In vivo heteromer formation. Expression of soluble betaA4-crystallin requires coexpression of a heteromeric partner

A novel missense variant c.71G > T (p.Gly24Val) of the CRYBA4 gene contributes to autosomal-dominant congenital cataract in a Chinese family

PURPOSE

To investigate the potential genetic defects in a five-generation Chinese family with autosomal dominant congenital cataract (ADCC).

METHODS

Whole exome sequencing was performed to search the variants in the candidate genes associated with congenital cataract. Sanger sequencing was used to validate the variants and examine their co-segregation in the patients and their relatives. The potential effect of the variants was analyzed using several bioinformatic methods and further examined through Western blotting and co-immunoprecipitation.

RESULTS

A missense variant c. 71 G > T (p. Gly24Val) in the CRYBA4 gene, a known ADCC candidate gene, was identified to be heterozygously present in the patients and co-segregate with cataract in the family. The mutation was absent in all of the searched databases, including our in-house exome sequences of 10,000 Chinese. The alignments of the amino acid sequences of CRYBA4 in a variety of species revealed that the amino acid residue Gly24 was evolutionarily highly conserved, and the in silico analysis predicted that the missense mutation of Gly24Val was damaging for the protein structure and function of CRYBA4. Then, the in vitro expression analysis further revealed that the Gly24Val mutation in CRYBA4 inhibited its binding with CRYBB1. The impaired interaction of β-crystallin proteins may affect their water-solubility and contribute to the formation of precipitates in lens fiber cells.

CONCLUSION

We identified a novel missense variant in the CRYBA4 gene as a pathogenic mutation of ADCC in a Chinese family. Our finding expanded the CRYBA4 variation spectrum associated with congenital cataracts.

Native Ambient Mass Spectrometry of an Intact Membrane Protein Assembly and Soluble Protein Assemblies Directly from Lens Tissue

Membrane proteins constitute around two-thirds of therapeutic targets but present a significant challenge for structural analysis due to their low abundance and solubility. Existing methods for structural analysis rely on over-expression and/or purification of the membrane protein, thus removing any links back to actual physiological environment. Here, we demonstrate mass spectrometry analysis of an intact oligomeric membrane protein directly from tissue. Aquaporin-0 exists as a 113 kDa tetramer, with each subunit featuring six transmembrane helices. We report the characterisation of the intact assembly directly from a section of sheep eye lens without sample pre-treatment. Protein identity was confirmed by mass measurement of the tetramer and subunits, together with top-down mass spectrometry, and the spatial distribution was determined by mass spectrometry imaging. Our approach allows simultaneous analysis of soluble protein assemblies in the tissue.

Native Ambient Mass Spectrometry of an Intact Membrane Protein Assembly and Soluble Protein Assemblies Directly from Lens Tissue

Membrane proteins constitute around two-thirds of therapeutic targets but present a significant challenge for structural analysis due to their low abundance and solubility. Existing methods for structural analysis rely on over-expression and/or purification of the membrane protein, thus removing any links back to actual physiological environment. Here, we demonstrate mass spectrometry analysis of an intact oligomeric membrane protein directly from tissue. Aquaporin-0 exists as a 113 kDa tetramer, with each subunit featuring six transmembrane helices. We report the characterisation of the intact assembly directly from a section of sheep eye lens without sample pre-treatment. Protein identity was confirmed by mass measurement of the tetramer and subunits, together with top-down mass spectrometry, and the spatial distribution was determined by mass spectrometry imaging. Our approach allows simultaneous analysis of soluble protein assemblies in the tissue.

