Chemical chaperones: a pharmacological strategy for disorders of protein folding and trafficking

Endoplasmic reticulum stress inhibition protects steatotic and non-steatotic livers in partial hepatectomy under ischemia-reperfusion

During partial hepatectomy, ischemia-reperfusion (I/R) is commonly applied in clinical practice to reduce blood flow. Steatotic livers show impaired regenerative response and reduced tolerance to hepatic injury. We examined the effects of tauroursodeoxycholic acid (TUDCA) and 4-phenyl butyric acid (PBA) in steatotic and non-steatotic livers during partial hepatectomy under I/R (PH+I/R). Their effects on the induction of unfolded protein response (UPR) and endoplasmic reticulum (ER) stress were also evaluated. We report that PBA, and especially TUDCA, reduced inflammation, apoptosis and necrosis, and improved liver regeneration in both liver types. Both compounds, especially TUDCA, protected both liver types against ER damage, as they reduced the activation of two of the three pathways of UPR (namely inositol-requiring enzyme and PKR-like ER kinase) and their target molecules caspase 12, c-Jun N-terminal kinase and C/EBP homologous protein-10. Only TUDCA, possibly mediated by extracellular signal-regulated kinase upregulation, inactivated glycogen synthase kinase-3β. This is turn, inactivated mitochondrial voltage-dependent anion channel, reduced cytochrome c release from the mitochondria and caspase 9 activation and protected both liver types against mitochondrial damage. These findings indicate that chemical chaperones, especially TUDCA, could protect steatotic and non-steatotic livers against injury and regeneration failure after PH+I/R.

Reducing the effects of intracellular accumulation of thermolabile collagen II mutants by increasing their thermostability in cell culture conditions

Mutations in collagen II are associated with spondyloepiphyseal dysplasia, a group of heritable diseases whose common features include aberrations of skeletal growth. The mechanisms through which mutations in collagen II affect the cartilaginous tissues are complex and include both intracellular and extracellular processes. One of those mechanisms involves cellular stress caused by excessive accumulation of misfolded collagen II mutants. We investigated whether stabilizing the structure of thermolabile R789C and R992C collagen II mutants would improve their secretion from cells, thereby reducing cellular stress and apoptosis. Employing glycerol and trimethylamine N-oxide (TMAO), chemicals that increase the thermostability of collagen triple helices, we demonstrated that those compounds function as chaperones and stabilize the R789C and R992C mutants, accelerate their secretion, and improve cell survival. Our study provides a scientific basis for considering misfolded triple helices of collagen mutants a target for reducing the deleterious effects caused by their excessive intracellular accumulation.

Linking endoplasmic reticulum stress to cell death in hepatocytes: roles of C/EBP homologous protein and chemical chaperones in palmitate-mediated cell death

Prolonged endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR) have been linked to apoptosis via several mechanisms, including increased expression of C/EBP homologous protein (Chop). Increased long-chain fatty acids, in particular saturated fatty acids, induce ER stress, Chop expression, and apoptosis in liver cells. The first aim of the present study was to determine the role of Chop in lipid-induced hepatocyte cell death and liver injury induced by a methionine-choline-deficient diet. Albumin-bound palmitate increased Chop gene and protein expression in a dose-dependent fashion in H4IIE liver cells. siRNA-mediated silencing of Chop in H4IIE liver cells reduced thapsigargin-mediated cell death by approximately 40% and delayed palmitate-mediated cell death, but only at high concentrations of palmitate (400-500 microM). Similar results were observed in primary hepatocytes isolated from Chop-knockout mice. Indices of liver injury were also not reduced in Chop-knockout mice provided a methionine-choline-deficient diet. To ascertain whether ER stress was linked to palmitate-induced cell death, primary hepatocytes were incubated in the absence or presence of the chemical chaperones taurine-conjugated ursodeoxycholic acid or 4-phenylbutyric acid. The presence of either of these chemical chaperones protected liver cells from palmitate-mediated ER stress and cell death, in part, via inhibition of JNK activation. These data suggest that ER stress is linked to palmitate-mediated cell death via mechanisms that include JNK activation.

Recovery of PEX1-Gly843Asp peroxisome dysfunction by small-molecule compounds

Zellweger spectrum disorder (ZSD) is a heterogeneous group of diseases with high morbidity and mortality caused by failure to assemble normal peroxisomes. There is no therapy for ZSD, but management is supportive. Nevertheless, one-half of the patients have a phenotype milder than classic Zellweger syndrome and exhibit a progressive disease course. Thus, patients would benefit if therapies became available and were instituted early. Recent reports indicate several interventions that result in partial peroxisome recovery in ZSD fibroblasts. To identify drugs that recover peroxisome functions, we expressed a GFP-peroxisome targeting signal 1 reporter in fibroblasts containing the common disease allele, PEX1-p.Gly843Asp. The GFP reporter remained cytosolic at baseline, and improvement in peroxisome functions was detected by the redistribution of the GFP reporter from the cytosol to the peroxisome. We established a high-content screening assay based on this phenotype assay and evaluated 2,080 small molecules. The cells were cultured in chemical for 2 days and then, were fixed and imaged by epifluorescent microscopy on a high-content imaging platform. We identified four compounds that partially recover matrix protein import, and we confirmed three using independent assays. Our results suggest that PEX1-p.G843D is a misfolded protein amenable to chaperone therapy.

The hepatic microcirculation: mechanistic contributions and therapeutic targets in liver injury and repair

The complex functions of the liver in biosynthesis, metabolism, clearance, and host defense are tightly dependent on an adequate microcirculation. To guarantee hepatic homeostasis, this requires not only a sufficient nutritive perfusion and oxygen supply, but also a balanced vasomotor control and an appropriate cell-cell communication. Deteriorations of the hepatic homeostasis, as observed in ischemia/reperfusion, cold preservation and transplantation, septic organ failure, and hepatic resection-induced hyperperfusion, are associated with a high morbidity and mortality. During the last two decades, experimental studies have demonstrated that microcirculatory disorders are determinants for organ failure in these disease states. Disorders include 1) a dysregulation of the vasomotor control with a deterioration of the endothelin-nitric oxide balance, an arterial and sinusoidal constriction, and a shutdown of the microcirculation as well as 2) an overwhelming inflammatory response with microvascular leukocyte accumulation, platelet adherence, and Kupffer cell activation. Within the sequelae of events, proinflammatory mediators, such as reactive oxygen species and tumor necrosis factor-alpha, are the key players, causing the microvascular dysfunction and perfusion failure. This review covers the morphological and functional characterization of the hepatic microcirculation, the mechanistic contributions in surgical disease states, and the therapeutic targets to attenuate tissue injury and organ dysfunction. It also indicates future directions to translate the knowledge achieved from experimental studies into clinical practice. By this, the use of the recently introduced techniques to monitor the hepatic microcirculation in humans, such as near-infrared spectroscopy or orthogonal polarized spectral imaging, may allow an early initiation of treatment, which should benefit the final outcome of these critically ill patients.

