Ghrelin, the same peptide for different functions: player or bystander?

Brain and kidney GHS-R1a underexpression is associated with changes in renal function and hemodynamics during neurogenic hypertension

Ghrelin is a peptide hormone whose effects are mediated by the growth hormone secretagogue receptor subtype 1a (GHS-R1a), mainly expressed in the brain but also in kidneys. The hypothesis herein raised is that GHS-R1a would be player in the renal contribution to the neurogenic hypertension pathophysiology. To investigate GHS-R1a role on renal function and hemodynamics, we used Wistar (WT) and spontaneously hypertensive rats (SHR). First, we assessed the effect of systemically injected vehicle, ghrelin, GHS-R1a antagonist PF04628935, ghrelin plus PF04628935 or GHS-R1a synthetic agonist MK-677 in WT and SHR rats housed in metabolic cages (24 h). Blood and urine samples were also analyzed. Then, we assessed the GHS-R1a contribution to the control of renal vasomotion and hemodynamics in WT and SHR. Finally, we assessed the GHS-R1a levels in brain areas, aorta, renal artery, renal cortex and medulla of WT and SHR rats using western blot. We found that ghrelin and MK-677 changed osmolarity parameters of SHR, in a GHS-R1a-dependent manner. GHS-R1a antagonism reduced the urinary Na⁺ and K⁺ and creatinine clearance in WT but not in SHR. Ghrelin reduced arterial pressure and increased renal artery conductance in SHR. GHS-R1a protein levels were decreased in the kidney and brain areas of SHR when compared to WT. Therefore, GHS-R1a role in the control of renal function and hemodynamics during neurogenic hypertension seem to be different, and this may be related to brain and kidney GHS-R1a downregulation.

Sharing Pathological Mechanisms of Insomnia and Osteoporosis, and a New Perspective on Safe Drug Choice

Lack of adequate sleep has become increasingly common in our 24/7 modern society. Reduced sleep has significant health consequences including metabolic and cardiovascular disorders, and mental problems including depression. In addition, although the increase in life expectancy has provided a dream of longevity to humans, the occurrence of osteoporosis is a big obstacle to this dream for both male and female. It is known that insomnia and bone health problems, which are very critical conditions in human life, interestingly, share a lot of pathogenesis in recent decades. Nevertheless, due to another side effects of the synthetic drugs being taken for the treatment of insomnia and osteoporosis, patients have substantial anxiety for the safety of drugs with therapeutic expectation. This review examines the pathogenesis shared by sleep and osteoporosis together and herbal medicine, which has recently been shown to be safe and efficacious in the treatment of both diseases other than synthetic drugs. We suggestions for how to treat osteoporosis. These efforts will be the first step toward enabling patients to have comfortable and safe prescriptions through a wide selection of therapeutic agents in the future.

Ghrelin increases memory consolidation through hippocampal mechanisms dependent on glutamate release and NR2B-subunits of the NMDA receptor

RATIONALE

Ghrelin (Ghr) is a peptide that participates in the modulation of several biological processes. Ghr administration into the hippocampus improves learning and memory in different memory tests. However, the possible mechanisms underlying this effect on memory have not yet been clarified.

OBJECTIVE

The purpose of the present work is to add new insights about the mechanisms by which Ghr modulates long-term memory consolidation in the hippocampus. We examined Ghr effects upon processes related to increased synaptic efficacy as presynaptic glutamate release and changes in the expression of the NR2B-subunits containing n-methyl-d-aspartate receptors (NMDAR), which are critical for LTP induction. We also attempted to determine the temporal window in which Ghr administration induces memory facilitation and if the described effects depend on GHS-R1a stimulation.

RESULTS

The present research demonstrated that Ghr increased glutamate release from hippocampal synaptosomes; intra-hippocampal Ghr administration increased NR2B-subunits expression in CA1 and DG subareas and also reversed the deleterious effects of the NR2B-subunit-specific antagonist, Ro 25-6981, upon memory consolidation and LTP generation in the hippocampus. These effects are likely to be the consequence of GHS-R1a activation.

