The mammary gland-specific marsupial ELP and eutherian CTI share a common ancestral gene

Elizabeth A Pharo,Marilyn B Renfree,Peter C Thomson,Kevin R Nicholas,Christophe M Lefèvre,Alison A De Leo

doi:10.1186/1471-2148-12-80

Abstract

BackgroundThe marsupial early lactation protein (ELP) gene is expressed in the mammary gland and the protein is secreted into milk during early lactation (Phase 2A). Mature ELP shares approximately 55.4% similarity with the colostrum-specific bovine colostrum trypsin inhibitor (CTI) protein. Although ELP and CTI both have a single bovine pancreatic trypsin inhibitor (BPTI)-Kunitz domain and are secreted only during the early lactation phases, their evolutionary history is yet to be investigated.ResultsTammar ELP was isolated from a genomic library and the fat-tailed dunnart and Southern koala ELP genes cloned from genomic DNA. The tammar ELP gene was expressed only in the mammary gland during late pregnancy (Phase 1) and early lactation (Phase 2A). The opossum and fat-tailed dunnart ELP and cow CTI transcripts were cloned from RNA isolated from the mammary gland and dog CTI from cells in colostrum. The putative mature ELP and CTI peptides shared 44.6%-62.2% similarity. In silico analyses identified the ELP and CTI genes in the other species examined and provided compelling evidence that they evolved from a common ancestral gene. In addition, whilst the eutherian CTI gene was conserved in the Laurasiatherian orders Carnivora and Cetartiodactyla, it had become a pseudogene in others. These data suggest that bovine CTI may be the ancestral gene of the Artiodactyla-specific, rapidly evolving chromosome 13 pancreatic trypsin inhibitor (PTI), spleen trypsin inhibitor (STI) and the five placenta-specific trophoblast Kunitz domain protein (TKDP1-5) genes.ConclusionsMarsupial ELP and eutherian CTI evolved from an ancestral therian mammal gene before the divergence of marsupials and eutherians between 130 and 160 million years ago. The retention of the ELP gene in marsupials suggests that this early lactation-specific milk protein may have an important role in the immunologically naïve young of these species.

Highlights

The marsupial early lactation protein (ELP) gene is expressed in the mammary gland and the protein is secreted into milk during early lactation (Phase 2A)
ELP/colostrum trypsin inhibitor (CTI) evolved from a common ancestral gene To determine whether the marsupial ELP gene was present in other species, we used multiple approaches
ELP/CTI transcripts were cloned from the mammary gland of the cow, opossum and fat-tailed dunnart and the dog CTI transcript was cloned from epithelial cells isolated from canine colostrum

Summary

Introduction

The marsupial early lactation protein (ELP) gene is expressed in the mammary gland and the protein is secreted into milk during early lactation (Phase 2A). The final phase is from day 200 to at least day 300 when the young suckles variably and begins to graze as well as maintaining a milk intake (Phase 3) [18] These phases are highly correlated with changes in milk composition and mammary gland gene expression [10,13,19]. Milk protein genes such as α-lactalbumin, βlactoglobulin (LGB), α-casein, β-casein and κ-casein are induced at parturition and expressed throughout lactation, whilst others are expressed and secreted in a phasespecific manner [13]. Lactation protein (ELP) is expressed during Phase 2A only [13,20,21], whey acidic protein (WAP) is Phase 2B-specific [22] and late lactation protein A and B are characteristic to late Phase 2B/ Phase 3 and Phase 3 respectively [23,24]

Methods

Results

Discussion

Conclusion