ATP is stored in lamellar bodies to activate vesicular P2X4 in an autocrine fashion upon exocytosis

Mechanosensitive channels TMEM63A and TMEM63B mediate lung inflation-induced surfactant secretion

Pulmonary surfactant is a lipoprotein complex lining the alveolar surface to decrease the surface tension and facilitate inspiration. Surfactant deficiency is often seen in premature infants and in children and adults with respiratory distress syndrome. Mechanical stretch of alveolar type 2 epithelial (AT2) cells during lung expansion is the primary physiological factor that stimulates surfactant secretion; however, it is unclear whether there is a mechanosensor dedicated to this process. Here, we show that loss of the mechanosensitive channels TMEM63A and TMEM63B (TMEM63A/B) resulted in atelectasis and respiratory failure in mice due to a deficit of surfactant secretion. TMEM63A/B were predominantly localized at the limiting membrane of the lamellar body (LB), a lysosome-related organelle that stores pulmonary surfactant and ATP in AT2 cells. Activation of TMEM63A/B channels during cell stretch facilitated the release of surfactant and ATP from LBs fused with the plasma membrane. The released ATP evoked Ca2+ signaling in AT2 cells and potentiated exocytic fusion of more LBs. Our study uncovered a vital physiological function of TMEM63 mechanosensitive channels in preparing the lungs for the first breath at birth and maintaining respiration throughout life.

Overstretching alveolar epithelial type II cells decreases surfactant secretion via actin polymerization and intracellular trafficking alteration

Pulmonary surfactant is essential for maintaining proper lung function. Alveolar epithelial type II (AE2) cells secrete surfactants via lamellar bodies (LBs). In tidal loading during each breath, the physiological cyclic stretching of AE2 cells promotes surfactant secretion. Excessive stretching inhibits surfactant secretion, which is considered to contribute to the development of lung damage. However, its precise mechanism remains unknown. This study tested whether actin polymerization and intracellular transport are required for pulmonary surfactant secretion and the association of actin polymerization and transport in identical human AE2-derived A549 cells using live-cell imaging, not in the bulk cells population. We found that overstretching approximately doubled actin polymerization into filaments (F-actin) and suppressed LB secretion by half in the fluorescent area ratio, compared with physiological stretching (F-actin: 1.495 vs 0.643 (P < 0.01); LB: 0.739 vs 0.332 (P < 0.01)). An inhibitor of actin polymerization increased LB secretion. Intracellular tracking using fluorescent particles revealed that cyclic stretching shifted the particle motion perpendicularly to the direction of stretching according to the orientation of the F-actin (proportion of perpendicular axis motion prior particle: 0h 40.12 % vs 2h 63.13 % (P < 0.01)), and particle motion was restricted over time in the cells subjected to overstretching, indicating that overstretching regulates intracellular transport dynamics (proportion of stop motion particle: 0h 1.01 % vs 2h 11.04 % (P < 0.01)). These findings suggest that overstretching changes secretion through the cytoskeleton: overstretching AE2 cells inhibits pulmonary surfactant secretion, at least through accelerating actin polymerization and decreasing intracellular trafficking, and the change in actin orientation would modulate intracellular trafficking.

A role for TMEM63 in the lung

Surfactants are essential for breathing. Although major progress has been made in the past half century toward an understanding of surfactant secretion mechanisms, the identity of the mechanosensor that couples breathing to surfactant secretion has remained elusive. In this issue of the JCI, Chen, Li, and colleagues provide evidence that the mechanosensor is the transmembrane 63 (TMEM63) ion channel. These findings open new avenues for future research into lung mechanobiology.

