Phoenixin: a novel peptide in rodent sensory ganglia

Chronic unpredictable stress (CUS) reduced phoenixin expression, induced abnormal sperm and testis morphology in male rats

Chronic stress caused by prolonged emotional pressure can lead to various physiological issues, including reproductive dysfunction. Although reproductive problems can also induce chronic stress, the impact of chronic stress-induced reproductive dysfunction remains contentious. This study investigates the effects of chronic unpredictable stress (CUS) on reproductive neuropeptides, sperm quality, and testicular morphology. Sixteen twelve-week-old Sprague Dawley rats were divided into two groups: a non-stress control group and a CUS-induced group. The CUS regimen involved various stressors over 28 days, with both groups undergoing behavioural assessments through sucrose-preference and forced-swim tests. Hypothalamic gene expression levels of CRH, PNX, GPR173, kisspeptin, GnRH, GnIH, and spexin neuropeptides were measured via qPCR, while plasma cortisol, luteinizing hormone (LH), and testosterone concentrations were quantified using ELISA. Seminal fluid and testis samples were collected for sperm analysis and histopathological evaluation, respectively. Results showed altered behaviours in CUS-induced rats, reflecting stress impacts. Hypothalamic corticotropin-releasing hormone (CRH) expression and plasma cortisol levels were significantly higher in CUS-induced rats compared to controls (p < 0.05). Conversely, phoenixin (PNX) expression decreased in the CUS group (p < 0.05), while kisspeptin, spexin, and gonadotropin-inhibitory hormone (GnIH) levels showed no significant differences between groups. Despite a significant increase in GnRH expression (p < 0.05), plasma LH and testosterone concentrations were significantly lower (p < 0.05) in CUS-induced rats. Histopathological analysis revealed abnormal testis morphology in CUS-induced rats, including disrupted architecture, visible interstitial spaces between seminiferous tubules, and absence of spermatogenesis. In conclusion, CUS affects reproductive function by modulating PNX and GnRH expression, influencing cortisol levels, and subsequently reducing plasma LH and testosterone concentrations. This study highlights the complex interplay between chronic stress and reproductive health, emphasizing the significant impact of stress on reproductive functions.

Phoenixin-14 may ameliorate testicular damage caused by torsion-detorsion by reducing oxidative stress and inflammation in prepubertal rats

The present study aimed to investigate the effects of Phoenixin-14 (PNX-14) on oxidative damage, inflammatory response, histopathological variations, and serum testosterone levels in testicular tissues. Forty-eight Wistar albino prepubertal male rats were divided into 4 groups (Sham, TTD, TT+PNX+TD, TTD+PNX) (n=12). The torsion period was 2 hours and the detorsion period was 24 hours in the testicular torsion/detorsion (TD) groups. A single PNX-14 (50 µg/kg) dose was injected into the rats in the TT+PNX TD group on the 90th minute of torsion, and it was injected into the rats in the TTD+PNX group at the beginning of detorsion. Oxidative damage in testicular tissues was determined based on superoxide dismutase (SOD), malondialdehyde (MDA), total antioxidant status (TAS) and total oxidant status (TOS), and inflammatory damage was determined based on TNF-α and IL-6 levels. Histopathological variations were investigated with the Periodic Acid Schiff (PAS) staining method in testicular tissues and analyzed based on Johnsen scores. Spermatogonia cells were examined immunohistochemically. Serum testosterone levels were determined with the enzyme-linked immunosorbent assay (ELISA). A significant increase in oxidative stress and inflammation parameters was determined in the TTD group when compared to the other groups (p<0.05). PNX-14 treatment led to a statistically significant decrease in these parameters and significantly repaired the TD damage in testicular tissue (p<0.05). Johnsen scoring revealed significant improvement in PNX-14 groups and an increase in spermatogonia count, supporting the biochemical findings (p<0.05). PNX-14 could be a potential therapeutic agent in testicular TD damage and further studies should be conducted to elucidate the present study findings.

Phoenixin-14 protects cardiac damages in a streptozotocin-induced diabetes mice model through SIRT3

BACKGROUND

Type I diabetes is a metabolic syndrome that severely impacts the normal lives of patients through its multiple complications, such as diabetic cardiomyopathy (DCM). Phoenixin-14 is a peptide found to be widely expressed in eukaryons with multiple protective properties, including anti-oxidative stress and anti-inflammatory effects. The present study aims to explore the potential therapeutic impacts of Phoenixin-14 on DCM.

METHODS

Type I diabetes was induced by treatment with a single dose of STZ (40 mg/kg body weight) intraperitoneally for 5 consecutive days. Mice were divided into four groups: the Control, Phoenixin-14, T1DM, and Phoenixin-14 +T1DM groups. The levels of myocardial injury markers were measured. Cardiac hypertrophy was assessed using wheat germ agglutinin (WGA) staining.

RESULTS

Phoenixin-14 was significantly downregulated in the cardiac tissue of diabetic mice. The myocardial injury and deteriorated cardiac function in diabetic mice induced by STZ were significantly ameliorated by Phoenixin-14, accompanied by the alleviation of cardiac hypertrophy. In addition, the severe oxidative stress and inflammation in diabetic mice were dramatically mitigated by Phoenixin-14. Lastly, the downregulated SIRT3 and upregulated p-FOXO3 in diabetic mice were pronouncedly reversed by Phoenixin-14. It is worth mentioning that compared to the Control, no significant changes to any of the investigated parameters in the present study were found in the Phoenixin-14-treated normal mice, suggesting that treatment with it has no side effects.

CONCLUSION

Our data revealed that Phoenixin-14 protected against cardiac damages in STZ-induced diabetes mice models.

Phoenixin-14 as a novel direct regulator of porcine luteal cell functions†

Phoenixin is a neuropeptide with a well-established role in the central regulation of reproductive processes; however, knowledge regarding its role in the ovary is limited. One of the main active phoenixin isoforms is phoenixin-14, which acts through G protein-coupled receptor 173. Our research hypothesis was that phoenixin-14 is expressed in porcine corpus luteum and exerts luteotropic action by affecting the endocrine function of luteal cells through G protein-coupled receptor 173 and protein kinase signaling. Luteal cells were cultured to investigate the effect of phoenixin-14 (1-1000 nM) on endocrine function. We showed that phoenixin-14 and G protein-coupled receptor 173 are produced locally in porcine corpus luteum and their levels change during the estrous cycle. We detected phoenixin-14 immunostaining in the cytoplasm and G protein-coupled receptor 173 in the cell membrane. Plasma phoenixin levels were highest during the early luteal phase. Interestingly, insulin, luteinizing hormone, progesterone, and prostaglandins decreased phoenixin-14 levels in luteal cells. Phoenixin-14 increased progesterone, estradiol, and prostaglandin E2 secretion, but decreased prostaglandin F2α, upregulated the expression of steroidogenic enzymes, and downregulated receptors for luteinizing hormone and prostaglandin. Also, phoenixin-14 increased the expression of G protein-coupled receptor 173 and the phosphorylation of extracellular signal-regulated kinase 1/2, protein kinase B, inhibited the phosphorylation of protein kinase A, and had mixed effect on AMP-activated protein kinase alpha and protein kinase C. G protein-coupled receptor 173 and extracellular signal-regulated kinase 1/2 mediated the effect of phoenixin-14 on endocrine function of luteal cells. Our results suggest that phoenixin is produced by porcine luteal cells and can be a new regulator of their function.

