Stress-dependent and gender-specific neuroregulatory roles of the apelin receptor in the hypothalamic-pituitary-adrenal axis response to acute stress

Sex and stress interactions in fear synchrony of mouse dyads

Socially coordinated threat responses support the survival of animal groups. Given their distinct social roles, males and females must differ in such coordination. Here, we report such differences during the synchronization of auditory-conditioned freezing in mouse dyads. To study the interaction of emotional states with social cues underlying synchronization, we modulated emotional states with prior stress or modified the social cues by pairing unfamiliar or opposite-sex mice. In same-sex dyads, males exhibited more robust synchrony than females. Stress disrupted male synchrony in a prefrontal cortex-dependent manner but enhanced it in females. Unfamiliarity moderately reduced synchrony in males but not in females. In dyads with opposite-sex partners, fear synchrony was resilient to both stress and unfamiliarity. Decomposing the synchronization process in the same-sex dyads revealed sex-specific behavioral strategies correlated with synchrony magnitude: following partners' state transitions in males and retroacting synchrony-breaking actions in females. Those were altered by stress and unfamiliarity. The opposite-sex dyads exhibited no synchrony-correlated strategy. These findings reveal sex-specific adaptations of socio-emotional integration defining coordinated behavior and suggest that sex-recognition circuits confer resilience to stress and unfamiliarity in opposite-sex dyads.

The regulatory effects of the apelin/APJ system on depression: A prospective therapeutic target

Depression is a debilitating neuropsychological disorder characterized by high incidence, high recurrence, high suicide, and high disability rates, which poses serious threats to human health and imposes heavy psychological and economic burdens on family and society. The pathogenesis of depression is extremely complex, and its etiology is multifactorial. Mounting evidence suggests that apelin and apelin receptor APJ, which compose the apelin/APJ system, are related to the development of depression. However, the specific mechanism is still unclear, and research in this area in human is still insufficient. Acceleration of research into the regulatory effects and underlying mechanisms of the apelin/APJ system in depression may identify attractive therapeutic targets and contribute to the development of novel intervention strategies against this devastating psychological disorder. In this review, we mainly discuss the regulatory effects of apelin/APJ system on depression and its potential therapeutic applications.

Immunological characterization of stroke-heart syndrome and identification of inflammatory therapeutic targets

Acute cardiac dysfunction caused by stroke-heart syndrome (SHS) is the second leading cause of stroke-related death. The inflammatory response plays a significant role in the pathophysiological process of cardiac damage. However, the mechanisms underlying the brain-heart interaction are poorly understood. Therefore, we aimed to analysis the immunological characterization and identify inflammation therapeutic targets of SHS. We analyzed gene expression data of heart tissue 24 hours after induction of ischemia stoke by MCAO or sham surgery in a publicly available dataset (GSE102558) from Gene Expression Omnibus (GEO). Bioinformatics analysis revealed 138 differentially expressed genes (DEGs) in myocardium of MCAO-treated compared with sham-treated mice, among which, immune and inflammatory pathways were enriched. Analysis of the immune cells infiltration showed that the natural killer cell populations were significantly different between the two groups. We identified five DIREGs, Aplnr, Ccrl2, Cdkn1a, Irak2, and Serpine1 and found that their expression correlated with specific populations of infiltrating immune cells in the cardiac tissue. RT-qPCR and Western blot methods confirmed significant changes in the expression levels of Aplnr, Cdkn1a, Irak2, and Serpine1 after MCAO, which may serve as therapeutic targets to prevent cardiovascular complications after stroke.

The investigation of apelin and apelin receptor expressions in mouse endometrium during peri-implantation period

BACKGROUND

Fertilization, pre-implantation embryo development, implantation, and decidualization are critical for a healthy pregnancy. Successful implantation requires a competent blastocyst and a receptive uterus. Apelin was purified from the bovine stomach in 1998. Apelin receptor (APJ) is a member of G protein-coupled receptors. Apelin/APJ system's physiological role was shown in cardiovascular system, immune response, stress response, fluid regulation, nutrient uptake, angiogenesis, and adipoinsular axis; however, whether apelin/APJ system plays a role in implantation is unknown. In our study, we aimed to evaluate the localization and expressions of the apelin/APJ system in the peri-implantation period mouse uterus.

METHODS

Uteri and implantation sites were collected from mice on the estrous phase and the 1st, 4th, 5th, 6th, and 8th days of pregnancy. Also, inter-implantation sites were collected from the 5th day of the pregnancy group. Localization and expressions of apelin and APJ were determined by immunohistochemistry and Western blot, respectively.

RESULTS

Apelin and APJ were expressed in the luminal and gland epithelium, the stroma of all experimental groups. Two isoforms of apelin-8 and 16 kDa were detected by Western blot. While apelin expression increased from the estrous to the 8th day of pregnancy, APJ expression increased from the estrous to the 4th day of pregnancy, reached the highest expression level, then decreased.

CONCLUSIONS

Our findings suggest that the apelin/APJ system might be involved in implantation and decidualization. Our findings will guide further studies and may help elucidate the underlying causes of implantation failure and pregnancy loss.

Apelin-13 Improves Cognitive Impairment and Repairs Hippocampal Neuronal Damage by Activating PGC-1α/PPARγ Signaling

Alzheimer's disease (AD) is a complex neurodegenerative disease that is prevalent around the world. Both Apelin-13 and proliferator-activated receptor-γ (PPARγ)/PPARγ co-activator 1α (PGC-1α) are regarded as candidate targets for treating AD. The investigation examined whether Apelin-13 exerts neuroprotective effects via PGC-1α/PPARγ signaling. In this study, Apelin-13 improved cognitive deficits in AD mice, while SR-18,292 (a PGC-1α inhibitor) interfered with the therapeutic effects of Apelin-13. Mechanistically, Apelin-13, PGC-1α and PPARγ were decreased in AD mice and oxygen-glucose deprivation (OGD)-induced neuronal cells. Apelin-13 bound to PGC-1α and negatively regulated the expression of PGC-1α and PPARγ. In turn, PGC-1α accelerated the accumulation of Apelin-13 and PPARγ. Additionally, neuronal apoptosis was inhibited, and the abundance of apoptosis-related proteins (Bax, Bcl-2, and cleaved caspase 3) was induced. The content of reactive oxygen species (ROS), malondialdehyde (MDA) and superoxide dismutase (SOD) fluctuated. The level of inflammatory factors (interleukin-6, IL-6, IL-10, tumor necrosis factor-α, TNF-α) was regulated. In short, Apelin-13 exerted anti-apoptosis, anti-oxidant stress and anti-inflammatory effects. Interestingly, PGC-1α silencing promoted neuronal apoptosis, oxidant stress and inflammation, and overexpression of PGC-1α exhibited the opposite. More importantly, inhibition of PGC-1α attenuated Apelin-13-enhanced cognitive impairment and neuronal damage. Therefore, our findings suggested that Apelin-13 exerted neuroprotective effects in part via the PGC-1α/PPARγ pathway.

