Disruption of layers 3 and 4 during development results in altered thalamocortical projections in ferret somatosensory cortex

Roots of the Malformations of Cortical Development in the Cell Biology of Neural Progenitor Cells

The cerebral cortex is a structure that underlies various brain functions, including cognition and language. Mammalian cerebral cortex starts developing during the embryonic period with the neural progenitor cells generating neurons. Newborn neurons migrate along progenitors’ radial processes from the site of their origin in the germinal zones to the cortical plate, where they mature and integrate in the forming circuitry. Cell biological features of neural progenitors, such as the location and timing of their mitoses, together with their characteristic morphologies, can directly or indirectly regulate the abundance and the identity of their neuronal progeny. Alterations in the complex and delicate process of cerebral cortex development can lead to malformations of cortical development (MCDs). They include various structural abnormalities that affect the size, thickness and/or folding pattern of the developing cortex. Their clinical manifestations can entail a neurodevelopmental disorder, such as epilepsy, developmental delay, intellectual disability, or autism spectrum disorder. The recent advancements of molecular and neuroimaging techniques, along with the development of appropriate in vitro and in vivo model systems, have enabled the assessment of the genetic and environmental causes of MCDs. Here we broadly review the cell biological characteristics of neural progenitor cells and focus on those features whose perturbations have been linked to MCDs.

The Ferret as a Model System for Neocortex Development and Evolution

The neocortex is the largest part of the cerebral cortex and a key structure involved in human behavior and cognition. Comparison of neocortex development across mammals reveals that the proliferative capacity of neural stem and progenitor cells and the length of the neurogenic period are essential for regulating neocortex size and complexity, which in turn are thought to be instrumental for the increased cognitive abilities in humans. The domesticated ferret, Mustela putorius furo, is an important animal model in neurodevelopment for its complex postnatal cortical folding, its long period of forebrain development and its accessibility to genetic manipulation in vivo. Here, we discuss the molecular, cellular, and histological features that make this small gyrencephalic carnivore a suitable animal model to study the physiological and pathological mechanisms for the development of an expanded neocortex. We particularly focus on the mechanisms of neural stem cell proliferation, neuronal differentiation, cortical folding, visual system development, and neurodevelopmental pathologies. We further discuss the technological advances that have enabled the genetic manipulation of the ferret in vivo. Finally, we compare the features of neocortex development in the ferret with those of other model organisms.

Building the Ferretome

Databases of structural connections of the mammalian brain, such as CoCoMac (cocomac.g-node.org) or BAMS (https://bams1.org), are valuable resources for the analysis of brain connectivity and the modeling of brain dynamics in species such as the non-human primate or the rodent, and have also contributed to the computational modeling of the human brain. Another animal model that is widely used in electrophysiological or developmental studies is the ferret; however, no systematic compilation of brain connectivity is currently available for this species. Thus, we have started developing a database of anatomical connections and architectonic features of the ferret brain, the Ferret(connect)ome, www.Ferretome.org. The Ferretome database has adapted essential features of the CoCoMac methodology and legacy, such as the CoCoMac data model. This data model was simplified and extended in order to accommodate new data modalities that were not represented previously, such as the cytoarchitecture of brain areas. The Ferretome uses a semantic parcellation of brain regions as well as a logical brain map transformation algorithm (objective relational transformation, ORT). The ORT algorithm was also adopted for the transformation of architecture data. The database is being developed in MySQL and has been populated with literature reports on tract-tracing observations in the ferret brain using a custom-designed web interface that allows efficient and validated simultaneous input and proofreading by multiple curators. The database is equipped with a non-specialist web interface. This interface can be extended to produce connectivity matrices in several formats, including a graphical representation superimposed on established ferret brain maps. An important feature of the Ferretome database is the possibility to trace back entries in connectivity matrices to the original studies archived in the system. Currently, the Ferretome contains 50 reports on connections comprising 20 injection reports with more than 150 labeled source and target areas, the majority reflecting connectivity of subcortical nuclei and 15 descriptions of regional brain architecture. We hope that the Ferretome database will become a useful resource for neuroinformatics and neural modeling, and will support studies of the ferret brain as well as facilitate advances in comparative studies of mesoscopic brain connectivity.

