The endoplasmic reticulum degradation pathway for mutant secretory proteins alpha1-antitrypsin Z and S is distinct from that for an unassembled membrane protein

Biomarkers Associated With Future Severe Liver Disease in Children With Alpha-1-Antitrypsin Deficiency

Background and Aims

Children with alpha-1-antitrypsin deficiency (AATD) exhibit a wide range of liver disease outcomes from portal hypertension and transplant to asymptomatic without fibrosis. Individual outcomes cannot be predicted. Liver injury in AATD is caused by the accumulation in hepatocytes of the mutant Z alpha-1-antitrypsin (AAT) protein, especially the toxic, intracellular polymerized conformation. AATD patients have trace Z polymer detectable in serum with unknown significance.

Methods

The Childhood Liver Disease Research Network is an NIH consortium for the study of pediatric liver diseases, including AATD. We obtained data and samples with the aim of identifying biomarkers predictive of severe AATD liver disease.

Results

We analyzed prospective AATD Childhood Liver Disease Research Network data and serum samples in 251 subjects from 2007 to 2015 for outcomes and Z polymer levels. Fifty-eight of 251 had clinically evident portal hypertension (CEPH) at enrollment, and 10 developed CEPH during follow-up. Higher Z AAT polymer levels were associated with existing CEPH (P = .01). In infants without CEPH, higher polymer levels were associated with future CEPH later in childhood, but total AAT was not predictive. Higher gamma-glutamyl transferase (GGT) in the first few months of life was also significantly associated with future CEPH, and risk-threshold GGT levels can be identified. A model was constructed to identify subjects at high risk of future CEPH by combining clinical GGT and polymer levels (area under the curve of 0.83; 95% confidence interval: 0.656-1.00, P = .019).

Conclusion

High circulating Z polymer levels and high GGT early in life are associated with future CEPH in AATD, and the use of predictive cutoffs may assist in future clinical trial design.

Probing of the reactive center loop region of alpha-1-antitrypsin by mutagenesis predicts new type-2 dysfunctional variants

Lung disease in alpha-1-antitrypsin deficiency (AATD) mainly results from insufficient control of the serine proteases neutrophil elastase (NE) and proteinase-3 due to reduced plasma levels of alpha-1-antitrypsin (AAT) variants. Mutations in the specificity-determining reactive center loop (RCL) of AAT would be predicted to minimally affect protein folding and secretion by hepatocytes but can impair anti-protease activity or alter the target protease. These properly secreted but dysfunctional 'type-2' variants would not be identified by common diagnostic protocols that are predicated on a reduction in circulating AAT. This has potential clinical relevance: in addition to the dysfunctional Pittsburgh and Iners variants reported previously, several uncharacterized RCL variants are present in genome variation databases. To prospectively evaluate the impact of RCL variations on secretion and anti-protease activity, here we performed a systematic screening of amino acid substitutions occurring at the AAT-NE interface. Twenty-three AAT variants that can result from single nucleotide polymorphisms in this region, including 11 present in sequence variation databases, were expressed in a mammalian cell model. All demonstrated unaltered protein folding and secretion. However, when their ability to form stable complexes with NE was evaluated by western blot, enzymatic assays, and a novel ELISA developed to quantify AAT-NE complexes, substrate-like and NE-binding deficient dysfunctional variants were identified. This emphasizes the ability of the RCL to accommodate inactivating substitutions without impacting the integrity of the native molecule and demonstrates that this class of molecule violates a generally accepted paradigm that equates circulating levels with functional protection of lung tissue.

Alpha-1 Antitrypsin Deficiency Liver Disease

Liver disease in homozygous ZZ alpha-1 antitrypsin (AAT) deficiency occurs due to the accumulation of large quantities of AAT mutant Z protein polymers in the liver. The mutant Z protein folds improperly during biogenesis and is retained within the hepatocytes rather than appropriately secreted. These intracellular polymers trigger an injury cascade, which leads to liver injury. However, the clinical liver disease is highly variable and not all patients with this same homozygous ZZ genotype develop liver disease. Evidence suggests that genetic determinants of intracellular protein processing, among other unidentified genetic and environmental factors, likely play a role in liver disease susceptibility. Advancements made in development of new treatment strategies using siRNA technology, and other novel approaches, are promising, and multiple human liver disease trials are underway.

Human placental proteomics and exon variant studies link AAT/SERPINA1 with spontaneous preterm birth

BACKGROUND

Preterm birth is defined as live birth before 37 completed weeks of pregnancy, and it is a major problem worldwide. The molecular mechanisms that lead to onset of spontaneous preterm birth are incompletely understood. Prediction and evaluation of the risk of preterm birth is challenging as there is a lack of accurate biomarkers. In this study, our aim was to identify placental proteins that associate with spontaneous preterm birth.

METHODS

We analyzed the proteomes from placentas to identify proteins that associate with both gestational age and spontaneous labor. Next, rare and potentially damaging gene variants of the identified protein candidates were sought for from our whole exome sequencing data. Further experiments we performed on placental samples and placenta-associated cells to explore the location and function of the spontaneous preterm labor-associated proteins in placentas.

RESULTS

Exome sequencing data revealed rare damaging variants in SERPINA1 in families with recurrent spontaneous preterm deliveries. Protein and mRNA levels of alpha-1 antitrypsin/SERPINA1 from the maternal side of the placenta were downregulated in spontaneous preterm births. Alpha-1 antitrypsin was expressed by villous trophoblasts in the placenta, and immunoelectron microscopy showed localization in decidual fibrinoid deposits in association with specific extracellular proteins. siRNA knockdown in trophoblast-derived HTR8/SVneo cells revealed that SERPINA1 had a marked effect on regulation of the actin cytoskeleton pathway, Slit-Robo signaling, and extracellular matrix organization.

CONCLUSIONS

Alpha-1 antitrypsin is a protease inhibitor. We propose that loss of the protease inhibition effects of alpha-1 antitrypsin renders structures critical to maintaining pregnancy susceptible to proteases and inflammatory activation. This may lead to spontaneous premature birth.

N-glycan processing selects ERAD-resistant misfolded proteins for ER-to-lysosome-associated degradation

Efficient degradation of by‐products of protein biogenesis maintains cellular fitness. Strikingly, the major biosynthetic compartment in eukaryotic cells, the endoplasmic reticulum (ER), lacks degradative machineries. Misfolded proteins in the ER are translocated to the cytosol for proteasomal degradation via ER‐associated degradation (ERAD). Alternatively, they are segregated in ER subdomains that are shed from the biosynthetic compartment and are delivered to endolysosomes under control of ER‐phagy receptors for ER‐to‐lysosome‐associated degradation (ERLAD). Demannosylation of N‐linked oligosaccharides targets terminally misfolded proteins for ERAD. How misfolded proteins are eventually marked for ERLAD is not known. Here, we show for ATZ and mutant Pro‐collagen that cycles of de‐/re‐glucosylation of selected N‐glycans and persistent association with Calnexin (CNX) are required and sufficient to mark ERAD‐resistant misfolded proteins for FAM134B‐driven lysosomal delivery. In summary, we show that mannose and glucose processing of N‐glycans are triggering events that target misfolded proteins in the ER to proteasomal (ERAD) and lysosomal (ERLAD) clearance, respectively, regulating protein quality control in eukaryotic cells.

