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First published online 9 August 2006
doi: 10.1242/dev.02501
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Review |
Division of Developmental Neurobiology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK.
e-mail: sang{at}nimr.mrc.ac.uk
SUMMARY
Although loss of midbrain dopaminergic neurons is associated with one of the most common human neurological disorders, Parkinson's disease, little is known about the specification of this neuronal subtype. Hence, the recent identification of major transcriptional determinants regulating the development of these neurons has brought much excitement and encouragement to this field. These new findings will help to elucidate the genetic program that promotes the generation of midbrain dopaminergic neurons. Importantly, these discoveries will also significantly advance efforts to differentiate stem cells into midbrain dopaminergic neurons that can be used for therapeutic use in treating Parkinson's disease.
Introduction
The midbrain dopaminergic (mDA) neurons (neurons that use dopamine as a
neurotransmitter) constitute about 75% of dopaminergic neurons in the adult
brain (Wallen and Perlmann,
2003
). The midbrain dopamine system has been intensively studied
because of its implication in diverse psychiatric and neurological disorders
(Hornykiewicz, 1978;
Carlsson, 2001). In addition,
midbrain dopaminergic neurons play key roles in the generation of pleasure,
and in the development of addictive behaviours such as drug abuse
(Chao and Nestler, 2004).
Our current understanding of the location of dopaminergic (DA) neurons in
the central nervous system (CNS) comes from immunohistochemical studies that
determine the expression pattern of the enzyme tyrosine hydroxylase (Th),
which is the rate-limiting enzyme of dopamine synthesis. Th+ mDA
neurons (historically called A8-A10) originate from the ventral part of a
domain of the brain that extends rostrally to the ventral
thalamus/hypothalamus border and caudally to the midbrain/hindbrain border
(Fig. 1)
(Bjorklund and Lindvall, 1984;
Dahlstrom and Fuxe, 1964;
Marin et al., 2005). These
neurons can be anatomically divided into three main subgroups. The lateral A9
group corresponds to neurons of the substantia nigra pars compacta (SNpc),
which mainly project into the dorsal striatum and form the nigro-striatal
pathway. This group of neurons is mainly involved in the control of movement,
as revealed by the resting tremor, rigidity, bradykinesia (abnormal slowing of
voluntary movement) and gait disturbance that is seen in individuals with
Parkinson's disease. These symptoms are due to the specific degeneration of
the SNpc neurons (Lang and Lozano,
1998). The medial A10 and lateral A8 neuron groups define the
ventral tegmental area (VTA) and retrorubal field of the midbrain,
respectively. A10 and A8 groups project to the ventromedial striatum, as part
of the mesocortical limbic system that is involved in emotional behaviour and
mechanisms of reward (Dahlstrom and Fuxe,
1964; Tzschentke and Schmidt,
2000). Dysregulation of DA transmission in the limbic system has
been linked to the development of drug addition
(Kelley and Berridge, 2002;
Wightman and Robinson, 2002)
and depression (Dailly et al.,
2004), and is thought to contribute to the psychotic symptoms of
schizoprenia (Sesack and Carr,
2002).
As for all neurons, the generation of mDA neurons from a neural progenitor
cell can be divided into distinct steps. At least three steps have been
recognized based on the expression of molecular markers: (1) regional
specification, (2) early and (3) late differentiation. Until recently, mDA
progenitors were poorly defined and not easily distinguishable from other CNS
progenitors. The recent discovery that certain transcription factors,
specifically Otx2, Lmx1a and Lmx1b (Lmx1a/b), Engrailed 1 and Engrailed 2
(En1/2), Msx1 and Msx2 (Msx1/2), Neurogenin 2 (Ngn2) and Mash1, are all
expressed in mDA progenitors has allowed these progenitors to be identified
for the first time by a combinatorial transcription factor code
(Simon et al., 2001;
Puelles et al., 2003;
Puelles et al., 2004;
Andersson et al., 2006a;
Andersson et al., 2006b;
Kele et al., 2006). These new
transcription factors provide additional markers with which to address the
issue of heterogeneity among mDA progenitors, and are candidate regulators for
promoting the specification and differentiation of progenitors into mDA
neurons. Important insights into the role of these transcription factors in
regulating the development of mDA neurons, which have arisen from loss- and
gain-of-function experiments in chick and mice, will be discussed in this
review. These new findings will underpin exciting research both in the stem
cell and developmental biology fields during the next few years.
Midbrain dopaminergic lineage
Molecular markers allow the distinction of three sequential cell
populations in the mDA lineage: progenitors, immature neurons and mature
neurons (see Fig. 2A,B). These
populations are generated during successive developmental steps, which are
called regional specification, early and late differentiation (see below).
Until recently, the only molecule known to mark specifically DA progenitors in
the midbrain was retinaldehyde dehydrogenase Aldh1a1 (Raldh1). Raldh1
catalyzes the oxidation of retinaldehyde into retinoic acid; however, the role
of Raldh1 in mDA progentiors remains to be discovered. Remarkably, during the
past two years several groups have shown that other transcription factors,
such as Otx2, Lmx1a/b, En1/2, Msx1/2, Ngn2 and Mash1, are also expressed in
these progenitors (Simon et al.,
2001; Puelles et al.,
2003; Puelles et al.,
2004; Andersson et al.,
2006a; Kele et al.,
2006; Andersson et al.,
2006b).
Despite this historical lack of information on molecular determinants of
mDA progenitors, several key transcription factors have been identified and
described that mark and regulate the development of postmitotic mDA neurons.
For example, the orphan nuclear receptor Nurr1 was found to be required not
for the generation, but for the maintenance of mDA neurons
(Zetterstrom et al., 1997;
Saucedo-Cadenas et al., 1998). It is also required for the expression of Th,
vesicular monoamine transporter 2 (Vmat2), dopamine transporter (DAT) and Ret
in mDA neurons (Wallen et al.,
1999; Wallen et al.,
2001; Smits et al.,
2003). By contrast, En1/2 homeodomain proteins are required, in
part, for the generation of mDA neurons and their survival
(Simon et al., 2001). En1/2
proteins have also been shown in vitro to be required cell autonomously in mDA
neurons for neuronal survival through their regulation of apoptosis
(Alberi et al., 2004). Pitx3 is
a paired homeodomain transcription factor that is required for the expression
of Th in a subset of mDA neurons (Maxwell
et al., 2005), and for the survival of primarily SNpc, but also
VTA, neurons (Hwang et al.,
2003; Nunes et al.,
2003; Smidt et al.,
2004; van den Munckhof et al.,
2003).
|
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).
