Melanocortin receptors as novel effectors of macrophage responses in inflammation

Alpha-Melanocyte-Stimulating Hormone Maintains Retinal Homeostasis after Ischemia/Reperfusion

Augmenting the natural melanocortin pathway in mouse eyes with uveitis or diabetes protects the retinas from degeneration. The retinal cells are protected from oxidative and apoptotic signals of death. Therefore, we investigated the effects of a therapeutic application of the melanocortin alpha-melanocyte-stimulating hormone (α-MSH) on an ischemia and reperfusion (I/R) model of retinal degenerative disease. Eyes were subjected to an I/R procedure and were treated with α-MSH. Retinal sections were histopathologically scored. Also, the retinal sections were immunostained for viable ganglion cells, activated Muller cells, microglial cells, and apoptosis. The I/R caused retinal deformation and ganglion cell loss that was significantly reduced in I/R eyes treated with α-MSH. While α-MSH treatment marginally reduced the number of GFAP-positive Muller cells, it significantly suppressed the density of Iba1-positive microglial cells in the I/R retinas. Within one hour after I/R, there was apoptosis in the ganglion cell layer, and by 48 h, there was apoptosis in all layers of the neuroretina. The α-MSH treatment significantly reduced and delayed the onset of apoptosis in the retinas of I/R eyes. The results demonstrate that therapeutically augmenting the melanocortin pathways preserves retinal structure and cell survival in eyes with progressive neuroretinal degenerative disease.

Effects of ACTH6-9-Pro-Gly-Pro Peptide on the Levels of Pro- and Anti-Inflammatory Cytokines in Wistar Rats under Conditions of Chronic Restraint Stress

We evaluated the effect of ACTH_6-9-Pro-Gly-Pro (ACTH_6-9-PGP) peptide on the serum levels of pro- and anti-inflammatory cytokines in experimental animals under conditions of chronic restraint stress. Stress exposure over 2 weeks led to an increase in the levels of IL-1β, IL-6, and IFNγ in rats. Daily intraperitoneal injection of ACTH_6-9-PGP in a dose of 5 μg/kg before stress exposure significantly decreased the levels of IL-6 and IFNγ by 4.8 and 49.3% respectively. Administration of the peptide in a dose of 50 μg/kg decreased the levels of IL-1β and IFNγ by 51.2 and 39.7%, respectively. However, no changes in the cytokine levels were observed after injection of the peptide in a dose of 500 μg/kg. Thus, ACTH_6-9-PGP administered in doses of 5 and 50 μg/kg prevented the stress-induced changes in the levels of pro- and inflammatory cytokines.

Hematopoietic-specific melanocortin 1 receptor signaling protects against nephrotoxic serum nephritis and mediates the beneficial effect of melanocortin therapy

The melanocortin hormone system has emerged as a novel therapeutic target for treating refractory glomerular diseases. However, the role of hematopoietic melanocortin 1 receptor (MC1R) signaling remains unknown. Upon insult by rabbit nephrotoxic serum, MC1R null-mutant mice developed more severe crescentic glomerulonephritis than wild-type mice, marked by aggravated proteinuria, kidney dysfunction and histologic lesions. Melanocortin therapy, using Repository Corticotropin Injection (Acthar Gel), the pan-melanocortin receptor agonist NDP-MSH, or the MC1R agonist MS05, ameliorated experimental nephritis in wild-type mice but this effect was blunted in null mice. Exacerbated experimental nephritis in null mice was associated with increased glomerular deposition of autologous IgG and C5b-9, in parallel with higher circulating levels of autologous IgG2c and IgG3. Additionally, the Th1 immune response was potentiated in null mice with experimental nephritis, accompanied by diminished kidney FoxP3+ regulatory T cells. Kidney infiltration of macrophages was also augmented by MC1R deficiency with an enhanced M1 polarization. Moreover, adoptive transfer of syngeneic bone marrow-derived cells from wild-type mice mitigated experimental nephritis in null mice and restored the beneficial efficacy of melanocortins. Mechanistically, MC1R was expressed by diverse subsets of kidney leukocytes, including macrophages, T and B lymphocytes, and was inversely associated with the NFκB pathway, a key player in immune responses. MS05 attenuated the production of rabbit IgG-specific IgG2c and IgG3 in cultured wild-type splenocytes, and promoted M2 polarization in M1-primed wild-type macrophages, associated with NFκB inhibition. In contrast, in null splenocytes or macrophages, this effect of MS05 was barely detectable, but was mimicked by an NFκB inhibitor. Thus, hematopoietic MC1R signaling attenuates experimental nephritis and mediates the beneficial effect of melanocortin therapy via, in part, regulating the immune response.

Molecular cloning and functional characterization of melanocortin-3 receptor in grass carp (Ctenopharyngodon idella)

The melanocortin-3-receptor (MC3R) plays an important role in mammals' food intake and energy homeostasis. However, its physiological role in bony fishes, such as grass carp, has not been well understood. This study reports the molecular cloning, tissue distribution, and pharmacological characterization of grass carp melanocortin-3-receptor (ciMC3R). Phylogenetic and chromosomal synteny analyses indicated that ciMC3R was closest to cyprinid fishes in evolution. Quantitative PCR experiments revealed that the mRNA of ciMC3R was highly expressed in the brain of grass carp. The cytological function of ciMC3R was investigated by the co-transfection of pcDNA3.1-ciMC3R and the signal-pathway-specific luciferase into the HEK293T cells. Results revealed that the four agonists, α-MSH, β-MSH, ACTH, and NDP-MSH, potentiate the activation of ciMC3R and further increase the production of cAMP and upregulate the MAPK/ERK signaling, respectively. Our study will provide basic data for exploring the physiological functions of grass carp MC3R, especially in energy homeostasis and food intake.

