Influence of molecular and chemical chaperones on protein folding

Protein folding inside the cell involves the Participation of accessory components known as molecular chaperones. In addition to their active participation in the folding process, molecular chaperones serve as a type of ‘quality control system’, recognizing, retaining and targeting misfolded proteins for their eventual degradation. It is now known that a number of human diseases arise as a consequence of specific point mutations or deletions within genes encoding essential proteins. In many cases these mutations/deletions are not so sever as to totally destroy the biological activity of the particular gene product. Rather, the mutations often result in only subtle folding abnormalities which lead to the newly synthesized protein being retained at the endoplasmic reticulum by the actions of the cellylar quality control system. In this short review article we discuss our recent studies showing that the protein folding defect associated with the most common mutation in patients with cystic fibriosis can be overcome by a novel strategy. As shown in the paper by Brown et al in this issue (Brown et al 1996), a number of different low molecular weight compounds, all known to stabilize proteins in their native conformation, are effective in rescuing the processing defect of the mutant cystic fibrosis transmembrane conductance regulator protein. We then discuss how these same compounds, which we now call chemical chaperones, also may prove to be effective in correcting a number of other protein folding abnormalities which constitute the underlying basis of a large number of different human diseases.     

Muscular dystrophies, dilated cardiomyopathy, lipodystrophy and neuropathy: the nuclear connection

An understanding of muscle structure and function is central to improving our knowledge of the group of muscle diseases referred to as muscular dystrophies. These diseases involve a progressive weakening and wasting of skeletal muscle, which can be associated with life-threatening cardiac arrhythmias. The vast majority of these diseases arise from defects in either cytoskeletal or structural proteins, resulting in a breakdown of muscle cell integrity. However, mutations in two nuclear proteins--emerin and lamin A/C--have also been demonstrated to give rise to a muscular dystrophy phenotype. In addition, mutations in lamin A/C can give rise to a dilated cardiomyopathy, a lipodystrophy or a neuropathy. It is far from clear how mutations in nuclear proteins can result in a dystrophy, or cause more than one clinically distinct disease. Understanding the functional role of nuclear proteins in causing these diseases will therefore provide novel insights into muscle function, and should hopefully provide new directions for treatment.     

The laminopathies: nuclear structure meets disease

Most inherited diseases are associated with mutations in a specific gene. Sometimes, mutations in two or more different genes result in diseases with a similar phenotype. Rarely do different mutations in the same gene result in a multitude of seemingly different and unrelated diseases. In the past three years, different mutations in LMNA, the gene encoding the A-type lamins, have been shown to be associated with at least six different diseases. These diseases and at least two others caused by mutations in other proteins associated with the nuclear lamina are collectively called the laminopathies. How different tissue-specific diseases arise from unique mutations in the LMNA gene, encoding almost ubiquitously expressed nuclear proteins, are providing tantalizing insights into the structural organization of the nucleus, its relation to nuclear function in different tissues and the involvement of the nuclear envelope in the development of disease.     

Mitochondrial protein import and human health and disease

The targeting and assembly of nuclear-encoded mitochondrial proteins are essential processes because the energy supply of humans is dependent upon the proper functioning of mitochondria. Defective import of mitochondrial proteins can arise from mutations in the targeting signals within precursor proteins, from mutations that disrupt the proper functioning of the import machinery, or from deficiencies in the chaperones involved in the proper folding and assembly of proteins once they are imported. Defects in these steps of import have been shown to lead to oxidative stress, neurodegenerative diseases, and metabolic disorders. In addition, protein import into mitochondria has been found to be a dynamically regulated process that varies in response to conditions such as oxidative stress, aging, drug treatment, and exercise. This review focuses on how mitochondrial protein import affects human health and disease.     

