CaMKII regulates pericyte loss in the retina of early diabetic mouse

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.

Unveiling the role of CaMKII in retinal degeneration: from biological mechanism to therapeutic strategies

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a family of broad substrate specificity serine (Ser)/threonine (Thr) protein kinases that play a crucial role in the Ca2+-dependent signaling pathways. Its significance as an intracellular Ca2+ sensor has garnered abundant research interest in the domain of neurodegeneration. Accumulating evidences suggest that CaMKII is implicated in the pathology of degenerative retinopathies such as diabetic retinopathy (DR), age-related macular degeneration (AMD), retinitis pigmentosa (RP) and glaucoma optic neuropathy. CaMKII can induce the aberrant proliferation of retinal blood vessels, influence the synaptic signaling, and exert dual effects on the survival of retinal ganglion cells and pigment epithelial cells. Researchers have put forth multiple therapeutic agents, encompassing small molecules, peptides, and nucleotides that possess the capability to modulate CaMKII activity. Due to its broad range isoforms and splice variants therapeutic strategies seek to inhibit specifically the CaMKII are confronted with considerable challenges. Therefore, it becomes crucial to discern the detrimental and advantageous aspects of CaMKII, thereby facilitating the development of efficacious treatment. In this review, we summarize recent research findings on the cellular and molecular biology of CaMKII, with special emphasis on its metabolic and regulatory mechanisms. We delve into the involvement of CaMKII in the retinal signal transduction pathways and discuss the correlation between CaMKII and calcium overload. Furthermore, we elaborate the therapeutic trials targeting CaMKII, and introduce recent developments in the zone of CaMKII inhibitors. These findings would enrich our knowledge of CaMKII, and shed light on the development of a therapeutic target for degenerative retinopathy.

Is CaMKII friend or foe for cell apoptosis in eye?: A narrative review

Ca2+/calmodulin-dependent protein kinase II (CaMKII) controls cell proliferation, differentiation, apoptosis, and other biological processes that have an essential role in eye diseases. However, it seems that previous studies have generated conflicting conclusions about the effect of CaMKII on cell apoptosis. In this review, we explore the positive and potentially deleterious effects of CaMKII on eye cell apoptosis. We can safely conclude that the early elevation of CaMKII could be viewed as a promoter of cell apoptosis. Overexpression of CaMKII by transfection or pretreatment with drugs helped combat cell apoptosis.

Protection of leukemia inhibitory factor against high-glucose-induced human retinal endothelial cell dysfunction

PURPOSE

In the study, we aimed to explore the mechanism of leukaemia inhibitory factor (LIF) affects hyperglycaemic induced retinopathy by regulating CaMKII-CREB pathway.

METHODS

Human retinal endothelial cell (HRECs) induced by high glucose to simulate one of the pathogenesis in the diabetic retinopathy (DR) model. After LIF treatment, cell viability was detected by CCK-8 and apoptosis was detected by flow cytometry. Angiogenesis was detected by in vitro tube formation. The expression levels of inflammatory, angiogenesis related proteins and CaMKII-CREB were detected by western blot. The gene level of angiogenesis was detected by qRT-PCR. HE staining was used to detect pathological changes of retinopathy in diabetic mice after LIF treatment.

RESULTS

Our results showed that LIF significantly increased hyperglycaemic-induced cell viability and inhibited apoptosis. Western blot results showed that LIF could down-regulate the expression levels of inflammatory cytokines such as IL-1β, IL-6 and TNF-α. In addition, angiogenesis of HRECs was inhibited by LIF in tubulisation experiments. LIF can down-regulate protein and gene levels of VEGF and HIF-1α via western blot and qRT-PCR. In diabetic mice induced by STZ, LIF could down-regulate the protein level of VEGF, HIF-1α, p-CaMKII and p-CREB, which suggest that LIF could inhibit retinal angiogenesis in diabetic mice. The results of HE staining showed that LIF could alleviate the damage of retinopathy in diabetic mice.

CONCLUSION

LIF could alleviate the damage of diabetic retinopathy by modulating the CaMKII/CREB signalling pathway to inhibit inflammatory response and angiogenesis.

