Growth and elongation within and along the axon

Abstract

Mechanical tension is a particularly effective stimulus for axonal elongation, but little is known about how it leads to the formation of new axon. To better understand this process, we examined the movement of axonal branch points, beads bound to the axon, and docked mitochondria while monitoring axonal width. We found these markers moved in a pattern that suggests elongation occurs by viscoelastic stretching and volume addition along the axon. To test the coupling between "lengthening" and "growth," we measured axonal width while forcing axons to grow and then pause by controlling the tension applied to the growth cone or to the cell body. We found axons thinned during high rates of elongation and thickened when the growth cones were stationary. These findings suggest that forces cause lengthening because they stretch the axon and that growth occurs, in a loosely coupled step, by volume addition along the axon.

Cytoplasmic dynein pushes the cytoskeletal meshwork forward during axonal elongation

“…Along the length of the axon, docked mitochondria slowly move forward at a rate of 1-50 mm/h, with a velocity profile that increases along the length of the axon (O'Toole et al, 2008). In previous studies, we have shown that this is paired with the forward advance of beads bound to the outside of the axon and axonal branch points (Lamoureux et al, 2010a), which together indicates that this slow movement reflects the bulk movement or stretching of the underlying cytoskeletal meshwork. Because this movement occurs at a rate of ,1000 times slower than fast transport, it is straightforward to distinguish between this 'low velocity transport' and fast axonal transport in kymographs (Miller and Sheetz, 2006).…”

“…By convention, the axonal cytoskeleton was considered to be stationary during elongation and it was thought that the new axon was formed by the addition of new material at the growth cone, either through cytoskeleton polymerization or the deposition of material by motor-driven transport (Goldberg and Burmeister, 1986;Dent and Gertler, 2003;Lowery and Van Vactor, 2009). Recently, however, there have been several studies of bulk translocation in multiple model systems, such as Aplysia growth cones (Lee and Suter, 2008), cultured chick sensory neurons (Miller and Sheetz, 2006;Lamoureux et al, 2010a) and Drosophila motoneurons in vivo (Roossien et al, 2013). They indicate that the classic findings of bulk microtubule translocation in Xenopus neurons (Reinsch et al, 1991;Chang et al, 1998) were not a species-specific phenomenon, but rather a broadly conserved mechanism for elongation.…”

Axon Stretch Growth: The Mechanotransduction of Neuronal Growth

“…In the second approach, forces are applied directly to the axon by pulling on the growth cone. Using glass needles, this latter approach has been used to model formation of new axon (Bernal, Pullarkat et al 2007; O'Toole, Lamoureux et al 2008;Lamoureux, Heidemann et al 2010), to identify force thresholds for elongation and retraction (Dennerll, Lamoureux et al 1989;Zheng, Lamoureux et al 1991;Lamoureux, Zheng et al 1992), and to analyze neurotransmitter clustering in axon terminals (Siechen, Yang et al 2009). A unique tissue engineering extension of this method was developed to produce large nerve constructs using an automated Axon Stretch Growth (ASG) bioreactor system (Iwata, Browne et al 2006;Pfister, Iwata et al 2006).…”

Using fly genetics to dissect the cytoskeletal machinery of neurons during axonal growth and maintenance

“…At the same time, there seems to be a steady and dynamic MT turnover, as suggested by continued MT polymerisation events in mature axons (Kollins et al, 2009). Such dynamics might allow for structural plasticity, such as shortening or extension of the axon shaft (Küppers-Munther et al, 2004;Lamoureux et al, 2010;Rajagopalan et al, 2010), pruning and regrowth of axons or formation of new side branches (Letourneau, 2009), which are required for rewiring processes during learning and memory formation or the regeneration of damaged axons (Bradke et al, 2012;Saxena and Caroni, 2007).…”