Microglial Activation Modulated by P2X4R in Ischemia and Repercussions in Alzheimer's Disease

There are over 80 million people currently living who have had a stroke. The ischemic injury in the brain starts a cascade of events that lead to neuronal death, inducing neurodegeneration which could lead to Alzheimer's disease (AD). Cerebrovascular diseases have been suggested to contribute to AD neuropathological changes, including brain atrophy and accumulation of abnormal proteins such as amyloid beta (Aβ). In patients older than 60 years, the incidence of dementia a year after stroke was significantly increased. Nevertheless, the molecular links between stroke and dementia are not clearly understood but could be related to neuroinflammation. Considering that activated microglia has a central role, there are brain-resident innate immune cells and are about 10-15% of glial cells in the adult brain. Their phagocytic activity is essential for synaptic homeostasis in different areas, such as the hippocampus. These cells polarize into phenotypes or subtypes: the pro-inflammatory M1 phenotype, or the immunosuppressive M2 phenotype. Phenotype M1 is induced by classical activation, where microglia secrete a high level of pro- inflammatory factors which can cause damage to the surrounding neuronal cells. Otherwise, M2 phenotype is the major effector cell with the potential to counteract pro-inflammatory reactions and promote repair genes expression. Moreover, after the classical activation, an anti-inflammatory and a repair phase are initiated to achieve tissue homeostasis. Recently it has been described the concepts of homeostatic and reactive microglia and they had been related to major AD risk, linking to a multifunctional microglial response to Aβ plaques and pathophysiology markers related, such as intracellular increased calcium. The upregulation and increased activity of purinergic receptors activated by ADP/ATP, specially P2X4R, which has a high permeability to calcium and is mainly expressed in microglial cells, is observed in diseases related to neuroinflammation, such as neuropathic pain and stroke. Thus, P2X4R is associated with microglial activation. P2X4R activation drives microglia motility via the phosphatidylinositol-3-kinase (PI3K)/Akt pathway. Also, these receptors are involved in inflammatory-mediated prostaglandin E2 (PGE2) production and induce a secretion and increase the expression of BDNF and TNF-α which could be a link between pathologies related to aging and neuroinflammation.

Endolysosomal Cation Channels and Lung Disease

Endolysosomal cation channels are emerging as key players of endolysosomal function such as endolysosomal trafficking, fusion/fission, lysosomal pH regulation, autophagy, lysosomal exocytosis, and endocytosis. Diseases comprise lysosomal storage disorders (LSDs) and neurodegenerative diseases, metabolic diseases, pigmentation defects, cancer, immune disorders, autophagy related diseases, infectious diseases and many more. Involvement in lung diseases has not been a focus of attention so far but recent developments in the field suggest critical functions in lung physiology and pathophysiology. Thus, loss of TRPML3 was discovered to exacerbate emphysema formation and cigarette smoke induced COPD due to dysregulated matrix metalloproteinase 12 (MMP-12) levels in the extracellular matrix of the lung, a known risk factor for emphysema/COPD. While direct lung function measurements with the exception of TRPML3 are missing for other endolysosomal cation channels or channels expressed in lysosome related organelles (LRO) in the lung, links between those channels and important roles in lung physiology have been established such as the role of P2X4 in surfactant release from alveolar epithelial Type II cells. Other channels with demonstrated functions and disease relevance in the lung such as TRPM2, TRPV2, or TRPA1 may mediate their effects due to plasma membrane expression but evidence accumulates that these channels might also be expressed in endolysosomes, suggesting additional and/or dual roles of these channels in cell and intracellular membranes. We will discuss here the current knowledge on cation channels residing in endolysosomes or LROs with respect to their emerging roles in lung disease.

Channels and Transporters of the Pulmonary Lamellar Body in Health and Disease

The lamellar body (LB) of the alveolar type II (ATII) cell is a lysosome-related organelle (LRO) that contains surfactant, a complex mix of mainly lipids and specific surfactant proteins. The major function of surfactant in the lung is the reduction of surface tension and stabilization of alveoli during respiration. Its lack or deficiency may cause various forms of respiratory distress syndrome (RDS). Surfactant is also part of the innate immune system in the lung, defending the organism against air-borne pathogens. The limiting (organelle) membrane that encloses the LB contains various transporters that are in part responsible for translocating lipids and other organic material into the LB. On the other hand, this membrane contains ion transporters and channels that maintain a specific internal ion composition including the acidic pH of about 5. Furthermore, P2X4 receptors, ligand gated ion channels of the danger signal ATP, are expressed in the limiting LB membrane. They play a role in boosting surfactant secretion and fluid clearance. In this review, we discuss the functions of these transporting pathways of the LB, including possible roles in disease and as therapeutic targets, including viral infections such as SARS-CoV-2.

