Gunnar Schulte: Frizzled Receptors and the GPCR Identity Question

Frizzled receptors are a family of 10 cell surface proteins that mediate Wnt signaling — one of the most fundamental pathways in vertebrate development, stem cell renewal, and tumor biology. Despite decades of study, the field remains divided on a foundational question: are frizzled receptors GPCRs? They carry the characteristic seven-transmembrane topology, but their ligands are large, lipid-modified proteins that require carrier molecules for transport, their canonical Wnt–beta-catenin signaling appears G-protein-independent, and almost no small molecule pharmacology exists for this subfamily.

Gunnar Schulte's lab at Karolinska Institute has spent years assembling the evidence on the other side — demonstrating G-protein coupling specificity across individual frizzled subtypes, detecting ligand-induced conformational changes through biosensor approaches that mirror what is seen in classical GPCRs, and showing that G-protein-dependent pathways downstream of frizzled activation are physiologically relevant.

For Schulte, the scarcity of pharmacological tools is not just a technical inconvenience — it is the central obstacle that has shaped every decision his lab has made since his first postdoc. The question of whether frizzled receptors are GPCRs is, in his view, inseparable from the question of whether they can ever be drugged.

About the Guest

Gunnar Schulte is a professor at the Department of Physiology and Pharmacology at Karolinska Institute in Stockholm, where he leads research on Wnt–frizzled receptor signaling. His lab investigates the molecular mechanisms of frizzled receptor activation, including G-protein coupling specificity across all 10 human frizzled subtypes, receptor conformational dynamics, and biosensor platforms for quantifying signaling selectivity.

Schulte's work helped establish that individual frizzled receptors are not interchangeable — each couples to distinct G-protein families in ways that challenge the field's conventional focus on the beta-catenin pathway alone. He began his scientific career studying adenosine receptor signaling during a PhD with Bertil Fredholm at Karolinska, and pivoted to frizzled receptors during a postdoctoral fellowship with Ernest Arenas, staying at the same institution ever since.

Scientific Themes of the Conversation

1. The GPCR identity question — what it takes to make the case, and why the field is still divided
   

2. G-protein coupling specificity across the 10 frizzled subtypes — each receptor is its own pharmacological entity
   

3. The Wnt ligand problem — lipid modifications, carrier proteins, and what makes these ligands so difficult to work with
   

4. Conformational dynamics as functional evidence — how biosensors are being used to argue for GPCR-like activation in frizzled
   

5. Pathway selectivity — how the receptor may choose between disheveled and G-protein signaling through conformational selection
   

6. The pharmacology gap — small molecules, repurposed Smoothened compounds, and what a druggable frizzled might eventually look like

Key Insights from the Conversation

1. Each frizzled receptor is its own pharmacological entity

The frizzled family is commonly discussed as a single target class, but Schulte's G-protein coupling data reveal that individual subtypes are functionally distinct. Frizzled 5 couples to Gq, frizzled 7 to Gs, frizzled 10 to G13 — and the pattern continues across the family without a simple organizing rule. Treating "frizzled" as a unified target, Schulte argues, is one reason the field has been slow to build pharmacology.

2. The Wnt ligand problem may be the field's deepest bottleneck

Wnt ligands are approximately 35–40 kDa, carry a lipid modification that prevents them from diffusing freely in aqueous medium, and require carrier proteins to reach their receptors. Fewer than one in 19 Wnt ligands is currently available in a biologically active, tagged form suitable for binding assays. Schulte describes spending a substantial portion of his lab budget on recombinant Wnts while still unable to run the systematic interaction mapping the field needs.

3. Conformational change is Schulte's strongest argument for GPCR identity

Using cpGFP-based conformational sensors inserted into frizzled receptors, Schulte's lab has shown that Wnt stimulation produces detectable conformational changes that parallel what is observed in classical GPCRs. For Schulte, this is not secondary evidence — it is the clearest demonstration that frizzled receptors operate by the same fundamental activation mechanism, and the clearest challenge to those who believe otherwise.

