Results StxB co-localizes with transferrin-positive endosomes We first sought to establish a time-line for retrograde traffic of StxB in green monkey kidney BSC-1 cells.. B Internalized St
Trang 1B subunit to reach the endoplasmic reticulum
Jenna McKenzie1, Ludger Johannes2,3, Tomohiko Taguchi4and David Sheff1
1 Department of Pharmacology, Carver College of Medicine, University of Iowa, Iowa City, IA, USA
2 Institut Curie, Centre de Recherche, Laboratoire Trafic, Signalisation et Ciblage Intracellulaires, Paris, France
3 CNRS UMR144, Paris, France
4 Department of Biochemistry, Osaka University Graduate School of Medicine, Japan
Shiga toxin (Stx) is a bacterial exotoxin responsible for
an estimated 165 million annual cases of severe
dysen-tery worldwide [1] The toxin attacks cytosolic targets
in mammalian cells To reach these targets, the toxin
navigates a retrograde pathway that passes sequentially
through the plasma membrane, endosomes, Golgi and
endoplasmic reticulum (ER) [2–5] Passage through the
Golgi appears to be rate limiting on this pathway,
resulting in prominent labeling of this organelle
How-ever, such prominent labeling may be misleading
Recycled transferrin was assumed to pass sequentially
through the early endosomes (EEs) and recycling
endo-somes (REs) based on prominent labeling of the RE at
later time points [5,6] It later became evident that the majority of transferrin actually bypasses the RE The same may be true for Golgi passage of Stx Empirical data supporting a requirement for passage through the Golgi is lacking Indeed, treatment with brefeldin A provides protection against the holotoxin, suggesting involvement of the Golgi However, that protection is incomplete, suggesting that Golgi passage may be favored but not required [7,8] Furthermore, other toxins, such as diphtheria toxin, bypass the Golgi and
ER by escaping the endosomal compartment directly into the cytosol [9] SV40 virus is internalized into a spe-cialized compartment which can communicate directly
Keywords
endosomes; Golgi; membrane traffic;
retrograde traffic; Shiga toxin
Correspondence
D Sheff, Department of Pharmacology,
Carver College of Medicine, University of
Iowa, Iowa City, IA 52242-2600, USA
Fax: +1 319 335 8930
Tel: +1 319 335 7705
E-mail: david-sheff@uiowa.edu
(Received 11 November 2008, revised 4
January 2009, accepted 7 January 2009)
doi:10.1111/j.1742-4658.2009.06890.x
Both Shiga holotoxin and the isolated B subunit, navigate a retrograde pathway from the plasma membrane to the endoplasmic reticulum (ER) of mammalian cells to deliver catalytic A subunits into the cytosol This route passes through early⁄ recycling endosomes and then through the Golgi Although passage through the endosomes takes only 30 min, passage through the Golgi is much slower, taking hours This suggests that Golgi passage is a key step in retrograde traffic However, there is no empirical data demonstrating that Golgi passage is required for the toxins to enter the ER In fact, an alternate pathway bypassing the Golgi is utilized by SV40 virus Here we find that blocking Shiga toxin B access to the entire Golgi with AlF4) treatment, temperature block or subcellular surgery prevented Shiga toxin B from reaching the ER This suggests that there is
no direct endosome to ER route available for retrograde traffic Curiously, when Shiga toxin B was trapped in endosomes, it entered the cytosol directly from the endosomal compartment Our results suggest that traffick-ing through the Golgi apparatus is required for Shiga toxin B to reach the
ER and that diversion into the Golgi may prevent toxin escape from endo-somes into the cytosol
Abbreviations
BFA, brefeldin A; EE, early endosome; ER, endoplasmic reticulum; MEM, minimal Eagle’s medium; PDI, protein disulfide isomerase; RE, recycling endosome; Stx, Shiga toxin; StxB, Shiga toxin B; Tfn, transferrin; TfnR, transferrin receptor; TGN, trans-Golgi network; WGA, wheatgerm agglutinin.
Trang 2with the ER, bypassing endosomes and Golgi [10].
