Ligation of CD40 in Human Müller Cells Induces P2X7 Receptor-Dependent Death of Retinal Endothelial Cells

Microvascular destabilization and intricated network of the cytokines in diabetic retinopathy: from the perspective of cellular and molecular components

Microvascular destabilization is the primary cause of the inner blood-retinal barrier (iBRB) breakdown and increased vascular leakage in diabetic retinopathy (DR). Microvascular destabilization results from the combinational effects of increased levels of growth factors and cytokines, involvement of inflammation, and the changed cell-to-cell interactions, especially the loss of endothelial cells and pericytes, due to hyperglycemia and hypoxia. As the manifestation of microvascular destabilization, the fluid transports via paracellular and transcellular routes increase due to the disruption of endothelial intercellular junctional complexes and/or the altered caveolar transcellular transport across the retinal vascular endothelium. With diabetes progression, the functional and the structural changes of the iBRB components, including the cellular and noncellular components, further facilitate and aggravate microvascular destabilization, resulting in macular edema, the neuroretinal damage and the dysfunction of retinal inner neurovascular unit (iNVU). Although there have been considerable recent advances towards a better understanding of the complex cellular and molecular network underlying the microvascular destabilization, some still remain to be fully elucidated. Recent data indicate that targeting the intricate signaling pathways may allow to against the microvascular destabilization. Therefore, efforts have been made to better clarify the cellular and molecular mechanisms that are involved in the microvascular destabilization in DR. In this review, we discuss: (1) the brief introduction of DR and microvascular destabilization; (2) the cellular and molecular components of iBRB and iNVU, and the breakdown of iBRB; (3) the matrix and cell-to-cell contacts to maintain microvascular stabilization, including the endothelial glycocalyx, basement membrane, and various cell-cell interactions; (4) the molecular mechanisms mediated cell-cell contacts and vascular cell death; (5) the altered cytokines and signaling pathways as well as the intricate network of the cytokines involved in microvascular destabilization. This comprehensive review aimed to provide the insights for microvascular destabilization by targeting the key molecules or specific iBRB cells, thus restoring the function and structure of iBRB and iNVU, to treat DR.

CCR1 mediates Müller cell activation and photoreceptor cell death in macular and retinal degeneration

Mononuclear cells are involved in the pathogenesis of retinal diseases, including age-related macular degeneration (AMD). Here, we examined the mechanisms that underlie macrophage-driven retinal cell death. Monocytes were extracted from patients with AMD and differentiated into macrophages (hMdɸs), which were characterized based on proteomics, gene expression, and ex vivo and in vivo properties. Using bioinformatics, we identified the signaling pathway involved in macrophage-driven retinal cell death, and we assessed the therapeutic potential of targeting this pathway. We found that M2a hMdɸs were associated with retinal cell death in retinal explants and following adoptive transfer in a photic injury model. Moreover, M2a hMdɸs express several CCRI (C-C chemokine receptor type 1) ligands. Importantly, CCR1 was upregulated in Müller cells in models of retinal injury and aging, and CCR1 expression was correlated with retinal damage. Lastly, inhibiting CCR1 reduced photic-induced retinal damage, photoreceptor cell apoptosis, and retinal inflammation. These data suggest that hMdɸs, CCR1, and Müller cells work together to drive retinal and macular degeneration, suggesting that CCR1 may serve as a target for treating these sight-threatening conditions.

