METAPLASTICITY IN A MOUSE MODEL OF ALZHEIMER’S DISEASE AND POSSIBLE THERAPEUTIC INTERVENTIONS

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder, causing loss of synaptic contacts and cognitive decline. It is widely believed that AD is initiated by synaptic dysfunction caused by production of Aβ peptides. Therefore, understanding how the progression of amyloidogenesis alters synaptic function is imperative in developing effective therapeutics for disease intervention. While many studies have focused on how Aβ peptides and the progression of AD affect frequency-dependent long-term potentiation and long-term depression (LTP/LTD), it is unclear whether synaptic dysfunction is at the level of induction or expression of synaptic plasticity mechanisms. Here we report that there is an age-dependent alteration in synaptic plasticity at the Schaffer collateral inputs to CA1 of APPswe;PS1deltaE9 AD transgenic (Tg) mice. Young pre-amyloidogenic Tgs showed enhanced LTP at the expense of LTD, while adult post-amyloidogenic Tgs showed enhanced LTD at the expense of LTP. The apparent shift in plasticity was mediated by altered LTP/LTD expression mechanisms, and in particular due to an absence of a normal age-dependent shift in pull-push metaplasticity. These results suggest that the main synaptic deficit in AD Tg mice is due to their inability to developmentally regulate LTP/LTD expression in accord with the pull-push metaplasticity model. Current AD therapeutics provide only temporary symptomatic relief, but need to strive to mitigate the long term progression of the disease by targeting the specific cellular mechanisms that become disrupted. Recognizing that cognitive decline during AD is correlated with loss of dendritic spine density, we examined two novel therapies that increase dendritic spine density through a Ras/ERK dependent mechanism. We found that the increase in dendritic spine density was better correlated with cognitive performance than the absolute magnitude of LTP. While these therapies were investigated in wild type mice, they both exhibit potential as drug candidates for AD treatment and warrant further studies to determine their effects in mouse models of AD. In summary, this project provides a novel mechanistic viewpoint in understanding the synaptic dysfunction seen in AD that can lead to the development of more effective therapeutics by specifically targeting the fundamental cellular mechanisms that are disrupted, such as pull-push metaplasticity

Phosphorylation of AMPA Receptors Is Required for Sensory Deprivation-Induced Homeostatic Synaptic Plasticity

Sensory experience, and the lack thereof, can alter the function of excitatory synapses in the primary sensory cortices. Recent evidence suggests that changes in sensory experience can regulate the synaptic level of Ca2+-permeable AMPA receptors (CP-AMPARs). However, the molecular mechanisms underlying such a process have not been determined. We found that binocular visual deprivation, which is a well-established in vivo model to produce multiplicative synaptic scaling in visual cortex of juvenile rodents, is accompanied by an increase in the phosphorylation of AMPAR GluR1 (or GluA1) subunit at the serine 845 (S845) site and the appearance of CP-AMPARs at synapses. To address the role of GluR1-S845 in visual deprivation-induced homeostatic synaptic plasticity, we used mice lacking key phosphorylation sites on the GluR1 subunit. We found that mice specifically lacking the GluR1-S845 site (GluR1-S845A mutants), which is a substrate of cAMP-dependent kinase (PKA), show abnormal basal excitatory synaptic transmission and lack visual deprivation-induced homeostatic synaptic plasticity. We also found evidence that increasing GluR1-S845 phosphorylation alone is not sufficient to produce normal multiplicative synaptic scaling. Our study provides concrete evidence that a GluR1 dependent mechanism, especially S845 phosphorylation, is a necessary pre-requisite step for in vivo homeostatic synaptic plasticity

METAPLASTICITY IN A MOUSE MODEL OF ALZHEIMER’S DISEASE AND POSSIBLE THERAPEUTIC INTERVENTIONS

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder, causing loss of synaptic contacts and cognitive decline. It is widely believed that AD is initiated by synaptic dysfunction caused by production of Aβ peptides. Therefore, understanding how the progression of amyloidogenesis alters synaptic function is imperative in developing effective therapeutics for disease intervention. While many studies have focused on how Aβ peptides and the progression of AD affect frequency-dependent long-term potentiation and long-term depression (LTP/LTD), it is unclear whether synaptic dysfunction is at the level of induction or expression of synaptic plasticity mechanisms. Here we report that there is an age-dependent alteration in synaptic plasticity at the Schaffer collateral inputs to CA1 of APPswe;PS1deltaE9 AD transgenic (Tg) mice. Young pre-amyloidogenic Tgs showed enhanced LTP at the expense of LTD, while adult post-amyloidogenic Tgs showed enhanced LTD at the expense of LTP. The apparent shift in plasticity was mediated by altered LTP/LTD expression mechanisms, and in particular due to an absence of a normal age-dependent shift in pull-push metaplasticity. These results suggest that the main synaptic deficit in AD Tg mice is due to their inability to developmentally regulate LTP/LTD expression in accord with the pull-push metaplasticity model. Current AD therapeutics provide only temporary symptomatic relief, but need to strive to mitigate the long term progression of the disease by targeting the specific cellular mechanisms that become disrupted. Recognizing that cognitive decline during AD is correlated with loss of dendritic spine density, we examined two novel therapies that increase dendritic spine density through a Ras/ERK dependent mechanism. We found that the increase in dendritic spine density was better correlated with cognitive performance than the absolute magnitude of LTP. While these therapies were investigated in wild type mice, they both exhibit potential as drug candidates for AD treatment and warrant further studies to determine their effects in mouse models of AD. In summary, this project provides a novel mechanistic viewpoint in understanding the synaptic dysfunction seen in AD that can lead to the development of more effective therapeutics by specifically targeting the fundamental cellular mechanisms that are disrupted, such as pull-push metaplasticity

