Cataract-Causing S93R Mutant Destabilized Structural Conformation of βB1 Crystallin Linking With Aggregates Formation and Cellular Viability

Cataract-causing mutations S78F and S78P of γD-crystallin decrease protein conformational stability and drive aggregation

Congenital cataract is the leading cause of childhood blindness, which primarily results from genetic factors. γD-crystallin is the most abundant γ-crystallin and is essential for maintaining lens transparency and refractivity. Numerous mutations in γD-crystallin have been reported with unclear pathogenic mechanism. Two different cataract-causing mutations Ser78Phe and Ser78Pro in γD-crystallin were previously identified at the same conserved Ser78 residue. In this work, firstly, we purified the mutants and characterized for the structural change using fluorescence spectroscopy, circular dichroism (CD) spectroscopy, and size-exclusion chromatography (SEC). Both mutants were prone to form insoluble precipitates when expressed in Escherichia coli strain BL21 (DE3) cells. Compared with wild-type (WT), both mutations caused structural disruption, increased hydrophobic exposure, decreased solubility, and reduced thermal stability. Next, we investigated the aggregation of the mutants at the cellular level. Overexpression the mutants in HLE-B3 and HEK 293T cells could induce aggresome formations. The environmental stresses (including heat, ultraviolet irradiation and oxidative stress) promoted the formation of aggregates. Moreover, the intracellular S78F and S78P aggregates could be reversed by lanosterol. Molecular dynamic simulation indicated that both mutations disrupted the structural integrity of Greek-key motif 2. Hence, our results reveal the vital role of conserved Ser78 in maintaining the structural stability, which can offer new insights into the mechanism of cataract formation.

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.