1 Departments of Anesthesiology, Psychiatry, Molecular Biology and Pharmacology,
School of Medicine, Washington University Pain Center, St Louis, MO 63110,
USA
2 Anatomy and Neurobiology, School of Medicine, Washington University Pain
Center, St Louis, MO 63110, USA
Author for correspondence (e-mail:
chenz{at}wustl.edu)
Accepted 15 February 2005
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Dcc, Early-born neuron, Migration, Primary afferents, Spinal cord, Mouse
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Another major event concurring is the projection of different classes of
primary sensory afferents within the spinal cord
(Brown, 1981;
Ozaki and Snider, 1997
). The
projections of distinct classes of primary afferents differ not only in their
final termination sites but also in their entry routes in the dorsal horn. For
example, proprioceptive afferents enter the spinal cord through the medial
region, whereas nociceptive afferents innervate laminae I-II of the dorsal
horn through the lateral region (Brown,
1981
; Ozaki and Snider,
1997
) (Fig. 1A). In
mice, the ETS transcription factor ER81 and a Runx transcription
factor Runx3 are required in the dorsal root ganglion (DRG) for the
projection of proprioceptive afferents in the spinal cord
(Arber et al., 2000
;
Inoue et al., 2002
;
Patel et al., 2003
).
Peripheral signaling such as NT3 is also important for the central patterning
of proprioceptive afferents (Lin et al.,
1998
; Patel et al.,
2003
). In addition, central target-derived signals are required as
well for primary afferent projection. For example, the transcription factor
Lmx1b is required specifically in the dorsal horn for the projection
of cutaneous afferents (Ding et al.,
2004
). Among axonal guidance molecules, Sema3a is one of
the most intensively studied molecules in the spinal cord, and may play a role
in repelling the projections of primary afferents
(He and Tessier-Lavigne, 1997
;
Kolodkin et al., 1997
).
Although the factors controlling the projections of primary afferents are
beginning to be understood, much less is known about the relationship between
neuronal migration and precise primary afferent projections during spinal cord
development.
|
In this study, we have used both loss- and gain-of-function approaches to address mechanisms underlying neuronal migration and primary afferent projections. We found that Dcc is both necessary and sufficient for directing the migration of developing spinal neurons towards the ventral spinal cord. Moreover, we showed that early-born neurons can repel nociceptive primary afferents through the activity of Sema3a in the developing spinal cord.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
BrdU labeling and detection by immunocytochemistry
Pregnant female mice derived from timed matings between wild-type mice or
Dcc+/ mice were given a single intraperitoneal
injection of BrdU (5 mg/ml solution in PBS and 60 µg/g of body weight) on
10.5 days postcoitum (dpc), 11.5 dpc and 12.5 dpc. After time periods of 1
day, 2 days, 3 days or 4 days, embryos were removed, genotyped and sectioned
as described (Chen et al.,
1998a; Chen et al.,
2001
; Roberts et al.,
1994
). The sections were stained sequentially with mouse antibody
against BrdU (Molecular Probes; 1:200), biotin-conjugated donkey anti-mouse
IgG (Jackson Immuno; 1:200), and Cy3-labeled streptavidin (Jackson Immuno;
1:1000) or Elite ABC kit (Vector; 1:200). For anti-BrdU antibody
detection, a nickel intensification technique was used. To quantify the
distribution of E10.5 BrdU-labeled neurons, Photoshop (Adobe) program was used
to divide the spinal cord into dorsal and ventral halves by the sulcus
limitans. BrdU-labeled neurons in the two halves were counted in ten sections
from four wild-type mice for each group.
Nissl staining, X-gal staining, in situ hybridization and immunocytochemistry
Nissl staining, X-gal staining, in situ hybridization and
immunocytochemistry were performed as described
(Chen et al., 1998b;
Chen et al., 2001
;
Wang et al., 1998
). Briefly,
embryos were fixed with 4% paraformaldehyde overnight at 4°C,
cryoprotected with 15% sucrose, and sectioned at a thickness of 12 µm. For
X-gal staining, the sections were incubated in X-gal buffer (5 mM potassium
ferrocyanide, 5 mM potassium ferricyanide, 0.2% Triton X-100, 1 mM
MgCl2 and 1 mg/ml X-gal in PBS). For immunocytochemistry, the
following antibodies were used: guinea pig (Gp) anti-Isl1 (gift from
T. Jessell; 1:4000), Gp anti-Lmx1b (gift from T. Jessel; 1:4000),
rabbit anti-Math1 (gift from J. Johnson; 1:500), rabbit anti-TrkA
(gift from L. Recharidt; 1:4000), rabbit anti-Phox2a (gift from J.
