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

WNT-induced association of Frizzled and LRP6 is not sufficient for the initiation of WNT/β-catenin signaling

The Wingless/Int-1 (WNT) signaling network is essential to orchestrate central physiological processes such as embryonic development and tissue homeostasis. In the currently held tenet, WNT/β-catenin signaling is initiated by WNT-induced recruitment of Frizzleds (FZDs) and LRP5/6 followed by the formation of a multiprotein signalosome complex. Here, we use bioluminescence resonance energy transfer (BRET) to show that different WNT paralogs dynamically trigger FZD-LRP6 association. While WNT-induced receptor interaction was independent of C-terminal LRP6 phosphorylation, it was allosterically modulated by binding of the phosphoprotein Dishevelled (DVL) to FZD. WNT-16B emerged as a ligand of particular interest, as it efficiently promoted FZD-LRP6 association but, unlike WNT-3A, did not lead to WNT/β-catenin signaling. Transcriptomic analysis further revealed distinct transcriptional fingerprints of WNT-3A and WNT-16B stimulation in HEK293 cells. Additionally, single-molecule tracking demonstrated that, despite increasing FZD5 and LRP6 confinement, WNT-16B stimulation did not result in formation of higher-order receptor clusters, in contrast to WNT-3A. Our results suggest that FZD-WNT-LRP5/6 complex formation alone is not sufficient for the initiation of WNT/β-catenin signaling. Instead, we propose a two-step model, where initial ligand-induced FZD-LRP6 association must be followed by receptor clustering into higher-order complexes and subsequent phosphorylation of LRP6 for efficient activation of WNT/β-catenin signaling.

Mechanistic insights into Wnt-β-catenin pathway activation and signal transduction

In multicellular organisms, Wnt proteins govern stem and progenitor cell renewal and differentiation to regulate embryonic development, adult tissue homeostasis and tissue regeneration. Defects in canonical Wnt signalling, which is transduced intracellularly by β-catenin, have been associated with developmental disorders, degenerative diseases and cancers. Although a simple model describing Wnt-β-catenin signalling is widely used to introduce this pathway and has largely remained unchanged over the past 30 years, in this Review we discuss recent studies that have provided important new insights into the mechanisms of Wnt production, receptor activation and intracellular signalling that advance our understanding of the molecular mechanisms that underlie this important cell-cell communication system. In addition, we review the recent development of molecules capable of activating the Wnt-β-catenin pathway with selectivity in vitro and in vivo that is enabling new lines of study to pave the way for the development of Wnt therapies for the treatment of human diseases.

Receptor dimers and biased ligands: Novel strategies for targeting G protein-coupled receptors

G protein-coupled receptors (GPCRs) are the largest superfamily of membrane receptors. They regulate physiological and pathological processes such as metabolic homeostasis, cell proliferation and differentiation, and the immune response, and are one of the most important classes of drug targets, being targeted by 30-40 % of marketed drugs. A growing number of studies continue to reveal the complexity of GPCRs, especially their ability to interact with each other to form higher-order structures such as homodimers and heterodimers, which have different functions than monomers, and are involved in disease development and progression. The existence of GPCR homodimers and heterodimers is opening up new directions in drug discovery and development to harness their therapeutic potential. Particularly striking is the ability of GPCR dimers to trigger unique biased signalling pathways, which exquisitely balance the relationship between therapeutic effects and side effects. By suppressing adverse reactions and enhancing beneficial drug effects, GPCR dimers provide an unprecedented opportunity to minimise side effects, maximise therapeutic efficacy and enhance safety. This review aims to highlight the latest research advances in GPCR dimerization and GPCR-biased signalling, focusing on the development of dimer-targeting and biased ligands as innovative drugs that will likely provide new strategies for treating GPCR-related diseases as well as a better understanding of drug design for compounds that target GPCRs. GPCRs will play an increasingly important role in precision medicine and personalised therapy, leading us towards a safer, more efficient and smarter pharmaceutical future.