P2X7 Is Involved in the Mouse Retinal Degeneration via the Coordinated Actions in Different Retinal Cell Types

Adenosine triphosphate (ATP) released from dying cells with high concentrations is sensed as a danger signal by the P2X7 receptor. Sodium iodate (NaIO₃) is an oxidative toxic agent, and its retinal toxicity has been used as the model of dry age-related macular degeneration (AMD). In this study, we used NaIO₃-treated mice and cultured retinal cells, including BV-2 microglia, 661W photoreceptors, rMC1 Müller cells and ARPE-19 retinal epithelial cells, to understand the pathological action of P2X7 in retinal degeneration. We found that NaIO₃ can significantly decrease the photoreceptor function by reducing a-wave and b-wave amplitudes in electroretinogram (ERG) analysis. Optical coherence tomography (OCT) analysis revealed the degeneration of retinal epithelium and ganglion cell layers. Interestingly, P2X7^-/- mice were protected from the NaIO₃-induced retinopathy and inflammatory NLRP3, IL-1β and IL-6 gene expression in the retina. Hematoxylin and eosin staining indicated that the retinal epithelium was less deteriorated in P2X7^-/- mice compared to the WT group. Although P2X7 was barely detected in 661W, rMC1 and ARPE-19 cells, its gene and protein levels can be increased after NaIO₃ treatment, leading to a synergistic cytotoxicity of BzATP [2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate tri(triethyleneammonium)salt] and NaIO₃ administration in ARPE-19 cells. In conclusion, the paracrine action of the ATP/P2X7 axis via cell-cell communication is involved in NaIO₃-induced retinal injury. Our results show that P2X7 antagonist might be a potential therapy in inflammation-related retinal degeneration.

The role of retinal Müller cells in diabetic retinopathy and related therapeutic advances

Diabetic retinopathy (DR) is a significant complication of diabetes. During the pathogenesis of retinal microangiopathy and neuronopathy, activated retinal Müller cells (RMCs) undergo morphological and structural changes such as increased expression of glial fibrillary acidic protein, disturbance of potassium and water transport regulation, and onset of production of a large number of inflammatory and vascular growth factors as well as chemokines. Evidently, activated RMCs are necessary for the pathogenesis of DR; therefore, exploring the role of RMCs in DR may provide a new target for the treatment thereof. This article reviews the mechanism of RMCs involvement in DR and the progress in related treatments.

Ferroptosis: a double-edged sword mediating immune tolerance of cancer

The term ferroptosis was put forward in 2012 and has been researched exponentially over the past few years. Ferroptosis is an unconventional pattern of iron-dependent programmed cell death, which belongs to a type of necrosis and is distinguished from apoptosis and autophagy. Actuated by iron-dependent phospholipid peroxidation, ferroptosis is modulated by various cellular metabolic and signaling pathways, including amino acid, lipid, iron, and mitochondrial metabolism. Notably, ferroptosis is associated with numerous diseases and plays a double-edged sword role. Particularly, metastasis-prone or highly-mutated tumor cells are sensitive to ferroptosis. Hence, inducing or prohibiting ferroptosis in tumor cells has vastly promising potential in treating drug-resistant cancers. Immunotolerant cancer cells are not sensitive to the traditional cell death pathway such as apoptosis and necroptosis, while ferroptosis plays a crucial role in mediating tumor and immune cells to antagonize immune tolerance, which has broad prospects in the clinical setting. Herein, we summarized the mechanisms and delineated the regulatory network of ferroptosis, emphasized its dual role in mediating immune tolerance, proposed its significant clinical benefits in the tumor immune microenvironment, and ultimately presented some provocative doubts. This review aims to provide practical guidelines and research directions for the clinical practice of ferroptosis in treating immune-resistant tumors.

CD40 Expressed in Endothelial Cells Promotes Upregulation of ICAM-1 But Not Pro-Inflammatory Cytokines, NOS2 and P2X7 in the Diabetic Retina

Purpose

CD40 is an upstream inducer of inflammation in the diabetic retina. CD40 is upregulated in retinal endothelial cells in diabetes. The purpose of this study was to determine whether expression of CD40 in endothelial cells is sufficient to promote inflammatory responses in the retina of diabetic mice.

Methods

Transgenic mice with CD40 expression restricted to endothelial cells (Trg-CD40 EC), transgenic control mice (Trg-Ctr), B6, and CD40^−/− mice were made diabetic using streptozotocin. Leukostasis was assessed using FITC-conjugated ConA. Pro-inflammatory molecule expression was examined by real-time PCR, immunohistochemistry, ELISA, or flow cytometry. Release of ATP was assessed by ATP bioluminescence.

