Fluorescence studies on the age related changes in bovine and human lens membrane structure

Inhibition of lens photodamage by UV-absorbing contact lenses

PURPOSE

To determine whether class 1 UV-blocking contact lenses protect against UVB radiation-induced damage in a human lens epithelial cell line (HLE B-3) and postmortem human lenses using a proteomics approach.

METHODS

HLE B-3 cells were exposed to 6.4 mW/cm(2) UVB radiation at 302 nm for 2 minutes (768 mJ/cm(2)) with or without covering by senofilcon A class 1 UV-blocking contact lenses or lotrafilcon A non-UV-blocking (lotrafilcon A has some UV-blocking ability, albeit minimal) contact lenses. Control cells were not exposed to UVB radiation. Four hours after treatment, cells were analyzed by two-dimensional difference gel electrophoresis and tandem mass spectrometry, and changes in protein abundance were quantified. F-actin and microtubule cytoskeletons were examined by fluorescence staining. In addition, human donor lenses were exposed to UVB radiation at 302 nm for 4 minutes (1536 mJ/cm(2)). Cortical and epithelial cell proteins were scraped from lens surfaces and subjected to the same protein analyses.

RESULTS

Senofilcon A lenses were beneficial for protecting HLE B-3 cells against UVB radiation-induced changes in caldesmon 1 isoform, lamin A/C transcript variant 1, DEAD (Asp-Glu-Ala-Asp) box polypeptide, β-actin, glyceraldehyde 3-phosphate dehydrogenase (G3PDH), annexin A2, triose phosphate isomerase, and ubiquitin B precursor. These contact lenses also prevented actin and microtubule cytoskeleton changes typically induced by UVB radiation. Conversely, non-UV-blocking contact lenses were not protective. UVB-irradiated human lenses showed marked reductions in αA-crystallin, αB-crystallin, aldehyde dehydrogenase 1, βS-crystallin, βB2-crystallin, and G3PDH, and UV-absorbing contact lenses significantly prevented these alterations.

CONCLUSIONS

Senofilcon A class 1 UV-blocking contact lenses largely prevented UVB-induced changes in protein abundance in lens epithelial cells and in human lenses.

Evidence for distinct cholesterol domains in fiber cell membranes from cataractous human lenses

Previous studies in our laboratory have provided direct evidence for the existence of distinct cholesterol domains within the plasma membranes of human ocular lens fiber cells. The fiber cell plasma membrane is unique in that it contains unusually high concentrations of cholesterol, with cholesterol to phospholipid (C/P) mole ratios ranging from 1 to 4. Since membrane cholesterol content is disturbed in the development of cataracts, it was hypothesized that perturbation of cholesterol domain structure occurs in cataracts. In this study, fiber cell plasma membranes were isolated from both normal (control) and cataractous lenses and assayed for cholesterol and phospholipid. Control and cataractous whole lens membranes had C/P mole ratios of 3.1 and 1.7, respectively. Small angle x-ray diffraction approaches were used to directly examine the structural organization of the cataractous lens plasma membrane versus control. Both normal and cataractous oriented membranes yielded meridional diffraction peaks corresponding to a unit cell periodicity of 34.0 A, consistent with the presence of immiscible cholesterol domains. However, comparison of diffraction patterns indicated that cataractous lens membranes contained more pronounced and better defined cholesterol domains than controls, over a broad range of temperature (5-40 degrees C) and relative humidity (52-92%) levels. In addition, diffraction analyses of the sterol-poor regions of cataractous membranes indicated increased membrane rigidity as compared with control membranes. Modification of the membrane lipid environment, such as by oxidative insult, is believed to be one potential mechanism for the formation of highly resolved cholesterol domains despite significantly reduced cholesterol content. The results of this x-ray diffraction study provide evidence for fundamental changes in the lens fiber cell plasma membrane structure in cataracts, including the presence of more prominent and highly ordered, immiscible cholesterol domains.

Ca(2+)-ATPase activity and lens lipid composition in reconstituted systems

Lens lipid composition and lipid hydrocarbon chain structure change with age, region and cataract. Since the lens Ca(2+)-ATPase pump is important to the maintenance of calcium homeostasis and lens clarity, muscle sarcoplasmic reticulum Ca(2+)-ATPase was reconstituted with bovine lens lipids and dihydrosphingomyelin, the rare and major phospholipid of the human lens. Ca(2+)-ATPase activity was found to be about 5 times lower when the pump was reconstituted into dihydrosphingomyelin or lens lipids compared to native sarcoplasmic reticulum lipids. The addition of cholesterol to levels ranging from 13-53 mole%, had no affect on reconstituted Ca(2+)-ATPase activity. Ca(2+)-ATPase activity correlated with the degree of hydrocarbon chain saturation. The greater levels of saturation are a consequence of the high sphingolipid content in the reconstituted systems. These data support the hypothesis that changes in lens lipid composition or structure could affect Ca(2+)-ATPase activity in human lenses. Because the mechanisms governing Ca(2+)-ATPase activity in vivo are much more complex than in these simple reconstituted systems, this study represents an initial step in the elucidation of the relationships of endogenous membrane lipid composition-structure and function.

