Lens fluorescence and metabolic control in type 1 diabetic patients: a 14 year follow up study

Protein posttranslational modification (PTM) by glycation: Role in lens aging and age-related cataractogenesis

Crystallins, the most prevalent lens proteins, have no turnover throughout the entire human lifespan. These long-lived proteins are susceptible to post-synthetic modifications, including oxidation and glycation, which are believed to be some of the primary mechanisms for age-related cataractogenesis. Thanks to high glutathione (GSH) and ascorbic acid (ASA) levels as well as low oxygen content, the human lens is able to maintain its transparency for several decades. Aging accumulates substantial changes in the human lens, including a decreased glutathione concentration, increased reactive oxygen species (ROS) formation, impaired antioxidative defense capacity, and increased redox-active metal ions, which induce glucose and ascorbic acid degradation and protein glycation. The glycated lens crystallins are either prone to UVA mediated free radical production or they attract metal ion binding, which can trigger additional protein oxidation and modification. This vicious cycle is expected to be exacerbated with older age or diabetic conditions. ASA serves as an antioxidant in the human lens under reducing conditions to protect the human lens from damage, but ASA converts to the pro-oxidative role and causes lens protein damage by ascorbylation in high oxidation or enriched redox-active metal ion conditions. This review is dedicated in honor of Dr. Frank Giblin, a great friend and superb scientist, whose pioneering and relentless work over the past 45 years has provided critical insight into lens redox regulation and glutathione homeostasis during aging and cataractogenesis.

Assessment of lens autofluorescence in patients with well-controlled essential hypertension and those with renal failure secondary to hypertension undergoing hemodialysis

PURPOSE

To compare the lens autofluorescence (AF) levels among patients with end-stage renal failure and undergoing hemodialysis secondary to hypertension, patients with well-controlled hypertension, and healthy controls.

METHOD

This study was a prospective, cross-sectional, comparative study conducted between February and April 2018. Two groups of patients and a group of healthy individuals were included in the study. The first group of patients included individuals with a renal insufficiency due to essential hypertension who underwent regular hemodialysis treatment (dialysis group). The second group included patients with well-controlled essential hypertension (hypertension group). Lens autofluorescence was measured via a scanning confocal lens fluorescence biomicroscope optical system for all participants. The measurement of fluorescence ratio is given as a numerical data. The AF results were compared in all groups.

RESULTS

The study included 87 individuals. There were 29 individuals (33.3 %) in the dialysis group, 30 (34.5 %) in the hypertension group, and 28 (32.2 %) in the healthy group. The mean fluorescence ratio(FR) was 0.20 ± 0.06, 0.20 ± 0.04, and 0.17 ± 0.04 in the dialysis, hypertension, and healthy groups respectively. There was a significant difference in the mean FR measurements between the three groups (p = .004). As a result of a binary comparison, mean FR values for patients in the dialysis group were higher (0.20 ± 0.06) than for healthy individuals (0.17 ± 0.04), which was statistically significant (p = .025). Mean FR measurements of hypertensive patients were higher (0.20 ± 0.04) than healthy individuals (0.17 ± 0.04), which was also statistically significant (p = .02). However, there was no statistically significant difference among the mean FR measurements between the hypertension and dialysis groups (p = .63).

CONCLUSION

We demonstrated that lens autofluorescence increased in patients with renal failure undergoing hemodialysis and those with well-controlled hypertension. The mean lens autofluorescence levels were significantly higher in both patient groups than the healthy control group.