Ventrolateral mDA neurons express Pitx3 prior to expressing Th, whereas the
dorsomedial mDA neurons express Th ahead of Pitx3 at E12.5. By E13.5, all
Th+ cells express Pitx3. Although this study indicates that mDA
neurons are a heterogenous population transiently at E12.5, it remains to be
determined whether their heterogenous connectivity patterns and resistance to
toxicity are already specified at the progenitor stage, or, alternatively,
only later in postmitotic neurons. Regional specification of mDA progenitor
Studies using rat and chick neural plate explants have demonstrated that
the floor plate-derived signal sonic hedgehog (Shh) and fibroblast growth
factor 8 (Fgf8) from the mid-hindbrain boundary are required for the induction
of DA neurons before E9.5 (Ye et al.,
1998). In addition, Wnt1
(Prakash et al., 2006) and
transforming growth factor-ß (Tgfß)
(Farkas et al., 2003) probably
also play a role in the patterning of midbrain progenitors. These signals
specify anteroposterior (AP) and dorsoventral (DV) identity, and result in the
activation of a combination of transcription factors, including Otx2, Lmx1a/b,
En1/2, Msx1/2, Ngn2 and Mash1, in a temporal sequence
(Fig. 2C). The expression of
Otx2, Lmx1b and En1/2 genes is already initiated by E9.0
(Ang et al., 1994;
Smidt et al., 2000).
Subsequently, Lmx1a and Msx1/2 expression turn on around E9.5, and Ngn2 and
Mash1 expression are not initiated until E10.75
(Andersson et al., 2006b). The
molecular mechanisms leading to the sequential activation of these genes is
not understood. Shh can induce Lmx1a and Msx1/2 expression in neural tube
explants of chick embryos (Andersson et
al., 2006b). However, these transcription factors are activated
endogenously in mouse embryos one day later than the initiation of Shh
expression (Echelard et al.,
1993). These results suggest that Shh signalling induces another
signal or factor that is required for the expression of Lmx1a and Msx1/2.
Based on their ventral midline position, mDA progenitors are presumed to be
derived from Shh+ glial-like floor plate cells. Thus, another step
in the specification of mDA progenitors requires the acquistion of neuronal
potential by floor plate progenitors, a step that is likely to involve the
activity of the proneural genes Ngn2 and Mash1
(Andersson et al., 2006a,
Andersson et al., 2006b;
Kele et al., 2006).
Differentiating mDA neurons
Early differentiation: generation of immature mDA neurons
Birthdating studies using tritiated thymidine incorporation demonstrate
that mDA progenitors exit the cell cycle and generate postmitotic immature mDA
neurons between E9.5 and E13.5 in mice
(Bayer et al., 1995). Immature
mDA neurons initiate Nurr1 expression
(Zetterstrom et al., 1997) and
En1/2 expression (Simon et al.,
2001; Alberi et al.,
2004) during this differentiation step
(Fig. 2B). In addition, mDA
progenitors, like many other CNS progenitors, downregulate Sox2 expression
while initiating the expression of general neuronal markers, such as
ßIII-tubulin, in postmitotic immature and mature mDA neurons
(Kele et al., 2006).
Late differentiation: generation of mature mDA neurons
From E11.0 onwards, immature mDA neurons continue to migrate radially on
radial glial fibres into the mantle zone, further differentiating into mDA
neurons (Kawano et al., 1995).
These neurons express Pitx3, Th and aromatic amino acid decarboxylase (Aadc),
in addition to the earlier markers expressed in immature mDA neurons
(Fig. 2B). Ngn2, however, is
not expressed in mature mDA neurons. Aadc converts DOPA into dopamine.
Aadc mRNA transcripts are thought to be expressed already in immature
mDA neurons (Smidt et al.,
2004).
Transcription factors required for mDA neuron development
The roles of transcription factors, such as Nurr1, En1/2 and Ptx3, acting
during the late differentiation step of the mDA lineage (described briefly
above, see Fig. 2B and
Table 1) have been extensively
reviewed elsewhere (Goridis and Rohrer,
2002; Riddle and Pollock,
2003; Wallen and Perlmann,
2003; Simeone,
2005; Smits et al.,
2006; Prakash and Wurst,
2006). However, the roles of the transcription factors that govern
the specification and early differentiation of mDA progenitors have only
recently started to emerge during the past few years and will be the focus of
this review (Table 1).
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;
Ang et al., 1994). Within the
nervous system, Otx2 expression is restricted to the forebrain and
midbrain between E8.5 and E12.5. In addition to these anterior brain regions,
expression is also detected in the rhombencephalon from E12.5 onwards
(Mallamaci et al., 1996).
Otx2 is required for the formation of the forebrain and midbrain as a
result of its role in the anterior visceral endoderm, where it functions to
restrict posterior fates (Mallamaci et
al., 1996; Perea-Gomez et al.,
2001) (reviewed by Simeone and
Acampora, 2001). Subsequently, Otx2 is also required for
positioning the expression of Wnt1 and Fgf8 at the midbrain boundary
(Brodski et al., 2003), and
for limiting the dorsal extent of Shh expression in the ventral midbrain
(Puelles et al., 2003).
Otx2 also has additional roles in mDA progenitor specification and
differentiation based on findings obtained from the phenotypical analyses of
Otx2 mutant mice. Several different conditional Otx2 mouse
mutants have been generated in an attempt to delete Otx2 specifically
at different developmental stages. For example, Otx2 was deleted in
the midbrain of En1cre;Otx2flox/flox embryos from E9.5
onwards (Puelles et al.,
2004). In these En1cre;Otx2flox/flox mutant
embryos, midbrain expression of Shh expands dorsally, whereas
Fgf8 expression, which is normally restricted to the anterior
hindbrain, shifts anteriorly into the midbrain
(Puelles et al., 2004).
