Ghrelin-attenuated cognitive dysfunction in streptozotocin-induced diabetic rats

Potential role of ghrelin in neuroprotection and cognitive function: implications for diabetic cognitive impairment

Ghrelin is a class of brain and intestinal peptides. It regulates food intake and body glucose levels and maintains cellular homeostasis. In recent years, research has revealed that ghrelin may positively impact learning and memory. Despite ghrelin's multiple functions in the central nervous system, its use as a therapeutic agent for neurologic dysfunction remains unclear. Diabetic cognitive impairment (DCI) is a severe neurological complication of diabetes mellitus. Its incidence is increasing as a comorbidity in endocrinology and neurology. Additionally, it is a risk factor for Alzheimer' s disease (AD). Ghrelin levels are altered in patients with diabetes mellitus combined with cognitive impairment. Furthermore, modulation of ghrelin levels improved cognitive function in rats with DCI. These findings suggest the potential therapeutic importance of ghrelin in the pathogenesis of DCI. This article presents a comprehensive review of the pathogenesis of DCI and its potential modulation by ghrelin and its mimics. Furthermore, this study elucidates the therapeutic prospects of ghrelin and its mimics for DCI, aiming to identify novel therapeutic targets and research avenues for the prevention and management of DCI in the future.

Ghrelin attenuates inflammation in diabetic lung disease by TLR4 pathway in vivo and in vitro

INTRODUCTION

Diabetic lung disease is already known as one of the diabetes complications, but report on its therapeutic strategy is rare. The present study aimed to add novel therapeutic strategy for diabetic lung disease, to reveal the protective effect of ghrelin on diabetic lung disease both in vivo and in vitro, and to discuss its probable molecular mechanism.

RESEARCH DESIGN AND METHODS

Diabetic mice and 16HBE cells were our research objects. We surveyed the effect of ghrelin on streptozotocin-induced lung tissue morphology changes by H&E staining. Furthermore, the changes of proinflammatory cytokines (interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α)) were detected by ELISA. To expound the molecular mechanism, we detected critical proteins of TLR4 pathway and observed their changes by immunohistochemistry (IHC), real-time PCR and western blot analysis in vivo and in vitro, respectively.

RESULTS

The results of H&E staining showed that pathological alterations of the lung induced by hyperglycemia were ameliorated by ghrelin. The results of ELISA demonstrated that the elevated levels of IL-1β and TNF-α induced by hyperglycemia turned to decrease in the lung after ghrelin treatment. In the results of IHC, real-time PCR and western blot analysis, we found that the TLR4 pathway was elevated by hyperglycemia or high glucose and is remarkably inhibited by the treatment of ghrelin both in vivo and in vitro.

CONCLUSIONS

Ghrelin could inhibit inflammation of diabetic lung disease by regulating the TLR4 pathway. This study might affect research on diabetic lung disease, and the therapeutic potential of ghrelin for diabetic lung disease is worth considering.

DOCK4 regulates ghrelin production in gastric X/A-like cells

PURPOSE

Ghrelin, a gastric hormone, provides a hunger signal to the central nervous system to stimulate food intake. Ghrelin also modulates neuroinflammatory and apoptotic processes. Dedicator of cytokinesis 4 (DOCK4), a guanine nucleotide exchange factor (GEF), is involved in the regulation of neuronal polarization and axon regeneration. However, the effect of DOCK4 on ghrelin production has not been explored.

METHODS

The expression of DOCK4 in human and mouse stomach was examined by immunohistochemical staining. The synthesis and secretion of ghrelin in Dock4 null mice were evaluated by real-time quantitative PCR, Western blot and ELISA. The effects of DOCK4 on ghrelin production in mHypoE-42 cells were measured by real-time quantitative PCR and Western blot.

RESULTS

We showed that DOCK4 was expressed in both human and mouse gastric ghrelin cells. The mRNA and protein levels of gastric ghrelin, as well as ghrelin secretion, were remarkably diminished in Dock4 null mice. Furthermore, we showed that overexpression of Dock4 significantly stimulated ghrelin expression, while siRNA knockdown of endogenous Dock4 resulted in a marked decrease of ghrelin in mHypoE-N42 cells.

CONCLUSIONS

Our results identify DOCK4 as a critical regulator for ghrelin production in gastric X/A-like cells.

Ghrelin Acylation-A Post-Translational Tuning Mechanism Regulating Adult Hippocampal Neurogenesis

Adult hippocampal neurogenesis—the generation of new functional neurones in the adult brain—is impaired in aging and many neurodegenerative disorders. We recently showed that the acylated version of the gut hormone ghrelin (acyl-ghrelin) stimulates adult hippocampal neurogenesis while the unacylated form of ghrelin inhibits it, thus demonstrating a previously unknown function of unacyl-ghrelin in modulating hippocampal plasticity. Analysis of plasma samples from Parkinson’s disease patients with dementia demonstrated a reduced acyl-ghrelin:unacyl-ghrelin ratio compared to both healthy controls and cognitively intact Parkinson’s disease patients. These data, from mouse and human studies, suggest that restoring acyl-ghrelin signalling may promote the activation of pathways to support memory function. In this short review, we discuss the evidence for ghrelin’s role in regulating adult hippocampal neurogenesis and the enzymes involved in ghrelin acylation and de-acylation as targets to treat mood-related disorders and dementia.

