Expression of P2X receptors on immune cells in the rat liver during postnatal development

Purinergic Signaling in Non-Parenchymal Liver Cells

Purinergic signaling has emerged as an important paracrine-autocrine intercellular system that regulates physiological and pathological processes in practically all organs of the body. Although this system has been thoroughly defined since the nineties, recent research has made substantial advances regarding its role in aspects of liver physiology. However, most studies have mainly targeted the entire organ, 70% of which is made up of parenchymal cells or hepatocytes. Because of its physiological role, the liver is exposed to toxic metabolites, such as xenobiotics, drugs, and fatty acids, as well as to pathogens such as viruses and bacteria. Under injury conditions, all cell types within the liver undergo adaptive changes. In this context, the concentration of extracellular ATP has the potential to increase dramatically. Indeed, this purinergic response has not been studied in sufficient detail in non-parenchymal liver cells. In the present review, we systematize the physiopathological adaptations related to the purinergic system in chronic liver diseases of non-parenchymal liver cells, such as hepatic stellate cells, Kupffer cells, sinusoidal endothelial cells, and cholangiocytes. The role played by non-parenchymal liver cells in these circumstances will undoubtedly be strategic in understanding the regenerative activities that support the viability of this organ under stressful conditions.

Deficiency of purinergic P2X4 receptor alleviates experimental autoimmune hepatitis in mice

Purinergic P2X4 receptor (P2X4R) has been shown to have immunomodulatory properties in infection, inflammation, and organ damage including liver regeneration and fibrosis. However, the mechanisms and pathophysiology associated with P2X4R during acute liver injury remain unknown. We used P2X4R-/- mice to explore the role of P2X4R in three different models of acute liver injury caused by concanavalin A (ConA), carbon tetrachloride, and acetaminophen. ConA treatment results in an increased expression of P2X4R in the liver of mice, which was positively correlated with higher levels of aspartate aminotransferase and alanine aminotransferase in the serum. However, P2X4R gene ablation significantly reduced the severity of acute hepatitis in mice caused by ConA, but not by carbon tetrachloride or acetaminophen. The protective benefits against immune-mediated acute hepatitis were achieved via modulating inflammation (Interleukin (IL)-1β, IL-6, IL-17A, interferon-γ, tumor necrosis factor-α), oxidative stress (malondialdehyde, superoxide dismutase, glutathione peroxidase, and catalase), apoptosis markers (Bax, Bcl-2, and Caspase-3), autophagy biomarkers (LC3, Beclin-1, and p62), and nucleotide oligomerization domain-likereceptorprotein 3(NLRP3) inflammasome-activated pyroptosis markers (NLRP3, Gasdermin D, Caspase-1, ASC, IL-1β). Additionally, administration of P2X4R antagonist (5-BDBD) or agonist (cytidine 5'-triphosphate) either improved or worsened ConA-induced autoimmune hepatitis, respectively. This study is the first to reveal that the absence of the P2X4 receptor may mitigate immune-mediated liver damage, potentially by restraining inflammation, oxidation, and programmed cell death mechanisms. And highlight P2X4 receptor is essential for ConA-induced acute hepatitis.

The P2X4 purinergic receptor impacts liver regeneration after partial hepatectomy in mice through the regulation of biliary homeostasis

Many regulatory pathways are involved in liver regeneration after partial hepatectomy (PH), to initiate growth, protect liver cells, and sustain remnant liver functions. Extracellular adenosine triphosphate rises in blood and bile after PH and contributes to liver regeneration, although purinergic receptors and mechanisms remain to be precisely explored. In this work we analyzed during regeneration after PH the involvement of P2X4 purinergic receptors, highly expressed in the liver. P2X4 receptor expression in the liver, liver histology, hepatocyte proliferation, plasma bile acid concentration, bile flow and composition, and lysosome distribution in hepatocytes were studied in wild-type and P2X4 knockout (KO) mice, before and after PH. P2X4 receptors were expressed in hepatocytes and Kupffer cells; in hepatocytes, P2X4 was concentrated in subcanalicular areas closely costained with lysosomal markers. After PH, delayed regeneration, hepatocyte necrosis, and cholestasis were observed in P2X4-KO mice. In P2X4-KO mice, post-PH biliary adaptation was impaired with a smaller increase in bile flow and HCO3 (-) biliary output, as well as altered biliary composition with reduced adenosine triphosphate and lysosomal enzyme release. In line with these data, lysosome distribution and biogenesis were altered in P2X4-KO compared with wild-type mice.

