Crystallins, genes and cataract

Making a head: Neural crest and ectodermal placodes in cranial sensory development

During development of the vertebrate sensory system, many important components like the sense organs and cranial sensory ganglia arise within the head and neck. Two progenitor populations, the neural crest, and cranial ectodermal placodes, contribute to these developing vertebrate peripheral sensory structures. The interactions and contributions of these cell populations to the development of the lens, olfactory, otic, pituitary gland, and cranial ganglia are vital for appropriate peripheral nervous system development. Here, we review the origins of both neural crest and placode cells at the neural plate border of the early vertebrate embryo and investigate the molecular and environmental signals that influence specification of different sensory regions. Finally, we discuss the underlying molecular pathways contributing to the complex vertebrate sensory system from an evolutionary perspective, from basal vertebrates to amniotes.

Identification of proteome markers for drug-induced liver injury in zebrafish embryos

The zebrafish embryo (ZFE) is a promising alternative non-rodent model in toxicology, and initial studies suggested its applicability in detecting hepatic responses related to drug-induced liver injury (DILI). Here, we hypothesize that detailed analysis of underlying mechanisms of hepatotoxicity in ZFE contributes to the improved identification of hepatotoxic properties of compounds and to the reduction of rodents used for hepatotoxicity assessment. ZFEs were exposed to nine reference hepatotoxicants, targeted at induction of steatosis, cholestasis, and necrosis, and effects compared with negative controls. Protein profiles of the individual compounds were generated using LC-MS/MS. We identified differentially expressed proteins and pathways, but as these showed considerable overlap, phenotype-specific responses could not be distinguished. This led us to identify a set of common hepatotoxicity marker proteins. At the pathway level, these were mainly associated with cellular adaptive stress-responses, whereas single proteins could be linked to common hepatotoxicity-associated processes. Applying several stringency criteria to our proteomics data as well as information from other data sources resulted in a set of potential robust protein markers, notably Igf2bp1, Cox5ba, Ahnak, Itih3b.2, Psma6b, Srsf3a, Ces2b, Ces2a, Tdo2b, and Anxa1c, for the detection of adverse responses.

Case Report: A de novo Variant of CRYGC Gene Associated With Congenital Cataract and Microphthalmia

Background: Congenital cataract is one of the most common causes of blindness in children. A rapid and accurate genetic diagnosis benefit the patients in the pediatric department. The current study aims to identify the genetic defects in a congenital cataract patient without a family history.

Case presentation: A congenital cataract patient with microphthalmia and nystagmus was recruited for this study. Trio-based whole-exome sequencing revealed a de novo variant (c.394delG, p.V132Sfs*15) in CRYGC gene. According to the American College of Medical Genetics and Genomics (ACMG) criteria, the variant could be annontated as pathogenic.

Conclusion: Our findings provide new knowledge of the variant spectrum of CRYGC gene and are essential for understanding the heterogeneity of cataracts in the Chinese population.

Congenital cataract-causing mutation βB1-L116P is prone to amyloid fibrils aggregation and protease degradation with low structural stability

Congenital cataract, a common disease with lens opacification, causes blindness in the newborn worldwide and is mainly caused by abnormal aggregation of crystallin. As the main structural protein in the mammalian lens, βB1-crystallin has an important role in the maintenance of lens transparency. Recently, the L116P mutation in βB1-CRY was found in a Chinese family with congenital nuclear cataracts, while its underlying pathogenic mechanism remains unclear. In the current study, the βB1 wild-type protein was purified, and the mutated form, βB1-L116P, was examined for examining the effect on structural stability and susceptibility against environmental stresses. Our results reveal low solubility and structural stability of βB1-L116P at physiological temperature, which markedly impaired the protein structure and the oligomerization of βB1-crystallin. Under guanidine hydrochloride-induced denaturing conditions, βB1-L116P mutation perturbed the protein unfolding process, making it prone to amyloid fibrils aggregation. More importantly, the L116P mutation increased susceptibility of βB1-crystallin against UV radiation. βB1-L116P overexpression led to the formation of more serious intracellular aggresomes under UV radiation or oxidative stress. Furthermore, the βB1-L116P mutation increased the sensitivity to the proteolysis process. These results indicate that the low structural stability, susceptibility to amyloid fibrils aggregation, and protease degradation of βB1-L116P may contribute to cataract development and associated symptoms.

Disruption of PIKFYVE causes congenital cataract in human and zebrafish

Congenital cataract, an ocular disease predominantly occurring within the first decade of life, is one of the leading causes of blindness in children. However, the molecular mechanisms underlying the pathogenesis of congenital cataract remain incompletely defined. Through whole-exome sequencing of a Chinese family with congenital cataract, we identified a potential pathological variant (p.G1943E) in PIKFYVE, which is located in the PIP kinase domain of the PIKFYVE protein. We demonstrated that heterozygous/homozygous disruption of PIKFYVE kinase domain, instead of overexpression of PIKFYVE^G1943E in zebrafish mimicked the cataract defect in human patients, suggesting that haploinsufficiency, rather than dominant-negative inhibition of PIKFYVE activity caused the disease. Phenotypical analysis of pikfyve zebrafish mutants revealed that loss of Pikfyve caused aberrant vacuolation (accumulation of Rab7⁺Lc3⁺ amphisomes) in lens cells, which was significantly alleviated by treatment with the V-ATPase inhibitor bafilomycin A1 (Baf-A1). Collectively, we identified PIKFYVE as a novel causative gene for congenital cataract and pinpointed the potential application of Baf-A1 for the treatment of congenital cataract caused by PIKFYVE deficiency.

Cataract-causing mutation R48C increases γA-crystallin susceptibility to oxidative stress and ultraviolet radiation

Among all congenital cataracts caused by genetic mutations, approximately half are caused by a mutation in crystallin genes, and accounts the leading cause of blindness in children globally. In this study, we investigated the underlying molecular mechanism of R48C mutation (c.142C > T; p.[Arg48Cys]) of γA-crystallin in a Mexican-Mestizo descent family causing congenital cataracts. We purified γA-crystallin wild-type (WT) and R48C mutant and compared their structural characteristics and biophysical properties by Spectroscopic experiments and environmental stress (oxidative stress, ultraviolet irradiation, pH disorders, thermal shock, or chemical denaturation). The R48C mutant did not affect the secondary and tertiary structure of monomer γA-crystallin, nor did it affect its stability to heat shock and chemicals. However, the R48C mutant destroys the oxidative stability of γA-crystallin, which makes the protein more prone to aggregation and precipitation under oxidative conditions. These might be the pathogenesis of γA-crystallin R48C mutant related to congenital cataract and help to develop anti-cataract strategies from the perspective of γA-crystallin.

Molecular and Genetic Mechanism of Non-Syndromic Congenital Cataracts. Mutation Screening in Spanish Families

Our purpose was to identify mutations responsible for non-syndromic congenital cataracts through the implementation of next-generation sequencing (NGS) in our center. A sample of peripheral blood was obtained from probands and willing family members and genomic DNA was extracted from leukocytes. DNA was analyzed implementing a panel (OFTv2.1) including 39 known congenital cataracts disease genes. 62 probands from 51 families were recruited. Pathogenic or likely pathogenic variants were identified in 32 patients and 25 families; in 16 families (64%) these were de novo mutations. The mutation detection rate was 49%. Almost all reported mutations were autosomal dominant. Mutations in crystallin genes were found in 30% of the probands. Mutations in membrane proteins were detected in seven families (two in GJA3 and five in GJA8). Mutations in LIM2 and MIP were each found in three families. Other mutations detected affected EPHA2, PAX6, HSF4 and PITX3. Variants classified as of unknown significance were found in 5 families (9.8%), affecting CRYBB3, LIM2, EPHA2, ABCB6 and TDRD7. Mutations lead to different cataract phenotypes within the same family.