Molecular advances in understanding inherited prion diseases

The prion diseases are neurodegenerative disorders that have attracted great interest because of the possible link between bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (CTD) in humans. Possible transmission of these diseases has been linked to a single protein termed the prion protein. This protein is an abnormal isoform of a normal synaptic glycoprotein. The majority of prion diseases does not appear to be caused by transmission of an infectious agent but occur spontaneously with no known cause. The strongest supporting evidence that the prion protein is the causative agent in prion disease comes from specific inheritable forms of prion disease which are linked to single point mutations in the prion protein gene. Paradoxically, these point mutations, although autosomal dominant with 100% penetrance do not lead to disease until late in life. Molecular techniques are now being used extensively to determine how these point-mutations alter the prion protein’s normal structure and activity. This review deals with the latest insights into how inherited mutations in the prion protein gene lead to neurodegenerative disease.     

Mutations in the extracellular domain and in the membrane-spanning domains interfere with nicotinic acetylcholine receptor maturation

The deg-3(u662) mutation is a degeneration-causing mutation in a Caenorhabditis elegans nicotinic acetylcholine receptor. In a large screen for mutations that suppress the deleterious effects of this mutation we identified 32 mutations in the deg-3 gene. Among these, 11 are missense mutations, affecting seven residues within the extracellular domain or the membrane-spanning domains. All of these mutations greatly reduce the degeneration-causing activity of deg-3(u662). All but one of these mutations cause defective localization of the DEG-3 protein, as seen in immunohistochemical analysis. Thus our screen identifies multiple residues within the nicotinic acetylcholine receptor needed for normal folding, assembly, or trafficking of this receptor. Interestingly, these mutations lead to distinct localization defects suggesting differences in their effect on DEG-3's maturation process. Specifically, mutations in the extracellular domain lead to a phenotype more severe than mutations in the membrane-spanning domains. Differences in the effects of the mutations are also predicted by homology-based modeling, showing that some mutations in the extracellular domain are likely to disrupt the native fold of the protein, while others are likely to disrupt trafficking.     

Mouse models of diseases of megakaryocyte and platelet homeostasis

Platelets are the small anuclear blood cells that are the product of megakaryocytopoiesis, the process of hematopoietic stem cell commitment to megakaryocyte production and the differentiation and maturation of these cells for platelet release. Deregulation or disruption of megakaryocytopoiesis can result in platelet deficiencies, the thrombocytopenias, with attendant risk of hemorrhage or thrombocytosis, a pathological excess of platelet numbers. Mouse models, particularly those engineered to carry genetic alterations modeling mutations associated with human disease, have provided important insights into megakaryocytopoiesis and deregulation of this process in disease. This review focuses on mouse models of diseases of altered megakaryocyte and platelet number, illustrating the profound contribution of these models in validating suspected roles of disease-associated genetic alterations, promoting discovery of new links between genetic mutations and specific diseases, and providing unique tools for better understanding of disease pathophysiology and progression, as well as resources to define drug action or develop new therapeutic strategies.     

Strategies for Correcting the AF508 CFTR Protein-Folding Defect

Many human diseases arise as a result of mutations within genes encoding essential proteins. In many cases, the mutations are not so severe as to render the protein biologically inactive. Rather, the mutations oftentimes result in only subtle protein-folding abnormalities. In the case of the CFTR protein, a mutation leading to the loss of a single amino acid is responsible for the diseased state in the majority of individuals with cystic fibrosis. Here the newly synthesized mutant CFTR protein, missing a phenylalanine residue at position 508 (F508 CFTR), is unable to transit from the endoplasmic reticulum to the plasma membrane, where it functions as a regulator of chloride transport. All of the available evidence indicate that the newly synthesized F508 CFTR protein adopts a slightly altered conformation and therefore is retained at the level of the endoplasmic reticulum, ostensibly by the actions of the cellular quality control system. Because the mutant protein is capable of functioning as a chloride channel, developing ways to elicit its release out of the ER and to the plasma membrane has important clinical implications. Herein, we discuss our recent studies showing that the protein folding defect associated with the F508 CFTR mutation, as well as a number of other temperature-sensitive mutations, can be overcome by strategies designed to influence protein folding inside the cell. Specifically we show that a number of low-molecular-weight compounds, all of which are known to stabilize proteins in their native conformation, are effective in rescuing the folding and/or processing defects associated with different mutations that oftentimes lead to human disease.     

