Plasma kallikrein mediates retinal vascular dysfunction and induces retinal thickening in diabetic rats

Rationale of Basic and Cellular Mechanisms Considered in Updating the Staging System for Diabetic Retinal Disease

Purpose

Hyperglycemia is a major risk factor for early lesions of diabetic retinal disease (DRD). Updating the DRD staging system to incorporate relevant basic and cellular mechanisms pertinent to DRD is necessary to better address early disease, disease progression, the use of therapeutic interventions, and treatment effectiveness.

Design

We sought to review preclinical and clinical evidence on basic and cellular mechanisms potentially pertinent to DRD that might eventually be relevant to update the DRD staging system.

Participants

Not applicable.

Methods

The Basic and Cellular Mechanisms Working Group (BCM-WG) of the Mary Tyler Moore Vision Initiative carefully and extensively reviewed available preclinical and clinical evidence through multiple iterations and classified these.

Main Outcome Measures

Classification was made into evidence grids, level of supporting evidence, and anticipated future relevance to DRD.

Results

A total of 40 identified targets based on pathophysiology and other parameters for DRD were grouped into concepts or evaluated as specific candidates. VEGFA, peroxisome proliferator-activated receptor-alpha related pathways, plasma kallikrein, and angiopoietin 2 had strong agreement as promising for use as biomarkers in diagnostic, monitoring, predictive, prognostic, and pharmacodynamic responses as well as for susceptibility/risk biomarkers that could underlie new assessments and eventually be considered within an updated DRD staging system or treatment, based on the evidence and need for research that would fit within a 2-year timeline. The BCM-WG found there was strong reason also to pursue the following important concepts regarding scientific research of DRD acknowledging their regulation by hyperglycemia: inflammatory/cytokines, oxidative signaling, vasoprotection, neuroprotection, mitophagy, and nutrients/microbiome.

Conclusion

Promising targets that might eventually be considered within an updated DRD staging system or treatment were identified. Although the BCM-WG recognizes that at this stage little can be incorporated into a new DRD staging system, numerous potential targets and important concepts deserve continued support and research, as they may eventually serve as biomarkers and/or therapeutic targets with measurable benefits to patients with diabetes.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

The Complement System as a Therapeutic Target in Retinal Disease

The complement cascade is a vital system in the human body's defense against pathogens. During the natural aging process, it has been observed that this system is imperative for ensuring the integrity and homeostasis of the retina. While this system is critical for proper host defense and retinal integrity, it has also been found that dysregulation of this system may lead to certain retinal pathologies, including geographic atrophy and diabetic retinopathy. Targeting components of the complement system for retinal diseases has been an area of interest, and in vivo, ex vivo, and clinical trials have been conducted in this area. Following clinical trials, medications targeting the complement system for retinal disease have also become available. In this manuscript, we discuss the pathophysiology of complement dysfunction in the retina and specific pathologies. We then describe the results of cellular, animal, and clinical studies targeting the complement system for retinal diseases. We then provide an overview of complement inhibitors that have been approved by the Food and Drug Administration (FDA) for geographic atrophy. The complement system in retinal diseases continues to serve as an emerging therapeutic target, and further research in this field will provide additional insights into the mechanisms and considerations for treatment of retinal pathologies.

Effectiveness of carbonic anhydrase inhibitor loaded nanoparticles in the treatment of diabetic retinopathy

Diabetic Retinopathy (DRP) is a disease consisting of all the structural and functional changes that develop in the retinal layer of the eye due to diabetes. DRP is the most important cause of blindness between the ages of 20-74 in the world, and the most successful standard treatment option in the treatment of DRP is intravitreal injections. To synthesize acetazolamide loaded nanoparticles to be applied intravitreal treatment of DRP and to examine thein vitroefficacy of the nanoparticles. ACZ loaded PHBV nanoparticles (PHBV-ACZ NPs) formulations were prepared. Nanoparticles with a particle size of 253.20 ± 0.55 nm. A DRP model was established and characterized in HRMEC cells. The effect of the nanoparticles on permeability has been investigated and carrier proteins in BRB due to the development of DRP has been investigated. To establish thein vitroDRP model, HRMEC was stimulated with Recombinant human 165 Vascular Endothelial Growth Factor (VEGF), thereby temporarily reducing the expression levels of endothelial junction proteins, increasing the number of intercellular spaces in the monolayers of HRMECs. It was determined that after the cells were exposed to Carbonic anhydrase inhibitors (CAI) loaded nanoparticles, permeability decreased and protein expression increased.

Tocotrienol-rich fraction reduces retinal inflammation and angiogenesis in rats with streptozotocin-induced diabetes

BACKGROUND

Diabetic retinopathy (DR) is the second commonest microvascular complication of diabetes mellitus. It is characterized by chronic inflammation and angiogenesis. Palm oil-derived tocotrienol-rich fraction (TRF), a substance with anti-inflammatory and anti-angiogenic properties, may provide protection against DR development. Therefore, in this study, we investigated the effect of TRF on retinal vascular and morphological changes in diabetic rats. The effects of TRF on the retinal expression of inflammatory and angiogenic markers were also studied in the streptozotocin (STZ)-induced diabetic rats.

METHODS

Male Sprague Dawley rats weighing 200-250 g were grouped into normal rats (N) and diabetic rats. Diabetes was induced by intraperitoneal injection of streptozotocin (55 mg/kg body weight) whereas N similarly received citrate buffer. STZ-injected rats with blood glucose of more than 20 mmol/L were considered diabetic and were divided into vehicle-treated (DV) and TRF-treated (DT) groups. N and DV received vehicle, whereas DT received TRF (100 mg/kg body weight) via oral gavage once daily for 12 weeks. Fundus images were captured at week 0 (baseline), week 6 and week 12 post-STZ induction to estimate vascular diameters. At the end of experimental period, rats were euthanized, and retinal tissues were collected for morphometric analysis and measurement of NFκB, phospho-NFκB (Ser536), HIF-1α using immunohistochemistry (IHC) and enzyme-linked immunosorbent assay (ELISA). Retinal inflammatory and angiogenic cytokines expression were measured by ELISA and real-time quantitative PCR.

RESULTS

TRF preserved the retinal layer thickness (GCL, IPL, INL and OR; p < 0.05) and retinal venous diameter (p < 0.001). TRF also lowered the retinal NFκB activation (p < 0.05) as well as expressions of IL-1β, IL-6, TNF-α, IFN-γ, iNOS and MCP-1 (p < 0.05) compared to vehicle-treated diabetic rats. Moreover, TRF also reduced retinal expression of VEGF (p < 0.001), IGF-1 (p < 0.001) and HIF-1α (p < 0.05) compared to vehicle-treated rats with diabetes.

CONCLUSION

Oral TRF provided protection against retinal inflammation and angiogenesis in rats with STZ-induced diabetes by suppressing the expression of the markers of retinal inflammation and angiogenesis.

Current and Novel Therapeutic Approaches for Treatment of Diabetic Macular Edema

Diabetic macular edema (DME) is a major ocular complication of diabetes mellitus (DM), leading to significant visual impairment. DME’s pathogenesis is multifactorial. Focal edema tends to occur when primary metabolic abnormalities lead to a persistent hyperglycemic state, causing the development of microaneurysms, often with extravascular lipoprotein in a circinate pattern around the focal leakage. On the other hand, diffusion edema is due to a generalized breakdown of the inner blood–retinal barrier, leading to profuse early leakage from the entire capillary bed of the posterior pole with the subsequent extravasation of fluid into the extracellular space. The pathogenesis of DME occurs through the interaction of multiple molecular mediators, including the overexpression of several growth factors, including vascular endothelial growth factor (VEGF), insulin-like growth factor-1, angiopoietin-1, and -2, stromal-derived factor-1, fibroblast growth factor-2, and tumor necrosis factor. Synergistically, these growth factors mediate angiogenesis, protease production, endothelial cell proliferation, and migration. Treatment for DME generally involves primary management of DM, laser photocoagulation, and pharmacotherapeutics targeting mediators, namely, the anti-VEGF pathway. The emergence of anti-VEGF therapies has resulted in significant clinical improvements compared to laser therapy alone. However, multiple factors influencing the visual outcome after anti-VEGF treatment and the presence of anti-VEGF non-responders have necessitated the development of new pharmacotherapies. In this review, we explore the pathophysiology of DME and current management strategies. In addition, we provide a comprehensive analysis of emerging therapeutic approaches to the treatment of DME.