The Apelin/APLNR system modulates tumor immune response by reshaping the tumor microenvironment

Apelin is an endogenous ligand of the Apelin receptor (APLNR), a seven-transmembrane G protein-coupled receptor, which is widely distributed in human tissue. The Apelin/APLNR system is involved in regulating several physiological and pathological processes. The Apelin expression is increased in a variety of cancer and the Apelin/APLNR system could regulate the development of tumors through mediating autophagy, apoptosis, pyroptosis, and other biological processes to regulate tumor cell proliferation, migration, and invasion. The Apelin/APLNR system also participates in immune response and immune regulation through PI3K-Akt, ERK-MAPK, and other signal pathways. The latest research points out that there is a negative regulatory relationship between APLNR and immune checkpoint PD-L1. In this review, we outline the significance of the Apelin/APLNR signaling pathway in tumorigenesis and its immune regulation. These endeavors provide new insights into the translational application of Apelin/APLNR in cancer and may contribute to the promotion of more effective treatments for cancers.

The first report on predictive comparative ligand-based multi-QSAR modeling analysis of 4-pyrimidinone and 2-pyridinone based APJ inhibitors

The APJ system participates in several major disorders including cancer. A multi-QSAR modeling study on some APJ inhibitors was performed for the first time. Some potential molecules were also designed based on the QSAR study conducted.

The Role of Apelin-APJ System in Diabetes and Obesity

Nowadays, diabetes and obesity are two main health-threatening metabolic disorders in the world, which increase the risk for many chronic diseases. Apelin, a peptide hormone, exerts its effect by binding with angiotensin II protein J receptor (APJ) and is considered to be linked with diabetes and obesity. Apelin and its receptor are widely present in the body and are involved in many physiological processes, such as glucose and lipid metabolism, homeostasis, endocrine response to stress, and angiogenesis. In this review, we summarize the literatures on the role of the Apelin-APJ system in diabetes and obesity for a better understanding of the mechanism and function of apelin and its receptor in the pathophysiology of diseases that may contribute to the development of new therapies.

Identification of a Hydroxypyrimidinone Compound () as a Potent APJ Receptor Agonist for the Potential Treatment of Heart Failure

This paper describes our continued efforts in the area of small-molecule apelin receptor agonists. Recently disclosed compound 2 showed an acceptable metabolic stability but demonstrated monodemethylation of the dimethoxyphenyl group to generate atropisomer metabolites in vitro. In this article, we extended the structure-activity relationship at the C2 position that led to the identification of potent pyrazole analogues with excellent metabolic stability. Due to the increased polarity at C2, the permeability for these compounds decreased. Further adjustment of the polarity by replacing the N1 2,6-dimethoxyphenyl group with a 2,6-diethylphenyl group and reoptimization for the potency of the C5 pyrroloamides resulted in potent compounds with improved permeability. Compound 21 displayed excellent pharmacokinetic profiles in rat, monkey, and dog models and robust pharmacodynamic efficacy in the rodent heart failure model. Compound 21 also showed an acceptable safety profile in preclinical toxicology studies and was selected as a backup development candidate for the program.

Study Progression of Apelin/APJ Signaling and Apela in Different Types of Cancer

Apelin is an endogenous ligand that binds to the G protein-coupled receptor angiotensin-like-receptor 1 (APJ). Apelin and APJ are widely distributed in organs and tissues and are involved in multiple physiological and pathological processes including cardiovascular regulation, neuroendocrine stress response, energy metabolism, etc. Additionally, apelin/APJ axis was found to play an important role in cancer development and progression. Apela is a newly identified endogenous ligand for APJ. Several studies have revealed the potential role of Apela in cancers. In this article, we review the current studies focusing on the role of apelin/APJ signaling and Apela in different cancers. Potential mechanisms by which apelin/APJ and Apela mediate the regulation of cancer development and progression were also mentioned. The Apelin/APJ signaling and Apela may serve as potential therapeutic candidates for treatment of cancer.

The role of autonomic pathways in peripheral apelin-induced gastrointestinal dysmotility: involvement of the circumventricular organs

NEW FINDINGS

What is the central question of this study? Are central autonomic pathways and circumventricular organs involved in apelin-induced inhibition of gut motility? What is the main finding and its importance? Peripherally administered apelin-13 inhibits gastric and colonic motor functions through sympathetic and parasympathetic autonomic pathways, which seems to be partly mediated by the apelin receptor in circumventricular organs.

ABSTRACT

Peripheral administration of apelin-13 has been shown to inhibit gastrointestinal (GI) motility, but the relevant mechanisms are incompletely understood. This study aimed to investigate (i) whether the apelin receptor (APJ) is expressed in circumventricular structures involved in autonomic functions, (ii) whether they are activated by peripherally administered apelin, (iii) the role of autonomic pathways in peripheral exogenous apelin-induced GI dysmotility, and (iv) the changes in apelin levels in the extracellular environment of the brain following its peripheral application. Ninety minutes after apelin-13 administration (300 μg kg^-1 , i.p.), gastric emptying (GE) and colon transit (CT) were measured in rats that underwent parasympathectomy and/or sympathectomy. Plasma and cerebrospinal fluid (CSF) samples were also collected from another group of rats that received apelin-13 or vehicle injection. The immunoreactivities for APJ and c-Fos in circumventricular organs (CVOs) were evaluated by immunohistochemistry. Compared with vehicle-treated rats, GE and CT were inhibited significantly by apelin-13 treatment, and were completely restored in animals that underwent the combination of parasympathectomy and sympathectomy and sympathectomy alone, respectively. Apelin concentrations were elevated in both plasma and CSF following peripheral administration of apelin-13. APJ expression was detected in area postrema (AP), subfornical organ and organum vasculosum of lamina terminalis, and c-Fos expression was observed in response to apelin injection. Apelin-induced c-Fos expression in AP was partially attenuated by pretreatment with the cholecystokinin-1 receptor antagonist lorglumide, whereas it was completely abolished in vagotomized rats. The present data suggest that APJ in CVOs could indirectly contribute to the inhibitory action of peripheral apelin on GI motor functions.

