Binding of Alpha-Crystallin to Cortical and Nuclear Lens Lipid Membranes Derived from a Single Lens

Cholesterol and Q147E Deamidation Modulates αA-Crystallin Membrane Binding Elucidating Protective Role of Lens Membrane Composition Changes With Aging

Purpose

The αA-Crystallin (αAc) binding with lens membranes increases with age and cataract formation. However, the role of lipids and cholesterol (Chol) in Q147E-αAc membrane binding remains unclear, which we aim to elucidate in this study.

Methods

We have used the electron paramagnetic resonance spin-labeling method to probe the Chol/ 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and Chol/ sphingomyelin (SM) membranes binding with wild-type (WT) and Q147E-αAc.

Results

Compared to WT-αAc, the Q147E mutant had increased binding to POPC and decreased binding to SM membranes without Chol. Adding 33 mol% Chol to the POPC and SM membranes decreased the binding of WT and, to a lesser degree, decreased the binding of Q147E-αAc to the membranes. Adding 60 mol% Chol completely inhibited Q147E mutant and WT binding to POPC membranes. However, 33 and 60 mol% Chol completely inhibited WT and Q147E mutant binding to SM membranes, respectively. WT and Q147E-αAc membrane binding decreased membrane mobility while increasing order and hydrophobicity near the headgroup.

Conclusions

In Chol-free membranes, the deamidated Q147E-αAc binds significantly more to the POPC membranes compared to WT, whereas WT binds significantly more to the SM membranes compared to Q147E-αAc. In contrast, for 33 mol% Chol-containing membranes, the deamidated Q147E-αAc binds significantly more to POPC and SM membranes than WT. Conversely, 60 mol% Chol-containing membranes completely inhibit WT and deamidated Q147E-αAc binding to POPC and SM membranes. These results suggest that increased Chol content of the lens membranes during aging protects against accumulation of modified proteins on the membrane associated with cataracts.

Effect of lens crystallins aggregation on cataract formation

Cataracts represent one of the leading causes of blindness globally. The World Health Organization's 2019World Report on Vision indicates that approximately 65.2 million individuals worldwide experience varying degrees of visual impairment or blindness attributable to cataracts. The prevalence of this condition is significantly increasing, largely due to the accelerated aging of the global population. The lens of the eye is primarily composed of crystallins, which are categorized into three families: α-, β-, and γ-crystallins. The highly ordered structure and interactions among these crystallins are crucial for maintaining lens transparency. Disruptions in the interactions within or between crystallins can compromise this delicate architecture, exposing hydrophobic surfaces that lead to crystallin aggregation and subsequent cataract formation. Currently, surgical intervention is the sole treatment for cataracts, and the cataract surgery rate in China remains considerably lower than that of developed nations. Investigating the mechanisms of crystallins interaction and aggregation is essential for understanding the molecular pathogenesis of cataract formation, which may inform the development of targeted therapies and preventative strategies. This paper reviews recent scientific advancements in the research field of lens crystallins aggregation and cataract formation.

Mechanical Properties of Eye Lens Cortical and Nuclear Membranes and the Whole Lens

Purpose

To elucidate the mechanical properties of the bovine lens cortical membrane (CM), the nuclear membrane (NM) containing cholesterol bilayer domains (CBDs), and whole bovine lenses.

Methods

The total lipids (lipids plus cholesterol) from the cortex and nucleus of a single bovine lens were isolated using the monophasic methanol extraction method. Supported CMs and NMs were prepared from total lipids extracted from the cortex and nucleus, respectively, using a rapid solvent exchange method and probe-tip sonication, followed by the fusion of unilamellar vesicles on a flat, freshly cleaved mica surface. Topographical images and force curves for the CMs and NMs were obtained via atomic force microscopy (AFM) in a fluid cell. Whole bovine lenses were affixed to custom-built glass Petri dishes, and an AFM was used to obtain force curves. Force curves were analyzed to estimate the breakthrough force, membrane stiffness (KA and Em), and lens stiffness (EL).

Results

The NMs containing CBDs exhibited significantly lower breakthrough force, KA, and Em than the CMs without CBDs. The Em values for CMs and NMs were significantly higher than the EL for the whole lens.

Conclusions

The significantly higher stiffness of the CM and NM compared to the stiffness of the whole lens suggests that slight modulation in CM and NM composition may play a crucial role in altering the overall lens stiffness. Furthermore, the NMs containing CBDs were less stiff than CMs without CBDs, suggesting that CBDs decrease lens membrane stiffness and possibly protect against lens hardening and presbyopia.

