Preparation Steps for Measurement of Reactivity in Mouse Retinal Arterioles Ex Vivo

Methods to measure blood flow and vascular reactivity in the retina

Disturbances of retinal perfusion are involved in the onset and maintenance of several ocular diseases, including diabetic retinopathy, glaucoma, and retinal vascular occlusion. Hence, knowledge on ocular vascular anatomy and function is highly relevant for basic research studies and for clinical judgment and treatment. The retinal vasculature is composed of the superficial, intermediate, and deep vascular layer. Detection of changes in blood flow and vascular diameter especially in smaller vessels is essential to understand and to analyze vascular diseases. Several methods to evaluate blood flow regulation in the retina have been described so far, but no gold standard has been established. For highly reliable assessment of retinal blood flow, exact determination of vessel diameter is necessary. Several measurement methods have already been reported in humans. But for further analysis of retinal vascular diseases, studies in laboratory animals, including genetically modified mice, are important. As for mice, the small vessel size is challenging requiring devices with high optic resolution. In this review, we recapitulate different methods for retinal blood flow and vessel diameter measurement. Moreover, studies in humans and in experimental animals are described.

Intraocular Pressure-Induced Endothelial Dysfunction of Retinal Blood Vessels Is Persistent, but Does Not Trigger Retinal Ganglion Cell Loss

Research has been conducted into vascular abnormalities in the pathogenesis of glaucoma, but conclusions remain controversial. Our aim was to test the hypothesis that retinal endothelial dysfunction induced by elevated intraocular pressure (IOP) persists after IOP normalization, further triggering retinal ganglion cell (RGC) loss. High intraocular pressure (HP) was induced in mice by episcleral vein occlusion (EVO). Retinal vascular function was measured via video microscopy in vitro. The IOP, RGC and their axons survival, levels of oxidative stress and inflammation as well as vascular pericytes coverage, were determined. EVO caused HP for two weeks, which returned to baseline afterwards. Mice with HP exhibited endothelial dysfunction in retinal arterioles, reduced density of RGC and their axons, and loss of pericytes in retinal arterioles. Notably, these values were similar to those of mice with recovered IOP (RP). Levels of oxidative stress and inflammation were increased in HP mice but went back to normal in the RP mice. Our data demonstrate that HP induces persistent endothelial dysfunction in retinal arterioles, which persists one month after RP. Oxidative stress, inflammation, and loss of pericytes appear to be involved in triggering vascular functional deficits. Our data also suggest that retinal endothelial dysfunction does not affect RGC and their axon survival.

In vivo analysis of noise dependent activation of white blood cells and microvascular dysfunction in mice

This article contains supporting information on data collection for the research article entitled “Aircraft noise exposure drives the activation of white blood cells and induces microvascular dysfunction in mice” by Eckrich et al. We found that noise-induced stress triggered microvascular dysfunction via involvement of innate immune-derived reactive oxygen species. In this article, we present the instrumentation of mice with dorsal skinfold chambers for in vivo microscopic imaging of blood flow, interaction of leukocytes with the vascular wall (also by fluorescent labelling of blood cells) and vessel diameter. In addition, we explain the preparation of cerebral arterioles for measurement of vascular reactivity in vitro.•

visualization of noise-dependent effects in dorsal skinfold chamber.

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in vivo microscopy of noise-dependent activation of white blood cells.

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analysis of noise-dependent microvascular dysfunction in dorsal skinfold chamber and cannulated cerebral arterioles.