CONCLUSION

During liver regeneration after PH, P2X4 contributes to the complex control of biliary homeostasis through mechanisms involving pericanalicular lysosomes, with a resulting impact on hepatocyte protection and proliferation. (Hepatology 2016;64:941-953).

Differential gene expression patterns and colocalization of ATP-gated P2X6/P2X4 ion channels during rat small intestine ontogeny

Gene coding for ATP-gated receptor ion channels (P2X1-7) has been associated with the developmental process in various tissues; among these ion channel subtypes, P2X6 acts as a physiological regulator of P2X4 receptor functions when the two receptors form heteroreceptors. The P2X4 receptor is involved in pain sensation, the inflammatory process, and body homeostasis by means of Mg(2+) absorption through the intestine. The small intestine is responsible for the absorption and digestion of nutrients; throughout its development, several gene expressions are induced that are related to nutrients received, metabolism, and other intestine functions. Previous work has shown a differential P2X4 and P2X6 protein distribution in the small intestine of newborn and adult rats; however, it is not well-known at what age the change in the relationship between the gene and protein expression occurs and whether or not these receptors are colocalized. In this work, we evaluate P2X4 and P2X6 gene expression patterns by qPCR from embryonic (E18, P0, P7, P17, P30) to adult age in rat gut, as well as P2X6/P2X4 colocalization using qRT-PCR and confocal immunofluorescence in proximal and distal small intestine sections. The results showed that P2X6 and P2X4 gene expression levels of both receptors decreased at the embryonic-perinatal transition, whereas from ages P17 to P30 (suckling-weaning transition) both receptors increased their gene expression levels. Furthermore, P2X4 and P2X6 proteins were expressed in a different way during rat small intestine development, showing a higher colocalization coefficient at age P30 in both intestine regions. Those results suggest that purinergic receptors may play a role in intestinal maturation, which is associated with age and intestinal region.

Purinergic regulation of the immune system

Cellular stress or apoptosis triggers the release of ATP, ADP and other nucleotides into the extracellular space. Extracellular nucleotides function as autocrine and paracrine signalling molecules by activating cell-surface P2 purinergic receptors that elicit pro-inflammatory immune responses. Over time, extracellular nucleotides are metabolized to adenosine, leading to reduced P2 signalling and increased signalling through anti-inflammatory adenosine (P1 purinergic) receptors. Here, we review how local purinergic signalling changes over time during tissue responses to injury or disease, and we discuss the potential of targeting purinergic signalling pathways for the immunotherapeutic treatment of ischaemia, organ transplantation, autoimmunity or cancer.

Purinergic signalling and development of the autonomic nervous system

Most early studies of the role of nucleotides in development have evidenced their crucial importance as carriers of energy in all organisms. However, an increasing number of studies are now available to suggest that purines and pyrimidines, acting as extracellular ligands specifically on receptors of the plasma membrane, may play a pivotal role throughout pre- and postnatal development in a wide variety of organisms including amphibians, birds, and mammals. Purinergic receptor expression and functions have been studied in the development of many organs, including the autonomic nervous system (ANS). Nucleotide receptors can induce a multiplicity of cellular signalling pathways via crosstalk with bioactive molecules acting on growth factors and neurotransmitter receptors which are fundamental for the development of a mature and functional ANS. Purines and pyrimidines may influence all the stages of neuronal development, including neural cell proliferation, migration, differentiation and phenotype determination of differentiated cells. Indeed, the normal development of the ANS is disturbed by dysfunction of purinergic signalling in animal models. To establish the primitive and fundamental nature of purinergic neurotransmission in the ontogeny of the ANS, in this review the roles of purines and pyrimidines as signalling molecules during embryological and postnatal development are considered.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Sterile inflammation in hepatic ischemia/reperfusion injury: present concepts and potential therapeutics

Ischemia and reperfusion (I/R) injury is an often unavoidable consequence of major liver surgery and is characterized by a sterile inflammatory response that jeopardizes the viability of the organ. The inflammatory response results from acute oxidative and nitrosative stress and consequent hepatocellular death during the early reperfusion phase, which causes the release of endogenous self-antigens known as damage-associated molecular patterns (DAMPs). DAMPs, in turn, are indirectly responsible for a second wave of reactive oxygen and nitrogen species (ROS and RNS) production by driving the chemoattraction of various leukocyte subsets that exacerbate oxidative liver damage during the later stages of reperfusion. In this review, the molecular mechanisms underlying hepatic I/R injury are outlined, with emphasis on the interplay between ROS/RNS, DAMPs, and the cell types that either produce ROS/RNS and DAMPs or respond to them. This theoretical background is subsequently used to explain why current interventions for hepatic I/R injury have not been very successful. Moreover, novel therapeutic modalities are addressed, including MitoSNO and nilotinib, and metalloporphyrins on the basis of the updated paradigm of hepatic I/R injury.

