JNK-interacting protein 3 mediates the retrograde transport of activated c-Jun N-terminal kinase and lysosomes

Altered expression of vesicular trafficking machinery in prostate cancer affects lysosomal dynamics and provides insight into the underlying biology and disease progression

BACKGROUND

This study focuses on the role of lysosomal trafficking in prostate cancer, given the essential role of lysosomes in cellular homoeostasis.

METHODS

Lysosomal motility was evaluated using confocal laser scanning microscopy of LAMP-1-transfected prostate cells and spot-tracking analysis. Expression of lysosomal trafficking machinery was evaluated in patient cohort databases and through immunohistochemistry on tumour samples. The roles of vesicular trafficking machinery were evaluated through over-expression and siRNA. The effects of R1881 treatment on lysosome vesicular trafficking was evaluated by RNA sequencing, protein quantification and fixed- and live-cell microscopy.

RESULTS

Altered regulation of lysosomal trafficking genes/proteins was observed in prostate cancer tissue, with significant correlations for co-expression of vesicular trafficking machinery in Gleason patterns. The expression of trafficking machinery was associated with poorer patient outcomes. R1881 treatment induced changes in lysosomal distribution, number, and expression of lysosomal vesicular trafficking machinery in hormone-sensitive prostate cancer cells. Manipulation of genes involved in lysosomal trafficking events induced changes in lysosome positioning and cell phenotype, as well as differential effects on cell migration, in non-malignant and prostate cancer cells.

CONCLUSIONS

These findings provide novel insights into the altered regulation and functional impact of lysosomal vesicular trafficking in prostate cancer pathogenesis.

An initial HOPS-mediated fusion event is critical for autophagosome transport initiation from the axon terminal

In neurons, macroautophagy/autophagy is a frequent and critical process. In the axon, autophagy begins in the axon terminal, where most nascent autophagosomes form. After formation, autophagosomes must initiate transport to exit the axon terminal and move toward the cell body via retrograde transport. During retrograde transport these autophagosomes mature through repetitive fusion events. Complete lysosomal cargo degradation occurs largely in the cell body. The precipitating events to stimulate retrograde autophagosome transport have been debated but their importance is clear: disrupting neuronal autophagy or autophagosome transport is detrimental to neuronal health and function. We have identified the HOPS complex as essential for early autophagosome maturation and consequent initiation of retrograde transport from the axon terminal. In yeast and mammalian cells, HOPS controls fusion between autophagosomes and late endosomes with lysosomes. Using zebrafish strains with loss-of-function mutations in vps18 and vps41, core components of the HOPS complex, we found that disruption of HOPS eliminates autophagosome maturation and disrupts retrograde autophagosome transport initiation from the axon terminal. We confirmed this phenotype was due to loss of HOPS complex formation using an endogenous deletion of the HOPS binding domain in Vps18. Finally, using pharmacological inhibition of lysosomal proteases, we show that initiation of autophagosome retrograde transport requires autophagosome maturation. Together, our data demonstrate that HOPS-mediated fusion events are critical for retrograde autophagosome transport initiation through promoting autophagosome maturation. This reveals critical roles for the HOPS complex in neuronal autophagy which deepens our understanding of the cellular pathology of HOPS-complex linked neurodegenerative diseases.Abbreviations: CORVET: Class C core vacuole/endosome tethering; gRNA: guide RNA; HOPS: homotypic fusion and protein sorting; pLL: posterior lateral line; Vps18: VPS18 core subunit of CORVET and HOPS complexes; Vps41: VPS41 subunit of HOPS complex.

ALS-Linked VapB P56S Mutation Alters Neuronal Mitochondrial Turnover at the Synapse

Mitochondrial population maintenance in neurons is essential for neuron function and survival. Contact sites between mitochondria and the endoplasmic reticulum (ER) are poised to regulate mitochondrial homeostasis in neurons. These contact sites can facilitate transfer of calcium and lipids between the organelles and have been shown to regulate aspects of mitochondrial dynamics. Vesicle-associated membrane protein-associated protein B (VapB) is an ER membrane protein present at a subset of ER-mitochondrial contact sites. A proline-to-serine mutation in VapB at amino acid 56 (P56S) correlates with susceptibility to amyotrophic lateral sclerosis (ALS) type 8. Given the relationship between failed mitochondrial health and neurodegenerative disease, we investigated the function of VapB in mitochondrial population maintenance. We demonstrated that transgenic expression of VapBP56S in zebrafish larvae (sex undetermined) increased mitochondrial biogenesis, causing increased mitochondrial population size in the axon terminal. Expression of wild-type VapB did not alter biogenesis but, instead, increased mitophagy in the axon terminal. Using genetic manipulations to independently increase mitochondrial biogenesis, we show that biogenesis is normally balanced by mitophagy to maintain a constant mitochondrial population size. VapBP56S transgenics fail to increase mitophagy to compensate for the increase in mitochondrial biogenesis, suggesting an impaired mitophagic response. Finally, using a synthetic ER-mitochondrial tether, we show that VapB's function in mitochondrial turnover is likely independent of ER-mitochondrial tethering by contact sites. Our findings demonstrate that VapB can control mitochondrial turnover in the axon terminal, and this function is altered by the P56S ALS-linked mutation.

Disruptions in axonal lysosome transport and its contribution to neurological disease

Lysosomes are central to the maintenance of protein and organelle homeostasis in cells. Optimal lysosome function is particularly critical for neurons which are long-lived, non-dividing and highly polarized with specialized compartments such as axons and dendrites with distinct architecture, cargo, and turnover requirements. In recent years, there has been a growing appreciation for the role played by axonal lysosome transport in regulating neuronal development, its maintenance and functioning. Perturbations to optimal axonal lysosome abundance leading to either strong accumulations or dearth of lysosomes are both linked to altered neuronal health and functioning. In this review we highlight how two critical regulators of axonal lysosome transport and abundance, the small GTPase Arl8 and the adaptor protein JIP3, aid in maintaining axonal lysosome homeostasis and how alterations to their levels and activity could contribute to neurodevelopmental and neurodegenerative diseases.

A decrease in Fkbp52 alters autophagosome maturation and A152T-tau clearance in vivo

The failure of the autophagy-lysosomal pathway to clear the pathogenic forms of Tau exacerbates the pathogenesis of tauopathies. We have previously shown that the immunophilin FKBP52 interacts both physically and functionally with Tau, and that a decrease in FKBP52 protein levels is associated with Tau deposition in affected human brains. We have also shown that FKBP52 is physiologically present within the lysosomal system in healthy human neurons and that a decrease in FKBP52 expression alters perinuclear lysosomal positioning and Tau clearance during Tau-induced proteotoxic stress in vitro. In this study, we generate a zebrafish fkbp4 loss of function mutant and show that axonal retrograde trafficking of Lamp1 vesicles is altered in this mutant. Moreover, using our transgenic HuC::mCherry-EGFP-LC3 line, we demonstrate that the autophagic flux is impaired in fkbp4 mutant embryos, suggesting a role for Fkbp52 in the maturation of autophagic vesicles. Alterations in both axonal transport and autophagic flux are more evident in heterozygous rather than homozygous fkbp4 mutants. Finally, taking advantage of the previously described A152T-Tau transgenic fish, we show that the clearance of pathogenic A152T-Tau mutant proteins is slower in fkbp4 +/- mutants in comparison to fkbp4 +/+ larvae. Altogether, these results indicate that Fkbp52 is required for the normal trafficking and maturation of lysosomes and autophagic vacuoles along axons, and that its decrease is sufficient to hinder the clearance of pathogenic Tau in vivo.

