Protein content in aqueous humor from patients with pseudoexfoliation (PEX) investigated by capillary LC MALDI-TOF/TOF MS

Changes in intraocular pressure during the first 24 h after transscleral cyclophotocoagulation

AIMS

To estimate the changes in intraocular pressure (IOP) during the first 24 h after transscleral cyclophotocoagulation (TCP).

METHODS

A prospective single-centre study, where patients with glaucoma destined for treatment with TCP were asked for participation. The IOP was measured prior to TCP and at 1, 2, 4, 6 and 24 h post-TCP. An IOP spike was defined as an elevation of IOP of ≥5 mmHg compared with baseline. The visual acuity (VA) was examined at baseline and after 24 h.

RESULTS

The mean IOP prior to TCP in 58 eyes of 58 patients was 26.2 (±8.9 SD) mmHg. Twenty-three eyes (40%) experienced an IOP spike at some examination timepoint during the first 24 h. The mean value of the IOP spike was 12.1 (±6.9) mmHg. Fifty-six per cent of the eyes with pseudoexfoliation glaucoma (PEXG) experienced an IOP spike, and 16% had an IOP spike ≥20 mmHg. The IOP was significantly reduced at the 24 h examination by 8.1 (±7.8) mmHg (n = 58). The VA 24 h after TCP was unchanged compared with baseline.

CONCLUSION

Clinically significant IOP spikes were common in the first 24 h post-TCP. Almost one in five eyes had an increase of 10 mmHg and in almost one in 10 eyes, the IOP increase was 20 mmHg or higher. Eyes with PEXG had a higher occurrence of IOP spikes and displayed a greater magnitude of IOP elevation. Prophylactic post-operative IOP-lowering medication should be considered to prevent further glaucoma damage.

Platelet Proteome Reveals Novel Targets for Hypercoagulation in Pseudoexfoliation Syndrome

Pseudoexfoliation syndrome (PEX) is characterized by the accumulation of abnormal extracellular matrix material in ocular and non-ocular tissues, including blood vessel walls. Clot-forming dysfunction might be responsible for venous thrombosis in PEX. We investigated global coagulation, the proteome, and functions of platelets in PEX patients and aimed to determine prognostic biomarkers for thrombosis risk in PEX. Peripheral blood was collected from PEX and retinal vein occlusion (RVO) patients, and age-sex matched controls. Viscoelastic hemostasis was evaluated by rotational thromboelastometry (ROTEM). Platelet markers (CD41, CD42, CD61, and CD62p) and endothelial markers (P-selectin, E-selectin, and von Willebrand factor) were investigated by flow cytometry and ELISA, respectively. The platelet proteome was analyzed by 2D fluorescence difference gel electrophoresis followed by mass spectrometry. Clot formation time (CFT) is significantly reduced in PEX patients compared to the controls (p < 0.05). P-selectin levels were higher in PEX patients than in controls (p < 0.05); E-selectin and von Willebrand factor remained unchanged. The monitorization of CFT by ROTEM, and soluble P-selectin, may help assess thrombotic risk in PEX patients. Proteomic analysis revealed differential expression of Profilin-1 in platelets. Profilin-1 regulates the stability of actin-cytoskeleton and may contribute to impaired platelet hemostatic functions. Increased P-selectin levels together with impaired coagulation dynamics might be responsible for the thrombotic events in PEX disease.

