Opposing effects of an F-box protein and the HSP90 chaperone network on microtubule stability and neurite growth in Caenorhabditis elegans

Molecular regulation of axon termination in mechanosensory neurons

Spatially and temporally accurate termination of axon outgrowth, a process called axon termination, is required for efficient, precise nervous system construction and wiring. The mechanosensory neurons that sense low-threshold mechanical stimulation or gentle touch have proven exceptionally valuable for studying axon termination over the past 40 years. In this Review, we discuss progress made in deciphering the molecular and genetic mechanisms that govern axon termination in touch receptor neurons. Findings across model organisms, including Caenorhabditis elegans, Drosophila, zebrafish and mice, have revealed that complex signaling is required for termination with conserved principles and players beginning to surface. A key emerging theme is that axon termination is mediated by complex signaling networks that include ubiquitin ligase signaling hubs, kinase cascades, transcription factors, guidance/adhesion receptors and growth factors. Here, we begin a discussion about how these signaling networks could represent termination codes that trigger cessation of axon outgrowth in different species and types of mechanosensory neurons.

An intraflagellar transport dependent negative feedback regulates the MAPKKK DLK-1 to protect cilia from degeneration

Primary cilia are specialized organelles supporting the development and function of cells and organisms. Intraflagellar transport (IFT) is essential for cilia formation, maintenance, and function. In C. elegans ciliated sensory neurons, IFT interacts with signaling molecules to generate distinct morphological and function features and also to maintain the integrity of cilia. Here, we report an IFT-dependent feedback control on the conserved MAPKKK DLK-1 in the ciliated sensory neurons. DLK proteins are widely known to act in synapse formation, axon regeneration, and degeneration, but their roles in other neuronal compartments are understudied. By forward genetic screening for altered expression of the endogenously tagged DLK-1 we identified multiple ift mutants showing increased DLK-1 accumulation in the defective sensory endings. We show that in response to acute IFT disruption, DLK-1 accumulates rapidly and reversibly. The expression levels of the transcription factor CEBP-1, known to act downstream of DLK-1 in the development and maintenance of synapses and axons, are also increased in the ciliated sensory neurons of ift mutants. Interestingly, the regulation of CEBP-1 expression shows sensory neuron-type dependency on DLK-1. Moreover, in the sensory neuron AWC, which has elaborate cilia morphology, up-regulated CEBP-1 represses DLK-1 at the transcription level, thereby dampening DLK-1 accumulation. Last, the IFT-dependent regulatory loop of DLK-1 and CEBP-1 offers neuroprotection in a cilia degeneration model. These findings uncover a surveillance mechanism in which tight control on the DLK-1 signaling protects cilia integrity in a context-specific manner.

Hsp70/Hsp90 Organising Protein (Hop): Coordinating Much More than Chaperones

The Hsp70/Hsp90 organising protein (Hop, also known as stress-inducible protein 1/STI1/STIP1) has received considerable attention for diverse cellular functions in both healthy and diseased states. There is extensive evidence that intracellular Hop is a co-chaperone of the major chaperones Hsp70 and Hsp90, playing an important role in the productive folding of Hsp90 client proteins, although recent evidence suggests that eukaryotic Hop is regulatory within chaperone complexes rather than essential. Consequently, Hop is implicated in many key signalling pathways, including aberrant pathways leading to cancer. Hop is also secreted, and it is now well established that Hop interacts with the prion protein, PrP^C, to mediate multiple signalling events. The intracellular and extracellular forms of Hop most likely represent two different isoforms, although the molecular determinants of these divergent functions are yet to be identified. There is also a growing body of research that reports the involvement of Hop in cellular activities that appear independent of either chaperones or PrP^C. While the various cellular functions of Hop have been described, its biological function remains elusive. However, recent knockout studies in mammals suggest that Hop has an important role in embryonic development. This review provides a critical overview of the latest molecular, cellular and biological research on Hop, critically evaluating its function in healthy systems and how this function is adapted in diseased states.

