Neurochemical Changes in the Mouse Hippocampus Underlying the Antidepressant Effect of Genetic Deletion of P2X7 Receptors

Evaluation of metabolomics-based urinary biomarker models for recognizing major depression disorder and bipolar disorder

BACKGROUND

Major depressive disorder (MDD) and bipolar disorder (BD) are psychiatric disorders with overlapping symptoms, leading to high rates of misdiagnosis due to the lack of biomarkers for differentiation. This study aimed to identify metabolic biomarkers in urine samples for diagnosing MDD and BD, as well as to establish unbiased differential diagnostic models.

METHODS

We utilized a metabolomics approach employing ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS) to analyze the metabolic profiles of urine samples from individuals with MDD (n = 50), BD (n = 12), and healthy controls (n = 50). The identification of urine metabolites was verified using MS data analysis tools and online metabolite databases.

RESULTS

Two diagnostic panels consisting of a combination of metabolites and clinical indicators were identified-one for MDD and another for BD. The discriminative capacity of these panels was assessed using the area under the receiver operating characteristic (ROC) curve, yielding an area under the curve (AUC) of 0.9084 for MDD and an AUC value of 0.9017 for BD.

CONCLUSIONS

High-resolution mass spectrometry-based assays show promise in identifying urinary biomarkers for depressive disorders. The combination of urine metabolites and clinical indicators is effective in differentiating healthy controls from individuals with MDD and BD. The metabolic pathway indicating oxidative stress is seen to significantly contribute to depressive disorders.

Targeting inflammasome complexes as a novel therapeutic strategy for mood disorders

INTRODUCTION

Inflammasome complexes, especially NLRP3, have gained great attention as a potential therapeutic target in mood disorders. NLRP3 triggers a caspase 1-dependent release of the inflammatory cytokines IL-1β and IL-18, and seems to interact with purinergic and kynurenine pathways, all of which are implicated in mood disorders development and progression.

AREAS COVERED

Emerging evidence supports NLRP3 inflammasome as a promising pharmacological target for mood disorders. We discussed the available evidence from animal models and human studies and provided a reflection on drawbacks and perspectives for this novel target.

EXPERT OPINION

Several studies have supported the involvement of NLRP3 inflammasome in MDD. However, most of the evidence comes from animal models. The role of NLRP3 inflammasome in BD as well as its anti-manic properties is not very clear and requires further exploration. There is evidence of anti-manic effects of P2×R7 antagonists associated with reduction in the brain levels of IL-1β and TNF-α in a murine model of mania. The involvement of other NLRP3 inflammasome expressing cells besides microglia, like astrocytes, and of other inflammasome complexes in mood disorders also deserves further investigation. Preclinical and clinical characterization of NLRP3 and other inflammasomes in mood disorders is needed before considering translational approaches, including clinical trials.

The Prevalence of Hyperuricemia and the Association Between Hyperuricemia and Age in Patients with Psychiatric Disorders to a General Hospital: A Cross-Section Study

Purpose

In clinical work, it has been found that the prevalence of hyperuricemia (HUA) is significantly higher in younger patients with psychiatric disorders, but there are few studies in this area. The present study aims to evaluate the prevalence of HUA and the relationship between the HUA and age in hospitalized patients with psychiatric disorders in the real world, and to provide a theoretical basis for clinical staff to pay attention to the metabolic indicators of younger patients and for future related studies.

Methods

This is a cross-sectional evaluation of a cohort of 1761 patients with psychiatric disorders of hospitalized. The categories of disorders designed for study included: Depression, Bipolar disorder, Schizophrenia, Anxiety, Obsessive-Compulsive disorder, Acute and transient psychotic disorder, Dissociative(conversion) disorders, Conduct disorders and Tic disorders. In addition, based on age, the participants are stratified into three groups. The authors used Kruskal-Wallis tests, chi-square tests, and multiple linear logistic regression to verify the relationship between HUA and age among hospitalized patients with psychiatric disorders.

Results

Overall, the estimated prevalence of HUA was 35.4%. The prevalence of HUA was significantly higher in individuals with 17 years and under compared to those with 45 years and above (P < 0.001). After adjusting for confounders, the prevalence of HUA remained higher at 17 years and under than at 45 years and above. Bipolar disorder can lead to an increased prevalence of HUA (P<0.05).

Conclusion

The prevalence of HUA was higher in hospitalized patients with psychiatric disorders, and the prevalence was inversely proportional to age.

P2X7 receptor inhibition alleviates mania-like behavior independently of interleukin-1β

Purinergic dysfunctions are associated with mania and depression pathogenesis. P2X7 receptor (P2X7R) mediates the IL-1β maturation via NLRP3 inflammasome activation. We tested in a mouse model of the subchronic amphetamine (AMPH)-induced hyperactivity whether P2X7R inhibition alleviated mania-like behavior through IL-1β. Treatment with JNJ-47965567, a P2X7R antagonist, abolished AMPH-induced hyperlocomotion in wild-type and IL-1α/β-knockout male mice. The NLRP3 inhibitor MCC950 failed to reduce AMPH-induced locomotion in WT mice, whereas the IL-1 receptor antagonist anakinra slightly increased it. AMPH increased IL-10, TNF-α, and TBARS levels, but did not influence BDNF levels, serotonin, dopamine, and noradrenaline content in brain tissues in either genotypes. JNJ-47965567 and P2rx7-gene deficiency, but not IL-1α/β-gene deficiency, attenuated AMPH-induced [3H]dopamine release from striatal slices. In wild-type and IL-1α/β-knockout female mice, JNJ-47965567 was also effective in attenuating AMPH-induced hyperlocomotion. This study suggests that AMPH-induced hyperactivity is modulated by P2X7Rs, but not through IL-1β.

Electroacupuncture regulates the P2X7R-NLRP3 inflammatory cascade to relieve decreased sensation on ocular surface of type 2 diabetic rats with dry eye

Dry eye (DE) is a prevalent ocular surface disease in patients with type 2 diabetes (T2DM). However, current medications are ineffective against decreased sensation on the ocular surface. While electroacupuncture (EA) effectively alleviates decreased sensation on ocular surface of DE in patients with T2DM, the neuroprotective mechanism remains unclear. This study explored the pathogenesis and therapeutic targets of T2DM-associated DE through bioinformatics analysis. It further investigated the underlying mechanism by which EA improves decreased sensation on the ocular surface of DE in rats with T2DM. Bioinformatic analysis was applied to annotate the potential pathogenesis of T2DM DE. T2DM and DE was induced in male rats. Following treatment with EA and fluorometholone, comprehensive metrics were assessed. Additionally, the expression patterns of key markers were studied. Key targets such as NLRP3, Caspase-1, and NOD-like receptor signaling may be involved in the pathogenesis of T2DM DE. EA treatment improved ocular measures. Furthermore, EA potently downregulated P2X7R, NLRP3, apoptosis-associated speck-like protein containing a CARD (ASC), and Caspase-1 expression within the trigeminal ganglion and spinal trigeminal nucleus caudalis. Targeted P2X7R antagonist (A-438079) and agonist (BzATP) employed as controls to decipher the biochemistry of the therapeutic effects of EA showed an anti-inflammatory effect with A-438079, while BzATP blocked the anti-inflammatory effect of EA. EA relieved DE symptoms and attenuated inflammatory damage to sensory nerve pathways in T2DM rats with DE. These findings suggest a crucial role of EA inhibition of the P2X7R-NLRP3 inflammatory cascade to provide these benefits.

Embers of the Past: Early Childhood Traumas Interact with Variation in P2RX7 Gene Implicated in Neuroinflammation on Markers of Current Suicide Risk

Both early childhood traumatic experiences and current stress increase the risk of suicidal behaviour, in which immune activation might play a role. Previous research suggests an association between mood disorders and P2RX7 gene encoding P2X7 receptors, which stimulate neuroinflammation. We investigated the effect of P2RX7 variation in interaction with early childhood adversities and traumas and recent stressors on lifetime suicide attempts and current suicide risk markers. Overall, 1644 participants completed questionnaires assessing childhood adversities, recent negative life events, and provided information about previous suicide attempts and current suicide risk-related markers, including thoughts of ending their life, death, and hopelessness. Subjects were genotyped for 681 SNPs in the P2RX7 gene, 335 of which passed quality control and were entered into logistic and linear regression models, followed by a clumping procedure to identify clumps of SNPs with a significant main and interaction effect. We identified two significant clumps with a main effect on current suicidal ideation with top SNPs rs641940 and rs1653613. In interaction with childhood trauma, we identified a clump with top SNP psy_rs11615992 and another clump on hopelessness containing rs78473339 as index SNP. Our results suggest that P2RX7 variation may mediate the effect of early childhood adversities and traumas on later emergence of suicide risk.

