Exploring the Role of P2X Receptors in Alzheimer's Disease.

Several studies have pointed to soluble oligomers of beta amyloid peptide (SOAß) as the principal neurotoxic agents responsible for the generation of synaptotoxic events that can explain the main symptoms of Alzheimer's disease (AD). Among the toxic features associated with SOAß, one of the most notorious is the formation of a non-selective pore-like structure in the plasma membrane, which may partly explain the overload of intracellular Ca2+. There is evidence that the pore causes leakage of key intracellular compounds, such as adenosine triphosphate (ATP), to the extracellular milieu. Extracellular ATP activates P2X receptors (P2XR), which are ligand-gated ion channels (LGICs) widely expressed in both neuron and glial cells and act as neuromodulators of synaptic activity by promoting Ca2+ entry and facilitating neurotransmitter release. There is abundant evidence correlating the overexpression of these receptors to neurodegenerative diseases, including AD, thus opening the possibility that P2XR could potentiate the toxic mechanisms induced by SOAß and contribute to intracellular Ca2+ overload in neurons and other mechanisms related to glial activation and inflammation. In this review, we correlate scientific evidence related to the main toxic effects induced by SOAß and those that are mediated by purinergic P2XR. The data suggest that these purinergic receptors participate in the deleterious cellular and molecular effects of SOAß that lead to the pathogenesis of AD. This information sheds light on the participation of new components in SOAß toxicity that could be interesting as pharmacological targets for the development of molecular or chemical compounds able to modulate them.

Integration of purinergic and angiotensin II receptor function in renal vascular responses and renal injury in angiotensin II-dependent hypertension.

Glomerular arteriolar vasoconstriction and tubulointerstitial injury are observed before glomerular damage occurs in models of hypertension. High interstitial ATP concentrations, caused by the increase in arterial pressure, alter renal mechanisms involved in the long-term control of blood pressure, autoregulation of glomerular filtration rate and blood flow, tubuloglomerular feedback (TGF) responses, and sodium excretion. Elevated ATP concentrations and augmented expression of P2X receptors have been demonstrated under a genetic background or induction of hypertension with vasoconstrictor peptides. In addition to the alterations of the microcirculation in the hypertensive kidney, the vascular actions of elevated intrarenal angiotensin II levels may be mitigated by the administration of broad purinergic P2 antagonists or specific P2Y12, P2X1, and P2X7 receptor antagonists. Furthermore, the prevention of tubulointerstitial infiltration with immunosuppressor compounds reduces the development of salt-sensitive hypertension, indicating that tubulointerstitial inflammation is essential for the development and maintenance of hypertension. Inflammatory cells also express abundant purinergic receptors, and their activation by ATP induces cytokine and growth factor release that in turn contributes to augment tubulointerstitial inflammation. Collectively, the evidence suggests a pathophysiological activation of purinergic P2 receptors in angiotensin-dependent hypertension. Coexistent increases in intrarenal angiotensin II and activates Ang II AT1 receptors, which interacts with over-activated purinergic receptors in a complex manner, suggesting convergence of their post-receptor signaling processes.

Potential Therapeutic Applications of P2 Receptor Antagonists: From Bench to Clinical Trials.

BACKGROUND: Extracellular purines and pyrimidines have important physiological functions in mammals. Purines and pyrimidines act on P1 and P2 purinergic receptors, which are widely expressed in the plasma membrane in various cell types. P2 receptors act as important therapeutic targets and are associated with several disorders, such as pain, neurodegeneration, cancer, inflammation, and thrombosis. However, the use of antagonists for P2 receptors in clinical therapy, with the exception of P2Y12, is a great challenge. Currently, many research groups and pharmaceutical companies are working on the development of specific antagonist molecules for each receptor subtype that could be used as new medicines to treat their respective disorders. OBJECTIVE: The present review compiles some interesting findings on the application of P2 receptor antagonists in different in vitro and in vivo experimental models as well as the progress of advanced clinical trials with these compounds. CONCLUSION: Despite all of the exciting results obtained on the bench, few antagonists of P2 receptors advanced to the clinical trials, and once they reach this stage, the effectiveness of the therapy is not guaranteed, as in the example of P2X7 antagonists. Despite this, P2Y12 receptor antagonists have a history of success and have been used in therapy for at least two decades to prevent thrombosis in patients at risk for myocardial infarctions. This breakthrough is the motivation for scientists to develop new drugs with antagonistic activity for the other P2 receptors; thus, in a matter of years, we will have an evolution in the field of purinergic therapy.

Heteromeric channels with different phenotypes are generated when coexpressing two P2X2 receptor isoforms.

