The document provides an overview of cytoskeletal proteins and their functions, as well as how bacterial pathogens interact with and manipulate the host cytoskeleton during infection. It discusses the major cytoskeletal components microfilaments and microtubules. It also reviews how bacteria can alter host actin and microtubule dynamics through various mechanisms to facilitate infection of both phagocytic and non-phagocytic cells. The complex pathways regulating cytoskeletal dynamics are targeted by pathogenic bacteria through different strategies to subvert the host cytoskeleton for their own benefit during the infection process.

1. Interaction of bacterial pathogens with
host cytoskeletal proteins during the
infection process
Molecular and Cellular Life Sciences
MSc thesis
By: Pier Paolo Posata
Supervision: Anna Akhmanova

2. Table of contents
Summary.....................................................................................................................................3
Cytoskeletal proteins and their functions....................................................................................3
Microfilaments ........................................................................................................................4
Vinculin and talin....................................................................................................................5
Arp2/3 complex and formins...................................................................................................5
Upstream regulators of actin filaments ...................................................................................5
Microtubules............................................................................................................................8
Cytoskeletal function in the immune system..............................................................................8
Bacterial infection in eukaryotic cells.........................................................................................9
Non-phagocytic cells...............................................................................................................9
Phagocytic cells.....................................................................................................................10
Bacterial systems controlling cytoskeletal dynamic.................................................................10
Bacterial manipulation of the host actin dynamics...................................................................11
Enterobacteria........................................................................................................................12
Other Gram-negative bacteria ...............................................................................................14
Gram-positive bacteria ..........................................................................................................15
Bacterial manipulation of host microtubule dynamics .............................................................16
Conclusions...............................................................................................................................16
References.................................................................................................................................18
2

3. Summary
The cytoskeleton is an essential cellular structure, which controls numerous subcellular
events. During the evolution process, bacteria have developed several strategies to interact
with cytoskeletal proteins and to manipulate their function to facilitate the infection. This
review provides an overview of the mechanisms that best-studied bacterial pathogens exploit
to alter the host cytoskeleton function.
Cytoskeletal proteins and their functions
The cytoskeleton is a network of filaments found in cell`s cytoplasm of the all three domains
of life: Archea, Bacteria and Eukaryota, and it is considered the skeleton of the cell. In
eukaryotic cells, three major types of filaments form the cytoskeleton: microfilaments (actin
filaments) and microtubules (tubulin filaments), which both are the best-known components,
and intermediate filaments. Prokaryotes have a similar structure composed by proteins
evolutionarily related to actin and tubulin (van den Ent, 2001). This filamentous structure
forms a matrix that supports the cell in numerous functions throughout the cell (Fig. 1). The
network of factors that makes up and regulates the cytoskeleton is composed by a variety of
conserved proteins which differ in function and structure depending on the organism, cell and
filament type. Cytoskeletal proteins are involved in structural support, cell shape,
internalization of extracellular materials, transport of substances in the cell, organelle
positioning, cell movement and cell division. Moreover, they form cell protrusions like cilia,
flagella, lamellipodia and podosomes which are essential for cell motility in eukaryotes and
prokaryotes (Becker, 2008; Wickstead, 2011).
Figure 1| Staining of microfilaments and microtubules. The two images show the positioning and
extension of microfilaments (left) and microtubules (right) throughout the cell: on the left F-actin is stained
with phalloidin (red), on the right tubulin is immunofluorescently stained for anti-alpha tubulin (green).
Microfilaments are located mainly along the cell membrane unlike microtubules, which extend entirely
throughout the cytoplasm connecting the cell membrane to the organelles and the nucleus.
3

4. Microfilaments
The single globular polymer of actin (G-actin) is the most abundant protein in eukaryotic and
it binds ATP leading to a linear polymer (F-actin) which forms the microfilament. The
microfilaments have a polarity which has been determined by the faster growth towards the
plus end of the filament. Towards the minus end of the filament, ATP-bound actin hydrolyzes
in ADP-bound actin which dissociates more rapidly from the filament than ATP-bound actin
at the plus end. This determines more stability at the plus end (Campellone, 2010) (Fig. 2).
Although this process is regulated by ATP hydrolysis, many other factors are involved: actin-
binding proteins, phospholipids, growth factors and regulatory proteins all constitute a big
network controlling actin organization in the cell. Some important actin-binding proteins are
described below. Most of the motility processes, like vesicle transport and cell contraction,
are regulated by myosin: the actin-dependent motor protein which proceeds along the
filaments. Myosin is composed of a head domain, a neck domain and a tail domain: the head
domain binds actin and
hydrolyzes ATP to generate a
force that promotes the
binding on the next actin
protein of the filament, the
neck domain is a linker and
the tail domain interacts with
cargos (Becker, 2008).
The capping protein (CapZ)
binds the ATP-bound actin on
the plus end and it stabilizes
the filaments, by arresting the
polymerization and the
depolymerization process.
Gelsolin is the most potent
actin-severing protein and it is
stimulated by calcium ions
which promote the gelsolin-
actin interaction. Profilin is
able to transfer G-actin
monomer to the filament
during the nucleation process:
this protein sequesters and
converts ADP-bound actin
into ATP-bound actin to
enhance the concentration of readily polymerizing G-actin. ADF/cofilin family has opposite
properties: it causes depolymerization of microfilaments because it is able to sever the minus
end of the filament (Fig. 2); in addition, its phosphorylation controls cytoskeletal dynamics
like membrane ruffling (Winder, 2005; Raghunathan, 1992; Arber, 1998; Sun, 1999).
Figure 2| Polymerization and depolymerization of the actin
filament (Nürnberg, 2011). G-actin binds ATP to form a nucleus
which grows into a long filament (F-actin). Along the filament, ATP-
bound actin is hydrolyzed in ADP-bound actin towards the minus
end which is less stable than the plus end of the filament. Therefore,
the filament polymerizes at the plus and it depolymerizes at the
minus end. Other proteins participate to this process: the capping
protein which blocks polymerization at the plus end, the ADF/cofilin
family which severs the filament at the minus end, and the profilin
protein which binds ADP-actin and enhances nucleotide exchange.
4

