Mutual diffusion of crystallin proteins at finite concentrations: a light-scattering study

Cataract-causing mutations L45P and Y46D promote γC-crystallin aggregation by disturbing hydrogen bonds network in the second Greek key motif

Congenital cataracts caused by genetic disorders are the primary cause of child blindness across the globe. In this work, we investigated the underlying molecular mechanism of two mutations, L45P and Y46D of γC-crystallin in two Chinese families causing nuclear congenital cataracts. Spectroscopic experiments were performed to determine structural differences between the wild-type (WT) and the L45P or Y46D mutant of γC-crystallin, and the structural stabilities of the WT and mutant proteins were measured under environmental stress (ultraviolet irradiation, pH disorders, oxidative stress, or chemical denaturation). The L45P and Y46D mutants had lower protein solubility and more hydrophobic residues exposed, making them prone to aggregation under environmental stress. The dynamic molecular simulation revealed that the L45P and Y46D mutations destabilized γC-crystallin by altering the hydrogen bonds network around the Trp residues in the second Greek key motif. In summary, L45P and Y46D mutants of γC-crystallin caused more hydrophobic residues to be solvent-exposed, lowered the solubility of γC-crystallin, and increased aggregation propensity under environmental stress. These might be the pathogenesis of γC-crystallin L45P and Y46D mutants related to congenital cataract.

Synthesis and application of PEGylated tracer particles for measuring protein solution viscosities using Dynamic Light Scattering-based microrheology

The measurement of flow properties, such as the zero shear viscosity, of protein solutions is of paramount importance for many applications such as pharmaceutical formulations, where the syringeability of physiologically effective doses is a key property. However, the determination of these properties with classical rheological methods is often challenging due to e.g. detrimental surface effects or simply the lack of sufficient material. A possible alternative is Dynamic Light Scattering-based microrheology, where the Brownian motion of tracer particles embedded in the protein solution is monitored to access the zero shear viscosity of the sample. The prime advantages of this method compared to classical rheology are the absence of disturbing surface effects and the up to two orders of magnitude smaller protein quantities needed for an entire concentration series. This Protocol provides a detailed description of the synthesis of sterically stabilized tracer particles with surface and overall particle properties specifically designed to investigate the viscosity of protein solutions up to concentrations close to the arrest transition. These particles are tailored to avoid protein-particle as well as particle-particle aggregation at various sample conditions and thus allow for an artifact-free application of Dynamic Light Scattering-based tracer microrheology to determine the flow behaviour of biological samples. The Protocol concludes with step by step instructions for the characterization of protein solutions using a combination of the tracer particles and an advanced dynamic light scattering technique yielding the concentration-dependent zero shear viscosity.

Proteomics: a tool to develop novel diagnostic methods and unravel molecular mechanisms of pediatric diseases

Proteomics is the study of the expression of changes and post-translational modifications (PTM) of proteins along a metabolic condition either normal or pathological. In the field of health, proteomics allows obtaining valuable data for treatment, diagnosis or pathophysiological mechanisms of different illnesses. To illustrate the aforementioned, we describe two projects currently being performed at the Instituto Nacional de Pediatría: The immuno-proteomic study of cow milk allergy and the Proteomic study of childhood cataract. Cow's milk proteins (CMP) are the first antigens to which infants are exposed and generate allergy in some of them. In Mexico, the incidence of CMP allergy has been estimated at 5-7%. Clinical manifestations include both gastrointestinal and extra-gastrointestinal symptoms, making its diagnosis extremely difficult. An inappropriate diagnosis affects the development and growth of children. The goals of the study are to identify the main immune-reactive CMP in Mexican pediatric population and to design more accurate diagnostic tools for this disease. Childhood cataract is a major ocular disease representing one of the main causes of blindness in infants; in developing countries, this disease promotes up to 27% of cases related to visual loss. From this group, it has been estimated that close to 60% of children do not survive beyond two years after vision lost. PTM have been pointed out as the main cause of protein precipitation at the crystalline and, consequently, clouding of this tissue. The study of childhood cataract represents an outstanding opportunity to identify the PTM associated to the cataract-genesis process.

