P2X7 receptor inhibition ameliorates dendritic spine pathology and social behavioral deficits in Rett syndrome mice

Biochemical and molecular determinants of the subclinical inflammatory mechanisms in Rett syndrome

To date, Rett syndrome (RTT), a genetic disorder mainly caused by mutations in the X-linked MECP2 gene, is increasingly considered a broad-spectrum pathology, instead of just a neurodevelopmental disease, due to the multitude of peripheral co-morbidities and the compromised metabolic pathways, affecting the patients. The altered molecular processes include an impaired mitochondrial function, a perturbed redox homeostasis, a chronic subclinical inflammation and an improper cholesterol metabolism. The persistent subclinical inflammatory condition was first defined ten years ago, as a previously unrecognized feature of RTT, playing a role in the pathology progress and modulation of phenotypical severity. In light of this, the present work aims at reviewing the current knowledge on the chronic inflammatory status and the altered immune/inflammatory functions in RTT, as well as investigating the emerging mechanisms underlying this condition with a special focus on the latest findings about inflammasome system, autoimmunity responses and intestinal micro- and mycobiota. On these bases, although further research is needed, future therapeutic strategies able to re-establish an adequate immune/inflammatory response could represent potential approaches for RTT patients.

Multidimensional Analysis of a Social Behavior Identifies Regression and Phenotypic Heterogeneity in a Female Mouse Model for Rett Syndrome

Regression is a key feature of neurodevelopmental disorders such as autism spectrum disorder, Fragile X syndrome, and Rett syndrome (RTT). RTT is caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2). It is characterized by an early period of typical development with subsequent regression of previously acquired motor and speech skills in girls. The syndromic phenotypes are individualistic and dynamic over time. Thus far, it has been difficult to capture these dynamics and syndromic heterogeneity in the preclinical Mecp2-heterozygous female mouse model (Het). The emergence of computational neuroethology tools allows for robust analysis of complex and dynamic behaviors to model endophenotypes in preclinical models. Toward this first step, we utilized DeepLabCut, a marker-less pose estimation software to quantify trajectory kinematics and multidimensional analysis to characterize behavioral heterogeneity in Het in the previously benchmarked, ethologically relevant social cognition task of pup retrieval. We report the identification of two distinct phenotypes of adult Het: Het that display a delay in efficiency in early days and then improve over days like wild-type mice and Het that regress and perform worse in later days. Furthermore, regression is dependent on age and behavioral context and can be detected in the initial days of retrieval. Together, the novel identification of two populations of Het suggests differential effects on neural circuitry, opens new avenues to investigate the underlying molecular and cellular mechanisms of heterogeneity, and designs better studies for stratifying therapeutics.

Neuronal connectivity, behavioral, and transcriptional alterations associated with the loss of MARK2

Neuronal connectivity is essential for adaptive brain responses and can be modulated by dendritic spine plasticity and the intrinsic excitability of individual neurons. Dysregulation of these processes can lead to aberrant neuronal activity, which has been associated with numerous neurological disorders including autism, epilepsy, and Alzheimer's disease. Nonetheless, the molecular mechanisms underlying aberrant neuronal connectivity remains unclear. We previously found that the serine/threonine kinase Microtubule Affinity Regulating Kinase 2 (MARK2), also known as Partitioning Defective 1b (Par1b), is important for the formation of dendritic spines in vitro. However, despite its genetic association with several neurological disorders, the in vivo impact of MARK2 on neuronal connectivity and cognitive functions remains unclear. Here, we demonstrate that loss of MARK2 in vivo results in changes to dendritic spine morphology, which in turn leads to a decrease in excitatory synaptic transmission. Additionally, loss of MARK2 produces substantial impairments in learning and memory, anxiety, and social behavior. Notably, MARK2 deficiency results in heightened seizure susceptibility. Consistent with this observation, RNAseq analysis reveals transcriptional changes in genes regulating synaptic transmission and ion homeostasis. These findings underscore the in vivo role of MARK2 in governing synaptic connectivity, cognitive functions, and seizure susceptibility.

