Genetic Otx2 mis-localization delays critical period plasticity across brain regions.

Accumulation of non-cell autonomous Otx2 homeoprotein in postnatal mouse visual cortex (V1) has been implicated in both the onset and closure of critical period (CP) plasticity. Here, we show that a genetic point mutation in the glycosaminoglycan recognition motif of Otx2 broadly delays the maturation of pivotal parvalbumin-positive (PV+) interneurons not only in V1 but also in the primary auditory (A1) and medial prefrontal cortex (mPFC). Consequently, not only visual, but also auditory plasticity is delayed, including the experience-dependent expansion of tonotopic maps in A1 and the acquisition of acoustic preferences in mPFC, which mitigates anxious behavior. In addition, Otx2 mis-localization leads to dynamic turnover of selected perineuronal net (PNN) components well beyond the normal CP in V1 and mPFC. These findings reveal widespread actions of Otx2 signaling in the postnatal cortex controlling the maturational trajectory across modalities. Disrupted PV+ network function and deficits in PNN integrity are implicated in a variety of psychiatric illnesses, suggesting a potential global role for Otx2 function in establishing mental health.

Increased dopaminergic innervation in the brain of conditional mutant mice overexpressing Otx2: effects on locomotor behavior and seizure susceptibility.

The homeobox-containing transcription factor Otx2 controls the identity, fate and proliferation of mesencephalic dopaminergic (mesDA) neurons. Transgenic mice, in which Otx2 was conditionally overexpressed by a Cre recombinase expressed under the transcriptional control of the Engrailed1 gene (En1(Cre/+); tOtx2(ov/+)), show an increased number of mesDA neurons during development. In adult mice, Otx2 is expressed in a subset of neurons in the ventral tegmental area (VTA) and its overexpression renders mesDA more resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-HCl (MPTP) neurotoxin. Here we further investigated the neurological consequences of the increased number of mesDA neurons in En1(Cre/+); tOtx2(ov/+) adult mice. Immunohistochemistry for the active, glycosylated form of the dopamine transporter (glyco-Dat) showed that En1(Cre/+); tOtx2(ov/+) adult mice display an increased density of mesocortical DAergic fibers, as compared to control animals. Increased glyco-Dat staining was accompanied by a marked hypolocomotion in En1(Cre/+); tOtx2(ov/+) mice, as detected in the open field test. Since conditional knockout mice lacking Otx2 in mesDA precursors (En1(Cre/+); Otx2(floxv/flox) mice) show a marked resistance to kainic acid (KA)-induced seizures, we investigated the behavioral response to KA in En1(Cre/+); tOtx2(ov/+) and control mice. No difference was observed between mutant and control mice, but En1(Cre/+); tOtx2(ov/+) mice showed a markedly different c-fos mRNA induction profile in the cerebral cortex and hippocampus after KA seizures, as compared to controls. Accordingly, an increased density of parvalbumin (PV)-positive inhibitory interneurons was detected in the deep layers of the frontal cortex of naïve En1(Cre/+); tOtx2(ov/+) mice, as compared to controls. These data indicate that Otx2 overexpression results in increased DAergic innervation and PV cell density in the fronto-parietal cortex, with important consequences on spontaneous locomotor activity and seizure-induced gene expression. Our results strengthen the notion that Otx2 mutant mouse models are a powerful genetic tool to unravel the molecular and behavioral consequences of altered development of the DAergic system.

Morphological organization of somatosensory cortex in Otx1(-/-) mice.

Knock-out Otx1 mice show brain hypoplasia, spontaneous epileptic seizures and abnormalities of the dorsal region of the neocortex. We investigated structural alterations in excitatory and inhibitory circuits in somatosensory cortex of Otx1(-/-) mice by immunocytochemistry using light, confocal and electron microscopy. Immunostaining for non-phosphorylated neurofilament SMI311 and subunit 1 of the NMDA receptor - used as markers of pyramidal neurons - showed reduced layer V pyramidal cells and ectopic pyramidal cells in layers II and III of the mutant cortex. Immunostaining for calcium-binding proteins calbindin, calretinin and parvalbumin - markers of non-overlapping types of GABAergic interneurons - showed no differences between wild-type and knock-out cortex for calbindin and calretinin neurons, while parvalbumin neurons were only patchily distributed in Otx1(-/-) cortex. The pattern of positivity of the GABAergic marker glutamic acid decarboxylase in Otx1(-/-) cortex was also altered and similar to that of parvalbumin. GABA transporter 1 immunoreactivity was greater in Otx1(-/-) than wild-type; quantitation of structures immunoreactive for this transporter in layer V showed that they were increased overall in Otx1(-/-) but the density of inhibitory terminals on pyramidal neurons in the same layer labeled with this transporter was similar to that in wild-type mice. No differences in the distribution or intensity of the glial markers GABA transporter 3 or glial fibrillary acidic protein were found. The defects found in the cortical GABAergic system of the Otx1(-/-) mouse can plausibly explain the cortical hyperexcitability that produces seizures in these animals.