The Importance of N186 in the Alpha-1-Antitrypsin Shutter Region Is Revealed by the Novel Bologna Deficiency Variant

Alpha-1-antitrypsin (AAT) deficiency causes pulmonary disease due to decreased levels of circulating AAT and consequently unbalanced protease activity in the lungs. Deposition of specific AAT variants, such as the common Z AAT, within hepatocytes may also result in liver disease. These deposits are comprised of ordered polymers of AAT formed by an inter-molecular domain swap. The discovery and characterization of rare variants of AAT and other serpins have historically played a crucial role in the dissection of the structural mechanisms leading to AAT polymer formation. Here, we report a severely deficient shutter region variant, Bologna AAT (N186Y), which was identified in five unrelated subjects with different geographical origins. We characterized the new variant by expression in cellular models in comparison with known polymerogenic AAT variants. Bologna AAT showed secretion deficiency and intracellular accumulation as detergent-insoluble polymers. Extracellular polymers were detected in both the culture media of cells expressing Bologna AAT and in the plasma of a patient homozygous for this variant. Structural modelling revealed that the mutation disrupts the hydrogen bonding network in the AAT shutter region. These data support a crucial coordinating role for asparagine 186 and the importance of this network in promoting formation of the native structure.

Alpha-1 antitrypsin deficiency liver disease

The clinical presentation of liver disease is highly variable in homozygous ZZ alpha-1 antitrypsin (AAT) deficiency, and not all patients with the homozygous ZZ genotype develop liver disease. Although not fully identified, there is likely a strong influence of genetic and environmental modifiers of the intracellular injury cascade and fibrotic response. Most ZZ children are well and remain undiagnosed. Of those who come to medical attention, the most common pediatric presentation is neonatal cholestatic hepatitis, sometimes referred to as "neonatal hepatitis syndrome". The gold standard for diagnosis of AAT deficiency is analysis of the AAT protein phenotype in the patient serum or the genotype of their DNA genome. Careful follow up of all diagnosed children is important. Heterozygotes for S and Z alleles of AAT (SZ) may develop progressive liver disease similar to ZZ patients and also require close monitoring. There is no specific treatment for AAT deficiency induced liver disease and current therapy remains supportive with management of complications. Rarely, patients require liver transplant and typically the patient outcomes are excellent. With improved understanding of liver injury mechanisms, new strategies for treatment are now being explored, including siRNA technology, molecules to modulate secretion, and enhancers of proteolysis.

Known Mutations at the Cause of Alpha-1 Antitrypsin Deficiency an Updated Overview of SERPINA1 Variation Spectrum

Alpha-1-Antitrypsin deficiency (AATD), caused by SERPINA1 mutations, is one of the most prevalent Mendelian disorders among individuals of European descend. However, this condition, which is characterized by reduced serum levels of alpha-1-antitrypsin (AAT) and associated with increased risks of pulmonary emphysema and liver disease in both children and adults, remains frequently underdiagnosed. AATD clinical manifestations are often correlated with two pathogenic variants, the Z allele (p.Glu342Lys) and the S allele (p.Glu264Val), which can be combined in severe ZZ or moderate SZ risk genotypes. Yet, screenings of AATD cases and large sequencing efforts carried out in both control and disease populations are disclosing outstanding numbers of rare SERPINA1 variants (>500), including many pathogenic and other likely deleterious mutations. Generally speaking, pathogenic variants can be subdivided into either loss- or gain-of-function according to their pathophysiological effects. In AATD, the loss-of-function is correlated with an uncontrolled activity of elastase by its natural inhibitor, the AAT. This phenomenon can result from the absence of circulating AAT (null alleles), poor AAT secretion from hepatocytes (deficiency alleles) or even from a modified inhibitory activity (dysfunctional alleles). On the other hand, the gain-of-function is connected with the formation of AAT polymers and their switching on of cellular stress and inflammatory responses (deficiency alleles). Less frequently, the gain-of-function is related to a modified protease affinity (dysfunctional alleles). Here, we revisit SERPINA1 mutation spectrum, its origins and population history with a greater emphasis on variants fitting the aforementioned processes of AATD pathogenesis. Those were selected based on their clinical significance and wider geographic distribution. Moreover, we also provide some directions for future studies of AATD clinically heterogeneity and comprehensive diagnosis.

The Autophagy Pathway: A Critical Route in the Disposal of Alpha 1-Antitrypsin Aggregates That Holds Many Mysteries

The maintenance of proteome homeostasis, or proteostasis, is crucial for preserving cellular functions and for cellular adaptation to environmental challenges and changes in physiological conditions. The capacity of cells to maintain proteostasis requires precise control and coordination of protein synthesis, folding, conformational maintenance, and clearance. Thus, protein degradation by the ubiquitin–proteasome system (UPS) or the autophagy–lysosomal system plays an essential role in cellular functions. However, failure of the UPS or the autophagic process can lead to the development of various diseases (aging-associated diseases, cancer), thus both these pathways have become attractive targets in the treatment of protein conformational diseases, such as alpha 1-antitrypsin deficiency (AATD). The Z alpha 1-antitrypsin (Z-AAT) misfolded variant of the serine protease alpha 1-antitrypsin (AAT) is caused by a structural change that predisposes it to protein aggregation and dramatic accumulation in the form of inclusion bodies within liver hepatocytes. This can lead to clinically significant liver disease requiring liver transplantation in childhood or adulthood. Treatment of mice with autophagy enhancers was found to reduce hepatic Z-AAT aggregate levels and protect them from AATD hepatotoxicity. To date, liver transplantation is the only curative therapeutic option for patients with AATD-mediated liver disease. Therefore, the development and discovery of new therapeutic approaches to delay or overcome disease progression is a top priority. Herein, we review AATD-mediated liver disease and the overall process of autophagy. We highlight the role of this system in the regulation of Z-variant degradation and its implication in AATD-medicated liver disease, including some open questions that remain challenges in the field and require further elucidation. Finally, we discuss how manipulation of autophagy could provide multiple routes of therapeutic benefit in AATD-mediated liver disease.

Intrahepatic heteropolymerization of M and Z alpha-1-antitrypsin

The α-1-antitrypsin (or alpha-1-antitrypsin, A1AT) Z variant is the primary cause of severe A1AT deficiency and forms polymeric chains that aggregate in the endoplasmic reticulum of hepatocytes. Around 2%-5% of Europeans are heterozygous for the Z and WT M allele, and there is evidence of increased risk of liver disease when compared with MM A1AT individuals. We have shown that Z and M A1AT can copolymerize in cell models, but there has been no direct observation of heteropolymer formation in vivo. To this end, we developed a monoclonal antibody (mAb2H2) that specifically binds to M in preference to Z A1AT, localized its epitope using crystallography to a region perturbed by the Z (Glu342Lys) substitution, and used Fab fragments to label polymers isolated from an MZ heterozygote liver explant. Glu342 is critical to the affinity of mAb2H2, since it also recognized the mild S-deficiency variant (Glu264Val) present in circulating polymers from SZ heterozygotes. Negative-stain electron microscopy of the Fab2H2-labeled liver polymers revealed that M comprises around 6% of the polymer subunits in the MZ liver sample. These data demonstrate that Z A1AT can form heteropolymers with polymerization-inert variants in vivo with implications for liver disease in heterozygous individuals.

Alpha-1 Antitrypsin Deficiency-Mediated Liver Toxicity: Why Do Some Patients Do Poorly? What Do We Know So Far?

Alpha-1 antitrypsin deficiency (AATD) is a rare genetic disease caused by mutations in the SERPINA1 gene and is associated with a decreased level of circulating alpha-1 antitrypsin (AAT). Among all the known mutations in the SERPINA1 gene, homozygous for the Z allele is well-known to result in both lung and liver disease. Unlike the lung injury that occurs in adulthood with the environment (notably, tobacco) as a co-factor, the hepatic damage is more complicated. Despite a common underlying gene mutation, the liver disease associated with AATD presents a considerable variability in the age-of-onset and severity, ranging from transient neonatal cholestasis (in early childhood) to cirrhosis and liver cancer (in childhood and adulthood). Given that all the cofactors- genetics and/or environmental- have not been fully identified, it is still impossible to predict which individuals with AATD may develop severe liver disease. The discovery of these modifiers represents the major challenge for the detection, diagnosis, and development of new therapies to provide alternative options to liver transplantation. The aim of this current review is to provide an updated overview of our knowledge on why some AATD patients associated with liver damage progress poorly.

