Current Medicinal Chemistry (v.19, #8)

Two important breakthroughs, in crystallography and in molecular simulations, moved forward investigations of the G-protein coupled receptors (GPCRs). Based on the recent crystal structures of GPCRs with agonists, and also on the results from very long simulations exceeding the microsecond frontier, it was possible to answer the fundamental questions about their multiple activation schemes in anticipation of a more effective drug design. The aim of this Hot Topic is to show the recent advancements in understanding the structure and, especially, the function of these highly elusive receptors. Recent studies of GPCRs changed the old dogma of a simple classification of the receptor ligands into agonists, antagonists and inverse agonists. Now, it is apparent that the same ligand can play diverse roles even in the same receptor acting as an agonist or an antagonist in different signaling pathways. Therefore, not a bare receptor but rather a receptor-ligand pair emerges as the smallest signaling unit. The scope of the reviews embraces recent achievements in the GPCR area which attempt to unravel the following pieces of the multithread puzzle of GPCRs: (1) The molecular evolution of the most populated class-A of GPCRs and possible causes of diversification of this huge family; (2) Structures of β-adrenergic receptors with agonists and antagonists including the structure of a newly crystallized β2AR-Gαβγ protein complex. β-Adrenergic receptors and the rhodopsin/opsin system are the bestcharacterized GPCRs also in terms of the activation schemes; (3) The ensemble of the GPCR active states and their contribution to the diversity of their signaling. Any changes in this signaling behavior have serious implications in health and disease; (4) The crosstalk between receptors in heteromers which can modulate signaling effects. Such physical interaction between receptors is known to exist but its functional implications are still elusive; (5) The action of molecular switches in GPCRs which perform the job of receptor activation; and (6) A description of results from very long time scale simulations which were subsequently validated by experiments. GPCRs are still mysterious in their activation scheme and multiple signaling but we are getting closer to better understanding of their secrets.

G protein coupled receptors (GPCRs), also called 7TM receptors, form a huge superfamily of membrane proteins that, upon activation by extracellular agonists, pass the signal to the cell interior. Ligands can bind either to extracellular N-terminus and loops (e.g. glutamate receptors) or to the binding site within transmembrane helices (Rhodopsin-like family). They are all activated by agonists although a spontaneous auto-activation of an empty receptor can also be observed. Biochemical and crystallographic methods together with molecular dynamics simulations and other theoretical techniques provided models of the receptor activation based on the action of so-called “molecular switches” buried in the receptor structure. They are changed by agonists but also by inverse agonists evoking an ensemble of activation states leading toward different activation pathways. Switches discovered so far include the ionic lock switch, the 3-7 lock switch, the tyrosine toggle switch linked with the nPxxy motif in TM7, and the transmission switch. The latter one was proposed instead of the tryptophan rotamer toggle switch because no change of the rotamer was observed in structures of activated receptors. The global toggle switch suggested earlier consisting of a vertical rigid motion of TM6, seems also to be implausible based on the recent crystal structures of GPCRs with agonists. Theoretical and experimental methods (crystallography, NMR, specific spectroscopic methods like FRET/BRET but also single-molecule-force-spectroscopy) are currently used to study the effect of ligands on the receptor structure, location of stable structural segments/domains of GPCRs, and to answer the still open question on how ligands are binding: either via ensemble of conformational receptor states or rather via induced fit mechanisms. On the other hand the structural investigations of homoand heterodimers and higher oligomers revealed the mechanism of allosteric signal transmission and receptor activation that could lead to design highly effective and selective allosteric or ago-allosteric drugs.

Class A or rhodopsin-like G-protein-coupled receptors (GPCRs) constitute the largest transmembrane receptor family of the human genome. Because of their biological and pharmaceutical importance, the evolutionary history of these receptors has been widely studied. Most studies agree on the classification of the 700 members of this family into a dozen of sub-families. However, the relationship between these sub-families remains controversial and the molecular processes that drove the evolution and diversification of such a large family have still to be determined. We review here the evolutionary analyses carried out on class A GPCRs either by phylogenetic methods or by multidimensional scaling (MDS). We detail the key molecular events driving the evolution of this receptor family. We analyze these events in view of the recently resolved crystal structures of GPCRs and we discuss the usefulness of evolutionary information to help molecular modeling.

Crosstalk between G protein-coupled receptors (GPCRs) is one of the key mechanisms used by the cell for integrating multiple signaling pathways. Functional crosstalk at the level of signaling pathways was initially thought to regulate receptor function. Importantly, the existence of GPCR heteromers demonstrates that direct physical interactions between GPCRs could also be behind the crosstalk phenomenon. Neurological disorders such as Parkinson's disease (PD) and schizophrenia have been linked to a dysfunctional communication between certain GPCRs. In this review, we discuss functional and physical crosstalk of the main GPCR families involved in the aforementioned disorders. In addition, we analyze the available structural information on physical crosstalk and highlight some strategies in drug discovery based on these crosstalk mechanisms.