Ghrelin mediated hippocampal neurogenesis

The stomach hormone, ghrelin, which is released during food restriction, provides a link between circulating energy state and adaptive brain function. The maintenance of such homeostatic systems is essential for an organism to survive and thrive, and accumulating evidence points to ghrelin being a key regulator of adult hippocampal neurogenesis and memory function. Aberrant neurogenesis is linked to cognitive decline in aging and neurodegeneration. Therefore, identifying endogenous metabolic factors that regulate new adult-born neuron formation is an important objective in understanding the link between nutritional status and CNS function. Here, we review current developments in our understanding of ghrelin's role in regulating neurogenesis and memory function.

Discovery of a dual-action small molecule that improves neuropathological features of Alzheimer's disease mice

Since Alzheimer’s disease (AD) is a multifaceted neurodegenerative disease, multitargeted therapeutic approaches are likely required for effective AD treatment. The importance of acid sphingomyelinase (ASM) activation in the various neuropathological features of AD is well-known. Therefore, in this study, we focused on identifying an efficient, direct inhibitor of ASM activity. We found that KARI 201 was a highly selective ASM activity inhibitor without any off-target effects. Through RNA-sequencing analysis in brains of AD mice, we also unexpectedly uncovered the role of KARI 201 as a ghrelin receptor agonist. This dual role of KARI 201 in neurons led to improvement of Aβ accumulation, neuroinflammation, synapse loss, hippocampal neurogenesis, and memory dysfunction in AD mice.

Alzheimer’s disease (AD) is characterized by complex, multifactorial neuropathology, suggesting that small molecules targeting multiple neuropathological factors are likely required to successfully impact clinical progression. Acid sphingomyelinase (ASM) activation has been recognized as an important contributor to these neuropathological features in AD, leading to the concept of using ASM inhibitors for the treatment of this disorder. Here we report the identification of KARI 201, a direct ASM inhibitor evaluated for AD treatment. KARI 201 exhibits highly selective inhibition effects on ASM, with excellent pharmacokinetic properties, especially with regard to brain distribution. Unexpectedly, we found another role of KARI 201 as a ghrelin receptor agonist, which also has therapeutic potential for AD treatment. This dual role of KARI 201 in neurons efficiently rescued neuropathological features in AD mice, including amyloid beta deposition, autophagy dysfunction, neuroinflammation, synaptic loss, and decreased hippocampal neurogenesis and synaptic plasticity, leading to an improvement in memory function. Our data highlight the possibility of potential clinical application of KARI 201 as an innovative and multifaceted drug for AD treatment.

Ghrelin treatment leads to dendritic spine remodeling in hippocampal neurons and increases the expression of specific BDNF-mRNA species

Ghrelin (Gr) is an orexigenic peptide that acts via its specific receptor, GHSR-1a distributed throughout the brain, being mainly enriched in pituitary, cortex and hippocampus (Hp) modulating a variety of brain functions. Behavioral, electrophysiological and biochemical evidence indicated that Gr modulates the excitability and the synaptic plasticity in Hp. The present experiments were designed in order to extend the knowledge about the Gr effect upon structural synaptic plasticity since morphological and quantitative changes in spine density after Gr administration were analyzed "in vitro" and "in vivo". The results show that Gr administered to hippocampal cultures or stereotactically injected in vivo to Thy-1 mice increases the density of dendritic spines (DS) being the mushroom type highly increased in secondary and tertiary extensions. Spines classified as thin type were increased particularly in primary extensions. Furthermore, we show that Gr enhances selectively the expression of BDNF-mRNA species.

MicroRNA-29b Modulates β-Secretase Activity in SH-SY5Y Cell Line and Diabetic Mouse Brain

Hyperglycemia is one of the major risk factors responsible for memory impairment in diabetes which may lead to Alzheimer's disease (AD) at a later stage. MicroRNAs are a class of non-coding RNAs that are found to play a role in diabetes. Downregulation of microRNA-29b in diabetes is well reported. Moreover, microRNA-29b is also reported to target the 3' UTR of β-secretase (BACE-1) enzyme which is involved in the formation of amyloid-beta (Aβ) in AD via cleavage of amyloid precursor protein (APP). Therefore, the present study was designed to elucidate whether microRNA-29b could be a link between diabetes and dementia. In the in vitro and in vivo diabetic model, we found downregulation of microRNA-29b due to hyperglycemia. After human microRNA-29b treatment, there was a significant improvement in the short-term and spatial memory in diabetic mice. Also, the human microRNA-29b treatment decreased oxidative stress and BACE-1 activity in diabetes. The present findings revealed that the downregulation of microRNA-29b in diabetes could be associated with memory impairment and increased BACE-1 activity. These results would give a future direction to study the role played by microRNAs in diabetes-associated memory impairment and hence aid in the development of therapeutics to treat the same.

