Structural and functional studies of D109A human αB-crystallin contributing to the development of cataract and cardiomyopathy diseases

Unraveling the impact of the p.R107L mutation on the structure and function of human αB-Crystallin: Implications for cataract formation

To date, several pathogenic mutations have been identified in the primary structure of human α-Crystallin, frequently involving the substitution of arginine with a different amino acid. These mutations can lead to the incidence of cataracts and myopathy. Recently, an important cataract-associated mutation has been reported in the functional α-Crystallin domain (ACD) of human αB-Crystallin protein, where arginine 107 (R107) is replaced by a leucine. In this study, we investigated the structure, chaperone function, stability, oligomerization, and amyloidogenic properties of the p.R107L human αB-Crystallin using a number of different techniques. Our results suggest that the p.R107L mutation can cause significant changes in the secondary, tertiary, and quaternary structures of αB-Crystallin. This cataractogenic mutation led to the formation of protein oligomers with larger sizes than the wild-type protein and reduced the chemical and thermal stability of the mutant chaperone. Both fluorescence and microscopic assessments indicated that this mutation significantly altered the amyloidogenic properties of human αB-Crystallin. Furthermore, the mutant protein indicated an attenuated in vitro chaperone activity. The molecular dynamics (MD) simulation confirmed the experimental results and indicated that p.R107L mutation could alter the proper conformation of human αB-Crystallin dimers. In summary, our results indicated that the p.R107L mutation could promote the formation of larger oligomers, diminish the stability and chaperone activity of human αB-Crystallin, and these changes, in turn, can play a crucial role in the development of cataract disorder.

Exploring the significance of potassium homeostasis in copper ion binding to human αB-Crystallin

αB-Crystallin (αB-Cry) is a small heat shock protein known for its protective role, with an adaptable structure that responds to environmental changes through oligomeric dynamics. Cu(II) ions are crucial for cellular processes but excessive amounts are linked to diseases like cataracts and neurodegeneration. This study investigated how optimal and detrimental Cu(II) concentrations affect αB-Cry oligomers and their chaperone activity, within the potassium-regulated ionic-strength environment. Techniques including isothermal titration calorimetry, differential scanning calorimetry, fluorescence spectroscopy, inductively coupled plasma atomic emission spectroscopy, cyclic voltammetry, dynamic light scattering, circular dichroism, and MTT assay were employed and complemented by computational methods. Results showed that potassium ions affected αB-Cry's structure, promoting Cu(II) binding at multiple sites and scavenging ability, and inhibiting ion redox reactions. Low concentrations of Cu(II), through modifications of oligomeric interfaces, induce regulation of surface charge and hydrophobicity, resulting in an increase in chaperone activity. Subunit dynamics were regulated, maintaining stable interfaces, thereby inhibiting further aggregation and allowing the functional reversion to oligomers after stress. High Cu(II) disrupted charge/hydrophobicity balance, sewing sizable oligomers together through subunit-subunit interactions, suppressing oligomer dissociation, and reducing chaperone efficiency. This study offers insights into how Cu(II) and potassium ions influence αB-Cry, advancing our understanding of Cu(II)-related diseases.

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).

Insights into the dual nature of αB-crystallin chaperone activity from the p.P39L mutant at the N-terminal region

The substitution of leucine to proline at position 39 (p.P39L) in human αB-crystallin (αB-Cry) has been associated with conflicting interpretations of pathogenicity in cataracts and cardiomyopathy. This study aimed to investigate the effects of the p.P39L mutation on the structural and functional features of human αB-Cry. The mutant protein was expressed in Escherichia coli (E. coli) and purified using anion exchange chromatography. We employed a wide range of spectroscopic analyses, gel electrophoresis, transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques to investigate the structure, function, stability, and fibrillation propensity of the mutant protein. The p.P39L mutation caused significant changes in the secondary, tertiary, and quaternary structures of human αB-Cry and increased the thermal stability of the protein. The mutant αB-Cry exhibited an increased chaperone activity and an altered oligomeric size distribution, along with an increased propensity to form amyloid aggregates. It is worth mentioning, increased chaperone activity has important positive and negative effects on damaged cells related to cataracts and cardiomyopathy, particularly by interfering in the process of apoptosis. Despite the apparent positive nature of the increased chaperone activity, it is also linked to adverse consequences. This study provides important insights into the effect of proline substitution by leucine at the N-terminal region on the dual nature of chaperone activity in human αB-Cry, which can act as a double-edged sword.

