In vitro and in vivo regulation of synaptogenesis by the novel antidepressant spadin

Serum potassium level is associated with serum neurofilament light chain in American adult population: a cross-sectional analysis of the 2013-2014 National Health and Nutrition Examination Survey

Objective

Serum neurofilament light chain (sNfL) is one of the most sensitive diagnostic biomarkers for a variety of neurodegenerative pathologies. Potassium, an essential electrolyte, plays a critical role in maintaining neuronal health and the proper functioning of the central nervous system (CNS). The aim of our research was to investigate the association between serum potassium level and sNfL levels.

Methods

Based on the National Health and Nutrition Examination Survey (NHANES) database, we analyzed data from the 2013 to 2014 NHANES. Serum potassium concentrations were measured via ion-selective electrode (ISE) technology. The levels of sNfL were measured using a sensitive immunoassay developed by Siemens Healthineers. Our researcher analyzed the association between potassium level in serum and sNfL in American persons using multivariate logistic regression analysis and smoothed curve fitting. The consistency of these results was examined in various population subgroups.

Results

A total of 1,670 participants were involved in our research, including 872 women (50.5%) and 798 men (49.5%). The median serum potassium concentration was 4.0 mmol/L and the median sNfL was 12.3 pg/ml. After adjusting for potential confounders in the full model, individuals with higher serum potassium concentrations had higher sNfL levels (Q3 vs. Q1, β = 2.86, 95% CI: 0.33-5.39, P = 0.027). There was a non-linear positive dose-response association between higher serum potassium concentrations and sNfL levels (P for non-linearity = 0.028). Based on the results of stratified analysis, the relationship was stronger in the low- and middle-income group, non-drinking and non-physical activity participants, and participants with hypertension and diabetes.

Interpretation

In the cohort of American adults, a greater serum potassium concentration was linked to a higher sNfL. However, causality still needs to be further verified.

TREK-1 inhibition promotes synaptic plasticity in the prelimbic cortex

Synaptic plasticity is one of the putative mechanisms involved in the maturation of the prefrontal cortex (PFC) during postnatal development. Early life stress (ELS) affects the shaping of cortical circuitries through impairment of synaptic plasticity supporting the onset of mood disorders. Growing evidence suggests that dysfunctional postnatal maturation of the prelimbic division (PL) of the PFC might be related to the emergence of depression. The potassium channel TREK-1 has attracted particular interest among many factors that modulate plasticity, concerning synaptic modifications that could underlie mood disorders. Studies have found that ablation of TREK-1 increases the resilience to depression, while rats exposed to ELS exhibit higher TREK-1 levels in the PL. TREK-1 is regulated by multiple intracellular transduction pathways including the ones activated by metabotropic receptors. In the hippocampal neurons, TREK-1 interacts with the serotonergic system, one of the main factors involved in the action of antidepressants. To investigate possible mechanisms related to the antidepressant role of TREK-1, we used brain slice electrophysiology to evaluate the effects of TREK-1 pharmacological blockade on synaptic plasticity at PL circuitry. We extended this investigation to animals subjected to ELS. Our findings suggest that in non-stressed animals, TREK-1 activity is required for the reduction of synaptic responses mediated by the 5HT1A receptor activation. Furthermore, we demonstrate that TREK-1 blockade promotes activity-dependent long-term depression (LTD) when acting in synergy with 5HT1A receptor stimulation. On the other hand, in ELS animals, TREK-1 blockade reduces synaptic transmission and facilitates LTD expression. These results indicate that TREK-1 inhibition stimulates synaptic plasticity in the PL and this effect is more pronounced in animals subjected to ELS during postnatal development.

Unlocking Therapeutic Synergy: Tailoring Drugs for Comorbidities such as Depression and Diabetes through Identical Molecular Targets in Different Cell Types

Research in the field of pharmacology aims to generate new treatments for pathologies. Nowadays, there are an increased number of chronic disorders that severely and durably handicap many patients. Among the most widespread pathologies, obesity, which is often associated with diabetes, is constantly increasing in incidence, and in parallel, neurodegenerative and mood disorders are increasingly affecting many people. For years, these pathologies have been so frequently observed in the population in a concomitant way that they are considered as comorbidities. In fact, common mechanisms are certainly at work in the etiology of these pathologies. The main purpose of this review is to show the value of anticipating the effect of baseline treatment of a condition on its comorbidity in order to obtain concomitant positive actions. One of the implications would be that by understanding and targeting shared molecular mechanisms underlying these conditions, it may be possible to tailor drugs that address both simultaneously. To this end, we firstly remind readers of the close link existing between depression and diabetes and secondly address the potential benefit of the pleiotropic actions of two major active molecules used to treat central and peripheral disorders, first a serotonin reuptake inhibitor (Prozac ®) and then GLP-1R agonists. In the second part, by discussing the therapeutic potential of new experimental antidepressant molecules, we will support the concept that a better understanding of the intracellular signaling pathways targeted by pharmacological agents could lead to future synergistic treatments targeting solely positive effects for comorbidities.