Neuronal Epac1 mediates retinal neurodegeneration in mouse models of ocular hypertension

Progressive loss of retinal ganglion cells (RGCs) leads to irreversible visual deficits in glaucoma. Here, we found that the level of cyclic AMP and the activity and expression of its mediator Epac1 were increased in retinas of two mouse models of ocular hypertension. Genetic depletion of Epac1 significantly attenuated ocular hypertension–induced detrimental effects in the retina, including vascular inflammation, neuronal apoptosis and necroptosis, thinning of ganglion cell complex layer, RGC loss, and retinal neuronal dysfunction. With bone marrow transplantation and various Epac1 conditional knockout mice, we further demonstrated that Epac1 in retinal neuronal cells (especially RGCs) was responsible for their death. Consistently, pharmacologic inhibition of Epac activity prevented RGC loss. Moreover, in vitro study on primary RGCs showed that Epac1 activation was sufficient to induce RGC death, which was mechanistically mediated by CaMKII activation. Taken together, these findings indicate that neuronal Epac1 plays a critical role in retinal neurodegeneration and suggest that Epac1 could be considered a target for neuroprotection in glaucoma.

Cathepsin D plays a role in endothelial-pericyte interactions during alteration of the blood-retinal barrier in diabetic retinopathy

Inflammation plays an important role in the pathogenesis of diabetic retinopathy. We have previously demonstrated the effect of cathepsin D (CD) on the mechanical disruption of retinal endothelial cell junctions and increased vasopermeability, as well as increased levels of CD in retinas of diabetic mice. Here, we have also examined the effect of CD on endothelial-pericyte interactions, as well as the effect of dipeptidyl peptidase-4 (DPP4) inhibitor on CD in endothelial-pericyte interactions in vitro and in vivo. Cocultured cells that were treated with pro-CD demonstrated a significant decrease in the expression of platelet-derived growth factor receptor-β, a tyrosine kinase receptor that is required for pericyte cell survival; N-cadherin, the key adherens junction protein between endothelium and pericytes; and increases in the vessel destabilizing agent, angiopoietin-2. The effect was reversed in cells that were treated with DPP4 inhibitor along with pro-CD. With pro-CD treatment, there was a significant increase in the phosphorylation of the downstream signaling protein, PKC-α, and Ca²⁺/calmodulin-dependent protein kinase II in endothelial cells and pericytes, which disrupts adherens junction structure and function, and this was significantly reduced with DPP4 inhibitor treatment. Increased CD levels, vasopermeability, and alteration in junctional-related proteins were observed in the retinas of diabetic rats, which were significantly changed with DPP4 inhibitor treatment. Thus, DPP4 inhibitors may be used as potential adjuvant therapeutic agents to treat increased vascular leakage observed in patients with diabetic macular edema.-Monickaraj, F., McGuire, P., Das, A. Cathepsin D plays a role in endothelial-pericyte interactions during alteration of the blood-retinal barrier in diabetic retinopathy.

Mechanisms of macular edema: Beyond the surface

Macular edema consists of intra- or subretinal fluid accumulation in the macular region. It occurs during the course of numerous retinal disorders and can cause severe impairment of central vision. Major causes of macular edema include diabetes, branch and central retinal vein occlusion, choroidal neovascularization, posterior uveitis, postoperative inflammation and central serous chorioretinopathy. The healthy retina is maintained in a relatively dehydrated, transparent state compatible with optimal light transmission by multiple active and passive systems. Fluid accumulation results from an imbalance between processes governing fluid entry and exit, and is driven by Starling equation when inner or outer blood-retinal barriers are disrupted. The multiple and intricate mechanisms involved in retinal hydro-ionic homeostasis, their molecular and cellular basis, and how their deregulation lead to retinal edema, are addressed in this review. Analyzing the distribution of junction proteins and water channels in the human macula, several hypotheses are raised to explain why edema forms specifically in the macular region. "Pure" clinical phenotypes of macular edema, that result presumably from a single causative mechanism, are detailed. Finally, diabetic macular edema is investigated, as a complex multifactorial pathogenic example. This comprehensive review on the current understanding of macular edema and its mechanisms opens perspectives to identify new preventive and therapeutic strategies for this sight-threatening condition.

Gene therapy for diabetic retinopathy: Are we ready to make the leap from bench to bedside?