The sterile immune response during hepatic ischemia/reperfusion

Hepatic ischemia and reperfusion elicits an immune response that lacks a microbial constituent yet poses a potentially lethal threat to the host. In this sterile setting, the immune system is alarmed by endogenous danger signals that are release by stressed and dying liver cells. The detection of these immunogenic messengers by sentinel leukocyte populations constitutes the proximal trigger for a self-perpetuating cycle of inflammation, in which consecutive waves of cytokines and chemokines orchestrate the influx of various leukocyte subsets that ultimately confer tissue destruction. This review focuses on the temporal organization of sterile hepatic inflammation, using surgery-induced trauma as a template disease state.

P2X7 receptor-mediated purinergic signaling promotes liver injury in acetaminophen hepatotoxicity in mice

Inflammation contributes to liver injury in acetaminophen (APAP) hepatotoxicity in mice and is triggered by stimulation of immune cells. The purinergic receptor P2X7 is upstream of the nod-like receptor family, pryin domain containing-3 (NLRP3) inflammasome in immune cells and is activated by ATP and NAD that serve as damage-associated molecular patterns. APAP hepatotoxicity was assessed in mice genetically deficient in P2X7, the key inflammatory receptor for nucleotides (P2X7-/-), and in wild-type mice. P2X7-/- mice had significantly decreased APAP-induced liver necrosis. In addition, APAP-poisoned mice were treated with the specific P2X7 antagonist A438079 or etheno-NAD, a competitive antagonist of NAD. Pre- or posttreatment with A438079 significantly decreased APAP-induced necrosis and hemorrhage in APAP liver injury in wild-type but not P2X7-/- mice. Pretreatment with etheno-NAD also significantly decreased APAP-induced necrosis and hemorrhage in APAP liver injury. In addition, APAP toxicity in mice lacking the plasma membrane ecto-NTPDase CD39 (CD39-/-) that metabolizes ATP was examined in parallel with the use of soluble apyrase to deplete extracellular ATP in wild-type mice. CD39-/- mice had increased APAP-induced hemorrhage and mortality, whereas apyrase also decreased APAP-induced mortality. Kupffer cells were treated with extracellular ATP to assess P2X7-dependent inflammasome activation. P2X7 was required for ATP-stimulated IL-1β release. In conclusion, P2X7 and exposure to the ligands ATP and NAD are required for manifestations of APAP-induced hepatotoxicity.

Role of purinergic P2X receptors in the control of liver homeostasis

It is now accepted that extracellular ATP and other nucleotides are potent signaling molecules, akin to neurotransmitters, hormones and lipid mediators. In the liver, several clues support a significant role for extracellular ATP-induced signaling pathways in the control of tissue homeostasis. First, ATP and other nucleotides are physiologically detected in extracellular fluids within the liver, including sinusoidal blood and intraductular bile, in various mammalian species including human and rodents. Moreover, finely tuned mechanisms of ATP release by different liver cell types have been described, under physiological cellular changes. In addition, most hepatic cells constitutively express, at the membrane level, several ATP-metabolizing ectoenzymes and ATP-sensitive receptors that modulate and transduce these mediator signals respectively. Finally, hepatic cells also express numerous membrane transporters that actively contribute to purinergic salvage pathways. Once released in the extracellular medium, unmetabolised ATP molecules can bind to purinergic P2X and P2Y receptors, and subsequently trigger various intracellular signal transduction pathways collectively referred to as purinergic signaling. In the liver, purinergic signaling has been shown to regulate key basic cellular functions, such as glucose/lipid metabolism, protein synthesis and ionic secretion, and homeostatic processes, such as cell cycle, inflammatory response and immunity. Whilst the functional relevance of P2Y receptors in liver physiology has been well documented, limited information is available regarding the potential role of hepatic P2X receptors in the modulation of liver homeostasis.

