Polycystin 2: A calcium channel, channel partner, and regulator of calcium homeostasis in ADPKD

Abstract

Polycystin 2 (PC2) is one of two main protein types responsible for the underlying etiology of autosomal dominant polycystic kidney disease (ADPKD), the most prevalent monogenic renal disease in the world. This debilitating and currently incurable condition is caused by loss-of-function mutations in PKD2 and PKD1, the genes encoding for PC2 and Polycystin 1 (PC1), respectively. Two-hit mutation events in these genes lead to renal cyst formation and eventual kidney failure, the main hallmarks of ADPKD. Though much is known concerning the physiological consequences and dysfunctional signaling mechanisms resulting from ADPKD development, to best understand the requirement of PC2 in maintaining organ homeostasis, it is important to recognize how PC2 acts under normal conditions. As such, an array of work has been performed characterizing the endogenous function of PC2, revealing it to be a member of the transient receptor potential (TRP) channel family of proteins. As a TRP protein, PC2 is a nonselective, cation-permeant, calcium-sensitive channel expressed in all tissue types, where it localizes primarily on the endoplasmic reticulum (ER), primary cilia, and plasma membrane. In addition to its channel function, PC2 interacts with and acts as a regulator of a number of other channels, ultimately further affecting intracellular signaling and leading to dysfunction in its absence. In this review, we describe the biophysical and physiological properties of PC2 as a cation channel and modulator of intracellular calcium channels, along with how these properties are altered in ADPKD.

Keywords: ADPKD; Calcium; Calcium channel; Polycystic kidney disease; Polycystin 2; TRP; TRPP1; TRPP2.

Figure 1:. Polycystin 2 is a membrane-bound transient receptor potential channel

A) Schematic depicting the topology of a full-length human PC2 monomer. PC2 contains six transmembrane helices and cytosolic N- and C-termini. Serine phosphorylation sites can be found in both termini; from N- to C-terminus: Ser76, 801, 812, and 829. The unique ‘tetragonal opening for polycystins’ (TOP, pink) domain between transmembrane regions 1–2 is rich in N-glycosylation sites: from N- to C-terminus: Asn299, 305, 328, 362, and 375. The characteristic voltage-sensing domain (VSD, yellow) of PC2 spans transmembrane helices 1–4. PC2’s pore-forming loop containing pore helices 1 and 2 (P, green) sits between transmembrane helices 5 and 6 (blue) and allows the transfer of Ca²⁺ from the ER lumen to the cytosol. The C-terminal tail of PC2 contains a Ca²⁺-binding EF hand, ER retention tag, and coiled-coil domain. Numbers depict amino acid locations. B) Structure of a truncated PC2 monomer (PDB: 5MKF) containing amino acids K215-K695, without the cytosolic N- or C-termini. Yellow highlights the VSD; pink highlights the TOP domain; green highlights the pore helices; blue highlights transmembrane helices 5–6. N-glycosylated residues within the TOP domain are labeled and highlighted in teal. C) Top-down (luminal) view of truncated PC2 (PDB: 5K47) highlighting the location of these same domains within a homotetrameric quaternary structure. Opaque coloring highlights a single monomer.

Figure 2:. Polycystin 2 interacts with ion channels to modulate intracellular Ca²⁺ signaling

Diagram depicting PC2/channel interactions in a healthy cell, and how normal Ca²⁺ signaling prevents the hyper-proliferation and -survival observed in PC2-deficient cells. PC2 acts to release ER Ca²⁺ through its intrinsic channel activity, or through its interactions with the InsP3R or RyR. The PC2/InsP3R interaction at the ER-mitochondrial interface serves to reduce the levels of mitochondrial Ca²⁺ uptake, preventing overload and apoptotic initiation. The PC2/PC1 and PC2/TRPV4 interactions on the primary cilium allow for mechanically stimulated Ca²⁺ influx. PC2/TRPC1 and PC2/TRPV4/TRPC1 heterocomplexes on the plasma membrane facilitate cation transport into the cytoplasm. The proper maintenance of cytosolic Ca²⁺ levels acts to limit the activity of AC5/6, thus regulating the cellular levels of cAMP. Without proper Ca²⁺ signaling, cAMP levels are enhanced, leading to the activation of PKA and numerous downstream pathways to promote cell proliferation and survival. Polycystin 2 (PC2); polycystin 1 (PC1); transient receptor potential cation channel subfamily V member 4 (TRPV4); transient receptor potential cation channel subfamily C member 1 (TRPC1); ryanodine receptor (RyR2); inositol 1,4,5-trisphosphate receptor (InsP3R); endoplasmic reticulum (ER); mitochondrion (Mito); adenylyl cyclase 5/6 (AC5/6); adenosine triphosphate (ATP); cyclic adenosine monophosphate (cAMP); protein kinase A (PKA).