Betulinic Acid Protects from Ischemia-Reperfusion Injury in the Mouse Retina

Ischemia/reperfusion (I/R) events are involved in the pathophysiology of numerous ocular diseases. The purpose of this study was to test the hypothesis that betulinic acid protects from I/R injury in the mouse retina. Ocular ischemia was induced in mice by increasing intraocular pressure (IOP) to 110 mm Hg for 45 min, while the fellow eye served as a control. One group of mice received betulinic acid (50 mg/kg/day p.o. once daily) and the other group received the vehicle solution only. Eight days after the I/R event, the animals were killed and the retinal wholemounts and optic nerve cross-sections were prepared and stained with cresyl blue or toluidine blue, respectively, to count cells in the ganglion cell layer (GCL) of the retina and axons in the optic nerve. Retinal arteriole responses were measured in isolated retinas by video microscopy. The levels of reactive oxygen species (ROS) were assessed in retinal cryosections and redox gene expression was determined in isolated retinas by quantitative PCR. I/R markedly reduced cell number in the GCL and axon number in the optic nerve of the vehicle-treated mice. In contrast, only a negligible reduction in cell and axon number was observed following I/R in the betulinic acid-treated mice. Endothelial function was markedly reduced and ROS levels were increased in retinal arterioles of vehicle-exposed eyes following I/R, whereas betulinic acid partially prevented vascular endothelial dysfunction and ROS formation. Moreover, betulinic acid boosted mRNA expression for the antioxidant enzymes SOD3 and HO-1 following I/R. Our data provide evidence that betulinic acid protects from I/R injury in the mouse retina. Improvement of vascular endothelial function and the reduction in ROS levels appear to contribute to the neuroprotective effect.

Aircraft noise exposure drives the activation of white blood cells and induces microvascular dysfunction in mice

Epidemiological studies showed that traffic noise has a dose-dependent association with increased cardiovascular morbidity and mortality. Whether microvascular dysfunction contributes significantly to the cardiovascular health effects by noise exposure remains to be established. The connection of inflammation and immune cell interaction with microvascular damage and functional impairment is also not well characterized. Male C57BL/6J mice or gp91phox^−/y mice with genetic deletion of the phagocytic NADPH oxidase catalytic subunit (gp91phox or NOX-2) were used at the age of 8 weeks, randomly instrumented with dorsal skinfold chambers and exposed or not exposed to aircraft noise for 4 days. Proteomic analysis (using mass spectrometry) revealed a pro-inflammatory phenotype induced by noise exposure that was less pronounced in noise-exposed gp91phox^−/y mice. Using in vivo fluorescence microscopy, we found a higher number of adhesive leukocytes in noise-exposed wild type mice. Dorsal microvascular diameter (by trend), red blood cell velocity, and segmental blood flow were also decreased by noise exposure indicating microvascular constriction. All adverse effects on functional parameters were normalized or improved at least by trend in noise-exposed gp91phox^−/y mice. Noise exposure also induced endothelial dysfunction in cerebral microvessels, which was associated with higher oxidative stress burden and inflammation, as measured using video microscopy. We here establish a link between a pro-inflammatory phenotype of plasma, activation of circulating leukocytes and microvascular dysfunction in mice exposed to aircraft noise. The phagocytic NADPH oxidase was identified as a central player in the underlying pathophysiological mechanisms.

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Noise exposure induces a pro-thrombo-inflammatory phenotype in mouse plasma.

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Aircraft noise increases leukocyte-endothelium interactions in dorsal microvessels.

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Noise decreases segmental blood flow/red blood cell velocity in dorsal microvessels.

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Noise increases cerebral microvascular dysfunction and oxidative stress.

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Nox2 deficiency (gp91phox^-/y) improves noise-induced adverse effects.

Ablation of lysozyme M-positive cells prevents aircraft noise-induced vascular damage without improving cerebral side effects

Aircraft noise induces vascular and cerebral inflammation and oxidative stress causing hypertension and cardiovascular/cerebral dysfunction. With the present studies, we sought to determine the role of myeloid cells in the vascular vs. cerebral consequences of exposure to aircraft noise. Toxin-mediated ablation of lysozyme M+ (LysM+) myeloid cells was performed in LysMCreiDTR mice carrying a cre-inducible diphtheria toxin receptor. In the last 4d of toxin treatment, the animals were exposed to noise at maximum and mean sound pressure levels of 85 and 72 dB(A), respectively. Flow cytometry analysis revealed accumulation of CD45+, CD11b+, F4/80+, and Ly6G-Ly6C+ cells in the aortas of noise-exposed mice, which was prevented by LysM+ cell ablation in the periphery, whereas brain infiltrates were even exacerbated upon ablation. Aircraft noise-induced increases in blood pressure and endothelial dysfunction of the aorta and retinal/mesenteric arterioles were almost completely normalized by ablation. Correspondingly, reactive oxygen species in the aorta, heart, and retinal/mesenteric vessels were attenuated in ablated noise-exposed mice, while microglial activation and abundance in the brain was greatly increased. Expression of phagocytic NADPH oxidase (NOX-2) and vascular cell adhesion molecule-1 (VCAM-1) mRNA in the aorta was reduced, while NFκB signaling appeared to be activated in the brain upon ablation. In sum, we show dissociation of cerebral and peripheral inflammatory reactions in response to aircraft noise after LysM+ cell ablation, wherein peripheral myeloid inflammatory cells represent a dominant part of the pathomechanism for noise stress-induced cardiovascular effects and their central nervous counterparts, microglia, as key mediators in stress responses.

