Requirement for LIM homeobox gene Isl1 in motor neuron generation reveals a motor neuron-dependent step in interneuron differentiation

The expression pattern of the transcription factor Phox2 delineates synaptic pathways of the autonomic nervous system

Many transcription factors, and most prominently among them, homeodomain proteins, are expressed in specific groups of cells in the developing nervous system in patterns that suggest their involvement in neural fate determination. How various aspects of neural identity are controlled by such transcription factors, or sets of them, is still mostly unknown. It has been shown previously that Phox2 is such a homeodomain protein, expressed exclusively in differentiated groups of neurons or their precursors, and that its expression correlated with that of the noradrenaline synthesis enzyme dopamine-beta-hydroxylase. Here we confirm this striking correlation at the single-cell level with the use of an anti-Phox2 antibody. Moreover, we uncover a second, nonmutually exclusive correlative clue to the Phox2 expression pattern: a high proportion of Phox2-expressing cells are involved in, or located in areas involved in, synaptic circuits, i.e., that of the medullary control reflexes of autonomic functions. This suggests that Phox2 could be involved in the establishment of these circuits.

floating head and masterblind regulate neuronal patterning in the roof of the forebrain

The epiphysial region of the dorsal diencephalon is the first site at which neurogenesis occurs in the roof of the zebrafish forebrain. We show that the homeobox containing gene floating head (flh) is required for neurogenesis to proceed in the epiphysis. In flh- embryos, the first few epiphysial neurons are generated, but beyond the 18 somite stage, neuronal production ceases. In contrast, in masterblind- (mbl-) embryos, epiphysial neurons are generated throughout the dorsal forebrain. We show that mbl is required to prevent the expression of flh in dorsal forebrain cells rostral to the epiphysis. Furthermore, epiphysial neurons are not ectopically induced in mbl-/flh- embryos, demonstrating that the epiphysial phenotype of mbl- embryos is mediated by ectopic Flh activity. We propose a role for Flh in linking the signaling pathways that regulate regional patterning to the signaling pathways that regulate neurogenesis.

Interactions of the LIM-domain-binding factor Ldb1 with LIM homeodomain proteins

The LIM homeodomain (LIM-HD) proteins, which contain two tandem LIM domains followed by a homeodomain, are critical transcriptional regulators of embryonic development. The LIM domain is a conserved cysteine-rich zinc-binding motif found in LIM-HD and LMO (rhombotin or Ttg) proteins, cytoskeletal components, LIM kinases and other proteins. LIM domains are protein-protein interaction motifs, can inhibit binding of LIM-HD proteins to DNA and can negatively regulate LIM-HD protein function. How LIM domains exert these regulatory effects is not known. We have now isolated a new LIM-domain-binding factor, Ldb1, on the basis of its ability to interact with the LIM-HD protein Lhx1 (Lim1). High-affinity binding by Ldb1 requires paired LIM domains and is restricted to the related subgroup of LIM domains found in LIM-HD and LMO proteins. The highly conserved Xenopus Ldb protein XLdb1, interacts with Xlim-1, the Xenopus orthologue of Lhx1. When injected into Xenopus embryos, XLdb1 (or Ldb1) can synergize with Xlim-1 in the formation of partial secondary axes and in activation of the genes encoding goosecoid (gsc), chordin, NCAM and XCG7, demonstrating a functional as well as a physical interaction between the two proteins.

Two critical periods of Sonic Hedgehog signaling required for the specification of motor neuron identity

Antibodies that block Sonic Hedgehog (SHH) signaling have been used to show that SHH activity is required for the induction of floor plate differentiation by the notochord and independently for the induction of motor neurons by both the notochord and midline neural cells. Motor neuron generation depends on two critical periods of SHH signaling: an early period during which naive neural plate cells are converted into ventralized progenitors and a late period that extends well into S phase of the final progenitor cell division, during which SHH drives the differentiation of ventralized progenitors into motor neurons. The ambient SHH concentration during the late period determines whether ventralized progenitors differentiate into motor neurons or interneurons, thus defining the pattern of neuronal cell types generated in the neural tube.

