Dlx5 and mef2 regulate a novel runx2 enhancer for osteoblast-specific expression

Area of Science:

• Molecular Biology
• Developmental Biology
• Genetics

Background:

• Runx2 is a critical transcription factor for osteoblast differentiation and chondrocyte maturation.
• Mesenchymal stem cell lineage determination to osteoblasts requires Runx2 expression.
• The distal promoter of Runx2, while expressed in osteoblasts, failed to direct reporter gene expression specifically to these cells.

Purpose of the Study:

• To identify regulatory regions responsible for osteoblast-specific Runx2 expression.
• To characterize the function and regulatory mechanisms of a newly discovered enhancer element.

Main Methods:

• Generation of bacterial artificial chromosome (BAC) green fluorescent protein (GFP) reporter mice for Runx2 locus.
• Serial deletion analysis of the Runx2 locus to pinpoint regulatory elements.
• Identification and characterization of a 343-bp enhancer located upstream of the Runx2 distal promoter.
• Analysis of transcription factor binding and activity using cell lines (MC3T3-E1, C3H10T1/2) and primary osteoblasts.
• Histone modification analysis of the enhancer region in primary osteoblasts.

Main Results:

• A 343-bp enhancer, approximately 30 kb upstream of the Runx2 distal promoter, was identified.
• This enhancer directed GFP expression specifically to osteoblasts in reporter mice.
• The enhancer sequence showed high conservation across multiple species (mouse, human, dog, horse, opossum, chicken).
• Dlx5 and Mef2 directly bound to the enhancer, with their binding sites being essential for osteoblast-specific expression.
• Several transcription factors (Dlx5, Mef2c, Tcf7, Ctnnb1, Sp7, Smad1, Sox6) synergistically upregulated enhancer activity, while Msx2 downregulated it.
• The enhancer exhibited typical epigenetic modifications associated with active enhancers, including H2A.Z and specific histone H3 modifications.

Conclusions:

• The identified 343-bp enhancer contains sufficient elements for directing Runx2 expression specifically to osteoblasts.
• Dlx5 and Mef2 play essential roles in the osteoblast-specific activation of this enhancer.
• A complex of transcription factors, including Dlx5, Mef2, Tcf7, Ctnnb1, Sp7, Smad1, and Sox6, forms an enhanceosome that regulates Runx2 expression.