Polycystins and partners: proposed role in mechanosensitivity

Abstract

Mutations of the two polycystins, PC1 and PC2, lead to polycystic kidney disease. Polycystins are able to form complexes with numerous families of proteins that have been suggested to participate in mechanical sensing. The proposed role of polycystins and their partners in the kidney primary cilium is to sense urine flow. A role for polycystins in mechanosensing has also been shown in other cell types such as vascular smooth muscle cells and cardiac myocytes. At the plasma membrane, polycystins interact with diverse ion channels of the TRP family and with stretch-activated channels (Piezos, TREKs). The actin cytoskeleton and its interacting proteins, such as filamin A, have been shown to be essential for these interactions. Numerous proteins involved in cell–cell and cell–extracellular matrix junctions interact with PC1 and/or PC2. These multimeric protein complexes are important for cell structure integrity, the transmission of force, as well as for mechanosensing and mechanotransduction. A group of polycystin partners are also involved in subcellular trafficking mechanisms. Finally, PC1 and especially PC2 interact with elements of the endoplasmic reticulum and are essential components of calcium homeostasis. In conclusion, we propose that both PC1 and PC2 act as conductors to tune the overall cellular mechanosensitivity.

Abbreviations

AC

adenylyl cyclase

ADPKD

autosomal dominant polycystic kidney disease

AJ

adherens junction

AP-1

activator protein 1

ARPKD

autosomal recessive polycystic kidney disease

ASIC

acid-sensing ion channel

CC

coiled-coil domain

CD2AP

CD2-associated protein

CHOP

C/EBP homologous protein

CLD

C-lectin-like domain

CP1

C-terminal PC1 peptide

CTT

C-terminal tail (PC1)

CSM

ciliary sorting motif

ECM

extracellular matrix

EF

calcium-binding EF-hand domain

EGFR

epidermal growth factor receptor

eIF2α

eukaryotic translation initiation factor 2α

ENaC

epithelial sodium channel

ER

endoplasmic reticulum

FAK

focal adhesion kinase

FAP

focal adhesion point

GPS

G-protein-coupled receptor proteolytic site

GSK3

glycogen synthase kinase 3

Hax‑1

HS-1-associated protein X-1

Id2

inhibitor of DNA binding 2

IFT57

intraflagellar transport 57

ILD

Ig-like domain

IP₃

inositol 1,4,5-trisphosphate

IP₃R

inositol 1,4,5-trisphosphate receptor

JAK

Janus kinase

K2P

two-pore K⁺ channel family

KAP3

kinesin-associated polypeptide 3

KIF 3A and 3B

kinesin family 3A and 3B

Kim1

kidney injury molecule 1

LRR

leucine-rich repeats

mDia1

mammalian Diaphanous 1

MEC

Caenorhabditis elegans mechanosensitive ion channel

MscS/MscL

mechanosensitive channel with small/large unitary conductance

mTOR

mammalian target of rapamycin

Nek8

NIMA-related kinase (NIMA is ‘Never In Mitosis, gene A’)

p130Cas

p130 Crk-associated substrate

PACS 1 and 2

phosphofurin acidic cluster sorting protein 1 and 2

PC1

polycystin-1 protein

PC2

polycystin-2 protein

PERK

pancreatic ER-resident eIF2α kinase

PIGEA14

polycystin-2 interactor, Golgi and endoplasmic reticulum-associated

PIP₂

phosphatidylinositol-4,5-bisphosphate

PKD1

polycystin-1 protein

Pkd1

gene encoding for PC1

Pkd2

gene encoding for PC2

PKHD1

polycystic kidney and hepatic disease 1 (gene encoding fibrocystin)

PKD1L

PKD2L, PKD-like genes

PLC

phospholipase C

PM

plasma membrane

pp60c-src

member of the c-src gene family of protein-tyrosine kinases

p97

AAA ATPase (ATPases Associated with diverse cellular Activities)

P100

transcriptional coactivator

pp125^FAK

focal adhesion kinase pp125

PP1α

protein phosphatase 1α

RGSs

regulators of G protein

RP2

retinitis pigmentosa 2

RPTP (σ and γ)

receptor protein tyrosine phosphatase

RyR

ryanodine receptor

SAC

stretch-activated channel (cationic)

SAK

stretch-activated channel (potassium selective)

Siah-1

Drosophila Seven in Absentia 1

SR

sarcoplasmic reticulum

STAT

signal transducer and activator of transcription

STIM1

stromal interaction molecule 1

TCF

transcription factor

TRAAK

TWIK-1-related arachidonic acid-stimulated K⁺ channel

TREK

TWIK-1-related K⁺ channel

TRPs

transient receptor potential ion channel superfamily

TRPCs

classic TRP family

TRPPs

polycystin TRP family

TRPP2

polycystin-2 protein

TRPVs

vanilloid receptor TRP family

TSC

tuberous sclerosis complex

TWIK

tandem of P domains in a weak inward rectifying K⁺ channel

Overview of mechanosensitivity

A large variety of cells are able to sense mechanical forces. The origin of the force can be external to the organism (touch, gravity, acceleration, sound) or internal (osmotic pressure, fluid flow, blood pressure, breathing, heart contraction, or any membrane deformation). The force can be high, as in chondrocytes, or modest, as in urine flow sensing (see for review Ernstrom & Chalfie, 2002).

Mechanosensitivity occurs in three steps. The first is mechanosensing, in which a cellular element detects the mechanical stimulus. The second is mechanotransduction, in which the mechanical signal is converted into a biochemical or biophysical signal. The third step is the downstream mechanoresponse, in which the various sensing and transduction signals are integrated over space and time (Vogel & Sheetz, 2006). Several families of proteins have been suggested that might participate in mechanosensing. Due to the variety of putative mechanosensors, various models explain how proteins are able to sense mechanical perturbation (Fig. 1; see for reviews Vogel & Sheetz, 2006; Brierley, 2010; Yoshimura & Sokabe, 2010; Nilius & Honoré, 2012). A transmembrane protein can sense mechanical stimuli through the change in tension in its surrounding lipid bilayer, referred to as the ‘Bilayer tension model’. In bacteria, the opening of the mechanosensitive channel MscL is thought to depend on the bilayer tension (Yoshimura & Sokabe, 2010). A protein can also sense mechanical tension through links to structural elements that can transmit force from the intracellular or extracellular side, or both: this is the ‘Tethered protein model’. In this model the mechanosensor is linked through interacting proteins to the extracellular matrix, focal adhesion points, cell–cell junctions, actin cytoskeleton, intermediate filaments, or microtubules. A typical example is the touch sensation in Caenorhabditis elegans, in which a complex of diverse MEC proteins localised on both sides of the membrane is transmitting force to the pore-forming subunits (Brierley, 2010). The way the proteins respond to the applied force is also diverse. A force applied to a mechanosensing protein can unfold it to expose cryptic peptide sequences that activate intracellular pathways, thus leading to mechanotransduction. This mechanism is termed the ‘Protein unfolding model’; it has been demonstrated for the extracellular matrix protein fibronectin (Gao et al. 2002). In all these cases it is difficult to determine if the mechanical forces can directly modulate the intrinsic activity of some proteins, such as enzyme activity, ligand–receptor interactions, or ion channel gating (‘Mechanosensitive protein activity model’), or if the mechanosensitive protein can indirectly activate a non-mechanosensitive protein leading to mechanotransduction (‘Adjacent mechanosensitive protein model’). The indirect activation can be through ligand release or through protein–protein interaction. Combinations of these various models are probably occurring in the mechanosensitive complexes.

