Adenosine 5' triphosphate evoked mobilization of intracellular calcium in central nervous system white matter of adult mouse optic nerve

Association between uric acid and cognitive dysfunction: A cross-sectional study with newly diagnosed, drug-naïve with bipolar disorder

BACKGROUND

Cognitive impairment is one of the major symptoms of individuals with bipolar disorder (BD). Purine system disorders may play an important role in cognitive dysfunction. So far, the relationship between cognitive deficits and purinergic metabolism in BD has been seldom discussed in previous studies. This study aims to explore its relevance and potential biological mechanisms.

METHODS

In this cross-sectional study, 205 first time diagnosed drug-naive individuals with BD and 97 healthy volunteers were recruited. The uric acid(UA) level was measured using automatic biochemical analyzer, and cognitive function was assessed by Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and Stroop color-word test. In addition, general information and clinical symptoms were collected and evaluated.

RESULTS

In this study, the UA level of BD group (U = 8475.000, p = 0.038) was found to be significantly higher than that of the healthy controls, but the scores of RBANS (t = -11.302, p < 0.001) and Stroop color-word test (t = -6.962, p < 0.001) were significantly lower than that of the healthy controls. In gender subgroup analysis, females had lower UA level and higher RBANS scores. In correlation analysis, the cognitive function of individuals with BD was found to present a significant negative correlation with UA level in attention (r = -0.23, p = 0.001) and delayed memory(r = -0.16, p = 0.022).

LIMITATIONS

This is a cross-sectional design.

CONCLUSION

Elevated UA levels may be a potential mechanism of cognitive impairment in BD. This provides a new possible strategy for the prevention and treatment of cognitive impairment in BD.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Inhibition of the P2X7 receptor reduces cystogenesis in PKD

The P2X7 receptor participates in purinergic signaling, which may promote the progression of ADPKD. We examined the effects of a P2X7 receptor antagonist and a P2X7 receptor agonist on cyst development in a zebrafish model of polycystic kidney disease in which we knocked down pkd2 by morpholinos. We used live wt-1b pronephric-specific GFP-expressing zebrafish embryos to directly observe changes in the pronephros. Exposure of pkd2-morphant zebrafish to a P2X7 receptor antagonist (oxidized ATP [OxATP]) significantly reduced the frequency of the cystic phenotype compared with either exposure to a P2X7 receptor agonist (BzATP) or with no treatment (P < 0.01). Histology confirmed improvement of glomerular cysts in OxATP-treated pkd2 morphants. OxATP also reduced p-ERK activity and cell proliferation in pronephric kidneys in pkd2 morphants. Inhibition of P2X7 with an additional specific antagonist (A-438079), and through morpholino-mediated knockdown of p2rx7, confirmed these effects. In conclusion, blockade of the P2X7 receptor reduces cyst formation via ERK-dependent pathways in a zebrafish model of polycystic kidney disease, suggesting that P2X7 antagonists may have therapeutic potential in ADPKD.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Functions of optic nerve glia: axoglial signalling in physiology and pathology

An early concept of glial function envisaged them as passive and unexcitable structural elements, much like the connective tissues of organs in the periphery. It is now known that glia have a widespread range of physiological roles and react to all forms of pathological insult. This paper reviews the major functions of oligodendrocytes and astrocytes, the main types of glia in the optic nerve, and examines novel NG2-glia, otherwise known as oligodendrocyte progenitor cells (OPCs). The major function of oligodendrocytes is to produce the myelin sheaths that insulate CNS axons, but they also have important roles in the establishment of nodes of Ranvier, the sites of action potential propagation, and axonal integrity. Astrocytes have multiple physiological and pathological functions, including potassium homeostasis and metabolism, and reactive astrogliosis in response to CNS insults. The bulk of NG2-glia are postmitotic complex cells, distinct from OPCs, and respond to any insult to the CNS by a rapid and stereotypic injury response. This may be their primary unction, but NG2-glia, or a subpopulation of NG2-expressing adult OPCs, also provide remyelinating oligodendrocytes following demyelination. Oligodendrocytes, astrocytes, and NG2-glia all contact axons at nodes of Ranvier and respond to glutamate, ATP, and potassium released during axonal electrical activity. Glutamate and ATP evoke calcium signalling in optic nerve glia and have dual roles in physiology and pathology, coupling glial functions to axonal activity during normal activity, but enhanced activation induces an injury response, as seen following injury, demyelination, and ischaemia.

