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and AT1A receptor mRNA within the rat subfornical organ during dehydration. Mol. Brain Res. 64: 151–164, 1999.
2. Blatteis, C. M., and E. Sehic. Fever—how may circulating pyrogens signal
the brain. News Physiol. Sci. 12: 1–9, 1997.
3. Bourque, C. W., S. H. R. Oliet, and D. Richard. Osmoreceptors, osmoreception and osmoregulation. Front. Neuroendocrinol. 15: 231–247, 1994.
4. Fitzsimons, J. T. Angiotensin, thirst, and sodium appetite. Physiol. Rev. 78:
583–686, 1998.
5. Gerstberger, R., and D. A. Gray. Fine structure, innervation, and functional
control of avian salt glands. Int. Rev. Cytol. 144: 129–215, 1993.
6. Hübschle, T., M. J. McKinley, and B. J. Oldfield. Efferent connections of the
lamina terminalis, the preoptic area and the insular cortex to submandibular and sublingual gland of the rat traced with pseudorabies virus.
Brain Res. 806: 219–231, 1998.
7. Jurzak, J., and H. A. Schmid. Vasopressin and sensory circumventricular
organs. Progr. Brain Res. 119: 221–245, 1998.
8. Lutz, T. A., M. Senn, J. Althaus, E. Delprete, F. Ehrensperger, and E. Scharrer. Lesion of the area postrema nucleus of the solitary tract (AP/NTS)
attenuates the anorectic effects of amylin and calcitonin gene-related peptide (CGRP) in rats. Peptides 19: 309–317, 1998.
9. McKinley, M. J., E. Badoer, and B. J. Oldfield. Intravenous angiotensin II
induces Fos-immunoreactivity in circumventricular organs of the lamina
terminalis. Brain Res. 594: 295–300, 1992.
10. Rauch, M., and H. A. Schmid. Functional evidence for subfornical organintrinsic conversion of angiotensin I to angiotensin II. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 276: R1630–R1638, 1999.
11. Riediger, T., M. Rauch, G. Jurat, and H. A. Schmid. Central nervous targets
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12. Riediger, T., H. A. Schmid, A. A. Young, and E. Simon. Pharmacological
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Intracellular Virus Trafficking Reveals Physiological
Characteristics of the Cytoskeleton
Robert P. Stidwill and Urs F. Greber
Virus particles that infect eukaryotic cells can take advantage of the cytoskeleton and associated
motors to translocate through the cytoplasm. Depending on the virus, motor proteins are
recruited or, alternatively, cytoskeletal elements are induced to polymerize onto viral
structures. Here we review recent advances toward understanding the roles of the
cytoskeleton in virus trafficking.
V
iruses depend on cells for genome replication and coat
component synthesis and assembly. They are, however, different from any known cellular structures. Viruses are not simply inert packages protecting their genome from physical stress;
they use the sophisticated cellular mechanisms to deliver their
genome into susceptible host cells and initiate replication.
Although mechanisms to attain these goals vary among different viruses, the very first step of an infection is universal for all
viruses, namely binding of an exterior virus component to a cell
surface receptor to gate entry. Typically, viruses enter their hosts
by receptor-mediated endocytosis or membrane fusion (for
R. P. Stidwill and U. F. Greber are in the Department of Zoology, University
of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland.
0886-1714/99 5.00 © 2000 Int. Union Physiol. Sci./Am.Physiol. Soc.
recent reviews and references therein, see Refs. 5 and 15). The
viral genome is then uncoated and replicated either in the
nucleus or in the cytoplasm. Replicated genomes and newly
synthesized coat components are subsequently brought together
to assemble new virions, which may then leave the infected cell
and initiate a new round of replication.
At different stages of the viral replication cycle particle
motility may be necessary. During entry, many DNA and RNA
viruses target their genomes over large distances to the host
nucleus. Others replicate in the cytoplasm but, nevertheless,
move from the entry site to the replication site. In certain
cases, viral movement has been observed after replication,
i.e., with viruses that do not release newly assembled particles
by lysing the host cell but leave the infected cell by a budding
or a fusion process through the plasma membrane. Passive
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CVOs. Combining these methods with electrophysiological
and histochemical analyses may help in attaining a degree of
(in)consistency of the data to permit supporting or rejecting
the idea of a putative interface function of a particular CVO
for a particular hormone.
