Expression of specific tau exons in normal and tumoral pancreatic acinar cells

Tau Loss of Function, by Deletion or Aggregation, Contributes to Peripheral Insulin Resistance

BACKGROUND

Several epidemiological data revealed an association between Alzheimer's disease (AD) and type 2 diabetes. Researchers concentrated on brain insulin resistance with little emphasis on the link between systemic insulin resistance and AD, despite the fact that the incidence of type 2 diabetes is higher in AD patients and that impairment in insulin signaling is a risk factor for AD.

OBJECTIVE

The goal of this study is to determine the role of systemic insulin resistance in the pathogenesis of Alzheimer's disease by evaluating the consequences of tau loss-of-function on peripheral insulin sensitivity.

METHODS

Primary hepatocytes isolated from transgenic mouse models (Tau KO, P301 L) and wild type mice (C57BL/6) were evaluated for their insulin sensitivity using glucose uptake assays as well as biochemical analysis of insulin signaling markers.

RESULTS

Our data show that tau deletion or loss of function promotes peripheral insulin resistance as seen in primary hepatocytes isolated from Tau KO and P301 L mice, respectively. Furthermore, exposure of wild-type primary hepatocytes to sub-toxic concentrations of tau oligomers results in a dose-dependent inhibition of glucose uptake, associated with downregulation of insulin signaling. Tau oligomers-induced inactivation of insulin signaling proteins was rescued by inhibition of p38 MAPK, suggesting the involvement of p38 MAPK.

CONCLUSIONS

This is the first study testing tau role in peripheral insulin resistance at the cellular level using multiple transgenic mouse models. Moreover, this study suggests that tau should be functional for insulin sensitivity, therefore, any loss of function by deletion or aggregation would result in insulin resistance.

Impaired Glucose Homeostasis in a Tau Knock-In Mouse Model

Alzheimer’s disease (AD) is the leading cause of dementia. While impaired glucose homeostasis has been shown to increase AD risk and pathological loss of tau function, the latter has been suggested to contribute to the emergence of the glucose homeostasis alterations observed in AD patients. However, the links between tau impairments and glucose homeostasis, remain unclear. In this context, the present study aimed at investigating the metabolic phenotype of a new tau knock-in (KI) mouse model, expressing, at a physiological level, a human tau protein bearing the P301L mutation under the control of the endogenous mouse Mapt promoter. Metabolic investigations revealed that, while under chow diet tau KI mice do not exhibit significant metabolic impairments, male but not female tau KI animals under High-Fat Diet (HFD) exhibited higher insulinemia as well as glucose intolerance as compared to control littermates. Using immunofluorescence, tau protein was found colocalized with insulin in the β cells of pancreatic islets in both mouse (WT, KI) and human pancreas. Isolated islets from tau KI and tau knock-out mice exhibited impaired glucose-stimulated insulin secretion (GSIS), an effect recapitulated in the mouse pancreatic β-cell line (MIN6) following tau knock-down. Altogether, our data indicate that loss of tau function in tau KI mice and, particularly, dysfunction of pancreatic β cells might promote glucose homeostasis impairments and contribute to metabolic changes observed in AD.

Microtubule-associated protein tau in murine kidney: role in podocyte architecture

Tau is a cytoskeletal protein that is expressed mainly in neurons and is involved in several cellular processes, such as microtubule stabilization, axonal maintenance, and transport. Altered tau metabolism is related to different tauopathies being the most important Alzheimer's disease where aberrant hyperphosphorylated and aggregated tau is found in the central nervous system. Here, we have analyzed that function in kidney by using tau knockout mice generated by integrating GFP-encoding cDNA into exon 1 of MAPT (here referred to as TauGFP/GFP). IVIS Lumina from PerkinElmer demonstrated GFP expression in the kidney. We then demonstrated by qPCR that the main tau isoform in the kidney is Tau4R. The GFP reporter allowed us to demonstrate that tau is found in the glomeruli of the renal cortex, and specifically in podocytes. This was further confirmed by immunohistochemistry. TauGFP/GFP mice present a podocyte cytoskeleton more dynamic as they contain higher levels of detyrosinated tubulin than wild-type mice. In addition, transmission electron microscopy studies demonstrated glomerular damage with a decrease in urinary creatinine. Our results prove that tau has an important role in kidney metabolism under normal physiological conditions.

