Abstracts - Society for Developmental Biology

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data suggest that Prdm1 may play arole in modulating endodermal derived signals required for TMJ formation in proximal
arch 1 and posterior endodermally derived structures.
Program/Abstract # 315
Reciprocal repression of Six1/Eya1 and Irx1 in the pre-placodal ectoderm, the embryonic precursor of cranial
sensory organs
Sullivan, Charles H., Grinnell Col Dept of Biol, Grinnell, United States; Neilson, Karen M.; Moody, Sally A. (George
Washington University Medical Center, Washington, DC, United States)
Irx1, an Iroquois family member, plays a key role in regulating expression of neural determination bHLH genes in the
neural plate. It also is expressed in the posterior region of the pre-placodal ectoderm (PPE), a precursor of cranial sensory
ganglia and the otocyst, but its role in placode neurogenesis has not been explored. We studied the relationship between
Irx1 and two other transcription factors (Six1 and Eya1) that are required for placode formation and maintain sensory
precursors in an immature state. Over-expression of Six1, either singly or in combination with Eya1, repressed Irx1
expression in the PPE at neurula stages and later in the otocyst. Use of a repressive construct (Six1EnR) demonstrated that
Six1 acts as a transcriptional repressor of Irx1. Interestingly, two mutant Six1 constructs that are identical to mutations in
the human BO/BOR syndrome, which is characterized by deafness, are as effective in repressing Irx1 as is wild type Six1,
indicating that the defect is not at this stage of development. Irx1 expression in the PPE lineage reciprocally reduced Six1
and Eya1, suggesting they need to be down-regulated for neural differentiation to proceed. Increased Irx1 expression also
caused a down-regulation in Fgf8 in the anterior neural ridge suggesting a role for this signaling pathway in PPE
formation. Because both the PPE and cranial neural crest are derived from a common precursor zone, we tested whether
Irx1 down-regulation of Six1/Eya1 expanded the neural crest. Irx1 expanded FoxD3 expression but repressed Sox9
expression in both the premigratory region and branchial arches. Increased Irx1 also repressed the expression of Sox9 in
the otic placode and resulted in abnormal otocyst development.
Program/Abstract # 316
Examining the role of C. elegans forkhead genes in neuron development
Nelms, Brian, Fisk University, Nashville, United States; Smith, Erica; Ridgeway, Naccolaine (Fisk University, Nashville,
TN, United States)
The nematode worm C. elegans is an attractive system for studying regulation of neuronal fate specification and
maintenance. The number of neurons (limited to just over 300) and transparent body allow for manageable observations
and manipulations of individual neurons, while the fundamental molecular biology is comparable to higher organisms
including humans. Using C. elegans as our model system, we hope to dissect the roles of one set of well-conserved genes,
the forkhead genes, many of which play important roles throughout development in invertebrates and vertebrates alike.
Among the C. elegans forkhead genes we are interested in, fkh-8, fkh-9, and fkh-10 seem to be expressed in dopaminergic
neurons. fkh-8 is most closely related to the mammalian FoxJ family, some members of which are also expressed in
dopamine-signaling neurons of the brain. We plan to further investigate the neuronal function of the FKH-8, FKH-9, and
FKH-10 transcription factors, also examining the potential for functional redundancy among these factors in C. elegans,
potentially giving insight into the neuronal role of their mammalian counterparts.
Program/Abstract # 317
Foxa genes in the development of the intervertebral disk
Maier, Jennifer; Lo, Yinting; Harfe, Brian, University of Florida, Gainesville, United States
Disk degeneration is a cause of chronic back pain; for which there are few effective treatments. Little is known about the
development of the intervertebral disk (IVD); knowledge which could lead to better treatments for degeneration, and
associated back pain. The IVD is composed of the gel-like nucleus pulposus (NP), the collagenous annulus fibrosus (AF),
and cartilaginous endplates. To study the development of the IVD, we used a Foxa1; Foxa2 knockout mouse. Foxa genes
are in the forkhead box family of transcription factors and are required for the development of many organs and for postnatal
life. They are expressed in the notochord, a structure that becomes the NP. Foxa1 is a null allele, Foxa2 is a floxed
conditional under the control of the tamoxifen-inducible ShhcreERT2 allele, allowing us to remove Foxa2 in tissues that
Shh is expressed. Giving tamoxifen to dams at E7.5 removed FOXA2 from the notochord by E9.5. Histology of Foxa1-/-;
Foxa2c/c; ShhcreERT2/+ embryos showed a severely deformed NP and vertebral abnormalities. Double mutants had
aberrant notochord to NP transition, shown by fate-mapping with a LacZ reporter allele. They also had dramatically
increased cell death in the tail of the embryo. In situ hybridization for notochord, neural tube and sclerotome markers have
been analyzed. Hedgehog signaling appears to be disturbed in double mutants, and dorso-ventral patterning of the neural