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The protracted and age-dependent degeneration of dopamine (DA)-producing neurons of the Substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) in the mammalian midbrain is a hallmark of human Parkinson's Disease (PD) and of certain genetic mouse models of PD, such as mice heterozygous for the homeodomain transcription factor Engrailed 1 (En1(+/-) mice). Neurotoxin-based animal models of PD, in contrast, are characterized by the fast and partly reversible degeneration of the SNc and VTA DA neurons. The secreted protein WNT1 was previously shown to be strongly induced in the neurotoxin-injured adult ventral midbrain (VM), and to protect the SNc and VTA DA neurons from cell death in this context. We demonstrate here that the sustained and ectopic expression of Wnt1 in the SNc and VTA DA neurons of En1(+/Wnt1) mice also protected these genetically affected En1 heterozygote (En1(+/-)) neurons from their premature degeneration in the adult mouse VM. We identified a developmental gene cascade that is up-regulated in the adult En1(+/Wnt1) VM, including the direct WNT1/β-catenin signaling targets Lef1, Lmx1a, Fgf20 and Dkk3, as well as the indirect targets Pitx3 (activated by LMX1A) and Bdnf (activated by PITX3). We also show that the secreted neurotrophin BDNF and the secreted WNT modulator DKK3, but not the secreted growth factor FGF20, increased the survival of En1 mutant dopaminergic neurons in vitro. The WNT1-mediated signaling pathway and its downstream targets BDNF and DKK3 might thus provide a useful means to treat certain genetic and environmental (neurotoxic) forms of human PD., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Understanding the molecular basis underlying the neurogenesis of mesencephalic-diencephalic Dopaminergic (mdDA) neurons is a major task fueled by their relevance in controlling locomotor activity and emotion and their involvement in neurodegenerative and psychiatric diseases. Increasing evidence suggests that mdDA neurons of the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA) represent two main distinct neuronal populations, which, in turn, include specific neuronal subsets. Relevant studies provided important results on mdDA neurogenesis, but, nevertheless, have not yet clarified how the identity of mdDA neuronal subtypes is established and, in particular, whether neurogenic factors may direct progenitors towards the differentiation of specific mdDA neuronal subclasses. The transcription factor Otx2 is required for the neurogenesis of mesencephalic DA (mesDA) neurons and to control neuron subtype identity and sensitivity to the MPTP neurotoxin in the adult VTA. Here we studied whether Otx2 is required in mdDA progenitors for the generation of specific mdDA neuronal subtypes. We found that although expressed in virtually all mdDA progenitors, Otx2 is required selectively for the differentiation of VTA neuronal subtypes expressing Ahd2 and/or Calb but not for those co-expressing Girk2 and glyco-Dat. Moreover, mild over-expression of Otx2 in SNpc progenitors and neurons is sufficient to rescue En1 haploinsufficiency-dependent defects, such as progressive loss and increased MPTP sensitivity of SNpc neurons. Collectively, these data suggest that mdDA progenitors exhibit differential sensitivity to Otx2, which selectively influences the generation of a large and specific subset of VTA neurons. In addition, these data suggest that Otx2 and En1 may share similar properties and control survival and vulnerability to MPTP neurotoxin respectively in VTA and SNpc., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Neurons usually migrate and differentiate in one particular encephalic vesicle. We identified a murine population of diencephalic neurons that colonized the telencephalic amygdaloid complex, migrating along a tangential route that crosses a boundary between developing brain vesicles. The diencephalic transcription factor OTP was necessary for this migratory behavior.

The vestibular portion of the inner ear, the three semicircular canals and their sensory cristae, is responsible for detecting angular head movements. It was proposed that sensory cristae induce formation of their non-sensory components, the semicircular canals. Here, we analyzed the inner ears of Foxg1(-/-) mouse mutants, which display vestibular defects that are in conflict with the above model. In Foxg1(-/-) ears, the lateral canal is present without the lateral ampulla, which houses the lateral crista. Our gene expression analyses indicate that at the time when canal specification is thought to occur, the prospective lateral crista is present, which could have induced lateral canal formation prior to its demise. Our genetic fate-mapping analyses indicate an improper separation between anterior and lateral cristae in Foxg1(-/-) mutants. Our data further suggest that a function of Foxg1 in the inner ear is to restrict sensory fate., (Copyright 2009 Wiley-Liss, Inc.)

