Pharmacological chaperone for α-crystallin partially restores transparency in cataract models

Drug delivery to the lens for the management of cataracts

Cataracts are one of the most prevalent diseases of the lens, affecting its transparency and are the leading cause of reversible blindness in the world. The clarity of the lens is essential for its normal physiological function of refracting light onto the retina. Currently there is no pharmaceutical treatment for prevention or cure of cataracts and surgery to replace the affected lens remains the gold standard in the management of cataracts. Pharmacological treatment for prevention of cataracts is hindered by many physiological barriers that must be overcome by a therapeutic agent to reach the avascular lens. Various therapeutic agents and formulation strategies are currently being investigated to prevent cataract formation as access to surgery is limited. This review provides a summary of recent research in the field of drug delivery to the lens for the management of cataracts including models used to study cataract treatments and discusses the future perspectives in the field.

Probing the changes in gene expression due to α-crystallin mutations in mouse models of hereditary human cataract

The mammalian eye lens expresses a high concentration of crystallins (α, β and γ-crystallins) to maintain the refractive index essential for lens transparency. Crystallins are long-lived proteins that do not turnover throughout life. The structural destabilization of crystallins by UV exposure, glycation, oxidative stress and mutations in crystallin genes leads to protein aggregation and development of cataracts. Several destabilizing mutations in crystallin genes are linked with human autosomal dominant hereditary cataracts. To investigate the mechanism by which the α-crystallin mutations Cryaa-R49C and Cryab-R120G lead to cataract formation, we determined whether these mutations cause an altered expression of specific transcripts in the lens at an early postnatal age by RNA-seq analysis. Using knock-in mouse models previously generated in our laboratory, in the present work, we identified genes that exhibited altered abundance in the mutant lenses, including decreased transcripts for Clic5, an intracellular water channel in Cryaa-R49C heterozygous mutant lenses, and increased transcripts for Eno1b in Cryab-R120G heterozygous mutant lenses. In addition, RNA-seq analysis revealed increased histones H2B, H2A, and H4 gene expression in Cryaa-R49C mutant lenses, suggesting that the αA-crystallin mutation regulates histone expression via a transcriptional mechanism. Additionally, these studies confirmed the increased expression of histones H2B, H2A, and H4 by proteomic analysis of Cryaa-R49C knock-in and Cryaa;Cryab gene knockout lenses reported previously. Taken together, these findings offer additional insight into the early transcriptional changes caused by Cryaa and Cryab mutations associated with autosomal dominant human cataracts, and indicate that the transcript levels of certain genes are affected by the expression of mutant α-crystallin in vivo.

Light-focusing human micro-lenses generated from pluripotent stem cells model lens development and drug-induced cataract in vitro

Cataracts cause vision loss and blindness by impairing the ability of the ocular lens to focus light onto the retina. Various cataract risk factors have been identified, including drug treatments, age, smoking and diabetes. However, the molecular events responsible for these different forms of cataract are ill-defined, and the advent of modern cataract surgery in the 1960s virtually eliminated access to human lenses for research. Here, we demonstrate large-scale production of light-focusing human micro-lenses from spheroidal masses of human lens epithelial cells purified from differentiating pluripotent stem cells. The purified lens cells and micro-lenses display similar morphology, cellular arrangement, mRNA expression and protein expression to human lens cells and lenses. Exposing the micro-lenses to the emergent cystic fibrosis drug Vx-770 reduces micro-lens transparency and focusing ability. These human micro-lenses provide a powerful and large-scale platform for defining molecular disease mechanisms caused by cataract risk factors, for anti-cataract drug screening and for clinically relevant toxicity assays.

Investigation of the radiotherapy-related changes in the eye lens using computed tomography entropy analysis

OBJECTIVE

The aim of this study was to apply texture analysis to investigate whether there was a change in the lens following radiotherapy.

PATIENTS AND METHOD

Patients who received radiotherapy (RT) for head and neck cancer or brain tumor were enrolled. Computed tomography (CT) images taken in the last month before RT and the most recent images after RT were compared. Entropy values were calculated using lens attenuation values. The lens doses were obtained from the dose-volume histogram data.

