LC3 binding to the scaffolding protein JIP1 regulates processive dynein-driven transport of autophagosomes

A high affinity RIM-binding protein/Aplip1 interaction prevents the formation of ectopic axonal active zones

Synaptic vesicles (SVs) fuse at active zones (AZs) covered by a protein scaffold, at Drosophila synapses comprised of ELKS family member Bruchpilot (BRP) and RIM-binding protein (RBP). We here demonstrate axonal co-transport of BRP and RBP using intravital live imaging, with both proteins co-accumulating in axonal aggregates of several transport mutants. RBP, via its C-terminal Src-homology 3 (SH3) domains, binds Aplip1/JIP1, a transport adaptor involved in kinesin-dependent SV transport. We show in atomic detail that RBP C-terminal SH3 domains bind a proline-rich (PxxP) motif of Aplip1/JIP1 with submicromolar affinity. Pointmutating this PxxP motif provoked formation of ectopic AZ-like structures at axonal membranes. Direct interactions between AZ proteins and transport adaptors seem to provide complex avidity and shield synaptic interaction surfaces of pre-assembled scaffold protein transport complexes, thus, favouring physiological synaptic AZ assembly over premature assembly at axonal membranes.

DOI:http://dx.doi.org/10.7554/eLife.06935.001

To pass on information, the neurons that make up the nervous system connect at structures known as synapses. Chemical messengers called neurotransmitters are released from one neuron, and travel across the synapse to trigger a response in the neighbouring cell. The formation of new synapses plays an important role in learning and memory, but many aspects of this process are not well understood.

In a specific region of the synapse called the active zone, a scaffold of proteins helps to release the neurotransmitters. These proteins are made in the cell body of the neuron, and are then transported to the end of the long, thin axons that protrude from the cell body. This presents a challenge for the cell, because the components of the active zone scaffold must be correctly targeted to the synapse at the end of the axon, ensuring the active zone scaffold assembles only at its proper location.

Siebert, Böhme et al. studied how some of the proteins that are found in the active zone scaffold of the fruit fly Drosophila are transported along axons. Labelling the proteins with fluorescent markers allowed their movement to be examined under a microscope in living Drosophila larvae. The results showed that two of the proteins—known as BRP and RBP—are transported along the axons together. Further investigation revealed that a transport adaptor protein called Aplip1, which binds to RBP, is required for this movement. Siebert, Böhme et al. established the structure of the part of RBP where this interaction occurs, and found that mutating this region causes premature active zone scaffold assembly in the axonal part of the neuron. The interaction between RBP and Aplip1 is very strong, and this helps to prevent the scaffold assembling before it has reached the correct part of the neuron. Exactly how the transport adaptor and active zone protein are separated once they reach their final destination (the synapse) remains to be discovered.

DOI:http://dx.doi.org/10.7554/eLife.06935.002

Microtubule motors transport phagosomes in the RPE, and lack of KLC1 leads to AMD-like pathogenesis

The degradation of phagosomes, derived from the ingestion of photoreceptor outer segment (POS) disk membranes, is a major role of the retinal pigment epithelium (RPE). Here, POS phagosomes were observed to associate with myosin-7a, and then kinesin-1, as they moved from the apical region of the RPE. Live-cell imaging showed that the phagosomes moved bidirectionally along microtubules in RPE cells, with kinesin-1 light chain 1 (KLC1) remaining associated in both directions and during pauses. Lack of KLC1 did not inhibit phagosome speed, but run length was decreased, and phagosome localization and degradation were impaired. In old mice, lack of KLC1 resulted in RPE pathogenesis that was strikingly comparable to aspects of age-related macular degeneration (AMD), with an excessive accumulation of RPE and sub-RPE deposits, as well as oxidative and inflammatory stress responses. These results elucidate mechanisms of POS phagosome transport in relation to degradation, and demonstrate that defective microtubule motor transport in the RPE leads to phenotypes associated with AMD.

