Brain development: integrins and the Reelin pathway

Cerebrospinal fluid exploratory proteomics and ketamine metabolite pharmacokinetics in human volunteers after ketamine infusion

Ketamine is a treatment for both refractory depression and chronic pain syndromes. In order to explore ketamine's potential mechanism of action and whether ketamine or its metabolites cross the blood brain barrier, we examined the pharmacokinetics of ketamine and its metabolites-norketamine (NK), dehydronorketamine (DHNK), and hydroxynorketamines (HNKs)-in cerebrospinal fluid (CSF) and plasma, as well as in an exploratory proteomic analysis in the CSF of nine healthy volunteers who received ketamine intravenously (0.5 mg/kg IV). We found that ketamine, NK, and (2R,6R;2S,6S)-HNK readily crossed the blood brain barrier. Additionally, 354 proteins were altered in the CSF in at least two consecutive timepoints (p < 0.01). Proteins in the classes of tyrosine kinases, cellular adhesion molecules, and growth factors, including insulin, were most affected, suggesting an interplay of altered neurotransmission, neuroplasticity, neurogenesis, synaptogenesis, and neural network functions following ketamine administration.

Cysteine-rich whey protein isolate (Immunocal®) ameliorates deficits in the GFAP.HMOX1 mouse model of schizophrenia

Schizophrenia is a neuropsychiatric disorder that features neural oxidative stress and glutathione (GSH) deficits. Oxidative stress is augmented in brain tissue of GFAP.HMOX1 transgenic mice which exhibit schizophrenia-relevant characteristics. The whey protein isolate, Immunocal® serves as a GSH precursor upon oral administration. In this study, we treated GFAP.HMOX1 transgenic mice daily with either Immunocal (33mg/ml drinking water) or equivalent concentrations of casein (control) between the ages of 5 and 6.5 months. Immunocal attenuated many of the behavioral, neurochemical and redox abnormalities observed in GFAP.HMOX1 mice. In addition to restoring GSH homeostasis in the CNS of the transgenic mice, the whey protein isolate augmented GSH reserves in the brains of wild-type animals. These results demonstrate that consumption of whey protein isolate augments GSH stores and antioxidant defenses in the healthy and diseased mammalian brain. Whey protein isolate supplementation (Immunocal) may constitute a safe and effective modality for the management of schizophrenia, an unmet clinical imperative.

Reelin promotes adhesion of multiple myeloma cells to bone marrow stromal cells via integrin β1 signaling

The close interaction between tumor cells and bone marrow stromal cells plays a crucial role in the tumorigenesis of multiple myeloma (MM). Reelin, an extracellular matrix protein, is found expressed in myeloma cells and is negatively associated with prognosis. We examined the role of Reelin in myeloma cell adhesion to bone marrow stromal cells and the signaling pathways involved. The results revealed that Reelin promoted the adhesion of myeloma cells to HS-5, a bone marrow stromal cell line, via the activation of β1 integrin. The resulting phosphorylation of focal adhesion kinase (FAK) led to the activation of Syk/STAT3 and Akt. Reelin's high affinity receptor ApoER2 indirectly modulated the adhesion of myeloma cells by promoting Reelin expression via Sp1. These findings indicate an important role for Reelin/integrin-β1-induced myeloma cell adhesion to bone marrow stromal cells and highlight the therapeutic potential of targeting Reelin/integrin/FAK axis.

Reelin promotes the adhesion and drug resistance of multiple myeloma cells via integrin β1 signaling and STAT3

Reelin is an extracellular matrix (ECM) protein that is essential for neuron migration and positioning. The expression of reelin in multiple myeloma (MM) cells and its association with cell adhesion and survival were investigated. Overexpression, siRNA knockdown, and the addition of recombinant protein of reelin were used to examine the function of reelin in MM cells. Clinically, high expression of reelin was negatively associated with progression-free survival and overall survival. Functionally, reelin promoted the adhesion of MM cells to fibronectin via activation of α5β1 integrin. The resulting phosphorylation of Focal Adhesion Kinase (FAK) led to the activation of Src/Syk/STAT3 and Akt, crucial signaling molecules involved in enhancing cell adhesion and protecting cells from drug-induced cell apoptosis. These findings indicate reelin's important role in the activation of integrin-β1 and STAT3/Akt pathways in multiple myeloma and highlight the therapeutic potential of targeting reelin/integrin/FAK axis.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

Extracellular matrix assembly: a multiscale deconstruction

The biochemical and biophysical properties of the extracellular matrix (ECM) dictate tissue-specific cell behaviour. The molecules that are associated with the ECM of each tissue, including collagens, proteoglycans, laminins and fibronectin, and the manner in which they are assembled determine the structure and the organization of the resultant ECM. The product is a specific ECM signature that is comprised of unique compositional and topographical features that both reflect and facilitate the functional requirements of the tissue.

