Evolution of lbx spinal cord expression and function.

Area of Science:

• Developmental Biology
• Neuroscience
• Genetics

Background:

• Ladybird homeobox (Lbx) transcription factors are essential for muscle and nervous system development across diverse animal species.
• Amniotes possess two Lbx genes, with Lbx1 being the sole member expressed in the spinal cord.
• Teleosts exhibit three lbx genes, prompting an investigation into their specific roles and evolutionary origins.

Purpose of the Study:

• To investigate the expression patterns and functional roles of zebrafish lbx1a, lbx1b, and lbx2 in spinal cord development.
• To determine the evolutionary conservation and divergence of Lbx gene functions, particularly in spinal interneuron specification.
• To elucidate the sequential roles of lbx1a and lbx1b in the development of specific spinal interneuron subtypes.

Main Methods:

• Comparative gene expression analysis of lbx genes across different species (zebrafish, Scyliorhinus canicula, Xenopus tropicalis, mouse).
• Zebrafish mutant generation (lbx1a, lbx1b, and lbx1a;lbx1b double mutants) to assess functional consequences.
• Quantification of excitatory and inhibitory spinal interneuron populations in wild-type and mutant zebrafish.

Main Results:

• Zebrafish lbx1a, lbx1b, and lbx2 are expressed in distinct spinal cell types, with lbx1a showing conserved expression in dI4-dI6 interneurons.
• Zebrafish lbx1a function in spinal interneuron development is conserved with mouse Lbx1, regulating inhibitory and excitatory interneuron numbers.
• lbx1b mutants exhibit reduced inhibitory interneurons, and lbx1a;lbx1b double mutants show phenotypes similar to lbx1a single mutants, suggesting sequential roles.

Conclusions:

• Zebrafish lbx1a expression in spinal interneurons is likely an ancestral trait, while lbx1b's spinal progenitor cell expression is a derived function.
• lbx2 spinal expression appears to be specific to the ray-finned lineage.
• Lbx1b and Lbx1a may act sequentially in the specification of dI4 and dI6 spinal interneurons, with Lbx1a being crucial for suppressing excitatory fates.