In vitro synthesis of M and Z forms of human alpha 1-antitrypsin

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.

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.

Molecular basis of alpha 1-antitrypsin deficiency and emphysema associated with the alpha 1-antitrypsin Mmineral springs allele

The Mmineral springs alpha 1-antitrypsin (alpha 1AT) allele, causing alpha 1AT deficiency and emphysema, is unique among the alpha 1AT-deficiency alleles in that it was observed in a black family, whereas most mutations causing alpha 1AT deficiency are confined to Caucasian populations of European descent. Immobilized pH gradient analysis of serum demonstrated that alpha 1AT Mmineral springs migrated cathodal to the normal M2 allele. Evaluation of Mmineral springs alpha 1AT as an inhibitor of neutrophil elastase, its natural substrate, demonstrated markedly lower than normal function. Characterization of the alpha 1AT Mmineral springs gene demonstrated that it differed from the common normal M1(Ala213) allele by a single-base substitution causing the amino acid substitution Gly-67 (GGG)----Glu-67 (GAG). Capitalizing on the fact that this mutation creates a polymorphism for the restriction endonuclease AvaII, family analysis demonstrated that the Mmineral springs alpha 1AT allele was transmitted in an autosomal-codominant fashion. Evaluation of genomic DNA showed that the index case was homozygous for the alpha 1AT Mmineral springs allele. Cytoplasmic blot analysis of blood monocytes of the Mmineral springs homozygote demonstrated levels of alpha 1AT mRNA transcripts comparable to those in cells of a normal M1 (Val213) homozygote control. Evaluation of in vitro translation of Mmineral springs alpha 1AT mRNA transcripts demonstrated a normal capacity to direct the translation of alpha 1AT. Evaluation of secretion of alpha 1AT by the blood monocytes by pulse-chase labeling with [35S]methionine, however, demonstrated less secretion by the Mmineral springs cells than normal cells. To characterize the posttranslational events causing the alpha 1AT-secretory defect associated with the alpha 1AT Mmineral springs gene, retroviral gene transfer was used to establish polyclonal populations of murine fibroblasts containing either a normal human M1 alpha 1AT cDNA or an Mmineral springs alpha 1AT cDNA and expressing comparable levels of human alpha 1AT mRNA transcripts. Pulse-chase labeling of these cells with [35S]methionine demonstrated less secretion of human alpha 1AT from the Mmineral springs cells than from the M1 cells, and evaluation of cell lysates also demonstrated lower amounts of intracellular human alpha 1AT in the Mmineral springs cells than in the normal M1 control cells. Thus, the Gly-67 --> Glu mutation that characterizes Mmineral springs causes reduced alpha 1AT secretion on the basis of aberrant posttranslational alpha 1AT biosynthesis by a mechanism distinct from that associated with the alpha 1AT Z allele, whereby intracellular aggregation of the mutant protein is etiologic of the alpha 1AT-secretory defect. Furthermore, for the alpha 1AT protein that does reach the circulation, this mutation markedly affects the ability of the molecule to inhibit neutrophil elastase; i.e., the alpha 1AT Mmineral springs allele predisposes to emphysema on the basis of serum apha 1AT deficiency coupled with alpha AT dysfunction.

Molecular basis of alpha 1-antitrypsin deficiency and emphysema associated with the alpha 1-antitrypsin Mmineral springs allele