Apolipoprotein E Deficiency Causes Endothelial Dysfunction in the Mouse Retina

Objective

Atherogenic lipoproteins may impair vascular reactivity consecutively causing tissue damage in multiple organs via abnormal perfusion and excessive reactive oxygen species generation. We tested the hypothesis that chronic hypercholesterolemia causes endothelial dysfunction and cell loss in the retina.

Methods

Twelve-month-old apolipoprotein E-deficient (ApoE-/-) mice and age-matched wild-type controls were used in this study (n = 8 per genotype for each experiment). Intraocular pressure, blood pressure, and ocular perfusion pressure were determined. Retinal arteriole responses were studied in vitro, and reactive oxygen and nitrogen species were quantified in the retinal and optic nerve cryosections. The expression of the lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) and the NADPH oxidase isoforms, NOX1, NOX2, and NOX4, were determined in retinal cryosections by immunofluorescence microscopy. Pro- and antioxidant redox genes were quantified in retinal explants by PCR. Moreover, cell number in the retinal ganglion cell layer and axon number in the optic nerve was calculated.

Results

Responses to the endothelium-dependent vasodilator, acetylcholine, were markedly impaired in retinal arterioles of ApoE-/- mice (P < 0.01). LOX-1 (P = 0.0007) and NOX2 (P = 0.0027) expressions as well as levels of reactive oxygen species (P = 0.0022) were increased in blood vessels but not in other retinal structures. In contrast, reactive nitrogen species were barely detectable in both mouse genotypes. Messenger RNA for HIF-1α, VEGF-A, NOX1, and NOX2, but also for various antioxidant redox genes was elevated in the retina of ApoE-/- mice. Total cell number in the retinal ganglion cell layer did not differ between ApoE-/- and wild-type mice (P = 0.2171). Also, axon number in the optic nerve did not differ between ApoE-/- and wild-type mice (P = 0.6435).

Conclusion

Apolipoprotein E deficiency induces oxidative stress and endothelial dysfunction in retinal arterioles, which may trigger hypoxia in the retinal tissue. Oxidative stress in nonvascular retinal tissue appears to be prevented by the upregulation of antioxidant redox enzymes, resulting in neuron preservation.

Retinal arteriole reactivity in mice lacking the endothelial nitric oxide synthase (eNOS) gene

Dysfunctional vascular endothelial nitric oxide synthase (eNOS) has been proposed to play a main pathophysiological role in various ocular diseases. The aim of the present study was to test the hypothesis that the chronic lack of eNOS impairs endothelium-dependent vasodilation in retinal arterioles. The relevance of eNOS for mediating vascular responses was studied in retinal vascular preparations from eNOS-deficient mice (eNOS-/-) and wild-type controls in vitro. Changes in luminal diameter in response to vasoactive agents were measured by videomicroscopy. The thromboxane mimetic, U46619, induced similar concentration-dependent constriction of retinal arterioles in eNOS-/- and wild-type mice. Responses to the endothelium-independent vasodilator, nitroprusside, did not differ between both mouse genotypes, either. In contrast, responses to the endothelium-dependent vasodilator, acetylcholine, were blunted in eNOS-/- mice. Non-isoform-selective blockade of either nitric oxide synthase (NOS) or cyclooxygenase (COX) alone did not affect responses to acetylcholine. However, combined blockade of both enzyme families markedly attenuated cholinergic vasodilation. Also, combined blockade of COX and neuronal NOS (nNOS) blunted acetylcholine-induced vasodilation, while combined COX and inducible NOS (iNOS) inhibition had no effect. Simultaneous NOS and COX-1 blockade did not affect cholinergic vasodilation, whereas combined NOS and COX-2 inhibition markedly reduced vasodilation to acetylcholine. These findings are the first to demonstrate that the chronic lack of eNOS is associated with moderate endothelial dysfunction in retinal arterioles. However, eNOS-deficiency is partially compensated by nNOS and COX-2 metabolites, which are reciprocally regulated.