Congress Abstracts - Society for Developmental Biology

The well-known regenerative abilities of planarian flatworms are attributed to a population of adult stem cells called neoblasts, which
proliferate and differentiate to produce all cell types in their bodies. A characteristic feature of neoblasts is the presence of large
cytoplasmic ribonucleoprotein (RNP) granules named chromatoid bodies. These organelles are structurally and molecularly similar to
RNP granules present in the germline of many organisms. As such, they contain symmetrical dimethylarginine (sDMA) methylation
substrates of Protein Arginine Methyltransferase 5, such as PIWI and SmB homologs, as well as Tudor domain-containing proteins
that bind to sDMA. This study shows that the Schmidtea mediterranea PIWI family members SMEDWI-1 and SMEDWI-3 are
required for localization of germinal histone H4 (gH4) transcripts to chromatoid bodies. Regulation of histone mRNA by chromatoid
body components may go beyond gH4, since transcripts of every major histone gene family were also found in these structures.
Additionally, gH4 mRNA levels increased upon inhibition of SMEDWI-1 and SMEDWI-3 levels by RNAi, suggesting the
involvement of these PIWIs in histone mRNA turnover. PIWI proteins are better known for silencing transposable elements via
piRNA-mediated mRNA turnover and genomic silencing. Similar mechanisms may be involved in the regulation of histone gene
expression in planarians, where neoblasts are the only proliferating somatic cells and others are terminally differentiated.
Program/Abstract # 38
System level reconstruction of brain development and function with light-sheet microscopy
Philipp Keller (HHMI- Janelia, USA)
In embryonic development of vertebrates and higher invertebrates, a single cell is transformed into a fully functional organism
comprising tens of thousands of cells and more. In a complex process of self-organization, these cells rapidly divide, migrate,
differentiate and form tissues and organs able to perform the most challenging tasks. The nervous system is a key component of the
developmental building plan that stands out in terms of size, complexity and function. However, very little is known about the
developmental dynamics of this complex system, since the technology to comprehensively record and computationally analyze in vivo
cell behavior in neural tissues is lacking. The overall objective of our research is to gain such quantitative experimental access, to
determine the fundamental rules governing neural development, and to systematically link development to the functional activation of
circuits in the nervous system. I will present our experimental approach based on light-sheet fluorescence microscopy, an emerging
imaging technology that achieves exceptionally high imaging speed and signal-to-noise ratio, while minimizing light exposure of the
specimen. This unique combination of capabilities makes light-sheet microscopes indispensable for the long-term in vivo imaging of
entire developing organisms. We are designing advanced implementations of scanned light-sheet fluorescence microscopy, such as the
SiMView technology framework for simultaneous multiview imaging [1], to systematically study the early development of entire fruit
fly, zebrafish and mouse embryos with cellular resolution. I will furthermore present strategies for automated large-scale image
processing, advanced specimen culturing techniques and new transgenic reporter lines. Together, these tools allow us to perform
whole-organism functional imaging and quantitatively analyze developmental lineages and their interrelationships in the entire animal
[2]. Our goal is to take advantage of these high-resolution data to attain a system-level understanding of cell fate decisions and how
they establish the dynamic architecture of neural tissues. In the long-term perspective, we will use this information for the
establishment and validation of a computer model of the developing nervous system. I envision that our quantitative approach to the
reconstruction of large neuronal system dynamics will provide critical insights into the properties of complex circuits and complement
ongoing large-scale electron microscopy analyses of static neuronal network architecture.
Program/Abstract # 39
Regulatory logic of pan-neuronal gene expression
Oliver Hobert, Inés Carrera, Nikolaos Stefanakis (Columbia, USA)
Cell fate decisions in the vertebrate nervous system are particularly complex as the nervous system is composed of a remarkably
complex assemblage of cell types. Although a lot is known about how specific transcription factors, or Terminal Selectors (TS),
specify different neuronal types by coregulating neuron-type specific terminal differentiation genes, much less is understood about the
regulatory programs that control the expression of those neuronal features shared by every neuron, pan-neuronal features. Addressing
this question is key to understanding how neuronal fate is determined. Using Caenorhabditis elegans as a model system, we are
dissecting the cis-regulatory logic of broadly expressed neuronal genes and also identifying those trans-acting factors that regulate
them. Dissectional analysis of cis-regulatory control elements of pan-neuronal genes shows a piecemeal regulation of gene expression
in different neuronal types as well as redundant elements. We find that TS are also able to regulate expression of isolated cisregulatory
modules of some pan-neuronal genes, although in TS mutants full promoters of these genes are not affected. We are
currently conducting genetic screens to identify these redundant transcription factors. Analysis of the temporal and spatial expression
of broadly expressed neuronal genes by fosmid reporters show expression in the nervous system as well as in other tissues. Progress
towards the understanding of how pan-neuronal gene expression is regulated will be presented.
Program/Abstract # 40
Coexpression of the homeogenes barhl2, otx2 and irx3 specifies the identity and properties of the Mid-Diencephalic Organizer
Béatrice Durand, Hugo Juraver-Geslin (IBENS/CNRS, France); José Luis Gómez-Skarmeta (CSIC/ Univ Pablo de Olavide, Spain)
The Mid-Diencephalic Organizer (MDO) secretes the morphogen Sonic Hedgehog (Shh) and controls growth and regionalization of
the caudal forebrain. Little is known about how the MDO compartment is established during growth and morphogenesis of the
forebrain or what controls MDO competence to express shh. In this study performed in amphibians, using in vivo loss- and gain- of
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