Exploring the Usability of α-MSH-SM-Liposome as an Imaging Agent to Study Biodegradable Bone Implants In Vivo

Novel biodegradable metal alloys are increasingly used as implant materials. The implantation can be accompanied by an inflammatory response to a foreign object. For studying inflammation in the implantation area, non-invasive imaging methods are needed. In vivo imaging for the implanted area and its surroundings will provide beneficiary information to understand implant-related inflammation and help to monitor it. Therefore, inflammation-sensitive fluorescent liposomes in rats were tested in the presence of an implant to evaluate their usability in studying inflammation. The sphingomyelin-containing liposomes carrying alpha-melanocyte-stimulating hormone (α-MSH)-peptide were tested in a rat bone implant model. The liposome interaction with implant material (Mg-10Gd) was analyzed with Mg-based implant material (Mg-10Gd) in vitro. The liposome uptake process was studied in the bone-marrow-derived macrophages in vitro. Finally, this liposomal tracer was tested in vivo. It was found that α-MSH coupled sphingomyelin-containing liposomes and the Mg-10Gd implant did not have any disturbing influence on each other. The clearance of liposomes was observed in the presence of an inert and biodegradable implant. The degradable Mg-10Gd was used as an alloy example; however, the presented imaging system offers a new possible use of α-MSH-SM-liposomes as tools for investigating implant responses.

Molecular cloning, tissue distribution, and pharmacologic function of melanocortin-3 receptor in common carp (Cyprinus carpio)

Melanocortin-3 receptor (MC3R) not only regulates energy homeostasis in animals, but also is an important regulator of inflammation. As one of the most widely farmed freshwater fish, common carp has attracted great interest for its feeding and inflammation regulation. In this study, we cloned the coding sequence (CDS) of common carp Mc3r (ccMc3r), examined its tissue expression profile, and investigated the function of this receptor in mediating downstream signaling pathways. The results showed that the CDS of ccMc3r was 975 bp, encoding a putative protein of 324 amino acids. Homology, phylogeny, and chromosomal synteny analyses revealed that ccMc3r is evolutionarily close to the orthologs of cyprinids. Quantitative real-time PCR (qPCR) indicated that ccMc3r was highly expressed in the brain and intestine. The luciferase reporter systems showed that four ligands, ACTH (1-24), α-MSH, β-MSH, and NDP-MSH, were able to activate the cAMP and MAPK/ERK signaling pathways downstream of ccMc3r with different potencies. For the cAMP signaling pathway, ACTH (1-24) had the highest activation potency; while for the MAPK/ERK signaling pathway, β-MSH had the greatest activation effect. In addition, we found that the four agonists were able to inhibit TNF-α-induced NF-κB signaling in approximately the same order of potency as cAMP signaling activation. This study may facilitate future studies on the role of Mc3r in common carp feed efficiency and immune regulation.

α-MSH is partially involved in the immunomodulation of Nile tilapia (Oreochromis niloticus) antibacterial immunity

α-Melanocyte-stimulating hormone (α-MSH) is a well-studied neuropeptide controlling skin and hair color. Besides, numerous immunomodulation roles of α-MSH were recorded in humans and mice. However, the regulatory effects of α-MSH in teleost immunity haven't been well elucidated. In this study, several precursor molecules of α-MSH (POMCs) and its receptors (MCRs) in Nile tilapia (Oreochromis niloticus) were characterized, and their expression characteristics and specific functions on antibacterial immunity were determined. Overall, POMCs and MCRs were principally detected in the brain, skin, and liver, and were remarkably promoted post Streptococcus agalactiae infection. However, tiny POMCs and MCRs were observed in tilapia immune organs (head kidney and spleen) or lymphocytes, and no evident immunomodulation effect was detected in vitro. Moreover, the in vivo challenge experiments revealed that α-MSH protects tilapia from bacterial infection by regulating responses in the brain and intestine. This study lays theoretical data for a deeper comprehension of the immunomodulation mechanisms of teleost α-MSH and the evolutional process of the vertebrate melanocortin system.

Role of the melanocortin system in zebrafish skin physiology

The agouti-signaling protein (ASIP) acts as both a competitive antagonist and inverse agonist of melanocortin receptors which regulate dorsal-ventral pigmentation patterns in fish. However, the potential role of ASIP in the regulation of additional physiological pathways in the skin is unknown. The skin plays a crucial role in the immune function, acting as a physical limitation against infestation and also as a chemical barrier due to its ability to synthesize and secrete mucus and many immune effector proteins. In this study, the putative role of ASIP in regulating the immune system of skin has been explored using a transgenic zebrafish model overexpressing the asip1 gene (ASIPzf). Initially, the structural changes in skin induced by asip1 overexpression were studied, revealing that the ventral skin of ASIPzf was thinner than that of wild type (WT) animals. A moderate hypertrophy of mucous cells was also found in ASIPzf. Histochemical studies showed that transgenic animals appear to compensate for the lower number of cell layers by modifying the mucus composition and increasing lectin affinity and mucin content in order to maintain or improve protection against microorganism adhesion. ASIPzf also exhibit higher protein concentration under crowding conditions suggesting an increased mucus production under stressful conditions. Exposure to bacterial lipopolysaccharide (LPS) showed that ASIPzf exhibit a faster pro-inflammatory response and increased mucin expression yet severe skin injures and a slight increase in mortality was observed. Electrophysiological measurements show that the ASIP1 genotype exhibits reduced epithelial resistance, an indicator of reduced tissue integrity and barrier function. Overall, not only are ASIP1 animals more prone to infiltration and subsequent infections due to reduced skin epithelial integrity, but also display an increased inflammatory response that can lead to increased skin sensitivity to external infections.

