Migration Speed of Cajal-Retzius Cells Modulated by Vesicular Trafficking Controls the Size of Higher-Order Cortical Areas

In the neocortex, higher-order areas are essential to integrate sensory-motor information and have expanded in size during evolution. How higher-order areas are specified, however, remains largely unknown. Here, we show that the migration and distribution of early-born neurons, the Cajal-Retzius cells (CRs), controls the size of higher-order areas in the mouse somatosensory, auditory, and visual cortex. Using live imaging, genetics, and in silico modeling, we show that subtype-specific differences in the onset, speed, and directionality of CR migration determine their differential invasion of the developing cortical surface. CR migration speed is cell autonomously modulated by vesicle-associated membrane protein 3 (VAMP3), a classically non-neuronal mediator of endosomal recycling. Increasing CR migration speed alters their distribution in the developing cerebral cortex and leads to an expansion of postnatal higher-order areas and congruent rewiring of thalamo-cortical input. Our findings thus identify novel roles for neuronal migration and VAMP3-dependent vesicular trafficking in cortical wiring.

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title: Patterning the cerebral cortex: traveling with morphogens.
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title: Modality-specific thalamocortical inputs instruct the identity of postsynaptic L4 neurons
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title: Cajal-Retzius cells in the mouse: transcription factors, neurotransmitters, and birthdays suggest a pallial origin.
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title: Cajal–Retzius cells and subplate neurons differentially express vesicular glutamate transporters 1 and 2 during development of mouse cortex
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title: A protein related to extracellular matrix proteins deleted in the mouse mutant reeler
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title: The reeler gene-associated antigen on cajal-retzius neurons is a crucial molecule for laminar organization of cortical neurons
abstract: In the neurological mutant mouse reeler, the histological organization of the neocortex develops abnormally and essentially results in an inversion of the relative positions of the cortical layers. The reeler mutation, therefore, provides an insight into the molecular mechanisms underlying the formation of the cortical layers. We have generated a monoclonal antibody (CR-50) that probes a distinct allelic antigen present in wild-type but not in reeler mutant mice. CR-50 reacted specifically with Cajal-Retzius neurons, one of the first cortical neurons to differentiate in the neocortex, but whose functional role is not known. When dissociated cerebral cortical cells were incubated with CR-50 in reaggregation culture, the genotype-dependent histogenetic assembly of wild-type cortical cells resembled that of reeler mutants. These findings revealed that the selective expression of a distinct molecule on Cajal-Retzius neurons is critical for the normal lamination of cortical neurons in the mammalian neocortex.

title: The reeler mouse: anatomy of a mutant.
abstract: Publisher Summary Reeler is a well-characterized autosomal mutation in the mouse that affects several aspects of brain development. Anatomical studies indicated that all major cortical structures of the brain are present, but appeared disorganized in reeler mutants. Strikingly, projection neurons are shown to connect properly to their ectopic targets. For example, thalamocortical projections terminated properly on neurons destined for cortical layer IV, even though these occupied abnormal positions in the reeler cortex. The analysis of the anatomical defects in reeler cortical structures implied that radially-migrating neurons in the neocortex, pyramidal neurons of the hippocampus, and Purkinje cells in the cerebellum were all direct or indirect Reelin targets. The chapter describes the most salient anatomical aspects of the reeler mutation, and recapitulates the key molecular discoveries. The studies of the reeler mutation have shed some light on many diverse aspects of brain development. Reeler mutant mice will continue to serve as invaluable models for neuronal migration, maturation, and synaptic circuitry, and the Reelin signaling pathway will continue to serve as a springboard for the investigation of the molecular events underlying these processes.

title: Disruption of neuronal migration and radial glia in the developing cerebral cortex following ablation of Cajal-Retzius cells.
abstract: Cortical neurons are generated within the proliferative layer and follow a strict 'inside-out' gradient of migration and positioning, which determines the characteristic layering and pattern of neural connections in the adult cerebral cortex. Thus, directional migration of postmitotic neuroblasts towards layer I and regulation of the radial glia phenotype subserving cortical migration are central issues in corticogenesis. Recent studies showing that the gene disrupted in the reeler mutation--reelin--is expressed in Cajal-Retzius cells have indicated a role for these pioneer neurons in cortical migration. We show here that ablation of Cajal-Retzius cells in layer I by local application of domoic acid in newborn mice arrests migration of the late-generated neurons, destined to cortical layers II-III, that have been labeled by 5-bromodeoxyuridine injections administered at E16. In addition, degeneration of Cajal-Retzius cells in newborn mice dramatically decreases the number of radial glial apical processes identified by nestin-immunostaining, but increases the number of maturing glial fibrillary acidic protein-positive astrocytes. These findings support an essential role for Cajal-Retzius cells in neuronal migration and corticogenesis, by regulating the identity and function of radial glia and the radial glia-to-astrocyte transformation.

title: Cajal-Retzius Cells Regulate the Radial Glia Phenotype in the Adult and Developing Cerebellum and Alter Granule Cell Migration
abstract: Studies on the reeler mutation have shown that pioneer Cajal-Retzius (CR) cells are involved in neuronal migration in the developing cortex. Here, we use grafting and coculture experiments to investigate the mechanisms by which CR cells govern migration. We show that transplantation of embryonic CR cells, but not other cortical neurons, into adult cerebella induces a transient rejuvenation of host Bergmann glia into a radial glia phenotype. Similarly, CR cells sustain the phenotype of developing radial glia in postnatal cerebellar slices and induce the organization of a glial scaffold inside the CR cell explants. Studies with semipermeable inserts show that these effects are mediated by diffusible signals. We also show that CR cells adjacent to the surface of cerebellar slices reverse the direction of the migration of granule cells. Finally, CR cells from reeler mutant embryos elicited similar effects. These observations imply a role for CR cells in the regulation of the radial glia phenotype, a key step for neuronal migration, and suggest that these pioneer neurons may also exert a chemoattractive influence on migrating neurons.

title: Reelin Signaling Specifies the Molecular Identity of the Pyramidal Neuron Distal Dendritic Compartment
abstract: The apical dendrites of many neurons contain proximal and distal compartments that receive synaptic inputs from different brain regions. These compartments also contain distinct complements of ion channels that enable the differential processing of their respective synaptic inputs, making them functionally distinct. At present, the molecular mechanisms that specify dendritic compartments are not well understood. Here, we report that the extracellular matrix protein Reelin, acting through its downstream, intracellular Dab1 and Src family tyrosine kinase signaling cascade, is essential for establishing and maintaining the molecular identity of the distal dendritic compartment of cortical pyramidal neurons. We find that Reelin signaling is required for the striking enrichment of HCN1 and GIRK1 channels in the distal tuft dendrites of both hippocampal CA1 and neocortical layer 5 pyramidal neurons, where the channels actively filter inputs targeted to these dendritic domains.

