Axon injury triggers EFA-6 mediated destabilization of axonal microtubules via TACC and doublecortin like kinase

Abstract

Axon injury triggers a series of changes in the axonal cytoskeleton that are prerequisites for effective axon regeneration. In Caenorhabditis elegans the signaling protein Exchange Factor for ARF-6 (EFA-6) is a potent intrinsic inhibitor of axon regrowth. Here we show that axon injury triggers rapid EFA-6-dependent inhibition of axonal microtubule (MT) dynamics, concomitant with relocalization of EFA-6. EFA-6 relocalization and axon regrowth inhibition require a conserved 18-aa motif in its otherwise intrinsically disordered N-terminal domain. The EFA-6 N-terminus binds the MT-associated proteins TAC-1/Transforming-Acidic-Coiled-Coil, and ZYG-8/Doublecortin-Like-Kinase, both of which are required for regenerative growth cone formation, and which act downstream of EFA-6. After injury TAC-1 and EFA-6 transiently relocalize to sites marked by the MT minus end binding protein PTRN-1/Patronin. We propose that EFA-6 acts as a bifunctional injury-responsive regulator of axonal MT dynamics, acting at the cell cortex in the steady state and at MT minus ends after injury.DOI: http://dx.doi.org/10.7554/eLife.08695.001

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

“…Although clearly related to other EFA6 family Arf6 GEFs, studies in C. elegans embryos revealed EFA-6 also functions as a MT destabilizing factor (O'Rourke et al 2010). Extensive analysis of EFA-6 in neurons supports the model that it restrains regeneration predominantly via inhibiting axonal MT dynamics (Chen et al 2015). Loss of function in EFA-6 results in elevated numbers of dynamic axonal MTs, and prevents the injury-triggered downregulation in MT dynamics.…”

“…Conversely, overexpression of EFA-6 blocks MT dynamics, an activity that can be localized to the intrinsically disordered N-terminus of EFA-6. Screens for EFA-6 interactors identified the MT-associated proteins TAC-1/TACC and ZYG-8/DCLK, each of which is required for axon regrowth (Chen et al 2015). Moreover, EFA-6 activity is dynamically regulated: after injury, EFA-6 translocates from the cell cortex to axonal puncta that contain the minus end marker PTRN-1.…”

“…Whereas analysis of cytoskeletal regulators in developmental guidance has focused on actin (see above), the MT cytoskeleton appears to play a prominent role in regenerative axon regrowth, and has been a focus of intensive analysis. Within seconds of injury, axonal MTs become depolymerized locally at the injury site, as visualized by MT plus end binding protein markers (Chen et al 2015). Over the next 2-3 hr, MTs undergo a series of changes, ultimately leading to an upregulation of dynamic MTs relative to the more stable MTs of the intact mature axon (Ghosh- Roy et al 2012).…”

“…Tissuespecific RNAi screens targeting early embryonic lethal genes might allow us to circumvent this problem. Tissue specific knockouts using Cre recombinase or CRISPR/Cas9 have recently been used to address the functions of essential genes in neurons (Chen et al 2015;Tian et al 2015).…”

Axon regeneration in C. elegans: Worming our way to mechanisms of axon regeneration

“…Interestingly, the functions of many of the identified genes are specific to axon regeneration and not development, as the PLM axons have mostly wild type morphology pre-injury. One of the characterized hits is the conserved Arf Guanine nucleotide Exchange Factor efa-6 , which inhibits axon regeneration by regulating microtubule dynamics (Chen et al, 2015). Interestingly, with some notable exceptions (e.g.…”