Secondary glaucoma: Toward interventions based on molecular underpinnings

Glaucoma is a heterogeneous group of progressive diseases that leads to irreversible blindness. Secondary glaucoma refers to glaucoma caused by a known underlying condition. Pseudoexfoliation and pigment dispersion syndromes are common causes of secondary glaucoma. Their respective deposits may obstruct the trabecular meshwork, leading to aqueous humor outflow resistance, ocular hypertension, and optic neuropathy. There are no disease-specific interventions available for either. Pseudoexfoliation syndrome is characterized by fibrillar deposits (pseudoexfoliative material) on anterior segment structures. Over a decade of multiomics analyses taken together with the current knowledge on pseudoexfoliative glaucoma warrant a re-think of mechanistic possibilities. We propose that the presence of nucleation centers (e.g., vitamin D binding protein), crosslinking enzymes (e.g., transglutaminase 2), aberrant extracellular matrix, flawed endocytosis, and abnormal aqueous-blood barrier contribute to the formation of proteolytically resistant pseudoexfoliative material. Pigment dispersion syndrome is characterized by abnormal iridolenticular contact that disrupts iris pigment epithelium and liberates melanin granules. Iris melanogenesis is aberrant in this condition. Cytotoxic melanogenesis intermediates leak out of melanosomes and cause iris melanocyte and pigment epithelium cell death. Targeting melanogenesis can likely decrease the risk of pigmentary glaucoma. Skin and melanoma research provides insights into potential therapeutics. We propose that specific prostanoid agonists and fenofibrates may reduce melanogenesis by inhibiting cholesterol internalization and de novo synthesis. Additionally, melatonin is a potent melanogenesis suppressor, antioxidant, and hypotensive agent, rendering it a valuable agent for pigmentary glaucoma. In pseudoexfoliative glaucoma, where environmental insults drive pseudoexfoliative material formation, melatonin's antioxidant and hypotensive properties may offer adjunct therapeutic benefits. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

The Ins and Outs of Clusterin: Its Role in Cancer, Eye Diseases and Wound Healing

Clusterin (CLU) is a glycoprotein originally discovered in 1983 in ram testis fluid. Rapidly observed in other tissues, it was initially given various names based on its function in different tissues. In 1992, it was finally named CLU by consensus. Nearly omnipresent in human tissues, CLU is strongly expressed at fluid-tissue interfaces, including in the eye and in particular the cornea. Recent research has identified different forms of CLU, with the most prominent being a 75-80 kDa heterodimeric protein that is secreted. Another truncated version of CLU (55 kDa) is localized to the nucleus and exerts pro-apoptotic activities. CLU has been reported to be involved in various physiological processes such as sperm maturation, lipid transportation, complement inhibition and chaperone activity. CLU was also reported to exert important functions in tissue remodeling, cell-cell adhesion, cell-substratum interaction, cytoprotection, apoptotic cell death, cell proliferation and migration. Hence, this protein is sparking interest in tissue wound healing. Moreover, CLU gene expression is finely regulated by cytokines, growth factors and stress-inducing agents, leading to abnormally elevated levels of CLU in many states of cellular disturbance, including cancer and neurodegenerative conditions. In the eye, CLU expression has been reported as being severely increased in several pathologies, such as age-related macular degeneration and Fuch's corneal dystrophy, while it is depleted in others, such as pathologic keratinization. Nevertheless, the precise role of CLU in the development of ocular pathologies has yet to be deciphered. The question of whether CLU expression is influenced by these disorders or contributes to them remains open. In this article, we review the actual knowledge about CLU at both the protein and gene expression level in wound healing, and explore the possibility that CLU is a key factor in cancer and eye diseases. Understanding the expression and regulation of CLU could lead to the development of novel therapeutics for promoting wound healing.

Comparative Proteomic Analysis of the Aqueous Humor from Patients with Pseudoexfoliation Syndrome

Purpose

The goal of this study was to pinpoint potential molecular pathways that may have contributed to the onset of pseudoexfoliation syndrome (PEX), a systemic illness associated with aging that has no known cause and is brought on by the deposition of fibrillary white flaky debris in ocular tissues.

Materials and methods

Protein pools representing each group were created using two-dimensional gel electrophoresis (2DE) in conjunction with a matrix-assisted laser desorption ionization-time of flight/time of flight (MALDI-TOF/TOF) mass spectrometer. Aqueous humor (AH) from patients with PEX and cataracts was also collected for a comprehensive study of the data; ingenuity pathway analysis (IPA) was used for the discovered proteins.

Results

In comparison to controls, 2DE showed that 10 sites in PEX patients had differently altered gene expression. Two of these proteins, transthyretin (TTR) and apolipoprotein A4 (ApoA4) were significantly overexpressed in PEX patients, but the remaining proteins were only mildly altered. The liver X receptor (LXR) and the retinoid X receptors (RXR) may play a crucial role in the pathophysiology of PEX according to IPA employing these 10 proteins.

Conclusion

The altered proteins, particularly ApoA4 and TTR, may be important in revealing the molecular process behind PEX, as anticipated by IPA.

How to cite this article

Toprak M, Yuksel N, Akpinar G, et al. Comparative Proteomic Analysis of the Aqueous Humor from Patients with Pseudoexfoliation Syndrome. J Curr Glaucoma Pract 2023;17(3):118-125.