The protective effects of Phoenixin-20 in tumor necrosis factor α (TNF-α)-induced cell senescence of rheumatoid arthritis fibroblast-like synoviocytes (FLS)

Rheumatoid arthritis (RA) is an age-related joint destruction disease that markedly impacts the normal life of patients. Currently, the clinical treatment strategies are far from satisfactory with severe side effects. Cellular senescence of fibroblast-like synoviocytes (FLS) has been reported to be involved in the pathological process of arthritis, which may provide an important research direction for RA treatment. Phoenixin-20 (PNX-20) is a peptide targeting G-protein-coupled receptor 173 (GPR173) with promising anti-inflammatory properties. Our study will probe into the function of PNX-20 on tumor necrosis factor α (TNF-α)- induced rheumatoid arthritis (RA) FLS cell senescence to provide a theoretical basis for treating RA with PNX-20. RA-FLSs were handled with 10 ng/mL TNF-α, followed by introducing Phoenixin-20 (10, 20 nM) or not for 7 days. Enhanced release of inflammatory cytokines, increased proportion of senescence-associated β-galactosidase (SA-β-gal) positive cells, and declined telomerase activity were all observed in TNF-α-treated RA-FLSs, accompanied by a noticeable decline in the p21 and p53 level, which were notably reversed by 10 and 20 nM PNX-20. Furthermore, the increased signal transducer and activator of transcription 6 (STAT6) level observed in TNF-α-treated RA-FLSs were signally repressed by PNX-20. Moreover, the impact of PNX-20 on TNF-α-induced cellular senescence in RA-FLSs was abrogated by the overexpression of STAT6. Collectively, PNX-20 protected the TNF-α-induced cell senescence in RA-FLSs by downregulating STAT6. Based on these findings, we speculate that PNX-20 might be a promising agent for the treatment of RA.

Distribution and Chemistry of Phoenixin-14, a Newly Discovered Sensory Transmission Molecule in Porcine Afferent Neurons

Phoenixin-14 (PNX), initially discovered in the rat hypothalamus, was also detected in dorsal root ganglion (DRG) cells, where its involvement in the regulation of pain and/or itch sensation was suggested. However, there is a lack of data not only on its distribution in DRGs along individual segments of the spinal cord, but also on the pattern(s) of its co-occurrence with other sensory neurotransmitters. To fill the above-mentioned gap and expand our knowledge about the occurrence of PNX in mammalian species other than rodents, this study examined (i) the pattern(s) of PNX occurrence in DRG neurons of subsequent neuromeres along the porcine spinal cord, (ii) their intraganglionic distribution and (iii) the pattern(s) of PNX co-occurrence with other biologically active agents. PNX was found in approximately 20% of all nerve cells of each DRG examined; the largest subpopulation of PNX-positive (PNX+) cells were small-diameter neurons, accounting for 74% of all PNX-positive neurons found. PNX+ neurons also co-contained calcitonin gene-related peptide (CGRP; 96.1%), substance P (SP; 88.5%), nitric oxide synthase (nNOS; 52.1%), galanin (GAL; 20.7%), calretinin (CRT; 10%), pituitary adenylate cyclase-activating polypeptide (PACAP; 7.4%), cocaine and amphetamine related transcript (CART; 5.1%) or somatostatin (SOM; 4.7%). Although the exact function of PNX in DRGs is not yet known, the high degree of co-localization of this peptide with the main nociceptive transmitters SP and CGRP may suggests its function in modulation of pain transmission.

Investigation of Serum Phoenixin-14 Concentration in Pregnant Women Diagnosed with Hyperemesis Gravidarum

OBJECTIVE

We aimed to investigate the relationship between the hyperemesis gravidarum (HG) and maternal serum phoenixin-14 (PNX-14) concentrations.

MATERIALS AND METHODS

This cross-sectional study was conducted with 88 pregnant women who applied to the Umraniye Training and Research Hospital Gynecology and Obstetrics Clinic between February 2022 and October 2022. The HG group consisted of 44 pregnant women diagnosed with HG between the 7th and 14th gestational weeks, and the control group consisted of 44 healthy pregnant women matched with the HG group in terms of age, BMI, and gestational week. Demographic characteristics, ultrasound findings, and laboratory outcomes were noted. The two groups were compared in terms of maternal serum PNX-14 concentrations.

RESULTS

Gestational age at blood sampling for PNX-14 was similar in both groups (p=1.000). While maternal serum PNX-14 concentration was 85.5 pg/ml in the HG group, it was 71.3 pg/ml in the control group (p=0.012). ROC analysis was performed to determine the value of maternal serum PNX-14 concentration in terms of predicting HG. AUC analysis of maternal serum PNX-14 for HG estimation was 0.656 (p=0.012, 95% CI=0.54-0.77). The optimal cutoff value for maternal serum PNX-14 concentration was determined as 79.81 pg/ml with 59% sensitivity and 59% specificity.

CONCLUSION

In this study, maternal serum PNX-14 concentration was found to be higher in pregnant women with HG, which indicates that high serum PNX-14 concentrations may have an anorexigenic effect on food intake in pregnancy. Concentrations of other PNX isoforms in HG and changes in PNX concentrations in pregnant women with HG who regained weight after treatment remain to be investigated.

Current state of phoenixin-the implications of the pleiotropic peptide in stress and its potential as a therapeutic target

Phoenixin is a pleiotropic peptide, whose known functions have broadened significantly over the last decade. Initially first described as a reproductive peptide in 2013, phoenixin is now recognized as being implicated in hypertension, neuroinflammation, pruritus, food intake, anxiety as well as stress. Due to its wide field of involvement, an interaction with physiological as well as psychological control loops has been speculated. It has shown to be both able to actively reduce anxiety as well as being influenced by external stressors. Initial rodent models have shown that central administration of phoenixin alters the behavior of the subjects when confronted with stress-inducing situations, proposing an interaction with the perception and processing of stress and anxiety. Although the research on phoenixin is still in its infancy, there are several promising insights into its functionality, which might prove to be of value in the pharmacological treatment of several psychiatric and psychosomatic illnesses such as anorexia nervosa, post-traumatic stress disorder as well as the increasingly prevalent stress-related illnesses of burnout and depression. In this review, we aim to provide an overview of the current state of knowledge of phoenixin, its interactions with physiological processes as well as focus on the recent developments in stress response and the possible novel treatment options this might entail.

Expression of phoenixin-14 and nesfatin-1 in the hypothalamo-pituitary-gonadal axis in the phases of the estrous cycle

Phoenixin-14 (PNX-14) and nucleobindin 2 (NUCB2)/nesfatin-1 are regulatory neuropeptides expressed in the hypothalamus. These neuropeptides can be effective in hormonal regulation of the hypothalamo-pituitary-gonadal (HPG) axis and reproductive functions. In the present study, the distribution of PNX-14 and NUCB2/nesfatin-1 in the hypothalamus, pituitary, ovary, and uterus tissues during the phases of the estrous cycle in female rats was investigated. Eighteen Wistar Albino rats determined among animals showing regular estrous cycle by vaginal smear method were divided into three groups: proestrus (Group I), estrus (Group II) and diestrus (Group III). Serum gonadotropin-releasing hormone (GnRH), plasma PNX-14, and NUCB2/nesfatin-1 concentrations were the highest, moderate, and lowest in estrus, diestrus, and proestrus phases, respectively. PNX-14 immunoreactivity in the supraoptic and arcuate nuclei of the hypothalamus and NUCB2/nesfatin-1 immunoreactivity in the paraventricular nuclei were particularly evident in the estrus phase. These neuropeptide immunoreactivities were decreased in different cells of anterior pituitary during proestrus compared with those during estrus and diestrus. PNX-14 immunoreactivity in the ovary, especially during the estrus phase, was diffuse and intense in the granulosa and luteal cells and oocytes, and it was few and weak in theca cells. In addition, NUCB2/nesfatin-1 immunoreactivity was abundant and strong in granulosa and luteal cells, theca and interstitial cells, and oocytes during estrus. In the estrus phase, PNX-14 immunoreactivity was strong in the glandular epithelial cells and stromal cells of the endometrium, also NUCB2/nesfatin-1 immunoreactivity was strong in the epithelial and glandular epithelial cells. As a result, when the estrous cycle was evaluated, it was concluded that the changes in the distribution of PNX-14 and NUCB2/nesfatin-1 at all phases were related to GnRH and that these neuropeptides showed the highest immunoreactivity especially in the HPG axis and uterus tissues of estrus rats.