Potential of Heterogeneous Compounds as Antidepressants: A Narrative Review

Depression is a globally widespread disorder caused by a complicated interplay of social, psychological, and biological factors. Approximately 280 million people are suffering from depression worldwide. Traditional frontline antidepressants targeting monoamine neurotransmitters show unsatisfactory effects. The development and application of novel antidepressants for dissimilar targets are on the agenda. This review characterizes the antidepressant effects of multiple endogenous compounds and/or their targets to provide new insight into the working mechanism of antidepressants. We also discuss perspectives and challenges for the generation of novel antidepressants.

Maternal DBP exposure promotes synaptic formation in offspring by activating astrocytes via the AKT/NF-κB/IL-6/JAK2/STAT3 signaling pathway

It has been demonstrated that activated astrocytes in the hypothalamus could disrupt GnRH secretion in offspring after maternal di-n-butyl phthalate (DBP) exposure, indicating that the effect of DBP on astrocyte activation and crosstalk between astrocytes and neurons is still worthy of further investigation. In this study, pregnant mice were intragastrically administered DBP dissolved in corn oil from gestational days (GD) 12.5-21.5. Maternal DBP exposure resulted in hippocampal astrocyte activation, abnormal synaptic formation, and reduced autonomic and exploratory behavior in offspring on postnatal day (PND) 22. Further studies identified that mono-n-butyl phthalate (MBP) induced astrocyte activation and proliferation by activating the AKT/NF-κB/IL-6/JAK2/STAT3 signaling pathway. Moreover, upregulated thrombospondin 1 (TSP1) in activated astrocytes regulated synaptic-related protein expression. This study highlights the neurotoxicity of maternal DBP exposure to offspring, which provides new insights into identifying potential molecular targets for the treatment of diseases related to neurological development disorders in children.

The Two-Pore Domain Potassium Channel TREK-1 Promotes Blood-Brain Barrier Breakdown and Exacerbates Neuronal Death After Focal Cerebral Ischemia in Mice

Earlier studies have shown the neuroprotective role of TWIK-related K⁺ channel 1 (TREK-1) in global cerebral and spinal cord ischemia, while its function in focal cerebral ischemia has long been debated. This study used TREK-1-deficient mice to directly investigate the role of TREK-1 after focal cerebral ischemia. First, immunofluorescence assays in the mouse cerebral cortex indicated that TREK-1 expression was mostly abundant in astrocytes, neurons, and oligodendrocyte precursor cells but was low in myelinating oligodendrocytes, microglia, or endothelial cells. TREK-1 deficiency did not affect brain weight and morphology or the number of neurons, astrocytes, or microglia but did increase glial fibrillary acidic protein (GFAP) expression in astrocytes of the cerebral cortex. The anatomy of the major cerebral vasculature, number and structure of brain micro blood vessels, and blood-brain barrier integrity were unaltered. Next, mice underwent 60 min of focal cerebral ischemia and 72 h of reperfusion induced by the intraluminal suture method. TREK-1-deficient mice showed less neuronal death, smaller infarction size, milder blood-brain barrier (BBB) breakdown, reduced immune cell invasion, and better neurological function. Finally, the specific pharmacological inhibition of TREK-1 also decreased infarction size and improved neurological function. These results demonstrated that TREK-1 might play a detrimental rather than beneficial role in focal cerebral ischemia, and inhibition of TREK-1 would be a strategy to treat ischemic stroke in the clinic.

Contribution of Neuronal and Glial Two-Pore-Domain Potassium Channels in Health and Neurological Disorders

Two-pore-domain potassium (K2P) channels are widespread in the nervous system and play a critical role in maintaining membrane potential in neurons and glia. They have been implicated in many stress-relevant neurological disorders, including pain, sleep disorder, epilepsy, ischemia, and depression. K2P channels give rise to leaky K⁺ currents, which stabilize cellular membrane potential and regulate cellular excitability. A range of natural and chemical effectors, including temperature, pressure, pH, phospholipids, and intracellular signaling molecules, substantially modulate the activity of K2P channels. In this review, we summarize the contribution of K2P channels to neuronal excitability and to potassium homeostasis in glia. We describe recently discovered functions of K2P channels in glia, such as astrocytic passive conductance and glutamate release, microglial surveillance, and myelin generation by oligodendrocytes. We also discuss the potential role of glial K2P channels in neurological disorders. In the end, we discuss current limitations in K2P channel researches and suggest directions for future studies.