Purinergic P2X7 receptor as a potential therapeutic target in depression

The elaborate mechanisms of depression have always been a research hotspot in recent years, and the pace of research has never ceased. The P2X7 receptor (P2X7R) belongs to one of the adenosine triphosphates (ATP)-gated cation channels that exist widely in brain tissues and play a prominent role in the regulation of depression-related pathology. To date, the role of purinergic P2X7R in the mechanisms underlying depression is not fully understood. In this review, we conclude that the purinergic receptor P2X7 is a potential therapeutic target for depression based on research results published over the past 5 years in Google Scholar and the National Library of Medicine (PubMed). Additionally, we introduced the functional characteristics of P2X7R and confirmed that excessive activation of P2X7R led to increased release of inflammatory cytokines, which eventually contributed to depression. Furthermore, the inhibition of P2X7R produced antidepressant-like effects in animal models of depression, further proving that P2X7R signalling mediates depression-like behaviours. Finally, we summarised related studies on drugs that exert antidepressant effects by regulating the expression of P2X7R. We hope that the conclusions of this review will provide information on the role of P2X7R in the neuropathophysiology of depression and novel therapeutic targets for the treatment of depression.

Neuroinflammation, memory, and depression: new approaches to hippocampal neurogenesis

As one of most common and severe mental disorders, major depressive disorder (MDD) significantly increases the risks of premature death and other medical conditions for patients. Neuroinflammation is the abnormal immune response in the brain, and its correlation with MDD is receiving increasing attention. Neuroinflammation has been reported to be involved in MDD through distinct neurobiological mechanisms, among which the dysregulation of neurogenesis in the dentate gyrus (DG) of the hippocampus (HPC) is receiving increasing attention. The DG of the hippocampus is one of two niches for neurogenesis in the adult mammalian brain, and neurotrophic factors are fundamental regulators of this neurogenesis process. The reported cell types involved in mediating neuroinflammation include microglia, astrocytes, oligodendrocytes, meningeal leukocytes, and peripheral immune cells which selectively penetrate the blood-brain barrier and infiltrate into inflammatory regions. This review summarizes the functions of the hippocampus affected by neuroinflammation during MDD progression and the corresponding influences on the memory of MDD patients and model animals.

The antidepressant effect of short- and long-term zinc exposition is partly mediated by P2X7 receptors in male mice

Background: As a member of the purinergic receptor family, divalent cation-regulated ionotropic P2X7 (P2rx7) plays a role in the pathophysiology of psychiatric disorders. This study aimed to investigate whether the effects of acute zinc administration and long-term zinc deprivation on depression-like behaviors in mice are mediated by P2X7 receptors. Methods: The antidepressant-like effect of elevated zinc level was studied using a single acute intraperitoneal injection in C57BL6/J wild-type and P2rx7 gene-deficient (P2rx7 -/-) young adult and elderly animals in the tail suspension test (TST) and the forced swim test (FST). In the long-term experiments, depression-like behavior caused by zinc deficiency was investigated with the continuous administration of zinc-reduced and control diets for 8 weeks, followed by the same behavioral tests. The actual change in zinc levels owing to the treatments was examined by assaying serum zinc levels. Changes in monoamine and brain-derived neurotrophic factor (BDNF) levels were measured from the hippocampus and prefrontal cortex brain areas by enzyme-linked immunosorbent assay and high-performance liquid chromatography, respectively. Results: A single acute zinc treatment increased the serum zinc level evoked antidepressant-like effect in both genotypes and age groups, except TST in elderly P2rx7 -/- animals, where no significant effect was detected. Likewise, the pro-depressant effect of zinc deprivation was observed in young adult mice in the FST and TST, which was alleviated in the case of the TST in the absence of functional P2X7 receptors. Among elderly mice, no pro-depressant effect was observed in P2rx7 -/- mice in either tests. Treatment and genotype changes in monoamine and BDNF levels were also detected in the hippocampi. Conclusion: Changes in zinc intake were associated with age-related changes in behavior in the TST and FST. The antidepressant-like effect of zinc is partially mediated by the P2X7 receptor.

Characterization of the central nervous system penetrant and selective purine P2X7 receptor antagonist JNJ-54175446 in patients with major depressive disorder

JNJ-54175446 is a selective purine P2X7 receptor (P2X7R) antagonist that attenuates microglial IL-1β/IL-18 release. In healthy volunteers, JNJ-54175446 suppressed peripheral interleukin (IL)-1β release, and attenuated dexamphetamine-induced improvements of mood and (visuo)motor performance in a human dexamphetamine-challenge paradigm. In depression, P2X7R inhibition may dampen immune-related dysregulation of mood. These results suggest that the impact of P2X7R inhibition is most prominent in situations where mood regulation is disrupted. Total sleep deprivation (TSD) results in an acute emotional perturbation, which yields a transient antidepressant effect. In the current study, TSD was applied as a behavioral challenge to investigate whether such effects could be modulated by JNJ-54175446. This was a double-blind, placebo-controlled, randomized study to assess the safety and pharmacokinetics of JNJ-54175446 and explore its effects in patients with single episode and recurrent major depressive disorder (MDD) (N = 69) and baseline total Inventory of Depressive Symptomatology Clinician Rated (IDS-C) > 30. Patients were randomized to receive JNJ-54175446 throughout the 10-day treatment period, placebo for days 1-3 followed by JNJ-54175446 or placebo throughout. All patients underwent 36 h of TSD starting on day three until the evening of day four. The early start group was hypothesized to experience a reduced effect from TSD whilst the late starting group was hypothesized to experience prolonged effects from the TSD. JNJ-54175446 was well-tolerated and adverse events were mild to moderate. JNJ-54175446 reduced IL-1β release by LPS-stimulated peripheral white blood cells in the presence of the P2X receptor agonist benzyl adenosine triphosphate (BzATP). JNJ-54175446 did not have a significant effect on mood as assessed using the Hamilton Depression Rating Scale, 17 items (HDRS17) and the Self-rated Quick Inventory of Depressive Symptoms (QIDS-SR). However, JNJ-54175446 blunted an acute reduction of anhedonia that occurred as a result of TSD, assessed by the Snaith-Hamilton Pleasure Scale (SHAPS) and the Probabilistic Instrumental Learning Task (PILT).

The neuroinflammatory astrocytic P2X7 receptor: Alzheimer's disease, ischemic brain injury, and epileptic state

INTRODUCTION

Astrocytes have previously been considered as cells supporting neuronal functions, but they are now recognized as active players in maintaining central nervous system (CNS) homeostasis. Astrocytes can communicate with other CNS cells, i.e. through the gliotransmitter ATP and P2X7 receptors (Rs).

AREAS COVERED

In this review, we will discuss how the P2X7R initiates the release of gliotransmitters and proinflammatory cytokines/chemokines, thereby establishing a dialog between astrocytes and neurons and, in addition, causing neuroinflammation. In astrocytes, dysregulation of P2X7Rs has been associated with neurodegenerative illnesses such as Alzheimer's disease (AD), as well as the consequences of cerebral ischemic injury and status epilepticus (SE).

EXPERT OPINION

Although all CNS cells are possible sources of ATP release, the targets of this ATP are primarily at microglial cells. However, astrocytes also contain ATP-sensitive P2X7Rs and have in addition the peculiar property over microglia to continuously interact with neurons via not only inflammatory mediators but also gliotransmitters, such as adenosine 5'-triphosphate (ATP), glutamate, γ-amino butyric acid (GABA), and D-serine. Cellular damage arising during AD, cerebral ischemia, and SE via P2X7R activation is superimposed upon the original disease, and their prevention by blood-brain barrier permeable pharmacological antagonists is a valid therapeutic option.

P2X7 receptor-mediated depression-like reactions arising in the mouse medial prefrontal cortex

Major depressive disorder is a frequent and debilitating psychiatric disease. We have shown in some of the acute animal models of major depressive disorder (tail suspension test and forced swim test) that depression-like behavior can be aggravated in mice by the microinjection into the medial prefrontal cortex of the P2X7R agonistic adenosine 5'-triphosphate or its structural analog dibenzoyl-ATP, and these effects can be reversed by the P2X7R antagonistic JNJ-47965567. When measuring tail suspension test, the prolongation of immobility time by the P2YR agonist adenosine 5'-[β-thio]diphosphate and the reduction of the adenosine 5'-(γ-thio)triphosphate effect by P2Y1R (MRS 2179) or P2Y12R (PSB 0739) antagonists, but not by JNJ-47965567, all suggest the involvement of P2YRs. In order to elucidate the localization of the modulatory P2X7Rs in the brain, we recorded current responses to dibenzoyl-ATP in layer V astrocytes and pyramidal neurons of medial prefrontal cortex brain slices by the whole-cell patch-clamp procedure; the current amplitudes were not altered in preparations taken from tail suspension test or foot shock-treated mice. The release of adenosine 5'-triphosphate was decreased by foot shock, although not by tail suspension test both in the hippocampus and PFC. In conclusion, we suggest, that in the medial prefrontal cortex, acute stressful stimuli cause supersensitivity of P2X7Rs facilitating the learned helplessness reaction.

