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

Rostral migratory stream neuroblasts turn and change directions in stereotypic patterns

Neuroblasts generated in the adult subventricular zone (SVZ) migrate through the rostral migratory stream (RMS) to the olfactory bulb (OB). Previous work uncovered motility ranging from straight to complex, but it was unclear if directional changes were stochastic or exhibited stereotypical patterns. Here, we provide the first in-depth two-photon time-lapse microscopy study of morphological and dynamic features that accompany turning and direction reversals in the RMS. We identified three specific kinds of turning (30-90 degrees): bending of the leading process proximal to the cell body (P-bending 47% of cases), bending of the distal leading process (D-bending 30%) or branching of the leading process or lamellipodium (23%). Bending and branching angles were remarkably constrained and were significantly different from one another. Cells reversed direction (> 90 degrees) through D-bendings (54%), branching (11%) or de novo growth of processes from the soma (23%), but not P-bending. Direction reversal was often composed of several iterations of D-bending or branching as opposed to novel modalities. Individual neuroblasts could turn or change direction in multiple patterns suggesting that the patterns are not specific for different lineages. These findings show that neuroblasts in the RMS use a limited number of distinct and constrained modalities to turn or reverse direction.

Cortical GABAergic interneurons transiently assume a sea urchin-like nonpolarized shape before axon initiation

Mature neurons polarize by extending an axon and dendrites. In vitro studies of dissociated neurons have demonstrated that axons are initiated from a nonpolarized stage. Dissociated hippocampal neurons form four to five minor neurites shortly after plating but then one of them starts to elongate rapidly to become the future axon, whereas the rest constitutes the dendrites at later stages. However, neuroepithelial cells as well as migrating neurons in vivo are already polarized, raising the possibility that mature neurons inherit the polarities of immature neurons of neuroepithelial or migrating neurons. Here we show that the axon of interneurons in mouse cortical explant emerges from a morphologically nonpolarized shape. The morphological maturation of cortical interneurons labeled by electroporation at an embryonic stage was analyzed by time-lapse imaging during the perinatal stage. In contrast to earlier stages, most interneurons at this stage show sea urchin-like nonpolarized shapes with alternately extending and retracting short processes. Abruptly, one of these processes extends to give rise to an outstandingly long axon-like process. Given that the interneurons exhibit typical polarized shapes during embryonic development, the present results suggest that axon-dendrite polarity develops from a nonpolarized intermediate stage.

Guiding neuronal cell migrations

Neuronal migration is, along with axon guidance, one of the fundamental mechanisms underlying the wiring of the brain. As other organs, the nervous system has acquired the ability to grow both in size and complexity by using migration as a strategy to position cell types from different origins into specific coordinates, allowing for the generation of brain circuitries. Guidance of migrating neurons shares many features with axon guidance, from the use of substrates to the specific cues regulating chemotaxis. There are, however, important differences in the cell biology of these two processes. The most evident case is nucleokinesis, which is an essential component of migration that needs to be integrated within the guidance of the cell. Perhaps more surprisingly, the cellular mechanisms underlying the response of the leading process of migrating cells to guidance cues might be different to those involved in growth cone steering, at least for some neuronal populations.

Understanding eukaryotic chemotaxis: a pseudopod-centred view

Current descriptions of eukaryotic chemotaxis and cell movement focus on how extracellular signals (chemoattractants) cause new pseudopods to form. This 'signal-centred' approach is widely accepted but is derived mostly from special cases, particularly steep chemoattractant gradients. I propose a 'pseudopod-centred' explanation, whereby most pseudopods form themselves, without needing exogenous signals, and chemoattractants only bias internal pseudopod dynamics. This reinterpretation of recent data suggests that future research should focus on pseudopod mechanics, not signal processing.

