Functional Diversification of Motor Neuron-specific Isl1 Enhancers during Evolution

PLoS Genet. 2015 Oct 8;11(10):e1005560. doi: 10.1371/journal.pgen.1005560. eCollection 2015 Oct.

Abstract

Functional diversification of motor neurons has occurred in order to selectively control the movements of different body parts including head, trunk and limbs. Here we report that transcription of Isl1, a major gene necessary for motor neuron identity, is controlled by two enhancers, CREST1 (E1) and CREST2 (E2) that allow selective gene expression of Isl1 in motor neurons. Introduction of GFP reporters into the chick neural tube revealed that E1 is active in hindbrain motor neurons and spinal cord motor neurons, whereas E2 is active in the lateral motor column (LMC) of the spinal cord, which controls the limb muscles. Genome-wide ChIP-Seq analysis combined with reporter assays showed that Phox2 and the Isl1-Lhx3 complex bind to E1 and drive hindbrain and spinal cord-specific expression of Isl1, respectively. Interestingly, Lhx3 alone was sufficient to activate E1, and this may contribute to the initiation of Isl1 expression when progenitors have just developed into motor neurons. E2 was induced by onecut 1 (OC-1) factor that permits Isl1 expression in LMCm neurons. Interestingly, the core region of E1 has been conserved in evolution, even in the lamprey, a jawless vertebrate with primitive motor neurons. All E1 sequences from lamprey to mouse responded equally well to Phox2a and the Isl1-Lhx3 complex. Conversely, E2, the enhancer for limb-innervating motor neurons, was only found in tetrapod animals. This suggests that evolutionarily-conserved enhancers permit the diversification of motor neurons.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Enhancer Elements, Genetic / genetics*
  • Evolution, Molecular
  • Gene Expression Regulation, Developmental
  • Homeodomain Proteins / biosynthesis*
  • Homeodomain Proteins / genetics
  • LIM-Homeodomain Proteins / biosynthesis*
  • LIM-Homeodomain Proteins / genetics
  • Lampreys / genetics
  • Mice
  • Motor Neurons / metabolism
  • Motor Neurons / physiology
  • Rhombencephalon / metabolism
  • Rhombencephalon / physiology
  • Spinal Cord / metabolism
  • Spinal Cord / physiology
  • Transcription Factors / biosynthesis*
  • Transcription Factors / genetics

Substances

  • Homeodomain Proteins
  • LIM-Homeodomain Proteins
  • Lhx3 protein
  • Phox2a protein, mouse
  • Transcription Factors
  • insulin gene enhancer binding protein Isl-1

Grant support

Support for MRS and this research was provided by grants from National Research Foundation of Korea (NRF-2013R1A1A2058548), the Cell Dynamics Research Center, National Research Foundation of Korea (2007-0056157), Intergrative Aging Research Center of Gwangju Institute of Science and Technology and Korea Health Industry Development Institute (HI14C3484). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.