Porcine alpha1,3-galactosyltransferase: tissue-specific and regulated expression of splicing isoforms

Allelic variation of the porcine alpha-1,3-galactosyltransferase 1 (GGTA1) gene

The alpha-1,3-galactosyltransferase 1 enzyme (GGTA1) produces the alpha-Gal epitopes, responsible for pig-to-human hyperacute xenograft rejection. Recently, efforts have been directed at inactivating the porcine GGTA1 gene in order to reduce hyperacute rejection. As very little is known about the genetic variability of this key gene among pig breeds, we investigated the variation in its nucleotide sequence, by amplification of the entire coding region with the use of polymerase chain reaction followed by DNA sequencing. Eight commercial pig populations were analysed and 17 single nucleotide polymorphisms (SNPs) were detected: 11 in intronic regions and 6 in the 3' untranslated region (UTR). No SNPs change the encoded protein; however, 8 of these SNPs may alter the transcriptional regulation and pre-mRNA splicing of GGTA1.

Functionally important glycosyltransferase gain and loss during catarrhine primate emergence

A glycosyltransferase, alpha1,3galactosyltransferase, catalyzes the terminal step in biosynthesis of Galalpha1,3Galbeta1-4GlcNAc-R (alphaGal), an oligosaccharide cell surface epitope. This epitope or antigenically similar epitopes are widely distributed among the different forms of life. Although abundant in most mammals, alphaGal is not normally found in catarrhine primates (Old World monkeys and apes, including humans), all of which produce anti-alphaGal antibodies from infancy onward. Natural selection favoring enhanced resistance to alphaGal-positive pathogens has been the primary reason offered to account for the loss of alphaGal in catarrhines. Here, we question the primacy of this immune defense hypothesis with results that elucidate the evolutionary history of GGTA1 gene and pseudogene loci. One such locus, GGTA1P, a processed (intronless) pseudogene (PPG), is present in platyrrhines, i.e., New World monkeys, and catarrhines but not in prosimians. PPG arose in an early ancestor of anthropoids (catarrhines and platyrrhines), and GGTA1 itself became an unprocessed pseudogene in the late catarrhine stem lineage. Strong purifying selection, denoted by low nonsynonymous substitutions per nonsynonymous site/synonymous substitutions per synonymous site values, preserved GGTA1 in noncatarrhine mammals, indicating that the functional gene product is subjected to considerable physiological constraint. Thus, we propose that a pattern of alternative and/or more beneficial glycosyltransferase activity had to first evolve in the stem catarrhines before GGTA1 inactivation could occur. Enhanced defense against alphaGal-positive pathogens could then have accelerated the replacement of alphaGal-positive catarrhines by alphaGal-negative catarrhines. However, we emphasize that positively selected regulatory changes in sugar chain metabolism might well have contributed in a major way to catarrhine origins.

Mechanisms of Gal(alpha)1-3Gal(beta)1-4GlcNAc-R (alphaGal) expression on porcine valve endothelial cells

OBJECTIVE

We have previously reported that porcine valve endothelium does not express immunodetectable levels of the carbohydrate Gal(alpha)1-3Galbeta1-4GlcNAc-R (known as alphaGal), suggesting that fresh porcine valve may be immunoprivileged. In this study, we further investigated the mechanisms of alphaGal expression on porcine valve endothelial cells.

METHODS

Primary cultures of porcine valvular endothelial cells were established and compared with porcine aortic endothelial cells and human vein endothelial cells. Immunoblotting, reverse transcriptase-polymerase chain reaction, and flow cytometry were used to compare the expression of alphaGal at both the protein and messenger RNA levels.

RESULTS

Porcine valvular endothelial cells grew rapidly on a gelatin substrate. Similar to our previous in vivo results, valve endothelial cells expressed alphaGal much less intensely than did aortic endothelial cells. Porcine aortic endothelial cells expressed an isolectin B4 (isolectin B4 lectin Bandeiraea simplicifolia) immunodetectable band at 135 kd that was not visible on porcine valve endothelial cells or on human vein endothelial cells. Reverse transcriptase-polymerase chain reaction documented three transcripts of the alphaGal gene that were identically expressed on porcine valve and aortic endothelial cells. Furthermore, flow cytometry showed an almost identical surface profile between porcine aortic and valve endothelial cells, in contrast with human vein endothelial cells.

CONCLUSIONS

Cultures of primary valve endothelial cells were established and exhibited similar phenotypic patterns in vitro to those we have previously documented in vivo. RNA and flow cytometric analyses documented no difference between the RNA expression and surface protein profile for alphaGal, although whole-cell extracts demonstrated an immunodetectable band on Western blotting that was present on aortic endothelial cells but not on valve endothelial cells. These findings clarify the mechanism of expression of alpha1,3galactosyltransferase gene expression in valve endothelial cells, suggesting that delayed rejection of fresh porcine cardiac valves may occur.

Molecular basis of evolutionary loss of the alpha 1,3-galactosyltransferase gene in higher primates

Galactose-alpha1,3-galactose (alphaGal) epitopes, the synthesis of which requires the enzyme product of alpha1,3-galactosyltransferase (alpha1,3GT), are sugar chains on the cell surface of most mammalian species. Notable exceptions are higher primates including Old World monkeys, apes, and humans. The alphaGal-negative species as well as mice with deletion of the alpha1,3GT gene produce abundant anti-alphaGal antibodies. The evolutionary loss of alphaGal epitopes has been attributed to point mutations in the coding region of the gene. Because no transcripts could be found in the higher primate species with Northern blot analysis, a potential alternative explanation has been loss of upstream regulation of the gene. Here, we have demonstrated that the rhesus promoter is functional. More importantly, a variety of full-length transcripts were detected with sensitive PCR-based methods in the tissues of rhesus monkeys, orangutans, and humans. Five crucial mutations were delineated in the coding region of the human and rhesus and three in the orangutan, any one of which could be responsible for inactivation of the alpha1,3GT gene. Two of the mutations were shared by all three higher primates. These findings, which elucidate the molecular basis for the evolutionary loss of alphaGal expression, may have implications in medical research.

