Ghrelin regulates hippocampal neurogenesis in adult mice

The catabolic - anabolic cycling hormesis model of health and resilience

A major goal of aging research is to identify ways of extending productive and disease-free lifespans. Here we present the catabolic - anabolic cycling hormesis (CACH) model for optimizing health. The CACH model is based on the concept that cells and organ systems respond to catabolic challenges in ways that bolster their resilience and that an anabolic recovery period is required to effectuate the benefits of the catabolic challenge. As two prominent real-world examples we highlight the literature on the molecular and cellular mechanisms by which physical exercise and intermittent fasting bolster cellular and organismal performance and resilience, and suppress disease processes. Over periods of weeks and months the CACH of exercise and fasting promote optimal health. The hormesis concept is integral to the CACH model and predicts an upper limit to the beneficial effects of catabolic - anabolic cycling that reflects a limit of biological plasticity. This paper extends the hormesis model of health by proposing that 1) it is comprised of two complementary phases: catabolic (adaptive stress responses and conservation of resources) and anabolic (growth and plasticity) and, 2) that CACH is metabolically integrated, quantitatively flexible and dynamically regulated. This model has important implications for future basic and translational research in the fields of aging and related disease processes.

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.

The 5:2 diet does not increase adult hippocampal neurogenesis or enhance spatial memory in mice

New neurones are generated throughout life in the mammalian brain in a process known as adult hippocampal neurogenesis (AHN). Since this phenomenon grants a high degree of neuroplasticity influencing learning and memory, identifying factors that regulate AHN may be important for ameliorating age-related cognitive decline. Calorie restriction (CR) has been shown to enhance AHN and improve memory, mediated by the stomach hormone, ghrelin. Intermittent fasting (IF), a dietary strategy offering more flexibility than conventional CR, has also been shown to promote aspects of AHN. The 5:2 diet is a popular form of IF; however, its effects on AHN are not well characterised. To address this, we quantified AHN in adolescent and adult wild-type and ghrelin-receptor-deficient mice following 6 weeks on a 5:2 diet. We report an age-related decline in neurogenic processes. However, the 5:2 diet does not increase AHN nor enhance memory performance, suggesting that this specific form of IF is ineffective in promoting brain plasticity to support learning.

Neurotrophic Factors and Dendritic Spines

Dendritic spines are highly dynamic structures that play important roles in neuronal plasticity. The morphologies and the numbers of dendritic spines are highly variable, and this diversity is correlated with the different morphological and physiological features of this neuronal compartment. Dendritic spines can change their morphology and number rapidly, allowing them to adapt to plastic changes. Neurotrophic factors play important roles in the brain during development. However, these factors are also necessary for a variety of processes in the postnatal brain. Neurotrophic factors, especially members of the neurotrophin family and the ephrin family, are involved in the modulation of long-lasting effects induced by neuronal plasticity by acting on dendritic spines, either directly or indirectly. Thereby, the neurotrophic factors play important roles in processes attributed, for example, to learning and memory.

Microbiota-Gut-Brain Axis Regulation of Adult Hippocampal Neurogenesis

The birth, maturation, and integration of new neurons in the adult hippocampus regulates specific learning and memory processes, responses to stress, and antidepressant treatment efficacy. This process of adult hippocampal neurogenesis is sensitive to environmental stimuli, including peripheral signals from certain cytokines, hormones, and metabolites, which can promote or hinder the production and survival of new hippocampal neurons. The trillions of microorganisms resident to the gastrointestinal tract, collectively known as the gut microbiota, also demonstrate the ability to modulate adult hippocampal neurogenesis. In doing so, the microbiota-gut-brain axis can influence brain functions regulated by adult hippocampal neurogenesis. Unlike the hippocampus, the gut microbiota is highly accessible to direct interventions, such as prebiotics, probiotics, and antibiotics, and can be manipulated by lifestyle choices including diet. Therefore, understanding the pathways by which the gut microbiota shapes hippocampal neurogenesis may reveal novel targets for non-invasive therapeutics to treat disorders in which alterations in hippocampal neurogenesis have been implicated. This review first outlines the factors which influence both the gut microbiome and adult hippocampal neurogenesis, with cognizance that these effects might happen either independently or due to microbiota-driven mechanisms. We then highlight approaches for investigating the regulation of adult hippocampal neurogenesis by the microbiota-gut-brain axis. Finally, we summarize the current evidence demonstrating the gut microbiota's ability to influence adult hippocampal neurogenesis, including mechanisms driven through immune pathways, microbial metabolites, endocrine signalling, and the nervous system, and postulate implications for these effects in disease onset and treatment.

"Sibling" battle or harmony: crosstalk between nesfatin-1 and ghrelin

Ghrelin was first identified as an endogenous ligand of the growth hormone secretagogue receptor (GHSR) in 1999, with the function of stimulating the release of growth hormone (GH), while nesfatin-1 was identified in 2006. Both peptides are secreted by the same kind of endocrine cells, X/A-like cells in the stomach. Compared with ghrelin, nesfatin-1 exerts opposite effects on energy metabolism, glucose metabolism, gastrointestinal functions and regulation of blood pressure, but exerts similar effects on anti-inflammation and neuroprotection. Up to now, nesfatin-1 remains as an orphan ligand because its receptor has not been identified. Several studies have shown the effects of nesfatin-1 are dependent on the receptor of ghrelin. We herein compare the effects of nesfatin-1 and ghrelin in several aspects and explore the possibility of their interactions.

Ghrelin Regulates Expression of the Transcription Factor Pax6 in Hypoxic Brain Progenitor Cells and Neurons

The nature of brain impairment after hypoxia is complex and recovery harnesses different mechanisms, including neuroprotection and neurogenesis. Experimental evidence suggests that hypoxia may trigger neurogenesis postnatally by influencing the expression of a variety of transcription factors. However, the existing data are controversial. As a proof-of-principle, we subjected cultured cerebral cortex neurons, cerebellar granule neurons and organotypic cerebral cortex slices from rat brains to hypoxia and treated these cultures with the hormone ghrelin, which is well-known for its neuroprotective functions. We found that hypoxia elevated the expression levels and stimulated nuclear translocation of ghrelin’s receptor GHSR1 in the cultured neurons and the acute organotypic slices, whereas ghrelin treatment reduced the receptor expression to normoxic levels. GHSR1 expression was also increased in cerebral cortex neurons of mice with induced experimental stroke. Additional quantitative analyses of immunostainings for neuronal proliferation and differentiation markers revealed that hypoxia stimulated the proliferation of neuronal progenitors, whereas ghrelin application during the phase of recovery from hypoxia counteracted these effects. At the mechanistic level, we provide a link between the described post-ischemic phenomena and the expression of the transcription factor Pax6, an important regulator of neural progenitor cell fate. In contrast to the neurogenic niches in the brain where hypoxia is known to increase Pax6 expression, the levels of the transcription factor in cultured hypoxic cerebral cortex cells were downregulated. Moreover, the application of ghrelin to hypoxic neurons normalised the expression levels of these factors. Our findings suggest that ghrelin stimulates neurogenic factors for the protection of neurons in a GHSR1-dependent manner in non-neurogenic brain areas such as the cerebral cortex after exposure to hypoxia.

