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

CAMSAP3 forms dimers via its α-helix domain that directly stabilize non-centrosomal microtubule minus ends

Microtubules are vital components of the cytoskeleton. Their plus ends are dynamic and respond to changes in cell morphology, whereas the minus ends are stable and serve a crucial role in microtubule seeding and maintaining spatial organization. In mammalian cells, the calmodulin-regulated spectrin-associated proteins (CAMSAPs), play a key role in directly regulating the dynamics of non-centrosomal microtubules minus ends. However, the molecular mechanisms are not yet fully understood. Our study reveals that CAMSAP3 forms dimers through its C-terminal α-helix; this dimerization not only enhances the microtubule-binding affinity of the CKK domain but also enables the CKK domain to regulate the dynamics of microtubules. Furthermore, CAMSAP3 also specializes in decorating at the minus end of microtubules through the combined action of the microtubule-binding domain (MBD) and the C-terminal α-helix, thereby achieving dynamic regulation of the minus ends of microtubules. These findings are crucial for advancing our understanding and treatment of diseases associated with non-centrosomal microtubules.

Dopey-dependent regulation of extracellular vesicles maintains neuronal morphology

Mature neurons maintain their distinctive morphology for extended periods in adult life. Compared to developmental neurite outgrowth, axon guidance, and target selection, relatively little is known of mechanisms that maintain the morphology of mature neurons. Loss of function in C. elegans dip-2, a member of the conserved lipid metabolic regulator Dip2 family, results in progressive overgrowth of neurites in adults. We find that dip-2 mutants display specific genetic interactions with sax-2, the C. elegans ortholog of Drosophila Furry and mammalian FRY. Combined loss of dip-2 and sax-2 results in failure to maintain neuronal morphology and elevated release of neuronal extracellular vesicles (EVs). By screening for suppressors of dip-2(0) sax-2(0) double mutant defects, we identified gain-of-function (gf) mutations in the conserved Dopey family protein PAD-1 and its associated phospholipid flippase TAT-5/ATP9A that restore normal neuronal morphology and normal levels of EV release to dip-2(0) sax-2(0) double mutants. Neuron-specific knockdown suggests that PAD-1(gf) can act cell autonomously in neurons. PAD-1(gf) displays increased association with the plasma membrane in oocytes and inhibits EV release in multiple cell types. Our findings uncover a novel functional network of DIP-2, SAX-2, PAD-1, and TAT-5 that maintains neuronal morphology and modulates EV release.

C. elegans touch receptor neurons direct mechanosensory complex organization via repurposing conserved basal lamina proteins

The sense of touch is conferred by the conjoint function of somatosensory neurons and skin cells. These cells meet across a gap filled by a basal lamina, an ancient structure found in metazoans. Using Caenorhabditis elegans, we investigate the composition and ultrastructure of the extracellular matrix at the epidermis and touch receptor neuron (TRN) interface. We show that membrane-matrix complexes containing laminin, nidogen, and the MEC-4 mechano-electrical transduction channel reside at this interface and are central to proper touch sensation. Interestingly, the dimensions and spacing of these complexes correspond with the discontinuous beam-like extracellular matrix structures observed in serial-section transmission electron micrographs. These complexes fail to coalesce in touch-insensitive extracellular matrix mutants and in dissociated neurons. Loss of nidogen reduces the density of mechanoreceptor complexes and the amplitude of the touch-evoked currents they carry. Thus, neuron-epithelium cell interfaces are instrumental in mechanosensory complex assembly and function. Unlike the basal lamina ensheathing the pharynx and body wall muscle, nidogen recruitment to the puncta along TRNs is not dependent upon laminin binding. MEC-4, but not laminin or nidogen, is destabilized by point mutations in the C-terminal Kunitz domain of the extracellular matrix component, MEC-1. These findings imply that somatosensory neurons secrete proteins that actively repurpose the basal lamina to generate special-purpose mechanosensory complexes responsible for vibrotactile sensing.

The outer kinetochore components KNL-1 and Ndc80 complex regulate axon and neuronal cell body positioning in the C. elegans nervous system

The KMN (Knl1/Mis12/Ndc80) network at the kinetochore, primarily known for its role in chromosome segregation, has been shown to be repurposed during neurodevelopment. Here, we investigate the underlying neuronal mechanism and show that the KMN network promotes the proper axonal organization within the C. elegans head nervous system. Postmitotic degradation of KNL-1, which acts as a scaffold for signaling and has microtubule-binding activities at the kinetochore, led to disorganized ganglia and aberrant placement and organization of axons in the nerve ring - an interconnected axonal network. Through gene-replacement approaches, we demonstrate that the signaling motifs within KNL-1, responsible for recruiting protein phosphatase 1, and activating the spindle assembly checkpoint are required for neurodevelopment. Interestingly, while the microtubule-binding activity is crucial to KMN's neuronal function, microtubule dynamics and organization were unaffected in the absence of KNL-1. Instead, the NDC-80 microtubule-binding mutant displayed notable defects in axon bundling during nerve ring formation, indicating its role in facilitating axon-axon contacts. Overall, these findings provide evidence for a noncanonical role for the KMN network in shaping the structure and connectivity of the nervous system in C. elegans during brain development.

CRMP/UNC-33 maintains neuronal microtubule arrays by promoting individual microtubule rescue

Microtubules (MTs) are intrinsically dynamic polymers. In neurons, staggered individual microtubules form stable, polarized acentrosomal MT arrays spanning the axon and dendrite to support long-distance intracellular transport. How the stability and polarity of these arrays are maintained when individual MTs remain highly dynamic is still an open question. Here we visualize MT arrays in vivo in C. elegans neurons with single microtubule resolution. We find that the CRMP family homolog, UNC-33, is essential for the stability and polarity of MT arrays in neurites. In unc-33 mutants, MTs exhibit dramatically reduced rescue after catastrophe, develop gaps in coverage, and lose their polarity, leading to trafficking defects. UNC-33 is stably anchored on the cortical cytoskeleton and forms patch-like structures along the dendritic shaft. These discrete and stable UNC-33 patches concentrate free tubulins and correlate with MT rescue sites. In vitro , purified UNC-33 preferentially associates with MT tips and increases MT rescue frequency. Together, we propose that UNC-33 functions as a microtubule-associated protein (MAP) to promote individual MT rescue locally. Through this activity, UNC-33 prevents the loss of individual MTs, thereby maintaining the coverage and polarity of MT arrays throughout the lifetime of neurons.

