1 Department of Biological Sciences, Tata Institute of Fundamental Research,
Homi Bhabha Road, Mumbai 400005, India
2 National Centre for Biological Sciences, TIFR, GKVK PO, Bangalore 560065,
India
* Author for correspondence (e-mail: veronica{at}tifr.res.in)
Accepted 20 January 2003
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SUMMARY |
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Key words: Olfactory lobe, Sensory neurons, Robo receptors, Slit, Glomerular patterning
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Introduction |
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The development of pattern within brain structures can be addressed at two
levels. First, what are the mechanisms by which individual sensory neurons and
their target interneurons find each other? Second, how are these synapses
located reproducibly within the three-dimensional architecture of the
neuropil? Studies in the Drosophila embryo have provided valuable
insights about how axonal scaffolds and synapses are organized within the axis
of the embryonic midline (Dickson,
2001; Grunvald and Klein,
2002
; Ziatic et al.,
2003
). This instruction involves a combinatorial expression of
receptors on the axonal surface, and attractive and repulsive ligands secreted
at the midline. It is becoming increasingly clear that similar principles
could apply in patterning more complex three-dimensional contours in adult
brains (Richards, 2002
;
Bagri et al., 2002
;
Hutson and Chien, 2002
;
Plump et al., 2002
).
In the olfactory system of both vertebrates and insects, neurons expressing
a given odorant receptor gene project with remarkable precision to specific
glomerular sites within the olfactory lobe. In rodents, the specificity of
axonal convergence is mediated by the odorant receptors
(Wang et al., 1998) in
collaboration with cell-surface molecules the Ephrin A proteins
(Cutforth et al., 2003
). In
Drosophila, the SH2/SH3 adaptor, Dreadlocks (Dock) and
serine/threonine kinase Pak, form part of a signaling module that is necessary
for the precise guidance of olfactory neurons to their glomerular targets
(Ang et al., 2003
). Dock and
Pak are known to be downstream of receptors that play key roles in axon
guidance, such as the Down Syndrome Cell Adhesion Molecule (Dscam)
(Schmucker et al., 2000
) and
the Roundabout (Robo) family (Fan et al.,
2003
)
Loss of Dscam function affects the development of olfactory
projections; neurons enter the lobe but terminate inappropriately at ectopic
sites (Hummel et al., 2003).
As Dscam can theoretically encode 38,000 isoforms by alternative
splicing, this locus provides an intriguing means by which axons can recognize
their postsynaptic partners. Interestingly, 68 combinations of transcripts
have been detected in olfactory neurons. The dendritic arborization of
projection neurons within specific glomeruli has been shown to occur prior to
the arrival of the sensory neurons into the lobe
(Jefferis et al., 2003
). The
mechanism of projection neuron patterning is therefore likely to be
independent of sensory neurons and has been shown to require the POU domain
transcription factors Drifter and Acj6
(Komiyama et al., 2003
). How
are sensory neuron arbors correctly positioned to allow interaction with
appropriate neural partners?
In this paper, we investigate the role played by Robo receptors as possible
guidance cues during formation of the olfactory map. A precedent for the role
of Robo/Slit mediated signaling in determining topographic projections exists
in the vomeronasal system of the rodent
(Knoll et al., 2003). We
demonstrate that a combinatorial code, defined by the domains and levels of
Robo receptors, patterns not only the location of glomeruli but also the
formation of the commissures which connect the two lobes. Our results from
loss-of-function and gain-of-function analysis suggest that Robo receptors
together with their ligand Slit are involved in positioning the arbors of the
olfactory neurons at defined sites within the lobe.
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Materials and methods |
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All fly stocks were grown on standard cornmeal medium at 25°C. White prepupae (0 hours after puparium formation; APF) were collected and allowed to develop on moist filter paper. This stage lasts 1 hour, hence the error in staging is ±30 minutes. Wild-type pupae take about 100 hours to eclose when grown at 25°C in our laboratory.