Perspectives on the Novel Multifunctional Nerve Guidance Conduits: From Specific Regenerative Procedures to Motor Function Rebuilding

Peripheral nerve injury potentially destroys the quality of life by inducing functional movement disorders and sensory capacity loss, which results in severe disability and substantial psychological, social, and financial burdens. Autologous nerve grafting has been commonly used as treatment in the clinic; however, its rare donor availability limits its application. A series of artificial nerve guidance conduits (NGCs) with advanced architectures are also proposed to promote injured peripheral nerve regeneration, which is a complicated process from axon sprouting to targeted muscle reinnervation. Therefore, exploring the interactions between sophisticated NGC complexes and versatile cells during each process including axon sprouting, Schwann cell dedifferentiation, nerve myelination, and muscle reinnervation is necessary. This review highlights the contribution of functional NGCs and the influence of microscale biomaterial architecture on biological processes of nerve repair. Progressive NGCs with chemical molecule induction, heterogenous topographical morphology, electroactive, anisotropic assembly microstructure, and self-powered electroactive and magnetic-sensitive NGCs are also collected, and they are expected to be pioneering features in future multifunctional and effective NGCs.

Axonal organelle buildup from loss of AP-4 complex function causes exacerbation of amyloid plaque pathology and gliosis in Alzheimer's disease mouse model

Lysosomes and related precursor organelles robustly build up in swollen axons that surround amyloid plaques and disrupted axonal lysosome transport has been implicated in worsening Alzheimer's pathology. Our prior studies have revealed that loss of Adaptor protein-4 (AP-4) complex function, linked primarily to Spastic Paraplegia (HSP), leads to a similar build of lysosomes in structures we term "AP-4 dystrophies". Surprisingly, these AP-4 dystrophies were also characterized by enrichment of components of APP processing machinery, β-site cleaving enzyme 1 (BACE1) and Presenilin 2. Our studies examining whether the abnormal axonal lysosome build up resulting from AP-4 loss could lead to amyloidogenesis revealed that the loss of AP-4 complex function in an Alzheimer's disease model resulted in a strong increase in size and abundance of amyloid plaques in the hippocampus and corpus callosum as well as increased microglial association with the plaques. Interestingly, we found a further increase in enrichment of the secretase, BACE1, in the axonal swellings of the plaques of Alzheimer model mice lacking AP-4 complex compared to those having normal AP-4 complex function, suggestive of increased amyloidogenic processing under this condition. Additionally, the exacerbation of plaque pathology was region-specific as it did not increase in the cortex. The burden of the AP-4 linked axonal dystrophies/AP-4 dystrophies was higher in the corpus callosum and hippocampus compared to the cortex, establishing the critical role of AP-4 -dependent axonal lysosome transport and maturation in regulating amyloidogenic amyloid precursor protein processing.

Molecular mechanism of dynein-dynactin complex assembly by LIS1

Cytoplasmic dynein is a microtubule motor vital for cellular organization and division. It functions as a ~4-megadalton complex containing its cofactor dynactin and a cargo-specific coiled-coil adaptor. However, how dynein and dynactin recognize diverse adaptors, how they interact with each other during complex formation, and the role of critical regulators such as lissencephaly-1 (LIS1) protein (LIS1) remain unclear. In this study, we determined the cryo-electron microscopy structure of dynein-dynactin on microtubules with LIS1 and the lysosomal adaptor JIP3. This structure reveals the molecular basis of interactions occurring during dynein activation. We show how JIP3 activates dynein despite its atypical architecture. Unexpectedly, LIS1 binds dynactin's p150 subunit, tethering it along the length of dynein. Our data suggest that LIS1 and p150 constrain dynein-dynactin to ensure efficient complex formation.

Structural and functional characterization of the IgSF21-neurexin2α complex and its related signaling pathways in the regulation of inhibitory synapse organization

The prevailing model behind synapse development and specificity is that a multitude of adhesion molecules engage in transsynaptic interactions to induce pre- and postsynaptic assembly. How these extracellular interactions translate into intracellular signal transduction for synaptic assembly remains unclear. Here, we focus on a synapse organizing complex formed by immunoglobulin superfamily member 21 (IgSF21) and neurexin2α (Nrxn2α) that regulates GABAergic synapse development in the mouse brain. We reveal that the interaction between presynaptic Nrxn2α and postsynaptic IgSF21 is a high-affinity receptor-ligand interaction and identify a binding interface in the IgSF21-Nrxn2α complex. Despite being expressed in both dendritic and somatic regions, IgSF21 preferentially regulates dendritic GABAergic presynaptic differentiation whereas another canonical Nrxn ligand, neuroligin2 (Nlgn2), primarily regulates perisomatic presynaptic differentiation. To explore mechanisms that could underlie this compartment specificity, we targeted multiple signaling pathways pharmacologically while monitoring the synaptogenic activity of IgSF21 and Nlgn2. Interestingly, both IgSF21 and Nlgn2 require c-jun N-terminal kinase (JNK)-mediated signaling, whereas Nlgn2, but not IgSF21, additionally requires CaMKII and Src kinase activity. JNK inhibition diminished de novo presynaptic differentiation without affecting the maintenance of formed synapses. We further found that Nrxn2α knockout brains exhibit altered synaptic JNK activity in a sex-specific fashion, suggesting functional linkage between Nrxns and JNK. Thus, our study elucidates the structural and functional relationship of IgSF21 with Nrxn2α and distinct signaling pathways for IgSF21-Nrxn2α and Nlgn2-Nrxn synaptic organizing complexes in vitro. We therefore propose a revised hypothesis that Nrxns act as molecular hubs to specify synaptic properties not only through their multiple extracellular ligands but also through distinct intracellular signaling pathways of these ligands.

Stat3 Has a Different Role in Axon Growth During Development Than It Does in Axon Regeneration After Injury

Signal transducer and activator of transcription 3 (STAT3) is essential for neural development and regeneration as a key transcription factor and mitochondrial activator. However, the mechanism of Stat3 in axon development and regeneration has not been fully understood. In this study, using zebrafish posterior lateral line (PLL) axons, we demonstrate that Stat3 plays distinct roles in PLL axon embryonic growth and regeneration. Our experiments indicate that stat3 is required for PLL axon extension. In stat3 mutant zebrafish, the PLL axon ends were stalled at the level of the cloaca, and expression of stat3 rescues the PLL axon growth in a cell-autonomous manner. Jak/Stat signaling inhibition did not affect PLL axon growth indicating Jak/Stat was dispensable for PLL axon growth. In addition, we found that Stat3 was co-localized with mitochondria in PLL axons and important for the mitochondrial membrane potential and ATPase activity. The PLL axon growth defect of stat3 mutants was mimicked and rescued by rotenone and DCHC treatment, respectively, which suggests that Stat3 regulates PLL axon growth through mitochondrial Stat3. By contrast, mutation of stat3 or Jak/Stat signaling inhibition retarded PLL axon regeneration. Meanwhile, we also found Schwann cell migration was also inhibited in stat3 mutants. Taken together, Stat3 is required for embryonic PLL axon growth by regulating the ATP synthesis efficiency of mitochondria, whereas Stat3 stimulates PLL axon regeneration by regulating Schwann cell migration via Jak/Stat signaling. Our findings show a new mechanism of Stat3 in axon growth and regeneration.