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.

Effect of unacylated ghrelin on peripheral nerve regeneration

Ghrelin is a circulating peptide hormone released by enteroendocrine cells of the gastrointestinal tract as two forms, acylated and unacylated. Acylated ghrelin (AG) binds to the growth hormone secretagogue receptor 1a (GHSR1a), thus stimulating food intake, growth hormone release, and gastrointestinal motility. Conversely, unacylated GHR (UnAG), through binding to a yet unidentified receptor, protects the skeletal muscle from atrophy, stimulates muscle regeneration, and protects cardiomyocytes from ischemic damage. Recently, interest about ghrelin has raised also among neuroscientists because of its effect on the nervous system, especially the stimulation of neurogenesis in spinal cord, brain stem, and hippocampus. However, few information is still available about its effectiveness on peripheral nerve regeneration. To partially fill this gap, the aim of this study was to assess the effect of UnAG on peripheral nerve regeneration after median nerve crush injury and after nerve transection immediately repaired by means of an end-to-end suture. To this end, we exploited FVB1 Myh6/Ghrl transgenic mice in which overexpression of the ghrelin gene (Ghrl) results in selective up-regulation of circulating UnAG levels, but not of AG. Regeneration was assessed by both functional evaluation (grasping test) and morphometrical analysis of regenerated myelinated axons. Results obtained lead to conclude that UnAG could have a role in development of peripheral nerves and during more severe lesions.

Ghrelin-Mediated Regeneration and Plasticity After Nervous System Injury

The nervous system is highly vulnerable to different factors which may cause injury followed by an acute or chronic neurodegeneration. Injury involves a loss of extracellular matrix integrity, neuronal circuitry disintegration, and impairment of synaptic activity and plasticity. Application of pleiotropic molecules initiating extracellular matrix reorganization and stimulating neuronal plasticity could prevent propagation of the degeneration into the tissue surrounding the injury. To find an omnipotent therapeutic molecule, however, seems to be a fairly ambitious task, given the complex demands of the regenerating nervous system that need to be fulfilled. Among the vast number of candidates examined so far, the neuropeptide and hormone ghrelin holds within a very promising therapeutic potential with its ability to cross the blood-brain barrier, to balance metabolic processes, and to stimulate neurorepair and neuroactivity. Compared with its well-established systemic effects in treatment of metabolism-related disorders, the therapeutic potential of ghrelin on neuroregeneration upon injury has received lesser appreciation though. Here, we discuss emerging concepts of ghrelin as an omnipotent player unleashing developmentally related molecular cues and morphogenic cascades, which could attenuate and/or counteract acute and chronic neurodegeneration.

Therapeutic effects of non-saponin fraction with rich polysaccharide from Korean red ginseng on aging and Alzheimer's disease

Biological aging provokes morbidity and several functional declines, causing older adults more susceptible to a variety of diseases than younger adults. In particular, aging is a major risk factor contributing to non-communicable diseases, such as neurodegenerative disorders. Alzheimer's disease (AD) is an aging-related neurodegenerative disease that is characterized by cognitive deficits and the formation of amyloid plaques formed by the accumulation of amyloid-β (Aβ) peptides. Non-saponin fraction with rich polysaccharide (NFP) from red ginseng, the largest fraction of the components of red ginseng, perform many biological activities. However, it has not been clarified whether the NFP from Korean red ginseng (KRG) has beneficial effects in the aging and AD. First, proteomics analysis was performed in aged brain to identify the effect of NFP on protein changes, and we confirmed that NFP induced changes in proteins related to the neuroprotective- and neurogenic-effects. Next, we investigated (1) the effects of NFP on AD pathologies, such as Aβ deposition, neuroinflammation, neurodegeneration, mitochondrial dysfunction, and impaired adult hippocampal neurogenesis (AHN), in 5XFAD transgenic mouse model of AD using immunostaining; (2) the effect of NFP on Aβ-mediated mitochondrial respiration deficiency in HT22 mouse hippocampal neuronal cells (HT22) using Seahorse XFp analysis; (3) the effect of NFP on cell proliferation using WST-1 analysis; and (4) the effect of NFP on Aβ-induced cognitive dysfunction in 5XFAD mouse model of AD using Y-maze test. Histological analysis indicated that NFP significantly alleviated the accumulation of Aβ, neuroinflammation, neuronal loss, and mitochondrial dysfunction in the subiculum of 5XFAD mouse model of AD. In addition, NFP treatment ameliorated mitochondrial deficits in Aβ-treated HT22 cells. Moreover, NFP treatment significantly increased the AHN and neuritogenesis of neural stem cells in both healthy and AD brains. Furthermore, NFP significantly increased cell proliferation in the HT22 cells. Finally, NFP administration significantly enhanced and restored the cognitive function of healthy and AD mice, respectively. Taken together, NFP treatment demonstrated changes in proteins involved in central nervous system organization/maintenance in aged brain and ameliorates AD pathology. Collectively, our findings suggest that NFP from KRG could be a potential therapeutic candidate for aging and AD treatments.

Platycodon grandiflorum Root Protects against Aβ-Induced Cognitive Dysfunction and Pathology in Female Models of Alzheimer's Disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by irreversible cognitive dysfunction. Amyloid beta (Aβ) peptide is an important pathological factor that triggers the progression of AD through accumulation and aggregation, which leads to AD-related pathologies that consequently affect cognitive functions. Interestingly, several studies have reported that Platycodon grandiflorum root extract (PGE), besides exhibiting other bioactive effects, displays neuroprotective, anti-neuroinflammatory, and cognitive-enhancing effects. However, to date, it is not clear whether PGE can affect AD-related cognitive dysfunction and pathogenesis. Therefore, to investigate whether PGE influences cognitive impairment in an animal model of AD, we conducted a Y-maze test using a 5XFAD mouse model. Oral administration of PGE for 3 weeks at a daily dose of 100 mg/kg significantly ameliorated cognitive impairment in 5XFAD mice. Moreover, to elucidate the neurohistological mechanisms underlying the PGE-mediated alleviative effect on cognitive dysfunction, we performed histological analysis of hippocampal formation in these mice. Histopathological analysis showed that PGE significantly alleviated AD-related pathologies such as Aβ accumulation, neurodegeneration, oxidative stress, and neuroinflammation. In addition, we observed a neuroprotective and antioxidant effect of PGE in mouse hippocampal neurons. Our findings suggest that administration of PGE might act as one of the therapeutic agents for AD by decreasing Aβ related pathology and ameliorating Aβ induced cognitive impairment.