Little is known about the cues controlling the generation of motoneuron populations in the mammalian ventral midbrain. We show that Otx2 provides the crucial anterior-posterior positional information for the generation of red nucleus neurons in the murine midbrain. Moreover, the homeodomain transcription factor Nkx6-1 controls the proper development of the red nucleus and of the oculomotor and trochlear nucleus neurons. Nkx6-1 is expressed in ventral midbrain progenitors and acts as a fate determinant of the Brn3a(+) (also known as Pou4f1) red nucleus neurons. These progenitors are partially dorsalized in the absence of Nkx6-1, and a fraction of their postmitotic offspring adopts an alternative cell fate, as revealed by the activation of Dbx1 and Otx2 in these cells. Nkx6-1 is also expressed in postmitotic Isl1(+) oculomotor and trochlear neurons. Similar to hindbrain visceral (branchio-) motoneurons, Nkx6-1 controls the proper migration and axon outgrowth of these neurons by regulating the expression of at least three axon guidance/neuronal migration molecules. Based on these findings, we provide additional evidence that the developmental mechanism of the oculomotor and trochlear neurons exhibits more similarity with that of special visceral motoneurons than with that controlling the generation of somatic motoneurons located in the murine caudal hindbrain and spinal cord.

Neurons that produce dopamine as a neurotransmitter constitute a heterogeneous group involved in the control of various behaviors and physiology. In mammals, dopaminergic neurons are found in distinct clusters mainly located in the ventral midbrain and the caudal forebrain [1]. Although much is known about midbrain dopaminergic neurons, development of diencephalic dopaminergic neurons is poorly understood. Here we demonstrate that Orthopedia (Otp) homeodomain protein is essential for the development of specific subsets of diencephalic dopaminergic neurons. Zebrafish embryos lacking Otp activity are devoid of dopaminergic neurons in the hypothalamus and the posterior tuberculum. Similarly, Otp-/- mouse [2, 3] embryos lack diencephalic dopaminergic neurons of the A11 group, which constitutes the diencephalospinal dopaminergic system. In both systems, Otp is expressed in the affected dopaminergic neurons as well as in potential precursor populations, and it might contribute to dopaminergic cell specification and differentiation. In fish, overexpression of Otp can induce ectopic tyrosine hydroxylase and dopamine transporter expression, indicating that Otp can specify aspects of dopaminergic identity. Thus, Otp is one of the few known transcription factors that can determine aspects of the dopaminergic phenotype and the first known factor to control the development of the diencephalospinal dopaminergic system.

One of the earliest organizational decisions in the development of the vertebrate brain is the division of the neural plate into Otx2-positive anterior and Gbx2-positive posterior territories. At the junction of these two expression domains, a local signaling center is formed, known as the midbrain-hindbrain boundary (MHB). This tissue coordinates or "organizes" the development of neighboring brain structures, such as the midbrain and cerebellum. Correct positioning of the MHB is thought to depend on mutual repression involving these two homeobox genes. Using a cell culture colocalization assay and coimmunoprecipitation experiments, we show that engrailed homology region 1 (eh1)-like motifs of both transcription factors physically interact with the WD40 domain of Groucho/Tle corepressor proteins. In addition, heat shock-induced expression of wild-type and mutant Otx2 and Gbx2 in medaka embryos demonstrates that Groucho is required for the repression of Otx2 by Gbx2. On the other hand, the repressive functions of Otx2 on Gbx2 do not appear to be dependent on corepressor interaction. Interestingly, the association of Groucho with Otx2 is also required for the repression of Fgf8 in the MHB. Therefore Groucho/Tle family members appear to regulate key aspects in the MHB development of the vertebrate brain.

In zebrafish, gbx1 and otx2 are among the earliest genes expressed in the neuroectoderm, dividing it into an anterior and a posterior domain with a common border that marks the midbrain-hindbrain boundary (MHB) primordium. Here, we describe the sequence and expression pattern of Gbx1 in mouse. The first transcripts are found at embryonic day 7.75 in the hindbrain. Later on, expression of Gbx1 is detectable in the hindbrain (rhombomeres 2 to 7), spinal cord, optic vesicles, and in the ventral telencephalon. In mouse, Gbx1 expression is not observed at the MHB as is the case during early zebrafish development. We suggest that an evolutionary switch occurred: in mouse Gbx2 is involved in the early specification of the MHB primordium, whereas in zebrafish, gbx1 is required instead of gbx2., (Copyright 2004 Wiley-Liss, Inc.)

Otx1 belongs to the paired class of homeobox genes and plays a pivotal role in brain development. Here, we show that Otx1 is expressed in hematopoietic pluripotent and erythroid progenitor cells. Moreover, bone marrow cells from mice lacking Otx1 exhibit a cell-autonomous impairment of the erythroid compartment. In agreement with these results, molecular analysis revealed decreased levels of erythroid genes that include the SCL and GATA-1 transcription factors. Accordingly, a gain of function of SCL rescues the erythroid deficiency in Otx1-/- mice. Taken together, our findings indicate a function for Otx1 in the regulation of blood cell production.