Triggering of Major Brain Disorders by Protons and ATP: The Role of ASICs and P2X Receptors

Adenosine triphosphate (ATP) is well-known as a universal source of energy in living cells. Less known is that this molecule has a variety of important signaling functions: it activates a variety of specific metabotropic (P2Y) and ionotropic (P2X) receptors in neuronal and non-neuronal cell membranes. So, a wide variety of signaling functions well fits the ubiquitous presence of ATP in the tissues. Even more ubiquitous are protons. Apart from the unspecific interaction of protons with any protein, many physiological processes are affected by protons acting on specific ionotropic receptors-acid-sensing ion channels (ASICs). Both protons (acidification) and ATP are locally elevated in various pathological states. Using these fundamentally important molecules as agonists, ASICs and P2X receptors signal a variety of major brain pathologies. Here we briefly outline the physiological roles of ASICs and P2X receptors, focusing on the brain pathologies involving these receptors.

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Neuroprotection of exercise: P2X4R and P2X7R regulate BDNF actions

The neurotrophin brain-derived neurotrophic factor (BDNF), which acts as a transducer, is responsible for improving cerebral stroke, neuropathic pain, and depression. Exercise can alter extracellular nucleotide levels and purinergic receptors in central nervous system (CNS) structures. This inevitably activates or inhibits the expression of BDNF via purinergic receptors, particularly the P2X receptor (P2XR), to alleviate pathological progression. In addition, the significant involvement of sensitive P2X4R in mediating increased BDNF and p38-MAPK for intracerebral hemorrhage and pain hypersensitivity has been reported. Moreover, archetypal P2X7R blockade induces mouse antidepressant-like behavior and analgesia by BDNF release. This review summarizes BDNF-mediated neural effects via purinergic receptors, speculates that P2X4R and P2X7R could be priming molecules in exercise-mediated changes in BDNF, and provides strategies for the protective mechanism of exercise in neurogenic disease.

The P2X7 receptor in mood disorders: Emerging target in immunopsychiatry, from bench to bedside

Psychiatric disorders are among the most burdensome disorders worldwide. Though therapies have evolved over the last decades, treatment resistance still affects many patients. Recently, neuroimmune systems have been identified as important factors of mood disorder biology. The underlying dysregulation in neuroimmune cross-talk is driven by genetic risk factors and accumulating adverse environmental influences like chronic psychosocial stress. These result in a cluster of proinflammatory cytokines and quantitative and functional changes of immune cell populations (e.g., microglia, monocytes, T cells), varying by disease entity and state. Among the emerging immune targets, purinergic signalling revolving around the membranous and ATP specific P2X7 receptor (P2X7R) has gained wider attention and clinical studies making use of antagonistic drugs are on-going. Still, no clinically meaningful applications have been identified so far. A major problem is the often overly simplified approach taken to translate findings from bench to bedside. Therefore, the present review focuses on purinergic signalling via P2X7R in the context of recent advances in immunopsychiatric mood disorder research. Our aim is to provide an overview of the current P2X7R-related findings, from bench to bedside. First, we summarize the characteristics of purinergic signalling and P2X7R, followed by a depiction of genetic and clinical data connecting P2X7R to mood disorders. We close with our perspective on current developments and discuss changes necessary to translate the evident potential of P2X7R signalling modulation into meaningful clinical application. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Major depressive disorder (mdd): emerging immune targets at preclinical level

BACKGROUND

Major depressive disorder is a mental health disorder that is characterized by a persistently low mood and loss of interest. MDD is affecting over 3.8% of the global population as a major health problem. Its etiology is complex, and involves the interaction between a number of factors, including genetic predisposition and the presence of environmental stresses.

AREAS COVERED

The role of the immune and inflammatory systems in depression has been gaining interest, with evidence suggesting the potential involvement of pro-inflammatory molecules like TNF, interleukins, prostaglandins, and other cytokines, among others, has been put forth. Along with this, the potential of agents, from NSAIDs to antibiotics, are being evaluated in therapy for depression. The current review will discuss emerging immune targets at the preclinical level.

EXPERT OPINION

With increasing evidence to show that immune and inflammatory mediators are implicated in MDD, increasing research toward their potential as drug targets is encouraged. At the same time, agents acting on these mediators and possessing anti-inflammatory potential are also being evaluated as future therapeutic options for MDD, and increasing focus toward non-conventional drugs which can act through these mechanisms is important as regards the future prospects of the use of anti-inflammatory agents in depression.

Therapeutic Potential of the Purinergic System in Major Depressive Disorder Associated with COVID-19

Neuroinflammation is closely related to the development of depression, since the latter is caused, among other factors, by inflammatory processes, mainly related to the activation of microglia and expression of specific genes, which occurs during the neuroinflammatory process. Thus, COVID-19 is an important risk factor for the development of depression, since in addition to generating the feeling of stress, which also increases the activity of the immune system, it is also the cause of pathological processes and physiological ones that lead to the development of neuroinflammation, microglial activation, gene expression dysfunction and decreased concentration of available serotonin. That said, drugs are being used to combat COVID-19 to reduce the oxidative stress presented in the disease. Thus, tramadol and fluoxetine are highlighted as drugs used, however, although they present some positive results, such as the reduction of pro-inflammatory cytokines, they are also associated with negative effects such as dependence, pulmonary, cardiac and brain impairment. From this, the purinergic system is highlighted in the literature as a possible therapeutic target. This is because its mechanisms are related to the regulation of microglia, astrocytes and the physiology of important neurotransmitters and hormones. Added to this, there is a modulation of inflammatory activity, especially with regard to the P2X7 receptors of this system. The latter is an important target for the treatment of depression and COVID-19, since positive results were obtained through the genetic exclusion of this receptor and the use of selective antagonists.

A Possible Causal Involvement of Neuroinflammatory, Purinergic P2X7 Receptors in Psychiatric Disorders

P2X7 receptors (Rs) are prominent members of the P2XR family, which after binding ATP, open non-selective cationic channels, thereby allowing the transmembrane passage of Na+, Ca²⁺, and K+. Long-lasting and repetitive stimulation of the receptor by its agonist leads to the formation of large membrane pores permeable for organic cations of up to 900 Da molecular size. These pores are believed to play a role in apoptosis and inflammation. P2X7Rs are located primarily at peripheral macrophages and microglial cells, the resident macrophages of the CNS. The coactivation of toll-like receptors 4 (TLR4) by lipopolysaccharide, a constituent of the cell membrane of gram-negative bacteria, and the P2X7R by ATP leads to the generation and release of the proinflammatory cytokines interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α. Together with the microglial release of chemokines, reactive oxygen and nitrogen species, proteases, and excitotoxic glutamate, these cytokines result in neurodegeneration. P2X7Rs were found not only to amplify various neurodegenerative illnesses, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis, but also to participate in a range of psychiatric diseases, such as major depression, bipolar disorder, schizophrenia, and autism spectrum disorder. Based on the prevention/reversal of neuroinflammation, pharmacological antagonists of P2X7Rs and their genetic deletion in animal experiments counteract these deleterious psychiatric conditions. Hence, brain penetrant P2X7R antagonists are potential therapeutics for psychiatric diseases, although the available evidence still needs to be extended and validated by further clinical data.

Astrocytes and major depression: The purinergic avenue

Major depressive disorder (MDD) is one of the most prevalent psychiatric illnesses worldwide which impairs the social functioning of the afflicted patients. Astrocytes promote homeostasis of the CNS and provide defense against various types of harmful influences. Increasing evidence suggests that the number, morphology and function of astrocytes are deteriorated in the depressed brain and the malfunction of the astrocytic purinergic system appears to participate in the pathophysiology of MDD. Adenosine 5'-triphosphate (ATP) released from astrocytes modulates depressive-like behavior in animal models and probably also clinical depression in patients. Astrocytes possess purinergic receptors, such as adenosine A_2A receptors (Rs), and P2X7, P2Y₁, and P2Y₁₁Rs, which mediate neuroinflammation, neuro(glio)transmission, and synaptic plasticity in depression-relevant areas of the brain (e.g. medial prefrontal cortex, hippocampus, amygdala nuclei). By contrast, astrocytic A₁Rs are neuroprotective and immunosuppressive. In the present review, we shall discuss the release of purines from astrocytes, and the expression/function of astrocytic purinergic receptors. Subsequently, we shall review in more detail novel evidence indicating that the dysregulation of astrocytic purinergic signaling actively contributes to the pathophysiology of depression and shall discuss possible therapeutic options based on knowledge recently acquired in this field.

