Cyclic Nucleotide Control of Microtubule Dynamics for Axon Guidance

Abstract

Graded distribution of intracellular second messengers, such as Ca 2ϩ and cyclic nucleotides, mediates directional cell migration, including axon navigational responses to extracellular guidance cues, in the developing nervous system. Elevated concentrations of cAMP or cGMP on one side of the neuronal growth cone induce its attractive or repulsive turning, respectively. Although effector processes downstream of Ca 2ϩ have been extensively studied, very little is known about the mechanisms that enable cyclic nucleotides to steer migrating cells. Here, we show that asymmetric cyclic nucleotide signaling across the growth cone mediates axon guidance via modulating microtubule dynamics and membrane organelle transport. In embryonic chick dorsal root ganglion neurons in culture, contact of an extending microtubule with the growth cone leading edge induces localized membrane protrusion at the site of microtubule contact. Such a contact-induced protrusion requires exocytosis of vesicle-associated membrane protein 7 (VAMP7)-positive vesicles that have been transported centrifugally along the microtubule. We found that the two cyclic nucleotides counteractively regulate the frequency of microtubule contacts and targeted delivery of VAMP7 vesicles: cAMP stimulates and cGMP inhibits these events, thereby steering the growth cone in the opposite directions. By contrast, Ca 2ϩ signals elicit no detectable change in either microtubule contacts or VAMP7 vesicle delivery during Ca 2ϩ -induced growth cone turning. Our findings clearly demonstrate growth cone steering machinery downstream of cyclic nucleotide signaling and highlight a crucial role of dynamic microtubules in leading-edge protrusion for cell chemotaxis.

Reciprocal link between cell biomechanics and exocytosis

“…Exocytosis and endocytosis also play crucial roles in growth cone chemotaxis . It has been suggested that axon turning is mediated by an asymmetric balance of exocytosis in the attraction side and endocytosis in the repulsion side .…”

“…For instance, Semaphorin‐3A repulsion inhibits VAMP2‐mediated exocytosis and triggers clathrin‐dependent endocytosis . The attractive signal cAMP is able to induce VAMP7‐exocytosis in the attraction side and the repulsive cGMP induces VAMP7‐exocytosis in the opposite side …”

“…Exocytosis and endocytosis also play crucial roles in growth cone chemotaxis. [103][104][105][106][107] It has been suggested that axon turning is mediated by an asymmetric balance of exocytosis in the attraction side and endocytosis in the repulsion side. [108][109][110] For instance, Semaphorin-3A repulsion inhibits VAMP2-mediated exocytosis 103 and triggers clathrin-dependent endocytosis.…”

“…104 The attractive signal cAMP is able to induce VAMP7-exocytosis in the attraction side and the repulsive cGMP induces VAMP7-exocytosis in the opposite side. 105 Therefore, it is tempting to speculate that membrane tension could be a master regulator of axon turning, which associate the proposed membrane trafficking-dependent turning model 110 with cytoskeletal dynamics. 111 Similar to the migrating cell ( Figure 1B Another important question is the precise contribution of exocytosis to cell mechanics.…”

FIGNL1 associates with KIF1Bβ and BICD1 to restrict dynein transport velocity during axon navigation

“…Two closely related cellular processes have been shown to be critical for GC navigational behavior: cytoskeletal (Dent et al, 2011; Cammarata et al, 2016) and membrane (Itofusa and Kamiguchi, 2011; Steketee and Goldberg, 2012) dynamics. Several studies have indeed reported an asymmetric transport of vesicles to the GC periphery, followed by their exocytosis in response to attractive guidance signals (Tojima et al, 2007, 2011; Akiyama et al, 2016). Conversely, polarized endocytosis has been observed in response to repulsive signals (Tojima et al, 2010).…”