Cytoplasmic Retention of Protein Phosphatase 2A Inhibitor 2 (I2PP2A) Induces Alzheimer-like Abnormal Hyperphosphorylation of Tau

Background: In Alzheimer brain, I-2(PP2A) is translocated from the neuronal nucleus to the cytoplasm and promotes abnormal hyperphosphorylation of Tau. Results: Inactivation of nuclear localization signal (NLS) causes retention of I-2(PP2A) in the cell cytoplasm, where it promotes Tau hyperphosphorylation by affecting PP2A signaling. Conclusion: Retention of I-2(PP2A) in cell cytoplasm results in Tau hyperphosphorylation. Significance: The study provides potential tools for investigating Tau-based therapeutics. Abnormal hyperphosphorylation of Tau leads to the formation of neurofibrillary tangles, a hallmark of Alzheimer disease (AD), and related tauopathies. The phosphorylation of Tau is regulated by protein phosphatase 2A (PP2A), which in turn is modulated by endogenous inhibitor 2 (I-2(PP2A)). In AD brain, I-2(PP2A) is translocated from neuronal nucleus to cytoplasm, where it inhibits PP2A activity and promotes abnormal phosphorylation of Tau. Here we describe the identification of a potential nuclear localization signal (NLS) in the C-terminal region of I-2(PP2A) containing a conserved basic motif, (RKR181)-R-179, which is sufficient for directing its nuclear localization. The current study further presents an inducible cell model (Tet-Off system) of AD-type abnormal hyperphosphorylation of Tau by expressing I-2(PP2A) in which the NLS was inactivated by (RKR181)-R-179 AAA along with (KR169)-K-168 AA mutations. In this model, the mutant NLS (mNLS)-I-2(PP2A) (I(2)(PP2A)AA-AAA) was retained in the cell cytoplasm, where it physically interacted with PP2A and inhibited its activity. Inhibition of PP2A was associated with the abnormal hyperphosphorylation of Tau, which resulted in microtubule network instability and neurite outgrowth impairment. Expression of mNLS-I-2(PP2A) activated CAMKII and GSK-3, which are Tau kinases regulated by PP2A. The immunoprecipitation experiments showed the direct interaction of I-2(PP2A) with PP2A and GSK-3 but not with CAMKII. Thus, the cell model provides insights into the nature of the potential NLS and the mechanistic relationship between I-2(PP2A)-induced inhibition of PP2A and hyperphosphorylation of Tau that can be utilized to develop drugs preventing Tau pathology.