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

VezA/vezatin facilitates proper assembly of the dynactin complex in vivo

Cytoplasmic dynein-mediated intracellular transport needs the multi-component dynactin complex for cargo binding and motor activation. However, the cellular factors involved in dynactin assembly remain unexplored. Here, we found in Aspergillus nidulans that the vezatin homolog VezA is important for dynactin assembly. VezA affects the microtubule plus-end accumulation of dynein before cargo binding and cargo-adapter-mediated dynein activation, two processes that both need dynactin. The dynactin complex contains multiple components, including p150, p50, and an Arp1 (actin-related protein 1) mini-filament associated with a pointed-end sub-complex. VezA physically interacts with the Arp1 mini-filament either directly or indirectly. Loss of VezA significantly decreases the amount of Arp1 pulled down with pointed-end proteins, as well as the protein levels of p50 and p150 in cell extract. Using various dynactin mutants, we further revealed that the dynactin assembly process must be highly coordinated. Together, these results shed light on dynactin assembly in vivo.

Unraveling the interplay of kinesin-1, tau, and microtubules in neurodegeneration associated with Alzheimer's disease

Alzheimer's disease (AD) is marked by the gradual and age-related deterioration of nerve cells in the central nervous system. The histopathological features observed in the brain affected by AD are the aberrant buildup of extracellular and intracellular amyloid-β and the formation of neurofibrillary tangles consisting of hyperphosphorylated tau protein. Axonal transport is a fundamental process for cargo movement along axons and relies on molecular motors like kinesins and dyneins. Kinesin's responsibility for transporting crucial cargo within neurons implicates its dysfunction in the impaired axonal transport observed in AD. Impaired axonal transport and dysfunction of molecular motor proteins, along with dysregulated signaling pathways, contribute significantly to synaptic impairment and cognitive decline in AD. Dysregulation in tau, a microtubule-associated protein, emerges as a central player, destabilizing microtubules and disrupting the transport of kinesin-1. Kinesin-1 superfamily members, including kinesin family members 5A, 5B, and 5C, and the kinesin light chain, are intricately linked to AD pathology. However, inconsistencies in the abundance of kinesin family members in AD patients underline the necessity for further exploration into the mechanistic impact of these motor proteins on neurodegeneration and axonal transport disruptions across a spectrum of neurological conditions. This review underscores the significance of kinesin-1's anterograde transport in AD. It emphasizes the need for investigations into the underlying mechanisms of the impact of motor protein across various neurological conditions. Despite current limitations in scientific literature, our study advocates for targeting kinesin and autophagy dysfunctions as promising avenues for novel therapeutic interventions and diagnostics in AD.

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.

Myelination by signaling through Arf guanine nucleotide exchange factor

During myelination, large quantities of proteins are synthesized and transported from the endoplasmic reticulum (ER)-trans-Golgi network (TGN) to their appropriate locations within the intracellular region and/or plasma membrane. It is widely believed that oligodendrocytes uptake neuronal signals from neurons to regulate the endocytosis- and exocytosis-mediated intracellular trafficking of major myelin proteins such as myelin-associated glycoprotein (MAG) and proteolipid protein 1 (PLP1). The small GTPases of the adenosine diphosphate (ADP) ribosylation factor (Arf) family constitute a large group of signal transduction molecules that act as regulators for intracellular signaling, vesicle sorting, or membrane trafficking in cells. Studies on mice deficient in Schwann cell-specific Arfs-related genes have revealed abnormal myelination formation in peripheral nerves, indicating that Arfs-mediated signaling transduction is required for myelination in Schwann cells. However, the complex roles in these events remain poorly understood. This review aims to provide an update on signal transduction, focusing on Arf and its activator ArfGEF (guanine nucleotide exchange factor for Arf) in oligodendrocytes and Schwann cells. Future studies are expected to provide important information regarding the cellular and physiological processes underlying the myelination of oligodendrocytes and Schwann cells and their function in modulating neural activity.

