Microglial ectonucleotidases: Identification and functional roles

Abstract

Microglial cells represent resident macrophages of the central nervous system where they resume a resting state of unknown physiological function. They are rapidly transformed into an activated state by acute pathological events affecting the nervous system. Activated microglia function in tissue repair and regeneration but may also enhance tissue damage. Cultured microglial cells express both P2Y and P2X receptors. Activation of the microglial P2X 7 receptor can result in the release of cytokines or also mediate cytotoxicity in microglial cells. Microglial cells have developed mechanisms for controlling the surrounding concentrations of free extracellular ATP via a surface-expressed ectonucleotidase chain. We analyzed the expression of the ectonucleotidases associated with resting microglia and microglia activated by two models of cerebral ischemia. Out of a plethora of enzymes with the potential to hydrolyze extracellular nucleotides, the major ectonucleotidases associated with microglia are NTPDase1, which hydrolyzes ATP and ADP, and ecto-5′-nucleotidase, which catalyzes the hydrolysis of AMP to adenosine. This is shown by enzyme histochemical analysis, immunocytochemistry, Northern hybridization, and the analysis of NTPDase1-deficient mice. Our results suggest that a major function of resting microglia could be in the lowering of interstitial levels of ATP acting either via autocrine pathways or in the ATP-involving signaling pathways between adjacent neurons and astrocytes. Enhanced activity of NTPDase1 may protect microglia, activated by various pathological settings, from overstimulation by ATP released from the injured tissue. The parallel increase in activity of ecto-5′-nucleotidase would facilitate the formation of the final hydrolysis product adenosine that exerts neuromodulatory and immunomodulatory actions and contributes to the protection of neurons. Drug Dev. Res. 53:208-217, 2001.

Immunocytochemical localization of NTPDases1 and 2 in the neural retina of mouse and zebrafish

“…NTPDase1 is usually associated with the microglia and the vasculature of the brain (Braun et al, 2000;Braun and Zimmermann, 2001). Accordingly, we observed conspicuous NTPDase1-IR in the choroid as well as in structures which look like intraretinal blood vessels in the outer and inner synaptic layers.…”

P2X purinergic receptor ligands: recently patented compounds

“…Ther. Patents (2010) 20 (5) pyridoxine, pyridoxine phosphate, pyridoxal, pyridoxamine and pyridoxamine phosphate were not active [105]. Yet another interesting patent reports the use of adenosine as A3 receptor agonist and NMDA receptor antagonists such as MK-801 and memantine to prevent P2X7R-induced processes in retinal ganglion cells [106].…”

“…Ther. Patents (2010) 20 (5) and pyrazole (compounds 14 --20) [48][49][50][51][52][53][54]. These compounds possess pIC 50 values between 7.3 and 8.8 at both receptors.…”

“…Ther. Patents (2010) 20 (5) antagonized by TNP-ATP (P2X1R and P2X3R antagonists) while having a slow desensitization property similar to that of P2X2R. P2X3R knockout and P2X2/3R double knockout mice were reported to have reduced pain-related behavior and marked urinary bladder hyporeflexia [33,34].…”

“…When cells are damaged, a considerable amount of ATP is released into the extracellular environment, where it functions as a fast-acting ligand for purinergic receptors before being quickly hydrolyzed by ecto-ATPases and ectonucleotidases [4][5][6]. In neurons, an even higher concentration of ATP is stored in synaptic vesicles and, contrary to the common view, ATP can be released in physiologically healthy cells by exocytosis and other non-lytic mechanisms as well as in response to cellular injury or death [6].…”