Assessment of the Acute Inhalation Toxicity of Airborne Particles by Exposing Cultivated Human Lung Cells at the Air-Liquid Interface

Here, we present a specially designed modular in vitro exposure system that enables the homogenous exposure of cultivated human lung cells at the ALI to ...

University of Munich

The melanocortin-4 receptor (MC4R) is a prototypical G protein-coupled receptor (GPCR) that plays a considerable role in controlling appetite and energy homeostasis. Signalling initiated by MC4R is orchestrated by multiple agonists, inverse agonism and by interactions with accessory proteins. The exact molecular events translating MC4R signalling into its physiological role, however, are not fully understood. This review is an attempt to summarize new aspects of MC4R signalling in the context of its recently discovered alternative G protein coupling, and to give a perspective on how future research could improve our knowledge about the intertwining molecular mechanisms that are responsible for the regulation of energy homeostasis by the melanocortin system.

Alternative G protein coupling and biased agonism: new insights into melanocortin-4 receptor signalling.

Signaling by heptahelical G protein-coupled receptors (GPCR) regulates many vital body functions. Consequently, dysfunction of GPCR signaling leads to pathologic states, and approximately 30% of all modern clinical drugs target GPCR. One decade ago, an entire new GPCR family was discovered, which was recently named MAS-related G protein-coupled receptors (MRGPR) by the HUGO Gene Nomenclature Committee. The MRGPR family consists of ∼40 members that are grouped into nine distinct subfamilies (MRGPRA to -H and -X) and are predominantly expressed in primary sensory neurons and mast cells. All members are formally still considered "orphan" by the Committee on Receptor Nomenclature and Drug Classification of the International Union of Basic and Clinical Pharmacology. However, several distinct peptides and amino acids are discussed as potential ligands, including β-alanine, angiotensin-(1-7), alamandine, GABA, cortistatin-14, and cleavage products of proenkephalin, pro-opiomelanocortin, prodynorphin, or proneuropeptide-FF-A. The full spectrum of biologic roles of all MRGPR is still ill-defined, but there is evidence pointing to a role of distinct MRGPR subtypes in nociception, pruritus, sleep, cell proliferation, circulation, and mast cell degranulation. This review article summarizes findings published in the last 10 years on the phylogenetic relationships, pharmacology, signaling, physiology, and agonist-promoted regulation of all MRGPR subfamilies. Furthermore, we highlight interactions between MRGPR and other hormonal systems, paying particular attention to receptor multimerization and morphine tolerance. Finally, we discuss the challenges the field faces presently and emphasize future directions of research. 

Pharmacology and signaling of MAS-related G protein-coupled receptors.

Serotonin 5-HT2C receptors (5-HT2CR) activate Gq proteins and are expressed in the central nervous system (CNS). 5-HT2CR regulate emotion, feeding, reward, or cognition and may serve as promising drug targets to treat psychiatric disorders or obesity. Owing to technical difficulties in isolating cells from the CNS and the lack of suitable cell lines endogenously expressing 5-HT2CR, our knowledge about this receptor subtype in native environments is rather limited. The hypothalamic mHypoA-2/10 cell line was recently established and resembles appetite-regulating hypothalamic neurons of the paraventricular nucleus (PVN), where 5-HT2CR have been detected in vivo. Therefore, we tested mHypoA-2/10 cells for endogenous 5-HT2CR expression. Serotonin or the 5-HT2CR preferential agonist WAY-161,503 initiated cAMP response element (CRE)-dependent gene transcription with EC50 values of 15.5 ± 9.8 and 1.1 ± 0.9 nM, respectively. Both responses were blocked by two unrelated 5-HT2CR-selective antagonists (SB-242,084, RS-102,221) but not by a 5-HT2AR (EMD-281,014) or 5-HT2BR (RS-127,455) antagonists. By single-cell calcium imaging, we found that serotonin and WAY-161,503 induced robust calcium transients, which were also blunted by both 5-HT2CR antagonists. Additionally we revealed, first, that 5-HT2CR induced CRE activation via protein kinase C (PKC)-mediated engagement of extracellular-regulated kinases-1/2 and, second, that intrinsic activity of WAY-161,503 was in the range of 0.3-0.5 compared with serotonin, defining the frequently used 5-HT2CR agonist as a partial agonist of endogenous 5-HT2CR. In conclusion, we have shown that hypothalamic mHypoA-2/10 cells endogenously express 5-HT2CR and thus are the first cell line in which to analyze 5-HT2CR pharmacology, signaling, and regulation in its natural environment.

Endogenous 5-HT2C Receptors Phosphorylate the cAMP Response Element Binding Protein via Protein Kinase C-Promoted Activation of Extracellular-Regulated Kinases-1/2 in Hypothalamic mHypoA-2/10 Cells.

Brain and muscle ARNT-like protein-1 (BMAL-1) is an important component of the cellular circadian clock. Proteins such as epidermal (EGF) or nerve growth factor (NGF) affect the cellular clock via extracellular signal-regulated kinases-1/2 (ERK-1/2) in NIH3T3 or neuronal stem cells, but no such data are available for the insulin-like growth factor-1 (IGF-1). The hypothalamus expresses receptors for all three growth factors, acts as a central circadian pacemaker, and releases hormones in a circadian fashion. However, little is known about growth factor-induced modulation of clock gene activity in hypothalamic cells. Here, we investigated effects of IGF-1, EGF, or NGF on the  promoter in two hypothalamic cell lines. We found that only IGF-1 but not EGF or NGF enhanced activity of the  promoter. Inhibition of ERK-1/2 activity did not affect IGF-1-induced  promoter activation and all three growth factors similarly phosphorylated ERK-1/2, questioning a role for ERK-1/2 in controlling  promoter activity. Of note, only IGF-1 induced sustained phosphorylation of glycogen synthase kinase-3β (GSK-3β). Moreover, the GSK-3β inhibitor lithium or siRNA-mediated GSK-3β knockdown diminished the effects of IGF-1 on the  promoter. When IGF-1 was used in the context of temperature cycles entraining hypothalamic clock gene expression to a 24-h rhythm, it shifted the phase of  promoter activity, indicating that IGF-1 functions as a zeitgeber for cellular hypothalamic circadian clocks. Our results reveal that IGF-1 regulates clock gene expression and that GSK-3β but not ERK-1/2 is required for the IGF-1-mediated regulation of the  promoter in hypothalamic cells.

Andreas Breit

Walther Straub Institute of Pharmacology and Toxicology

University of Munich

Assessment of the Acute Inhalation Toxicity of Airborne Particles by Exposing Cultivated Human Lung Cells at the Air-Liquid Interface

Andreas Breit

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Walther Straub Institute of Pharmacology and Toxicology

University of Munich

Assessment of the Acute Inhalation Toxicity of Airborne Particles by Exposing Cultivated Human Lung Cells at the Air-Liquid Interface

Walther Straub Institute of Pharmacology and Toxicology