Tau Pathology

                               Tau Pathology:

Tau protein:

Tau was first discovered in the middle of the 1970s as a factor promoting tubulin polymerization (Weingarten et al., 1975). Tau is a neuronal protein essentially located within the axonal compartment. Its structure makes it essential for the organization,stabilization, and dynamics of microtubules. 
             Tau is special and with most biophysical methods, such as X-ray crystallography, not analyzable. Neither heat nor acid can harm the protein. Whereas most proteins fold to adopt the structure necessary for their function, tau can do it in the absence of folded structure, is very flexible and changes its form very rapidly.

With Nuclear Magnetic Resonance Spectroscopy the scientists where able to shed light on the structural properties of tau and followed its fast motions. For the first time detailed investigations of structural changes from a large almost unfolded protein where conducted. "The financial support was granted by the DFG Research Center "Molecular Physiology of the Brain" (CMPB) in Göttingen, the Volkswagen foundation and an institute overlapping Max Planck Society project, ‘Toxic protein conformation’ ", says Christian Griesinger, head of the department of NMR-based structural biology at the Max Planck Institute.

          

             Tau protein structure. Image: Max Planck Institute 

"We can directly observe which modules of the tau protein bind to microtubules. If the protein is equipped with more phosphates than usual we can see that in this case the binding becomes significantly weaker. Tau and microtubule proteins can no longer interact" summarizes Zweckstetter. As a direct consequence the transport along the microtubule "tracks" is disturbed and nerve cell endings do not grow.

The interplay with binding partners is also possibly broken down. "We now hold the tau protein in our hands and are able to look at the interaction with its binding partners in the cell in a very detailed way".

        Tau is a low molecular weight component of cytoskeletal structures and is known as one of the microtubule-associated proteins (MAPs). Neuronal MAPs consisting of tau and MAP2 regulate the assembly of microtubules (MTs). Although tau and MAP2 are thought to have similar functions,intracellular localization of tau largely differs from that of MAP2. The mRNAs encoding tau proteins are expressed predominantly in neurons, where these tau proteins are localized mostly to axons of the CNS and PNS under normal physiologic conditions.The primary sequence of tau can be subdivided in an amino-terminal region followed by a proline-rich domain, the microtubule-binding repeat motifs, and the carboxy-terminal tail (Fig. 4.2b). Regarding the primary structure, the polypeptide sequences encoded by exons 2/3 add to tau acidity, whereas exon 10 encodes a positively charged sequence that adds to the basic character of tau. More generally, the amino-terminal region has a pI of 3.8 followed by the proline domain, which has a pI of 11.4, and the carboxy-terminal region is also positively charged with a pI of 10.8. Tau is rather a dipole with two domains with opposite charge, which can be modulated by posttranslational modifications. Structural analysis of human tau using several biophysical methods showed that in solution, tau behaves as an unfolded protein (Schweers et al., 1994).

However, functions of tau are distributed both in the amino- and the repeat-domains.

The amino-terminal region together with the proline-rich domain is referred to as the projection domain. This unstructured and negatively charged region detaches from the surface of microtubules (Hirokawa et al., 1988) and can interact with the plasma membrane or cytoskeletal proteins (Brandt et al., 1995). 

                Tau may also contribute to the spacing in between microtubule lattice and to the parallel-ordered organization of microtubules in axons (Chen et al., 1992). The spacing may be dependent upon the presence of additional amino-terminal sequences such as exons 2, 3, or 4A. The latter is included only in the spinal cord in peripheral nerve tissue (Georgieff et al., 1993). More recently, using a biophysical assay, Rosenberg and colleagues suggest that the amino-terminal regions of two tau molecules, each one individually binding to a microtubule, form an electrostatic “zipper” (Rosenberget al., 2008). The amino-terminal region of tau also interacts with a growing panel of proteins including motor proteins such as kinesin-1 (Utton et al., 2005) and dynactin/dynein complex (Magnani et al., 2007), SH3 containing tyrosine kinases such as the phospholipase C-gamma 1, or the p85α subunit of PI3K (Reynolds et al.,2008). Recently, it has been established that tau regulates the motility of dynein and kinesin motor proteins by an isoform-dependent mechanism (Dixit et al., 2008). Indeed, the shortest tau isoform lacking exons 2, 3, and 10 impedes the motility of both kinesin and dynein, whereas the longest tau isoforms with all exons have less effect on motor protein motility (Dixit et al., 2008). Thus, the axonal transport of vesicles may be finely tuned by the ratio of tau isoforms expressed. In neurodegenerative disorders, a modified pattern of tau isoform expression/ratio, due to tau aggregation for instance, may profoundly affect the axonal transport and could possibly lead to neurodegeneration (Crosby, 2003). Interactors of tau proteins may also regulate tau function. Tau has been shown to interact with thioredoxinlike protein RdCVFL in the retinal neurons (Fridlich et al., 2009). One isoform of RdCVFL having the thioredoxin-like function likely regulates the phosphorylation and degradation of tau.