Applications of MALDI-MS/MS-Based Proteomics in Biomedical Research

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is one of the most widely used techniques in proteomics to achieve structural identification and characterization of proteins and peptides, including their variety of proteoforms due to post-translational modifications (PTMs) or protein–protein interactions (PPIs). MALDI-MS and MALDI tandem mass spectrometry (MS/MS) have been developed as analytical techniques to study small and large molecules, offering picomole to femtomole sensitivity and enabling the direct analysis of biological samples, such as biofluids, solid tissues, tissue/cell homogenates, and cell culture lysates, with a minimized procedure of sample preparation. In the last decades, structural identification of peptides and proteins achieved by MALDI-MS/MS helped researchers and clinicians to decipher molecular function, biological process, cellular component, and related pathways of the gene products as well as their involvement in pathogenesis of diseases. In this review, we highlight the applications of MALDI ionization source and tandem approaches for MS for analyzing biomedical relevant peptides and proteins. Furthermore, one of the most relevant applications of MALDI-MS/MS is to provide “molecular pictures”, which offer in situ information about molecular weight proteins without labeling of potential targets. Histology-directed MALDI-mass spectrometry imaging (MSI) uses MALDI-ToF/ToF or other MALDI tandem mass spectrometers for accurate sequence analysis of peptide biomarkers and biological active compounds directly in tissues, to assure complementary and essential spatial data compared with those obtained by LC-ESI-MS/MS technique.

Analyses of Pseudoexfoliation aqueous humor lipidome

Pseudoexfoliation syndrome (PEX) is a systemic disorder that manifests as fluffy, proteinaceous fibrillar material throughout the body. In the eye such deposits result in glaucoma (PEXG), due to impeding aqueous...

Clusterin, other extracellular chaperones, and eye disease

Proteostasis refers to all the processes that maintain the correct expression level, location, folding and turnover of proteins, essential to organismal survival. Both inside cells and in body fluids, molecular chaperones play key roles in maintaining proteostasis. In this article, we focus on clusterin, the first-recognized extracellular mammalian chaperone, and its role in diseases of the eye. Clusterin binds to and inhibits the aggregation of proteins that are misfolded due to mutations or stresses, clears these aggregating proteins from extracellular spaces, and facilitates their degradation. Clusterin exhibits three main homeostatic activities: proteostasis, cytoprotection, and anti-inflammation. The so-called "protein misfolding diseases" are caused by aggregation of misfolded proteins that accumulate pathologically as deposits in tissues; we discuss several such diseases that occur in the eye. Clusterin is typically found in these deposits, which is interpreted to mean that its capacity as a molecular chaperone to maintain proteostasis is overwhelmed in the disease state. Nevertheless, the role of clusterin in diseases involving such deposits needs to be better defined before therapeutic approaches can be entertained. A more straightforward case can be made for therapeutic use of clusterin based on its proteostatic role as a proteinase inhibitor, as well as its cytoprotective and anti-inflammatory properties. It is likely that clusterin works together in this way with other extracellular chaperones to protect the eye from disease, and we discuss several examples. We end this article by predicting future steps that may lead to development of clusterin as a biological drug.

Aqueous humor metabolite profile of pseudoexfoliation glaucoma is distinctive

Pseudoexfoliation (PEX) is a known cause of secondary open angle glaucoma. PEX glaucoma is associated with structural and metabolic changes in the eye. Despite similarities, PEX and primary open angle glaucoma (POAG) may have differences in the composition of metabolites. We analyzed the metabolites of the aqueous humor (AH) of PEX subjects sequentially first using nuclear magnetic resonance (1H NMR: HSQC and TOCSY), and subsequently with liquid chromatography tandem mass spectrometry (LC-MS/MS) implementing isotopic ratio outlier analysis (IROA) quantification. The findings were compared with previous results for POAG and control subjects analyzed using identical sequential steps. We found significant differences in metabolites between the three conditions. Principle component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) indicated clear grouping based on the metabolomes of the three conditions. We used machine learning algorithms and a percentage set of the data to train, and utilized a different or larger dataset to test whether a trained model can correctly classify the test dataset as PEX, POAG or control. Three different algorithms: linear support vector machines (SVM), deep learning, and a neural network were used for prediction. They all accurately classified the test datasets based on the AH metabolome of the sample. We next compared the AH metabolome with known AH and TM proteomes and genomes in order to understand metabolic pathways that may contribute to alterations in the AH metabolome in PEX. We found potential protein/gene pathways associated with observed significant metabolite changes in PEX.

