Netrin 1 and Dcc regulate oligodendrocyte process branching and membrane extension via Fyn and RhoA

Abstract

The molecular mechanisms underlying the elaboration of branched processes during the later stages of oligodendrocyte maturation are not well understood. Here we describe a novel role for the chemotropic guidance cue netrin 1 and its receptor deleted in colorectal carcinoma (Dcc) in the remodeling of oligodendrocyte processes. Postmigratory, premyelinating oligodendrocytes express Dcc but not netrin 1, whereas mature myelinating oligodendrocytes express both. We demonstrate that netrin 1 promotes process extension by premyelinating oligodendrocytes in vitro and in vivo. Addition of netrin 1 to mature oligodendrocytes in vitro evoked a Dcc-dependent increase in process branching. Furthermore, expression of netrin 1 and Dcc by mature oligodendrocytes was required for the elaboration of myelin-like membrane sheets. Maturation of oligodendrocyte processes requires intracellular signaling mechanisms involving Fyn, focal adhesion kinase (FAK), neuronal Wiscott-Aldrich syndrome protein (N-WASP) and RhoA; however, the extracellular cues upstream of these proteins in oligodendrocytes are poorly defined. We identify a requirement for Src family kinase activity downstream of netrin-1-dependent process extension and branching. Using oligodendrocytes derived from Fyn knockout mice, we demonstrate that Fyn is essential for netrin-1-induced increases in process branching. Netrin 1 binding to Dcc on mature oligodendrocytes recruits Fyn to a complex with the Dcc intracellular domain that includes FAK and N-WASP, resulting in the inhibition of RhoA and inducing process remodeling. These findings support a novel role for netrin 1 in promoting oligodendrocyte process branching and myelin-like membrane sheet formation. These essential steps in oligodendroglial maturation facilitate the detection of target axons, a key step towards myelination.

PIKEis essential for oligodendroglia development and CNS myelination

“…Given that netrin 1 also regulates OPC migration and differentiation (34), PIKE-L may be an executor of netrin 1 to promote OL differentiation. In addition, netrin 1 is required to induce OL branching and formation of myelin-like sheet in vitro (35), which fits with our observation that PIKE-L is necessary for OL complexity development. We also found that the amount of QKI proteins is reduced in PIKE −/− OLs.…”

Inhibition of CNS Remyelination by the Presence of Semaphorin 3A

“…However, the same group subsequently reported that netrin-1 does not affect the differentiation of immature or mature OPCs in vitro (observation period, 24 h). Moreover, data included in our paper demonstrate that netrin-1 is able to exert positive effects on process formation, branching, and myelin-like sheet formation (Rajasekharan et al, 2009). The temporal switch from netrin-1-mediated chemorepulsion and process inhibition to process elaboration occurring in OPCs was subsequently shown to be dependent on RhoA signaling (Rajasekharan et al, 2009).…”

Protein-tyrosine Phosphatase α Acts as an Upstream Regulator of Fyn Signaling to Promote Oligodendrocyte Differentiation and Myelination

“…Oligosphere-derived OPCs isolated from PTP␣ Ϫ/Ϫ mouse embryos likewise exhibit an OL differentiation defect as deter- Fyn is activated during OPC differentiation, and this is critical for morphological differentiation, maturation, and CNS myelination (9,10,25,59). Several upstream molecules stimulate Fyn activity in this process, including the ligand-receptor interactions of extracellular matrix components like vitronectin and fibronectin with ␤1 integrins, laminin 2 binding to ␣6␤1 integrin, and the laminin family member netrin 1 and its receptor Dcc (12)(13)(14). Other receptors such as FcR␥ (upon cross-linking of bound immunoglobulin G) (11) and the PTP CD45 (32) can also promote Fyn activation during OL differentiation/myelination.…”

“…Diverse Fyn-FAK signaling mechanisms that regulate these RhoGTPases during OL differentiation have been described. For example, laminin stimulation induces Fyn-FAK-Rac1/Cdc42 signaling in OL differentiation (19), whereas netrin-1 stimulates recruitment of Fyn to the netrin-1 receptor Dcc that is complexed with FAK and thereby promotes the inhibition of Rho without affecting Rac1/ Cdc42 (14). The ablated Fyn-FAK to RhoGTPase signaling in PTP␣-deficient cells, comprising impaired activation of Rac1 and Cdc42 and defective inhibition of Rho, is likely a major defect contributing to their reduced process extension and maturation.…”

“…In vitro studies have linked Fyn activation or inhibition to several stimuli that, respectively, induce or inhibit OL differentiation and maturation. Fyn is required for and activated in OL differentiation by serum withdrawal, IGF-1, ␤1 integrin stimulation (for example, by laminin binding to ␣6␤1 integrin), netrin-1 interaction with the receptor Dcc, and antibody-mediated cross-linking of MAG or FcR␥ (8,[10][11][12][13][14][15]. Recently, co-stimulation of an integrin-contactin complex in OLs was found to amplify Fyn activation and promote myelination (16).…”