Despite these changes in AP and DV patterning molecules, a small domain of
midbrain tissue develops normally. Within this domain, expression of the
homeodomain protein Nkx2.2 expands ventrally into presumptive DA progenitors
around E9.5, indicating that Otx2 is required for the repression of
Nkx2.2 in these progenitors
(Prakash et al., 2006).
Interestingly, serotonergic neurons are generated ectopically in these
Otx2 conditional mutants at the expense Th+ mDA neurons.
Loss of mDA neurons is directly linked to the abnormal expression of Nkx2.2 in
mDA progenitors, as the presence of ectopic serotonergic neurons and the
reduction in the number of mDA neurons are rescued in
En1cre;Otx2flox/flox;Nkx2.2-/- embryos
(Prakash et al., 2006). The
ectopic expression of Nkx2.2 in En1cre;Otx2flox/flox
embryos is also associated with a loss of expression of Wnt1, which indicates
that Otx2 might regulate Wnt1 expression indirectly via its
repressive effects on Nkx2.2 in mDA progenitors at E12.5. Accordingly,
Wnt1 expression is recovered in
En1cre;Otx2flox/flox;Nkx2.2-/- embryos that
lack both Otx2 and Nkx2.2. It is noteworthy that
Wnt1 may also be required upstream of Otx2, as ectopic
expression of Wnt1 under the control of the En1 promoter
leads to an upregulation of Otx2 in the floor plate of the rostral
hindbrain of En1Wnt1/+ embryos
(Prakash et al., 2006).
A different role for Otx2 in mDA progenitors was identified from
studies of Nestin-Cre;Otx2flox/flox embryos
(Vernay et al., 2005). In
these conditional mutants, loss of Otx2 protein from E10.5 onwards results in
loss of expression of the proneural genes Ngn2 and Mash1 in
ventral mDA progenitors. Subsequently, mDA neurons are missing at the ventral
midline of the midbrain. In addition, Nkx2.2 expression expands ventrally into
the ventricular zone adjacent to the red nucleus neurons that are reduced in
size in Nestin-Cre;Otx2flox/flox embryos
(Vernay et al., 2005). Red
nucleus neurons normally lie dorsal to mDA neurons, and are implicated in the
control of locomotion (reviewed by
Kennedy, 1990). This loss of
red nucleus neurons is fully rescued in
Nestin-Cre;Otx2flox/flox;Nkx2.2-/- embryos, but
the loss of mDA neurons is not. Taken together, these results indicate that
Otx2, presumably via regulating the expression of Ngn2 and
Mash1 (see below), is also required for the generation of mDA
neurons. A later role for Otx2 in regulating neurogenesis in mDA
progenitors seems contradictory to the observation that mDA neuronal
development is recovered in
En1cre;Otx2flox/flox;Nkx2.2-/- mutant embryos,
despite the loss of Otx2 in mDA progenitors prior to neuronal
differentiation in these triple mutants at E9.5. In order to reconcile these
two findings, I propose that the requirement for Otx2 in regulating
Ngn2 expression in mDA progenitors may be bypassed by changes in Shh
expression in the ventral midbrain of
En1cre;Otx2flox/flox;Nkx2.2-/- mutant embryos
(Prakash et al., 2006). This
is because Shh can influence the expression of Ngn2, possibly via its effect
on Lmx1a (see below). Alternatively, or in addition, the recovered Wnt1
expression in En1cre;Otx2flox/flox;Nkx2.2-/-
mutants is not completely normal and this may also affect Ngn2 expression in
these mutants (Prakash et al.,
2006).
Lmx1a and Lmx1b
Lmx1a and Lmx1b, members of the family of LIM homeodomain transcription
factors, play important roles in the developing brain. Lmx1b is expressed in
the midbrain from E8.0 onwards (Smidt et
al., 2000), but this expression becomes restricted by E9.5 to the
roof plate, the mid-hindbrain boundary and the ventral midbrain, including the
floor plate. By contrast, Lmx1a expression begins at E9.5 in the ventral
midbrain and then progressively expands dorsally
(Andersson et al., 2006b).
Double antibody labelling studies have revealed that at E9.5, Lmx1b expression
encompasses more cells in the ventral midbrain than does Lmx1a, but that by
E10.5 the expression domains of the two genes largely coincide. Because the
expression of Lmx1a in progenitors directly overlies a region where
Th+ neurons develop at E11.5, Lmx1a expression has been proposed to
mark the dorsal boundary of mDA progenitors
(Andersson et al., 2006b),
whereas the initial expression of Lmx1b is believed to include progenitors for
other cell types as well. This interpretation seems likely but awaits
confirmation by lineage- and fate-mapping studies.
Loss-of-function studies have shown that Lmx1b is required for the
maintenance of Th+ mDA neurons. Th+ neurons are found in
Lmx1b-/- mutants up to E16.0, although they fail to
express the paired homeodomain transcription factor Pitx3
(Smidt et al., 2000). This
differentiation defect results in the eventual loss of DA neurons in
Lmx1b-/- embryos. Although this phenotype has been
interpreted as demonstrating a role for Lmx1b in the maintenance of
DA neurons, Lmx1b is initially broadly expressed in the midbrain; and
earlier defects in patterning of the mid-hindbrain region may contribute to
the loss of Th+ neurons at later stages. In addition,
Lmx1b.1 and Lmx1b.2 genes are required for maintenance of
the mid-hindbrain organizer and for the expression of Wnt1, Wnt3a, Wnt10b,
Pax8 and Fgf8 in zebrafish embryos
(O'Hara et al., 2005).
Therefore, whether the mDA phenotype of Lmx1b mutant mouse embryos is
due to an intrinsic role in the mDA lineage or to earlier patterning functions
in the midbrain remains to be clarified. A detailed analysis of Lmx1b
functions in mDA progenitors and neurons will be important to elucidate its
roles at different developmental stages.
Positional cloning has identified Lmx1a as being the gene affected
in the spontaneous mouse mutant dreher
(Millonig et al., 2000). The
point mutation in Lmx1a in dreher mice results in the loss
of the caudal roof plate. In addition, overexpression studies have shown that
Lmx1a can induce the expression of roof plate markers and of
components of roof plate signalling
(Chizhikov and Millen, 2004b),
thus demonstrating that Lmx1a is both required and sufficient to
induce roof plate formation.
