A lag in intracellular degradation of mutant alpha 1-antitrypsin correlates with the liver disease phenotype in homozygous PiZZ alpha 1-antitrypsin deficiency

Protein aggregation as a cause for disease

The ability of proteins to fold into a defined and functional conformation is one of the most fundamental processes in biology. Certain conditions, however, initiate misfolding or unfolding of proteins. This leads to the loss of functional protein or it can result in a wide range of diseases. One group of diseases, which includes Alzheimer's, Parkinson's, Huntington's disease, and the transmissible spongiform encephalopathies (prion diseases), involves deposition of aggregated proteins. Normally, such protein aggregates are not found in properly functioning biological systems, because a variety of mechanisms inhibit their formation. Understanding the nature of these protective mechanisms together with the understanding of factors reducing or deactivating the natural protection machinery will be crucial for developing strategies to prevent and treat these disastrous diseases.

Intracellular inclusions containing mutant alpha1-antitrypsin Z are propagated in the absence of autophagic activity

Mutant alpha(1)-antitrypsin Z (alpha(1)-ATZ) protein, which has a tendency to form aggregated polymers as it accumulates within the endoplasmic reticulum of the liver cells, is associated with the development of chronic liver injury and hepatocellular carcinoma in hereditary alpha(1)-antitrypsin (alpha(1)-AT) deficiency. Previous studies have suggested that efficient intracellular degradation of alpha(1)-ATZ is correlated with protection from liver disease in alpha(1)-AT deficiency and that the ubiquitin-proteasome system accounts for a major route, but not the sole route, of alpha(1)-ATZ disposal. Yet another intracellular degradation system, autophagy, has also been implicated in the pathophysiology of alpha(1)-AT deficiency. To provide genetic evidence for autophagy-mediated disposal of alpha(1)-ATZ, here we used cell lines deleted for the Atg5 gene that is necessary for initiation of autophagy. In the absence of autophagy, the degradation of alpha(1)-ATZ was retarded, and the characteristic cellular inclusions of alpha(1)-ATZ accumulated. In wild-type cells, colocalization of the autophagosomal membrane marker GFP-LC3 and alpha(1)-ATZ was observed, and this colocalization was enhanced when clearance of autophagosomes was prevented by inhibiting fusion between autophagosome and lysosome. By using a transgenic mouse with liver-specific inducible expression of alpha(1)-ATZ mated to the GFP-LC3 mouse, we also found that expression of alpha(1)-ATZ in the liver in vivo is sufficient to induce autophagy. These data provide definitive evidence that autophagy can participate in the quality control/degradative pathway for alpha(1)-ATZ and suggest that autophagic degradation plays a fundamental role in preventing toxic accumulation of alpha(1)-ATZ.

Accumulation of mutant alpha1-antitrypsin Z in the endoplasmic reticulum activates caspases-4 and -12, NFkappaB, and BAP31 but not the unfolded protein response

In alpha(1)-antitrypsin (alpha1AT) deficiency, a polymerogenic mutant form of the secretory glycoprotein alpha1AT, alpha1ATZ, is retained in the endoplasmic reticulum (ER) of liver cells. It is not yet known how this results in liver injury in a subgroup of deficient individuals and how the remainder of deficient individuals escapes liver disease. One possible explanation is that the "susceptible" subgroup is unable to mount the appropriate protective cellular responses. Here we examined the effect of mutant alpha1ATZ on several potential protective signaling pathways by using cell lines with inducible expression of mutant alpha1AT as well as liver from transgenic mice with liver-specific inducible expression of mutant alpha1AT. The results show that ER retention of polymerogenic mutant alpha1ATZ does not result in an unfolded protein response (UPR). The UPR can be induced in the presence of alpha1ATZ by tunicamycin excluding the possibility that the pathway has been disabled. In striking contrast, ER retention of nonpolymerogenic alpha1AT mutants does induce the UPR. These results indicate that the machinery responsible for activation of the UPR can distinguish the physical characteristics of proteins that accumulate in the ER in such a way that it can respond to misfolded but not relatively ordered polymeric structures. Accumulation of mutant alpha1ATZ does activate specific signaling pathways, including caspase-12 in mouse, caspase-4 in human, NFkappaB, and BAP31, a profile that was distinct from that activated by nonpolymerogenic alpha1AT mutants.

Grp78, Grp94, and Grp170 interact with alpha1-antitrypsin mutants that are retained in the endoplasmic reticulum

In alpha1-antitrypsin (alpha1-AT) deficiency, a mutant form of alpha1-AT polymerizes in the endoplasmic reticulum (ER) of liver cells resulting in chronic hepatitis and hepatocellular carcinoma by a gain of toxic function mechanism. Although some aspects of the cellular response to mutant alpha1-AT Z have been partially characterized, including the involvement of several proteasomal and nonproteasomal mechanisms for disposal, other parts of the cellular response pathways, particularly the chaperones with which it interacts and the signal transduction pathways that are activated, are still not completely elucidated. The alpha1-AT Z molecule is known to interact with calnexin, but, according to one study, it does not interact with Grp78. To carry out a systematic search for the chaperones with which alpha1-AT Z interacts in the ER, we used chemical cross-linking of several different genetically engineered cell systems. Mutant alpha1-AT Z was cross-linked with Grp78, Grp94, calnexin, Grp170, UDP-glucose glycoprotein:glucosyltransferase, and two unknown proteins of approximately 110-130 kDa. Sequential immunoprecipitation/immunoblot analysis and coimmunoprecipitation techniques demonstrated each of these interactions without chemical cross-linking. The same chaperones were found to interact with two nonpolymerogenic alpha1-AT mutants that are retained in the ER, indicating that these interactions are not specific for the alpha1-AT Z mutant. Moreover, sucrose density gradient centrifugation studies suggest that approximately 85% of alpha1-AT Z exists in heterogeneous soluble complexes with multiple chaperones and approximately 15% in extremely large polymers/aggregates devoid of chaperones. Agents that perturb the synthesis and/or activity of ER chaperones such as tunicamycin and calcium ionophore A23187, have different effects on the solubility and degradation of alpha1-AT Z as well as on its residual secretion.

