Abstracts - Society for Developmental Biology

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delayed and stunted as monitored by expression of Tbx5, the earliest known forelimb marker. Rdh10 mutant hearts exhibit
ectopic expression of not only Fgf8 but also Isl1, an FGF8-target gene. Elimination of Isl1 function prevented heart
enlargement in RA-deficient embryos, but did not rescue forelimb initiation. Our results show that defects inforelimb
initiation during RA deficiency are not due to heart enlargement, but are likely caused by excess FGF signaling in the
developing trunk (derived from either the heart or caudal progenitor zone) that prevents initiation of Tbx5 expression in
lateral plate mesoderm where the forelimb field normally arises.
Program/Abstract # 354
Spot, a new mouse model for Hirschsprung disease and Waardenburg-Shah syndrome
Bergeron, Karl-Frederik, UQAM Sciences Biologiques, Montreal, United States; Silversides, David W. (Centre de
recherche en reproduction animale (CRRA), St-Hyacinthe, Canada); Pilon, Nicolas (UQAM, Montreal, Canada)
Hirschsprung disease (HSCR), or aganglionic megacolon, is a human developmental disorder of the enteric nervous system
(ENS). HSCR has an incidence of 1 in 5000 live births and is characterized by a lack of ENS ganglions in the colon,
leading to a neuromuscular defect that prevents intestinal motility. ENS ganglions are entirely derived from neural crest
cells (NCC). This transient, multipotent cell population gives rise to diverse cell lineages during vertebrate development by
undergoing extensive migration, proliferation and differentiation. In HSCR patients, defects in any of these key
developmental processes result in a failure of NCC to completely colonize the intestines. HSCR can also occur in
association with other features, as in Waardenburg-Shah syndrome (WS) where it is observed in combination with
pigmentation abnormalities and sensorineural deafness. The genetics of HSCR is complex and almost 80% of its
heritability is still unexplained. A random insertional transgene mutation screen was performed in our lab to identify new
genes/loci important for NCC development in the mouse. This screen yielded a mouse line named “Spot” that displays
pigmentation abnormalities and, in F2 litters, a phenotype reminiscent of WS that includes aganglionic megacolon as well
as balance problems indicative of inner ear defects. Gut colonization by fluorescently-labeled enteric NCC is severely
delayed in Spot embryos. We are now examining the inner ear of Spot animals, the sensory epithelia of which are partly
derived from NCC. A genomic library screen recently placed the Spot transgene insertion site on mouse chromosome
10D2, in a region syntenic to human chromosome 12q15 and devoid of any previously known HSCR-associated locus.
Program/Abstract # 355
cAMP promotes retinal midline crossing by regulating Nrp1 expression
Dell, Alison, University of Pennsylvania, Philadelphia, United States; Xu, Hong (Nanchang University, Nanchang,
China); Raper, Jonathan (University of Pennsylvania, Philadelphia, PA, United States)
The projection of axons to their targets is a key step in establishing neural circuits. Axons navigate through a complex
environment by interpreting attractive and repellent cues including Semaphorins (Semas). We previously showed that
axons modulate their response to repellents via a G protein coupled pathway. To examine the role of G protein signaling in
axon guidance in vivo, we used GAL4/UAS to drive the expression of highly selective dominant negative (DN)
heterotrimeric G proteins in retinal neurons of embryonic zebrafish. Among these, we expressed a DN construct targeting
GαS. Retinal ganglion cell (RGC) axons normally cross at the ventral midline and project to the contralateral tectum.
dNGαS expressing RGCs misproject to the ipsilateral tectum. GαS is expressed in RGCs as axons approach and cross the
midline. Constitutively active GαS mRNA rescues RGC misprojections in dnGαS transgenics. Thus GαS, canonically
known for activating cAMP, is required for reliable midline crossing of RGC axons. dnGαS induced RGC misprojections
phenocopy morphantsfor adenylyl cyclase 1 (AC1), AC8, or the Sema co-receptor Nrp1a. AC8 morpholino (MO) and
dnGαS synergize to produce ipsilateral misprojections. A likely common downstream effector forthe two is cAMP. We
hypothesized that reduced cAMP leads to a decrease in Nrp1 expression. AC8 and Nrp1a MOs induce a synergistic
increase in RGC misprojections. QPCR reveals that AC8 MO or dnGαS decrease levels of both Nrp1a and Nrp1b mRNAs.
We propose that reduced cAMP levels in dnGαSor AC8 MO embryos cause axon guidance errors by inhibiting Nrp1
expression, thus making axons insensitive to Sema signaling near the midline that promotes midline crossing.
Program/Abstract # 356
Tetraspanin18 restricts neural crest migration by stabilizing epithelial Cadherin6B
Fairchild, Corinne L., Gammill, Laura, Univ of Minnesota-Twin Cities Genetics, Cell Bio, Development, Minneapolis,
United States
Unlike typical neuroepithelial cells in the developing central nervous system, neural crest cells (NCCs) undergo an
epithelial tomesenchymal transition (EMT), detach from the neural tube, and migrate to give rise to diverse structures, such
as the peripheral nervous system. Regulation of neural crest EMT is incompletely understood. Tetraspanin18 (Tspan18) is
a member of the tetraspanin family of transmembrane proteins that have been implicated in cell signaling, motility and