The multimeric structure of polycystin-2 (TRPP2): structural-functional correlates of homo- and hetero-multimers with TRPC1

Identification of the structural motif responsible for trimeric assembly of the C-terminal regulatory domains of polycystin channels PKD2L1 and PKD2

Polycystin 2-type cation channels PKD2 and PKD2L1 interact with polycystin 1-type proteins PKD1 and PKD1L3 respectively, to form receptor-cation-channel complexes. The PKD2L1-PKD1L3 complex perceives sour taste, whereas disruption of the PKD2-PKD1 complex, responsible for mechanosensation, leads to development of ADPKD (autosomal-dominant polycystic kidney disease). Besides modulating channel activity and related signalling events, the CRDs (C-terminal regulatory domains) of PKD2 and PKD2L1 play a central role in channel oligomerization. The present study investigates the aggregation state of purified full-length PKD2L1-CRD as well as truncations of CRDs from PKD2 channels. Far- and near-UV CD spectroscopy show that the full-length PKD2L1 CRD (PKD2L1-198) and the truncated PKD2 CRD (PKD2-244) are alpha-helical with no beta-sheet, the alpha-helix content agrees with sequence-based predictions, and some of its aromatic residues are in an asymmetric environment created at least by partially structured regions. Additionally, the CRD truncations exhibit an expected biochemical function by binding Ca2+ in a physiologically relevant range with Kd values of 2.8 muM for PKD2-244 and 0.51 muM for PKD2L1-198. Complimentary biophysical and biochemical techniques establish that truncations of the PKD2 and PKD2L1 CRDs are elongated molecules that assemble as trimers, and the trimeric aggregation state is independent of Ca2+ binding. Finally, we show that a common coiled-coil motif is sufficient and necessary to drive oligomerization of the PKD2 and PKD2L1 CRD truncations under study. Despite the moderate sequence identity (39%) between CRDs of PKD2 and PKD2L1, they both form trimers, implying that trimeric organization of CRDs may be true of all polycystin channels.

A polycystin-2 (TRPP2) dimerization domain essential for the function of heteromeric polycystin complexes

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in two genes, PKD1 and PKD2, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Earlier work has shown that PC1 and PC2 assemble into a polycystin complex implicated in kidney morphogenesis. PC2 also assembles into homomers of uncertain functional significance. However, little is known about the molecular mechanisms that direct polycystin complex assembly and specify its functions. We have identified a coiled coil in the C-terminus of PC2 that functions as a homodimerization domain essential for PC1 binding but not for its self-oligomerization. Dimerization-defective PC2 mutants were unable to reconstitute PC1/PC2 complexes either at the plasma membrane (PM) or at PM-endoplasmic reticulum (ER) junctions but could still function as ER Ca(2+)-release channels. Expression of dimerization-defective PC2 mutants in zebrafish resulted in a cystic phenotype but had lesser effects on organ laterality. We conclude that C-terminal dimerization of PC2 specifies the formation of polycystin complexes but not formation of ER-localized PC2 channels. Mutations that affect PC2 C-terminal homo- and heteromerization are the likely molecular basis of cyst formation in ADPKD.

TRPC channels are necessary mediators of pathologic cardiac hypertrophy

Pathologic hypertrophy of the heart is regulated through membrane-bound receptors and intracellular signaling pathways that function, in part, by altering Ca(2+) handling and Ca(2+)-dependent signaling effectors. Transient receptor potential canonical (TRPC) channels are important mediators of Ca(2+)-dependent signal transduction that can sense stretch or activation of membrane-bound receptors. Here we generated cardiac-specific transgenic mice that express dominant-negative (dn) TRPC3, dnTRPC6, or dnTRPC4 toward blocking the activity of the TRPC3/6/7 or TRPC1/4/5 subfamily of channels in the heart. Remarkably, all three dn transgenic strategies attenuated the cardiac hypertrophic response following either neuroendocrine agonist infusion or pressure-overload stimulation. dnTRPC transgenic mice also were partially protected from loss of cardiac functional performance following long-term pressure-overload stimulation. Importantly, adult myocytes isolated from hypertrophic WT hearts showed a unique Ca(2+) influx activity under store-depleted conditions that was not observed in myocytes from hypertrophied dnTRPC3, dnTRPC6, or dnTRPC4 hearts. Moreover, dnTRPC4 inhibited the activity of the TRPC3/6/7 subfamily in the heart, suggesting that these two subfamilies function in coordinated complexes. Mechanistically, inhibition of TRPC channels in transgenic mice or in cultured neonatal myocytes significantly reduced activity in the calcineurin-nuclear factor of activated T cells (NFAT), a known Ca(2+)-dependent hypertrophy-inducing pathway. Thus, TRPC channels are necessary mediators of pathologic cardiac hypertrophy, in part through a calcineurin-NFAT signaling pathway.