Physiological and pathological functions of βB2-crystallins in multiple organs: a systematic review

Crystallins, the major constituent proteins of mammalian lenses, are significant not only for the maintenance of eye lens stability, transparency, and refraction, but also fulfill various physiopathological functions in extraocular tissues. βB2-crystallin, for example, is a multifunctional protein expressed in the human retina, brain, testis, ovary, and multiple tumors. Mutations in the βB2 crystallin gene or denaturation of βB2-crystallin protein are associated with cataracts, ocular pathologies, and psychiatric disorders. A prominent role for βB2-crystallins in axonal growth and regeneration, as well as in dendritic outgrowth, has been demonstrated after optic nerve injury. Studies in βB2-crystallin-null mice revealed morphological and functional abnormalities in testis and ovaries, indicating βB2-crystallin contributes to male and female fertility in mice. Interestingly, although pathogenic significance remains obscure, several studies identified a clear correlation between βB2 crystallin expression and the prognosis of patients with breast cancer, colorectal cancer, prostate cancer, renal cell carcinoma, and glioblastoma in the African American population. This review summarizes the physiological and pathological functions of βB2-crystallin in the eye and other organs and tissues and discusses findings related to the expression and potential role of βB2-crystallin in tumors.

Chemical Properties Determine Solubility and Stability in βγ-Crystallins of the Eye Lens

βγ-Crystallins are the primary structural and refractive proteins found in the vertebrate eye lens. Because crystallins are not replaced after early eye development, their solubility and stability must be maintained for a lifetime, which is even more remarkable given the high protein concentration in the lens. Aggregation of crystallins caused by mutations or post-translational modifications can reduce crystallin protein stability and alter intermolecular interactions. Common post-translational modifications that can cause age-related cataracts include deamidation, oxidation, and tryptophan derivatization. Metal ion binding can also trigger reduced crystallin solubility through a variety of mechanisms. Interprotein interactions are critical to maintaining lens transparency: crystallins can undergo domain swapping, disulfide bonding, and liquid-liquid phase separation, all of which can cause opacity depending on the context. Important experimental techniques for assessing crystallin conformation in the absence of a high-resolution structure include dye-binding assays, circular dichroism, fluorescence, light scattering, and transition metal FRET.

Biochemical characterization of G64W mutant of acidic beta-crystallin 4

Crystallins are structural proteins in the lens that last a lifetime with little turnover. Deviant in crystallins can cause rare but severe visual impairment, namely, congenital cataracts. It is reported that several mutations in the acidic β-crystallin 4 (CRYBA4) are related to congenital cataracts. However, the pathogenesis of these mutants is not well understood at molecular level. Here we evaluate the biochemical properties of wild type CRYBA4 (CRYBA4^WT) and a pathogenic G64W mutant (CRYBA4^G64W) including protein folding, polymerization state and protein stability. Furthermore, we explore the differences in their interactions with α-crystallin A (CRYAA) and basic β-crystallin 1 (CRYBB1) via yeast two-hybrid and pull-down assay in vitro, through which we find that G64W mutation leads to protein misfolding, decreases protein stability, blocks its interaction with CRYBB1 but maintains its interaction with CRYAA. Our results deepen our understanding of the pathogenesis of congenital cataracts.

Human βB2-Crystallin Forms a Face-en-Face Dimer in Solution: An Integrated NMR and SAXS Study

βγ-Crystallins are long-lived eye lens proteins that are crucial for lens transparency and refractive power. Each βγ-crystallin comprises two homologous domains, which are connected by a short linker. γ-Crystallins are monomeric, while β-crystallins crystallize as dimers and multimers. In the crystal, human βB2-crystallin is a domain-swapped dimer while the N-terminally truncated βB1-crystallin forms a face-en-face dimer. Combining and integrating data from multi-angle light scattering, nuclear magnetic resonance, and small-angle X-ray scattering of full-length and terminally truncated human βB2-crystallin in solution, we show that both these βB2-crystallin proteins are dimeric, possess C2 symmetry, and are more compact than domain-swapped dimers. Importantly, no inter-molecular paramagnetic relaxation enhancement effects compatible with domain swapping were detected. Our collective experimental results unambiguously demonstrate that, in solution, human βB2-crystallin is not domain swapped and exhibits a face-en-face dimer structure similar to the crystal structure of truncated βB1-crystallin.

