On 2020-12-15 18:38:48, user Angelica Gaona wrote:
BI598 Group 3 Paper Review<br />
Jamie Dela Cruz1, Angélica Gaona1, John Axiotakis1, Guangmei Liu2
1Senior undergraduate in Neurobiology, Boston University. 21st-year PhD student in Neurobiology, Boston University.
Microglial activation results in neuron-type-specific increase in mPFC GABAergic transmission and abnormal behavior in mice Binliang Tang, Jinxiang Jiang, Lei Wang, Afzal Misrani, Qingwei Huo, Yuanyuan Han, Cheng Long, Li Yang doi: https://doi.org/10.1101/202...
Introduction<br />
We are university students taking an upper-level neurobiology course (with professor Cruz-Martín) that centers on understanding neural circuits and modern research techniques through in-depth discussions of recent literature. To fully immerse ourselves in current scientific discourse, we have written this review of the manuscript from Tang et al. posted on biorxiv.org (version: June 14, 2020)
Summary<br />
Microglia are known to mediate activity-dependent synaptic plasticity and neurogenesis in the fully mature CNS and to release inflammatory cytokines and neuroprotective factors in response to inflammatory signals. However, there is currently little understanding of how microglia-associated neuroinflammation regulates neuronal activity. To investigate this, Tang et al. used a single-dose injection of lipopolysaccharide (LPS), a proinflammatory cytokine inducer, to induce acute neuroinflammation and therefore microglial activation in the medial prefrontal cortex (mPFC) of 1-2 month-old male and female mice. They performed a variety of electrophysiological, biochemical, and behavioral evaluations two hours after the LPS injection (referring to these mice as 2h-LPS mice). To study the effects of blocking microglial activation, they also had a subset of mice undergo a minocycline pretreatment, injecting them with minocycline once daily for three days and performing the LPS challenge on the third day. They found that activating microglia leads to significantly increased miniature inhibitory (but not excitatory) response in the mPFC pyramidal neurons. Accordingly, minocycline alleviated the LPS-induced abnormal mIPSCs and the associated abnormal protein expression and behavior. Therefore, this paper shows that acute neuroinflammation via activated microglia affects GABAergic synaptic transmission in the mPFC and proposes minocycline as a possible treatment for neuroinflammation-induced abnormal neural function and behavior. Overall, we recommend that the authors provide more clarification for certain steps in their procedure, such as their reasoning for choosing the given timepoints, compounds, and concentrations. We also suggest that the authors provide some stratification of their measurements within the mPFC and reconsider the direct claims of minocycline’s specificity. Additionally, our review asks the authors to tweak certain figures for improved clarity and to provide further comment on their findings concerning mIPSC half-width and open field test results.
In Figure 1, the authors looked at how systemic inflammation affects synaptic transmission in the mPFC. They performed whole-cell patch clamp recordings of mEPSCs and mIPSCs in acute in vitro mPFC cortical slices of mice injected with 0.5 mg/kg LPS or PBS (control). There were no significant differences in the amplitude and frequency of mEPSCs observed in control and experimental mice, but LPS-treated mice showed significantly increased mIPSC amplitude and frequency (Figure 1C-F). This is visually shown through example traces of mEPSCs and mIPSCs from control and 2h-LPS mice (Figure 1A-B).
Given this observed increase in mIPSC amplitude and frequency, the authors suspected that there were alterations in the pre- and postsynaptic activities of mPFC pyramidal neurons in 2h-LPS mice. For Figure 2, the authors recorded IPSCs evoked by single and paired electrical stimulation (Figure 2A). mPFC pyramidal neurons in 2h-LPS mice exhibited an increased eIPSC amplitude in response to a single stimulation and had significantly lower paired-pulse ratios, suggesting that treatment with LPS leads to GABAergic changes at the pre- and post-synaptic level (Figure 2B-C). The authors also wanted to see what other presynaptic alterations underlie the increased mIPSC frequency in the mPFC of 2h-LPS mice, so they evaluated GABA levels in the mPFC using a brain-homogenate puff assay. After inducing IPSCs in the mPFC pyramidal neurons of normal C57 mice by puffing with mPFC lysates (the supernatants of mPFC tissue from 2h-LPS mice or controls) in whole-cell patch-clamp experiments, Tang et al. found that the amplitude of puff-evoked IPSC was higher in response to LPS-mPFC supernatant than control supernatant (Figure 2D-F). This suggests that there was an increased amount of GABA in the mPFC of LPS mice, partly contributing to the increased mIPSC frequency of pyramidal neurons.