Modulation of Canine Melanocortin-3 and -4 Receptors by Melanocortin-2 Receptor Accessory Protein 1 and 2

The neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), have crucial roles in regulating energy homeostasis. The melanocortin-2 receptor accessory proteins (MRAPs, MRAP1 and MRAP2) have been shown to regulate neural MCRs in a species-specific manner. The potential effects of MRAP1 and MRAP2 on canine neural MCRs have not been investigated before. Herein, we cloned canine (c) MC3R and identified one canine MRAP2 splice variant, MRAP2b, with N-terminal extension of cMRAP2a. Canine MC3R showed higher maximal responses to five agonists than those of human MC3R. We further investigated the modulation of cMRAP1, cMRAP2a, and cMRAP2b, on cMC3R and cMC4R pharmacology. For the cMC3R, all MRAPs had no effect on trafficking; cMRAP1 significantly decreased B_max whereas cMRAP2a and cMRAP2b significantly increased B_max. Both MRAP1 and MRAP2a decreased R_maxs in response to α-MSH and ACTH; MRAP2b only decreased α-MSH-stimulated cAMP generation. For the MC4R, MRAP1 and MRAP2a increased cell surface expression, and MRAP1 and MRAP2a increased B_maxs. All MRAPs had increased affinities to α-MSH and ACTH. MRAP2a increased ACTH-induced cAMP levels, whereas MRAP2b decreased α-MSH- and ACTH-stimulated cAMP production. These findings may lead to a better understanding of the regulation of neural MCRs by MRAP1 and MRAP2s.

Regulation of Melanocortin-3 and -4 Receptors by Isoforms of Melanocortin-2 Receptor Accessory Protein 1 and 2

The neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play essential non-redundant roles in the regulation of energy homeostasis. Interaction of neural MCRs and melanocortin-2 receptor accessory proteins (MRAPs, MRAP1 and MRAP2) is suggested to play pivotal roles in MC3R and MC4R signaling. In the present study, we identified two new human (h) MRAP2 splice variants, MRAP2b (465 bp open reading frame) and MRAP2c (381 bp open reading frame). Human MRAP2s are different in C-termini. We investigated the effects of five isoforms of MRAPs, hMRAP1a, hMRAP1b, hMRAP2a, hMRAP2b, and hMRAP2c, on MC3R and MC4R pharmacology. At the hMC3R, hMRAP1a and hMRAP2c increased and hMRAP1b decreased the cell surface expression. hMRAP1a increased affinity to ACTH. Four MRAPs (hMRAP1a, hMRAP1b, hMRAP2a, and hMRAP2c) decreased the maximal responses in response to α-MSH and ACTH. For hMC4R, hMRAP1a, hMRAP2a, and hMRAP2c increased the cell surface expression of hMC4R. Human MRAP1b significantly increased affinity to ACTH while MRAP2a decreased affinity to ACTH. Human MRAP1a increased ACTH potency. MRAPs also affected hMC4R basal activities, with hMRAP1s increasing and hMRAP2s decreasing the basal activities. In summary, the newly identified splicing variants, hMRAP2b and hMRAP2c, could regulate MC3R and MC4R pharmacology. The two MRAP1s and three MRAP2s had differential effects on MC3R and MC4R trafficking, binding, and signaling. These findings led to a better understanding of the regulation of neural MCRs by MRAP1s and MRAP2s.

Functional characterization of melanocortin-3 receptor in rainbow trout (Oncorhynchus mykiss)

The melanocortin-3 receptor (MC3R) is an important regulator of energy homeostasis and inflammation in mammals. However, its function in teleost fish needs to be further explored. In this study, we characterized rainbow trout MC3R (rtMC3R), which encoded a putative protein of 331 amino acids. Phylogenetic and chromosomal synteny analyses showed that rtMC3R was closely related to bony fishes. Quantitative PCR (qPCR) revealed that the transcripts of rtMC3R were highly expressed in the brain and muscle. The cellular function of rtMC3R was further verified by the signal-pathway-specific luciferase reporter assays. Four agonists such as α-MSH, β-MSH, ACTH (1-24), and NDP-MSH can active rtMC3R, increasing the production of intracellular cAMP and upregulating MAPK/ERK signals. Moreover, we found that rtMC3R stimulated with α-MSH and NDP-MSH can significantly inhibit the NF-κB signaling pathway. This research will be helpful for further studies on the function of MC3R in rainbow trout, especially the role of energy metabolism and immune regulation.

Functional Characterization of Melanocortin-3 Receptor in a Hibernating Cavefish Onychostoma macrolepis

Simple Summary

In this study we isolated and characterized a gene called omMc3r from a hibernating cavefish Onychostoma macrolepis. This gene was confirmed by our study to be involved in the regulation of signal pathways related to energy balance and food efficiency. These results can provide clues for exploring the adaptive mechanisms of fish, especially cavefish, with respect to nutrient-poor conditions.