Figure 1. Models of proteins mechanosensitivity

Applied force is indicated by a red arrow, mechanotransduction by a magenta arrow; activated mechanosensor is highlighted in yellow. A, ‘Bilayer tension model’ with the force sensed through the lipid bilayer proposed, for example, in the bacteria mechanosensitive channel MscL (Yoshimura & Sokabe, 2010). B, ‘Tethered protein models’, with accessory proteins linking to the ECM or cytoskeleton, as described for MEC proteins in the touch sensation in Caenorhabditis elegans (Brierley, 2010). C, ‘Protein unfolding model’ with the unravelling of cryptic sites under mechanical stress, demonstrated for the extracellular matrix protein fibronectin (Gao et al. 2002). D, ‘Mechanosensitive protein activity’ with endogenous activity (enzyme, ligand, ion channel) directly activated by the perceived force, E, ‘Adjacent mechanosensitive model’ in which a primary mechanosensor indirectly activates another partner for mechanotransduction.

The role of ion channels in mechanosensing has been studied for more than 30 years (Sachs, 1986; Morris, 1992; Delmas & Coste, 2013), and some criteria for an ion channel to be considered as mechanically gated have been proposed (Ernstrom & Chalfie, 2002; Arnadottir & Chalfie, 2010). These criteria could be generalised to any other mechanosensing protein. First, the expression of the protein should match within time and within space to the sensory function of the mechanosensory organ. Second, removal of the protein should invalidate the sensory response. Third, a mutation of the protein leading to changed properties, such as permeability for an ion channel, should modify the mechanosensing of the organ or cell. Fourth, a mechanical response should be recorded when the protein is heterologously expressed in another cell type.

Mechanosensation is widely distributed in cellular compartments and implicates numerous interacting protein complexes such as desmosomes, adherens junctions, integrins, extracellular matrix, focal adhesion points, membrane receptors and transporters, as well as actin microfilaments, microtubules and intermediate filament networks. Many of these proteins are linked to signalling systems that involve cytosolic mediators, calcium homeostasis, or transcription factors (for review see De et al. 2010; Bukoreshtliev et al. 2013).

It is very interesting to note that a family of proteins, the polycystins, physically interact with most of the mechanosensing proteins mentioned above. A direct mechanical activation of polycystins has never been demonstrated. Nevertheless, many different research groups have shown that polycystins and interacting partners are involved in mechanosensing, demonstrated in renal flow sensing by Nauli et al. (2003) and Jin et al. (2013), and two other groups, Xu et al. (2007) and Köttgen et al. (2008), and in renal mechanoprotection (Peyronnet et al. 2012). It has also been demonstrated in vascular pressure sensing by Sharif-Naeini et al. (2009) and by another group, Narayanan et al. (2013), as well as in vascular flow sensing (Nauli et al. 2008), in blood–brain barrier mechanical injury (Berrout et al. 2012), and in nodal flow sensing (McGrath et al. 2003). Here, we will review the functional interactions between polycystins and their partners (indicated in bold) and their putative role in mechanosensing and mechanotransduction.

Polycystin mutations lead to polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited disorder affecting almost 1 in 400 to 1 in 1000 individuals (Gabow, 1993). The disease is characterised by the presence of cysts in the kidney, liver and pancreas, as well as cardiovascular abnormalities such as mitral valve prolapse, hypertension, and intracranial aneurysm. Proliferation and apoptosis are also known to be affected in ADPKD (Boletta et al. 2000). ADPKD is associated with mutations in Pkd1 or Pkd2 genes encoding polycystin‑1 (PC1, PKD1) and polycystin‑2 (PC2, TRPP2) (Bichet et al. 2006). Polycystins are present in the kidneys, and in several other tissues, including blood vessels, heart, liver, pancreas and brain. PC1 and PC2 are proposed to form an ion channel complex (Hanaoka et al. 2000). Inactivation of Pkd1 in young mice (before postnatal day 13) leads to robust cyst formation, whereas inactivation in older mice (after day 14 and until 3–6 months of age) leads to a strongly delayed cyst formation with diminished severity (Lantinga-van Leeuwen et al. 2007; Piontek et al. 2007). This shows the existence of a developmental switch important for understanding the onset of the disease in the kidney. In ADPKD, only 1% of the tubules become cystic, nevertheless it ultimately leads to total kidney failure. By a mechanism of compression and/or obstruction, the cystic tubules are thought to induce tubular dilatation in the neighbouring non-cystic tubules. A model in which an increase in intrarenal mechanical stress can lead to apoptosis has thus been proposed (Grantham et al. 2011). The polycystins and polycystin-like family is divided into two structural classes. The members of the first are 11-transmembrane domain proteins (PKD1, PKD1L1, PKD1L2), and the second are 6-transmembrane domain proteins belonging to the TRPP family (PKD2, PKD2L1, PKD2L2) (Clapham et al. 2012). The other form of polycystic disease (ARPKD), which is recessive, is more severe. It presents large kidneys often observed in utero or during the neonatal period. It is due to a mutation in the PKHD1 gene encoding fibrocystin, a molecule localised at the primary cilia. The precise function of fibrocystin is still unknown (for review see Harris & Torres, 2009).

Polycystin structure, localisation and pharmacology

The predicted structures of PC1 and PC2 are schematised in Fig. 2 (for review see Bichet et al. 2006). PC1 is a large integral membrane protein of 460 kDa, with a long N-terminal extracellular region of up to 3000 amino acids, 11 predicted transmembrane segments and a short intracellular C-terminal region of 200 amino acids. The N-terminal extracellular region contains many protein motifs, including two cysteine-flanked leucine-rich repeats (LRRs), 16 Ig-like domains (ILDs or PKD repeats), a G-protein-coupled receptor proteolytic site (GPS), and a C-lectin-like domain (CLD). The C-terminal intracellular region contains a G protein binding site (G) and a coiled-coil domain (CC). Thus PC1 is involved in protein–protein and protein–carbohydrate interactions, including ligand-binding sites and adhesive domains, suggesting that it can interact with elements on the extracellular side of the cell.

