Spontaneous Thalamic Activity Modulates the Cortical Innervation of the Primary Visual Nucleus of the Thalamus.

Sensory processing relies on the correct development of thalamocortical loops. Visual corticothalamic axons (CTAs) invade the dorsolateral geniculate nucleus (dLGN) of the thalamus in early postnatal mice according to a regulated program that includes activity-dependent mechanisms. Spontaneous retinal activity influences the thalamic incursion of CTAs, yet the perinatal thalamus also generates intrinsic patterns of spontaneous activity whose role in modulating afferent connectivity remains unknown. Here, we found that patterned spontaneous activity in the dLGN contributes to proper spatial and temporal innervation of CTAs. Disrupting patterned spontaneous activity in the dLGN delays corticogeniculate innervation under normal conditions and upon eye enucleation. The delayed innervation was evident throughout the first two postnatal weeks but resumes after eye-opening, suggesting that visual experience is necessary for the homeostatic recovery of corticogeniculate innervation.

Input-dependent segregation of visual and somatosensory circuits in the mouse superior colliculus.

Whereas sensory perception relies on specialized sensory pathways, it is unclear whether these pathways originate as modality-specific circuits. We demonstrated that somatosensory and visual circuits are not by default segregated but require the earliest retinal activity to do so. In the embryo, somatosensory and visual circuits are intermingled in the superior colliculus, leading to cortical multimodal responses to whisker pad stimulation. At birth, these circuits segregate, and responses switch to unimodal. Blocking stage I retinal waves prolongs the multimodal configuration into postnatal life, with the superior colliculus retaining a mixed somato-visual molecular identity and defects arising in the spatial organization of the visual system. Hence, the superior colliculus mediates the timely segregation of sensory modalities in an input-dependent manner, channeling specific sensory cues to their appropriate sensory pathway.

Spontaneous activity in developing thalamic and cortical sensory networks.

Developing sensory circuits exhibit different patterns of spontaneous activity, patterns that are related to the construction and refinement of functional networks. During the development of different sensory modalities, spontaneous activity originates in the immature peripheral sensory structures and in the higher-order central structures, such as the thalamus and cortex. Certainly, the perinatal thalamus exhibits spontaneous calcium waves, a pattern of activity that is fundamental for the formation of sensory maps and for circuit plasticity. Here, we review our current understanding of the maturation of early (including embryonic) patterns of spontaneous activity and their influence on the assembly of thalamic and cortical sensory networks. Overall, the data currently available suggest similarities between the developmental trajectory of brain activity in experimental models and humans, which in the future may help to improve the early diagnosis of developmental disorders.

Prenatal activity from thalamic neurons governs the emergence of functional cortical maps in mice.

The mammalian brain's somatosensory cortex is a topographic map of the body's sensory experience. In mice, cortical barrels reflect whisker input. We asked whether these cortical structures require sensory input to develop or are driven by intrinsic activity. Thalamocortical columns, connecting the thalamus to the cortex, emerge before sensory input and concur with calcium waves in the embryonic thalamus. We show that the columnar organization of the thalamocortical somatotopic map exists in the mouse embryo before sensory input, thus linking spontaneous embryonic thalamic activity to somatosensory map formation. Without thalamic calcium waves, cortical circuits become hyperexcitable, columnar and barrel organization does not emerge, and the somatosensory map lacks anatomical and functional structure. Thus, a self-organized protomap in the embryonic thalamus drives the functional assembly of murine thalamocortical sensory circuits.

Impact of thalamocortical input on barrel cortex development.

The development of cortical maps requires the balanced interaction between genetically determined programs and input/activity-dependent signals generated spontaneously or triggered from the environment. The somatosensory pathway of mice provides an excellent scenario to study cortical map development because of its highly organized cytoarchitecture, known as the barrel field. This precise organization makes evident even small alterations in the cortical map layout. In this review, we will specially focus on the thalamic factors that control barrel field development. We will summarize the role of thalamic input integration and identity, neurotransmission and spontaneous activity in cortical map formation and early cross-modal plasticity.

Prenatal thalamic waves regulate cortical area size prior to sensory processing.

The cerebral cortex is organized into specialized sensory areas, whose initial territory is determined by intracortical molecular determinants. Yet, sensory cortical area size appears to be fine tuned during development to respond to functional adaptations. Here we demonstrate the existence of a prenatal sub-cortical mechanism that regulates the cortical areas size in mice. This mechanism is mediated by spontaneous thalamic calcium waves that propagate among sensory-modality thalamic nuclei up to the cortex and that provide a means of communication among sensory systems. Wave pattern alterations in one nucleus lead to changes in the pattern of the remaining ones, triggering changes in thalamic gene expression and cortical area size. Thus, silencing calcium waves in the auditory thalamus induces Rorß upregulation in a neighbouring somatosensory nucleus preluding the enlargement of the barrel-field. These findings reveal that embryonic thalamic calcium waves coordinate cortical sensory area patterning and plasticity prior to sensory information processing.

Contact repulsion controls the dispersion and final distribution of Cajal-Retzius cells.

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.

Actomyosin contraction at the cell rear drives nuclear translocation in migrating cortical interneurons.