Phase 1 Dose-Escalation Study of Plasma Kallikrein Inhibitor THR-149 for the Treatment of Diabetic Macular Edema

Purpose

The purpose of this study was to evaluate the safety and preliminary efficacy of a single intravitreal injection of 3 dose levels of THR-149 in adults with center-involved diabetic macular edema (DME).

Methods

A phase 1, open-label, multicenter 3 + 3 dose-esclation study with 3-month follow-up. The primary endpoint was the incidence of dose-limiting toxicities (DLTs) up to and including the Day 14 visit. Additional key endpoints included the incidence of (serious) adverse events ([S]AEs), mean change from baseline in best-corrected visual acuity (BCVA) and central subfield thickness (CST), and additional imaging parameters on widefield fluorescein angiography and optical coherence tomography (OCT) angiography.

Results

Twelve subjects were treated: 3 subjects received THR-149 0.005 mg, 3 received 0.022 mg and 6 received 0.13 mg. Baseline ocular characteristics were balanced between subjects at each dose level. There were no DLTs or ocular SAEs, and all subjects completed the study. Six subjects experienced a total of 10 AEs in the study eye; 1 case of mild anterior chamber inflammation was deemed related to THR-149 and/or the injection procedure. Mean change from Baseline in BCVA was +7.5 Early Treatment of Diabetic Retinopathy Study (ETDRS) letters on Day 14, and +6.4 ETDRS letters by Month 3. CST was variable, and mean CST change from baseline was +30.0 µm at Month 3. There were no clinically meaningful changes in imaging parameters.

Conclusions

THR-149 was safe and well tolerated; preliminary efficacy in terms of BCVA improvement was observed.

Translational Relevance

This work bridges the gap between basic research and clinical care by providing first in human safety and preliminary efficacy data, supporting the further investigation of THR-149 as a potential treatment for DME.

Evans blue staining to detect deep blood vessels in peripheral retina for observing retinal pathology in early-stage diabetic rats

AIM

To observe and compare the statistical significance of superficial and deep vascular leakage in the pathological changes of the diabetic rats retina after the Evans blue (EB) perfusion, and utilize the modified whole-retina spreading method to make the slides while protecting the periphery of the retina.

METHODS

The Sprague-Dawley (SD) rats were randomly divided into 6 groups. Each group named as the normal groups for 4, 8, and 12wk and the diabetic groups for 4, 8, and 12wk. The EB was injected into the cardiovascular system of the rats at the different time points. The retina of each group was obtained for observation.

RESULTS

The superficial vascular leakage was found in all 6 groups. The size of leakage area of superficial retinal blood vessels was (0.54±0.23)%, (0.65±0.11)%, and (0.58±0.10)% in normal group. No notable leakage was found in the deep blood vessels [(0.03±0.04)%, (0.03±0.05)%, and (0.03±0.05)%]. The deep retinal vascular leakage was found in the peripheral retina of diabetic rats. The size of leakage area of superficial retinal blood vessels in diabetic group were (0.53±0.22)%, (0.69±0.16)%, and (0.52±0.11)%. The leakage areas of deep blood vessels were (0.54±0.50)%, (1.42±0.16)%, and (1.80±0.07)% at 4, 8, and 12wk, respectively. There was a statistically difference of the leakage area between the 8th week and the 4th week of diabetes group (P=0.003). The statistically significant difference between the diabetes and the control groups was noted at 4wk and 8wk (P<0.001).

CONCLUSION

The main retinal pathological changes of early-stage diabetic rats are the vascular leakage of the periphery of deep retina. Diabetic rats modeled after 8wk have semi-quantitative statistical difference compared with the normal rats, thus early intervention treatment research can start at this time point.

Targeting Plasma Kallikrein With a Novel Bicyclic Peptide Inhibitor (THR-149) Reduces Retinal Thickening in a Diabetic Rat Model

Purpose

To investigate the effect of plasma kallikrein (PKal)-inhibition by THR-149 on preventing key pathologies associated with diabetic macular edema (DME) in a rat model.

Methods

Following streptozotocin-induced diabetes, THR-149 or its vehicle was administered in the rat via either a single intravitreal injection or three consecutive intravitreal injections (with a 1-week interval; both, 12.5 µg/eye). At 4 weeks post-diabetes, the effect of all groups was compared by histological analysis of Iba1-positive retinal inflammatory cells, inflammatory cytokines, vimentin-positive Müller cells, inwardly rectifying potassium and water homeostasis-related channels (Kir4.1 and AQP4, respectively), vascular leakage (fluorescein isothiocyanate-labeled bovine serum albumin), and retinal thickness.

Results

Single or repeated THR-149 injections resulted in reduced inflammation, as depicted by decreasing numbers and activation state of immune cells and IL-6 cytokine levels in the diabetic retina. The processes of reactive gliosis, vessel leakage, and retinal thickening were only significantly reduced after multiple THR-149 administrations. Individual retinal layer analysis showed that repeated THR-149 injections significantly decreased diabetes-induced thickening of the inner plexiform, inner nuclear, outer nuclear, and photoreceptor layers. At the glial-vascular interface, reduced Kir4.1-channel levels in the diabetic retina were restored to control non-diabetic levels in the presence of THR-149. In contrast, little or no effect of THR-149 was observed on the AQP4-channel levels.

Conclusions

These data demonstrate that repeated THR-149 administration reduces several DME-related key pathologies such as retinal thickening and neuropil disruption in the diabetic rat. These observations indicate that modulation of the PKal pathway using THR-149 has clinical potential to treat patients with DME.

Recent Advancements in the Medical Treatment of Diabetic Retinal Disease

Diabetic retinal disease remains one of the most common complications of diabetes mellitus (DM) and a leading cause of preventable blindness. The mainstay of management involves glycemic control, intravitreal, and laser therapy. However, intravitreal therapy commonly requires frequent hospital visits and some patients fail to achieve a significant improvement in vision. Novel and long-acting therapies targeting a range of pathways are warranted, while evidence to support optimal combinations of treatments is currently insufficient. Improved understanding of the molecular pathways involved in pathogenesis is driving the development of therapeutic agents not only targeting visible microvascular disease and metabolic derangements, but also inflammation and accelerated retinal neurodegeneration. This review summarizes the current and emerging treatments of diabetic retinal diseases and provides an insight into the future of managing this important condition.

Kinins and Their Receptors as Potential Therapeutic Targets in Retinal Pathologies

The kallikrein-kinin system (KKS) contributes to retinal inflammation and neovascularization, notably in diabetic retinopathy (DR) and neovascular age-related macular degeneration (AMD). Bradykinin type 1 (B1R) and type 2 (B2R) receptors are G-protein-coupled receptors that sense and mediate the effects of kinins. While B2R is constitutively expressed and regulates a plethora of physiological processes, B1R is almost undetectable under physiological conditions and contributes to pathological inflammation. Several KKS components (kininogens, tissue and plasma kallikreins, and kinin receptors) are overexpressed in human and animal models of retinal diseases, and their inhibition, particularly B1R, reduces inflammation and pathological neovascularization. In this review, we provide an overview of the KKS with emphasis on kinin receptors in the healthy retina and their detrimental roles in DR and AMD. We highlight the crosstalk between the KKS and the renin-angiotensin system (RAS), which is known to be detrimental in ocular pathologies. Targeting the KKS, particularly the B1R, is a promising therapy in retinal diseases, and B1R may represent an effector of the detrimental effects of RAS (Ang II-AT1R).

Diabetic retinopathy and diabetic macular oedema pathways and management: UK Consensus Working Group

The management of diabetic retinopathy (DR) has evolved considerably over the past decade, with the availability of new technologies (diagnostic and therapeutic). As such, the existing Royal College of Ophthalmologists DR Guidelines (2013) are outdated, and to the best of our knowledge are not under revision at present. Furthermore, there are no other UK guidelines covering all available treatments, and there seems to be significant variation around the UK in the management of diabetic macular oedema (DMO). This manuscript provides a summary of reviews the pathogenesis of DR and DMO, including role of vascular endothelial growth factor (VEGF) and non-VEGF cytokines, clinical grading/classification of DMO vis a vis current terminology (of centre-involving [CI-DMO], or non-centre involving [nCI-DMO], systemic risks and their management). The excellent UK DR Screening (DRS) service has continued to evolve and remains world-leading. However, challenges remain, as there are significant variations in equipment used, and reproducible standards of DMO screening nationally. The interphase between DRS and the hospital eye service can only be strengthened with further improvements. The role of modern technology including optical coherence tomography (OCT) and wide-field imaging, and working practices including virtual clinics and their potential in increasing clinic capacity and improving patient experiences and outcomes are discussed. Similarly, potential roles of home monitoring in diabetic eyes in the future are explored. The role of pharmacological (intravitreal injections [IVT] of anti-VEGFs and steroids) and laser therapies are summarised. Generally, IVT anti-VEGF are offered as first line pharmacologic therapy. As requirements of diabetic patients in particular patient groups may vary, including pregnant women, children, and persons with learning difficulties, it is important that DR management is personalised in such particular patient groups. First choice therapy needs to be individualised in these cases and may be intravitreal steroids rather than the standard choice of anti-VEGF agents. Some of these, but not all, are discussed in this document.