Dopey-dependent regulation of extracellular vesicles maintains neuronal morphology

Mature neurons maintain their distinctive morphology for extended periods in adult life. Compared to developmental neurite outgrowth, axon guidance, and target selection, relatively little is known of mechanisms that maintain mature neuron morphology. Loss of function in C. elegans DIP-2, a member of the conserved lipid metabolic regulator Dip2 family, results in progressive overgrowth of neurites in adults. We find that dip-2 mutants display specific genetic interactions with sax-2, the C. elegans ortholog of Drosophila Furry and mammalian FRY. Combined loss of DIP-2 and SAX-2 results in severe disruption of neuronal morphology maintenance accompanied by increased release of neuronal extracellular vesicles (EVs). By screening for suppressors of dip-2 sax-2 double mutant defects we identified gain-of-function (gf) mutations in the conserved Dopey family protein PAD-1 and its associated phospholipid flippase TAT-5/ATP9A. In dip-2 sax-2 double mutants carrying either pad-1(gf) or tat-5(gf) mutation, EV release is reduced and neuronal morphology across multiple neuron types is restored to largely normal. PAD-1(gf) acts cell autonomously in neurons. The domain containing pad-1(gf) is essential for PAD-1 function, and PAD-1(gf) protein displays increased association with the plasma membrane and inhibits EV release. Our findings uncover a novel functional network of DIP-2, SAX-2, PAD-1, and TAT-5 that maintains morphology of neurons and other types of cells, shedding light on the mechanistic basis of neurological disorders involving human orthologs of these genes.

The response of Dual-leucine zipper kinase (DLK) to nocodazole: Evidence for a homeostatic cytoskeletal repair mechanism

Genetic and pharmacological perturbation of the cytoskeleton enhances the regenerative potential of neurons. This response requires Dual-leucine Zipper Kinase (DLK), a neuronal stress sensor that is a central regulator of axon regeneration and degeneration. The damage and repair aspects of this response are reminiscent of other cellular homeostatic systems, suggesting that a cytoskeletal homeostatic response exists. In this study, we propose a framework for understanding DLK mediated neuronal cytoskeletal homeostasis. We demonstrate that low dose nocodazole treatment activates DLK signaling. Activation of DLK signaling results in a DLK-dependent transcriptional signature, which we identify through RNA-seq. This signature includes genes likely to attenuate DLK signaling while simultaneously inducing actin regulating genes. We identify alterations to the cytoskeleton including actin-based morphological changes to the axon. These results are consistent with the model that cytoskeletal disruption in the neuron induces a DLK-dependent homeostatic mechanism, which we term the Cytoskeletal Stress Response (CSR) pathway.

Patronin regulates presynaptic microtubule organization and neuromuscular junction development in Drosophila

Synapses are fundamental components of the animal nervous system. Synaptic cytoskeleton is essential for maintaining proper neuronal development and wiring. Perturbations in neuronal microtubules (MTs) are correlated with numerous neuropsychiatric disorders. Despite discovering multiple synaptic MT regulators, the importance of MT stability, and particularly the polarity of MT in synaptic function, is still under investigation. Here, we identify Patronin, an MT minus-end-binding protein, for its essential role in presynaptic regulation of MT organization and neuromuscular junction (NMJ) development. Analyses indicate that Patronin regulates synaptic development independent of Klp10A. Subsequent research elucidates that it is short stop (Shot), a member of the Spectraplakin family of large cytoskeletal linker molecules, works synergistically with Patronin to govern NMJ development. We further raise the possibility that normal synaptic MT polarity contributes to proper NMJ morphology. Overall, this study demonstrates an unprecedented role of Patronin, and a potential involvement of MT polarity in synaptic development.

The response of Dual-Leucine Zipper Kinase (DLK) to nocodazole: evidence for a homeostatic cytoskeletal repair mechanism

Genetic and pharmacological perturbation of the cytoskeleton enhances the regenerative potential of neurons. This response requires Dual-leucine Zipper Kinase (DLK), a neuronal stress sensor that is a central regulator of axon regeneration and degeneration. The damage and repair aspects of this response are reminiscent of other cellular homeostatic systems, suggesting that a cytoskeletal homeostatic response exists. In this study, we propose a framework for understanding DLK mediated neuronal cytoskeletal homeostasis. We demonstrate that a) low dose nocodazole treatment activates DLK signaling and b) DLK signaling mitigates the microtubule damage caused by the cytoskeletal perturbation. We also perform RNA-seq to discover a DLK-dependent transcriptional signature. This signature includes genes likely to attenuate DLK signaling while simultaneously inducing actin regulating genes and promoting actin-based morphological changes to the axon. These results are consistent with the model that cytoskeletal disruption in the neuron induces a DLK-dependent homeostatic mechanism, which we term the Cytoskeletal Stress Response (CSR) pathway.

Sculpting the dendritic landscape: Actin, microtubules, and the art of arborization

Dendrites are intricately designed neuronal compartments that play a vital role in the gathering and processing of sensory or synaptic inputs. Their diverse and elaborate structures are distinct features of neuronal organization and function. Central to the generation of these dendritic arbors is the neuronal cytoskeleton. In this review, we delve into the current progress toward our understanding of how dendrite arbors are generated and maintained, focusing on the role of the actin and microtubule cytoskeleton.

Patronin/CAMSAP promotes reactivation and regeneration of Drosophila quiescent neural stem cells

The ability of stem cells to switch between quiescent and proliferative states is crucial for maintaining tissue homeostasis and regeneration. Drosophila quiescent neural stem cells (qNSCs) extend a primary protrusion that is enriched in acentrosomal microtubules and can be regenerated upon injury. Arf1 promotes microtubule growth, reactivation (exit from quiescence), and regeneration of qNSC protrusions upon injury. However, how Arf1 is regulated in qNSCs remains elusive. Here, we show that the microtubule minus-end binding protein Patronin/CAMSAP promotes acentrosomal microtubule growth and quiescent NSC reactivation. Patronin is important for the localization of Arf1 at Golgi and physically associates with Arf1, preferentially with its GDP-bound form. Patronin is also required for the regeneration of qNSC protrusion, likely via the regulation of microtubule growth. Finally, Patronin functions upstream of Arf1 and its effector Msps/XMAP215 to target the cell adhesion molecule E-cadherin to NSC-neuropil contact sites during NSC reactivation. Our findings reveal a novel link between Patronin/CAMSAP and Arf1 in the regulation of microtubule growth and NSC reactivation. A similar mechanism might apply to various microtubule-dependent systems in mammals.

Lipid droplets and peroxisomes are co-regulated to drive lifespan extension in response to mono-unsaturated fatty acids

Dietary mono-unsaturated fatty acids (MUFAs) are linked to longevity in several species. But the mechanisms by which MUFAs extend lifespan remain unclear. Here we show that an organelle network involving lipid droplets and peroxisomes is critical for MUFA-induced longevity in Caenorhabditis elegans. MUFAs upregulate the number of lipid droplets in fat storage tissues. Increased lipid droplet number is necessary for MUFA-induced longevity and predicts remaining lifespan. Lipidomics datasets reveal that MUFAs also modify the ratio of membrane lipids and ether lipids-a signature associated with decreased lipid oxidation. In agreement with this, MUFAs decrease lipid oxidation in middle-aged individuals. Intriguingly, MUFAs upregulate not only lipid droplet number but also peroxisome number. A targeted screen identifies genes involved in the co-regulation of lipid droplets and peroxisomes, and reveals that induction of both organelles is optimal for longevity. Our study uncovers an organelle network involved in lipid homeostasis and lifespan regulation, opening new avenues for interventions to delay aging.

