The Rho kinase pathway regulates mouse adult neural precursor cell migration.

Adult neural precursor cells (NPCs) in the subventricular zone (SVZ) normally migrate via the rostral migratory stream (RMS) to the olfactory bulb (OB). Following neural injury, they also migrate to the site of damage. This study investigated the role of Rho-dependent kinase (ROCK) on the migration of NPCs in vitro and in vivo. In vitro, using neurospheres or SVZ explants, inhibition of ROCK using Y27632 promoted cell body elongation, process protrusion, and migration, while inhibiting NPC chain formation. It had no effect on proliferation, apoptosis, or differentiation. Both isoforms of ROCK were involved. Using siRNA, knockdown of both ROCK1 and ROCK2 was required to promote NPC migration and morphological changes; knockdown of ROCK2 alone was partially effective, with little/no effect of knockdown of ROCK1 alone. In vivo, infusion of Y27632 plus Bromodeoxyuridine (BrdU) into the lateral ventricle for 1 week reduced the number of BrdU-labeled NPCs in the OB compared with BrdU infusion alone. However, ROCK inhibition did not affect the tangential-to-radial switch of NPC migration, as labeled cells were present in all OB layers. The decrease in NPC number at the OB was not attributed to a decrease in NPCs at the SVZ. However, ROCK inhibition decreased the density of BrdU-labeled cells in the RMS and increased the distribution of these cells to ectopic brain regions, such as the accessory olfactory nucleus, where the majority differentiated into neurons. These findings suggest that ROCK signaling regulates NPC migration via regulation of cell-cell contact and chain migration.

The role of intracellular calcium and RhoA in neuronal migration.

The successive steps involved in cell migration include extension of the leading process, followed by translocation of the soma and retraction of the trailing process. These events require the coordinated activity of various intracellular signaling mechanisms. Recent evidence suggests that the intracellular distribution of calcium ions and of the Rho guanosine triphosphatase (RhoGTPase), RhoA, are important components of these mechanisms. During migration, the growth cone of the leading process senses guidance cues present in the extracellular environment. These cues, acting through appropriate receptors on the growth cone, induce changes in the concentration of calcium, both in the growth cone and in the soma. These changes in the distribution of calcium cause a redistribution of intracellular RhoA and the eventual translocation of the soma. The trailing process is retracted as the cell moves forward.

Convergent extension, planar-cell-polarity signalling and initiation of mouse neural tube closure.

Planar-cell-polarity (PCP) signalling is necessary for initiation of neural tube closure in higher vertebrates. In mice with PCP gene mutations, a broad embryonic midline prevents the onset of neurulation through wide spacing of the neural folds. In order to evaluate the role of convergent extension in this defect, we vitally labelled the midline of loop-tail (Lp) embryos mutant for the PCP gene Vangl2. Injection of DiI into the node, and electroporation of a GFP expression vector into the midline neural plate, revealed defective convergent extension in both axial mesoderm and neuroepithelium, before the onset of neurulation. Chimeras containing both wild-type and Lp-mutant cells exhibited mainly wild-type cells in the midline neural plate and notochordal plate, consistent with a cell-autonomous disturbance of convergent extension. Inhibitor studies in whole-embryo culture demonstrated a requirement for signalling via RhoA-Rho kinase, but not jun N-terminal kinase, in convergent extension and the onset of neural tube closure. These findings identify a cell-autonomous defect of convergent extension, requiring PCP signalling via RhoA-Rho kinase, during the development of severe neural tube defects in the mouse.

Suppressor of cytokine signaling 2 regulates neuronal differentiation by inhibiting growth hormone signaling.

The intracellular mechanisms that determine the response of neural progenitor cells to growth factors and regulate their differentiation into either neurons or astrocytes remain unclear. We found that expression of SOCS2, an intracellular regulator of cytokine signaling, was restricted to mouse progenitor cells and neurons in response to leukemia inhibitory factor (LIF)-like cytokines. Progenitors lacking SOCS2 produced fewer neurons and more astrocytes in vitro, and Socs2(-/-) mice had fewer neurons and neurogenin-1 (Ngn1)-expressing cells in the developing cortex, whereas overexpression of SOCS2 increased neuronal differentiation. We also report that growth hormone inhibited Ngn1 expression and neuronal production, and this action was blocked by SOCS2 overexpression. These findings indicate that SOCS2 promotes neuronal differentiation by blocking growth hormone-mediated downregulation of Ngn1.