The tammar wallaby: a model system to examine domain-specific delivery of milk protein bioactives

The role of milk extends beyond simply providing nutrition to the suckled young. Milk has a comprehensive role in programming and regulating growth and development of the suckled young, and provides a number of potential autocrine factors so that the mammary gland functions appropriately during the lactation cycle. This central role of milk is best studied in animal models such as marsupials that have evolved a different lactation strategy to eutherians and allow researchers to more easily identify regulatory mechanisms that are not as readily apparent in eutherian species. For example, the tammar wallaby (Macropus eugenii) has evolved with a unique reproductive strategy of a short gestation, birth of an altricial young and a relatively long lactation during which the mother progressively changes the composition of the major, and many of the minor components of milk. Consequently, in contrast to eutherians, there is a far greater investment in development of the young during lactation and it is likely that many of the signals that regulate development of eutherian embryos in utero are delivered by the milk. This requires the co-ordinated development and function of the mammary gland since inappropriate timing of these signalling events may result in either limited or abnormal development of the young, and potentially a higher incidence of mature onset disease. Milk proteins play a significant role in these processes by providing timely presentation of signalling molecules and antibacterial protection for the young and the mammary gland at times when there is increased susceptibility to infection. This review describes studies exploiting the unique reproductive strategy of the tammar wallaby to investigate the role of several proteins secreted at specific times during the lactation cycle and that are correlated with potential roles in the young and mammary gland. Interestingly, alternative splicing of some milk protein genes has been utilised by the mammary gland to deliver domain-specific functions at specific times during lactation.