These studies showed that CD40 is an inducer of ATP release in Mller glia (7). in microglia/macrophages purinergic receptor-dependent inflammatory responses that drive the development of retinopathy. The CD40-ATP-P2X7 pathway not only amplifies inflammation but also induces death of retinal endothelial cells, an event key to the development of capillary degeneration and retinal ischemia. Taken together, CD40 expressed in non-hematopoietic cells is sufficient to mediate inflammation and tissue pathology as well as cause death of retinal endothelial cells. This process likely contributes to development of degenerate capillaries, a hallmark of diabetic and ischemic retinopathies. Blockade of signaling pathways downstream of CD40 operative in non-hematopoietic cells may offer a novel means of treating diabetic and ischemic retinopathies. revealed that human and rodent Mller glia are unable to secrete these pro-inflammatory cytokines in response to CD40 ligation even though these cells react to CD40 stimulation (CCL2 secretion and ICAM-1 protein upregulation) (7). This apparent discrepancy raised Sildenafil citrate the possibility that CD40 in Mller glia acts on bystander microglia/macrophages to promote expression of TNF- and IL-1. Testing whether Mller glia activated by Rabbit Polyclonal to DP-1 CD40 induce IL-1 and TNF- production in bystander monocytes/macrophages was done by adding human CD154 to human CD40+ Mller glia incubated with CD40? human monocytic cells (to avoid the effects of direct CD40 ligation on these cells), or by adding human CD154 to human CD40-expressing mouse Mller glia incubated with mouse macrophages (human CD154 does not stimulate mouse CD40 expressed in macrophages) (7). While Mller glia and monocyte/macrophages failed to secrete TNF- and IL-1 in Sildenafil citrate response to CD154, addition of CD154 to the co-culture of these cells triggered TNF- and IL-1 production (7). The studies have an correlate since diabetic mice that express CD40 restricted to Mller glia upregulate TNF- protein levels in microglia/macrophages but not in Mller glia Sildenafil citrate while the latter cells upregulate CCL2 protein levels (7). Taken together, these studies revealed that Mller glia activated by CD40 induce pro-inflammatory responses in bystander microglia/macrophages. The CD40-ATP-P2X7 Pathway and Inflammatory Responses in Bystander Microglia/Macrophages ATP functions not only as a neurotransmitter for neurons but can also be secreted by non-excitable cells (72, 73). Moreover, various cell types express P2 purinergic receptors. These receptors are divided into ATP-gated ionotropic P2X receptors and metabotropic, G protein-coupled P2Y receptors (72, 73). The seven subtypes of P2X receptors are ligand-gated channels permeable to Ca2+, Na+, and K+. P2X7 receptor is characterized by the ability to form large trans-membrane pores in response to repetitive or prolonged exposure to ATP (72, 73). P2X7 receptor is key for IL-1 and TNF- secretion by microglia/macrophages stimulated with ATP (74, 75). Indeed, secretion of ATP by astrocytes may cause P2X7-dependent microglial activation that would drive neuroinflammatory and degenerative disorders (76). and studies were conducted to determine whether CD40 acts through ATP-P2X7 signaling to induce cytokine production in bystander myeloid cells. These studies showed that CD40 is an inducer of ATP release in Mller glia (7). Moreover, purinergic signaling explains TNF- and IL-1 secretion in bystander monocytes/macrophages incubated with Mller glia activated by CD40. Blockade of the P2X7 receptor either by pharmacologic approaches, knockdown of P2X7 or the use of macrophages from mice results in marked inhibition of TNF- and IL-1 secretion (7). In addition, a purinergic receptor ligand (Bz-ATP) enhances cytokine production by monocytic cells (7). As described above, studies in diabetic transgenic mice that express CD40 only in Mller glia revealed that TNF- is expressed in a distinct compartmentmicroglia/macrophages (7). Moreover, P2X7 receptor mRNA levels are enhanced in the retinas of diabetic mice and P2X7 receptor protein expression is increased in microglia/macrophages from these animals (7). This is relevant since increased levels of P2X7 receptor facilitate the effects of the receptor (77). Mice treated with the P2X7 receptor inhibitor BBG as well as mice are protected from diabetes-induced upregulation of IL-1 and TNF-.