Consequences of Inhibiting Amyloid Precursor Protein Processing Enzymes on Synaptic Function and Plasticity

Alzheimer's disease (AD) is a neurodegenerative disease, one of whose major pathological hallmarks is the accumulation of amyloid plaques comprised of aggregated β-amyloid (Aβ) peptides. It is now recognized that soluble Aβ oligomers may lead to synaptic dysfunctions early in AD pathology preceding plaque deposition. Aβ is produced by a sequential cleavage of amyloid precursor protein (APP) by the activity of β- and γ-secretases, which have been identified as major candidate therapeutic targets of AD. This paper focuses on how Aβ alters synaptic function and the functional consequences of inhibiting the activity of the two secretases responsible for Aβ generation. Abnormalities in synaptic function resulting from the absence or inhibition of the Aβ-producing enzymes suggest that Aβ itself may have normal physiological functions which are disrupted by abnormal accumulation of Aβ during AD pathology. This interpretation suggests that AD therapeutics targeting the β- and γ-secretases should be developed to restore normal levels of Aβ or combined with measures to circumvent the associated synaptic dysfunction(s) in order to have minimal impact on normal synaptic function

Abnormal synaptic function at MF to CA3 synapses in young BACE1 KOs.

<p>(A) Young BACE1 KOs (black circles) displayed larger PPF ratio (especially at 25 and 50 msec ISIs) compared to WTs (white circles). Top panel: Representative field potential traces following paired-pulse stimulation at 50 msec ISI. *P<0.001, two-way ANOVA; Fisher’s PLSD post hoc test P<0.001 between the two genotypes. Bottom right: BACE1 heterozygotes (HET) showed an intermediate phenotype between WT and KO at 50 msec ISI (one-way ANOVA, P<0.02; Dunnett’s multiple comparison test: *P<0.05 compared to WT). (B) Absence of mossy fiber LTP in young BACE1 KOs. Left: Summary graph plotting changes in normalized field potential against time. The arrow depicts when HFS (100 Hz, 1 sec×3) was delivered. Note that KOs (black circles) showed no LTP 60 minutes after LTP induction compared to WTs (white circles). Middle: Superimposed representative field potential traces taken from WTs and KOs at times indicated in the left panel. Right: BACE1 HETs showed an intermediate level of mfLTP (one-way ANOVA, P<0.02; Dunnett’s multiple comparison test: *P<0.05 compared to WT).</p

Reduced frequency of mEPSCs in CA3 PYRs of BACE1 KOs.

<p>(A) Representative mEPSC traces from CA3 PYRs in WTs and KOs. (B) BACE1 KOs showed significantly decreased mEPSCs frequency in CA3 PYRs compared to WTs. mEPSC frequency of each cell is shown as open circles. *t-test: P<0.01. (C) Amplitude of mEPSCs in CA3 PYRs was not altered in KOs. Left: The cumulative probability curve of KO mEPSC amplitudes (black dotted line) superimposed with that of WT (gray solid line) (K–S test, P>0.5). Right: Average mEPSC traces from PYRs of the two groups.</p

Normal mEPSCs recorded from SL-INTs of BACE1 KOs.

<p>(A) Representative mEPSC traces from SL-INTs of WTs and KOs. (B) There was no change in the frequency of mEPSCs from SL-INTs. mEPSC frequency of each cell is shown as open circles. (C) Amplitude of mEPSCs in SL-INTs was not altered in KOs. Left: The cumulative probability curve of KO mEPSC amplitudes (black dotted line) superimposed with that of WT (gray solid line) (K–S test, P = 0.2). Right: Average mEPSC traces from SL-INTs of the two groups.</p

Reduced frequency of mIPSCs in CA3 PYRs of BACE1 KOs.

<p>(A) Representative mIPSC traces from CA3 PYRs in WTs and KOs. (B) BACE1 KOs showed significantly reduced mIPSC frequency in CA3 PYRs compared to WTs. Open circles are mIPSC frequency of individual cells *t test: P<0.05. (C) Amplitude of mIPSCs in CA3 PYRs was not altered in KOs. Left: The cumulative probability curve of KO mIPSC amplitudes (black dotted line) superimposed with that of WT (gray solid line) (K–S test, P>0.7). Right: Average mIPSC traces from PYRs of the two groups.</p

Increased paired-pulse facilitation of eEPSCs at MF inputs to CA3 PYRs in BACE1 KOs.

<p>(A) A diagram showing the CA3 circuitry. Mossy fibers (MF) form glutamatergic excitatory synapses onto CA3 pyramidal cells (PYRs) as well as onto inhibitory SL-INTs in the stratum lucidum; SL-INTs project GABAergic synapses onto CA3 PYRs. The grey circle highlights the monosynaptic excitatory synapses of MF terminals onto CA3 PYRs. (B) Representative evoked EPSC traces from CA3 PYR following paired-pulse stimulation at 50 msec ISI before and after DCG-IV application in WTs and KOs. (C) PPF ratio was significantly increased in BACE1 KOs at 50 msec ISI compared to WTs. PPF ratio of individual cells are overlayed on the bar graph as open circles. *t-test: P<0.01. (D) Immunohistochemical labeling of a representative biocytin filled CA3 PYR. Left: DAPI staining of the CA3 subfield where a biocytin filled pyramidal cell was located. Stratum pyramidale (SP) and stratum lucidum (SL) borders are marked as white dotted lines. Middle: Biocytin staining of a recorded CA3 PYR. Right: Overlay of DAPI (blue) and biocytin (green). Scale: 40 μm.</p