There may even be alternative retrograde pathways
between the endosomes and ER that either include or
bypass the Golgi, where the majority of traffic
nor-mally passes through the Golgi To investigate these
possibilities, we examined the fate of Stx where access
to the Golgi was blocked
Stx is secreted by Shigella dysenteriae It is highly
homologous to the Shiga-like toxins (also termed
vero-toxins) secreted by enterohemorrhagic strains of
Esc-herichia coli Stx is a member of the A-B5 family of
toxins, which are composed of one enzymatic A
sub-unit, noncovalently bound to a B subunit composed of
a homopentamer of B fragments [11] The Stx A
sub-unit is an rRNA N-glycosidase, which stops protein
synthesis and causes cell death [12] The A subunit
must be delivered to the host-cell cytosol to encounter
its ribosomal substrate To reach this destination, it is
carried by a homopentameric B subunit (StxB) along a
retrograde pathway from the plasma membrane
through the EE⁄ RE to the Golgi and the ER Stx
takes advantage of trafficking through the Golgi to
facilitate cleavage and activation of the catalytic
A subunit by trans-Golgi network (TGN) resident
furin protease [13] The catalytic domain remains
attached to the anchor domain by a disulfide bridge
that is cleaved when the complex enters the cytosol
Entry of the catalytic A subunit into the cytosol is via
retrotranslocation [14–16] The B subunit initially gains
entry to cells by binding the neutral glycosphingolipid,
globotriaosyl ceramide (Gb3 or CD77) at the cell
surface [17] Bound toxin is endocytosed via both
clathrin-dependent and -independent mechanisms and
is delivered to EEs [18–20] There is no known protein
receptor for Stx B subunit (StxB), and the mechanism
by which it is recruited into clathrin-coated pits
remains unknown StxB binding to Gb3 at the cell
sur-face induces changes in plasma membrane topology
resulting in the formation of tubular invaginations that
facilitate internalization [21] It remains to be
deter-mined whether this toxin-induced pathway or
clathrin-mediated endocytosis is predominant in normal cells
In both cases, newly internalized StxB appears to be
delivered to EEs StxB binding is not a passive process
Binding and endocytosis of the toxin is accompanied
by activation of Syk kinase and activation of
microtu-bule networks, which facilitate transport into the cell
[22,23]
Passage of Stx through the EEs⁄ REs is well
docu-mented and involves many proteins that are now being
identified [3,9,24] Two Rab GTPases, Rab11a and
Rab6A¢, regulate retrograde traffic of Stx from the
EEs⁄ REs to the Golgi, suggesting that this is a
regulated vesicular trafficking process [25,26] In addi-tion, components of the retromer complex, specifically sorting nexins 1 and 2 and Vps26 are required for
traf-fic of Stx through the endosomes, but it is still unclear
if this mediates an intra-endosomal step or if they are required for delivery to the TGN [27–29] Delivery to the TGN does appear to involve the GARP complex, first identified in yeast as mediator of retrograde traffic into the Golgi [30] It is clear that Stx does not pass through the late endosomes [24] Instead, direct trans-port to the TGN is mediated by syntaxin 5, syntaxin 6, and syntaxin 16, a pathway that is shared by the endogenous protein TGN38, which cycles between Golgi and plasma membrane via REs [31,32] Unlike TGN38, traffic of StxB from endosomes to Golgi is dependent upon the Golgin, GCC185 [31,33–35] Here we examine the traffic of StxB which follows the same route as the holotoxin through the retrograde trafficking pathway [15,36] We perturbed access to the Golgi by an AlF4 ) treatment, temperature block and subcellular surgery to examine whether there exit routes for StxB to bypass the Golgi while trafficking from endosomes to ER Using these systems, we deter-mined that Golgi transit is required for trafficking to the ER
Results
StxB co-localizes with transferrin-positive endosomes
We first sought to establish a time-line for retrograde traffic of StxB in green monkey kidney BSC-1 cells These cells were selected due to their distinct endo-somal and Golgi morphologies that allow ready visual identification Like HeLa cells, different strains of BSC-1 cells show different affinities for StxB Our lab-oratory strain (a gift from I Mellman) binds StxB readily Another strain reported by Spooner et al [37] does not For co-localization studies, cells were infected with adenovirus containing human transferrin receptor (TfnR), a well-studied marker of the endo-cytic recycling pathway [5,38] This infection did not alter the morphology of internalized StxB observed in uninfected cells (not shown) Cells were labeled on ice with both Cy3–StxB and Alexa 488–transferrin (Tfn) for 30 min Internalization of both labels was per-formed at 37C in label-free medium for the indicated times (Fig 1A) After 5 min, Tfn was in peripheral puncta representing EEs (Fig 1A; 5 min) [5] StxB co-localized with Tfn throughout the EEs This suggested that although internalization of StxB may be through clathrin-dependent or -independent mechanisms,
