Organization and biology of the porcine serum amyloid A (SAA) gene cluster: isoform specific responses to bacterial infection

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.

Figure 1. Porcine SAA gene cluster organization.

The top line indicates a 60(CU633234.2) onto which the four SAA mRNA reference sequences (RefSeqs) from GenBank, NCBI, are positioned. Arrows show reading frame direction. Light blue is untranslated regions (UTRs). Dark blue is coding domains. Accession numbers are given below the RefSeqs. The SAA3 mRNA RefSeq derives directly from DQ367410.1, published by Chang et al. as SAA2. The other RefSeqs (i.e. the model RefSeqs) derive from NCBI’s automated mRNA prediction methods based on genomic DNA (CU633234.2). *Upstream part of the SAA1 model RefSeq conflicts with the conserved structure of SAA genes in general, and this transcript was not detected in the present study. See discussion in the text.

Figure 2. Relative gene expression levels of SAA2 and SAA3 in different tissues for each infection experiment.

All samples (n = 3 in each group) are shown relative to control spleen tissue, which is set to a mean expression of 1. Please note that Y axis is log10 scale.