Crowd Control: Effects of Physical Crowding on Cargo Movement in Healthy and Diseased Neurons

Abstract

High concentration of cytoskeletal filaments, organelles, and proteins along with the space constraints due to the axon's narrow geometry lead inevitably to intracellular physical crowding along the axon of a neuron. Local cargo movement is essential for maintaining steady cargo transport in the axon, and this may be impeded by physical crowding. Molecular motors that mediate active transport share movement mechanisms that allow them to bypass physical crowding present on microtubule tracks. Many neurodegenerative diseases, irrespective of how they are initiated, show increased physical crowding owing to the greater number of stalled organelles and structural changes associated with the cytoskeleton. Increased physical crowding may be a significant factor in slowing cargo transport to synapses, contributing to disease progression and culminating in the dying back of the neuronal process. This review explores the idea that physical crowding can impede cargo movement along the neuronal process. We examine the sources of physical crowding and strategies used by molecular motors that might enable cargo to circumvent physically crowded locations. Finally, we describe sub-cellular changes in neurodegenerative diseases that may alter physical crowding and discuss the implications of such changes on cargo movement.

Keywords: axonal transport; cytoskeleton; molecular motors; neurodegeneration; neuron; organelles; physical crowding.

FIGURE 1

Schematic representation of sources of physical crowding in the axon. Magnified view of features that contribute to crowding shown in insets. (A) Actin cortical rings, deep actin. (B) Neurofilaments can physically crowd the neuron by excluding organelles and small molecules. (C) High concentration of soluble proteins and narrow axonal geometry can lead to a local increase in viscosity. (D) Stalled cargo can physically impede the movement of motile cargo and diffusive proteins. (E) The MT bundle can exclude organelles but may allow diffusion of small proteins.

FIGURE 2

Electron micrograph of myelinated axons from Central Nervous System (CNS) and Peripheral Nervous System (PNS). (Top panel) Some myelinated axons in the CNS have mitochondria (M) that can be up to half the diameter of the neuronal process. In the magnified view on the top right, we can see a number of microtubules (solid arrows) and neurofilaments (arrowheads). (Bottom panel) Small caliber axons in the Remak bundles (R) can be as thin as 200 nm. We also see the axon filled with mitochondria (M), microtubules (solid arrows), neurofilaments (arrowheads) and vesicles. Reprinted with permission from Frontiers in Neuroscience, 12, (2018) p. 467 (Stassart et al., 2018).

FIGURE 3

Strategies to maintain cargo movement that can bypass physical crowding. (A) Multiple active motors can increase pulling force or reverse. (B) Motors can switch to a MT track that is less crowded. (C) Macromolecular complexes can be transported by transient association with organelles interspersed potentially with diffusion. (D) Switch to an actin track from a MT track. (E) Small soluble proteins can preferentially diffuse deep within the axon (arrows indicate diffusion) rich in MTs and other cytoskeletal elements. Some MAPs can also diffuse along MTs. These proteins can evade crowding near the axonal cortex.

FIGURE 4

Defects observed in neurodegenerative diseases. (A) Defective transport or increased physical crowding leads to increase in stalled cargo by disengaging motor-based transport as indicated by arrows. (B) Increased aggregation of cytosolic proteins lead to increased viscosity in the entire neuron. (C) MLOs may undergo a phase transition from a fluid to a gel-like state which further crowds the neuron as illustrated by the arrow. (D) Discontinuous coverage of MTs throughout the axon or (E) collapsed MT bundle lead to jamming of cargo due to unavailability of tracks and defects in cytosolic protein diffusion respectively in the neuronal process.

FIGURE 5

Sub-cellular events after injury/disease leading to axonal swellings. (i.A) Large tensile or torsional forces can buckle the cytoskeleton of the axon leading to local swelling of the axonal membrane. (i.B) Axonal swellings are temporarily locally uncrowded and allow free diffusion of proteins as indicated by lighter green. (i.C) Eventual accumulation at the time scale of minutes of organelles and destabilization of MTs in the swelling can lead to a loss of the movement of organelles and proteins illustrated by a darker shade of green. (i.D) Axonal swelling further may act as a locally crowded region promoting aggregation of proteins that later in life may predispose the neuron to degeneration. (ii.A) In the case of demyelinating diseases, external factors lead to a redistribution of mitochondria along the axons. (ii.B) Increase in mitochondria in a region may lead to increased crowding in its vicinity that reduces movement of organelles and cytosolic proteins as illustrated by a darker shade of green. (ii.C) Reduced movement of organelles and proteins at a region leads to swelling of the plasma membrane. (ii.D) Axonal swelling filled with organelles can lead to increased disruption of transport.