Current understanding of the molecular and cellular pathology of diabetic retinopathy

Diabetes mellitus has profound effects on multiple organ systems; however, the loss of vision caused by diabetic retinopathy might be one of the most impactful in a patient's life. The retina is a highly metabolically active tissue that requires a complex interaction of cells, spanning light sensing photoreceptors to neurons that transfer the electrochemical signal to the brain with support by glia and vascular tissue. Neuronal function depends on a complex inter-dependency of retinal cells that includes the formation of a blood-retinal barrier. This dynamic system is negatively affected by diabetes mellitus, which alters normal cell-cell interactions and leads to profound vascular abnormalities, loss of the blood-retinal barrier and impaired neuronal function. Understanding the normal cell signalling interactions and how they are altered by diabetes mellitus has already led to novel therapies that have improved visual outcomes in many patients. Research highlighted in this Review has led to a new understanding of retinal pathophysiology during diabetes mellitus and has uncovered potential new therapeutic avenues to treat this debilitating disease.

Complement, a Therapeutic Target in Diabetic Kidney Disease

Currently available treatments of diabetic kidney disease (DKD) remain limited despite improved understanding of DKD pathophysiology. The complement system is a central part of innate immunity, but its dysregulated activation is detrimental and results in systemic diseases with overt inflammation. Growing evidence suggests complement activation in DKD. With existent drugs and clinical success of treating other kidney diseases, complement inhibition has emerged as a potential novel therapy to halt the progression of DKD. This article will review DKD, the complement system's role in diabetic and non-diabetic disease, and the potential benefits of complement targeting therapies especially for DKD patients.

Retinol binding protein 3 is increased in the retina of patients with diabetes resistant to diabetic retinopathy

The Joslin Medalist Study characterized people affected with type 1 diabetes for 50 years or longer. More than 35% of these individuals exhibit no to mild diabetic retinopathy (DR), independent of glycemic control, suggesting the presence of endogenous protective factors against DR in a subpopulation of patients. Proteomic analysis of retina and vitreous identified retinol binding protein 3 (RBP3), a retinol transport protein secreted mainly by the photoreceptors, as elevated in Medalist patients protected from advanced DR. Mass spectrometry and protein expression analysis identified an inverse association between vitreous RBP3 concentration and DR severity. Intravitreal injection and photoreceptor-specific overexpression of RBP3 in rodents inhibited the detrimental effects of vascular endothelial growth factor (VEGF). Mechanistically, our results showed that recombinant RBP3 exerted the therapeutic effects by binding and inhibiting VEGF receptor tyrosine phosphorylation. In addition, by binding to glucose transporter 1 (GLUT1) and decreasing glucose uptake, RBP3 blocked the detrimental effects of hyperglycemia in inducing inflammatory cytokines in retinal endothelial and Müller cells. Elevated expression of photoreceptor-secreted RBP3 may have a role in protection against the progression of DR due to hyperglycemia by inhibiting glucose uptake via GLUT1 and decreasing the expression of inflammatory cytokines and VEGF.

Discovery and development of plasma kallikrein inhibitors for multiple diseases

Plasma kallikrein (PKal) belongs to the family of trypsin-like serine proteases. The expression of PKal is associated with multiple physiological systems or pathways such as coagulation pathway, platelet aggregation process, kallikrein-kinin system, renin-angiotensin system and complement pathway. On the basis of PKal's multiple physiological functions, it has been considered as a potential target for several diseases including hereditary angioedema, microvascular complications of diabetes mellitus and cerebrovascular disease. Up to now, many PKal inhibitors have been identified and a few of them have reached clinical trials or market. This review summarizes the development of small molecule and peptide PKal inhibitors having different scaffolds and discusses their structure-activity relationship and selectivity. We hope this review facilitates a comprehensive understanding of the types of PKal inhibitors developed to tackle different manifestations of PKal-associated diseases.

The effects of anti-VEGF and kinin B1 receptor blockade on retinal inflammation in laser-induced choroidal neovascularization

BACKGROUND AND PURPOSE

Age-related macular degeneration (AMD) is a complex neurodegenerative disease treated by anti-VEGF intravitreal injections. As inflammation is potentially involved in retinal degeneration, the pro-inflammatory kallikrein-kinin system is a possible alternative pharmacological target. Here, we investigated the effects of anti-VEGF and anti-B₁ receptor treatments on the inflammatory mechanisms in a rat model of choroidal neovascularization (CNV).

EXPERIMENTAL APPROACH

Immediately after laser-induced CNV, Long-Evans rats were treated by eye-drop application of a B₁ receptor antagonist (R-954) or by intravitreal injection of B₁ receptor siRNA or anti-VEGF antibodies. Effects of treatments on gene expression of inflammatory mediators, CNV lesion regression and integrity of the blood-retinal barrier was measured 10 days later in the retina. B₁ receptor and VEGF-R2 cellular localization was assessed.

KEY RESULTS

The three treatments significantly inhibited the CNV-induced retinal changes. Anti-VEGF and R-954 decreased CNV-induced up-regulation of B₁ and B₂ receptors, TNF-α, and ICAM-1. Anti-VEGF additionally reversed up-regulation of VEGF-A, VEGF-R2, HIF-1α, CCL2 and VCAM-1, whereas R-954 inhibited gene expression of IL-1β and COX-2. Enhanced retinal vascular permeability was abolished by anti-VEGF and reduced by R-954 and B₁ receptor siRNA treatments. Leukocyte adhesion was impaired by anti-VEGF and B₁ receptor inhibition. B₁ receptors were found on astrocytes and endothelial cells.

CONCLUSION AND IMPLICATIONS

B₁ receptor and VEGF pathways were both involved in retinal inflammation and damage in laser-induced CNV. The non-invasive, self-administration of B₁ receptor antagonists on the surface of the cornea by eye drops might be an important asset for the treatment of AMD.

Investigational plasma kallikrein inhibitors for the treatment of diabetic macular edema: an expert assessment

Introduction: Plasma kallikrein is a  mediator of vascular leakage and inflammation. Activation of plasma kallikrein can induce features of diabetic macular edema (DME) in preclinical models. Human vitreous shows elevated plasma kallikrein levels in patients with DME. Because of the incomplete response of some patients to anti-VEGF agents, and the treatment burden associated with frequent dosing, there is still considerable need for VEGF-independent targeted pathways.Areas covered: This review covers the role of plasma kallikrein in the pathogenesis of DME and the therapeutic potential of plasma kallikrein inhibitors. It discusses early clinical studies of plasma kallikrein pathway modulation for DME, which have been associated with some improvement in visual acuity but with limited improvement in macular edema. This review also highlights KVD001, which is furthest along the development pathway, THR-149, which has recently completed a phase 1 study, and oral agents under development.Expert opinion: Plasma kallikrein inhibitors have a potential role in the treatment of DME, with mixed functional/anatomic results in early clinical trials. Given the large unmet need in DME treatment, further studies are warranted.

The effect of Shengpuhuang-tang on retinal inflammation in streptozotocin-induced diabetic rats by NF-κB pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Diabetic retinopathy (DR) is a terrible microvascular disorder causing blindness. Retinal inflammation is the early stage in DR, which is believed to play a crucial role in the development of it. Shengpuhuang-tang (ST), a traditional herbal formula, which has effective treatment of fundus bleeding disorder. ST exerts protective effects against DR in rats, but its underlying mechanism of this efficacy remains unknown. Thus, the objective of this study is to examine the mechanism and the efficacy of ST on retinal inflammation in streptozotocin-induced diabetic rats.

MATERIALS AND METHODS

The administration of ST was initiated at 4 weeks after diabetes induction and continued for 12 weeks. Retinal vessel permeability was evaluated by using FITC-dextran and Evans blue. Retinal leukostasis was evaluated with FITC-coupled concanavalin A lectin (ConA). Moreover, western blotting was performed to detect TNF-α, ICAM-1 and the relative expression levels of IκBα, IKKβ, and p65 in vivo.