CONCLUSION

According to the results above mentioned and previous findings, we can hypothesize some of the mechanisms by which Ghr modulates memory consolidation. At presynaptic level, Ghr stimulates glutamate release, probably by enhancing [Ca(2+)]i. At postsynaptic level, the glutamate released activates NMDAR while Ghr also mediates effects directly activating its specific receptors and increases NR2B-subunit expression.

'As above, so below' examining the interplay between emotion and the immune system

While the concept of a palpable relationship between our mental and physical well-being is certainly not new, it is only in the light of modern scientific research that we have begun to realize how deeply connected our emotional and immune states may be. We begin this review with a series of studies demonstrating how four fundamental emotional responses: anger, anxiety, mirth and relaxation are able modulate cytokine production and cellular responses to a variety of immune stimuli. These modulations are shown to be either detrimental or beneficial to a patient's health dependent on the context and duration of the emotion. We also discuss the reverse, highlighting research demonstrating how the loss of key immune cells such as T lymphocytes in clinical and animal studies can negatively impact both emotional well-being and cognition. Additionally, to give a more complete picture of the manifold pathways that link emotion and the immune system, we give a brief overview of the influence the digestive system has upon mental and immunological health. Finally, throughout this review we attempt to highlight the therapeutic potential of this burgeoning field of research in both the diagnosis and treatment of immune and disorders. As well as identifying some of the key obstacles the field must address in order to put this potential into practice.

Predictive value of early decreased plasma ghrelin level for three-month cognitive deterioration in patients with mild traumatic brain injury

The orexigenic hormone, ghrelin, is tightly linked to cognition impairment in neurodegenerative disorders. No previous studies have investigated the early ghrelin concentration change in patients with mild traumatic brain injury (mTBI) and it's relationship to cognitive deterioration. This study was performed to investigate the early plasma ghrelin concentrations in patients with mTBI and to explore the relationship between ghrelin and cognitive deterioration. Plasma ghrelin concentrations of 118 adults after acute mTBI were determined by enzyme-linked immunosorbent assay. Forty patients (33.9%) had cognitive deterioration three months after mTBI. Plasma ghrelin levels were significantly lower in mTBI patients with cognitive deterioration than patients without cognitive deterioration (38.8±4.5 pg/mL vs 50.8±7.7 pg/mL, P<0.001). Decreased Plasma ghrelin level was identified as an independent predictor for three-month cognitive deterioration after mTBI (odds ratio, 0.746; 95% confidence interval, 0.651-0.856; P<0.001). Plasma ghrelin level was negatively associated with serum adrenocorticotrophin hormone level (t=-6.854, P<0.001) and age (t=-6.112, P<0.001). A plasma ghrelin level of 41.6 pg/mL predicted three-month cognitive deterioration after mTBI with the optimal sensitivity (85.9%) and specificity (80.0%) values (area under curve, 0.904; 95% confidence interval, 0.852-0.957; P<0.001). The predictive value of ghrelin was bigger than that of serum adrenocorticotrophin hormone level (area under curve, 0.638; 95% confidence interval, 0.536-0.741; P=0.014) and age (area under curve, 0.638; 95% confidence interval, 0.536-0.741; P=0.014) for three-month cognitive deterioration after mTBI.

The expression of GHS-R in primary neurons is dependent upon maturation stage and regional localization

Ghrelin is a hormone with a crucial role in the regulation of appetite, regulation of inflammation, glucose metabolism and cell proliferation. In the brain ghrelin neurons are located in the cortex (sensorimotor area, cingular gyrus), and the fibres of ghrelin neurons in hypothalamus project directly to the dorsal vagal complex (DVC). Ghrelin binds the growth hormone secretagogue receptor (GHS-R) a G-protein-coupled receptor with a widespread tissue distribution, indeed these receptors are localized both in nonnervous, organs/tissues (i.e. adipose tissue, myocardium, adrenals, gonads, lung, liver, arteries, stomach, pancreas, thyroid, and kidney) as well as in central nervous system (CNS) and higher levels of expression in the pituitary gland and the hypothalamus and lower levels of expression in other organs, including brain. A GHS-R specific monoclonal antibody has been developed and characterized and through it we demonstrate that GHS-R is expressed in primary neurons and that its expression is dependent upon their developmental stage and shows differences according to the brain region involved, with a more pronounced expression in hippocampal rather than cortical neurons. A characterization of GHS-R within the central nervous system is of extreme importance in order to gain insights on its role in the modulation of neurodegenerative events such as Alzheimer’s disease.

