Activation and inhibition of G protein-coupled receptors by cell-penetrating membrane-tethered peptides

New Horizons in Therapeutic Antibody Discovery

Antibody drugs have become an increasingly significant component of the therapeutic landscape. Their success has been driven by some of their unique properties, in particular their very high specificity and selectivity, in contrast to the off-target liabilities of small molecules (SMs). Antibodies can bring additional functionality to the table with their ability to interact with the immune system, and this can be further manipulated with advances in antibody engineering. This review summarizes what antibody therapeutics have achieved to date and what opportunities and challenges lie ahead. The target landscape for large molecules (LMs) versus SMs and some of the challenges for antibody drug development are discussed. Effective penetration of membrane barriers and intracellular targeting is one challenge, particularly across the highly resistant blood-brain barrier. The expanding pipeline of antibody-drug conjugates offers the potential to combine SM and LM modalities in a variety of creative ways, and antibodies also offer exciting potential to build bi- and multispecific molecules. The ability to pursue more challenging targets can also be further exploited but highlights the need for earlier screening in functional cell-based assays. I discuss how this might be addressed given the practical constraints imposed by high-throughput screening sample type and process differences in antibody primary screening.

Development and characterization of pepducins as Gs-biased allosteric agonists

The β2-adrenergic receptor (β2AR) is a prototypical G protein-coupled receptor that mediates many hormonal responses, including cardiovascular and pulmonary function. β-Agonists used to combat hypercontractility in airway smooth muscle stimulate β2AR-dependent cAMP production that ultimately promotes airway relaxation. Chronic stimulation of the β2AR by long acting β-agonists used in the treatment of asthma can promote attenuated responsiveness to agonists and an increased frequency of fatal asthmatic attacks. β2AR desensitization to β-agonists is primarily mediated by G protein-coupled receptor kinases and β-arrestins that attenuate receptor-Gs coupling and promote β2AR internalization and degradation. A biased agonist that can selectively stimulate Gs signaling without promoting receptor interaction with G protein-coupled receptor kinases and β-arrestins should serve as an advantageous asthma therapeutic. To identify such molecules, we screened ∼50 lipidated peptides derived from the intracellular loops of the β2AR, known as pepducins. This screen revealed two classes of Gs-biased pepducins, receptor-independent and receptor-dependent, as well as several β-arrestin-biased pepducins. The receptor-independent Gs-biased pepducins operate by directly stimulating G protein activation. In contrast, receptor-dependent Gs-biased pepducins appear to stabilize a Gs-biased conformation of the β2AR that couples to Gs but does not undergo G protein-coupled receptor kinase-mediated phosphorylation or β-arrestin-mediated internalization. Functional studies in primary human airway smooth muscle cells demonstrate that Gs-biased pepducins are not subject to conventional desensitization and thus may be good candidates for the development of next generation asthma therapeutics. Our study reports the first Gs-biased activator of the β2AR and provides valuable tools for the study of β2AR function.

Pilot the pulse: controlling the multiplicity of receptor dynamics

G protein-coupled receptors (GPCRs) are involved in almost every (patho)physiological process, which explains their importance as drug targets. GPCRs have long been regarded as on/off-switches, which is reflected by direct activation or blockade of these receptors through the majority of marketed GPCR drugs. In recent years, however, our view of GPCRs has changed dramatically. GPCRs are now appreciated as integrative and highly dynamic signaling machines which can adopt numerous distinct conformations enabling them to initiate a highly ramified signaling network. We argue here that it may be possible to chemically encode distinct signaling profiles into ligands by rational ligand design. We exemplify our hypothesis by fine-tuning partial and biased agonism, thereby exploiting two new principles of GPCR modulation - dynamic and dualsteric ligand binding. We propose that the emerging understanding of the multiplicity of receptor dynamics will eventually lead to rationally designed new drugs which pilot the pulse; in other words, that stabilize distinct receptor states to fine-tune GPCR signaling.

A pepducin derived from the third intracellular loop of FPR2 is a partial agonist for direct activation of this receptor in neutrophils but a full agonist for cross-talk triggered reactivation of FPR2

We recently described a novel receptor cross-talk mechanism in neutrophils, unique in that the signals generated by the PAF receptor (PAFR) and the ATP receptor (P2Y₂R) transfer formyl peptide receptor 1 (FPR1) from a desensitized (non-signaling) state back to an actively signaling state (Forsman H et al., PLoS One, 8:e60169, 2013; Önnheim K, et al., Exp Cell Res, 323∶209, 2014). In addition to the G-protein coupled FPR1, neutrophils also express the closely related receptor FPR2. In this study we used an FPR2 specific pepducin, proposed to work as an allosteric modulator at the cytosolic signaling interface, to determine whether the cross-talk pathway is utilized also by FPR2. The pepducin used contains a fatty acid linked to a peptide sequence derived from the third intracellular loop of FPR2, and it activates as well as desensensitizes this receptor. We now show that neutrophils desensitized with the FPR2-specific pepducin display increased cellular responses to stimulation with PAF or ATP. The secondary PAF/ATP induced response was sensitive to FPR2-specific inhibitors, disclosing a receptor cross-talk mechanism underlying FPR2 reactivation. The pepducin induced an activity in naïve cells similar to that of a conventional FPR2 agonist, but with lower potency (partial efficacy), meaning that the pepducin is a partial agonist. The PAF- or ATP-induced reactivation was, however, much more pronounced when neutrophils had been desensitized to the pepducin as compared to cells desensitized to conventional agonists. The pepducin should thus in this respect be classified as a full agonist. In summary, we demonstrate that desensitized FPR2 can be transferred back to an actively signaling state by receptor cross-talk signals generated through PAFR and P2Y₂R, and the difference in agonist potency with respect to pepducin-induced direct receptor activation and cross-talk reactivation of FPR2 puts the concept of functional selectivity in focus.