Altered Purinergic Signaling in Neurodevelopmental Disorders: Focus on P2 Receptors

With the umbrella term 'neurodevelopmental disorders' (NDDs) we refer to a plethora of congenital pathological conditions generally connected with cognitive, social behavior, and sensory/motor alterations. Among the possible causes, gestational and perinatal insults have been demonstrated to interfere with the physiological processes necessary for the proper development of fetal brain cytoarchitecture and functionality. In recent years, several genetic disorders caused by mutations in key enzymes involved in purine metabolism have been associated with autism-like behavioral outcomes. Further analysis revealed dysregulated purine and pyrimidine levels in the biofluids of subjects with other NDDs. Moreover, the pharmacological blockade of specific purinergic pathways reversed the cognitive and behavioral defects caused by maternal immune activation, a validated and now extensively used rodent model for NDDs. Furthermore, Fragile X and Rett syndrome transgenic animal models as well as models of premature birth, have been successfully utilized to investigate purinergic signaling as a potential pharmacological target for these diseases. In this review, we examine results on the role of the P2 receptor signaling in the etiopathogenesis of NDDs. On this basis, we discuss how this evidence could be exploited to develop more receptor-specific ligands for future therapeutic interventions and novel prognostic markers for the early detection of these conditions.

Rapid effects of valproic acid on the fetal brain transcriptome: Implications for brain development and autism

There is an increased incidence of autism among the children of women who take the anti-epileptic, mood stabilizing drug, valproic acid (VPA) during pregnancy; moreover, exposure to VPA in utero causes autistic-like symptoms in rodents and non-human primates. Analysis of RNAseq data obtained from fetal mouse brains 3 hr after VPA administration revealed that VPA significantly [p(FDR) ≤ 0.025] increased or decreased the expression of approximately 7,300 genes. No significant sex differences in VPA-induced gene expression were observed. Expression of genes associated with neurodevelopmental disorders such as autism as well as neurogenesis, axon growth and synaptogenesis, GABAergic, glutaminergic and dopaminergic synaptic transmission, perineuronal nets, and circadian rhythms was dysregulated by VPA. Moreover, expression of 400 autism risk genes was significantly altered by VPA as was expression of 247 genes that have been reported to play fundamental roles in the development of the nervous system, but are not linked to autism by GWAS. The goal of this study was to identify mouse genes that are: (a) significantly up- or down-regulated by VPA in the fetal brain and (b) known to be associated with autism and/or to play a role in embryonic neurodevelopmental processes, perturbation of which has the potential to alter brain connectivity in the postnatal and adult brain. The set of genes meeting these criteria provides potential targets for future hypothesis-driven approaches to elucidating the proximal underlying causes of defective brain connectivity in neurodevelopmental disorders such as autism.

P2 receptor-mediated signaling in the physiological and pathological brain: From development to aging and disease

The purinergic pathway mediates both pro-inflammatory and anti-inflammatory responses, whereas the breakdown of adenosine triphosphate (ATP) is in a critical equilibrium. Under physiological conditions, extracellular ATP is maintained at a nanomolar concentration. Whether released into the medium following tissue damage, inflammation, or hypoxia, ATP is considered a clear indicator of cell damage and a marker of pathological conditions. In this overview, we provide an update on the participation of P2 receptor-mediated purinergic signaling in normal and pathological brain development, with special emphasis on neurodevelopmental psychiatric disorders. Since purinergic signaling is ubiquitous, it is not surprising that it plays a prominent role in developmental processes and pathological alterations. The main aim of this review is to conceptualize the time-dependent dynamic changes in the participation of different players in the purinome in shaping the normal and aberrant developmental patterns and diseases of the central nervous system over one's lifespan.

KW-2449 and VPA exert therapeutic effects on human neurons and cerebral organoids derived from MECP2-null hESCs

BACKGROUND

Rett syndrome (RTT), mainly caused by mutations in methyl-CpG binding protein 2 (MECP2), is one of the most prevalent neurodevelopmental disorders in girls. However, the underlying mechanism of MECP2 remains largely unknown and currently there is no effective treatment available for RTT.

METHODS

We generated MECP2-KO human embryonic stem cells (hESCs), and differentiated them into neurons and cerebral organoids to investigate phenotypes of MECP2 loss-of-function, potential therapeutic agents, and the underlying mechanism by transcriptome sequencing.

RESULTS

We found that MECP2 deletion caused reduced number of hESCs-derived neurons and simplified dendritic morphology. Moreover, MECP2-KO cortical organoids exhibited fewer neural progenitor cells and neurons at day 60. Electrophysiological recordings showed that MECP2 deletion altered synaptic activity in organoids. Transcriptome analysis of organoids identified many genes in the PI3K-AKT pathway downregulated following MECP2 deletion. Treatment with either KW-2449 or VPA, small molecules for the activation of PI3K-AKT signaling pathway, alleviated neuronal deficits and transcriptome changes in MECP2-KO human neuronal models.

