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

Ginsenoside Rg1 Regulates Immune Microenvironment and Neurological Recovery After Spinal Cord Injury Through MYCBP2 Delivery via Neuronal Cell-Derived Extracellular Vesicles

Spinal cord injury (SCI) is a severe neurological condition that frequently leads to significant sensory, motor, and autonomic dysfunction. This study sought to delineate the potential mechanistic underpinnings of extracellular vesicles (EVs) derived from ginsenoside Rg1-pretreated neuronal cells (Rg1-EVs) in ameliorating SCI. These results demonstrated that treatment with Rg1-EVs substantially improved motor function in spinal cord-injured mice. Rg1-EVs enhance microglial polarization toward the M2 phenotype and repressed oxidative stress, thereby altering immune responses and decreasing inflammatory cytokine secretion. Moreover, Rg1-EVs substantially diminish reactive oxygen species accumulation and enhanced neural tissue repair by regulating mitochondrial function. Proteomic profiling highlighted a significant enrichment of MYCBP2 in Rg1-EVs, and functional assays confirmed that MYCBP2 knockdown counteracted the beneficial effects of Rg1-EVs in vitro and in vivo. Mechanistically, MYCBP2 is implicated in the ubiquitination and degradation of S100A9, thereby promoting microglial M2-phenotype polarization and reducing oxidative stress. Overall, these findings substantiated the pivotal role of Rg1-EVs in neuronal protection and functional recovery following SCI through MYCBP2-mediated ubiquitination of S100A9. This research offers novel mechanistic insights into therapeutic strategies against SCI and supports the clinical potential of Rg1-EVs.

Regulation of the Activity of the Dual Leucine Zipper Kinase by Distinct Mechanisms

The dual leucine zipper kinase (DLK) alias mitogen-activated protein 3 kinase 12 (MAP3K12) has gained much attention in recent years. DLK belongs to the mixed lineage kinases, characterized by homology to serine/threonine and tyrosine kinase, but exerts serine/threonine kinase activity. DLK has been implicated in many diseases, including several neurodegenerative diseases, glaucoma, and diabetes mellitus. As a MAP3K, it is generally assumed that DLK becomes phosphorylated and activated by upstream signals and phosphorylates and activates itself, the downstream serine/threonine MAP2K, and, ultimately, MAPK. In addition, other mechanisms such as protein-protein interactions, proteasomal degradation, dephosphorylation by various phosphatases, palmitoylation, and subcellular localization have been shown to be involved in the regulation of DLK activity or its fine-tuning. In the present review, the diverse mechanisms regulating DLK activity will be summarized to provide better insights into DLK action and, possibly, new targets to modulate DLK function.

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in Caenorhabditis elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signalling effector that controls diverse cellular functions.

Axon development is regulated at genetic and proteomic interfaces between the integrin adhesome and the RPM-1 ubiquitin ligase signaling hub

Integrin signaling plays important roles in development and disease. An adhesion signaling network called the integrin adhesome has been principally defined using bioinformatics and proteomics. To date, the adhesome has not been studied using integrated proteomic and genetic approaches. Here, proteomic studies in C. elegans identified physical associations between the RPM-1 ubiquitin ligase signaling hub and numerous adhesome components including Talin, Kindlin and beta-integrin. C. elegans RPM-1 is orthologous to human MYCBP2, a prominent player in nervous system development associated with a neurodevelopmental disorder. Using neuron-specific, CRISPR loss-of-function strategies, we show that core adhesome components affect axon development and interact genetically with RPM-1. Mechanistically, Talin opposes RPM-1 in a functional 'tug-of-war' on growth cones that is required for accurate axon termination. Thus, our findings orthogonally validate the adhesome via multi-component genetic and physical interfaces with a key neuronal signaling hub and identify new links between the adhesome and brain disorders.

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in C. elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signaling effector that controls diverse cellular functions.

Dysfunction of ubiquitin protein ligase MYCBP2 leads to cell resilience in human breast cancers

Breast cancer is the most common type of cancer among women worldwide, and it is estimated that 294 000 new diagnoses and 37 000 deaths will occur each year in the United States alone by 2030. Large-scale genomic studies have identified a number of genetic loci with alterations in breast cancer. However, identification of the genes that are critical for tumorgenicity still remains a challenge. Here, we perform a comprehensive functional multi-omics analysis of somatic mutations in breast cancer and identify previously unknown key regulators of breast cancer tumorgenicity. We identify dysregulation of MYCBP2, an E3 ubiquitin ligase and an upstream regulator of mTOR signaling, is accompanied with decreased disease-free survival. We validate MYCBP2 as a key target through depletion siRNA using in vitro apoptosis assays in MCF10A, MCF7 and T47D cells. We demonstrate that MYCBP2 loss is associated with resistance to apoptosis from cisplatin-induced DNA damage and cell cycle changes, and that CHEK1 inhibition can modulate MYCBP2 activity and caspase cleavage. Furthermore, we show that MYCBP2 knockdown is associated with transcriptomic responses in TSC2 and in apoptosis genes and interleukins. Therefore, we show that MYCBP2 is an important genetic target that represents a key node regulating multiple molecular pathways in breast cancer corresponding with apparent drug resistance in our study.

Optimized protocol for in vivo affinity purification proteomics and biochemistry using C. elegans

We present an optimized protocol for in vivo affinity purification proteomics and biochemistry using the model organism C. elegans. We describe steps for target tagging, large-scale culture, affinity purification using a cryomill, mass spectrometry and validation of candidate binding proteins. Our approach has proven successful for identifying protein-protein interactions and signaling networks with verified functional relevance. Our protocol is also suitable for biochemical evaluation of protein-protein interactions in vivo. For complete details on the use and execution of this protocol, please refer to Crawley et al.,1 Giles et al.,2 and Desbois et al.3.