5. Vinculin and talin
Vinculin and talin are other important proteins of the actin filament which are involved in
focal adhensions through integrin binding. Integrins are trasmembrane receptors which can be
a connection either between the cell and the extracellular matrix or between lymphocytes to
other cells. These two proteins interact to each other and they anchor F-actin to the cell
membrane. As they are located to the proximity of the membrane, they interact also with
phosphatidylinositol 4,5-bisphosphate (PIP2) on the internal surface of the cell membrane
and this may regulates membrane rearrangement during pathogen internalization. Moreover,
they regulate both receptor clustering and signal trasduction, therefore they are the
connection between cytoskeleton and pathways regulated by extracellular factors
(Humphries, 2007; Yin, 2003).
Arp2/3 complex and formins
In addition to these proteins, there are two groups of proteins which function as a universal
actin-nucleating machine and they regulate F-actin shape. These are the Arp2/3 complex and
the formin group of proteins. Arp2/3 complex is composed by seven subunits, which are
conserved among eukaryotes, and it has two main functions. The first one is to start
polymerizing actin upon activation by upstream signal; the second is to cross-link actin
filaments enabling the formation of Y-braches with a branch angle of 70°. In this way,
Arp2/3 complex controls actin shape promoting an actin patch pattern. This complex seems
essential for proper cellular functions that involve actin filaments, like the formation of
phagocytic cups and lamellipodia (Welch, 2002). Differently from the Arp2/3 complex,
formins assemble F-actin in a more linear way, like a cable. These proteins interact with
signaling molecules and they are involved in cell polarity and cell migration function
promoting actin polymerization from the plus end (Evangelista, 2003). Both Arp2/3 complex
and formins are regulated by other factors which may determine actin shape in relation to the
cell function.
Upstream regulators of actin filaments
Most of the actin-binding protein discussed so far are regulated by nucleation-promoting
factors (NPFs) which determine actin structure and function. NPFs of class I include five
groups of proteins: Wiskott-Aldrich Syndrome protein (WASP) and neuronal-enriched
homologue of WASP (N-WASP), WASP and Scar homologue (WASH), WASP family
Verprolin-homologous (WAVE), WASP homologue associated with actin, membranes and
microtubules (WHAMM) and junction-mediating regulatory protein (JMY). All these NPFs
of class I bind the Arp2/3 complex at the C-terminus, initiating actin nucleation and driving
F-actin branching. They are involved in several cell functions: for example, they play a role
in host cell membrane defense mechanisms against bacterial pathogens like membrane
ruffling, membrane invagination, phagocytosis, receptor-mediated endocytosis and vesicle
formation (Rottner, 2010; Campellone, 2010). WASPs and N-WASPs represent a convergent
point of different signaling pathways to control actin dynamics, and their activity may also be
regulated by post-translational modifications like phosphorylation (Stradal, 2004). Moreover,
5

6. it has been reported that WASP is important for an adequate function of the immune system
and the dysfunction of this protein leads to abnormal morphology of cells, phagocytic and
chemotactic defect (Thrasher, 2002). WASH plays a role in lamellipodia and endosomes: it is
able to associate with other six proteins, which interact with CapZ and stabilize actin
filaments (Rottner, 2010). In addition, the ubiquitination of WASH through the E3 RING
ubiquitin ligase regulates endosomal protein trafficking supporting F-actin nucleation (Hao,
2013). WAVE and WHAMM also form a complex analogous to WASH. WAVE
accumulation at the membrane protrusion is necessary for Arp2/3 complex-dependent actin
assembly. WHAMM is associated with microtubules, and its activity can be controlled by
post-translational modifications, oligomerization, or membrane binding. These findings
might unveil new pathways regarding actin-microtubule interactions. Differently from other
NPFs, JMY has a dual function because it is involved in both Arp2/3-dependent nucleation
and Arp2/3-independent nucleation. Moreover, it has been implicated in p53 response as well
as cell motility. Therefore, it has opened the possibility that very different cellular processes,
like apoptosis or cell cycle regulation, might be correlated to actin assembly for motility
processes (Rottner, 2010; Campellone, 2010). In addition to the NPFs of class I, there are
NPFs of class II, like cortactin which is involved in actin-branch formation and stabilization.
This monomeric protein also activates the Arp2/3 complex and it undergoes post-translational
modifications in response to extracellular signals like bacterial invasion or growth factors.
Cortactin is involved in endocytic traffic, formation of intercellular junctions, it is very active
in lamellipodia during chemotaxis, and it inhibits contractility at the podosomes (Cosen-
Binker, 2006). The network of NPF interactions is very complicated and many aspects are
still unclear, however they are very important to connect several cellular functions to the
Arp2/3-dependent actin nucleation.
Upstream of NPFs, cytoskeletal dynamics are regulated by three GTP-binding proteins
belonging to the Ras superfamily and to the Rho GTPase family: Rac, Rho and Cdc42. Rho
GTPases are G proteins which transmit extracellular signals inside the cell, and they are
directly connected to membrane receptors. These proteins are involved in important cell
functions like cellular shape maintenance, phagocytosis, cytokinesis and intracellular
transport (Nobes, 1995; Allen, 1997). Rac is responsible for regulating the actin meshwork
formation below the cell membrane to generate lamellipodia and membrane ruffles; in
addition, Rac is essential for the regulation of the NADPH oxidase in phagocytes which is
very important for bacterial killing (Ridley, 1995). Rho activation initiates the development
of stress fibers creating contractile actin-myosin filaments, and Cdc42 leads to the formation
of actin-rich surface protrusions called filopodia. These three Rho GTPases regulate each
other`s activity: Cdc42 activates Rac (revealing that filopodia are associated with
lamillipodia), and Rac activates Rho (Hall, 1998); furthermore, Rho GTPases are able to
down-regulate each other activity, as described in Alberts textbook (Alberts, 2014). Rho
GTPases bind and hydrolyze GTP to GDP through a process in which three different
regulators are involved: guanine nucleotide exchange factors (GEFs), GTPase-activating
proteins (GAPs) and guanine nucleotide dissociation inhibitors (GDIs). When Rho GTPases
bind GDP, GEFs are activated, and subsequently they stimulate GDP release allowing GTP
binding. GTP induces conformational changes which lead to effector protein binding and
downstream signal activation. On the other hand, the signaling pathway is switched off by
6