Scrutinizing the Therapeutic Promise of Purinergic Receptors Targeting Depression

Antidepressant use has resulted in a variety of negative consequences, including permanent brain damage and erectile dysfunction. So, the purpose lies in developing something more productive with minimal side effects and consequently improved efficacy. A growing body of evidences indicated a remarkable purinergic signalling system, which helped in dealing with this complication. This has been found to be a powerful formula in dealing with psychiatric disorders. P1 (adenosine), P2X, and P2Y (ATP) are the receptors, involved in the pathology as well as exhibiting the therapeutic action by triggering the purinergic pathway. It was found that A2A and P2X7 receptors specifically were involved and recognized as possible targets for treating depression. Further, the development of biomarkers for the diagnosis of depression has also been attributed to accelerate the process. One such biomarker includes serum uric acid. Many clinical studies reveal the importance of antagonizing P2X7 and A2A receptors, for promising research in understanding the molecular premises of depression. However, further investigations are still needed to be done to open several unfolded mysteries for a better and safe upshot. The selective antagonists for A2A and P2X7 receptors may have antidepressant effects showing positive results, in agreement with non-clinical testing. In this review, efforts are being devoted to the targeted receptors in bringing out antidepressant effects with a possible link involving depression and defined purinergic signalling. Additionally, the overview of various receptors, including their functions and distribution, is being explored in a representative way along with the biomarkers involved.

Gallic Acid Alleviates Visceral Pain and Depression via Inhibition of P2X7 Receptor

Chronic visceral pain can occur in many disorders, the most common of which is irritable bowel syndrome (IBS). Moreover, depression is a frequent comorbidity of chronic visceral pain. The P2X7 receptor is crucial in inflammatory processes and is closely connected to developing pain and depression. Gallic acid, a phenolic acid that can be extracted from traditional Chinese medicine, has been demonstrated to be anti-inflammatory and anti-depressive. In this study, we investigated whether gallic acid could alleviate comorbid visceral pain and depression by reducing the expression of the P2X7 receptor. To this end, the pain thresholds of rats with comorbid visceral pain and depression were gauged using the abdominal withdraw reflex score, whereas the depression level of each rat was quantified using the sucrose preference test, the forced swimming test, and the open field test. The expressions of the P2X7 receptor in the hippocampus, spinal cord, and dorsal root ganglion (DRG) were assessed by Western blotting and quantitative real-time PCR. Furthermore, the distributions of the P2X7 receptor and glial fibrillary acidic protein (GFAP) in the hippocampus and DRG were investigated in immunofluorescent experiments. The expressions of p-ERK1/2 and ERK1/2 were determined using Western blotting. The enzyme-linked immunosorbent assay was utilized to measure the concentrations of IL-1β, TNF-α, and IL-10 in the serum. Our results demonstrate that gallic acid was able to alleviate both pain and depression in the rats under study. Gallic acid also reduced the expressions of the P2X7 receptor and p-ERK1/2 in the hippocampi, spinal cords, and DRGs of these rats. Moreover, gallic acid treatment decreased the serum concentrations of IL-1β and TNF-α, while raising IL-10 levels in these rats. Thus, gallic acid may be an effective novel candidate for the treatment of comorbid visceral pain and depression by inhibiting the expressions of the P2X7 receptor in the hippocampus, spinal cord, and DRG.

P2X7 Receptor-Related Genetic Mouse Models – Tools for Translational Research in Psychiatry

Depression is a common psychiatric disorder and the leading cause of disability worldwide. Although treatments are available, only about 60% of treated patients experience a significant improvement in disease symptoms. Numerous clinical and rodent studies have identified the purinergic P2X7 receptor (P2X7R) as one of the genetic factors potentially contributing to the disease risk. In this respect, genetically engineered mouse models targeting the P2X7R have become increasingly important in studying designated immunological features and subtypes of depression in vivo. This review provides an overview of the P2X7R -related mouse lines currently available for translational psychiatric research and discusses their strengths, weaknesses, and potentials.

Beyond Seizure Control: Treating Comorbidities in Epilepsy via Targeting of the P2X7 Receptor

Epilepsy is one of the most common chronic diseases of the central nervous system (CNS). Treatment of epilepsy remains, however, a clinical challenge with over 30% of patients not responding to current pharmacological interventions. Complicating management of treatment, epilepsy comes with multiple comorbidities, thereby further reducing the quality of life of patients. Increasing evidence suggests purinergic signalling via extracellularly released ATP as shared pathological mechanisms across numerous brain diseases. Once released, ATP activates specific purinergic receptors, including the ionotropic P2X7 receptor (P2X7R). Among brain diseases, the P2X7R has attracted particular attention as a therapeutic target. The P2X7R is an important driver of inflammation, and its activation requires high levels of extracellular ATP to be reached under pathological conditions. Suggesting the therapeutic potential of drugs targeting the P2X7R for epilepsy, P2X7R expression increases following status epilepticus and during epilepsy, and P2X7R antagonism modulates seizure severity and epilepsy development. P2X7R antagonism has, however, also been shown to be effective in treating conditions most commonly associated with epilepsy such as psychiatric disorders and cognitive deficits, which suggests that P2X7R antagonisms may provide benefits beyond seizure control. This review summarizes the evidence suggesting drugs targeting the P2X7R as a novel treatment strategy for epilepsy with a particular focus of its potential impact on epilepsy-associated comorbidities.

High, in Contrast to Low Levels of Acute Stress Induce Depressive-like Behavior by Involving Astrocytic, in Addition to Microglial P2X7 Receptors in the Rodent Hippocampus

Extracellular adenosine 5'-triphosphate (ATP) in the brain is suggested to be an etiological factor of major depressive disorder (MDD). It has been assumed that stress-released ATP stimulates P2X7 receptors (Rs) at the microglia, thereby causing neuroinflammation; however, other central nervous system (CNS) cell types such as astrocytes also possess P2X7Rs. In order to elucidate the possible involvement of the MDD-relevant hippocampal astrocytes in the development of a depressive-like state, we used various behavioral tests (tail suspension test [TST], forced swim test [FST], restraint stress, inescapable foot shock, unpredictable chronic mild stress [UCMS]), as well as fluorescence immunohistochemistry, and patch-clamp electrophysiology in wild-type (WT) and genetically manipulated rodents. The TST and FST resulted in learned helplessness manifested as a prolongation of the immobility time, while inescapable foot shock caused lower sucrose consumption as a sign of anhedonia. We confirmed the participation of P2X7Rs in the development of the depressive-like behaviors in all forms of acute (TST, FST, foot shock) and chronic stress (UCMS) in the rodent models used. Further, pharmacological agonists and antagonists acted in a different manner in rats and mice due to their diverse potencies at the respective receptor orthologs. In hippocampal slices of mice and rats, only foot shock increased the current responses to locally applied dibenzoyl-ATP (Bz-ATP) in CA1 astrocytes; in contrast, TST and restraint depressed these responses. Following stressful stimuli, immunohistochemistry demonstrated an increased co-localization of P2X7Rs with a microglial marker, but no change in co-localization with an astroglial marker. Pharmacological damage to the microglia and astroglia has proven the significance of the microglia for mediating all types of depression-like behavioral reactions, while the astroglia participated only in reactions induced by strong stressors, such as foot shock. Because, in addition to acute stressors, their chronic counterparts induce a depressive-like state in rodents via P2X7R activation, we suggest that our data may have relevance for the etiology of MDD in humans.