Altered migratory behavior of interneurons in a model of cortical dysplasia: the influence of elevated GABAA activity

Appropriate function of the neocortex depends on timely generation and migration of cells produced in the germinal zones of the neocortex and ganglionic eminence (GE). Failure to accurately complete migration results in cortical dysplasia, a developmental syndrome implicated in many neurologic disorders. We developed a model of cortical dysplasia in ferrets involving administration of methylaxozymethanol acetate (MAM), an antimitotic, to pregnant ferrets on gestational day 33, leading to dramatic reduction of layer 4 in the neocortex. Here, using time-lapse video imaging, we investigate dynamic behavior of migrating cells arising from the GE and cortical ventricular zone (CVZ) in ferrets and the role of GABAA activity. Treatment with MAM significantly reduced migration speed and the relative proportion of cells arising from the GE demonstrating exploratory behavior. To a lesser extent, the behavior of cells leaving the CVZ was affected. Pharmacologic inhibition of GABAA receptors (GABAAR) improved the speed of migration and exploratory ability of migrating MAM-treated cells arising from the GE. Additionally, the expression of α2 and α3 subunits of GABAAR and the potassium chloride co-transporter (KCC2) increased in the neocortex of MAM-treated animals. After MAM treatment, increases in endogenous KCC2 and GABAAR combine to alter the dynamic properties and exploratory behavior of migrating interneurons in ferrets. We show a direct correlation between increased GABAA and KCC2 expression with impaired migration and ability to explore the environment.

Targeted disruption of layer 4 during development increases GABAA receptor neurotransmission in the neocortex

Cortical dysplasia (CD) associates with clinical pathologies, including epilepsy and mental retardation. CD results from impaired migration of immature neurons to their cortical targets, leading to clustering of neural cells and changes in cortical properties. We developed a CD model by administering methylazoxymethanol (MAM), an anti-mitotic, to pregnant ferrets on embryonic day 33; this leads to reduction in cortical thickness in addition to redistribution and increased expression of GABAA receptors (GABAAR). We evaluated the impact of MAM treatment on GABAAR-mediated synaptic transmission in postnatal day 0-1 neurons, leaving the ganglionic eminence (GE) and in layer 2/3 pyramidal cells of postnatal day 28-38 ferrets. Embryonic day 33 MAM treatment significantly increases the amplitude and frequency of spontaneous GABAAR-mediated inhibitory postsynaptic currents (IPSCs) in the cells leaving the GE. In older MAM-treated animals, the amplitude and frequency of GABAAR-mediated spontaneous IPSCs in layer 2/3 pyramidal cells is increased, as are the amplitude and frequency of miniature IPSCs. The kinetics of GABAAR opening also altered following treatment with MAM. Western blot analysis shows that the expression of the GABAAα3R and GABAAγ2R subunits amplified in our model animals. We did not observe any significant change in the passive properties of either the layer 2/3 pyramidal cells or cells leaving the GE after MAM treatment. These observations reinforce the idea that synaptic neurotransmission through GABAAR enhances following treatment with MAM and coincides with our finding of increased GABAAαR expression within the upper cortical layers. Overall, we demonstrate that small amounts of toxins delivered during corticogenesis can result in long-lasting changes in ambient expression of GABAAR that influence intrinsic neuronal properties.