3-Hydroxykynurenine bound to eye lens proteins induces oxidative modifications in crystalline proteins through a type I photosensitizing mechanism

Photosensitized reactions mediated by endogenous chromophores have been associated with the etiology of age-related cataract disease. Endogenous chromophores such as 3-hydroxykynurenine (3OHKN) can be found in both free form, and bound to crystallin proteins. However, their efficiency in generating photo-induced oxidative modifications on eye lens proteins is not completely understood. In this work, the efficiency and photodynamic activity of 3OHKN bound to both lysine (3OHKN-Lys) and bovine lens proteins (3OHKN-BLP) was assessed and compared with the photosensitizing activity of the major chromophore arising from glucose degradation (GDC). The photosensitizing activity of 3OHKN-Lys, 3OHKN-BLP and GDC was characterized by measurement of singlet oxygen quantum yields, O₂ consumption, SDS-PAGE and amino acid analysis of the photo-oxidized proteins. Singlet oxygen quantum yields under 20% O₂ atmosphere were 0.02, 0.01, and 0.27 for 3OHKN-Lys, 3OHKN-BLP and GDC, respectively. O₂ consumption by photosensitized reactions was more efficient for 3OHKN-BLP, with the extent of O₂ consumption being ∼28% higher than for 3OHKN-Lys and GDC under both 5 and 20% O₂. SDS-PAGE showed that protein crosslinking is dependent on the O₂ concentration, and more extensive at 5 than 20% O₂. GDC and 3OHKN-Lys were the most efficient crosslinkers at 20 and 5% O₂, respectively. Amino acid analysis of the irradiated proteins showed consumption of Trp, His, Tyr and Phe, and formation of kynurenine (from Trp), methionine sulfoxide (from Met) and DOPA (from Tyr). Kynurenine formation was dependent on the O₂ concentration with higher amounts detected at 5 than 20% O₂ for 3OHKN-BLP and 3OHKN-Lys, with 3OHKN-BLP the most efficient sensitizer. Our results suggest that 3OHKN-BLP can elicit photo-oxidative damage mainly by a type I photosensitizing mechanism, with this likely to be the most prevalent pathway at the low physiologic O₂ concentrations in the eye lens.

Assessment of advanced glycation end-products as a biomarker of diabetic outcomes

There are substantial differences in the onset and severity of diabetes complications that are not fully explained by HbA1c levels and other risk factors. HbA1c is the gold standard for assessing metabolic control, but has limited value to identify patients at risk of developing diabetic complications. The main disadvantage of HbA1c is that it does not provide information about glycemic variability and does not reflect long-term exposure to hyperglycemia. One of the main pathogenetic mechanisms of diabetic complications is the generation and accumulation of advanced glycation end-products (AGEs). Based on its fluorescence properties, AGEs may be measured in tissues such as the skin or lens. These non-invasive measurements of AGE accumulation may be considered as promising biomarkers of late diabetic complications, and our objective is to summarize the available evidence supporting this statement. However, further translational research and prospective clinical trials are needed before these new biomarkers may be incorporated into clinical practice.

The assessment of autofluorescence of the crystalline lens in diabetic patients and healthy controls: can it be used as a screening test?

Background

Our purpose was to demonstrate if measuring lens autofluorescence (AF) with a scanning confocal biomicroscope may be used to identify subjects with undiagnosed type II diabetes mellitus (DM), and hence, for it to be used as a marker for the severity of diabetic retinopathy in diabetic patients.

Patients and methods

In this cross-sectional, comparative study, lens AF was measured with scanning confocal lens fluorescence biomicroscope in diabetic and healthy groups. Full ophthalmological examination was performed. Blood tests of fasting plasma glucose, and glycosylated hemoglobin were also analyzed. The correlation between lens AF results and blood tests was evaluated in both groups. The cutoff value for the diagnosis of DM using lens AF was investigated.

Results

The study included 191 subjects with a mean age of 52.09±6.75 years. One hundred and seven (56.0%) subjects were female, and 84 (44.0%) were male. Eighty-two (42.9%) patients had type II DM, and 109 (57.1%) subjects self-reported as normal. The fluorescence ratio (FR) values ranged from 0.09 to 0.46 (0.23±0.06) in the total group. Mean FR measurements of diabetic subjects were significantly higher (0.27±0.06) than those without DM (0.20±0.05), (p=0.001). A statistically significant correlation was found between glycosylated hemoglobin, fasting plasma glucose, and FR. The cutoff point for the FR according to the presence of DM was found to be 0.24 and above (p=0.001), with a sensitivity of 71.95% and a specificity of 80.73%.

