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Structural Biology of Cell Surface Receptor Recognition and Activation
Summary: K. Christopher Garcia studies the structure and function of cell surface receptor recognition and activation, in the immune and nervous systems.
Extracellular information is communicated to intracellular mediators by activation of membrane-embedded proteins on the cell surface called receptors. Receptors represent the gateway through which the cell senses and responds to its environment. Most physiologically important processes are, at some level, initiated by the engagement of an extracellular molecule with the extracellular regions of cell surface receptors in a highly specific fashion, in order to activate intracellular signal transduction cascades and subsequent genetic programs. Cell surface receptors are also the major therapeutic entry points for drug development by the pharmaceutical industry.
My laboratory is investigating structural and functional aspects of cell surface receptor recognition and activation, in receptor-ligand systems with direct relevance to human health and disease. Our 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. How is the static aspect of recognition linked with the dynamic aspects of activation? We hope to use this structural and mechanistic information to inform new strategies to modulate receptor activation in human disease conditions. Our work targets receptors of biomedical relevance, where there is future potential for applications in drug design.
The mechanisms of cell surface receptor activation are highly diverse and poorly understood. We currently have an appreciation for only a small fraction of the potential receptor-ligand interaction paradigms across the human genome. We have endeavored to characterize novel receptor activation mechanisms by determining crystal structures of extracellular receptor-ligand complexes. The structural studies have been complemented by companion biochemical and functional experiments to probe the relevance of the structural information to the proteins in their native environments. We have described four new paradigms for recognition and activation of cytokine receptors, vasoactive hormone receptors, neurotrophin receptors, and T cell receptors (TCRs), involving both receptor oligomerization and conformational change induced by their ligands. These receptors play critical roles in cancer (gp130), autoimmunity (TCR), blood pressure regulation (natriuretic peptide receptor), and neural growth and repair (neurotrophin and Nogo receptors). My lab will continue to investigate the diversity of receptor activation mechanisms by targeting new receptor systems for which structural information does not exist. However, the long-term vision for my lab is to probe more deeply the systems for which we have gained structural insight, by carrying our studies into the membrane, to examine entire receptor molecules fully loaded with both extracellular ligand and intracellular adapter molecules.
Thematically, my laboratory is focusing on "shared" receptors—receptors that are activated by a variety of structurally diverse ligands and give rise to both unique and redundant signaling outcomes. We are particularly interested in receptors at the interface between immunity, neurobiology, and microbial pathogenesis. For instance, we are studying TCRs involved in the demyelinating disease multiple sclerosis, immunoregulatory cytokine receptors that are also neurotrophic factors, a neurotrophin receptor that is the entry receptor for rabies virus, and promiscuous G protein–coupled chemokine receptors that also serve as HIV coreceptors. One emerging theme since the advent of genomics is that receptors that were previously thought to have a restricted function within a particular system turn out to have broad tissue distributions and functions in seemingly disparate areas. Thus, as my lab is technically interfacial across disciplines, we also study molecules that bridge disciplines.
As one example, the shared cytokine receptor gp130 is a signaling receptor for more than 10 different cytokine ligands with critical functions in both the immune and nervous systems. What is the structural basis for this cross-reactivity, and how can the engagement of the different cytokines lead to qualitatively different signaling outcomes? So far, our studies have shown that gp130 cross-reacts with different ligands through a uniquely accommodating surface chemistry in its structurally rigid binding site, rather than through conformational change. It also appears that ligand recognition by gp130 is coupled to a bending of the receptor as it enters the membrane, in order to position the intracellular segments optimally for signaling. Our ongoing studies of the gp130 system include incorporating a second shared receptor, LIFR (the leukemia inhibitory factor receptor), into the heterodimeric signaling complex with gp130 to determine the basis for assembly of an asymmetric signaling complex. We are also interested in visualizing, potentially through electron microscopy, the architecture of the entire gp130 receptor in both quiescent and activated forms.
In the nervous system, the p75 neurotrophin receptor is a shared receptor for all neurotrophins, as well as acting as a signaling receptor in several different membrane complexes. For the normal growth and development of the mammalian nervous system, as well as for repair of damaged neurons, p75 function is critical. In that it is a member of the trimeric death receptor family, and yet is activated by dimeric ligands, p75 presents an enigma. We are attempting to understand how structurally unrelated ligands can bind and activate p75, inducing a spectrum of unique downstream signaling cascades. A recent crystal structure we determined of p75 complexed with nerve growth factor revealed a potentially novel structural mechanism explaining how this receptor seems to functionally engage a wide range of different coreceptors, such as Nogo receptor. In ongoing experiments, we are addressing the structural coordination of p75 with other receptor systems.
My lab also has a long-standing interest in structural aspects of T cell recognition of peptide-MHC (major histocompatibility complex). Currently we are focused on the apparent ability of TCRs to react with any MHC molecule. This property raises the question of whether there is a recognition code between TCR and MHC molecules that has so far been elusive. To determine whether conserved "footprints" might emerge from several structures of highly related TCR-pMHC complexes, we are studying TCRs derived from transgenic mice that utilize only a restricted subset of TCR variable-region genes. In tandem, we are attempting to reconstitute a functional TCR signaling complex on insect cells to assess the contribution of T cell coreceptors to this recognition process.
Finally, although we have made significant progress in structural studies of receptor extracellular domain complexes, our foray into membrane proteins represents a new challenge. The vast majority of receptors are multiple transmembrane helix proteins that must be studied in a lipid environment. Therefore, we will begin expression and structural studies of multipass membrane proteins in tandem with our ongoing studies of soluble receptor complexes.
This work is also supported by grants from the National Institutes of Health and the W.M. Keck Foundation.
Last updated: November 19, 2007
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