Abstract

Melanocortin-3 receptor (MC3R) plays an important role in the energy homeostasis of animals under different nutritional conditions. Onychostoma macrolepis is a hibernating cavefish found in the northern part of the Yangtze River, and its adaptation to a nutrient-poor environment has attracted growing interest. In this study, we characterized the protein structure of Onychostoma macrolepis Mc3r (omMc3r), examined its tissue distribution, and investigated its function in mediating cellular signaling. We showed that the CDS of omMc3r is 978 bp, encoding a putative protein of 325 amino acids. Homology and phylogenetic analyses indicated that omMc3r is evolutionary close to cyprinids. Real-time quantitative PCR (RT-qPCR) revealed that omMc3r was highly expressed in the liver and brain. The functions of omMc3r to mediate ligands activating downstream signaling have also been confirmed by using signal pathway-specific reporters. The four agonists α-MSH, β-MSH, NDP-MSH, and ACTH (1–24) can all activate the cAMP and MAPK/ERK signaling pathway, albeit with different potency orders. The “primitive” ligand ACTH (1–24) had the highest potency on the cAMP signaling pathway, while the synthetic ligand NDP-MSH had the highest activation effect on the MAPK/ERK signaling pathway. This research will lay the foundation for studying the energy regulation mechanism of cavefish in an oligotrophic environment.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

Topmouth culter melanocortin-3 receptor: regulation by two isoforms of melanocortin-2 receptor accessory protein 2

Melanocortin-3 receptor (MC3R) is a regulator of energy homeostasis, and interaction of MC3R and melanocortin-2 receptor accessory protein 2 (MRAP2) plays a critical role in MC3R signaling of mammals. However, the physiological roles of MC3R in teleosts are not well understood. In this study, qRT-PCR was used to measure gene expression. Radioligand binding assay was used to study the binding properties of topmouth culter MC3R (caMC3R). Intracellular cAMP generation was determined by RIA, and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the CNS. All agonists could bind and stimulate caMC3R to increase dose dependently intracellular cAMP accumulation. Compared to human MC3R, culter MC3R showed higher constitutive activity, higher efficacies, and Rmax to alpha-melanocyte-stimulating hormone (α-MSH), des-α-MSH, and adrenocorticotrophic hormone. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, Bmax, and Rmax of caMC3R. Expression analysis suggested that MRAP2a and MRAP2b might be more important in regulating MC3R/MC4R signaling during larval period, and reduced mc3r, mc4r, and pomc expression might be primarily involved in modulation of MC3R/MC4R in adults. These data indicated that the cloned caMC3R was a functional receptor. MRAP2a and MRAP2b had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormones might play different roles in regulating culter development and growth.

Alpha-MSH Targeted Liposomal Nanoparticle for Imaging in Inflammatory Bowel Disease (IBD)

Background:

The purpose of our study was to find a novel targeted imaging and drug delivery vehicle for inflammatory bowel disease (IBD). IBD is a common and troublesome disease that still lacks effective therapy and imaging options. As an attempt to improve the disease treatment, we tested αMSH for the targeting of nanoliposomes to IBD sites. αMSH, an endogenous tridecapeptide, binds to the melanocortin-1 receptor (MC1-R) and has anti-inflammatory and immunomodulating effects. MC1-R is found on macrophages, neutrophils and the renal tubule system. We formulated and tested a liposomal nanoparticle involving αMSH in order to achieve a specific targeting to the inflamed intestines.

Methods:

NDP-αMSH peptide conjugated to Alexa Fluor™ 680 was linked to the liposomal membrane via NSuccinyl PE and additionally loaded into the lumen of the liposomes. Liposomes without the αMSH-conjugate and free NDP-αMSH were used as a control. The liposomes were also loaded with ICG to track them. The liposomes were tested in DSS treated mice, which had received DSS via drinking water order to develop a model IBD. Inflammation severity was assessed by the Disease Activity Index (DAI) score and ex vivo histological CD68 staining of samples taken from different parts of the intestine. The liposome targeting was analyzed by analyzing the ICG and ALEXA 680 fluorescence in the intestine compared to the biodistribution.

Results:

NPD-αMSH was successfully labeled with Alexa and retained its biological activity. Liposomes were identified in expected regions in the inflamed bowel regions and in the kidneys, where MC1-R is abundant. In vivo liposome targeting correlated with the macrophage concentration at the site of the inflammation supporting the active targeting of the liposomes through αMSH. The liposomal αMSH was well tolerated by animals.

Conclusions:

This study opens up the possibility to further develop an αMSH targeted theranostic delivery to different clinically relevant applications in IBD inflammation but also opens possibilities for use in other inflammations like lung inflammation in Covid 19.

The Resolution Approach to Cystic Fibrosis Inflammation

Despite the high expectations associated with the recent introduction of CFTR modulators, airway inflammation still remains a relevant clinical issue in cystic fibrosis (CF). The classical anti-inflammatory drugs have shown very limited efficacy, when not being harmful, raising the question of whether alternative approaches should be undertaken. Thus, a better knowledge of the mechanisms underlying the aberrant inflammation observed in CF is pivotal to develop more efficacious pharmacology. In this respect, the observation that endogenous proresolving pathways are defective in CF and that proresolving mediators, physiologically generated during an acute inflammatory reaction, do not completely suppress inflammation, but promote resolution, tissue healing and microbial clearance, without compromising immune host defense mechanisms, opens interesting therapeutic scenarios for CF. In this mini-review, we present the current knowledge and perspectives of proresolving pharmacology in CF, focusing on the specialized proresolving lipid mediators and selected peptides.