Alpha 1-Antitrypsin Deficiency: A Disorder of Proteostasis-Mediated Protein Folding and Trafficking Pathways

Human cells express large amounts of different proteins continuously that must fold into well-defined structures that need to remain correctly folded and assemble in order to ensure their cellular and biological functions. The integrity of this protein balance/homeostasis, also named proteostasis, is maintained by the proteostasis network (PN). This integrated biological system, which comprises about 2000 proteins (chaperones, folding enzymes, degradation components), control and coordinate protein synthesis folding and localization, conformational maintenance, and degradation. This network is particularly challenged by mutations such as those found in genetic diseases, because of the inability of an altered peptide sequence to properly engage PN components that trigger misfolding and loss of function. Thus, deletions found in the ΔF508 variant of the Cystic Fibrosis (CF) transmembrane regulator (CFTR) triggering CF or missense mutations found in the Z variant of Alpha 1-Antitrypsin deficiency (AATD), leading to lung and liver diseases, can accelerate misfolding and/or generate aggregates. Conversely to CF variants, for which three correctors are already approved (ivacaftor, lumacaftor/ivacaftor, and most recently tezacaftor/ivacaftor), there are limited therapeutic options for AATD. Therefore, a more detailed understanding of the PN components governing AAT variant biogenesis and their manipulation by pharmacological intervention could delay, or even better, avoid the onset of AATD-related pathologies.

Heteropolymerization of α-1-antitrypsin mutants in cell models mimicking heterozygosity

The most common genotype associated with severe α-1-antitrypsin deficiency (AATD) is the Z homozygote. The Z variant (Glu342Lys) of α-1-antitrypsin (AAT) undergoes a conformational change and is retained within the endoplasmic reticulum (ER) of hepatocytes leading to the formation of ordered polymeric chains and inclusion bodies. Accumulation of mutated protein predisposes to cirrhosis whilst plasma AAT deficiency leads to emphysema. Increased risk of liver and lung disease has also been reported in heterozygous subjects who carry Z in association with the milder S allele (Glu264Val) or even with wild-type M. However, it is unknown whether Z AAT can co-polymerize with other AAT variants in vivo. We co-expressed two AAT variants, each modified by a different tag, in cell models that replicate AAT deficiency. We used pull-down assays to investigate interactions between co-expressed variants and showed that Z AAT forms heteropolymers with S and with the rare Mmalton (Phe52del) and Mwurzburg (Pro369Ser) mutants, and to a lesser extent with the wild-type protein. Heteropolymers were recognized by the 2C1 mAb that binds to Z polymers in vivo. There was increased intracellular accumulation of AAT variants when co-expressed with Z AAT, suggesting a dominant negative effect of the Z allele. The molecular interactions between S and Z AAT were confirmed by confocal microscopy showing their colocalization within dilated ER cisternae and by positivity in Proximity Ligation Assays. These results provide the first evidence of intracellular co-polymerization of AAT mutants and contribute to understanding the risk of liver disease in SZ and MZ heterozygotes.

Characterisation of a type II functionally-deficient variant of alpha-1-antitrypsin discovered in the general population

Lung disease in alpha-1-antitrypsin deficiency (AATD) results from dysregulated proteolytic activity, mainly by neutrophil elastase (HNE), in the lung parenchyma. This is the result of a substantial reduction of circulating alpha-1-antitrypsin (AAT) and the presence in the plasma of inactive polymers of AAT. Moreover, some AAT mutants have reduced intrinsic activity toward HNE, as demonstrated for the common Z mutant, as well as for other rarer variants. Here we report the identification and characterisation of the novel AAT reactive centre loop variant Gly349Arg (p.G373R) present in the ExAC database. This AAT variant is secreted at normal levels in cellular models of AATD but shows a severe reduction in anti-HNE activity. Biochemical and molecular dynamics studies suggest it exhibits unfavourable RCL presentation to cognate proteases and compromised insertion of the RCL into β-sheet A. Identification of a fully dysfunctional AAT mutant that does not show a secretory defect underlines the importance of accurate genotyping of patients with pulmonary AATD manifestations regardless of the presence of normal levels of AAT in the circulation. This subtype of disease is reminiscent of dysfunctional phenotypes in anti-thrombin and C1-inibitor deficiencies so, accordingly, we classify this variant as the first pure functionally-deficient (type II) AATD mutant.

Alpha-1-Antitrypsin Deficiency Liver Disease

In homozygous ZZ alpha-1-antitrypsin (AAT) deficiency, the liver synthesizes large quantities of AAT mutant Z, which folds improperly during biogenesis and is retained within the hepatocytes and directed into intracellular proteolysis pathways. These intracellular polymers trigger an injury cascade, which can lead to liver injury. This is highly variable and not all patients develop liver disease. Although not fully described, there is likely a strong influence of genetic and environmental modifiers of the injury cascade and of the fibrotic response. With improved understanding of liver injury mechanisms, new strategies for treatment are now being explored.

Therapeutic Genome Editing With CRISPR/Cas9 in a Humanized Mouse Model Ameliorates α1-antitrypsin Deficiency Phenotype

α1-antitrypsin (AAT) is a circulating serine protease inhibitor secreted from the liver and important in preventing proteolytic neutrophil elastase associated tissue damage, primarily in lungs. In humans, AAT is encoded by the SERPINA1 (hSERPINA1) gene in which a point mutation (commonly referred to as PiZ) causes aggregation of the miss-folded protein in hepatocytes resulting in subsequent liver damage. In an attempt to rescue the pathologic liver phenotype of a mouse model of human AAT deficiency (AATD), we used adenovirus to deliver Cas9 and a guide-RNA (gRNA) molecule targeting hSERPINA1. Our single dose therapeutic gene editing approach completely reverted the phenotype associated with the PiZ mutation, including circulating transaminase and human AAT (hAAT) protein levels, liver fibrosis and protein aggregation. Furthermore, liver histology was significantly improved regarding inflammation and overall morphology in hSERPINA1 gene edited PiZ mice. Genomic analysis confirmed significant disruption to the hSERPINA1 transgene resulting in a reduction of hAAT protein levels and quantitative mRNA analysis showed a reduction in fibrosis and hepatocyte proliferation as a result of editing. Our findings indicate that therapeutic gene editing in hepatocytes is possible in an AATD mouse model.

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α1-antitrypsin (AAT) is a circulating protein secreted from the liver and important in preventing tissue damage in lungs.

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We used CRISPR/Cas9 to disrupt the gene of a mutant version of the protein to reverse liver pathology in a mouse model of human AAT deficiency (AATD)

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Our gene editing approach reverted the AATD pathology and genomic analysis confirmed significant disruption to the gene.

In an attempt to treat a pathologic liver disease called α1-antitrypsin deficiency (AATD) in a mouse model of the human disease, we used CRISPR/Cas9 technology to remove a mutant form of hSERPINA1, which causes AATD in the mouse. Our gene editing approach reverted the disease associated with the mutated gene and we saw a reversal in liver fibrosis and mutant protein aggregation. Our findings in a mouse model indicate that therapeutic gene removal, by editing out a mutated form of the gene, in liver cells is possible.

Liver Disease in Alpha-1 Antitrypsin Deficiency: Current Approaches and Future Directions

Purpose of Review

The aim of the study is to review the liver disease caused by alpha-1 antitrypsin deficiency (A1ATD), including pathogenesis, epidemiology, diagnostic testing, and recent therapeutic developments.