Fine-tuning of neurogenesis is essential for the evolutionary expansion of the cerebral cortex

We used several animal models to study global and regional cortical surface expansion: The lissencephalic mouse, gyrencephalic normal ferrets, in which the parietal cortex expands more than the temporal cortex, and moderately lissencephalic ferrets, showing a similar degree of temporal and parietal expansion. We found that overall cortical surface expansion is achieved when specific events occur prior to surpragranular layer formation. (1) The subventricular zone (SVZ) shows substantial growth, (2) the inner SVZ contains an increased number of outer radial glia and intermediate progenitor cells expressing Pax6, and (3) the outer SVZ contains a progenitor cell composition similar to the combined VZ and inner SVZ. A greater parietal expansion is also achieved by eliminating the latero-dorsal neurogenic gradient, so that neurogenesis displays a similar developmental degree between parietal and temporal regions. In contrast, mice or lissencephalic ferrets show more advanced neurogenesis in the temporal region. In conclusion, we propose that global and regional cortical surface expansion rely on similar strategies consisting in altering the timing of neurogenic events prior to the surpragranular layer formation, so that more progenitor cells, and ultimately more neurons, are produced. This hypothesis is supported by findings from a ferret model of lissencephaly obtained by transiently blocking neurogenesis during the formation of layer IV.

Reelin is essential for neuronal migration but not for radial glial elongation in neonatal ferret cortex

Numerous functions related to neuronal migration are linked to the glycoprotein reelin. Reelin also elongates radial glia, which are disrupted in mutant reeler mice. Our lab developed a model of cortical dysplasia in ferrets that shares features with the reeler mouse, including impaired migration of neurons into the cerebral cortex and disrupted radial glia. Explants of normal ferret cortex in coculture with dysplastic ferret cortex restore the deficits in this model. To determine if reelin is integral to the repair, we used explants of P0 mouse cortex either of the wild type (WT) or heterozygous (het) for the reelin gene, as well as P0 reeler cortex (not containing reelin), in coculture with organotypic cultures of dysplastic ferret cortex. This arrangement revealed that all types of mouse cortical explants (WT, het, reeler) elongated radial glia in ferret cortical dysplasia, indicating that reelin is not required for proper radial glial morphology. Migration of cells into ferret neocortex, however, did not improve with explants of reeler cortex, but was almost normal after pairing with WT or het explants. We also placed an exogenous source of reelin in ferret cultures at the pial surface to reveal that migrating cells move toward the reelin source in dysplastic cortex; radial glia in these cultures were also improved toward normal. Our results demonstrate that the normotopic position of reelin is important for proper neuronal positioning, and that reelin is capable of elongating radial glial cells but is not the only radialization factor.

Migration of transplanted neural progenitor cells in a ferret model of cortical dysplasia

Although altered gene expression clearly causes failure of the neocortex to form properly, many causes of neocortical dysplasia arise from environmental or unknown factors. Our lab studies a model of cortical dysplasia induced by injection of methylazoxymethanol (MAM) into pregnant ferrets on embryonic day 33 (E33), which shares many features of neocortical dysplasia in humans. E33 MAM treatment results in characteristic deficits that include dramatic reduction of layer 4 in somatosensory cortex, widespread termination of thalamic afferents, and altered distribution of GABAergic elements. We determined the ability of immature cells to migrate into MAM-treated cortex using ferret neural progenitor cells obtained at E27 and E33 and mouse neural progenitor cells obtained at E14. When these cells were transplanted into organotypic cultures obtained from normal and E33 MAM-treated ferret cortex prepared on postnatal day 0 (P0), all progenitor cells migrated similarly in both hosts, preferentially residing in the upper cortical plate. The site of transplantation was significant, however, so that injections into the ventricular zone were more likely to reach the cortical plate than transplants into the intermediate zone. When similar cells were transplanted into ferret kits, approximately P7-P9, and allowed to survive for 2-4 weeks, the donor cells migrated differently and also reached distinct destinations in normal and MAM-treated hosts. MAM-treated cortex was more permissive to invasion by donor cells as they migrated to widespread aspects of the cortex, whereas transplants in normal host cortex were more restricted. E27 neural progenitor cells populated more cortical layers than later born E33 neural progenitor cells, suggesting that the fate of transplanted cells is governed by a combination of extrinsic and intrinsic factors.

