Transcriptional networks regulating neuronal identity in the developing spinal cord.

The spinal cord is composed of anatomically distinct classes of neurons that perform sensory and motor functions. Because of its relative simplicity, the spinal cord has served as an important system for defining molecular mechanisms that contribute to the assembly of circuits in the central nervous system. At early embryonic stages, the neural tube contains multipotential cells whose identity becomes specified by cell-to-cell signaling. This review will focus on the progress made in understanding the transcriptional networks that become activated by these cell-cell interactions, with particular emphasis on the neurons that contribute to locomotor control. Remarkably, many of the transcription factors implicated in neuronal specification in the spinal cord are found to inhibit transcription, which has led to a 'derepression' model for cell fate specification in the developing spinal cord.

Cracking the transcriptional code for cell specification in the neural tube.

The bHLH repressor Olig2 participates in the transcriptional code governing cell fate specification in the ventral spinal cord. By temporally selective interactions with other transcription factors, Olig2 first directs motor neuron fate and later switches to promoting oligodendrocyte production.

Distinct roles of nerve and muscle in postsynaptic differentiation of the neuromuscular synapse.

The development of chemical synapses is regulated by interactions between pre- and postsynaptic cells. At the vertebrate skeletal neuromuscular junction, the organization of an acetylcholine receptor (AChR)-rich postsynaptic apparatus has been well studied. Much evidence suggests that the nerve-derived protein agrin activates muscle-specific kinase (MuSK) to cluster AChRs through the synapse-specific cytoplasmic protein rapsyn. But how postsynaptic differentiation is initiated, or why most synapses are restricted to an 'end-plate band' in the middle of the muscle remains unknown. Here we have used genetic methods to address these issues. We report that the initial steps in postsynaptic differentiation and formation of an end-plate band require MuSK and rapsyn, but are not dependent on agrin or the presence of motor axons. In contrast, the subsequent stages of synaptic growth and maintenance require nerve-derived agrin, and a second nerve-derived signal that disperses ectopic postsynaptic apparatus.

Topographic mapping from the retina to the midbrain is controlled by relative but not absolute levels of EphA receptor signaling.

Topographic maps are a fundamental feature of sensory representations in nervous systems. The formation of one such map, defined by the connection of ganglion cells in the retina to their targets in the superior colliculus of the midbrain, is thought to depend upon an interaction between complementary gradients of retinal EphA receptors and collicular ephrin-A ligands. We have tested this hypothesis by using gene targeting to elevate EphA receptor expression in a subset of mouse ganglion cells, thereby producing two intermingled ganglion cell populations that express distinct EphA receptor gradients. We find that these two populations form separate maps in the colliculus, which can be predicted as a function of the net EphA receptor level that a given ganglion cell expresses relative to its neighbors.

Genetic and epigenetic mechanisms contribute to motor neuron pathfinding.

Many lines of evidence indicate that genetically distinct subtypes of motor neurons are specified during development, with each type having characteristic properties of axon guidance and cell-body migration. Motor neuron subtypes express unique combinations of LIM-type homeodomain factors that may act as intrinsic genetic regulators of the cytoskeletal events that mediate cell migration, axon navigation or both. Although experimentally displaced motor neurons can pioneer new routes to their targets, in many cases the axons of motor neurons in complete isolation from their normal territories passively follow stereotypical pathways dictated by the environment. To investigate the nonspecific versus genetically controlled regulation of motor connectivity we forced all motor neurons to express ectopically a LIM gene combination appropriate for the subgroup that innervates axial muscles. Here we show that this genetic alteration is sufficient to convert the cell body settling pattern, gene-expression profile and axonal projections of all motor neurons to that of the axial subclass. Nevertheless, elevated occupancy of the axial pathway can override their genetic program, causing some axons to project to alternative targets.

Transcriptional mechanisms in the development of motor control.

Considerable progress has been made in understanding how combinatorially expressed transcription factors control the development of neuronal subtypes in the fly and vertebrate central nervous systems. The mode of action of many of these factors has been conserved from invertebrates to vertebrates throughout evolution, such as the formation and regulation of specific transcriptional complexes, the utilization of repressors for maintaining specificity, and the use of phosphorylation as an important means for transiently altering transcriptional activity.

Active suppression of interneuron programs within developing motor neurons revealed by analysis of homeodomain factor HB9.

Sonic hedgehog (Shh) specifies the identity of both motor neurons (MNs) and interneurons with morphogen-like activity. Here, we present evidence that the homeodomain factor HB9 is critical for distinguishing MN and interneuron identity in the mouse. Presumptive MN progenitors and postmitotic MNs express HB9, whereas interneurons never express this factor. This pattern resembles a composite of the avian homologs MNR2 and HB9. In mice lacking Hb9, the genetic profile of MNs is significantly altered, particularly by upregulation of Chx10, a gene normally restricted to a class of ventral interneurons. This aberrant gene expression is accompanied by topological disorganization of motor columns, loss of the phrenic and abducens nerves, and intercostal nerve pathfinding defects. Thus, MNs actively suppress interneuron genetic programs to establish their identity.

