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Noncanonical contribution of microglial transcription factor NR4A1 to post-stroke recovery through TNF mRNA destabilization [1]
['Pinyi Liu', 'Department Of Neurology', 'Nanjing Drum Tower Hospital', 'Affiliated Hospital Of Medical School', 'The State Key Laboratory Of Pharmaceutical Biotechnology', 'Nanjing University', 'Nanjing', 'Yan Chen', 'Zhi Zhang', 'Zengqiang Yuan']
Date: 2023-08
Microglia-mediated neuroinflammation is involved in various neurological diseases, including ischemic stroke, but the endogenous mechanisms preventing unstrained inflammation is still unclear. The anti-inflammatory role of transcription factor nuclear receptor subfamily 4 group A member 1 (NR4A1) in macrophages and microglia has previously been identified. However, the endogenous mechanisms that how NR4A1 restricts unstrained inflammation remain elusive. Here, we observed that NR4A1 is up-regulated in the cytoplasm of activated microglia and localizes to processing bodies (P-bodies). In addition, we found that cytoplasmic NR4A1 functions as an RNA-binding protein (RBP) that directly binds and destabilizes Tnf mRNA in an N6-methyladenosine (m 6 A)-dependent manner. Remarkably, conditional microglial deletion of Nr4a1 elevates Tnf expression and worsens outcomes in a mouse model of ischemic stroke, in which case NR4A1 expression is significantly induced in the cytoplasm of microglia. Thus, our study illustrates a novel mechanism that NR4A1 posttranscriptionally regulates Tnf expression in microglia and determines stroke outcomes.
Given that NR4A1 suppresses proinflammatory response in macrophages and microglia, we investigated its underlying regulatory mechanisms in microglia. Intriguingly, we found that the transcription factor NR4A1 exerted a novel function as an RNA-binding protein (RBP) in microglia. That is, cytoplasmic NR4A1 was up-regulated and localized to processing bodies (P-bodies) after microglial activation, directly binding and promoting the destabilization of Tnf mRNA in an N6-methyladenosine (m 6 A)-dependent manner. Moreover, trichostatin A (TSA), a pan-histone deacetylase (HDAC) inhibitor, increased NR4A1 expression and accelerated Tnf mRNA decay in microglia. Similar to in vitro findings, the expression of NR4A1 was remarkably induced, mainly in the cytoplasm of microglia, in a middle cerebral artery occlusion (MCAO) mouse model and stroke patients. Global and conditional microglial knockout of Nr4a1-enhanced Tnf expression and remarkably exacerbated ischemic brain injury. This study identified a key role of microglial NR4A1 as an RBP that posttranscriptionally regulates Tnf expression, providing a promising target for stroke treatment.
NR4A1 (also referred to as Nur77, TR3 and NGFI-B and encoded by the gene Nr4a1) is a member of the nuclear hormone receptor superfamily, and the expression of NR4A1, as an immediate early gene, is rapidly induced in response to various stimuli [ 8 ]. Similar to other classical nuclear hormone receptors, NR4A1 contains a transactivation domain, DNA-binding domain (DBD), and ligand-binding domain (LBD). In peripheral tissues, NR4A1 acts as an anti-inflammatory molecule in macrophages, and Nr4a1-deficiency enhances the polarization of macrophages into a proinflammatory phenotype and exacerbates atherosclerosis [ 9 , 10 ]. It was recently found that NR4A1 is also key for the down-regulation of isocitrate dehydrogenase and the prevention of succinate accumulation, which induces metabolic reprogramming and attenuates chronic inflammation [ 11 ]. In the mouse brain, NR4A1 protects dopaminergic neurons in a rodent model of Parkinson’s disease and delays the onset of clinical symptoms of experimental autoimmune encephalomyelitis (EAE) by inhibiting microglial activation and proinflammatory gene expression [ 12 , 13 ]. It has also been reported that NR4A1 in myeloid cells could recruit the corepressor CoREST to the Th (tyrosine hydroxylase) promoter, repressing the production of norepinephrine, which limited the progression of EAE [ 14 ].