RESULTS

A total of 55 lenses were evaluated. The mean Hounsfield Unit value of the lenses was 66.14±12.16 before RT and 72.02±9.12 after RT (p = 0.007). The mean entropy value was 1.87±0.31 before RT and this reduced to 1.31±0.34 after RT (p < 0.001), respectively. As time increased, the difference in entropy also increased (p = 0.007). A correlation close to statistical significance was determined between the entropy difference and minimum, maximum and mean lens radiation dose (p = 0.052, p = 0.052, p = 0.063, respectively). The entropy difference was significantly reduced in the >4 Gy group (p = 0.046).

CONCLUSION

Study results indicated that the entropy values in the lens were signifcantly changed after radiotherapy and the degree of the change associated with dose and time.

Epigenetic Treatment of Neurodegenerative Ophthalmic Disorders: An Eye Toward the Future

Eye disease is one of the primary medical conditions that requires attention and therapeutic intervention in ageing populations worldwide. Further, the global burden of diabetes and obesity, along with heart disease, all lead to secondary manifestations of ophthalmic distress. Therefore, there is increased interest in developing innovative new approaches that target various mechanisms and sequelae driving conditions that result in adverse vision. The research challenge is even greater given that the terrain of eye diseases is difficult to landscape into a single therapeutic theme. This report addresses the burden of eye disease due to mitochondrial dysfunction, including antioxidant, autophagic, epigenetic, mitophagic, and other cellular processes that modulate the biomedical end result. In this light, we single out lipoic acid as a potent known natural activator of these pathways, along with alternative and potentially more effective conjugates, which together harness the necessary potency, specificity, and biodistribution parameters required for improved therapeutic outcomes.

Recent Advances in Understanding Werner Syndrome

Aging, the universal phenomenon, affects human health and is the primary risk factor for major disease pathologies. Progeroid diseases, which mimic aging at an accelerated rate, have provided cues in understanding the hallmarks of aging. Mutations in DNA repair genes as well as in telomerase subunits are known to cause progeroid syndromes. Werner syndrome (WS), which is characterized by accelerated aging, is an autosomal-recessive genetic disorder. Hallmarks that define the aging process include genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulation of nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. WS recapitulates these hallmarks of aging and shows increased incidence and early onset of specific cancers. Genome integrity and stability ensure the normal functioning of the cell and are mainly guarded by the DNA repair machinery and telomeres. WRN, being a RecQ helicase, protects genome stability by regulating DNA repair pathways and telomeres. Recent advances in WS research have elucidated WRN’s role in DNA repair pathway choice regulation, telomere maintenance, resolution of complex DNA structures, epigenetic regulation, and stem cell maintenance.

Paradigm shifts, peregrinations and pixies in ophthalmology

Human ingenuity is challenged by defending vision, our highest bandwidth sense. Special challenges are presented by the replacement or repair of highly specialized but scarce tissue within the constraints of transparency, tissue shape and alignment, tissue borders and pressure maintenance. Many, mostly destructive, surgical procedures were developed prior to an understanding of underlying pathophysiology. For a number of conditions, both reconstructive and destructive procedures co-exist, yet there are few guidelines as to the better approach. Because the consequences of these procedures may take many years to surface (consistent with a stem cell role in long-term tissue maintenance), guidance may be provided by the elucidation of underlying principles from these approaches. Illustrative examples from clinical, basic research and biotechnology, particularly relating to pterygium, ocular surface squamous neoplasia, dry-eye syndrome, corneal rehabilitation and replacement, cataract surgery, strabismus surgery and bionic eye research, are described. An unexpected consequence of bionic device development has been an appreciation of the sophistication of tissues being replaced, given the limitations of available biomaterials. Examples of how this has provided insights into ocular disease will be illustrated. Stem cell and biomaterial technologies are starting to impact at a time when cost-effectiveness is under scrutiny. Both efficacy and cost will need to be considered as these interventions are introduced. It appears that the paradigm shift rate is accelerating and there is evidence of this in ophthalmology. Lessons learned from the areas of destructive versus reconstructive surgery and the limitations of development of bionic replacements will be used to illustrate how new procedures and technologies can be developed.