Mechanisms of lysosomal tubulation and sorting driven by LRRK2

Lysosomes are dynamic cellular structures that adaptively remodel their membrane in response to stimuli, including membrane damage. Lysosomal dysfunction plays a central role in the pathobiology of Parkinson's disease (PD). Gain-of-function mutations in Leucine-rich repeat kinase 2 (LRRK2) cause familial PD and genetic variations in its locus increase the risk of developing the sporadic form of the disease. We previously uncovered a process we term LYTL (LYsosomal Tubulation/sorting driven by LRRK2), wherein membrane-damaged lysosomes generate tubules sorted into mobile vesicles. Subsequently, these vesicles interact with healthy lysosomes. LYTL is orchestrated by LRRK2 kinase activity, via the recruitment and phosphorylation of a subset of RAB GTPases. Here, we summarize the current understanding of LYTL and its regulation, as well as the unknown aspects of this process.

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.

Opposing actions of JIP4 and RILPL1 provide antagonistic motor force to dynamically regulate membrane reformation during lysosomal tubulation/sorting driven by LRRK2

Lysosomes are dynamic cellular structures that adaptively remodel their membrane in response to stimuli, including membrane damage. We previously uncovered a process we term LYTL (LYsosomal Tubulation/sorting driven by Leucine-Rich Repeat Kinase 2 [LRRK2]), wherein damaged lysosomes generate tubules sorted into mobile vesicles. LYTL is orchestrated by the Parkinson's disease-associated kinase LRRK2 that recruits the motor adaptor protein and RHD family member JIP4 to lysosomes via phosphorylated RAB proteins. To identify new players involved in LYTL, we performed unbiased proteomics on isolated lysosomes after LRRK2 kinase inhibition. Our results demonstrate that there is recruitment of RILPL1 to ruptured lysosomes via LRRK2 activity to promote phosphorylation of RAB proteins at the lysosomal surface. RILPL1, which is also a member of the RHD family, enhances the clustering of LRRK2-positive lysosomes in the perinuclear area and causes retraction of LYTL tubules, in contrast to JIP4 which promotes LYTL tubule extension. Mechanistically, RILPL1 binds to p150Glued, a dynactin subunit, facilitating the transport of lysosomes and tubules to the minus end of microtubules. Further characterization of the tubulation process revealed that LYTL tubules move along tyrosinated microtubules, with tubulin tyrosination proving essential for tubule elongation. In summary, our findings emphasize the dynamic regulation of LYTL tubules by two distinct RHD proteins and pRAB effectors, serving as opposing motor adaptor proteins: JIP4, promoting tubulation via kinesin, and RILPL1, facilitating tubule retraction through dynein/dynactin. We infer that the two opposing processes generate a metastable lysosomal membrane deformation that facilitates dynamic tubulation events.

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.

Driving autophagy - the role of molecular motors

Most of the vesicular transport pathways inside the cell are facilitated by molecular motors that move along cytoskeletal networks. Autophagy is a well-explored catabolic pathway that is initiated by the formation of an isolation membrane known as the phagophore, which expands to form a double-membraned structure that captures its cargo and eventually moves towards the lysosomes for fusion. Molecular motors and cytoskeletal elements have been suggested to participate at different stages of the process as the autophagic vesicles move along cytoskeletal tracks. Dynein and kinesins govern autophagosome trafficking on microtubules through the sequential recruitment of their effector proteins, post-translational modifications and interactions with LC3-interacting regions (LIRs). In contrast, myosins are actin-based motors that participate in various stages of the autophagic flux, as well as in selective autophagy pathways. However, several outstanding questions remain with regard to how the dominance of a particular motor protein over another is controlled, and to the molecular mechanisms that underlie specific disease variants in motor proteins. In this Review, we aim to provide an overview of the role of molecular motors in autophagic flux, as well as highlight their dysregulation in diseases, such as neurodegenerative disorders and pathogenic infections, and ageing.