The netrin protein family

Abstract

The name netrin is derived from the Sanskrit Netr, meaning 'guide'. Netrins are a family of extracellular proteins that direct cell and axon migration during embryogenesis. Three secreted netrins (netrins 1, 3 and 4), and two glycosylphosphatidylinositol (GPI)-anchored membrane proteins, netrins G1 and G2, have been identified in mammals. The secreted netrins are bifunctional, acting as attractants for some cell types and repellents for others. Receptors for the secreted netrins include the Deleted in Colorectal Cancer (DCC) family, the Down's syndrome cell adhesion molecule (DSCAM), and the UNC-5 homolog family: Unc5A, B, C and D in mammals. Netrin Gs do not appear to interact with these receptors, but regulate synaptic interactions between neurons by binding to the transmembrane netrin G ligands NGL1 and 2. The chemotropic function of secreted netrins has been best characterized with regard to axon guidance during the development of the nervous system. Extending axons are tipped by a flattened, membranous structure called the growth cone. Multiple extracellular guidance cues direct axonal growth cones to their ultimate targets where synapses form. Such cues can be locally derived (short-range), or can be secreted diffusible cues that allow target cells to signal axons from a distance (long-range). The secreted netrins function as short-range and long-range guidance cues in different circumstances. In addition to directing cell migration, functional roles for netrins have been identified in the regulation of cell adhesion, the maturation of cell morphology, cell survival and tumorigenesis.

Figure 1

The netrin family of proteins. (a) Netrins are members of the laminin superfamily. A phylogenetic tree based on human protein sequences illustrates the relationship between netrin and laminin family members. (b) Laminin 1 is a heterotrimer composed of α (blue), β (green), and γ (turquoise) chains. The amino-terminal VI and V domains of netrins 1 to 3 (red) are homologous with the γ chain of laminin 1. These domains in netrins 4, G1 and G2 are more similar to the β chain of laminin 1. (c) Domain organization of the netrin family members. Netrins 1 to 4 are secreted proteins and contain a carboxy-terminal C-domain (C), whereas netrins G1 and G2 are linked to the plasma membrane by a GPI linker (magenta).

Figure 2

Canonical netrin receptors and their mechanism of action. (a) The DCC/neogenin family, the Unc5 homologs (Unc5A-Unc5D), and DSCAM are receptors for netrin 1. DB, DCC-binding domain; DD, death domain; Ig, immunoglobulin domain; FNIII, fibronectin type III repeat; TSP, thrombospondin type-I module; ZU5, domain homologous to part of Zona Occludens-1. (b) Signal transduction components that act downstream of DCC and Unc5 homologs. Netrin 1 binding to DCC results in the recruitment of intracellular signaling molecules associated with reorganization of the actin cytoskeleton. Netrin binding to DCC induces homodimer formation, which is the active form of the receptor. These proteins include members of the Rho family of GTPases (RhoA, Rac1, Cdc42), Src-family kinases (Src-1, pFyn), the serine/threonine kinase Pak1, the Wiskott-Aldrich syndrome related protein family (NWASP), and the Arp2/3 actin-binding complex. Proteins are shape and color-coded according to family. The tyrosine phosphatase Shp2 and the tyrosine kinase Src-1 have been implicated in Unc5 function, but signaling downstream of the Unc5 receptors is relatively poorly understood. Within neuronal growth cones, activation of protein kinase A (PKA, red star) recruits DCC to the plasma membrane from an intracellular pool of vesicles. Endocytosis of Unc5A is triggered by activation of protein kinase C (PKC, green star). Activation of PLC by DCC leads to release of calcium from intracellular calcium stores. DAG, diacylglycerol; FAK, pFAK, focal adhesion kinase; IP3, inositol trisphosphate; LARG, leukemia associated Rho guanine nucleotide exchange factor; MAP1B, microtubule associated protein 1B; Nck, an adaptor protein; PIP, phosphatidylinositol phosphate; PTP1α, protein phosphatase 1α; RGM, repulsive guidance molecule. RGM is a GPI-linked cell-membrane-associated ligand for neogenin that is not related to netrins. RGM is thought to signal growth-cone collapse through a complex of neogenin and Unc5B. The double-headed arrow with the question mark indicates that it is not clear how netrin-1 may interact with the Unc5B, neogenin, and RGM complex. For further discussion of the mechanisms regulating netrin signaling and receptor trafficking, see [34,79].