Alpha-Smooth Muscle Actin Expression and Parafoveal Blood Flow Pathways Are Altered in Preclinical Diabetic Retinopathy

Development of Immunostaining Protocols for 3D Visualization of Pericytes in Human Retinal Flatmounts

Vascular pericytes are widely present across the human body and crucial in regulating vascular flow, permeability, and homeostasis. In the human retina, pericytes are important for forming and maintaining the blood-retinal barrier, as well as for autoregulation of blood flow. Pericyte loss has been implicated in various pathological conditions. Visualization of pericytes by immunofluorescence (IF) staining provides valuable information on pericyte number, morphology, location, and on expression of anatomic and functional markers. However, species-specific differences in pericyte marker expression exist. In this study, we aimed to develop a novel IF co-staining protocol to detect the pericyte markers NG2, PDGFRβ, αSMA, CD13, and RFC1 in human retinal flatmounts. Unlike retinal sections, retinal flatmounts enable 3D visualization of pericyte distribution across the entire vascular network. Key optimizations included tailoring the fixation method, blocking buffer composition and antibody solvent, as well as using jasplakinolide to enhance αSMA detection. Our protocol successfully enabled double staining of NG2 and PDGFRβ, as well as αSMA and PDGFRβ, whereas CD13 and RFC1 expression was not detectable in human retinal flatmounts. This novel 3D IF protocol enhances in situ visualization of human retinal pericytes, enabling accurate studies of their role in vascular health and disease to aid targeted therapy development.

Predictors of Peripheral Retinal Non-Perfusion in Clinically Significant Diabetic Macular Edema

Background/Objectives: Diabetic macular edema (DME) is a significant cause of vision loss. The development of peripheral non-perfusion (PNP) might be associated with the natural course, severity, and treatment of DME. The present study seeks to understand the predictive power of central macular changes and clinico-demographic features for PNP in patients with clinically significant DME. Methods: A prospective study using contemporaneous multi-modal retinal imaging was performed. In total, 48 eyes with DME from 33 patients were enrolled. Demographic, clinical history, laboratory measures, ultrawide field photography, fluorescein angiography, optical coherence tomography (OCT), and OCT angiography results were acquired. Anatomic and vascular features of the central macula and peripheral retina were quantified from retinal images. Separate (generalized) linear mixed models were used to assess differences between PNP present and absent groups. Mixed effects logistic regression was used to assess which features have predictive power for PNP. Results: Variables with significant differences between eyes with and without PNP were insulin use (p = 0.0001), PRP treatment (p = 0.0003), and diffuse fluorescein leakage (p = 0.013). Importantly, there were no significant differences for any of the macular vascular metrics including vessel density (p = 0.15) and foveal avascular zone (FAZ) area (p = 0.58 and capillary tortuosity (p = 0.55). Features with significant predictive power (all p < 0.001) were subretinal fluid, FAZ eccentricity, ellipsoid zone disruption, past anti-VEGF therapy, insulin use, and no ischemic heart disease. Conclusions: In the setting of DME, macular vascular changes did not predict the presence of PNP. Therefore, in order to detect peripheral non-perfusion in DME, our results implicate the importance of peripheral retinal vascular imaging.

Interpericyte Tunneling Nanotubes Are Nonuniformly Distributed in the Human Macula

Purpose

Pericyte-to-pericyte communication via interpericyte tunneling nanotubes (IP-TNTs) is an important mechanism by which spatial and temporal precision in neurovascular coupling is achieved. This study quantifies the distribution and morphologic characteristics of IP-TNTs in the normal human macula.

Methods

Ultra high-resolution, three-dimensional microscopic imaging of 11 perfusion-labeled normal human donor eyes was performed. Immunofluorescent markers for collagen IV, glial fibrillary acidic protein, nuclei, α-smooth muscle actin and phalloidin were used to distinguish IP-TNTs from perfused/nonperfused capillaries and glia processes. IP-TNT length, diameter and density in each capillary plexus was quantified and compared.