The comparative analysis of CVOs as targets has shown
that a particular function may be dominant in one species or
class but weakly represented, or even virtually nonexistent,
in other species or classes. This caveat has to be observed
generally in the analysis of autonomic control, as exemplified
by the role of the hypothalamus as a thermosensor (15). Thus
conclusions based on even the most convincing set of data
obtained in one species in vivo or in vitro, and even more so
in primary cell cultures or cultured cell lines, should be
extrapolated only with caution.
diffusion of particles as large as 50 nm or more is generally
restricted by the properties of the cytoplasm. If a virus or any
cellular structure larger than ~50 nm requires directional
intracellular transport, energy-consuming cellular components, such as the cytoskeleton and its associated motors, or
factors regulating assembly or disassembly of the cytoskeleton
are needed. There are three structurally and functionally distinct cytoskeletal elements: actin, microtubules (MTs), and
several classes of intermediate filaments. Although actin and
MTs are involved in cellular motility processes, intermediate
filaments seem to have largely scaffolding functions. Cytoskeleton-based motile functions include intracellular vesicle
transport, chromosome movement during cell division,
cytokinesis, the movement of transmembrane cell surface
receptors, and the maintenance and modulation of cell shape
during cell motility and phagocytosis.
In the following sections, we will show how viruses take
advantage of the microtubule and actin systems for directional and nondirectional trafficking within the cytoplasm of
an infected cell, employing either cytoskeleton-dependent
motor proteins or the polymerization/depolymerization of
the cytoskeleton as a driving force. Owing to space restrictions, we will not discuss the numerous investigations
describing cytoskeletal changes or alterations in organelle
motilities during viral infections but will instead refer to a
recent review on the subject (2).
MT-based viral motilities
MTs are polymers of α- and β-tubulin heterodimers typically forming hollow tubes of 13 protofilaments (1). A third
major tubulin isoform, γ-tubulin, serves to organize this network by nucleating MTs on MT-organizing centers (MTOCs),
such as the centrosome. MTs have an outer diameter of
25–30 nm and an inner diameter of ~15 nm. MTs are polar68
News Physiol. Sci. • Volume 15 • April 2000
ized structures with a fast-growing plus end and a slow-growing minus end. In fibroblastic cells, the great majority of MT
minus ends is found at the MTOC near the nucleus, whereas
the plus ends extend to the cell periphery. In polarized
epithelial cells lacking a classical MTOC, MTs are generally
arranged parallel to the apical-basal axis, with the minus
ends at the apex of the cell. MT-associated proteins, so-called
MAPs, have been found to promote tubulin polymerization
and to stabilize MTs. MTs can mediate cellular motility by
two different mechanisms: dynamic assembly and disassembly reactions, which allow pushing or retraction of cargo, or
by serving as a track for bidirectional transport catalyzed by
tubulin-activated ATPases. These ATPases are classified into
two families: the plus end-directed kinesins and the minus
end-directed dyneins (6). Both motors are multiprotein complexes with several cargo- and MT-binding domains.
Viral capsids have been observed in close association with
MTs shortly after infection of fibroblastic cells. In the case of
herpes simplex virus type 1 (HSV-1) and adenovirus type 2
(Ad2), the associations with MTs are most probably functional
because pharmacological depolymerization of the MTs abolishes intracellular virus transport but does not affect virus
entry into the cytosol (12, 13). In the case of Ad2, entry into a
susceptible host cell occurs by virus attachment to a highaffinity surface receptor, termed Coxsackie virus-adenovirus
receptor (CAR; for review, see Ref. 5 and Fig. 1). The virus is
taken up by integrin-dependent endocytosis, escapes from
early endosomes by acid-assisted disruption, and then associates with MTs. Examples of Ad2 associations with MTs in
cultured cells are shown in Figs. 2 and 3. At 30–40 min
postinfection, fluorophore-labeled virus particles colocalized
in >95% of the cases with MTs and were often found at the
MTOC, as shown by confocal microscopy (Fig. 2; see also references in Ref. 13). At higher resolution using electron
microscopy, adenoviruses are found closely associated with
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FIGURE 1. Organelles and molecules involved in adenovirus entry. Adenovirus gains entry into nonpolarized cells by binding to a high-affinity fiber receptor,
termed Coxsackie virus-adenovirus receptor (CAR). Virus then interacts with secondary surface receptors, integrins, and endocytoses via clathrin-coated pits
(reviewed in Ref. 4). In cytosol (~15 min postinfection), it interacts with microtubule (MT)-associated motor proteins and is transported either to MT plus end or
MT minus end. Minus end-directed motor is dynein/dynactin, whereas plus end-directed motor is presently unknown. Dynein/dynactin is required for virus transport to nuclear pore complexes (NPCs), where adenovirus type 2 (Ad2) binds and releases its DNA genome for import into nucleoplasm. This import reaction
starts at ~60 min postinfection and requires O-linked N-acetylglucosamine (GlcNAc)-modified glycoproteins of NPC and nuclear envelope calcium. After
120–180 min postinfection, viral DNA is detected in nucleoplasm.
