TWH regulates the development of subsets of spinal cord neurons

Intracellular Communication among Morphogen Signaling Pathways during Vertebrate Body Plan Formation

During embryonic development in vertebrates, morphogens play an important role in cell fate determination and morphogenesis. Bone morphogenetic proteins (BMPs) belonging to the transforming growth factor-β (TGF-β) family control the dorsal-ventral (DV) patterning of embryos, whereas other morphogens such as fibroblast growth factor (FGF), Wnt family members, and retinoic acid (RA) regulate the formation of the anterior-posterior (AP) axis. Activation of morphogen signaling results in changes in the expression of target genes including transcription factors that direct cell fate along the body axes. To ensure the correct establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated by a fine-tuning of morphogen signaling. In this review, we focus on the interplay of various intracellular regulatory mechanisms and discuss how communication among morphogen signaling pathways modulates body axis formation in vertebrate embryos.

Foxb1 Regulates Negatively the Proliferation of Oligodendrocyte Progenitors

Oligodendrocyte precursor cells (OPC), neurons and astrocytes share a neural progenitor cell (NPC) in the early ventricular zone (VZ) of the embryonic neuroepithelium. Both switch to produce either of the three cell types and the generation of the right number of them undergo complex genetic regulation. The components of these regulatory cascades vary between brain regions giving rise to the unique morphological and functional heterogeneity of this organ. Forkhead b1 (Foxb1) is a transcription factor gene expressed by NPCs in specific regions of the embryonic neuroepithelium. We used the mutant mouse line Foxb1-Cre to analyze the cell types derived from Fobx1-expressing NPCs (the Foxb1 cell lineage) from two restricted regions, the medulla oblongata (MO; hindbrain) and the thalamus (forebrain), of normal and Foxb1-deficient mice. Foxb1 cell lineage derivatives appear as clusters in restricted regions, including the MO (hindbrain) and the thalamus (forebrain). Foxb1-expressing NPCs produce mostly oligodendrocytes (OL), some neurons and few astrocytes. Foxb1-deficient NPCs generate mostly OPC and immature OL to the detriment of neurons, astrocytes and mature OL. The axonal G-ratio however is not changed. We reveal Foxb1 as a novel modulator of neuronal and OL generation in certain restricted CNS regions. Foxb1 biases NPCs towards neuronal generation and inhibits OPC proliferation while promoting their differentiation.

The forkhead transcription factor FoxB1 regulates the dorsal-ventral and anterior-posterior patterning of the ectoderm during early Xenopus embryogenesis

The formation of the dorsal-ventral (DV) and anterior-posterior (AP) axes, fundamental to the body plan of animals, is regulated by several groups of polypeptide growth factors including the TGF-β, FGF, and Wnt families. In order to ensure the establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated. However, the molecular mechanisms responsible for these interactions remain unclear. Here, we demonstrate that the forkhead box transcription factor FoxB1, which is upregulated by the neuralizing factor Oct-25, plays an important role in the formation of the DV and AP axes. Overexpression of FoxB1 promoted neural induction and inhibited BMP-dependent epidermal differentiation in ectodermal explants, thereby regulating the DV patterning of the ectoderm. In addition, FoxB1 was also found to promote the formation of posterior neural tissue in both ectodermal explants and whole embryos, suggesting its involvement in embryonic AP patterning. Using knockdown analysis, we found that FoxB1 is required for the formation of posterior neural tissues, acting in concert with the Wnt and FGF pathways. Consistent with this, FoxB1 suppressed the formation of anterior structures via a process requiring the function of XWnt-8 and eFGF. Interestingly, while downregulation of FoxB1 had little effect on neural induction, we found that it functionally interacted with its upstream factor Oct-25 and plays a supportive role in the induction and/or maintenance of neural tissue. Our results suggest that FoxB1 is part of a mechanism that fine-tunes, and leads to the coordinated formation of, the DV and AP axes during early development.

