ABSTRACT
Under physiological conditions, adenosine triphosphate (ATP) is present at low levels in the extracellular milieu, being massively released by stressed or dying cells. Once outside the cells, ATP and related nucleotides/nucleoside generated by ectonucleotidases mediate a high evolutionary conserved signaling system: the purinergic signaling, which is involved in a variety of pathological conditions, including inflammatory diseases. Extracellular ATP has been considered an endogenous adjuvant that can initiate inflammation by acting as a danger signal through the activation of purinergic type 2 receptors-P2 receptors (P2Y G-protein coupled receptors and P2X ligand-gated ion channels). Among the P2 receptors, the P2X7 receptor is the most extensively studied from an immunological perspective, being involved in both innate and adaptive immune responses. P2X7 receptor activation induces large-scale ATP release via its intrinsic ability to form a membrane pore or in association with pannexin hemichannels, boosting purinergic signaling. ATP acting via P2X7 receptor is the second signal to the inflammasome activation, inducing both maturation and release of pro-inflammatory cytokines, such as IL-1ß and IL-18, and the production of reactive nitrogen and oxygen species. Furthermore, the P2X7 receptor is involved in caspases activation, as well as in apoptosis induction. During adaptive immune response, P2X7 receptor modulates the balance between the generation of T helper type 17 (Th17) and T regulatory (Treg) lymphocytes. Therefore, this receptor is involved in several inflammatory pathological conditions. In infectious diseases and cancer, P2X7 receptor can have different and contrasting effects, being an angel or a demon depending on its level of activation, cell studied, type of pathogen, and severity of infection. In neuroinflammatory and neurodegenerative diseases, P2X7 upregulation and function appears to contribute to disease progression. In this review, we deeply discuss P2X7 receptor dual function and its pharmacological modulation in the context of different pathologies, and we also highlight the P2X7 receptor as a potential target to treat inflammatory related diseases.
ABSTRACT
Tissue accumulation of high amounts of D-2-hydroxyglutaric acid (DGA) and l-2-hydroxyglutaric acid (LGA) is the biochemical hallmark of the inherited neurometabolic disorders D-2-hydroxyglutaric aciduria (DHGA) and l-2-hydroxyglutaric aciduria (LHGA), respectively. Patients affected by DHGA predominantly present neurological and cardiomuscular symptoms, while those with LHGA have mainly severe neurological symptoms. Lactic aciduria and/or lactic acidemia may also occur in both disorders, suggesting mitochondrial dysfunction. We have previously reported that cytochrome c oxidase (COX) activity is severely inhibited by DGA in rat cerebral cortex and human skeletal muscle. In the present study, we initially evaluated the role of DGA and LGA on the mitochondrial respiratory chain complex activities, as well as CO2 on production in cardiac and skeletal muscle from 30-day-old Wistar rats. DGA significantly inhibited COX and ATP synthase (F0F1-ATP synthase) activities, in contrast to the other activities of the respiratory chain enzymes which were not affected by DGA in both muscular tissues. In addition, CO2 production was also markedly reduced by DGA in rat skeletal and cardiac muscles. On the other hand, LGA did not interfere with any of the respiratory chain complex activities studied, neither with CO2 generation. We also measured mitochondrial respiratory parameters in rat brain mitochondrial preparations in the presence of DGA and LGA. Both metabolites significantly lowered the respiratory control ratio in the presence of glutamate/malate and succinate. Since the metabolites stimulated oxygen consumption in state IV and compromised ATP formation, it can be presumed that these organic acids might act as endogenous uncouplers of mitochondria respiration. Moreover, COX activity linked to TMPD-ascorbate was significantly reduced by DGA in the brain mitochondrial enriched fractions. Finally, DGA and LGA reduced cell viability of rat cerebral cortex slices, as determined by the MTT assay. In case our in vitro data also occur in vivo, it may be presumed that impairment of energy metabolism may contribute to the understanding of the clinical features mainly of patients affected by DHGA.