motility at 30 min or later after infection and frequently
switches from one direction to the other (13). In contrast, two
unrelated endosomal viruses, reovirus and canine parvo virus,
have been reported to be transported towards the MTOC in
endosomes in a MT-dependent manner (4, 14). The underlying
mechanisms for the differential behaviors of these viruses are
not known. It is, however, possible that MTs and associated proteins (MAPs) themselves can influence the prevalent direction
of trafficking since a cell line stably transfected with the MTbinding domain of MAP4 (linked to green fluorescence protein)
supported minus end-directed transport of both cytosolic and
endosomal adenovirus particles more efficiently than plus enddirected transport compared with the nontransfected mother
cell line (13). Whether this result is due to increased MT stability in the MAP4-overexpressing cells is presently unknown.
Actin-based motility
Although dynamic MTs have been implicated in vesicular
trafficking, there is presently no solid evidence for a role of
MT polymerization and depolymerization in viral transport.
In contrast, dynamic modulations of the actin cytoskeleton
are known to be involved in both viral and cellular motilities.
Unlike MTs, actin filaments generally do not originate at a
common organizing center. At steady state, the globular
monomeric form of actin (G actin) is in dynamic equilibrium
with filamentous actin (F actin). Polymerization and depolymerization of monomers occurs at both ends of the filament,
albeit at different rates, thereby producing a polarized actin
FIGURE 2. Confocal laser scanning micrograph of Texas red-labeled Ad2 on
MTs in a PtK2 (Potorous tridactylis) kidney cell 40 min postinfection. A single
optical section of virus particles from Texas red channel was contrast
enhanced and inverted to show virions as black dots. Image processing was
performed on a Macintosh computer using public domain NIH Image program (developed at U. S. National Institutes of Health and available on Internet at http://rsb.info.nih.gov/nih-image/). Corresponding optical section of
immunostained MTs in FITC channel was contrast enhanced and then
processed using bas relief filter function of NIH Image. This makes MTs appear
as three-dimensional structures. These two images were then merged as layers
in Adobe Photoshop 5.0. Arrow points to juxtanuclear MT-organizing center.
For further details, see Ref. 12.
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MTs, often only separated by a distance of 20–50 nm from a
nearby MT (Fig. 3). Immunoelectron microscopy experiments
of HSV-1-infected Vero cells have identified the dynein motor
protein near incoming viral capsids in the vicinity of MTs (12).
The dynein motor is part of a large multiprotein complex,
called dynein-dynactin, which represents the major minus
end-directed machinery of interphase cells. Like Ad2, incoming HSV-1 capsids have been found in close proximity to the
MTOC of control cells but not to cells treated with the
MT-depolymerizing agent nocodazole, suggesting that dynein
is involved in minus end-directed transport of cytosolic
HSV-1 capsids. Similar results have been obtained with a
retrovirus, human foamy virus (HFV), which transports its
reverse-transcribed DNA genome into the nucleus independent of nuclear envelope breakdown (10). Functional involvement of the dynein motor has recently been demonstrated for
minus end-directed transport of Ad2 in live cells (13). In this
study, a dynactin component, called dynamitin, has been
overexpressed in target cells to disrupt the dynein/dynactin
complex. Earlier work by other groups had demonstrated that
these conditions inhibited most of the minus end-directed cellular trafficking in interphase cells. Infection experiments using
fluorophore-tagged Ad2 particles showed that these conditions also severely inhibited minus end-directed transport of
Ad2 (13). Time-lapse microscopy, together with electron microscopy and confocal laser scanning microscopy experiments in
control cells, has revealed that single cytosolic Ad2 particles
move to either the minus or the plus end of MTs in quasilinear directions at peak speeds of 2–3 µm/s, indicative of
dynein- and kinesin-type motors. These bipolar motilities
strictly depended on intact MTs and did not require highly
dynamic MTs, implying that the MTs serve as tracks to mediate motor-driven virus transport rather than inducing viral
movements by polymerization and depolymerization events
at MT ends. Depending on the cell line, a minus end-directed
population speed of 1–10 µm/min was determined. Surprisingly, in cells lacking functional dynein/dynactin, the frequency of plus end-directed Ad2 transport was significantly
increased and, together with the reduction of the minus enddirected transport, resulted in a net virus transport toward the
cell periphery. These data imply that Ad2 engages with both
types of MT-dependent motors, the minus end-directed
dynein/dynactin complex and an unknown plus end-directed
activity. Although the purpose of the plus end-directed movement is presently unknown, it could be required for virus exit
from an infected cell, as suggested for Gag proteins of murine
leukemia virus (MuLV), a retrovirus replicating in the cell
nucleus (8). MuLV Gag proteins have been shown to interact
with a plus end-directed kinesin molecule implicated in trafficking from the perinuclear MTOC to the plasma membrane.
