The emerging role of heterodimerisation and interacting proteins in ghrelin receptor function

Ghrelin suppresses apoptosis and autophagy in osteoarthritis synovial cells by modulating the ADORA2B/PI3K/Akt/mTOR signaling pathway

Given the pivotal role that apoptosis and autophagy play in the pathogenesis of osteoarthritis (OA), the current study aims to examine the regulatory effects of ghrelin on these processes via the ADORA2B/PI3K/Akt/mTOR signaling pathway. Serum levels of ghrelin were measured in both OA patients and healthy controls using an ELISA kit. Cell proliferation was evaluated through the Cell Counting Kit-8 (CCK-8) assay, while Western blot analysis was utilized to determine the expression levels of autophagy-related proteins (LC3II/I, BECLIN-1) and apoptosis markers (BAX, Bcl-2), as well as to assess the activation status of the PI3K/Akt/mTOR signaling pathway in OA synovial cells. These analyses were performed under conditions of ADORA2B and mTOR silencing, as well as in control settings. The results revealed that ghrelin expression was significantly reduced in the serum of OA patients. Furthermore, ghrelin was found to enhance synovial cell proliferation while simultaneously inhibiting apoptosis and autophagy, as evidenced by lowered expression levels of LC3/I, BECLIN-1, and BAX, alongside an increase in Bcl-2 expression. This modulation occurred through the regulation of the PI3K/Akt/mTOR signaling pathway mediated by ADORA2B. These findings underscore the role of ghrelin in the progression of osteoarthritis by influencing synovial cell activity through the ADORA2B/PI3K/Akt/mTOR pathway, thus laying the groundwork for investigating targeted therapeutic strategies in clinical practice.

Ghrelin in Depression: A Promising Therapeutic Target

Depression is a widespread disease affecting over 300 million individuals of various ethnicities and socioeconomic backgrounds globally. It frequently strikes early in life and becomes a chronic or recurring lifelong illness. Out of the various hypotheses for the pathophysiology of depression, the gut-brain axis and stress hypothesis are the ones that need to be researched, as psychological stress impairs one or more pathways of the brain-gut axis and is likely to cause brain-gut axis dysfunction and depression. A dysfunctional reciprocal gut-brain relationship may contribute to many diseases, including inflammatory disorders, abnormal stress responses, impaired behavior, and metabolic changes. The hormone ghrelin is a topic of interest concerning the gut-brain axis as it interacts with the gut-brain axis indirectly via the central nervous system or via crossing the blood-brain barrier. Ghrelin release is also affected by the gut microbes, which has also been discussed in the review. This review elaborates on Ghrelin's role in depression and its effect on various aspects like neurogenesis, HPA axis, and neuroinflammation. Furthermore, this review focuses on ghrelin as a potential target for alleviation of depressive symptoms.

Syndecans modulate ghrelin receptor signaling

Ghrelin is a gut hormone that enhances food intake and growth hormone secretion through its G-protein coupled receptor, the growth hormone secretagogue receptor (GHSR). Recently, we have shown that ghrelin interacts with syndecans (SDCs), a family of membrane proteins known to modulate hypothalamic appetite signaling. Here, we investigated whether SDCs impact ghrelin signaling at GHSR by assessing ghrelin-induced intracellular Ca2+ mobilization (iCa2+) and inositol phosphate 1 (IP1) production in HEK293 cells. Compared with controls, the overexpression of SDCs dose-dependently increased the maximum iCa2+ response two- to four-fold, without affecting EC50. The IP1 response was similarly amplified by SDCs, but it also indicated that they reduce constitutive (ghrelin-independent) activity of GHSR. These enhanced responses occurred despite a SDC dose-dependent reduction in plasma membrane GHSR levels. Although ghrelin-stimulated Gαq activation was unaltered by SDC1 expression, it failed to restore iCa2+ responsiveness in GNAQ/11 knockout cells, indicating dependence on Gαq/11, not another Gα subunit. This suggests that SDCs modulate either signaling downstream of Gαq/11 or quenching of β-arrestin2 recruitment to GHSR. Indeed, expression of SDCs at levels that only modestly suppress cell surface receptor reduced ghrelin-induced β-arrestin2 recruitment by ∼80%. SDC co-expression also delayed the peak β-arrestin2 response. However, peak β-arrestin2 recruitment follows the peak iCa2+ response, making it unclear whether reduced β-arrestin2 recruitment potentiated Ca2+ signaling. Altogether, SDCs enhanced iCa2+/IP1 and reduced β-arrestin2 recruitment by GHSR in response to ghrelin, likely by modulating signaling downstream of Gαq. This could be a novel mechanism through which SDCs affect metabolism and obesity.

