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First published online October 12, 2006
doi: 10.1242/10.1242/dev.02606
,
1 Biozentrum, University of Basel, CH-4056 Basel, Switzerland.
2 Institut de Génétique et de Biologie Moléculaire et
Cellulaire, UMR 7104, CNRS/INSERM/ULP, BP 10142, F-67404 Illkirch Cedex, CU de
Strasbourg, France.
3 Institute of Genetics, University of Mainz, D-55099 Mainz, Germany.
Author for correspondence (e-mail:
frank.hirth{at}unibas.ch)
Accepted 31 August 2006
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SUMMARY |
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 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key words: Drosophila, Brain development, Neuromere, DV patterning, ventral nervous system defective, Hox gene, labial, Neuroblast, Programmed cell death
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INTRODUCTION |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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;
von Ohlen and Doe, 2000).
Thus, ventral nervous system defective (vnd) is expressed in
ventral neuroectodermal cells (Jimenez et
al., 1995; Mellerick and
Nirenberg, 1995), while intermediate neuroblasts
defective (ind) expression is restricted to intermediate
neuroectodermal cells (Weiss et al.,
1998) and muscle specific homeobox (msh) is
expressed in lateral neuroectodermal cells
(D'Alessio and Frasch, 1996;
Isshiki et al., 1997). These
neuroectodermal cells in turn give rise to the three columns of ventral,
intermediate and lateral neuroblasts. The different fates of the individual
neuroblasts that form in different dorsoventral columns are influenced by
expression of the corresponding columnar genes (reviewed by
Skeath, 1999;
Skeath and Thor, 2003).
The functional role of the columnar genes is exemplified by vnd,
an Nkx2-type homeobox gene, which is continuously expressed within
the developing VNC from cellularization until the completion of embryonic
development. In vnd loss-of-function mutants, ventral neuroblasts are
absent or mis-specified. These precursor cell defects correlate with loss or
mis-specification of neuronal progeny, errors in axonal pathfinding and an
overall reduced number of cells in the developing VNC of vnd mutant
embryos. Thus, in vnd loss-of-function mutants, commissures are
fused, ventral unpaired medial neurons show pathfinding defects and midline
glia are reduced in number. Conversely, overexpression of vnd can
lead to transformations in the identity of intermediate and lateral
neuroblasts (Chu et al., 1998;
McDonald et al., 1998;
Mellerick and Modica,
2002).
In contrast to the situation in the embryonic VNC, much less is known about
the expression and function of the columnar gene vnd in the
developing brain of Drosophila. A recent study on early brain
neurogenesis shows that vnd is expressed in the procephalic
neuroectoderm as well as in subsets of identified brain neuroblasts
(Urbach and Technau, 2003a;
Urbach and Technau, 2003b).
However, information on the expression and function of vnd during
later embryonic brain development is lacking.
Here, we analyse the role of the columnar patterning gene vnd during embryonic brain development of Drosophila. Using immunocytochemistry, we first map the expression of vnd and show that it is confined to subsets of neural cells in the developing protocerebrum, deuterocerebrum and tritocerebrum. We then carry out a functional analysis of vnd focussed on the intercalary segment and tritocerebral neuromeres, and show that vnd is essential for the formation of the tritocerebrum in the developing brain. Thus, loss of vnd function leads to a gap-like brain phenotype that is due to the defective formation of a subset of tritocerebral neuroblast and the subsequent loss of neural tissue in the mutant domain. Moreover, we show that this loss of neural tissue is associated with increased apoptotic activity, resulting in the loss of the tritocerebral commissure and the longitudinal connectives that normally run through this neuromere, whereas blocking apoptosis in vnd-null mutant embryos results in partial restoration of tritocerebral cells and axon tracts. These findings suggest that the dorsoventral patterning gene vnd is essential for development and identity specification of tritocerebral neural lineages in embryonic brain development of Drosophila.
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MATERIALS AND METHODS |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) was used, balanced over FM7,
ftz-lacZ. Homozygous null mutants were identified by the absence of
ftz-lacZ. To analyse the development of the tritocerebral
lab expression territory in the vnd mutant background, we
used the line 7.31 lab-lacZ/7.31 lab-lacZ
(Tremml and Bienz, 1992)
crossed into vnd6. To identify vnd expression in
former lab-expressing tritocerebral cells in the labial
mutant background, we used line 7.31 lab-lacZ/7.31 lab-lacZ;
labvd1/TM3, hb-lacZ
(Tremml and Bienz, 1992).