Extracellular matrix assembly: a multiscale deconstruction

The biochemical and biophysical properties of the extracellular matrix (ECM) dictate tissue-specific cell behaviour. The molecules that are associated with the ECM of each tissue, including collagens, proteoglycans, laminins and fibronectin, and the manner in which they are assembled determine the structure and the organization of the resultant ECM. The product is a specific ECM signature that is comprised of unique compositional and topographical features that both reflect and facilitate the functional requirements of the tissue.

Extracellular matrix assembly: a multiscale deconstruction

The biochemical and biophysical properties of the extracellular matrix (ECM) dictate tissue-specific cell behaviour. The molecules that are associated with the ECM of each tissue, including collagens, proteoglycans, laminins and fibronectin, and the manner in which they are assembled determine the structure and the organization of the resultant ECM. The product is a specific ECM signature that is comprised of unique compositional and topographical features that both reflect and facilitate the functional requirements of the tissue.

Reelin controls neuronal positioning by promoting cell-matrix adhesion via inside-out activation of integrin α5β1

Birthdate-dependent neuronal layering is fundamental to neocortical functions. The extracellular protein Reelin is essential for the establishment of the eventual neuronal alignments. Although this Reelin-dependent neuronal layering is mainly established by the final neuronal migration step called "terminal translocation" beneath the marginal zone (MZ), the molecular mechanism underlying the control by Reelin of terminal translocation and layer formation is largely unknown. Here, we show that after Reelin binds to its receptors, it activates integrin α5β1 through the intracellular Dab1-Crk/CrkL-C3G-Rap1 pathway. This intracellular pathway is required for terminal translocation and the activation of Reelin signaling promotes neuronal adhesion to fibronectin through integrin α5β1. Since fibronectin is localized in the MZ, the activated integrin α5β1 then controls terminal translocation, which mediates proper neuronal alignments in the mature cortex. These data indicate that Reelin-dependent activation of neuronal adhesion to the extracellular matrix is crucial for the eventual birth-date-dependent layering of the neocortex.

Interaction between Reelin and Notch signaling regulates neuronal migration in the cerebral cortex

Neuronal migration is a fundamental component of brain development whose failure is associated with various neurological and psychiatric disorders. Reelin is essential for the stereotypical inside-out sequential lamination of the neocortex, but the molecular mechanisms of its action still remain unclear. Here we show that regulation of Notch activity plays an important part in Reelin-signal-dependent neuronal migration. We found that Reelin-deficient mice have reduced levels of the cleaved form of Notch intracellular domain (Notch ICD) and that loss of Notch signaling in migrating neurons results in migration and morphology defects. Further, overexpression of Notch ICD mitigates the laminar and morphological abnormalities of migrating neurons in Reeler. Finally, our in vitro biochemical studies show that Reelin signaling inhibits Notch ICD degradation via Dab1. Together, our results indicate that neuronal migration in the developing cerebral cortex requires a Reelin-Notch interaction.

C3G regulates cortical neuron migration, preplate splitting and radial glial cell attachment

Neuronal migration is integral to the development of the cerebral cortex and higher brain function. Cortical neuron migration defects lead to mental disorders such as lissencephaly and epilepsy. Interaction of neurons with their extracellular environment regulates cortical neuron migration through cell surface receptors. However, it is unclear how the signals from extracellular matrix proteins are transduced intracellularly. We report here that mouse embryos lacking the Ras family guanine nucleotide exchange factor, C3G (Rapgef1, Grf2), exhibit a cortical neuron migration defect resulting in a failure to split the preplate into marginal zone and subplate and a failure to form a cortical plate. C3G-deficient cortical neurons fail to migrate. Instead, they arrest in a multipolar state and accumulate below the preplate. The basement membrane is disrupted and radial glial processes are disorganised and lack attachment in C3G-deficient brains. C3G is activated in response to reelin in cortical neurons, which, in turn, leads to activation of the small GTPase Rap1. In C3G-deficient cells, Rap1 GTP loading in response to reelin stimulation is reduced. In conclusion, the Ras family regulator C3G is essential for two aspects of cortex development, namely radial glial attachment and neuronal migration.

