Neuronal activity regulates extracellular tau in vivo

Abstract

Tau is primarily a cytoplasmic protein that stabilizes microtubules. However, it is also found in the extracellular space of the brain at appreciable concentrations. Although its presence there may be relevant to the intercellular spread of tau pathology, the cellular mechanisms regulating tau release into the extracellular space are not well understood. To test this in the context of neuronal networks in vivo, we used in vivo microdialysis. Increasing neuronal activity rapidly increased the steady-state levels of extracellular tau in vivo. Importantly, presynaptic glutamate release is sufficient to drive tau release. Although tau release occurred within hours in response to neuronal activity, the elimination rate of tau from the extracellular compartment and the brain is slow (half-life of ∼11 d). The in vivo results provide one mechanism underlying neuronal tau release and may link trans-synaptic spread of tau pathology with synaptic activity itself.

Figure 1.

Depolarization increases tau in ISF. (A–C) Microdialysis experiments were performed in hippocampi of wild-type mice. After baseline collection, the regular perfusion buffer was switched to high K⁺ perfusion buffer (administration indicated by gray box). After 1 h, the buffer was switched back to normal perfusion buffer (wash out) and ISF collection was continued. Glucose (A), lactate (B), and tau (C) in ISF were measured. (n = 5; *, P < 0.05; **, P < 0.01). For mice studied in A–C, each mouse was investigated independently. Any treatment effects were compared with baseline values within each mouse. Error bars represent SEM. (D) Representative EEG trace from one of three mice during depolarization with high K⁺ perfusion buffer.

Figure 2.

Neuronal activity and synaptic activity increase tau in ISF. (A) Representative EEG trace from one of three mice during depolarization with high K⁺ perfusion buffer. 25 µM PTX was delivered directly into the hippocampus of wild-type mice via reverse microdialysis (indicated by dashed line). (B–D) Glucose (B), lactate (C), and tau (D) in ISF was measured (n = 6/group). (E) Hippocampal tissue tau levels after PTX reverse microdialysis (n = 6/group). (F and G) 25 µM PTX was delivered directly into the hippocampus of P301S human tau transgenic mice via reverse microdialysis for 6 h (n = 5). After 6-h washout, the mice were given an i.p. injection of 325 mg/kg pilocarpine hydrochloride (indicated by arrows). Human tau (F) and LDH (G) activity in ISF was measured. The mean LDH activity was compared between groups (H; *, P < 0.05). (I) Representative EEG trace from one of three mice during various doses of NMDA infusion. (J) The effects of various doses of NMDA delivered by reverse microdialysis on ISF tau was measured (n = 6; *, P < 0.05; ***, P < 0.001). (K) The effects of the highly selective mGluR2/3 antagonist LY341495 (administered by reverse microdialysis) on ISF tau was measured (n = 6 for LY341495, n = 7 for vehicle). Error bars represent SEM. For mice studied in B–D, F–H, and J–K, each mouse was investigated independently. Any treatment effects were compared with baseline values within each mouse.

Figure 3.

TTX does not cause an acute decrease of ISF tau. After baseline ISF tau collection, 5 µM TTX was delivered via reverse microdialysis (indicated by dashed line). The effect of TTX on ISF tau (A; n = 10) and ISF endogenous Aβx-40 (B; n = 7; *, P < 0.05) is shown. The effect of preadministration of TTX on ISF tau in the presence of 0.4 µM NMDA (C; n = 6–7 per group; *, P < 0.05) is also shown. Both TTX and NMDA were administered by reverse microdialysis in C. Error bars represent SEM. Each mouse studied in A–C was investigated independently. Any treatment effects were compared with baseline values within each mouse.

Figure 4.

The turnover rate of tau in hippocampus and ISF is low. Human tau expression in anti-aggregant mice was switched off for the indicated lengths of time by doxycycline, and human tau (black circle) and murine tau (white square) levels in hippocampus were measured by ELISA (A; n = 5–7/group/time point; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001). Human tau (black circle) and lactate (white square) in ISF after switch-off were measured by ELISA (B; n = 4–5/group/time point; **, P < 0.01). The plot of the common logarithm of percent tau in hippocampal lysates or ISF versus time was linear in both groups studied. The slope from the linear regressions from log (% tau) versus time was used to calculate the half-life (t_1/2) of elimination for tau from hippocampus (C) and ISF (D). Each mouse studied in A–D was investigated independently. Any treatment effects were compared with baseline values obtained from mice studied at time zero. Error bars represent SEM.