Results

IP-TNTs were present in all capillary plexuses. IP-TNTs bridged capillary segments within and between capillary plexuses but did not connect capillaries to arterioles or venules. Mean length of IP-TNTs was 72.6 ± 39.5µm (range 14.0 to 202 µm) and mean diameter was 1.0 ± 0.42µm. IP-TNT length was non-normally distributed with a right-skewed distribution and 43% were 'short' (<55µm). Diameters were greater in the "long" (1.13 ± 0.44 µm) than "short" (0.82 ± 0.33 µm; P < 0.001) IP-TNTs. Density of IP-TNTs was greater in the superficial vascular plexus (3.80 ± 0.69 per 500 µm2) compared to the intermediate (1.85 ± 0.80 per 500 µm2; P < 0. 0001) and deep capillary plexus (1.58 ± 0.84 per 500 µm2; P < 0.0001). No significant difference in IP-TNT density was found between the four macula quadrants (P = 0.98).

Conclusions

The distribution of IP-TNTs in the human macula is non-uniform and is associated with the compartmentalized nature of retinal energy consumption and microvascular perfusion. The nonuniform properties of IP-TNTs may predispose distinct vascular beds to injury in conditions such as diabetes.

Changes in foveal avascular zone parameters in individuals with prediabetes compared to normoglycemic controls: a systematic review and meta-analysis

To investigate changes in foveal avascular zone parameters in individuals with prediabetes compared to normoglycemic controls. PUBMED, Scopus and Cochrane Library were searched for published articles comparing the foveal avascular zone between prediabetic individuals and normoglycemic controls as assessed by optical coherence tomography angiography (OCTA). Standardised Mean Difference (SMD) with 95% confidence interval (CI) was computed for the comparison. A total of seven studies were included in our analysis, 6 provided data for the superficial capillary plexus from 345 eyes of individuals with prediabetes and 347 eyes of controls and 4 provided data on the deep capillary plexus from 285 eyes from individuals with prediabetes and 325 eyes of controls. Foveal avascular zone (FAZ) area in the superficial capillary plexus was enlarged in individuals with prediabetes compared to normoglycemic controls (SMD = 0.23, 95% CI = 0.03-0.44, p = 0.03, I2 = 27%, 6 studies). There was no statistically significant change in the deep capillary plexus FAZ area between the two groups (SMD = 1.14, 95% CI = -0.06-2.34, p = 0.06, I2 = 97%, 4 studies). FAZ area in the superficial capillary plexus was larger in individuals diagnosed with prediabetes compared to normoglycemic controls. This finding suggests that prediabetes could induce retinal microvascular changes before the onset of clinical diabetes. More original studies are needed to validate the results of the current meta-analysis.

Current understanding of subclinical diabetic retinopathy informed by histology and high-resolution in vivo imaging

The escalating incidence of diabetes mellitus has amplified the global impact of diabetic retinopathy. There are known structural and functional changes in the diabetic retina that precede the fundus photography abnormalities which currently are used to diagnose clinical diabetic retinopathy. Understanding these subclinical alterations is important for effective disease management. Histology and high-resolution clinical imaging reveal that the entire neurovascular unit, comprised of retinal vasculature, neurons and glial cells, is affected in subclinical disease. Early functional manifestations are seen in the form of blood flow and electroretinography disturbances. Structurally, there are alterations in the cellular components of vasculature, glia and the neuronal network. On clinical imaging, changes to vessel density and thickness of neuronal layers are observed. How these subclinical disturbances interact and ultimately manifest as clinical disease remains elusive. However, this knowledge reveals potential early therapeutic targets and the need for imaging modalities that can detect subclinical changes in a clinical setting.

Modulation of Extracellular Matrix Composition and Chronic Inflammation with Pirfenidone Promotes Scar Reduction in Retinal Wound Repair

Wound repair in the retina is a complex mechanism, and a deeper understanding of it is necessary for the development of effective treatments to slow down or even prevent degenerative processes leading to photoreceptor loss. In this study, we harnessed a laser-induced retinal degeneration model (532-nm laser photocoagulation with 300 μm spot size, 60 ms duration and 60 mV pulse), enabling a profound molecular elucidation and a comprehensive, prolonged observation of the wound healing sequence in a murine laser-induced degeneration model (C57BL/6J mice, 6-12 weeks) until day 49 post-laser. Our observations included the expression of specific extracellular matrix proteins and myofibroblast activity, along with an analysis of gene expression related to extracellular matrix and adhesion molecules through RNA measurements. Furthermore, the administration of pirfenidone (10 mg/kg via drinking water), an anti-inflammatory and anti-fibrotic compound, was used to modulate scar formation after laser treatment. Our data revealed upregulated collagen expression in late regenerative phases and sustained inflammation in the damaged tissue. Notably, treatment with pirfenidone was found to mitigate scar tissue formation, effectively downregulating collagen production and diminishing the presence of inflammatory markers. However, it did not lead to the regeneration of the photoreceptor layer.

