Growth-associated protein-43 is required for commissural axon guidance in the developing vertebrate nervous system

Growth-associated protein-43 (GAP-43) is a major growth cone protein whose phosphorylation by PKC in response to extracellular guidance cues can regulate F-actin behavior. Here we show that 100% of homozygote GAP-43 (-/-) mice failed to form the anterior commissure (AC), hippocampal commissure (HC), and corpus callosum (CC) in vivo. Instead, although midline fusion was normal, selective fasciculation between commissural axons was inhibited, and TAG-1-labeled axons tangled bilaterally into Probst's bundles. Moreover, their growth cones had significantly smaller lamellas and reduced levels of F-actin in vitro. Likewise, 100% of GAP-43 (+/-) mice with one disrupted allele also showed defects in HC and CC, whereas the AC was unaffected. Individual GAP-43 (+/-) mice could be assigned to two groups based on the amount that PKC phosphorylation of GAP-43 was reduced in neocortical neurons. In mice with approximately 1% phosphorylation, the HC and CC were absent, whereas in mice with approximately 10% phosphorylation, the HC and CC were smaller. Both results suggest that PKC-mediated signaling in commissural axons may be defective. However, although Probst's bundles formed consistently at the location of the glial wedge, both GAP-43 (-/-) and GAP-43 (+/+) cortical axons were still repulsed by Slit-2 in vitro, precluding failure of this deflective signal from the glial wedge as the source of the phenotype. Nonetheless, the data show that a functional threshold of GAP-43 is required for commissure formation and suggests that failure to regulate F-actin in commissural growth cones may be related to inhibited PKC phosphorylation of GAP-43.

Structural brain abnormalities in adult males with clefts of the lip and/or palate

PURPOSE

To evaluate brain morphology of adult males with nonsyndromic clefts of the lip and/or palate (NSCLP) in comparison to a matched healthy control group.

METHODS

Brain structure was measured using quantitative analysis of magnetic resonance images.

RESULTS

Subjects with NSCLP had significant abnormalities in brain morphology consisting of abnormally enlarged anterior regions of the cerebrum, and decreased volumes of the posterior cerebrum and cerebellum. Overall, the most severely affected region was the left temporal lobe. Furthermore, these structural abnormalities were directly related to cognitive dysfunction.

CONCLUSIONS

These findings highlight the important relationship and interplay between face and brain development.

Hydranencephaly in an infant with vascular malformations

Hydranencephaly is a condition in which cerebral hemispheres are absent and reduced to fluid-filled sacs in a normal skull. Numerous causes have been proposed. We report a male infant with hydranencephaly and congenital vascular malformations (port wine stains, generalized nevus flammeus, anomalous retinal vessels, and absent internal carotid flow). Magnetic resonance imaging of the brain showed absence of most of the cerebrum except for small portions of the occipital cortex and thalami. Magnetic resonance angiography showed flow within the vertebral and basilar arteries without internal carotid intracranial flow above the internal carotid petrous and cavernous portion. This is a report of cutaneous and retinal malformations associated with hydranencephaly. Vascular malformations of larger vessels (e.g., webbing of the carotid arteries and an absent internal carotid arterial system) have been observed in other infants with hydranencephaly, and are proposed to lead to brain destruction. The case reported herein supports the role of primary vascular malformations in the development of some cases of hydranencephaly.

PAX6 haploinsufficiency causes cerebral malformation and olfactory dysfunction in humans

PAX6 is widely expressed in the central nervous system. Heterozygous PAX6 mutations in human aniridia cause defects that would seem to be confined to the eye. Magnetic resonance imaging (MRI) and smell testing reveal the absence or hypoplasia of the anterior commissure and reduced olfaction in a large proportion of aniridia cases, which shows that PAX6 haploinsuffiency causes more widespread human neuro developmental anomalies.