Lens Biology and Biochemistry

The primary function of the lens resides in its transparency and ability to focus light on the retina. These require both that the lens cells contain high concentrations of densely packed lens crystallins to maintain a refractive index constant over distances approximating the wavelength of the light to be transmitted, and a specific arrangement of anterior epithelial cells and arcuate fiber cells lacking organelles in the nucleus to avoid blocking transmission of light. Because cells in the lens nucleus have shed their organelles, lens crystallins have to last for the lifetime of the organism, and are specifically adapted to this function. The lens crystallins comprise two major families: the βγ-crystallins are among the most stable proteins known and the α-crystallins, which have a chaperone-like function. Other proteins and metabolic activities of the lens are primarily organized to protect the crystallins from damage over time and to maintain homeostasis of the lens cells. Membrane protein channels maintain osmotic and ionic balance across the lens, while the lens cytoskeleton provides for the specific shape of the lens cells, especially the fiber cells of the nucleus. Perhaps most importantly, a large part of the metabolic activity in the lens is directed toward maintaining a reduced state, which shelters the lens crystallins and other cellular components from damage from UV light and oxidative stress. Finally, the energy requirements of the lens are met largely by glycolysis and the pentose phosphate pathway, perhaps in response to the avascular nature of the lens. Together, all these systems cooperate to maintain lens transparency over time.

Functions of crystallins in and out of lens: roles in elongated and post-mitotic cells

The vertebrate lens evolved to collect light and focus it onto the retina. In development, the lens grows through massive elongation of epithelial cells possibly recapitulating the evolutionary origins of the lens. The refractive index of the lens is largely dependent on high concentrations of soluble proteins called crystallins. All vertebrate lenses share a common set of crystallins from two superfamilies (although other lineage specific crystallins exist). The α-crystallins are small heat shock proteins while the β- and γ-crystallins belong to a superfamily that contains structural proteins of uncertain function. The crystallins are expressed at very high levels in lens but are also found at lower levels in other cells, particularly in retina and brain. All these proteins have plausible connections to maintenance of cytoplasmic order and chaperoning of the complex molecular machines involved in the architecture and function of cells, particularly elongated and post-mitotic cells. They may represent a suite of proteins that help maintain homeostasis in such cells that are at risk from stress or from the accumulated insults of aging.

Lens β-crystallins: the role of deamidation and related modifications in aging and cataract

Crystallins are the major proteins in the lens of the eye and function to maintain transparency of the lens. Of the human crystallins, α, β, and γ, the β-crystallins remain the most elusive in their structural significance due to their greater number of subunits and possible oligomer formations. The β-crystallins are also heavily modified during aging. This review focuses on the functional significance of deamidation and the related modifications of racemization and isomerization, the major modifications in β-crystallins of the aged human lens. Elucidating the role of these modifications in cataract formation has been slow, because they are analytically among the most difficult post-translational modifications to study. Recent results suggest that many amides deamidate to similar extent in normal aged and cataractous lenses, while others may undergo greater deamidation in cataract. Mimicking deamidation at critical structural regions induces structural changes that disrupt the stability of the β-crystallins and lead to their aggregation in vitro. Deamidations at the surface disrupt interactions with other crystallins. Additionally, the α-crystallin chaperone is unable to completely prevent deamidated β-crystallins from insolubilization. Therefore, deamidation of β-crystallins may enhance their precipitation and light scattering in vivo contributing to cataract formation. Future experiments are needed to quantify differences in deamidation rates at all Asn and Gln residues within crystallins from aged and cataractous lenses, as well as racemization and isomerization which potentially perturb protein structure greater than deamidation alone. Quantitative data is greatly needed to investigate the importance of these major age-related modifications in cataract formation.

