Multiple roles of ephrins during the formation of thalamocortical projections: maps and more

The functional architecture of the cerebral cortex is based on intrinsic connections that precisely link neurons from distinct cortical laminae as well as layer-specific afferent and efferent projections. Experimental strategies using in vitro assays originally developed by Friedrich Bonhoeffer have suggested that positional cues confined to individual layers regulate the assembly of local cortical circuits and the formation of thalamocortical projections. One of these wiring molecules is ephrinA5, a ligand for Eph receptor tyrosine kinases. EphrinA5 and Eph receptors exhibit highly dynamic expression patterns in distinct regions of the cortex and thalamus during early and late stages of thalamocortical and cortical circuit formation. In vitro assays suggest that ephrinA5 is a multifunctional wiring molecule for different populations of cortical and thalamic axons. Additionally, the expression patterns of ephrinA5 during cortical development are consistent with this molecule regulating, in alternative ways, specific components of thalamic and cortical connectivity. To test this directly, the organization of thalamocortical projections was examined in mice lacking ephrinA5 gene expression. The anatomical studies in ephrinA5 knockout animals revealed a miswiring of limbic thalamic projections and changes in neocortical circuits that were predicted from the expression pattern and the in vitro analysis of ephrinA5 function.     

Early thalamocortical tract guidance and topographic sorting of thalamic projections requires LIM-homeodomain gene Lhx2

The thalamocortical tract is the primary source of sensory information to the cerebral cortex, but the mechanisms regulating its pathfinding are not completely understood. LIM-homeodomain (LIM-HD) gene Lhx2 has been proposed to participate in a combinatorial "code" to regulate dorsal thalamic patterning and also the topography of thalamocortical projections. Here, we report that Lhx2-/- embryos exhibit a gross disruption in the early development of the thalamocortical tract, such that thalamic axons are unable to enter the ventral telencephalon. A possible cause for this deficit is a severe reduction of "pioneer" cells in the mutant ventral telencephalon that constitutes a putative mechanism for guiding the entry of the thalamocortical tract into this structure in vivo. However, in vitro, the thalamocortical tract is able to enter the ventral telencephalon, and this permitted an examination of whether thalamocortical topography is normal in the Lhx2 mutant. Contrary to hypotheses that proposed a cell-autonomous role for Lhx2 in the thalamus, Lhx2-/- thalamic explants generate a normal topography of projections in control ventral telencephalic preparations. This is consistent with our findings of normal patterning of the Lhx2 mutant dorsal thalamus using a wide array of markers. In the reverse experiment, however, control thalamic explants display aberrant topography in Lhx2-/- telencephalic preparations. This perturbation is restricted to projections from caudal thalamic explants, while rostral and middle explants project normally. Thus Lhx2 is required for multiple steps in thalamocortical tract pathfinding, but these functions appear localized in the ventral telencephalon rather than in the dorsal thalamic neurons. Furthermore, the absence of Lhx2 in the ventral telencephalon selectively disrupts a subset of thalamic axon topography, indicating a specific rather than a general perturbation of cues in this structure.     

Interdigitated paralemniscal and lemniscal pathways in the mouse barrel cortex

Primary sensory cortical areas receive information through multiple thalamic channels. In the rodent whisker system, lemniscal and paralemniscal thalamocortical projections, from the ventral posteromedial nucleus (VPM) and posterior medial nucleus (POm) respectively, carry distinct types of sensory information to cortex. Little is known about how these separate streams of activity are parsed and integrated within the neocortical microcircuit. We used quantitative laser scanning photostimulation to probe the organization of functional thalamocortical and ascending intracortical projections in the mouse barrel cortex. To map the thalamocortical projections, we recorded from neocortical excitatory neurons while stimulating VPM or POm. Neurons in layers (L)4, L5, and L6A received dense input from thalamus (L4, L5B, and L6A from VPM; and L5A from POm), whereas L2/3 neurons rarely received thalamic input. We further mapped the lemniscal and paralemniscal circuits from L4 and L5A to L2/3. Lemniscal L4 neurons targeted L3 within a column. Paralemniscal L5A neurons targeted a superficial band (thickness, 60 μm) of neurons immediately below L1, defining a functionally distinct L2 in the mouse barrel cortex. L2 neurons received input from lemniscal L3 cells and paralemniscal L5A cells spread over multiple columns. Our data indicate that lemniscal and paralemniscal information is segregated into interdigitated cortical layers.     

