EphA4-dependent axon retraction and midline localization of Ephrin-B3 are disrupted in the spinal cord of mice lacking mDia1 and mDia3 in combination

Dual spatio-temporal regulation of axon growth and microtubule dynamics by RhoA signaling pathways

RhoA plays a crucial role in neuronal polarization, where its action restraining axon outgrowth has been thoroughly studied. We now report that RhoA has not only an inhibitory but also a stimulatory effect on axon development depending on when and where exerts its action and the downstream effectors involved. In cultured hippocampal neurons, FRET imaging revealed that RhoA activity selectively localized in growth cones of undifferentiated neurites, whereas in developing axons it displayed a biphasic pattern, being low in nascent axons and high in elongating ones. RhoA-Rho kinase (ROCK) signaling prevented axon initiation but had no effect on elongation, whereas formin inhibition reduced axon extension without significantly altering initial outgrowth. In addition, RhoA-mDia signaling promoted axon elongation by stimulating growth cone microtubule stability and assembly, as opposed to RhoA-ROCK signaling, which restrained growth cone microtubule assembly and protrusion.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

EphA4 may contribute to microvessel remodeling in the hippocampal CA1 and CA3 areas in a mouse model of temporal lobe epilepsy

Unclustered and pre-clustered ephrin-A5-Fc have identical anti-epileptic effects in the dentate gyrus of hippocampus in a mouse model of temporal lobe epilepsy (TLE), and act through alleviating ephrin receptor A4 (EphA4)-mediated neurogenesis and angiogenesis. However, the effects of ephrin-A5-Fcs on EphA4 and angiogenesis in Cornu Ammonis (CA)1 and CA3 areas remain unclear. In the present study, male C57BL/6 mice underwent pilocarpine-induced TLE. The expression of EphA4 and ephrin-A5 proteins was analyzed by immunohistochemistry, and the mean density and diameter of platelet endothelial cell adhesion molecule-1-labeled microvessels in CA1 and CA3 were calculated in the absence or presence of two types of ephrin-A5-Fc intrahippocampal infusion. Microvessels perpendicular to the pyramidal cell layer decreased; however, microvessels that traversed the layer increased, and became distorted and fragmented. The mean densities and diameters of microvessels gradually increased and remained greater than those in the control group at 56 days post-status epilepticus (SE). The upregulation of EphA4 and ephrin-A5 proteins began at 7 days and was maintained until 28 days, subsequently decreasing slightly at 56 days post-SE. Blockade of EphA4 by unclustered-ephrin-A5-Fc effected a reduction in the mean density and mean diameter of microvessels in the CA1 and CA3 areas; conversely, activation of EphA4 by clustered-ephrin-A5-Fc induced an increase in these values. Ephrin-A5 ligand binding to EphA4 receptor may contribute to angiogenesis during epileptogenesis in the hippocampal CA1 and CA3 areas.