Aberrant expression of the microtubule-associated protein tau is an independent prognostic feature in prostate cancer

Background

Microtubule-associated protein Tau (MAPT) overexpression has been linked to poor prognosis and decreased response to taxane-based therapies in several cancer types, but its relevance in prostate cancer is unknown.

Methods

In this study, MAPT expression was analyzed by immunohistochemistry on a tissue microarray containing 17,747 prostate cancers.

Results

MAPT was absent in normal prostate epithelial cells but detectable in 1004 (8.2%) of 12,313 interpretable cancers. Its expression was associated with advanced tumor stage, high Gleason grade, positive lymph nodes, and early biochemical recurrence (p < 0.0001 each). For example, MAPT was found in 3.6% of 2072 Gleason ≤3 + 3 cancers but in 14.4% of 704 Gleason ≥4 + 4 cancers. High-level MAPT staining was also linked to TMPRSS2:ERG fusions (p < 0.0001). MAPT staining was seen in 15.2 and 16% of cancers with TMPRSS2:ERG fusion detected by immunohistochemistry and fluorescence in-situ hybridization, but in only 3.5 and 3.9% of cancers without ERG staining or ERG rearrangements. Moreover, an association was found between MAPT expression and PTEN deletions, with 19% MAPT positivity in 948 PTEN deleted cancers but only 7% MAPT positivity in 3895 tumors with normal PTEN copy numbers (p < 0.0001). Multivariate analysis revealed that the prognostic value of MAPT was independent from established parameters. Conventional large section analyses showed intratumoral MAPT heterogeneity in all three analyzed cancers.

Conclusions

The results of our study identify MAPT, as a moderate prognostic marker in prostate cancer, whose clinical impact, however, may be limited due to the rarity and heterogeneity of its expression.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5390-1) contains supplementary material, which is available to authorized users.

Secondary structures transition of tau protein with intrinsically disordered proteins specific force field

Microtubule-associated Tau protein plays a key role in assembling microtubule and modulating the functional organization of the neuron and developing axonal morphology, growth, and polarity. The pathological Tau can aggregate into cross-beta amyloid as one of the hallmarks for Alzheimer's disease (AD). Therefore, one of the top priorities in AD research is to figure out the structural model of Tau aggregation and to screen the inhibitors. The latest generation intrinsically disordered protein specific force field ff14IDPSFF significantly improved the distributions of heterogeneous conformations for intrinsically disordered proteins (IDPs). Here, the molecular dynamics (MD) simulations with three force fields of ff14SB, ff14IDPs, and ff14IDPSFF were employed to investigate the secondary structures transition of Tau (267-312) fragment. The results indicate that ff14IDPSFF can generate more heterogeneous conformers, and the predicted secondary structural distribution is closer to that of the experimental observation. In addition, predicted secondary chemical shifts from ff14IDPSFF are the most approach to those of experiment. Secondary structures transition kinetics for Tau(267-312) with ff14IDPSFF shows that the secondary structures were gradually transformed from α-helix to β-strand and the β-strand located at the regions of the residues 274-280 and residues 305-311. Besides, the driving force for the secondary structures transition of Tau(267-312) is mainly hydrophobic interactions which located at hexa-peptides ²⁷⁵ VQIINK²⁸⁰ and ³⁰⁶ VQIVYK³¹¹ . Secondary structure transition of Tau protein can give insight into the aggregation mechanism for AD.