RESULTS

The results showed that the retinal inflammation in streptozotocin-induced diabetic rats was significantly decreased by ST. ST could decreased the expression levels of TNF-α, ICAM-1 and inhibited the expression of p-IKKβ, p-p65 and IκBα. It could also inhibited the nuclear transfer of p65.

CONCLUSIONS

In conclusion, these data suggested that ST may have potential treatment strategies against early stage of diabetic retinopathy through NF-κB pathway.

Prolonged activated partial thromboplastin time due to plasma prekallikrein deficiency: a case study and literature review on its clinical significance

: Activated partial thromboplastin time is a test assessing the intrinsic and common pathways of the coagulation cascade. We presented an asymptomatic case with isolated activated partial thromboplastin time prolongation. After excluding coagulation factor deficiency and lupus anticoagulant, the patient was diagnosed with plasma prekallikrein (PPK) deficiency. We reviewed the literature regarding effects of PPK deficiency which could have both antithrombotic and prothrombotic effects. At the moment, research supports that PPK deficiency in healthy adults rarely causes bleeding as it is not a major contributor of hemastasis; whereas in adults with multiple comorbidities or with predominant systemic inflammation, effects of PPK deficiency remain debatable. Further research is needed to clarify impacts of PPK deficiency in clinical settings.

Roles of Drug Transporters in Blood-Retinal Barrier

Blood-retinal barrier (BRB) includes inner BRB (iBRB) and outer BRB (oBRB), which are formed by retinal capillary endothelial (RCEC) cells and by retinal pigment epithelial (RPE) cells in collaboration with Bruch's membrane and the choriocapillaris, respectively. Functions of the BRB are to regulate fluids and molecular movement between the ocular vascular beds and retinal tissues and to prevent leakage of macromolecules and other potentially harmful agents into the retina, keeping the microenvironment of the retina and retinal neurons. These functions are mainly attributed to absent fenestrations of RCECs, tight junctions, expression of a great diversity of transporters, and coverage of pericytes and glial cells. BRB existence also becomes a reason that systemic administration for some drugs is not suitable for the treatment of retinal diseases. Some diseases (such as diabetes and ischemia-reperfusion) impair BRB function via altering tight junctions, RCEC death, and transporter expression. This chapter will illustrate function of BRB, expressions and functions of these transporters, and their clinical significances.

Activation of Factor XII and Kallikrein-Kinin System Combined with Neutrophil Extracellular Trap Formation in Diabetic Retinopathy

BACKGROUND

In diabetic retinopathy (DR), neutrophil extracellular traps (NET) and kallikrein-kinin system are considered as contributing factors. However, the detail activation mechanisms has not been fully understood. Since the NET could provide negative-charged surface for factor XII activation and the activated factor XII (XIIa) can initiate kallikrein-kinin system, this study investigated whether patients with DR show activation of NET, factor XII and kallikrein-kinin system.

METHODS

The markers related to NET (DNA-histone complex) and kallikrein-kinin system (high-molecular-weight kininogen, prekallikrein, bradykinin) and factor XIIa were measured in 253 patients with diabetes. To access ex vivo effect of glucose, DNA-histone complex and factor XIIa were measured in whole blood stimulated by glucose.

RESULTS

The circulating level of DNA-histone complex and factor XIIa were significantly higher in patients with DR than those without DR. In logistic regression analysis, DNA-histone complex, factor XIIa, and high-molecular-weight kininogen were the risk factors of DR. In recursive partitioning analysis, among patients with diabetes duration less than 10 years, patients with high level of DNA-histone complex (>426 AU) showed high risk of DR. In ex vivo experiment, glucose significantly elevated both DNA-histone complex and factor XIIa.

CONCLUSION

Our findings suggest that activation of factor XII and kallikrein-kinin system combined with NET formation actively occur in patients with DR and circulating levels of DNA-histone complex, factor XIIa and HMWK can be potential biomarkers to estimate the risk of DR. Strategies against factor XII activation may be beneficial to inhibit DR.

Retinal proteome associated with bradykinin-induced edema

The plasma kallikrein-stimulated generation of bradykinin (BK) has been implicated in diabetic macular edema (DME). This study characterizes the effects of BK on the ultrastructure and proteome of the rat retina. The effects of intravitreal injection of BK on retinal thickness and vascular ultrastructure in Sprague Dawley rats were analyzed and compared with the effects of VEGF using spectral-domain optical coherence tomography. At 24 h post intravitreal injection of BK or saline vehicle retina were harvested and solubilized proteins were analyzed by mass spectrometry-based proteomics. Proteins were identified using X!Tandem and spectral counts were used as a semiquantitative measurement of protein abundance. Proteins identified from retinal extracts were annotated by Gene Ontology (GO) slim terms and compared with a human DME vitreous proteome. Intravitreal injection of BK and VEGF induced transient increases in retinal thickness of 46 μm (24.6%, p = 0.015) and 39 μm (20.3%, p = 0.004), respectively at 24 h, which were resolved to baseline thicknesses at 96 h post injection. BK and VEGF also increased retinal vessel diameters and tortuosity at 24 h post intravitreal injection. Proteomic analyses identified 1757 non-redundant proteins in the rat retina, including 1739 and 1725 proteins from BK- and saline control-injected eyes, respectively. Eighteen proteins, including two proteins associated with intercellular junctions, filamin A and actinin alpha 4, were decreased by at least 50% (p < 0.05) in retina from BK-injected eyes compare with retina from eyes injected with saline. In addition, 32 proteins were increased by > 2-fold (p < 0.05) in retina from BK-injected eyes. Eight proteins, including complement C3, were identified to be increased in both BK-stimulated rat retina and in human DME vitreous. Western blot analysis showed that Complement 3 levels in vitreous from BK-injected eyes in rats and clinical DME samples were increased by 6.6-fold (p = 0.039) and 4.3-fold (p = 0.02), compared with their respective controls. In summary, this study identifies protein changes in rat retina that are associated with BK-induced retinal thickening, including 8 proteins that were previously reported to be increased in the human DME vitreous proteome.

Pancreatic kallikrein protects against diabetic retinopathy in KK Cg-Ay/J and high-fat diet/streptozotocin-induced mouse models of type 2 diabetes

AIMS/HYPOTHESIS

Many studies have shown that tissue kallikrein has effects on diabetic vascular complications such as nephropathy, cardiomyopathy and neuropathy, but its effects on diabetic retinopathy are not fully understood. Here, we investigated the retinoprotective role of exogenous pancreatic kallikrein and studied potential mechanisms of action.

METHODS

We used KK Cg-A^y/J (KKAy) mice (a mouse model of spontaneous type 2 diabetes) and mice with high-fat diet/streptozotocin (STZ)-induced type 2 diabetes as our models. After the onset of diabetes, both types of mice were injected intraperitoneally with either pancreatic kallikrein (KKAy + pancreatic kallikrein and STZ + pancreatic kallikrein groups) or saline (KKAy + saline and STZ + saline groups) for 12 weeks. C57BL/6J mice were used as non-diabetic controls for both models. We analysed pathological changes in the retina; evaluated the effects of pancreatic kallikrein on retinal oxidative stress, inflammation and apoptosis; and measured the levels of bradykinin and B1 and B2 receptors in both models.

RESULTS

In both models, pancreatic kallikrein improved pathological structural features of the retina, increasing the thickness of retinal layers, and attenuated retinal acellular capillary formation and vascular leakage (p < 0.05). Furthermore, pancreatic kallikrein ameliorated retinal oxidative stress, inflammation and apoptosis in both models (p < 0.05). We also found that the levels of bradykinin and B1 and B2 receptors were increased after pancreatic kallikrein in both models (p < 0.05).

CONCLUSIONS/INTERPRETATION

Pancreatic kallikrein can protect against diabetic retinopathy by activating B1 and B2 receptors and inhibiting oxidative stress, inflammation and apoptosis. Thus, pancreatic kallikrein may represent a new therapeutic agent for diabetic retinopathy.