Homozygous null mutants were identified by the absence of hb-lacZ.
For comparison with the wild-type situation, 7.31 lab-lacZ was
crossed back to wild type. 7.31 lab-lacZ shows cytoplasmic
distribution of ßgal and reflects endogenous lab
expression with additional ectopic expression patterns in the deutocerebral
anlage (Hirth et al., 1998;
Hirth et al., 2001). The
UAS/Gal4 transcriptional activation system
(Brand and Perrimon, 1993) was
used in order to perform overexpression experiments. As a Gal4-driver line, we
used the sca::Gal4 (Klaes et al.,
1994; Sprecher et al.,
2004) which in the embryonic CNS is active from neurectodermal
stage to early neural lineage formation. For the ubiquitous block of apoptosis
within neural lineages of vnd6 mutant embryos, we used the
sca::Gal4 driver line in order to activate UAS::p35
transcription (Mergliano and Minden,
2003). Embryos were staged according to Campos-Ortega and
Hartenstein (Campos-Ortega and Hartenstein,
1997).
Immunocytochemistry and TUNEL assay
Embryos were dechorionated, fixed, immunostained, flattened and staged
according to previously published protocols
(Patel, 1994;
Therianos et al., 1995;
Urbach et al., 2003). Primary
antibodies were rabbit anti-Deadpan [1:300
(Bier et al., 1992); kindly
provided by H. Vaessin], rabbit anti-HRP [FITC-conjugated; 1:100
(Jan and Jan, 1982); Jackson
Immunoresearch], rabbit anti-LAB at 1:100 (F.H., unpublished), rat anti-LAB at
1:500 (F.H., unpublished), mouse anti-NRT at 1:20 (BP106 antibody, DSHB),
rabbit anti-VND at 1:200 (McDonald et al.,
1998) (kindly provided by C. Q. Doe), rabbit anti-ßGAL
1:200-1:400 (Milan Analytika), mouse anti-ßGAL 1:50 (DSHB), mouse
anti-Fasciclin II 1:5 (Lin and Goodman,
1994), rat anti-ELAV 1:30 (DSHB), mouse anti-PROS 1:4
(Spana and Doe, 1995), mouse
anti-REPO 1:20 (DSHB), mouse anti-Engrailed [4D9,1:6
(Patel et al., 1989); DSHB].
Secondary antibodies used for confocal microscopic analysis were Alexa-488,
Alexa-568 and Alexa-647 antibodies generated in goat (Molecular Probes), all
at 1:150 dilution. Secondary antibodies used for flat-mount preparations
analyzed using Nomarski optics were either biotinylated or alkaline
phosphatase-conjugated antibodies generated in goat all at 1:500 (Dianova).
Apoptotic activity was assayed by TUNEL analysis using a commercial TUNEL kit
(ApoTag, Oncor) as previously described
(Richter et al., 1998) with
the following modifications. After fixation, embryos were washed in PBT for
2x5 minutes, then washed in Equilibration Buffer (from the ApoTaq kit)
for 2 minutes. Embryos were incubated in the working strength TdT mixture
(from the ApoTaq kit) for 1 hour at 37°C. After incubation, supernatant
was removed and embryos were washed for 2x2 minutes with Stop/Wash
solution (from the ApoTaq kit), and subsequently washed for 3x2 minutes,
2x30 minutes in PBT before starting immunolabelling. Embryos were
mounted in Vectashield H-1000 (Vector).
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). Figures were arranged and labelled using Adobe
Photoshop. |
RESULTS |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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)]. During later stages of embryogenesis, vnd
expression is also seen in specific cell clusters within these brain
neuromeres. Thus, at late stage 12, a large expression domain is seen in the
protocerebral neuromere (Fig.
1A,B) and two smaller expression domains are observed in the
deutocerebrum and in the tritocerebrum. Although the tritocerebral and
deuterocerebral vnd expression clusters are in close proximity to
each other, they do not overlap. Towards the end of embryogenesis, at
embryonic stage 15, expression of vnd is still visible in these three
neuromeric domains (Fig. 1C,D).
Throughout embryonic neurogenesis, we find vnd expression in
neuroblasts, ganglion mother cells and neurons as judged by immunolabelling
with anti-PROS and neuron-specific anti-ELAV antibodies. Immunolabelling with
glia-specific anti-REPO antibody indicates that none of the glia cells of the
embryonic brain express vnd (data not shown). vnd-expressing
cells are also seen in the neuromeres of the suboesophageal ganglion and the
VNC, as well as in peripheral sense organs; these vnd-expressing
cells will not be considered further in this report.
