Functional dissection of the N-terminal extracellular domains of Frizzled 6 reveals their roles for receptor localization and Dishevelled recruitment

Frizzleds act as dynamic pharmacological entities

The Frizzled family of transmembrane receptors (FZD1-10) belongs to the class F of G protein-coupled receptors (GPCRs). FZDs bind to and are activated by Wingless/Int1 (WNT) proteins. The WNT/FZD signaling system regulates crucial aspects of developmental biology and stem-cell regulation. Dysregulation of WNT/FZD communication can lead to developmental defects and diseases such as cancer and fibrosis. Recent insight into the activation mechanisms of FZDs has underlined that protein dynamics and conserved microswitches are essential for FZD-mediated information flow and build the basis for targeting these receptors pharmacologically. In this review, we summarize recent advances in our understanding of FZD activation, and how novel concepts merge and collide with existing dogmas in the field.

Identification of Novel FZD4 Mutations in Familial Exudative Vitreoretinopathy and Investigating the Pathogenic Mechanisms of FZD4 Mutations

Purpose

The purpose of this study is to report five novel FZD4 mutations identified in familial exudative vitreoretinopathy (FEVR) and to analyze and summarize the pathogenic mechanisms of 34 of 96 reported missense mutations in FZD4.

Methods

Five probands diagnosed with FEVR and their family members were enrolled in the study. Ocular examinations and targeted gene panel sequencing were conducted on all participants. Plasmids, each carrying 29 previously reported FZD4 missense mutations and five novel mutations, were constructed based on the selection of mutations from each domain of FZD4. These plasmids were used to investigate the effects of mutations on protein expression levels, Norrin/β-catenin activation capacity, membrane localization, norrin binding ability, and DVL2 recruitment ability in HEK293T, HEK293STF, and HeLa cells.

Results

All five novel mutations (S91F, V103E, C145S, E160K, C377F) responsible for FEVR were found to compromise Norrin/β-catenin activation of FZD4 protein. After reviewing a total of 34 reported missense mutations, we categorized all mutations based on their functional changes: signal peptide mutations, cysteine mutations affecting disulfide bonds, extracellular domain mutations influencing norrin binding, transmembrane domain (TM) 1 and TM7 mutations impacting membrane localization, and intracellular domain mutations affecting DVL2 recruitment.

Conclusions

We expanded the spectrum of FZD4 mutations relevant to FEVR and experimentally demonstrated that missense mutations in FZD4 can be classified into five categories based on different functional changes.

The dark sides of the GPCR tree - research progress on understudied GPCRs

A large portion of the human GPCRome is still in the dark and understudied, consisting even of entire subfamilies of GPCRs such as odorant receptors, class A and C orphans, adhesion GPCRs, Frizzleds and taste receptors. However, it is undeniable that these GPCRs bring an untapped therapeutic potential that should be explored further. Open questions on these GPCRs span diverse topics such as deorphanisation, the development of tool compounds and tools for studying these GPCRs, as well as understanding basic signalling mechanisms. This review gives an overview of the current state of knowledge for each of the diverse subfamilies of understudied receptors regarding their physiological relevance, molecular mechanisms, endogenous ligands and pharmacological tools. Furthermore, it identifies some of the largest knowledge gaps that should be addressed in the foreseeable future and lists some general strategies that might be helpful in this process.

Pathway selectivity in Frizzleds is achieved by conserved micro-switches defining pathway-determining, active conformations

The class Frizzled of G protein-coupled receptors (GPCRs), consisting of ten Frizzled (FZD1-10) paralogs and Smoothened, remains one of the most enigmatic GPCR families. This class mediates signaling predominantly through Disheveled (DVL) or heterotrimeric G proteins. However, the mechanisms underlying pathway selection are elusive. Here we employ a structure-driven mutagenesis approach in combination with an extensive panel of functional signaling readouts to investigate the importance of conserved state-stabilizing residues in FZD5 for signal specification. Similar data were obtained for FZD4 and FZD10 suggesting that our findings can be extrapolated to other members of the FZD family. Comparative molecular dynamics simulations of wild type and selected FZD5 mutants further support the concept that distinct conformational changes in FZDs specify the signal outcome. In conclusion, we find that FZD5 and FZDs in general prefer coupling to DVL rather than heterotrimeric G proteins and that distinct active state micro-switches in the receptor are essential for pathway selection arguing for conformational changes in the receptor protein defining transducer selectivity.