Microglia are the resident macrophages in the brain, which exert critical physiological functions during development and homeostasis, such as scanning of the brain microenvironment, remodeling of synapses, and support for the development of oligodendrocytes [ 1 , 2 ]. In most neurological diseases, however, microglia respond rapidly to brain injuries and undergo distinct morphological and transcriptional changes [ 3 , 4 ]. In ischemic stroke, microglia are first activated after ischemia, releasing proinflammatory cytokines and chemokines to initiate early proinflammatory response and trigger the subsequent infiltration of peripheral immune cells to damage the brain [ 5 – 7 ]. Thus, a stronger mechanistic understanding of the regulation of neuroinflammation by microglia during ischemic stroke could pave the way for a novel therapy that would reduce ischemic brain injury.
Furthermore, to mimic microglial activation in ischemic stroke in vitro, we induced neuronal injury by incubating neurons in oxygen-glucose deprivation (OGD) conditions and then treated WT and Nr4a1 -/- primary microglia with neuron-conditioned media (NCM) from control or OGD-exposed neurons ( S6A Fig ). Our results revealed that the expression of Tnf, Il1b, and Il6 mRNA was robustly up-regulated and that the levels of anti-inflammatory factors, such as Arg1 and Mrc1, were markedly decreased in Nr4a1 -/- microglia compared to WT microglia ( S6B Fig ). Consistently, the concentrations of TNF-α, IL-6, and IL-1β were significantly higher in the supernatants of OGD-NCM-treated Nr4a1 -/- microglia than in the supernatants of OGD-NCM-treated WT microglia ( S6C Fig ). In addition, we also found that culture medium from OGD-NCM-treated Nr4a1 -/- microglia exacerbated neuronal death compared to that from OGD-NCM-treated WT microglia, as demonstrated by the decrease in the ratio of Calcein-AM + /propidium iodide (PI) + cells ( S7A–S7C Fig ). The CCK-8 assay further confirmed that OGD-NCM-treated Nr4a1 -/- microglia-conditioned medium reduced the viability of neurons ( S7D Fig ). Collectively, these results indicate that microglial NR4A1 suppresses Tnf expression and alleviates microglia-potentiated neuronal damage after stroke.
(A–D) mRNA levels of several inflammatory factors in ischemic hemispheres from Tmem119-CreERT2; Nr4a1 fl/fl mice and Nr4a1 fl/fl mice 6 h and 24 h after MCAO (n = 3 for sham mice, n = 5 for MCAO mice per group). (E) Representative images of Nissl-stained tissues from Tmem119-CreERT2; Nr4a1 fl/fl mice and Nr4a1 fl/fl mice 3 days after MCAO. (F) Quantification of the infarct size, as measured by Nissl staining (n = 9 for Nr4a1 fl/fl + Saline mice, n = 11 for Nr4a1 fl/fl + ETA mice, n = 11 for Tmem119-CreERT2; Nr4a1 fl/fl + Saline mice, n = 10 for Tmem119-CreERT2; Nr4a1 fl/fl + ETA mice). (G–I) Neurological deficits were aggravated in Tmem119-CreERT2; Nr4a1 fl/fl mice compared to Nr4a1 fl/fl mice 1 day and 3 days after MCAO and rescued by ETA, as determined by the mNSS test ( G ) (n = 9 for Nr4a1 fl/fl + Saline mice, n = 11 for Nr4a1 fl/fl + ETA mice, n = 11–12 for Tmem119-CreERT2; Nr4a1 fl/fl + Saline mice, n = 10 for Tmem119-CreERT2; Nr4a1 fl/fl + ETA mice), foot fault test ( H ) (n = 9 for Nr4a1 fl/fl + Saline mice, n = 11 for Nr4a1 fl/fl + ETA mice, n = 10–12 for Tmem119-CreERT2; Nr4a1 fl/fl + Saline mice, n = 9–10 for Tmem119-CreERT2; Nr4a1 fl/fl + ETA mice) and grip strength test ( I ) (n = 9 for Nr4a1 fl/fl + Saline mice, n = 11 for Nr4a1 fl/fl + ETA mice, n = 11–12 for Tmem119-CreERT2; Nr4a1 fl/fl + Saline mice, n = 10 for Tmem119-CreERT2; Nr4a1 fl/fl + ETA mice). Data are presented as mean ± SEM. In ( A–D ), two-way ANOVA with post hoc Bonferroni’s test. In ( F ), one-way ANOVA with post hoc Bonferroni’s test. In ( G–I ), two-way ANOVA with post hoc Bonferroni’s test (day 3 after MCAO) *P < 0.05; **P < 0.01; ***P < 0.001. The underlying data for this figure can be found in S1 Data . MCAO, middle cerebral artery occlusion.