Sortilin-derived peptides promote pancreatic beta-cell survival through CREB signaling pathway

Deterioration of insulin secretion and pancreatic beta-cell mass by inflammatory attacks is one of the main pathophysiological features of type 2 diabetes (T2D). Therefore, preserving beta-cell mass and stimulating insulin secretion only in response to glucose for avoiding the hypoglycemia risks, are the most state-of-the-art option for the treatment of T2D. In this study we tested two correlated hypothesis that 1/ the endogenous peptide released from sortilin, known as PE, that stimulates insulin secretion only in response to glucose, protects beta-cells against death induced by cytokines, and 2/ Spadin and Mini-Spadin, two synthetic peptides derived from PE, that mimic the effects of PE in insulin secretion, also provide beneficial effect on beta-cells survival. We show that PE and its derivatives by inducing a rise of intracellular calcium concentration by depolarizing the membrane protect beta-cells against death induced by Interleukin-1β. Using biochemical, confocal imaging and cell biology techniques, we reveal that the protective effects of PE and its derivatives rely on the activation of the CaM-Kinase pathway, and on the phosphorylation and activation of the transcription factor CREB. In addition, Mini-Spadin promotes beta-cell proliferation, suggesting its possible regenerative effect. This study highlights new possible roles of PE in pancreatic beta-cell survival and its derivatives as pharmacological tools against diabetes.

Genetic and pharmacological inhibition of two-pore domain potassium channel TREK-1 alters depression-related behaviors and neuronal plasticity in the hippocampus in mice

Introduction

The two‐pore domain potassium channel TREK‐1 is a member of background K⁺ channels that are thought to provide baseline regulation of membrane excitability. Recent studies have highlighted the putative role of TREK‐1 in the action of antidepressants, and its antagonists might be potentially effective antidepressants. However, the mechanisms underlying the actions of TREK‐1 are not yet fully understood.

Methods

The expression of TREK‐1 was examined in a mouse model of chronic unpredictable mild stress (CUMS) using immunoblotting. Neuron‐specific genetic manipulation of TREK‐1 was performed through adeno‐associated virus. Behavioral tests were performed to evaluate depression‐related behaviors. Electrophysiological recordings were used to evaluate synaptic plasticity. Golgi staining was used to examine neuroplasticity.

Results

TREK‐1 expression was increased in the mouse hippocampus after CUMS. Knockdown of TREK‐1 in hippocampal neurons significantly attenuated depressive‐like behaviors and prevented the decrease of CUMS‐induced synaptic proteins in mice. Further examination indicated that neuron‐specific knockdown of TREK‐1 in the hippocampus prevented stress‐induced impairment of glutamatergic synaptic transmission in the CA1 region. Moreover, chronic TREK‐1 inhibition protected against CUMS‐induced depressive‐like behaviors and impairment of synaptogenesis in the hippocampus.

Conclusion

Our results indicate a role for TREK‐1 in the modulation of synaptic plasticity in a mouse model of depression. These findings will provide insight into the pathological mechanism of depression and further evidence for a novel target for antidepressant treatment.

Involvement of Phosphodiesterase 2A Activity in the Pathophysiology of Fragile X Syndrome

The fragile X mental retardation protein (FMRP) is an RNA-binding protein involved in translational regulation of mRNAs that play key roles in synaptic morphology and plasticity. The functional absence of FMRP causes the fragile X syndrome (FXS), the most common form of inherited intellectual disability and the most common monogenic cause of autism. No effective treatment is available for FXS. We recently identified the Phosphodiesterase 2A (Pde2a) mRNA as a prominent target of FMRP. PDE2A enzymatic activity is increased in the brain of Fmr1-KO mice, a recognized model of FXS, leading to decreased levels of cAMP and cGMP. Here, we pharmacologically inhibited PDE2A in Fmr1-KO mice and observed a rescue both of the maturity of dendritic spines and of the exaggerated hippocampal mGluR-dependent long-term depression. Remarkably, PDE2A blockade rescued the social and communicative deficits of both mouse and rat Fmr1-KO animals. Importantly, chronic inhibition of PDE2A in newborn Fmr1-KO mice followed by a washout interval, resulted in the rescue of the altered social behavior observed in adolescent mice. Altogether, these results reveal the key role of PDE2A in the physiopathology of FXS and suggest that its pharmacological inhibition represents a novel therapeutic approach for FXS.

TREK-1 Null Impairs Neuronal Excitability, Synaptic Plasticity, and Cognitive Function

TREK-1, a two-pore-domain K+ channel, is highly expressed in the central nervous system. Although aberrant expression of TREK-1 is implicated in cognitive impairment, the cellular and functional mechanism underlying this channelopathy is poorly understood. Here we examined TREK-1 contribution to neuronal morphology, excitability, synaptic plasticity, and cognitive function in mice deficient in TREK-1 expression. TREK-1 immunostaining signal mainly appeared in hippocampal pyramidal neurons, but not in astrocytes. TREK-1 gene knockout (TREK-1 KO) increases dendritic sprouting and the number of immature spines in hippocampal CA1 pyramidal neurons. Functionally, TREK-1 KO increases neuronal excitability and enhances excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs). The increased EPSCs appear to be attributed to an increased release probability of presynaptic glutamate and functional expression of postsynaptic AMPA receptors. TREK-1 KO decreased the paired-pulse ratio and severely occluded the long-term potentiation (LTP) in the CA1 region. These altered synaptic transmission and plasticity are associated with recognition memory deficit in TREK-1 KO mice. Although astrocytic expression of TREK-1 has been reported in previous studies, TREK-1 KO does not alter astrocyte membrane K+ conductance or the syncytial network function in terms of syncytial isopotentiality. Altogether, TREK-1 KO profoundly affects the cellular structure and function of hippocampal pyramidal neurons. Thus, the impaired cognitive function in diseases associated with aberrant expression of TREK-1 should be attributed to the failure of this K+ channel in regulating neuronal morphology, excitability, synaptic transmission, and plasticity.