Role of neuroepithelial Sonic hedgehog in hypothalamic patterning

The hypothalamus is a region of the diencephalon with particularly complex patterning. Sonic hedgehog (Shh), encoding a protein with key developmental roles, shows a peculiar and dynamic diencephalic expression pattern. Here, we use transgenic strategies and in vitro experiments to test the hypothesis that Shh expressed in the diencephalic neuroepithelium (neural Shh) coordinates tissue growth and patterning in the hypothalamus. Our results show that neural Shh coordinates anteroposterior and dorsoventral patterning in the hypothalamus and in the diencephalon-telencephalon junction. Neural Shh also coordinates mediolateral hypothalamic patterning, since it is necessary for the lateral hypothalamus to attain proper size and is required for the specification of hypocretin/orexin cells. Finally, neural Shh is necessary to maintain expression of differentiation markers including survival factor Foxb1.

Transcriptional networks in the early development of sensory-motor circuits

The emergence of coordinated locomotor behaviors in vertebrates relies on the establishment of selective connections between discrete populations of neurons present in the spinal cord and peripheral nervous system. The assembly of the circuits necessary for movement presumably requires the generation of many unique cell types to accommodate the intricate connections between motor neurons, sensory neurons, interneurons, and muscle. The specification of diverse neuronal subtypes is mediated largely through networks of transcription factors that operate within progenitor and postmitotic cells. Selective patterns of transcription factor expression appear to define the cell-type-specific cellular programs that govern the axonal guidance decisions and synaptic specificities of neurons, and may lay the foundation through which innate motor behaviors are genetically predetermined. Recent studies on the developmental programs that specify two highly diverse neuronal classes-spinal motor neurons and proprioceptive sensory neurons-have provided important insights into the molecular strategies used in the earliest phases of locomotor circuit assembly. This chapter reviews progress toward elucidating the early transcriptional networks that define neuronal identity in the locomotor system, focusing on the pathways controlling the specific connections of motor neurons and sensory neurons in the formation of simple reflex circuits.

Genetic ablation of the mammillary bodies in the Foxb1 mutant mouse leads to selective deficit of spatial working memory

Mammillary bodies and the mammillothalamic tract are parts of a classic neural circuitry that has been implicated in severe memory disturbances accompanying Korsakoff's syndrome. However, the specific role of mammillary bodies in memory functions remains controversial, often being considered as just an extension of the hippocampal memory system. To study this issue we used mutant mice with a targeted mutation in the transcription factor gene Foxb1. These mice suffer perinatal degeneration of the medial and most of the lateral mammillary nuclei, as well as of the mammillothalamic bundle. Foxb1 mutant mice showed no deficits in such hippocampal-dependent tasks as contextual fear conditioning and social transmission of food preference. They were also not impaired in the spatial reference memory test in the radial arm maze. However, Foxb1 mutants showed deficits in the task for spatial navigation within the Barnes maze. Furthermore, they showed impairments in spatial working memory tasks such as the spontaneous alternation and the working memory test in the radial arm maze. Thus, our behavioural analysis of Foxb1 mutants suggests that the medial mammillary nuclei and mammillothalamic tract play a role in a specific subset of spatial tasks, which require combined use of both spatial and working memory functions. Therefore, the mammillary bodies and the mammillothalamic tract may form an important route through which the working memory circuitry receives spatial information from the hippocampus.

Neonatal lethality, dwarfism, and abnormal brain development in Dmbx1 mutant mice

Dmbx1 encodes a paired-like homeodomain protein that is expressed in developing neural tissues during mouse embryogenesis. To elucidate the in vivo role of Dmbx1, we generated two Dmbx1 mutant alleles. Dmbx1- lacks the homeobox and Dmbx1z is an insertion of a lacZ reporter gene. Dmbx1z appears to be a faithful reporter of Dmbx1 expression during embryogenesis and after birth. Dmbx1-lacZ expression was detected in the superior colliculus, cerebellar nuclei, and subpopulations of the medulla oblongata and spinal cord. Some Dmbx1 homozygous mutant mice died during the neonatal period, while others survived to adulthood; however, their growth was impaired. Both heterozygous and homozygous mutant offspring from Dmbx1 homozygous mutant females exhibited a low survival rate and poor growth. However, even wild-type pups fostered onto Dmbx1 homozygous mutant females grew poorly, suggesting a Dmbx1-dependent nursing defect. Dmbx1 mutant mice had an aberrant Dmbx1-lacZ expression pattern in the nervous system, indicating that they had abnormal brain development. These results demonstrate that Dmbx1 is required for postnatal survival, growth, and brain development.

