Assembly and bundling of marginal band microtubule protein: role of tau

The active Hsc70/tau complex can be exploited to enhance tau turnover without damaging microtubule dynamics

The pathological accumulation of abnormally hyperphosphorylated and aggregated tau, a neuronal microtubule (MT)-associated protein that functions to maintain MT stability, is implicated in a number of hereditary and sporadic neurodegenerative diseases including frontotemporal dementia and Alzheimer's disease. Targeting tau for the treatment of these diseases is an area of intense interest and toward that end, modulation of cellular molecular chaperones is a potential therapeutic target. In particular, the constitutive Hsp70 isoform, Hsc70, seems highly interconnected with tau, preserving tau protein levels and synergizing with it to assemble MTs. But the relationship between tau and Hsc70, as well as the impact of this interaction in neurons and its therapeutic implications remain unknown. Using a human dominant negative Hsc70 that resembles isoform selective inhibition of this important chaperone, we found for the first time that Hsc70 activity is required to stimulate MT assembly in cells and brain. However, surprisingly, active Hsc70 also requires active tau to regulate MT assembly in vivo, suggesting that tau acts in some ways as a co-chaperone for Hsc70 to coordinate MT assembly. This was despite tau binding to Hsc70 as substrate, as determined biochemically. Moreover, we show that while chronic Hsc70 inhibition damaged MT dynamics, intermittent treatment with a small molecule Hsp70 inhibitor lowered tau in brain tissue without disrupting MT integrity. Thus, in tauopathies, where MT injury would be detrimental to neurons, the unique relationship of tau with the Hsc70 machinery can be exploited to deplete tau levels without damaging MT networks.

Specific serine-proline phosphorylation and glycogen synthase kinase 3β-directed subcellular targeting of stathmin 3/Sclip in neurons

During nervous system development, neuronal growth, migration, and functional morphogenesis rely on the appropriate control of the subcellular cytoskeleton including microtubule dynamics. Stathmin family proteins play major roles during the various stages of neuronal differentiation, including axonal growth and branching, or dendritic development. We have shown previously that stathmins 2 (SCG10) and 3 (SCLIP) fulfill distinct, independent and complementary regulatory roles in axonal morphogenesis. Although the two proteins have been proposed to display the four conserved phosphorylation sites originally identified in stathmin 1, we show here that they possess distinct phosphorylation sites within their specific proline-rich domains (PRDs) that are differentially regulated by phosphorylation by proline-directed kinases involved in the control of neuronal differentiation. ERK2 or CDK5 phosphorylate the two proteins but with different site specificities. We also show for the first time that, unlike stathmin 2, stathmin 3 is a substrate for glycogen synthase kinase (GSK) 3β both in vitro and in vivo. Interestingly, stathmin 3 phosphorylated at its GSK-3β target site displays a specific subcellular localization at neuritic tips and within the actin-rich peripheral zone of the growth cone of differentiating hippocampal neurons in culture. Finally, pharmacological inhibition of GSK-3β induces a redistribution of stathmin 3, but not stathmin 2, from the periphery toward the Golgi region of neurons. Stathmin proteins can thus be either regulated locally or locally targeted by specific phosphorylation, each phosphoprotein of the stathmin family fulfilling distinct and specific roles in the control of neuronal differentiation.

Ferritin associates with marginal band microtubules

We characterized chicken erythrocyte and human platelet ferritin by biochemical studies and immunofluorescence. Erythrocyte ferritin was found to be a homopolymer of H-ferritin subunits, resistant to proteinase K digestion, heat stable, and contained iron. In mature chicken erythrocytes and human platelets, ferritin was localized at the marginal band, a ring-shaped peripheral microtubule bundle, and displayed properties of bona fide microtubule-associated proteins such as tau. Red blood cell ferritin association with the marginal band was confirmed by temperature-induced disassembly-reassembly of microtubules. During erythrocyte differentiation, ferritin co-localized with coalescing microtubules during marginal band formation. In addition, ferritin was found in the nuclei of mature erythrocytes, but was not detectable in those of bone marrow erythrocyte precursors. These results suggest that ferritin has a function in marginal band formation and possibly in protection of the marginal band from damaging effects of reactive oxygen species by sequestering iron in the mature erythrocyte. Moreover, our data suggest that ferritin and syncolin, a previously identified erythrocyte microtubule-associated protein, are identical. Nuclear ferritin might contribute to transcriptional silencing or, alternatively, constitute a ferritin reservoir.