Diversity and pattern in the developing spinal cord

The generation of distinct neuronal cell types in appropriate numbers and at precise positions underlies the assembly of neural circuits that encode animal behavior. Despite the complexity of the vertebrate central nervous system, advances have been made in defining the principles that control the diversification and patterning of its component cells. A combination of molecular genetic, biochemical, and embryological assays has begun to reveal the identity and mechanism of action of molecules that induce and pattern neural tissue and the role of transcription factors in establishing generic and specific neuronal fates. Some of these advances are discussed here, focusing on the spinal cord as a model system for analyzing the molecular control of central nervous system development in vertebrates.

Nuclear LIM interactor, a rhombotin and LIM homeodomain interacting protein, is expressed early in neuronal development

LIM domain-containing transcription factors, including the LIM-only rhombotins and LIM-homeodomain proteins, are crucial for cell fate determination of erythroid and neuronal lineages. The zinc-binding LIM domains mediate protein-protein interactions, and interactions between nuclear LIM proteins and transcription factors with restricted expression patterns have been demonstrated. We have isolated a novel protein, nuclear LIM interactor (NLI), that specifically associates with a single LIM domain in all nuclear LIM proteins tested. NLI is expressed in the nuclei of diverse neuronal cell types and is coexpressed with a target interactor islet-1 (Isl1) during the initial stages of motor neuron differentiation, suggesting the mutual involvement of these proteins in the differentiation process. The broad range of interactions between NLI and LIM-containing transcription factors suggests the utilization of a common mechanism to impart unique cell fate instructions.

Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function

Targeted gene disruption in the mouse shows that the Sonic hedgehog (Shh) gene plays a critical role in patterning of vertebrate embryonic tissues, including the brain and spinal cord, the axial skeleton and the limbs. Early defects are observed in the establishment or maintenance of midline structures, such as the notochord and the floorplate, and later defects include absence of distal limb structures, cyclopia, absence of ventral cell types within the neural tube, and absence of the spinal column and most of the ribs. Defects in all tissues extend beyond the normal sites of Shh transcription, confirming the proposed role of Shh proteins as an extracellular signal required for the tissue-organizing properties of several vertebrate patterning centres.

Transcription factor genes and the developing eye: a genetic perspective

We review the current knowledge of transcription factors in mammallan eye development. The 14 transcription factors presently known to be required for eye formation are examined in some detail, incorporating data from both humans and rodents. Aspects of the biochemistry, expression patterns, genetics, mutant phenotypes, and biological insights acquired from the examination of loss-of-function mutations are summarized. The other 32 tissue-restricted transcription factors that are currently known to be expressed in the developing or mature mammallan eye are tabulated, together with the timing and site of their ocular expression; the requirement for most of these genes in the eye is unknown. Contributions to mammallan eye development from the study of the genetics of the Drosophila eye are discussed briefly. Identification of the entire cohort of transcription factors required for eye development is an essential first step towards understanding the mechanisms underlying eye morphogenesis and differentiation, and the molecular basis of inherited eye abnormalities in humans.

Molecular genetic approaches to nociceptor development and function

The activation of peripheral nociceptors is the subject of intense scrutiny, because of its significance in pain regulation. Genetic approaches, including homology cloning, difference cloning and transgenic manipulation of mice are providing useful insights into nociceptor function. Recent work suggests that transcriptional regulators (for example, islet-I), which are expressed relatively selectively in sensory neurones, play a crucial role in defining cellular phenotype. Difference cloning has identified genes which encode both ligand-gated and voltage-gated ion channels expressed by small-diameter sensory neurones. The role of inflammatory mediators such as NGF in regulating nociceptor function has been clarified in mis-expression and deletion studies. An understanding of the mechanisms that regulate gene expression in nociceptors should provide new ways to manipulate nociceptor sensitivity, with potential significance for pain therapy.

Patterning cascades in the neural tube. Neural development

The vertebrate central nervous system comprises an intricate array of neurons generated in a highly organized way. Examination of the genes expressed and required at early stages of neural differentiation reveals that a coordinated signalling cascade transforms progenitor cells into discrete neuronal subsets.