The unfolded protein response and its relevance to connective tissue diseases

The unfolded protein response (UPR) has evolved to counter the stresses that occur in the endoplasmic reticulum (ER) as a result of misfolded proteins. This sophisticated quality control system attempts to restore homeostasis through the action of a number of different pathways that are coordinated in the first instance by the ER stress-senor proteins IRE1, ATF6 and PERK. However, prolonged ER-stress-related UPR can have detrimental effects on cell function and, in the longer term, may induce apoptosis. Connective tissue cells such as fibroblasts, osteoblasts and chondrocytes synthesise and secrete large quantities of proteins and mutations in many of these gene products give rise to heritable disorders of connective tissues. Until recently, these mutant gene products were thought to exert their effect through the assembly of a defective extracellular matrix that ultimately disrupted tissue structure and function. However, it is now becoming clear that ER stress and UPR, because of the expression of a mutant gene product, is not only a feature of, but may be a key mediator in the initiation and progression of a whole range of different connective tissue diseases. This review focuses on ER stress and the UPR that characterises an increasing number of connective tissue diseases and highlights novel therapeutic opportunities that may arise.

Knitting and untying the protein network: modulation of protein ensembles as a therapeutic strategy

Proteins constitute the working machinery and structural support of all organisms. In performing a given function, they must adopt highly specific structures that can change with their level of activity, often through the direct or indirect action of other proteins. Indeed, proteins typically function within an ensemble, rather than individually. Hence, they must be sufficiently flexible to interact with each other and execute diverse tasks. The discovery that errors within these groups can ultimately cause disease has led to a paradigm shift in drug discovery, from an emphasis on single protein targets to a holistic approach whereby entire ensembles are targeted.

Genetic diseases of connective tissues: cellular and extracellular effects of ECM mutations

Tissue-specific extracellular matrices (ECMs) are crucial for normal development and tissue function, and mutations in ECM genes result in a wide range of serious inherited connective tissue disorders. Mutations cause ECM dysfunction by combinations of two mechanisms. First, secretion of the mutated ECM components can be reduced by mutations affecting synthesis or by structural mutations causing cellular retention and/or degradation. Second, secretion of mutant protein can disturb crucial ECM interactions, structure and stability. Moreover, recent experiments suggest that endoplasmic reticulum (ER) stress, caused by mutant misfolded ECM proteins, contributes to the molecular pathology. Targeting ER stress might offer a new therapeutic strategy.

Endoplasmic reticulum stress plays a central role in development of leptin resistance

Leptin has not evolved as a therapeutic modality for the treatment of obesity due to the prevalence of leptin resistance in a majority of the obese population. Nevertheless, the molecular mechanisms of leptin resistance remain poorly understood. Here, we show that increased endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR) in the hypothalamus of obese mice inhibits leptin receptor signaling. The genetic imposition of reduced ER capacity in mice results in severe leptin resistance and leads to a significant augmentation of obesity on a high-fat diet. Moreover, we show that chemical chaperones, 4-phenyl butyric acid (PBA), and tauroursodeoxycholic acid (TUDCA), which have the ability to decrease ER stress, act as leptin-sensitizing agents. Taken together, our results may provide the basis for a novel treatment of obesity.

Modulation of the activity of newly synthesized human phenylalanine hydroxylase mutant proteins by low-molecular-weight compounds

Phenylketonuria, the most frequent disorder of amino acid metabolism, is caused by a deficient activity of human phenylalanine hydroxylase (hPAH). Rescue of the enzyme activity of several recombinant hPAH mutant forms (I65T, R261Q, R270K and V388M) by low molecular weight compounds namely glycerol, trimethylamine N-oxide (TMAO) and sodium 4-phenylbutyrate (4-PB) was investigated using a prokaryotic expression model. The studied compounds were added to the culture medium, in a concentration dependent manner, simultaneously to induction of protein expression. Among the tested molecules glycerol and TMAO were able to increase the enzyme activity of the studied mutant proteins. Furthermore, a decrease in aggregates and a recovery of the active tetrameric and dimeric forms were detected. Since the addition of the studied compounds to the medium did not change the expression level of E. Coli molecular chaperones we postulate that glycerol and TMAO rescue results from a direct stabilizing effect of the newly synthesized mutant hPAH enzymes.

Protein quality control: the who's who, the where's and therapeutic escapes

In cells the quality of newly synthesized proteins is monitored in regard to proper folding and correct assembly in the early secretory pathway, the cytosol and the nucleoplasm. Proteins recognized as non-native in the ER will be removed and degraded by a process termed ERAD. ERAD of aberrant proteins is accompanied by various changes of cellular organelles and results in protein folding diseases. This review focuses on how the immunocytochemical labeling and electron microscopic analyses have helped to disclose the in situ subcellular distribution pattern of some of the key machinery proteins of the cellular protein quality control, the organelle changes due to the presence of misfolded proteins, and the efficiency of synthetic chaperones to rescue disease-causing trafficking defects of aberrant proteins.

The improvement effect of L-Lys as a chemical chaperone on STZ-induced diabetic rats, protein structure and function

BACKGROUND

L-Lysine (L-Lys) has been known as an inhibitor of protein glycation; however, its long-term use for diabetes treatment considering different aspects of diabetic complication is not seen in the literature. In addition, the effect of L-Lys, as a chemical chaperone, was considered on protein folding and activity.