Alpha-1-antitrypsin deficiency: a new paradigm for hepatocellular carcinoma in genetic liver disease

Liver disease in alpha-1-antitrypsin (alpha1AT) deficiency is caused by a gain-of-toxic function mechanism engendered by the accumulation of a mutant glycoprotein in the endoplasmic reticulum (ER). The extraordinary degree of variation in phenotypical expression of this liver disease is believed to be determined by genetic modifiers and/or environmental factors that influence the intracellular disposal of the mutant glycoprotein or the signal transduction pathways that are activated. Recent investigations suggest that a specific repertoire of signaling pathways are involved, including the autophagic response, mitochondrial- and ER-caspase activation, and nuclear factor kappaB (NFkappaB) activation. Whether activation of these signaling pathways, presumably to protect the cell, inadvertently contributes to liver injury or perhaps protects the cell from one injury and, in so doing, predisposes it to another type of injury, such as hepatocarcinogenesis, is not yet known. Recent studies also suggest that hepatocytes with marked accumulation of alpha1ATZ, globule-containing hepatocytes, engender a cancer-prone state by surviving with intrinsic damage and by chronically stimulating in 'trans' adjacent relatively undamaged hepatocytes that have a selective proliferative advantage. Further, this paradigm may apply to other genetic and infectious liver diseases that are predisposed to hepatocellular carcinoma.

Genome-environment interactions in the molecular pathogenesis of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a heart muscle disease characterized by impaired contractility and dilation of the ventricles. In a subset of DCM patients, classical inheritance patterns occur (familial DCM), which have led to the identification of specific genomic loci and gene defects causing monogenic DCM subtypes. In the majority of DCM patients, however, there is no evidence for a monogenic etiology of the disorder (sporadic DCM), and in the absence of other recognizable etiological factors, these cases were classified as "idiopathic". Recent research suggests that cardiotropic viruses are important environmental factors in the pathogenesis of "idiopathic" cases and that DCM commonly results from interactions between genetic and environmental factors, whereas "pure" genetic forms are rather rare. Regarding genetics, the clinical cardiomyopathic phenotype associated with single gene defects may be highly variable for unknown reasons. Furthermore, a novel class of genetic defects was identified recently which provide a molecular basis for abnormal reactions of cardiomyocytes to environmental stress. These defects are paradigms of specific molecular links between genome and environment during the pathogenesis of DCM. Regarding environmental factors, a recent molecular virological study based on myocardial biopsies in a large series of sporadic DCM patients has detected cardiac viral infections in the majority of patients, with a broad spectrum of virus species being involved. Apparently, DCM does not only occur as a late sequela of acute viral myocarditis, but also in patients without clinical history of cardiac viral disease. Cardiotropic viruses thus emerge as prevalent environmental factors which may cause or influence the course of DCM in a large fraction of cases. Synopsis of current data suggests that a comprehensive picture of DCM pathogenesis can only be drawn if both genetic and environmental pathogenetic factors are considered. The course of cardiac viral infections depends strongly on genetic host factors and may range from rapid and complete virus elimination or silencing without clinical symptoms, to rapidly progressive or fatal disease. Viruses interact not only with genetically heterogenous host systems of virus uptake, migration, and antiviral immunity, but, due to their prevalence in DCM hearts, are also likely to encounter multiple structural proteins of cardiac cells known to be defective in familial DCM. The combined knowledge on DCM-associated gene defects and viruses therefore suggests in-depth studies on genome-environment interactions in DCM pathogenesis which may underlie the high clinical variability observed both in monogenic and virus-associated DCM and have implications for the clinical management of DCM patients.

Alpha-1-antitrypsin associated panniculitis: the MS variant

Over 90 mutant alleles of the alpha-1-antitrypsin (AAT) gene are recognized and classified by mobility on an acid starch gel. The four major categories include: F=fast, M=medium, S=slow, Z=very slow. Among 41 reported cases of AAT panniculitis, most have the ZZ phenotype with AAT levels below normal. We report two cases of AAT panniculitis with MS phenotype and normal AAT levels. In addition, we review the pathophysiology, epidemiology, and extracutaneous manifestations of AAT disease and propose a diagnostic algorithm for ulcerative panniculitis. A 42-year-old man presented with a solitary plaque on the left thigh exacerbated by trauma or excessive activity. The lesion frequently suppurated with a yellowish oily material. Twenty years before, he had fractured his left femur which was repaired with a metal plate. X-rays, histology with special stains for organisms, and cultures were negative. AAT phenotype was MS and AAT value was normal. A 43-year-old woman presented with multiple plaques on the proximal extremities which suppurated with exercise or trauma. AAT phenotype was MS and AAT level was normal. Histologic exam for both patients showed a dense neutrophilic infiltrate with septal and lobular panniculitis and areas of necrobiosis in the lower reticular dermis.

Comparison of the properties of rare variants of alpha1-proteinase inhibitor expressed in COS-1 cells and assessment of their potential as risk factors in human disease

Among the more than 75 known variants of alpha(1)-proteinase inhibitor, a sub-population of rare, point mutations causing single amino acid replacements have been identified and classified as "at risk" alleles for development of pulmonary disease. In most cases, it is not clear how the amino acid replacements typical of these variants change the properties of the inhibitor to increase risk of disease in the affected individuals. To begin to address this question, we mutagenized a wild type alpha(1)-proteinase inhibitor cDNA to encode a panel of eight different point mutants reported to be associated with increased risk for development of pulmonary disease. These variants were then expressed in COS-l cells transiently transfected with plasmids containing the altered cDNAs. The effects of the mutations on the rates of secretion, cellular location, intracellular degradation, activity, stability, and tendency to aggregate were determined. Results of these studies show that, in some cases, the mutations affect the rate of secretion, the activity or both of these properties of alpha(1)-proteinase inhibitor in a manner consistent with its designation as an "at-risk" allele. In other cases, the mutations do not significantly change the properties of the inhibitor, suggesting that these may be normal variants and that their expression may not increase the risk of disease.

Liver histology of an afibrinogenemic patient with the Bbeta-L353R mutation showing no evidence of hepatic endoplasmic reticulum storage disease (ERSD); comparative study in COS-1 cells of the intracellular processing of the Bbeta-L353R fibrinogen vs. the ERSD-associated gamma-G284R mutant

BACKGROUND

Type I fibrinogen deficiencies (hypofibrinogenemia and afibrinogenemia) are rare congenital disorders characterized by low or unmeasurable plasma fibrinogen antigen levels. Their genetic bases are represented by mutations within the three fibrinogen genes. Among the 11 reported missense mutations, a few have been characterized by expression studies and found to have an impaired fibrinogen assembly and/or secretion. Histopathological analyses were previously reported in two hypofibrinogenemic cases with discernible hepatic disease, revealing that both underlying mutations (gamma-Gly284Arg and gamma-Arg375Trp) were associated with hepatic fibrinogen endoplasmic reticulum storage disease (ERSD).

