Basic Sciences Research

Maslen Laboratory

The major research focus is on the determining the molecular genetic basis of congenital heart defects. A current interest in the laboratory is the association between the cirrin gene and cardiac malformations known as endocardial cushion defects. The cirrin gene encodes a novel extracellular protein that is expressed in the developing heart. We have determined that deletion of the cirrin gene correlates with the occurrence of endocardial cushion defects in the cytogenetic disorder known as 3p- syndrome. Investigations are now underway to determine if cirrin gene mutations give rise to congenital heart defects of endocardial origin. Additional studies include determining the function of cirrin through the development of a cirrin knockout mouse as a model, and discovering interacting proteins using genetic and biochemical approaches. Identification of the cirrin biochemical pathway will lead to an understanding of the role of cirrin in normal heart development and the pathogenesis of heart malformations.

Another area of interest is the role of the elastin fiber system in the developing heart. Elastic microfibrils are macromolecular structures found in most connective tissues. They make up a multifunctional network that contributes to the elasticity of tissues, but has unknown functions in organ development. Two heritable disorders of elastic microfibrils, Marfan syndrome and congenital contractural arachnodactyly (CCA), provide insight into the structure and functions of elastic microfibrils and their different roles in heart development. Individuals with Marfan syndrome have normal heart development, but frequently manifest cardiovascular malfunction later in life. By contrast, about 15% of individuals with CCA are born with cardiac malformations. Comparative studies that reveal the molecular mechanisms behind these closely related disorders will clarify the role of the elastic microfibril system in heart development.

Heart and Smooth Muscle Development in Drosophila and Mice

My lab studies heart and smooth muscle development in Drosophila and mice. We take advantage of the power of developmental genetics in Drosophila and evolutionary conservation of developmental mechansims to understand mammalian organogenesis. We have developed a method to identify targets of transcriptional regulation by the homeodomain protein Tinman. Tinman is a conserved determinant of cardiogenesis. Homozygous fly mutants have no heart or smooth muscle. Heterozygous human mutants have been identified with a broad spectrum of congenital heart lesions.

We identified a novel, secreted signaling protein called Jelly belly (Jeb) as a target of Tinman regulation. In jeb mutants no mature smooth muscles develop. Smooth muscle precursors are normally specified but they fail to migrate and differentiate. Jeb is synthesized in cells immediately adjacent to the smooth muscle precursors, secreted from them and taken up by the smooth muscle precursors.

We have recently identified a high affinity receptor for Jeb that belongs to the insulin receptor superfamily. It is the Drosophila homologue of the human proto-oncogene Anaplastic Lymphoma Kinase. The normal function of this receptor has been unknown. We are performing mutagenesis experiments to determine the mechanism of ligand binding and receptor activation. The identification of this receptor as the Jeb receptor makes clear that this signaling system is conserved in mammals as well. We are exploiting this conservation to extend our work on Jeb signaling in Drosophila to mammalian development and disease.

Transcriptional Regulation of Development in Drosophila

During development, cells maintain or change their specific identities by activating or silencing specific genes in response to the signals received from both cell-cell contacts and the environment. The interaction of the transcriptional coactivator CBP with a number of different signal-responsive transcriptional activators mediates and integrates the input from several different signaling pathways into a specific transcriptional response. CBP functions as a bridge between the activated transcription factors and the basal transcriptional machinery. In addition, it has an intrinsic acetyltransferase activity that is hypothesized to modify the chromatin surrounding the activated promoter making it more accessible to protein complexes that stimulate transcription. We have cloned Drosophila CBP (dCBP) and generated mutations in this gene in an effort to understand how the various signals are integrated at the level of transcription to elicit specific cellular responses.

Two proteins that interact with dCBP are the transcription factor cubitus interruptus that mediates hedgehog signaling during development and a SIR2-like deacetylase. The fact that dCBP has histone acetyltransferase activity suggests that this interaction may be critical for the maintenance of gene silencing during development. We are using genetic and molecular techniques to determine how these two interactions activate/silence transcription in response to various developmental signaling systems. The mutational analysis of dCBP also demonstrated that this factor is required for the S phase checkpoint of the cell cycle. These results suggest that dCBP and its mammalian counterpart may function as signal-responsive chromatin modifying proteins rather than solely as transcriptional coactivators.