Using photoactivatable GFP to track axonal transport kinetics

Spectrin condensates provide a nidus for assembling the periodic axonal structure

Coordinated assembly of individual components into higher-order structures is a defining theme in biology, but underlying principles are not well-understood. In neurons, α/β spectrins, adducin, and actinfilaments assemble into a lattice wrapping underneath the axonal plasma membrane, but mechanistic events leading to this periodic axonal structure (PAS) are unclear. Visualizing PAS components in axons as they develop, we found focal patches in distal axons containing spectrins and adducin (but sparse actin filaments) with biophysical properties reminiscent of biomolecular condensation. Overexpressing spectrin-repeats - constituents of α/β-spectrins - in heterologous cells triggered condensate formation, and preventing association of βII-spectrin with actin-filaments/membranes also facilitated condensation. Finally, overexpressing condensate-triggering spectrin repeats in neurons before PAS establishment disrupted the lattice, presumably by competing with innate assembly, supporting a functional role for biomolecular condensation. We propose a condensation-assembly model where PAS components form focal phase-separated condensates that eventually unfurl into a stable lattice-structure by associating with subplasmalemmal actin. By providing local 'depots' of assembly parts, biomolecular condensation may play a wider role in the construction of intricate cytoskeletal structures.

Imaging Diversity in Slow Axonal Transport

The polarized morphology of neurons necessitates the delivery of proteins synthesized in the soma along the length of the axon to distal synapses; critical for sustaining communication between neurons. This constitutive and dynamic process of protein transport along axons termed "axonal transport" was initially characterized by classic pulse-chase radiolabeling studies which identified two major rate components: a fast component and a slow component. Early radiolabeling studies indicated "cohesive co-transport" of slow transport cargos. However, this approach could not be used to visualize or provide mechanistic insights on this highly dynamic process. The advent of fluorescent and photoactivatable imaging probes have now enabled real-time imaging of axonal transport. Conventional fluorescent probes have helped visualize and characterize the molecular mechanisms of transport of vesicular proteins. These proteins typically move in the fast component of axonal transport and appear as "punctate structures" along axons. However, a large majority of transported proteins that move in the slow component of transport, typically show a "uniform diffusive glow" along axons when tagged to conventional fluorescent probes. This makes it challenging to unequivocally track them in real time. Our lab has used photoactivatable fluorescent probes to tag three individual cytosolic proteins moving in the slow component of axonal transport, and identified three distinct modes of transport along axons. Our data from these experiments argue against the prevailing hypothesis based on classic radiolabeling studies, which suggested that all slow-transport proteins may move along the axon as one large macromolecular protein complex. Although other labs have started using photoactivation to study axonal transport of cytosolic proteins, this technique remains largely under-utilized. Here, we describe the detailed protocols to image and analyze axonal transport of three typical slow-component cargoes along axons of cultured hippocampal neurons.

Clathrin packets move in slow axonal transport and deliver functional payloads to synapses

In non-neuronal cells, clathrin has established roles in endocytosis, with clathrin cages enclosing plasma membrane infoldings, followed by rapid disassembly and reuse of monomers. However, in neurons, clathrin is conveyed in slow axonal transport over days to weeks, and the underlying transport/targeting mechanisms, mobile cargo structures, and even its precise presynaptic localization and physiologic role are unclear. Combining live imaging, photobleaching/conversion, mass spectrometry, electron microscopy, and super-resolution imaging, we found that unlike in dendrites, where clathrin cages rapidly assemble and disassemble, in axons, clathrin and related proteins organize into stable "transport packets" that are unrelated to endocytosis and move intermittently on microtubules, generating an overall slow anterograde flow. At synapses, multiple clathrin packets abut synaptic vesicle (SV) clusters, and clathrin packets also exchange between synaptic boutons in a microtubule-dependent "superpool." Within synaptic boundaries, clathrin is surprisingly dynamic, continuously exchanging between local clathrin assemblies, and its depletion impairs SV recycling. Our data provide a conceptual framework for understanding clathrin trafficking and presynaptic targeting that has functional implications.

Processive flow by biased polymerization mediates the slow axonal transport of actin

Chakrabarty et al. propose a model in which slow axonal transport of actin can occur by a biased polymerization of actin filaments along the axon shaft without the involvement of microtubules (MTs) or MT-based motors. These dynamics are distinct from polymer sliding—the canonical mechanism thought to convey cytoskeletal cargoes in slow transport.

Classic pulse-chase studies have shown that actin is conveyed in slow axonal transport, but the mechanistic basis for this movement is unknown. Recently, we reported that axonal actin was surprisingly dynamic, with focal assembly/disassembly events (“actin hotspots”) and elongating polymers along the axon shaft (“actin trails”). Using a combination of live imaging, superresolution microscopy, and modeling, in this study, we explore how these dynamic structures can lead to processive transport of actin. We found relatively more actin trails elongated anterogradely as well as an overall slow, anterogradely biased flow of actin in axon shafts. Starting with first principles of monomer/filament assembly and incorporating imaging data, we generated a quantitative model simulating axonal hotspots and trails. Our simulations predict that the axonal actin dynamics indeed lead to a slow anterogradely biased flow of the population. Collectively, the data point to a surprising scenario where local assembly and biased polymerization generate the slow axonal transport of actin without involvement of microtubules (MTs) or MT-based motors. Mechanistically distinct from polymer sliding, this might be a general strategy to convey highly dynamic cytoskeletal cargoes.

