Expression of the ghrelin and neurotensin systems is altered in the temporal lobe of Alzheimer's disease patients

Systems-Level G Protein-Coupled Receptor Therapy Across a Neurodegenerative Continuum by the GLP-1 Receptor System

With our increasing appreciation of the true complexity of diseases and pathophysiologies, it is clear that this knowledge needs to inform the future development of pharmacotherapeutics. For many disorders, the disease mechanism itself is a complex process spanning multiple signaling networks, tissues, and organ systems. Identifying the precise nature and locations of the pathophysiology is crucial for the creation of systemically effective drugs. Diseases once considered constrained to a limited range of organ systems, e.g., central neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington' disease (HD), the role of multiple central and peripheral organ systems in the etiology of such diseases is now widely accepted. With this knowledge, it is increasingly clear that these seemingly distinct neurodegenerative disorders (AD, PD, and HD) possess multiple pathophysiological similarities thereby demonstrating an inter-related continuum of disease-related molecular alterations. With this systems-level appreciation of neurodegenerative diseases, it is now imperative to consider that pharmacotherapeutics should be developed specifically to address the systemic imbalances that create the disorders. Identification of potential systems-level signaling axes may facilitate the generation of therapeutic agents with synergistic remedial activity across multiple tissues, organ systems, and even diseases. Here, we discuss the potentially therapeutic systems-level interaction of the glucagon-like peptide 1 (GLP-1) ligand-receptor axis with multiple aspects of the AD, PD, and HD neurodegenerative continuum.

Estrogen: a master regulator of bioenergetic systems in the brain and body

Estrogen is a fundamental regulator of the metabolic system of the female brain and body. Within the brain, estrogen regulates glucose transport, aerobic glycolysis, and mitochondrial function to generate ATP. In the body, estrogen protects against adiposity, insulin resistance, and type II diabetes, and regulates energy intake and expenditure. During menopause, decline in circulating estrogen is coincident with decline in brain bioenergetics and shift towards a metabolically compromised phenotype. Compensatory bioenergetic adaptations, or lack thereof, to estrogen loss could determine risk of late-onset Alzheimer's disease. Estrogen coordinates brain and body metabolism, such that peripheral metabolic state can indicate bioenergetic status of the brain. By generating biomarker profiles that encompass peripheral metabolic changes occurring with menopause, individual risk profiles for decreased brain bioenergetics and cognitive decline can be created. Biomarker profiles could identify women at risk while also serving as indicators of efficacy of hormone therapy or other preventative interventions.

Neurodegenerative disease and obesity: what is the role of weight loss and bariatric interventions?

Neurodegenerative diseases are amongst the leading causes of worldwide disability, morbidity and decreased quality of life. They are increasingly associated with the concomitant worldwide epidemic of obesity. Although the prevalence of both AD and PD continue to rise, the available treatment strategies to combat these conditions remain ineffective against an increase in global neurodegenerative risk factors. There is now epidemiological and mechanistic evidence associating obesity and its related disorders of impaired glucose homeostasis, type 2 diabetes mellitus and metabolic syndrome with both AD and PD. Here we describe the clinical and molecular relationship between obesity and neurodegenerative disease. Secondly we outline the protective role of weight loss, metabolic and caloric modifying interventions in the context of AD and PD. We conclude that the application of caloric restriction through dietary changes, bariatric (metabolic) surgery and gut hormone therapy may offer novel therapeutic strategies against neurodegenerative disorders. Investigating the protective mechanisms of weight loss, metabolic and caloric modifying interventions can increase our understanding of these major public health diseases and their management.

