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European Journal of Cell Biology 87 (2008) 631–640
0171-9335/$ - se
doi:10.1016/j.ej
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(F.N. van Leeu
www.elsevier.de/ejcb
Interplay between TRP channels and the cytoskeleton
in health and disease
Kristopher Clarka,b, Jeroen Middelbeekb, Frank N. van Leeuwenb,�
aUniversity of Dundee, MRC Protein Phosphorylation Unit, James Black Centre, Dow Street, Dundee DD1 5EH, Scotland, UKbLaboratory of Pediatric Oncology, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre,
PO Box 9101, 6500 HB Nijmegen, The Netherlands
Received 7 January 2008; received in revised form 17 January 2008; accepted 18 January 2008
Abstract
Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasisand cell volume regulation. In addition, TRP channels are considered universal integrators of sensory informationrequired for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigationsexploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channelsoperate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows thatmammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesionvia ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplaybetween TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiologicalsystems.r 2008 Elsevier GmbH. All rights reserved.
Keywords: TRP; Channels; Actin; Tubulin; Myosin; Cytoskeleton; Mechanotransduction
Introduction
The actomyosin cytoskeleton is a large network ofstructural, motor and signaling proteins that coordi-nates a plethora of cellular functions including celldivision, adhesion and migration. Proper function of thevarious cytoskeletal components requires the assemblyof the individual entities into macromolecular com-plexes. Notably, bi-directional relationships are estab-lished where cytoskeletal-associated proteins affect
e front matter r 2008 Elsevier GmbH. All rights reserved.
cb.2008.01.009
ing author. Tel.: +3124 366 6203;
6352.
ess: [email protected]
wen).
actomyosin remodeling and contractility while thecytoskeleton regulates the activity of the associatedprotein complex.
At the plasma membrane, integral membrane proteinsare intimately connected to the actomyosin cytoskeleton.Integrins are the most extensively studied paradigm forthe relationship between the cytoskeleton and transmem-brane proteins. These adhesion molecules are physicallyassociated with the actomyosin cytoskeleton via linkerproteins such as a-actinin and talin to provide anchorageand structural integrity to the cell (Arnaout et al., 2007;Delon and Brown, 2007). Upon activation, integrinstransmit intracellular signals that regulate organizationand contractility of the actomyosin cytoskeleton, whichin turn affects the architecture of cell–extracellular
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matrix (ECM) adhesion structures (Clark et al., 2007;DeMali et al., 2003). Recent investigations demonstratethat, by similarity to integrins, ion-transport proteinsincluding TRP channels also form large macromolecularcomplexes linked to the actomyosin cytoskeleton (Clarket al., 2006; Goel et al., 2005; Tang et al., 2000). Here, wewill focus on novel findings describing this closeassociation between TRP channels and the actomyosincytoskeleton.
Fig. 1. TRP channels are present in macromolecular com-
plexes associated with the actomyosin cytoskeleton: (A) the
Drosophila TRP/TRPL signalplex is organized around the
multiple PDZ domain-containing scaffold protein INAD
(PDZ domains are numbered 1–5). Note how INAD recruits
rhodopsin, TRP/TRPL, PLC, PKC and NINAC (myosin III)
to the complex. Interactions of the signalplex with cortical
actin are not only mediated by NINAC but also involve
Dmoesin, which connects TRP/TRPL channels to actin
filaments. (B) Mammalian TRP channels such as TRPV4 or
TRPM7 may directly interact with either actin filaments or
myosin II. (C) By similarity to Drosophila TRP/TRPL,
mammalian TRP channels such as TRPC4 and TRPC5
interact with the PDZ-containing scaffold NHERF, which
links these channels to the actin cytoskeleton via ERM
proteins.
