A tau-related protein of 130 kDa is present in Alzheimer brain

Platelets, a reliable source for peripheral Alzheimer's disease biomarkers?

Peripheral biomarkers play an indispensable role in quick and reliable diagnoses of any kind of disease. With the population ageing, the number of people suffering from age-related diseases is expected to rise dramatically over the coming decades. In particular, all types of cognitive deficits, such as Alzheimer's disease, will increase. Alzheimer's disease is characterised mainly by coexistence of amyloid plaques and neurofibrillary tangles in brain. Reliable identification of such molecular characteristics antemortem, however, is problematic due to restricted availability of appropriate sample material and definitive diagnosis is only possible postmortem. Currently, the best molecular biomarkers available for antemortem diagnosis originate from cerebrospinal fluid. Though, this is not convenient for routine diagnosis because of the required invasive lumbar puncture. As a consequence, there is a growing demand for additional peripheral biomarkers in a more readily accessible sample material. Blood platelets, due to shared biochemical properties with neurons, can constitute an attractive alternative as discussed here. This review summarises potential platelet Alzheimer's disease biomarkers, their role, implication, and alteration in the disease. For easy comparison of their performance, the Hedge effect size was calculated whenever data were available.

The association of tissue transglutaminase with human recombinant tau results in the formation of insoluble filamentous structures

To determine possible mechanisms by which NFTs are formed in Alzheimer's disease (AD), we investigated the ability of tissue transglutaminase (TGase) to convert human recombinant tau proteins into insoluble filamentous structures. TGase derived from guinea pig liver was activated by calcium to catalyze the in vitro cross-linking of the largest soluble recombinant tau isoform (htau40) into insoluble complexes as determined by electrophoresis following incubation in 4 M urea and SDS. The TGase-catalyzed formation of these insoluble complexes occurred within 15 min to 24 h and the decreased migration of the insoluble material correlated with increased calcium concentrations ranging from 2 mM to 50 mM when analyzed electrophoretically. TGase-treated human recombinant tau formed filamentous structures in vitro that were immunoreactive with antibodies to tau and TGase. These structures retained the insoluble characteristics typical of AD PHF/NFTs. Immunolabeling with the TGase antibody revealed that TGase is associated with the filaments formed from human recombinant tau in vitro as well as with PHFs isolated from NFTs from AD brains. These novel findings support an in vitro model for investigating the biophysical changes that occur in converting soluble tau proteins into an insoluble matrix consistent with the insoluble PHFs/NFTs which may contribute to neuronal degeneration and cell death in the AD brain.

Tubulin domains for the interaction of microtubule associated protein DMAP-85 from Drosophila melanogaster

The interaction of microtubule associated proteins (MAPs) with the microtubule system has been characterized in depth in neuronal cells from various mammalian species. These proteins interact with well-defined domains within the acidic tubulin carboxyl-terminal regulatory region. However, there is little information on the mechanisms of MAPs-tubulin interactions in nonmammalian systems. Recently, a novel tau-like protein designated as DMAP-85 has been identified in Drosophila melanogaster, and the regulation of its interactions with cytoskeletal elements was analyzed throughout different developmental stages of this organism. In this report, the topographic domains involved in the binding of DMAP-85 with tubulin heterodimer were investigated. Affinity chromatography of DMAP-85 in matrixes of taxol-stabilized microtubules showed the reversible interaction of DMAP-85 with domains on the microtubular surface. Co-sedimentation studies using the subtilisin-treated tubulin (S-tubulin) indicated the lack of association of DMAP-85 to this tubulin moiety. Moreover, studies on affinity chromatography of the purified 4 kDa C-terminal tubulin peptide bound to an affinity column, confirmed that DMAP-85 interacts directly with this regulatory domain on tubulin subunits. Further studies on sequential affinity chromatography using a calmodulin affinity column followed by the microtubule column confirmed the similarities in the interaction behaviour of DMAP-85 with that of tau. DMAP-85 associated to both calmodulin and the microtubular polymer. These studies support the idea that the carboxyl-terminal region on tubulin constitutes a common binding domain for most microtubule-interacting proteins.

Dephosphorylation of tau protein from Alzheimer's disease patients

Tau protein-prepared post-mortem from brains of Alzheimer's disease patients was treated with protein phosphatase 1 catalytic subunit, 2A catalytic subunit, and 2B (calcineurin). Dephosphorylation was monitored by immunoblotting with two monoclonal antibodies, TAU-1 and SMI31, which recognize in tau the dephospho- and phospho-states, respectively, of proline-directed protein kinase phosphorylation sites. Out of the three enzymes tested, protein phosphatase 2A was only effective in dephosphorylating tau at these Alzheimer-type epitopes.

Heterogeneity of the high molecular weight tau proteins in N115 neuroblastoma cells

The sequence of a high molecular weight (HMW) tau cDNA cloned from a neuroblastoma N115 library contains, in addition to the C- and N-terminal and middle regions present in the low molecular weight mouse brain tau proteins, a 711-bp nonhomologous domain (exon 4a) and a region of 198 bp corresponding to exon 6 of the tau gene. Protein immunoblot analysis, performed with antibodies specific either for a sequence present in the N-terminal region of all the tau variants or for exon 4a revealed several bands suggesting that more than one tau form is expressed in this cell line. Northern blot experiments performed with a number of cDNA probes spanning domains common and uncommon to low molecular weight and HMW tau allowed the identification of four tau transcripts differing in the size of their coding and noncoding regions. All these transcripts contain the sequence encoded by exon 6, but two of them lack exon 4a. As shown by RNase protection assays, the N-terminal region of these transcripts is also variable and contains either exon 1, or exons 1 and 2, or exons 1-3. Yet all these HMW tau forms contain four homologous repeats in their C-terminal domain both in the differentiated and nondifferentiated cells, i.e., have adult characteristics. In conclusion, the data reported in this article demonstrate that several HMW tau variants are expressed in neuroblastoma N115 cells and that the transition between immature to mature tau forms occurring during brain development is not required for neurite outgrowth during morphological differentiation of this cell line.

High and low molecular weight tau proteins are differentially expressed from a single gene

Both high and low molecular weight (HMW and LMW) tau proteins are expressed in the immature and adult mouse spinal cord. Northern blot analysis, performed with probes complementary to domains common and uncommon to the LMW and HMW entities, suggested that HMW tau proteins found in the immature mouse spinal cord are not translated from the single transcript of 6 kb expressed at these stages, but are transported within this nervous structure by axons arising in the periphery. In contrast, another minor transcript of 8 kb was detected in the adult mouse spinal cord by a HMW tau specific probe, suggesting that a small fraction of the HMW tau forms present in adulthood are translated within mouse spinal cord neurons. LMW spinal cord tau forms are encoded by mRNAs of 6 kb that contain three and four homologous repeats at immature and mature stages, respectively, whereas adult HMW entities contain four repeats. PCR analysis performed with mouse genomic DNA also showed that the nonhomologous region specific for HMW tau is a single exon. Southern blot and gene mapping showed that the same gene, located on the murine chromosome 11, encodes all the LMW and HMW tau variants. All these tau forms, therefore, are produced by an alternative splicing mechanism that is neuron-specific and developmentally regulated.