Gene name - pointed
Synonyms - Ets-2
Cytological map position - 49E
Function - transcription factor
Symbol - pnt
Genetic map position - 3-79.0
Classification - ETS family
Cellular location - nuclear ation -
Pointed is required for the differentiation of glial cells in the ventral nerve cord known as the CNS, and is also required downstream of Ras in the development of the eye. This discussion will deal with the role of pointed in glial differentiation. Glia are companion cells for neurons, providing a substrate for axon growth as well as nourishment, protection and insulation for mature neurons. The central nervous system has two embryonic origins: the mesectoderm, which gives rise to the ventral midline and the neuroectoderm, which gives rise to the CNS proper. Pointed is involved in glial differentiation in both these systems.
Mutations of single-minded, a gene required for the proper formation of the ventral midline, delete the glial cells that express pointed. These cells known as midline glia are absent in pointed mutants. One of two Pointed transcripts, P2, is involved in this functional determination of midline glia. pointed also affects longitudinal glia, another group of glial cells only a few cells away from midline glia. They are not part of the ventral midline, but instead form a part of the CNS proper. pointed mutants result in a misplacement of longitudinal glia, rather their deletion. Glia are required for axon guidance, and a defect in glia results in defective axon guidance. In these cases the pointed P1 transcript is implicated. Proper axon guidance is necessary for the formation of commissures and longitudinal connectives. pointed mutants give rise to fused commissures and thinner than normal longitudinal connectives.
Misplacement of longitudinal glia and improper axon guidance are only part of the disruptive effects caused by pointed mutation. Specific neural cells termed MP2 normally express the antigen Mab22C10. In pointed mutants, MP2 neurons fail to make this antigen. Thus a mutation affecting glial cells results in improper neural functioning. It is possible that the Mab22C10 antigen acts to engender proper axon guidance (Klaes, 1994).
Therefore pointed is involved in glial survival, axon transport dependent on glia, and glial nourishment of neurons. The separate involvement of pointed in the CNS and in the midline, points to the complexity of tissue specific regulation of gene expression.
In the ventral nerve cord of Drosophila most axons are organized in a simple, ladder-like pattern. Two segmental commissures connect the hemisegments along the mediolateral axis and two longitudinal connectives connect individual neuromeres along the anterior-posterior axis. Cells located at the midline of the developing CNS first guide commissural growth cones toward and across the midline. The first growth cones navigate toward the anterior most ventral unpaired median (VUM) cell and thus pioneer the prospective posterior commissure. Only when the posterior commissure is established, the anterior commissure forms. In later stages, midline glial cells, migrating toward the posterior, are required to separate anterior and posterior commissures into distinct axon bundles. The VUM neurons reside ventral to the posterior commissure and project in a characteristic axon-bundle to the anterior commissure. Migration of two midline glial cells occurs along these cell processes. To unravel the genes underlying the formation of axon pattern in the embryonic ventral nerve cord, a saturating ethylmethane sulfonate mutagenesis was conducted, screening for mutations that disrupt this process. Subsequent genetic and phenotypic analyses support a sequential model of axon pattern formation in the embryonic ventral nerve cord. Specification of midline cell lineages is brought about by the action of segment polarity genes. Five genes are necessary for the establishment of the commissures. Two gene functions are required for the initial formation of commissural tracts, in addition to the function of commissureless, the netrin genes, and the netrin receptor encoded by the frazzled gene. Over 20 genes appear to be required for correct development of the midline glial cells which are necessary for the formation of distinct segmental commissures (Hummel, 1999a).
The analysis of mutations reveals two major phenotypic classes: the pointed and the tramtrack groups. pointed and tramtrack mediate different aspects of glial development. In pointed mutants no glial differentiation occurs, whereas ectopic pointed expression results in ectopic glial differentiation. tramtrack, in contrast, does not interfere with actual glial cell differentiation but appears to be required for the repression of neuronal differentiation in these cells. The pointed group consists of pointed itself, rhomboid, kastchen, klotzchen, kette, schmalspur, mochte gern, spitz, Star, cabrio and kubel. Mutations in eight other genes lead to an axon phenotype initially described for tramtrack. In tramtrack-type mutation (tramtrack, shroud, disembodied, spook, shade, shadow, phantom, and rippchen) commissures appear fused, but in contrast to pointed group mutations, connectives are not affected (Hummell, 1999a).
