Organization and Biology of the Porcine Serum Amyloid A (SAA) Gene Cluster: Isoform Specific Responses to Bacterial Infection

Helle G Olsen,Ole L Nielsen,Peter M H Heegaard,Tine Iburg,Henrik E Jensen,Kerstin Skovgaard,Páll S Leifsson,Prasun K Datta

doi:10.1371/journal.pone.0076695

Helle G Olsen, Ole L Nielsen + Show 6 more

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https://doi.org/10.1371/journal.pone.0076695

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Abstract

Serum amyloid A (SAA) is a prominent acute phase protein. Although its biological functions are debated, the wide species distribution of highly homologous SAA proteins and their uniform behavior in response to injury or inflammation in itself suggests a significant role for this protein. The pig is increasingly being used as a model for the study of inflammatory reactions, yet only little is known about how specific SAA genes are regulated in the pig during acute phase responses and other responses induced by pro-inflammatory host mediators. We designed SAA gene specific primers and quantified the gene expression of porcine SAA1, SAA2, SAA3, and SAA4 by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) in liver, spleen, and lung tissue from pigs experimentally infected with the Gram-negative swine specific bacterium Actinobacillus pleuropneumoniae, as well as from pigs experimentally infected with the Gram-positive bacterium Staphylococcus aureus. Our results show that: 1) SAA1 may be a pseudogene in pigs; 2) we were able to detect two previously uncharacterized SAA transcripts, namely SAA2 and SAA4, of which the SAA2 transcript is primarily induced in the liver during acute infection and presumably contributes to circulating SAA in pigs; 3) Porcine SAA3 transcription is induced both hepatically and extrahepatically during acute infection, and may be correlated to local organ affection; 4) Hepatic transcription of SAA4 is markedly induced in pigs infected with A. pleuropneumoniae, but only weakly in pigs infected with S. aureus. These results for the first time establish the infection response patterns of the four porcine SAA genes which will be of importance for the use of the pig as a model for human inflammatory responses, e.g. within sepsis, cancer, and obesity research.

Highlights

Serum amyloid A (SAA) is a small apolipoprotein and a major positive acute phase protein in most vertebrate species [1]
In addition to the previously characterized porcine SAA3 transcript [32], we identified two previously uncharacterized porcine SAA transcripts, namely SAA2 and SAA4, while no transcript corresponding to the porcine SAA1 gene was detected
The PCR products obtained from the three different assays used to target the putative SAA1 transcript (Table 1) turned out to be 100% identical to SAA2 DNA sequence, while only 97% identical to the SAA1 mRNA model reference sequences (RefSeqs)

Summary

Introduction

Serum amyloid A (SAA) is a small apolipoprotein and a major positive acute phase protein in most vertebrate species [1]. The biological function of SAA is debated [4,5], the magnitude of hepatic transcription and synthesis of SAA during the early phase of acute inflammation [6,7], and its high degree of evolutionary conservation across species [8], imply the function of SAA to be remarkably important. Human SAA1, SAA2, and SAA3P genes are more related to each other than each gene is related to its murine orthologous counterpart, and vice versa [8], indicating subsequent species-specific homogenization of the SAA genes by concerted evolution [8,11]. All genes share the same four-exon three-intron structure [16]

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