Microglial Inflammatory-Metabolic Pathways and Their Potential Therapeutic Implication in Major Depressive Disorder

Increasing evidence supports the notion that neuroinflammation plays a critical role in the etiology of major depressive disorder (MDD), at least in a subset of patients. By virtue of their capacity to transform into reactive states in response to inflammatory insults, microglia, the brain's resident immune cells, play a pivotal role in the induction of neuroinflammation. Experimental studies have demonstrated the ability of microglia to recognize pathogens or damaged cells, leading to the activation of a cytotoxic response that exacerbates damage to brain cells. However, microglia display a wide range of responses to injury and may also promote resolution stages of inflammation and tissue regeneration. MDD has been associated with chronic priming of microglia. Recent studies suggest that altered microglial morphology and function, caused either by intense inflammatory activation or by senescence, may contribute to depression and associated impairments in neuroplasticity. In this context, modifying microglia phenotype by tuning inflammatory pathways might have important translational relevance to harness neuroinflammation in MDD. Interestingly, it was recently shown that different microglial phenotypes are associated with distinct metabolic pathways and analysis of the underlying molecular mechanisms points to an instrumental role for energy metabolism in shaping microglial functions. Here, we review various canonical pro-inflammatory, anti-inflammatory and metabolic pathways in microglia that may provide new therapeutic opportunities to control neuroinflammation in brain disorders, with a strong focus on MDD.

The Purinergic System as a Target for the Development of Treatments for Bipolar Disorder

The neurobiological and neurochemical mechanisms underlying the pathophysiology of bipolar disorder are complex and not yet fully understood. From circadian disruption to neuroinflammation, many pathways and signaling molecules are important contributors to bipolar disorder development, some specific to a disease subtype or a cycling episode. Pharmacological agents for bipolar disorder have shown only partial efficacy, including mood stabilizers and antipsychotics. The purinergic hypothesis for bipolar disorder emerges in this scenario as a promising target for further research and drug development, given its role in neurotransmission and neuroinflammation that results in behavioral and mood regulation. Here, we review the basic concepts of purinergic signaling in the central nervous system and its contribution to bipolar disorder pathophysiology. Allopurinol and novel P2X7 receptor antagonists are promising candidates for treating bipolar disorder. We further explore currently available pharmacotherapies and the emerging new purinergic targets for drug development in bipolar disorder.

Ion Channel Dysfunction and Neuroinflammation in Migraine and Depression

Migraine and major depression are debilitating disorders with high lifetime prevalence rates. Interestingly these disorders are highly comorbid and show significant heritability, suggesting shared pathophysiological mechanisms. Non-homeostatic function of ion channels and neuroinflammation may be common mechanisms underlying both disorders: The excitation-inhibition balance of microcircuits and their modulation by monoaminergic systems, which depend on the expression and function of membrane located K⁺, Na⁺, and Ca⁺² channels, have been reported to be disturbed in both depression and migraine. Ion channels and energy supply to synapses not only change excitability of neurons but can also mediate the induction and maintenance of inflammatory signaling implicated in the pathophysiology of both disorders. In this respect, Pannexin-1 and P2X7 large-pore ion channel receptors can induce inflammasome formation that triggers release of pro-inflammatory mediators from the cell. Here, the role of ion channels involved in the regulation of excitation-inhibition balance, synaptic energy homeostasis as well as inflammatory signaling in migraine and depression will be reviewed.

P2X7 Receptor as a Potential Target for Major Depressive Disorder

Major depressive disorder (MDD) is a common mental disorder. Although the genetic, biochemical, and psychological factors have been related to the development of MDD, it is generally believed that a series of pathological changes in the brain caused by chronic stress is the main cause of MDD. However, the specific mechanisms underlying chronic stress-induced MDD are largely undermined. Recent investigations have found that increased pro-inflammatory cytokines and changes in the inflammatory pathway in the microglia cells in the brain are the potential pathophysiological mechanism of MDD. P2X7 receptor (P2X7R) and its mediated signaling pathway play a key role in microglia activation. The present review aimed to present and discuss the accumulating data on the role of P2X7R in MDD. Firstly, we summarized the research progress in the correlation between P2X7R and MDD. Subsequently, we presented the P2X7R mediated microglia activation in MDD and the role of P2X7R in increased blood-brain barrier (BBB) permeability caused by chronic stress. Lastly, we also discussed the potential mechanism underlying-P2X7R expression changes after chronic stress. In conclusion, P2X7R is a key molecule regulating the activation of microglia. Chronic stress activates microglia in the hippocampus by secreting interleukin- 1β (IL-1β) and other inflammatory cytokines, and increasing the BBB permeability, thus promoting the occurrence and development of MDD, which indicated that P2X7R might be a promising therapeutic target for MDD.

The potential of the P2X7 receptor as a therapeutic target in a sub-chronic PCP-induced rodent model of schizophrenia

OBJECTIVE

We studied the role of the P2X7 receptor on cognitive dysfunction in a mouse model of schizophrenia.

METHODS

An adult mouse model was established by treatment with phencyclidine (PCP), an N-methyl-D-aspartate (NMDA) receptor antagonist. Young mice were divided into three groups: 1) the control (saline-injected) group; 2) experimental 5 mg/kg PCP-injected group; and 3) experimental 10 mg/kg PCP-injected group. The mice were subjected to the open-field and Morris water maze tests at 7 weeks. After intraperitoneal injection of the P2X7 receptor antagonist JNJ-47965567, the behaviour tests were performed again. Samples were taken after testing. The P2X7 receptor protein and mRNA expression levels were detected by immunohistochemistry, Western blotting and PCR.

RESULTS

This study revealed that the infant sub-chronic PCP mice model showed severe spatial learning and memory impairment in the Morris water maze and schizophrenia-like symptoms (hypermotor behaviour) in the open-field test. The P2X7 receptor protein was highly expressed in the sub-chronic PCP mouse model and more highly expressed in the hippocampus than the prefrontal lobe. After the P2X7 receptor was blocked with JNJ-47965567, P2X7 receptor protein and mRNA expression in the frontal lobe were significantly increased, and the spatial memory impairment and hypermotor behaviour induced by PCP were reversed.

CONCLUSION

PCP-induced cognitive impairment can be significantly improved by antagonizing the P2X7 receptor. Therefore, we believe that the P2X7 receptor plays an important role in the cognition of schizophrenic-like mice.

P2RX7 gene variation mediates the effect of childhood adversity and recent stress on the severity of depressive symptoms

The P2X purinoceptor 7 (P2RX7) mediates inflammatory microglial responses and is implicated in neuroimmune mechanisms of depression and neurodegenerative disorders. A number of studies suggest that psychosocial stress may precipitate depression through immune activation. Genetic association studies of P2RX7 variants with depression have been inconclusive. However, nearly all studies have focused on only one single-nucleotide polymorphism (SNP) and have not considered interaction with psychosocial stress. We investigated the effect of several variations in P2RX7 gene using a clumping method in interaction with early adversities and recent stress on depression severity. 1752 subjects provided information on childhood adversities, recent life events, and current depression severity. Participants were genotyped for 681 SNPs in the P2RX7 gene, 335 of them passed quality control and were entered into linear regression models followed by a clumping procedure for main effect and interactions. No significant main effect was observed. Rs74892325 emerged as a top SNP for interaction with childhood adversities and rs61953400 for interaction with recent life events. Our study is the first to investigate several variants in the P2RX7 gene and in interaction with two types of stress, extending our understanding of neuroinflammation in depression, and supporting that the majority of genes influence depression by enhancing sensitivity to stressors.

Brain immune cells characterization in UCMS exposed P2X7 knock-out mouse

BACKGROUND

Several lines of evidence suggest that neuroinflammation might be a key neurobiological mechanism of depression. In particular, the P2X7 receptor (P2X7R), an ATP-gated ion channel involved in activation of the pro-inflammatory interleukin IL-1β, has been shown to be a potential new pharmacological target in depression. The aim of this study was to explore the impact of unpredictable chronic mild stress (UCMS) on behavioural changes, hippocampal neurogenesis, and cellular characterisation of brain immune cells, in P2X7R Knock-Out (KO) mice.

METHODS

P2X7R KO and wild-type (WT) mice were subjected to a 6-week UCMS protocol and received a conventional oral antidepressant (15 mg.kg^-1 fluoxetine) or water per os. The mice then underwent behavioural tests consisting of the tail suspension test (TST), the elevated plus maze (EPM) test, the open field test, the splash test and the nest building test (week 7). Doublecortin immunostaining (DCX) of brain slices was used to assess neurogenesis in the dentate gyrus. Iba1 and TMEM119 immunostaining was used to characterise brain immune cells, Iba1 as a macrophage marker (including microglial cells) and TMEM119 as a potential specific resident microglial cells marker.

