tetraurelia
(AJ537442), Toxoplasma gondii (P53476), Dictyostelium discoideum (AA052255), Caenorhabditis elegans (X16797), Drosophila melanogaster
(NP_523625), Mus musculus (NP_033739), and Homo sapiens (AAH16045) were aligned using the CLUSTALW program. Identical residues are
shaded (black), while lesser conserved positions are labeled greyish.
The Journal of Histochemistry & Cytochemistry
Actin in Paramecium
1547
ary basal bodies of the outer cell surface and along the
oral cavity, the outline of the oral cavity, and on the
cytoproct. This structure is identified by its “posterovental” position, size, and shape (Allen 1988). The
degree of coincidence (yellow) on basal bodies and in
the oral cavity may vary; e.g., it is higher in Figures 3A
and 3B than in Figures 3C and 3D. The gradient of coincidence in Figure 3A indicates some differential positioning of the respective antigens along the z-axis.
Figure 3B shows the occurrence of actin around
vesicles and vacuoles of various sizes, whereas the position of the red-labeled structures may suggest coincidence with elements of the osmoregulatory system—
aspects that have not been followed in any more
detail. Figure 3D documents more clearly a cortical
actin layer and actin filaments throughout the cytoplasm, frequently in a radial arrangement, and sometimes with local concentration.
We used conventional LM analysis to analyze immuno-FITC labeling of cilia with anti-actin ABs (Figure 4), thus taking advantage of a thicker optical
section layer. While intracellular details are largely
blurred, ciliary basal bodies and cilia on the outer cell
surface are clearly labeled. This may also apply to cilia
in the oral cavity, although this is not resolved in Figure 4.
Figure 2 Western blot using affinity-purified anti-actin (Paramecium type 1-1) ABs showing a prominent band of 43 kD in the homogenate and in the fractions indicated, except trichocysts. This
band represents preferably structure-bound actin (100,000 g pellet) and appears also in isolated cortex and ciliary fractions. Note a
fainter band of 40 kD in the homogenate and in the 100,000 g
supernatant and pellet, which both contain further weak bands of
lower mass (possibly degradation products or cross-reacting actinrelated proteins). Right lane: molecular mass markers.
generated by partial proteolysis during fractionation.
None of the bands were visible when Western blots
were produced with the corresponding preimmune
sera or in controls with the second AB alone (data not
shown).
Immunofluorescence Labeling
To account for some variability in the immunofluorescence staining, we present typical extremes of CLSM
images from double labeling experiments (Figures 3A–
3D), with mouse anti-actin FITC-ABs and rabbit anticentrin Texas Red-ABs, the latter specific for the centrosome in Dictyostelium (Daunderer et al. 2001) and
basal bodies in Paramecium. This is in contrast to the
pattern obtained by the monoclonal AB 20H5 against
centrin from Chlamydomonas (Sanders and Salisbury
1994) which in Paramecium brilliantly stains not only
basal bodies but also the infraciliary lattice (Klotz et
al. 1997; Beisson et al. 2001). Labeling with both antiactin and anti-centrin ABs in part coincides with cili-
Comparative Analysis of CLSM and Immunogold
EM Labeling
For most results achieved by CLSM analysis, we find
equivalents in the immunogold EM analyses (Figures
5 to 11), as specified below and summarized in Table
1. Off-cell background is low [2.15 gold grains per
m2 0.85 (SEM)], as it is on irrelevant structures,
such as mitochondria, trichocyst contents, and alveolar sacs (2.2, 1.4, and 0.3 gold grains per m2, respectively).
After postembedding labeling, gold granules are
scattered, yet with specific concentration zones over
the cytosolic compartment. This holds for the cell cortex (Figures 6 to 8) with its ciliary basal bodies, as
well as for regions adjacent to the oral cavity, including a zone enriched in ciliary basal bodies (Figure
10A) and a zone enriched in recycling vesicles (discoidal vesicles) dedicated to phagosome formation (Figure 10B). It also holds for regions deeper inside the
cytoplasm where elements of vesicle trafficking are enriched (Figure 11). Cilia are also labeled at the EM
level (Figures 5 and 10, Table 1), just as with the other
methods used (Figures 2 and 4). In sum, there is good
agreement between LM and EM labeling. Because the
cytoproct shows up rarely, we were unable to analyze
it at the EM level.