The Potential Roles of Ghrelin in Metabolic Syndrome and Secondary Symptoms of Alzheimer's Disease

Although the major causative factors of Alzheimer's disease (AD) are the accumulation of amyloid β and hyperphosphorylated tau, AD can also be caused by metabolic dysfunction. The major clinical symptom of AD is cognitive dysfunction. However, AD is also accompanied by various secondary symptoms such as depression, sleep-wake disturbances, and abnormal eating behaviors. Interestingly, the orexigenic hormone ghrelin has been suggested to have beneficial effects on AD-related metabolic syndrome and secondary symptoms. Ghrelin improves lipid distribution and alters insulin sensitivity, effects that are hypothesized to delay the progression of AD. Furthermore, ghrelin can relieve depression by enhancing the secretion of hormones such as serotonin, noradrenaline, and orexin. Moreover, ghrelin can upregulate the expression of neurotrophic factors such as brain-derived neurotrophic factor and modulate the release of proinflammatory cytokines such as tumor necrosis factor α and interleukin 1β. Ghrelin alleviates sleep-wake disturbances by increasing the levels of melatonin, melanin-concentrating hormone. Ghrelin reduces the risk of abnormal eating behaviors by increasing neuropeptide Y and γ-aminobutyric acid. In addition, ghrelin increases food intake by inhibiting fatty acid biosynthesis. However, despite the numerous studies on the role of ghrelin in the AD-related pathology and metabolic disorders, there are only a few studies that investigate the effects of ghrelin on secondary symptoms associated with AD. In this mini review, our purpose is to provide the insights of future study by organizing the previous studies for the role of ghrelin in AD-related pathology and metabolic disorders.

Chronic ghrelin administration suppresses IKK/NF-κB/BACE1 mediated Aβ production in primary neurons and improves cognitive function via upregulation of PP1 in STZ-diabetic rats

Diabetic rats display cognition impairments accompanied by activation of NF-κB signalling and increased Aβ expression. Ghrelin has been suggested to improve cognition in diabetic rats. In this study, we investigated the role of ghrelin on cognition and NF-κB mediated Aβ production in diabetic rats. A diabetic rat model was established with streptozotocin (STZ) injection, and diabetic rats were intracerebroventricularly administered with ghrelin or (D-lys3)-GHRP-6 (DG). Our results showed that diabetic rats had cognition impairment in the Morris water maze test, accompanied by the higher expression of Aβ in the hippocampus. Western blot analysis showed that diabetic rats exhibited significantly decreased levels of GHSR-1a and protein phosphatase 1 (PP1) in the hippocampus and increased activation of the IKK/NF-κB/BACE1 pathway. Chronic ghrelin administration upregulated hippocampal PP1 expression, suppressed IKK/NF-κB/BACE1 mediated Aβ production, and improved cognition in STZ-induced diabetic rats. These effects were reversed by DG. Then, primary rat hippocampal neurons were isolated and treated with high glucose, followed by Ghrelin and DG, PP1 or IKK. Similar to the in vivo results, high glucose suppressed the expression levels of GHSR-1a and PP1, activated the IKK/NF-κB/BACE1 pathway, increased Aβ production. Ghrelin suppressed IKK/NF-κB/BACE1 induced Aβ production. This improvement was reversed by DG and a PP1 antagonist and was enhanced by the IKK antagonist. Our findings indicated that chronic ghrelin administration can suppress IKK/NF-κB/BACE1 mediated Aβ production in primary neurons with high glucose treatment and improve the cognition via PP1 upregulation in diabetic rats.

Ghrelin-Mediated Hippocampal Neurogenesis: Implications for Health and Disease

There is a close relationship between cognition and nutritional status, however, the mechanisms underlying this relationship require elucidation. The stomach hormone, ghrelin, which is released during food restriction, provides a link between circulating energy state and adaptive brain function. The maintenance of such homeostatic systems is essential for an organism to thrive and survive, and accumulating evidence points to ghrelin being key in promoting adult hippocampal neurogenesis and memory. Aberrant neurogenesis is linked to cognitive decline in ageing and neurodegeneration. Therefore, identifying endogenous metabolic factors that regulate new adult-born neurone formation is an important objective in understanding the link between nutritional status and central nervous system (CNS) function. Here, we review current developments in our understanding of ghrelin's role in regulating neurogenesis and memory function.

