Revisiting Netrin-1: One Who Guides (Axons)

Abstract

Proper patterning of the nervous system requires that developing axons find appropriate postsynaptic partners; this entails microns to meters of extension through an extracellular milieu exhibiting a wide range of mechanical and chemical properties. Thus, the elaborate networks of fiber tracts and non-fasciculated axons evident in mature organisms are formed via complex pathfinding. The macroscopic structures of axon projections are highly stereotyped across members of the same species, indicating precise mechanisms guide their formation. The developing axon exhibits directionally biased growth toward or away from external guidance cues. One of the most studied guidance cues is netrin-1, however, its presentation in vivo remains debated. Guidance cues can be secreted to form soluble or chemotactic gradients or presented bound to cells or the extracellular matrix to form haptotactic gradients. The growth cone, a highly specialized dynamic structure at the end of the extending axon, detects these guidance cues via transmembrane receptors, such as the netrin-1 receptors deleted in colorectal cancer (DCC) and UNC5. These receptors orchestrate remodeling of the cytoskeleton and cell membrane through both chemical and mechanotransductive pathways, which result in traction forces generated by the cytoskeleton against the extracellular environment and translocation of the growth cone. Through intracellular signaling responses, netrin-1 can trigger either attraction or repulsion of the axon. Here we review the mechanisms by which the classical guidance cue netrin-1 regulates intracellular effectors to respond to the extracellular environment in the context of axon guidance during development of the central nervous system and discuss recent findings that demonstrate the critical importance of mechanical forces in this process.

Keywords: DCC; UNC5; axon guidance; chemotaxis; growth cone; haptotaxis; netrin-1.

FIGURE 1

Axon guidance cues attract or repulse growth cones, and can be presented adhered to the extracellular matrix or soluble. The data accumulated over the last 30 years suggest that netrin-1 acts both as an attractive and repulsive cue, and may function both as a soluble, chemotactic cue and as a substrate-adhered, haptotactic cue.

FIGURE 2

Diagrams of the known netrin-1 receptor dimers and their signaling functions. Netrin-1 binding simultaneously to two DCC molecules induces DCC homodimers, which similarly occurs for neogenin. If netrin is substrate bound, this could crosslink DCC dimers to the extracellular matrix. However, the signaling result of neogenin dimers is not currently known. Whether netrin-1 also links neogenin:neogenin, UNC5:DCC, or UNC5:DSCAM dimers to the extracellular matrix is also unknown, but represents potential mechanotransductive mechanisms in netrin-1 signaling. Structural studies have not been conducted on the UNC5:DSCAM dimer, therefore whether a single netrin-1 molecule binds both receptors is unknown.

FIGURE 3

Summary of known “switches” between attractive and repulsive function of netrin-1. Switches span from receptor concentrations at the plasma membrane, cytoplasmic second messenger status, extracellular netrin-1 concentrations, and the presence of other extracellular cues.

FIGURE 4

The concentration of netrin-1 may switch attraction to repulsion responses. Attractive and repulsive forces generated by DCC and UNC5 dimers are represented as green and red arrows, respectively, leading to an axon outgrowth vector (black arrows). As the concentration of netrin-1 begins to increase, more DCC homodimers are recruited. At higher netrin-1 concentrations, the promiscuous receptor binding site on netrin begins to recruit UNC5 to dimers, as this binding site has a lower affinity for UNC5 than for DCC. At the highest concentrations of netrin-1, saturation of receptors with ligand may result in receptors being maintained as monomers as opposed to dimerization, preventing downstream signaling that requires a dimer. By virtue of the concentration dependence of attraction and repulsion responses, axons are guided into a concentration in which forces are balanced. However, this may not be a stable position as receptor trafficking, intracellular secondary messengers and extracellular conditions can alter growth cone sensitivity to netrin-1.

