The DLK-1 kinase promotes mRNA stability and local translation in C. elegans synapses and axon regeneration

Wallenda/DLK protein levels are temporally downregulated by Tramtrack69 to allow R7 growth cones to become stationary boutons

Dual leucine zipper kinase (DLK) promotes growth cone motility and must be restrained to ensure normal development. PHR (Pam/Highwire/RPM-1) ubiquitin ligases therefore target DLK for degradation unless axon injury occurs. Overall DLK levels decrease during development, but how DLK levels are regulated within a developing growth cone has not been examined. We analyzed the expression of the fly DLK Wallenda (Wnd) in R7 photoreceptor growth cones as they halt at their targets and become presynaptic boutons. We found that Wnd protein levels are repressed by the PHR protein Highwire (Hiw) during R7 growth cone halting, as has been observed in other systems. However, as R7 growth cones become boutons, Wnd levels are further repressed by a temporally expressed transcription factor, Tramtrack69 (Ttk69). Previously unobserved negative feedback from JNK also contributes to Wnd repression at both time points. We conclude that neurons deploy additional mechanisms to downregulate DLK as they form stable, synaptic connections. We use live imaging to probe the effects of Wnd and Ttk69 on R7 bouton development and conclude that Ttk69 coordinates multiple regulators of this process.

An evolutionarily conserved mechanism for cAMP elicited axonal regeneration involves direct activation of the dual leucine zipper kinase DLK

A broadly known method to stimulate the growth potential of axons is to elevate intracellular levels of cAMP, however the cellular pathway(s) that mediate this are not known. Here we identify the Dual Leucine-zipper Kinase (DLK, Wnd in Drosophila) as a critical target and effector of cAMP in injured axons. DLK/Wnd is thought to function as an injury ‘sensor’, as it becomes activated after axonal damage. Our findings in both Drosophila and mammalian neurons indicate that the cAMP effector kinase PKA is a conserved and direct upstream activator of Wnd/DLK. PKA is required for the induction of Wnd signaling in injured axons, and DLK is essential for the regenerative effects of cAMP in mammalian DRG neurons. These findings link two important mediators of responses to axonal injury, DLK/Wnd and cAMP/PKA, into a unified and evolutionarily conserved molecular pathway for stimulating the regenerative potential of injured axons.

DOI:http://dx.doi.org/10.7554/eLife.14048.001

Adult mammals typically cannot repair damage to the nerve fibers in their brain or spinal cord. This is because these nerve cells cannot generally grow new nerve fibers. However this inability to regenerate nerve fibers is not set in stone. Instead, it can be unlocked by a second injury in nerves elsewhere in the body, the so-called “peripheral nervous system”. This process relies on an enzyme called DLK, which becomes activated in damaged nerve fibers.

But how does DLK ‘sense’ damage to nerve fibers? Injuring the peripheral nervous system causes the levels of a molecule called cAMP to increase in the damaged nerve cells, and the elevated cAMP levels stimulate the nerve fibers to regenerate. However, it was not known if cAMP activates DLK, or if the two act independently of each other.

By looking at the regeneration of damaged nerve fibers in fruit fly larvae, Hao et al. now show that the cAMP and DLK signaling pathways are clearly linked. Further experiments with nerve cells from mice and human cells revealed more detail about this link. Together the results showed that another enzyme called PKA activates DLK directly when cAMP levels are high. These findings reveal a unified pathway that is the key to unlocking the regenerative potential of injured nerve fibers, which has been conserved for hundreds of millions of years of evolution. Further work could now ask if the DLK enzyme is involved in the other known roles of cAMP signaling in nerve cells; or if cAMP and PKA activate DLK in other forms of nerve damage, including injuries where nerve fibers normally fail to regenerate.

DOI:http://dx.doi.org/10.7554/eLife.14048.002

Endocannabinoid signaling regulates regenerative axon navigation in Caenorhabditis elegans via the GPCRs NPR-19 and NPR-32

The axon regeneration ability of neurons depends on the interplay of factors that promote and inhibit regeneration. In Caenorhabditis elegans, axon regeneration is promoted by the JNK MAP kinase (MAPK) pathway. Previously, we found that the endocannabinoid anandamide (AEA) inhibits the axon regeneration response of motor neurons after laser axotomy by suppressing the JNK signaling pathway. Here, we show that the G-protein-coupled receptors (GPCRs) NPR-19 and NPR-32 inhibit axon regeneration in response to AEA. Furthermore, we show that sensory neuron expression of the nape-1 gene, which encodes an enzyme synthesizing AEA, causes the regenerating motor axons to avoid sensory neurons and this avoidant response depends on NPR-19 and NPR-32. These results indicate that the navigation of regenerating axons is modulated by the action of AEA on NPR-19/32 GPCRs.

The MADD-3 LAMMER Kinase Interacts with a p38 MAP Kinase Pathway to Regulate the Display of the EVA-1 Guidance Receptor in Caenorhabditis elegans

The proper display of transmembrane receptors on the leading edge of migrating cells and cell extensions is essential for their response to guidance cues. We previously discovered that MADD-4, which is an ADAMTSL secreted by motor neurons in Caenorhabditis elegans, interacts with an UNC-40/EVA-1 co-receptor complex on muscles to attract plasma membrane extensions called muscle arms. In nematodes, the muscle arm termini harbor the post-synaptic elements of the neuromuscular junction. Through a forward genetic screen for mutants with disrupted muscle arm extension, we discovered that a LAMMER kinase, which we call MADD-3, is required for the proper display of the EVA-1 receptor on the muscle’s plasma membrane. Without MADD-3, EVA-1 levels decrease concomitantly with a reduction of the late-endosomal marker RAB-7. Through a genetic suppressor screen, we found that the levels of EVA-1 and RAB-7 can be restored in madd-3 mutants by eliminating the function of a p38 MAP kinase pathway. We also found that EVA-1 and RAB-7 will accumulate in madd-3 mutants upon disrupting CUP-5, which is a mucolipin ortholog required for proper lysosome function. Together, our data suggests that the MADD-3 LAMMER kinase antagonizes the p38-mediated endosomal trafficking of EVA-1 to the lysosome. In this way, MADD-3 ensures that sufficient levels of EVA-1 are present to guide muscle arm extension towards the source of the MADD-4 guidance cue.