Trang 3they converge on the EEs [39,40] After 10 min, StxB
and Tfn co-localized in both the peripheral EEs and a
perinuclear organelle, identified by Tfn pulses as the
RE (Fig 1A; 10 min) [41] After 30 min, Tfn primarily
labeled the endosomes, although the signal was weaker
due to recycling of Tfn into the media, whereas StxB
had entered a separate perinuclear structure (Fig 1A;
30 min) This structure had the appearance of a Golgi
ribbon in these cells The difference in localization was
more obvious after 45 min (Fig 1A; 45 min arrow
indicates transferrin-containing endosomes) At the
later times, Tfn had recycled out of the endosomes and was no longer clearly visible although StxB remained
in Golgi morphology (Fig 1A; 60, 120 min) [38,42]
At 180 min, the internalized StxB took on a lacy appearance typical of the ER (Fig 1A), suggesting that a substantial amount of the toxin had been ered to the ER [43] Thus, endocytosed StxB was deliv-ered into the endocytic recycling pathway within
5 min, was transferred to perinuclear endosomes within 10–20 min, and then was delivered to the Golgi within 30–45 min of internalization
A
B
C
Fig 1 Trafficking of StxB in BSC-1 cells Cy-3 StxB was bound to BSC-1 cells on ice and internalized at 37 C for the times shown (A) StxB passes through Tfn-positive endosomes Alexa 488 Tfn and StxB bound to BSC-1 cells expressing human Tfn receptor on ice and then warmed for times shown Both co-localized up to 20 min By 45 min Tfn (green) and StxB (red) had separated Arrow indicates perinuclear endosome StxB remained in a Golgi-like ribbon for the remainder of the Tfn ⁄ StxB time-course 60–180 min (B) Internalized StxB (red) with cells fixed and immunolabeled for Golgi marker GM130 (green) Note co-localization (yellow) (C) Internalized StxB (red) with cells fixed and immunolabeled for ER marker PDI (green) Note co-localization at 240 min Inset is indicated area magnified Bars = 10 l M
Trang 4StxB is delayed in the Golgi before entering the ER
We next characterized the passage of StxB through the
Golgi of BSC-1 cells under normal cell culture
condi-tions (Fig 1B,C) The distribution of StxB at various
time points was compared with that of the cis⁄ medial
Golgi marker GM130, or the ER marker protein
disul-fide isomerase (PDI) [44,45] Cells were labeled with
StxB as before and fixed for immunofluorescence StxB
initially partly co-localized with the cis⁄ medial Golgi
marker GM130 after 20 min (Fig 1B), and
co-localiza-tion increased up to 120 min (Fig 1B) This confirmed
that StxB passes from the transferrin-positive
endo-somes to the Golgi rather than to another
compart-ment such as late endosomes [24] Passage through the
Golgi was slow, as observed elsewhere [2] To
deter-mine how long it took for StxB to enter the ER, we
internalized StxB for up to 4 h and labeled the cells
for the ER marker, PDI (Fig 1C) StxB remained in a
perinuclear ribbon (Golgi, as shown by co-localization
in Fig 1B) up to 120 min StxB began to co-localize
with PDI at 150 min (not shown) and 180 min (not
shown) By 240 min (Fig 1C), StxB was localized to
the ER as shown by co-localization with the ER
resi-dent, PDI These data support the observation that
passage through the Golgi is the slowest step in the
retrograde pathway, requiring up to 120 min [46]
Taken together, Fig 1A–C established a normal
time-course of StxB traffic in BSC-1 cells We used this
time-course as a basis for our further experiments
Passage through the Golgi is required for StxB to
reach the ER in cytoplasts
We wished to test directly if passage through the
Gol-gi⁄ TGN was required for StxB entry into the ER To
accomplish this, we made use of subcellular surgery to
create cytoplasts lacking a Golgi apparatus [47]
Peripheral extensions of adherent BSC-1 cells were
cleaved using a glass micro-pipette to create cytoplasts
(peripheral areas lacking a nucleus) and karyoplasts,
containing the nucleus, the Golgi apparatus and the
REs [48] Cytoplasts generated in this manner lack a
Golgi apparatus, and importantly, cannot regenerate
one [47] By contrast, cytoplasts can regenerate
func-tional REs from peripheral EEs, as we have previously