Drosophila TRP channels in phototransduction
The photoresponse in the Drosophila eye involves theinflux of Ca2+ ions followed by rapid depolarization ofthe photoreceptor cells. Genetic screens aimed atcharacterizing this process have led to the identificationof more than 30 genes required for phototransduction inDrosophila (Wang and Montell, 2007). A spontaneousmutation in the trp locus generated a distinctivephenotype where illumination of photoreceptor cellsled to a transient response. Ca2+ influx in trp mutantphotoreceptors was significantly diminished and,although the cells depolarized, recovery to baselinewas accelerated and therefore referred to as ‘transientreceptor potential’ (trp) (see Minke and Cook (2002) forhistorical perspective). These findings led to the identi-fication of ‘‘Transient Receptor Potential’’, the foundingmember of this channel superfamily (Montell andRubin, 1989). A closely related channel, known asTRPL, is responsible for the small current remaining intrp flies. Deletion of both TRP and TRPL abolishes theresponse of photoreceptors to light (Niemeyer et al.,1996).
Biochemical characterization of the signaling path-ways controlling the function of TRP and TRPLdemonstrated that several genes identified in geneticscreens encode proteins that are physically connectedinto a macromolecular complex called the signalplex(Montell, 2005). The central component of this complexis the scaffold protein ‘inactivation no afterpotential D’(INAD), which contains five PDZ domains (the name‘‘PDZ’’ derives from the first three proteins in whichthese domains were identified: PSD-95, a proteininvolved in signaling at the post-synaptic density;DLG, the Drosophila discs large protein; and ZO-1,the zonula occludens 1 protein) that mediate directinteractions with rhodopsin, TRP, TRPL, phospholi-pase C (PLC/NORPA), protein kinase C (PKC/INAC),and myosin III (NINAC) (Fig. 1A). The clustering ofsignaling proteins into a single complex is thought to benecessary for proper localization of the components andefficient transmission and termination of signals, whichis extremely rapid (�20ms) in the case of phototrans-duction (Wang and Montell, 2007). These studies alsorevealed that TRP and TRPL channels are indirectly
connected to the actomyosin cytoskeleton via INAD.The primary role of the nonconventional myosinNINAC in phototransduction is to efficiently terminatethe photoresponse (Porter et al., 1992). Moreover,NINAC prevents the reactivation of TRP and TRPLchannels once illumination of the photoreceptors hasceased (Li et al., 1998). The mechanism responsible forthe negative feedback between NINAC and TRPchannels is, however, poorly understood. Targeting ofcalmodulin to rhabdomers by NINAC is necessary butnot sufficient as a mutation in NINAC, which abrogatesits association with INAD, has no effect on localizationof calmodulin to the rhabdomers but remains inefficientin terminating the response to light (Porter et al., 1993,
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1995; Wes et al., 1999). At this moment, it appears thatthe association between TRP and TRPL with theactomyosin cytoskeleton is sufficient to close thechannels after illumination of photoreceptor cells hasended (Wes et al., 1999). Notably, the motor activity ofNINAC is also required for rapid termination of theresponse (Wes et al., 1999), and thus changes inmechanical force exerted by the cytoskeleton onto thesignalplex may contribute to terminating phototrans-duction.
In addition to controlling the gating of channels inphotoreceptor cells, NINAC also regulates the move-ment of TRPL channels within the cell (Bahner et al.,2002; Meyer et al., 2006). TRPL channels localize to theplasma membrane in the rhabdomers in dark-raisedflies. However, upon exposure to light, TRPL channelsare endocytosed and translocated to the cell body(Bahner et al., 2002). Although this translocation eventrequires NINAC (Meyer et al., 2006), it is unlikely thatNINAC directly transports the vesicles from therhabdomer to the cell body. A detailed characterizationof the motor activity of class III myosins demonstratedthat NINAC is a so-called ‘nonprocessive’ motor,similar to brush border myosin-I, and hence incapableof transporting vesicles (Komaba et al., 2003). Instead,it has been suggested that secondary effects of NINACdeletion on the cytoskeleton may contribute to mis-localization of TRPL (Meyer et al., 2006). Irrespectiveof the precise mechanism, these findings underscore theimportance of the actomyosin cytoskeleton both incontrolling the gating of TRP channels as well as in theirlocalization.