Clonal analysis
Clones of robo mutants were generated using the mosaic analysis
with repressible cell marker (MARCM) method described by Lee and Luo
(Lee and Luo, 1999). Stocks of
FRT40A robo21/CyO FRT40A and
robo31/CyO were generated using standard
recombination. Males of genotype Or-22a-Gal4;robo21 (or
robo31) FRT40A/CyO;ey-Flp/+ were crossed to
Tub-Gal80 FRT40A/CyO;UNG12/TM6-Tb females.
Non-CyO, non-Tb flies were dissected and stained with
antibodies against GFP and mAbnc82 (see below). GFP expressing neurons are
homozygous for the mutation. To ascertain the efficiency of eye-Flp,
we crossed eye-Flp/TM6-Tb flies with
Act5C>CD2>Gal4, UAS-nlsGFP. All non-Tb flies
showed GFP expression in the entire eye antennal disc. The brains of these
animals did not show any clones in the cells around the olfactory lobes,
establishing that eye-Flp generates clones only in the sensory
neurons of the olfactory system.
Immunohistochemistry
Dissection and antibody staining of pupal and adult brain whole mounts were
carried out as described previously
(Jhaveri et al., 2000). Where
mAbnc82 was to be used, the protocol of Laissue et al.
(Laissue et al., 1999
) was
followed. The primary antibodies used were anti-Robo (1:10 from Corey
Goodman), anti-Robo2 (1:200 from Corey Goodman), anti-Robo3 (1:100
Developmental Studies Hybridoma Bank at University of Iowa), mouse anti-Slit
(1:50 from Spryos Artavonis-Tsakonis), mAbnc82 (1:10 from E. Buchner), rabbit
anti-GFP (1:10,000 from Molecular Probes) and rabbit anti-Repo (1:5,000 from
S. Susinder). Secondary antibodies used were Alexa 488- and Alexa 568-coupled
goat anti-mouse and anti-rabbit IgG (1:200 Molecular Probes). Labeled samples
were mounted in anti-fading agent, Vectashield (Vector Laboratories) imaged on
BioRad Radiance 2000 at 1 µm intervals; data were processed using Confocal
Assistant 4.2 and Adobe Photoshop 5.5.
In order to quantitate staining intensity, the regions of interest on 0.7 or 1 µm sections were demarcated and pixel intensity was estimated using Image J software. The cumulative intensity over the volume of the glomerulus (for Robo experiments) or the entire lobe (for Slit experiments) was estimated.
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Results |
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|
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Sensory neurons begin to invade the lobe from about 25 hours APF and the
first signs of glomerular organization become apparent by around 36 hours APF
(Jhaveri et al., 2000;
Jefferis et al., 2003
).
Glomerular formation occurs sequentially and by 60 hours APF most of the
glomeruli have formed. The entry of glial cell processes and concomitant
increase in lobe volume, results in some re-organization of glomerular
position and the adult pattern (Fig.
2J) can only be discerned by about 80 hours APF. Robo and Robo3
are enriched in subsets of sensory neurons as they terminate within the lobe
(Fig. 2G-I). Robo is detected
in most axons, although at differing levels
(Fig. 2G,I), while Robo3 is
strongly enriched in terminals within a smaller number of glomeruli
(Fig. 2H,I). A comparison of
stained 60 hour APF lobes (Fig.
2H,I) with the adult glomerular map
(Fig. 2J) suggests that
Robo3-expressing neurons tend to preferentially target more dorsomedial
locations. An estimation of Robo and Robo3 immunoreactivity in identified
glomeruli supports the idea of a combinatorial code in determining sensory
neuron position (Table 1).
|
Loss of Robo function reveal a function in olfactory neuron targeting and lobe organization
The MARCM method (Lee and Luo,
1999) combined with ey-FLP
(Newsome et al., 2000
)
generates large patches of homozygous tissue in the eye-antennal disc
(Ang et al., 2003
;
Jefferis et al., 2003
). As
flip-out occurs early, phenotypes generated in mature neurons result from a
lack of gene function from the beginning of differentiation. We generated
clones of robo21 and robo31 and
examined targeting of a small number of sensory neurons marked by the
Or22a-Gal4 transgene. Sensory neurons expressing Or22a normally
project to glomerulus designated DM2 (Fig.