WNT stimulation induces dynamic conformational changes in the Frizzled-Dishevelled interaction

Frizzleds (FZDs) are G protein-coupled receptors (GPCRs) that bind to WNT family ligands. FZDs signal through multiple effector proteins, including Dishevelled (DVL), which acts as a hub for several downstream signaling pathways. To understand how WNT binding to FZD stimulates intracellular signaling and influences downstream pathway selectivity, we investigated the dynamic changes in the FZD₅-DVL2 interaction elicited by WNT-3A and WNT-5A. Ligand-induced changes in bioluminescence resonance energy transfer (BRET) between FZD₅ and DVL2 or the isolated FZD-binding DEP domain of DVL2 revealed a composite response consisting of both DVL2 recruitment and conformational dynamics in the FZD₅-DVL2 complex. The combination of different BRET paradigms enabled us to identify ligand-dependent conformational dynamics in the FZD₅-DVL2 complex and distinguish them from ligand-induced recruitment of DVL2 or DEP to FZD₅. The observed agonist-induced conformational changes at the receptor-transducer interface suggest that extracellular agonist and intracellular transducers cooperate through transmembrane allosteric interaction with FZDs in a ternary complex reminiscent of that of classical GPCRs.

Functional role of the Frizzled linker domain in the Wnt signaling pathway

The Wnt signaling pathway plays a critical role in the developmental and physiological processes of metazoans. We previously reported that the Frizzled4 (FZD4) linker domain plays an important role in Norrin binding and signaling. However, the question remains whether the FZD linker contributes to Wnt signaling in general. Here, we show that the FZD linker is involved in Wnt binding and affects downstream Wnt signaling. A FZD4 chimera, in which the linker was swapped with that of the non-canonical receptor FZD6, impairs the binding with WNT3A and suppresses the recruitment of LRP6 and Disheveled, resulting in reduced canonical signaling. A similar effect was observed for non-canonical signaling. A FZD6 chimera containing the FZD1 linker showed reduced WNT5A binding and impaired signaling in ERK, JNK, and AKT mediated pathways. Altogether, our results suggest that the FZD linker plays an important role in specific Wnt binding and intracellular Wnt signaling.

The C-terminal of the α1b-adreneroceptor is a key determinant for its structure integrity and biological functions

The C-terminal of G protein-coupled receptors is now recognized as being important for G protein activation and signaling function. To detect the role of C-terminal tail in receptor activation, we used the α1b-AR, which has a long C-terminal of 164 amino acids. We constructed the intramolecular FRET sensors, in which the C-terminal was truncated to 10 (∆C-10), 20 (∆C-20), 30 (∆C-30), 50 (∆C-50), 70 (∆C-70), or 90 (∆C-90). The truncated mutants of ∆C-10, ∆C-20, or ∆C-30 cannot induce FRET signal changes and downstream ERK1/2 phosphorylation. However, the truncated mutants of ∆C-50, ∆C-70, or ∆C-90 induce significant FRET signal changes and downstream ERK1/2 phosphorylation, especially ∆C-90. This is particularly true in the case of the ∆C-90, ∆C-70, or ∆C-50 which retained the potential phosphorylation sites (Ser401, Ser404, Ser408, or Ser410). The ∆C-90 showed an increase in agonist-induced FRET signal changes and ERK1/2 phosphorylation in PKC- or endocytosis-dependent and EGFR-, src-, or β-arrestin2-independent.

Residue 6.43 defines receptor function in class F GPCRs

The class Frizzled of G protein-coupled receptors (GPCRs), consisting of ten Frizzled (FZD1-10) subtypes and Smoothened (SMO), remains one of the most enigmatic GPCR families. While SMO relies on cholesterol binding to the 7TM core of the receptor to activate downstream signaling, underlying details of receptor activation remain obscure for FZDs. Here, we aimed to investigate the activation mechanisms of class F receptors utilizing a computational biology approach and mutational analysis of receptor function in combination with ligand binding and downstream signaling assays in living cells. Our results indicate that FZDs differ substantially from SMO in receptor activation-associated conformational changes. SMO manifests a preference for a straight TM6 in both ligand binding and functional readouts. Similar to the majority of GPCRs, FZDs present with a kinked TM6 upon activation owing to the presence of residue P6.43. Functional comparison of FZD and FZD P6.43F mutants in different assay formats monitoring ligand binding, G protein activation, DVL2 recruitment and TOPflash activity, however, underlines further the functional diversity among FZDs and not only between FZDs and SMO.

Quantitative Profiling of WNT-3A Binding to All Human Frizzled Paralogues in HEK293 Cells by NanoBiT/BRET Assessments

The WNT signaling system governs critical processes during embryonic development and tissue homeostasis, and its dysfunction can lead to cancer. Details concerning selectivity and differences in relative binding affinities of 19 mammalian WNTs to the cysteine-rich domain (CRD) of their receptors—the ten mammalian Frizzleds (FZDs)—remain unclear. Here, we used eGFP-tagged mouse WNT-3A for a systematic analysis of WNT interaction with every human FZD paralogue in HEK293A cells. Employing HiBiT-tagged full-length FZDs, we studied eGFP-WNT-3A binding kinetics, saturation binding, and competition binding with commercially available WNTs in live HEK293A cells using a NanoBiT/BRET-based assay. Further, we generated receptor chimeras to dissect the contribution of the transmembrane core to WNT-CRD binding. Our data pinpoint distinct WNT-FZD selectivity and shed light on the complex WNT-FZD binding mechanism. The methodological development described herein reveals yet unappreciated details of the complexity of WNT signaling and WNT-FZD interactions, providing further details with respect to WNT-FZD selectivity.