Transcriptome of the GSH-Depleted Lens Reveals Changes in Detoxification and EMT Signaling Genes, Transport Systems, and Lipid Homeostasis

Purpose

To understand the effects of glutathione (GSH)-deficiency on genetic processes that regulate lens homeostasis and prevent cataractogenesis.

Methods

The transcriptome of lens epithelia and fiber cells was obtained from C57BL/6 LEGSKO (lens GSH-synthesis knockout) and buthionine sulfoximine (BSO)-treated LEGSKO mice and compared to C57BL/6 wild-type mice using RNA-Seq. Transcriptomic data were confirmed by qPCR and Western blot/ELISA on a subset of genes.

Results

RNA-Seq results were in excellent agreement with qPCR (correlation coefficients 0.87-0.94 and P < 5E-6 for a subset of 36 mRNAs). Of 24,415 transcripts mapped to the mouse genome, 441 genes showed significantly modulated expression. Pathway analysis indicated major changes in epithelial-mesenchymal transition (EMT) signaling, visual cycle, small molecule biochemistry, and lipid metabolism. GSH-deficient lenses showed upregulation of detoxification genes, including Aldh1a1, Aldh3a1 (aldehyde dehydrogenases), Mt1, Mt2 (metallothioneins), Ces1g (carboxylesterase), and Slc14a1 (urea transporter UT-B). Genes in canonical EMT pathways, including Wnt10a, showed upregulation in lens epithelia samples. Severely GSH-deficient lens epithelia showed downregulation of vision-related genes (including crystallins). The BSO-treated LEGSKO lens epithelia transcriptome has significant correlation (r = 0.63, P < 0.005) to that of lens epithelia undergoing EMT. Protein expression data correlated with transcriptomic data and confirmed EMT signaling activation.

Conclusions

These results show that GSH-deficiency in the lens leads to expression of detoxifying genes and activation of EMT signaling, in addition to changes in transport systems and lipid homeostasis. These data provide insight into the adaptation and consequences of GSH-deficiency in the lens and suggest that GSH plays an important role in lenticular EMT pathology.

Chaperone activity of human small heat shock protein-GST fusion proteins

Small heat shock proteins (sHsps) are a ubiquitous part of the machinery that maintains cellular protein homeostasis by acting as molecular chaperones. sHsps bind to and prevent the aggregation of partially folded substrate proteins in an ATP-independent manner. sHsps are dynamic, forming an ensemble of structures from dimers to large oligomers through concentration-dependent equilibrium dissociation. Based on structural studies and mutagenesis experiments, it is proposed that the dimer is the smallest active chaperone unit, while larger oligomers may act as storage depots for sHsps or play additional roles in chaperone function. The complexity and dynamic nature of their structural organization has made elucidation of their chaperone function challenging. HspB1 and HspB5 are two canonical human sHsps that vary in sequence and are expressed in a wide variety of tissues. In order to determine the role of the dimer in chaperone activity, glutathione-S-transferase (GST) was genetically linked as a fusion protein to the N-terminus regions of both HspB1 and HspB5 (also known as Hsp27 and αB-crystallin, respectively) proteins in order to constrain oligomer formation of HspB1 and HspB5, by using GST, since it readily forms a dimeric structure. We monitored the chaperone activity of these fusion proteins, which suggest they primarily form dimers and monomers and function as active molecular chaperones. Furthermore, the two different fusion proteins exhibit different chaperone activity for two model substrate proteins, citrate synthase (CS) and malate dehydrogenase (MDH). GST-HspB1 prevents more aggregation of MDH compared to GST-HspB5 and wild type HspB1. However, when CS is the substrate, both GST-HspB1 and GST-HspB5 are equally effective chaperones. Furthermore, wild type proteins do not display equal activity toward the substrates, suggesting that each sHsp exhibits different substrate specificity. Thus, substrate specificity, as described here for full-length GST fusion proteins with MDH and CS, is modulated by both sHsp oligomeric conformation and by variations of sHsp sequences.

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.