Aplnr knockout mice display sex-specific changes in conditioned fear

The G-protein-coupled receptor APLNR and its ligands apelin and ELABELA/TODDLER/apela comprise the apelinergic system, a signaling pathway that is critical during development and physiological homeostasis. Targeted regulation of the receptor has been proposed to treat several important diseases including heart failure, pulmonary arterial hypertension and metabolic syndrome. The apelinergic system is widely expressed within the central nervous system (CNS). However, the role of this system in the CNS has not been completely elucidated. Utilizing an Aplnr knockout mouse model, we report here results from tests of sensory ability, locomotion, reward preference, social preference, learning and memory, and anxiety. We find that knockout of Aplnr leads to significant effects on acoustic startle response and sex-specific effects on conditioned fear responses without significant changes in baseline anxiety. In particular, male Aplnr knockout mice display enhanced context- and cue-dependent fear responses. Our results complement previous reports that exogenous Apelin administration reduced conditioned fear and freezing responses in rodent models, and future studies will explore the therapeutic benefit of APLNR-targeted drugs in rodent models of PTSD.

Apelin Receptor Signaling Protects GT1-7 GnRH Neurons Against Oxidative Stress In Vitro

Hypothalamic-pituitary-adrenal (HPA) axis regulates stress response in the body and abnormal increase in oxidative stress contributes to the various disease pathogenesis. Although hypothalamic distribution of Apelin receptor (APLNR) has been studied, the potential regulatory role in hormone releasing function of hypothalamus in response to stress is not well elucidated yet. To determine whether APLNR is involved in the protection of the hypothalamus against oxidative stress, gonadotropin-releasing hormone (GnRH) cells were used as an in vitro model system. GT1-7 mouse hypothalamic neuronal cell line was subjected to H₂O₂ and hypoxia induced oxidative stress under various circumstances including APLNR overexpression, knockdown and knockout. Overexpression and activation of APLNR in GnRH producing neurons caused an increase in cell proliferation under oxidative stress. In addition, blockage of APLNR function by siRNA reduced GnRH release. Activation of APLNR initiated AKT kinase pathway as a proliferative response against hypoxic culture conditions and blocked apoptosis. Although expression and activation of APLNR have not been related to GnRH neuron differentiation during development, positive contribution of activated APLNR signaling to GnRH release in mouse embryonic stem cell derived GnRH neurons was observed in the present study. Sustained overexpression and complete deletion of APLNR in mouse embryonic stem cell derived GnRH neurons reduced GnRH release in vitro. The present findings suggest that expression and activation of APLNR in GnRH releasing GT1-7 neurons might induce a protective mechanism against oxidative stress induced cell death and APLNR signaling may play a role in GnRH neurons.

Interaction between stress hormones and phagocytic cells and its effect on the health status of dairy cows: A review

Dairy cows are exposed to various stressors during their production cycle that makes them more susceptible to various diseases. Phagocytes (neutrophils and macrophages) are important soldiers of the innate immune system. Neutrophils are the first responders to an inflammatory response and stress and kill pathogens by generating reactive oxygen species and by the release of various antimicrobial peptides, enzymes, neutrophil extracellular trap formation, etc. Macrophages, the other phagocytes, are also the cleanup crew for the innate immune system that removes debris, pathogens, and dead neutrophils later on after an inflammatory response. The neuroendocrine system along with phagocytes exhibits an immunomodulatory potential during stressful conditions. Neuroendocrine system directly affects the activity of phagocytes by communicating bidirectionally through shared receptors and messenger molecules such as hormones, neurotransmitters, or cytokines. Different immune cells may show variable responses to each hormone. Short time exposure to stress can be beneficial, but repeated or extended exposure to stress may be detrimental to the overall health and well-being of an animal. Although some stresses associated with farming practices in dairy cows are unavoidable, better understanding of the interactions occurring between various stress hormones and phagocytic cells can help to reduce stress, improve productivity and animal welfare. This review highlights the role played by various stress hormones in modulating phagocytic cell performance of dairy cattle under inflammatory conditions.

Serum Levels of Chemerin, Apelin, and Adiponectin in Relation to Clinical Symptoms, Quality of Life, and Psychological Factors in Irritable Bowel Syndrome

BACKGROUND

Adipokines have endocrine roles in metabolism and immunity. Dysregulation of adipokine levels is associated with several diseases with chronic inflammation. We aimed to assess the serum concentrations of chemerin, apelin, and adiponectin in irritable bowel syndrome (IBS). Furthermore, we evaluated the possible association of these adipokines with clinical symptoms, quality of life (QoL), and psychological factors.

MATERIALS AND METHODS

In this case-control study, 114 male and female IBS patients were recruited from outpatient clinics. Along with the IBS patients, 114 sex and age-matched healthy volunteers were recruited. Patients filled in the questionnaires of the IBS severity scoring system (IBSSS), gastrointestinal (GI) and somatic symptoms, IBS specific QoL (IBS-QoL), and psychological disorders, and went to the lab for blood sampling.

RESULTS

Serum levels of both adiponectin and apelin were significantly (P=0.04, 0.03, respectively) lower, whereas chemerin was significantly (P=0.01) higher in IBS patients. Chemerin was higher in IBS-D compared with both IBS-C and IBS-A, while apelin and adiponectin were not different between subtypes. After adjustments for confounders only, chemerin had a positive association with IB severity scoring system and GI symptoms. Furthermore, chemerin had positive associations, whereas apelin and adiponectin had inverse associations with somatic symptoms and psychological factors. There were no significant associations between adipokines including chemerin, apelin, and adiponectin, and IBS-QoL.

CONCLUSIONS

Chemerin had significant associations with both the severity of clinical symptoms and psychological factors in IBS; thus, it could be considered as a potential therapeutic target in these patients; however, further studies are needed.