Proteomics of pseudoexfoliation materials in the anterior eye segment

Pseudoexfoliation syndrome (PEX) is characterized by the production of white extracellular fluffy clumps of microfibrillar material that aggregates in various organs throughout the body but is known to cause disease in the eye. The accumulation of PEX material (PEXM) in the anterior segment ocular structures is believed to cause an increase in intraocular pressure (IOP) resulting in pseudoexfoliation glaucoma (PEXG). The onset of PEXG is often bilateral but asymmetric-one eye often presents with glaucoma prior to the other eye. Proteomics has been used to identify key proteins involved in PEXM formation with the end goal of developing effective treatments for PEX and PEXG which may act through inhibiting the formation of the PEX aggregates. To date, a variety of proteins with various molecular functions have been identified from extracted anterior segment structures and fluids, such as aqueous humor (AH) and blood serum of patients affected by PEX. From past studies, some proteins identified in AH, lens capsule epithelium, iris tissue, and blood serum samples include vitamin D binding protein (GC), apolipoprotein A4 (APOA4), lysyl oxidase like-1 (LOXL1), complement C3, beta-crystalline B1, and B2, and antithrombin-III (SERPINC1). Each of these proteins have been observed in eyes with PEX at varying levels within the different eye structures. In this review, we further examine the anterior segment ocular proteomics of PEXM from past studies to better understand the mechanism of PEX and PEXG development. Both genetic and environmental risk factors have been implicated to be involved in the development of PEX and PEXG. This field is at an early stage of investigation identifying how these factors modify proteins both at the expression and functional level to cause changes leading to the pathophysiology of PEX glaucoma.

Molecular Biology of Exfoliation Syndrome

Exfoliation syndrome (XFS) is a common age-related matrix process resulting from excessive production and disordered assembly of elastic microfibrillar components into highly cross-linked fibrillary aggregates throughout the anterior eye segment and various organ systems. The underlying molecular pathophysiology involves a complex interplay of profibrotic protagonists including growth factors, proteolytic enzymes and inhibitors, proinflammatory cytokines, chaperones, and dysregulated stress response pathways including insufficient autophagy. Interaction between individual genetic predisposition and stress factors is a plausible theory explaining the development of XFS in the aging individual. Genome-wide association studies have identified robust genetic associations with LOXL1, CACNA1A, and 5 additional genes including POMP and TMEM136, which provide new biological insights into the pathology of XFS and highlight a role for abnormal matrix cross-linking processes, Ca channel deficiency, blood-aqueous barrier dysfunction, and abnormal ubiquitin-proteasome signaling in XFS pathophysiology. However, the exact pathophysiological mechanisms, the functional role of genetic risk variants, and gene-environment interactions still remain to be characterized.

The Proteome of Cataract Markers: Focus on Crystallins

Cataract is a major cause of blindness worldwide. It is characterized by lens opacification and is accompanied by extensive posttranslational modifications (PTMs) in various proteins. PTMs play an essential role in lens opacification. Several PTMs have been described in proteins isolated from relatively old human lenses, including phosphorylation, deamidation, racemization, truncation, acetylation, and methylation. An overwhelming majority of previous cataract proteomic studies have exclusively focused on crystallin proteins, which are the most abundant proteome components of the lens. To investigate the proteome of cataract markers, this chapter focuses on the proteomic research on the functional relevance of the major PTMs in crystallins of human cataractous lenses. Elucidating the role of these modifications in cataract formation has been a challenging task because they are among the most difficult PTMs to study analytically. The proteomic status of some amides presents similar properties in normal aged and cataractous lenses, whereas some may undergo greater PTMs in cataract. Therefore, it is of great importance to review the current proteomic research on crystallins, the major protein markers in different types of cataract, to elucidate the pathogenesis of this major human-blinding condition.

A Role for Clusterin in Exfoliation Syndrome and Exfoliation Glaucoma?