METHODS

The streptozotocin-induced diabetic rats were used as a model of diabetes. Normal and diabetic rats were studied for 5 months with and without 0.1% of L-Lys in drinking water. Serum glucose, advanced glycation end product (AGEs), haemoglobin A(1C) (HbA(1c)), triglyceride, total cholesterol, HDL-cholesterol, antioxidant activity, advanced oxidation protein products, fasting insulin level and body weight were determined at 4-week intervals. Heat shock protein (HSP)70, Lecithin: cholesterol acyl transferase (LCAT) and paraoxonase activity were determined 1 week after diabetes induction (time 0), and after 3 and 5 months. The structure of glycated and normal serum albumin (Alb) in the presence and absence of L-Lys was also investigated in an in vitro study using spectrofluorometry and circular dichroism (CD).

RESULTS

We found that L-Lysine therapy prevented diabetic- induced increases in Glc, AGE, HbA(1c), triglyceride, total- and LDL- cholesterol, and it caused an increase in the decreased antioxidant capacity, HDL-c, HDL functionality and HSP70. L-Lys had no effect on serum insulin level. The conformation of Alb changed due to glycation and L-Lys retained it similar to the native.

CONCLUSIONS

L-Lys, not only as an inhibitor of glycation but also as a chemical chaperone and a protein chaperone inducer, causes effective changes in many parameters of the model animals. However, it is not enough to achieve complete improvement.

Genetic Abnormalities of Surfactant Metabolism

Pulmonary surfactant is the complex mixture of lipids and proteins needed to reduce alveolar surface tension at the air-liquid interface and prevent alveolar collapse at the end of expiration. It has been recognized for almost 50 years that a deficiency in surfactant production due to pulmonary immaturity is the principal cause of the respiratory distress syndrome (RDS) observed in prematurely born infants.1 Secondary surfactant deficiency due to injury to the cells involved in its production and functional inactivation of surfactant is also important in the pathophysiology of acute respiratory distress syndrome (ARDS) observed in older children and adults.2,3 In the past 15 years, it has been recognized that surfactant deficiency may result from genetic mechanisms involving mutations in genes encoding critical components of the surfactant system or proteins involved in surfactant metabolism.4,5 Although rare, these single gene disorders provide important insights into normal surfactant metabolism and into the genes in which frequently occurring allelic variants may be important in more common pulmonary diseases.

A counterintuitive approach to treat enzyme deficiencies: use of enzyme inhibitors for restoring mutant enzyme activity

Pharmacological chaperone therapy is an emerging counterintuitive approach to treat protein deficiencies resulting from mutations causing misfolded protein conformations. Active-site-specific chaperones (ASSCs) are enzyme active-site directed small molecule pharmacological chaperones that act as a folding template to assist protein folding of mutant proteins in the endoplasmic reticulum (ER). As a result, excessive degradation of mutant proteins in the ER-associated degradation (ERAD) machinery can be prevented, thus restoring enzyme activity. Lysosomal storage disorders (LSDs) are suitable candidates for ASSC treatment, as the levels of enzyme activity needed to prevent substrate storage are relatively low. In addition, ASSCs are orally active small molecules and have potential to gain access to most cell types to treat neuronopathic LSDs. Competitive enzyme inhibitors are effective ASSCs when they are used at sub-inhibitory concentrations. This whole new paradigm provides excellent opportunity for identifying specific drugs to treat a broad range of inherited disorders. This review describes protein misfolding as a pathophysiological cause in LSDs and provides an overview of recent advances in the development of pharmacological chaperone therapy for the diseases. In addition, a generalized guidance for the design and screening of ASSCs is also presented.

Effect of spermine on lipid profile and HDL functionality in the streptozotocin-induced diabetic rat model

This study aimed to investigate the effect of spermine (Spm) as a chemical chaperone and glycation inhibitor on the lipid profile and HDL functionality in the short- and long-term treatment of the STZ-induced diabetic rats. Male Wistar rats were divided into 4 groups (control, n=7; diabetic, n=9). Two groups (named 2 and 3) were injected intraperitoneally with streptozotocin. Control rats (named 1 and 4) were injected with vehicle alone. The treatment of diabetic and control animals (groups 3 and 4) with 60 micromol/l of Spm in drinking water was begun. The study continued up to the end of the fifth month. The serum glucose and insulin level, AGE formation, lipid profile, paraoxonase 1 (PON1), and lecithin: cholesterol acyl transferase (LCAT) activities were measured. Significantly lower plasma PON1, and LCAT activities and higher serum AGE, TG, TC and LDL-c, and lower HDL-c were seen in diabetic rats as compared to control groups (P<0.01). The increased AGE, TG, TC and LDL-c levels in diabetic groups decreased gradually after receiving Spm. In addition, due to Spm administration, an increase in the HDL-c level was observed after the first month of the experiment (P<0.01). The increase in the PON1 and LCAT activities in the diabetic group that received Spm was significant after the second and the forth month of the experiment, P<0.02 and P<0.05, respectively. In conclusion, spermine administration attenuated the changed parameters to near normal values in diabetic rats. Spermine, despite a lack of significant changes on glucose metabolism and insulin secretion, was found to improve diabetes complications.

Thiamine diphosphate binds to intermediates in the assembly of adenovirus fiber knob trimers in Escherichia coli

Assembly of the adenovirus (Ad) homotrimeric fiber protein is nucleated by its C-terminal knob domain, which itself can trimerize when expressed as a recombinant protein fragment. The non-interlocked, globular structure of subunits in the knob trimer implies that trimers assemble from prefolded monomers through a dimer intermediate, but these intermediates have not been observed and the mechanism of assembly therefore remains uncharacterized. Here we report that expression of the Ad serotype 2 (Ad2) knob was toxic for thi- strains of Escherichia coli, which are defective in de novo synthesis of thiamine (vitamin B1). Ad2 knob trimers isolated from a thi+ strain copurified through multiple chromatography steps with a small molecule of mass equivalent to that of thiamine diphosphate (ThDP). Mutant analysis did not implicate any specific site for ThDP binding. Our results suggest that ThDP may associate with assembly intermediates and become trapped in assembled trimers, possibly within one of several large cavities that are partially solvent-accessible or buried completely within the trimer interior.

