3 DISTINCT AXONAL-TRANSPORT RATES FOR TAU, TUBULIN, AND OTHER MICROTUBULE-ASSOCIATED PROTEINS - EVIDENCE FOR DYNAMIC INTERACTIONS OF TAU WITH MICROTUBULES IN-VIVO

Microtubule-associated proteins (MAPs), such as tau, modulate neuronal shape and process outgrowth by influencing the stability and organiza tion of microtubules. The dynamic nature of MAP-microtubule interactio ns in vivo, however, is poorly understood. Here, we have assessed the stability of these interactions by investigating the synthesis and axo plasmic transport of tau in relation to that of tubulin and other MAPs within retinal ganglion cells of normal adult mice in vivo. Using imm unoprecipitation and Western blot analysis with anti-tau monoclonal an d polyclonal antibodies, we unequivocally identified in optic axons a family of 50-60 kDa tau isoforms and a second 90-95 kDa tau family, th e members of which were shown to contain the domain of tau encoded by exon 4A. To measure the rates of translocation of tau proteins in vivo , we injected mice with S-35-methionine intravitreously and, after 6-3 0 d, quantitated the radiolabeled tau isoforms immunoprecipitated from eight consecutive 1.1 mm segments of the nerve and optic tract and se parated by electrophoresis. Linear regression analysis of protein tran sport along optic axons showed that the tau isoforms advanced at a rat e of 0.2-0.4 mm/d, and other radiolabeled MAPs, identified by their as sociation with taxol-stabilized microtubules, moved three- to fivefold more rapidly. By contrast, tubulins advanced at 0.1-0.2 mm/d, signifi cantly more slowly than tau or other MAPs. These studies establish tha t tau is not cotransported with tubulin or microtubules, indicating th at associations of tau with microtubules within axons are not as stabl e as previously believed. Our findings also reveal differences among v arious MAPs in their interactions with microtubules and provide eviden ce that assembly and reorganization of the microtubule network is an a ctive process even after axons establish connections and fully mature.