Aerobic metabolism in the bovine lens

Uncoupling proteins in the mitochondrial defense against oxidative stress

Oxidative stress is a major component of most major retinal diseases. Many extrinsic anti-oxidative strategies have been insufficient at counteracting one of the predominant intrinsic sources of reactive oxygen species (ROS), mitochondria. The proton gradient across the inner mitochondrial membrane is a key driving force for mitochondrial ROS production, and this gradient can be modulated by members of the mitochondrial uncoupling protein (UCP) family. Of the UCPs, UCP2 shows a widespread distribution and has been shown to uncouple oxidative phosphorylation, with concomitant decreases in ROS production. Genetic studies using transgenic and knockout mice have documented the ability of increased UCP2 activity to provide neuroprotection in models of a number of diseases, including retinal diseases, indicating that it is a strong candidate for a therapeutic target. Molecular studies have identified the structural mechanism of action of UCP2 and have detailed the ways in which its expression and activity can be controlled at the transcriptional, translational and posttranslational levels. These studies suggest a number of ways in control of UCP2 expression and activity can be used therapeutically for both acute and chronic conditions. The development of such therapeutic approaches will greatly increase the tools available to combat a broad range of serious retinal diseases.

The effects of toxicological agents on the optics and mitochondria of the lens and the mitochondria of the corneal epithelium

This review describes how the morphology and distribution of the mitochondria of the epithelium and the superficial fibre layers of the lens were studied using confocal scanning laser microscopy. This research was correlated with an effort to use the optical properties of the intact lens in culture as a proxy for the cornea in measuring ocular toxicity. In turn, this work led to the confocal study of the in vitro and then the in vivo cornea and their possible use in using confocal microscopy to evaluate the effect of various treatments on the integrity of the surface of the eye. Finally, confocal examination of the mitochondria of the lens has provided an avenue to the study of mitochondrial dynamics.

The lens of the eye as a focusing device and its response to stress

The continued peripheral growth of the lens, resulting in the concentration of older tissue toward the center, has the important optical consequence of producing a lens of variable refractive index. An approach consisting of the projection of fine laser beams through excised lenses in physiological solution has been used for in vitro study of lens optical quality. By varying the separation of the incident beams and/or the wavelength characteristics of the laser used, lens refractive properties and relative transparency may be examined. In the review provided, these optical properties are correlated to lens suture anatomy, lens mitochondrial morphology and function and the function of lens heat shock proteins. In addition, lens spherical aberration is evaluated as a function of accommodation. This work can be highlighted as follows: Mammalian lens suture morphology has a direct impact on lens optical function and, while suture structure of mammalian and avian lenses are very different, they both show an age-related deterioration in morphology and focusing ability. The distribution and appearance of mitochondria of the lens epithelium and superficial fiber cells are similar in all vertebrates. Lens mitochondrial integrity is correlated to lens focusing ability, suggesting a correlation between lens optical properties and lens metabolic function. The induction of cold cataract measured optically in cultured mammalian lenses is enhanced by thermal (heat) shock and this effect is prevented by inhibiting heat shock protein production. Finally, lens accommodative function can be studied by measuring lens refractive change using a physiological model involving an intact accommodative apparatus.