NMR-detected brownian dynamics of αB-crystallin over a wide range of concentrations

Knowledge about the global translational and rotational motion of proteins under crowded conditions is highly relevant for understanding the function of proteins in vivo. This holds in particular for human αB-crystallin, which is strongly crowded in vivo and inter alia responsible for preventing cataracts. Quantitative information on translational and rotational diffusion is not readily available, and we here demonstrate an approach that combines pulsed-field-gradient NMR for translational diffusion and proton T1ρ/T2 relaxation-time measurements for rotational diffusion, thus overcoming obstacles encountered in previous studies. The relaxation times measured at variable temperature provide a quantitative measure of the correlation function of protein tumbling, which cannot be approximated by a single exponential, because two components are needed for a minimal and adequate description of the data. We find that at high protein concentrations, rotational diffusion is decoupled from translational diffusion, the latter following the macroscopic viscosity change almost quantitatively, resembling the behavior of spherical colloids. Analysis of data reported in the literature shows that well-packed globular proteins follow a scaling relation between the hydrodynamic radius and the molar mass, Rh ∼ M(1/d), with a fractal dimension of d ∼ 2.5 rather than 3. Despite its oligomeric nature, Rh of αB-crystallin as derived from both NMR methods is found to be fully consistent with this relation.

Overview of the Lens

In order to accomplish its function of transmitting and focusing light, the crystalline lens of the vertebrate eye has evolved a unique cellular structure and protein complement. These distinct adaptations have provided a rich source of scientific discovery ranging from biochemistry and genetics to optics and physics. In addition, because of these adaptations, lens cells persist for the lifetime of an organism, providing an excellent model of the aging process. The chapters dealing with the lens will demonstrate how the different aspects of lens biology and biochemistry combine in this singular refractive organ to accomplish its critical role in the visual system.

Structural and biochemical characterization of the childhood cataract-associated R76S mutant of human γD-crystallin

Although a number of γD-crystallin mutations are associated with cataract formation, there is not a clear understanding of the molecular mechanism(s) that lead to this protein deposition disease. As part of our ongoing studies on crystallins, we investigated the recently discovered Arg76 to Ser (R76S) mutation that is correlated with childhood cataract in an Indian family. We expressed the R76S γD-crystallin protein in E. coli, characterized it by CD, fluorescence, and NMR spectroscopy, and determined its stability with respect to thermal and chemical denaturation. Surprisingly, no significant biochemical or biophysical differences were observed between the wild-type protein and the R76S variant, except a lowered pI (6.8 compared to the wild-type value of 7.4). NMR assessment of the R76S γD-crystallin solution structure, by RDCs, and of its motional properties, by relaxation measurements, also revealed a close resemblance to wild-type crystallin. Further, kinetic unfolding/refolding experiments for R76S and wild-type protein showed similar degrees of off-pathway aggregation suppression by αB-crystallin. Overall, our results suggest that neither structural nor stability changes in the protein are responsible for the R76S γD-crystallin variant's association with cataract. However, the change in pI and the associated surface charge or the altered nature of the amino acid could influence interactions with other lens protein species.

Intermolecular interactions of IgG1 monoclonal antibodies at high concentrations characterized by light scattering

Light scattering intensity measurements of solutions of two purified monoclonal antibodies were performed over a wide range of concentrations (0.5-275 mg/mL) and ionic strengths (0.02 to 0.6 M). Despite extensive sequence homology between these mAbs, alteration of ∼20 amino acids in the complementarity determining regions resulted in different net intermolecular interactions and responses to solution ionic strength. The concentration dependence of scattering was analyzed by comparison with the predictions of three models, allowing for intermolecular interaction of various types. In order of increasing complexity, the three models account for: (1) steric repulsions (simple hard-sphere model), (2) steric repulsion with short-ranged attractive interactions of varying magnitude (adhesive hard-sphere model), and (3) steric and nonsteric repulsive interactions between several species whose relative concentrations may change as a function of total protein concentration as dictated by equilibrium self-association (effective hard-sphere mixture model). Simple scattering models of noninteracting and adhesive hard-sphere species permitted qualitative interpretation of contributions from excluded volume, electrostatic, and van der Waals interactions on net mAb interactions at high concentration as a function of ionic strength. mAb2 electrostatic interactions were repulsive, whereas mAb1 interactions were net attractive at low ionic strengths, attributed to an anisotropic distribution of molecular charge. The effective hard-sphere mixture model can account quantitatively for the dependence of scattering for both antibodies over the entire concentration range and at salt concentrations exceeding 40 mM. This analysis showed that at high ionic strength both mAbs self-associate weakly to form dimer with an affinity that varies little with salt concentration at concentrations exceeding 75 mM. In addition, mAb1 appears to self-associate further to form oligomers with stoichiometry of 4-6 and an affinity that declines substantially with increasing ionic strength. All three models lead to the conclusion that at high concentrations repulsive interactions are predominantly due to excluded volume, whereas additional features are salt-dependent and reflect a substantial electrostatic contribution to intermolecular interactions of both mAbs.

