Hydroimidazolone modification of the conserved Arg12 in small heat shock proteins: studies on the structure and chaperone function using mutant mimics

Site-Specific Glycation of Human Heat Shock Protein (Hsp27) Enhances Its Chaperone Activity

Non-enzymatic posttranslational modifications are believed to affect at least 30% of human proteins, commonly termed glycation. Many of these modifications are implicated in various pathological conditions, e.g., cataract, diabetes, neurodegenerative diseases, and cancer. Chemical protein synthesis enables access to full-length proteins carrying site-specific modifications. One such modification, argpyrimidine (Apy), has been detected in human small heat shock protein Hsp27 and closely related proteins in patient-derived tissues. Thus far, studies have looked into only artificial mixtures of Apy modifications, and only one has analyzed Apy188. We were interested in understanding the impact of such individual Apy modifications on five different arginine sites within the crucial N-terminal domain of Hsp27. By combining protein semisynthesis with biochemical assays on semisynthetic Hsp27 analogues with single-point Apy modification at those sites, we have shown how a seemingly minimal modification within this region results in dramatically altered functional attributes.

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.

Substrate Protein Interactions and Methylglyoxal Modifications Reduce the Aggregation Propensity of Human Alpha-A-Crystallin G98R Mutant

The G98R mutation in αA-crystallin is associated with presenile cataract development in humans. Previous studies have indicated that mutant proteins altered structure, decreased stability, increased oligomeric size, loss of chaperone-like activity, and susceptibility to proteolysis could be contributing factors to cataract formation. To evaluate the effect of substrate protein interactions with the mutant protein on cataract formation, we have performed chaperone assays with alcohol dehydrogenase (ADH), citrate synthase (CS), and βB2-crystallin (βB2), and analyzed the reaction mixtures by multi-angle light scattering (MALS) analysis. It appears that αAG98R protein initially gets stabilized upon interaction with substrate proteins. Analysis of the chaperone-client protein complexes revealed that wild-type αA-crystallin interacts with substrate proteins to form compact complexes leading to a slight increase in oligomeric mass, whereas αAG98R forms less compact and high molecular weight complexes with the substrate, and the resulting complexes continue to increase in size over time. As a result, the soluble complexes formed initially by the mutant protein begin to scatter light and precipitate. We found that the stability and chaperone activity of the αAG98R can be improved by modifying the protein with low concentrations (50 µM) of methylglyoxal (MGO). Incubation of αAG98R protein (1 mg/ml) under aseptic conditions for 30 days at 37°C resulted in precipitation of the mutant protein. In contrast, mutant protein incubations carried out with 50 µM MGO remained soluble and transparent. SDS-PAGE analysis showed gradual autolysis of the mutant protein in the absence of MGO. The average molar mass of the mutant protein oligomers changed from 7,258 ± 12 kDa to 3,950 ± 08 kDa within 60 min of incubation with MGO. There was no further significant change in the molar mass of mutant protein when tested on day 7 of MGO treatment. Our data suggest that the initial stabilization of αAG98R by substrate proteins could delay congenital cataracts' appearance, and the uncontrolled long-term interaction amongst mutant subunits and substrate proteins could be the rationale behind presenile cataracts formation. The results also demonstrate the potential benefit of low concentrations of MGO in stabilizing mutant chaperone protein(s).

Relationship between the Structure and Chaperone Activity of Human αA-Crystallin after Its Modification with Diabetes-Associated Oxidative Agents and Protective Role of Antioxidant Compounds

The study was aimed to evaluate the impact of peroxynitrite (PON, oxidative stress agent in diabetes), methylglyoxal (MGO, diabetes-associated reactive carbonyl compound), and their simultaneous application on the structural and functional features of human αA-crystallin (αA-Cry) using various spectroscopy techniques. Additionally, the surface tension and oligomer size distribution of the treated and untreated protein were tested using tensiometric analysis and dynamic light scattering, respectively. Our results indicated that the reaction of PON and MGO with human αA-Cry leads to the formation of new chromophores, alterations in the secondary to quaternary protein structure, reduction in the size of protein oligomers, and significant enhancement in the chaperone activity of αA-Cry. To reverse the effects of the tested compounds, ascorbic acid and glutathione (main components of lens antioxidant defense system) were applied. As expected, the two antioxidant compounds significantly prevented formation of high molecular weight aggregates of αA-Cry (according to SDS-PAGE). Our results suggest that the lens antioxidant defense system, in particular, glutathione, may provide a strong protection against rapid incidence and progression of diabetic cataract by preventing the destructive reactions of highly reactive DM-associated metabolites.

