Vitreous fluid biomarkers

Recent Advances and Perspectives in Relation to the Metabolomics-Based Study of Diabetic Retinopathy

Diabetic retinopathy (DR), a prevalent microvascular complication of diabetes, is a major cause of acquired blindness in adults. Currently, a clinical diagnosis of DR primarily relies on fundus fluorescein angiography, with a limited availability of effective biomarkers. Metabolomics, a discipline dedicated to scrutinizing the response of various metabolites within living organisms, has shown noteworthy advancements in uncovering metabolic disorders and identifying key metabolites associated with DR in recent years. Consequently, this review aims to present the latest advancements in metabolomics techniques and comprehensively discuss the principal metabolic outcomes derived from analyzing blood, vitreous humor, aqueous humor, urine, and fecal samples.

Retinal fluid is associated with cytokines of aqueous humor in age-related macular degeneration using automatic 3-dimensional quantification

Background: To explain the biological role of cytokines in the eye and the possible role of cytokines in the pathogenesis of neovascular age-related macular degeneration (nAMD) by comparing the correlation between cytokine of aqueous humor concentration and optical coherence tomography (OCT) retinal fluid. Methods: Spectral-domain OCT (SD-OCT) images and aqueous humor samples were collected from 20 nAMD patient's three clinical visits. Retinal fluid volume in OCT was automatically quantified using deep learning--Deeplabv3+. Eighteen cytokines were detected in aqueous humor using the Luminex technology. OCT fluid volume measurements were correlated with changes in aqueous humor cytokine levels using Pearson's correlation coefficient (PCC). Results: The patients with intraretinal fluid (IRF) showed significantly lower levels of cytokines, such as C-X-C motif chemokine ligand 2 (CXCL2) (p = 0.03) and CXCL11 (p = 0.009), compared with the patients without IRF. And the IRF volume was negatively correlated with CXCL2 (r = -0.407, p = 0.048) and CXCL11 (r = -0.410, p = 0.046) concentration in the patients with IRF. Meanwhile, the subretinal fluid (SRF) volume was positively correlated with vascular endothelial growth factor (VEGF) concentration (r = 0.299, p = 0.027) and negatively correlated with interleukin (IL)-36β concentration (r = -0.295, p = 0.029) in the patients with SRF. Conclusion: Decreased level of VEGF was associated with decreased OCT-based retinal fluid volume in nAMD patients, while increased levels of CXCL2, CXCL11, and IL-36β were associated with decreased OCT-based retinal fluid volume in nAMD patients, which may suggest a role for inflammatory cytokines in retinal morphological changes and pathogenesis of nAMD patients.

Protein Biomarkers in Uveitis

The diseases affecting the retina or uvea (iris, ciliary body, or choroid) generate changes in the biochemical or protein composition of ocular fluids/tissues due to disruption of blood-retinal barrier. Ocular infections and inflammations are sight-threatening diseases associated with various infectious and non-infectious etiologies. Several etiological entities cause uveitis, a complex intraocular inflammatory disease. These causes of uveitis differ in different populations due to geographical, racial, and socioeconomic variations. While clinical appearance is sufficiently diagnostic in many diseases, some of the uveitic entities manifest nonspecific or atypical clinical presentation. Identification of biomarkers in such diseases is an important aid in their diagnostic armamentarium. Different diseases and their different severity states release varying concentrations of proteins, which can serve as biomarkers. Proteomics is a high throughput technology and a powerful screening tool for serum biomarkers in various diseases that identifies proteins by mass spectrometry and helps to improve the understanding of pathogenesis of a disease. Proteins determine the biological state of a cell. Once identified as biomarkers, they serve as future diagnostic and pharmaceutical targets. With a potential to redirect the diagnosis of idiopathic uveitis, ocular proteomics provide a new insight into the pathophysiology and therapeutics of various ocular inflammatory diseases. Tears, aqueous and vitreous humor represent potential repositories for proteomic biomarkers discovery in uveitis. With an extensive proteomics work done on animal models of uveitis, various types of human uveitis are being subjected to proteome analysis for biomarker discovery in different ocular fluids (vitreous, aqueous, or tears).

Osteopontin in vitreous and idiopathic epiretinal membranes

PURPOSE

To investigate osteopontin (OPN) expression in vitreous and in related idiopathic epiretinal membranes (ERMs), with respect to VEGF-A, IL8, MIP1α, IL6, and IL33, and correlate OPN expression with disease staging.

METHODS

Fifteen (15) vitreous and allied ERMs were collected at the time of therapeutic vitreoretinal surgery. Additional 5 vitreous and 10 ERMs (historical collection) were used. Biochemical and molecular analysis of OPN was performed in clear vitreous, vitreal pelleted cells, and ERMs. Double-immunofluorescence analysis (OPN - GFAP and OPN - αSMA) was performed on paraffin and whole-mounted ERMs. Vitreal OPN levels were correlated to those of VEGF-A, IL8, MIP1α, IL6, and IL33.