Amyloid β-Sheet Secondary Structure Identified in UV-Induced Cataracts of Porcine Lenses using 2D IR Spectroscopy

Cataracts are formed by the aggregation of crystallin proteins in the eye lens. Many in vitro studies have established that crystallin proteins precipitate into aggregates that contain amyloid fibers when denatured, but there is little evidence that ex vivo cataracts contain amyloid. In this study, we collect two-dimensional infrared (2D IR) spectra on tissue slices of porcine eye lenses. As shown in control experiments on in vitro αB- and γD-crystallin, 2D IR spectroscopy can identify the highly ordered β-sheets typical of amyloid secondary structure even if the fibers themselves are too short to be resolved with TEM. In ex vivo experiments of acid-treated tissues, characteristic 2D IR features are observed and fibers >50nm in length are resolved by transmission electron microscopy (TEM), consistent with amyloid fibers. In UV-irradiated lens tissues, fibers are not observed with TEM, but highly ordered β-sheets of amyloid secondary structure is identified from the 2D IR spectra. The characteristic 2D IR features of amyloid β-sheet secondary structure are created by as few as four or five strands and so identify amyloid secondary structure even if the aggregates themselves are too small to be resolved with TEM. We discuss these findings in the context of the chaperone system of the lens, which we hypothesize sequesters small aggregates, thereby preventing long fibers from forming. This study expands the scope of heterodyned 2D IR spectroscopy to tissues. The results provide a link between in vitro and ex vivo studies and support the hypothesis that cataracts are an amyloid disease.

Recent Insights into the Role of Unfolded Protein Response in ER Stress in Health and Disease

Unfolded stress response (UPR) is a conserved cellular pathway involved in protein quality control to maintain homeostasis under different conditions and disease states characterized by cell stress. Although three general schemes of and genes induced by UPR are rather well-established, open questions remain including the precise role of UPR in human diseases and the interactions between different sensor systems during cell stress signaling. Particularly, the issue how the normally adaptive and pro-survival UPR pathway turns into a deleterious process causing sustained endoplasmic reticulum (ER) stress and cell death requires more studies. UPR is also named a friend with multiple personalities that we need to understand better to fully recognize its role in normal physiology and in disease pathology. UPR interacts with other organelles including mitochondria, and with cell stress signals and degradation pathways such as autophagy and the ubiquitin proteasome system. Here we review current concepts and mechanisms of UPR as studied in different cells and model systems and highlight the relevance of UPR and related stress signals in various human diseases.

Cataract-associated P23T γD-crystallin retains a native-like fold in amorphous-looking aggregates formed at physiological pH

Cataracts cause vision loss through the large-scale aggregation of eye lens proteins as a result of ageing or congenital mutations. The development of new treatments is hindered by uncertainty about the nature of the aggregates and their mechanism of formation. We describe the structure and morphology of aggregates formed by the P23T human γD-crystallin mutant associated with congenital cataracts. At physiological pH, the protein forms aggregates that look amorphous and disordered by electron microscopy, reminiscent of the reported formation of amorphous deposits by other crystallin mutants. Surprisingly, solid-state NMR reveals that these amorphous deposits have a high degree of structural homogeneity at the atomic level and that the aggregated protein retains a native-like conformation, with no evidence for large-scale misfolding. Non-physiological destabilizing conditions used in many in vitro aggregation studies are shown to yield qualitatively different, highly misfolded amyloid-like fibrils.

Aggregation of eye lens proteins leads to cataracts, a major cause of blindness. Here the authors use solid state NMR to probe the structure of γD-crystallin eye lens proteins aggregates, which are found to retain a native-like conformation.

Shaping proteostasis at the cellular, tissue, and organismal level

The proteostasis network (PN) regulates protein synthesis, folding, transport, and degradation to maintain proteome integrity and limit the accumulation of protein aggregates, a hallmark of aging and degenerative diseases. In multicellular organisms, the PN is regulated at the cellular, tissue, and systemic level to ensure organismal health and longevity. Here we review these three layers of PN regulation and examine how they collectively maintain cellular homeostasis, achieve cell type-specific proteomes, and coordinate proteostasis across tissues. A precise understanding of these layers of control has important implications for organismal health and could offer new therapeutic approaches for neurodegenerative diseases and other chronic disorders related to PN dysfunction.