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.

Glycation-mediated inter-protein cross-linking is promoted by chaperone-client complexes of α-crystallin: Implications for lens aging and presbyopia

Lens proteins become increasingly cross-linked through nondisulfide linkages during aging and cataract formation. One mechanism that has been implicated in this cross-linking is glycation through formation of advanced glycation end products (AGEs). Here, we found an age-associated increase in stiffness in human lenses that was directly correlated with levels of protein-cross-linking AGEs. α-Crystallin in the lens binds to other proteins and prevents their denaturation and aggregation through its chaperone-like activity. Using a FRET-based assay, we examined the stability of the αA-crystallin-γD-crystallin complex for up to 12 days and observed that this complex is stable in PBS and upon incubation with human lens-epithelial cell lysate or lens homogenate. Addition of 2 mm ATP to the lysate or homogenate did not decrease the stability of the complex. We also generated complexes of human αA-crystallin or αB-crystallin with alcohol dehydrogenase or citrate synthase by applying thermal stress. Upon glycation under physiological conditions, the chaperone-client complexes underwent greater extents of cross-linking than did uncomplexed protein mixtures. LC-MS/MS analyses revealed that the levels of cross-linking AGEs were significantly higher in the glycated chaperone-client complexes than in glycated but uncomplexed protein mixtures. Mouse lenses subjected to thermal stress followed by glycation lost resilience more extensively than lenses subjected to thermal stress or glycation alone, and this loss was accompanied by higher protein cross-linking and higher cross-linking AGE levels. These results uncover a protein cross-linking mechanism in the lens and suggest that AGE-mediated cross-linking of α-crystallin-client complexes could contribute to lens aging and presbyopia.

Methylglyoxal, a Highly Reactive Dicarbonyl Compound, in Diabetes, Its Vascular Complications, and Other Age-Related Diseases

The formation and accumulation of methylglyoxal (MGO), a highly reactive dicarbonyl compound, has been implicated in the pathogenesis of type 2 diabetes, vascular complications of diabetes, and several other age-related chronic inflammatory diseases such as cardiovascular disease, cancer, and disorders of the central nervous system. MGO is mainly formed as a byproduct of glycolysis and, under physiological circumstances, detoxified by the glyoxalase system. MGO is the major precursor of nonenzymatic glycation of proteins and DNA, subsequently leading to the formation of advanced glycation end products (AGEs). MGO and MGO-derived AGEs can impact on organs and tissues affecting their functions and structure. In this review we summarize the formation of MGO, the detoxification of MGO by the glyoxalase system, and the biochemical pathways through which MGO is linked to the development of diabetes, vascular complications of diabetes, and other age-related diseases. Although interventions to treat MGO-associated complications are not yet available in the clinical setting, several strategies to lower MGO have been developed over the years. We will summarize several new directions to target MGO stress including glyoxalase inducers and MGO scavengers. Targeting MGO burden may provide new therapeutic applications to mitigate diseases in which MGO plays a crucial role.

The impact of different mutations at arginine141 on the structure, subunit exchange dynamics and chaperone activity of Hsp16.3

Hsp16.3, a molecular chaperone, plays a vital role in the growth and survival of Mycobacterium tuberculosis inside the host. We previously reported that deletion of three amino acid residues (¹⁴² STN¹⁴⁴ ) from C-terminal extension (CTE) of Hsp16.3 triggers its structural perturbation and increases its chaperone activity, which reaches its apex upon the deletion of its entire CTE (¹⁴¹ RSTN¹⁴⁴ ). Thus, we hypothesized that Arg141 (R141) and Ser142 (S142) in the CTE of Hsp16.3 possibly hold the key in maintaining its native-like structure and chaperone activity. To test this hypothesis, we generated two deletion mutants in which R141 and S142 were deleted individually (Hsp16.3ΔR141 and Hsp16.3ΔS142) and three substitution mutants in which R141 was replaced by lysine (Hsp16.3R141K), alanine (Hsp16.3R141A), and glutamic acid (Hsp16.3R141E), respectively. Hsp16.3ΔS142 or Hsp16.3R141K mutant has native-like structure and chaperone activity. Deletion of R141 from the CTE (Hsp16.3ΔR141) perturbs the secondary and tertiary structure, lowers the subunit exchange dynamics and decreases the chaperone activity of Hsp16.3. But, the substitution of R141 with alanine (Hsp16.3R141A) or glutamic acid (Hsp16.3R141E) perturbs its secondary and tertiary structure. Surprisingly, such charge tampering of R141 enhances the subunit exchange dynamics and chaperone activity of Hsp16.3. Interestingly, neither the deletion of R141/S142 nor the substitution of R141 with lysine, alanine and glutamic acid affects the oligomeric mass/size of Hsp16.3. Overall, our study suggests that R141 (especially the positive charge on R141) plays a crucial role in maintaining the native-like structure as well as in regulating subunit exchange dynamics and chaperone activity of Hsp16.3.