Evolution of crystallins for a role in the vertebrate eye lens

The camera eye lens of vertebrates is a classic example of the re-engineering of existing protein components to fashion a new device. The bulk of the lens is formed from proteins belonging to two superfamilies, the α-crystallins and the βγ-crystallins. Tracing their ancestry may throw light on the origin of the optics of the lens. The α-crystallins belong to the ubiquitous small heat shock proteins family that plays a protective role in cellular homeostasis. They form enormous polydisperse oligomers that challenge modern biophysical methods to uncover the molecular basis of their assembly structure and chaperone-like protein binding function. It is argued that a molecular phenotype of a dynamic assembly suits a chaperone function as well as a structural role in the eye lens where the constraint of preventing protein condensation is paramount. The main cellular partners of α-crystallins, the β- and γ-crystallins, have largely been lost from the animal kingdom but the superfamily is hugely expanded in the vertebrate eye lens. Their structures show how a simple Greek key motif can evolve rapidly to form a complex array of monomers and oligomers. Apart from remaining transparent, a major role of the partnership of α-crystallins with β- and γ-crystallins in the lens is to form a refractive index gradient. Here, we show some of the structural and genetic features of these two protein superfamilies that enable the rapid creation of different assembly states, to match the rapidly changing optical needs among the various vertebrates.

βB1-crystallin: thermodynamic profiles of molecular interactions

Background

β-Crystallins are structural proteins maintaining eye lens transparency and opacification. Previous work demonstrated that dimerization of both βA3 and βB2 crystallins (βA3 and βB2) involves endothermic enthalpy of association (∼8 kcal/mol) mediated by hydrophobic interactions.

Methodology/Principal Findings

Thermodynamic profiles of the associations of dimeric βA3 and βB1 and tetrameric βB1/βA3 were measured using sedimentation equilibrium. The homo- and heteromolecular associations of βB1 crystallin are dominated by exothermic enthalpy (−13.3 and −24.5 kcal/mol, respectively).

Conclusions/Significance

Global thermodynamics of βB1 interactions suggest a role in the formation of stable protein complexes in the lens via specific van der Waals contacts, hydrogen bonds and salt bridges whereas those β-crystallins which associate by predominately hydrophobic forces participate in a weaker protein associations.

Truncated human betaB1-crystallin shows altered structural properties and interaction with human betaA3-crystallin

The purpose of the study was to determine the effects of truncation of various regions of betaB1-crystallin on its structural properties and stability of heterooligomers formed by wild-type (WT) betaB1 or its deletion mutants with WT betaA3-crystallin. For these analyses, seven deletion mutants of betaB1-crystallin were generated with the following sequential deletions of either N-terminal arm [betaB1(59-252)], N-terminal arm + motif I [betaB1(99-252)], N-terminal arm + motif I + motif II [betaB1(144-252)], N-terminal arm + motif I + motif II + connecting peptide [betaB1(149-252)], C-terminal extension [betaB1(1-234)], C-terminal extension plus motif IV [betaB1(1-190)], or C-terminal extension + motif III + motif IV [betaB1(1-148)]. The betaB1-crystallin became water insoluble on the deletion of C-terminal extension and subsequent deletions of the C-terminal domain (C-terminal extension plus motifs III and IV) while it remained partially soluble on the deletion of the N-terminal domain (N-terminal arm plus motifs I and II). However, circular dichroism spectral analysis showed that the deletion of the N-terminal domain but not the C-terminal domain exhibited relatively greater structural changes in the crystallin. The deletion of the C-terminal domain resulted in a greater exposure and disturbance in the microenvironment of Trp-100, Trp-123, and Trp-126 (localized in the motif II), suggesting a relatively greater role of the C-terminal domain than the N-terminal domain in the structural stability of the crystallin. The deletion of the N-terminal extension in betaB1 resulted in maximum exposure of hydrophobic patches and compact structure and in a maximum loss of subunit exchange with WT betaA3-crystallin compared to deletion of either the C-terminal extension, the N-terminal domain, or the C-terminal domain. The thermal stability results of the heterooligomer of betaB1- plus betaA3-crystallins suggested that oligomers lose their stability on deletion of the C-terminal domain. Together, the results suggested that the N-terminal arm of betaB1-crystallin plays a major role in interaction with betaA3-crystallin during heterooligomer formation, and the solubility of betaB1-crystallin per se and that of the heterooligomer with betaA3-crystallin are dependent on the intact C-terminal domain of betaB1-crystallin.