7. GAPs which hydrolyze GTP to GDP and inactivate Rho GTPases. GDIs are cytosolic
proteins which have the property to form a complex with inactive Rho GTPases. In this way,
they block GEFs activity preventing GTP binding to Rho GTPases. GDIs phosphorylation
induces complex disassembly and cycle of Rho GTPases from the cytosol to the plasma
membrane, where Rho GTPases are activated by GEFs (Fig.3). Upon Rho GTPaes activation,
effector proteins are fundamental to transmit the signal and lead to cytoskeletal
rearrangements. Best-studied effector proteins are p21-activated kinases (PAKs) and Rho-
associated coiled-coil-containing protein kinases (ROCK-I and ROCK-II). PAKs bind Rac1
and Cdc42, ROCK binds Rho (Ellenbroek, 2007). Most of effector proteins are kinases,
however N-WASP is considered an effector protein that binds directly Cdc42 and it regulates
actin dynamics without having a kinase activity (Rohatgi, 1999).
Pathways that regulate actin dynamics are very complex and still far from being completely
understood. The study of bacterial infection might help to elucidate them. During the
infection process bacteria alter host cytoskeletal dynamics, and they might have developed
strategies to target each of these proteins. The figure below represents a schematic depiction
of the actin-regulatory pathway (Fig. 3).
Plasma membrane
Extracellular space
Cytoplasm
GTP
GDP
Rho GTPase
GEF GAP
GDI
Receptor
Ligand
GTP
Effector
Rho GTPase
NPF
Arp2/3
complex
Formins
Figure 3| A schematic representation of the actin-regulatory pathway. An extracellular stimulus activates the
Rho GTase close to the plasma membrane. The Rho GTPase is regulated by three proteins: GEF, GAP and GDI.
GEF promotes GTP binding and leads to the interaction with an effector which triggers the downstream pathway.
Many aspects are still unknown about this pathway, however NPF activation induces cytoskeletal rearrangements
by interacting with actin-binding proteins. Different NPFs interact with either Arp2/3 complex leading to Y-
braches of the actin filaments, or formins leading to a linear structure.
Actin filaments
Downstream
pathways
GEF
7

8. Microtubules
Microtubules are the biggest filaments of the cytoskeleton and they are involved in several
functions like cell structure maintenance, cell division, formation of cilia or flagella and
intracellular transport. They are composed of dimers of α- and β-tubulin which polymerize at
both microtubule ends with different velocity. The plus end, where β-tubulin is exposed,
shows in vitro a faster tendency to extend the filament compared to the minus end, where α-
tubulin is exposed. Both parts of the tubulin dimer bind GTP, which can only be hydrolyzed
when it is bound to β-tubulin (Desai, 1997). It seems that the docking of β-tubulin with α-
tubulin promotes GTP hydrolysis which drives microtubule dynamics, however the
mechanisms of microtubule polymerization and depolymerization are not clear yet (Howard,
2003). Heterodimer polymerization creates protofilaments which expand throughout the
cytoplasm forming a hollow tube. There are two microtubule-dependent motor proteins,
kinesin and dynein, which transport vesicles and organelles. They both use ATP hydrolysis
for mechanical energy: kinesin to move towards the plus end (anterograde transport) and
dynein to move towards the minus end (retrograde transport). The plus end is usually oriented
towards the periphery of the cell and the minus end towards the center of the cell (Becker,
2008). In addition to motor proteins, tubulin interacts with several proteins called microtubule
associated proteins (MAPs) which control microtubule organization. MAPs can be regulated
by the microtubule-affinity-regulating-kinase (MARK) protein through phosphorylation.
MAP1 is found in axons and neuronal dendrites, and it binds different part of the microtubule
or the plasma membrane, regulating spacing of the microtubule structure (Mandelkow, 1995;
Wickstead, 2011). MAP2, MAP4 and tau stabilize microtubule lattice. MAP2 and tau are
mainly found in neurons and tau may also regulate microtubule bundling (Dehmelt, 2004).
MAP4 is found in many tissues and regulates mitotic microtubule dynamics (Ookata, 1995).
Furthermore, there are many novel MAPs which have been identified: for example, katanin
and spastin are microtubule-severing proteins which require ATP, XMAP215 promotes
microtubule polymerization, STOP binds the calcium-binding messenger calmodulin and it is
also required for microtubule stabilization (Mandelkow, 1995). All these proteins and many
more are essential for cell lifespan and cell function, therefore they can be targeted by
bacteria to enhance the infection process.
Cytoskeletal function in the immune system
Many bacteria like Salmonella interact with the cytoskeleton of non-phagocytic cell, like
epithelial cells, in order to enhance internalization, penetrate tissues and escape from immune
system (Galán, 1999). However, many bacteria need also to impair the immune system
function for a successful infection, therefore they need to manipulate the cytoskeleton of
immune system cells. Numerous studies have shown the importance of cytoskeleton in
different white blood cells like T cells, B cells, NK cells, macrophages and neutrophils
(Burkhardt, 2013). Firstly, microfilament network proteins are involved in the activation of
lymphocytes: actin remodeling is important to stimulate B-cell receptor and T-cell receptor
with their downstream signaling (Song, 2013; Matalon, 2013). In addition, the interface
between antigen-presenting cell (APC) and the T-cell, called immunological synapse, is
mediated by cross-talk between actin and microtubules (Soares, 2013). Secondly, actin
8