Acylated Ghrelin as a Multi-Targeted Therapy for Alzheimer's and Parkinson's Disease

Much thought has been given to the impact of Amyloid Beta, Tau and Alpha-Synuclein in the development of Alzheimer's disease (AD) and Parkinson's disease (PD), yet the clinical failures of the recent decades indicate that there are further pathological mechanisms at work. Indeed, besides amyloids, AD and PD are characterized by the culminative interplay of oxidative stress, mitochondrial dysfunction and hyperfission, defective autophagy and mitophagy, systemic inflammation, BBB and vascular damage, demyelination, cerebral insulin resistance, the loss of dopamine production in PD, impaired neurogenesis and, of course, widespread axonal, synaptic and neuronal degeneration that leads to cognitive and motor impediments. Interestingly, the acylated form of the hormone ghrelin has shown the potential to ameliorate the latter pathologic changes, although some studies indicate a few complications that need to be considered in the long-term administration of the hormone. As such, this review will illustrate the wide-ranging neuroprotective properties of acylated ghrelin and critically evaluate the hormone's therapeutic benefits for the treatment of AD and PD.

The Potential Role of Gut Peptide Hormones in Autism Spectrum Disorder

Gut peptide hormones are one group of secretory factors produced from gastrointestinal endocrine cells with potent functions in modulating digestive functions. In recent decades, they have been found across different brain regions, many of which are involved in autism-related social, emotional and cognitive deficits. Clinical studies have revealed possible correlation between those hormones and autism spectrum disorder pathogenesis. In animal models, gut peptide hormones modulate neurodevelopment, synaptic transmission and neural plasticity, explaining their behavioral relevance. This review article will summarize major findings from both clinical and basic research showing the role of gut peptide hormones in mediating autism-related neurological functions, and their potential implications in autism pathogenesis. The pharmaceutical value of gut hormones in alleviating autism-associated behavioral syndromes will be discussed to provide new insights for future drug development.

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 in Alzheimer's disease: Pathologic roles and therapeutic implications

Ghrelin, which has many important physiological roles, such as stimulating food intake, regulating energy homeostasis, and releasing insulin, has recently been studied for its roles in a diverse range of neurological disorders. Despite the several functions of ghrelin in the central nervous system, whether it works as a therapeutic agent for neurological dysfunction has been unclear. Altered levels and various roles of ghrelin have been reported in Alzheimer's disease (AD), which is characterized by the accumulation of misfolded proteins resulting in synaptic loss and cognitive decline. Interestingly, treatment with ghrelin or with the agonist of ghrelin receptor showed attenuation in several cases of AD-related pathology. These findings suggest the potential therapeutic implications of ghrelin in the pathogenesis of AD. In the present review, we summarized the roles of ghrelin in AD pathogenesis, amyloid beta (Aβ) homeostasis, tau hyperphosphorylation, neuroinflammation, mitochondrial deficit, synaptic dysfunction and cognitive impairment. The findings from this review suggest that ghrelin has a novel therapeutic potential for AD treatment. Thus, rigorously designed studies are needed to establish an effective AD-modifying strategy.

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.

Calorie restriction activates new adult born olfactory-bulb neurones in a ghrelin-dependent manner but acyl-ghrelin does not enhance subventricular zone neurogenesis

The ageing and degenerating brain show deficits in neural stem/progenitor cell (NSPC) plasticity that are accompanied by impairments in olfactory discrimination. Emerging evidence suggests that the gut hormone ghrelin plays an important role in protecting neurones, promoting synaptic plasticity and increasing hippocampal neurogenesis in the adult brain. In the present study, we investigated the role of ghrelin with respect to modulating adult subventricular zone (SVZ) NSPCs that give rise to new olfactory bulb (OB) neurones. We characterised the expression of the ghrelin receptor, growth hormone secretagogue receptor (GHSR), using an immunohistochemical approach in GHSR-eGFP reporter mice to show that GHSR is expressed in several regions, including the OB but not in the SVZ of the lateral ventricle. These data suggest that acyl-ghrelin does not mediate a direct effect on NSPC in the SVZ. Consistent with these findings, treatment with acyl-ghrelin or genetic silencing of GHSR did not alter NSPC proliferation within the SVZ. Similarly, using a bromodeoxyuridine pulse-chase approach, we show that peripheral treatment of adult rats with acyl-ghrelin did not increase the number of new adult-born neurones in the granule cell layer of the OB. These data demonstrate that acyl-ghrelin does not increase adult OB neurogenesis. Finally, we investigated whether elevating ghrelin indirectly, via calorie restriction (CR), regulated the activity of new adult-born cells in the OB. Overnight CR induced c-Fos expression in new adult-born OB cells but not in developmentally born cells, whereas neuronal activity was absent following re-feeding. These effects were not present in ghrelin^-/- mice, suggesting that adult-born cells are uniquely sensitive to changes in ghrelin mediated by fasting and re-feeding. In summary, ghrelin does not promote neurogenesis in the SVZ and OB; however, new adult-born OB cells are activated by CR in a ghrelin-dependent manner.

Brain accessibility delineates the central effects of circulating ghrelin

Ghrelin is a hormone produced in the gastrointestinal tract that acts via the growth hormone secretagogue receptor. In the central nervous system, ghrelin signalling is able to recruit different neuronal targets that regulate the behavioural, neuroendocrine, metabolic and autonomic effects of the hormone. Notably, several studies using radioactive or fluorescent variants of ghrelin have found that the accessibility of circulating ghrelin into the mouse brain is both strikingly low and restricted to some specific brain areas. A variety of studies addressing central effects of systemically injected ghrelin in mice have also provided indirect evidence that the accessibility of plasma ghrelin into the brain is limited. Here, we review these previous observations and discuss the putative pathways that would allow plasma ghrelin to gain access into the brain together with their physiological implications. Additionally, we discuss some potential features regarding the accessibility of plasma ghrelin into the human brain based on the observations reported by studies that investigate the consequences of ghrelin administration to humans.

[Effect of Ghrelin on Memory Impairment in a Rat Model of Vascular Dementia]

PURPOSE

The purpose of this study was to identify the effect of ghrelin on memory impairment in a rat model of vascular dementia induced by chronic cerebral hypoperfusion.

METHODS

Randomized controlled groups and the posttest design were used. We established the representative animal model of vascular dementia caused by bilateral common carotid artery occlusion and administered 80 μg/kg ghrelin intraperitoneally for 4 weeks. First, behavioral studies were performed to evaluate spatial memory. Second, we used molecular biology techniques to determine whether ghrelin ameliorates the damage to the structure and function of the white matter and hippocampus, which are crucial to learning and memory.

