Nadean L. Brown

Background: During vertebrate lens development, the lens placode in the embryonic ectoderm invaginates into a lens vesicle, which then separates from the surface epithelium, followed by two waves of fiber cell differentiation. In the mouse, multiple labs have shown that Jag1‐Notch signaling is critically required during the second wave of lens fiber cell formation. However, Notch signaling appears to play no obvious role during lens induction or morphogenesis, although multiple pathway genes are expressed at these earlier stages. Results: Here, we explored functions for Notch signaling specifically during early lens development, by using the early‐acting AP2α‐Cre driver to delete Jag1 or Rbpj. We found that Jag1 and Rbpj are not required during lens induction, but are necessary for proper lens vesicle separation from the surface ectoderm. Conclusions: We conclude that precise levels of Notch signaling are essential during lens vesicle morphogenesis. In addition, AP2α‐Cre‐mediated deletion of Rbpj resulted in embryos with cardiac outflow tract and liver deformities, and perinatal lethality. Developmental Dynamics 241:493–504, 2012.

Vertebrate retinal progenitor cells (RPCs) are pluripotent, but pass through competence states that progressively restrict their developmental potential (Cepko et al., 1996; Livesey and Cepko, 2001; Cayouette et al., 2006). In the rodent eye, seven retinal cell classes differentiate in overlapping waves, with RGCs, cone photoreceptors, horizontals, and amacrines forming predominantly before birth, and rod photoreceptors, bipolars, and Müller glia differentiating postnatally. Both intrinsic and extrinsic factors regulate each retinal cell type (for review, see Livesey and Cepko, 2001). Here, we conditionally deleted the transcription factorRbpj, a critical integrator of multipleNotchsignals (Jarriault et al., 1995; Honjo, 1996; Kato et al., 1997; Han et al., 2002), during prenatal mouse retinal neurogenesis. Removal ofRbpjcaused reduced proliferation, premature neuronal differentiation, apoptosis, and profound mispatterning. To determine the cell autonomous requirements forRbpjduring RGC and cone formation, we marked Cre-generated retinal lineages with GFP expression, which showed thatRbpjautonomously promotes RPC mitotic activity, and suppresses RGC and cone fates. In addition, the progressive loss ofRbpj−/− RPCs resulted in a diminished progenitor pool available for rod photoreceptor formation. This circumstance, along with the overproduction ofRbpj−/− cones, revealed that photoreceptor development is under homeostatic regulation. Finally, to understand how theNotchpathway regulates the simultaneous formation of multiple cell types, we compared the RGC and cone phenotypes ofRbpjtoNotch1(Jadhav et al., 2006b; Yaron et al., 2006),Notch3, andHes1mutants. We found particular combinations ofNotchpathway genes regulate the development of each retinal cell type.

Activation of the bHLH factor Math5 (Atoh7) is an initiating event for mammalian retinal neurogenesis, as it is critically required for retinal ganglion cell formation. However, the cis‐regulatory elements and trans‐acting factors that control Math5 expression are largely unknown. Using a combination of transgenic mice and bioinformatics, we identified a phylogenetically conserved regulatory element that is required to activate Math5 transcription during early retinal neurogenesis. This element drives retinal expression in vivo, in a cross‐species transgenic assay. Previously, Pax6 was shown to be necessary for the initiation of Math5 mRNA expression. We extend this finding by showing that the Math5 retinal enhancer also requires Pax6 for its activation, via Pax6 binding to a highly conserved binding site. In addition, our data reveal that other retinal factors are required for accurate regulation of Math5 by Pax6. genesis 47:175–187, 2009.

Pax6 regulates eye development in many animals. In addition, Pax6 activates atonal transcription factors in both invertebrate and vertebrate eyes. Here, we investigate the roles of Pax6 and atonal during embryonic development of Limulus polyphemus rudimentary lateral, medial and ventral eyes, and the initiation of lateral ommatidial eye and medial ocelli formation. Limulus eye development is of particular interest because these animals hold a unique position in arthropod phylogeny and possess multiple eye types. Furthermore, the molecular underpinnings of eye development have yet to be investigated in chelicerates. We characterized a Limulus Pax6 gene, with multiple splice products and predicted protein isoforms, and one atonal homologue. Unexpectedly, neither gene is expressed in the developing eye types examined, although both genes are present in the lateral sense organ, a structure of unknown function. Developmental Dynamics 237:2209–2219, 2008.