Oculo-palatal-cerebral syndrome: a second case

Oculo-palato-cerebral syndrome is an extremely rare disorder consisting of low birth weight, microcephaly, short stature, persistent hyperplastic primary vitreous, microphthalmia, large ears, small hands and feet, cleft palate, joint hypermobility, developmental delay, and cerebral atrophy. There has been one report of a consanguineous family with three affected children, suggesting autosomal recessive inheritance. We report on the second case of this disorder. Our patient, a 2-year-old boy, had growth delay, microcephaly, bilateral persistent hyperplastic primary vitreous with right microphthalmia, long ears with thickened helices, small hands and feet, highly arched palate, joint hypermobility, hypoplastic nails, frontal cerebral atrophy and thinning of the corpus callosum on brain magnetic resonance imaging, and mild developmental delay. He has much milder features than those seen in the previously reported cases.

[Development and the developmental disorders of human brain. I. Early development of the cerebrum]

The recent discovery of many genes that regulate brain development is revolutionizing our knowledge of neuroembryology and, moreover, our understanding of how gene defects cause human birth defects. The first 8 weeks of the development of the cerebrum can be subdivided into 23 stages, with early development of mostly the spinal cord and the brain stem. Regionalization of the brain has been related to genes that play a part in it. A characteristic developmental disorder for this early phase in the development of the forebrain is holoprosencephaly, a brain patterning disorder. Numerous genes play a part in its occurrence; abnormal function of signal factors as well as of transcription factors may lead to holoprosencephaly.

Expression of leukemia inhibitory factor in the ganglionic eminence of the human fetal brain after bleedings

Bleedings in the ganglionic eminence (GE) being a prominent domain of the telencephalic proliferative zone is a frequent complication of preterm infants. Such bleedings may induce cellular responses in the vicinity of the lesion. Using immunohistochemistry, this study demonstrates for the first time that leukemia inhibitory factor (LIF) belonging to the family of neuropoetic cytokines is expressed in GE cells up to 200 microm from the lesion (six brains, gestational age 22-26 weeks); whereas control brains do not exhibit any LIF-immunoreactive cells in the GE. The expression of LIF in cases with bleedings may interfere with normal developmental processes taking place within the ganglionic eminence.

Developmental toxicity of the topoisomerase inhibitor, etoposide, in rabbits after intravenous administration

The developmental effect of the topoisomerase inhibitor, etoposide, was investigated in pregnant rabbits given intravenous doses during early organogenesis. Does received 0, 0.25, 0.5, 1, or 2 mg/kg/day on days 7 through 9 of gestation. Fetal parameters were evaluated on day 28 of gestation. Live fetuses were examined for gross, visceral, and skeletal malformations and variations. In addition, telencephalon in embryos 8 h following the final treatment was examined histologically. No change in general condition was observed in any does, but a significant decrease in body weight gain during the pregnancy and enlargement of the liver resulting from marked fatty change were observed in does treated with etoposide at 2 mg/kg/day. Etoposide had neither lethal nor growth retarded effects on embryos/fetuses. However, axial skeletal malformation and extra ribs had a low incidence but were significant in the group treated with etoposide at 2 mg/kg/day, whereas no significant increases in external malformations in term fetuses nor in pyknotic cells in the ventricular zone of telencephalon in embryos were noticed in any etoposide-treated groups. It was concluded that anatomical defects (skeletal malformation or variation) in rabbits were induced by intravenous etoposide treatment during early organogenesis and that they occurred in the presence of maternal toxicity.