Specificity and Plasticity of Thalamocortical Connections in Sema6A Mutant Mice

The establishment of connectivity between specific thalamic nuclei and cortical areas involves a dynamic interplay between the guidance of thalamocortical axons and the elaboration of cortical areas in response to appropriate innervation. We show here that Sema6A mutants provide a unique model to test current ideas on the interactions between subcortical and cortical guidance mechanisms and cortical regionalization. In these mutants, axons from the dorsal lateral geniculate nucleus (dLGN) are misrouted in the ventral telencephalon. This leads to invasion of presumptive visual cortex by somatosensory thalamic axons at embryonic stages. Remarkably, the misrouted dLGN axons are able to find their way to the visual cortex via alternate routes at postnatal stages and reestablish a normal pattern of thalamocortical connectivity. These findings emphasize the importance and specificity of cortical cues in establishing thalamocortical connectivity and the spectacular capacity of the early postnatal cortex for remapping initial sensory representations.     

Development and plasticity of cortical areas and networks

The development of cortical layers, areas and networks is mediated by a combination of factors that are present in the cortex and are influenced by thalamic input. Electrical activity of thalamocortical afferents has a progressive role in shaping cortex. For early thalamic innervation and patterning, the presence of activity might be sufficient; for features that develop later, such as intracortical networks that mediate emergent responses of cortex, the spatiotemporal pattern of activity often has an instructive role. Experiments that route projections from the retina to the auditory pathway alter the pattern of activity in auditory thalamocortical afferents at a very early stage and reveal the progressive influence of activity on cortical development. Thus, cortical features such as layers and thalamocortical innervation are unaffected, whereas features that develop later, such as intracortical connections, are affected significantly. Surprisingly, the behavioural role of 'rewired' cortex is also influenced profoundly, indicating the importance of patterned activity for this key aspect of cortical function.     

Screening for genes that wire the cerebral cortex

Thalamocortical projections convey visual, somatosensory and auditory information to the cerebral cortex. A recent report in Neural Development shows how a forward genetic screen has enabled the identification of novel mutations affecting specific decision points of thalamocortical axon pathfinding.     

Ephrins regulate the formation of terminal axonal arbors during the development of thalamocortical projections

The development of connections between thalamic afferents and their cortical target cells occurs in a highly precise manner. Thalamic axons enter the cortex through deep cortical layers, then stop their growth in layer 4 and elaborate terminal arbors specifically within this layer. The mechanisms that underlie target layer recognition for thalamocortical projections are not known. We compared the growth pattern of thalamic explants cultured on membrane substrates purified from cortical layer 4, the main recipient layer for thalamic axons, and cortical layer 5, a non-target layer. Thalamic axons exhibited a reduced growth rate and an increased branching density on their appropriate target membranes compared with non-target substrate. When confronted with alternating stripes of both membrane substrates, thalamic axons grew preferentially on their target membrane stripes. Enzymatic treatment of cortical membranes revealed that growth, branching and guidance of thalamic axons are independently regulated by attractive and repulsive cues differentially expressed in distinct cortical layers. These results indicate that multiple membrane-associated molecules collectively contribute to the laminar targeting of thalamic afferents. Furthermore, we found that interfering with the function of Eph tyrosine kinase receptors and their ligands, ephrins, abolished the preferential branching of thalamic axons on their target membranes, and that recombinant ephrin-A5 ligand elicited a branch-promoting activity on thalamic axons. We conclude that interactions between Eph receptors and ephrins mediate branch formation of thalamic axons and thereby may play a role in the establishment of layer-specific thalamocortical connections.     

Nonoverlapping Thalamocortical Connections to Normal and Deprived Primary Somatosensory Cortex for Similar Forelimb Receptive Fields in Chronic Spinal Cats

The fluorescent dye retrograde tracing technique, using fast blue in combination with fluorogold, was used to examine thalamocortical projections from the ventrobasal complex to primary somatosensory cortex in chronic spinal cats that sustained T12 cord transection at 2 weeks of age. Following cord transection at this age, it has been shown that forelimb afferents can excite the deprived hindlimb projection zone, in addition to the region of somatosensory cortex that they normally occupy (McKinley et al, 1987). These two regions of cortex are separated by over 10 mm, thus facilitating the determination of whether the forelimb representation in “hindlimb cortex” is derived from the sector of the ventrobasal complex of the thalamus representing the forelimb, hindlimb, or both. Injections of the two dyes into separate regions of the cortex that were excited by the same peripheral forelimb receptive fields produced single labeling of two nonoverlapping clusters of thalamic neurons. This finding suggests that the projections for these two areas are independent and distinct, and indicates that altered thalamocortical projections do not contribute the critical component underlying reorganizational changes observed at the cortical level after spinal cord transection. It is hypothesized that the degree of reorganization required to achieve the magnitude of change observed in the cortex must occur below the level of the thalamocortical relay.     