9. rearrangements are essential for NK cells because they control several functions like immune
integrin-mediated signaling and transport of lytic granules and cytokines (Lagrue, 2013).
Besides lymphocytes, actin pathways are used by macrophages and neutrophils for
phagocytosis and chemotaxis. Several NPFs like WASP or WAVE and GTP-binding
proteins are recruited beneath the particle detected by the receptor during phagocytosis. F-
actin is assembled and generates a phagocytic cup that extends around the particle and grows
into a pseudopod which wraps and internalizes the particle (Rougerie, 2013). In addition,
actin plays a role in chemotaxis: Rho GTPases and myosin regulators contribute to formation
of F-actin protrusions towards the site of infection (Fukata, 2003; Li, 2010). As it is shown
that cytoskeleton plays important function in the immune system, in this review we will
discuss bacterial cytoskeletal interaction with both epithelial and immune system cells.
Bacterial infection in eukaryotic cells
Many types of bacteria modulate host cytoskeletal dynamics to enhance their fitness inside
the human or animal host. This relies on different bacterial mechanisms which differ
depending on phagocytic or non-phagocytic host cells. In the target cell, new structures are
formed and other structures are demolished to enhance the infection process.
Non-phagocytic cells
Epithelial cells do not have the ability to engulf large particles (non-phagocytic cells),
however these types of cells play important roles in protection against pathogens; they make
up a barrier that divides the host`s interior from the external environment, they secrete
mucous and enzymes, and they initiate and regulate the mucosal inflammatory and immune
response (Kagnoff, 1997). Epithelial cells form different structures which involve the
cytoskeleton and could be targeted by bacteria. Tight junctions are a complex of proteins that
create sealing strands composed mainly by claudins and occludins which may interact with
actin filaments. This complex promotes formation of a properly sealed epithelial cell layer.
Other structures involved in epithelial cell adhesion are desmosomes and gap junctions: the
former creates a bridge between two epithelial cells exploiting transmembrane receptors
(cadherins) and anchoring intermediate filaments, the latter forms a connection between
epithelial cells that allows the passage of molecules and the transmission of signals. Cilia are
another important structure of epithelial cells. These organelles contain microtubules; they
form long protuberances which sense chemical, thermal or mechanical conditions of the
extracellular environment and can induce movement of fluid along epithelial surface (Becker,
2008).
Bacterial interactions with epithelial cells mainly have the purpose to promote penetration
inside the cell. This is necessary in some bacteria which can only replicate within the host
cell (obligate intracellular pathogens), and it is preferred in some others which create an
opportune niche in the host. Firstly, bacteria need to invade epithelial cells of the mucosal
surface that are the first site of interaction with the host. This may occur at the
gastrointestinal, genitourinary, and respiratory tracts which are the main sites of bacterial
infection in human. Bacteria have developed two different strategies to induce engulfment
into epithelial cells: a “zipper” and a “trigger” type mechanism. The former exploits direct
9

