ATP-Dependent Chromatin Remodeling in Development

The Interface of Signal Transduction and Chromatin Regulation As the pathways that carry information from the cell membrane to the nucleus are elucidated a paradox has became apparent: only about a dozen generic signaling pathways appear to be responsible for carrying nearly all the information from the cell membrane to the nucleus to result in the development of the embryo and cellular responses to the environment.Where does the specificity of signaling then arise? At least part of the answer appears to be the preexisting chromatin context in which the signal is received. Inspired by work in yeast implicating ATP-dependent chromatin remodeling in mating type switching, several years ago we isolated a chromatin remodeling complex similar to the ATP-dependent yeast SWI/SNF complex and cloned the genes that encode the subunits of this complex. These subunits are encoded by gene families and each member appears to be able to associate with each member of the subunits families. This combinatorial assembly leads to a predicted 196 different complexes.

While recent data indicate that these complexes have very different functions, the true meaning of this combinatorial diversity is still to be understood. In addition, mammalian Brg complexes have b-actin as an intrinsic subunit, while the yeast SWI/SNF complex does not. Finally, mammalian BAF complexes have 5 subunits that encode DNA-binding domains suggesting that the mechanism of recruitment may be by direct DNA binding rather than transcription factor recruitment as in the case of the yeast complex.

The questions we would like to answer include: What is the meaning of combinatorial assembly of these complexes and do the combinatorially assembled complexes have different developmental functions?

What is the role of b-actin in the complexes and does it interface with the diverse signaling pathways controlling phosphoinositols and actin polymerization.

Finally, we would like to understand the mechanism of recruitment of the complex to its target genes and weather DNA binding by mammalian BAF complexes is critical to target gene activation.

Neural Specific Chromatin Remodeling

ABOVE: bBAF53b in neurons (rt) and glia (lf) costained with Tuj and GAFP notice the lack of nuclear staining in the glia(rught), yet robust staining of nuclei in neurons(left).

Recently we have discovered that mammalian neurons contain a specific class of chromatin remodeling complexes based on the neural specific expression of BAF53b, an actin related protein resembling ARP3 or 4. This complex is expressed post-mitotically in neurons and seems to be combinatorially assembled. We hope to understand why neurons are the only cell type to have a specific chromatin remodeling complex and how this complex functions in development of the vertebrate nervous system.