Thermal and acid denaturation of bovine lens α-crystallin

Promotion of Protein Solubility and Reduction in Stiffness in Human Lenses by Aggrelyte-1: Implications for Reversing Presbyopia

With aging, human lenses lose the ability to focus on nearby objects due to decreases in accommodative ability, a condition known as presbyopia. An increase in stiffness or decrease in lens elasticity due to protein aggregation and insolubilization are the primary reasons for presbyopia. In this study, we tested aggrelyte-1 (S,N-diacetyl glutathione diethyl ester) for its ability to promote protein solubility and decrease the stiffness of lenses through its dual property of lysine acetylation and disulfide reduction. Treatment of water-insoluble proteins from aged human lenses (58-75 years) with aggrelyte-1 significantly increased the solubility of those proteins. A control compound that did not contain the S-acetyl group (aggrelyte-1C) was substantially less efficient in solubilizing water-insoluble proteins. Aggrelyte-1-treated solubilized protein had significant amounts of acetyllysine, as measured by Western blotting and LC-MS/MS. Aggrelytes increased the protein-free thiol content in the solubilized protein. Aged mouse (7 months) and human (44-66 years) lenses treated with aggrelyte-1 showed reduced stiffness accompanied by higher free thiol and acetyllysine levels compared with those treated with aggrelyte-1C or untreated controls. Our results suggested that aggrelyte-1 reduced lens stiffness through acetylation followed by disulfide reduction. This proof-of-concept study paves the way for developing aggrelyte-1 and related compounds to reverse presbyopia.

Conformational Changes of α-Crystallin Proteins Induced by Heat Stress

α-crystallin is a major structural protein in the eye lenses of vertebrates that is composed of two relative subunits, αA and αB crystallin, which function in maintaining lens transparency. As a member of the small heat-shock protein family (sHsp), α-crystallin exhibits chaperone-like activity to prevent the misfolding or aggregation of critical proteins in the lens, which is associated with cataract disease. In this study, high-purity αA and αB crystallin proteins were expressed from E. coli and purified by affinity and size-exclusion chromatography. The size-exclusion chromatography experiment showed that both αA and αB crystallins exhibited oligomeric complexes in solution. Here, we present the structural characteristics of α-crystallin proteins from low to high temperature by combining circular dichroism (CD) and small-angle X-ray scattering (SAXS). Not only the CD data, but also SAXS data show that α-crystallin proteins exhibit transition behavior on conformation with temperature increasing. Although their protein sequences are highly conserved, the analysis of their thermal stability showed different properties in αA and αB crystallin. In this study, taken together, the data discussed were provided to demonstrate more insights into the chaperone-like activity of α-crystallin proteins.

Modulation of the Structure and Stability of Novel Camel Lens Alpha-Crystallin by pH and Thermal Stress

Alpha-crystallin protein performs structural and chaperone functions in the lens and comprises alphaA and alphaB subunits at a molar ratio of 3:1. The highly complex alpha-crystallin structure challenges structural biologists because of its large dynamic quaternary structure (300−1000 kDa). Camel lens alpha-crystallin is a poorly characterized molecular chaperone, and the alphaB subunit possesses a novel extension at the N-terminal domain. We purified camel lens alpha-crystallin using size exclusion chromatography, and the purity was analyzed by gradient (4−12%) sodium dodecyl sulfate−polyacrylamide gel electrophoresis. Alpha-crystallin was equilibrated in the pH range of 1.0 to 7.5. Subsequently, thermal stress (20−94 °C) was applied to the alpha-crystallin samples, and changes in the conformation and stability were recorded by dynamic multimode spectroscopy and intrinsic and extrinsic fluorescence spectroscopic methods. Camel lens alpha-crystallin formed a random coil-like structure without losing its native-like beta-sheeted structure under two conditions: >50 °C at pH 7.5 and all temperatures at pH 2.0. The calculated enthalpy of denaturation, as determined by dynamic multimode spectroscopy at pH 7.5, 4.0, 2.0, and 1.0 revealed that alpha-crystallin never completely denatures under acidic conditions or thermal denaturation. Alpha-crystallin undergoes a single, reversible thermal transition at pH 7.5. The thermodynamic data (unfolding enthalpy and heat capacity change) and chaperone activities indicated that alpha-crystallin does not completely unfold above the thermal transition. Camels adapted to live in hot desert climates naturally exhibit the abovementioned unique features.