Melittin, a honeybee venom derived peptide for the treatment of chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is the most prevalent neurological complication of cancer treatment which involves sensory and motor nerve dysfunction. Severe CIPN has been reported in around 5% of patients treated with single and up to 38% of patients treated with multiple chemotherapeutic agents. Present medications available for CIPN are the use of opioids, nonsteroidal anti-inflammatory agents, and tricyclic antidepressants, which are only marginally effective in treating neuropathic symptoms. In reality, symptom reappears after these drugs are discontinued. The pathogenesis of CIPN has not been sufficiently recognized and methods for the prevention and treatment of CIPN remain vulnerable to therapeutic problems. It has witnessed that the present medicines available for the disease offer only symptomatic relief for the short term and have severe adverse side effects. There is no standard treatment protocol for preventing, reducing, and treating CIPN. Therefore, there is a need to develop curative therapy that can be used to treat this complication. Melittin is the main pharmacological active constituent of honeybee venom and has therapeutic values including in chemotherapeutic-induced peripheral neuropathy. It has been shown that melittin and whole honey bee venom are effective in treating paclitaxel and oxaliplatin-induced peripheral neuropathy. The use of melittin against peripheral neuropathy caused by chemotherapy has been limited despite having strong therapeutic efficacy against the disease. Melittin mediated haemolysis is the key reason to restrict its use. In our study, it is found that α-Crystallin (an eye lens protein) is capable of inhibiting melittin-induced haemolysis which gives hope of using an appropriate combination of melittin and α-Crystallin in the treatment of CIPN. The review summarizes the efforts made by different research groups to address the concern with melittin in the treatment of chemotherapeutic-induced neuropathy. It also focuses on the possible approaches to overcome melittin-induced haemolysis.

The genetic landscape of crystallins in congenital cataract

Background

The crystalline lens is mainly composed of a large family of soluble proteins called the crystallins, which are responsible for its development, growth, transparency and refractive index. Disease-causing sequence variants in the crystallins are responsible for nearly 50% of all non-syndromic inherited congenital cataracts, as well as causing cataract associated with other diseases, including myopathies. To date, more than 300 crystallin sequence variants causing cataract have been identified.

Methods

Here we aimed to identify the genetic basis of disease in five multi-generation British families and five sporadic cases with autosomal dominant congenital cataract using whole exome sequencing, with identified variants validated using Sanger sequencing. Following bioinformatics analysis, rare or novel variants with a moderate to damaging pathogenicity score, were filtered out and tested for segregation within the families.

Results

We have identified 10 different heterozygous crystallin variants. Five recurrent variants were found: family-A, with a missense variant (c.145C>T; p.R49C) in CRYAA associated with nuclear cataract; family-B, with a deletion in CRYBA1 (c.272delGAG; p.G91del) associated with nuclear cataract; and family-C, with a truncating variant in CRYGD (c.470G>A; W157*) causing a lamellar phenotype; individuals I and J had variants in CRYGC (c.13A>C; T5P) and in CRYGD (c.418C>T; R140*) causing unspecified congenital cataract and nuclear cataract, respectively. Five novel disease-causing variants were also identified: family D harboured a variant in CRYGC (c.179delG; R60Qfs*) responsible for a nuclear phenotype; family E, harboured a variant in CRYBB1 (c.656G>A; W219*) associated with lamellar cataract; individual F had a variant in CRYGD (c.392G>A; W131*) associated with nuclear cataract; and individuals G and H had variants in CRYAA (c.454delGCC; A152del) and in CRYBB1 (c.618C>A; Y206*) respectively, associated with unspecified congenital cataract. All novel variants were predicted to be pathogenic and to be moderately or highly damaging.

Conclusions

We report five novel variants and five known variants. Some are rare variants that have been reported previously in small ethnic groups but here we extend this to the wider population and record a broader phenotypic spectrum for these variants.

Inherited cataracts: molecular genetics, clinical features, disease mechanisms and novel therapeutic approaches

Cataract is the most common cause of blindness in the world; during infancy and early childhood, it frequently results in visual impairment. Congenital cataracts are phenotypically and genotypically heterogeneous and can occur in isolation or in association with other systemic disorders. Significant progress has been made in identifying the molecular genetic basis of cataract; 115 genes to date have been found to be associated with syndromic and non-syndromic cataract and 38 disease-causing genes have been identified to date to be associated with isolated cataract. In this review, we briefly discuss lens development and cataractogenesis, detail the variable cataract phenotypes and molecular mechanisms, including genotype-phenotype correlations, and explore future novel therapeutic avenues including cellular therapies and pharmacological treatments.

Transcriptional profiling of single fiber cells in a transgenic paradigm of an inherited childhood cataract reveals absence of molecular heterogeneity

Our recent single-cell transcriptomic analysis has demonstrated that heterogeneous transcriptional activity attends molecular transition from the nascent to terminally differentiated fiber cells in the developing mouse lens. To understand the role of transcriptional heterogeneity in terminal differentiation and the functional phenotype (transparency) of this tissue, here we present a single-cell analysis of the developing lens, in a transgenic paradigm of an inherited pathology, known as the lamellar cataract. Cataracts hinder transmission of light into the eye. Lamellar cataract is the most prevalent bilateral childhood cataract. In this disease of early infancy, initially, the opacities remain confined to a few fiber cells, thus presenting an opportunity to investigate early molecular events that lead to cataractogenesis. We used a previously established paradigm that faithfully recapitulates this disease in transgenic mice. About 500 single fiber cells, manually isolated from a 2-day-old transgenic lens were interrogated individually for the expression of all known 17 crystallins and 78 other relevant genes using a Biomark HD (Fluidigm). We find that fiber cells from spatially and developmentally discrete regions of the transgenic (cataract) lens show remarkable absence of the heterogeneity of gene expression. Importantly, the molecular variability of cortical fiber cells, the hallmark of the WT lens, is absent in the transgenic cataract, suggesting absence of specific cell-type(s). Interestingly, we find a repetitive pattern of gene activity in progressive states of differentiation in the transgenic lens. This molecular dysfunction portends pathology much before the physical manifestations of the disease.

Alzheimer's Disease Is Responsible for Progressive Age-Dependent Differential Expression of Various Protein Cascades in Retina of Mice

Alzheimer's disease (AD) is a devastating neurodegenerative disease associated with cognitive impairments and memory loss usually in aged people. In the past few years, it has been detected in the eye retina, manifesting the systematic spread of the disease. This might be used for biomarker discovery for early detection and treatment of the disease. Here, we have described the proteomic alterations in retina of 2, 4, and 6 months old 3×Tg-AD mice by using iTRAQ (isobaric tags for relative and absolute quantification) proteomics technology. Out of the total identified proteins, 121 (71 up- and 50 down-regulated), 79 (51 up- and 28 down-regulated), and 153 (37 up- and 116 down-regulated) significantly differentially expressed proteins (DEPs) are found in 2, 4, and 6 month's mice retina (2, 4, and 6 M), respectively. Seventeen DEPs are found common in these three groups with consistent expression behavior or opposite expression in the three groups. Bioinformatics analysis of these DEPs highlighted their involvement in vital AD-related biological phenomenon. To further prompt the results, four proteins from 2 M group, three from 4 M, and four from 6 M age groups are successfully validated with Western blot analysis. This study confirms the retinal involvement of AD in the form of proteomic differences and further explains the protein-based molecular mechanisms, which might be a step toward biomarker discovery for early detection and treatment of the disease.