Pancreas dorsal lobe agenesis and abnormal islets of Langerhans in Hlxb9-deficient mice.

In most mammals the pancreas develops from the foregut endoderm as ventral and dorsal buds. These buds fuse and develop into a complex organ composed of endocrine, exocrine and ductal components. This developmental process depends upon an integrated network of transcription factors. Gene targeting experiments have revealed critical roles for Pdx1, Isl1, Pax4, Pax6 and Nkx2-2 (refs 3,4,5,6,7, 8,9,10). The homeobox gene HLXB9 (encoding HB9) is prominently expressed in adult human pancreas, although its role in pancreas development and function is unknown. To facilitate its study, we isolated the mouse HLXB9 orthologue, Hlxb9. During mouse development, the dorsal and ventral pancreatic buds and mature beta-cells in the islets of Langerhans express Hlxb9. In mice homologous for a null mutation of Hlxb9, the dorsal lobe of the pancreas fails to develop. The remnant Hlxb9-/- pancreas has small islets of Langerhans with reduced numbers of insulin-producing beta-cells. Hlxb9-/- beta-cells express low levels of the glucose transporter Glut2 and homeodomain factor Nkx 6-1. Thus, Hlxb9 is key to normal pancreas development and function.

LIM homeodomain factors Lhx3 and Lhx4 assign subtype identities for motor neurons.

The circuits that control movement are comprised of discrete subtypes of motor neurons. How motor neuron subclasses develop and extend axons to their correct targets is still poorly understood. We show that LIM homeodomain factors Lhx3 and Lhx4 are expressed transiently in motor neurons whose axons emerge ventrally from the neural tube (v-MN). Motor neurons develop in embryos deficient in both Lhx3 and Lhx4, but v-MN cells switch their subclass identity to become motor neurons that extend axons dorsally from the neural tube (d-MN). Conversely, the misexpression of Lhx3 in dorsal-exiting motor neurons is sufficient to reorient their axonal projections ventrally. Thus, Lhx3 and Lhx4 act in a binary fashion during a brief period in development to specify the trajectory of motor axons from the neural tube.

Formation of Rathke's pouch requires dual induction from the diencephalon.

Targeted disruption of the homeobox gene T/ebp (Nkx2.1, Ttf1, Titf1) in mice results in ablation of the pituitary. Paradoxically, while T/ebp is expressed in the ventral diencephalon during forebrain formation, it is not expressed in Rathke's pouch or in the pituitary gland at any time of embryogenesis. Examination of pituitary development in the T/ebp homozygous null mutant embryos revealed that a pouch rudiment is initially formed but is eliminated by programmed cell death before formation of a definitive pouch. In the diencephalon of the mutant, Bmp4 expression is maintained, whereas Fgf8 expression is not detectable. These data and additional genetic and molecular observations suggest that Rathke's pouch develops in a two-step process that requires at least two sequential inductive signals from the diencephalon. First, BMP4 is required for induction and formation of the pouch rudiment, a role confirmed by analysis of Bmp4 homozygous null mutant embryos. Second, FGF8 is necessary for activation of the key regulatory gene Lhx3 and subsequent development of the pouch rudiment into a definitive pouch. This study provides firm molecular genetic evidence that morphogenesis of the pituitary primordium is induced in vivo by signals from the adjacent diencephalon.

Xenopus TFIIIA gene transcription is dependent on cis-element positioning and chromatin structure.

The Xenopus TFIIIA gene is transcribed very efficiently in oocytes. In addition to a TATA element at -30, we show that from -425 to +7 the TFIIIA gene contains only two positive cis elements centered at -267 (element 1) and -230 (element 2). This arrangement of the cis elements in the TFIIIA gene is striking because these two elements are positioned very close to each other yet separated from the TATA element by approximately 190 nucleotides. We show that the 190-nucleotide spacing between the TATA element and the upstream cis elements (elements 1 and 2) is critical for efficient transcription of the gene in oocytes and that a nucleosome is positioned in this intervening region. This nucleosome may act positively on TFIIIA transcription in oocytes by placing transcription factors bound at elements 1 and 2 in a favorable position relative to the transcription complex at the TATA element.

The nuclear LIM domain interactor NLI mediates homo- and heterodimerization of LIM domain transcription factors.

