July 3, 2015

Columbia University Medical Center | Long-term Memories Are Maintained by Prion-like Proteins

The molecules that maintain long-term memories in mice are a normal version of prion* proteins and work the same way as mechanisms in prions that cause mad cow disease, Creutzfeld-Jakob disease in humans, and other degenerative brain diseases. That’s the conclusion of research from the lab of Nobel-winning neuroscientist Eric Kandel, MD, of Columbia University Medical Center (CUMC).

When long-term memories are created in the hippocampus of the brain, new connections are made between neurons to store the memory. But those physical connections must be maintained for a memory to persist, or else they will disintegrate and the memory will disappear within days. Many researchers have searched for molecules that maintain long-term memory, but their identity has remained elusive.

Kandel and Kausik Si, Ph.D., an associate investigator at the Stowers Institute, first identified functional prions in the giant sea slug (Aplysia) in 2003 and found they might contribute to the maintenance of memory storage. And in 2012, they found a major clue from a study in fruit flies: oligomer (self-copying clusters) of a synapse protein are an essential ingredient for the formation of their long-term memory.

CPEB3 prion-like protein is key

CPEB3 protein is a functional prion that interacts with the actin cytoskeleton (credit: Joseph S. Stephan et al./Cell Reports)

More recently, the Kandel laboratory found evidence for the critical role of a similar prion-like protein called CPEB3 for maintaining long-term memories in mice, and probably in other mammals. That research is described in four papers published in Neuron and Cell Reports (listed below; three of them are open-access).

In one of many experiments, described in the paper by Luana Fioriti et al., the researchers challenged mice to repeatedly navigate a maze, allowing the animals to create a long-term memory. But when the researchers knocked out the animal’s CPEB3 gene two weeks after the memory was made, the memory disappeared.

The researchers then discovered how CPEB3 works inside the neurons to maintain long-term memories. “Like disease-causing prions, functional prions come in two varieties, a soluble form and a form that creates aggregates,” said. Kandel. In contrast, functional prion proteins can play a physiological role in the cell and do not contribute to disease.

“When we learn something and form long-term memories, new synaptic connections are made, the soluble prions in those synapses are converted into aggregated prions. The aggregated prions turn on protein synthesis necessary to maintain the memory.”

As long as these aggregates are present, Kandel says, long-term memories persist. Prion aggregates renew themselves by continually recruiting newly made soluble prions into the aggregates. “This ongoing maintenance is crucial,” said Dr. Kandel. “It’s how you remember, for example, your first love for the rest of your life.”

A similar protein exists in humans, suggesting that the same mechanism is at work in the human brain, but more research is needed. “It’s possible that it has the same role in memory, but until this has been examined, we won’t know,” said Dr. Kandel. “There are probably other regulatory components involved,” he added. “Long-term memory is a complicated process, so I doubt this is the only important factor.”

Kandel is University Professor & Kavli Professor of Brain Science, co-director of Columbia’s Mortimer B. Zuckerman Mind Brain Behavior Institute, director of the Kavli Institute for Brain Science, and senior investigator, Howard Hughes Medical Institute at CUMC.

* Prions (protein infectious particles) are a unique class of proteins. Unlike other proteins, they both self-propagate and induce other proteins to take on their alternative shape. When prions form in a cell (in a neuron in the case), they cause damage by grouping together in sticky aggregates that disrupt cellular processes. Prion aggregates are highly stable and accumulate in infected tissue, causing tissue damage and cell death. The dying cell releases the prion proteins, which are then taken up by other cells – and are thus considered infectious. These abnormal proteins are known to cause mad cow disease (bovine spongiform encephalopathy). They also have been linked to a variety of neurodegenerative diseases, including Alzheimer’s, Parkinson’s, and Huntington’s.

Abstract of The Persistence of Hippocampal-Based Memory Requires Protein Synthesis Mediated by the Prion-like Protein CPEB3

Consolidation of long-term memories depends on de novo protein synthesis. Several translational regulators have been identified, and their contribution to the formation of memory has been assessed in the mouse hippocampus. None of them, however, has been implicated in the persistence of memory. Although persistence is a key feature of long-term memory, how this occurs, despite the rapid turnover of its molecular substrates, is poorly understood. Here we find that both memory storage and its underlying synaptic plasticity are mediated by the increase in level and in the aggregation of the prion-like translational regulator CPEB3 (cytoplasmic polyadenylation element-binding protein). Genetic ablation of CPEB3 impairs the maintenance of both hippocampal long-term potentiation and hippocampus-dependent spatial memory. We propose a model whereby persistence of long-term memory results from the assembly of CPEB3 into aggregates. These aggregates serve as functional prions and regulate local protein synthesis necessary for the maintenance of long-term memory.

