Immunology, Structural Biology
Dr. Garcia is also a professor of molecular and cellular physiology and of structural biology at Stanford University School of Medicine.
Structural Biology and Protein Engineering of Cell Surface Receptor Signaling
K. Christopher Garcia studies the structural, mechanistic, and functional aspects of receptor-ligand interactions that play important roles in mammalian biology and human disease. To do this, he blends traditional structural approaches with ligand engineering and discovery. His goal is to paint a detailed mechanistic picture – from the outside to the inside of a cell – of how ligand binding is structurally coupled to receptor activation, and to exploit this information to manipulate signaling with engineered ligands, potentially resulting in therapeutics.
Garcia’s laboratory focuses on “shared” receptors, sometimes referred to as “pleiotropic.” These are receptors that can be activated by several structurally diverse ligands, giving rise to unique and redundant signaling outcomes. Garcia’s team has described new paradigms for recognition and activation of a range of shared receptors involved in adaptive immunity, neural signaling, and development.
For example, the researchers have shown that the cytokine receptor gp130 cross-reacts with different ligands through a uniquely accommodating surface chemistry within its structurally rigid binding site. Garcia also has a long-standing interest in T cell recognition of the peptide-MHC (major histocompatibility complex). His team is now looking at the question of whether there is a coevolved, germline-derived recognition code between T cell receptors and MHC molecules.
Garcia’s structural studies have also provided a snapshot of how developmentally important Wnt proteins bind to the Frizzled family of receptors. Wnts have long been considered a potential drug target for cancers, and knowing their structure provides a huge advantage when trying to develop compounds that will bind to Wnts.
A growing interest in the Garcia lab is receptor “deorphanization,” or matching receptors to their ligands. Using proteomics, Garcia’s group has discovered many new receptor-ligand pairs in Drosophila melanogaster. One of these new receptor-ligand families, the DIP-DPR network, appears to play a major role in wiring of the fly brain. Garcia hopes to escalate this work to a genome-wide receptor deorphanization program.
This work is also supported by grants from the National Institutes of Health.
K. Christopher Garcia studies the structure and function of cell surface receptors involved in human health and disease. What distinguishes Garcia’s approach is the close integration of structural biology with protein engineering, cell signaling, and in vivo studies. This approach allows Garcia to exploit structural and mechanistic information to manipulate receptor signaling, often in therapeutically relevant manners. He focuses on “shared” receptors that play important roles in immunity, neurobiology, developmental biology, and microbial pathogenesis. These receptors can recognize and bind to several different molecules, or ligands, which often elicit unique responses. Although this is a common phenomenon in biological signaling, until recently, researchers puzzled over how a single molecule could recognize so many different binding partners.
While others puzzled, Garcia pursued the question with extraordinary tenacity, bringing to bear his broad knowledge of many areas of science. Combining structural studies with biochemical and biophysical experiments, Garcia’s team has identified new paradigms for recognition and activation of a variety of receptors that play critical roles in autoimmunity (T cell receptor and peptide-MHC), cancer (cytokines such as IL-2 and IL-6), stem cell biology (Wnt and Notch), neural growth and repair (p75 neurotrophin and Nogo receptors), viral pathogenesis (chemokine GPCR), and blood pressure regulation (ANP receptor).
Each of these receptors acts in a unique way. For example, the T cell receptor (TCR) interacts with an antigenic peptide and the major histocompatibility complex (MHC) in a manner that represents the convergence of 400 million years of coevolution by our cellular immune system. Garcia and his colleagues determined the structure of the first complete TCR and its complex with peptide-MHC. One recent advance from the Garcia group is the finding that TCRs appear to be more specific for their peptide-MHC ligands than previously realized. This finding led to the group developing a novel means of identifying peptide antigens for orphan TCRs, such as those derived from tumors. Garcia’s subsequent biophysical studies have shown that activation of the TCR is achieved through a complex combination of conformational changes and kinetic discrimination. However, identifying the precise molecular basis of TCR recognition and activation, as well as other lineages of immune receptors, remains a long-term challenge for the Garcia lab.
The Wnt and Notch systems stand as central receptor-ligand systems in developmental biology and cancer. Garcia’s lab determined structures of both Wnt/Frizzled and Notch/ligand (Jagged and Delta) complexes, and determined their unique mechanisms for linking ligand binding to receptor activation using ligand engineering. One interesting conclusion from this work on two completely different receptor systems is that both rely on post-translational modifications for ligand engagement – lipidation for Wnt, and O-glycosylation for Notch. These are rare examples of post-translational chemistry influencing the specificity of a receptor-ligand pair. Garcia’s group is studying the concept that post-translational mechanisms are used in these primordial systems as a primitive means of “tuning” ligand interactions in a way that negates the need for amino acid changes.
Garcia’s goal is to probe these systems more deeply, working to examine entire receptor molecules, before and after activation, loaded with their full complements of extracellular ligands and intracellular adapter molecules. Understanding the many ways in which the relatively simple act of ligand binding prompts conformational change and ultimately activates receptors should help researchers design drugs that target disease-related receptors.