 Have a look at these two girls. Can you see any similarities? I can’t - and yet, they’re twin sisters. Just like these sisters, there are proteins whose folding or functional relationships can only be discerned through sophisticated analysis based on a deep understanding of the molecular evolution of protein structures. The cytokine IL-33 is an excellent example of a long-lost molecular sibling; this is the story of how it was found. Today, IL-33 and its receptor, ST2, are the focus of intense research in both academia and industry. But as recently as 2005, ST2 was an intriguing but orphan IL-1 family receptor whose function remained elusive since its cloning 16 years earlier by Shin-Ichi Tominaga’s group. Investigators at several biotechnology companies (notably Millennium - now Takeda) deduced that dysregulation of ST2 was likely tied to chronic autoimmune and inflammatory diseases, and sought to block its activity with antibodies. 4th & Aspen's principal consultant, Fernando Bazan, recalls: “Although research discerned the expression pattern of ST2, and gleaned that it was involved in certain Th2-like immune responses, the field could not move forward without identifying the physiological ligand and understanding the triggering of ST2 signaling inside cells.” Yet, despite many efforts, the ST2 ligand stubbornly eluded identification. | Fernando and his team at the DNAX Research Institute finally cracked the case, based on the logic that since ST2 is a distant member of the IL-1 receptor family, it should engage a β-trefoil fold cytokine from the IL-1 family. Using structural insights and the power of expanding genomic databases, Fernando iteratively refined his computational searches until a candidate molecule was fished out of a genomic screen of canine trauma-induced genes. This protein, and its deduced human homologue, lacked a signal peptide, but instead possessed a hallmark pro-domain similar to non-classically secreted IL-1 cytokines, as well as a predicted β-trefoil domain. This discovery was the newest member of the IL-1 family, christened IL-33. Fernando and his colleagues rapidly cloned and expressed the globular domain of IL-33 and showed that the cytokine binds and signals through ST2. “Finally finding the ligand for ST2 permitted us and others to show that inflammation is exacerbated when you stimulate ST2-bearing cells with recombinant IL-33,” Fernando recalls. The team’s seminal publication in Immunity has been cited over 850 times, and sparked research that attests to IL-33’s diverse roles in a range of disorders including infection, cardiovascular disease, and kidney disease. As for its role in inflammation, “we now know that the Th2 connection is likely due to the stimulation by IL-33 of a very specialized class of IL-33 responsive immune cells called innate lymphoid cells or ILCs, which then secrete Th2-like cytokines,” says Fernando. “That means that the field now has new cellular targets for therapeutic efforts above and beyond the discrete IL-33 signaling complex, benefiting people suffering from a wide range of debilitating diseases.” Fernando continued to characterize IL-33 with colleagues at Genentech and Stanford, culminating in publications in Structure and Nature Structural & Molecular Biology. This work has led to a comprehensive picture of IL-1 family receptor complexes. | So, what can the IL-33 story teach us? Just like those twin girls in the picture look nothing like each other, in the protein world as well, it's important to focus on deeper evolutionary links to gain clues about the relationships between molecules, complexes and signaling networks. And armed with that kind of information, therapeutics R&D is a lot less guesswork. |
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