Regulation of P2X 7 nucleotide receptor function in human monocytes by extracellular ions and receptor density

Macrophages are unique innate immune cells that play an integral role in the defense of the host by virtue of their ability to recognize, engulf, and kill pathogens while sending out danger signals via cytokines to recruit and activate inflammatory cells. It is becoming increasingly clear that purinergic signaling events are essential components of the macrophage response to pathogen challenges and disorders such as sepsis may be, at least in part, regulated by these important sensors. The activation of the P2X 7 receptor is a powerful event in the regulation of the caspase-1 inflammasome. We provide evidence that the inflammasome activation requires "priming" of macrophages prior to ATP activation of the P2X 7 R. Inhibition of the inflammasome activation by the tyrosine kinase inhibitor, AG126, suggests regulation by phosphorylation. Finally, the P2X 7 R may also be activated by other elements of the host response such as the antimicrobial peptide LL-37, which adds a new, physiologically relevant agonist to the P2X 7 R pathway. Therapeutic approaches to inflammation and sepsis will certainly be enhanced by an increased understanding of how purinergic receptors modulate the inflammasomes.

Section: Human Monocytes Express P2x 7 Receptorssupporting

confidence: 81%

P2X7 receptor and macrophage function

Wewers

Sarkar

2009

“…In line with their data, we also observed that these ionic conditions resulted in an increased agonist affinity, such that 100 μM ATP was sufficient for the activation of a nonselective pore by P2X 7 receptors. Although it is not known why ATP induces a pore in macrophages and not in monocytes in normal NaCl-containing medium, the number of cell surface P2X 7 receptors, which is significantly higher in macrophages than in monocytes, seems to play a critical role [37]. To our surprise, NAD + , in contrast to ATP, failed to induce the nonselective pore that typifies activated P2X 7 receptors.…”

Section: Engagement Of P2x 4 Receptorsmentioning

confidence: 54%

“…However as described earlier, monocytes, in contrast to monocyte-derived macrophages, do not form pores in [46,47]. Considering the crucial role of ionic compositions of the extracellular medium in P2X 7 receptor activation by ATP [48][49][50][51], Gudipaty et al [37] showed that replacement of extracellular Na + and Cl − with K + and nonhalide anions strongly facilitated ATP-dependent pore formation in monocytes. In line with their data, we also observed that these ionic conditions resulted in an increased agonist affinity, such that 100 μM ATP was sufficient for the activation of a nonselective pore by P2X 7 receptors.…”

Section: Engagement Of P2x 4 Receptorsmentioning

confidence: 88%

“…The pore is permeable to molecules with molecular masses up to 900 Da, irrespective of the charge [36]. To measure P2X 7 -dependent pore formation, monocytes were suspended in potassium glutamate basic salt solution [37] containing ethidium bromide before ATP and NAD + were added at different concentrations for 15 min at 37°C. Ethidium is a 340 Da hydrophilic molecule normally excluded by intact plasma membranes.…”

Section: Engagement Of P2x 7 Receptorsmentioning

confidence: 99%

See 1 more Smart Citation

Involvement of P2X receptors in the NAD+-induced rise in [Ca2+]i in human monocytes

Grahnert

Klein

Hauschildt

2009

In the present study, we show that the extracellular addition of nicotinamide adenine dinucleotide (NAD + ) induces a transient rise in [Ca 2+ ] i in human monocytes caused by an influx of extracellular calcium. The NAD + -induced Ca 2+ response was prevented by adenosine triphosphate (ATP), suggesting the involvement of ATP receptors. Of the two subtypes of ATP receptors (P2X and P2Y), the P2X receptors were considered the most likely candidates. By the use of subtype preferential agonists and antagonists, we identified P2X 1 , P2X 4 , and P2X 7 receptors being engaged in the NAD + -induced rise in [Ca 2+ ] i . Among the P2X receptor subtypes, the P2X 7 receptor is unique in facilitating the induction of nonselective pores that allow entry of ethidium upon stimulation with ATP. In monocytes, opening of P2X 7 receptor-dependent pores strongly depends on specific ionic conditions. Measuring pore formation in response to NAD + , we found that NAD + unlike ATP lacks the ability to induce this pore-forming response. Whereas as little as 100 μM ATP was sufficient to activate the nonselective pore, NAD + at concentrations up to 2 mM had no effect. Taken together, these data indicate that despite similarities in the action of extracellular NAD + and ATP there are nucleotide-specific variations. So far, common and distinct features of the two nucleotides are only beginning to be understood.

“…Additional studies are needed to examine whether expression of P2X7 is regulated and how such regulation might contribute to bone cell function. In this regard, exposure to certain stimuli has been found to enhance P2X7 expression in various cell types [89][90][91]. On the other hand, high concentrations of ATP appear to mediate internalization of P2X7 in osteoclast-like cells [59].…”

Section: Prospects and Conclusionmentioning

confidence: 99%

Expression, signaling, and function of P2X7 receptors in bone

Grol

Panupinthu

Korcok

et al. 2009

Nucleotides released from cells in response to mechanical stimulation or injury may serve as paracrine regulators of bone cell function. Extracellular nucleotides bind to multiple subtypes of P2 receptors on osteoblasts (the cells responsible for bone formation) and osteoclasts (cells with the unique ability to resorb mineralized tissues). Both cell lineages express the P2X7 receptor subtype. The skeletal phenotype of mice with targeted disruption of P2rx7 points to interesting roles for this receptor in the regulation of bone formation and resorption, as well as the response of the skeleton to mechanical stimulation. This paper reviews recent work on the expression of P2X7 receptors in bone, their associated signal transduction mechanisms and roles in regulating bone formation and resorption. Areas for future research in this field are also discussed.