Purification and Functional Characterization of the C-Terminal Domain of the β-Actin-Binding Protein AIM1 In Vitro

The protein absent in melanoma 1 (AIM1) is a member of the βγ-crystal lens superfamily that is associated with the development of multiple cancers. The binding of AIM1 to β-actin affects the migration and invasion of prostate cancer epithelial cells. The C-terminus of AIM1 is required for the β-actin interaction. However, the characteristics of AIM1 in vitro and the interaction mode between AIM1 and β-actin remain unknown. We describe novel methods to prepare pure recombinant AIM1 and identify possible binding modes between AIM1 and β-actin; we also obtain the crystal of the first two βγ-crystallin domains of AIM1 (g1g2) for future structural biology research. We first express and purify AIM1 after cloning the sequence into a modified pET-28a_psp expression vector. Next, we define the minimum unit formed by the βγ-crystallin domain repeats that bound to β-actin and perform its physiological function. Finally, we made the structural model of the AIM1 g1g2 that can be used to guide future biomedical investigations and prostate cancer research.

Spatial Analysis of Single Fiber Cells of the Developing Ocular Lens Reveals Regulated Heterogeneity of Gene Expression

The developing eye lens presents an exceptional paradigm for spatial transcriptomics. It is composed of highly organized long, slender transparent fiber cells, which differentiate from the edges of the anterior epithelium of the lens (equator), attended by high expression of crystallins, which generates transparency. Every fiber cell, therefore, is an optical unit whose refractive properties derive from its gene activity. Here, we probe this tangible relationship between the gene activity and the phenotype by studying the expression of all known 17 crystallins and 77 other non-crystallin genes in single fiber cells isolated from three states/regions of differentiation, allowing us to follow molecular progression at the single-cell level. The data demonstrate highly variable gene activity in cortical fibers, interposed between the nascent and the terminally differentiated fiber cell transcription. These data suggest that the so-called stochastic, highly heterogeneous gene activity is a regulated intermediate in the realization of a functional phenotype.

Inhibition of the Expression of the Small Heat Shock Protein αB-Crystallin Inhibits Exosome Secretion in Human Retinal Pigment Epithelial Cells in Culture

Exosomes carry cell type-specific molecular cargo to extracellular destinations and therefore act as lateral vectors of intercellular communication and transfer of genetic information from one cell to the other. We have shown previously that the small heat shock protein αB-crystallin (αB) is exported out of the adult human retinal pigment epithelial cells (ARPE19) packaged in exosomes. Here, we demonstrate that inhibition of the expression of αB via shRNA inhibits exosome secretion from ARPE19 cells indicating that exosomal cargo may have a role in exosome biogenesis (synthesis and/or secretion). Sucrose density gradient fractionation of the culture medium and cellular extracts suggests continued synthesis of exosomes but an inhibition of exosome secretion. In cells where αB expression was inhibited, the distribution of CD63 (LAMP3), an exosome marker, is markedly altered from the normal dispersed pattern to a stacked perinuclear presence. Interestingly, the total anti-CD63(LAMP3) immunofluorescence in the native and αB-inhibited cells remains unchanged suggesting continued exosome synthesis under conditions of impaired exosome secretion. Importantly, inhibition of the expression of αB results in a phenotype of the RPE cell that contains an increased number of vacuoles and enlarged (fused) vesicles that show increased presence of CD63(LAMP3) and LAMP1 indicating enhancement of the endolysosomal compartment. This is further corroborated by increased Rab7 labeling of this compartment (RabGTPase 7 is known to be associated with late endosome maturation). These data collectively point to a regulatory role for αB in exosome biogenesis possibly via its involvement at a branch point in the endocytic pathway that facilitates secretion of exosomes.

Bioinformatics Analysis of Potential Candidates for Therapy of TDRD7 Deficiency-Induced Congenital Cataract

AIMS

The aim of this study was to identify potential candidates and explore the possible mechanism in congenital cataract induced by tudor domain-containing 7 (TDRD7) deficiency.

METHODS

The gene expression profile GSE25812 generated from 18 samples was downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) between disease and normal groups were identified. Then, gene ontology and pathway enrichment analysis of DEGs were performed. The protein-protein interaction (PPI) network and transcription factor (TF) regulatory network were constructed. The modules in the PPI network were identified. Significant target genes were selected from the TF regulatory network.

RESULTS

A total of 329 DEGs were obtained, and downregulated DEGs were significantly enriched in biological processes including defense response and immune response. In the PPI network, high-degree genes of complement component 1, q subcomponent, A/B/C chain (C1QA/C1QB/C1QC), lymphocyte antigen 86 (LY86) and neuroblastoma RAS viral oncogene homolog (NRAS) were identified. From the TF regulatory network, the heat shock 27 kDa protein 1 (HSPB1) was the target of the estrogen receptor 1, and LY86 was the target of the v-myc avian myelocytomatosis viral oncogene homolog.

CONCLUSION

HSPB1, NRAS, immune response, defense response and the related genes LY86, C1QA/C1QB/C1QC may play an important role in the development of congenital cataract induced by TDRD7 deficiency. However, further experiments are still needed.

Small heat-shock proteins: important players in regulating cellular proteostasis

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of post-translational modifications in sHsp chaperone function, particularly in the context of disease.

Expression of the HSF4 DNA binding domain-EGFP hybrid gene recreates early childhood lamellar cataract in transgenic mice

PURPOSE

The clinical management of cataracts in infancy involves surgical removal of the lens to ensure transmission of light to the retina, which is essential for normal neural development of the infant. This surgery, however, entails a lifelong follow-up and impaired vision. To our knowledge, no animal models recapitulate human lamellar opacities, the most prevalent form of early childhood cataracts. We present data on the recreation of the human lamellar cataract phenotype in transgenic mice.

METHODS

Mutations in the DNA binding domain (DBD) of the heat shock transcription factor 4 (HSF4) are known to be associated with early childhood autosomal dominant lamellar cataract. We used bacterial artificial chromosome (BAC) transgenesis to express a hybrid gene: Hsf4 (DBD)-enhanced green fluorescent protein (EGFP), by recombineering EGFP sequences into the DBD of the Hsf4 gene, to interfere with the DNA binding properties of Hsf4.

RESULTS

We recapitulated the human lamellar cataract, in its temporal as well as spatial presentation, within the transgenic mouse lens. This phenotype was reproduced faithfully using four different BACs, indicating that EGFP can be used to target transcription factor function in transgenic mice. Molecular and cell biological examination of early postnatal transgenic lens reveals impairment of secondary fiber cell differentiation.

CONCLUSIONS

Recreation of the human lamellar cataract phenotype in mice allows investigation of this human pathology at a level not possible previously and points to the relevance of fiber cell heterogeneity dictated by fiber cell-specific gene activity in the biogenesis of the lamellar cataract.

A collaborative visual analytics suite for protein folding research

Molecular dynamics (MD) simulation is a crucial tool for understanding principles behind important biochemical processes such as protein folding and molecular interaction. With the rapidly increasing power of modern computers, large-scale MD simulation experiments can be performed regularly, generating huge amounts of MD data. An important question is how to analyze and interpret such massive and complex data. One of the (many) challenges involved in analyzing MD simulation data computationally is the high-dimensionality of such data. Given a massive collection of molecular conformations, researchers typically need to rely on their expertise and prior domain knowledge in order to retrieve certain conformations of interest. It is not easy to make and test hypotheses as the data set as a whole is somewhat "invisible" due to its high dimensionality. In other words, it is hard to directly access and examine individual conformations from a sea of molecular structures, and to further explore the entire data set. There is also no easy and convenient way to obtain a global view of the data or its various modalities of biochemical information. To this end, we present an interactive, collaborative visual analytics tool for exploring massive, high-dimensional molecular dynamics simulation data sets. The most important utility of our tool is to provide a platform where researchers can easily and effectively navigate through the otherwise "invisible" simulation data sets, exploring and examining molecular conformations both as a whole and at individual levels. The visualization is based on the concept of a topological landscape, which is a 2D terrain metaphor preserving certain topological and geometric properties of the high dimensional protein energy landscape. In addition to facilitating easy exploration of conformations, this 2D terrain metaphor also provides a platform where researchers can visualize and analyze various properties (such as contact density) overlayed on the top of the 2D terrain. Finally, the software provides a collaborative environment where multiple researchers can assemble observations and biochemical events into storyboards and share them in real time over the Internet via a client-server architecture. The software is written in Scala and runs on the cross-platform Java Virtual Machine. Binaries and source code are available at http://www.aylasoftware.org and have been released under the GNU General Public License.

