On January 27, a study first summarized in our SfN conference news, on post-translational modifications of the protein tau, appeared in JBC online. First author Diane Cripps, working with Austin Yang at University of Southern California, Los Angeles, and collaborators report full details of a systematic tandem mass spec analysis of tau protein isolated from Alzheimer brain samples. The researchers not only characterize hyperphosphorylation of human tau, but also report that three residues in the microtubule-binding site of soluble tau tend to become ubiquitinated early in the disease. This suggests that ubiquitination might play a role in controlling the stability of microtubules, and it implicates an age-related slowdown of the ubiquitin-proteasome system in the formation of tangles. Read our related conference story for a broad update on recent developments on tau.—Gabrielle Strobel

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  1. This is a well-performed study that mapped phosphorylation sites and polyubiquitination sites of paired helical fragment (PHF)-tau that was affinity-purified from the heat-treated soluble fraction of AD brain using monoclonal antibody MC-1. The authors confirmed many phosphorylation sites of PHF-tau reported by previous studies. They also identified five additional potential phosphorylation sites (i.e., Ser68, Thr71, Thr111, Ser113, and Thr414) in addition to the previously reported 37 sites on serine and threonine residues (see Gong et al., 2006, for the list of phosphorylation sites of PHF-tau). This study also supports a new hypothesis that abnormal hyperphosphorylation of tau is an early event that may be the key to the initiation of neurodegeneration (Alonso and Iqbal 2005; Gong et al 2006).

    Because polyubiquitin is found attached to the soluble PHF-tau, Cripps et al. proposed that polyubiquitination might also be an early modification event. However, abnormal hyperphosphorylation of tau had been observed in AD brain before polymerization into PHF, and this pool of hyperphosphorylated tau is not modified by ubiquitin (Bancher et al., 1991; Kopke et al., 1993). It is also possible that the soluble PHF-tau used in Cripps’s study might be derived from fragmented PHFs/NFTs (neurofibrillary tangles) that actually represent the late stage of tau pathology. In any case, we have shown that abnormal hyperphosphorylation is more likely to occur before being modified by polyubiquitination (Gong et al., 2005).

    View all comments by Cheng-Xin Gong

References

News Citations

  1. SfN: Return of the Other—Tau Is Back, Part 3

Further Reading

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Primary Papers

  1. . Alzheimer disease-specific conformation of hyperphosphorylated paired helical filament-Tau is polyubiquitinated through Lys-48, Lys-11, and Lys-6 ubiquitin conjugation. J Biol Chem. 2006 Apr 21;281(16):10825-38. PubMed.