RESULTS

High OPN levels were observed in vitreal samples, and OPN transcripts were amplified in vitreal cells and related ERMs. OPN immunoreactivity was found in ERMs, mainly in GFAP-bearing (Muller cells) and to a less extend in αSMA-expressing (myofibroblasts) cells. OPN levels were highest at early stages of ERM formation and positively correlated to VEGF-A and MIP1α.

CONCLUSIONS

High OPN levels in vitreous, OPN transcripts in vitreal cells/ERMs, OPN immunoreactivity in activated Müller cells and contractile myofibroblasts, as well as the correlation with VEGF-A and MIP1α fulfill the potential involvement of OPN in both inflammation and tissue remodeling that takes part in vitreoretinal interface disorders. The highest OPN levels at early stages of ERM formation would prospect OPN as a potential biomarker for disease severity.

Proteomic Biomarkers of Retinal Inflammation in Diabetic Retinopathy

Diabetic retinopathy (DR), a sight-threatening neurovasculopathy, is the leading cause of irreversible blindness in the developed world. DR arises as the result of prolonged hyperglycemia and is characterized by leaky retinal vasculature, retinal ischemia, retinal inflammation, angiogenesis, and neovascularization. The number of DR patients is growing with an increase in the elderly population, and therapeutic approaches are limited, therefore, new therapies to prevent retinal injury and enhance repair are a critical unmet need. Besides vascular endothelial growth factor (VEGF)-induced vascular proliferation, several other mechanisms are important in the pathogenesis of diabetic retinopathy, including vascular inflammation. Thus, combining anti-VEGF therapy with other new therapies targeting these pathophysiological pathways of DR may further optimize treatment outcomes. Technological advancements have allowed for high-throughput proteomic studies examining biofluids such as aqueous humor, vitreous humor, tear, and serum. Many DR biomarkers have been identified, especially proteins involved in retinal inflammatory processes. This review attempts to summarize the proteomic biomarkers of DR-associated retinal inflammation identified over the last several years.

A Simple Spontaneous Vitreal Reflux Collecting Procedure During Intravitreal Injection: Set-Up and Validation Studies

AIM

To set-up a simple technique for collecting spontaneous vitreal reflux (VR) in patients undergoing intravitreal injection. Both total protein concentration and vascular endothelial growth factor (VEGF)/Interleukin 13 (IL13) levels were used to validate the technique.

METHODS

Sixty consecutive patients with neovascular age-related macular degeneration (nAMD, vitreal reflux drop, VR) and 10 patients underwent vitrectomy for macular hole (whole vitreous removal) were enrolled for the study as controls. Thirty-three out of 60 patients were also subjected to tear sampling. VR sampling was performed after the intravitreal injection. Four sampling tools (10 Schirmer strips, 10 microsponges, 20 millipore filters; 20 micropipettes) were tested. Analysis of protein concentration/composition was performed between VR samples and vitreous samples to analyze the difference. The concentration of VEGF and IL 13 levels between cases and control samples were compared.

RESULTS

Millipore and micropipette techniques allowed the collection of higher protein concentrations in VR samples, comparison of both protein concentrations revealed no significant difference in the protein profile. However, the micropipette sampling was found easier to perform and did not require additional protein extraction from a solid support (membrane). Indeed, tear proteins and drug contaminants were not detected in micropipette samples. Increased VEGF levels were detected in naive VR group and to a less extend in VR group of nAMD patients undergoing intravitreal injection, with respect to the controls (macular holes). No significant differences in IL13 levels were quantified in nAMD sub-groups, as compared to naive and controls.

CONCLUSIONS

Overall, we provide evidence for a safe method for sampling VR at the end of intravitreal injection. This procedure might represent an interesting approach either for the prognosis of disease or monitoring the efficacy of intravitreal therapy.

Elevated vitreous body glial fibrillary acidic protein in retinal diseases

Purpose

Increased expression of glial fibrillary acidic protein (GFAP) is a characteristic of gliotic activation (Müller cells and astrocytes) in the retina. This study assessed vitreous body GFAP levels in various forms of retinal pathology.

Methods

This prospective study included 82 patients who underwent vitrectomy (46 retinal detachments (RDs), 13 macular hole (MHs), 15 epiretinal glioses (EGs), 8 organ donors). An established enzyme–linked immunosorbent assay (ELISA, SMI26) was used for quantification of GFAP.

Results

The highest concentration of vitreous body GFAP in organ donors was 20 pg/mL and it was used as the cutoff. A significant proportion of patients suffering from RD (65 %) to EG (53 %) had vitreous body GFAP levels above this cutoff when compared to organ donors (0 %, p < 0.0001, p = 0.0194, respectively, Fisher’s exact test) and MH (8 %, p < 0.0001, p = 0.0157, respectively). In RD and EG, vitreous body GFAP levels were correlated with axial length (R = 0.69, R = 0.52, p < 0.05 for both).

Conclusions

The data suggest that human vitreous body GFAP is a protein biomarker for glial activation in response to retinal pathologies. Vitreous body GFAP levels may be of interest as a surrogate outcome for experimental treatment strategies in translational studies.