Mutations and mechanisms in congenital and age-related cataracts

The crystalline lens plays an important role in the refractive vision of vertebrates by facilitating variable fine focusing of light onto the retina. Loss of lens transparency, or cataract, is a frequently acquired cause of visual impairment in adults and may also present during childhood. Genetic studies have identified mutations in over 30 causative genes for congenital or other early-onset forms of cataract as well as several gene variants associated with age-related cataract. However, the pathogenic mechanisms resulting from genetic determinants of cataract are only just beginning to be understood. Here, we briefly summarize current concepts pointing to differences in the molecular mechanisms underlying congenital and age-related forms of cataract.

Protein-Remodeling Factors As Potential Therapeutics for Neurodegenerative Disease

Protein misfolding is implicated in numerous neurodegenerative disorders including amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, and Huntington's disease. A unifying feature of patients with these disorders is the accumulation of deposits comprised of misfolded protein. Aberrant protein folding can cause toxicity through a loss or gain of protein function, or both. An intriguing therapeutic approach to counter these disorders is the application of protein-remodeling factors to resolve these misfolded conformers and return the proteins to their native fold and function. Here, we describe the application of protein-remodeling factors to alleviate protein misfolding in neurodegenerative disease. We focus on Hsp104, Hsp110/Hsp70/Hsp40, NMNAT, and HtrA1, which can prevent and reverse protein aggregation. While many of these protein-remodeling systems are highly promising, their activity can be limited. Thus, engineering protein-remodeling factors to enhance their activity could be therapeutically valuable. Indeed, engineered Hsp104 variants suppress neurodegeneration in animal models, which opens the way to novel therapeutics and mechanistic probes to help understand neurodegenerative disease.

Structural Basis for the Interaction of a Human Small Heat Shock Protein with the 14-3-3 Universal Signaling Regulator

By interacting with hundreds of protein partners, 14-3-3 proteins coordinate vital cellular processes. Phosphorylation of the small heat shock protein, HSPB6, within its intrinsically disordered N-terminal domain activates its interaction with 14-3-3, ultimately triggering smooth muscle relaxation. After analyzing the binding of an HSPB6-derived phosphopeptide to 14-3-3 using isothermal calorimetry and X-ray crystallography, we have determined the crystal structure of the complete assembly consisting of the 14-3-3 dimer and full-length HSPB6 dimer and further characterized this complex in solution using fluorescence spectroscopy, small-angle X-ray scattering, and limited proteolysis. We show that selected intrinsically disordered regions of HSPB6 are transformed into well-defined conformations upon the interaction, whereby an unexpectedly asymmetric structure is formed. This structure provides the first atomic resolution snapshot of a human small HSP in functional state, explains how 14-3-3 proteins sequester their regulatory partners, and can inform the design of small-molecule interaction modifiers to be used as myorelaxants.

Lanosterol Suppresses the Aggregation and Cytotoxicity of Misfolded Proteins Linked with Neurodegenerative Diseases

Accumulation of misfolded or aberrant proteins in neuronal cells is linked with neurodegeneration and other pathologies. Which molecular mechanisms fail and cause inappropriate folding of proteins and what is their relationship to cellular toxicity is not well known. How does it happen and what are the probable therapeutic or molecular approaches to counter them are also not clear. Here, we demonstrate that treatment of lanosterol diminishes aberrant proteotoxic aggregation and mitigates their cytotoxicity via induced expression of co-chaperone CHIP and elevated autophagy. The addition of lanosterol not only reduces aggregation of mutant bonafide misfolded proteins but also effectively prevents accumulation of various mutant disease-prone proteotoxic proteins. Finally, we observed that lanosterol mitigates cytotoxicity in cells, mediated by different stress-inducing agents. Taken together, our present results suggest that upregulation of cellular molecular chaperones, primarily using small molecules, can probably offer an efficient therapeutic approach in the future against misfolding of different disease-causing proteins and neurodegenerative disorders. Graphical Abstract ᅟ.

Screening methods for identifying pharmacological chaperones

This review highlights recent screening methods for identifying pharmacological chaperones, which are small-molecules capable of rescuing misfolded proteins.