CONCLUSIONS

These findings suggest that KW-2449 and VPA might be promising drugs for RTT treatment.

Single channel properties of pannexin-1 and connexin-43 hemichannels and P2X7 receptors in astrocytes cultured from rodent spinal cords

Astrocytes express surface channels involved in purinergic signaling. Among these channels, pannexin-1 (Px1) and connexin-43 (Cx43) hemichannels (HCs) release ATP that acts directly, or through its derivatives, on neurons and glia via purinergic receptors. Although HCs are functional, that is, open and close under physiological and pathological conditions, single channel properties of Px1 HCs in astrocytes have not been defined. Here, we developed a dual voltage clamp technique in HeLa cells expressing human Px1-YFP, and then applied this system to rodent spinal astrocytes to compare their single channel properties with other surface channels, that is, Cx43 HCs and P2X7 receptors (P2X7Rs). Channels were recorded in cell attached patches and evoked with ramp cycles applied through another pipette in whole cell voltage clamp. The mean unitary conductances of Px1 HCs were comparable in HeLa Px1-YFP cells and spinal astrocytes, ~42 and ~48 pS, respectively. Based on their unitary conductance, voltage-dependence, and unitary activity after pharmacological and gene silencing, Px1 HCs in astrocytes could be distinguished from Cx43 HCs and P2X7Rs. Channel activity of Px1 HCs and P2X7Rs was greater than that of Cx43 HCs in control astrocytes during ramps. Unitary activity of Px1 HCs was decreased and that of Cx43 HCs and P2X7Rs increased in astrocytes treated with fibroblast growth factor 1 (FGF-1). In summary, we resolved single channel properties of three different surface channels involved in purinergic signaling in spinal astrocytes, which were differentially modulated by FGF-1, a growth factor involved in neurodevelopment, inflammation and repair.

Pattern decorrelation in the mouse medial prefrontal cortex enables social preference and requires MeCP2

Sociability is crucial for survival, whereas social avoidance is a feature of disorders such as Rett syndrome, which is caused by loss-of-function mutations in MECP2. To understand how a preference for social interactions is encoded, we used in vivo calcium imaging to compare medial prefrontal cortex (mPFC) activity in female wild-type and Mecp2-heterozygous mice during three-chamber tests. We found that mPFC pyramidal neurons in Mecp2-deficient mice are hypo-responsive to both social and nonsocial stimuli. Hypothesizing that this limited dynamic range restricts the circuit's ability to disambiguate coactivity patterns for different stimuli, we suppressed the mPFC in wild-type mice and found that this eliminated both pattern decorrelation and social preference. Conversely, stimulating the mPFC in MeCP2-deficient mice restored social preference, but only if it was sufficient to restore pattern decorrelation. A loss of social preference could thus indicate impaired pattern decorrelation rather than true social avoidance.

Monocyte NLRP3-IL-1β Hyperactivation Mediates Neuronal and Synaptic Dysfunction in Perioperative Neurocognitive Disorder

Perioperative neurocognitive disorder may develop in vulnerable patients following major operation. While neuroinflammation is linked to the cognitive effects of surgery, how surgery and immune signaling modulate neuronal circuits, leading to learning and memory impairment remains unknown. Using in vivo two-photon microscopy, Ca²⁺ activity and postsynaptic dendritic spines of layer 5 pyramidal neurons in the primary motor cortex of a mouse model of thoracic surgery are imaged. It is found that surgery causes neuronal hypoactivity, impairments in learning-dependent dendritic spine formation, and deficits in multiple learning tasks. These neuronal and synaptic alterations in the cortex are mediated by peripheral monocytes through the NLRP3 inflammasome-dependent IL-1β production. Depleting peripheral monocytes or inactivating NLRP3 inflammasomes before surgery reduces levels of IL-1β and ameliorates neuronal and behavioral deficits in mice. Furthermore, adoptive transfer of IL-1β-producing myeloid cells from mice undertaking thoracic surgery is sufficient to induce neuronal and behavioral deficits in naïve mice. Together, these findings suggest that surgery leads to excessive NLRP3 activation in monocytes and elevated IL-1β signaling, which in turn causes neuronal hypoactivity and perioperative neurocognitive disorder.

