Molecular mechanisms behind learning and memory identified

Molecular mechanisms behind learning and memory identified

Summary: The results reveal the molecular mechanism of acetylcholine in learning and memory.

Source: Fujita Health University

Patients with Alzheimer’s disease (AD) have lower levels of the neuromodulator acetylcholine (ACh) in their brains. Donepezil, an AD drug, increases ACh levels in the brain and improves learning deficits associated with AD.

Now, researchers have identified the intracellular signaling cascade by which ACh regulates aversive learning, a major preliminary test for AD drugs.

The researchers also found that donepezil activated this signaling cascade to regulate aversive learning. The results indicate the potential of the signaling cascade as drug targets.

Acetylcholine (ACh) is a neuromodulator that plays a central role in aversive learning, that is, rapid conditioning to an unpleasant smell, taste, or touch. These learning functions take place in cells called D2 receptor-expressing medium spiny neurons (D2R-MSN) that are located in the striatum/nucleus accumbens (NAc) of the brain. ACh levels increase in the NAc during aversive learning experiences.

Previous studies have shown that ACh acts on D2R-MSNs through a receptor called M1 muscarinic receptor (M1R), which in turn activates the downstream signaling molecule called protein kinase C (PKC).

However, until now, the exact intracellular signaling mechanism by which ACh influences aversive learning has not been clear, which has limited the development of AD therapeutic strategies that directly target intracellular ACh signaling. .

Recently, in a new study published in Molecular psychiatryresearchers from the laboratory of Professor Kozo Kaibuchi at Fujita Health University (FHU), have elucidated the molecular mechanisms of ACh for learning and memory.

“This is the first time this has been done in the 45 years since the cholinergic hypothesis of AD was established. Our study also led us to understand the intracellular mechanism of donepezil and its effect on learning and This exciting discovery opens doors to new therapeutic strategies for AD,” says Assistant Professor Yukie Yamahashi, lead author of the study.

Molecular signaling cascades are facilitated by a process called phosphorylation, which involves the addition of phosphate groups to certain substrate molecules by kinases in cells. To study phosphorylation, the research team used a technique called kinase-oriented phosphoproteomics analysis, which was developed by Professor Kozo Kaibuchi, the corresponding author of the study.

The research team confirmed the role of ACh in PKC stimulation after monitoring phosphorylation events after ACh binding to M1Rs in streak/NAc sliced ​​mice ex vivo. Subsequently, phosphoproteomic analysis was performed, which yielded 116 candidate PKC substrates, including “β-PIX”, the activator of a protein called “small GTPase Rac”.

“We found that PKC phosphorylates and activates β-PIX downstream of ACh, which in turn activates a kinase called PAK, a downstream target of Rac. We then examined the involvement of the identified ACh-M1R-PKC-Rac-β-PIX-PAK cascade in aversive learning and aversive memory using passive avoidance tests in mice,” explains the Dr Yamahashi. Finally, the researchers also found that donepezil activated the cascade to enhance aversive learning.

“This study constitutes the first evidence of the intracellular mechanisms of donepezil that regulate learning and memory,” says Dr. Yamahashi.

Their findings agree well with a recent study from Professor Kaibuchi’s lab published in the Journal of Neurochemistry. The study’s first author, Dr. Md. Omar Faruk, received the Mark A. Smith Prize from the International Society for Neurochemistry (ISN).

The study showed the involvement of the “voltage-gated KCNQ2 potassium channel” – which was identified as another PKC substrate candidate in the above phosphoproteomics analysis – in aversive learning. In fact, PKC directly phosphorylates KCNQ2 at threonine 217, the phosphorylation site previously reported for possible involvement in modulating its channel activity. In addition, administration of donepezil also enhanced the phosphorylation event in the NAc.

This shows a diagram of the study
Upon an aversive stimulus (electric foot shock), acetylcholine activates PAK kinase through the M1R-PKC cascade to facilitate synaptic plasticity. It also enhances PKC-mediated KCNQ2 phosphorylation to stimulate neuronal excitability, which then increases neuronal firing in response to glutamatergic input. Activation of PAK and KCNQ2-mediated pathways leads to aversive behavior. Credit: Kozo Kaibuchi and Yukie Yamahashi of Fujita Health University

The team’s findings directly imply that the signaling cascade, M1R-PKC-β-PIX-PAK, is involved in recognition memory and associative learning. This is very important because the cascade itself provides a platform for screening AD drugs in development.

“While we focused only on β-PIX and elucidated the M1R-PKC-PAK pathway, our phosphoproteomics data revealed many other PKC substrates – presynaptic proteins and postsynaptic scaffold proteins to name a few. , which are recorded in a database called Kinase-Associated Neural PHOspho-Signalization (KANPHOS) (https://kanphos.neuroinf.jp/).

“We are only seeing the tip of the iceberg and believe that future research may produce new signal transduction mechanisms in other areas of the brain,” says Dr. Yamahashi, regarding the future prospects of their research. .

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About this learning and memory research news

Author: Press office
Source: Fujita Health University
Contact: Press Office – Fujita Health University
Image: Image credited to Kozo Kaibuchi and Yukie Yamahashi of Fujita Health University

Original research: Free access.
“Acetylcholine pathway phosphoproteomics enables discovery of PKC-β-PIX-Rac1-PAK cascade as a stimulating signal for aversive learning” by Yukie Yamahashi et al. Molecular psychiatry


Summary

Phosphoproteomics of the acetylcholine pathway enables the discovery of the PKC-β-PIX-Rac1-PAK cascade as a stimulating signal for aversive learning

Acetylcholine is an essential neuromodulator for learning and memory. Donepezil, a cholinesterase inhibitor, increases levels of acetylcholine in the brain and improves learning disabilities associated with Alzheimer’s disease (AD).

Acetylcholine activates striatal dopamine receptor/nucleus accumbens medium spiny neurons expressing medium spiny neurons (D2R-MSN), which regulate aversive learning through the M1 muscarinic receptor (M1R). However, how acetylcholine stimulates learning beyond the M1Rs remains unresolved.

Here, we found that acetylcholine stimulated protein kinase C (PKC) in mouse striatal/nucleus accumbens. Our original kinase-oriented phosphoproteomics analysis revealed 116 candidate PKC substrates, including the activator Rac1 β-PIX. Acetylcholine induced phosphorylation and activation of β-PIX, thereby stimulating the effector Rac1 p21-activated kinase (PAK).

The aversive stimulus activated the M1R-PKC-PAK pathway in D2R-MSN mice. Specific D2R-MSN expression of PAK mutants by the Cre-Flex system regulated dendritic spine structural plasticity and aversive learning. Donepezil induced PAK activation in cumulative D2R-MSNs and in the CA1 region of the hippocampus and enhanced D2R-MSN-mediated aversive learning.

These results demonstrate that acetylcholine stimulates M1R-PKC-β-PIX-Rac1-PAK signaling in D2R-MSNs for aversive learning and implicate the therapeutic potential of the cascade for AD, as aversive learning is utilized. to pre-screen for AD medications.

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