Targeting the anionic region of human protease-activated receptor 4 inhibits platelet aggregation and thrombosis without interfering with hemostasis

BACKGROUND

Human platelet activation and aggregation is a complex process. To date, many therapies have been developed targeting proteins that mediate this process to prevent unwanted activation. However, the current standard of care for acute coronary syndromes still has limitations, including bleeding risk.

OBJECTIVE

To evaluate the protease-activated receptor 4 (PAR4) anionic cluster as a viable antiplatelet target by using a polyclonal antibody (CAN12).

METHODS

We used western blotting, aggregation and secretion ex vivo to evaluate the ability of CAN12 to interact with PAR4 and inhibit platelet activation. The effects of CAN12 in vivo were evaluated with the Rose Bengal arterial thrombosis model and two models of hemostasis.

RESULTS

CAN12 was able to interact with human PAR4 and delay PAR4 cleavage. In addition, CAN12 inhibited thrombin-induced human platelet aggregation and secretion in a dose-dependent manner. The specificity of CAN12 was agonist-dependent. In vivo, we determined that CAN12 was able to inhibit arterial thrombosis, and, using two independent methods, we found that CAN12 did not influence hemostasis.

CONCLUSION

Targeting the extracellular anionic cluster on PAR4 is a viable novel strategy as an antiplatelet therapy.

Biased and g protein-independent signaling of chemokine receptors

Biased signaling or functional selectivity occurs when a 7TM-receptor preferentially activates one of several available pathways. It can be divided into three distinct forms: ligand bias, receptor bias, and tissue or cell bias, where it is mediated by different ligands (on the same receptor), different receptors (with the same ligand), or different tissues or cells (for the same ligand–receptor pair). Most often biased signaling is differentiated into G protein-dependent and β-arrestin-dependent signaling. Yet, it may also cover signaling differences within these groups. Moreover, it may not be absolute, i.e., full versus no activation. Here we discuss biased signaling in the chemokine system, including the structural basis for biased signaling in chemokine receptors, as well as in class A 7TM receptors in general. This includes overall helical movements and the contributions of micro-switches based on recently published 7TM crystals and molecular dynamics studies. All three forms of biased signaling are abundant in the chemokine system. This challenges our understanding of “classic” redundancy inevitably ascribed to this system, where multiple chemokines bind to the same receptor and where a single chemokine may bind to several receptors – in both cases with the same functional outcome. The ubiquitous biased signaling confers a hitherto unknown specificity to the chemokine system with a complex interaction pattern that is better described as promiscuous with context-defined roles and different functional outcomes in a ligand-, receptor-, or cell/tissue-defined manner. As the low number of successful drug development plans implies, there are great difficulties in targeting chemokine receptors; in particular with regard to receptor antagonists as anti-inflammatory drugs. Un-defined and putative non-selective targeting of the complete cellular signaling system could be the underlying cause of lack of success. Therefore, biased ligands could be the solution.

Targeting the transient receptor potential vanilloid type 1 (TRPV1) assembly domain attenuates inflammation-induced hypersensitivity

The transient receptor potential channel vanilloid type 1 (TRPV1) is a non-selective cation channel expressed in sensory neurons of the dorsal root and trigeminal ganglia. TRPV1 is a polymodal channel activated by noxious heat, capsaicin, and protons. As a sensor for noxious stimuli, TRPV1 channel has been described as a key contributor to pain signaling. To form a functional channel, TRPV1 subunits must assemble into tetramers, and several studies have identified the TRPV1 C terminus as an essential element in subunit association. Here we combined biochemical assays with electrophysiology and imaging-based bimolecular fluorescence complementation (BiFC) and bioluminescence resonance energy transfer (BRET) in live cells to identify a short motif in the C-terminal tail of the TRPV1 subunit that governs channel assembly. Removing this region through early truncation or targeted deletion results in loss of subunit association and channel function. Importantly, we found that interfering with TRPV1 subunit association using a plasma membrane-tethered peptide attenuated mechanical and thermal hypersensitivity in two mouse models of inflammatory hyperalgesia. This represents a novel mechanism to disrupt TRPV1 subunit assembly and hence may offer a new analgesic tool for pain relief.

Antibacterial activity of pepducins, allosterical modulators of formyl peptide receptor signaling

Pepducins containing a fatty acid linked to an amino acid sequence derived from cytosolic parts of a G-protein-coupled receptor (GPCR) constitute a new group of lipopeptide tools in GPCR studies. Pepducins corresponding to the third intracellular loop of formyl peptide receptor 2 (FPR2) activate human neutrophils, and we show here that, in addition, these allosteric modulators of receptor activity also kill bacteria. The functional dualism of FPR2 pepducins could potentially be explored as a novel class of antibacterial drugs with immunomodulatory properties.