2016 EMBO Chemical Biology Conference

The full breadth of the field: The 2016 EMBO Chemical Biology Conference, covering topics from tool development to biological applications and from computational drug design to synthetic chemistry, took place in Heidelberg from 31st August to 3rd September.

Novel phenotypes and loci identified through clinical genomics approaches to pediatric cataract

Pediatric cataract is highly heterogeneous clinically and etiologically. While mostly isolated, cataract can be part of many multisystem disorders, further complicating the diagnostic process. In this study, we applied genomic tools in the form of a multi-gene panel as well as whole-exome sequencing on unselected cohort of pediatric cataract (166 patients from 74 families). Mutations in previously reported cataract genes were identified in 58% for a total of 43 mutations, including 15 that are novel. GEMIN4 was independently mutated in families with a syndrome of cataract, global developmental delay with or without renal involvement. We also highlight a recognizable syndrome that resembles galactosemia (a fulminant infantile liver disease with cataract) caused by biallelic mutations in CYP51A1. A founder mutation in RIC1 (KIAA1432) was identified in patients with cataract, brain atrophy, microcephaly with or without cleft lip and palate. For non-syndromic pediatric cataract, we map a novel locus in a multiplex consanguineous family on 4p15.32 where exome sequencing revealed a homozygous truncating mutation in TAPT1. We report two further candidates that are biallelically inactivated each in a single cataract family: TAF1A (cataract with global developmental delay) and WDR87 (non-syndromic cataract). In addition to positional mapping data, we use iSyTE developmental lens expression and gene-network analysis to corroborate the proposed link between the novel candidate genes and cataract. Our study expands the phenotypic, allelic and locus heterogeneity of pediatric cataract. The high diagnostic yield of clinical genomics supports the adoption of this approach in this patient group.

BAG3 Is a Modular, Scaffolding Protein that physically Links Heat Shock Protein 70 (Hsp70) to the Small Heat Shock Proteins

Small heat shock proteins (sHsps) are a family of ATP-independent molecular chaperones that are important for binding and stabilizing unfolded proteins. In this task, the sHsps have been proposed to coordinate with ATP-dependent chaperones, including heat shock protein 70 (Hsp70). However, it is not yet clear how these two important components of the chaperone network are linked. We report that the Hsp70 co-chaperone, BAG3, is a modular, scaffolding factor to bring together sHsps and Hsp70s. Using domain deletions and point mutations, we found that BAG3 uses both of its IPV motifs to interact with sHsps, including Hsp27 (HspB1), αB-crystallin (HspB5), Hsp22 (HspB8), and Hsp20 (HspB6). BAG3 does not appear to be a passive scaffolding factor; rather, its binding promoted de-oligomerization of Hsp27, likely by competing for the self-interactions that normally stabilize large oligomers. BAG3 bound to Hsp70 at the same time as Hsp22, Hsp27, or αB-crystallin, suggesting that it might physically bring the chaperone families together into a complex. Indeed, addition of BAG3 coordinated the ability of Hsp22 and Hsp70 to refold denatured luciferase in vitro. Together, these results suggest that BAG3 physically and functionally links Hsp70 and sHsps.

Unraveling the CHIP:Hsp70 complex as an information processor for protein quality control

The CHIP:Hsp70 complex stands at the crossroads of the cellular protein quality control system. Hsp70 facilitates active refolding of misfolded client proteins, while CHIP directs ubiquitination of misfolded client proteins bound to Hsp70. The direct competition between CHIP and Hsp70 for the fate of misfolded proteins leads to the question: how does the CHIP:Hsp70 complex execute triage decisions that direct misfolded proteins for either refolding or degradation? The current body of literature points toward action of the CHIP:Hsp70 complex as an information processor that takes inputs in the form of client folding state, dynamics, and posttranslational modifications, then outputs either refolded or ubiquitinated client proteins. Herein we examine the CHIP:Hsp70 complex beginning with the structure and function of CHIP and Hsp70, followed by an examination of recent studies of the interactions and dynamics of the CHIP:Hsp70 complex.