Figure 2. Predicted structures of PC1 (PKD1) (left panel) and of PC2 (TRPP2) (right panel)

The plasma membrane is depicted in grey. Abbreviations: C, C-terminus; CC, coiled-coil domain; CLD, C-lectin-like domain; CSM, ciliary sorting motif; EF, calcium-binding EF-hand domain; ER, ER retention domain; G, G protein binding site; GPS, G-protein-coupled receptor proteolytic site; ILD, Ig-like domains (PKD repeats); LRR, leucine-rich repeats; N, N-terminus.

PC2 is also predicted to be an integral membrane protein of 110 kDa with six transmembrane-spanning segments. Both the N- and C-termini are intracellular. The C-terminus contains a calcium-binding EF-hand domain (EF), an endoplasmic reticulum (ER) retention domain, and a coiled-coil domain (CC). The N-terminus contains a ciliary sorting motif (CSM) localised in residues 6, 7 and 9. Triptolide, a diterpene, is proposed to activate PC2 calcium release activity (Leuenroth et al. 2007). Another activator is MR3, a polyclonal anti-hPC1 antibody (Delmas et al. 2004). PC2 channel activity is inhibited by an anti-PC2 antibody, Ca²⁺, La³⁺, Gd³⁺ and amiloride (Gonzalez-Perrett et al. 2001).

While PC1 is localised in the plasma membrane (Palsson et al. 1996), PC2 is localised within the cell, although it can also be translocated to the plasma membrane in the presence of PC1 (Hanaoka et al. 2000). PC2 is a calcium channel located in the ER, and is a member of the TRP sensory channel family (Gonzalez-Perrett et al. 2001; Koulen et al. 2002). PC1 is probably a multimodal sensor but a channel activity has also been proposed (Babich et al. 2004). PC1 and PC2 associate together through their coiled-coil domains located in their cytoplasmic C-termini (Qian et al. 1997; Tsiokas et al. 1997) but they also have many other partners in various subcellular localisations described in the following text.

Polycystins and their associated proteins

Partners within the primary cilium

In the kidney, the mechanical response that is elicited by urine flow is linked to an increase in intracellular calcium (Praetorius et al. 2003, Praetorius & Spring 2003). This response probably requires the primary cilium, a sensory organelle protruding from the apical membrane of many cell types, including renal epithelium, and connected with the cortical actin cytoskeleton. The involvement of the primary cilium in flow sensing was also demonstrated in human mesenchymal cells (Hoey et al. 2011, 2012). The primary cilium is composed of ordered microtubules, some associated motor proteins (IFTs, kinesins) and various other interacting proteins. Its genesis and function depend on intraflagellar transport (IFT) involving kinesins and dynein motors (Verhey et al. 2011). PC1 and PC2 are localised at the primary cilium of murine renal epithelial cells (Pazour et al. 2002; Yoder et al. 2002). The large extracellular domain of PC1 is proposed to be the mechanosensor detecting the flow. Its activation is thought to lead to mechanotransduction by opening the PC2 channel giving rise to a calcium entry that subsequently triggers an intracellular calcium release through ryanodine or inositol 1,4,5-trisphosphate (IP₃) receptors in the ER, depending on cell type (Nauli et al. 2003; Jin et al. 2013). This view has recently been challenged by Dr Clapham's group who demonstrate that the main calcium channel within the primary cilia is composed of a PKD1L1–PKD2L1 heteromeric complex (DeCaen et al. 2013). Nevertheless, this complex does not present strong mechanosensitivity, thus, it cannot be ruled out that the mechanosensitivity is brought to the PKD1L1–PKD2L1 complex by PC1 and/or PC2. Interestingly, osmolytes such as urea (a major component of urine) or sorbitol may change the mechanical properties of PC1, and thus modulate its mechanosensing properties (Ma et al. 2010). Some studies have shown a relation between flow sensing and signalling pathways related to cystogenesis. Thus a defect in flow sensing by the primary cilium has been proposed to result in cyst formation in polycystic kidneys (for review see Kotsis et al. 2013). A speculative link between flow, calcium entry and cell cycle is inversin, a molecule that binds to calmodulin in the absence of calcium (Morgan et al. 2002). Another elegant model proposes that urine flow, acting on the primary cilium, can influence the centrosome localisation and therefore the plane of cell division within the renal tubule. A defect in the cilium would then provoke an inappropriate mechanoresponse with some disoriented cell division and incorrect planar cell polarity, ultimately leading to cystogenesis (Fischer et al. 2006). Kinesin-2 is a heterotrimer formed by KIF3A, KIF3B and KAP3, and is involved in primary cilium growth and the cell cycle (Verhey et al. 2011). The trimer also interacts with fibrocystin, a protein mutated in ARPKD. PC2 directly interacts with KIF3A (Li et al. 2006) and with KIF3B (Wu et al. 2006). These studies show that the three proteins, PC2, fibrocystin, and kinesin‑2, form a complex in the primary cilium and in the perinuclear cytoplasm of renal epithelial cells. The role of the complex is not clear but fibrocystin is able to stimulate PC2 channel activity in the presence of KIF3B. It is noteworthy that mutations in any of these proteins lead to a similar polycystic phenotype (for KIF3A see Lin et al. 2003). Nonetheless, some other studies have challenged the importance of the primary cilium in cystogenesis. For example, the depletion of TRPV4 leads to the absence of flow-induced calcium transient but TRPV4-deficient mice and zebrafish do not present renal cysts (Köttgen et al. 2008).

Another mechanism that could regulate PC2 mechanosensing comes from its interaction with EGFR and with PLC-γ2. In the primary cilium, EGFR activation by its ligand, EGF, leads to a PLC-γ2-dependent reduction of phosphatidylinositol-4,5-bisphosphate (PIP₂) pool size. Similar to its effects on TRPV1, PIP₂ negatively regulates PC2 channel activity. Thus, EGFR activation is proposed to reduce the threshold of PC2 activation by various stimuli, including fluid shear stress, in the primary cilium (Ma et al. 2005).