Identification of interaction sites between human betaA3- and alphaA/alphaB-crystallins by mammalian two-hybrid and fluorescence resonance energy transfer acceptor photobleaching methods

Our recent study has shown that betaA3-crystallin along with betaB1- and betaB2-crystallins were part of high molecular weight complex obtained from young, old, and cataractous lenses suggesting potential interactions between alpha- and beta-crystallins (Srivastava, O. P., Srivastava, K., and Chaves, J. M. (2008) Mol. Vis. 14, 1872-1885). To investigate this further, this study was carried out to determine the interaction sites of betaA3-crystallin with alphaA- and alphaB-crystallins. The study employed a mammalian two-hybrid method, an in vivo assay to determine the regions of betaA3-crystallin that interact with alphaA- and alphaB-crystallins. Five regional truncated mutants of betaA3-crystallin were generated using specific primers with deletions of N-terminal extension (NT) (named betaA3-NT), N-terminal extension plus motif I (named betaA3-NT + I), N-terminal extension plus motifs I and II (named betaA3-NT + I + II), motif III plus IV (named betaA3-III + IV), and motif IV (named betaA3-IV). The mammalian two-hybrid studies were complemented with fluorescence resonance energy transfer acceptor photobleaching studies using the above described mutant proteins, fused with DsRed (Red) and AcGFP fluorescent proteins. The results showed that the motifs III and IV of betaA3-crystallin were interactive with alphaA-crystallin, and motifs II and III of betaA3-crystallin primarily interacted with alphaB-crystallin.

Protein-protein interactions involving congenital cataract T5P gammaC-crystallin mutant: a confocal fluorescence microscopy study

The human lens crystallin gene CRYGC T5P is associated with Coppock-like cataract and has a phenotype of a dust-like opacity of the fetal lens nucleus and deep cortical region. Previous in vitro mutation studies indicate that the protein has changed conformation, solubility, and stability, which may make it susceptible to aggregation, as seen in cataractous lens and cell culture expression. To investigate the mechanisms leading to these events, we studied protein-protein interactions using confocal fluorescence resonance energy transfer (FRET) microscopy. The method detects protein-protein interactions in the natural environment of living cells. Crystallin genes (CRYGC T5P, CRYGC, and CRYAA) were fused to either the green fluorescence protein (GFP) or red fluorescence protein (DsRED or RFP) vector. Each of the following GFP-RFP (donor-acceptor) plasmid pairs was cotransfected into HeLa cells: gammaC-gammaC, gammaC-gammaCT5P, gammaCT5P-gammaCT5P, alphaA-gammaC, and alphaA-gammaCT5P. After culture, confocal fluorescence cell images were taken. Protein-protein interactions in the form of net FRET were evaluated. The confocal fluorescence images show that cells expressing T5P gammaC-crystallin contain many protein aggregates, but cells co-expressing with either gammaC- or alphaA-crystallin reduce the aggregation considerably. FRET determination indicates that gammaCT5P-gammaCT5P shows less protein-protein interaction than either gammaC-gammaC or gammaC-gammaCT5P. Cotransfection with alphaA-crystallin (alphaA-gammaC or alphaA-T5PgammaC) increases nFRET compared with gammaC-gammaC or gammaC-T5PgammaC. Our results demonstrate that T5P gammaC-crystallin shows more protein aggregates and less protein-protein interaction than WT gammaC-crystallin. Chaperone alphaA-crystallin can rescue T5P gammaC-crystallin from aggregation through increased protein interaction. The formation of congenital cataract may be due to reduced protein-protein interactions and increased aggregation from an insufficient amount of alpha-crystallin for protection.