Conclusion

Measuring AF of human lens as an indirect evidence of increased advanced glycaton end products may helpful in detecting impaired glucose metabolism. Our results show highly significant correlation between possibility of DM and FR.

Non-invasive tryptophan fluorescence measurements as a novel method of grading cataract

Development of non-invasive treatments for cataract calls for a sensitive diagnostic assay. We conducted a study to test whether the ratio of folded tryptophan to non-tryptophan fluorescence emission (F-factor) may be used for grading cataracts in human lenses. The F-factor was measured on aspirated lens material from eyes undergoing femtosecond laser assisted cataract surgery (FLACS) and was compared to a preoperative optical grading of cataract using Scheimpflug imaging. The preoperative optical grading allocated the cataracts to 1 of 4 categories according to the density of the cataract. All cataracts were age-related. Lens material from 16 eyes of 14 patients was included in the study. Cataracts were preoperatively graded in categories 1, 2 and 3. No lenses were category 4. For nuclear cataracts mean values of F-factor were 52.9 (SD 12.2), 61.7 (SD 5.3) and 75.7 (SD 8.9) for categories 1, 2 and 3 respectively. Linear regression on F-factor as a function of preoperative grading category showed increasing values of F-factor with increasing preoperative grading category, R² = 0.515. Our experiment showed that preoperative optical grading of cataracts by Scheimpflug imaging may correlate to measures of tryptophan and non-tryptophan fluorescence in human lenses. Based on our results we find that measuring the ratio between tryptophan- and non-tryptophan fluorescence may be a future tool for grading cataracts, but further research is needed.

Measurement of Lens Autofluorescence Can Distinguish Subjects With Diabetes From Those Without

Lens autofluorescence is increased in patients with diabetes mellitus, but clinical application has been limited by the lack of an instrument suitable for routine clinical use. We investigate possible uses of a new scanning confocal biomicroscope (1) to identify subjects with undiagnosed type 2 diabetes and (2) as a marker for the progression of diabetes. One hundred seventy-eight subjects self-reported as normal and 53 subjects physician-diagnosed with diabetes or prediabetes were recruited. Measurements were collected using a ClearPath DS-120 Lens Fluorescence Biomicroscope calibrated with standards traceable to National Institute of Standards and Technology (NIST). Fluorescence intensities were corrected for age by subtracting the value expected from a regression of intensity versus age for normal subjects. This "fluorescence deviation" showed progressively higher values for normal, prediabetes, type 2 diabetes, and type 1 diabetes and a high degree of predictability of diabetes diagnosis. A receiver operating characteristics curve was used to determine sensitivity and specificity for prediction of diabetes type 2. At a fluorescence deviation of 2500, a sensitivity of 67% at 94% specificity was observed detection of type 2 diabetes. The progressively higher fluorescence deviations are consistent with the physiological mechanisms of accumulation of fluorescent advanced glycation end products as the subject ages. The sensitivity and specificity performance of the lens autofluorescence test for type 2 diabetes is comparable to the performance of glucose threshold tests. The statistically significant difference between fluorescence deviations of normal and type 2 diabetes supports the feasibility of lens autofluorescence to screen subjects for undiagnosed type 2 diabetes. Ophthalmic practices are points of care at which there may be a public health benefit for screening patients for undiagnosed diabetes.

Microvascular retinopathy in subjects without diabetes: the Inter99 Eye Study

PURPOSE

Retinal vascular lesions such as microaneurysms and haemorrhages, while typical of diabetic retinopathy, are also seen in subjects without diabetes where they are associated with elevated cardiovascular mortality. In theory, these lesions could be a consequence of past hyperglycaemia. We examined the prevalence and risk factors for retinopathy, including lens fluorescence, a biomarker of cumulative life-time glycaemia in adults without diabetes.

