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POU4F3 Pioneer Activity Enables ATOH1 to Drive Diverse Mechanoreceptor Differentiation Through a Feed-Forward Epigenetic Mechanism

ABSTRACT: During embryonic development, hierarchical cascades of transcription factors interact with lineage specific chromatin structures to control the sequential steps in the differentiation of specialized cell types. While examples of transcription factor cascades have been well documented, the mechanisms underlying developmental changes in accessibility of cell type-specific enhancers remain poorly understood. Here we show that the transcriptional ‘master regulator’, ATOH1 – which is necessary for the differentiation of two distinct mechanoreceptor cell types, hair cells in the inner ear, and Merkel cells of the epidermis – is unable to access much of its target enhancer network in the progenitor populations of either cell type when it first appears, imposing a block to further differentiation. This block is overcome by a feed-forward mechanism in which ATOH1 first stimulates expression of POU4F3, which subsequently acts as a pioneer factor to provide access to closed ATOH1 enhancers, allowing hair cell and Merkel cell differentiation to proceed. Our analysis also indicates the presence of both shared and divergent ATOH1/POU4F3-dependent, enhancer networks in hair cells and Merkel cells. These cells share a deep developmental lineage relationship, deriving from their common epidermal origin, and suggesting that this feed-forward mechanism preceded the evolutionary divergence of these very different mechanoreceptive cell types.

ORGANISM(S): Mus musculus

PROVIDER: GSE150391 | GEO | 2021/07/09

REPOSITORIES: GEO

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Project description:Neural basic helix-loop-helix (bHLH) transcription factors are important for the differentiation and cell type specification of neurons. They are thought to share direct downstream targets in their common role as neuronal differentiation factors, but have distinct targets with respect to their cell type specific roles. Little is known about distinct cell-type specific bHLH targets as previous work did not distinguish these from common targets. Based on previous genetic evidence, we hypothesize that bHLH transcription factors have unique targets for their function in regulating neuronal sub-type specification. Atoh1 (Math1) is a bHLH transcription factor that specifies different cell types of the proprioceptive pathway in mammals such as the dorsal interneuron 1 population of the developing neural tube. Using microarray analyses of neighboring specific bHLH sorted populations from developing mouse neural tubes, we determine transcripts unique to the Atoh1-derived population and not those common to bHLH transcription factors in related neural progenitor populations. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) experiments of native tissue followed by enhancer reporter analyses identified five direct cell-type specific targets of Atoh1 in vivo: Klf7, Rab15, Rassf4, Selm, and Smad7, along with their Atoh1-responsive enhancers. These Atoh1 targets were found from native tissue in the appropriate developmental context and have diverse functions that range from transcription factors to regulators of endocytosis and signaling pathways. Only Rab15 and Selm are expressed across several different Atoh1-specified cell types including external granule cells (EGL) in the developing cerebellum, hair cells of the inner ear, and Merkel cells, demonstrating that even within Atoh1 lineages, not all Atoh1 specific targets are shared. Our work establishes on a molecular level that the neuronal differentiation bHLH transcription factors also have distinct targets for their roles in neuronal sub-type specification. From this work, we can begin to address how bHLH transcription factors are able to specify unique cell types and initiate programs that organize neuronal diversity. Gene expression analysis: Two samples, Atoh1-GFP and dNeurog1-GFP, were analyzed. Two biological replicates of each. Atoh1-GFP transgenics are mice with GFP inserted into a BAC that drives GFP to the Atoh1-derived population of the developing neural tube marking the dorsal interneuron 1 population (Raft et al. Development 2007). dNeurog1-GFP transgenics are mice with a Neurog1 enhancer that drives GFP to the dorsal part of the developing neural tube marking the dorsal interneuron 2 population (Nakada et al. Dev Bio 2004). Chip-seq analysis: Series GSE22111

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POU4F3 Pioneer Activity Enables ATOH1 to Drive Diverse Mechanoreceptor Differentiation Through a Feed-Forward Epigenetic Mechanism

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