c-Myc affects mRNA translation, cell proliferation and progenitor cell function in the mammary gland

BACKGROUND: The oncoprotein c-Myc has been intensely studied in breast cancer and mouse mammary tumor models, but relatively little is known about the normal physiological role of c-Myc in the mammary gland. Here we investigated functions of c-Myc during mouse mammary gland development using a conditional knockout approach. RESULTS: Generation of c-mycfl/fl mice carrying the mammary gland-specific WAPiCre transgene resulted in c-Myc loss in alveolar epithelial cells starting in mid-pregnancy. Three major phenotypes were observed in glands of mutant mice. First, c-Myc-deficient alveolar cells had a slower proliferative response at the start of pregnancy, causing a delay but not a block of alveolar development. Second, while milk composition was comparable between wild type and mutant animals, milk production was reduced in mutant glands, leading to slower pup weight-gain. Electron microscopy and polysome fractionation revealed a general decrease in translational efficiency. Furthermore, analysis of mRNA distribution along the polysome gradient demonstrated that this effect was specific for mRNAs whose protein products are involved in milk synthesis. Moreover, quantitative reverse transcription-polymerase chain reaction analysis revealed decreased levels of ribosomal RNAs and ribosomal protein-encoding mRNAs in mutant glands. Third, using the mammary transplantation technique to functionally identify alveolar progenitor cells, we observed that the mutant epithelium has a reduced ability to repopulate the gland when transplanted into NOD/SCID recipients. CONCLUSION: We have demonstrated that c-Myc plays multiple roles in the mouse mammary gland during pregnancy and lactation. c-Myc loss delayed, but did not block proliferation and differentiation in pregnancy. During lactation, lower levels of ribosomal RNAs and proteins were present and translation was generally decreased in mutant glands. Finally, the transplantation studies suggest a role for c-Myc in progenitor cell proliferation and/or survival

c-Myc affects mRNA translation, cell proliferation and progenitor cell function in the mammary gland

Abstract Background The oncoprotein c-Myc has been intensely studied in breast cancer and mouse mammary tumor models, but relatively little is known about the normal physiological role of c-Myc in the mammary gland. Here we investigated functions of c-Myc during mouse mammary gland development using a conditional knockout approach. Results Generation of c-mycfl/fl mice carrying the mammary gland-specific WAPiCre transgene resulted in c-Myc loss in alveolar epithelial cells starting in mid-pregnancy. Three major phenotypes were observed in glands of mutant mice. First, c-Myc-deficient alveolar cells had a slower proliferative response at the start of pregnancy, causing a delay but not a block of alveolar development. Second, while milk composition was comparable between wild type and mutant animals, milk production was reduced in mutant glands, leading to slower pup weight-gain. Electron microscopy and polysome fractionation revealed a general decrease in translational efficiency. Furthermore, analysis of mRNA distribution along the polysome gradient demonstrated that this effect was specific for mRNAs whose protein products are involved in milk synthesis. Moreover, quantitative reverse transcription-polymerase chain reaction analysis revealed decreased levels of ribosomal RNAs and ribosomal protein-encoding mRNAs in mutant glands. Third, using the mammary transplantation technique to functionally identify alveolar progenitor cells, we observed that the mutant epithelium has a reduced ability to repopulate the gland when transplanted into NOD/SCID recipients. Conclusion We have demonstrated that c-Myc plays multiple roles in the mouse mammary gland during pregnancy and lactation. c-Myc loss delayed, but did not block proliferation and differentiation in pregnancy. During lactation, lower levels of ribosomal RNAs and proteins were present and translation was generally decreased in mutant glands. Finally, the transplantation studies suggest a role for c-Myc in progenitor cell proliferation and/or survival. See related minireview by Evan et al: http://jbiol.com/content/8/8/77</p

Competition and Cooperation among Relational Memory Representations.

Mnemonic processing engages multiple systems that cooperate and compete to support task performance. Exploring these systems' interaction requires memory tasks that produce rich data with multiple patterns of performance sensitive to different processing sub-components. Here we present a novel context-dependent relational memory paradigm designed to engage multiple learning and memory systems. In this task, participants learned unique face-room associations in two distinct contexts (i.e., different colored buildings). Faces occupied rooms as determined by an implicit gender-by-side rule structure (e.g., male faces on the left and female faces on the right) and all faces were seen in both contexts. In two experiments, we use behavioral and eye-tracking measures to investigate interactions among different memory representations in both younger and older adult populations; furthermore we link these representations to volumetric variations in hippocampus and ventromedial PFC among older adults. Overall, performance was very accurate. Successful face placement into a studied room systematically varied with hippocampal volume. Selecting the studied room in the wrong context was the most typical error. The proportion of these errors to correct responses positively correlated with ventromedial prefrontal volume. This novel task provides a powerful tool for investigating both the unique and interacting contributions of these systems in support of relational memory

Stimuli and design in the novel context-dependent relational memory test.

<p>(a) General display information and resulting regions of interest. (b) Study phase design and timing information. (c) Test phase design and timing information.</p

Eye-movement data.

<p>Proportion of viewing to rooms on the correct (green) and incorrect (blue) side of the building. Dark bars indicate face placements to the studied room on a given side whereas colored (green or blue) bars indicate face placements to any of the remaining eight unstudied rooms. Data are presented for both the Young Adult (a) and Older Adult (b) groups for both the gray building and color building viewing period; standard error bars are shown.</p

Behavioral performance.

<p>Proportion of face placement to rooms on the correct (green) and incorrect (blue) side of the building. Dark bars indicate face placements to the studied room on a given side whereas colored (green or blue) bars indicate face placements to any of the remaining eight unstudied rooms. Data are presented for both the Young Adult and Older Adult groups and standard error bars are shown.</p

Behavior and volumetric relationships.

<p><i>(</i>a) The correlation between bilateral hippocampal volume and preference for placing the face in the studied room as a proportion of all face placements to context-correct (green) and context-incorrect (blue) sides of the building. (b) The correlation between bilateral ventromedial prefrontal cortex volume and both correct novel face placements (purple) and Triad Scores (studied room placements on the context-incorrect side of the building divided by studied room placements to the context-correct side of the building; teal). All behavioral data were <i>z</i>-scored for presentation purposes.</p

Behavioral performance.

<p>Proportion of face placement to rooms on the correct (green) and incorrect (blue) side of the building. Dark bars indicate face placements to the studied room on a given side whereas colored (green or blue) bars indicate face placements to any of the remaining eight unstudied rooms. Data are presented for both the Study1x and Study3x groups and standard error bars are shown.</p