A recent breakthrough study by the groups of Ericson and Perlmann has
identified Lmx1a as a crucial determinant of mDA neuron fate
development (Andersson et al.,
2006b). In this study, these researchers showed that the
overexpression of Lmx1a in the ventral midbrain of chick embryos
promoted the generation of DA neurons over that of other neuronal subtypes.
The induction of DA neurons in chick embryos was preceded by a
re-specification of progenitor cells, as indicated by the activation of Msx1
and the repression of Nkx6.1 expression. Importantly, the activity of
Lmx1a is context dependent, as Lmx1a can only induce ectopic
DA neurons in ventral, and not in dorsal, midbrain cells. This context
dependence suggests a role for additional factors that are specifically
expressed in ventral midbrain cells. Alternatively, Lmx1a might not
be able to inhibit dorsal differentiation programs and thus cannot convert
dorsal midbrain cells to a more ventral fate. It is noteworthy that in this
context, the overexpression of Foxa2 and Gli in transgenic
mice results in the generation of ectopic Th+ neurons in the dorsal
midbrain (Sasaki and Hogan,
1994; Hynes et al.,
1997). In these transgenic embryos, ectopic DA neurons were found
adjacent to dorsal sites of Shh expression. These results indicate
that some dorsal midbrain progenitors are competent to acquire a mDA fate in
mouse embryos at developmental stages equivalent to those of the chick
experiments discussed above (Sasaki and
Hogan, 1994; Hynes et al.,
1997). Taken together, these results favour the hypothesis that
Lmx1a alone is not sufficient to induce mDA neurons, and that it
functions cooperatively with ventral factors induced by the Shh pathway
(Fig. 3).
|
).
Impressively, Lmx1a promotes Th expression in up to 80% of
ßIII-tubulin+ neurons that were derived from ES cells.
Importantly, these neurons also expressed other mDA neuron markers, such as
Pitx3, Lmx1b, En1/2 and DAT. By contrast, mock-transfected ES cells produced
Th+ neurons that were mostly GABA+ and
Pitx3-.
A complementary loss-of-function study by RNA interference also supports a
role for Lmx1a in the generation of DA neurons in chick embryos
(Andersson et al., 2006b). In
these experiments, Lmx1a knockdown by siRNA electroporation resulted
in a loss of DA neurons, which was not compensated for by unperturbed
expression of Lmx1b. This result was surprising, as studies in mice
have demonstrated a role for Lmx1b in the development of mDA neurons
(Smidt et al., 2000);
Lmx1b can also partially rescue roof plate formation in
dreher mice (Chizhikov and
Millen, 2004a). One way to reconcile these results is to propose
that the two genes have overlapping roles in mDA development, with
Lmx1a being perhaps more efficient at the specification step, while
Lmx1b is required for later differentiation events in the DA lineage.
This hypothesis is consistent with the observation that Lmx1b is much
less efficient than Lmx1a at promoting mDA neuron differentiation in
ES cells (Andersson et al.,
2006b). It is also possible that the requirements for
Lmx1a and Lmx1b genes are different for the two species.
Whether Lmx1a and Lmx1b have unique and/or redundant roles
in the development of mDA neurons in mice awaits further studies of Lmx1a,
Lmx1a;Lmx1b double mutant embryos and of Lmx1b conditional
mutants.
Msx1 and Msx2
The mouse Msx genes, Msx1, Msx2 and Msx3, encode
homeodomain transcription factors that share 98% homology in the homeodomain
and function as transcriptional repressors (reviewed by
Ramos and Robert, 2005).
Msx1 and Msx2 are expressed in DA progenitors in the ventral
midbrain (Andersson et al.,
2006b), in addition to in the roof plate and adjacent cells in the
dorsal neural tube and neural crest, as well as in many other sites where
epithelial-mesenchymal inductive interactions occur, such as in the limbs and
tooth buds, heart, branchial arches and in craniofacial processes.
Msx3, by contrast, is expressed exclusively in the dorsal aspect of
the neural tube in the mouse, caudal to the mid-hindbrain boundary
(Shimeld et al., 1996;
Wang et al., 1996).
Msx1-/- embryos exhibit a 40% reduction in the normal
number of mDA neurons, probably as a result of the downregulation of
Ngn2 expression (Andersson et al.,
2006b). This partial reduction of mDA neurons in Msx1
mutants may be due to compensation by Msx2, a possibility that
remains to be addressed by the analysis of Msx1;Msx2 double mutants.
In addition, Msx1 is able, and is required, to repress
Nkx6.1 expression in ventral midbrain progenitors
(Andersson et al., 2006b)
(Fig. 3). Premature expression
of Msx1 in the midbrain in transgenic mice also leads to the
precocious expression of Ngn2 and Nurr1, and to the downregulation of Shh in
the floor plate, indicating that Msx1 sets the timing of mDA neuron
generation possibly by inducing Ngn2 expression in ventral midbrain
progenitors (Andersson et al.,
2006b). Given that Msx genes normally function as repressors,
Msx1 may regulate the activity of a repressor of Ngn2 in mDA
progenitors.
Ngn2 and Mash1
Proneural basic helix-loop-helix genes are crucial regulators of
neurogenesis and of subtype specification in many areas of the nervous system.
In the ventral midbrain, the proneural genes Mash1, Ngn2 and
Ngn1 show an intricate pattern of expression. Ngn2 and
Mash1 are expressed in mDA progenitors, whereas Ngn1, Ngn2
and Mash1 are co-localized in the ventricular zone more dorsally
(Kele et al., 2006).
Ngn2 is also expressed in newly born postmitotic immature mDA neurons
immediately adjacent to the ventricular zone at the ventral midline.
Ngn2 is required for the generation of Nurr1+ immature mDA
neurons, and probably also for their subsequent differentiation into
Th+ mature mDA neurons
(Andersson et al., 2006a;
Kele et al., 2006). Although
Mash1 by itself is not required for mDA neuron development, the loss
of both Mash1 and Ngn2 in Mash1;Ngn2 double mutant
mouse embryos leads to a greater loss of mDA neurons than occurs in
Ngn2 single mutants (Fig.