Recent Findings

Therapeutic approaches target several intracellular pathways to reduce the cytotoxic effects of the misfolded mutant globular protein (ATZ) on the hepatocyte. These include promoting ATZ transport out of the endoplasmic reticulum (ER), enhancing ATZ degradation, and preventing ATZ globule-aggregation.

Summary

A1ATD is the leading genetic cause of liver disease among children. It is a protein-folding disorder in which toxic insoluble ATZ proteins aggregate in the ER of hepatocytes leading to inflammation, fibrosis, cirrhosis, and increased risk of hepatocellular carcinoma. The absence of the normal A1AT serum protein also predisposes patients to pan lobar emphysema as adults. At this time, the only approved therapy for A1ATD-associated liver disease is orthotopic liver transplantation, which is curative. However, there has been significant recent progress in the development of small molecule therapies with potential both to preserve the native liver and prevent hepatotoxicity.

Pathophysiology of Alpha-1 Antitrypsin Deficiency Liver Disease

Classical alpha-1 antitrypsin (a1AT) deficiency is an autosomal recessive disease associated with an increased risk of liver disease in adults and children, and with lung disease in adults (Teckman and Jain, Curr Gastroenterol Rep 16(1):367, 2014). The vast majority of the liver disease is associated with homozygosity for the Z mutant allele, the so-called PIZZ. These homozygous individuals synthesize large quantities of a1AT mutant Z protein in the liver, but the mutant protein folds improperly during biogenesis and approximately 85% of the molecules are retained within the hepatocytes rather than appropriately secreted. The resulting low, or "deficient," serum level leaves the lungs vulnerable to inflammatory injury from uninhibited neutrophil proteases. Most of the mutant Z protein molecules retained within hepatocytes are directed into intracellular proteolysis pathways, but some molecules remain in the endoplasmic reticulum for long periods of time. Some of these molecules adopt an unusual aggregated or "polymerized" conformation (Duvoix et al., Rev Mal Respir 31(10):992-1002, 2014). It is thought that these intracellular polymers trigger a cascade of intracellular injury which can lead to end-organ liver injury including chronic hepatitis, cirrhosis, and hepatocellular carcinoma (Lindblad et al., Hepatology 46(4):1228-1235, 2007). The hepatocytes with the largest accumulations of mutant Z polymers undergo apoptotic death and possibly other death mechanisms. This intracellular death cascade appears to involve ER stress, mitochondrial depolarization, and caspase cleavage, and is possibly linked to autophagy and redox injury. Cells with lesser burdens of mutant Z protein proliferate to maintain the liver cell mass. This chronic cycle of cell death and regeneration activates hepatic stellate cells and initiates the process of hepatic fibrosis. Cirrhosis and hepatocellular carcinoma then result in some patients. Since not all patients with the same homozygous PIZZ genotype develop end-stage disease, it is hypothesized that there is likely to be a strong influence of genetic and environmental modifiers of the injury cascade and of the fibrotic response.

Deficient and Null Variants of SERPINA1 Are Proteotoxic in a Caenorhabditis elegans Model of α1-Antitrypsin Deficiency

α1-antitrypsin deficiency (ATD) predisposes patients to both loss-of-function (emphysema) and gain-of-function (liver cirrhosis) phenotypes depending on the type of mutation. Although the Z mutation (ATZ) is the most prevalent cause of ATD, >120 mutant alleles have been identified. In general, these mutations are classified as deficient (<20% normal plasma levels) or null (<1% normal levels) alleles. The deficient alleles, like ATZ, misfold in the ER where they accumulate as toxic monomers, oligomers and aggregates. Thus, deficient alleles may predispose to both gain- and loss-of-function phenotypes. Null variants, if translated, typically yield truncated proteins that are efficiently degraded after being transiently retained in the ER. Clinically, null alleles are only associated with the loss-of-function phenotype. We recently developed a C. elegans model of ATD in order to further elucidate the mechanisms of proteotoxicity (gain-of-function phenotype) induced by the aggregation-prone deficient allele, ATZ. The goal of this study was to use this C. elegans model to determine whether different types of deficient and null alleles, which differentially affect polymerization and secretion rates, correlated to any extent with proteotoxicity. Animals expressing the deficient alleles, Mmalton, Siiyama and S (ATS), showed overall toxicity comparable to that observed in patients. Interestingly, Siiyama expressing animals had smaller intracellular inclusions than ATZ yet appeared to have a greater negative effect on animal fitness. Surprisingly, the null mutants, although efficiently degraded, showed a relatively mild gain-of-function proteotoxic phenotype. However, since null variant proteins are degraded differently and do not appear to accumulate, their mechanism of proteotoxicity is likely to be different to that of polymerizing, deficient mutants. Taken together, these studies showed that C. elegans is an inexpensive tool to assess the proteotoxicity of different AT variants using a transgenic approach.

Alpha-1 antitrypsin and liver disease: mechanisms of injury and novel interventions

α-1-Antitrypsin (α1AT) is a serum glycoprotein synthesized in the liver. The majority of patients with α1AT deficiency liver disease are homozygous for the Z mutant of α1AT (called ZZ or 'PIZZ'). This mutant gene directs the synthesis of an abnormal protein which folds improperly during biogenesis. Most of these mutant Z protein molecules undergo proteolysis; however, some of the mutant protein accumulates in hepatocytes. Hepatocytes with the largest mutant protein burdens undergo apoptosis, causing compensatory hepatic proliferation. Cycles of hepatocyte injury, cell death and compensatory proliferation results in liver disease ranging from mild asymptomatic enzyme elevations to hepatic fibrosis, cirrhosis and hepatocellular carcinoma. There is a high variability in clinical disease presentation suggesting that environmental and genetic modifiers are important. Management of α1AT liver disease is based on standard supportive care and liver transplant. However, increased understanding of the cellular mechanisms of liver injury has led to new clinical trials.

Advances in alpha-1-antitrypsin deficiency liver disease

Alpha-1-antitrypsin (a1AT) deficiency is a common, but under-diagnosed, genetic disease. In the classical form, patients are homozygous for the Z mutant of the a1AT gene (called ZZ or PIZZ), which occurs in 1 in 2,000-3,500 births. The mutant Z gene directs the synthesis of large quantities of the mutant Z protein in the liver, which folds abnormally during biogenesis and accumulates intracellularly, rather than being efficiently secreted. The accumulation mutant Z protein within hepatocytes causes liver injury, cirrhosis, and hepatocellular carcinoma via a cascade of chronic hepatocellular apoptosis, regeneration, and end organ injury. There is no specific treatment for a1AT-associated liver disease, other than standard supportive care and transplantation. There is high variability in the clinical manifestations among ZZ homozygous patients, suggesting a strong influence of genetic and environmental modifiers. New insights into the biological mechanisms of intracellular injury have led to new, rational therapeutic approaches.

Antisense oligonucleotide treatment ameliorates alpha-1 antitrypsin-related liver disease in mice

Alpha-1 antitrypsin deficiency (AATD) is a rare genetic disease that results from mutations in the alpha-1 antitrypsin (AAT) gene. The mutant AAT protein aggregates and accumulates in the liver leading to AATD liver disease, which is only treatable by liver transplant. The PiZ transgenic mouse strain expresses a human AAT (hAAT) transgene that contains the AATD-associated Glu342Lys mutation. PiZ mice exhibit many AATD symptoms, including AAT protein aggregates, increased hepatocyte death, and liver fibrosis. In the present study, we systemically treated PiZ mice with an antisense oligonucleotide targeted against hAAT (AAT-ASO) and found reductions in circulating levels of AAT and both soluble and aggregated AAT protein in the liver. Furthermore, AAT-ASO administration in these animals stopped liver disease progression after short-term treatment, reversed liver disease after long-term treatment, and prevented liver disease in young animals. Additionally, antisense oligonucleotide treatment markedly decreased liver fibrosis in this mouse model. Administration of AAT-ASO in nonhuman primates led to an approximately 80% reduction in levels of circulating normal AAT, demonstrating potential for this approach in higher species. Antisense oligonucleotides thus represent a promising therapy for AATD liver disease.

