Xiu-Tang Cheng

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.

Efficient degradation of autophagic vacuoles (AVs) generated at axon terminals by mature lysosomes enriched in the cell body represents an exceptional challenge that neurons face in maintaining cellular homeostasis. Here, we discuss our recent findings revealing a lipid-mediated impairment of lysosome transport to distal axons contributing to axonal AV accumulation in the neurodegenerative lysosomal storage disorder Niemann-Pick disease type C (NPC). Using transmission electron microscopy, we observed a striking buildup of endocytic and autophagic organelles in NPC dystrophic axons, indicating defects in the clearance of organelles destined for lysosomal degradation. We further revealed that elevated cholesterol on NPC lysosome membranes abnormally sequesters motor-adaptors of axonal lysosome delivery, resulting in impaired anterograde lysosome transport into distal axons that disrupts maturation of axonal AVs during their retrograde transport route. Together, our study demonstrates a mechanism by which altered membrane lipid composition compromises axonal lysosome trafficking and positioning and shows that lowering lysosomal lipid levels rescues lysosome transport into NPC axons, thus reducing axonal autophagic stress at early stages of NPC disease.

In this study, Kulkarni et al. elegantly revealed that synaptic activity has a robust impact on the motility and degradative capacity of dendritic autophagic vacuoles (AVs) in cultured hippocampal neurons. Using live-cell imaging combined with pharmacological stimulating paradigms, Kulkarni et al. examined AV motility in both axons and dendrites before and following neuronal depolarization. The authors found an intriguing phenotype: depolarization and subsequently elevated synaptic activation reduces AV motility in dendrites, but not in axons, whereas silencing neuronal activity reverses AV motility. This activity-induced regulation allows neurons to enrich AVs at postsynaptic compartments for local degradation to maintain cellular homeostasis. These findings were supported by increased acidic AVs at dendritic compartments upon neuronal activation. This study by the Maday group establishes an intriguing regulation of neuronal AV dynamics, thus ensuring compartment-restricted autophagic degradation in response to changes in synaptic activity, although its underlying physiological relevance to the maintenance or regulation of synaptic function remains unclear. Given both autophagic and lysosomal trafficking are affected by synaptic activity with differing motility patterns, it would be of interest to explore the trafficking mechanisms that regulate these two separate pathways. It would be even more compelling to investigate whether enhanced dendritic delivery of mature lysosomes contributes to the increased degradative capacity of AVs at local dendritic compartments. Perhaps the most direct follow-up study would be to determine whether an altered synaptic proteome or autophagic stress, by either disrupting activity-induced local autophagic dynamics or blocking AV degradation, leads to synaptic dysfunction. Mechanistic insight into this fundamental process will conceptually advance our understanding of the emerging homeostatic and proteomic regulation of synaptic transmission.

Mitochondria are essential in long axons to provide metabolic support and sustain neuron integrity. A healthy mitochondrial pool is maintained by biogenesis, transport, mitophagy, fission, and fusion, but how these events are regulated in axons is not well defined. Here, we show that the Drosophila glutathione S-transferase (GST) Gfzf prevents mitochondrial hyperfusion in axons. Gfzf loss altered redox balance between glutathione (GSH) and oxidized glutathione (GSSG) and initiated mitochondrial fusion through the coordinated action of Mfn and Opa1. Gfzf functioned epistatically with the thioredoxin peroxidase Jafrac1 and the thioredoxin reductase 1 TrxR-1 to regulate mitochondrial dynamics. Altering GSH:GSSG ratios in mouse primary neurons in vitro also induced hyperfusion. Mitochondrial changes caused deficits in trafficking, the metabolome, and neuronal physiology. Changes in GSH and oxidative state are associated with neurodegenerative diseases like Alzheimer's. Our demonstration that GSTs are key in vivo regulators of axonal mitochondrial length and number provides a potential mechanistic link. Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.