Interestingly, an alteration in mechanical stimuli can induce some proteolytic cleavages of the intracellular C-terminal domain of PC1. The released C-terminal fragments of various sizes can induce a mechanoresponse. The release of a fragment of about 34 kDa, called CTT and probably made of the full PC1 C-terminal cytosolic tail, was shown in in vivo models, in which urine flow was stopped (unilateral ureteral ligation) or in which the primary cilium was absent (knockout of KIF3A) (Chauvet et al. 2004). Subsequently, CTT was able to activate the activator protein 1 (AP‑1) pathway which is involved in proliferation, transformation and apoptosis (Chauvet et al. 2004). The activation of the AP‑1 pathway is mediated by G proteins and PKC (Arnould et al. 1998; Parnell et al. 1998) and is regulated by PC2 (Chauvet et al. 2004). The cleavage and release of a shorter fragment of about 17 kDa, corresponding to the C-terminal half of PC1, was also shown in in vitro models by changing the flow intensity onto cells in culture (Low et al. 2006). This fragment interacts with the coactivator P100 and they subsequently activate the transcription factor STAT6. Moreover, the nuclear expression of P100, STAT6 and the small PC1 C-terminus fragment is increased in polycystic epithelial cells (Low et al. 2006). The STATs are a large family of transcription factors, with a role in cell growth, that could be activated by tyrosine kinases of the JAK family. PC1 is also constitutively bound to JAK2 and can activate the STAT pathway in the presence of PC2. This mechanism may explain how mutation of either Pkd1 or Pkd2 can lead to abnormal growth (Bhunia et al. 2002). More recently, it has been demonstrated that γ-secretase is involved in the cleavage of PC1 and that the CTT fragment regulates the Wnt pathway (Merrick et al. 2012). CTT also directly interacts with CHOP and TCF, two transcription factors of the Wnt pathway (Merrick et al. 2012). The Wnt signalling pathway regulates the level of β‑catenin, which is involved in cellular adhesion. The cleaved PC1 C-terminus interacts and stabilises β‑catenin. This could be relevant to ADPKD because the gene expression of β-catenin and the Wnt pathway elements are elevated in renal cystic tissue (Kim et al. 1999b; Lal et al. 2008). A peptide of PC1 C-terminus, called CP1, is also a regulator of the mTOR pathway, which regulates protein synthesis and cell growth through a direct interaction with TSC2 (Dere et al. 2010). Another cleavage site, called GPS, is located on the N-terminal domain of PC1. The released N-terminal fragment can be tethered to the C-terminal fragment in a non-covalent way (Qian et al. 2002). Both fragments could thus regulate each other, although this remains hypothetical. All these results enable speculation about a novel mechanism for mechanotransduction by polycystins. The detection of flow by the PC1/PC2 complex within the primary cilium could lead to the cleavage of a PC1 C-terminus, which is then transduced into changes in gene expression and activation of signalling pathways in order to get a mechanoresponse.

Thus, polycystins are involved in multiprotein complexes within the primary cilium. The exact role of each partner is still poorly understood but the overall role in flow mechanosensing and mechanotransduction has been proposed by different groups.

Partners present at the plasma membrane

At the plasma membrane, the major players in mechanosensation are the mechanosensitive channels. Among the K2P family, or two pore background K⁺ channels, there is a well-defined subfamily of stretch-activated potassium channels (SAKs). The first members were cloned in Professor Lazdunski's laboratory almost two decades ago: TREK‑1 (Fink et al. 1996) and TRAAK (Fink et al. 1998). TREK‑1 and TRAAK mechanosensitivity was then demonstrated (Patel et al. 1998; Maingret et al. 1999). The third member is TREK‑2 (Bang et al. 2000). The heterologous expression gives rise to a background current in several cell types. They are multimodal channels sensitive to a wide variety of physical (stretch, heat and acidosis) and chemical stimuli (polyunsaturated fatty acids, volatile anaesthetics and lysophospholipids). These channels, and notably TREK‑1, were proposed to play a role in pain perception, general anaesthesia, depression, neuroprotection and vascular physiology (for reviews see Patel & Honore 2002; Dedman et al. 2009; Mathie et al. 2010; Noel et al. 2011).

The molecular identity of the SACs (stretch-activated channels, cationic type) is still under debate (see for review Nilius & Honoré, 2012). Recently, two new proteins, Piezo1 and Piezo2, have been described as SACs. Using RNA interference knockdown Dr Patapoutian's team has shown that piezos are needed for mechanical perception in dorsal root ganglia neurons (Coste et al. 2010). They also demonstrated mechanically activated currents upon heterologous expression (Coste et al. 2010) and this was confirmed by other groups (Gottlieb et al. 2012; Peyronnet et al. 2013). Piezo proteins are pore-forming subunits as shown by bilayer reconstitution experiments (Coste et al. 2012). Two distinct Piezo2 mutations lead to an increase channel activity. These mutations are observed in patients with distal arthrogryposis type 5, a disease characterised by generalised dominant contractures, restrictive lung disease, and knee ligaments default (Coste et al. 2013). Piezo1 and 2 are both widely expressed and are very promising for understanding the role of SACs in mechanosensation.

The other eukaryotic channel families that fulfil most (but not all) the above criteria for mechanosensing are the degenerins and ENaCs (MEC-4, MEC-10, ENaC, ASIC) (see for review Drummond et al. 2008), many TRPs such as TRPA, NompC, TRPN1, TRP‑4, TRPV2 and TRPV4 (O'Neil & Heller, 2005), OSM‑9, TRPM4 and TRPM7 (Sharif-Naeini et al. 2008), PC2, LOV‑1 (Caenorhabditis elegans PC1 homologue), TRPC1 and TRPC6 (Sharif-Naeini et al. 2010), TRPY1 (Yin & Kuebler, 2010), the MscS-like (MSL2, MSL3, MSL9, MSL10, MSC1; Arnadottir & Chalfie, 2010).

Polycystins in association with other proteins were often proposed to form an ion channel complex. The physical assembly of the ion channel subunits PC2 and TRPC1 has been shown (Tsiokas et al. 1999). The stoichiometry is 2:2 and the subunits are alternating (Kobori et al. 2009). In the kidney, PC2 and TRPC1 are proposed to form a new heteromeric channel, with biophysical and pharmacological properties distinct from each protein alone (Bai et al. 2008). PC2 and TRPV4 can also form an ion channel complex with a putative 2:2 stoichiometry and alternating subunit arrangement (Stewart et al. 2010). The authors have demonstrated that flow-induced calcium current is due to this heteromultimeric channel in the primary cilium of collecting duct cells in culture (Köttgen et al. 2008). Moreover a C-terminal tail-truncated PC2 mutant, called 697fsX, is able to form an active calcium channel when bound to TRPC3 or TRPC7 (Miyagi et al. 2009).

