Congress Abstracts - Society for Developmental Biology

were colocalized along the epidermis and in the dorsal regions of the neural tube, in agreement with the RECK in situ data. Overall,
these results suggest that RECK is temporally and spatially restricted during early Xenopus laevis development and is colocalized with
MT1-MMP during neurulation.
Program/Abstract # 503
Withdrawn
Program/Abstract # 504
The role of Retinoic acid signaling in tectal laminar formation
Kukreja, Shweta (Indian Institute of Technology Kanpur, India)
Retinoic acid (RA) is critical for morphogenesis and differentiation of many developing organs and tissues. In RA reporter mice high
RA activity was detected in the developing visual system, i.e. the retina, optic tract, and the retino-recipient structure in the mid brain,
superior colliculus(SC). SC is functionally homologous to the tectum in chick which comprises of 16 distinct cellular and fibrous
layers, giving it a laminated structure. The development of these laminae occurs as a result of cell proliferation in the neuroepithelium,
and three distinct waves of radial migration of differentiated neurons and glia. In accordance with the presence of RA activity in
mouse SC, we found the expression of RA synthesizing enzyme ALDH1A2 in a transient lamina in developing chick tectum from
embryonic day 6 onwards. Expression of RA degrading enzyme Cyp26B1 was found in the ventricular layer. The onset of expression
of RA synthesizing and degrading enzymes coincides with the late migratory wave and with the innervation of tectum by retinal
axons. The spatial and temporal expression of source and sink of RA in the tectum suggests that it may have a role in development of
the laminar structure of the tectum. In our studies, we have observed that disruption of RA signalling in tectum by virus mediated
misexpression of dominant negative receptor RAR alpha at early stages of the chicken embryo leads to thinning of the tectum. We
found that this is primarily the result of severe lamination defects as characterized by expression of lamina specific genes. We are
further investigating the mechanisms and intermediary players through which these defects occur, and the arborization defects of the
RGC axons in the disrupted lamina.
Program/Abstract # 505
The Role of DCC for Mitral Cell Axon Guidance in Zebrafish
Horne, Jack; Sheth, Ruchi (Pace U, USA)
In vertebrates, the majority of olfactory sensory information is transferred to the brain through two basic relays: (i) first, from
olfactory sensory neurons through the olfactory nerve to the glomeruli of the olfactory bulb; (ii) second, through the axon projections
of mitral cells of the olfactory bulb to multiple higher brain targets in the telencephalon. While much is known about the guidance
mechanisms of olfactory sensory axons, relatively little is known about the second relay, the axon guidance of mitral cells. This is an
extremely important question as the molecular mechanisms that control this axon guidance process determine how olfactory sensory
information is relayed and processed by the brain. Here we begin to characterize these mechanisms by assessing the role of DCC for
mitral cell axon guidance through loss-of-function analysis in zebrafish. In vivo electroporation, in combination with a gal4 transgenic
line, was used to temporally and spatially target DCC loss-of-function. Knockdown of DCC function at 24 hpf using anti-DCC siRNA
oligonucletides led to significant alterations in the mitral cell axon projections at 5 to 7 dpf. The three-dimensional structure of the
mitral cell axon projection was visualized by confocal imaging of a membrane-localized YFP, which was targeted to mitral cells by
co-electroporation with the loss-of-function siRNA oligonucleotides. Embryos targeted with anti-DCC siRNA showed a significant
decrease in the number of midline-crossing commissural axons. These results suggest that netrin-DCC plays a role in midline targeting
of mitral cell commissural axons.
Program/Abstract # 506
The Role of Heparan Sulphotransferase Enzymes Hs2st and Hs6st1 in Corpus Callosum Development
Clegg, James; Pratt, Thomas (U of Edinburgh, UK)
Heparan sulphate proteoglycans (HSPGs) are complex macromolecules that are found at the cell surface and form part of the
extracellular matrix. HSPGs are known to play a crucial role in the modulation of a number of different cell-cell signalling pathways
during development; these include the signalling of axon guidance molecules such as Slit/Robo. HSPGs consist of a core protein to
which heparan sulphate side chains are added, these side chains are extensively modified by the addition and removal of sulphate
groups at specific positions on the carbohydrate ring. It has been proposed that the differential sulphation of these HS side chains may
change the ability of cells or axons to respond to particular signalling molecules. This may help explain how a relatively small number
of axon guidance molecules are able to guide the enormous number of axons found within the brain. The corpus callosum (CC) is a
large axon tract which links the two cerebral hemispheres. Previously we have shown that in mice which lack the HS modifying
enzymes Hs2st and Hs6st1 the CC fails to form, demonstrating that the HS sulphation provided by these two enzymes is required for
the guidance of callosal axons. It remains unclear however whether this defect is primarily caused by a loss of HS sulphation at the
growth cone of the callosal axons or whether the change in sulphation affects the midline environment through which these axons
navigate. To answer this question we have used conditional mutagenesis to specifically ablate the expression of Hs2st or Hs6st1 in
callosal projection neurons or populations of cells at the telencephalic midline and examined the effect this has on signalling and the
development of the CC.
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