Deprivation amblyopia and congenital hereditary cataract

Amblyopia is a neurodevelopmental disorder of vision associated with decreased visual acuity, poor or absent stereopsis, and suppression of information from one eye.(1,2) Amblyopia may be caused by strabismus (strabismic amblyopia), refractive error (anisometropic amblyopia), or deprivation from obstructed vision (deprivation amblyopia). 1 In the developed world, amblyopia is the most common cause of childhood visual impairment, 3 which reduces quality of life 4 and also almost doubles the lifetime risk of legal blindness.(5, 6) Successful treatment of amblyopia greatly depends on early detection and treatment of predisposing disorders such as congenital cataract, which is the most common cause of deprivational amblyopia. Understanding the genetic causes of congenital cataract leads to more effective screening tests, early detection and treatment of infants and children who are at high risk for hereditary congenital cataract.

A gene-specific non-enhancer sequence is critical for expression from the promoter of the small heat shock protein gene αB-crystallin

BACKGROUND

Deciphering of the information content of eukaryotic promoters has remained confined to universal landmarks and conserved sequence elements such as enhancers and transcription factor binding motifs, which are considered sufficient for gene activation and regulation. Gene-specific sequences, interspersed between the canonical transacting factor binding sites or adjoining them within a promoter, are generally taken to be devoid of any regulatory information and have therefore been largely ignored. An unanswered question therefore is, do gene-specific sequences within a eukaryotic promoter have a role in gene activation? Here, we present an exhaustive experimental analysis of a gene-specific sequence adjoining the heat shock element (HSE) in the proximal promoter of the small heat shock protein gene, αB-crystallin (cryab). These sequences are highly conserved between the rodents and the humans.

RESULTS

Using human retinal pigment epithelial cells in culture as the host, we have identified a 10-bp gene-specific promoter sequence (GPS), which, unlike an enhancer, controls expression from the promoter of this gene, only when in appropriate position and orientation. Notably, the data suggests that GPS in comparison with the HSE works in a context-independent fashion. Additionally, when moved upstream, about a nucleosome length of DNA (-154 bp) from the transcription start site (TSS), the activity of the promoter is markedly inhibited, suggesting its involvement in local promoter access. Importantly, we demonstrate that deletion of the GPS results in complete loss of cryab promoter activity in transgenic mice.

CONCLUSIONS

These data suggest that gene-specific sequences such as the GPS, identified here, may have critical roles in regulating gene-specific activity from eukaryotic promoters.

Ca2+-binding motif of βγ-crystallins

βγ-Crystallin-type double clamp (N/D)(N/D)XX(S/T)S motif is an established but sparsely investigated motif for Ca(2+) binding. A βγ-crystallin domain is formed of two Greek key motifs, accommodating two Ca(2+)-binding sites. βγ-Crystallins make a separate class of Ca(2+)-binding proteins (CaBP), apparently a major group of CaBP in bacteria. Paralleling the diversity in βγ-crystallin domains, these motifs also show great diversity, both in structure and in function. Although the expression of some of them has been associated with stress, virulence, and adhesion, the functional implications of Ca(2+) binding to βγ-crystallins in mediating biological processes are yet to be elucidated.

Thermal stress induced aggregation of aquaporin 0 (AQP0) and protection by α-crystallin via its chaperone function

Aquaporin 0 (AQP0) formerly known as membrane intrinsic protein (MIP), is expressed exclusively in the lens during terminal differentiation of fiber cells. AQP0 plays an important role not only in the regulation of water content but also in cell-to-cell adhesion of the lens fiber cells. We have investigated the thermal stress-induced structural alterations of detergent (octyl glucoside)-solubilized calf lens AQP0. The results show an increase in the amount of AQP0 that aggregated as the temperature increased from 40°C to 65°C. α-Crystallin, molecular chaperone abundantly present in the eye lens, completely prevented the AQP0 aggregation at a 1∶1 (weight/weight) ratio. Since α-crystallin consists of two gene products namely αA- and αB-crystallins, we have tested the recombinant proteins on their ability to prevent thermal-stress induced AQP0 aggregation. In contrast to the general observation made with other target proteins, αA-crystallin exhibited better chaperone-like activity towards AQP0 compared to αB-crystallin. Neither post-translational modifications (glycation) nor C-terminus truncation of AQP0 have any appreciable effect on its thermal aggregation properties. α-Crystallin offers similar protection against thermal aggregation as in the case of the unmodified AQP0, suggesting that αcrystallin may bind to either intracellular loops or other residues of AQP0 that become exposed during thermal stress. Far-UV circular dichroism studies indicated a loss of αhelical structures when AQP0 was subjected to temperatures above 45°C, and the presence of α-crystallin stabilized these secondary structures. We report here, for the first time, that α-crystallin protects AQP0 from thermal aggregation. Since stress-induced structural perturbations of AQP0 may affect the integrity of the lens, presence of the molecular chaperone, α-crystallin (particularly αA-crystallin) in close proximity to the lens membrane is physiologically relevant.

Cell-type-dependent access of HSF1 and HSF4 to αB-crystallin promoter during heat shock

Epithelial cells and fibroblasts both express heat shock transcription factors, HSF1 and HSF4, yet they respond to heat shock differentially. For example, while HSP70 is induced in both cell types, the small heat shock protein, αB-crystallin gene (CRYAB) that contains a canonical heat shock promoter, is only induced in fibroblasts. A canonical heat shock promoter contains three or more inverted repeats of the pentanucleotide 5'-nGAAn-3' that make the heat shock element. It is known that, in vitro, promoter architecture (the order and spacing of these repeats) impacts the interaction of various heat shock transcription factors (HSFs) with the heat shock promoter, but in vivo relevance of these binding preferences so far as the expression is concerned is poorly understood. In this report, we first establish cell-type-dependent differential expression of CRYAB in four established cell lines and then working with adult human retinal pigment epithelial cells and NIH3T3 fibroblasts and employing chromatin immunoprecipitation, attempt to relate expression to promoter occupancy by HSF1 and HSF4. We show that HSF4 occupies only CRYAB and not HSP70 promoter in epithelial cells, while HSF1 occupies only HSP70 promoter in both cell types, and cryab promoter, only in heat shocked fibroblasts; HSF4, on the other hand, is never seen on these two promoters in NIH3T3 fibroblasts. This comparative analysis with CRYAB and HSP70 demonstrates that differential heat shock response is controlled by cell-type-dependent access of HSFs (HSF1 and HSF4) to specific promoters, independent of the promoter architecture.