Alternatively, plus end-directed motility may be needed for
entry into polarized cells, which appear to be a target for adenoviruses in vivo.
How directional trafficking of viral or cellular structures is
regulated by the cell is presently unknown. It is interesting to
note, however, that in contrast to wild-type Ad2, a mutant Ad2
particle (ts-1), which is unable to escape from endosomes, has
no clear preference for either plus end- or minus end-directed
filament. Filament polarity can also be determined on isolated actin in the electron microscope after decoration with
proteolytic myosin fragments, which results in the formation
of a typical arrowhead structure. The pointed end of this
structure corresponds to the slow-growing end (or minus end)
and the barbed end to the fast-growing end (or plus end). The
cellular functions of actin are to a large extent regulated by
actin-associated proteins. Monomer sequestering proteins
can modulate the amount of available G actin for polymerization. F actin severing or stabilizing proteins either create
or maintain minus and plus ends, or capping proteins block
filament ends, thereby preventing subunit addition or loss.
Other actin-binding proteins organize filaments into bundles,
which can span large areas of the cell (stress fibers), or into
networks predominantly found in the cell cortex. A wellknown group of F actin-associated proteins is the myosin
family of motor proteins. Myosins (with one possible exception) are plus end-directed motors characterized by a similar
actin-binding motor domain. They are implicated in (among
other actions) vesicular transport, generation of tension, and
receptor translocation in the plane of the plasma membrane.
A major question in cell motility is how actin establishes
and modulates the shape of cells and gives rise to cell protrusions, like microvilli, filopodia, and lamellipodia. How
does actin generate vectorial forces required for cell movements? Such forces can, in principle, arise when G actin is
recruited to the plasma membrane, incorporated into the plus
end of an actin filament, and thus extends a filament further
toward the periphery. The use of bacterial and viral model
systems has, rather unexpectedly, shed light on the underlying
mechanisms. Microorganisms, such as Listeria monocytogenes, Shigella flexneri, Rickettsia, and vaccinia virus use cellular actin to propel themselves through an infected host cell
(3, 7). Cellular components, such as vasodilatator-stimulated
phosphoprotein (VASP), the multiprotein actin-related multiprotein (Arp)2/3 complex, and cofilin, have been suggested to
be involved in bacterial actin-based motility. It has been sug70
News Physiol. Sci. • Volume 15 • April 2000
gested that vaccinia virus spreads by mimicking the signaling
pathways that are involved in actin polymerization at the
plasma membrane (3).
One of the most spectacular interactions of virions with
host cell actin has been detected with vaccinia virus, a large
enveloped DNA virus of the Poxviridae family replicating in
the cytoplasm (2, 9). During virion morphogenesis in fibroblastic cells, intracellular mature virus (IMV) is wrapped by
additional cellular membranes that contain viral glycoproteins to form the intracellular enveloped virus (IEV). Some of
these viral envelope proteins then nucleate the formation of
actin tails, which propel IEV particles with a trailing tail of
F actin through the cytoplasm and occasionally to the cell surface at velocities of several micrometers per minute. Once the
virus has reached the plasma membrane, actin polymerization persists and virus particles are found at the tip of large
microvilli-like protrusions. These protrusions can be engulfed
by neighboring cells and thus support virus dissemination.
Immunolocalization experiments revealed VASP, profilin, and
the actin-crosslinking proteins α-actinin, filamin, and fimbrin
in the actin-rich trails of motile virions, suggesting that they
might have a role in this actin polymerization-driven motility.
