Vitellogenesis and the vitellogenin gene family.

Vitellogenin is synthesized under estrogen control in the liver, extensively modified, transported to the ovary, and there processed to the yolk proteins lipovitellin and phosvitin. In the frog Xenopus laevis there are at least four distinct but related vitellogenin genes. The two genes A1 and A2 have a 95 percent sequence homology in their messenger RNA coding regions, and contain 33 introns that interrupt the coding region (exons) at homologous positions. Sequences and lengths of analogous introns differ, and many introns contain repetitive DNA elements. The introns in these two genes that have apparently arisen by duplication have diverged extensively by events that include deletions, insertions, and probably duplications. Rapid evolutionary change involving rearrangements and the presence of repeated DNA suggests that the bulk of the sequences within introns may not have any specific function

Xrx1 controls proliferation and neurogenesis in Xenopus anterior neural plate

In Xenopus neuroectoderm, posterior cells start differentiating at the end of gastrulation, while anterior cells display an extended proliferative period and undergo neurogenesis only at tailbud stage. Recent studies have identified several important components of the molecular pathways controlling posterior neurogenesis, but little is known about those controlling the timing and positioning of anterior neurogenesis. We investigate the role of Xrx1, a homeobox gene required for eye and anterior brain development, in the control of proliferation and neurogenesis of the anterior neural plate. Xrx1 is expressed in the entire proliferative region of the anterior neural plate delimited by cells expressing the neuronal determination gene X-ngnr-1, the neurogenic gene X-Delta-1, and the cell cycle inhibitor p27Xic1. Positive and negative signals position Xrx1 expression to this region. Xrx1 is activated by chordin and Hedgehog gene signaling, which induce anterior and proliferative fate, and is repressed by the differentiation-promoting activity of neurogenin and retinoic acid. Xrx1 is required for anterior neural plate proliferation and, when overexpressed, induces proliferation, inhibits X-ngnr-1, X-Delta-1 and N-tubulin and counteracts X-ngnr-1- and retinoic acid-mediated differentiation. We find that Xrx1 does not act by increasing lateral inhibition but by inducing the antineurogenic transcriptional repressors Xhairy2 and Zic2, and by repressing p27Xic1. The effects of Xrx1 on proliferation, neurogenesis and gene expression are restricted to the most rostral region of the embryo, implicating this gene as an anterior regulator of neurogenesis

Cloning and developmental expression of zebrafish pdzrn3

Pdzrn3, a member of the PDZRN/SEMCAP/LNX protein family containing a RING finger and two PDZ domains, has been implicated in myoblasts and osteoblasts differentiation. However, its spatio-temporal expression pattern during embryonic development has not been defined. Here, we describe the cloning and expression pattern of pdzrn3 during zebrafish development. We found that, besides being expressed in several mesodermal structures, this gene displays specific expression in the central nervous system including rhombomere 1, ventral retina, thalamus and motor neurons, indicating a still undescribed function during neural development. In particular, absence of expression of pdzrn3 in the ventral retina of noi mutant suggests a role for this gene in regulating fasciculation and/or navigation of retinal ganglion cells axons

Motoneuron fate specification revealed by patterned LIM homeobox gene expression in embryonic zebrafish

Development121124117-4125DEVP