The circadian system in cystic fibrosis mice is regulated by histone deacetylase 6

Disordered sleep experienced by people with cystic fibrosis (CF) suggest a possible disruption in circadian regulation being associated with the loss of cystic fibrosis transmembrane conductance regulator (Cftr) function. To test this hypothesis, circadian regulation was assessed in an F508del/F508del CF mouse model. CF mice exhibited significant alterations in both timing of locomotor activity and in mean activity per hour in both light-dark (LD) and dark-dark (DD) photoperiods compared with wild-type (WT) controls. It was also noted that in DD periodicity increased in CF mice, whereas shortening in WT mice as is expected. CF mice also exhibited altered timing of circadian gene expression and a reduction of melatonin production at all time points. Mechanistically, the role of microtubules in regulating these outcomes was explored. Mice lacking expression of tubulin polymerization promoting protein (Tppp) effectively mimicked CF mouse phenotypes with each measured outcome. Depleting expression of the microtubule regulatory protein histone deacetylase 6 (Hdac6) from CF mice (CF/Hdac6) resulted in the reversal of each phenotype to WT profiles. These data demonstrate an innate disruption of circadian regulation in CF mice and identify a novel microtubule-related mechanism leading to this disruption that can be targeted for therapeutic intervention.

The MAP3Ks DLK and LZK Direct Diverse Responses to Axon Damage in Zebrafish Peripheral Neurons

Mitogen-activated protein kinase kinase kinases (MAP3Ks) dual leucine kinase (DLK) and leucine zipper kinase (LZK) are essential mediators of axon damage responses, but their responses are varied, complex, and incompletely understood. To characterize their functions in axon injury, we generated zebrafish mutants of each gene, labeled motor neurons (MNs) and touch-sensing neurons in live zebrafish, precisely cut their axons with a laser, and assessed the ability of mutant axons to regenerate in larvae, before sex is apparent in zebrafish. DLK and LZK were required redundantly and cell autonomously for axon regeneration in MNs but not in larval Rohon-Beard (RB) or adult dorsal root ganglion (DRG) sensory neurons. Surprisingly, in dlk lzk double mutants, the spared branches of wounded RB axons grew excessively, suggesting that these kinases inhibit regenerative sprouting in damaged axons. Uninjured trigeminal sensory axons also grew excessively in mutants when neighboring neurons were ablated, indicating that these MAP3Ks are general inhibitors of sensory axon growth. These results demonstrate that zebrafish DLK and LZK promote diverse injury responses, depending on the neuronal cell identity and type of axonal injury.SIGNIFICANCE STATEMENT The MAP3Ks DLK and LZK are damage sensors that promote diverse outcomes to neuronal injury, including axon regeneration. Understanding their context-specific functions is a prerequisite to considering these kinases as therapeutic targets. To investigate DLK and LZK cell-type-specific functions, we created zebrafish mutants in each gene. Using mosaic cell labeling and precise laser injury we found that both proteins were required for axon regeneration in motor neurons but, unexpectedly, were not required for axon regeneration in Rohon-Beard or DRG sensory neurons and negatively regulated sprouting in the spared axons of touch-sensing neurons. These findings emphasize that animals have evolved distinct mechanisms to regulate injury site regeneration and collateral sprouting, and identify differential roles for DLK and LZK in these processes.

Molecular chaperone Hsp90 protects KCBP from degradation by proteasome in Dunaliella salina cells

Kinesin-like calmodulin-binding protein (KCBP) is a unique kinesin with half kinesin and half myosin, with kinesin motor domain at C-terminus and myosin tail homology region 4 (MyTH4) and band 4.1, ezrin, radixin, moesin (FERM) domains at N-terminus. The special structure endows KCBP multi-intracellular functions, including cell division, trichome morphogenesis in plants, and flagellar function in algae. However, little is known about the molecular mechanism underlying these functions. Here, we identified a molecular chaperone Hsp90 as a novel binding partner with KCBP in Dunaliella salina using a yeast two-hybrid screen. Further analysis showed that Hsp90 interacted with both the N-terminal and C-terminal of DsKCBP. Since Hsp90 was involved in the stability and proteolytic turnover of numerous proteins, whether Hsp90 regulated the degradation of DsKCBP was investigated. Our results showed that both Hsp90 and DsKCBP presented in the purified proteasome, and the interaction of DsKCBP-Hsp90 was inhibited upon Hsp90 inhibitor geldanamycin treatment. The level of DsKCBP proteins was diminished remarkably indicating that the disassociation of DsKCBP from Hsp90 accelerated the degradation of the former. Furthermore, immunofluorescence results showed that the localization of DsKCBP at basal body and flagella was disappeared by Hsp90 inhibition. The increased mRNA level of DsKCBP during flagellar assembly was not obvious by geldanamycin treatment. These data provided evidence that Hsp90 protected DsKCBP from degradation by proteasome and was involved in the role of DsKCBP in flagellar assembly.