demonstrated Recycling of Tfn in cytoplasts is
com-plete and follows the same kinetics in cytoplasts as in
whole cells [6] Cytoplasts and karyoplasts were labeled
with StxB and Tfn for 5 min at 37C (rather than on
ice to avoid releasing the cytoplast from the coverslip)
and both ligands were chased into the cytoplasts for
various times (Fig 2A) After 10 min, StxB
co-local-ized with Tfn in endosomal structures (Fig 2A; 10 min yellow arrow) After 30 min, Tfn and StxB continued to co-localize with Tfn in endosomes (compare Fig 2A;
30 min to Fig 1A) Because Tfn recycles out of cytoplasts at longer StxB internalization times (120 min), it was necessary to add Tfn to the media for 5 min and chase in unlabeled media for the final
25 min of the assay before fixation to illuminate the endocytic pathway Surprisingly, after 120 min, although the majority StxB (red arrows) remained inside the cytoplasts, it did not co-localize with endo-somal structures (labeled with Tfn, green arrow) Rather, it appeared in a diffuse cytosolic-like pattern (red arrows, Fig 2A; 120 min) To ensure that Golgi was not inadvertently included in the cytoplasts, we immunolabeled cytoplasts for GM130 and found it to
be absent from the cytoplast, but readily visible in the karyoplast (Fig 2B) To identify which compartment the StxB had entered, we chased StxB into cytoplasts for 120 min and labeled the plasma membrane (wheat-germ agglutinin, WGA; Fig 2C), ER (PDI; Fig 2D), and cytosol (Rho GDI; Fig 2E) StxB (red arrows) did not co-localize with WGA (green arrows; Fig 2C) and thus had not recycled to the plasma membrane Nor did it co-localize with the ER marker, even when allowing 240 min for co-localization with PDI (green arrows; Fig 2D) However, StxB did co-localize with cytosolic GDI (yellow arrow; Fig 2E), suggesting that StxB was in the cytosol The GDI immunolabel required methanol fixation, which causes a grainy cast
to cytosolic proteins Cytosolic depletion using SLO or saponin proved unfeasible as treated cytoplasts detached from the coverslip Together, these results suggest that when the Golgi was absent, StxB did not enter the ER Furthermore, under these conditions the toxin was able to escape the endosomes directly into the cytosol At no time was an ER morphology or co-localization with PDI of StxB observed While it is possible that some remnant Golgi, below the threshold
of visualization, was present in the cytoplast, it was clearly insufficient to mediate StxB traffic to the ER This phenomenon may occur to some extent during normal transit of the endosomes, although the amount
of toxin available for escape may be minimal as the toxin passes rapidly through the endosomes to the Golgi It may, however, correspond to the brefel-din A-resistant toxicity reported elsewhere [8]
Aluminum fluoride traps StxB and Tfn in perinuclear endosomes
We wished to confirm the requirement for Golgi pas-sage and to quantify escape of StxB from endosomes
Trang 5into the cytosol However, cytoplasts are extremely
small, and must be made individually, making
frac-tionation impossible Therefore, we used a
pharmaco-logical approach Aluminum fluoride (AlF4)) is an
activator of small GTPases and is well-documented to
block recycling of Tfn from the RE [5,49] Although
the precise target of AlF4) at the RE is not known,
the effect of this drug on Tfn recycling is immediate
and remarkably specific to recycling out of the RE in
nonpolar cells and to basolateral recycling from the
RE in polarized cells [5] Treatment for > 1 h results
in dispersal of the Golgi although both the TGN and
the ER remain functional [50,51] Because StxB
co-localized extensively with Tfn in perinuclear REs,
we suspected that AlF4 ) might also block retrograde
StxB from the endosomes to the TGN just as it did
for recycling traffic to the plasma membrane
Fortu-itously, both StxB and Tfn were trapped together in
the REs following AlF4) treatment (Fig 3) This was
especially apparent after 60 min, when Tfn would
normally have recycled out of the cell, and StxB would normally have moved to the Golgi Both remained in the endosomes of treated cells after 60 and even
120 min (Fig 3; 60 min, 120 min, yellow arrows) Although this result is based on the fortunate effects
of AlF4)treatment on these two pathways, it does not necessarily imply that the same drug target is involved
in both retrograde and recycling pathways It does, however, present a unique opportunity As in the cytoplast, StxB is prevented from reaching the Golgi, and it is trapped inside of the endosomes This allowed
us to quantify the consequences of trapping StxB in endosomes