The stochastic search dynamics of interneuron migration

Migration is a dynamic process in which a cell searches the environment and translates acquired information into somal advancement. In particular, interneuron migration during development is accomplished by two distinct processes: the extension of neurites tipped with growth cones; and nucleus translocation, termed nucleokinesis. The primary purpose of our study is to investigate neurite branching and nucleokinesis using high-resolution time-lapse confocal microscopy and computational modeling. We demonstrate that nucleokinesis is accurately modeled by a spring-dashpot system and that neurite branching is independent of the nucleokinesis event, and displays the dynamics of a stochastic birth-death process. This is in contrast to traditional biological descriptions, which suggest a closer relationship between the two migratory mechanisms. Our models are validated on independent data sets acquired using two different imaging protocols, and are shown to be robust to alterations in guidance cues and cellular migratory mechanisms, through treatment with brain-derived neurotrophic factor, neurotrophin-4, and blebbistatin. We postulate that the stochastic branch dynamics exhibited by interneurons undergoing guidance-directed migration permit efficient exploration of the environment.

Random versus directionally persistent cell migration

Directional migration is an important component of cell motility. Although the basic mechanisms of random cell movement are well characterized, no single model explains the complex regulation of directional migration. Multiple factors operate at each step of cell migration to stabilize lamellipodia and maintain directional migration. Factors such as the topography of the extracellular matrix, the cellular polarity machinery, receptor signalling, integrin trafficking, integrin co-receptors and actomyosin contraction converge on regulation of the Rho family of GTPases and the control of lamellipodial protrusions to promote directional migration.

Migration of cortical interneurons relies on branched leading process dynamics

Migrating cells typically reach their targets in response to a relatively wide variety of extracellular molecules. Somehow surprisingly, most cells transduce these extracellular signals into a relatively homogeneous set of cellular changes that allow them to accurately find their target position. Here we summarize the characterization of the migratory behaviour of cortical interneurons in their journey to the cerebral cortex, which seems to represent a novel type of cellular adaptation during directional guidance. Similar to other migrating cells, cortical interneurons are highly polarized cells, with a prominent leading process and a short trailing process. However, the leading process of migrating interneurons continuously branches during the migratory cycle of these cells. Leading process branches are generated in response to the extracellular environment, and seem to serve as the main mechanism that determines the migratory direction for the cell. For each migratory cycle, the branch that is best oriented towards an attractive guidance cue will become stabilized, which in turn will allow the subcellular organelles and the nucleus to progress in the right direction. This migratory process is under the strict control, among several other molecules, of members from the small Rho GTPases family proteins. Pharmacological blocking of ROCKI/II abrogates the formation of leading process branches in migrating interneurons. The resulting cells, with a single leading process, do not efficiently modify their orientation in response to extracellular guidance cues, and so they fail to complete their migration.

The embryonic preoptic area is a novel source of cortical GABAergic interneurons

GABA-containing (GABAergic) interneurons play an important role in the function of the cerebral cortex. Through mostly inhibitory mechanisms, interneurons control hyperexcitability and synchronize and shape the spatiotemporal dynamics of cortical activity underlying various brain functions. Studies over the past 10 years have demonstrated that, in most mammals, interneurons originate during development from the subcortical telencephalon--the subpallium--and reach the cerebral cortex through tangential migration. Until now, interneurons have been demonstrated to derive exclusively from two subpallial regions, the medial ganglionic eminence and the caudal ganglionic eminence. Here, we show that another subpallial structure, the preoptic area, is a novel source of cortical GABAergic interneurons in the mouse. In utero labeling and genetic lineage-tracing experiments demonstrate that neurons born in this region migrate to the neocortex and hippocampus, where they differentiate into a distinct population of GABAergic interneurons with relatively uniform neurochemical, morphological, and electrophysiological properties.

Molecular regulation of neuronal migration during neocortical development

Neocortex, a distinct six-layered neural structure, is one of the most exquisite nerve tissues in the human body. Proper assembly of neocortex requires precise regulation of neuronal migration and abnormalities can result in severe neurological diseases. Three major types of neuronal migration have been implicated in corticogenesis: radial migration of excitatory neuron precursors and tangential migration of interneurons as well as Cajal-Retzius cells. In the past several years, significant progress has been made in understanding how these parallel events are regulated and coordinated during corticogenesis. New insights have been gained into regulation of radial neuron migration by the well-known Reelin signal. New pathways have also been identified that regulate radial as well as tangential migration. Equally important, better understandings have been obtained on the cellular and molecular mechanics of cell migration by both projection neurons and interneurons. These findings have not only enhanced our understanding of normal neuron migration but also revealed insights into the etiologies of several neurological diseases where these processes go awry.