The mammalian chromatin---remodeling BAF complex controls diverse biological processes, often in response to environmental signals. Such signals are well known to regulate BAF recruitment to target genes. For example, upon TLR4 signaling in macrophages, the BAF complex is recruited to many TLR4 target genes, where it remodels their chromatin to promote gene expression. Unexpectedly, we have found that BAF recruitment is insufficient for remodeling a subset of TLR4 target genes; a second signal, delivered by calcium/calmodulin (CaM), is additionally required. CaM directly binds the BAF complex to stimulate remodeling without affecting BAF recruitment. This is the first time CaM is shown to regulate a chromatin remodeler, and the first time a signaling pathway is known to regulate a remodeler independently of recruitment. In this grant, we will address how CaM stimulates remodeling (Aim 1), which is currently a total mystery. Our hypothesis is that some repressors of remodeling are tethered to CaM---dependent BAF target genes, and CaM uses the BAF complex to hitchhike to the target genes, where it inactivates the repressor to allow chromatin remodeling. We will test these ideas in macrophages, using molecular genetic methods. This grant will also test the idea that the CaM---BAF signaling pathway found in macrophages is of general importance, given that both CaM and BAF are widely expressed (Aim 2). Our preliminary data suggest the CaM---BAF pathway indeed operates in T cells, which we will further explore using mice bearing point mutations that block CaM---BAF interaction. Our study will shed light on the mechanisms, generality and biological importance of the novel Calcium---CaM---BAF pathway, and of interest to both signaling and chromatin fields.
Environmental signals produce biological effects often by altering gene expression, and it is of crucial importance to understand how the signals are relayed to the target genes. We have found a novel, potentially ubiquitous signal transduction pathway, and will define the mechanism of signal transduction and immunological function of this pathway, which has obvious medical implications.