αA-crystallin and αB-crystallin reside in separate subcellular compartments in the developing ocular lens

αA-Crystallin (αA) and αB-crystallin (αB), the two prominent members of the small heat shock family of proteins are considered to be two subunits of one multimeric protein, α-crystallin, within the ocular lens. Outside of the ocular lens, however, αA and αB are known to be two independent proteins, with mutually exclusive expression in many tissues. This dichotomous view is buoyed by the high expression of αA and αB in the lens and their co-fractionation from lens extracts as one multimeric entity, α-crystallin. To understand the biological function(s) of each of these two proteins, it is important to investigate the biological basis of this perceived dichotomy; in this report, we address the question whether αA and αB exist as independent proteins in the ocular lens. Discontinuous sucrose density gradient fractionation and immunoconfocal localization reveal that in early developing rat lens αA is a membrane-associated small heat shock protein similar to αB but with remarkable differences. Employing an established protocol, we demonstrate that αB predominantly sediments with rough endoplasmic reticulum, whereas αA fractionates with smooth membranes. These biochemical observations were corroborated with immunogold labeling and transmission electron microscopy. Importantly, in the rat heart also, which does not contain αA, αB fractionates with rough endoplasmic reticulum, suggesting that αA has no influence on the distribution of αB. These data demonstrate presence of αA and αB in two separate subcellular membrane compartments, pointing to their independent existence in the developing ocular lens.

Secretion of αB-Crystallin via exosomes: New clues to the function of human retinal pigment epithelium

αB-crystallin (αB) is an archetypical small heat shock protein whose physiological function is not clearly defined. The interest in this protein arises from its well-established but poorly understood association with a myriad of neurodegenerative diseases, cancer and cardiomyopathies. The discovery of the secretion of αB from human adult retinal pigment epithelial cells (ARPE19) via exosomes not only points to the involvement of this protein in lateral transfer of information between cells in the visual system but also to the status of this protein as a potential ligand that may activate or modulate immune and stress responses, normal growth and oncogenic pathways. Retinal pigment epithelium (RPE) is a single layer of polarized cells that supports photoreceptor physiology and function. We have initiated investigations on understanding the origin of the elevated levels of αB in extracellular sub-retinal proteolipid deposits (known as "drusen") associated with the death of photoreceptor neurons in age-related macular degeneration (AMD). Here we discuss the potential implications of the presence and transport of αB in exosomes across cell membranes in RPE.

Small heat shock protein αA-crystallin prevents photoreceptor degeneration in experimental autoimmune uveitis

The small heat shock protein, αA-crystallin null (αA−/−) mice are known to be more prone to retinal degeneration than the wild type mice in Experimental Autoimmune Uveoretinitis (EAU). In this report we demonstrate that intravenous administration of αA preserves retinal architecture and prevents photoreceptor damage in EAU. Interestingly, only αA and not αB-crystallin (αB), a closely related small heat shock protein works, pointing to molecular specificity in the observed retinal protection. The possible involvement of αA in retinal protection through immune modulation is corroborated by adaptive transfer experiments, (employing αA−/− and wild type mice with EAU as donors and Rag2−/− as the recipient mice), which indicate that αA protects against the autoimmune challenge by modulating the systemic B and T cell immunity. We show that αA administration causes marked reduction in Th1 cytokines (TNF-α, IL-12 and IFN-γ), both in the retina and in the spleen; notably, IL-17 was only reduced in the retina suggesting local intervention. Importantly, expression of Toll-like receptors and their associated adaptors is also inhibited suggesting that αA protection, against photoreceptor loss in EAU, is associated with systemic suppression of both the adaptive and innate immune responses.

Methionine sulfoxide reductase A: Structure, function and role in ocular pathology

Methionine is a highly susceptible amino acid that can be oxidized to S and R diastereomeric forms of methionine sulfoxide by many of the reactive oxygen species generated in biological systems. Methionine sulfoxide reductases (Msrs) are thioredoxin-linked enzymes involved in the enzymatic conversion of methionine sulfoxide to methionine. Although MsrA and MsrB have the same function of methionine reduction, they differ in substrate specificity, active site composition, subcellular localization, and evolution. MsrA has been localized in different ocular regions and is abundantly expressed in the retina and in retinal pigment epithelial (RPE) cells. MsrA protects cells from oxidative stress. Overexpression of MsrA increases resistance to cell death, while silencing or knocking down MsrA decreases cell survival; events that are mediated by mitochondria. MsrA participates in protein-protein interaction with several other cellular proteins. The interaction of MsrA with α-crystallins is of utmost importance given the known functions of the latter in protein folding, neuroprotection, and cell survival. Oxidation of methionine residues in α-crystallins results in loss of chaperone function and possibly its antiapoptotic properties. Recent work from our laboratory has shown that MsrA is co-localized with αA and αB crystallins in the retinal samples of patients with age-related macular degeneration. We have also found that chemically induced hypoxia regulates the expression of MsrA and MsrB2 in human RPE cells. Thus, MsrA is a critical enzyme that participates in cell and tissue protection, and its interaction with other proteins/growth factors may provide a target for therapeutic strategies to prevent degenerative diseases.

Transcriptional response of marine medaka (Oryzias javanicus) on exposure to toxaphene

Differential gene expression profiles were established from the head and liver tissues of the marine medaka fish (Oryzias javanicus) after its exposure to toxaphene, a persistent organic pollutant and insecticide, using differential display polymerase chain reaction. Twenty-seven differentially expressed genes were identified, which were associated with the cytoskeleton, development, metabolism, nucleic acid/protein binding, and signal transduction. Among these genes, those encoding molecular biomarkers known to be involved in metabolism, ATP hydrolysis, and protein regulation were strongly induced at the transcription level, and genes encoding one structural protein subunit or involved in lipid metabolism were strongly downregulated. The same trends in gene expression changes were observed with real-time PCR analysis of 12 selected clones. The genes identified could be used as molecular biomarkers of biological responses to polychlorinated camphene contamination in aquatic environments.

Heat shock proteins in the human eye

Heat shock proteins (Hsps) are believed to primarily protect and maintain cell viability under stressful conditions such as those occurring during thermal and oxidative challenges chiefly by refolding and stabilizing proteins. Hsps are found throughout the various tissues of the eye where they are thought to confer protection from disease states such as cataract, glaucoma, and cancer. This minireview summarizes the placement, properties, and roles of Hsps in the eye and aims to provide a better comprehension of their function and involvement in ocular disease pathogenesis.

AlphaB-crystallin is found in detergent-resistant membrane microdomains and is secreted via exosomes from human retinal pigment epithelial cells

αB-crystallin (αB) is known as an intracellular Golgi membrane-associated small heat shock protein. Elevated levels of this protein have been linked with a myriad of neurodegenerative pathologies including Alzheimer disease, multiple sclerosis, and age-related macular degeneration. The membrane association of αB has been known for more than 3 decades, yet its physiological import has remained unexplained. In this investigation we show that αB is secreted from human adult retinal pigment epithelial cells via microvesicles (exosomes), independent of the endoplasmic reticulum-Golgi protein export pathway. The presence of αB in these lipoprotein structures was confirmed by its susceptibility to digestion by proteinase K only when exosomes were exposed to Triton X-100. Transmission electron microscopy was used to localize αB in immunogold-labeled intact and permeabilized microvesicles. The saucer-shaped exosomes, with a median diameter of 100-200 nm, were characterized by the presence of flotillin-1, α-enolase, and Hsp70, the same proteins that associate with detergent-resistant membrane microdomains (DRMs), which are known to be involved in their biogenesis. Notably, using polarized adult retinal pigment epithelial cells, we show that the secretion of αB is predominantly apical. Using OptiPrep gradients we demonstrate that αB resides in the DRM fraction. The secretion of αB is inhibited by the cholesterol-depleting drug, methyl β-cyclodextrin, suggesting that the physiological function of this protein and the regulation of its export through exosomes may reside in its association with DRMs/lipid rafts.

