Polycystin-2 is an essential ion channel subunit in the primary cilium of the renal collecting duct epithelium

Abstract

Mutations in the polycystin genes, PKD1 or PKD2, results in Autosomal Dominant Polycystic Kidney Disease (ADPKD). Although a genetic basis of ADPKD is established, we lack a clear understanding of polycystin proteins’ functions as ion channels. This question remains unsolved largely because polycystins localize to the primary cilium – a tiny, antenna-like organelle. Using a new ADPKD mouse model, we observe primary cilia that are abnormally long in cells associated with cysts after conditional ablation of Pkd1 or Pkd2. Using primary cultures of collecting duct cells, we show that polycystin-2, but not polycystin-1, is a required subunit for the ion channel in the primary cilium. The polycystin-2 channel preferentially conducts K+ and Na+; intraciliary Ca2+, enhances its open probability. We introduce a novel method for measuring heterologous polycystin-2 channels in cilia, which will have utility in characterizing PKD2 variants that cause ADPKD.

The ion channel function of polycystin‐1 in the polycystin‐1/polycystin‐2 complex

“…Thirdly, our results show that the PC1/PC2_AA channel has a larger or more flexible pore, and higher permeability to most monovalent cations tested in this study (i.e., Na + , Li + , Cs + , DMA + , DEA + , and TEA + ) when compared to PC2_AA alone (Fig 5). The fact that the homomeric PC2 channel has low Ca 2+ permeability [11,22] while the PC1/PC2 channel complex has much higher Ca 2+ permeability indicates their very distinct physiological roles and thus contributions to ADPKD. Collectively, these results tell us that the heteromeric PC1/PC2 channel and the homomeric PC2 channel are two distinct channels that likely have their unique functions.…”

“…How these mutations lead to ADPKD is unclear, and the molecular mechanism of the function of the PC1/PC2 complex is largely unknown. The Ca 2+ conductance by PC2 or PC1/PC2 complex is also hotly debated [7,[9][10][11][12][13][14][15][16]. However, thus far the in vivo stimuli of this complex remain unknown, although previous studies suggested fluid flow as the potential stimulus [8] and Wnt proteins as candidate ligands [9].…”

“…This complex shares overall structural similarity to the homotetrameric PC2 channel, with the last two transmembrane domains (S10-S11) of PC1 directly participating in forming the presumed channel pore [42]. PC2_F640P gave rise to robust whole-oocyte currents when expressed in Xenopus laevis oocytes [11,22,43]. This is mainly due to the lack of knowledge on the channel activation mechanism, making functional analysis very challenging.…”

Regulation of ciliary membrane protein trafficking and signalling by kinesin motor proteins

“…renal cortical collecting duct cells [139,140], where they were initially proposed to form a receptor-channel complex [141] that functions as a mechanosensitive Ca 2+ channel activated by fluid flow [142]. This hypothesis has been challenged by others [143,144], and recent work from David Clapham's lab using primary cultures of collecting duct cells isolated from kidneys of a new ADPKD mouse model, indicated that PC2, but not PC1, constitutes an essential ion channel subunit in the primary cilium that preferentially conducts K + and Na + and is potentiated by intraciliary Ca 2+ [145]. This study also showed that ciliary trafficking of PC2 occurs independently of PC1, contradicting several earlier reports [16,145], but in agreement with the recently solved homotetrameric structure of PC2 [146,147].…”

Role of calcium in adult onset polycystic kidney disease

“…Mutations in the pkd1 and pdk2 genes are associated with ADPKD. These genes encode the polycystins, PC1 & 2, the latter, referred to as TRPP2 (transient receptor potential polycystic), functions as a non-selective cation channel[3] [4]. Malfunction of either PC1 or PC2 leads to cyst formation [5]…”