Maternal P2X7 receptor inhibition prevents autism-like phenotype in male mouse offspring through the NLRP3-IL-1β pathway

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition caused by interactions of environmental and genetic factors. Recently we showed that activation of the purinergic P2X7 receptors is necessary and sufficient to convert maternal immune activation (MIA) to ASD-like features in male offspring mice. Our aim was to further substantiate these findings and identify downstream signaling pathways coupled to P2X7 upon MIA. Maternal treatment with the NLRP3 antagonist MCC950 and a neutralising IL-1β antibody during pregnancy counteracted the development of autistic characteristics in offspring mice. We also explored time-dependent changes of a widespread cytokine and chemokine profile in maternal blood and fetal brain samples of poly(I:C)/saline-treated dams. MIA-induced increases in plasma IL-1β, RANTES, MCP-1, and fetal brain IL-1β, IL-2, IL-6, MCP-1 concentrations are regulated by the P2X7/NLRP3 pathway. Offspring treatment with the selective P2X7 receptor antagonist JNJ47965567 was effective in the prevention of autism-like behavior in mice using a repeated dosing protocol. Our results highlight that in addition to P2X7, NLRP3, as well as inflammatory cytokines, may also be potential biomarkers and therapeutic targets of social deficits and repetitive behaviors observed in autism spectrum disorder.

Analysis of P2X7-Induced Neuronal Branching

P2X7 receptors regulate different aspects of neuronal development, including neurogenesis, dendritic outgrowth, and axonal elongation. Primary neuronal culture is a widely used model system in neuroscience as it enables to study molecular and cellular events caused by the activation of different ion channels, receptors, and transporters under controlled conditions. Primary neuronal cultures derived from normal and genetically modified mouse models can be used with a wide array of molecular biological, anatomical, and functional techniques such as RNA sequencing, western blots, immunostaining, Ca²⁺ imaging, and electrophysiology. In addition, they can also be genetically manipulated relatively easily. Moreover, cells can survive for multiple weeks if they are properly maintained and thus the development and maturation of individual neurons and their morphological properties can be studied under different conditions. Here, we present a protocol for the isolation and culturing of primary hippocampal cells from embryonic mouse hippocampal tissue (embryonic days 17.5-18.5). The neurons are plated in poly-L-lysine/laminin coated coverslips, where astroglia proliferation is controlled for the proper study of individual primary neurons. To investigate the development of dendrites and axons, as a good correlate of neuron morphology, we present a transfection protocol, which allows us to fill the whole neuron with a fluorescent protein. Subsequently, we perform tracing and analysis of dendritic branching by Sholl analysis using Neurolucida tracing Software (MBF Bioscience).

PRG-1 relieves pain and depressive-like behaviors in rats of bone cancer pain by regulation of dendritic spine in hippocampus

Rationale: Pain and depression, which tend to occur simultaneously and share some common neural circuits and neurotransmitters, are highly prevalent complication in patients with advanced cancer. Exploring the underlying mechanisms is the cornerstone to prevent the comorbidity of chronic pain and depression in cancer patients. Plasticity-related gene 1 (PRG-1) protein regulates synaptic plasticity and brain functional reorganization during neuronal development or after cerebral lesion. Purinergic P2X7 receptor has been proposed as a therapeutic target for various pain and neurological disorders like depression in rodents. In this study, we investigated the roles of PRG-1 in the hippocampus in the comorbidity of pain and depressive-like behaviors in rats with bone cancer pain (BCP). Methods: The bone cancer pain rat model was established by intra-tibial cell inoculation of SHZ-88 mammary gland carcinoma cells. The animal pain behaviors were assessed by measuring the thermal withdrawal latency values by using radiant heat stimulation and mechanical withdrawal threshold by using electronic von Frey anesthesiometer, and depressive-like behavior was assessed by sucrose preference test and forced swim test. Alterations in the expression levels of PRG-1 and P2X7 receptor in hippocampus were separately detected by using western blot, immunofluorescence and immunohistochemistry analysis. The effects of intra-hippocampal injection of FTY720 (a PRG-1/PP2A interaction activator), PRG-1 overexpression or intra-hippocampal injection of A438079 (a selective competitive P2X₇ receptor antagonist) were also observed. Results: Carcinoma intra-tibia injection caused thermal hyperalgesia, mechanical allodynia and depressive-like behaviors in rats, and also induced the deactivation of neurons and dendritic spine structural anomalies in the hippocampus. Western blot, immunofluorescence and immunohistochemistry analysis showed an increased expression of PRG-1 and P2X₇ receptor in the hippocampus of BCP rats. Intra-hippocampal injection of FTY720 or A438079 attenuated both pain and depressive-like behaviors. Furthermore, overexpression of PRG-1 in hippocampus has similar analgesic efficacy to FTY720. In addition, they rescued neuron deactivation and dendritic spine anomalies. Conclusion: The results suggest that both PRG-1 and P2X₇ receptor in the hippocampus play important roles in the development of pain and depressive-like behaviors in bone cancer condition in rats by dendritic spine regulation via P2X₇R/PRG-1/PP2A pathway.