Since we mainly focused on the role of NR4A1 in microglia, conditional microglial Nr4a1-knockout mice line (Tmem119-CreERT2; Nr4a1 fl/fl ) was generated and subjected to MCAO ( S4A and S4B Fig ). We found that the ischemic hemispheres of Tmem119-CreERT2; Nr4a1 fl/fl mice at 24 h after stroke expressed more Tnf mRNA than those from Nr4a1 fl/fl mice, whereas the mRNA expression of Il1b, Il6, Il10, and Tgfb1 did not show any significant changes ( Figs 6A–6D and S5A ). Consistently, the protein level of TNF-α was also up-regulated in Tmem119-CreERT2; Nr4a1 fl/fl mice at 24 h after stroke ( S5B–S5F Fig ). Moreover, Tmem119-CreERT2; Nr4a1 fl/fl mice phenocopied Nr4a1 -/- mice and showed a larger infarct size and worse neurological deficits than those in Nr4a1 fl/fl littermates after stroke, while Etanercept (ETA, a dimeric Fc-fusion protein that specifically target TNF-α) treatment significantly reduced the infarct size and attenuated neurological deficits in Tmem119-CreERT2; Nr4a1 fl/fl mice, suggesting that increased TNF-α was the major contributing factor that mediate pro-damaging effect of Nr4a1-knockout in microglia ( Fig 6E–6I ).
Next, the infarct size was evaluated by Nissl staining of brain slices 3 days after stroke. We found that the infarct size was significantly larger in Nr4a1 -/- mice than in WT mice ( Fig 5E and 5F ). In addition, several behavioral tests were conducted to explore the effects of NR4A1 on neurological deficits after stroke. Notably, Nr4a1 -/- mice had more severe neurological deficits than WT controls, as assessed by the modified neurological severity score (mNSS) test, the rotarod test, total distance traveled, and mean velocity 1, 3, and 7 days after stroke ( Fig 5G–5J ). Taken together, these data suggest that Nr4a1-knockout significantly promoted Tnf expression and exacerbated brain injury in experimental ischemic stroke.
(A–D) Relative mRNA levels of several inflammatory factors in the ipsilateral hemispheres of WT and Nr4a1 -/- mice 1 day, 3 days, and 7 days after MCAO compared with those in the contralateral hemispheres (n = 4 mice in each group). (E) Representative images of Nissl staining of tissues from WT and Nr4a1 -/- mice 3 days after MCAO. (F) Quantification of the infarct size, as measured by Nissl staining (n = 10 for WT mice, n = 5 for Nr4a1 -/- mice). (G–J) Sensorimotor deficits were aggravated in Nr4a1 -/- mice compared to WT mice 1 day, 3 days, and 7 days after MCAO, as determined by the mNSS test ( G ) (n = 15–38 for WT mice, n = 14–28 for Nr4a1 -/- mice), rotarod test ( H ) (n = 14–18 for WT mice, n = 7–16 for Nr4a1 -/- mice), total distance traveled ( I ) (n = 12–30 for WT mice, n = 9–21 for Nr4a1 -/- mice), and mean velocity ( J ) (n = 12–40 for WT mice, n = 9–28 for Nr4a1 -/- mice). Data are presented as mean ± SEM. In ( A–D ) and ( F ), two-tailed unpaired Student’s t test. In ( G–J ), two-way ANOVA. *P < 0.05; **P < 0.01; ***P < 0.001. The underlying data for this figure can be found in S1 Data . MCAO, middle cerebral artery occlusion; WT, wild-type.
To investigate whether NR4A1 impacts disease progression, experimental ischemic stroke was induced in WT and Nr4a1 -/- mice by MCAO. We first explored the putative effect of NR4A1 on the expression profiles of inflammatory factors after ischemia. Compared to those from WT mice, brain tissues from Nr4a1 -/- mice exhibited higher mRNA expression of Tnf (1 day: p < 0.05, 7 days: p < 0.01) ( Fig 5A ). Although Il1b expression also elevated at day 7 post-stroke ( Fig 5B ), there was no significant difference in the mRNA levels of Il6 and Il10 between Nr4a1 -/- mice and WT mice ( Fig 5C and 5D ).