The Apelin/APJ System in Psychosis and Neuropathy

Apelin, an endogenous neuropeptide, has been identified as the cognate ligand for the G-protein-coupled receptor APJ. Apelin, APJ messenger RNA, and protein are widely expressed in the central nervous system and peripheral tissues of humans and animals. The apelin/APJ system has been implicated in diverse physiological and pathological processes. The present article reviews the progress of the latest research investigating the apelin/APJ system in pain, depression, anxiety, memory, epilepsy, neuroprotection, stroke, and brain injury and protection, and highlights its promising potential as a therapeutic target for treatment of psychosis and neuropathy.

GABA-A receptor modulating steroids in acute and chronic stress; relevance for cognition and dementia?

Cognitive dysfunction, dementia and Alzheimer's disease (AD) are increasing as the population worldwide ages. Therapeutics for these conditions is an unmet need. This review focuses on the role of the positive GABA-A receptor modulating steroid allopregnanolone (APα), it's role in underlying mechanisms for impaired cognition and of AD, and to determine options for therapy of AD. On one hand, APα given intermittently promotes neurogenesis, decreases AD-related pathology and improves cognition. On the other, continuous exposure of APα impairs cognition and deteriorates AD pathology. The disparity between these two outcomes led our groups to analyze the mechanisms underlying the difference. We conclude that the effects of APα depend on administration pattern and that chronic slightly increased APα exposure is harmful to cognitive function and worsens AD pathology whereas single administrations with longer intervals improve cognition and decrease AD pathology. These collaborative assessments provide insights for the therapeutic development of APα and APα antagonists for AD and provide a model for cross laboratory collaborations aimed at generating translatable data for human clinical trials.

Anxiolytic impact of Apelin-13 in a rat model of Alzheimer's disease: Involvement of glucocorticoid receptor and FKBP5

Apelin-13 is known to be one of the predominant neuropeptides with marked protective role in circuits involved in mood disturbances. The most putative hypothesis in pathophysiology of Alzheimer's disease (AD) is Amyloid beta (Aβ) aggregation which interrupt proper function of hypothalamic-pituitary-adrenal (HPA) axis and are associated with anxiety. Here, we assessed the potential anxiolytic effect of Apelin-13 in a rodent cognitive impairment model induced by intrahippocampal Aβ 25-35 administration. We evaluated the memory impairment and anxiogenic behavior using shuttle box and Elevated plus maze apparatuses. We also measured the glucocorticoid receptor (GR) and FK506 binding protein 51 (FKBP5) expression as important markers showing the proper feedback mechanism within the HPA axis. Our findings showed that Aβ 25-35 administration induced memory impairment and anxiety behaviors. Apelin-13 exerted the anxiolytic effects and provided protection against Aβ 25-35 -induced passive avoidance memory impairment. Moreover, Apelin-13 caused an increase in GR and a decrease in FKBP5 expression levels in Aβ 25-35 treated animals. Taken together, these findings showed the anxiolytic effect of Apelin-13. This effect at least in part, may be mediated through the regulation of GR and FKBP5 expression levels which have a pivotal role in the appropriate negative feedback mechanism within the HPA axis. These data suggest that Apelin-13 might be considered as a potential neuropeptide defense that reduces anxiety along with neuroprotective effect against the Aβ 25-35 -induced injury.

The Role of Apelin in the Functioning of the Reproductive System

Adipokine apelin through the apelin receptors activates a wide range of signaling cascades in the target cells and controls their growth, differentiation, apoptosis, and energy metabolism. In the recent years, the evidence has been obtained that all components of the hypothalamic-pituitary-gonad axis, in which apelin and its receptor are expressed, are targets of apelin. In the hypothalamus, apelin modulates the activity of the melanocortin and ghrelin systems and indirectly affects the production of gonadoliberin. In the ovaries, it controls the growth and maturation of the follicles, stimulates the angiogenesis, and affects the basal and stimulated by the other factors steroidogenic activity in follicular cells. The changes in the apelin signaling system are closely associated with dysfunctions of the female reproductive system, such as polycystic ovary syndrome, endometriosis, and cancer. Information on the regulation of the male reproductive system by apelin is limited to animal studies showing the effect of apelin on the hypothalamic components of the gonad axis. The participation of apelin in the regulation of the reproductive system opens up the broad opportunities for the development of new approaches for the correction of abnormalities in this system and for the treatment of infertility.

Distinct Proteomic, Transcriptomic, and Epigenetic Stress Responses in Dorsal and Ventral Hippocampus

BACKGROUND

Acutely stressful experiences can trigger neuropsychiatric disorders and impair cognitive processes by altering hippocampal function. Although the intrinsic organization of the hippocampus is highly conserved throughout its long dorsal-ventral axis, the dorsal (anterior) hippocampus mediates spatial navigation and memory formation, whereas the ventral (posterior) hippocampus is involved in emotion regulation. To understand the molecular consequences of stress, detailed genome-wide screens are necessary and need to distinguish between dorsal and ventral hippocampal regions. While transcriptomic screens have become a mainstay in basic and clinical research, proteomic methods are rapidly evolving and hold even greater promise to reveal biologically and clinically relevant biomarkers.

METHODS

Here, we provide the first combined transcriptomic (RNA sequencing) and proteomic (sequential window acquisition of all theoretical mass spectra [SWATH-MS]) profiling of dorsal and ventral hippocampus in mice. We used three different acute stressors (novelty, swim, and restraint) to assess the impact of stress on both regions.

RESULTS

We demonstrated that both hippocampal regions display radically distinct molecular responses and that the ventral hippocampus is particularly sensitive to the effects of stress. Separately analyzing these structures greatly increased the sensitivity to detect stress-induced changes. For example, protein interaction cluster analyses revealed a stress-responsive epigenetic network around histone demethylase Kdm6b restricted to the ventral hippocampus, and acute stress reduced methylation of its enzymatic target H3K27me3. Selective Kdm6b knockdown in the ventral hippocampus led to behavioral hyperactivity/hyperresponsiveness.

CONCLUSIONS

These findings underscore the importance of considering dorsal and ventral hippocampus separately when conducting high-throughput molecular analyses, which has important implications for fundamental research as well as clinical studies.