Most of the neurons of the ventral nerve cord send out long projecting axons that cross the midline. In the Drosophila CNS, cells of the midline give rise to neuronal and glial lineages with different functions during the establishment of the commissural pattern. The development of midline cells is fairly well understood. In the developing ventral neural cord, 7-8 midline progenitor cells per abdominal segment generate about 26 glial and neuronal cells, i.e. 3-4 midline glial cells, 2 MP1 neurons, 6 VUM neurons, 2 UMI neurons, as well as the median neuroblast and its support cells. The VUM neurons comprise motoneurons as well as interneurons, which project through the anterior and posterior commissures. Genetic studies indicate that the VUM neurons are involved in the initial attraction of commissural growth cones. The MP1 neurons are ipsilateral projecting interneurons, which participate in the formation of specific longitudinal axon pathways. The median neuroblast divides during larval and pupal stages. Contrary to what occurs in the grasshopper CNS, the Drosophila median neuroblast does not generate midline glial cells. In Drosophila, the midline glial cells develop from a set of 2-3 progenitors located in the anterior part of each segment. A function of the midline glial cells during the maturation of the segmental commissures has been found, such that two midline glial cells migrate along cell processes of the VUM-midline neurons to separate anterior and posterior axon commissures. If this migration is blocked, a typical fused commissure phenotype develops. Toward the end of embryogenesis, midline glial cells are required for the formation of individual fascicles within the commissures (Hummel, 1999b and references).
The formation of distinct anterior and posterior commissures requires intercalating migration of midline glial cells. This migration depends on a neuron-glia interaction at the midline. Thus perturbation of midline glial migration could be either due to cell autonomous defects in the midline glia itself or could be due to defects in the interacting midline neurons. Many genes have been identified that appear to be required for midline glia migration and show fused commissure phenotypes upon mutation. Based on additional phenotypic similarities, 12 of these genes have been placed in the so called pointed group. In order to define further functional relationships of the pointed group genes, a number of double mutant combinations were examined. pointed is expressed and required in the midline glial cells. It is expected that embryos homozygous mutant for both pointed and other members of the pointed group would show a pointed-like CNS phenotype if both gene functions are required only in the glial cells. pointed kästchen or pointed schmalspur or kästchen klötzchen double mutant embryos show a slightly enhanced fused commissure phenotype, as compared to the phenotypes of the single mutants. In pointed klötzchen double mutant embryos a more severe fused commissure phenotype is observed. In order to correlate these axonal phenotypes with the number and location of midline glial cells, single and double mutant embryos were analyzed with different enhancer trap markers. In pointed embryos the midline glial cells fail to migrate at all. In contrast, partial midline glial cell migration combined with a slight reduction of the glial cell number, is observed in stage 16 kästchen, schmalspur and klötzchen embryos. In embryos double mutant for kästchen klötzchen or pointed schmalspur or pointed klötzchen reduced numbers of midline glial cells are detected. This reduction corresponds well to the strength of the fusion of commissures. In the most extreme case only a few glial cells are present per embryo. In addition these cells express the midline glial enhancer trap marker at low levels. Thus, it is concluded that klötzchen, kästchen and schmalspur mainly function in the midline glial cells but compared to pointed have weaker effects on glial differentiation (Hummel, 1999a). In contrast a qualitatively different phenotype is observed in pointed kette double mutant embryos. Here commissures fail to develop in most neuromeres, suggesting that kette does not function in the midline glial cells but rather might act in the midline neurons. To date the only other gene known to be required for the development of midline neurons is orthodenticle (otd), which encodes a transcription factor expressed in these cells. In otd mutant embryos the posterior commissures fail to develop. Interestingly, double mutant embryos for otd and pointed show a phenotype comparable to the pointed kette mutant. Furthermore, otd;kette double mutant embryos display an otd-like phenotype. This supports the notion that kette acts in the midline neurons as does orthodenticle. The failure of midline glial cell migration in kette mutant embryos would then be a non cell autonomous consequence of a defect in the midline neurons (Hummel, 1999b).