Dynamic fibrillar assembly of αB-crystallin induced by perturbation of the conserved NT-IXI motif resolved by cryo-EM

αB-crystallin is an archetypical member of the small heat-shock proteins (sHSPs) vital for cellular proteostasis and mitigating protein misfolding diseases. Gaining insights into the principles defining their molecular organization and chaperone function have been hindered by intrinsic dynamic properties and limited high-resolution structural analysis. To disentangle the mechanistic underpinnings of these dynamical properties, we mutated a conserved IXI-motif located within the N-terminal (NT) domain of human αB-crystallin. This resulted in a profound structural transformation, from highly polydispersed caged-like native assemblies into a comparatively well-ordered helical fibril state amenable to high-resolution cryo-EM analysis. The reversible nature of the induced fibrils facilitated interrogation of functional effects due to perturbation of the NT-IXI motif in both the native-like oligomer and fibril states. Together, our investigations unveiled several features thought to be key mechanistic attributes to sHSPs and point to a critical significance of the NT-IXI motif in αB-crystallin assembly, dynamics and chaperone activity.

The structural and functional consequences of melatonin and serotonin on human αB-crystallin and their dual role in the eye lens transparency

Crystallins are the major soluble lens proteins, and α-crystallin, the most important protective protein of the eye lens, has two subunits (αA and αB) with chaperone activity. αB-crystallin (αB-Cry) with a relatively wide tissue distribution has an innate ability to interact effectively with the misfolded proteins, preventing their aggregation. Melatonin and serotonin have also been identified in relatively high concentrations in the lenticular tissues. This study investigated the effect of these naturally occurring compounds and medications on the structure, oligomerization, aggregation, and chaperone-like activity of human αB-Cry. Various spectroscopic methods, dynamic light scattering (DLS), differential scanning calorimetry (DSC), and molecular docking have been used for this purpose. Based on our results, melatonin indicates an inhibitory effect on the aggregation of human αB-Cry without altering its chaperone-like activity. However, serotonin decreases αB-Cry oligomeric size distribution by creating hydrogen bonds, decreases its chaperone-like activity, and at high concentrations increases protein aggregation.

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.

Role of ATP-Small Heat Shock Protein Interaction in Human Diseases

Adenosine triphosphate (ATP) is an important fuel of life for humans and Mycobacterium species. Its potential role in modulating cellular functions and implications in systemic, pulmonary, and ocular diseases is well studied. Plasma ATP has been used as a diagnostic and prognostic biomarker owing to its close association with disease’s progression. Several stresses induce altered ATP generation, causing disorders and illnesses. Small heat shock proteins (sHSPs) are dynamic oligomers that are dominantly β-sheet in nature. Some important functions that they exhibit include preventing protein aggregation, enabling protein refolding, conferring thermotolerance to cells, and exhibiting anti-apoptotic functions. Expression and functions of sHSPs in humans are closely associated with several diseases like cataracts, cardiovascular diseases, renal diseases, cancer, etc. Additionally, there are some mycobacterial sHSPs like Mycobacterium leprae HSP18 and Mycobacterium tuberculosis HSP16.3, whose molecular chaperone functions are implicated in the growth and survival of pathogens in host species. As both ATP and sHSPs, remain closely associated with several human diseases and survival of bacterial pathogens in the host, therefore substantial research has been conducted to elucidate ATP-sHSP interaction. In this mini review, the impact of ATP on the structure and function of human and mycobacterial sHSPs is discussed. Additionally, how such interactions can influence the onset of several human diseases is also discussed.