Debra A. Kendall - US grants

Many proteins which are synthesized in the cytoplasm of cells are ultimately found in noncytoplasmic locations. The correct targeting and transport of proteins must occur across the endoplasmic reticulum membrane, the peroxisomal membrane, the membranes of mitochondria and chloroplasts and those of bacterial cells. The unifying feature between secreted proteins in all systems is the requirement for a signal peptide. The long-term objectives of this work are to determine the structural requirements of signal peptides that are necessary for protein secretion and to elucidate how these physical properties interface a protein to be exported with the secretion pathway. The earmark of a signal peptide is a cluster of hydrophobic residues in its central, hydrophobic core region. This structural feature plays a role in the membrane insertion and translocation steps of secretion and may provide a critical recognition element linking the preprotein to the secretion pathway. The specific aims of the proposed research are to determine the characteristics of an optimal signal peptide hydrophobic core unit and how the properties of this domain interface with other signal peptide subsegments during different stages of the secretion process. The role of this domain in SecA and SecY interactions and in differentiating SecB-dependent and independent pathways will then be examined. For this purpose, a systematic series of mutants of the Escherichia coli alkaline phosphatase gene will be produced. The mutant signal sequences will be designed to amplify certain physical traits to test the roles of conformation, length, hydrophobicity and overall topology. These will be evaluated in vivo for the extent to which different steps of the secretion process are accomplished. Representative series will then be used for in vitro analyses to establish direct interactions between signal peptides with particular properties and the Sec machinery. Biochemical analyses and direct-binding studies with synthetic signal peptides are designed with the aim of establishing the same hierarchy for binding in vitro as we observe for function in vivo. The features of the hydrophobic core domain which are amenable to change in response to environment and those which are universally critical for secretion and are thus, conserved, will be highlighted through a comparative analysis between E. coli and the thermophile, Thermotoga neapolitana. The movement of proteins across membranes is vital to the health of all cells. Understanding the structural features of signal peptides which enhance correct compartmentalization in bacteria will be useful for probing their eukaryotic counterparts. The principles which evolve can be applied to the tissue-specific targeting of therapeutic agents and the design of vehicles to transport other proteins, including eukaryotic proteins, into the E. coli periplasm for subsequent isolation.

DESCRIPTION (provided by applicant): The human cannabinoid receptor one (CB1) binds ?9-tetrahydrocannabinol, the psychoactive component of Cannabis sativa L., and other cannabimimetic compounds. It is a G-protein coupled receptor (GPCR) that is associated with the central nervous system and exerts its effects primarily via coupling to Gi/Go proteins. The pharmacological effects of CB1 agonists include analgesia, inhibition of nausea, appetite stimulation, antiemetic activity and bronchial dilation while inverse agonists attenuate excessive eating disorders. Any therapeutic strategy targeting CB1 will require that we have a firm understanding of the structural features of the receptor that impact key points in its life cycle: ER integration and cell surface expression, receptor activation, and desensitization and internalization. In Aim 1, we will examine the basis for the weak ER translocation of the CB1 amino terminus (N-tail). The structure of the N-terminus will be determined and accessory proteins and their recognition motifs in the N- and C-terminus will be identified. The role of these motifs in cellular localization including sematodendritic and axonal membrane surface localization in neurons will be examined. In Aim 2, we will build on our identification of structural elements of the receptor critical for distinguishing agonist and inverse agonist interactions and fully map the key contact points in the TM domain and the extracellular region of CB1 that are involved. We will define residues critical for poising the ligand- independent equilibrium of CB1 atypically toward activation and those responsible for the interconversion of this intermediate receptor state to the resting and activated forms of the receptor. In Aim 3, we will take advantage of novel receptor mutants that model different structural states of the receptor to examine linkages between receptor activation, desensitization, and cellular localization. Mutants that model the inactive and active forms of CB1 will provide tools for analyzing the consequences of prolonged treatment with inverse agonists and agonists. We will utilize our expertise in developing structural analyses and binding assays with purified components to examine the molecular basis of these processes with emphasis on the carboxyl terminus (C-tail) of the receptor. In the course of this work we will identify determinants that enhance the cell surface expression of CB1 and strategies for the large-scale preparation of domains of the receptor, and their structural analysis, which could be applied to structural studies of other GPCRs and membrane proteins in general. RELEVANCE: The cannabinoid receptor one (CB1) is a G-protein coupled receptor that is associated with the central nervous system. Research activities that have focused on the role of CB1 in signal transduction have underscored its enormous potential as a target for therapeutic agents. The goal of this project is to understand the structural features that influence key stages in the life cycle of CB1 including cell surface expression, receptor activation, and internalization and ultimately impact its cell surface exposure so that it will be accessible for therapeutic strategies.