Central apelin administration and restraint stress induce hypothalamic cholecystokinin release via the APJ receptor

Exposure to an acute stressor induces up-regulation of apelin and cholecystokinin (CCK) in the hypothalamic paraventricular nucleus (PVN), which is the key brain centre integrating the stress-induced alterations in neuroendocrine, autonomic and behavioural functions. We tested the hypothesis that the release of CCK from the PVN is increased by centrally administered or stress-induced up-regulated endogenous apelin via the APJ receptor. Additionally, the effect of hypothalamic CCK on autonomic outflow was investigated under basal and stressed conditions. In vivo brain microdialysis was performed in rats that received (i) intra-PVN administration of apelin-13 or (ii) acute restraint stress (ARS). For chemical stimulation of the neurones in the PVN, a high concentration of KCl was applied by reverse microdialysis. CCK-8 levels in microdialysates were quantified by an enzyme immunoassay. The immunoreactivity of the APJ receptor and CCK was detected by immunofluorescence in hypothalamic sections. Heart rate variability was assessed in rats that received PVN stimulation or ARS following pre-administration of vehicle or CCK1 receptor antagonist lorglumide. Both intra-PVN exogenous apelin-13 and ARS increased the CCK-8 levels in dialysates significantly. The ARS-induced elevations in CCK levels were reversed by intra-PVN pre-administration of the APJ receptor antagonist F13A. Within the PVN, robust APJ receptor expression was detected on the CCK-producing mediocellular cells, in addition to the parvocellular neurones in the periventricular region. Dual immunoreactivity of APJ/CCK was observed in magnocellular cells to a lesser degree. Both exogenous apelin and ARS increased the CCK immunoreactivity markedly within the PVN, which was diminished significantly by F13A. Sympathetic tonus was increased markedly both by PVN stimulation and ARS, which was attenuated by lorglumide. These results revealed the interaction between apelin and CCK in the brain, suggesting that hypothalamic CCK may contribute to the apelin-induced alterations in autonomic outflow under stressed conditions.

Deletion of exchange proteins directly activated by cAMP (Epac) causes defects in hippocampal signaling in female mice

Previous studies demonstrate essential roles for the exchange proteins directly activated by cAMP 1 and 2 (Epac1 and Epac2; here collectively referred to as Epac) in the brain. In the hippocampus, Epac contributes to the control of neuronal growth and differentiation and has been implicated in memory and learning as well as in anxiety and depression. In the present study we address the hypothesis that Epac affects hippocampal cellular responses to acute restraint stress. Stress causes activation of the hypothalamus-pituitary-adrenal (HPA)-axis, and glucocorticoid receptor (GR) signaling is essential for proper feedback regulation of the stress response, both in the brain and along the HPA axis. In the hippocampus, GR expression is regulated by cAMP and the brain enriched micro RNA miR-124. Epac has been associated with miR-124 expression in hippocampal neurons, but not in regulation of GR. We report that hippocampal expression of Epac1 and Epac2 increased in response to acute stress in female wild type mice. In female mice genetically deleted for Epac, nuclear translocation of GR in response to restraint stress was significantly delayed, and moreover, miR-124 expression was decreased in these mice. Male mice lacking Epac also showed abnormalities in miR-124 expression, but the phenotype was less profound than in females. Serum corticosterone levels were slightly altered immediately after stress in both male and female mice deleted for Epac. The presented data indicate that Epac1 and Epac2 are involved in controlling cellular responses to acute stress in the mouse hippocampus and provide novel insights into the underlying transcriptional and signaling networks. Interestingly, we observe sex specific differences when Epac is deleted. As the incidence and prevalence of stress-related diseases are higher in women than in men, the Epac knockout models might serve as genetic tools to further elucidate the cellular mechanisms underlying differences between male and female with regard to regulation of stress.

Novel transmitters in brain stem vagal neurocircuitry: new players on the pitch

The last few decades have seen a major increase in the number of neurotransmitters and neuropeptides recognized as playing a role in brain stem neurocircuits, including those involved in homeostatic functions such as stress responsiveness, gastrointestinal motility, feeding, and/or arousal/wakefulness. This minireview will focus on the known physiological role of three of these novel neuropeptides, i.e., apelin, nesfatin-1, and neuropeptide-S, with a special emphasis on their hypothetical roles in vagal signaling related to gastrointestinal motor functions.

Xiaoyaosan Improves Depressive-Like Behaviors in Mice through Regulating Apelin-APJ System in Hypothalamus

Background: The apelin-APJ system has been considered to play a crucial role in HPA axis function, and how the traditional Chinese compound prescription Xiaoyaosan regulates the apelin-APJ system as a supplement to treat depressive disorders. Objective: To investigate the depression-like behaviors and expression of apelin and APJ in hypothalamus of chronic unpredictable mild stress (CUMS) mice and study whether these changes related to the regulation of Xiaoyaosan. Methods: 60 adult C57BL/6J mice were randomly divided into four groups, including control group, CUMS group, Xiaoyaosan treatment group and fluoxetine treatment group. Mice in the control group and CUMS group received 0.5 mL physiological saline once a day by intragastric administration. Mice in two treatment groups received Xiaoyaosan (0.25 g/kg/d) and fluoxetine (2.6 mg/kg/d), respectively. After 21 days of modeling with CUMS, the expression of apelin and APJ in hypothalamus were measured by real-time fluorescence quantitative PCR, western blot and immunohistochemical staining. The physical condition, body weight, food intake and behavior tests such as open field test, sucrose preference test and force swimming test were measured to evaluate depressive-like behaviors. Results: In this study, significant behavioral changes were found in CUMS-induced mice, meanwhile the expressions of apelin and APJ in the hypothalamus were changed after modeling. The body weight, food-intake and depressive-like behaviors in CUMS-induced mice could be improved by Xiaoyaosan treatment which is similar with the efficacy of fluoxetine, while the expressions of apelin and APJ in hypothalamus were modified by Xiaoyaosan. Conclusions: The data suggest that apelin-APJ system changes in the hypothalamus may be a target of depressive disorders, and the beneficial effects of Chinese compound prescription Xiaoyaosan on depressive-like behaviors may be mediated by the apelin-APJ system.