METHODS

Cross-sectional population-based study of 711 subjects without diabetes (WHO 1999 criteria) aged 30-60 years, including oral glucose tolerance testing, clinical and laboratory examinations, non-invasive ocular lens fluorometry and seven-field fundus photography.

RESULTS

  Retinopathy was present in 8.3% (CI(95) 6.3-10.3%) of subjects. Higher systolic blood pressure (SBP) (p = 0.032), increasing body mass index (BMI) (p = 0.014) and wider waist circumference (p = 0.014) were significantly associated with retinopathy after adjusting for age and sex. Retinopathy was not significantly related to long-term, short-term or current glycaemia (lens fluorescence, HbA(1c) , fasting plasma glucose). In the multivariate analysis, the odds ratio (OR) for retinopathy in subjects with SBP ≥160 mmHg compared to subjects with SBP <130 mmHg was 2.68 (CI(95) 1.07-6.70, p = 0.036) and in subjects with BMI ≥30 compared to subjects with BMI < 25 the OR for retinopathy was 2.14 (CI(95) 1.01-4.57, p = 0.049) when adjusting for both variables, age, sex, the presence of impaired glucose tolerance and impaired fasting glucose.

CONCLUSION

 Retinopathy in subjects without diabetes was associated with hypertension and obesity. The study found no evidence that microvascular retinopathy in non-diabetic subjects was a consequence of past hyperglycaemia.

Simultaneous noninvasive clinical measurement of lens autofluorescence and rayleigh scattering using a fluorescence biomicroscope

BACKGROUND

Lens autofluorescence increases with the age of the subject, and the fluorophores responsible are associated with cataract, retinopathy, and other complications of diabetes. We built a scanning confocal lens fluorescence biomicroscope suitable for routine clinical measurement of lens autofluorescence and light scattering and report data from 127 healthy subjects.

METHOD

The fluorescence biomicroscope focuses a beam of light from a blue light-emitting diode on the lens and measures fluorescent green light and blue scattered light using a sensitive silicon photomultiplier. The system includes a target fixation light and a video camera for alignment and automatic pupil tracking. Under software control, a volume of measurement is scanned from behind the posterior lens capsule, through the lens to the aqueous humor, and then back again. Software computes the average ratio of lens autofluorescence to scattered light in the central portion of the lens. Self-reported healthy nondiabetic subjects were examined by an optometrist; if their eyes were healthy and without significant cataract, they were entered into the study.

RESULTS

Valid lens autofluorescence data were collected from 127 subjects between 21 and 70 years of age. A linear model for lens autofluorescence intensity with age was highly statistically significant, and the improvement in fit for higher-order polynomial models was not statistically significant. The ratio of lens autofluorescence to light scatter was also calculated; regression analysis showed significant curvature for the relationship of the fluorescence ratio to age, so a nonlinear model was used to estimate the mean ratio of autofluorescence to scatter and its prediction intervals as a function of age.

CONCLUSIONS

Our observation of a strongly significant linear regression of fluorescence intensity with age of the subjects agrees with the results from previous studies, as does a nonlinear model for the fluorescence ratio. The fluorescence biomicroscope enables the clinician to identify patients with fluorescence ratio significantly higher than expected for their age.

Cumulative glycemia and microangiopathy in subjects with impaired glucose regulation in the Inter99 study

AIMS

To assess cumulative glycemia, microvascular characteristics, and associated risk factors for diabetes in subjects with impaired glucose regulation.

METHODS

Cross-sectional, population-based study comprising systemic characteristics in 6487 participants and ocular characteristics in 970 participants.