Circulating messenger for neuroprotection induced by molecular hydrogen

Molecular hydrogen (H2) showed protection against various kinds of oxidative-stress-related diseases. First, it was reported that the mechanism of therapeutic effects of H2was antioxidative effect due to inhibition of the most cytotoxic reactive oxygen species, hydroxy radical (•OH). However, after chronic administration of H2in drinking water, oxidative-stress-induced nerve injury is significantly attenuated even in the absence of H2. It suggests indirect signaling of H2and gastrointestinal tract is involved. Indirect effects of H2could be tested by giving H2water only before nerve injury, as preconditioning. For example, preconditioning of H2for certain a period (∼7 days) in Parkinson’s disease model mice shows significant neuroprotection. As the mechanism of indirect effect, H2in drinking water induces ghrelin production and release from the stomach via β1-adrenergic receptor stimulation. Released ghrelin circulates in the body, being transported across the blood–brain barrier, activates its receptor, growth-hormone secretagogue receptor. H2-induced upregulation of ghrelin mRNA is also shown in ghrelin-producing cell line, SG-1. These observations help with understanding the chronic effects of H2and raise intriguing preventive and therapeutic options using H2.

Salidroside prevents diabetes‑induced cognitive impairment via regulating the Rho pathway

In previous years, it has been found that Rhodiola has a wide range of pharmacological effects in diseases of the cardiovascular system, as it can remove superoxide anions and hydroxyl radicals in chemical reactions. Behavioral assessment was used to measure cognitive impairment. Inflammation, oxidative stress and caspase‑3 activity were measured using commercial kits. Western blot analysis was used to measure Rho/Rho‑associated kinase (ROCK)/sirtuin 1 (SIRT1)/nuclear factor (NF)‑κB protein expression. The objective of the present study was to investigate the protective effect of salidroside on diabetes and diabetes‑induced cognitive impairment. The results of the study demonstrated that salidroside prevented cognitive impairment, decreased serum blood glucose levels and increased body weight, reduced fasting blood glucose levels and blood lipid levels, and inhibited oxidative stress, inflammation and nerve cell apoptosis in the diabetic rat model. Salidroside suppressed ROCK/ SIRT1 NF‑κB pathway and protein expression in the diabetic rats. These data showed that salidroside prevented diabetes‑induced cognitive impairment by regulating the Rho/ROCK/SIRT1/NF‑κB pathway.

Gastrointestinal hormones in regulation of memory

The connection between the gastrointestinal hormones and the brain has been established many years ago. This relation is termed the gut-brain axis (GBA). The GBA is a bidirectional communication which not only regulates gastrointestinal homeostasis but is also linked with higher emotional and cognitive functions. Hypothalamus plays a critical role in the regulation of energy metabolism, nutrient partitioning and control of feeding behaviors. Various gut hormones are released inside the gastrointestinal tract on food intake. These hormones act peripherally and influence the different responses of the tissues to the food intake, but do also have effects on the brain. The hypothalamus, in turn, integrates visceral function with limbic system structures such as hippocampus, amygdala, and cerebral cortex. The hippocampus has been known for its involvement in the cognitive function and the modulation of synaptic plasticity. This review aims to establish the role of various gut hormones in learning and memory, through the interaction of various receptors in the hippocampus. Understanding their role in memory can also aid in finding novel therapeutic strategies for the treatment of the neurological disorders associated with memory dysfunctions.

Association of plasma ghrelin levels and ghrelin rs4684677 polymorphism with mild cognitive impairment in type 2 diabetic patients

BACKGROUND AND AIMS

People with insulin resistance and type 2 diabetes mellitus (T2DM) are at increased risks of cognitive impairment. We aimed to investigate the association of plasma ghrelin levels and ghrelin rs4684677 polymorphism with mild cognitive impairment (MCI) in T2DM patients.

RESULTS

In addition to elevated glycosylated hemoglobin (HbA1c), fasting blood glucose (FBG) and homeostasis model assessment of insulin resistance (HOMA-IR), T2DM patients with MCI had decreased plasma ghrelin levels compared with their healthy-cognition subjects (all p < 0.05). Further logistic regression analysis showed that ghrelin level was one of independent factors for MCI in T2DM patients (p < 0.05). Moreover, partial correlation analysis demonstrated that ghrelin levels were positively associated with the scores of Montreal Cognitive Assessment (r = 0.196, p = 0.041) and Auditory Verbal Learning Test-delayed recall (r = 0.197, p = 0.040) after adjustment for HbA1c, FBG and HOMA-IR, wherein the latter represented episodic memory functions. No significant differences were found for the distributions of genotype and allele of ghrelin rs4684677 polymorphism between MCI and control group.

MATERIALS AND METHODS

A total of 218 T2DM patients, with 112 patients who satisfied the MCI diagnostic criteria and 106 who exhibited healthy cognition, were enrolled in this study. Demographic characteristics, clinical variables and cognitive performances were extensively assessed. Plasma ghrelin levels and ghrelin rs4684677 polymorphism were also determined.

CONCLUSIONS

Our results suggest that decreased ghrelin levels are associated with MCI, especially with episodic memory dysfunction in T2DM populations.