Pepducin targeting the C-X-C chemokine receptor type 4 acts as a biased agonist favoring activation of the inhibitory G protein

Short lipidated peptide sequences derived from various intracellular loop regions of G protein-coupled receptors (GPCRs) are named pepducins and act as allosteric modulators of a number of GPCRs. Recently, a pepducin selectively targeting the C-X-C chemokine receptor type 4 (CXCR4) was found to be an allosteric agonist, active in both cell-based assays and in vivo. However, the precise mechanism of action of this class of ligands remains poorly understood. In particular, given the diversity of signaling effectors that can be engaged by a given receptor, it is not clear whether pepducins can show biased signaling leading to functional selectivity. To explore the ligand-biased potential of pepducins, we assessed the effect of the CXCR4 selective pepducin, ATI-2341, on the ability of the receptor to engage the inhibitory G proteins (Gi1, Gi2 and Gi3), G13, and β-arrestins. Using bioluminescence resonance energy transfer-based biosensors, we found that, in contrast to the natural CXCR4 ligand, stromal cell-derived factor-1α, which promotes the engagement of the three Gi subtypes, G13 and the two β-arrestins, ATI-2341 leads to the engagement of the Gi subtypes but not G13 or the β-arrestins. Calculation of the transduction ratio for each pathway revealed a strong negative bias of ATI-2341 toward G13 and β-arrestins, revealing functional selectivity for the Gi pathways. The negative bias toward β-arrestins results from the reduced ability of the pepducin to promote GPCR kinase-mediated phosphorylation of the receptor. In addition to revealing ligand-biased signaling of pepducins, these findings shed some light on the mechanism of action of a unique class of allosteric regulators.

Recent progress of cell-penetrating peptides as new carriers for intracellular cargo delivery

The plasma membrane as a selectively permeable barrier of living cells is essential to cell survival and function. In many cases, however, the efficient passage of exogenous bioactive molecules through the plasma membrane remains a major hurdle for intracellular delivery of cargoes. During the last two decades, the potential of peptides for drug delivery into cells has been highlighted by the discovery of numerous cell-penetrating peptides (CPPs). CPPs serving as carriers can successfully intracellular transport cargoes such as siRNA, nucleic acids, proteins, small molecule therapeutic agents, quantum dots and MRI contrast agents. This review mainly introduces recent advances of CPPs as new carriers for the development of cellular imaging, nuclear localization, pH-sensitive and thermally targeted delivery systems. In particular, we highlight the exploiting of the synergistic effects of targeting ligands and CPPs. What's more, the classification and cellular uptake mechanisms of CPPs are briefly discussed as well.

FRET-based imaging of transbilayer movement of pepducin in living cells by novel intracellular bioreductively activatable fluorescent probes

To elucidate the mechanisms of direct transmembrane penetration of pepducins, which are artificial lipopeptide G protein-coupled receptor (GPCR) modulators, we developed two types of FRET-based probes, Pep13-FL-SS-Dab (13) targeting the inner leaflet of the lipid bilayer and Pep13-Dab-SS-FL (14) targeting the cytosol, respectively. They are composed of a pepducin moiety and a fluorescent switch component consisting of 5(6)-carboxyfluorescein (FAM) as a fluorophore and dabcyl as a quencher connected through disulfide bond linkage. When they are internalized into the cytosol, intracellular glutathione can cleave the disulfide bond to release the quencher, which results in a turn-on fluorescence signal. Using these probes, we performed live cell imaging of transbilayer movements of pepducins on MCF-7 cells for the first time. The results suggested that the lipid moiety of the probes facilitated pepducin flipping across and tethering to the membrane. The present study raises the possibility of applying the probe architecture for direct intracellular drug delivery.

Toward drugs for protease-activated receptor 2 (PAR2)

PAR2 has a distinctive functional phenotype among an unusual group of GPCRs called protease activated receptors, which self-activate after cleavage of their N-termini by mainly serine proteases. PAR2 is the most highly expressed PAR on certain immune cells, and it is activated by multiple proteases (but not thrombin) in inflammation. PAR2 is expressed on many types of primary human cells and cancer cells. PAR2 knockout mice and PAR2 agonists and antagonists have implicated PAR2 as a promising target in inflammatory conditions; respiratory, gastrointestinal, metabolic, cardiovascular, and neurological dysfunction; and cancers. This article summarizes salient features of PAR2 structure, activation, and function; opportunities for disease intervention via PAR2; pharmacological properties of published or patented PAR2 modulators (small molecule agonists and antagonists, pepducins, antibodies); and some personal perspectives on limitations of assessing their properties and on promising new directions for PAR2 modulation.

Modulation of PAR(1) signalling by benzimidazole compounds

BACKGROUND AND PURPOSE

Recently, a small molecule (Q94) was reported to selectively block PAR(1) /Gα(q) interaction and signalling. Here, we describe the pharmacological properties of Q94 and two analogues that share its benzimidazole scaffold (Q109, Q89). Q109 presents a modest variation from Q94 in the substituent group at the 2-position, while Q89 has quite different groups at the 1- and 2-positions.

EXPERIMENTAL APPROACH

Using human microvascular endothelial cells, we examined intracellular Ca(2+) mobilization and inositol 1,4,5-trisphosphate accumulation as well as isoprenaline- or forskolin-stimulated cAMP production in response to thrombin.

KEY RESULTS

Q89 (10 µM) produced a leftward shift in the thrombin-mediated intracellular Ca(2+) mobilization concentration-response curve while having no effect on the E(max) . Both Q94 (10 µM) and Q109 (10 µM) reduced intracellular Ca(2+) mobilization, leading to a decrease in E(max) and an increase in EC(50) values. Experiments utilizing receptor-specific activating peptides confirmed that Q94 and Q109 were selective for PAR(1) as they did not alter the Ca(2+) response mediated by a PAR(2) activating peptide. Consistent with our Ca(2+) results, micromolar concentrations of either Q94 or Q109 significantly reduced thrombin-induced inositol 1,4,5-trisphosphate production. Neither Q94 nor Q109 diminished the inhibitory effects of thrombin on cAMP production, indicating they inhibit signalling selectively through the G(q) pathway. Our results also suggest the 1,2-disubstituted benzimidazole derivatives act as 'allosteric agonists' of PAR(1) .

