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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2009.11.003 Byline: Min Deng (a), Ling Pan (a), Xiaoling Xie (a), Lin Gan (a)(b)(c) Keywords: LIM; LMO; LMO4; Otic vesicle; Otocyst; Vestibular morphogenesis; Inner ear development; Transcription factor Abstract: During development, compartmentalization of an early embryonic structure produces blocks of cells with distinct properties and developmental potentials. The auditory and vestibular components of vertebrate inner ears are derived from defined compartments within the otocyst during embryogenesis. The vestibular apparatus, including three semicircular canals, saccule, utricle, and their associated sensory organs, detects angular and linear acceleration of the head and relays the information through vestibular neurons to vestibular nuclei in the brainstem. How the early developmental events manifest vestibular structures at the molecular level is largely unknown. Here, we show that LMO4, a LIM-domain-only transcriptional regulator, is required for the formation of semicircular canals and their associated sensory cristae. Targeted disruption of Lmo4 resulted in the dysmorphogenesis of the vestibule and in the absence of three semicircular canals, anterior and posterior cristae. In Lmo4-null otocysts, canal outpouches failed to form and cell proliferation was reduced in the dorsolateral region. Expression analysis of the known otic markers showed that Lmo4 is essential for the normal expression of Bmp4, Fgf10, Msx1, Isl1, Gata3, and Dlx5 in the dorsolateral domain of the otocyst, whereas the initial compartmentalization of the otocyst remains unaffected. Our results demonstrate that Lmo4 controls the development of the dorsolateral otocyst into semicircular canals and cristae through two distinct mechanisms: regulating the expression of otic specific genes and stimulating the proliferation of the dorsolateral part of the otocyst. Author Affiliation: (a) University of Rochester Flaum Eye Institute, University of Rochester, Rochester, NY 14642, USA (b) Center for Neural Development and Disease, University of Rochester, Rochester, NY 14642, USA (c) Department of Neurobiology and Anatomy, University of Rochester, Rochester, NY 14642, USA Article History: Received 26 November 2008; Revised 4 November 2009; Accepted 4 November 2009

In mice, all of the six retinal neuron types are generated from common multipotent retinal progenitors, and their differentiation from progenitors is regulated by both extrinsic and intrinsic factors. Previously, we showed that targeted deletion of the atonal (ato) homologue math5 blocked the differentiation of most retinal ganglion cells (RGCs), revealing an essential role for math5 in RGC differentiation. In this study, we used the Cre-loxP recombination system to trace the fate of math5-expressing cells in retina. Our results demonstrated that math5 expression was associated with the differentiation of multiple retinal neuron types, including RGCs, photoreceptor, horizontal, and amacrine cells, implying that math5 expression alone is not sufficient to determine the RGC fate. Math5 expression was restricted to postmitotic cells in developing retina, suggesting that cell fate commitment of retinal neurons occurs after the terminal mitosis. The insufficiency of and requirement for math5 in RGC differentiation indicates that, like ato in the development of Drosophila R8 photoreceptors, math5 plays a role in determining the RGC competence state of retinal progenitors and that additional positive and negative factors are required in determining RGC fate. Furthermore, we show that loss of Math5 function severely reduced the RGC expression of the transcription factors Brn-3b, Gfi-1, Isl-1, Isl-2, Nscl-1, Nscl-2, and RPF-1, suggesting that Math5 expression is required to activate a comprehensive transcription network of RGC differentiation. Keywords: math5; Retina; Cell lineage; Neurogenesis; Retinal ganglion cells; Basic helix-loop-helix (bHLH)

An orthodenticle-related protein-encoding cDNA clone SpOtx from the sea urchin Strongylocentrotus purpuratus exhibits an orthodenticle homeobox but differs from its mouse and fly equivalents in sequence. Band-shift analysis reveals that SpOtx, with a binding specificity for the TAATCC/T sequence of DNA, binds to the Spec2a gene that contains multiple sites with this motif. The dividing embryo exhibits SpOtx transcripts initially in all cells, which is limited to oral endoderm and ectoderm cells in the later stages.

Data from site-directed mutagenesis and bandshift analysis on the RSR enhancer element of Strongylocentrotus purpuratus Spec2a gene indicates that the three otx binding sites in the enhancer and a fourth otx site upstream regulate Spec2a expression in mesenchyme and aboral ectoderm cells. Mutations in the RSR enhancer eliminates the activity of the otx binding sites. A study using LacZ reporter gene indicates that two of the otx sites are present within a 76-bp DNA fragment and that the two DNA elements in the otx site overlapping position modulate Spec2a transcription.

In vertebrate embryonic development, segmentation arises through a process that subdivides the paraxial mesoderm of the segmental plate into repeating metameric units called somites. How do cells of the segmental plate recognize their position and form discrete somite units? Recent studies indicate that this process involves in Notch signaling pathway, including the Notch-1, Delta-1 and Delta-3, RBPJ-Kappa, Presenilin-1 and Lunatic fringe. Identification and characterization of a group of the hairy and E(spl) related genes in Drosophila suggest that these proteins act as nuclear effectors for the Notch signaling pathway. Recently, an avian hairy like gene, c-hairy1 has been identified and shown to be expressed in the presomitic mesoderm in a cyclic fashion with temporal periodicity corresponding to the formation time of one somite. Here we report the cloning and expression analysis of a novel mouse hairy and E(spl) related subfamily, called hairy and E(spl) related gene (hesr) -1, -2, and -3

While the mammalian retina is well understood at the anatomical and physiological levels, little is known about the mechanisms that give rise to the retina's highly ordered pattern or its diverse neuronal cell types. Previous investigations have shown that gene disruption of the POU-IV class transcription factor Brn-3b (Brn-3.2) resulted in the loss of most retinal ganglion cells in retinas of postnatal mice. Here, we used lacZ and human placental alkaline phosphatase genes knocked into the brn-3b locus to follow the fate of brn-3b-mutant cells in the developing retina. We found that Brn-3b was not required for the initial commitment of retinal ganglion cell fate or for the migration of ganglion cells to the ganglion cell layer. However, Brn-3b was essential for the normal differentiation of retinal ganglion cells; without it, the cells underwent enhanced apoptosis. Retinal ganglion cells lacking brn-3b extended processes at the appropriate time in development, but these processes were disorganized, resulting in a thinner optic nerve. Explanted retinas from brn-3b-null embryos also extended processes when cultured in vitro, but the processes were shorter and less bundled than in wild-type retinas. Ultrastructural and marker analyses showed that the processes of mutant ganglion cells had dendritic rather than axonal features, suggesting that mutant cells formed dendrites in place of axons. These results suggest that Brn-3b regulates the activity of genes whose products play essential roles in the formation of retinal ganglion cell axons. Key Words: brn-3 genes; POU domain factors; retinal ganglion cells; retina development.