General Research Interests
My research seeks to understand the basic principles of how vertebrate embryos develop. Understanding embryonic development is of fundamental importance to general biological knowledge and to medicine. Many basic principles of biochemistry and cellular biology have been revealed by embryology, and very often these principles have proven relevant to understanding the mechanisms of disease. The causes of a variety of birth defects, which are obviously of embryonic origin, have been uncovered through the study of 'model' animals such as mice, flies, worms and frogs. The molecular origins of diseases such as cancer, which is characterized by uncontrolled growth and abnormal differentiation, are also much better understood today through the efforts of molecular embryology. Underscoring this connection is the realization over the past decade that many of the regulatory mechanisms that govern embryonic development are the same as those that control the normal function of adult cells. Furthermore, many advances in biotechnology are fueled by new methods developed and applied to embryonic systems. As the various model organism (fly, mouse, fish and human) genome projects progress over the next few years, embryology is poised to play an increasingly important role in puzzling-out the function of newly discovered genes. Below are summarized several areas of research ongoing in my laboratory. Relevant publications are also cited. A very nice, comprehensive review of Xenopus pattern formation and the role of growth factors can be found in a review by Harland and Gerhart, "Formation and Function of Spemann's Organizer" in the Annual Review of Cell and Developmental Biology , vol 13, 611-667 (1997). Also, check out this link to a useful Xenopus research site: http://vize222.zo.utexas.edu/
1. The Transforming Growth Factor-ß (TGFß) Family in Embryonic Development
A large portion of my lab's research effort is focused on the study of TGFß growth factors in developing vertebrate embryos of the frog, Xenopus laevis. Growth factors are substances that are secreted by cells and that act on neighboring cells to instruct them to differentiate, proliferate or halt their growth. We are interested in how members of the TGFß family, such as Vg1, activin and bone morphogenetic proteins, or BMPs, function to control differentiation and growth of tissues, particularly tissues that form blood, muscle, heart, nervous system and the head. Using the frog embryo we seek to characterize the role of TGFß growth factors in controlling tissue differentiation and embryonic patterning in the mesodermal, endodermal and ectodermal embryonic germ layers. We have also been characterizing the biochemical mechanisms of how various TGFß factors trigger their effects in cells. To this end we have identified, and continue to pursue, new growth factors and components of the TGFß signal transduction machinery that determine how cells to respond to TGFß factors.
a) BMPs and blood
We have discovered recently that Bone Morphogenetic Proteins (BMPs) directly trigger blood differentiation. This finding has fueled our work on early aspects of hematopoiesis in vertebrates, and we are presently working to discover new factors that control cellular responses to BMPs and blood formation. In collaboration with Dr. Peter Gergen we have also been studying the function of another transcription factor in Xenopus blood formation. This gene, named XAML, is related to the mammalian Acute Myologenous Leukemia (AML) gene, a gene that is critical for normal blood differentiation and which may function in blood vessel formation as well. We aim to eventually understand if and how XAML is regulated by TGFß growth factors
b) Activin and Vg1 signaling in head induction.
Recent work in the lab has revealed that signals through Smad2 are required for head formation in the Xenopus embryo, most likely by functioning in head organizer induction (Hoodless et al., 1999; reference below). Past work from my postdoctoral also showed that Vg1 signals can induce heads and a body axis in ventralized embryos (Thomsen and Melton, 1993; below). Other work in the Xenopus field also indicates that ectopic heads are induced when wnt and BMP signals are inhibited in the mesoderm. The preceise nature of the signals or molecular determinants for head formation in the absence of wnt and BMP signals are not known. On the hypothesis that endogenous Vg1 or activin signals may be at the root of head induction we are now in the process of exploring the identity of genes that are activated in response to Vg1 signals. We eventually explore whether Vg1-inducible genes function in head and dorsal axis formation.
c) Smads in signal transduction and embryonic patterning
In the past few years work in Drosophila and nematodes led to the discovery of a group of proteins, named the Smads, that function in signal transduction in the TGFß superfamily. Smads are critical determinants in how cells respond to TGFß factors, and mutations in a variety of human and mouse Smads trigger cancer. Presently there are about 10 Smads identified in vertebrates and distinct subsets of Smads function to transduce signals from particular TGFß receptors. For a good recent review of Smad functions see Whitman, "Smads and early developmental signaling by the TGFß superfamily", in Genes and Development vol. 12, 2445-2462 (1998).
