Overexpression of tau in a nonneuronal cell induces long cellular processes.

Neurons are polarized cells presenting two distinct compartments, dendrites and an axon. Dendrites can be distinguished from the axon by the presence of rough endoplasmic reticulum (RER). The mechanism by which the structure and distribution of the RER is maintained in these cells is poorly understood. In the present study, we investigated the role of the dendritic microtubuleassociated protein-2 (MAP2) in the RER membrane positioning by comparing their distribution in brain subcellular fractions and in primary hippocampal cells and by examining the MAP2-microtubule interaction with RER membranes in vitro. Subcellular fractionation of rat brain revealed a high MAP2 content in a subfraction enriched with the endoplasmic reticulum markers ribophorin and p63. Electron microscope morphometry confirmed the enrichment of this subfraction with RER membranes. In cultured hippocampal neurons, MAP2 and p63 were found to concomitantly compartmentalize to the dendritic processes during neuronal differentiation. Protein blot overlays using purified MAP2c protein revealed its interaction with p63, and immunoprecipitation experiments performed in HeLa cells showed that this interaction involves the projection domain of MAP2. In an in vitro reconstitution assay, MAP2-containing microtubules were observed to bind to RER membranes in contrast to microtubules containing tau, the axonal MAP. This binding of MAP2c microtubules was reduced when an anti-p63 antibody was added to the assay. The present results suggest that MAP2 is involved in the association of RER membranes with microtubules and thereby could participate in the differential distribution of RER membranes within a neuron.Neurons are polarized cells that present two distinct compartments, dendrites and an axon. These compartments can be distinguished by their morphology; dendrites are multiple and taper, whereas the axon is unique and its diameter is uniform (1-4). Dendrites and axon also differ by their membranous organelle composition; RER 1 is found in the somato-dendritic compartment but not in the axon, and the number of free ribosomes is a lot higher in dendrites than in the axon (1, 2, 5). The cytoskeletal elements are also distinctly distributed in these neuronal compartments; the number of microtubules is higher in dendrites than in the axon, whereas the opposite is noted for neurofilaments (1, 2, 6).Microtubules are involved in the establishment of cell polarity. Notably, the microtubules of dendrites and axon contain different microtubule-associated proteins (MAPs); the microtubule-associated protein-2 (MAP2) is found in dendrites, whereas tau is present only in the axon (6 -8). The contribution of these MAPs to the establishment of neuronal polarity has been well documented. Tau contributes to the axonal differentiation in primary neuronal cultures, whereas MAP2 is involved in the differentiation of minor neurites, the neuronal processes that become dendrites, and in the maintenance of dendrites in adult neurons (9 -14). Despite the fact that MAP2 and tau are...

supporting

confidence: 59%

Section: Preparation Of a Fraction Enriched In Rough Microsomes (Rm) mentioning

confidence: 99%

Interaction of Microtubule-associated Protein-2 and p63

Farah

Liazoghli

Perreault

et al. 2005

“…Overexpression of tau in non-neuronal cells induces the formation of long neurite-like processes (52,53), whereas inhibiting tau expression using antisense tau resulted in the loss of neurite outgrowth and extension (54). In mouse cortical cells expression of wild type tau resulted in the formation of long processes/neurites, unlike the R406W mutant tau-expressing cells, which were much shorter, stubby projections.…”

Section: Cellular Localization Of Wild Type and Mutant R406wmentioning

confidence: 99%

Mutant (R406W) Human Tau Is Hyperphosphorylated and Does Not Efficiently Bind Microtubules in a Neuronal Cortical Cell Model