Antimicrobial neuropeptides and their therapeutic potential in vertebrate brain infectious disease

The defense mechanisms of the vertebrate brain against infections are at the forefront of immunological studies. Unlike other body parts, the brain not only fends off pathogenic infections but also minimizes the risk of self-damage from immune cell induced inflammation. Some neuropeptides produced by either nerve or immune cells share remarkable similarities with antimicrobial peptides (AMPs) in terms of size, structure, amino acid composition, amphiphilicity, and net cationic charge. These similarities extend to a wide range of antibacterial activities demonstrated in vitro, effectively protecting nerve tissue from microbial threats. This review systematically examines 12 neuropeptides, pituitary adenylate cyclase-activating peptide (PACAP), vasoactive intestinal peptide (VIP), α-melanocyte stimulating hormone (α-MSH), orexin-B (ORXB), ghrelin, substance P (SP), adrenomedullin (AM), calcitonin-gene related peptide (CGRP), urocortin-II (UCN II), neuropeptide Y (NPY), NDA-1, and catestatin (CST), identified for their antimicrobial properties, summarizing their structural features, antimicrobial effectiveness, and action mechanisms. Importantly, the majority of these antimicrobial neuropeptides (9 out of 12) also possess significant anti-inflammatory properties, potentially playing a key role in preserving immune tolerance in various disorders. However, the connection between this anti-inflammatory property and the brain's infection defense strategy has rarely been explored. Our review suggests that the combined antimicrobial and anti-inflammatory actions of neuropeptides could be integral to the brain's defense strategy against pathogens, marking an exciting direction for future research.

The Effect of Copper Nanoparticles on Liver Metabolism Depends on the Type of Dietary Fiber

BACKGROUND/OBJECTIVES

A diet enriched with copper nanoparticles (CuNPs) exhibits a wide range of effects on liver metabolism, both positive and negative. Dietary fibers are the key components that may affect the absorption of minerals, including copper, and change their impact on organisms.

METHODS

Therefore, this study investigated whether and how supplementation with different sources of dietary fiber (cellulose, pectin, inulin, and psyllium) affects the function of CuNPs in the liver of male Wistar rats.

RESULTS

The results showed that CuNPs at different doses had varying effects on lipid metabolism and inflammation in the liver. Specifically, higher doses of CuNPs were associated with increased lipid accumulation and the activation of pro-inflammatory mechanisms. However, combining CuNPs with dietary fibers, such as psyllium and inulin, was beneficial in mitigating the effects of the examined nanoparticles, leading to reduced fat, cholesterol, and triglycerides in the liver. Combining psyllium with CuNPs showed the most substantial effect on liver metabolism and inflammation parameters. Furthermore, hepatic histology analyses showed that adding psyllium to the diet with CuNPs reduces changes associated with fat accumulation and mononuclear cell infiltration. The observed beneficial changes in the liver may have been related to a reduction in the gene expression level of sterol regulatory element-binding protein 1 and peroxisome proliferator-activated receptor gamma and cyclooxygenase-2.

CONCLUSIONS

In conclusion, enriching the diet with dietary fibers such as psyllium can regulate the action of CuNPs, thereby improving lipid metabolism and reducing inflammation in the liver.

Sites and mechanisms of action of colokinetics at dopamine, ghrelin and serotonin receptors in the rodent lumbosacral defecation centre

Agonists of dopamine D2 receptors (D2R), 5-hydroxytryptamine (5-HT, serotonin) receptors (5-HTR) and ghrelin receptors (GHSR) activate neurons in the lumbosacral defecation centre, and act as 'colokinetics', leading to increased propulsive colonic motility, in vivo. In the present study, we investigated which neurons in the lumbosacral defecation centre express the receptors and whether dopamine, serotonin and ghrelin receptor agonists act on the same lumbosacral preganglionic neurons (PGNs). We used whole cell electrophysiology to record responses from neurons in the lumbosacral defecation centre, following colokinetic application, and investigated their expression profiles and the chemistries of their neural inputs. Fluorescence in situ hybridisation revealed Drd2, Ghsr and Htr2C transcripts were colocalised in lumbosacral PGNs of mice, and immunohistochemistry showed that these neurons have closely associated tyrosine hydroxylase and 5-HT boutons. Previous studies showed that they do not receive ghrelin inputs. Whole cell electrophysiology in adult mice spinal cord revealed that dopamine, serotonin, α-methylserotonin and capromorelin each caused inward, excitatory currents in overlapping populations of lumbosacral PGNs. Furthermore, dopamine caused increased frequency of both IPSCs and EPSCs in a cohort of D2R neurons. Tetrodotoxin blocked the IPSCs and EPSCs, revealing a post-synaptic excitatory action of dopamine. In lumbosacral PGNs of postnatal day 7-14 rats, only dopamine's postsynaptic effects were observed. Furthermore, inward, excitatory currents evoked by dopamine were reduced by the GHSR antagonist, YIL781. We conclude that lumbosacral PGNs are the site where the action of endogenous ligands of D2R and 5-HT2R converge, and that GHSR act as a cis-modulator of D2R expressed by the same neurons. KEY POINTS: Dopamine, 5-hydroxytryptamine (5-HT, serotonin) and ghrelin (GHSR) receptor agonists increase colorectal motility and have been postulated to act at receptors on parasympathetic preganglionic neurons (PGNs) in the lumbosacral spinal cord. We aimed to determine which neurons in the lumbosacral spinal cord express dopamine, serotonin and GHSR receptors, their neural inputs, and whether agonists at these receptors excite them. We show that dopamine, serotonin and ghrelin receptor transcripts are contained in the same PGNs and that these neurons have closely associated tyrosine hydroxylase and serotonin boutons. Whole cell electrophysiology revealed that dopamine, serotonin and GHSR receptor agonists induce an inward excitatory current in overlapping populations of lumbosacral PGNs. Dopamine-induced excitation was reversed by GHSR antagonism. The present study demonstrates that lumbosacral PGNs are the site at which actions of endogenous ligands of dopamine D2 receptors and 5-HT type 2 receptors converge. Ghrelin receptors are functional, but their role appears to be as modulators of dopamine effects at D2 receptors.

