Lack of the peroxiredoxin 6 gene causes impaired spatial memory and abnormal synaptic plasticity

Abstract

Peroxiredoxin 6 (PRDX6) is expressed dominantly in the astrocytes and exerts either neuroprotective or neurotoxic effects in the brain. Although PRDX6 can modulate several signaling cascades involving cognitive functions, its physiological role in spatial memory has not been investigated yet. This study aims to explore the function of the Prdx6 gene in spatial memory formation and synaptic plasticity. We first tested Prdx6^-/- mice on a Morris water maze task and found that their memory performance was defective, along with reduced long-term potentiation (LTP) in CA3-CA1 hippocampal synapses recorded from hippocampal sections of home-caged mice. Surprisingly, after the probe test, these knockout mice exhibited elevated hippocampal LTP, higher phosphorylated ERK1/2 level, and decreased reactive astrocyte markers. We further reduced ERK1/2 phosphorylation by administering MEK inhibitor, U0126, into Prdx6^-/- mice before the probe test, which reversed their spatial memory deficit. This study is the first one to report the role of PRDX6 in spatial memory and synaptic plasticity. Our results revealed that PRDX6 is necessary for maintaining spatial memory by modulating ERK1/2 phosphorylation and astrocyte activation.

Keywords: Long-term potentiation; Neuroinflammation; Peroxiredoxin 6; Reactive astrocyte; Spatial memory.

Fig. 1

Prdx6^−/− mice displayed impaired performance of spatial reference memory in the Morris water maze (MWM) task. a Timeline of MWM task and sample collection. b Escape latency to platform, and c mean swimming velocity during visible platform trial (n = 13–14 mice/group). d Learning curve during 5 days of training. e Representation of the time spent throughout the water maze using a heat map (upper panel) and illustration of the swimming pattern (lower panel) during the probe test. f Percent time spent in each quadrant during probe test. All data are presented as mean ± SEM. ^#p < 0.05, mixed-design repeated measure ANOVA followed by Bonferroni’s post hoc test with unpaired Student’s t test for individual differences between groups within each quadrant

Fig. 2

Normal locomotor functions and anxiety-like behaviors in Prdx6^−/− mice. a Time remained on an acceleration rota rod (sec) before falling (n = 9 mice/group) and b mean rotational velocity (rpm) at the time of falling. c Schematic of zones in the open field arena. d Number of entries into the center of an open field arena (n = 8 mice/group). e and f Percent time spent in the center (e) and outer (f) areas in the open field arena. g Schematic of light/dark transfer test. h Time spent in the light compartment (n = 9 mice /group). i Number of entries into the light compartment. j Percent risk assessment during 10 min of exploration. k Distance traveled, and l moving speed in the light compartment during light/dark transfer test. All data are presented as mean ± SEM., unpaired Student’s t test following a normal distribution

Fig. 3

Decline of hippocampal LTP in Prdx6^−/− mice recorded from basal condition. a Schematic representation of LTP depicting stimulation and recording in hippocampal CA1-CA3 synapse. b Input–output curve of fEPSP slope (n = 8 slices/5 mice in each group). c Average fEPSP plotted against time in minutes (n = 5 mice/group). d Example traces representing the average of 3 sweeps. e–g The average of fEPSP slopes from the last 10 min of the first hour and third hour after HFS of Prdx6^+/+ and Prdx6^−/− mice were calculated and plotted. Normalized fEPSP slope (%) for baseline (e), first hour (f) and third hour (g) of hippocampal LTP. All data are presented as mean ± SEM. ^#p < 0.05. Unpaired Student’s t test following a normal distribution and mixed-design repeated measure ANOVA followed by Bonferroni’s post hoc test with unpaired Student’s t test for individual differences between groups within training day

Fig. 4

Enhanced hippocampal LTP in Prdx6^−/− mice after probe test. a Schematic representation of LTP depicting stimulation and recording in hippocampal CA1-CA3 synapse after the completion of probe test. b Input–output curve of fEPSP slope (n = 8 slices/4–5 mice in each group). c Average fEPSP plotted against time in minutes (n = 4–5 mice/group). d and e Normalized fEPSP slope (%) for baseline (d) and first hour (e) of hippocampal LTP. All data are presented as mean ± SEM. ^#p < 0.05, unpaired Student’s t-test following a normal distribution

Fig. 5

Prdx6^−/− mice showed an increase in MAPK signaling in the hippocampus at the time of memory retrieval. a Schematic of total protein collection from the hippocampus immediately after the probe test. b–f Representative western blot and quantification of PSD95 (b), pERK1/2 (c), cPLA2 (d), pAkt1 ser473 (e), and pCaMKIIα/β (f) in the hippocampi collected immediately after the probe test (n = 8 mice/group). All data are presented as mean ± SEM. ^#p < 0.05, unpaired Student’s t-test following a normal distribution and Mann-Whitney U test following a non normally distributed

Fig. 6

Reducing phosphorylated ERK1/2 level rescued spatial memory impairment of Prdx6^−/− mice. a MEK inhibitor (U0126) injection before probe test. b Escape latency to platform and c mean swimming velocity during visible platform trial (n = 10–12 mice/group). d Learning curve during 5 days of training. e Percent time spent in each quadrant during the probe test. All data are presented as mean ± SEM. ^#p < 0.05, two-way measure ANOVA followed by Bonferroni’s post hoc test with one-way ANOVA for multiple comparison of percent time spent in target quadrant

Fig. 7

Astrocytic activation and proinflammatory cytokine level in the hippocampus of Prdx6^−/− mice. a Schematic of total protein collection from the hippocampus immediately after the probe test. b–d Representative western blot and quantification of GFAP (b), TNFα (c), and IL-6 (d) in the hippocampus immediately after the probe test (n = 8 mice/group). All data are presented as mean ± SEM. ^#p < 0.05, unpaired Student’s t-test following a normal distribution