Gli3 is required for Emx gene expression during dorsal telencephalon development

Dentate gyrus and hippocampus as centers for spatial learning, memory and emotional behaviour have been the focus of much interest in recent years. The molecular information on its development, however, has been relatively poor. To date, only Emx genes were known to be required for dorsal telencephalon development. Here, we report on forebrain development in the extra toes (Xt(J)) mouse mutant which carries a null mutation of the Gli3 gene. This defect leads to a failure to establish the dorsal di-telencephalic junction and finally results in a severe size reduction of the neocortex. In addition, Xt(J)/Xt(J) mice show absence of the hippocampus (Ammon's horn plus dentate gyrus) and the choroid plexus in the lateral ventricle. The medial wall of the telencephalon, which gives rise to these structures, fails to invaginate during embryonic development. On a molecular level, disruption of dorsal telencephalon development in Xt(J)/Xt(J) embryos correlates with a loss of Emx1 and Emx2 expression. Furthermore, the expression of Fgf8 and Bmp4 in the dorsal midline of the telencephalon is altered. However, expression of Shh, which is negatively regulated by Gli3 in the spinal cord, is not affected in the Xt(J)/Xt(J) forebrain. This study therefore implicates Gli3 as a key regulator for the development of the dorsal telencephalon and implies Gli3 to be upstream of Emx genes in a genetic cascade controlling dorsal telencephalic development.

Knockout of REST/NRSF shows that the protein is a potent repressor of neuronally expressed genes in non-neural tissues

The protein repressor element 1 silencing transcription factor/neuron restrictive silencer factor (REST/NRSF) is a negative regulator of neuronal genes that contain a particular DNA sequence, the neuron restrictive silencer element (NRSE). REST is expressed ubiquitously in non-neural tissues but is down-regulated in neural precursors and turned off in postmitotic neurons, suggesting that it can act both to prevent extraneural expression of certain genes and to delay the differentiation of neuronal subtypes. In a recent paper, Chen et al.(1) describe the production of a null mutant for REST in mice and the mosaic inactivation of REST function in chicken embryos. Knockout of REST led to malformations in several non-neural tissues, as well as apoptosis and embryonic lethality in mice. In addition, the expression of several REST target genes was derepressed in non-neural tissues and in neural progenitors in both mouse and chicken embryos. These studies clearly demonstrate that active repression of tissue-specific genes is required for proper tissue differentiation during embryonic development.

Ectopic bone morphogenetic proteins 5 and 4 in the chicken forebrain lead to cyclopia and holoprosencephaly

Proper dorsal-ventral patterning in the developing central nervous system requires signals from both the dorsal and ventral portions of the neural tube. Data from multiple studies have demonstrated that bone morphogenetic proteins (BMPs) and Sonic hedgehog protein are secreted factors that regulate dorsal and ventral specification, respectively, within the caudal neural tube. In the developing rostral central nervous system Sonic hedgehog protein also participates in ventral regionalization; however, the roles of BMPs in the developing brain are less clear. We hypothesized that BMPs also play a role in dorsal specification of the vertebrate forebrain. To test our hypothesis we implanted beads soaked in recombinant BMP5 or BMP4 into the neural tube of the chicken forebrain. Experimental embryos showed a loss of the basal telencephalon that resulted in holoprosencephaly (a single cerebral hemisphere), cyclopia (a single midline eye), and loss of ventral midline structures. In situ hybridization using a panel of probes to genes expressed in the dorsal and ventral forebrain revealed the loss of ventral markers with the maintenance of dorsal markers. Furthermore, we found that the loss of the basal telencephalon was the result of excessive cell death and not a change in cell fates. These data provide evidence that BMP signaling participates in dorsal-ventral patterning of the developing brain in vivo, and disturbances in dorsal-ventral signaling result in specific malformations of the forebrain.

[Anatomic MRI study of commissural agenesis and dysplasia of the Telencephalon (Agenesis of the corpus callosum and related anomalies). Clinical correlations and morphogenetic interpretation]

A series of 78 patients presenting with agenesis of the cerebral commissures and properly investigated with MR imaging, was reviewed and analyzed morphologically. Results were compared with descriptive data from the literature, and with the developmental models proposed. From this, a model of a-commissural brain is described: the lamina terminalis would be homologous to a telencephalic anterior medullary velum of which the commissure would be the anterior commissure, and the lamina of white matter described as the Probst's and the fornical bundles, would be homologous to a posterior medullary velum having become a medial medullary velum due to the division of the prosecephalon into two cerebral hemispheres, and of which the commissure would be the (posterior) calloso-hippocampal commissure. Also, the comparison with the model establishes that in the actual malformations, defects of the cingulum and of at least some of the intralobaroccipital association bundles are observed beside the commissural defect. Such a model would reclassify these disorders, distinguishing the "simple" commissural defects, complete or segmental, global or dissociated, without or with a ventricular expansion, from more complex forms with multicystic defects, adding major dysplastic lesions of the dura mater, leptomeninges and parenchyma, to the commissural defects. Paradoxically, the latter group seems to be clinically less severe than the "simple agenesis" group, of which prognosis (including the neurologic and intellectual disorders as well as the associated pathologies) is generally very poor; this should be seriously considered since the antenatal diagnosis of these malformations is made routinely with ultrasonography and MRI.