Apelin in Reproductive Physiology and Pathology of Different Species: A Critical Review

Apelin has been isolated from the bovine stomach extracts as an endogenous ligand of the previously orphan receptor APJ. Expression of the apelinergic system (apelin and APJ) was described in many organs where pleiotropic effects like regulation of food intake, body weight, or cardiovascular and immune function were described. Recent studies have shown that apelin also plays an important role in the regulation of female and male reproduction. Some data showed that the gene and protein of apelin/APJ are expressed in the hypothalamic-pituitary-gonad (HPG) axis tissue. Thus, apelin is synthesized locally in the hypothalamus, pituitary, ovaries, and testis of many species and has autocrine and/or paracrine effects. Most research indicates that apelin has an inhibitory effect on gonadotropin secretion and participates in the direct regulation of steroidogenesis, cell proliferation, and apoptosis in gonads. The article summarizes also results of a series of recent studies on the effect of apelin on reproduction pathology, like polycystic ovarian syndrome, endometriosis, and ovarian cancer. Many of these pathologies are still in critical need of therapeutic intervention, and recent studies have found that apelin can be targets in reproductive pathological states.

Apelin-APJ system is responsible for stress-induced increase in atrial natriuretic peptide expression in rat heart

BACKGROUND

The cardiovascular system is a primary target of stress and stress is the most important etiologic factor in cardiovascular diseases. Stressors increase expressions of atrial natriuretic peptide (ANP) and apelin in cardiac tissue.

AIM

The aim of the present study was to investigate whether stress-induced apelin has an effect on the expression of ANP in the right atrium of rat heart.

METHODS

The rats were divided into the control, stress and F13A+stress groups. In the stress and F13A+stress groups, the rats were subjected to water immersion and restraint stress (WIRS) for 6h. In the F13A+stress group, apelin receptor antagonist F13A, was injected intravenously immediately before application of WIRS. The plasma samples were obtained for the measurement of corticosterone and atrial natriuretic peptide. The atrial samples were used for immunohistochemistry and western blot analysis.

RESULTS

F13A administration prevented the rise of plasma corticosterone and ANP levels induced by WIRS. While WIRS application increased the expressions of apelin, HIF-1α and ANP in atrial tissue, while F13A prevented the stress-induced increase in the expression of HIF-1α and ANP.

CONCLUSION

Stress-induced apelin induces ANP expression in atrial tissue and may play a role in cardiovascular homeostasis by increasing ANP expression under WIRS conditions.

Apelin/APJ system and cancer

Apelin is an endogenous ligand of the apelin receptor (APJ), a seven-transmembrane G protein-coupled receptor. Apelin/APJ system has a wide tissue distribution in the brain as well as in peripheral organs including heart, lung, vessels, and adipose tissue. Apelin/APJ was involved in regulating cardiac and vascular function, heart development, and vascular smooth muscle cell proliferation. In this article, we summarize the role of apelin/APJ system on lung cancer, gastroesophageal and colonic cancer, hepatocellular carcinoma, prostate cancer, endometrial cancer, oral squamous cell carcinoma, brain cancer, and tumor neoangiogenesis. Apelin/APJ may be a potential anticancer therapeutic target.

Apelin-13 Protects PC12 Cells from Corticosterone-Induced Apoptosis Through PI3K and ERKs Activation

It is widely accepted that environmental stress is a risk factor for mental disorders. Glucocorticoid hormones play a vital role in the regulation of physiological response to stress. High concentrations of corticosterone can induce cellular damage in PC12 cells, which possess typical neuronal features. Apelin and its receptor APJ are widely distributed in the central nervous system including limbic structures involved in stress responses. Previous studies have suggested that apelin has a neuroprotective function. However, the effect of apelin on corticosterone-induced neuronal damage remains to be elucidated. In the present study, we explored the potential protective activity of apelin-13 in PC12 cells treated with corticosterone and its underling mechanisms. The viability of the cells, the apoptosis of the cells, the level of phosphorylation of Akt (p-Akt) and extracellular signal-regulated kinases (p-ERKs) and cleaved caspase-3 expression were detected by MTT, Hoechst staining and flow cytometer assays and Western blotting. Results showed that corticosterone induced cells viability loss, cell apoptosis, down-regulation of p-Akt and p-ERKs and up-regulation of cleaved caspase-3. The effects induced by corticosterone were attenuated by apelin-13 pretreatment. Furthermore, apelin-13-mediated anti-viability loss, antiapoptosis and caspase-3 suppression activities were blocked by specific inhibitors of phosphatidylinositol 3-kinase (PI3K) (LY294002) and ERKs (PD98059). The data suggest that apelin-13 protects PC12 cells from corticosterone-induced apoptosis through activating PI3K/Akt and ERKs signaling pathways.

Apelin-13 exerts antidepressant-like and recognition memory improving activities in stressed rats

Apelin is the endogenous ligand for the G-protein-coupled receptor (APJ). The localization of APJ in limbic structures suggests a potential role for apelin in emotional processes. However, the role of apelin in the regulation of stress-induced responses such as depression and memory impairment is largely unknown. In the present study, we evaluated the role of apelin-13 in the regulation of stress-induced depression and memory impairment in rats. We report that repeated intracerebroventricular injections of apelin-13 reversed behavioral despair (immobility) in the forced swim (FS) test, a model widely used for the selection of new antidepressant agents. Apelin-13 also reversed behavioral deficits (escape failure) in the learned helplessness test. The magnitude of the antiimmobility and anti-escape failure effects of apelin-13 was comparable to that of imipramine, a classic antidepressant used as a positive control. Rats exposed to FS stress showed memory performance impairment in the novel object recognition test, and this impairment was improved by apelin-13 treatment. Apelin-13 did not affect recognition memory performance in non-stressed rats. Furthermore, the pretreatment of LY294002 (PI3K inhibitors) or PD98059 (ERK1/2 inhibitor) blocked apelin-13-mediated activities in FS-stressed rats. These findings suggest that apelin-13 exerts antidepressant-like and recognition memory improving activities through activating PI3K and ERK1/2 signaling pathways in stressed rats.

