Neurofilaments form flexible bundles during neuritogenesis in culture and in mature axons in situ

• Published in: Journal of neuroscience research Vol. 97; no. 10; pp. 1306 - 1318
• Main Authors: Lee, Sangmook, Eyer, Joel, Letournel, Franck, Boumil, Edward, Hall, Garth, Shea, Thomas B.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.10.2019; Wiley
• Subjects: Axon guidance; axonal cytoskeleton; Axonogenesis; Axons; Bundles; Bundling; Cell culture; Cytoskeleton; Flexibility; Life Sciences; neurofilament; Neurofilaments; pathfinding; remodeling; RRID:AB_2314901; RRID:AB_2315331; RRID:AB_521686; pathfinding; remodeling; RRID:AB_2314901; neurofilament; RRID:AB_521686; axonal cytoskeleton; RRID:AB_2315331; Remodeling
• ISSN: 0360-4012; 1097-4547
• DOI: 10.1002/jnr.24482

Neurofilaments (NFs) undergo cation‐dependent phospho‐mediated associations with each other and other cytoskeletal elements that support axonal outgrowth. Progressive NF‐NF associations generate a resident, bundled population that undergoes exchange with transporting NFs. We examined the properties of bundled NFs. Bundles did not always display a fully linear profile but curved and twisted at various points along the neurite length. Bundles retracted faster than neurites and retracted bundles did not expand following extraction with Triton, indicating that they coiled passively rather than due to pressure from the cell. Bundles consisted of helically wound NFs, which may provide flexibility necessary for turning of growing axons during pathfinding. Interactions between NFs and other cytoskeletal elements may be disrupted en masse during neurite retraction or regionally during remodeling. It is suggested that bundles within long axons that cannot be fully retracted into the soma could provide maintain proximal support yet still allow more distal flexibility for remodeling and changing direction during pathfinding. Prior studies demonstrate that neurofilaments (NFs) undergo cation‐dependent phospho‐mediated associations with each other and other cytoskeletal elements that support axonal outgrowth. Some neurofilaments (NFs) undergo lateral associations leading to formation of bundles in culture and in situ. Bundles are flexible, which may accommodate pathfinding and remodeling. They can undergo retraction independently of overall neurite retraction suggesting that interactions between NFs and other cytoskeletal elements may be disrupted en masse during neurite retraction or regionally during remodeling. Bundles are observed following expression of GFP‐tagged NF‐H in axonal neurites of NB2a/d1 cells (a) and murine sciatic nerve (b) Ultrastructural examination of negative stained preparations (c) and thin sectioning (d) suggest helical coiling (arrows; not all coils are indicated). Panel e depicts a bundle undergoing retraction faster than that of the overall neurite (arrow indicates the neuritic tip; numbers indicate minutes of image capture.