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To delineate the region affected in vnd mutants in more detail, we
studied the expression of engrailed (en), which in the
wild-type embryonic brain is located in several small clusters of cells that
demarcate the posterior boundary of the brain neuromeres
(Schmidt-Ott and Technau,
1992; Hirth et al.,
1995). The b1 en-stripe (or en head spot)
delimits the posterior protocerebrum (several en cells are also seen
more anteriorly in the protocerebrum as the secondary head spot), the b2
en-stripe (or en antennal stripe) delimits the posterior
deutocerebrum, and the b3 en-stripe (or en intercalary
stripe) delimits the posterior tritocerebrum
(Fig. 2E). In late vnd
mutant brain (embryonic stage 13 onwards), only the b1 en-stripe and
the secondary head spot are visible; neither the b2 en-stripe nor the
b3 en-stripe can be identified
(Fig. 2F). This supports the
observation that major parts of the embryonic tritocerebrum and parts of the
deutocerebrum are lacking in the vnd mutant. In addition to the cell
loss defect in the tritocerebral/deutocerebral brain region, a less marked
reduction in overall size of the protocerebrum is also seen in vnd
mutant embryos. Moreover the organization of the suboesophageal ganglion and
the VNC is affected in the vnd mutant (see also Mellerick and Modica,
2001). These latter two phenomena were not studied further.
At the gross histological level, the vnd mutant brain phenotype
described above is, in part, reminiscent of the mutant brain phenotype
observed for the Hox gene labial (lab). In lab-null
mutants, tritocerebral cells are generated and positioned correctly; however,
these cells fail to differentiate into neurons and marked axogenesis defects
occur, including the disruption of longitudinal connectives and lack of the
tritocerebral commissure (Hirth et al.,
1998; Page, 2000;
Hirth et al., 2001). As
lab and vnd also show overlapping expression in a subset of
tritocerebral neuroblasts (Urbach and
Technau, 2003a), these findings suggest that
lab-expressing tritocerebral neuroblasts are affected in vnd
mutant embryos. To investigate this, we focussed on the developing
tritocerebrum, and specifically on the lab expression domain of this
neuromere, and first determined whether loss of vnd function affects
formation of lab-expressing neuroblasts.
Defective tritocerebral neuroblast formation in vnd mutants
During the early phase of brain neurogenesis, the lab-expressing
neuroectodermal domain gives rise to 15 neuroblasts, which include all of the
tritocerebral neuroblasts and two deutocerebral neuroblasts
(Urbach and Technau, 2003a).
By stage 11, all of these neuroblasts are present and express lab;
they include a ventral group of tritocerebral neuroblasts, Tv1-Tv5, a more
dorsal group of tritocerebral neuroblasts, Td1-Td8, and two deutocerebral
neuroblasts, Dv2 and Dv4 (Fig.
3A,B). In the wild type, the most ventral part of the
neuroectodermal domain, from which the tritocerebral neuroblasts Tv1-Tv5 and
the two deutocerebral neuroblasts originate, dynamically co-expresses
lab and vnd between stages 8 and 11
(Urbach and Technau,
2003a).
In vnd mutants this ventral-most part of the
lab-expressing domain appears to be reduced in size and accordingly,
the number of lab-expressing neuroblasts that derive from this brain
region is diminished (Fig.
3C,D). Generally only four to six large rounded cells are observed
that co-express lab and the neuroblast-specific marker Deadpan (this
may be a slight underestimate as a few enlarged rounded cells in
sub-ectodermal position lacking Deadpan expression are sometimes observed in
this region). Based on the expression of molecular markers indicative of
dorsal neuroblasts [e.g. ladybird early, empty spiracles, wingless
(Urbach and Technau, 2003a;
Urbach et al., 2006)], this
reduction in lab-expressing neuroblasts appears to affect
preferentially ventral neuroblasts of the tritocerebrum and adjacent part of
the deutocerebrum. These data imply that vnd is required for the
formation of a ventral subset of lab-expressing neuroblasts in the
developing tritocerebrum.