We are studying how Smad activities affect development and cell differentiation, particularly with respect to the biochemistry of Smads. In frog embryos, various mutated Smads cause loss of blood, head, heart and muscle tissues, indicating a critical role for these factors in differentiaton and embryonic patterning. In addition to performing critical functions in developing embryos of many species, TGFß growth factors have a direct role in the control of human cell growth and cancer. It has been shown in mice and humans that many cancers are associated with defects in the molecular components of TGFß response systems, such as the Smads. My lab presently has an ongoing collaboration with the labs of Drs. Jeff Wrana and Lillian Attisano (University of Toronto) to study biochemical mechanisms of TGFß regulation in cancer cells and normal mammalian cells
2. Smurfs: a new brand of ubiquitin ligases that affect Smads and patterning
In our studies of Smads we have recently discovered a unique set of genes that we named Smurfs. Smurfs are members of a class of proteins known as ubiquitin ligases which in general target proteins for ubiquitination and degradation via the proteosome. For a good review of ubiquitination and ubiquitin ligases see Hershko & Ciechanover, "The ubiquitin system" in Annual Review of Biochemistry, vol. 67, 425-479 (1998). The particular Smurfs we have discovered trigger the degradation of Smad proteins, which causes changes in responsiveness to different TGFß growth factors and has dramatic effects on pattern formation. For example, Smurf1 targets Smads that function in BMP signaling, rendering cells non-responsive to these factors, and that results in dorsalization of mesoderm and neuralization of the ectoderm. Our discovery of Smurf-1, now (Feb '99) submitted for publication, is a novel finding in the TGFß field and it should have a wide impact in embryology and other basic and clinically-oriented research on TGFß growth factors.
3. T-box Transcription factors in patterning and Heart Development
Besides TGFß factors, my lab studies transcription factors that control gene expression and cell differentiation. One group of such factors are T-box genes that regulate early development and organogenesis. Presently we are studying a gene that goes by the name of Brat, or VgT, and it functions in early embryos to trigger differentiaton of mesoderm, such as blood, muscle, heart and bone, and endoderm (gut). We are presently investigating the expression of endogenous Brat protein in embryos and are isolating genes that are regulated directly by Brat. We are also studying the function of another T-box gene, named Tbx5, in heart development and have discovered that it is necessary for heart formation. Tbx-5 is clinically very important because mutations in the human Tbx5 gene cause heart defects. We are now pursuing studies to determine whether Tbx5 regulates cardiac cell differentiation and to identify heart-specific genes that are controlled by Tbx5. Our studies of Tbx5 in frog embryonic development should help shed light on the role this gene plays in normal heart development and how defects in Tbx5 may lead to abnormal heart formation in humans.
Pertinent Publications from the lab
P. D. Vize and G. H. Thomsen. (1994). Vg1 and regional specification in vertebrates: a new role for an old molecule. Trends in Genetics 10: 371-376.
G. H. Thomsen. (1997). Antagonism within and around the Spemann organizer: BMP inhibitors in vertebrate body patterning. Trends in Genetics 13: 209-211.
Embryonic Patterning by TGFß Growth factors and Smad Signal Transducers
P. Hoodless, T. Tsukazaki, S. Nishimatsu, L. Attisano, J. Wrana and G. H. Thomsen. (1999). Dominant-negative Smad2 mutants inhibit activin/Vg1 signaling and disrupt axis formation in Xenopus. Developmental Biology 207: 364-379.
G. H. Thomsen. (1996). Xenopus mothers against decapentaplegic is an embryonic ventralizing agent that acts downstream of the BMP-2/4 receptor. Development 122: 2359-2366.