The Microtubule-Associated Protein Tau and Its Relevance for Pancreatic Beta Cells

Structural and biochemical alterations of the microtubule-associated protein tau (MAPT) are associated with degenerative disorders referred to as tauopathies. We have previously shown that MAPT is present in human islets of Langerhans, human insulinomas, and pancreatic beta-cell line models, with biophysical similarities to the pathological MAPT in the brain. Here, we further studied MAPT in pancreatic endocrine tissue to better understand the mechanisms that lead to functional dysregulation of pancreatic beta cells. We found upregulation of MAPT protein expression in human insulinomas when compared to human pancreatic islets of Langerhans and an imbalance between MAPT isoforms in insulinomas tissue. We cloned one 3-repeat domain MAPT and transduced this into a beta-cell derived rodent cell line Rin-5F. Proliferation experiments showed higher growth rates and metabolic activities of cells overexpressing MAPT protein. We observed that a MAPT overexpressing cell line demonstrates altered insulin transcription, translation, and insulin secretion rates. We found the relative insulin secretion rates were significantly decreased in a MAPT overexpressing cell line and these findings could be confirmed using partial MAPT knock-down cell lines. Our findings support that MAPT may play an important role in insulin granule trafficking and indicate the importance of balanced MAPT phosphorylation and dephosphorylation for adequate insulin release.

[The testicular microtubule-associated protein Tau: Where, when during spermatogenesis?]

The Tau protein (Tubulin Associated Unit) is a phosphoprotein of the microtubule-associated protein family (MAPs). Its role is the regulation of the microtubule polymerization. The Tau protein is naturally present in brain, heart, muscle, lung, kidney, pancreas and liver. An expression of Tau protein and RNA messengers was also highlighted in the testis that is an organ rich in microtubules. The role of microtubules is essential in the stabilization of the cellular shape and in cell divisions. In the testis, Tau protein could be involved in the division process of the spermatogenesis by acting on the microtubular dynamics in the arrangement of the spermatozoon polarity. This review synthesizes the current knowledge, the localization and the main functions of the Tau protein focused on the testis. The localization and the potential roles of the Tau protein during the spermatogenesis are discussed by emphasizing the link with the microtubular structures of seminiferous tubules.

Tau regulates the localization and function of End-binding proteins 1 and 3 in developing neuronal cells

The axonal microtubule-associated protein tau is a well-known regulator of microtubule stability in neurons. However, the putative interplay between tau and End-binding proteins 1 and 3 (EB1/3), the core microtubule plus-end tracking proteins, has not been elucidated yet. Here, we show that a cross-talk between tau and EB1/3 exists in developing neuronal cells. Tau and EBs partially colocalize at extending neurites of N1E-115 neuroblastoma cells and axons of primary hippocampal neurons, as shown by confocal immunofluorescence analyses. Tau down-regulation leads to a reduction of EB1/3 comet length, as observed in shRNA-stably depleted neuroblastoma cells and TAU-/- neurons. EB1/3 localization depends on the expression levels and localization of tau protein. Over-expression of tau at high levels induces EBs relocalization to microtubule bundles at extending neurites of N1E-115 cells. In differentiating primary neurons, tau is required for the proper accumulation of EBs at stretches of microtubule bundles at the medial and distal regions of the axon. Tau interacts with EB proteins, as shown by immunoprecipitation in different non-neuronal and neuronal cells and in whole brain lysates. A tau/EB1 direct interaction was corroborated by in vitro pull-down assays. Fluorescence recovery after photobleaching assays performed in neuroblastoma cells confirmed that tau modulates EB3 cellular mobility. In summary, we provide evidence of a new function of tau as a direct regulator of EB proteins in developing neuronal cells. This cross-talk between a classical microtubule-associated protein and a core microtubule plus-end tracking protein may contribute to the fine-tuned regulation of microtubule dynamics and stability during neuronal differentiation. We describe here a novel function for tau as a direct regulator of End binding (EB) proteins in differentiating neuronal cells. EB1/3 cellular mobility and localization in extending neurites and axons is modulated by tau levels and localization. We provide new evidence of the interplay between classical microtubule-associated proteins (MAPs) and "core" microtubule plus-end tracking proteins (+TIPs) during neuronal development.