Ephrin-as guide the formation of functional maps in the visual cortex

Ephrin-As and their receptors, EphAs, are expressed in the developing cortex where they may act to organize thalamic inputs. Here, we map the visual cortex (V1) in mice deficient for ephrin-A2, -A3, and -A5 functionally, using intrinsic signal optical imaging and microelectrode recording, and structurally, by anatomical tracing of thalamocortical projections. V1 is shifted medially, rotated, and compressed and its internal organization is degraded. Expressing ephrin-A5 ectopically by in utero electroporation in the lateral cortex shifts the map of V1 medially, and expression within V1 disrupts its internal organization. These findings indicate that interactions between gradients of EphA/ephrin-A in the cortex guide map formation, but that factors other than redundant ephrin-As are responsible for the remnant map. Together with earlier work on the retinogeniculate map, the current findings show that the same molecular interactions may operate at successive stages of the visual pathway to organize maps.     

Sleep-active cells in the cerebral cortex and their role in slow-wave activity

We recently identified neurons in the cerebral cortex that become activated during sleep episodes with high slow-wave activity (SWA). The distinctive properties of these neurons are the ability to produce nitric oxide and their long-range projections within the cortex. In this review, we discuss how these characteristics of sleep-active cells could be relevant to SWA production in the cortex. We also discuss possible models of the role of nNOS cells in SWA production.     

Making cortex in a dish: In vitro corticopoiesis from embryonic stem cells

The cerebral cortex is arguably the most complex structure in the mammalian brain. It develops through the coordinated generation of dozens of neuronal subtypes, but the mechanisms involved in this daunting process of cell diversification remain poorly understood. We recently described a novel pathway by which mouse embryonic stem (ES) cells, cultured in the absence of any added morphogen but in the presence of a Sonic Hedgehog inhibitor, can recapitulate the major milestones of cortical development observed in vivo. In this system cortical-like progenitors seem to follow an intrinsic pathway to generate a surprisingly diverse repertoire of neurons that display most salient features of bona fide cortical pyramidal neurons. When grafted into the cerebral cortex in vivo, these neuronal populations develop patterns of axonal projections highly similar to those of native cortical neurons. The discovery of intrinsic corticogenesis, from stem cells to cortical circuits, sheds new light on the mechanisms of neuronal specification, and may open new venues for the modelling of cortical development and diseases, and for the rational design of brain repair strategies.     

Sleep-active cells in the cerebral cortex and their role in slow-wave activity

We recently identified neurons in the cerebral cortex that become activated during sleep episodes with high slow-wave activity (SWA). The distinctive properties of these neurons are the ability to produce nitric oxide and their long-range projections within the cortex. In this review, we discuss how these characteristics of sleep-active cells could be relevant to SWA production in the cortex. We also discuss possible models of the role of nNOS cells in SWA production.

     

The relationship between thalamocortical connections and stimulus-evoked metabolic activity in the ventroposterior nucleus of the monkey.

Although a highly organized system of reciprocal projections exists between the cerebral cortex and the thalamus, the relationship of the thalamocortical projections to functional activity remains unclear. This study attempts to identify the correlation between thalamic relay cells and functional activity evoked in the ventroposterior nucleus (VP) of cynomolgus and squirrel monkeys. Wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) was iontophoretically injected into physiologically determined sites in the somatosensory cortex, resulting in retrogradely labeled cells and anterogradely labeled terminals in corresponding somatosensory thalamic regions. In the same animals, 2-deoxyglucose (2DG) experiments were carried out 2 days later, using the somatic stimuli identified as best exciting the cortical neurons. Stimulation to the limbs produced crescent-shaped clusters of metabolic label arranged in a somatotopically organized fashion in the ventral posterior lateral nucleus (VPL). Following WGA:HRP injections into area 3b, the stimulus-evoked 2DG label was colocalized with the retrograde and anterograde tracer. This finding suggests that the location of stimulus-evoked metabolic activity can be predicted by the presence of transported WGA:HRP clusters.     