Regionalisation of anterior neuroectoderm and its competence in responding to forebrain and midbrain inducing activities depend on mutual antagonism between OTX2 and GBX2.

The anterior neural ridge (ANR), and the isthmic organiser (IsO) represent two signalling centres possessing organising properties necessary for forebrain (ANR) as well as midbrain and rostral hindbrain (IsO) development. An important mediator of ANR and IsO organising property is the signalling molecule FGF8. Previous work has indicated that correct positioning of the IsO and Fgf8 expression in this domain is controlled by the transcription factors Otx2 and Gbx2. In order to provide novel insights into the roles of Otx2 and Gbx2, we have studied mutant embryos carrying different dosages of Otx2, Otx1 and Gbx2. Embryos deficient for both OTX2 and GBX2 proteins (hOtx1(2)/hOtx1(2); Gbx2(-/-)) show abnormal patterning of the anterior neural tissue, which is evident at the presomite-early somite stage prior to the onset of Fgf8 neuroectodermal expression. Indeed, hOtx1(2)/hOtx1(2); Gbx2(-/-) embryos exhibit broad co-expression of early forebrain, midbrain and rostral hindbrain markers such as hOtx1, Gbx2, Pax2, En1 and Wnt1 and subsequently fail to activate forebrain and midbrain-specific gene expression. In this genetic context, Fgf8 is expressed throughout the entire anterior neural plate, thus indicating that its activation is independent of both OTX2 and GBX2 function. Analysis of hOtx1(2)/hOtx1(2); Gbx2(-/-) and Otx1(+/-); Otx2(+/-) mutant embryos also suggests that FGF8 cannot repress Otx2 without the participation of GBX2. Finally, we report that embryos carrying a single strong hypomorphic Otx2 allele (Otx2(lambda)) in an Otx2 and Gbx2 null background (Otx2(lambda)/-; Gbx2(-/-)) recover both the headless phenotype exhibited by Otx2(lambda)/- embryos and forebrain- and midbrain-specific gene expression that is not observed in hOtx1(2)/hOtx1(2); Gbx2(-/-) mutants. Together, these data provide novel genetic evidence indicating that OTX2 and GBX2 are required for proper segregation of early regional identities anterior and posterior to the mid-hindbrain boundary (MHB) and for conferring competence to the anterior neuroectoderm in responding to forebrain-, midbrain- and rostral hindbrain-inducing activities.

OTD/OTX2 functional equivalence depends on 5' and 3' UTR-mediated control of Otx2 mRNA for nucleo-cytoplasmic export and epiblast-restricted translation.

How gene activity is translated into phenotype and how it can modify morphogenetic pathways is of central importance when studying the evolution of regulatory control mechanisms. Previous studies in mouse have suggested that, despite the homeodomain-restricted homology, Drosophila orthodenticle (otd) and murine Otx1 genes share functional equivalence and that translation of Otx2 mRNA in epiblast and neuroectoderm might require a cell type-specific post-transcriptional control depending on its 5' and 3' untranslated sequences (UTRs). In order to study whether OTD is functionally equivalent to OTX2 and whether synthesis of OTD in epiblast is molecularly dependent on the post-transcriptional control of Otx2 mRNA, we generated a first mouse model (otd(2)) in which an Otx2 region including 213 bp of the 5' UTR, exons, introns and the 3' UTR was replaced by an otd cDNA and a second mutant (otd(2FL)) replacing only exons and introns of Otx2 with the otd coding sequence fused to intact 5' and 3' UTRs of Otx2. otd(2) and otd(2FL) mRNAs were properly transcribed under the Otx2 transcriptional control, but mRNA translation in epiblast and neuroectoderm occurred only in otd(2FL) mutants. Phenotypic analysis revealed that visceral endoderm (VE)-restricted translation of otd(2) mRNA was sufficient to rescue Otx2 requirement for early anterior patterning and proper gastrulation but it failed to maintain forebrain and midbrain identity. Importantly, epiblast and neuroectoderm translation of otd(2FL) mRNA rescued maintenance of anterior patterning as it did in a third mouse model replacing, as in otd(2FL), exons and introns of Otx2 with an Otx2 cDNA (Otx2(2c)). The molecular analysis has revealed that Otx2 5' and 3' UTR sequences, deleted in the otd(2) mRNA, are required for nucleo-cytoplasmic export and epiblast-restricted translation. Indeed, these molecular impairments were completely rescued in otd(2FL) and Otx2(2c) mutants. These data provide novel in vivo evidence supporting the concept that during evolution pre-existing gene functions have been recruited into new developmental pathways by modifying their regulatory control.