Proteomic analysis of Caenorhabditis elegans wound model reveals novel molecular players involved in repair

Wound repair is a multistep process which involves coordination of multiple molecular players from different cell types and pathways. Though the cellular processes that are taking place in order to repair damage is already known, molecular players involved in crucial pathways are still scarce. In this regard, the present study intends to uncover crucial players that are involved in the central repair events through proteomics approach which included 2-D GE and LC-MS/MS using Caenorhabditis elegans wound model. Initial gel-based 2-D GE and following protein-protein interaction (PPI) network analyses revealed active role of calcium signaling, acetylcholine transport and serotonergic neurotransmitter pathways. Further, gel-free LC-MS/MS and following PPI network analyses revealed the incidence of actin nucleation at the initial hours immediately after injury. Further by visualizing the PPI network and the interacting players, pink-1, a mitochondrial Serine/threonine-protein kinase which is known to regulate mitochondrial dynamics, was found to be the central player in facilitating the mitochondrial fission and its role was further verified using qPCR analysis and pink-1 transgenic worms. Overall, the study delivers new insights from crucial regulatory pathways and central players involved in wound repair using high throughput proteomic approaches and the mass spectrometry Data (PXD024629/PXD024744) are available via ProteomeXchange. SIGNIFICANCE.

Epistatic, synthetic, and balancing interactions among tubulin missense mutations affecting neurite growth in Caenorhabditis elegans

Mutations in tubulins affect microtubule (MT) dynamics and functions during neuronal differentiation and their genetic interaction provides insights into the regulation of MT functions. We previously used Caenorhabditis elegans touch receptor neurons to analyze the cellular impact of tubulin mutations and reported the phenotypes of 67 tubulin missense mutations, categorized into three classes: loss-of-function (lf), antimorphic (anti), and neomorphic (neo) alleles. In this study, we isolated 54 additional tubulin alleles through suppressor screens in sensitized backgrounds that caused excessive neurite growth. These alleles included 32 missense mutations not analyzed before, bringing the total number of mutations in our collection to 99. Phenotypic characterization of these newly isolated mutations identified three new types of alleles: partial lf and weak neo alleles of mec-7/β-tubulin that had subtle effects and strong anti alleles of mec-12/α-tubulin. We also discovered complex genetic interactions among the tubulin mutations, including the suppression of neo mutations by intragenic lf and anti alleles, additive and synthetic effects between mec-7 neo alleles, and unexpected epistasis, in which weaker neo alleles masked the effects of stronger neo alleles in inducing ectopic neurite growth. We also observed balancing between neo and anti alleles, whose respective MT-hyperstablizing and -destabilizing effects neutralized each other.

Homeostatic Roles of the Proteostasis Network in Dendrites

Cellular protein homeostasis, or proteostasis, is indispensable to the survival and function of all cells. Distinct from other cell types, neurons are long-lived, exhibiting architecturally complex and diverse multipolar projection morphologies that can span great distances. These properties present unique demands on proteostatic machinery to dynamically regulate the neuronal proteome in both space and time. Proteostasis is regulated by a distributed network of cellular processes, the proteostasis network (PN), which ensures precise control of protein synthesis, native conformational folding and maintenance, and protein turnover and degradation, collectively safeguarding proteome integrity both under homeostatic conditions and in the contexts of cellular stress, aging, and disease. Dendrites are equipped with distributed cellular machinery for protein synthesis and turnover, including dendritically trafficked ribosomes, chaperones, and autophagosomes. The PN can be subdivided into an adaptive network of three major functional pathways that synergistically govern protein quality control through the action of (1) protein synthesis machinery; (2) maintenance mechanisms including molecular chaperones involved in protein folding; and (3) degradative pathways (e.g., Ubiquitin-Proteasome System (UPS), endolysosomal pathway, and autophagy. Perturbations in any of the three arms of proteostasis can have dramatic effects on neurons, especially on their dendrites, which require tightly controlled homeostasis for proper development and maintenance. Moreover, the critical importance of the PN as a cell surveillance system against protein dyshomeostasis has been highlighted by extensive work demonstrating that the aggregation and/or failure to clear aggregated proteins figures centrally in many neurological disorders. While these studies demonstrate the relevance of derangements in proteostasis to human neurological disease, here we mainly review recent literature on homeostatic developmental roles the PN machinery plays in the establishment, maintenance, and plasticity of stable and dynamic dendritic arbors. Beyond basic housekeeping functions, we consider roles of PN machinery in protein quality control mechanisms linked to dendritic plasticity (e.g., dendritic spine remodeling during LTP); cell-type specificity; dendritic morphogenesis; and dendritic pruning.