Biochemical consequences of mutations causing the GM2 gangliosidoses

The hydrolysis of GM2-ganglioside is unusual in its requirements for the correct synthesis, processing, and ultimate combination of three gene products. Whereas two of these proteins are the alpha- (HEXA gene) and beta- (HEXB) subunits of beta-hexosaminidase A, the third is a small glycolipid transport protein, the GM2 activator protein (GM2A), which acts as a substrate specific co-factor for the enzyme. A deficiency of any one of these proteins leads to storage of the ganglioside, primarily in the lysosomes of neuronal cells, and one of the three forms of GM2-gangliosidosis, Tay-Sachs disease, Sandhoff disease or the AB-variant form. Studies of the biochemical impact of naturally occurring mutations associated with the GM2 gangliosidoses on mRNA splicing and stability, and on the intracellular transport and stability of the affected protein have provided some general insights into these complex cellular mechanisms. However, such studies have revealed little in the way of structure-function information on the proteins. It appears that the detrimental effect of most mutations is not specifically on functional elements of the protein, but rather on the proteins' overall folding and/or intracellular transport. The few exceptions to this generalization are missense mutations at two codons in HEXA, causing the unique biochemical phenotype known as the B1-variant, and one codon in both the HEXB and GM2A genes. Biochemical characterization of these mutations has led to the localization of functional residues and/or domains within each of the encoded proteins.     

蛋白质动态平衡网络维稳机制的研究进展

蛋白质是构成生命体的基础,其在体内有多种存在形式,而这些形式之间如何维持动态平衡对细胞发挥正常功能来说至关重要。诸多因素会影响蛋白质稳态,包括一些内外源性的刺激、翻译过程中出现的问题以及一些调控因子的作用等。当错误折叠的蛋白在细胞中不断积累时,会导致蛋白质稳态失衡并无法发挥正常功能进而引发相关疾病。同时在错误发生后,机体的许多“监控”就会感知这些异常,并通过多种途径来帮助蛋白质维持稳态。本文旨在综述蛋白质动态平衡网络之间错综复杂的关系,以及各种因素在其中发挥的主要作用,并为一些由蛋白合成异常导致疾病的研究等提供新的思路。     

Protein folding in the post-genomic era

Protein folding is a topic of fundamental interest since it concerns the mechanisms by which the genetic message is translated into the three-dimensional and functional structure of proteins. In these post-genomic times, the knowledge of the fundamental principles are required in the exploitation of the information contained in the increasing number of sequenced genomes. Protein folding also has practical applications in the understanding of different pathologies and the development of novel therapeutics to prevent diseases associated with protein misfolding and aggregation. Significant advances have been made ranging from the Anfinsen postulate to the "new view" which describes the folding process in terms of an energy landscape. These new insights arise from both theoretical and experimental studies. The problem of folding in the cellular environment is briefly discussed. The modern view of misfolding and aggregation processes that are involved in several pathologies such as prion and Alzheimer diseases. Several approaches of structure prediction, which is a very active field of research, are described.     

Plasticity and steric strain in a parallel beta-helix: rational mutations in the P22 tailspike protein