Anti-Semaphorin 4D Rescues Motor, Cognitive, and Respiratory Phenotypes in a Rett Syndrome Mouse Model

Rett syndrome is a neurodevelopmental disorder caused by mutations of the methyl-CpG binding protein 2 gene. Abnormal physiological functions of glial cells contribute to pathogenesis of Rett syndrome. Semaphorin 4D (SEMA4D) regulates processes central to neuroinflammation and neurodegeneration including cytoskeletal structures required for process extension, communication, and migration of glial cells. Blocking SEMA4D-induced gliosis may preserve normal glial and neuronal function and rescue neurological dysfunction in Rett syndrome. We evaluated the pre-clinical therapeutic efficacy of an anti-SEMA4D monoclonal antibody in the Rett syndrome Mecp2T158A transgenic mouse model and investigated the contribution of glial cells as a proposed mechanism of action in treated mice and in primary glial cultures isolated from Mecp2T158A/y mutant mice. SEMA4D is upregulated in neurons while glial fibrillary acidic protein and ionized calcium binding adaptor molecule 1-positive cells are upregulated in Mecp2T158A/y mice. Anti-SEMA4D treatment ameliorates Rett syndrome-specific symptoms and improves behavioural functions in both pre-symptomatic and symptomatic cohorts of hemizygous Mecp2T158A/y male mice. Anti-SEMA4D also reduces astrocyte and microglia activation in vivo. In vitro experiments demonstrate an abnormal cytoskeletal structure in mutant astrocytes in the presence of SEMA4D, while anti-SEMA4D antibody treatment blocks SEMA4D–Plexin B1 signaling and mitigates these abnormalities. These results suggest that anti-SEMA4D immunotherapy may be an effective treatment option to alleviate symptoms and improve cognitive and motor function in Rett syndrome.

P2X7-deficiency improves plasticity and cognitive abilities in a mouse model of Tauopathy

Alzheimer's disease is the most common form of dementia characterized by intracellular aggregates of hyperphosphorylated Tau protein and extracellular accumulation of amyloid β (Aβ) peptides. We previously demonstrated that the purinergic receptor P2X7 (P2X7) plays a major role in Aβ-mediated neurodegeneration but the relationship between P2X7 and Tau remained overlooked. Such a link was supported by cortical upregulation of P2X7 in patients with various type of frontotemporal lobar degeneration, including mutation in the Tau-coding gene, MAPT, as well as in the brain of a Tauopathy mouse model (THY-Tau22). Subsequent phenotype analysis of P2X7-deficient Tau mice revealed the instrumental impact of this purinergic receptor. Indeed, while P2X7-deficiency had a moderate effect on Tau pathology itself, we observed a significant reduction of microglia activation and of Tau-related inflammatory mediators, particularly CCL4. Importantly, P2X7 deletion ultimately rescued synaptic plasticity and memory impairments of Tau mice. Altogether, the present data support a contributory role of P2X7 dysregulation on processes governing Tau-induced brain anomalies. Due to the convergent role of P2X7 blockade in both Aβ and Tau background, P2X7 inhibitors might prove to be ideal candidate drugs to curb the devastating cognitive decline in Alzheimer's disease and Tauopathies.

Learning-dependent dendritic spine plasticity is impaired in spontaneous autoimmune encephalomyelitis

Cognitive impairment is often observed in multiple sclerosis and its animal models, experimental autoimmune encephalomyelitis (EAE). Using mice with immunization-induced EAE, we have previously shown that the stability of cortical synapses is markedly decreased before the clinical onset of EAE. In this study, we examined learning-dependent structural synaptic plasticity in a spontaneous EAE model. Transgenic mice expressing myelin basic protein-specific T cell receptor genes develop EAE spontaneously at around 8 weeks of age. Using in vivo two-photon microscopy, we found that the elimination and formation rates of postsynaptic dendritic spines in somatosensory and motor cortices increased weeks before detectable signs of EAE and remained to be high during the disease onset. Despite the elevated basal spine turnover, motor learning-induced spine formation was reduced in presymptomatic EAE mice, in line with their impaired ability to retain learned motor skills. Additionally, we found a substantial elevation of IFN-γ mRNA in the brain of 4-week-old presymptomatic mice, and treatment of anti-IFN-γ antibody reduced dendritic spine elimination in the cortex. Together, these findings reveal synaptic instability and failure to form new synapses after learning as early brain pathology of EAE, which may contribute to cognitive and behavioral deficits seen in autoimmune diseases.