Potentially epileptogenic dysfunction of cortical NMDA- and GABA-mediated neurotransmission in Otx1-/- mice.

Knockout Otx1 mice present a microcephalic phenotype mainly due to reduced deep neocortical layers and spontaneous recurrent seizures. We investigated the excitable properties of layer V pyramidal neurons in neocortical slices prepared from Otx1-/- mice and age-matched controls. The qualitative firing properties of the neurons of Otx1-/- mice were identical to those found in wild-type controls, but the proportion of intrinsically bursting (IB) neurons was significantly smaller. This is in line with the lack of the Otx1 gene contribution to the generation and differentiation of neurons destined for the deep neocortical layers, in which IB neurons are located selectively in wild-type rodents. The pyramidal neurons recorded in Otx1-/- mice responded to near-threshold electrical stimulation of the underlying white matter, with aberrant polysynaptic excitatory potentials often leading to late action potential generation. When the strength of the stimulus was increased, the great majority of the Otx1-/- neurons (78%) responded with a prominent biphasic inhibitory postsynaptic potential that was significantly larger than that observed in the wild-type mice, and was often followed by complex postinhibitory depolarizing events. Both late excitatory postsynaptic potentials and postinhibitory excitation were selectively suppressed by NMDA receptor antagonists, but not by AMPA antagonists. We conclude that the cortical abnormalities of Otx1-/- neocortex due to a selective loss of large projecting neurons lead to a complex rearrangement of local circuitry, which is characterized by an excess of N-methyl-d-aspartate-mediated polysynaptic excitation that is counteracted by GABA-mediated inhibition in only a limited range of stimulus intensity. Prominent postsynaptic inhibitory potentials may also act as a further pro-epileptogenic event by synchronizing abnormal excitatory potentials.

Forebrain and midbrain development requires epiblast-restricted Otx2 translational control mediated by its 3' UTR.

Otx genes play an important role in brain development. Previous mouse models suggested that the untranslated regions (UTRs) of Otx2 mRNA may contain regulatory element(s) required for its post-transcriptional control in epiblast and neuroectoderm. In order to study this, we have perturbed the 3' UTR of Otx2 by inserting a small fragment of DNA from the lambda phage. Otx2(lambda) mutants exhibited proper gastrulation and normal patterning of the early anterior neural plate, but from 8.5 days post coitum they developed severe forebrain and midbrain abnormalities. OTX2 protein levels in Otx2(lambda) mutants were heavily reduced in the epiblast, axial mesendoderm and anterior neuroectoderm but not in the visceral endoderm. At the molecular level, we found out that the ability of the Otx2(lambda) mRNA to form efficient polyribosome complexes was impaired. Sequence analysis of the Otx2-3' UTR revealed a 140 bp long element that is present only in vertebrate Otx2 genes and conserved in identity by over 80%. Our data provide experimental evidence that murine brain development requires accurate translational control of Otx2 mRNA in epiblast and neuronal progenitor cells. This leads us to hypothesise that this control might have important evolutionary implications.

Otx genes in evolution: are they involved in instructing the vertebrate brain morphology?