The Knockdown of TREK-1 in Hippocampal Neurons Attenuate Lipopolysaccharide-Induced Depressive-Like Behavior in Mice

TWIK-related potassium channel-1 (TREK-1) is broadly expressed in the brain and involved in diverse brain diseases, such as seizures, ischemia, and depression. However, the cell type-specific roles of TREK-1 in the brain are largely unknown. Here, we generated a Cre-dependent TREK-1 knockdown (Cd-TREK-1 KD) transgenic mouse containing a gene cassette for Cre-dependent TREK-1 short hairpin ribonucleic acid to regulate the cell type-specific TREK-1 expression. We confirmed the knockdown of TREK-1 by injecting adeno-associated virus (AAV) expressing Cre into the hippocampus of the mice. To study the role of hippocampal neuronal TREK-1 in a lipopolysaccharide (LPS)-induced depression model, we injected AAV-hSyn-BFP (nCTL group) or AAV-hSyn-BFP-Cre (nCre group) virus into the hippocampus of Cd-TREK-1 KD mice. Interestingly, the immobility in the tail suspension test after LPS treatment did not change in the nCre group. Additionally, some neurotrophic factors (BDNF, VEGF, and IGF-1) significantly increased more in the nCre group compared to the nCTL group after LPS treatment, but there was no difference in the expression of their receptors. Therefore, our data suggest that TREK-1 in the hippocampal neurons has antidepressant effects, and that Cd-TREK-1 KD mice are a valuable tool to reveal the cell type-specific roles of TREK-1 in the brain.

First evidence of protective effects on stroke recovery and post-stroke depression induced by sortilin-derived peptides

Post-stroke depression (PSD) is the most common mood disorder following stroke with high relevance for outcome and survival of patients. The TREK-1 channel represents a crucial target in the pathogenesis of stroke and depression. Spadin and its short analog mini-spadin were reported to display potent antidepressant properties. We investigated the therapeutic effects of mini-spadin in a mouse model of focal ischemia and PSD. To activate TREK-1 and induce neuroprotection a single low dose of mini-spadin (0.03 μg/kg) was intraperitoneally injected 30  min after the onset of ischemia, once a day during 7 days post-ischemia. Then, to inhibit TREK-1 and induce antidepressant effect, the peptide was injected at higher concentration (3 μg/kg) once a day for 4 days/week until the sacrifice of animals. Electrophysiological studies showed that mini-spadin had a biphasic action on TREK-1. At low doses, the channel activity was increased whereas at higher doses it was inhibited. Mini-spadin prevented the loss of body weight and the delayed dopaminergic degeneration in substantia nigra and improved the motor and cognitive ischemia-induced deficits. Moreover, mini-spadin prevented PSD analyzed in the Forced Swim (FST) and Novelty Suppressed Feeding (NSF) tests. Finally, enhanced neurogenesis and synaptogenesis contributed to the beneficial effects of mini-spadin against stroke and PSD. This work reveals the first evidence that the modulation of TREK-1 channels in the early and chronic phases of stroke as well as the stimulation of brain plasticity by mini-spadin could play a key role in its brain protective effects against stroke and its deleterious consequences such as PSD.

Altered detrusor contractility and voiding patterns in mice lacking the mechanosensitive TREK-1 channel

BACKGROUND

Previously published results from our laboratory identified a mechano-gated two-pore domain potassium channel, TREK-1, as a main mechanosensor in the smooth muscle of the human urinary bladder. One of the limitations of in vitro experiments on isolated human detrusor included inability to evaluate in vivo effects of TREK-1 on voiding function, as the channel is also expressed in the nervous system, and may modulate micturition via neural pathways. Therefore, in the present study, we aimed to assess the role of TREK-1 channel in bladder function and voiding patterns in vivo by using TREK-1 knockout (KO) mice.

METHODS

Adult C57BL/6 J wild-type (WT, N = 32) and TREK-1 KO (N = 33) mice were used in this study. The overall phenotype and bladder function were evaluated by gene and protein expression of TREK-1 channel, in vitro contractile experiments using detrusor strips in response to stretch and pharmacological stimuli, and cystometry in unanesthetized animals.

RESULTS

TREK-1 KO animals had an elevated basal muscle tone and enhanced spontaneous activity in the detrusor without detectable changes in bladder morphology/histology. Stretch applied to isolated detrusor strips increased the amplitude of spontaneous contractions by 109% in the TREK-1 KO group in contrast to a 61% increase in WT mice (p ≤ 0.05 to respective baseline for each group). The detrusor strips from TREK-1 KO mice also generated more contractile force in response to electric field stimulation and high potassium concentration in comparison to WT group (p ≤ 0.05 for both tests). However, cystometric recordings from TREK-1 KO mice revealed a significant increase in the duration of the intermicturition interval, enhanced bladder capacity and increased number of non-voiding contractions in comparison to WT mice.

