Development of inhibition by ephrin‐A5 on outgrowth of embryonic spinal motor neurites

Abstract

The spinal motor pool maps systematically onto the surface of muscles. This map is detectable in rat embryonic muscles, and is partially restored after reinnervation. Recent evidence shows that either overexpression or deletion of the ephrin-A5 gene significantly disrupts the map, suggesting that ephrin-A5 plays a critical role in the formation of this topography. Several studies have demonstrated that ephrin-A5 is a repulsive molecule in the nervous system, including the neuromuscular system. To examine the development of sensitivity of ventral spinal axons to this inhibitory ligand, slices of E11 to E15 embryonic rat spinal cords were cocultured with membranes derived from ephrin-A5-expressing cell lines. We detected a progressive expression of inhibition by ephrin-A5 between E11 and E15. By E15, rostral and caudal spinal neurites showed clear differences in responsiveness to the ephrin-A5 ligand. Further, we found that at this age caudal neurites are more sensitive to changes of ephrin-A5 concentration along a gradient. In addition, growth cones of caudal, more than rostral, neurites tended to assume a collapsed shape in the presence of the ligand. These results demonstrate a progressive development of sensitivity to ephrin-A5, and suggest a divergence in this sensitivity between rostral and caudal spinal cord neurites. These results provide further insight into how subtle rostrocaudal differences in the development of sensitivity to ephrin-A5 may explain, in part, neuromuscular topography.

Differential expression of Eph receptors and ephrins in the cochlear ganglion and eighth cranial nerve of the chick embryo

“…Gradients of Eph receptor and ephrin expression have been shown to be essential for the formation of topographic maps in the visual system (Cheng et al, 1995;Drescher et al, 1995;Feldheim et al, 1998Feldheim et al, , 2000Flanagan and Vanderhaeghen, 1998;Yates et al, 2001;Hindges et al, 2002;Mann et al, 2002b), the hippocamposeptal pathway (Gao et al, 1996;Yue et al, 2002), thalamic projections to sensory cortical areas (Dufour et al, 2003), and spinal cord projections to precise targets within muscles (Feng et al, 2000;Wang et al, 2001) or limbs (Helmbacher et al, 2000;Eberhart et al, 2002). These studies demonstrate the role of Eph proteins in the formation of topographic maps in a variety of neural pathways.…”

Topographic Specificity within Membranes of a Single Muscle DetectedIn Vitro

“…This overall inhibitory role of ephrin As is based on three observations. First, both cervical and lumbar spinal cord neurites avoid lanes containing ephrin A5, especially caudal neurites (Wang et al, 1999(Wang et al, , 2001. Second, we report that the avoidance of muscle membranes from a distant region of the neuraxis is reduced in ephrin A2, A5 mutant mice.…”

“…In an effort to define the molecular mechanisms underlying these topographic maps, we have previously reported the preferential growth of cervical spinal neurites (rostral) on rostral muscle membranes. In contrast, lumbar neurites (caudal) preferred growth on caudal muscle membranes (Wang et al, 1999(Wang et al, , 2001Feng et al 2000). Although instructive, this experimental design did not reproduce the in vivo situation wherein neurites within a single motor pool are able to detect topographic differences within a single appropriate target muscle.…”

“…Caudal spinal cord neurites, and to a lesser extent rostral neurites avoided growth on membranes expressing the ligand ephrin A5 (Wang et al 2001). Moreover, in vivo physiological studies revealed that overexpression of ephrin A5 or deletion of the genes for ephrin A2 and A5 degrade the neuromuscular map.…”

EFN-4/Ephrin functions in LAD-2/L1CAM-mediated axon guidance in Caenorhabditis elegans

“…The specificity of these connections requires accurate axon pathfinding, which is controlled by many molecules, including growth factors, adhesion molecules, and long-and short-range guidance cues and their receptors. An important class of guidance cues are the ephrins, which are either glycosyl-phosphatidylinositol (GPI) linked or single-pass transmembrane proteins (Helmbacher et al, 2000;Luria et al, 2008;Wang et al, 2001). In mammals, the five GPI-linked 'A' class ephrins and the three transmembrane 'B' class ephrins act as short-range guidance cues that signal via their cognate A or B class Eph receptor tyrosine kinases.…”