Download The role of Pitx3 in survival of midbrain dopaminergic neurons

J Neural Transm (2006) [Suppl] 70: 57–60
# Springer-Verlag 2006
The role of Pitx3 in survival of midbrain dopaminergic neurons
S. M. Smits and M. P. Smidt
Rudolf Magnus Institute of Neuroscience, Department of Pharmacology and Anatomy,
University Medical Center Utrecht, Utrecht, The Netherlands
Summary. Dopamine belongs to the most
intensively studied neurotransmitters of the
brain, because of its implications in psychiatric and neurological disorders. Although,
clinical relevance of midbrain dopaminergic
(mDA) neurons is well recognized and
dopaminergic dysfunction may have a genetic component, the genetic cascades underlying developmental processes are still
largely unknown. With the advances in molecular biology, mDA neurons and their
involvement in psychiatric and neurological disorders are now subject of studies that
aim to delineate the fundamental neurobiology of these neurons. These studies are
concerned with developmental processes,
cell-specific gene expression and regulation,
molecular pharmacology, and genetic association of dopamine-related genes and mDAassociated disorders. Several transcription
factors implicated in the post-mitotic mDA
development, including Nurr1, Lmx1b,
Pitx3, and En1=En2 have contributed to the
understanding of how mDA neurons are
generated in vivo. Furthermore, these studies provide insights into new strategies for
future therapies of Parkinson’s Disease (PD)
using stem cells for engineering DA neurons in vitro. Here, we will discuss the
role of Pitx3 in molecular mechanisms involved in the regional specification, neuronal specification and differentiation of mDA
neurons.
The heterogeneity of midbrain
dopaminergic (mDA) neurons
The mDA system (A8–A10 cell groups) is
involved in many brain functions including
motor control, reward, emotional and motivated behavior, and is of clinical importance
because of its implication in neurological and
psychiatric disorders. The A9 cell group
located in the substantia nigra pars compacta
(SNc) has preferred projections to the dorsal
striatum forming the nigrostriatal pathway,
which is involved in the control of movement. The mDA system further includes the
ventral tegmental area (VTA), located in the
A10 group and the retrorubral field located in
the A8 group. Dopamine neurons of the VTA
with their efferents to the nucleus accumbens, other limbic brain areas and the cortex form the mesolimbic=cortical pathways,
and are involved in the control of emotional
behaviors and reward. In addition, specific
mDA subpopulations have been described
based on pharmacology, gene expression and
electrophysiological properties. A neuropathological enigma is posed by the selective
degeneration of the SNc dopamine neurons
in PD and many animal models of PD,
whereas mDA neurons in the VTA are largely spared. Gene expression profile studies of
discrete adult mDA subpopulations revealed
distinct molecular features that might underlie the differential susceptibility (Grimm
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S. M. Smits and M. P. Smidt
et al., 2004; Greene et al., 2005). Therefore,
one may expect that the difference between
dopamine neurons of the SNc and those of
the VTA roots in the molecular make-up of
these neurons, which might originate from
subpopulation-specific developmental pathways. Interestingly, the recent discovery of a
brain phenotype in Pitx3-deficient mice indicates that Pitx3 drives molecular pathways
that are essential for the development and=or
survival of specific mDA subsets (Smidt
et al., 2004a, b). Thus, these data suggest that
differentiation of specific mDA subpopulations is controlled by different developmental
pathways=factors, or that different subpopulations differentially respond to the same
factor.
Pitx3 and its role in mDA development
The development of mDA neurons follows a
number of stages marked by distinct events.
After preparation of the region by signals
that provide induction and patterning, cascades of transcription factors involved in
specification and differentiation enroll towards fully matured mDA neurons (Hynes
and Rosenthal, 1999). Molecular studies into
the developmental pathways of these neurons
and analysis of mutant animals defective in
mDA development have identified several
key transcription factors, including Nurr1,
Lmx1b and En1=En2, with a function in
specification of transmitter identity, neuronal identity and survival of mDA neurons
(Smidt et al., 2004a; Perlmann and WallenMackenzie, 2004; Simon et al., 2004).
The paired-like homeodomain transcription factor Pitx3 is uniquely expressed in the
brain in post-mitotic mDA neurons during the
late differentiation phase from E11.5 onwards,
and its expression is conserved among species including human. Genetic analysis of
the Aphakia (ak) mouse mutant revealed deletions in the Pitx3 gene, causing the ablation
of Pitx3 expression (Smidt et al., 2004a, b).
These Pitx3-deficient (ak) mice display neu-
roanatomical alterations in the mDA system
from E12.5 onwards, characterized by the
absence of mDA neurons in the SNc, whereas mDA neurons in the VTA and the most
lateral tip of the SNc are largely spared
(Smidt et al., 2004a, b). As a consequence of
the neuronal loss in the SNc, connections
to the dorsal striatum are virtually absent
resulting in a dramatic decrease of dopamine.
