Novel DLK-independent neuronal regeneration in Caenorhabditis elegans shares links with activity-dependent ectopic outgrowth

Abstract

During development, a neuron transitions from a state of rapid growth to a stable morphology, and neurons within the adult mammalian CNS lose their ability to effectively regenerate in response to injury. Here, we identify a novel form of neuronal regeneration, which is remarkably independent of DLK-1/DLK, KGB-1/JNK, and other MAPK signaling factors known to mediate regeneration in Caenorhabditis elegans, Drosophila, and mammals. This DLK-independent regeneration in C. elegans has direct genetic and molecular links to a well-studied form of endogenous activity-dependent ectopic axon outgrowth in the same neuron type. Both neuron outgrowth types are triggered by physical lesion of the sensory dendrite or mutations disrupting sensory activity, calcium signaling, or genes that restrict outgrowth during neuronal maturation, such as SAX-1/NDR kinase or UNC-43/CaMKII. These connections suggest that ectopic outgrowth represents a powerful platform for gene discovery in neuronal regeneration. Moreover, we note numerous similarities between C. elegans DLK-independent regeneration and lesion conditioning, a phenomenon producing robust regeneration in the mammalian CNS. Both regeneration types are triggered by lesion of a sensory neurite via reduction of neuronal activity and enhanced by disrupting L-type calcium channels or elevating cAMP. Taken as a whole, our study unites disparate forms of neuronal outgrowth to uncover fresh molecular insights into activity-dependent control of the adult nervous system's intrinsic regenerative capacity.lesion conditioning | axon regeneration | femtosecond laser ablation | DLK-1 | activity-dependent ectopic axon outgrowth O ne of the principal goals of modern neuroscience is the comprehensive understanding and therapeutic application of neuronal regeneration (1). This goal is particularly relevant in the case of the mammalian central nervous system (CNS), which regenerates poorly. Efforts to enhance axon regeneration generally fall into two broad categories: eliminating or blocking nonpermissive extrinsic inhibitors or promoting a neuron's intrinsic regenerative capacity (2). Although much research in previous decades focused on the extrinsic angle, recent encouraging developments, particularly in invertebrate models, increasingly examine the cell intrinsic control of regeneration. Studies demonstrate that axon regeneration recruits or recapitulates mechanisms involved in a diverse range of biological processes, including synapse formation, stress response, apoptosis, and development.During development, neuronal electrical activity acts as a common intracellular feedback mechanism to establish appropriate connections and modulate outgrowth (3). Subsequently, a neuron transitions from a state of rapid growth to a stable morphology, and neurons within the adult mammalian CNS lose their ability to effectively regenerate in response to injury. A striking exception to this paradigm is lesion conditioning, a phenomenon exemplified by the dorsal root ganglion (DRG), where peripheral axon damag...

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

“…Regrowth responses diminish distally from the soma. Some chemosensory dendrites exhibit regenerative responses after dendrite injury (dendrotomy) (Wu et al 2007); moreover, a recent report suggests that dendrotomy alters the axonal response to injury in ASJ neurons (Chung et al 2016) (see below).…”

“…However, in neurons such as ALM, dlk-1 null mutants display partial regrowth after axotomy, indicating the existence of DLK-1-independent regrowth pathways. Moreover, axotomized chemosensory neurons such as ASJ display regenerative regrowth in response to dendrotomy that is independent of the DLK-1 and KGB-1/JNK cascades (Chung et al 2016). In these neurons, inhibition of neuronal activity by a variety of methods, including mutation of Ca 2+ channels, altered membrane potential, or sensory transduction, can promote DLK-1-independent regeneration after axotomy.…”

Dendrites actively restrain axon outgrowth and regeneration

“…Mutation of dlk-1 also eliminated regeneration after laser cutting of ASJ axons. Surprisingly, however, Chung et al (5) found that simultaneous cutting of the ASJ axon and dendrite restores the regenerative capacity of ASJ axons even in dlk-1 mutants. Thus, dendrite lesion appears to activate a novel type of Dlkindependent axon regrowth.…”

“…The effect of the preconditioning lesion has been thought to result from the first lesion driving neuronal de-differentiation and resetting its fate to something more like a developing neuron (4), but the precise mechanisms involved have remained unclear. In PNAS, Chung et al (5) use an elegant combination of genetics, laser ablations, and pharmacology to demonstrate that dendrites actively repress regenerative outgrowth in functionally mature neurons through a pathway that is independent of the well-conserved dual leucine zipper kinase (DLK)-regulated regeneration cascade (Fig. 1).…”

“…Chung et al (5) investigate sensory regulation of regeneration in Caenorhabditis elegans ASJ sensory neurons. These neurons are bilaterally symmetrical and bipolar: from each ASJ neuronal cell body projects a single dendrite with sensory cilia and an axonal connection to the nerve ring (central ganglion of the nematode nervous system), and each compartment of the cell can be easily resolved by microscopy.…”