LIM domain-containing transcription factors are required for embryonic survival and for the determination of many cell types. The combinatorial expression of the LIM homeodomain proteins Isl1, Isl2, Lhx1, and Lhx3 in subsets of developing motor neurons correlates with the future organization of these neurons into motor columns with distinct innervation targets, implying a functional role for LIM homeodomain protein combinations in the specification of neuronal identity. NLI is a widely expressed, dimeric protein that has been shown to specifically interact with the LIM domains of LIM domain-containing transcription factors. The present studies demonstrate that NLI mediates homo- and heteromeric complex formation between LIM domain transcription factors, requiring both the N-terminal dimerization and C-terminal LIM interaction domains of NLI. Although the interaction between most LIM homeodomain proteins is dependent on NLI, a direct interaction between the LIM domains of Lhx3 and the homeodomains of Isl1 and Isl2 was also observed. This interaction was disrupted by NLI, demonstrating that the conformational state of Lhx3-Isl1/Isl2 complexes is modified by NLI. Evidence indicating that NLI facilitates long range enhancer-promoter interactions suggests that NLI-dependent LIM domain transcription factor complexes are involved in communication between transcriptional control elements.

Independent requirement for ISL1 in formation of pancreatic mesenchyme and islet cells.

The mammalian pancreas is a specialized derivative of the primitive gut endoderm and controls many homeostatic functions through the activity of its component exocrine acinar and endocrine islet cells. The LIM homeodomain protein ISL1 is expressed in all classes of islet cells in the adult and its expression in the embryo is initiated soon after the islet cells have left the cell cycle. ISL1 is also expressed in mesenchymal cells that surround the dorsal but not ventral evagination of the gut endoderm, which together comprise the pancreatic anlagen. To define the role of ISL1 in the development of the pancreas, we have now analysed acinar and islet cell differentiation in mice deficient in ISL1 function. Dorsal pancreatic mesenchyme does not form in ISL1-mutant embryos and there is an associated failure of exocrine cell differentiation in the dorsal but not the ventral pancreas. There is also a complete loss of differentiated islet cells. Exocrine, but not endocrine, cell differentiation in the dorsal pancreas can be rescued in vitro by provision of mesenchyme derived from wild-type embryos. These results indicate that ISL1, by virtue of its requirement for the formation of dorsal mesenchyme, is necessary for the development of the dorsal exocrine pancreas, and also that ISL1 function in pancreatic endodermal cells is required for the generation of all endocrine islet cells.

Differential expression of LIM homeobox genes among motor neuron subpopulations in the developing chick brain stem.

During development of the chick brain stem, cranial motor neuron subpopulations differentiate at distinct axial levels and extend their axons along specific pathways into the periphery. Differences in phenotype and axonal trajectory of these neuronal populations might be governed by the expression of different repertoires of transcription factors. In 2- to 7-day chick embryos, we find that genes of the LIM homeobox family are expressed differentially among cranial motor nuclei. Whereas Islet-1 is expressed by motor neurons of all cranial nerves, Islet-2 is expressed only in nuclei that contain somatic motor neurons and transiently in a specialized population of contralateral vestibuloacoustic efferent neurons. Lim-3 is expressed in the hypoglossal and accessory abducens nuclei only, and Lim-1 and Lim-2 are not expressed by cranial motor neurons. Our findings are consistent with a role of these transcription factors in determining neuronal phenotype and axonal pathfinding.

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

Motor neuron differentiation is accompanied by the expression of a LIM homeodomain transcription factor, Islet1 (ISL1). To assess the involvement of ISL1 in the generation of motor neurons, we analyzed cell differentiation in the neural tube of embryos in which ISL1 expression has been eliminated by gene targeting. Motor neurons are not generated without ISL1, although many other aspects of cell differentiation in the neural tube occur normally. A population of interneurons that express Engrailed1 (EN1), however, also fails to differentiate in Isl1 mutant embryos. The differentiation of EN1+ interneurons can be induced in both wild-type and mutant neural tissue by regions of the neural tube that contain motor neurons. These results show that ISL1 is required for the generation of motor neurons and suggest that motor neuron generation is required for the subsequent differentiation of certain interneurons.

Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes.

Motor neurons located at different positions in the embryonic spinal cord innervate distinct targets in the periphery, establishing a topographic neural map. The topographic organization of motor projections depends on the generation of subclasses of motor neurons that select specific paths to their targets. We have cloned a family of LIM homeobox genes in chick and show here that the combinatorial expression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasses of motor neurons that segregate into columns in the spinal cord and select distinct axonal pathways. These genes are expressed prior to the formation of distinct motor axon pathways and before motor columns appear. Our results suggest that LIM homeobox genes contribute to the generation of motor neuron diversity and may confer subclasses of motor neurons with the ability to select specific axon pathways, thereby initiating the topographic organization of motor projections.