Loss-of-function variants in MYCBP2 cause neurobehavioural phenotypes and corpus callosum defects

The corpus callosum is a bundle of axon fibres that connects the two hemispheres of the brain. Neurodevelopmental disorders that feature dysgenesis of the corpus callosum as a core phenotype offer a valuable window into pathology derived from abnormal axon development. Here, we describe a cohort of eight patients with a neurodevelopmental disorder characterized by a range of deficits including corpus callosum abnormalities, developmental delay, intellectual disability, epilepsy and autistic features. Each patient harboured a distinct de novo variant in MYCBP2, a gene encoding an atypical really interesting new gene (RING) ubiquitin ligase and signalling hub with evolutionarily conserved functions in axon development. We used CRISPR/Cas9 gene editing to introduce disease-associated variants into conserved residues in the Caenorhabditis elegans MYCBP2 orthologue, RPM-1, and evaluated functional outcomes in vivo. Consistent with variable phenotypes in patients with MYCBP2 variants, C. elegans carrying the corresponding human mutations in rpm-1 displayed axonal and behavioural abnormalities including altered habituation. Furthermore, abnormal axonal accumulation of the autophagy marker LGG-1/LC3 occurred in variants that affect RPM-1 ubiquitin ligase activity. Functional genetic outcomes from anatomical, cell biological and behavioural readouts indicate that MYCBP2 variants are likely to result in loss of function. Collectively, our results from multiple human patients and CRISPR gene editing with an in vivo animal model support a direct link between MYCBP2 and a human neurodevelopmental spectrum disorder that we term, MYCBP2-related developmental delay with corpus callosum defects (MDCD).

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

Autophagy protein ULK1 interacts with and regulates SARM1 during axonal injury

Autophagy is a cellular catabolic pathway generally thought to be neuroprotective. However, autophagy and in particular its upstream regulator, the ULK1 kinase, can also promote axonal degeneration. We examined the role and the mechanisms of autophagy in axonal degeneration using a mouse model of contusive spinal cord injury (SCI). Consistent with activation of autophagy during axonal degeneration following SCI, autophagosome marker LC3, ULK1 kinase, and ULK1 target, phospho-ATG13, accumulated in the axonal bulbs and injured axons. SARM1, a TIR NADase with a pivotal role in axonal degeneration, colocalized with ULK1 within 1 h after SCI, suggesting possible interaction between autophagy and SARM1-mediated axonal degeneration. In our in vitro experiments, inhibition of autophagy, including Ulk1 knockdown and ULK1 inhibitor, attenuated neurite fragmentation and reduced accumulation of SARM1 puncta in neurites of primary cortical neurons subjected to glutamate excitotoxicity. Immunoprecipitation data demonstrated that ULK1 physically interacted with SARM1 in vitro and in vivo and that SAM domains of SARM1 were necessary for ULK1-SARM1 complex formation. Consistent with a role in regulation of axonal degeneration, in primary cortical neurons ULK1-SARM1 interaction increased upon neurite damage. Supporting a role for autophagy and ULK1 in regulation of SARM1 in axonal degeneration in vivo, axonal ULK1 activation and accumulation of SARM1 were both decreased after SCI in Becn1^+/- autophagy hypomorph mice compared to wild-type (WT) controls. These findings suggest a regulatory crosstalk between autophagy and axonal degeneration pathways, which is mediated through ULK1-SARM1 interaction and contributes to the ability of SARM1 to accumulate in injured axons.

An atypical ubiquitin ligase at the heart of neural development and programmed axon degeneration

The degeneration of nerve fibres following injury was first described by Augustus Waller over 170 years ago. Initially assumed to be a passive process, it is now evident that axons respond to insult via regulated cellular signaling events resulting in their programmed degeneration. Pro-survival and pro-degenerative factors have been identified and their regulatory mechanisms are beginning to emerge. The ubiquitin system has been implicated in the pro-degenerative process and a key component is the ubiquitin E3 ligase MYCBP2 (also known as PHR1). Ubiquitin E3 ligases are tasked with the transfer of the small protein modifier ubiquitin to substrates and consist of hundreds of members. They can be classified as single subunit systems or as multi-subunit complexes. Their catalytic domains can also be assigned to three general architectures. Hints that MYCBP2 might not conform to these established formats came to light and it is now clear from biochemical and structural studies that MYCBP2 is indeed an outlier in terms of its modus operandi. Furthermore, the unconventional way in which MYCBP2 transfers ubiquitin to substrates has been linked to neurodevelopmental and pro-degenerative function. Herein, we will summarize these research developments relating to the unusual features of MYCBP2 and postulate therapeutic strategies that prevent Wallerian degeneration. These have exciting potential for providing relief from pathological neuropathies and neurodegenerative diseases.

Ubiquitin ligase activity inhibits Cdk5 to control axon termination

The Cdk5 kinase plays prominent roles in nervous system development, plasticity, behavior and disease. It also has important, non-neuronal functions in cancer, the immune system and insulin secretion. At present, we do not fully understand negative regulatory mechanisms that restrict Cdk5. Here, we use Caenorhabditis elegans to show that CDK-5 is inhibited by the RPM-1/FSN-1 ubiquitin ligase complex. This atypical RING ubiquitin ligase is conserved from C. elegans through mammals. Our finding originated from unbiased, in vivo affinity purification proteomics, which identified CDK-5 as a putative RPM-1 substrate. CRISPR-based, native biochemistry showed that CDK-5 interacts with the RPM-1/FSN-1 ubiquitin ligase complex. A CRISPR engineered RPM-1 substrate ‘trap’ enriched CDK-5 binding, which was mediated by the FSN-1 substrate recognition module. To test the functional genetic relationship between the RPM-1/FSN-1 ubiquitin ligase complex and CDK-5, we evaluated axon termination in mechanosensory neurons and motor neurons. Our results indicate that RPM-1/FSN-1 ubiquitin ligase activity restricts CDK-5 to control axon termination. Collectively, these proteomic, biochemical and genetic results increase our understanding of mechanisms that restrain Cdk5 in the nervous system.