Diabetic retinopathy (DR), a chronic and progressive complication of diabetes mellitus, is a sight-threatening disease characterized in the early stages by neuronal and vascular dysfunction in the retina, and later by neovascularization that further damages vision. A major contributor to the pathology is excess production of vascular endothelial growth factor (VEGF), a growth factor that induces formation of new blood vessels and increases permeability of existing vessels. Despite the recent availability of effective treatments for the disease, including laser photocoagulation and therapeutic VEGF antibodies, DR remains a significant cause of vision loss worldwide. Existing anti-VEGF agents, though generally effective, are limited by their short therapeutic half-lives, necessitating frequent intravitreal injections and the risk of attendant adverse events. Management of DR with gene therapies has been proposed for several years, and pre-clinical studies have yielded enticing findings. Gene therapy holds several advantages over conventional treatments for DR, such as a longer duration of therapeutic effect, simpler administration, the ability to intervene at an earlier stage of the disease, and potentially fewer side-effects. In this review, we summarize the current understanding of the pathophysiology of DR and provide an overview of research into DR gene therapies. We also examine current barriers to the clinical application of gene therapy for DR and evaluate future prospects for this approach.

Curcumin Inhibits Neuronal Loss in the Retina and Elevates Ca²⁺/Calmodulin-Dependent Protein Kinase II Activity in Diabetic Rats

PURPOSE

To determine whether curcumin offers neuroprotection to minimize the apoptosis of neural cells in the retina of diabetic rats.

METHODS

Streptozotocin (STZ)-induced diabetic rats and control rats were used in this study. A subgroup of STZ-induced diabetic rats were treated with curcumin for 12 weeks. Retinal histology, apoptosis of neural cells in the retina, electroretinograms, and retinal glutamate content were evaluated after 12 weeks. Retinal levels of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), phospho-CaMKII (p-CaMKII), and cleaved caspase-3 were determined by Western blot analysis.

RESULTS

The amplitudes a-wave, b-wave, and oscillatory potential were reduced by diabetes, but curcumin treatment suppressed this reduction of amplitudes. Curcumin also prevented cell loss from the outer nuclear, inner nuclear, and ganglion cell layers. Apoptosis of retinal neurons was detected in diabetic rats. The concentration of glutamate in the retina was higher in diabetic rats, but was significantly reduced in the curcumin-treated group. Furthermore, p-CaMKII and cleaved caspase-3 expression were upregulated in the diabetic retina, but reduced in curcumin-treated rats.

CONCLUSIONS

Curcumin attenuated diabetes-induced apoptosis in retinal neurons by reducing the glutamate level and downregulating CaMKII. Thus, curcumin might be used to prevent neuronal damage in the retina of patients with diabetes mellitus.

Combined CpG and poly I:C stimulation of monocytes results in unique signaling activation not observed with the individual ligands

Toll-like receptors (TLRs) bind to components of microbes, activate cellular signal transduction pathways and stimulate innate immune responses. Previously, we have shown in chicken monocytes that the combination of CpG, the ligand for TLR21 (the chicken equivalent of TLR9), and poly I:C, the ligand for TLR3, results in a synergistic immune response. In order to further characterize this synergy, kinome analysis was performed on chicken monocytes stimulated with either unmethylated CpG oligodeoxynucleotides (CpG) and polyinosinic-polycytidylic acid (poly I:C) individually or in combination for either 1h or 4h. The analysis was carried out using chicken species-specific peptide arrays to study the kinase activity induced by the two ligands. The arrays are comprised of kinase target sequences immobilized on an array surface. Active kinases phosphorylate their respective target sequences, and these phosphorylated peptides are then visualized and quantified. A significant number of peptides exhibited altered phosphorylation when CpG and poly I:C were given together, that was not observed when either CpG or poly I:C was given separately. The unique, synergistic TLR agonist affected peptides represent protein members of signaling pathways including calcium signaling pathway, cytokine-cytokine receptor interaction and Endocytosis at the 1h time point. At the 4h time point, TLR agonist synergy influenced pathways included Adipocytokine signaling pathway, cell cycle, calcium signaling pathway, NOD-like receptor signaling pathway and RIG-I-like receptor signaling pathway. Using nitric oxide (NO) production as the readout, TLR ligand synergy was also investigated using signaling protein inhibitors. A number of inhibitors were able to inhibit NO response in cells given CpG alone but not in cells given both CpG and poly I:C, as poly I:C alone does not elicit a significant NO response. The unique peptide phosphorylation induced by the combination of CpG and poly I:C and the unique signaling protein requirements for synergy determined by inhibitor assays both show that synergistic signaling is not a simple addition of TLR pathways. A set of secondary pathways activated by the ligand combination are proposed, leading to the activation of cAMP response element-binding protein (CREB), nuclear factor κB (NFκB) and ultimately of inducible nitric oxide synthase (iNOS). Since many microbes can stimulate more than one TLR, this synergistic influence on cellular signaling may be an important consideration for the study of immune response and what we consider to be the canonical TLR signaling pathways.