StxB leaks into the cytosol when trapped at endosomes
StxB trapped in the endosomes of AlF4)-treated cells took on a diffuse cytosolic appearance at later time points following internalization (Fig 3; 120 min, red
E
D
C
Fig 2 StxB cannot access the ER in BSC-1 cytoplasts BSC-1 cells were manually cut with a glass needle to create karyoplasts (k) contain-ing both the nucleus and Golgi and cytoplasts (c) All cytoplasts and karyoplasts were labeled with Cy-3 StxB (red) that was internalized for times shown (A) Shiga and Tfn (green) internalized together for 10 min then chased for 10 or 30 min For 120 min, Tfn was internalized for the final 25 min (B) Cytoplast with StxB (red) immunolabeled for Golgi marker GM130 (green) (C) Cytoplast stained for plasma membrane with wheat germ agglutinin (green) Note that the cytoplast has moved next to the karyoplast but the two remain separate (D) Cytoplast labeled for ER marker PDI (green), at various times of StxB (red) internalization Note exclusion of StxB from ER (E) Cytoplast labeled for cytosolic marker GDI (green) note co-localization (yellow) with StxB (red) Insets are cytoplasts presented in single channels with larger inset showing a magnified view of the combined channels Red arrows indicate StxB, green arrows indicate other compartment markers as indicated Bars = 10 l M
Trang 6arrows) RE-associated versus peripheral fluorescence
was measured using NIH image in 20 cells; 20 ± 7%
was found in the periphery The diffuse material did
not have an ER morphology, however, AlF4) is
known to disperse the medial Golgi in some cells after
extended treatment (> 120 min) [50] To determine
which cellular compartment StxB had entered, we
per-formed a series of co-localizations (Fig 4) The
distri-bution of StxB was compared to the cis⁄ medial Golgi
marker GM130 in BSC-1 cells in the presence of
AlF4) Although dispersed, the Golgi remained clearly
visible as structures surrounding StxB-labeled
endo-somes at times up to 240 min (Fig 4A) GM130 did
not co-localize with the StxB in punctate (endosomal)
structures nor did it co-localize with the diffuse StxB
(Fig 4A) This confirmed both that AlF4 ) treatment
prevented access to the Golgi and that StxB was not in
the fragmented Golgi
It was possible that AlF4) treatment may have
allowed StxB to bypass the Golgi and enter the ER
However, despite changes in ER morphology (Fig 4B),
StxB did not co-localize with the ER marker PDI,
even at 240 min (Fig 4B) As with the cytoplasts, StxB
prevented from reaching the Golgi by AlF4) did not
access the ER A fraction of the internalized StxB
appeared to escape the endosomes as had occurred in
the cytoplasts This suggested that StxB alone could escape endosomes if it was not sequestered into the Golgi It was not completely clear if the extra-endoso-mal StxB was in a membrane or cytosolic fraction
It was also unclear if endosomal escape was specific for StxB or resulted from AlF4)treatment altering the endosomal membranes to allow escape of all cargo
To differentiate between these possibilities, we used
a cell fractionation approach to separate cytosol from membrane-bound organelles Iodinated StxB was bound to BSC-1 cells on ice, washed, then warmed in the presence or absence of AlF4)to initiate internaliza-tion Cells were harvested after 120 min of chase, then homogenized in a ball-bearing cell homogenizer so as
to recover intact organelles Membrane and cytosolic fractions were separated via Opti-prep step-gradients
In these experiments, (Fig 5A) cytosol was collected from the top of the gradient, and all membranes were collected from an Optiprep cushion at the bottom of the gradient As a control for rupture of endosomes,
125I-labeled Tfn was bound to the cell surface and then chased into the endosomal compartments of control cells Because Tfn normally recycles rapidly out of the cell, it was chased for 20 min in control cells or for
120 min in AlF4)-treated cells This ensured that Tfn would be in the REs [5,6] Because Tfn is released
Fig 3 Aluminum fluoride traps Stx in endosomes Both StxB (red) and Tfn (green) were bound to BSC-1 cells on ice Both were internalized
at 37 C for times shown in the presence of AlF 4 ) Yellow arrows indicate where both Tfn and StxB have been trapped in a perinuclear
endosome Red arrows indicate StxB in diffuse distribution Inset is magnification of indicated area Bar = 10 l M
Trang 7B
Fig 4 Aluminum fluoride traps StxB in endosomes (A) StxB (red) bound to BSC-1 cells and internalized for times shown in the presence of AlF4) Cells were stained for Golgi marker GM130 (green) Note diffuse StxB Red arrows indicate StxB in endosomal structure, Green arrows indicate the Golgi (B) Cells labeled as in A but stained for ER marker PDI (green) Green arrows indicate ER structures ER morphol-ogy is altered (compare with Fig 1) Bar = 10 l M