In Figure 3, the authors investigate whether LPS treatment affects synaptic transmission of mPFC GABAergic interneurons using a GAD67+/GFP knock-in mouse line to conduct whole-cell patch recordings in mPFC GABAergic interneurons (Figure 3A). However, there were no significant differences in the amplitude and frequency of mEPSCs and mIPSCs in the GABAergic interneurons. Overall, this suggests that LPS potentiates IPSCs in the mPFC only when the postsynaptic cells are glutamatergic pyramidal neurons (inhibitory-excitatory synapse), not when the postsynaptic cells are GABAergic interneurons (inhibitory-inhibitory synapse).
In Figure 4, the authors investigate whether there was greater involvement and activation of postsynaptic GABAaRs in the mPFC of LPS mice, how LPS affects other factors related to GABA signaling and degradation, and if LPS affects levels of BDNF. The researchers performed a Western blot to determine the levels of GABAAR?1, GABAAR?2, GABAAR?5, GS, vGAT, GABA-T, SSADH, KCC2, BDNF, TrkB, and pTrkB. They also performed an RT-PCR to look at mRNA levels of GABAAR?1 and GABAAR?2. The mPFC of LPS mice showed increased levels of GABAAR?1, GABAAR?2, GS, and vGAT protein, significantly reduced levels of BDNF and pTrkB, and no significant differences between controls and LPS mice for the other compounds (Figure 4A, C-D). The decrease in GS and vGAT protein suggests that there is an increase in GABA synthesis and vesicle loading. The RT-PCR showed that there were increased levels of mRNA expression of GABAAR?1 and GABAAR?2, suggesting that those two subunits are transcriptionally regulated by LPS (Figure 4B). Next, the authors wanted to see if microglial activation could be the cause of the observed LPS-induced alterations, so the aforementioned Western blot analyses also looked at the protein levels in the mPFC of mice who were pretreated with minocycline or PBS once a day for 3 days, with an LPS injection on the third day. In these mice, the levels of GABAAR?1 and GABAAR?2 proteins and mRNA were not significantly different from those in controls (Figure 4A-B). Western blotting also showed that the expression vGAT, BDNF, and pTrkB were also brought to control levels in LPS mice with minocycline pretreatment (Figure 4C-D). These results indicated that LPS-induced microglial activation played a role in the abnormal expression of the mentioned proteins and the subsequent changes in GABAergic synaptic transmission. To visualize whether the 2h-LPS treatment activated microglia in the mPFC, Tang et al. conducted an immunofluorescent staining with anti-Iba1 antibody. They found that the amount of Iba1 signal was significantly higher and the soma sizes of Iba1+ cells were significantly larger in the mPFC of LPS mice but not in the control mice or the LPS+minocycline pre-treatment mice (Figure 4E-F). This suggests that there was more microglia activation in the LPS and that minocycline was sufficient to prevent that effect in LPS mice. Additionally, this shows that microglia could play a role in the transcriptional regulation of GABAergic receptors. On the other hand, Supplementary Figure 1 looks at whether LPS affects the expression of glutamatergic excitation-related proteins, but the Western blot found that the expression of NMDAR1, NMDAR2B, and GluR1 were not significantly different between the control and LPS mice.
Figure 5 looks at whether minocycline pretreatment also affects the mIPSC amplitude in the mPFC pyramidal neurons of LPS-treated mice. After giving mice the same minocycline treatment described previously, whole-cell patch clamp recordings were used to capture mIPSC data. The authors found that a minocycline pretreatment reduced mIPSC amplitude and frequency in the mPFC of LPS mice, actually normalizing the differences between control and LPS mice, but had no effect on controls (Figure 5A-D). Additionally, LPS increased the mIPSC half-width, but pretreatment with minocycline inhibited that effect (Figure 5E). All in all, these results suggest that LPS-induced microglial activation leads to the LPS potentiation of mPFC inhibitory synaptic activity in mPFC pyramidal neurons.
