Cataract-linked mutation R188H promotes βB2-crystallin aggregation and fibrillization during acid denaturation

Heteromeric formation with βA3 protects the low thermal stability of βB1-L116P

BACKGROUND/AIMS

Congenital cataract is the leading cause of visual disability and blindness in childhood. βB1-crystallin (CRYBB1) comprises about 1/10th of crystallin structural proteins, forming heteromers to maintain lens transparency. We previously reported a CRYBB1 mutation (c.347T>C, p.L116P) affecting 16 patients in a congenital nuclear cataract family. In this study, we investigate the underlying pathogenic mechanism of βB1-L116P.

METHODS

Protein isolation, size-exclusion chromatography, spectroscopy, Uncle stability screens and molecular dynamics simulations were used to assess βA3- and βB1-crystallin thermal stability, structural properties and heteromer formation.

RESULTS

Cells that overexpressed βB1-L116P tended to form aggregates and precipitations under heat-shock stress. Thermal denaturation and time-dependent turbidity experiments showed that thermal stability was significantly impaired. Moreover, protein instability appeared to increase with elevated concentrations detected by the Uncle system. Additionally, βA3 had a relative protective effect on βB1-L116P after heteromers were formed, although βA3 was relatively unstable and was usually protected by basic β-crystallins. Molecular dynamic simulations revealed that L116P mutation altered the hydrophobic residues at the surface around the mutant site, providing solvents more access to the internal and hydrophobic parts of the protein.

CONCLUSIONS

Decreased βB1-crystallin thermal stability in the presence of the cataract-related L116P mutation contributes significantly to congenital cataract formation. Moreover, its formation of heteromers with βA3 protects against the low thermal stability of βB1-L116P.

Cataract-causing variant Q70P damages structural stability of βB1-crystallin and increases its tendency to form insoluble aggregates

Congenital cataract is the primary cause of childhood blindness worldwide. As the predominant structural protein, βB1-crystallin plays an important role in maintaining lens transparency and cellular homeostasis. Numerous cataract-causing mutations of βB1-crystallin have been identified with unclear pathogenic mechanism. We previously identified the mutation Q70P (Q to P at residue position 70) of βB1-crystallin linked to congenital cataract in a Chinese family. In this work, we investigated the potential molecular mechanism of βB1-Q70P in the congenital cataract at the molecular, protein, and cellular levels. We purified recombinant βB1 wild-type (WT) and Q70P proteins and compared their structural characteristics and biophysical properties by spectroscopic experiments under physiological temperature and environmental stresses (ultraviolet irradiation, heat stress, oxidative stress). Notably, βB1-Q70P significantly changed the structures of βB1-crystallin and exhibited lower solubility at physiological temperature. Meanwhile, βB1-Q70P was prone to aggregation in eukaryotic and prokaryotic cells, and was more sensitive to environmental stresses, along with impaired cellular viability. Furthermore, the molecular dynamics simulation indicated that the mutation Q70P damaged secondary structures and hydrogen bond network of βB1-crystallin, which were essential for the first Greek-key motif. This study delineated the pathological mechanism of βB1-Q70P and provided novel insights into treatment and prevention strategies for cataract-associated βB1 mutations.

Conformational stability of the deamidated and mutated human βB2-crystallin

Previous studies propose that genetic mutations and post-translational modifications in protein crystallins promote protein aggregation and are considered significant risk factors for cataract formation. The βB2-crystallin (HβB2C) forms a high proportion of proteins in the human eye lens. Different congenital mutations and post-translational deamidations in βB2-crystallin have been reported and linked to cataract formation. In this work, we employed extensive all-atom molecular dynamics simulations to evaluate the conformational stability of deamidated and mutated HβB2C. Our results show critical changes in the protein surface and its native contacts due to a modification in the conformational equilibrium of these proteins. The double deamidated (Q70E/Q162E) and single deamidated (Q70E) impact the well compact conformation of the HβB2C. These post-translational modifications allow the exposure of the protein hydrophobic interface, which lead to the exposure of electronegative residues. On the other hand, our mutational studies showed that the S143F mutation modifies the hydrogen-bond network of an antiparallel β-sheet, unfolding the C-terminal domain. Interestingly, the chain termination mutation (Q155X) does not unfold the N-terminal domain. However, the resultant conformation is more compact and avoids the exposure of the hydrophobic interface. Our results provide valuable information about the first steps of HβB2C unfolding in the presence of deamidated amino acids that have been reported to appear during aging. The findings reported in this work are essential for the general knowledge of the initial steps in the cataract formation mechanism, which may be helpful for the further development of molecules with pharmacological potential against cataract disease.