Autophagy in synaptic development, function, and pathology

In the nervous system, neurons contact each other to form neuronal circuits and drive behavior, relying heavily on synaptic connections. The proper development and growth of synapses allows functional transmission of electrical information between neurons or between neurons and muscle fibers. Defects in synapse-formation or development lead to many diseases. Autophagy, a major determinant of protein turnover, is an essential process that takes place in developing synapses. During the induction of autophagy, proteins and cytoplasmic components are encapsulated in autophagosomes, which fuse with lysosomes to form autolysosomes. The cargoes are subsequently degraded and recycled. However, aberrant autophagic activity may lead to synaptic dysfunction, which is a common pathological characteristic in several disorders. Here, we review the current understanding of autophagy in regulating synaptic development and function. In addition, autophagy-related synaptic dysfunction in human diseases is also summarized.

Massive accumulation of luminal protease-deficient axonal lysosomes at Alzheimer's disease amyloid plaques

Through a comprehensive analysis of organellar markers in mouse models of Alzheimer's disease, we document a massive accumulation of lysosome-like organelles at amyloid plaques and establish that the majority of these organelles reside within swollen axons that contact the amyloid deposits. This close spatial relationship between axonal lysosome accumulation and extracellular amyloid aggregates was observed from the earliest stages of β-amyloid deposition. Notably, we discovered that lysosomes that accumulate in such axons are lacking in multiple soluble luminal proteases and thus are predicted to be unable to efficiently degrade proteinaceous cargos. Of relevance to Alzheimer's disease, β-secretase (BACE1), the protein that initiates amyloidogenic processing of the amyloid precursor protein and which is a substrate for these proteases, builds up at these sites. Furthermore, through a comparison between the axonal lysosome accumulations at amyloid plaques and neuronal lysosomes of the wild-type brain, we identified a similar, naturally occurring population of lysosome-like organelles in neuronal processes that is also defined by its low luminal protease content. In conjunction with emerging evidence that the lysosomal maturation of endosomes and autophagosomes is coupled to their retrograde transport, our results suggest that extracellular β-amyloid deposits cause a local impairment in the retrograde axonal transport of lysosome precursors, leading to their accumulation and a blockade in their further maturation. This study both advances understanding of Alzheimer's disease brain pathology and provides new insights into the subcellular organization of neuronal lysosomes that may have broader relevance to other neurodegenerative diseases with a lysosomal component to their pathology.

Axonal autophagosomes recruit dynein for retrograde transport through fusion with late endosomes

Late endosome-loaded dynein–snapin complexes drive amphisome retrograde transport upon fusion of autophagosomes with late endosomes in distal axons.

Efficient degradation of autophagic vacuoles (AVs) via lysosomes is an important cellular homeostatic process. This is particularly challenging for neurons because mature acidic lysosomes are relatively enriched in the soma. Although dynein-driven retrograde transport of AVs was suggested, a fundamental question remains how autophagosomes generated at distal axons acquire dynein motors for retrograde transport toward the soma. In this paper, we demonstrate that late endosome (LE)–loaded dynein–snapin complexes drive AV retrograde transport in axons upon fusion of autophagosomes with LEs into amphisomes. Blocking the fusion with syntaxin17 knockdown reduced recruitment of dynein motors to AVs, thus immobilizing them in axons. Deficiency in dynein–snapin coupling impaired AV transport, resulting in AV accumulation in neurites and synaptic terminals. Altogether, our study provides the first evidence that autophagosomes recruit dynein through fusion with LEs and reveals a new motor–adaptor sharing mechanism by which neurons may remove distal AVs engulfing aggregated proteins and dysfunctional organelles for efficient degradation in the soma.

Signalling scaffolds and local organization of cellular behaviour

Cellular responses to environmental cues involve the mobilization of GTPases, protein kinases and phosphoprotein phosphatases. The spatial organization of these signalling enzymes by scaffold proteins helps to guide the flow of molecular information. Allosteric modulation of scaffolded enzymes can alter their catalytic activity or sensitivity to second messengers in a manner that augments, insulates or terminates local cellular events. This Review examines the features of scaffold proteins and highlights examples of locally organized groups of signalling enzymes that drive essential physiological processes, including hormone action, heart rate, cell division, organelle movement and synaptic transmission.