Dielectric response to thermal denaturation of lenses in healthy and diabetic rabbits

The relative permittivity and conductivity of healthy and alloxane-induced diabetic rabbits lenses were measured over a frequency range of 500 Hz to 100 kHz in an electric field and at temperatures from 25 to 150 °C. The dielectric spectra for both tissues showed two separate relaxations with a characteristic frequency of around 4 and 25 kHz assigned to the cortical and nuclear zones, respectively. These two dispersions are due to the interfacial polarization at the surface of the α-crystallin molecules. The denaturation temperature for the non-diabetic lens and the diabetic lens is approximately 70 and 80 °C, respectively. Moreover, the relative permittivity and conductivity values are higher in the diabetic lens than in the non-diabetic tissue at the same temperature and frequency. Our dielectric studies provide a better understanding of the thermal stability of crystallin-water complexes in normal and diseased human lenses.

Protection of ζ-crystallin by α-crystallin under thermal stress

Cataract is one of the major causes of blindness worldwide. Several factors including post-translational modification, thermal and solar radiations promote cataractogenesis. The camel lens proteins survive very harsh desert conditions and resist cataractogenesis. The folding and aggregation mechanism of camel lens proteins are poorly characterized. The camel lens contains three ubiquitous crystallins (α-, β-, and γ-crystallin) and a novel protein (ζ-crystallin) in large amounts. In this study, a sequence similarity search of camel α-crystallin with that of other organisms showed that the camel αB-crystallin consists of an extended N-terminal domain. Our results indicate that camel α-crystallin efficiently prevented aggregation of ζ-crystallin, with or without an obligate cofactor up to 89 °C. It performed a quick and efficient holdase function irrespective of the unfolding stage or aggregation. Camel α-crystallin exhibits approximately 20% chaperone activity between 30 and 40 °C and is completely activated above 40 °C. Camel α-crystallin underwent a single reversible thermal transition without loss of β-sheet secondary structure. Intrinsic tryptophan fluorescence and ANS binding experiments revealed two transitions which corresponded to activation of its chaperone function. In contrast to earlier studies, camel α-crystallin completely protected lens proteins during thermal stress.

Structure and function of α-crystallins: Traversing from in vitro to in vivo

BACKGROUND

The two α-crystallins (αA- and αB-crystallin) are major components of our eye lenses. Their key function there is to preserve lens transparency which is a challenging task as the protein turnover in the lens is low necessitating the stability and longevity of the constituent proteins. α-Crystallins are members of the small heat shock protein family. αB-crystallin is also expressed in other cell types.

SCOPE OF THE REVIEW

The review summarizes the current concepts on the polydisperse structure of the α-crystallin oligomer and its chaperone function with a focus on the inherent complexity and highlighting gaps between in vitro and in vivo studies.

MAJOR CONCLUSIONS

Both α-crystallins protect proteins from irreversible aggregation in a promiscuous manner. In maintaining eye lens transparency, they reduce the formation of light scattering particles and balance the interactions between lens crystallins. Important for these functions is their structural dynamics and heterogeneity as well as the regulation of these processes which we are beginning to understand. However, currently, it still remains elusive to which extent the in vitro observed properties of α-crystallins reflect the highly crowded situation in the lens.

GENERAL SIGNIFICANCE

Since α-crystallins play an important role in preventing cataract in the eye lens and in the development of diverse diseases, understanding their mechanism and substrate spectra is of importance. To bridge the gap between the concepts established in vitro and the in vivo function of α-crystallins, the joining of forces between different scientific disciplines and the combination of diverse techniques in hybrid approaches are necessary. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Impact of structural stability of cold adapted Candida antarctica lipase B (CaLB): in relation to pH, chemical and thermal denaturation

The effect of pH on the conformational behavior of Candida antartica lipase B (CaLB) has been monitored by spectroscopic and calorimetric studies.

Control of the structural stability of α-crystallin under thermal and chemical stress: the role of carnosine

The structural properties of α-crystallin, the major protein of the eye lens of mammals, in aqueous solution are investigated by means of small angle X-ray and dynamic light scattering. The research interest is devoted in particular to the effect of carnosine in protecting the protein under stress conditions, like temperature increase and presence of denaturant (guanidinium-HCl). The results suggest that carnosine interacts, through mechanisms involving hydrophobic interactions, with α-crystallin and avoids the structural changes in the quaternary structure induced by thermal and chemical stress. It is also shown that, if mediated by carnosine, the self-aggregation of α-crystallin induced by the denaturant at higher temperature can be controlled and even partially reversed. Therefore, carnosine is effective in preserving the structural integrity of the protein, suggesting the possibility of new strategies of intervention for preventing or treating pathologies related to protein aggregation, like cataracts.