Identification, characterisation and expression analysis of natural killer receptor genes in Chlamydia pecorum infected koalas (Phascolarctos cinereus)
Morris, KM, Mathew, M, Waugh, C, Ujvari, B, Timms, P, Polkinghorne, A & Belov, K 2015, BMC Genomics, vol. 16, no 796.
The aim of this study was to characterise genes within the receptor clusters of natural killer (NK) cells of the koala. Analysis with quantitative polymerase chain reaction (qPCR) found that CLEC4E was upregulated in response to Chlamydia pecorum infection.
In this study, analysis of available koala transcriptomes identified four genes, CLEC1B, CLEC4E, PRF1, NCR3, which were used in the development of qPCR assays to analyse the expression of receptors and other genes during C. pecorum infection. Both NCR3 and PRF1 full-length transcripts were identified in the koala transcriptomes, with PRF1 also identified in most vertebrate species. In mammals, NCR3 has only been identified in the genomes of the human, koala, tammar wallaby and rat. However, orthologues of NCR3 were identified in reptilian genomes, such as from turtles, pythons and alligators, suggested to be genetic evidence of the ancient origin of NCR3 dating back to the Mesozoic Era. Full-length transcripts of CLEC1B and CLEC4E were also found in the koala transcriptomes and had at least 99% bootstrap support during phylogenetic analyses against opossum and Tasmanian devil genomes. Preliminary analysis of the four genes in cohorts of uninfected, infected and previously-infected koalas observed that CLEC4E was significantly differentially expressed, with a four-fold increase in expression in the infected koalas compared to the uninfected koalas.
In humans, NCR3 and PRF1 have been linked to the response against chlamydial infection. While PRF3 is conserved across the multiple genomes investigated in this study, the evolutionary history of NCR3 is complex, evidently originating from the last common ancestor between mammals and reptiles and was lost in many mammalian genomes. Preliminary qPCR assays have identified an upregulation of all four genes in diseased koalas, but only CLEC4E was statistically significant. CLEC4E recognises and is induced by lipopolysaccharide, which is a receptor ligand found in Chlamydia. Hence, the upregulation of CLEC4E would have been necessary for the induction of the immune response against C. pecorum. The statistical non-significance of the other genes, while exhibiting a trend of upregulation in the diseased koalas, could have been due to high biological variability across the samples. As such, the authors suggest a larger cohort would be beneficial for future studies, with samples taken at infected sites for characterisation of the immune response at the site of infection.
This study has characterised genes involved in the NK immune response in the koala and developed qPCR assays which could be useful in further research to examine genes of interest during chlamydial infection.
Summarised by Daniel Chew
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