In vivo imaging of endothelial injury in choriocapillaris during endotoxin-induced uveitis

VEGFR-2 adhesive nanoprobes reveal early diabetic retinopathy in vivo

Diabetic retinopathy (DR) is a debilitating organ manifestation of diabetes. Absent of early diagnosis and intervention, vision tends to drastically and irreversibly decline. Previously, we showed higher vascular endothelial growth factor receptor 2 (VEGFR-2) expression in diabetic microvessels, and the suitability of this molecule as a biomarker for early DR diagnosis. However, a hurdle to translation remained generation of biodegradable nanoprobes that are sufficiently bright for in vivo detection. Here, an adhesive fluorescent nanoprobe with high brightness was developed using biodegradable materials. To achieve that, a fluorophore with bulky hydrophobic groups was encapsulated in the nanoparticles to minimize fluorophore π-π stacking, which diminishes brightness at higher loading contents. The nanoprobe selectively targeted the VEGFR-2 under dynamic flow conditions. Upon systemic injection, the nanoprobes adhered in the retinal microvessels of diabetic mice and were visualized as bright spots in live retinal microscopy. Histology validated the in vivo results and showed binding of the nanoprobes to the microvascular endothelium and firmly adhering leukocytes. Leukocytes were found laden with nanoprobes, indicating the potential for payload transport across the blood-retinal barrier. Our results establish the translational potential of these newly generated nanoprobes in early diagnosis of DR.

In contrast to Western diet, a plant-based, high-fat, low-sugar diet does not exacerbate retinal endothelial injury in streptozotocin-induced diabetes

Controversy remains about how diet affects the vascular endothelial dysfunction associated with disordered insulin-glucose homeostasis. It is postulated that the type and level of certain macronutrients contribute to endothelial dysfunction in vascular diabetes complications. However, it is not well understood how specific macronutrients affect the molecular inflammatory response under conditions of hyperglycemia. Here, we examined retinal microvascular endothelial injury in streptozotocin (STZ)-diabetic rats fed a laboratory Western diet (WD). WD, characterized by its high content of saturated fat, cholesterol, and sugar, significantly increased retinal leukocyte accumulation and endothelial injury in the STZ-diabetic rats. Suppression of endothelial NF-κB signaling in the STZ model reduced the WD-induced increase in leukocyte accumulation. To isolate the effect of dietary fat, we generated high-fat diets with varying fatty acid balance and type. These diets contained moderate amounts of carbohydrates but no sugar. We found that neither high levels of saturated or unsaturated fats per se increased retinal leukocyte accumulation and endothelial injury in the STZ-diabetic rat model but that the combination of high levels of dietary cholesterol with specific saturated fatty acids that are abundant in WD exacerbated leukocyte accumulation and endothelial injury in the retinas of STZ-diabetic rats.-Barakat, A., Nakao, S., Zandi, S., Sun, D., Schmidt-Ullrich, R., Hayes, K. C., Hafezi-Moghadam, A. In contrast to Western diet, a plant-based, high-fat, low-sugar diet does not exacerbate retinal endothelial injury in streptozotocin-induced diabetes.

Cathepsin B-mediated CD18 shedding regulates leukocyte recruitment from angiogenic vessels

Cathepsin B (CtsB) contributes to atherosclerosis and cancer progression by processing the extracellular matrix and promoting angiogenesis. Although CtsB was reported to promote and reduce angiogenesis, there is no mechanistic explanation that reconciles this apparent discrepancy. CtsB cleaves CD18 from the surface of immune cells, but its contribution to angiogenesis has not been studied. We developed an in vivo technique for visualization of immune cell transmigration from corneal vessels toward implanted cytokines. Wild-type (WT) leukocytes extravasated from limbal vessels, angiogenic stalks, and growing tip vessels and migrated toward the cytokines, indicating immune competence of angiogenic vessels. Compared to WT leukocytes, CtsB^-/- leukocytes accumulated in a higher number in angiogenic vessels, but extravasated less toward the implanted cytokine. The accumulated CtsB^-/- leukocytes in angiogenic vessels expressed more CD18. CD18^-/- leukocytes extravasated later than WT leukocytes. However, once extravasated, CD18^-/- leukocytes transmigrated more rapidly than their WT counterparts. These results suggest that, although CD18 facilitates efficient extravasation, outside of the vessel CD18 interaction with the extracellular matrix, it reduced transmigration velocity. Our results reveal an unexpected role for CtsB in leukocyte extravasation and transmigration, which advances our understanding of the complex contribution of CtsB to angiogenesis.-Nakao, S., Zandi, S., Sun, D., Hafezi-Moghadam, A. Cathepsin B-mediated CD18 shedding regulates leukocyte recruitment from angiogenic vessels.