3A) (Vosshall et al.,
2000
) and cross-over to the contralateral lobe in the
inter-antennal commissure (arrow in Fig.
3A).
|
As Robo is expressed rather generally in olfactory neurons, we decided to
study loss-of-function by targeted misexpression of antagonists of signaling
(Dickson, 2001), rather than in
clones. SG18.1-Gal4 expresses in a large fraction of olfactory
neurons thus revealing most of the glomeruli
(Fig. 3G, asterisks) as well as
the antennal commissure (arrow in Fig.
3J). Ectopic expression of commissureless (comm)
(Kidd et al., 1998) using SG18.1-Gal4 resulted in disorganization of
glomerular patterning (compare Fig. 3H with
3G) with a weak effect on the commissure (arrow in
Fig. 3H). Comm has been shown
to downregulate Robo, although its effect on Robo2 and Robo3 is less well
understood (Rajagopalan et al.,
2000a
; Rajagopalan et al.,
2000b
). The phenotype of Comm ectopic expression suggests that
Robo is necessary for determining sensory neuron position within the lobe.
Abelson kinase (Abl) phosphorylates the CC0 and CC1 domains of Robo, thus
antagonizing signaling (Bashaw et al.,
2000
). Ectopic expression of either Abl or a constitutively active
Dcdc42v12 completely abolishes glomerular formation
(Fig. 3I,K). Sensory neurons
expressing Dcdc42v12 (SG18.1::Dcdc42v12) show an
attraction for the midline and terminate there forming `glomerular-like'
structures at the midline (Fig.
3L, asterisks). Results from loss-of-function clones predict such
a phenotype for robo2 nulls. Constitutive activation of Dcdc42 is
known to affect cytosketal dynamics generally, and could phenocopy a
loss-of-function of all Robo receptors
(Fritz and VanBerkum,
2002
).
Ectopic expression demonstrates that levels and location of Robo receptor expression are important for three-dimensional patterning of sensory terminals
We ectopically expressed Robo in sensory neurons to test whether the
domains and levels of receptors are instructive in positioning of sensory
terminals within the lobe. SG18.1::GFP was used to drive Robo in
olfactory neurons; the positions and morphology of glomeruli could be
visualized by GFP. Robo is expressed endogenously in all olfactory neurons and
the small increase in level caused by driving a single copy of the
UAS-robo transgene did not significantly alter lobe morphology (not
shown). Higher levels achieved by driving three copies of the transgene
abrogated glomerular formation (Fig.
4A). Changing the nature of the Robo code by misexpressing Robo3,
however, resulted in a dorsomedial shift of projections
(Fig. 4B). The commissure forms
normally when either Robo or Robo3 are misexpressed (red arrows in
Fig. 4A,B). Ectopic expression
of Robo2, however, completely abolishes commissure formation with a less
severe effect on glomerular morphology
(Fig. 4C).
|
Results presented above argue that sensory neuron positioning within the
lobe is determined by signaling through the Robo receptors. Reduction of Slit
levels suppress the effect of receptor overexpression, demonstrating that the
phenotypes are mediated through endogenous ligand. In this case, alteration of
the geometry of the Slit gradient by misexpression would be expected to alter
terminal positioning of sensory neurons. We drove high Slit expression in
glial cells around and within the lobe using loco-Gal4 (data
not shown) (Jhaveri et al.,
2000). Staining of the adult lobes in these animals with an
antibody against the synaptic marker mAbnc82 revealed the presence of ectopic
glomeruli outside the normal lobe circumference (broken lines in
Fig. 4H,I). Increasing Slit
levels further using multiple copies of the transgene led to more severe
effects.
