AFM study for morphological characteristics and biomechanical properties of human cataract anterior lens capsules

An AFM Approach Applied in a Study of α-Crystallin Membrane Association: New Insights into Lens Hardening and Presbyopia Development

The lens of the eye loses elasticity with age, while α-crystallin association with the lens membrane increases with age. It is unclear whether there is any correlation between α-crystallin association with the lens membrane and loss in lens elasticity. This research investigated α-crystallin membrane association using atomic force microscopy (AFM) for the first time to study topographical images and mechanical properties (breakthrough force and membrane area compressibility modulus (K_A), as measures of elasticity) of the membrane. α-Crystallin extracted from the bovine lens cortex was incubated with a supported lipid membrane (SLM) prepared on a flat mica surface. The AFM images showed the time-dependent interaction of α-crystallin with the SLM. Force spectroscopy revealed the presence of breakthrough events in the force curves obtained in the membrane regions where no α-crystallin was associated, which suggests that the membrane’s elasticity was maintained. The force curves in the α-crystallin submerged region and the close vicinity of the α-crystallin associated region in the membrane showed no breakthrough event within the defined peak force threshold, indicating loss of membrane elasticity. Our results showed that the association of α-crystallin with the membrane deteriorates membrane elasticity, providing new insights into understanding the molecular basis of lens hardening and presbyopia.

Membrane elasticity modulated by cholesterol in model of porcine eye lens-lipid membrane

Experimental evidence shows that the eye lens loses its elasticity dramatically with age. It has also been reported that the cholesterol (Chol) content in the eye lens fiber cell plasma membrane increases significantly with age. High Chol content leads to the formation of cholesterol bilayer domains (CBDs) in the lens membrane. The role of high Chol associated with lens elasticity is unclear. The purpose of this research is to investigate the membrane elasticity of the model of porcine lens-lipid (MPLL) membrane with increasing Chol content to elucidate the role of high Chol in lens membrane elasticity. In this study, we used atomic force microscopy (AFM) to study the mechanical properties (breakthrough force and area compressibility modulus (K_A)) of the MPLL membrane with increasing Chol content where K_A is the measure of membrane elasticity. We varied Chol concentration in Chol/MPLL membrane from 0 to ∼71 mol%. Supported Chol/MPLL membranes were prepared by fusion of small unilamellar vesicles (SUVs) on top of a flat mica surface. SUVs of the Chol/MPLL lipid mixture were prepared with the rapid solvent exchange method followed by probe-tip sonication. For the Chol/MPLL mixing ratio of 0, AFM image showed the formation of two distinct phases of the membrane, i.e., liquid-disordered phase (l_d) and solid-ordered phase (s_o) membrane. However, with Chol/MPLL mixing ratio of 0.5 and above, only liquid-ordered phase (l_o) membrane was formed. Also, two distinct breakthrough forces corresponding to l_d and s_o were observed for Chol/MPLL mixing ratio of 0, whereas only one breakthrough force was observed for membranes with Chol/MPLL mixing ratio of 0.5 and above. No significant difference in the membrane surface roughness was measured with increasing Chol content for these membranes; however, breakthrough force and K_A for l_o membrane increased when Chol/MPLL mixing ratio was increased from 0.5 to 1. Interestingly above the Chol/MPLL mixing ratio of 1, both breakthrough force and K_A decreased, indicating the formation of CBDs. Furthermore, these results showed that membrane elasticity increases at high Chol content, suggesting that high Chol content in lens membrane might be responsible for maintaining lens membrane elasticity.

Pathological Changes of the Anterior Lens Capsule

The anterior lens capsule (ALC), as the thickest basement membrane in the body, is an acellular, soft, smooth, transparent membrane secreted by lens epithelial cells. The ALC has its unique biomechanical properties to serve as a barrier and separate the lens from infectious viruses and bacteria together with the posterior capsule and pericapsular membrane. However, the biomechanical and ultrastructural properties of the ALC can be changed under certain conditions. Here, we provide a brief review of the pathological changes of the ALC in several eye disorders, including cataract, aniridia, climatic droplet keratopathy, exfoliation syndrome, true exfoliation syndrome, Alport syndrome, and silicone oil tamponade.