RESULTS

Ghrelin improved the spatial memory impairment in the Y-maze and Morris water maze test. In the white matter, demyelination and atrophy of the corpus callosum were significantly decreased in the ghrelin-treated group. In the hippocampus, ghrelin increased the length of hippocampal microvessels and reduced the microvessels pathology. Further, we confirmed angiogenesis enhancement through the fact that ghrelin treatment increased vascular endothelial growth factor (VEGF)-related protein levels, which are the most powerful mediators of angiogenesis in the hippocampus.

CONCLUSION

We found that ghrelin affected the damaged myelin sheaths and microvessels by increasing angiogenesis, which then led to neuroprotection and improved memory function. We suggest that further studies continue to accumulate evidence of the effect of ghrelin. Further, we believe that the development of therapeutic interventions that increase ghrelin may contribute to memory improvement in patients with vascular dementia.

The protective effects of Ghrelin/GHSR on hippocampal neurogenesis in CUMS mice

Ghrelin is an orexigenic hormone that also plays an important role in mood disorders. Our previous studies demonstrated that ghrelin administration could protect against depression-like behaviors of chronic unpredictable mild stress (CUMS) in rodents. However, the mechanism related to the effect of ghrelin on CUMS mice has yet to be revealed. This article shows that ghrelin (5 nmol/kg/day for 2 weeks, i.p.) decreased depression-like behaviors induced by CUMS and increased hippocampal integrity (neurogenesis and spine density) measured via Ki67, 5-bromo-2-deoxyuridine (BrdU), doublecortin (DCX) labeling and Golgi-cox staining, which were decreased under CUMS. The behavioral phenotypes of Growth hormone secretagogue receptor (Ghsr)-null and wild type (WT) mice were evaluated under no stress condition and after CUMS exposure to determine the effect of Ghsr knockout on the behavioral phenotypes and stress susceptibility of mice. Ghsr-null mice exhibited depression-like behaviors under no stress condition. CUMS induced similar depression- and anxiety-like behavioral manifestations in both Ghsr-null and WT mice. A similar pattern of behavioral changes was observed after hippocampal GHSR knockdown. Additionally, both Ghsr knockout as well as CUMS exhibited deleterious effects on neurogenesis and spine density in the dentate gyrus (DG). Besides, CCK8 assay and 5-Ethynyl-2'-deoxyuridine (EdU) incorporation assay showed that ghrelin has a proliferative effect on primary cultured hippocampal neural stem cells (NSCs) and this proliferation was blocked by D-Lys3-GHRP-6 (DLS, the antagonist of GHSR, 100 μM) pretreatment. Ghrelin-induced proliferation is associated with the inhibition of G1 arrest, and this inhibition was blocked by LY294002 (specific inhibitor of PI3K, 20 μM). Furthermore, the in vivo data displayed that LY294002 (50 nmol, i.c.v.) can significantly block the antidepressant-like action of exogenous ghrelin treatment. All these results suggest that ghrelin/GHSR signaling maintains the integrity of hippocampus and has an inherent neuroprotective effect whether facing stress or not.

Regulation of Memory Function by Feeding-Relevant Biological Systems: Following the Breadcrumbs to the Hippocampus

The hippocampus (HPC) controls fundamental learning and memory processes, including memory for visuospatial navigation (spatial memory) and flexible memory for facts and autobiographical events (declarative memory). Emerging evidence reveals that hippocampal-dependent memory function is regulated by various peripheral biological systems that are traditionally known for their roles in appetite and body weight regulation. Here, we argue that these effects are consistent with a framework that it is evolutionarily advantageous to encode and recall critical features surrounding feeding behavior, including the spatial location of a food source, social factors, post-absorptive processing, and other episodic elements of a meal. We review evidence that gut-to-brain communication from the vagus nerve and from feeding-relevant endocrine systems, including ghrelin, insulin, leptin, and glucagon-like peptide-1 (GLP-1), promote hippocampal-dependent spatial and declarative memory via neurotrophic and neurogenic mechanisms. The collective literature reviewed herein supports a model in which various stages of feeding behavior and hippocampal-dependent memory function are closely linked.

Intracerebroventricular Ghrelin Administration Increases Depressive-Like Behavior in Male Juvenile Rats

Major depressive disorder (MDD) is arguably the largest contributor to the global disease and disability burden, but very few treatment options exist for juvenile MDD patients. Ghrelin is the principal hunger-stimulating peptide, and it has also been shown to reduce depressive-like symptoms in adult rodents. We examined the effects of intracerebroventricular (icv) injection of ghrelin on depressive-like behavior. Moreover, we determined whether ghrelin increased neurogenesis in the hippocampus. Ghrelin (0.2-nM, 0.5-nM, and 1.0-nM) was administered acutely by icv injection to juvenile rats to determine the most effective dose (0.5-nM) by a validated feeding behavior test and using the forced swim test (FST) as an indicator of depressive-like behavior. 0.5-nM ghrelin was then administered icv against an artificial cerebrospinal fluid (aCSF) vehicle control to determine behavioral changes in the tail suspension test (TST) as an indicator of depressive-like behavior. Neurogenesis was investigated using a mitogenic paradigm, as well as a neurogenic paradigm to assess whether ghrelin altered neurogenesis. Newborn hippocampal cells were marked using 5′-bromo-2′-deoxyuridine (BrdU) administered intraperitoneally (ip) at either the end or the beginning of the experiment for the mitogenic and neurogenic paradigms, respectively. We found that ghrelin administration increased immobility time in the TST. Treatment with ghrelin did not change mitogenesis or neurogenesis. These results suggest that ghrelin administration does not have an antidepressant effect in juvenile rats. In contrast to adult rodents, ghrelin increases depressive-like behavior in male juvenile rats. These results highlight the need to better delineate differences in the neuropharmacology of depressive-like behavior between juvenile and adult rodents.

Homeobox B4 gene expression is upregulated by ghrelin through PI3-kinase signaling pathway in rat's bone marrow stromal cells

OBJECTIVE

Ghrelin, a 28 amino acid peptide, has diverse physiological roles. Phosphatidylino-sitol-bisphosphate 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) are involved in some of the recognized actions of ghrelin. It has been shown that ghrelin upregulates HOXB4 gene expression but the real mechanism of this effect is not clear.

METHODS

Rat bone marrow stromal cells (BMSCs) were cultured in DMEM. BMSCs were treated with ghrelin (100 μM) for 48 h. Real-time PCR for HOXB4 was performed from Control (untreated BMSCs), BG (BMSCs treated with 100 µM ghrelin), PD (BMSCs treated with 10 µM PD98059, a potent inhibitor of mitogen-activated protein kinase, and 100 µM ghrelin), LY (BM-SCs treated with 10 µM LY294002, a strong inhibitor of phosphoinositide 3-kinase, and 100 µM ghrelin) and SY (BMSCs treated with 10 µM LY294002 plus 10 µM PD98059, and 100 µM ghrelin) groups. Relative gene expression changes were determined using Relative expression software tool 9 (REST 9).