P2Y and P2X purinoceptor mediated Ca2+ signalling in glial cell pathology in the central nervous system

Activation of purinoceptors by extracellular ATP is an important component of the glial response to injury in the central nervous system (CNS). ATP has been shown to evoke raised cytosolic [Ca(2+)] in astrocytes, oligodendrocytes, and microglia, the three major glial cell types in the CNS. Glial cells express a heterogenous collection of metabotropic P2Y and ionotropic P2X purinoceptors, which respectively mobilise Ca(2+) from intracellular stores and trigger Ca(2+) influx across the plasmalemma. It is likely that different receptors have distinct roles in glial cell physiology and pathology. Our studies on optic nerve glia in situ indicate that P2Y(1) and P2Y(2/4) receptors are activated at low ATP concentrations, suggesting they are the predominant purinoceptors mediating physiological Ca(2+) signalling. Glia also express P2X(1) and P2X(3) purinoceptors, which mediate fast, rapidly desensitising current and may also be important in signalling. At high concentrations, such as occur in CNS injury, ATP induces large and prolonged increases in glial [Ca(2+)](i) with a primary role for P2Y purinoceptors and inositol trisphosphate (IP(3))-dependent release of Ca(2+) from intracellular stores. In addition, we found that high concentrations of ATP activated a significant P2X component that did not desensitise or saturate and was dependent on extracellular Ca(2+). These are characteristic properties of the P2X(7) subtype, and we provide in situ evidence that application of the P2X(7) receptor agonist benzoyl-benzoyl ATP (BzATP) evokes raised [Ca(2+)](i) in optic nerve glia, and that the dye YO-PRO-1, which passes through pore-forming P2X(7) receptors, is taken up by astrocytes, oligodendrocytes and microglia. Glia also express P2X(2) and P2X(4) receptors that are also pore-forming in the presence of sustained high ATP concentrations and which may also be important in the glial injury response. There is evidence that activation of P2 purinoceptors is a key step in triggering reactive changes in glial cells, including expression of immediate early genes, induction of extracellular signal regulated kinase and cyclooxygenase-2, synthesis of phospholipase A(2), release of arachidonic acid, production of prostaglandins and release of interleukins. We show that the ATP-mediated increase in glial [Ca(2+)](i) is potentiated by arachidonic acid and reduced by the inhibition of phospholipase A(2) inhibition. Together, the results implicate ATP as a primary signalling molecule in glial cells and indicate specific roles for P2Y and P2X purinoceptors in glial cell pathology.

Changes in P2Y and P2X purinoceptors in reactive glia following axonal degeneration in the rat optic nerve

Purinoceptors have been shown to be important in mediating Ca(2+) signalling in glial cells and it has been proposed that they may have a role in their response to injury. To investigate this, the glial response to adenosine 5'triphosphate (ATP) was measured in situ, in optic nerves from juvenile rats that were enucleated at postnatal day (P) 1; age-matched normal nerves were used as controls. The optic nerve is a typical central nervous system (CNS) white matter tract containing axons and glial cells, but not neurones or synapses. Following neonatal enucleation, axons degenerate and oligodendrocytes do not develop, so that the optic nerve is populated predominantly by reactive astrocytes, with a minor population of activated microglia. Application of 1 mM ATP evoked a large and rapid increase in glial [Ca(2+)](i) in fura-2 ratiometric whole nerve recordings from normal and gliotic axon-free nerves. Significantly, the response to ATP had a prolonged duration in gliotic axon-free nerves and there was as shift in the agonist rank order of potency from ATP = ADP > UTP >> alpha,beta-metATP to ATP > ADP = UTP = alpha,beta-metATP. The results indicate an in situ role for ATP signalling in reactive astrocytes, via metabotropic P2Y(1) and P2Y(2/4) purinoceptors and ionotropic P2X purinoreceptors. The change in the purinoceptor profile following axon degeneration suggests a special role for P2X purinoceptors in mediating the glial reaction to CNS injury.

P2X and P2Y purinoreceptors mediate ATP-evoked calcium signalling in optic nerve glia in situ

It is known that ATP acts as an extracellular messenger mediating Ca2+ signalling in glial cells. Here, the mechanisms involved in the ATP-evoked increase in glial [Ca2+]i were studied in situ, in the acutely isolated rat optic nerve. ATP and agonists for P2X (a,b-metATP) and P2Y (2MeSATP) purinoreceptors triggered raised glial [Ca2+]i, and there was no significant difference between cells identified morphologically as astrocytes and oligodendrocytes. Dose-response curves indicated that P2Y receptors were activated at nanomolar concentrations, whereas P2X purinoreceptors were only activated above 10 microM. The rank order of potency for several agonists indicated optic nerve glia expressed heterogeneous purinoreceptors, with P2Y1< or = P2Y2/4< or = P2X. The ATP evoked increase in [Ca2+]i was reversibly blocked by the P2X/Y purinoreceptor antagonist suramin (100 microM) and markedly reduced by thapsigargin (10 microM), which blocks IP3-dependent release of Ca2+ from intracellular stores. Removal of extracellular Ca2+ reduced the ATP evoked increase in [Ca2+]i and completely blocked its recovery, indicating that refilling of intracellular stores was ultimately dependent on Ca2+ influx from the extracellular milieu. The results implicate ATP as an important signal in CNS white matter astrocytes and oligodendrocytes in situ, and indicate that metabotropic P2Y purinoreceptors mobilize intracellular Ca2+ at physiological concentrations of ATP, whereas ionotropic P2X purinoreceptors induce Ca2+ influx across the plasmalemma only at high concentrations of ATP, such as occur following CNS injury.