Protective effect of syringic acid via restoring cells biomechanics and organelle structure in human lens epithelial cells

We have previously reported that syringic acid (SA) extracted from D. aurantiacum var. denneanum (kerr) may be used to prevent diabetic cataract (DC). However, the underlying mechanisms through which SA prevents DC in human lens epithelial cells (HLECs) remained unclear. In the present study, we employed single-molecule optics technologies, including transmission electron microscopy (TEM), atomic force microscopy (AFM), laser scanning confocal microscopy (LSCM) and Raman spectroscopy, to monitor the effect of SA on HLECs biomechanics and organelle structure in real-time. TEM suggested that SA improved the ultrastructure of HLECs with regard to nuclear chromatin condensation and reducing mitochondrial swelling and degeneration, which may aid in the maintenance of HLECs integrity in the presence of glucose. AFM revealed a reduced surface roughness and stiffness following SA treatment, suggesting an improved viscoelasticity of HELCs. Raman spectrometry and LSCM further revealed that these changes were related to a modification of cell liquidity and cytoskeletal structure by SA. Taken together, these results provide insights into the effects of SA on the biomechanics of HLECs and further strengthen the evidence for its potential use as a novel therapeutic strategy for DC prevention.

Multimodal quantitative optical elastography of the crystalline lens with optical coherence elastography and Brillouin microscopy

Assessing the biomechanical properties of the crystalline lens can provide crucial information for diagnosing disease and guiding precision therapeutic interventions. Existing noninvasive methods have been limited to global measurements. Here, we demonstrate the quantitative assessment of the elasticity of crystalline lens with a multimodal optical elastography technique, which combines dynamic wave-based optical coherence elastography (OCE) and Brillouin microscopy to overcome the drawbacks of individual modalities. OCE can provide direct measurements of tissue elasticity rapidly and quantitatively, but it is a challenge to image transparent samples such as the lens because this technique relies on backscattered light. On the other hand, Brillouin microscopy can map the longitudinal modulus with micro-scale resolution in transparent samples. However, the relationship between Brillouin-deduced modulus and Young's modulus is not straightforward and sample dependent. By combining these two techniques, we can calibrate Brillouin measurements with OCE, based on the same sample, allowing us to completely map the Young's modulus of the crystalline lens. The combined system was first validated with tissue-mimicking gelatin phantoms of varying elasticities (N = 9). The OCE data was used to calibrate the Brillouin shift measurements and subsequently map the Young's modulus of the phantoms. After validation, OCE and Brillouin measurements were performed on ex-vivo porcine lenses (N = 6), and the Young's modulus of the lenses was spatially mapped. The results show a strong correlation between Young's moduli measured by OCE and longitudinal moduli measured by Brillouin microscopy. The correlation coefficient R was 0.98 for the phantoms and 0.94 for the lenses, respectively. The mean Young's modulus of the anterior and posterior lens was 1.98 ± 0.74 kPa and 2.93 ± 1.13 kPa, respectively, and the Young's modulus of the lens nucleus was 11.90 ± 2.94 kPa. The results presented in this manuscript open a new way for truly quantitative biomechanical mapping of optically transparent (or low scattering) tissues in 3D.

[Atomic force microscopy in the study of anterior eye segment structures]

The application of atomic force microscopy (AFM) in ophthalmology is considered with respect of the classical anatomical division of the eye into anterior and posterior segments. The review presents the findings of published research studies that involved AFM as a method for assessing anatomical and functional condition of the structures of anterior eye segment. One significant line of research is related to the use of AFM technologies for evaluation of morphological and biomechanical characteristics of various parts of the eye lens. That topic draws interest due to the need of furthering the understanding of cataractogenesis, as well as the significance of the anatomical and functional characteristics of the lens capsule in the mechanism of accommodation and modern phaco surgery.

A simplified approach for the determination of fitting constants in Oliver-Pharr method regarding biological samples

The atomic force microscopy (AFM) nanoindentation regarding biological samples is a challenging procedure. Biological samples at the nanoscale can be considered as purely elastic materials under the condition that the indentation depth is very small and the indenter is smooth. However, the indenters that are commonly used are pyramidal and in several cases the indentation depths are big comparing to the dimensions of the tip apex. Hence, pyramidal indenters usually cause a permanent damage to the sample. In this case, the best model that can be applied for the data processing is the Oliver-Pharr model which takes into account the elastic-plastic behavior of the sample. The Oliver-Pharr model is based on the fitting of the unloading load-indentation data to a power law equation. In this paper a simplified procedure which ensures the accurate fitting of the unloading load-indentation data to the Oliver-Pharr model is presented and validated on experimental data obtained from a human glioma cell line. It should be noted that the proposed method can be also applied for the data fitting in the case of purely elastic response.