Figure 9 represents experiments with digitonin-permeabilized cells, showing AB-gold labeling in the narrow subplasmalemmal space between the plasmalemma
The Journal of Histochemistry & Cytochemistry
1548
Kissmehl, Sehring, Wagner, Plattner
Figure 3 Colocalization of actin and centrin (yellow) by CLSM using mouse ABs against actin (green) and rabbit ABs against centrin (red).
Two deciliated cells (A,B vs C,D) showing extreme situations of labeling are presented. (A,C) are superficial sections; (B,D) are median focal
planes. Note colocalization at basal bodies in top-most focal planes (arrowheads), on the cytoproct (cp) and in parts of the oral cavity (oc).
Basal bodies located in layers outside the optical section are preferably red (A) or green (C), thus suggesting a layered arrangement of actin
and centrin in these regions of the cell. Note occurrence of actin in the outermost cortex layer particularly in (D, arrowheads) as well as of
interior actin clusters probably associated with vacuoles (v in B) and as filament bundles indicated by arrows (D). (B) displays centrin staining
at two conspicuous sites where the osmoregulatory system is located (asterisks) and actin labeling associated with large vacuoles (v). Bars
10 m.
and the outer side of alveolar sacs, while there is only
spurious label occasionally seen after mere section labeling (Figures 5 and 7). Apart from this aspect, little
label only is seen in the cell cortex with permeabilized
cells (Figure 9). While digitonin permeabilization may
be more appropriate than section labeling to show the
presence of some actin in the very narrow outermost
cytosolic space, particularly when enhanced by additional postembedding labeling (Figure 9), it may cause
a serious overall loss of antigen. The abundance of
Figure 4 Conventional anti-actin AB-fluorescence image of a cell permeabilized under conditions preserving cilia. (A) superficial, and (B)
median plane. Note labeling of cilia (ci) in (A) and of their basal bodies (bb) in (A,B), of specks and strands in (B), and of the presumable oral
cavity (oc) in (A,B). Bars 10 m.
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Actin in Paramecium
cortical label after postembedding labeling justifies reliance in this study mainly on the postembedding procedure for further evaluation. Concomitantly, all figures presented with the exception of Figure 9 were
obtained by this method.
The Journal of Histochemistry & Cytochemistry
Specification of Results Obtained with
Postembedding Labeling
Beyond the general labeling of the cytosolic compartment of the cell cortex (Figures 5 to 8), we recognize
that gold granules are enriched to a variable extent in
a variety of structures.
The cytoplasm of cell surface ridges, typical of ciliated protozoa, are labeled (Figures 5 and 6). This also
holds for the cytoplasm surrounding the tips of the
elongate trichocyst organelles, as shown in cross-section (Figures 5 and 6) and in longitudinal section (Figure 7). The gold label associated with cortical basal
bodies is somewhat variable and may in part sit inside
this structure, as shown particularly in Figure 8B,
where it shows up below the basal plate (Figure 8A).
Gold label also occurs adjacent to cortical basal bodies, e.g., in the filamentous mass in Figure 6. This material is associated with the origin of a kinodesmal
fiber emanating from a basal body from where the
infraciliary lattice also emanates. From there, these filament bundles pass near adjacent trichocyst tips (Figure 5), as established by Allen (1971,1988). Although
the bulk of the latter filament system is made of centrin (Beisson et al. 2001), some actin clearly appears
to be associated with it. Gold label also surrounds
ghosts from discharged trichocysts (Figure 6).
Table 1 summarizes labeling densities on a quantitative level (gold grains per m2). These are, in decreasing magnitude, as follows: 301.0 Au/m2 for cytoplasmic regions around oral cavity and around food
vacuoles, 141.5 for cell surface ridges, 111.9 for immediate surroundings of trichocysts, between 89.5 and
95.6 for infraciliary lattice, ciliary basal bodies, and
cilia, followed by cortical cytoplasm (37.8) and the
complex formed by the plasma membrane and the
outer alveolar sacs membrane (25.9 Au/m2). For statistics, see Table 1.