10. binding between the host outer-membrane and the bacterial ligand to induce filopodia
formation around the bacteria. The latter induces membrane ruffling and cytoskeletal
rearrangements in the host cell through molecules secreted by bacteria. Once internalized,
bacteria may prevent the fusion of their vacuole with the host lysosome and survive inside the
vacuole, or lyse it and live in the cytosol. Intracellularly, bacteria induce cytoskeletal
rearrangements which may lead to assembly and disassembly of cellular structures. They
could promote the formation of protrusion towards the adjacent cell, they could regulate actin
pathway to create pedestals that promote internalization of more bacteria, they could target
cytoskeletal protein involved in cell junction causing their disassembly, and they could
exploit actin protein machinery to move within the cytosol (Finlay, 1997). All these changes
lead to bacterial dispersion within tissues.
Phagocytic cells
In addition to epithelial cells, bacteria may also manipulate cytoskeletal dynamics of
phagocytic cells, like macrophages and neutrophils, to prevent killing. In these cells, bacteria
exploit proteins which have an opposite function compared to epithelial cells: instead of
promoting actin polymerization for enhancing internalization, they promote actin disassembly
to prevent phagocytosis. Firstly, some bacteria can escape from phagocytosis exploiting
proteins which target actin-binding proteins involved in the formation of phagocytic cup. In
this way, phagocytes are not able to form a large actin meshwork capable of encircling the
bacteria. Secondly, bacteria can also prevent chemotaxis inhibiting filopodia formation
towards the site of infection (Li, 2010). Moreover, the hyper-activation of actin pathways
might lead to the induction of apoptosis in macrophages (Monack, 1996). If the bacterium is
not able to escape form phagocytosis, it might be able also to survive intracellularly.
Phagocytic cells have several antimicrobial peptides which are activated when the lysosome
fuse with the phagosome. Bacteria have developed several strategies to survive within
phagocytes that are still under investigation. Some bacteria prevent activation of lysosomal
hydrolytic enzymes by blocking the incorporation into the phagosome membrane of the
adenosine triphosphatase which is responsible of reducing vesicle pH that activates the
enzymes. Other bacteria might interfere with the anterograde vesicle export pathway altering
Rho GTPase activity; in this way, they could survive and shield within the vacuole. In
addition, it might be possible that bacteria might bind to surface receptors that do not target
the phagosome to become a lysosome (Sturgill-Koszycki, 1994; Finlay, 1997).
Bacterial systems controlling cytoskeletal dynamic.
Bacteria have developed several strategies to modulate host cytoskeletal proteins for their
advantage in phagocytic and non-phagocytic cells (Finlay, 2005; Zhou, 1999). All these
strategies rely on three different bacterial mechanisms: the type III secretion system (TTSS),
outer-membrane proteins and toxins (Fig. 4). TTSS is a type of injectosome that provides a
passage into the host cytoplasm. This machinery translocates effector proteins which interfere
with actin dynamic pathways and with microtubule stabilization (Bhavsar, 2007). TTSS is
assembled on bacterial membrane and a hollow tube, called the needle, penetrates the host
cell membrane. Subsequently, effector proteins and pore-forming factors are produced and
10

11. transported towards the bacterial membrane to punch it. Once the connection is formed,
effector proteins are injected inside the host cytoplasm to modulate actin dynamics. This
structure is very efficient and it is found only in Gram-negative bacteria which may use it to
transfer tens to hundreds effectors (Mota, 2005; Abe, 2005). A system that translocates an
effector directly inside the host cell has also been described for the Gram-positive
Streptococcus pyogenes and it might be the equivalent of TTSS (Madden, 2001). Secondly,
bacteria harbor outer-membrane proteins that interact with host cell receptors inducing
cytoskeletal rearrangements via signal transduction. Finally, bacteria may secrete toxins into
the environment which target the host cell, and subsequently are internalized to alter host
cytoskeletal protein network. These toxins are mainly involved in the modulation of the Rho
GTPase family (Boquet, 2003).
Bacterial manipulation of the host actin dynamics
Bacterial interactions with host cytoskeleton occur to promote bacterial entry into the host
cell and to facilitate the infection process. Many bacteria have been reported to subvert actin
dynamics using different strategies. On this review, we describe these strategies exploited by
the most studied bacteria.
Figure 4| The three strategies exploited by bacteria to induce cytoskeletal rearrangements. Different types
of effectors are injected inside the host cell through the type III secretion system (TTSS) which connects the
bacteriumand the host cell cytosol perforating the plasma membrane (1). Bacterial proteins on the membrane
surface interact with host receptors triggering signaling pathways which lead to cytoskeletal remodeling (2).
Bacteria release toxins into the environment to modulate cytoskeletal dynamics (3).
1 2 3
Plasma membrane
Extracellular
space
Cytoplasm
TTSS Receptor-receptor
interaction
Toxin-receptor
interaction
ACTIN/MICROTUBULE –
REGULATORY PATHWAY
Bacterium
11

12. Enterobacteria
Salmonella belongs to enterobacteria, it is a pathogen of man and animals, which translocates
through TTSS five effectors that interact with F-actin inducing membrane ruffling and
internalization into non-phagocytic cells (Patel, 2005) (Fig. 5). Firstly, Salmonella transports
into the host cell the SopE protein which is an efficient guanine nucleotide exchange factor
(GEF) essential for Rho GTPase regulation (Rudolph, 1999; Friebel 2001). Secondly, the
effector Sop B is responsible for increasing the activity of the inositol polyphosphatase
(Zhou, 2001), which is responsible of cytoskeletal rearrangements, although the mechanisms
remain unclear. Both Sop proteins induce activation of the Rho GTPases (Cdc42 and Rac)
and membrane ruffling (Patel, 2005). In addition, Salmonella translocates two actin-binding
proteins: SipC and SipA. SipC is a part of the injectosome TTSS (Scherer, 2000) and it is
capable of supporting actin nucleation and bundling (Hayward, 1999). SipA stabilizes F-actin
and inhibit cofilin and gelsolin-mediated actin depolymerization (McGhie, 2001). Moreover,
SipA increases the activity of the actin-binding protein T-plastin which supports actin in the
bundling process (Zhou, 1999). Sip proteins are very important because they regulate actin
nucleation independently of Rho GTPases and the Arp2/3 complex (Hayward, 2002). The
fifth effector called SptP has different functions because it supports the host cell to recovery
from Salmonella actin modifications. Once internalized, Salmonella does not need to alter F-
actin anymore, therefore the bacterium injects SptP that antagonizes SopE and SopB function
and blocks actin remodeling. In this way, the host cell does not undergo any further damages
and the cellular architecture is preserved (Patel, 2005; Stebbins, 2000). In this case, actin
remodeling is exploited by Salmonella, as well as other enterobacteria like Shigella, Yersinia
and E.coli, to create a phagocytic cup that internalizes the bacterium to promote infection.
Shigella is a human pathogen closely related to Salmonella which also translocates proteins
with similar functions for host actin regulation (Tran Van Nhieu, 2000). IpaC is translocated
first and it has the same function of Salmonella SipC (Blocker, 1999). It mediates Rho
GTPase activation together with VirA which may also lead to microtubule destabilization
through Rac pathway (Rottner, 2005, Yoshida, 2002). IpgD is another homologue of a
Salmonella effector (SopB), and it is involved in the dephosphorylation of PIP2. This event
leads to a reduced interaction between the plasma membrane and the actin filaments, and
Figure 5| Salmonella-induced
modulation of actin filaments (Pizarro-
Cerdá, 2006). Salmonella TTSS injects
five effectors which have the function to
facilitate the internalization of the bacteria
modulating actin dynamics. Sop E and
SopB regulate Rho GTPases through
insositol metabolism. SipA and SipC
promote actin bundling through direct
binding to F-actin. SptP is an antagonist of
SopE and SopB and it promotes actin
depolymerization.
12