Brunet; 1:4000), rabbit anti-Pax2 (Zymed Lab; 1:400), mouse anti-2H3
(DSHB; 1:50), goat anti-Dcc (Santa Cruz; 1:200), rabbit
anti-Peripherin 57K (Chemicon; 1:2000), rabbit anti-Calbindin 28K (Chemicon;.
1:2000) and mouse anti-Parvalbumin (Chemicon; 1:2000). For in situ
hybridization, the hybridization signals were visualized upon nitro blue
tetrazolium chloride and 5-bromo-4-chloro-3-indolyl phosphate staining.
DiI labeling
DiI labeling was performed as previously described
(Ding et al., 2004). Briefly,
to label the projection of DRG axons to the spinal cord, DiI crystals
(Molecular Probes) were placed in the DRG. The tissues were kept in 4%
paraformaldehyde at 37°C for 2-6 days before they were sectioned on a
vibratome at a thickness of 50-100 µm. Labeling was observed with
epifluorescent or laser confocal microscopy.
In utero electroporation of Dcc and Sema3a
In utero electroporation was performed according to previous procedures
(Saito and Nakatsuji, 2001).
Briefly, pregnant wild-type mice at 12.5 dpc were anesthetized with sodium
pentobarbital (40 mg/kg), followed by the exposure of the uterus and cutting
of the uterine wall on both horns along the antiplacental side. A mixture of
pCAGGS-Dcc or pCAGGS-Sema3a with pCAGGS-EGFP
(Niwa et al., 1991
) (2 µl;
1 µg/µl for each vector) was injected into the central canal of the
spinal cord. PCAGGS-EGFP was injected into different embryos and they were
served as control. After the injection, square electric pulses were delivered
by the use of an Electro Square Porator (ECM830) at a rate of one pulse per
second (voltage 25 V, 5 pulses, 50 ms pulses) to embryos by holding the yolk
sac with forceps-type electrodes. Expression of electroporated exogenous genes
was examined by in situ hybridization and immunohistochemistry as described
above. For counting electroporated cells (EGFP/Dcc and EGFP only) in
the spinal cord, 7-10 sections from each embryo were counted (four embryos for
EGFP and seven embryos for EGFP/Dcc).
Quantification of TrkA+ fibers in the dorsal horn of Sema3a electroporated spinal cord
The areas of TrkA-positive staining were quantified by the use of software
(MetaMorph, 6.2; Universal Imaging Corporation, Downingtown, PA, USA) and a
comparison between the electroporated and contralateral sides was performed
using Student's t-test. Intensity of a pixel above 110 was considered
to be positive and areas of TrkA-positive fibers were measured by the
software. Areas are indicated in µm2. Twenty-five images from
five electroporated embryos were used for the quantification.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
By contrast, when BrdU was injected at E11.5, BrdU+ cells were
found in the dorsal horn of the spinal cord at E13.5
(Fig. 1E). Similarly, when BrdU
was injected at E12.5, BrdU+ cells were also found in the dorsal
horn at E14.5 (Fig. 1F). Taken
together, our study demonstrates that early-born neurons (born prior to E11.5)
migrate ventrally, and do not contribute to the formation of the superficial
layer of the dorsal spinal cord. The superficial dorsal horn neurons comprise
only those born at E11.5 or afterwards. Although these results are in
consistent with previous molecular marker studies
(Gross et al., 2002), we, for
the first time, clearly demonstrate that early-born neurons do not contribute
to the formation of the superficial dorsal horn of the spinal cord.
