The pioneer transcription factor Pax7 contains two DNA binding domains (DBD), a paired and a homeo domain. Previous work on Pax7 and the related Pax3 showed that each DBD binds a cognate DNA sequence, thus defining two targets of binding and possibly modalities of action. Genomic targets of Pax7 pioneer action leading to chromatin opening are enriched for composite DNA target sites containing juxtaposed sites for both paired and homeo domains. The present work investigated the implication of the DBDs in pioneer action. We show that the composite sequence is a higher affinity binding site and that efficient binding to this site involves both DBDs of the same Pax7 molecule. This binding is not sensitive to cytosine methylation of the DNA sites consistent with pioneer action within nucleosomal heterochromatin. Introduction of single amino acid mutations in either paired or homeo domain that impair binding to cognate DNA sequences showed that both DBDs must be intact for pioneer action. In contrast, only the paired domain is required for low affinity binding of heterochromatin sites. Thus, Pax7 pioneer action on heterochromatin requires unique protein:DNA interactions that are more complex compared to its simpler DNA binding modalities at accessible enhancer target sites.

Pioneer transcription factors direct cell differentiation by deploying new enhancer repertoires through their unique ability to target and initiate remodelling of closed chromatin. The initial steps of their action remain undefined although pioneers were shown to interact with nucleosomal target DNA and with some chromatin remodelling complexes. We now define the sequence of events that provide pioneers with their unique abilities. Chromatin condensation exerted by linker histone H1 is the first constraint on pioneer recruitment, and this establishes the initial speed of chromatin remodelling. The first step of pioneer action involves recruitment of the LSD1 H3K9me2 demethylase for removal of this repressive mark, as well as recruitment of the MLL complex for deposition of the H3K4me1 mark. Further progression of pioneer action requires passage through cell division, and this involves dissociation of pioneer targets from perinuclear lamins. Only then, the SWI/SNF remodeling complex and the coactivator p300 are recruited, leading to nucleosome displacement and enhancer activation. Thus, the unique features of pioneer actions are those occurring in the lamin-associated compartment of the nucleus. This model is consistent with much prior work that showed a dependence on cell division for establishment of new cell fates.

Pioneer transcription factors have key roles in development as master regulators of cell fate specification. Only a small fraction of all transcription factors have the pioneer ability that confers access to target genomic DNA sites embedded in so-called “closed” heterochromatin. This ability to seek and bind target sites within the silenced portion of the epigenome is the basis for their role in changing cell fate. Upon binding heterochromatin sites, pioneer factors trigger remodelling of chromatin from a repressed into an active organization. This action is typically exerted at enhancer regulatory sequences, thus allowing activation of new gene subsets. During pituitary development, the only pioneer with a well-documented role is Pax7 that specifies the intermediate lobe melanotrope cell fate. In this review, a particular focus is placed on this Pax7 function but its properties are also considered within the general context of pioneer factor action. Given their potent activity to reprogram gene expression, it is not surprising that many pioneers are associated with tumor development. Over-expression or chromosomal translocations leading to production of chimeric pioneers have been implicated in different cancers. We review here the current knowledge on the mechanism of pioneer factor action.

The pioneer transcription factor Pax7 contains two DNA binding domains (DBD), a paired and a homeo domain. Previous work on Pax7 and the related Pax3 had shown that each DBD can bind a cognate DNA sequence, thus defining two targets of binding and possibly modalities of action. Genomic targets of Pax7 pioneer action leading to chromatin opening are enriched for composite DNA target sites containing juxtaposed binding sites for both paired and homeo domains. The present work investigated the implication of both DBDs in pioneer action. We now show that the composite sequence is a higher affinity Pax7 binding site compared to either paired or homeo binding sites and that efficient binding to this site involves both DBDs. We also show that a Pax7 monomer binds composite sites and that methylation of cytosines within the binding site does not affect binding, which is consistent with pioneer action exerted at methylated DNA sites within nucleosomal heterochromatin. Finally, introduction of single amino acid mutations in either the paired or homeo domain that impair binding to cognate DNA sequences showed that both DBDs must be intact for pioneer action. In contrast, only the paired domain is required for low affinity binding of heterochromatin sites. Thus, Pax7 pioneer action on heterochromatin requires unique protein:DNA interactions that are more complex compared to its simpler DNA binding modalities at accessible enhancer target sites.Significance StatementPioneer transcription factors have the unique ability to recognize DNA target sites within closed heterochromatin and to trigger chromatin opening. Only a fraction of the heterochromatin recruitment sites of pioneers are subject to chromatin opening. The molecular basis for this selectivity is unknown and the present work addressed the importance of DNA sequence affinity for selection of sites to open. The pioneering ability of the pioneer factor Pax7 is not strictly determined by affinity or DNA sequence of binding sites, nor by number or methylation status of DNA sites. Mutation analyses showed that recruitment to heterochromatin is primarily dependent on the Pax7 paired domain whereas the ability to open chromatin requires both paired and homeo DNA binding domains.