Mitochondria are cellular power plants that supply most of the ATP required in the brain to power neuronal growth, function, and regeneration. Given their extremely polarized structures and extended long axons, neurons face an exceptional challenge to maintain energy homeostasis in distal axons, synapses, and growth cones. Anchored mitochondria serve as local energy sources; therefore, the regulation of mitochondrial trafficking and anchoring ensures that these metabolically active areas are adequately supplied with ATP. Chronic mitochondrial dysfunction is a hallmark feature of major aging‐related neurodegenerative diseases, and thus, anchored mitochondria in aging neurons need to be removed when they become dysfunctional. Investigations into the regulation of microtubule (MT)‐based trafficking and anchoring of axonal mitochondria under physiological and pathological circumstances represent an important emerging area. In this short review article, we provide an updated overview of recent in vitro and in vivo studies showing (1) how mitochondria are transported and positioned in axons and synapses during neuronal developmental and maturation stages, and (2) how altered mitochondrial motility and axonal energy deficits in aging nervous systems link to neurodegeneration and regeneration in a disease or injury setting. We also highlight a major role of syntaphilin (SNPH) as a key MT‐based regulator of axonal mitochondrial trafficking and anchoring in mature neurons.

The formation and maintenance of synapses require long-distance delivery of newly synthesized synaptic proteins from the soma to distal synapses, raising the fundamental question of whether impaired transport is associated with neurodevelopmental disorders such as autism. We previously revealed that syntabulin acts as a motor adapter linking kinesin-1 motor and presynaptic cargos. Here, we report that defects in syntabulin-mediated transport and thus reduced formation and maturation of synapses are one of core synaptic mechanisms underlying autism-like synaptic dysfunction and social behavioral abnormalities. Syntabulin expression in the mouse brain peaks during the first 2 weeks of postnatal development and progressively declines during brain maturation. Neurons from conditional syntabulin^−/− mice (stb cKO) display impaired transport of presynaptic cargos, reduced synapse density and active zones, and altered synaptic transmission and long-term plasticity. Intriguingly, stb cKO mice exhibit core autism-like traits, including defective social recognition and communication, increased stereotypic behavior, and impaired spatial learning and memory. These phenotypes establish a new mechanistic link between reduced transport of synaptic cargos and impaired maintenance of synaptic transmission and plasticity, contributing to autism-associated behavioral abnormalities. This notion is further confirmed by the human missense variant STB-R178Q, which is found in an autism patient and loses its adapter capacity for binding kinesin-1 motors. Expressing STB-R178Q fails to rescue reduced synapse formation and impaired synaptic transmission and plasticity in stb cKO neurons. Altogether, our study suggests that defects in syntabulin-mediated transport mechanisms underlie the synaptic dysfunction and behavioral abnormalities that bear similarities to autism.

Chronic mitochondrial stress is associated with major neurodegenerative diseases; and thus, the recovery of those mitochondria constitutes a critical step of energy maintenance in early stages of neurodegeneration. Our recent study provides the first lines of evidence showing that the MUL1-MFN2 pathway acts as an early checkpoint to maintain mitochondrial integrity by regulating mitochondrial morphology and interplay with the endoplasmic reticulum (ER). This mechanism ensures that degradation through mitophagy is restrained in neurons under early stress conditions. MUL1 deficiency increases MFN2 activity, triggering the first phase of mitochondrial hyperfusion and acting as an antagonist of ER-mitochondria (ER-Mito) tethering. Reduced ER-Mito interplay enhances the cytoplasmic Ca²⁺ load that induces the DNM1L/Drp1-dependent second phase of mitochondrial fragmentation and mitophagy. Our study provides new mechanistic insights into neuronal mitochondrial maintenance under stress conditions. Identifying this pathway is particularly relevant because chronic mitochondrial dysfunction and altered ER-Mito contacts have been reported in major neurodegenerative diseases.

