On 2018-12-16 23:05:03, user BU_Fall_NE598_Group2 wrote:
Summary: <br />
DeNardo et al. were able to identify changes in cortical activity regarding the standard model of memory consolidation over 28 days. The authors use a new TRAP2 mouse line to address their question regarding the role of the prelimbic cortex (PL) for memory consolidation and retrieval. By identifying and quantifying TRAPed cells at specific timepoints, DeNardo and colleagues were able to elucidate the dynamic changes in cortical memory trace ensembles over time. Specifically, by analyzing contributions of TRAPed cells during 1, 7, or 14 day memory retrieval, researchers found that ensembles TRAPed at later time points had higher contributions to 28 day remote memory retrieval. These findings support the standard model of memory consolidation, that increased cortical ensemble recruitment occurs in the first two weeks after learning when a memory is “transferred” from the hippocampus to the cortex.
DeNardo and colleagues were able to complete these experiments with a new TRAP2 mouse line. Researchers explained how they generated a new strategy for ‘targeted recombination in active populations’ (TRAP), termed TRAP2. The TRAP method harnesses an immediate early gene locus to drive expression of a tamoxifen-inducible CreER; neuron activation in the presence of tamoxifen allows genetic recombination to permanently express an effector gene. The previous version of TRAP was found to disrupt endogenous Fos, a marker of neural activity, and also did not adequately infect several brain regions. This inhibited researchers from being able to relate the activity of cortical neurons during learning/recent memory retrieval to their function in remote memory.
In Figure 1, the authors provide schematics for the TRAP2 design and provide various measures used to characterize the transgenic mouse line. Here, the researchers show that the TRAP2 mouse line is effective in a variety of brain regions based on quantification of Fos expression. Figure 2 provides characterization of PL activation patterns during fear conditioning and memory retrieval at the aforementioned timepoints and demonstrates how cells TRAPed at later time points have higher contributions to remote memory retrieval.
To establish a causal role for TRAPed PL neurons in remote memory retrieval, researchers employed chemogenetic and optogenetic manipulations, as displayed in Figure 3. Chemogenetic inhibition of the PL during fear conditioning, followed by later optogenetic activation of 14 day recall TRAPed ensembles, did not produce light-induced freezing, as in animals who were not inhibited during fear conditioning. Figure 4 includes data from a whole-brain analysis. Following photostimulation of TRAPed neurons in a mouse’s homecage, researchers evaluated Fos expression in a variety of brain regions to gain a better understanding of the brain-wide memory network from a cellular perspective.
Merits:<br />
Overall, this paper is effective in communicating how prelimbic cortical neuron ensembles change with time. The authors provide convincing evidence through a variety of techniques that support their claims and conclusions.
Additionally, this paper offers an improved tool, TRAP2, with corresponding characterization in a transgenic mouse line. DeNardo et. al. make a convincing argument for the adoption of TRAP2 over the original TRAP1.
TRAP2 can be effectively used alongside immunological tools such as iDISCO+, as well as computational methods such as t-distributed stochastic neighbor embedding (t-SNE) to investigate memory circuits on a brain-wide scale.
Specific Critiques:<br />
Figure 1 explains the design and characterization of TRAP2 by comparing cell fluorescence in the presence or absence of fear conditioning. Although the authors include a detailed comparison of TRAP1 vs. TRAP2 in Extended Data Figures 2 and 3, including such a comparison in a main text figure will more strongly emphasize the need for this new tool. Additionally, there is no figure depicting the endogenous expression of Fos of TRAP1. Including this would provide more validation of DeNardo et al’s claim that TRAP2, unlike TRAP1, does not disrupt the endogenous Fos pathway.
When characterizing the effectiveness of TRAP2 labelling vs. TRAP1 labelling in Extended Data Figure 3b, the authors do not explain the considerable non-zero tdTomato labelling in the absence of 4OHT. The authors also do not show if TRAP1 has the same basal tdTomato expression in the absence of 4OHT or if this leakiness is a result of the new mechanism of expression. Furthermore, since this basal expression is nonzero, all results should be normalized to appropriate controls in the absence of 4OHT.
The schematic included in Figure 2B of one brain hemisphere with what appears to be the merged image from Figure 2C laid over the PL should be identified and explained in the legend. We assume that the bigger red dots are merged TRAP/Fos puncta and the green dots represent Fos expression, but it is unclear. Alternatively, the figure could be expanded in Figure 2C or removed entirely.
The authors indicate in their Methods section that a software called FreezeFrame was used to quantify mouse freezing behavior, except during optogenetic experiments where wires occluded the view and manual observation was used. In order to verify that manual methods closely matched those of automated freeze detection, matching controls should be performed on mice (without optogenetic cables) using both methods simultaneously--ideally, both methods should capture the exact same time points of freezing. Furthermore, specific criteria used during manual observation of freezing should be listed in the methods.
