Memory: Can fearlessness come in a tiny package?
A molecule called microRNA-153 helps to prevent rats associating new environments with fear.
- Dartmouth College, United States
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
- Version of Record published: February 6, 2017 (version 1)
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
-
- 872
- views
-
- 104
- downloads
-
- 0
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
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)
Memory: Can fearlessness come in a tiny package?
eLife 6:e24575.
https://doi.org/10.7554/eLife.24575
Further reading
-
- Cell Biology
Elastic cartilage constitutes a major component of the external ear, which functions to guide sound to the middle and inner ears. Defects in auricle development cause congenital microtia, which affects hearing and appearance in patients. Mutations in several genes have been implicated in microtia development, yet, the pathogenesis of this disorder remains incompletely understood. Here, we show that Prrx1 genetically marks auricular chondrocytes in adult mice. Interestingly, BMP-Smad1/5/9 signaling in chondrocytes is increasingly activated from the proximal to distal segments of the ear, which is associated with a decrease in chondrocyte regenerative activity. Ablation of Bmpr1a in auricular chondrocytes led to chondrocyte atrophy and microtia development at the distal part. Transcriptome analysis revealed that Bmpr1a deficiency caused a switch from the chondrogenic program to the osteogenic program, accompanied by enhanced protein kinase A activation, likely through increased expression of Adcy5/8. Inhibition of PKA blocked chondrocyte-to-osteoblast transformation and microtia development. Moreover, analysis of single-cell RNA-seq of human microtia samples uncovered enriched gene expression in the PKA pathway and chondrocyte-to-osteoblast transformation process. These findings suggest that auricle cartilage is actively maintained by BMP signaling, which maintains chondrocyte identity by suppressing osteogenic differentiation.
-
- Cancer Biology
- Cell Biology
Internalization from the cell membrane and endosomal trafficking of receptor tyrosine kinases (RTKs) are important regulators of signaling in normal cells that can frequently be disrupted in cancer. The adrenal tumor pheochromocytoma (PCC) can be caused by activating mutations of the rearranged during transfection (RET) receptor tyrosine kinase, or inactivation of TMEM127, a transmembrane tumor suppressor implicated in trafficking of endosomal cargos. However, the role of aberrant receptor trafficking in PCC is not well understood. Here, we show that loss of TMEM127 causes wildtype RET protein accumulation on the cell surface, where increased receptor density facilitates constitutive ligand-independent activity and downstream signaling, driving cell proliferation. Loss of TMEM127 altered normal cell membrane organization and recruitment and stabilization of membrane protein complexes, impaired assembly, and maturation of clathrin-coated pits, and reduced internalization and degradation of cell surface RET. In addition to RTKs, TMEM127 depletion also promoted surface accumulation of several other transmembrane proteins, suggesting it may cause global defects in surface protein activity and function. Together, our data identify TMEM127 as an important determinant of membrane organization including membrane protein diffusability and protein complex assembly and provide a novel paradigm for oncogenesis in PCC where altered membrane dynamics promotes cell surface accumulation and constitutive activity of growth factor receptors to drive aberrant signaling and promote transformation.