Blame the signaling: Role of cAMP for the resolution of inflammation

A complex intracellular signaling governs different cellular responses in inflammation. Extracellular stimuli are sensed, amplified, and transduced through a dynamic cellular network of messengers converting the first signal into a proper response: production of specific mediators, cell activation, survival, or death. Several overlapping pathways are coordinated to ensure specific and timely induction of inflammation to neutralize potential harms to the tissue. Ideally, the inflammatory response must be controlled and self-limited. Resolution of inflammation is an active process that culminates with termination of inflammation and restoration of tissue homeostasis. Comparably to the onset of inflammation, resolution responses are triggered by coordinated intracellular signaling pathways that transduce the message to the nucleus. However, the key messengers and pathways involved in signaling transduction for resolution are still poorly understood in comparison to the inflammatory network. cAMP has long been recognized as an inducer of anti-inflammatory responses and cAMP-dependent pathways have been extensively exploited pharmacologically to treat inflammatory diseases. Recently, cAMP has been pointed out as coordinator of key steps of resolution of inflammation. Here, we summarize the evidence for the role of cAMP at inducing important features of resolution of inflammation.

Cyclic AMP Regulates Key Features of Macrophages via PKA: Recruitment, Reprogramming and Efferocytosis

Macrophages are central to inflammation resolution, an active process aimed at restoring tissue homeostasis following an inflammatory response. Here, the effects of db-cAMP on macrophage phenotype and function were investigated. Injection of db-cAMP into the pleural cavity of mice induced monocytes recruitment in a manner dependent on PKA and CCR2/CCL2 pathways. Furthermore, db-cAMP promoted reprogramming of bone-marrow-derived macrophages to a M2 phenotype as seen by increased Arg-1/CD206/Ym-1 expression and IL-10 levels (M2 markers). Db-cAMP also showed a synergistic effect with IL-4 in inducing STAT-3 phosphorylation and Arg-1 expression. Importantly, db-cAMP prevented IFN-γ/LPS-induced macrophage polarization to M1-like as shown by increased Arg-1 associated to lower levels of M1 cytokines (TNF-α/IL-6) and p-STAT1. In vivo, db-cAMP reduced the number of M1 macrophages induced by LPS injection without changes in M2 and Mres numbers. Moreover, db-cAMP enhanced efferocytosis of apoptotic neutrophils in a PKA-dependent manner and increased the expression of Annexin A1 and CD36, two molecules associated with efferocytosis. Finally, inhibition of endogenous PKA during LPS-induced pleurisy impaired the physiological resolution of inflammation. Taken together, the results suggest that cAMP is involved in the major functions of macrophages, such as nonphlogistic recruitment, reprogramming and efferocytosis, all key processes for inflammation resolution.

Effect of BMS-470539 on lipopolysaccharide-induced neutrophil activation

Background

BMS-470539, a recently introduced selective agonist of the melanocortin 1 receptor, is known to have anti-inflammatory properties. In this study, we investigated the effects of BMS-470539 on lipopolysaccharide (LPS)-induced inflammatory responses and delayed apoptosis with its signaling pathways in human neutrophils.

Methods

Isolated human neutrophils were incubated with various concentrations of BMS-470539 (1, 10, and 100 µM) in the presence or absence of LPS (100 ng/ml), and the expression of pro-inflammatory cytokines, such as tumor necrosis factor alpha, interleukin (IL)-6, and IL-1β, were assessed. The effects of BMS-470539 on the expression of mitogen-activated protein kinases (MAPKs), such as p38, extracellular-signal-regulated kinase 1/2, and c-Jun N-terminal kinase, and the expression of nuclear factor kappa B (NF-κB) in LPS-stimulated human neutrophils, were evaluated by enzyme-linked immunosorbent assay. Neutrophil apoptosis was also measured by fluorescence-activated cell sorting (annexin V/propidium iodide) in LPS-stimulated neutrophils under treatment with BMS-470539.

Results

BMS-470539 attenuated LPS-induced expression of pro-inflammatory cytokines, and phosphorylation of MAPKs and NF-κB. LPS stimulation reduced neutrophil apoptosis compared to the controls; however, BMS-470539 significantly inhibited the reduction of neutrophil apoptosis.

Conclusions

BMS-470539 can suppress the inflammatory responses of LPS-stimulated neutrophils by inhibition of MAPK pathways or NF-κB pathway, and it can also inhibit LPS-delayed neutrophil apoptosis.