Results

Diabetic B6 and Trg-CD40 EC mice exhibited increased retinal mRNA levels of ICAM-1, higher ICAM-1 expression in endothelial cells, and increased leukostasis. These responses were not detected in diabetic mice that lacked CD40 (CD40^−/− and Trg-Ctr). Diabetic B6 but not Trg-CD40 EC mice upregulated TNF-α, IL-1β, and NOS2 mRNA levels. CD40 stimulation in retinal endothelial cells upregulated ICAM-1 but not TNF-α, IL-1β, or NOS2. CD40 ligation did not trigger ATP release by retinal endothelial cells or pro-inflammatory cytokine production in bystander myeloid cells. In contrast to diabetic B6 mice, diabetic Trg-CD40 EC mice did not upregulate P2X₇ mRNA levels in the retina.

Conclusions

Endothelial cell CD40 promotes ICAM-1 upregulation and leukostasis. In contrast, endothelial cell CD40 does not lead to pro-inflammatory cytokine and NOS2 upregulation likely because it does not activate purinergic-mediated pro-inflammatory molecule expression by myeloid cells or induce expression of these pro-inflammatory molecules in endothelial cells.

P2X7 Receptors: An Untapped Target for the Management of Cardiovascular Disease

Chronic low-grade inflammation contributes to the development of several diseases, including cardiovascular disease. Adequate strategies to target inflammation in cardiovascular disease are in their infancy and remain an avenue of great interest. The purinergic receptor P2X7 is a ubiquitously expressed receptor that predominately mediates inflammation and cellular death. P2X7 is a ligand-gated cation channel that is activated in response to high concentrations of extracellular ATP, triggering the assembly and activation of the NLRP3 (nuclear oligomerization domain like receptor family pyrin domain containing 3) inflammasome and subsequent release of proinflammatory cytokines IL (interleukin)-1β and IL-18. Increased P2X7 activation and IL-1β and IL-18 concentrations have been implicated in the development of many cardiovascular conditions including hypertension, atherosclerosis, ischemia/reperfusion injury, and heart failure. P2X7 receptor KO (knockout) mice exhibit a significant attenuation of the inflammatory response, which corresponds with reduced disease severity. P2X7 antagonism blunts blood pressure elevation in hypertension and progression of atherosclerosis in animal models. IL-1β and IL-18 inhibition has shown efficacy in clinical trials reducing major adverse cardiac events, including myocardial infarction, and heart failure. With several P2X7 antagonists available with proven safety margins, P2X7 antagonism could represent an untapped potential for therapeutic intervention in cardiovascular disorders.

The purinergic P2X7 receptor as a potential drug target to combat neuroinflammation in neurodegenerative diseases

Neurodegenerative diseases (NDDs) represent a huge social burden, particularly in Alzheimer's disease (AD) in which all proposed treatments investigated in murine models have failed during clinical trials (CTs). Thus, novel therapeutic strategies remain crucial. Neuroinflammation is a common pathogenic feature of NDDs. As purinergic P2X7 receptors (P2X7Rs) are gatekeepers of inflammation, they could be developed as drug targets for NDDs. Herein, we review this challenging hypothesis and comment on the numerous studies that have investigated P2X7Rs, emphasizing their molecular structure and functions, as well as their role in inflammation. Then, we elaborate on research undertaken in the field of medicinal chemistry to determine potential P2X7R antagonists. Subsequently, we review the state of neuroinflammation and P2X7R expression in the brain, in animal models and patients suffering from AD, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, multiple sclerosis, and retinal degeneration. Next, we summarize the in vivo studies testing the hypothesis that by mitigating neuroinflammation, P2X7R blockers afford neuroprotection, increasing neuroplasticity and neuronal repair in animal models of NDDs. Finally, we reviewed previous and ongoing CTs investigating compounds directed toward targets associated with NDDs; we propose that CTs with P2X7R antagonists should be initiated. Despite the high expectations for putative P2X7Rs antagonists in various central nervous system diseases, the field is moving forward at a relatively slow pace, presumably due to the complexity of P2X7Rs. A better pharmacological approach to combat NDDs would be a dual strategy, combining P2X7R antagonism with drugs targeting a selective pathway in a given NDD.