In most animals, the physical meeting of the pre- and post-synaptic membranes of the neuromuscular junction occurs via axonal extension towards the muscle. In nematodes, however, motor axons do not extend towards the muscle and instead form a dorsal and ventral cord with relatively few branches. To make the physical connection, the body wall muscles extend membrane projections called muscle arms to the motor axons within the dorsal and ventral cords. Through previous genetic and biochemical analyses with the nematode C. elegans, we identified a neuronally-expressed muscle arm chemoattractant (MADD-4) and its muscle-expressed co-receptor complex (UNC-40/EVA-1). Here, we report our discovery of madd-3, which encodes a LAMMER kinase that is expressed in muscles to regulate muscle arm extension. Genetic analyses revealed that MADD-3 may inhibit a p38 MAP kinase pathway whose normal function is to decrease the abundance of the EVA-1 receptor. In the presence of MADD-3, the activity of the p38 pathway is relatively low, and EVA-1 levels are consequently relatively high. Without MADD-3, the p38 pathway is freed to decrease the abundance of EVA-1. The relationships that we have uncovered between MADD-3, the p38 Map Kinase pathway, and the EVA-1 receptor provide one explanation for the muscle arm defects observed in madd-3 mutants.

Regulation of Microtubule Dynamics in Axon Regeneration: Insights from C. elegans

The capacity of an axon to regenerate is regulated by its external environment and by cell-intrinsic factors. Studies in a variety of organisms suggest that alterations in axonal microtubule (MT) dynamics have potent effects on axon regeneration. We review recent findings on the regulation of MT dynamics during axon regeneration, focusing on the nematode Caenorhabditis elegans. In C. elegans the dual leucine zipper kinase (DLK) promotes axon regeneration, whereas the exchange factor for Arf6 (EFA-6) inhibits axon regeneration. Both DLK and EFA-6 respond to injury and control axon regeneration in part via MT dynamics. How the DLK and EFA-6 pathways are related is a topic of active investigation, as is the mechanism by which EFA-6 responds to axonal injury. We evaluate potential candidates, such as the MT affinity-regulating kinase PAR-1/MARK, in regulation of EFA-6 and axonal MT dynamics in regeneration.

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

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.

The PHR proteins: intracellular signaling hubs in neuronal development and axon degeneration

During development, a coordinated and integrated series of events must be accomplished in order to generate functional neural circuits. Axons must navigate toward target cells, build synaptic connections, and terminate outgrowth. The PHR proteins (consisting of mammalian Phr1/MYCBP2, Drosophila Highwire and C. elegans RPM-1) function in each of these events in development. Here, we review PHR function across species, as well as the myriad of signaling pathways PHR proteins regulate. These findings collectively suggest that the PHR proteins are intracellular signaling hubs, a concept we explore in depth. Consistent with prominent developmental functions, genetic links have begun to emerge between PHR signaling networks and neurodevelopmental disorders, such as autism, schizophrenia and intellectual disability. Finally, we discuss the recent and important finding that PHR proteins regulate axon degeneration, which has further heightened interest in this fascinating group of molecules.

Context Specificity of Stress-activated Mitogen-activated Protein (MAP) Kinase Signaling: The Story as Told by Caenorhabditis elegans

Stress-associated p38 and JNK mitogen-activated protein (MAP) kinase signaling cascades trigger specific cellular responses and are involved in multiple disease states. At the root of MAP kinase signaling complexity is the differential use of common components on a context-specific basis. The roundwormCaenorhabditis eleganswas developed as a system to study genes required for development and nervous system function. The powerful genetics ofC. elegansin combination with molecular and cellular dissections has led to a greater understanding of how p38 and JNK signaling affects many biological processes under normal and stress conditions. This review focuses on the studies revealing context specificity of different stress-activated MAPK components inC. elegans.

Axon Self-Destruction: New Links among SARM1, MAPKs, and NAD+ Metabolism

Wallerian axon degeneration is a form of programmed subcellular death that promotes axon breakdown in disease and injury. Active degeneration requires SARM1 and MAP kinases, including DLK, while the NAD+ synthetic enzyme NMNAT2 prevents degeneration. New studies reveal that these pathways cooperate in a locally mediated axon destruction program, with NAD+ metabolism playing a central role. Here, we review the biology of Wallerian-type axon degeneration and discuss the most recent findings, with special emphasis on critical signaling events and their potential as therapeutic targets for axonopathy.

Timing mechanisms in neuronal pathfinding, synaptic reorganization, and neuronal regeneration

Precise temporal control of neuro differentiation and post-differentiation events are necessary for the creation of appropriate wiring diagram in the brain. To make advances in the treatment of neurodevelopmental and neurodegenerative disorders, and traumatic brain injury, it is important to understand these mechanisms. Caenorhabditis elegans has emerged as a revolutionary tool for the study of neural circuits due to its genetic homology to vertebrates and ease of genetic manipulation. microRNA (miRNA), a ubiquitous class of small non-coding RNA, that inhibits the expression of target genes, has emerged as an important timing control molecule through research conducted on C. elegans. This review will focus on the temporal control of neurodifferentiation and post-differentiation events exerted by two conserved miRNAs, lin-4 and let-7. We summarize recent findings on the role of lin-4 as a timing regulator controlling transition of sequential events in neuronal pathfinding and synaptic remodeling, and the role of let-7 as a timing regulator that limits the regeneration potential of post-differentiated AVM neurons as they age.

Palmitoylation controls DLK localization, interactions and activity to ensure effective axonal injury signaling

Dual leucine-zipper kinase (DLK) is critical for axon-to-soma retrograde signaling following nerve injury. However, it is unknown how DLK, a predicted soluble kinase, conveys long-distance signals and why homologous kinases cannot compensate for loss of DLK. Here, we report that DLK, but not homologous kinases, is palmitoylated at a conserved site adjacent to its kinase domain. Using short-hairpin RNA knockdown/rescue, we find that palmitoylation is critical for DLK-dependent retrograde signaling in sensory axons. This functional importance is because of three novel cellular and molecular roles of palmitoylation, which targets DLK to trafficking vesicles, is required to assemble DLK signaling complexes and, unexpectedly, is essential for DLK's kinase activity. By simultaneously controlling DLK localization, interactions, and activity, palmitoylation ensures that only vesicle-bound DLK is active in neurons. These findings explain how DLK specifically mediates nerve injury responses and reveal a novel cellular mechanism that ensures the specificity of neuronal kinase signaling.