Expression and functional characterization of P2X receptors in mouse alveolar macrophages

Alveolar macrophages (AM) are crucial for pulmonary host defense, and evidence emerges that ATP-gated P2X receptors are involved in inflammatory processes. This study focuses on the expression and functional characterization of P2X receptors in AM from mouse. In RT-PCR experiments, transcripts encoding the P2X₁, P2X₃, P2X₄, P2X₅, and P2X₇ receptors were detected. In whole-cell patch-clamp recordings, ATP (1 mM) evoked an inward current (mouse and human AM) that was reversible upon washout, and the reversal potential was ~5 mV, indicating the activation of a non-selective conductance-a fingerprint of P2X receptors. Further characterization (mouse AM) revealed that the current was not desensitized by a second ATP application. The ATP-induced current was increased by the removal of extracellular Ca²⁺ (in human and mouse AM), and EC₅₀ in mouse AM were determined with ~1 mM ATP, in the presence as well as in the absence of extracellular Ca²⁺. Pharmacological characterization of mouse AM revealed that the effect was augmented by BzATP and pre-application with ivermectin, but no effect with α,β-meATP was observed. Further, the ATP effect was reduced by PPADS (300 μM), brilliant blue G (5 μM), and about A438079 (10 μM). Although different P2X receptor transcripts were detected in mouse AM, the observed functional and pharmacological characteristics indicate primarily the participation of P2X₄ and P2X₇ receptors as mediators of the ATP-induced ion current in mouse AM. These suggestions were confirmed by experiments with AM from P2X₇ -deficient animals, indicating a contribution of P2X₄ and P2X₇ receptors in pulmonary immune function.

Extracellular nucleosides and nucleotides regulate liver functions via a complex system of membrane proteins

Nucleosides and nucleotides are now considered as extracellular signalling molecules, like neurotransmitters and hormones. Hepatic cells, amongst other cells, ubiquitously express specific transmembrane receptors that transduce the physiological signals induced by extracellular nucleosides and nucleotides, as well as various cell surface enzymes that regulate the levels of these mediators in the extracellular medium. Here, we cover various aspects of the signalling pathways initiated by extracellular nucleosides and nucleotides in the liver, and discuss their overall impact on hepatic physiology.

ATP release after partial hepatectomy regulates liver regeneration in the rat

BACKGROUND & AIMS

Paracrine interactions are critical to liver physiology, particularly during regeneration, although physiological involvement of extracellular ATP, a crucial intercellular messenger, remains unclear. The physiological release of ATP into extracellular milieu and its impact on regeneration after partial hepatectomy were investigated in this study.

METHODS

Hepatic ATP release after hepatectomy was examined in the rat and in human living donors for liver transplantation. Quinacrine was used for in vivo staining of ATP-enriched compartments in rat liver sections and isolated hepatocytes. Rats were treated with an antagonist for purinergic receptors (Phosphate-6-azo(benzene-2,4-disulfonic acid), PPADS), and liver regeneration after hepatectomy was analyzed.

RESULTS

A robust and transient ATP release due to acute portal hyperpressure was observed immediately after hepatectomy in rats and humans. Clodronate liposomal pre-treatment partly inhibited ATP release in rats. Quinacrine-stained vesicles, co-labeled with a lysosomal marker in liver sections and isolated hepatocytes, were predominantly detected in periportal areas. These vesicles significantly disappeared after hepatectomy, in parallel with a decrease in liver ATP content. PPADS treatment inhibited hepatocyte cell cycle progression after hepatectomy, as revealed by a reduction in bromodeoxyuridine incorporation, phosphorylated histone 3 immunostaining, cyclin D1 and A expression and immediate early gene induction.

CONCLUSION

Extracellular ATP is released immediately after hepatectomy from hepatocytes and Kupffer cells under mechanical stress and promotes liver regeneration in the rat. We suggest that in hepatocytes, ATP is released from a lysosomal compartment. Finally, observations made in living donors suggest that purinergic signalling could be critical for human liver regeneration.

Expression of P2 nucleotide receptors varies with age and sex in murine brain microglia

Microglia are implicated in multiple neurodegenerative disorders, many of which display sexual dimorphisms and have symptom onsets at different ages. P2 purinergic receptors are critical for regulating various microglial functions, but little is known about how their expression varies with age or sex. Therefore, comprehensive information about purinergic receptor expression in normal microglia, in both sexes, over age is necessary if we are to better understand their roles in the healthy and diseased CNS. We analyzed the expression of all fourteen rodent P2X and P2Y receptors in CD11b⁺ cells freshly-isolated from the brains of C57Bl/6 mice at five different ages ranging from postnatal day 3 to 12 months, in males and females, using quantitative RT-PCR. We also compared purinergic receptor expression in microglia freshly-isolated from 3 day-old pups to that in primary neonatal microglial cultures created from mice of the same age. We observed patterns in P2 receptor expression with age, most notably increased expression with age and age-restricted expression. There were also several receptors that showed sexually dimorphic expression. Lastly, we noted that in vitro culturing of neonatal microglia greatly changed their P2 receptor expression profiles. These data represent the first complete and systematic report of changes in purinergic receptor expression of microglia with age and sex, and provide important information necessary for accurate in vitro modeling of healthy animals.