Cdk5 is an atypical cyclin dependent kinase and an important player in nervous system development, plasticity, and disease. Decades of research has focused on understanding how Cdk5 is activated. In contrast, we know much less about the genetic and molecular mechanisms that restrict Cdk5 activity. Here, we examined how Cdk5 is inhibited in the nervous system using the model organism C. elegans. Our results indicate that the RPM-1/FSN-1 E3 ubiquitin ligase complex inhibits Cdk5 to control termination of axon growth. Our finding that ubiquitin ligase activity restricts Cdk5 in the nervous system in vivo now opens up the interesting possibility that ubiquitin ligase activity might regulate Cdk5 in other cellular contexts and disease settings.

Organization of Presynaptic Autophagy-Related Processes

Brain synapses pose special challenges on the quality control of their protein machineries as they are far away from the neuronal soma, display a high potential for plastic adaptation and have a high energy demand to fulfill their physiological tasks. This applies in particular to the presynaptic part where neurotransmitter is released from synaptic vesicles, which in turn have to be recycled and refilled in a complex membrane trafficking cycle. Pathways to remove outdated and damaged proteins include the ubiquitin-proteasome system acting in the cytoplasm as well as membrane-associated endolysosomal and the autophagy systems. Here we focus on the latter systems and review what is known about the spatial organization of autophagy and endolysomal processes within the presynapse. We provide an inventory of which components of these degradative systems were found to be present in presynaptic boutons and where they might be anchored to the presynaptic apparatus. We identify three presynaptic structures reported to interact with known constituents of membrane-based protein-degradation pathways and therefore may serve as docking stations. These are (i) scaffolding proteins of the cytomatrix at the active zone, such as Bassoon or Clarinet, (ii) the endocytic machinery localized mainly at the peri-active zone, and (iii) synaptic vesicles. Finally, we sketch scenarios, how presynaptic autophagic cargos are tagged and recruited and which cellular mechanisms may govern membrane-associated protein turnover in the presynapse.

Historical perspective and progress on protein ubiquitination at glutamatergic synapses

Transcription-translation coupling leads to the production of proteins that are key for controlling essential neuronal processes that include neuronal development and changes in synaptic strength. Although these events have been a prevailing theme in neuroscience, the regulation of proteins via posttranslational signaling pathways are equally relevant for these neuronal processes. Ubiquitin is one type of posttranslational modification that covalently attaches to its targets/substrates. Ubiquitination of proteins play a key role in multiple signaling pathways, the predominant being removal of its substrates by a large molecular machine called the proteasome. Here, I review 40 years of progress on ubiquitination in the nervous system at glutamatergic synapses focusing on axon pathfinding, synapse formation, presynaptic release, dendritic spine formation, and regulation of postsynaptic glutamate receptors. Finally, I elucidate emerging themes in ubiquitin biology that may challenge our current understanding of ubiquitin signaling in the nervous system.

Deubiquitinating enzyme amino acid profiling reveals a class of ubiquitin esterases

Ubiquitination involves the covalent attachment of the protein ubiquitin to substrates. It can be reversed by the action of deubiquitinating enzymes (DUBs), thereby providing an important layer of regulation. Originally believed to be restricted to lysine residues, it is emerging that additional amino acids, including serine, threonine and cysteine, are also modified. It remains unknown which DUBs might target these unusual sites for deubiquitination. Herein, we develop representative model substrates and screen 53 DUBs for non-lysine activity, thereby providing important insights into DUB function. Strikingly, we find that a poorly studied DUB class has potent and highly selective serine/threonine activity. These findings suggest that non-lysine ubiquitination rivals the regulatory sophistication of its conventional counterpart and might serve distinct cellular functions.

The reversibility of ubiquitination by the action of deubiquitinating enzymes (DUBs) serves as an important regulatory layer within the ubiquitin system. Approximately 100 DUBs are encoded by the human genome, and many have been implicated with pathologies, including neurodegeneration and cancer. Non-lysine ubiquitination is chemically distinct, and its physiological importance is emerging. Here, we couple chemically and chemoenzymatically synthesized ubiquitinated lysine and threonine model substrates to a mass spectrometry-based DUB assay. Using this platform, we profile two-thirds of known catalytically active DUBs for threonine esterase and lysine isopeptidase activity and find that most DUBs demonstrate dual selectivity. However, with two anomalous exceptions, the ovarian tumor domain DUB class demonstrates specific (iso)peptidase activity. Strikingly, we find the Machado–Joseph disease (MJD) class to be unappreciated non-lysine DUBs with highly specific ubiquitin esterase activity rivaling the efficiency of the most active isopeptidases. Esterase activity is dependent on the canonical catalytic triad, but proximal hydrophobic residues appear to be general determinants of non-lysine activity. Our findings also suggest that ubiquitin esters have appreciable cellular stability and that non-lysine ubiquitination is an integral component of the ubiquitin system. Its regulatory sophistication is likely to rival that of canonical ubiquitination.

Multiple signaling pathways are essential for synapse formation induced by synaptic adhesion molecules

Little is known about the cellular signals that organize synapse formation. To explore what signaling pathways may be involved, we employed heterologous synapse formation assays in which a synaptic adhesion molecule expressed in a nonneuronal cell induces pre- or postsynaptic specializations in cocultured neurons. We found that interfering pharmacologically with microtubules or actin filaments impaired heterologous synapse formation, whereas blocking protein synthesis had no effect. Unexpectedly, pharmacological inhibition of c-jun N-terminal kinases (JNKs), protein kinase-A (PKA), or AKT kinases also suppressed heterologous synapse formation, while inhibition of other tested signaling pathways-such as MAP kinases or protein kinase C-did not alter heterologous synapse formation. JNK and PKA inhibitors suppressed formation of both pre- and postsynaptic specializations, whereas AKT inhibitors impaired formation of post- but not presynaptic specializations. To independently test whether heterologous synapse formation depends on AKT signaling, we targeted PTEN, an enzyme that hydrolyzes phosphatidylinositol 3-phosphate and thereby prevents AKT kinase activation, to postsynaptic sites by fusing PTEN to Homer1. Targeting PTEN to postsynaptic specializations impaired heterologous postsynaptic synapse formation induced by presynaptic adhesion molecules, such as neurexins and additionally decreased excitatory synapse function in cultured neurons. Taken together, our results suggest that heterologous synapse formation is driven via a multifaceted and multistage kinase network, with diverse signals organizing pre- and postsynaptic specializations.