Embryonic and Postnatal Development of the Layer I–Directed (“Matrix”) Thalamocortical System in the Rat

Inputs to the layer I apical dendritic tufts of pyramidal cells are crucial in "top-down" interactions in the cerebral cortex. A large population of thalamocortical cells, the "matrix" (M-type) cells, provides a direct robust input to layer I that is anatomically and functionally different from the thalamocortical input to layer VI. The developmental timecourse of M-type axons is examined here in rats aged E (embryonic day) 16 to P (postnatal day) 30. Anterograde techniques were used to label axons arising from 2 thalamic nuclei mainly made up of M-type cells, the Posterior and the Ventromedial. The primary growth cones of M-type axons rapidly reached the subplate of dorsally situated cortical areas. After this, interstitial branches would sprout from these axons under more lateral cortical regions to invade the overlying cortical plate forming secondary arbors. Moreover, retrograde labeling of M-type cell somata in the thalamus after tracer deposits confined to layer I revealed that large numbers of axons from multiple thalamic nuclei had already converged in a given spot of layer I by P3. Because of early ingrowth in such large numbers, interactions of M-type axons may significantly influence the early development of cortical circuits.

Alteration of interneuron migration in a ferret model of cortical dysplasia

During cerebral cortical development, gamma-aminobutyric acidergic (GABAergic) interneurons arise from a different site than projection neurons. GABAergic cells are generated in the subpallial ganglionic eminence (GE), while excitatory projection neurons arise from the neocortical ventricular zone. Our laboratory studies a model of cortical dysplasia that displays specific disruption of GABAergic mechanisms and an alteration in the overall balance of excitation in the neocortex. To produce this model, the birth of neurons on a specific gestational day in ferrets (embryonic day 33 [E33]) is interrupted by injection of the antimitotic methylazoxymethanol (MAM). We hypothesized that migration of interneurons might be disrupted in this cortical dysplasia paradigm. We observed that although interneurons migrate into the neocortex in both normal and dysplastic cortex, the migrating cells become disoriented over time after E33 MAM treatment. Coculture experiments using normal GE and MAM-treated cortex (and vice versa) demonstrate that cues dictating proper orientation of migrating interneurons arise from the cortex and are not intrinsic to the migrating cells. As a consequence, interneurons in mature brains of MAM-treated animals are abnormally distributed. We report that GABA(A) receptor activation is crucial to the proper positioning of interneurons migrating into the cortex from the GE in normal and MAM-treated animals.

Cellular therapies in motor neuron diseases

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are prototypical motor neuron diseases that result in progressive weakness as a result of motor neuron dysfunction and death. Though much work has been done in both diseases to identify the cellular mechanisms of motor neuron dysfunction, once motor neurons have died, one of potential therapies to restore function would be through the use of cellular transplantation. In this review, we discuss potential strategies whereby cellular therapies, including the use of stem cells, neural progenitors and cells engineered to secrete trophic factors, may be used in motor neuron diseases. We review pre-clinical data in rodents with each of these approaches and discuss advances and regulatory issues regarding the use of cellular therapies in human motor neuron diseases.

Neocortical neuronal arrangement in Miller Dieker syndrome

Miller Dieker syndrome (MDS, type I lissencephaly) is a neuronal migration disorder, which is caused by deletions along the short arm of chromosome 17 (17p13.3). Recent studies would suggest that the cortical lamination in MDS is inverted, based on morphological criteria. The present neuropathological study examines the cerebral cortex from a 33-week old fetus with MDS using both neuronal and laminar-specific markers. These expression studies demonstrate a relatively preserved cortex and cortical lamination, overlying a layer of immature neurons in MDS brain. The findings are consistent with both a migratory and proliferative defect, giving rise to lissencephaly. Moreover, characterization of such rare human malformations of cortical development by immunohistochemical techniques will provide a greater understanding of the underlying mechanisms.