While the abundant filament bundles located in the
cytoplasm around the oral cavity are made of materials other than actin (see “Discussion”), the distinct labeling in between such bundles (Figure 10A) again indicates association with actin. As in the cell cortex,
some label may be associated with ciliary basal bodies
around the oral cavity. Furthermore, we find intense
labeling of cytosolic regions enriched in vesicles accumulated near the cytopharynx (Figure 10B). Many are
oblong and thus represent discoidal vesicles known to
serve membrane recycling from the cytoproct, i.e., formation of new phagocytic vacuoles (Fok and Allen
1988; Allen and Fok 2000). In these domains of the
cell, less labeling is seen immediately below the cell
membrane than between the adjacent round and discoidal vesicles.
Deeper inside the cell, small vesicles of different diameters are embedded in considerably labeled cytosol,
frequently in close association with a large vacuole
(Figures 11A and 11B). This arrangement suggests
their identity either as lysosomes or as acidosomes in
typical arrangement with phagosomes. These interpretations are suggested by the work of Allen and Fok
(2000); e.g., considering the flat shape of the large
vacuole indicating an early biogenetic stage of a food
vacuole. Figure 11B shows association of actin label
with parallel microtubular aggregates, the gold label
unilaterally concentrated at sites where microtubules
enter the section plane. Also in Figure 11B, a heavily
labeled “trail” is in direct extension of the adjacent
microtubular bundle. This indicates involvement of
actin in phago-lysosomal vesicle trafficking, although
after the preparation protocol required for immunogold analysis, distinct filaments are difficult to recognize. However, some of these gold aggregates may
be the equivalent of the fluorescent strands visualized
by anti-actin ABs in Figure 3.
Discussion
Background from Previous Work
Occurrence of most actin in Paramecium in structurebound form contrasts with the abundance of monomeric actin in Apicomplexa (Sibley 2004), including
Toxoplasma (Poupel et al. 2000; Wetzel et al. 2003).
This makes fluorescence labeling studies with F-actin–
specific drugs feasible. In Paramecium, phalloidin,
heavy meromyosin, and DNaseI have clearly revealed
labeling of the cell cortex, particularly of ciliary basal
bodies (Tiggemann and Plattner 1981; Kersken et al.
1986a,b). Phalloidin also has labeled the nascent food
vacuole (Kersken et al. 1986a,b). Concomitantly, cytochalasin B has been reported to inhibit formation of
phagocytic vacuoles (Allen and Fok 1983,1985; Fok
and Allen 1988). It also inhibits docking of trichocysts
(Beisson and Rossignol 1975), and it even can detach
docked trichocysts from the cell surface (Pape and
Plattner 1990). When phagocytosis has been analyzed
with different F-actin–disrupting drugs and analogs,
respectively, the requirement of concentrations well
above those used with mammalian cells has been confirmed (Beisson and Rossignol 1975; Pape and Plattner 1990; Zackroff and Hufnagel 1998). This is in line
with the low sensitivity of F-actin in other ciliates. In
total, these data are all compatible with our current
results obtained with ABs against the original Paramecium antigen.
Previous attempts to localize actin in Paramecium
have led to controversies. One discrepancy concerned
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Kissmehl, Sehring, Wagner, Plattner
The Journal of Histochemistry & Cytochemistry
Actin in Paramecium
1551
Figure 6 Similar situation as in Figure 5, but in addition with more distinct label around a trichocyst tip (tt) and a ghost (gh) from a released trichocyst, and much less in diffuse association (frames 1, 2) with two basal bodies (bb) from which typically kinodesmal fibers (kf)
originate. Cell surface ridges (r) are also labeled. Note almost absence of label outside the cell or inside alveolar sacs (as), the trichocyst tip
and ghost, as well as in a mitochondrion (m). Bar 0.1 m.
the composition of cortical filament bundles, notably
of the infraciliary lattice emanating from ciliary basal
bodies. While the bulk of this filament system has
been established as centrin (Beisson et al. 2001), this
does not necessarily preclude association of centrin filaments with actin, as we can show. Recall that widely
different affinity stains for actin, including heavy meromyosin, have resulted in cortical labeling in Paramecium (Tiggemann and Plattner 1981; Kersken et al.
