Loeken, Mary R., Ph.D.
The overall goal of the Loeken lab is to understand how genes are regulated and how abnormal gene expression impacts disease states. Our research focuses on embryo gene expression during pregnancies of women with pregestational diabetes. Diabetic embryopathy is one of the least studied diabetic complications, and, while this complication may be particularly challenging for therapeutic intervention, research on this topic is necessary in order to design better diagnostic tools and novel treatment strategies. Moreover, because the embryo is comprised exclusively of stem and progenitor cells, understanding how abnormal fuel metabolism disturbs cell fate and differentiation potential may be aid in understanding how stem and progenitor cells involved in tissue repair may be compromised during diabetes.
We employ a mouse model of diabetic pregnancy that was developed in our laboratory to elucidate the biochemical and molecular pathways by which increased glucose transport to the embryo disturbs expression of critical developmental control genes, and how deficient gene expression causes congenital malformations. We also use mouse embryonic stem cells (ESC) that can be induced to form neuronal precursors to model embryonic neuroepithelium and neural crest, tissues that are prone to malformation in human diabetic pregnancy. A cell culture model is valuable because cells can be grown in quantity and to homogeneity, making possible some molecular and biochemical studies that would be difficult or impossible using embryos.
We have recently begun to investigate whether genes that are essential for islet differentiation could be involved in fetal programming of impaired glucose tolerance in the offspring of diabetic mothers. We have also begun studies with a cancer model as an additional approach to understand the function of the gene we have studied in association with diabetic embryopathy.
Role of Pax3 and p53 in diabetic embryopathy:
For the past several years, we have studied the embryonic gene, Pax3, which is essential for neural tube closure and formation of neural crest-derived structures, such as the outflow tracts (aorta and pulmonary arteries) of the heart. We showed that maternal diabetes (specifically, hyperglycemia) inhibits Pax3 expression. Because embryos with nonfunctional Pax3 alleles develop neural tube and cardiac outflow tract defects, two of the most common defects during diabetic embryopathy, with 100% penetrance, inadequate expression of Pax3 is, therefore, sufficient to explain how these defects arise. We have elucidated several steps of a pathway in which glucose is delivered to the embryo in high concentrations during episodes of hyperglycemia, glucose efficiently enters the embryo cells via the Glut2 glucose transporter, and several glucose-metabolizing processes are activated. Increased glucose metabolism induces hypoxic and oxidative stress, which stimulates activity of the enzyme, AMP-activated protein kinase (AMPK). AMPK inhibits expression of Pax3. Because Pax3 is only expressed in certain structures of the embryo that are prone to malformation, this can explain how hyperglycemia that is experienced by the entire embryo leads to specific types of defects. In the absence of sufficient Pax3 production, neuroepithelial and neural crest cells undergo apoptosis, thereby causing malformation of the tissues that derive from these cells. We showed that the p53 tumor suppressor protein is needed for apoptosis and malformation in the absence of Pax3, suggesting that Pax3 is only needed during neural tube closure and development of neural crest-dependent structures to inhibit p53 function.
Previously, we did not consider that Pax3 was specifically regulated by glucose, but rather, considered that suppressed Pax3 expression was part of the “collateral damage” of “glucose toxicity” that was mediated by oxidative stress. However, as a result of recent research from our lab and others, we now hypothesize that Pax3 is regulated by the transition from highly glycolytic to increasingly aerobic metabolism that occurs as embryonic cells normally differentiate. Thus, increased glucose metabolism may block the metabolic signals that induce expression of Pax3. We recently showed that Pax3 inhibits p53 function by stimulating Mdm2-mediated degradation of p53. Therefore, Pax3 may be induced in order to suppress activity of p53, which promotes oxidative metabolism, as well as differentiation and cessation of proliferation. Pax3 may be a member of a class of genes that become activated in stem and progenitor cells by metabolism, but whose function is to titrate oxidative activity, and along with it, terminal differentiation and senescence, during organogenesis or tissue renewal.
Our ongoing studies investigate the mechanisms by which Pax3 is regulated during normal development and how oxidative stress inhibits this process, how Pax3 stimulates p53 degradation, and how Pax3 and p53 interact to regulate differentiation. We are also interested in broader regulation of progenitor cells by maternal diabetes during islet development, and the significance of Pax3 regulation of p53 in normal and pathological conditions.
Mary Loeken, Ph.D., is an Investigator in the Section on Islet Cell Biology and Regenerative Medicine at Joslin and an Associate Professor of Medicine at Harvard Medical School. She received her doctorate in Reproductive Endocrinology at the University of Maryland School of Medicine and did postdoctoral training at the National Cancer Institute's Laboratory of Molecular Virology before coming to Joslin. In 1992 she was named a Capps Scholar in Diabetes Research at Harvard Medical School, which also awarded her a Scholars in Medicine Award in 1998. She is an expert on the study of birth defects resulting from diabetic pregnancy and has served on study sections for the NIH, the Juvenile Diabetes Research Foundation, and the American Diabetes Association, and on the Editorial Board for the journal, Diabetes. She is an Associate Member of the Diabetic Pregnancy Study Group of the European Association for the Study of Diabetes.
Page last updated: October 21, 2014