Interaction of the developmental regulator SALL1 with UBE2I and SUMO-1

Mutations in the SALL1 gene on chromosome 16q12.1 cause Townes-Brocks syndrome (TBS). This autosomal dominantly inherited disorder is characterized by typical malformations of the thumbs, the ears, and the anus, and also commonly affects the kidneys and other organ systems. SALL1 has recently been shown to localize to chromocenters and other heterochromatin foci in murine fibroblasts and to interact with the telomere-repeat-binding factor TRF1/PIN2. Here, we show that the ubiquitin-conjugating enzyme 2I (UBE2I), the human homolog of S. cerevisiae UBC9, and the small ubiquitin-like modifier-1 (SUMO-1) interact with SALL1 in the yeast two-hybrid system. The interaction of SALL1 and UBE2I was confirmed in a glutathione S-transferase (GST) pull-down experiment. In an in vitro assay, it could be demonstrated that SALL1 is covalently modified by at least two SUMO-1 molecules in the presence of UBA2/AOS1 and UBE2I. Mutation of lysine 1086 of SALL1 to arginine abrogates SALL1 sumoylation, suggesting the presence of a polymeric SUMO-1 chain in the wild type state.     

The role of Wnt genes in vertebrate development.

Over the past decade, many potential candidates for molecules involved in pattern formation in the vertebrate embryo have been identified. Manipulation of the expression of some of these factors has generated fascinating results that have allowed investigators to address their roles in embryogenesis. One such family consists of a group of putative cell signaling molecules related to the proto-oncogene Wnt-1. An accumulating body of evidence suggests that the Wnt-family plays a major role in several aspects of vertebrate development.     

Cilia in vertebrate development and disease

Through the combined study of model organisms, cell biology, cell signaling and medical genetics we have significantly increased our understanding of the structure and functions of the vertebrate cilium. This ancient organelle has now emerged as a crucial component of certain signaling and sensory perception pathways in both developmental and homeostatic contexts. Here, we provide a snapshot of the structure, function and distribution of the vertebrate cilium and of the pathologies that are associated with its dysfunction.     

The role of Hox genes during vertebrate limb development

The potential role of Hox genes during vertebrate limb development was brought into focus by gene expression analyses in mice (P Dolle, JC Izpisua-Belmonte, H Falkenstein, A Renucci, D Duboule, Nature 1989, 342:767-772), at a time when limb growth and patterning were thought to depend upon two distinct and rather independent systems of coordinates; one for the anterior-to-posterior axis and the other for the proximal-to-distal axis (see D Duboule, P Dolle, EMBO J 1989, 8:1497-1505). Over the past years, the function and regulation of these genes have been addressed using both gain-of-function and loss-of-function approaches in chick and mice. The use of multiple mutations either in cis-configuration in trans-configuration or in cis/trans configurations, has confirmed that Hox genes are essential for proper limb development, where they participate in both the growth and organization of the structures. Even though their molecular mechanisms of action remain somewhat elusive, the results of these extensive genetic analyses confirm that, during the development of the limbs, the various axes cannot be considered in isolation from each other and that a more holistic view of limb development should prevail over a simple cartesian, chess grid-like approach of these complex structures. With this in mind, the functional input of Hox genes during limb growth and development can now be re-assessed.     

Homeobox genes and development of the vertebrate CNS

The discovery of homeobox genes in vertebrates may allow analysis of a basic problem in developmental neurobiology: how regional differences in CNS organization are specified during development. This view is based on the roles defined for homologous genes in Drosophila development, and is supported by studies of the patterns of homeobox gene expression in vertebrate embryos. Homeobox genes comprise a multigene family, members of which are expressed in different spatially restricted domains along the anterior-posterior axis of the CNS. These observations are consistent with homeobox genes having roles in the positional specification of CNS organization, and experimental tests of this should be forthcoming shortly.     

Transforming growth factor-beta-related genes in Drosophila and vertebrate development.

The Drosophila decapentaplegic gene, the Xenopus activin genes and the genes encoding the mouse bone morphogenetic proteins are transforming growth factor-beta-related genes whose roles in development are the focus of current studies. They exhibit elaborate patterns of expression during development, and the protein products have potent effects on the differentiation of specific cell types.     

Regulation of vertebrate eye development by Rx genes

The paired-like homeobox-containing gene Rx has a critical role in the eye development of several vertebrate species including Xenopus, mouse, chicken, medaka, zebrafish and human. Rx is initially expressed in the anterior neural region of developing embryos, and later in the retina and ventral hypothalamus. Abnormal regulation or function of Rx results in severe abnormalities of eye formation. Overexpression of Rx in Xenopus and zebrafish embryos leads to overproliferation of retinal cells. A targeted elimination of Rx in mice results in a lack of eye formation. Mutations in Rx genes are the cause of the mouse mutation eyeless (ey1), the medaka temperature sensitive mutation eyeless (el) and the zebrafish mutation chokh. In humans, mutations in Rx lead to anophthalmia. All of these studies indicate that Rx genes are key factors in vertebrate eye formation. Because these results cannot be easily reconciled with the most popular dogmas of the field, we offer our interpretation of eye development and evolution.     

SALL4 is directly activated by TCF/LEF in the canonical Wnt signaling pathway

The SALL4 promoter has not yet been characterized. Animal studies showed that SALL4 is downstream of and interacts with TBX5 during limb and heart development, but a direct regulation of SALL4 by TBX5 has not been demonstrated. For other SAL genes, regulation within the Shh, Wnt, and Fgf pathways has been reported. Chicken csal1 expression can be activated by a combination of Fgf4 and Wnt3a or Wnt7a. Murine Sall1 enhances, but Xenopus Xsal2 represses, the canonical Wnt signaling. Here we describe the cloning and functional analysis of the SALL4 promoter. Within a minimal promoter region of 31bp, we identified a consensus TCF/LEF-binding site.The SALL4 promoter was strongly activated not only by LEF1 but also by TCF4E. Mutation of the TCF/LEF-binding site resulted in decreased promoter activation. Our results demonstrate for the first time the direct regulation of a SALL gene by the canonical Wnt signaling pathway.     

Regulation of R7 and R8 differentiation by the spalt genes

Photoreceptor development begins in the larval eye imaginal disc, where eight distinct photoreceptor cells (R1-R8) are sequentially recruited into each of the developing ommatidial clusters. Final photoreceptor differentiation, including rhabdomere formation and rhodopsin expression, is completed during pupal life. During pupation, spalt was previously proposed to promote R7 and R8 terminal differentiation. Here we show that spalt is required for proper R7 differentiation during the third instar larval stage since the expression of several R7 larval markers (prospero, enhancer of split mdelta0.5, and runt) is lost in spalt mutant clones. In R8, spalt is not required for cell specification or differentiation in the larval disc but promotes terminal differentiation during pupation. We show that spalt is necessary for senseless expression in R8 and sufficient to induce ectopic senseless in R1-R6 during pupation. Moreover, misexpression of spalt or senseless is sufficient to induce ectopic rhodopsin 6 expression and partial suppression of rhodopsin 1. We demonstrate that spalt and senseless are part of a genetic network, which regulates rhodopsin 6 and rhodopsin 1. Taken together, our results suggest that while spalt is required for R7 differentiation during larval stages, spalt and senseless promote terminal R8 differentiation during pupal stages, including the regulation of rhodopsin expression.