Abstract of SUMOylation Is an Inhibitory Constraint that Regulates the Prion-like Aggregation and Activity of CPEB3

Protein synthesis is crucial for the maintenance of long-term-memory-related synaptic plasticity. The prion-like cytoplasmic polyadenylation element-binding protein 3 (CPEB3) regulates the translation of several mRNAs important for long-term synaptic plasticity in the hippocampus. Here, we provide evidence that the prion-like aggregation and activity of CPEB3 is controlled by SUMOylation. In the basal state, CPEB3 is a repressor and is soluble. Under these circumstances, CPEB3 is SUMOylated in hippocampal neurons both in vitro and in vivo. Following neuronal stimulation, CPEB3 is converted into an active form that promotes the translation of target mRNAs, and this is associated with a decrease of SUMOylation and an increase of aggregation. A chimeric CPEB3 protein fused to SUMO cannot form aggregates and cannot activate the translation of target mRNAs. These findings suggest a model whereby SUMO regulates translation of mRNAs and structural synaptic plasticity by modulating the aggregation of the prion-like protein CPEB3.

Abstract of The CPEB3 Protein Is a Functional Prion that Interacts with the Actin Cytoskeleton

The mouse cytoplasmic polyadenylation element-binding protein 3 (CPEB3) is a translational regulator implicated in long-term memory maintenance. Invertebrate orthologs of CPEB3 inAplysia and Drosophila are functional prions that are physiologically active in the aggregated state. To determine if this principle applies to the mammalian CPEB3, we expressed it in yeast and found that it forms heritable aggregates that are the hallmark of known prions. In addition, we confirm in the mouse the importance of CPEB3’s prion formation for CPEB3 function. Interestingly, deletion analysis of the CPEB3 prion domain uncovered a tripartite organization: two aggregation-promoting domains surround a regulatory module that affects interaction with the actin cytoskeleton. In all, our data provide direct evidence that CPEB3 is a functional prion in the mammalian brain and underline the potential importance of an actin/CPEB3 feedback loop for the synaptic plasticity underlying the persistence of long-term memory.

Abstract of MicroRNA-22 Gates Long-Term Heterosynaptic Plasticity in Aplysia through Presynaptic Regulation of CPEB and Downstream Targets

The maintenance phase of memory-related long-term facilitation (LTF) of synapses between sensory and motor neurons of the gill-withdrawal reflex of Aplysia depends on a serotonin (5-HT)-triggered presynaptic upregulation of CPEB, a functional prion that regulates local protein synthesis at the synapse. The mechanisms whereby serotonin regulates CPEB levels in presynaptic sensory neurons are not known. Here, we describe a sensory neuron-specific microRNA 22 (miR-22) that has multiple binding sites on the mRNA of CPEB and inhibits it in the basal state. Serotonin triggers MAPK/Erk-dependent downregulation of miR-22, thereby upregulating the expression of CPEB, which in turn regulates, through functional CPE elements, the presynaptic expression of atypical PKC (aPKC), another candidate regulator of memory maintenance. Our findings support a model in which the neurotransmitter-triggered downregulation of miR-22 coordinates the regulation of genes contributing synergistically to the long-term maintenance of memory-related synaptic plasticity.

references:
Luana Fioriti, Cory Myers, Yan-You Huang, Xiang Li, Joseph S. Stephan, Pierre Trifilieff, Luca Colnaghi, Stylianos Kosmidis, Bettina Drisaldi, Elias Pavlopoulos, Eric R. Kandel. The Persistence of Hippocampal-Based Memory Requires Protein Synthesis Mediated by the Prion-like Protein CPEB3. Neuron, 2015; 86 (6): 1433 DOI: 10.1016/j.neuron.2015.05.021
Bettina Drisaldi, Luca Colnaghi, Luana Fioriti, Nishta Rao, Cory Myers, Anna M. Snyder, Daniel J. Metzger, Jenna Tarasoff, Edward Konstantinov, Paul E. Fraser, James L. Manley, Eric R. Kandel. SUMOylation Is an Inhibitory Constraint that Regulates the Prion-like Aggregation and Activity of CPEB3. Cell Reports, 2015; 11 (11): 1694 DOI: 10.1016/j.celrep.2015.04.061 (open access)
Joseph S. Stephan, Luana Fioriti, Nayan Lamba, Luca Colnaghi, Kevin Karl, Irina L. Derkatch, Eric R. Kandel. The CPEB3 Protein Is a Functional Prion that Interacts with the Actin Cytoskeleton. Cell Reports, 2015; 11 (11): 1772 DOI: 10.1016/j.celrep.2015.04.060 (open access)
Ferdinando Fiumara, Priyamvada Rajasethupathy, Igor Antonov, Stylianos Kosmidis, Wayne S. Sossin, Eric R. Kandel. MicroRNA-22 Gates Long-Term Heterosynaptic Plasticity in Aplysia through Presynaptic Regulation of CPEB and Downstream Targets. Cell Reports, 2015; 11 (12): 1866 DOI: 10.1016/j.celrep.2015.05.034 (open access)
related:
Long-term memories are maintained by prion-like proteins

http://www.kurzweilai.net/long-term-memories-in-mice-are-maintained-by-prion-like-proteins