Disruption of layer 4 development alters laminar processing in ferret somatosensory cortex

Treatment with the anti-mitotic agent methylazoxymethanol (MAM) on embryonic day 33 (E33) in ferrets changes features of somatosensory cortex. These include dramatic reduction of cells in layer 4, and altered distributions of thalamocortical afferent terminations and GABA(A) receptors. To determine the effect of the relative absence of layer 4 on processing of sensory stimuli we used current source-density profiles to assess laminar activity patterns. Nearly synchronous activation occurs across all layers in treated animals, which contrasts with the normal cortical activation pattern of initial sinks in layer 4. This change after MAM treatment is consistent with the absence of layer 4 cells and widespread termination of thalamocortical afferents. Using periodic stimulation at 'flutter' frequency, layer 4 neurons in normal somatosensory cortex fire reproducibly to the stimulus rate; the capacity for entrainment is best for layer 4 and weaker in the extragranular layers. The capacity to encode periodic sensory stimuli is disrupted in MAM-treated somatosensory cortex; after an initial response to the onset of periodic stimuli, neurons in all cortical layers show weak entrainment. Neural responses to sensory drive in E33 MAM-treated cortex are also embedded in levels of neural activity substantially above those in normal somatosensory cortex. Sustained stimulation additionally reveals different capacities in each layer for improved signal-to-noise ratios, with layer 4 neurons in normal animals exhibiting the most improved signaling over time. We conclude that normal thalamic terminations, an intact layer 4 and subsequent intracortical processing are integral to proper encoding of stimulus features.

Experimentally-induced microencephaly: effects on cortical neurons

Genetic and epigenetic factors may alter the normal development of cerebral cortex, producing laminar and cellular abnormalities and heterotopiae, major causes of juvenile, drug-resistant epilepsy. Experimentally-induced migration disorders provide interesting insights in the mechanisms of the determination of neuronal phenotype and connectivity, of congenital cortical dysgenesis and the pathophysiology of associated neurological disorders, such as epilepsy. We investigated the effects of E14 administration of methylazoxymethanol acetate (MAM), which induces microencephaly by ablating dividing cells. Brains from newborn and adult rats were reacted for NADPH-d and CO histochemistry. Moreover, callosally-projecting neurons were retrogradely labeled with DiI at P9 or with BDA in adults. MAM-treated rats displayed a remarkable reduction in cortical thickness, mainly due to reduction in layer IV and in supragranular layers. Heterotopic nodules appeared in the supragranular layers and in the hippocampus. CO-positive barrels in somatosensory cortex were almost absent. The distribution of NADPH-d-positive neurons was regular, but they were rare in heterotopic nodules. Callosally-projecting neurons displayed abnormal orientation of the apical dendrite and increase in the basal dendritic length. Alterations in the dendritic arborization of pyramidal neurons may be one of the substrates for the increased sensitivity to drugs which induce epileptic seizures in these animals.

Influence of radial glia and Cajal-Retzius cells in neuronal migration

Normal development of cerebral cortex depends on proper sequential genesis of cortical neurons and glia. Disruption of corticogenesis in ferret by short-term arresting of cell division using injections of methylazoxy methanol (MAM) leads to a specific constellation of effects, including disruption and early differentiation of radial glia into astrocytes and disorganization of reelin-containing Cajal-Retzius cells. We hypothesize that early interference of normal cortical development removes a factor instrumental in maintaining radial glia in their normal elongated shape. In support of this idea, coculture of MAM-treated slices with normal cortical plate restores radial glia and Cajal-Retzius cells to their normal positions. Recently, we found that conditioned medium obtained from normal organotypic cultures returned radial glia toward their normal morphology only in a fraction of 30-50 kDa molecular weight (MW). To assess whether restoring this factor would also improve effective migration into the cortical plate of E24 MAM-treated animals, we conducted experiments using cocultures of normal cortical plate with organotypic cultures of MAM-treated cortex, which received prior BrdU injections. In both the normal and E24 MAM-treated/normal cortical plate coculture, a greater percentage of BrdU positive cells migrated effectively into the cortical plate. We suggest that early interruption of cell division eliminates a population of cells and a factor important for maintaining proper cortical development, specifically providing cues maintaining elongation of radial glia.