Neuronal Autophagy: Regulations and Implications in Health and Disease

Autophagy is a major degradative pathway that plays a key role in sustaining cell homeostasis, integrity, and physiological functions. Macroautophagy, which ensures the clearance of cytoplasmic components engulfed in a double-membrane autophagosome that fuses with lysosomes, is orchestrated by a complex cascade of events. Autophagy has a particularly strong impact on the nervous system, and mutations in core components cause numerous neurological diseases. We first review the regulation of autophagy, from autophagosome biogenesis to lysosomal degradation and associated neurodevelopmental/neurodegenerative disorders. We then describe how this process is specifically regulated in the axon and in the somatodendritic compartment and how it is altered in diseases. In particular, we present the neuronal specificities of autophagy, with the spatial control of autophagosome biogenesis, the close relationship of maturation with axonal transport, and the regulation by synaptic activity. Finally, we discuss the physiological functions of autophagy in the nervous system, during development and in adulthood.

Axonal transport of autophagosomes is regulated by dynein activators JIP3/JIP4 and ARF/RAB GTPases

Neuronal autophagosomes form and engulf cargos at presynaptic sites in the axon and are then transported to the soma to recycle their cargo. Autophagic vacuoles (AVs) mature en route via fusion with lysosomes to become degradatively competent organelles; transport is driven by the microtubule motor protein cytoplasmic dynein, with motor activity regulated by a sequential series of adaptors. Using lysate-based single-molecule motility assays and live-cell imaging in primary neurons, we show that JNK-interacting proteins 3 (JIP3) and 4 (JIP4) are activating adaptors for dynein that are regulated on autophagosomes and lysosomes by the small GTPases ARF6 and RAB10. GTP-bound ARF6 promotes formation of the JIP3/4-dynein-dynactin complex. Either knockdown or overexpression of RAB10 stalls transport, suggesting that this GTPase is also required to coordinate the opposing activities of bound dynein and kinesin motors. These findings highlight the complex coordination of motor regulation during organelle transport in neurons.

Pioneer statoacoustic neurons guide neuroblast behaviour during otic ganglion assembly

Cranial ganglia are aggregates of sensory neurons that mediate distinct types of sensation. The statoacoustic ganglion (SAG) develops into several lobes that are spatially arranged to connect appropriately with hair cells of the inner ear. To investigate the cellular behaviours involved in the 3D organization of the SAG, we use high-resolution confocal imaging of single-cell, labelled zebrafish neuroblasts (NBs), photoconversion, photoablation, and genetic perturbations. We show that otic NBs delaminate out of the otic epithelium in an epithelial-mesenchymal transition-like manner, rearranging apical polarity and primary cilia proteins. We also show that, once delaminated, NBs require RhoGTPases in order to perform active migration. Furthermore, tracking of recently delaminated NBs revealed their directed migration and coalescence around a small population of pioneer SAG neurons. These pioneer SAG neurons, not from otic placode origin, populate the coalescence region before otic neurogenesis begins and their ablation disrupts delaminated NB migratory pathways, consequentially affecting SAG shape. Altogether, this work shows for the first time the role of pioneer SAG neurons in orchestrating SAG development.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

Axonal transport of autophagosomes is regulated by dynein activators JIP3/JIP4 and ARF/RAB GTPases

Neuronal autophagosomes, "self-eating" degradative organelles, form at presynaptic sites in the distal axon and are transported to the soma to recycle their cargo. During transit, autophagic vacuoles (AVs) mature through fusion with lysosomes to acquire the enzymes necessary to breakdown their cargo. AV transport is driven primarily by the microtubule motor cytoplasmic dynein in concert with dynactin and a series of activating adaptors that change depending on organelle maturation state. The transport of mature AVs is regulated by the scaffolding proteins JIP3 and JIP4, both of which activate dynein motility in vitro. AV transport is also regulated by ARF6 in a GTP-dependent fashion. While GTP-bound ARF6 promotes the formation of the JIP3/4-dynein-dynactin complex, RAB10 competes with the activity of this complex by increasing kinesin recruitment to axonal AVs and lysosomes. These interactions highlight the complex coordination of motors regulating organelle transport in neurons.

shox2 is required for vestibular statoacoustic neuron development

Homeobox genes act at the top of genetic hierarchies to regulate cell specification and differentiation during embryonic development. We identified the short stature homeobox domain 2 (shox2) transcription factor that is required for vestibular neuron development. shox2 transcripts are initially localized to the otic placode of the developing inner ear where neurosensory progenitors reside. To study shox2 function, we generated CRISPR-mediated mutant shox2 fish. Mutant embryos display behaviors associated with vestibular deficits and showed reduced number of anterior statoacoustic ganglion neurons that innervate the utricle, the vestibular organ in zebrafish. Moreover, a shox2-reporter fish showed labeling of developing statoacoustic ganglion neurons in the anterior macula of the otic vesicle. Single cell RNA-sequencing of cells from the developing otic vesicle of shox2 mutants revealed altered otic progenitor profiles, while single molecule in situ assays showed deregulated levels of transcripts in developing neurons. This study implicates a role for shox2 in development of vestibular but not auditory statoacoustic ganglion neurons.

Common mechanisms underlying axonal transport deficits in neurodegenerative diseases: a mini review

Many neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis are characterized by the accumulation of pathogenic proteins and abnormal localization of organelles. These pathological features may be related to axonal transport deficits in neurons, which lead to failures in pathological protein targeting to specific sites for degradation and organelle transportation to designated areas needed for normal physiological functioning. Axonal transport deficits are most likely early pathological events in such diseases and gradually lead to the loss of axonal integrity and other degenerative changes. In this review, we investigated reports of mechanisms underlying the development of axonal transport deficits in a variety of common neurodegenerative diseases, such as Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease and Huntington's disease to provide new ideas for therapeutic targets that may be used early in the disease process. The mechanisms can be summarized as follows: (1) motor protein changes including expression levels and post-translational modification alteration; (2) changes in microtubules including reducing stability and disrupting tracks; (3) changes in cargoes including diminished binding to motor proteins. Future studies should determine which axonal transport defects are disease-specific and whether they are suitable therapeutic targets in neurodegenerative diseases.

NudC regulated Lis1 stability is essential for the maintenance of dynamic microtubule ends in axon terminals

In the axon terminal, microtubule stability is decreased relative to the axon shaft. The dynamic microtubule plus ends found in the axon terminal have many functions, including serving as a docking site for the Cytoplasmic dynein motor. Here, we report an unexplored function of dynein in microtubule regulation in axon terminals: regulation of microtubule stability. Using a forward genetic screen, we identified a mutant with an abnormal axon terminal structure owing to a loss of function mutation in NudC. We show that, in the axon terminal, NudC is a chaperone for the protein Lis1. Decreased Lis1 in nudc axon terminals causes dynein/dynactin accumulation and increased microtubule stability. Microtubule dynamics can be restored by pharmacologically inhibiting dynein, implicating excess dynein motor function in microtubule stabilization. Together, our data support a model in which local NudC-Lis1 modulation of the dynein motor is critical for the regulation of microtubule stability in the axon terminal.

