Otx2 regulates subtype specification and neurogenesis in the midbrain

Temporal and spatial delineation of mouse Otx2 functions by conditional self-knockout

To identify the independent spatial and temporal activities of the essential developmental gene the Otx2, the germline mutation of which is lethal at embryonic day 8.5, we floxed one allele and substituted the other with an inducible CreER recombinase gene. This makes 'trans' self-knockout possible at any developmental stage. The transient action of tamoxifen pulses allows time-course mutation. We demonstrate efficient temporal knockout and demarcate spatio-temporal windows in which Otx2 controls the head, brain structures and body development.

Genetic networks controlling the development of midbrain dopaminergic neurons

Recent data have substantially advanced our understanding of midbrain dopaminergic neuron development. Firstly, a Wnt1-regulated genetic network, including Otx2 and Nkx2-2, and a Shh-controlled genetic cascade, including Lmx1a, Msx1 and Nkx6-1, have been unravelled, acting in parallel or sequentially to establish a territory competent for midbrain dopaminergic precursor production at relatively early stages of neural development. Secondly, the same factors (Wnt1 and Lmx1a/Msx1) appear to regulate midbrain dopaminergic and/or neuronal fate specification in the postmitotic progeny of these precursors by controlling the expression of midbrain dopaminergic-specific and/or general proneural factors at later stages of neural development. For the first time, early inductive events have thus been linked to later differentiation processes in midbrain dopaminergic neuron development. Given the pivotal importance of this neuronal population for normal function of the human brain and its involvement in severe neurological and psychiatric disorders such as Parkinson's Disease, these advances open new prospects for potential stem cell-based therapies. We will summarize these new findings in the overall context of midbrain dopaminergic neuron development in this review.

Persistent and high levels of Hes1 expression regulate boundary formation in the developing central nervous system

The developing central nervous system is partitioned into compartments by boundary cells, which have different properties than compartment cells, such as forming neuron-free zones, proliferating more slowly and acting as organizing centers. We now report that in mice the bHLH factor Hes1 is persistently expressed at high levels by boundary cells but at variable levels by non-boundary cells. Expression levels of Hes1 display an inverse correlation to those of the proneural bHLH factor Mash1, suggesting that downregulation of Hes1 leads to upregulation of Mash1 in non-boundary regions,whereas persistent and high Hes1 expression constitutively represses Mash1 in boundary regions. In agreement with this notion, in the absence of Hes1 and its related genes Hes3 and Hes5, proneural bHLH genes are ectopically expressed in boundaries, resulting in ectopic neurogenesis and disruption of the organizing centers. Conversely, persistent Hes1 expression in neural progenitors prepared from compartment regions blocks neurogenesis and reduces cell proliferation rates. These results indicate that the mode of Hes1 expression is different between boundary and non-boundary cells, and that persistent and high levels of Hes1 expression constitutively repress proneural bHLH gene expression and reduce cell proliferation rates,thereby forming boundaries that act as the organizing centers.

Strategies to unravel molecular codes essential for the development of meso-diencephalic dopaminergic neurons

Understanding the development of neuronal systems has become an important asset in the attempt to solve complex questions about neuropathology as found in Parkinson's disease, schizophrenia and other complex neuronal diseases. The development of anatomical and functional divergent structures in the brain is achieved by a combination of early anatomical patterning and highly coordinated neuronal migration and differentiation events. Fundamental to the existence of divergent structures in the brain is the early region-specific molecular programming. Neuronal progenitors located along the neural tube can still adapt many different identities. Their exact position in the developing brain, however, determines early molecular specification by region-specific signalling molecules. These signals determine time and region-specific expression of early regulatory genes, leading to neuronal differentiation. Here, we focus on a well-described neuronal group, the meso-diencephalic dopaminergic neurons, of which heterogeneity based on anatomical position could account for the difference in vulnerability of specific subgroups as observed in Parkinson's disease. The knowledge of their molecular coding helps us to understand how the meso-diencephalic dopaminergic system is built and could provide clues that unravel mechanisms associated with the neuropathology in complex diseases such as Parkinson's disease.

Otx2 controls identity and fate of glutamatergic progenitors of the thalamus by repressing GABAergic differentiation

GABAergic and glutamatergic neurons modulate inhibitory and excitatory networks in the CNS, and their impairment may cause neurological and psychiatric disorders. Thus, understanding the molecular mechanisms that control neurotransmitter phenotype and identity of excitatory and inhibitory progenitors has considerable relevance. Here we investigated the consequence of Otx2 (orthodenticle homolog) ablation in glutamatergic progenitors of the dorsal thalamus (referred to as thalamus). We report that Otx2 is cell-autonomously required in these progenitors to repress GABAergic differentiation. Our data indicate that Otx2 may prevent GABAergic fate switch by repressing the basic helix-loop-helix gene Mash1 (mammalian achaete-schute homolog) in progenitors expressing Ngn2 (neurogenin homolog). The lack of Otx2 also resulted in the activation of Pax3 (paired box gene), Pax7, and Lim1 (Lin-11/Isl-1/Mec-3), three genes normally coexpressed with Mash1 and GABAergic markers in the pretectum, thus suggesting that thalamic progenitors lacking Otx2 exhibit marker similarities with those of the pretectum. Furthermore, Otx2 ablation gave rise to a marked increase in proliferating activity of thalamic progenitors and the formation of hyperplastic cell masses. Thus, this study provides evidence for a novel and crucial role of Otx2 in the molecular mechanism by which identity and fate of glutamatergic precursors are established in the thalamus. Our data also support the concept that proper assignment of identity and fate of neuronal precursors occurs through the suppression of alternative differentiation programs.