CONCLUSIONS AND IMPLICATIONS

The Q94 and Q109 benzimidazole derivatives represent a novel scaffold for the development of new PAR(1) inhibitors and provide a starting point to develop dual signalling pathway-selective positive/negative modulators of PAR(1) .

Development of highly potent protease-activated receptor 2 agonists via synthetic lipid tethering

Protease-activated receptor-2 (PAR₂) is a G-protein coupled receptor (GPCR) associated with a variety of pathologies. However, the therapeutic potential of PAR₂ is limited by a lack of potent and specific ligands. Following proteolytic cleavage, PAR₂ is activated through a tethered ligand. Hence, we reasoned that lipidation of peptidomimetic ligands could promote membrane targeting and thus significantly improve potency and constructed a series of synthetic tethered ligands (STLs). STLs contained a peptidomimetic PAR₂ agonist (2-aminothiazol-4-yl-LIGRL-NH₂) bound to a palmitoyl group (Pam) via polyethylene glycol (PEG) linkers. In a high-throughput physiological assay, these STL agonists displayed EC₅₀ values as low as 1.47 nM, representing a ∼200 fold improvement over the untethered parent ligand. Similarly, these STL agonists were potent activators of signaling pathways associated with PAR₂: EC₅₀ for Ca(2+) response as low as 3.95 nM; EC₅₀ for MAPK response as low as 9.49 nM. Moreover, STLs demonstrated significant improvement in potency in vivo, evoking mechanical allodynia with an EC₅₀ of 14.4 pmol. STLs failed to elicit responses in PAR2(-/-) cells at agonist concentrations of >300-fold their EC₅₀ values. Our results demonstrate that the STL approach is a powerful tool for increasing ligand potency at PAR₂ and represent opportunities for drug development at other protease activated receptors and across GPCRs.

G Protein-Coupled Receptors in cancer: biochemical interactions and drug design

G Protein-Coupled Receptors (GPCRs) share the same topology made of seven-transmembrane segments and represent the largest family of membrane receptors. Initially associated with signal transduction in differentiated cells, GPCRs and heterotrimeric G proteins were shown to behave as proto-oncogenes whose overexpression or activating mutations confer transforming properties. The first part of this review focuses on the link between biochemical interactions of a GPCR with other receptors, such as dimerization or multiprotein complexes, and their oncogenic properties. Alteration of these interactions or deregulation of transduction cascades can promote uncontrolled cell proliferation or cell transformation that leads to tumorigenicity and malignancy. The second part concerns the design of drugs specifically targeting these complex interactions and their promise in cancer therapy.

Challenges in design and characterization of ligand-targeted drug delivery systems

Targeting of therapeutic agents to molecular markers expressed on the surface of cells requiring clinical intervention holds promise to improve specificity of delivery, enhancing therapeutic effects while decreasing potential damage to healthy tissues. Drug targeting to cellular receptors involved in endocytic transport facilitates intracellular delivery, a requirement for a number of therapeutic goals. However, after several decades of experimental design, there is still considerable controversy on the practical outcome of drug targeting strategies. The plethora of factors contributing to the relative efficacy of targeting makes the success of these approaches hardly predictable. Lack of fully specific targets, along with selection of targets with spatial and temporal expression well aligned to interventional requirements, pose difficulties to this process. Selection of adequate sub-molecular target epitopes determines accessibility for anchoring of drug conjugates and bulkier drug carriers, as well as proper signaling for uptake within the cell. Targeting design must adapt to physiological variables of blood flow, disease status, and tissue architecture by accommodating physicochemical parameters such as carrier composition, functionalization, geometry, and avidity. In many cases, opposite features need to meet a balance, e.g., sustained circulation versus efficient targeting, penetration through tissues versus uptake within cells, internalization within endocytic compartment to avoid efflux pumps versus accessibility to molecular targets within the cytosol, etc. Detailed characterization of these complex physiological factors and design parameters, along with a deep understanding of the mechanisms governing the interaction of targeted drugs and carriers with the biological environment, are necessary steps toward achieving efficient drug targeting systems.

Modulation of the cannabinoid receptors by hemopressin peptides

Changes in the endocannabinoid system are implicated in numerous diseases, making it an attractive target for pharmaceutical development. The endocannabinoid receptors have traditionally been thought to act through the effects of lipophilic messengers called cannabinoids. The exciting finding of endocannabinoid system modulation by the nonapeptide hemopressin and its N-terminal extensions has highlighted the complexity of cannabinoid biology and pharmacology and sparked interest for therapeutic purposes. However, many questions surrounding the generation and regulation of the hemopressin peptides, the self-assembly of hemopressin and the potential for drug development based on hemopressin remain and are discussed in this review.

[Development of non-hormonal regulators of adenylyl cyclase signaling system on the basis of peptides, derivatives of the third intracellular loop of somatostatin receptors]

In the majority of the serpentine type receptors the third intracellular loop (ICL-3) is responsible for interaction with heterotrimeric G-proteins and for transduction of hormonal signal to the enzymes, generators of the second messengers. It was found that the peptides corresponding to ICL-3 influence functional activity of hormonal signaling systems in the absence of the hormone and, in consequence, can be considered as prototypes for the development of selective regulators of these systems. We have originally synthesized peptides corresponding to C-terminal regions 255-269 and 240 254 of ICL-3 of type 1 and 2 rat somatostatin receptors (Som1R and Som2R). Micromolar concentrations of these peptides activated Gi-proteins and inhibited forskolin-stimulated activity of adenylyl cyclase (AC) in rat brain tissues. The peptide 255-269 of Som1R is a selective antagonist of Som1R, and the peptide 240-254 of Som2R is an agonist of Som1R. So, the peptide 255-269 of Som1R decreased the regulatory effects of somatostatin and selective Som1R-agonist CH-275 realized via the receptor homologous to them, while the peptide 240-254 of Som2R, on the contrary, increased AC inhibitory action of CH-275. Both peptides insignificantly influenced regulatory effects of the Som2R-agonist octreotide. Summing up, the peptides studied by us are selective regulators of somatostatin-sensitive AC system. Using the peptides it was shown that ICL-3 of Som1R and Som2R includes the main molecular determinants that are responsible for activation of Gi-proteins and regulation of AC system by somatostatin and its analogues.