             The carboxy-terminal region, which is the basic region of tau protein, is characterized by the presence of three or four repeat motifs, depending on the inclusion or not of the exon 10 encoding sequence. The repeat motifs corresponding with the microtubule-binding domains. Apart from binding to microtubules, the repeatdomains of tau interact with the histone deacetylase 6 (HDAC6), and HDAC6 is suggested to regulate tau phosphorylation. The inhibition of the proteasome activity by MG132 enhances the interaction of HDAC6 with tau. Interestingly, in this condition

HDAC6–tau complexes are observed in perinuclear aggresomes (Ding et al., 2008). 

            The proteasomal degradation of tau is also sensitive to chaperone and cochaperones Hsc70 and Bcl2-associated athanogene-1 (BAG-1), respectively, which are interactors with tau proteins (Elliott et al., 2007; 2009). The repeat-domains also interact more strongly with apolipoprotein E3 than the E4 isovariant (Huang et al., 1995). Apolipoprotein E4 genotype is a major risk factor in AD (for review, see Lambert and Amouyel, 2007). The interaction of tau and ApoE is reduced by phosphorylation of tau at serine 262 residue. The functionality of the tau–ApoE interaction remains unknown. However, a triple transgenic mouse model of AD crossed with a knock-in ApoE4 mouse showed a strong influence of ApoE4 upon the topographical distribution of amyloid deposits, but surprisingly, the tau pathology was strongly reduced (Oddo et al., 2008). Together, tau interacting partners may profoundly regulate its function but also modulate the tau pathology. The physiologic and pathologic function of tau is also regulated by posttranslational modifications such as phosphorylation. 

                             Tau Phosphorylation

                                       

            Tau microtubule-associated proteins are phosphoproteins (Butler and Shelanski, 1986). There are 85 potential phosphorylation sites on the longest tau isoform (see Fig. 4.3). Phosphorylation sites were characterized using phospho-dependent tauantibodies, phospho-peptide mapping, mass spectrometry, or NMR. According to the latest extensive analysis of tau phosphorylation and that of a previously published review, 71 among the 85 putative phosphorylation sites can be phosphorylated in physiologic or pathologic conditions (for review, see Hanger et al., 2009). Most of the phosphorylation sites surround the microtubule-binding domains in the proline-rich region and carboxy-terminal region of tau. A total of more than 20 protein kinases can phosphorylate tau proteins. This include four groups of protein kinases: (1) the proline-directed protein kinases (PDPK), which phosphorylate tau on serines or threonines that are followed by a proline residue. This group includes cyclin-dependent kinase 2 and 5, mitogen-activated protein kinase (MAPK), and several stress-activated protein kinases (SAPKs) (Goedert et al., 1997; Buee-Scherrer and Goedert, 2002). (2) The non- PDPK group includes tau-tubulin kinases 1 and 2, casein kinases 1, 2, and 1δ, Dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A), the phosphorylase kinase (Paudel, 1997), Rho kinase (Amano et al., 2003), and protein kinase A (PKA), protein kinase B (PKB/AKT), protein kinase C (PKC), protein kinase N (PKN) (for review, see Sergeant et al., 2008). (3) The third group includes protein kinases that phosphorylate tau on serine or threonine residues followed or not by a proline. Glycogen protein kinases (GSK3α and GSK3β) belong to this group and have a recognition motif (SXXXS or SXXXD/E) (Hanger et al., 1992). This group also includes mitogen and stress-activated protein kinase MSK1, which belongs to the AGC kinases group AGC kinases group (for protein kinases A, G, and C). AGC kinases preferentially phosphorylate serine and threonine residues that lie in RXRXXS/T motifs (Virdee et al., 2007). The S6 kinases p70 and p90 (RSK1 and RSK2) also phosphorylate tau as well as serumand glucocorticoid-induced kinase-1 (SGK1) . (4) The fourth group corresponds with tyrosine protein kinases such as Src kinases, C-abl, and c-met (Kinoshita et al., 2008). (The phosphorylation sites for each kinase are represented on the longest tau isoform in Fig. 4.3; see also Diane Hanger’s Web site,

