Process Outgrowth of Oligodendrocytes Is Promoted by Interaction of Fyn Kinase with the Cytoskeletal Protein Tau

Abstract

Fyn kinase plays an important role during myelination and has been shown to promote morphological differentiation of cultured oligodendrocytes. We analyzed the downstream targets of Fyn kinase in oligodendrocytes. Because process outgrowth and wrapping of axons involve cytoskeletal rearrangement, we focused on cytoskeletal proteins linked to Fyn. Here we demonstrate that Fyn binds to the cytoskeletal proteins Tau and ␣-Tubulin in oligodendrocytes. Tau interacts with the Fyn SH3 domain whereas ␣-Tubulin binds to the Fyn SH2 and SH3 domains. To study the function of the Fyn-Tau interaction in oligodendrocytes, we designed a Tau deletion mutant that would compete with endogenous Tau-Fyn binding in transfected cells. The mutant Tau protein binds to the Fyn SH3 domain but lacks the microtubuli interaction domain and thus cannot bind to microtubuli. In the presence of the mutant Tau protein, a reduction of the process number and process length in oligodendroglial cells was observed. This effect is likely to be caused by interference with the Fyn-Tau-microtubuli cascade rather than inactivation of the kinase, because Fyn bound to the mutant Tau retains activity. A similar inhibition of process outgrowth was observed when oliogodendroglial cells were cultured in the presence of Fumonisin B1, an inhibitor of sphingolipid synthesis that prevents the formation of rafts. Because ligation of the cell adhesion molecule F3 on oligodendrocytes leads to activation of Fyn kinase localized in rafts, these findings suggest that recruitment of Tau and Tubulin to activated Fyn kinase in rafts is an important step in the initiation of myelination. Key words: myelination; oligodendrocytes; Fyn kinase; cytoskeleton; Tau; glycosphingolipid-rich raftsThe myelination of axons by oligodendrocytes involves the coordinated recognition of the axonal surface, ensheathment of the axonal process, and ultimately compaction of the wrappings of oligodendroglial membrane to generate the myelin sheath. The interplay of the adhesion molecules expressed by oligodendrocytes and axons as well as the downstream signal transduction cascades mediating these complex cellular interactions still remain primarily unelucidated (Pedraza et al., 2001).Substantial evidence exists that Fyn expressed by oligodendrocytes plays an important role in myelination. Mice deficient for Fyn kinase are hypomyelinated (Umemori et al., 1994;). In addition, knock-in mice expressing a kinase-defective Fyn protein that cannot be activated because of a mutation in the ATP-binding site of Fyn show severe myelination defects . Inhibition of the Fyn activity in cultured oligodendrocytes using kinase inhibitors or dominant negative Fyn constructs inhibited the process outgrowth of the cells (Osterhout et al., 1999). We have shown that in oligodendrocytes [as in neurons (Zisch et al., 1995)] the F3 adhesion molecule is complexed to the Src-family tyrosine kinase Fyn and that this association takes place within glycosphingolipid (gsl)/cholesterol-rich microdomains (Kramer et al., 1999), ge...

Tau Potentiates Nerve Growth Factor-induced Mitogen-activated Protein Kinase Signaling and Neurite Initiation without a Requirement for Microtubule Binding

“…The CP17-reactive Tau was most likely located in the lipid raft fraction of the cells because solubilization of the cells for the immunoprecipitation required the use of a stringent buffer to recover all of the Tau; solubilization with 1% Triton X-100 resulted in much of the NGF-induced Tau pelleting before the immunoprecipitation (data not shown). Lipid raft-associated Tau has been previously reported (43)(44)(45)(46). These data suggested a novel function for Thr-231-phosphorylated Tau in response to NGF.…”

Absence of Tau triggers age‐dependent sciatic nerve morphofunctional deficits and motor impairment

“…In line with previous studies demonstrating an absence of axonal deficits in neurons of Tau−/− animals (Yuan et al ., 2008; Vossel et al ., 2010), there were no differences in mRNA levels of transcription factors that critically regulate myelination process and/or maintenance, for example, Krox20 and Oct6 (data not shown), suggesting no gross changes in myelin gene regulation of Tau−/− Schwann cells. Furthermore, supporting the involvement of Tau and other cytoskeletal elements in myelination process, previous evidence suggests that Tau strongly colocalizes with MBP in distal tips of oligodendrocytes (LoPresti et al ., 1995; Muller et al ., 1997), suggesting that transportation and/or local MBP translation may require microtubule cytoskeleton and might be controlled by Tau–Fyn interaction (Klein et al ., 2002). As recent evidence suggests that Tau is responsible to locate Fyn in spines and Fyn has a well‐described role in myelination of both CNS and PNS (Kramer‐Albers & White, 2011), the above Tau–Fyn–MBP complex could be disrupted in the absence of Tau protein, affecting myelination signaling and process.…”

Src-family tyrosine kinase fyn phosphorylates phosphatidylinositol 3-kinase enhancer-activating Akt, preventing its apoptotic cleavage and promoting cell survival

“…For example, fyn is essential for myelin development in the central nervous system (CNS) and mediates learning and memory. 13,14,28,29 Conceivably, fyn-mediated PIKE-A phosphorylation might regulate its biological functions in the brain. In summary, our data provide compelling evidence that active fyn is a physiological tyrosine kinase for PIKE-A.…”