Quinacrine is not a vital fluorescent probe for vesicular ATP storage

Quinacrine, a fluorescent amphipathic amine, has been used as a vital fluorescent probe to visualize vesicular storage of ATP in the field of purinergic signaling. However, the mechanism(s) by which quinacrine represents vesicular ATP storage remains to be clarified. The present study investigated the validity of the use of quinacrine as a vial fluorescent probe for ATP-storing organelles. Vesicular nucleotide transporter (VNUT), an essential component for vesicular storage and ATP release, is present in very low density lipoprotein (VLDL)-containing secretory vesicles in hepatocytes. VNUT gene knockout (Vnut^-/-) or clodronate treatment, a VNUT inhibitor, disappeared vesicular ATP release (Tatsushima et al., Biochim Biophys Acta Molecular Basis of Disease 2021, e166013). Upon incubation of mice's primary hepatocytes, quinacrine accumulates in a granular pattern into the cytoplasm, sensitive to 0.1-μM bafilomycin A₁, a vacuolar ATPase (V-ATPase) inhibitor. Neither Vnut^-/- nor treatment of clodronate affected quinacrine granular accumulation. In vitro, quinacrine is accumulated into liposomes upon imposing inside acidic transmembranous pH gradient (∆pH) irrespective of the presence or absence of ATP. Neither ATP binding on VNUT nor VNUT-mediated uptake of ATP was affected by quinacrine. Consistently, VNUT-mediated uptake of quinacrine was negligible or under the detection limit. From these results, it is concluded that vesicular quinacrine accumulation is not due to a consequence of its interaction with ATP but due to ∆pH-driven concentration across the membranes as an amphipathic amine. Thus, quinacrine is not a vital fluorescent probe for vesicular ATP storage.

P2X4 Receptors Mediate Ca2+ Release from Lysosomes in Response to Stimulation of P2X7 and H1 Histamine Receptors

The P2X4 purinergic receptor is targeted to endolysosomes, where it mediates an inward current dependent on luminal ATP and pH. Activation of P2X4 receptors was previously shown to trigger lysosome fusion, but the regulation of P2X4 receptors and their role in lysosomal Ca²⁺ signaling are poorly understood. We show that lysosomal P2X4 receptors are activated downstream of plasma membrane P2X7 and H₁ histamine receptor stimulation. When P2X4 receptors are expressed, the increase in near-lysosome cytosolic [Ca²⁺] is exaggerated, as detected with a low-affinity targeted Ca²⁺ sensor. P2X4-dependent changes in lysosome properties were triggered downstream of P2X7 receptor activation, including an enlargement of lysosomes indicative of homotypic fusion and a redistribution of lysosomes towards the periphery of the cell. Lysosomal P2X4 receptors, therefore, have a role in regulating lysosomal Ca²⁺ release and the regulation of lysosomal membrane trafficking.

Structural and Functional Features of the P2X4 Receptor: An Immunological Perspective