Characterization of channel catfish (Ictalurus punctatus) melanocortin-3 receptor reveals a potential network in regulation of energy homeostasis

The melanocortin-3 receptor (MC3R) is known to be involved in regulation of energy homeostasis, regulating feed efficiency and nutrient partitioning in mammals. Its physiological roles in non-mammalian vertebrates, especially economically important aquaculture species, are not well understood. Channel catfish (Ictalurus punctatus) is the main freshwater aquaculture species in North America. In this study, we characterized the channel catfish MC3R. The mc3r of channel catfish encoded a putative protein (ipMC3R) of 367 amino acids. We transfected HEK293T cells with ipMC3R plasmid for functional studies. Five agonists, including adrenocorticotropin, α-melanocyte stimulating hormone (α-MSH), β-MSH, [Nle⁴, D-Phe⁷]-α-MSH, and D-Trp⁸-γ-MSH, were used in the pharmacological studies. Our results showed that ipMC3R bound β-MSH with higher affinity and D-Trp⁸-γ-MSH with lower affinity compared with human MC3R. All agonists could stimulate ipMC3R and increase intracellular cAMP production with sub-nanomolar potencies. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation could also be triggered by ipMC3R. The ipMC3R exhibited constitutive activities in both cAMP and ERK1/2 pathways, and Agouti-related protein served as an inverse agonist at ipMC3R, potently inhibiting the high basal cAMP level. Moreover, we showed that melanocortin receptor accessory protein 2 (MRAP2) preferentially modulated ipMC3R in cAMP production rather than ERK1/2 activation. Our study will assist further investigation of the physiological roles of the ipMC3R, especially in energy homeostasis, in channel catfish.

Probing the Role of Melanocortin Type 1 Receptor Agonists in Diverse Immunological Diseases

Background: The melanocortin α-melanocyte stimulating hormone (α-MSH), an endogenous peptide with high affinity for the melanocortin 1 receptor (MC1r), has demonstrated prevention and reversal of intestinal and ocular inflammation in animal models. Preclinical studies were performed to determine whether two MC1r receptor agonists, PL-8177 and PL-8331, exhibit actions and efficacy similar to α-MSH in preventing and reversing intestinal and ocular inflammation.

Methods: Both PL-8177 and PL-8331 were assessed in a Eurofins LeadProfilingScreen selectivity panel including 72 in vitro assays. PL-8177 and PL-8331 were evaluated in an in vitro assay using human whole blood stimulated by lipopolysaccharide to determine inhibition of tumor necrosis factor alpha (TNF-α); for comparison, adrenocorticotropic hormone (ACTH) and α-MSH were used as positive controls. PL-8177, dosed at 0.5, 1.5, and 5.0 μg, was assessed in a cannulated rat model of dinitrobenzene sulfonic acid (DNBS)-induced bowel inflammation versus vehicle and oral sulfasalazine. PL-8177 was also dosed at 0.3 mg/kg/mouse injected intraperitoneally versus untreated controls and α-MSH treatment in mice with experimental autoimmune uveitis (EAU). PL-8331 at 3 doses, 3 times daily, was evaluated in a murine model of scopolamine-induced dry eye disease (SiccaSystem^TM model), versus twice-daily Restasis^® and Xiidra^®.

Results: Both PL-8177 and PL-8331 demonstrated no significant activity at the 1 μm concentration in any of the 72 in vitro assays. PL-8177 and PL-8331 inhibited lipopolysaccharide-induced TNF-α to a similar degree as ACTH and α-MSH. In the DNBS rat model of bowel inflammation, PL-8177 was significantly superior to untreated controls at all 3 doses (P < 0.05) in reducing bowel inflammation parameters, with effects similar to sulfasalazine. In the murine EAU model, PL-8177 significantly reduced retinal inflammation scores versus untreated controls (P = 0.0001) over 3–5 weeks, and to a similar degree as α-MSH. In the murine scopolamine-induced model of dry eye disease, PL-8331 reduced corneal fluorescein staining scores at all doses, significantly (P = 0.02) for the highest dose (1 × 10^-5 mg⋅mL^-1), and similarly to Restasis^®; Xiidra^® demonstrated no effect.

Conclusion: The MC1r receptor agonists PL-8177 and PL-8331 exhibited actions similar to those of α-MSH in preventing and reversing intestinal and ocular inflammation in preclinical disease models.

The Cholinergic Anti-Inflammatory Response and the Role of Macrophages in HIV-Induced Inflammation

Macrophages are phagocytic immune cells that protect the body from foreign invaders and actively support the immune response by releasing anti- and proinflammatory cytokines. A seminal finding revolutionized the way macrophages are seen. The expression of the neuronal alpha7 nicotinic acetylcholine receptor (α7-nAChR) in macrophages led to the establishment of the cholinergic anti-inflammatory response (CAR) in which the activation of this receptor inactivates macrophage production of proinflammatory cytokines. This novel neuroimmune response soon began to emerge as a potential target to counteract inflammation during illness and infection states. Human immunodeficiency virus (HIV)-infected individuals suffer from chronic inflammation that persists even under antiretroviral therapy. Despite the CAR’s importance, few studies involving macrophages have been performed in the HIV field. Evidence demonstrates that monocyte-derived macrophages (MDMs) recovered from HIV-infected individuals are upregulated for α7-nAChR. Moreover, in vitro studies demonstrate that addition of an HIV viral constituent, gp120_IIIB, to uninfected MDMs also upregulates the α7-nAChR. Importantly, contrary to what was expected, activation of upregulated α7-nAChRs in macrophages does not reduce inflammation, suggesting a CAR disruption. Although it is reasonable to consider this receptor as a pharmacological target, additional studies are necessary since its activity seems to differ from that observed in neurons.