scAAV2-Mediated Expression of Thioredoxin 2 and C3 Transferase Prevents Retinal Ganglion Cell Death and Lowers Intraocular Pressure in a Mouse Model of Glaucoma

Elevated intraocular pressure (IOP) in glaucoma causes retinal ganglion cell (RGC) loss and damage to the optic nerve. Although IOP is controlled pharmacologically, no treatment is available to restore retinal and optic nerve function. In this paper, we aimed to develop a novel gene therapy for glaucoma using an AAV2-based thioredoxin 2 (Trx2)-exoenzyme C3 transferase (C3) fusion protein expression vector (scAAV2-Trx2-C3). We evaluated the therapeutic effects of this vector in vitro and in vivo using dexamethasone (DEX)-induced glaucoma models. We found that scAAV2-Trx2-C3-treated HeLa cells had significantly reduced GTP-bound active RhoA and increased phosphor-cofilin Ser3 protein expression levels. scAAV2-Trx2-C3 was also shown to inhibit oxidative stress, fibronectin expression, and alpha-SMA expression in DEX-treated HeLa cells. NeuN immunostaining and TUNEL assay in mouse retinal tissues was performed to evaluate its neuroprotective effect upon RGCs, whereas changes in mouse IOP were monitored via rebound tonometer. The present study showed that scAAV2-Trx2-C3 can protect RGCs from degeneration and reduce IOP in a DEX-induced mouse model of glaucoma, while immunohistochemistry revealed that the expression of fibronectin and alpha-SMA was decreased after the transduction of scAAV2-Trx2-C3 in murine eye tissues. Our results suggest that AAV2-Trx2-C3 modulates the outflow resistance of the trabecular meshwork, protects retinal and other ocular tissues from oxidative damage, and may lead to the development of a gene therapeutic for glaucoma.

Variability in Capillary Perfusion Is Increased in Regions of Retinal Ischemia Due to Branch Retinal Vein Occlusion

Purpose

To investigate alterations in macular perfusion variability due to branch retinal vein occlusion (BRVO) using a novel approach based on optical coherence tomography angiography (OCTA) coefficient of variation (CoV) analysis.

Methods

Thirteen eyes of 13 patients with macular ischemia due to BRVO were studied. Multiple consecutive en face OCTA images were acquired. Bias field correction, spatial alignment, and normalization of intensities across the images were performed followed by pixelwise computation of standard deviation divided by the mean to generate a CoV map. Region of interest-based CoV values, derived from this map, for arterioles, venules, and the microvasculature were compared between regions with macular ischemia and control areas of the same eye. Control areas were regions of the same macula that were not affected by the BRVO and had normal retinal vascular structure as seen on multimodal imaging and normal retinal vascular density measurements as quantified using OCTA.

Results

CoV increased by a mean value of 17.6% within the microvasculature of ischemic regions compared to the control microvasculature (P < 0.0001). CoV measurements of microvasculature were consistently greater in the ischemic area of all 13 eyes compared to control. There were no differences in CoV measurements between ischemic and control areas for arterioles (P = 0.13) and venules (P = 1.0).

Conclusions

Greater variability in microvasculature perfusion occurs at sites of macular ischemia due to BRVO. We report a novel way for quantifying macular perfusion variability using OCTA. This technique may have applicability for studying the pathophysiology of other retinal vascular diseases.