Phoenixin 14 ameloriates pancreatic injury in streptozotocin-induced diabetic rats by alleviating oxidative burden

Phoenixin-14 (PNX) is a neuropeptide that has been shown to prevent oxidative damage and stimulates insulin secretion. We investigated the effects of PNX on pancreatic injury induced by streptozotocin (STZ), and nicotinamide (NAD). Male Sprague-Dawley rats, in control (C) and diabetic (STZ) groups, were treated with either saline, or PNX (0.45 nmol/kg, or 45 nmol/kg) daily for 3 days 1 week after STZ injection. Fasting blood glucose (FBG) and gastric emptying rate (GER) were measured. Tissue and blood samples were collected. PNX treatments prevented pancreatic damage and β cell loss. Increased luminol and lucigenin levels in the pancreas, ileum and liver tissues of STZ groups were alleviated by PNX treatment in pancreatic and ileal tissues. PNX0.45 decreased FBG without any change in insulin blood level and pancreatic mRNA. GER increased in all diabetic rats while PNX0.45 delayed GER only in the C group. PNX diminishes pancreatic damage and lowers FBG by reducing oxidative load.

Regulation and physiological functions of phoenixin

Phoenixin is a newly discovered neuropeptide generated from small integral membrane protein 20. Phoenixin is a ligand for the G protein-coupled receptor 173 (GPR173) and has been detected in central and peripheral tissues of human, rats, mice, bovine, and zebrafish. It was initially involved in regulating reproductive function by stimulating the luteinizing hormone release from pituitary cells by increasing the level of gonadotropin-releasing hormone. Recently, many functions of phoenixin have been generalized, including regulation of food intake, memory, Alzheimer’s disease, anxiety, inflammation, neuronal and microglial activity, energy metabolism and body fluid balance, cardiovascular function, and endocrine activity. In addition, the interaction between phoenixin and nesfatin-1 have been revealed. The present article summarized the latest research progress on physiological function of phoenixin, suggesting that it is a potential target for novel drug development and clinical application.

Super-conserved receptors expressed in the brain: biology and medicinal chemistry efforts

The super-conserved receptors expressed in the brain (SREB) constitute a family of orphan G protein-coupled receptors that include GPR27 (SREB1), GPR85 (SREB2) and GPR173 (SREB3). Their sequences are highly conserved in vertebrates, and they are almost exclusively expressed in the central nervous system. This family of receptors has attracted much attention due to their putative physiological functions and their potential as novel drug targets. The SREB family has been postulated to play important roles in a wide range of different diseases, including pancreatic β-cell insulin secretion and regulation, schizophrenia, autism and atherosclerosis. This review intends to provide a comprehensive overview of the SREB family and its recent advances in biology and medicinal chemistry.

Liver and muscle-specific effects of phoenixin-20 on the insulin-like growth factor system mRNAs in zebrafish

OBJECTIVE

Phoenixin-20 (Pnx-20) is a bioactive peptide with endocrine-like actions in vertebrates. Recent studies suggest Pnx-20 promotes growth hormone/insulin-like growth factors (Gh/Igf) axis, an important endocrine regulator of growth in mammals and fish.

DESIGN

In this research, we determined whether Pnx-20 affects the different members of the Igf family, its binding proteins and receptors (Igf-system) in zebrafish liver and muscle.

RESULTS

In vivo administration of Pnx-20 downregulated igfs, igf receptors (igfrs) and igf binding protein (igfbp) 5 mRNA expression in the liver of male and female zebrafish at both 1 and 6 h post-intraperitoneal (IP) injection. Interestingly, this effect occurred at a relatively earlier timepoint in female zebrafish suggesting sex-specific differences in Pnx-20 action. Besides, either 6 or 24 h in vitro incubations with Pnx-20 downregulated the expression of all igfs, igfrs and igfbp5 mRNAs (except igf2a) analyzed in a zebrafish liver cell (ZFL) line. Moreover, siRNA-mediated knockdown of Pnx-20 upregulated all Igf-system mRNAs analyzed in ZFL cells. Together, these results (both in vivo and in vitro) revealed a general suppressive action for both endogenous and exogenous Pnx-20 on the hepatic Igf-system of zebrafish. In contrast, a general sex-specific upregulation of the Igf-system mRNAs analyzed was found in the muscle of Pnx-20 injected fish. Future research should explore the sex- and time-differences observed in the present study.

CONCLUSIONS

Collectively, this research shows that Pnx-20 is a tissue-specific regulator of the liver (suppressor) and muscle (stimulant) Igf signaling in both male and female zebrafish.

Pre-metazoan origin of neuropeptide signalling

Neuropeptides are a diverse class of signaling molecules in metazoans. They occur in all animals with a nervous system and also in neuron-less placozoans. However, their origin has remained unclear because no neuropeptide shows deep homology across lineages, and none have been found in sponges. Here, we identify two neuropeptide precursors, phoenixin (PNX) and nesfatin, with broad evolutionary conservation. By database searches, sequence alignments, and gene-structure comparisons, we show that both precursors are present in bilaterians, cnidarians, ctenophores, and sponges. We also found PNX and a secreted nesfatin precursor homolog in the choanoflagellate Salpingoeca rosetta. PNX, in particular, is highly conserved, including its cleavage sites, suggesting that prohormone processing occurs also in choanoflagellates. In addition, based on phyletic patterns and negative pharmacological assays, we question the originally proposed GPR-173 (SREB3) as a PNX receptor. Our findings revealed that secreted neuropeptide homologs derived from longer precursors have premetazoan origins and thus evolved before neurons.

Inflammatory Stress Induced by Intraperitoneal Injection of LPS Increases Phoenixin Expression and Activity in Distinct Rat Brain Nuclei

Due to phoenixin’s role in restraint stress and glucocorticoid stress, as well as its recently shown effects on the inflammasome, we aimed to investigate the effects of lipopolysaccharide (LPS)-induced inflammatory stress on the activity of brain nuclei-expressing phoenixin. Male Sprague Dawley rats (n = 6/group) were intraperitoneally injected with either LPS or control (saline). Brains were processed for c-Fos and phoenixin immunohistochemistry and the resulting slides were evaluated using ImageJ software. c-Fos was counted and phoenixin was evaluated using densitometry. LPS stress significantly increased c-Fos expression in the central amygdaloid nucleus (CeM, 7.2-fold), supraoptic nucleus (SON, 34.8 ± 17.3 vs. 0.0 ± 0.0), arcuate nucleus (Arc, 4.9-fold), raphe pallidus (RPa, 5.1-fold), bed nucleus of the stria terminalis (BSt, 5.9-fold), dorsal motor nucleus of the vagus nerve (DMN, 89-fold), and medial part of the nucleus of the solitary tract (mNTS, 121-fold) compared to the control-injected group (p < 0.05). Phoenixin expression also significantly increased in the CeM (1.2-fold), SON (1.5-fold), RPa (1.3-fold), DMN (1.3-fold), and mNTS (1.9-fold, p < 0.05), leading to a positive correlation between c-Fos and phoenixin in the RPa, BSt, and mNTS (p < 0.05). In conclusion, LPS stress induces a significant increase in activity in phoenixin immunoreactive brain nuclei that is distinctively different from restraint stress.