Small-angle X-ray scattering studies of the intact eye lens: effect of crystallin composition and concentration on microstructure

BACKGROUND

The cortex and nucleus of eye lenses are differentiated by both crystallin protein concentration and relative distribution of three major crystallins (alpha, beta, and gamma). Here, we explore the effects of composition and concentration of crystallins on the microstructure of the intact bovine lens (37 degrees C) along with several lenses from Antarctic fish (-2 degrees C) and subtropical bigeye tuna (18 degrees C).

METHODS

Our studies are based on small-angle X-ray scattering (SAXS) investigations of the intact lens slices where we study the effect of crystallin composition and concentration on microstructure.

RESULTS

We are able to distinguish the nuclear and cortical regions by the development of a characteristic peak in the intensity of scattered X-rays. For both the bovine and fish lenses, the peak corresponds to that expected for dense suspensions of alpha-crystallins.

CONCLUSIONS

The absence of the scattering peak in the nucleus indicates that there is no characteristic wavelength for density fluctuations in the nucleus although there is liquid-like order in the packing of the different crystallins. The loss in peak is due to increased polydispersity in the sizes of the crystallins and due to the packing of the smaller gamma-crystallins in the void space of alpha-crystallins.

GENERAL SIGNIFICANCE

Our results provide an understanding for the low turbidity of the eye lens that is a mixture of different proteins. This will inform design of optically transparent suspensions that can be used in a number of applications (e.g., artificial liquid lenses) or to better understand human diseases pathologies such as cataract.

Dynamic light scattering study on phase separation of a protein-water mixture: application on cold cataract development in the ocular lens

We present a detailed dynamic light scattering study of the phase separation in the ocular lens emerging during cold cataract development. Cold cataract is a phase separation effect that proceeds via spinodal decomposition of the lens cytoplasm with cooling. The intensity autocorrelation functions of the lens protein content are analyzed with the aid of two methods, providing information on the populations and dynamics of the scattering elements associated with cold cataract. It is found that the temperature dependence of many measurable parameters changes appreciably at the characteristic temperature approximately 16+/-1 degrees C which is associated with the onset of cold cataract. By extending the temperature range of this work to previously inaccessible regimes, i.e., well below the phase separation or coexistence curve at Tcc, we have been able to accurately determine the temperature dependence of the collective and self-diffusion coefficients of proteins near the spinodal. The analysis showed that the dynamics of proteins bears some resemblance to the dynamics of structural glasses, where the apparent activation energy for particle diffusion increases below Tcc, indicating a highly cooperative motion. Application of ideas developed for studying the critical dynamics of binary protein-solvent mixtures, as well as the use of a modified Arrhenius equation, enabled us to estimate the spinodal temperature Tsp of the lens nucleus. The applicability of dynamic light scattering as a noninvasive, early-diagnostic tool for ocular diseases is also demonstrated in light of the findings of the present paper.

Congenital cataracts and their molecular genetics

Cataract can be defined as any opacity of the crystalline lens. Congenital cataract is particularly serious because it has the potential for inhibiting visual development, resulting in permanent blindness. Inherited cataracts represent a major contribution to congenital cataracts, especially in developed countries. While cataract represents a common end stage of mutations in a potentially large number of genes acting through varied mechanisms in practice most inherited cataracts have been associated with a subgroup of genes encoding proteins of particular importance for the maintenance of lens transparency and homeostasis. The increasing availability of more detailed information about these proteins and their functions and is making it possible to understand the pathophysiology of cataracts and the biology of the lens in general.

