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

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.

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.

The effect of amino acid substitutions at position 342 on the secretion of human alpha 1-antitrypsin from Xenopus oocytes

A glutamic acid to lysine change in the Z variant of human alpha 1-antitrypsin is associated with a failure to secrete the protein from synthesising cells. The block in export of the protein may be caused either by the loss of an acidic residue or the introduction of a basic one at this point in the polypeptide chain. Site-directed mutagenesis has been used to construct novel alpha 1-antitrypsin mutants which show that the side chain interactions from Glu-342 are not obligatory for protein export and it is rather the introduction of a basic residue at this point which produces the intracellular accumulation of the protein.

Expression of yeast invertase in oocytes from Xenopus laevis. Secretion of active enzyme differing in glycosylation

In an effort to understand factors that control glycosylation of proteins and processing of carbohydrate chains, invertase from Saccharomyces cerevisiae was expressed in a heterologous system. Microinjection of invertase-specific in vitro transcripts into oocytes from Xenopus laevis resulted in synthesis, glycosylation and secretion of enzymatically active invertase. It was found that although the number of carbohydrate chains acquired is the same as in yeast, the carbohydrate processing is different. This is consistent with the notion that the usage of a glycosylation site is determined by the protein part, whereas subsequent processing occurs in a host-dependent manner. Both, high-mannose and complex type glycans, most likely tri- and tetra-antennary structures, were synthesized in oocytes. The data obtained suggests that in this system the core chains of yeast invertase remain high-mannose type, whereas the more extensively processed polymannose chains are modified to complex oligosaccharides. In the presence of the glycosylation inhibitor, tunicamycin, and the glucosidase processing inhibitor, methyldeoxynojirimycin, secretion of invertase is significantly decreased, whereas in the presence of the mannosidase inhibitor, deoxymannojirimycin, no influence of secretion is seen. This may suggest that glycosylation of invertase is important for early secretion events. Expression of invertase lacking the leader sequence results in loss of glycosylation and secretion in oocytes. This indicates that yeast signals for secretion are functional in this higher eukaryote.

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.

Neonatal hepatitis induced by alpha 1-antitrypsin: a transgenic mouse model

Transgenic mouse lineages were established that carry the normal (M) or mutant (Z) alleles of the human alpha 1-antitrypsin (alpha 1-Pi) gene. All of the alpha 1-Pi transgenic mice expressed the human protein in the liver, cartilage, gut, kidneys, lymphoid macrophages, and thymus. The human M-allele protein was secreted normally into the serum. However, the human Z-allele protein accumulated in several cell types, but particularly in hepatocytes, and was found in serum in tenfold lower concentrations than the M-allele protein. Mice in one lineage carrying the mutant Z allele expressed high levels of human alpha 1-Pi RNA and displayed significant runting (50% of normal weight) in the neonatal period. This lineage was found to have alpha 1-Pi-induced liver pathology in the neonatal period, concomitant with the accumulation of human Z protein in diastase-resistant cytoplasmic globules that could be revealed in the Periodic acid-Schiff reaction (PAS). The phenotype of mice in the strain expressing high levels of the Z allele is remarkably similar to human neonatal hepatitis, and this strain may prove to be a useful animal model for studying this disease.

Expression of normal and mutant human pre-pro-insulins in Xenopus oocytes

Conveniently situated PstI sites were used to delete a major segment from the C-peptide coding region of a human pre-pro-insulin cDNA. The resultant mutant cDNA encoded a protein with the structure: pre-peptide B chain--Arg-Arg-Glu-Ala-Glu-Asp-Leu-Gln-Lys-Arg-A chain. Normal and mutant human pre-pro-insulin cDNAs were used as templates for the synthesis of mRNA in a reaction catalysed by T7 RNA polymerase. The mRNAs were then microinjected into Xenopus oocytes to determine the effect of the deletion on the secretion of pro-insulin. When normal pre-pro-insulin mRNA was microinjected, pre-pro-insulin was processed to pro-insulin, which in turn was secreted into the media. When the mutant pre-pro-insulin mRNA was microinjected, however, mutant pro-insulin could be detected in the oocytes but at a much lower level than the normal pro-insulin. No mutant pro-insulin could be detected in the media. The stability of the mRNAs in the oocytes was investigated by microinjecting [32P]mRNA. 24 and 48 h after microinjection, the recovery of [33P]mRNA from the oocytes was 95 and 24% and 20 and 16% of that injected, for the normal and mutant mRNAs, respectively. In a cell-free translation system supplemented with dog pancreatic microsomal membranes, the pre-peptide was cleaved from the normal pre-pro-insulin but not from the mutant pre-pro-insulin. These results suggest that C-peptide plays an important role in the segregation of pro-insulin within and transport through the cellular secretory pathway.