title: Timing of cortical interneuron migration is influenced by the cortical hem.
abstract: Cerebral cortical γ-aminobutyric acid (GABA)ergic interneurons originate from the basal forebrain and migrate into the cortex in 2 phases. First, interneurons cross the boundary between the developing striatum and the cortex to migrate tangentially through the cortical primordium. Second, interneurons migrate radially to their correct neocortical layer position. A previous study demonstrated that mice in which the cortical hem was genetically ablated displayed a massive reduction of Cajal-Retzius (C-R) cells in the neocortical marginal zone (MZ), thereby losing C-R cell-generated reelin in the MZ. Surprisingly, pyramidal cell migration and subsequent layering were almost normal. In contrast, we find that the timing of migration of cortical GABAergic interneurons is abnormal in hem-ablated mice. Migrating interneurons both advance precociously along their tangential path and switch prematurely from tangential to radial migration to invade the cortical plate (CP). We propose that the cortical hem is responsible for establishing cues that control the timing of interneuron migration. In particular, we suggest that loss of a repellant signal from the medial neocortex, which is greatly decreased in size in hem-ablated mice, allows the early advance of interneurons and that reduction of another secreted molecule from C-R cells, the chemokine SDF-1/CXCL12, permits early radial migration into the CP.

title: Morphology, input–output relations and synaptic connectivity of Cajal–Retzius cells in layer 1 of the developing neocortex of CXCR4-EGFP mice
abstract: Layer 1 (L1) neurons, in particular Cajal–Retzius (CR) cells are among the earliest generated neurons in the neocortex. However, their role and that of L1 GABAergic interneurons in the establishment of an early cortical microcircuit are still poorly understood. Thus, the morphology of whole-cell recorded and biocytin-filled CR cells was investigated in postnatal day (P) 7–11 old CXCR4-EGFP mice where CR cells can be easily identified by their fluorescent appearance. Confocal-, light- and subsequent electron microscopy was performed to investigate their developmental regulation, morphology, synaptic input–output relationships and electrophysiological properties. CR cells reached their peak in occurrence between P4 to P7 and from thereon declined to almost complete disappearance at P14 by undergoing selective cell death through apoptosis. CR cells formed a dense and long-range horizontal network in layer 1 with a remarkable high density of synaptic boutons along their axons. They received dense GABAergic and non-GABAergic synaptic input and in turn provided synaptic output preferentially with spines or shafts of terminal tuft dendrites of pyramidal neurons. Interestingly, no dye-coupling between CR cells with other cortical neurons was observed as reported for other species, however, biocytin-labeling of individual CR cells leads to co-staining of L1 end foot astrocytes. Electrophysiologically, CR cells are characterized by a high input resistance and a characteristic firing pattern. Increasing depolarizing currents lead to action potential of decreasing amplitude and increasing half width, often terminated by a depolarization block. The presence of membrane excitability, the high density of CR cells in layer 1, their long-range horizontal axonal projection together with a high density of synaptic boutons and their synaptic input–output relationship suggest that they are an integral part of an early cortical network important not only in layer 1 but also for the establishment and formation of the cortical column.

title: Spontaneous GABAergic postsynaptic currents in Cajal–Retzius cells in neonatal rat cerebral cortex
abstract: Cajal–Retzius cells are among the first neurons appearing during corticogenesis, and play an important role in the establishment of cortical lamination. The variety of neurotransmitter receptors recently found on these cells imply that they are integrated in the neonatal cortical network. To investigate the presence and properties of spontaneous synaptic activity we performed whole‐cell patch‐clamp recordings from visually identified and biocytin‐labelled Cajal–Retzius cells in a tangential slice preparation of neonatal rat cerebral cortex (postnatal days P0–P5). Spontaneous postsynaptic currents (sPSCs) could be observed in about 23% of the cells using a pipette solution containing 136 mm Cl–. The sPSCs occurred at a low frequency (0.07 ± 0.07 Hz, n = 42 cells), had an average amplitude of 24.3 ± 12.4 pA (n = 415 events) and could not be divided in subpopulations according to their amplitude distribution or kinetic properties. The sPSCs were blocked by the GABAA antagonist bicuculline (100 µm), while the glutamatergic antagonists (±)‐2‐amino‐5‐phosphonopentatonic acid (APV, 30 µm) and 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX, 10 µm), as well as tetrodotoxin (1–2 µm), a blocker of voltage‐gated sodium‐currents, had no significant effect on sPSCs. The incidence rate of sPSCs declined within the age of the rats and no sPSCs could be observed after P4. These results suggest that Cajal–Retzius cells transiently receive action potential‐independent and GABAA receptor‐mediated spontaneous synaptic input, which may contribute to the refinement of cortical circuits.

title: Electrophysiological and morphological properties of Cajal–Retzius cells with different ontogenetic origins
abstract: The different origins of Cajal-Retzius cells (CRc) as well as their diverse molecular profile suggest that this cell type may represent different neuronal subpopulations. In order to investigate whether CRc from different origins show distinct functional or morphological characteristics we used transgenic Dbx1(cre);ROSA26(YFP) mice in which two subpopulations of CRc, originating from the septum and ventral pallium (VP) at the pallial-subpallial border (PSB), were permanently labeled by yellow fluorescent protein (YFP) expression. Electrophysiological properties of YFP(+) and YFP(-) CRc were investigated by whole-cell patch-clamp recordings, while a thorough somatodendritic and axonal reconstruction of the biocytin labeled CRc was subsequently performed using a Neurolucida system. Our experiments revealed that no significant differences in resting membrane potential, input resistance or capacitance, hyperpolarization activated currents and most action potentials properties could be observed between YFP(+) and YFP(-) CRc. Both YFP(+) and YFP(-) CRc displayed spontaneous and carbachol-induced GABAergic postsynaptic currents with similar properties and comparable NMDA-receptor mediated glutamatergic inward currents that were equally affected by the NR2B specific antagonist ifenprodil. Morphological reconstructions revealed that dendritic and axonal parameters are similar between YFP(+) and YFP(-) CRc, while the dendritic compartment of YFP(+) CRc was slightly larger. In summary, no considerable differences in functional and morphological properties between YFP(+) and YFP(-) CRc could be observed in this study. These observations suggest that CRc of different ontogenic origins display comparable functional properties in the early postnatal cortex and therefore perform similar functions within the transient neuronal networks of the developing cortex.