Will perturbation of Robo levels in specific neurons result in changes in their three-dimensional organization?
Our model proposes that olfactory neurons traveling in the outer nerve
layer possess a different combination of Robo receptors that respond to Slit
by branching into the lobe and arborizing at specific positions. In order to
understand this positional code, we selected a Gal4 line that would allow us
to drive expression in a set of neurons projecting to identified glomeruli
from early during development. lz-Gal4;UAS-GFP labels two
glomeruli DM6 and DL3 during development and in the adult
brain (Fig. 5A-D), thus
providing a means to examine the location of selective sensory neurons when
the combinations of Robo are altered. We found that a change in the levels of
any of the three Robo receptors, caused by misexpression using the
lz-Gal4 driver, altered the positions of these identified terminals
(Fig. 5E-P). The phenotypes
showed variable expressivity; however, we were able to categorize preferred
positions for the terminals in each treatment
(Fig. 5D,H,L,P;
Table 2).
|
|
The ectopic `glomeruli' produced by alterations in the Robo code showed a
normal organization of cellular elements
(Fig. 5Q-V). In the wild type,
terminal branches of sensory neurons remain at the periphery of each
glomerulus (Fig. 5Q,S).
Dendritic arbors of the lobe interneurons, filled the entire glomerulus as
seen by GFP driven by GH146-Gal4
(Jefferis et al., 2001)
(Fig. 5R, green) or the synapse
specific marker mAbnc82 (Fig.
5Q',R',S red). Glomeruli produced by misexpression of
any of the Robo receptors also showed a similar organization as evidenced by
mAbnc82 staining (Fig.
5T-V).
Our expression and genetic data allows us to propose the model summarized in Fig. 6. Neurons arriving at the olfactory lobe in the antennal nerve express Robo, and those expressing high levels of Robo3 additionally decussate onto the medial side of the outer nerve layer (Fig. 6A). The position of an axon in the nerve layer is influenced by Slit levels, although the identity of the cells that contribute Slit still needs to be elucidated. Several regions of Slit expression have been detected in the brain, although the cells at the midline express highest levels. Robo2, which is expressed at very low levels in all sensory neurons, is elevated after they cross the midline thereby preventing re-crossing. Later during pupation (Fig. 6B), sensory axons branch into the lobe and terminate at distinctive positions regulated by their unique Robo code in response to Slit levels. This allows short-range interactions with the dendritic arbors of projection neurons leading to formation of glomeruli.
|
![]() |
Discussion |
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The mechanism by which attractive and repulsive cues act to shape neural
architecture is most elegantly demonstrated in the midline of the
Drosophila embryo. A combinatorial expression of Robo receptors
respond to the diffusible ligand Slit to dictate positioning of axons within
longitudinal tracts (Rajgopalan et al., 2000a; Rajgopalan et al., 2000b;
Simpson et al., 2000a;
Simpson et al., 2000b
). At the
midline, commissural neurons downregulate receptor expression by Comm-mediated
protein degradation (Rosenzweig and
Garrity, 2002
). Similar principles guide the selection and shaping
of axon fascicles in the retinotectal system of the zebrafish, as well as
several major pathways in mammalian brains
(Hutson and Chien, 2002
;
Plump et al., 2002
;
Bagri et al., 2002
). The
general principle of growth cone repulsion by Slit-mediated Robo signaling
appears to be a conserved theme in circuit design
(Richards, 2002
).
Can a similar mechanism be exploited to determine the location of synapses
in three-dimensional space? Ziatic et al.