K. Eppert, S. W. Scherer, H. Ozcelik, R. Pirone, P. Hoodless, H. Kim, L. Tsui, B. Bapat, S. Gallinger, I. Andrulis, G. H. Thomsen, J. L. Wrana and L. Attisano. (1996). MADR2 maps to 18q21 and encodes a TGFß regulated MAD-related protein that is functionally mutated in colorectal carcinoma. Cell 86: 543-552.
A. Hemmati-Brivanlou and G. H. Thomsen. (1995). Ventral mesodermal patterning in Xenopus embryos: The expression patterns and activities of BMP-2 and BMP-4. Developmental Genetics 17: 78-89.
Smurfs in Ubiquitination and Pattern Formation - My lab's newest area of research.
H. Zhu, S. Abdollah, P. Kavsak, J. Wrana and G. H. Thomsen. Ubiquitination in TGFß signaling and development: A Smad ubiquitin ligase targets the BMP pathway and affects embryonic pattern formation. (submitted).
T-Box genes and Heart formation
M. E. Horb and G. H. Thomsen. (1999). Tbx5 is essential for vertebrate heart formation. Development 126: 1739-1751.
M. E. Horb and G. H. Thomsen. (1997). A vegetally localized T-box transcription factor in Xenopus eggs specifies mesoderm and endoderm and is essential for embryonic mesoderm formation. Development 124: 1689-1698.
W. D. Tracey, M. E. Pepling, M. E. Horb, G. H. Thomsen and J. P. Gergen. (1998). A Xenopus homologue of AML-1 reveals unexpected patterning mechanisms leading to the formation of embryonic blood. Development 125: 1371-1380.
S. Nishimatsu and G. H. Thomsen. (1998). Ventral mesoderm induction and patterning by BMP heterodimers in Xenopus embryos. Mechanisms of Development 74: 75-88.
Fertilization Mechanisms, in collaboration with Dr. W. J. Lennarz
J. Tian, H. Gong, G. H. Thomsen and W. J. Lennarz. (1997). Xenopus laevis sperm-Egg adhesion is regulated by modifications in the sperm receptor and vitelline envelope. Developmental Biology 187: 143-153.
J. Tiang, H. Gong, G. H. Thomsen and W. J. Lennarz. (1997) Xenopus laevis gamete interactions: Identification of sperm binding proteins in the egg vitelline envelope. Journal of Cell Biology 136: 1099-1108.
J. Tian, G. H. Thomsen, H. Gong and W. J. Lennarz. (1997). Xenopus Cdc6 confers sperm binding competence to mature oocytes in the absence of meiotic maturation. Proceedings of the National Acadademy of Sciences, USA 94:10729-10734.
Other relevant papers on embryonic TGFß function from my postdoc years
G. H. Thomsen and D. A. Melton. (1993). Processed Vg1 protein is an axial mesoderm inducer in Xenopus. Cell 74: 433-441.
C. Dohrman, A. Hemmati-Brivanlou, G. H. Thomsen, A. Fields, T. Wolff, and D. Melton. (1993). Expression of activin mRNA during early development in Xenopus laevis. Developmental Biology 157: 474-483.
K. Wharton, G. H. Thomsen and W. Gelbart. (1991). Drosophila 60A gene, a new TGF-ß family member, is closely related to human bone morphogenetic proteins. Proceedings of the National Acadademy of Sciences, USA 88: 9214-9218.
G. H. Thomsen, T. Woolf, M. Whitman, S. Sokol, J. Vaughn, W. Vale and D. Melton. (1990). Activins are expressed early in Xenopus embryogenesis and can induce axial mesoderm and anterior structures. Cell 63: 485-493.
E. Mitrani, K. Ziv, G. H. Thomsen, Y. Shimoni, D. A. Melton and A. Bril. (1990). Activin can induce the formation of axial structures and is expressed in the hypoblast of the chick. Cell 63: 495-501.