TRP/TRPL channels also actively reshape the acto-myosin cytoskeleton in photoreceptor cells. Exposure tolight leads to a redistribution of cortical actin away fromthe rhabdomer and into the cell body, a process that isabrogated in trp mutant flies (Kosloff et al., 2003).Moreover, both TRP and TRPL channels interactdirectly with Dmoesin which provides another physicallink with the cytoskeleton (Fig. 1A) (Chorna-Ornanet al., 2005). ERM domain proteins (ERM is anacronym for ezrin, radixin and moesin, the first threeproteins in which this domain was recognized.) likeDmoesin are primarily involved in linking plasmamembrane proteins to the actomyosin cytoskeleton(Hughes and Fehon, 2007). Notably, the interactionbetween the TRP channels and Dmoesin is highlydynamic. In dark-raised flies, Dmoesin is phosphory-lated and associated with TRP and TRPL (Chorna-Ornan et al., 2005). Illumination of photoreceptors leadsto a rapid dephosphorylation of Dmoesin, whichsubsequently dissociates from the channels and translo-cates into the cell body (Chorna-Ornan et al., 2005).Deletion of trp prevents this light-induced translocationof Dmoesin, which is essential for the maintenance ofthe photoreceptor cells (Chorna-Ornan et al., 2005). It is
still unclear how TRP affects Dmoesin localization, orhow its translocation impacts on TRP/TRPL channelactivity. One model proposes that a phosphatase isactivated by Ca2+-mediated influx through TRP, whichdephosphorylates Dmoesin and promotes the dissocia-tion of Dmoesin and its translocation (Chorna-Ornanet al., 2005). Future work in this area will provide novelinsights into the mechanisms that TRP channels employto regulate cytoskeletal dynamics and vice versa.
Association of mammalian TRP channels with
the cytoskeleton
The human genome encodes 27 TRP channelswhich have been implicated in a variety of sensoryfunctions including temperature sensing, hearing, vision,smell, taste, and touch, as well as important ion-transport mechanisms involved in regulating cell volumeand maintaining ion homeostasis (Clapham, 2003;Venkatachalam and Montell, 2007). Based on homol-ogy, members of this protein superfamily have beenfurther classified into 6 categories: canonical, TRPC;vanilloid, TRPV; melastatin, TRPM; ankyrin, TRPA;polycystin, TRPP; and mucolipin, TRPML (Ramseyet al., 2006). All TRP channels possess 6 transmembranedomains that form the channel flanked by cytosolicN- and C-terminal tails. Within these cytosolic regions,several domains that could mediate the assembly ofmacromolecular complexes have been identified includ-ing PDZ-binding motifs, coiled-coil domains andankyrin repeats (Ramsey et al., 2006). Moreover, liketheir counterparts in Drosophila, several mammalianTRP channels interact with the actomyosin cytoskeleton.
The association between TRP channels and thecytoskeleton can be mediated via different mechanisms.Since ankyrin repeats are involved in linking transmem-brane proteins to the cytoskeleton (Bennett and Baines,2001), it was originally thought that TRP (TRPC,TRPV and TRPA) channels containing these repeatswould bind directly to the cytoskeleton. However, theseankyrin repeats appear to be primarily involved in thehomo- and heterotetramerization of the TRP channels,which is essential for channel function (Schindl andRomanin, 2007). Mutation or deletion of ankyrinrepeats abolishes the assembly of several TRP channels.The available evidence suggests that mammalian TRPchannels either directly associate with the cytoskeletonor, by similarity to Drosophila TRP, via scaffoldproteins. For instance, TRPV4 and TRPM7 appear toassociate with the actomyosin cytoskeleton by interact-ing with actin and nonmuscle myosin II, respectively(Clark et al., 2006; Ramadass et al., 2007) (Fig. 1B).On the other hand, TRPC4 and TRPC5 interactwith a PDZ domain in ‘Na+/H+ exchanger regulatory
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factor’ (NHERF), which associates with the cytoskeletonvia proteins of the ERM family (Tang et al., 2000)(Fig. 1C). Other examples include TRPP2 and TRPP3,which interact with a-actinin (Li et al., 2005, 2007),while TRPC1 binds a1-syntrophin of the dystrophin-based cytoskeleton (Vandebrouck et al., 2007).