Met signaling is required for recruitment of motor neurons to PEA3-positive motor pools

Motor neurons in the spinal cord are grouped into motor pools, each of which innervates a single muscle. The ETS transcription factor PEA3 is a marker of a few such motor pools. Here, we show that pea3 is first induced by GDNF in a caudal subset of the motor neurons that will constitute the pea3+ population. Expansion of the pea3 domain subsequently occurs by recruitment of neurons from more anterior segments. Signaling by Met, the HGF receptor, is required for the rostral expansion of the pea3 domain, while the onset of pea3 expression is independent of met function. met expression is observed in pioneer neurons but does not precede that of pea3 in recruited neurons. We provide genetic evidence for a non-cell-autonomous function of met during the recruitment process. We propose the presence of a relay mechanism allowing cells induced by peripheral signals to recruit more anterior neurons to adopt the same motor pool-related phenotype.

Decisions on life and death: FOXO Forkhead transcription factors are in command when PKB/Akt is off duty

Forkhead transcription factors of the FOXO family are important downstream targets of protein kinase B (PKB)/Akt, a kinase shown to play a decisive role in cell proliferation and cell survival. Direct phosphorylation by PKB/Akt inhibits transcriptional activation by FOXO factors, causing their displacement from the nucleus into the cytoplasm. Work from recent years has shown that this family of transcription factors regulates the expression of a number of genes that are crucial for the proliferative status of a cell, as well as a number of genes involved in programmed cell death. As such, these transcription factors appear to play an essential role in many of the effects of PKB/Akt on cell proliferation and survival. Indeed, in cells of the hematopoietic system, mere activation of a FOXO factor is sufficient to activate a variety of proapoptotic genes and to trigger apoptosis. In contrast, in most other cell types, activation of FOXO blocks cellular proliferation and drives cells into a quiescent state. In such cell types, FOXO factors also provide the protective mechanisms that are required to adapt to the altered metabolic state of quiescent cells. Thus, as PKB/Akt signaling is switched off, FOXO factors take over to determine the fate of a cell, long-term survival in a quiescent state, or programmed cell death. This review summarizes our current understanding of the mechanisms by which PKB/Akt and FOXO factors regulate these decisions.

Role of EphA4 in defining the position of a motoneuron pool within the spinal cord

The correct assembly of the neural circuits that control movement requires the development of topographically organized pools of motoneurons within the spinal cord. The generation of a diverse array of motoneuron subtypes, which express differing transcription factors and cell-surface receptors, allows different motoneuron pools to be segregated to specific positions during development. In this investigation, we show that the Eph receptor tyrosine kinase, EphA4, appears to be important for the correct localization of a motoneuron pool to a specific position in the spinal cord. In the spinal cord of mice deficient in EphA4, the motoneuron pool that innervates the tibialis anterior muscle of the hindlimb is caudally displaced by approximately one vertebral segment. However, despite the abnormal position of the tibialis anterior motoneuron pool in the spinal cord of EphA4-deficient animals, the motoneurons of this pool still project to the tibialis anterior muscle of the hindlimb correctly. Additional analyses of other limb innervating motoneuron pools in the cervical and lumbar enlargements of the spinal cord of EphA4-deficient animals revealed them to be located in the appropriate segmental positions. Furthermore, we show that EphA4 does not appear to be important for spinal motoneuron survival as stereological quantification of the number of motoneurons present in the sciatic motoneuron pool of EphA4-deficient animals demonstrated these motoneurons to be present in the correct numbers. These observations suggest an important role for EphA4 in regulating the position of a specific motoneuron pool within the spinal cord.