Murine Otx1 and Drosophila otd genes share conserved genetic functions required in invertebrate and vertebrate brain development

Despite the obvious differences in anatomy between invertebrate and vertebrate brains, several genes involved in the development of both brain types belong to the same family and share similarities in expression patterns. Drosophila orthodenticle (otd) and murine Otx genes exemplify this, both in terms of expression patterns and mutant phenotypes. In contrast, sequence comparison of OTD and OTX gene products indicates that homology is restricted to the homeodomain suggesting that protein divergence outside the homeodomain might account for functional differences acquired during brain evolution. In order to gain insight into this possibility, we replaced the murine Otx1 gene with a Drosophila otd cDNA. Strikingly, epilepsy and corticogenesis defects due to the absence of Otx1 were fully rescued in homozygous otd mice. A partial rescue was also observed for the impairments of mesencephalon, eye and lachrymal gland. In contrast, defects of the inner ear were not improved suggesting a vertebrate Otx1-specific function involved in morphogenesis of this structure. Furthermore, otd, like Otx1, was able to cooperate genetically with Otx2 in brain patterning, although with reduced efficiency. These data favour an extended functional conservation between Drosophila otd and murine Otx1 genes and support the idea that conserved genetic functions required in mammalian brain development evolved in a primitive ancestor of both flies and mice.

Telencephalosynapsis (synencephaly) and rhombencephalosynapsis with posterior fossa ventriculocele ('Dandy-Walker cyst'): an unusual aberrant syngenetic complex

Agenesis of the cerebellar vermis (paleocerebellar agenesis) with fusion of the cerebellar hemispheres (rhombencephalosynapsis) is a rare malformation of the central nervous system (CNS). Its combination with synencephaly (telencephalosynapsis), telencephalic ventricular aplasia, aqueductal atresia and cystic fourth ventricle has not yet been described, as far as we know. Here, we report this combination in a 23-weeks' gestation male fetus who was aborted to a 24-year-old diabetic mother. In this fetus with cerebral and cerebellar hemispheric fusion, vermian agenesis was associated with a Dandy-Walker-like posterior fossa cyst, in spite of the fusion of the hypoplastic cerebellar hemispheres. The CNS malformations were further accompanied by dysmorphic facial stigmata such as unilateral atresia of the external ear, ocular hypertelorism and a broad nasal bridge. Preaxial polydactyly and contractures of the upper limbs were the only associated non-cranial abnormalities. Cytogenetic studies revealed a numerically and structurally normal male (46, XY). The malformation complex described in this fetus of a mother with antedating pregnancy diabetes appears to represent a previously undescribed aberrant syngenetic CNS phenotype, some basic teratogenetic aspects of which will be discussed in this paper.

Dysgenesis of the corpus callosum and associated telencephalic anomalies: MRI

We analysed the MRI findings in 23 patients with callosal dysgenesis in relation to their associated telencephalic anomalies to investigate the morphological significance of the development of Probst's bundles and the anterior commissure in congenital callosal dysgenesis. We classified callosal dysgenesis into three types: total defect (9 patients), partial defect (7) and hypoplasia (7). Associated anomalies were observed in 15 patients, including migration disorder (8 patients), micrencephaly (5), and lipoma (2). The remaining 8 patients had no associated anomalies. Probst's bundles were not identified in 4 patients with a severe migration disorder. An absent or hypoplastic anterior commissure was observed in 9 of the 16 patients with callosal defect and all 7 of those with callosal hypoplasia. Colpocephaly and keyhole dilatation of the temporal horns were seen in 16 and 21 patients, respectively. Callosal dysgenesis may occur not only through a defect in the callosal anlage, but also from impaired growth of axonal fibres projecting from the cerebral isocortex. Therefore, associated telencephalic anomalies may be responsible for additional features in callosal dysgenesis. Consequently, identification of Probst's bundles and the anterior commissure may be important when assessing cortical development in patients with callosal dysgenesis.