(A) Immunoblot analysis of NR4A1 in contralateral and peri-infarct tissue 1 day, 3 days, and 7 days after MCAO. (B) Quantification of immunoblot analysis of NR4A1 expression in contralateral and peri-infarct tissue 1 day, 3 days, and 7 days after MCAO showing that brain ischemia induced NR4A1 expression in peri-infarct tissue (n = 3 mice in each group). (C) Representative images of the contralateral and peri-infarct cortices of mice 1 day, 3 days, and 7 days after MCAO showing localization of NR4A1 (green) in Iba1 + (red) microglia. The inset shows a digitally magnified field of a single cell from the image. Scale bar, 30 μm. (D) Quantification of the percentage of NR4A1 + microglia showing that MCAO-induced NR4A1 expression in microglia in the peri-infarct cortex (n = 4 mice in each group). (E) Images of human brain samples from 2 non-stroke patients and 2 stroke patients. The inset shows a digitally magnified field of a single cell from the image. Stroke-induced NR4A1 expression in human microglia. Scale bar, 50 μm. Data are presented as mean ± SEM. In ( B ) and ( D ), one-way ANOVA with post hoc Dunnett’s test. *P < 0.05; **P < 0.01; ***P < 0.001. The underlying data for this figure can be found in S1 Data . The original blot for this figure can be found in S1 Raw Images . MCAO, middle cerebral artery occlusion.
Microglia in the ischemic penumbra region release proinflammatory cytokines to initiate early inflammatory response [ 30 , 31 ]. To characterize the expression pattern of NR4A1 in ischemic stroke, we used MCAO as a mouse model to mimic ischemic stroke in human and found that NR4A1 expression was significantly elevated in the penumbra at day 1 and day 3 post-stroke ( Fig 4A and 4B ). Moreover, the number of NR4A1 + microglia surrounding the infarct area peaked 1 to 3 days after MCAO ( Fig 4C and 4D ). Specifically, NR4A1 was predominantly located in the cytoplasm of microglia, but not in the nucleus ( Fig 4C ). To assess whether this expression pattern can also be observed in stroke patients, we analyzed NR4A1 expression in postmortem brain sections obtained from patients after ischemic stroke by immunofluorescence staining. Our data showed that in the peri-infarct tissue, more NR4A1 was localized in microglia in stroke patients than in non-stroke controls ( Fig 4E ). Thus, we found that microglial NR4A1 was significantly up-regulated in the cytoplasm at the acute phase after ischemic stroke, suggesting cytoplasmic NR4A1 might play an important role in the pathophysiology of ischemic stroke.
To determine whether Tnf mRNA bears m 6 A sites, we used SRAMP software to predict m 6 A sites in Tnf mRNA [ 29 ]. We identified 4 very high- or high-confidence m 6 A sites in the CDS region and 3′ UTR of Tnf mRNA ( Fig 3E ), and m 6 A-RIP-qPCR analysis confirmed that m 6 A modification indeed occurs in the CDS and 3′ UTR of Tnf mRNA ( Fig 3F ). To further confirm whether NR4A1 directly bind the m 6 A-modified Tnf mRNA, we performed PAR-CLIP and found that NR4A1 could bind to the m 6 A-modified regions in the CDS and 3′ UTR of Tnf mRNA ( S3C Fig ). We next inserted the WT 5′ UTR, CDS, or 3′ UTR of Tnf mRNA into a firefly luciferase reporter. As expected, NR4A1 overexpression significantly reduced the luciferase activity of the WT CDS and 3′ UTR but did not affect that of the 5′ UTR ( Fig 3G ). Furthermore, mutations in the m 6 A sites of the CDS and 3′ UTR (A829, A1061+A1104) partly reversed the effect of NR4A1 on the luciferase activity of the WT CDS and 3′ UTR ( Fig 3H–3J ). In addition, we also overexpressed NR4A1-NES in Mettl3-knockout BV2 cells ( S3D Fig ). The results showed that cytoplasmic NR4A1 had no effect on the expression level of Tnf in m 6 A-deficient BV2 cells ( S3E Fig ). Taken together, our data indicate that m 6 A modification of Tnf mRNA is the potential mechanism accounting for cytoplasmic NR4A1-mediated Tnf mRNA degradation.