1986a,b), as well as in Tetrahymena (Méténier 1984).
Theoretically, previous LM and preembedding-EM localization studies could have faced the problem of sol-
Figure 5 Postembedding immunogold labeling in the cell cortex. Note almost absence of background outside the cell and within membrane-bound organelles such as alveolar sacs (as), trichocyst tips (tt), mitochondria (m), and a Golgi field (go). This is in contrast to the occurrence of clear, though scattered, labeling of cytoplasmic ridges (r) typical of the Paramecium cell surface, around a trichocyst tip (top) and
close to a trichocyst attachment site (ta), in a cilium (ci), in a ciliary basal body (bb), and along filamentous materials emanating from there
(rectangle), probably infraciliary lattice. Note a single gold grains (arrowheads) on the complex formed by the cell membrane and the outer
side of an alveolar sac. Bar 0.1 m.
The Journal of Histochemistry & Cytochemistry
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Kissmehl, Sehring, Wagner, Plattner
Figure 7 Similar situation as in Figures 5 and 6, but with more clearly visible label (rectangle) particularly surrounding a longitudinally cut
trichocyst tip (tt) and occasional label (arrowheads) in the very narrow subplasmalemmal space between the plasma membrane (pm) and the
outer alveolar sacs membrane (oam). Note absence of label from the off-cell region, alveolar sacs (as), and mitochondria (m). Bar 0.1 m.
uble antigen relocation and even loss during permeabilization. This would not easily be possible with the
postembedding immuno-EM labeling procedure used
now. Another hint to real cortical F-actin localization in Paramecium came from the in vivo labeling
by injection of fluorescent phalloidin (Kersken et al.
1986a,b), resulting first in cortical labeling and, over
longer time periods, in disappearance from the cortex
and re-assembly as thick trans-cellular filament bundles of a type not previously seen. Conversely, aberrant phalloidin binding by F-actin formed by some
isoforms may preclude labeling (Hirono et al. 1989),
while such forms may bind actin-specific ABs.
Additional Functional Aspects Derived
from This Study
Cortical F-actin is generally required for cyclosis—an
actomyosin-based process (Shimmen and Yokota 2004).
This is a permanent ongoing process also in Parame-
cium (Sikora et al. 1979), where it serves the delivery
of trichocysts to the cell cortex (Aufderheide 1977)
and the cycling of phago-lysosomal elements through
the cell body (Fok and Allen 1988,1990; Allen and
Fok 2000). Myosins occur in Paramecium (Cohen et
al. 1987), just as in other protists (Gavin 2001).
Our present EM analysis verifies that in the Paramecium cell cortex, actin is enriched at ciliary basal
bodies, as discussed above on the LM level. From
there it emanates to the infraciliary lattice and around
trichocyst docking sites. The association of actin with
ciliary basal bodies has led to the description of the
“basal body cage,” particularly in Tetrahymena (Hoey
and Gavin 1992), where association with myosin has
been demonstrated (Garcés et al. 1995). The loose arrangement of gold label within and around basal bodies, as we see it here, suggests that during permeabilization for LM analysis, F-actin emanating from basal
bodies may collapse to a compact arrangement. In
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Actin in Paramecium
1553
Figure 8 Postembedding immunogold labeling of ciliary basal bodies (bb) located on the outer cell surface, in longitudinal (A) and in crosssection (B), with additional label on diffuse materials surrounding the basal body (framed in A). Note again absence of label on irrelevant
structures, such as alveolar sacs (as), mitochondria (m) and a trichocyst body (tb). Bars 0.1 m.
sum, a more loosely arranged cortical F-actin in
conjunction with myosin may underlie cytoplasmic
streaming and possibly trichocyst docking. Concomitantly, inhibition of trichocyst docking by cytochala-
sin B (Beisson and Rossignol 1975) would be compatible with both actin-based transport by cyclosis
and enrichment of actin around trichocyst tips (this
study).