Age-related changes in platelet microviscosity and aggregation in rats

1. To study possible changes in platelet microviscosity in aged animals, 1,6-diphenyl-1,3,5-hexatriene (DPH) was used as a nonpolar probe embedded in thrice-washed platelets from young, adult and aged rats. With the known values of maximum limiting anisotropy and the structural parameter of DPH and by estimating the steady state of fluorescence anisotropy and the average fluorescence of lifetime, we applied the Perrin equation to calculate the microviscosity. 2. We measured platelet aggregation, platelet lipids and platelet polyunsaturated fatty acids to determine any causal relationship between these parameters. Platelet aggregation, the platelet molar ratio of cholesterol to phospholipid ([C]/[PL]) and platelet microviscosity increased with age (P < 0.05) and were correlated with one another (P < 0.05). 3. Age-dependent increases in the steady state of fluorescence anisotropy, order parameters and the short component of fluorescence lifetime of the fluorophore were expressed as functions of variables, such as microviscosity or the [C]/[PL] ratio. 4. Platelet concentrations of arachidonic and eicosapentaenoic acids increased with age, but were not associated with aggregation. Age-related changes in microviscosity and the [C]/[PL] ratio seemed to be determinants affecting biophysical properties of platelet aggregation.

Temperature induced structural changes of beta-crystallin and sphingomyelin binding

The study of the binding of alpha-crystallin to membranes is potentially important for understanding the function of alpha-crystallin in the ocular lens and the formation of cataracts. Using fluorescence probes, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3 -phosphoethanolamine, triethylammonium salt (NBD-PE) and (1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid, dipotassium salt (bis-ANS), the temperature dependence of the binding of alpha-crystallin to sphingomyelin liposomes, and the structural changes of alpha-crystallin and sphingomyelin induced by temperature were studied. The influence of the binding of alpha-crystallin on the mobility of the head group region of liposomes of sphingomyelin was dependent on the thermal history of alpha-crystallin. Binding of alpha-crystallin to sphingomyelin caused a decrease in the anisotropy of the fluorophore NBD-PE at or below 37 degrees C. However, when alpha-crystallin or the mixture of alpha-crystallin/sphingomyelin were preincubated near the secondary structure phase transition temperature of 60 degrees C, an increase of the anisotropy of NBD-PE (decrease of lipid head group mobility) was observed when measured at 22 degrees C or 37 degrees C. An inflection near 47 degrees C in the curve of fluorescence anisotropy of bis-ANS pre-incorporated into the alpha-crystallin corresponded to a 3 degrees or 4 degrees structural change of alpha-crystallin. alpha-Crystallin either increases or decreases the flexibility of the head group of sphingomyelin liposomes depending on its structure.

Spectroscopic studies on the interaction of calf lens membranes with crystallins

The interaction of crystallins with lens membranes and liposomes was studied by fluorescence and circular dichroism (CD) measurements. Two extrinsic fluorescence probes ANS (1-anilino-naphthalene-8-sulfonic acid) and DPH (1,6-diphenyl, 1,3,5-hexatriene) were used to detect the binding and to explore the binding site. The ANS fluorescence intensity is greater in membranes than in liposomes, but is reverse for DPH. Among alpha, beta and gamma-crystallins, only alpha c-crystallin decreased the ANS fluorescence intensity in membranes, indicating a binding between membranes and alpha c-crystallin. The binding site appears to be at the polar-apolar interface in membrane protein (MIP26) and alpha c-crystallin. Fluorescence polarization measurements show that lipid bilayer becomes less mobile with alpha c-crystallin binding. The change in the near UV CD due to the binding also indicates a decreased freedom of rotation of aromatic amino acid residues either in MIP26 or in alpha-crystallin.

Interaction and aggregation of lens crystallins

Crystallins of the normal eye lens have a specific structure that maintains lens transparency. This structure reportedly arises from a short-range order. The age-related post-translational changes and high molecular weight (HMW) aggregation disrupt the normal structure and decrease lens transparency. In the current study, we have carried out fluorescence measurements (polarization and extrinsic probe binding) in an attempt to understand protein interaction and aggregation. Based on polarization measurements of dansyl chloride (DCl) labeled gamma-crystallin, there is a hetero-interaction among crystallins (alpha-gamma, beta-gamma, etc.) but no self-interaction in gamma-crystallin (gamma-gamma) (less than 30 mg ml-1). Alpha-crystallin is rich in hydrophobic region and possibly has a self-interaction through hydrophobic interaction. Hydrophobic probes, ANS (1-anilinonaphthalene-8-sulfonic acid) and DPH (1,6-diphenyl 1,3,5-hexatriene) measurements show that alpha-crystallin is a very hydrophobic protein. HMW alpha-crystallin appears to be even more hydrophobic, indicating that the post-translational unfolding exposes more hydrophobic groups which facilitate HMW aggregation. The HMW aggregation results from highly hydrophobic proteins which tend to become thermodynamically more stable.