This manuscript heavily relies on one kind of behavioral output (freezing) to measure the effects of TRAPed cells. Testing the function of TRAPed cells with other behavioral assays to measure the occurrence of anxiety or fear following photostimulation would strengthen the manuscript. Consider elevated plus maze or open field tests to determine how TRAPed PL cells influence behavior.
Minor Concerns: <br />
In general, none of the schematics or figure legends are very clear or informative about the experiments that were performed. Specific explanations in text and within the figure legends are needed so that readers less familiar with the TRAP technology and memory consolidation experiments can also glean the importance of these results.
The authors could also look into improving the quality of the images used for histology. Increasing the magnification of these images and retaining the original fluorescent colors (Figure 2C) would permit better comparison across the TRAP, Fos, and merge images.
There was also some discrepancy between optimal cFos expression. Brains were collected an hour after behavior to determine cFos expression. However, previous research has shown that a 90 minute window is most effective for maximal Fos expression.
For experiments presented in Figure 3, the authors inject an AAV expression hM4Di (bilaterally) in addition to Cre-conditional ChR2-eYFP (unilaterally) into the same region (PL). Authors should expand on their reasoning for only injecting the ChR2 unilaterally. Additionally, authors should take into consideration the possibility of confounds for viral expression/effectiveness, as well as cytotoxicity, when injecting a cocktail of viruses into the same brain region. Furthermore, with so many different mechanisms at play simultaneously (e.g. transgenic mice, TRAP mechanism, ChR2 optogenetic activation, DREADD hM4D expression, CNO injection), one has to wonder if the amount of potential variables in the system could vastly confound the results.
In the last paragraph of pg 2, the authors claim that “reactivating TRAPed cells during presentation of the CS+ and CS- was not sufficient to increase freezing above the level of the tones” and make reference to Figure 3D, as well as Extended Data Figure 7a and b. First of all, it is unclear what “above the level of the tones” means, as all three plots show results in response to tones so there is no comparison to a “non-tone” condition that could produce such a statement. Secondly, although in Extended Data Figure 7a and b, ChR2 stimulation does not increase the amount of freezing in the CS- or CS+ altered context states, in Figure 3D, ChR2 stimulation clearly has a significant impact on freezing under the CS+ normal condition, as p-values are all below 0.0001. Thus, the authors need to revise their conclusion made here or better explain the claim they are making.
In Figure 4h, the authors claim that principal component analysis can distinguish two distinct populations along PC2; however, the separation chosen seems rather arbitrary and the two groups identified do not seem to represent distinct populations. A more refined analysis may be needed to truly claim that these groups are significantly different.
Future Directions: <br />
Calcium imaging could be used to record dynamic cell activity during behavior. This would provide more information on the temporal dynamics of activation, as well as how the ensemble is modified overtime. Researchers could more closely track the remapping of ensembles and determine if reflected behavioral changes in real time. Tracking such modifications over time could give researchers a better idea of how to approach this circuit from a molecular perspective.
Although it is not the focus of the paper, further characterization of the TRAP2 mechanism may be needed in order to understand the particular ensembles that are being targeted by TRAP. Is there a minimal activity level that needs to be sustained in order for cells to be TRAPed (i.e. is the TRAP2 method biased towards hyperactive neurons)? Perhaps cells that do not meet a threshold activation level may be important for memory consolidation but may contribute to the network in different ways (i.e. sporadic, low frequency activity, inhibited activity, etc.) than tonically active cells or cells that increase their firing rates.
As briefly mentioned in the abstract and discussion, the hippocampus is crucial for memory formation and consolidation. In future studies, it would be compelling to further investigate the dynamic changes of hippocampal ensembles during consolidation and remote memory retrieval. Repeating these experiments targeting the dorsal dentate gyrus, known for encoding context-specific memories, would allow for a deeper understanding of the hippocampus’ role in this neural circuit.
Additionally, quantifications of TRAPed cells in the hippocampus at the same time points used in PL experiments could give rise to evidence supporting the standard consolidation theory between the hippocampus and cortex. Ideally, the hippocampus would show an initially high number of TRAPed cells, decreasing over the 28 day time-course, while the PL would show an initially low level of TRAPed cells, with increased ensemble size at later time points.
The infralimbic cortex (IL) has interactions with the prelimbic cortex (PL). It would be interesting for the researchers to analyze the influence of IL activation on the PL, such as whether the inhibition of IL would influence the PL and significantly influence fear conditioning behavior in mice. A protein specifically expressed in IL could be inhibited in function, and its effects could be analyzed to determine whether this projection pathway also plays a role in fear conditioning and memory in mice.
Future studies could investigate if these findings observed here for fear memory consolidation and retrieval holds true for memories of a positive valence. Repeating the experiments by tagging a positive memory such as female exposure would provide evidence to better characterize dynamic cortical changes for consolidation and retrieval.