Frizzled receptors: gatekeepers of Wnt signaling in development and disease

Frizzled (FZD) receptors are a subset of G-protein-coupled receptors (GPCRs), the largest class of human cell surface receptors and a major target of FDA-approved drugs. Activated by Wnt ligands, FZDs regulate key cellular processes such as proliferation, differentiation, and polarity, positioning them at the intersection of developmental biology and disease, including cancer. Despite their significance, FZD signaling remains incompletely understood, particularly in distinguishing receptor-specific roles across canonical and non-canonical Wnt pathways. Challenges include defining ligand-receptor specificity, elucidating signal transduction mechanisms, and understanding the influence of post translational modifications and the cellular context. Structural dynamics, receptor trafficking, and non-canonical signaling contributions also remain areas of active investigation. Recent advances in structural biology, transcriptomics, and functional genomics are beginning to address these gaps, while emerging therapeutic approaches-such as small-molecule modulators and antibodies-highlight the potential of FZDs as drug targets. This review synthesizes current insights into FZD receptor biology, examines ongoing controversies, and outlines promising directions for future research and therapeutic development.

A Putative Frizzled 7-Targeting Compound Acts as a Firefly Luciferase Inhibitor

The Frizzled family (FZD1-10) of G protein-coupled receptors regulates WNT signaling mediating proliferative input. Dysregulation of FZD7 and exaggerated WNT/β-catenin signaling is frequently observed in intestinal cancers. Therefore, it is attractive to develop therapeutics targeting FZD7 for cancer treatment. Structure-based virtual screening has identified compound 28, which inhibited WNT/β-catenin signaling based on the luciferase-based reporter gene TOPFlash assay. However, upon pharmacological validation, compound 28 rather acts as a potent Firefly luciferase (Fluc) inhibitor (IC50 = 30 nM), matching the reported IC50 for compound 28-mediated inhibition in the TOPFlash assay. Moreover, we employed Fluc-independent assays, a FZD7-focused bioluminescence resonance energy transfer biosensor and quantitative PCR, to emphasize the inability of compound 28 to inhibit the WNT-3A-induced conformational dynamics in FZD7 and transcription of Axin2, a WNT target gene. Thus, we underline the importance of counter screens to validate compounds that interfere with the detection technology used for compound screening.

Roles of miRNAs in regulating ovarian cancer stemness

Ovarian cancer is one of the gynaecology malignancies with the highest mortality rate. Ovarian cancer stem cell (CSC) is a subpopulation of ovarian cancer cells with increased self-renewability, aggression, metastatic potentials, and resistance to conventional anti-cancer therapy. The emergence of ovarian CSC is a critical factor that promotes treatment resistance and frequent relapse among ovarian cancer patients, leading to poor clinical outcomes. MicroRNA (miRNA) is a short, non-protein-coding RNA that regulates ovarian CSC development. Although multiple original research articles have discussed the CSC-regulatory roles of different miRNAs in ovarian cancer, there is a deficiency of a review article that can summarize the findings from different research papers. To narrow the gap in the literature, this review aimed to provide an up-to-date summary of the CSC-regulatory roles of various miRNAs in modulating ovarian cancer cell stemness. This review will begin by giving an overview of ovarian CSC and the pathways responsible for driving its appearance. Next, the CSC-regulatory roles of miRNAs in controlling ovarian CSC development will be discussed. Overall, more than 60 miRNAs have been reported to play CSC-regulatory roles in the development and progression of ovarian cancer. By targeting various downstream targets, these miRNAs can control the signaling activities of PI3K/AKT, EGFR/ERK, WNT/ß-catenin, NF-kß, Notch, Hippo/YAP, EMT, and DNA repair pathways. Hence, these CSC-modulatory miRNAs have the potential to be used as prognostic biomarkers in predicting the clinical outcomes of ovarian cancer patients. Targeting CSC-promoting miRNAs or increasing the expressions of CSC-repressing miRNAs can help slow ovarian cancer progression. However, more in-depth functional and clinical trials must be carried out to evaluate the suitability, safety, sensitivity, and specificity of these CSC-regulating miRNAs as prognostic biomarkers or therapeutic targets.