Although the reduction in tritocerebral neuroblast number seen in vnd mutants can account for some of the tritocerebral defects, this mechanism alone is unlikely to be the exclusive cause for the massive cell loss phenotype observed in the late embryonic vnd mutant brain. This is because a dorsal subset of the lab-expressing tritocerebral neuroblasts, as well as large number of lab-expressing neural progeny are generated in the tritocerebrum of stage 11 vnd mutant brains (compare Fig. 3B with 3D). Hence, in addition to defective neuroblast formation, other phenomena must be responsible for the gap-like phenotype observed in vnd mutant brains, implying that vnd is required also later in embryogenesis - either by acting directly on lab expression in tritocerebral cells or through a lab-independent requirement.
vnd and lab in tritocerebral neuromere formation
To investigate this, we first determined whether vnd and
lab show overlapping expression during later stages of tritocerebral
neuromere formation. Immunocytochemical analysis indicates that a partial
overlap of vnd and lab expression persists in the
differentiating tritocerebrum throughout embryogenesis and is prominent in the
ventral region (according to neuraxis) of this neuromere
(Fig. 4A,B). Next, we analysed
lab expression in late vnd loss-of-function mutant brains.
Owing to extensive cell loss in the vnd mutant tritocerebrum, this
analysis was limited to the remaining strand of cells that interconnects the
protocerebrum and the remaining part of the deutocerebrum with the
suboesophageal ganglion. Despite the extensive cell loss seen in vnd
mutant brains, remaining cells of the interconnecting strand do show
lab expression (Fig.
4C,D).
We next investigated whether expression of vnd occurs in the
lab mutant tritocerebrum by studying lab loss-of-function
mutants. For this, we took advantage of the fact that in lab-null
mutants, cells in the tritocerebral mutant domain are generated and can be
visualized by a 7.31 lab-lacZ reporter construct
(Tremml and Bienz, 1992;
Hirth et al., 1998).
Surprisingly, despite the lack of expression of neuronal differentiation
markers in cells of the lab mutant domain (see also
Hirth et al., 1998),
vnd is expressed normally and shows partial overlap with
tritocerebral lab mutant cells, as visualized by the
lab-specific reporter construct
(Fig. 4F). This indicates that
expression of vnd is not affected by the absence of lab
during late stages of embryonic brain development.
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|
;
Nassif et al., 1998).] In
early stage 12 vnd mutants, a significant increase in the number of
TUNEL-positive apoptotic cells is seen associated with the tritocerebral
lab-expressing domain when compared with wild type and this
correlates with a decrease in the number of lab-expressing cells in
this region (Fig. 5H).
Accordingly, by late stage 12, a significant reduction of the number of
anti-LAB positive cells has occurred in vnd mutants when compared
with wild type (Fig. 5I,J).
Thus, on average, 20 TUNEL-positive apoptotic cells are identifiable within
the vnd mutant tritocerebral lab-expression domain when
compared with 10 apoptotic cells in wild type
(Fig. 5K). These findings
suggest that marked reduction of tritocerebral lab-expressing cells
occurs as early as embryonic stage 12 and that this reduction is caused by
increased apoptosis of neural tissue.
In order to further substantiate these observations, we next determined
whether blocking apoptosis can prevent cell loss in the vnd mutant
brain. For this, we used the UAS/Gal4 transcriptional activation system
(Brand and Perrimon, 1993) in
order to perform misexpression experiments in vnd mutant embryos. For
the ubiquitous block of apoptosis from neurectodermal stage to early neural
lineage formation, we used the sca::Gal4 driver line
(Sprecher et al., 2004) in
order to activate UAS::p35 transcription, an inhibitor of cell-death
effector caspases (Mergliano and Minden,
2003), in a vnd-null mutant background. When compared
with wild type (Fig. 6A) and
vnd mutant brain (Fig.
6B), ubiquitous block of apoptosis leads to a remarkable
restoration of HRP-immunoreactive tissue in the vnd mutant
tritocerebrum (Fig. 6C).
Notably, and in contrast to the vnd mutant situation, descending
longitudinal connectives that transverse the tritocerebrum are detectable
(Fig. 6C, arrow). Moreover, a
pronounced lab-expression domain is observed in the cell death
prevented vnd mutant tritocerebrum (compare
Fig. 6D with
Fig. 4B). These data suggest
that lab and vnd act in a genetically independent manner in
tritocerebral neuromere development: the expression of lab appears to
be largely unaffected by the absence of vnd as long as apoptosis is
prevented.