Fig 5 StxB trapped in the endosomes leaks into the cytosol (A) Quantification of StxB in cytosol 125 I-labeled StxB or 125 I-labeled Tfn inter-nalized into BSC-1 cells expressing transferrin receptor for 120 min Cells were harvested and homogenized Total cytosol was separated from total membranes using an Optiprep 0 ⁄ 8 ⁄ 25% step gradient Average values for the percent of each ligand in the cytosol with and with-out AlF4)are shown Error bars are SD *Significant change n = 15 for StxB conditions, n = 9 for Tfn conditions (B) Cell fractionation of BSC-1 organelles to identify those containing internalized ligands on preformed linear 8–25% Optiprep gradients Top of gradient is to the left (I)125I-labeled StxB internalized in the absence (dark red, closed circles) or presence (orange, open circles) of AlF 4 )for 1 h Bar 1
indi-cates cytosolic fractions Bars 2 and 3 indicate endosome or Golgi associated peaks in treated cells Bar 4 indiindi-cates a peak of membrane bound StxB found only in treated cells (II) Positions in the gradient of cytosol (red), plasma membrane (orange), lysosomes (yellow), REs (green), Golgi (light blue), ER (dark blue) and cell debris (purple) Supporting data are given in Fig S1 (III)125I-labeled Tfn internalized for
25 min in control cells to locate RE (at 1 h it is recycled out of the cell), Bar 6 (dark blue closed squares) and 1 h in AlF4)treated cells (treat-ment prevents recycling) (light blue, open squares) Note that some Tfn remains at the plasma membrane Bar 5 (n = 1, results typical).
Trang 8from its receptor at the neutral pH of the gradient, it
acts as a sensitive indicator of endosomal rupture
dur-ing handldur-ing It also serves as a specific indicator of
changes in endosomal fragility due to AlF4 )treatment
Figure 5A shows that there was no difference in
cyto-solic transferrin between control and AlF4)-treated
cells (Fig 5A; Tfn and Tfn + AlF4)) Thus,
endo-somes were not made more fragile by the drug
treat-ment By contrast, the amount of StxB found in the
cytosol was significantly larger in the presence of
AlF4) (Fig 5A; StxB vs StxB + AlF4)) The
differ-ence was statistically significant with P < 0.0001
(Stu-dent’s t-test) This cytosolic escape was not observed
when StxB was internalized for only 20 min (data not
shown), a time at which StxB remained in endosomes
and was not visualized in the cytosol in intact cells
Cytosolic StxB accounted for 12% of the
radiola-beled StxB, but analysis of cell fluorescence had found
that 25% of internalized StxB was not in the
endo-somes To resolve this difference, we utilized a
differ-ent density gradidiffer-ent protocol to fractionate organelles
within the cell 125I-labeled StxB cell homogenates
from control and AlF4)-treated cells were applied to
preformed linear 8–25% Optiprep gradients We have
previously described the use of these gradients for the
fractionation of cellular organelles [5] Gradients were
characterized by locating fractions containing alkaline
phosphodiesterase activity (plasma membrane),
B-hex-osaminidase activity (lysosomes) radiolabeled Tfn
internalized for 25 min (REs), added phenol red
(cyto-sol), GM130 (Golgi) and PDI (ER) The position of
the peak activity for each organelle is shown in the
color-coded bar in Fig 5B (II), and in Fig S1 StxB
internalized for 60 min in control cells co-localized
with Golgi and REs (which could not be readily
distin-guished in this gradient) However, AlF4 ) treatment
shifted the distribution of both StxB and Tfn into a
doublet of peaks at, and just below, the density of
REs (Fig 5B) In this representative experiment, 9%
of StxB was observed in the cytosol in AlF-treated
cells compared with 3% in control cells Also, 24% of
StxB was observed in a dense fraction (compared with
8% in control cells) that did not co-segregate with any
of the characterized organelles Although this could
not be identified, we speculate that it may represent
transport vesicles derived from the endosomes, unable
to reach the Golgi This fraction would account for
the difference between non-endosomal StxB observed
microscopically and that observed in the cytosolic
frac-tion of step gradients above
These results suggest that AlF4 ) can block
retro-grade traffic at the endosomes and that StxB is able to
escape the endosomes to the cytosol Taken together
with the cytoplast results, this suggests that when StxB