As mentioned above, mutations in Pkd1 or Pkd2 genes can also lead to cardiac and vascular defects (Kim et al. 2000; Wu et al. 2000; Boulter et al. 2001). Two major mechanosensing mechanisms occur in the vascular system. An increase in blood flow is sensed by the endothelial cells which leads to vasorelaxation through NO release (Ayajiki et al. 1996). Additionally, an increase in blood pressure is sensed by the smooth muscle cells and induces vasocontraction, or ‘myogenic response’, originally described in 1902 (Bayliss, 1902). The primary cilium is not restricted to renal epithelium; it is also found on the luminal side of endothelial cells in blood vessels. PC1 is also expressed in endothelial cells (Ibraghimov-Beskrovnaya et al. 1997) and PC2 is present in the primary cilium of vascular endothelial cells and is proposed to transmit extracellular shear stress. The endothelial activated polycystin‑2 could transmit the extracellular shear stress to induce an increase in intracellular NO biosynthesis leading to smooth muscle dilatation. This mechanism, similar to renal flow sensing, is proposed by the same group to explain the flow-induced vascular relaxation (AbouAlaiwi et al. 2009). PC2 is also involved, together with TRPC1, in calcium influx leading to mechanical injury of blood–brain barrier endothelial cells (Berrout et al. 2012). Filamin A is an actin crosslinking protein that interacts with PC2. The PC1/PC2 ratio regulates SAC activity via filamin A and the actin cytoskeletal network. Filamin A also inhibits PC2 channel activity through an actin-dependent mechanism (Sharif-Naeini et al. 2009; Wang et al. 2012). A role for PC2 in pressure sensing by vascular smooth muscle cells and in myogenic tone was demonstrated in mesenteric arteries (Sharif-Naeini et al. 2009) as well as in cerebral arteries (Narayanan et al. 2013).

PC2 regulates the stretch-activated potassium channels, or SAKs, in a filamin A- and actin-dependent manner. The stretch sensitivity of TREK‑1, TREK‑2 and TRAAK is strongly inhibited by the mutant PC2-740X. Interestingly, although these K2P channels are polymodal and can therefore be activated by different stimuli, only their mechanosensitivity is altered (Peyronnet et al. 2012). A direct interaction of PC2 with TREK‑1 was demonstrated (Peyronnet et al. 2012). It is proposed that polycystins, through the modulation of K2P channel gating, can protect kidney epithelial cells against apoptosis in response to the mechanical stress induced by cyst growth. This is, to our knowledge, the first functional relationship between mechanotransduction and mechanoprotection (Peyronnet et al. 2012).

PC2, through its N-terminus, is also interacting with Piezo1. The article by Peyronnet et al. demonstrates that renal SACs strongly depend on Piezo1 expression and that PC2 negatively regulates the stretch sensitivity of this channel (Peyronnet et al. 2013). This paper, together with previous work by the same team (Sharif-Naeini et al. 2009; Peyronnet et al. 2012), shows that PC2 is a universal regulator of mechanosensitive ion channels, ranging from non-selective cationic SAC to potassium-selective SAK channels, and in various cell types (renal epithelium, vascular smooth muscle and immortalised cell lines). This regulation involves the cytoskeleton and filamin A, but the exact regulatory mechanisms remain to be determined.

Partners involved in adhesion and cell structure

Three groups of molecules are crucially involved in the mechanical properties of cells and tissues. First, the extracellular matrix (ECM) forms a structural network around the cells. It is a defining feature of the physical properties of a tissue. It also has an integrative role in extracellular signalling. Second, adherens junctions and desmosome complexes serve as physical connections between cells and play an important role in force transmission. Third, the cytoskeleton is made of actin filaments, microtubules and intermediate filaments. They all play a crucial role in controlling cellular stiffness and in regulating and transducing mechanical stimuli to the cells (for review see De et al. 2010; Bukoreshtliev et al. 2013).

The formation of a polarised renal tubule depends on sequential events, including cell–cell association, epithelial junction formation, polarity acquisition, and finally planar polarity. In the adult kidney, PC1 is confined mainly to epithelial cells, and in culture it localises at lateral cell junctions, suggesting a role in cell–cell interactions (Ibraghimov-Beskrovnaya et al. 1997). Polycystins are able to associate and interact with many elements of focal adhesion points and extracellular matrix (reviewed by Drummond, 2011). The Ig-like domain of PC1 is proposed to form homophilic interactions with other PC1 and Ig-like domain-containing proteins of neighbouring cells and thus play a role in intercellular adhesion (Ibraghimov-Beskrovnaya et al. 2000). PC1 is able to bind to many extracellular matrix proteins such as collagen I, II and IV, fibronectin and laminin (Weston et al. 2001; Malhas et al. 2002). Cell adhesion to the extracellular matrix leads to the transient formation of focal clusters with the recruitment of structural and signal transduction proteins including integrins, vinculin, paxillin and the focal adhesion kinase pp125^FAK (Wilson et al. 1999). In human ADPKD kidneys the extracellular matrix is abnormal; the main features are banded collagen and unique blebs or spheroids (Wilson et al. 1986). It has been demonstrated that PC1 associates with the following focal adhesion proteins; α-actinin, FAK, paxillin, p130Cas, pp60c-src, pp125^FAK, talin and vinculin in normal fetal collecting tubule epithelia (Wilson et al. 1999; Geng et al. 2000). The association with pp125^FAK is lost in ADPKD epithelia. An overproduction of PC1, α2-integrin, vinculin and paxillin and a decreased expression of pp125^FAK are also observed in ADPKD renal epithelial cells (Wilson et al. 1999). CD2AP is an adapter protein regulating spatial and temporal assembly of signalling complexes, notably focal adhesion complexes. CD2AP and PC2 associate via their C-termini, suggesting a role of CD2AP in the association of PC2 with intracellular multimeric complexes (Lehtonen et al. 2000). Of note, interacting PC1 and PC2 were also found at the dense plaques in aortic smooth muscle cells. These plaques are involved in the interaction between the cytoskeleton, plasma membrane and ECM. They are the equivalent of the focal adhesion complexes in epithelial cells (Qian et al. 2003). In conclusion, polycystins associate with extracellular matrix and focal adhesion proteins, which are transmitting the mechanical forces from outside the cell, but the exact regulatory role of polycystins on force transmission is still unknown.

The adherens junctions couple individual cells to maintain tissue structure and function. They are composed of transmembrane adhesion receptors (cadherins, nectins) and their associated catenins linked to actin filaments (see for review Niessen & Gottardi, 2008). PC1 associates with E-cadherin, and α-, β- and γ-catenins in a multiprotein complex (Huan & van Adelsberg, 1999; Geng et al. 2000). E-cadherin promotes a Ca²⁺-dependent cell–cell adhesion through homophilic interactions with other E-cadherins of adjacent cells. E-cadherin associates with catenins in order to stabilise cellular adhesion and is also involved in the modulation of signal transduction (Roitbak et al. 2004). In renal epithelial cells from ADPKD patients, this multiprotein complex is disrupted, with a depletion of PC1 and E-cadherin from the plasma membrane. Interestingly, in ADPKD cells, N-cadherin was found to substitute for the loss of E-cadherin. It is proposed that this substitution is an attempt to maintain the adherens junction structure (Roitbak et al. 2004). E-cadherin also modulates cell proliferation and cell polarity by directing a cytoskeletal network, containing ankyrin and spectrin, to the basolateral membrane (Huan & van Adelsberg, 1999). The recruitment of E-cadherin to the membrane is inhibited by annexin A5, a Ca²⁺- and phospholipid-binding protein that interacts with the LRR domain of PC1 (Markoff et al. 2007). Thus, PC1 is suggested to play a role in adherens junction formation both by mediating initial cell–cell association and regulating the recruitment of E-cadherin but not in the maintenance of the junction (Streets et al. 2009). Interestingly, this could explain the developmental switch mentioned above and the reason why the consequence of early polycystin disruption is much more severe than later disruption.