Krishnamurthy

Johnson

2004

Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) is an autosomal dominant neurodegenerative disorder caused by mutations in the gene that encodes for tau, a microtubule-binding protein. Neuropathologically the disease is characterized by extensive neuronal loss in the frontal and temporal lobes and the filamentous accumulation of hyperphosphorylated tau. The R406W missense mutation was originally described in an American and a Dutch family. Although R406W tau is hyperphosphorylated in FTDP-17 cases, R406W tau expressed in cell model systems has not shown increased phosphorylation. The purpose of this study was to establish a neuronal model system in which the phosphorylation of R406W tau is increased and thus more representative of the in vivo situation. To accomplish this goal immortalized mouse cortical cells that express low levels of endogenous tau were stably transfected with human wild type or R406W tau. In this neuronal model R406W tau was more highly phosphorylated at numerous epitopes and showed decreased microtubule binding compared with wild type tau, an effect that could be reversed by dephosphorylation. In addition the expression of R406W tau in the cortical cells resulted in increased cell death as compared with wild type tau-expressing cells when the cells were exposed to an apoptotic stressor. These results indicate that in an appropriate cellular context R406W tau is hyperphosphorylated, which leads to decreased microtubule binding. Furthermore, expression of R406W tau sensitized cells to apoptotic stress, which may contribute to the neuronal cell loss that occurs in this FTDP-17 tauopathy.Frontotemporal dementia (FTD) 1 refers to a group of neurological disorders that are characterized clinically by progressive behavioral changes and neuropathologically by neuronal loss in the frontal and temporal lobes and the presence of filamentous deposits of abnormally hyperphosphorylated tau protein in neurons and/or glial cells (1-4). FTDs occur mainly as sporadic cases but also as familial forms with an autosomal dominant mode of inheritance and age-dependent penetrance (1). In 1998 it was demonstrated that several familial FTDs were due to mutations in the tau gene and have been referred to as frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) (5-7). Since these initial discoveries, additional tau mutations have been described in frontotemporal dementia families, and presently over 25 mutations in the tau gene have been identified (8, 9). They include missense mutations in coding regions, amino acid deletions, and intronic mutations in the region following exon 10. The mutations may be divided into two main groups, those that affect alternative splicing of exon 10, leading to changes in the ratio of tau mRNAs with or without exon 10, and thus the proportion of tau with three microtubule binding repeats versus tau with four microtubule binding repeats, and missense mutations.Tau is a microtubule-associated protein that facilitates microtubule assembly an...

“…Finally, 4-repeat tau is a significantly stronger stabilizer of microtubule dynamics, both in vitro (12) and in cells. 2 The FTDP-17 mutations introduce amino acid substitutions in or near to the MT binding domain or alter the pattern of tau RNA splicing without altering the primary sequence of the encoded tau protein (16 -18, 27). These latter mutations result in an increase in the expression ratio of 4-repeat tau to 3-repeat tau in adult neurons.…”

mentioning

confidence: 99%

“…[1][2][3][4][5][6]. Tau is also expressed in glial cells (7), although its role (8) there are less defined.…”

mentioning

confidence: 99%

Microtubule-dependent Oligomerization of Tau

Makrides

Shen

Bhatia

et al. 2003

The accumulation of abnormal tau filaments is a pathological hallmark of many neurodegenerative diseases. In 1998, genetic analyses revealed a direct linkage between structural and regulatory mutations in the tau gene and the neurodegenerative disease, frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Importantly, the FTDP-17 phenotype is transmitted in a dominant rather than a recessive manner. However, the underlying molecular mechanisms causing disease remain uncertain. The most common molecular mechanism generating dominant phenotypes is the loss of function of a multimeric complex containing both mutant and wild-type subunits. Therefore, we sought to determine whether tau might normally function as a multimer. We co-incubated 35 S-radiolabeled tau and biotinylated tau with taxol stabilized microtubules, at very low molar ratios of tau to tubulin. Subsequent covalent cross-linking followed by affinity-precipitation of the biotinylated tau revealed the formation of microtubule-dependent tau oligomers. We next used atomic force microscopy to independently assess this conclusion. Our results are consistent with the hypothesis that tau forms oligomers upon binding to microtubules. In addition to providing insights into normal tau action, our findings lead us to propose that one mechanism by which mutations in tau may cause cell death is through the formation of tau complexes containing mutant tau molecules in association with wild-type tau. These wildtype-mutant tau complexes may possess altered biological and/or biophysical properties that promote onset of the FTDP-17 phenotype, including neuronal cell death by either altering normal tau-mediated regulation of microtubule-dependent cellular functions and/or promoting the formation of pathological tau aggregates.The microtubule-associated protein tau, localized predominantly in the cell bodies and axons of neuronal cells, is necessary for the establishment of neuronal cell polarity and axon outgrowth, for axonal transport, and the maintenance of axonal morphology (for example, see Refs. 1-6). Tau is also expressed in glial cells (7), although its role (8) there are less defined.Mechanistically, tau is well known to stimulate MT 1 polymerization, to stabilize MTs, and to modulate MT dynamics (9 -12). Since MT dynamics must be tightly regulated for cells to function and remain viable (e.g. see Refs. 13 and 14), it follows that tau action must also be finely regulated in cells.Alternatively, abnormal tau behavior is often associated with neurodegenerative diseases. In Alzheimer's disease, FTDP-17 and a large number of additional "tauopathies," necrotic neurons possess abnormal pathological fibers composed primarily of hyperphosphorylated and dysfunctional tau (for a recent review, see Ref. 15). Until recently, the relationship between tau and these various diseases was only correlative. However, in 1998, several groups reported a direct genetic linkage between mutations in the tau gene and FTDP-17, a group of related neurodegenerative con...