(Ziatic et al., 2003) have
chosen projections from the chordotonal organs specified by Ato to demonstrate
how Robo3 can specify location of sensory arbors in the central nervous
system. The terminals act as substrate upon which synaptic interactions with
second-order neurons are built leading to formation of connectivity. A similar
process also appears to operate in giant fiber system of adult
Drosophila, where Robo receptors have been shown to play key roles in
synapse formation (Godenschwege et al.,
2002
). We envisage a comparable developmental strategy in the
olfactory lobe of the Drosophila adult. The correct spatial and
temporal regulation of Robo receptors and their response to Slit levels
determines the positioning of neurons and branching of terminal arbors at
specified positions within the lobe. This facilitates short-range interactions
with appropriate projection neurons which are already present within the
developing olfactory lobe (Jefferis et
al., 2003
).
Role of the secreted ligand Slit
Although the source and nature of the Slit gradient within the developing
olfactory system remains obscure, we have demonstrated that an alteration in
levels by ectopic expression leads to aberrant lobe patterning. Slit exerts
growth cone repulsion at the Drosophila embryonic midline through its
action on Robo receptors, as well as by silencing the attractive cues of
Netrins by action on the DCC receptor
(Stein and Tessier-Lavigne,
2001; Giger and Kolodkin,
2001
). In the cortex of the mammalian brain, Slit is widely and
dynamically expressed and exerts a major role on dendritic growth and
branching (Zinn and Sun, 1999
;
Whitford et al., 2002
).
Studies on cortical cultures demonstrated that concentrations of Slit strongly
affect dendritic length as well as number of branch-points per cell. Hence,
the complex geometry of a Slit gradient during development and its ability to
induce attraction, repulsion, as well as branching, could provide a means to
sculpt axon pathways in three-dimensional space.
Multiple signaling systems instruct formation of a precise functional olfactory map
The stereotypic location of each glomerulus within the lobe, and the
connectivity within and between glomeruli, are important aspects of
information processing in these functional units
(Rodrigues, 1988;
Galazia and Menzel, 2000
;
Fiala et al., 2002
;
Ng et al., 2002
;
Wang et al., 2003
). The
relatively conserved pattern of olfactory glomeruli within different
individuals suggests a robust developmental program for their formation.
The effects of Robo loss of function are incompletely penetrant and suggest
that multiple signals must exist for targeting of neurons. We propose that the
Robo receptors act in concert with DSCAM isoforms to guide sensory neurons to
the 50 unique positions within the three dimensional architecture of the
lobe (Hummel et al., 2003
).
DSCAM and also Robo signaling could converge onto the Dock/Pak module,
mutations in which also result in targeting defects
(Ang et al., 2003
;
Fan et al., 2003
). An accurate
spatiotemporal estimation of receptors, ligands and signaling cascades across
the developing olfactory lobe, together with mathematical modeling could
provide valuable insights about the determination of glomerular patterns. Such
knowledge is likely to be more generally applicable in the analysis of more
complex brain structures including those of vertebrates.
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ACKNOWLEDGMENTS |
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Ang, L. H., Kim, J., Stepensky, V. and Hing, H.
(2003). Dock and Pak regulate olfactory axon pathfinding in
Drosophila. Development
130,1307
-1316.