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NudC, a dynein regulator, is crucial for axon terminal structure

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NudC mutation leads to a near complete loss of Lis1 protein in axon terminals

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Lis1 deficits cause accumulation of dynein and cargo in axon terminals

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Local elevation of dynein increases axon terminal microtubule stability

Transport-dependent maturation of organelles in neurons

Some organelles show a spatial gradient of maturation along the neuronal process where more mature organelles are found closer to the cell body. This gradient is set up by progressive maturation steps that are aided by differential organelle distribution as well as transport. Autophagosomes and endosomes mature as they acquire lysosomal membrane proteins and decrease their luminal pH as they are retrogradely transported towards the cell body. The acquisition of lysosomal proteins along the neuronal processes likely occurs through fusion or membrane exchange events with Golgi-derived donor transport carriers that are transported anterogradely from the cell body. The mechanisms by which endosomes and autophagosomes mature might be applicable to other organelles that are transported along neuronal processes. Defects in axonal transport may also contribute to the accumulation of immature organelles in neurons. Such accumulations have been seen in neurons of neurodegenerative models.

The ARSACS disease protein sacsin controls lysosomal positioning and reformation by regulating microtubule dynamics

Autosomal recessive spastic ataxia of Charlevoix-Saguenay is a fatal brain disorder featuring cerebellar neurodegeneration leading to spasticity and ataxia. This disease is caused by mutations in the SACS gene that encodes sacsin, a massive 4579-amino acid protein with multiple modular domains. However, molecular details of the function of sacsin are not clear. Here, using live cell imaging and biochemistry, we demonstrate that sacsin binds to microtubules and regulates microtubule dynamics. Loss of sacsin function in various cell types, including knockdown and KO primary neurons and patient fibroblasts, leads to alterations in lysosomal transport, positioning, function, and reformation following autophagy. Each of these phenotypic changes is consistent with altered microtubule dynamics. We further show the effects of sacsin are mediated at least in part through interactions with JIP3, an adapter for microtubule motors. These data reveal a new function for sacsin that explains its previously reported roles and phenotypes.

JIP3 interacts with dynein and kinesin-1 to regulate bidirectional organelle transport

The MAP kinase and motor scaffold JIP3 prevents excess lysosome accumulation in axons of vertebrates and invertebrates. How JIP3's interaction with dynein and kinesin-1 contributes to organelle clearance is unclear. We show that human dynein light intermediate chain (DLIC) binds the N-terminal RH1 domain of JIP3, its paralog JIP4, and the lysosomal adaptor RILP. A point mutation in RH1 abrogates DLIC binding without perturbing the interaction between JIP3's RH1 domain and kinesin heavy chain. Characterization of this separation-of-function mutation in Caenorhabditis elegans shows that JIP3-bound dynein is required for organelle clearance in the anterior process of touch receptor neurons. Unlike JIP3 null mutants, JIP3 that cannot bind DLIC causes prominent accumulation of endo-lysosomal organelles at the neurite tip, which is rescued by a disease-associated point mutation in JIP3's leucine zipper that abrogates kinesin light chain binding. These results highlight that RH1 domains are interaction hubs for cytoskeletal motors and suggest that JIP3-bound dynein and kinesin-1 participate in bidirectional organelle transport.

Loss of MAPK8IP3 Affects Endocytosis in Neurons

Perturbations in endo-lysosomal trafficking pathways are linked to many neurodevelopmental and neurodegenerative diseases. Of relevance to our current study, MAPK8IP3/JIP3, a brain enriched putative adaptor between lysosomes and motors has been previously implicated as a key regulator of axonal lysosome transport. Since de novo variants in MAPK8IP3 have recently been linked to a neurodevelopmental disorder with intellectual disability, there is a need to better understand the functioning of this protein in human neurons. To this end, using induced neurons (i3Neurons) derived from human iPSCs lacking MAPK8IP3, we demonstrate that loss of hMAPK8IP3 affects endocytic uptake in neurons but does not affect the proteolytic activity of lysosomes in neuronal cell bodies. Our findings indicate that MAPK8IP3 may be a regulator of bulk endocytosis in neurons and that altered endocytic uptake may play a role in MAPK8IP3-linked neurodevelopmental disorders.

Axonal transport of late endosomes and amphisomes is selectively modulated by local Ca2+ efflux and disrupted by PSEN1 loss of function

Dysfunction and mistrafficking of organelles in autophagy- and endosomal-lysosomal pathways are implicated in neurodegenerative diseases. Here, we reveal selective vulnerability of maturing degradative organelles (late endosomes/amphisomes) to disease-relevant local calcium dysregulation. These organelles undergo exclusive retrograde transport in axons, with occasional pauses triggered by regulated calcium efflux from agonist-evoked transient receptor potential cation channel mucolipin subfamily member 1 (TRPML1) channels-an effect greatly exaggerated by exogenous agonist mucolipin synthetic agonist 1 (ML-SA1). Deacidification of degradative organelles, as seen after Presenilin 1 (PSEN1) loss of function, induced pathological constitutive "inside-out" TRPML1 hyperactivation, slowing their transport comparably to ML-SA1 and causing accumulation in dystrophic axons. The mechanism involved calcium-mediated c-Jun N-terminal kinase (JNK) activation, which hyperphosphorylated dynein intermediate chain (DIC), reducing dynein activity. Blocking TRPML1 activation, JNK activity, or DIC1B serine-80 phosphorylation reversed transport deficits in PSEN1 knockout neurons. Our results, including features demonstrated in Alzheimer-mutant PSEN1 knockin mice, define a mechanism linking dysfunction and mistrafficking in lysosomal pathways to neuritic dystrophy under neurodegenerative conditions.

A Model of Discovery: The Role of Imaging Established and Emerging Non-mammalian Models in Neuroscience

Rodents have been the dominant animal models in neurobiology and neurological disease research over the past 60 years. The prevalent use of rats and mice in neuroscience research has been driven by several key attributes including their organ physiology being more similar to humans, the availability of a broad variety of behavioral tests and genetic tools, and widely accessible reagents. However, despite the many advances in understanding neurobiology that have been achieved using rodent models, there remain key limitations in the questions that can be addressed in these and other mammalian models. In particular, in vivo imaging in mammals at the cell-resolution level remains technically difficult and demands large investments in time and cost. The simpler nervous systems of many non-mammalian models allow for precise mapping of circuits and even the whole brain with impressive subcellular resolution. The types of non-mammalian neuroscience models available spans vertebrates and non-vertebrates, so that an appropriate model for most cell biological questions in neurodegenerative disease likely exists. A push to diversify the models used in neuroscience research could help address current gaps in knowledge, complement existing rodent-based bodies of work, and bring new insight into our understanding of human disease. Moreover, there are inherent aspects of many non-mammalian models such as lifespan and tissue transparency that can make them specifically advantageous for neuroscience studies. Crispr/Cas9 gene editing and decreased cost of genome sequencing combined with advances in optical microscopy enhances the utility of new animal models to address specific questions. This review seeks to synthesize current knowledge of established and emerging non-mammalian model organisms with advances in cellular-resolution in vivo imaging techniques to suggest new approaches to understand neurodegeneration and neurobiological processes. We will summarize current tools and in vivo imaging approaches at the single cell scale that could help lead to increased consideration of non-mammalian models in neuroscience research.