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.

Identification and characterization of six β-crystallin gene mutations associated with congenital cataract in Chinese families

Background

This study aims to identify the underlying genetic defects of β‐crystallin (CRYB) genes responsible for congenital cataracts in a group of Chinese families.

Methods

Detailed family history and clinical data of six Chinese families with autosomal dominant congenital cataracts were recorded. Targeted exome sequencing was applied to detect the underlying genetic defects for the families. Generated variants were confirmed by PCR and sanger sequencing. Afterward, bioinformatic analysis through several computational predictive programs was performed to assess impacts of mutations on protein structure and function.

Results

A total of 53 participants (23 affected and 30 unaffected) from six unrelated Chinese families were recruited. Cataract phenotypes covered nuclear, total, posterior polar, pulverulent, snowflake‐like, and zonular. Through targeted exome sequencing, six mutations in four β‐crystallin genes were revealed which included five missense mutations CRYBB1 p.Q70P, CRYBB2 p.E23Q, CRYBB2 p.A49V, CRYBB2 R188C, CRYBA4 p.M14K and one splice mutation CRYBB3 c.75+1 G>A. In silico results predicted pathogenic for all four missense variants except variant CRYBB2‐p.A49V yielded results as tolerant. The CRYBB3 c.75+1 G>A splice site mutation was predicted to be deleterious by leading to a broken splice site, a premature stop codon, and subsequently resulting in a short peptide of 113 amino acids, which may affect protein features.

Conclusion

The obtained results expanded mutational and phenotype spectrum of β‐crystallin genes and offer clues for pathogenesis of congenital cataracts. The data also demonstrated that targeted exome sequencing is valuable for providing molecular diagnostic information for congenital cataract patients.

Cataract-causing mutations L45P and Y46D impair the thermal stability of γC-crystallin

Crystallin gene mutations are responsible for about half of the congenital cataract caused by genetic disorders. L45P and Y46D mutations of γC-crystallin have been reported in patients with nuclear congenital cataract. In this study, we explored the thermal stability of wild type (WT), L45P, and Y46D mutants of γC-crystallin at low and high concentrations, as well as the effect of αA-crystallin on the thermal stability of mutants. Spectroscopic experiments were used to monitor the structural changes on temperature-gradient and time-course heating process. Intermediate morphologies were determined through cryo-electron microscopy. The thermal stability of WT and mutants at concentrations ranging up to hundreds of milligrams were assessed via the UNcle multifunctional protein stability analysis system. The results showed that L45P and Y46D mutations impaired the thermal stability of γC-crystallin at low (0.2 mg/mL) and high concentrations (up to 200 mg/mL). Notably, with increase in protein concentration, the thermal stability of L45P and Y46D mutants of γC-crystallin simultaneously decreased. Thermal stability of L45P and Y46D mutants could be rescued by αA-crystallin in a concentration-dependent manner. The dramatic decrease in thermal stability of γC-crystallin caused by L45P and Y46D mutations contributed to congenital cataract in the mature human lens.

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.

Molecular genetics of congenital cataracts

Congenital cataracts, the most common cause of visual impairment and blindness in children worldwide, have diverse etiologies. According to statistics analysis, about one quarter of congenital cataracts caused by genetic defects. Various mutations of more than one hundred genes have been identified in hereditary cataracts so far. In this review, we briefly summarize recent developments about the genetics, molecular mechanisms, and treatments of congenital cataracts. The studies of these pathogenic mutations and molecular genetics is making it possible for us to comprehend the underlying mechanisms of cataractogenesis and providing new insights into the preventive, diagnostic and therapeutic approaches of cataracts.

