Interplay of BAF and MLL4 promotes cell type-specific enhancer activation

Area of Science:

• Epigenetics
• Molecular Biology
• Cellular Differentiation

Background:

• Cell type-specific enhancers are regulated by lineage-determining transcription factors (LDTFs) and chromatin modifiers.
• The SWI/SNF chromatin remodeling complex BAF and MLL4 (KMT2D) are known to be involved in enhancer activation.
• The precise interplay between BAF and MLL4 in this process is not fully understood.

Purpose of the Study:

• To investigate the functional relationship between the BAF complex and MLL4 in activating cell type-specific enhancers.
• To elucidate the role of BAF and MLL4 in the context of adipogenesis, a model for cell differentiation.

Main Methods:

• Utilized adipogenesis as a model system to study enhancer activation.
• Depleted key subunits of the BAF complex (SMARCA4, SMARCB1) and MLL4 to assess their necessity for differentiation.
• Examined the colocalization and reciprocal regulation of BAF and MLL4 on active enhancers.
• Focused on enhancer activation mediated by the pioneer transcription factor C/EBPβ.

Main Results:

• Identified BAF as the predominant SWI/SNF complex cooperating with MLL4 and LDTFs on active enhancers during adipogenesis.
• Demonstrated that BAF is essential for cell differentiation, while the promoter-enriched SWI/SNF complex PBAF is not.
• Showed that BAF and MLL4 reciprocally regulate each other's binding to active enhancers.
• Provided direct evidence for an interdependent relationship between BAF and MLL4 in activating cell type-specific enhancers, even without full differentiation.

Conclusions:

• BAF and MLL4 exhibit a reciprocal regulatory relationship essential for activating cell type-specific enhancers.
• This interaction forms a positive feedback loop that promotes LDTF-dependent enhancer activation and facilitates cell differentiation.
• The findings highlight a critical mechanism for precise gene regulation during cell fate determination.