Ectodermal Organ Development Is Regulated by a microRNA-26b-Lef-1-Wnt Signaling Axis

Abstract

The developmental role of <i>Lef-1</i> in ectodermal organs has been characterized using <i>Lef-1</i> murine knockout models. We generated a <i>Lef-1</i> conditional over-expression (COEL) mouse to determine the role of <i>Lef-1</i> expression in epithelial structures at later stages of development after endogenous expression switches to the mesenchyme. <i>Lef-1</i> over expression (OE) in the oral epithelium creates a new dental epithelial stem cell niche that significantly increases incisor growth. These data indicate that <i>Lef-1</i> expression is switched off in the dental epithelial at early stages to maintain the stem cell niche and regulate incisor growth. Bioinformatics analyses indicated that <i>miR-26b</i> expression increased coinciding with decreased <i>Lef-1</i> expression in the dental epithelium. We generated a murine model over-expressing <i>miR-26b</i> that targets endogenous <i>Lef-1</i> expression and <i>Lef-1</i>-related developmental mechanisms. <i>miR-26b</i> OE mice have ectodermal organ defects including a lack of incisors, molars, and hair similar to the <i>Lef-1</i> null mice. <i>miR-26b</i> OE rescues the <i>Lef-1</i> OE phenotype demonstrating a critical genetic and developmental role for <i>miR-26b</i> in the temporal and spatial expression of <i>Lef-1</i> in epithelial tissues. <i>Lef-1</i> expression regulates Wnt signaling and Wnt target genes as well as cell proliferation mechanisms, while <i>miR-26b</i> OE reduced the levels of Wnt target gene expression. The extra stem cell compartment in the <i>COEL</i> mice expressed <i>Lef-1</i> suggesting that <i>Lef-1</i> is a stem cell factor, which was absent in the <i>miR-26b OE/COEL</i> rescue mice. This is the first demonstration of a microRNA OE mouse model that has ectodermal organ defects. These findings demonstrate that the levels of <i>Lef-1</i> are critical for development and establish a role for <i>miR-26b</i> in the regulation of ectodermal organ development through the control of <i>Lef-1</i> expression and an endogenous stem cell niche.