Abeta(1-42) induces abnormal alternative splicing of tau exons 2/3 in NGF-induced PC12 cells

Protein tau plays a pivotal role in the pathophysiology of Alzheimer's disease, where its hyperphos-phorylation promotes aggregation and microtubule destabilization. Tau undergoes alternative splicing which generates six isoforms in the human brain, due to inclusion/exclusion of exons 2, 3 and 10. Dysregulation of the splicing process of tau exon 10 is sufficient to cause tauopathy and has shown to be influenced by beta-amyloid peptides, but splicing of other exons is less studied. We studied the effects of beta-amyloid(42) in the alternative splicing of tau exons 2/3 and 6, using untreated and Nerve Growth Factor-induced PC12 cells. Beta-amyloid exposure caused formed cell processes to retract in differentiated cells and altered the expression of exons 2/3 in both undifferentiated and differentiated cells. Expression of exon 6 was repressed in undifferentiated cells only. Our results suggest that beta-amyloid interferes with the splicing process of exons 2/3, favoring their exclusion and thus the expression of immature tau isoforms that are less efficient in stabilizing microtubules and may also be more prone to hyperphosphorylation. The molecular mechanism for this amyloid-tau interaction remains to be determined, but may have potential implications for the understanding of the underlying neuropathological processes in Alzheimer's disease.

Intermediate filament-associated cytolinker plectin 1c destabilizes microtubules in keratinocytes

The transition of microtubules (MTs) from an assembled to a disassembled state plays an essential role in several cellular functions. While MT dynamics are often linked to those of actin filaments, little is known about whether intermediate filaments (IFs) have an influence on MT dynamics. We show here that plectin 1c (P1c), one of the multiple isoforms of the IF-associated cytolinker protein plectin, acts as an MT destabilizer. We found that MTs in P1c-deficient (P1c(-/-)) keratinocytes are more resistant toward nocodazole-induced disassembly and display increased acetylation. In addition, live imaging of MTs in P1c(-/-), as well as in plectin-null, cells revealed decreased MT dynamics. Increased MT stability due to P1c deficiency led to changes in cell shape, increased velocity but loss of directionality of migration, smaller-sized focal adhesions, higher glucose uptake, and mitotic spindle aberrations combined with reduced growth rates of cells. On the basis of ex vivo and in vitro experimental approaches, we suggest a mechanism for MT destabilization in which isoform-specific binding of P1c to MTs antagonizes the MT-stabilizing and assembly-promoting function of MT-associated proteins through an inhibitory function exerted by plectin's SH3 domain. Our results open new perspectives on cytolinker-coordinated IF-MT interaction and its physiological significance.

Microtubule-associated protein tau in human prostate cancer cells: isoforms, phosphorylation, and interactions

Tau is a microtubule-associated protein whose function has been investigated primarily in neurons. Recently, tau expression has been correlated with increased drug resistance in various cancers of non-neuronal tissues. In this report, we investigate the tau expressed in cancerous prostate lines ALVA-31, DU 145, and PC-3. Prostate cancer tau is heat-stable and highly phosphorylated, containing many of the modifications identified in Alzheimer's disease brain tau. RT-PCR and phosphatase treatment indicated that all six alternatively spliced adult brain tau isoforms are expressed in ALVA-31 cells, and isoforms containing exon 6 as well as high molecular weight tau isoforms containing either exon 4A or a larger splice variant of exon 4A are also present. Consistent with its hyperphosphorylated state, a large proportion of ALVA-31 tau does not bind to microtubules, as detected by confocal microscopy and biochemical tests. Finally, endogenous ALVA-31 tau can interact with the p85 subunit of phosphatidylinositol 3-kinase, as demonstrated by co-immunoprecipitations and in vitro protein-binding assays. Our results suggest that tau in prostate cancer cells does not resemble that from normal adult brain and support the hypothesis that tau is a multifunctional protein.