Ghrelin ameliorates nerve growth factor Dysmetabolism and inflammation in STZ-induced diabetic rats

Diabetic encephalopathy is characterized by cognitive impairment and neuroinflammation, deficient neurotrophic support, and neuronal and synaptic loss. Ghrelin, a 28 amino acid peptide, is associated with neuromodulation and cognitive improvement, which has been considered as a potential protective agent for several neurodegenerative diseases. Here we sought to investigate the role of ghrelin in preventing diabetic-related neuropathology. We found that ghrelin attenuated astrocytic activation and reduced levels of interleukin-6 and tumor necrosis factor-α in streptozotocin-induced diabetic rats. In addition, ghrelin inhibited p38 mitogen-associated protein kinase activation. The upregulation of nerve growth factor (NGF) precursor and matrix metalloproteinase (MMP)-9 and downregulation of mature NGF and MMP-7 in the diabetic brain were reversed by ghrelin. Treatment with ghrelin elevated synaptophysin expression and synaptic density in diabetic rats. Taken together, our results demonstrate that ghrelin ameliorates diabetes-related neurodegeneration by preventing NGF dysmetabolism and synaptic degeneration through regulating MMP levels as well as inhibiting neuroinflammation.

Angiotensin-(1-7) administration attenuates Alzheimer's disease-like neuropathology in rats with streptozotocin-induced diabetes via Mas receptor activation

Diabetes mellitus (DM) is associated with cognitive deficits and an increased risk of Alzheimer's disease (AD). Recently, a newly identified heptapeptide of the renin-angiotensin system (RAS), angiotensin-(1-7) [Ang-(1-7)], was found to protect against brain damage. This study investigated the effects of Ang-(1-7) on diabetes-induced cognitive deficits. Sprague-Dawley rats were randomly divided into four groups. Diabetes was induced via single i.p. streptozotocin (STZ) injections. Ten weeks after diabetes induction, rats in each group received an intracerebral-ventricular (ICV) infusion of either vehicle, Ang-(1-7) alone, or Ang-(1-7)+A779 daily for two weeks. At the end of the study, Morris water maze (MWM) tests were performed to test cognitive functions before the rats were euthanized. Ang-(1-7) treatment significantly reduced escape latencies in diabetic rats in acquisition trials and markedly enhanced platform area crossing frequency and time spent in the target quadrant in probe trials (3.0±0.39 vs. 1.0±0.33, 39.39±1.11% vs. 25.62±3.07%, respectively, P<0.01). Ang-(1-7) treatment ameliorated damage to the ultrastructure of hippocampal synapses, reduced the expression of hippocampal phospho-tau at Ser396 (P<0.01), Ser404 (P<0.01) and Ser202/Thr205 (P<0.05), and decreased amyloid-β oligomer and both soluble and insoluble β-amyloid peptide 1-42 (Aβ 1-42) and Aβ 1-40 levels (P<0.01). These protective effects were significantly reversed by the co-administration of A779. These findings show that Ang-(1-7) is a promising therapeutic target for diabetes-induced cognitive impairment. The neuroprotective effects of Ang-(1-7) were mainly through Mas receptor (MasR) activation.

Ghrelin is a possible new predictor associated with executive function in patients with type 2 diabetes mellitus

AIMS/INTRODUCTION

The aim of the present research was to study the ghrelin level, executive function and their possible association in patients with type 2 diabetes mellitus.

MATERIALS AND METHODS

A total of 370 people were recruited between March 2015 and March 2016 in this study. Among them, 212 participants were patients with type 2 diabetes mellitus and 158 participants were included as the control group. Their blood sample was analyzed for the level of ghrelin and other clinical indexes. Cognitive function was measured by the Montreal Cognitive Assessment, and executive function was evaluated by the Wisconsin Card Sorting Test.

RESULTS

In the type 2 diabetes mellitus group, age, years of education, duration of diabetes, fasting blood glucose, glycated hemoglobin, hypertension and waist-to-hip ratio were correlated with total Montreal Cognitive Assessment scores. No association was found between ghrelin level and total Montreal Cognitive Assessment score in patients with type 2 diabetes mellitus. However, ghrelin was found to be a significant predictor for executive function impairment measured by the Wisconsin Card Sorting Test in patients with type 2 diabetes mellitus.

CONCLUSIONS

The level of serum ghrelin might be a biomarker of executive function and become a strong predictor of executive function impairment in patients with type 2 diabetes mellitus. Ghrelin might have a potential protective effect against cognitive function impairment in type 2 diabetes patients.

