Mechanistic advances in axon pathfinding
Introduction
Formation of appropriate neural networks requires that axons navigate to and connect with appropriate targets. Defining how the growth cone at the tip of the extending axon accomplishes its vast repertoire of responses associated with establishing neuronal connectivity remains an area of intense research. Growth cone motility and axonal pathfinding involve spatial and temporal remodeling of growth cone architecture, resulting in mechanotransduction and motility (Figure 1). Attractive and repulsive guidance cues interact with receptors on the axon surface and promote or constrain growth, respectively [1]. Diffusion of soluble chemotactic cues influence axon dynamics over long distances, whereas interaction of the axon with adherent, haptotactic guidance cues influences dynamics locally. These cues activate a host of diverse signaling pathways, which results in cytoskeletal and membrane remodeling within the growth cone. Here, we review recent work on axonal pathfinding, with attention to the combinatorial synergy of pathways and parallels to morphogenesis of the dendritic spine.
Section snippets
Guidance cues direct the growth cone
Axon pathfinding begins with the guidance cues that direct growth. Over the last 30 + years, a host of guidance cues have been identified, classically divided into four families: semaphorins, ephrins, netrins, and slits. Identification of additional cues such as sonic hedgehog (Shh), Wnt, and growth factors provides additional complexity and control to axonal wiring [2]. This list continues to grow, for example, recent work showed that the extracellular domains of the trans-synaptic adhesion
The growth cone transduces mechanical force
The classic ‘clutch hypothesis’ of motility was first proposed three decades ago [26,27]. In this model, neurons are linked to the extracellular matrix (ECM) by adhesion molecules. In turn, adhesion molecules are connected to the actomyosin network by ‘clutch proteins’. Contraction of the actin cytoskeleton by myosin motors generates force, which is then transmitted to the ECM, facilitates motility (Figure 1b). In non-neuronal cells, talin and FAK are understood as the clutch between integrins
Cytoskeleton reorganization drives growth cone motility
Growth cone morphology, and in turn motility, are controlled by reorganization of the actin and microtubule cytoskeletons. Although neuronal studies have focused on Actβ, six mammalian genes encode actin [36]. Actα, Actβ, and Actγ all localize to growth cone filopodia in cultured murine motor neurons. Knockdown of any isoform decreases filopodia initiation. Loss of Actα or Actγ decreases filopodial dynamics whereas Actβ knockdown decreases growth cone size and motility [37]. Actin-based myosin
Calcium regulates growth cone signaling
Calcium signaling is essential for regulating the actin and microtubule cytoskeletons and ultimately axon guidance [55,56] (Figure 1e). For example, the actin-severing protein cofilin is activated downstream of the calcium-dependent phosphatase calcineurin in response to serotonin [57]. In serotonin-treated Aplysia bag neurons, cofilin activation is enhanced downstream of PKC and myosin II, which modulates actin density, traction stress in the growth cone, and neurite outgrowth. This suggests
Membrane trafficking delivers plasma membrane material during axon outgrowth
Unlike cytoskeleton remodeling or calcium signaling, membrane trafficking plays an underappreciated role in axon guidance. As highly polarized cells, neurons face the unique challenge of directionally targeting cargo to the growing axon. In particular, growth cone motility during neurite outgrowth is concomitant with increased plasma membrane surface area [66]. Classic hypotheses propose insertion and remodeling of membrane material occurs via calcium-regulated exocytosis and membrane flow [67,
Parallels between growth cones and dendritic spines
As cytoskeletal rearrangements dominate both growth cone motility and synaptogenesis, we asked whether pathways in the growth cone are conserved in dendritic spines (Figure 2). During synapse formation, filopodia grow and retract from dendrites, with the potential to form contacts with axons, and mature into actin-rich dendritic spines that receive synaptic transmission [75]. Similar to filopodia in the growth cone, dendritic filopodia are dynamic structures that must respond to signals in the
Conclusions
Although significant, unanswered questions remain regarding growth cone signaling and motility, a multitude of knowledge has accumulated over the past century. A theme of increasing complexity, specificity, and synergy emerges, likely lending exquisite tunability to growth cone navigation. As research continues, we expect additional convergence and specificity of pathways and cytoskeletal regulation to be revealed. Additional parallels and differences between the development of spines and
Conflict of interest statement
Nothing declared.
Acknowledgements
Funding from the National Institutes of Health supported this research: including R01GM108970 (SLG), and F31NS113381 (LEM).
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