GDNF acts through PEA3 to regulate cell body positioning and muscle innervation of specific motor neuron pools

Target innervation by specific neuronal populations involves still incompletely understood interactions between central and peripheral factors. We show that glial cell line-derived neurotrophic factor (GDNF), initially characterized for its role as a survival factor, is present early in the plexus of the developing forelimb and later in two muscles: the cutaneus maximus and latissimus dorsi. In the absence of GDNF signaling, motor neurons that normally innervate these muscles are mispositioned within the spinal cord and muscle invasion by their axons is dramatically reduced. The ETS transcription factor PEA3 is normally expressed by these motor neurons and fails to be induced in most of them in GDNF signaling mutants. Thus, GDNF acts as a peripheral signal to induce PEA3 expression in specific motor neuron pools thereby regulating both cell body position and muscle innervation.

Mechanisms and molecules in motor neuron specification and axon pathfinding

The vertebrate nervous system performs the most complex functions of any organ system. This feat is mediated by dedicated assemblies of neurons that must be precisely connected to one another and to peripheral tissues during embryonic development. Motor neurons, which innervate muscle and regulate autonomic functions, form an integral part of this neural circuitry. The first part of this review describes the remarkable progress in our understanding of motor neuron differentiation, which is arguably the best understood model of neuronal differentiation to date. During development, the coordinate actions of inductive signals from adjacent non-neural tissues initiate the differentiation of distinct motor neuron subclasses, with specific projection patterns, at stereotypical locations within the neural tube. Underlying this specialisation is the expression of specific homeodomain proteins, which act combinatorially to confer motor neurons with both their generic and subtype-specific properties. Ensuring that specific motor neuron subtypes innervate the correct target structure, however, requires precise motor axon guidance mechanisms. The second half of this review focuses on how distinct motor neuron subtypes pursue highly specific projection patterns by responding differentially to spatially discrete attractive and repulsive molecular cues. The tight link between motor neuron specification and axon pathfinding appears to be established by the dominant role of homeodomain proteins in dictating the ways that navigating motor axons interpret the plethora of guidance cues impinging on growth cones.

Specification of neuronal fates in the ventral neural tube

The generation of distinct classes of neurons at defined positions is a fundamental step in the development of the vertebrate central nervous system. Recent work has begun to reveal the extracellular signals and transcriptional mediators that direct the pattern of generation of distinct neuronal subtypes in the neural tube. This work has provided a framework to understand the patterning of the ventral neural tube and is permitting molecular analyses of the assembly of functional neuronal circuits.

The winged helix gene, Foxb1, controls development of mammary glands and regions of the CNS that regulate the milk-ejection reflex

The ability to lactate is a process restricted to mammals and is necessary for the survival of nonhuman mammals. Female mice carrying a null mutation in the winged helix transcription factor Foxb1 (previously Mf3/Fkh5/TWH) have lactation defects on inbred genetic backgrounds. To determine the cellular basis of the Foxb1 lactation defect we have inserted a tau-lacZ lineage marker into the locus to follow the fate of Foxb1 expressing cells. This approach has revealed that Foxb1 is expressed in epithelial cells of developing and adult mammary glands as well as previously described regions of the central nervous system. Mammary glands from C57BL/6 Foxb1-/- mice have incomplete lobuloalveolar development. In addition, the tau-lacZ lineage label was used to determine that the mammillothalamic tract was lost in all Foxb1-/- mice. Finally, morphological defects in the inferior colliculi of the midbrain in Foxb1-/- mice correlate with the inability to lactate, suggesting that the midbrain defect, but not the loss of the mammillothalamic tract, may be responsible for the lactation defect.

Neuronal specification in the spinal cord: inductive signals and transcriptional codes

Neural circuits are assembled with remarkable precision during embryonic development, and the selectivity inherent in their formation helps to define the behavioural repertoire of the mature organism. In the vertebrate central nervous system, this developmental program begins with the differentiation of distinct classes of neurons from progenitor cells located at defined positions within the neural tube. The mechanisms that specify the identity of neural cells have been examined in many regions of the nervous system and reveal a high degree of conservation in the specification of cell fate by key signalling molecules.