RESULTS

HOXB4 gene has been overexpressed in ghrelin-treated BMSCs (p<0.05). PI3K inhi-bition by LY294002 significantly downregulated the ghrelin-induced overexpression of HOXB4 (p<0.05).

CONCLUSION

We can conclude that ghrelin, through PI3K/Akt pathway, may improve BMSC transplantation potency by reducing its apoptosis. Moreover, upregulating HOXB4 in BMSC and its possible differentiation to HSCs might in the future open the doors to new treatment for hematologic disorders. Therefore, activating the PI3K/Akt pathway, instead of using a non-specific inducer, could be the principal point to increase the efficiency of BMSC-based cell therapies in the future.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

Food consumption increases cell proliferation in the python brain

Pythons are model organisms for investigating physiological responses to food intake. While systemic growth in response to food consumption is well documented, what occurs in the brain is currently unexplored. In this study, male ball pythons (Python regius) were used to test the hypothesis that food consumption stimulates cell proliferation in the brain. We used 5-bromo-12'-deoxyuridine (BrdU) as a cell-birth marker to quantify and compare cell proliferation in the brain of fasted snakes and those at 2 and 6 days after a meal. Throughout the telencephalon, cell proliferation was significantly increased in the 6 day group, with no difference between the 2 day group and controls. Systemic postprandial plasticity occurs quickly after a meal is ingested, during the period of active digestion; however, the brain displays a surge of cell proliferation after most digestion and absorption is complete.

Ghrelin Upregulates Hoxb4 Gene Expression in Rat Bone Marrow Stromal Cells

OBJECTIVES

Ghrelin is a peptide which has a proliferative and antiapoptotic effect in many cells including bone marrow stromal cells (BMSCs). Homeobox protein B4 (HOXB4) is a transcription factor involved in stem cell regeneration and survival. The aim of the study was to find out the efect of ghrelin on Hoxb4 expression in BMSCs.

MATERIALS AND METHODS

In this experimental study, rat BMSCs were cultivated in Dulbecco's Modified Eagle Medium (DMEM). Passage three BMSCs were treated with ghrelin 100 μM for 48 hours. Real-time polymerase chain reaction (PCR) was carried out from the untreated BMSCs (B), BMSCs treated with 125 μM H₂O₂ (BH), BMSCs treated with 100 μM ghrelin then 125 μM H₂O₂ (BGH) and BMSCs treated with 100 μM ghrelin (BG) groups. For immunofluorescence, cells were incubated with an anti-HOXB4 monoclonal antibody. Primary antibodies were visualized using the Fluorescein isothiocyanate (FITC) method. All data are presented as mean ± SEM and P<0.05 was considered as statistical significant.

RESULTS

Hoxb4 expression significantly increased in the BG compared with BH and BGH groups. Furthermore, 100 μM ghrelin, increased the mean of HOXB4 positive immunoreactive cells compared to the BH group.

CONCLUSIONS

Ghrelin probably enhances proliferation and viability of BMSCs through Hoxb4 upregulation. However, the signaling pathway and other biological outcomes of this effect should be elucidated in different stem cells.

Ghrelin accelerates the cartilagic differentiation of rabbit mesenchymal stem cells through the ERK1/2 pathway

Mesenchymal stem cells (MSCs) can differentiate into chondroblasts, adipocytes, or cartilage under appropriate stimulation. Identifying a mechanism triggering the differentiation of MSCs into cartilage may help develop novel therapeutic approaches for treating heterotopic ossification, the pathological formation of lamellar bone in soft tissue outside the skeleton that can lead to debilitating immobility. Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor, stimulates growth hormone secretion, and has both orexigenic and adipogenic effects. This study sought to understand the potential involvement of the ERK1/2 signaling pathway in the ghrelin-induced growth of rat MSCs (rMSCs). We applied various concentrations of ghrelin to cultured rMSCs by observing the changes in the phosphorylation state of ERK1/2, p38, JNK as well as the type II collagen expression levels by western blot. The highest expression level for both type II collagen was obtained with 600 ng/mL ghrelin at 24 h. We found that the ghrelin-induced differentiation of rMSCs into cartilage was promoted primarily by the ERK1/2 pathway. Our study suggests that ghrelin induced differentiation of rMSCs into cartilage primarily through the ERK1/2 pathway.

Nutritional Factors Affecting Adult Neurogenesis and Cognitive Function

Adult neurogenesis, a complex process by which stem cells in the hippocampal brain region differentiate and proliferate into new neurons and other resident brain cells, is known to be affected by many intrinsic and extrinsic factors, including diet. Neurogenesis plays a critical role in neural plasticity, brain homeostasis, and maintenance in the central nervous system and is a crucial factor in preserving the cognitive function and repair of damaged brain cells affected by aging and brain disorders. Intrinsic factors such as aging, neuroinflammation, oxidative stress, and brain injury, as well as lifestyle factors such as high-fat and high-sugar diets and alcohol and opioid addiction, negatively affect adult neurogenesis. Conversely, many dietary components such as curcumin, resveratrol, blueberry polyphenols, sulforaphane, salvionic acid, polyunsaturated fatty acids (PUFAs), and diets enriched with polyphenols and PUFAs, as well as caloric restriction, physical exercise, and learning, have been shown to induce neurogenesis in adult brains. Although many of the underlying mechanisms by which nutrients and dietary factors affect adult neurogenesis have yet to be determined, nutritional approaches provide promising prospects to stimulate adult neurogenesis and combat neurodegenerative diseases and cognitive decline. In this review, we summarize the evidence supporting the role of nutritional factors in modifying adult neurogenesis and their potential to preserve cognitive function during aging.

Less is more: Caloric regulation of neurogenesis and adult brain function

Calorie intake is essential for regulating normal physiological processes and is fundamental to maintaining life. Indeed, both extremes of calorie intake result in increased morbidity and mortality. In this review, we discuss the effect of calorie intake on adult brain function, with an emphasis on the beneficial effects of mild calorie restriction. Recent findings relating to the regenerative and protective effects of the gastrointestinal hormone, ghrelin, suggest that it may underlie the beneficial effects of calorie restriction. We discuss the putative cellular mechanisms underlying the action of ghrelin and their possible role in supporting healthy brain ageing.