Unraveling the mechanisms of synapse formation and axon regeneration: the awesome power of C. elegans genetics

Since Caenorhabditis elegans was chosen as a model organism by Sydney Brenner in 1960’s, genetic studies in this organism have been instrumental in discovering the function of genes and in deciphering molecular signaling network. The small size of the organism and the simple nervous system enable the complete reconstruction of the first connectome. The stereotypic developmental program and the anatomical reproducibility of synaptic connections provide a blueprint to dissect the mechanisms underlying synapse formation. Recent technological innovation using laser surgery of single axons and in vivo imaging has also made C. elegans a new model for axon regeneration. Importantly, genes regulating synaptogenesis and axon regeneration are highly conserved in function across animal phyla. This mini-review will summarize the main approaches and the key findings in understanding the mechanisms underlying the development and maintenance of the nervous system. The impact of such findings underscores the awesome power of C. elegans genetics.

Doublecortin-Like Kinases Promote Neuronal Survival and Induce Growth Cone Reformation via Distinct Mechanisms

After axotomy, neuronal survival and growth cone re-formation are required for axon regeneration. We discovered that doublecortin-like kinases (DCLKs), members of the doublecortin (DCX) family expressed in adult retinal ganglion cells (RGCs), play critical roles in both processes, through distinct mechanisms. Overexpression of DCLK2 accelerated growth cone re-formation in vitro and enhanced the initiation and elongation of axon re-growth after optic nerve injury. These effects depended on both the microtubule (MT)-binding domain and the serine-proline-rich (S/P-rich) region of DCXs in-cis in the same molecules. While the MT-binding domain is known to stabilize MT structures, we show that the S/P-rich region prevents F-actin destabilization in injured axon stumps. Additionally, while DCXs synergize with mTOR to stimulate axon regeneration, alone they can promote neuronal survival possibly by regulating the retrograde propagation of injury signals. Multifunctional DCXs thus represent potential targets for promoting both survival and regeneration of injured neurons.

Life without Food and the Implications for Neurodegeneration

Food availability determines developmental rate, behavior, and survival of animals. Animals that enter diapause or hibernate in response to lack of food have a double advantage: they are able to adapt to environmental and cellular challenges and survive to these challenges for a prolonged time. The metabolic and physiological adaptations that make possible diapause and hibernation also provide a favorable cellular environment for tissue protection. This review highlights the benefits of dormancy on neuronal protection in the model organism Caenorhabditis elegans and small mammals such as squirrels. Additionally, I discuss the link between metabolic restructuring occurring in diapause and changes in gene expression with the increased capacity of diapausing animals to protect neurons from degeneration and potentially foster their regeneration.

Axon Regeneration Is Regulated by Ets-C/EBP Transcription Complexes Generated by Activation of the cAMP/Ca2+ Signaling Pathways

The ability of specific neurons to regenerate their axons after injury is governed by cell-intrinsic regeneration pathways. In Caenorhabditis elegans, the JNK and p38 MAPK pathways are important for axon regeneration. Axonal injury induces expression of the svh-2 gene encoding a receptor tyrosine kinase, stimulation of which by the SVH-1 growth factor leads to activation of the JNK pathway. Here, we identify ETS-4 and CEBP-1, related to mammalian Ets and C/EBP, respectively, as transcriptional activators of svh-2 expression following axon injury. ETS-4 and CEBP-1 function downstream of the cAMP and Ca2+-p38 MAPK pathways, respectively. We show that PKA-dependent phosphorylation of ETS-4 promotes its complex formation with CEBP-1. Furthermore, activation of both cAMP and Ca2+ signaling is required for activation of svh-2 expression. Thus, the cAMP/Ca2+ signaling pathways cooperatively activate the JNK pathway, which then promotes axon regeneration.

Developmental Function of the PHR Protein RPM-1 Is Required for Learning in Caenorhabditis elegans

The PAM/Highwire/RPM-1 (PHR) proteins are signaling hubs that function as important regulators of neural development. Loss of function in Caenorhabditis elegans rpm-1 and Drosophila Highwire results in failed axon termination, inappropriate axon targeting, and abnormal synapse formation. Despite broad expression in the nervous system and relatively dramatic defects in synapse formation and axon development, very mild abnormalities in behavior have been found in animals lacking PHR protein function. Therefore, we hypothesized that large defects in behavior might only be detected in scenarios in which evoked, prolonged circuit function is required, or in which behavioral plasticity occurs. Using quantitative approaches in C. elegans, we found that rpm-1 loss-of-function mutants have relatively mild abnormalities in exploratory locomotion, but have large defects in evoked responses to harsh touch and learning associated with tap habituation. We explored the nature of the severe habituation defects in rpm-1 mutants further. To address what part of the habituation circuit was impaired in rpm-1 mutants, we performed rescue analysis with promoters for different neurons. Our findings indicate that RPM-1 function in the mechanosensory neurons affects habituation. Transgenic expression of RPM-1 in adult animals failed to rescue habituation defects, consistent with developmental defects in rpm-1 mutants resulting in impaired habituation. Genetic analysis showed that other regulators of neuronal development that function in the rpm-1 pathway (including glo-4, fsn-1, and dlk-1) also affected habituation. Overall, our findings suggest that developmental defects in rpm-1 mutants manifest most prominently in behaviors that require protracted or plastic circuit function, such as learning.

Neurodegeneration and microtubule dynamics: death by a thousand cuts

Microtubules form important cytoskeletal structures that play a role in establishing and maintaining neuronal polarity, regulating neuronal morphology, transporting cargo, and scaffolding signaling molecules to form signaling hubs. Within a neuronal cell, microtubules are found to have variable lengths and can be both stable and dynamic. Microtubule associated proteins, post-translational modifications of tubulin subunits, microtubule severing enzymes, and signaling molecules are all known to influence both stable and dynamic pools of microtubules. Microtubule dynamics, the process of interconversion between stable and dynamic pools, and the proportions of these two pools have the potential to influence a wide variety of cellular processes. Reduced microtubule stability has been observed in several neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS), and tauopathies like Progressive Supranuclear Palsy. Hyperstable microtubules, as seen in Hereditary Spastic Paraplegia (HSP), also lead to neurodegeneration. Therefore, the ratio of stable and dynamic microtubules is likely to be important for neuronal function and perturbation in microtubule dynamics might contribute to disease progression.