Palmitoylation couples the kinases DLK and JNK3 to facilitate prodegenerative axon-to-soma signaling

Dual leucine-zipper kinase (DLK; a MAP3K) mediates neuronal responses to diverse injuries and insults through the c-Jun N-terminal kinase (JNK) family of mitogen-activated protein kinases (MAPKs). Here, we identified two ways through which DLK is coupled to the neural-specific isoform JNK3 to control prodegenerative signaling. JNK3 catalyzed positive feedback phosphorylation of DLK that further activated DLK and locked the DLK-JNK3 module in a highly active state. Neither homologous MAP3Ks nor a homologous MAPK could support this positive feedback loop. Unlike the related JNK1 isoform JNK2 and JNK3 promote prodegenerative axon-to-soma signaling and were endogenously palmitoylated. Moreover, palmitoylation targeted both DLK and JNK3 to the same axonal vesicles, and JNK3 palmitoylation was essential for axonal retrograde signaling in response to optic nerve crush injury in vivo. These findings provide previously unappreciated insights into DLK-JNK signaling relevant to neuropathological conditions and answer long-standing questions regarding the selective prodegenerative roles of JNK2 and JNK3.

RUFY3 and RUFY4 are ARL8 effectors that promote coupling of endolysosomes to dynein-dynactin

The small GTPase ARL8 associates with endolysosomes, leading to the recruitment of several effectors that couple endolysosomes to kinesins for anterograde transport along microtubules, and to tethering factors for eventual fusion with other organelles. Herein we report the identification of the RUN- and FYVE-domain-containing proteins RUFY3 and RUFY4 as ARL8 effectors that promote coupling of endolysosomes to dynein-dynactin for retrograde transport along microtubules. Using various methodologies, we find that RUFY3 and RUFY4 interact with both GTP-bound ARL8 and dynein-dynactin. In addition, we show that RUFY3 and RUFY4 promote concentration of endolysosomes in the juxtanuclear area of non-neuronal cells, and drive redistribution of endolysosomes from the axon to the soma in hippocampal neurons. The function of RUFY3 in retrograde transport contributes to the juxtanuclear redistribution of endolysosomes upon cytosol alkalinization. These studies thus identify RUFY3 and RUFY4 as ARL8-dependent, dynein-dynactin adaptors or regulators, and highlight the role of ARL8 in the control of both anterograde and retrograde endolysosome transport.

Neuronal endolysosomal transport and lysosomal functionality in maintaining axonostasis

Lysosomes serve as degradation hubs for the turnover of endocytic and autophagic cargos, which is essential for neuron function and survival. Deficits in lysosome function result in progressive neurodegeneration in most lysosomal storage disorders and contribute to the pathogenesis of aging-related neurodegenerative diseases. Given their size and highly polarized morphology, neurons face exceptional challenges in maintaining cellular homeostasis in regions far removed from the cell body where mature lysosomes are enriched. Neurons therefore require coordinated bidirectional intracellular transport to sustain efficient clearance capacity in distal axonal regions. Emerging lines of evidence have started to uncover mechanisms and signaling pathways regulating endolysosome transport and maturation to maintain axonal homeostasis, or “axonostasis,” that is relevant to a range of neurologic disorders. In this review, we discuss recent advances in how axonal endolysosomal trafficking, distribution, and lysosomal functionality support neuronal health and become disrupted in several neurodegenerative diseases.

JIP3 links lysosome transport to regulation of multiple components of the axonal cytoskeleton

Lysosome axonal transport is important for the clearance of cargoes sequestered by the endocytic and autophagic pathways. Building on observations that mutations in the JIP3 (MAPK8IP3) gene result in lysosome-filled axonal swellings, we analyzed the impact of JIP3 depletion on the cytoskeleton of human neurons. Dynamic focal lysosome accumulations were accompanied by disruption of the axonal periodic scaffold (spectrin, F-actin and myosin II) throughout each affected axon. Additionally, axonal microtubule organization was locally disrupted at each lysosome-filled swelling. This local axonal microtubule disorganization was accompanied by accumulations of both F-actin and myosin II. These results indicate that transport of axonal lysosomes is functionally interconnected with mechanisms that control the organization and maintenance of the axonal cytoskeleton. They have potential relevance to human neurological disease arising from JIP3 mutations as well as for neurodegenerative diseases associated with the focal accumulations of lysosomes within axonal swellings such as Alzheimer’s disease.

Rafiq et al. report that disruption of JIP3-dependent control of axonal lysosome transport in human neurons results in unexpected changes to the organization of multiple cytoskeletal proteins. This study provides new insights that improve our understanding of intellectual disabilities caused by mutations in JIP3, and are relevant for neurodegenerative diseases associated with accumulations of lysosomes such as the Alzheimer’s disease

Doublecortin and JIP3 are neural-specific counteracting regulators of dynein-mediated retrograde trafficking

Mutations in the microtubule (MT)-binding protein doublecortin (DCX) or in the MT-based molecular motor dynein result in lissencephaly. However, a functional link between DCX and dynein has not been defined. Here, we demonstrate that DCX negatively regulates dynein-mediated retrograde transport in neurons from Dcx^-/y or Dcx^-/y;Dclk1^-/- mice by reducing dynein's association with MTs and disrupting the composition of the dynein motor complex. Previous work showed an increased binding of the adaptor protein C-Jun-amino-terminal kinase-interacting protein 3 (JIP3) to dynein in the absence of DCX. Using purified components, we demonstrate that JIP3 forms an active motor complex with dynein and its cofactor dynactin with two dyneins per complex. DCX competes with the binding of the second dynein, resulting in a velocity reduction of the complex. We conclude that DCX negatively regulates dynein-mediated retrograde transport through two critical interactions by regulating dynein binding to MTs and regulating the composition of the dynein motor complex.

A Compendium of Information on the Lysosome

For a long time, lysosomes were considered as mere waste bags for cellular constituents. Thankfully, studies carried out in the past 15 years were brimming with elegant and crucial breakthroughs in lysosome research, uncovering their complex roles as nutrient sensors and characterizing them as crucial multifaceted signaling organelles. This review presents the scientific knowledge on lysosome physiology and functions, starting with their discovery and reviewing up to date ground-breaking discoveries highlighting their heterogeneous functions as well as pending questions that remain to be answered. We also review the roles of lysosomes in anti-cancer drug resistance and how they undergo a series of molecular and functional changes during malignant transformation which lead to tumor aggression, angiogenesis, and metastases. Finally, we discuss the strategy of targeting lysosomes in cancer which could lead to the development of new and effective targeted therapies.

RUSC2 and WDR47 oppositely regulate kinesin-1-dependent distribution of ATG9A to the cell periphery

Autophagy-related protein 9 (ATG9) is a transmembrane protein component of the autophagy machinery that cycles between the trans-Golgi network (TGN) in the perinuclear area and other compartments in the peripheral area of the cell. In mammalian cells, export of the ATG9A isoform from the TGN into ATG9A-containing vesicles is mediated by the adaptor protein 4 (AP-4) complex. However, the mechanisms responsible for the subsequent distribution of these vesicles to the cell periphery are unclear. Herein we show that the AP-4-accessory protein RUSC2 couples ATG9A-containing vesicles to the plus-end-directed microtubule motor kinesin-1 via an interaction between a disordered region of RUSC2 and the kinesin-1 light chain. This interaction is counteracted by the microtubule-associated protein WDR47. These findings uncover a mechanism for the peripheral distribution of ATG9A-containing vesicles involving the function of RUSC2 as a kinesin-1 adaptor and WDR47 as a negative regulator of this function.

c-Jun N-Terminal Kinases in Alzheimer's Disease: A Possible Target for the Modulation of the Earliest Alterations

Given the highly multifactorial origin of Alzheimer's disease (AD) neuropathology, disentangling and orderly knowing mechanisms involved in sporadic onset are arduous. Nevertheless, when the elements involved are dissected into smaller pieces, the task becomes more accessible. This review aimed to describe the link between c-Jun N-terminal Kinases (JNKs), master regulators of many cellular functions, and the early alterations of AD: synaptic loss and dysregulation of neuronal transport. Both processes have a role in the posterior cognitive decline observed in AD. The manuscript focuses on the molecular mechanisms of glutamatergic, GABA, and cholinergic synapses altered by the presence of amyloid-β aggregates and hyperphosphorylated tau, as well as on several consequences of the disruption of cellular processes linked to neuronal transport that is controlled by the JNK-JIP (c-jun NH2-terminal kinase (JNK)-interacting proteins (JIPs) complex, including the transport of AβPP or autophagosomes.