? DESCRIPTION (provided by applicant): The cannabinoid receptor 1 (CB1) is a G-protein coupled receptor (GPCR) that regulates neural transmission and other physiological processes. Considerable excitement surrounds the therapeutic potential of CB1 including to regulate appetite, for the relief of neuropathic pain, and to prevent relapse to the use of drugs of abuse. CB1 is activated by the endogenous ligands arachidonoyl ethanolamide (AEA) and 2-arachidonoyl glycerol (2-AG) and various synthetic and plant-derived ligands (e.g. 9 tetrahydrocannabinol; THC). These CB1 agonists transduce signals to downstream effectors primarily via Gi/o coupling. Moreover, since CB1 also exhibits ligand-independent, constitutive activity, compounds able to block this activity provide vital tools to probe the profile of constitutive levels of signal transduction and the consequence of their inhibition. Furthermore, recent evidence suggests that pregnenolone, a CB1 ligand with inverse agonist activity, is elevated following prolonged THC use illustrating an endogenous mechanism to counterbalance CB1 over- activity. The goal of this project is to profile the molecular-level signal transduction patterns of the basally active and inactive forms of CB1 induced by inverse agonists including first generation, endogenous, and novel compounds, to establish biased-inverse agonism for the first time. While biased agonism is now well recognized and changing the GPCR drug discovery landscape, biased inverse agonism has not been examined. CB1 is an ideal candidate to address this issue and this is critical knowledge for the longer-term goal of developing these compounds for the treatment of overeating behaviors and addiction. In this project, we will (1) contrast CB1 constitutive activity and CB1 inactive forms using inverse agonists of diverse structure. We will identify the downstream CB1 signaling patterns in the presence and absence of robust inverse agonists using global arrays to identify alterations in kinase phosphorylation, mRNA, and miRNA expression (critical for mRNA regulation) in basally active and inverse agonist treated cells. CB1 activity will be correlated with subcellular localization of the receptor in its basal and differentially inactive forms. We will (2) exploit ne benzhydrylpiperazine and coumarin scaffolds to yield novel potent and chemically diverse CB1 inverse agonists. We will synthesize these derivatives to increase potency and characterize their signal transduction patterns. Recently, we identified an inverse agonist of CB1 with a Ki = 220 nM that lacks the heteroaromatic central core common to CB1 inverse agonists. This new scaffold presents the opportunity to develop peripherally active CB1 inverse agonists for treating obesity without the unwanted anxiogenic and depressogenic side effects associated with centrally active CB1 inverse agonists. By characterizing a structurally diverse set of inverse agonists, differential effects on cell responses will be observed to guide the development of future therapeutics.