Green-fluorescent-protein-expressing mice as models for the study of axonal growth and regeneration in vitro

The culture of hippocampal-entorhinal brain slices is a widely used model for studying neuronal differentiation, axon growth and pathfinding in vitro. The application of tracers (e.g. biocytin) is a well-established method for studying single or multiple neurons and their extensions in this model. For quantifying the growth of high numbers of axons after lesion, however, genetically expressed enhanced green fluorescent protein (EGFP) has proven particularly useful for labeling living axons in vivo and in vitro. Here, we introduce several EGFP-expressing mouse lines which improve the organotypic brain slice model. The questions addressed determine which mouse line to use: beta-actin-EGFP mice for labeling all cells and their extensions; Tau-EGFP mice for labeling the axoplasma; or Thy-1.2-EGFP mice for labeling the axonal membrane. Cocultures of EGFP-positive entorhinal cortex explants with EGFP-negative hippocampal explants allow the monitoring of fluorescent axons growing into the hippocampus in an easily quantifiable manner.

Brain-specific change in alternative splicing of Tau exon 6 in myotonic dystrophy type 1

Alternative splicing is altered in myotonic dystrophy of type 1 (DM1), a syndrome caused by an increase of CTG triplet repeats in the 3' untranslated region of the myotonic dystrophy protein kinase gene. Previously, we reported the preferential skipping of Tau exon 2 in DM1 brains. In this study, we analyze the alternative splicing of Tau exon 6 which can be inserted in three different forms (c, p and d) depending on the 3' splice site used. In fact, inclusion of exon 6c decreases in DM1 brains compared to control brains whereas inclusion of 6d increases. Alteration of exon 6 splicing was not observed in DM1 muscle although this exon was inserted in RNAs from normal muscle and DM1 splicing alterations were first described in this organ. In contrast, alteration of exon 2 of Tau mRNA was observed in both muscle and brain. However, co-transfections of a minigene containing exon 6 with CELF or MBNL1 cDNAs, two splicing factor families suspected to be involved in DM1, showed that they influence exon 6 splicing. Altogether, these results show the importance of determining all the exons and organs targeted by mis-splicing to determine the dysregulation mechanisms of mis-splicing in DM1.

Reprogramming of tau alternative splicing by spliceosome-mediated RNA trans-splicing: implications for tauopathies

Frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) is caused by mutations in the gene encoding the microtubule-associated protein, tau. Some FTDP-17 mutations affect exon 10 splicing. To correct aberrant exon 10 splicing while retaining endogenous transcriptional control, we evaluated the feasibility of using spliceosome-mediated RNA trans-splicing (SMaRT) to reprogram tau mRNA. We designed a pre-trans-splicing molecule containing human tau exons 10 to 13 and a binding domain complementary to the 3' end of tau intron 9. A minigene comprising tau exons 9, 10, and 11 and minimal flanking intronic sequences was used as a target. RT-PCR analysis of SH-SY5Y cells or COS cells cotransfected with a minigene and a pre-trans-splicing molecule using primers to opposite sides of the predicted splice junction generated products containing exons 9 to 13. Sequencing of the chimeric products showed that an exact exon 9-exon 10 junction had been created, thus demonstrating that tau RNA can be reprogrammed by trans-splicing. Furthermore, by using the same paradigm with a minigene containing full-length intronic sequences, we show that cis-splicing exclusion of exon 10 can be by-passed by trans-splicing and that conversion of exon 10(-) tau RNA into exon 10(+) tau RNA could be achieved with approximately 34% efficiency. Our results demonstrate that an alternatively spliced exon can be replaced by trans-splicing and open the way to novel therapeutic applications of SMaRT for tauopathies and other disorders linked to aberrant alternative splicing.

