Studies in both vertebrates and invertebrates have defined signaling pathways, such as Notch, Wnt and BMP as well as transcription factors such as the HLH and Sox families that are essential for neurogenesis and neural differentiation. These and other regulators appear to operate within a developmental context, which is in part defined by a neural chromatin landscape regulating genomic accessibility to ubiquitous signaling and developmental pathways. Recent studies have shown that neurons contain a highly specific polymorphic family of ATP-dependent chromatin remodeling complexes, which resemble yeast SWI/SNF complexes. ATP- dependent chromatin remodeling complexes are thought to use the energy of ATP hydrolysis to control transitions between stable epigenetic states by exchanging or mobilizing nucleosomes thereby altering histone codes. Neural progenitors have a distinct complex (npBAF) distinguished by its subunit composition. The npBAF complex is essential for self renewal of neural progenitor/stem cells. Near the last mitotic division of neurons, two of the subunits (BAF45a and BAF53a) are removed from npBAF complexes and replaced by homologous subunits (BAF45b and BAF53b) to give rise to a new family of neural-specific complexes (neural BAF or nBAF), which are required for dendrite morphogenesis and other post mitotic functions. This exchange of ATP-dependent chromatin remodeling complexes also occurs in the course of directed differentiation of ES cells to neurons. Our studies are designed to understand the mechanism and biologic meaning of the switch between these two essential epigenetic states. Because the expression of the neural progenitor subunits of the complexes (BAF45a and BAF53a) is mutually exclusive with the expression of the subunits of the neural complexes (BAF45b and BAF53b), negative feedback mechanisms are a possible contributor to the switch. We will define the intrinsic regulators that bring about this essential switch in subunit composition. In the second specific aim we will use this information to define the extrinsic regulators using an unbiased genetic screen. We will then test the role of these polymorphic complexes and their regulators by preparing mice carrying a conditional deletion of the neural-specific subunits and also mice in which the subunit composition is altered. One family of subunits (BAF45) contains two PHD domains and a Kruppel domain and hence might contribute to the specific retargeting of these complexes to different genomic sites at different developmental stages. The role of these subunits will be tested using the conditional mutant mice described above as well as transgenic animals in which the deleted subunits are replaced with chimeric genes. To help understanding targeting and the mechanism of action of these neural specific ATP-dependent chromatin remodeling complexes we will develop a generally useful technique for precise temporal targeting of proteins that interact with DNA or chromatin and use it to understand the actions of the neural progenitor and neural chromatin remodeling complexes. At the conclusion of our studies we will have defined the mechanisms used by an essential epigenetic switch to regulate specific aspects of neural development. These studies should contribute to a fundamental understanding of neural development and also help with the goal of production of specific cell types from pluripotent cells for uses in regenerative medicine.
One of the major challenges of modern biology and medicine is to repair damaged or diseased tissues. Avenues to accomplish this have grown from an understanding of the fundamental processes controlling the development of the embryo. One of these processes is the control of accessibility of our genetic material to regulatory mechanisms that allow an orderly use of genes to make tissues and organs such as the heart, lungs, immune system and brain. We hope to understand how the accessibility of genetic material is controlled during the formation of the brain and how it differs from other tissues. Our studies might provide new avenues for the production of tissues and cells for regeneration of diseased or damaged tissues.
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