Congress Abstracts - Society for Developmental Biology

Program/Abstract # 507
Slit1a Promotes Axon-Glial Interations to Facilitate Post-Optic Commissure Formation in Zebrafish
Park, Jin Sook (Smith College, USA)
Bilaterally symmetrical organisms must be able to communicate between the two hemispheres of the brain and body. In order to
achieve proper neural connections between the two halves of the central nervous system, axons must navigate across the midline,
forming a commissure. It is known that the Slit-Roundabout signaling system plays a crucial guidance mechanism involved in
commissure formation. Historically, Slit-Robo signaling has been known to function to repel path-finding axons; however, we propose
a new function to this guidance system, in which slit1a may promote axon-astroglial interactions to facilitate post-optic commissure
(POC) formation in the zebrafish forebrain. We show that the cells of the diencephalic glial bridge express slit1a, and global
misexpression of slit1a causes ectopic wandering POC axons. We and others have shown that crossing by POC axons in the you-too
(Gli2DR) mutant is greatly reduced, which is in part due to expanded midline expression of the known repellents slit2 and slit3. We
show here that global misexpression of slit1a in the you-too (Gli2DR) mutant can rescue POC midline crossing. Importantly, POC
axons are always associated with astroglial cells even when either is displaced from their normal positions. This suggests POC axons
and astroglial cells exhibit positive and potentially necessary interactions for commissure formation. We hypothesize that Slit1a-Robo
signaling mediates this positive interaction to facilitate POC axons crossing over the prefigured diencephalic glial bridge. We have
demonstrated that loss of Robo1 function by morpholino knockdown in the context of slit1a misexpression is capable of eliminating
the association of wandering POC axons with astroglial cells. These results suggest that slit1a functions distinctly from slit2 or slit3 to
positively promote association of POC axons with astroglial cells in a Robo1-dependent manner during commissure formation in the
zebrafish forebrain. We are currently testing the role of other Roundabout receptors in mediating axon-astroglial interactions during
POC formation.
Program/Abstract # 508
The PCP factor Prickle1b and transcriptional repressor Rest function within facial branchiomotor neurons to regulate their
migration during zebrafish development
Love, Crystal E. (U Chicago, USA); Sirotkin, Howard (Stony Brook, USA); Prince, Victoria (U Chicago, USA)
Facial branchiomotor neurons (FBMNs) migrate from rhombomere (r) 4 to r6 in the hindbrain during development. These neurons fail
to migrate out of r4 in embryos lacking Prickle1b (Pk1b), a component of the planar cell polarity pathway. Localization of Pk1b to the
nucleus of FBMNs is required for their proper migration, which suggests a PCP-independent role of Pk1b in this process. Pk1b has
additionally been shown to interact with RE1-silencing transcription factor (Rest), a transcriptional repressor of neuronal maturation
genes, and is required for the translocation of Rest to the nucleus in vitro. Rest is depleted from the nucleus of FBMNs in Pk1bdeficient
embryos, suggesting Pk1b is similarly important for Rest nuclear localization in these neurons. Consistent with this
hypothesis, we find FBMN migration is disrupted in mutant embryos expressing a nonfunctional Rest protein, and this phenotype is
enhanced in maternal-zygotic mutant embryos. Using Tol2-mediated transgenesis to either derepress or activate Rest targets
specifically within FBMNs, we show that Rest functions within the neurons, rather than in the surrounding neuroepithelium, to
mediate migration. Our data also suggest that FBMNs mature too early when Rest function is disrupted in these neurons, including the
inappropriate expression of Rest target genes. We propose a model in which Pk1b localizes Rest to the nucleus of FBMNs, with Rest
in turn functioning to repress target genes in the FBMNs, and thus maintain their immature, migratory state.
Program/Abstract # 509
Migration of Cajal-Retzius cells in the olfactory region of the developing telencephalon
Frade, Daniela; Varela-Echavarria, Alfredo (UNAM-Queretaro, Mexico)
The mammalian cortex is organized into layers which depend for their organization on the glycoprotein Reelin secreted by Cajal-
Retzius cells (CR) located in the outermost layer of the developing telencephalon. CR cells originate mainly in the cortical hem in the
dorsal midline of the telencephalon, but are also known or proposed to be generated from other domains. One of the proposed domains
is the lateral telencephalic region near the boundary between the pallium and subpallium in the ventral pallium (VP) and another one
is located in the rostral region of the rostral cortex. Migration routes of cells in these regions have been little studied. It has been
proposed that CR cells originating from the VP migrate to the dorsal cortex while the cells generated in the rostral region are
positioned in the rostral cortex. We have analyzed the migration of cells in both regions during the stages where CR cells are being
generated. By injecting a fluorescent tracer we observed that labeled cells along the anteroposterior extent of the VP migrate in ventral
direction while cells from the rostral domain migrate caudally along the olfactory region and dorsally to the dorsal cortex. Moreover,
we have observed that about one fourth of the migrating cells express Reelin. These results reveal novel routes of migration of
uncharacterized cells including a Reelin-expressing population in the developing telencephalon.
Program/Abstract # 510
Neogenin/RGMa signaling may regulate polarized migration during neural convergent extension in Danio rerio
Olmo, Valerie; Jayachandran, Pradeepa; Brewster, Rachel (U Maryland-Baltimore County, USA)
The neural tube, the precursor of the CNS, is formed by morphogenetic changes that transform the neuroepithelium into a tubular
structure during neurulation. Though aspects of neurulation vary by species, neural convergent extension (NCE), a very early stage of
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