Hsc70 chaperone activity is required for the cytosolic slow axonal transport of synapsin

Using proteomics, live microscopy, and superresolution microscopy, Ganguly et al. offer insight into the molecular composition of cytosolic cargo complexes conveyed in slow axonal transport, identifying the heat shock protein Hsc70 as a major regulator of this transport.

Soluble cytosolic proteins vital to axonal and presynaptic function are synthesized in the neuronal soma and conveyed via slow axonal transport. Our previous studies suggest that the overall slow transport of synapsin is mediated by dynamic assembly/disassembly of cargo complexes followed by short-range vectorial transit (the “dynamic recruitment” model). However, neither the composition of these complexes nor the mechanistic basis for the dynamic behavior is understood. In this study, we first examined putative cargo complexes associated with synapsin using coimmunoprecipitation and multidimensional protein identification technology mass spectrometry (MS). MS data indicate that synapsin is part of a multiprotein complex enriched in chaperones/cochaperones including Hsc70. Axonal synapsin–Hsc70 coclusters are also visualized by two-color superresolution microscopy. Inhibition of Hsc70 ATPase activity blocked the slow transport of synapsin, disrupted axonal synapsin organization, and attenuated Hsc70–synapsin associations, advocating a model where Hsc70 activity dynamically clusters cytosolic proteins into cargo complexes, allowing transport. Collectively, our study offers insight into the molecular organization of cytosolic transport complexes and identifies a novel regulator of slow transport.

Axonal actin in action: Imaging actin dynamics in neurons

Actin is a highly conserved, key cytoskeletal protein involved in numerous structural and functional roles. In neurons, actin has been intensively investigated in axon terminals-growth cones-and dendritic spines, but details about actin structure and dynamics in axon shafts have remained obscure for decades. A major barrier in the field has been imaging actin. Actin exists as soluble monomers (G-actin) as well as actin filaments (F-actin), and labeling actin with conventional fluorescent probes like GFP/RFP typically leads to a diffuse haze that makes it difficult to discern kinetic behaviors. In a recent publication, we used F-actin selective probes to visualize actin dynamics in axons, resolving striking actin behaviors that have not been described before. However, using these probes to visualize actin dynamics is challenging as they can cause bundling of actin filaments; thus, experimental parameters need to be strictly optimized. Here we describe some practical methodological details related to using these probes for visualizing F-actin dynamics in axons.

Visualizing APP and BACE-1 approximation in neurons yields insight into the amyloidogenic pathway

Cleavage of APP (amyloid precursor protein) by BACE-1 (β-site APP cleaving enzyme-1) is the rate-limiting step in amyloid-beta (Aβ) production and a neuropathologic hallmark of Alzheimer's disease (AD); thus physical approximation of this substrate-enzyme pair is a critical event with broad biological and therapeutic implications. Despite much research, neuronal locales of APP/BACE-1 convergence and APP-cleavage remain unclear. Here we report an optical assay – based on fluorescence complementation – to visualize in-cellulo APP/BACE-1 interactions as a simple on/off signal. Combined with other assays tracking the fate of internalized APP in hippocampal neurons, we found that APP/BACE-1 interact in both biosynthetic and endocytic compartments; particularly along recycling-microdomains such as dendritic spines and presynaptic boutons. In axons, APP and BACE-1 are co-transported, and also interact during transit. Finally, our assay reveals that the AD-protective “Icelandic” mutation greatly attenuates APP/BACE-1 interactions, suggesting a mechanistic basis for protection. Collectively, the data challenge canonical models and provide concrete insights into long-standing controversies in the field.

A dynamic formin-dependent deep F-actin network in axons

Although actin at neuronal growth cones is well-studied, much less is known about actin organization and dynamics along axon shafts and presynaptic boutons. Using probes that selectively label filamentous-actin (F-actin), we found focal "actin hotspots" along axons-spaced ∼3-4 µm apart-where actin undergoes continuous assembly/disassembly. These foci are a nidus for vigorous actin polymerization, generating long filaments spurting bidirectionally along axons-a phenomenon we call "actin trails." Super-resolution microscopy reveals intra-axonal deep actin filaments in addition to the subplasmalemmal "actin rings" described recently. F-actin hotspots colocalize with stationary axonal endosomes, and blocking vesicle transport diminishes the actin trails, suggesting mechanistic links between vesicles and F-actin kinetics. Actin trails are formin-but not Arp2/3-dependent and help enrich actin at presynaptic boutons. Finally, formin inhibition dramatically disrupts synaptic recycling. Collectively, available data suggest a two-tier F-actin organization in axons, with stable "actin rings" providing mechanical support to the plasma membrane and dynamic "actin trails" generating a flexible cytoskeletal network with putative physiological roles.