Otx genes play a relevant role in specification, maintenance and patterning of anterior neuroectoderm. OTX1 and OTX2 proteins share extensive codogenic similarity even though in OTX1 these regions of homology are separated by stretches of amino acid insertions. From 1 to 3 somites stage onwards, Otx1 and Otx2 are largely coexpressed, but only Otx2 is expressed during gastrulation. To determine whether OTX1 and OTX2 gene products share common biochemical properties, mouse models replacing Otx1 with Otx2 and vice versa have been generated. These studies have indicated a remarkable functional equivalence between the two proteins. Nevertheless, it was still debated whether OTX1 is functionally equivalent to OTX2 in early anterior neuroectoderm. To address this issue we generated a new mouse model (hOtx1(2FL)) replacing only the coding sequence and introns of Otx2 with the human Otx1 codogenic sequence. hOtx1(2FL/2FL) and hOtx1(2FL/-) mice were viable, fertile and exhibited an apparently normal behaviour. hOtx1 mRNA was correctly transcribed under the Otx2 transcriptional control and, similarly, the hOTX1 protein was properly distributed and quantitatively very similar if not identical to that of OTX2. Patterning and regionalisation of forebrain and midbrain were unaffected as revealed by the expression of diagnostic genes which are highly sensitive to reduction of OTX proteins, such as Fgf8, Pax2 and Gbx2.

Organizing centers emit signaling molecules that specify different neuronal cell types at precise positions along the anterior-posterior (A-P) and dorsal-ventral (D-V) axes of neural tube during development. Here we report that reduction in Otx proteins near the alar-basal plate boundary (ABB) of murine midbrain resulted in a dorsal shift of Shh expression, and reduction in Otx proteins at the midbrain-hindbrain boundary (MHB) resulted in an anterior expansion of the Fgf8 domain. Our data thus indicate that an Otx dose-dependent repressive effect coordinates proper positioning of Shh and Fgf8 expression. Furthermore, this control is effective for conferring proper cell identity in the floor-plate region of midbrain and does not require an Otx2-specific property. We propose that this mechanism may provide both A-P and D-V positional information to neuronal precursors located within the midbrain.

Otx1 and Otx2, the murine homologs of the Drosophila orthodenticle gene, play a remarkable role in specification and regionalization of forebrain and midbrain. Recently, genetic approaches have indicated that OTD, OTX1 and OTX2 have retained reciprocal functional equivalence in evolution, whereas their regulatory control has been remarkably modified. This suggests that during the evolution of the vertebrate brain, regulatory changes modulating the transcriptional and translational control of pre-existing gene functions might have favored the establishment of new morphogenetic pathways.

We have isolated a paired-type homeobox gene Dmbx1, previously known as Atx (Development 128 (2001) 4789), from chick and mouse. Sequence similarity reveals that this gene is highly related to the Otx genes. Expression of Dmbx1 commences during gastrulation, when transcripts are detected in a crescent around the anterior neural plate. As development progresses, Dmbx1 marks the prospective midbrain and pretectum. Dmbx1 shares its caudal border of expression with Otx2, while expression is sharply delimited rostrally by the synencephalic-parencephalic boundary, later becoming restricted to the posterior synencephalon. At later stages, Dmbx1 is expressed in dynamic domains of the hindbrain and spinal cord. Additional sites of expression comprise stomodeal ectoderm and foregut endoderm, presomitic mesoderm, and the nasal pit., (Copyright 2002 Elsevier Science Ireland Ltd.)

Over the past few years, a huge amount of work has provided mouse mutants for many genes required for regionalization of the developing brain. This remarkable work now offers the opportunity of unmasking new and unexpected gene functions that underlie a complex network of molecular interactions.

Background: Homeobox genes of the orthodenticle (otd)/Otx family have conserved roles in the embryogenesis of head and brain. Gene replacement experiments show that the Drosophila otd gene and orthologous mammalian Otx genes are functionally equivalent, in that overexpression of either gene in null mutants of Drosophila or mouse can restore defects in cephalic and brain development. This suggests that otd and Otx genes control a comparable subset of downstream target genes in either organism. Here we use quantitative transcript imaging to analyze this equivalence of otd and Otx gene action at a genomic level., Results: Oligonucleotide arrays representing 13,400 annotated Drosophila genes were used to study differential gene expression in flies in which either the Drosophila otd gene or the human Otx2 gene was overexpressed. Two hundred and eighty-seven identified transcripts showed highly significant changes in expression levels in response to otd overexpression, and 682 identified transcripts showed highly significant changes in expression levels in response to Otx2 overexpression. Among these, 93 showed differential expression changes following overexpression of either otd or Otx2, and for 90 of these, comparable changes were observed under both experimental conditions. We postulate that these transcripts are common downstream targets of the fly otd gene and the human Otx2 gene in Drosophila., Conclusion: Our experiments indicate that approximately one third of the otd-regulated transcripts also respond to overexpression of the human Otx2 gene in Drosophila. These common otd/Otx2 downstream genes are likely to represent the molecular basis of the functional equivalence of otd and Otx2 gene action in Drosophila.