Hypocretin (orexin) induces calcium transients in single spines postsynaptic to identified thalamocortical boutons in prefrontal slice

In vivo, thalamocortical axons are susceptible to the generation of terminal spikes which antidromically promote bursting in the thalamus. Although neurotransmitters could elicit such ectopic action potentials at thalamocortical boutons, this hypothesis has never been confirmed. Prefrontal cortex is the cortical area most implicated in arousal and is innervated by thalamic neurons that are unusual since they burst rhythmically during waking. We show that a neurotransmitter critical for alertness, hypocretin (orexin), directly excites prefrontal thalamocortical synapses in acute slice. This TTX-sensitive activation of thalamic axons was demonstrated electrophysiologically and by two-photon sampling of calcium transients at single spines in apposition to thalamic boutons anterogradely labeled in vivo. Spines receiving these long-range projections constituted a unique population in terms of the presynaptic excitatory action of hypocretin. By this mechanism, the hypocretin projection to prefrontal cortex may play a larger role in prefrontal or "executive" aspects of alertness and attention than previously anticipated.     

Axon extension and retraction by leech neurons: severing early projections to peripheral targets prevents normal retraction of other projections

During leech embryogenesis, interactions between homologous neurons in neighboring segments lead to the selective retraction of longitudinal axonal projections by midbody AP and AE neurons, which maintain lateral axonal projections to the periphery. Results of experiments reported here show that disconnecting the lateral projections from the periphery rescues the projections normally fated to retract. We propose that these neurons normally progress through two states during early development, one in which they are insensitive to interactions with their homologs (state A) and a second in which they are sensitive (state B). Establishment of lateral connections with their targets triggers the switch from state A to state B; cutting these projections puts neurons back to state A.     

Emerging roles of neural stem cells in cerebral cortex development and evolution

Expansion and folding of the cerebral cortex are landmark features of mammalian brain evolution, which are recapitulated during embryonic development. Neural stem cells and their derived germinal cells are coordinated during cerebral cortex development to produce the appropriate amounts and types of neurons. This process is further complicated in gyrencephalic species, where newborn neurons must disperse in the tangential axis to expand the cerebral cortex in surface area. Here, we review advances that have been made over the last decade in understanding the nature and diversity of telencephalic neural stem cells and their roles in cortical development, and we discuss recent progress on how newly identified types of cortical progenitor cell populations may have evolved to drive the expansion and folding of the mammalian cerebral cortex.     

Regional differences in the stratified transitional field and the honeycomb matrix of the developing human cerebral cortex

The neurons of the cerebral cortex originate in the proliferative neuroepithelium and settle in the cortical plate during embryonic development. Interposed between these two sites is a large transitional field. We have earlier demonstrated experimentally in rats with ³H-thymidine autoradiography that this transitional field is a stratified structure composed of discrete layers of migrating and sojourning cells, and fiber bands. Here we show that the different layers of the stratified transitional field are identifiable without experimental procedures in the developing human cerebral cortex and that there are conspicuous regional differences in its stratification. At the peak of its development, the stratified transitional field contains three fibrous bands in an inside-out order: the commissural fibers of the corpus callosum, the thalamocortical and corticofugal projection fibers, and the expanding white matter. There are regional differences in the thickness of these fibrous layers as well as in the number and configuration of the perikaryal layers. This preview focuses on laminar differences of the transitional fields of the agranular frontal lobe and the granular parietal and occipital lobes. At the latter sites, but not in the frontal lobe, there is a distinctive multi-layered band, the honeycomb matrix, where radially oriented fiber columns are sandwiched between two perikaryal sublayers and are separated from one another by radially oriented cells. We postulate that the radial fiber columns of the honeycomb matrix are composed of topographically organized thalamocortical fibers and that the unspecified young neurons acquire their enduring topographic identity by making selective contacts with tagged fibers here before they resume their radial or tangential migration to the cortical plate.     

Convergence between projections from different thalamic nuclei to columns of the rat somatosensory cortex

Primuline fluorochrome retrograde transport technique was used to investigate sources of thalamocortical projections to a single rat somatosensory cortex column connected with the projection of the C₃ vibrissa. Labeled cells were identified in eight different thalamic nuclei: two specific, five nonspecific, and one association nucleus. Labeled neurons differed in the degree of stain accumulated as well as cell numbers and density of distribution from one nucleus to another, indicative of the different arborization patterns of their axons within the cortex. Highest numbers of heavily stained cells as well as highest density of distribution were observed in the ventral thalamic nucleus. The convergence seen between different thalamocortical inputs on to a single somatosensory cortex column explains the functional differences observed between neurons belonging to the same column and makes the formation of functionally distinct neuronal groupings appear possible on this structural basis.Neurocybernetics Research Institute, Rostov-on-Don. Translated from Neirofiziologiya, Vol. 21, No. 2, pp. 168–174, March–April, 1989.