Fenofibrate-Loaded Biodegradable Nanoparticles for the Treatment of Experimental Diabetic Retinopathy and Neovascular Age-Related Macular Degeneration

Fenofibrate is a peroxisome proliferator-activated receptor α (PPARα) agonist and has been shown to have therapeutic effects on diabetic retinopathy (DR). However, the effects of fenofibrate through systemic administration are not as potent as desired due to inefficient drug delivery to the retina. The present study aimed to explore the sustained therapeutic effects of fenofibrate-loaded biodegradable nanoparticles (NP) on both DR and neovascular age-related macular degeneration (AMD). Fenofibrate was successfully encapsulated into poly(lactic- co-glycolic acid) (PLGA) NP (Feno-NP), and Feno-NP were optimized by varying polymer composition to achieve high drug loading and prolonged drug release. The Feno-NP made of PLGA 34 kDa demonstrated a drug content of 6% w/w and a sustained drug release up to 60 days in vitro. Feno-NP (PLGA 34 kDa) was selected for following in vivo studies, and one single intravitreal (IVT) injection of Feno-NP into rat eyes with a 30G fine needle maintained sustained fenofibric acid drug level in the eye for more than 60 days. The efficacy of Feno-NP in DR and neovascular AMD was investigated using streptozotocin (STZ)-induced diabetic rats, laser-induced choroidal neovascularization (CNV) rats, and very low-density lipoprotein receptor knockout ( Vldlr ^-/-) mice. Therapeutic effects of Feno-NP were evaluated by measuring electroretinogram (ERG), retinal vascular leakage, leukostasis, CNV size, and retinal levels of vascular endothelial growth factor (VEGF) and intracellular adhesion molecule-1 (ICAM-1). In diabetic rats, Feno-NP ameliorated retinal dysfunctions, reduced retinal vascular leakage, inhibited retinal leukostasis, and downregulated the overexpression of VEGF and ICAM-1 at 8 weeks after one IVT injection. In addition, Feno-NP reduced retinal vascular leakage and CNV formation in both CNV rats and Vldlr ^-/- mice. Moreover, no toxicity of Feno-NP or Blank-NP to retinal structure and function was detected. Feno-NP exhibited good physiochemical characteristics and controlled drug release profile, conferring prolonged beneficial effects on DR and neovascular AMD.

Recent advances in the pathogenesis of microvascular complications in diabetes

Millions of people worldwide have diabetes, which is diagnosed by fasting blood glucose levels exceeding 126 mg/dL. Regardless of the type of diabetes, prolonged hyperglycemia is damaging to several organs including eyes, kidneys, nerve, and/or heart. The damages are associated with a high risk of morbidity and mortality. Diabetes has been implicated in ischemia in the microvasculature of the target tissues, which occurs due to the insufficient perfusion of tissues. The resulting occlusion and pain affect the quality of life. Multiple therapeutic approaches have been proposed for a long time to overcome these vascular complications. Apart from systemically controlling high glucose levels, other therapeutic strategies are not well understood. In this review, we summarize the recent literature for biochemical/cellular targets that are being utilized for the treatment of diabetic microvascular diseases. These targets, which are closely associated with mitochondrial dysfunction, include the polyol and diacylglycerol-protein kinase C pathways, oxidative stress, non-enzymatic glycation and the formation of advanced glycation end products, and immune dysregulation/inflammation.

Expression, distribution and function of kinin B1 receptor in the rat diabetic retina

BACKGROUND AND PURPOSE

The kinin B₁ receptor contributes to vascular inflammation and blood-retinal barrier breakdown in diabetic retinopathy (DR). We investigated the changes in expression, cellular localization and vascular inflammatory effect of B₁ receptors in retina of streptozotocin diabetic rats.

EXPERIMENTAL APPROACH

The distribution of B₁ receptors on retinal cell types was investigated by immunocytochemistry. Effects of B₁ receptor agonist, R-838, and antagonist, R-954, on retinal leukocyte adhesion, gene expression of kinin and VEGF systems, B₁ receptor immunoreactivity, microgliosis and capillary leakage were measured. Effect of B₁ receptor siRNA on gene expression was also assessed.

KEY RESULTS

mRNA levels of the kinin and VEGF systems were significantly enhanced at 2 weeks in streptozotocin (STZ)-retina compared to control-retina and were further increased at 6 weeks. B₁ receptor mRNA levels remained increased at 6 months. B₁ receptor immunolabelling was detected in vascular layers of the retina, on glial and ganglion cells. Intravitreal R-838 amplified B₁ and B₂ receptor gene expression, B₁ receptor levels (immunodetection), leukostasis and vascular permeability at 2 weeks in STZ-retina. Topical application (eye drops) of R-954 reversed these increases in B₁ receptors, leukostasis and vascular permeability. Intravitreal B₁ receptor siRNA inhibited gene expression of kinin and VEGF systems in STZ-retina. Microgliosis was unaffected by R-838 or R-954 in STZ-retina.

CONCLUSION AND IMPLICATIONS

Our results support the detrimental role of B₁ receptors on endothelial and glial cells in acute and advanced phases of DR. Topical application of the B₁ receptor antagonist R-954 seems a feasible therapeutic approach for the treatment of DR.

Diabetic Microvascular Disease: An Endocrine Society Scientific Statement

Both type 1 and type 2 diabetes adversely affect the microvasculature in multiple organs. Our understanding of the genesis of this injury and of potential interventions to prevent, limit, or reverse injury/dysfunction is continuously evolving. This statement reviews biochemical/cellular pathways involved in facilitating and abrogating microvascular injury. The statement summarizes the types of injury/dysfunction that occur in the three classical diabetes microvascular target tissues, the eye, the kidney, and the peripheral nervous system; the statement also reviews information on the effects of diabetes and insulin resistance on the microvasculature of skin, brain, adipose tissue, and cardiac and skeletal muscle. Despite extensive and intensive research, it is disappointing that microvascular complications of diabetes continue to compromise the quantity and quality of life for patients with diabetes. Hopefully, by understanding and building on current research findings, we will discover new approaches for prevention and treatment that will be effective for future generations.

Oxidative stress in the optic nerve and cortical visual area of steptozotocin-induced diabetic Wistar rats: Blockade with a selective bradykinin B1 receptor antagonist

The role of bradykinin B₁ receptors on the oxidative stress as measured by the levels of Na+/K+ ATPase activity, malondialdehyde (MDA) and glutathione (GSH) in male Wistar rat optic nerve and visual cortex area 1 and 4weeks after STZ treatment was studied. Rats were divided into 4 groups (n=6-7): 1. Controls (non-diabetics); 2. Diabetics (65mg/kg streptozotocin, STZ); 3. Diabetics injected with B₁ antagonist R-954 (2mg/Kg) during the last 3days of a one week period; 4. Diabetics injected with B₁ antagonist R-954 (2mg/Kg) during the last 3days of a 4week period. The results showed that plasma glucose levels increased by up to 4 fold in diabetic rats 1 or 4weeks following the STZ treatment. R-954 treatment did significantly decrease blood glucose levels. Levels of MDA was increased in the plasma of the 1 and 4week diabetic animals whereas the GSH levels were decreased. Both markers returned to normal following R-954 treatment. Na+/K+ ATPase activity significantly decreased in the optic nerve and visual cortex of diabetic rats at 1 and 4weeks but returned to normal following R-954 treatment. MDA levels increased markedly at 1 and 4weeks compared with control levels in the optic nerve but slightly in the visual cortex and returned to control levels in both tissues following R-954 treatment. GSH levels decreased in both tissues at 1 and 4weeks compared with control levels. Following administration of the selective BKB₁R antagonist R-954, the levels of GSH returned to normal in both tissues of the 1 and 4week diabetic animals. These results showed that the inducible BKB₁ receptors are associated with the oxidative stress in the optic nerve and cortical visual area of diabetic rats and suggested that BKB₁-R antagonist R-954 could have a beneficial role in the treatment of diabetic retinopathy.

Effects of plasma kallikrein deficiency on haemostasis and thrombosis in mice: Murine Ortholog of the Fletcher Trait

Plasma kallikrein is a multifunctional serine protease involved in contact activation of coagulation. Deficiency in humans is characterised by prolonged activated partial thromboplastin time (aPTT); however, the balance between thrombosis and haemostasis is not fully understood. A study of plasma kallikrein-deficient mice revealed increased aPTT, without prolonged bleeding time. Prekallikrein antisense oligonucleotide (ASO) treatment in mice suggested potential for a positive therapeutic index. The current goal was to further define the role of plasma kallikrein in coagulation. Blood pressure and heart rate were normal in plasma kallikrein-deficient mice, and mice were completely protected from occlusion (100 ± 1.3% control flow) in 3.5% FeCl3 -induced arterial thrombosis versus heterozygotes (20 ± 11.4%) and wild-type littermates (8 ± 0%). Vessels occluded in 8/8 wild-type, 7/8 heterozygotes, and 0/8 knockouts. Anti-thrombotic protection was less pronounced in 5% FeCl3-induced arterial injury. Integrated blood flow was 8 ± 0% control in wild-type and heterozygotes, and significantly (p<0.01) improved to 43 ± 14.2% in knockouts. The number of vessels occluded was similar in all genotypes. Thrombus weight was significantly reduced in knockouts (−47%) and heterozygotes (−23%) versus wild-type in oxidative venous thrombosis. Average tail bleeding time increased modestly in knockout mice compared to wild-type. Average renal bleeding times were similar in all genotypes. These studies confirm and extend studies with prekallikrein ASO, and demonstrate that plasma kallikrein deletion prevents occlusive thrombus formation in mice with a minimal role in provoked bleeding. Additional support for the significance of the intrinsic pathway in the coagulation cascade is provided, as well as for a potential new anti-thrombotic approach.

