On 2019-11-06 20:42:01, user Gabriela Rodriguez wrote:
BI 598 Group 5: Stephanie Yemane, Alex Terzibachian & Gabriela A. Rodríguez-Morales
Review written by undergraduate and graduate students from Boston University as requirement from the BI598 class
Summary:
The complement system, pathway that works alongside the immune system, is activated by the deposition of C1q, a protein complex that binds antigen-antibody complexes tagging synapses for elimination by cleaving C3 into C3a, which recruits phagocytic cells, and C3b which facilitates phagocytosis via the microglia-specific complement receptor 3. The deposition of the complement system has been shown during disease, in this paper, Hammond et. al. tried to test whether there is excess production of the complement system in the hippocampus of a multiple sclerosis mouse model, EAE, and if complement-dependent synapse loss is a source of degeneration in EAE.
To answer this question, the authors first aimed to characterize the change in complement production through quantitative analysis of C1qa, C3, and mRNA in Figure 1. Using Western blot analysis, researchers found that EAE mice had significantly increased expression of C1q and uncleaved C3 protein compared to sham mice. Through qPCR analysis of mRNA expression in the hippocampus, EAE mice were found to have significantly increased C1qa and C3 expression. qPCR was also used to analyze the expression of complement proteins in CD11b+ microglia/myeloid cells, EAE mice displayed significantly increased C3 expression, but no difference for C1qa expression.
Afterwards, they wanted to localize the expression of C1q and C3 in the hippocampus of EAE mice using immunohistochemistry analysis of hippocampal sections. In Figure 2, EAE mice were found to have varied increases of fluorescence in the hippocampus compared to sham. EAE brains were identified to have C1q localized in high density punctate regions. Postsynaptic marker PSD95 was used, where it was found that both EAE and sham brains had co-localization of C1q to synapses and dendrites, but not all C1q had overlapping localization with PSD95. Next, they analyzed the expression of complement proteins in different hippocampal regions. EAE were found to have no insignificant changes in complement protein expression across the striatum radiatum, lacunosum moleculare, and dentate molecular layer.
To examine if loss of C1q or C3 could protect against the EAE-induced motor impairment, the authors used a pre-mixed emulsion containing MOG in CFA containing heat-activated mycobacterium tuberculosis H37RA in order to immunize WT, C1qKO and C3KO for EAE. Results showed a significant decrease in EAE-induced motor deficits on C3KO mice during the peak disease phase and chronic phase while the C1qKO showed no significant difference in motor deficits compared to WT mice. These results suggest that the alternative complement pathway plays an important role in EAE white matter.
The authors then tried to show that C1qa and C3 knockout mice have synapse loss that’s correlated with EAE in the CA1-SR region of the hippocampus. They only focused on the CA1-SR region because they were previously able to demonstrate that there was a significant synapse elimination in the CA1-SR layer. In figure 4, they look at how Homer1 and PSD95 puncta in the CA1-SR are affected in WT, C1q KO and C3 KO mice in both Sham and EAE transfected mice. They do so by immuno-staining both postsynaptic markers (Homer1 and PSD95). C1qa and C3 KO mice result in partial protection of against EAE-induced synaptic death. Data processed was a normalized amount of present PSD95 and Homer1 puncta. C1qa KO shows a larger loss of Homer1 in the EAE mice compared to sham mice. Whereas, C3 KO shows relatively no difference in loss of Homer1, when comparing EAE and sham mice. The same was seen when looking at PSD95 puncta density, as C1qa KO showed some protection against EAE-induced synaptic death, but C3 KO showed stronger protection. Hence, knockout of C3 proved to be a lot more efficient at preventing synapse loss.
Finally, they looked at decreased amount of microglia activation in C3 KO mice with EAE compared to WT EAE mice. Loss of C1qa had a slight effect on microglia activation induced by EAE. They did so by looking at morphometric parameters of microglial activation. To do so, they measure the surface area/volume ratio and the skeletal length/volume ratio in figure five. They immuno-stained the microglia protein IBA1. WT EAE mice displayed increased expression of IBA1 by microglia and thicker/shorter processes in microglial morphology, which is associated with a functional microglial phenotype. Both WT and C1qa KO EAE mice showed similar increases in IBA1 volume and intensity compared to their respective sham mice. Yet C3 KO EAE mice showed no significant increase in IBA1 volume or intensity when compared to the C3 KO sham mice. Similar results were obtained when looking at the surface value/volume ratio and skeletal length/volume ratio. C1qa KO and WT EAE mice showed similar decreases in microglial morphology measurements, when compared to their respective sham controls. However, C3 KO EAE mice showed an insignificant decrease in the two morphologies. In conclusion, this study provides evidence that may suggest that genetic loss of C1q and C3 provides protective effects against grey matter synapse loss and microglial activation, making the complement pathway a possible therapeutic target for MS.
Merits:
The authors were able to provide enough evidence to suggest that C3 might be a protein of interest when working with multiple sclerosis-related symptoms which opens a new door into possible therapeutic applications of C3 in multiple sclerosis. Another strength of the paper is the use of the EAE animal model, this model has been proven to replicate many of the clinical and pathophysiological features of multiple sclerosis, making it a better experimental design than a mouse model only exhibiting motor deficits.
Major Criticisms:
Figures 1B-D should include individual data points on the bar graphs as these figures all had an N that was 11 or less; error bars displayed are rather large, and it may give more information to also display individual data points.
There was no reference for how many mice had successful EAE immunization. How many mice were immunized? What percentage of immunized mice displayed this increase in complement expression?