Autophagy in neuronal cells: general principles and physiological and pathological functions

Autophagy delivers cytoplasmic components and organelles to lysosomes for degradation. This pathway serves to degrade nonfunctional or unnecessary organelles and aggregate-prone and oxidized proteins to produce substrates for energy production and biosynthesis. Macroautophagy delivers large aggregates and whole organelles to lysosomes by first enveloping them into autophagosomes that then fuse with lysosomes. Chaperone-mediated autophagy (CMA) degrades proteins containing the KFERQ-like motif in their amino acid sequence, by transporting them from the cytosol across the lysosomal membrane into the lysosomal lumen. Autophagy is especially important for the survival and homeostasis of postmitotic cells like neurons, because these cells are not able to dilute accumulating detrimental substances and damaged organelles by cell division. Our current knowledge on the autophagic pathways and molecular mechanisms and regulation of autophagy will be summarized in this review. We will describe the physiological functions of macroautophagy and CMA in neuronal cells. Finally, we will summarize the current evidence showing that dysfunction of macroautophagy and/or CMA contributes to neuronal diseases. We will give an overview of our current knowledge on the role of autophagy in aging neurons, and focus on the role of autophagy in four types of neurodegenerative diseases, i.e., amyotrophic lateral sclerosis and frontotemporal dementia, prion diseases, lysosomal storage diseases, and Parkinson's disease.

Promoting autophagic clearance: viable therapeutic targets in Alzheimer's disease

Many neurodegenerative disorders are characterized by the aberrant accumulation of aggregate-prone proteins. Alzheimer's disease (AD) is associated with the buildup of β-amyloid peptides and tau, which aggregate into extracellular plaques and neurofibrillary tangles, respectively. Multiple studies have linked dysfunctional intracellular degradation mechanisms with AD pathogenesis. One such pathway is the autophagy-lysosomal system, which involves the delivery of large protein aggregates/inclusions and organelles to lysosomes through the formation, trafficking, and degradation of double-membrane structures known as autophagosomes. Converging data suggest that promoting autophagic degradation, either by inducing autophagosome formation or enhancing lysosomal digestion, provides viable therapeutic strategies. In this review, we discuss compounds that can augment autophagic clearance and may ameliorate disease-related pathology in cell and mouse models of AD. Canonical autophagy induction is associated with multiple signaling cascades; on the one hand, the best characterized is mammalian target of rapamycin (mTOR). Accordingly, multiple mTOR-dependent and mTOR-independent drugs that stimulate autophagy have been tested in preclinical models. On the other hand, there is a growing list of drugs that can enhance the later stages of autophagic flux by stabilizing microtubule-mediated trafficking, promoting lysosomal fusion, or bolstering lysosomal enzyme function. Although altering the different stages of autophagy provides many potential targets for AD therapeutic interventions, it is important to consider how autophagy drugs might also disturb the delicate balance between autophagosome formation and lysosomal degradation.

Cholesterol-mediated activation of acid sphingomyelinase disrupts autophagy in the retinal pigment epithelium

How autophagy is regulated in the postmitotic retinal pigment epithelium (RPE) is unclear. Visual cycle metabolites and cholesterol that accumulate in the RPE inhibit autophagic flux by activating acid sphingomyelinase (ASMase). Increased ceramide promotes tubulin acetylation, which prevents autophagosome traffic. ASMase inhibition restores RPE autophagy.