The Regulation of Phoenixin: A Fascinating Multidimensional Peptide

The phoenixin (PNX) peptide is linked to the control of reproduction, food intake, stress, and inflammation. However, little is known about what regulates its gene and protein expression, information that is critical to understand the physiological role of PNX. In this review, we summarize what is known about the transcriptional control of Pnx and its receptor Gpr173. A main function of PNX is as a positive regulator of the hypothalamic-pituitary-gonadal axis, but there is a lack of research on its control by reproductive hormones and peptides. PNX is also associated with food intake, and its expression is linked to feeding status, fatty acids, and glucose. It is influenced by environmental and hormonal-induced stress. The regulation of Pnx in most contexts remains an enigma, in part due to conflicting and negative results. An extensive analysis of the response of the Pnx gene to factors related to reproduction, metabolism, stress, and inflammation is required. Analysis of the Pnx promoter and epigenetic regulation must be considered to understand how this level of control contributes to its pleiotropic effects. PNX is now linked to a broad range of functions, but more research on its gene regulation is required to understand its place in overall physiology and therapeutic potential.

Phoenixin as a New Target in the Development of Strategies for Endometriosis Diagnosis and Treatment

Small integral membrane protein 20/phoenixin (SMIM20/PNX) and its receptor GPR173 (G Protein-Coupled Receptor 173) play a role in the regulation of the hypothalamic–pituitary–gonadal axis (HPG). The aim of the study was to determine PNX, FSH, LH, and 17β-estradiol association in women with endometriosis, and the expression of SMIM20/PNX signaling via GPR173. Serum PNX, FSH, LH, and 17β-estradiol concentrations were measured by enzyme and electrochemiluminescence immunoassay. SMIM20/PNX and GPR173 expression in the eutopic and ectopic endometrium was assessed by qPCR and immunohistochemistry. Reduced PNX level, increased LH/FSH ratio and elevated 17β-estradiol concentration were found in patients with endometriosis. No differences in SMIM20 expression were observed between the studied endometria. GPR173 expression was lower in ectopic than in eutopic endometria. SMIM20 expression was mainly restricted to stroma. GPR173 was detected in some eutopic and ectopic stromal cells and in eutopic glandular epithelial cells. Discriminant analysis indicates the diagnostic relevance of PNX and LH/FSH ratio in patients with endometriosis. In women with endometriosis, reduced PNX levels and GPR173 expression may be responsible for HPG axis dysregulation. These new insights may contribute to a better understanding of the pathophysiology of endometriosis and provide the basis for a new strategy for diagnosis and treatment of endometriosis.

Role of the Novel Peptide Phoenixin in Stress Response and Possible Interactions with Nesfatin-1

The novel peptide phoenixin was shown to be involved in several physiological processes ranging from reproduction to food intake. Interest in this protein has steadily increased over the last few years and its known implications have become much broader, playing a role in glucose homeostasis, anxiety, nociception, and pruritus. Phoenixin is expressed in a multitude of organs such as the small intestine, pancreas, and in the hypothalamus, as well as several other brain nuclei influencing numerous physiological functions. Its highly conserved amino-acid sequence amongst species leads to the assumption, that phoenixin might be involved in essential physiological functions. Its co-expression and opposing functionality to the extensively studied peptide nesfatin-1 has given rise to the idea of a possible counterbalancing role. Several recent publications focused on phoenixin’s role in stress reactions, namely restraint stress and lipopolysaccharide-induced inflammation response, in which also nesfatin-1 is known to be altered. This review provides an overview on the phoenixins and nesfatin-1 properties and putative effects, and especially highlights the recent developments on their role and interaction in the response to response.

Phoenixin-14 reduces the frequency of interictal-like events in mice brain slices

Phoenixin-14 (PNX-14) has a wide bioactivity in the central nervous system. Its role in the hypothalamus has been investigated, and it has been reported that it is involved in the regulation of excitability in hypothalamic neurons. However, its role in the regulation of excitability in entorhinal cortex and the hippocampus is unknown. In this study, we investigated whether i. PNX-14 induces any synchronous discharges or epileptiform activity and ii. PNX-14 has any effect on already initiated epileptiform discharges. We used 350 µm thick acute horizontal hippocampal-entorhinal cortex slices obtained from 30- to 35-day-old mice. Extracellular field potential recordings were evaluated in the entorhinal cortex and hippocampus CA1 region. Bath application of PNX-14 did not initiate any epileptiform activity or abnormal discharges. 4-Aminopyridine was applied to induce epileptiform activity in the slices. We found that 200 nM PNX-14 reduced the frequency of interictal-like events in both the entorhinal cortex and hippocampus CA1 region which was induced by 4-aminopyridine. Furthermore, PNX-14 led to a similar suppression in the total power of local field potentials of 1-120 Hz. The frequency or the duration of the ictal events was not affected. These results exhibited for the first time that PNX-14 has a modulatory effect on synchronized neuronal discharges which should be considered in future therapeutic approaches.

Mouse gastric mucosal endocrine cells are sources and sites of action of Phoenixin-20

Energy homeostasis is is determined by food intake and energy expenditure, which are partly regulated by the cross-talk between central and peripheral hormonal signals. Phoenixin (PNX) is a recently discovered pleiotropic neuropeptide with isoforms of 14 (PNX-14) and 20 (PNX-20) amino acids. It is a potent reproductive peptide in vertebrates, regulating the hypothalamo-pituitary-gonadal axis (HPG). It has been identified as a regulator of food intake during light phase when injected intracerebroventricularly in rats. In addition, plasma levels of PNX also increased after food intake in rats, suggesting that it might have possible roles in energy homeostasis. We hypothesized that gut is a source and site of action of PNX in mice. Immunoreactivity for PNX and its putative receptor, super-conserved receptor expressed in brain (SREB3; also known as the G-protein coupled receptor 173/GPR 173) was found in the stomach and intestine of male C57/BL6 J mice, and in MGN3-1 (mouse stomach endocrine) cells and STC-1 (mouse enteroendocrine) cells. In MGN3-1 cells, PNX-20 significantly upregulated ghrelin (10 nM) and ghrelin-O-acyl transferase (GOAT) mRNAs (1000 nM) at 6 h. In STC-1 cells, it significantly suppressed CCK (100 nM) at 2 h. No effects were found on other intestinal hormones tested (glucagon like peptide-1, glucose dependent insulinotropic polypeptide, and peptide YY). Together, these results indicate that PNX-20 is produced in the gut, and it could act directly on gut cells to regulate metabolic hormones.