The most sophisticated cell–cell junctions are the desmosomes. They have been proposed to be implicated in mechanosensation due to their abundance in tissues experiencing mechanical stress. They have a structural function but are also involved in cell signalling. They are made up of three gene superfamilies, the desmosomal cadherins, the armadillo family (plakoglobin, plakophilins) and the plakins (desmoplakin, plectin) linking the complex to intermediate filaments (see for review Green & Gaudry, 2000). PC1 is colocalised with the desmosome junction components desmoplakins I and II (Scheffers et al. 2000). Moreover, the cytoplasmic tail of PC1 binds directly to the intermediate filament proteins vimentin, cytokeratins 8 and 18, and desmin through the coiled-coil motif of each partner (Xu et al. 2001). As a consequence, a severe mispolarisation of desmosomal elements to both the apical and basolateral domains is observed in cells from ADPKD cysts (Russo et al. 2005; Silberberg et al. 2005). Interestingly, the intercellular adhesions between these diseased cells are significantly more fragile and more sensitive to shear stress (Silberberg et al. 2005).

These junctional complexes are tightly linked to actin microfilaments. PC2 channel activity can be stimulated by hydrostatic or osmotic pressure, and this stimulatory effect is regulated by the cortical actin cytoskeleton that links the PC2 channel to the plasma membrane (Montalbetti et al. 2005). PC2 also directly associates with many actin-binding proteins; filamin A (Sharif-Naeini et al. 2009), tropomyosin‑1 (Li et al. 2003a), cardiac troponin I (Li et al. 2003b), α‑actinin (Li et al. 2005) and Hax‑1 (Gallagher et al. 2000). The precise role of these associations is not clear, but an involvement in cell–cell adhesions, proliferation and migration, in cell morphogenesis and stability, and in angiogenesis is proposed by the authors. A member of the RhoA GTPase-binding formin homology protein family called mDia1 has a role in the organisation of the cytoskeleton, in cytokinesis, and in signal transduction. The interaction of mDia1 with PC2 is observed at the mitotic spindle of dividing cells, and a role in cell division is proposed (Rundle et al. 2004).

Overall, these results suggest that PC1 and PC2 are part of multiprotein complexes crucial to cell-to-extracellular matrix, and cell-to-cell interactions. These complexes are important for the transmission of forces between cells and extra- to intracellular mechanotransduction.

Partners localised in the ER

Many studies have demonstrated that a mechanical stimulation leads to an increase in intracellular calcium in mouse L cells, in ligament, in osteoblast and in myoblast (Bukoreshtliev et al. 2013). There is also a close relation between calcium homeostasis and mechanosensing in the heart (Ter Keurs, 2011). A link between mechanical sensing and calcium homeostasis within the primary cilium has already been discussed above. The induced calcium signalling has been proposed to be due to PC2 as an ER calcium release channel by the group of Dr Nauli (for review see Jones & Nauli, 2012). PC2 does not seem to need the presence of PC1 for its channel activity. Nevertheless, a functional coupling between the PC1-dependent channel activity, at the cell surface, and PC2, at the ER membrane, has been proposed by the group of Dr Somlo (Koulen et al. 2002). Polycystins are strongly implicated in calcium homeostasis by affecting the resting cytosolic calcium concentration, reducing the expression level of the Ca²⁺-ATPase SERCA2a, and inhibiting the passive Ca²⁺ leak from the endoplasmic reticulum (Mekahli et al. 2012). Moreover, they interact with the two major calcium-release channels, IP₃ receptors in epithelial cells and ryanodine receptors in cardiomyocytes. PC1 interacts with the IP₃ receptors, IP₃Rs, to decrease calcium release (Li et al. 2009). However, it has also been shown that the PC2–IP₃R interaction can enhance calcium release (Li et al. 2005). Both studies were performed in Xenopus oocytes. A more recent analysis performed in permeabilised epithelial cells demonstrated that both PC1 and PC2 are needed to amplify the IP₃-induced calcium release and that they are inactive alone (Mekahli et al. 2012). The channel activity of PC2 is negatively regulated by a direct interaction with syntaxin‑5 that could be involved in calcium homeostasis (Geng et al. 2008). Similarly, in the heart, PC2 interacts, through its C-terminal domain, with the ryanodine receptor RyR2. The consequence is an inhibition of RyR2 single channel currents, demonstrating the ability of PC2 to modulate the release of Ca²⁺ from the cardiac sarcoplasmic reticulum stores (Anyatonwu et al. 2007). Finally, a PC1 C-terminus deletion product directly interacts with the ER Ca²⁺ sensor STIM1 and reduces the store-operated Ca²⁺ entry that replenishes ER calcium levels (Woodward et al. 2010).

Thus PC1 and PC2 strongly interfere with calcium homeostasis. A flow-induced increase of calcium has been demonstrated in the primary cilium and the involvement of polycystins has been proposed. Nevertheless, the relation between mechanosensing by polycystins and reticular calcium signalling remains to be established. Of note, it has recently been demonstrated that the polycystin-like proteins PKD1L1 and PKD2L1 form a ciliary calcium channel. The same group also challenges the fact that variation in ciliary calcium concentration directly leads to a variation of global cytoplasmic concentration (DeCaen et al. 2013; Delling et al. 2013).

Other polycystin-interacting partners

Many other polycystin partners have been described but they do not seem to be directly involved in mechanosensing. Nevertheless, we speculate that their role in the regulation of polycystins could be important for PC1 or PC2 regulatory activity on mechanosensing.

PC1 associates with nephrocystin‑1, a cytoplasmic adapter molecule (Wodarczyk et al. 2010). PC1 signalling is also regulated by the direct interaction with the phosphatases RPTPσ and RPTPγ both in cilia and in adherens junctions (Boucher et al. 2011), and with PP1α (Parnell et al. 2012). PC1, through its C-terminus, also associates with the raft marker flotillin‑2. The localisation of PC1, and the multiprotein complex, in a large cholesterol-containing microdomain, is suggested to promote the association with other regulatory proteins such as G-proteins, kinases and phosphatases (Roitbak et al. 2005). The C-terminal part of PC1 is also able to inhibit the degradation of RGS7, a negative regulator of G protein signalling (Kim et al. 1999a). Furthermore, the C-terminus of PC1 can bind and activate G proteins, suggesting that it may function as a heterotrimeric G-protein-coupled receptor (Parnell et al. 1998; Delmas et al. 2002, 2004). The PC1 C-terminal domain also associates with two different regions of the Na⁺-K⁺-ATPase α-subunit (Pagel & Bodmer, 2003; Zatti et al. 2005).

