Chick Lrrn2, a novel downstream effector of Hoxb1 and Shh, functions in the selective targeting of rhombomere 4 motor neurons.
BACKGROUND
Capricious is a Drosophila adhesion molecule that regulates specific targeting of a subset of motor neurons to their muscle target. We set out to identify whether one of its vertebrate homologues, Lrrn2, might play an analogous role in the chick.


RESULTS
We have shown that Lrrn2 is expressed from early development in the prospective rhombomere 4 (r4) of the chick hindbrain. Subsequently, its expression in the hindbrain becomes restricted to a specific group of motor neurons, the branchiomotor neurons of r4, and their pre-muscle target, the second branchial arch (BA2), along with other sites outside the hindbrain. Misexpression of the signalling molecule Sonic hedgehog (Shh) via in ovo electroporation results in upregulation of Lrrn2 exclusively in r4, while the combined expression of Hoxb1 and Shh is sufficient to induce ectopic Lrrn2 in r1/2. Misexpression of Lrrn2 in r2/3 results in axonal rerouting from the r2 exit point to the r4 exit point and BA2, suggesting a direct role in motor axon guidance.


CONCLUSIONS
Lrrn2 acts downstream of Hoxb1 and plays a role in the selective targeting of r4 motor neurons to BA2.
Andreae
Laura C
LC
MRC Centre for Developmental Neurobiology, King's College London, New Hunt's House, Guy's Campus, London SE1 1UL, UK. laura.andreae@kcl.ac.uk
Lumsden
Andrew
A
Gilthorpe
Jonathan D
JD
D000367:Age Factors; D000818:Animals; D015816:Cell Adhesion Molecules, Neuronal / Q000235:genetics / Q000378:metabolism*; D002465:Cell Movement; D002642:Chick Embryo; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D053823:Hedgehog Proteins / Q000378:metabolism*; D018398:Homeodomain Proteins / Q000378:metabolism*; D009046:Motor Neurons / Q000378:metabolism*; D012249:Rhombencephalon / Q000166:cytology* / Q000196:embryology; D015398:Signal Transduction / Q000502:physiology
D015816:Cell Adhesion Molecules, Neuronal; C101121:HOXB1 homeodomain protein; D053823:Hedgehog Proteins; D018398:Homeodomain Proteins