Memory: Can fearlessness come in a tiny package?
Contextual fear conditioning is a process that occurs when a painful or frightening stimulus happens within a specific context, and it can cause an individual to fear the context even when the stimulus is removed. In humans, it is thought that contextual fear conditioning can contribute to anxiety and post-traumatic stress disorder, so understanding how the brain forms “associative memories” that link a context to a traumatic event has been the subject of research for many years (Maren et al., 2013).
In rodents, we can study the neurobiological mechanisms responsible for the formation of such memories by pairing a painful stimulus with a new environment. If a rodent experiences an electrical shock after being placed in a new cage, it will associate that cage with the electrical shock. Thus, when the rodent is placed in the cage after this association has been established, it will become “frozen with fear” even if no shock is delivered.
Two regions of the brain – the amygdala and the hippocampus – have major roles in the formation of fear-associated memories. The experience of fear increases neuronal activity in several regions of the amygdala, and learning about a new environment increases activity in the hippocampus (Phillips and LeDoux, 1992). Stable associative memories are formed by changing the strength of the connections between neurons – called synapses – in these two regions.
To communicate across synapses, the presynaptic neuron releases neurotransmitters from membrane-enclosed compartments called vesicles in a process called exocytosis. The neurotransmitter molecules then travel across the synapse and bind to receptors on the surface of the postsynaptic neuron. The strength of the synapse can be changed by altering the ability of the presynaptic neuron to release neurotransmitters, or by altering the availability of the receptors on the postsynaptic neuron (Kessels and Malinow, 2009; Nicoll and Schmitz, 2005). Now, in eLife, Danesh Moazed of Harvard Medical School and colleagues – including Rebecca Mathew and Antonis Tatarakis as joint first authors – report that the synapses responsible for the formation of fear-associated memories are kept in check by a tiny molecule called microRNA-153 (Mathew et al., 2016).
MicroRNAs are short RNA molecules (Lee et al., 1993; Wightman et al., 1993) that interfere with the ability of messenger RNA molecules to encode proteins (Selbach et al., 2008). Mathew et al. – who are based at Harvard and a number of other institutes in the United States – identified a set of 21 microRNAs whose production is increased by fear conditioning in rats. In particular, they found that learning to associate fear with a new environment caused the expression of microRNA-153 to increase by a factor of approximately four in a part of the hippocampus called the dentate gyrus.
To determine whether microRNA-153 has a role in the formation of fear-associated memories Mathew et al. reduced its production in the hippocampus and performed fear conditioning experiments. They found that rats that were deficient in microRNA-153 froze more often in the cage where they had experienced an electrical shock. Thus, it appears that microRNA-153 decreases the formation of fear-associated memories.
To determine how microRNA-153 inhibits the formation of fear memories, Mathew et al. analyzed all of the genes that they had predicted would be regulated by fear-induced microRNAs. This sample included a large proportion of the genes involved in vesicle exocytosis, and microRNA-153 targeted a large number of these genes. Further investigation revealed that fear conditioning reduced the expression of the exocytosis-related genes, and microRNA-153 knockdown increased their expression.
Genes regulated by microRNA-153 (such as Snap25 and Pclo) control synaptic strength by regulating both presynaptic vesicle exocytosis and postsynaptic receptor trafficking (Jurado et al., 2013; Südhof, 2013). By demonstrating in vitro that manipulating the expression of microRNA-153 can also regulate these processes, Mathew et al. conclude that microRNA-153 counteracts the formation of associative memories during fear conditioning by decreasing the strength of synapses.
The results also lead to a number of new questions. Does microRNA-153 regulate the activity of the hippocampus more generally? And is microRNA-153 expression regulated in other brain regions, such as the amygdala, to modulate other aspects of fear?