Regulation of tissue oxygen levels in the mammalian lens

Opacification of the lens nucleus is a major cause of blindness and is thought to result from oxidation of key cellular components. Thus, long-term preservation of lens clarity may depend on the maintenance of hypoxia in the lens nucleus. We mapped the distribution of dissolved oxygen within isolated bovine lenses and also measured the rate of oxygen consumption (QO2) by lenses, or parts thereof. To assess the contribution of mitochondrial metabolism to the lens oxygen budget, we tested the effect of mitochondrial inhibitors on (QO2) and partial pressure of oxygen (PO2). The distribution of mitochondria was mapped in living lenses by 2-photon microscopy. We found that a steep gradient of PO2 was maintained within the tissue, leading to PO2 < 2 mmHg in the core. Mitochondrial respiration accounted for approximately 90% of the oxygen consumed by the lens; however, PO2 gradients extended beyond the boundaries of the mitochondria-containing cell layer, indicating the presence of non-mitochondrial oxygen consumers. Time constants for oxygen consumption in various regions of the lens and an effective oxygen diffusion coefficient were calculated from a diffusion-consumption model. Typical values were 3 x 10(-5) cm(2) s(-1) for the effective diffusion coefficient and a 5 min time constant for oxygen consumption. Surprisingly, the calculated time constants did not differ between differentiating fibres (DF) that contained mitochondria and mature fibres (MF) that did not. Based on these parameters, DF cells were responsible for approximately 88% of lens oxygen consumption. A modest reduction in tissue temperature resulted in a marked decrease in (QO2) and the subsequent flooding of the lens core with oxygen. This phenomenon may be of clinical relevance because cold, oxygen-rich solutions are often infused into the eye during intraocular surgery. Such procedures are associated with a strikingly high incidence of postsurgical nuclear cataract.

Effect of metabolic inhibitors and anaerobiosis on rat lens

The effect of the inhibitors of aerobic metabolism on the transport of rubidium and ATP content in rat lens has been studied under organ culture system. The inhibitors used were malonate, monofluoroacetate, cyanide, antimycin A, 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), 2,4-dinitrophenol (DNP) and dicumarol. All these compounds inhibited the transport of rubidium, reflecting a decrease in the ability of the lens to transport potassium. The levels of ATP also decreased. The results were similar to those obtained under anaerobic conditions where the lenses were incubated in a nitrogen atmosphere without the inhibitors. The degree of inhibition observed by the metabolic inhibitors as well as by anaerobiosis indicate that the rat lens depends substantially on aerobic oxidation for its energy needs, unlike bovine and rabbit lenses studied before.

Mechanisms of senile cataract formation

Research on the mechanisms of lens opacification during the past 20 years has revealed a number of individual, identifiable cataractogenic stresses in man. They include osmotic cataract formation (diabetic, galactosemic and hypoglycemic cataracts), radiation cataracts (X-irradiation, near U.V. radiation and microwave radiation) and in senile cataract the conversion of soluble low molecular weight cytoplasmic proteins to soluble high molecular weight aggregates, insoluble phases, and insoluble membrane-protein matrices. Oxidative stress has emerged as a common denominator of many changes in senile cataract. As we increase our understanding of these mechanisms, we may be able to intervene therapeutically to delay or prevent human cataract formation in man.

Overview of the current attempts toward the medical treatment of cataract

A variety of agents are currently available that claim to either prevent, delay, or reverse cataracts associated with aging (senile cataracts), radiation, or diabetes and galactosemia (sugar cataracts). Senile cataract therapy includes formulation containing inorganic salts, nutritional supplements, natural product extracts, sulfhydryl, and sulfonic acid containing compounds and miscellaneous redox and nonsteroidal anti-inflammatory compounds. Agents associated with the treatment of radiation cataracts include anti oxidants and free radial scavengers. Aldose reductase inhibitors have been effective in the prevention of sugar cataracts. A summary of these agents and their potential ocular effects are presented.

Calmodulin binds to chick lens gap junction protein in a calcium-independent manner

A biochemically active conjugate of calmodulin and tetramethylrhodamine isothiocyanate (CaM-RITC) was synthesized. When incubated with sections of chick lens, this conjugate bound to the surface membranes of lens fiber cells in the presence of absence of calcium. Incubation of lens sections with antibodies to gap junction protein of lens completely blocked the binding of the conjugate to cell membranes, whereas serum from nonimmunized animals or antibodies to others lens proteins reduced the binding only slightly. By means of a gel overlay procedure, 125I-labeled calmodulin was found to bind to the gap junction protein of lens, also in a calcium-independent manner. These results support the concept that calmodulin may interact with and regulate gap junctions in living cells.