Imbalanced post- and extrasynaptic SHANK2A functions during development affect social behavior in SHANK2-mediated neuropsychiatric disorders

Mutations in SHANK genes play an undisputed role in neuropsychiatric disorders. Until now, research has focused on the postsynaptic function of SHANKs, and prominent postsynaptic alterations in glutamatergic signal transmission have been reported in Shank KO mouse models. Recent studies have also suggested a possible presynaptic function of SHANK proteins, but these remain poorly defined. In this study, we examined how SHANK2 can mediate electrophysiological, molecular, and behavioral effects by conditionally overexpressing either wild-type SHANK2A or the extrasynaptic SHANK2A(R462X) variant. SHANK2A overexpression affected pre- and postsynaptic targets and revealed a reversible, development-dependent autism spectrum disorder-like behavior. SHANK2A also mediated redistribution of Ca²⁺-permeable AMPA receptors between apical and basal hippocampal CA1 dendrites, leading to impaired synaptic plasticity in the basal dendrites. Moreover, SHANK2A overexpression reduced social interaction and increased the excitatory noise in the olfactory cortex during odor processing. In contrast, overexpression of the extrasynaptic SHANK2A(R462X) variant did not impair hippocampal synaptic plasticity, but still altered the expression of presynaptic/axonal signaling proteins. We also observed an attention-deficit/hyperactivity-like behavior and improved social interaction along with enhanced signal-to-noise ratio in cortical odor processing. Our results suggest that the disruption of pre- and postsynaptic SHANK2 functions caused by SHANK2 mutations has a strong impact on social behavior. These findings indicate that pre- and postsynaptic SHANK2 actions cooperate for normal neuronal function, and that an imbalance between these functions may lead to different neuropsychiatric disorders.

Activation of MAP3K DLK and LZK in Purkinje cells causes rapid and slow degeneration depending on signaling strength

The conserved MAP3K Dual-Leucine-Zipper Kinase (DLK) and Leucine-Zipper-bearing Kinase (LZK) can activate JNK via MKK4 or MKK7. These two MAP3Ks share similar biochemical activities and undergo auto-activation upon increased expression. Depending on cell-type and nature of insults DLK and LZK can induce pro-regenerative, pro-apoptotic or pro-degenerative responses, although the mechanistic basis of their action is not well understood. Here, we investigated these two MAP3Ks in cerebellar Purkinje cells using loss- and gain-of function mouse models. While loss of each or both kinases does not cause discernible defects in Purkinje cells, activating DLK causes rapid death and activating LZK leads to slow degeneration. Each kinase induces JNK activation and caspase-mediated apoptosis independent of each other. Significantly, deleting CELF2, which regulates alternative splicing of Map2k7, strongly attenuates Purkinje cell degeneration induced by LZK, but not DLK. Thus, controlling the activity levels of DLK and LZK is critical for neuronal survival and health.

Multitasking: Dual Leucine Zipper-Bearing Kinases in Neuronal Development and Stress Management

The dual leucine zipper-bearing kinase (DLK) and leucine zipper-bearing kinase (LZK) are evolutionarily conserved MAPKKKs of the mixed-lineage kinase family. Acting upstream of stress-responsive JNK and p38 MAP kinases, DLK and LZK have emerged as central players in neuronal responses to a variety of acute and traumatic injuries. Recent studies also implicate their function in astrocytes, microglia, and other nonneuronal cells, reflecting their expanding roles in the multicellular response to injury and in disease. Of particular note is the potential link of these kinases to neurodegenerative diseases and cancer. It is thus critical to understand the physiological contexts under which these kinases are activated, as well as the signal transduction mechanisms that mediate specific functional outcomes. In this review we first provide a historical overview of the biochemical and functional dissection of these kinases. We then discuss recent findings on regulating their activity to enhance cellular protection following injury and in disease, focusing on but not limited to the nervous system.

Loss of highwire Protects Against the Deleterious Effects of Traumatic Brain Injury in Drosophila Melanogaster

Traumatic brain injury is a major global cause of death and disability. Axonal injury is a major underlying mechanism of TBI and could represent a major therapeutic target. We provide evidence that targeting the axonal death pathway known as Wallerian degeneration improves outcome in a Drosophila Melanogaster model of high impact trauma. This cell-autonomous neurodegenerative pathway is initiated following axon injury, and in Drosophila, involves activity of the E3 ubiquitin ligase highwire. We demonstrate that a loss-of-function mutation in the highwire gene rescues deleterious effects of a traumatic injury, including-improved functional outcomes, lifespan, survival of dopaminergic neurons, and retention of synaptic proteins. This data suggests that highwire represents a potential therapeutic target in traumatic injury.

Roles of the HUWE1 ubiquitin ligase in nervous system development, function and disease

Huwe1 is a highly conserved member of the HECT E3 ubiquitin ligase family. Here, we explore the growing importance of Huwe1 in nervous system development, function and disease. We discuss extensive progress made in deciphering how Huwe1 regulates neural progenitor proliferation and differentiation, cell migration, and axon development. We highlight recent evidence indicating that Huwe1 regulates inhibitory neurotransmission. In covering these topics, we focus on findings made using both vertebrate and invertebrate in vivo model systems. Finally, we discuss extensive human genetic studies that strongly implicate HUWE1 in intellectual disability, and heighten the importance of continuing to unravel how Huwe1 affects the nervous system.