How Zebrafish Can Drive the Future of Genetic-based Hearing and Balance Research

Over the last several decades, studies in humans and animal models have successfully identified numerous molecules required for hearing and balance. Many of these studies relied on unbiased forward genetic screens based on behavior or morphology to identify these molecules. Alongside forward genetic screens, reverse genetics has further driven the exploration of candidate molecules. This review provides an overview of the genetic studies that have established zebrafish as a genetic model for hearing and balance research. Further, we discuss how the unique advantages of zebrafish can be leveraged in future genetic studies. We explore strategies to design novel forward genetic screens based on morphological alterations using transgenic lines or behavioral changes following mechanical or acoustic damage. We also outline how recent advances in CRISPR-Cas9 can be applied to perform reverse genetic screens to validate large sequencing datasets. Overall, this review describes how future genetic studies in zebrafish can continue to advance our understanding of inherited and acquired hearing and balance disorders.

Sequential dynein effectors regulate axonal autophagosome motility in a maturation-dependent pathway

Autophagy is a degradative pathway required to maintain homeostasis. Neuronal autophagosomes form constitutively at the axon terminal and mature via lysosomal fusion during dynein-mediated transport to the soma. How the dynein–autophagosome interaction is regulated is unknown. Here, we identify multiple dynein effectors on autophagosomes as they transit along the axons of primary neurons. In the distal axon, JIP1 initiates autophagosomal transport. Autophagosomes in the mid-axon require HAP1 and Huntingtin. We find that HAP1 is a dynein activator, binding the dynein–dynactin complex via canonical and noncanonical interactions. JIP3 is on most axonal autophagosomes, but specifically regulates the transport of mature autolysosomes. Inhibiting autophagosomal transport disrupts maturation, and inhibiting autophagosomal maturation perturbs the association and function of dynein effectors; thus, maturation and transport are tightly linked. These results reveal a novel maturation-based dynein effector handoff on neuronal autophagosomes that is key to motility, cargo degradation, and the maintenance of axonal health.

Cytoskeletal regulation guides neuronal trafficking to effectively supply the synapse

The development and proper function of the brain requires the formation of highly complex neuronal circuitry. These circuits are shaped from synaptic connections between neurons and must be maintained over a lifetime. The formation and continued maintenance of synapses requires accurate trafficking of presynaptic and postsynaptic components along the axon and dendrite, respectively, necessitating deliberate and specialized delivery strategies to replenish essential synaptic components. Maintenance of synaptic transmission also requires readily accessible energy stores, produced in part by localized mitochondria, that are tightly regulated with activity level. In this review, we focus on recent developments in our understanding of the cytoskeletal environment of axons and dendrites, examining how local regulation of cytoskeletal dynamics and organelle trafficking promotes synapse-specific delivery and plasticity. These new insights shed light on the complex and coordinated role that cytoskeletal elements play in establishing and maintaining neuronal circuitry.

Overlapping roles of JIP3 and JIP4 in promoting axonal transport of lysosomes in human iPSC-derived neurons

The dependence of neurons on microtubule-based motors for the movement of lysosomes over long distances raises questions about adaptations that allow neurons to meet these demands. Recently, JIP3/MAPK8IP3, a neuronally enriched putative adaptor between lysosomes and motors, was identified as a critical regulator of axonal lysosome abundance. In this study, we establish a human induced pluripotent stem cell (iPSC)-derived neuron model for the investigation of axonal lysosome transport and maturation and show that loss of JIP3 results in the accumulation of axonal lysosomes and the Alzheimer’s disease–related amyloid precursor protein (APP)-derived Aβ42 peptide. We furthermore reveal an overlapping role of the homologous JIP4 gene in lysosome axonal transport. These results establish a cellular model for investigating the relationship between lysosome axonal transport and amyloidogenic APP processing and more broadly demonstrate the utility of human iPSC–derived neurons for the investigation of neuronal cell biology and pathology.

FOXO Regulates Neuromuscular Junction Homeostasis During Drosophila Aging

The transcription factor foxo is a known regulator of lifespan extension and tissue homeostasis. It has been linked to the maintenance of neuronal processes across many species and has been shown to promote youthful characteristics by regulating cytoskeletal flexibility and synaptic plasticity at the neuromuscular junction (NMJ). However, the role of foxo in aging neuromuscular junction function has yet to be determined. We profiled adult Drosophila foxo- null mutant abdominal ventral longitudinal muscles and found that young mutants exhibited morphological profiles similar to those of aged wild-type flies, such as larger bouton areas and shorter terminal branches. We also observed changes to the axonal cytoskeleton and an accumulation of late endosomes in foxo null mutants and motor neuron-specific foxo knockdown flies, similar to those of aged wild-types. Motor neuron-specific overexpression of foxo can delay age-dependent changes to NMJ morphology, suggesting foxo is responsible for maintaining NMJ integrity during aging. Through genetic screening, we identify several downstream factors mediated through foxo-regulated NMJ homeostasis, including genes involved in the MAPK pathway. Interestingly, the phosphorylation of p38 was increased in the motor neuron-specific foxo knockdown flies, suggesting foxo acts as a suppressor of p38/MAPK activation. Our work reveals that foxo is a key regulator for NMJ homeostasis, and it may maintain NMJ integrity by repressing MAPK signaling.

Lysosome Function and Dysfunction in Hereditary Spastic Paraplegias

Hereditary Spastic Paraplegias (HSPs) are a genetically diverse group of inherited neurological diseases with over 80 associated gene loci. Over the last decade, research into mechanisms underlying HSPs has led to an emerging interest in lysosome dysfunction. In this review, we highlight the different classes of HSPs that have been linked to lysosome defects: (1) a subset of complex HSPs where mutations in lysosomal genes are causally linked to the diseases, (2) other complex HSPs where mutation in genes encoding membrane trafficking adaptors lead to lysosomal defects, and (3) a subset of HSPs where mutations affect genes encoding proteins whose function is primarily linked to a different cellular component or organelle such as microtubule severing and Endoplasmic Reticulum-shaping, while also altering to lysosomes. Interestingly, aberrant axonal lysosomes, associated with the latter two subsets of HSPs, are a key feature observed in other neurodegenerative diseases such as Alzheimer's disease. We discuss how altered lysosome function and trafficking may be a critical contributor to HSP pathology and highlight the need for examining these features in the cortico-spinal motor neurons of HSP mutant models.