CONCLUSIONS

Our results provide evidence that global down-regulation of TREK-1 channels has dual effects on detrusor contractility and micturition patterns in vivo. The observed differences are likely due to expression of TREK-1 channel not only in detrusor myocytes but also in afferent and efferent neural pathways involved in regulation of micturition which may underly the "mixed" voiding phenotype in TREK-1 KO mice.

Role of TREK-1 in Health and Disease, Focus on the Central Nervous System

TREK-1 is the most studied background K_2P channel. Its main role is to control cell excitability and maintain the membrane potential below the threshold of depolarization. TREK-1 is multi-regulated by a variety of physical and chemical stimuli which makes it a very promising and challenging target in the treatment of several pathologies. It is mainly expressed in the brain but also in heart, smooth muscle cells, endocrine pancreas, and prostate. In the nervous system, TREK-1 is involved in many physiological and pathological processes such as depression, neuroprotection, pain, and anesthesia. These properties explain why many laboratories and pharmaceutical companies have been focusing their research on screening and developing highly efficient modulators of TREK-1 channels. In this review, we summarize the different roles of TREK-1 that have been investigated so far in attempt to characterize pharmacological tools and new molecules to modulate cellular functions controlled by TREK-1.

Sortilin in Glucose Homeostasis: From Accessory Protein to Key Player?

The pharmacological properties and physiological roles of the type I receptor sortilin, also called neurotensin receptor-3, are various and complex. Sortilin is involved in important biological functions from neurotensin and pro-Nerve Growth Factor signaling in the central nervous system to regulation of glucose and lipid homeostasis in the periphery. The peripheral functions of sortilin being less extensively addressed, the focus of the current review is to discuss recent works describing sortilin-induced molecular mechanisms regulating blood glucose homeostasis and insulin signaling. Thus, an overview of several roles ascribed to sortilin in diabetes and other metabolic diseases are presented. Investigations on crucial cellular pathways involved in the protective effect of sortilin receptor on beta cells, including recent discoveries about regulation of cell fate, are also detailed. In addition, we provide a special focus on insulin secretion regulation involving complexes between sortilin and neurotensin receptors. The last section comments on the future research areas which should be developed to address the function of new effectors of the sortilin system in the endocrine apparatus.

The Involvement of Sortilin/NTSR3 in Depression as the Progenitor of Spadin and Its Role in the Membrane Expression of TREK-1

The molecular identification of sortilin, also called neurotensin receptor-3, from three different biochemical approaches already predicted the involvement of the protein in numerous biological and cellular functions. The first important observation was that sortilin is synthesized as a precursor that is converted to a mature protein after cleavage by the protein convertase furin in late Golgi compartments. This maturation leads to the formation of a 44 amino acid peptide, the propeptide (PE). The release of this peptide when matured sortilin reached the plasma membrane remained to be demonstrated. Sortilin has been also shown to be shedded by matrix metalloproteases releasing a large extracellular fragment identified as soluble sortilin. Therefore, sortilin has been shown to interact with several proteins and receptors confirming its role in the sorting of cellular components to the plasma membrane and/or to the lysosomal pathway. Interestingly, sortilin physically interacts with the two pore domain potassium channel TREK-1 and the PE as well as its synthetic analog spadin is able to block the activation of TREK-1 highlighting their role in the depression pathology. The present review describes the advance of research that led to these results and how both the soluble form of sortilin and the sortilin-derived PE have been detected in human serum and whose levels are affected in patients with major depressive disorder (MDD). The use of spadin as an antidepressant and the further role of soluble sortilin and of sortilin-derived PE as potential biomarkers during depression statement and/or remission of the pathology are considered and discussed in this review.

Identification and characterization of two zebrafish Twik related potassium channels, Kcnk2a and Kcnk2b

KCNK2 is a 2 pore domain potassium channel involved in maintaining cellular membrane resting potentials. Although KCNK2 is regarded as a mechanosensitive ion channel, it can also be gated chemically. Previous research indicates that KCNK2 expression is particularly enriched in neuronal and cardiac tissues. In this respect, KCNK2 plays an important role in neuroprotection and has also been linked to cardiac arrhythmias. KCNK2 has subsequently become an attractive pharmacologic target for developing preventative/curative strategies for neuro/cardio pathophysiological conditions. Zebrafish represent an important in vivo model for rapidly analysing pharmacological compounds. We therefore sought to identify and characterise zebrafish kcnk2 to allow this model system to be incorporated into therapeutic research. Our data indicates that zebrafish possess two kcnk2 orthologs, kcnk2a and kcnk2b. Electrophysiological analysis of both zebrafish Kcnk2 orthologs shows that, like their human counterparts, they are activated by different physiological stimuli such as mechanical stretch, polyunsaturated fatty acids and intracellular acidification. Furthermore, both zebrafish Kcnk2 channels are inhibited by the human KCNK2 inhibitory peptide spadin. Taken together, our results demonstrate that both Kcnk2a and Kcnk2b share similar biophysiological and pharmacological properties to human KCNK2 and indicate that the zebrafish will be a useful model for developing KCNK2 targeting strategies.