The role of ghrelin in neuroprotection after ischemic brain injury

Ghrelin, a gastrointestinal peptide with a major role in regulating feeding and metabolism, has recently been investigated for its neuroprotective effects. In this review we discuss pre-clinical evidence suggesting ghrelin may be a useful therapeutic in protecting the brain against injury after ischemic stroke. Specifically, we will discuss evidence showing ghrelin administration can improve neuronal cell survival in animal models of focal cerebral ischemia, as well as rescue memory deficits. We will also discuss its proposed mechanisms of action, including anti-apoptotic and anti-inflammatory effects, and suggest ghrelin treatment may be a useful intervention after stroke in the clinic.

Role of ghrelin system in neuroprotection and cognitive functions: implications in Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial progressive neurodegenerative disorder characterized by loss of memory and cognitive deficits, strongly influenced by the metabolic status, in which the impairment of neuropeptides/neurotransmitters systems has been previously observed. Ghrelin is a multifunctional hormone produced in a wide variety of tissues, which has been associated with the progression of obesity and metabolic syndrome, but has been also linked to neuromodulation, neuroprotection and memory and learning processes. In addition, ghrelin system also acts in an autocrine/paracrine fashion where the majority of its components [ghrelin variants (native ghrelin, In1-ghrelin), acylation enzyme (GOAT) and receptors (GHS-Rs)] are expressed in the different regions of central nervous system. In spite of all these pieces of information strongly suggesting a close association between ghrelin system and AD, which could be of pathophysiological relevance, few studies have been addressed to clarify this relationship. In this work, the role of ghrelin system in neuroprotection, memory consolidation and learning is reviewed, and its influence in AD, as well as the regulation of its expression in the brain of AD patients, is discussed.

A novel human ghrelin variant (In1-ghrelin) and ghrelin-O-acyltransferase are overexpressed in breast cancer: potential pathophysiological relevance

The human ghrelin gene, which encodes the ghrelin and obestatin peptides, contains 5 exons (Ex), with Ex1-Ex4 encoding a 117 amino-acid (aa) preproprotein that is known to be processed to yield a 28-aa (ghrelin) and/or a 23-aa (obestatin) mature peptides, which possess biological activities in multiple tissues. However, the ghrelin gene also encodes additional peptides through alternative splicing or post-translational modifications. Indeed, we previously identified a spliced mRNA ghrelin variant in mouse (In2-ghrelin-variant), which is regulated in a tissue-dependent manner by metabolic status and may thus be of biological relevance. Here, we have characterized a new human ghrelin variant that contains Ex0-1, intron (In) 1, and Ex2 and lacks Ex3-4. This human In1-ghrelin variant would encode a new prepropeptide that conserves the first 12aa of native-ghrelin (including the Ser3-potential octanoylation site) but has a different C-terminal tail. Expression of In1-variant was detected in 22 human tissues and its levels were positively correlated with those of ghrelin-O-acyltransferase (GOAT; p = 0.0001) but not with native-ghrelin expression, suggesting that In1-ghrelin could be a primary substrate for GOAT in human tissues. Interestingly, levels of In1-ghrelin variant expression in breast cancer samples were 8-times higher than those of normal mammary tissue, and showed a strong correlation in breast tumors with GOAT (p = 0.0001), ghrelin receptor-type 1b (GHSR1b; p = 0.049) and cyclin-D3 (a cell-cycle inducer/proliferation marker; p = 0.009), but not with native-ghrelin or GHSR1a expression. Interestingly, In1-ghrelin variant overexpression increased basal proliferation of MDA-MB-231 breast cancer cells. Taken together, our results provide evidence that In1-ghrelin is a novel element of the ghrelin family with a potential pathophysiological role in breast cancer.

Cognitive enhancing effects of ghrelin receptor agonists

RATIONALE

Ghrelin, the endogenous ligand for the growth hormone secretagogue receptor, has been shown to play a role in multiple physiological processes including appetite regulation, metabolism and, more recently, dendritic spine architecture, long-term potentiation and cognition.