Characterization of the G protein-coupled receptor family SREB across fish evolution

The SREB (Super-conserved Receptors Expressed in Brain) family of G protein-coupled receptors is highly conserved across vertebrates and consists of three members: SREB1 (orphan receptor GPR27), SREB2 (GPR85), and SREB3 (GPR173). Ligands for these receptors are largely unknown or only recently identified, and functions for all three are still beginning to be understood, including roles in glucose homeostasis, neurogenesis, and hypothalamic control of reproduction. In addition to the brain, all three are expressed in gonads, but relatively few studies have focused on this, especially in non-mammalian models or in an integrated approach across the entire receptor family. The purpose of this study was to more fully characterize sreb genes in fish, using comparative genomics and gonadal expression analyses in five diverse ray-finned (Actinopterygii) species across evolution. Several unique characteristics were identified in fish, including: (1) a novel, fourth euteleost-specific gene (sreb3b or gpr173b) that likely emerged from a copy of sreb3 in a separate event after the teleost whole genome duplication, (2) sreb3a gene loss in Order Cyprinodontiformes, and (3) expression differences between a gar species and teleosts. Overall, gonadal patterns suggested an important role for all sreb genes in teleost testicular development, while gar were characterized by greater ovarian expression that may reflect similar roles to mammals. The novel sreb3b gene was also characterized by several unique features, including divergent but highly conserved amino acid positions, and elevated brain expression in puffer (Dichotomyctere nigroviridis) that more closely matched sreb2, not sreb3a. These results demonstrate that SREBs may differ among vertebrates in genomic structure and function, and more research is needed to better understand these roles in fish.

Modulatory effect of olanzapine on SMIM20/phoenixin, NPQ/spexin and NUCB2/nesfatin-1 gene expressions in the rat brainstem

Background

Phoenixin, spexin and nesfatin-1 belong to a family of newly discovered multifunctional neuropeptides that play regulatory roles in several brain structures and modulate the activity of important neural networks. However, little is known about their expression and action at the level of brainstem. The present work was, therefore, focused on gene expression of the aforementioned peptides in the brainstem of rats chronically treated with olanzapine, a second generation antipsychotic drug.

Methods

Studies were carried out on adult, male Sprague–Dawley rats that were divided into 2 groups: control and experimental animals treated with olanzapine (28-day-long intraperitoneal injection, at dose 5 mg/kg daily). All individuals were killed under anesthesia and the brainstem excised. Total mRNA was isolated from homogenized samples of both structures and the RT-PCR method was used for estimation of related SMIM20/phoenixin, NPQ/spexin and NUCB2/nesfatin-1 gene expression.

Results

Long-term treatment with olanzapine is reflected in qualitatively different changes in expression of examined neuropeptides mRNA in the rat brainstem. Olanzapine significantly decreased NPQ/spexin mRNA expression, but increased SMIM20/phoenixin mRNA level in the rat brainstem; while NUCB2/nesfatin-1 mRNA expression remained unchanged.

Conclusions

Olanzapine can affect novel peptidergic signaling in the rat brainstem. This may cautiously suggest the presence of an alternative mode of its action.

phoenixin(smim20), a gene coding for a novel reproductive ligand, is expressed in the brain of adult zebrafish

Gonadotropin-releasing hormone (GnRH) is a highly conserved neuroendocrine decapeptide that is essential for the onset of puberty and the maintenance of the reproductive state. In addition to its role as hypothalamic releasing hormone, GnRH has multiple functions including modulator of neural activity within the nervous system and of resulting behaviors. These multiple functions are reflected by the existence of multiple isoforms. Despite its importance as a critical hypothalamic releasing hormone, the gnrh1 gene has been lost in zebrafish, and its reproductive function is not compensated for by other GnRH isoforms (GnRH2 and GnRH3), suggesting that, surprisingly, zebrafish do not use any of the GnRH peptides to control reproduction and fertility. Previously we proposed that Phoenixin/SMIM20, a novel peptide identified in mammals and the ligand for the orphan GPR173, is a potential candidate to control the initiation of sexual development and fertility in the zebrafish. Here we confirm the sequence of the zebrafish phoenixin/smim20 gene and by RT-PCR show that it is expressed early in development through adulthood. Subsequently we show that phoenixin/smim20 is expressed in the adult brain including the regions of the hypothalamus important in the control of fertility and reproduction.

Phoenixin: More than Reproductive Peptide

Phoenixin (PNX) neuropeptide is a cleaved product of the Smim20 protein. Its most common isoforms are the 14- and 20-amino acid peptides. The biological functions of PNX are mediated via the activation of the GPR173 receptor. PNX plays an important role in the central nervous system (CNS) and in the female reproductive system where it potentiates LH secretion and controls the estrus cycle. Moreover, it stimulates oocyte maturation and increases the number of ovulated oocytes. Nevertheless, PNX not only regulates the reproduction system but also exerts anxiolytic, anti-inflammatory, and cell-protective effects. Furthermore, it is involved in behavior, food intake, sensory perception, memory, and energy metabolism. Outside the CNS, PNX exerts its effects on the heart, ovaries, adipose tissue, and pancreatic islets. This review presents all the currently available studies demonstrating the pleiotropic effects of PNX.

Distribution and chemical coding of phoenixin-immunoreactive nerve structures in the spinal cord of the pig

Phoenixin (PNX) is a newly described peptide found in both neural and non-neural tissues. Until now, no attempts have been made to investigate the expression of PNX in the nervous system of animals other than laboratory rodents, in which an enzyme immunoassay revealed the highest quantity of the substance in the spinal cord. Since the domestic pig, due to its anatomical and histological resemblance to humans, is often used as an animal model in biomedical investigations, the present study was designed to examine PNX-immunoreactivity in the spinal cords of female pigs (n=5). The spinal cords were dissected and divided into the cervical, thoracic, lumbar, sacral and coccygeal segments, which were sectioned transversally into 10-μm-thick serial sections. The sections from each spinal cord segment were processed for double-labelling immunohistochemistry using antibodies against PNX in a mixture with those against calcitonin gene-related peptide (CGRP), substance P (SP) or choline acetyltransferase (CHAT). The PNX-immunoreactivity had a similar distribution in the grey matter of all the spinal cord sections examined and was mainly observed in varicose nerve fibres (NF) that formed a dense plexus in laminae I and II of the dorsal horn. Nearly all of the PNX-immunoreactive NF stained also for CGRP or SP and, interestingly, many of them were CHAT-positive. The present study has provided for the first time the detailed information on the arrangement and chemical features of nerve structures expressing PNX-immunoreactivity in the spinal cord of a large mammal. The exact function of PNX in the spinal cord is not known yet. However, the distribution pattern and immunohistochemical characteristics of PNX-IR NF clearly suggest that this peptite most likely plays a role in spinal noxious signalling.

Levels of the neuropeptide phoenixin-14 and its receptor GRP173 in the hypothalamus, ovary and periovarian adipose tissue in rat model of polycystic ovary syndrome

Phoenixin (PNX) is a newly discovered peptide produced by proteolytic cleavage of a small integral membrane protein 20 (Smim20), which acts as an important regulator of energy homeostasis and reproduction. Since dysfunction of reproduction is characteristic in polycystic ovarian syndrome (PCOS), the role of PNX in pathogenesis of PCOS needs further investigation. The objective of this study was to determine expression of Smim20, PNX-14 and its receptor GRP173 in the hypothalamus, ovary and periovarian adipose tissue (PAT) of letrozole induced PCOS rats. Phosphorylation of extracellular signal-regulated kinase (ERK1/2), protein kinases A (PKA) and B (Akt) were also estimated. We observed that PCOS rats had high weight gain and a number of ovarian cyst, high levels of testosterone, luteinizing hormone and PNX-14, while low estradiol. Smim20 mRNA expression was higher in the ovary and PAT, while PNX-14 peptide production was higher only in the ovary of PCOS rat. Moreover, in PCOS rats Gpr173 level was lower in PAT but at the protein level increased only in the ovary. Depending on the tissues, kinases phosphorylation were significantly differ in PCOS rats. Our results showed higher levels of PNX-14 in PCOS rats and indicated some novel findings regarding the mechanisms of PCOS pathophysiology.