Mdivi-1 Protects Adult Rat Hippocampal Neural Stem Cells against Palmitate-Induced Oxidative Stress and Apoptosis

Palmitate concentrations in type 2 diabetic patients are higher than in healthy subjects. The prolonged elevation of plasma palmitate levels induces oxidative stress and mitochondrial dysfunction in neuronal cells. In this study, we examined the role of mdivi-1, a selective inhibitor of mitochondrial fission protein dynamin-regulated protein 1 (Drp1), on the survival of cultured hippocampal neural stem cells (NSCs) exposed to high palmitate. Treatment of hippocampal NSCs with mdivi-1 attenuated palmitate-induced increase in cell death and apoptosis. Palmitate exposure significantly increased Drp1 protein levels, which were prevented by pretreatment of cells with mdivi-1. We found that cytosolic Drp1 was translocated to the mitochondria when cells were exposed to palmitate. In contrast, palmitate-induced translocation of Drp1 was inhibited by mdivi-1 treatment. We also investigated mdivi-1 regulation of apoptosis at the mitochondrial level. Mdivi-1 rescued cells from palmitate-induced lipotoxicity by suppressing intracellular and mitochondrial reactive oxygen species production and stabilizing mitochondrial transmembrane potential. Mdivi-1-treated cells showed an increased Bcl-2/Bax ratio, prevention of cytochrome c release, and inhibition of caspase-3 activation. Our data suggest that mdivi-1 protects hippocampal NSCs against lipotoxicity-associated oxidative stress by preserving mitochondrial integrity and inhibiting mitochondrial apoptotic cascades.

Ghrelin produces antidepressant-like effect in the estrogen deficient mice

Recent evidence shows that ghrelin plays an important role in depression. However, it was little known whether ghrelin produces antidepressant-like effect in the ovariectomized mice. The present study was aimed to investigate the antidepressant-like effects of the ghrelin in ovariectomized mice. In the forced swim test, ghrelin significantly decreased immobility time, reversing the “depressive-like” effect observed in ovariectomized mice, and this effect was reversed by the tamoxifen. In addition, immunohistochemical study indicated that ghrelin treatment reversed the reductions in c-Fos expression induced by ovariectomy. An estrogen antagonist tamoxifen also antagonized the effect of ghrelin on the c-Fos expression. Furthermore, the western blotting indicated that brain-derived neurotrophic factor (BDNF) in the hippocampus, but not phosphorylated cAMP response element-binding protein (pCREB)/CREB in the frontal cortex, were affected by ghrelin treatment. Ghrelin treatment significantly increased BrdU expression. Therefore, these findings suggest that ghrelin produces antidepressant-like effects in ovariectomized mice, and estrogen receptor may be involved in the antidepressant-like effects of the ghrelin.

Favorable Impact on Stress-Related Behaviors by Modulating Plasma Butyrylcholinesterase

In the last decade, it has become clear that the neuropeptide “ghrelin” and its principal receptor have a large impact on anxiety and stress. Our recent studies have uncovered a link between plasma butyrylcholinesterase (BChE) and ghrelin. BChE actually turns out to be the key regulator of this peptide. This article reviews our recent work on manipulating ghrelin levels in mouse blood and brain by long term elevation of BChE, leading to sustained decrease of ghrelin. That effect in turn was found to reduce stress-induced aggression in group caged mice. Positive consequences were fewer bite wounds and longer survival times. No adverse effects were observed. Further exploration may pave the way for BChE-based treatment of anxiety in humans.

New neurons in adult brain: distribution, molecular mechanisms and therapies

"Are new neurons added in the adult mammalian brain?" "Do neural stem cells activate following CNS diseases?" "How can we modulate their activation to promote recovery?" Recent findings in the field provide novel insights for addressing these questions from a new perspective. In this review, we will summarize the current knowledge about adult neurogenesis and neural stem cell niches in healthy and pathological conditions. We will first overview the milestones that have led to the discovery of the classical ventricular and hippocampal neural stem cell niches. In adult brain, new neurons originate from proliferating neural precursors located in the subventricular zone of the lateral ventricles and in the subgranular zone of the hippocampus. However, recent findings suggest that new neuronal cells can be added to the adult brain by direct differentiation (e.g., without cell proliferation) from either quiescent neural precursors or non-neuronal cells undergoing conversion or reprogramming to neuronal fate. Accordingly, in this review we will also address critical aspects of the newly described mechanisms of quiescence and direct conversion as well as the more canonical activation of the neurogenic niches and neuroblast reservoirs in pathological conditions. Finally, we will outline the critical elements involved in neural progenitor proliferation, neuroblast migration and differentiation and discuss their potential as targets for the development of novel therapeutic drugs for neurodegenerative diseases.

Acyl ghrelin improves cognition, synaptic plasticity deficits and neuroinflammation following amyloid β (Aβ1-40) administration in mice

Ghrelin is a metabolic hormone that has neuroprotective actions in a number of neurological conditions, including Parkinson's disease (PD), stroke and traumatic brain injury. Acyl ghrelin treatment in vivo and in vitro also shows protective capacity in Alzheimer's disease (AD). In the present study, we used ghrelin knockout (KO) and their wild-type littermates to test whether or not endogenous ghrelin is protective in a mouse model of AD, in which human amyloid β peptide 1-40 (Aβ_1-40 ) was injected into the lateral ventricles i.c.v. Recognition memory, using the novel object recognition task, was significantly impaired in ghrelin KO mice and after i.c.v. Aβ_1-40 treatment. These deficits could be prevented by acyl ghrelin injections for 7 days. Spatial orientation, as assessed by the Y-maze task, was also significantly impaired in ghrelin KO mice and after i.c.v. Aβ_1-40 treatment. These deficits could be prevented by acyl ghrelin injections for 7 days. Ghrelin KO mice had deficits in olfactory discrimination; however, neither i.c.v. Aβ_1-40 treatment, nor acyl ghrelin injections affected olfactory discrimination. We used stereology to show that ghrelin KO and Aβ_1-40 increased the total number of glial fibrillary acidic protein expressing astrocytes and ionised calcium-binding adapter expressing microglial in the rostral hippocampus. Finally, Aβ_1-40 blocked long-term potentiation induced by high-frequency stimulation and this effect could be acutely blocked with co-administration of acyl ghrelin. Collectively, our studies demonstrate that ghrelin deletion affects memory performance and also that acyl ghrelin treatment may delay the onset of early events of AD. This supports the idea that acyl ghrelin treatment may be therapeutically beneficial with respect to restricting disease progression in AD.

Neurogenic Effects of Ghrelin on the Hippocampus

Mammalian neurogenesis continues throughout adulthood in the subventricular zone of the lateral ventricle and in the subgranular zone of the dentate gyrus in the hippocampus. It is well known that hippocampal neurogenesis is essential in mediating hippocampus-dependent learning and memory. Ghrelin, a peptide hormone mainly synthesized in the stomach, has been shown to play a major role in the regulation of energy metabolism. A plethora of evidence indicates that ghrelin can also exert important effects on neurogenesis in the hippocampus of the adult brain. The aim of this review is to discuss the current role of ghrelin on the in vivo and in vitro regulation of neurogenesis in the adult hippocampus. We will also discuss the possible role of ghrelin in dietary restriction-induced hippocampal neurogenesis and the link between ghrelin-induced hippocampal neurogenesis and cognitive functions.