Central apelin mediates stress-induced gastrointestinal motor dysfunction in rats

Apelin, an endogenous ligand for APJ receptor, has been reported to be upregulated in paraventricular nucleus (PVN) following stress. Central apelin is known to stimulate release of corticotropin-releasing factor (CRF) via APJ receptor. We tested the hypothesis that stress-induced gastrointestinal (GI) dysfunction is mediated by central apelin. We also assessed the effect of exogenous apelin on GI motility under nonstressed (NS) conditions in conscious rats. Prior to solid gastric emptying (GE) and colon transit (CT) measurements, APJ receptor antagonist F13A was centrally administered under NS conditions and following acute stress (AS), chronic homotypic stress (CHS), and chronic heterotypic stress (CHeS). Plasma corticosterone was assayed. Strain gage transducers were implanted on serosal surfaces of antrum and distal colon to record postprandial motility. Stress exposure induced coexpression of c-Fos and apelin in hypothalamic PVN. Enhanced hypothalamic apelin and CRF levels in microdialysates were detected following AS and CHeS, which were negatively and positively correlated with GE and CT, respectively. Central F13A administration abolished delayed GE and accelerated CT induced by AS and CHeS. Central apelin-13 administration increased the plasma corticosterone and inhibited GE and CT by attenuating antral and colonic contractions. The inhibitory effect elicited by apelin-13 was abolished by central pretreatment of CRF antagonist CRF9-41 in antrum, but not in distal colon. Central endogenous apelin mediates stress-induced changes in gastric and colonic motor functions through APJ receptor. The inhibitory effects of central exogenous apelin-13 on GI motility appear to be partly CRF dependent. Apelin-13 inhibits colon motor functions through a CRF-independent pathway.

Long-term continuous allopregnanolone elevation causes memory decline and hippocampus shrinkage, in female wild-type B6 mice

Chronic stress in various forms increases the risk for cognitive dysfunction, dementia and Alzheimer's disease. While the pathogenesis behind these findings is unknown, growing evidence suggests that chronic increase in neurosteroid levels, such as allopregnanolone, is part of the mechanism. We treated wild-type C57BL/6J mice with allopregnanolone for 5months, using osmotic pumps. This treatment led to moderately increased levels of allopregnanolone, equivalent to that of mild chronic stress. After an interval of no treatment for 1month, female mice showed impaired learning and memory function in the Morris water maze (MWM) in combination with diminished hippocampus weight and increased cerebellum weight, both correlating to MWM performance. Male mice showed a minor reduction in memory function and no differences in brain structure. We conclude that chronic allopregnanolone elevation can lead to cognitive dysfunction and negative brain alterations. We suggest that allopregnanolone could play a key role in the pathogenesis of stress-induced cognitive disturbances and perhaps dementia.

Effects of Embryo Transfer on Emotional Behaviors in C57BL/6 Mice

Microbiologic standardization plays a key role in the management of animal facilities because contamination of stock could affect the health status and wellbeing of animals and thereby induce artifacts in biomedical research. One common method to avoid the dissemination of pathogens is embryo transfer (ET). Although disturbances in the perinatal environment may cause long-lasting effects on the behavior and physiology of mouse offspring, the influences of ET during this sensitive phase have not yet been addressed. Our study investigated the effects of various components of ET (anesthesia, surgery, recipient strain) on the behavior of dams (exploration, nest-building) and offspring (nest-building, exploration, anxiety, and social and depressive-like behaviors). For ET, the donor strain C57BL/6N and a standard protocol were used. Whereas treatment with anesthesia-analgesia did not affect maternal behavior, female offspring demonstrated overall effects on weight gain and corticosterone levels. Compared with naturally delivered female offspring, dams obtained through ET demonstrated decreased exploration and nest-building. In addition, female ET-derived offspring had enhanced levels of anxiety and increased social interest. Furthermore, ET-derived dams obtained by using NMRI as the recipient strain showed increased exploratory behavior compared with that of dams obtained by using C57 mice as recipients. Compared with using C57 as recipients, both sexes of offspring transferred into NMRI recipients weighed more, and female mice showed a depressive-like phenotype. Our findings suggest that ET, now considered to be a routine procedure in animal husbandry, bears the risk of introducing artifacts.

Hypothalamic-Pituitary-Adrenal Axis Programming after Recurrent Hypoglycemia during Development

Permanent brain injury is a complication of recurrent hypoglycemia during development. Recurrent hypoglycemia also has adverse consequences on the neuroendocrine system. Hypoglycemia-associated autonomic failure, characterized by ineffective glucose counterregulation during hypoglycemia, is well described in children and adults on insulin therapy for diabetes mellitus. Whether recurrent hypoglycemia also has a programming effect on the hypothalamus-pituitary-adrenal cortex (HPA) axis has not been well studied. Hypoglycemia is a potent stress that leads to increased glucocorticoid secretion in all age groups, including the perinatal period. Other conditions associated with exposure to excess glucocorticoid in the perinatal period have a programming effect on the HPA axis activity. Limited animal data suggest the possibility of similar programming effect after recurrent hypoglycemia in the postnatal period. The age at exposure to hypoglycemia likely determines the HPA axis response in adulthood. Recurrent hypoglycemia in the early postnatal period likely leads to a hyperresponsive HPA axis, whereas recurrent hypoglycemia in the late postnatal period lead to a hyporesponsive HPA axis in adulthood. The age-specific programming effects may determine the neuroendocrine response during hypoglycemia and other stressful events in individuals with history of recurrent hypoglycemia during development.

The apelin receptor: physiology, pathology, cell signalling, and ligand modulation of a peptide-activated class A GPCR

The apelin receptor (AR or APJ) is a class A (rhodopsin-like) G-protein-coupled receptor with wide distribution throughout the human body. Activation of the AR by its cognate peptide ligand, apelin, induces diverse physiological effects including vasoconstriction and dilation, strengthening of heart muscle contractility, angiogenesis, and regulation of energy metabolism and fluid homeostasis. Recently, another endogenous peptidic activator of the AR, Toddler/ELABELA, was identified as having a crucial role in zebrafish (Danio rerio) embryonic development. The AR is also implicated in pathologies including cardiovascular disease, diabetes, obesity, and cancer, making it a promising therapeutic target. Despite its established importance, the precise roles of AR signalling remain poorly understood. Moreover, little is known about the mechanisms of peptide-AR activation. Additional complexity arises from modulation of the AR by 2 endogenous peptide ligands, both with multiple bioactive isoforms of variable length and distribution. The various apelin and Toddler/ELABELA isoforms may also produce distinct cellular effects. Further complexity arises through formation of functionally distinct heterodimers between the AR and other G-protein-coupled receptors. This minireview outlines key (patho)physiological actions of the AR, addresses what is known about signal transduction downstream of AR activation, and concludes by discussing unique properties of the endogenous peptidic ligands of the AR.

