Kenan Christopher Garcia

Structural Basis of Wnt Recognition by Frizzled

Dissecting Wnt/Fz Interaction Wnt proteins activate the transmembrane receptor Frizzled (Fz) to initiate pathways central to vertebrate and invertebrate development. Wnts are palmitoylated, which has complicated structural and functional characterization. Janda et al. (p. 59, published online 31 May; see the Perspective by Bienz and He) achieved coexpression and purification of Xenopus Wnt8 (XWnt8) with mouse Fz8 cysteine rich domain (CRD) and determined a crystal structure of the complex. Wnt binds to the Fz8 CRD at two distinct sites with the lipid group playing a key role in the first interface. Both interfaces involve conserved amino acids, which may explain the known pleiotropy of the Wnt/Fz interaction. The structure of the morphogen Wnt bound to its receptor provides a basis for understanding Wnt’s functional pleiotropy. Wnts are lipid-modified morphogens that play critical roles in development principally through engagement of Frizzled receptors. The 3.25 angstrom structure of Xenopus Wnt8 (XWnt8) in complex with mouse Frizzled-8 (Fz8) cysteine-rich domain (CRD) reveals an unusual two-domain Wnt structure, not obviously related to known protein folds, resembling a “hand” with “thumb” and “index” fingers extended to grasp the Fz8-CRD at two distinct binding sites. One site is dominated by a palmitoleic acid lipid group projecting from serine 187 at the tip of Wnt’s thumb into a deep groove in the Fz8-CRD. In the second binding site, the conserved tip of Wnt’s “index finger” forms hydrophobic amino acid contacts with a depression on the opposite side of the Fz8-CRD. The conservation of amino acids in both interfaces appears to facilitate ligand-receptor cross-reactivity, which has important implications for understanding Wnt’s functional pleiotropy and for developing Wnt-based drugs for cancer and regenerative medicine.

Structural basis for Notch1 engagement of Delta-like 4

An interaction that guides cell fate Notch signaling is important in cell fate determination in mammals. Signaling is initiated when the extracellular domain of the transmembrane Notch protein on one cell binds to a surface ligand on another cell. Luca et al. report the crystal structure of the interacting regions of Notch and the Delta-like ligand DLL-4. The Notch protein is modified by O-linked glycan addition, and this is required for signaling. The structure shows two interaction interfaces. A glycan anchors the less conserved interface, which potentially provides a flexible way of regulating Notch interactions during development. Science, this issue p. 847 O-linked glycosylation regulates a receptor-ligand interaction that is important in cell fate determination. Notch receptors guide mammalian cell fate decisions by engaging the proteins Jagged and Delta-like (DLL). The 2.3 angstrom resolution crystal structure of the interacting regions of the Notch1-DLL4 complex reveals a two-site, antiparallel binding orientation assisted by Notch1 O-linked glycosylation. Notch1 epidermal growth factor–like repeats 11 and 12 interact with the DLL4 Delta/Serrate/Lag-2 (DSL) domain and module at the N-terminus of Notch ligands (MNNL) domains, respectively. Threonine and serine residues on Notch1 are functionalized with O-fucose and O-glucose, which act as surrogate amino acids by making specific, and essential, contacts to residues on DLL4. The elucidation of a direct chemical role for O-glycans in Notch1 ligand engagement demonstrates how, by relying on posttranslational modifications of their ligand binding sites, Notch proteins have linked their functional capacity to developmentally regulated biosynthetic pathways.

Design of a superior cytokine antagonist for topical ophthalmic use

IL-1 is a key inflammatory and immune mediator in many diseases, including dry-eye disease, and its inhibition is clinically efficacious in rheumatoid arthritis and cryopyrin-associated periodic syndromes. To treat ocular surface disease with a topical biotherapeutic, the uniqueness of the site necessitates consideration of the agent’s size, target location, binding kinetics, and thermal stability. Here we chimerized two IL-1 receptor ligands, IL-1β and IL-1Ra, to create an optimized receptor antagonist, EBI-005, for topical ocular administration. EBI-005 binds its target, IL-1R1, 85-fold more tightly than IL-1Ra, and this increase translates to an ∼100-fold increase in potency in vivo. EBI-005 preserves the affinity bias of IL-1Ra for IL-1R1 over the decoy receptor (IL-1R2), and, surprisingly, is also more thermally stable than either parental molecule. This rationally designed antagonist represents a unique approach to therapeutic design that can potentially be exploited for other β-trefoil family proteins in the IL-1 and FGF families.

Instructive roles for cytokine-receptor binding parameters in determining signaling and functional potency.