Extracellular nucleotides are important mediators of activation, triggering various responses through plasma membrane P2 and P1 receptors. P2 receptors are further subdivided into ionotropic P2X receptors and G protein-coupled P2Y receptors. P2X4 is an ATP-gated cation channel broadly expressed in most tissues of the body. Within the P2X family, P2X4 has a unique subcellular distribution, being preferentially localized in lysosomes. In these organelles, high ATP concentrations do not trigger P2X4 because of the low pH. However, when the pH increases to 7.4, P2X4 can be stimulated by intra-lysosomal ATP, which is in its active, tetra-anionic form. Elucidation of P2X4, P2X3 and P2X7 structures has shed some light on the functional differences between these purinergic receptors. The potential interaction between P2X4 and P2X7 has been extensively studied. Despite intensive effort, it has not been possible yet to determine whether P2X4 and P2X7 interact as heterotrimers or homotrimers at the plasma membrane. However, several publications have shown that functional interactions between P2X4 and P2X7 do occur. Importantly, these studies indicate that P2X4 potentiates P2X7-dependent activation of inflammasomes, leading to increased release of IL-1β and IL-18. The role of P2X4 in various diseases could be beneficial or deleterious even though the pathophysiological mechanisms involved are still poorly defined. However, in diseases whose physiopathology involves activation of the NLRP3 inflammasome, P2X4 was found to exacerbate severity of disease. The recent production of monoclonal antibodies specific for the human and mouse P2X4, some of which are endowed with agonist or antagonist properties, raises the possibility that they could be used therapeutically. Analysis of single nucleotide polymorphisms of the human P2RX4 gene has uncovered the association of P2RX4 gene variants with susceptibility to several human diseases.

Role of the C5a-C5a receptor axis in the inflammatory responses of the lungs after experimental polytrauma and hemorrhagic shock

Singular blockade of C5a in experimental models of sepsis is known to confer protection by rescuing lethality and decreasing pro-inflammatory responses. However, the role of inhibiting C5a has not been evaluated in the context of sterile systemic inflammatory responses, like polytrauma and hemorrhagic shock (PT + HS). In our presented study, a novel and highly specific C5a L-aptamer, NoxD21, was used to block C5a activity in an experimental murine model of PT + HS. The aim of the study was to assess early modulation of inflammatory responses and lung damage 4 h after PT + HS induction. NoxD21-treated PT + HS mice displayed greater polymorphonuclear cell recruitment in the lung, increased pro-inflammatory cytokine levels in the bronchoalveolar lavage fluids (BALF) and reduced myeloperoxidase levels within the lung tissue. An in vitro model of the alveolar-capillary barrier was established to confirm these in vivo observations. Treatment with a polytrauma cocktail induced barrier damage only after 16 h, and NoxD21 treatment in vitro did not rescue this effect. Furthermore, to test the exact role of both the cognate receptors of C5a (C5aR1 and C5aR2), experimental PT + HS was induced in C5aR1 knockout (C5aR1 KO) and C5aR2 KO mice. Following 4 h of PT + HS, C5aR2 KO mice had significantly reduced IL-6 and IL-17 levels in the BALF without significant lung damage, and both, C5aR1 KO and C5aR2 KO PT + HS animals displayed reduced MPO levels within the lungs. In conclusion, the C5aR2 could be a putative driver of early local inflammatory responses in the lung after PT + HS.

Purinergic receptors in airway hydration

Airway epithelial purinergic receptors control key components of the mucociliary clearance (MCC), the dominant component of pulmonary host defense. In healthy airways, the periciliary liquid (PCL) is optimally hydrated, thus acting as an efficient lubricant layer over which the mucus layer moves by ciliary force. When the hydration of the airway surface decreases, the mucus becomes hyperconcentrated, the PCL collapses, and the "thickened" mucus layer adheres to cell surfaces, causing plaque/plug formation. Mucus accumulation is a major contributing factor to the progression of chronic obstructive lung diseases such as cystic fibrosis (CF) and chronic bronchitis (CB). Mucus hydration is regulated by finely tuned mechanisms of luminal Cl^- secretion and Na⁺ absorption with concomitant osmotically driven water flow. These activities are regulated by airway surface liquid (ASL) concentrations of adenosine and ATP, acting on airway epithelial A_2B and P2Y₂ receptors, respectively. The goal of this article is to provide an overview of our understanding of the role of purinergic receptors in the regulation of airway epithelial ion/fluid transport and the mechanisms of nucleotide release and metabolic activities that contribute to airway surface hydration in healthy and chronically obstructed airways.