Phoenixin influences the excitability of nucleus of the solitary tract neurones, effects which are modified by environmental and glucocorticoid stress

Phoenixin (PNX) is a neuropeptide shown to play roles in the control of reproduction. The nucleus of the solitary tract (NTS), a critical autonomic integrating centre in the hindbrain, is one of many areas with dense expression of PNX. Using coronal NTS slices obtained from male Sprague-Dawley rats, the present study characterised the effects of PNX on both spike frequency and membrane potential of NTS neurones. Extracellular recordings demonstrated that bath-applied 10 nmol L^-1 PNX increased the firing frequency in 32% of NTS neurones, effects which were confirmed with patch-clamp recordings showing that 50% of NTS neurones tested depolarised in response to application of the peptide. Surprisingly, the responsiveness to PNX in NTS neurones then declined suddenly to 9% (P < 0.001). This effect was subsequently attributed to stress associated with construction in our animal care facility because PNX responsiveness was again observed in slices from rats delivered and maintained in a construction-free facility. We then examined whether this loss of PNX responsiveness could be replicated in rats placed on a chronic stress regimen involving ongoing corticosterone (CORT) treatment in the construction-free facility. Slices from animals treated in this way showed a similar lack of neuronal responsiveness to PNX (9.1 ± 3.9%) within 2 weeks of CORT treatment. These effects were specific to PNX responsiveness because CORT treatment had no effect on the responsiveness of NTS neurones to angiotensin II. These results are the first to implicate PNX with respect to directly controlling the excitability of NTS neurones and also provide intriguing data showing the plasticity of these effects associated with environmental and glucocorticoid stress levels of the animal.

Restraint stress increases the expression of phoenixin immunoreactivity in rat brain nuclei

Phoenixin is a recently discovered peptide, which has been associated with reproduction, anxiety and food intake. Based on a considerable co-localization it has been linked to nesfatin-1, with a possible antagonistic mode of action. Since nesfatin-1 is known to play a role in anxiety and the response to stress, this study aims to investigate the effects of a well-established psychological stress model, restraint stress, on phoenixin-expressing brain nuclei and phoenixin expression in rats. Male Sprague-Dawley rats were subjected to restraint stress (n = 8) or left undisturbed (control, n = 6) and the brains processed for c-Fos- and phoenixin immunohistochemistry. The number of c-Fos expressing cells was counted and phoenixin expression assessed semiquantitatively. Restraint stress significantly increased c-Fos expression in the dorsal motor nucleus of vagus nerve (DMN, 52-fold, p < 0.001), raphe pallidus (RPa, 15-fold, p < 0.001), medial part of the nucleus of the solitary tract (mNTS, 16-fold, p < 0.001), central amygdaloid nucleus, medial division (CeM, 9-fold, p = 0.01), supraoptic nucleus (SON, 9-fold, p < 0.001) and the arcuate nucleus (Arc, 2.5-fold, p < 0.03) compared to control animals. Also phoenixin expression significantly increased in the DMN (17-fold, p < 0.001), RPa (2-fold, p < 0.001) and mNTS (1.6-fold, p < 0.001) with positive correlations between c-Fos and phoenixin (r = 0.74-0.85; p < 0.01) in these nuclei. This pattern of activation suggests an involvement of phoenixin in response to restraint stress. Whether phoenixin mediates stress effects or is activated in a counterbalancing fashion will have to be further investigated.

Phoenixin-20 Stimulates mRNAs Encoding Hypothalamo-Pituitary-Gonadal Hormones, is Pro-Vitellogenic, and Promotes Oocyte Maturation in Zebrafish

Phoenixin-20 (PNX-20) is a bioactive peptide with hormone-like actions in vertebrates. In mammals, PNX stimulates hypothalamo-pituitary-gonadal hormones and regulate reproductive processes. Our immunohisto/cytochemical studies show PNX-like and the putative PNX receptor, SREB3-like immunoreactivity in the gonads of zebrafish, and in zebrafish liver (ZFL) cells. Intraperitoneal injection of zebrafish PNX-20 upregulates mRNAs encoding both salmon gonadotropin-releasing hormone (GnRH), and chicken GnRH-II and kisspeptin and its receptor in zebrafish hypothalamus. Similarly, luteinizing hormone receptor mRNA expression in the testis, follicle-stimulating hormone receptor in the ovary, and the kisspeptin system were upregulated in the gonads of PNX-20 injected fish. We also observed the upregulation of genes involved in the sex steroidogenic pathway (cyp11a1, cyp17a1, 17βhsd, cyp19a1a) in the gonads of PNX-20 administered fish. PNX-20 upregulates the expression of vitellogenin isoforms and estrogen receptor (esr2a and 2b) mRNAs in ZFL cells in vitro. Meanwhile, siRNA-mediated knockdown of PNX-20 resulted in the downregulation of all vitellogenin transcripts, further suggesting its possible role in vitellogenesis. PNX-20 treatment resulted in a significant increase in germinal vesicle breakdown in zebrafish follicles in vitro. Collectively, these results provide strong evidence for PNX-20 effects on the HPG axis and liver to promote reproduction in zebrafish.

Phoenixin-20 suppresses food intake, modulates glucoregulatory enzymes, and enhances glycolysis in zebrafish

Phoenixin is a 20-amino acid peptide (PNX-20) cleaved from the small integral membrane protein 20 (SMIM20), with multiple biological roles in mammals. However, its role in nonmammalian vertebrates is poorly understood. This research aimed to determine whether PNX-20 influences feeding and metabolism in zebrafish. The mRNAs encoding SMIM20 and its putative receptor, super conserved receptor expressed in brain 3 (SREB3), are present in both central and peripheral tissues of zebrafish. Immunohistochemical analysis confirmed the presence of PNX-like immunoreactivity in the gut and in zebrafish liver (ZFL) cell line. We also found that short-term fasting (7 days) significantly decreased smim20 mRNA expression in the brain, gut, liver, gonads, and muscle, which suggests a role for PNX-20 in food intake regulation. Indeed, single intraperitoneal injection of 1,000 ng/g body wt PNX-20 reduced feeding in both male and female zebrafish, likely in part by enhancing hypothalamic cart and reducing hypothalamic/gut preproghrelin mRNAs. Furthermore, the present results demonstrated that PNX-20 modulates the expression of genes involved in glucose transport and metabolism in ZFL cells. In general terms, such PNX-induced modulation of gene expression was characterized by the upregulation of glycolytic genes and the downregulation of gluconeogenic genes. A kinetic study of the ATP production rate from both glycolytic and mitochondrial pathways demonstrated that PNX-20-treated ZFL cells exhibited significantly higher ATP production rate associated with glycolysis than control cells. This confirms a positive role for PNX-20 on glycolysis. Together, these results indicate that PNX-20 is an anorexigen with important metabolic roles in zebrafish.