OBJECTIVE

The objective of this study was to investigate the liver histology in an afibrinogenemic patient, homozygous for the Bbeta-Leu353Arg mutation, and to study the intracellular processing of the mutant protein.

PATIENTS AND METHODS

Liver histology was evaluated by light microscopy, electron microscopy and immunocytochemistry. Intracellular processing of mutant fibrinogen was analyzed by pulse-chase labeling and immunoprecipitation experiments. Messenger RNA levels were determined by real-time reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

The histopathological characterization of the liver showed no signs of fibrinogen accumulation, a difference from the previously reported findings in two hypofibrinogenemic kindreds with ERSD. To evaluate whether the Bbeta-Leu353Arg mutation and the ERSD-associated gamma-Gly284Arg mutation affected intracellular fibrinogen trafficking differently, both mutant proteins were expressed in COS-1 cells. Bbeta-Leu353Arg led to a more severe secretion defect, but no differences that could explain phenotype-genotype correlation were found in the intracellular processing. Endoglycosidase-H analysis demonstrated a secretion block before translocation to the Golgi medial stacks. Real-time RT-PCR studies showed normal levels of the Bbeta mRNA in the patient's liver.

CONCLUSIONS

The results confirm that Bbeta-Leu353Arg is associated with impaired fibrinogen secretion, but not with hepatic ERSD.

His36Pro point-mutated proteolipid protein retained in the endoplasmic reticulum of oligodendrocytes in theShaking pup

The shaking pup (shp) is a canine mutation that affects the myelin protein proteolipid protein (PLP) and its smaller and less abundant isoform, DM20, with proline replacing histidine(36), resulting in a severe myelin deficiency in the central nervous system. We present evidence that the mutation leads to disrupted trafficking of the shp PLP/DM20 within oligodendrocytes. Immunohistochemical studies revealed significantly reduced levels of PLP/DM20 and other major myelin components such as myelin basic protein (MBP), myelin associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in shp myelin. The distribution of shp PLP/DM20 proteins were altered and mostly retained in perinuclear cytoplasm and proximal processes, which co-localized with distended rough endoplasmic reticulum (RER) within oligodendrocytes. No abnormal accumulation of MAG, MBP, or CNP in the cell body was found. These results suggest that mutated PLP/DM20 in the shp could be selectively retained in RER, causing disruption of their translocation to the periphery to myelinate axons.

The unfolded protein response--a stress signaling pathway of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a factory for folding and maturation of newly synthesized transmembrane and secretory proteins. The ER provides stringent quality control systems to ensure that only correctly folded proteins exit the ER and unfolded or misfolded proteins are retained and ultimately degraded. A number of biochemical and physiological stimuli can change ER homeostasis, impose stress to the ER, and subsequently lead to accumulation of unfolded or misfolded proteins in the ER lumen. The ER has evolved stress response signaling pathways collectively called the unfolded protein response (UPR) to cope with the accumulation of unfolded or misfolded proteins. This review summarizes our understanding of the UPR signaling developed in the recent years.

Alpha-1-antitrypsin deficiency: diagnosis and treatment

Alpha-1-antitrypsin (AT) deficiency was first described in the late 1960s in patients with severe pulmonary emphysema. The recognition of AT deficiency as a cause of emphysema then led to what is still the prevailing theory for the pathogenesis of emphysema, the protease-antiprotease theory. Soon it was found that AT deficiency accounted for a significant number of cases of neonatal liver disease that were previously categorized as idiopathic. We now know that AT deficiency is the most common genetic cause of neonatal liver disease and the most frequent diagnosis necessitating liver transplantation. It has also been shown to cause chronic liver disease, cryptogenic cirrhosis, and hepatocellular carcinoma in adults never previously known to have liver disease in infancy or childhood. Observations indicate that genetic traits unlinked to the AT gene or environmental factors predispose to or protect AT-deficient individuals from liver disease.

Role of quality control pathways in human diseases involving protein misfolding

Advances in connecting phenotype to genotype have led to new insights regarding the basis of human disease. Many inherited diseases are now known to arise due to specific mutations within a gene that then lead to a protein product unable to assume a stable conformation within the cell. Cellular machineries serving as "quality control monitors" recognize and target such abnormally folded proteins for rapid destruction. As a consequence, specific biochemical pathways requiring the protein of interest are adversely affected and lead to the disease phenotype. Yet in other cases, upon its misfolding the particular protein quickly aggregates, leading to the formation of inclusion bodies that eventually lead to cell demise. In what follows I discuss some classic examples of human diseases known to arise due to mutations that lead to altered protein folding, abnormal protein maturation and/or protein aggregation. In many cases simply altering the protein folding environment within the cell, via molecular or pharmacological approaches, can effectively rescue the maturation and stability of the mutant protein and thereby reduce the onset and/or progression of the disease phenotype. These new insights regarding the mechanisms underlying the disease phenotype, as well as new approaches to correct the protein folding defect, will undoubtedly prove to have a tremendous impact on clinical medicine.

Quality control in the apoA-I secretory pathway

From a total of 47 known apolipoprotein A-I (apoA-I) mutations, only 18 are linked to low plasma HDL apoA-I concentrations, and 78% of these map to apoA-I helices 6 and 7 (residues 143-186). Gene transfer and transgenic mouse studies have shown that several helix 6 apoA-I mutations have reduced hepatic HDL production. Our objective was to examine the impact of helix 6 modifications on intracellular biosynthetic processing and secretion of apoA-I. Cells were transfected with wild-type or mutant apoA-I, radiolabeled with [(35)S]Met/Cys, and then placed in unlabeled medium for up to 4 h. Results show that >90% of newly synthesized wild-type apoA-I was secreted by 60 min. Over the same length of time, only 20% of helix 6 deletion mutant (Delta 6 apoA-I) was secreted, whereas 80% remained cell associated. Microscopic and biochemical studies revealed that cell-associated Delta 6 apoA-I was located predominantly within the cytoplasm as lipid-protein inclusions, whereas wild-type apoA-I was localized in the endoplasmic reticulum/Golgi. Results using other helix deletions or helix 6 substitution mutations indicated that only complete removal of helix 6 resulted in massive cytoplasmic accumulation. These data suggest that alterations in native apoA-I conformation can lead to aberrant trafficking and accumulation of apolipoprotein-phospholipid structures. Thus, conformation-dependent alterations in intracellular trafficking and turnover may underlie the reduced plasma HDL concentrations observed in individuals harboring deletion mutations within helix 6.