Non-coding ribonucleic acid-mediated CAMSAP1 upregulation leads to poor prognosis with suppressed immune infiltration in liver hepatocellular carcinoma

Liver hepatocellular carcinoma (LIHC) is well-known for its unfavorable prognosis due to the lack of reliable diagnostic and prognostic biomarkers. Calmodulin-regulated spectrin-associated protein 1 (CAMSAP1) is a non-centrosomal microtubule minus-end binding protein that regulates microtubule dynamics. This study aims to investigate the specific role and mechanisms of CAMSAP1 in LIHC. We performed systematical analyses of CAMSAP1 and demonstrated that differential expression of CAMSAP1 is associated with genetic alteration and DNA methylation, and serves as a potential diagnostic and prognostic biomarker in some cancers, especially LIHC. Further evidence suggested that CAMSAP1 overexpression leads to adverse clinical outcomes in advanced LIHC. Moreover, the AC145207.5/LINC01748-miR-101–3p axis is specifically responsible for CAMSAP1 overexpression in LIHC. In addition to the previously reported functions in the cell cycle and regulation of actin cytoskeleton, CAMSAP1-related genes are enriched in cancer- and immune-associated pathways. As expected, CAMSAP1-associated LIHC is infiltrated in the suppressed immune microenvironment. Specifically, except for immune cell infiltration, it is significantly positively correlated with immune checkpoint genes, especially CD274 (PD-L1), and cancer-associated fibroblasts. Prediction of immune checkpoint blockade therapy suggests that these patients may benefit from therapy. Our study is the first to demonstrate that besides genetic alteration and DNA methylation, AC145207.5/LINC01748-miR-101-3p-mediated CAMSAP1 upregulation in advanced LIHC leads to poor prognosis with suppressed immune infiltration, representing a potential diagnostic and prognostic biomarker as well as a promising immunotherapy target for LIHC.

CAMSAP2 organizes a γ-tubulin-independent microtubule nucleation centre through phase separation

Microtubules are dynamic polymers consisting of αβ-tubulin heterodimers. The initial polymerization process, called microtubule nucleation, occurs spontaneously via αβ-tubulin. Since a large energy barrier prevents microtubule nucleation in cells, the γ-tubulin ring complex is recruited to the centrosome to overcome the nucleation barrier. However, a considerable number of microtubules can polymerize independently of the centrosome in various cell types. Here, we present evidence that the minus-end-binding calmodulin-regulated spectrin-associated protein 2 (CAMSAP2) serves as a strong nucleator for microtubule formation by significantly reducing the nucleation barrier. CAMSAP2 co-condensates with αβ-tubulin via a phase separation process, producing plenty of nucleation intermediates. Microtubules then radiate from the co-condensates, resulting in aster-like structure formation. CAMSAP2 localizes at the co-condensates and decorates the radiating microtubule lattices to some extent. Taken together, these in vitro findings suggest that CAMSAP2 supports microtubule nucleation and growth by organizing a nucleation centre as well as by stabilizing microtubule intermediates and growing microtubules.

Cells are able to hold their shape thanks to tube-like structures called microtubules that are made of hundreds of tubulin proteins. Microtubules are responsible for maintaining the uneven distribution of molecules throughout the cell, a phenomenon known as polarity that allows cells to differentiate into different types with various roles.

A protein complex called the γ-tubulin ring complex (γ-TuRC) is necessary for microtubules to form. This protein helps bind the tubulin proteins together and stabilises microtubules. However, recent research has found that in highly polarized cells such as neurons, which have highly specialised regions, microtubules can form without γ-TuRC. Searching for the proteins that could be filling in for γ-TuRC in these cells some evidence has suggested that a group known as CAMSAPs may be involved, but it is not known how.

To characterize the role of CAMSAPs, Imasaki, Kikkawa et al. studied how one of these proteins, CAMSAP2, interacts with tubulins. To do this, they reconstituted both CAMSAP2 and tubulins using recombinant biotechnology and mixed them in solution. These experiments showed that CAMSAP2 can help form microtubules by bringing together their constituent proteins so that they can bind to each other more easily. Once microtubules start to form, CAMSAP2 continues to bind to them, stabilizing them and enabling them to grow to full size.

These results shed light on how polarity is established in cells such as neurons, muscle cells, and epithelial cells. Additionally, the ability to observe intermediate structures during microtubule formation can provide insights into the processes that these structures are involved in.

Localized glucose import, glycolytic processing, and mitochondria generate a focused ATP burst to power basement-membrane invasion

Invasive cells use transient, energy-consuming protrusions to breach basement membrane (BM) barriers. Using the ATP sensor PercevalHR during anchor cell (AC) invasion in Caenorhabditis elegans, we show that BM invasion is accompanied by an ATP burst from mitochondria at the invasive front. RNAi screening and visualization of a glucose biosensor identified two glucose transporters, FGT-1 and FGT-2, which bathe invasive front mitochondria with glucose and facilitate the ATP burst to form protrusions. FGT-1 localizes at high levels along the invasive membrane, while FGT-2 is adaptive, enriching most strongly during BM breaching and when FGT-1 is absent. Cytosolic glycolytic enzymes that process glucose for mitochondrial ATP production cluster with invasive front mitochondria and promote higher mitochondrial membrane potential and ATP levels. Finally, we show that UNC-6 (netrin), which polarizes invasive protrusions, also orients FGT-1. These studies reveal a robust and integrated energy acquisition, processing, and delivery network that powers BM breaching.

Microtubule Anchoring: Attaching Dynamic Polymers to Cellular Structures

Microtubules are dynamic, filamentous polymers composed of α- and β-tubulin. Arrays of microtubules that have a specific polarity and distribution mediate essential processes such as intracellular transport and mitotic chromosome segregation. Microtubule arrays are generated with the help of microtubule organizing centers (MTOC). MTOCs typically combine two principal activities, the de novo formation of microtubules, termed nucleation, and the immobilization of one of the two ends of microtubules, termed anchoring. Nucleation is mediated by the γ-tubulin ring complex (γTuRC), which, in cooperation with its recruitment and activation factors, provides a template for α- and β-tubulin assembly, facilitating formation of microtubule polymer. In contrast, the molecules and mechanisms that anchor newly formed microtubules at MTOCs are less well characterized. Here we discuss the mechanistic challenges underlying microtubule anchoring, how this is linked with the molecular activities of known and proposed anchoring factors, and what consequences defective microtubule anchoring has at the cellular and organismal level.

A proximity labeling protocol to probe proximity interactions in C. elegans

Enzyme-catalyzed proximity labeling (PL) has emerged as a critical approach for identifying protein-protein proximity interactions in cells; however, PL techniques were not historically practical in living multicellular organisms due to technical limitations. Here, we present a protocol for applying PL to living C. elegans using the biotin ligase mutant enzyme TurboID. We demonstrated PL in a tissue-specific and region-specific manner by focusing on non-centrosomal MTOCs (ncMTOCs) of intestinal cells. This protocol is useful for targeted in vivo protein network profiling. For complete details on the use and execution of this protocol, please refer to Sanchez et al. (2021).