The Mmineral springs alpha 1-antitrypsin (alpha 1AT) allele, causing alpha 1AT deficiency and emphysema, is unique among the alpha 1AT-deficiency alleles in that it was observed in a black family, whereas most mutations causing alpha 1AT deficiency are confined to Caucasian populations of European descent. Immobilized pH gradient analysis of serum demonstrated that alpha 1AT Mmineral springs migrated cathodal to the normal M2 allele. Evaluation of Mmineral springs alpha 1AT as an inhibitor of neutrophil elastase, its natural substrate, demonstrated markedly lower than normal function. Characterization of the alpha 1AT Mmineral springs gene demonstrated that it differed from the common normal M1(Ala213) allele by a single-base substitution causing the amino acid substitution Gly-67 (GGG)----Glu-67 (GAG). Capitalizing on the fact that this mutation creates a polymorphism for the restriction endonuclease AvaII, family analysis demonstrated that the Mmineral springs alpha 1AT allele was transmitted in an autosomal-codominant fashion. Evaluation of genomic DNA showed that the index case was homozygous for the alpha 1AT Mmineral springs allele. Cytoplasmic blot analysis of blood monocytes of the Mmineral springs homozygote demonstrated levels of alpha 1AT mRNA transcripts comparable to those in cells of a normal M1 (Val213) homozygote control. Evaluation of in vitro translation of Mmineral springs alpha 1AT mRNA transcripts demonstrated a normal capacity to direct the translation of alpha 1AT. Evaluation of secretion of alpha 1AT by the blood monocytes by pulse-chase labeling with [35S]methionine, however, demonstrated less secretion by the Mmineral springs cells than normal cells. To characterize the posttranslational events causing the alpha 1AT-secretory defect associated with the alpha 1AT Mmineral springs gene, retroviral gene transfer was used to establish polyclonal populations of murine fibroblasts containing either a normal human M1 alpha 1AT cDNA or an Mmineral springs alpha 1AT cDNA and expressing comparable levels of human alpha 1AT mRNA transcripts. Pulse-chase labeling of these cells with [35S]methionine demonstrated less secretion of human alpha 1AT from the Mmineral springs cells than from the M1 cells, and evaluation of cell lysates also demonstrated lower amounts of intracellular human alpha 1AT in the Mmineral springs cells than in the normal M1 control cells. Thus, the Gly-67 --> Glu mutation that characterizes Mmineral springs causes reduced alpha 1AT secretion on the basis of aberrant posttranslational alpha 1AT biosynthesis by a mechanism distinct from that associated with the alpha 1AT Z allele, whereby intracellular aggregation of the mutant protein is etiologic of the alpha 1AT-secretory defect. Furthermore, for the alpha 1AT protein that does reach the circulation, this mutation markedly affects the ability of the molecule to inhibit neutrophil elastase; i.e., the alpha 1AT Mmineral springs allele predisposes to emphysema on the basis of serum apha 1AT deficiency coupled with alpha AT dysfunction.

Repair of the secretion defect in the Z form of alpha 1-antitrypsin by addition of a second mutation

Homozygous inheritance of the Z-type mutant form of the alpha 1-antitrypsin (alpha 1AT) gene results in the most common form of alpha 1AT deficiency, a human hereditary disease associated with a high risk for the development of emphysema and an increased incidence of neonatal hepatitis. The alpha 1AT-synthesizing cells of individuals with the Z gene have normal alpha 1AT messenger RNA levels, but alpha 1AT secretion is markedly reduced secondary to accumulation of newly synthesized alpha 1AT in the rough endoplasmic reticulum. Crystallographic analysis of alpha 1AT predicts that in normal alpha 1AT, a negatively charged Glu342 is adjacent to positively charged Lys290. Thus the Glu342----Lys342 Z mutation caused the loss of a normal salt bridge, resulting in the intracellular aggregation of the Z molecule. The prediction was made that a second mutation in the alpha 1AT genet that changed the positively charged Lys290 to a negatively charged Glu290 would correct the secretion defect. When the second mutation was added to the Z-type complementary DNA, the resulting gene directed the synthesis and secretion of amounts of alpha 1AT similar to that directed by the normal alpha 1AT complementary DNA in an in vitro eukaryotic expression system. This suggests the possibility that a human hereditary disease can be corrected by inserting an additional mutation in the same gene.

Molecular basis of alpha-1-antitrypsin deficiency

Alpha-1-antitrypsin (A1AT) deficiency is an autosomal hereditary disorder associated with a major reduction in serum A1AT levels. Clinically, A1AT deficiency is associated with emphysema in adults and, less commonly, liver disease in neonates. A1AT is a 52-kDa, 394-amino acid, single-chain glycoprotein normally present in serum at 150 to 350 mg/dl. The A1AT gene, composed of seven exons dispersed over 12 kb of chromosomal segment 14q31-32.3, is expressed in hepatocytes and mononuclear phagocytes. The A1AT protein, a member of the class of protease inhibitor proteins known as serpins (serine protease inhibitors), is a globular molecule composed of nine alpha-helices and three beta-pleated sheets. The major function of A1AT is to inhibit neutrophil elastase; A1AT does so through an active site centered around Met358 contained within an external stressed loop on the surface of the molecule. A1AT is a highly pleomorphic protein with greater than 75 variants determined at the protein and/or gene level. These variants can be categorized into four groups according to their serum A1AT level and function: normal, deficient, dysfunctional, and absent. There are two important salt bridges within the A1AT molecule (Glu342-Lys290; Glu263-Lys387); a mutation in the A1AT gene causing disruption of either salt bridge causes distinct molecular pathology resulting in reduced serum A1AT levels. Clinically relevant variants can be distinguished by a combination of isoelectric focusing of serum, restriction fragment length analysis of genomic DNA, oligonucleotide probes, and direct sequencing of the variant A1AT genes.