Fig. 2. Hypothetical models describing how mechanosensitive
channels may respond to mechanical force: (A) changes in cell
volume affect tension within the lipid bilayer. Such forces
could be transmitted to the transmembrane domains of TRP
channels causing channel opening. (B) Mechanosensitive
channels that associate with the cytoskeleton may be sensitive
to changes in actomyosin-based tension, acting as a spring.
(C) Mechanical forces are sensed at specialized cell–matrix
adhesions involving a complex of integrins, cytoskeletal
elements and TRP channels. Traction forces exerted by the
ECM trigger integrin activation, accompanied by the recruit-
ment and activation of adaptor and signaling molecules.
Activation of this integrin signaling complex leads to increased
actomyosin-based tension. Thus, a combination of traction
forces applied externally by the ECM and cytoskeletal counter
forces on the inside may lead to channel opening. In this
particular setting, the integrin complex is the primary sensor.
(D) Mechanical forces may initially be sensed by G protein-
coupled receptors (GPCR), as has been shown for the
bradykinin receptor. Signals downstream of these receptors
subsequently activate the channel, for instance by activation of
specific kinases or PLC. Obviously, combinations of these
different modes of activation may also take place, or serve to
amplify the initial response.
Regulation of TRP channel activity by the
cytoskeleton
Cell surface expression
The identification of interacting partners for TRPchannels has led to important insights into their regu-lation, and it appears that the actomyosin cytoskeletonplays a prominent role in controlling channel activity.One mechanism by which actomyosin may affectchannel function is by modulating the interactionsbetween TRP channels and their regulatory proteins.For instance, some TRP channels localized within theplasma membrane will associate under specific condi-tions with proteins residing in the endoplasmic reticu-lum and this interaction requires an intact actomyosincytoskeleton (Lopez et al., 2006; Mehta et al., 2003).Alternatively, the cytoskeleton can regulate the accu-mulation of channels within the plasma membrane.Depolymerization of actin induces the internalization ofseveral members of the TRPC family (Itagaki et al.,2004; Lockwich et al., 2001) and inhibition of contrac-tility impairs the plasma membrane localization ofTRPC5 (Shimizu et al., 2006). Moreover, mutationsthat interfere with the association of TRPC4 withNHERF and ERM proteins prevent its delivery tothe cell surface (Mery et al., 2002). In addition toregulating subcellular localization of TRP channels,the actomyosin cytoskeleton may also play a role indirectly regulating channel activity, for instance duringmechanotransduction.
Mechanotransduction
The opening of several TRP channels is triggered bythe application of mechanical force (Pedersen andNilius, 2007). However, the molecular mechanismsunderlying this process are unclear, and it remains achallenge to classify channels as responding directly orindirectly to mechanical force (Christensen and Corey,2007). The mechanoreceptor may be the channel itself(Fig. 2A and B), but in other situations changes inmechanical force may be initially interpreted by otherreceptors such as integrins or G protein-coupledreceptors (GPCR) (Fig. 2C and D). For instance, itwas recently discovered that the bradykinin receptor canactivate downstream signaling cascades in response to
fluid shear and hypotonic stress independently of ligandbinding (Fig. 2D) (Chachisvilis et al., 2006), which couldlead to the subsequent opening of TRPM7 channels(Langeslag et al., 2007).