Dccis expressed in early-born neurons of the spinal cord
To study the molecular basis for the control of ventral migration of
early-born neurons, we have examined expression of Dcc in the spinal
cord. Dcc was most abundantly expressed in the region lining along
the VZ of the neural tube between E10.5 and E11.5
(Fig. 2A). Dcc was not
detected in DRG neurons. In the developing neural tube, Dcc is
expressed in dI1-2 neurons (Helms and
Johnson, 1998) (Fig.
2A,B). Double immunocytochemical staining of Dcc and
Isl1 (dI3 marker) showed that Dcc was also colocalized with
Isl1, except in a few Isl1+ neurons
(Lee and Jessell, 1999
)
(Fig. 2B,C). Isl1
staining was detected in the nuclei, while Dcc expression was
enriched in the axons and membranes (Fig.
2C). In addition, Dcc was co-expressed with
Lmx1b (dI5 marker) (Fig.
2D,E), and Pax2 (dI4 and dI6 marker)
(Fig. 2F-H)
(Burrill et al., 1997
;
Chen et al., 2001
). At E12.5,
Dcc was detected in the dorsal side of the spinal cord, probably in
the migrating early-born neurons (see Fig. S1A in the supplementary material).
At E13.5 and E14.5, Dcc was not detected in the superficial dorsal
horn of the spinal cord where late-born neurons were present (see Fig. S1B,C
in the supplementary material). Thus, Dcc is expressed in all classes
of early-born dorsal interneurons, but not in late-born neurons.
|
|
To rule out the possibility that the ectopic presence of Isl1+ and Math1+ cells was due to a cell fate switch from late-born neurons to early-born ones, BrdU was injected into 10.5 dpc pregnant mice. At this stage, cells incorporating BrdU should be early-born neurons in the dorsal neural tube. Unlike wild-type embryos, whose early-born neurons labeled by BrdU had settled in the intermediate region and ventral horn, but were not present in the superficial dorsal horn by E14.5 (Fig. 3I), a cohort of BrdU+ cells was detected in the medial superficial dorsal horn of Dcc/ mutants (Fig. 3J). When the spinal cord of Dcc/ mutants labeled with BrdU at E11.5 was examined at E14.5, an area was devoid of BrdU+ cells in the medial part of laminae I-II of Dcc/ mutants (Fig. 3L), which was different from wild-type embryos whose superficial layer comprised evenly distributed BrdU+ cells (Fig. 3K). These results suggest that many of early-born cells aberrantly migrated to the medial superficial dorsal horn and failed to migrate ventrally in Dcc/ mutants.
We next sought to determine whether the migration of all classes or only
subsets of early-born neurons were impaired and how early the defects might
have occurred in Dcc/ mutants. At E10.5, a
stream of Math1+ cells were lining the dorsal-most VZ,
suggesting that Math1+ cells migrated to their destination
(Fig. 4A). However,
Dcc/ mutant Math1+ cells
remained in the region adjacent to the roof plate and few
Math1+ cells were found at more ventral regions of the
dorsal neural tube (Fig. 4B).
Examination of FoxD3 (dI2 marker) expression showed a defect similar
to Math1 staining pattern in Dcc/
mutants at E10.5 (Fig. 4C,D)
(Gross et al., 2002). Although
no apparent defect was found in Isl1 staining pattern at E10.5, more
Isl1+ cells were accumulated in the dorsal aspect of the
neural tube of Dcc/ mutant compared with the
control at E11.5 (Fig. 4E,F).
Double staining of Pax2 (dI4 and dI6 marker) and Lmx1b (dI5
marker) also revealed an aberrant distribution of Lmx1b+
cells in Dcc/ mutants
(Fig. 4G,H). A few
Lmx1b+ cells were found in more dorsal regions of the
neural tube. This result was further confirmed by Phox2a (dI5 marker)
staining in Dcc/ mutant embryos
(Ding et al., 2004
). A few
Phox2a+ cells appeared to have migrated aberrantly along
the dorsal direction (Fig.
4I,J). In addition, there was no change in numbers of cells
labeled by each markers examined between wild-type and
Dcc/ mutant mice. Taken together, our
results indicate that a majority of early-born cells depend on
Dcc-mediated signaling pathway for their ventral migration.
|
Dcc promotes ventral migration and prevents dorsolateral migration of late-born neurons
To test whether Dcc controls the migratory behavior of spinal cord
neurons directly, we introduced Dcc expression vectors into the mouse
dorsal spinal cord by in utero electroporation
(Saito and Nakatsuji, 2001).