RESULTS

Lens fluorescence, a quantitative index of life-long cumulative glycemia, was increased by 7.5% (CI(95) 0.37-15.1%) in subjects with impaired fasting glucose, by 13.0% (CI(95) 5.5-21%) in subjects with combined impaired fasting glucose and impaired glucose tolerance (IFG+IGT), and by 11.8% (CI(95) 6.8-17.1%) in subjects with screen-detected diabetes compared to normoglycemic subjects, adjusted for age, sex, and smoking. The prevalences of microalbuminuria and retinopathy were significantly increased in subjects with screen-detected diabetes after adjusting for age, sex and systolic blood pressure. The prevalences of associated risk factors for diabetes were elevated in all categories of abnormal glucose regulation compared to normoglycemic subjects.

CONCLUSIONS

Life-long cumulative glycemia, microangiopathy, and associated risk factors for diabetes were significantly elevated in subjects with abnormal glucose metabolism, most prominently in subjects with IFG+IGT and in subjects with screen-detected diabetes. These results provide the first objective evidence that cumulative glycemic load is increased at the earliest stage of impaired glucose regulation.

Multiphoton spectral microscopy for imaging and quantification of tissue glycation

Tissue glycation from diabetes and aging can result in complications such as renal failure, blindness, nerve damage and vascular diseases. In this work, we applied multiphoton microscopy for imaging and characterizing the extent of tissue glycation. The characteristic features of multiphoton autofluorescence (MPAF) and second harmonic generation (SHG) images as well as MPAF spectra of glycated bovine skin, cornea and aorta were acquired. The analysis of MPAF intensity change accompanying the glycation process shows that collagen is more responsive to the formation of autofluorescent advanced glycation endproducts (AGE) than elastic fibers. Changes in spectral features were also used to estimate the rate of glycation in tissues with intrinsic AF. Our study shows that multiphton imaging may be used for the in vitro investigation of the effects of tissue glycation and that this approach may be used for monitoring AGE formation in the clinical setting.

Lens autofluorescence is not increased at high altitude

PURPOSE

To study the relation between ambient environmental ultraviolet radiation exposure and lens fluorescence.

METHODS

Non-invasive lens fluorometry measurements were compared in healthy Bolivian and Danish subjects. Background ultraviolet radiation was 4.5 times higher in Bolivia than in Denmark.

RESULTS

No significant differences in lens fluorescence or transmittance were found between Bolivian and Danish volunteers.

CONCLUSION

Age-corrected lens fluorescence and transmittance were comparable for healthy participants living at high altitude near the equator and healthy volunteers living at sea level at 55 degrees northern latitude. These results suggest that lens ageing, as assessed by lens autofluorometry, is independent of exposure to ultraviolet radiation.

The clinical relevance of assessing advanced glycation endproducts accumulation in diabetes

Cardiovascular disease is the major cause of morbidity and mortality associated with diabetes. There is increasing evidence that advanced glycation endproducts (AGEs) play a pivotal role in atherosclerosis, in particular in diabetes. AGE accumulation is a measure of cumulative metabolic and oxidative stress, and may so represent the "metabolic memory". Furthermore, increased AGE accumulation is closely related to the development of cardiovascular complications in diabetes. This review article will focus on the clinical relevance of measuring AGE accumulation in diabetic patients by focusing on AGE formation, AGEs as predictors of long-term complications, and interventions against AGEs.

Glycation Products as Markers and Predictors of the Progression of Diabetic Complications

The structure of a growing number of glycation and advanced glycation end products has been elucidated. Measuring these products can be used to assess cumulative glycemic and glycoxidative damage in diabetes and other chronic conditions. The predictive power of a given glycation product can be tested in large prospective studies that evaluate the risk of developing diabetic micro- and macrovascular disease over years following the quantitative determination of that marker. This article provides a comprehensive review of the field, comparing the merits of each marker, whether in skin, serum, or other tissue. Several conclusions are drawn, one of which identifies skin glycation products as powerful predictors of the risk of developing diabetic complications.