Fighting against depression with TREK-1 blockers: Past and future. A focus on spadin

Depression is a devastating mood disorder and a leading cause of disability worldwide. Depression affects approximately one in five individuals in the world and represents heavy economic and social burdens. The neurobiological mechanisms of depression are not fully understood, but evidence highlights the role of monoamine neurotransmitter balance. Several antidepressants (ADs) are marketed to treat depression and related mood disorders. However, despite their efficacy, they remain nonspecific and unsafe because they trigger serious adverse effects. Therefore, developing new molecules for new targets in depression has become a real necessity. Eight years ago, spadin was described as a natural peptide with AD properties. This 17-amino acid peptide blocks TREK-1 channels, an original target in depression. Compared to the classical AD drugs such as fluoxetine, which requires 3-4 weeks for the AD effect to manifest, spadin acts rapidly within only 4 days of treatment. The AD properties are associated with increased neurogenesis and synaptogenesis in the brain. Despite the advantages of this fast-acting AD, the in vivo stability is weak and does not last for >7 h. The present review summarizes different strategies such as retro-inverso strategy, cyclization, and shortening the spadin sequence that has led to the development and optimization of spadin as an AD. Shortened spadin analogs present increased inhibition potency for TREK-1, an improved AD activity, and prolonged in vivo bioavailability. Finally, we also discuss about other inhibitors of TREK-1 channels with a proven efficacy in treating depression in the clinic, such as fluoxetine.

Altered Trek-1 Function in Sortilin Deficient Mice Results in Decreased Depressive-Like Behavior

The background potassium channel TREK-1 has been shown to be a potent target for depression treatment. Indeed, deletion of this channel in mice resulted in a depression resistant phenotype. The association of TREK-1 with the sorting protein sortilin prompted us to investigate the behavior of mice deleted from the gene encoding sortilin (Sort1^−/−). To characterize the consequences of sortilin deletion on TREK-1 activity, we combined behavioral, electrophysiological and biochemical approaches performed in vivo and in vitro. Analyses of Sort1^−/− mice revealed that they display: (1) a corticosterone-independent anxiety-like behavior, (2) a resistance to depression as demonstrated by several behavioral tests, and (3) an increased activity of dorsal raphe nucleus neurons. All these properties were associated with TREK-1 action deficiency consequently to a decrease of its cell surface expression and to the modification of its electrophysiological activity. An increase of BDNF expression through activation of the furin-dependent constitutive pathway as well as an increase of the activated BDNF receptor TrkB were in agreement with the decrease of depressive-like behavior of Sort1^−/− mice. Our results demonstrate that the TREK-1 expression and function are altered in the absence of sortilin confirming the importance of this channel in the regulation on the mood as a crucial target to treat depression.

Corticotropin-Releasing Hormone Suppresses Synapse Formation in the Hippocampus of Male Rats via Inhibition of CXCL5 Secretion by Glia

Corticotropin-releasing hormone (CRH) is believed to play a critical role in stress-induced synaptic formation and modification. In the current study, we explored the mechanisms underlying CRH modulation of synaptic formation in the hippocampus by using various models in vitro. In cultured hippocampal slices, CRH treatment decreased synapsin I and postsynaptic density protein 95 (PSD95) levels via CRH receptor type 1 (CRHR1). In isolated hippocampal neurons, however, it increased synapsin I-labeled presynaptic terminals and PSD95-labeled postsynaptic terminals via CRHR1. Interestingly, the inhibitory effect of CRH on synapsin I-labeled and PSD95-labeled terminals occurred in the model of neuron-glia cocultures. These effects were prevented by CRHR1 antagonist. Moreover, treatment of the neurons with the media of CRH-treated glia led to a decrease in synaptic terminal formation. The media collected from CRH-treated glial cells with CRHR1 knockdown did not show an inhibitory effect on synaptic terminals in hippocampal neurons. Unbiased cytokine array coupled with confirmatory enzyme-linked immunosorbent assay revealed that CRH suppressed C-X-C motif chemokine 5 (CXCL5) production in glia via CRHR1. Administration of CXCL5 reversed the inhibitory effects of CRH-treated glia culture media on synaptic formation. Our data suggest that CRH suppresses synapse formation through inhibition of CXCL5 secretion from glia in the hippocampus. Our study indicates that glia-neuron intercommunication is one of the mechanisms responsible for neuronal circuit remodeling during stress.