Regulation of alpha1,3galactosyltransferase expression in pig endothelial cells. Implications for xenotransplantation

The disaccharide galactose(alpha)1,3 galactose (the alphaGal epitope) is the major xenoantigen responsible for the hyperacute vascular rejection occurring in pig-to-primates organ transplantation. The synthesis of the alphaGal epitope is catalyzed by the enzyme alpha1,3-galactosyltransferase (alpha1,3GalT). To be able to control porcine alpha1,3GalT gene expression specifically, we have analyzed the upstream portion of the alpha1,3GalT gene, and identified the regulatory sequences. Porcine alpha1,3GalT transcripts were detected by 5' RACE analysis, and the corresponding genomic sequences were isolated from a phage library. The porcine alpha1,3GalT gene consists of at least 10 different exons, four of which contain 5' untranslated sequence. Four distinct promoters, termed A-D, drive alpha1,3GalT gene transcription in porcine cells. A combination of alternative promoter usage and alternative splicing produces a series of transcripts that differ in their 5' portion, but encode the same protein. Promoters A-C have been isolated, and functionally characterized using luciferase reporter gene assays in transfected porcine endothelial cells (PEC-A). Promoter preference in porcine endothelial cells was estimated on the basis of relative transcript levels as determined by real-time quantitative PCR. More than 90% of the alpha1,3GalT transcripts in PEC-A cells originate from promoter B, which has characteristics of a housekeeping gene promoter. While promoter preference remains unchanged, alpha1,3GalT mRNA levels increase by 50% in 12 h upon tumour necrosis factor alpha-activation of PEC-A cells. However, the magnitude of this change induced by inflammatory conditions could be insufficient to affect cell surface alpha1,3-galactosylation.

Isolation of the regulatory regions and genomic organization of the porcine alpha1,3-galactosyltransferase gene

BACKGROUND

Alpha1,3-galactosyltransferase (alpha1,3GT) is an enzyme that produces carbohydrate chains termed alphaGal epitopes found in most mammals, although some species of higher primates, including human, are notable exceptions. The evolutionary origin of the lost alpha1,3GT enzyme activity is not yet known, although it has been suggested that the promoter activity of this gene in the ancestors of higher primates was inactivated.

METHODS

We used 5'-or 3'-RACE, GenomeWalking, reverse transcriptase polymerase chain reaction (RT-PCR) and dual Luciferase reporter assay for identification of the full-length cDNA, which includes the transcription initiation site and the promoter region of porcine alpha1,3GT gene.

RESULTS

The region around exon 1 is guanine and cytosine (GC)-rich (about 70%), comprising a CpG island spanning more than 1.5 kbp. The 5'-flanking region of exon 1 contains multiple transcription factor consensus motifs, including GC-box, SP1, AP2, and GATA-box sites, in the absence of TATA or CAAT-box sequences. The entire gene consists of three 5' noncoding and six coding region exons spanning more than 52 kbp. Detailed analysis of alpha1,3GT transcripts revealed two major alternative splicing patterns in the 5'-untranslated region (5'-UTR) and evidence for minor splicing activity that occurs in a tissue-specific manner. Interspecies comparison of 5'-UTR shows minimal homology between porcine and murine sequences except for exon 2, which suggests that the regulatory regions differ among species.

CONCLUSIONS

These observations have important implications for experiments involving genetic manipulation of the alpha1,3GT gene in transgenic animals in terms of promoter utilization, and particularly in genetically engineering cells for the animal cloning technology by nuclear transfer.

Intracellular expression in pig cells of anti-alpha1,3galactosyltransferase single-chain FV antibodies reduces Gal alpha1,3Gal expression and inhibits cytotoxicity mediated by anti-Gal xenoantibodies

BACKGROUND

The carbohydrate structure Gal alpha1,3Gal expressed on pig cells is the major antigen recognized by xenoreactive natural antibodies in the higher primates. In xenotransplantation, natural antibodies binding to that structure initiate hyperacute rejection, and the anti-Gal alpha1,3Gal antibodies that are elicited probably take part in later phases of vascularized graft rejection. This epitope also appears to be involved in innate cellular responses. Inactivation of alpha1,3 galactosyltransferase in transgenic pigs would certainly lead to the success of xenotransplantation, but gene knockout in pigs is not feasible yet.

METHODS

As a novel strategy to inhibit alpha1,3 galactosylation, we generated recombinant single-chain Fv (ScFv) antibodies directed against pig alpha1,3-galactosyltransferase and evaluated the effect of their intracellular expression on enzyme activity and Gal alpha1,3Gal expression.

RESULTS

After in vitro transfection in pig cells, the scFv antibody anti-pig alpha1,3-galactosyltransferase reduced the amount or function of enzyme by up to 70% as evidenced by immunofluorescence and measurement of cell-associated activity. Consequently, Gal alpha1,3Gal on cell membranes was reduced to the same extent. This led to a profound (more than 90%) reduction in the cytotoxicity involving anti-Gal antibodies and complement.

CONCLUSION

Although not sufficient to knock out the overall human anti-pig natural xenoreactivity, intracellular expression of the scFv antibody anti-alpha1,3-galactosyltransferase in pig cells significantly decreases the amount of Gal alpha1,3Gal and could be important to protect cells from elicited antibodies as well as from innate effectors.