Methylglyoxal modification reduces the sensitivity of hen egg white lysozyme to stress-induced aggregation: Insight into the anti-amyloidogenic property of α-dicarbonyl compound

The reactive α-oxoaldehyde, methylglyoxal reacts with different proteins to form Advanced Glycation End Products (AGEs) through Maillard reaction. Its level increases significantly in diabetic condition. Here, we have investigated the effect of different concentrations of methylglyoxal (200-400 µM) on the monomeric protein, hen egg white lysozyme (HEWL) following incubation for 3 weeks. Reaction of methylglyoxal with HEWL induced considerable changes in tertiary structure of the protein, but no significant alteration in secondary structure, as evident from different spectroscopic and biophysical studies. Interestingly, methylglyoxal modification was found to enhance the thermal stability of the protein and reduce its sensitivity to stress-induced aggregation. Finally, peptide mass fingerprinting revealed modification of arginine (Arg-45, Arg-14, Arg-68 or Arg-72) and lysine (Lys-116) residues of the protein to AGE adducts, namely, hydroimidazolone, tetrahydropyrimidine, and carboxyethyllysine. Methylglyoxal-derived AGE adducts (MAGE) appear to be responsible for the observed changes in protein. As demonstrated in the present study, the findings may highlight a possible therapeutic potential of the α-oxoaldehyde against protein misfolding and conformational disorder.Communicated by Ramaswamy H. Sarma.

Effect of Glyoxal Modification on a Critical Arginine Residue (Arg-31α) of Hemoglobin: Physiological Implications of Advanced Glycated end Product an in vitro Study

BACKGROUND

Non-enzymatic protein glycation is involved in structure and stability changes that impair protein functionality, resulting in several human diseases, such as diabetes and amyloidotic neuropathies (Alzheimer's disease, Parkinson's disease and Andrade's syndrome). Glyoxal, an endogenous reactive oxoaldehyde, increases in diabetes and reacts with several proteins to form advanced glycation end products through Maillard-like reaction.

OBJECTIVE

Human hemoglobin, the most abundant protein in blood cells is subjected to nonenzymatic modification by reactive oxoaldehydes in diabetic condition. In the present study, the effect of a low concentration of glyoxal (5 μM) on hemoglobin (10 μM) has been investigated following a period of 30 days incubation in vitro.

METHODS

Different techniques, mostly biophysical and spectroscopic (e.g. circular dichroism, differential scanning calorimetric study, dynamic light scattering, mass spectrometry, etc.) were used to study glyoxal-induced changes of hemoglobin.

RESULTS

Glyoxal-treated hemoglobin exhibits decreased absorbance around 280 nm, decreased fluorescence and reduced surface hydrophobicity compared to normal hemoglobin. Glyoxal treatment enhances the stability of hemoglobin and lowers its susceptibility to thermal aggregation compared to control hemoglobin as seen by different studies. Finally, peptide mass fingerprinting study showed glyoxal to modify an arginine residue of α-chain of hemoglobin (Arg-31α) to hydroimidazolone.

CONCLUSION

Increased level of glyoxal in diabetes mellitus as well as its high reactivity may cause modifications of the heme protein. Thus, considering the significance of glyoxal-induced protein modification under physiological conditions, the observation appears clinically relevant in terms of understanding hydroimidazolone-mediated protein modification under in vivo conditions.

Connexin hemichannels mediate glutathione transport and protect lens fiber cells from oxidative stress

Elevated oxidized stress contributes to lens cataracts, and gap junctions play important roles in maintaining lens transparency. As well as forming gap junctions, connexin (Cx) proteins also form hemichannels. Here, we report a new mechanism whereby hemichannels mediate transport of reductant glutathione into lens fiber cells and protect cells against oxidative stress. We found that Cx50 (also known as GJA8) hemichannels opened in response to H2O2 in lens fiber cells but that transport through the channels was inhibited by two dominant-negative mutants in Cx50, Cx50P88S, which inhibits transport through both gap junctions and hemichannels, and Cx50H156N, which only inhibits transport through hemichannels and not gap junctions. Treatment with H2O2 increased the number of fiber cells undergoing apoptosis, and this increase was augmented with dominant-negative mutants that disrupted both hemichannels formed from Cx46 (also known as GJA3) and Cx50, while Cx50E48K, which only impairs gap junctions, did not have such an effect. Moreover, hemichannels mediate uptake of glutathione, and this uptake protected lens fiber cells against oxidative stress, while hemichannels with impaired transport had less protective benefit from glutathione. Taken together, these results show that oxidative stress activates connexin hemichannels in the lens fiber cells and that hemichannels likely protect lens cell against oxidative damage through transporting extracellular reductants.