FHX.L and FHX.S, two isoforms of the human fork-head factor FHX (FOXJ2) with differential activity

Many biological phenomena are dependent on mechanisms that fine-tune the expression levels of particular genes. This can be achieved by altering the relative activity of a single transcription factor, by post-translational modifications or by interaction with regulatory molecules. An alternative mechanism is based on competition between two or more differently active isoforms of the same transcription factor. We found that FHX, a recently characterized human fork-head transcriptional activator, may show such a mechanism for balancing its activity by expressing two differently sized isoforms, FHX.S and FHX.L, encoded by a single gene located on human chromosome 12. FHX. L and FHX.S showed different transcriptional capacities, the larger form, FHX.L, behaving as the more potent transactivator. A transactivation domain of the acidic type present only in FHX.L would account for this functional difference. The relative concentrations of these two FHX isoforms appear to vary in a number of cell types, a circumstance that may regulate the final activity of this transcription factor.

Coordinate roles for LIM homeobox genes in directing the dorsoventral trajectory of motor axons in the vertebrate limb

Motor neurons extend axons along specific trajectories, but the molecules that control their pathfinding remain poorly defined. We show that two LIM homeodomain transcription factors, Lim1 and Lmx1b, control the initial trajectory of motor axons in the developing mammalian limb. The expression of Lim1 by a lateral set of lateral motor column (LMC) neurons ensures that their axons select a dorsal trajectory in the limb. In a complementary manner, the expression of Lmx1b by dorsal limb mesenchymal cells controls the dorsal and ventral axonal trajectories of medial and lateral LMC neurons. In the absence of these two proteins, motor axons appear to select dorsal and ventral trajectories at random. Thus, LIM homeodomain proteins act within motor neurons and cells that guide motor axons to establish the fidelity of a binary choice in axonal trajectory.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Winged helix transcription factor Foxb1 is essential for access of mammillothalamic axons to the thalamus

Our aim was to study the mechanisms of brain histogenesis. As a model, we have used the role of winged helix transcription factor gene Foxb1 in the emergence of a very specific morphological trait of the diencephalon, the mammillary axonal complex. Foxb1 is expressed in a large hypothalamic neuronal group (the mammillary body), which gives origin to a major axonal bundle with branches to thalamus, tectum and tegmentum. We have generated mice carrying a targeted mutation of Foxb1 plus the tau-lacZ reporter. In these mutants, a subpopulation of dorsal thalamic ventricular cells “thalamic palisade” show abnormal persistence of Foxb1 transcriptional activity; the thalamic branch of the mammillary axonal complex is not able to grow past these cells and enter the thalamus. The other two branches of the mammillary axonal complex (to tectum and tegmentum) are unaffected by the mutation. Most of the neurons that originate the mammillothalamic axons suffer apoptosis after navigational failure. Analysis of chimeric brains with wild-type and Foxb1 mutant cells suggests that correct expression of Foxb1 in the thalamic palisade is sufficient to rescue the normal phenotype. Our results indicate that Foxb1 is essential for diencephalic histogenesis and that it exerts its effects by controlling access to the target by one particular axonal branch.

Minimal phenotype of mice homozygous for a null mutation in the forkhead/winged helix gene, Mf2

Mf2 (mesoderm/mesenchyme forkhead 2) encodes a forkhead/winged helix transcription factor expressed in numerous tissues of the mouse embryo, including paraxial mesoderm, somites, branchial arches, vibrissae, developing central nervous system, and developing kidney. We have generated mice homozygous for a null mutation in the Mf2 gene (Mf2(lacZ)) to examine its role during embryonic development. The lacZ allele also allows monitoring of Mf2 gene expression. Homozygous null mutants are viable and fertile and have no major developmental defects. Some mutants show renal abnormalities, including kidney hypoplasia and hydroureter, but the penetrance of this phenotype is only 40% or lower, depending on the genetic background. These data suggest that Mf2 can play a unique role in kidney development, but there is functional redundancy in this organ and other tissues with other forkhead/winged helix genes.