Suppression of arterial thrombosis without affecting hemostatic parameters with a cell-penetrating PAR1 pepducin

BACKGROUND

Thrombin-dependent platelet activation is heightened in the setting of percutaneous coronary intervention and may cause arterial thrombosis with consequent myocardial necrosis. Given the high incidence of adverse effects in patients with acute coronary syndromes, there remains an unmet need for the development of new therapeutics that target platelet activation without unduly affecting hemostasis. The thrombin receptor, PAR1, has recently emerged as a promising new target for therapeutic intervention in patients with acute coronary syndromes.

METHODS AND RESULTS

We report the development of a first-in-class intracellular PAR1 inhibitor with optimized pharmacokinetic properties for use during percutaneous coronary intervention in patients with acute coronary syndromes. PZ-128 is a cell-penetrating pepducin inhibitor of PAR1 that targets the receptor-G-protein interface on the inside surface of platelets. The structure of PZ-128 closely resembles the predicted off-state of the corresponding juxtamembrane region of the third intracellular loop of PAR1. The onset of action of PZ-128 was rapid and suppressed PAR1 aggregation and arterial thrombosis in guinea pigs and baboons and strongly synergized with oral clopidogrel. There was full recovery of platelet function by 24 hours. Importantly, PZ-128 had no effect on bleeding or coagulation parameters in primates or in blood from patients undergoing percutaneous coronary intervention.

CONCLUSIONS

Based on the efficacy data in nonhuman primates with no noted adverse effects on hemostasis, we anticipate that the rapid onset of platelet inhibition and reversible properties of PZ-128 are well suited to the acute interventional setting of percutaneous coronary intervention and may provide an alternative to long-acting small-molecule inhibitors of PAR1.

Physiology and emerging biochemistry of the glucagon-like peptide-1 receptor

The glucagon-like peptide-1 (GLP-1) receptor is one of the best validated therapeutic targets for the treatment of type 2 diabetes mellitus (T2DM). Over several years, the accumulation of basic, translational, and clinical research helped define the physiologic roles of GLP-1 and its receptor in regulating glucose homeostasis and energy metabolism. These efforts provided much of the foundation for pharmaceutical development of the GLP-1 receptor peptide agonists, exenatide and liraglutide, as novel medicines for patients suffering from T2DM. Now, much attention is focused on better understanding the molecular mechanisms involved in ligand induced signaling of the GLP-1 receptor. For example, advancements in biophysical and structural biology techniques are being applied in attempts to more precisely determine ligand binding and receptor occupancy characteristics at the atomic level. These efforts should better inform three-dimensional modeling of the GLP-1 receptor that will help inspire more rational approaches to identify and optimize small molecule agonists or allosteric modulators targeting the GLP-1 receptor. This article reviews GLP-1 receptor physiology with an emphasis on GLP-1 induced signaling mechanisms in order to highlight new molecular strategies that help determine desired pharmacologic characteristics for guiding development of future nonpeptide GLP-1 receptor activators.

Matrix metalloprotease-1a promotes tumorigenesis and metastasis

Matrix metalloprotease-1 (MMP1), a collagenase and activator of the G protein-coupled protease activated receptor-1 (PAR1), is an emerging new target implicated in oncogenesis and metastasis in diverse cancers. However, the functional mouse homologue of MMP1 in cancer models has not yet been clearly defined. We report here that Mmp1a is a functional MMP1 homologue that promotes invasion and metastatic progression of mouse lung cancer and melanoma. LLC1 (Lewis lung carcinoma) and primary mouse melanoma cells harboring active BRAF express high levels of endogenous Mmp1a, which is required for invasion through collagen. Silencing of either Mmp1a or PAR1 suppressed invasive stellate growth of lung cancer cells in three-dimensional matrices. Conversely, ectopic expression of Mmp1a conferred an invasive phenotype in epithelial cells that do not express endogenous Mmp1a. Consistent with Mmp1a acting as a PAR1 agonist in an autocrine loop, inhibition or silencing of PAR1 resulted in a loss of the Mmp1a-driven invasive phenotype. Knockdown of Mmp1a on tumor cells resulted in significantly decreased tumorigenesis, invasion, and metastasis in xenograft models. Together, these data demonstrate that cancer cell-derived Mmp1a acts as a robust functional homologue of MMP1 by conferring protumorigenic and metastatic behavior to cells.