Lack of effect of oral 4-phenylbutyrate on serum alpha-1-antitrypsin in patients with alpha-1-antitrypsin deficiency: a preliminary study

OBJECTIVE

In homozygotes with ZZ genotype alpha-1-antitrypsin (alpha1AT) deficiency, mutant alpha1ATZ protein (alpha1ATZ) accumulates in hepatocytes, rather than being secreted into the blood. Homozygous individuals experience emphysema as a result of reduced levels of circulating alpha1AT in the lung with which to inhibit connective tissue breakdown. Homozygotes may also experience liver disease from the accumulation of alpha1ATZ within hepatocytes, which causes liver damage. A previous study indicated that the compound 4-phenylbutyrate (4-PBA) mediated a significant increase in release of alpha1ATZ from cells in tissue culture and in a mouse model of alpha1AT deficiency. The authors hypothesized that 4-PBA could be used to treat both the liver and lung disease of humans with alpha1AT deficiency.

METHODS

In this preliminary, open label study the authors evaluated the effect of 14 days of oral 4-PBA therapy on alpha1AT blood levels in 10 patients with alpha1AT deficiency.

RESULTS

There was no significant increase in alpha1AT blood level associated with 4-PBA administration. Symptomatic and metabolic side effects were significant.

CONCLUSION

4-PBA did not increase alpha1AT blood levels in humans with alpha1AT deficiency in this preliminary trial.

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.

Mitochondrial autophagy and injury in the liver in alpha 1-antitrypsin deficiency

Homozygous, PIZZ alpha(1)-antitrypsin (alpha(1)-AT) deficiency is associated with chronic liver disease and hepatocellular carcinoma resulting from the toxic effects of mutant alpha(1)-anti-trypsin Z (alpha(1)-ATZ) protein retained in the endoplasmic reticulum (ER) of hepatocytes. However, the exact mechanism(s) by which retention of this aggregated mutant protein leads to cellular injury are still unknown. Previous studies have shown that retention of mutant alpha(1)-ATZ in the ER induces an intense autophagic response in hepatocytes. In this study, we present evidence that the autophagic response induced by ER retention of alpha(1)-ATZ also involves the mitochondria, with specific patterns of both mitochondrial autophagy and mitochondrial injury seen in cell culture models of alpha(1)-AT deficiency, in PiZ transgenic mouse liver, and in liver from alpha(1)-AT-deficient patients. Evidence for a unique pattern of caspase activation was also detected. Administration of cyclosporin A, an inhibitor of mitochondrial permeability transition, to PiZ mice was associated with a reduction in mitochondrial autophagy and injury and reduced mortality during experimental stress. These results provide evidence for the novel concept that mitochondrial damage and caspase activation play a role in the mechanism of liver cell injury in alpha(1)-AT deficiency and suggest the possibility of mechanism-based therapeutic interventions.

What We Owe to α1‐Antitrypsin and to Carl‐Bertil Laurell

The archetypal status of alpha(1)-antitrypsin in biology and medicine grew from the finding, thirty years ago, by Carl-Bertil Laurell, of the association of its deficiency with emphysema. In biology, alpha(1)-antitrypsin now provides the model for both the structure and the remarkable mechanism of the serpin protease inhibitors that control the key proteolytic pathways of the body. In medicine, the plasma deficiency of alpha(1)-antitrypsin has drawn attention to protease-antiprotease imbalance as a contributory cause of chronic obstructive pulmonary disease. But even more significantly, the finding that the common genetic deficiency of alpha(1)-antitrypsin was also associated with the development of liver cirrhosis introduced the new entity of the conformational diseases. The proposal that the same general mechanism was responsible for the best known of the conformational diseases, the common late-onset dementias, was controversial. It was vindicated however by the recent finding that a mutation, which results in the liver aggregation of alpha(1)-antitrypsin, also results in a typical late-onset dementia when it occurs in a brain-specific homologue of alpha(1)-antitrypsin. The extensive development of such diverse fields of studies, each based on alpha(1)-antitrypsin, is a measure of the encouragement Laurell gave to younger colleagues in the field. It also reflects the great advantage of linked contributions from clinical as well as basic sciences. Time after time, scientific controversies and deadlocks have been solved by landmark clinical cases, which have revealed unexpected findings and insights, within and beyond the fields of study.

Evolving questions and paradigm shifts in endoplasmic-reticulum-associated degradation (ERAD)

ER-associated degradation (ERAD) is a component of the protein quality control system, ensuring that aberrant polypeptides cannot transit through the secretory pathway. This is accomplished by a complex sequence of events in which unwanted proteins are selected in the ER and exported to the cytosol for degradation by the proteasome. Given that protein quality control can be essential for cell survival, it is not surprising that ERAD is linked to numerous disease states. Here we review the molecular mechanisms of ERAD, its role in metabolic regulation and biomedical implications, and the unanswered questions regarding this process.

Elucidation of the molecular logic by which misfolded alpha 1-antitrypsin is preferentially selected for degradation

The exocytic pathway provides a physical route through which newly synthesized secretory and membrane proteins are deployed to the eukaryote cell surface. For newly synthesized alpha1-antitrypsin (AAT), the modification of its asparagine-linked oligosaccharides by a slow-acting mannosidase partitions the misfolded monomer into the proteasomal degradation pathway. Herein, we asked whether, and how, modification by endoplasmic reticulum mannosidase I (ERManI) contributes to the preferential selection of the misfolded AAT monomer for proteasomal degradation. Transiently expressed mutant and WT AAT variants underwent rapid destabilization in response to an artificially elevated ERManI concentration in the murine hepatoma cell line, Hepa1a. Based on the mannosidase- and lactacystin-sensitive properties of intracellular turnover, a stochastic model is proposed in which the delayed onset of the glycan modification, relative to the duration of nonnative protein structure, coordinates the preferential degradation of the misfolded monomer and spares the native molecule from destruction. Newly synthesized endogenous transferrin underwent degradation in response to an elevated concentration of ERManI, whereas the nonglycosylated secretory glycoprotein albumin was not affected. Taken together, these findings indicate that efficient conformational maturation might function as the initial quality control standard for a broad population of glycoproteins.