Molecular advances in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is the most common form of inherited kidney disease that results in renal failure. The understanding of the pathogenesis of ADPKD has advanced significantly since the discovery of the 2 causative genes, PKD1 and PKD2. Dominantly inherited gene mutations followed by somatic second-hit mutations inactivating the normal copy of the respective gene result in renal tubular cyst formation that deforms the kidney and eventually impairs its function. The respective gene products, polycystin-1 and polycystin-2, work together in a common cellular pathway. Polycystin-1, a large receptor molecule, forms a receptor-channel complex with polycystin-2, which is a cation channel belonging to the TRP family. Both polycystin proteins have been localized to the primary cilium, a nonmotile microtubule-based structure that extends from the apical membrane of tubular cells into the lumen. Here we discuss recent insights in the pathogenesis of ADPKD including the genetics of ADPKD, the properties of the respective polycystin proteins, the role of cilia, and some cell-signaling pathways that have been implicated in the pathways related to PKD1 and PKD2.

Constitutive activity of the human TRPML2 channel induces cell degeneration

The mucolipin (TRPML) ion channel proteins represent a distinct subfamily of channel proteins within the transient receptor potential (TRP) superfamily of cation channels. Mucolipin 1, 2, and 3 (TRPML1, -2, and -3, respectively) are channel proteins that share high sequence homology with each other and homology in the transmembrane domain with other TRPs. Mutations in the TRPML1 protein are implicated in mucolipidosis type IV, whereas mutations in TRPML3 are found in the varitint-waddler mouse. The properties of the wild type TRPML2 channel are not well known. Here we show functional expression of the wild type human TRPML2 channel (h-TRPML2). The channel is functional at the plasma membrane and characterized by a significant inward rectification similar to other constitutively active TRPML mutant isoforms. The h-TRPML2 channel displays nonselective cation permeability, which is Ca(2+)-permeable and inhibited by low extracytosolic pH but not Ca(2+) regulated. In addition, constitutively active h-TRPML2 leads to cell death by causing Ca(2+) overload. Furthermore, we demonstrate by functional mutation analysis that h-TRPML2 shares similar characteristics and structural similarities with other TRPML channels that regulate the channel in a similar manner. Hence, in addition to overall structure, all three TRPML channels also share common modes of regulation.

The transient receptor potential channels TRPP2 and TRPC1 form a heterotetramer with a 2:2 stoichiometry and an alternating subunit arrangement

There is functional evidence that polycystin-2 (TRPP2) interacts with other members of the transient receptor potential family, including TRPC1 and TRPV4. Here we have used atomic force microscopy to study the structure of the TRPP2 homomer and the interaction between TRPP2 and TRPC1. The molecular volumes of both Myc-tagged TRPP2 and V5-tagged TRPC1 isolated from singly transfected tsA 201 cells indicated that they assembled as homotetramers. The molecular volume of the protein isolated from cells expressing both TRPP2 and TRPC1 was intermediate between the volumes of the two homomers, suggesting that a heteromer was being formed. The distribution of angles between pairs of anti-Myc antibodies bound to TRPP2 particles had a large peak close to 90 degrees and a smaller peak close to 180 degrees , consistent with the assembly of TRPP2 as a homotetramer. In contrast, the corresponding angle distributions for decoration of the TRPP2-TRPC1 heteromer by either anti-Myc or anti-V5 antibodies had predominant peaks close to 180 degrees . This decoration pattern indicates a TRPP2:TRPC1 subunit stoichiometry of 2:2 and an alternating subunit arrangement.

Conditional mutation of Pkd2 causes cystogenesis and upregulates beta-catenin

Loss of polycystin-2 (PC2) in mice (Pkd2(-/-)) results in total body edema, focal hemorrhage, structural cardiac defects, abnormal left-right axis, hepatorenal and pancreatic cysts, and embryonic lethality. The molecular mechanisms by which loss of PC2 leads to these phenotypes remain unknown. We generated a model to allow targeted Pkd2 inactivation using the Cre-loxP system. Global inactivation of Pkd2 produced a phenotype identical to Pkd2(-/-) mice with undetectable PC2 protein and perinatal lethality. Using various Cre mouse lines, we found that kidney, pancreas, or time-specific deletion of Pkd2 led to cyst formation. In addition, we developed an immortalized renal collecting duct cell line with inactive Pkd2; these cells had aberrant cell-cell contact, ciliogenesis, and tubulomorphogenesis. They also significantly upregulated beta-catenin, axin2, and cMyc. Our results suggest that loss of PC2 disrupts normal behavior of renal epithelial cells through dysregulation of beta-catenin-dependent signaling, revealing a potential role for this signaling pathway in PC2-associated ADPKD.

Structural and molecular basis of the assembly of the TRPP2/PKD1 complex

Mutations in PKD1 and TRPP2 account for nearly all cases of autosomal dominant polycystic kidney disease (ADPKD). These 2 proteins form a receptor/ion channel complex on the cell surface. Using a combination of biochemistry, crystallography, and a single-molecule method to determine the subunit composition of proteins in the plasma membrane of live cells, we find that this complex contains 3 TRPP2 and 1 PKD1. A newly identified coiled-coil domain in the C terminus of TRPP2 is critical for the formation of this complex. This coiled-coil domain forms a homotrimer, in both solution and crystal structure, and binds to a single coiled-coil domain in the C terminus of PKD1. Mutations that disrupt the TRPP2 coiled-coil domain trimer abolish the assembly of both the full-length TRPP2 trimer and the TRPP2/PKD1 complex and diminish the surface expression of both proteins. These results have significant implications for the assembly, regulation, and function of the TRPP2/PKD1 complex and the pathogenic mechanism of some ADPKD-producing mutations.