Direct interactions between calcitonin-like receptor (CLR) and CGRP-receptor component protein (RCP) regulate CGRP receptor signaling

Calcitonin gene-related peptide (CGRP) is a neuropeptide with multiple neuroendocrine roles, including vasodilation, migraine, and pain. The receptor for CGRP is a G protein-coupled receptor (GPCR) that requires three proteins for function. CGRP binds to a heterodimer composed of the GPCR calcitonin-like receptor (CLR) and receptor activity-modifying protein (RAMP1), a single transmembrane protein required for pharmacological specificity and trafficking of the CLR/RAMP1 complex to the cell surface. In addition, the CLR/RAMP1 complex requires a third protein named CGRP-receptor component protein (RCP) for signaling. Previous studies have demonstrated that depletion of RCP from cells inhibits CLR signaling, and in vivo studies have demonstrated that expression of RCP correlates with CLR signaling and CGRP efficacy. It is not known whether RCP interacts directly with CLR to exert its effect. The current studies identified a direct interaction between RCP and an intracellular domain of CLR using yeast two-hybrid analysis and coimmunoprecipitation. When this interacting domain of CLR was expressed as a soluble fusion protein, it coimmunoprecipitated with RCP and inhibited signaling from endogenous CLR. Expression of this dominant-negative domain of CLR did not significantly inhibit trafficking of CLR to the cell surface, and thus RCP may not have a chaperone function for CLR. Instead, RCP may regulate CLR signaling in the cell membrane, and direct interaction between RCP and CLR is required for CLR activation. To date, RCP has been found to interact only with CLR and represents a novel neuroendocrine regulatory step in GPCR signaling.

Cell-penetrating peptides: classes, origin, and current landscape

With more than ten new FDA approvals since 2001, peptides are emerging as an important therapeutic alternative to small molecules. However, unlike small molecules, peptides on the market today are limited to extracellular targets. By contrast, cell-penetrating peptides (CPPs) can target intracellular proteins and also carry other cargoes (e.g. other peptides, small molecules or proteins) into the cell, thus offering great potential as future therapeutics. In this review I present a classification scheme for CPPs based on their physical-chemical properties and origin, and I provide a general framework for understanding and discovering new CPPs.

Turning receptors on and off with intracellular pepducins: new insights into G-protein-coupled receptor drug development

G-protein-coupled receptors (GPCRs) are a large family of remarkably versatile membrane proteins that are attractive therapeutic targets because of their involvement in a vast range of normal physiological processes and pathological diseases. Upon activation, intracellular domains of GPCRs mediate signaling to G-proteins, but these domains have yet to be effectively exploited as drug targets. Cell-penetrating lipidated peptides called pepducins target specific intracellular loops of GPCRs and have recently emerged as effective allosteric modulators of GPCR activity. The lipid moiety facilitates translocation across the plasma membrane, where pepducins then specifically modulate signaling of their cognate receptor. To date, pepducins and related lipopeptides have been shown to specifically modulate the activity of diverse GPCRs and other membrane proteins, including protease-activated receptors (PAR1, PAR2, and PAR4), chemokine receptors (CXCR1, CXCR2, and CXCR4), sphingosine 1-phosphate receptor-3 (S1P3), the melanocortin-4 receptor, the Smoothened receptor, formyl peptide receptor-2 (FPR2), the relaxin receptor (LGR7), G-proteins (Gα(q/11/o/13)), muscarinic acetylcholine receptor and vanilloid (TRPV1) channels, and the GPIIb integrin. This minireview describes recent advances made using pepducin technology in targeting diverse GPCRs and the use of pepducins in identifying potential novel drug targets.

Protease-activated receptor 1 (PAR1) coupling to G(q/11) but not to G(i/o) or G(12/13) is mediated by discrete amino acids within the receptor second intracellular loop

Protease-activated receptor 1 (PAR1) is an unusual GPCR that interacts with multiple G protein subfamilies (G(q/11), G(i/o), and G(12/13)) and their linked signaling pathways to regulate a broad range of pathophysiological processes. However, the molecular mechanisms whereby PAR1 interacts with multiple G proteins are not well understood. Whether PAR1 interacts with various G proteins at the same, different, or overlapping binding sites is not known. Here we investigated the functional and specific binding interactions between PAR1 and representative members of the G(q/11), G(i/o), and G(12/13) subfamilies. We report that G(q/11) physically and functionally interacts with specific amino acids within the second intracellular (i2) loop of PAR1. We identified five amino acids within the PAR1 i2 loop that, when mutated individually, each markedly reduced PAR1 activation of linked inositol phosphate formation in transfected COS-7 cells (functional PAR1-null cells). Among these mutations, only R205A completely abolished direct G(q/11) binding to PAR1 and also PAR1-directed inositol phosphate and calcium mobilization in COS-7 cells and PAR1-/- primary astrocytes. In stark contrast, none of the PAR1 i2 loop mutations disrupted direct PAR1 binding to either G(o) or G(12), or their functional coupling to linked pertussis toxin-sensitive ERK phosphorylation and C3 toxin-sensitive Rho activation, respectively. In astrocytes, our findings suggest that PAR1-directed calcium signaling involves a newly appreciated G(q/11)-PLCε pathway. In summary, we have identified key molecular determinants for PAR1 interactions with G(q/11), and our findings support a model where G(q/11), G(i/o) or G(12/13) each bind to distinct sites within the cytoplasmic regions of PAR1.

Targeting proteinase-activated receptors: therapeutic potential and challenges

Proteinase-activated receptors (PARs), a family of four seven-transmembrane G protein-coupled receptors, act as targets for signalling by various proteolytic enzymes. PARs are characterized by a unique activation mechanism involving the proteolytic unmasking of a tethered ligand that stimulates the receptor. Given the emerging roles of these receptors in cancer as well as in disorders of the cardiovascular, musculoskeletal, gastrointestinal, respiratory and central nervous system, PARs have become attractive targets for the development of novel therapeutics. In this Review we summarize the mechanisms by which PARs modulate cell function and the roles they can have in physiology and diseases. Furthermore, we provide an overview of possible strategies for developing PAR antagonists.