Neuropeptides in gut-brain axis and their influence on host immunity and stress

In recent decades, neuropeptides have been found to play a major role in communication along the gut-brain axis. Various neuropeptides are expressed in the central and peripheral nervous systems, where they facilitate the crosstalk between the nervous systems and other major body systems. In addition to being critical to communication from the brain in the nervous systems, neuropeptides actively regulate immune functions in the gut in both direct and indirect ways, allowing for communication between the immune and nervous systems. In this mini review, we discuss the role of several neuropeptides, including calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase-activating polypeptide (PACAP), corticotropin-releasing hormone (CRH) and phoenixin (PNX), in the gut-brain axis and summarize their functions in immunity and stress. We choose these neuropeptides to highlight the diversity of peptide communication in the gut-brain axis.

Contingent role of phoenixin and nesfatin-1 on secretions of the male reproductive hormones

Phoenixin (PNX) and nesfatin-1 are localised in the hypothalamus and the pituitary gland. Moreover, the most of the PNX-expressing neurons in the hypothalamus also co-express nesfatin-1. These outcomes may suggest that there is an interaction between PNX and nesfatin-1, at least in terms of neuroendocrine-mediated regulations. Hence, the study was planned to find out the effects of centrally delivered PNX and nesfatin-1 on male sex hormones or to show the interactive association of intracerebroventricularly (ICV) injected PNX+nesfatin-1 combination on the release of male hormones. PNX and nesfatin-1, single or together, were delivered ICV to different male Wistar Albino rat groups. Both PNX and nesfatin-1 induced a significant enhancement in plasma FSH, LH and testosterone without inducing any alteration in plasma GnRH in the rats. The central combinatorial treatment of both the neuropeptides produced a more potent rise in male plasma hormone levels than treating with single neuropeptide. In summary, our preliminary data show that centrally delivered PNX and nesfatin-1 can affect plasma male hormone levels. Moreover, that the combinatorial treatment with both the neuropeptides in male rats leading to a more potent effect on the plasma male hormone levels might suggest that both these neuropeptides act synergistically in terms of regulation of male HPGA.

The role of phoenixin in behavior and food intake

The recently discovered peptide phoenixin was initially implicated in reproduction as a regulator of gonadotropin-releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release from the pituitary. Subsequently, various functions of phoenixin have been demonstrated including mediation of itching sensation, stimulation of vasopressin secretion, stimulation of white adipogenesis and hypothalamic nutrient sensing. Subsequently, additional actions of phoenixin have been described, namely effects on behavior. A systematic search of four data bases was performed and original articles selected accordingly. The present systematic review will present the current knowledge on the effects of phoenixin on different behaviors such as anxiety and food intake as well as cognition. Lastly, gaps in knowledge will be mentioned to stimulate further research.

Neuroendocrinology of reproduction: Is gonadotropin-releasing hormone (GnRH) dispensable?

Gonadotropin releasing hormone (GnRH) is a highly conserved neuroendocrine decapeptide that is essential for the onset of puberty and the maintenance of the reproductive state. First identified in mammals, the GnRH signaling pathway is found in all classes of vertebrates; homologues of GnRH have also been identified in invertebrates. In addition to its role as a hypothalamic releasing hormone, GnRH has multiple functions including modulating neural activity within specific regions of the brain. These various functions are mediated by multiple isoforms, which are expressed at diverse locations within the central nervous system. Here we discuss the GnRH signaling pathways in light of new reports that reveal that some vertebrate genomes lack GnRH1. Not only do other isoforms of GnRH not compensate for this gene loss, but elements upstream of GnRH1, including kisspeptins, appear to also be dispensable. We discuss routes that may compensate for the loss of the GnRH1 pathway.

Phoenixin-14 stimulates differentiation of 3T3-L1 preadipocytes via cAMP/Epac-dependent mechanism

Phoenixin-14 (PNX) is a newly discovered peptide produced by proteolytic cleavage of the small integral membrane protein 20 (Smim20). Previous studies showed that PNX is involved in controlling reproduction, pain, anxiety and memory. Furthermore, in humans, PNX positively correlates with BMI suggesting a potential role of PNX in controlling fat accumulation in obesity. Since the influence of PNX on adipose tissue formation has not been so far demonstrated, we investigated the effects of PNX on proliferation and differentiation of preadipocytes using 3T3-L1 and rat primary preadipocytes. We detected Smim20 and Gpr173 mRNA in 3T3-L1 preadipocytes as well as in rat primary preadipocytes. Furthermore, we found that PNX peptide is produced and secreted from 3T3-L1 and rat primary adipocytes. PNX increased 3T3-L1 preadipocytes proliferation and viability. PNX stimulated the expression of adipogenic genes (Pparγ, C/ebpβ and Fabp4) in 3T3-L1 adipocytes. 3T3-L1 preadipocytes differentiated in the presence of PNX had increased lipid content. Stimulation of cell proliferation and differentiation by PNX was also confirmed in rat preadipocytes. PNX failed to induce AKT phosphorylation, however, PNX increased cAMP levels in 3T3-L1 cells. Suppression of Epac signalling attenuated PNX-induced Pparγ expression without affecting cell proliferation. Our data show that PNX stimulates differentiation of 3T3-L1 and rat primary preadipocytes into mature adipocytes via cAMP/Epac-dependent pathway. In conclusion our data shows that phoenixin promotes white adipogenesis, thereby may be involved in controlling body mass regulation.

The phoenixins: From discovery of the hormone to identification of the receptor and potential physiologic actions

Using a series of classical protein purification techniques, coupled with more modern molecular approaches, a family of neuropeptides, the Phoenixins, was identified to be produced in brain and heart, and to bind selectively in pituitary gland, ovary and brain. These same binding sites were revealed, using a novel receptor identification strategy, to express the orphan G protein-coupled receptor, GPR173, the expression of which was required for the actions of phoenixin both in vivo and in vitro. In fact, studies using small interfering RNA molecules to compromise GPR173 expression revealed the physiologic relevance of the initially reported pharmacologic actions of the peptides. Those include not only the reproductive actions of the peptides in brain and pituitary gland, but also a CNS site of action in the maintenance of fluid and electrolyte homeostasis. Additional pharmacologic actions of the phoenixins have been described and the race is on to establish the physiologic relevance of those actions as well as the therapeutic potential of phoenixin analogs.

Phoenixin-A Pleiotropic Gut-Brain Peptide

Phoenixin is a recently discovered brain peptide initially thought to be restricted to reproductive functions. The subsequent identification of phoenixin’s expression in peripheral tissues was accompanied by the description of several other actions of this hormone, such as effects on behavior, sensory perception, memory retention, the cardiovascular system as well as food intake, pointing towards a pleiotropic role of this peptide. The present review will discuss the present knowledge on phoenixin and the signaling involved as well as highlight gaps in knowledge to stimulate further research.

Phoenixin: a novel brain-gut-skin peptide with multiple bioactivity

In this brief review we summarize the current fndings relative to the discovery of a small peptide ligand, phoenixin (PNX). Using a bioinformatic approach, two novel peptides PNX-14 and PNX-20 containing 14 and 20 amino acids, respectively, were isolated from diverse tissues including the brain, heart, lung and stomach. Mass spectrometry analysis identified a major and minor peak corresponding to PNX-14 and PNX-20, in rat or mouse spinal cord extracts. With the use of a rabbit polyclonal antiserum, phoenixin immunoreactivity (irPNX) was detected in discrete areas of the rodent brain including several hypothalamic subnuclei and dorsal motor nucleus of the vagus. In addition, irPNX was detected in a population of sensory ganglion cells including dorsal root ganglion, nodose ganglion and trigeminal ganglion, and in cell processes densely distributed to the superficial layers of the dorsal horn, nucleus of the solitary tract and spinal trigeminal tract. irPNX cell processes were also detected in the skin and myenteric plexus, suggesting a brain-gut and/or brain-skin connection. Pharmacological studies show that PNX-14 injected subcutaneously to the nape of the neck of mice provoked dose-dependent repetitive scratching bouts directed to the back of the neck with the hindpaws. Our result suggests that the peptide PNX-14 and/or PNX-20, may serve as one of the endogenous signal molecules transducing itch sensation. Additionally, results from other laboratories show that exogenous PNX may affect a number of diverse behaviors such as memory formation, depression, reproduction, food-intake and anxiolytic-like behaviors.