Rescue of degradation-prone mutants of the FK506-rapamycin binding (FRB) protein with chemical ligands

We recently reported that certain mutations in the FK506-rapamycin binding (FRB) domain disrupt its stability in vitro and in vivo (Stankunas et al. Mol. Cell, 2003, 12, 1615). To determine the precise residues that cause instability, we calculated the folding free energy (Delta G) of a collection of FRB mutants by measuring their intrinsic tryptophan fluorescence during reversible chaotropic denaturation. Our results implicate the T2098L point mutation as a key determinant of instability. Further, we found that some of the mutants in this collection were destabilized by up to 6 kcal mol(-1) relative to the wild type. To investigate how these mutants behave in cells, we expressed firefly luciferase fused to FRB mutants in African green monkey kidney (COS) cell lines and mouse embryonic fibroblasts (MEFs). When unstable FRB mutants were used, we found that the protein levels and the luminescence intensities were low. However, addition of a chemical ligand for FRB, rapamycin, restored luciferase activity. Interestingly, we found a roughly linear relationship between the Delta G of the FRB mutants calculated in vitro and the relative chemical rescue in cells. Because rapamycin is capable of simultaneously binding both FRB and the chaperone, FK506-binding protein (FKBP), we next examined whether FKBP might contribute to the protection of FRB mutants. Using both in vitro experiments and a cell-based model, we found that FKBP stabilizes the mutants. These findings are consistent with recent models that suggest damage to intrinsic Delta G can be corrected by pharmacological chaperones. Further, these results provide a collection of conditionally stable fusion partners for use in controlling protein stability.

Chemical chaperone-mediated protein folding: stabilization of P22 tailspike folding intermediates by glycerol

Polyol co-solvents such as glycerol increase the thermal stability of proteins. This has been explained by preferential hydration favoring the more compact native over the denatured state. Although polyols are also expected to favor aggregation by the same mechanism, they have been found to increase the folding yields of some large, aggregation-prone proteins. We have used the homotrimeric phage P22 tailspike protein to investigate the origin of this effect. The folding of this protein is temperature-sensitive and limited by the stability of monomeric folding intermediates. At non-permissive temperature (≥35°C), tailspike refolding yields were increased significantly in the presence of 1–4 mglycerol. At low temperature, tailspike refolding is prevented when folding intermediates are destabilized by the addition of urea. Glycerol could offset the urea effect, suggesting that the polyol acts by stabilizing crucial folding intermediates and not by increasing solvent viscosity. The stabilization effect of glycerol on tailspike folding intermediates was confirmed in experiments using a temperature-sensitive folding mutant protein, by fluorescence measurements of subunit folding kinetics, and by temperature up-shift experiments. Our results suggest that the chemical chaperone effect of polyols observed in the folding of large proteins is due to preferential hydration favoring structure formation in folding intermediates.

Opioid receptor pharmacological chaperones act by binding and stabilizing newly synthesized receptors in the endoplasmic reticulum

Accumulating evidence has indicated that membrane-permeable G protein-coupled receptor ligands can enhance cell surface targeting of their cognate wild-type and mutant receptors. This pharmacological chaperoning was thought to result from ligand-mediated stabilization of immature receptors in the endoplasmic reticulum (ER). In the present study, we directly tested this hypothesis using wild-type and mutant forms of the human delta-opioid receptor as models. ER-localized receptors were isolated by expressing the receptors in HEK293 cells under tightly controlled tetracycline induction and blocking their ER export with brefeldin A. The ER-retained delta-opioid receptor precursors were able to bind [(3)H]diprenorphine with high affinity, and treatment of cells with an opioid antagonist naltrexone led to a 2-fold increase in the number of binding sites. After removing the transport block, the antagonist-mediated increase in the number of receptors was detectable at the cell surface by flow cytometry and cell surface biotinylation assay. Importantly, opioid ligands, both antagonists and agonists, were found to stabilize the ER-retained receptor precursors in an in vitro heat inactivation assay and the treatment enhanced dissociation of receptor precursors from the molecular chaperone calnexin. Thus, we conclude that pharmacological chaperones facilitate plasma membrane targeting of delta-opioid receptors by binding and stabilizing receptor precursors, thereby promoting their release from the stringent ER quality control.

Transcriptional regulation by protein kinase A in Cryptococcus neoformans

A defect in the PKA1 gene encoding the catalytic subunit of cyclic adenosine 5'-monophosphate (cAMP)-dependent protein kinase A (PKA) is known to reduce capsule size and attenuate virulence in the fungal pathogen Cryptococcus neoformans. Conversely, loss of the PKA regulatory subunit encoded by pkr1 results in overproduction of capsule and hypervirulence. We compared the transcriptomes between the pka1 and pkr1 mutants and a wild-type strain, and found that PKA influences transcript levels for genes involved in cell wall synthesis, transport functions such as iron uptake, the tricarboxylic acid cycle, and glycolysis. Among the myriad of transcriptional changes in the mutants, we also identified differential expression of ribosomal protein genes, genes encoding stress and chaperone functions, and genes for secretory pathway components and phospholipid synthesis. The transcriptional influence of PKA on these functions was reminiscent of the linkage between transcription, endoplasmic reticulum stress, and the unfolded protein response in Saccharomyces cerevisiae. Functional analyses confirmed that the PKA mutants have a differential response to temperature stress, caffeine, and lithium, and that secretion inhibitors block capsule production. Importantly, we also found that lithium treatment limits capsule size, thus reinforcing potential connections between this virulence trait and inositol and phospholipid metabolism. In addition, deletion of a PKA-regulated gene, OVA1, revealed an epistatic relationship with pka1 in the control of capsule size and melanin formation. OVA1 encodes a putative phosphatidylethanolamine-binding protein that appears to negatively influence capsule production and melanin accumulation. Overall, these findings support a role for PKA in regulating the delivery of virulence factors such as the capsular polysaccharide to the cell surface and serve to highlight the importance of secretion and phospholipid metabolism as potential targets for anti-cryptococcal therapy.