Interestingly, vaccinia virus has also been found to promote cell movement, as shown in so-called “wound healing”
experiments (11). In contrast to the motility of intracellular
virus particles, induction of cell migration did not require
expression of late vaccinia genes but depended solely on
early genes. Expression of late genes in the absence of particle production, on the other hand, induced the formation of
growth cone-like lamellipodia-derived projections almost as
long as the cell body. The data suggest that poxviruses can be
a valid model system to study cell movement.
Conclusions
Two intracellular transport systems, the MTs and the actin
cytoskeleton, are exploited by viral pathogens in quite different
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FIGURE 3. Electron micrograph of incoming Ad2 near a MT filament. TC7 cells (African green monkey kidney cells) grown on a glass coverslip were infected with
Ad2 for 40 min, washed briefly with PBS in absence of divalent cations, and then fixed in 2% glutaraldehyde containing 0.2% tannic acid, embedded in Epon,
and processed for thin-section electron microscopy as described (12).
Outlook
In the past several years, numerous studies of virus-host
interactions were initiated to understand the regulation of
cellular and viral motilities. Prospects for this work on virus
interactions with the cytoskeleton are promising, since virus
particles represent a highly versatile tool that can be manipulated experimentally in many ways before an infection is
initiated. Viruses are not only the most efficient gene-delivery
vehicles known today but, owing to their relatively simple
design, they also offer yet another elegant approach toward
understanding complex cellular processes, such as regulatory
circuits designed to control intracellular trafficking of large
complexes and cell motility. One goal for immediate future
investigations will be to elucidate virus-cytoskeleton interactions in the predominant natural target cells, namely polarized epithelial cells or nerve cells, which both have a particular MT and actin architecture distinct from fibroblastic cells.
From a medical point of view, it is reasonable to propose that
any successful synthetic or semisynthetic gene transfer system will have to incorporate elements facilitating intracellular motility to ensure high efficiency of gene expression.
We are grateful to Karin Boucke (University of Zürich, Switzerland) for Fig.
3, Michael Way (EMBL, Heidelberg, Germany) for valuable discussions, and
Beate Sodeik (Hannover, Germany) for suggestions about sample preparation
for electron microscopy. Work from the Greber laboratory was supported by
the Swiss National Science Foundation and the Kanton of Zürich.
References
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2. Cudmore, S., I. Reckmann, and M. Way. Viral manipulations of the actin
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Superti-Furga, and M. Way. Actin-based motility of vaccinia virus mimics
receptor kinase signaling. Nature 401: 926–929, 2000.
4. Georgi, A., C. Mottola-Hartshorn, A. Warner, B. Fields, and L. B. Chen.
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Wiley & Sons, 1998, p. 89–114.
6. Hirokawa, N. Kinesin and dynein superfamily proteins and the mechanism of organelle transport. Science 279: 519–526, 1998.
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8. Kim, W., Y. Tang, Y. Okada, T. A. Torrey, S. K. Chattopadhyay, M. Pfleiderer,
F. G. Falkner, F. Dorner, W. Choi, N. Hirokawa, and H. C. Morse III. Binding of murine leukemia virus Gag polyproteins to KIF4, a microtubulebased motor protein. J. Virol. 72: 6898–6901, 1998.
9. Roper, R. L., E. J. Wolffe, A. Weisberg, and B. Moss. The envelope protein
encoded by the A33R gene is required for formation of actin-containing
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4192–4204, 1998.
10. Saib, A., F. Puvion-Dutilleul, M. Schmid, J. Peries, and H. de The. Nuclear
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J. Virol. 72: 1235–1243, 1998.
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U. F. Greber. Microtubule-dependent minus and plus end-directed
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ways. Certain viruses, such as adenovirus and herpes virus,
bind MT-associated motor proteins and use MTs as tracks
for long-distance vectorial translocation within the host
cell. In these cases, little is known about the modulations of
directionality, the nature and regulation of virus/motor protein interactions, and possible linker proteins between
motor and cargo. Other viruses, like vaccinia (and most
likely also baculovirus) have acquired the ability to use
actin dynamics by promoting actin polymerization onto
their coat and thus creating a jet-like propulsion that allows
an apparently randomly directed vectorial migration in
the host cytoplasm. Considering the versatility of viruses, it
would not be surprising to find evidence for the utilization
of both types of motilities by one and the same virus type,
thus considerably expanding this virus’ repertoire of motility. Possibly, we will in the future also find viruses that are
using myosins to translocate on actin filaments or that utilize MT polymerization and depolymerization to achieve
intracellular transport.