Emerging evidence of a link between the polycystins and the mTOR pathways

Abstract

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disease characterized by the formation of renal cysts. This disease can be caused by mutations in two genes, PKD1 and PKD2, which encode polycystin-1 (PC-1) and -2 (PC-2), respectively.PC-1 is a large plasma membrane receptor involved in the regulation of several biological functions and signaling pathways, and PC-2 is a calcium channel of the TRP family. The two proteins associate in a complex to prevent cyst formation, but the precise mechanism(s) involved remain largely unknown.This review will focus on recent advances in our understanding of the functions of polycystins and their role in signal transduction.Increased activity of the mammalian target of rapamycin (mTOR) kinase has been observed in cysts found in ADPKD tissues. Rapamycin has been shown to have beneficial effects in rodent models of polycystic kidney disease, prompting the initiation of pilot clinical trials with human patients. Furthermore, a direct role for PC-1 in the regulation of cell growth (size) via mTOR has recently been demonstrated.Major advancements in the study of mTOR biology have highlighted that this kinase exists in association with two different complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). The mTORC1 complex regulates cell growth (size), proliferation, translation and autophagy, and mTORC2 regulates the actin cytoskeleton and apoptosis. Interestingly, mTORC2 has been shown to contain the kinase responsible for the phosphorylation of Akt at Serine 473. Previous studies have shown that PC-1 controls the PI 3-kinase/Akt cascade to regulate apoptosis and the actin cytoskeleton, suggesting that this receptor might regulate mTOR at several levels.This review aims to discuss three different, inter-related themes emerging from the literature: (i) studies performed in our and other laboratories collectively suggest that PC-1 might be able to differentially regulate the two mTOR complexes; (ii) several studies point to genetic and functional cross-talk between the PKD and TSC genes, although the molecular details remain obscure; and (iii) studies performed in mammals and in the unicellular algae Chlamidomonas Reinhardtii might highlight a link between cilia, regulation of cell size and regulation of the cell cycle.

Figure 1

Schematic representation of the polycystins. A. Polycystin-1 is a large plasma membrane receptor that undergoes a series of cleavage events to generate several different species co-existing within the same cell and most likely carrying out distinct functions. The protein exists as an uncleaved polypeptide of 4302 amino acids (aa) (I) and can be cleaved at its G-protein coupled proteolytic site, generating an N-terminal fragment (NTF) that can be released (II) or remain tethered to the C-terminal fragment (CTF) (III) [48]. Two additional products generated by cleavage at yet-to-be-identified sites release either the entire C-terminal tail (IV) [52] or the last 112 aa (V) [54]. B and C. PC-1 and PC-2 have been shown to interact through coiled-coil domains located in their cytoplasmic C-terminal tail. The precise localization and topology of the complex remains to be determined. The two proteins might co-localize at the plasma membrane, where PC-2 would regulate calcium influx from the extracellular compartment (A) [39]. This might occur in some subcellular compartments such as the primary cilium. Alternatively, the plasma membrane pool of PC-1 might interact with the endoplasmic reticulum (ER) pool of PC-2, regulating its calcium release from the ER (B) [60].

Figure 2

Overview of the two mammalian targets of rapamycin (mTOR) complexes and their potential regulation by the polycystins. A. Schematic overview of the composition of mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) and cross-talk between them. mTORC1 contains mTOR, raptor, GβL/mLST, PRAS40 and DEP domains interactor of mTOR (DEPTOR). It can be regulated by a variety of activating or inhibitory cascades, as well as by amino acids capable of associating with Rag-GTP, leading to its association with mTORC1 to enhance its activity. One of the effectors of mTORC1, S6K1/2, regulates a negative feed-back loop at several levels. It is able to regulate insulin signalling by phosphorylating and inducing the degradation of IRS [111-113], and PDGF signalling by regulation of PDGF receptor levels [110]. In addition, S6K1/2 can phosphorylate rictor [114]. mTORC2 contains mTOR, Rictor, GβL/mLST, mSin and Protor. mTORC2 can phosphorylate Akt at Serine 473, regulating its specificity towards different substrates. Akt, in turn, can phosphorylate Tuberin (TSC2), potentially placing mTORC1 downstream of mTORC2 (see text). mTORC2 can also phosphorylate SGK1. B. Schematic representation of the effect of PC-1 on the two mTOR complexes. PC-1 has been described to inhibit the mTORC1 complex [87] whereas mTORC2 seems to be upregulated, since Akt phosphorylation at Serine 473 is enhanced by overexpression of PC-1 [70,81]. The role of PC-2 in the regulation of these cascades and their precise mechanism of regulation remain to be clarified.

Figure 3

Functional cross-talk between the TSC2 gene product, Tuberin and the PKD1 gene product, polycystin-1. Evidence published to date suggests that PC-1 trafficking from the Golgi compartment to cell-cell junctions requires Tuberin [122]. The role of Tuberin in PC-1 trafficking to the primary cilium was not investigated. In addition, PC-1 can regulate the mTORC1 cascade by regulating the phosphorylation and activity of Tuberin (A) [87]. Furthermore, a physical interaction was described between the portion of the PC-1 cytoplasmic C-terminal tail most proximal to the last transmembrane domain and Tuberin [26]. The PC-1/Tuberin interaction might be necessary for the correct trafficking of PC-1 or for regulation of mTORC1 (B, see text). Future studies should focus on providing experimental evidence of the significance of this interaction. One final possibility is that PC-1 and Tuberin cross-talk at several levels.