By Phoebe Ingraham Renda
Photography by Rayni Shiring, University of Pittsburgh
Cell signaling is how cells communicate at the molecular level to coordinate biological processes that shape our development, maintain our health and sometimes contribute to illness. That communication is an intricate blur of molecular motion, where molecules collide, bind and separate in fleeting but highly specific ways that seem chaotic to an unfamiliar eye—much like a pinball game.
Biology’s Skill Shot
Think of a pinball game in action. To a novice, the ball seems to move chaotically, bouncing off bumpers, rolling through chutes, activating lights, ringing bells and scoring points. But, to a pinball wizard, that point-earning movement is nonrandom and governed by the laws of physics and the structural layout of the game’s playfield—the surface inside a pinball machine where the game takes place.
In both cases, what seems chaotic is actually an orchestrated process directed by structure and physics. A pinball machine's design ensures the ball can physically hit specific targets to score points and—to achieve maximum points—those targets and actions need to be completed in a specific sequence as detailed in the game’s rule card.
Outside of a cell, signaling ligands, like the ball, navigate through the extracellular space—the equivalent of a pinball playfield—which is a supportive network surrounding cells that guides ligands to receptors on the cell surface. Like the playfield, cellular structures facilitate ligand movement with precise timing and aim to direct them to the right receptors in the correct order to trigger signaling pathways and transmit a signal to the cell’s interior. Just as the ball's path is influenced by the placement of bumpers, ramps and chutes, the ligands' journey is directed by the location and atomic-level structure of coreceptors and signaling receptors on the cell’s surface. Similar to the rule card, cell signaling studies have provided insight into the sequences of signaling interactions and events that describe how a ligand binds to and assembles its receptors into an active signaling complex (a group of proteins that work together), which needs to take place to "score points” and drive key cellular outcomes.
Like each pinball machine, every signaling process is unique—each has different rules, geometries and objectives.
Introducing TGF-β Pinball Wizard Andrew Hinck
Andrew Hinck, professor of structural biology, University of Pittsburgh School of Medicine, has been mastering the “gameplay” of TGF-β signaling for more than 30 years. Known for his exceptional ability to visualize and think in terms of atoms, he has detailed TGF-β's structure and physics with remarkable precision.
Andrew Hinck, professor of structural biology, University of Pittsburgh School of Medicine, standing next to the 800 MHz NMR spectrometer optimized for protein studies in the Biomedical Science Tower 3, University of Pittsburgh.
His journey into the world of TGF-β pinball began when he was a postdoctoral fellow at the National Institutes of Health, and then, with establishment of his own laboratory a few years later, he began to impress both peers and spectators with his mastery.
Hinck's favorite part of TGF-β is understanding the way it manages to orchestrate the cooperative assembly of its receptors into an active signaling complex, where he has achieved some of the highest levels of expertise.
Hinck has earned a reputation as one of the top players in the field, as he codirects Pitt’s Molecular Biophysics and Structural Biology graduate program and has published more than 110 research papers. His most recent publication in Nature Communications was 15 years in the making and details, for the first time, the way betaglycan helps gather and assemble the TGF-β signaling complex to boost signal transmission.
Off the TGF-β research floor, Hinck is known for his dedication to the research community, sharing his knowledge and skills with fellow researchers in the public research domain. His work detailing the structures of TGF-β signaling components, and how those structures work together, sheds light on how the “game” works. With structural and mechanistic knowledge, researchers—like Pitt cancer immunologist Greg M. Delgoffe—can better interpret clinical data and design targeted therapies, like therapies that leverage biology and harness the power of a patient’s own immune system to effectively fight cancer.
Let’s Play TGF-β Pinball
In the intricate TGF-β signaling pinball game, a key element is the coreceptor betaglycan, which works like the flippers to keep the game going. In the “game,” betaglycan catches TGF-β from outside the cell (extracellular space) and helps build the signaling complex around it, which boosts TGF-β signal transmission.
Betaglycan is essential for life—embryos cannot develop without it—but also plays a villainous role as a powerful ally to cancer. TGF-β signaling has immune-suppressing functions, which cancerous cells use to cloak themselves from immune system surveillance—making them impenetrable to immunotherapies. This survival paradox highlights the complexity of TGF-β signaling and presents a significant challenge for researchers aiming to “master the game.”
| Pinball Element | Gameplay Role | TGF-β Signaling Element | Signaling “Gameplay” Role |
|---|---|---|---|
| Plunger | Gets the ball going | Integrins/Proteases | Release TGF-β from its inhibitory prodomain |
| Ball | Earns points and completes game objectives by hitting targets | TGF-β | Signaling ligand |
| Playfield | Area in which the pinball moves around to hit targets | Extracellular space | The area where the TGF-β diffuses to interact with receptors on the cell surface to initiate signaling |
| Flippers | Keep the ball in play and aims it toward targets | Betaglycan | The coreceptor that binds TGF-β and promotes subsequent binding of the signal receptors |
| Target banks | Point-earning target that often spells out words to activate the next game mode | TGF-β receptor type I (TGFBR1) | The final receptor that must bind to initiate signaling |
| Ramps | Point-earning target | TGF-β receptor type II (TGFBR2) | The first receptor that is recruited after TGF-β is captured by betaglycan |
| Bumpers | Elements that interfere with the ball’s motion | Thermal fluctuations | Enable the displacement of betaglycan as TGFBR2 and TGFBR1 bind |
Mastering the Game
In the world of structural biology, researchers like Hinck meticulously reverse engineer the layout and precise structure of the TGF-β signaling "playfield.” Using the "TGF-β signaling rule card" as a guide, Hinck studies the interactions between the ball (ligand) and the playfield elements (receptors and other proteins) to piece together how these elements are arranged and how they physically work together to achieve each objective.
To map out these configurations, Hinck and other structural biologists employ techniques like X-ray crystallography, which reveals protein structures based on how their shadows look from different angles, and CryoEM, which creates 3D images by taking very high-resolution pictures of the protein from multiple perspectives. Advanced computer technology, like Coot, then helps identify how the protein's chemical sequence fits into that 3D structure.
Through this detailed structural detective work, Hinck can visualize where and how the ligand interacts with specific receptors to orchestrate TGF-β signaling events. With this knowledge, researchers can design drugs that mimic these interactions or block unwanted signaling processes.
Researchers have already used the known structures of TGF-β bound to receptors (TGFBR1 and TGFBR2) to develop agents that are being tested in clinical trials as adjuncts—additional treatments used alongside the primary therapy—to enhance the effectiveness of cancer immunotherapies.
While structural insights into how betaglycan binds to TGF-β haven’t been leveraged in the same way yet, Hinck and his team are beginning to explore its potential.
“When facing a disease that is as complex, varied and challenging to treat as cancer, we need to think and approach the development of effective therapies, not as a quick match, but as a tournament—maybe even a championship,” says Hinck.