Quantification of Dendritic Spines Remodeling under Physiological Stimuli and in Pathological Conditions

Numerous brain diseases are associated with abnormalities in morphology and density of dendritic spines, small membranous protrusions whose structural geometry correlates with the strength of synaptic connections. Thus, the quantitative analysis of dendritic spines remodeling in microscopic images is one of the key elements towards understanding mechanisms of structural neuronal plasticity and bases of brain pathology. In the following article, we review experimental approaches designed to assess quantitative features of dendritic spines under physiological stimuli and in pathological conditions. We compare various methodological pipelines of biological models, sample preparation, data analysis, image acquisition, sample size, and statistical analysis. The methodology and results of relevant experiments are systematically summarized in a tabular form. In particular, we focus on quantitative data regarding the number of animals, cells, dendritic spines, types of studied parameters, size of observed changes, and their statistical significance.

Cadmium exposure triggers osteoporosis in duck via P2X7/PI3K/AKT-mediated osteoblast and osteoclast differentiation

Cadmium is a common environmental pollutant that accumulates in the bone and kidneys and causes severe health and social problems. However, the effects of Cd on the occurrence of osteoporosis and its mechanism of action in this process are unclear. To test whether Cd-induced osteoporosis is mediated via P2X7/PI3K/AKT signaling, duck bone marrow mesenchymal stem cells (BMSCs) and bone marrow macrophage cells (BMMs) were treated with Cd for 5 days, and duck embryos were treated with Cd_. Micro-CT analysis indicated that Cd-induced osteoporosis occurs in vivo, and histopathology and immunohistochemical analyses also revealed that Cd induced bone damage and the downregulation of osteogenic and bone resorption-related proteins. Cd exposure significantly inhibited the differentiation of BMSCs and BMMs into osteoblasts and osteoclasts in vitro, and promoted osteoblast and osteoclast apoptosis. Cd exposure significantly downregulated the P2X7/PI3K/AKT signaling pathway in vivo and in vitro, and inhibition of this signaling pathway significantly aggravated osteoblast and osteoclast differentiation. Cd exposure also upregulated the OPG/RANKL ratio in vivo and in vitro, further inhibiting osteoclast differentiation. These results demonstrate that Cd causes osteoporosis in duck by inhibiting P2X7/PI3K/AKT signaling and increasing the OPG/RANKL ratio. These results establish a previously unknown mechanism of Cd-induced osteoporosis.

Role of Conserved Residues and F322 in the Extracellular Vestibule of the Rat P2X7 Receptor in Its Expression, Function and Dye Uptake Ability

Activation of the P2X7 receptor results in the opening of a large pore that plays a role in immune responses, apoptosis, and many other physiological and pathological processes. Here, we investigated the role of conserved and unique residues in the extracellular vestibule connecting the agonist-binding domain with the transmembrane domain of rat P2X7 receptor. We found that all residues that are conserved among the P2X receptor subtypes respond to alanine mutagenesis with an inhibition (Y51, Q52, and G323) or a significant decrease (K49, G326, K327, and F328) of 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP)-induced current and permeability to ethidium bromide, while the nonconserved residue (F322), which is also present in P2X4 receptor, responds with a 10-fold higher sensitivity to BzATP, much slower deactivation kinetics, and a higher propensity to form the large dye-permeable pore. We examined the membrane expression of conserved mutants and found that Y51, Q52, G323, and F328 play a role in the trafficking of the receptor to the plasma membrane, while K49 controls receptor responsiveness to agonists. Finally, we studied the importance of the physicochemical properties of these residues and observed that the K49R, F322Y, F322W, and F322L mutants significantly reversed the receptor function, indicating that positively charged and large hydrophobic residues are important at positions 49 and 322, respectively. These results show that clusters of conserved residues above the transmembrane domain 1 (K49-Y51-Q52) and transmembrane domain 2 (G326-K327-F328) are important for receptor structure, membrane expression, and channel gating and that the nonconserved residue (F322) at the top of the extracellular vestibule is involved in hydrophobic inter-subunit interaction which stabilizes the closed state of the P2X7 receptor channel.