Tracheal motile cilia in mice require CAMSAP3 for formation of central microtubule pair and coordinated beating

Motile cilia of multiciliated epithelial cells undergo synchronized beating to produce fluid flow along the luminal surface of various organs. Each motile cilium consists of an axoneme and a basal body (BB), which are linked by a “transition zone” (TZ). The axoneme exhibits a characteristic 9+2 microtubule arrangement important for ciliary motion, but how this microtubule system is generated is not yet fully understood. Here we show that calmodulin-regulated spectrin-associated protein 3 (CAMSAP3), a protein that can stabilize the minus-end of a microtubule, concentrates at multiple sites of the cilium–BB complex, including the upper region of the TZ or the axonemal basal plate (BP) where the central pair of microtubules (CP) initiates. CAMSAP3 dysfunction resulted in loss of the CP and partial distortion of the BP, as well as the failure of multicilia to undergo synchronized beating. These findings suggest that CAMSAP3 plays pivotal roles in the formation or stabilization of the CP by localizing at the basal region of the axoneme and thereby supports the coordinated motion of multicilia in airway epithelial cells.

Cyst formation in proximal renal tubules caused by dysfunction of the microtubule minus-end regulator CAMSAP3

Epithelial cells organize an ordered array of non-centrosomal microtubules, the minus ends of which are regulated by CAMSAP3. The role of these microtubules in epithelial functions, however, is poorly understood. Here, we show that the kidneys of mice in which Camsap3 is mutated develop cysts at the proximal convoluted tubules (PCTs). PCTs were severely dilated in the mutant kidneys, and they also exhibited enhanced cell proliferation. In these PCTs, epithelial cells became flattened along with perturbation of microtubule arrays as well as of certain subcellular structures such as interdigitating basal processes. Furthermore, YAP and PIEZO1, which are known as mechanosensitive regulators for cell shaping and proliferation, were activated in these mutant PCT cells. These observations suggest that CAMSAP3-mediated microtubule networks are important for maintaining the proper mechanical properties of PCT cells, and its loss triggers cell deformation and proliferation via activation of mechanosensors, resulting in the dilation of PCTs.

Molecular mechanisms that mediate dendrite morphogenesis

Neurons develop dendritic morphologies that bear cell type-specific features in dendritic field size and geometry, branch placement and density, and the types and distributions of synaptic contacts. Dendritic patterns influence the types and numbers of inputs a neuron receives, and the ways in which neural information is processed and transmitted in the circuitry. Even subtle alterations in dendritic structures can have profound consequences on neuronal function and are implicated in neurodevelopmental disorders. In this chapter, I review how growing dendrites acquire their exquisite patterns by drawing examples from diverse neuronal cell types in vertebrate and invertebrate model systems. Dendrite morphogenesis is shaped by intrinsic and extrinsic factors such as transcriptional regulators, guidance and adhesion molecules, neighboring cells and synaptic partners. I discuss molecular mechanisms that regulate dendrite morphogenesis with a focus on five aspects of dendrite patterning: (1) Dendritic cytoskeleton and cellular machineries that build the arbor; (2) Gene regulatory mechanisms; (3) Afferent cues that regulate dendritic arbor growth; (4) Space-filling strategies that optimize dendritic coverage; and (5) Molecular cues that specify dendrite wiring. Cell type-specific implementation of these patterning mechanisms produces the diversity of dendrite morphologies that wire the nervous system.

Molecular mechanisms governing axonal transport: a C. elegans perspective

Axonal transport is integral for maintaining neuronal form and function, and defects in axonal transport have been correlated with several neurological diseases, making it a subject of extensive research over the past several years. The anterograde and retrograde transport machineries are crucial for the delivery and distribution of several cytoskeletal elements, growth factors, organelles and other synaptic cargo. Molecular motors and the neuronal cytoskeleton function as effectors for multiple neuronal processes such as axon outgrowth and synapse formation. This review examines the molecular mechanisms governing axonal transport, specifically highlighting the contribution of studies conducted in C. elegans, which has proved to be a tractable model system in which to identify both novel and conserved regulatory mechanisms of axonal transport.

cnd-1/NeuroD1 Functions with the Homeobox Gene ceh-5/Vax2 and Hox Gene ceh-13/labial To Specify Aspects of RME and DD Neuron Fate in Caenorhabditis elegans

Identifying the mechanisms behind neuronal fate specification are key to understanding normal neural development in addition to neurodevelopmental disorders such as autism and schizophrenia. In vivo cell fate specification is difficult to study in vertebrates. However, the nematode Caenorhabditis elegans, with its invariant cell lineage and simple nervous system of 302 neurons, is an ideal organism to explore the earliest stages of neural development. We used a comparative transcriptome approach to examine the role of cnd-1/NeuroD1 in C. elegans nervous system development and function. This basic helix-loop-helix transcription factor is deeply conserved across phyla and plays a crucial role in cell fate specification in both the vertebrate nervous system and pancreas. We find that cnd-1 controls expression of ceh-5, a Vax2-like homeobox class transcription factor, in the RME head motorneurons and PVQ tail interneurons. We also show that cnd-1 functions redundantly with the Hox gene ceh-13/labial in defining the fate of DD1 and DD2 embryonic ventral nerve cord motorneurons. These data highlight the utility of comparative transcriptomes for identifying transcription factor targets and understanding gene regulatory networks.

CAMSAP1 breaks the homeostatic microtubule network to instruct neuronal polarity

The establishment of axon/dendrite polarity is fundamental for neurons to integrate into functional circuits, and this process is critically dependent on microtubules (MTs). In the early stages of the establishment process, MTs in axons change dramatically with the morphological building of neurons; however, how the MT network changes are triggered is unclear. Here we show that CAMSAP1 plays a decisive role in the neuronal axon identification process by regulating the number of MTs. Neurons lacking CAMSAP1 form a multiple axon phenotype in vitro, while the multipolar-bipolar transition and radial migration are blocked in vivo. We demonstrate that the polarity regulator MARK2 kinase phosphorylates CAMSAP1 and affects its ability to bind to MTs, which in turn changes the protection of MT minus-ends and also triggers asymmetric distribution of MTs. Our results indicate that the polarized MT network in neurons is a decisive factor in establishing axon/dendritic polarity and is initially triggered by polarized signals.

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.