Autophagy-lysosome dysfunction is involved in Aβ deposition in STZ-induced diabetic rats

β-Amyloid (Aβ) deposition has a central role in the pathogenesis of Alzheimer disease (AD). Previous studies have indicated that as a risk factor for AD, diabetes mellitus (DM) could induce Aβ deposition in the brain, but the mechanism is not fully elucidated. Autophagy-lysosome is a cellular pathway involved in protein and organelle degradation. In the present study, we used streptozotocin (STZ)-induced diabetic rats to investigate whether autophagy-lysosome is related to Aβ_1-42 clearance in DM. We found that DM rats had a longer escape latency and less frequent entry into the target zone than that of the control group (p<0.05) in the Morris water maze test. Meanwhile, hippocampal neuron damage and apoptosis (p<0.05) were found in the DM rats. The Aβ_1-42 expression in the hippocampus significantly increased in the DM group compared with the control group (p<0.05). The markers of autophagy, beclin-1 and LC3 II, were increased (p<0.05), whereas LC3 I was decreased (p<0.05), and the ratio of LC3 II / I was increased as the time advanced (p<0.01). LAMP1 and LAMP2, which are the markers of lysosome function, were decreased in the hippocampus of DM rats (p<0.05). The Aβ_1-42 deposition was correlated with beclin-1, LC3 II, and LC3 I positively (p<0.05), but with LAMP1 and LAMP2 negatively (p<0.05). These findings indicate that DM activated autophagy, but lysosome function was impaired. Autophagy-lysosome dysfunction may be involved in the Aβ deposition in diabetic cognitive impairment.

Potential effects of current drug therapies on cognitive impairment in patients with type 2 diabetes

Type 2 diabetes mellitus is a complex metabolic disease that can cause serious damage to various organs. Among the best-known complications, an important role is played by cognitive impairment. Impairment of cognitive functioning has been reported both in type 1 and 2 diabetes mellitus. While this comorbidity has long been known, no major advances have been achieved in clinical research; it is clear that appropriate control of blood glucose levels represents the best current (although unsatisfactory) approach in the prevention of cognitive impairment. We have focused our attention on the possible effect on the brain of antidiabetic drugs, despite their effects on blood glucose levels, giving a brief rationale on the mechanisms (e.g. GLP-1, BDNF, ghrelin) that might be involved. Indeed, GLP-1 agonists are currently clinically studied in other neurodegenerative diseases (i.e. Parkinson's and Alzheimer's disease); furthermore, also other antidiabetic drugs have proven efficacy in preclinical studies. Overall, promising results are already available and finding new intervention strategies represents a current need in this field of research.

Short-term calorie restriction enhances adult hippocampal neurogenesis and remote fear memory in a Ghsr-dependent manner

The beneficial effects of calorie restriction (CR) have been described at both organismal and cellular levels in multiple organs. However, our understanding of the causal mediators of such hormesis is poorly understood, particularly in the context of higher brain function. Here, we show that the receptor for the orexigenic hormone acyl-ghrelin, the growth hormone secretagogue receptor (Ghsr), is enriched in the neurogenic niche of the hippocampal dentate gyrus (DG). Acute elevation of acyl-ghrelin levels by injection or by overnight CR, increased DG levels of the neurogenic transcription factor, Egr-1. Two weeks of CR increased the subsequent number of mature newborn neurons in the DG of adult wild-type but not Ghsr(-/-) mice. CR wild-type mice also showed improved remote contextual fear memory. Our findings suggest that Ghsr mediates the beneficial effects of CR on enhancing adult hippocampal neurogenesis and memory.

Ghrelin ameliorates impaired angiogenesis of ischemic myocardium through GHSR1a-mediated AMPK/eNOS signal pathway in diabetic rats

OBJECTIVES

Ghrelin, an endogenous ligand of the growth hormone secretagogue receptor (GHSR), has been found to stimulate angiogenesis in vivo and in vitro. However, the effect and the corresponding mechanisms of ghrelin on impaired myocardial angiogenesis in diabetic and myocardial infarction (MI) rat model are still unknown.

METHODS

In the present study, adult SD rats were randomly divided into 4 groups: control, DM, DM+ghrelin, DM+ghrelin+[D-Lys3]-GHRP-6 groups. DM was induced by streptozotocin (STZ) 60 mg/kg body weight. 12 weeks post STZ injection all groups were subjected to MI, which was induced by ligation left anterior descending artery (LAD). Ghrelin and [D-Lys3]-GHRP-6 were administered via intraperitoneal injection at the doses 200 μg/kg and 50mg/kg for 4 weeks, respectively. Left ventricular function, microvascular density (MVD), myocardial infarct size, the expression of hypoxia-inducible factor (HIF1α), vascular endothelial growth factor (VEGF), fetal liver kinase-1 (Flk-1) and fms-like tyrosine kinase-1 (Flt-1), AMPK and endothelial nitric oxide synthase (eNOS) phosphorylation were examined.

RESULTS

Compared with the DM group, left ventricular ejection fraction (LVEF), fractional shortening (FS), and MVD were increased, whereas myocardial infarct size decreased remarkably in DM+ghrelin group. For the mechanism study, we found that ghrelin promoted the HIF1α, VEGF, Flk-1 and Flt-1 expression, AMPK and eNOS phosphorylation in diabetic rats. However, the above biochemical events in ghrelin treated diabetic rats were completely inhibited by GHSR-1a blocker [D-Lys3]-GHRP-6.

