Nkx6 proteins specify one zebrafish primary motoneuron subtype by regulating late islet1 expression
2007
The ability of animals to carry out their normal behavioral repertoires
requires exquisitely precise matching between specific motoneuron subtypes and
the muscles they innervate. However, the molecular mechanisms that regulate
motoneuron subtype specification remain unclear. Here, we use individually
identified zebrafish primary motoneurons to describe a novel role for Nkx6 and
Islet1 proteins in the specification of vertebrate motoneuron subtypes. We
show that zebrafish primary motoneurons express two related Nkx6 transcription
factors. In the absence of both Nkx6 proteins, the CaP motoneuron subtype
develops normally, whereas the MiP motoneuron subtype develops a more
interneuron-like morphology. In the absence of Nkx6 function, MiPs exhibit
normal early expression of islet1 , which is required for motoneuron
formation; however, they fail to maintain islet1 expression.
Misexpression of islet1 RNA can compensate for loss of Nkx6 function,
providing evidence that Islet1 acts downstream of Nkx6. We suggest that Nkx6
proteins regulate MiP development at least in part by maintaining the
islet1 expression that is required both to promote the MiP subtype
and to suppress interneuron development.
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