Large protein complexes retained in the ER are dislocated by non-COPII vesicles and degraded by selective autophagy

Multisubunit protein complexes are assembled in the endoplasmic reticulum (ER). Existing pools of single subunits and assembly intermediates ensure the efficient and rapid formation of complete complexes. While being kinetically beneficial, surplus components must be eliminated to prevent potentially harmful accumulation in the ER. Surplus single chains are cleared by the ubiquitin-proteasome system. However, the fate of not secreted assembly intermediates of multisubunit proteins remains elusive. Here we show by high-resolution double-label confocal immunofluorescence and immunogold electron microscopy that naturally occurring surplus fibrinogen Aα-γ assembly intermediates in HepG2 cells are dislocated together with EDEM1 from the ER to the cytoplasm in ER-derived vesicles not corresponding to COPII-coated vesicles originating from the transitional ER. This route corresponds to the novel ER exit path we have previously identified for EDEM1 (Zuber et al. Proc Natl Acad Sci USA 104:4407-4412, 2007). In the cytoplasm, detergent-insoluble aggregates of fibrinogen Aα-γ dimers develop that are targeted by the selective autophagy cargo receptors p62/SQSTM1 and NBR1. These aggregates are degraded by selective autophagy as directly demonstrated by high-resolution microscopy as well as biochemical analysis and inhibition of autophagy by siRNA and kinase inhibitors. Our findings demonstrate that different pathways exist in parallel for ER-to-cytoplasm dislocation and subsequent proteolytic degradation of large luminal protein complexes and of surplus luminal single-chain proteins. This implies that ER-associated protein degradation (ERAD) has a broader function in ER proteostasis and is not limited to the elimination of misfolded glycoproteins.

The Lhs1/GRP170 chaperones facilitate the endoplasmic reticulum-associated degradation of the epithelial sodium channel

The epithelial sodium channel, ENaC, plays a critical role in maintaining salt and water homeostasis, and not surprisingly defects in ENaC function are associated with disease. Like many other membrane-spanning proteins, this trimeric protein complex folds and assembles inefficiently in the endoplasmic reticulum (ER), which results in a substantial percentage of the channel being targeted for ER-associated degradation (ERAD). Because the spectrum of factors that facilitates the degradation of ENaC is incomplete, we developed yeast expression systems for each ENaC subunit. We discovered that a conserved Hsp70-like chaperone, Lhs1, is required for maximal turnover of the ENaC α subunit. By expressing Lhs1 ATP binding mutants, we also found that the nucleotide exchange properties of this chaperone are dispensable for ENaC degradation. Consistent with the precipitation of an Lhs1-αENaC complex, Lhs1 holdase activity was instead most likely required to support the ERAD of αENaC. Moreover, a complex containing the mammalian Lhs1 homolog GRP170 and αENaC co-precipitated, and GRP170 also facilitated ENaC degradation in human, HEK293 cells, and in a Xenopus oocyte expression system. In both yeast and higher cell types, the effect of Lhs1 on the ERAD of αENaC was selective for the unglycosylated form of the protein. These data establish the first evidence that Lhs1/Grp170 chaperones can act as mediators of ERAD substrate selection.

Diagnosis and management of patients with α1-antitrypsin (A1AT) deficiency

Alpha(1)-antitrypsin (A1AT) deficiency is an autosomal codominant disease that can cause chronic liver disease, cirrhosis, and hepatocellular carcinoma in children and adults and increases risk for emphysema in adults. The development of symptomatic disease varies; some patients have life-threatening symptoms in childhood, whereas others remain asymptomatic and healthy into old age. As a result of this variability, patients present across multiple disciplines, including pediatrics, adult medicine, hepatology, genetics, and pulmonology. This can give physicians the mistaken impression that the condition is less common than it actually is and can lead to fragmented care that omits critical interventions commonly performed by other specialists. We sought to present a rational approach for hepatologists to manage adult patients with A1AT deficiency.

Activating transcription factor 6 limits intracellular accumulation of mutant α(1)-antitrypsin Z and mitochondrial damage in hepatoma cells

α(1)-Antitrypsin is a serine protease inhibitor secreted by hepatocytes. A variant of α(1)-antitrypsin with an E342K (Z) mutation (ATZ) has propensity to form polymers, is retained in the endoplasmic reticulum (ER), is degraded by both ER-associated degradation and autophagy, and causes hepatocyte loss. Constant features in hepatocytes of PiZZ individuals and in PiZ transgenic mice expressing ATZ are the formation of membrane-limited globular inclusions containing ATZ and mitochondrial damage. Expression of ATZ in the liver does not induce the unfolded protein response (UPR), a protective mechanism aimed to maintain ER homeostasis in the face of an increased load of proteins. Here we found that in hepatoma cells the ER E3 ligase HRD1 functioned to degrade most of the ATZ before globular inclusions are formed. Activation of the activating transcription factor 6 (ATF6) branch of the UPR by expression of spliced ATF6(1-373) decreased intracellular accumulation of ATZ and the formation of globular inclusions by a pathway that required HRD1 and the proteasome. Expression of ATF6(1-373) in ATZ-expressing hepatoma cells did not induce autophagy and increased the level of the proapoptotic factor CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) but did not lead to apoptotic DNA fragmentation. Expression of ATF6(1-373) did not cause inhibition of protein synthesis and prevented mitochondrial damage induced by ATZ expression. It was concluded that activation of the ATF6 pathway of the UPR limits ATZ-dependent cell toxicity by selectively promoting ER-associated degradation of ATZ and is thereby a potential target to prevent hepatocyte loss in addition to autophagy-enhancing drugs.

Mechanisms underlying the cellular clearance of antitrypsin Z: lessons from yeast expression systems

The most frequent cause of α(1)-antitrypsin (here referred to as AT) deficiency is homozygosity for the AT-Z allele, which encodes AT-Z. Such individuals are at increased risk for liver disease due to the accumulation of aggregation-prone AT-Z in the endoplasmic reticulum of hepatocytes. However, the penetrance and severity of liver dysfunction in AT deficiency is variable, indicating that unknown genetic and environmental factors contribute to its occurrence. There is evidence that the rate of AT-Z degradation may be one such contributing factor. Through the use of several AT-Z model systems, it is now becoming appreciated that AT-Z can be degraded through at least two independent pathways. One model system that has contributed significantly to our understanding of the AT-Z disposal pathway is the yeast, Saccharomyces cerevisiae.

Intracellular processing of alpha1-antitrypsin

α(1)-Antitrypsin (AAT) secreted from hepatocytes is an inhibitor of neutrophil elastase. Its normal circulating concentration functions to maintain the elasticity of the lung by preventing the hydrolytic destruction of elastin fibers. Severely diminished circulating concentrations of AAT, resulting from the impaired secretion of genetic variants that exhibit distinct polypeptide folding defects, can function as an etiologic agent for the development of chronic obstructive pulmonary disease. In addition, the inappropriate accumulation of structurally aberrant AAT within the hepatocyte endoplasmic reticulum can contribute to the etiology of liver disease. This article focuses on the discovery and characterization of a biosynthetic quality control system that contributes to the secretion of AAT by first facilitating its proper structural maturation, and then by orchestrating the selective elimination of those molecules that fail to attain structural maturation. Mechanistic elucidation of these interconnected quality control events recently led to the identification of an underlying genetic modifier capable of accelerating the onset of end-stage liver disease by impairing the efficiency of an initial step in the protein disposal process.