Cataract: Window for systemic disorders

Cataract is the leading cause of visual handicap throughout the world, and almost all elderly individuals develop lens opacities. Epidemiological studies have shown that nuclear cataracts in young adults are associated with higher mortality. Many cataractogenic stressors induce endoplasmic reticulum (ER) stress, which in turn induces the unfolded protein response (UPR). The UPR can damage or kill a wide range of cell types and may be involved in many human diseases. We hypothesize that a cataract can be considered a window that can indicate the presence of systemic disorders. This is important because cataract is easily detected during a routine ocular examination. The slightest opacity in any region of the lenses, especially in younger patients, may be a sign of systemic disorders. Earlier detection of systemic disorders can save the lives of patients. If our hypothesis is correct, then elimination of known ER/cataractogenic stressors from individuals with cataracts should be the one of the first steps for treatments of the systemic disorders. We discuss the potential risk factors and beneficial effects of removal of such risk factors in patients with early cataracts. All patients with cataract should be referred for comprehensive medical examination.

Retinal ischemic injury rescued by sodium 4-phenylbutyrate in a rat model

Retinal ischemia is a common cause of visual impairment for humans and animals. Herein, the neuroprotective effects of phenylbutyrate (PBA) upon retinal ischemic injury were investigated using a rat model. Retinal ganglion cells (RGCs) were retrograde labeled with the fluorescent tracer fluorogold (FG) applied to the superior collicoli of test Sprague-Dawley rats. High intraocular pressure and retinal ischemia were induced seven days subsequent to such FG labeling. A dose of either 100 or 400 mg/kg PBA was administered intraperitoneally to test rats at two time points, namely 30 min prior to the induction of retinal ischemia and 1 h subsequent to the cessation of the procedure inducing retinal ischemia. The test-rat retinas were collected seven days subsequent to the induction of retinal ischemia, and densities of surviving RGCs were estimated by counting FG-labeled RGCs within the retina. Histological analysis revealed that ischemic injury caused the loss of retinal RGCs and a net decrease in retinal thickness. For PBA-treated groups, almost 100% of the RGCs were preserved by a pre-ischemia treatment with PBA (at a dose of either 100 or 400 mg/kg), while post-ischemia treatment of RGCs with PBA did not lead to the preservation of RGCs from ischemic injury by PBA as determined by the counting of whole-mount retinas. Pre-ischemia treatment of RGCs with PBA (at a dose of either 100 or 400 mg/kg) significantly reduced the level of ischemia-associated loss of thickness of the total retina, especially the inner retina, and the inner plexiform layer of retina. Besides, PBA treatment significantly reduced the ischemia-induced loss of cells in the ganglion-cell layer of the retina. Taken together, these results suggest that PBA demonstrates a marked neuroprotective effect upon high intraocular pressure-induced retinal ischemia when the PBA is administered prior to ischemia induction.

ABCA3 deficiency: neonatal respiratory failure and interstitial lung disease

ABCA3 is a member of the ATP Binding Cassette family of proteins, transporters that hydrolyze ATP in order to move substrates across biological membranes. Mutations in the gene encoding ABCA3 have been found in children with severe neonatal respiratory disease and older children with some forms of interstitial lung disease. This review summarizes current knowledge concerning clinical, genetic, and pathologic features of the lung disease associated with mutations in the ABCA3 gene, and also briefly reviews some other forms of childhood interstitial lung diseases that have their antecedents in the neonatal period and may also have a genetic basis.

Sulfonylureas correct trafficking defects of disease-causing ATP-sensitive potassium channels by binding to the channel complex

ATP-sensitive potassium (K(ATP)) channels mediate glucose-induced insulin secretion by coupling metabolic signals to beta-cell membrane potential and the secretory machinery. Reduced K(ATP) channel expression caused by mutations in the channel proteins: sulfonylurea receptor 1 (SUR1) and Kir6.2, results in loss of channel function as seen in congenital hyperinsulinism. Previously, we reported that sulfonylureas, oral hypoglycemic drugs widely used to treat type II diabetes, correct the endoplasmic reticulum to the plasma membrane trafficking defect caused by two SUR1 mutations, A116P and V187D. In this study, we investigated the mechanism by which sulfonylureas rescue these mutants. We found that glinides, another class of SUR-binding hypoglycemic drugs, also markedly increased surface expression of the trafficking mutants. Attenuating or abolishing the ability of mutant SUR1 to bind sulfonylureas or glinides by the following mutations: Y230A, S1238Y, or both, accordingly diminished the rescuing effects of the drugs. Interestingly, rescue of the trafficking defects requires mutant SUR1 to be co-expressed with Kir6.2, suggesting that the channel complex, rather than SUR1 alone, is the drug target. Observations that sulfonylureas also reverse trafficking defects caused by neonatal diabetes-associated Kir6.2 mutations in a way that is dependent on intact sulfonylurea binding sites in SUR1 further support this notion. Our results provide insight into the mechanistic and structural basis on which sulfonylureas rescue K(ATP) channel surface expression defects caused by channel mutations.

Glutamate receptors and endoplasmic reticulum quality control: looking beneath the surface

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. The cellular regulation of glutamate receptor (GluR) ion channel function and expression is important for maintaining or adjusting target cell excitability to meet ever-changing demands, for example, in relation to developmental or use-dependent synaptic plasticity. Dysregulation of GluR function or expression may be a contributing factor in certain forms of epilepsy, stroke/ischemia, head trauma, cognitive impairments, and neurodegenerative disease. Recent years have seen substantial progress in understanding how GluRs operate in terms of their structural and functional properties, their synaptic targeting and membrane anchoring by PDZ-domain proteins, and their activity-dependent cycling at the plasma membrane. Yet precious little is known about the earliest events in GluR biogenesis or the mechanisms in place to ensure the GluRs that reach the cell surface are processed, folded, and oligomerized in an appropriate manner. Indeed, only a minor fraction of the GluR content of cells is expressed at any given time on the cell surface, whereas most of the remaining receptors exist in the endoplasmic reticulum (ER). The functional competence and significance of the ER fraction of receptors are presently unknown, but they are generally thought to represent immature, unassembled, or improperly assembled subunits. Some are ultimately destined for insertion in the plasma membrane. Others may be targeted for proteosomal degradation. Still others might provide a latent pool of fully functional receptors that can be recruited to enhance cell excitability in response to specific signals or under pathological conditions. This review will explore the structural and functional elements that regulate GluR assembly and export from the ER.