http://cnr.iop.kcl.ac.uk/hangerlab/tautable.) These different groups share or have specific structural functions. The PDPK and GSK families are involved in modulation of tau binding to microtubules (Thr231 and Ser396) and also in the pathologic process of aggregation (for review, see Buee et al., 2000). The second group includes a number of kinases involved in signal transduction where tau may act as a linker or modulator.              In contrast, microtubuleaffinity regulating kinase (MARK) kinases are strictly involved in the regulation of tau binding to microtubules by phosphorylating specific motifs (KXGS) within the microtubule-binding domains (R1–R4 encoded by exons 9–12) (Mandelkow et al., 2004). Finally, regarding the fourth group, studies have determined that human tau Tyr18 and Tyr29 are phosphorylated by the src family tyrosine kinase fyn (Williamson et al., 2002; Lee et al., 2004). The same proline-rich region of tau proteins is likely involved in the interaction with phospholipase C-γ (PLC-γ) isozymes (Hwang et al., 1996; Jenkins and Johnson, 1998). Hwang and colleagues have demonstrated in vitro that tau proteins complex specifically with the SH3domain of PLC-γ and enhance PLC-γ activity in the presence of unsaturated fatty acids such as arachidonic acid. These results suggest that in cells that express tau proteins, receptors coupled to cytosolic phospholipase A2 may activate PLC-γ indirectly, in the absence of the usual tyrosine phosphorylation, through the hydrolysis of phosphatidylcholine to generate arachidonic acid (Hwang et al., 1996; Jenkins and Johnson, 1998). Altogether, these data indicate that tau proteins may also play a role in the signal transduction pathway involving PLC-γ (for review, see Rhee, 2001; Rhee, 2001). Interestingly, tau is phosphorylated by c-met (Kinoshita et al., 2008), the tyrosine kinase receptor of hepatocyte growth and scatter factor (HGF/SF) (Sergeant et al., 2000). This tyrosine receptor is also proteolyzed by the same proteases as the amyloid protein precursor (APP) (Foveau et al., 2009). Yet, it remains unknown whether HGF/SF could also modulate tau function. 
Tau-tubulin kinase 1 (TTBK1) phosphorylates tau on tyrosine 197 and therefore belongs to the fourth group of tau protein kinases (Sato et al., 2006). TTBK1 shows high similarity with tau-tubulin kinase 2 (TTBK2) that is supposed to phosphorylate tau at serine 208 and 210 as TTBK1 (Tomizawa et al., 2001; Kitano-Takahashi et al., 2007). Notably, a very recent study from Holden and collaborators has discovered a mutation of TTBK2 gene associated with spinocerebellar ataxia of type 11. Neuropathologic examination of the brain demonstrated the accumulation of tau in degenerating neurons of the medullary tegmentum of brain stem (Houlden et al., 2007). TTBK1 and TTBK2 belong to the casein kinase 1 (CK1) phylogenetic tree, and TTBK1 shows more than 30% similarity with CK1δ. Although the phosphorylation sites were characterized using mass spectrometry, TTBK1 and CK1δ do not phosphorylate tau at identical residues. Tau is phosphorylated by CK1δ on 37 residues, not including a tyrosine residue. Only serines 198, 208, and 210 are phosphorylatable by TTBK1 and CK1δ. Finally, kinases may also regulate the posttranscriptional maturation of tau RNAs. Thus, several kinases, including GSK3β, DYRK1A, and CLK2 or p25/CDK5 indirectly modulate the splicing of tau exon 10 through the phosphorylation of splicing factors (Hartmann et al., 2001; Hernandez et al., 2004; Shi et al., 2008). Phosphorylation or dephosphorylation of tau may also contribute to the cell localization of tau. Phosphorylation of tau by GSK3β regulates its axonal transport by reducing its interaction with kinesin (Cuchillo-Ibanez et al., 2008). In sharp contrast, dephosphorylated tau is located to the cell nucleus (Loomis et al., 1990) and is suggested to contribute to nucleolar organization (Sjoberg et al., 2006) and may also contribute to chromosome stability (Rossi et al., 2008).