Dr. Dongbin Xu from the Children’s Hospital of Philadelphia (CHOP) will kindly share with us two independent stories, which cover molecular regulation of apoptosis and mechanism study of Cornelia de Lange Syndrome. As a major approach, animal model system with Drosophila and induced pluripotent stem (iPS) cells will be discussed.
(1) Identification of Essential Apoptotic Genes in Drosophila melanogaster
Apoptosis is a normal physiological cell suicide process which is essential for tissue homeostasis and normal development of metazoans. Misregulation of apoptosis is associated with many developmental defects and human diseases. The genes involved in the regulation and execution of apoptosis are highly conserved in humans and flies. Caspases are the executioners of cell suicide. Because of the unavailability of specific fly mutants, the developmental function of many caspase genes and genetic relationship between caspases and apoptotic components were undefined in Drosophila. We isolated several mutant alleles of the initiator caspase gene dronc, the effector casase drICE, and the Mediator component Cyclin C from the GMR-hid eyFLP/FRT screens which is designed to isolate mutants of recessive cell death genes in Drosophila melanogaster. Characterization of these mutants defined that they are essential for developmental cell death in Drosophila. dronc is required for most, but not all, cell death in Drosophila. drICE is required for apoptosis in many cells and it shares redundancy with another effector caspase gene, dcp-1, in a subset of cells in Drosophila. The genetic relationship between caspases and other apoptotic components was established through mutant analysis. We also found that Cyclin C and its kinase partner Cdk8 are required for prompt transcriptional induction of dronc in cell killing contexts. This suggests transcriptional activation of the initial caspase in apoptotic responses. In short, we defined the essential pro-apoptoic function of dronc, drICE, and Cyclin C in Drosophila and provided evidences to support a novel mechanism for regulation of dronc transcription. In the long run, these studies will help us decipher the complicated regulatory mechanism of cell death in humans.
(2) Investigation of Cdls-associated Neuronal Defects in Drosophila and iPS Cells
Cornelia de Lange Syndrome (CdLS) is a dominant genetic disease characterized by a striking constellation of birth defects including malformations of many organ systems, growth delay, and mental retardation. Approximately 50% of CdLS probands have mutations in the NIPBL/Scc2 gene. The NIPBL protein is required for the dynamic association of the multi-protein cohesin complex with sister chromatids during cell division. About 6% CdLS probands have mutations in other two essential cohesin components, SMC1 and SMC3I. While the canonical role of cohesin has been reported in the regulation of sister chromatid cohesion and segregation during mitosis and meiosis, recent research has indicated that the pathogenic mechansim by which disruption of cohesin function results in the CdLS phenotpye is through cohesin's more recently described role as a regulator of gene expression. Two recent studies in Drosophila demonstrated that cohesin plays a postmitotic function in axon pruning during neuronal development with evidience suggesting that this is mediated through regulating the expression of an ecdyson receptor EcR-B1. While the potential applicability of these findings towards understanding cohesin's contribution to cognitive defcits in humans is exciting there are still many question that remain to be answered: (A) It has not been tested if the analagous NIPBL (nipped-b in Drosophila) or SMC1A (smc1 in Drosophila) mutations identified in CdLS probands also cause neuronal defects and (B) It has not been investigated if the differentiation defects and correlated dysregulated cohesin target genes seen in Drosophila are analagous to what is occuring in the central nervous system of CdLS probands. Based on current findings, we hypothesize that CdLs-associated mutations cause postmitotic differentiation defects in the central nervous system through dysregulated expression of a conserved subset of target genes which is directly related to the cognitive phenotypes seen in inidividuals with CdLS. We plan to test this hypothesis through an animal model system using Drosophila and a human model system using induced pluripotent stem (iPS) cells. Using these two model systems will allow for a synergistic approach towards understanding the developmental impact of cohesin disruption in the developing central nervous system as well as providing the reagents to examine potential therapeutic modalities for correcting identified regulatory disruptions. Research plans and current progression will be presented.
Want to know more? Please bring your curiosity, questions and come join us at BRBII/III Room 801, 6-7:30pm Friday (Nov. 20)!