Dissimilarity in the Contributions of the N-Terminal Domain Hydrophobic Core to the Structural Stability of Lens β/γ-Crystallins

Vertebrate lens β/γ-crystallins share a conserved tertiary structure consisting of four Greek-key motifs divided into two globular domains. Numerous inherited mutations in β/γ-crystallins have been linked to cataractogenesis. In this research, the folding mechanism underlying cataracts caused by the I21N mutation in βB2 was investigated by comparing the effect of mutagenesis on the structural features and stability of four β/γ-crystallins, βB1, βB2, γC, and γD. Our results showed that the four β/γ-crystallins differ greatly in solubility and stability against various stresses. The I21N mutation greatly impaired βB2 solubility and native structure as well as its stability against denaturation induced by guanidine hydrochloride, heat treatment, and ultraviolet irradiation. However, the deleterious effects were much weaker for mutations at the corresponding sites in βB1, γC, and γD. Molecular dynamics simulations indicated that the introduction of a nonnative hydrogen bond contributed to twisting Greek-key motif I outward, which might direct the misfolding of the I21N mutant of βB2. Meanwhile, partial hydration of the hydrophobic interior of the domain induced by the mutation destabilized βB1, γC, and γD. Our findings highlight the importance of nonnative hydrogen bond formation and hydrophobic core hydration in crystallin misfolding caused by inherited mutations.

A Triple Mutation of BetaB2-Crystallin is Necessary to Develop Cataract and Glaucoma

Crystallins are the predominant structural proteins in the lens that are evolutionarily related to stress proteins. There are two main crystallin gene families: α-crystallins and β/γ-crystallins. α- and β-crystallins were first considered to be lens-specific, but were recently recognized also as neuronal and retinal proteins. While in the ocular lens they are responsible for the maintenance of the transparency, their function in neurons is obviously different - regulating various protective mechanisms in degenerative conditions of the central nervous system. We recently reported the correlation between a gene conversion leading to a triple mutation in the betaB2-crystallin protein and a phenotype of familial congenital cataract with a high familial incidence also of primary open angle glaucoma. Congenital cataract is the leading cause of childhood blindness and progressive neuro degeneration of the optic nerve in glaucoma accounts as the leading cause of blindness worldwide. Altered solubility and stability of crystallin proteins cause cataract formation and are directly linked to a decrease in their protective function. Thus in this study, we evaluated the functional consequences of the mutations associated with this gene conversion on beta B2-crystallin protein biochemical properties in retinal neurons. We found that only the occurrence of the triple mutation leads to decreased solubility and formation of aggregates, which as we previously demonstrated, is associated with mislocalization to the mitochondria along with decreased mitochondrial function in retinal neurons and lens epithelial cells. Our data strongly support a significant role for beta B2-crystallin in both lenticular and retinal ocular tissues and warrant further analysis of its regulation and its impact not only in cataract formation but also in retinal neurodegenerative diseases.

Two novel mutations identified in ADCC families impair crystallin protein distribution and induce apoptosis in human lens epithelial cells

Congenital cataract (CC) is a clinical and genetically heterogeneous eye disease that primarily causes lens disorder and even amblyopic blindness in children. As the mechanism underlying CC is genetically inherited, identification of CC-associated gene mutations and their role in protein distribution are topics of both pharmacological and biological research. Through physical and ophthalmic examinations, two Chinese pedigrees with autosomal dominant congenital cataract (ADCC) were recruited for this study. Mutation analyses of CC candidate genes by next-generation sequencing (NGS) and Sanger sequencing revealed a novel missense mutation in CRYBB2 (p.V146L) and a deletion mutation in CRYAA (p.116_118del). Both mutations fully co-segregated were not observed in unaffected family members or in 100 unrelated healthy controls. The CRYBB2 missense mutation disrupts the distribution of CRYBB2 in human lens epithelial cells (HLEpiCs), and the CRYAA deletion mutation causes hyperdispersion of CRYAA. Furthermore, these two crystallin mutations result in aberrant expression of unfolded protein response (UPR) marker genes as well as apoptosis in HLEpiCs. Collectively, these findings broaden the genetic spectrum of ADCC.