Consilience and unity in ocular anterior segment research

In his beautiful book, Consilience: The Unity of Knowledge, the eminent biologist Edward O Wilson, advocates the need for integration and reconciliation across the sciences. He defines consilience as "literally a 'jumping together' of knowledge with a linking of facts ... to create a common groundwork of explanation". It is the premise of this paper that as much as basic biomedical research is in need of data generation using the latest available techniques- unifying available knowledge is just as critical. This involves the necessity to resolve contradictory findings, reduce silos, and acknowledge complexity. We take the cornea and the lens as case studies of our premise. Specifically, in this perspective, we discuss the conflicting and fragmented information on protein aggregation, oxidative damage, and fibrosis. These are fields of study that are integrally tied to anterior segment research. Our goal is to highlight the vital need for Wilson's consilience and unity of knowledge which in turn should lead to enhanced rigor and reproducibility, and most importantly, to greater understanding and not simply knowing.

Association of Alpha-Crystallin with Human Cortical and Nuclear Lens Lipid Membrane Increases with the Grade of Cortical and Nuclear Cataract

Eye lens α-crystallin has been shown to become increasingly membrane-bound with age and cataract formation; however, to our knowledge, no studies have investigated the membrane interactions of α-crystallin throughout the development of cataracts in separated cortical membrane (CM) and nuclear membrane (NM) from single human lenses. In this study, four pairs of human lenses from age-matched male and female donors and one pair of male lenses ranging in age from 64 to 73 years old (yo) were obtained to investigate the interactions of α-crystallin with the NM and CM throughout the progression of cortical cataract (CC) and nuclear cataract (NC) using the electron paramagnetic resonance spin-labeling method. Donor health history information (diabetes, smoker, hypertension, radiation treatment), sex, and race were included in the data analysis. The right eye lenses CM and NM investigated were 64 yo male (CC: 0), 68 yo male (CC: 3, NC: 2), 73 yo male (CC: 1, NC: 2), 68 yo female (CC: 3, NC: 2), and 73 yo female (CC: 1, NC: 3). Similarly, left eye lenses CM and NM investigated were 64 yo male (CC: 0), 68 yo male (CC: 3, NC: 2), 73 yo male (CC: 2, NC: 3), 68 yo female (CC: 3, NC: 2), and 73 yo female (CC: 1, NC: 3). Analysis of α-crystallin binding to male and female eye lens CM and NM revealed that the percentage of membrane surface occupied (MSO) by α-crystallin increases with increasing grade of CC and NC. The binding of α-crystallin resulted in decreased mobility, increased order, and increased hydrophobicity on the membrane surface in male and female eye lens CM and NM. CM mobility decreased with an increase in cataracts for both males and females, whereas the male lens NM mobility showed no significant change, while female lens NM showed increased mobility with an increase in cataract grade. Our data shows that a 68 yo female donor (long-term smoker, pre-diabetic, and hypertension; grade 3 CC) showed the largest MSO by α-crystallin in CM from both the left and right lens and had the most pronounced mobility changes relative to all other analyzed samples. The variation in cholesterol (Chol) content, size and amount of cholesterol bilayer domains (CBDs), and lipid composition in the CM and NM with age and cataract might result in a variation of membrane surface mobility, membrane surface hydrophobicity, and the interactions of α-crystallin at the surface of each CM and NM. These findings provide insight into the effect of decreased Chol content and the reduced size and amount of CBDs in the cataractous CM and NM with an increased binding of α-crystallin with increased CC and NC grade, which suggests that Chol and CBDs might be a key component in maintaining lens transparency.

Interaction of βL- and γ-Crystallin with Phospholipid Membrane Using Atomic Force Microscopy

Highly concentrated lens proteins, mostly β- and γ-crystallin, are responsible for maintaining the structure and refractivity of the eye lens. However, with aging and cataract formation, β- and γ-crystallin are associated with the lens membrane or other lens proteins forming high-molecular-weight proteins, which further associate with the lens membrane, leading to light scattering and cataract development. The mechanism by which β- and γ-crystallin are associated with the lens membrane is unknown. This work aims to study the interaction of β- and γ-crystallin with the phospholipid membrane with and without cholesterol (Chol) with the overall goal of understanding the role of phospholipid and Chol in β- and γ-crystallin association with the membrane. Small unilamellar vesicles made of Chol/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (Chol/POPC) membranes with varying Chol content were prepared using the rapid solvent exchange method followed by probe tip sonication and then dispensed on freshly cleaved mica disk to prepare a supported lipid membrane. The βL- and γ-crystallin from the cortex of the bovine lens was used to investigate the time-dependent association of βL- and γ-crystallin with the membrane by obtaining the topographical images using atomic force microscopy. Our study showed that βL-crystallin formed semi-transmembrane defects, whereas γ-crystallin formed transmembrane defects on the phospholipid membrane. The size of semi-transmembrane defects increases significantly with incubation time when βL-crystallin interacts with the membrane. In contrast, no significant increase in transmembrane defect size was observed in the case of γ-crystallin. Our result shows that Chol inhibits the formation of membrane defects when βL- and γ-crystallin interact with the Chol/POPC membrane, where the degree of inhibition depends upon the amount of Chol content in the membrane. At a Chol/POPC mixing ratio of 0.3, membrane defects were observed when both βL- and γ-crystallin interacted with the membrane. However, at a Chol/POPC mixing ratio of 1, no association of γ-crystallin with the membrane was observed, which resulted in a defect-free membrane, and the severity of the membrane defect was decreased when βL-crystallin interacted with the membrane. The semi-transmembrane or transmembrane defects formed by the interaction of βL- and γ-crystallin on phospholipid membrane might be responsible for light scattering and cataract formation. However, Chol suppressed the formation of such defects in the membrane, likely maintaining lens membrane homeostasis and protecting against cataract formation.

