The mesencephalic dopamine (mesDA) system is involved in the control of movement and behavior. The expression of Pitx3 in the brain is restricted to the mesDA system and the gene is induced relatively late, at E11.5, a time when tyrosine hydroxylase (Th) gene expression is initiated. We show here that, in the Pitx3-deficient aphakia(ak) mouse mutant, the mesDA system is malformed. Owing to the developmental failure of mesDA neurons in the lateral field of the midbrain,mesDA neurons are not found in the SNc and the projections to the caudate putamen are selectively lost. However, Pitx3 is expressed in all mesDA neurons in control animals. Therefore, mesDA neurons react specifically to the loss of Pitx3. Defects of motor control where not seen in the ak mice,suggesting that other neuronal systems compensate for the absence of the nigrostriatal pathway. However, an overall lower activity was observed. The results suggest that Pitx3 is specifically required for the formation of the SNc subfield at the onset of dopaminergic neuron differentiation.

Selective neuronal loss in the substantia nigra (SNc), as described for Parkinson's disease (PD) in humans and for Pitx3 deficiency in mice,highlights the existence of neuronal subpopulations. As yet unknown subset-specific gene cascades might underlie the observed differences in neuronal vulnerability. We identified a developmental cascade in mice in which Ahd2 (Aldh1a1) is under the transcriptional control of Pitx3. Interestingly, Ahd2 distribution is restricted to a subpopulation of the meso-diencephalic dopaminergic (mdDA) neurons that is affected by Pitx3 deficiency. Ahd2 is involved in the synthesis of retinoic acid(RA), which has a crucial role in neuronal patterning, differentiation and survival in the brain. Most intriguingly, restoring RA signaling in the embryonic mdDA area counteracts the developmental defects caused by Pitx3 deficiency. The number of tyrosine hydroxylase-positive (TH+)neurons was significantly increased after RA treatment in the rostral mdDA region of Pitx3-/- embryos. This effect was specific for the rostral part of the developing mdDA area, and was observed exclusively in Pitx3-/- embryos. The effect of RA treatment during the critical phase was preserved until later in development, and our data suggest that RA is required for the establishment of proper mdDA neuronal identity. This positions Pitx3 centrally in a mdDA developmental cascade linked to RA signaling. Here, we propose a novel mechanism in which RA is involved in mdDA neuronal development and maintenance, providing new insights into subset-specific vulnerability in PD.

The mesencephalic dopaminergic (mesDA) system is involved in many brain functions including motor control and motivated behaviour, and is of clinical importance because of its implication in psychiatric disorders and Parkinson's disease. Nurr1, a member of the nuclear hormone receptor superfamily of transcription factors, is essential for establishing the dopaminergic phenotype, because expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, requires Nurr1. In addition, Nurr1 plays an important role in the maintenance of mesDA neurons. Neonatal Nurr1 knockout mice lack expression of the dopamine transporter (DAT), the vesicular monoamine transporter 2 (VMAT2) and l-aromatic amino acid decarboxylase (AADC) in addition to TH specifically in mesDA neurons. It is unclear whether the lack of expression of these dopaminergic markers is caused by a maintenance defect or whether the induction of these markers depends on Nurr1 expression. To address this problem, the expression of DAT, VMAT2 and AADC was analysed at embryonic day 12.5 and 14.5. Here we demonstrate that induction of VMAT2 and DAT specifically in mesDA neurons requires Nurr1 expression, whereas AADC expression in mesDA neurons is induced independently of Nurr1 function.

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The homeobox gene Pitx3 plays an important part in the development and function of vertebrate midbrain dopaminergic neurons. Re-localization of the genetic defect in the mouse mutant aphakia to the Pitx3 locus, together with the subsequent identification of two deletions causing the gene to be silent, has been the hallmark of several studies into the role of Pitx3. In this review, we summarize the data and reflect on the role of Pitx3 in the development of dopamine neurons in the midbrain. The data indicate that Pitx3 is essential for the survival of dopamine neurons located in the substantia nigra compacta during development. Molecular analysis of the underlying mechanisms might provide new insights for understanding the selective degeneration observed in Parkinson patients.