Implication of Neuronal Versus Microglial P2X4 Receptors in Central Nervous System Disorders

The P2X4 receptor (P2X4) is an ATP-gated cation channel that is highly permeable to Ca²⁺ and widely expressed in neuronal and glial cell types throughout the central nervous system (CNS). A growing body of evidence indicates that P2X4 plays key roles in numerous central disorders. P2X4 trafficking is highly regulated and consequently in normal situations, P2X4 is present on the plasma membrane at low density and found mostly within intracellular endosomal/lysosomal compartments. An increase in the de novo expression and/or surface density of P2X4 has been observed in microglia and/or neurons during pathological states. This review aims to summarize knowledge on P2X4 functions in CNS disorders and provide some insights into the relative contributions of neuronal and glial P2X4 in pathological contexts. However, determination of the cell-specific functions of P2X4 along with its intracellular and cell surface roles remain to be elucidated before its potential as a therapeutic target in multiple disorders can be defined.

Mechanical stretch activates piezo1 in caveolae of alveolar type I cells to trigger ATP release and paracrine stimulation of surfactant secretion from alveolar type II cells

Secretion of pulmonary surfactant in the alveoli of the lungs is essential to maintain lung function. Stretching of alveoli during lung inflation is the main trigger for surfactant secretion. Yet, the molecular mechanisms how mechanical distension of alveoli results in surfactant secretion are still elusive. The alveolar epithelium consists of alveolar epithelial type I (ATI) and surfactant secreting type II (ATII) cells. ATI, but not ATII cells, express caveolae, small plasma membrane invaginations that can respond to plasma membrane stresses and serve mechanotransductive roles. Within this study, we investigated the role of caveolae as mechanosensors in the alveolus. We generated a human caveolin-1 knockout ATI cell (hAELVi^cav-/- ) using CRISPR/Cas9. Wildtype (hAELVi^wt ) and hAELVi^cav-/- cells grown on flexible membranes responded to increasing stretch amplitudes with rises in intracellular Ca²⁺ . The response was less frequent and started at higher stretch amplitudes in hAELVi^cav-/- cells. Stretch-induced Ca²⁺ -signals depended on Ca²⁺ -entry via piezo1 channels, localized within caveolae in hAELVi^wt and primary ATI cells. Ca²⁺ -entry via piezo1 activated pannexin-1 hemichannels resulting in ATP release from ATI cells. ATP release was reduced in hAELVi^cav-/- cells. In co-cultures resembling the alveolar epithelium, released ATP stimulated Ca²⁺ signals and surfactant secretion from neighboring ATII cells when co-cultured with hAELVi^wt but not hAELVi^cav-/- cells. In summary, we propose that caveolae in ATI cells are mechanosensors within alveoli regulating stretch-induced surfactant secretion from ATII cells.

Resolving the Ionotropic P2X4 Receptor Mystery Points Towards a New Therapeutic Target for Cardiovascular Diseases

Adenosine triphosphate (ATP) is a primordial versatile autacoid that changes its role from an intracellular energy saver to a signaling molecule once released to the extracellular milieu. Extracellular ATP and its adenosine metabolite are the main activators of the P2 and P1 purinoceptor families, respectively. Mounting evidence suggests that the ionotropic P2X4 receptor (P2X4R) plays pivotal roles in the regulation of the cardiovascular system, yet further therapeutic advances have been hampered by the lack of selective P2X4R agonists. In this review, we provide the state of the art of the P2X4R activity in the cardiovascular system. We also discuss the role of P2X4R activation in kidney and lungs vis a vis their interplay to control cardiovascular functions and dysfunctions, including putative adverse effects emerging from P2X4R activation. Gathering this information may prompt further development of selective P2X4R agonists and its translation to the clinical practice.