Autophagy is inhibited by ubiquitin ligase activity in the nervous system

Autophagy is an intracellular catabolic process prominent in starvation, aging and disease. Neuronal autophagy is particularly important, as it affects the development and function of the nervous system, and is heavily implicated in neurodegenerative disease. Nonetheless, how autophagy is regulated in neurons remains poorly understood. Using an unbiased proteomics approach, we demonstrate that the primary initiator of autophagy, the UNC-51/ULK kinase, is negatively regulated by the ubiquitin ligase RPM-1. RPM-1 ubiquitin ligase activity restricts UNC-51 and autophagosome formation within specific axonal compartments, and exerts effects broadly across the nervous system. By restraining UNC-51 activity, RPM-1 inhibits autophagosome formation to affect axon termination, synapse maintenance and behavioral habituation. These results demonstrate how UNC-51 and autophagy are regulated subcellularly in axons, and unveils a mechanism for restricting initiation of autophagy across the nervous system. Our findings have important implications beyond nervous system development, given growing links between altered autophagy regulation and neurodegenerative diseases.

FBXO45-MYCBP2 regulates mitotic cell fate by targeting FBXW7 for degradation

Cell fate decision upon prolonged mitotic arrest induced by microtubule-targeting agents depends on the activity of the tumor suppressor and F-box protein FBXW7. FBXW7 promotes mitotic cell death and prevents premature escape from mitosis through mitotic slippage. Mitotic slippage is a process that can cause chemoresistance and tumor relapse. Therefore, understanding the mechanisms that regulate the balance between mitotic cell death and mitotic slippage is an important task. Here we report that FBXW7 protein levels markedly decline during extended mitotic arrest. FBXO45 binds to a conserved acidic N-terminal motif of FBXW7 specifically under a prolonged delay in mitosis, leading to ubiquitylation and subsequent proteasomal degradation of FBXW7 by the FBXO45-MYCBP2 E3 ubiquitin ligase. Moreover, we find that FBXO45-MYCBP2 counteracts FBXW7 in that it promotes mitotic slippage and prevents cell death in mitosis. Targeting this interaction represents a promising strategy to prevent chemotherapy resistance.

The ubiquitin ligase PHR promotes directional regrowth of spinal zebrafish axons

To reconnect with their synaptic targets, severed axons need to regrow robustly and directionally along the pre-lesional trajectory. While mechanisms directing axonal regrowth are poorly understood, several proteins direct developmental axon outgrowth, including the ubiquitin ligase PHR (Mycbp2). Invertebrate PHR also limits regrowth of injured axons, whereas its role in vertebrate axonal regrowth remains elusive. Here we took advantage of the high regrowth capacity of spinal zebrafish axons and observed robust and directional regrowth following laser transection of spinal Mauthner axons. We found that PHR directs regrowing axons along the pre-lesional trajectory and across the transection site. At the transection site, initial regrowth of wild-type axons was multidirectional. Over time, misdirected sprouts were corrected in a PHR-dependent manner. Ablation of cyfip2, known to promote F-actin-polymerization and pharmacological inhibition of JNK reduced misdirected regrowth of PHR-deficient axons, suggesting that PHR controls directional Mauthner axonal regrowth through cyfip2- and JNK-dependent pathways.

The E3 ligase Highwire promotes synaptic transmission by targeting the NAD-synthesizing enzyme dNmnat

The ubiquitin ligase Highwire restrains synaptic growth and promotes evoked neurotransmission at NMJ synapses in Drosophila Highwire regulates synaptic morphology by downregulating the MAP3K Wallenda, but excess Wallenda signaling does not account for the decreased presynaptic release observed in highwire mutants. Hence, Highwire likely has a second substrate that inhibits neurotransmission. Highwire targets the NAD+ biosynthetic and axoprotective enzyme dNmnat to regulate axonal injury responses. dNmnat localizes to synapses and interacts with the active zone protein Bruchpilot, leading us to hypothesize that Highwire promotes evoked release by downregulating dNmnat. Here, we show that excess dNmnat is necessary in highwire mutants and sufficient in wild-type larvae to reduce quantal content, likely via disruption of active zone ultrastructure. Catalytically active dNmnat is required to drive defects in evoked release, and depletion of a second NAD+ synthesizing enzyme is sufficient to suppress these defects in highwire mutants, suggesting that excess NAD+ biosynthesis is the mechanism inhibiting neurotransmission. Thus, Highwire downregulates dNmnat to promote evoked synaptic release, suggesting that Highwire balances the axoprotective and synapse-inhibitory functions of dNmnat.

Machine learning in schizophrenia genomics, a case-control study using 5,090 exomes

Our hypothesis is that machine learning (ML) analysis of whole exome sequencing (WES) data can be used to identify individuals at high risk for schizophrenia (SCZ). This study applies ML to WES data from 2,545 individuals with SCZ and 2,545 unaffected individuals, accessed via the database of genotypes and phenotypes (dbGaP). Single nucleotide variants and small insertions and deletions were annotated by ANNOVAR using the reference genome hg19/GRCh37. Rare (predicted functional) variants with a minor allele frequency ≤1% and genotype quality ≥90 including missense, frameshift, stop gain, stop loss, intronic, and exonic splicing variants were selected. A file containing all cases and controls, the names of genes with variants meeting our criteria, and the number of variants per gene for each individual, was used for ML analysis. The supervised machine-learning algorithm used the patterns of variants observed in the different genes to determine which subset of genes can best predict that an individual is affected. Seventy percent of the data was used to train the algorithm and the remaining 30% of data (n = 1,526) was used to evaluate its efficiency. The supervised ML algorithm, gradient boosted trees with regularization (eXtreme Gradient Boosting implementation) was the best performing algorithm yielding promising results (accuracy: 85.7%, specificity: 86.6%, sensitivity: 84.9%, area under the receiver-operator characteristic curve: 0.95). The top 50 features (genes) of the algorithm were analyzed using bioinformatics resources for new insights about the pathophysiology of SCZ. This manuscript presents a novel predictor which could potentially enable studies exploring disease-modifying intervention in the early stages of the disease.