Liquid-liquid phase separation and static light scattering of concentrated ternary mixtures of bovine alpha and gammaB crystallins

We have used light scattering, turbidimetry, and thermodynamic analysis to study the phase diagram of concentrated aqueous mixtures of the bovine lens proteins, gammaB crystallin, and alpha crystallin. We find that dilute alpha crystallin raises the phase separation temperature of concentrated gammaB crystallin, while more concentrated alpha crystallin suppresses phase separation. Very concentrated alpha/gammaB mixtures can reversibly cloud above 37 degrees C, even though gammaB alone phase separates only below temperatures near 0 degrees C, and alpha does not phase separate. At the scattering vector magnitude used, high-concentration alpha/gammaB mixtures scatter less light than the weighted average of their component alpha and gammaB solutions, while low-concentration alpha/gammaB mixtures scatter more than such a weighted average. We use a mean-field thermodynamic analysis of such ternary mixtures to show that the observed light scattering and phase boundaries of alpha and gammaB crystallin mixtures give evidence for prominent local fluctuations of relative protein composition. In the single phase, these fluctuations scatter comparatively little light, but are associated with enhanced thermodynamic instability. By applying this analysis to the experimental tie lines we estimate the magnitude of the saddlelike component of the free energy near the aqueous-gammaB critical point.

Structure formation by polysaccharides in concentrated solution

Molecular structure determination in concentrated solutions is generally considered unfeasible. This conclusion, drawn from experience with linear flexible chains, relates to the complete interpenetration of coils and their ensuing entanglement. Such deep interpenetration is prevented by the obstacles of branching units in branched macromolecules. The present study with branched (and one linear cellulose) polysaccharides demonstrates that different architectures can be distinguished via their different repulsive interactions. The procedure displayed in this article permits the estimation of true molar mass and true radius of gyration at finite concentration c. It furthermore allows, within upper and lower limits, an estimation of the growing size, when association takes place. Results from starches, glycogen, dextran, and cellulose, in aqueous media are discussed. The cellulose was dissolved in a recently developed aqueous cadmium complex. Association can lead to gel formation and/or to phase separation. The former is characterized by repulsive interactions which only slightly decrease when the gel point is approached. Phase separation, on the other hand, results from a decrease of the repulsive interaction induced, for instance, by another polymer. The effect is demonstrated with starches containing different amounts of amylose, where pure amylose forms unstable solutions and precipitates in time as a semicrystalline solid.

Structural basis of eye lens transparency: light scattering by concentrated solutions of bovine alpha-crystallin proteins

Short range order of the crystallins does account for the transparency of the eye lens. To explain the solution structure of this highly concentrated protein solution on a quantitative basis, the hydrodynamic structure and the interparticle interactions of the proteins have to be known. For that purpose, the light scattering of concentrated solutions of alpha-crystallin has been studied. Starting from the detailed knowledge of the solution parameters of alpha-crystallin in diluted solutions, the structure of concentrated solutions up to 360 mg/ml has been studied using light scattering. Our results indicate that subtle changes in the macromolecular structure such as optical anisotropy or structural asymmetry for part of the alpha-crystallins, which results in solute light-scattering heterogeneity, can dramatically increase the light scattering by the alpha-crystallins and cause solution opacity.

Self-similarity properties of alpha-crystallin supramolecular aggregates

The supramolecular aggregation of alpha-crystallin, the major protein of the eye lens, was investigated by means of static and dynamic light scattering. The aggregation was induced by generating heat-modified alpha-crystallin forms and by stabilizing the clusters with calcium ions. The kinetic pattern of the aggregation and the structural features of the clusters can be described according to the reaction limited cluster-cluster aggregation theory previously adopted for the study of colloidal particles aggregation systems. Accordingly, the average mass and the hydrodynamic radius of alpha-crystallin supramolecular aggregates grow exponentially in time. The structure factor of the clusters is typical of fractal aggregates. A fractal dimension df approximately 2.15 was determined, indicating a low probability of sticking together of the primitive aggregating particles. As a consequence, the slow-forming clusters assemble a rather compact structure. The basic units forming the fractal aggregates were found to have a radius about twice (approximately 17 nm) that of the native protein and 5.3 times its size, which is consistent with an intermediate molecular assembly corresponding to the already known high molecular weight forms of alpha-crystallin.

