Forces to Drive Neuronal Migration Steps

Abstract

To establish and maintain proper brain architecture and elaborate neural networks, neurons undergo massive migration. As a unique feature of their migration, neurons move in a saltatory manner by repeating two distinct steps: extension of the leading process and translocation of the cell body. Neurons must therefore generate forces to extend the leading process as well as to translocate the cell body. In addition, neurons need to switch these forces alternately in order to orchestrate their saltatory movement. Recent studies with mechanobiological analyses, including traction force microscopy, cell detachment analyses, live-cell imaging, and loss-of-function analyses, have begun to reveal the forces required for these steps and the molecular mechanics underlying them. Spatiotemporally organized forces produced between cells and their extracellular environment, as well as forces produced within cells, play pivotal roles to drive these neuronal migration steps. Traction force produced by the leading process growth cone extends the leading processes. On the other hand, mechanical tension of the leading process, together with reduction in the adhesion force at the rear and the forces to drive nucleokinesis, translocates the cell body. Traction forces are generated by mechanical coupling between actin filament retrograde flow and the extracellular environment through clutch and adhesion molecules. Forces generated by actomyosin and dynein contribute to the nucleokinesis. In addition to the forces generated in cell-intrinsic manners, external forces provided by neighboring migratory cells coordinate cell movement during collective migration. Here, we review our current understanding of the forces that drive neuronal migration steps and describe the molecular machineries that generate these forces for neuronal migration.

Keywords: actomyosin; adhesion force; dynein; mechanical tension; mechanobiology; neuronal migration; shootin1; traction force.

FIGURE 1

A Mechanical model for neuronal migration. (A) Neurons migrate in a saltatory manner by repeating the two distinct steps: leading process extension and somal translocation. (B) The driving force for leading process extension (white arrow) is produced as a counter force of the traction force on the adhesive substrate produced by the growth cone (yellow arrow). Somal translocation is likely to be driven by multiple forces, including mechanical tension along the leading process (black arrows), a decrease in the adhesion force at the cell body (smaller green arrow), and pushing (red arrow) and pulling (blue arrow) forces exerted on the nucleus. In addition, forces provided by neighboring cells (brown arrow) coordinate cell movement during collective migration.

FIGURE 2

Generation of traction force for leading process extension by actin–adhesion coupling. (A) Schema of traction force microscopy to monitor force generated by migrating neurons. Neurons are cultured on polyacrylamide gel coated with adhesive substrates such as L1-CAM; fluorescent beads are embedded in the gel. Traction forces under the cell (yellow arrow) are monitored by visualizing force-induced deformation of the gel, which is reflected by the movement of beads under the neuron (blue arrows). (B) Force mapping of a migrating neuron. Differential interference contrast (DIC, upper panels) and fluorescence (lower panels) time-lapse images of a migrating olfactory interneuron (see Supplementary Video S1). The original and displaced positions of the beads are indicated by green and red, respectively, while the bead displacements are indicated by cyan rectangles. Yellow arrows in DIC images indicate the magnitude and direction of traction forces. Dashed lines indicate the boundary of the cell. The kymographs (lower right) along the axis of bead displacement (pink arrows) at the boxed areas 1 and 2 of the neuron show movement of beads. Note that the gel under the cell body deformed forward during the somal translocation step (white arrows in the bottom DIC image and box 2). Modified from Minegishi et al. (2018) (This work is licensed under the CC BY license, https://creativecommons.org/licenses/by/4.0/) with permission. (C) Fluorescent speckle image of HaloTag-actin at the leading process growth cone of an olfactory interneuron, and kymograph of the boxed area at 3 s intervals (right) (see Supplementary Video S2). The dashed line indicates the retrograde flow of speckles. Reproduced from Minegishi et al. (2018) with permission. (D) Molecular machinery for generation of traction force in migrating olfactory interneurons. At the leading process growth cone, shootin1b mediates actin–adhesion coupling, through its interactions with cortactin and L1-CAM. This coupling generates traction force under the growth cone (yellow arrow). The driving force for leading process extension (forward white arrow) is generated as a counterforce to the traction forces exerted on the adhesive substrate. Scale bars: 5 μm.

FIGURE 3

Multiple forces that cooperate for somal translocation. (A) Leading process extension (white arrows) increases the mechanical tension along the leading process (black arrows), which in turn pulls the cell body for somal translocation. In addition, actomyosin contraction at the proximal region of the leading process (red arrows) increases the tension along the leading process. (B) Decrease in the adhesion force at the cell body (smaller green arrow) propels somal translocation. Adhesion receptors are transported from the cell body to the leading process via endocytic pathways (black arrows), resulting in an increase in the adhesion force at the leading process and a decrease in adhesion force at the cell body (smaller green arrow). (C) Actomyosin, which may be anchored to the cell cortex, contracts at the rear of the nucleus (small red arrows), thereby squeezing the nucleus and generating pushing force (large red arrow) for nucleokinesis. (D) The dynein complex mechanically interacts with the nucleus via the LINC complex, and its movement (yellow arrows) along perinuclear microtubules generates pulling force (blue arrow) for nucleokinesis. (E) In the swelling of the proximal part of the leading process, the dynein motor complex may be immobilized on a cellular component via an anchoring molecule. The force of dynein movement (yellow arrow) slides microtubules forward, thereby pulling (blue arrow) the centrosome forward.