Micromorphology analysis of the anterior human lens capsule

PURPOSE

This study aimed to quantify the three-dimensional micromorphology of the surface of the human lens capsule as a function of age.

METHODS

Imaging experiments were conducted on whole human lenses received from eight human cadavers (donor age range: 30-88 years). Imaging was performed with an atomic force microscope (AFM) in contact mode in fluid. The porosity and surface roughness were quantified from the height images obtained. A novel approach, based on stereometric and fractal analysis of three-dimensional surfaces developed for use in conjunction with AFM data, was also used to analyze the surface microtexture as a function of age.

RESULTS

The AFM images obtained depict a highly ordered fibrous structure at the surface of the lens capsule, although the overall structure visually changes with age. Porosity and roughness were quantified for each image and analyzed as a function of donor age. The interfibrillar spacing revealed an increasing trend with age, although this result was not significant (p = 0.110). The root mean square (RMS) deviation and average deviation significantly decreased with increasing age (p<0.001 for both). The fractal analysis provided quantitative values for 29 amplitude, hybrid, functional, and spatial parameters. All the hybrid parameters decreased with age, although not significantly. Of the functional parameters, the surface bearing index increased significantly with age (p = 0.017) and the summit height exhibited a decreasing trend with age (p = 0.298). Of the spatial parameters, the dominant radial wavelength trend moved toward an increase with age (p = 0.103) and the cross-hatch angle tended toward a decrease with age (p = 0.213).

CONCLUSIONS

Significant changes in the three-dimensional surface microtexture of the human lens capsule were found with age, although more experiments on a larger dataset are needed to conclude this with certainty. The analyzed AFM images demonstrate a fractal nature of the surface, which is not considered in classical surface statistical parameters. The surface fractal dimension may be useful in ophthalmology for quantifying human lens architectural changes associated with different disease states to further our understanding of disease evolution.

Fractal analysis of AFM images of the surface of Bowman's membrane of the human cornea

The objective of this study is to further investigate the ultrastructural details of the surface of Bowman's membrane of the human cornea, using atomic force microscopy (AFM) images. One representative image acquired of Bowman's membrane of a human cornea was investigated. The three-dimensional (3-D) surface of the sample was imaged using AFM in contact mode, while the sample was completely submerged in optisol solution. Height and deflection images were acquired at multiple scan lengths using the MFP-3D AFM system software (Asylum Research, Santa Barbara, CA), based in IGOR Pro (WaveMetrics, Lake Oswego, OR). A novel approach, based on computational algorithms for fractal analysis of surfaces applied for AFM data, was utilized to analyze the surface structure. The surfaces revealed a fractal structure at the nanometer scale. The fractal dimension, D, provided quantitative values that characterize the scale properties of surface geometry. Detailed characterization of the surface topography was obtained using statistical parameters, in accordance with ISO 25178-2: 2012. Results obtained by fractal analysis confirm the relationship between the value of the fractal dimension and the statistical surface roughness parameters. The surface structure of Bowman's membrane of the human cornea is complex. The analyzed AFM images confirm a fractal nature of the surface, which is not taken into account by classical surface statistical parameters. Surface fractal dimension could be useful in ophthalmology to quantify corneal architectural changes associated with different disease states to further our understanding of disease evolution.

Optical coherence elastography for tissue characterization: a review

Optical coherence elastography (OCE) represents the frontier of optical elasticity imaging techniques and focuses on the micro-scale assessment of tissue biomechanics in 3D that is hard to achieve with traditional elastographic methods. Benefit from the advancement of optical coherence tomography, and driven by the increasing requirements in nondestructive biomechanical characterization, this emerging technique recently has experienced a rapid development. In this paper, we start with the description of the mechanical contrast that has been employed by OCE and review the state-of-the-art techniques based on the reported applications and discuss the current technical challenges, emphasizing the unique role of OCE in tissue mechanical characterization. The position of OCE among other elastography techniques.