4. A Smoothened compound became the first pharmacological handle on frizzled

SAG1.3, a small molecule agonist originally developed for the related receptor Smoothened, was shown by Schulte's lab to act as a weak partial agonist at frizzled 6. Every chemical modification so far has converted it into an antagonist rather than a stronger agonist. But the result was the first demonstration that frizzled receptors have a pharmacologically accessible binding pocket — a conclusion that was not obvious from the initial frizzled 4 crystal structure, which suggested the pocket was too small for small molecule targeting.

5. The class F CRISPR knockout cell line changed what questions could be asked

Working in cells that endogenously express multiple frizzled subtypes makes it nearly impossible to attribute a signaling event to a single receptor. The development of a HEK cell line with all class F receptors knocked out — generated by Benoît Vanhollebeke's lab in Belgium and shared with the field — gave Schulte's group the clean background needed to study individual frizzled subtypes in isolation. Combined with porcupine inhibitors to suppress endogenous Wnt secretion, this system has become what Schulte describes as one of the most important practical advances the field has seen in years.

6. Disheveled may regulate access to both G-protein and beta-catenin pathways through conformational selection

Disheveled (DVL), the scaffolding protein downstream of frizzled, is not simply a beta-catenin pathway component. Schulte's lab has preliminary evidence that disheveled undergoes conformational selection at the receptor — analogous to the way G proteins adopt receptor-stabilized conformations before activating. This would mean the receptor itself determines whether a signal flows through disheveled or through a G protein depending on its conformational state, a form of pathway selectivity that remains to be fully mapped in the frizzled system.

7. A PhD lesson in pharmacological humility that lasted 25 years

During his thesis seminar, Schulte presented data attributing an unexpected signaling connection to a kinase pathway he had dissected with pharmacological inhibitors. His supervisor, Bertil Fredholm, asked a single question: had he checked whether those inhibitors were also antagonists at the adenosine receptors under study? Several were — the signal Schulte had mapped was an off-target receptor effect, not the kinase pathway he had concluded. He describes the experience as formative, and the lesson as one that has quietly shaped his approach to chemical probes ever since.

Episode Timeline

Timestamps are AI-generated from the transcript and may not reflect the final edited video exactly.

00:00 Introduction

00:48 From Berlin to Stockholm — a career redirected by armed robbery

08:30 The adenosine receptor talk that pulled Schulte into GPCRs

11:14 From adenosine to frizzled: the postdoc decision that defined the lab

17:43 Frizzled receptors: 10 subtypes, 19 ligands, almost no drugs

20:22 When Wnt–frizzled signaling goes wrong — cancer, fibrosis, nail dysplasia

28:23 G-protein coupling specificity across the frizzled family

34:55 SAG1.3: repurposing a Smoothened compound to activate frizzled 6

40:51 Where beta-arrestin and disheveled fit in Wnt signaling

47:01 The class F CRISPR knockout cell line that changed everything

53:43 Conformational change as the strongest argument for GPCR identity

01:03:11 Career advice: explore widely, then commit to a niche

Selected Quotes

About Dr. Gunnar Schulte

Gunnar Schulte is a Professor in receptor pharmacology and research group leader for the section Receptor Biology and Signaling at the Department of Physiology and Pharmacology. He has a background in biochemistry from the Free University in Berlin/Germany and a Ph.D. in molecular pharmacology (supervisor: Bertil B Fredholm; 1998-2002) from Karolinska Institutet.

As a postdoc, he trained first with Ernest Arenas (Karolinska Institutet, Molecular Neurobiology; 2003-2005) and later with Roger J Summers (Monash University, Melbourne Australia, GPCR pharmacology; 2006) before starting his independent research team "Receptor Biology & Signalling" in 2008. Gunnar Schulte is also the scientific secretary of the Swedish Society for Medical Research (SSMF) and a member of the editorial board/editorial advisory board of Molecular Pharmacology, British Journal of Pharmacology, Pharmacological Reviews, and The Journal of Biological Chemistry.

General Research Interest: The focus in the Schulte lab is on Frizzled signaling and pharmacology aiming to understand the role of WNT/Frizzled signaling in biology, physiology, and disease. Most importantly my research team tries to understand underlying mechanisms of WNT-receptor interaction, the relevance of receptor dynamics, and receptor complex composition for signal initiation and specification. The ultimate aim is to use the new knowledge to find, create and optimize Frizzled-targeting small molecule drugs to improve future therapies of human disease.

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