Binding of βL-Crystallin with Models of Animal and Human Eye Lens-Lipid Membrane

Several discoveries show that with age and cataract formation, β-crystallin binds with the lens membrane or associates with other lens proteins, which bind with the fiber cell plasma membrane, accompanied by light scattering and cataract formation. However, how lipids (phospholipids and sphingolipids) and cholesterol (Chol) influence β-crystallin binding to the membrane is unclear. This research aims to elucidate the role of lipids and Chol in the binding of β-crystallin to the membrane and the membrane's physical properties (mobility, order, and hydrophobicity) with β-crystallin binding. We used electron paramagnetic resonance (EPR) spin-labeling methods to investigate the binding of βL-crystallin with a model of porcine lens-lipid (MPLL), model of mouse lens-lipid (MMLL), and model of human lens-lipid (MHLL) membrane with and without Chol. Our results show that βL-crystallin binds with all of the investigated membranes in a saturation manner, and the maximum parentage of the membrane surface occupied (MMSO) by βL-crystallin and the binding affinity (Ka) of βL-crystallin to the membranes followed trends: MMSO (MPLL) > MMSO (MMLL) > MMSO (MHLL) and Ka (MHLL) > Ka (MMLL) ≈ Ka (MPLL), respectively, in which the presence of Chol reduces the MMSO and Ka for all membranes. The mobility near the headgroup regions of the membranes decreases with an increase in the binding of βL-crystallin; however, the decrease is more pronounced in the MPLL and MMLL membranes than the MHLL membrane. In the MPLL and MMLL membranes, the membranes become slightly ordered near the headgroup with an increase in βL-crystallin binding compared to the MHLL membrane. The hydrophobicity near the headgroup region of the membrane increases with βL-crystallin binding; however, the increase is more pronounced in the MPLL and MMLL membranes than the MHLL membrane, indicating that βL-crystallin binding creates a hydrophobic barrier for the passage of polar molecules, which supports the barrier hypothesis in cataract formation. However, in the presence of Chol, there is no significant increase in hydrophobicity with βL-crystallin binding, suggesting that Chol prevents the formation of a hydrophobic barrier, possibly protecting against cataract formation.

Membrane Models and Experiments Suitable for Studies of the Cholesterol Bilayer Domains

Cholesterol (Chol) is an essential component of animal cell membranes and is most abundant in plasma membranes (PMs) where its concentration typically ranges from 10 to 30 mol%. However, in red blood cells and Schwann cells, PMs Chol content is as high as 50 mol%, and in the PMs of the eye lens fiber cells, it can reach up to 66 mol%. Being amphiphilic, Chol molecules are easily incorporated into the lipid bilayer where they affect the membrane lateral organization and transmembrane physical properties. In the aqueous phase, Chol cannot form free bilayers by itself. However, pure Chol bilayer domains (CBDs) can form in lipid bilayer membranes with the Chol content exceeding 50 mol%. The range of Chol concentrations surpassing 50 mol% is less frequent in biological membranes and is consequently less investigated. Nevertheless, it is significant for the normal functioning of the eye lens and understanding how Chol plaques form in atherosclerosis. The most commonly used membrane models are unilamellar and multilamellar vesicles (MLVs) and supported lipid bilayers (SLBs). CBDs have been observed directly using confocal microscopy, X-ray reflectometry and saturation recovery electron paramagnetic resonance (SR EPR). Indirect evidence of CBDs has also been reported by using atomic force microscopy (AFM) and fluorescence recovery after photobleaching (FRAP) experiments. The overall goal of this review is to demonstrate the advantages and limitations of the various membrane models and experimental techniques suitable for the detection and investigation of the lateral organization, function and physical properties of CBDs.