Synapse maintenance is impacted by ATAT-2 tubulin acetyltransferase activity and the RPM-1 signaling hub

Synapse formation is comprised of target cell recognition, synapse assembly, and synapse maintenance. Maintaining established synaptic connections is essential for generating functional circuitry and synapse instability is a hallmark of neurodegenerative disease. While many molecules impact synapse formation generally, we know little about molecules that affect synapse maintenance in vivo. Using genetics and developmental time course analysis in C.elegans, we show that the α-tubulin acetyltransferase ATAT-2 and the signaling hub RPM-1 are required presynaptically to maintain stable synapses. Importantly, the enzymatic acetyltransferase activity of ATAT-2 is required for synapse maintenance. Our analysis revealed that RPM-1 is a hub in a genetic network composed of ATAT-2, PTRN-1 and DLK-1. In this network, ATAT-2 functions independent of the DLK-1 MAPK and likely acts downstream of RPM-1. Thus, our study reveals an important role for tubulin acetyltransferase activity in presynaptic maintenance, which occurs via the RPM-1/ATAT-2 pathway.

RPM-1 and DLK-1 regulate pioneer axon outgrowth by controlling Wnt signaling

Axons must correctly reach their targets for proper nervous system function, although we do not fully understand the underlying mechanism, particularly for the first 'pioneer' axons. In C. elegans, AVG is the first neuron to extend an axon along the ventral midline, and this pioneer axon facilitates the proper extension and guidance of follower axons that comprise the ventral nerve cord. Here, we show that the ubiquitin ligase RPM-1 prevents the overgrowth of the AVG axon by repressing the activity of the DLK-1/p38 MAPK pathway. Unlike in damaged neurons, where this pathway activates CEBP-1, we find that RPM-1 and the DLK-1 pathway instead regulate the response to extracellular Wnt cues in developing AVG axons. The Wnt LIN-44 promotes the posterior growth of the AVG axon. In the absence of RPM-1 activity, AVG becomes responsive to a different Wnt, EGL-20, through a mechanism that appears to be independent of canonical Fz-type receptors. Our results suggest that RPM-1 and the DLK-1 pathway regulate axon guidance and growth by preventing Wnt signaling crosstalk.

Correlation of Crystallin Expression and RGC Susceptibility in Experimental Glaucoma Rats of Different Ages

PURPOSE

Glaucoma is one of the leading causes of blindness worldwide with age being an important risk factor. However, the pathogenesis remains poorly understood. Aim of this study was to focus on age-dependent molecular changes in an experimental animal model of glaucoma.

METHODS

Intraocular pressure was elevated in Sprague-Dawley rats aged 3, 14, and 47 weeks for a period of 7 weeks by episcleral vein cauterization. Ganglion cell loss was monitored by an immunohistochemical staining of the Brain-specific homeobox/POU (Pit-1, Oct-2, Unc-86) domain protein 3A positive cells in retinal flat-mounts and spectral-domain optical coherence tomography measuring the retinal nerve fiber layer thickness. Molecular protein alterations were analyzed using a comprehensive mass spectrometric proteomics approach of the retina and vitreous body.

RESULTS

While juvenile animals did not show a significant loss of retinal ganglion cells due to intraocular pressure elevation, adolescent animals showed a decrease up to 26% (p < 0.05). A shift of retinal crystallin protein expression levels within all protein-family subclasses (α, β, γ) could be observed in the youngest animal group (p < 0.05), while the upregulation of crystallin proteins in older animals was less striking. In addition, numerous crystallin proteins were also detected in the vitreous body.

CONCLUSION

These results provide insights of a potential correlation of age-related glaucomatous damage and the absence of crystallin proteins in the retina.

BMP signaling downstream of the Highwire E3 ligase sensitizes nociceptors

A comprehensive understanding of the molecular machinery important for nociception is essential to improving the treatment of pain. Here, we show that the BMP signaling pathway regulates nociception downstream of the E3 ubiquitin ligase highwire (hiw). hiw loss of function in nociceptors caused antagonistic and pleiotropic phenotypes with simultaneous insensitivity to noxious heat but sensitized responses to optogenetic activation of nociceptors. Thus, hiw functions to both positively and negatively regulate nociceptors. We find that a sensory reception-independent sensitization pathway was associated with BMP signaling. BMP signaling in nociceptors was up-regulated in hiw mutants, and nociceptor-specific expression of hiw rescued all nociception phenotypes including the increased BMP signaling. Blocking the transcriptional output of the BMP pathway with dominant negative Mad suppressed nociceptive hypersensitivity that was induced by interfering with hiw. The up-regulated BMP signaling phenotype in hiw genetic mutants could not be suppressed by mutation in wallenda suggesting that hiw regulates BMP in nociceptors via a wallenda independent pathway. In a newly established Ca²⁺ imaging preparation, we observed that up-regulated BMP signaling caused a significantly enhanced Ca²⁺ signal in the axon terminals of nociceptors that were stimulated by noxious heat. This response likely accounts for the nociceptive hypersensitivity induced by elevated BMP signaling in nociceptors. Finally, we showed that 24-hour activation of BMP signaling in nociceptors was sufficient to sensitize nociceptive responses to optogenetically-triggered nociceptor activation without altering nociceptor morphology. Overall, this study demonstrates the previously unrevealed roles of the Hiw-BMP pathway in the regulation of nociception and provides the first direct evidence that up-regulated BMP signaling physiologically sensitizes responses of nociceptors and nociception behaviors.

Although pain is a universally experienced sensation that has a significant impact on human lives and society, the molecular mechanisms of pain remain poorly understood. Elucidating these mechanisms is particularly important to gaining insight into the clinical development of currently incurable chronic pain diseases. Taking an advantage of the powerful genetic model organism Drosophila melanogaster (fruit flies), we unveil the Highwire-BMP signaling pathway as a novel molecular pathway that regulates the sensitivity of nociceptive sensory neurons. Highwire and the molecular components of the BMP signaling pathway are known to be widely conserved among animal phyla, from nematode worms to humans. Since abnormal sensitivity of nociceptive sensory neurons can play a critical role in the development of chronic pain conditions, a deeper understanding of the regulation of nociceptor sensitivity has the potential to advance effective therapeutic strategies to treat difficult pain conditions.