Triciribine attenuates pathological neovascularization and vascular permeability in a mouse model of proliferative retinopathy

Proliferative retinopathies are the leading cause of irreversible blindness in all ages, and there is a critical need to identify novel therapies. We investigated the impact of triciribine (TCBN), a tricyclic nucleoside analog and a weak Akt inhibitor, on retinal neurovascular injury, vascular permeability, and inflammation in oxygen-induced retinopathy (OIR). Post-natal day 7 (P7) mouse pups were subjected to OIR, and treated (i.p.) with TCBN or vehicle from P14-P16 and compared with age-matched, normoxic, vehicle or TCBN-treated controls. P17 retinas were processed for flat mounts, immunostaining, Western blotting, and qRT-PCR studies. Fluorescein angiography, electroretinography, and spectral domain optical coherence tomography were performed on days P21, P26, and P30, respectively. TCBN treatment significantly reduced pathological neovascularization, vaso-obliteration, and inflammation marked by reduced TNFα, IL6, MCP-1, Iba1, and F4/80 (macrophage/microglia markers) expression compared to the vehicle-treated OIR mouse retinas. Pathological expression of VEGF (vascular endothelial growth factor), and claudin-5 compromised the blood-retinal barrier integrity in the OIR retinas correlating with increased vascular permeability and neovascular tuft formation, which were blunted by TCBN treatment. Of note, there were no changes in the retinal architecture or retinal cell function in response to TCBN in the normoxia or OIR mice. We conclude that TCBN protects against pathological neovascularization, restores blood-retinal barrier homeostasis, and reduces retinal inflammation without adversely affecting the retinal structure and neuronal function in a mouse model of OIR. Our data suggest that TCBN may provide a novel therapeutic option for proliferative retinopathy.

Region-related and layer-specific permeability of the iris vasculature with morphological mechanism: A novel understanding of blood-aqueous barrier

The permeability of iris blood vessels has an important role in maintaining aqueous humor (AH) homeostasis, contributing to variation in iris volume and probably the pathogenesis of angle closure glaucoma. This study investigates the permeability of the iris microvasculature to plasma-derived protein and correspond it with the morphologic characteristics of vascular mural cells (MCs). Twenty-two enucleated porcine eyes were used in this study. 12 eyes were micro-perfused with vehicle alone as control or with FITC-albumin as a marker of protein leakage and histological sections subsequently made to examine for FITC-albumin presence. The other 10 eyes were immunolabeled via micro-perfusion for αSMA and VE-cadherin to investigate their topographic distribution in the porcine iris vasculature, and to cross correspond with the locations of FITC-albumin deposits. Distribution of FITC-signals exhibited a site-dependent pattern and time-dependent change in the iris. Fluorescence was initially detected around capillaries in the superficial and deep layer of the iris microvascular network. The pupillary region and the iris root retained more fluorescent signal than the iridal ciliary region. At low magnification, αSMA labelling displayed a regional variation which was inversely correlated with vascular permeability. At the cellular level, αSMA labeling corresponded with vascular MCs distribution in the iris vascular network. The correspondence between iris microvascular permeability to FITC-albumin and the pattern of αSMA distribution and MCs coverage adds to the understanding of the elements comprising the blood-aqueous barrier with implications for the bio-mechanics of iris volume change.

Studies of the retinal microcirculation using human donor eyes and high-resolution clinical imaging: Insights gained to guide future research in diabetic retinopathy

The microcirculation plays a key role in delivering oxygen to and removing metabolic wastes from energy-intensive retinal neurons. Microvascular changes are a hallmark feature of diabetic retinopathy (DR), a major cause of irreversible vision loss globally. Early investigators have performed landmark studies characterising the pathologic manifestations of DR. Previous works have collectively informed us of the clinical stages of DR and the retinal manifestations associated with devastating vision loss. Since these reports, major advancements in histologic techniques coupled with three-dimensional image processing has facilitated a deeper understanding of the structural characteristics in the healthy and diseased retinal circulation. Furthermore, breakthroughs in high-resolution retinal imaging have facilitated clinical translation of histologic knowledge to detect and monitor progression of microcirculatory disturbances with greater precision. Isolated perfusion techniques have been applied to human donor eyes to further our understanding of the cytoarchitectural characteristics of the normal human retinal circulation as well as provide novel insights into the pathophysiology of DR. Histology has been used to validate emerging in vivo retinal imaging techniques such as optical coherence tomography angiography. This report provides an overview of our research on the human retinal microcirculation in the context of the current ophthalmic literature. We commence by proposing a standardised histologic lexicon for characterising the human retinal microcirculation and subsequently discuss the pathophysiologic mechanisms underlying key manifestations of DR, with a focus on microaneurysms and retinal ischaemia. The advantages and limitations of current retinal imaging modalities as determined using histologic validation are also presented. We conclude with an overview of the implications of our research and provide a perspective on future directions in DR research.