Abstract
BackgroundCapricious 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.ResultsWe 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.ConclusionLrrn2 acts downstream of Hoxb1 and plays a role in the selective targeting of r4 motor neurons to BA2.
Highlights
Capricious is a Drosophila adhesion molecule that regulates specific targeting of a subset of motor neurons to their muscle target
Lrrn2 is expressed in presumptive r4 territory from early stages Lrrn2 is first expressed in chick embryos at Hamburger and Hamilton stage 4 (HH4) in a triangular region around Hensen's node (Figure 1A)
Expression is activated in the neural plate at HH5 (Figure 1B), becoming more robust by HH7, when expression extends to posterior regions (Figure 1C)
Summary
Capricious is a Drosophila adhesion molecule that regulates specific targeting of a subset of motor neurons to their muscle target. The key regulators of AP patterning for the posterior central nervous system (CNS) are the Hox genes, which are expressed in a segmental fashion in the hindbrain and have anterior expression boundaries that correspond to specific rhombomere boundaries [1,2,3,4]. Much as their Drosophila homologues confer segment-specific identity upon the body segments of the fly embryo, the vertebrate Hox genes confer segment-specific identity upon rhombomeres: gain or loss of function results in homeotic transformations [5,6,7,8,9,10]. Shh is expressed by the floor plate and notochord, and regulates the expression of a (page number not for citation purposes)
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