PAM forms an atypical SCF ubiquitin ligase complex that ubiquitinates and degrades NMNAT2

PHR (PAM/Highwire/RPM-1) proteins are conserved RING E3 ubiquitin ligases that function in developmental processes, such as axon termination and synapse formation, as well as axon degeneration. At present, our understanding of how PHR proteins form ubiquitin ligase complexes remains incomplete. Although genetic studies indicate NMNAT2 is an important mediator of PHR protein function in axon degeneration, it remains unknown how PHR proteins inhibit NMNAT2. Here, we decipher the biochemical basis for how the human PHR protein PAM, also called MYCBP2, forms a noncanonical Skp/Cullin/F-box (SCF) complex that contains the F-box protein FBXO45 and SKP1 but lacks CUL1. We show FBXO45 does not simply function in substrate recognition but is important for assembly of the PAM/FBXO45/SKP1 complex. Interestingly, we demonstrate a novel role for SKP1 as an auxiliary component of the target recognition module that enhances binding of FBXO45 to NMNAT2. Finally, we provide biochemical evidence that PAM polyubiquitinates NMNAT2 and regulates NMNAT2 protein stability and degradation by the proteasome.

Distinct homeostatic modulations stabilize reduced postsynaptic receptivity in response to presynaptic DLK signaling

Synapses are constructed with the stability to last a lifetime, yet sufficiently flexible to adapt during injury. Although fundamental pathways that mediate intrinsic responses to neuronal injury have been defined, less is known about how synaptic partners adapt. We have investigated responses in the postsynaptic cell to presynaptic activation of the injury-related Dual Leucine Zipper Kinase pathway at the Drosophila neuromuscular junction. We find that the postsynaptic compartment reduces neurotransmitter receptor levels, thus depressing synaptic strength. Interestingly, this diminished state is stabilized through distinct modulations to two postsynaptic homeostatic signaling systems. First, a retrograde response normally triggered by reduced receptor levels is silenced, preventing a compensatory enhancement in presynaptic neurotransmitter release. However, when global presynaptic release is attenuated, a postsynaptic receptor scaling mechanism persists to adaptively stabilize this diminished neurotransmission state. Thus, the homeostatic set point of synaptic strength is recalibrated to a reduced state as synapses acclimate to injury.

Activity-based E3 ligase profiling uncovers an E3 ligase with esterification activity

Ubiquitination is initiated by transfer of ubiquitin (Ub) from a ubiquitin-activating enzyme (E1) to a ubiquitin-conjugating enzyme (E2), producing a covalently linked intermediate (E2-Ub) ¹ . Ubiquitin ligases (E3s) of the 'really interesting new gene' (RING) class recruit E2-Ub via their RING domain and then mediate direct transfer of ubiquitin to substrates ² . By contrast, 'homologous to E6-AP carboxy terminus' (HECT) E3 ligases undergo a catalytic cysteine-dependent transthiolation reaction with E2-Ub, forming a covalent E3-Ub intermediate^3,4. Additionally, RING-between-RING (RBR) E3 ligases have a canonical RING domain that is linked to an ancillary domain. This ancillary domain contains a catalytic cysteine that enables a hybrid RING-HECT mechanism ⁵ . Ubiquitination is typically considered a post-translational modification of lysine residues, as there are no known human E3 ligases with non-lysine activity. Here we perform activity-based protein profiling of HECT or RBR-like E3 ligases and identify the neuron-associated E3 ligase MYCBP2 (also known as PHR1) as the apparent single member of a class of RING-linked E3 ligase with esterification activity and intrinsic selectivity for threonine over serine. MYCBP2 contains two essential catalytic cysteine residues that relay ubiquitin to its substrate via thioester intermediates. Crystallographic characterization of this class of E3 ligase, which we designate RING-Cys-relay (RCR), provides insights into its mechanism and threonine selectivity. These findings implicate non-lysine ubiquitination in cellular regulation of higher eukaryotes and suggest that E3 enzymes have an unappreciated mechanistic diversity.

The HECT Family Ubiquitin Ligase EEL-1 Regulates Neuronal Function and Development

Genetic changes in the HECT ubiquitin ligase HUWE1 are associated with intellectual disability, but it remains unknown whether HUWE1 functions in post-mitotic neurons to affect circuit function. Using genetics, pharmacology, and electrophysiology, we show that EEL-1, the HUWE1 ortholog in C. elegans, preferentially regulates GABAergic presynaptic transmission. Decreasing or increasing EEL-1 function alters GABAergic transmission and the excitatory/inhibitory (E/I) balance in the worm motor circuit, which leads to impaired locomotion and increased sensitivity to electroshock. Furthermore, multiple mutations associated with intellectual disability impair EEL-1 function. Although synaptic transmission defects did not result from abnormal synapse formation, sensitizing genetic backgrounds revealed that EEL-1 functions in the same pathway as the RING family ubiquitin ligase RPM-1 to regulate synapse formation and axon termination. These findings from a simple model circuit provide insight into the molecular mechanisms required to obtain E/I balance and could have implications for the link between HUWE1 and intellectual disability.

A MIG-15/JNK-1 MAP kinase cascade opposes RPM-1 signaling in synapse formation and learning

The Pam/Highwire/RPM-1 (PHR) proteins are conserved intracellular signaling hubs that regulate synapse formation and axon termination. The C. elegans PHR protein, called RPM-1, acts as a ubiquitin ligase to inhibit the DLK-1 and MLK-1 MAP kinase pathways. We have identified several kinases that are likely to form a new MAP kinase pathway that suppresses synapse formation defects, but not axon termination defects, in the mechanosensory neurons of rpm-1 mutants. This pathway includes: MIG-15 (MAP4K), NSY-1 (MAP3K), JKK-1 (MAP2K) and JNK-1 (MAPK). Transgenic overexpression of kinases in the MIG-15/JNK-1 pathway is sufficient to impair synapse formation in wild-type animals. The MIG-15/JNK-1 pathway functions cell autonomously in the mechanosensory neurons, and these kinases localize to presynaptic terminals providing further evidence of a role in synapse development. Loss of MIG-15/JNK-1 signaling also suppresses defects in habituation to repeated mechanical stimuli in rpm-1 mutants, a behavioral deficit that is likely to arise from impaired glutamatergic synapse formation. Interestingly, habituation results are consistent with the MIG-15/JNK-1 pathway functioning as a parallel opposing pathway to RPM-1. These findings indicate the MIG-15/JNK-1 pathway can restrict both glutamatergic synapse formation and short-term learning.

