Abstract
The aberrant accumulation of tau protein is a pathological hallmark of a class of neurodegenerative diseases known as tauopathies, including Alzheimer's disease and related dementias. On the basis of previous observations that tau is a direct substrate of histone deacetylase 6 (HDAC6), we sought to map all HDAC6-responsive sites in tau and determine how acetylation in a site-specific manner affects tau's biophysical properties in vitro. Our findings indicate that several acetylation sites in tau are responsive to HDAC6 and that acetylation on Lys-321 (within a KCGS motif) is both essential for acetylation-mediated inhibition of tau aggregation in vitro and a molecular tactic for preventing phosphorylation on the downstream Ser-324 residue. To determine the functional consequence of this HDAC6-regulated phosphorylation event, we examined tau's ability to promote microtubule assembly and found that phosphorylation of Ser-324 interferes with the normal microtubule-stabilizing function of tau. Tau phosphorylation of Ser-324 (pSer-324) has not previously been evaluated in the context of tauopathy, and here we observed increased deposition of pSer-324–positive tau both in mouse models of tauopathy and in patients with Alzheimer's disease. These findings uncover a novel acetylation–phosphorylation switch at Lys-321/Ser-324 that coordinately regulates tau polymerization and function. Because the disease relevance of this finding is evident, additional studies are needed to examine the role of pSer-324 in tau pathobiology and to determine whether therapeutically modulating this acetylation–phosphorylation switch affects disease progression in vivo.
Highlights
The aberrant accumulation of tau protein is a pathological hallmark of a class of neurodegenerative diseases known as tauopathies, including Alzheimer’s disease and related dementias
We confirmed that robust deacetylation of tau was observed in the presence of histone deacetylase 6 (HDAC6), which was prevented upon the addition of ACY-738
Acetylation sites were deemed responsive to HDAC6 if the number of observations in the sample with p300 exceeded that in the negative reaction for both analysis techniques; the number of observations was reduced in the sample with both p300 and HDAC6 relative to p300 alone for both analysis techniques; and the number of observations was increased in the presence of HDAC6 inhibition relative to the sample with p300 and HDAC6 (Table 1)
Summary
Mass spectrometry was utilized to identify acetylated residues following in vitro acetylation/deacetylation reactions in the presence or absence of the HDAC6 inhibitor ACY-738 (see supplemental Fig. S1, confirming acetylation status of four reactions included in the analysis). Information after the site of modification represents the number of observations upon analysis using collision-induced dissociation, followed by the number of observations using higher-energy collisional dissociation. Of particular relevance to tauopathy, we show that phosphorylation of Ser-324 inhibits tau function and is detected within inclusions in mouse models of tauopathy and in patients with AD. These findings suggest that a more thorough assessment of the relationship between acetylated Lys-321, hyperphosphorylated Ser-324, and disease progression/tau toxicity is warranted
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