Pitx3 is required for development of substantia nigra dopaminergic neurons

Abstract

Dopaminergic (DA) neurons of substantia nigra in the midbrain control voluntary movement, and their degeneration is the cause of Parkinson's disease. The complete set of genes required to specifically determine the development of midbrain DA subgroups is not known yet. We report here that mice lacking the bicoidrelated homeoprotein Pitx3 fail to develop DA neurons of the substantia nigra. Other mesencephalic DA neurons of the ventral tegmental area and retrorubral field are unaltered in their dopamine expression and histological organization. These data suggest that Pitx3-dependent gene expression is specifically required for the differentiation of DA progenitors within the mesencephalic DA system. M idbrain dopaminergic (DA) cells contribute to the control of voluntary movement, cognition, and emotional behavior. Degeneration of these cells results in Parkinson's disease, and aberrant dopamine neurotransmitter signaling is implicated in schizophrenia and addictive behavioral disorders (1-3). Within the midbrain there are three subgroups of DA neurons, the ventral tegmental area (VTA͞A10), substantia nigra (SN͞ A9), and retrorubral field (RRF͞A8), that together constitute the mesencephalic (mes) system (4). Those DA cells in the SN that innervate the striatum preferentially degenerate in Parkinson's disease, whereas other DA subgroups regulate emotional and reward behaviors.Transcription factors regulate the differentiation of midbrain DA neuron precursors. Two transcription factors have been shown to contribute to different aspects of mesDA neuronal differentiation. One of these, the orphan nuclear hormone receptor Nurr1, is required for maturation of mesDA neuron precursors. Mice harboring null alleles of Nurr1 do not express tyrosine hydroxylase (TH), which catalyzes the initial step of dopamine neurotransmitter biosynthesis (5-11). The other factor, LIM homeodomain transcription factor Lmx1b, contributes partially to the specification of mesDA neuronal progenitors beginning on embryonic day 12.5 in the mouse but is not essential for TH gene expression (12).A third transcription factor, the bicoid-related homeodomain-containing transcription factor Pitx3, which is also known as Ptx3, has been implicated in the development of DA neurons. Pitx3 gene expression is restricted to the developing eye and DA progenitor cells from embryonic day 11 throughout adult life in mice (13,14). In the brain, Pitx3 mRNA localizes specifically to the SN and VTA (14). A reduction in Pitx3 mRNA levels is observed in the ventral midbrain of Lmx1b knockout mice, 6-hydroxydopamine-lesioned rats, and Parkinson's patients (8,11,12,14). Yet, Pitx3 expression is maintained in the ventral midbrain of Nurr1 null mutant embryos (12). These data have been interpreted to suggest that Pitx3 contributes to the combinatorial code defined by multiple transcription factors that establish specification and differentiation of midbrain DA progenitors (12). Here we report that Pitx3 is essential for the development of neurons specific to the SN. The D...

Motor deficits and altered striatal gene expression in aphakia (ak) mice

“…These expression changes alter over time, despite no worsening in motor phenotype. In contrast to previously described studies (Nunes et al, 2003), both male and female ak mice in this study were significantly lighter than age-matched wt mice at all ages examined. PD patients exhibit lower body weight in comparison to age-matched subjects (Chen et al, 2003).…”

“…The Pitx3 −/− or aphakia (ak) mouse is a promising model of striatal denervation in PD as it exhibits the cell specificity of neurodegeneration observed in humans (Hwang et al, 2003;Nunes et al, 2003;Smidt et al, 2004), similar neuroadaptive phenomena at the level of the striatum (Smits et al, 2005;van den Munckhof et al, 2006), and locomotor deficits that are rescued by levodopa van den Munckhof et al, 2006). Pitx3 is a member of the pituitary family of bicoid type homeobox transcription factors (Semina et al, 1997).…”

“…In homozygous ak mice, genomic deletions within the promoter and exon1 of the Pitx3 gene (Rieger et al, 2001;Semina et al, 2000) result in incomplete lens development and aphakia. Undetectable expression of Pitx3 protein in the midbrain is accompanied by failed development of approximately 90% of SNc neurons, and severe reduction of dorsal striatal dopaminergic innervation (Hwang et al, 2003;Nunes et al, 2003;Smidt et al, 2004;van den Munckhof et al, 2003). The ak mouse retains expression of Pitx3 in skeletal muscle (L'Honoré et al, 2007).…”

“…Heterozygous ak mice are normal (Nunes et al, 2003). In homozygote ak mice, SNc cell loss (particularly in the ventral nigra) is almost complete by birth, whereas the ventral tegmental area (VTA) undergoes progressive postnatal cell loss (Nunes et al, 2003;van den Munckhof et al, 2003) so that approximately 50% of this region is lost by 100 days (van den Munckhof et al, 2003).…”

“…In homozygote ak mice, SNc cell loss (particularly in the ventral nigra) is almost complete by birth, whereas the ventral tegmental area (VTA) undergoes progressive postnatal cell loss (Nunes et al, 2003;van den Munckhof et al, 2003) so that approximately 50% of this region is lost by 100 days (van den Munckhof et al, 2003). Ak mice exhibit relatively subtle behavioral and motor deficits (Hwang et al, 2003;van den Munckhof et al, 2003;, and some conflicting data exist in this regard (Smidt et al, 2004;Hwang et al, 2005).…”