Multi-crystallin complexes exist in the water-soluble high molecular weight protein fractions of aging normal and cataractous human lenses

The purpose of the study was to identify non-covalently held complexes that exist in the water-soluble high molecular weight (WS-HMW) protein fractions of normal human lenses of 20-year-old and 60- to 70-year-old, and in the age-matched 60- to 70-year-old cataractous lenses. The WS protein fractions were prepared from five pooled normal lenses of 20-year-old donors or five pooled lenses of 60- to 70-year-old donors or four pooled cataractous lenses (with nuclear opacity) of 60- to 70-year-old donors. Each WS protein fraction was subjected to size-exclusion chromatography using an Agarose A 5m column to recover the void volume WS-HMW protein fraction. A method known as blue-native polyacrylamide gel electrophoresis (BN-PAGE), which allows the isolation of large multi-protein complexes (MPCs) in their native state for further characterization, was used to separate such complexes from individual WS-HMW protein fractions. The protein species that existed as a complex were excised from a gel and trypsin-digested, and the amino acid sequences of the tryptic fragments analyzed by electrospray tandem mass spectrometry (ES-MS/MS). After the second-dimensional sodium dodecyl sulfate-PAGE during BN-PAGE, protein complexes containing a total of 16, 12, and 24 species with M(r) between 10 and 90 kDa were identified in the HMW protein fractions of normal lenses of 20-year-old, 60- to 70-year-old and cataractous lenses of 60- to 70-year-old donors, respectively. Based on the amino acid sequences of tryptic peptides of individual protein species in the complexes by the ES-MS/MS method, the presence of alpha-, beta-, and gamma-crystallin species along with beaded filament proteins (filensin and phakinin) was observed in the 20-year-old normal lenses. The 60- to 70-year-old normal lenses contained filensin and aldehyde dehydrogenase in addition to the above crystallins. Similarly, the age-matched cataractous lenses also contained the above crystallins and aldehyde dehydrogenase but lacked beaded filament proteins. Protein complexes, held mostly via non-covalent bonding, were seen in the WS-HMW proteins of 20-year-old normal, 60- to 70-year-old normal, and 60- to 70-year-old cataractous lenses. The complexes in the normal lenses were made of alpha-, beta-, and gamma-crystallin species, beaded filament proteins (filensin and/or phakinin), and aldehyde dehydrogenase. The complexes in the age-matched cataractous lenses also contained these crystallins, and aldehyde dehydrogenase, but not the beaded filament proteins. Further, the crystallin fragments were greater in number in the cataractous lenses compared to the age-matched normal lenses. During multi-angle light scattering (MALS), the HMW proteins from cataractous lenses exhibited species with lower molecular weight range than age-matched normal lenses. The HMW protein preparations from both normal and cataractous lenses showed spherical structures on electron microscopic analysis.

Protein-protein interactions among human lens acidic and basic beta-crystallins

Human lens beta-crystallin contains four acidic (betaA1-->betaA4) and three basic (betaB1-->betaB3) subunits. They oligomerize in the lens, but it is uncertain which subunits are involved in the oligomerization. We used a two-hybrid system to detect protein-protein interactions systematically. Proteins were also expressed for some physicochemical studies. The results indicate that all acidic-basic pairs (betaA-betaB) except betaA4-betaBs pairs show strong hetero-molecular interactions. For acidic or basic pairs, only two pairs (betaA1-betaA1 and betaA3-betaA3) show strong self-association. betaA2 and betaA4 show very weak self-association, which arises from their low solubility. Confocal fluorescence microscopy shows enormous protein aggregates in betaA2- or betaA4-crystallin transfected cells. However, coexpression with betaB2-crystallin decreased both the number and size of aggregates. Circular dichroism indicates subtle differences in conformation among beta-crystallins that may have contributed to the differences in interactions.