The Impact of Ghrelin in Metabolic Diseases: An Immune Perspective

Obesity and insulin resistance have reached epidemic proportions. Obesogenic conditions are associated with increased risk for the development of other comorbidities and obesity-related diseases. In metabolic disorders, there is chronic low-grade inflammation induced by the activation of immune cells, especially in metabolic relevant organs such as white adipose tissue (WAT). These immune cells are regulated by environmental and systemic cues. Ghrelin is a peptide secreted mainly by X/A-like gastric cells and acts through the growth hormone secretagogue receptor (GHS-R). This receptor is broadly expressed in the central nervous system (CNS) and in several cell types, including immune cells. Studies show that ghrelin induces an orexigenic state, and there is increasing evidence implicating an immunoregulatory role for ghrelin. Ghrelin mainly acts on the innate and adaptive immune systems to suppress inflammation and induce an anti-inflammatory profile. In this review, we discuss the immunoregulatory roles of ghrelin, the mechanisms by which ghrelin acts and potential pharmacological applications for ghrelin in the treatment of obesity-associated inflammatory diseases, such as type 2 diabetes (T2D).

Ghrelin gene products rescue cultured adult rat hippocampal neural stem cells from high glucose insult

Adult hippocampal neurogenesis is decreased in type 2 diabetes, and this impairment appears to be important in cognitive dysfunction. Previous studies suggest that ghrelin gene products (acylated ghrelin (AG), unacylated ghrelin (UAG) and obestatin (OB)) promote neurogenesis. Therefore, we hypothesize that ghrelin gene products may reduce the harmful effects of high glucose (HG) on hippocampal neural stem cells (NSCs). The aim of this study was to investigate the role of these peptides on the survival of cultured hippocampal NSCs exposed to HG insult. Treatment of hippocampal NSCs with AG, UAG or OB inhibited HG-induced cell death and apoptosis. Exposure of cells to the growth hormone secretagogue receptor 1a antagonist abolished the protective effects of AG against HG toxicity, whereas those of UAG or OB were preserved. All three peptides attenuated HG-induced decrease in BrdU-labeled and phosphohistone-H3-labeled cells. We also investigated the effects of ghrelin gene products on the regulation of apoptosis at the mitochondrial level. AG, UAG or OB rescued hippocampal NSCs from HG insult by inhibiting intracellular and mitochondrial reactive oxygen species generation and stabilizing mitochondrial transmembrane potential. In addition, cells treated with ghrelin gene products showed an increased Bcl-2 and decreased Bax levels, thereby increasing the Bcl-2/Bax ratio, inhibiting cytochrome c release and preventing caspase-3 activation. Finally, AG-, UAG- or OB-mediated protection was dependent on the activities of adenosine monophosphate-activated protein kinase/uncoupling protein 2 pathway. Our data indicate that ghrelin gene products may act as survival factors that preserve mitochondrial function and inhibit oxidative stress-induced apoptosis.

Interactive effects of ghrelin and ketamine on forced swim performance: Implications for novel antidepressant strategies

The efficacy of ketamine to alleviate depressive symptoms has promoted a wealth of research exploring alternate therapeutic targets for depression. Given the caveats of ketamine treatment taken together with the increasingly greater emphasis on combinatorial therapeutic approaches to depression, we sought to asses whether the hypothalamic "hunger hormone", ghrelin, would augment the effects of ketamine. Indeed, ghrelin has recently been found to possess antidepressant potential and may be especially effective against the metabolic and feeding deficits observed with depression. Two studies were performed: 1. mice were given an intraperitoneal injection of ghrelin (80μg/kg) or saline, followed by a saline or a low or high dose of ketamine (5 or 10mg/kg) and 2. mice received 10mg/kg of ketamine together with saline or the ghrelin receptor antagonist JMV2959 (3 or 6mg/kg) and Forced Swim Test (FST) performance was assessed. In both studies, ketamine alone reduced FST immobility. Similarly, ghrelin alone reduced swim immobility suggesting an antidepressant-like response. However, ghrelin did not augment the impact of ketamine when co-administered and in fact, it appeared to antagonize its actions at the lower dose. As well, JMV2959 did not significantly influence FST performance. These data confirm the antidepressant-like effects of ketamine and further suggest that ghrelin might have similar properties. Yet, our results caution against combinatorial treatment with these agents, probably owing to unexpected allosteric or other antagonist actions.

Ghrelin regulates cell cycle-related gene expression in cultured hippocampal neural stem cells

We have previously demonstrated that ghrelin stimulates the cellular proliferation of cultured adult rat hippocampal neural stem cells (NSCs). However, little is known about the molecular mechanisms by which ghrelin regulates cell cycle progression. The purpose of this study was to investigate the potential effects of ghrelin on cell cycle regulatory molecules in cultured hippocampal NSCs. Ghrelin treatment increased proliferation assessed by CCK-8 proliferation assay. The expression levels of proliferating cell nuclear antigen and cell division control 2, well-known cell-proliferating markers, were also increased by ghrelin. Fluorescence-activated cell sorting analysis revealed that ghrelin promoted progression of cell cycle from G0/G1 to S phase, whereas this progression was attenuated by the pretreatment with specific inhibitors of MEK/extracellular signal-regulated kinase 1/2, phosphoinositide 3-kinase/Akt, mammalian target of rapamycin, and janus kinase 2/signal transducer and activator of transcription 3. Ghrelin-induced proliferative effect was associated with increased expression of E2F1 transcription factor in the nucleus, as determined by Western blotting and immunofluorescence. We also found that ghrelin caused an increase in protein levels of positive regulators of cell cycle, such as cyclin A and cyclin-dependent kinase (CDK) 2. Moreover, p27(KIP1) and p57(KIP2) protein levels were reduced when cell were exposed to ghrelin, suggesting downregulation of CDK inhibitors may contribute to proliferative effect of ghrelin. Our data suggest that ghrelin targets both cell cycle positive and negative regulators to stimulate proliferation of cultured hippocampal NSCs.

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.

Driving the need to feed: Insight into the collaborative interaction between ghrelin and endocannabinoid systems in modulating brain reward systems

Independent stimulation of either the ghrelin or endocannabinoid system promotes food intake and increases adiposity. Given the similar distribution of their receptors in feeding associated brain regions and organs involved in metabolism, it is not surprising that evidence of their interaction and its importance in modulating energy balance has emerged. This review documents the relationship between ghrelin and endocannabinoid systems within the periphery and hypothalamus (HYP) before presenting evidence suggesting that these two systems likewise work collaboratively within the ventral tegmental area (VTA) to modulate non-homeostatic feeding. Mechanisms, consistent with current evidence and local infrastructure within the VTA, will be proposed.