title: Segregation and Coactivation of Developing Neocortical Layer 1 Neurons
abstract: Layer 1 in the developing cerebral cortex is populated by two basic neuronal cell types, Cajal–Retzius (CR) cells and non-CR cells. We generated transgenic mice in which green fluorescent protein (GFP) was driven by the promoter of metabotropic glutamate receptor subtype 2 and expressed specifically in CR cells during cortical development. On the basis of the precise identification of CR cells with GFP fluorescence, we pursued developmental changes and synaptic mechanisms of both CR and non-CR cells during the postnatal period. Immunostaining in combination with GFP fluorescence imaging showed that GFP and reelin, a protein involved in corticogenesis, completely overlap in CR cells at postnatal day 0. At the subsequent postnatal stage, reelin-positive neurons are segregated and categorized into GFP-positive/GABA-negative CR cells and GFP-negative/GABA-positive non-CR cells. Individual and simultaneous whole-cell recordings of CR and non-CR cells in developing cerebral slices revealed that spontaneous and electrically evoked postsynaptic currents (sPSCs and ePSCs) measured in CR and non-CR cells are differentially mediated by GABAA receptors versus GABAA, AMPA, and NMDA receptors, respectively. Furthermore, CR and non-CR cells show synchronized repetitive barrages of sPSCs that reflect a network-driven activity in the developing cerebral cortex. These findings imply that the layer 1 neurons dynamically change and play a distinct and integral role in the postnatal developing neocortex.

title: Generation of Reelin-Positive Marginal Zone Cells from the Caudomedial Wall of Telencephalic Vesicles
abstract: An early and fundamental step of the laminar organization of developing neocortex is controlled by the developmental programs that critically depend on the activities of reelin-positive cells in the marginal zone. However, the ontogeny of reelin-positive cells remained elusive. To gain insights into the spatial and temporal regulation of reelin-positive marginal zone cell development, we used a transgenic mouse line in which we defined the green fluorescent protein (GFP) transgene as a novel reliable molecular marker of reelin-positive marginal zone cells from the early stages of their development. We further used exo utero electroporation-mediated gene transfer that allows us to mark progenitor cells and monitor the descendants in the telencephalon in vivo. We show here the generation of reelin-positive marginal zone cells from the caudomedial wall of telencephalic vesicles, including the cortical hem, where the prominent expression of GFP is initially detected. These neurons tangentially migrate at the cortical marginal zone and are distributed throughout the entire neocortex in a caudomedial-high to rostrolateral-low gradient during the dynamic developmental period of corticogenesis. Therefore, our findings on reelin-positive marginal zone cells, in addition to the cortical interneurons, add to the emerging view that the neocortex consists of neuronal subtypes that originate from a focal source extrinsic to the neocortex, migrate tangentially into the neocortex, and thereby underlie neural organization of the neocortex.

title: Multiple origins of Cajal-Retzius cells at the borders of the developing pallium
abstract: Cajal-Retzius cells are critical in cortical lamination, but very little is known about their origin and development. The homeodomain transcription factor Dbx1 is expressed in restricted progenitor domains of the developing pallium: the ventral pallium (VP) and the septum. Using genetic tracing and ablation experiments in mice, we show that two subpopulations of Reelin+ Cajal-Retzius cells are generated from Dbx1-expressing progenitors. VP- and septum-derived Reelin+ neurons differ in their onset of appearance, migration routes, destination and expression of molecular markers. Together with reported data supporting the generation of Reelin+ cells in the cortical hem, our results show that Cajal-Retzius cells are generated at least at three focal sites at the borders of the developing pallium and are redistributed by tangential migration. Our data also strongly suggest that distinct Cajal-Retzius subtypes exist and that their presence in different territories of the developing cortex might contribute to region-specific properties.

title: Massive loss of Cajal-Retzius cells does not disrupt neocortical layer order
abstract: Cajal-Retzius (CR) cells, the predominant source of reelin in developing neocortex, are thought to be essential for the inside out formation of neocortical layers. Fate mapping revealed that a large population of neocortical CR cells arises from the cortical hem. To investigate the function of CR cells, we therefore genetically ablated the hem. Neocortical CR cells were distributed beneath the pial surface in control mice, but were virtually absent in hem-ablated mice from embryonic day (E) 10.5 until birth. CR cells derived from other sources did not invade the neocortical primordium to compensate for hem loss. We predicted that neocortical layers would be inverted in hem-ablated animals, as in reeler mice, deficient in reelin signaling. Against expectation, layers showed the standard order. Low levels of reelin in the cortical primordium, or diffusion of reelin from other sites, may have allowed lamination to proceed. Our findings indicate, however, that the sheet of reelin-rich CR cells that covers the neocortical primordium is not required to direct layer order.

title: A Novel Role for Dbx1-Derived Cajal-Retzius Cells in Early Regionalization of the Cerebral Cortical Neuroepithelium
abstract: Patterning of the cerebral cortex during embryogenesis depends not only on passive diffusion of morphogens but also on signal delivery by Cajal-Retzius neurons that migrate over long distances.

title: Contact Repulsion Controls the Dispersion and Final Distribution of Cajal-Retzius Cells
abstract: Cajal-Retzius (CR) cells play a fundamental role in the development of the mammalian cerebral cortex. They control the formation of cortical layers by regulating the migration of pyramidal cells through the release of Reelin. The function of CR cells critically depends on their regular distribution throughout the surface of the cortex, but little is known about the events controlling this phenomenon. Using time-lapse video microscopy in vivo and in vitro, we found that movement of CR cells is regulated by repulsive interactions, which leads to their random dispersion throughout the cortical surface. Mathematical modeling reveals that contact repulsion is both necessary and sufficient for this process, which demonstrates that complex neuronal assemblies may emerge during development through stochastic events. At the molecular level, we found that contact repulsion is mediated by Eph/ephrin interactions. Our observations reveal a mechanism that controls the even distribution of neurons in the developing brain.

title: The Tetanus Neurotoxin‐Sensitive and Insensitive Routes to and from the Plasma Membrane: Fast and Slow Pathways?
abstract: Intracellular membrane trafficking in eukaryotes involves the budding of vesicles from a donor compartment, their translocation, and subsequent fusion with a target membrane. This last step has been shown to involve SNARE proteins, classified into two categories, vesicular (v)‐SNAREs and target (t)‐SNAREs. It is the pairing of v‐ and t‐SNAREs that is responsible for bringing the lipid bilayers together for membrane fusion. Key to the discovery of SNAREs is the sensitivity of their neuronal synaptic prototypes, which mediate the release of neurotransmitters, to clostridial neurotoxins. In this review, we focus on tetanus neurotoxin‐sensitive and tetanus neurotoxin‐insensitive v‐SNAREs, in particular synaptobrevin and cellubrevin, both tetanus neurotoxin‐sensitive and Tetanus neurotoxin‐ Insensitive Vesicle‐Associated Membrane Protein (TI‐VAMP, also called VAMP7). The brevins are characterized by an RD sequence in the middle of their SNARE motif whereas TI‐VAMP has an RG sequence. These two categories of exocytic v‐SNAREs define two important routes to and from the plasma membrane: one sensitive, the other insensitive to tetanus neurotoxin. We also discuss the central role of the endosomal system that could be considered, as already suggested for Rab proteins, as a mosaic of v‐SNAREs, thus raising the question of whether or not these two routes can merge, and if so, how and where.