The authors postulated that BDNF downregulation may have resulted in the enhanced IPSC in the mPFC of 2h-LPS mice, so they studied this in Figure 6. Tang et al. first looked at the dose-response effect of LPS on the mIPSCs of mPFC pyramidal neurons. They then recorded mIPSCs after preincubation with aCSF containing 0, 20, or 50 ng/ml of BDNF followed by a perfusion with 0 or 50ng/ml LPS. Any dosage of LPS incubation significantly increased the amplitude and frequency of mIPSCs, and this was visually shown by representative traces at different doses of LPS incubation (Figure 6A-C). However, preincubation with 20 ng/ml of BDNF was sufficient to significantly reduce the amplitude but not frequency of mIPSCs in the mPFC mice with the LPS treatment (Figure 6F-G). They show that a higher dose, 50 ng/ml, of BDNF was needed to prevent the increase in both mIPSC amplitude and frequency in mPFC pyramidal neurons.
In Figure 7, the authors investigated whether mice treated with LPS would show abnormal behavioral phenotypes. In other words, would they show different levels of anxiety and depression? To test this, mice given 2h-LPS treatment were subject to the open field test, elevated plus maze, tail suspension test, and sucrose consumption observations. 2h-LPS showed a significantly lower total distance and center time in the OFT (Figure 7A, C-D), decreased open arm entries in EPM (Figure 7B, E), and increased immobility in the OFT (Figure 7F). They also consumed significantly less sucrose (Figure 7G). Minocycline pretreatment, however, was able to partially rescue these deficits in the 2h-LPS mice. These results indicate that microglia activated during early inflammation may have important pathological effects and that blocking microglial activation could completely or partly reduce inflammation-induced abnormal behavior.
The authors wrap up with Figure 8, a schematic depicting the proposed underlying mechanisms of abnormal GABAergic synaptic transmission and behavior after microglial activation. They describe their results in the context of related literature about minocycline, BDNF, neurotoxic factors, and LPS challenge protocols. Lastly, they conclude by postulating that components of the investigated GABAergic system may be studied in the future as possible therapeutic targets for inflammatory diseases.
To supplement their conclusions, Figure S1 shows that the levels of excitatory receptors in the mPFCs of control and LPS-treatment mice were not significantly different. Figure S2 looks at whether astrocytes were also activated by LPS treatment; the immunofluorescent staining image suggests that levels of astrocyte activation were comparable between controls and LPS mice. To see if levels of pro-inflammatory cytokines were increased in LPS mice, the authors performed an RT-PCR of TNF-?, IL-1? and IL-6 for Figure S3, showing that mRNA levels for these cytokines were elevated.
Major Criticisms<br />
In Figure 1 we thought that there were a few places that could use improvement or clarification. To go into detail, we would like you to comment on why LPS was used instead of poly I:C. Poly I:C is a common molecule that is also used to generate an immune response, thus can you please comment on what was the logic behind using LPS instead. In addition, we would also like to know if you believe that injecting poly I:C in place of LPS would result in a similar phenotype. We would also like to know why the specific concentration of LPS was chosen and if you have any literature supporting that choice. In terms of the results relating to the mEPSCs, the paper indicated that there were not many differences documented in amplitude and frequency in between groups. However, we believe that the results shown in panel C and D could due to recording from a heterogeneous population. That is, it could be that the mEPSC is affecting certain layers of the cortex differently. Thus, we recommend that you sort through the layers and record from each layer respectively or that there should be a certain layer that you focus on. However, if you decide to focus on a certain layer there should be supporting literature given that cites why this layer is optimal. There also should be a comparative measure between VIP and SST interneurons, as that would help with the understanding of mEPSCs results. We would also like to see you comment on whey the mPFC was chosen specifically as it is highly variable. Would you expect to see the same result from the mPFC as the somatosensory cortex and what would those results entail?
The last major criticism we have for Figure 1 pertains to the age chosen for this experiment. Epidemiological studies have indicated that during gestation of the mice is when you would see the biggest impact due to infection on the development of schizophrenia (Waterhouse et al. 2016). We believe that the experiment would have been best conducted during MIA since the model indicates that infection in this time period Is what increases the probability of schizophrenia. We would like you to comment on why the age of 1 month chosen in specific if the MIA model is so well established and supports a different age? In addition, why was the experiment conducted 2 hours after LPS injection? One of the hallmarks of schizophrenia is that it leads to long lasting effects, because of that we believe that LPS and its effect should be recorded for additional days, weeks, and even months in order to emulate a true schizophrenic model. The receptors that react to LPS at E12.5 could be very different from 1 month in life.