CONCLUSIONS

These results suggest that administration of ghrelin ameliorates impaired angiogenesis in diabetic MI rats. And these beneficial effects derive from regulating GHSR1a-mediated AMPK/eNOS signal pathway by upregulating of HIF1α, VEGF and its receptors Flk-1, Flt-1 expressions.

Activation of Hippocampal CREB by Rolipram Partially Recovers Balance Between TNF-α and IL-10 Levels and Improves Cognitive Deficits in Diabetic Rats

Diabetes damages the central nervous system, inducing cognitive dysfunction and structural changes, known as diabetic encephalopathy (DE). Some research suggests that the pathogenesis of DE may involve an inflammatory imbalance in the nervous system, along with β-amyloid deposition, similar to Alzheimer's disease. Less data have been yet provided to prove that mechanism. The aim of this study was to evaluate the influence of diabetes on the expression of TNF-α, IL-10, and cAMP response element-binding protein (CREB)/phosphorylated CREB (pCREB). Moreover, we investigated whether rolipram can improve memory, suppress the inflammatory response, and improve balance of CREB/pCREB in the hippocampus of diabetic rats. We used a 4-week high-fat diet and a low dose of streptozocin (30 mg/kg) to induce diabetes with hyperinsulinemia and hyperglycemia. Cognitive impairment was induced over a period of 4 months, and rolipram treatment was concomitantly given. Cognitive impairment was evaluated with the Morris water maze test. We also assessed expression of the pro-inflammatory cytokine TNF-α and the anti-inflammatory cytokine IL-10. We found that memory in rats with long-term diabetes was impaired. Treatment with rolipram increased expression of CREB and pCREB, reduced the inflammatory reaction (decreased TNF-α levels and increased IL-10 levels), and prevented cognitive impairment in these diabetic animals. This present study suggests that rolipram improves cognitive function by activating the CREB signaling pathway and alleviating neuroinflammation in type 2 diabetic rats. Rolipram may have therapeutic potential in DE.

Increased Spontaneous Central Bleeding and Cognition Impairment in APP/PS1 Mice with Poorly Controlled Diabetes Mellitus

Alzheimer’s disease (AD) and vascular dementia (VaD) are the most common causes of dementia, and borderlines are blurred in many cases. Aging remains the main risk factor to suffer dementia; however, epidemiological studies reveal that diabetes may also predispose to suffer AD. In order to further study this relationship, we have induced hypoinsulinemic diabetes to APPswe/PS1dE9 (APP/PS1) mice, a classical model of AD. APP/PS1 mice received streptozotocin (STZ) ip at 18 weeks of age, when AD pathology is not yet established in this animal model. Cognition was evaluated at 26 weeks of age in the Morris water maze and the new object discrimination tests. We observed that STZ-induced episodic and working memory impairment was significantly worsened in APP/PS1 mice. Postmortem assessment included brain atrophy, amyloid-beta and tau pathology, spontaneous bleeding, and increased central inflammation. Interestingly, in APP/PS1-STZ diabetic mice, we detected a shift in Aβ soluble/insoluble levels, towards more toxic soluble species. Phospho-tau levels were also increased in APP/PS1-STZ mice, accompanied by an exacerbated inflammatory process, both in the close proximity to senile plaque (SP) and in SP-free areas. The presence of hemorrhages was significantly higher in APP/PS1-STZ mice, and although pericytes and endothelium were only partially affected, it remains possible that blood-brain barrier alterations underlie observed pathological features. Our data support the implication of the diabetic process in AD and VaD, and it is feasible that improving metabolic control could delay observed central pathology.

Ghrelin in psychiatric disorders - A review

Ghrelin is a 28-amino-acid peptide hormone, first described in 1999 and broadly expressed in the organism. As the only known orexigenic hormone secreted in the periphery, it increases hunger and appetite, promoting food intake. Ghrelin has also been shown to be involved in various physiological processes being regulated in the central nervous system such as sleep, mood, memory and reward. Accordingly, it has been implicated in a series of psychiatric disorders, making it subject of increasing investigation, with knowledge rapidly accumulating. This review aims at providing a concise yet comprehensive overview of the role of ghrelin in psychiatric disorders. Ghrelin was consistently shown to exert neuroprotective and memory-enhancing effects and alleviated psychopathology in animal models of dementia. Few human studies show a disruption of the ghrelin system in dementia. It was also shown to play a crucial role in the pathophysiology of addictive disorders, promoting drug reward, enhancing drug seeking behavior and increasing craving in both animals and humans. Ghrelin's exact role in depression and anxiety is still being debated, as it was shown to both promote and alleviate depressive and anxiety-behavior in animal studies, with an overweight of evidence suggesting antidepressant effects. Not surprisingly, the ghrelin system is also implicated in eating disorders, however its exact role remains to be elucidated. Its widespread involvement has made the ghrelin system a promising target for future therapies, with encouraging findings in recent literature.