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

RNA imaging in living cells - methods and applications

Numerous types of transcripts perform multiple functions in cells, and these functions are mainly facilitated by the interactions of the RNA with various proteins and other RNAs. Insight into the dynamics of RNA biosynthesis, processing and cellular activities is highly desirable because this knowledge will deepen our understanding of cell physiology and help explain the mechanisms of RNA-mediated pathologies. In this review, we discuss the live RNA imaging systems that have been developed to date. We highlight information on the design of these systems, briefly discuss their advantages and limitations and provide examples of their numerous applications in various organisms and cell types. We present a detailed examination of one application of RNA imaging systems: this application aims to explain the role of mutant transcripts in human disease pathogenesis caused by triplet repeat expansions. Thus, this review introduces live RNA imaging systems and provides a glimpse into their various applications.

Glutamate Stimulates Local Protein Synthesis in the Axons of Rat Cortical Neurons by Activating α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors and Metabotropic Glutamate Receptors

Glutamate is the principal excitatory neurotransmitter in the mammalian CNS. By analyzing the metabolic incorporation of azidohomoalanine, a methionine analogue, in newly synthesized proteins, we find that glutamate treatments up-regulate protein translation not only in intact rat cortical neurons in culture but also in the axons emitting from cortical neurons before making synapses with target cells. The process by which glutamate stimulates local translation in axons begins with the binding of glutamate to the ionotropic AMPA receptors and metabotropic glutamate receptor 1 and members of group 2 metabotropic glutamate receptors on the plasma membrane. Subsequently, the activated mammalian target of rapamycin (mTOR) signaling pathway and the rise in Ca(2+), resulting from Ca(2+) influxes through calcium-permeable AMPA receptors, voltage-gated Ca(2+) channels, and transient receptor potential canonical channels, in axons stimulate the local translation machinery. For comparison, the enhancement effects of brain-derived neurotrophic factor (BDNF) on the local protein synthesis in cortical axons were also studied. The results indicate that Ca(2+) influxes via transient receptor potential canonical channels and activated the mTOR pathway in axons also mediate BDNF stimulation to local protein synthesis. However, glutamate- and BDNF-induced enhancements of translation in axons exhibit different kinetics. Moreover, Ca(2+) and mTOR signaling appear to play roles carrying different weights, respectively, in transducing glutamate- and BDNF-induced enhancements of axonal translation. Thus, our results indicate that exposure to transient increases of glutamate and more lasting increases of BDNF would stimulate local protein synthesis in migrating axons en route to their targets in the developing brain.

The back and forth of axonal injury and repair after stroke

PURPOSE OF REVIEW

The axon plays a central role in both the injury and repair phases after stroke. This review highlights emerging principles in the study of axonal injury in stroke and the role of the axon in neural repair after stroke.

RECENT FINDINGS

Ischemic stroke produces a rapid and significant loss of axons in the acute phase. This early loss of axons results from a primary ischemic injury that triggers a wave of calcium signaling, activating proteolytic mechanisms and downstream signaling cascades. A second progressive phase of axonal injury occurs during the subacute period and damages axons that survive the initial ischemic insult but go on to experience a delayed axonal degeneration driven in part by changes in axoglial contact and axonal energy metabolism. Recovery from stroke is dependent on axonal sprouting and reconnection that occurs during a third degenerative/regenerative phase. Despite this central role played by the axon, comparatively little is understood about the molecular pathways that contribute to early and subacute axonal degeneration after stroke. Recent advances in axonal neurobiology and signaling suggest new targets that hold promise as potential molecular therapeutics including axonal calcium signaling, axoglial energy metabolism and cell adhesion as well as retrograde axonal mitogen-activated protein kinase pathways. These novel pathways must be modeled appropriately as the type and severity of axonal injury vary by stroke subtype.

SUMMARY

Stroke-induced injury to axons occurs in three distinct phases each with a unique molecular underpinning. A wealth of new data about the molecular organization and molecular signaling within axons is available but not yet robustly applied to the study of axonal injury after stroke. Identifying the spatiotemporal patterning of molecular pathways within the axon that contribute to injury and repair may offer new therapeutic strategies for the treatment of stroke.

Dynamic microtubules drive circuit rewiring in the absence of neurite remodeling

A striking neuronal connectivity change in C. elegans involves the coordinated elimination of existing synapses and formation of synapses at new locations, without altering neuronal morphology. Here, we investigate the tripartite interaction between dynamic microtubules (MTs), kinesin-1, and vesicular cargo during this synapse remodeling. We find that a reduction in the dynamic MT population in motor neuron axons, resulting from genetic interaction between loss of function in the conserved MAPKKK dlk-1 and an α-tubulin mutation, specifically blocks synapse remodeling. Using live imaging and pharmacological modulation of the MT cytoskeleton, we show that dynamic MTs are increased at the onset of remodeling and are critical for new synapse formation. DLK-1 acts during synapse remodeling, and its function involves MT catastrophe factors including kinesin-13/KLP-7 and spastin/SPAS-1. Through a forward genetic screen, we identify gain-of-function mutations in kinesin-1 that can compensate for reduced dynamic MTs to promote synaptic vesicle transport during remodeling. Our data provide in vivo evidence supporting the requirement of dynamic MTs for kinesin-1-dependent axonal transport and shed light on the role of the MT cytoskeleton in facilitating neural circuit plasticity.

Neuronal responses to stress and injury in C. elegans

The nervous system plays critical roles in the stress response. Animals can survive and function under harsh conditions, and resist and recover from injuries because neurons perceive and respond to various stressors through specific regulatory mechanisms. Caenorhabditis elegans has served as an excellent model to discover fundamental mechanisms underlying the neuronal response to stress. The basic physiological processes that C. elegans exhibits under stress conditions are similar to those observed in higher organisms. Many molecular pathways activated by environmental and cellular stresses are also conserved. In this review, we summarize major findings in examining neuronal responses to hypoxia, oxidative stress, osmotic stress, and traumatic injury. These studies from C. elegans have provided novel insights into our understanding of neuronal responses to stress at the molecular, cellular, and circuit levels.