P2 Purinergic Signaling in the Distal Lung in Health and Disease

The distal lung provides an intricate structure for gas exchange in mammalian lungs. Efficient gas exchange depends on the functional integrity of lung alveoli. The cells in the alveolar tissue serve various functions to maintain alveolar structure, integrity and homeostasis. Alveolar epithelial cells secrete pulmonary surfactant, regulate the alveolar surface liquid (ASL) volume and, together with resident and infiltrating immune cells, provide a powerful host-defense system against a multitude of particles, microbes and toxicants. It is well established that all of these cells express purinergic P2 receptors and that purinergic signaling plays important roles in maintaining alveolar homeostasis. Therefore, it is not surprising that purinergic signaling also contributes to development and progression of severe pathological conditions like pulmonary inflammation, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and pulmonary fibrosis. Within this review we focus on the role of P2 purinergic signaling in the distal lung in health and disease. We recapitulate the expression of P2 receptors within the cells in the alveoli, the possible sources of ATP (adenosine triphosphate) within alveoli and the contribution of purinergic signaling to regulation of surfactant secretion, ASL volume and composition, as well as immune homeostasis. Finally, we summarize current knowledge of the role for P2 signaling in infectious pneumonia, ALI/ARDS and idiopathic pulmonary fibrosis (IPF).

Physiopathological roles of vesicular nucleotide transporter (VNUT), an essential component for vesicular ATP release

Vesicular nucleotide transporter (VNUT) is the last identified member of the SLC17 organic anion transporter family, which plays a central role in vesicular storage in ATP-secreting cells. The discovery of VNUT demonstrated that, despite having been neglected for a long time, vesicular ATP release represents a major pathway for purinergic chemical transmission, which had been mainly attributed to ATP permeation channels. This article summarizes recent advances in our understanding of the mechanism of VNUT and its physiopathological roles as well as the development of inhibitors. Regulating the activity and/or the expression of VNUT represents a new and promising therapeutic strategy for the treatment of multiple diseases.

Nanoparticle-Mediated Therapeutic Application for Modulation of Lysosomal Ion Channels and Functions

Applications of nanoparticles in various fields have been addressed. Nanomaterials serve as carriers for transporting conventional drugs or proteins through lysosomes to various cellular targets. The basic function of lysosomes is to trigger degradation of proteins and lipids. Understanding of lysosomal functions is essential for enhancing the efficacy of nanoparticles-mediated therapy and reducing the malfunctions of cellular metabolism. The lysosomal function is modulated by the movement of ions through various ion channels. Thus, in this review, we have focused on the recruited ion channels for lysosomal function, to understand the lysosomal modulation through the nanoparticles and its applications. In the future, lysosomal channels-based targets will expand the therapeutic application of nanoparticles-associated drugs.

Type 2 secretory cells are primary source of ATP release in mechanically stretched lung alveolar cells

Extracellular ATP and its metabolites are potent paracrine modulators of lung alveolar cell function, including surfactant secretion and fluid transport, but the sources and mechanism of intra-alveolar ATP release remain unclear. To determine the contribution of gas-exchanging alveolar type 1 (AT1) and surfactant-secreting type 2 (AT2) cells to stretch-induced ATP release, we used quantitative real-time luminescence ATP imaging and rat primary alveolar cells cultured on silicon substrate for 2-7 days. When cultured on solid support, primary AT2 cells progressively transdifferentiated into AT1-like cells with ~20% of cells showing AT1 phenotype by day 2-3 (AT2:AT1 ≈ 4:1), while on day 7, the AT2:AT1 cell ratio was reversed with up to 80% of the cells displaying characteristics of AT1 cells. Stretch (1 s, 5-35%) induced ATP release from AT2/AT1 cell cultures, and it was highest on days 2 and 3 but declined in older cultures. ATP release tightly correlated with the number of remaining AT2 cells in culture, consistent with ~10-fold lower ATP release by AT1 than AT2 cells. ATP release was unaffected by inhibitors of putative ATP channels carbenoxolone and probenecid but was significantly diminished in cells loaded with calcium chelator BAPTA. These pharmacological modulators had similar effects on stretch-induced intracellular Ca²⁺ responses measured by Fura2 fluorescence. The study revealed that AT2 cells are the primary source of stretch-induced ATP release in heterocellular AT2/AT1 cell cultures, suggesting similar contribution in intact alveoli. Our results support a role for calcium-regulated mechanism but not ATP-conducting channels in ATP release by alveolar epithelial cells.