Translational vitreous proteomics

The vitreous is an extracellular matrix that is still poorly understood. Although many constituents and characteristics have been previously determined, there are many attributes still being discovered. Currently, using protein arrays, MS, and bioinformatics, the vitreous provides a wealth of knowledge regarding ocular diseases and potential targets for personalized therapeutics.

In-depth mass spectrometric mapping of the human vitreous proteome

Mapping of proteins involved in normal eye functions is a prerequisite to identify pathological changes during eye disease processes. We therefore analysed the proteome of human vitreous by applying in-depth proteomic screening technologies. For ethical reasons human vitreous samples were obtained by vitrectomy from “surrogate normal patients” with epiretinal gliosis that is considered to constitute only negligible pathological vitreoretinal changes. We applied different protein prefractionation strategies including liquid phase isoelectric focussing, 1D SDS gel electrophoresis and a combination of both and compared the number of identified proteins obtained by the respective method. Liquid phase isoelectric focussing followed by SDS gel electrophoresis increased the number of identified proteins by a factor of five compared to the analysis of crude unseparated human vitreous. Depending on the prefractionation method proteins were subjected to trypsin digestion either in-gel or in solution and the resulting peptides were analysed on a UPLC system coupled online to an LTQ Orbitrap XL mass spectrometer. The obtained mass spectra were searched against the SwissProt database using the Mascot search engine. Bioinformatics tools were used to annotate known biological functions to the detected proteins. Following this strategy we examined the vitreous proteomes of three individuals and identified 1111 unique proteins. Besides structural, transport and binding proteins, we detected 261 proteins with known enzymatic activity, 51 proteases, 35 protease inhibitors, 35 members of complement and coagulation cascades, 15 peptide hormones, 5 growth factors, 11 cytokines, 47 receptors, 30 proteins of visual perception, 91 proteins involved in apoptosis regulation and 265 proteins with signalling activity. This highly complex mixture strikingly differs from the human plasma proteome. Thus human vitreous fluid seems to be a unique body fluid. 262 unique proteins were detected which are present in all three patient samples indicating that these might represent the constitutive protein pattern of human vitreous. The presented catalogue of human vitreous proteins will enhance our understanding of physiological processes in the eye and provides the groundwork for future studies on pathological vitreous proteome changes.

Differential accumulation of secreted AbetaPP metabolites in ocular fluids

Amyloid-beta (Abeta) accumulates in several types of retinal degeneration and in Alzheimer's disease (AD), but its source has been unclear. We detected the neuronal 695 amino acid form of amyloid-beta protein precursor (AbetaPP) in the normal retina and AbetaPP751 in the retinal pigment epithelium (RPE) and anterior eye tissues. Similar to the brain, alpha- and beta-secretases cleaved AbetaPP to soluble derivatives (sAbetaPP) alpha or beta and membrane-bound C-terminal fragments alpha or beta in the retina and RPE. Levels of sAbetaPP were particularly high in the vitreous and low in aqueous humor revealing a molecular barrier for AbetaPP. In contrast, Abeta40 and Abeta42 levels were only 50% lower in the aqueous than the vitreous humor, indicating relatively barrier-free movement of Abeta. These studies demonstrated a relatively high yield of AbetaPP and Abeta in the ocular fluids, which may serve as a trackable marker for AD. In addition, failure of free clearance from the eye may trigger retina degeneration in a manner similar to Abeta-related neurodegeneration in AD.

Activated VEGF receptor shed into the vitreous in eyes with wet AMD: a new class of biomarkers in the vitreous with potential for predicting the treatment timing and monitoring response

OBJECTIVE

To examine whether phosphorylated vascular endothelial growth factor (VEGF) receptors shed into the vitreous reflect the ongoing retinal and choroidal signal pathway activity in wet age-related macular degeneration (AMD).

METHODS

Vitreous samples obtained immediately prior to anti-VEGF injection from 11 patients with choroidal neovascularization were analyzed using reverse-phase microarrays. Two patients had samples collected at the time of injection and 1 month later. Samples from 5 patients were collected prior to vitrectomy for macular hole, epiretinal membrane, or retinal detachment.

RESULTS

Phosphorylated forms of VEGF receptor (VEGFR Y996 and Y1175), platelet-derived growth factor receptor beta (PDGFRbeta Y716 and Y751), and c-KIT (Y703) were present in the vitreous. A significant difference in PDGFRbeta Y751 (P < .002), VEGFR Y996 (P < .04), and VEGFR Y1175 (P < .006), but not c-KIT Y703 (P < .05) or PDGFRbeta Y716 (P < .96), was noted for the responders to treatment (n = 5) compared with nonresponders (n = 6) and controls (n = 5).

CONCLUSIONS

Vitreous levels of activated receptors constitute a new class of biomarkers. Activated forms of VEGF and PDGF receptors, previously not known to exist in the vitreous, correlate with response to anti-VEGF therapy. These findings could provide the basis for the development of individualized treatment and discovery of new therapeutic targets.