[Intrinsic and extrinsic mechanisms regulating neuronal dendrite morphogenesis]

Neurons are the structural and functional unit of the nervous system. Precisely regulated dendrite morphogenesis is the basis of neural circuit assembly. Numerous studies have been conducted to explore the regulatory mechanisms of dendritic morphogenesis. According to their action regions, we divide them into two categories: the intrinsic and extrinsic regulators of neuronal dendritic morphogenesis. Intrinsic factors are cell type-specific transcription factors, actin polymerization or depolymerization regulators and regulators of the secretion or endocytic pathways. These intrinsic factors are produced by neuron itself and play an important role in regulating the development of dendrites. The extrinsic regulators are either secreted proteins or transmembrane domain containing cell adhesion molecules. They often form receptor-ligand pairs to mediate attractive or repulsive dendritic guidance. In this review, we summarize recent findings on the intrinsic and external molecular mechanisms of dendrite morphogenesis from multiple model organisms, including Caenorhabditis elegans, Drosophila and mice. These studies will provide a better understanding on how defective dendrite development and maintenance are associated with neurological diseases.

Nucleating microtubules in neurons: Challenges and solutions

The highly polarized morphology of neurons is crucial for their function and involves formation of two distinct types of cellular extensions, the axonal and dendritic compartments. An important effector required for the morphogenesis and maintenance and thus the identity of axons and dendrites is the microtubule cytoskeleton. Microtubules in axons and dendrites are arranged with distinct polarities, to allow motor-dependent, compartment-specific sorting of cargo. Despite the importance of the microtubule cytoskeleton in neurons, the molecular mechanisms that generate the intricate compartment-specific microtubule configurations remain largely obscure. Work in other cell types has identified microtubule nucleation, the de novo formation of microtubules, and its spatio-temporal regulation as essential for the proper organization of the microtubule cytoskeleton. Whereas regulation of microtubule nucleation usually involves microtubule organizing centers such as the centrosome, neurons seem to rely largely on decentralized nucleation mechanisms. In this review, I will discuss recent advances in deciphering nucleation mechanisms in neurons, how they contribute to the arrangement of microtubules with specific polarities, and how this affects neuron morphogenesis. While this work has shed some light on these important processes, we are far from a comprehensive understanding. Thus, to provide a coherent model, my discussion will include both well-established mechanisms and mechanisms with more limited supporting data. Finally, I will also highlight important outstanding questions for future investigation.

Reducing INS-IGF1 signaling protects against non-cell autonomous vesicle rupture caused by SNCA spreading

Aging is associated with a gradual decline of cellular proteostasis, giving rise to devastating protein misfolding diseases, such as Alzheimer disease (AD) or Parkinson disease (PD). These diseases often exhibit a complex pathology involving non-cell autonomous proteotoxic effects, which are still poorly understood. Using Caenorhabditis elegans we investigated how local protein misfolding is affecting neighboring cells and tissues showing that misfolded PD-associated SNCA/α-synuclein is accumulating in highly dynamic endo-lysosomal vesicles. Irrespective of whether being expressed in muscle cells or dopaminergic neurons, accumulated proteins were transmitted into the hypodermis with increasing age, indicating that epithelial cells might play a role in remote degradation when the local endo-lysosomal degradation capacity is overloaded. Cell biological and genetic approaches revealed that inter-tissue dissemination of SNCA was regulated by endo- and exocytosis (neuron/muscle to hypodermis) and basement membrane remodeling (muscle to hypodermis). Transferred SNCA conformers were, however, inefficiently cleared and induced endo-lysosomal membrane permeabilization. Remarkably, reducing INS (insulin)-IGF1 (insulin-like growth factor 1) signaling provided protection by maintaining endo-lysosomal integrity. This study suggests that the degradation of lysosomal substrates is coordinated across different tissues in metazoan organisms. Because the chronic dissemination of poorly degradable disease proteins into neighboring tissues exerts a non-cell autonomous toxicity, this implies that restoring endo-lysosomal function not only in cells with pathological inclusions, but also in apparently unaffected cell types might help to halt disease progression.Abbreviations: AD: Alzheimer disease; BM: basement membrane; BWM: body wall muscle; CEP: cephalic sensilla; CLEM: correlative light and electron microscopy; CTNS-1: cystinosin (lysosomal protein) homolog; DA: dopaminergic; DAF-2: abnormal dauer formation; ECM: extracellular matrix; FLIM: fluorescence lifetime imaging microscopy; fps: frames per second; GFP: green fluorescent protein; HPF: high pressure freezing; IGF1: insulin-like growth factor 1; INS: insulin; KD: knockdown; LMP: lysosomal membrane permeabilization; MVB: multivesicular body; NOC: nocodazole; PD: Parkinson disease; RFP: red fluorescent protein; RNAi: RNA interference; sfGFP: superfolder GFP; SNCA: synuclein alpha; TEM: transmission electron microscopy; TNTs: tunneling nanotubes; TCSPC: time correlated single photon counting; YFP: yellow fluorescent protein.

Microtubule Minus-End Binding Protein CAMSAP2 and Kinesin-14 Motor KIFC3 Control Dendritic Microtubule Organization

Neuronal dendrites are characterized by an anti-parallel microtubule organization. The mixed oriented microtubules promote dendrite development and facilitate polarized cargo trafficking; however, the mechanism that regulates dendritic microtubule organization is still unclear. Here, we found that the kinesin-14 motor KIFC3 is important for organizing dendritic microtubules and to control dendrite development. The kinesin-14 motor proteins (Drosophila melanogaster Ncd, Saccharomyces cerevisiae Kar3, Saccharomyces pombe Pkl1, and Xenopus laevis XCTK2) are characterized by a C-terminal motor domain and are well described to organize the spindle microtubule during mitosis using an additional microtubule binding site in the N terminus [1-4]. In mammals, there are three kinesin-14 members, KIFC1, KIFC2, and KIFC3. It was recently shown that KIFC1 is important for organizing axonal microtubules in neurons, a process that depends on the two microtubule-interacting domains [5]. Unlike KIFC1, KIFC2 and KIFC3 lack the N-terminal microtubule binding domain and only have one microtubule-interacting domain, the motor domain [6, 7]. Thus, in order to regulate microtubule-microtubule crosslinking or sliding, KIFC2 and KIFC3 need to interact with additional microtubule binding proteins to connect two microtubules. We found that KIFC3 has a dendrite-specific distribution and interacts with microtubule minus-end binding protein CAMSAP2. Depletion of KIFC3 or CAMSAP2 results in increased microtubule dynamics during dendritic development. We propose a model in which CAMSAP2 anchors KIFC3 at microtubule minus ends and immobilizes microtubule arrays in dendrites.

Structural determinants of microtubule minus end preference in CAMSAP CKK domains

CAMSAP/Patronins regulate microtubule minus-end dynamics. Their end specificity is mediated by their CKK domains, which we proposed recognise specific tubulin conformations found at minus ends. To critically test this idea, we compared the human CAMSAP1 CKK domain (HsCKK) with a CKK domain from Naegleria gruberi (NgCKK), which lacks minus-end specificity. Here we report near-atomic cryo-electron microscopy structures of HsCKK- and NgCKK-microtubule complexes, which show that these CKK domains share the same protein fold, bind at the intradimer interprotofilament tubulin junction, but exhibit different footprints on microtubules. NMR experiments show that both HsCKK and NgCKK are remarkably rigid. However, whereas NgCKK binding does not alter the microtubule architecture, HsCKK remodels its microtubule interaction site and changes the underlying polymer structure because the tubulin lattice conformation is not optimal for its binding. Thus, in contrast to many MAPs, the HsCKK domain can differentiate subtly specific tubulin conformations to enable microtubule minus-end recognition.