To investigate if channels with different stoichiometry are formed from P2X2 receptor isoforms during their heterologous co-expression. The two-electrode voltage-clamp technique was used to measured ATP induced currents in Xenopus laevis oocytes. We used a mutant (P2X2-2bm) because its ATP sensitivity is lower than P2X2-2b receptors, which highlights the differences with its splice variant P2X2-1a.Currents through homomeric channels had significantly different Hill coefficients. P2XR are trimeric proteins with three agonist binding sites; therefore, only two homomeric and two heteromeric stoichiometries are possible when both P2X2 isoforms are coexpressed, the heteromeric channels might be formed by: i) 2(P2X2-1a)+1(P2X2-2bm); or ii) 1(P2X2-1a)+2(P2X2-2bm). Because P2X2 channels open when two binding sites are occupied, these stoichiometries are expected to have different ATP sensitivities. Thus, co-expressing both P2X2 isoforms, two oocyte populations were distinguished based on their sensitivity to ATP and Hill coefficients. For the first population (P2X2-1a like), the ATP EC50 and the Hill coefficient were not different than those of homomeric P2X2-1a channels similarly, for the second population (P2X2-2bm like), these variables were also not different than for those of homomeric P2X2-2bm channels. Various findings indicate that homomeric channel expression is not responsible for such differences. Our observations indicate that two heteromeric channels can be assembled from two P2X2 receptor isoforms. Our data support a current model, according to which, ATP activation of two subunits can open P2X2 channel. However, PPADS appears to bind to all three subunits in order to inhibit ATP effects on P2X2 receptors.

ATP leakage induces P2XR activation and contributes to acute synaptic excitotoxicity induced by soluble oligomers of ß-amyloid peptide in hippocampal neurons.

Recent studies suggest that the toxic effects of Aß can be attributed to its capability to insert in membranes and form pore-like structures, which are permeable to cations and molecules such as ATP. Our working hypothesis is that Aß increases extracellular ATP causing activation of P2X receptors and potentiating excitatory synaptic activity. We found that soluble oligomers of ß-amyloid peptide increased cytosolic Ca(2+) 4-fold above control (415 ± 28% of control). Also, ATP leakage (157 ± 10% of control) was independent of extracellular Ca(2+), suggesting that ATP traveled from the cytosol through an Aß pore-mediated efflux and not from exocytotic mechanisms. The subsequent activation of P2XR by ATP can contribute to the cytosolic Ca(2+) increase observed with Aß. Additionally, we found that ß-amyloid oligomers bind preferentially to excitatory neurons inducing an increase in excitatory synaptic current frequency (248.1 ± 32.7%) that was blocked by the use of P2XR antagonists such as PPADS (Aß + PPADS: 110.9 ± 18.35%) or Apyrase plus DPCPX (Aß + inhibitors: 98.97 ± 17.4%). Taken together, we suggest that Aß induces excitotoxicity by binding preferentially to excitatory neuron membranes forming a non-selective pore and by increasing intracellular calcium by itself and through P2XR activation by extracellular ATP leading to an augmention in mEPSC activity. All these effects were blocked with a non-specific P2XR antagonist, indicating that part of the neurotoxicity of Aß is mediated by P2XR activation and facilitation of excitatory neurotransmitter release. These findings suggest that P2XR can be considered as a potential new target for the development of drugs or pharmacological tools to treat Alzheimer's disease. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Modulation of the neuronal network activity by P2X receptors and their involvement in neurological disorders.

ATP is a key energetic molecule, fundamental to cell function, which also has an important role in the extracellular milieu as a signaling molecule, acting as a chemoattractant for immune cells and as a neuro- and gliotransmitter. The ionotropic P2X receptors are members of an ATP-gated ion channels family. These ionotropic receptors are widely expressed through the body, with 7 subunits described in mammals, which are arranged in a trimeric configuration with a central pore permeable mainly to Ca(2+) and Na(+). All 7 subunits are expressed in different brain areas, being present in neurons and glia. ATP, through these ionotropic receptors, can act as a neuromodulator, facilitating the Ca(2+)-dependent release of neurotransmitters, inducing the cross-inhibition between P2XR and GABA receptors, and exercising by this way a modulation of synaptic plasticity. Growing evidence shows that P2XR play an important role in neuronal disorders and neurodegenerative diseases, like Parkinson's and Alzheimer's disease; this role involves changes on P2XR expression levels, activation of key pathways like GSK3ß, APP processing, oxidative stress and inflammatory response. This review is focused on the neuromodulatory function of P2XR on pathophysiological conditions of the brain; the recent evidence could open a window to a new therapeutic target.