title: Cellubrevin and synaptobrevins: similar subcellular localization and biochemical properties in PC12 cells
abstract: There is strong evidence to indicate that proteins of the synaptobrevin family play a key role in exocytosis. Synaptobrevin 1 and 2 are expressed at high concentration in brain where they are localized on synaptic vesicles. Cellubrevin, a very similar protein, has a widespread tissue distribution and in fibroblasts is localized on endosome-derived, transferin receptor-positive vesicles. Since brain cellubrevin is not detectable in synaptic vesicles, we investigated whether cellubrevin and the synaptobrevins are differentially targeted when co-expressed in the same cell. We report that in the nervous system cellubrevin is expressed at significant levels only by glia and vascular cells. However, cellubrevin is coexpressed with the two synaptobrevins in PC12 cells, a neuroendocrine cell line which contains synaptic vesicle-like microvesicles. In PC12 cells, cellubrevin has a distribution very similar to that of synaptobrevin 1 and 2. The three proteins are targeted to neurites which exclude the transferrin receptor and are enriched in synaptic-like microvesicles and dense-core granules. They are recovered in the synaptic-like microvesicle peak of glycerol velocity gradients, have a similar distribution in isopycnic fractionation and are coprecipitated by anti-synaptobrevin 2 immunobeads. Finally, cellubrevin, like the synaptobrevins, interact with the neuronal t-SNAREs syntaxin 1 and SNAP-25. These results suggest that cellubrevin and the synaptobrevins have similar function and do not play a specialized role in constitutive and regulated exocytosis, respectively.

title: Tetanus toxin-mediated cleavage of cellubrevin impairs exocytosis of transferrin receptor-containing vesicles in CHO cells
abstract: Cellubrevin is a member of the synaptobrevin/VAMP family of SNAREs, which has a broad tissue distribution. In fibroblastic cells it is concentrated in the vesicles which recycle transferrin receptors but its role in membrane trafficking and fusion remains to be demonstrated. Cellubrevin, like the synaptic vesicle proteins synaptobrevins I and II, can be cleaved by tetanus toxin, a metallo-endoprotease which blocks neurotransmitter release. However, nonneuronal cells are unaffected by the toxin due to lack of cell surface receptors for its heavy chain. To determine whether cellubrevin cleavage impairs exocytosis of recycling vesicles, we tested the effect of tetanus toxin light chain on the release of preinternalized transferrin from streptolysin-O-perforated CHO cells. The release was found to be temperature and ATP dependent as well as NEM sensitive. Addition of tetanus toxin light chain, but not of a proteolytically inactive form of the toxin, resulted in a partial inhibition of transferrin release which correlated with the toxin-mediated cleavage of cellubrevin. The residual release of transferrin occurring after complete cellubrevin degradation was still ATP dependent. Our results indicate that cellubrevin plays an important role in the constitutive exocytosis of vesicles which recycle plasmalemma receptors. The incomplete inhibition of transferrin release produced by the toxin suggests the existence of a cellubrevin-independent exocytotic mechanism, which may involve tetanus toxin-insensitive proteins of the synaptobrevin/VAMP family.

title: [Tetanus neurotoxin-mediated cleavage of cellubrevin impairs epithelial cell migration and integrin-dependent cell adhesion].
abstract: A role for endocytosis and exocytosis in cell migration has been proposed but not yet demonstrated. Here, we show that cellubrevin (Cb), an early endosomal v-SNARE, mediates trafficking in the lamellipod of migrating epithelial cells and partially colocalizes with markers of focal contacts. Expression of tetanus neurotoxin, which selectively cleaves Cb, significantly reduced the speed of migrating epithelial cells. Furthermore, expression of tetanus neurotoxin enhanced the adhesion of epithelial cells to collagen, laminin, fibronectin, and E-cadherin; altered spreading on collagen; and impaired the recycling of β1 integrins. These results suggest that Cb-dependent membrane trafficking participates in cell motility through the regulation of cell adhesion.

title: Lamellipodium extension and membrane ruffling require different SNARE-mediated trafficking pathways
abstract: BackgroundIntracellular membrane traffic is an essential component of the membrane remodeling that supports lamellipodium extension during cell adhesion. The membrane trafficking pathways that contribute to cell adhesion have not been fully elucidated, but recent studies have implicated SNARE proteins. Here, the functions of several SNAREs (SNAP23, VAMP3, VAMP4 and syntaxin13) are characterized during the processes of cell spreading and membrane ruffling.ResultsWe report the first description of a SNARE complex, containing SNAP23, syntaxin13 and cellubrevin/VAMP3, that is induced by cell adhesion to an extracellular matrix. Impairing the function of the SNAREs in the complex using inhibitory SNARE domains disrupted the recycling endosome, impeded delivery of integrins to the cell surface, and reduced haptotactic cell migration and spreading. Blocking SNAP23 also inhibited the formation of PMA-stimulated, F-actin-rich membrane ruffles; however, membrane ruffle formation was not significantly altered by inhibition of VAMP3 or syntaxin13. In contrast, membrane ruffling, and not cell spreading, was sensitive to inhibition of two SNAREs within the biosynthetic secretory pathway, GS15 and VAMP4. Consistent with this, formation of a complex containing VAMP4 and SNAP23 was enhanced by treatment of cells with PMA. The results reveal a requirement for the function of a SNAP23-syntaxin13-VAMP3 complex in the formation of lamellipodia during cell adhesion and of a VAMP4-SNAP23-containing complex during PMA-induced membrane ruffling.ConclusionsOur findings suggest that different SNARE-mediated trafficking pathways support membrane remodeling during ECM-induced lamellipodium extension and PMA-induced ruffle formation, pointing to important mechanistic differences between these processes.