An Antimicrobial Peptide and Its Neuronal Receptor Regulate Dendrite Degeneration in Aging and Infection

Infections have been identified as possible risk factors for aging-related neurodegenerative diseases, but it remains unclear whether infection-related immune molecules have a causative role in neurodegeneration during aging. Here, we reveal an unexpected role of an epidermally expressed antimicrobial peptide, NLP-29 (neuropeptide-like protein 29), in triggering aging-associated dendrite degeneration in C. elegans. The age-dependent increase of nlp-29 expression is regulated by the epidermal tir-1/SARM-pmk-1/p38 MAPK innate immunity pathway. We further identify an orphan G protein-coupled receptor NPR-12 (neuropeptide receptor 12) acting in neurons as a receptor for NLP-29 and demonstrate that the autophagic machinery is involved cell autonomously downstream of NPR-12 to transduce degeneration signals. Finally, we show that fungal infections cause dendrite degeneration using a similar mechanism as in aging, through NLP-29, NPR-12, and autophagy. Our findings reveal an important causative role of antimicrobial peptides, their neuronal receptors, and the autophagy pathway in aging- and infection-associated dendrite degeneration.

A tensile trilayered cytoskeletal endotube drives capillary-like lumenogenesis

Unicellular tubes are components of internal organs and capillaries. It is unclear how they meet the architectural challenge to extend a centered intracellular lumen of uniform diameter. In an RNAi-based Caenorhabditis elegans screen, we identified three intermediate filaments (IFs)-IFA-4, IFB-1, and IFC-2-as interactors of the lumenal membrane-actin linker ERM-1 in excretory-canal tubulogenesis. We find that IFs, generally thought to affect morphogenesis indirectly by maintaining tissue integrity, directly promote lumenogenesis in this capillary-like single-cell tube. We show that ERM-1, ACT-5/actin, and TBB-2/tubulin recruit membrane-forming endosomal and flux-promoting canalicular vesicles to the lumen, whereas IFs, themselves recruited to the lumen by ERM-1 and TBB-2, restrain lateral vesicle access. IFs thereby prevent cystogenesis, equilibrate the lumen diameter, and promote lumen forward extension. Genetic and imaging analyses suggest that IFB-1/IFA-4 and IFB-1/IFC-2 polymers form a perilumenal triple IF lattice, sandwiched between actin and helical tubulin. Our findings characterize a novel mechanism of capillary-like lumenogenesis, where a tensile trilayered cytoskeletal endotube transforms concentric into directional growth.

Expecto Patronin for slow and persistent minus end microtubule growth in dendrites

Microtubule plus ends are highly dynamic in neurons, while minus ends are often capped and stable. In this issue, Feng et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201810155) demonstrate that in dendrites, free minus ends undergo slow and processive growth mediated by the minus end-binding protein Patronin.

Microtubule minus-end regulation at a glance

Microtubules are cytoskeletal filaments essential for numerous aspects of cell physiology. They are polarized polymeric tubes with a fast growing plus end and a slow growing minus end. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge on the dynamics and organization of microtubule minus ends. Several factors, including the γ-tubulin ring complex, CAMSAP/Patronin, ASPM/Asp, SPIRAL2 (in plants) and the KANSL complex recognize microtubule minus ends and regulate their nucleation, stability and interactions with partners, such as microtubule severing enzymes, microtubule depolymerases and protein scaffolds. Together with minus-end-directed motors, these microtubule minus-end targeting proteins (-TIPs) also control the formation of microtubule-organizing centers, such as centrosomes and spindle poles, and mediate microtubule attachment to cellular membrane structures, including the cell cortex, Golgi complex and the cell nucleus. Structural and functional studies are starting to reveal the molecular mechanisms by which dynamic -TIP networks control microtubule minus ends.

Patronin-mediated minus end growth is required for dendritic microtubule polarity

Feng et al. describe persistent neuronal microtubule minus end growth that depends on the CAMSAP protein Patronin and is needed for dendritic minus-end-out polarity.

Microtubule minus ends are thought to be stable in cells. Surprisingly, in Drosophila and zebrafish neurons, we observed persistent minus end growth, with runs lasting over 10 min. In Drosophila, extended minus end growth depended on Patronin, and Patronin reduction disrupted dendritic minus-end-out polarity. In fly dendrites, microtubule nucleation sites localize at dendrite branch points. Therefore, we hypothesized minus end growth might be particularly important beyond branch points. Distal dendrites have mixed polarity, and reduction of Patronin lowered the number of minus-end-out microtubules. More strikingly, extra Patronin made terminal dendrites almost completely minus-end-out, indicating low Patronin normally limits minus-end-out microtubules. To determine whether minus end growth populated new dendrites with microtubules, we analyzed dendrite development and regeneration. Minus ends extended into growing dendrites in the presence of Patronin. In sum, our data suggest that Patronin facilitates sustained microtubule minus end growth, which is critical for populating dendrites with minus-end-out microtubules.

A CCRK and a MAK Kinase Modulate Cilia Branching and Length via Regulation of Axonemal Microtubule Dynamics in Caenorhabditis elegans

The diverse morphologies of primary cilia are tightly regulated as a function of cell type and cellular state. CCRK- and MAK-related kinases have been implicated in ciliary length control in multiple species, although the underlying mechanisms are not fully understood. Here, we show that in C. elegans, DYF-18/CCRK and DYF-5/MAK act in a cascade to generate the highly arborized cilia morphologies of the AWA olfactory neurons. Loss of kinase function results in dramatically elongated AWA cilia that lack branches. Intraflagellar transport (IFT) motor protein localization, but not velocities, in AWA cilia is altered upon loss of dyf-18. We instead find that axonemal microtubules are decorated by the EBP-2 end-binding protein along their lengths and that the tubulin load is increased and tubulin turnover is reduced in AWA cilia of dyf-18 mutants. Moreover, we show that predicted microtubule-destabilizing mutations in two tubulin subunits, as well as mutations in IFT proteins predicted to disrupt tubulin transport, restore cilia branching and suppress AWA cilia elongation in dyf-18 mutants. Loss of dyf-18 is also sufficient to elongate the truncated rod-like unbranched cilia of the ASH nociceptive neurons in animals carrying a microtubule-destabilizing mutation in a tubulin subunit. We suggest that CCRK and MAK activity tunes cilia length and shape in part via modulation of axonemal microtubule stability, suggesting that similar mechanisms may underlie their roles in ciliary length control in other cell types.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Mechanisms that regulate morphogenesis of a highly branched neuron in C. elegans

The shape of an individual neuron is linked to its function with axons sending signals to other cells and dendrites receiving them. Although much is known of the mechanisms for axonal outgrowth, the striking complexity of dendritic architecture has hindered efforts to uncover pathways that direct dendritic branching. Here we review the results of an experimental strategy that exploits the power of genetic analysis and live cell imaging of the PVD sensory neuron in C. elegans to reveal key molecular drivers of dendrite morphogenesis.