Methylglyoxal and Small Heat Shock Proteins

Methylglyoxal is a highly reactive dicarbonyl compound formed during glucose metabolism and able to modify phospholipids, nucleic acids, and proteins belonging to the so-called dicarbonyl proteome. Small heat shock proteins participating in protection of the cell against different unfavorable conditions can be modified by methylglyoxal. The probability of methylglyoxal modification is increased in the case of distortion of glucose metabolism (diabetes), in the case of utilization of glycolysis as the main source of energy (malignancy), and/or at low rate of modified protein turnover. We have analyzed data on modification of small heat shock protein HspB1 in different tumors and under distortion of carbohydrate metabolism. Data on the effect of methylglyoxal modification on stability, chaperone-like activity, and antiapoptotic activity of HspB1 were analyzed. We discuss data on methylglyoxal modifications of lens α-crystallins. The mutual dependence and mutual effects of methylglyoxal modification and other posttranslational modifications of lens crystallins are analyzed. We conclude that although there is no doubt that the small heat shock proteins undergo methylglyoxal modification, the physiological significance of this process remains enigmatic, and new experimental approaches should be developed for understanding how this type of modification affects functioning of small heat shock proteins in the cell.

Glyoxal-induced modification enhances stability of hemoglobin and lowers iron-mediated oxidation reactions of the heme protein: An in vitro study

Glyoxal, a reactive α-oxoaldehyde, increases in diabetic condition. It reacts with different proteins to form advanced glycation end products (AGEs) following Maillard-like reaction. Considering the significance of AGE-mediated protein modification by glyoxal, here we have investigated the in vitro effect of the reactive α-oxoaldehyde (10, 20μM) on the heme protein hemoglobin (HbA₀) (100μM) after incubation for one week at 25°C. In comparison with HbA₀, glyoxal-treated HbA₀ exhibited decreased absorbance around 280nm, reduced intrinsic fluorescence and lower surface hydrophobicity. Glyoxal treatment was found to increase the stability of HbA₀ without significant perturbation of the secondary structure of the heme protein. In addition, H₂O₂-mediated iron release and subsequent iron-mediated oxidative (Fenton) reactions were found to be lower in presence of glyoxal-treated HbA₀ compared to HbA₀. Mass spectrometric studies revealed modification of arginine residues of HbA₀ (Arg-31α, Arg-40β) to hydroimidazolone adducts. AGE-induced modifications thus appear to be associated with the observed changes of the heme protein. Considering the increased level of glyoxal in diabetes mellitus as well as its high reactivity, glyoxal-derived AGE adducts might thus be associated with modifications of the protein including physiological significance.

Small heat shock proteins in ageing and age-related diseases

Small heat shock proteins (sHSPs) are gatekeepers of cellular homeostasis across species, preserving proteome integrity under stressful conditions. Nonetheless, recent evidence suggests that sHSPs are more than molecular chaperones with merely auxiliary role. In contrast, sHSPs have emerged as central lifespan determinants, and their malfunction has been associated with the manifestation of neurological disorders, cardiovascular disease and cancer malignancies. In this review, we focus on the role of sHSPs in ageing and age-associated diseases and highlight the most prominent paradigms, where impairment of sHSP function has been implicated in human pathology.

