Figure 1:
Diagram of tau protein, its domains, sites of phosphorylation and mutations. The near-N-terminal inserts (exons 2, 3, grey) and the second of the 4 repeats (exon 10) can be absent due to alternative splicing. The 'assembly domain' comprises the C-terminal half; it binds to microtubules and stabilizes them, whereas the N-terminal 'projection domain' protrudes from the microtubule surface. The regions flanking the repeats are rich in Ser-Pro or Thr-Pro motifs which can be phosphorylated by proline-directed kinases and thus form epitopes of antibodies diagnostic of Alzheimer tau. Ser214 and Ser262 (in the KXGS motif of the first repeat) are non-proline motifs phosphorylated in AD and in vitro by PKA or MARK; this causes the detachment of tau from microtubules. Some of the mutations linked to dementias with tauopathy are indicated, others are silent on the protein level.
|
Figure 2:
Model showing a possible link between axonal transport, microtubules and tau in Alzheimer's disease. Transport relies on microtubules as tracks, stabilized by the ties of tau protein. Phosphorylation (P) at crucial sites detaches tau from microtubules, leading to the breakdown of microtubules and the accumulation of tau aggregated into paired helical filaments (PHFs).
|
Expression of tau protein induces process formation in SF9 cells  Figure 3: The Sf9 cell bodies have a round shape, with diameter of 20 µm. After expression of tau protein they develop a single process per cell, with uniform diameter of 1-2 µm and up to 100 µm long. |
Mutations of KXGS motifs in the repeat domain of tau inhibit process formation in SF9 cells(a)    (b)    Figure 4: Sf9 cells were transfected with (a) htau23, (b) mutant KXGA/R1/3/4, and immunostained 60 hours after infection with antibodies DM1A (for tubulin, left) and K9JA (for tau, right). a) Transfection with htau23 leads to many processes, but b) when all three KXGS motifs are changed into KXGA the formation of processes is almost completely inhibited. |
Inhibition of intracellular trafficking of vesicles and organelles by tau protein (Ebneth et al., J. Cell Biol. 143:777-794, 1998). Figure 5: Overexpression of tau protein in CHO cells: Note the accumulation of mitochondria (yellow/green) in the cell center (right), whereas normal cells show mitochondria dispersed throughout the cell (left). This is explained by the inhibition of kinesin-dependent transport along microtubules (red) towards the cell periphery (see diagram Fig.8). |
 Figure 6: Overexpression of tau protein in neuroblastoma N2a cells: Mitochondria accumulate in the cell body and are almost completely excluded from the cell processes (right) because tau protein inhibits anterograde transport, whereas normal N2a cells (left) contain mitochondria throughout the cell process. This explains why elevated tau protein makes cell processes vulnerable. |
 Figure 7: Overexpression of tau protein in CHO cells leads to the retraction of the endoplasmic reticulum from the cell periphery (right), whereas the ER extends nearly to the plasma membrane in control cells (left). |
 Figure 8: Model of the effects of tau protein on intracellular transport along microtubules: Microtubules have a polar arrangement, originating from the MTOC (microtubule-organizing center) near the nucleus (minus-end) to the cell periphery (plus-ends) and into the axons in the case of neurons (not shown). The transport towards the plus-end is achieved by kinesin-like motors, the tranport towards the minus-ends by dynein. Overexpression of tau protein leads to the preferential inhibition of plus-end directed transport. As a result, minus-end directed transport becomes dominant, mitochondria, peroxisomes, or intermediate filaments accumulate at the cell center, the endoplasmic reticulum retracts from the plasma membrane, and cell processes become vulnerable due to insufficient supply of proteins, membranes, or energy metabolism. |
Structure and assembly of Alzheimer paired helical filaments (PHF) Figure 9: Electron micrographs of paired helical filaments. The filaments consist mainly of tau-protein and can be reassembled from recombinant tau protein in vitro. This allows one to study the assembly mechanism and to search for inhibitors of this pathological process. Dimerization of tau by oxidation of SH groups is an important intermediate step. The assembly can be accelerated by polyanionic cofactors (such as RNA or acidic peptides). (a) PHF from Alzheimer brain tissue, (b) PHF assembled from htau23, the smallest isoform of human tau proteins. |