Systematic identification of 3'-UTR regulatory elements in activity-dependent mRNA stability in hippocampal neurons

Ongoing neuronal activity during development and plasticity acts to refine synaptic connections and contributes to the induction of plasticity and ultimately long-term memory storage. Activity-dependent, post-transcriptional control of mRNAs occurs through transport to axonal and dendritic compartments, local translation and mRNA stability. We have identified a mechanism that contributes to activity-dependent regulation of mRNA stability during synaptic plasticity in rat hippocampal neurons. In this study, we demonstrate rapid, post-transcriptional control over process-enriched mRNAs by neuronal activity. Systematic analysis of the 3'-UTRs of destabilized transcripts, identifies enrichment in sequence motifs corresponding to microRNA (miRNA)-binding sites. The miRNAs that were identified, miR-326-3p/miR-330-5p, miR-485-5p, miR-666-3p and miR-761 are predicted to regulate networks of genes important in plasticity and development. We find that these miRNAs are developmentally regulated in the hippocampus, many increasing by postnatal day 14. We further find that miR-485-5p controls NGF-induced neurite outgrowth in PC12 cells, tau expression and axonal development in hippocampal neurons. miRNAs can function at the synapse to rapidly control and affect short- and long-term changes at the synapse. These processes likely occur during refinement of synaptic connections and contribute to the induction of plasticity and learning and memory.

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

Nerve injury can lead to axonal regeneration, axonal degeneration, and/or neuronal cell death. Remarkably, the MAP3K dual leucine zipper kinase, DLK, promotes each of these responses, suggesting that DLK is a sensor of axon injury. In Drosophila, mutations in proteins that stabilize the actin and microtubule cytoskeletons activate the DLK pathway, suggesting that DLK may be activated by cytoskeletal disruption. Here we test this model in mammalian sensory neurons. We find that pharmacological agents designed to disrupt either the actin or microtubule cytoskeleton activate the DLK pathway, and that activation is independent of calcium influx or induction of the axon degeneration program. Moreover, activation of the DLK pathway by targeting the cytoskeleton induces a pro-regenerative state, enhancing axon regeneration in response to a subsequent injury in a process akin to preconditioning. This highlights the potential utility of activating the DLK pathway as a method to improve axon regeneration. Moreover, DLK is required for these responses to cytoskeletal perturbations, suggesting that DLK functions as a key neuronal sensor of cytoskeletal damage.

Neuronal regeneration in C. elegans requires subcellular calcium release by ryanodine receptor channels and can be enhanced by optogenetic stimulation

Regulated calcium signals play conserved instructive roles in neuronal repair, but how localized calcium stores are differentially mobilized, or might be directly manipulated, to stimulate regeneration within native contexts is poorly understood. We find here that localized calcium release from the endoplasmic reticulum via ryanodine receptor (RyR) channels is critical in stimulating initial regeneration following traumatic cellular damage in vivo. Using laser axotomy of single neurons in Caenorhabditis elegans, we find that mutation of unc-68/RyR greatly impedes both outgrowth and guidance of the regenerating neuron. Performing extended in vivo calcium imaging, we measure subcellular calcium signals within the immediate vicinity of the regenerating axon end that are sustained for hours following axotomy and completely eliminated within unc-68/RyR mutants. Finally, using a novel optogenetic approach to periodically photo-stimulate the axotomized neuron, we can enhance its regeneration. The enhanced outgrowth depends on both amplitude and temporal pattern of excitation and can be blocked by disruption of UNC-68/RyR. This demonstrates the exciting potential of emerging optogenetic technology to beneficially manipulate cell physiology in the context of neuronal regeneration and indicates a link to the underlying cellular calcium signal. Taken as a whole, our findings define a specific localized calcium signal mediated by RyR channel activity that stimulates regenerative outgrowth, which may be dynamically manipulated for beneficial neurotherapeutic effects.

Regulatory roles of RNA binding proteins in the nervous system of C. elegans

Neurons have evolved to employ many factors involved in the regulation of RNA processing due to their complex cellular compartments. RNA binding proteins (RBPs) are key regulators in transcription, translation, and RNA degradation. Increasing studies have shown that regulatory RNA processing is critical for the establishment, functionality, and maintenance of neural circuits. Recent advances in high-throughput transcriptomics have rapidly expanded our knowledge of the landscape of RNA regulation, but also raised the challenge for mechanistic dissection of the specific roles of RBPs in complex tissues such as the nervous system. The C. elegans genome encodes many RBPs conserved throughout evolution. The rich analytic tools in molecular genetics and simple neural anatomy of C. elegans offer advantages to define functions of genes in vivo at the level of a single cell. Notably, the discovery of microRNAs has had transformative effects to the understanding of neuronal development, circuit plasticity, and neurological diseases. Here we review recent studies unraveling diverse roles of RBPs in the development, function, and plasticity of C. elegans nervous system. We first summarize the general technologies for studying RBPs in C. elegans. We then focus on the roles of several RBPs that control gene- and cell-type specific production of neuronal transcripts.

MAP kinase cascades regulating axon regeneration in C. elegans

Mitogen-activated protein kinase (MAPK) signaling cascades are activated by diverse stimuli such as growth factors, cytokines, neurotransmitters and various types of cellular stress. Our evolving understanding of these signal cascades has been facilitated by genetic analyses and physiological characterization in model organisms such as the nematode Caenorhabditis elegans. Genetic and biochemical studies in C. elegans have shed light on the physiological roles of MAPK cascades in the control of cell fate decision, neuronal function and immunity. Recently it was demonstrated that MAPK signaling is also important for axon regeneration in C. elegans, and the use of C. elegans as a model system has significantly advanced our understanding of the largely conserved molecular mechanisms underlying axon regeneration. This review summarizes our current understanding of the role and regulation of MAPK signaling in C. elegans axon regeneration.

Exposure to mitochondrial genotoxins and dopaminergic neurodegeneration in Caenorhabditis elegans

Neurodegeneration has been correlated with mitochondrial DNA (mtDNA) damage and exposure to environmental toxins, but causation is unclear. We investigated the ability of several known environmental genotoxins and neurotoxins to cause mtDNA damage, mtDNA depletion, and neurodegeneration in Caenorhabditis elegans. We found that paraquat, cadmium chloride and aflatoxin B₁ caused more mitochondrial than nuclear DNA damage, and paraquat and aflatoxin B₁ also caused dopaminergic neurodegeneration. 6-hydroxydopamine (6-OHDA) caused similar levels of mitochondrial and nuclear DNA damage. To further test whether the neurodegeneration could be attributed to the observed mtDNA damage, C. elegans were exposed to repeated low-dose ultraviolet C radiation (UVC) that resulted in persistent mtDNA damage; this exposure also resulted in dopaminergic neurodegeneration. Damage to GABAergic neurons and pharyngeal muscle cells was not detected. We also found that fasting at the first larval stage was protective in dopaminergic neurons against 6-OHDA-induced neurodegeneration. Finally, we found that dopaminergic neurons in C. elegans are capable of regeneration after laser surgery. Our findings are consistent with a causal role for mitochondrial DNA damage in neurodegeneration, but also support non mtDNA-mediated mechanisms.

