Congress Abstracts - Society for Developmental Biology

Program/Abstract # 98
A gradient of sulfated proteoglycans is required for dorsal-ventral skeletal patterning in sea urchin embryos
Cynthia Bradham, Finnegan Hewitt, Michael Piacentino, Christy Li, Jia Yu, Evan Bardot, David Lee, Hajerah Hameeduddin, Arlene
Reyna, Oliver Chung, James Chaves, Patrick Ferrell, Ian Murray, Matthew Tse, Ah Ra Cho, Amanda Core, Jasmin Coulomb-
Huntington (Boston U, USA); Albert Poustka (Max-Planck Inst. Molec Genet, Germany)
The sea urchin larval skeleton is secreted by primary mesenchyme cells (PMCs) in response to instructive ectodermal cues. During
gastrulation, PMCs become organized in a vegetal ring of cells with additional cells forming ventrolateral cords that extend animally,
in response to these cues. This PMC pattern gives rise to the skeletal pattern. We identified mutliple ectodermal genes responsible for
skeletal patterning using an RNA-seq-based screen. Among them is SLC26a2 (SLC), a sulfate transporter required for synthesis of
sulfated proteoglycans (sPGs). LvSLC loss of function (LOF) results in loss of the ventral and animal (VA) skeletal elements, and
perturbs PMC positioning in VA regions. SLC is expressed in the VA ectoderm, and its expression profile correlates well with sPGs,
which are enriched both ventrally and animally. The ventral-to-dorsal sPG gradient is strongly diminished in SLC LOF embryos.
These results indicate that sPGs function to attract PMCs to VA positions. Interestingly, LOF for Notch2, another validated candidate,
promotes the opposite distribution of PMCs, such that they accumulate in VA regions at the expense of dorsal, despite the ventral
expression domain for LvNotch2. This suggests that Notch2 signaling promotes the opposite effect, directing a subset of PMCs
dorsally, away from sPGs. Consistent with this interpretation, combined LOF for both SLC and Notch2 strongly perturbs PMC
localization and blocks skeleton formation. Together these data suggest a model in which VA-expressed SLC promotes local sPG
synthesis that in turns serves as an attractive cue for PMCs, while VA-expressed Notch2 functions as a switch that induces a subset of
PMCs to instead migrate dorsally, away from sPG cues.
Program/Abstract # 99
Nodal Morphogen Interpretation
Alex Schier (Harvard, USA)
Morphogens are long-range signaling molecules that pattern developing tissues in a concentration-dependent manner. The graded
activity of morphogens within tissues exposes cells to different signal levels and leads to region-specific transcriptional responses and
cell fates. To determine how morphogen gradients are established and interpreted, we study morphogens belonging to the Nodal
family. Nodal signals induce and maintain cell fates in embryos and embryonic stem cells. It is poorly understood how dynamic Nodal
signaling is interpreted by responding cells to generate different cell types. I will discuss our recent studies that determine how signal
concentration, signaling duration, and cellular history underlie the interpretation of Nodal signals.
Program/Abstract # 100
Bucky ball interacts with RNA binding proteins to pattern the oocyte and follicle cells in zebrafish
Florence L. Marlow, Amanda Heim, Odelya Hartung, Sophie Rothhämel, Andreas Jenny (Albert Einstein Coll. Med., USA)
In vertebrates the first asymmetries are established during oogenesis along the animal-vegetal axis, but the underlying molecular
mechanisms are poorly understood. Bucky ball (Buc) was identified in zebrafish as a novel vertebrate specific regulator of oocyte
polarity, acting through unknown molecular interactions. Our studies show that endogenous Buc protein localizes to the Balbiani
body, a conserved, asymmetric structure in oocytes that requires Buc for its formation and that asymmetric distribution of Buc in
oocytes precedes Balbiani body formation, defining Buc as the earliest marker of oocyte polarity in zebrafish. Through a transgenic
strategy, we determined that proper localization of Buc is essential for oocyte polarity. Moreover, analyses of mosaic ovaries
indicates that oocyte pattern determines, likely via a close range signal or direct cell contact, the number of animal pole specific
micropylar cells that are associated with an egg. We demonstrate Buc protein interacts with Rbpms2, a conserved RNA binding
protein localized to the Balbiani body. Cumulatively, our results are consistent with a model whereby Buc establishes oocyte polarity
through interactions with RNA binding proteins, initiating a feedback amplification mechanism in which Buc protein recruits RNA
binding proteins that in turn recruit buc RNA and other RNAs to the Balbiani body.
Program/Abstract # 101
Target-specific robustness to Hedgehog production levels in the Drosophila wing disc
Marcos Nahmad, Arthur Lander (UC Irvine, USA)
Developmental patterns of gene expression are generally highly precise and reproducible despite variability of certain genetic and
environmental parameters. Self-enhanced ligand degradation has been proposed as a general mechanism to provide robustness of
morphogen gradients to changes in ligand production. The Hedgehog (Hh) signaling pathway self-limits its range of signaling by
transcriptionally activating its own receptor, Patched (Ptc), and thereby self-promoting ligand degradation. Previous theoretical studies
suggest that signal-dependent-upregulation of Ptc may provide robustness to variability in Hh levels, but such studies presume that Hh
target genes are a direct readout of the steady-state distribution of the Hh gradient. However, as was recently shown in the wing
imaginal disc of Drosophila, not all target genes respond to steady-state Hh levels; dynamics of the Hh gradient also play a role in
conveying positional information in this system. Here we show experimentally that steady-state-determined Hh targets (e.g. collier
and ptc) are more robust to changes in Hh dosage than transient-determined Hh targets (such as dpp). Using mathematical modeling,
we show that this result is in agreement with a dynamic model of Hh signal interpretation. Surprisingly, we provide quantitative
evidence that the observed lack of robustness in determining the width of the dpp pattern does not affect the phenotype of the adult
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