Previous mouse models have indicated that Otx1 and Otx2 play an important role in brain and sense organ development and, together with the Drosophila orthodenticle (otd) gene, they share a high degree of reciprocal functional equivalence. Interestingly, mouse models replacing the same region of the Otx2 locus with Otx1, otd or lacZ genes have revealed the existence of a differential post-transcriptional control between the visceral endoderm (VE) and epiblast cells. Indeed Otx1, otd or lacZ mRNA were transcribed in both tissues but translated only in the VE. Embryos lacking OTX1 or OTD proteins in the epiblast and derived tissues, such as the neuroectoderm and axial mesendoderm (AME), fail to maintain the anterior identity and result in a headless phenotype. This finding leads us to hypothesise that, during evolution, the specification of the vertebrate-type brain may have required epiblast cells to translate Otx2 mRNA in order to establish maintenance properties. The establishment of this regulatory control might have been reflected into a remarkable reorganisation of the rostral CNS architecture and might have represented an important event in the evolution of the vertebrate head. Current data suggest that the Otx2 replaced region and in particular the 3' untranslated region (UTR), may contain regulatory element(s) necessary to translate and/or stabilise Otx2 mRNA in epiblast and its derivatives.

Otx genes are required for tissue specification in the developing eye.

Patterning of the vertebrate eye appears to be controlled by the mutual regulation and the progressive restriction of the expression domains of a number of genes initially co-expressed within the eye anlage. Previous data suggest that both Otx1 and Otx2 might contribute to the establishment of the different eye territories. Here, we have analysed the ocular phenotype of mice carrying different functional copies of Otx1 and Otx2 and we show that these genes are required in a dose-dependent manner for the normal development of the eye. Thus, all Otx1(-/-); Otx2(+/-) and 30% of Otx1(+/-); Otx2(+/-) genotypes presented consistent and profound ocular malformation, including lens, pigment epithelium, neural retina and optic stalk defects. During embryonic development, optic vesicle infolding was severely altered and the expression of pigment epithelium-specific genes, such as Mitf or tyrosinase, was lost. Lack of pigment epithelium specification was associated with an expansion of the prospective neural retina and optic stalk territories, as determined by the expression of Pax6, Six3 and Pax2. Later in development the presumptive pigment epithelium region acquired features of mature neural retina, including the generation of Islet1-positive neurones. Furthermore, in Otx1(-/-); Otx2(+/-) mice neural retina cell proliferation, cell differentiation and apoptotic cell death were also severely affected. Based on these findings we propose a model in which Otx gene products are required for the determination and differentiation of the pigment epithelium, co-operating with other eye patterning genes in the determination of the specialised tissues that will constitute the mature vertebrate eye.

Multicomponent targeted intervention to prevent delirium in hospitalized older patients: what is the economic value?

INTRODUCTION: Delirium, or acute confusional state, is a common and serious occurrence among hospitalized older persons. Current estimates suggest that delirium complicates hospital stays for more than 2.3 million older persons each year, involving more than 17.5 million hospital days and accounting for more than $4 billion (1994 dollars) of Medicare expenditures. A 40% reduction was recently reported in the risk for delirium among hospitalized older persons receiving a multicomponent targeted risk factor intervention (MTI) strategy to prevent delirium, compared with subjects receiving usual hospital care.1 Before recommending that this preventive strategy be implemented in clinical practice, however, the cost implications must be thoroughly examined as well. METHODS: The present analysis performs net cost evaluations of the MTI for the prevention of delirium among hospitalized patients. Hospital charge and cost-to-charge ratio data are linked to a database of 852 subjects, who were treated with MTI or usual care. Multivariable regression methods were used to help isolate the impact of MTI on hospital costs. These results were then combined with our earlier work on the impact of the MTI on delirium prevention to assess the cost effectiveness of this intervention. RESULTS: The MTI significantly reduced nonintervention costs among subjects at intermediate risk for developing delirium, but not among subjects at high risk. When MTI intervention costs were included, MTI had no significant effect on overall health care costs in the intermediate risk cohort, but raised overall costs in the high risk group. CONCLUSIONS: Because the MTI prevented delirium in the intermediate risk group without raising costs, the conclusion reached is that it is a cost effective treatment option for patients at intermediate risk for developing delirium. In contrast, the results suggest that the MTI is not cost effective for subjects at high risk.

Otx genes in the development and evolution of the vertebrate brain.

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the orthologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group, including the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, is mostly involved in fundamental processes of anterior neural patterning. In mouse, Drosophila and intermediate species otd/Otx genes have shown a remarkable similarity in expression pattern suggesting that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. In order to verify this hypothesis, a series of mouse models have been generated in which the functions of the murine Otx genes were: (i) fully inactivated, (ii) replaced with each other, and (iii) replaced with the Drosophila otd gene. The data obtained highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system and lead to hypothesize that modification of their regulatory control may have influenced the morphogenesis and evolution of the brain.

