Tau impacts on growth-factor-stimulated actin remodeling

Abstract

The microtubule-associated protein tau interacts with the SH3 domain of non-receptor Src family protein tyrosine kinases. A potential consequence of the SH3 interaction is the upregulation of tyrosine kinase activity. Here we investigated the activation of Src or Fyn by tau, both in vitro and in vivo. Tau increased the kinase activity in in vitro assays and in transfected COS7 cells. In plateletderived growth factor (PDGF)-stimulated fibroblasts, tau appeared to prime Src for activation following PDGF stimulation, as reflected by changes in Src-mediated actin rearrangements. In addition, while fibroblasts normally recovered actin stress fibers by 5-7 hours after PDGF stimulation, tau-expressing cells showed sustained actin breakdown. Microtubule association by tau was not required for the observed changes in actin morphology. Inhibition of Src kinases or a mutant deficient in Src interaction reduced the effects, implicating Src family protein tyrosine kinases as a mediator of the effects of tau on actin rearrangements. Our results provide evidence that the interaction of tau with Src upregulates tyrosine kinase activity and that this interaction allows tau to impact on growth-factor-induced actin remodeling. Journal of Cell Science Tau impacts on actin remodeling Results Tau affects SFK activityWe have previously shown through in vitro binding assays that human tau interacted with the SH3 domain of Fyn and Src (Bhaskar et al., 2005;Lee et al., 1998). Based on the crystal structure of Src, SH3 ligands are thought to 'unclamp' an inactive form of Src, enhancing its activation (Xu et al., 1999;Xu et al., 1997). Our first indication that tau might function in this manner came from experiments initially performed to examine the ability of tyrosine-phosphorylated tau to associate with microtubules. Purified tau and Fyn were incubated in an in vitro kinase reaction to first generate tyrosinephosphorylated tau. The reaction was then added to taxolstabilized microtubules. Upon examining the microtubule pellet for tyrosine-phosphorylated tau, we discovered that the taxol-stabilized tubulin was also tyrosine phosphorylated, presumably by the Fyn that had been carried over from the kinase reaction. Moreover, although Fyn alone was capable of phosphorylating taxol-stabilized microtubules, the presence of tau greatly enhanced the phosphorylation (Fig. 1A, compare lanes 4 and 8).However, because tau binds to microtubules, a possible explanation for the enhanced tubulin phosphorylation was that tau was simply bringing Fyn to the microtubules and facilitating tubulin phosphorylation without affecting Fyn activity. This would predict that a fragment of tau lacking microtubule-binding activity would be incapable of enhancing the phosphorylation. We therefore tested an N-terminal fragment of tau that binds to Fyn SH3 but lacks the microtubule-binding repeats [amino acids 1-184 (Lee et al., 1998)]. By using the same protocol in which the tau fragment was incubated with Fyn and then added to taxol-stabilized microtubules, we found ...

Membrane‐bound β‐amyloid oligomers are recruited into lipid rafts by a fyn‐dependent mechanism

“…Tau can also induce alterations in the actin cytoskeleton, resulting in subsequent neurodegeneration; this is potentiated by A␤ (26). Furthermore, tau has been reported to enhance the kinase activity of fyn in tau-mediated actin rearrangement (27). Taken together, fyn and tau may act in concert to link external A␤ accumulation to cytoskeletal dysregulation and our observations reported here are consistent with this occurring in lipid rafts.…”

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

“…Because process initiation in Tau-depleted cell lines could not be restored by providing a microtubule stabilizing agent, these findings suggested that a Tau function other than microtubule stabilization was involved in neurite initiation. This was further investigated by testing a phosphomimetic Tau mutant, S262D/S356D, which has a significantly reduced affinity for microtubules both in vitro and in cells (29,(32)(33)(34) (supplemental Fig. S1).…”

“…Plasmids expressing the 0N3R human Tau mutants S262D/S356D and T231D/S235D have been previously described (29).…”

Tau Aggregation and Toxicity in Tauopathic Neurodegenerative Diseases

“…Third, tau can enter into reactions other than aggregation, including binding reactions with the SH3 domains of various signal transduction enzymes [56], with other microtubule associated proteins [1], and with the active sites of proteases leading to tau fragmentation [2,48]. These interactions can generate toxicity unrelated to aggregation and therefore complicate interpretation of modeling studies.…”