Control of cell pattern in the neural tube: motor neuron induction by diffusible factors from notochord and floor plate.

The identity of cell types generated along the dorsoventral axis of the neural tube depends on inductive signals that derive from both mesodermal and neural cells. To define the nature of these signals, we have analyzed the differentiation of cells in neural plate explants. Motor neurons and neural crest cells differentiate in vitro from appropriate regions of the neural plate, indicating that the specification of cell fate along the dorsoventral axis of the neural tube begins at the neural plate stage. Motor neuron differentiation can be induced by a diffusible factor that derives initially from the notochord and later from floor plate cells. By contrast, floor plate induction requires contact with the notochord. Thus, the identity and patterning of neural cell types appear to involve distinct contact-mediated and diffusible signals from the notochord and floor plate.

Characterization of a Xenopus oocyte factor that binds to a developmentally regulated cis-element in the TFIIIA gene.

TFIIIA mRNA expression is developmentally regulated during Xenopus oogenesis. We have identified a positive-acting cis-element in the TFIIIA gene located between -671 and -629 with respect to the mRNA initiation site, termed element 3. Element 3 contributes to the efficient transcription of the TFIIIA gene in stage III oocytes, but is inactive in the normal context of the TFIIIA gene in stage VI oocytes. A trans-acting factor, termed B3, in immature ovary extracts binds to a region within element 3 containing four repeats of the consensus nucleotide sequence 5' (T/A)GGTTACT. We discuss how the developmental regulation of B3-element 3 function could be achieved in the context of the TFIIIA gene during oogenesis.

Regulation of the Xenopus laevis transcription factor IIIA gene during oogenesis and early embryogenesis: negative elements repress the O-TFIIIA promoter in embryonic cells.

Expression of the Xenopus laevis transcription factor IIIA (TFIIIA) gene is developmentally regulated. In this study we have used defined nucleotide mutations to map cis-elements involved in transcriptional regulation of the promoter for oocyte-TFIIIA (O-TFIIIA) in stage II-IV oocytes, stage VI oocytes, and tail bud embryos. During oogenesis O-TFIIIA mRNA levels decline 5- to 10-fold, and during early embryogenesis O-TFIIIA mRNA levels decline approximately 10(6)-fold per cell. In stage II-IV oocytes we find evidence for at least three distinct positive-acting cis-elements that contribute to the efficient expression of O-TFIIIA. These elements are located between -1800 to -425, -280 to -235, and -235 to -220. The most distal cis-element(s) appears to be developmentally regulated during oogenesis, since deletion of nucleotide sequences from -1800 to -425 does not reduce O-TFIIIA expression in stage VI oocytes. However, the two cis-elements located between -280 to -235 and -235 to -220 are required for the efficient expression of O-TFIIIA in stage VI oocytes. In tail bud embryos we find evidence for several developmentally regulated positive and negative cis-elements involved in O-TFIIIA expression. The positive-acting cis-elements are located between -159 to -110 and -110 to -58. The negative-acting cis-elements are found at positions -425 to -350 and -200 to -159. In addition to the developmentally regulated elements controlling O-TFIIIA gene expression in tail bud embryos, the positive-acting cis-elements active during oogenesis (located between -280 to -235 and -235 to -220) are also active during early embryogenesis. Thus, transcription from the O-TFIIIA promoter appears to be regulated by a combination of constitutive positive factors and developmentally regulated positive and negative factors during oogenesis and early embryogenesis.

pOEV: a Xenopus oocyte protein expression vector.

We have constructed a Xenopus laevis oocyte expression vector, pOEV, which allows cloned DNA to be transcribed and translated directly in the oocyte. Since proteins translated in oocytes are post-translationally modified according to conserved eukaryotic signals, these cells offer a convenient system for performing structural and functional analyses of cloned genes. pOEV can be used for direct analysis of proteins encoded by cloned cDNAs without preparing mRNA in vitro, simplifying existing protocols for translating proteins in oocytes with a very high translational yield. Transcription of the vector in oocytes is driven by the promoter for the TFIIIA gene, which can generate 1-2 ng (per oocyte within 2 days) of stable mRNA template for translation. The vector also contains SP6 and T7 promoters for in vitro transcription to make mRNA and hybridization probes. DNA clones encoding chloramphenicol acetyltransferase (CAT) were injected into oocyte germinal vesicles and CAT protein accumulated in the cell over a 2- to 4-day period. We found that the concentration of DNA injected affected protein yields; surprisingly relatively low concentrations in the range 25-50 pg DNA per oocyte gave maximum yields of CAT protein. When as little as 5 pg of pOEV DNA is injected we typically expressed 40 fmol of CAT protein per oocyte, after 4-day incubations. In addition, we have shown that this system is amenable to the expression of nuclear and membrane proteins.