In summary, the genetic analyses show that pointed, kästchen, schmalspur and klötzchen act in the midline glia, whereas kette acts in the midline neurons. Only when the development of neurons and glial cells are disrupted in the midline as in otd pointed or pointed;kette double mutant embryos do no commissures develop. However, in klötzchen;kette or kästchen;kette double mutant embryos, a fused commissure phenotype is found. This supports the previous notion that mutations in klötzchen or kästchen have weaker effects on midline glia development than does pointed (Hummel, 1999b).
How are anterior and posterior commissures separated? Beside guiding commissural axons, the midline glial cells are required for the shaping of the two segmental commissures. The formation of two distinct segmental commissures in each segment requires an intercalating migration of two of the 4-6 midline glial cells. This migration is preceded by a neuron-glia interaction at the midline. A surprisingly high number of genes appear to be required for this process. To be routed into the appropriate commissures, axons that cross the midline have to differentiate between the dorsal and ventral side of the combined signals represented by the migrating midline glial cells and by the extending VUM neurons. Thus, some mutations which lead to a fused commissure phenotype could stem from glial cells defects where the dorsal and ventral sides cannot be distinguished any more. Subsequently anterior commissure neurons would grow on the dorsal side of the CNS next to the posterior commissure. However, following migration, the midline glial cells would then be located below the segmental commissures. Some of the mutations will not affect glial migration in the first place but will rather control the general differentiation of midline glial cells, as for example pointed does. Since neuronal migration along glial processes in the vertebrate system depends on several receptor systems, it appears possible that some components have been identified that are required specifically for glial migration along neuronal processes; this also occurs in the visual system of Drosophila. Based on the double mutant analysis, at least one of the genes, kette, appears to be required in the midline neurons. Indeed, a P-element induced kette mutation has been isolated that shows a specific beta-galactosidase expression in the VUM-neurons, suggesting that kette is expressed in neuronal midline cells. Here kette could either control neuronal development or could be involved in the actual neuron-glia interaction at the midline. But why do commissures fail to develop in pointed;kette double mutant embryos? If attraction of first commissural growth cones is mediated by signals emanating from the midline neurons, this would imply that neuronal differentiation at the midline is more defective in the double mutant, when compared to either of the single mutants. As a consequence, neuronal differentiation as well as glial differentiation in the midline must depends on pointed function. In the CNS, pointed is expressed only in glial cells and in pointed mutant embryos the VUM neurons are present and appear to project in their normal pattern. However, they fail to properly differentiate and do not express orthodenticle at high levels. It is presently unknown whether this disruption of VUM glia differentiation is due to a lack of pointed in the midline glia or depends on pointed function in the VUM support glial cells. Based on this evidence it can also be deduced that klötzchen and kästchen do not influence the development of the midline neurons but encode new components regulating glial development (Hummel, 1999b).
The following model is proposed for commissure formation. The initial growth of commissural growth cones towards the midline in stage 12 embryos is guided by an attractive signal expressed by the midline neurons. Presumably, this attraction is mediated by early Netrin expression in the midline neurons or alternatively by the action of a Schizo/Weniger attractive system. At this early developmental stage the midline glial cells are elongated in shape, contacting the epidermis with their basal side and are assumed to send out cellular processes contacting the VUM-midline neurons at the dorsal side of the nervous system. The midline glial cells express a repulsive signal that is conveyed to lateral axons via the Robo receptor and/or the karussell gene product. This repulsive function restricts the first axons to cross the midline just anterior of the VUM neurons. The midline glial cells also express a contact dependent permissive guidance cue helping the axons to cross the midline. Subsequently, neuron-glia interaction at the midline results in the migration of the midline glial cells along processes of the VUM neurons (Hummel, 1999b).
Bases in 5' UTR - 1046
Exons - four: the first is not shared with transcript #2 and encodes 229 amino acids.