title: Recycling Endosome Membrane Incorporation into the Leading Edge Regulates Lamellipodia Formation and Macrophage Migration
abstract: In comparison to our knowledge of the recycling of adhesion receptors and actin assembly, exactly how the cell controls its surface membrane to form a lamellipodium during migration is poorly understood. Here, we show the recycling endosome membrane is incorporated into the leading edge of a migrating cell to expand lamellipodia membrane. We have identified the SNARE complex that is necessary for fusion of the recycling endosome with the cell surface, as consisting of the R‐SNARE VAMP3 on the recycling endosome partnering with the surface Q‐SNARE Stx4/SNAP23, which was found to translocate and accumulate on the leading edge of migrating cells. Increasing VAMP3‐mediated fusion of the recycling endosome with the surface increased membrane ruffling, while inhibition of VAMP3‐mediated fusion showed that incorporation of the recycling endosome is necessary for efficient lamellipodia formation. At the same time, insertion of this recycling endosome membrane also delivers its cargo integrin α5β1 to the cell surface. The loss of this extra membrane for lamellipodia expansion and delivery of cargo in cells resulted in macrophages with a diminished capacity to effectively migrate. Thus, the recycling endosome membrane is incorporated into the leading edge and this aids expansion of the lamellipodia and simultaneously delivers integrins necessary for efficient cell migration.

title: Regulation of integrin endocytic recycling and chemotactic cell migration by syntaxin 6 and VAMP3 interaction
abstract: Integrins are the primary receptors of cells adhering to the extracellular matrix, and play key roles in various cellular processes including migration, proliferation and survival. The expression and distribution of integrins at the cell surface is controlled by endocytosis and recycling. The present study examines the function of syntaxin 6 (STX6), a t-SNARE located in the trans-Golgi network, in integrin trafficking. STX6 is overexpressed in many types of human cancer. We show that depletion of STX6 inhibits chemotactic cell migration and the delivery of the laminin receptor &agr;3&bgr;1 integrin to the cell surface, whereas STX6 overexpression stimulates chemotactic cell migration, integrin delivery, and integrin-initiated activation of focal adhesion kinase. These data indicate that STX6 plays a rate-limiting role in cell migration and integrin trafficking. In STX6-depleted cells, &agr;3&bgr;1 integrin is accumulated in recycling endosomes that contain the v-SNARE VAMP3. Importantly, we show that STX6 and VAMP3 form a v-/t-SNARE complex, VAMP3 is required in &agr;3&bgr;1 integrin delivery to the cell surface, and endocytosed &agr;3&bgr;1 integrin traffics to both VAMP3 and STX6 compartments. Collectively, our data suggest a new integrin trafficking pathway in which endocytosed integrins are transported from VAMP3-containing recycling endosomes to STX6-containing trans-Golgi network before being recycled to the plasma membrane.

title: VAMP3 regulates podosome organisation in macrophages and together with Stx4/SNAP23 mediates adhesion, cell spreading and persistent migration.
abstract: The ability of cells to adhere, spread and migrate is essential to many physiological processes, particularly in the immune system where cells must traffic to sites of inflammation and injury. By altering the levels of individual components of the VAMP3/Stx4/SNAP23 complex we show here that this SNARE complex regulates efficient macrophage adhesion, spreading and migration on fibronectin. During cell spreading this complex mediates the polarised exocytosis of VAMP3-positive recycling endosome membrane into areas of membrane expansion, where VAMP3's surface partner Q-SNARE complex Stx4/SNAP23 was found to accumulate. Lowering the levels of VAMP3 in spreading cells resulted in a more rounded cell morphology and most cells were found to be devoid of the typical ring-like podosome superstructures seen normally in spreading cells. In migrating cells lowering VAMP3 levels disrupted the polarised localisation of podosome clusters. The reduced trafficking of recycling endosome membrane to sites of cell spreading and the disorganised podosome localisation in migrating macrophages greatly reduced their ability to persistently migrate on fibronectin. Thus, this important SNARE complex facilitates macrophage adhesion, spreading, and persistent macrophage migration on fibronectin through the delivery of VAMP3-positive membrane with its cargo to expand the plasma membrane and to participate in organising adhesive podosome structures.

title: Chronic Blockade of Glutamate Receptors Enhances Presynaptic Release and Downregulates the Interaction between Synaptophysin-Synaptobrevin–Vesicle-Associated Membrane Protein 2
abstract: During development of neuronal circuits, presynaptic and postsynaptic functions are adjusted in concert, to optimize interneuronal signaling. We have investigated whether activation of glutamate receptors affects presynaptic function during synapse formation, when constitutive synaptic vesicle recycling is downregulated. Using primary cultures of hippocampal neurons as a model system, we have found that chronic exposure to both NMDA and non-NMDA glutamate receptor blockers during synaptogenesis produces an increase in miniature EPSC (mEPSC) frequency, with no significant changes in mEPSC amplitude or in the number of synapses. Enhanced synaptic vesicle recycling, selectively in glutamatergic nerve terminals, was confirmed by the increased uptake of antibodies directed against the lumenal domain of synaptotagmin. No increased uptake was detected in neuronal cultures grown in the chronic presence of TTX, speaking against an indirect effect caused by decreased electrical activity. Enhanced mEPSC frequency correlated with a reduction of synaptophysin-synaptobrevin–vesicle-associated membrane protein 2 (VAMP2) complexes detectable by immunoprecipitation. Intracellular perfusion with a peptide that inhibits the binding of synaptophysin to synaptobrevin–VAMP2 induced a remarkable increase of mEPSC frequency in control but not in glutamate receptor blocker-treated neurons. These findings suggest that activation of glutamate receptors plays a role in the downregulation of the basal rate of synaptic vesicle recycling that accompanies synapse formation. They also suggest that one of the mechanisms through which this downregulation is achieved is an increased interaction of synaptophysin with synaptobrevin–VAMP2.

title: The vesicular SNARE Synaptobrevin is required for Semaphorin 3A axonal repulsion
abstract: Semaphorin 3A-mediated signaling and axonal repulsion in the mouse brain require Synaptobrevin-dependent vesicular traffic.

title: Attractive axon guidance involves asymmetric membrane transport and exocytosis in the growth cone
abstract: Asymmetric elevation of the Ca2+ concentration in the growth cone can mediate both attractive and repulsive axon guidance. Ca2+ signals that are accompanied by Ca2+-induced Ca2+ release (CICR) trigger attraction, whereas Ca2+ signals that are not accompanied by CICR trigger repulsion. The molecular machinery downstream of Ca2+ signals, however, remains largely unknown. Here we report that asymmetric membrane trafficking mediates growth cone attraction. Local photolysis of caged Ca2+, together with CICR, on one side of the growth cone of a chick dorsal root ganglion neuron facilitated the microtubule-dependent centrifugal transport of vesicles towards the leading edge and their subsequent vesicle-associated membrane-protein 2 (VAMP2)–mediated exocytosis on the side with an elevated Ca2+ concentration. In contrast, Ca2+ signals without CICR had no effect on the vesicle transport. Furthermore, pharmacological inhibition of VAMP2-mediated exocytosis prevented growth cone attraction, but not repulsion. These results strongly suggest that growth cone attraction and repulsion are driven by distinct mechanisms, rather than using the same molecular machinery with opposing polarities.