The cytoplasmic rhodopsin-protein interface: potential for drug discovery

The mammalian dim-light photoreceptor rhodopsin is a prototypic G protein coupled receptor (GPCR), interacting with the G protein, transducin, rhodopsin kinase, and arrestin. All of these proteins interact with rhodopsin at its cytoplasmic surface. Structural and modeling studies have provided in-depth descriptions of the respective interfaces. Overlap and thus competition for binding surfaces is a major regulatory mechanism for signal processing. Recently, it was found that the same surface is also targeted by small molecules. These ligands can directly interfere with the binding and activation of the proteins of the signal transduction cascade, but they can also allosterically modulate the retinal ligand binding pocket. Because the pocket that is targeted contains residues that are highly conserved across Class A GPCRs, these findings imply that it may be possible to target multiple GPCRs with the same ligand(s). This is desirable for example in complex diseases such as cancer where multiple GPCRs participate in the disease networks.

New strategies to develop novel pain therapies: addressing thermoreceptors from different points of view

One approach to develop successful pain therapies is the modulation of dysfunctional ion channels that contribute to the detection of thermal, mechanical and chemical painful stimuli. These ion channels, known as thermoTRPs, promote the sensitization and activation of primary sensory neurons known as nociceptors. Pharmacological blockade and genetic deletion of thermoTRP have validated these channels as therapeutic targets for pain intervention. Several thermoTRP modulators have progressed towards clinical development, although most failed because of the appearance of unpredicted side effects. Thus, there is yet a need to develop novel channel modulators with improved therapeutic index. Here, we review the current state-of-the art and illustrate new pharmacological paradigms based on TRPV1 that include: (i) the identification of activity-dependent modulators of this thermoTRP channel; (ii) the design of allosteric modulators that interfere with protein-protein interaction involved in the functional coupling of stimulus sensing and gate opening; and (iii) the development of compounds that abrogate the inflammation-mediated increase of receptor expression in the neuronal surface. These new sites of action represent novel strategies to modulate pathologically active TRPV1, while minimizing an effect on the TRPV1 subpopulation involved in physiological and protective roles, thus increasing their potential therapeutic use.

The down regulation of neutrophil oxidative metabolism by S100A8 and S100A9: implication of the protease-activated receptor-2

S100A8 and S100A9 regulate polymorphonuclear neutrophils (PMNs) recruitment and represent 40% of PMN cytosolic protein weight. We have shown that S100A8/S100A9 inhibit PMN oxidative metabolism. The present study was designed to elucidate the mechanisms of this anti-oxidative effect. We hypothesized that the protease activated receptor-2 (PAR-2) played a role in the down-regulation of PMN oxidative metabolism by S100A8/S100A9. Freshly isolated PMNs were tested for their ability to oxidize dichlorofluorescin-diacetate. Functional inhibition of PAR-2 with ENMD-1068, the pepducin P2pal-21 or an antibody directed at PAR-2 cleavage/activation site, resulted in a significant inhibition of S100A8 and S100A9 anti-oxidative effect. Conversely, the controlled activation of PAR-2 potentiated S100 anti-oxidative effect. Taken together, the data indicate that the anti-oxidative effect of S100A8/A9 is initiated by PAR-2 activation. S100A8/S100A9 may therefore dampen inflammation without interfering with its initial strength. This finding opens translational possibilities to limit deleterious PMN activation with a dual PAR-2/S100 strategy.

Targeting CXCR4 with cell-penetrating pepducins in lymphoma and lymphocytic leukemia

The chemokine receptor CXCR4, which normally regulates stromal stem cell interactions in the bone marrow, is highly expressed on a variety of malignant hematologic cells, including lymphoma and lymphocytic leukemias. A new treatment concept has arisen wherein CXCR4 may be an effective therapeutic target as an adjunct to treatment of hematologic neoplasms with chemo- and immunotherapy. In the present study, we developed pepducins, cell-penetrating lipopeptide antagonists of CXCR4, to interdict CXCL12-CXCR4 transmembrane signaling to intracellular G-proteins. We demonstrate that pepducins targeting the first (i1) or third (i3) intracellular loops of CXCR4 completely abrogate CXCL12-mediated cell migration of lymphocytic leukemias and lymphomas. Stromal-cell coculture protects lymphoma cells from apoptosis in response to treatment with the CD20-targeted Ab rituximab. However, combination treatment with CXCR4 pepducins and rituximab significantly increases the apoptotic effect of rituximab. Furthermore, treatment of mice bearing disseminated lymphoma xenografts with pepducins alone or in combination with rituximab significantly increased their survival. These data demonstrate that CXCL12-CXCR4 signaling can be effectively inhibited by cell-penetrating pepducins, which represents a potential new treatment strategy for lymphoid malignancies.

Ionic channels as targets for drug design: a review on computational methods

Ion channels are involved in a broad range of physiological and pathological processes. The implications of ion channels in a variety of diseases, including diabetes, epilepsy, hypertension, cancer and even chronic pain, have signaled them as pivotal drug targets. Thus far, drugs targeting ion channels were developed without detailed knowledge of the molecular interactions between the lead compounds and the target channels. In recent years, however, the emergence of high-resolution structures for a plethora of ion channels paves the way for computer-assisted drug design. Currently, available functional and structural data provide an attractive platform to generate models that combine substrate-based and protein-based approaches. In silico approaches include homology modeling, quantitative structure-activity relationships, virtual ligand screening, similarity and pharmacophore searching, data mining, and data analysis tools. These strategies have been frequently used in the discovery and optimization of novel molecules with enhanced affinity and specificity for the selected therapeutic targets. In this review we summarize recent applications of in silico methods that are being used for the development of ion channel drugs.

