Congress Abstracts - Society for Developmental Biology

communicates with downstream machinery such as actomyosin to coordinate cell behaviors. We approached the question by focusing
on CE during notochord formation in Xenopus laevis embryos. We found that actomyosin predominantly accumulates at specific
topologies of cell-cell junctions along the medio-lateral axis. Live imaging shows that the actomyosin accumulation pulses
synchronously with cell edge shrinkage; and we found that the shrinking edges have higher cortical tension than other edges.
Interestingly, tracking photoactivated actin biosensors revealed that actin localization is spatially restricted by septin, a cytoskeletal
element that acts as a partition protein in various cell types. Knockdown of septin disrupts the selective accumulation of actomyosin at
specific cell junctions, leading to a disturbance of the spatially-restricted cortical tension needed for medio-lateral intercalation.
Finally, we found that the PCP pathway is required for septin localization and polarized actomyosin function. Together, we conclude
that the PCP pathway regulates septin to generate discrete domains of active actomyosin along cell edges to coordinate medio-lateral
intercalation during CE.
Program/Abstract # 67
The dynamic puzzle of cell shape and polarity in plant morphogenesis
Yara Elena Sanchez Corrales, Matthew Hartley (John Innes Ctr, UK); Jop van Rooij(Utrecht U, Netherlands); Enrico Coen, Stan
Marée, Verônica Grieneisen, (John Innes Ctr, UK)
Although considerable progress has been made in identifying genes that control cell polarity, it is still unclear how they work together
to generate cells with particular shapes. We also have limited understanding about how cell polarity, cell shape and cell growth are
temporally and spatially related to ensure a correct organ development. Plant cells offer an excellent system to address these questions
because, in contrast to animal cells, they cannot move during development; therefore, plant cells are restricted inside an organ. To
understand cell shape and polarity coordination within a growing tissue, we here focus on the development of pavement cells, an
intricate system in which the full complexity of polarity and shape change comes together at many levels. Pavement cells show a
characteristic jigsaw puzzle-like shape in a non-homogenous spatial pattern within a growing leaf. Combining live-imaging techniques
and novel quantitative shape analytic tools with computational and mathematical modelling techniques we have captured cell size and
shape changes over time and extracted quantitative information that will be directly compared with our modelling efforts to capture
cell polarity and multicellular behaviour. Our results show that local changes at individual cell-level can lead to global patterns, and
we hypothesize on the mechanisms of this regulation.
Program/Abstract # 68
Sp6 and Sp8 transcription factors are necessary mediators of WNT/ß-Catenin function in the limb ectoderm
Marian Ros, Endika Haro, Irene Delgado (U de Cantabria, Spain); Yoshihiko Yamada (NIH, USA); Ahmed Mansouri (MPI for
Biophysical Chemistry, Germany); Kerby Oberg (Loma Linda U, USA)
The apical ectodermal ridge (AER) is a specialized epithelium located at the distal dorso-ventral (DV) rim of the developing limb that
is crucial for limb bud development. The induction of the AER is a complex process that relies on intricate interactions among the
FGF, WNT, and BMP signaling pathways operating within the ectoderm and between the ectoderm and mesoderm of the early limb
bud. Furthermore, induction of the AER is linked to the establishment of DV patterning. Sp6 and Sp8 are two members of the
Specificity Protein family of transcription factors that are expressed in the limb bud ectoderm and function downstream of WNT/ β
Catenin signaling and upstream of Fgf8. Their individual genetic inactivations result in a mild syndactyly phenotype for Sp6 and limb
truncation, due to the premature regression of the AER, for Sp8. To investigate a possible functional redundancy between Sp6 and
Sp8, we generated double Sp6;Sp8 null mutants. We also generated Sp6-null; Sp8-conditional mutants using a Sp8 floxed allele with
the Ap2-Cre and with the Msx2-Cre deleter lines. Our results show that double Sp6;Sp8 mutants are tetra-amelic. Initial budding
occurs, but Fgf8 and Bmp4 are not activated in the limb ectoderm and the dorsal marker Wnt7a persists throughout the limb bud
ectoderm. The phenotype of mutants bearing a single functional copy of Sp6 (Sp6 +/- ;Sp8 -/- ) is indistinguishable from that of the double
mutants, whereas the presence of a single functional allele of Sp8 (Sp6 -/- ;Sp8 +/- ) results in a Split Hand Foot Malformation phenotype.
We conclude that Sp6 and Sp8 work in a redundant manner as indispensable mediators of WNT/ β Catenin signaling in the limb
ectoderm and that their function links the Proximo-distal and DV axes.
Program/Abstract # 69
Coordinating organogenesis: Insights from the Drosophila eye
Tiffany Cook, Mark Charlton-Perkins, John Mast (Cincinnati Children's Hosp., USA); Elke Buschbeck (U Cincinnati, USA)
The Drosophila compound eye is a powerful system to study molecular networks controlling the generation of a complex organ. This
is due to its regular crystalline architecture and its easy electrophysiological assessment, which allow ready detection of morphological
abnormalities and visual dysfunction. We previously showed that two direct cell-restricted Ras and Notch (N) target genes, the
transcription factors Prospero (Pros) and dPax2, antagonize each other to define neuronal vs non-neuronal (R7 photoreceptor [PR] vs
lens-secreting cone cells [CCs]) fates, but cooperate during CC specification and lens formation. We further demonstrated that Pros
and dPax2 mediate these functions through positive feedback with Ras and Notch signaling, respectively. Combining genetic,
transcriptomic, cell biological, and electrophysiological approaches, we have now defined two novel non-autonomous CC functions
that also require Pros and dPax2. Early in eye development, differential Ras/Pros and N/dPax2 activities oppositely control the
formation of the two essential retinal pigmented epithelia (RPE) cell types: 2° and 3° pigment cells. Later, Pros and dPax2 in CCs
cooperate to regulate PR structure and function, respectively. These new CC functions involve interplay with both cell adhesion and
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