Importance of eye lens α-crystallin heteropolymer with 3:1 αA to αB ratio: stability, aggregation, and modifications

Chaperone-like activity (CLA) of α-crystallin is essential not only for the maintenance of eye lens transparency but also in the biology of other tissues. Eye lens α-crystallin is a heteropolymer composed of two homologous subunits, αA and αB, and in most vertebrates they are present in a ratio of 3:1. The structural and functional significance of this specific ratio of α-crystallin subunits is of considerable interest in understanding its role in the eye lens transparency. Previously, we have shown that although at physiologically relevant conditions αB-crystallin has greater CLA, under stress conditions such as elevated temperatures α-crystallin heteropolymer with 3:1 αA to αB ratio displayed higher CLA (Srinivas et al., Biochem. J., 2008, 414, 453 - 460). Herein, we provide a rationale for the existence of α-crystallin heteropolymer with 3:1 αA to αB ratio in terms of structural stability, aggregation pattern, and susceptibility to posttranslational modifications that could explain the importance of the heteropolymer of α-crystallin in the eye lens. We demonstrate that αA-crystallin is not only more stable but also imparts stability to the heteropolymer by preventing the aggregation of αB-crystallin at higher temperatures by using differential scanning calorimetry, size-exclusion chromatography, and denaturant-induced unfolding methods. Further, the physiological significance of heteropolymer with higher proportion of αA subunit is substantiated by using a heteropolymer with mutant (F71L) αA-crystallin and the susceptibility of 3:1 heteropolymer to glycation-induced modifications. Thus, the existence of 3:1 heteropolymer might be vital for the eye lens transparency under diverse conditions to prevent cataract.

Regulation of the members of the mammalian heat shock factor family

Regulation of gene expression is fundamental in all living organisms and is facilitated by transcription factors, the single largest group of proteins in humans. For cell- and stimulus-specific gene regulation, strict control of the transcription factors themselves is crucial. Heat shock factors are a family of transcription factors best known as master regulators of induced gene expression during the heat shock response. This evolutionary conserved cellular stress response is characterized by massive production of heat shock proteins, which function as cytoprotective molecular chaperones against various proteotoxic stresses. In addition to promoting cell survival under stressful conditions, heat shock factors are involved in the regulation of life span and progression of cancer and they are also important for developmental processes such as gametogenesis, neurogenesis and maintenance of sensory organs. Here, we review the regulatory mechanisms steering the activities of the mammalian heat shock factors 1–4.

Characterization of O-phosphohydroxyproline in rat {alpha}-crystallin A

Post-translational modifications have major importance for the structure and function of a multiplicity of proteins. Phosphorylation is a widespread phenomenon among eukaryotic proteins. Whereas O-phosphorylation on the side chains of serine, threonine, and tyrosine in proteins is well known and has been studied extensively, to our knowledge the endogenous phosphorylation of hydroxyproline has not previously been reported. In the present work, we provide evidence for the first time that O-phosphohydroxyproline (Hyp(P)) is a proteinogenic amino acid. To detect Hyp(P) in proteins we generated a Hyp(P)-specific polyclonal antibody. We could identify Hyp(P) in various proteins by Western blot analysis, and we characterized the sequence position of Hyp(P) in the protein α-crystallin A by electrospray ionization-tandem mass spectrometry. Our experiments clearly demonstrate hydroxylation and subsequent phosphorylation of a proline residue in α-crystallin A in the eye as well as in heart tissue of rat.

Modulation of alpha-crystallin chaperone activity: a target to prevent or delay cataract?

Cataract, loss of eye lens transparency, is the leading cause of blindness worldwide. alpha-Crystallin, initially known as one of the major structural proteins of the eye lens, is composed of two homologous subunits alphaA- and alphaB-crystallins. It is convincingly established now that alpha-crystallin functions like a chaperone and plays a decisive role in the maintenance of eye lens transparency. The functional ability of alpha-crystallin subunits is to act in cooperation as molecular chaperones to prevent the cellular aggregation and/or inactivation of client proteins under variety of stress conditions. However, chaperone-like activity of alpha-crystallin could be deteriorated or lost during aging or under certain clinical conditions because of various genetic and environmental factors. This review will focus specifically on relevance of alpha-crystallin chaperone function to lens transparency. In particular, we reviewed the studies that demonstrate the modulation of alpha-crystallin chaperone-like activity and discussed the possibility of chaperone-like activity of alpha-crystallin as a potential target to prevent or delay the cataractogenesis.

AlphaB-crystallin: a Golgi-associated membrane protein in the developing ocular lens

PURPOSE

All crystallins have non-crystallin catalytic functions. Because catalytic functions do not require large concentrations of protein, as are seen in the lens, there is a perception of dichotomy in the catalytic/physiological function of crystallins within and outside the lens. The status of alphaB-crystallin, a ubiquitously expressed small heat shock protein (and a crystallin) in the ocular lens, was investigated.

METHODS

Discontinuous sucrose density gradients were used for fractionation of Golgi membranes and vesicles. Light microscopy and confocal microscopy were used for immunolocalization in cultured cells and the native lens.

RESULTS

alphaB-crystallin is highly organized, as indicated by its polar presence in the apical Golgi in lens epithelium and in the perinuclear Golgi streaks in differentiating lens fiber cells. Assessment of the distribution of alphaB-crystallin in Golgi-enriched and vesicular fractions (characterized by the presence of Golgi membrane protein GM130 and vesicle coat protein gammaCOP) in the developing lens reveal a gradual transition from Golgi to vesicular fraction, concomitant with the appearance of alphaB-crystallin as a "soluble" protein.

CONCLUSIONS

These data demonstrate that alphaB-crystallin, known to be a soluble protein, starts life as a Golgi-associated membrane protein in the fetal and early postnatal lens and that the developmentally controlled physical state of the Golgi determines the status of this protein in the lens. These findings also show the similarity in the localization/physiological function of alphaB-crystallin within and outside the ocular lens and suggest that non-crystallin/catalytic function is an innate component of the expression of a crystallin in the lens.

Significance of α-crystallin heteropolymer with a 3:1 αA/αB ratio: chaperone-like activity, structure and hydrophobicity

The small heat-shock protein α-crystallin isolated from the eye lens exists as a large (700 kDa) heteropolymer composed of two subunits, αA and αB, of 20 kDa each. Although trace amounts of αA-crystallin are found in other tissues, non-lenticular distribution of α-crystallin is dominated by the αB homopolymer. In most vertebrate lens, the molar ratio of αA to αB is generally 3:1. However, the importance of this ratio in the eye lens is not known. In the present study, we have investigated the physiological significance of the 3:1 ratio by determining the secondary/tertiary structure, hydrophobicity and chaperone-like activity of αA- and αB-homopolymers and heteropolymers with different ratios of αA to αB subunits. Although, under physiologically relevant conditions, the αB-homopolymer (37–40 °C) has shown relatively higher activity, the αA-homopolymer or the heteropolymer with a higher αA proportion (3:1 ratio) has shown greater chaperone-like activity at elevated temperatures (>50 °C) and also upon structural perturbation. Furthermore, higher chaperone activity at elevated temperatures as well as upon structural perturbation is mainly mediated through increased hydrophobicity of αA. Although homopolymers and heteropolymers of α-crystallin did not differ in their secondary structure, changes in tertiary structure due to structural perturbations upon pre-heating are mediated predominantly by αA. Interestingly, the heteropolymer with higher αA proportion (3:1) or the αA-homopolymer seems to be better chaperones in protecting lens β- and γ-crystallins at both normal and elevated temperatures. Thus lens might have favoured a combination of these qualities to achieve optimal protection under both native and stress (perturbed) conditions for which the heteropolymer with αA to αB in the 3:1 ratio appears to be better suited.

