Alpha-secretase inhibition impairs Group I metabotropic glutamate receptor-mediated protein synthesis, long-term potentiation and long-term depression

Abstract

Group I metabotropic glutamate receptors (Gp1-mGluRs) exert a host of effects on cellular functions, including enhancement of protein synthesis and the associated facilitation of long-term potentiation (LTP) and induction of long-term depression (LTD). However, the complete cascades of events mediating these events are not fully understood. Gp1-mGluRs trigger α-secretase cleavage of amyloid precursor protein, producing soluble amyloid precursor protein-α (sAPPα), a known regulator of LTP. However, the α-cleavage of APP has not previously been linked to Gp1-mGluR's actions. Using rat hippocampal slices, we found that the α-secretase inhibitor tumour necrosis factor-alpha protease inhibitor-1, which inhibits both disintegrin and metalloprotease 10 (ADAM10) and 17 (ADAM17) activity, blocked or reduced the ability of the Gp1-mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) to stimulate protein synthesis, metaplastically prime future LTP and elicit sub-maximal LTD. In contrast, the specific ADAM10 antagonist GI254023X did not affect the regulation of plasticity, suggesting that ADAM17 but not ADAM10 is involved in mediating these effects of DHPG. However, neither drug affected LTD that was strongly induced by either high-concentration DHPG or paired-pulse synaptic stimulation. Our data suggest that moderate Gp1-mGluR activation triggers α-secretase sheddase activity targeting APP or other membrane-bound proteins as part of a more complex signalling cascade than previously envisioned. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Keywords: alpha-secretase; hippocampus; metabotropic glutamate receptors; protein synthesis; synaptic plasticity.

Figure 1.

Regulation of protein synthesis by DHPG and sAPPα, as assessed using the puromycin-based SUnSET technique. (a) Representative western blot illustrating the enhancement of puromycin incorporation in CA1 mini-slices across the various treatment groups, as well as an α-tubulin band separately shown for the same blot and used as a loading control. (b) Quantification showing that 30 min DHPG (20 µM) increased the puromycin immunoreactivity almost twofold. The effect was completely abolished by co-administration of TAPI-1 (20 µM), which by itself did not affect basal protein synthesis. (c) Representative western blot as in (a). (d) Quantification showing that sAPPα (30 min, 1 nM) enhanced protein synthesis by almost threefold. The global mGluR antagonist LY341495 (100 µM) did not significantly inhibit the effect of sAPPα, nor did it affect basal protein synthesis. *p < 0.05, **p < 0.01, ****p < 0.0001 (Tukey’s post hoc tests) compared to untreated control slices. Cont, control CA1 mini-slices; D+T, combined DHPG and TAPI-1 treatment; LY, LY341495; M, marker lane; S+LY, combined sAPPα and LY341495 treatment. Data are mean ± s.e.m.

Figure 2.

Block of DHPG priming of LTP in area CA1 of hippocampal slices by TAPI-1. (a) Plot showing the time course of the effects of DHPG (20 µM, 10 min) on LTP induced by theta-burst stimulation in CA1 stratum radiatum, along with the effects of the α-secretase inhibitors. Representative waveforms shown at the top are averages of 10 sweeps taken just before the TBS (dotted lines) and at the end of the experiment (solid lines). Calibration bars: 1 mV, 10 ms. (b) Quantification of the early potentiation response revealed that TAPI-1 (20 µM) completely blocked the enhancement of LTP by DHPG, while GI254023X (20 µM) had no effect. *p < 0.05; (Tukey’s post hoc tests). Data are mean ± s.e.m.

Figure 3.

Impairment of DHPG-induced moderate LTD by TAPI-1. (a) Plot showing the time course of the effects of DHPG on hippocampal field potentials evoked in CA1 stratum radiatum, along with the effects of the α-secretase inhibitors. Representative waveforms at the top are averages of 10 sweeps taken just before the time of DHPG administration (dotted lines) and at the end of the experiment (solid lines). Calibration bars: 1 mV, 10 ms. (b) Bar chart showing the quantification of the drug effects on the LTD, delivered for the times and concentrations as in figure 2. TAPI-1 but not GI254023X significantly impaired the DHPG-induced LTD. *p < 0.05; **p < 0.01 (Dunnett’s post hoc tests). Data are mean ± s.e.m.

Figure 4.

Lack of effect of α-secretase inhibitors on strongly induced LTD. (a, b) Lack of effect of GI254023X on LTD and associated PPF change (c) induced by a high concentration of DHPG (100 µM). Representative waveforms are averages of 10 sweeps taken just before the time of DHPG administration (dotted lines) and at the end of the experiment (solid lines). Calibration bars: 1 mV, 10 ms. (d, e) Lack of effect of GI254023X administration throughout the experiment on strongly induced LTD and the associated PPF change (f). Waveforms as in (a). (g, h) Lack of effect of TAPI-1 on strongly induced LTD and associated PPF change (i). Data are mean ± s.e.m.

Figure 5.

Lack of effect of TAPI-1 on synaptically evoked mGluR-LTD. (a) mGluR-LTD was evoked in stratum radiatum by 1200 pairs of pulses (50 ms ISI) delivered at 1 Hz. (b) A combination of both TAPI-1 and GI254023X failed to impair the LTP induction or persistence. (c,d) There was also no effect on the degree of PPF change caused by the PP-LFS. Waveforms are as in previous figures. Data are mean ± s.e.m.