13. subsequently promotes the extension of the filaments towards the formation of membrane
filopodia which enclose bacteria (Niebuhr, 2002). IpaA is similar to SptP function but
interacts with vinculin: the complex IpaA-vinculin binds F-actin inducing depolymerization
of the focal adhesion filaments (Bourdet-Sicard, 1999). Although the Salmonella and Shigella
upstream pathways are well studied, the mechanisms that regulate downstream actin
rearrangement, in order to create phagocytic cups from Rho GTPases regulation, are still not
well understood.
Differently from Salmonella and Shigella, Yersinia does not exploit the TTSS to interact with
F-actin for the first stage of infection. This bacterium has an outer-membrane invasin protein
which acts as a virulence factor interacting with integrin receptor. This interaction modulates
the actin cytoskeleton through Rho GTPases and enhances bacterial internalization into non-
phagocytic cells (Dersch, 1999). It is important to notice that Yersinia uses this mechanisms
only to penetrate epithelial layers. Once it is located in the extracellular space, it secretes
through TTSS several effectors which have anti-phagocytic properties (Pizarro- Cerdá, 2006).
Protein kinase A (YpkA) targets Rho GTPases inhibiting nucleotide exchange for Rac1 and
RhoA in vitro, inducing cytoskeleton disruption (Prehna, 2006). Yop effectors are involved in
dephosphorylation of protein complexes essential for actin assembly to form focal adhesions.
In this way, YopH blocks macrophages and neutrophil phagocytosis inducing paralysis of
microfilaments (Hamid, 1999; Grosdent, 2002). Moreover, Yop effectors control Rho
GTPases inhibiting their functions and reducing actin remodeling (Cornelis, 2002). The
expression of these effectors is related to the
highly pathogenic Yersinia species for
humans like Y. pestis, which exploits these
effectors to escape from the immune system
(Navarro, 2007).
As the other enterobacteria, pathogenic
strains of E.coli like enteropathogenic E.coli
(EPEC) and enterohemorrhagic E. coli
(EHEC) transfer effectors, which modulate
the host cytoskeleton. A very well studied
example of EPEC/EHEC effector is Tir.
This effector is delivered into the host cell
via TTSS where it anchors the host plasma
membrane and binds the bacterial outer-
membrane protein intimin for bacterial
docking. Subsequently, host kinases activate
Tir phosphorylation and the host protein
Nck. This host adaptor protein recruits
WASP and Arp2/3 complex to elongate actin
filaments and form a “pedestal” which may
evolve into filopodia to create a phagocytic
cup (Gruenheid, 2001; Kenny, 2002) (Fig.
6). In addition, pedestals can shift along the
Figure 6| E.coli modulation of actin filaments via
Tir effector (Pizarro-Cerdá, 2006). Tir is injected
inside the host cell to enhance bacterial adhesion to
host plasma membrane. Tir interacts with bacterial
outer-membrane protein intimin and it is
phosphorylated by Nck which recruits N-Wasp and
Arp 2/3. These two proteins induce actin
polymerization and pedestal formation.
13