RESULTS

After a 6-week UCMS exposure, P2X7R KO mice exhibited less deterioration of their coat state, spent a significantly smaller amount of time immobile in the TST and spent a larger amount of time in the open arms of the EPM. As expected, adult ventral hippocampal neurogenesis was significantly decreased by UCMS in WT mice, while P2X7R KO mice maintained ventral hippocampal neurogenesis at similar levels in both control and UCMS conditions. In stress-related brain regions, P2X7R KO mice also exhibited less recruitment of Iba1+/TMEM119+ and Iba1+/TMEM119- cells in the brain. The ratio between these two staining patterns revealed that brain immune cells were mostly composed of Iba1+/TMEM119+ cells (87 to 99%), and this ratio was affected neither by P2X7R genetic depletion nor by antidepressant treatment.

DISCUSSION

Behavioural patterns, neurogenesis levels and density of brain immune cells in P2X7R KO mice after exposure to UCMS significantly differed from control conditions. Brain immune cells were mostly increased in brain regions known to be sensitive to UCMS exposure in WT but not in P2X7R KO mice. Considering Iba1+/TMEM119- staining might characterize peripheral immune cells, the ratio between Iba1+/TMEM119+ cells and IBA1+/TMEM119- cells, suggests that the rate of peripheral immune cells recruitment may not be modified neither by P2X7R gene expression nor by antidepressant treatment.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

Serotonergic neurons in the treatment of mood disorders: The dialogue with astrocytes

Astrocytes were traditionally regarded as cells important to neuronal activity, providing both metabolic and structural supports. Recent evidence suggests that they may also play a crucial role in the control of higher brain functions. In keeping with this hypothesis, it is now well accepted that astrocytes contribute to stress but also react to antidepressant drugs as they express serotonergic transporters and receptors. However, the downstream mechanisms leading to the fine-tuned regulation of mood are still unknown. This chapter pays attention to the role of astrocytes in the regulation of emotional behavior and related serotonergic neurotransmission. In particular, it gives a current state of the clinical and preclinical evidence showing that astrocytes respond to environmental conditions and antidepressant drugs through the release of gliotransmitters and neurotrophic factors which in turn, influence serotonergic tone in discrete brain areas. This state-of-the-art review aims at demonstrating the remarkable potential for novel therapeutic antidepressant strategies targeting these glial cells.

Oligodendrocyte lineage cells and depression

Depression is a common mental illness, affecting more than 300 million people worldwide. Decades of investigation have yielded symptomatic therapies for this disabling condition but have not led to a consensus about its pathogenesis. There are data to support several different theories of causation, including the monoamine hypothesis, hypothalamic-pituitary-adrenal axis changes, inflammation and immune system alterations, abnormalities of neurogenesis and a conducive environmental milieu. Research in these areas and others has greatly advanced the current understanding of depression; however, there are other, less widely known theories of pathogenesis. Oligodendrocyte lineage cells, including oligodendrocyte progenitor cells and mature oligodendrocytes, have numerous important functions, which include forming myelin sheaths that enwrap central nervous system axons, supporting axons metabolically, and mediating certain forms of neuroplasticity. These specialized glial cells have been implicated in psychiatric disorders such as depression. In this review, we summarize recent findings that shed light on how oligodendrocyte lineage cells might participate in the pathogenesis of depression, and we discuss new approaches for targeting these cells as a novel strategy to treat depression.

P2X7 Receptor-Dependent Layer-Specific Changes in Neuron-Microglia Reactivity in the Prefrontal Cortex of a Phencyclidine Induced Mouse Model of Schizophrenia

Background: It has been consistently reported that the deficiency of the adenosine triphosphate (ATP) sensitive purinergic receptor P2X7 (P2X7R) ameliorates symptoms in animal models of brain diseases.

Objective: This study aimed to investigate the role of P2X7R in rodent models of acute and subchronic schizophrenia based on phencyclidine (PCP) delivery in animals lacking or overexpressing P2X7R, and to identify the underlying mechanisms involved.

Methods: The psychotomimetic effects of acute i.p. PCP administration in C57Bl/6J wild-type, P2X7R knockout (P2rx7^−/−) and overexpressing (P2X7-EGFP) young adult mice were quantified. The medial prefrontal cortex (mPFC) of P2rx7^−/− and heterozygous P2X7-EGFP acutely treated animals was characterized through immunohistochemical staining. The prefrontal cortices of young adult P2rx7^−/− and P2rx7^tg/+ mice were examined with tritiated dopamine release experiments and the functional properties of the mPFC pyramidal neurons in layer V from P2rx7^−/− mice were assessed by patch-clamp recordings. P2rx7^−/− animals were subjected to a 7 days subchronic systemic PCP treatment. The animals working memory performance and PFC cytokine levels were assessed.

Results: Our data strengthen the hypothesis that P2X7R modulates schizophrenia-like positive and cognitive symptoms in NMDA receptor antagonist models in a receptor expression level-dependent manner. P2X7R expression leads to higher medial PFC susceptibility to PCP-induced circuit hyperactivity. The mPFC of P2X7R knockout animals displayed distinct alterations in the neuronal activation pattern, microglial organization, specifically around hyperactive neurons, and were associated with lower intrinsic excitability of mPFC neurons.

Conclusions: P2X7R expression exacerbated PCP-related effects in C57Bl/6J mice. Our findings suggest a pleiotropic role of P2X7R in the mPFC, consistent with the observed behavioral phenotype, regulating basal dopamine concentration, layer-specific neuronal activation, intrinsic excitability of neurons in the mPFC, and the interaction of microglia with hyperactive neurons. Direct measurements of P2X7R activity concerning microglial ramifications and dynamics could help to further elucidate the molecular mechanisms involved.

Characterisation of the pharmacodynamic effects of the P2X7 receptor antagonist JNJ-54175446 using an oral dexamphetamine challenge model in healthy males in a randomised, double-blind, placebo-controlled, multiple ascending dose trial

BACKGROUND

This is the first report of the pharmacodynamic (PD) effects of the selective, potent and brain-penetrant P2X7 receptor (P2X7R) antagonist JNJ-54175446. Activation of the P2X7R, an adenosine triphosphate-gated ion channel, leads to the production of pro-inflammatory cytokines, which have been linked to neuroinflammation and play a role in the pathogenesis of mood disorders. Previous clinical studies with JNJ-54175446 demonstrated peripheral target engagement of JNJ-54175446 by assessing ex vivo lipopolysaccharide (LPS)-stimulated cytokine production. Blood-brain barrier penetration and a clear dose-receptor occupancy relationship was demonstrated using positron emission tomography.

AIMS

The objectives of this double-blind, placebo-controlled, translational study were to assess the safety and tolerability of administering multiple doses of JNJ-54175446 and to explore its PD effects using a dexamphetamine challenge.

METHODS

Subjects (N = 64) were randomised to either JNJ-54175446 (50-450 mg; n = 48) or placebo (n = 16) and underwent a baseline oral 20 mg dexamphetamine challenge followed by 11 consecutive days q.d. dosing with JNJ-54175446/placebo and a randomised crossover dexamphetamine/placebo challenge.

RESULTS

At all doses tested, JNJ-54175446 was well tolerated and suppressed the ex vivo LPS-induced release of cytokines. At doses ⩾100 mg, JNJ-54175446 attenuated dexamphetamine-induced increases in locomotion and enhanced the mood-elevating effects of dexamphetamine, suggesting that a dose that is approximately twice as high is needed to obtain a central PD response compared to the dose needed for maximum peripheral occupancy.

CONCLUSION

Overall, the observed pharmacological profile of JNJ-54175446 in the dexamphetamine challenge paradigm is compatible with a potential mood-modulating effect.

The P2X7 Receptor: Central Hub of Brain Diseases

The P2X7 receptor is a cation channel activated by high concentrations of adenosine triphosphate (ATP). Upon long-term activation, it complexes with membrane proteins forming a wide pore that leads to cell death and increased release of ATP into the extracellular milieu. The P2X7 receptor is widely expressed in the CNS, such as frontal cortex, hippocampus, amygdala and striatum, regions involved in neurodegenerative diseases and psychiatric disorders. Despite P2X7 receptor functions in glial cells have been extensively studied, the existence and roles of this receptor in neurons are still controversially discussed. Regardless, P2X7 receptors mediate several processes observed in neuropsychiatric disorders and brain tumors, such as activation of neuroinflammatory response, stimulation of glutamate release and neuroplasticity impairment. Moreover, P2X7 receptor gene polymorphisms have been associated to depression, and isoforms of P2X7 receptors are implicated in neuropsychiatric diseases. In view of that, the P2X7 receptor has been proposed to be a potential target for therapeutic intervention in brain diseases. This review discusses the molecular mechanisms underlying P2X7 receptor-mediated signaling in neurodegenerative diseases, psychiatric disorders, and brain tumors. In addition, it highlights the recent advances in the development of P2X7 receptor antagonists that are able of penetrating the central nervous system.