Lysosomal degradation of protein aggregates and damaged organelles is essential for maintaining cellular homeostasis. This process in neurons is challenging due to their highly polarized architecture. While enzymatically active degradative lysosomes are enriched in the cell body, their trafficking and degradation capacity in axons remain elusive. We recently characterized the axonal delivery of degradative lysosomes by applying a set of fluorescent probes that selectively label active forms of lysosomal hydrolases on cortical neurons in microfluidic devices. We revealed that soma-derived degradative lysosomes rapidly influx into distal axons and target to autophagosomes and Parkinson disease-related SNCA/α-synuclein cargos for local degradation. Disrupting axon-targeted delivery of degradative lysosomes induces axonal autophagic stress. We demonstrate that the axon is an active compartment for local degradation, establishing a foundation for future investigations into axonal lysosome trafficking and functionality in neurodegenerative diseases and lysosomal storage disorders associated with axonal pathology and macroautophagy/autophagy stress.

Chronic mitochondrial stress associates with major neurodegenerative diseases. Recovering stressed mitochondria constitutes a critical step of mitochondrial quality control and thus energy maintenance in early stages of neurodegeneration. Here, we reveal Mul1-Mfn2 pathway that maintains neuronal mitochondrial integrity under stress conditions. Mul1 deficiency increases Mfn2 activity that triggers the first phasic mitochondrial hyperfusion and also acts as an ER-Mito tethering antagonist. Reduced ER-Mito coupling leads to increased cytoplasmic Ca²⁺load that activates calcineurin and induces the second phasic Drp1-dependent mitochondrial fragmentation and mitophagy. Overexpressing Mfn2, but not Mfn1, mimics Mul1-deficient phenotypes, while expressing PTPIP51, an ER-Mito anchoring protein, suppresses Parkin-mediated mitophagy. Thus, by regulating mitochondrial morphology and ER-Mito contacts, Mul1-Mfn2 pathway plays an early checkpoint role in maintaining mitochondrial integrity. Our study provides new mechanistic insights into neuronal mitochondrial maintenance under stress conditions, which is relevant to several major neurodegenerative diseases associated with mitochondrial dysfunction and altered ER-Mito interplay.

Mutations in PTEN-induced kinase 1 (PINK1) and valosin-containing protein (VCP) have long been reported to be involved in the pathogenesis of familial Parkinson’s disease (PD). While the role of mitochondria-targeted PINK1 has been well characterized in mitochondrial quality control (mitophagy), little is known regarding the role of cytosolic PINK1 in neuronal health and degeneration. In this elegant study, Wang et al. reported an unexpected role of neuronal PINK1 in promoting dendritic growth and branching. The authors proposed that mitochondria release processed PINK1 into the cytosolic pool as a pro-growth signal to promote neuronal differentiation. By using an unbiased proteomic screen for PINK1 binding proteins, they discovered a new PINK1-VCP-protein kinase A (PKA)-p47 signaling pathway. They found that cytosolic PINK1 physically interacts with both VCP-p47 complex and PKA. Through PINK1 scaffolding with the complex, PINK1 phosphorylates and activates PKA, which in turn results in p47 phosphorylation. They further showed that phospho-p47 can promote dendritic arborization. Suppressing any factor in this pathway reduces dendritic complexity. This fits well with emerging evidence that VCP and p47 are involved in spinogenesis. While this study provides some important mechanistic insights as to how cytosolic PINK1 contributes to neuronal morphogenesis in developing neurons, it was mainly based on protein overexpression in neuroblastoma cell lines and immature cultured neurons. Thus, the roles of endogenous levels of cytosolic PINK1 in a well-mature neuronal system remain unclear. Given that mutations in PINK1 and VCP are known to cause Parkinson’s disease and frontotemporal dementia, it would be of immediate interest to investigate whether the same signaling pathway also acts to protect mature or aging neurons under physiological stress or at early stages of pathological conditions in disease-related models.