Bagri, A., Marin, O., Plump, A. S., Mak, J., Pleasure, S. J., Rubenstein, J. L. and Tessier-Lavigne, M. (2002). Slit proteins prevent midline crossing and determine the dorso-ventral position of the major axonal pathways in the mammalian forebrain. Neuron 33,233 -248.[Medline]
Bashaw, G. J., Kidd, T., Murray, D., Pawson, T. and Goodman, C. S. (2000). Repulsive axon guidance: abelson and enabled play opposing roles downstream of the roundabout receptor.Cell 101,703 -715.[Medline]
Bhalerao, S., Sen, A., Stocker, R. and Rodrigues, V. (2003). Olfactory neurons expressing identified receptor genes project to subsets of glomeruli within the antennal lobe of Drosophila melanogaster. J. Neurobiol. 54,577 -592.[CrossRef][Medline]
Buck, L. (2000). The molecular architecture of odor and pheromone sensing in mammals. Cell 100,611 -618.[Medline]
Cutforth, T., Moring, L., Mendelsohn, M., Nemes, A., Shah, N. M., Kim, M. M., Frisen, J. and Axel, R. (2003). Axonal Ephrin-As and odorant receptors: Coordinate determination of the olfactory sensory map. Cell 114,311 -322.[Medline]
Dickson, B. (2001). Rho GTPase in growth cone guidance. Curr. Opin. Neurobiol. 11,103 -110.[CrossRef][Medline]
Fan, X., Labrador, J. P., Hing, H. and Bashaw, G. J. (2003). Slit stimulation recruits Dock and Pak to the roundabout receptor and increases Rac activity to regulate axon repulsion at the CNS midline. Neuron 401,113 -127.
Fiala, A., Spall, T., Diegelmann, S., Eisermann, B., Sachse, S., Devaud, J. M., Buchner, E. and Galizia, C. G. (2002). Genetically expressed cameleon in Drosophila melanogaster is used to visualize olfactory information in projection neurons. Curr. Biol. 12,1877 -1884.[CrossRef][Medline]
Fritz, J. L. and VanBerkum, M. F. A. (2002). Regulation of Rho family GTPases is required to prevent axons from crossing the midline. Dev. Biol. 252, 46-58.[CrossRef][Medline]
Galazia, C. G. and Menzel, R. (2000). Odour perception in honeybees: coding information in glomerular patterns. Curr. Opin. Neurobiol. 10,504 -510.[CrossRef][Medline]
Gao, Q., Yuan, B. and Chess, A. (2000). Convergent projections of Drosophila olfactory neurons to specific glomeruli in the antennal lobe. Nat. Neurosci. 3, 780-785.[CrossRef][Medline]
Giger, R. J. and Kolodkin, A. L. (2001). Silencing the Siren: guidance cue hierarchies at the CNS midline. Cell 105,1 -4.[Medline]
Godenschwege, T. A., Simpson, J. H., Shan, X., Bashaw, G. J.,
Goodman, C. S. and Murphey, R. K. (2002). Ectopic expression
in the giant fiber system of Drosophila reveals distinct roles for
roundabout (robo) Robo2 and Robo3 in dendritic guidance and synaptic
connectivity. J. Neurosci.
22,3117
-3129.
Grunvald, I. C. and Klein, R. (2002). Axon guidance: receptor complexes and signaling mechanisms. Curr. Opin. Neurobiol. 12,250 -259.[CrossRef][Medline]
Hassan, B. A., Bermingham, N. A., He, Y., Sun, Y., Jan, Y. N., Zoghbi, H. Y. and Bellen, H. (2000). Atonal regulates neurite arborization but does not act as a proneural gene in the Drosophila brain. Neuron 25,549 -561.[Medline]
Hummel, T., Vasconcelos, M. L., Clemens, J. C., Fishilevich, Y., Vosshall, L. B. and Zipursky, S. L. (2003). Axonal targeting of olfactory receptor neurons in Drosophila is controlled by Dscam. Neuron 37,221 -231.[Medline]
Hutson, L. D. and Chien, C. B. (2002). Pathfinding and error correction by retinal axons: the role of astray/robo2. Neuron 33,205 -217.[Medline]
Jan, Y. N. and Jan, L. Y. (1994). Genetic control of cell fate specification in the peripheral nervous system. Annu. Rev. Genet. 28,373 -393.[CrossRef][Medline]
Jefferis, G. S. X. E., Marin, E. C., Stocker, R. F. and Luo. L. (2001). Target neuron prespecification in the olfactory map of Drosophila. Nature 414,204 -208.[Medline]
Jefferis, G. S. X. E., Marin, E., Watts, R. J. and Luo, L. (2002). Development of neuronal connectivity in Drosophila antennal lobes and mushroom bodies. Curr. Opin. Neurobiol. 12,80 -86.[CrossRef][Medline]
Jefferis, G. S. X. E., Vyas, R. M., Berdnik, D., Ramaekers, A., Stocker, R. F., Tanaka, N. K., Ito, K. and Luo, L. (2003). Developmental origin of wiring specificity in the olfactory system of Drosophila. Development 131,117 -130.[Medline]
Jhaveri, D. and Rodrigues, V. (2002). Sensory
neurons of the Atonal lineage pioneer the formation of glomeruli within the
adult Drosophila olfactory lobe. Development
129,1251
-1260.