Beta1 integrins in radial glia but not in migrating neurons are essential for the formation of cell layers in the cerebral cortex

Radial glial cells in the cerebral cortex serve as progenitors for neurons and glia and guide the migration of cortical neurons. The integrin alpha3beta1 is thought to mediate interactions of migrating neurons with radial glial cells and to function as a receptor for the reelin signaling molecule. Here, we challenge this view and demonstrate that beta1 integrins in migrating neurons are not essential for the formation of cell layers in the cerebral cortex. Cortical cell layers also form normally in mice deficient in the integrin alpha3beta1. However, we provide evidence that beta1 integrins in radial glia control the morphological differentiation of both glia and neurons. We conclude that beta1 integrins in radial glia are required for the proper development of the cerebral cortex, whereas beta1 integrins in migrating neurons are not essential for glial-guided migration and reelin signaling.

The c-Jun N-terminal kinase activator dual leucine zipper kinase regulates axon growth and neuronal migration in the developing cerebral cortex

Mammalian corticogenesis substantially depends on migration and axon projection of newborn neurons that are coordinated by a yet unidentified molecular mechanism. Dual leucine zipper kinase (DLK) induces activation of c-Jun N-terminal kinase (JNK), a molecule that regulates morphogenesis in various organisms. We show here, using gene targeting in mice, that DLK is indispensable for establishing axon tracts, especially those originating from neocortical pyramidal neurons of the cerebrum. Direct and quantitative analysis of radial migration of pyramidal neurons using slice culture and a time-lapse imaging system revealed that acceleration around the subplate was affected by DLK gene disruption and by administration of a JNK inhibitor. Phosphorylation of JNK substrates, including c-Jun and doublecortin, and of JNK itself at the activation loop were partially affected in brains of DLK-deficient mouse embryos. These data suggest that DLK plays a significant role in the coordinated regulation of radial migration and axon projection by modulating JNK activity.

Human disorders of cortical development: from past to present

Epilepsy and mental retardation, originally of unknown cause, are now known to result from many defects including cortical malformations, neuronal circuitry disorders and perturbations of neuronal communication and synapse function. Genetic approaches in combination with MRI and related imaging techniques continually allow a re-evaluation and better classification of these disorders. Here we review our current understanding of some of the primary defects involved, with insight from recent molecular biology advances, the study of mouse models and the results of neuropathology analyses. Through these studies the molecular determinants involved in the control of neuron number, neuronal migration, generation of cortical laminations and convolutions, integrity of the basement membrane at the pial surface, and the establishment of neuronal circuitry are being elucidated. We have attempted to integrate these results with the available data concerning, in particular, human brain development, and to emphasize the limitations in some cases of extrapolating from rodent models. Taking such species differences into account is clearly critical for understanding the pathophysiological mechanisms associated with these disorders.

Localization of the scaffolding protein RACK1 in the developing and adult mouse brain

RACK1 is a multifunctional scaffolding protein known to be involved in the regulation of various signaling cascades in the central nervous system (CNS). In order to gain insight into the neurological functions of RACK1, we examined the expression of RACK1 mRNA and protein during gestation and in the adult mouse brain. Several expression patterns were observed. At embryonic day 11.5 (E11.5), RACK1 is expressed in a high-dorsal to low-ventral gradient throughout the brain. At E13.5, RACK1 is most abundant in the telencephalon. In the developing cortical primordium, RACK1 protein is expressed in a high-rostromidline to low-caudolateral gradient that appears to be regulated post-transcriptionally. At E18.5, RACK1 is expressed most abundantly in layers 1-4 of the cortex, striatum, hippocampus, dentate gyrus and specific thalamic nuclei. In the adult mouse, RACK1 is ubiquitously expressed in neuronal perikarya in most brain regions, with relatively higher levels in hippocampus, olfactory bulb, cortex and cerebellum. Subcellular staining was detected mainly in the cell bodies and extending into dendrites, whereas RACK1 was not present significantly in axonal fibers or nuclei. We also determined brain regions in which RACK1 interacts with one of its binding partners, the betaII isoform of protein kinase C (betaIIPKC). We found that betaIIPKC had a much more restricted expression pattern than RACK1 and overlapped with the scaffolding protein only in certain regions, including the CA1 area of the hippocampus, cerebellum and striatum. Our results suggest an important role for RACK1 during CNS development and support multiple functions of the protein in the adult brain.