CEPU-1, an immunoglobulin superfamily molecule, has cell adhesion activity and shows dynamic expression patterns in chick embryonic spinal cord

In an attempt to isolate novel molecules involved in motoneuron differentiation and target muscle innervation during embryogenesis, we performed mRNA differential display analysis by comparing cDNAs of motoneurons purified by immunopanning from different portions along the rostro-caudal axis of chick embryonic spinal cord, and cloned an immunoglobulin superfamily protein named C30. By sequence comparison, C30 was shown to be an alternatively spliced isoform of CEPU-1, which was formerly reported as a member of the immunoglobulin superfamily specifically expressed in cerebellar Purkinje cells (Spaltmann and Brummendorf, 1996, J. Neurosci. 16, 1770-1779). We analyzed the expression pattern of CEPU-1 both at the mRNA and protein levels in the spinal cord of the chick embryo. Until stage 23, CEPU-1 was expressed faintly in the ventral part of the neural tube but gradually it became localized to a specific group of cells. In the motor column, CEPU-1 was expressed transiently in many columnar layers. A C30-transfected cell line showed Ca(2+)-independent cell-cell binding activity. These results suggest a role for CEPU-1 in specific axon guidance and/or fasciculation of motoneurons during development.

Patterning motoneurons in the vertebrate nervous system

Vertebrate motoneurons show considerable diversity in their soma locations, axonal trajectories and innervation targets. Results from studies of a variety of vertebrate species as well as fruit-flies are elucidating the mechanisms by which this diversity is generated. Motoneuron subpopulations appear to be defined by combinations of transcription factor genes expressed in distinct spatiotemporal patterns in both motoneuron progenitors and postmitotic motoneurons. Notochord-derived signals can induce motoneuron formation, paraxial-mesoderm-derived signals can pattern motoneuron subpopulations along the rostrocaudal body axis, and local signals within the neural tube can regulate the number and time at which motoneurons form. Additional, later signals can promote formation of proper central circuitry and motoneuron survival. The identification of the genes and signals responsible for regulating these processes should help to provide a more-detailed understanding of motoneuron patterning.

The fork head transcription factor Fkh5/Mf3 is a developmental marker gene for superior colliculus layers and derivatives of the hindbrain somatic afferent zone

Fork head-5 (Fkh5; also known as Mf3 and TWH) is a transcription factor of the winged helix family. As part of an extended project to understand the function of this protein in the developing mouse brain, in the present work we have used Fkh5/Mf3 expression as a marker to study the development of the midbrain and hindbrain. In the midbrain, Fkh5/Mf3 is expressed in the superior colliculus, in the ventricular layer of the inferior colliculus and in the isthmus. In the superior colliculus, Fkh5/Mf3 is expressed by cells of layers 4a and 4c since early in development. In the hindbrain, Fkh5/Mf3 is a longitudinal marker (as opposed to a transverse or rhombomeric one), since it labels nuclei belonging to the somatic afferent zone (ventral cochlear nucleus, cuneate and external cuneate nuclei, principal and spinal nuclei of the trigeminal). In addition, Fkh5/Mf3 is expressed by the developing endopiriform nucleus and by the olivary pretectal nucleus. The results suggest that Fkh5/Mf3 has an early role in the lamination of the tectum and in the longitudinal differentiation of the hindbrain.

FAST-2 is a mammalian winged-helix protein which mediates transforming growth factor beta signals

The mechanisms by which transforming growth factor beta (TGF-beta) and related ligands regulate transcription remain poorly understood. The winged-helix (WH) transcription factor fork head activin signal transducer 1 (FAST-1) was identified as a mediator of activin signaling in Xenopus embryos (X. Chen, M. J. Rubock, and M. Whitman, Nature 383:691-696, 1996). We have cloned a novel WH gene from the mouse which shares many properties with FAST-1. We find that this gene, which we call FAST-2, is able to mediate transcriptional activation by TGF-beta. FAST-2 also interacts directly with Smad2, a cytoplasmic protein which is translocated to the nucleus in response to TGF-beta, and forms a multimeric complex with Smad2 and Smad4 on the activin response element, a high-affinity binding site for FAST-1. Analysis of the sequences of FAST-1 and FAST-2 reveals substantial protein sequence divergence compared to known vertebrate orthologs in the WH family. This suggests that FAST-2 represents a new WH gene related to FAST-1, which functions to mediate TGF-beta signals in mammals. We have also examined the structure of the FAST-2 gene and find that it overlaps with a kinesin motor protein gene. The genes are transcribed in opposite orientations, and their transcripts overlap in the 3' untranslated region.