Dcc- and Egfp-expressing vectors were electroporated into
the spinal cords of E12.5 mouse embryos and the migratory behavior of
electroporated neurons was examined at E14.5. In four control embryos that
received EGFP only, EGFP+ cells were detected predominantly in the
electroporated dorsal horn and few EGFP+ cells were found in the
ventral horn of the spinal cord at E14.5
(Fig. 5A,C). A few
EGFP+ cells were present in the intermediate region surrounding the
central canal of the spinal cord (Fig.
5A). Notably, EGFP+ cells were present in laminae I-II
(Fig. 5C). Strikingly, in the
spinal cord that expressed Dcc and Egfp, some
EGFP+ cells were found in the ventral horn of the spinal cord
(Fig. 5B). Moreover,
EGFP+ cells were hardly detected in laminae I-II
(Fig. 5D). Double-staining of
Egfp and Dcc indicated that Dcc and Egfp
were colocalized in electroporated cells
(Fig. 5E-G). Quantitative
analysis indicated that there was no significant difference between the number
of EGFP+ cells and Egfp/Dcc+ cells in
electroporated spinal cords (Fig.
5I). However, the number of EGFP/DCC+ cells was
markedly increased in the ventral horn and decreased in laminae I-II of
electroporated spinal cords compared with the spinal cords electroporated with
EGFP only (Fig. 5I,J). We
observed this phenotype in five of seven electroporated embryos. Taken
together, our results indicate that Dcc affects the migration of
late-born neurons by promoting their ventral migration and preventing their
dorsolateral migration, suggesting that expression of Dcc is
sufficient to re-direct the migration of late-born neurons, which do not
normally express Dcc.
|
|
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In Dcc/ mutants, some early-born neurons
appear to undergo normal ventral migration. Several factors may account for
this observation. First, not all early-born neurons express Dcc
(Fig. 2). Second, Dcc
is not the only receptor for netrins, and it may functionally be redundant
with other Dcc family members such as neogenin genes or Unc5h3
receptors (Ackerman and Knowles,
1998; Engelkamp,
2002
; Keino-Masu et al.,
1996
). Indeed, the observation that a similar defect found in
Unc5h3 mutant mice supports such a possibility (see Fig. S2 in the
supplementary material). Thus, multiple receptors for netrins may guide the
migration of early-born neurons synergistically. In
Dcc/ mutants, the migration of late-born
neurons appears to be normal, despite their proximity to early-born neurons
during their migration from the VZ to the superficial dorsal horn, consistent
with weak or no expression of Dcc in these neurons (see Fig. S1 in
the supplementary material). Furthermore, although late-born neurons that take
up exogenous Dcc fail to settle in the superficial layer of the
dorsal horn, other non-electroporated late-born neurons exhibit no apparent
aberrant migration. Thus, Dcc appears to function in a cell
autonomous manner.
The simplest explanation for how Dcc controls ventral migration is
that it allows early-born neurons to respond to Ntn1, which is
expressed in the floorplate and perhaps also as a ventral-to-dorsal gradient
in the spinal cord (Kennedy et al.,
1994). In this aspect, the function of Dcc is reminiscent
of its role in ventral projection of commissural neurons in the spinal cord
(Kennedy et al., 1994
).
Moreover, that overexpression of Dcc in late-born neurons altered
their migratory behavior suggests that an intracellular signal transduction
pathway that can be activated by Dcc may also be present in these
neurons.
Our study reveals an unexpected role of early-born neurons in the
patterning of primary afferents in the spinal cord. Interestingly, not all
early-born neurons are required for the projection of proprioceptive
afferents. For example, no obvious defects were found in the projections of
proprioceptive afferents in mice lacking Math1
(Bermingham et al., 2001). In
Lmx1b mutants, the specification of dI5 neurons is impaired, but
proprioceptive afferent do project to the ventral horn
(Ding et al., 2004
;
Gross et al., 2002
;
Muller et al., 2002
).