Purinergic Signaling and Related Biomarkers in Depression

It is established that purinergic signaling can shape a wide range of physiological functions, including neurotransmission and neuromodulation. The purinergic system may play a role in the pathophysiology of mood disorders, influencing neurotransmitter systems and hormonal pathways of the hypothalamic-pituitary-adrenal axis. Treatment with mood stabilizers and antidepressants can lead to changes in purinergic signaling. In this overview, we describe the biological background on the possible link between the purinergic system and depression, possibly involving changes in adenosine- and ATP-mediated signaling at P1 and P2 receptors, respectively. Furthermore, evidence on the possible antidepressive effects of non-selective adenosine antagonist caffeine and other purinergic modulators is reviewed. In particular, A2A and P2X7 receptors have been identified as potential targets for depression treatment. Preclinical studies highlight that both selective A2A and P2X7 antagonists may have antidepressant effects and potentiate responses to antidepressant treatments. Consistently, recent studies feature the possible role of the purinergic system peripheral metabolites as possible biomarkers of depression. In particular, variations of serum uric acid, as the end product of purinergic metabolism, have been found in depression. Although several open questions remain, the purinergic system represents a promising research area for insights into the molecular basis of depression.

Pathological ATPergic Signaling in Major Depression and Bipolar Disorder

The mood disorders, major depression (MD) and bipolar disorder (BD), have a high lifetime prevalence in the human population and accordingly generate huge costs for health care. Efficient, rapidly acting, and side-effect-free pharmaceuticals are hitherto not available, and therefore, the identification of new therapeutic targets is an imperative task for (pre)clinical research. Such a target may be the purinergic P2X7 receptor (P2X7R), which is localized in the central nervous system (CNS) at microglial and neuroglial cells mediating neuroinflammation. MD and BD are due to neuroinflammation caused in the first line by the release of the pro-inflammatory cytokine interleukin-1β (IL-1β) from the microglia. IL-1β in turn induces the secretion of corticotropin-releasing hormone (CRH) and in consequence the secretion of adrenocorticotropic hormone (ACTH) and cortisol, which together with a plethora of further cytokines/chemokines lead to mood disorders. A number of biochemical/molecular biological measurements including the use of P2X7R- or IL-1β-deficient mice confirmed this chain of events. More recent studies showed that a decrease in the astrocytic release of ATP in the prefrontal cortex and hippocampus is a major cause of mood disorders. It is an attractive hypothesis that compensatory increases in P2X7Rs in these areas of the brain are the immediate actuators of MD and BD. Hence, blood-brain barrier-permeable P2X7R antagonists may be promising therapeutic tools to improve depressive disorders in humans.

Antidepressant-like effect induced by P2X7 receptor blockade in FSL rats is associated with BDNF signalling activation

BACKGROUND

P2X7 receptors (P2X7R) are ligand-gated ion channels activated by adenosine 5'-triphosphate (ATP), which are involved in processes that are dysfunctional in stress response and depression, such as neurotransmitter release, and neuroimmune response. Genetic and pharmacological inhibition of the P2X7R induce antidepressant-like effects in animals exposed to stress. However, the effect of P2X7R antagonism in an animal model of depression based on selective breeding has not previously been studied, and the mechanism underling the antidepressant-like effect induced by the P2X7R blockade remains unknown.

AIMS

The present study aimed to: (1) determine whether P2X7R blockade induces antidepressant-like effects in the Flinders Sensitive Line (FSL) rats and, (2) investigate whether brain-derived neurotrophic factor (BDNF) signalling in the frontal cortex and hippocampus is involved in this effect.

METHODS

FSL and the control Flinders Resistant Line (FRL) rats were treated with vehicle or the P2X7R antagonist A-804598 (3, 10 or 30 mg/Kg/day) for 1 or 7 days before being exposed to the forced swim test (FST). After the behavioural test, animals were decapitated, their brains were removed and the frontal cortex, ventral and dorsal hippocampus were dissected for BDNF signalling analysis.

RESULTS

We found that repeated treatment with A-804598 (30 mg/Kg) reduced the immobility time in the FST and activated the BDNF signalling in the ventral hippocampus of FSL rats.

CONCLUSIONS

P2X7R blockade induces an antidepressant-like effect associated with increased levels of BDNF-AKT-p70 S6 kinase in the ventral hippocampus, which may be mediated by tropomyosin-related kinase B (TRKB) receptor activation supporting the notion of P2X7R antagonism as a potential new antidepressant strategy.

ATP/P2X7r axis mediates the pathological process of allergic asthma by inducing M2 polarization of alveolar macrophages

Recent studies revealed that macrophages are polarized towards the M2 phenotype in an ovalbumin (OVA)-induced asthmatic model. Alveolar macrophages (AMs) are immune barriers in alveoli to various pathogens in the respiratory tract; AMs suppress Th2 cell proliferation, inhibit interleukin (IL)-4, IL-5, and IL-13 secretion, and protect against airway hyperresponsiveness in allergic asthma. However, the polarization status and effects of different types of AMs in the pathogenesis of asthma are not known. ATP/P2X7r, expressed mainly on macrophages and dendritic cells, is associated with acute and chronic asthmatic airway inflammation and Th2 immune responses in mice and humans and functions by activating the NLRP3 inflammasome complex and inducing proinflammatory cytokine release (IL-1β and IL-18). Therefore, we evaluated the association between the ATP/P2X7r axis and different types of AMs in the pathology of allergic asthma. A murine AM-depleted asthma model was established by administration of clodronate-encapsulated liposomes, and M1-or M2-AMs were adoptively transferred to confirm the effects of different AMs in allergic asthma. Brilliant Blue G and BzATP were administered to OVA/HDM-induced mice in vivo. Lipopolysaccharide + OVA, ATP, Brilliant Blue G, and BzATP were used to stimulate AMs isolated from control and asthmatic mice. We found that selective depletion of AMs aggravated lung inflammation in asthmatic mice. Further, M2-type AMs may play a key role in mediating asthmatic inflammatory responses via the adoptive transfer of M2-type AMs to AM-depleted asthmatic mice, and the phenotype of AMs differentiated to M2 type in asthma. P2X7r expression in M2-type AMs was higher than that in M1-type AMs. Activating P2X7r induced polarization of M2-type AMs and inhibited polarization of M1-type AMs, while blockage of P2X7r had the opposite effect. The ATP/P2X7r axis may participate in the pathogenesis of asthma by mediating the M2-type AM polarization.

Reduced P2X receptor levels are associated with antidepressant effect in the learned helplessness model

Purinergic receptors, especially P2RX, are associated to the severity of symptoms in patients suffering from depressive and bipolar disorders, and genetic deletion or pharmacological blockade of P2RX7 induces antidepressant-like effect in preclinical models. However, there is scarce evidence about the alterations in P2RX7 or P2RX4 levels and in behavioral consequences induced by previous exposure to stress, a major risk factor for depression in humans. In the present study, we evaluated the effect of imipramine (IMI) on P2RX7 and P2RX4 levels in dorsal and ventral hippocampus as well as in the frontal cortex of rats submitted to the pretest session of learned helplessness (LH) paradigm. Repeated, but not acute administration of IMI (15 mg/kg ip) reduced the levels of both P2RX7 and P2RX4 in the ventral, but not in dorsal hippocampus or frontal cortex. In addition, we tested the effect of P2RX7/P2RX4 antagonist brilliant blue G (BBG: 25 or 50 mg/kg ip) on the LH paradigm. We observed that repeated (7 days) but not acute (1 day) treatment with BBG (50 mg) reduced the number of failures to escape the shocks in the test session, a parameter mimicked by the same regimen of IMI treatment. Taken together, our data indicates that pharmacological blockade or decrease in the expression of P2RX7 is associated to the antidepressant-like behavior observed in the LH paradigm after repeated drug administration.

Mitochondria: A Connecting Link in the Major Depressive Disorder Jigsaw

Background

Depression is a widespread phenomenon with varying degrees of pathology in different patients. Various hypotheses have been proposed for the cause and continuance of depression. Some of these include, but not limited to, the monoamine hypothesis, the neuroendocrine hypothesis, and the more recent epigenetic and inflammatory hypotheses.

Objective

In this article, we review all the above hypotheses with a focus on the role of mitochondria as the connecting link. Oxidative stress, respiratory activity, mitochondrial dynamics and metabolism are some of the mitochondria-dependent factors which are affected during depression. We also propose exogenous ATP as a contributing factor to depression.