Initial behavioural analysis of ak mice revealed inconsistent reports on their motor
impairments. Although it was stated that ak
mice display the akinetic subtype of PD and
motor deficits that are reversed by L-DOPA
(van den Munckhof et al., 2003; Hwang et al.,
2005), we and others observed no characteristic neurological PD symptoms in ak mice
(Hwang et al., 2003; Nunes et al., 2003;
Smidt et al., 2004a, b).
The mechanism by which Pitx3 influences
specifically the survival of SNc mDA neurons
is unknown and intriguing. Post-mitotic mDA
neurons start to express Pitx3 at the most ventral position of the developing midbrain after
they have migrated ventrally from the neuroepithelium. Therefore, Pitx3 is not directly
involved in the proliferation and=or migration
of young mDA neurons, but rather in the
terminal differentiation and maintenance. A
possible explanation for the selective vulnerability, observed in ak mice may be that Pitx3
is not expressed in all mDA neurons. However, we and others found complete overlap
between Pitx3 and tyrosine hydroxylase (TH),
the key enzyme in dopamine synthesis (Smidt
et al., 2004a, b; Zhao et al., 2004). Thus, although all mDA neurons depend on identical
signals for their early specification, the specification of neuronal fate of mDA subsets is
probably maintained, in part, by independent
regulatory cascades.
Origin and specification
of mDA neurons
Specification of neuronal fates begins with
the acquisition of anterior-posterior (A=P)
The role of Pitx3 in survival of midbrain dopaminergic neurons
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Fig. 1. Schematic representation of the anterior=posterior (A) and dorsal=ventral (B) patterning of the brain
and the emergence of mDA neurons (red) with specific identity to regional molecular coding. A Drawing of
an E12.5 mouse brain in a sagittal plane showing the location of fully differentiated mDA neurons (red) in
specific brain segments (M-P3). B Drawing of an E12.5 mouse midbrain in a coronal plane, showing the
ventral localization of fully differentiated mDA neurons. Neurons are born in the ventricular zone across
specific longitudinal domains (floor plate (FP, green), basal plate (BP, blue) or alar plate (AP, yellow)) and
migrate ventrally (arrows) where they adopt the full dopaminergic phenotype and start to express Pitx3.
Aq aqueduct; H hindbrain; M midbrain; MHB mid-hindbrain border (Isthmus); P1-3 prosomere 1–3; RD rostral
diencephalon; Tel telencepalon
and dorsal-ventral (D=V) patterning in restricted domains of the neuronal plate
(Fig. 1). D=V patterning causes longitudinal
subdivisions in the brain (floor plate, basal
plate and alar plate), whereas A=P patterning
leads to neuromeric domains (forebrain, midbrain, isthmus and hindbrain; Puelles, 2001).
The commitment of neuronal identity by a
molecular code within progenitor cells in
the ventricular zone and region-specific developmental cascades ultimately results in
induction of distinct neuronal cell types,
including mDA neurons (Fig. 1). In human
embryos, mDA neurons in the SNc and
VTA originate independently across several
neuromeric domains and longitudinal subdivisions, and thus are not primarily unitary
(Verney et al., 2001). Thus, the developmental origin of mDA neurons with respect to
the longitudinal subdivisions and neuromeric domains in the brain, and the molecular
codes within mDA subsets might determine
the distinct features of these cells.
Concluding remarks
It becomes more and more clear that the
mDA system harbors a multitude of specific functional neuronal units exemplified by
region-specific molecular codes during development and in the adult. The role of Pitx3
in the development of SNc mDA neurons
might link molecular codes to survival of
mDA subsets, which can be exploited in the
treatment of PD. Recently, it was shown that
Pitx3 facilitates differentiation of mouse embryonic stem cells into the A9 cell group of
mDA neurons, without affecting the total
number of dopamine neurons (Chung et al.,
2005), illustrating the importance of identifying the appropriate signals and factors that
influence normal mDA development. Until
now no molecular target genes of Pitx3 are
identified and molecular processes initiated
by Pitx3 remain unidentified. Therefore, further investigation is warranted to elucidate
the role of Pitx3 in mDA neuronal development and maintenance.
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S. M. Smits and M. P. Smidt: The role of Pitx3 in survival of midbrain dopaminergic neurons
Note added in proof
Recent development in specification of the dopamine
neurons of the SNc and VTA have led to the new
nomenclature of these neurons as the meso-diencephalic dopamine (mdDA) neurons. This is highlighted
in the following recent review: Smits et al., 2006 in
‘‘Progress in Neurobiology’’.
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Author’s address: M. P. Smidt, Rudolf Magnus
Institute of Neuroscience, Universiteitsweg 100, 3584
CG Utrecht, The Netherlands, e-mail: m.p.smidt@
med.uu.nl