G protein coupled receptors (GPCRs) are a large eukaryotic protein family of transmembrane receptors that react to a signal coming from the extracellular environment to generate an intracellular response through the activation of a signal transduction pathway mediated by a heterotrimeric G protein. Their diversity, dictated by the multiplicity of stimuli to which they respond and by the variety of intracellular signalling pathways they activate, make them one of the most prominent families of validated pharmacological targets in biomedicine. In recent years, major breakthroughs in structure determination of GPCRs have given new stimuli to the exploration of the biology of these proteins, providing a structural basis to understand the molecular origin of GPCR mechanisms of action. Based on the information coming from these structural studies, a number of recent in silico investigations used molecular dynamics (MD) simulations to contribute to our knowledge of GPCRs. In this review, we will focus on investigations that, taking advantage of the tremendous progress in both hardware and software, made testable hypotheses that have been validated by subsequent structural studies. These stateof- the-art molecular simulations highlight the potential of microsecond MD simulations as a valuable tool in GPCR structural biology and biophysics.

G protein-coupled receptors (GPCRs) play critical roles in cellular signal transduction and are important targets for therapeutics. Although these receptors have been intensely studied for quite some time, our understanding about their mechanism of action is still incomplete. GPCR activity has traditionally been viewed within the context of two-state models where the receptor is in equilibrium between a single inactive state and a single active state. This framework is too simple and restrictive to accommodate more recent observations made on these receptors, which instead point to a situation where the receptor can adopt several different active conformational substates with distinct functional effects. Structural and functional evidence for this emerging view is presented in this review. Implications of this emerging view in rationalizing diseased states and in drug discovery are also discussed.

The understanding of β2-adrenergic receptor (β2AR) interactions with ligands as well as the mechanism of receptor activation changed radically from 2007, when the first crystallographic structure of the receptor was reported. Since then numerous crystallographic studies described interactions with all main classes of β2AR ligands and with G proteins, which provided a great insight into the molecular structure of the receptor. However, molecular mechanisms of receptor activations remain to be determined. Functional research supported the concept of ligand - directed signaling at β-adrenoceptors. Agonist can activate alternative signaling pathways with different capacities and trigger cellular effects. It indicates that agonists nominally belonging to the same class may bind to and/or stabilize different active conformations of the receptor which are selectively recognized by signaling proteins in the allosteric manner.

RAS/RAF/MEK Inhibitors in Oncology by P. Rusconi (1164-1176).
The RAS/RAF/MEK signaling pathway plays a central role in mediating both proliferation and survival of cancer cells. These proteins are a group of serine/threonine kinases activated in response to a variety of extracellular stimuli and mediate signal transduction from the cell surface towards both nuclear and cytosolic targets. In combination with several other signaling pathways, they can differentially alter phosphorylation status of the transcription factors. A controlled regulation of these cascades is involved in cell proliferation and differentiation, whereas an unregulated activation of these kinases can result in oncogenesis. Dysregulation of the RAS/RAF/MEK pathway has been detected in more than 30% of human tumors, however mutations in the MEK1 and MEK2 genes are seldom, so that hyperactivation of MEK1/2 usually results from gain-of-function mutations in RAS and/or B-RAF. In addition, alteration of the pathways is often associated with drug resistance in the clinic, such as the case of K-RAS mutant expressing tumors. Since RAS protein is a difficult target, alternative ways altering post-translational modifications using farnesyl transferase inhibitors have been adopted. Drug discovery programs have therefore largely focused on B-RAF and MEK. In this review we will discuss the most promising strategies developed to target these kinases and the most recent inhibitors facing the preclinical and clinical setting, also considering their structure-activity relationship (SAR).

HIV-1 integrase is one of the three viral enzymes essential to HIV replication. Consequently the development of therapeutics targeting this enzyme has been a major focus of antiretroviral research over the past two decades. Several classes of integrase inhibitors have been identified; of these the diketoacids (DKAs) show greatest promise: raltegravir (Merck & Co) has been approved by the US Food and Drug Administration (FDA) for HIV-1 therapy, while elvitegravir (Gilead Sciences/ Japan Tobacco) has reached phase III clinical trials. This review considers the development of DKA-based inhibitors from early screening studies through to the release of raltegravir. SAR data collated from numerous studies are compared and analysed, shedding light on the geometric and electronic requirements for effective binding to HIV-1 integrase. This information will in turn aid the rational design of future generations of integrase inhibitors.