is trapped within the endosomes, it can ‘escape’ into the cytosol, as previously suggested for dendritic cells and macrophages [52,53] A similar escape has also been observed for the Stx A subunit [8]
A temperature block separates StxB and Tfn
It remained possible that StxB was equally capable of entering the cytosol from any organelle along the ret-rograde pathway We tested this possibility by using a temperature block to trap StxB within the TGN for a time In HeLa cells, it has been reported that maintain-ing the cells at 20 C traps StxB along with Tfn in the endosomes [24] We too observed this effect in HeLa cells (Fig S2) However, reducing the temperature to
20C in BSC-1 cells had a surprising and useful effect Both Tfn and StxB were bound to BSC-1 cells and internalized at 20 C for various lengths of time (Fig 6A) As expected, Tfn did not recycle out of the endosomes, but remained in the perinuclear region After 30 min, StxB appeared to co-localize with Tfn in the majority of cells However, after 60 min, StxB sep-arated into another perinuclear structure that did not co-localize with Tfn This distribution was maintained
up to 180 min We suspected that this other structure might be part of the Golgi due to the ribbon-like appearance Fortunately, in BSC-1 cells, the TGN and cis⁄ medial Golgi can be discriminated visually (although there is slight overlap) using the TGN marker TGN46 and the cis⁄ medial marker GM130 (Fig 6B) We therefore compared the localization of StxB with that of TGN46 and GM130 after 120 and
180 min of internalization at 20C There was striking co-localization of StxB with the TGN marker at both time points (Fig 6C) suggesting that at 20C in BSC-1 cells, StxB was trapped in the TGN This was very dif-ferent from the situation at 37C (Fig 6D) where StxB co-localized with both TGN and cis⁄ medial Golgi
in as little as 90 min
We wished to confirm that we were seeing co-locali-zation in the TGN and not at ER exit sites StxB inter-nalized at 20C co-localized with TGN46 but clearly did not co-localize with the ER exit site marker Sec31, again suggesting that StxB was trapped specifically at the TGN at this temperature (Fig 6E)
These results suggested that BSC-1 cells, unlike HeLa cells, hold StxB in the TGN at 20C This difference between HeLa and BSC-1 cells provided a natural experiment in BSC-1 cells to test if StxB could escape into the cytosol when it was held in the TGN instead of the endosomes Notably, no escape of StxB into the cytosol was seen in the cells held at 20C
Trang 9even after 180 min (Fig 6C; red box and E) Taken
together these results suggest that StxB cannot escape
the TGN into the cytosol, and that escape may be
dependent upon some property of the endosomes such
as low pH found in EEs or membrane composition [54,55]
A
B
C
Fig 6 A 20 C temperature block traps Stx in the TGN in BSC-1 cells (A) StxB (red) and Tfn (green) were internalized for times shown at
20 C Yellow arrows indicate co-localization at earlier times, red arrows indicate StxB differently distributed than Tfn (B) anti-GM130 for cis ⁄ medial Golgi (green) and anti-TGN46 for TGN (red) are visually resolved, two BSC-1 cells are shown (C) StxB (red) co-localizes with TGN46 (blue) but not with GM130 (green) when internalized at 20 C Purple arrows indicate co-localization of StxB and TGN46 Red bor-dered inset shows magnification of cytosol in a cell adjacent to the labeled Golgi Note lack of cytosolic StxB (D) Same as (C) but internal-ized at 37 C Yellow arrow indicates partial co-localization of GM130 and StxB (E) StxB (red) co-localizes with TGN46 (blue) but not with ER exit point marker Sec31 (green) when internalized at 20 C Upper insets are enlarged versions of regions indicated Lower insets are Sec31 (green), StxB (red) and a merge of only Sec31 and StxB Purple arrows indicate co-localization of StxB and TGN46 Green and red arrows indicate locations of ER exit sites and StxB respectively Bar = 10 l M
Trang 10Stx is an A–B5 toxin that binds to the plasma
mem-brane lipid Gb3 In this respect it is like cholera toxin
and SV40 virus, both of which bind to the ganglioside
GM1 [56,57] Both toxins, and the virus are
trans-ported to the ER after internalization [58] However,
whereas cholera toxin appears to pass through
endo-somes and the Golgi, SV40 bypasses both the normal
endocytic organelles and the Golgi, trafficking directly
from a specialized endocytic population to the ER
[10] These examples demonstrate that binding to a
glycolipid receptor, and even trafficking to the ER do
not guarantee passage through the Golgi
Further-more, even if passage through the Golgi is a normal
component of the retrograde pathway, it does not