Neurodegeneration in diabetic retinopathy: Potential for novel therapies

The complex pathology of diabetic retinopathy (DR) affects both vascular and neural tissue. The characteristics of neurodegeneration are well-described in animal models but have more recently been confirmed in the clinical setting, mostly by using non-invasive imaging approaches such as spectral domain optical coherence tomography (SD-OCT). The most frequent observations report loss of tissue in the nerve fiber layer and inner plexiform layer, confirming earlier findings from animal models. In several cases the reduction in inner retinal layers is reported in patients with little evidence of vascular lesions or macular edema, suggesting that degenerative loss of neural tissue in the inner retina can occur after relatively short durations of diabetes. Animal studies also suggest that neurodegeneration leading to retinal thinning is not limited to cell death and tissue loss but also includes changes in neuronal morphology, reduced synaptic protein expression and alterations in neurotransmission, including changes in expression of neurotransmitter receptors as well as neurotransmitter release, reuptake and metabolism. The concept of neurodegeneration as an early component of DR introduces the possibility to explore alternative therapies to prevent the onset of vision loss, including neuroprotective therapies and drugs targeting individual neurotransmitter systems, as well as more general neuroprotective approaches to preserve the integrity of the neural retina. In this review we consider some of the evidence for progressive retinal neurodegeneration in diabetes, and explore potential neuroprotective therapies.

Structure-Dependent Hematological Effects of Per- and Polyfluoroalkyl Substances on Activation of Plasma Kallikrein-Kinin System Cascade

Per- and polyfluoroalkyl substances (PFASs) are a global concern because of their ubiquitous occurrence and high persistence in human blood, and increasing amounts of unidentified fluorinated compounds are now becoming new exposure issues. This study aims to investigate the structure-related effects of PFASs on the activation of the plasma kallikrein-kinin system (KKS). The effects of 20 PFASs and the related long-chain aliphatic compounds were screened, and their binding affinities for the initial zymogen, Hagmen factor XII (FXII) in the KKS, were evaluated by molecular docking analysis. PFASs were demonstrated to activate the KKS in a structure-dependent mode. More specifically, PFASs with longer carbon chain length, higher fluorine atom substitution degree, and terminal acid group exhibited relatively higher activities in activating the KKS. The binding affinities of PFASs with FXII determined their capabilities for inducing KKS activation. The alternative binding modes of PFASs with FXII, together with van der Waals and hydrogen bonds, specifically accommodated the distinctive chemical structures. To our knowledge, PFASs, for the first time, were found to induce the activation of the KKS in plasma, and their chemical structure-related effects would be extremely important for risk assessment on emerging PFASs in addition to the listing in Stockholm Convention.

Retinopathy with central oedema in an INS C94Y transgenic pig model of long-term diabetes

AIMS/HYPOTHESIS

Diabetic retinopathy is a severe complication of diabetes mellitus that often leads to blindness. Because the pathophysiology of diabetic retinopathy is not fully understood and novel therapeutic interventions require testing, there is a need for reliable animal models that mimic all the complications of diabetic retinopathy. Pig eyes share important anatomical and physiological similarities with human eyes. Previous studies have demonstrated that INS ^C94Y transgenic pigs develop a stable diabetic phenotype and ocular alterations such as cataracts. The aim of this study was to conduct an in-depth analysis of pathological changes in retinas from INS ^C94Y pigs exposed to hyperglycaemia for more than 2 years, representing a chronic diabetic condition.

METHODS

Eyes from six INS ^C94Ypigs and six age-matched control littermates were analysed via histology and immunohistochemistry. For histological analyses of retinal (layer) thickness, sections were stained with H&E or Mallory's trichrome. For comparison of protein expression patterns and vessel courses, sections were stained with different antibodies in immunohistochemistry. Observed lesions were compared with reported pathologies in human diabetic retinopathy.

RESULTS

INS ^C94Ypigs developed several signs of diabetic retinopathy similar to those seen in humans, such as intraretinal microvascular abnormalities, symptoms of proliferative diabetic retinopathy and central retinal oedema in a region that is cone rich, like the human macula.

CONCLUSIONS/INTERPRETATION

The INS ^C94Ypig is an interesting model for studying the pathophysiology of diabetic retinopathy and for testing novel therapeutic strategies.

The Effects of the Contact Activation System on Hemorrhage

The contact activation system (CAS) exerts effects on coagulation via multiple mechanisms, which modulate both the intrinsic and extrinsic coagulation cascades as well as fibrinolysis and platelet activation. While the effects of the CAS on blood coagulation measured as activated partial thromboplastin time shortening are well documented, genetic mutations that result in deficiencies in the expression of either plasma prekallikrein (PPK) or factor XII (FXII) are not associated with spontaneous bleeding or increased bleeding risk during surgery. Deficiencies in these proteins are often undiagnosed for decades and detected later in life during routine coagulation assays without an apparent clinical phenotype. Increased interest in the CAS as a potentially safe target for antithrombotic therapies has emerged, in large part, from studies on animal models with provoked thrombosis, which have shown that deficiencies in PPK or FXII can reduce thrombus formation without increasing bleeding. Gene targeting and pharmacological studies in healthy animals have confirmed that PPK and FXII blockade does not cause coagulopathies. These findings support the conclusion that CAS is not required for hemostasis. However, while deficiencies in FXII and PPK do not significantly affect bleeding associated with peripheral wounds, recent reports have demonstrated that these proteins can promote hemorrhage in the retina and brain. Intravitreal injection of plasma kallikrein (PKal) induces retinal hemorrhage and intracerebral injection of PKal increases intracranial bleeding. PPK deficiency and PKal inhibition ameliorates hematoma formation following cerebrovascular injury in diabetic animals. Moreover, both PPK and FXII deficiency are protective against intracerebral hemorrhage caused by tissue plasminogen activator-mediated thrombolytic therapy in mice with thrombotic middle cerebral artery occlusion. Thus, while the CAS is not required for hemostasis, its inhibition may provide an opportunity to reduce hemorrhage in the retina and brain. Characterization of the mechanisms and potential clinical implications associated with the effects of the CAS on hemorrhage requires further consideration of the effects of PPK and FXII on hemorrhage beyond their putative effects on coagulation cascades. Here, we review the experimental and clinical evidence on the effects of the CAS on bleeding and hemostatic mechanisms.

Plasma Kallikrein Mediates Vascular Endothelial Growth Factor-Induced Retinal Dysfunction and Thickening

PURPOSE

Plasma kallikrein is a serine protease and circulating component of inflammation, which exerts clinically significant effects on vasogenic edema. This study examines the role of plasma kallikrein in VEGF-induced retinal edema.

METHODS

Intravitreal injections of VEGF and saline vehicle were performed in plasma prekallikrein-deficient (KLKB1-/-) and wild-type (WT) mice, and in both rats and mice receiving a selective plasma kallikrein inhibitor, VA999272. Retinal vascular permeability (RVP) and retinal thickness were measured by Evans blue permeation and optical coherence tomography, respectively. The retinal kallikrein kinin system was examined by Western blotting and immunohistochemistry. Retinal neovascularization was investigated in KLKB1-/- and WT mice subjected to oxygen-induced retinopathy.

RESULTS

Vascular endothelial growth factor-induced RVP and retinal thickening were reduced in KLKB1-/- mice by 68% and 47%, respectively, compared to VEGF responses in WT mice. Plasma kallikrein also contributes to TNFα-induced retinal thickening, which was reduced by 52% in KLKB1-/- mice. Systemic administration of VA999272 reduced VEGF-induced retinal thickening by 57% (P < 0.001) in mice and 53% (P < 0.001) in rats, compared to vehicle-treated controls. Intravitreal injection of VEGF in WT mice increased plasma prekallikrein in the retina, which was diffusely distributed throughout the inner and outer retinal layers. Avascular and neovascular areas induced by oxygen-induced retinopathy were similar in WT and KLKB1-/- mice.