In Figure 2 it was noted the experiment was done in the mPFC, again we would like a bit more information on why that specific location was chosen. The mPFC is a very disorganized area and it seems like the hippocampus may have been a more appropriate area since neuronal pathways can be easily isolated for stimulation. Likewise, we also wondered how you were able to stimulate the same pathways in the control and LPS condition. If the case was that the electrodes were recorded in different pathways in the LPS and control condition, we would expect that it may have led to variability and changed the results. For panel A we would have liked to see an output-input curve (current vs. AP)that attributed how much LPS was injected and how much different cell types are engaged.There is still a possibility that some of the effects in release probability are due to differences in excitability between control and LPS condition.
The last major criticism relating to Figure 2 is pertaining to panel E. In this panel we see that the timescale is almost 5 times longer and the amplitude is two times higher than in panel B and C. We wanted you to comment on why that is and how you can attribute the response to evoke GABAA. That is, we believe that it is possible that more than GABAA was evoked by the large response and recommend blocking other GABA receptors if it is not done so already. We would also like to see a positive and negative control. A possible negative control is using Puffs of aCSF. Likewise a positive control can be looking at different concentration of puff aCSF to evaluate how much GABA needs to be release to get the response that is presented in panel E. A curve would aid in supplementing the findings found using puff aCSF and in indicating the results in panel E are physiologically possible.
In Figure 3 we believe that the analysis is too general in referring to the mPFC and that the experiment should refer to a certain cell type. The interneurons recorded in Figure 3 are too heterogeneous which again may there be no indicated changes in mIPSC and mEPSC. Referring to a criticism made before for Figure 1,it is imperative that the experiment documents changes in the mIPSCs and mEPSC at different time scales The crux of Figure 4 it would seem is minocycline and its ability to ameliorate the effects of LPS by specific inhibition of microglia. While this effect may be possible it is far from the only documented role minocycline plays. Its inhibitory effects in particular are widespread pertaining to myriad enzymes including, iNOS, caspase-3, and p38 MAPK. In C6 glioma cells apoptosis could be induced by minocycline’s inhibition of caspase-3 under the condition that autophagy was also inhibited (Liu et al., 2011). In cerebellar granule neurons (CGN) and glia minocycline has also been shown to inhibit iNOS expression and is related to similar reduced phosphorylation of p38 MAPK in CGN, the consolidated effects of which are believed to block MPTP neurotoxicity (Diguet et al., 2003). Even in microglia the effects are diverse such as reduced production of IL-1?, IL-6, TNF and nerve growth factor in amyloid precursor protein in models of Alzheimer's disease (Seabrook et al., 2006). Thus it seems unbefitting of minocycline to be labeled a microglial inhibitor or to consider the effects it has on the brain as such unless such claims can be further substantiated with more experimentation. The data from the Iba stain was good but could be quantified to better accentuate the figure, elements such as the branching and intensity of the Iba signal for each microglia could be quantified and the microglia could also have been co-marked with common activation markers such as LN3 and Gal-3. Further elaboration on connection of assayed elements like BDNF and the alpha-5 subunit would be helpful as well. BDNF is known to relate to GABA through its effect to regulate the KCC2 symporter which facilitates the inhibitory effect of GABA particularly during the developmental phase, and its ability to phosphorylate TrkB which has an effect on inhibitory drive though its effect on this circuit isn’t clearly defined.
In Figure 6, mIPSCs were recorded in mPFC slices under different concentrations of LPS, so we were wondering if you also tried the same concentration ranges on mEPSCs. If not, we can speculate that the absent effect of LPS on mEPSCs might be due to too low concentration. Or LPS may need longer time to take an effect on the mEPSCs of mPFC than mIPSCs, which also links back to the reason of selecting 2-h LPS treatment duration. So, we suggest you do the same different concentrations and time points of LPS treatment.
In Figure 8, the TrkB receptor and the conversion of glutamate to glutamine by GS are depicted in a microglia, not an astrocyte. Although the authors state that there were unchanged levels of astrocytic activation in the study and provide an immunofluorescent staining showing levels of astrocyte activation in control and LPS mice, this claim does not have quantitative data to support it. We suggest that the authors provide a quantification of astrocyte activation, especially given that a study of mice hippocampi found that postnatal LPS exposure inhibited GABAAR expression through astrocyte activation and the subsequent downregulation of the BDNF-TrkB signaling pathway (Liang et al., 2019).