A fresh look at an ancient receptor family: emerging roles for low density lipoprotein receptors in synaptic plasticity and memory formation

The well-known family of low-density lipoprotein receptors represents a collection of ancient membrane receptors that have been remarkably conserved throughout evolution. These multifunctional receptors, known to regulate cholesterol transport, are becoming increasingly interesting to the neuroscience community due to their ability to transduce a diversity of extracellular signals across the membrane in the adult CNS. Their roles in modulating synaptic plasticity and necessity in hippocampus-specific learning and memory have recently come to light. In addition, genetic, biochemical and behavioral studies have implicated these signaling systems in a number of human neurodegenerative and neuropsychiatric disorders involving loss of cognitive ability, such as Alzheimer's disease, schizophrenia and autism. This review describes the known functions of these receptors and discusses their potential role in processes of synaptic regulation and memory formation.

Deficits in motor coordination with aberrant cerebellar development in mice lacking testicular orphan nuclear receptor 4

Since testicular orphan nuclear receptor 4 (TR4) was cloned, its physiological function has remained largely unknown. Throughout postnatal development, TR4-knockout (TR4-/-) mice exhibited behavioral deficits in motor coordination, suggesting impaired cerebellar function. Histological examination of the postnatal TR4-/- cerebellum revealed gross abnormalities in foliation; specifically, lobule VII in the anterior vermis was missing. Further analyses demonstrated that the laminations of the TR4-/- cerebellar cortex were changed, including reductions in the thickness of the molecular layer and the internal granule layer, as well as delayed disappearance of the external granule cell layer (EGL). These lamination irregularities may result from interference with granule cell proliferation within the EGL, delayed inward migration of postmitotic granule cells, and a higher incidence of apoptotis. In addition, abnormal development of Purkinje cells was observed in the postnatal TR4-/- cerebellum, as evidenced by aberrant dendritic arborization and reduced calbindin staining intensity. Expression of Pax-6, Sonic Hedgehog (Shh), astrotactin (Astn), reelin, and Cdk-5, genes correlated with the morphological development of the cerebellum, is reduced in the developing TR4-/- cerebellum. Together, our findings suggest that TR4 is required for normal cerebellar development.

Altered morphological and electrophysiological properties of Cajal-Retzius cells in cerebral cortex of embryonic Presenilin-1 knockout mice

Mutations of Presenilin-1 are the major cause of familial Alzheimer's disease. Presenilin-1 knockout (PS1-/-) mice develop severe cortical dysplasia related to human type 2 lissencephaly. This overmigration syndrome has been attributed to the premature loss of Cajal-Retzius cells (CRcs), pioneer neurons required for the termination of radial neuronal migration. To elucidate the potential cellular mechanisms responsible for this premature neuronal loss, we investigated the morphological and electrophysiological properties of visually identified CRcs of wild-type (WT) and PS1-/- mouse brains at embryonic day 16.5. The density of CRcs was substantially reduced in the cerebral cortex of PS1-/-. In PS1-/- CRcs the number of axonal branches was significantly increased to 12.5 +/- 4.9 (n = 8; WT, 4.0 +/- 1.4, n = 12), while no differences in dendritic branching and total length of dendritic and axonal compartments were observed. Additionally, the resting membrane potential of PS1-/- CRcs was significantly depolarized (-48.3 +/- 1.7 mV; n = 23) in contrast to WT CRcs (-57.9 +/- 2.1 mV; n = 38). Active membrane properties were not affected by PS1 deficiency. CRcs of both genotypes showed spontaneous postsynaptic currents that could be completely blocked by 100 microM bicuculline and were unaffected by glutamatergic antagonists, suggesting that they were mediated by GABAA receptors. These results demonstrate that axonal branching and resting membrane potential of CRcs was affected in embryonic cerebral cortex of PS1-/- mice. The depolarized membrane potential observed in PS1-/- CRcs may increase the susceptibility to neuronal death, thus facilitating the premature loss of CRcs in PS1-/- mice.