Production, characterization, and application of a monoclonal antibody specific for the extracellular domain of human P2X7R

This paper focuses on the production of a high-affinity monoclonal antibody (mAb) that can efficiently detect and block purinergic ligand-gated ion channel 7 receptor (P2X7R). To achieve this goal, the extracellular domain of human P2X7R, P2X7R-ECD, was used as an immunogen for BALB/c mice, inducing them to produce spleen lymphocytes that were subsequently fused with myeloma cells. Screening of the resultant hybridoma clones resulted in the selection of one stable positive clone that produced a qualified mAb, named 4B3A4. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated that the purity of the purified 4B3A4 mAb was above 85%, with prominent bands corresponding to molecular weights of 55 kDa (heavy chain) and 25 kDa (light chain), and the BCA assay showed that the concentration of the purified 4B3A4 mAb was 0.3 mg/mL. Western blot analysis revealed that the 4B3A4 mAb could specifically recognize and bind both P2X7R-ECD and the full-length P2X7R protein. Laser scanning confocal microscopy (LSCM) revealed that the 4B3A4 mAb specifically bound to P2X7R on the membrane of human peripheral blood mononuclear cells (PBMCs). P2X7R expression was significantly different between healthy individuals and people with certain cancers as determined by flow cytometry (FCM). In addition, the 4B3A4 mAb significantly reduced ATP-stimulated Ca²⁺ entry and YO-PRO-1 uptake, which indicated that the 4B3A4 mAb effectively blocked P2X7R activity. These data indicate that the 4B3A4 mAb can be further used as not only an antibody to detect cell surface P2X7R but also as a therapeutic antibody to target P2X7R-related signaling pathways.

Endothelial Dysfunction in Diabetic Retinopathy

Diabetic retinopathy (DR) is a diabetic complication which affects retinal function and results in severe loss of vision and relevant retinal diseases. Retinal vascular dysfunction caused by multifactors, such as advanced glycosylation end products and receptors, pro-inflammatory cytokines and chemokines, proliferator-activated receptor-γ disruption, growth factors, oxidative stress, and microRNA. These factors promote retinal endothelial dysfunction, which results in the development of DR. In this review, we summarize the contributors in the pathophysiology of DR for a better understanding of the molecular and cellular mechanism in the development of DR with a special emphasis on retinal endothelial dysfunction.

The CD40-ATP-P2X 7 Receptor Pathway: Cell to Cell Cross-Talk to Promote Inflammation and Programmed Cell Death of Endothelial Cells

Extracellular adenosine 5′-triphosphate (ATP) functions not only as a neurotransmitter but is also released by non-excitable cells and mediates cell–cell communication involving glia. In pathological conditions, extracellular ATP released by astrocytes may act as a “danger” signal that activates microglia and promotes neuroinflammation. This review summarizes in vitro and in vivo studies that identified CD40 as a novel trigger of ATP release and purinergic-induced inflammation. The use of transgenic mice with expression of CD40 restricted to retinal Müller glia and a model of diabetic retinopathy (a disease where the CD40 pathway is activated) established that CD40 induces release of ATP in Müller glia and triggers 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.