Molecular imaging reveals elevated VEGFR-2 expression in retinal capillaries in diabetes: a novel biomarker for early diagnosis

Diabetic retinopathy (DR) is a microvascular complication of diabetes and a leading cause of vision loss. Biomarkers and methods for early diagnosis of DR are urgently needed. Using a new molecular imaging approach, we show up to 94% higher accumulation of custom designed imaging probes against vascular endothelial growth factor receptor 2 (VEGFR-2) in retinal and choroidal vessels of diabetic animals (P<0.01), compared to normal controls. More than 80% of the VEGFR-2 in the diabetic retina was in the capillaries, compared to 47% in normal controls (P<0.01). Angiography in rabbit retinas revealed microvascular capillaries to be the location for VEGF-A-induced leakage, as expressed by significantly higher rate of fluorophore spreading with VEGF-A injection when compared to vehicle control (26±2 vs. 3±1 μm/s, P<0.05). Immunohistochemistry showed VEGFR-2 expression in capillaries of diabetic animals but not in normal controls. Macular vessels from diabetic patients (n=7) showed significantly more VEGFR-2 compared to nondiabetic controls (n=5) or peripheral retinal regions of the same retinas (P<0.01 in both cases). Here we introduce a new approach for early diagnosis of DR and VEGFR-2 as a molecular marker. VEGFR-2 could become a key diagnostic target, one that might help to prevent retinal vascular leakage and proliferation in diabetic patients.

Recent advances in ophthalmic molecular imaging

The aim of molecular imaging techniques is the visualization of molecular processes and functional changes in living animals and human patients before morphological changes occur at the cellular and tissue level. Ophthalmic molecular imaging is still in its infancy and has mainly been used in small animals for pre-clinical research. The goal of most of these pre-clinical studies is their translation into ophthalmic molecular imaging techniques in clinical care. We discuss various molecular imaging techniques and their applications in ophthalmology.

In vivo molecular imaging in retinal disease

There is an urgent need for early diagnosis in medicine, whereupon effective treatments could prevent irreversible tissue damage. The special structure of the eye provides a unique opportunity for noninvasive light-based imaging of ocular fundus vasculature. To detect endothelial injury at the early and reversible stage of adhesion molecule upregulation, some novel imaging agents that target retinal endothelial molecules were generated. In vivo molecular imaging has a great potential to impact medicine by detecting diseases or screening disease in early stages, identifying extent of disease, selecting disease and patient-specific therapeutic treatment, applying a directed or targeted therapy, and measuring molecular-specific effects of treatment. Current preclinical findings and advances in instrumentation such as endoscopes and microcatheters suggest that these molecular imaging modalities have numerous clinical applications and will be translated into clinical use in the near future.

VAP-1-mediated M2 macrophage infiltration underlies IL-1β- but not VEGF-A-induced lymph- and angiogenesis

Vascular adhesion protein-1 (VAP-1) contributes to inflammatory and angiogenic diseases, including cancer and age-related macular degeneration. It is expressed in blood vessels and contributes to inflammatory leukocyte recruitment. The cytokines IL-1β and vascular endothelial growth factor A (VEGF-A) modulate angiogenesis, lymphangiogenesis, and leukocyte infiltration. The lymphatic endothelium expresses intercellular adhesion molecule-1 and vascular adhesion molecule-1, which facilitate leukocyte transmigration into the lymphatic vessels. However, whether lymphatics express VAP-1 and whether they contribute to cytokine-dependent lymph- and angiogenesis are unknown. We investigated the role of VAP-1 in IL-1β- and VEGF-A-induced lymph- and angiogenesis using the established corneal micropocket assay. IL-1β increased VAP-1 expression in the inflamed cornea. Our in vivo molecular imaging revealed significantly higher VAP-1 expression in neovasculature than in the preexisting vessels. VAP-1 was expressed in blood but not lymphatic vessels in vivo. IL-1β-induced M2 macrophage infiltration and lymph- and angiogenesis were blocked by VAP-1 inhibition. In contrast, VEGF-A-induced lymph- and angiogenesis were unaffected by VAP-1 inhibition. Our results indicate a key role for VAP-1 in lymph- and angiogenesis-related macrophage recruitment. VAP-1 might become a new target for treatment of inflammatory lymph- and angiogenic diseases, including cancer.

Noninvasive molecular imaging reveals role of PAF in leukocyte-endothelial interaction in LPS-induced ocular vascular injury

Uveitis is a systemic immune disease and a common cause of blindness. The eye is an ideal organ for light-based imaging of molecular events underlying vascular and immune diseases. The phospholipid platelet-activating factor (PAF) is an important mediator of inflammation, the action of which in endothelial and immune cells in vivo is not well understood. The purpose of this study was to investigate the role of PAF in endothelial injury in uveitis. Here, we use our recently introduced in vivo molecular imaging approach in combination with the PAF inhibitors WEB 2086 (WEB) and ginkgolide B (GB). The differential inhibitory effects of WEB and GB in reducing LPS-induced endothelial injury in the choroid indicate an important role for PAF-like lipids, which might not require the PAF receptor for their signaling. P-selectin glycoprotein ligand-1-mediated rolling of mouse leukocytes on immobilized P-selectin in our autoperfused microflow chamber assay revealed a significant reduction in rolling velocity on the cells' contact with PAF. Rolling cells that came in contact with PAF rapidly assumed morphological signs of cell activation, indicating that activation during rolling does not require integrins. Our results show a key role for PAF in mediating endothelial and leukocyte activation in acute ocular inflammation. Our in vivo molecular imaging provides a detailed view of cellular and molecular events in the complex physiological setting.