Syntelencephaly in an infant of a diabetic mother

Here we report on an infant of a diabetic mother (IDM) with midline interhemispheric "fusion" (MIF), or syntelencephaly. This is a rare anomaly characterized by segmental failure of cleavage of the cerebral hemispheres and other brain structures in the posterior frontal and parietal regions, with a normal interhemispheric fissure anterior and posterior to the "fused" region. While there is obvious overlap with holoprosencephaly (HPE), this condition differs from HPE in that the midline "fusion" in MIF is complete but segmental, while the structural brain anomalies seen in the HPE spectrum progress smoothly in severity in a posterior to anterior "fusion." However, while it is apparent that there are key distinctions between MIF and HPE, in all likelihood they arise from a similar pathogenetic mechanisms. We therefore suggest that MIF is a distinct variant of the HPE spectrum of midline brain anomalies. Given the known increased incidence of HPE in IDMs, MIF is likely a maternal diabetes-associated malformation.

Atelencephalic aprosencephaly

Absence of the telencephalon and diencephalon characterizes the syndrome of aprosencephaly, while in atelencephaly, only the telencephalon is absent. Atelencephalic aprosencephaly is characterized by the presence of at least a rudimentary diencephalon. Embryologically, aprosencephaly is thought to occur after the optic vesicles form but before the cerebral vesicles appear. The syndrome is quite rare, with only 10 cases previously reported. We describe two fetuses with atelencephalic aprosencephaly. A 25-week estimated gestational age fetus was born to first-cousin parents and had a prenatal ultrasonographic diagnosis of anencephaly. The second, a 19-week estimated gestational age fetus, was thought to have semilobar holoprosencephaly by prenatal ultrasound. At autopsy, neuropathologic examination in both cases showed virtual absence of the cerebral hemispheres with an incomplete diencephalon. Microscopic examination in one case revealed disorganized neuropil with a proliferative vasculopathy. The optic globes were completely formed and attached to hypoplastic optic nerves, but retinal dysplasia was apparent histologically in both cases, and bilateral colobomata were present in one case. The findings in these cases demonstrate a spectrum of congenital variations that lie between the syndromes of atelencephaly and aprosencephaly, underscoring the complexity of the congenital anomalies.

The arrest of luteinizing hormone-releasing hormone neuronal migration in the genetic arhinencephalic mouse embryo (Pdn/Pdn)

From previous observations, it was suggested that non-attachment of the olfactory nerve to the telencephalon blocked the induction of the olfactory bulbs in genetic arhinencephalic mouse embryos (Pdn/Pdn). The olfactory nerve ends in a tangle beneath the forebrain in these embryos. From these observations, we speculated that the migration of luteinizing hormone-releasing hormone (LHRH) neurons might be disturbed in the olfactory nerve. A mass of LHRH neurons was observed in the end of the olfactory nerve fibers, but LHRH neurons were found in the hypothalamus in Pdn/Pdn embryos on day 16 of gestation. Narrow by-paths were found between the olfactory nerve and the forebrain, and the migration of LHRH neurons through these by-paths was observed in Pdn/Pdn embryos on day 13 of gestation. From the reports that a gene deleted in the arhinencephalic syndrome (Kallmann's syndrome) shares homology with neural cell adhesion molecules (N-CAM), it was speculated that non-attachment of the olfactory nerve in the Pdn/Pdn embryo might be associated with abnormalities of N-CAM. The axon fibers of the olfactory nerve reacted specifically with anti-N-CAM IgG both in +/- (+/+ and/or Pdn/+) and Pdn/Pdn on day 11.5 and 12, but not on day 13 and 16 of gestation. The axon fibers of the olfactory nerve were positive to anti-N-CAM IgG specifically just during the developmental period that the olfactory nerve fibers attached to the telencephalon. It is still not clear whether non-attachment of the olfactory nerve may be associated with N-CAM or not from the present observations.