PC2 interacts with the adenylyl cyclase AC5/6 and may explain the dysregulation of cAMP signalling observed in ADPKD (Choi et al. 2011). PC2 is mostly present in the endoplasmic reticulum (90%) and in the primary cilium. The association with PC1 leads to an expression at the plasma membrane (Hanaoka et al. 2000), but other PC2 partners also regulate its trafficking and fate. The phosphorylation of a PC2 N-terminal site by GSK3 is important to its trafficking to the plasma membrane but not to the primary cilium (Streets et al. 2006). Other partners, PACS1, PACS2 and PIGEA14 are important to the routing of PC2 between ER, Golgi and the plasma membrane (Köttgen et al. 2005; Morick et al. 2013). The intraflagellar transport protein IFT57 and the serine/threonine kinase Nek8 directly bind PC2 and are important to PC2 localisation to the primary cilium. IFT57 is also required for assembly of the cilium (Jurczyk et al. 2004; Sohara et al. 2008). PC2 also specifically interacts with Kim1, a single-pass transmembrane protein also overexpressed in ADPKD cells (Kuehn et al. 2007), as well as with RP2, a protein mutated in retinitis pigmentosa (Hurd et al. 2010), and Id2, a negative transcription factor that regulates the cell cycle (Li et al. 2005).

Various processes can lead to the degradation of proteins. PC1 associates with Siah-1 (Kim et al. 2004), and PC2 associates with ATPase p97 (Liang et al. 2008a), PERK and eIF2α (Liang et al. 2008b), which are components of these degradation processes.

In conclusion, these interactions could have structural or developmental roles, and the intracellular localisation of polycystins could be crucial to their function. These various mechanisms of regulation, trafficking and degradation could help the cells to adapt to their mechanical environment by controlling the fate, in time and space, of polycystins and their partners, but this remains hypothetical.

Conclusion

Polycystins associate and interact with numerous proteins as summarised in Tables 1 and 2. Many of these proteins are involved in mechanosensation or mechanotransduction as summarised in Figs 3 and 4. PC1 interacts with many proteins localised at focal adhesion points, adherens junctions, desmosomes and in the primary cilium (Fig. 3), while PC2 interacts more specifically with ion channels at the plasma membrane, with proteins of the actin microfilament network and in the primary cilium (Fig. 4). PC2 trafficking is also highly regulated by various partners. We could speculate that the interacting proteins involved in adhesion and cell structure play a role in mechanosensation through the ‘Protein unfolding model’ or through the ‘Tethered protein model’. The proteins localised at the plasma membrane are more likely to involve the ‘Bilayer tension model’ (Fig. 1). We propose that both PC1 and PC2 act as conductors to tune the overall mechanosensitivity of the cells. In many cases, any defects in the expression level of PC1 or PC2 will lead to structural abnormalities (cyst formation, aneurysms, mitral valve prolapse) or to mechanosensing defects (myogenic tone, flow sensing). Of note, there are often no clear differences in abnormalities driven by a PC1 defect with those driven by a PC2 defect. The importance of envisioning the whole cell in mechanosensation studies is nicely illustrated in epithelial renal cells, in which the flow sensing by ion channels located in the primary cilium (see above) can be modulated by the overall mechanical state of the entire cell, implicating the integrins, the actin microfilaments, and the microtubules (Alenghat et al. 2004).

Table 1. List of all PC1 partners

PC1 partners	PC1 domain	Localisation	Proposed role	References
α-Actinin	Unknown	FAP	Regulation of FAP	(Geng et al. 2000)
Annexin A5	N-terminus (ter) LRR	PM, extracellular side	Formation of AJ	(Markoff et al. 2007)
Catenins (α-, β-, γ-)	C-ter (with β-catenins)	Adherens junction	Formation of AJ and signalling	(Huan & van Adelsberg, 1999; Geng et al. 2000)
CHOP	C-ter	Intracellular	Regulation of apoptosis and proliferation	(Merrick et al. 2012)
Collagen I	N-ter LRR	ECM	Cell proliferation	(Malhas et al. 2002)
Collagen I, II and IV	N-ter CLD	ECM	ECM to cell signal transduction	(Weston et al. 2001)
Cytokeratins 8 and 18	C-ter	Desmosome complex	Structural, storage, signalling functions	(Xu et al. 2001)
Desmin	C-ter	Desmosome complex	Structural, storage, signalling functions	(Xu et al. 2001)
E-cadherin	Unknown	Adherens junction	Formation of AJ and signalling	(Huan & van Adelsberg, 1999; Geng et al. 2000)
FAK	Unknown	FAP	Regulation of FAP	(Geng et al. 2000)
Fibronectin	N-ter LRR	ECM	Cell proliferation	(Malhas et al. 2002)
Flotillin-2	C-ter	PM	Signalling	(Roitbak et al. 2005)
G-proteins	C-ter	PM	Transduction	(Parnell et al. 1998; Delmas et al. 2002)
γ-Secretase	C-ter	C-ter cleavage	Transduction	(Merrick et al. 2012)
IP3R	C-ter	ER	Calcium release from the ER	(Li et al. 2009)
JAK2	C-ter	Intracellular	Cell cycle regulation	(Bhunia et al. 2002)
Laminin	N-ter LRR	ECM	Cell proliferation	(Malhas et al. 2002)
Na^+–K^+-ATPase	C-ter	PM	Na^+ and K^+ flux in renal epithelium	(Pagel & Bodmer, 2003; Zatti et al. 2005)
Nephrocystin-1 (NPHP1)	C-ter polyproline motif	Cell–cell and cell–ECM junctions, primary cilium	Resistance to apoptosis	(Wodarczyk et al. 2010)
PC1	N-ter ILD	PM, extracellular side	Cell–cell interaction	(Ibraghimov-Beskrovnaya et al. 2000)
PC2	C-ter CC	PM/ER	Ion channel complex	(Qian et al. 1997; Tsiokas et al. 1997)
P100	C-ter	Primary cilium	Transduction	(Low et al. 2006)
p130Cas	Unknown	FAP	Regulation of FAP	(Geng et al. 2000)
Paxillin	Unknown	FAP	Regulation of integrin-mediated cell adhesion	(Wilson et al. 1999; Geng et al. 2000)
PP1α	C-ter	Intracellular	Regulation of PC1 signalling	(Parnell et al. 2012)
pp125^FAK	Unknown	FAP	Regulation of integrin-mediated cell adhesion	(Wilson et al. 1999)
pp60c-src	Unknown	FAP	Regulation of FAP	(Geng et al. 2000)
RGS7	C-ter	Intracellular	Negative regulation of G protein signalling	(Kim et al. 1999a)
RPTPσ	First extracellular domain	Primary cilium and adherens junction	Regulation of PC1 signalling	(Boucher et al. 2011)
RPTPγ	C-ter	Primary cilium and adherens junction	Regulation of PC1 signalling	(Boucher et al. 2011)
Siah-1	C-ter	Intracellular	PC1 degradation	(Kim et al. 2004)
STIM1	N-ter	ER	Inhibition of store-operated Ca^2+ entry	(Woodward et al. 2010)
Talin	Unknown	FAP	Regulation of FAP	(Geng et al. 2000)
TCF	C-ter	Intracellular	Regulation of apoptosis and proliferation	(Merrick et al. 2012)
TSC2	C-ter	Intracellular	Regulation of the mTOR pathway	(Dere et al. 2010)
Vimentin	C-ter CC	Desmosome complex	Structural, storage, signalling functions	(Xu et al. 2001)
Vinculin	Unknown	FAP	Regulation of integrin-mediated cell adhesion	(Wilson et al. 1999; Geng et al. 2000)

• The first column describes PC1 domain involved in the association, the second column the main localisation of the association within the cell, the third column the proposed role for the association, and the last column contains the references.