Rapamycin reduces intrahepatic alpha-1-antitrypsin mutant Z protein polymers and liver injury in a mouse model

Alpha-1-antitrypsin (a1AT) deficiency is caused by homozygosity for the a1AT mutant Z gene and occurs in one in 2000 Americans. The Z mutation confers an abnormal conformation on the a1AT mutant Z protein, resulting in accumulation within the endoplasmic reticulum of hepatocytes and chronic liver injury. Autophagy is one of several proteolytic mechanisms activated to cope with this hepatocellular protein burden, and is likely important in disposal of the unique polymerized conformation of the a1AT mutant Z protein, which is thought to be especially injurious to the cell. Recent data indicate that rapamycin may more efficiently upregulate autophagy when given in weekly dose pulses, as compared with a daily regimen. Therefore, we evaluated the effect of rapamycin on PiZ mice, a well-characterized model which recapitulates human a1AT liver disease. Daily dosing had no effect on autophagy, on accumulation of a1AT mutant Z protein or on liver injury. Weekly dosing of rapamycin did increase autophagic activity, as shown by increased numbers of autophagic vacuoles. This was associated with reduction in the intrahepatic accumulation of a1AT mutant Z protein in the polymerized conformation. Markers of hepatocellular injury, including cleavage of caspase 12 and hepatic fibrosis, were also decreased. In conclusion, this is the first report of a successful in vivo method for reduction of intrahepatic a1AT mutant Z polymerized protein. Application of this finding may be therapeutic in patients with a1AT deficiency by reducing the intracellular burden of the polymerized, mutant Z protein and by reducing the progression of liver injury.

Gp78, an ER associated E3, promotes SOD1 and ataxin-3 degradation

Superoxide dismutase-1 (SOD1) and ataxin-3 are two neurodegenerative disease proteins in association with familial amyotrophic lateral sclerosis and Machado-Joseph disease/spinocerebellar ataxia type 3. Both normal and mutant types of SOD1 and ataxin-3 are degraded by the proteasome. It was recently reported that these two proteins are associated with the endoplasmic reticulum (ER). Mammalian gp78 is an E3 ubiquitin ligase involved in ER-associated degradation (ERAD). Here, we show that gp78 interacts with both SOD1 and ataxin-3. Overexpression of gp78 promotes the ubiquitination and degradation of these two proteins, whereas knockdown of gp78 stabilizes them. Moreover, gp78 represses aggregate formation of mutant SOD1 and protect cells against mutant SOD1-induced cell death. Furthermore, gp78 is increased in cells transfected with these two mutant proteins as well as in ALS mice. Thus, our results suggest that gp78 functions in the regulation of SOD1 and ataxin-3 to target them for ERAD.

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.

Alpha-1-antitrypsin deficiency: diagnosis, pathophysiology, and management

Alpha-1-antitrypsin deficiency is a relatively common but under-recognized genetic disease in which individuals homozygous for the mutant Z disease-associated allele are at risk for the development of liver disease and emphysema. The protein product of the mutant Z gene is synthesized in hepatocytes but accumulates intracellularly rather than being appropriately secreted. The downstream effects of the intracellular accumulation of the mutant Z protein include the formation of unique protein polymers, activation of autophagy, mitochondrial injury, endoplasmic reticulum stress, and caspase activation, which subsequently progress in a cascade, causing chronic hepatocellular injury. The variable clinical presentations among affected individuals suggest an important contribution of genetic and environmental disease modifiers, which are only now being identified. The heterozygous carrier state for the mutant Z gene, found in 1.5% to 3% of the population, is not itself a common cause of liver injury but may be a modifier gene for other liver diseases.

Antigen presentation and the ubiquitin-proteasome system in host-pathogen interactions

Relatively small genomes and high replication rates allow viruses and bacteria to accumulate mutations. This continuously presents the host immune system with new challenges. On the other side of the trenches, an increasingly well-adjusted host immune response, shaped by coevolutionary history, makes a pathogen's life a rather complicated endeavor. It is, therefore, no surprise that pathogens either escape detection or modulate the host immune response, often by redirecting normal cellular pathways to their advantage. For the purpose of this chapter, we focus mainly on the manipulation of the class I and class II major histocompatibility complex (MHC) antigen presentation pathways and the ubiquitin (Ub)-proteasome system by both viral and bacterial pathogens. First, we describe the general features of antigen presentation pathways and the Ub-proteasome system and then address how they are manipulated by pathogens. We discuss the many human cytomegalovirus (HCMV)-encoded immunomodulatory genes that interfere with antigen presentation (immunoevasins) and focus on the HCMV immunoevasins US2 and US11, which induce the degradation of class I MHC heavy chains by the proteasome by catalyzing their export from the endoplasmic reticulum (ER)-membrane into the cytosol, a process termed ER dislocation. US2- and US11-mediated subversion of ER dislocation ensures proteasomal degradation of class I MHC molecules and presumably allows HCMV to avoid recognition by cytotoxic T cells, whilst providing insight into general aspects of ER-associated degradation (ERAD) which is used by eukaryotic cells to purge their ER of defective proteins. We discuss the similarities and differences between the distinct pathways co-opted by US2 and US11 for dislocation and degradation of human class I MHC molecules and also a putatively distinct pathway utilized by the murine herpes virus (MHV)-68 mK3 immunoevasin for ER dislocation of murine class I MHC. We speculate on the implications of the three pathogen-exploited dislocation pathways to cellular ER quality control. Moreover, we discuss the ubiquitin (Ub)-proteasome system and its position at the core of antigen presentation as proteolysis and intracellular trafficking rely heavily on Ub-dependent processes. We add a few examples of manipulation of the Ub-proteasome system by pathogens in the context of the immune system and such diverse aspects of the host-pathogen relationship as virus budding, bacterial chromosome integration, and programmed cell death, to name a few. Finally, we speculate on newly found pathogen-encoded deubiquitinating enzymes (DUBs) and their putative roles in modulation of host-pathogen interactions.

Mutants of neuroserpin that cause dementia accumulate as polymers within the endoplasmic reticulum

The dementia familial encephalopathy with neuroserpin inclusion bodies (FENIB) is caused by the accumulation of mutant neuroserpin within neurons (Davis, R. L., Shrimpton, A. E., Holohan, P. D., Bradshaw, C., Feiglin, D., Sonderegger, P., Kinter, J., Becker, L. M., Lacbawan, F., Krasnewich, D., Muenke, M., Lawrence, D. A., Yerby, M. S., Shaw, C.-M., Gooptu, B., Elliott, P. R., Finch, J. T., Carrell, R. W., and Lomas, D. A. (1999) Nature 401, 376-379), but little is known about the trafficking of wild type and mutant neuroserpins. We have established a cell model to study the processing of wild type neuroserpin and the Syracuse (S49P) and Portland (S52R) mutants that cause FENIB. Here we show that Syracuse and Portland neuroserpin are retained soon after their synthesis in the endoplasmic reticulum and that the limiting step in their processing is the transport to the Golgi complex. This is in contrast to the wild type protein, which is secreted into the culture medium. Mutant neuroserpin is retained within the endoplasmic reticulum as polymers, similar to those isolated from the intraneuronal inclusions in the brains of individuals with FENIB. Remarkably, the Portland mutant showed faster accumulation and slower secretion compared with the Syracuse mutant, in keeping with the more severe clinical phenotype found in patients with the Portland variant of neuroserpin. Both mutant and wild type neuroserpin were partially degraded by proteasomes. Taken together, our results provide further understanding of how cells handle defective but ordered mutant proteins and provide strong support for a common mechanism of disease caused by mutants of the serine protease inhibitor superfamily.