Control of adult neurogenesis by programmed cell death in the mammalian brain

The presence of neural stem cells (NSCs) and the production of new neurons in the adult brain have received great attention from scientists and the public because of implications to brain plasticity and their potential use for treating currently incurable brain diseases. Adult neurogenesis is controlled at multiple levels, including proliferation, differentiation, migration, and programmed cell death (PCD). Among these, PCD is the last and most prominent process for regulating the final number of mature neurons integrated into neural circuits. PCD can be classified into apoptosis, necrosis, and autophagic cell death and emerging evidence suggests that all three may be important modes of cell death in neural stem/progenitor cells. However, the molecular mechanisms that regulate PCD and thereby impact the intricate balance between self-renewal, proliferation, and differentiation during adult neurogenesis are not well understood. In this comprehensive review, we focus on the extent, mechanism, and biological significance of PCD for the control of adult neurogenesis in the mammalian brain. The role of intrinsic and extrinsic factors in the regulation of PCD at the molecular and systems levels is also discussed. Adult neurogenesis is a dynamic process, and the signals for differentiation, proliferation, and death of neural progenitor/stem cells are closely interrelated. A better understanding of how adult neurogenesis is influenced by PCD will help lead to important insights relevant to brain health and diseases.

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's Role in the Hypothalamic-Pituitary-Adrenal Axis Stress Response: Implications for Mood Disorders

Ghrelin is a stomach hormone normally associated with feeding behavior and energy homeostasis. Recent studies highlight that ghrelin targets the brain to regulate a diverse number of functions, including learning, memory, motivation, stress responses, anxiety, and mood. In this review, we discuss recent animal and human studies showing that ghrelin regulates the hypothalamic-pituitary-adrenal axis and affects anxiety and mood disorders, such as depression and fear. We address the neural sites of action through which ghrelin regulates the hypothalamic-pituitary-adrenal axis and associated stress-induced behaviors, including the centrally projecting Edinger-Westphal nucleus, the hippocampus, amygdala, locus coeruleus, and the ventral tegmental area. Stressors modulate many behaviors associated with motivation, fear, anxiety, depression, and appetite; therefore, we assess the potential role for ghrelin as a stress feedback signal that regulates these associated behaviors. Finally, we briefly discuss important areas for future research that will help us move closer to potential ghrelin-based therapies to treat stress responses and related disorders.

Effects of neuroleptics administration on adult neurogenesis in the rat hypothalamus

BACKGROUND

Among many factors influencing adult neurogenesis, pharmacological modulation has been broadly studied. It is proven that neuroleptics positively affect new neuron formation in canonical neurogenic sites - subgranular zone of the hippocampal dentate gyrus and subventricular zone of the lateral ventricles. Latest findings suggest that adult neurogenesis also occurs in several additional regions like the hypothalamus, amygdala, neocortex and striatum. As the hypothalamus is considered an important target of neuroleptics, a hypothesis can be made that these substances are able to modulate local neural proliferation.

METHODS

Experiments were performed on adult male rats injected for 28 days or 1 day by three neuroleptics: olanzapine, chlorpromazine and haloperidol. Immunohistochemistry was used to determine expression of proliferation marker (Ki-67) and the marker of neuroblasts - doublecortin (DCX) - which may inform about drug influence on adult neurogenesis at the level of the hypothalamus.

RESULTS

It was shown that a single injection of antipsychotics causes significant decrease in hypothalamic DCX expression, but after chronic treatment with chlorpromazine, but not olanzapine, there is an increase in the number of newly formed neuroblasts. Haloperidol has the opposite effect - its long-term administration decreases the number of DCX-positive cells. Cell proliferation levels (Ki-67 expression) increase after long-term drug administration, whereas their single doses do not have any modulatory effect on proliferation potential.

CONCLUSIONS

Our results throw a new light on the neuroleptics mechanism of action. They also support the potential role of antipsychotics as a factor that can modulate hypothalamic neurogenesis with putative clinical applications.

The P7C3 class of neuroprotective compounds exerts antidepressant efficacy in mice by increasing hippocampal neurogenesis

Augmenting hippocampal neurogenesis represents a potential new strategy for treating depression. Here we test this possibility by comparing hippocampal neurogenesis in depression-prone ghrelin receptor (Ghsr)-null mice to that in wild-type littermates and by determining the antidepressant efficacy of the P7C3 class of neuroprotective compounds. Exposure of Ghsr-null mice to chronic social defeat stress (CSDS) elicits more severe depressive-like behavior than in CSDS-exposed wild-type littermates, and exposure of Ghsr-null mice to 60% caloric restriction fails to elicit antidepressant-like behavior. CSDS resulted in more severely reduced cell proliferation and survival in the ventral dentate gyrus (DG) subgranular zone of Ghsr-null mice than in that of wild-type littermates. Also, caloric restriction increased apoptosis of DG subgranular zone cells in Ghsr-null mice, although it had the opposite effect in wild-type littermates. Systemic treatment with P7C3 during CSDS increased survival of proliferating DG cells, which ultimately developed into mature (NeuN+) neurons. Notably, P7C3 exerted a potent antidepressant-like effect in Ghsr-null mice exposed to either CSDS or caloric restriction, while the more highly active analog P7C3-A20 also exerted an antidepressant-like effect in wild-type littermates. Focal ablation of hippocampal stem cells with radiation eliminated this antidepressant effect, further attributing the P7C3 class antidepressant effect to its neuroprotective properties and resultant augmentation of hippocampal neurogenesis. Finally, P7C3-A20 demonstrated greater proneurogenic efficacy than a wide spectrum of currently marketed antidepressant drugs. Taken together, our data confirm the role of aberrant hippocampal neurogenesis in the etiology of depression and suggest that the neuroprotective P7C3-compounds represent a novel strategy for treating patients with this disease.

Ghrelin is required for dietary restriction-induced enhancement of hippocampal neurogenesis: lessons from ghrelin knockout mice

Neurogenesis occurs in the adult hippocampus and is enhanced by dietary restriction (DR), and neurogenesis enhancement is paralleled by circulating ghrelin level enhancement. We have previously reported that ghrelin modulates adult neurogenesis in the hippocampus. In order to investigate the possible role of ghrelin in DR-induced hippocampal neurogenesis in adult mice, ghrelin knockout (GKO) mice and wild-type (WT) mice were maintained for 3 months on DR or ad libitum (AL) diets. Protein levels of ghrelin in the stomach and the hippocampus were increased by DR in WT mice. One day after BrdU administration, the number of BrdU-labeled cells in the hippocampal dentate gyrus was decreased in GKO mice maintained on the AL diet. DR failed to alter the proliferation of progenitor cells in both WT and GKO mice. Four weeks after BrdU injection, the number of surviving cells in the dentate gyrus was decreased in AL-fed GKO mice. DR increased survival of newborn cells in WT mice, but not in GKO mice. Levels of brain-derived neurotrophic factor protein in the hippocampus were similar between WT and GKO mice, and were increased by DR both in WT and GKO mice. These results suggest that elevated levels of ghrelin during DR may have an important role in the enhancement of neurogenesis induced by DR.