In figure 2E there is no quantification of C1q or PSD95 puncta, or quantification of how many overlap; the panel displaying the merged fluorescence is quite unclear and without quantification is not supportive of the hypothesis. The panel showing an image of C1q KO mouse hippocampus is rather dark, doing a DAPI stain to show that the structural integrity is not compromised, and that the WT and KO brains are comparable.
Using the C3d antibody for figure 2G doesn’t quite make sense as it detects the active and inactive forms, the cleaved and full length forms of C3, respectively. Using a marker that identifies the inactive form of C3 doesn’t indicate a good marker for analyzing the activity of the complement pathway – as the protein must be cleaved in order to be active. Additionally, this figure should include DAPI staining as well to prove that the sections are comparable. Why were C3/C3d panels not magnified, but C1q were? Is there more significance in visualizing C1q fluorescence?For figures I and J, there is also no quantification of the individual puncta, and the percentage of overlapping puncta.
In figure 3, the authors failed to provide comparison of experimental EAE animals with the sham mice regarding the clinical score for motor deficits across days post immunization. Another major criticism for figure 3 is the fact that they failed to provide a measure for complement deposition levels, specially during the increase in motor deficits.
A major criticism for figure four is that their data only represents one time-point. This doesn’t allow for analysis of how Homer 1 and PSD95 are affected over a period of time. Another major error was that they only decided to look at SR. They had a reason to only look there, but it would have made it clearer that its only specific to SR if they also looked at other regions such as SP, SO and SLM of the hippocampus and noticed no difference. A third error was that the differences in Homer1 and PSD95 found were so minimal that they were basically insignificant. This makes their conclusions seem exaggerated, as the discovery barely had any evidence to back it up.
A major criticism for figure 5 is the fact that they only decided to tag IBA1 in order to measure microglia density. This would not be enough to measure microglia density, as EAE immunization is not the only thing that results in increased IBA1 expression by microglia.
Minor Criticisms:
Minor criticisms include some punctuation errors in addition to referring to protein C1q as “C1q” and C1qa”. As well as inconsistencies in test references (i.e. “t-test” v. “t test”). N sizes were rather small, and thus don’t hold enough power to display significant differences; include more mice in order to prove there is no difference – begin by doubling the N per gender group and see if significant differences arise. Otherwise, include in the manuscript that not enough mice were included in the study to determine if there was a significant effect.
In figure 3, the n values for all three experimental groups are quite different, C3KO EAE mouse cohort being the one with the least amount of subjects. In this experiment the C3KO EAE group have an approximate difference of 10 to 17 animals in comparison with the other two experimental groups. This difference begs the question if the robust decrease in motor deficits are actually due to the C3KO or to the power difference.
One minor criticism in figure 5 is the location of the figure. It would be better to put it before figure 2, as it would be good to look at the change in morphology prior to behavior. To do so, they could even integrate it with figure 1, as that’s where they first start looking into C1qa and C3.
Future Directions:
Using CD11b as a marker for microglia/myeloid cells is not necessarily the most accurate, as it is not specifically for microglia but it is also a marker for monocytes and macrophages. Use a different marker specific for microglia, like TM119, and see if the CD11b overlaps in order to determine that it is specifically microglia/myeloid that are being observed.
The results section mentions that the increased expression of complement proteins in EAE mice could be due to other proteins. But, no other markers were used in order to determine if they were different from microglia/myeloid cells. How do we know another inflammatory response had been upregulated, or a different mechanism was utilized in the absence of the complement proteins? Using markers like TNF-alpha, IL-2, and IL-6 – which are pro-inflammatory markers for microglia responses. Or markers Arg1 and Ym1 which are markers for maintained inflammation response, which can identify other response mechanisms. It would strengthen the hypothesis of the increased phagocytosis by microglia if general markers like Arg1 and Ym1 were used to identify if there were inflammatory responses that did not overlap with the CD11b+ microglia/myeloid cells. Additionally, analyzing complement levels in mice at more time points post-immunization would show the progression of degeneration.
Methods for this Figure 2 included confocal microscopy which has limitations in resolution, electron microscopy would provide the resolution needed to analyze the co-localization of these proteins. If complement protein levels at the synapse are to be analyzed, using synaptosome enrichment of both EAE and sham mouse hippocampi in order to selectively observe pre/post-synaptic cleft areas and the proteins expressed. Without quantification of the puncta in panels D-J, localization of proteins cannot be directly addressed or compared across brain regions, or across EAE and sham mice. For all experiments in figure 2 that examine co-localization of proteins, merged panels should be pixel shifted in order to confirm that the location of proteins is specific and ordered.
As a future direction for the experiment shown in figure 3, it would be useful to use a conditional C3KO on the brain areas related to motor activity, like the motor cortices, striatum and GP in order to better identify the area of interest for the C3-related protective effect that reduces motor deficits. Furthermore, providing a cognitive score curve for the different KO groups alongside the motor curve would provide further characterization of the possible protective effects of these conditions against MS-related symptomatology.
To strengthen the research shown in figure 4, the authors should have shown a progression of the synaptic density over the course of the 26 days post immunization. They could have done so by picking out three different dates and looked at how the density of Homer1 and PSD95 are in comparison to the previous days.
One way to build upon figure five is to use more markers other than just IBA1 to measure microglial density. This would have made their data be better supported. Another future direction that would allow for a better study of the morphological changes of microglia, they could have used done a sholl analysis. This would have allowed them to record the number of intersections at various distances from the cell body, which would look at how complex and arborized the microglia is.