The Mechanism of the Gut-Brain Axis in Regulating Food Intake

With the increasing prevalence of energy metabolism disorders such as diabetes, cardiovascular disease, obesity, and anorexia, the regulation of feeding has become the focus of global attention. The gastrointestinal tract is not only the site of food digestion and absorption but also contains a variety of appetite-regulating signals such as gut-brain peptides, short-chain fatty acids (SCFAs), bile acids (BAs), bacterial proteins, and cellular components produced by gut microbes. While the central nervous system (CNS), as the core of appetite regulation, can receive and integrate these appetite signals and send instructions to downstream effector organs to promote or inhibit the body's feeding behaviour. This review will focus on the gut-brain axis mechanism of feeding behaviour, discussing how the peripheral appetite signal is sensed by the CNS via the gut-brain axis and the role of the central "first order neural nuclei" in the process of appetite regulation. Here, elucidation of the gut-brain axis mechanism of feeding regulation may provide new strategies for future production practises and the treatment of diseases such as anorexia and obesity.

The effects and mechanisms of ghrelin upon angiogenesis in human coronary artery endothelial cells under hypoxia

Ghrelin, an endogenous ligand of the growth hormone secretagogue receptor (GHSR), has been found to stimulate angiogenesis both in vivo and in vitro. However, the effect of ghrelin upon angiogenesis, and the corresponding mechanisms of ghrelin therein, in human coronary artery endothelial cells (HCAECs) under hypoxia is still unknown. Our study found that ghrelin significantly increased HCAECs proliferation, migration, in vitro angiogenesis, and microvessel sprouting from the aortic ring under hypoxic conditions. The ghrelin-induced angiogenic process was accompanied by vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), angiopoietin-2 (Ang-2) and endothelial-specific receptor tyrosine kinase (Tie2) expressions. In addition, this angiogenic effect was almost completely inhibited by Ang-2 RNAi and Tie2 RNAi. Pretreatment with the GHSR1a blocker [D-Lys3]-GHRP-6 abolished ghrelin-induced VEGF, Ang-1, Ang-2 and Tie2 expressions and in vitro angiogenesis. In conclusion, this is the first demonstration that ghrelin stimulates HCAECs in vitro angiogenesis through GHSR1a-mediated VEGF, Ang-1, Ang-2 and Tie2 pathways under hypoxic conditions. It indicated that ghrelin might play an important role in myocardial angiogenesis after ischemic injury.

Role of the ghrelin system in alcohol use disorder and alcohol-associated liver disease: A narrative review

Unhealthy alcohol consumption is a global health problem. Adverse individual, public health, and socioeconomic consequences are attributable to harmful alcohol use. Epidemiological studies have shown that alcohol use disorder (AUD) and alcohol-associated liver disease (ALD) are the top two pathologies among alcohol-related diseases. Consistent with the major role that the liver plays in alcohol metabolism, uncontrolled drinking may cause significant damage to the liver. This damage is initiated by excessive fat accumulation in the liver, which can further progress to advanced liver disease. The only effective therapeutic strategies currently available for ALD are alcohol abstinence or liver transplantation. Any molecule with dual-pronged effects at the central and peripheral organs controlling addictive behaviors and associated metabolic pathways are a potentially important therapeutic target for treating AUD and ALD. Ghrelin, a hormone primarily derived from the stomach, has such properties, and regulates both behavioral and metabolic functions. In this review, we highlight recent advances in understanding the peripheral and central functions of the ghrelin system and its role in AUD and ALD pathogenesis. We first discuss the correlation between blood ghrelin concentrations and alcohol use or abstinence. Next, we discuss the role of ghrelin in alcohol-seeking behaviors and finally its role in the development of fatty liver by metabolic regulations and organ crosstalk. We propose that a better understanding of the ghrelin system could open an innovative avenue for improved treatments for AUD and associated medical consequences, including ALD.