[Show abstract][Hide abstract] ABSTRACT: Sonic Hedgehog (Shh) signaling is an important determinant of vertebrate retinal ganglion cell (RGC) development. In mice, there are two major RGC populations: (1) the Islet2-expressing contralateral projecting (c)RGCs, which both produce and respond to Shh; and (2) the Zic2-expressing ipsilateral projecting RGCs (iRGCs), which lack Shh expression. In contrast to cRGCs, iRGCs, which are generated in the ventrotemporal crescent (VTC) of the retina, specifically express Boc, a cell adhesion molecule that acts as a high-affinity receptor for Shh. In Boc(-/-) mutant mice, the ipsilateral projection is significantly decreased. Here, we demonstrate that this phenotype results, at least in part, from the misspecification of a proportion of iRGCs. In Boc(-/-) VTC, the number of Zic2-positive RGCs is reduced, whereas more Islet2/Shh-positive RGCs are observed, a phenotype also detected in Zic2 and Foxd1 null embryos. Consistent with this observation, organization of retinal projections at the dorsal lateral geniculate nucleus is altered in Boc(-/-) mice. Analyses of the molecular and cellular consequences of introducing Shh into the developing VTC and Zic2 and Boc into the central retina indicate that Boc expression alone is insufficient to fully activate the ipsilateral program and that Zic2 regulates Shh expression. Taking these data together, we propose that expression of Boc in cells from the VTC is required to sustain Zic2 expression, likely by regulating the levels of Shh signaling from the nearby cRGCs. Zic2, in turn, directly or indirectly, counteracts Shh and Islet2 expression in the VTC and activates the ipsilateral program.
Journal of Neuroscience 05/2013; 33(20):8596-607. · 6.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Sox2 transcription factor is active in stem/progenitor cells throughout the developing vertebrate central nervous system. However, its conditional deletion at E12.5 in mouse causes few brain developmental problems, with the exception of the postnatal loss of the hippocampal radial glia stem cells and the dentate gyrus. We deleted Sox2 at E9.5 in the telencephalon, using a Bf1-Cre transgene. We observed embryonic brain defects that were particularly severe in the ventral, as opposed to the dorsal, telencephalon. Important tissue loss, including the medial ganglionic eminence (MGE), was detected at E12.5, causing the subsequent impairment of MGE-derived neurons. The defect was preceded by loss of expression of the essential ventral determinants Nkx2.1 and Shh, and accompanied by ventral spread of dorsal markers. This phenotype is reminiscent of that of mice mutant for the transcription factor Nkx2.1 or for the Shh receptor Smo. Nkx2.1 is known to mediate the initial activation of ventral telencephalic Shh expression. A partial rescue of the normal phenotype at E14.5 was obtained by administration of a Shh agonist. Experiments in Medaka fish indicate that expression of Nkx2.1 is regulated by Sox2 in this species also. We propose that Sox2 contributes to Nkx2.1 expression in early mouse development, thus participating in the region-specific activation of Shh, thereby mediating ventral telencephalic patterning induction.
Development 03/2013; 140(6):1250-61. · 6.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cdon and Boc, and their Drosophila homologues Ihog and Boi, are evolutionary conserved transmembrane glycoproteins belonging to a subgroup of the Immunoglobulin superfamily of cell adhesion molecules (CAMs). Initially isolated in vertebrates as CAMs that link cadherin function with MAPK signaling in myoblast differentiation, they have thereafter been shown to act as essential receptors for the Hedgehog (Hh) family of secreted proteins. They associate with both ligand and other Hh receptor components, including Ptch and Gas1, thus forming homo- and heteromeric complexes. In Drosophila, they are also involved in ligand processing and release from Hh producing cells. Cdon/Boc and Ihog/Boi can substitute one another and play redundant functions is some contexts. In addition, Boc, but not Cdon, mediates axon guidance information provided by Hh in specific neuronal populations, whereas mutations in the CDON cause holoprosencephaly, a human congenital anomaly defined by forebrain midline defects prominently associated with diminished Hh pathway activity.
The international journal of biochemistry & cell biology 02/2012; 44(5):698-702. · 4.89 Impact Factor