Rescuing the traffic-deficient mutants of rat mu-opioid receptors with hydrophobic ligands

Deletion of a sequence near the fifth transmembrane domain (258RLSKV262, i3-1 mutant) and a motif residing at the proximal carboxyl tail (344KFCTR348, C-2 mutant) resulted in mu-opioid receptor mutants that were poorly expressed on the surface of transfected human embryonic kidney 293 cells. Treatment with the opioid antagonist naloxone, the agonist etorphine, and other hydrophobic ligands enhanced cell surface expression of i3-1 and C-2 mutants. The observed enhancement was time- and concentration-dependent, required the ligands to be membrane permeable, and was not the result of the reversal of the constitutive activities of the mutant receptors. The binding of the ligands resulted in the trafficking of the mutant receptors retained in the endoplasmic reticulum to the cell surface. The cell surface-expressed mutant C-2, but not i3-1, fully retained ability to mediate inhibition of adenylyl cyclase activity. Furthermore, the Golgi-disturbing agents brefeldin A and monensin completely blocked naloxone-enhanced expression of i3-1 and C-2 mutants. Results of these studies suggest that intracellular interactions of agonist and antagonist with mutant receptors can serve as chaperones in the trafficking of the mutants to the cell surface.

Endoplasmic reticulum calcium storage and release in cells expressing misfolded growth hormone

AIM

Deletion of amino acids 32-71 in human growth hormone (Delta 32-71-GH) causes severe autosomal dominant GH deficiency. These experiments test whether retention of Delta 32-71-GH in the endoplasmic reticulum (ER) lumen leads to aberrant Ca(2+) regulation.

DESIGN

COS cells were transfected with Delta 32-71-GH, wild-type-GH or empty plasmid, and the ability of the cells to release Ca(2+) from the ER and transmit a Ca(2+) signal to the cytoplasm was investigated using cytoplasmic Ca(2+) dyes and ER-targeted cameleon Ca(2+) reporters.

RESULTS

Resting free Ca(2+), the rate of Ca(2+) release from the ER and the size of the ionophore-releasable ER Ca(2+) pool were not altered by Delta 32-71-GH. Stimulation of endogenous Ca(2+)-mobilizing receptors for histamine and thrombin resulted in similar changes in cytoplasmic and ER Ca(2+) in cells expressing wild-type and Delta 32-71-GH.

CONCLUSION

Ca(2+) regulation is preserved despite retention of misfolded GH in the ER.

The C264Y missense mutation in the extracellular domain of L1 impairs protein trafficking in vitro and in vivo

The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, performs important functions in the developing and adult nervous system and is implicated in neuronal migration and survival, elongation, fasciculation and pathfinding of axons, and synaptic plasticity. This view is in line with the fact that mutations in the L1 gene result in severe neurological syndromes in humans. Patients with missense mutations in the extracellular domain of L1 often develop severe phenotypes. Here, we characterized in vitro and in vivo the missense mutation C264Y, which is located in the extracellular domain of L1 and causes a severe phenotype in humans. Transfection studies in vitro demonstrate that L1 carrying this missense mutation is not expressed at the cell surface but instead is located intracellularly, most likely within the endoplasmic reticulum. Lack of cell surface expression of L1 with a C264Y mutation was confirmed in a transgenic mouse line expressing the C264Y mutation under the control of the L1 promoter in an L1-deficient background. Analysis of these transgenic mice indicates that they represent functional null mutants, phenotypically indistinguishable from L1-deficient mice. These observations corroborate the view that impaired cell surface expression of mutated variants of L1 is a potential explanation for the high number of severe pathogenic mutations identified within the human L1 gene.

The C264Y missense mutation in the extracellular domain of L1 impairs protein trafficking in vitro and in vivo

The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, performs important functions in the developing and adult nervous system and is implicated in neuronal migration and survival, elongation, fasciculation and pathfinding of axons, and synaptic plasticity. This view is in line with the fact that mutations in the L1 gene result in severe neurological syndromes in humans. Patients with missense mutations in the extracellular domain of L1 often develop severe phenotypes. Here, we characterized in vitro and in vivo the missense mutation C264Y, which is located in the extracellular domain of L1 and causes a severe phenotype in humans. Transfection studies in vitro demonstrate that L1 carrying this missense mutation is not expressed at the cell surface but instead is located intracellularly, most likely within the endoplasmic reticulum. Lack of cell surface expression of L1 with a C264Y mutation was confirmed in a transgenic mouse line expressing the C264Y mutation under the control of the L1 promoter in an L1-deficient background. Analysis of these transgenic mice indicates that they represent functional null mutants, phenotypically indistinguishable from L1-deficient mice. These observations corroborate the view that impaired cell surface expression of mutated variants of L1 is a potential explanation for the high number of severe pathogenic mutations identified within the human L1 gene.

Fasting in alpha1-antitrypsin deficient liver: constitutive [correction of consultative] activation of autophagy

Alpha1-antitrypsin (alpha1-AT) deficiency causes severe liver injury in a subgroup of patients. Liver injury is thought to be caused by retention of a polymerized mutant alpha1-ATZ molecule in the endoplasmic reticulum (ER) of hepatocytes and is associated with an intense autophagic response. However, there is limited information about what physiologic stressors might influence liver injury. In this study, we examined the effect of fasting in the PiZ mouse model of alpha1-AT deficiency, because fasting is a well-characterized physiological stressor and a known stimulus for autophagy. Results show that there is a marked increase in fat accumulation and in alpha1-AT-containing globules in the liver of the PiZ mouse induced by fasting. Although fasting induced a marked autophagic response in wild-type mice, the autophagic response was already activated in PiZ mice and did not further increase with fasting. PiZ mice also had a significantly decreased tolerance for prolonged fasting compared with wild-type mice (PiZ mice 0% survival of 72-h fast; wild-type 100% survivial). These results demonstrate an altered response to stress in the alpha1-AT-deficient liver, including inability to further increase an activated autophagic response, a developmental state-specific increase in alpha1-AT-containing globules, and increased mortality.

Serpinopathies and the conformational dementias

The serpin superfamily of serine proteinase inhibitors has a central role in controlling proteinases in many biological pathways in a wide range of species. The inhibitory function of the serpins involves a marked conformational transition, but this inherent molecular flexibility also renders the serpins susceptible to point mutations that result in aberrant intermolecular linkage and polymer formation. The effects of such protein aggregation are cumulative, with a progressive loss of cellular function that results in diseases as diverse as cirrhosis and emphysema. The recent recognition that mutations in a serpin can also result in late-onset dementia provides insights into changes that underlie other conformational diseases, such as the amyloidoses, the prion encephalopathies and Huntington and Alzheimer diseases.

