RHGF-1/PDZ-RhoGEF and retrograde DLK-1 signaling drive neuronal remodeling on microtubule disassembly

Abstract

Neurons remodel their connectivity in response to various insults, including microtubule disruption. How neurons sense microtubule disassembly and mount remodeling responses by altering genetic programs in the soma are not well defined. Here we show that in response to microtubule disassembly, the Caenorhabditis elegans PLM neuron remodels by retracting its synaptic branch and overextending the primary neurite. This remodeling required RHGF-1, a PDZ-Rho guanine nucleotide exchange factor (PDZ-RhoGEF) that was associated with and inhibited by microtubules. Independent of the myosin light chain activation, RHGF-1 acted through Rho-dependent kinase LET-502/ROCK and activated a conserved, retrograde DLK-1 MAPK (DLK-1/dual leucine zipper kinase) pathway, which triggered synaptic branch retraction and overgrowth of the PLM neurite in a dose-dependent manner. Our data represent a neuronal remodeling paradigm during development that reshapes the neural circuit by the coordinated removal of the dysfunctional synaptic branch compartment and compensatory extension of the primary neurite.axon retraction | axon regeneration | Rho signaling | DLK | microtubules T he connectivity of neuronal circuits is established first through a highly dynamic phase of addition or elimination of axon branches and synapses, followed by a maintenance phase in which axon and dendritic branches show remarkable stability during the lifetime of postmitotic neurons. Disruption of neuronal circuits elicits remodeling responses that eliminate dysfunctional neurites or synapses and may stimulate the regeneration of injured axons. To enable these remodeling responses, neurons need to sense the insults and signal to the effectors that execute either retraction or extension of the neuronal structures. A well-studied effector for neuronal remodeling is the dual leucine-zipper kinase DLK, whose activation is required for both Wallerian degeneration and axon regeneration after injury, depending on the species and contexts examined (1-4). The DLK protein level is primarily controlled by proteasomal turnover that depends on Highwire/RPM-1, a ubiquitin E3 ligase that targets DLK for degradation (5). The Caenorhabditis elegans DLK, DLK-1, is activated by calcium influx during axon injury that triggers the dissociation of an inhibitory DLK-1 isoform (6). A recent report suggests that C. elegans dlk-1 is a direct transcriptional target of the FoxO transcription factor DAF-16 in axon regeneration (7). Despite the extensive characterization of DLK, little is known about how neurons sense various insults and translate them into DLK-activating signals.Microtubules are a major neuronal cytoskeleton component that shapes and maintains synapses and axons (8, 9). Mutations in Ankyrin and α-spectrin, two membrane skeletal proteins that organize microtubules at presynaptic sites, triggered synaptic bouton shrinkage and axon terminal retraction at Drosophila neuromuscular junction, highlighting the central importance of microtubules in the maintenance of synapses and axon b...

Cytoskeletal disruption activates the DLK/JNK pathway, which promotes axonal regeneration and mimics a preconditioning injury

“…Mutation of the spectraplakin short stop , an actin-microtubule cross-linker, or knockdown of subunits of TCP1, a chaperonin complex that helps fold both actin and tubulin, both lead to activation of the DLK pathway. In addition, mutations disrupting microtubules in C. elegans lead to DLK-dependent changes in levels of neuronal proteins and neuronal morphology (Bounoutas et al, 2011; Chen et al, 2014; Marcette et al, 2014). These findings lead us to hypothesize that cytoskeletal injury may be a signal that activates the DLK pathway.…”

An axonal stress response pathway: degenerative and regenerative signaling by DLK

“…Mutations that inhibit DLK signaling rescue the synaptic defects associated with mutations in the kinesin unc-104 [24••]. Other genetic interaction studies in invertebrate peripheral neurons suggest that DLK mediates changes in neuronal morphology caused by mutations that impair cytoskeletal structure [47,52–55]. Hence DLK signaling appears responsible for both neuronal plasticity and for major pathologies associated with defects in cytoskeleton and axonal transport.…”

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

“…Calcium signaling triggers dissociation of an inhibitory DLK-1 isoform to activate DLK-1 function (Yan and Jin, 2012). In addition to calcium signaling, DLK-1 is activated by colchicine-induced microtubule disassembly via RHGF-1 (PDZ-RhoGEF) (Bounoutas et al, 2011; Chen et al, 2014), and DLK-1 function requires palmitoylation (Holland et al, 2016). …”