Our laboratory is broadly interested in understanding the biochemistry, subcellular organization and dynamics of receptor-mediated signaling systems in eukaryotic cells. Cell signaling in complex cells typically exhibits precise spatiotemporal control, and such organization is essential to acute signaling. Dynamic reorganization of the cellular signaling machinery occurs throughout life, and contributes to physiological homeostasis and plasticity occurring over a wide range of time scales. In addition, these processes can be profoundly altered under pathological conditions, and influenced in interesting ways by acute or chronic administration of clinically relevant drugs.

Much of our lab's work has focused on the subcellular organization and membrane trafficking of seven-transmembrane signaling receptors (7TMRs), traditionally called G protein-coupled receptors (GPCRs). 7TMRs comprise the largest family of signaling receptors, and individual members of this family function in the control of virtually all aspects of animal physiology.  7TMRs are also molecular targets of a wide range of therapeutic and illicit drugs. We are also interested in other types of signaling receptor, such as glutamate-regulated ion channels that function at neuronal synapses, and membrane proteins such as connexins that function in direct cell-cell communication.

Our studies have contributed to understanding how particular signaling receptors, and other cellular proteins with which they interact, are organized and functionally regulated in individual cells. In essence they reveal an intricate choreography of regulated membrane trafficking processes, which contribute to dynamic reorganization of the cell's signaling machinery under both physiological and pathological conditions.  In many cases this reorganization by membrane traffic occurs in a highly receptor-specific manner, and we are intrigued that - in some cases- dynamic reorganization of the same signaling system can occur differentially in response to physiological compared to pharmacological stimulation. 

We believe that such processes of regulated membrane trafficking, besides contributing at a fundamental level to  diverse aspects of physiological homeostasis and plasticity, are important to elucidating pathological processes and  the cellular basis of drug action. It has emerged relatively recently that various signaling receptors, while they are delivered and reorganized largely via the cell's 'core' membrane trafficking machinery, can produce 'customized' effects by exerting local control over the function or dynamics of this machinery. Thus we think that understanding the regulated trafficking of particular signaling receptors can inform us, more generally, about the relationship between conserved membrane trafficking mechanisms and their relationship to the diversity of 'cargo' that they transport. Main scientific goals of our present work include (1) defining individual steps of receptor membrane trafficking, (2) determining points of regulation by physiological and pharmacological stimuli, and (3) elucidating key regulated trafficking events at the level of biochemical mechanism. Our methods include a variety of biochemical, molecular biological, immunochemical, pharmacological, and microscopic imaging techniques.

We are particularly interested in understanding functional consequences of the regulatory processes that we study. A rich background in receptor cell biology suggests that endocytic trafficking of receptors plays a fundamental role in regulating the intensity of signaling responses elicited by specific inputs. Two main effects have been established, and are readily understood in terms of classical receptor theory: (1) a change in the maximal response that can be achieved by a saturating concentration of agonist (sometimes called ''system efficacy" of a particular signaling response) and (2) a change in the effective potency with which a particular agonist can produce a given signaling response. Both of these effects are thought to occur in vivo, and may contribute to clinically important phenomena such as tachyphylaxis and tolerance to various drugs. An evolving area of research suggests, in addition, a distinct role of regulated trafficking in qualitatively "switching" receptor signaling specificity from one downstream effector pathway to another.

Despite close relationships between mechanism and function apparent from studies of cultured cells, and extensive effort by many groups, it can be quite challenging to unambiguously link specific cellular regulatory mechanisms elucidated in individual cells to salient physiological and behavioral effects occurring in neural circuits and in vivo. This challenge gets even more interesting when one begins to consider pharmacological and pathological effects.

There are some systems in which the connections are rather straightforward.  For example, certain receptors that mediate synaptic neurotransmission (by functioning as ligand-gated ion channels) are regulated in an activity-dependent manner by endocytic membrane trafficking. Studies carried out in collaboration with colleagues in Roger Nicoll's and Rob Malenka's labs established a direct relationship between regulated endocytic removal and exocytic insertion of one class of such receptors - the AMPA-type glutamate receptors- and distinct processes of synaptic depression and potentiation, respectively, occurring in the hippocampus. Motivated by this simple and direct relationship, our laboratory and others have been working to understand in detail the processes underlying endocytic receptor removal from, and exocytic delivery to, particular surface domains of dendrites.  This is a fundamental problem in membrane cell biology, and is particularly intriguing because it scales directly to the question of how neural circuits are organized and controlled.

Understanding the physiological consequences of membrane traffic to the diverse signaling activities of seven-transmembrane receptors has continued to pose fascinating challenges. As a first step, we have focused on defining precisely whether, and how, particular regulatory mechanisms defined in cell culture models (or in cell-free systems) occur in the context of native cell types and tissues. These studies have verified the occurrence of regulated endocytic trafficking of a number of receptors in vivo, yielded further insight regarding differences among the regulatory effects of certain drugs, and revealed that the cell biology of receptor regulation occurring in physiologically relevant CNS neurons is even more precisely controlled than observed in heterologous cell models. These efforts have also motivated us to apply new imaging methods to examine the dynamics of receptor trafficking and signaling with improved spatiotemporal resolution, and in differentiated membrane domains.  Efforts in this direction have stimulated, and been greatly aided by, terrific collaborations with colleagues at UCSF and elsewhere.

One recent direction in the lab is the application of chemical genetic approaches as a means to extend pharmacology and to precisely manipulate defined receptor regulatory mechanisms.  We are working toward application of these approaches to ex vivo tissue preparations as well as to intact animals. Goals of this fledgling effort are to further move mechanistic cell biology toward the in vivo realm, and to develop improved tools for assessing the potential of specific receptor regulatory mechanisms as future therapeutic targets.

Lab publications