Cloning of a novel insulin-regulated ghrelin transcript in prostate cancer

Ghrelin is a multifunctional hormone, with roles in stimulating appetite and regulating energy balance, insulin secretion and glucose homoeostasis. The ghrelin gene locus (GHRL) is highly complex and gives rise to a range of novel transcripts derived from alternative first exons and internally spliced exons. The wild-type transcript encodes a 117 amino acid preprohormone that is processed to yield the 28 amino acid peptide ghrelin. Here, we identified insulin-responsive transcription corresponding to cryptic exons in intron 2 of the human ghrelin gene. A transcript, termed in2c-ghrelin (intron 2-cryptic), was cloned from the testis and the LNCaP prostate cancer cell line. This transcript may encode an 83 amino acid preproghrelin isoform that codes for ghrelin, but not obestatin. It is expressed in a limited number of normal tissues and in tumours of the prostate, testis, breast and ovary. Finally, we confirmed that in2c-ghrelin transcript expression, as well as the recently described in1-ghrelin transcript, is significantly upregulated by insulin in cultured prostate cancer cells. Metabolic syndrome and hyperinsulinaemia have been associated with prostate cancer risk and progression. This may be particularly significant after androgen deprivation therapy for prostate cancer, which induces hyperinsulinaemia, and this could contribute to castrate-resistant prostate cancer growth. We have previously demonstrated that ghrelin stimulates prostate cancer cell line proliferation in vitro. This study is the first description of insulin regulation of a ghrelin transcript in cancer and should provide further impetus for studies into the expression, regulation and function of ghrelin gene products.

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.

The anti-apoptotic role of neurotensin

The neuropeptide, neurotensin, exerts numerous biological functions, including an efficient anti-apoptotic role, both in the central nervous system and in the periphery. This review summarizes studies that clearly evidenced the protective effect of neurotensin through its three known receptors. The pivotal involvement of the neurotensin receptor-3, also called sortilin, in the molecular mechanisms of the anti-apoptotic action of neurotensin has been analyzed in neuronal cell death, in cancer cell growth and in pancreatic beta cell protection. The relationships between the anti-apoptotic role of neurotensin and important physiological and pathological contexts are discussed in this review.

Semagacestat, a γ-secretase inhibitor, activates the growth hormone secretagogue (GHS-R1a) receptor

OBJECTIVES

Semagacestat, is a γ-secretase inhibitor, which belongs to a class of drugs that are being developed as therapeutic agents for Alzheimer's disease (AD). This study aims to evaluate another potential effect of semagacestat, namely its ability to stimulate the growth hormone secretagogue receptor (GHS-R1a), which may also contribute to its therapeutic efficacy.

METHODS

The GHS-R1a-activating potential of semagacestat and its synthetic precursor was assessed in an in vitro calcium mobilization assay in cells expressing the GHS-R1a receptor and compared with that of the endogenous peptide GHS-R1a agonist, acyl-ghrelin, as well as the non-peptidyl synthetic GHS-R1a agonist, MK0677. In addition, semagacestat-mediated cellular trafficking of the GHS-R1a receptor, expressed as an enhanced green fluorescent protein tagged fusion protein, was analysed.

KEY FINDINGS

Semagacestat and its precursor were shown to activate the GHS-R1a receptor, as demonstrated by an increased GHS-R1a-mediated intracellular calcium influx. Moreover, a synergistic GHS-R1a receptor activation was shown following a combined exposure to ghrelin and semagacestat. In addition, GHS-R1a receptor internalization was observed upon exposure to semagacestat and its precursor.

CONCLUSION

These data suggest a novel molecular mechanism of action for semagacestat via modest GHS-R1a receptor activation. Studies focusing on the relative functional consequence of such effects in vivo are now warranted.

Glucagon-like peptide-2 directly regulates hypothalamic neurons expressing neuropeptides linked to appetite control in vivo and in vitro