14. membrane: this process involves other actin-binding proteins involved in actin
depolymerization like ADF/cofilin family and gelsolin (Shaner, 2005, Bhavsar, 2007), and it
might also require the binding to other receptors, as it has been reviewed for integrins
(Frankel, 1996). In addition to Tir, EPEC/EHEC secrete ~21 effectors and at least other four
are supposed to interact with cytoskeleton: EspB, EspF, EspH and EspG (Dean, 2009). These
effectors have several functions and their role in virulence has still to be elucidated. Some
studies described that EspF and EspB induce anti-phagocytic effect: EspF interacts mainly
with WASP (Cheng, 2008), and EspB blocks myosin-actin interactions (Lizumi, 2007). On
the other hand, EspH induce filopodia (Tu, 2003), and EspG seems to interact also with
tubulin, inducing microtubule disruption (Hardwidge, 2005). Furthermore, EPEC/EHEC may
modulate actin cytoskeleton also through virulence factors. Despite most bacteria exploit
toxins to inactivate Rho GTPases and block actin nucleation, the uropathogenic E. coli
(UPEC) releases CNF1 toxin to translocate into the host cell and regulate Rho GTPase
activation by controlling ubiquitin post-translational modifications (Pei, 2001; Doye, 2002).
In this way, CNF1 induce a transient activation of Rho GTPases but its function is still to be
clarified (Doye, 2002).
Other Gram-negative bacteria
In addition to enterobacteria, other Gram-negative bacteria have been shown to interact with
the host cytoskeleton. Pathogenic Neisseria species subvert actin dynamics through outer-
membrane proteins Opa and Opc (Merz, 1997). These proteins interact with host receptors
(integrins) inducing bacterial engulfment into epithelial cells (Merz, 2000). The mechanism
that promotes Neisseria internalization is different and unclear because does not exploit any
effectors and because the alteration of the downstream signaling cascade upon Opa/Opc
binding is still unknown.
On the other hand, Pseudomonas exploits TTSS to inject ExoT and ExoS effectors which
inhibits bacterial engulfment in epithelial cells and macrophages. They both block signal
transduction targeting GEFs, and therefore down-regulating Rho GTPases (Garrity-Ryan,
2000; Ganesan, 1999; Würtele, 2001). As RhoGTPases are involved in T-cell activation
(Caron, 1998), ExoT/ExoS might interfere with host immunity by blocking actin
polymerization essential for immunological synapse formation. Moreover, Pseudomonas
translocates PopB and PopD effectors, homologues of Yersinia Yop effectors, which cause F-
actin disassembly. This may lead to the host membrane pore formation to inject TTSS
effectors inside the cytosol (Frithz-Lindsten, 1998).
Unlike the bacteria described thus far, Helicobacter pylori exploits another secretion system
to modulate host cytoskeleton: the type IV secretion system (T4SS) which is functionally
related to TTSS (Christie, 2000). This secretion system is used very rarely by bacteria to
translocate effectors, however the human pathogen Helicobacter pylori is one of the few
bacteria that exploits T4SS to inject CagA and VacA proteins. CagA induces
dephosphorylation of the NPF cortactin (Weaver, 2001). Therefore, CagA activation and
actin rearrangements seem to correlate (Selbach, 2003). VacA has several functions on
epithelial cells, but interestingly it interferes with antigenic peptide presentation on B cells
and T cells activation (Molinari, 1998); this could be due to alteration of the actin protein
14

15. network. In addition, Helicobacter harbors two virulence factors, called BabA and SabA,
which interact with host receptors and might lead to cytoskeletal remodeling (Rottner, 2005).
This type of virulence factors are called adhesins and they are exploited by several bacteria,
like Helicobacter and Chlamydia, to attract host cell and promote the development of
filopodia.
In Chlamydia, the activation of host cell receptors via adhesins regulates actin filaments
through the phosphorylation of PIP2 which involves the activation of the GEF of Rac
(Carabeo, 2011). Chlamydia transports via TTSS an important effector essential for infection:
Tarp. This protein was shown to have many actin binding sites, therefore it is very likely that
it could promote actin elongation (Jewett, 2010). Furthermore, Chlamydia seems to be able to
induce actin disassembly: a new effector (CT694) might sever F-actin to G-actin and control
host membrane ruffling (Hower, 2009).
Gram-positive bacteria
Thus far, we reported only Gram-negative bacterial interaction with host cytoskeleton. Much
less is known about Gram-positive bacteria of which Listeria is the best-studied example.
Like other Gram-positive, Listeria does not exploit the TTSS but uses two virulence factors
called internalin A (InlA) and internalin B (InlB). InlA interacts with host cell by binding E-
cadherin receptors which interact with actin filaments and are essential for forming cell
junctions (Geiger, 1992). E-cadherin recruits catenin proteins which promote Listeria uptake
via Rho GTPase modulation (Pizarro-Cerdá, 2006). Moreover, host myosin is also essential
for InlA-mediated internalization because it produces the right cell tension to surround the
bacterium (Sousa, 2004). InlB strategy shares similarities with growth factor-regulated
pathway (Cossart, 1998), in fact InlB binds the growth factor receptor Met (Shen, 2000) and
leads to increasing amount of phosphatidylinositol (Ireton, 1996). As shown previously, this
induces actin rearrangements either promoting actin nucleation via the WAVE and the
Arp2/3 complex or controlling actin depolymerization via cofilin modulation (Bierne, 2001).
Remarkably, InlB seems to mimic the binding of the physiological Met receptor ligand to
regulate the downstream pathway and induce cytoskeletal rearrangements (Li, 2005). In
addition, actin polymerization is regulated by another bacterial surface protein ActA that
stimulates Arp 2/3 protein complex (Welch, 1997) and seems to have a strong homology with
vinculin protein (Domann, 1992). This virulence factor is very important because it initiates
the process that leads to the formation of the Listeria comet tail: a long structure of growing
actin filaments which acts as a force that pushes the bacterium throughout the cytosol of the
infected cell. In this way, Listeria controls actin network proteins to penetrate deeper in
epithelial cells and to spread the infection. The discovery of these comet tails and the
underlying molecular mechanisms is a spectacular example of how studying bacterial
infection helps us to understand cytoskeletal dynamics (Lambrechts, 2008).
Streptococcus pyogenes is another Gram-positive bacterium which might promote
cytoskeletal rearrangements. It has been studied that this bacterium exploits the hyaluronic
acid of its capsule to bind human receptor CD44. This interaction might lead to actin
disruption and enhance intracellular junction disassembly to facilitate Streptococcus
pyogenes passage between epithelial cells (Cywes, 2001).
15