By means of genetic screens, a great number of mutations that affect the folding and stability of the tailspike protein from Salmonella phage P22 have been identified. Temperature-sensitive folding (tsf) mutations decrease folding yields at high temperature, but hardly affect thermal stability of the native trimeric structure when assembled at low temperature. Global suppressor (su) mutations mitigate this phenotype. Virtually all of these mutations are located in the central domain of tailspike, a large parallel beta-helix. We modified tailspike by rational single amino acid replacements at three sites in order to investigate the influence of mutations of two types: (1) mutations expected to cause a tsf phenotype by increasing the side-chain volume of a core residue, and (2) mutations in a similar structural context as two of the four known su mutations, which have been suggested to stabilize folding intermediates and the native structure by the release of backbone strain, an effect well known for residues that are primarily evolved for function and not for stability or folding of the protein. Analysis of folding yields, refolding kinetics and thermal denaturation kinetics in vitro show that the tsf phenotype can indeed be produced rationally by increasing the volume of side chains in the beta-helix core. The high-resolution crystal structure of mutant T326F proves that structural rearrangements only take place in the remarkably plastic lumen of the beta-helix, leaving the arrangement of the hydrogen-bonded backbone and thus the surface of the protein unaffected. This supports the notion that changes in the stability of an intermediate, in which the beta-helix domain is largely formed, are the essential mechanism by which tsf mutations affect tailspike folding. A rational design of su mutants, on the other hand, appears to be more difficult. The exchange of two residues in the active site expected to lead to a drastic release of steric strain neither enhanced the folding properties nor the stability of tailspike. Apparently, side-chain interactions in these cases overcompensate for backbone strain, illustrating the extreme optimization of the tailspike protein for conformational stability. The result exemplifies the view arising from the statistical analysis of the distribution of backbone dihedral angles in known three-dimensional protein structures that the adoption of straight phi/psi angles other than the most favorable ones is often caused by side-chain interactions. Proteins 2000;39:89-101.     

Chemical and pharmacological chaperones as new therapeutic agents

Proteins that are exported from the cell, or targeted to the cell surface or other organelles, are synthesised and assembled in the endoplasmic reticulum and then delivered to their destinations. Point mutations - the most common cause of human genetic diseases - can inhibit folding and assembly of the protein in the endoplasmic reticulum. The unstable or partially folded mutant protein does not undergo trafficking and is usually rapidly degraded. A potential therapy for protein misfolding is to correct defective protein folding and trafficking using pharmacological chaperones. Pharmacological chaperones are substrates or modulators that appear to function by directly binding to the partially folded biosynthetic intermediate to stabilise the protein and allow it to complete the folding process to yield a functional protein. Initial clinical studies with pharmacological chaperones have successfully reduced clinical symptoms of disease. Therefore, pharmacological chaperones show great promise as a new class of therapeutic agents that can be specifically tailored for a particular genetic disease.     

Mapping Approaches to Gene Identification in Humans

Although a number of human genes that cause disease have been traced through the defective product, most genetic defects are recognized only by phenotype. When the biochemical defect is unknown, a gene can be located only through molecular approaches based on coinheritance (genetic linkage) of the disease phenotype with a particular allele of a polymorphic DNA marker that has already been mapped to a specific chromosomal region. Linkage studies in affected families have already localized genes for several important diseases, including cystic fibrosis. Finding a genetic linkage in families in which a disease segregates requires that the human genetic map have a large number of polymorphic markers; when the map is dense enough, any disease gene can be located by linkage to a known marker. Many DNA segments with a high degree of polymorphism are being found and mapped as markers in normal reference pedigrees. Genetic linkage mapping has implications even broader than its application to prenatal diagnosis or therapeutic strategy; analyzing mutations in important genes will illuminate basic mechanisms in molecular biology and the early events that lead to cancer and other disorders.     

Missense mutations of dual oxidase 2 (DUOX2) implicated in congenital hypothyroidism have impaired trafficking in cells reconstituted with DUOX2 maturation factor