OBJECTIVE

The objective of this study was to determine the effects of two structurally non-peptide ghrelin receptor agonists (GSK894490A and CP-464709-18) on rodent cognition.

METHODS

All experiments were performed in male Lister hooded rats. Effects of the test compounds on rat cognitive performance was determined using the novel object recognition test, a modified water maze paradigm and a scopolamine-induced deficit in cued fear conditioning. These tests were chosen as they each probe a relatively independent cognitive domain and therefore potentially have differing underlying neural substrates.

RESULTS

Both compounds significantly improved performance in the novel object recognition and modified water maze tests but were unable to attenuate a scopolamine deficit in cued fear conditioning.

CONCLUSIONS

These results demonstrate that the small-molecule ghrelin receptor agonists profiled here readily cross the blood/brain barrier and elicit pro-cognitive effects in recognition and spatial learning and memory tests. Based on these observations, the central ghrelin receptor would appear to be a chemically tractable receptor and perhaps should be considered as a new drug target for therapeutic approaches to treat diseases affecting cognition.

Cardiac, skeletal, and smooth muscle regulation by ghrelin

Ghrelin, mainly secreted from gastric mucosa, is the endogenous ligand for the growth hormone secretagogue receptor and induces a potent release of growth hormone. Ghrelin is widely expressed in different tissues and therefore has both endocrine and paracrine/autocrine effects. In this chapter, we summarize: (1) structure and distribution of ghrelin and its receptors; (2) myocardial effects of ghrelin, describing its acute and chronic actions on cardiac function; (3) ghrelin effects on smooth muscle, namely vascular smooth muscle, intraocular and gastrointestinal smooth muscle; and (4) skeletal actions of ghrelin. Ghrelin has a potent vasodilator effect, thereby reducing cardiac afterload and increasing cardiac output. In models of heart failure and myocardial ischemia, ghrelin administration has beneficial effects. At smooth muscle, ghrelin modulates vascular tone, increases gut transit, and relaxes iris muscles. In the skeletal muscle, ghrelin regulates resting membrane potential. In conclusion, there are increasing evidences that ghrelin is a peptide with paracrine actions that can modulate cardiac, smooth, and skeletal muscle functions.

Secretion of ghrelin from rat stomach ghrelin cells in response to local microinfusion of candidate messenger compounds: a microdialysis study

Ghrelin is produced by A-like cells (ghrelin cells) in the mucosa of the acid-producing part of the stomach. The mobilization of ghrelin is stimulated by nutritional deficiency and suppressed by nutritional abundance. In an attempt to identify neurotransmitters and regulatory peptides that may contribute to the physiological, nutrient-related regulation of ghrelin secretion, we challenged the ghrelin cells in situ with a wide variety of candidate messengers, including known neurotransmitters (e.g. acetylcholine, catecholamines), candidate neurotransmitters (e.g. neuropeptides), local tissue hormones (e.g. serotonin, histamine, bradykinin, endothelin), circulating gut hormones (e.g. gastrin, CCK, GIP, neurotensin, PYY, secretin) and other circulating hormones/regulatory peptides (e.g. calcitonin, glucagon, insulin, PTH). Microdialysis probes were placed in the submucosa of the acid-producing part of the rat stomach. Three days later, the putative messenger compounds were administered via the microdialysis probe (reverse microdialysis) at a screening dose of 0.1 mmol l(-1) for regulatory peptides and 0.1 and 1 mmol l(-1) for amines and amino acids. The rats were awake during the experiments. The resulting microdialysate ghrelin concentration was monitored continuously for 3 h (radioimmunoassay), thereby revealing stimulators or inhibitors of ghrelin secretion. Dose-response curves were constructed for each candidate messenger that significantly (p<0.05) affected ghrelin mobilization at the screening dose. Peptides that showed a (non-significant) tendency to affect ghrelin release at the screening dose were also given at a dose of 0.3 or 1 mmol l(-1). Adrenaline, noradrenaline, endothelin and secretin stimulated ghrelin release, while somatostatin and GRP inhibited. Whether these agents act directly or indirectly on the ghrelin cells remains to be investigated. All other candidate messengers were without measurable effects, including acetylcholine, serotonin, histamine, GABA, aspartic acid, glutamic acid, glycine, VIP, PACAP, CGRP, substance P, NPY, PYY, PP, gastrin, CCK, GIP, insulin, glucagon, GLP and glucose.