Therefore, at least dI1 and dI5 neurons are dispensable for the projections of
proprioceptive afferents.
In Dcc/ mutants, early-born neurons are
mispositioned in the medial superficial dorsal horn to which proprioceptive
afferents aberrantly project. This suggests that early-born neurons contain a
chemoattractant necessary for directing the projection of proprioceptive
afferents. Although Sema3a is most notable for its chemorepellent
function, it may also function as a chemoattractant in the nervous system
(Pasterkamp and Kolodkin,
2003; Polleux et al.,
2000
). However, the previous finding that Sema3a has
little influence on NT3-responsive axons raises the possibility that other
unknown chemoattractant(s) may be involved
(Messersmith et al., 1995
). On
the other hand, the observation that TrkA+ afferents are
excluded from the region where early-born neurons are present argues that
early-born neurons have a repulsive function for cutaneous afferents. Several
lines of evidence suggest that early-born neurons may exert this function
through Sema3a. First, Sema3a expression is abnormally
detected in the region where early-born neurons are ectopically located in the
medial superficial dorsal horn (Fig.
8). In addition, forced expression of Sema3a in the
dorsal horn by in utero electroporation repels TrkA+ primary
afferents (Fig. 8). Therefore,
our study indicates that Sema3a repels cutaneous afferents in vivo,
which is in consistent with studies performed in chicken and rat spinal cord
(Fu et al., 2000
;
Pasterkamp et al., 2000
;
Shepherd et al., 1997
;
Tang et al., 2004
). It further
offers an explanation for why TrkA+ afferents avoid the midline
region of the dorsal horn where early-born neurons are present as their entry
route. In both chick and mouse spinal cord, the entry of cutaneous primary
afferents into the dorsal side of the spinal cord occurs only after a waiting
period that is coincident with the abundant presence of early-born neurons and
Sema3a at the dorsal side of the spinal cord
(Shepherd et al., 1997
). Thus,
a spatial regulated distribution of Sema3a+ in early-born
neurons may have a key role in preventing TrkA+ primary afferents
from entering the dorsal spinal cord prematurely. Together, our results
demonstrate that early-born neurons possess dual roles in guiding the
projections of primary afferents: (1) to steer the initial ingrowth of
proprioceptive afferents towards the ventral horn; and (2) to repel the
nociceptive afferents from the midline region and from the deep dorsal
horn.
Our results suggest that an early-born neurons-derived signal is crucial
for the initial ingrowth of proprioceptive afferent along the midline and
toward the intermediate region within the spinal cord. As Dcc is not
expressed in DRG neurons, our result is in contrast with previous studies that
showed that factors guiding the ventral projection of proprioceptive afferents
are most likely to reside in the proprioceptive neurons in the DRG
(Arber et al., 2000;
Inoue et al., 2002
;
Lin et al., 1998
;
Patel et al., 2003
).
Interestingly, in the absence of Dcc, some proprioceptive afferents
did manage to project to the intermediate region as well as the ventral horn
of the spinal cord at later stages, albeit in much reduced number
(Fig. 7). Whether a delay of
ventral projection of proprioceptive afferents reflects a partial defect of
early-born neurons or a compensation mechanism remains unknown. Nevertheless,
because a majority of early-born neurons no longer migrate further once they
reach the intermediate region, other signals may come into play for further
elongation of those afferents. Together with the findings that transcription
factor such as Er81 and Runx3 are required in DRG neurons
for proprioceptive afferent projection, our data suggest that coordinated
signals derived from both primary afferents and their central target neurons
within the spinal cord are essential for the projections of the primary
afferents.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
Footnotes |
---|
Supplementary material for this article is available at http://dev.biologists.org/cgi/content/full/132/9/2047/DC1
* Present address: Laboratory of Neural Development, Institute of
Neuroscience, Chinese Academy of Sciences, 320 Yue Yang Road, Shanghai,
200031, PR China
Present address: Department of Neurology, Northwestern University Feinberg
School of Medicine, 303 E Chicago Avenue, Ward 10-185, Chicago, IL 60611,
USA
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Ackerman, S. L. and Knowles, B. B. (1998). Cloning and mapping of the UNC5C gene to human chromosome 4q21-q23. Genomics 52,205 -208.[CrossRef][Medline]
Arber, S., Ladle, D. R., Lin, J. H., Frank, E. and Jessell, T. M. (2000). ETS gene Er81 controls the formation of functional connections between group Ia sensory afferents and motor neurons. Cell 101,485 -498.[CrossRef][Medline]
Bermingham, N. A., Hassan, B. A., Wang, V. Y., Fernandez, M., Banfi, S., Bellen, H. J., Fritzsch, B. and Zoghbi, H. Y. (2001). Proprioceptor pathway development is dependent on Math1. Neuron 30,411 -422.[CrossRef][Medline]
Brown, A. G. (1981). Organization of the Spinal Cord. The Anatomy and Physiology of Identified Neurons. Berlin: Springer-Verlag.