Different models have been developed to explain theopening of TRP channels in direct response to varioustypes of mechanical force (Orr et al., 2006). Certain TRPchannels involved in cell volume regulation are activatedby osmotic shock. Since changes in osmotic pressureaffect cell volume, forces could be transferred within the
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lipid bilayer to the transmembrane domains of TRPchannels leading to the regulation of channel activity(Pedersen and Nilius, 2007) (Fig. 2A). TRPC1 is a primeexample as it can be directly activated by mechanicalstretch in reconstituted liposomes devoid of anycytoskeletal components (Maroto et al., 2005). On theother hand, adherent cells sense mechanical forces atspecialized cell–matrix adhesion sites (Clark et al.,2007). Adhesion molecules, such as integrins, promoteactomyosin-based contractility, which could regulate thegating of cytoskeleton-associated TRP channels (Orret al., 2006) (Fig. 2C). Although this hypothesis requiresfurther experimentation, channels like TRPM7 areprime candidates. TRPM7 localizes to sites of cell–ECMadhesion where it is connected to the adjacent acto-myosin cytoskeleton (Clark et al., 2006; Su et al., 2006).Moreover, TRPM7 is delivered to the cell surface andactivated in response to mechanical forces (Oanceaet al., 2006; Numata et al., 2007a, b). The accumulationof TRPM7 channels in the plasma membrane may helpamplify the signal. Finally, TRPM7 is important for thedevelopment and maintenance of mechanosensitiveorgans (Elizondo et al., 2005). It will be important todetermine whether channels like TRPM7, which associ-ate with the actomyosin cytoskeleton, require an intactF-actin network in order to sense mechanical forces.
Regulation of cytoskeletal dynamics by TRP
channels
Activation of TRP channels leads to remodeling ofthe actomyosin cytoskeleton. The influx of ions,primarily Ca2+, will activate a wide range of Ca2+-dependent proteins, which include protein kinases,phosphatases and proteases. For instance, Ca2+ influxthrough TRPC1 activates the phosphatase calcineurin,which relays the signal to Slingshot (Wen et al., 2007).Slingshot dephosphorylates cofilin, which thereby pro-motes actin filament severing (Huang et al., 2006).Alternatively, TRPM7-mediated Ca2+ influx at sites ofcell adhesion activates calpains, which cleave compo-nents of focal adhesions such as FAK (Su et al., 2006).The ubiquitous nature of Ca2+ signaling pathwayssuggests that TRP channels can influence cell behaviorin many different ways. Other factors likely to beimportant for TRP channel function are subcellularlocalization of the channel, local concentrations of Ca2+
and duration of the Ca2+ signal.The cytoskeleton is composed of three major fila-
ment systems known as F-actin microfilaments, micro-tubules and intermediate filaments. Not only do TRPchannels associate with the actomyosin (microfilament)cytoskeleton but certain TRP channels also interactwith microtubules. Of particular interest is TRPV1,
which directly binds to tubulin in a Ca2+-dependentmanner via its C-terminus (Goswami et al., 2004,2007a). Activation of TRPV1 leads to a rapid disas-sembly of microtubules but not of the other cytoskeletalfilaments (Goswami et al., 2006, 2007b). Moreover,overexpression of TRPV1 leads to the initiation andelongation of filopodia (Goswami and Hucho, 2007).Future work may reveal the extent of TRP channels incontrolling the different cytoskeletal elements as well aspermit a classification of microfilament-, microtubule-,and intermediate filament-associated TRP channels.
Ion-transport-dependent and -independent
functions of TRP channels
Although the primary mode of action for TRPchannels is through ion influx, recent findings suggestthat these proteins may also affect cell behavior by ion-transport-independent mechanisms. In the Drosophila
eye, TRP plays an important scaffolding functionwhereby the localization of INAD and therefore itsassociated proteins (Fig. 1A) depends on the presence ofTRP (Li and Montell, 2000; Tsunoda et al., 2001).Deletion of TRP or mutation of the INAD-binding siteleads to a mislocalization of several components of thesignalplex and defects in phototransduction (Li andMontell, 2000; Tsunoda et al., 2001; Wang et al., 2005).Notably, some mammalian ion-transport proteins, suchas the Na+–H+ exchanger NHE1 (which is not a TRPchannel), also have ion-transport-independent func-tions. It was shown that NHE1 binds directly to ERMproteins. Mutations which affect ERM binding but notion translocation impair organization of the actomyosincytoskeleton and the formation of focal adhesions aswell as cell polarity (Denker and Barber, 2002; Denkeret al., 2000). Thus, NHE1 influences cell migration bycoordinating the recruitment of proteins into a largemacromolecular complex to the leading edge of the cell.In addition to these ion-independent effects of NHE1,channel activity is also important for cell adhesion andmigration (Stock et al., 2008).
