Abstracts - Society for Developmental Biology

99
Program/Abstract # 301
Dual roles for canonical and non-canonical Wnt signaling in craniofacial development and patterning.
Alexander, Courtney;Piloto, Sarah; Schilling, Thomas, University of California, Irvine, United States)
Craniofacial development is a complex process that requires signaling from the pharyngeal arch epithelia to regulate the
patterning of the skeletal precursor cells, neural crest cells (NCC), into distinct dorsal and ventral elements. While the Wnt
signaling pathway has been implicated in multiple processes of NCC development the exact nature of Wnt signaling in
craniofacial patterning has not been fully characterized. Using transgenic zebrafish lines to overexpress a dominant
negative Tcf3, (Tg(hsp70I:tcf3-GFP), and ectopically express the canonical Wnt inhibitor dickkopf1 (dkk1),
(Tg(hsp70i:dkk1-GFP), we have repressed Wnt signaling in a temporal manner. Loss of Wnt signaling in dntcf3 embryos
results in reduction of ventral cartilage elements, proliferation defects, and loss of dorsal-ventral (DV) patterning genes
such as hand2, dlx3b, dlx5a, and nkx3.2. This closely resembles the DV patterning defects seen following loss of Bmp
signaling. In addition, hs-dkk1 embryos present a unique craniofacial phenotype – clefting of the mandible at the ventral
midline and stacking defects that resemble non-canonical Wnt mutants. Dkk1 is expressed exclusively in the pharyngeal
arch endoderm and transplantation of hs-dkk1+ endoderm into control embryos is sufficient to phenocopy mandibular
clefting. Therefore, Wnt signaling regulates mandibular development through both canonical and non-canonical signaling
pathways – we propose that canonical signaling interacts with Bmp signals from the ectoderm to control DV patterning,
while non-canonical Wnt signaling from the endoderm regulates mandibular growth and morphogenesis.
Program/Abstract # 302
The levels of Sox21 alter its function in neurogenesis
Whittington, Niteace C.; Cunningham, Doreen; Casey, Elena S., Georgetown University, Washington, United States
Neurogenesis, the progression from neural progenitor to committed neuron, is a tightly regulated process that is
fundamental for development ofthe central nervous system (CNS). Members of the SoxB transcription factor family play
critical roles in this process. Whereas SoxB1 proteins, which act as transcriptional activators, are required for induction
and maintenance of a proliferating neural progenitor population, the closely related SoxB2 proteins function as repressors
and are proposed to inhibit SoxB1 targets to control the progression from progenitor to neuron. To determine the
mechanism of action of the SoxB2 proteins, we are characterizing the function of the SoxB2 protein, Sox21, in primary
neurogenesis in the African clawed frog Xenopus laevis. Our gain of function assays showed that rather than promoting
differentiation, both Xenopus and chick Sox21 expand the neural progenitor domain and prohibit neuronal differentiation,
indicating that Sox21 enables progenitors to stay in the cell cycle longer. However our loss of function assays
demonstrated that the decrease in Sox21 reduced neuron formation while progenitors remained unaffected. Our gain and
loss of function analyses together suggest that Sox21 plays more than one role in neurogenesis, where a threshold level is
required for differentiation of neurons from progenitors but a high concentration of Sox21 inhibits neurogenesis and
instead promotes proliferation. Thus like other Sox proteins, Sox21 functions in a dose dependent manner. Since Sox
protein target specificity and function are dependent on partner protein interactions, we propose that when expressed at
different levels, Sox21 interacts with different partners and therefore has different functions.
Program/Abstract # 303
Relationship between Calcium activity, neurotransmitter phenotype, and expression of the transcription factor
Ptf1a in the developing Xenopus laevis retina
Allen, Chelsea; Saha, Margaret (The College of William and Mary, Williamsburg, VA, United States)
To establish a mature visual system, retinal neurons must acquire the appropriate neurotransmitter fate. While the
mechanisms regulating retinal cell type identity are well studied, the processes by which retinal cells acquire a particular
neurotransmitter phenotype are less well known. The role of transcription factors in retinal cell specification has been
extensively studied, but there has been far less focus on the role of activity, particularly the role of early calcium activity.
In this study, we employ Xenopus laevis embryos as a model to investigate the possible correlations among calcium
activity, transcription factor expression, and neurotransmitter phenotype at the individual cell level. Specifically, we
hypothesize that early calcium activity in the developing retina modulates transcription factor expression thereby inducing
a specific neurotransmitter phenotype. We predict that specific patterns of calcium activity will correlate with the
coexpression of neurotransmitter phenotype markers and transcription factors. To test this hypothesis, we are utilizing in
vivo calcium imaging to analyze the calcium activity of individual retinal cells from primary retinal cell culture from early
developmental stages of Xenopus embryos. In situ hybridization is then performed using neurotransmitter phenotype
markers for GABA and glutamate. Additionally, we are assessing calcium activity for cells expressing Ptf1a, a