Mechanistic advances in axon pathfinding

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Abstract

The development of a functional nervous system entails establishing connectivity between appropriate synaptic partners. During axonal pathfinding, the developing axon navigates through the extracellular environment, extending toward postsynaptic targets. In the early 1900s, Ramon y Cajal suggested that the growth cone, a specialized, dynamic, and cytoskeletal-rich structure at the tip of the extending axon, is guided by chemical cues in the extracellular environment. A century of work supports this hypothesis and introduced myriad guidance cues and receptors that promote a variety of growth cone behaviors including extension, pause, collapse, retraction, turning, and branching. Here, we highlight research from the last two years regarding pathways implicated 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).

References (91)

  • K.T. Chan et al.

    Regulation of adhesion dynamics by calpain-mediated proteolysis of focal adhesion kinase (FAK)

    J Biol Chem

    (2010)
  • T. Tojima

    Attractive axon guidance involves asymmetric membrane transport and exocytosis in the growth cone

    Nat Neurosci

    (2007)
  • T. Verreet et al.

    Syntaphilin-mediated docking of mitochondria at the growth cone is dispensable for axon elongation in vivo

    eNeuro

    (2019)
  • C.-W. He et al.

    Wnt signalling in the development of axon, dendrites and synapses

    Open Biol

    (2018)
  • F. Mills

    Cadherins mediate cocaine-induced synaptic plasticity and behavioral conditioning

    Nat Neurosci

    (2017)
  • P. Hotulainen

    Defining mechanisms of actin polymerization and depolymerization during dendritic spine morphogenesis

    J Cell Biol

    (2009)
  • A. Chazeau

    Nanoscale segregation of actin nucleation and elongation factors determines dendritic spine protrusion

    EMBO J

    (2014)
  • L.A. Lowery et al.

    The trip of the tip: understanding the growth cone machinery

    Nat Rev Mol cell Biol

    (2009)
  • A.L. Kolodkin et al.

    Mechanisms and molecules of neuronal wiring: a primer

    Cold Spring Harb Perspect Biol

    (2011)
  • N.V. Vysokov

    The mechanism of regulated release of lasso/teneurin-2

    Front Mol Neurosci

    (2016)
  • N.V. Vysokov

    Proteolytically released Lasso/teneurin-2 induces axonal attraction by interacting with latrophilin-1 on axonal growth cones

    Elife

    (2018)
  • A.M. Taylor et al.

    Passive microfluidic chamber for long-term imaging of axon guidance in response to soluble gradients

    Lab Chip

    (2015)
  • K.L.W. Sun

    Netrins: versatile extracellular cues with diverse functions

    Development

    (2011)
  • G. Ahmed

    Draxin inhibits axonal outgrowth through the netrin receptor DCC

    J Neurosci

    (2011)
  • H. Song

    Conversion of neuronal growth cone responses from repulsion to attraction by cyclic nucleotides | lisa McKerracher - academia.edu

    Science

    (1998)
  • N.P. Boyer et al.

    Revisiting netrin-1: one who guides (axons)

    Front Cell Neurosci

    (2018)
  • S.G. Varadarajan

    Netrin1 produced by neural progenitors, not floor plate cells, is required for axon guidance in the spinal cord

    Neuron

    (2017)
  • C. Dominici

    Floor-plate-derived netrin-1 is dispensable for commissural axon guidance

    Nature

    (2017)
  • J.A. Moreno-Bravo et al.

    Synergistic activity of Floor-Plate- and ventricular-zone-derived netrin-1 in spinal cord commissural axon guidance

    Neuron

    (2019)
  • Z. Wu

    Long-range guidance of spinal commissural axons by Netrin1 and sonic hedgehog from midline floor plate cells

    Neuron

    (2019)
  • S.G. Varadarajan

    Netrin1 produced by neural progenitors, not floor plate cells, is required for axon guidance in the spinal cord

    Neuron

    (2017)
  • T.E. Kennedy et al.

    Axon guidance by diffusible chemoattractants: a gradient of netrin protein in the developing spinal cord

    J Neurosci

    (2006)
  • E. Stein et al.

    Hierarchical organization of guidance receptors: silencing of netrin attraction by slit through a Robo/DCC receptor complex

    Science

    (2001)
  • I. Dudanova et al.

    Integration of guidance cues: parallel signaling and crosstalk

    Trends Neurosci

    (2013)
  • S. Poliak

    Synergistic integration of netrin and ephrin axon guidance signals by spinal motor neurons

    Elife

    (2015)
  • L.-P. Croteau et al.

    Ephrin-A5 potentiates netrin-1 axon guidance by enhancing Neogenin availability

    Sci Rep

    (2019)
  • J. Ferent

    Boc acts via numb as a shh-dependent endocytic platform for Ptch1 internalization and shh-mediated axon guidance

    Neuron

    (2019)
  • S. Makihara

    Polarized dock activity drives shh-mediated axon guidance

    Dev Cell

    (2018)
  • T. Mitchison et al.

    Cytoskeletal dynamics and nerve growth

    Neuron

    (1988)
  • D.G. Jay

    The clutch hypothesis revisited: ascribing the roles of actin-associated proteins in filopodial protrusion in the nerve growth cone

    J Neurobiol

    (2000)
  • K. Abe

    Grip and slip of L1-CAM on adhesive substrates direct growth cone haptotaxis

    Proc Natl Acad Sci

    (2018)
  • T. Shimada

    Shootin1 interacts with actin retrograde flow and L1-CAM to promote axon outgrowth

    J Cell Biol

    (2008)
  • Y. Kubo

    Shootin1-cortactin interaction mediates signal-force transduction for axon outgrowth

    J Cell Biol

    (2015)
  • S. Sugio et al.

    Transient receptor potential vanilloid 2 activation by focal mechanical stimulation requires interaction with the actin cytoskeleton and enhances growth cone motility

    FASEB J

    (2017)
  • P.C. Kerstein et al.

    Mechanochemical regulation of growth cone motility

    Front Cell Neurosci

    (2015)
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