The role of Otx2 in organizing the anterior patterning in mouse.

Understanding the molecular mechanism controlling induction and maintenance of signals required for specifying anterior territory (forebrain and midbrain) of the central nervous system is a major task of molecular embryology. The current view indicates that in mouse, early specification of the anterior patterning is established at the beginning of gastrulation by the anterior visceral endoderm, while maintenance and refinement of the early specification is under the control of epiblast-derived tissues corresponding to the axial mesendoderm and rostral neuroectoderm. In vertebrates a remarkable amount of data has been collected on the role of genes contributing to brain morphogenesis. Among these genes,the orthodenticle group is defined bythe Drosophila orthodenticle and the vertebrate Otx1 and Otx2 genes, which contain a bicoid-like homeodomain. Mouse models and chimera experiments have provided strong evidence that Otx2 plays an important role in the specification and maintenance of the rostral neuroectoderm destined to become forebrain and midbrain. In evolutionary terms, some of these findings lead us to hypothesize a fascinating and crucial contribution of the Otx genes to the genetic program underlying the establishment of the mammalian brain.

Otx genes in brain morphogenesis.

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the homologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group includes the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, which are mostly involved in fundamental processes of anterior neural patterning. These genes encode transcription factors that recognise specific target sequences through the DNA binding properties of the homeodomain. In Drosophila, mutations of otd cause the loss of the anteriormost head neuromere where the gene is transcribed, suggesting that it may act as a segmentation "gap" gene. In mouse embryos, the expression patterns of Otx1 and Otx2 have shown a remarkable similarity with the Drosophila counterpart. This suggested that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. To verify this hypothesis a series of mouse models have been generated in which the functions of the murine genes were: (i) fully inactivated, (ii) replaced with each others, (iii) replaced with the Drosophila otd gene. Otx1-/- mutants suffer from epilepsy and are affected by neurological, hormonal, and sense organ defects. Otx2-/- mice are embryonically lethal, they show gastrulation impairments and fail in specifying anterior neural plate. Analysis of the Otx1-/-; Otx2+/- double mutants has shown that a minimal threshold level of the proteins they encode is required for the correct positioning of the midbrain-hindbrain boundary (MHB). In vivo otd/Otx reciprocal gene replacement experiments have provided evidence of a general functional equivalence among otd, Otx1 and Otx2 in fly and mouse. Altogether these data highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system (CNS) and lead to hypothesize that modification of their regulatory control may have influenced morphogenesis and evolution of the brain.

The role of Otx and Otp genes in brain development.

Over the last ten years, many genes involved in the induction, specification and regionalization of the brain have been identified and characterized at the functional level through a series of animal models. Among these genes, both Otx1 and Otx2, two murine homologues of the Drosophila orthodenticle (otd) gene which encode transcription factors, play a pivotal role in the morphogenesis of the rostral brain. Classical knock-out studies have revealed that Otx2 is fundamental for the early specification and subsequent maintenance of the anterior neural plate, whereas Otx1 is mainly necessary for both normal corticogenesis and sense organ development. A minimal threshold of both gene products is required for correct patterning of the fore-midbrain and positioning of the isthmic organizer. A third gene, Orthopedia (Otp) is a key element of the genetic pathway controlling development of the neuroendocrine hypothalamus. This review deals with a comprehensive analysis of the Otx1, Otx2 and Otp functions, and with the possible evolutionary implications suggested by the models in which the Otx genes are reciprocally replaced or substituted by the Drosophila homologue, otd.

Patterning of the mammalian cochlea.

The mammalian cochlea is sophisticated in its function and highly organized in its structure. Although the anatomy of this sense organ has been well documented, the molecular mechanisms underlying its development have remained elusive. Information generated from mutant and knockout mice in recent years has increased our understanding of cochlear development and physiology. This article discusses factors important for the development of the inner ear and summarizes cochlear phenotypes of mutant and knockout mice, particularly Otx and Otx2. We also present data on gross development of the mouse cochlea.

Synaptic properties of neocortical neurons in epileptic mice lacking the Otx1 gene.