Bioelectric regulation of innate immune system function in regenerating and intact Xenopus laevis

Two key inputs that regulate regeneration are the function of the immune system, and spatial gradients of transmembrane potential (Vmem). Endogenous bioelectric signaling in somatic tissues during regenerative patterning is beginning to be understood, but its role in the context of immune response has never been investigated. Here, we show that Vmem levels modulate innate immunity activity in Xenopus laevis embryos. We developed an assay in which X. laevis embryos are infected with a uropathogenic microorganism, in the presence or absence of reagents that modify Vmem, prior to the ontogenesis of the adaptive immune system. General depolarization of the organism's Vmem by pharmacological or molecular genetic (ion channel misexpression) methods increased resistance to infection, while hyperpolarization made the embryos more susceptible to death by infection. Hyperpolarized specimens harbored a higher load of infectious microorganisms when compared to controls. We identified two mechanisms by which Vmem mediates immune function: serotonergic signaling involving melanocytes and an increase in the number of primitive myeloid cells. Bioinformatics analysis of genes whose transcription is altered by depolarization revealed a number of immune system targets consistent with mammalian data. Remarkably, amputation of the tail bud potentiates systemic resistance to infection by increasing the number of peripheral myeloid cells, revealing an interplay of regenerative response, innate immunity, and bioelectric regulation. Our study identifies bioelectricity as a new mechanism by which innate immune response can be regulated in the context of infection or regeneration. Vmem modulation using drugs already approved for human use could be exploited to improve resistance to infections in clinical settings.

Neuropeptides and Microglial Activation in Inflammation, Pain, and Neurodegenerative Diseases

Microglial cells are responsible for immune surveillance within the CNS. They respond to noxious stimuli by releasing inflammatory mediators and mounting an effective inflammatory response. This is followed by release of anti-inflammatory mediators and resolution of the inflammatory response. Alterations to this delicate process may lead to tissue damage, neuroinflammation, and neurodegeneration. Chronic pain, such as inflammatory or neuropathic pain, is accompanied by neuroimmune activation, and the role of glial cells in the initiation and maintenance of chronic pain has been the subject of increasing research over the last two decades. Neuropeptides are small amino acidic molecules with the ability to regulate neuronal activity and thereby affect various functions such as thermoregulation, reproductive behavior, food and water intake, and circadian rhythms. Neuropeptides can also affect inflammatory responses and pain sensitivity by modulating the activity of glial cells. The last decade has witnessed growing interest in the study of microglial activation and its modulation by neuropeptides in the hope of developing new therapeutics for treating neurodegenerative diseases and chronic pain. This review summarizes the current literature on the way in which several neuropeptides modulate microglial activity and response to tissue damage and how this modulation may affect pain sensitivity.

Mechanisms underlying heterologous skin scaffold-mediated tissue remodeling

Biocompatibility of two newly developed porcine skin scaffolds was assessed after 3, 14, 21 and 90 days of implantation in rats. Both scaffolds showed absence of cells, preservation of ECM and mechanical properties comparable to non-decellularised skin before implantation. Host cell infiltration was much prominent on both scaffolds when compared to Permacol (surgical control). At day 3, the grafts were surrounded by polymorphonuclear cells, which were replaced by a notable number of IL-6-positive cells at day 14. Simultaneously, the number of pro-inflammatory M1-macrophage was enhanced. Interestingly, a predominant pro-remodeling M2 response, with newly formed vessels, myofibroblasts activation and a shift on the type of collagen expression was sequentially delayed (around 21 days). The gene expression of some trophic factors involved in tissue remodeling was congruent with the cellular events. Our findings suggested that the responsiveness of macrophages after non-crosslinked skin scaffolds implantation seemed to intimately affect various cell responses and molecular events; and this range of mutually reinforcing actions was predictive of a positive tissue remodeling that was essential for the long-standing success of the implants. Furthermore, our study indicates that non-crosslinked biologic scaffold implantation is biocompatible to the host tissue and somehow underlying molecular events involved in tissue repair.

Improved cutaneous wound healing after intraperitoneal injection of alpha-melanocyte-stimulating hormone

Skin wound healing is a complex process involving many types of cells and molecules and often results in scar tissue formation in adult mammals. However, scarless healing occurs in foetal skin and minimal scars may occur after cutaneous healing in the adult with reduced inflammation. Alpha-melanocyte-stimulating hormone (α-MSH) is widely distributed within the central nervous system and in other body regions, such as the skin, and has strong anti-inflammatory activity. The aim in the present experiments was to learn whether intraperitoneal (i.p) injection of α-MSH just before skin wounds antagonize inflammation and improves skin wound healing in adult mice. C57BL/6 young adult mice received an i.p. injection of 1 mg/kg of α-MSH and, 30 min later, two circular through-and-through holes (6.5 mm diameter) were made in their dorsal skin under anaesthesia. Control mice were wounded after vehicle injection. The wound healing process was analysed macroscopically and microscopically at 3, 7, 40 and 60 days. Skin samples were fixed in formalin, embedded in paraffin, sectioned at 5 μm, stained with H&E or toluidine blue for cell analysis or Gomori's trichrome for extracellular matrix (ECM) analysis. Other samples were fixed in DMSO+methanol, embedded in paraplast and incubated with anti-CD45, antismooth muscle actin, anticollagen-I and anticollagen-III for immunofluorescence analysis. Alpha-MSH significantly reduced the number of leucocytes, mast cells and fibroblasts at 3 and 7 days after injury. On days 40 and 60, α-MSH reduced scar area and improved the organization of the collagen fibres indicating that it may direct the healing into a more-regenerative/less-scarring pathway.