4), suggesting that Mash1 can partially compensate for
the loss of Ngn2 function in mDA progenitors. Accordingly, this
results in a further rescue of Th+ neurons
(Fig. 4) in
Ngn2KIMash1/Mash1 embryos that express Mash1
under the control of the Ngn2 promoter
(Kele et al., 2006).
Ngn2 has a role in regulating generic neuronal, as well as
subtype-specific, differentiation programs in other parts of the CNS (reviewed
by Bertrand et al., 2002). The
reduced number of Th+ neurons in Ngn2 mutants is due to a
failure in neuronal production, as suggested by the loss of
ßIII-tubulin+ neurons, which lie directly beneath mDA
progenitors (Kele et al.,
2006). Moreover, the loss of two markers of proneural activity,
Dll1, a Notch ligand, and Hes5, an effector of Notch
signalling, in the ventral midbrain is consistent with a role for
Ngn2 in regulating the generic aspect of neurogenesis
(Kele et al., 2006). In other
parts of the CNS, the role of Ngn2 in subtype specification has been
demonstrated by the inability of other classes of proneural genes to
compensate for Ngn2 activity (reviewed by
Bertrand et al., 2002).
Mash1 is able to compensate partially for Ngn2 function, as
60% of the normal number of mDA neurons are generated in
Ngn2KIMash1/KIMash1 embryos. This partial compensation
suggests some unique role for Ngn2 in specification of the mDA
neuronal subtype. In addition, the expression of Ngn2, but not Mash1, in
postmitotic DA neurons is consistent with an additional and unique role for
Ngn2 in regulating later differentiation steps in immature mDA
neurons. Complementary gain-of-function studies performed by electroporating
Ngn2 in the dorsal midbrain of mouse embryos have demonstrated a role
for Ngn2 in promoting the migration of newborn neurons from the
ventricular zone to the mantle zone and in inducing expression of the general
neuronal marker ßIII-tubulin. However, these studies show that
Ngn2 alone is insufficient to promote the ectopic expression of DA
neuron markers and the generation of ectopic DA neurons
(Kele et al., 2006). This
finding is not surprising because Ngn2 is known to function in other
parts of the CNS in a context-dependent manner, working cooperatively with
other transcription factors (Bertrand et
al., 2002). Further insights into the function of Ngn2 in
mDA neuron development will come from the identification of its
transcriptional targets and cofactors in both mDA progenitors and immature mDA
neurons.
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DA neuron development via distinct transcription factor networks
Besides the neurons in the A8 (retrorubral area), A9 (SNpc) and A10 (VTA)
areas of the midbrain discussed so far, some catecholaminergic neurons in the
forebrain are also dopaminergic. They are found in areas A11 to A15
(diencephalic and hypothalamic groups), A16 (periglomerular cells in the
olfactory bulb), and A17 (interplexiform cells in the retina; see
Fig. 1). Very little is known
about the specification of the DA fate in these groups, except for the A13 and
A16 groups. The A13 DA neurons reside in the alar plate of neuromere segment
p3. Recent studies demonstrate that A13 progenitors express homeodomain
transcription factors Dlx1/2 and Pax6, and differentiate into
Pax6+/Islet+/Th+ neurons
(Andrews et al., 2003;
Mastick and Andrews, 2001). In
Dlx1/Dlx2 double mutant mice, neurons generated by A13 progenitors
fail to express Pax6, Islet1 and Th. These results thus identify a role for
Dlx1/2 in the specification of A13 DA neurons. Pax6,
however, is not required for A13 progenitors to differentiate into
Th+ neurons. By contrast, Pax6 is a molecular determinant of A16
progenitors that is required for the neurogenesis and subtype determination of
olfactory bulb periglomerular A16 DA neurons. These programs also differ from
the one that promotes mDA differentiation, indicating that distinct programs
are involved in specifying DA differentiation at different CNS positions. In
support of this hypothesis, lateral tuberal hypothalamic DA neurons in the
forebrain of chick embryos express Nkx2.1 and Msx1 homeodomain proteins, and
have been shown to depend on the expression of the homeodomain transcription
factor Six3 in progenitors for their development
(Ohyama et al., 2005). By
contrast, Six3 is not expressed in A13 and mDA progenitors.
The distinct differentiation programs for CNS DA neurons highlight the
importance of identifying a DA neuron by its developmental history in addition
to its neurotransmitter phenotype. Crucial parameters affecting the success of
a transplantation therapy for the treatment of Parkinson's disease include
good graft integration and functional reinnervation (reviewed by
Bjorklund and Isacson, 2002;
Lindvall and Bjorklund, 2004;
Snyder and Olanow, 2005).
Recent findings indicate that grafted SNpc and VTA DA neurons differ in their
axon projection patterns in the DA-denervated forebrain of adult mice,
suggesting that mDA neuronal subtypes display distinct responses to axon
guidance cues and target recognition mechanisms regulating reinnervation in
the forebrain (Thompson et al.,
2005). These and earlier studies
(Hudson et al., 1994;
Zuddas et al., 1991) indicate
that the success of transplantation therapies will be strongly influenced by
the type of DA neurons used in these procedures. Therefore, elucidating the
molecular determinants that regulate mDA neuron differentiation from CNS
progenitor cells in vivo should facilitate the generation of specific
populations of DA neurons from stem cells that will be useful for
transplantation therapies.
Conclusions
Although some of the key molecular players required in specifying neural
progenitors towards a mDA differentiation program are now known, their precise
functions remain to be deciphered. Gain-of-function studies suggest that the
major determinants, such as Otx2, Lmx1a/b, Msx1/2, Ngn2 and
Mash1, are likely to act in a combinatorial manner to promote the mDA
fate, as the ectopic expression of some of these genes individually fails to
promote the differentiation of dorsal midbrain progenitors into DA neurons
(Kele et al., 2006;
Andersson et al., 2006b).
Future work will shed light on the specific combinatorial interactions of
transcription factors that govern the differentiation of midbrain progenitors
into mDA neurons, and will also lead to the discovery of downstream components
of these regulatory networks. Besides these transcription factors, members of
the forkhead/winged helix transcription factor family, Foxa1 and Foxa2, are
also expressed in mDA progenitors (Puelles
et al., 2003), and their role in the development of mDA neurons is
currently being investigated in our laboratory. In addition, the molecules
responsible for the generation of SNpc versus VTA DA neurons remain unknown.
