Function and regulation of TRPP2 at the plasma membrane

Abstract

The vast majority (approximately 99%) of all known cases of autosomal dominant polycystic kidney disease (ADPKD) are caused by naturally occurring mutations in two separate, but genetically interacting, loci, pkd1 and pkd2. pkd1 encodes a large multispanning membrane protein (PKD1) of unknown function, while pkd2 encodes a protein (TRPP2, polycystin-2, or PKD2) of the transient receptor potential (TRP) superfamily of ion channels. Biochemical, functional, and genetic studies support a model in which PKD1 physically interacts with TRPP2 to form an ion channel complex that conveys extracellular stimuli to ionic currents. However, the molecular identity of these extracellular stimuli remains elusive. Functional studies in cell culture show that TRPP2 can be activated in response to mechanical cues (fluid shear stress) and/or receptor tyrosine kinase (RTK) and G protein-coupled receptor (GPCR) activation at the cell surface. Recent genetic studies in Chlamydomonas reinhardtii show that CrPKD2 functions in a pathway linking cell-cell adhesion and Ca(2+) signaling. The mode of activation depends on protein-protein interactions with other channel subunits and auxiliary proteins. Therefore, understanding the mechanisms underlying the molecular makeup of TRPP2-containing complexes is critical in delineating the mechanisms of TRPP2 activation and, most importantly, the mechanisms by which naturally occurring mutations in pkd1 or pkd2 lead not only to ADPKD, but also to other defects reported in model organisms lacking functional TRPP2. This review focuses on the molecular assembly, function, and regulation of TRPP2 as a cell surface cation channel and discusses its potential role in Ca(2+) signaling and ADPKD pathophysiology.

Fig. 1.

Diagram illustrating the membrane topology of PKD1 and TRPP2. LRR-LDL, leucine-rich repeats and low-density lipoprotein homology domains; PKD, polycystic kidney disease domain; C-type lectin: C-type lectin homology domain; REJ, receptor of egg jelly; GPS, G protein-coupled receptor (GPCR) proteolytic site.

Fig. 2.

Receptor- and store-operated Ca²⁺ signaling in nonexcitable cells. Activation of GPCRs or receptor tyrosine kinases (RTKs) results in the activation of phospholipase C-β (PLC-β) or -γ (PLC-γ) isoforms, respectively. Activated PLCs catalyze the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP₂) to inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ binds and activates IP₃ receptors (IP₃Rs) in the endoplasmic reticulum (ER) membrane, resulting in the release of Ca²⁺ from the intracellular stores. Depletion of Ca²⁺ from the ER triggers the translocation of ER-resident STIM1 to the vicinity of the plasma membrane (PM) to activate store-operated Ca²⁺ channels (SOC). Receptor-operated channels (ROC) are activated by second messengers other than store depletion (i.e., reduction of PIP₂, production of DAG, etc).

Fig. 3.

Function of TRPP2 as a ROC at the PM of LLC-PK₁ kidney epithelial cells. Activation of epidermal growth factor (EGF) receptor (EGFR) by EGF results in the activation of PLC-γ2 and conversion of PIP₂ to IP₃ and DAG. In this cell line, EGF-induced IP₃ is not sufficient to activate intracellular Ca²⁺ release from the ER and subsequent activation of SOCs. However, EGF-induced activation of PIP₂ breakdown results in the release of PIP₂-mediated inhibition of TRPP2. In this case, TRPP2 behaves as a bona fide ROC.

Fig. 4.

Activation of TRPP2 by mammalian homolog of Diaphanous related formin 1 (mDia1). At resting membrane potentials, autoinhibited mDia1 (red) binds and blocks TRPP2 activity. In response to EGF or membrane depolarization, mDia1 switches from the autoinhibited state to the activated state, releasing the block on TRPP2. Predicted pore-forming region in TRPP2 is shown in green. Formin homology domains 1 and 2 (FH1 and FH2) are shown as red cylinders.

Fig. 5.

Hypothetical regulation TRPP2 by EGF in the cilium in vivo. TRPP2 colocalizes with PIP₂ and EGFR along the cilium, where it is kept insensitive to flow stimulation due to PIP₂-mediated inhibition. Liberated EGF from prepro-EGF at the apical surface of the PM through the action of locally acting proteases releases TRPP2 from PIP₂-mediated inhibition, generating a gradient in sensitized TRPP2 with a higher level of sensitization at the base (shown as yellow) and a lower level of sensitization at the tip of the cilium (shown as white). Highly sensitized TRPP2 at the base of the cilium mediates efficient mechanotransduction secondary to fluid flow changes.

Fig. 6.

Hypothetical role of TRPP2 in mechanotransduction. Cilium bending in response to fluid shear stress may activate GPCR(s) in a ligand-independent fashion, probably through mechanical stretching of the PM and/or associated cytoskeletal structures (arrows). Activation of GPCR would result in intracellular Ca²⁺ release from the ER through the IP₃ pathway and activation of SOCs. ROCs, including plasma membrane TRPP2, could also be activated, resulting in a massive and long-lasting increase in intracellular Ca²⁺ concentration. Dashed arrow denotes changes in membrane tension in response to cilium bending.