Serum Ghrelin Levels in patients with Hashimoto's thyroiditis

OBJECTIVE

Hypothyroidism is associated with changes in appetite and body weight. Ghrelin is an orexigenic peptide, and it stimulates appetite and increases food intake. However, the potential relationship between circulating ghrelin levels, hypothyroidism, and thyroid antibodies has not been adequately studied.

DESIGN

Forty-seven patients with hypothyroidism due to Hashimoto's thyroiditis and 48 euthyroid subjects were enrolled in the study. Thyroid hormones and antibodies, insulin, glucose, ghrelin levels, and lipid parameters were measured in all the subjects.

MAIN OUTCOME

Hypothyroid group showed significantly decreased serum levels of ghrelin and ghrelin=body mass index (BMI) compared to euthyroid group (31.9 +/- 21.5 pg/mL vs. 50.5 +/- 34.8 pg/mL, p < 0.001; and 1.24 +/- 0.93 vs. 2.12 +/- 1.53, p < 0.0001). In hypothyroid group, 6 months after treatment, ghrelin levels and ghrelin/BMI remained lower than euthyroid group (33.2 +/- 21.1 pg/mL vs. 50.5 +/- 34.8 pg/mL, p < 0.001; and 1.27 +/- 0.86 vs. 2.12 +/- 1.53, p < 0.0001). Ghrelin levels were decreased in hypothyroid patients with high thyroid peroxidase antibody (TPOAb) titre compared to hypothyroid patients with low TPOAb titre (19.1 +/- 23.1 pg/ mL vs. 35.3 +/- 17.4 pg/mL, p < 0.01). Ghrelin levels correlated positively with free triiodothyronine (FT3) and free thyroxine (FT4), and negatively with age, thyroglobulin antibody (TAb), TPOAb, total cholesterol (T-C), low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), and triglycerides (TG) in hypothyroid group. In euthyroid group, circulating ghrelin levels correlated negatively with age, FT3, FT4, TG, and VLDL-C levels. No significant correlation was observed between ghrelin and homeostasis model assessment for insulin resistance (HOMA-IR) and between ghrelin and quantitative insulin sensitivity check index (QUICKI) in both groups. Regression analysis revealed that FT3 level is the most important predictor of ghrelin levels.

CONCLUSION

Thyroid hormones and antibodies seem to have a potential effect on serum ghrelin levels in patients with hypothyroidism.

Frailty

Frailty is a common condition in older people. It now can be objectively defined by the Fried criteria. When recognized, early intervention should begin with the institution of endurance, resistance, and balance exercises. In men with testosterone deficiency a trial of testosterone replacement should be considered. Vitamin D deficiency needs to be recognized and treated. Appropriate treatment of underlying diseases, such as anemia, diabetes mellitus, and congestive heart failure, are a key management principle. In people who have frailty aggressive health promotion and disease prevention techniques can lead to an inhibition of the downward spiral to disability.

One ancestor, several peptides post-translational modifications of preproghrelin generate several peptides with antithetical effects

Preproghrelin is the polypeptide precursor of ghrelin. First discovered in gastric extract as a growth hormone releasing peptide and food intake modulator, it has more recently been revealed to have other physiological aspects. The fine molecular mechanisms of ghrelin biosynthesis show that this peptide is but one piece of a puzzle which contains many other peptides obtained from alternative splicing of the same gene or from extensive post-translational modifications. Recent developments have shown that pro-ghrelin cleavage generates another active peptide named obestatin with an intriguingly subtle but opposite physiological action to ghrelin. Noteworthy, and similar to ghrelin, which requires post-translational processing close to its amino terminus by acylation, the biological activity of the ghrelin-associated peptide obestatin also depends on modification, but by amidation at its carboxyl terminus. In this review we will summarize the steps which led to the identification of pre-proghrelin gene products and will examine the significance and perspectives of the different peptides generated from the same ancestor gene.