Burrill, J. D., Moran, L., Goulding, M. D. and Saueressig,
H. (1997). PAX2 is expressed in multiple spinal cord
interneurons, including a population of EN1+ interneurons that require PAX6
for their development. Development
124,4493
-4503.
Caspary, T. and Anderson, K. V. (2003). Patterning cell types in the dorsal spinal cord: what the mouse mutants say. Nat. Rev. Neurosci. 4,289 -297.[CrossRef][Medline]
Chan, S. S., Zheng, H., Su, M. W., Wilk, R., Killeen, M. T., Hedgecock, E. M. and Culotti, J. G. (1996). UNC-40, a C. elegans homolog of DCC (Deleted in Colorectal Cancer), is required in motile cells responding to UNC-6 netrin cues. Cell 87,187 -195.[CrossRef][Medline]
Chen, H., Lun, Y., Ovchinnikov, D., Kokubo, H., Oberg, K. C., Pepicelli, C. V., Gan, L., Lee, B. and Johnson, R. L. (1998a). Limb and kidney defects in Lmx1b mutant mice suggest an involvement of LMX1B in human nail patella syndrome. Nat. Genet. 19,51 -55.[CrossRef][Medline]
Chen, Z. F., Paquette, A. J. and Anderson, D. J. (1998b). NRSF/REST is required in vivo for repression of multiple neuronal target genes during embryogenesis. Nat. Genet. 20,136 -142.[CrossRef][Medline]
Chen, Z. F., Rebelo, S., White, F., Malmberg, A. B., Baba, H., Lima, D., Woolf, C. J., Basbaum, A. I. and Anderson, D. J. (2001). The paired homeodomain protein DRG11 is required for the projection of cutaneous sensory afferent fibers to the dorsal spinal cord. Neuron 31,59 -73.[CrossRef][Medline]
Ding, Y. Q., Yin, J., Kania, A., Zhao, Z. Q., Johnson, R. L. and
Chen, Z. F. (2004). Lmx1b controls the differentiation and
migration of the superficial dorsal horn neurons of the spinal cord.
Development 131,3693
-3703.
Dodd, J., Morton, S. B., Karagogeos, D., Yamamoto, M. and Jessell, T. M. (1988). Spatial regulation of axonal glycoprotein expression on subsets of embryonic spinal neurons. Neuron 1,105 -116.[CrossRef][Medline]
Engelkamp, D. (2002). Cloning of three mouse Unc5 genes and their expression patterns at mid-gestation. Mech. Dev. 118,191 -197.[CrossRef][Medline]
Fazeli, A., Dickinson, S. L., Hermiston, M. L., Tighe, R. V., Steen, R. G., Small, C. G., Stoeckli, E. T., Keino-Masu, K., Masu, M., Rayburn, H. et al. (1997). Phenotype of mice lacking functional Deleted in colorectal cancer (Dcc) gene. Nature 386,796 -804.[CrossRef][Medline]
Fearon, E. R., Cho, K. R., Nigro, J. M., Kern, S. E., Simons, J. W., Ruppert, J. M., Hamilton, S. R., Preisinger, A. C., Thomas, G., Kinzler, K. W. et al. (1990). Identification of a chromosome 18q gene that is altered in colorectal cancers. Science 247, 49-56.[Medline]
Finger, J. H., Bronson, R. T., Harris, B., Johnson, K.,
Przyborski, S. A. and Ackerman, S. L. (2002). The netrin 1
receptors Unc5h3 and Dcc are necessary at multiple choice points for the
guidance of corticospinal tract axons. J. Neurosci.
