Human Z alpha 1-antitrypsin accumulates intracellularly and stimulates lysosomal activity when synthesised in the Xenopus oocyte

Molecular Mechanisms Responsible for Increased Vulnerability of the Ageing Oocyte to Oxidative Damage

In their midthirties, women experience a decline in fertility, coupled to a pronounced increase in the risk of aneuploidy, miscarriage, and birth defects. Although the aetiology of such pathologies are complex, a causative relationship between the age-related decline in oocyte quality and oxidative stress (OS) is now well established. What remains less certain are the molecular mechanisms governing the increased vulnerability of the aged oocyte to oxidative damage. In this review, we explore the reduced capacity of the ageing oocyte to mitigate macromolecular damage arising from oxidative insults and highlight the dramatic consequences for oocyte quality and female fertility. Indeed, while oocytes are typically endowed with a comprehensive suite of molecular mechanisms to moderate oxidative damage and thus ensure the fidelity of the germline, there is increasing recognition that the efficacy of such protective mechanisms undergoes an age-related decline. For instance, impaired reactive oxygen species metabolism, decreased DNA repair, reduced sensitivity of the spindle assembly checkpoint, and decreased capacity for protein repair and degradation collectively render the aged oocyte acutely vulnerable to OS and limits their capacity to recover from exposure to such insults. We also highlight the inadequacies of our current armoury of assisted reproductive technologies to combat age-related female infertility, emphasising the need for further research into mechanisms underpinning the functional deterioration of the ageing oocyte.

The endosomal protein-sorting receptor sortilin has a role in trafficking α-1 antitrypsin

Up to 1 in 3000 individuals in the United States have α-1 antitrypsin deficiency, and the most common cause of this disease is homozygosity for the antitrypsin-Z variant (ATZ). ATZ is inefficiently secreted, resulting in protein deficiency in the lungs and toxic polymer accumulation in the liver. However, only a subset of patients suffer from liver disease, suggesting that genetic factors predispose individuals to liver disease. To identify candidate factors, we developed a yeast ATZ expression system that recapitulates key features of the disease-causing protein. We then adapted this system to screen the yeast deletion mutant collection to identify conserved genes that affect ATZ secretion and thus may modify the risk for developing liver disease. The results of the screen and associated assays indicate that ATZ is degraded in the vacuole after being routed from the Golgi. In fact, one of the strongest hits from our screen was Vps10, which can serve as a receptor for the delivery of aberrant proteins to the vacuole. Because genome-wide association studies implicate the human Vps10 homolog, sortilin, in cardiovascular disease, and because hepatic cell lines that stably express wild-type or mutant sortilin were recently established, we examined whether ATZ levels and secretion are affected by sortilin. As hypothesized, sortilin function impacts the levels of secreted ATZ in mammalian cells. This study represents the first genome-wide screen for factors that modulate ATZ secretion and has led to the identification of a gene that may modify disease severity or presentation in individuals with ATZ-associated liver disease.

Review: alpha 1-antitrypsin deficiency associated liver disease

alpha 1-Antitrypsin (alpha 1-AT) deficiency is the most common genetic cause of liver disease in children and genetic disease for which children undergo liver transplantation. It also causes cirrhosis and hepatocellular carcinoma in adults. Studies by Sveger in Sweden have shown that only a subgroup of the population with homozygous PiZZ alpha 1-AT deficiency develop clinically significant liver injury. Other studies have shown that the mutant alpha 1-AT Z molecule undergoes polymerization in the endoplasmic reticulum and that a subpopulation of alpha 1-AT-deficient individuals may be susceptible to liver injury because they also have a trait that reduces the efficiency by which the mutant alpha 1-AT Z molecule is degraded in the endoplasmic reticulum.