Reduction of experimental diabetic vascular leakage and pericyte apoptosis in mice by delivery of αA-crystallin with a recombinant adenovirus

AIMS/HYPOTHESIS

The study aimed to evaluate the efficacy of recombinant adenovirus expressing αA-crystallin (Ad-αAc-Gfp) in reducing pericyte loss within retinal vasculature in early diabetes.

METHODS

Diabetes was induced by streptozotocin injection into C57BL/6 mice. Ad-αAc-Gfp was delivered by intravitreous injection to the right eyes of mice 2 weeks before induction of diabetes. Vascular leakage was determined by fluorescent angiography, Evans Blue leakage assay and leucocyte adhesion test. Production of αA-crystallin was analysed by immunoblotting and double immunostaining and pericyte loss was analysed by pericyte count.

RESULTS

Vessel leakage and pericyte loss were observed in the streptozotocin-induced diabetic retina. Decreased abundance of αA-crystallin in retinas 2 and 6 months after the induction of diabetes was confirmed by two-dimensional electrophoretic analysis, immunoblotting and RT-PCR. Double immunofluorescence staining for αA-crystallin and NG2 chondroitin sulphate proteoglycan revealed that αA-crystallin was predominantly produced in the retinal pericyte and that the number of αA-crystallin-producing pericytes decreased in the diabetic retina. Retinal infection with Ad-αAc-Gfp led to decreased pericyte loss and vascular leakage compared with control.

CONCLUSIONS/INTERPRETATION

Intravitreal delivery of Ad-αAc-Gfp protects against vascular leakage in the streptozotocin-induced model of diabetes. This effect is associated with the inhibition of diabetic retinal pericyte loss in early diabetes, suggesting that αA-crystallin has a role in preventing the pathogenesis of early diabetic retinopathy.

Calcium entry mediates hyperglycemia-induced apoptosis through Ca(2+)/calmodulin-dependent kinase II in retinal capillary endothelial cells

PURPOSE

Hyperglycemia-induced vascular cell apoptosis is a seminal early event in diabetic retinopathy. Prolonged hyperglycemia is known to increase intracellular cytosolic free calcium ([Ca(2+)]i) in retinal vascular endothelial cells (RECs), suggesting that [Ca(2+)]i is a critical trigger for microvascular degeneration. This study aims to elucidate Ca(2+)-dependent signaling mechanisms that mediate hyperglycemia-induced apoptosis in RECs.

METHODS

A cultured macaque choroid-retinal endothelial cell line (RF/6A) was incubated in normal glucose (NG), NG plus the Ca(2+) entry blocker 2-aminoethoxydiphenyl borate (2-APB), high glucose (HG), or HG plus either 2-APB, the c-jun N-terminal kinase (JNK) inhibitor SP600125, or the calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93. Changes in [Ca(2+)]i evoked by adenosine 5'-triphosphate (ATP) were measured in fluo-3/AM-loaded RF/6A cells by confocal microscopy. The mitochondrial membrane potential (ΔΨm) and apoptosis were assessed by flow cytometry. Expression levels of CaMKII, phosphorylated CaMKII (p-CaMKII), c-Jun N-terminal kinase (JNK), phosphorylated JNK (p-JNK), the death receptor (Fas), and cytochrome c were detected by western blotting analysis.

RESULTS

Prolonged exposure to HG (96 h) potentiated ATP-evoked Ca(2+) entry as well as CaMKII phosphorylation and RF/6A cell apoptosis. Enhanced apoptosis was blocked by 2-APB and KN93. Furthermore, HG increased JNK phosphorylation and Fas expression, and both responses were partially blocked by 2-APB and KN93, while the JNK inhibitor SP600125 partially reduced HG-induced Fas expression. In addition, HG depolarized the ΔΨm and triggered the release of mitochondrial cytochrome c. These early signs of mitochondria-dependent apoptosis were partially reversed by 2-APB and KN93.

CONCLUSIONS

HG-induced apoptosis in RF/6A cells depends on Ca(2+) entry and CaMKII activation, leading to the activation of both Fas-dependent and mitochondria-dependent apoptosis pathways. The CaMKII-JNK-Fas pathway is involved in HG-evoked apoptosis of RECs.