Characterization of the cellular microstructure of ocular lens using 2D power law analysis

Power law analysis provides a quantitative method for characterization of spatial fluctuations in the cellular microstructure of the ocular lens. In the power law analysis, Fourier components of the spatial fluctuations are computed, and the relationship between the amplitude, A, and spatial frequency, f, of the components is defined by a power law function: [formula, see text]. The exponent of the function, beta, defines the scaling of the amplitude of the Fourier components as a function of spatial frequency. We performed two-dimensional power law analysis on electron micrographs of lens cells ranging from transparent to opaque. We identified two values of power law exponent, beta, for the spatial fluctuations of all lens cells, one for low- and a second for high-spatial frequencies. In the low-spatial frequency region, the value of beta was in the range of 0.53 to 1.33, for transparent and opaque cells. In the high-spatial frequency region, the value of beta increased from 2.78 for transparent lens cells to 3.60 for opaque lens cells. The power law analysis provides a new method for quantitative characterization of the spatial fluctuations in the microstructure of transparent and opaque lens cells.

Quantitative analysis of the lens cell microstructure in selenite cataract using a two-dimensional Fourier analysis

Using two-dimensional (2-D) Fourier methods, we analysed the cellular microstructure of three rat lenses: normal transparent, selenite-induced cataractous and selenite-treated plus a phase separation inhibitor (PSI) to prevent cataract. 2-D Fourier analysis of electron micrographs of the lens cells quantified the dimensions of the spatial fluctuations in electron density of the lens cell microstructure. The 2-D Fourier spectra of the transparent normal and PSI-treated lens cells were remarkably similar while those of the opaque selenite-treated lens cells were dramatically different. In the opaque cells the contributions of large Fourier components (larger than half the wavelength of light) in the 2-D Fourier spectra were much greater than in the transparent cells. The results of the 2-D Fourier analysis of electron micrographs are consistent with the theory of transparency of the eye.

Light scattering by bovine alpha-crystallin proteins in solution: hydrodynamic structure and interparticle interaction

We have studied diluted bovine eye lens alpha-crystallin solutions by using light scattering. The protein particles were modeled as hard spheres, showing electrostatic repulsion, due to surplus electric charges, and weak attractive interaction. The repulsive potential VR is defined by the radius of the particles, the Debye length kappa-1, and the number of charges at the Gouy layer; the attractive potential has been described by the London-van der Waals potential and is defined by the Hamaker constant A. We have used the diluted gas approximation and the one component macrofluid model to relate the experimental static factor Ki to the theoretical expression of the interaction potential V(x). This resulted in a Hamaker constant A of 0.06 +/- 0.01 KBT and an effective charge q ranging from 18 +/- 1 at low ionic strength (omega = 0.0022 M) to 50 +/- 5 at high ionic strength (omega = 0.1472 M).

Calcium decreases transparency of homogenate from lens cortex and has no effect on nucleus

We studied the effects of calcium on transparency in homogenates of cortical and nuclear cells from calf lenses. Calcium was mixed into samples of homogenate to final concentrations between 0 and 50 mM and the transparency of the calcium-treated homogenates was measured using laser transmittance. In the presence of 10 mM calcium, the transmittance of cortical homogenate decreased 50% while the nuclear homogenate lost less than 4% transmittance after 24 h at 37 degrees C. To better understand the contribution of cytoplasm and membranes to opacity, the nuclear and cortical homogenates were centrifuged to separate membranes from the cytoplasm. When 10 mM calcium was added to cortical homogenate which was then centrifuged, the transmittance of the membrane fraction decreased nearly 60%, while the fraction without membrane decreased only 10%. The strong effect of calcium on the membrane fraction was accompanied by an increase in specific gravity of membranes from 1.23 to 1.32. Ten- and 20 mM calcium had no effect on transparency of membranes or cytoplasm separated from nuclear homogenate, and 50 mM calcium produced only a slight opacity. The results indicate that an effect on membrane-protein interactions may be important in the loss of transparency produced by calcium in cells of lens cortex.