Ubiquitin-dependent proteolysis: its role in human diseases and the design of therapeutic strategies

Protein degradation is one of the tactics employed by the cell for irreversibly inactivating proteins. In eukaryotes, ATP-dependent protein degradation in the cytoplasm and nucleus is carried out by the 26S proteasome. Most proteins are targeted to the 26S proteasome by covalent attachment of a multi-ubiquitin chain. A key component of the enzyme cascade that results in attachment of the multi-ubiquitin chain to the target or labile protein is the ubiquitin ligase that controls the specificity of the ubiquitination reaction. Defects in ubiquitin-dependent proteolysis have been shown to result in a variety of human diseases, including cancer, neurodegenerative diseases, and metabolic disorders. This review focuses on the role of ubiquitin-dependent degradation in human disease and potential clinical applications that are being developed to exploit the cells natural proteolytic machinery to treat diseases.

Organizational diversity among distinct glycoprotein endoplasmic reticulum-associated degradation programs

Protein folding and quality control in the early secretory pathway function as posttranslational checkpoints in eukaryote gene expression. Herein, an aberrant form of the hepatic secretory protein alpha1-antitrypsin was stably expressed in a human embryonic kidney cell line to elucidate the mechanisms by which glycoprotein endoplasmic reticulum-associated degradation (GERAD) is administered in cells from higher eukaryotes. After biosynthesis, genetic variant PI Z underwent alternative phases of secretion and degradation, the latter of which was mediated by the proteasome. Degradation required release from calnexin- and asparagine-linked oligosaccharide modification by endoplasmic reticulum mannosidase I, the latter of which occurred as PI Z was bound to the molecular chaperone grp78/BiP. That a distinct GERAD program operates in human embryonic kidney cells was supported by the extent of PI Z secretion, apparent lack of polymerization, inability of calnexin to participate in the degradation process, and sequestration of the glycoprotein folding sensor UDP-glucose:glycoprotein glucosyltransferase in the Golgi complex. Because UDP-glucose:glycoprotein glucosyltransferase sustains calnexin binding, its altered distribution is consistent with a GERAD program that hinders the reentry of substrates into the calnexin cycle, allowing grp78/BiP to partner with a lectin, other than calnexin, in the recognition of a two-component GERAD signal to facilitate substrate recruitment. How the processing of a mutant protein, rather than the mutation itself, can contribute to disease pathogenesis, is discussed.

Coexpression and accumulation of ubiquitin +1 and ZZ proteins in livers of children with alpha(1)-antitrypsin deficiency

The ZZ variant of alpha(1)-antitrypsin deficiency (AATD) is well known to cause liver damage and cirrhosis in some affected children. Ubiquitin abnormality was recently shown to be significant in AATD in childhood cirrhosis. Molecular misreading (MM), defined as faulty transcription of genomic information from DNA into mRNA, as well as its translation into mutant proteins, has been documented in many pathologic processes where aggregation of abnormal proteins occurs. The misread protein, ubiquitin-B(+1) (UBB(+1)), was recently identified in the hallmarks of various neurological disorders. The objective of this study was to determine whether MM of ubiquitin occurs in AATD. Twelve explanted liver specimens from AATD-affected children with cirrhosis were retrieved from archival sources, along with 10 control liver specimens obtained from autopsies of age-matched children with no clinical, gross anatomic, or histologic evidence of liver disease. Double immunofluorescence studies using rabbit polyclonal antibodies against UBB(+1) and AAT were performed on consecutively sectioned tissue. UBB(+1) immunoreactivity was colocalized with AAT in all cirrhotic AATD livers. The control livers were consistently negative. Ubiquitin MM is prominent in AATD-affected cirrhotic livers. This indicates that for children with AATD and cirrhosis, ubiquitin MM is a necessary cofactor to the aggregation of mutant ZZ isoform of AATD.

Mechanisms of liver injury relevant to pediatric hepatology

Hepatocyte injury and necrosis from many causes may result in pediatric liver disease. Influenced by other cell types in the liver, by its unique vascular arrangements, by lobular zonation, and by contributory effects of sepsis, reactive oxygen species and disordered hepatic architecture, the hepatocyte is prone to injury from exogenous toxins, from inborn errors of metabolism, from hepatotrophic viruses, and from immune mechanisms. Experimental studies on cultured hepatocytes or animal models must be interpreted with caution. Having discussed general concepts, this review describes immune mechanisms of liver injury, as seen in autoimmune hepatitis, hepatitis B and C infection, the anticonvulsant hypersensitivity syndrome, and autoimmune polyendocrinopathy. Of the monogenic disorders causing significant liver injury in childhood, alpha-1 antitrypsin deficiency and Niemann-Pick C disease demonstrate the effect of endoplasmic or endosomal retention of macromolecules. Tyrosinemia illustrates how understanding the biochemical defect leads to understanding cell injury, extrahepatic porphyric effects, oncogenesis, pharmacological intervention, and possible stem cell therapy. Pathogenesis of cirrhosis in galactosemia remains incompletely understood. In hereditary fructose intolerance, phosphate sequestration causes ATP depletion. Recent information about mitochondrial disease, NASH, disorders of glycosylation, Wilson's disease, and the progressive familial intrahepatic cholestases is discussed.

Gamma371 Thr-->Ile substitution in the fibrinogen gammaD domain causes hypofibrinogenaemia

Six members of a family with hypofibrinogenaemia had fibrinogen concentrations ranging from 0.5 to 1.1 mg/ml and, after sequencing the entire coding region and the intron exon boundaries of all three fibrinogen genes, a single heterozygous ACT-->ATT mutation was identified in the gamma gene. This novel mutation was not detected in normal family members or unrelated controls. The gamma371 Thr-->Ile substitution occurs at a conserved threonine in the gammaD domain, but molecules containing the new isoleucine were not present in circulating fibrinogen. The evidence for this was that purified gamma chains had a normal mass of 48375 Da compared to a control of 48374 Da, and tryptic peptide maps were entirely normal. The mutation predicts a mass increase of 12 Da in peptide T-36, but on mass mapping only the normal [M+2H] ion was detected, at 948 m/z. There was no new signal at 954 m/z that would indicate expression of variant chains. Also the normal 948 m/z signal was at the same intensity in digests from the proposita and controls. Crystal structures show a hydrogen bond from the threonine hydroxyl to the main chain and this case suggests this bond is critical in maintaining the structure of the gammaD domain.