The function of small heat-shock proteins and their implication in proteostasis

All organisms rely on a conserved cellular machinery supporting and controlling the life cycle of proteins: the proteostasis network. Within this network, the main players that determine the fate of proteins are molecular chaperones, the ubiquitin–proteasome and the lysosome–autophagy systems. sHsps (small heat-shock proteins) represent one family of molecular chaperones found in all domains of life. They prevent irreversible aggregation of unfolded proteins and maintain proteostasis by stabilizing promiscuously a variety of non-native proteins in an ATP-independent manner. In the cellular chaperone network, sHsps act as the first line of defence and keep their substrates in a folding-competent state until they are refolded by downstream ATP-dependent chaperone systems. Besides this interaction with unfolding substrates upon stress, sHsps show a different mode of binding for specific clients which are also recognized under physiological conditions. In vertebrates, sHsps are especially needed to maintain the refractive index of the eye lens. Additionally, sHsps are linked to a broad variety of diseases such as myopathies and neuropathies. The most striking feature of sHsps is their ability to form dynamic ensembles of higher oligomers. The activity of sHsps is regulated by changes in the composition of the ensembles.

Pharmacological approaches to restoring lens transparency: Real world applications

Cataract is the most common cause of blindness and a major cause of visual impairment worldwide. As the world's population ages, cataract-induced visual impairment is of increasing prevalence, and treatment is limited to those with access to surgical care. While cataracts are mainly a disease of the elderly, infantile cataracts lead to lifelong visual impairment if untreated. Even in those with surgical treatment early in life, visual prognosis is often guarded. Consequently, there is an increasing impetus for alternative therapeutic modalities. Makley and Zhao utilize two different experimental approaches to identify novel pharmacological substances able to improve lens transparency by reducing aggregation of crystalline proteins. These data support an alternative to surgical correction that may be applied to adult patients without access to surgical care as well as address the unique challenges of infantile cataracts.

From structural biology to designing therapy for inborn errors of metabolism

At the SSIEM Symposium in Istanbul 2010, I presented an overview of protein structural approaches in the study of inborn errors of metabolism (Yue and Oppermann 2011). Five years on, the field is going strong with new protein structures, uncovered catalytic functions and novel chemical matters for metabolic enzymes, setting the stage for the next generation of drug discovery. This article aims to update on recent advances and lessons learnt on inborn errors of metabolism via the protein-centric approach, citing examples of work from my group, collaborators and co-workers that cover diverse pathways of transsulfuration, cobalamin and glycogen metabolism. Taking into consideration that many inborn errors of metabolism result in the loss of enzyme function, this presentation aims to outline three key principles that guide the design of small molecule therapy in this technically challenging field: (1) integrating structural, biochemical and cell-based data to evaluate the wide spectrum of mutation-driven enzyme defects in stability, catalysis and protein-protein interaction; (2) studying multi-domain proteins and multi-protein complexes as examples from nature, to learn how enzymes are activated by small molecules; (3) surveying different regions of the enzyme, away from its active site, that can be targeted for the design of allosteric activators and inhibitors.

Heat shock proteins as potential targets for protective strategies in neurodegeneration

Protein aggregates are hallmarks of nearly all age-related neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and several polyglutamine diseases such as Huntington's disease and different forms of spinocerebellar ataxias (SCA; SCA1-3, SCA6, and SCA7). The collapse of cellular protein homoeostasis can be both a cause and a consequence of this protein aggregation. Boosting components of the cellular protein quality control system has been widely investigated as a strategy to counteract protein aggregates or their toxic consequences. Heat shock proteins (HSPs) play a central part in regulating protein quality control and contribute to protein aggregation and disaggregation. Therefore, HSPs are viable targets for the development of drugs aimed at reducing pathogenic protein aggregates that are thought to contribute to the development of so many neurodegenerative disorders.