International Union of Basic and Clinical Pharmacology CXV: The Class F of G Protein-Coupled Receptors

The class F of G protein-coupled receptors (GPCRs) consists of 10 Frizzleds (FZD1-10) and Smoothened (SMO). FZDs bind and are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, and SMO is indirectly activated by the Hedgehog (Hh) family of morphogens acting on the transmembrane protein Patched. The advance of our understanding of FZDs and SMO as dynamic transmembrane receptors and molecular machines, which emerged during the past 14 years since the first-class F GPCR IUPHAR nomenclature report, justifies an update. This article focuses on the advances in molecular pharmacology and structural biology providing new mechanistic insight into ligand recognition, receptor activation mechanisms, signal initiation, and signal specification. Furthermore, class F GPCRs continue to develop as drug targets, and novel technologies and tools such as genetically encoded biosensors and CRISP/Cas9 edited cell systems have contributed to refined functional analysis of these receptors. Also, advances in crystal structure analysis and cryogenic electron microscopy contribute to the rapid development of our knowledge about structure-function relationships, providing a great starting point for drug development. Despite the progress, questions and challenges remain to fully understand the complexity of the WNT/FZD and Hh/SMO signaling systems. SIGNIFICANCE STATEMENT: The recent years of research have brought about substantial functional and structural insight into mechanisms of activation of Frizzleds and Smoothened. While the advance furthers our mechanistic understanding of ligand recognition, receptor activation, signal specification, and initiation, broader opportunities emerge that allow targeting class F GPCRs for therapy and regenerative medicine employing both biologics and small molecule compounds.

The 75-Year Anniversary of the Department of Physiology and Pharmacology at Karolinska Institutet-Examples of Recent Accomplishments and Future Perspectives

Karolinska Institutet is a medical university encompassing 21 departments distributed across three departmental or campus groups. Pharmacological research has a long and successful tradition at the institute with a multitude of seminal findings in the areas of neuronal control of vasodilatation, cardiovascular pharmacology, neuropsychopharmacology, receptor pharmacology, and pharmacogenomics that resulted in, among many other recognitions, two Nobel prizes in Physiology and Medicine, one in 1970 to Ulf von Euler for his discovery of the processes involved in storage, release, and inactivation of neurotransmitters and the other in 1982 to Sune Bergström and Bengt Samuelsson for their work on prostaglandins and the discovery of leukotrienes. Pharmacology at Karolinska Institutet has over the last decade been ranked globally among the top 10 according to the QS World University Ranking. With the Department of Physiology and Pharmacology now celebrating its 75-year anniversary, we wanted to take this as an opportunity to showcase recent research achievements and how they paved the way for current activities at the department. We emphasize examples from preclinical and clinical research where the dpartment's integrative environment and robust infrastructure have successfully facilitated the translation of findings into clinical applications and patient benefits. The close collaboration between preclinical scientists and clinical researchers across various disciplines, along with a strong network of partnerships within the department and beyond, positions us to continue leading world-class pharmacological research at the Department of Physiology and Pharmacology for decades to come. SIGNIFICANCE STATEMENT: Pharmacological research at Karolinska Institutet has a long and successful history. Given the 75-year anniversary of the Department of Physiology and Pharmacology, this perspective provides an overview of recent departmental achievements and future trajectories. For these developments, interdisciplinary and intersectoral collaborations and a clear focus on result translation are key elements to continue its legacy of world-leading pharmacological research.

Class-Wide Analysis of Frizzled-Dishevelled Interactions Using BRET Biosensors Reveals Functional Differences among Receptor Paralogs

Wingless/Int-1 (WNT) signaling is mediated by WNT binding to 10 Frizzleds (FZD1-10), which propagate the signal inside the cell by interacting with different transducers, most prominently the phosphoprotein Dishevelled (DVL). Despite recent progress, questions about WNT/FZD selectivity and paralog-dependent differences in the FZD/DVL interaction remain unanswered. Here, we present a class-wide analysis of the FZD/DVL interaction using the DEP domain of DVL as a proxy in bioluminescence resonance energy transfer (BRET) techniques. Most FZDs engage in a constitutive high-affinity interaction with DEP. Stimulation of unimolecular FZD/DEP BRET sensors with different ligands revealed that most paralogs are dynamic in the FZD/DEP interface, showing distinct profiles in terms of ligand selectivity and signal kinetics. This study underlines mechanistic differences in terms of how allosteric communication between FZDs and their main signal transducer DVL occurs. Moreover, the unimolecular sensors represent the first receptor-focused biosensors to surpass the requirements for high-throughput screening, facilitating FZD-targeted drug discovery.