Neuronal migration is a complex process requiring the coordinated interaction of cytoskeletal components and regulated by calcium signaling among other factors. Migratory neurons are polarized cells in which the largest intracellular organelle, the nucleus, has to move repeatedly. Current views support a central role for pulling forces that drive nuclear movement. The participation of actomyosin driven forces acting at the nucleus rear has been suggested, however its precise contribution has not been directly addressed. By analyzing interneurons migrating in cortical slices of mouse brains, we have found that nucleokinesis is associated with a precise pattern of actin dynamics characterized by the initial formation of a cup-like actin structure at the rear nuclear pole. Time-lapse experiments show that progressive actomyosin contraction drives the nucleus forward. Nucleokinesis concludes with the complete contraction of the cup-like structure, resulting in an actin spot at the base of the retracting trailing process. Our results demonstrate that this actin remodeling requires a threshold calcium level provided by low-frequency spontaneous fast intracellular calcium transients. Microtubule stabilization with taxol treatment prevents actin remodeling and nucleokinesis, whereas cells with a collapsed microtubule cytoskeleton induced by nocodazole treatment, display nearly normal actin dynamics and nucleokinesis. In summary, the results presented here demonstrate that actomyosin forces acting at the rear side of the nucleus drives nucleokinesis in tangentially migrating interneurons in a process that requires calcium and a dynamic cytoskeleton of microtubules.

Biased selection of leading process branches mediates chemotaxis during tangential neuronal migration.

Current models of chemotaxis during neuronal migration and axon guidance propose that directional sensing relies on growth cone dynamics. According to this view, migrating neurons and growing axons are guided to their correct targets by steering the growth cone in response to attractive and repulsive cues. Here, we have performed a detailed analysis of the dynamic behavior of individual neurons migrating tangentially in telencephalic slices using high-resolution time-lapse videomicroscopy. We found that cortical interneurons consistently display branched leading processes as part of their migratory cycle, a feature that seems to be common to many other populations of GABAergic neurons in the brain and spinal cord. Analysis of the migratory behavior of individual cells suggests that interneurons respond to chemoattractant signals by generating new leading process branches that are better aligned with the source of the gradient, and not by reorienting previously existing branches. Moreover, experimental evidence revealed that guidance cues influence the angle at which new branches emerge. This model is further supported by pharmacological experiments in which inhibition of branching blocked chemotaxis, suggesting that this process is an essential component of the mechanism controlling directional guidance. These results reveal a novel guidance mechanism during neuronal migration that might be extensively used in brain development.

Chemokine signaling controls intracortical migration and final distribution of GABAergic interneurons.

Functioning of the cerebral cortex requires the coordinated assembly of circuits involving glutamatergic projection neurons and GABAergic interneurons. Although much is known about the migration of interneurons from the subpallium to the cortex, our understanding of the mechanisms controlling their precise integration within the cortex is still limited. Here, we have investigated in detail the behavior of GABAergic interneurons as they first enter the developing cortex by using time-lapse videomicroscopy, slice culture, and in utero experimental manipulations and analysis of mouse mutants. We found that interneurons actively avoid the cortical plate for a period of approximately 48 h after reaching the pallium; during this time, interneurons disperse tangentially through the marginal and subventricular zones. Perturbation of CXCL12/CXCR4 signaling causes premature cortical plate invasion by cortical interneurons and, in the long term, disrupts their laminar and regional distribution. These results suggest that regulation of cortical plate invasion by GABAergic interneurons is a key event in cortical development, because it directly influences the coordinated formation of appropriate glutamatergic and GABAergic neuronal assemblies.

Neurons in motion: same principles for different shapes?

The special conformation of the developing nervous system, in which progenitor zones are largely confined to the lumen of the neural tube, places neuronal migration as one of the most fundamental processes in brain development. Previous studies have shown that different neuronal types adopt distinct morphological modes of migration in the developing brain, indicating that neuronal migration might be a diverse process. Here, we review recent data on the molecular mechanisms underlying neuronal migration that suggest that similar signaling principles are responsible for the frequently variable morphology of different types of migrating neuron. According to this idea, the same basic molecular mechanisms found in other cell types, such as fibroblasts, might have been adapted to the special morphological needs of migrating neurons in different regions of the developing brain.

Polarized increase of calcium and nucleokinesis in tangentially migrating neurons.

Cortical interneurons originate from the ganglionic eminences and reach their final position in the cortex via tangential migratory routes. The mechanisms of this migration are poorly understood. Here we have performed confocal time-lapse analysis of cell movement in the intermediate zone of embryonic mouse cortical slices in order to directly visualize their mode of migration. Tangentially migrating neurons moved by nucleokinesis, characterized by active phases of discontinuous advances of the nucleus followed by periods of quiescence. Dissociated cells from the ganglionic eminences also showed nucleokinesis associated with an increase of intracellular calcium, [Ca(2+)](i) Calcium elevation was greatest in the proximal region of the leading process, a zone with a wide distribution of gamma-tubulin. General increases in [Ca(2+)](i) elicited by microperfussion with neurotransmitters did not elicit nucleokinesis. These results show that tangential migration uses nucleokinesis, a cell-intrinsic process in which calcium signalling is local, directional and highly regulated.