Autophagy is an essential mechanism for clearing damaged organelles and proteins within the cell. As with neurodegenerative diseases, dysfunctional autophagy could contribute to blinding diseases such as macular degeneration. However, precisely how inefficient autophagy promotes retinal damage is unclear. In this study, we investigate innate mechanisms that modulate autophagy in the retinal pigment epithelium (RPE), a key site of insult in macular degeneration. High-speed live imaging of polarized adult primary RPE cells and data from a mouse model of early-onset macular degeneration identify a mechanism by which lipofuscin bisretinoids, visual cycle metabolites that progressively accumulate in the RPE, disrupt autophagy. We demonstrate that bisretinoids trap cholesterol and bis(monoacylglycero)phosphate, an acid sphingomyelinase (ASMase) cofactor, within the RPE. ASMase activation increases cellular ceramide, which promotes tubulin acetylation on stabilized microtubules. Live-imaging data show that autophagosome traffic and autophagic flux are inhibited in RPE with acetylated microtubules. Drugs that remove excess cholesterol or inhibit ASMase reverse this cascade of events and restore autophagosome motility and autophagic flux in the RPE. Because accumulation of lipofuscin bisretinoids and abnormal cholesterol homeostasis are implicated in macular degeneration, our studies suggest that ASMase could be a potential therapeutic target to ensure the efficient autophagy that maintains RPE health.

Axonal transport: cargo-specific mechanisms of motility and regulation

Axonal transport is essential for neuronal function, and many neurodevelopmental and neurodegenerative diseases result from mutations in the axonal transport machinery. Anterograde transport supplies distal axons with newly synthesized proteins and lipids, including synaptic components required to maintain presynaptic activity. Retrograde transport is required to maintain homeostasis by removing aging proteins and organelles from the distal axon for degradation and recycling of components. Retrograde axonal transport also plays a major role in neurotrophic and injury response signaling. This review provides an overview of axonal transport pathways and discusses their role in neuronal function.

The role of rab proteins in neuronal cells and in the trafficking of neurotrophin receptors

Neurotrophins are a family of proteins that are important for neuronal development, neuronal survival and neuronal functions. Neurotrophins exert their role by binding to their receptors, the Trk family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to receptors triggers a complex series of signal transduction events, which are able to induce neuronal differentiation but are also responsible for neuronal maintenance and neuronal functions. Rab proteins are small GTPases localized to the cytosolic surface of specific intracellular compartments and are involved in controlling vesicular transport. Rab proteins, acting as master regulators of the membrane trafficking network, play a central role in both trafficking and signaling pathways of neurotrophin receptors. Axonal transport represents the Achilles' heel of neurons, due to the long-range distance that molecules, organelles and, in particular, neurotrophin-receptor complexes have to cover. Indeed, alterations of axonal transport and, specifically, of axonal trafficking of neurotrophin receptors are responsible for several human neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis and some forms of Charcot-Marie-Tooth disease. In this review, we will discuss the link between Rab proteins and neurotrophin receptor trafficking and their influence on downstream signaling pathways.

Integrated regulation of motor-driven organelle transport by scaffolding proteins

Intracellular trafficking pathways, including endocytosis, autophagy, and secretion, rely on directed organelle transport driven by the opposing microtubule motor proteins kinesin and dynein. Precise spatial and temporal targeting of vesicles and organelles requires the integrated regulation of these opposing motors, which are often bound simultaneously to the same cargo. Recent progress demonstrates that organelle-associated scaffolding proteins, including Milton/TRAKs (trafficking kinesin-binding protein), JIP1, JIP3 (JNK-interacting proteins), huntingtin, and Hook1, interact with molecular motors to coordinate activity and sustain unidirectional transport. Scaffolding proteins also bind to upstream regulatory proteins, including kinases and GTPases, to modulate transport in the cell. This integration of regulatory control with motor activity allows for cargo-specific changes in the transport or targeting of organelles in response to cues from the complex cellular environment.

Directing traffic and autophagy in axonal transport

In this issue of Developmental Cell, Fu et al. (2014) address what determines persistent directional movement along microtubules of organelles capable of bidirectional transit. They show that retrograde axonal autophagosome transport is mediated by the scaffolding protein JIP1, which not only inhibits anterograde movement but may also promote autophagosome maturation.