Molecular analysis of the Pi*Z allele in patients with liver disease

Alpha1-antitrypsin (AAT) is the main protease inhibitor in human plasma. There are more than 75 variants of this protein that differ from each other by their isoelectric point. Most of these alleles cause a reduction in AAT levels; the most common allele is Pi*Z. The main complications related to the Pi*Z allele are obstructive pulmonary disease and liver disease. Some Pi*Z allele carriers present cholestatic jaundice and cirrhosis. The Z type is associated with a secretion defect, which leads to deficiency of AAT and to the formation of intrahepatocytic inclusions in affected subjects. The diagnosis of AAT deficiency can be made by different techniques, including molecular analysis, although the final diagnosis should be done in conjunction with demonstration of the periodic acid-Schiff-positive globules on liver biopsy. In this study, specimens of 29 patients with cryptogenic cirrhosis between age 1 month and 18 years, and of 100 controls were submitted to polymerase chain reaction followed by digestion with TaqI enzyme. Five of the 29 patients had undergone liver transplantation. Three patients were heterozygous for the Pi*Z allele, and two were homozygous (allele frequency = 12.07%; 7/58). Among the controls, who represented the population of Porto Alegre, 1 in 100 individuals was heterozygous for the Pi*Z allele, resulting in an allele frequency of 0.5% (1/200). The high frequency of Pi*Z alleles among the patients indicates the usefulness of AAT molecular testing in children with cholestatic jaundice and cirrhosis.

[Degree of association between serum levels and genotype in alpha-1-antitrypsin deficiency. Clinical usefulness]

AIM

To determine the degree of association between serum alpha-1-antitrypsin levels and its phenotypes as well as its clinical expression.

PATIENTS AND METHODS

The alpha-1-antitrypsin genotype was identified using polymerase chain reaction followed by restriction enzyme digest in 212 patients in whom serum alpha-1-antitrypsin determination had been requested. The reasons for the request, the existence of pulmonary or liver disease, clinical diagnoses and functional repercussions were analyzed.

RESULTS

Two hundred and twelve patients were evaluated (68% males; mean age: 34 20 years). In 23 patients (10.8%) a deficiency variant was found (one or two M alleles were lacking) and in 8 patients (3.8%) the genotype was ZZ. All patients with MM genotype had alpha-1-antitrypsin levels of 75 mg/dl or higher while none of the patients with ZZ genotype had levels higher than 40 ml/dl. All the patients with ZZ genotype showed alterations: 3 had pulmonary emphysema, 1 had chronic obstructive pulmonary disease and 4 had hypertransaminasemia. One patient with pulmonary emphysema had severe respiratory insufficiency while in the remaining patients with respiratory problems, respiratory insufficiency was slight or moderate. None of the patients with hypertransaminasemia showed echographic signs of portal hypertension or clinical or laboratory signs of reduced liver function.

CONCLUSIONS

There is a close association between alpha-1-antitrypsin levels and the different genotypes. Consequently, in basal conditions with serum alpha-1-antitrypsin levels higher than 75 mg/dl genotyping is not required. The functional repercussions of deficiency variants in young adults is slight.

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.

Misfolded growth hormone causes fragmentation of the Golgi apparatus and disrupts endoplasmic reticulum-to-Golgi traffic

In some individuals with autosomal dominant isolated growth hormone deficiency, one copy of growth hormone lacks amino acids 32-71 and is severely misfolded. We transfected COS7 cells with either wild-type human growth hormone or Δ32-71 growth hormone and investigated subcellular localization of growth hormone and other proteins. Δ32-71 growth hormone was retained in the endoplasmic reticulum, whereas wild-type hormone accumulated in the Golgi apparatus. When cells transfected with wild-type or Δ32-71 growth hormone were dually stained for growth hormone and the Golgi markers β-COP, membrin or 58K, wild-type growth hormone was colocalized with the Golgi markers, but β-COP, membrin and 58K immunoreactivity was highly dispersed or undetectable in cells expressing Δ32-71 growth hormone. Examination of α-tubulin immunostaining showed that the cytoplasmic microtubular arrangement was normal in cells expressing wild-type growth hormone, but microtubule-organizing centers were absent in nearly all cells expressing Δ32-71 growth hormone. To determine whether Δ32-71 growth hormone would alter trafficking of a plasma membrane protein, we cotransfected the cells with the thyrotropin-releasing hormone (TRH) receptor and either wild-type or Δ32-71 growth hormone. Cells expressing Δ32-71 growth hormone, unlike those expressing wild-type growth hormone, failed to show normal TRH receptor localization or binding. Expression of Δ32-71 growth hormone also disrupted the trafficking of two secretory proteins, prolactin and secreted alkaline phosphatase. Δ32-71 growth hormone only weakly elicited the unfolded protein response as indicated by induction of BiP mRNA. Pharmacological induction of the unfolded protein response partially prevented deletion mutant-induced Golgi fragmentation and partially restored normal TRH receptor trafficking. The ability of some misfolded proteins to block endoplasmic reticulum-to-Golgi traffic may explain their toxic effects on host cells and suggests possible strategies for therapeutic interventions.

A naturally occurring nonpolymerogenic mutant of alpha 1-antitrypsin characterized by prolonged retention in the endoplasmic reticulum

The classical form of alpha 1-antitrypsin (alpha 1-AT) deficiency is associated with a mutant alpha 1-ATZ molecule that polymerizes in the endoplasmic reticulum (ER) of liver cells. A subgroup of individuals homozygous for the protease inhibitor (PI) Z allele develop chronic liver injury and are predisposed to hepatocellular carcinoma. In this study we evaluated the primary structure of alpha 1-AT in a family in which three affected members had severe liver disease associated with alpha 1-AT deficiency. We discovered that one sibling was a compound heterozygote with one PI Z allele and a second allele, the PI Z + saar allele, bearing the mutation that characterizes alpha 1-ATZ as well as the mutation that characterizes alpha 1-AT Saarbrucken (alpha 1-AT saar). The mutation in PI saar introduces a premature termination codon resulting in an alpha 1-AT protein truncated for 19 amino acids at its carboxyl terminus. Studies of a second sib with severe liver disease and other living family members did not reveal the presence of the alpha 1-AT saar mutation and therefore do not substantiate a role for this mutation in the liver disease phenotype of this family. However, studies of alpha 1-AT saar and alpha 1-ATZ + saar expressed in heterologous cells show that there is prolonged intracellular retention of these mutants even though they do not have polymerogenic properties. These results therefore have important implications for further understanding the fate of mutant alpha 1-AT molecules, the mechanism of ER retention, and the pathogenesis of liver injury in alpha 1-AT deficiency.