Hepatic alpha 1-antitrypsin mRNA content in cirrhosis with normal and abnormal protease inhibitor phenotypes

We quantitated alpha 1-antitrypsin mRNA in normal, alpha 1-antitrypsin-deficient cirrhotic and biliary cirrhotic livers using two-dimensional electrophoretograms of [35S]methionine-labeled translational products of total hepatic RNA and RNA/DNA hybridization. alpha 1-Antitrypsin precursor product was identified by immunoprecipitation. The relative abundance of alpha 1-antitrypsin product from normal (0.989 +/- 0.197), cirrhotic (0.956 +/- 0.062) and alpha 1-antitrypsin deficient (0.818 +/- 0.12) livers was not significantly different. Although (RNA/DNA) was decreased in the PiZZ cirrhotic livers compared to normal (0.56 +/- 0.045 vs. 0.95 +/- 0.225), it equaled that found in the PiM cirrhotic livers (0.56 +/- 0.055). The concentration of alpha 1-antitrypsin mRNA [relative abundance X (RNA/DNA)], while decreased in PiZZ compared to normal liver, is thus no different in PiZZ cirrhotics than in PiM cirrhotics. We confirmed this observation by quantitation of the alpha 1-antitrypsin mRNA using an alpha 1-antitrypsin genomic probe. By RNA/DNA hybridization, alpha 1-antitrypsin mRNA was equal in PiM cirrhotic and PiZZ cirrhotic (38.48 +/- 4.5 vs. 31.93 +/- 2.1), but significantly decreased from noncirrhotic PiM liver (58.36 +/- 12.7). We conclude that alpha 1-antitrypsin mRNA is decreased in cirrhosis of any etiology, and this decrease appears to represent a general response of the liver to injury. Since the decreased alpha 1-antitrypsin mRNA in PiM cirrhotics is associated with normal serum alpha 1-antitrypsin levels, it is unlikely that the decreased alpha 1-antitrypsin mRNA in PiZZ cirrhotics accounts for their decreased serum levels.

Expression of the alpha-1-antitrypsin gene in mononuclear phagocytes of normal and alpha-1-antitrypsin-deficient individuals

To evaluate the contribution of mononuclear phagocytes, and particularly alveolar macrophages, to alpha-1-antitrypsin (alpha 1AT) production in normal and alpha 1AT-deficient individuals, Northern analysis with a human alpha 1AT complementary DNA was used to demonstrate that alpha 1AT messenger RNA (mRNA) can be detected in liver, blood monocytes, and alveolar macrophages. Quantification of alpha 1AT mRNA expression demonstrated that: (a) type PiMM monocytes and alveolar macrophages expressed, respectively, 200-fold and 70-fold less alpha 1AT mRNA per cell than the liver; (b) the level of expression of the alpha 1AT gene was increased during the in vitro maturation of blood monocytes; and (c) blood monocyte and alveolar macrophage levels of expression of the alpha 1AT gene were the same in PiMM and PiZZ individuals. However, the amount of newly synthesized alpha 1AT secreted by ZZ alveolar macrophages was 10 times lower than that secreted by MM alveolar macrophages. Thus, mononuclear phagocytes of PiZZ individuals express a secretory defect in alpha 1AT in a fashion similar to hepatocytes. Not only do mononuclear phagocytes provide a readily accessible cell to evaluate the regulation of alpha 1AT gene expression, but these cells may contribute to the levels of alpha 1AT present in the lower respiratory tract in the normal and ZZ states.

Immunological and chemical properties of mouse alpha 1-protease inhibitors

We previously described the isolation and purification of two similar alpha 1-protease inhibitors from mouse plasma termed alpha 1-PI(E) and alpha 1-PI(T) because of their respective affinities for elastase and trypsin. Some of the biochemical and immunological properties of these proteins are reported. Both are acidic glycoproteins with pI's of 4.1-4.2. The plasma half-life of each inhibitor, determined after administration of the 125I-protein, is approximately 4 h both in normal mice and in mice after induction of the acute phase reaction. The two proteins have almost identical amino acid compositions and similar CNBr peptide maps. Tryptic maps, however, are considerably different. Reverse-phase chromatography separated alpha 1-PI(E) into three distinct isoforms, each eluting with approximately 60% acetonitrile. Under these conditions alpha 1-PI(T) shows a single peak, clearly different from those of alpha 1-PI(E). The three alpha 1-PI(E) isoforms have the same molecular weights on sodium dodecyl sulfate-gel electrophoresis and the same tripeptide sequence at their N-terminus, and appear to be immunologically identical. Polyclonal, monospecific antibodies to each native inhibitor, prepared in rabbits, showed no cross-reactivity when tested by functional assay or crossed immunoelectrophoresis. Interestingly, each antibody recognized epitopes on the C-terminal portion of its respective antigen. These studies confirm that alpha 1-PI(E) and alpha 1-PI(T), although highly similar, are products of different genes. Like human alpha 1-PI, the two mouse inhibitors are partially inactivated by mild oxidation with chloramine-T, losing all elastase inhibitor and lesser amounts of antichymotryptic and antitryptic activity. However, unlike the human protein, neither alpha 1-PI(E) nor alpha 1-PI(T) was found to have a methionine residue at its P1 site.