The axonal endolysosomal and autophagic systems

Neurons, because of their elaborate morphology and the long distances between distal axons and the soma as well as their longevity, pose special challenges to autophagy and to the endolysosomal system, two of the main degradative routes for turnover of defective proteins and organelles. Autophagosomes sequester cytoplasmic or organellar cargos by engulfing them into their lumen before fusion with degradative lysosomes enriched in neuronal somata and participate in retrograde signaling to the soma. Endosomes are mainly involved in the sorting, recycling, or lysosomal turnover of internalized or membrane-bound macromolecules to maintain axonal membrane homeostasis. Lysosomes and the multiple shades of lysosome-related organelles also serve non-degradative roles, for example, in nutrient signaling and in synapse formation. Recent years have begun to shed light on the distinctive organization of the autophagy and endolysosomal systems in neurons, in particular their roles in axons. We review here our current understanding of the localization, distribution, and growing list of functions of these organelles in the axon in health and disease and outline perspectives for future research.

Doublecortin facilitates the elongation of the somatic Golgi apparatus into proximal dendrites

Mutations in the doublecortin (DCX) gene, which encodes a microtubule (MT)-binding protein, cause human cortical malformations, including lissencephaly and subcortical band heterotopia. A deficiency in DCX and DCX-like kinase 1 (DCLK1), a functionally redundant and structurally similar cognate of DCX, decreases neurite length and increases the number of primary neurites directly arising from the soma. The underlying mechanism is not completely understood. In this study, the elongation of the somatic Golgi apparatus into proximal dendrites, which have been implicated in dendrite patterning, was significantly decreased in the absence of DCX/DCLK1. Phosphorylation of DCX at S47 or S327 was involved in this process. DCX deficiency shifted the distribution of CLASP2 proteins to the soma from the dendrites. In addition to CLASP2, dynein and its cofactor JIP3 were abnormally distributed in DCX-deficient neurons. The association between JIP3 and dynein was significantly increased in the absence of DCX. Down-regulation of CLASP2 or JIP3 expression with specific shRNAs rescued the Golgi phenotype observed in DCX-deficient neurons. We conclude that DCX regulates the elongation of the Golgi apparatus into proximal dendrites through MT-associated proteins and motors.

Retrograde Mitochondrial Transport Is Essential for Organelle Distribution and Health in Zebrafish Neurons

In neurons, mitochondria are transported by molecular motors throughout the cell to form and maintain functional neural connections. These organelles have many critical functions in neurons and are of high interest as their dysfunction is associated with disease. While the mechanics and impact of anterograde mitochondrial movement toward axon terminals are beginning to be understood, the frequency and function of retrograde (cell body directed) mitochondrial transport in neurons are still largely unexplored. While existing evidence indicates that some mitochondria are retrogradely transported for degradation in the cell body, the precise impact of disrupting retrograde transport on the organelles and the axon was unknown. Using long-term, in vivo imaging, we examined mitochondrial motility in zebrafish sensory and motor axons. We show that retrograde transport of mitochondria from axon terminals allows replacement of the axon terminal population within a day. By tracking these organelles, we show that not all mitochondria that leave the axon terminal are degraded; rather, they persist over several days. Disrupting retrograde mitochondrial flux in neurons leads to accumulation of aged organelles in axon terminals and loss of cell body mitochondria. Assays of neural circuit activity demonstrated that disrupting mitochondrial transport and function has no effect on sensory axon terminal activity but does negatively impact motor neuron axons. Taken together, our work supports a previously unappreciated role for retrograde mitochondrial transport in the maintenance of a homeostatic distribution of mitochondria in neurons and illustrates the downstream effects of disrupting this process on sensory and motor circuits.

SIGNIFICANCE STATEMENT Disrupted mitochondrial transport has been linked to neurodegenerative disease. Retrograde transport of this organelle has been implicated in turnover of aged organelles through lysosomal degradation in the cell body. Consistent with this, we provide evidence that retrograde mitochondrial transport is important for removing aged organelles from axons; however, we show that these organelles are not solely degraded, rather they persist in neurons for days. Disrupting retrograde mitochondrial transport impacts the homeostatic distribution of mitochondria throughout the neuron and the function of motor, but not sensory, axon synapses. Together, our work shows the conserved reliance on retrograde mitochondrial transport for maintaining a healthy mitochondrial pool in neurons and illustrates the disparate effects of disrupting this process on sensory versus motor circuits.

Mitochondrial dysfunction interferes with neural crest specification through the FoxD3 transcription factor

The neural crest is an important group of cells with pluripotency and migratory ability that is crucially involved in tissue and cell specification during development. Craniofacial shaping, sensory neurons, body asymmetry, and pigmentation are linked to neural crest functionality. Despite its prominent role in embryogenesis, neural crest specification as well as the possible part mitochondria play in such a process remains unclarified. Mitochondria are important organelles not only for respiration, but also for regulation of cell proliferation, differentiation and death. Modulation of mitochondrial fitness and depletion of mitochondrial ATP synthesis has been shown to down-regulate Wnt signaling, both in vitro and in vivo. Since Wnt signaling is one of the crucial players during neural crest induction/specification, we hypothesized a signaling cascade connecting mitochondria to embryonic development and neural crest migration and differentiation. Here, by using pharmacological and genetic modulators of mitochondrial function, we provide evidence that a crosstalk between mitochondrial energy homeostasis and Wnt signaling is important in the development of neural crest-derived tissues. Furthermore, our results highlight the possibility to modulate neural crest cell specification by tuning mitochondrial metabolism via FoxD3, an important transcription factor that is regulated by Wnt. FoxD3 ensures the correct embryonic development and contributes to the maintenance of cell stemness and to the induction of epithelial-to-mesenchymal transition. In summary, our work offers new insights into the molecular mechanism of action of FoxD3 and demonstrates that mitochondrial fitness is linked to the regulation of this important transcription factor via Wnt signaling in the context of neural crest specification.

ARL8 Relieves SKIP Autoinhibition to Enable Coupling of Lysosomes to Kinesin-1

Long-range movement of organelles within the cytoplasm relies on coupling to microtubule motors, a process that is often mediated by adaptor proteins. In many cases, this coupling involves organelle- or adaptor-induced activation of the microtubule motors by conformational reversal of an autoinhibited state. Herein, we show that a similar regulatory mechanism operates for an adaptor protein named SKIP (also known as PLEKHM2). SKIP binds to the small guanosine triphosphatase (GTPase) ARL8 on the lysosomal membrane to couple lysosomes to the anterograde microtubule motor kinesin-1. Structure-function analyses of SKIP reveal that the C-terminal region comprising three pleckstrin homology (PH) domains interacts with the N-terminal region comprising ARL8- and kinesin-1-binding sites. This interaction inhibits coupling of lysosomes to kinesin-1 and, consequently, lysosome movement toward the cell periphery. We also find that ARL8 does not just recruit SKIP to the lysosomal membrane but also relieves SKIP autoinhibition, promoting kinesin-1-driven, anterograde lysosome transport. Finally, our analyses show that the largely disordered middle region of SKIP mediates self-association and that this self-association enhances the interaction of SKIP with kinesin-1. These findings indicate that SKIP is not just a passive connector of lysosome-bound ARL8 to kinesin-1 but is itself subject to intra- and inter-molecular interactions that regulate its function. We anticipate that similar organelle- or GTPase-induced conformational changes could regulate the activity of other kinesin adaptors.