The effects of UV irradiation on collagen D-band revealed by atomic force microscopy

The objective of this paper was to investigate the influence of UV irradiation on collagen D-band periodicity by using the AFM imaging and nanoindentation methods. It is well known than UV irradiation is one of the main factors inducing destabilization of collagen molecules. Due to the human's skin chronic exposure to sun light, the research concerning the influence of UV radiation on collagen is of great interest. The impact of UV irradiation on collagen can be studied in nanoscale using Atomic Force Microscopy (AFM). AFM is a powerful tool as far as surface characterization is concerned, due to its ability to relate high resolution imaging with mechanical properties. Hence, high resolution images of individual collagen fibrils and load-displacement curves on the overlapping and gap regions, under various time intervals of UV exposure, were obtained. The results demonstrated that the UV rays affect the height level differences between the overlapping and gap regions. Under various time intervals of UV exposure, the height difference between overlaps and gaps reduced from ~3.7 nm to ~0.8 nm and the fibril diameters showed an average of 8-10% reduction. In addition, the irradiation influenced the mechanical properties of collagen fibrils. The Young's modulus values were reduced per 66% (overlaps) and 61% (gaps) compared to their initial values. The observed alterations on the structural and the mechanical properties of collagen fibrils are probably a consequence of the polypeptide chain scission due to the impact of the UV irradiation.

Analysis of femtosecond laser assisted capsulotomy cutting edges and manual capsulorhexis using environmental scanning electron microscopy

Purpose. To investigate the structure and irregularity of the capsulotomy cutting edges created by two femtosecond (FS) laser platforms in comparison with manual continuous circular capsulorhexis (CCC) using environmental scanning electron microscopy (eSEM). Methods. Ten anterior capsulotomies were obtained using two different FS laser cataract platforms (LenSx, n = 5, and Victus, n = 5). In addition, five manual CCC (n = 5) were obtained using a rhexis forceps. The specimens were imaged by eSEM (FEI Quanta 400, OR, USA). Objective metrics, which included the arithmetic mean deviation of the surface (Sa) and the root-mean-square deviation of the surface (Sq), were used to evaluate the irregularity of both the FS laser capsulotomies and the manual CCC cutting edges. Results. Several microirregularities were shown across the FS laser capsulotomy cutting edges. The edges of manually torn capsules were shown, by comparison of Sa and Sq values, to be smoother (P < 0.05) than the FS laser capsulotomy edges. Conclusions. Work is needed to understand whether the FS laser capsulotomy edge microirregularities, not seen in manual CCC, may act as focal points for the concentration of stress that would increase the risk of capsular tear during phacoemulsification as recently reported in the literature.

Effects of pressure and electrical charge on macromolecular transport across bovine lens basement membrane

Molecular transport through the basement membrane is important for a number of physiological functions, and dysregulation of basement membrane architecture can have serious pathological consequences. The structure-function relationships that govern molecular transport in basement membranes are not fully understood. The basement membrane from the lens capsule of the eye is a collagen IV-rich matrix that can easily be extracted and manipulated in vitro. As such, it provides a convenient model for studying the functional relationships that govern molecular transport in basement membranes. Here we investigate the effects of increased transmembrane pressure and solute electrical charge on the transport properties of the lens basement membrane (LBM) from the bovine eye. Pressure-permeability relationships in LBM transport were governed primarily by changes in diffusive and convective contributions to solute flux and not by pressure-dependent changes in intrinsic membrane properties. The solute electrical charge had a minimal but statistically significant effect on solute transport through the LBM that was opposite of the expected electrokinetic behavior. The observed transport characteristics of the LBM are discussed in the context of established membrane transport modeling and previous work on the effects of pressure and electrical charge in other basement membrane systems.

Measurements of elastic modulus for human anterior lens capsule with atomic force microscopy: the effect of loading force

The purpose of the study was to appraise the effect of loading force magnitude on the determination of the elastic modulus of the anterior lens capsule through atomic force microscopy. Four human anterior lens capsules taken during phacoemulsification cataract surgery were studied, free of epithelial cells, with atomic force microscopy. For the experiment, five different indentation loading forces were applied to near areas of the specimen. Experimental data was exported and analyzed according to the Hertz model to obtain the Young's modulus with regards to the elastic behavior of the material. Force-distance curves were acquired by applying a load of 2, 5, 10, 20 and 30 nN. When examining the results it was evident that determination of Young's modulus of the anterior lens capsule is dependent on the loading force concerning the examined range. Loading forces of 10 and 20 nN led to results without significant difference (p > 0.05) and more reproducible (coefficients of variation 12.4 and 11.7 %, respectively).