Jhaveri, D., Sen, A. and Rodrigues, V. (2000). Mechanisms underlying olfactory neuronal connectivity in Drosophila the Atonal lineage organizes the periphery while sensory neurons and glia pattern the olfactory lobe. Dev. Biol. 226, 73-87.[CrossRef][Medline]
Kidd, T., Bland, K. S. and Goodman, C. S. (1999). Slit is the midline repellent for the Robo receptor in Drosophila. Cell 96,785 -794.[Medline]
Knoll, B., Schmidt, H., Andrews, W., Guthrie, S., Pini, A.,
Sundaresan, V. and Drescher, U. (2003). On the topographic
targeting of basal vomeronasal axons through Slit-mediated chemorepulsion.
Development 130,5073
-5082.
Komiyama, T., Johnston, W. A., Luo, L. and Jefferies, G. S. X. E. (2003). From lineage to wiring specificity: POU domain transcription factors control precise connections of Drosophila olfactory projection neurons. Cell 112,157 -167.[Medline]
Laissue, P. P., Reiter, C., Hiesinger, P. R., Halter, S., Fischbach, K. F. and Stocker, R. F. (1999). Three-dimensional reconstruction of the antennal lobe in Drosophila melanogaster. J. Comp. Neurol. 22,543 -552.
Lebetsky, T., Chang, T., Hartenstein, V. and Banerjee, U.
(2000). Specification of Drosophila hematopoietic lineage by
conserved transcription factors. Science
288,146
-149.
Lee, T. and Luo, L. (1999). Mosaic analysis with a repressible cell marker for studies of gene function in neuronal morphogenesis. Neuron 22,451 -461.[Medline]
Marin, E. C., Jefferis, G. S. X. E., Komiyama. T., Zhu, H. and Luo, L. (2002). Representation of the glomerular olfactory map in the Drosophila brain. Cell 109,243 -255.[Medline]
Newsome, T. P., Asling, B. and Dickson, B. J.
(2000). Analysis of Drosophila photoreceptor axon guidance in
eye-specific mosaics. Development
127,851
-860.