22,10346
-10356.
Fu, S. Y., Sharma, K., Luo, Y., Raper, J. A. and Frank, E. (2000). SEMA3A regulates developing sensory projections in the chicken spinal cord. J. Neurobiol. 45,227 -236.[CrossRef][Medline]
Gross, M. K., Dottori, M. and Goulding, M. (2002). Lbx1 specifies somatosensory association interneurons in the dorsal spinal cord. Neuron 34,535 -549.[CrossRef][Medline]
He, Z. and Tessier-Lavigne, M. (1997). Neuropilin is a receptor for the axonal chemorepellent Semaphorin III. Cell 90,739 -751.[CrossRef][Medline]
Helms, A. W. and Johnson, J. E. (1998).
Progenitors of dorsal commissural interneurons are defined by MATH1
expression. Development
125,919
-928.
Helms, A. W. and Johnson, J. E. (2003). Specification of dorsal spinal cord interneurons. Curr. Opin. Neurobiol. 13,42 -49.[CrossRef][Medline]
Huang, E. J., Wilkinson, G. A., Farinas, I., Backus, C., Zang,
K., Wong, S. L. and Reichardt, L. F. (1999). Expression of
Trk receptors in the developing mouse trigeminal ganglion: in vivo evidence
for NT-3 activation of TrkA and TrkB in addition to TrkC.
Development 126,2191
-2203.
Inoue, K., Ozaki, S., Shiga, T., Ito, K., Masuda, T., Okado, N., Iseda, T., Kawaguchi, S., Ogawa, M., Bae, S. C. et al. (2002). Runx3 controls the axonal projection of proprioceptive dorsal root ganglion neurons. Nat. Neurosci. 5, 946-954.[CrossRef][Medline]
Keino-Masu, K., Masu, M., Hinck, L., Leonardo, E. D., Chan, S. S., Culotti, J. G. and Tessier-Lavigne, M. (1996). Deleted in colorectal cancer (DCC) encodes a netrin receptor. Cell 87,175 -185.[CrossRef][Medline]
Kennedy, T. E., Serafini, T., de la Torre, J. R. and Tessier-Lavigne, M. (1994). Netrins are diffusible chemotropic factors for commissural axons in the embryonic spinal cord. Cell 78,425 -435.[CrossRef][Medline]
Kolodkin, A. L., Levengood, D. V., Rowe, E. G., Tai, Y. T., Giger, R. J. and Ginty, D. D. (1997). Neuropilin is a semaphorin III receptor. Cell 90,753 -762.[CrossRef][Medline]
Lee, K. J. and Jessell, T. M. (1999). The specification of dorsal cell fates in the vertebrate central nervous system. Annu. Rev. Neurosci. 22,261 -294.[CrossRef][Medline]
Lee, K. J., Mendelsohn, M. and Jessell, T. M.
(1998). Neuronal patterning by BMPs: a requirement for GDF7 in
the generation of a discrete class of commissural interneurons in the mouse
spinal cord. Genes Dev.
12,3394
-3407.
Lee, K. J., Dietrich, P. and Jessell, T. M. (2000). Genetic ablation reveals that the roof plate is essential for dorsal interneuron specification. Nature 403,734 -740.[CrossRef][Medline]
Lin, J. H., Saito, T., Anderson, D. J., Lance-Jones, C., Jessell, T. M. and Arber, S. (1998). Functionally related motor neuron pool and muscle sensory afferent subtypes defined by coordinate ETS gene expression. Cell 95,393 -407.[CrossRef][Medline]
Luo, Y., Raible, D. and Raper, J. A. (1993). Collapsin: a protein in brain that induces the collapse and paralysis of neuronal growth cones. Cell 75,217 -227.[CrossRef][Medline]
Messersmith, E. K., Leonardo, E. D., Shatz, C. J., Tessier-Lavigne, M., Goodman, C. S. and Kolodkin, A. L. (1995). Semaphorin III can function as a selective chemorepellent to pattern sensory projections in the spinal cord. Neuron 14,949 -959.[CrossRef][Medline]
Muller, T., Brohmann, H., Pierani, A., Heppenstall, P. A., Lewin, G. R., Jessell, T. M. and Birchmeier, C. (2002). The homeodomain factor lbx1 distinguishes two major programs of neuronal differentiation in the dorsal spinal cord. Neuron 34,551 -562.[CrossRef][Medline]
Murase, S. and Horwitz, A. F. (2002). Deleted
in colorectal carcinoma and differentially expressed integrins mediate the
directional migration of neural precursors in the rostral migratory stream.