Congenital hydrocephalus associated with anomalies of midline telencephalic structures. A case report

Hydrocephalus is classified into communicating, if cerebrospinal fluid (CSF) can flow freely from the ventricle to the subarachnoid space, and into non-communicating, if it cannot. The cause of hydrocephalus is diverse, either congenital or acquired conditions causing obstruction to the flow of CSF. The location of the lesion is more important than the size or nature of the lesion. We describe an unusual case of congenital communicating hydrocephalus associated with multiple malformations of midline telencephalic structures. This was a premature male baby of 30 weeks gestational age. The pregnancy was terminated after revealing a hydrocephalus by ultrasonography. The cerebral hemispheres showed marked thinning of parenchyme with dilated lateral ventricles, communication of lateral and third ventricles with dilated foramina of Monro, absence of the septum pellucidum, and hypoplasia or focal agenesis of posterior portion of corpus callosum with dorsal dilation of the third ventricle. The right fornix appeared as a single thick midline cord in its approximately normal position and the hippocampi were poorly developed, especially in the left side. The left fornix was rudimentary.

Telencephalic dysgenesis associated with presumptive maternal carbon monoxide intoxication in the first trimester of pregnancy

We report the association of telencephalic dysgeneses (expected to occur around week 6 of gestation) with presumptive maternal carbon monoxide intoxication. This case supports the hypothesis that carbon monoxide intoxication at critical periods of human brain development may lead not only to decreased brain size and hypoxic-ischemic lesions, but also to dysgeneses.

Mediobasal prosencephalic defects, including holoprosencephaly and cyclopia, in relation to the development of the human forebrain

Four very early synophthalmic embryos were studied in serial sections and reconstructed graphically by the point-plotting method. Three belonged to stage 16 (5 weeks) and one to stages 19/20 (7 weeks). Recently completed accounts and reconstructions of the normal brains of staged human embryos served as controls for comparison with the abnormal examples. The embryos shared in common: holoprosencephaly, arhinencephaly sensu stricto (absence of olfactory nerve fibers, bulbs, and tracts), presence of a proboscis, synophthalmia with two lens vesicles, a retarded telencephalic wall, absence of the mediobasal part of the telencephalon (the future septal area and the commissural plate: future anterior commissure and corpus callosum), irregularity of the diencephalon, mensural changes in the brain, absence of the rostral part of the notochord and consequent cranial defects, and small ganglia of the cranial nerves. Where it could be determined (at least in the three less advanced specimens), the adenohypophysial primordium was either small and isolated or was absent; a tentorial condensation appeared to be missing; and disturbances of the primordia of the orbital muscles and their innervation were noted. The corpus striatum is single and corresponds to only the diencephalic part (medial eminence) of normal embryos. Interference with induction by the prechordal plate at or before stage 8 (18 days) would be expected to affect the future mediobasal part of the neural plate (median prosencephalic dysgenesis) and the future optic primordium (cyclopia sensu stricto). Insufficient formation of material from the prechordal plate would account for disorders of the orbital musculature and, possibly, for inadequacy of the tentorium cerebelli. Disturbance a couple of days later (stage 9) would result in synophthalmia. Cyclopia and synophthalmia entail arhinencephaly and holoprosencephaly, both of which may arise independently. Defective distribution of the cephalic mesenchyme points to a derangement of the mesencephalic neural crest (stages 10 and 11), causing such features as an incomplete chondrocranium and reduction in size of the ganglia of the cranial nerves. Failure of bilateral division of the telencephalon would occur at or before 4 weeks (stages 13 and 14). It is concluded that all the above conditions arise during the first 4 postovulatory weeks.