Mutations which impede loop/sheet polymerization enhance the secretion of human alpha 1-antitrypsin deficiency variants

alpha 1-Antitrypsin plasma deficiency variants which form hepatic inclusion bodies within the endoplasmic pathway include the common Z variant (Glu342-->Lys) and the rarer alpha 1-antitrypsin Siiyama (Ser53-->Phe). It has been proposed that retention of both abnormal proteins is accompanied by a common mechanism of loop-sheet polymerization with the insertion of the reactive center loop of one molecule into a beta-pleated sheet of another. We have compared the biosynthesis, glycosylation, and secretion of normal, Z and Siiyama variants of alpha 1-antitrypsin using Xenopus oocytes. Siiyama and Z alpha 1-antitrypsin both duplicated the secretory defect seen in hepatocytes that results in decreased plasma alpha 1-antitrypsin levels. Digestion with endoglycosidase H localized both variants to a pre-Golgi compartment. The mutation Phe51-->Leu abolished completely the intracellular blockage of Siiyama alpha 1-antitrypsin and reduced significantly the retention of Z alpha 1-antitrypsin. The secretory properties of M and Z alpha 1-antitrypsin variants containing amino acid substitutions designed to decrease loop mobility and sheet insertion were investigated. A reduction in intracellular levels of Z alpha 1-antitrypsin was achieved with the replacement of P11/12 alanines by valines. Thus a decrease in Z and Siiyama alpha 1-antitrypsin retention was observed with mutations which either closed the A sheet or decreased loop mobility at the loop hinge region.

Conformational changes in serpins and the mechanism of alpha 1-antitrypsin deficiency

alpha 1-Antitrypsin is a member of the serine proteinase inhibitor, serpin, family of protease inhibitors, which have their reactive centers situated on a mobile peptide loop. This reactive loop can adopt varied conformations and perturbations of molecular structure to allow the pathological linking of the loop of one molecule to a beta-pleated sheet of another. This linkage has been shown to be the cause of the polymerization and aggregation within the hepatocyte of the common Z mutant of antitrypsin. The occurrence of loop-sheet polymerization has been confirmed with other deficiency variants of antitrypsin that accumulate in the liver (Mmalton, Siiyama) and also shown to occur in pathological mutants of C1-inhibitor and antithrombin. Deductive evidence indicates that the loop is inserted into the A-sheet of the next molecule, but recent structural findings raise the possibility of insertion into the C-sheet. This detail of loop-sheet polymerization is important for the design of strategies to interfere with insertion and hence lesson the accumulation of Z antitrypsin that is responsible for associated liver damage.

The molecular genetics of alpha 1 antitrypsin deficiency

The human serum protein alpha 1-antitrypsin is the major source of antiprotease activity found in the blood. The protein is synthesised primarily by liver cells but, to a lesser extent, by at least one other cell type. Expression of the gene has provided a paradigm for studies on transcriptional regulation in liver and of tissue-specific promoter activity. The pleiomorphic nature of the gene has given rise to a variety of alpha 1-antitrypsin variants some of which are clinically important. These abnormal variants may be poorly synthesised, rapidly degraded or inefficiently secreted; studies on the molecular mechanisms which underly these events are providing interesting insights into the general processes of protein transport and intracellular protein degradation.

Alpha 1-antitrypsin deficiency and liver disease

Alpha 1-antitrypsin (alpha 1AT) deficiency, one of the most common lethal hereditary disorders among Caucasians, is associated with emphysema in adults, while in children it is associated with liver disease. Produced in the liver and released into the plasma, alpha 1AT serves as the body's major inhibitor of neutrophil elastase, a powerful proteolytic enzyme capable of degrading extracellular structural proteins. The pathogenesis of the liver disease associated with alpha 1AT deficiency is not as well understood, but is clearly linked to specific mutations in coding exons of the alpha 1AT gene, and the resulting accumulation of alpha 1AT within hepatocytes. At present, therapy for the liver disease associated with alpha 1AT deficiency is symptomatic, with liver transplantation as a last resort. New strategies are being developed to suppress the accumulation of alpha 1AT by transferring the normal gene into the liver.

Biosynthesis and clearance of prothrombin in warfarin-treated rats

The steady-state concentration of abnormal plasma prothrombin in warfarin-treated rats (10 mg/kg) was found to be approx. 6% of the plasma prothrombin level in normal rats. The clearance of abnormal plasma prothrombin in warfarin-treated rats was studied using either cycloheximide, to inhibit the synthesis, or vitamin K, to block the appearance of abnormal prothrombin in plasma. The clearance of abnormal plasma prothrombin corresponded to a half-life of approx. 6 h, which is similar to the half-life of normal plasma prothrombin. The de novo synthesis of prothrombin in warfarin-treated and normal rats was compared by measuring the incorporation of [3H]leucine into plasma prothrombin 90 min after an intravenous injection of the isotope. In warfarin-treated rats, accumulated prothrombin precursor was carboxylated and transported into circulation by injecting vitamin K 30 min after isotope administration. On comparing the incorporation of [3H]leucine into plasma prothrombin in warfarin-treated and normal rats, no significant difference in the de novo synthesis was detected. Our results suggest that the secretion of prothrombin in warfarin-treated rats is decreased to 6% of the normal rate. As the de novo synthesis is not affected by warfarin treatment, more than 90% of the newly synthesized prothrombin appears to be degraded intracellularly.