Increased serum levels of sortilin-derived propeptide after electroconvulsive therapy in treatment-resistant depressed patients

PURPOSE

Sortilin-derived propeptide (PE) and its synthetic analog spadin show strong antidepressant activity in rodents and, therefore, could be used as a biomarker to evaluate the clinical efficacy of antidepressant treatments. The aim of this study was to determine whether electroconvulsive therapy (ECT) modulates serum PE concentration in patients with treatment-resistant depression (TRD).

PATIENTS AND METHODS

Forty-five patients with major depressive disorder, who met the Diagnostic and Statistical Manual of Mental Disorders-IV criteria, were selected for this study.

RESULTS

We did not observe any difference in the PE levels between TRD patients and controls (z=0.10, P=0.92), but we found a strong significant increase between the PE levels measured just before (T0) and about 1 month (T2) after ECT (z=-2.82, P=0.005). A significant difference between T0 and T2 was observed only in responders (z=-2.59, P=0.01), whereas no effect was found in nonresponders (z=-1.27, P=0.20). Interestingly, we found a significant correlation between the increase in PE levels and decrease in Montgomery -Åsberg Depression Rating Scale scores for the total patient sample (P=0.03).

CONCLUSION

This study indicates for the first time that ECT affects serum PE concentration in responders and, therefore, could contribute to the evaluation of the therapy success.

Adult hippocampal neurogenesis: an important target associated with antidepressant effects of exercise

Depression is a prevalent devastating mental disorder that affects the normal life of patients and brings a heavy burden to whole society. Although many efforts have been made to attenuate depressive/anxiety symptoms, the current clinic antidepressants have limited effects. Scientists have long been making attempts to find some new strategies that can be applied as the alternative antidepressant therapy. Exercise, a widely recognized healthy lifestyle, has been suggested as a therapy that can relieve psychiatric stress. However, how exercise improves the brain functions and reaches the antidepressant target needs systematic summarization due to the complexity and heterogeneous feature of depression. Brain plasticity, especially adult neurogenesis in the hippocampus, is an important neurophysiology to facilitate animals for neurogenesis can occur in not only humans. Many studies indicated that an appropriate level of exercise can promote neurogenesis in the adult brains. In this article, we provide information about the antidepressant effects of exercise and its implications in adult neurogenesis. From the neurogenesis perspective, we summarize evidence about the effects of exercise in enhancing neurogenesis in the hippocampus through regulating growth factors, neurotrophins, neurotransmitters and metabolism as well as inflammations. Taken together, a large number of published works indicate the multiple benefits of exercise in the brain functions of animals, particularly brain plasticity like neurogenesis and synaptogenesis. Therefore, a new treatment method for depression therapy can be developed by regulating the exercise activity.

Shortened Spadin Analogs Display Better TREK-1 Inhibition, In Vivo Stability and Antidepressant Activity

Depression is a devastating mental disorder that affects 20% of the population worldwide. Despite their proven efficacy, antidepressants present a delayed onset of action and serious adverse effects. Seven years ago, we described spadin (PE 12-28) as a promising endogenous peptide with antidepressant activity. Spadin specifically blocks the TREK-1 channel. Previously, we showed in vivo that, spadin activity disappeared beyond 7 h after administration. In order to improve in vivo spadin stability and bioavailability, we screened spadin analogs and derivatives. From the study of spadin blood degradation products, we designed a 7 amino-acid peptide, PE 22-28. In vitro studies on hTREK-1/HEK cells by using patch-clamp technique, showed that PE 22-28 displayed a better specificity and affinity for TREK-1 channel compared to spadin, IC₅₀ of 0.12 nM vs. 40–60 nM for spadin. In the same conditions, we also pointed out that different modifications of its N or C-terminal ends maintained or abolished TREK-1 channel activity without affecting PE 22-28 affinity. In vivo, the antidepressant properties of PE 22-28 and its derivatives were demonstrated in behavioral models of depression, such as the forced swimming test. Mice treated with spadin-analogs showed a significant reduction of the immobility time. Moreover, in the novelty suppressed feeding test after a 4-day sub-chronic treatment PE 22-28 reduced significantly the latency to eat the food pellet. PE 22-28 and its analogs were able to induce neurogenesis after only a 4-day treatment with a prominent effect of the G/A-PE 22-28. On mouse cortical neurons, PE 22-28 and its derivatives enhanced synaptogenesis measured by the increase of PSD-95 expression level. Finally, the action duration of PE 22-28 and its analogs was largely improved in comparison with that of spadin, up to 23 h instead of 7 h. Taken together, our results demonstrated that PE 22-28 and its derivatives represent other promising molecules that could be an alternative to spadin in the treatment of depression.

Serum sortilin-derived propeptides concentrations are decreased in major depressive disorder patients

BACKGROUND

Despite intense research on mechanisms underlying the depressive pathophysiology, reliable biomarkers to assess antidepressant treatment response are still lacking. Since the sortilin-derived propeptide (PE) displays potent antidepressant activities and can be measured in the blood of rodents, we wondered whether in human its seric level can vary between patients affected by major depressive disorder (MDD) and healthy controls and after antidepressant treatment.