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

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.

The microtubule minus-end-binding protein patronin/PTRN-1 is required for axon regeneration in C. elegans

Precise regulation of microtubule (MT) dynamics is increasingly recognized as a critical determinant of axon regeneration. In contrast to developing neurons, mature axons exhibit noncentrosomal microtubule nucleation. The factors regulating noncentrosomal MT architecture in axon regeneration remain poorly understood. We report that PTRN-1, the C. elegans member of the Patronin/Nezha/calmodulin- and spectrin-associated protein (CAMSAP) family of microtubule minus-end-binding proteins, is critical for efficient axon regeneration in vivo. ptrn-1-null mutants display generally normal developmental axon outgrowth but significantly impaired regenerative regrowth after laser axotomy. Unexpectedly, mature axons in ptrn-1 mutants display elevated numbers of dynamic axonal MTs before and after injury, suggesting that PTRN-1 inhibits MT dynamics. The CKK domain of PTRN-1 is necessary and sufficient for its functions in axon regeneration and MT dynamics and appears to stabilize MTs independent of minus-end localization. Whereas in developing neurons, PTRN-1 inhibits activity of the DLK-1 mitogen-activated protein kinase (MAPK) cascade, we find that, in regeneration, PTRN-1 and DLK-1 function together to promote axonal regrowth.

Neuronal development in Caenorhabditis elegans is regulated by inhibition of an MLK MAP kinase pathway

We show that loss-of-function mutations in kinases of the MLK-1 pathway (mlk-1, mek-1, and kgb-1/jnk) function cell-autonomously in neurons to suppress defects in synapse formation and axon termination caused by rpm-1 loss of function. Our genetic analysis also suggests that the phosphatase PPM-1, like RPM-1, is a potential inhibitor of kinases in the MLK-1 pathway.

Axon plasticity in the mammalian central nervous system after injury

It is widely recognized that severed axons in the adult central nervous system (CNS) have limited capacity to regenerate. However, mounting evidence from studies of CNS injury response and repair is challenging the prevalent view that the adult mammalian CNS is incapable of structural reorganization to adapt to an altered environment. Animal studies demonstrate the potential to achieve significant anatomical repair and functional recovery following CNS injury by manipulating axon growth regulators alone or in combination with activity-dependent strategies. With a growing understanding of the cellular and molecular mechanisms regulating axon plasticity, and the availability of new experimental tools to map detour circuits of functional importance, directing circuit rewiring to promote functional recovery may be achieved.

Systematic analyses of rpm-1 suppressors reveal roles for ESS-2 in mRNA splicing in Caenorhabditis elegans

The PHR (Pam/Highwire/RPM-1) family of ubiquitin E3 ligases plays conserved roles in axon patterning and synaptic development. Genetic modifier analysis has greatly aided the discovery of the signal transduction cascades regulated by these proteins. In Caenorhabditis elegans, loss of function in rpm-1 causes axon overgrowth and aberrant presynaptic morphology, yet the mutant animals exhibit little behavioral deficits. Strikingly, rpm-1 mutations strongly synergize with loss of function in the presynaptic active zone assembly factors, syd-1 and syd-2, resulting in severe locomotor deficits. Here, we provide ultrastructural evidence that double mutants, between rpm-1 and syd-1 or syd-2, dramatically impair synapse formation. Taking advantage of the synthetic locomotor defects to select for genetic suppressors, previous studies have identified the DLK-1 MAP kinase cascade negatively regulated by RPM-1. We now report a comprehensive analysis of a large number of suppressor mutations of this screen. Our results highlight the functional specificity of the DLK-1 cascade in synaptogenesis. We also identified two previously uncharacterized genes. One encodes a novel protein, SUPR-1, that acts cell autonomously to antagonize RPM-1. The other affects a conserved protein ESS-2, the homolog of human ES2 or DGCR14. Loss of function in ess-2 suppresses rpm-1 only in the presence of a dlk-1 splice acceptor mutation. We show that ESS-2 acts to promote accurate mRNA splicing when the splice site is compromised. The human DGCR14/ES2 resides in a deleted chromosomal region implicated in DiGeorge syndrome, and its mutation has shown high probability as a risk factor for schizophrenia. Our findings provide the first functional evidence that this family of proteins regulate mRNA splicing in a context-specific manner.

PI3K-GSK3 signalling regulates mammalian axon regeneration by inducing the expression of Smad1

In contrast to neurons in the central nervous system, mature neurons in the mammalian peripheral nervous system (PNS) can regenerate axons after injury, in part, by enhancing intrinsic growth competence. However, the signalling pathways that enhance the growth potential and induce spontaneous axon regeneration remain poorly understood. Here we reveal that phosphatidylinositol 3-kinase (PI3K) signalling is activated in response to peripheral axotomy and that PI3K pathway is required for sensory axon regeneration. Moreover, we show that glycogen synthase kinase 3 (GSK3), rather than mammalian target of rapamycin, mediates PI3K-dependent augmentation of the growth potential in the PNS. Furthermore, we show that PI3K-GSK3 signal is conveyed by the induction of a transcription factor Smad1 and that acute depletion of Smad1 in adult mice prevents axon regeneration in vivo. Together, these results suggest PI3K-GSK3-Smad1 signalling as a central module for promoting sensory axon regeneration in the mammalian nervous system.

Regulatory mechanisms underlying the differential growth of dendrites and axons

A typical neuron is comprised of an information input compartment, or the dendrites, and an output compartment, known as the axon. These two compartments are the structural basis for functional neural circuits. However, little is known about how dendritic and axonal growth are differentially regulated. Recent studies have uncovered two distinct types of regulatory mechanisms that differentiate dendritic and axonal growth: dedicated mechanisms and bimodal mechanisms. Dedicated mechanisms regulate either dendritespecific or axon-specific growth; in contrast, bimodal mechanisms direct dendritic and axonal development in opposite manners. Here, we review the dedicated and bimodal regulators identified by recent Drosophila and mammalian studies. The knowledge of these underlying molecular mechanisms not only expands our understanding about how neural circuits are wired, but also provides insights that will aid in the rational design of therapies for neurological diseases.