The amino acid substitutions of human alpha 1-antitrypsin M3, X and Z

alpha 1-Antitrypsin has been isolated from individuals with inherited genetic variants M3, X and Z. A fragmentation and peptide mapping system is described which together with amino acid and sequence analyses revealed the substitutions in M3 at 376 of Glu to Asp, in X at 204 of Glu to Lys and in the physiologically innocent Z a mutation at 213 of Val to Ala. The latter represents a second amino acid substitution in the Z protein.

Disruption of the Lys-290--Glu-342 salt bridge in human alpha 1-antitrypsin does not prevent its synthesis and secretion

The object of this work was to test the hypothesis that failure to secrete the Z variant of human alpha 1-antitrypsin is related to the loss of a particular structural feature, the Lys-290 to Glu-342 salt bridge. Oligonucleotide-directed mutagenesis was used to disrupt the salt bridge by substituting a glutamic acid for lysine at residue 290. RNA transcripts prepared from this mutant DNA and from the normal cDNA were both able to direct the synthesis of protein in a cell-free system and after injection into Xenopus oocytes. Furthermore, the constructed mutant alpha 1-antitrypsin was secreted as readily as the normal inhibitor.

Alpha 1-antitrypsin deficiency--a defect in secretion

A naturally occurring point mutation in the human alpha 1-antitrypsin gene leads to the synthesis of a variant of the protein which is poorly secreted from hepatocytes. This Z mutation codes for a glutamic acid to lysine substitution at residue 342 in the polypeptide chain. The mutant protein is correctly translocated into the lumen of the endoplasmic reticulum and core glycosylated but inefficiently transported beyond the ER compartment. Experiments using Xenopus oocytes as a surrogate secretory cell show that abberant secretion of the variant is not confined to hepatocytes and glycosylation of the polypeptide is not obligatory for the block in secretion. Site-directed mutagenesis can be used to examine the effect of natural mutations on protein structure and the relationship between structure and intracellular transport.

Human alpha 1-antitrypsin expression in Xenopus oocytes. Secretion of the normal (PiM) and abnormal (PiZ) forms

Injection of equivalent amounts of normal (PiMM) or abnormal (PiZZ) alpha 1-antitrypsin mRNA into Xenopus oocytes resulted in secretion of both the normal and abnormal alpha 1-antitrypsin. A much lower proportion of the abnormal protein was secreted, and the Z alpha 1-antitrypsin that was not secreted accumulated within the cell in a high-mannose form. The time taken for secretion of the normal and abnormal proteins was identical. Both the secreted and intracellular alpha 1-antitrypsin synthesized by oocytes were functionally active.

Alpha 1-antitrypsin and serum albumin mRNA accumulation in normal, acute phase and ZZ human liver

Alpha 1-Antitrypsin and albumin mRNA levels of 4 human livers were assessed using a newly sequenced cDNA clone of the carboxyterminal third of alpha 1-antitrypsin and a previously cloned albumin cDNA sequence. The relative concentration of alpha 1-antitrypsin mRNA was the same in poly(A)-containing RNA isolated from acute phase (MM) and alpha1-antitrypsin deficient (ZZ) individuals. In the acute phase liver relative to the normal (MM) liver, total RNA extracts showed a marked decrease in albumin mRNA concentration but no increase in alpha 1-antitrypsin mRNA. The ZZ liver showed decreased total and poly(A)-containing RNA content but the same proportion of alpha 1-antitrypsin to albumin mRNA as in the normal (MM) liver. This supports other evidence that ZZ alpha 1-antitrypsin deficiency is due to a defect in polypeptide processing (secretion) rather than a deficiency in mRNA accumulation.