Figure 9 Combination of pre- and postembedding immunolabeling shows label in the narrow subplasmalemmal space (at/between arrowheads) between the plasma membrane (pm) and the outer alveolar sacs (as) membrane (oam), with little background on irrelevant structures outside the cytosolic compartment. Note deformation of the cell surface membrane complex (pm/oam), with some label attached particularly in regions with a “grazing” section plane, due to the permeabilization applied. This cell has been digitonized during aldehyde
fixation for impregnation with primary AB and IgG-Au5 and embedded for incubation with the same ABs in sequence. Bar 0.1 m.
The Journal of Histochemistry & Cytochemistry
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Kissmehl, Sehring, Wagner, Plattner
The Journal of Histochemistry & Cytochemistry
Actin in Paramecium
Assembly of F-actin around nascent phagosomes is
well established, not only in mammalian cells but also
in Paramecium cells (Allen and Fok 1983; Fok and
Allen 1988). In detail, fusion of acidosomes with the
nascent food vacuole depends on F-actin (Fok et al.
1987), as does maturation along the phago-lysosomal
pathway, where multiple fusion/fission processes occur (Allen and Fok 1985; Allen et al. 1995). Interestingly, in our study, gold labeling immediately below
the cytopharyngeal plasma membrane is less intense
than between the closely packed globular and discoidal vesicles slightly below. This can be seen in line
with the following reports. In Dictyostelium, F-actin
prevents clustering of endosomal vacuoles (Drengk et
al. 2003). Alternatively, in yeast, actin is required for
Ca2-mediated vacuole interaction leading to fusion
(Merz and Wickner 2004). The final step of this cycle
in Paramecium, exocytotic release of spent phagolysosomes, can also be inhibited by cytochalasin B
(Allen and Fok 1985). In agreement with this previous
work, the site of phagosome formation, vacuoles of
different size, and the cytoproct are clearly labeled
with anti-actin ABs in our CLSM and EM pictures.
Therefore, the fine filaments described at the cytoproct by Cohen et al. (1984) are, at least to some extent, F-actin. However, centrin also occurs at the cytoproct, according to the CLSM pictures presented in
Figure 3.
At the EM level, we see that the cytosolic compartment around large and small vacuoles is frequently heavily labeled (even when filaments are difficult to discern due to faint contrast resulting from
preparation for immuno-EM analysis). This holds,
e.g., for domains with clearly visible microtubule
bundles deep inside the cell and for regions with discoidal vesicles approaching the cytopharynx. The latter are delivered along microtubule rails, using dynein as a motor (Schroeder et al. 1990). Therefore,
actin at these sites may serve not as a motor, but
rather as a kind of scaffold. In sum, apart from association with non-actin filaments (see below), we see
that actin is also associated with the second cytoskeletal element, the microtubules. This agrees with functional data obtained by combined drug application
(Fok et al. 1985).
Label also occurs around the oral cavity outside the
site of phagosome formation in the cytopharynx. Such
filaments are known not to represent actin, either in
1555
Paramecium (Clérot et al. 2001), or in other ciliates
(Viguès et al. 1999). In these regions, F-actin may
again serve structuring of these firmly established subcellular domains and/or vesicle trafficking. Interestingly, co-assembly of polymerizing actin with other
filament components from Tetrahymena can be produced in vitro (Mitchell and Zimmerman 1985).
Vesicles deeper inside the cytoplasm, often close to
a large phagosome, are also surrounded by gold label.
All this reflects that actin is present throughout the
cell in LM analyses, frequently as strands. Actin may
thus participate directly or indirectly in vesicle trafficking, including cyclosis.
Not only ciliary basal bodies, but also the ciliary
shaft, are labeled by anti-actin ABs. Labeling of cilia
has been reported previously based on peroxidasebased preembedding immunostaining in Paramecium
(Tiggemann and Plattner 1981) and in quail oviducts
(Sandoz et al. 1982). Because this method is subject to
redistribution artifacts (Plattner and Zingsheim 1983),
we considered a re-analysis by Western blots and by
the postembedding EM methodology to be necessary.