Result

Literature review shows that pro-inflammatory markers are elevated in depressive individuals. The cause for elevated levels of cytokines in depression is not completely understood. We propose exogenous ATP activates purinergic receptors which in turn increase the levels of various pro-inflammatory factors in the pathophysiology of depression.

Conclusion

Mitochondria are integral to the function of neurons and undergo dysfunction in major depressive disorder patients. This dysfunction is reflected in all the various hypotheses that have been proposed for depression. Among the newer targets identified, which also involve mitochondria, includes the role of exogenous ATP. The diversity of purinergic receptors, and their differential expression among various individuals in the population, due to genetic and environmental (prenatal) influences, may influence the susceptibility and severity of depression. Identifying specific receptors involved and using patient-specific purinergic receptor antagonist may be an appropriate therapeutic course in the future.

Association of purinergic receptor P2RX7 gene polymorphisms with depression symptoms

INTRODUCTION

The activation of the ATP-gated P2RX7 (purinergic receptor P2X, ligand-gated ion channel, 7) produces microglial activation, a process which has been demonstrated in depression, bipolar disorder, and schizophrenia. Emerging data over the last years highlighted the importance of P2X7 cation channel as a potential drug target for these central nervous system disorders. The Gln460Arg (rs2230912) polymorphism of the P2RX7 gene has been widely studied in mood disorders, however the results are still controversial. Therefore, we aimed to investigate the C-terminal region of this gene in major depressive and bipolar disorders (MDD and BD) by studying possibly functional, non-synonymous polymorphisms within a 7 kb long region around the Gln460Arg, including Ala348Thr (rs1718119), Thr357Ser (rs2230911), and Glu496Ala (rs3751143) variants. Since Gln460Arg is located at the 3' end of the P2RX7 gene, we included additional, potentially functional single nucleotide polymorphisms (SNPs) from the 3' untranslated region (UTR), which can be in linkage with Gln460Arg. Based on in silico search, we chose two SNPs in putative microRNA target sites which are located in consecutive positions: rs1653625 and rs1718106.

METHODS

P2RX7 SNPs from the C-terminal region were selected based on previous functional assays, 3' UTR variants were chosen using PolymiRTS and Patrocles databases. The genotyping of the non-synonymous SNPs was carried out by pre-designed TaqMan^® kits, while the 3' UTR variants were analyzed by PCR-RFLP method. Case-control analyses were carried out between 315 inpatients with acute major depressive episode (195 MDD, 120 BD) and 406 healthy control subjects. The two subscales of the Hospital Anxiety and Depression Scale (HADS) self-report questionnaire were used for quantitative analyses, including an additional, "at-risk" population of 218 patients with diabetes mellitus. The in vitro reporter gene assays were carried out on HEK and SK-N-FI cells transiently transfected with pMIR vector constructs containing the P2RX7 3' UTR downstream of the luciferase gene.

RESULTS

Haplotype analysis indicated a relatively high linkage between the analyzed P2RX7 SNPs. Our case-control study did not yield any association between P2RX7 gene variants and depression. However, dimensional analyses showed significant associations of the HADS depression severity scores with Gln460Arg (rs2230912) and Ala348Thr (rs1718119) in the depressed and diabetic patient groups. In the in vitro experiments, the P2RX7 3' UTR constructs with the lowest predicted binding efficiency to their miRNAs showed the highest expression of the gene. The combination of the depression-associated P2RX7 C-terminal and 3' UTR SNPs contributed to the highest depression severity score in the haplotype analysis.

CONCLUSION

Based on our findings, we propose that a P2RX7 haplotype combination of the Gln460Arg and neighboring SNPs contribute to the observed genetic association with depressive symptoms.

P2X7 Receptor Signaling in Stress and Depression

Stress exposure is considered to be the main environmental cause associated with the development of depression. Due to the limitations of currently available antidepressants, a search for new pharmacological targets for treatment of depression is required. Recent studies suggest that adenosine triphosphate (ATP)-mediated signaling through the P2X7 receptor (P2X7R) might play a prominent role in regulating depression-related pathology, such as synaptic plasticity, neuronal degeneration, as well as changes in cognitive and behavioral functions. P2X7R is an ATP-gated cation channel localized in different cell types in the central nervous system (CNS), playing a crucial role in neuron-glia signaling. P2X7R may modulate the release of several neurotransmitters, including monoamines, nitric oxide (NO) and glutamate. Moreover, P2X7R stimulation in microglia modulates the innate immune response by activating the NLR family pyrin domain containing 3 (NLRP3) inflammasome, consistent with the neuroimmune hypothesis of MDD. Importantly, blockade of P2X7R leads to antidepressant-like effects in different animal models, which corroborates the findings that the gene encoding for the P2X7R is located in a susceptibility locus of relevance to depression in humans. This review will discuss recent findings linked to the P2X7R involvement in stress and MDD neuropathophysiology, with special emphasis on neurochemical, neuroimmune, and neuroplastic mechanisms.

Antidepressant-like Effects Induced by Chronic Blockade of the Purinergic 2X7 Receptor through Inhibition of Non-like Receptor Protein 1 Inflammasome in Chronic Unpredictable Mild Stress Model of Depression in Rats

Objective

Purinergic 2X7 receptor (P2X7R) activation is known to be involved in pathogenesis of depression. Our aims were to investigate P2X7R-activated inflammasome pathways in parallel with induction of depression and to test the antidepressant-like effects of the selective P2X7R antagonist Brilliant Blue G (BBG) in a rat model of chronic unpredictable mild stress (CUMS).

Methods

Male Wistar albino rats were divided into control, CUMS, CUMS+BBG25 (25 mg/kg/day) and CUMS+BBG50 (50 mg/kg/day) groups (n=10 for each group). Various stressors were applied to rats for 6 weeks to establish the CUMS model and daily BBG treatment was started at the end of 3rd week. Sucrose preference test and forced swim test (FST) were performed to assess antidepressant-like effects. Brain samples were obtained for real-time polymerase chain reaction and immunohistochemistry analysis.

Results

In FST, duration of immobility was reduced in the CUMS+BBG50 group. Also, BBG treatment significantly enhanced sucrose preference. While NLRP3 gene expression levels were unchanged in rats exposed to the CUMS protocol, expression levels of other inflammasome pathway factors NLRP1, caspase-1, ASC, NF-κB, IL-1β, IL-6 and P2X7R were increased. BBG treatment reduced expression levels of these factors. Likewise, Iba-1 and GFAP immunoreactivities were enhanced by the CUMS protocol and this action was reversed by BBG treatment.

Conclusion

Chronic administration of BBG in CUMS model results in antidepressant-like activity in a dose dependent manner. Molecular and histological results show that these effects might be at least partially related to the suppression of inflammasome-related neuroinflammatory responses and suggest involvement of NLRP1 in depression.

P2X7 Receptors Drive Poly(I:C) Induced Autism-like Behavior in Mice

Maternal immune activation (MIA) is a principal environmental risk factor contributing to autism spectrum disorder (ASD), which compromises fetal brain development at critical periods of pregnancy and might be causally linked to ASD symptoms. We report that endogenous activation of the purinergic ion channel P2X7 (P2rx7) is necessary and sufficient to transduce MIA to autistic phenotype in male offspring. MIA induced by poly(I:C) injections to P2rx7 WT mouse dams elicited an autism-like phenotype in their offspring, and these alterations were not observed in P2rx7-deficient mice, or following maternal treatment with a specific P2rx7 antagonist, JNJ47965567. Genetic deletion and pharmacological inhibition of maternal P2rx7s also counteracted the induction of IL-6 in the maternal plasma and fetal brain, and disrupted brain development, whereas postnatal P2rx7 inhibition alleviated behavioral and morphological alterations in the offspring. Administration of ATP to P2rx7 WT dams also evoked autistic phenotype, but not in KO dams, implying that P2rx7 activation by ATP is sufficient to induce autism-like features in offspring. Our results point to maternal and offspring P2rx7s as potential therapeutic targets for the early prevention and treatment of ASD.SIGNIFICANCE STATEMENT Autism spectrum disorder (ASD) is a neurodevelopmental psychiatric disorder caused by genetic and environmental factors. Recent studies highlighted the importance of perinatal risks, in particular, maternal immune activation (MIA), showing strong association with the later emergence of ASD in the affected children. MIA could be mimicked in animal models via injection of a nonpathogenic agent poly(I:C) during pregnancy. This is the first report showing the key role of a ligand gated ion channel, the purinergic P2X7 receptor in MIA-induced autism-like behavioral and biochemical features. We show that genetic or pharmacological inhibition of both maternal and offspring P2X7 receptors could reverse the compromised brain development and autistic phenotype pointing to new possibilities for prevention and treatment of ASD.