Neuroglian
As early as six hours, the long alternatively spliced form of Neuroglian is expressed on the surface of specific CNS and PNS neurons, and a few PNS support cells. The longitudinal strip of expression found by seven hours in the CNS prefigures the location where the longitudinal axons will form, and appears even before the longitudinal glia have migrated into this same position (Hortsch, 1990). Between 11 and 12 hours of development, the short alternatively spliced form of Neuroglian is expressed in glia, and a variety of non-neuronal tissues, including trachea, hindgut, salivary gland and muscle.
The longitudinal glia (LG), progeny of a single glioblast, form a scaffold that presages the formation of longitudinal tracts in the ventral
nerve cord (VNC) of the Drosophila embryo. The LG are used as a substrate during the extension of the first axons of the longitudinal
tract. The differentiation of the LG has been examined in six mutations in which the longitudinal tracts are absent, displaced, or
interrupted to determine whether the axon tract malformations may be attributable to disruptions in the LG scaffold. Embryos mutant
for the gene prospero have no longitudinal tracts, and glial differentiation remains arrested at a preaxonogenic state. Two mutants of
the Polycomb group also lacked longitudinal tracts; here the glia fail to form an oriented scaffold, but cytological differentiation of
the LG is unperturbed. The longitudinal tracts in embryos mutant for slit fuse at the VNC midline and scaffold formation is
normal, except that it is medially displaced. Longitudinal tracts have intersegmental interruptions in embryos mutant for hindsight and
midline. In hindsight, there are intersegmental gaps in the glial scaffold. In midline, the glial scaffold retracts after initial extension.
LG morphogenesis during axonogenesis is abnormal in midline. Commitment to glial identity and glial differentiation also occurs
before scaffold formation. In all mutants examined, the early distribution of the glycoprotein Neuroglian is perturbed. This is
indicative of early alterations in VNC pattern present before LG scaffold formation begins. Therefore, some changes in scaffold
formation may reflect changes in the placement and differentiation of other cells of the VNC. In all mutants, alterations in
scaffold formation precedes longitudinal axon tract formation (Jacobs, 1993).
The lack of widespread axonal defects in the CNS of neurotactin mutants suggests that the function of Nrt in CNS morphogenesis
might be largely replaced by functionally related molecules. If so, embryos lacking Nrt as well as one of these other
molecules may display synergistic mutant phenotypes. To test this possibility, embryos lacking function of
both nrt and one of several genes encoding neural CAMs were examined.
Embryos of some double mutant combinations of neurotactin and other genes
encoding adhesion/signaling molecules, including neuroglian, derailed , and kekkon1, display phenotypic synergy.
This result provides evidence for functional cooperativity in vivo between the adhesion and signaling pathways
controlled by neurotactin and the other three genes (Speicher, 1998).
Neuroglian (Nrg) is a Drosophila neural CAM related to several vertebrate CAMs, though most closely to mouse L1. Two forms of Nrg that differ in their cytoplasmic domains and patterns of expression are
known . The long Nrg isoform is neural-specific; it is initially (early stage 12) found in a
fraction of CNS neurons, but during stage 13 it can be detected in most (and probably all)
differentiating neurons. The short Nrg isoform is expressed by glia, is widely expressed in other tissues,
and is probably expressed throughout the entire CNS. nrg1, a loss-of-function mutation
for both Nrg forms, is lethal and causes motor neuron pathfinding defects, but the overall CNS structure of mutant embryos
looks normal. Furthermore, unlike nrt5 embryos, no
defects are detected with mAb 1D4 in nrg1 embryos. In contrast, nrg1; nrt5 double mutant
embryos have a severe CNS phenotype. With mAb BP102, thinning or complete interruption of longitudinal
connectives, as well as fusion of commissures are observed. Fas II fascicles exhibit similar
abnormalities as those observed in nrt5 embryos, albeit with a much higher expressivity and penetrance. Most notably,
interruptions of the longitudinal axon bundles are frequent, as are misguidance phenotypes.
Double mutant embryos, like single nrt- embryos, also show a local constriction of the ventral nerve cord with a
variable expressivity. This defect may be a consequence of the impaired axogenesis and condensation of the nerve cord. No
defects outside the CNS are evident in the double mutants (Speicher, 1998).
Using mAb 22C10 (see Futsch), which recognizes a subset of neurons,
and mAb 1D4, the behavior of several identified pioneer axons were examined during early stages of axogenesis in nrg1;nrt5 embryos.The pioneer axon of the intersegmental nerve, aCC, as well as the
pioneer axon of the segmental nerve, establish their correct pathways. Likewise, the axons of the U neurons follow the aCC
pathway correctly. In contrast, in 37 of 128 cases (29%), the axons of the dMP2 and MP1 neurons, pioneers of the MP1
pathway, do not normally defasciculate from the aCC axon and turn to the posterior; instead, they either become and remain stalled or they delay their extension for a considerable time.
Other axons showing misguidance phenotypes are those of the six ventral
unpaired medial (VUM) neurons. In the wild type, the VUM axons initially fasciculate together before splitting into two
fascicles that grow laterally on either side of the midline, passing the RP2 neuron and fasciculating with the corresponding
anterior aCC axon. In 19 of 128 (15%)
double mutant segments, the fascicle of VUM axons either does not split or splits into more than two fascicles, each joining
a different aCC axon, including that of the same hemisegment. The first two axons of the vMP2 pathway, pCC
(the pioneer) and vMP2, grow correctly in most hemisegments; only in 4 of 128 cases was a misrouted vMP2
axon observed. Anomalies in the trajectory of the SP1 axon are also observed, though rarely. nrg1; nrt5 embryos also
display, due to slight mispositioning of cells, a somewhat irregular appearance of what is normally a highly stereotyped
pattern of neurons. However, the relative positions of neurons are maintained (Speicher, 1998).
It seems most likely that the phenotypes of nrg1; nrt5 embryos result from a direct requirement for these two CAMs
during axogenesis, and not as a secondary consequence of a previous requirement during neurogenesis. Thus, expression
of the nuclear proteins Eve, Ftz, and En, markers of the specification of subsets of neurons that are arranged in
characteristic patterns, is found to be
normal in nrg1; nrt5 embryos between stages 12 and 16. This suggests that a failure of proper cell fate
determination does not cause the axonal mutant phenotype. Likewise, glial cells expressing Repo, a specific marker for
most of the CNS glia, form at the correct time and place and in normal numbers in nrg1;
nrt5 embryos. The longitudinal glia (LG), which could provide a matrix for longitudinal axon extension, migrate and arrange normally in the double mutant,
prefiguring the longitudinal connectives. It is from stage 14 onward, when the LG normally stretch in the anterior-posterior
direction and enwrap the connectives, that gaps in the LG begin to appear, overlapping with gaps in the connectives. It is most likely, therefore, that this LG phenotype in late mutant embryos is a consequence, rather than the
origin, of the interruptions observed along the axonal connectives (Speicher, 1998).
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Neuroglian:
Biological Overview
| Evolutionary Homologs
| Regulation
| Developmental Biology
date revised: 25 May 2008
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