Understanding the structural and functional changes and biochemical pathomechanism of the cardiomyopathy-associated p.R123W mutation in human αB-crystallin

αB-crystallin, a member of the small heat shock protein (sHSP) family, is expressed in diverse tissues, including the eyes, brain, muscles, and heart. This protein plays a crucial role in maintaining eye lens transparency and exhibits holdase chaperone and anti-apoptotic activities. Therefore, structural and functional changes caused by genetic mutations in this protein may contribute to the development of disorders like cataract and cardiomyopathy. Recently, the substitution of arginine 123 with tryptophan (p.R123W mutation) in human αB-crystallin has been reported to trigger cardiomyopathy. In this study, human αB-crystallin was expressed in Escherichia coli (E. coli), and the missense mutation p.R123W was created using site-directed mutagenesis. Following purification via anion exchange chromatography, the structural and functional properties of both proteins were investigated and compared using a wide range of spectroscopic and microscopic methods. The p.R123W mutation induced significant alterations in the secondary, tertiary, and quaternary structures of human αB-crystallin. This pathogenic mutation resulted in an increased β-sheet structure and formation of protein oligomers with larger sizes compared to the wild-type protein. The mutant protein also exhibited reduced chaperone activity and lower thermal stability. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) demonstrated that the p.R123W mutant protein is more prone to forming amyloid aggregates. The structural and functional changes observed in the p.R123W mutant protein, along with its increased propensity for aggregation, could impact its proper functional interaction with the target proteins in the cardiac muscle, such as calcineurin. Our results provide an explanation for the pathogenic intervention of p.R123W mutant protein in the occurrence of hypertrophic cardiomyopathy (HCM).

Investigating the role of double mutations R12C/P20R, and R12C/R69C on structure, chaperone-like activity, and amyloidogenic properties of human αB-crystallin

α-crystallin is a structurally essential small heat shock protein (sHSP) with a chaperone-like activity which maintains transparency of the lenticular tissues during a period of time that is as long as human life. α-crystallin is a multimeric protein consisting of αA and αB subunits, with 57 % homology. The CRYAB gene on chromosome 11 encodes human αB-crystallin (αB-Cry), which contains 175 amino acid residues. In the current study, the cataractogenic mutations R12C, P20R, R69C, and double mutations R12C/P20R and R12C/P20R were embedded into the human CRYAB gene. Following successful expression in the prokaryotic system and purification, a number of spectroscopic techniques, gel electrophoresis, dynamic light scattering (DLS), and transmission electron microscopy (TEM) were applied to assess the role of these mutations on the structure, amyloidogenicity, and biological function of human αB-Cry. The created mutations caused significant changes in the structure, and oligomeric state of human αB-Cry. These mutations, particularly R12C, R12C/P20R, and R12C/R69C, dramatically enhanced the tendency of this protein for the amyloid fibril formation and reduced its chaperone-like activity. Since double mutations R12C/P20R and R12C/P20R were able to intensely change the protein's structure and chaperone function, it can be suggested that they may play a destructive role in a cumulative manner. Our findings indicated that the simultaneous presence of two pathogenic mutations may have a cumulative destructive impacts on the structure and function of human αB-Cry and this observation is likely related to the disease severity of the mutated proteins.

Analysis of the data on titration of native and peroxynitrite modified αA- and αB-crystallins by Cu2+-ions

The interaction of αA- and αB-crystallins with Cu²⁺ ion modulates their structure and chaperone-like activity which is important for lens transparency. Theoretical analysis of the dependences of fluorescence intensity of native αA- and αB-crystallins and αA- and αB-crystallins modified by peroxynitrite on concentration of Cu²⁺ ions has been carried out. It has been shown that one subunit of native αA-crystallin contains two equivalent Cu²⁺-binding sites. The microscopic dissociation constant for Cu²⁺–αA-crystallin complex (K_diss) was found to be equal to 9.7 µM. For peroxynitrite modified αA-crystallin the K_diss value is equal to 17 µM. One subunit of native αB-crystallin contains two non-equivalent Cu²⁺-binding sites. The corresponding microscopic dissociation constants for Cu²⁺–αB-crystallin complexes (K₁ and K₂) were found to be equal to 0.94 and 36 µM. For peroxynitrite modified αB-crystallin the K₁ and K₂ values are equal to 4.3 and 70 µM, respectively.

Kinetic data analysis of chaperone-like activity of Wt, R69C and D109H αB-crystallins

The α-Crystallin (α-Cry) functions as a molecular chaperone, preventing the formation of stress-induced protein aggregation which is important for maintenance of lens transparency. The kinetic data of Wt, R69C and D109H αB-Crys chaperone-like activity were obtained by UV–Vis spectroscopy in both thermal- and chemical-induced aggregation methods. The data were analyzed using physical parameters describing the aggregation process including t* (the characteristic of the stage of nucleation), and t_0.5 (the characteristic of the stage of aggregate growth) and I_lim (the limiting value of the light scattering intensity). Parameter t* is duration of the lag phase and the lower t* value is associated with the higher rate of the nucleation stage. Also, the lower values of t_0.5 indicated the higher rate of aggregate growth stage. The change in parameter I_lim in the presence of chaperones can be connected with the change in the size of protein aggregates. These data are related to the research article entitled “Structural and functional characterization of D109H and R69C mutant versions of human αB-crystallin: the biochemical pathomechanism underlying cataract and myopathy development” [1].

Structural and functional characterization of D109H and R69C mutant versions of human αB-crystallin: The biochemical pathomechanism underlying cataract and myopathy development

In human αB-crystallin (αB-Cry), the highly conserved residues arginine 69 (R69) and aspartate 109 (D109) are located within a critical motif of α-crystallin domain (ACD), contributing to the subunit interactions and oligomeric assembly. Recently, two missense mutations (R69C and D109H) in human αB-Cry have been reported to cause congenital cataract and myopathy disorders. We used various spectroscopic techniques, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), gel electrophoresis and transmission electron microscopy (TEM) to show how these mutations cause significant changes in structure, amyloidogenic feature and biological function of human αB-Cry. These pathogenic mutations resulted in the important alterations of the secondary, tertiary and oligomeric (quaternary) structures of human αB-Cry. The missense mutations were also capable to significantly increase the amyloidogenic propensity of human αB-Cry and to diminish the chaperone-like activity of this protein. The above mentioned changes were observed more noticeably after D109H mutation. The detrimental effects of D109H mutation may be due to the loss of salt bridge with R120 in the dimeric interface, flagging the anti-aggregation ability of αB-Cry chaperone. In conclusion, the R69C and D109H mutations displayed a significant damaging effect on the structure and chaperone function of human αB-Cry which could be considered as their biochemical pathomechanisms in development of congenital cataract and myopathy disorders.