SNpc and VTA DA neurons settle in different positions along the mediolateral
axis of the midbrain and have distinct axon projection patterns. A major
challenge for the future is to discover the molecules that are responsible for
these distinct migratory and axon growth behaviours. The ability to generate
DA neurons from neural progenitor/stem cells of the SNpc subtype and to purify
them based on a specific set of molecular markers will lead to significant
progress in stem cell-based therapies for Parkinson's disease.
ACKNOWLEDGMENTS
I am extremely grateful to Stella Uwabor for help with references, to Anna Ferri and Wai Han Yau for help with illustrations, and to members of the lab for critical reading of this manuscript. Work in my laboratory is funded by Parkinson's Disease Society, UK and the Medical Research Council.
REFERENCES
Alberi, L., Sgado, P. and Simon, H. H. (2004).
Engrailed genes are cellautonomously required to prevent apoptosis in
mesencephalic dopaminergic neurons. Development
131,3229
-3236.
Andersson, E., Jensen, J. B., Parmar, M., Guillemot, F. and
Bjorklund, A. (2006a). Development of the mesencephalic
dopaminergic neuron system is compromised in the absence of neurogenin 2.
Development 133,507
-516.
Andersson, E., Tryggvason, U., Deng, Q., Friling, S.,
Alekseenko, Z., Robert, B., Perlmann, T. and Ericson, J.
(2006b). Identification of intrinsic determinants of midbrain
dopamine neurons. Cell
124,393
-405.[CrossRef][Medline]
Andrews, G. L., Yun, K., Rubenstein, J. L. and Mastick, G.
S. (2003). Dlx transcription factors regulate differentiation
of dopaminergic neurons of the ventral thalamus. Mol. Cell
Neurosci. 23,107
-120.[CrossRef][Medline]
Ang, S. L., Conlon, R. A., Jin, O. and Rossant, J.
(1994). Positive and negative signals from mesoderm regulate the
expression of mouse Otx2 in ectoderm explants.
Development 120,2979
-2989.[Abstract]
Bayer, S. A., Wills, K. V., Triarhou, L. C. and Ghetti, B.
(1995). Time of neuron origin and gradients of neurogenesis in
midbrain dopaminergic neurons in the mouse. Exp. Brain
Res. 105,191
-199.[Medline]
Bertrand, N., Castro, D. S. and Guillemot, F.
(2002). Proneural genes and the specification of neural cell
types. Nat. Rev. Neurosci.
3, 517-530.[CrossRef][Medline]
Bjorklund, A. and Lindvall, O. (1984).
Dopamine-containing systems in the CNS. In Handbook of Chemical
Neuroanatomy. Amsterdam: Elsevier.
Bjorklund, L. M. and Isacson, O. (2002).
Regulation of dopamine cell type and transmitter function in fetal and stem
cell transplantation for Parkinson's disease. Prog. Brain
Res. 138,411
-420.[Medline]
Brodski, C., Weisenhorn, D. M., Signore, M., Sillaber, I.,
Oesterheld, M., Broccoli, V., Acampora, D., Simeone, A. and Wurst, W.
(2003). Location and size of dopaminergic and serotonergic cell
populations are controlled by the position of the midbrain-hindbrain
organizer. J. Neurosci.
23,4199
-4207.
Carlsson, A., Waters, N., Holm-Waters, S., Tedroff, J., Nilsson,
M. and Carlsson, M. l. (2001). Interactions between
monoamines, glutamate and GABA in schizopherenia: new evidence.
Annu. Rev. Pharmacol. Toxicol.
41,237
-260.[CrossRef][Medline]
Chao, J. and Nestler, E. J. (2004). Molecular
neurobiology of drug addition. Annu. Rev. Med.
55,113
-132.[CrossRef][Medline]
Chizhikov, V. V. and Millen, K. J. (2004a).
Control of roof plate development and signaling by Lmx1b in the caudal
vertebrate CNS. J. Neurosci.
24,5694
-5703.
Chizhikov, V. V. and Millen, K. J. (2004b).
Control of roof plate formation by Lmx1a in the developing spinal cord.
Development 131,2693
-2705.
Dahlstrom, A. and Fuxe, K. (1964). Evidence for
the existence of monoamine-containing neurons in the central nervous system.
I. Demonstration of monoamines in the cell bodies of brain stem neurons.
Acta Physiol. Scand. 62,231
-255.
Dailly, E., Chenu, F., Renard, C. E. and Bourin, M.
(2004). Dopamine, depression and antidepressants.
Fundam. Clin. Pharmacol.
18,601
-607.[CrossRef][Medline]
Echelard, Y., Epstein, D. J., St-Jaques, B., Shen, L., Mohler,
J., McMahon, J. A. and McMahon, A. P. (1993). Sonic hedgehog,
a member of a family of putative signalling molecules, is implicated in the
regulation of CNS polarity. Cell
75,1417
-1430.[CrossRef][Medline]
Farkas, L. M., Dunker, N., Roussa, E., Unsicker, K. and
Krieglstein, K. (2003). Transforming growth factor-beta(s)
are essential for the development of midbrain dopaminergic neurons in vitro
and in vivo. J. Neurosci.
23,5178
-5186.
Goridis, C. and Rohrer, H. (2002).
Specification of catecholaminergic and serotonergic neurons. Nat.
Rev. Neurosci. 3,531
-541.[CrossRef][Medline]
Hornykiewicz, O. (1978). Psychopharmacological
implications of dopamine and dopamine antagonists: a critical evaluation of
current evidence. Neuroscience
3, 773-783.[CrossRef][Medline]
Hudson, J. L., Bickford, P., Johansson, M., Hoffer, B. J. and
Stromberg, I. (1994). Target and neurotransmitter specificity
of fetal central nervous system transplants: importance for functional
reinnervation. J. Neurosci.
14,283
-290.[Abstract]
Hwang, D. Y., Ardayfio, P., Kang, U. J., Semina, E. V. and Kim,
K. S. (2003). Selective loss of dopaminergic neurons in the
substantia nigra of Pitx3-deficient aphakia mice. Brain Res. Mol.