Microtubule regulators act in the nervous system to modulate fat metabolism and longevity through DAF-16 in C. elegans

Microtubule (MT) regulation is involved in both neuronal function and the maintenance of neuronal structure, and MT dysregulation appears to be a general downstream indicator and effector of age‐related neurodegeneration. But the role of MTs in natural aging is largely unknown. Here, we demonstrate a role of MT regulators in regulating longevity. We find that loss of EFA‐6, a modulator of MT dynamics, can delay both neuronal aging and extend the lifespan of C. elegans. Through the use of genetic mutants affecting other MT‐regulating genes in C. elegans, we find that loss of MT stabilizing genes (including ptrn‐1 and ptl‐1) shortens lifespan, while loss of MT destabilizing gene hdac‐6 extends lifespan. Via the use of tissue‐specific transgenes, we further show that these MT regulators can act in the nervous system to modulate lifespan. Through RNA‐seq analyses, we found that genes involved in lipid metabolism were differentially expressed in MT regulator mutants, and via the use of Nile Red and Oil Red O staining, we show that the MT regulator mutants have altered fat storage. We further find that the increased fat storage and extended lifespan of the long‐lived MT regulator mutants are dependent on the DAF‐16/FOXO transcription factor. Our results suggest that neuronal MT status might affect organismal aging through DAF‐16‐regulated changes in fat metabolism, and therefore, MT‐based therapies might represent a novel intervention to promote healthy aging.

Patronin governs minus-end-out orientation of dendritic microtubules to promote dendrite pruning in Drosophila

Class IV ddaC neurons specifically prune larval dendrites without affecting axons during Drosophila metamorphosis. ddaCs distribute the minus ends of microtubules (MTs) to dendrites but the plus ends to axons. However, a requirement of MT minus-end-binding proteins in dendrite-specific pruning remains completely unknown. Here, we identified Patronin, a minus-end-binding protein, for its crucial and dose-sensitive role in ddaC dendrite pruning. The CKK domain is important for Patronin’s function in dendrite pruning. Moreover, we show that both patronin knockdown and overexpression resulted in a drastic decrease of MT minus ends and a concomitant increase of plus-end-out MTs in ddaC dendrites, suggesting that Patronin stabilizes dendritic minus-end-out MTs. Consistently, attenuation of Klp10A MT depolymerase in patronin mutant neurons significantly restored minus-end-out MTs in dendrites and thereby rescued dendrite-pruning defects. Thus, our study demonstrates that Patronin orients minus-end-out MT arrays in dendrites to promote dendrite-specific pruning mainly through antagonizing Klp10A activity.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that minor issues remain unresolved (see decision letter).

Microtubule control of functional architecture in neurons

Neurons are exquisitely polarized cells whose structure and function relies on microtubules. Microtubules in signal-receiving dendrites and signal-sending axons differ in their organization and microtubule-associated proteins. These differences, coupled with microtubule post-translational modifications, combine to locally regulate intracellular transport, morphology, and function. Recent discoveries provide new insight into the regulation of non-centrosomal microtubule arrays in neurons, the relationship between microtubule acetylation and mechanosensation, and the spatial patterning of microtubules that regulates motor activity and cargo delivery in axons and dendrites. Together, these new studies bring us closer to understanding how microtubule function is locally tuned to match the specialized tasks associated with signal reception and transmission.

Atlastin-1 regulates morphology and function of endoplasmic reticulum in dendrites

Endoplasmic reticulum (ER) is characterized by interconnected tubules and sheets. Neuronal ER adopts specific morphology in axons, dendrites and soma. Here we study mechanisms underlying ER morphogenesis in a C. elegans sensory neuron PVD. In PVD soma and dendrite branch points, ER tubules connect to form networks. ER tubules fill primary dendrites but only extend to some but not all dendritic branches. We find that the Atlastin-1 ortholog, atln-1 is required for neuronal ER morphology. In atln-1 mutants with impaired GTPase activity, ER networks in soma and dendrite branch points are reduced and replaced by tubules, and ER tubules retracted from high-order dendritic branches, causing destabilized microtubule in these branches. The abnormal ER morphology likely causes defects in mitochondria fission at dendritic branch points. Mutant alleles of Atlastin-1 found in Hereditary Spastic Paraplegia (HSP) patients show similar ER phenotypes, suggesting that neuronal ER impairment contributes to HSP disease pathogenesis.

The molecular mechanisms that achieve ER morphology in neurites are not well understood. This study uses a forward genetic approach to demonstrate that atln-1 is required for neuronal ER morphology and that C. elegans atln-1 mutants exhibit defects in mitochondria fission at dendritic branch points and abnormalities in protein homeostasis.

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.

Loss of CAMSAP3 promotes EMT via the modification of microtubule-Akt machinery

Epithelial-to-mesenchymal transition (EMT) plays pivotal roles in a variety of biological processes, including cancer invasion. Although EMT involves alterations of cytoskeletal proteins such as microtubules, the role of microtubules in EMT is not fully understood. Microtubule dynamics are regulated by microtubule-binding proteins, and one such protein is CAMSAP3, which binds the minus-end of microtubules. Here, we show that CAMSAP3 is important to preserve the epithelial phenotypes in lung carcinoma cells. Deletion of CAMSAP3 in human lung carcinoma-derived cell lines showed that CAMSAP3-deficient cells acquired increased mesenchymal features, mostly at the transcriptional level. Analysis of the mechanisms underlying these changes demonstrated that tubulin acetylation was dramatically increased following CAMSAP3 removal, leading to the upregulation of Akt proteins (also known as protein kinase B proteins, hereafter Akt) activity, which is known to promote EMT. These findings suggest that CAMSAP3 functions to protect lung carcinoma cells against EMT by suppressing Akt activity via microtubule regulation and that CAMSAP3 loss promotes EMT in these cells.This article has an associated First Person interview with the first author of the paper.

CAMSAP3 maintains neuronal polarity through regulation of microtubule stability

Each neuron forms a single axon and multiple dendrites, and this configuration is important for wiring the brain. How only a single axon extends from a neuron, however, remains unknown. This study demonstrates that CAMSAP3, a protein that binds the minus-end of microtubules, preferentially localizes along axons in hippocampal neurons. Remarkably, mutations of CAMSAP3 lead to production of multiple axons in these neurons. In attempts to uncover mechanisms underlying this abnormal axon extension, the authors found that CAMSAP3-anchored microtubules escape from acetylation, a process mediated by α-tubulin acetyltransferase-1, and depletion of this enzyme abolishes abnormal axon formation in CAMSAP3 mutants. These findings reveal that CAMSAP3 controls microtubule dynamics, preventing tubulin acetylation; this mechanism is required for single-axon formation.