It is known only from flagella of the green alga,
Chlamydomonas (Mitchell 2000; Hayashi et al. 2001;
Hirono et al. 2003), that actin is mandatory for normal beat activity. This may apply also to cilia of Tetrahymena, whose 14S axonemal dynein binds actin
(Muto et al. 1994). More details on the role of actin in
cilia remain to be elucidated.
Another poorly understood aspect concerns coupling of cortical calcium stores to the cell membrane.
With mammalian cells, one of the molecules considered to establish such connections, particularly for
store-operated Ca2-influx, is actin (Patterson et al.
1999; Rosado and Sage 2000; Kunzelmann-Marche et
al. 2001; Wang et al. 2002). Interestingly, we find
gold label that may be associated with the narrow
subplasmalemmal space not only using a variation of
the general labeling procedure that faciliates access of
ABs (Figure 9), but also, though to a lesser extent, using postembedding labeling (Figures 5 and 7). This becomes evident particularly after statistical evaluation
(Table 1). Although cytochalasin B application did not
change concomittant Ca2 signals (Mohamed et al.
2003), we keep this question open because the different actin isoforms found in Paramecium (Kissmehl et
al., in preparation) may have different drug sensitivities.
Figure 10 Label around the oral cavity, in a region enriched in ciliary basal bodies (bb) in (A) or in vesicles (B). These represent, at least in
part, discoidal vesicles (dv) known to recycle membranes for nascent phagosome formation, which may be assisted by round vesicles (rv) as
discussed in the text. A particularly densely labeled domain in (A) is framed. Some label is located between unlabeled fibrous material (fm,
not actin-type). Also note some label on basal bodies (bb) and within some cilia (ci). In (B), the label is scattered between the discoidal vesicles (dv). In (A) and (B), a 1-m-thick layer below the oral cavity plasma membrane is heavily labeled, starting at a distance from the
plasma membrane. Bars 0.1 m.
The Journal of Histochemistry & Cytochemistry
1556
Kissmehl, Sehring, Wagner, Plattner
1557
Actin in Paramecium
Table 1 Labeling density (gold grains/m2) achieved with anti-actin AB/gold conjugate over different structural components,
background [2.15 0.85 (SEM), n 6, determined off cell, or in the lumen of food vacuoles] subtracted
Structure analyzed
The Journal of Histochemistry & Cytochemistry
Cilia
Basal bodies
Plasma membrane/outer alveolar membrane complex
Cell surface “ridges”
Alveolar sacs contents
Cortical cytoplasm
Infraciliary lattice
Surroundings of trichocyst tips
Trichocyst contents
Mitochondria
Small vesicles associated with oral cavity and around food vacuoles
Cytoplasmic regions around oral cavity and around food vacuoles
Gold grains/mm2
6SEM
n
89.5
91.0
25.9
141.5
0.3
37.8
95.6
111.9
1.4
2.2
7.0
301.0
25.5
24.9
4.1
56.8
0.3
4.9
6.2
27.0
0.8
1.0
7.0
7.6
21
12
14
6
21
5
3
7
8
11
63
2
SEM standard error of the mean; n number of structural components analyzed.
Our present immunogold EM analysis largely depends on the preparation schedule used, whereas we
obtained no such clear-cut labeling pattern with other
approaches (data not shown). The current approach
implied rapid injection (spraying) of cells in 0C aldehyde fixative, containing high formaldehyde and very
low glutaraldehyde concentrations, followed by low
temperature embedding and UV polymerization at
35C. This can considerably restrict diffusion of
macromolecules and, even more, of filamentous aggregates. Therefore, we consider the current approach,
elaborated on a (semi-)quantitative basis, more reliable
than some previous attempts to localize actin in such
cells.
Acknowledgments
We gratefully acknowledge the kind help of Dr R. Gräf
(University of Munich) for a gift of anti-centrin ABs, as well
as the help of our group members, of Dr Joachim Hentschel
with the quenched-flow preparations, and the skillful technical assistance of Ms Lauretta Schade in the EM documentation. We thank Dr Claudia Stuermer for access to the CLSM
and Ms Sylvia Hannbeck von Hanwehr for technical help in
the CLSM analysis, as well as Ms Doris Bliestle for electronic
image processing. Supported by grants from the Deutsche
Forschungsgemeinschaft to HP.
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