SkpA restrains synaptic terminal growth during development and promotes axonal degeneration following injury

The Wallenda (Wnd)/dual leucine zipper kinase (DLK)-Jnk pathway is an evolutionarily conserved MAPK signaling pathway that functions during neuronal development and following axonal injury. Improper pathway activation causes defects in axonal guidance and synaptic growth, whereas loss-of-function mutations in pathway components impairs axonal regeneration and degeneration after injury. Regulation of this pathway is in part through the E3 ubiquitin ligase Highwire (Hiw), which targets Wnd/DLK for degradation to limit MAPK signaling. To explore mechanisms controlling Wnd/DLK signaling, we performed a large-scale genetic screen in Drosophila to identify negative regulators of the pathway. Here we describe the identification and characterization of SkpA, a core component of SCF E3 ubiquitin ligases. Mutants in SkpA display synaptic overgrowth and an increase in Jnk signaling, similar to hiw mutants. The combination of hypomorphic alleles of SkpA and hiw leads to enhanced synaptic growth. Mutants in the Wnd-Jnk pathway suppress the overgrowth of SkpA mutants demonstrating that the synaptic overgrowth is due to increased Jnk signaling. These findings support the model that SkpA and the E3 ligase Hiw function as part of an SCF-like complex that attenuates Wnd/DLK signaling. In addition, SkpA, like Hiw, is required for synaptic and axonal responses to injury. Synapses in SkpA mutants are more stable following genetic or traumatic axonal injury, and axon loss is delayed in SkpA mutants after nerve crush. As in highwire mutants, this axonal protection requires Nmnat. Hence, SkpA is a novel negative regulator of the Wnd-Jnk pathway that functions with Hiw to regulate both synaptic development and axonal maintenance.

RPM-1 is localized to distinct subcellular compartments and regulates axon length in GABAergic motor neurons

Background

The PAM/Highwire/RPM-1 (PHR) proteins are conserved signaling proteins that regulate axon length and synapse formation during development. Loss of function in Caenorhabditis elegans rpm-1 results in axon termination and synapse formation defects in the mechanosensory neurons. An explanation for why these two phenotypes are observed in a single neuronal cell has remained absent. Further, it is uncertain whether the axon termination phenotypes observed in the mechanosensory neurons of rpm-1 mutants are unique to this specific type of neuron, or more widespread defects that occur with loss of function in rpm-1.

Results

Here, we show that RPM-1 is localized to both the mature axon tip and the presynaptic terminals of individual motor neurons and individual mechanosensory neurons. Genetic analysis indicated that GABAergic motor neurons, like the mechanosensory neurons, have both synapse formation and axon termination defects in rpm-1 mutants. RPM-1 functions in parallel with the active zone component SYD-2 (Liprin) to regulate not only synapse formation, but also axon termination in motor neurons. Our analysis of rpm-1−/−; syd-2−/− double mutants also revealed a role for RPM-1 in axon extension. The MAP3K DLK-1 partly mediated RPM-1 function in both axon termination and axon extension, and the relative role of DLK-1 was dictated by the anatomical location of the neuron in question.

Conclusions

Our findings show that axon termination defects are a core phenotype caused by loss of function in rpm-1, and not unique to the mechanosensory neurons. We show in motor neurons and in mechanosensory neurons that RPM-1 is localized to multiple, distinct subcellular compartments in a single cell. Thus, RPM-1 might be differentially regulated or RPM-1 might differentially control signals in distinct subcellular compartments to regulate multiple developmental outcomes in a single neuron. Our findings provide further support for the previously proposed model that PHR proteins function to coordinate axon outgrowth and termination with synapse formation.

RPM-1 uses both ubiquitin ligase and phosphatase-based mechanisms to regulate DLK-1 during neuronal development

The Pam/Highwire/RPM-1 (PHR) proteins are key regulators of neuronal development that function in axon extension and guidance, termination of axon outgrowth, and synapse formation. Outside of development, the PHR proteins also regulate axon regeneration and Wallerian degeneration. The PHR proteins function in part by acting as ubiquitin ligases that degrade the Dual Leucine zipper-bearing Kinase (DLK). Here, we show that the Caenorhabditis elegans PHR protein, Regulator of Presynaptic Morphology 1 (RPM-1), also utilizes a phosphatase-based mechanism to regulate DLK-1. Using mass spectrometry, we identified Protein Phosphatase Magnesium/Manganese dependent 2 (PPM-2) as a novel RPM-1 binding protein. Genetic, transgenic, and biochemical studies indicated that PPM-2 functions coordinately with the ubiquitin ligase activity of RPM-1 and the F-box protein FSN-1 to negatively regulate DLK-1. PPM-2 acts on S874 of DLK-1, a residue implicated in regulation of DLK-1 binding to a short, inhibitory isoform of DLK-1 (DLK-1S). Our study demonstrates that PHR proteins function through both phosphatase and ubiquitin ligase mechanisms to inhibit DLK. Thus, PHR proteins are potentially more accurate and sensitive regulators of DLK than originally thought. Our results also highlight an important and expanding role for the PP2C phosphatase family in neuronal development.

The molecular mechanisms that govern formation of functional synaptic connections are central to brain development and function. We have used the nematode C. elegans to explore the mechanism of how the intracellular signaling protein RPM-1 regulates neuronal development. Using a combination of proteomic, genetic, transgenic, and biochemical approaches we have shown that RPM-1 functions through a PP2C phosphatase, PPM-2, to regulate the activity of a MAP3 kinase, DLK-1. Our results indicate that a combination of PPM-2 phosphatase activity and RPM-1 ubiquitin ligase activity inhibit DLK-1.

Axon regeneration in C. elegans

Single axon transection by laser surgery has made Caenorhabditis elegans a new model for axon regeneration. Multiple conserved molecular signaling modules have been discovered through powerful genetic screening. In vivo imaging with single cell and axon resolution has revealed unprecedented cellular dynamics in regenerating axons. Information from C. elegans has greatly expanded our knowledge of the molecular and cellular mechanisms of axon regeneration.