automatically follow that this pathway is exclusive of
other routes
We sought to determine whether retrograde traffic
of StxB required passage through the Golgi to reach
the ER A morphological examination of retrograde
traffic, as performed here, suggests that StxB appears
to progress sequentially from the plasma membrane to
the endosomes to the Golgi and then to the ER
How-ever, just as the majority of Tfn actually bypasses the
RE, a fraction of StxB may actually bypass the Golgi
[5] Alternatively, the Golgi transit route could be
pre-ferred and mask a lower flux endosome to ER route
We used cytoplasts to physically separate endosomes
and ER from the Golgi in an isolated piece of living
cells We had previously determined that these
cytop-lasts are able to regenerate a fully functional endocytic
system with both EEs and REs [6] They also contain
ER, as demonstrated here However, they are unable
to regenerate a Golgi, which allowed us to test directly
whether Golgi transit was required for StxB to
pro-gress from endosomes to the ER [47] In the absence
of a Golgi, StxB was unable to access the ER in
cytop-lasts Had a slower or lower flux pathway connected
the endosomes directly to the ER, we would have seen
StxB in the ER of the cytoplasts Although it remains
possible that such a pathway exists for some
endo-genous proteins, it is clearly not accessed by StxB
Our findings raise the question of why StxB should
take such a circuitous path from plasma membrane to
cytosol Other toxins such as diphtheria toxin and
anthrax toxin take advantage of low endosomal pH to
penetrate the endosomal membrane and enter the
cyto-sol directly [59] Our results may shed some light on the
host–pathogen interaction that has developed around
the retrograde traffic of StxB In cytoplasts, StxB was
trapped at the endosomes There are several possible
alternative routes that would be available out of the
EE compartments The most obvious would be to recycle out of the endosomes and return to the plasma membrane along with Tfn However, StxB did not reappear at the plasma membrane in any visible amount A second possibility would be for StxB to be shunted into late endosomes (also found in cytoplasts) [6] However, we did not see co-localization with lyso-somal proteins (J McKenzie & D Sheff, unpublished observations) To our surprise, StxB appeared to enter the cytosol directly from the endosomes in cytoplasts Because cytoplasts are extremely small and must be formed manually, we were unable to perform biochemi-cal analysis or cell fractionation on these preparations However, we were fortunate in finding that AlF4) treat-ment blocked exit of StxB from the endosomes Despite being a generalized inhibitor of GTPase function in the endocytic recycling pathway, AlF4) is specific for egress from REs [5] Using AlF4 ), we were able to con-firm that 12% of internalized StxB accessed the cytosol directly from the endosomes This result is inconsistent with prior findings that a small percentage of Stx
A subunit (5%) can effectively reach its target even when traffic along the postendosomal retrograde path-way is impaired by disruption of the Golgi with brefel-din A [8] It is also consistent with the prior finbrefel-ding that 10% of cell-associated StxB reaches the cytosol in human monocyte derived macrophages [53] This is particularly relevant because monocytes-derived macro-phages internalize StxB, but do not support retrograde traffic of StxB from endosomes to the Golgi [52] Fur-thermore, our finding that StxB trapped in the TGN could not enter the cytosol, suggests that endocytic membranes are particularly susceptible to penetration
by StxB Thus toxicity in the monocyte-derived macro-phages may paradoxically result from the inability of the cell to transport toxin out of the endosomes and into the Golgi (provided that the toxin would then not
be able to exit the Golgi)
Penetration of the endosomal membrane by StxB was surprising in light of the normal retrograde path-way taken by StxB However, endosomal escape is known not only for bacterial toxins such as diphtheria toxin, but also for fibroblast growth factor following endocytosis [54, 60–62] Such translocation may repre-sent another physiological pathway that is subverted
by StxB when the Golgi pathway is unavailable Clearly StxB cannot be present in the cytosol while membrane bound This suggested that the toxin was able to dissociate from Gb3 while inside the endosome Such dissociation is unlikely to be a result of low pH because acid wash of bound StxB does not remove it from the plasma membrane However, StxB can bind
up to 15 Gb3 molecules, mediated through three