Dual oxidase 2 (DUOX2), a reduced NAD phosphate:O2 oxidoreductase flavoprotein, is a component of the thyrocyte H2O2 generator required for hormone synthesis at the apical plasma membrane. We recently identified a specific DUOX2 maturation factor (DUOXA2) that is necessary and sufficient for expression of functional DUOX2 in mammalian cell lines. We have now used a DUOXA2 reconstituted system to provide the first characterization of natural DUOX2 missense variants (Q36H, R376W, D506N) at the molecular level, analyzing their impact on H2O2 generation, trafficking, stability, folding, and DUOXA2 interaction. The Q36H and R376W mutations completely prevent routing of DUOX2 to the cell surface. The mutant proteins are predominantly present as core N-glycosylated, thiol-reduced folding intermediates, which are retained by the quality control system within the endoplasmic reticulum (ER) as indicated by increased complexation with the lectin calnexin. D506N displays a partial deficiency phenotype with reduced surface expression of a mutant protein with normal intrinsic activity in generating H2O2. D506N N-glycan moieties are not subject to normal modification in the Golgi apparatus, suggesting that nonnative protein can escape the quality control in the ER. Oxidative folding of DUOX2 in the ER appears to be the rate-limiting step in the maturation of DUOX2, but is not facilitated by DUOXA2. Rather, DUOXA2 allows rapid ER exit of folded DUOX2 or enhanced degradation of mutant DUOX2 proteins not competent for ER exit. DUOXA2 may thus be part of a secondary quality control system specific for DUOX2.     

Cellular pathophysiology of cystic kidney disease: insight into future therapies.

Polycystic kidney disease (PKD) is a developmental kidney disorder which can be inherited as either an autosomal dominant trait, with an incidence of 1:50 to 1:1000, or as an autosomal recessive trait with an incidence of 1:6,000 to 1:40,000. Three different genes have now been cloned that are associated with mutations that cause PKD. Two of these are linked to the most common forms of the dominant disease while the third is associated with the orpk mouse model of recessive polycystic kidney disease. Advances in understanding the molecular genetics of PKD have been paralleled by new insights into the cellular pathophysiology of cyst formation and progressive enlargement. Current data suggest that a number of PKD proteins may interact in a complex, which when disrupted by mutations in PKD genes may lead to altered epithelial proliferative activity, secretion, and cell matrix biology. The identification of a unique cystic epithelial phenotype presents new opportunities for targeted therapies. These include targeted gene therapy, gene complementation, and specific immunological or pharmacological interruption of growth factor pathways.     

How mitochondrial damage affects cell function

The pathophysiology of mitochondrial DNA (mtDNA) diseases is caused by increased cell death and dysfunction due to the accumulation of mutations to mtDNA. While the disruption of oxidative phosphorylation is central to mtDNA diseases, many other factors, such as Ca²⁺ dyshomeostasis, increased oxidative stress and defective turnover of mitochondrial proteins, may also contribute. The relative importance of these processes in causing cell dysfunction and death is uncertain. It is also unclear whether these damaging processes lead to the disease phenotype through affecting cell function, increasing cell death or a combination of both. These uncertainties limit our understanding of mtDNA disease pathophysiology and our ability to develop rational therapies. Here, we outline how the accumulation of mtDNA mutations can lead to cell dysfunction by altering oxidative phosphorylation, Ca²⁺ homeostasis, oxidative stress and protein turnover and discuss how these processes affect cell function and susceptibility to cell death. A better understanding of these processes will eventually clarify why particular mtDNA mutations cause defined syndromes in some cases but not in others and why the same mutation can lead to different phenotypes.     

Voltage-gated calcium channels in genetic diseases

Voltage-gated calcium channels (VGCCs) mediate calcium entry into excitable cells in response to membrane depolarization. During the past decade, our understanding of the gating and functions of VGCCs has been illuminated by the analysis of mutations linked to a heterogeneous group of genetic diseases called "calcium channelopathies". Calcium channelopathies include muscular, neurological, cardiac and vision syndromes. Recent data suggest that calcium channelopathies result not only from electrophysiological defects but also from altered alpha(1)/Ca(V) subunit protein processing, including folding, posttranslational modifications, quality control and trafficking abnormalities. Overall, functional analyses of VGCC mutations provide a more comprehensive view of the corresponding human disorders and offer important new insights into VGCC function. Ultimately, the understanding of these pathogenic channel mutations should lead to improved treatments of such hereditary diseases in humans.