Effect of methylglyoxal modification on the structure and properties of human small heat shock protein HspB6 (Hsp20)

Human small heat shock protein HspB6 (Hsp20) was modified by metabolic α-dicarbonyl compound methylglyoxal (MGO). At low MGO/HspB6 molar ratio, Arg13, Arg14, Arg27, and Arg102 were the primary sites of MGO modification. At high MGO/HspB6 ratio, practically, all Arg and Lys residues of HspB6 were modified. Both mild and extensive MGO modification decreased susceptibility of HspB6 to trypsinolysis and prevented its heat-induced aggregation. Modification by MGO was accompanied by formation of small quantities of chemically crosslinked dimers and did not dramatically affect quaternary structure of HspB6. Mild modification by MGO did not affect whereas extensive modification decreased interaction of HspB6 with HspB1. Phosphorylation of HspB6 by cyclic adenosine monophosphate (cAMP)-dependent protein kinase was inhibited after mild modification and completely prevented after extensive modification by MGO. Chaperone-like activity of HspB6 measured with subfragment 1 of skeletal myosin was enhanced after MGO modifications. It is concluded that Arg residues located in the N-terminal domain of HspB6 are easily accessible to MGO modification and that even mild modification by MGO affects susceptibility to trypsinolysis, phosphorylation by cAMP-dependent protein kinase, and chaperone-like activity of HspB6.

Structure, Chaperone Activity, and Aggregation of Wild-Type and R12C Mutant αB-Crystallins in the Presence of Thermal Stress and Calcium Ion - Implications for Role of Calcium in Cataract Pathogenesis

The current study was performed with the aim to evaluate the chaperoning ability, structural features, and aggregation propensity of wild-type and R12C mutant αB-crystallins (αB-Cry) under thermal stress and in the presence of calcium ion. The results of different spectroscopic analyses suggest that wild-type and mutant αB-Cry have dissimilar secondary and tertiary structures. Moreover, αB-Cry indicates slightly improved chaperone activity upon the R12C mutation. Thermal stress and calcium, respectively, enhance and reduce the extent of solvent-exposed hydrophobic surfaces accompanying formation of ordered and non-ordered aggregate entities in both proteins. Compared to the wild-type protein, the R12C mutant counterpart shows significant resistance against thermal and calcium-induced aggregation. In addition, in the presence of calcium, significant structural variation was accompanied by reduction in the solvent-exposed hydrophobic patches and attenuation of chaperone activity in both proteins. Additionally, gel mobility shift assay indicates the intrinsic propensity of R12C mutant αB-Cry for disulfide bridge-mediated protein dimerization. Overall, the results of this study are of high significance for understanding the molecular details of different factors that are involved in the pathomechanism of cataract disorders.

Role of Subunit Exchange and Electrostatic Interactions on the Chaperone Activity of Mycobacterium leprae HSP18

Mycobacterium leprae HSP18, a major immunodominant antigen of M. leprae pathogen, is a small heat shock protein. Previously, we reported that HSP18 is a molecular chaperone that prevents aggregation of different chemically and thermally stressed client proteins and assists refolding of denatured enzyme at normal temperature. We also demonstrated that it can efficiently prevent the thermal killing of E. coli at higher temperature. However, molecular mechanism behind the chaperone function of HSP18 is still unclear. Therefore, we studied the structure and chaperone function of HSP18 at normal temperature (25°C) as well as at higher temperatures (31–43°C). Our study revealed that the chaperone function of HSP18 is enhanced significantly with increasing temperature. Far- and near-UV CD experiments suggested that its secondary and tertiary structure remain intact in this temperature range (25–43°C). Besides, temperature has no effect on the static oligomeric size of this protein. Subunit exchange study demonstrated that subunits of HSP18 exchange at 25°C with a rate constant of 0.018 min^-1. Both rate of subunit exchange and chaperone activity of HSP18 is found to increase with rise in temperature. However, the surface hydrophobicity of HSP18 decreases markedly upon heating and has no correlation with its chaperone function in this temperature range. Furthermore, we observed that HSP18 exhibits diminished chaperone function in the presence of NaCl at 25°C. At elevated temperatures, weakening of interactions between HSP18 and stressed client proteins in the presence of NaCl results in greater reduction of its chaperone function. The oligomeric size, rate of subunit exchange and structural stability of HSP18 were also found to decrease when electrostatic interactions were weakened. These results clearly indicated that subunit exchange and electrostatic interactions play a major role in the chaperone function of HSP18.

Differential role of arginine mutations on the structure and functions of α-crystallin

BACKGROUND

α-Crystallin is a major protein of the eye lens in vertebrates. It is composed of two subunits, αA- and αB-crystallin. α-Crystallin is an oligomeric protein having these two subunits in 3:1 ratio. It belongs to small heat shock protein family and exhibits molecular chaperone function, which plays an important role in maintaining the lens transparency. Apart from chaperone function, both subunits also exhibit anti-apoptotic property. Comparison of their primary sequences reveals that αA- and αB-crystallin posses 13 and 14 arginine residues, respectively. Several of them undergo mutations which eventually lead to various eye diseases such as congenital cataract, juvenile cataract, and retinal degeneration. Interestingly, many arginine residues of these subunits are modified during glycation and even some are truncated during aging. All these facts indicate the importance of arginine residues in α-crystallin.