Integrin and the Reelin-Dab1 pathway: a sticky affair?

It has been repeatedly suggested that integrin is essential for neuronal migration. A new study proposes a link between Disabled-1 (Dab1) phosphorylation and alpha3 integrin signalling that is thought to drive the timely detachment of migrating neurons from the guiding radial glia fibers during early corticogenesis. This proposal however is hardly compatible with time-lapse anatomical investigations and genetic studies on integrin or with recent works which indicate direct reelin signalling to radial fibers.

The central fragment of Reelin, generated by proteolytic processing in vivo, is critical to its function during cortical plate development

Reelin is a large extracellular protein that controls cortical development. It binds to lipoprotein receptors very-low-density lipoprotein receptor and apolipoprotein-E receptor type 2, thereby inducing phosphorylation of the adapter Dab1. In vivo, Reelin is cleaved into three fragments, but their respective function is unknown. Here we show the following: (1) the central fragment is necessary and sufficient for receptor binding in vitro and for Dab1 phosphorylation in neuronal cultures; (2) Reelin does not bind the protocadherin cadherin-related neuronal receptor (CNR1) as reported previously; (3) Reelin and its central fragment are equally able to rescue the reeler phenotype in a slice culture assay; and (4) anti-receptor antibodies can induce Dab1 phosphorylation but do not correct the reeler phenotype in slices. These observations show that the function of Reelin is critically dependent on the central fragment generated by processing but primarily independent of interactions with CNR1 and on the N-terminal region. They also indicate that events acting in parallel to Dab1 phosphorylation might be required for full activity.

Missplicing resulting from a short deletion in the reelin gene causes reeler-like neuronal disorders in the mutant shaking rat Kawasaki

The shaking rat Kawasaki (SRK) is an autosomal recessive mutant that exhibits reeler-like abnormal locomotor behaviors. The murine reeler mutants arise from several mutations in the specific gene called reelin, which result in defects of Reelin expression or secretion in the cerebral cortex and other regions of CNS. To address the issue of whether the SRK mutation also arises from a mutation in reelin, we analyzed the reelin gene in SRK. Northern analysis of reelin mRNA from normal rats showed that rat reelin was expressed as a approximately 12-kb transcript in both the cerebrum and the cerebellum, whereas reelin expression was markedly reduced in the SRK brains. In situ hybridization analysis showed that reelin mRNA in the SRK brains was expressed in Cajal-Retzius cells in the marginal zone of the cerebral cortex and outer granular cells in the cerebellar cortex in similar manners to normal controls, but its expression was considerably reduced. On Western blotting and immunohistochemical analyses using antibodies specific for the Reelin protein, no immunoproduct was recognized in the cerebral and cerebellar cortices. From the cDNA sequences, we found a 64-base heterologous sequence in SRK reelin, which contains a termination codon in the reading frame. Furthermore, genomic DNA analysis revealed that a 10-base deletion, which contains a predicted splice donor site, occurred in the SRK genomic reelin gene, resulting in "read through" into the following intron in SRK. Thus, the SRK mutation is another type of mutation that lacks expression of the functional Reelin protein and, therefore, causes the reeler phenotype.

Receptor protein tyrosine phosphatase alpha is essential for hippocampal neuronal migration and long-term potentiation

Despite clear indications of their importance in lower organisms, the contributions of protein tyrosine phosphatases (PTPs) to development or function of the mammalian nervous system have been poorly explored. In vitro studies have indicated that receptor protein tyrosine phosphatase alpha (RPTPalpha) regulates SRC family kinases, potassium channels and NMDA receptors. Here, we report that absence of RPTPalpha compromises correct positioning of pyramidal neurons during development of mouse hippocampus. Thus, RPTPalpha is a novel member of the functional class of genes that control radial neuronal migration. The migratory abnormality likely results from a radial glial dysfunction rather than from a neuron-autonomous defect. In spite of this aberrant development, basic synaptic transmission from the Schaffer collateral pathway to CA1 pyramidal neurons remains intact in Ptpra(-/-) mice. However, these synapses are unable to undergo long-term potentiation. Mice lacking RPTPalpha also underperform in the radial-arm water-maze test. These studies identify RPTPalpha as a key mediator of neuronal migration and synaptic plasticity.