Genetic and molecular analyses of motoneuron development

Motoneurons have distinct identities and muscle targets. Recent classical and molecular genetic studies in flies and vertebrates have begun to elucidate how motoneuron identities and target specificities are established. Many of the same molecules participate in the guidance of both vertebrate and fly motor axons. It is less clear, however, whether the same molecular mechanisms establish vertebrate and fly motoneuron identities.

The human forkhead protein FREAC-2 contains two functionally redundant activation domains and interacts with TBP and TFIIB

Forkhead-related activator 2 (FREAC-2) is a human transcription factor expressed in lung and placenta that binds to cis-elements in several lung-specific genes. We have identified the parts of FREAC-2 responsible for trans-activation and found two functionally redundant activation domains on the C-terminal side of the DNA binding forkhead domain. Activation domain 1 consists of the most C-terminal 23 amino acids of FREAC-2 and contains a sequence motif conserved in an activation domain of another forkhead protein, FREAC-1. Activation domain 2 is built up by three synergistic subdomains in the central part of the FREAC-2 protein. FREAC-2 was shown to interact in vitro with TBP and TFIIB. The target site for FREAC-2 on TBP was localized to the N-terminal repeat in the core domain of TBP. TFIIB binds FREAC-2 close to the cleft between its two globular domains. The part of FREAC-2 that binds TBP was mapped to 21 amino acids in the C-terminal end of the forkhead domain. This sequence is well conserved among forkhead proteins, raising the possibility that interaction with TBP may be a general characteristic of this family of transcription factors. Overexpression of TFIIB potentiates activation by FREAC-2 in a manner dependent on the FREAC-2 activation domains. Nuclear localization of FREAC-2 was found to depend on sequences from both ends of the forkhead domain.

In situ hybridization with 33P-labeled RNA probes for determination of cellular expression patterns of liver transcription factors in mouse embryos

Murine hepatocyte nuclear factor-3beta (HNF-3beta) protein is a member of a large family of developmentally regulated transcription factors that share homology in the winged helix/fork head DNA binding domain and that participate in embryonic pattern formation. HNF-3beta also mediates cell-specific transcription of genes important for the function of hepatocytes, intestinal and bronchiolar epithelium, and pancreatic acinar cells. We have previously identified a hepatocyte and pancreatic cut-homeodomain transcription factor, HNF-6, which is required for HNF-3beta promoter activity. In this study, we used in situ hybridization studies of stage-specific embryos to demonstrate that HNF-6 and its target gene, HNF-3beta, are coexpressed in the foregut endoderm and in the pancreatic and hepatic diverticulum. More detailed analysis of HNF-6 and HNF-3beta's developmental expression patterns provides evidence of colocalization in hepatocytes, intestinal epithelium, and pancreatic ductal epithelium and exocrine acinar cells. In support of the role of HNF-6 in regulating HNF-3beta expression in developing hepatocytes, their liver expression levels are both transiently reduced between 14 and 15 days of gestation. At day 18 of gestation and in adult pancreas, HNF-6 and HNF-3beta transcripts remain colocalized in the exocrine acinar cells, but their expression patterns diverge in endocrine cells. HNF-3beta expression is restricted to the endocrine cells of the islets of Langerhans, whereas the ductal epithelium expresses HNF-6. We discuss these expression patterns with respect to specification of hepatocytes and differentiation of the endocrine and exocrine pancreas.

The winged helix transcription factor Hfh2 is expressed in neural crest and spinal cord during mouse development

The embryonic neural crest is a unique group of cells that gives rise to the peripheral nervous system as well as many other cell types. Most of the work describing the behavior and specification of these cells has been done in the avian system, a system amenable to experimental manipulations such as tissue grafting, cell transplantation and lineage tracing. This work has been greatly facilitated by the use of molecular markers of neural crest cells such as HNK-1 and slug, markers that are not yet available for the study of mouse neural crest. Here we demonstrate that Hfh2 (HNF3 forkhead homologue 2), a member of the 'winged helix' or 'forkhead' transcription factor gene family, is expressed in premigratory and migrating neural crest cells in the early mouse embryo and in motorneuron progenitors in the developing spinal cord. Using linkage analysis we have localized the Hfh2 gene to chromosome 4 at 44.91 cM from the centromere.