Ng, M., Roorda, R. D., Lima, S. Q., Zemelman, B. V., Morcillo, P. and Meisenbock, G. (2002). Transmission of olfactory information between three populations of neurons in the antennal lobe of the fly. Neuron 36,463 -474.[Medline]
Plump, A. S., Erskine, L., Sabatier, C., Brose, K., Epstein, C. J., Goodman, C. S., Mason, C. A. and Tessier-Lavigne, M. (2002). Slit1 and Slit2 cooperate to prevent premature midline crossing of retinal axons in the mouse visual system. Neuron 33,219 -232.[Medline]
Rajagopalan, S., Nicolas, E., Vivancos, V., Berger, J. and Dickson, B. J. (2000a). Crossing the midline: Roles and regulation of Robo receptors. Neuron 26,767 -777.[CrossRef]
Rajagopalan, S., Vivancos, V., Nicolas, E. and Dickson, B. J. (2000b). Selecting a longitudinal pathway: Robo receptors specify the lateral position of axons in the Drosophila CNS. Cell 103,1033 -1045.[Medline]
Richards, L. J. (2002). Surrounded by Slit-how forebrain commissural axons can be led astray. Neuron 33,153 -155.[CrossRef][Medline]
Rodrigues, V. (1988). Spatial coding of olfactory information in the antennal lobe of Drosophila melanogaster.Brain Res. 453,299 -307.[CrossRef][Medline]
Rosenzweig, M. and Garrity, P. (2002). Axon targeting meets protein trafficking: Comm takes Robo to the cleaners. Dev. Cell 3,301 -302.[CrossRef][Medline]
Rothberg, J. M., Jacobs, J. R., Goodman, C. S. and Artavanis-Tsaklonas, S. (1990). Slit: an extracellular protein necessary for development of midline glia and commissural axons pathways contains both EGF and LRR domains. Genes Dev. 4,2169 -2187.[Abstract]
Schmucker, D., Clemens, J. C., Shu, H., Worby, C. A., Xiao, J., Muda, M., Dixon, J. E. and Zipursky, S. L. (2000). Drosophila Dscam is an axon guidance receptor exhibiting extraordinary molecular diversity. Cell 101,671 -684.[Medline]
Shymala, B. V. and Chopra, A. (1999). Drosophila melanogaster chemosensory and muscle development: identification and properties of a novel allele of scalloped and a new locus SG18.1, in a Gal4 enhancer trap screen. J. Genet. 78,87 -97.
Simpson, J. H., Simpson, J. H., Bland, K. S., Fetter, R. D. and Goodman, C. S. (2000a). Short-range and long-range guidance by Slit and its Robo receptors: a combinatorial code of Robo receptors control lateral position. Cell 103,1019 -1032.[Medline]
Simpson, J. H., Kidd, T., Bland, K. S. and Goodman, C. S. (2000b). Short-range and long-range guidance by Slit and its Robo receptors: Robo and Robo2 play distinct roles in midline guidance. Neuron 28,753 -766.[Medline]
Stein, E. and Tessier-Lavigne, M. (2001).
Hierarchical organization of guidance receptors: Silencing of netrin
attraction by slit through a Robo/DCC receptor complex.
Science 291,1928
-1938.
Stocker, R. F., Lienhard, M. C., Borst, A. and Fischbach, K. F. (1990). Neuronal architecture of the antennal lobe in Drosophila melanogaster. Cell Tissue Res. 262, 9-34.[Medline]
Stocker, R. F. (1994). The organization of the chemosensory system in Drosophila melanogaster: a review. Cell Tissue Res. 275,3 -26.[CrossRef][Medline]
Vosshall, L. B., Wong, A. M. and Axel, R. (2000). An olfactory sensory map in the fly brain. Cell 102,147 -159.[Medline]
Wang, F., Nemes, A., Mendelsohn, M. and Axel, R. (1998). Odorant receptors govern the formation of a precise topographic map. Cell 93, 47-60.[Medline]
Wang, J. W., Wong, A. M., Flores, J., Vosshall, L. B. and Axel, R. (2003). Two-photon calcium imaging reveals an odor-evoked map of activity in the fly brain. Cell 112,271 -282.[Medline]
Whitford, K. I., Marillat, V., Stein, E., Goodman, C. S., Tessier-Lavigne, M., Chedotal, A. and Ghosh, A. (2002). Regulation of cortical dendrite development by Slit-Robo interactions. Neuron 33,47 -61.[Medline]
Wong, A. M., Wang, J. W. and Axel, R. (2002). Spatial representation of the glomerular map in the Drosophila protocerebrum. Cell 109,229 -241.[Medline]
Ziatic, M., Landraf, M. and Bate, M. (2003). Genetic specification of axonal arbors: atonal regulates robo3 to position terminal branches in the Drosophila nervous system. Neuron 37, 41-51.[Medline]
Zinn, K. and Sun, Q. (1999). Slit branches out: a secreted protein mediates both attractive and repulsive axon guidance. Cell 97,1 -4.[Medline]
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