Korean J Physiol Pharmacol.  2015 Nov;19(6):515-522. 10.4196/kjpp.2015.19.6.515.

Mind Bomb-2 Regulates Hippocampus-dependent Memory Formation and Synaptic Plasticity

• Kim S
• Kim T
• Lee HR
• Kong YY
• Kaang BK

• KMID: 2070791
• DOI: http://doi.org/10.4196/kjpp.2015.19.6.515

Abstract

Notch signaling is a key regulator of neuronal fate during embryonic development, but its function in the adult brain is still largely unknown. Mind bomb-2 (Mib2) is an essential positive regulator of the Notch pathway, which acts in the Notch signal-sending cells. Therefore, genetic deletion of Mib2 in the mouse brain might help understand Notch signaling-mediated cell-cell interactions between neurons and their physiological function. Here we show that deletion of Mib2 in the mouse brain results in impaired hippocampal spatial memory and contextual fear memory. Accordingly, we found impaired hippocampal synaptic plasticity in Mib2 knock-out (KO) mice; however, basal synaptic transmission did not change at the Schaffer collateral-CA1 synapses. Using western blot analysis, we found that the level of cleaved Notch1 was lower in Mib2 KO mice than in wild type (WT) littermates after mild foot shock. Taken together, these data suggest that Mib2 plays a critical role in synaptic plasticity and spatial memory through the Notch signaling pathway.

Keyword

Hippocampus; Memory; Mind bomb-2; Notch signaling; Synaptic plasticity

MeSH Terms

Adult
Animals
Blotting, Western
Brain
Embryonic Development
Female
Foot
Hippocampus
Humans
Memory*
Mice
Neurons
Plastics*
Pregnancy
Shock
Synapses
Synaptic Transmission
Plastics

Figure

• 

  Fig. 1 Mib2 KO mice show impaired hippocampus-dependent learning and memory. (A) WT and Mib2 KO mice showed normal levels of basal anxiety in the open field test (WT, n=8; KO, n=11; two-way ANOVA, interaction between genotype and zone, F2,17=1.082, p=0.350). (B) The total distance moved during the open field test was similar in WT and Mib2 KO mice (WT, n=8; KO, n=11; unpaired t-test, p=0.562). (C) Mib2 KO mice showed a normal level of anxiety in the elevated plus maze (WT, n=7; KO, n=8; two-way ANOVA, interaction between genotype and arms, F1,13=0.002, p=0.966). (D) Learning curve during 5 training days of the Morris water maze task shows the latency required for the mice to reach the platform (WT, n=9; KO, n=9; two-way ANOVA, genotype, F1,16=7.191, *p<0.05). (E) Freezing levels of WT and Mib2 KO mice before (pre-training) and after (retrieval) contextual fear conditioning (WT, n=8; KO, n=11; two-way ANOVA, interaction between genotype and conditioning, F1,17=14.39, **p<0.01). All graphs represent mean±SEM. Abbreviations: WT, wild type; KO, Knockout.

• 

  Fig. 2 General electrophysiological characteristics of Mib2 KO mice. (A) Input-output relationship at the SC-CA1 synapses was not different between Mib2 KO and WT littermates (WT, n=8; KO, n=8; two-way ANOVA, p=0.9993). (B) Paired pulse ratio was normal in Mib2 KO mice (WT, n=9; KO, n=8; two-way ANOVA, p=0.8546). (C) NMDAR-dependent LTD was similar in WT and Mib2 KO mice (WT, n=6; KO, n=9; unpaired t-test, p=0.8788). Abbreviations: SC, Schaffer collaterals; WT, wild type; KO, knock-out; LTD, long-term depression; NMDAR, N-methyl-D-aspartate receptor.

• 

  Fig. 3 E-LTP and L-LTP are selectively impaired in Mib2 KO mouse depending on the induction protocol. (A) TBS-induced E-LTP was impaired in Mib2 KO mice (WT, n=12; KO, n=9; unpaired t-test, *p<0.05). (B) HFS-induced E-LTP at the SC-CA1 synapses was not different in WT and Mib2 KO mice (WT, n=7; KO, n=8; unpaired t-test, p=0.9001). (C) The potentiation level of HFS-induced L-LTP was different during the last 5 minutes in WT and Mib2 KO mice (WT, n=3; KO, n=4; unpaired t-test, *p<0.05). (D) The potentiation level of TBS-induced L-LTP was different during the last 5 minutes in WT and Mib2 KO mice (WT, n=11; KO, n=9; unpaired t-test, p=0.2498). Abbreviations: SC, Schaffer collaterals; WT, wild type; KO, knock-out; E-LTP, early-phase long-term potentiation; L-LTP, late-phase long-term potentiation; TBS, theta burst stimulation; HFS, high frequency stimulation

• 

  Fig. 4 Mib2 KO mice have a normal GluN2B level, but a reduced Notch signaling. (A) Protein expression of Mib2, but not Mib1, was downregulated in Mib2 KO mouse hippocampi. (B~D) pGluN2B and GluN2B levels were comparable in Mib2 KO and WT littermates. (C) pGluN2B levels normalized to the total GluN2B levels (WT, n=5; KO, n=4; unpaired t-test, p=0.490). (D) GluN2B levels normalized with tubulin (WT, n=5; KO, n=4; unpaired t-test, p=0.494). (E, F) Cleaved Notch1 level in the hippocampus was comparable in WT and Mib2 KO littermates in the home cage condition. Mib2 KO mice had a lower level of cleaved Notch1 than their WT littermates after a mild foot shock (WT, n=5; KO, n=4; unpaired t-test, *p<0.05). Abbreviations: WT, wild type; KO, knock-out; GluN2B, GluRepsilon2; SM, size marker.

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