Cytoprotective signaling by activated protein C requires protease-activated receptor-3 in podocytes

The cytoprotective effects of activated protein C (aPC) are well established. In contrast, the receptors and signaling mechanism through which aPC conveys cytoprotection in various cell types remain incompletely defined. Thus, within the renal glomeruli, aPC preserves endothelial cells via a protease-activated receptor-1 (PAR-1) and endothelial protein C receptor-dependent mechanism. Conversely, the signaling mechanism through which aPC protects podocytes remains unknown. While exploring the latter, we identified a novel aPC/PAR-dependent cytoprotective signaling mechanism. In podocytes, aPC inhibits apoptosis through proteolytic activation of PAR-3 independent of endothelial protein C receptor. PAR-3 is not signaling competent itself as it requires aPC-induced heterodimerization with PAR-2 (human podocytes) or PAR-1 (mouse podocytes). This cytoprotective signaling mechanism depends on caveolin-1 dephosphorylation. In vivo aPC protects against lipopolysaccharide-induced podocyte injury and proteinuria. Genetic deletion of PAR-3 impairs the nephroprotective effect of aPC, demonstrating the crucial role of PAR-3 for aPC-dependent podocyte protection. This novel, aPC-mediated interaction of PARs demonstrates the plasticity and cell-specificity of cytoprotective aPC signaling. The evidence of specific, dynamic signaling complexes underlying aPC-mediated cytoprotection may allow the design of cell type specific targeted therapies.

Diversity and modularity of G protein-coupled receptor structures

G protein-coupled receptors (GPCRs) comprise the most 'prolific' family of cell membrane proteins, which share a general mechanism of signal transduction, but greatly vary in ligand recognition and function. Crystal structures are now available for rhodopsin, adrenergic, and adenosine receptors in both inactive and activated forms, as well as for chemokine, dopamine, and histamine receptors in inactive conformations. Here we review common structural features, outline the scope of structural diversity of GPCRs at different levels of homology, and briefly discuss the impact of the structures on drug discovery. Given the current set of GPCR crystal structures, a distinct modularity is now being observed between the extracellular (ligand-binding) and intracellular (signaling) regions. The rapidly expanding repertoire of GPCR structures provides a solid framework for experimental and molecular modeling studies, and helps to chart a roadmap for comprehensive structural coverage of the whole superfamily and an understanding of GPCR biological and therapeutic mechanisms.

Protease-activated receptor-2 modulates protease-activated receptor-1-driven neointimal hyperplasia

OBJECTIVE

Emerging evidence suggests that protease-activated receptors-1 and -2 (PAR1 and PAR2) can signal together in response to proteases found in the rapidly changing microenvironment of damaged blood vessels. However, it is unknown whether PAR1 and PAR2 promote or mitigate the hyperplastic response to arterial injury. Using cell-penetrating PAR1 pepducins and mice deficient in PAR1 or PAR2, we set out to determine the respective contributions of the receptors to hyperplasia and phenotypic modulation of smooth muscle cells (SMCs) in response to arterial injury.

METHODS AND RESULTS

SMCs were strongly activated by PAR1 stimulation, as evidenced by increased mitogenesis, mitochondrial activity, and calcium mobilization. The effects of chronic PAR1 stimulation following vascular injury were studied by performing carotid artery ligations in mice treated with the PAR1 agonist pepducin, P1pal-13. Histological analysis revealed that PAR1 stimulation caused striking hyperplasia, which was ablated in PAR1(-/-) and, surprisingly, PAR2(-/-) mice. P1pal-13 treatment yielded an expression pattern consistent with a dedifferentiated phenotype in carotid artery SMCs. Detection of PAR1-PAR2 complexes provided an explanation for the hyperplastic effects of the PAR1 agonist requiring the presence of both receptors.

CONCLUSIONS

We conclude that PAR2 regulates the PAR1 hyperplastic response to arterial injury leading to stenosis.

Direct interaction between an allosteric agonist pepducin and the chemokine receptor CXCR4

Cell surface heptahelical G protein-coupled receptors (GPCRs) mediate critical cellular signaling pathways and are important pharmaceutical drug targets. (1) In addition to traditional small-molecule approaches, lipopeptide-based GPCR-derived pepducins have emerged as a new class of pharmaceutical agents. (2, 3) To better understand how pepducins interact with targeted receptors, we developed a cell-based photo-cross-linking approach to study the interaction between the pepducin agonist ATI-2341 and its target receptor, chemokine C-X-C-type receptor 4 (CXCR4). A pepducin analogue, ATI-2766, formed a specific UV-light-dependent cross-link to CXCR4 and to mutants with truncations of the N-terminus, the known chemokine docking site. These results demonstrate that CXCR4 is the direct binding target of ATI-2341 and suggest a new mechanism for allosteric modulation of GPCR activity. Adaptation and application of our findings should prove useful in further understanding pepducin modulation of GPCRs as well as enable new experimental approaches to better understand GPCR signal transduction.

The best of both worlds? Bitopic orthosteric/allosteric ligands of g protein-coupled receptors

It is now acknowledged that G protein-coupled receptors, the largest class of drug targets, adopt multiple active states that can be preferentially stabilized by orthosteric ligands or allosteric modulators, thus giving rise to the phenomenon of pathway-biased signaling. In the past few years, researchers have begun to explore the potential of linking orthosteric and allosteric pharmacophores to yield bitopic hybrid ligands. This approach is an extension of the more traditional bivalent ligand concept and shares some of the same challenges, including the choice and role of the linker between the two pharmacophores and the validation of mechanism of action. Nonetheless, the promise of bitopic ligands is the generation of novel chemical tools that have improved affinity and/or selectivity profiles. Previously identified functionally selective compounds (and medicines) also may act via a bitopic mechanism, suggesting that the phenomenon is more widespread than currently appreciated.