To characterize in more detail the extent of neural tissue restoration of
the vnd mutant brain phenotype resulting from block of apoptosis, we
carried out immunocytochemistry using antibodies against Fasciclin II (FAS2)
(Lin and Goodman, 1994) and
ELAV. In the wild-type embryonic brain, anti-FAS2 immunostaining labels a
number of early differentiating neurons, as well as axon tracts
(Fig. 7A), whereas anti-ELAV
labels differentiating postmitotic neurons
(Fig. 7B). In the vnd
mutant tritocerebrum, both FAS2 and ELAV-positive cells, as well as
FAS2-immunoreactive longitudinal axon tracts, are severely perturbed or
lacking (Fig. 7C,D; arrow).
When compared with wild-type and vnd mutant brain, ubiquitous block
of apoptosis significantly reduces the gap-like defects observed in
vnd loss-of-function mutants. Thus, FAS2-immunoreactive longitudinal
connectives are detectable and form a continuous band along the
anteroposterior neuraxis. However, neural fibres contributing to longitudinal
axon tracts display fasciculation defects and appear only loosely bundled when
compared with wild type (Fig.
7E, arrow). Moreover, a large number of ELAV-positive cells are
detectable in the cell death prevented vnd mutant tritocerebrum
(Fig. 7F, arrow). These data
provide further evidence that the gap-like brain defect observed in
vnd mutants is largely due to increased apoptotic activity and the
subsequent loss of neural cells in the tritocerebral domain.
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DISCUSSION |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). In the
case of vnd, detailed studies have shown that vnd is
required for specification of the ventral neuroectodermal column and specific
neuroblasts, and that absence or misspecification of ventral neuroblasts
correlates with the loss or mis-specification of neural progeny. For example,
the aCC/pCC and dMP2/vMP2 neurons are lost, and the RP2 neuron is frequently
absent in vnd mutant embryos, resulting in axonal pathfinding defects
and defective commissure formation in the developing VNC
(Jimenez et al., 1995;
Chu et al., 1998;
McDonald et al., 1998;
Mellerick and Modica,
2002).
In this study, we show that the vnd gene is also required during
embryonic brain development. Starting at early stages of neuroectoderm and
brain neuroblast formation, vnd expression is seen in the developing
neuromeres of the embryonic brain (Urbach
and Technau, 2003b; Urbach et
al., 2006) and in the subsequent course of embryogenesis,
vnd expression is seen in specific clusters of differentiating neural
cells in each brain neuromere. For an investigation of the functional role of
vnd in brain development, we focused on the tritocerebral neuromeres,
where our mutant analysis suggests that vnd acts at least during two
important steps in its development: precursor cell development and neural
progeny maintenance. Thus, early in neurogenesis, a ventral subset of
tritocerebral neuroblasts are lacking in vnd mutants, suggesting that
vnd is required for the formation of neuroblasts in the developing
tritocerebrum. Later in embryogenesis, vnd mutants display a severe
loss of neural tissue, together with axonal patterning defects in the
tritocerebrum. This gap-like phenotype is associated with increased apoptotic
activity, and blocking apoptosis in vnd-null mutant embryos results
in partial restoration of tritocerebral cells and axon tracts.
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; Chu et al.,
1998; McDonald et al.,
1998; Mellerick and Modica,
2002), vnd activity is required for proper formation and
identity specification of tritocerebral neural lineages. Accordingly, our
observation of increased apoptotic activity in the vnd mutant
tritocerebrum might be a consequence of identity changes imposed on
vnd deficient neural lineages. Indeed, vnd loss-of-function
embryos are characterized by the ectopic expression of dorsal marker genes
such as Ems and Msh in residual ventral neuroblasts of the vnd mutant
tritocerebrum (Urbach et al.,
2006). It is therefore conceivable that ectopic activity of genes
such as Ems and Msh, together with the absence of vnd activity
ultimately impose identity changes on residual tritocerebral vnd
mutant lineages that are incompatible with their survival and eventually lead
to cell death. In this sense, the gap-like brain phenotype observed in late
vnd mutant embryos can be regarded as a consequence both of defects
in neuroblast formation and of improper identity specification of
tritocerebral neural lineages during embryonic brain development.
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; Urbach and
Technau, 2004) and subsequently in neural cells of the posterior
tritocerebrum (this study), expression of vnd appears unaffected in
lab mutant cells. Conversely, vnd does not act on
lab expression as the complete absence of lab expression in
vnd mutants (with the exception of a rare thin strand of neuronal
cells) reflects a secondary defect because of the absence of cells that
normally express lab. This independent genetic activity of
vnd and lab is further supported by the fact that blocking
apoptosis restores tritocerebral lab expression in vnd-null
mutant embryos.