title: Relevance of Exocytotic Glutamate Release from Retinal Glia
abstract: Glial cells release molecules that influence brain development, function, and disease. Calcium-dependent exocytosis has been proposed as potential release mechanism in astroglia, but the physiological relevance of "gliotransmission" in vivo remains controversial. We focused on the impact of glial exocytosis on sensory transduction in the retina. To this end, we generated transgenic mice to block exocytosis by Cre recombinase-dependent expression of the clostridial botulinum neurotoxin serotype B light chain, which cleaves vesicle-associated membrane protein 1-3. Ubiquitous and neuronal toxin expression caused perinatal lethality and a reduction of synaptic transmission thus validating transgene function. Toxin expression in Müller cells inhibited vesicular glutamate release and impaired glial volume regulation but left retinal histology and visual processing unaffected. Our model to study gliotransmission in vivo reveals specific functions of exocytotic glutamate release in retinal glia.

title: Patterning of frontal cortex subdivisions by Fgf17
abstract: The frontal cortex (FC) is the seat of higher cognition. The genetic mechanisms that control formation of the functionally distinct subdivisions of the FC are unknown. Using a set of gene expression markers that distinguish subdivisions of the newborn mouse FC, we show that loss of Fgf17 selectively reduces the size of the dorsal FC whereas ventral/orbital FC appears normal. These changes are complemented by a rostral shift of sensory cortical areas. Thus, Fgf17 functions similar to Fgf8 in patterning the overall neocortical map but has a more selective role in regulating the properties of the dorsal but not ventral FC.

title: A local Wnt-3a signal is required for development of the mammalian hippocampus.
abstract: The mechanisms that regulate patterning and growth of the developing cerebral cortex remain unclear. Suggesting a role for Wnt signaling in these processes, multiple Wnt genes are expressed in selective patterns in the embryonic cortex. We have examined the role of Wnt-3a signaling at the caudomedial margin of the developing cerebral cortex, the site of hippocampal development. We show that Wnt-3a acts locally to regulate the expansion of the caudomedial cortex, from which the hippocampus develops. In mice lacking Wnt-3a, caudomedial cortical progenitor cells appear to be specified normally, but then underproliferate. By mid-gestation, the hippocampus is missing or represented by tiny populations of residual hippocampal cells. Thus, Wnt-3a signaling is crucial for the normal growth of the hippocampus. We suggest that the coordination of growth with patterning may be a general role for Wnts during vertebrate development.

title: Early B-cell factors 2 and 3 (EBF2/3) regulate early migration of Cajal–Retzius cells from the cortical hem
abstract: Cajal–Retzius (CR) cells play a crucial role in the formation of the cerebral cortex, yet the molecules that control their development are largely unknown. Here, we show that Ebf transcription factors are expressed in forebrain signalling centres—the septum, cortical hem and the pallial–subpallial boundary—known to generate CR cells. We identified Ebf2, through fate mapping studies, as a novel marker for cortical hem- and septum-derived CR cells. Loss of Ebf2 in vivo causes a transient decrease in CR cell numbers on the cortical surface due to a migratory defect in the cortical hem, and is accompanied by upregulation of Ebf3 in this and other forebrain territories that produce CR cells, without affecting proper cortical lamination. Accordingly, using in vitro preparations, we demonstrated that both Ebf2 and Ebf3, singly or together, control the migration of CR cells arising in the cortical hem. These findings provide evidence that Ebfs directly regulate CR cell development.

title: Meninges control tangential migration of hem-derived Cajal-Retzius cells via CXCL12/CXCR4 signaling
abstract: Cajal-Retzius cells are critical in the development of the cerebral cortex, but little is known about the mechanisms controlling their development. Three focal sources of Cajal-Retzius cells have been identified in mice—the cortical hem, the ventral pallium and the septum—from where they migrate tangentially to populate the cortical surface. Using a variety of tissue culture assays and in vivo manipulations, we demonstrate that the tangential migration of cortical hem–derived Cajal-Retzius cells is controlled by the meninges. We show that the meningeal membranes are a necessary and sufficient substrate for the tangential migration of Cajal-Retzius cells. We also show that the chemokine CXCL12 secreted by the meninges enhances the dispersion of Cajal-Retzius cells along the cortical surface, while retaining them within the marginal zone in a CXCR4-dependent manner. Thus, the meningeal membranes are fundamental in the development of Cajal-Retzius cells and, hence, in the normal development of the cerebral cortex.

title: Sema3E/PlexinD1 regulates the migration of hem-derived Cajal-Retzius cells in developing cerebral cortex.
abstract: During the development of the cerebral cortex, Cajal-Retzius (CR) cells settle in the preplate and coordinate the precise growth of the neocortex. Indeed, CR cells migrate tangentially from specific proliferative regions of the telencephalon (for example, the cortical hem (CH)) to populate the entire cortical surface. This is a very finely tuned process regulated by an emerging number of factors that has been sequentially revealed in recent years. However, the putative participation of one of the major families of axon guidance molecules in this process, the Semaphorins, was not explored. Here we show that Semaphorin-3E (Sema3E) is a natural negative regulator of the migration of PlexinD1-positive CR cells originating in the CH. Our results also indicate that Sema3E/PlexinD1 signalling controls the motogenic potential of CR cells in vitro and in vivo. Indeed, absence of Sema3E/PlexinD1 signalling increased the migratory properties of CR cells. This modulation implies negative effects on CXCL12/CXCR4 signalling and increased ADF/Cofilin activity.