Ion channels as therapeutic antibody targets

It is now well established that antibodies have numerous potential benefits when developed as therapeutics. Here, we evaluate the technical challenges of raising antibodies to membrane-spanning proteins together with enabling technologies that may facilitate the discovery of antibody therapeutics to ion channels. Additionally, we discuss the potential targeting opportunities in the anti-ion channel antibody landscape, along with a number of case studies where functional antibodies that target ion channels have been reported. Antibodies currently in development and progressing towards the clinic are highlighted.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

P2X7 as a scavenger receptor for innate phagocytosis in the brain

The P2X7 receptor has been widely studied for its ATP-induced pro-inflammatory effect, but in the absence of a ligand, P2X7 has a second function as a scavenger receptor, which is active in the development of the human brain. The scavenger activity of P2X7 is only evident in the absence of serum but is fully active in cerebrospinal fluid. P2X7 on the cell surface is present as a membrane complex, and an attachment to non-muscle myosin of the cytoskeleton is required for particle engulfment. Selective antagonists of P2X7 pro-inflammatory function have little effect on phagocytosis, but inheritance of a variant haplotype spanning the P2RX7 and P2RX4 genes has been associated with loss of P2X7-mediated phagocytosis. Recent studies in mice suggest that the innate phagocytosis mediated by P2X7 receptors declines with ageing. Thus, defective P2X7-mediated phagocytosis may contribute to age-related neuro-degenerative diseases including Alzheimer's disease, age-related macular degeneration and primary progressive multiple sclerosis.

Sweet Stress: Coping With Vascular Dysfunction in Diabetic Retinopathy

Oxidative stress plays key roles in the pathogenesis of retinal diseases, such as diabetic retinopathy. Reactive oxygen species (ROS) are increased in the retina in diabetes and the antioxidant defense system is also compromised. Increased ROS stimulate the release of pro-inflammatory cytokines, promoting a chronic low-grade inflammation involving various signaling pathways. An excessive production of ROS can lead to retinal endothelial cell injury, increased microvascular permeability, and recruitment of inflammatory cells at the site of inflammation. Recent studies have started unraveling the complex crosstalk between retinal endothelial cells and neuroglial cells or leukocytes, via both cell-to-cell contact and secretion of cytokines. This crosstalk is essential for the maintenance of the integrity of retinal vascular structure. Under diabetic conditions, an aberrant interaction between endothelial cells and other resident cells of the retina or invading inflammatory cells takes place in the retina. Impairment in the secretion and flow of molecular signals between different cells can compromise the retinal vascular architecture and trigger angiogenesis. In this review, the synergistic contributions of redox-inflammatory processes for endothelial dysfunction in diabetic retinopathy will be examined, with particular attention paid to endothelial cell communication with other retinal cells.

The P2X7 purinergic receptor: An emerging therapeutic target in cardiovascular diseases

The P2X7 purinergic receptor, a calcium permeable cationic channel, is activated by extracellular ATP. Most studies show that P2X7 receptor plays an important role in the nervous system diseases, immune response, osteoporosis and cancer. Mounting evidence indicates that P2X7 receptor is also associated with cardiovascular disease. For example, the P2X7 receptor activated by ATP can attenuate myocardial ischemia-reperfusion injury. By contrast, inhibition of P2X7 receptor decreases arrhythmia after myocardial infarction, prolongs cardiac survival after a long term heart transplant, alleviates the dilated cardiomyopathy and the autoimmune myocarditis process. The P2X7 receptor also mitigates vascular diseases including atherosclerosis, hypertension, thrombosis and diabetic retinopathy. This review focuses on the latest research on the role and therapeutic potential of P2X7 receptor in cardiovascular diseases.

CD40, a Novel Inducer of Purinergic Signaling: Implications to the Pathogenesis of Experimental Diabetic Retinopathy

Diabetic retinopathy is a leading complication of diabetes. Death of capillary cells with resulting capillary degeneration is a central feature of this disease. Chronic low-grade inflammation has been linked to the development of retinal capillary degeneration in diabetes. CD40 is an upstream inducer of a broad range of inflammatory responses in the diabetic retina and is required for death of retinal capillary cells. Recent studies uncovered CD40 as a novel inducer of purinergic signaling and identified the CD40-ATP-P2X₇ pathway as having a key role in the induction of inflammation in the diabetic retina and programmed cell death of retinal endothelial cells.