Aggregation of Trp > Glu point mutants of human gamma-D crystallin provides a model for hereditary or UV-induced cataract

Numerous mutations and covalent modifications of the highly abundant, long-lived crystallins of the eye lens cause their aggregation leading to progressive opacification of the lens, cataract. The nature and biochemical mechanisms of the aggregation process are poorly understood, as neither amyloid nor native-state polymers are commonly found in opaque lenses. The βγ-crystallin fold contains four highly conserved buried tryptophans, which can be oxidized to more hydrophilic products, such as kynurenine, upon UV-B irradiation. We mimicked this class of oxidative damage using Trp→Glu point mutants of human γD-crystallin. Such substitutions may represent a model of UV-induced photodamage-introduction of a charged group into the hydrophobic core generating "denaturation from within." The effects of Trp→Glu substitutions were highly position dependent. While each was destabilizing, only the two located in the bottom of the double Greek key fold-W42E and W130E-yielded robust aggregation of partially unfolded intermediates at 37°C and pH 7. The αB-crystallin chaperone suppressed aggregation of W130E, but not W42E, indicating distinct aggregation pathways from damage in the N-terminal vs C-terminal domain. The W130E aggregates had loosely fibrillar morphology, yet were nonamyloid, noncovalent, showed little surface hydrophobicity, and formed at least 20°C below the melting temperature of the native β-sheets. These features are most consistent with domain-swapped polymerization. Aggregation of partially destabilized crystallins under physiological conditions, as occurs in this class of point mutants, could provide a simple in vitro model system for drug discovery and optimization.

Small molecules, both dietary and endogenous, influence the onset of lens cataracts

How the lens ages successfully is a lesson in biological adaption and the emergent properties of its complement of cells and proteins. This living tissue contains some of the oldest proteins in our bodies and yet they remain functional for decades, despite exposure to UV light, to reactive oxygen species and all the other hazards to protein function. This remarkable feat is achieved by a shrewd investment in very stable proteins as lens crystallins, by providing a reservoir of ATP-independent protein chaperones unequalled by any other tissue and by an oxidation-resistant environment. In addition, glutathione, a free radical scavenger, is present in mM concentrations and the plasma membranes contain oxidation-resistant sphingolipids what compromises lens function as it ages? In this review, we examine the role of small molecules in the prevention or causation of cataracts, including those associated with diet, metabolic pathways and drug therapy (steroids).

Systems biology of lens development: A paradigm for disease gene discovery in the eye

Over the past several decades, the biology of the developing lens has been investigated using molecular genetics-based approaches in various vertebrate model systems. These efforts, involving target gene knockouts or knockdowns, have led to major advances in our understanding of lens morphogenesis and the pathological basis of cataracts, as well as of other lens related eye defects. In particular, we now have a functional understanding of regulators such as Pax6, Six3, Sox2, Oct1 (Pou2f1), Meis1, Pnox1, Zeb2 (Sip1), Mab21l1, Foxe3, Tfap2a (Ap2-alpha), Pitx3, Sox11, Prox1, Sox1, c-Maf, Mafg, Mafk, Hsf4, Fgfrs, Bmp7, and Tdrd7 in this tissue. However, whether these individual regulators interact or their targets overlap, and the significance of such interactions during lens morphogenesis, is not well defined. The arrival of high-throughput approaches for gene expression profiling (microarrays, RNA-sequencing (RNA-seq), etc.), which can be coupled with chromatin immunoprecipitation (ChIP) or RNA immunoprecipitation (RIP) assays, along with improved computational resources and publically available datasets (e.g. those containing comprehensive protein-protein, protein-DNA information), presents new opportunities to advance our understanding of the lens tissue on a global systems level. Such systems-level knowledge will lead to the derivation of the underlying lens gene regulatory network (GRN), defined as a circuit map of the regulator-target interactions functional in lens development, which can be applied to expedite cataract gene discovery. In this review, we cover the various systems-level approaches such as microarrays, RNA-seq, and ChIP that are already being applied to lens studies and discuss strategies for assembling and interpreting these vast amounts of high-throughput information for effective dispersion to the scientific community. In particular, we discuss strategies for effective interpretation of this new information in the context of the rich knowledge obtained through the application of traditional single-gene focused experiments on the lens. Finally, we discuss our vision for integrating these diverse high-throughput datasets in a single web-based user-friendly tool iSyTE (integrated Systems Tool for Eye gene discovery) - a resource that is already proving effective in the identification and characterization of genes linked to lens development and cataract. We anticipate that application of a similar approach to other ocular tissues such as the retina and the cornea, and even other organ systems, will significantly impact disease gene discovery.