Blood vessel endothelial VEGFR-2 delays lymphangiogenesis: an endogenous trapping mechanism links lymph- and angiogenesis

Angio- and lymphangiogenesis are inherently related processes. However, how blood and lymphatic vessels regulate each other is unknown. This work introduces a novel mechanism explaining the temporal and spatial relation of blood and lymphatic vessels. Vascular endothelial growth factor-A (VEGF-A) surprisingly reduced VEGF-C in the supernatant of blood vessel endothelial cells, suggesting growth factor (GF) clearance by the growing endothelium. The orientation of lymphatic sprouting toward angiogenic vessels and away from exogenous GFs was VEGF-C dependent. In vivo molecular imaging revealed higher VEGF receptor (R)-2 in angiogenic tips compared with normal vessels. Consistently, lymphatic growth was impeded in the angiogenic front. VEGF-C/R-2 complex in the cytoplasm of VEGF-A-treated endothelium indicated that receptor-mediated internalization causes GF clearance from the extracellular matrix. GF clearance by receptor-mediated internalization is a new paradigm explaining various characteristics of lymphatics.

Novel molecular imaging approach for subclinical detection of iritis and evaluation of therapeutic success

There is an urgent need for early diagnosis in medicine, whereupon effective treatments could prevent irreversible tissue damage. Acute anterior chamber inflammation is the most common form of uveitis and a major cause of vision loss. The proximity of the iris vasculature to the light-permeable cornea and its involvement in ocular inflammation make it an ideal target for noninvasive molecular imaging. To accomplish this, carboxylated fluorescent microspheres (MSs) were conjugated with recombinant P-selectin glycoprotein ligand-1 and systemically injected in endotoxin-induced uveitic animals. MS adhesion in the microcirculation of the anterior and posterior chamber was visualized by intravital microscopy and scanning laser ophthalmoscopy. In iritic animals, significantly higher numbers of recombinant P-selectin glycoprotein ligand-1-conjugated MSs adhered to the endothelium (P = 0.03) matching the increase in leukocyte adhesion. Conjugated MSs specifically interacted with firmly adhering leukocytes, allowing quantification of the endogenous immune response. Topical eye drop treatment with dexamethasone (P < 0.01) or cyclosporine A (P < 0.01) significantly lowered MS adhesion in iris vessels. Surprisingly, topical dexamethasone significantly reduced MS interaction in the fundus vessels (P < 0.01), while cyclosporine A did not. In vivo MS accumulation preceded clinical signs of anterior uveitis and leukocyte adhesion in iris vasculature. This work introduces noninvasive subclinical detection of endothelial injury in the iris vasculature, providing a unique opportunity for quantifying vascular injury and immune response in vivo.

Superior sensitivity of novel molecular imaging probe: simultaneously targeting two types of endothelial injury markers

The need remains great for early diagnosis of diseases. The special structure of the eye provides a unique opportunity for noninvasive light-based imaging of fundus vasculature. To detect endothelial injury at the early and reversible stage of adhesion molecule up-regulation, we generated novel imaging agents that target two distinct types of endothelial molecules, a mediator of rolling, P-selectin, and one that mediates firm adhesion, ICAM-1. Interactions of these double-conjugated fluorescent microspheres (MSs) in retinal or choroidal microvasculature were visualized in live animals by scanning laser ophthalmoscopy. The new imaging agents showed significantly higher sensitivity for detection of endothelial injury than singly conjugated MSs (rPSGL-1- or alpha-ICAM-1-conjugated), both in terms of rolling (P<0.01) and firm adhesion (P<0.01). The rolling flux of alpha-ICAM-1-conjugated MSs did not differ in EIU animals, whereas double-conjugated MSs showed significantly higher rolling flux (P<0.01), revealing that ICAM-1 in vivo supports rolling, once MS interaction with the endothelium is initiated. Double-conjugated MSs specifically detected firmly adhering leukocytes (P<0.01), allowing in vivo quantification of immune response. Antiinflammatory treatment with dexamethasone led to reduced leukocyte accumulation (P<0.01) as well as MS interaction (P<0.01), which suggests that treatment success and resolution of inflammation is quantitatively reflected with this molecular imaging approach. This work introduces novel imaging agents for noninvasive detection of endothelial injury in vivo. Our approach may be developed further to diagnose human disease at a much earlier stage than currently possible.