Alpha1-antitrypsin deficiency. 4: Molecular pathophysiology

The molecular basis of alpha(1)-antitrypsin deficiency is reviewed and is shown to be due to the accumulation of mutant protein as ordered polymers within the endoplasmic reticulum of hepatocytes. The current goals are to determine the cellular response to polymeric alpha(1)-antitrypsin and to develop therapeutic strategies to block polymerisation in vivo.

Endoplasmic reticulum quality control of unassembled iron transporter depends on Rer1p-mediated retrieval from the golgi

Endoplasmic reticulum (ER) quality control is a conserved process by which misfolded or unassembled proteins are selectively retained in the endoplasmic reticulum (ER). Failure in oligomerization of multisubunit membrane proteins is one of the events that triggers ER quality control. The transmembrane domains (TMDs) of unassembled subunits are determinants of ER retention in many cases, although the mechanism of the TMD-mediated sorting of unassembled subunits remains elusive. We studied a yeast iron transporter complex on the cell surface as a new model system for ER quality control. When Fet3p, a transmembrane subunit, is not assembled with the other membrane subunit, Ftr1p, unassembled Fet3p is exclusively localized to the ER at steady state. The TMD of Fet3p contains a determinant for this process. However, pulse-chase analysis and in vitro budding assays indicate that unassembled Fet3p rapidly escapes from the ER. Furthermore, Rer1p, a retrieval receptor for ER-resident membrane proteins in the Golgi, is responsible for the TMD-dependent ER retrieval of unassembled Fet3p. These findings provide clear evidence that the ER quality control of unassembled membrane proteins can be achieved by retrieval from the Golgi and that Rer1p serves as a specific sorting receptor in this process.

Differential requirements of novel A1PiZ degradation deficient (ADD) genes in ER-associated protein degradation

In the eukaryotic cell, a protein quality control process termed endoplasmic reticulum-associated degradation (ERAD) rids the ER of aberrant proteins and unassembled components of protein complexes that fail to reach a transport-competent state. To identify novel genes required for ERAD, we devised a rapid immunoassay to screen yeast lacking uncharacterized open reading frames that were known targets of the unfolded protein response (UPR), a cellular response that is induced when aberrant proteins accumulate in the ER. Six genes required for the efficient degradation of the Z variant of the alpha1-proteinase inhibitor (A1PiZ), a known substrate for ERAD, were identified, and analysis of other ERAD substrates in the six A1PiZ-degradation-deficient (add) mutants suggested diverse requirements for the Add proteins in ERAD. Finally, we report on bioinformatic analyses of the new Add proteins, which will lead to testable models to elucidate their activities.

An HRD/DER-independent ER quality control mechanism involves Rsp5p-dependent ubiquitination and ER-Golgi transport

We have identified a new pathway of ER-associated degradation in Saccharomyces cerevisiae that functions separately from the HRD/DER pathway comprised of Hrd1p, Hrd3p, Der1p, and Ubc7p. This pathway, termed Hrd1p independent-proteolysis (HIP), is capable of recognizing and degrading both lumenal (CPY* and PrA*), and integral membrane proteins (Sec61-2p) that misfold in the ER. CPY* overexpression likely saturates the HRD/DER pathway and activates the HIP pathway, so the slowed degradation kinetics of CPY* in a hrd1 Delta strain is restored to a wild-type rate when CPY* is overexpressed. Substrates of HIP require vesicular trafficking between the ER and Golgi apparatus before degradation by the ubiquitin-proteasome system. Ubiquitination of HIP substrates does not involve the HRD/DER pathway ubiquitin ligase Hrd1p, but instead uses another ubiquitin ligase, Rsp5p. HIP is regulated by the unfolded protein response as Ire1p is necessary for the degradation of CPY* when overexpressed, but not when CPY* is expressed at normal levels. Both the HIP and HRD/DER pathways contribute to the degradation of CPY*, and only by eliminating both is CPY* degradation completely blocked.

The proteasome participates in degradation of mutant alpha 1-antitrypsin Z in the endoplasmic reticulum of hepatoma-derived hepatocytes

Because retention of mutant alpha(1)-antitrypsin (alpha(1)-AT) Z in the endoplasmic reticulum (ER) is associated with liver disease in alpha(1)-AT-deficient individuals, the mechanism by which this aggregated glycoprotein is degraded has received considerable attention. In previous studies using stable transfected human fibroblast cell lines and a cell-free microsomal translocation system, we found evidence for involvement of the proteasome in degradation of alpha(1)-ATZ (Qu, D., Teckman, J. H., Omura, S., and Perlmutter, D. H. (1996) J. Biol. Chem. 271, 22791-22795). In more recent studies, Cabral et al. (Cabral, C. M., Choudhury, P., Liu, Y., and Sifers, R. N. (2000) J. Biol. Chem. 275, 25015-25022) found that degradation of alpha(1)-ATZ in a stable transfected murine hepatoma cell line was inhibited by tyrosine phosphatase inhibitors, but not by the proteasomal inhibitor lactacystin and concluded that the proteasome was only involved in ER degradation of alpha(1)-ATZ in nonhepatocytic cell types or in cell types with levels of alpha(1)-AT expression that are substantial lower than that which occurs in hepatocytes. To examine this important issue in further detail, in this study we established rat and murine hepatoma cell lines with constitutive and inducible expression of alpha(1)-ATZ. In each of these cell lines degradation of alpha(1)-ATZ was inhibited by lactacystin, MG132, epoxomicin, and clasto-lactacystin beta-lactone. Using the inducible expression system to regulate the relative level of alpha(1)-ATZ expression, we found that lactacystin had a similar inhibitory effect on degradation of alpha(1)-ATZ at high and low levels of alpha(1)-AT expression. Although there is substantial evidence that other mechanisms contribute to ER degradation of alpha(1)-ATZ, the data reported here indicate that the proteasome plays an important role in many cell types including hepatocytes.

Degradation of human thyroperoxidase in the endoplasmic reticulum involves two different pathways depending on the folding state of the protein

Human thyroperoxidase (hTPO), a type I transmembrane glycoprotein, plays a key role in thyroid hormone synthesis. In a previous paper (Fayadat, L., Niccoli, P., Lanet, J., and Franc, J. L. (1998) Endocrinology 139, 4277-4285) we established that after the synthesis, only 15-20% of the hTPO molecules were recognized by a monoclonal antibody (mAb15) directed against a conformational structure and that only 2% were able to reach the cell surface. In the present study using pulse-chase experiments in the presence or absence of protease inhibitors followed by immunoprecipitation procedures with monoclonal antibodies recognizing unfolded or partially folded hTPO forms we show that: (i) unfolded hTPO forms are degraded by the proteasome and (ii) partially folded hTPO forms are degraded by other proteases. It was also established upon incubating endoplasmic reticulum (ER) membranes in vitro that the degradation of the partially folded hTPO was carried out by serine and cysteine integral ER membrane proteases. These data provide valuable insights into the quality control mechanisms whereby the cells get rid of misfolded or unfolded proteins. Moreover, this is the first study describing a protein degradation process involving two distinct degradation pathways (proteasome and ER cysteine/serine proteases) at the ER level, depending on the folding state of the protein.