Structural basis of frizzled 7 activation and allosteric regulation

Frizzleds (ten paralogs: FZD1-10) belong to the class F of G protein-coupled receptors (GPCRs), which remains poorly understood despite its crucial role in multiple key biological functions including embryonic development, stem cell regulation, and homeostasis in the adult. FZD7, one of the most studied members of the family, is more specifically involved in the migration of mesendoderm cells during the development and renewal of intestinal stem cells in adults. Moreover, FZD7 has been highlighted for its involvement in tumor development predominantly in the gastrointestinal tract. This study reports the structure of inactive FZD7, without any stabilizing mutations, determined by cryo-electron microscopy (cryo-EM) at 1.9 Å resolution. We characterize a fluctuating water pocket in the core of the receptor important for FZD7 dynamics. Molecular dynamics simulations are used to investigate the temporal distribution of those water molecules and their importance for potential conformational changes in FZD7. Moreover, we identify lipids interacting with the receptor core and a conserved cholesterol-binding site, which displays a key role in FZD7 association with a transducer protein, Disheveled (DVL), and initiation of downstream signaling and signalosome formation.

The emerging understanding of Frizzled receptors

The Wnt signaling pathway is a huge network governing development and homeostasis, dysregulation of which is associated with a myriad of human diseases. The Frizzled receptor (FZD) family comprises receptors for Wnt ligands, which indispensably mediate Wnt signaling jointly with a variety of co-receptors. Studies of FZDs have revealed that 10 FZD subtypes play diverse roles in physiological processes. At the same time, dysregulation of FZDs is also responsible for various diseases, in particular human cancers. Enormous attention has been paid to the molecular understanding and targeted therapy of FZDs in the past decade. In this review, we summarize the latest research on FZD structure, function, regulation and targeted therapy, providing a basis for guiding future research in this field.

Role of the Ror family receptors in Wnt5a signaling

Ror-family receptors, Ror1 and Ror2, are type I transmembrane proteins that possess an extracellular cysteine-rich domain, which is conserved throughout the Frizzled-family receptors and is a binding site for Wnt ligands. Both Ror1 and Ror2 function primarily as receptors or co-receptors for Wnt5a to activate the β-catenin-independent, non-canonical Wnt signaling, thereby regulating cell polarity, migration, proliferation, and differentiation depending on the context. Ror1 and Ror2 are expressed highly in many tissues during embryogenesis but minimally or scarcely in adult tissues, with some exceptions. In contrast, Ror1 and Ror2 are expressed in many types of cancers, and their high expression often contributes to the progression of the disease. Therefore, Ror1 and Ror2 have been proposed as potential targets for the treatment of the malignancies. In this review, we provide an overview of the regulatory mechanisms of Ror1/Ror2 expression and discuss how Wnt5a-Ror1/Ror2 signaling is mediated and regulated by their interacting proteins.

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.

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.

Rho family small GTPase Rif regulates Wnt5a-Ror1-Dvl2 signaling and promotes lung adenocarcinoma progression

Rho in filopodia (Rif), a member of the Rho family of small GTPases, induces filopodia formation primarily on the dorsal surface of cells; however, its function remains largely unclear. Here, we show that Rif interacts with Ror1, a receptor for Wnt5a that can also induce dorsal filopodia. Our immunohistochemical analysis revealed a high frequency of coexpression of Ror1 and Rif in lung adenocarcinoma. Lung adenocarcinoma cells cultured on Matrigel established front-rear polarity with massive filopodia on their front surfaces, where Ror1 and Rif were accumulated. Suppression of Ror1 or Rif expression inhibited cell proliferation, survival, and invasion, accompanied by the loss of filopodia and cell polarity in vitro, and prevented tumor growth in vivo. Furthermore, we found that Rif was required to activate Wnt5a-Ror1 signaling at the cell surface leading to phosphorylation of the Wnt signaling pathway hub protein Dvl2, which was further promoted by culturing the cells on Matrigel. Our findings reveal a novel function of Rif in mediating Wnt5a-Ror1-Dvl2 signaling, which is associated with the formation of polarized filopodia on 3D matrices in lung adenocarcinoma cells.