title: Steering Neuronal Growth Cones by Shifting the Imbalance between Exocytosis and Endocytosis
abstract: Extracellular molecular cues guide migrating growth cones along specific routes during development of axon tracts. Such processes rely on asymmetric elevation of cytosolic Ca2+ concentrations across the growth cone that mediates its attractive or repulsive turning toward or away from the side with Ca2+ elevation, respectively. Downstream of these Ca2+ signals, localized activation of membrane trafficking steers the growth cone bidirectionally, with endocytosis driving repulsion and exocytosis causing attraction. However, it remains unclear how Ca2+ can differentially regulate these opposite membrane-trafficking events. Here, we show that growth cone turning depends on localized imbalance between exocytosis and endocytosis and identify Ca2+-dependent signaling pathways mediating such imbalance. In embryonic chicken dorsal root ganglion neurons, repulsive Ca2+ signals promote clathrin-mediated endocytosis through a 90 kDa splice variant of phosphatidylinositol-4-phosphate 5-kinase type-1γ (PIPKIγ90). In contrast, attractive Ca2+ signals facilitate exocytosis but suppress endocytosis via Ca2+/calmodulin-dependent protein kinase II (CaMKII) and cyclin-dependent kinase 5 (Cdk5) that can inactivate PIPKIγ90. Blocking CaMKII or Cdk5 leads to balanced activation of both exocytosis and endocytosis that causes straight growth cone migration even in the presence of guidance signals, whereas experimentally perturbing the balance restores the growth cone's turning response. Remarkably, the direction of this resumed turning depends on relative activities of exocytosis and endocytosis, but not on the type of guidance signals. Our results suggest that navigating growth cones can be redirected by shifting the imbalance between exocytosis and endocytosis, highlighting the importance of membrane-trafficking imbalance for axon guidance and, possibly, for polarized cell migration in general.

title: Polarized Signaling Endosomes Coordinate BDNF-Induced Chemotaxis of Cerebellar Precursors
abstract: During development, neural precursors migrate in response to positional cues such as growth factor gradients. However, the mechanisms that enable precursors to sense and respond to such gradients are poorly understood. Here we show that cerebellar granule cell precursors (GCPs) migrate along a gradient of brain-derived neurotrophic factor (BDNF), and we demonstrate that vesicle trafficking is critical for this chemotactic process. Activation of TrkB, the BDNF receptor, stimulates GCPs to secrete BDNF, thereby amplifying the ambient gradient. The BDNF gradient stimulates endocytosis of TrkB and associated signaling molecules, causing asymmetric accumulation of signaling endosomes at the subcellular location where BDNF concentration is maximal. Thus, regulated BDNF exocytosis and TrkB endocytosis enable precursors to polarize and migrate in a directed fashion along a shallow BDNF gradient.

title: Confinement and Low Adhesion Induce Fast Amoeboid Migration of Slow Mesenchymal Cells
abstract: The mesenchymal-amoeboid transition (MAT) was proposed as a mechanism for cancer cells to adapt their migration mode to their environment. While the molecular pathways involved in this transition are well documented, the role of the microenvironment in the MAT is still poorly understood. Here, we investigated how confinement and adhesion affect this transition. We report that, in the absence of focal adhesions and under conditions of confinement, mesenchymal cells can spontaneously switch to a fast amoeboid migration phenotype. We identified two main types of fast migration--one involving a local protrusion and a second involving a myosin-II-dependent mechanical instability of the cell cortex that leads to a global cortical flow. Interestingly, transformed cells are more prone to adopt this fast migration mode. Finally, we propose a generic model that explains migration transitions and predicts a phase diagram of migration phenotypes based on three main control parameters: confinement, adhesion, and contractility.

title: EGF-receptor-mediated mammary epithelial cell migration is driven by sustained ERK signaling from autocrine stimulation
abstract: EGF family ligands are synthesized as membrane-anchored precursors whose proteolytic release yields mature diffusible factors that can activate cell surface receptors in autocrine or paracrine mode. Expression of these ligands is altered in pathological states and in physiological processes, such as development and tissue regeneration. Despite the widely documented biological importance of autocrine EGF signaling, quantitative relationships between protease-mediated ligand release and consequent cell behavior have not been rigorously investigated. We thus explored the relationship between autocrine EGF release rates and cell behavioral responses along with activation of ERK, a key downstream signal, by expressing chimeric ligand precursors and modulating their proteolytic shedding using a metalloprotease inhibitor in human mammary epithelial cells. We found that ERK activation increased monotonically with increasing ligand release rate despite concomitant downregulation of EGF receptor levels. Cell migration speed was directly related to ligand release rate and proportional to steady-state phospho-ERK levels. Moreover, migration speed was significantly greater for autocrine stimulation compared with exogenous stimulation, even at comparable phospho-ERK levels. By contrast, cell proliferation rates were approximately equivalent at all ligand release rates and were similar regardless of whether the ligand was presented endogenously or exogenously. Thus, in our mammary epithelial cell system, migration and proliferation are differentially sensitive to the mode of EGF ligand presentation.

title: How does Reelin control neuronal migration and layer formation in the developing mammalian neocortex?
abstract: The mammalian neocortex has a laminar structure that develops in a birth-date-dependent "inside-out" pattern. Its layered structure is established by neuronal migration accompanied by sequential changes in migratory mode regulated by several signaling cascades. Although Reelin was discovered about two decades ago and is one of the best known molecules that is indispensable to the establishment of the "inside-out" neuron layers, the cellular and molecular functions of Reelin in layer formation are still largely unknown. In this review article, we summarize our recent understanding of Reelin's functions during neuronal migration. Reelin acts in at least two different steps of neuronal migration: the final step of neuronal migration (somal/terminal translocation) just beneath the marginal zone (MZ) and the regulation of cell polarity step when the neurons change their migratory mode from multipolar migration to locomotion. During the translocation mode, Reelin activates integrin α5β1 through an intracellular pathway that triggers the translocation and activates N-cadherin in concert with the nectin-afadin system. Reelin is also involved in the termination of neuronal migration by degrading Dab1 via the SOCS7-Cullin5-Rbx2 system, and Reelin has been found to induce the birth-date-dependent neuronal aggregation in vivo. Based on these findings, we hypothesize that the molecular function of Reelin during neuronal migration is to control cell-adhesiveness during development by regulating the expression/activation of cell adhesion molecules.

title: Reelin Controls Progenitor Cell Migration in the Healthy and Pathological Adult Mouse Brain
abstract: Understanding the signals that control migration of neural progenitor cells in the adult brain may provide new therapeutic opportunities. Reelin is best known for its role in regulating cell migration during brain development, but we now demonstrate a novel function for reelin in the injured adult brain. First, we show that Reelin is upregulated around lesions. Second, experimentally increasing Reelin expression levels in healthy mouse brain leads to a change in the migratory behavior of subventricular zone-derived progenitors, triggering them to leave the rostral migratory stream (RMS) to which they are normally restricted during their migration to the olfactory bulb. Third, we reveal that Reelin increases endogenous progenitor cell dispersal in periventricular structures independently of any chemoattraction but via cell detachment and chemokinetic action, and thereby potentiates spontaneous cell recruitment to demyelination lesions in the corpus callosum. Conversely, animals lacking Reelin signaling exhibit reduced endogenous progenitor recruitment at the lesion site. Altogether, these results demonstrate that beyond its known role during brain development, Reelin is a key player in post-lesional cell migration in the adult brain. Finally our findings provide proof of concept that allowing progenitors to escape from the RMS is a potential therapeutic approach to promote myelin repair.