Bases in 3' UTR - 233
Exons - eight exons, spanning over 50 kb. The first five are not shared with transcript #1 and encode 324 amino acids.
Bases in 3' UTR - 233
Pointed has an ETS oncogene domain and a second evolutionally conserved domain, the pointed domain, present in the N-terminal region of P2 but not P1. The pointed domain is shared with murine ETS 1, GA binding protein alpha, and other ETS homologs (Klambt, 1993).
Homology of Pointed ETS domain to mouse or human ELK 1 is 95% in the central ETS domain (Klambt, 1993).
Genetic analysis of lin-1 loss-of-function mutations suggests that lin-1 controls multiple cell-fate decisions during Caenorhabditis elegans development and is negatively regulated by a conserved receptor tyrosine kinase-Ras-ERK mitogen-activated protein (MAP) kinase signal transduction pathway. LIN-1 protein contains an ETS domain and presumably regulates transcription. The vertebrate proteins Elk-1, SAP-1a, and Net/ERP/SAP-2 are classified as members of the Elk subfamily of ETS proteins because they share three regions of significant sequence conservation: an N-terminal ETS domain, a centrally positioned B box, and a C-terminal C box. Based on the positions and sequences of their ETS domains and the positions and sequences of regions similar to the C box, it is proposed that LIN-1 and Drosophila Aop are both members of the Elk subfamily. The ETS domain of LIN-1 shares more sequence identity with the ETS domain of human Elk-1 (67% identity) and human SAP-1a (61% identity) than with any other ETS domain. Likewise, the ETS domain of Aop is most similar to the ETS domain of Elk-1 (51% identity). The ETS domains of LIN-1 and Aop are somewhat less similar (41% identity). LIN-1 (441 residues), Elk-1 (428 residues), SAP-1a (453 residues) and Net (409 residues) are similarly sized and have ETS domains similarly positioned in the N-terminal region. By comparison, Aop (688 residues) is larger and has more residues N-terminal to the ETS domain, which is located near the center of the protein. However, the number of residues C-terminal to the ETS domain is similar among all five proteins analyzed. Sequence similarity outside the ETS domain provides further evidence that ETS proteins are members of a subfamily. By studying the C termini of these proteins, it was found that LIN-1, Elk-1, SAP-1a, and Net each have the sequence FQFP, while Aop has the sequence FQFHP. In Elk-1, SAP-1a, and Net, the FQFP sequence is at the end of the C box. The C boxes of ELK-1, SAP-1a, and Net are characterized by five or six S/TP sequences, which are potential MAP kinase phosphorylation sites. In the corresponding regions, LIN-1 has five S/TP sequences and Aop has three. Elk-1, SAP-1a, and Net have additional identities in the C box that are not conserved in LIN-1 and Aop. These observations suggest that LIN-1 and Aop contain divergent C boxes. Thus, lin-1, aop, elk-1, sap-1, and net appear to be derived from an ancestral gene that encoded a protein with an N-terminal ETS domain and a C-terminal C box (Jacobs, 1998 and references).
The Pointed (PNT) domain and an adjacent mitogen-activated protein (MAP) kinase phosphorylation site are defined by sequence conservation among a subset of ets transcription factors and are implicated in two regulatory strategies: protein interactions and posttranslational modifications, respectively. By using NMR, the structure of a 110-residue fragment of murine Ets-1 has been determined that includes the PNT domain and MAP kinase site. The Ets-1 PNT domain forms a monomeric five-helix bundle. The architecture is distinct from that of any known DNA- or protein-binding module, including the helix-loop-helix fold proposed for the PNT domain of the ets protein TEL. The MAP kinase site is in a highly flexible region of both the unphosphorylated and phosphorylated forms of the Ets-1 fragment. Phosphorylation alters neither the structure nor monomeric state of the PNT domain. These results suggest that the Ets-1 PNT domain functions in heterotypic protein interactions and support the possibility that target recognition is coupled to structuring of the MAP kinase site (Slupsky, 1998).
date revised: 12 December 99
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