CONCLUSIONS

Vascular endothelial growth factor increases extravasation of plasma kallikrein into the retina, and plasma kallikrein is required for the full effects of VEGF on RVP and retinal thickening in rodents. Systemic plasma kallikrein inhibition may provide a therapeutic opportunity to treat VEGF-induced retina edema.

Plasma Kallikrein Inhibitors in Cardiovascular Disease: An Innovative Therapeutic Approach

Plasma prekallikrein is the liver-derived precursor of the trypsin-like serine protease plasma kallikrein, and circulates in plasma bound to high molecular weight kininogen. Plasma prekallikrein is activated to plasma kallikrein by activated factor XII or prolylcarboxypeptidase. Plasma kallikrein regulates the activity of multiple proteolytic cascades in the cardiovascular system such as the intrinsic pathway of coagulation, the kallikrein-kinin system, the fibrinolytic system, the renin-angiotensin system, and the complement pathways. As such, plasma kallikrein plays a central role in the pathogenesis of thrombosis, inflammation, and blood pressure regulation. Under physiological conditions, plasma kallikrein serves as a cardioprotective enzyme. However, its increased plasma concentration or hyperactivity perpetuates cardiovascular disease (CVD). In this article, we review the biochemistry and cell biology of plasma kallikrein and summarize data from preclinical and clinical studies that have established important functions of this serine protease in CVD states. Finally, we propose plasma kallikrein inhibitors as a novel class of drugs with potential therapeutic applications in the treatment of CVDs.

Effect of circulating tissue factor on hypercoagulability in type 2 diabetes mellitus studied by rheometry and dielectric blood coagulometry

BACKGROUND

Hypercoagulability in type 2 diabetes mellitus (T2DM) patients increases their risk of cardiovascular diseases.

OBJECTIVE

The aim of this work was to investigate the hypercoagulation mechanism in T2DM patients in terms of circulating tissue factor (TF).

METHODS

Whole blood coagulation tests by damped oscillation rheometry and dielectric blood coagulometry (DBCM) were performed.

RESULTS

The average coagulation time was significantly shorter for T2DM patients than for healthy controls. In vitro addition of either anti-TF or anti-activated factor VII (FVIIa) antibody to hypercoagulable blood samples prolonged coagulation times for one group of patients, while coagulation times remained short for another group. The levels of circulating TF were estimated in the former group by measuring the coagulation times for blood samples from healthy subjects with addition of various concentrations of TF and comparing them with the coagulation times for the group. The results indicated that the levels of circulating TF were on the order of subpicomolar at most.

CONCLUSIONS

Circulating TF is at least partially responsible for a hypercoagulable group of T2DM patients, while an abnormality in the intrinsic coagulation pathway probably occurs in the other group.

The Role of Plasma Kallikrein-Kinin Pathway in the Development of Diabetic Retinopathy: Pathophysiology and Therapeutic Approaches

Diabetic retinal disease is characterized by a series of retinal microvascular changes and increases in retinal vascular permeability that lead to development of diabetic retinopathy (DR) and diabetic macular edema (DME), respectively. Current treatment strategies for DR and DME are mostly limited to vascular endothelial growth factor (VEGF) inhibitors and laser photocoagulation. These treatment modalities are not universally effective in all patients, and potential side effects persist in a significant portion of patients. The plasma kallikrein-kinin system (KKS) is one of the pathways that has been identified in the vitreous in proliferative DR and DME. Preclinical studies have shown that the activation of intraocular KKS induces retinal vascular permeability, vasodilation, and retinal thickening. Proteomic analysis from vitreous of eyes with DME has shown that KKS and VEGF pathways are potentially independent biologic pathways. Furthermore, proteins associated with DME in the vitreous were significantly more correlated with the KKS pathway compared to VEGF pathway. Preclinical experiments on diabetic animals showed that inhibition of KKS components was found to be an effective approach to decrease retinal vascular permeability. An initial phase I human trial of a novel plasma kallikrein inhibitor for the treatment of DME is currently ongoing to test the safety of this approach and serves as an initial step in the translation of basic science discovery into an innovative clinical intervention.

Diabetes enhances the efficacy of AAV2 vectors in the retina: therapeutic effect of AAV2 encoding vasoinhibin and soluble VEGF receptor 1

Adeno-associated virus (AAV) vector-mediated delivery of inhibitors of blood-retinal barrier breakdown (BRBB) offers promise for the treatment of diabetic macular edema. Here, we demonstrated a reversal of blood-retinal barrier pathology mediated by AAV type 2 (AAV2) vectors encoding vasoinhibin or soluble VEGF receptor 1 (sFlt-1) when administered intravitreally to diabetic rats. Efficacy and safety of the AAV2 vasoinhibin vector were tested by monitoring its effect on diabetes-induced changes in the retinal vascular bed and thickness, and in the electroretinogram (ERG). Also, the transduction of AAV2 vectors and expression of AAV2 receptors and co-receptors were compared between the diabetic and the non-diabetic rat retinas. AAV2 vasoinhibin or AAV2 sFlt-1 vectors were injected intravitreally before or after enhanced BRBB due to diabetes induced by streptozotocin. The BRBB was examined by the Evans blue method, the vascular bed by fluorescein angiography, expression of the AAV2 EGFP reporter vector by confocal microscopy, and the AAV2 genome, expression of transgenes, receptors, and co-receptors by quantitative PCR. AAV2 vasoinhibin and sFlt-1 vectors inhibited the diabetes-mediated increase in BRBB when injected after, but not before, diabetes was induced. The AAV2 vasoinhibin vector decreased retinal microvascular abnormalities and the diabetes-induced reduction of the B-wave of the ERG, but it had no effect in non-diabetic controls. Also, retinal thickness was not altered by diabetes or by the AAV2 vasoinhibin vector. The AAV2 genome, vasoinhibin and sFlt-1 transgenes, and EGFP levels were higher in the retinas from diabetic rats and were associated with an elevated expression of AAV2 receptors (syndecan, glypican, and perlecan) and co-receptors (fibroblast growth factor receptor 1, αvβ5 integrin, and hepatocyte growth factor receptor). We conclude that retinal transduction and efficacy of AAV2 vectors are enhanced in diabetes, possibly due to their elevated cell entry. AAV2 vectors encoding vasoinhibin and sFlt-1 may be desirable gene therapeutics to target diabetic retinopathy and macular edema.

The contact activation and kallikrein/kinin systems: pathophysiologic and physiologic activities

The contact activation system (CAS) and kallikrein/kinin system (KKS) are older recognized biochemical pathways that include several proteins that skirt the fringes of the blood coagulation, fibrinolytic, complement and renin-angiotensin fields. These proteins initially were proposed as part of the hemostatic pathways because their deficiencies are associated with prolonged clinical assays. However, the absence of bleeding states with deficiencies of factor XII (FXII), prekallikrein (PK) and high-molecular-weight kininogen indicates that the CAS and KKS do not contribute to hemostasis. Since the discovery of the Hageman factor 60 years ago much has been learned about the biochemistry, cell biology and animal physiology of these proteins. The CAS is a pathophysiologic surface defense mechanism against foreign proteins, organisms and artificial materials. The KKS is an inflammatory response mechanism. Targeting their activation through FXIIa or plasma kallikrein inhibition when blood interacts with the artificial surfaces of modern interventional medicine or in acute attacks of hereditary angioedema restores vascular homeostasis. FXII/FXIIa and products that arise with PK deficiency also offer novel ways to reduce arterial and venous thrombosis without an effect on hemostasis. In summary, there is revived interest in the CAS and KKS due to better understanding of their activities. The new appreciation of these systems will lead to several new therapies for a variety of medical disorders.