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.

Plasma protein and liver mRNA levels of two closely related murine alpha 1-protease inhibitors during the acute phase reaction

Plasma levels of alpha 1-PI(T) and alpha 1-PI(E), two closely related murine alpha 1-protease inhibitors, having affinities for trypsin and elastase, respectively, were compared to changes in specific liver mRNA levels after induction of the acute-phase reaction by subcutaneous injection of turpentine. In earlier, qualitative experiments an increase in plasma levels of alpha 1-PI(E), but not alpha 1-PI(T), during the acute-phase reaction had been shown. It is now shown that stimulation of plasma alpha 1-PI(E) levels reaches a maximum of 35-50% above baseline 12 h after induction of the acute-phase response using either a functional or immunological assay to measure protease inhibitor activity. Consistent with earlier observations, little or no change in plasma levels of alpha 1-PI(T) is seen. Determination of mRNA levels in the mouse liver specific for alpha 1-PI(E) and alpha 1-PI(T) was accomplished using a cell-free translation system followed by immunoprecipitation of the 35S-labeled protease inhibitors. The apparent Mr's of alpha 1-PI(E) and alpha 1-PI(T) synthesized in vitro are 42K and 46K, respectively. Apparent Mr's of the native proteins in plasma are 55K and 65K. Unexpectedly, mRNA levels for both alpha 1-PI(E) and alpha 1-PI(T) were found to increase after induction of the acute-phase reaction. Maximal stimulation for both mRNAs was approximately 300% and occurred 9 h after turpentine administration. Under these conditions, levels of translatable albumin mRNA in the mouse liver decreased to 40% of baseline in 6-9 h.

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

Microinjection of human liver mRNA from a patient homozygous for alpha 1-antitrypsin deficiency (PiZZ) into Xenopus oocytes led to a 2--10-fold increase in lysosomal activity. Stimulation of lysosomal activity was not observed when mRNA from a normal human liver (alpha 1-antitrypsin PiMM), or water was injected into the oocyte. This lysosomal activity was oocyte derived and was not due to translation products of the human liver mRNA. Thus a protein that accumulates intracellularly in the secretory pathway is capable of stimulating lysosomal activity.

Structural and functional characterization of the abnormal Z alpha 1-antitrypsin isolated from human liver

alpha 1-Antitrypsin has been isolated from liver inclusion bodies of a subject with a homozygous Z deficiency. The inhibitor was recovered in a fully active form by extraction in high salt at either pH 2.0 or pH 8.0. Carbohydrate analysis indicated a protein in the 'high mannose' form, and this was collaborated by its sensitivity to endo-beta N-glucosaminidase. These data suggest that the abnormal alpha 1-antitrypsin is blocked in the secretory pathway prior to its entrance into the Golgi, and that this blockage is not due to a gross misfolding of the polypeptide.

Xenopus oocytes can synthesise but do not secrete the Z variant of human alpha 1-antitrypsin

Human liver mRNA was prepared from a patient homozygous for alpha 1-antitrypsin deficiency (PiZZ) and from a normal subject (PiMM). Both liver RNAs were microinjected into Xenopus oocytes and alpha 1-antitrypsin identified by immunoprecipitation. The normal M variant of alpha 1-antitrypsin is synthesised and secreted by Xenopus oocytes, the abnormal Z protein is not secreted and an intracellular form accumulates in the oocytes. In the presence of tunicamycin an unglycosylated form of M alpha 1-antitrypsin appears in the incubation medium but no corresponding unglycosylated version of the Z protein is secreted.

Translation and processing of normal (PiMM) and abnormal (PiZZ) human alpha 1-antitrypsin

Human liver mRNA isolated from subjects phenotyped as homozygous PiMM or PiZZ alpha 1-antitrypsin, was translated in a reticulocyte cell-free system, and alpha 1-antitrypsin identified by immunoprecipitation. In the presence of dog pancreas membranes the translated alpha 1-antitrypsin appeared as a larger product. Treatment with endo-beta-N-glucosaminidase yielded a protein smaller than the reticulocyte translated product, presumably due to removal of the N-terminal signal sequence by membranes and sugar residues by endo-beta-N-glucosaminidase. Quantitation of alpha 1-antitrypsin translated from PiMM and PiZZ livers suggests that both mRNA species were present at the same cellular concentration, and that processing to the core glycosylation stage proceeded at identical rates.