Brain Res. 114,123
-131.[Medline]
Hynes, M., Stone, D. M., Dowd, M., Pitts-Meek, S., Goddard, A.,
Gurney, A. and Rosenthal, A. (1997). Control of cell pattern
in the neural tube by the zinc finger transcription factor and oncogene Gli-1.
Neuron 19,15
-26.[CrossRef][Medline]
Kawano, H., Ohyama, K., Kawamura, K. and Nagatsu, I.
(1995). Migration of dopaminergic neurons in the embryonic
mesencephalon of mice. Brain Res. Dev. Brain Res.
26,101
-113.
Kele, J., Simplicio, N., Ferri, A. L., Mira, H., Guillemot, F.,
Arenas, E. and Ang, S. L. (2006). Neurogenin 2 is required
for the development of ventral midbrain dopaminergic neurons.
Development 133,495
-505.
Kelley, A. E. and Berridge, K. C. (2002). The
neuroscience of natural rewards: relevance to addictive drugs. J.
Neurosci. 22,3306
-3311.
Kennedy, P. R. (1990). Corticospinal,
rubrospinal and rubro-olivary projections: a unifying hypothesis.
Trends Neurosci. 13,474
-479.[CrossRef][Medline]
Lang, A. E. and Lozano, A. M. (1998).
Parkinson's disease. Second of two parts. N. Engl. J.
Med. 339,1130
-1143.
Lindvall, O. and Bjorklund, A. (2004). Cell
therapy in Parkinson's disease. NeuroRx
1, 382-393.
Mallamaci, A., Di Blas, E., Briata, P., Boncinelli, E. and
Corte, G. (1996). OTX2 homeoprotein in the developing central
nervous system and migratory cells of the olfactory area. Mech.
Dev. 58,165
-178.[CrossRef][Medline]
Marin, F., Herrero, M. T., Vyas, S. and Puelles, L.
(2005). Ontogeny of tyrosine hydroxylase mRNA expression in mid-
and forebrain: neuromeric pattern and novel positive regions. Dev.
Dyn. 234,709
-717.[CrossRef][Medline]
Mastick, G. S. and Andrews, G. L. (2001). Pax6
regulates the identity of embryonic diencephalic neurons. Mol.
Cell. Neurosci. 17,190
-207.[CrossRef][Medline]
Maxwell, S. L., Ho, H. Y., Kuehner, E., Zhao, S. and Li, M.
(2005). Pitx3 regulates tyrosine hydroxylase expression in the
substantia nigra and identifies a subgroup of mesencephalic dopaminergic
progenitor neurons during mouse development. Dev.
Biol. 282,467
-479.[CrossRef][Medline]
Millonig, J. H., Millen, K. J. and Hatten, M. E.
(2000). The mouse Dreher gene Lmx1a controls formation of the
roof plate in the vertebrate CNS. Nature
403,764
-769.[CrossRef][Medline]
Nunes, I., Tovmasian, L. T., Silva, R. M., Burke, R. E. and
Goff, S. P. (2003). Pitx3 is required for development of
substantia nigra dopaminergic neurons. Proc. Natl. Acad. Sci.
USA 100,4245
-4250.
O'Hara, F. P., Beck, E., Barr, L. K., Wong, L. L., Kessler, D.
S. and Riddle, R. D. (2005). Zebrafish Lmx1b.1 and Lmx1b.2
are required for maintenance of the isthmic organizer.
Development 132,3163
-3173.
Ohyama, K., Ellis, P., Kimura, S. and Placzek, M.
(2005). Directed differentiation of neural cells to hypothalamic
dopaminergic neurons. Development
132,5185
-5197.
Perea-Gomez, A., Lawson, K. A., Rhinn, M., Zakin, L., Brulet,
P., Mazan, S. and Ang, S. L. (2001). Otx2 is required for
visceral endoderm movement and for the restriction of posterior signals in the
epiblast of the mouse embryo. Development
128,753
-765.[Abstract]
Prakash, N. and Wurst, W. (2006). Development
of dopaminergic neurons in the mammalian brain. Cell Mol. Life
Sci. 63,187
-206.[CrossRef][Medline]
Prakash, N., Brodski, C., Naserke, T., Puelles, E., Gogoi, R.,
Hall, A., Panhuysen, M., Echevarria, D., Sussel, L., Weisenhorn, D. M. et
al. (2006). A Wnt1-regulated genetic network controls the
identity and fate of midbrain-dopaminergic progenitors in vivo.
Development 133,89
-98.
Puelles, E., Acampora, D., Lacroix, E., Signore, M., Annino, A.,
Tuorto, F., Filosa, S., Corte, G., Wurst, W., Ang, S. L. et al.
(2003). Otx dose-dependent integrated control of antero-posterior
and dorso-ventral patterning of midbrain. Nat.
Neurosci. 6,453
-460.[Medline]
Puelles, E., Annino, A., Tuorto, F., Usiello, A., Acampora, D.,
Czerny, T., Brodski, C., Ang, S. L., Wurst, W. and Simeone, A.
(2004). Otx2 regulates the extent, identity and fate of neuronal
progenitor domains in the ventral midbrain.
Development 131,2037
-2048.
Puelles, L. and Rubenstein, J. L. (2003).
Forebrain gene expression domains and the evolving prosomeric model.
Trends Neurosci. 26,469
-476.[CrossRef][Medline]
Ramos, C. and Robert, B. (2005). msh/Msx gene
family in neural development. Trends Genet.
21,624
-632.[CrossRef][Medline]
Riddle, R. and Pollock, J. D. (2003). Making
connections: the development of mesencephalic dopaminergic neurons.
Brain Res. Dev. Brain Res.
147, 3-21.[Medline]
Sasaki, H. and Hogan, B. L. (1994). HNF-3 beta
as a regulator of floor plate development. Cell
76,103
-115.[CrossRef][Medline]
Saucedo-Cardenas, O., Quintana-Hau, J., Le, W., Smidt, M., Cox,
J., Mayo, F. D., Burbach, J. and Conneely, O. (1998). Nurr1
is essential for the induction of the dopaminergic phenotype and the survival
of ventral mesencephalic late dopaminergic precursor neurons. Proc.