Ghrelin: a link between ageing, metabolism and neurodegenerative disorders

Along with the increase in life expectancy over the last century comes the increased risk for development of age-related disorders, including metabolic and neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's diseases. These chronic disorders share two main characteristics: 1) neuronal loss in motor, sensory or cognitive systems, leading to cognitive and motor decline; and 2) a strong correlation between metabolic changes and neurodegeneration. In order to treat them, a better understanding of their complexity is required: it is necessary to interpret the neuronal damage in light of the metabolic changes, and to find the disrupted link between the peripheral organs governing energy metabolism and the CNS. This review is an attempt to present ghrelin as part of molecular regulatory interface between energy metabolism, neuroendocrine and neurodegenerative processes. Ghrelin takes part in lipid and glucose metabolism, in higher brain functions such as sleep-wake state, learning and memory consolidation; it influences mitochondrial respiration and shows neuroprotective effect. All these make ghrelin an attractive target for development of biomarkers or therapeutics for prevention or treatment of disorders, in which cell protection and recruitment of new neurons or synapses are needed.

Ghrelin inhibits high glucose-induced PC12 cell apoptosis by regulating TLR4/NF-κB pathway

Ghrelin has a protective effect on diabetic encephalopathy. To expound the protective mechanism, we investigated the effects of ghrelin on high glucose-induced cell apoptosis and intracellular signaling in cultured PC12, which is a suitable model for studying neuronal cell death. High glucose-induced PC12 apoptosis was significantly inhibited by co-treatment of ghrelin. Sustaining inflammatory response is one of the molecular mechanisms of diabetic encephalopathy and TLR4 signaling has close relationship with inflammatory response. But there is no report about the biologic role of toll-like receptor 4/nuclear factor-κB (TLR4/NF-κB) signaling in controlling high glucose-induced PC12 apoptosis by ghrelin. In this study, we found that TLR4/NF-κB pathway was activated by high glucose stimulation in PC12 and significantly alleviated by the co-treatment of ghrelin. From these findings, we made the conclusion that ghrelin could attenuate the symptoms of diabetic encephalopathy, which alleviates inflammatory reaction of diabetic encephalopathy by regulating TLR4/NF-κB pathway.

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.

Altered expression of histone and synaptic plasticity associated genes in the hippocampus of streptozotocin-induced diabetic mice

Accumulating evidence indicates that hyper-glycaemia is deleterious to brain function, in particular to the hippocampus. It is thought this hippocampal dysfunction may contribute to hyperglycaemia related cognitive impairment, such as that which manifests with diabetes. In the present study, we investigated the effects of diabetes-related hyperglycaemia on hippocampal gene expression, in order to identify potential mechanisms that might be associated with the cognitive dysfunction that develops with diabetes mellitus. Genome-wide gene expression profiling was carried out on the hippocampi from streptozotocin (STZ)-induced diabetic mice, and from vehicle treated control mice. Genes of interest that satisfied expression fold-change and statistical criteria, and that were considered to be potentially associated with cognitive function, were further tested by real time, quantitative polymerase chain reaction (qPCR) analysis. We found that STZ-induced diabetes resulted in decreased hippocampal expression of genes involved in epigenetic regulation and synaptic plasticity, for example, histone deacetylases and glycogen synthase kinase 3 beta (HDACs and GSK3β). We also found increased expression of genes involved in signalling cascades related to cell growth, cell survival and energy metabolism, such as neurotropic tyrosine kinase receptor type 2, apolipoprotein E, and protein tyrosine phosphatase receptor type (Ntrk2, APOE, PTPRT). To our knowledge this is the first study to demonstrate a gene expression profile implicating epigenetic modifications and alterations of synaptic plasticity associated genes in diabetes mellitus. The present study will improve our understanding of the neural mechanisms that might underpin diabetes-related cognitive dysfunction.

Obestatin alleviates subarachnoid haemorrhage-induced oxidative injury in rats via its anti-apoptotic and antioxidant effects

OBJECTIVE

The aim was to investigate the putative anti-inflammatory and anti-apoptotic effect of obestatin in a rat model of subarachnoidal haemorrhage (SAH).

METHODS

To induce SAH, rats were injected with 0.3 mL blood into their cisterna magna. At 48 hours rats were decapitated after neurological examination. Blood-brain barrier (BBB) permeability, brain water content, oxidative stress markers and histological analysis were done in brain tissue.

RESULTS

The results showed that neurological examination scores were increased in the SAH group and, moreover, BBB permeability was impaired and oedema formed. SAH resulted in increased levels of plasma tumour necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6 levels and caspase-3 activity. Lipid peroxidation and protein oxidation levels and myeloperoxidase activity were all increased in the brain tissue, with concomitant decreases in antioxidant enzymes. On the other hand, SAH-induced neurological impairment and oxidative brain injury were ameliorated in the obestatin-treated group.

CONCLUSION

The present study provides the first evidence that peripheral administration of obestatin exerts potent anti-inflammatory and neuroprotective effects in SAH-induced oxidative damage by maintaining a balance in oxidant-antioxidant status through the augmentation of endogenous antioxidants and the inhibition of pro-inflammatory mediators.