The microRNA-processing enzyme Dicer is dispensable for somite segmentation but essential for limb bud positioning

Area of Science:

• Developmental Biology
• Molecular Biology
• Genetics

Background:

• MicroRNAs (miRNAs) are crucial regulators of gene expression, initially identified for their role in developmental timing.
• The enzyme Dicer processes precursor miRNAs into mature forms, enabling their regulatory functions.
• Mesoderm development, including somitogenesis and limb bud formation, involves complex temporal and spatial patterning.

Purpose of the Study:

• To investigate the role of miRNAs, processed by Dicer, in mouse embryonic mesoderm development, specifically focusing on somitogenesis and limb bud positioning.
• To determine if Dicer inactivation affects the segmentation process of somites or the formation and positioning of limb buds.
• To elucidate the molecular mechanisms by which miRNAs might regulate genes involved in limb bud development.

Main Methods:

• Utilized a T (Brachyury)-Cre mouse line to achieve conditional inactivation of the Dicer gene in the mesoderm lineage.
• Analyzed Dicer mutant embryos for defects in anterior-posterior axis formation, somitogenesis, and limb bud development.
• Examined gene expression patterns critical for limb positioning, including Hand2, Tbx3, and Gli3, in wild-type and mutant embryos.
• Performed in vitro experiments to validate miRNA-mediated regulation of target genes (e.g., mir-363 on Hand2/Tbx3).

Main Results:

• Dicer inactivation in mouse mesoderm resulted in a reduced anterior-posterior axis and increased embryonic cell death.
• Somite segmentation (number) was unaffected, but somite size was reduced and later somites were caudalized.
• Limb bud positioning was significantly altered, with posterior shifting and delayed initiation of hindlimb bud formation.
• Expression of key limb positioning genes (Hand2, Tbx3, Gli3) was perturbed in the prospective limb bud field of Dicer mutants.
• In vitro studies confirmed that specific miRNAs (e.g., mir-363) can directly regulate Hand2 and Tbx3 expression.

Conclusions:

• Dicer-dependent miRNA processing is dispensable for the fundamental process of somite segmentation in mouse embryonic mesoderm.
• However, Dicer-mediated miRNA activity is essential for the correct positioning and timely initiation of limb bud development.
• This study highlights a critical role for miRNAs in regulating gene expression critical for patterning lateral plate mesoderm derivatives, such as limb buds.