Four-dimensional migratory coordinates of GABAergic interneurons in the developing mouse cortex

We have used time-lapse multiphoton microscopy to map the migration and settling pattern of GABAergic interneurons that originate in the ganglionic eminence of the ventral forebrain and incorporate into the neocortex of the cerebral hemispheres. Imaging of the surface of the cerebral hemispheres in both explant cultures and brains of living mouse embryos revealed that GABAergic interneurons migrating within the marginal zone originate from three different sources and migrate via distinct and independent streams. After reaching their areal destination, interneurons descend into the underlying cortex to assume positions with isochronically generated, radially derived neurons. The dynamics and pattern of cell migration in the marginal zone (see movies, available at www.jneurosci.org) suggest that the three populations of interneurons respond selectively to distinct local cues for directing their migration to the appropriate areas and layers of the neocortex. This approach opens a new avenue for study of normal and abnormal neuronal migration in their native environment and indicate that interneurons have specific programs for their areal and laminar deployment.

Development of the brain: a vital role for cerebrospinal fluid

There has been considerable recent progress in understanding the processes involved in brain development. An analysis of a number of neurological conditions, together with our studies of the hydrocephalic Texas (H-Tx) rat, presents an important role for cerebrospinal fluid (CSF) in the developmental process. The fluid flow is essentially one-way and the location of the choroid plexuses in the lateral, third, and fourth ventricles allows for the possibility of new components being added to the fluid at these points. The role of the fourth ventricular CSF is particularly interesting since this is added to the fluid downstream of the cerebral hemisphere germinal epithelium (the main site of cortical cell proliferation and differentiation) and is destined for the basal cisterns and subarachnoid space suggesting different target cells to those within the ventricular system. Moreover, other sources of additions to the CSF exist, notably the subcommissural organ, which sits at the opening of the third ventricle into the cerebral aqueduct and is the source of Reisner's fibre, glycoproteins, and unknown soluble proteins. In this paper a model for the role of CSF is developed from studies of the development of the cortex of the H-Tx rat. We propose that CSF is vital in controlling development of the nervous system along the whole length of the neural tube and that the externalisation of CSF during development is essential for the formation of the layers of neurones in the cerebral cortex.

A radialization factor in normal cortical plate restores disorganized radial glia and disrupted migration in a model of cortical dysplasia

Treatment of pregnant ferrets on embryonic day 24 (E24) with the antimitotic methylazoxy methanol (MAM) leads to a specific constellation of effects in newborn kits, which include a very thin and poorly laminated neocortex, disruption of radial glial cell morphology with early differentiation into astrocytes, and abnormal positioning of Cajal-Retzius cells. We suggest that MAM treatment on E24 results in this model of cortical dysplasia by eliminating a population of cells that produce a factor capable of maintaining radial glia in their normal morphology. The abnormal radial glia, either alone or in combination with other abnormal features, are likely to prevent proper migration into the cortical plate. To test the possibility that normal cortex can provide the missing substance that influences radial glia, slices of E24 MAM-treated cortex were removed at postnatal day 0 (P0) and cultured adjacent to explants of P0 normal cortical plate. By labelling a small number of cells with injections of fluorescent dextrans into the cultured slices, we found that abnormal radial glia in MAM treated slices cocultured adjacent to normal cortical plate were restored toward normal, in comparison to E24 MAM treated slices cultured alone and in other control conditions. We also found that abnormally positioned Cajal-Retzius cells move into the marginal zone and that neurons are able to migrate into the cortical plate more effectively in the coculture condition. These data indicate that normal cortical plate of ferrets contains a factor causing radial glia to maintain their elongated morphology; the improved position of radial glia encourages repositioning of Cajal-Retzius cells and improved neuronal migration into the cortical plate.

Integrins in development: moving on, responding to, and sticking to the extracellular matrix

Integrins are cell surface receptors of the extracellular matrix present in all animals. Genetic analysis in worms, flies, and vertebrates has revealed integrin involvement in key developmental processes, and we focus here on examples of integrin functions that are comparable across these model organisms. Integrins contribute to cell movement by providing traction to migrating cells, through assembly of extracellular matrices that can serve as tracks for migration, and by transmitting guidance signals that direct cells or cell processes to their targets. Integrins also participate in signaling events that govern tissue differentiation and organogenesis. Finally, adhesion by integrin-mediated junctions allows tissues to withstand mechanical load and is essential for tissue integrity.