Role of ubiquitin in proteasomal degradation of mutant alpha(1)-antitrypsin Z in the endoplasmic reticulum

A delay in intracellular degradation of the mutant alpha(1)-antitrypsin (alpha(1)AT)Z molecule is associated with greater retention within the endoplasmic reticulum (ER) and susceptibility to liver disease in a subgroup of patients with alpha(1)AT deficiency. Recent studies have shown that alpha(1)ATZ is ordinarily degraded in the ER by a mechanism that involves the proteasome, as demonstrated in intact cells using human fibroblast cell lines engineered for expression of alpha(1)ATZ and in a cell-free microsomal translocation assay system programmed with purified alpha(1)ATZ mRNA. To determine whether the ubiquitin system is required for proteasomal degradation of alpha(1)ATZ and whether specific components of the ubiquitin system can be implicated, we have now used two approaches. First, we overexpressed a dominant-negative ubiquitin mutant (UbK48R-G76A) by transient transfection in the human fibroblast cell lines expressing alpha(1)ATZ. The results showed that there was marked, specific, and selective inhibition of alpha(1)ATZ degradation mediated by UbK48R-G76A, indicating that the ubiquitin system is at least in part involved in ER degradation of alpha(1)ATZ. Second, we subjected reticulocyte lysate to DE52 chromatography and tested the resulting well-characterized fractions in the cell-free system. The results showed that there were both ubiquitin-dependent and -independent proteasomal mechanisms for degradation of alpha(1)ATZ and that the ubiquitin-conjugating enzyme E2-F1 may play a role in the ubiquitin-dependent proteasomal mechanism.

Signal transduction from the endoplasmic reticulum to the cell nucleus

The endoplasmic reticulum (ER) serves several important functions. Cholesterol, an essential component of cellular membranes, is synthesized on the ER surface. Inside the organelle, proteins destined for secretion or transport to the cell surface are folded and become glycosylated. Because these processes are essential for cell viability, a disturbance in ER function presents significant stress to the cell. In response to ER stress, three distinct signal transduction pathways can be activated. Two of these, the unfolded protein response and the ER-overload response, respond to disturbances in protein processing. The third, the sterol regulatory cascade, is activated by depletion of cholesterol. This review summarizes the recent advances in our understanding of these ER-nuclear signal transduction pathways. In addition, it points to novel regulatory mechanisms discovered in these pathways, which may be widely used in other systems.

Disruption of disulfide bonds is responsible for impaired secretion in human complement factor H deficiency

Factor H, a secretory glycoprotein composed of 20 short consensus repeat modules, is an inhibitor of the complement system. Previous studies of inherited factor H deficiency revealed single amino acid substitutions at conserved cysteine residues, on one allele arginine for cysteine 518 (C518R) and on the other tyrosine for cysteine 941 (C941Y) (Ault, B. H., Schmidt, B. Z., Fowler, N. L., Kashtan, C. E., Ahmed, A. E., Vogt, B. A., and Colten, H. R. (1997) J. Biol. Chem. 272, 25168-25175). To ascertain if the phenotype, impaired secretion of factor H, is due to the C518R substitution or the C941Y substitution and to ascertain the mechanism by which secretion is impaired, we studied COS-1 and HepG2 cells transfected with wild type and several mutant factor H molecules. The results showed markedly impaired secretion of both C518R and C941Y factor H as well as that of factor H molecules bearing alanine or arginine substitutions at the Cys518-Cys546 disulfide bond (C518A, C546A, C546R, C518A-C546A). In each case, mutant factor H was retained in the endoplasmic reticulum and degraded relatively slowly as compared with most other mutant secretory and membrane proteins that are retained in the endoplasmic reticulum. These data indicate that impaired secretion of the naturally occurring C518R and C941Y mutant factor H proteins is due to disruption of framework-specific disulfide bonds in factor H short consensus repeat modules.

Oligosaccharide modification in the early secretory pathway directs the selection of a misfolded glycoprotein for degradation by the proteasome

The role of conformation-based quality control in the early secretory pathway is to eliminate misfolded polypeptides and unassembled multimeric protein complexes from the endoplasmic reticulum, ensuring the deployment of only functional molecules to distal sites. The intracellular fate of terminally misfolded human alpha1-antitrypsin was examined in hepatoma cells to identify the functional role of asparagine-linked oligosaccharide modification in the selection of glycoproteins for degradation by the cytosolic proteasome. Proteasomal degradation required physical interaction with the molecular chaperone calnexin. Altered sedimentation of intracellular complexes following treatment with the specific proteasome inhibitor lactacystin, and in combination with mannosidase inhibition, revealed that the removal of mannose from attached oligosaccharides abrogates the release of misfolded alpha1-antitrypsin from calnexin prior to proteasomal degradation. Intracellular turnover was arrested with kifunensine, implicating the participation of endoplasmic reticulum mannosidase I in the disposal process. Accelerated degradation occurred in a mannosidase-independent manner and was arrested by lactacystin, in response to the posttranslational inhibition of glucosidase II, demonstrating that the attenuated removal of glucose from attached oligosaccharides functions as the underlying rate-limiting step in the proteasome-mediated pathway. A model is proposed in which the removal of mannose from multiple attached oligosaccharides directs calnexin in the selection of misfolded alpha1-antitrypsin for degradation by the proteasome.

The requirement for molecular chaperones during endoplasmic reticulum-associated protein degradation demonstrates that protein export and import are mechanistically distinct

Polypeptide import into the yeast endoplasmic reticulum (ER) requires two hsp70s, Ssa1p in the cytosol and BiP (Kar2p) in the ER lumen. After import, aberrant polypeptides may be exported to the cytoplasm for degradation by the proteasome, and defects in the ER chaperone calnexin (Cne1p) compromise their degradation. Both import and export require BiP and the Sec61p translocation complex, suggesting that import and export may be mechanistically related. We now show that the cne1Delta and two kar2 mutant alleles exhibit a synthetic interaction and that the export and degradation of pro-alpha factor is defective in kar2 mutant microsomes. Pulse-chase analysis indicates that A1PiZ, another substrate for degradation, is stabilized in the kar2 strains at the restrictive temperature. Because two of the kar2 mutants examined are proficient for polypeptide import, the roles of BiP during ER protein export and import differ, indicating that these processes must be mechanistically distinct. To examine whether Ssa1p drives polypeptides from the ER and is also required for degradation, we assembled reactions using strains either containing a mutation in SSA1 or in which the level of Ssa1p could be regulated. We found that pro-alpha factor and A1PiZ were degraded normally, indicating further that import and export are distinct and that other cytosolic factors may pull polypeptides from the ER.

Endoplasmic reticulum quality control of asialoglycoprotein receptor H2a involves a determinant for retention and not retrieval

The human asialoglycoprotein receptor H2a subunit contains a charged pentapeptide, EGHRG, in its ectodomain that is the only sequence absent from the H2b alternatively spliced variant. H2b exits the endoplasmic reticulum (ER) even when singly expressed, whereas H2a gives rise to a cleaved soluble secreted ectodomain fragment; uncleaved membrane-bound H2a molecules are completely retained and degraded in the ER. We have inserted the H2a pentapeptide into the sequence of the H1 subunit (H1i5), which caused complete ER retention but, unexpectedly, no degradation. This suggests that the pentapeptide is a determinant for ER retention not colocalizing in H2a with the determinant for degradation. The state of sugar chain processing and the ER localization of H1i5, which was unchanged at 15 degrees C or after treatment with nocodazole, indicate ER retention and not retrieval from the cis-Golgi or the intermediate compartment. H1i5 folded similarly to H1, and both associated to calnexin. However, whereas H1 dissociated with a half time of 45 min, H1i5 remained bound to the chaperone for prolonged periods. The correct global folding of H2a and H1i5 and of other normal precursors and unassembled proteins and the true ER retention, and not exit and retrieval, suggest a difference in their quality control mechanism compared with that of misfolded proteins, which does involve retrieval. However, both pathways may involve calnexin.