Mathematical modeling of the cellular responses to cytokine variants of differing binding affinities may help design better therapies. Modeling cytokine behavior The use of cytokines, such as interleukin-2 (IL-2) or IL-13, as therapies has been hampered by the fact that many cytokines share receptor subunits on different cell types. Moraga et al. generated recombinant variants of IL-13 with a wide range of binding affinities for the IL-13 receptor. Mathematical modeling of the correlation between the receptor binding affinities of the variants and the extents to which they differentially stimulated early and late cellular responses highlighted aspects of receptor-ligand binding properties that should aid in the development of more effective cytokine therapies. Cytokines dimerize cell surface receptors to activate signaling and regulate many facets of the immune response. Many cytokines have pleiotropic effects, inducing a spectrum of redundant and distinct effects on different cell types. This pleiotropy has hampered cytokine-based therapies, and the high doses required for treatment often lead to off-target effects, highlighting the need for a more detailed understanding of the parameters controlling cytokine-induced signaling and bioactivities. Using the prototypical cytokine interleukin-13 (IL-13), we explored the interrelationships between receptor binding and a wide range of downstream cellular responses. We applied structure-based engineering to generate IL-13 variants that covered a spectrum of binding strengths for the receptor subunit IL-13Rα1. Engineered IL-13 variants representing a broad range of affinities for the receptor exhibited similar potencies in stimulating the phosphorylation of STAT6 (signal transducer and activator of transcription 6). Delays in the phosphorylation and nuclear translocation of STAT6 were only apparent for those IL-13 variants with markedly reduced affinities for the receptor. From these data, we developed a mechanistic model that quantitatively reproduced the kinetics of STAT6 phosphorylation for the entire spectrum of binding affinities. Receptor endocytosis played a key role in modulating STAT6 activation, whereas the lifetime of receptor-ligand complexes at the plasma membrane determined the potency of the variant for inducing more distal responses. This complex interrelationship between extracellular ligand binding and receptor function provides the foundation for new mechanism-based strategies that determine the optimal cytokine dose to enhance therapeutic efficacy.

Abstract 3629: Overcoming macrophage immunosuppression in small cell lung cancer with high-affinity SIRPa variants

CD47 allows cancer cells to evade the immune system by signaling through SIRPa, an inhibitory receptor on macrophages. Therapies that block CD47 convert tumor-promoting macrophages to a tumoricidal state within the tumor microenvironment. We recently developed next-generation CD47 antagonists by engineering the extracellular domain of SIRPa. As single-domain polypeptides, these “high-affinity SIRPa variants” have an affinity for human CD47 (KD) as low as 11.1 pM, approximately 50,000-fold improved over wild-type SIRPa. By themselves, the high-affinity SIRPa variants are inert and therefore non-toxic in mouse and primate studies. However, when combined with tumor-specific antibodies, the high-affinity SIRPa variants act as immunotherapeutic adjuvants to antibody therapies by maximizing the ability of macrophages to destroy cancer cells. In our current study, we hypothesized these novel CD47-blocking agents could be applied to the treatment of small cell lung cancer (SCLC), a cancer with poor prognosis for which no clinically-approved antibodies or immunotherapies exist. We examined a panel of human SCLC samples and found all samples tested expressed high levels of CD47 on their surface. Using purified macrophages in vitro, we found that CD47-blocking therapies were able to induce macrophage phagocytosis of SCLC cell lines and primary patient samples. As a proof-of-concept, treatment of mice bearing primary SCLC tumors with CD47-blocking antibodies was able to inhibit tumor growth and significantly prolong survival. To identify novel SCLC antigens that can be targeted in combination with the high-affinity SIRPa variants, we screened SCLC samples by high-throughput flow cytometry using LEGENDScreen comprehensive antibody arrays. We identified several new and established therapeutic targets on the surface of SCLC cells, including CD99, CD56, CD166, CD326, and CD164. We identified antibodies to these antigens that could elicit macrophage phagocytosis in vitro, validating these antigens as targets for immune-based therapies. The ability of these antibodies to induce phagocytosis was dramatically enhanced when combined with the high-affinity SIRPa variants. Future studies will test these immunotherapeutic combinations in vivo against SCLC samples to develop novel therapeutic combinations for patients. We propose this strategy as a universal method to identify new tumor antigens and overcome macrophage immunosuppression within the tumor microenvironment. Citation Format: Kipp Weiskopf, Peter J. Schnorr, Nadine Jahchan, Aaron M. Ring, Roy L. Maute, Anne K. Volkmer, Jens-Peter Volkmer, Kenan C. Garcia, Julien Sage, Irving L. Weissman. Overcoming macrophage immunosuppression in small cell lung cancer with high-affinity SIRPa variants. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3629. doi:10.1158/1538-7445.AM2014-3629