A Wnt1-regulated genetic network controls the identity and fate of midbrain-dopaminergic progenitors in vivo

Midbrain neurons synthesizing the neurotransmitter dopamine play a central role in the modulation of different brain functions and are associated with major neurological and psychiatric disorders. Despite the importance of these cells, the molecular mechanisms controlling their development are still poorly understood. The secreted glycoprotein Wnt1 is expressed in close vicinity to developing midbrain dopaminergic neurons. Here, we show that Wnt1 regulates the genetic network, including Otx2 and Nkx2-2, that is required for the establishment of the midbrain dopaminergic progenitor domain during embryonic development. In addition, Wnt1 is required for the terminal differentiation of midbrain dopaminergic neurons at later stages of embryogenesis. These results identify Wnt1 as a key molecule in the development of midbrain dopaminergic neurons in vivo. They also suggest the Wnt1-controlled signaling pathway as a promising target for new therapeutic strategies in the treatment of Parkinson's disease.

Expression of the Otx2 homeobox gene in the developing mammalian brain: embryonic and adult expression in the pineal gland

Otx2 is a vertebrate homeobox gene, which has been found to be essential for the development of rostral brain regions and appears to play a role in the development of retinal photoreceptor cells and pinealocytes. In this study, the temporal expression pattern of Otx2 was revealed in the rat brain, with special emphasis on the pineal gland throughout late embryonic and postnatal stages. Widespread high expression of Otx2 in the embryonic brain becomes progressively restricted in the adult to the pineal gland. Crx (cone-rod homeobox), a downstream target gene of Otx2, showed a pineal expression pattern similar to that of Otx2, although there was a distinct lag in time of onset. Otx2 protein was identified in pineal extracts and found to be localized in pinealocytes. Total pineal Otx2 mRNA did not show day-night variation, nor was it influenced by removal of the sympathetic input, indicating that the level of Otx2 mRNA appears to be independent of the photoneural input to the gland. Our results are consistent with the view that pineal expression of Otx2 is required for development and we hypothesize that it plays a role in the adult in controlling the expression of the cluster of genes associated with phototransduction and melatonin synthesis.

Ventral midbrain glia express region-specific transcription factors and regulate dopaminergic neurogenesis through Wnt-5a secretion

Glial cells have been classically described as supporting cells for neurons. Recently, additional roles during neural development have begun to emerge. Here, we report that ventral midbrain glia, including astrocytes and radial glia, are the source of signals required by neural precursors to acquire a dopaminergic phenotype. We found that ventral midbrain glia, but not cortical glia, secrete high levels of the glycolipoprotein Wnt-5a, express region-specific transcription factors such as Pax-2, En-1 and Otx-2 and increase the differentiation of cortical or ventral midbrain Nurr1 precursors into tyrosine hydroxylase-positive neurons. Moreover, blocking experiments using a Wnt-5a blocking antibody indicated that the effects of ventral midbrain glia on Nurr1-positive neural precursors are partially mediated by Wnt-5a. Thus, our results identify Wnt-5a as an important component of the dopaminergic inductive activity of the ventral midbrain glia.

Developmental origin and fate of meso-diencephalic dopamine neurons

Specific vulnerability of substantia nigra compacta neurons as compared to ventral tegmental area neurons, as emphasized in Parkinson's disease, has been studied for many years and is still not well understood. The molecular codes and mechanisms that drive development of these structures have recently been studied through the use of elegant genetic ablation experiments. The data suggested that specific genes at specific anatomical positions in the ventricular zone are crucial to drive development of young neurons into the direction of the dopaminergic phenotype. In addition, it has become clear the these dopaminergic neurons are present in the diencephalon and in the mesencephalon and that they may contain a specific molecular signature that defines specific subsets in terms of position and function. The data indicate that these specific subsets may explain the specific response of these neurons to toxins and genetic ablation.

Temporal regulation of Cre recombinase activity in neural stem cells

Neural stem cells are known to give rise to distinct subtypes of neurons and glial cells over time by changing their competency. However, precise characterization of neural stem cells at various developmental stages remains to be performed. For such analysis, a tool to manipulate neural stem cells at different time points is necessary. Here, we generated transgenic mice that express Cre-ER(T2) in the ventricular zone of the developing nervous system under the control of the nestin promoter and enhancer (Nes-CreER(T2)). In mice expressing Cre-ER(T2) at appropriate levels, Cre recombinase activity was mostly inactive but efficiently activated by tamoxifen within 1 day. When such mice were crossed with the ROSA-26 or Z/EG reporter mice, neural stem cells were permanently labeled after administration of tamoxifen. Thus, Nes-CreER(T2) mice offer a powerful tool to manipulate neural stem cells genetically at desired time points.