The forkhead/winged helix gene Mf1 is disrupted in the pleiotropic mouse mutation congenital hydrocephalus

Mf1 encodes a forkhead/winged helix transcription factor expressed in many embryonic tissues, including prechondrogenic mesenchyme, periocular mesenchyme, meninges, endothelial cells, and kidney. Homozygous null Mf1lacZ mice die at birth with hydrocephalus, eye defects, and multiple skeletal abnormalities identical to those of the classical mutant, congenital hydrocephalus. We show that congenital hydrocephalus involves a point mutation in Mf1, generating a truncated protein lacking the DNA-binding domain. Mesenchyme cells from Mf1lacZ embryos differentiate poorly into cartilage in micromass culture and do not respond to added BMP2 and TGFbeta1. The differentiation of arachnoid cells in the mutant meninges is also abnormal. The human Mf1 homolog FREAC3 is a candidate gene for ocular dysgenesis and glaucoma mapping to chromosome 6p25-pter, and deletions of this region are associated with multiple developmental disorders, including hydrocephaly and eye defects.

A human forkhead/winged-helix transcription factor expressed in developing pulmonary and renal epithelium

Members of the forkhead/winged-helix transcription factor family play crucial roles during vertebrate development. A human hepatocyte nuclear factor/forkhead homolog (HFH)-4 cDNA encoding a 421-amino acid protein was isolated from a human fetal lung cDNA library. By Southern blot analysis of human-rodent somatic cell hybrid genomic DNA, the human HFH-4 gene localizes to chromosome 17q23-qter. This is the locus of another forkhead/winged-helix gene, the interleukin enhancer binding factor gene. RNA blot analysis revealed a 2.5-kilobase human HFH-4 transcript in fetal lung, kidney, and brain as well as in adult reproductive tissues, lung, and brain. By in situ hybridization, HFH-4 expression is associated with differentiation of the proximal pulmonary epithelium, starting during the pseudoglandular stage of human lung development. During human renal morphogenesis, HFH-4 is expressed in the developing epithelial cells of the ureteric duct, glomerulus, and epithelial vesicles. The unique pattern of HFH-4 expression during human fetal development suggests a role for this forkhead/winged-helix factor during pulmonary and renal epithelial development.

Mouse Mesenchyme forkhead 2 (Mf2): expression, DNA binding and induction by sonic hedgehog during somitogenesis

Cloning and sequencing of mouse Mf2 (mesoderm/mesenchyme forkhead 2) cDNAs revealed an open reading frame encoding a putative protein of 492 amino acids which, after in vitro translation, binds to a DNA consensus sequence. Mf2 is expressed at high levels in the ventral region of newly formed somites, in sclerotomal derivatives, in lateral plate and cephalic mesoderm and in the first and second branchial arches. Other regions of mesodermal expression include the developing tongue, meninges, nose, whiskers, kidney, genital tubercule and limb joints. In the nervous system Mf2 is transcribed in restricted regions of the mid- and forebrain. In several tissues, including the early somite, Mf2 is expressed in cell populations adjacent to regions expressing sonic hedgehog (Shh) and in explant cultures of presomitic mesoderm Mf2 is induced by Shh secreted by COS cells. These results suggest that Mf2, like other murine forkhead genes, has multiple roles in embryogenesis, possibly mediating the response of cells to signaling molecules such as SHH.

Motoneuron differentiation, survival and synaptogenesis

The motoneuron is the central neuron whose development is best understood. Recent research has provided much new information about the molecules involved in aspects of motoneuron development first outlined by classic embryology studies. Over the past year, progress has been particularly apparent in the following areas: motoneuron induction and control of motoneuron identity; factors that guide motor axon outgrowth; neurotrophic factors for motoneurons; and early steps in the formation of the neuromuscular junction.