(A) Binding motif identified by HOMER with NR4A1-binding peaks. (B) The distribution (left) and enrichment (right) of NR4A1-binding peaks identified by RIP-seq. (C, D) Immunoblot analysis of NR4A1 in the cytoplasmic fraction of ATP+LPS-treated primary microglia pulled down by m 6 A-containing or unmethylated oligonucleotides. (E) SRAMP software prediction of the m 6 A sites in Tnf mRNA. (F) m 6 A-RIP-qPCR analysis of m 6 A enrichment in the CDS and 3′ UTR of Tnf mRNA in BV2 cells (n = 3 biological repeats in each group). (G) Luciferase activities of the 5′ UTR, CDS, and 3′ UTR of Tnf in HEK293T cells overexpressing NR4A1 or empty vector (n = 3 biological repeats in each group). (H) m 6 A site mutations in the CDS and 3′ UTR of Tnf mRNA. (I, J) Relative luciferase activities of WT or mutant CDS of Tnf ( I ) and WT or mutant 3′ UTR of Tnf ( J ) in HEK293T cells overexpressing NR4A1 (normalized to Fluc/Rluc in HEK293T cells with empty vector) (n = 3 biological repeats in each group). Data are presented as mean ± SEM. In ( F ), ( G ), ( I ), two-tailed unpaired Student’s t test. In ( J ), one-way ANOVA with post hoc Dunnett’s test. *P < 0.05; **P < 0.01; ***P < 0.001. The underlying data for this figure can be found in S1 Data . The original blot for this figure can be found in S1 Raw Images . RIP-seq, RNA-binding protein immunoprecipitation sequencing; WT, wild-type.
m 6 A modification, one of the key mechanisms through which mRNAs are posttranscriptionally regulated, impacts fundamental aspects of RNA and plays critical roles in various biological processes, including inflammation [ 23 – 25 ]. In addition, several m 6 A readers (YTHDF2, IGF2BPs, etc.) were previously reported to be localized in cytoplasmic granules and regulate mRNA stability [ 26 , 27 ]. Here, we found that an “AGACA” sequence, which is consistent with the “DRACH” m 6 A core motif (D = G/A/U, R = G/A, H = A/U/C), was enriched in NR4A1-binding peaks ( Fig 3A ) [ 28 ]. Moreover, NR4A1-binding sites were highly enriched around stop codons and in 3′ UTR, coinciding with the distribution of m 6 A sites ( Fig 3B ) [ 28 ]. Thus, we hypothesized that NR4A1 can promote Tnf mRNA degradation in an m 6 A-dependent manner. To verify our hypothesis, we used a methylated RNA probe and an unmethylated RNA probe (“GGACU” and “AGACA” motif) for RNA pull-down and found that endogenous NR4A1 showed higher binding affinity for the methylated probe than for the unmethylated probe ( S3A , S3C and S3D Fig ). Moreover, RNA electrophoretic mobility shift assay (REMSA) also validated that recombinant NR4A1 preferentially bound to the methylated probe ( S3B Fig ), suggesting that NR4A1 is a potential m 6 A-binding protein.
However, we also found that NR4A1-overexpression could suppress the activity of Tnf promoter by luciferase assay ( S2G Fig ), which was consistent with the previous study [ 22 ], indicating that NR4A1 functions as a transcription factor to regulate Tnf expression. Thus, to investigate the function of cytoplasmic NR4A1, we overexpressed wildtype-NR4A1 and NR4A1 with 3XNES (nuclear export sequence) in BV2 cells ( S2H–S2J Fig ). We found that wildtype-NR4A1 down-regulated Tnf, Il1b, and Il6 after ATP+LPS stimulation, whereas cytoplasmic NR4A1 (NR4A1-NES) specifically regulated the expression level of Tnf mRNA and promoted its degradation ( S2K–S2M Fig ). Collectively, these data demonstrate that cytoplasmic NR4A1 specifically promotes Tnf mRNA degradation through directly binding to Tnf mRNA in microglia.
We also used TSA, a pan-histone deacetylase (HDAC) inhibitor that has been reported to induce NR4A1 expression, to pharmacologically up-regulate NR4A1 in microglia ( Fig 2C and 2D ) [ 21 ]. We found that TSA treatment significantly suppressed Tnf mRNA expression in primary microglia ( Fig 2E ). However, Nr4a1 knockout significantly attenuated the reduction of Tnf mediated by TSA ( Fig 2E and 2F ). Furthermore, we investigated whether TSA suppresses Tnf expression via the mechanism described above. We examined mRNA stability after TSA treatment and found that TSA remarkably promoted the mRNA degradation of Tnf, but this effect was not observed in Nr4a1-knockout microglia ( Fig 2G and 2H ).
