Discoveries

Preparation of the Mammary Gland for Lactation

September 15, 2017

During pregnancy, a subgroup of cells in the mammary gland begins to rapidly proliferate and to differentiate into milk-producing cells. These changes occur, in part, as a result of stimulation by the hormone prolactin, which activates the STAT5 transcription factor, a key activator of milk protein-encoding genes. However, a number of other signaling pathways are also involved, among them the neuregulins (NRGs), which bind to and activate ErbB4, a member of the epidermal growth factor receptor (EGFR) family. Upon NRG binding, ErbB4 forms a dimer with a second EGFR family member, and a frequent partner is ErbB3. This led Vanderbilt Basic Sciences investigator Rebecca Cook and her laboratory to investigate a possible role for ErbB3 in mammary gland differentiation during pregnancy. They created a genetically engineered mouse model that selectively lacks ErbB3 expression in the mammary gland. They found reduced proliferation and increased death of cells in the gland during mid-pregnancy, as compared to what is observed in the glands of wild-type mice. As a result, fewer milk-producing cells formed, and offspring of the ErbB3-deficient mice failed to gain weight normally after birth. Further studies in these mice and in a mammary epithelial cell line revealed failure of normal STAT5 activation in the absence of ErbB3, explaining the reduction in milk-producing cells. In addition, normal activation of the enzyme Akt did not occur in ErbB3-deficient cells, leading to a failure of signaling that promotes cell survival. The findings suggest that ErbB3 plays a key role in the growth, differentiation, and survival of breast cells during pregnancy and lactation, and that it does this by promoting the activation of two pathways – the STAT5 pathway leading to growth and differentiation, and the Akt pathway that enhances cell survival. Thus, ErbB3 can be added to the signaling network that is required to prepare the mammary gland for lactation after birth of the newborn. The work is published in the journal Breast Cancer Research [M. M. Williams, et al., (2017) Breast Cancer Res., 19, 105].

Figure reproduced under the Creative Commons Attribution 4.0 International License from M. M. Williams, et al., (2017) Breast Cancer Res., 19, 105.

Path to Successful Cytokinesis

August 14, 2017

When we think of mitosis, the intricate process of DNA replication followed by precise alignment of the duplicated chromosomes and their distribution to the daughter cells comes immediately to mind. However, equally important is the process of cytokinesis by which the remainder of the cell’s contents are equally divided and distributed. In many cell types, a contractile ring (CR) forms around the equator of the dividing cell, defining a plane along which cytokinesis will occur. Constriction of this ring is ultimately responsible for splitting the cell’s contents as the chromosomes move towards their respective newly forming nuclei. Despite considerable research, however, the mechanisms that dictate formation and constriction of the CR remain a mystery. Now, work from Vanderbilt Basic Sciences investigator Kathy Gould and her laboratory shows that the yeast protein Efr3 is intricately involved in CR function in dividing yeast. Previous work had shown that Efr3 works together with a partner protein Ypp1 to form a platform for the phosphatidylinositol-4 kinase Stt4 at the plasma membrane. This enables Stt4 to increase the levels of phosphatidylinositol 4-phosphate (PI(4)P) and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in the membrane. (PI(4,5)P2) serves as a binding site for a number of CR-associated proteins, and levels of the phospholipid are elevated in the region of the assembling CR. The investigators showed that deletion of Efr3 results in reduced levels of PI(4)P and (PI(4,5)P2) in the membrane, and although the CR appears to form normally, it slides away from its correct central position during constriction, resulting in two unequally divided daughter cells. Multiple genetic studies showed that Efr3 functions to maintain CR position by a mechanism distinct from that of other proteins known to be important in CR function. Then, further work demonstrated that Myo51, a type V myosin, is required to observe abnormal division in Efr3-deficient cells. The findings suggest that Myo51 exerts perpendicular forces on the CR as it constricts and that Efr3 is required to prevent slipping of the CR away from the center in the presence of these forces. The discovery of these myosin-dependent forces in CR function provides new insight into the process of cytokinesis. The work is published in the Journal of Cell Biology [C. E. Snider et al., (2017) J. Cell Biol., published online August 7, DOI: 10.1083/jcb.201705070].

 

Newly Discovered Critical Step in B Cell Development

July 27, 2017

Histone deacetylases (HDACs) are enzymes that remove the acetyl groups from acetylated lysine residues of histones. The reaction produces a positively charged, unmodified lysine residue that enables the histone to interact more tightly with negatively charged DNA. In general, this promotes packaging of DNA into chromatin, thereby suppressing its replication, transcription, or repair. HDAC3 serves this function as part of the corepressor complex NCoR/SMRT, which is known to play an important role in heterochromatin maintenance in some cells and tissues. Among these HDAC3-dependent cells, as discovered by Vanderbilt Basic Sciences investigator Scott Hiebert, his laboratory, and his collaborator Srividya Bhaskara (University of Utah), are the B cells of the immune system. This discovery resulted from studies using genetically engineered mice in which HDAC3 expression was selectively deleted in developing B cells. A key step in early B cell differentiation is the splicing together of three segments in the gene for the immunoglobulin heavy chain, a process known as VDJ recombination. The resultant heavy chain gene is then expressed as part of a B cell receptor, which serves as a key signaling molecule to spur further differentiation. In the absence of HDAC3, however, heavy chain gene recombination did not occur, and a stark absence of mature B cells in the mutant mice resulted. The investigators found that recombinations involving gene segments that were far apart on the chromosome were the most likely to be blocked by the HDAC3 deficiency. Reintroduction of wild-type HDAC3 into the deficient mice restored B-cell development, but this was not the case for reintroduction of a catalytically inactive mutant HDAC3. The findings demonstrate that HDAC3 is a key component in B cell development through its regulation of gene recombination and that HDAC3 catalytic activity is required for it to execute this function. The work is published in the journal Proceedings of the National Academy of Sciences USA [K.R. Stengel, et al., (2017) Proc. Natl. Acad. Sci. USA, published online July 24, DOI: 10.1073/pnas.1701610114]. 

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