SCOPE OF REVIEW

In this review, we will emphasize the recent in vitro and in vivo findings related to congenital cataract causing arginine mutations in α-crystallin.

MAJOR CONCLUSIONS

Congenital cataract causing arginine mutations alters the structure and decreases the chaperone function of α-crystallin. These mutations also affect the lens morphology and phenotypes. Interestingly, non-natural arginine mutations (generated for mimicking the glycation and truncation environment) improve the chaperone function of α-crystallin which may play an important role in maintaining the eye lens transparency during aging.

GENERAL SIGNIFICANCE

The neutralization of positive charge on the guanidino group of arginine residues is not always detrimental to the functionality of α-crystallin. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

The role of methylglyoxal and the glyoxalase system in diabetes and other age-related diseases

The formation and accumulation of advanced glycation endproducts (AGEs) are related to diabetes and other age-related diseases. Methylglyoxal (MGO), a highly reactive dicarbonyl compound, is the major precursor in the formation of AGEs. MGO is mainly formed as a byproduct of glycolysis. Under physiological circumstances, MGO is detoxified by the glyoxalase system into D-lactate, with glyoxalase I (GLO1) as the key enzyme in the anti-glycation defence. New insights indicate that increased levels of MGO and the major MGO-derived AGE, methylglyoxal-derived hydroimidazolone 1 (MG-H1), and dysfunctioning of the glyoxalase system are linked to several age-related health problems, such as diabetes, cardiovascular disease, cancer and disorders of the central nervous system. The present review summarizes the mechanisms through which MGO is formed, its detoxification by the glyoxalase system and its effect on biochemical pathways in relation to the development of age-related diseases. Although several scavengers of MGO have been developed over the years, therapies to treat MGO-associated complications are not yet available for application in clinical practice. Small bioactive inducers of GLO1 can potentially form the basis for new treatment strategies for age-related disorders in which MGO plays a pivotal role.

Characterization of rice small heat shock proteins targeted to different cellular organelles

Small heat shock proteins (sHSPs) are a family of ATP-independent molecular chaperones which prevent cellular protein aggregation by binding to misfolded proteins. sHSPs form large oligomers that undergo drastic rearrangement/dissociation in order to execute their chaperone activity in protecting substrates from stress. Substrate-binding sites on sHSPs have been predominantly mapped on their intrinsically disordered N-terminal arms. This region is highly variable in sequence and length across species, and has been implicated in both oligomer formation and in mediating chaperone activity. Here, we present our results on the functional and structural characterization of five sHSPs in rice, each differing in their subcellular localisation, viz., cytoplasm, nucleus, chloroplast, mitochondria and peroxisome. We performed activity assays and dynamic light scattering studies to highlight differences in the chaperone activity and quaternary assembly of sHSPs targeted to various organelles. By cloning constructs that differ in the length and sequence of the tag in the N-terminal region, we have probed the sensitivity of sHSP oligomer assembly and chaperone activity to the length and amino acid composition of the N-terminus. In particular, we have shown that the incorporation of an N-terminal tag has significant consequences on sHSP quaternary structure.

Therapeutic potential of α-crystallin

BACKGROUND

The findings that α-crystallins are multi-functional proteins with diverse biological functions have generated considerable interest in understanding their role in health and disease. Recent studies have shown that chaperone peptides of α-crystallin could be delivered into cultured cells and in experimental animals with beneficial effects against protein aggregation, oxidation, inflammation and apoptosis.

SCOPE OF REVIEW

In this review, we will summarize the latest developments on the therapeutic potential of α-crystallins and their functional peptides.

MAJOR CONCLUSIONS

α-Crystallins and their functional peptides have shown significant favorable effects against several diseases. Their targeted delivery to tissues would be of great therapeutic benefit. However, α-crystallins can also function as disease-causing proteins. These seemingly contradictory functions must be carefully considered prior to their therapeutic use.