Sugar metabolism in the crystalline lens

Research on the sugar metabolism of the crystalline lens, past and preent, is reviewed. The chief energy source in the lens is the Embden-Meyerhof pathway; respiration and oxidative phosphorylation become more important as the lens ages. The function of the alpha-glycerophosphate cycle is not fully understood. The mechanisms involved in cataract formation, including those of hypoglycemic cataract and osmotic cataracts, are discussed. Sugar cataracts can be delayed or prevented with such aldose reductase inhibitors as flavonoids. By inhibiting aldose reductase, the formation and accumulation of sugar alcohols is stopped. This approach may be useful as a medical therapy for human diabetic senile cataracts.

Relative contents of sodium, potassium, water and dry matter in human senile cataractous lenses in relation to anterior capsular/subcapsular opacity

In 49 human senile cataractous lenses the sodium and potassium concentrations of the lens water as well as the water and dry weight percentages were examined. It was found justifiable to classify the lenses into three categories on the basis of correlated biomicroscopic and biochemical findings: 1. Immature cataractous lenses without anterior capsular/subcapsular opacity (ac-sco) were characterized by low CNaL+, high CKL+ and low sums of CNaL++CKL+. 2. Immature cataractous lenses with ac-sco were characterized by intermediate value of CNaL+ and CKL+, as well as high sums of CNAL++CKL+. 3. Totally opaque lenses (these lenses had 80-100% of ac-sco) were characterized by high CNaL+, low CKL+, high sums of CNaL++CKL+, high water, and low dry weight percentages. It was found that in immature cataractous lenses increasing extension of ac-sco was correlated to increasing CNAL+ and increasing ratios of CNAL+/CNAA+ as well as to decreasing CKL+ and decreasing ratios of CKL+/CKA+. The sums of CNaL++CKA+ increased. There was a correlation of the extent of ac=sco to the water and dry weight percentages of the immature senile cataractous lenses with ac-sco, viz. a negative correlation for water and a positive one for the dry weight. However, these latter two correlations may be due to chance significances, the level of significance being only greater than P greater than 0.02 in both instances. Lenses which were estimated to have larger than or equal to 30% of ac-sco were found to be more opaque than lenses with less than or equal to 25% of ac-sco.

The metabolism of amino acids in the bovine lens. Their oxidation as a source of energy

1. The metabolism by the bovine lens of nine (14)C-labelled l-amino acids was studied. These were: alanine, aspartate, glutamate, leucine, lysine, proline, serine, tyrosine and tryptophan. 2. All were taken up by the tissue and incorporated into protein. 3. Aspartate and glutamate, although poorly taken up, were readily metabolized to CO(2). Radioactivity from glutamate was also found in glutathione, glutamine, proline and ophthalmic acid. Aspartate was converted into glutamate, glutathione, proline, alanine and lactate. 4. Alanine was largely converted into lactate, which was released into the medium, but incorporation of radioactivity into CO(2), glutamate, glutathione, aspartate and lipids also occurred. 5. Radioactivity from leucine was detected in CO(2), lipids, glutamate, glutathione, proline and glutamine. 6. Lysine was only slightly broken down by the bovine lens; radioactivity was observed in CO(2), glutamate, glutathione, proline and two unidentified compounds. 7. Proline was metabolized to glutamate from which CO(2), glutathione and glutamine were formed. Hydroxyproline in the capsule collagen was labelled. 8. Radioactivity from serine was found in CO(2), lipids, glutathione, glycine, cystine, ATP, lactate and three unidentified compounds, one of which was probably taurine. 9. Neither tyrosine nor tryptophan were metabolized by the bovine lens. 10. The ability of the lens to metabolize amino acids was also shown by measurement of NH(3) production: more NH(3) was formed when glucose was absent from the incubation medium. 11. These experiments suggest that oxidation of amino acids is a source of energy for the lens.