It is also important to note that Mathew et al. found a total of 21 microRNAs whose production increased as a result of fear conditioning. Based on their sequence, these microRNAs are predicted to target genes involved in a number of processes: vesicle fusion, neuronal development, long-term potentiation, neurotransmission and synaptogenic adhesion. Thus, figuring out how these microRNAs influence memory formation is likely to involve a number of mechanisms that were not investigated by Mathew et al. Finally, as we gain insight into the roles that microRNAs play, it may be possible to leverage the properties of these tiny molecules to develop new treatments for anxiety and other disorders.
References
-
The contextual brain: implications for fear conditioning, extinction and psychopathologyNature Reviews Neuroscience 14:417–428.https://doi.org/10.1038/nrn3492
-
Synaptic plasticity at hippocampal mossy fibre synapsesNature Reviews Neuroscience 6:863–876.https://doi.org/10.1038/nrn1786
-
Differential contribution of amygdala and hippocampus to cued and contextual fear conditioningBehavioral Neuroscience 106:274–285.https://doi.org/10.1037/0735-7044.106.2.274
Article and author information
Author details
Publication history
Copyright
© 2017, Luikart
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 876
- views
-
- 104
- downloads
-
- 0
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Further reading
-
- Cell Biology
Overnutrition engenders the expansion of adipose tissue and the accumulation of immune cells, in particular, macrophages, in the adipose tissue, leading to chronic low-grade inflammation and insulin resistance. In obesity, several proinflammatory subpopulations of adipose tissue macrophages (ATMs) identified hitherto include the conventional ‘M1-like’ CD11C-expressing ATM and the newly discovered metabolically activated CD9-expressing ATM; however, the relationship among ATM subpopulations is unclear. The ER stress sensor inositol-requiring enzyme 1α (IRE1α) is activated in the adipocytes and immune cells under obesity. It is unknown whether targeting IRE1α is capable of reversing insulin resistance and obesity and modulating the metabolically activated ATMs. We report that pharmacological inhibition of IRE1α RNase significantly ameliorates insulin resistance and glucose intolerance in male mice with diet-induced obesity. IRE1α inhibition also increases thermogenesis and energy expenditure, and hence protects against high fat diet-induced obesity. Our study shows that the ‘M1-like’ CD11c+ ATMs are largely overlapping with but yet non-identical to CD9+ ATMs in obese white adipose tissue. Notably, IRE1α inhibition diminishes the accumulation of obesity-induced metabolically activated ATMs and ‘M1-like’ ATMs, resulting in the curtailment of adipose inflammation and ensuing reactivation of thermogenesis, without augmentation of the alternatively activated M2 macrophage population. Our findings suggest the potential of targeting IRE1α for the therapeutic treatment of insulin resistance and obesity.
-
- Cell Biology
Understanding how the brain controls nutrient storage is pivotal. Transient receptor potential (TRP) channels are conserved from insects to humans. They serve in detecting environmental shifts and in acting as internal sensors. Previously, we demonstrated the role of TRPγ in nutrient-sensing behavior (Dhakal et al., 2022). Here, we found that a TRPγ mutant exhibited in Drosophila melanogaster is required for maintaining normal lipid and protein levels. In animals, lipogenesis and lipolysis control lipid levels in response to food availability. Lipids are mostly stored as triacylglycerol in the fat bodies (FBs) of D. melanogaster. Interestingly, trpγ deficient mutants exhibited elevated TAG levels and our genetic data indicated that Dh44 neurons are indispensable for normal lipid storage but not protein storage. The trpγ mutants also exhibited reduced starvation resistance, which was attributed to insufficient lipolysis in the FBs. This could be mitigated by administering lipase or metformin orally, indicating a potential treatment pathway. Gene expression analysis indicated that trpγ knockout downregulated brummer, a key lipolytic gene, resulting in chronic lipolytic deficits in the gut and other fat tissues. The study also highlighted the role of specific proteins, including neuropeptide DH44 and its receptor DH44R2 in lipid regulation. Our findings provide insight into the broader question of how the brain and gut regulate nutrient storage.