The role of chaperone-assisted folding and quality control in inborn errors of metabolism: protein folding disorders

Molecular chaperones are present in the various compartments of the cell and assist the folding of newly synthesized proteins. Compared to wild-type proteins, missense mutant proteins are generally synthesized in a normal fashion, but may be impaired in their folding. A broad array of diseases that are due to misfolding of mutant proteins may be labelled conformational diseases: aggregation diseases, such as Alzheimer disease; diseases caused by negative dominance from misfolded structural proteins, such as hypertrophic cardiomyopathy; and disorders where the misfolded protein is degraded by intracellular proteases. Many metabolic disorders belong to this last category, where the so-called protein quality control systems, comprising chaperones and proteases, attempt to eliminate folding intermediates or misfolded proteins. On the basis of in vitro experiments with a limited number of missense mutations identified in patients with phenylalanine hydroxylase and fatty acid oxidation deficiencies, we discuss the cellular fate of missense mutant proteins. We find that the balance between folding to functional conformers, retention (holding) and degradation of folding intermediates or misfolded proteins is dependent on the nature of the mutation and on the efficiency of the quality control. For example, low temperature may promote formation of functional conformers, while elevated temperature usually promotes retention and degradation. We conclude that disorders caused by many missense mutations are complex diseases in which the mutation itself is a necessary major primary component, but that its effect may be modified by cellular conditions and possibly by genetic variations in the quality control systems. We suggest that this new knowledge about cell handling may open new avenues of understanding of the cell pathology and treatment of patients with metabolic disorders.

Alpha(1)-Antitrypsin Deficiency

Most of the care of liver disease in alpha(1)-antitrypsin (alpha(1)-AT) deficiency involves supportive management for complications of chronic liver disease including gastrointestinal bleeding, ascites, edema, encephalopathy, coagulation disturbances, spontaneous bacterial peritonitis, and hepatorenal syndrome. Some of these patients will have manifestations of cholestatic injury, including pruritus, hypercholesterolemia, and steatorrhea with fat-soluble vitamin deficiencies. The major challenge for the clinician taking care of these patients is the timing of referral for liver transplantation therapy. Timing of such referral is a relatively straightforward decision in alpha(1)-AT-deficient patients with progressive liver dysfunction. Some patients have nonprogressive or slowly progressing liver disease even after the development of cirrhosis or portal hypertension. Timing of liver transplantation in these patients should not be based simply on the presence of cirrhosis, portal hypertension or mild liver synthetic dysfunction, but rather on the basis of a subjective judgment by the hepatologist, patient, and family that manifestations of liver disease are interfering with overall life functioning.

Chronic liver disease in heterozygous alpha1-antitrypsin deficiency PiZ

BACKGROUND/AIMS

The contribution of the heterozygous state PiZ of alpha1-antitrypsin deficiency (AATD) to the pathogenesis of chronic liver disease is debated. We analyzed whether patients with this genetic defect carrying a single PiZ gene are at increased risk for developing chronic liver disease.

METHODS

1847 consecutive biopsy cases and 1030 autopsy cases of Caucasian adults were screened immunohistochemically for PiZ deposits. The zygosity status was analyzed by single-strand conformational polymorphism (SSCP) and by sequencing DNA extracted from paraffin-embedded tissue.

RESULTS

All analyzed biopsy cases were heterozygous for the PiZ mutation. The biopsy group revealed a significantly higher rate of PiZ-positive cases (3.4%) than the autopsy group (1.8%) (p=0.019). PiZ deposits ranged from scarce granules to extensive globular inclusions as in homozygous AATD of PiZ type. The extent of PiZ deposits correlated well with the inflammatory activity and stage of fibrosis. Cirrhotic livers contained globular PiZ deposits significantly more often than the biopsies with minor fibrosis. PiZ-positive biopsies from patients without concurrent liver disease (n= 26) revealed only minor fibrosis in the age group between 20 and 39 years, but significantly more severe fibrosis and significantly more PiZ deposits in the older age groups. Biopsies with concurrent liver disease (n=28) presented with significantly more severe inflammation and fibrosis, and more PiZ deposits than the cases without concurrent liver disease.

CONCLUSIONS

Patients with heterozygous AATD of PiZ type bear an increased risk for chronic liver disease. If at all, this genetic defect will become clinically relevant only in middle-aged or old adults. It rarely causes liver cirrhosis even without concurrent liver disease. It can aggravate or can be aggravated by advanced coexistent chronic liver diseases. PiZ immunohistochemistry is an easy, highly specific method to detect this metabolic defect on liver biopsies.

Retention of mutant alpha(1)-antitrypsin Z in endoplasmic reticulum is associated with an autophagic response

Although there is evidence for specific subcellular morphological alterations in response to accumulation of misfolded proteins in the endoplasmic reticulum (ER), it is not clear whether these morphological changes are stereotypical or if they depend on the specific misfolded protein retained. This issue may be particularly important for mutant secretory protein alpha(1)-antitrypsin (alpha(1)AT) Z because retention of this mutant protein in the ER can cause severe target organ injury, the chronic hepatitis/hepatocellular carcinoma associated with alpha(1)AT deficiency. Here we examined the morphological changes that occur in human fibroblasts engineered for expression and ER retention of mutant alpha(1)ATZ and in human liver from three alpha(1)AT-deficient patients. In addition to marked expansion and dilatation of ER, there was an intense autophagic response. Mutant alpha(1)ATZ molecules were detected in autophagosomes by immune electron microscopy, and intracellular degradation of alpha(1)ATZ was partially reduced by chemical inhibitors of autophagy. In contrast to mutant CFTRDeltaF508, expression of mutant alpha(1)ATZ in heterologous cells did not result in the formation of aggresomes. These results show that ER retention of mutant alpha(1)ATZ is associated with a marked autophagic response and raise the possibility that autophagy represents a mechanism by which liver of alpha(1)AT-deficient patients attempts to protect itself from injury and carcinogenesis.