The presence of TRP channels (e.g. TRPC4–6 (Goelet al., 2005; Tang et al., 2000)) in large macromolecularcomplexes suggests that, by similarity to NHE1, thesemay also contribute to the proper architecture of thecomplex by acting as scaffolds. When these TRPchannels contain domains which can exert enzymaticactivities, these may directly affect cell behavior(Venkatachalam and Montell, 2007). Interestingly, theregulation of filopodia by TRPV1 occurs independentlyof channel activity (Goswami and Hucho, 2007).However, more studies are required to determine themechanism underlying this phenomenon. Moreover,TRPM7, which encodes a TRP channel fused to an
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a-kinase, was shown to phosphorylate myosin II heavychain accompanied by cytoskeletal relexation. Thesecytoskeletal effects were largely kinase dependent, as acatalytically inactive mutant of TRPM7 maintainednormal channel function but failed to induce relaxationof the actomyosin cytoskeleton (Clark et al., 2006).Notably, a TRPM7 kinase-dead mutant could no longerinteract with actomyosin. However, since the interactionbetween the kinase and its substrate is facilitated byCa2+ influx (Clark et al., 2006), it remains unclearwhether channel activity is completely dispensable as isthe case for NHE1. These studies indicate the need tofurther characterize the macromolecular complexessurrounding mammalian TRP channels, which willfurther help explore a role for TRP channels asmolecular scaffolds.
(Patho)physiological relevance of interplay
between TRP channels and cytoskeleton
The close functional association between TRP chan-nels and the cytoskeleton is relevant to a number ofphysiological processes in mammalian cells and organsystems. Defects in the ability of TRP channels tointeract with the cytoskeleton contribute to the patho-genesis of human disease. Here, we provide a fewexamples in which TRP channels and the cytoskeletonconverge to regulate (patho)physiological functions.
Hearing
To interpret the information stored in sound waves,the inner ear converts it into an electrical signal byactivating Ca2+ influx through an unknown channel(Corey, 2006). Activation of the channel is thought tooccur directly via mechanical forces. The deflection ofstereocilia in response to sound waves will induce theopening of channels at the cell surface (Fettiplace, 2006).Since the bending of cilia is performed by thecytoskeleton, it is not surprising that several mutationsin myosins have been detected in patients withnonsyndromic deafness (Vollrath et al., 2007). Interest-ingly, several TRP channels (TRPV4, TRPA1, TRPN,and TRPML3) have been proposed as candidates for themechanically gated channel within the inner ear (Corey,2006; Cuajungco et al., 2007). For instance, TRPV4knockout mice have impaired hearing but the phenotypeis milder than expected (Tabuchi et al., 2005). Morerecently, mutations in TRPML3 which cause the mousevaritint-waddler phenotype were found to lead todeafness (Di Palma et al., 2002). However, thesemutations constitutively activate the channel and Ca2+
overload may influence the response and viability of thecells (Grimm et al., 2007; Kim et al., 2007). Moreover,
Grimm et al. (2007) were unable to activate TRPML3by applying mechanical force to the cells. Although themolecular composition of this channel complex awaitsidentification, the involvement of both cytoskeletalelements and mechanically gated TRP channels in theperception of sound provides an ideal model to furtherexplore the relationship between the cytoskeleton andTRP channels.
Vascular endothelial cells
Endothelial cells lining the blood vessels provide abarrier, the permeability of which is regulated by variousfactors including pro-inflammatory mediators such asthrombin and histamine. The integrity of the endothelialbarrier is maintained by a balance between actomyosincontractility and cell–cell adhesions (Kwan et al., 2007).Pro-inflammatory factors increase vascular permeabil-ity, leading to vascular edema by affecting this balance.An essential event leading to vascular dysfunction is arise in intracellular Ca2+ concentrations that activatecytoskeletal tension, actomyosin remodeling and disas-sembly of cell–cell interactions (Kwan et al., 2007). Theinflux of Ca2+ is mediated by several TRP channels thatrequire the actomyosin cytoskeleton for activation andalso signal to the cytoskeleton. For instance, TRPC4localizes to sites of cell–cell adhesion in pulmonaryendothelial cells, where it associates with protein 4.1,which links TRPC4 to the underlying cytoskeleton(Cioffi et al., 2005). Importantly, the link with thecytoskeleton is required for activation of TRPC4 (Cioffiet al., 2005). Knocking out TRPC4 in mice reduces thethrombin-induced Ca2+ influx which was associatedwith a lack of actin-stress fiber formation and areduction in microvascular permeability (Tiruppathiet al., 2002). Since vascular endothelial cells are alsosubjected to mechanical forces in the form of shear stressand express several mechanosensitive TRP channels(Kwan et al., 2007; Yao and Garland, 2005), these cellsprovide another excellent model system to study the bi-directional signaling cascades between TRP channelsand the actomyosin cytoskeleton.