Role of osmolytes as chemical chaperones during the refolding of aminoacylase

The refolding and reactivation of aminoacylase is particularly difficult because of serious off-pathway aggregation. The effects of 4 osmolytes--dimethylsulphoxide, glycerol, proline, and sucrose--on the refolding and reactivation of guanidine-denatured aminoacylase were studied by measuring aggregation, enzyme activity, intrinsic fluorescence spectra, 1-anilino-8-naphthalenesulfonate (ANS) fluorescence spectra, and circular dichroism (CD) spectra. The results show that all the osmolytes not only inhibit aggregation but also recover the activity of aminoacylase during refolding in a concentration-dependent manner. In particularly, a 40% glycerol concentration and a 1.5 mol/L sucrose concentration almost completely suppressed the aminoacylase aggregation. The enzyme activity measurements revealed that the influence of glycerol is more significant than that of any other osmolyte. The intrinsic fluorescence results showed that glycerol, proline, and sucrose stabilized the aminoacylase conformation effectively, with glycerol being the most effective. All 4 kinds of osmolytes reduced the exposure of the hydrophobic surface, indicating that osmolytes facilitate the formation of protein hydrophobic collapse. The CD results indicate that glycerol and sucrose facilitate the return of aminoacylase to its native secondary structure. The results of this study suggest that the ability of the various osmolytes to facilitate the refolding and renaturation of aminoacylase is not the same. A survey of the results in the literature, as well as those presented here, suggests that although the protective effect of osmolytes on protein activity and structure is equal for different osmolytes, the ability of osmolytes to facilitate the refolding of various proteins differs from case to case. In all cases, glycerol was found to be the best stabilizer and a folding aid.

Reclamation of proteins from the cellular scrap heap

A growing number of diseases have been associated with protein misfolding. Thus, strategies that use small molecules to adjust folding tendencies have therapeutic potential. However, progress in this area has been hampered by an insufficient description of the molecular underpinnings of protein instability within the cell. In a recent report, a chemical approach was taken to probe the mechanism by which Gaucher disease associated mutations in glucocerebrosidase destabilize that enzyme and lead to its destruction. These studies provide a blueprint for the design of "chemical chaperones" for the exploration of cellular protein homeostasis and the treatment of misfolding diseases.

Protein misfolding disorders: pathogenesis and intervention

Newly synthesized proteins in the living cell must go through a folding process to attain their functional structure. To achieve this in an efficient fashion, all organisms, including humans, have evolved a large set of molecular chaperones that assist the folding as well as the maintenance of the functional structure of cellular proteins. Aberrant proteins, the result of production errors, inherited or acquired amino acid substitutions or damage, especially oxidative modifications, can in many cases not fold correctly and will be trapped in misfolded conformations. To rid the cell of misfolded proteins, the living cell contains a large number of intracellular proteases, e.g. the proteasome, which together with the chaperones comprise the cellular protein quality control systems. Many inherited disorders due to amino acid substitutions exhibit loss-of-function pathogenesis because the aberrant protein is eliminated by one of the protein quality control systems. Examples are cystic fibrosis and phenylketonuria. However, not all aberrant proteins can be eliminated and the misfolded protein may accumulate and form toxic oligomeric and/or aggregated inclusions. In this case the loss of function may be accompanied by a gain-of-function pathogenesis, which in many cases determines the pathological and clinical features. Examples are Parkinson and Huntington diseases. Although a number of strategies have been tried to decrease the amounts of accumulated and aggregated proteins, a likely future strategy seems to be the use of chemical or pharmacological chaperones with specific effects on the misfolded protein in question. Positive examples are enzyme enhancement in a number of lysosomal disorders.

Retention of Mutant Low Density Lipoprotein Receptor in Endoplasmic Reticulum (ER) Leads to ER Stress

Familial hypercholesterolemia is an autosomal dominant disease caused by mutations in the gene encoding the low density lipoprotein receptor (LDLR). More than 50% of these mutations lead to receptor proteins that are completely or partly retained in the endoplasmic reticulum (ER). The mechanisms involved in the intracellular processing and retention of mutant LDLR are poorly understood. In the present study we show that the G544V mutant LDLR associates with the chaperones Grp78, Grp94, ERp72, and calnexin in the ER of transfected Chinese hamster ovary cells. Retention of the mutant LDLR was shown to cause ER stress and activation of the unfolded protein response. We observed a marked increase in the activity of two ER stress sensors, IRE1 and PERK. These results show that retention of mutant LDLR in ER induces cellular responses, which might be important for the clinical outcome of familial hypercholesterolemia.

Cellular abnormalities linked to endoplasmic reticulum dysfunction in cerebrovascular disease—therapeutic potential

Unfolded proteins accumulate in the lumen of the endoplasmic reticulum (ER) as part of the cellular response to cerebral hypoxia/ischemia and also to the overexpression of the mutant genes responsible for familial forms of degenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyothrophic lateral sclerosis, and Huntington's disease, as well as other disorders that are caused by an expanded CAG repeat. This accumulation arises from an imbalance between the load of proteins that need to be folded and processed in the ER lumen and the ER folding/processing capacity. To withstand such potentially lethal conditions, stress responses are activated that includes the shutdown of translation to reduce the ER work load and the activation of the expression of genes coding for proteins involved in the folding and processing reactions, to increase folding/processing capacity. In transient cerebral ischemia, ER stress-induced suppression of protein synthesis is believed to be too severe to permit sufficient activation of the genetic arm of the ER stress response. Mutations associated with Alzheimer's disease down-regulate the ER stress response and make cells more vulnerable to conditions associated with ER stress. When the functioning of the ER is severely impaired and affected cells can no longer withstand these stressful conditions, programmed cell death is induced, including a mitochondria-driven apoptotic pathway. Raising the resistance of cells to conditions that interfere with ER functions and activating the degradation and refolding of unfolded proteins accumulated in the ER lumen are possible strategies for blocking the pathological process leading to cell death at an early stage.

