Progressive changes in milk protein gene expression and prolactin binding during lactation in the tammar wallaby (Macropus eugenii)

Abstract

Changes in milk protein gene expression and specific prolactin binding were quantified in mammary tissue from the tammar wallaby (Macropus eugenii) at different stages of lactation. The transition from early (phase 2) lactation to late (phase 3) lactation was characterized by the induction of the gene for late lactation protein, a novel whey protein. During the same period, the levels of beta-lactoglobulin and beta-casein gene expression increased, whereas there was no change in the levels of expression of alpha-lactalbumin and alpha-casein genes. Prolactin binding in the mammary gland doubled during the latter half of phase 2 of lactation but declined significantly during the transition to phase 3 of lactation. These changes in prolactin binding resulted from changes in the number of receptors and not from a change in the affinity of the receptor for prolactin. Treatment of membranes with concanavalin A increased the number of prolactin-binding sites by 40% in membranes from phase 2 mammary tissue but decreased binding by 40% in membranes from phase 3 tissue, indicating that significant changes had occurred in the membranes of cells during this period. The tammar wallaby can secrete phase 2 and phase 3 milk from adjacent mammary glands (asynchronous concurrent lactation) and the developmental changes in milk protein gene expression and prolactin binding observed during lactation were reflected in these individual glands. Taken collectively, these findings suggest that mammary development and milk secretion in the tammar wallaby are regulated by both endocrine and local (intramammary) mechanisms.

Differential expression of milk protein genes during lactation in the common brushtail possum (Trichosurus vulpecula)

“…Similarly, concentration of -lactalbumin in the milk of the ringtail possum is constant throughout lactation . Levels of -lactoglobulin mRNA in the wallaby increased by 50% during phase 3 to reach a peak at day 239 before decreasing again in late lactation (Bird et al 1994). This increase, however, was not observed in the brushtail possum.…”

“…The level of -lactalbumin mRNA observed in the mammary gland therefore does not appear to be directly related to the concentration of the -lactalbumin protein in possum milk. In the tammar wallaby, levels of -lactalbumin protein and mRNA remained constant throughout lactation , Bird et al 1994. Similarly, concentration of -lactalbumin in the milk of the ringtail possum is constant throughout lactation .…”

“…Although there appeared to be considerable variation between animals in the levels of casein mRNA, the ratio of -casein to -casein mRNA remained close to 1:1 in all animals tested, suggesting tight regulation of casein gene transcription in the possum. In the wallaby -casein RNA levels were constant throughout lactation, whereas the level of -casein RNA increased by 80% during phase 3 of lactation, reaching a peak at day 239 before decreasing later in lactation (Bird et al 1994). This increase in -casein RNA was reflected by an increase in -casein protein concentration in wallaby milk (Nicholas et al 1995).…”

“…RNA levels (means ...) of eight milk protein genes in phase 2 (days 1-120 of lactation, n=26) and phase 3 (days 120-190, n=9) lactation protein RNA was expressed at low levels on day 172 of lactation in the wallaby, but reached a peak on day 239 (Bird et al 1994). Expression of the late lactation protein in both the possum and the wallaby coincides with the transition from phase 2 to 3 of lactation.…”

Evolution of Lactation: Ancient Origin and Extreme Adaptations of the Lactation System

“…In particular, lactation stage-specific genes, such as early lactation protein (ELP), mid-late whey acidic protein (WAP), and late-lactation proteins (LLP-A and LLP-B) have been characterized in the tammar and other marsupial species (6,19,27,28,69,70,88,96). With the exception of WAP, which is also found in milk of many eutherians (37) but not in humans, goat, or ewe (32), all of these phase-specific milk proteins are marsupial specific and have not been found in eutherian or monotreme milk.…”