J. Neurosci. 22,3568
-3579.
Niwa, H., Yamamura, K. and Miyazaki, J. (1991). Efficient selection for high-expression transfectants with a novel eukaryotic vector. Gene 108,193 -199.[CrossRef][Medline]
Ozaki, S. and Snider, W. D. (1997). Initial trajectories of sensory axons toward laminar targets in the developing mouse spinal cord. J. Comp. Neurol. 380,215 -229.[CrossRef][Medline]
Pasterkamp, R. J. and Kolodkin, A. L. (2003). Semaphorin junction: making tracks toward neural connectivity. Curr. Opin. Neurobiol. 13, 79-89.[CrossRef][Medline]
Pasterkamp, R. J., Giger, R. J., Baker, R. E., Hermens, W. T. and Verhaagen, J. (2000). Ectopic adenoviral vector-directed expression of Sema3A in organotypic spinal cord explants inhibits growth of primary sensory afferents. Dev. Biol. 220,129 -141.[CrossRef][Medline]
Patel, T. D., Kramer, I., Kucera, J., Niederkofler, V., Jessell, T. M., Arber, S. and Snider, W. D. (2003). Peripheral NT3 signaling is required for ETS protein expression and central patterning of proprioceptive sensory afferents. Neuron 38,403 -416.[CrossRef][Medline]
Polleux, F., Morrow, T. and Ghosh, A. (2000). Semaphorin 3A is a chemoattractant for cortical apical dendrites. Nature 404,567 -573.[CrossRef][Medline]
Qian, Y., Shirasawa, S., Chen, C. L., Cheng, L. and Ma, Q.
(2002). Proper development of relay somatic sensory neurons and
D2/D4 interneurons requires homeobox genes Rnx/Tlx-3 and Tlx-1.
Genes Dev. 16,1220
-1233.
Roberts, C. W., Shutter, J. R. and Korsmeyer, S. J. (1994). Hox11 controls the genesis of the spleen. Nature 368,747 -749.[CrossRef][Medline]
Saba, R., Nakatsuji, N. and Saito, T. (2003).
Mammalian BarH1 confers commissural neuron identity on dorsal cells in the
spinal cord. J. Neurosci.
23,1987
-1991.
Saito, T. and Nakatsuji, N. (2001). Efficient gene transfer into the embryonic mouse brain using in vivo electroporation. Dev. Biol. 240,237 -246.[CrossRef][Medline]
Schwarting, G. A., Kostek, C., Bless, E. P., Ahmad, N. and
Tobet, S. A. (2001). Deleted in colorectal cancer (DCC)
regulates the migration of luteinizing hormone-releasing hormone neurons to
the basal forebrain. J. Neurosci.
21,911
-919.
Shepherd, I. T., Luo, Y., Lefcort, F., Reichardt, L. F. and
Raper, J. A. (1997). A sensory axon repellent secreted from
ventral spinal cord explants is neutralized by antibodies raised against
collapsin-1. Development
124,1377
-1385.
Tang, X. Q., Tanelian, D. L. and Smith, G. M.
(2004). Semaphorin3A inhibits nerve growth factor-induced
sprouting of nociceptive afferents in adult rat spinal cord. J.
Neurosci. 24,819
-827.
Wang, H. U., Chen, Z. F. and Anderson, D. J. (1998). Molecular distinction and angiogenic interaction between embryonic arteries and veins revealed by ephrin-B2 and its receptor Eph-B4. Cell 93,741 -753.[CrossRef][Medline]