B-RAF kinase drives developmental axon growth and promotes axon regeneration in the injured mature CNS

Intraneuronal activation of B-RAF kinase is sufficient to drive the growth of peripheral axon projections and enables robust regenerative axon growth in the injured optic nerve.

Activation of intrinsic growth programs that promote developmental axon growth may also facilitate axon regeneration in injured adult neurons. Here, we demonstrate that conditional activation of B-RAF kinase alone in mouse embryonic neurons is sufficient to drive the growth of long-range peripheral sensory axon projections in vivo in the absence of upstream neurotrophin signaling. We further show that activated B-RAF signaling enables robust regenerative growth of sensory axons into the spinal cord after a dorsal root crush as well as substantial axon regrowth in the crush-lesioned optic nerve. Finally, the combination of B-RAF gain-of-function and PTEN loss-of-function promotes optic nerve axon extension beyond what would be predicted for a simple additive effect. We conclude that cell-intrinsic RAF signaling is a crucial pathway promoting developmental and regenerative axon growth in the peripheral and central nervous systems.

Signaling regulations of neuronal regenerative ability

Different from physiological axon growth during development, a major limiting factor for successful axon regeneration is the poor intrinsic regenerative capacity in mature neurons in the adult mammalian central nervous system (CNS). Recent studies identified several molecular pathways, including PTEN/mTOR, Jak/STAT, DLK/JNK, providing important probes in investigating the mechanisms by which the regenerative ability is regulated. This review will summarize these recent findings and speculate their implications.

S6 kinase inhibits intrinsic axon regeneration capacity via AMP kinase in Caenorhabditis elegans

The ability of axons to regrow after injury is determined by the complex interplay of intrinsic growth programs and external cues. In Caenorhabditis elegans mechanosensory neuron, axons exhibit robust regenerative regrowth following laser axotomy. By surveying conserved metabolic signaling pathways, we have identified the ribosomal S6 kinase RSKS-1 as a new cell-autonomous inhibitor of axon regeneration. RSKS-1 is not required for axonal development but inhibits axon regrowth after injury in multiple neuron types. Loss of function in rsks-1 results in more rapid growth cone formation after injury and accelerates subsequent axon extension. The enhanced regrowth of rsks-1 mutants is partly dependent on the DLK-1 MAPK cascade. An essential output of RSKS-1 in axon regrowth is the metabolic sensor AMP kinase, AAK-2. We further show that the antidiabetic drug phenformin, which activates AMP kinase, can promote axon regrowth. Our data reveal a new function for an S6 kinase acting through an AMP kinase in regenerative growth of injured axons.

Independent control of aging and axon regeneration

Axon regeneration capacity often declines with age. One might assume that loss of regeneration is an obvious consequence of organismal aging. However, in the latest issue of Neuron, Byrne et al. (2014) demonstrate that regeneration ability and aging are regulated cell-autonomously within neurons, and can be decoupled.

Axon regeneration genes identified by RNAi screening in C. elegans

Axons of the mammalian CNS lose the ability to regenerate soon after development due to both an inhibitory CNS environment and the loss of cell-intrinsic factors necessary for regeneration. The complex molecular events required for robust regeneration of mature neurons are not fully understood, particularly in vivo. To identify genes affecting axon regeneration in Caenorhabditis elegans, we performed both an RNAi-based screen for defective motor axon regeneration in unc-70/β-spectrin mutants and a candidate gene screen. From these screens, we identified at least 50 conserved genes with growth-promoting or growth-inhibiting functions. Through our analysis of mutants, we shed new light on certain aspects of regeneration, including the role of β-spectrin and membrane dynamics, the antagonistic activity of MAP kinase signaling pathways, and the role of stress in promoting axon regeneration. Many gene candidates had not previously been associated with axon regeneration and implicate new pathways of interest for therapeutic intervention.

PTRN-1, a microtubule minus end-binding CAMSAP homolog, promotes microtubule function in Caenorhabditis elegans neurons

In neuronal processes, microtubules (MTs) provide structural support and serve as tracks for molecular motors. While it is known that neuronal MTs are more stable than MTs in non-neuronal cells, the molecular mechanisms underlying this stability are not fully understood. In this study, we used live fluorescence microscopy to show that the C. elegans CAMSAP protein PTRN-1 localizes to puncta along neuronal processes, stabilizes MT foci, and promotes MT polymerization in neurites. Electron microscopy revealed that ptrn-1 null mutants have fewer MTs and abnormal MT organization in the PLM neuron. Animals grown with a MT depolymerizing drug caused synthetic defects in neurite branching in the absence of ptrn-1 function, indicating that PTRN-1 promotes MT stability. Further, ptrn-1 null mutants exhibited aberrant neurite morphology and synaptic vesicle localization that is partially dependent on dlk-1. Our results suggest that PTRN-1 represents an important mechanism for promoting MT stability in neurons.

DOI:http://dx.doi.org/10.7554/eLife.01498.001

Microtubules are tiny tubular structures made from many copies of proteins called tubulins. Microtubules have a number of important roles inside cells: they are part of the cytoskeleton that provides structural support for the cell; they help to pull chromosomes apart during cell division; and they guide the trafficking of proteins and molecules around inside the cell. Most microtubules are relatively unstable, undergoing continuous dis-assembly and re-assembly in response to the needs of the cell. The microtubules in the branches of nerve cells are an exception, remaining relatively stable over time. Now Richardson et al. and, independently, Marcette et al., have shown that a protein called PTRN-1 has an important role in stabilizing the microtubules in the nerve cells of nematode worms.

By tagging the PTRN-1 proteins with fluorescent molecules, Richardson et al. were able to show that these proteins were present along the length of the microtubules within the nerve cells. Further work showed that the PTRN-1 proteins stabilize the microtubule filaments within the branches of these nerve cells and also hold them in position.

Richardson et al. also found that worms that had been genetically modified to prevent them from producing PTRN-1 failed to traffic certain molecules to the synapses between nerve cells. Moreover, these mutants also had problems with the branching of their nerve cells; however, these defects were relatively mild, which suggests that other molecules and proteins act in parallel with PTRN-1 to stabilize microtubules in nerve cells. Further work should be able to identify these factors and elucidate how they work together to stabilize the microtubules in nerve cells.

DOI:http://dx.doi.org/10.7554/eLife.01498.002