Table 2. List of all PC2 partners

PC2 partners	PC2 domain	Localisation	Proposed role	References
α-Actinin	N-ter and C-ter	Actin microfilament complex	Cell adhesion, proliferation and migration	(Li et al. 2005)
AC5/6	C-ter	Primary cilium	cAMP signalling	(Choi et al. 2011)
CD2AP	C-ter	ER, perinuclear, FAP	Formation of a multimeric complex	(Lehtonen et al. 2000)
EGFR	C-ter	Primary cilium	Regulation of PC2 channel activity	(Ma et al. 2005)
eIF2α	Unknown	ER	Cell proliferation	(Liang et al. 2008b)
Filamin A	N-ter and C-ter	Cytoskeleton	SAC regulation	(Sharif-Naeini et al. 2009; Wang et al. 2012)
GSK3	N-ter (Ser^76, Ser^812)	Intracellular	PC2 trafficking to the plasma membrane	(Streets et al. 2006)
Hax-1	C-ter	Actin microfilament complex	Cyst formation	(Gallagher et al. 2000)
Id2	Unknown	Intracellular	Regulation of cell cycle	(Li et al. 2005)
IFT57	Unknown	Primary cilium	Primary cilium assembly; trafficking	(Jurczyk et al. 2004)
IP3R	C-ter	ER	Calcium release from the ER	(Li et al. 2005)
KIF3A	Unknown	Primary cilium	Regulation of primary cilium sensory function	(Li et al. 2006)
KIF3B	Unknown	Primary cilium	Regulation of PC2 channel activity	(Wu et al. 2006)
Kim1	N-ter CSM	Primary cilium	Chemosensing	(Kuehn et al. 2007)
mDia1	C-ter	Mitotic spindle	Cell division	(Rundle et al. 2004)
Nek8	Unknown	Primary cilium	Trafficking to the primary cilium	(Sohara et al. 2008)
p97	Unknown	ER	PC2 degradation	(Liang et al. 2008a)
PACS1, PACS2	C-ter	Intracellular, ER	PC2 trafficking between ER, Golgi and plasma membrane	(Köttgen et al. 2005)
PC1	C-ter CC	PM/ER	Ion channel complex	(Qian et al. 1997; Tsiokas et al. 1997)
PERK	Unknown	ER	Cell proliferation	(Liang et al. 2008b)
Piezo1	N-ter	PM	SAC complex	(Peyronnet et al. 2013)
PIGEA14	C-ter	Intracellular	PC2 trafficking to the plasma membrane	(Morick et al. 2013)
PLC-γ2	N-ter	Primary cilium	Regulation of PC2 channel activity	(Ma et al. 2005)
RP2	Unknown	Primary cilium	Ciliogenesis and primary cilium signalling	(Hurd et al. 2010)
RyR2	C-ter	SR	Calcium release from the SR	(Anyatonwu et al. 2007)
Syntaxin-5	N-ter (amino acids 5–72)	ER, Golgi apparatus	Regulation of PC2 channel activity	(Geng et al. 2008)
TREK-1	Unknown	PM	SAK complex	(Peyronnet et al. 2012)
Troponin I	C-ter	Actin microfilament complex	Angiogenesis	(Li et al. 2003b)
Tropomyosin-1	C-ter	Actin microfilament complex	Cyst formation and growth	(Li et al. 2003a)
TRPC1	C-ter and a transmembrane segment	PM	SAC complex	(Tsiokas et al. 1999)
TRPC3, TRPC7	C-ter tail-truncated mutant 697fsX	PM	Receptor-activated Ca^2+ channel	(Miyagi et al. 2009)
TRPV4	Unknown	PM	SAC complex	(Stewart et al. 2010)

• Same as Table 1 with PC2 partners.

Figure 3. Localisation of PC1 partners with a putative role in mechanosensing

PC1 interacts with proteins localised mostly at focal adhesion points, adherens junctions, desmosomes and in the primary cilium. Localisation is indicated in black and bold, the partners are indicated in black, and the cell is schematised in light orange.

Figure 4. Localisation of PC2 partners with a putative role in mechanosensing

PC2 interacts more specifically with ion channels at the plasma membrane, with proteins of the actin microfilament network and in the primary cilium. Localisation is indicated in black and bold, the partners are indicated in black, and the cell is schematised in light orange.

In conclusion, the challenge for future work on mechanosensing proteins is to take into account the overall mechanical properties of cells and tissues. A better understanding of how polycystins exert their influence on numerous mechanosensing mechanisms will help develop improved therapeutic strategies against ADPKD and related pathologies.

Biography

• 

  Fabrice Duprat (left) obtained his PhD at the University of Nice–Sophia Antipolis (France). He is now an INSERM senior researcher working at the Institute of Molecular and Cellular Pharmacology (IPMC, CNRS) in Sophia Antipolis. His main research interests are ion channels and their role in the nervous system, the kidneys and the arteries. He is currently addressing the role of polycystins in renal and vascular mechanosensing. He is an electrophysiologist by training and has a large experience in imaging. Kevin Retailleau (right) obtained his PhD at the University of Angers (France). He is a postdoctoral fellow, funded by Fondation de Recherche sur l’Hypertension Artérielle (FRHTA), also working at IPMC. His main research interest is vascular physiology, and he is now addressing the molecular mechanisms of mechanotransduction in resistance arteries.

Additional information

Competing interests

The authors have no conflicts of interest to disclose.

Funding

We thank l’Institut National de la Santé et de la Recherche Médical (INSERM) and Fondation de Recherche sur l’Hypertension Artérielle (FRHTA) for their financial support.

Acknowledgements

We gratefully thank Drs R. S. Naeini, D. J. Peters, P. Delmas, A. J. Patel and S. Demolombe for critically reviewing the manuscript, F. Aguila for graphical design, and Dr C. Duprat for language correction.