Trehalose and glycerol stabilize and renature yeast inorganic pyrophosphatase inactivated by very high temperatures

A number of naturally occurring small organic molecules, primarily involved in maintaining osmotic pressure in the cell, display chaperone-like activity, stabilizing the native conformation of proteins, and protecting them from various kinds of stress. Most of them are sugars, polyols, amino acids or methylamines. Similar to molecular chaperones, most of these compounds have no substrate specificity, but some specifically stabilize certain proteins. In the present work, the capacity of trehalose and glycerol, two well-known osmolytes, to stabilize and renature inorganic pyrophosphatase is demonstrated. Both trehalose and glycerol significantly protect pyrophosphatase against thermoinactivation achieved by incubating the enzyme at temperatures up to 95 degrees C, and allow the enzyme already inactivated in the presence of these osmolytes to renature upon incubation at low temperatures. To the best of our knowledge, there are no data on the effects of these compounds on renaturation of thermoinactivated proteins. The correlation between the recovery of enzyme activity and structural changes indicated by fluorescence spectroscopy contribute to better understanding of the protein stabilization mechanism.

Trehalose and 6-aminohexanoic acid stabilize and renature glucose-6-phosphate dehydrogenase inactivated by glycation and by guanidinium hydrochloride

A number of naturally occurring small organic molecules, primarily involved in maintaining osmotic pressure in the cell, display chaperone-like activity, stabilizing the native conformation of proteins and protecting them from various kinds of stress. Most of them are sugars, polyols, amino acids or methylamines. In addition to their intrinsic protein-stabilizing activity, these small organic stress molecules regulate the activity of some molecular chaperones, and may stabilize the folded state of proteins involved in unfolding or in misfolding diseases, such as Alzheimer's and Parkinson's diseases, or alpha1-antitrypsin deficiency and cystic fibrosis, respectively. Similar to molecular chaperones, most of these compounds have no substrate specificity, but some specifically stabilize certain proteins, e.g., 6-aminohexanoic acid (AHA) stabilizes apolipoprotein A. In the present work, the capacity of 6-aminohexanoic acid to stabilize non-specifically other proteins is demonstrated. Both trehalose and AHA significantly protect glucose-6-phosphate dehydrogenase (G6PD) against glycation-induced inactivation, and renatured enzyme already inactivated by glycation and by guanidinium hydrochloride (GuHCl). To the best of our knowledge, there are no data on the effect of these compounds on protein glycation. The correlation between the recovery of enzyme activity and structural changes indicated by fluorescence spectroscopy and Western blotting contribute to better understanding of the protein stabilization mechanism.

Molecular mechanism of a temperature-sensitive phenotype in peroxisomal biogenesis disorder

Peroxisomal biogenesis disorders include Zellweger syndrome and milder phenotypes, such as neonatal adrenoleukodystrophy (NALD). Our previous study of a NALD patient with a marked deterioration by a fever revealed a mutation (Ile326Thr) within a SH3 domain of PEX13 protein (Pex13p), showing a temperature-sensitive (TS) phenotype in peroxisomal biogenesis. Clinical TS phenotypes also have been reported in several genetic diseases, but the molecular mechanisms still remain to be clarified. The immunofluorescent staining with anti-Pex13p antibody also revealed TS phenotype of the I326T mutant protein itself in the patient cells. Protease digestion of the recombinant Pex13p-SH3 domain showed an increase of protease susceptibility, suggesting a problem of mutant protein fold. Conformational analyses against urea denaturation using urea gradient gel electrophoresis or fluorescence emission from tryptophan residue revealed that the mutant protein should be easily unfolded. Far-UV circular dichroism (CD) spectra demonstrated that both wild-type and the mutant protein have antiparallel beta-sheets as their secondary structure with slightly different extent. The thermal unfolding profiles measured by CD showed a marked lower melting temperature for I326T protein compared with that of wild-type protein. Analysis of the protein 3D-structure indicated that the Ile326 should be a core residue for folding kinetics and the substitution of Ile326 by threonine should directly alter the kinetic equilibrium, suggesting a marked increase of the unfolded molecules when the patient had a high fever. Structural analyses of the protein in the other genetic diseases could provide an avenue for better understanding of genotype-phenotype correlations.

ER retention and degradation as the molecular basis underlying Gaucher disease heterogeneity

Gaucher disease (GD), an autosomal recessive disease, is characterized by accumulation of glucosylceramide mainly in cells of the reticuloendothelial system, due to mutations in the acid beta-glucocerebrosidase gene. Some of the patients suffer from neurological symptoms (type 2 and type 3 patients), whereas patients with type 1 GD do not present neurological signs. The disease is heterogeneous even among patients with the same genotype, implicating that a mutation in the glucocerebrosidase gene is required to cause GD but other factors play an important role in the manifestation of the disease. Glucocerebrosidase is a lysosomal enzyme, synthesized on endoplasmic reticulum (ER)-bound polyribosomes and translocated into the ER. Following N-linked glycosylations, it is transported to the Golgi apparatus, from where it is trafficked to the lysosomes. In this study, we tested glucocerebrosidase protein levels, N-glycans processing and intracellular localization in skin fibroblasts derived from patients with GD. Our results strongly suggest that mutant glucocerebrosidase variants present variable levels of ER retention and undergo ER-associated degradation in the proteasomes. The degree of ER retention and proteasomal degradation is one of the factors that determine GD severity.

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Mistargeted MRPΔF728 mutant is rescued by intracellular GSH

The most common cystic fibrosis-causing mutation is the deletion of the widely conserved phenylalanine 508 (DeltaF508) of CFTR. The mutant is unable to fold correctly and to transit to the plasma membrane. MRP1 belongs to the same subfamily of ABC proteins as CFTR and confers resistance to a wide range of chemotherapeutic drugs. By analogy, phenylalanine 728 was deleted in MRP1. Our results shown that MRPDeltaF728 is correctly targeted to the plasma membrane, actively transports doxorubicin (DOX) and vincristine (VCR) and shares a structure identical to MRP1. Intracellular GSH depletion however results in a mistargeted mutant that is retained into the cytoplasm, while in the same conditions wild-type MRP1 is correctly routed to the plasma membrane. The GSH-protein complex could adopt a stable conformation protected against proteolytic degradation and correctly targeted to the plasma membrane.