Phoenixin: uncovering its receptor, signaling and functions

Phoenixin (PNX) is a newly discovered peptide that has been linked to reproductive function, both in the hypothalamus and pituitary. This review will focus on the most recent discoveries related to this novel neuropeptide. Initially, it was found that PNX increased gonadotropin releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release from pituitary cells. Importantly, knockdown of PNX in female rats extended the estrous cycle by 2.3 days. Using novel hypothalamic cell lines, we found that PNX has a stimulatory role on kisspeptin (Kiss) and GnRH gene expression and secretion. The PNX receptor was uncovered using siRNA knockdown of GPR173, an orphan receptor postulated to bind PNX. We have found that the PNX-R signaling through protein kinase A (PKA) in hypothalamic neurons. Althuogh a number of studies demonstrate that PNX plays an important role in reproductive function, there is also evidence that it may have other functions, regulating the heart, feeding, memory, and anxiety, both in the brain and the periphery.

Phoenixin-14: detection and novel physiological implications in cardiac modulation and cardioprotection

Phoenixin-14 (PNX) is a newly identified peptide co-expressed in the hypothalamus with the anorexic and cardioactive Nesfatin-1. Like Nesfatin-1, PNX is able to cross the blood-brain barrier and this suggests a role in peripheral modulation. Preliminary mass spectrography data indicate that, in addition to the hypothalamus, PNX is present in the mammalian heart. This study aimed to quantify PNX expression in the rat heart, and to evaluate whether the peptide influences the myocardial function under basal condition and in the presence of ischemia/reperfusion (I/R). By ELISA the presence of PNX was detected in both hypothalamus and heart. In plasma of normal, but not of obese rats, the peptide concentrations increased after meal. Exposure of the isolated and Langendorff perfused rat heart to exogenous PNX induces a reduction of contractility and relaxation, without effects on coronary pressure and heart rate. As revealed by immunoblotting, these effects were accompanied by an increase of Erk1/2, Akt and eNOS phosphorylation. PNX (EC50 dose), administered after ischemia, induced post-conditioning-like cardioprotection. This was revealed by a smaller infarct size and a better systolic recovery with respect to those detected on hearts exposed to I/R alone. The peptide also activates the cardioprotective RISK and SAFE cascades and inhibits apoptosis. These effects were also observed in the heart of obese rats. Our data provide a first evidence on the peripheral activity of PNX and on its direct cardiomodulatory and cardioprotective role under both normal conditions and in the presence of metabolic disorders.

Phoenixin-14 injected intracerebroventricularly but not intraperitoneally stimulates food intake in rats

Phoenixin, a recently discovered 20-amino acid peptide was implicated in reproduction. However, the expression in food intake-regulatory nuclei such as the paraventricular nucleus, the arcuate nucleus and the nucleus of the solitary tract suggests an implication of phoenixin in food intake regulation. Therefore, we investigated the effects of phoenixin-14, the shorter form of phoenixin, on food intake following intracerebroventricular (icv) and intraperitoneal (ip) injection in ad libitum fed male Sprague-Dawley rats. Phoenixin-14 injected icv (0.2, 1.7 or 15nmol/rat) during the light phase induced a dose-dependent increase of light phase food intake reaching significance at a minimum dose of 1.7 nmol/rat (+72%, p<0.05 vs. vehicle) used for all further analyses. Assessment of the food intake microstructure showed an icv phoenixin-14-induced increase in meal size (+51%), meal duration (+157%), time spent in meals (+182%) and eating rate (+123%), while inter-meal intervals (-42%) and the satiety ratio (-64%) were decreased compared to vehicle (p<0.05). When injected icv during the dark phase, no modulation of food intake was observed (p>0.05). The light phase icv phoenixin-14-induced increase of water intake did not reach statistical significance compared to vehicle (+136%, p>0.05). The increase of food intake following icv phoenixin-14 was not associated with a significant alteration of grooming behavior (0.4-fold, p=0.377) or locomotion (6-fold, p=0.066) compared to vehicle. When injected ip at higher doses (0.6, 5nmol/kg or 45nmol/kg body weight) during the light phase, phoenixin-14 did not affect food intake (p>0.05). In summary, phoenixin-14 exerts a centrally-mediated orexigenic effect.

Phoenixin is negatively associated with anxiety in obese men

Phoenixin was recently identified in the rat hypothalamus and initially implicated in reproductive functions. A subsequent study described an anxiolytic effect of the peptide. The aim of the study was to investigate a possible association of circulating phoenixin with anxiety in humans. We therefore enrolled 68 inpatients with a broad spectrum of psychometrically measured anxiety (GAD-7). We investigated men since a menstrual cycle dependency of phoenixin has been assumed. Obese subjects were enrolled since they often report psychological comorbidities. In addition, we also assessed depressiveness (PHQ-9) and perceived stress (PSQ-20). Plasma phoenixin levels were measured using a commercial ELISA. First, we validated the ELISA kit performing a spike-and-recovery experiment showing a variance of 6.7±8.8% compared to the expected concentrations over the whole range of concentrations assessed, while a lower variation of 1.6±0.8% was observed in the linear range of the assay (0.07-2.1ng/ml). We detected phoenixin in the circulation of obese men at levels of 0.68±0.50ng/ml. These levels showed a negative association with anxiety scores (r=-0.259, p=0.043), while no additional associations with other psychometric parameters were observed. In summary, phoenixin is present in the human circulation and negatively associated with anxiety in obese men, a population often to report comorbid anxiety.

Escitalopram affects spexin expression in the rat hypothalamus, hippocampus and striatum

BACKGROUND

Spexin (SPX) is a recently discovered neuropeptide that exhibits a large spectrum of central and peripheral regulatory activity, especially when considered as a potent anorexigenic factor. It has already been proven that antidepressants, including selective serotonin reuptake inhibitors (SSRI), can modulate peptidergic signaling in various brain structures. Despite these findings, there is so far no information regarding the influence of treatment with the SSRI antidepressant escitalopram on brain SPX expression.

METHODS

In this current study we measured SPX mRNA and protein expression in the selected brain structures (hypothalamus, hippocampus and striatum) of rats chronically treated with a 10mg/kg dose of escitalopram using quantitative Real-Time PCR and immunohistochemistry.

RESULTS

Strikingly, long-term (4 week) drug treatment led to the downregulation of SPX expression in the rat hypothalamus. This supports the hypothesis that SPX may be involved in the hypothalamic serotonin-dependent actions of SSRI antidepressants and possibly also in the central mechanism of body mass increase. Conversely, SPX expression increased in the hippocampus and striatum.

CONCLUSIONS

This is the first report of the effects of a neuropsychiatric medication on SPX expression in animal brain. Our findings shed a new light on the pharmacology of antidepressants and may contribute to a better understanding of the alternative mechanisms responsible for antidepressant action.