METHODS

By using a specific dosing method, characterized by structure-recognition analysis with various synthesized PE analogues, we conducted a translational study to test whether blood levels of PE are under pathophysiological regulation and could serve as biomarkers of the depression state.

RESULTS

The serum concentration of PE, a peptide displaying potent antidepressant activities in rodents, is decreased in patients affected by major depressive disorder (MDD) when compared to healthy non-psychiatric controls cohort (p=0.035). Interestingly, pharmacological antidepressant treatments restore normal PE levels.

LIMITATIONS

The limitation of the study concerns the relatively small patient samples that could negatively affect the likelihood that a nominally statistically significant finding actually reflects a true effect.

CONCLUSIONS

The longitudinal quantification of the serum PE concentration could assist psychiatrists in the diagnosis of antidepressant response efficacy, and the need to modify the therapeutic strategy.

Sortilin derived propeptide regulation during adipocyte differentiation and inflammation

In this work, we aimed to correlate the expression of sortilin with the production of sortilin-derived propeptide (PE) during adipocyte differentiation, insulin resistance and inflammation. We also investigated the effect of spadin, a shorter analogue of PE that exerts a potent antidepressant in mice, on adipocyte functions. During adipogenesis, insulin resistance and inflammation, we measured the mRNA and protein expression of sortilin, by quantitative PCR and Western-blot, and quantified the expression of PE by a specific dosing method. We observed that the production of PE was correlated with the sortilin expression during adipogenesis. Immunostaining experiments allowed to visualize the co-localization of sortilin, PE and VAMP2 in 3T3-L1 adipocytes. TNFα treatment induced insulin resistance and a decrease of sortilin expression (mRNA and protein), correlated with the decrease of the PE production. By contrast, treatment with dexamethasone, which also induced insulin resistance, was without effect on sortilin expression and PE production. As a putative bioactive peptide, we have evaluated its autocrine effect by the use of spadin on 3T3-L1 adipocytes by performing glucose uptake and signalling experiments. Any effect was measured on adipocytes indicating that the use of spadin as an antidepressant would have no side effects on adipocyte physiology.

Fluoxetine Protection in Decompression Sickness in Mice is Enhanced by Blocking TREK-1 Potassium Channel with the "spadin" Antidepressant

In mice, disseminated coagulation, inflammation, and ischemia induce neurological damage that can lead to death. These symptoms result from circulating bubbles generated by a pathogenic decompression. Acute fluoxetine treatment or the presence of the TREK-1 potassium channel increases the survival rate when mice are subjected to an experimental dive/decompression protocol. This is a paradox because fluoxetine is a blocker of TREK-1 channels. First, we studied the effects of an acute dose of fluoxetine (50 mg/kg) in wild-type (WT) and TREK-1 deficient mice (knockout homozygous KO and heterozygous HET). Then, we combined the same fluoxetine treatment with a 5-day treatment protocol with spadin, in order to specifically block TREK-1 activity (KO-like mice). KO and KO-like mice were regarded as antidepressed models. In total, 167 mice (45 WT_cont 46 WT_flux 30 HET_flux and 46 KO_flux) constituting the flux-pool and 113 supplementary mice (27 KO-like 24 WT_flux2 24 KO-like_flux 21 WT_cont2 17 WT_{no dive}) constituting the spad-pool were included in this study. Only 7% of KO-TREK-1 treated with fluoxetine (KO_flux) and 4% of mice treated with both spadin and fluoxetine (KO-like_flux) died from decompression sickness (DCS) symptoms. These values are much lower than those of WT control (62%) or KO-like mice (41%). After the decompression protocol, mice showed significant consumption of their circulating platelets and leukocytes. Spadin antidepressed mice were more likely to exhibit DCS. Nevertheless, mice which had both blocked TREK-1 channels and fluoxetine treatment were better protected against DCS. We conclude that the protective effect of such an acute dose of fluoxetine is enhanced when TREK-1 is inhibited. We confirmed that antidepressed models may have worse DCS outcomes, but concomitant fluoxetine treatment not only decreased DCS severity but increased the survival rate.

Potentiation of Calcium Influx and Insulin Secretion in Pancreatic Beta Cell by the Specific TREK-1 Blocker Spadin

Inhibition of the potassium channels TREK-1 by spadin (SPA) is currently thought to be a promising therapeutic target for the treatment of depression. Since these channels are expressed in pancreatic β-cells, we investigated their role in the control of insulin secretion and glucose homeostasis. In this study, we confirmed the expression of TREK-1 channels in the insulin secreting MIN6-B1 β-cell line and in mouse islets. We found that their blockade by SPA potentiated insulin secretion induced by potassium chloride dependent membrane depolarization. Inhibition of TREK-1 by SPA induced a decrease of the resting membrane potential (ΔV_m ~ 12 mV) and increased the cytosolic calcium concentration. In mice, administration of SPA enhanced the plasma insulin level stimulated by glucose, confirming its secretagogue effect observed in vitro. Taken together, this work identifies SPA as a novel potential pharmacological agent able to control insulin secretion and glucose homeostasis.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.