Glucagon-like peptide-2 (GLP-2), a proglucagon-derived peptide, has been postulated to affect appetite at the level of the hypothalamus. To gain better insight into this process, a degradation-resistant GLP-2 analog, human (Gly(2))GLP-2(1-33) [h(Gly(2))GLP-2] was intracerebroventricularly injected into mice to examine its action on food and water intake and also activation of hypothalamic anorexigenic α-melanocyte-stimulating hormone/proopiomelanocortin, neurotensin, and orexigenic neuropeptide Y, and ghrelin neurons. Central h(Gly(2))GLP-2 administration significantly suppressed food and water intake with acute weight loss at 2 h. Further, central h(Gly(2))GLP-2 robustly induced c-Fos activation in the hypothalamic arcuate, dorsomedial, ventromedial, paraventricular, and the lateral hypothalamic nuclei. We found differential colocalization of neuropeptides with c-Fos in specific regions of the hypothalamus. To assess whether hypothalamic neuropeptides are directly regulated by GLP-2 in vitro, we used an adult-derived clonal, immortalized hypothalamic cell line, mHypoA-2/30, that endogenously expresses functional GLP-2 receptors (GLP-2R) and two of the feeding-related neuropeptides linked to GLP-2R activation in vivo: neurotensin and ghrelin. Treatment with h(Gly(2))GLP-2 stimulated c-Fos expression and phosphorylation of cAMP response element-binding protein/activating transcription factor-1. In addition, treatment with h(Gly(2))GLP-2 significantly increased neurotensin and ghrelin mRNA transcript levels by 50 and 95%, respectively, at 24 h after treatment in protein kinase A-dependent manner. Taken together, these findings implicate the protein kinase A pathway as the means by which GLP-2 can up-regulate hypothalamic neuropeptide mRNA levels and provide evidence for a link between central GLP-2R activation and specific hypothalamic neuropeptides involved in appetite regulation.

Ghrelin and leptin secretion in patients with moderate Alzheimer's disease

BACKGROUND

Weight loss is a characteristic finding of patients with Alzheimer's disease (AD). It seems that precedes cognitive impairment by some years, but the underlying causes are not fully understood. Ghrelin and leptin are involved in energy homeostasis, and may be implicated in weight losing observed in these patients.

OBJECTIVE

To examine the potential relationship between ghrelin and leptin levels and weight loss in patients with AD.

DESIGN

The study included 27 patients (10 men and 17 women) with AD of moderate severity, and 23 controls (10 males and 13 females), matched for age and BMI. Body fat and lean mass content were assessed using a portable apparatus. Cognitive function was assessed with the Mini-Mental State Examination. Basal serum samples for the measurement of leptin, ghrelin, insulin and glucose were obtained, and serum ghrelin, insulin and glucose were measured after a 75-gr glucose load in both groups.

RESULTS

Patients with Alzheimer Disease (AD) have lower lean mass content compared to controls. Basal ghrelin and leptin is similar in patients with AD and controls. The area-under-the-curve for ghrelin (AUC) is lower in male patients with AD compared to control males, while no difference was observed between females AD and controls.

CONCLUSION

Male patients with AD, in contrast with female patients, fail to maintain a normal energy homeostasis even in the early stages of the disease, as shown by the decreased lean mass content in males AD compared to controls. Disruption of the normal compensatory modulation of ghrelin secretion might contribute to the metabolic changes observed in male patients with AD.

Genetic association between ghrelin polymorphisms and Alzheimer's disease in a Japanese population

BACKGROUND/AIMS

Ghrelin has been reported to enter the hippocampus and to bind to the neurons of the hippocampal formation. This peptide also affects neuronal glucose uptake and decreases tau hyperphosphorylation. There is increasing evidence suggesting an association between ghrelin and Alzheimer's disease (AD) pathology. The aim of this study was to investigate whether single nucleotide polymorphisms (SNPs) of the ghrelin gene are associated with AD.

METHODS

The SNPs were genotyped using TaqMan technology and were analyzed using a case-control study design. Our case-control dataset consisted of 182 AD patients and 143 age-matched controls.

RESULTS

Hardy-Weinberg equilibrium and linkage disequilibrium analyses suggest that the region in and around the gene is highly polymorphic. One SNP, rs4684677 (Leu90Gln), showed a marginal association with age of AD onset. We did not detect any association between the other SNPs of the ghrelin gene and AD.

CONCLUSION

There have been few genetic studies on the relationship between circulating ghrelin and functional SNPs. Further multifactorial studies are needed to clarify the relationship between ghrelin and AD.