P2X4 and lysosome fusion

Similar to other members of the P2X receptor family, the P2X4 receptor at the plasma membrane forms a highly Ca²⁺ permeable, non-selective cation channel that is activated by extracellular ATP. Yet, P2X4 differs from the other subtypes, as it is predominantly localized on late endosomal, lysosomal and/or lysosome-related organelles. It is targeted there by virtue of tyrosine-based and di-leucine like trafficking motifs contained within its C-terminal and N-terminal regions respectively. The physiological role of the stable intracellular expression of P2X4 in acidic compartments has been a long-standing puzzle. Recent evidence, however, points to a dual role in the regulation of ion fluxes across lysosomal membranes to control lysosome membrane fusion and in the re-sensitization of receptors exposed to extracellular ATP.

Targeting P2X4 and P2X7 receptors in multiple sclerosis

Multiple sclerosis (MS) is a chronic disease of the central nervous system characterized by massive infiltration of immune cells, demyelination, and axonal loss. However, spontaneous myelin repair can occur during the course of the disease. A major component of this regenerative process is a robust innate immune response consisting of infiltrating macrophages and brain microgliosis. Therefore, specifically targeting myeloid cells could be an attractive therapeutic approach. Purinergic receptors control not only immune cell function together with the activation of microglia and astrocytes, but also neuronal and oligodendroglial survival in the pathology. Thus, targeting these receptors can modulate a whole variety of responses. In this review, we will summarize recent findings highlighting the potential of P2X4 and P2X7 as therapeutic targets for MS.

Mechanosensitive ATP release in the lungs: New insights from real-time luminescence imaging studies

Extracellular ATP and other nucleotides are important autocrine/paracrine mediators that stimulate purinergic receptors and regulate diverse processes in the normal lungs. They are also associated with pathogenesis of a number of respiratory diseases and clinical complications including acute respiratory distress syndrome and ventilator induced lung injury. Mechanical forces are major stimuli for cellular ATP release but precise mechanisms responsible for this release are still debated. The present review intends to provide the current state of knowledge of the mechanisms of ATP release in the lung. Putative pathways of the release, including the contribution of cell membrane injury and cell lysis are discussed addressing their strength, weaknesses and missing evidence that requires future study. We also provide an overview of the recent technical advances in studying cellular ATP release in vitro and ex vivo. Special attention is given to new insights into lung ATP release obtained with the real-time luminescence ATP imaging. This includes recent data on stretch-induced mechanosensitive ATP release in a model and primary cells of lung alveoli in vitro as well as inflation-induced ATP release in airspaces and pulmonary blood vessels of lungs, ex vivo.

Microglia P2X4 receptors as pharmacological targets for demyelinating diseases

Pharmacological activation of the P2X4 receptor expressed by brain microglia may provide a novel avenue to promote remyelination and improve clinical symptoms in experimental autoimmune encephalomyelitis and potentially in multiple sclerosis.

Inflammation-induced upregulation of P2X4 expression augments mucin secretion in airway epithelia

Mucus clearance provides an essential innate defense mechanism to keep the airways and lungs free of particles and pathogens. Baseline and stimulated mucin secretion from secretory airway epithelial cells need to be tightly regulated to prevent mucus hypersecretion and mucus plugging of the airways. It is well established that extracellular ATP is a potent stimulus for regulated mucus secretion. Previous studies revealed that ATP acts via metabotropic P2Y₂ purinoreceptors on goblet cells. Extracellular ATP, however, is also a potent agonist for ionotropic P2X purinoreceptors. Expression of several P2X isoforms has been reported in airways, but cell type-specific expression and the function thereof remained elusive. With this study, we now provide evidence that P2X₄ is the predominant P2X isoform expressed in secretory airway epithelial cells. After IL-13 treatment of either human primary tracheal epithelial cells or mice, P2X₄ expression is upregulated in vitro and in vivo under conditions of chronic inflammation, mucous metaplasia, and hyperplasia. Upregulation of P2X₄ is strongest in MUC5AC-positive goblet cells. Moreover, activation of P2X₄ by extracellular ATP augments intracellular Ca²⁺ signals and mucin secretion, whereas Ca²⁺ signals and mucin secretion are dampened by inhibition of P2X₄ receptors. These data provide new insights into the purinergic regulation of mucin secretion and add to the emerging picture that P2X receptors modulate exocytosis of large secretory organelles and secretion of macromolecular vesicle cargo.