GAD isoforms exhibit distinct spatiotemporal expression patterns in the developing mouse lens: correlation with Dlx2 and Dlx5

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter of the adult nervous system and its biosynthetic enzyme glutamic acid decarboxylase (GAD) are abundantly expressed in the embryonic nervous system and are involved in the modulation of cell proliferation, migration, and differentiation. Here we describe for the first time the expression of GABA and embryonic and adult GAD isoforms in the developing mouse lens. We show that the GAD isoforms are sequentially induced with specific spatiotemporal profiles: GAD65 and embryonic GAD isoforms prevail in primary fibers, while GAD67 is the predominant GAD expressed in the postnatal secondary fibers. This pattern correlates well with the expression of Dlx2 and Dlx5, known as upstream regulators of GAD. GABA and GAD are most abundant at the tips of elongating fibers and are absent from organelle-free cells, suggesting their involvement is primarily in shaping of the cytoskeleton during fiber elongation stages.

Distinct embryonic expression and localization of CBP and p300 histone acetyltransferases at the mouse alphaA-crystallin locus in lens

Mouse alphaA-crystallin gene encodes the most abundant protein of the mammalian lens. Expression of alphaA-crystallin is regulated temporally and spatially during lens development with initial expression in the lens vesicle followed by strong upregulation in the differentiating primary lens fibers. Lens-specific expression of alphaA-crystallin is mediated by DNA-binding transcription factors Pax6, c-Maf and CREB bound to its promoter region. Its 5'-distal enhancer, DCR1, mediates regulation of alphaA-crystallin via FGF signaling, while its 3'-distal enhancer, DCR3, functions only in elongated primary lens fibers via other lens differentiation pathways. DCR1 and DCR3 establish outside borders of a lens-specific chromatin region marked by histone H3 K9 acetylation. Here, we identified CREB-binding protein (CBP) and p300 as major histone acetyltransferases (HATs) associated in vivo with the mouse alphaA-crystallin locus. Both HATs are expressed in embryonic lens. Expression of CBP in primary lens fiber cells coincides with alphaA-crystallin. In the chromatin of lens epithelial cells, chromatin immunoprecipitations (ChIPs) show that the alphaA-crystallin promoter is notably devoid of any significant presence of CBP and p300, though DCR1 and a few other regions show the presence of these HATs. In the chromatin obtained from newborn lens, CBP was localized specifically at the promoter region with about ten times higher abundance compared to the entire alphaA-crystallin locus. In contrast, p300 is distributed more evenly across the entire locus. Analysis of total histone H3 and H3 K9 acetylation revealed potential lower density of nucleosomes 2 kb upstream from the promoter region. Collectively, our data suggest that moderate level of alphaA-crystallin gene expression in lens epithelial cells does not require the presence of CBP and p300 in the promoter. However, the lens-specific chromatin domain contains both promoter localized CBP on the "background" of locus-spread presence of CBP and p300.

Chaperone-like activity and hydrophobicity of alpha-crystallin

alpha-Crystallin, a prominent member of small heat shock protein (sHsp) family and a major structural protein of the eye lens is a large polydisperse oligomer of two isoforms, alphaA- and alphaB-crystallins. Numerous studies have demonstrated that alpha-crystallin functions like a molecular chaperone in preventing the aggregation of various proteins under a wide range of stress conditions. The molecular chaperone function of alpha-crystallin is thus considered to be vital in the maintenance of lens transparency and in cataract prevention. alpha-Crystallin selectively interacts with non-native proteins thereby preventing them from aggregation and helps maintain them in a folding competent state. It has been proposed and generally accepted that alpha-crystallin suppresses the aggregation of other proteins through the interaction between hydrophobic patches on its surface and exposed hydrophobic sites of partially unfolded substrate protein. However, a quantifiable relationship between hydrophobicity and chaperone-like activity remains a matter to be concerned about. On an attentive review of studies on alpha-crystallin chaperone-like activity, particularly the studies that have direct or indirect implications to hydrophobicity and chaperone-like activity, we found several instances wherein the correlation between hydrophobicity and its chaperone-like activity is paradoxical. We thus attempted to provide an overview on the role of hydrophobicity in chaperone-like activity of alpha-crystallin, the kind of evaluation done for the first time.

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

Crystallization and preliminary X-ray crystallographic investigations on a betagamma-crystallin domain of absent in melanoma 1 (AIM1), a protein from Homo sapiens

AIM1g1 is a single betagamma-crystallin domain from the protein absent in melanoma 1 (AIM1), which appears to play a role in the suppression of melanomas. This domain is known to bind calcium and its structure would help in identifying calcium-coordinating sites in vertebrate crystallins, which have hitherto been believed to have lost this ability during evolution. Crystallization of this domain was performed by the hanging-drop vapour-diffusion method. Crystals diffracted to a maximum resolution of 1.86 A and were found to belong to space group P6(1) or P6(5), with unit-cell parameters a = b = 54.98, c = 59.73 A. Solvent-content analysis indicated the presence of one monomer per asymmetric unit.

Alphab-crystallin-assisted reactivation of glucose-6-phosphate dehydrogenase upon refolding

Alphab-crystallin, a small heat-shock protein has been shown to prevent the aggregation of other proteins under various stress conditions. We have investigated the role of alphaB-crystallin in the reactivation of denaturant [GdmCl (guanidinium chloride)]-inactivated G6PD (glucose-6-phosphate dehydrogenase). Studies indicate that unfolding and inactivation of G6PD by GdmCl proceeds via formation of a molten globule-like state at low concentrations of GdmCl, which was characterized by having maximum surface hydrophobicity and no catalytic activity. At high concentrations of GdmCl, G6PD was completely unfolded, which upon dilution-induced refolding yielding 35% of original activity. In contrast, no activity was recovered when G6PD was refolded from a molten globule-like state. Interestingly, refolding of completely unfolded G6PD in the presence of alphaB-crystallin resulted in 70% gain of the original activity, indicating that alphaB-crystallin assisted in enhanced refolding of G6PD. Intriguingly, alphaB-crystallin was unable to reactivate G6PD from a molten globule-like state. Size-exclusion chromatography data indicate that alphaB-crystallin-assisted reactivation of completely unfolded G6PD is concomitant with the restoration of the native structure of G6PD. Nonetheless, alphaB-crystallin failed to reactivate G6PD from preformed aggregates. Moreover, methylglyoxal-modified alpha-crystallin, which occurs in aged and diabetic cataract lenses, was less efficient in the reactivation of denaturant inactivated G6PD. Diminished chaperone-like activity of alpha-crystallin due to post-translational modifications may thus result in the accumulation of aggregated/inactivated proteins.

Detection of gender differences in rat lens proteins using 2-D-DIGE

The glass-like transparency of the human eye lens is achieved by the tight packing of abundant crystallin proteins. However, the precise role of the accessory non-crystallin proteins is not well understood. We have carried out 2-DE mapping of these proteins in rat lens. This showed the presence of the high molecular weight filamentous structural proteins spectrin, filensin, tubulin, vimentin, actin and phakinin as well as several forms of potential crystallin oligomers comprised of alphaA, betaB1, betaA1 and betaA4 chains. Other proteins that were present include, heat shock protein 71, WD repeat protein 1, and several enzymes including alpha-enolase, pyruvate kinase, transketolase and aldose reductase. 2-D-DIGE analysis revealed several expression differences between the lens proteomes of male and female rats. Female rat lenses contained lower levels of aldose reductase, increased proteolyic fragments of the structural proteins filensin, vimentin and phakinin and higher levels of potential alphaA, betaB1 and betaA1 crystallin oligomers. Taken together these findings suggest that there are potential differences in oxidative stress regulation between male and female rat lenses, which may have implications on susceptibility to cataract formation. Future studies aimed at elucidating pre-cataractic changes in the non-crystallin proteins described here may facilitate identification of novel markers involved in cataractogenesis.