GENERAL SIGNIFICANCE

αA and αB-Crystallin are members of the small heat shock protein family. These proteins exhibit molecular chaperone and anti-apoptotic activities. The core crystallin domain within these proteins is largely responsible for these prosperities. Recent studies have identified peptides within the crystallin domain of both α- and αB-crystallins with remarkable chaperone and anti-apoptotic activities. Administration of α-crystallin or their functional peptides has shown substantial inhibition of pathologies in several diseases. However, α-crystallins have been shown to promote disease-causing pathways. These two sides of the proteins are discussed in this review. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Influence of nonenzymatic posttranslational modifications on constitution, oligomerization and receptor binding of S100A12

This study examined the effect of methylglyoxal (MGO)-derived nonenzymatic posttranslational modifications (nePTMs) on the binding affinity of S100A12 to its natural receptor for advanced glycation end-products (RAGE). Binding of MGO-modified S100A12 to RAGE decreased significantly with increasing MGO concentration and incubation time. Ca²⁺-induced S100A12 hexamerization was impaired only at higher MGO concentrations indicating that the loss of affinity is not predominantly caused by disturbance of ligand oligomerization. nePTM mapping showed carboxyethylation of lysine (CEL) and the N-terminus without preferential modification sites. Besides, hydroimidazolone, hemiaminals, argpyrimidine, and tetrahydropyrimidine rapidly formed at R21. Even at the highest modification rate, hexamerization of synthesized CEL-S100A12 was unaffected and RAGE-binding only slightly impaired. Thus, nePTMs at R21 seem to be the major cause of MGO-induced impairment of S100A12 oligomerization and RAGE binding.

Acetylation of Gly1 and Lys2 promotes aggregation of human γD-crystallin

The human lens contains three major protein families: α-, β-, and γ-crystallin. Among the several variants of γ-crystallin in the human lens, γD-crystallin is a major form. γD-Crystallin is primarily present in the nuclear region of the lens and contains a single lysine residue at the second position (K2). In this study, we investigated the acetylation of K2 in γD-crystallin in aging and cataractous human lenses. Our results indicated that K2 is acetylated at an early age and that the amount of K2-acetylated γD-crystallin increased with age. Mass spectrometric analysis revealed that in addition to K2, glycine 1 (G1) was acetylated in γD-crystallin from human lenses and in γD-crystallin acetylated in vitro. The chaperone ability of α-crystallin for acetylated γD-crystallin was lower than that for the nonacetylated protein. The tertiary structure and the microenvironment of the cysteine residues were significantly altered by acetylation. The acetylated protein exhibited higher surface hydrophobicity, was unstable against thermal and chemical denaturation, and exhibited a higher propensity to aggregate at 80 °C in comparison to the nonacetylated protein. Acetylation enhanced the GdnHCl-induced unfolding and slowed the subsequent refolding of γD-crystallin. Theoretical analysis indicated that the acetylation of K2 and G1 reduced the structural stability of the protein and brought the distal cysteine residues (C18 and C78) into close proximity. Collectively, these results indicate that the acetylation of G1 and K2 residues in γD-crystallin likely induced a molten globule-like structure, predisposing it to aggregation, which may account for the high content of aggregated proteins in the nucleus of aged and cataractous human lenses.

Human resistin, a proinflammatory cytokine, shows chaperone-like activity

Resistin, a cysteine-rich adipocytokine, proposed as a link between obesity and diabetes in mice, was shown as a proinflammatory molecule in humans. We earlier reported that human resistin (hRes), a trimer, was resistant to heat and urea denaturation, existed in an oligomeric polydispersed state, and showed a concentration-dependent conformational change. These properties and an intimate correlation of hRes expression with cellular stress prompted us to investigate hRes as a possible chaperone. Here, we show that recombinant human resistin was able to protect the heat-labile enzymes citrate synthase and Nde1 from thermal aggregation and inactivation and was able to refold and restore their enzymatic activities after heat/guanidinium chloride denaturation. Furthermore, recombinant human resistin could bind misfolded proteins only. Molecular dynamics-based association-dissociation kinetics of hRes subunits pointed to resistin being a molecular chaperone. Bis-ANS, which blocks surface hydrophobicity, abrogated the chaperone activity of hRes, establishing the importance of surface hydrophobicity for chaperone activity. Replacement of Phe49 with Tyr (F49YhRes), a critical residue within the hydrophobic patch of hRes, although it could prevent thermal aggregation of citrate synthase and Nde1, was unable to refold and restore their activities. Treatment of U937 cells with tunicamycin/thapsigargin resulted in reduced hRes secretion and concomitant localization in the endoplasmic reticulum. Escherichia coli transformants expressing hRes could be rescued from thermal stress, pointing to hRes's chaperone-like function in vivo. HeLa cells transfected with hRes showed protection from thapsigargin-induced apoptosis. In conclusion, hRes, an inflammatory protein, additionally exhibited chaperone-like properties, suggesting a possible link between inflammation and cellular stress.

