Abstract
Axonal growth cones are guided by molecular cues in the extracellular environment. The mechanisms of combinatorial integration of guidance signals at the growth cone cell membrane are still being unravelled. Limb-innervating axons of vertebrate spinal lateral motor column (LMC) neurons are attracted to netrin-1 via its receptor, Neogenin, and are repelled from ephrin-A5 through its receptor EphA4. The presence of both cues elicits synergistic guidance of LMC axons, but the mechanism of this effect remains unknown. Using fluorescence immunohistochemistry, we show that ephrin-A5 increases LMC growth cone Neogenin protein levels and netrin-1 binding. This effect is enhanced by overexpressing EphA4 and is inhibited by blocking ephrin-A5-EphA4 binding. These effects have a functional consequence on LMC growth cone responses since bath addition of ephrin-A5 increases the responsiveness of LMC axons to netrin-1. Surprisingly, the overexpression of EphA4 lacking its cytoplasmic tail, also enhances Neogenin levels at the growth cone and potentiates LMC axon preference for growth on netrin-1. Since netrins and ephrins participate in a wide variety of biological processes, the enhancement of netrin-1 signalling by ephrins may have broad implications.
Highlights
During nervous system assembly, neuronal wiring is specified by a rather limited set of axon guidance cues deployed at axonal trajectory decision points[1]
Ephrin ligands expressed in the limb repel lateral motor column (LMC) axons through their cognate receptors expressed in LMC axons: medial LMC axons express EphB tyrosine kinase receptors and are repelled from ephrin-Bs expressed in the dorsal limb, whereas lateral LMC axons express EphA receptors, including EphA4, and are repelled from ephrin-As, including ephrin-A5, in the ventral limb[8,9,10,11]
At the time of their growth into the limb mesenchyme, chick lateral LMC axons respond synergistically to the presence of ephrin-A5 and netrin-1: while these axons are insensitive to low concentrations of either netrin-1 or ephrin-A5, when challenged simultaneously with stripes containing low concentrations of netrin-1 and ephrin-A5, lateral LMC axons exhibit a robust growth on netrin-1 stripes[18]
Summary
Neuronal wiring is specified by a rather limited set of axon guidance cues deployed at axonal trajectory decision points[1]. The extension of the axons of the lateral LMC into the dorsal limb, and axons of the medial LMC into the ventral limb[4,5] is specified by a number of axon guidance cues, including members of the ephrin and netrin protein families[6,7]. In thalamocortical (TC) growth cones, Slit[1] signalling via Robo[1] and FLRT3 raises the levels of DCC allowing netrin-1 attraction, such that in the absence of Slit[1], TC axons are unresponsive towards netrin-123,24 These and above studies suggest that netrin-1 signalling through its attractive receptors depends on the action of other axon guidance signals, prompting us to examine the mechanism of netrin-1 and ephrin-A integration by LMC axons. This effect occurs in the absence of the intracellular signalling tail of EphA4, demonstrating that lateral LMC axon repulsion from ephrin-A5 and sensitization to netrin-1 occur through molecularly distinct pathways
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