The synergistic combination of the farnesyl transferase inhibitor lonafarnib and paclitaxel enhances tubulin acetylation and requires a functional tubulin deacetylase

Farnesyl transferase (FT) inhibitors (FTI) are anticancer agents developed to target oncogenic Ras proteins by inhibiting Ras farnesylation. FTIs potently synergize with paclitaxel and other microtubule-stabilizing drugs; however, the mechanistic basis underlying this synergistic interaction remains elusive. Here we show that the FTI lonafarnib affects the microtubule cytoskeleton resulting in microtubule bundle formation, increased microtubule stabilization and acetylation, and suppression of microtubule dynamics. Notably, treatment with the combination of low doses of lonafarnib with paclitaxel markedly enhanced tubulin acetylation (a marker of microtubule stability) as compared with either drug alone. This synergistic effect correlated with FT inhibition and was accompanied by a synergistic increase in mitotic arrest and cell death. Mechanistically, we show that the combination of lonafarnib and paclitaxel inhibits the in vitro deacetylating activity of the only known tubulin deacetylase, histone deacetylase 6 (HDAC6). In addition, the lonafarnib/taxane combination is synergistic only in cells lines expressing the wild-type HDAC6, but not a catalytic-mutant HDAC6, revealing that functional HDAC6 is required for the synergy of lonafarnib with taxanes. Furthermore, tubacin, a specific HDAC6 inhibitor, synergistically enhanced tubulin acetylation in combination with paclitaxel, similar to the combination of lonafarnib and paclitaxel. Taken together, these data suggest a relationship between FT inhibition, HDAC6 function, and cell death, providing insight into the putative molecular basis of the lonafarnib/taxane synergistic antiproliferative combination.

Tau gene alternative splicing: expression patterns, regulation and modulation of function in normal brain and neurodegenerative diseases

Organization of cytoskeletal elements is critical for cellular migration and maintenance of morphology. Tau protein, which binds to and organizes microtubules, is instrumental in forming and maintaining the neuronal axon. Disturbances in tau expression result in disruption of the neuronal cytoskeleton and formation of pathological tau structures (neurofibrillary tangles, NFTs) found in brains of dementia sufferers. Null tau mice, although viable, exhibit developmental and cognitive defects and transgenic mice which overexpress tau develop severe neuropathies. The neuron-specific tau transcript produces multiple isoforms by intricately regulated alternative splicing. These isoforms modulate tau function in normal brain. Moreover, aberrations in tau splicing regulation directly cause several neurodegenerative diseases. Thus, tau splicing regulation is vital to neuronal health and correct brain function. This review briefly presents our cumulative knowledge of tau splicing-cis elements and trans factors which influence it at the RNA level, its effect on the structure and roles of the tau protein and its repercussions on neuronal morphology and neurodegeneration.

Tau protein as a differential biomarker of tauopathies

Microtubule-associated Tau proteins are the basic component of intraneuronal and glial inclusions observed in many neurological disorders, the so-called tauopathies. Many etiological factors, phosphorylation, splicing, and mutations, relate Tau proteins to neurodegeneration. Molecular analysis has revealed that hyperphosphorylation and abnormal phosphorylation might be one of the important events in the process leading to tau intracellular aggregation. Specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution, could characterize five main classes of tauopathies. A direct correlation has been established between the regional brain distribution of tau pathology and clinical symptoms; for instance progressive involvement of neocortical areas is well correlated to the severity of dementia in Alzheimer's disease, overall suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. Recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies. Overall, a better knowledge of the etiological factors responsible for the aggregation of tau proteins in brain diseases is essential for development of future differential diagnosis and therapeutic strategies. They would hopefully find their application against Alzheimer's disease but also in all neurological disorders for which a dysfunction of Tau biology has been identified.

Transcriptional regulation of the mouse microtubule-associated protein tau

The microtubule-associated protein (MAP) tau is found primarily in neurons and errors in its regulation are associated with Alzheimer's disease and other neurodegenerative disorders. Tau expression is transcriptionally regulated and tissue-specific. In this study, starting with a approximately 7500-bp fragment from the mouse tau gene, which includes tau exon -1, we define regions preferentially conferring tissue-specific expression. Furthermore, gel shift assays indicate that transcriptional regulators SP-1 and AP-2 are important for basal expression but not necessary for neuron-specific expression of the tau transcript.