PURPOSE: The murine homeobox-containing Otx gene is required for correct nervous system and sense organ development. Otx1-/1 mice obtained by replacing Otx with the lac Z gene show developmental abnormalities of the cerebellum, mesencephalon, and cerebral cortex associated with spontaneous epileptic seizures (1). The epileptogenic mechanisms accounting for these seizures were investigated by means of electrophysiological recordings made from neocortical slices. METHODS: The 400-microm slices were prepared from the somatosensory cortex of Otx1-/- and Otx1+/+ mice, and the current clamp intracellular recordings were obtained from layer V pyramidal neurons by means of pipettes containing K+ acetate 1.5 mol/L and biocytin 2% (pH 7.3). RESULTS: Synaptic responses could be evoked in the neocortical pyramidal neurons by electrically stimulating the underlying white matter. gamma-Aminobutyric acid A/B-mediated inhibitory postsynaptic potentials were more pronounced in the Otx1-/- than in the control pyramidal neurons from the earliest postnatal period; multisynaptic excitatory postsynaptic potentials were significantly more expressed in the Otx1-/- mice also at the end of the first postnatal month, when they were only rarely encountered in controls. CONCLUSION: Excessive excitatory amino acid-mediated synaptic driving may lead to a hyperexcitable condition that is responsible for the epileptic manifestations occurring in Otx1-/- mice. This excess of excitation is not counteracted by well-developed gamma-aminobutyric acid activity, which seems to be involved in the synchronization of cell discharges. Our ongoing and more extensive comparative analysis of the mutants and controls should help to clarify the way in which the putative rearrangement taking place in Otx1-/- neocortex may lead to the excitatory hyperinnervation of layer V pyramidal neurons.

Genetic and molecular roles of Otx homeodomain proteins in head development.

Insights into the molecular mechanisms underlying neural development in vertebrates come from the cloning and the functional analysis of genes which are involved in the molecular pathways leading to neural induction, tissue specification and regionalization of the brain. Among them, transcription factors belonging to the orthodenticle family (Otx1, Otx2) play an important role during early and later events required for proper brain development. To better understand their functions, several mouse mutants have been generated by homologous recombination. Their analysis clearly indicates that Otx1 is involved in corticogenesis, sense organ development and pituitary functions, while Otx2 is necessary earlier in development, for the correct anterior neural plate specification and organisation of the primitive streak. A molecular mechanism depending on a precise threshold of OTX proteins is necessary for the correct positioning of the isthmic region and for anterior brain patterning. Finally, vertebrate Otx genes share functional equivalence with the Drosophila homologue otd, indicating that the genetic mechanisms underlying pattern formation in insect and mammalian brain development are evolutionarily conserved.

Progressive impairment of developing neuroendocrine cell lineages in the hypothalamus of mice lacking the Orthopedia gene.

Development of the neuroendocrine hypothalamus is characterized by a precise series of morphogenetic milestones culminating in terminal differentiation of neurosecretory cell lineages. The homeobox-containing gene Orthopedia (Otp) is expressed in neurons giving rise to the paraventricular (PVN), supraoptic (SON), anterior periventricular (aPV), and arcuate (ARN) nuclei throughout their development. Homozygous Otp(-/-) mice die soon after birth and display progressive impairment of crucial neuroendocrine developmental events such as reduced cell proliferation, abnormal cell migration, and failure in terminal differentiation of the parvocellular and magnocellular neurons of the aPV, PVN, SON, and ARN. Moreover, our data provide evidence that Otp and Sim1, a bHLH-PAS transcription factor that directs terminal differentiation of the PVN, SON, and aPV, act in parallel and are both required to maintain Brn2 expression which, in turn, is required for neuronal cell lineages secreting oxytocin (OT), arginine vasopressin (AVP), and corticotropin-releasing hormone (CRH).

Otx genes in corticogenesis and brain development.

A large number of genes have been isolated in the last few years that seem to be involved in the processes of induction, specification and regionalization of the brain. The generation of mouse mutant models for such genes has provided a powerful tool of analysis into their in-vivo properties. Among these genes, Otx1 and Otx2 play a crucial role in several processes of brain morphogenesis. Otx2 is required in the early specification of the neuroectoderm destined to become the fore-midbrain, Otx1 is required mainly for correct corticogenesis, and both genes co-operate in patterning the developing brain through a gene-dosage dependent mechanism. Furthermore, an extensive functional equivalence occurs between the mammalian Otx genes and their Drosophila homolog, orthodenticle. This appears particularly fascinating when considering the huge differences between the brains of mice and flies. This review deals with the major aspects related to the involvement of Otx1 and Otx2 in the development of the mammalian brain and, in particular, of the cerebral cortex.