Metabolic dysfunction in Alzheimer's disease and related neurodegenerative disorders

Alzheimer's disease and other related neurodegenerative diseases are highly debilitating disorders that affect millions of people worldwide. Efforts towards developing effective treatments for these disorders have shown limited efficacy at best, with no true cure to this day being present. Recent work, both clinical and experimental, indicates that many neurodegenerative disorders often display a coexisting metabolic dysfunction which may exacerbate neurological symptoms. It stands to reason therefore that metabolic pathways may themselves contain promising therapeutic targets for major neurodegenerative diseases. In this review, we provide an overview of some of the most recent evidence for metabolic dysregulation in Alzheimer's disease, Huntington's disease, and Parkinson's disease, and discuss several potential mechanisms that may underlie the potential relationships between metabolic dysfunction and etiology of nervous system degeneration. We also highlight some prominent signaling pathways involved in the link between peripheral metabolism and the central nervous system that are potential targets for future therapies, and we will review some of the clinical progress in this field. It is likely that in the near future, therapeutics with combinatorial neuroprotective and 'eumetabolic' activities may possess superior efficacies compared to less pluripotent remedies.

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.

Epigenetic control of somatostatin and cortistatin expression by β amyloid peptide

β Amyloid, present in senile plaques, has been related largely to neuronal loss in the brain of patients with Alzheimer's disease. However, how neurons respond to β amyloid insults is still poorly understood. Here we show that β amyloid increases somatostatin and cortistatin gene expression mainly through an increase in histone 3 lysine 4 methylation (H3K4me3), a modification associated with transcriptional activation. Somatostatin and cortistatin partially decreased β amyloid toxicity in primary cortical neurons in culture. Thus we suggest that neurons respond to β amyloid insults by releasing somatostatin and cortistatin, which will act as a protective agent against β amyloid toxicity. Our results suggest a relevant function for both neuropeptides against β amyloid toxicity, providing new insights into Alzheimer's disease.

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.

Investigation of the presence of ghrelin in the central nervous system of the rat and mouse

Ghrelin and ghrelin receptor agonist have effects on central neurons in many locations, including the hypothalamus, caudal brain stem, and spinal cord. However, descriptions of the distributions of ghrelin-like immunoreactivity in the CNS in published work are inconsistent. We have used three well-characterized anti-ghrelin antibodies, an antibody to the unacylated form of ghrelin, and a ghrelin peptide assay in rats, mice, ghrelin knockout mice, and ghrelin receptor reporter mice to re-evaluate ghrelin presence in the rodent CNS. The stomach served as a positive control. All antibodies were effective in revealing gastric endocrine cells. However, no specific staining could be found in the brain or spinal cord. Concentrations of antibody 10 to 30 times those effective in the stomach bound to nerve cells in rat and mouse brain, but this binding was not reduced by absorbing concentrations of ghrelin peptide, or by use of ghrelin gene knockout mice. Concentrations of ghrelin-like peptide, detected by enzyme-linked immunosorbent assay in extracts of hypothalamus, were 1% of gastric concentrations. Ghrelin receptor-expressing neurons had no adjacent ghrelin immunoreactive terminals. It is concluded that there are insignificant amounts of authentic ghrelin in neurons in the mouse or rat CNS and that ghrelin receptor-expressing neurons do not receive synaptic inputs from ghrelin-immunoreactive nerve terminals in these species.

Ghrelin axis genes, peptides and receptors: recent findings and future challenges

The ghrelin axis consists of the gene products of the ghrelin gene (GHRL), and their receptors, including the classical ghrelin receptor GHSR. While it is well-known that the ghrelin gene encodes the 28 amino acid ghrelin peptide hormone, it is now also clear that the locus encodes a range of other bioactive molecules, including novel peptides and non-coding RNAs. For many of these molecules, the physiological functions and cognate receptor(s) remain to be determined. Emerging research techniques, including proteogenomics, are likely to reveal further ghrelin axis-derived molecules. Studies of the role of ghrelin axis genes, peptides and receptors, therefore, promises to be a fruitful area of basic and clinical research in years to come.