Deconvolution of WNT-induced Frizzled conformational dynamics with fluorescent biosensors

The G protein-coupled receptors Frizzled_1-10 (FZD_1-10) act as molecular checkpoints mediating intracellular signaling induced by 19 mammalian, secreted Wingless/Int-1 lipoglycoproteins (WNTs). Despite the vital roles of these signaling components in health and disease, our knowledge about WNT/FZD selectivity, and the mechanisms of receptor activation and intracellular signal propagation by individual ligand/receptor pairs is limited due to the current lack of suitable biophysical techniques. Here, we developed fluorescence-based biosensors that detect WNT-induced FZD conformational changes in living cells in order to assess WNT action via FZDs at the most proximal level, i.e. the receptor conformation. By testing a panel of recombinant ligands on conformational biosensors representing all four homology clusters of FZDs, we discover yet unappreciated selectivities of WNTs to their receptors and, surprisingly, identify distinct ligand-induced receptor conformations. Furthermore, we demonstrate that FZDs can undergo conformational changes upon WNT binding without being dependent on the WNT co-receptors LRP5/6. This sensor toolbox provides an advanced platform for a thorough investigation of the 190 possible WNT/FZD pairings and for future screening campaigns targeting synthetic FZD ligands. Furthermore, our findings shed new light on the complexity of the WNT/FZD signaling system and have substantial implications for our understanding of fundamental biological processes including embryonal development and tumorigenesis.

Volumetric Compression Induces Intracellular Crowding to Control Intestinal Organoid Growth via Wnt/β-Catenin Signaling

Enormous amounts of essential intracellular events are crowdedly packed inside picoliter-sized cellular space. However, the significance of the physical properties of cells remains underappreciated because of a lack of evidence of how they affect cellular functionalities. Here, we show that volumetric compression regulates the growth of intestinal organoids by modifying intracellular crowding and elevating Wnt/β-catenin signaling. Intracellular crowding varies upon stimulation by different types of extracellular physical/mechanical cues and leads to significant enhancement of Wnt/β-catenin signaling by stabilizing the LRP6 signalosome. By enhancing intracellular crowding using osmotic and mechanical compression, we show that expansion of intestinal organoids was facilitated through elevated Wnt/β-catenin signaling and greater intestinal stem cell (ISC) self-renewal. Our results provide an entry point for understanding how intracellular crowdedness functions as a physical regulator linking extracellular physical cues with intracellular signaling and potentially facilitate the design of engineering approaches for expansion of stem cells and organoids.

Structural insight into small molecule action on Frizzleds

WNT-Frizzled (FZD) signaling plays a critical role in embryonic development, stem cell regulation and tissue homeostasis. FZDs are linked to severe human pathology and are seen as a promising target for therapy. Despite intense efforts, no small molecule drugs with distinct efficacy have emerged. Here, we identify the Smoothened agonist SAG1.3 as a partial agonist of FZD₆ with limited subtype selectivity. Employing extensive in silico analysis, resonance energy transfer- and luciferase-based assays we describe the mode of action of SAG1.3. We define the ability of SAG1.3 to bind to FZD₆ and to induce conformational changes in the receptor, recruitment and activation of G proteins and dynamics in FZD–Dishevelled interaction. Our results provide the proof-of-principle that FZDs are targetable by small molecules acting on their seven transmembrane spanning core. Thus, we provide a starting point for a structure-guided and mechanism-based drug discovery process to exploit the potential of FZDs as therapeutic targets.

WNT-Frizzled (FZD) signaling plays a critical role in embryonic development, tissue homeostasis and human disease but no small molecule drugs targeting FZD with distinct efficacy have emerged so far. Here, authors identify the Smoothened agonist SAG1.3 as a partial agonist for FZD6 with limited subtype selectivity.