Adiponectin is an abundant plasma protein secreted from adipocytes. Its role in energy homeostasis is well-known, including the regulation of hydrocarbons and lipids metabolism as well as the improvement of insulin resistance. It has been thought to be a key molecule in the development of type 2 diabetes mellitus and metabolic syndrome, which are epidemiological targets for preventing cardiovascular disease. In addition to beneficial metabolic effects, adiponectin seems to have anti-inflammatory, anti-atherosclerotic and vasoprotective actions. Furthermore, adiponectin affects signalling in myocardial cells and exerts beneficial actions on the heart after pressure overload and ischemia-reperfusion injury. The ability of adiponectin to reduce insulin resistance in conjunction with its antiinflammatory and cardioprotective properties makes this adipocytokine a promising therapeutic target. On clinical interest, agents that enhance endogenous adiponectin production or action have potential for the treatment of cardiovascular disease. Management strategies that increase adiponectin levels include weight reduction, Mediterranean diet, thiazolidinediones, antihypertensive and lipid lowering drugs. Current knowledge on the main actions of adiponectin and therapeutic approaches for cardiovascular disease is summarized in this review.

Arterial hypertension is a well-known disease with a worldwide high prevalence and impaired prognosis with respect to normotensive subjects, due to increased cardiovascular mortality and morbidity. Blood pressure levels over range can be successfully controlled with adequate treatment, but more than 10% of hypertensive people have their blood pressure uncontrolled despite a therapeutic regimen of 3 or more antihypertensive drugs. These patients, named to have resistant hypertension, have a worse cardiovascular prognosis than controlled hypertensive subjects. Twenty-four hour-ambulatory blood pressure monitoring (ABPM) reveals that at least one third of these patients have indeed white-coat resistant hypertension, a rather more benign entity. In view of this evidence, performance of 24h-ABPM is mandatory and to document the occurrence of subclinical target organ damage in this population before the development of cardiovascular disease is needed. This would help the physician to more rigorously implement adequate measures to control hypertension. On the other hand, the definition itself of the disease implies that conventional pharmacological treatment is not effective enough for these patients to reach normal blood pressure values. To treat resistant hypertensives, recent reports pay attention to the need to recover traditional treatments -either non-pharmacologic such as strict sodium diet restriction or pharmacologic such as the use of aldosterone receptor blockers - or to implement those treatments that are novelties, such as renal sympathetic nervous system ablation or carotid barorreceptors stimulation. This review focuses on outlining the current evidence about the diagnostic confirmation of resistant hypertension, the need to characterize these patients through 24h-ABPM, to identify the presence of subclinical target organ damage, and to deal with not only classical but also novel treatment approaches for blood pressure control.

Testicular torsion or torsion of the spermatic cord is a surgical emergency in which misdiagnosis and inappropriate treatment can lead to male infertility. Events occurring during testicular torsion and detorsion are representative of an ischemia-reperfusion injury observed in other organs. The two most important factors determining testicular damage are the degree of twisting and the early onset of a surgical treatment to counter-rotate both testis and spermatic cord for inducing reperfusion. The damage from reperfusion is more severe than that induced by ischemia and several mechanisms are implicated in the development of testicular damage following torsion and detorsion. However, these mechanisms have not yet been fully clarified and, as a consequence, there is still a strong need to identify specific pharmacological treatment to limit the damage triggered by the reperfusion procedures. Ischemia and reperfusion of testis result in elevated production of reactive oxygen species (ROS), activate mitogen activated protein kinases (MAPKs) and PPARβ/δ receptor, induce transcription factors and growth factors including NF-κB and IL-1β . This pathological cascade is responsible for the testicular atrophy, decreased blood flow and impaired spermatogenesis. Several pharmacological approaches have been characterized as promising therapeutic agents for the management of testicular torsion and may be useful to ameliorate the sequel of this disease.

Fatty acid biosynthesis is essential for bacterial survival. In recent years, components of this biosynthetic pathway have aroused wide concern. β-Ketoacyl-acyl carrier protein synthase III (FabH) is a particularly attractive target which catalyzes the initial step of fatty acid biosynthesis. In this review, fatty acid biosynthesis, recent advances in the research of FabH as well as related inhibitors are reviewed. Finally, we also discuss the prospect and developmental trend of FabH inhibitors as anti-bacterial agents.

DNA topoisomerase I is required for DNA relaxation during a variety of cellular functions. The identification of camptothecins as specific enzyme poisons and their clinical efficacy have stimulated extensive efforts to exploit topoisomerase I as a therapeutic target for cancer. However, several limitations of camptothecins, such as low solubility and stability, high toxicity, and the occurrence of resistance, have encouraged the development of non-camptothecin topoisomerase I inhibitors. Different natural and synthetic compounds (e.g., indolocarbazoles, dibenzonaphthyridine and indenoisoquinoline) have been extensively studied as alternatives to camptothecins and have been proved to be promising therapeutic agents. In this review, we comparatively evaluate the preclinical results obtained with the different non-camptothecin poisons proposed thus far as topoisomerase I inhibitors, with special reference to cellular pharmacology, and discuss the perspective for their use in the clinical setting.