16. Bacterial manipulation of host microtubule dynamics
As mentioned previously, some bacterial effectors may also subvert microtubule dynamics
suggesting that bacteria might regulate vesicle transport and organelle positioning; this is the
case of the Shigella VirA and the E.coli EspG which disrupt microtubules (Yoshida, 2002;
Hardwidge, 2005). Other effectors like Salmonella SseF and SseG were found to induce
microtubule bundling and to be responsible for vesicle aggregation along microtubule (Kuhle,
2004). Hypothetically, Salmonella might block the fusion of phagosome with the lysosome,
and therefore survive intracellularly. Moreover, Salmonella seems to interact with the host
kinesin via two effectors: SifA and PipB2. These two proteins exploit the host protein SKIP
to regulate the kinesin both positively and negatively in order to control the direction of the
Salmonella-containing vacuole towards the interior of epithelial cells (Haglund, 2011). On
the other hand, the bacteria Campylobacter jejuni has been found to interact with dynein (Hu,
1999), which, in epithelial cells, targets vesicles towards the apical side of the cell unlike
kinesin. The interaction with dynein is difficult to explain for promoting bacterial infection,
therefore it is still to be elucidated. In addition, microtubule dynamics can be regulated also
by toxins secreted by bacteria: Clostridium difficile toxin CDT increases bacterial adherence
to the cell surface by inducing dense microtubule meshwork which wraps the bacteria
(Schwan, 2009). Less is known about signal transduction pathways alteration, which leads to
microtubule rearrangements by bacteria. In fact, it is unlikely that the binding of bacterial
outer-membrane protein with host receptors may alter pathways connected with microtubule
proteins as it occurs with actin network. However, there is still much to discover about
microtubule dynamics and microtubule-dependent pathways, and the study of bacterial
effectors might help.
Conclusions
In the infection process, one of the main difficulties for bacteria is to penetrate through the
host cell membrane and reach the host cytoskeleton. Therefore, many of bacterial-host
cytoskeleton interactions rely on the effector proteins. Although TTSS contact to host cell
membrane is still a critical issue, this machinery can easily penetrate the host cell membrane
and inject effectors. We showed that pathogens can deliver many effectors which regulate
host cytoskeletal dynamics in different ways, for example either enhancing or inhibiting
phagocytosis. However, how these effectors are regulated is still unknown. It seems very
unlikely that the 40 TTSS effectors of E.coli (Tobe, 2006) could be secreted independently
inside the cell. On the contrary, bacteria could be able to regulate their effectors depending on
the cell type. In this way, bacteria would secrete effectors that promote phagocytic cup
formation in epithelial cells (Rougerie, 2013), and they would promote the inhibition of
internalization in phagocytic cells (Hamid, 1999). Studying these effectors has been useful to
discover many cytoskeletal pathways and to elucidate actin and microtubule dynamics. In
fact, the study of bacterial infection has revealed many regulators of cytoskeletal dynamics
that were unknown earlier.
In addition to effectors, bacteria can mimic the ligand effect on the transmembrane receptor
through virulence factors. This strategy induces cytoskeletal rearrangements through the
activation of the receptor signal transduction, and it is mainly used by bacteria which do not
16

17. form the TTSS machinery, like the Gram-positive bacteria Listeria (Geiger, 1992). However,
some Gram-negative bacteria, like Yersinia, prefer to exploit this strategy instead of TTSS, to
increase the contact with host cell and to promote internalization (Dersch, 1999). In fact,
TTSS might be very efficient in host cell membrane penetration but it might be less efficient
in promoting bacterial-cell contact. On the other hand, bacteria, like E.coli, exploit TTSS to
secrete an effector (Tir) which binds to the host outer-membrane to enhance bacterial
adhesion and to induce internalization (Gruenheid, 2001). This system exploits both
penetration efficiency of TTSS and bacterial-host receptor interactions to have a very
successful infection. In addition, effector proteins, which alter host pathways, might lead to
the activation of the apoptotic pathway through hyper-activation of actin dynamics (Gourlay,
2006; Weinrauch, 1999). This could be beneficial for the microbe causing phagocytes killing
on the one hand, and it could prevent intracellular bacteria proliferation causing elimination
of epithelial infected cells on the other. Therefore, bacterial-host receptor interaction strategy
might be less detrimental for the host cell inducing a minor alteration of the host cytoskeleton
compared to actin-binding effectors. This strategy could be exploited at the first stage of
infection by less pathogenic bacteria which want to create a niche in the host. In the evolution
process, other bacteria could have could have developed efficient strategies, like Tir, that
combines TTSS and receptors interaction to support virulence in the host.
Thus far, we reported that most of the bacterial interactions with cytoskeleton during the
infection process occur between bacterial effectors and actin network proteins; less is known
about microtubule and much more has to be investigated. However, microfilament proteins
might be an easier bacterial target than microtubule because they are located on the outer
limit of the cell, they interact with the cell membrane as well as cell receptors, and they are
involved in cell phagocytosis and other function of the immune system. Therefore, the
regulation of microfilament dynamics by bacteria might lead to very severe infections.
To conclude, the study of the interaction of bacterial pathogens with host cytoskeletal
proteins during the infection process is considered of great importance because it might help
to understand both the common and diverse molecular mechanisms of bacterial virulence and
new cytoskeletal dynamics.
17

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