PROJECT SUMMARY The classic model of G protein coupled receptor (GPCR) activation centers on ligand binding, G protein activation, and signal transduction via G protein-mediated signaling events. This paradigm has been called into question however, with the finding that some ligands ?including endogenous ligands and therapeutic agents? have a preference for beta-arrestin mediated pathways. However, the information flow from receptor activation to signaling cascades and the mechanism of beta-arrestin signaling are not well understood. This proposal will elucidate, at the molecular level, the fundamental mechanisms controlling ligand-specific induction of beta- arrestin signaling with two highly clinically relevant GPCRs, the cannabinoid 2 receptor (CB2R) and the mu opioid receptor (MOR). These human receptors bind the plant-derived cannabinoids and opioids leading to psychostimulant effects and reduction of pain. Precise control of receptor activation and signaling is critical to obtain only the desired therapeutic results, however, and not the undesired side effects such as tolerance, drug abuse and dependence. Substantial preliminary studies identified ligand-specific dwell times, i.e. the time receptors are clustered into clathrin coated pits with beta-arrestins before endocytosis, as a mechanism controlling beta-arrestin signaling. This trafficking event can be chemically and genetically modulated to selectively control beta-arrestin signaling, providing novel therapeutic strategies. This project will combine state-of-the-art live cell imaging technologies (total internal reflection fluorescence and spinning disk microscopies), and biochemical approaches to determine if ligand-specific dwell times are a general event controlling beta-arrestin signaling.  Multiple ligands for these receptors will be investigated in heterologous systems and in cells endogenously expressing the receptors. Preliminary results in primary cultures strongly support our hypothesis that long dwell times correlate with beta-arrestin signaling. The aims are to: (1) examine endocytosis of the CBR2 and MOR at the single endocytic pit level, and (2) define the impact on cellular mechanisms of CB2R and MOR mediated beta-arrestin signaling, including whether endocytic dwell times can modulate these pathways. Results will provide a physiological role for the previously described variability in endocytic dwell times. These findings may be extended to future drug discovery efforts, including for other GPCRs, to rationally design therapeutic agents with specific outcomes in areas intractable via current technology.

? DESCRIPTION (provided by applicant): The cannabinoid receptor CB1 is a G-protein coupled receptor (GPCR) that regulates neural transmission and other physiological processes. CB1 is activated by endogenous ligands (e.g. arachidonoyl ethanolamide (AEA) and 2-arachidonoyl glycerol (2-AG)) and various synthetic and plant-derived ligands that bind to the receptor orthosteric site. These CB1 orthosteric ligands transduce signals to downstream effectors primarily via Gi/o coupling, but Gs and arrestin coupling are also possible. The biological responses are implicated in many therapeutic applications, such as substance addiction, pain and inflammation, memory, and feeding and energy balance. Recently, allosteric modulators for GPCRs were discovered. Whereas positive allosteric modulators (PAMs) enhance the functional efficacy of orthosteric agonists, negative allosteric modulators (NAMs) non-competitively decrease the activity of the receptor. These modulators bind GPCR regions topographically distinct from the orthosteric ligand binding sites and thereby exert their modulatory effects in a highly subtype specific manner. CB1 NAMs offer exciting opportunities for developing therapeutics for drug addiction and metabolic syndromes and PAMs open a new pathway to treat pain-related conditions with a reduced incidence of unwanted psychotropic effects. However, the underlying mechanisms for CB1 allosteric modulation, including for allosteric-modulation biased downstream signaling, remain elusive. We have designed and synthesized highly potent PAMs, such as LDK 1256 (KB = 89 nM), and with high allostery, such as LDK1258 (?=24.5), for enhanced agonist binding. We also have identified NAMs that are the only known modulators for reducing agonist binding to CB1. We have discovered that the CB1 allosteric modulator ORG27569 is coupled to an arrestin signaling pathway, which represents the first example of a CB1 biased allosteric modulator. In this project, we will synthesize compounds through optimizing the properties of identified allosteric modulators, develop new scaffolds and identify pharmacophoric groups that improve the equilibrium dissociation constants and cooperativity factors. These compounds will be tested for impact on agonist and inverse agonist binding. We will also elucidate their G-protein and arrestin coupling and downstream molecular-level activities. In an iterative process, key CB1 modulators will be fine tuned via structure-activity relationship (SAR) analysis. We will evaluate potent allosteric modulators for their impact on CB1 pharmacological responses in vivo. This effort involves assessing key CB1 allosteric modulators versus orthosteric compounds in functional assays including in the cannabinoid tetrad and drug discrimination assays. We will also test lead ligands in mouse models of neuropathic pain, feeding, and nicotine reward. The overall goal of this work is to identify new PAMs and NAMs, elucidate their molecular level profiles, and test lead ligands in animal models. These efforts are critical for elucidating the basis of CB1 allosteric modulation so that ultimately highly specific responses can be attained via therapeutic agents.