Curtis R. Altmann

Neural patterning in the vertebrate embryo.

The embryonic central nervous system (CNS) is patterned along its antero-posterior, dorsal-ventral, and left-right axes. Along the dorsal-ventral axis, cell fate determination occurs during and following neural tube closure and involves the action of two opposing signaling pathways: SHH ventrally from the notochord and BMP/GDF dorsally from the boundary of neural and nonneural ectoderm and later from the roof plate. In addition, Wnt and retinoic acid signaling have been shown to act in dorsal-ventral patterning; however, their roles are understood in less detail. Along the antero-posterior axis, signals divide the neural tube into four major divisions: forebrain, midbrain, hindbrain, and spinal cord, and these differences can be detected soon after the formation of the neural plate. The FGF, Wnt, and retinoic acid signaling pathways have been implicated in the caudalization of neural tissue. Boundaries of Hox gene expression are observed along the anteroposterior axis and have been suggested to be involved in establishing different identities in the hindbrain and spinal cord. Complex gene expression patterns in the brain suggest the development of neuromeres dividing the brain into different regions that are elaborated further during development. Patterning along the left-right axis occurs concurrently with antero-posterior and dorsal-ventral patterning during gastrulation. A leading candidate for initiating asymmetry is activin, which acts through Nodal and Lefty before any morphological differences are observed. The big challenge will be understanding how these diverse signaling pathways interact both temporally and spatially to generate the complex adult nervous system.

Cell fate specification and competence by Coco, a maternal BMP, TGFbeta and Wnt inhibitor.

Patterning of the pre-gastrula embryo and subsequent neural induction post-gastrulation are very complex and intricate processes of which little, until recently, has been understood. The earliest decision in neural development, the choice between epidermal or neural fates, is regulated by bone morphogenetic protein (BMP) signaling within the ectoderm. Inhibition of BMP signaling is sufficient for neural induction. Many secreted BMP inhibitors are expressed exclusively within the organizer of the Xenopus gastrula embryo and therefore are predicted to act as bona fide endogenous neural inducers. Other cell-autonomous inhibitors of the BMP pathway are more widely expressed, such as the inhibitory Smads, Smad6 and Smad7. In this report we describe the biological and biochemical characterization of 51-B6, a novel member of Cerberus/Dan family of secreted BMP inhibitors, which we identified in a screen for Smad7-induced genes. This gene is expressed maternally in an animal to vegetal gradient, and its expression levels decline rapidly following gastrulation. In contrast to known BMP inhibitors, 51-B6 is broadly expressed in the ectoderm until the end of gastrulation. The timing, pattern of expression, and activities of this gene makes it unique when compared to other BMP/TGFβ/Wnt secreted inhibitors which are expressed only zygotically and maintained post-gastrulation. We propose that a function of 51-B6 is to block BMP and TGFβ signals in the ectoderm in order to regulate cell fate specification and competence prior to the onset of neural induction. In addition, we demonstrate that 51-B6 can act as a neural inducer and induce ectopic head-like structures in neurula staged embryos. Because of this embryological activity, we have renamed this clone Coco, after the Spanish word meaning head.

Gene profiling during neural induction in Xenopus laevis: regulation of BMP signaling by post-transcriptional mechanisms and TAB3, a novel TAK1-binding protein

The earliest decision in vertebrate neural development is the acquisition of a neural identity by embryonic ectodermal cells. The default model for neural induction postulates that neural fate specification in the vertebrate embryo occurs by inhibition of epidermal inducing signals in the gastrula ectoderm. Bone morphogenetic proteins (BMPs) act as epidermal inducers, and all identified direct neural inducers block BMP signaling either intra- or extracellularly. Although the mechanism of action of the secreted neural inducers has been elucidated, the relevance of intracellular BMP inhibitors in neural induction is not clear. In order to address this issue and to identify downstream targets after BMP inhibition, we have monitored the transcriptional changes in ectodermal explants neuralized by Smad7 using a Xenopus laevis 5000-clone gastrula-stage cDNA microarray. We report the identification and initial characterization of 142 genes whose transcriptional profiles change in the neuralized explants. In order to address the potential involvement during neural induction of genes identified in the array, we performed gain-of-function studies in ectodermal explants. This approach lead to the identification of four genes that can function as neural inducers in Xenopus and three others that can synergize with known neural inducers in promoting neural fates. Based on these studies, we propose a role for post-transcriptional control of gene expression during neural induction in vertebrates and present a model whereby sustained BMP inhibition is promoted partly through the regulation of TGFβ activated kinase (TAK1) activity by a novel TAK1-binding protein (TAB3).

Gene Expression Profiling of Osteoclast Differentiation by Combined Suppression Subtractive Hybridization (SSH) and cDNA Microarray Analysis

Bone homeostasis is maintained by the balanced action of bone-forming osteoblasts and bone-resorbing osteoclasts. Multinucleated, mature osteoclasts develop from hematopoietic stem cells via the monocyte-macrophage lineage, which also give rise to macrophages and dendritic cells. Despite their distinct physiologic roles in bone and the immune system, these cell types share many molecular and biochemical features. To provide insights into how osteoclasts differentiate and function to control bone metabolism, we employed a systematic approach to profile patterns of osteoclast-specific gene expression by combining suppression subtractive hybridization (SSH) and cDNA microarray analysis. Here we examined how gene expression profiles of mature osteoclast differ from macrophage or dendritic cells, how gene expression profiles change during osteoclast differentiation, and how Mitf, a transcription factor critical for osteoclast maturation, affects the gene expression profile. This approach revealed a set of genes coordinately regulated for osteoclast function, some of which have previously been implicated in several bone diseases in humans.

The POU homeobox protein Oct-1 regulates radial glia formation downstream of Notch signaling

Radial glia cells function as guide cells for neuronal migration and a source of neural progenitor cells, and play a crucial role for the development of the central nervous system. To date, several signals have been demonstrated to promote the formation of radial glia cells and Notch signaling is one such signal. However, the mechanism of the signaling hierarchy of radial glia developmental cascade promoted by Notch signaling still remains incomplete. Here we show that Notch signaling promotes Xenopus radial glia formation and that the Notch activation is sufficient for radial glia formation prior to neural tube closure. Moreover, we have identified Oct-1 (POU2f1), a POU transcription factor, as a downstream target of Notch signaling by microarray based screen. We demonstrate that the expression of Oct-1 in the brain is regulated by Notch signaling and that Oct-1 is sufficient and necessary for radial glia formation. Together, Oct-1 is a downstream effector of Notch signaling during radial glia formation.

Cloning and developmental expression of Baf57 in Xenopus laevis

Mammalian and Drosophila homologues of Baf57 have been previously isolated as being a subunit of SWI/SNF-like chromatin remodeling complexes. Here, we report the cloning and developmental expression of Xenopus Baf57. We isolated XBaf57 by using an expression cloning approach to identify novel modulators of Xenopus Smad7. XBaf57 co-operates with XSmad7 by increasing the expression of neural markers in ectodermal explants. XBaf57 is expressed in the ectoderm and pre-involuting mesoderm during gastrula stages and in the central nervous system during neurula and tailbud stages. These results raise the possibility that XBaf57 (or XBaf57-containing chromatin remodelling complexes) may be involved in the process of neural induction during Xenopus embryonic development.

Genomics and embryology in amphibians

A report on the Eighth Biannual Xenopus Conference, Estes Park, Colorado, August 16-20, 2000.