Cataract-associated P23T γD-crystallin retains a native-like fold in amorphous-looking aggregates formed at physiological pH

Cataracts cause vision loss through the large-scale aggregation of eye lens proteins as a result of ageing or congenital mutations. The development of new treatments is hindered by uncertainty about the nature of the aggregates and their mechanism of formation. We describe the structure and morphology of aggregates formed by the P23T human γD-crystallin mutant associated with congenital cataracts. At physiological pH, the protein forms aggregates that look amorphous and disordered by electron microscopy, reminiscent of the reported formation of amorphous deposits by other crystallin mutants. Surprisingly, solid-state NMR reveals that these amorphous deposits have a high degree of structural homogeneity at the atomic level and that the aggregated protein retains a native-like conformation, with no evidence for large-scale misfolding. Non-physiological destabilizing conditions used in many in vitro aggregation studies are shown to yield qualitatively different, highly misfolded amyloid-like fibrils.

Aggregation of eye lens proteins leads to cataracts, a major cause of blindness. Here the authors use solid state NMR to probe the structure of γD-crystallin eye lens proteins aggregates, which are found to retain a native-like conformation.

Congenital microcornea-cataract syndrome-causing mutation X253R increases βB1-crystallin hydrophobicity to promote aggregate formation

The high solubility and lifelong stability of crystallins are crucial to the maintenance of lens transparency and optical properties. Numerous crystallin mutations have been linked to congenital cataract, which is one of the leading causes of newborn blindness. Besides cataract, several crystallin mutations have also been linked to syndromes such as congenital microcornea-cataract syndrome (CMCC). However, the molecular mechanism of CMCC caused by crystallin mutations remains elusive. In the present study, we investigated the mechanism of CMCC caused by the X253R mutation in βB1-crystallin. The exogenously expressed X253R proteins were prone to form p62-negative aggregates in HeLa cells, strongly inhibited cell proliferation and induced cell apoptosis. The intracellular X253R aggregates could be successfully redissolved by lanosterol but not cholesterol. The extra 26 residues at the C-terminus of βB1-crystallin introduced by the X253R mutation had little impact on βB1-crystallin structure and stability, but increased βB1-crystallin hydrophobicity and decreased its solubility. Interestingly, the X253R mutant fully abolished the aggregatory propensity of βB1- and βA3/βB1-crystallins at high temperatures, suggesting that X253R was an aggregation-inhibition mutation of β-crystallin homomers and heteromers in dilute solutions. Our results suggest that an increase in hydrophobicity and a decrease in solubility might be responsible for cataractogenesis induced by the X253R mutation, while the cytotoxic effect of X253R aggregates might contribute to the defects in ocular development. Our results also highlight that, at least in some cases, the aggregatory propensity in dilute solutions could not fully mimic the behaviours of mutated proteins in the crowded cytoplasm of the cells.

Cataract-causing mutation S228P promotes βB1-crystallin aggregation and degradation by separating two interacting loops in C-terminal domain

β/γ-Crystallins are predominant structural proteins in the cytoplasm of lens fiber cells and share a similar fold composing of four Greek-key motifs divided into two domains. Numerous cataract-causing mutations have been identified in various β/γ-crystallins, but the mechanisms underlying cataract caused by most mutations remains uncharacterized. The S228P mutation in βB1-crystallin has been linked to autosomal dominant congenital nuclear cataract. Here we found that the S228P mutant was prone to aggregate and degrade in both of the human and E. coli cells. The intracellular S228P aggregates could be redissolved by lanosterol. The S228P mutation modified the refolding pathway of βB1-crystallin by affecting the formation of the dimeric intermediate but not the monomeric intermediate. Compared with native βB1-crystallin, the refolded S228P protein had less packed structures, unquenched Trp fluorophores and increased hydrophobic exposure. The refolded S228P protein was prone to aggregate at the physiological temperature and decreased the protective effect of βB1-crystallin on βA3-crystallin. Molecular dynamic simulation studies indicated that the mutation decreased the subunit binding energy and modified the distribution of surface electrostatic potentials. More importantly, the mutation separated two interacting loops in the C-terminal domain, which shielded the hydrophobic core from solvent in native βB1-crystallin. These two interacting loops are highly conserved in both of the N- and C-terminal domains of all β/γ-crystallins. We propose that these two interacting loops play an important role in the folding and structural stability of β/γ-crystallin domains by protecting the hydrophobic core from solvent access.