Natl. Acad. Sci. USA 95,4013
-4018.
Sesack, S. R. and Carr, D. B. (2002). Selective
prefrontal cortex inputs to dopamine cells: implications for schizophrenia.
Physiol. Behav. 77,513
-517.[CrossRef][Medline]
Shimeld, S. M., McKay, I. J. and Sharpe, P. T.
(1996). The murine homeobox gene Msx-3 shows highly restricted
expression in the developing neural tube. Mech. Dev.
55,201
-210.[CrossRef][Medline]
Simeone, A. (2005). Genetic control of a
dopaminergic neuron differentiation. Trends Neurosci.
28, 62-65.[CrossRef][Medline]
Simeone, A. and Acampora, D. (2001). The role
of Otx2 in organizing the anterior patterning in mouse. Int. J.
Dev. Biol. 45,337
-345.[Medline]
Simeone, A., Acampora, D., Mallamaci, A., Stornaiuolo, A.,
D'Apice, M. R., Nigro, V. and Boncinelli, E. (1993). A
vertebrate gene related to orthodenticle contains a homeodomain of the bicoid
class and demarcates anterior neuroectoderm in the gastrulating mouse embryo.
EMBO J. 12,2735
-2747.[Medline]
Simon, H. H., Saueressig, H., Wurst, W., Goulding, M. D. and
O'Leary, D. D. (2001). Fate of midbrain dopaminergic neurons
controlled by the engrailed genes. J. Neurosci.
21,3126
-3134.
Smidt, M. P., Asbreuk, C. H., Cox, J. J., Chen, H., Johnson, R.
L. and Burbach, J. P. (2000). A second independent pathway
for development of mesencephalic dopaminergic neurons requires Lmx1b.
Nat. Neurosci. 3,337
-341.[CrossRef][Medline]
Smidt, M. P., Smits, S. M., Bouwmeester, H., Hamers, F. P., van
der Linden, A. J., Hellemons, A. J., Graw, J. and Burbach, J. P.
(2004). Early developmental failure of substantia nigra dopamine
neurons in mice lacking the homeodomain gene Pitx3.
Development 131,1145
-1155.
Smits, S. M., Ponnio, T., Conneely, O. M., Burbach, J. P. H. and
Smidt, M. P. (2003). Involvement of Nurr1 in specifying the
neurotransmitter identity of ventral midbrain dopaminergic neurons.
Eur. J. Neurosci. 18,1731
-1738.[CrossRef][Medline]
Smits, S. M., Burbach, J. P. and Smidt, M. P.
(2006). Developmental origin and fate of meso-diencephalic
dopamine neurons. Prog. Neurobiol.
78, 1-16.[CrossRef][Medline]
Snyder, B. J. and Olanow, C. W. (2005). Stem
cell treatment for Parkinson's disease: an update for 2005. Curr.
Opin. Neurol. 18,376
-385.[Medline]
Thompson, L., Barraud, P., Andersson, E., Kirik, D. and
Bjorklund, A. (2005). Identification of dopaminergic neurons
of nigral and ventral tegmental area subtypes in grafts of fetal ventral
mesencephalon based on cell morphology, protein expression, and efferent
projections. J. Neurosci.
25,6467
-6477.
Tzschentke, T. M. and Schmidt, W. J. (2000).
Functional relationship among medial prefrontal cortex, nucleus accumbens, and
ventral tegmental area in locomotion and reward. Crit. Rev.
Neurobiol. 14,131
-142.[Medline]
van den Munckhof, P., Luk, K. C., Ste-Marie, L., Montgomery, J.,
Blanchet, P. J., Sadikot, A. F. and Drouin, J. (2003). Pitx3
is required for motor activity and for survival of a subset of midbrain
dopaminergic neurons. Development
130,2535
-2542.
Vernay, B., Koch, M., Vaccarino, F., Briscoe, J., Simeone, A.,
Kageyama, R. and Ang, S. L. (2005). Otx2 regulates subtype
specification and neurogenesis in the midbrain. J.
Neurosci. 25,4856
-4867.
Wallen, A. and Perlmann, T. (2003).
Transcriptional control of dopamine neuron development. Ann. N. Y.
Acad. Sci. 991,48
-60.[Medline]
Wallen, A., Zetterstrom, R. H., Solomin, L., Arvidsson, M.,
Olson, L. and Perlmann, T. (1999). Fate of mesencephalic
AHD2-expressing dopamine progenitor cells in NURR1 mutant mice.
Exp. Cell Res. 253,737
-746.[CrossRef][Medline]
Wallen, A. A., Castro, D. S., Zetterstrom, R. H., Karlen, M.,
Olson, L., Ericson, J. and Perlmann, T. (2001). Orphan
nuclear receptor nurr1 is essential for ret expression in midbrain dopamine
neurons and in the brain stem. Mol. Cell. Neurosci.
18,649
-663.[CrossRef][Medline]
Wang, W., Chen, X., Xu, H. and Lufkin, T.
(1996). Msx3: a novel murine homologue of the Drosophila msh
homeobox gene restricted to the dorsal embryonic central nervous system.
Mech. Dev. 58,203
-215.[CrossRef][Medline]
Wightman, R. M. and Robinson, D. L. (2002).
Transient changes in mesolimbic dopamine and their association with `reward'.
J. Neurochem. 82,721
-735.[CrossRef][Medline]
Ye, W., Shimamura, K., Rubenstein, J. L., Hynes, M. A. and
Rosenthal, A. (1998). FGF and Shh signals control
dopaminergic and serotonergic cell fate in the anterior neural plate.
Cell 93,755
-766.[CrossRef][Medline]
Zetterstrom, R. H., Solomin, L., Jansson, L., Hoffer, B. J.,
Olson, L. and Perlmann, T. (1997). Dopamine neuron agenesis
in Nurr1-deficient mice. Science
276,248
-250.
Zuddas, A., Corsini, G. U., Barker, J. L., Kopin, I. J. and Di
Porzio, U. (1991). Specific reinnervation of lesioned mouse
striatum by grafted mesencephalic dopaminergic neurons. Eur. J.
Neurosci. 3,72
-85.[CrossRef][Medline]
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