Developmental regulation of plasticity in the forepaw representation of ferret somatosensory cortex

This study characterized the spatiotemporal responses in ferret somatosensory cortex after sensory deprivation at different phases of cortical development. We hypothesized that cortical responses to stimulation of intact superficial radial nerve in adults will vary systematically according to maturation of thalamocortical relationships at the time of an ulnar nerve transection. Depending on the age of the animal at the time of the lesion, we found differential effects on the spatial distribution of the short- and long-latency components of the cortical response. In animals lesioned at postnatal days 5-7, when thalamic projections are not yet stabilized and layer 4 is not yet formed, we found that initial (short-latency) cortical responses are widespread and fragmented. Ulnar nerve transections performed at postnatal day 20 or 21, when thalamocortical afferents are more stabilized and layer 4 is clearly identifiable, yield moderate expansions in the distribution of short- and long-latency components of the cortical response. Nerve lesions in adults lead to a wider distribution of long-latency cortical activity. Neonatal lesions broaden the spatial distribution and increase the latency of the initial cortical response; interruption of nerve input in older juveniles alters both the early and later components; and nerve lesions in adult animals expand the distribution of later cortical activity only. These findings demonstrate correlation between developmental phase at the time sensory input is interrupted and the latency of affected components of the cortical response. This supports the hypothesis that differential response changes are regulated by functional reorganization of thalamocortical connections after neonatal lesions and alteration of corticocortical dynamics after adult lesions.

N-cadherin regulates ingrowth and laminar targeting of thalamocortical axons

Thalamocortical axons are precisely targeted to cortical layer IV, but the identity of specific molecules that govern the establishment of laminar specificity in the thalamocortical projection has been elusive. In this study, we test the role of N-cadherin, a homophilic cell adhesion molecule, in laminar targeting of thalamocortical axons using cocultured thalamic and cortical slice explants exposed to N-cadherin function-blocking antibodies or inhibitory peptides. In untreated cocultures, labeled thalamocortical axons normally grow to and stop in layer IV, forming terminal-like arbors. In the N-cadherin-blocked cocultures, thalamic axons reach layer IV by growing through deep layers at the same rate as those in the untreated cocultures, but instead of terminating in layer IV, they continue growing uninterruptedly through layer IV and extend into supragranular layers to reach the outermost cortical edge, where some form terminal-like arbors in this aberrant laminar position. In cocultures in which the cortical slice is taken at an earlier maturational stage, one that corresponds to a time when thalamic axons are normally growing through deep layers before the emergence of layer IV from the cortical plate, thalamic axon ingrowth through deep layers is significantly attenuated by N-cadherin blocking reagents. These data indicate that N-cadherin has multifaceted roles in establishing the thalamocortical projection, governing aspects of both thalamic axon ingrowth and laminar targeting by acting as a layer IV stop signal, which progressively change in parallel with the maturational state of the 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.

Wiring of the brain by a range of guidance cues

During development of the central nervous system, growth cones navigate along specific pathways, recognize their targets and then form synaptic connections by elaborating terminal arbors. To date, a number of developmental and in vitro studies have characterized the nature of the guidance cues that underlie various types of axonal behavior, from initial outgrowth to synapse formation, including pathway selection, polarized growth, orientated growth, termination and branching. New approaches in molecular biology have identified several types of guidance cues, most of which are likely to act as local cues. Moreover, recent studies have indicated that axonal responsiveness to guidance cues changes dynamically, which appears to be elicited by environmental factors encountered by the navigating growth cones. This article addresses what molecular cues are responsible for guidance mechanisms including axonal responsiveness, focusing on axonal behavior in the developmental stages.

Layer-specific thalamocortical innervation in organotypic cultures is prevented by substances that alter neural activity

Cortical layer IV is the major target of thalamocortical axons and many previous studies have shown that the development of this layer-specific innervation can be modelled in vitro by organotypic cocultures of thalamus and cortex. The mechanisms causing thalamic axons to terminate in layer IV are unknown. We used these in vitro models to test the possibility that neural activity plays a part in this termination process by adding substances that raise or lower levels of neural activity to the cocultures. We found that addition of tetrodotoxin or 2-amino-5-phosphonovalerate, to block activity, or potassium, to raise it, all interfered with termination in layer IV. These findings suggest that termination in layer IV requires neural activity at an appropriate level in the thalamocortical system. They also add support to recent findings that show that the importance of neural activity in development may extend to an earlier period than thought previously, to include the correct targeting of axons as well as the later refinement of connections.

