NB 7-3 details
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In grasshopper, the NB 7-3 lineage was initially described by Paul Taghert and Corey Goodman (1984). Their work with this group of cells began with their identification of two serotonergic cells (S1 and S2) per hemisegment. Subsequently, Lucifer Yellow dye fills of NB 7-3 confirmed its identity as the progenitor of the S1 and S2 cells and revealed the rest of the grasshopper lineage. The NB 7-3 neurons can be distinguished on the basis of birth order, serotonin immunoreactivity and position relative to the midline.S1, the most medial of the two anterior cells of the clone, is derived from the first GMC of the lineage and expresses serotonin uniformly and strongly. Its more lateral sibling, S2, expresses serotonin everywhere but in segments T3 and A1. Their cousin, S3, derived from the second daughter (GMC) of NB 7-3, expresses serotonin in the first thoracic segment only (Taghert and Goodman, 1984). The identity of the sibling of S3 is unknown.
Drosophila studies have revealed a great deal about the expression of other molecular markers in this lineage. NB 7-3 expresses eagle (egl) (Higashijima et al, 1996; Dittrich et al, 1997); engrailed (en) (Lundell et al, 1996); zinc-finger homeodomain -2 (zfh-2) (Lundell and Hirsh, 1992); POU-homeodomain protein-1 (pdm-1) (Billin et al, 1991; Dick et al, 1991; Bhat et al, 1995); huckebein (hkb) (Lundell et al, 1996; Dittrich et al, 1997); Klumpfuss (Klu) (Yang et al, 1997); seven-up-lacZ (svp-lacZ) (Doe, 1992; Broadus et al, 1995) and gooseberry distal (gsb-d) (Skeath et al, 1995). In addition, some neurons in the lineage have been shown to express serotonin and dopa-decarboxylase (Ddc) (Beall and Hirsh, 1987; Konrad and Marsh, 1987) and islet, a transcription factor believed to directly regulate serotonin and Ddc expression in a segment-specific and cell-specific manner (Thor and Thomas, 1997). Lundell and Hirsh (1992) most recently showed that eagle is required for the specification of serotonergic neurons in the 7-3 lineage. Combining the results from the Taghert and Goodman study with the Lundell and Hirsh study, it is clear that S1 and EW1 are homologous cells, as are S2 and EW2, and S3 and EW3. Lundell and Hirsh describe EW1 as expressing eagle, engrailed and serotonin/Ddc; EW2 as expressing eagle, engrailed, zfh-2, pdm1 and serotonin/Ddc; EW3 as expressing only eagle, engrailed and zfh-2.
Higashijima et al (1997), had demonstrated that the anterior cells in the lineage, the EW cells, project axons anteriorly by embryonic stage 13. Bossing et al (1996) described the 7-3 lineage in Drosophila as consisting of an obligatory epidermal subclone of 4-9 cells, and a neural subclone consisting of (usually) four cells.The neuronal sub-clone consists of 3 interneurons projecting contralaterally via the posterior commissure and a putative motorneuron. We observe a similar neural clone.
A. Motoneurons
The relatively small GW motoneuron (5.3 um; n=8) is positioned to the lateral side of the clone, and projects out the posterior root of the ISN, joins the SNd, and contacts muscles 15,16 and 17 from the external embryonic surface before forming a fork-like endings in the clefts that separate these three muscles (Fig. 7-3B,C)
B. Interneurons:
We observe 3 local interneurons (EW1-3 or S1-3), slightly larger than the GW motoneuron, that project across the posterior commissure before forming large synaptic complexes spanning as much as 50 um in the contralateral connective. Thoracic clones produce more complex arborizations than abdominal clones (Fig. 7-3 B,C). In addition, there is always a small axonless cell (3.7 um; n=11); (Fig. 7-3, asterisk). We never saw the EW cells to be arranged as a horizontal row of three cells, as described by Lundell and Hirsh (1998); this is probably because the clones we examined were never younger than stage 15. The EW cells we examined were arranged in a triangular configuration (see Fig 7-3); it is therefore likely that the EW3 cell moves to a more medial and posterior position during stages 13-15. The EW neurons are always uniform in size and are, on average, 10-15% larger in cell diameter, than the GW cell.