Elliptical versus circular erythrocyte marginal bands: isolation, shape conversion, and mechanical properties

Differentiation of nucleated erythrocytes involves transformation from spheroids to flattened discoids to mature flattened ellipsoids. The marginal band (MB) of microtubules is required for this process and continues to play a role in maintaining mature ellipsoidal cell shape. One hypothesis for MB function is that cell ellipticity is generated and maintained by asymmetric application of force across a flexible, circular MB frame by the membrane skeleton or other transverse elements. This is based on an earlier finding that isolated erythrocyte MBs are much more circular than MBs in situ. However, our present studies of salamander erythrocyte MBs isolated by a detergent-based method challenge this hypothesis. Most of these isolated MBs are initially elliptical, even though they lack transverse material (= E-MBs). They can be stabilized in that form for long periods and can be converted experimentally into the circular form (= C-MBs) by extended incubation in isolation medium or by treatment with elastase or subtilisin. We have tested an alternative hypothesis for generation and maintenance of ellipsoidal MBs, one based on intrinsic differential bending resistance and supported by construction of models. Using laser microsurgical transection to compare mechanical responses of isolated E-MBs and C-MBs, we have found their behavior to be quite different. Whereas C-MBs linearize, most E-MBs do not, instead retaining considerable curvature. These results are incompatible with the differential bending resistance hypothesis, which predicts both C-MB and E-MB linearization. However, they are consistent with a third model, in which material bound to the MB stabilizes it in the mature ellipsoidal form, and indicate that the mechanism for maintenance of MB ellipticity differs from that involved in its generation.

Tau interacts with src-family non-receptor tyrosine kinases

Tau and other microtubule-associated proteins promote the assembly and stabilization of neuronal microtubules. While each microtubule-associated protein has distinct properties, their in vivo roles remain largely unknown. Tau is important in neurite outgrowth and axonal development. Recently, we showed that the amino-terminal region of tau, which is not involved in microtubule interactions, is important in NGF induced neurite outgrowth in PC12 cells. Here we report that a proline rich sequence in the amino terminus of tau interacts with the SH3 domains of fyn and src non-receptor tyrosine kinases. Tau and fyn were co-immunoprecipitated from human neuroblastoma cells and co-localization of tau and fyn was visualized in co-transfected NIH3T3 cells. Co-transfection of tau and fyn also resulted in an alteration in NIH3T3 cell morphology, consistent with an in vivo interaction. Fyn-dependent tyrosine phosphorylation of tau occurred in transfected cells and tyrosine phosphorylated tau was identified in human neuroblastoma cells as well. Our data suggest that tau is involved in signal transduction pathways. An interaction between tau and fyn may serve as a mechanism by which extracellular signals influence the spatial distribution of microtubules. The tyrosine phosphorylation of tau by fyn may also have a role in neuropathogenesis, as fyn is upregulated in Alzheimer's disease.

Tau-like proteins in the nervous system of goldfish

This report describes the presence of a group of tau-like proteins (TLPs) in goldfish central nervous system. The TLPs were immunoreactive with antibodies that recognized the carboxy-terminal domain of mammalian tau, but not with antibodies that recognized the amino-terminus. The TLPs of goldfish exhibited the basic properties of tau proteins including neuronal specificity, structural heterogeneity, heat stability and the ability to co-assemble with tubulin. We propose that TLPs may represent a precursor of tau, that share the microtubule binding domain and the carboxy-terminal domain with mammalian tau proteins. In contrast the amino-terminus of the TLPs is much shorter and may represent a more variable domain of tau proteins.

Microtubule-associated protein 2c reorganizes both microtubules and microfilaments into distinct cytological structures in an actin-binding protein-280-deficient melanoma cell line

The emergence of processes from cells often involves interactions between microtubules and microfilaments. Interactions between these two cytoskeletal systems are particularly apparent in neuronal growth cones. The juvenile isoform of the neuronal microtubule-associated protein 2 (MAP2c) is present in growth cones, where we hypothesize it mediates interactions between microfilaments and microtubules. To approach this problem in vivo, we used the human melanoma cell, M2, which lacks actin-binding protein-280 (ABP-280) and forms membrane blebs, which are not seen in wild-type or ABP-transfected cells. The microinjection of tau or mature MAP2 rescued the blebbing phenotype; MAP2c not only caused cessation of blebbing but also induced the formation of two distinct cellular structures. These were actin-rich lamellae, which often included membrane ruffles, and microtubule-bearing processes. The lamellae collapsed after treatment with cytochalasin D, and the processes retracted after treatment with colchicine. MAP2c was immunocytochemically visualized in zones of the cell that were devoid of tubulin, such as regions within the lamellae and in association with membrane ruffles. In vitro rheometry confirmed that MAP2c is an efficient actin gelation protein capable of organizing actin filaments into an isotropic array at very low concentrations; tau and mature MAP2 do not share this rheologic property. These results suggest that MAP2c engages in functionally specific interactions not only with microtubules but also with microfilaments.

Microtubules and microtubule-associated proteins

Microtubule research is becoming increasingly diverse, reflecting the many isoforms and modifications of tubulin and the many proteins with which microtubules interact. Recent advances are particularly visible in four areas: microtubule motor proteins (their structures, stepping modes, and forces); microtubule nucleation (the roles of centrosomes and gamma-tubulin); tubulin folding (mediated by cytoplasmic chaperones); and the expanding list of microtubule-associated proteins, knowledge of their phosphorylation states, and information on their effects on microtubule dynamics.