P2X receptors are differentially expressed on vasopressin- and oxytocin-containing neurons in the supraoptic and paraventricular nuclei of rat hypothalamus

In the present study, the distribution of P2X receptor protein and colocalization of P2X receptors with vasopressin and oxytocin in the supraoptic and paraventricular nuclei of rat hypothalamus was studied using double-labeling fluorescence immunohistochemistry. The results showed that vasopressin-containing neurons expressed P2X(2), P2X(4), P2X(5) and P2X(6) receptor and oxytocin-containing neurons expressed P2X(2), P2X(4) and P2X(5) receptors in the supraoptic nucleus. In the paraventricular nucleus, vasopressin-containing neurons expressed P2X(4), P2X(5) and P2X(6) receptors, while oxytocin-containing neurons expressed P2X(4) receptors. This study provides the first evidence that P2X receptor subunits are differentially expressed on vasopressin- and oxytocin-containing neurons in the supraoptic and paraventricular nuclei, and hence, provides a substantial neuroanatomical basis for possible functional interactions between the purinergic and vasopressinergic systems, and the purinergic and oxytocinergic systems in the rat hypothalamus.

Expression of P2X5 receptors in the mouse CNS

P2X receptors are ATP-gated cationic channels composed of seven known subunits (P2X1-7) which are involved in different functions in neural tissue. The present study investigates the P2X5 receptor expression pattern in the mouse CNS using immunohistochemistry and in situ hybridization histochemistry. The specificity of the immunostaining has been verified by pre-absorption, Western blot and in situ hybridization methods. Heavy P2X5 receptor immunostaining was observed in the mitral cells of the olfactory bulb; cerebral cortex; globus pallidum, anterior cortical amygdaloid nucleus, amygdalohippocampal area of subcortical telencephalon; anterior nuclei, anteroventral nucleus, ventrolateral nucleus of thalamus; supraoptic nucleus, ventromedial nucleus, arcuate nucleus of hypothalamus; substantia nigra of midbrain; pontine nuclei, mesencephalic trigeminal nucleus, motor trigeminal nucleus, ambiguous nucleus, inferior olive, hypoglossal nucleus, dorsal motor vagus nucleus, area postrema of hindbrain; Purkinje cells of cerebellum; and spinal cord. The identification of extensive P2X5 receptor immunoreactivity and mRNA distribution within the CNS of the mouse demonstrated here is consistent with a role for extracellular ATP acting as a fast neurotransmitter.

Dendritic cells, the liver, and transplantation

Interstitial liver dendritic cells (DCs) exhibit phenotypic diversity and functional plasticity. They play important roles in both innate and adaptive immunity. Their comparatively low inherent T cell stimulatory ability and the outcome of their interactions with CD4(+) and CD8(+) T cells, as well as with natural killer (NK) T cells and NK cells within the liver, may contribute to regulation of hepatic inflammatory responses and liver allograft outcome. Liver DCs migrate in the steady state and after liver transplantation to secondary lymphoid tissues, where the outcome of their interaction with antigen-specific T cells determines the balance between tolerance and immunity. Systemic and local environmental factors that are modulated by ischemia-reperfusion injury, liver regeneration, microbial infection, and malignancy influence hepatic DC migration, maturation, and function. Current research in DC biology is providing new insights into the role of these important antigen-presenting cells in the complex events that affect liver transplant outcome.

Rat hepatocytes express functional P2X receptors

Extracellular ATP regulates many hepatic functions by stimulating purinergic receptors. Only the G protein-coupled P2Y receptors have been studied in hepatocytes. We investigated the functional expression of P2X receptors, the ATP-gated channels in rat hepatocytes. P2X4 and P2X7 transcripts and proteins were detected by RT-PCR and by both Western blotting and immunocytochemistry. High concentrations of ATP, and 2'-and 3'-O-(4-benzoylbenzoyl)-ATP the preferring agonist of P2X7, induced membrane blebbing and significant uptake of 4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]-1-[3-(triethylammonio)propyl]diiodide, both of which were inhibited by oxidised ATP, a blocker of P2X receptors. These results provide evidence that P2X4 and P2X7 receptors are expressed and functional on rat hepatocytes, possibly playing an important role in the purinergic signaling complex in these cells.