Thus, although the lab and vnd mutant brain phenotypes
result in comparable axonal patterning defects (loss of the tritocerebral
commissure and perturbation of the longitudinal connectives that normally run
through this neuromere), their mode of action within the developing
tritocerebrum is discriminable. Our results suggest that vnd is
required for the specification of neural lineages within the developing
tritocerebral neuromere, whereas the Hox gene lab appears to be
independently required for the specification of neuronal identity within the
same territory during later stages (Hirth
et al., 1998; Page,
2000; Hirth et al.,
2001; Sprecher et al.,
2004). This indicates that the activity of the columnar gene
vnd is integrated into pattern formation along the anteroposterior
neuraxis by ensuring proper formation and development of tritocerebral neural
lineages that subsequently become further specified by the activity of the Hox
gene lab.
vnd/Nkx2 genes in brain development and evolution
The Drosophila columnar gene vnd belongs to the highly
conserved Nkx2 class of transcription factors that have been found in
various animals, including mammals (Harvey,
1996; Cornell and von Ohlen,
2000). Notably, the vnd/Nkx2 family of genes is
exceptionally well conserved, both in terms of expression and function. Thus,
the vertebrate homologues of vnd are expressed in the neural plate,
or tube, in topologically similar positions as is vnd in the
Drosophila ventral neuroectoderm and in the absence of
vnd/Nkx2 genes, ventral-most cells in the spinal cord and the
Drosophila VNC are missing or transformed
(Cornell and von Ohlen, 2000;
Rallu et al., 2002). Moreover,
this evolutionary conservation in expression and function of vnd/Nkx2
genes appears to apply to some extent to brain development. A comparison of
the anteroposterior order of vnd/Nkx2 gene expression in the early
embryonic brains of Drosophila and mouse reveals remarkable
similarities (Urbach and Technau,
2003b; Urbach and Technau,
2004).
In terms of function, genetic knockouts in mice have shown that
Nkx2 genes appear to play a crucial role in patterning and neuronal
specification during embryonic development of the telencephalon and hindbrain.
Nkx2.1 mutant mice display numerous brain patterning defects: the
entire pituitary is missing (Kimura et
al., 1996); the number of cortical interneurons is halved; there
is a complete absence of TrkA-expressing cells in the developing
telencephalon; and the ventral-most aspect of the telencephalon (the medial
ganglionic eminence) becomes trans-fated to that of the adjacent more
dorsolateral ganglionic eminence (Sussel
et al., 1999). Thus, comparable with the role of vnd
during Drosophila brain development (this study)
(Urbach et al., 2006),
Nkx2.1 is involved in pattern formation and in cell fate
determination during embryonic brain development in mice
(Rallu et al., 2002).
In addition, recent studies have shown that Nkx2.2 is involved in
neural lineage specification in the developing hindbrain. In particular, the
sequential generation of visceral motoneurons and serotonergic neurons from a
common pool of neural progenitors located in the ventral hindbrain crucially
depend on the integrated activities of Nkx2.2- and Hox1/2-class homeodomain
proteins (Pattyn et al.,
2003a; Pattyn et al.,
2003b). An important function of these proteins is to coordinate
the spatial and temporal activation of the homeodomain protein Phox2b, which
in turn acts as a binary switch in the selection of motor neuron or
serotonergic neuronal fate (Pattyn et al.,
2003a; Samad et al.,
2004). De-repressive activity of Nkx2.2 at or in vicinity of
Pbx/Hox-binding sites proximal to the Phox2b enhancer enhances transcriptional
activation of Phox2b by Hox1 and Pbx factors
(Samad et al., 2004). These
data suggest that comparable with the integrated activity of vnd and
lab in Drosophila brain neuromere specification, integrated
activity of the Nkx2.2 and Hox1/2 proteins is involved in the specification of
segmental neural lineages. Thus, integration of anteroposterior and
dorsoventral patterning systems by homeodomain transcription factors of the
Hox and vnd/Nkx2 genes might represent an ancestral feature
of insect and mammalian brain development
(Hirth et al., 2003).
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ACKNOWLEDGMENTS |
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Footnotes |
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Present address: MRC Centre for Neurodegeneration Research, King's College
London, De Crespigny Park, London SE5 8AF, UK
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REFERENCES |
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TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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