title: Dab1 and reelin participate in a common signal pathway that controls intestinal crypt/villus unit dynamics
abstract: The myofibroblasts placed underneath the epithelium of the rodent small intestine express reelin, and the reelin absence modifies both the morphology and the cell renewal processes of the crypt–villus unit. In the developing central nervous system, the reelin effects are mediated by the disabled‐1 (Dab1) protein. The present work explores whether Dab1 mediates the reelin control of the crypt–villus unit dynamics by examining in the mouse small intestine the consequences of the absence of (i) Dab1 (scrambler mutation) on crypt–villus unit cell renewal processes and morphology and (ii) reelin (reeler mutation) on the intestinal expression of Dab1.

title: Active Wnt proteins are secreted on exosomes
abstract: Wnt signalling has important roles during development and in many diseases. As morphogens, hydrophobic Wnt proteins exert their function over a distance to induce patterning and cell differentiation decisions. Recent studies have identified several factors that are required for the secretion of Wnt proteins; however, how Wnts travel in the extracellular space remains a largely unresolved question. Here we show that Wnts are secreted on exosomes both during Drosophila development and in human cells. We demonstrate that exosomes carry Wnts on their surface to induce Wnt signalling activity in target cells. Together with the cargo receptor Evi/WIs, Wnts are transported through endosomal compartments onto exosomes, a process that requires the R-SNARE Ykt6. Our study demonstrates an evolutionarily conserved functional role of extracellular vesicular transport of Wnt proteins.

title: The cortical hem regulates the size and patterning of neocortex
abstract: The cortical hem, a source of Wingless-related (WNT) and bone morphogenetic protein (BMP) signaling in the dorsomedial telencephalon, is the embryonic organizer for the hippocampus. Whether the hem is a major regulator of cortical patterning outside the hippocampus has not been investigated. We examined regional organization across the entire cerebral cortex in mice genetically engineered to lack the hem. Indicating that the hem regulates dorsoventral patterning in the cortical hemisphere, the neocortex, particularly dorsomedial neocortex, was reduced in size in late-stage hem-ablated embryos, whereas cortex ventrolateral to the neocortex expanded dorsally. Unexpectedly, hem ablation also perturbed regional patterning along the rostrocaudal axis of neocortex. Rostral neocortical domains identified by characteristic gene expression were expanded, and caudal domains diminished. A similar shift occurs when fibroblast growth factor (FGF) 8 is increased at the rostral telencephalic organizer, yet the FGF8 source was unchanged in hem-ablated brains. Rather we found that hem WNT or BMP signals, or both, have opposite effects to those of FGF8 in regulating transcription factors that control the size and position of neocortical areas. When the hem is ablated a necessary balance is perturbed, and cerebral cortex is rostralized. Our findings reveal a much broader role for the hem in cortical development than previously recognized, and emphasize that two major signaling centers interact antagonistically to pattern cerebral cortex.

title: Radial glia regulate Cajal-Retzius cell positioning in the early embryonic cerebral cortex.
abstract: The organization of neocortex, along its radial axis, into a six-layered structure is one of the most exquisite features of the brain. Because of their strategic localization in the marginal zone, and their expression of reelin, a signal that controls spatial ordering of cortical layers, Cajal-Retzius (C-R) cells play a crucial role in cortical patterning along this axis. Yet, it remains less well understood how C-R cell targeting itself is regulated. At the onset of corticogenesis when C-R cells first arrive in the cortex via tangential migration, radial glia (RG) are the main cell type present. This suggests that RG may play a role in C-R cell localization. To test this, we used genetic approaches to perturb RG scaffold during early corticogenesis. We found that disrupting RG endfoot adhesion to basal lamina consistently results in C-R cell displacement. These displacements do not appear to result from primary defects in neural progenitor cell proliferation, deficits in the meninges or basement membrane, or cell autonomous defects in C-R cells. Instead, they show close temporal and spatial correlation with RG endfoot retraction. Moreover, ablation of RG via cell cycle blockade similarly results in local displacement of C-R cells. These lines of evidence thus indicate that, during early corticogenesis, RG play a primary role in regulating spatial targeting of C-R cells. Since RG are also neural progenitors as well as neuronal migration scaffolds, these findings suggest that, during nervous system development, neuroepithelial stem cells may not only be responsible for generating a diverse array of neuronal cell types and facilitating their radial migration. They may also, through regulating the placement of guidepost cells, coordinate spatial patterning of the nervous system along its radial axis.

title: A revised view of sensory cortical parcellation
abstract: Traditional cortical parcellation schemes have emphasized the presence of sharply defined visual, auditory, and somatosensory domains populated exclusively by modality-specific neurons (i.e., neurons responsive to sensory stimuli from a single sensory modality). However, the modality-exclusivity of this scheme has recently been challenged. Observations in a variety of species suggest that each of these domains is subject to influences from other senses. Using the cerebral cortex of the rat as a model, the present study systematically examined the capability of individual neurons in visual, auditory, and somatosensory cortex to be activated by stimuli from other senses. Within the major modality-specific domains, the incidence of inappropriate (i.e., nonmatching) and/or multisensory neurons was very low. However, at the borders between each of these domains a concentration of multisensory neurons was found whose modality profile matched the representations in neighboring cortices and that were able to integrate their cross-modal inputs to give rise to enhanced and/or depressed responses. The results of these studies are consistent with some features of both the traditional and challenging views of cortical organization, and they suggest a parcellation scheme in which modality-specific cortical domains are separated from one another by transitional multisensory zones.

title: Evidence for two organizational plans within the somatic sensory‐motor cortex of the rat
abstract: Previous electrophysiological mapping of somatic sensory (SI) and motor (MI) cortex in the rat has shown that these functional regions are completely overlapping in the hindlimb (HL) area. Partially separate in the forelimb area, and completel separate in the face area. The present studies were designed to show how the sensory and motor thalamic relay nuclei priject onto separate or overlapping subdivisions of SI and MI cortex. The experimental sequence involved physiological mapping of one subdivision of SI or MI cortex and then injecting horseradish peroxidase (HRP) as a retrograde cell marker to back‐fill the thalamic neurons that project to the identified area of cortex. The central issue was to compare the projection of the lemniscal‐recipient ventrobasal nucleus (VB) and the cerebellar‐recipient ventrolateral nucleus (VL) to the hindlimb and face subdivision of SI and MI cortex. Various authors, however have divided the ventral nucleus differently using Nisslstained transverse sections through the thalamus. Therefore, tritiated amino acids were also injected into the deep cerebellar nuclei and the distribution of cerebellar fiber terminals in the ventral nucleus was used as another criterion to identify the border between VB and VL.