P2X4 receptor re-sensitization depends on a protonation/deprotonation cycle mediated by receptor internalization and recycling

KEY POINTS

Re-sensitization of P2X₄ receptors depends on a protonation/de-protonation cycle Protonation and de-protonation of the receptors is achieved by internalization and recycling of P2X₄ receptors via acidic compartments Protonation and de-protonation occurs at critical histidine residues within the extracellular loop of P2X₄ receptors Re-sensitization is blocked in the presence of the receptor agonist ATP ABSTRACT: P2X₄ receptors are members of the P2X receptor family of cation-permeable, ligand-gated ion channels that open in response to the binding of extracellular ATP. P2X₄ receptors are implicated in a variety of biological processes, including cardiac function, cell death, pain sensation and immune responses. These physiological functions depend on receptor activation on the cell surface. Receptor activation is followed by receptor desensitization and deactivation upon removal of ATP. Subsequent re-sensitization is required to return the receptor into its resting state. Desensitization and re-sensitization are therefore crucial determinants of P2X receptor signal transduction and responsiveness to ATP. However, the molecular mechanisms controlling desensitization and re-sensitization are not fully understood. In the present study, we provide evidence that internalization and recycling via acidic compartments is essential for P2X₄ receptor re-sensitization. Re-sensitization depends on a protonation/de-protonation cycle of critical histidine residues within the extracellular loop of P2X₄ receptors that is mediated by receptor internalization and recycling. Interestingly, re-sensitization under acidic conditions is completely revoked by receptor agonist ATP. Our data support the physiological importance of the unique subcellular distribution of P2X₄ receptors that is predominantly found within acidic compartments. Based on these findings, we suggest that recycling of P2X₄ receptors regulates the cellular responsiveness in the sustained presence of ATP.

Excessive Extracellular ATP Desensitizes P2Y2 and P2X4 ATP Receptors Provoking Surfactant Impairment Ending in Ventilation-Induced Lung Injury

Stretching the alveolar epithelial type I (AT I) cells controls the intercellular signaling for the exocytosis of surfactant by the AT II cells through the extracellular release of adenosine triphosphate (ATP) (purinergic signaling). Extracellular ATP is cleared by extracellular ATPases, maintaining its homeostasis and enabling the lung to adapt the exocytosis of surfactant to the demand. Vigorous deformation of the AT I cells by high mechanical power ventilation causes a massive release of extracellular ATP beyond the clearance capacity of the extracellular ATPases. When extracellular ATP reaches levels >100 μM, the ATP receptors of the AT II cells become desensitized and surfactant impairment is initiated. The resulting alteration in viscoelastic properties and in alveolar opening and collapse time-constants leads to alveolar collapse and the redistribution of inspired air from the alveoli to the alveolar ducts, which become pathologically dilated. The collapsed alveoli connected to these dilated alveolar ducts are subject to a massive strain, exacerbating the ATP release. After reaching concentrations >300 μM extracellular ATP acts as a danger-associated molecular pattern, causing capillary leakage, alveolar space edema, and further deactivation of surfactant by serum proteins. Decreasing the tidal volume to 6 mL/kg or less at this stage cannot prevent further lung injury.

A role for P2X4 receptors in lysosome function

Murrell-Lagnado provides insight into new research revealing the physiological role of lysosomal P2X₄ channels.