Expression and distribution of distinct variants of E-MAP-115 during proliferation and differentiation of human intestinal epithelial cells

Epithelial cell proliferation and differentiation occur concomitant with striking remodeling of the cytoskeleton. Microtubules (MTs) play important roles in these processes, during which the MTs themselves are reorganized and stabilized by microtubule-associated proteins (MAPs). Among the proteins classified as structural MAPs, E-MAP-115 (also named ensconsin) is preferentially expressed in cells of epithelial origin. The aims of this study were, first, to determine if E-MAP-115, like other MAPs, is expressed as different isoforms during differentiation and, second, to perform a detailed analysis of the expression and distribution of any E-MAP-115 variants detected in intestinal epithelial cells during their polarization/differentiation. It was our expectation that these data would help us to develop hypotheses concerning the role of this MAP in epithelial development. We report the expression of three E-MAP-115 transcripts encoding isoforms of 115, 105, and 95 kDa; two display an expression gradient inverse to the third one as Caco-2 cells progress from proliferation through the stages of differentiation. To monitor the proteins produced from each transcript, we used purified polyclonal antibodies against synthetic peptides contained within the 115, 105, and 95 kDa isoforms to assay proliferating and differentiating CaCo-2 cells. Our results indicate that the expression and MT-binding capacity of the 115, 105, and 95 kDa isoforms vary upon proliferation/differentiation of the cells. E-MAP-115 proteins colocalize with MTs in proliferative and differentiated Caco-2 cells; in vivo, they are expressed in both crypt and villus epithelial cells where they are mainly concentrated at the apical pole of the cells.

Correction of alternative splicing of tau in frontotemporal dementia and parkinsonism linked to chromosome 17

Mutations in the human tau gene cause frontotemporal dementia and Parkinsonism associated with chromosome 17 (FTDP-17). One of the major disease mechanisms in FTDP-17 is the increased inclusion of tau exon 10 during pre-mRNA splicing. Here we show that modified oligonucleotides directed against the tau exon 10 splice junctions suppress inclusion of tau exon 10. The effect is mediated by the formation of a stable pre-mRNA-oligonucleotide hybrid, which blocks access of the splicing machinery to the pre-mRNA. Correction of tau splicing occurs in a tau minigene system and in endogenous tau RNA in neuronal pheochromocytoma cells and is specific to exon 10 of the tau gene. Antisense oligonucleotide-mediated exclusion of exon 10 has a physiological effect by increasing the ratio of protein lacking the microtubule-binding domain encoded by exon 10. As a consequence, the microtubule cytoskeleton becomes destabilized and cell morphology is altered. Our results demonstrate that alternative splicing defects of tau as found in FTDP-17 patients can be corrected by application of antisense oligonucleotides. These findings provide a tool to study specific tau isoforms in vivo and might lead to a novel therapeutic strategy for FTDP-17.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

The cytoskeleton of digestive epithelia in health and disease

The mammalian cell cytoskeleton consists of a diverse group of fibrillar elements that play a pivotal role in mediating a number of digestive and nondigestive cell functions, including secretion, absorption, motility, mechanical integrity, and mitosis. The cytoskeleton of higher-eukaryotic cells consists of three highly abundant major protein families: microfilaments (MF), microtubules (MT), and intermediate filaments (IF), as well as a growing number of associated proteins. Within digestive epithelia, the prototype members of these three protein families are actins, tubulins, and keratins, respectively. This review highlights the important structural, regulatory, functional, and unique features of the three major cytoskeletal protein groups in digestive epithelia. The emerging exciting biological aspects of these protein groups are their involvement in cell signaling via direct or indirect interaction with a growing list of associated proteins (MF, MT, IF), the identification of several disease-causing mutations (IF, MF), the functional role that they play in protection from environmental stresses (IF), and their functional integration via several linker proteins that bridge two or potentially all three of these groups together. The use of agents that target specific cytoskeletal elements as therapeutic modalities for digestive diseases offers potential unique areas of intervention that remain to be fully explored.