Interactions between TrkB signaling and serotonin excess in the developing murine somatosensory cortex: a role in tangential and radial organization of thalamocortical axons

Mice lacking monoamine oxidase A (MAOA) display high levels of brain serotonin during the first postnatal week, causing an exuberant outgrowth of thalamocortical axons (TCAs) in layer IV of the somatosensory cortex (S1). We asked whether this exuberance is attributable to abnormal TrkB signaling, because modulation of TrkB signaling during a critical period dramatically influences the segregation and the morphology of TCAs in layer IV of the visual cortex. Using in situ hybridization and ELISA immunoassays, we showed that the levels of trkB mRNA and BDNF and neurotrophin-4 (NT-4) proteins are normal in the thalamus and the cortex of mice lacking MAOA during barrel field formation. Because the release of BDNF and NT-4 could be abnormal in MAOA knock-out (KO) mice, we tested whether abnormal TrkB signaling is required for TCA exuberance in MAOA-KO mice by generating mice lacking both trkB and MAOA. Surprisingly, these mice exhibited more severe phenotypes than those found in MAOA-KO mice: a widespread tangential expansion of TCAs in layer IV of the cortex, resulting in a fusion of all sensory representations and a radial expansion of TCAs in layers II-III of the cortex. Careful examination of mice lacking trkB alone revealed subtle alterations of TCAs, with abnormal invasion of layer III. This study reveals the following: (1) expression of trkB, BDNF, and NT-4 are not modulated by an excess of serotonin during barrel formation, (2) TrkB signaling limits branching of TCAs in inappropriate supragranular cortical layers, and (3) serotonin and TrkB signaling act together to cluster thalamocortical axons in layer IV.

Cellular and molecular basis for the formation of lamina-specific thalamocortical projections

The neocortex is composed of a characteristic layered structure, which is a basis of extrinsic and intrinsic cortical connections. In recent years the cellular and molecular mechanisms, which are responsible for the formation of lamina-specific connections, have been explored by extensive molecular and in vitro studies. This article attempts to address what cell-cell interactions are required for axonal targeting and what molecules regulate cellular events, focusing upon the development of the thalamocortical projection.

Laminar specific alterations of thalamocortical projections in organotypic cultures following layer 4 disruption in ferret somatosensory cortex

The developing neocortex influences the growth of thalamocortical projections. Layer 4 in particular receives the majority of input from the thalamus and is important in instructing thalamic afferents to terminate. Previous in vivo experiments demonstrated that disruption of layer 4 during corticogenesis in ferret somatosensory cortex by application of methylazoxy methanol acetate (MAM) prevents proper termination of thalamic afferents in appropriate cortical regions. To further explore the role of layer 4 in thalamocortical development, we prepared organotypic cocultures consisting of normal gestational day 0 (P0) ferret thalamus paired with normal, embryonic day 33 (E33), or E38 MAM-treated cortex obtained from ferrets at either P0 or P7. Injection of MAM on E33 disrupts layer 4 formation, whereas similar injections on E38 interfere with layer 2 formation. The cocultures grew together for a number of days, then discrete injections of either fluorescent dextrans or 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) were made into the thalamic piece. The labeled thalamic afferents that grew into the cortical slice were analysed and the sites of their terminations quantified after 3, 5, or 7-10 days in culture (DIC). Our results varied somewhat with the amount of time in culture, but the preponderance of thalamic fibers in normal cortex terminated in layer 4, whereas their counterparts in E33 MAM-treated cortex grew beyond the cortical plate and many fibers terminated inappropriately within lower cortical layers or white matter. Terminal distribution of thalamic fibers in E38 MAM-treated cortex looked similar to normal. These results demonstrate that the cells of layer 4 provide thalamic afferents with important positional and termination cues.