Chemical chaperones protect from effects of apoptosis-inducing mutation in carbonic anhydrase IV identified in retinitis pigmentosa 17

Carbonic anhydrase (CA) IV is a glycosylphosphotidylinositol-anchored enzyme highly expressed on the plasma face of microcapillaries and especially strongly expressed in the choriocapillaris of the human eye. In collaboration with scientists at the University of Cape Town (Rondebosch, South Africa), we recently showed that the R14W mutation in the signal sequence of CA IV, which they identified in patients with the retinitis pigmentosa (RP) 17 form of autosomal dominant RP, results in accumulation of unfolded protein in the endoplasmic reticulum (ER), leading to ER stress, the unfolded protein response, and apoptosis in a large fraction of transfected COS-7 cells expressing mutant, but not wild-type, CA IV. Here we present experiments showing that several well characterized CA inhibitors largely prevent the adverse effects of expressing R14W CA IV in transfected COS-7 cells. Specifically, CA inhibitors prevent the accelerated turnover of the mutant protein, the up-regulation of Ig-binding protein, double-stranded RNA-regulated protein kinase-like ER kinase, and CCAAT/enhancer-binding protein homologous protein (markers of the unfolded protein response and ER stress), the inhibition of production of other secretory proteins expressed from COS-7-transfecting plasmids, and the induction of apoptosis, all characteristics of transfected cells expressing R14W CA IV. Furthermore, treatment with 4-phenylbutyric acid, a nonspecific chemical chaperone used in other protein-folding disorders, also dramatically reduces the apoptosis-inducing effect of expressing R14W CA IV cDNA in transfected COS-7 cells. These experiments suggest a promising approach to treatment of RP17 that might delay the onset or possibly prevent this autosomal dominant form of RP.

Apoptosis-inducing signal sequence mutation in carbonic anhydrase IV identified in patients with the RP17 form of retinitis pigmentosa

Genetic and physical mapping of the RP17 locus on 17q identified a 3.6-megabase candidate region that includes the gene encoding carbonic anhydrase IV (CA4), a glycosylphosphatidylinositol-anchored protein that is highly expressed in the choriocapillaris of the human eye. By sequencing candidate genes in this region, we identified a mutation that causes replacement of an arginine with a tryptophan (R14W) in the signal sequence of the CA4 gene at position -5 relative to the signal sequence cleavage site. This mutation was found to cosegregate with the disease phenotype in two large families and was not found in 36 unaffected family members or 100 controls. Expression of the mutant cDNA in COS-7 cells produced several findings, suggesting a mechanism by which the mutation can explain the autosomal dominant disease. In transfected COS-7 cells, the R14W mutation (i) reduced the steady-state level of carbonic anhydrase IV activity expressed by 28% due to a combination of decreased synthesis and accelerated turnover; (ii) led to up-regulation of immunoglobulin-binding protein, double-stranded RNA-regulated protein kinase-like ER kinase, and CCAAT/enhancer-binding protein homologous protein, markers of the unfolded protein response and endoplasmic reticulum stress; and (iii) induced apoptosis, as evidenced by annexin V binding and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling staining, in most cells expressing the mutant, but not the WT, protein. We suggest that a high level of expression of the mutant allele in the endothelial cells of the choriocapillaris leads to apoptosis, leading in turn to ischemia in the overlying retina and producing autosomal dominant retinitis pigmentosa.

Sulfonylureas correct trafficking defects of ATP-sensitive potassium channels caused by mutations in the sulfonylurea receptor

The pancreatic ATP-sensitive potassium (K(ATP)) channel, a complex of four sulfonylurea receptor 1 (SUR1) and four potassium channel Kir6.2 subunits, regulates insulin secretion by linking metabolic changes to beta-cell membrane potential. Sulfonylureas inhibit K(ATP) channel activities by binding to SUR1 and are widely used to treat type II diabetes. We report here that sulfonylureas also function as chemical chaperones to rescue K(ATP) channel trafficking defects caused by two SUR1 mutations, A116P and V187D, identified in patients with congenital hyperinsulinism. Sulfonylureas markedly increased cell surface expression of the A116P and V187D mutants by stabilizing the mutant SUR1 proteins and promoting their maturation. By contrast, diazoxide, a potassium channel opener that also binds SUR1, had no effect on surface expression of either mutant. Importantly, both mutant channels rescued to the cell surface have normal ATP, MgADP, and diazoxide sensitivities, demonstrating that SUR1 harboring either the A116P or the V187D mutation is capable of associating with Kir6.2 to form functional K(ATP) channels. Thus, sulfonylureas may be used to treat congenital hyperinsulinism caused by certain K(ATP) channel trafficking mutations.

Assessing the severity of the small inframe deletion mutation in the alpha-subunit of beta-hexosaminidase A found in the Turkish population by reproducing it in the more stable beta-subunit

GM(2) gangliosidoses are a group of panethnic lysosomal storage diseases in which GM(2) ganglioside accumulates in the lysosome due to a defect in one of three genes, two of which encode the alpha- or beta-subunits of beta- N -acetylhexosaminidase (Hex) A. A small inframe deletion mutation in the catalytic domain of the alpha-subunit of Hex has been found in five Turkish patients with infantile Tay-Sachs disease. To date it has not been detected in other populations and is the only mutation to be found in exon 10. It results in detectable levels of inactive alpha-protein in its precursor form. Because the alpha- and beta-subunits share 60% sequence identity, the Hex A and Hex B genes are believed to have arisen from a common ancestral gene. Thus the subunits must share very similar three-dimensional structures with conserved functional domains. Hex B, the beta-subunit homodimer is more stable than the heterodimeric Hex A, and much more stable than Hex S, the alpha homodimer. Thus, mutations that completely destabilize the alpha-subunit can often be partially rescued if expressed in the aligned positions in the beta-subunit. To better understand the severity of the Turkish HEXA mutation, we reproduced the 12 bp deletion mutation (1267-1278) in the beta-subunit cDNA. Western blot analysis of permanently transfected CHO cells expressing the mutant detected only the pro-form of the beta-subunit coupled with a total lack of detectable Hex B activity. These data indicate that the deletion of the four amino acids severely affects the folding of even the more stable beta-subunit, causing its retention in the endoplasmic reticulum and ultimate degradation.