Proteome and phosphoproteome profiling reveals the regulation mechanism of hibernation in a freshwater leech (Whitmania pigra)

Hibernation is an energy-saving and adaptive strategy adopted by leech, an important medicinal resource in Asia, to survive low temperature. Reversible protein phosphorylation (RPP) plays a key role in the regulation of mammalian hibernation processes but has never been documented in freshwater invertebrate such as leech. In this study, we detected the effects of hibernation on the proteome and phosphoproteome of the leech Whitmania pigra. A total of 2184 proteins and 2598 sites were quantified. Deep-hibernation resulted in 85 up-regulated and 107 down-regulated proteins and 318 up-regulated and 204 down-regulated phosphosites using a 1.5-fold threshold (P<0.05). Proteins involved in protein digestion and absorption, amino acid metabolism and N-glycan biosynthesis were significantly down-regulated during deep-hibernation. However, proteins involved in maintaining cell structure stability in hibernating animals were up-regulated. Differentially phosphorylated proteins provided the first global picture of a shift in energy metabolism, protein synthesis, cytoprotection and signaling during deep hibernation. Furthermore, AMP-activated protein kinase and protein kinase C play major roles in the regulation of these functional processes. These data significantly improve our understanding of the regulatory mechanisms of leech hibernation processes and provides substantial candidate phosphorylated proteins that could be important for functionally adapt in freshwater animals. SIGNIFICANCE: The leech Whitmania pigra as an important medicinal resource in Asia is an excellent model freshwater invertebrate for studies of environmentally-induced hibernation. The present study provides the first quantitative proteomics and phosphoproteomic analysis of leech hibernation using isobaric tag based TMT labeling and high-resolution mass spectrometry. These data significantly improve our understanding of the regulatory mechanisms when ectotherm animals face environmental stress and provides substantial candidate phosphorylated proteins that could be important for functionally adapt in freshwater animals.

A Conserved Role for Vezatin Proteins in Cargo-Specific Regulation of Retrograde Axonal Transport

Active transport of organelles within axons is critical for neuronal health. Retrograde axonal transport, in particular, relays neurotrophic signals received by axon terminals to the nucleus and circulates new material among enpassant synapses. A single motor protein complex, cytoplasmic dynein, is responsible for nearly all retrograde transport within axons: its linkage to and transport of diverse cargos is achieved by cargo-specific regulators. Here, we identify Vezatin as a conserved regulator of retrograde axonal transport. Vertebrate Vezatin (Vezt) is required for the maturation and maintenance of cell-cell junctions and has not previously been implicated in axonal transport. However, a related fungal protein, VezA, has been shown to regulate retrograde transport of endosomes in hyphae. In a forward genetic screen, we identified a loss-of-function mutation in the Drosophila vezatin-like (vezl) gene. We here show that vezl loss prevents a subset of endosomes, including signaling endosomes containing activated BMP receptors, from initiating transport out of motor neuron terminal boutons. vezl loss also decreases the transport of endosomes and dense core vesicles, but not mitochondria, within axon shafts. We disrupted vezt in zebrafish and found that vezt loss specifically impairs the retrograde axonal transport of late endosomes, causing their accumulation in axon terminals. Our work establishes a conserved, cargo-specific role for Vezatin proteins in retrograde axonal transport.

JNK3 as Therapeutic Target and Biomarker in Neurodegenerative and Neurodevelopmental Brain Diseases

The c-Jun N-terminal kinase 3 (JNK3) is the JNK isoform mainly expressed in the brain. It is the most responsive to many stress stimuli in the central nervous system from ischemia to Aβ oligomers toxicity. JNK3 activity is spatial and temporal organized by its scaffold protein, in particular JIP-1 and β-arrestin-2, which play a crucial role in regulating different cellular functions in different cellular districts. Extensive evidence has highlighted the possibility of exploiting these adaptors to interfere with JNK3 signaling in order to block its action. JNK plays a key role in the first neurodegenerative event, the perturbation of physiological synapse structure and function, known as synaptic dysfunction. Importantly, this is a common mechanism in many different brain pathologies. Synaptic dysfunction and spine loss have been reported to be pharmacologically reversible, opening new therapeutic directions in brain diseases. Being JNK3-detectable at the peripheral level, it could be used as a disease biomarker with the ultimate aim of allowing an early diagnosis of neurodegenerative and neurodevelopment diseases in a still prodromal phase.

Excess active P13K rescues huntingtin-mediated neuronal cell death but has no effect on axonal transport defects

High levels of oxidative stress is detected in neurons affected by many neurodegenerative diseases, including huntington's disease. Many of these diseases also show neuronal cell death and axonal transport defects. While nuclear inclusions/accumulations likely cause cell death, we previously showed that cytoplasmic axonal accumulations can also contribute to neuronal death. However, the cellular mechanisms responsible for activating cell death is unclear. One possibility is that perturbations in normal axonal transport alter the function of the phosphatidylinositol 3-kinase (PI3K)-protein kinase B (AKT)-pathway, a signal transduction pathway that promotes survival/growth in response to extracellular signals. To test this proposal in vivo, we expressed active PI3K in the context of pathogenic huntingtin (HTT-138Q) in Drosophila larval nerves, which show axonal transport defects and neuronal cell death. We found that excess expression of active P13K significantly suppressed HTT-138Q-mediated neuronal cell death, but had no effect on HTT-138Q-mediated axonal transport defects. Expression of active PI3K also rescued Paraquat-mediated cell death. Further, increased levels of pSer9 (inactive) glycogen synthase kinase 3β was seen in HTT-138Q-mediated larval brains, and in dynein loss of function mutants, indicating the modulation of the pro-survival pathway. Intriguingly, proteins in the PI3K/AKT-pathway showed functional interactions with motor proteins. Taken together our observations suggest that proper axonal transport is likely essential for the normal function of the pro-survival PI3K/AKT-signaling pathway and for neuronal survival in vivo. These results have important implications for targeting therapeutics to early insults during neurodegeneration and death.

The Endolysosomal System and Proteostasis: From Development to Degeneration

How do neurons adapt their endolysosomal system to address the particular challenge of membrane transport across their elaborate cellular landscape and to maintain proteostasis for the lifetime of the organism? Here we review recent findings that address this central question. We discuss the cellular and molecular mechanisms of endolysosomal trafficking and the autophagy pathway in neurons, as well as their role in neuronal development and degeneration. These studies highlight the importance of understanding the basic cell biology of endolysosomal trafficking and autophagy and their roles in the maintenance of proteostasis within the context of neurons, which will be critical for developing effective therapies for various neurodevelopmental and neurodegenerative disorders.

Retrograde Ret signaling controls sensory pioneer axon outgrowth

The trafficking mechanisms and transcriptional targets downstream of long-range neurotrophic factor ligand/receptor signaling that promote axon growth are incompletely understood. Zebrafish carrying a null mutation in a neurotrophic factor receptor, Ret, displayed defects in peripheral sensory axon growth cone morphology and dynamics. Ret receptor was highly enriched in sensory pioneer neurons and Ret51 isoform was required for pioneer axon outgrowth. Loss-of-function of a cargo adaptor, Jip3, partially phenocopied Ret axonal defects, led to accumulation of activated Ret in pioneer growth cones, and reduced retrograde Ret51 transport. Jip3 and Ret51 were also retrogradely co-transported, ultimately suggesting Jip3 is a retrograde adapter of active Ret51. Finally, loss of Ret reduced transcription and growth cone localization of Myosin-X, an initiator of filopodial formation. These results show a specific role for Ret51 in pioneer axon growth, and suggest a critical role for long-range retrograde Ret signaling in regulating growth cone dynamics through downstream transcriptional changes.