Annual Report 2011 - Center for Advanced Biotechnology and ...

Diverse developmental roles of Foxn4. In our previous studies, we have shown that
Foxn4 is transiently expressed in a subset of progenitors during retinogenesis and
spinal neurogenesis. Targeted inactivation and gain-of-function analyses have
revealed an essential function of Foxn4 in the generation of amacrine and horizontal
cells during retinal development as well as in specifying the V2b interneurons during
spinal cord development. In collaboration with Drs. Stavros Malas (University of
Cyprus) and William Richardson (University College London), we have now used
genetic fate mapping and loss-of-function analysis to show that the transcription
factor Sox1 is expressed in and required for a third type of p2-derived interneuron,
which we named V2c. These are close relatives of V2b interneurons, and, in the
absence of Sox1, they switch to the V2b fate. The absence of Foxn4 results in a
complete loss of Sox1-expressing V2c neurons, indicating that Foxn4 is necessary for
specifying not only V2b but also V2c interneuron subtypes. In another collaboration
with Dr. Jeffrey Goldberg (University of Miami, Miller School of Medicine), we
observed a developmental delay in the distribution of RGC (retinal ganglion cell)
projections to the superior colliculus. Moreover, RGC axons failed to penetrate into
the retinorecipient layers in Foxn4 mutant mice. Foxn4 was not expressed by RGCs
and was undetectable in the superior colliculus itself. Thus, Foxn4 may be indirectly
required for RGC distal axon patterning. Aside from its neural expression, we have
also shown that during murine lung development Foxn4 is expressed in proximal
airways by a subpopulation of postmitotic epithelial cells which are distinct from
basal and ciliated cells. Foxn4 inactivation causes dilated alveoli, thinned alveolar
walls and reduced septa in the distal lung but no overt gross alterations in proximal
airways, suggesting that Foxn4 may have a non-cell-autonomous role critical for
alveologenesis during lung development. Together, our data suggest that Foxn4 may
play multiple cell-autonomous and non-cell-autonomous roles during epithelial cell
development.
Brn3a regulates dorsal root ganglion sensory neuron specification and axonal
projection into the spinal cord. The sensory neurons of the dorsal root ganglia
(DRG) must project accurately to their central targets to convey proprioceptive,
nociceptive and mechanoreceptive information to the spinal cord. How these different
sensory modalities and central connectivities are specified and coordinated still
remains unclear. Given the expression of the POU homeodomain transcription factors
Brn3a and Brn3b in DRG and spinal cord sensory neurons, we analyzed subtype
specification of DRG and spinal cord sensory neurons as well as DRG central
projections in Brn3a and Brn3b single and double mutant mice. Inactivation of either
or both genes causes no gross abnormalities in early spinal cord neurogenesis;
however, in Brn3a single and Brn3a;Brn3b double mutant mice, sensory afferent
axons from the DRG fail to form normal trajectories in the spinal cord. The TrkA +
cutaneous nociceptive afferents stay outside the dorsal horn and fail to extend into the
spinal cord, while the projections of TrkC + proprioceptive afferents into the ventral
horn are also impaired. Moreover, Brn3a mutant DRGs are defective in sensory
neuron specification, as marked by the excessive generation of TrkB + and TrkC +
neurons as well as TrkA + /TrkB + and TrkA + /TrkC + double positive cells at early
embryonic stages. At later stages in the mutant, TrkB + , TrkC + and parvalbumin +
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