Expression of eph-family receptor tyrosine kinases and ephrins in the tadpole of the frog Xenopus laevis, and possible roles in the development of retinotectal topography
<p>Assembling a nervous system requires exquisite specificity in the construction of neuronal connectivity. One method by which such specificity is implemented is the presence of chemical cues within the tissues, differentiating one region from another, and the presence of receptors for tho...
| Summary: | <p>Assembling a nervous system requires exquisite specificity in the construction of
neuronal connectivity. One method by which such specificity is implemented is
the presence of chemical cues within the tissues, differentiating one region from
another, and the presence of receptors for those cues on the surface of neurons
and their axons that are navigating within this cellular environment.</p>
<p>Connections from one part of the nervous system to another often take the form
of a topographic mapping. One widely studied model system that involves such
a mapping is the vertebrate retinotectal projection-the set of connections
between the eye and the optic tectum of the midbrain, which is the primary
visual center in non-mammals and is homologous to the superior colliculus in
mammals. In this projection the two-dimensional surface of the retina is mapped
smoothly onto the two-dimensional surface of the tectum, such that light from
neighboring points in visual space excites neighboring cells in the brain. This
mapping is implemented at least in part via differential chemical cues in
different regions of the tectum.</p>
<p>The Eph family of receptor tyrosine kinases and their cell-surface ligands, the
ephrins, have been implicated in a wide variety of processes, generally involving
cellular movement in response to extracellular cues. In particular, they possess
expression patterns-i.e., complementary gradients of receptor in retina and
ligand in tectum- and in vitro and in vivo activities and phenotypes-i.e.,
repulsive guidance of axons and defective mapping in mutants,
respectively-consistent with the long-sought retinotectal chemical mapping
cues.</p>
<p>The tadpole of Xenopus laevis, the South African clawed frog, is advantageous
for in vivo retinotectal studies because of its transparency and manipulability.
However, neither the expression patterns nor the retinotectal roles of these
proteins have been well characterized in this system. We report here
comprehensive descriptions in swimming stage tadpoles of the messenger RNA
expression patterns of eleven known Xenopus Eph and ephrin genes, including
xephrin-A3, which is novel, and xEphB2, whose expression pattern has not
previously been published in detail. We also report the results of in vivo protein
injection perturbation studies on Xenopus retinotectal topography, which were
negative, and of in vitro axonal guidance assays, which suggest a previously
unrecognized attractive activity of ephrins at low concentrations on retinal
ganglion cell axons. This raises the possibility that these axons find their correct
targets in part by seeking out a preferred concentration of ligands appropriate to
their individual receptor expression levels, rather than by being repelled to
greater or lesser degrees by the ephrins but attracted by some as-yet-unknown
cue(s).</p> |
|---|