Taghert and Goodman (1984) demonstrated intricate axonal morphologies for the S cells. The branching pattern they describe for thoracic S1 cells most closely resembles what we find for both thoracic and abdominal clones in Drosophila. Drosophila interneuronal projections do not bifurcate prior to forming synaptic connections in the neuropil, in contrast to the situation they describe in Schistocerca. EW neurites simply cross the midline in the posterior commissure and formed large synaptic complexes spanning as much as 50 microns in the contralateral longitudinal connective. The extent of this arborization varied by age and by segment.
C. Glia and Other Cells:
The Technau lab describes the clone as containing an obligate epidermal sub-clone. We find the epidermal cells to be present in only about half of our clones (n=4/11) (see Fig 7-3D). In one case, we also saw a glial cell (see Fig 7-3D). This glial cell resembled the segmental nerve glia that is frequently derived from the NB 1-1 clone and NB 7-1 clone, and appeared to be in contact with the GW motoneuron as the SNd branch leaves the ISN (see Fig 7-3D).
References:
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Bhat, K.M., Poole, S.J., and Schedl, P. (1995). The miti-mere and pdm1 genes collaborate during specification of the RP2/sib lineage in Drosophila neurogenesis. Mol Cell Biol 15(8): 4052-63.
Billin, A.N., cockerill, K.A., and Poole, S.J., (1991). Isolation of a family of Drosophila POU domain genes expressed in early development. Mech Dev 34(2-3):75-84.
Bossing, T., Udolph, G., Doe, C. Q., and Technau, G. M. (1996). The Embryonic CNS lineages of Drosophila melanogaster I. Neuroblast lineages derived from the ventral half of the neurectoderm. Dev Biol 179: 41-64.
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Dick, T., Tang, X.H., Yeo, S.L., and Chia, W. (1991). Two closely linked Drosophila POU domain genes are expressed in neuroblasts and sensory elements. PNAS 88(17): 7645-9.
Dittrich, R., Bossing, T., Gould, A.P., Technau, G.M., and Urban, J. (1997). The differentiation of the serotonergic neurons in the Drosophila ventral nerve cord depedns on the combined function of the zinc finger proteins Eagle and Huckebein. Development 124(13): 2515-25.
Doe, C. Q. (1992) Molecular markers for identified neuroblasts and ganglion mother cells in the Drosophila central nervous system. Development 116: 855-863.
Higashijima, S-i., Shishido, E., Matsuzaki, M., and Saigo, K. (1996). eagle, a member of the steroid receptor gene superfamily is expressed in a subset of neuroblasts and regulates the fate of their putative progeny in the Drosophila melanogaster CNS. Development 122: 527-36.
Konrad, K.D., and Marsh, J.L. (1987). Developmental expression and spatial distribution of dopa decarboxylase in Drosophila. Dev Biol 122(1): 172-85.
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Lundell, M. J., and Hirsh, J. (1998). eagle is required for the specification of serotonin neurons and other neuroblast 7-3 progeny in the Drosophila CNS. Development 125(3): 463-72.
Skeath, J. B., Zhang, Y., Holmgren, R., Carroll, S. B., and Doe, C. Q. (1995). Specification of neuroblast identity in the Drosophila embryonic central nervous system by gooseberry-distal. Nature 376: 427-430.
Taghert, P. M. and Goodman, C. S. (1984). Cell determination and differentiation of identified serotonin-immunoreactive neurons in the grasshopper embryo J Neurosci 4: 989-1000.
Thor, S., and Thomas, J. B. (1997). The Drosophila islet gene governs axon path-finding and neurotransmitter identity. Neuron 18: 397-409.
Yang, X., Bahri, S., Klein, T., and Chia, W. (1997). Klumpfuss, a putative Drosophila zinc finger transcription factor, acts to differentiate between the identities of two secondary precursor cells within one neuroblast lineage. Genes Dev 11(11):1396-1408.