FIGURE 5

Model of known signaling pathways and interactions downstream of DCC in netrin-1 dependent axon guidance. DCC interacts with several enzymes and adaptor proteins and components of the actin and microtubule cytoskeletons in the absence of netrin-1, forming a “primed” signaling complex, which can rapidly initiate responses to ligand binding. Netrin-1 increases valency through multimerization of DCC homodimers. This clustering brings intracellular domains of the receptors into close apposition, forming a scaffold for the recruitment and activation of several proteins. Solid green arrows denote direct steps in activation, whereas dashed green arrows are known connections that may have intermediates. These pathways together modify the intracellular environment to promote directional axon outgrowth in response to netrin-1. AKAP79, A kinase anchoring protein 79; ARP2/3, actin-related protein 2/3 complex; CDC42, cell division control protein 42; DCC, deleted in colorectal cancer; DOCK1, dedicator of cytokinesis 1; ELK1, ETS transcription factor; ERK, mitogen activated protein kinase; ERM, ezrin-radixin-moesin; FAK, focal adhesion kinase; FN3, fibronectin type 3 domain; Ig, immunoglobulin domain; JNK1, c-Jun N-terminal kinase 1; MARCKS, myristoylated alanine-rich C kinase substrate; MEK, mitogen activated protein kinase kinase; MENA, mammalian enabled; MYOII, myosin II; MYOX, unconventional myosin X; NCK1, NCK adaptor protein 1; N-WASP, neuronal Wiskott–Aldrich syndrome protein; PAK1, protein associated kinase 1; PKA, protein kinase A; PKB, protein kinase B; PKC, protein kinase C; PLCγ, phospholipase C gamma; RAC1, Ras-related C3 botulinum toxin substrate 1; RHOA, Ras homolog gene family member A; ROCK, Rho associated protein kinase; SFKs, Src family kinases; SNAP25, synaptosomal associated protein 25; SYTX1, syntaxin-1; TIAM1, T-lymphoma invasion and metastasis protein 1; TRIM9, tripartite motif protein 9; TRIO, triple domain functional protein; VAMP2, vesicle associated membrane protein 2; VAMP7, vesicle associated membrane protein 7; VASP, vasodilator stimulated phosphoprotein.

FIGURE 6

Theoretical model of FAK activation by mechanical forces (adapted from Moore et al., 2012). In the inactive state, the N-terminal FERM domain of FAK interacts with the kinase domain, preventing activation. Upon netrin-1 binding, tension on the actin cytoskeleton (through actin treadmilling and motor protein function) is transduced through ezrin-radixin-moesin (ERM) proteins to the N-terminus of FAK. The immobilization of the C-terminus of FAK through interaction with DCC, which is adhered to the extracellular matrix through its interaction with netrin-1, leads to increased mechanical force across FAK. This causes a conformational change that exposes the kinase domain, allowing autophosphorylation and subsequent activation of FAK.

FIGURE 7

Summary of known signaling pathways downstream of UNC5 in repulsive netrin-1 signaling. Interaction between the intracellular DB domain of UNC5 and the P1 domain of DCC produces a scaffold similar to that in DCC homodimers. FAK and Src are phosphorylated and activated as in attractive netrin-1 signaling, however, the functional outcomes are different. CDC42, cell division control protein 42; CRMP, collapsin response mediating protein; DB, DCC-binding domain; DD, death domain; FAK, focal adhesion kinase; FMOs, Flavin monooxygenases; Ig, immunoglobulin domain; MAX2, more axillary growth 2; PLEKHH1, pleckstrin homology MyTH and FERM domain containing protein H1; RAC1, Ras-related C3 botulinum toxin substrate 1; RHOA, Ras homolog gene family member A; SHP2, protein tyrosine phosphatase 2C; SRC, proto-oncogene tyrosine-protein kinase Src; TRIO, triple domain functional protein; TSP, thrombospondin type 1 domain; ZU5, ZO-1/Unc5 domain.

FIGURE 8

Summary of known interactions linking the transmembrane receptors DCC and UNC5 to the membrane and cytoskeleton. Both receptors are connected to the extracellular matrix through interaction with netrin-1. DCC, deleted in colorectal cancer; ERM, ezrin-radixin-moesin; ICD, intracellular domain; MARCKS, myristoylated alanine-rich C kinase substrate; MYOX, unconventional myosin X; TRIM9, tripartite motif protein 9; UNC5, uncoordinated locomotion 5; VASP, vasodilator stimulated phosphoprotein.

FIGURE 9

Model for fasciculation induced by draxin-netrin-1 interaction with DCC (adapted from Liu et al., 2018). Interaction between draxin and netrin-1 potentially links together DCC receptors from adjacent axons, causing fasciculation. This could additionally lead to higher-order multimers of DCC, which would induce attractive signaling cascades along with linking axons.