Maintenance of olfactory neurogenesis requires HSF1, a major heat shock transcription factor in mice

Heat shock transcription factors (HSFs) play roles not only in heat shock response but also in development of the reproductive organs, brain, and lens. Here, we analyzed sensory organs and found abnormalities of the olfactory epithelium in adult HSF1-null mice, which is developmentally related to the lens. The olfactory epithelium was normal until postnatal 3 weeks but was not maintained later than 4 weeks in HSF1-null mice. The olfactory epithelium was atrophied with increased cell death of olfactory sensory neurons. Analysis of the epithelium revealed that induction of HSP expression and reduction of LIF expression are lacking in adult HSF1-null mice. We found that DNA binding activity of HSF1 is induced in the olfactory epithelium later than 4 weeks and that HSF1 binds directly to Lif gene and inhibits its expression. HSF4 has opposing effects on LIF expression and olfactory neurogenesis. These data indicate that HSF1 is required for the precise expression of Hsp and cytokine genes that is obligatory for maintenance of olfactory neurogenesis in adult mice and suggest that stress-related processes are involved in its maintenance.

Elevated expression of alphaA- and alphaB-crystallins in streptozotocin-induced diabetic rat

alpha-Crystallin, a predominant protein of the ocular lens, is composed of two subunits, alphaA and alphaB. Of these, alphaB-crystallin has been shown to present widely in non-lenticular tissues while alphaA-crystallin is largely lens-specific. Although, expression of alphaB-crystallin is elevated under various stress and pathological conditions, yet its physiological significance remained unknown. Some studies suggest that the expression of alphaB-crystallin gene is related to oxidative stress. Persistent hyperglycemia during uncontrolled diabetes is known to cause oxidative stress, which has been implicated in various secondary complications of diabetes. Hence, expression of alphaA- and alphaB-crystallins in various tissues of streptozotocin (STZ)-induced diabetic Wistar-NIN rats was investigated by RT-PCR and immunoblotting. While expression of alphaB-crystallin was noted in the wide range of tissues examined in the study, alphaA-crystallin expression was detected only in lens and retina. Interestingly, alphaB-crystallin expression was elevated in lens, heart, muscle, and brain, but decreased in adipose tissue of diabetic rats compared to control rats. alphaA-Crystallin expression was increased in retina of diabetic rat. Increased oxidative stress appears to be a major stimulus for the enhanced expression of alphaA- and alphaB-crystallins in the tissues of diabetic rats and elevated expression of alpha-crystallin may have a protective role against metabolic stress. Interestingly, feeding of curcumin, a dietary antioxidant, to diabetic rats attenuated the enhanced expression of alphaB-crystallin. The results indicate that elevated expression of alpha-crystallins in some tissues may have implications in pathophysiology of diabetic complications.

Tissue-specific regulation of the mouse alphaA-crystallin gene in lens via recruitment of Pax6 and c-Maf to its promoter

Pax6 is a lineage-restricted DNA-binding transcription factor regulating the formation of mammalian organs including brain, eye and pancreas. Pax6 plays key roles during the initial formation of lens lineage, proliferation of lens progenitor and precursor cells and their terminal differentiation. In addition to Pax6, lens fiber cell differentiation is regulated by c-Maf, Prox1 and Sox1. Crystallins are essential lens structural proteins required for light refraction and transparency. Mouse alphaA-crystallin represents about 17% of all crystallins at the protein level and ranks as one of the most abundant tissue-specific proteins. Lens-specific expression of this gene is regulated at the level of transcription. A promoter fragment of -88 to +46 is capable of driving lens-specific expression in transgenic mouse. Here we provide data suggesting that this lens-specific promoter fragment is comprised of multiple Pax6 and Maf-binding sites. Site-directed mutagenesis of regions within these sites resulted in partially or completely reduced promoter activities in lens cells. Co-transfections using Pax6 and c-Maf alone revealed moderate and strong activations of this promoter, respectively. In contrast to synergistic activation of alphaB-crystallin by Pax6 and c-Maf, Pax6 has a neutral effect on c-Maf-mediated alphaA-crystallin promoter activation. Chromatin immunoprecipitations established in vivo interactions of Pax6 and c-Maf with the alphaA-crystallin promoter in lens cells. Collectively, the present data support a molecular model in which tissue-specific expression of alphaA-crystallin is regulated by recruitment of Pax6 and c-Maf, two proteins regulating multiple processes of lens differentiation, to its promoter. In addition, the data suggest a molecular model of temporal and spatial regulation of alphaB, alphaA and gamma-crystallin genes in mouse embryonic lens by using variants of the Pax6/Maf regulatory module.

Regulation of gene expression by Pax6 in ocular cells: a case of tissue-preferred expression of crystallins in lens

Lens development is an excellent model for genetic and biochemical studies of embryonic induction, cell cycle regulation, cellular differentiation and signal transduction. Differentiation of lens is characterized by lens-preferred expression and accumulation of water-soluble proteins, crystallins. Crystallins are required for light transparency, refraction and maintenance of lens integrity. Here, we review mechanisms of lens-preferred expression of crystallin genes by employing synergism between developmentally regulated DNA-binding transcription factors: Pax6, c-Maf, MafA/L-Maf, MafB, NRL, Sox2, Sox1, RARbeta/RXRbeta, RORalpha, Prox1, Six3, gammaFBP-B and HSF2. These factors are differentially expressed in lens precursor cells, lens epithelium and primary and secondary lens fibers. They exert their function in combination with ubiquitously expressed factors (e.g. AP-1, CREB, pRb, TFIID and USF) and co-activators/chromatin remodeling proteins (e.g. ASC-2 and CBP/p300). A special function belongs to Pax6, a paired domain and homeodomain-containing protein, which is essential for lens formation. Pax6 is expressed in lens progenitor cells before the onset of crystallin expression and it serves as an important regulatory factor required for expression of c-Maf, MafA/L-Maf, Six3, Prox1 and retinoic acid signaling both in lens precursor cells and the developing lens. The roles of these factors are illustrated by promoter studies of mouse alphaA-, alphaB-, gammaF- and guinea pig zeta-crystallins. Pax6 forms functional complexes with a number of transcription factors including the retinoblastoma protein, pRb, MafA, Mitf and Sox2. We present novel data showing that pRb antagonizes Pax6-mediated activation of the alphaA-crystallin promoter likely by inhibiting binding of Pax6 to DNA.

Insights into Hydrophobicity and the Chaperone-like Function of αA- and αB-crystallins

Alpha-crystallin, composed of two subunits, alphaA and alphaB, has been shown to function as a molecular chaperone that prevents aggregation of other proteins under stress conditions. The exposed hydrophobic surfaces of alpha-crystallins have been implicated in this process, but their exact role has not been elucidated. In this study, we quantify the hydrophobic surfaces of alphaA- and alphaB-crystallins by isothermal titration calorimetry using 8-anilino-1-napthalenesulfonic acid (ANS) as a hydrophobic probe and analyze its correlation to the chaperone potential of alphaA- and alphaB-crystallins under various conditions. Two ANS binding sites, one with low and another with high affinity, were clearly detected, with alphaB showing a higher number of sites than alphaA at 30 degrees C. In agreement with the higher number of hydrophobic sites, alphaB-crystallin demonstrated higher chaperone activity than alphaA at this temperature. Thermodynamic analysis of ANS binding to alphaA- and alphaB-crystallins indicates that high affinity binding is driven by both enthalpy and entropy changes, with entropy dominating the low affinity binding. Interestingly, although the number of ANS binding sites was similar for alphaA and alphaB at 15 degrees C, alphaA was more potent than alphaB in preventing aggregation of the insulin B-chain. Although there was no change in the number of high affinity binding sites of alphaA and alphaB for ANS upon preheating, there was an increase in the number of low affinity sites of alphaA and alphaB. Preheated alphaA, in contrast to alphaB, exhibited remarkably enhanced chaperone activity. Our results indicate that although hydrophobicity appears to be a factor in determining the chaperone-like activity of alpha-crystallins, it does not quantitatively correlate with the chaperone function of alpha-crystallins.