The multifunctional protein clusterin (CLU) is a secreted glycoprotein ubiquitously expressed throughout the body, including in the eye. Its primary function is to act as an extracellular molecular chaperone, preventing the precipitation and aggregation of misfolded extracellular proteins. Clusterin is commonly identified at fluid-tissue interfaces, and has been identified in most body fluids. It is a component of exfoliation material, and CLU mRNA is reduced in eyes with exfoliation syndrome compared with controls. SNPs located in the CLU genomic region have been associated with Alzheimer disease (AD) at the genome-wide level and several CLU SNPs located in an apparent regulatory region have been nominally associated with XFS/XFG in Caucasians with European ancestry and in south Indians. Interestingly, clusterin associates with altered elastic fibers in human photoaged skin and prevents UV-induced elastin aggregation in vitro. In light of the known geographic risk factors for XFS/XFG, which could include UV light, investigations of CLU-geographic interactions could be of interest. Future studies investigating rare CLU variation and other complex interactions including gene-gene interactions in XFS/XFG cases and controls may also be fruitful. Although CLU has been considered as a therapeutic target in AD, cancer and dry eye, a role for clusterin in XFS/XFG needs to be better defined before therapeutic approaches involving CLU can be entertained.

Clusterin in the eye: An old dog with new tricks at the ocular surface

The multifunctional protein clusterin (CLU) was first described in 1983 as a secreted glycoprotein present in ram rete testis fluid that enhanced aggregation ('clustering') of a variety of cells in vitro. It was also independently discovered in a number of other systems. By the early 1990s, CLU was known under many names and its expression had been demonstrated throughout the body, including in the eye. Its homeostatic activities in proteostasis, cytoprotection, and anti-inflammation have been well documented, however its roles in health and disease are still not well understood. CLU is prominent at fluid-tissue interfaces, and in 1996 it was demonstrated to be the most highly expressed transcript in the human cornea, the protein product being localized to the apical layers of the mucosal epithelia of the cornea and conjunctiva. CLU protein is also present in human tears. Using a preclinical mouse model for desiccating stress that mimics human dry eye disease, the authors recently demonstrated that CLU prevents and ameliorates ocular surface barrier disruption by a remarkable sealing mechanism dependent on attainment of a critical all-or-none concentration in the tears. When the CLU level drops below the critical all-or-none threshold, the barrier becomes vulnerable to desiccating stress. CLU binds selectively to the ocular surface subjected to desiccating stress in vivo, and in vitro to LGALS3 (galectin-3), a key barrier component. Positioned in this way, CLU not only physically seals the ocular surface barrier, but it also protects the barrier cells and prevents further damage to barrier structure. CLU depletion from the ocular surface epithelia is seen in a variety of inflammatory conditions in humans and mice that lead to squamous metaplasia and a keratinized epithelium. This suggests that CLU might have a specific role in maintaining mucosal epithelial differentiation, an idea that can now be tested using the mouse model for desiccating stress. Most excitingly, the new findings suggest that CLU could serve as a novel biotherapeutic for dry eye disease.

Proteins in aqueous humor from cataract patients with and without pseudoexfoliation syndrome

The aim of this study was to investigate the protein content in aqueous humor in eyes with and without pseudoexfoliations (PEX) and to evaluate the quantitative proteomics method, isobaric tagging for relative and absolute protein quantification (iTRAQ), in combination with two separation methods followed by matrix-assisted Laser desorption/ionization (MALDI) mass spectrometry and tandem mass spectrometry (MS/MS). During cataract surgery, samples of aqueous humor were collected from 20 eyes with PEX and from 18 control eyes. The relative concentrations of proteins in the pooled samples of ten PEX eyes and eight controls were evaluated after trypsin digestion and Labeling of the peptides with (iTRAQ) reagent. Two separation methods, Liquid chromatography (LC) and capillary electrophoresis (CE) were used, followed by MALDI mass spectrometry and MS/MS. Furthermore, 1D gel electrophoresis was performed on the remaining ten pooled PEX samples and ten control samples. The gel material was separated by nano-liquid chromatography (nano-LC) followed by Linear-ion-trap quadrupole Fourier transformation ion cyclotron resonance (FT-ICR). Fifty four proteins were identified in the LC runs and 24 with CE. The relative concentrations of beta-crystallines B2 and S were raised and those of angiotensinogen and osteopontin Lowered in the PEX sample compared to the control. The trends regarding beta-crystallines B2, angiotensinogen and osteopontin were confirmed by the 1D gel electrophoresis.