Kidney function
Podocytes are an integral component of the kidneyglomerulus required for the ultrafiltration of plasma.These cells extend actin-rich foot processes which areorganized into an interdigitating network where eachneighboring foot process is connected by a multiproteincomplex signal transduction unit called the slit dia-phragm (Faul et al., 2007). The underlying actomyosincytoskeleton defines the morphology of the podocyte,and organization of the actin network is required forproper kidney function (Faul et al., 2007). Thus,
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proteins involved in the dynamic regulation of thepodocyte cytoskeleton are important components torenal (dys)function. For instance, a-actinin-4 knockoutmice die early after birth due to kidney failure, whilemutations in a-actinin-4 have been associated with thepathogenesis of focal segmental glomerulosclerosis (Koset al., 2003; Yao et al., 2004). Interestingly, TRPC6 mayaffect function of the slit diaphragm by inducing theremodeling of the actomyosin cytoskeleton. TRPC6localizes to the glomerular slit diaphragm, where itassociates with core components such as nephrin andpodocin as well as the cytoskeleton (Moller et al., 2007;Reiser et al., 2005). Moreover, TRPC6 overexpressionleads to the reorganization of the actomyosin cytoskeletonin podocytes and also to proteinuria (Moller et al.,2007). Recently, activating mutations in TRPC6 werediscovered in patients suffering from familial focalsegmental glomerulosclerosis (Reiser et al., 2005; Winnet al., 2005).
Another group of channels of particular interest in thekidney are the TRPP family members. In particular,mutations in TRPP2 underlie polycystic kidney disease(Hsu et al., 2007). In contrast to TRPC6, TRPP2 islocalized within the cilia of renal epithelial cells (Nauliet al., 2003). TRPP2 is linked to the actomyosincytoskeleton via a-actinin and tropomyosin-1 (Li et al.,2003, 2005). TRPP2 also associates with microtubuleswhich increase channel activity (Li et al., 2006). SinceTRPP2 is a central component of the mechanosensorfound in the cilia of kidney epithelial cells (Nauli et al.,2003), the association of the channel with differentcytoskeletal elements may play an integral role incontrolling its activity. Understanding the complexrelationship between TRP channels and the cytoskeletonmay help identify the molecular mechanisms underlyingkidney disease.
Concluding remarks
Recent work has demonstrated that mammalianchannels are organized into macromolecular complexeslinked to the cytoskeleton. Since the characterization ofthe signalplex was essential to decipher the molecularmechanisms underlying phototransduction in Drosophi-
la, defining the macromolecular complexes surroundingmammalian TRP channels will be pivotal to under-standing the role of this protein superfamily in healthand disease. To further explore the interplay betweenTRP channels and the cytoskeleton, several physiologi-cal systems such as stereocilia in the inner ear, podocytesand renal epithelial cells as well as vascular endothelialcells will provide excellent model systems. These studiesmay reveal novel mechanisms by which the cytoskeletoncontrols channel activity and clarify how TRP channels
influence cell behavior independently of the classicalion-transport pathways. Such efforts will certainlyfurther our understanding of the molecular mechanismsunderlying the pathogenesis of channelopathies.
Acknowledgments
We thank M. Langeslag for critically reviewing thismanuscript. The work is financially supported by agrant from the Dutch Cancer Society (KUN 2007-3733).
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