Homeostatic plasticity can be induced and expressed to restore synaptic strength at neuromuscular junctions undergoing ALS-related degeneration

Amyotrophic lateral sclerosis (ALS) is debilitating neurodegenerative disease characterized by motor neuron dysfunction and progressive weakening of the neuromuscular junction (NMJ). Hereditary ALS is strongly associated with variants in the human C9orf72 gene. We have characterized C9orf72 pathology at the Drosophila NMJ and utilized several approaches to restore synaptic strength in this model. First, we demonstrate a dramatic reduction in synaptic arborization and active zone number at NMJs following C9orf72 transgenic expression in motor neurons. Further, neurotransmission is similarly reduced at these synapses, consistent with severe degradation. However, despite these defects, C9orf72 synapses still retain the ability to express presynaptic homeostatic plasticity, a fundamental and adaptive form of NMJ plasticity in which perturbation to postsynaptic neurotransmitter receptors leads to a retrograde enhancement in presynaptic release. Next, we show that these endogenous but dormant homeostatic mechanisms can be harnessed to restore synaptic strength despite C9orf72 pathogenesis. Finally, activation of regenerative signaling is not neuroprotective in motor neurons undergoing C9orf72 toxicity. Together, these experiments define synaptic dysfunction at NMJs experiencing ALS-related degradation and demonstrate the potential to activate latent plasticity as a novel therapeutic strategy to restore synaptic strength.

RPM-1 regulates axon termination by affecting growth cone collapse and microtubule stability

Axon termination is essential for efficient and accurate nervous system construction. At present, relatively little is known about how growth cone collapse occurs prior to axon termination in vivo Using the mechanosensory neurons of C. elegans, we found collapse prior to axon termination is protracted, with the growth cone transitioning from a dynamic to a static state. Growth cone collapse prior to termination is facilitated by the signaling hub RPM-1. Given the prominence of the cytoskeleton in growth cone collapse, we assessed the relationship between RPM-1 and regulators of actin dynamics and microtubule stability. Our results reveal several important findings about how axon termination is orchestrated: (1) RPM-1 functions in parallel to RHO-1 and CRMP/UNC-33, but is suppressed by the Rac isoform MIG-2; (2) RPM-1 opposes the function of microtubule stabilizers, including tubulin acetyltransferases; and (3) genetic epistasis suggests the microtubule-stabilizing protein Tau/PTL-1 potentially inhibits RPM-1. These findings provide insight into how growth cone collapse is regulated during axon termination in vivo, and suggest that RPM-1 signaling destabilizes microtubules to facilitate growth cone collapse and axon termination.

Axon Termination, Pruning, and Synaptogenesis in the Giant Fiber System of Drosophila melanogaster Is Promoted by Highwire

The ubiquitin ligase Highwire has a conserved role in synapse formation. Here, we show that Highwire coordinates several facets of central synapse formation in the Drosophila melanogaster giant fiber system, including axon termination, axon pruning, and synaptic function. Despite the similarities to the fly neuromuscular junction, the role of Highwire and the underlying signaling pathways are distinct in the fly's giant fiber system. During development, branching of the giant fiber presynaptic terminal occurs and, normally, the transient branches are pruned away. However, in highwire mutants these ectopic branches persist, indicating that Highwire promotes axon pruning. highwire mutants also exhibit defects in synaptic function. Highwire promotes axon pruning and synaptic function cell-autonomously by attenuating a mitogen-activated protein kinase pathway including Wallenda, c-Jun N-terminal kinase/Basket, and the transcription factor Jun. We also show a novel role for Highwire in non-cell autonomous promotion of synaptic function from the midline glia. Highwire also regulates axon termination in the giant fibers, as highwire mutant axons exhibit severe overgrowth beyond the pruning defect. This excessive axon growth is increased by manipulating Fos expression in the cells surrounding the giant fiber terminal, suggesting that Fos regulates a trans-synaptic signal that promotes giant fiber axon growth.

Defining Minimal Binding Regions in Regulator of Presynaptic Morphology 1 (RPM-1) Using Caenorhabditis elegans Neurons Reveals Differential Signaling Complexes

The intracellular signaling protein regulator of presynaptic morphology 1 (RPM-1) is a conserved regulator of synapse formation and axon termination in Caenorhabditis elegans RPM-1 functions in a ubiquitin ligase complex with the F-box protein FSN-1 and functions through the microtubule binding protein RAE-1. Using a structure-function approach and positive selection for transgenic C. elegans, we explored the biochemical relationship between RPM-1, FSN-1, and RAE-1. This led to the identification of two new domains in RPM-1 that are sufficient for binding to FSN-1, called FSN-1 binding domain 2 (FBD2) and FBD3. Furthermore, we map the RAE-1 binding domain to a much smaller region of RPM-1. Point mutations in RPM-1 that reduce binding to RAE-1 did not affect FSN-1 binding, indicating that RPM-1 utilizes different biochemical mechanisms to bind these molecules. Analysis of RPM-1 protein complexes in the neurons of C. elegans elucidated two further discoveries: FSN-1 binds to RAE-1, and this interaction is not mediated by RPM-1, and RPM-1 binding to FSN-1 and RAE-1 reduces FSN-1·RAE-1 complex formation. These results indicate that RPM-1 uses different mechanisms to recruit FSN-1 and RAE-1 into independent signaling complexes in neurons.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.