Plasma Kallikrein-Kinin System as a VEGF-Independent Mediator of Diabetic Macular Edema

This study characterizes the kallikrein-kinin system in vitreous from individuals with diabetic macular edema (DME) and examines mechanisms contributing to retinal thickening and retinal vascular permeability (RVP). Plasma prekallikrein (PPK) and plasma kallikrein (PKal) were increased twofold and 11.0-fold (both P < 0.0001), respectively, in vitreous from subjects with DME compared with those with a macular hole (MH). While the vascular endothelial growth factor (VEGF) level was also increased in DME vitreous, PKal and VEGF concentrations do not correlate (r = 0.266, P = 0.112). Using mass spectrometry-based proteomics, we identified 167 vitreous proteins, including 30 that were increased in DME (fourfold or more, P < 0.001 vs. MH). The majority of proteins associated with DME displayed a higher correlation with PPK than with VEGF concentrations. DME vitreous containing relatively high levels of PKal and low VEGF induced RVP when injected into the vitreous of diabetic rats, a response blocked by bradykinin receptor antagonism but not by bevacizumab. Bradykinin-induced retinal thickening in mice was not affected by blockade of VEGF receptor 2. Diabetes-induced RVP was decreased by up to 78% (P < 0.001) in Klkb1 (PPK)-deficient mice compared with wild-type controls. B2- and B1 receptor-induced RVP in diabetic mice was blocked by endothelial nitric oxide synthase (NOS) and inducible NOS deficiency, respectively. These findings implicate the PKal pathway as a VEGF-independent mediator of DME.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

Thrombosis and Hemorrhage in Diabetic Retinopathy: A Perspective from an Inflammatory Standpoint

Retinal ischemia and hemorrhage are hallmarks of worsening diabetic retinopathy, which can lead to neovascularization, macular edema, and severe vision loss. Although diabetes alters expression of clotting factors and their activities, and increases retinal microthromboses, the effects of thrombotic processes on the pathogenesis of diabetic retinopathy are not fully understood. In addition to the roles of coagulation and fibrinolytic cascades in thrombosis and hemostasis, components in these systems also mediate multiple effects on the vasculature that promote inflammation. Plasma kallikrein, thrombin, and urokinase are increased in diabetic retinopathy, and exert proinflammatory effects that contribute to retinal vascular dysfunction. The accumulation and activation of these and additional coagulation factors in the vitreous due to hemorrhage and chronic retinal injury in the diabetic retina may contribute to worsening of retinal inflammation and capillary dysfunction, which lead to retinal ischemia and edema. Further understanding of the role for specific coagulation factors in diabetic retinopathy may suggest new therapeutic opportunities for this vision-threatening disease.

Critical insights into the beneficial and protective actions of the kallikrein-kinin system

Hypertension is characterized by an imbalance between the renin-angiotensin system (RAS) and the kallikrein-kinin system (KKS). Angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin II AT-1 receptor antagonists (also known as sartans or ARBs) are potent modulators of these systems and are highly effective as first-line treatments for hypertension, diabetic nephropathies, and diseases of the brain and coronary arteries. However, these agents are mechanistically distinct and should not be considered interchangeable. In this mini-review, we provide novel insights into the often neglected roles of the KKS in the beneficial, protective, and reparative actions of ACEIs. Indeed, ACEIs are the only antihypertensive drugs that properly reduce the imbalance between the RAS and the KKS, thereby restoring optimal cardiovascular homeostasis and significantly reducing morbidity and the risk of all-cause mortality among individuals affected by hypertension and other cardiovascular diseases.

Vasoinhibins regulate the inner and outer blood-retinal barrier and limit retinal oxidative stress

Vasoinhibins are prolactin fragments present in the retina, where they have been shown to prevent the hypervasopermeability associated with diabetes. Enhanced bradykinin (BK) production contributes to the increased transport through the blood-retina barrier (BRB) in diabetes. Here, we studied if vasoinhibins regulate BRB permeability by targeting the vascular endothelium and retinal pigment epithelium (RPE) components of this barrier. Intravitreal injection of BK in male rats increased BRB permeability. Vasoinhibins prevented this effect, as did the B2 receptor antagonist Hoe-140. BK induced a transient decrease in mouse retinal and brain capillary endothelial monolayer resistance that was blocked by vasoinhibins. Both vasoinhibins and the nitric oxide (NO) synthase inhibitor L-NAME, but not the antioxidant N-acetyl cysteine (NAC), blocked the transient decrease in bovine umbilical vein endothelial cell (BUVEC) monolayer resistance induced by BK; this block was reversed by the NO donor DETANONOate. Vasoinhibins also prevented the BK-induced actin cytoskeleton redistribution, as did L-NAME. BK transiently decreased human RPE (ARPE-19) cell monolayer resistance, and this effect was blocked by vasoinhibins, L-NAME, and NAC. DETANONOate reverted the blocking effect of vasoinhibins. Similar to BK, the radical initiator Luperox induced a reduction in ARPE-19 cell monolayer resistance, which was prevented by vasoinhibins. These effects on RPE resistance coincided with actin cytoskeleton redistribution. Intravitreal injection of vasoinhibins reduced the levels of reactive oxygen species (ROS) in retinas of streptozotocin-induced diabetic rats, particularly in the RPE and capillary-containing layers. Thus, vasoinhibins reduce BRB permeability by targeting both its main inner and outer components through NO- and ROS-dependent pathways, offering potential treatment strategies against diabetic retinopathies.

The kallikrein system in retinal damage/protection

Kallikrein is a serine protease involved in the kallikrein-kinnin system. Kallikrein is derived from the blood plasma or tissue, and is correlated with aggravation and improvement in eye diseases, such as, glaucoma, diabetic retinopathy, age-related macular degeneration, and ocular ischemic syndrome. The plasma kallikrein stimulates retinal vascular permeability and intraocular hemorrhage. On the other hand, we had reported that the tissue kallikrein normalizes retinal vasopermeability and inhibited retinal neovascularization and retinal ischemic injury. The protective mechanisms of the tissue-derived kallikrein include the cleavage of vascular endothelial growth factor (VEGF), which suggests that the tissue kallikrein could be potentially-effective against any disease involving the VEGF production.

Inhibition of plasma kallikrein by a highly specific active site blocking antibody

Plasma kallikrein (pKal) proteolytically cleaves high molecular weight kininogen to generate the potent vasodilator and the pro-inflammatory peptide, bradykinin. pKal activity is tightly regulated in healthy individuals by the serpin C1-inhibitor, but individuals with hereditary angioedema (HAE) are deficient in C1-inhibitor and consequently exhibit excessive bradykinin generation that in turn causes debilitating and potentially fatal swelling attacks. To develop a potential therapeutic agent for HAE and other pKal-mediated disorders, we used phage display to discover a fully human IgG1 monoclonal antibody (DX-2930) against pKal. In vitro experiments demonstrated that DX-2930 potently inhibits active pKal (K_i = 0.120 ± 0.005 nm) but does not target either the zymogen (prekallikrein) or any other serine protease tested. These findings are supported by a 2.1-Å resolution crystal structure of pKal complexed to a DX-2930 Fab construct, which establishes that the pKal active site is fully occluded by the antibody. DX-2930 injected subcutaneously into cynomolgus monkeys exhibited a long half-life (t_½ ∼12.5 days) and blocked high molecular weight kininogen proteolysis in activated plasma in a dose- and time-dependent manner. Furthermore, subcutaneous DX-2930 reduced carrageenan-induced paw edema in rats. A potent and long acting inhibitor of pKal activity could be an effective treatment option for pKal-mediated diseases, such as HAE.

Exendin-4 alleviates retinal vascular leakage by protecting the blood-retinal barrier and reducing retinal vascular permeability in diabetic Goto-Kakizaki rats

The breakdown of the inner endothelial blood-retinal barrier (BRB) and subsequent retinal vascular leakage are the main causes of vision loss due to diabetic retinopathy (DR). Exendin-4 (E4) is a long-acting agonist of the glucagon-like peptide 1 hormone receptor (GLP-1R) that is widely used in clinics and has shown a neuroprotective effect. Our previous studies demonstrated the protective effect of E4 in early experimental DR; however, the molecular and cellular mechanisms that mediate this protective effect are not fully known. The BRB plays a key role in DR. We speculated that E4 may exert its protective effects on the BRB. To test this hypothesis, E4 (0.1 μg/2 μL/eye) or vehicle were intravitreally injected into diabetic Goto-Kakizaki(GK) rats and control animals. The results revealed that E4 significantly inhibited the reductions in electroretinogram (ERG) amplitudes in the GK rats, particularly in the b-wave and oscillatory potentials (OPs). E4 upregulated retinal GLP-1R expression and downregulated the expressions of placental growth factor (PLGF) and vascular endothelial growth factor (VEGF) via the ERK and AKT/PKB pathways. Decreases in tight junction protein (i.e., claudin-5 and occludin) expression and increases in Evans blue permeation (EBP) were inhibited by E4. Similar results were also found in primary rat Müller cells in high glucose concentration cultures in vitro. We conclude that E4 may protect the BRB from diabetic insults by decreasing PLGF and ICAM-1 expression and maintaining the integrity of the BRB. Thus, E4 treatment may be an effective therapeutic approach for DR.