Effect of calcium on the calf lens cytoplasm

Opacification was induced in calf lens cytoplasmic extracts by addition of calcium. The sample turbidity was shown to increase with calcium molarity, incubation time and temperature and to decrease with the protein cytoplasmic concentration. Although this turbidity was enhanced when membrane fragments were left over in the cytoplasmic extracts, it did show up in the absence of any detectable vesicular fragment. Scattering techniques (X-ray and light) showed that the calcium-induced opacification is linked to enhanced light scattering, which results from the formation of additional scatterers, a few tens of nm in diameter. Additional structures were indeed visualized by freeze-fracture electron microscopy (FFEM): they appear as molecular clusters with diameters ranging from 20 to 90 nm, made of densely packed particles, with heterogeneous sizes. The turbidity expected from these clusters, as well as the expected variation of turbidity with cytoplasmic concentration, was calculated to be in agreement with the measurements. When compared with cold cataract, these results illustrate that similar opacities may result from completely different biophysical mechanisms.

Calf lens alpha-crystallin quaternary structure. A three-layer tetrahedral model

Calf lens alpha-crystallins are polydisperse globular particles made of a large number of two types of subunits, A and B, both of molecular weight congruent to 20,000. alpha-Crystallin populations consisting on average of 40 subunits or more were subjected to various changes in pH, ionic strength, temperature and urea concentration. Modifications in quaternary structure induced by variation of these physicochemical parameters were followed by means of X-ray and quasi-elastic light-scattering and quantified in terms of weight average molecular weight (M), radius of gyration (Rg) and hydrodynamic radius (Rh). High-pressure liquid chromatography was used as a control of polydispersity. Increasing the pH, decreasing the ionic strength and incubating at temperatures from 20 degrees C to 45 degrees C all resulted in the formation of particles of decreasing M, Rg and Rh values. These effects are cumulative. All monomodal alpha-crystallin populations encountered in this study, which covers a wide range of sizes and molecular weights, may be accounted for by a three-layer model with partial filling up of the layers. Applying basic principles of symmetry and postulating specific contacts between protein subunits to construct this three-layer model leads to tetrahedral symmetry, with 12, 24 and 24 sites in the first, second and third layers, respectively. Variations in probabilities of site occupancy account for both the observed quaternary structure modifications and the intrinsic polydispersity of alpha-crystallins

In vivo measurement of the aging rabbit lens using quasielastic light scattering

We have been able to analyze the autocorrelation function of the light scattered from the rabbit lens in vivo in terms of a two component, exponential fit. We have measured the intensity and the decay rate associated with each of these two components as a function of age and position in the lens. As far as is possible we have interpreted these results in terms of molecular changes in the state of association of the protein constituents in the aging rabbit lens. These studies suggest that the method of quasielastic light scattering spectroscopy can provide a useful probe of the protein modifications that occur in both normal and cataractous human lenses.

Photon correlation spectroscopy and light scattering of eye lens proteins at high concentrations

The bovine eye lens protein, alpha L crystallin, has been studied with photon correlation spectroscopy and statical light scattering in the concentration range up to 200 g/l in different solvent conditions. At higher concentration (c greater than 70 g/l) the scattering behavior is quite complicated, which results in nonexponential correlation functions. Three methods have been used for the analysis of these correlation functions, namely, cumulant analysis, sum of two exponentials analysis, and exponential sampling method. These methods resulted in very similar results. The highly concentrated solutions contain two scattering entities: the single alpha L crystallin and a rather heterogeneous population of large clusters. The statical light-scattering experiments can be interpreted in the same way and gave consistent results for the dimensions of the large scattering units. The formation of these clusters, which are strong light scatterers, is superimposed on an increasing degree of correlation between the bulk of the alpha L-crystallins, resulting in a net decrease of light scattering as a function of concentration.