Variation in the lack of polyadenylation of the rat milk protein mRNAs during the lactation cycle

Gene expression in the mammary gland of the tammar wallaby during the lactation cycle reveals conserved mechanisms regulating mammalian lactation

The tammar wallaby (Macropus eugenii), an Australian marsupial, has evolved a different lactation strategy compared with eutherian mammals, making it a valuable comparative model for lactation studies. The tammar mammary gland was investigated for changes in gene expression during key stages of the lactation cycle using microarrays. Differentially regulated genes were identified, annotated and subsequent gene ontologies, pathways and molecular networks analysed. Major milk-protein gene expression changes during lactation were in accord with changes in milk-protein secretion. However, other gene expression changes included changes in genes affecting mRNA stability, hormone and cytokine signalling and genes for transport and metabolism of amino acids and lipids. Some genes with large changes in expression have poorly known roles in lactation. For instance, SIM2 was upregulated at lactation initiation and may inhibit proliferation and involution of mammary epithelial cells, while FUT8 was upregulated in Phase 3 of lactation and may support the large increase in milk volume that occurs at this point in the lactation cycle. This pattern of regulation has not previously been reported and suggests that these genes may play a crucial regulatory role in marsupial milk production and are likely to play a related role in other mammals.

Translational regulation of milk protein synthesis at secretory activation

Studies conducted since the 1970s have revealed that the production of milk proteins in the mammary gland under the influence of lactogenic hormones (insulin, prolactin, and glucocorticoids) is regulated at multiple levels. Whereas earlier studies concentrated on transcriptional regulation and stabilization of milk protein mRNAs, more recent studies have revealed that translation of milk protein mRNAs is also dependent on lactogenic hormones. A general stimulation of translation in mammary epithelial cells is caused by amino acids (as signaling molecules) or by phosphorylation of the translational regulator 4E-BP1 in a synergistic response to signals from insulin and prolactin. However, a selective enhancement of milk protein mRNA translation is caused by cytoplasmic polyadenylation of mRNA, again in a synergistic response to these two hormones. Preliminary evidence indicates that the latter effect depends on the existence of a cytoplasmic polyadenylation element (CPE) in milk protein mRNAs and phosphorylation of its binding protein, CPEB. Experiments in whole animals, organ explants, and cell culture have shown that the poly(A) length of milk protein mRNAs changes as a function of the lactation cycle. Interestingly, cytoplasmic polyadenylation is likely to be responsible for the selective hormone-dependent enhancement of both translation and stability of milk protein mRNAs.

Efficiency of utilization of the simian virus 40 late polyadenylation site: effects of upstream sequences

The late polyadenylation signal of simian virus 40 functions with greater efficiency than the early polyadenylation signal, in turn affecting steady-state mRNA levels. Two chloramphenicol acetyltransferase (CAT) transient expression vectors, pL-EPA and pL-LPA, that differ only in their polyadenylation signals were constructed by using the early and late polyadenylation signals, respectively. In transfections of Cos, CV-1P, or HeLa cells and subsequent Northern blot analysis of CAT-specific RNA, approximately five times more steady-state CAT mRNA was produced in transfections with pL-LPA than with pL-EPA. The basis for this difference was not related to the specific promoter used or to RNA stability. Overall, the difference in steady-state mRNA levels derived from the two plasmids appeared to be attributable to intrinsic properties of the two polyadenylation signals, resulting in distinctly different cleavage and polyadenylation efficiencies. Additionally, we found that the utilization of the late polyadenylation site was dramatically reduced by deletion of sequences between 48 and 29 nucleotides 5' of the AAUAAA hexanucleotide. This reduction of mRNA levels was shown not to be caused by altered stability of mutant precursor RNAs or mRNAs, suggesting that these upstream sequences constitute an element of the late polyadenylation signal and may cause, at least to some extent, the greater efficiency of utilization of the late polyadenylation site.

Efficiency of utilization of the simian virus 40 late polyadenylation site: effects of upstream sequences

The late polyadenylation signal of simian virus 40 functions with greater efficiency than the early polyadenylation signal, in turn affecting steady-state mRNA levels. Two chloramphenicol acetyltransferase (CAT) transient expression vectors, pL-EPA and pL-LPA, that differ only in their polyadenylation signals were constructed by using the early and late polyadenylation signals, respectively. In transfections of Cos, CV-1P, or HeLa cells and subsequent Northern blot analysis of CAT-specific RNA, approximately five times more steady-state CAT mRNA was produced in transfections with pL-LPA than with pL-EPA. The basis for this difference was not related to the specific promoter used or to RNA stability. Overall, the difference in steady-state mRNA levels derived from the two plasmids appeared to be attributable to intrinsic properties of the two polyadenylation signals, resulting in distinctly different cleavage and polyadenylation efficiencies. Additionally, we found that the utilization of the late polyadenylation site was dramatically reduced by deletion of sequences between 48 and 29 nucleotides 5' of the AAUAAA hexanucleotide. This reduction of mRNA levels was shown not to be caused by altered stability of mutant precursor RNAs or mRNAs, suggesting that these upstream sequences constitute an element of the late polyadenylation signal and may cause, at least to some extent, the greater efficiency of utilization of the late polyadenylation site.

Cardiac mRNA levels during the development and growth of the rat

Developmental variations of RNA content and of total mRNA activity were examined in fetal, neonatal and adult rats. The amount of cardiac RNA extractable with the LiCl urea extraction procedure decreased from 3 mg/g tissue in the 18-day-old fetal heart to 1 mg/g in the 30-day-old heart and then to 0.5 mg/g in 100-day-old animals. Translations in the reticulocyte lysate translation system showed a similar pattern for total mRNA activity. It is shown that over this entire period both RNA content and mRNA activity vary as an exponential function of the growth rate of the heart and an explanation for this relationship is proposed. An examination of the translation products of the abundant mRNAs shows that the development of the heart is characterized by a general reduction in all mRNAs.

N1-monoacetylation abolishes the inhibitory effect of spermine and spermidine in the reticulocyte lysate translation system

At optimum magnesium, the translation of rat heart mRNA in the nuclease treated rabbit reticulocyte lysate system was inhibited by low concentrations of spermidine or spermine but not of putrescine. Spermidine and spermine cause a general reduction in the translation of all the heart mRNAs since no differential effects were observed when the translation products were examined by gel electrophoresis. Spermine was a five times more potent inhibitor than spermidine but no inhibition was obtained with N1-acetylspermidine or N1-acetylspermine. Since analyses of endogenous polyamines demonstrate that the inhibitory concentrations of spermine could be obtained by converting a small fraction of the endogenous spermidine to spermine, these results indicate that interconversions of the polyamines might be a sensitive regulatory mechanism for protein synthesis.