Alterations in the profile of blood neutrophil membrane receptors caused by in vivo adrenocorticotrophic hormone actions

Elevated levels of adrenocorticotrophic hormone (ACTH) mobilize granulocytes from bone marrow into the blood, although these neutrophils are refractory to a full migratory response into inflamed tissues. Here, we investigated the dependence of glucocorticoid receptor activation and glucocorticoid-regulated protein annexin A1 (ANXA1) on ACTH-induced neutrophilia and the phenotype of blood neutrophil after ACTH injection, focusing on adhesion molecule expressions and locomotion properties. ACTH injection (5 μg ip, 4 h) induced neutrophilia in wild-type (WT) mice and did not alter the elevated numbers of neutrophils in RU-38486 (RU)-pretreated or ANXA1(-/-) mice injected with ACTH. Neutrophils from WT ACTH-treated mice presented higher expression of Ly6G⁺ANXA1(high), CD18(high), CD62L(high), CD49(high), CXCR4(high), and formyl-peptide receptor 1 (FPR1(low)) than those observed in RU-pretreated or ANXA1(-/-) mice. The membrane phenotype of neutrophils collected from WT ACTH-treated mice was paralleled by elevated fractions of rolling and adherent leukocytes to the cremaster postcapillary venules together with impaired neutrophil migration into inflamed air pouches in vivo and in vitro reduced formyl-methionyl-leucyl-phenylalanine (fMLP) or stromal-derived factor-1 (SDF-1α)-induced chemotaxis. In an 18-h senescence protocol, neutrophils from WT ACTH-treated mice had a higher proportion of ANXAV(low)/CXCR4(low), and they were less phagocytosed by peritoneal macrophages. We conclude that alterations on HPA axis affect the pattern of membrane receptors in circulating neutrophils, which may lead to different neutrophil phenotypes in the blood. Moreover, ACTH actions render circulating neutrophils to a phenotype with early reactivity, such as in vivo leukocyte-endothelial interactions, but with impaired locomotion and clearance.

Resolution of inflammation: an integrated view

Resolution of inflammation is a coordinated and active process aimed at restoration of tissue integrity and function. This review integrates the key molecular and cellular mechanisms of resolution. We describe how abrogation of chemokine signalling blocks continued neutrophil tissue infiltration and how apoptotic neutrophils attract monocytes and macrophages to induce their clearance. Uptake of apoptotic neutrophils by macrophages reprograms macrophages towards a resolving phenotype, a key event to restore tissue homeostasis. Finally, we highlight the therapeutic potential that derives from understanding the mechanisms of resolution.

Control of myeloid cell trafficking in resolution

Following tissue injury or microbial invasion, neutrophils are robustly recruited to inflammatory loci, which is a hallmark of the host inflammatory response. This event initiates a series of processes required to activate resolution, including recruitment of monocytes, clearance of microbes, cellular debris and apoptotic neutrophils, the egress of phagocytes and, ultimately, regain of tissue homeostasis. Substantial evidence now signifies that resolution of inflammation is a highly coordinated, active process dictated by the spatial-temporal generation of proresolving mediators that act on specific receptors to modulate cell and tissue reactivity. This review will focus on the mediators, targets and pathways initiated to orchestrate resolution. Importantly, disruption of the key processes involved in inflammatory resolution could result in delayed restoration of tissue homeostasis, leading to fibrosis and/or persistent inflammation.

Resolution of inflammation: mechanisms and opportunity for drug development

Inflammation is a beneficial host reaction to tissue damage and has the essential primary purpose of restoring tissue homeostasis. Inflammation plays a major role in containing and resolving infection and may also occur under sterile conditions. The cardinal signs of inflammation dolor, calor, tumor and rubor are intrinsically associated with events including vasodilatation, edema and leukocyte trafficking into the site of inflammation. If uncontrolled or unresolved, inflammation itself can lead to further tissue damage and give rise to chronic inflammatory diseases and autoimmunity with eventual loss of organ function. It is now evident that the resolution of inflammation is an active continuous process that occurs during an acute inflammatory episode. Successful resolution requires activation of endogenous programs with switch from production of pro-inflammatory towards pro-resolving molecules, such as specific lipid mediators and annexin A1, and the non-phlogistic elimination of granulocytes by apoptosis with subsequent removal by surrounding macrophages. These processes ensure rapid restoration of tissue homeostasis. Here, we review recent advances in the understanding of resolution of inflammation, highlighting the pharmacological strategies that may interfere with the molecular pathways which control leukocyte survival and clearance. Such strategies have proved beneficial in several pre-clinical models of inflammatory diseases, suggesting that pharmacological modulation of the resolution process may be useful for the treatment of chronic inflammatory diseases in humans.

Pharmacological strategies to resolve acute inflammation

The inflammatory response is a physiological process that has the major role of restoring tissue homeostasis. However, uncontrolled or unresolved inflammation may cause tissue damage and contribute to the pathogenesis of chronic inflammatory and autoimmune diseases. Current pharmacological therapies to treat inflammatory maladies focus on inhibition of the productive phase of the inflammatory response including inhibition of leukocyte influx. Resolution of inflammation is an active process, which relies on the production of pro-resolving molecules and activation of intracellular pathways. Here, we will discuss mechanisms and therapeutic potential of pharmacological strategies, which accelerate resolution in animal models of acute inflammation by mimicking or inducing natural pathways of resolution phase of inflammation.