Acetylation of lysine 92 improves the chaperone and anti-apoptotic activities of human αB-crystallin

αB-Crystallin is a chaperone and an anti-apoptotic protein that is strongly expressed in many tissues, including the lens, retina, heart, and kidney. In the human lens, several lysine residues in αB-crystallin are acetylated. We have previously shown that such acetylation is predominant at lysine 92 (K92) and lysine 166 (K166). We have investigated the effect of lysine acetylation on the structure and functions of αB-crystallin by the specific introduction of an N(ε)-acetyllysine (AcK) mimic at K92. The introduction of AcK slightly altered the secondary and tertiary structures of the protein. The introduction of AcK also resulted in an increase in the molar mass and hydrodynamic radius of the protein, and the protein became structurally more open and more stable than the native protein. The acetyl protein acquired higher surface hydrophobicity and exhibited 25-55% higher chaperone activity than the native protein. The acetyl protein had more client protein binding per subunit of the protein and higher binding affinity relative to that of the native protein. The acetyl protein was at least 20% more effective in inhibiting chemically induced apoptosis than the native protein. Molecular modeling suggests that acetylation of K92 makes the "α-crystallin domain" more hydrophobic. Together, our results reveal that the acetylation of a single lysine residue in αB-crystallin makes the protein structurally more stable and improves its chaperone and anti-apoptotic activities. Our findings suggest that lysine acetylation of αB-crystallin is an important chemical modification for enhancing αB-crystallin's protective functions in the eye.

Future potential for anti-infectives from bacteria - how to exploit biodiversity and genomic potential

The early stages of antibiotic development include the identification of novel hit compounds. Since actinomycetes and myxobacteria are still the most important natural sources of active metabolites, we provide an overview on these producers and discuss three of the most promising approaches toward finding novel anti-infectives from microorganisms. These are defined as the use of biodiversity to find novel producers, the variation of culture conditions and induction of silent genes, and the exploitation of the genomic potential of producers via "genome mining". Challenges that exist beyond compound discovery are outlined in the last section.

Methylglyoxal, obesity, and diabetes

Methylglyoxal (MG) is a highly reactive compound derived mainly from glucose and fructose metabolism. This metabolite has been implicated in diabetic complications as it is a strong AGE precursor. Furthermore, recent studies suggested a role for MG in insulin resistance and beta-cell dysfunction. Although several drugs have been developed in the recent years to scavenge MG and inhibit AGE formation, we are still far from having an effective strategy to prevent MG-induced mechanisms. This review summarizes the mechanisms of MG formation, detoxification, and action. Furthermore, we review the current knowledge about its implication on the pathophysiology and complications of obesity and diabetes.

The combined effect of acetylation and glycation on the chaperone and anti-apoptotic functions of human α-crystallin

N(ε)-acetylation occurs on select lysine residues in α-crystallin of the human lens and alters its chaperone function. In this study, we investigated the effect of N(ε)-acetylation on advanced glycation end product (AGE) formation and consequences of the combined N(ε)-acetylation and AGE formation on the function of α-crystallin. Immunoprecipitation experiments revealed that N(ε)-acetylation of lysine residues and AGE formation co-occurs in both αA- and αB-crystallin of the human lens. Prior acetylation of αA- and αB-crystallin with acetic anhydride (Ac(2)O) before glycation with methylglyoxal (MGO) resulted in significant inhibition of the synthesis of two AGEs, hydroimidazolone (HI) and argpyrimidine. Similarly, synthesis of ascorbate-derived AGEs, pentosidine and N(ε)-carboxymethyl lysine (CML), was inhibited in both proteins by prior acetylation. In all cases, inhibition of AGE synthesis was positively related to the degree of acetylation. While prior acetylation further increased the chaperone activity of MGO-glycated αA-crystallin, it inhibited the loss of chaperone activity by ascorbate-glycation in both proteins. BioPORTER-mediated transfer of αA- and αB-crystallin into CHO cells resulted in significant protection against hyperthermia-induced apoptosis. This effect was enhanced in acetylated and MGO-modified αA- and αB-crystallin. Caspase-3 activity was reduced in α-crystallin transferred cells. Glycation of acetylated proteins with either MGO or ascorbate produced no significant change in the anti-apoptotic function. Collectively, these data demonstrate that lysine acetylation and AGE formation can occur concurrently in α-crystallin of human lens, and that lysine acetylation improves anti-apoptotic function of α-crystallin and prevents ascorbate-mediated loss of chaperone function.