Familial Exudative Vitreoretinopathy-Related Disease-Causing Genes and Norrin/β-Catenin Signal Pathway: Structure, Function, and Mutation Spectrums

Familial exudative vitreoretinopathy (FEVR) is a hereditary ocular disorder characterized by incomplete vascularization/abnormality of peripheral retina. Four of the identified disease-causing genes of FEVR were NDP, FZD4, LRP5, and TSPAN12, the protein coded by which were the components of the Norrin/β-catenin signal pathway. In this review, we summarized and discussed the spectrum of mutations involving these four genes. By the end of 2017, the number of FEVR causing mutations reported for NDP, FZD4, LRP5, and TSPAN12 was, respectively, 26, 121, 58, and 40. Three most frequently reported mutations were c. 362G > A (p.R121Q) of NDP, c. 313A > G (p.M105V), and c.1282_1285delGACA (p.D428SfsX2) of FZD4. Mutations have a tendency to cluster in some “hotspots” domains which may be responsible for protein interactions.

Molecular Pharmacology of Class F Receptor Activation

The class Frizzled (FZD) or class F of G protein-coupled receptors consists of 10 FZD paralogues and Smoothened (SMO). FZDs coordinate wingless/Int-1 signaling and SMO mediates Hedgehog signaling. Class F receptor signaling is intrinsically important for embryonic development and its dysregulation leads to diseases, including diverse forms of tumors. With regard to the importance of class F signaling in human disease, these receptors provide an attractive target for therapeutics, exemplified by the use of SMO antagonists for the treatment of basal cell carcinoma. Here, we review recent structural insights in combination with a more detailed functional understanding of class F receptor activation, G protein coupling, conformation-based functional selectivity, and mechanistic details of activating cancer mutations, which will lay the basis for further development of class F-targeting small molecules for human therapy. SIGNIFICANCE STATEMENT: Stimulated by recent insights into the activation mechanisms of class F receptors from structural and functional analysis of Frizzled and Smoothened, we aim to summarize what we know about the molecular details of ligand binding, agonist-driven conformational changes, and class F receptor activation. A better understanding of receptor activation mechanisms will allow us to engage in structure- and mechanism-driven drug discovery with the potential to develop more isoform-selective and potentially pathway-selective drugs for human therapy.

WNT-3A-induced β-catenin signaling does not require signaling through heterotrimeric G proteins

The network of Wingless/Int-1 (WNT)-induced signaling pathways includes β-catenin-dependent and -independent pathways. β-Catenin regulates T cell factor/lymphoid enhancer-binding factor (TCF/LEF)-mediated gene transcription, and in response to WNTs, β-catenin signaling is initiated through engagement of a Frizzled (FZD)/LDL receptor-related protein 5/6 (LRP5/6) receptor complex. FZDs are G protein-coupled receptors, but the question of whether heterotrimeric G proteins are involved in WNT/β-catenin signaling remains unanswered. Here, we investigate whether acute activation of WNT/β-catenin signaling by purified WNT-3A requires functional signaling through heterotrimeric G proteins. Using genome editing, we ablated expression of G_s/G_olf/G_q/G₁₁/G₁₂/G₁₃/G_z in HEK293 (ΔG7) cells, leaving the expression of pertussis toxin (PTX)-sensitive G_i/o proteins unchanged, to assess whether WNT-3A activates WNT/β-catenin signaling in WT and ΔG7 cells devoid of functional G protein signaling. We monitored WNT-3A-induced activation by detection of phosphorylation of LDL receptor-related protein 6 (LRP6), electrophoretic mobility shift of the phosphoprotein Dishevelled (DVL), β-catenin stabilization and dephosphorylation, and TCF-dependent transcription. We found that purified, recombinant WNT-3A efficiently induces WNT/β-catenin signaling in ΔG7 cells in both the absence and presence of G_i/o-blocking PTX. Furthermore, cells completely devoid of G protein expression, so called Gα-depleted HEK293 cells, maintain responsiveness to WNT-3A with regard to the hallmarks of WNT/β-catenin signaling. These findings corroborate the concept that heterotrimeric G proteins are not required for this FZD- and DVL-mediated signaling branch. Our observations agree with previous results arguing for FZD conformation-dependent functional selectivity between DVL and heterotrimeric G proteins. In conclusion, WNT/β-catenin signaling through FZDs does not require the involvement of heterotrimeric G proteins.

Structural insight into Class F receptors - What have we learnt regarding agonist-induced activation?

Class F receptors, including the ten Frizzleds (FZD_1-10 ) and SMO, mediate the effects of WNTs and hedgehog proteins and belong to the superfamily of G protein-coupled receptors (GPCRs). While the recent, high-resolution insight into mechanisms of GPCR activation provides a better understanding of receptor activation in Class A, B and C GPCRs, it remains unclear how Class F receptors bind their ligands, how ligand binding is translated to receptor activation and how signal initiation and specification are achieved. Here, we summarize recent efforts in elucidating Class F receptor structure and activation mechanisms and critically discuss the progress made in this area. A better understanding of the activation mechanisms of Class F receptors is required to engage in mechanism-based and structure-guided drug discovery to exploit the large therapeutic potential of targeting these receptors pharmacologically.