The molecular mechanisms that guide each neuron to become polarized, forming a single axon and multiple dendrites, remain unknown. Here we show that CAMSAP3 (calmodulin-regulated spectrin-associated protein 3), a protein that regulates the minus-end dynamics of microtubules, plays a key role in maintaining neuronal polarity. In mouse hippocampal neurons, CAMSAP3 was enriched in axons. Although axonal microtubules were generally acetylated, CAMSAP3 was preferentially localized along a less-acetylated fraction of the microtubules. CAMSAP3-mutated neurons often exhibited supernumerary axons, along with an increased number of neurites having nocodazole-resistant/acetylated microtubules compared with wild-type neurons. Analysis using cell lines showed that CAMSAP3 depletion promoted tubulin acetylation, and conversely, mild overexpression of CAMSAP3 inhibited it, suggesting that CAMSAP3 works to retain nonacetylated microtubules. In contrast, CAMSAP2, a protein related to CAMSAP3, was detected along all neurites, and its loss did not affect neuronal polarity, nor did it cause increased tubulin acetylation. Depletion of α-tubulin acetyltransferase-1 (αTAT1), the key enzyme for tubulin acetylation, abolished CAMSAP3 loss-dependent multiple-axon formation. These observations suggest that CAMSAP3 sustains a nonacetylated pool of microtubules in axons, interfering with the action of αTAT1, and this process is important to maintain neuronal polarity.

Tissue-specific degradation of essential centrosome components reveals distinct microtubule populations at microtubule organizing centers

Non-centrosomal microtubule organizing centers (ncMTOCs) are found in most differentiated cells, but how these structures regulate microtubule organization and dynamics is largely unknown. We optimized a tissue-specific degradation system to test the role of the essential centrosomal microtubule nucleators γ-tubulin ring complex (γ-TuRC) and AIR-1/Aurora A at the apical ncMTOC, where they both localize in Caenorhabditis elegans embryonic intestinal epithelial cells. As at the centrosome, the core γ-TuRC component GIP-1/GCP3 is required to recruit other γ-TuRC components to the apical ncMTOC, including MZT-1/MZT1, characterized here for the first time in animal development. In contrast, AIR-1 and MZT-1 were specifically required to recruit γ-TuRC to the centrosome, but not to centrioles or to the apical ncMTOC. Surprisingly, microtubules remain robustly organized at the apical ncMTOC upon γ-TuRC and AIR-1 co-depletion, and upon depletion of other known microtubule regulators, including TPXL-1/TPX2, ZYG-9/ch-TOG, PTRN-1/CAMSAP, and NOCA-1/Ninein. However, loss of GIP-1 removed a subset of dynamic EBP-2/EB1-marked microtubules, and the remaining dynamic microtubules grew faster. Together, these results suggest that different microtubule organizing centers (MTOCs) use discrete proteins for their function, and that the apical ncMTOC is composed of distinct populations of γ-TuRC-dependent and -independent microtubules that compete for a limited pool of resources.

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

Signaling through the dual leucine zipper-bearing kinase (DLK) is required for injured neurons to initiate new axonal growth; however, activation of this kinase also leads to neuronal degeneration and death in multiple models of injury and neurodegenerative diseases. This has spurred current consideration of DLK as a candidate therapeutic target, and raises a vital question: in what context is DLK a friend or foe to neurons? Here, we review our current understanding of DLK's function and mechanisms in regulating both regenerative and degenerative responses to axonal damage and stress in the nervous system.

PTRN-1/CAMSAP promotes CYK-1/formin-dependent actin polymerization during endocytic recycling

Cargo sorting and membrane carrier initiation in recycling endosomes require appropriately coordinated actin dynamics. However, the mechanism underlying the regulation of actin organization during recycling transport remains elusive. Here we report that the loss of PTRN-1/CAMSAP stalled actin exchange and diminished the cytosolic actin structures. Furthermore, we found that PTRN-1 is required for the recycling of clathrin-independent cargo hTAC-GFP The N-terminal calponin homology (CH) domain and central coiled-coils (CC) region of PTRN-1 can synergistically sustain the flow of hTAC-GFP We identified CYK-1/formin as a binding partner of PTRN-1. The N-terminal GTPase-binding domain (GBD) of CYK-1 serves as the binding interface for the PTRN-1 CH domain. The presence of the PTRN-1 CH domain promoted CYK-1-mediated actin polymerization, which suggests that the PTRN-1-CH:CYK-1-GBD interaction efficiently relieves autoinhibitory interactions within CYK-1. As expected, the overexpression of the CYK-1 formin homology domain 2 (FH2) substantially restored actin structures and partially suppressed the hTAC-GFP overaccumulation phenotype in ptrn-1 mutants. We conclude that the PTRN-1 CH domain is required to stimulate CYK-1 to facilitate actin dynamics during endocytic recycling.

The Arabidopsis SPIRAL2 Protein Targets and Stabilizes Microtubule Minus Ends

The contribution of microtubule tip dynamics to the assembly and function of plant microtubule arrays remains poorly understood. Here, we report that the Arabidopsis SPIRAL2 (SPR2) protein modulates the dynamics of the acentrosomal cortical microtubule plus and minus ends in an opposing manner. Live imaging of a functional SPR2-mRuby fusion protein revealed that SPR2 shows both microtubule plus- and minus-end tracking activity in addition to localization at microtubule intersections and along the lattice. Analysis of microtubule dynamics showed that cortical microtubule plus ends rarely undergo catastrophe in the spr2-2 knockout mutant compared to wild-type. In contrast, cortical microtubule minus ends in spr2-2 depolymerized at a much faster rate than in wild-type. Destabilization of the minus ends in spr2-2 caused a significant decrease in the lifetime of microtubule crossovers, which dramatically reduced the microtubule-severing frequency and inhibited light-induced microtubule array reorientation. Using in vitro reconstitution experiments combined with single-molecule imaging, we found that recombinant SPR2-GFP intrinsically localizes to microtubule minus ends, where it binds stably and inhibits their dynamics. Together, our data establish SPR2 as a new type of microtubule tip regulator that governs the length and lifetime of microtubules.

Cargo crowding at actin-rich regions along axons causes local traffic jams

Steady axonal cargo flow is central to the functioning of healthy neurons. However, a substantial fraction of cargo in axons remains stationary up to several minutes. We examine the transport of precursors of synaptic vesicles (pre-SVs), endosomes and mitochondria in Caenorhabditis elegans touch receptor neurons, showing that stationary cargo are predominantly present at actin-rich regions along the neuronal process. Stationary vesicles at actin-rich regions increase the propensity of moving vesicles to stall at the same location, resulting in traffic jams arising from physical crowding. Such local traffic jams at actin-rich regions are likely to be a general feature of axonal transport since they also occur in Drosophila neurons. Repeated touch stimulation of C. elegans reduces the density of stationary pre-SVs, indicating that these traffic jams can act as both sources and sinks of vesicles. This suggests that vesicles trapped in actin-rich regions are functional reservoirs that may contribute to maintaining robust cargo flow in the neuron. A video abstract of this article can be found at: Video S1; Video S2.