The protective effect of apelin against water-immersion and restraint stress-induced gastric damage

The aim of the present study was to investigate the gastroprotective effect of apelin on water-immersion and restraint stress (WIRS)-induced gastric lesions. Male Wistar rats were divided into four groups: control, WIRS, F13A + WIRS and F13A. APJ receptor antagonist F13A was administered to rats to determine the influence of apelin on stress-induced gastric injury. WIRS administered for 6 h resulted in the development of gastric mucosal lesions accompanied by a significant increase in plasma corticosterone. WIRS increased the concentration of 4-hydroxynonenol (4-HNE) + malondialdehyde (MDA) and the expression of apelin and hypoxia inducible factor-1α (HIF-1α) in gastric mucosa. In addition, WIRS reduced the mucosal blood flow and gastric prostaglandin E(2) (PGE(2)) concentration. Plasma corticosterone, which was increased due to stress, was significantly decreased in the F13A + WIRS group. Gastric lesions and the 4-HNE + MDA concentration were also higher in the F13A + WIRS compared to the WIRS group. We conclude that apelin has a gastroprotective effect against stress-induced lesions possibly by reducing lipid peroxidation in gastric mucosa.

Mechanical allodynia induced by traumatic brain injury is independent of restraint stress

BACKGROUND

This study identifies the relationship between a test for post-traumatic headache and a marker for acute stress in rodent models of traumatic brain injury.

NEW METHOD

C57BL/6 mice and Sprague Dawley rats were divided into Controlled Cortical Impact (CCI) injury, craniotomy (CR), and incision groups. Periorbital and paw allodynia were evaluated using the von Frey test prior to injury and up to four weeks post-operatively. Serum corticosterone was evaluated in groups with and without mild restraint.

RESULTS

Periorbital and forepaw thresholds, but not hindpaw thresholds, were reduced in CCI and CR mice compared to incision (p<0.0001 and p<0.01). In contrast to mice, reduced periorbital and forepaw periorbital thresholds were found in CCI rats but not CR rats compared to incision (p<0.0001). Right periorbital thresholds were reduced compared to left thresholds for both rat and mouse at one week (p<0.01), but there were no side differences for forepaw thresholds. Hindpaw thresholds did not change from baseline values for any groups of mice or rats. In mice serum corticosterone levels were increased at one, two and four weeks post-CCI and CR, while the levels for rats were not different from incision (p<0.0001). Corticosterone levels were not different in mice subjected to restraint compared to no restraint.

COMPARISON WITH EXISTING METHODS

This study presents novel data for allodynia in a rat model of TBI, and differences among mouse and rat species.

CONCLUSIONS

Mechanical allodynia occurs independent of evoked restraint stress, while hypothalamic pituitary adrenal axis activity is dependent on head trauma and species.

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

The apelin receptor (APJ; gene symbol APLNR) is a member of the G protein-coupled receptor gene family. Neural gene expression patterns of APJ, and its cognate ligand apelin, in the brain implicate the apelinergic system in the regulation of a number of physiological processes. APJ and apelin are highly expressed in the hypothalamo-neurohypophysial system, which regulates fluid homeostasis, in the hypothalamic-pituitary-adrenal axis, which controls the neuroendocrine response to stress, and in the forebrain and lower brainstem regions, which are involved in cardiovascular function. Recently, apelin, synthesised and secreted by adipocytes, has been described as a beneficial adipokine related to obesity, and there is growing awareness of a potential role for apelin and APJ in glucose and energy metabolism. In this review we provide a comprehensive overview of the structure, expression pattern and regulation of apelin and its receptor, as well as the main second messengers and signalling proteins activated by apelin. We also highlight the physiological and pathological roles that support this system as a novel therapeutic target for pharmacological intervention in treating conditions related to altered water balance, stress-induced disorders such as anxiety and depression, and cardiovascular and metabolic disorders.

Regulation of feeding behavior, gastrointestinal function and fluid homeostasis by apelin

Apelin was first identified and characterized from bovine stomach extracts as an endogenous ligand for the APJ receptor. Apelin/APJ system is abundantly present in peripheral tissues and central nervous system. Apelin plays a broad role in regulating physiological and pathological functions. Recently, many reports have showed the effects of apelin on feeding behavior, however the results are inconsistent, due to different administration routes, animal species, forms of apelin, etc. Apelin has been involved in stimulating gastric cell proliferation, cholecystokinin (CCK) secretion, histamine release, gastric acid and bicarbonate secretion, and regulation of gastrointestinal motility. In addition, apelin produced regulatory effects on drinking behavior, diuresis, arginine vasopressin (AVP) release and glucocorticoids secretion. This article reviews the role of apelin on feeding behavior, gastrointestinal function and fluid homeostasis.

Associations of common variants at APLN and hypertension in Chinese subjects with and without diabetes

Background. Apelin, the endogenous ligand for the APJ receptor, has a potent hypotensive effect via a nitric oxide-dependent mechanism in vivo. The aim of the study was to investigate the association between the common variants of apelin gene (APLN) and hypertension, which was reported recently in a Chinese Han population with and without diabetes. Methods. Three single nucleotide polymorphisms (SNPs) on APLN were genotyped in 3156 diabetic patients and 3736 nondiabetic individuals. For non-diabetic subjects, 1779 were enrolled in stage 1 and 1757 were recruited for validation. A meta-analysis combining the two stages was carried out to obtain the overall effect. Results. In diabetic patients, no significant associations of the three SNPs with hypertension were observed. In contrast, we found that rs2235306 was associated with hypertension in non-diabetic males after adjusting for covariates (OR = 1.19, P = 0.039) while rs2235307 and rs3115759 displayed no evidence of association in both genders. One haplotype, C-C-A, also showed an association with hypertension (OR = 1.47, P = 0.032) only in men. However, analysis in stage 2 and meta-analysis did not support these findings. Conclusions. We conclude that common variants on APLN are not associated with the prevalence of hypertension in the Chinese.