(A) RIP analysis of NR4A1-bound Tnf mRNA in untreated and ATP+LPS-treated BV2 cells (n = 3 biological repeats in each group). (B) RNA stability assay of Tnf mRNA in ATP+LPS-activated WT and Nr4a1 -/- primary microglia at the indicated time points after actinomycin D treatment (n = 4 biological repeats in each group). (C, D) mRNA ( C ) (n = 3 biological repeats in each group) and protein ( D ) levels of Nr4a1 in unstimulated and ATP+LPS-activated primary microglia treated with DMSO or TSA (50 nM). (E) Tnf mRNA levels in unstimulated and ATP+LPS-treated primary microglia treated with DMSO or TSA (50 nM) (n = 3 biological repeats in each group). (F) Relative reduction in Tnf expression induced by TSA treatment in ATP+LPS-treated WT and Nr4a1 -/- primary microglia (calculated from E ) (n = 3 biological repeats in each group). (G, H) RNA stability assay of Tnf mRNA in ATP+LPS-treated WT and Nr4a1 -/- primary microglia treated with TSA at the indicated time points after actinomycin D treatment (n = 3 biological repeats in each group). Data are presented as mean ± SEM. In ( A ), ( C ), ( E ), two-way ANOVA with post hoc Bonferroni’s test. In ( B ), ( G ), ( H ), two-way ANOVA. In ( F ), two-tailed unpaired Student’s t test. *P < 0.05; **P < 0.01; ***P < 0.001. The underlying data for this figure can be found in S1 Data . The original blot for this figure can be found in S1 Raw Images . RBP, RNA-binding protein; TSA, trichostatin A; WT, wild-type.
Since P-bodies are cytoplasmic granules that mediate RNA decay, and because NR4A1 has 2 highly conserved Cys 4 zinc fingers in its DBD, which might have RNA-binding capacity, we hypothesized that NR4A1 might function as an RBP that regulates the stability of mRNA [ 19 , 20 ]. We performed RBP immunoprecipitation sequencing (RIP-seq) to determine NR4A1-RNA associations. The RIP-seq data showed that NR4A1 bound to several transcripts that are related to inflammatory response ( S2 Table ). Additionally, since NR4A1 is reported to be capable of suppressing proinflammatory response in microglia, it was reasonable to assume that NR4A1 might regulate the mRNA stability of proinflammatory factors. Accordingly, RIP-qPCR analysis confirmed that NR4A1 directly bound Tnf, Il1b, Il6, and Il10 mRNA, and these interactions substantially increased after ATP+LPS treatment ( Figs 2A and S2A–S2C ). We next found that Nr4a1 knockout led to significantly greater stabilization of Tnf mRNA in activated microglia treated with actinomycin D, a commonly used transcription inhibitor, than in wild-type (WT) microglia, but did not affect Il1b, Il6, or Il10 ( Figs 2B and S2D–S2F ).
To investigate how NR4A1 functions in activated microglia, we screened for proteins that interact with NR4A1 in ATP+LPS-treated BV2 cells overexpressing FLAG-NR4A1 by MS/MS. Intriguingly, 26 out of 632 identified proteins were components of cytoplasmic granules, including P-bodies ( Fig 1C and S1 Table ). In addition, GO analysis suggested that NR4A1-interacting proteins are predominantly enriched in the cytoplasm and possess RNA-binding capacity, which also indicated microglial NR4A1 might function in cytoplasmic RNA granules ( Fig 1D and 1E ). Furthermore, Immunofluorescence staining revealed that NR4A1 colocalized with DCP1A, a key component of P-bodies ( Fig 1F ). To confirm the direct interaction between NR4A1 and P-bodies, we performed in situ proximity ligation assays and obtained consistent results ( Fig 1G and 1H ). Additionally, we confirmed that ATP+LPS enhanced the interactions between FLAG-NR4A1 and the P-body marker DCP1A by coimmunoprecipitation (Co-IP) ( Fig 1I ), suggesting that NR4A1 in activated microglia might function in P-bodies.
(A) Immunoblot analysis of NR4A1 in WT and Nr4a1 -/- primary microglia treated with or without ATP (1 mM) + LPS (100 ng/ml). (B) Immunoblot analysis of NR4A1 expression in the cytoplasmic and nuclear fractions of untreated and ATP+LPS-treated primary microglia. (C) Co-IP-MS/MS analysis of Flag-NR4A1-interacting proteins. (D, E) Representative GO terms of cellular components and molecular functions enriched in Flag-NR4A1-interacting proteins. (F) Representative images of untreated and ATP+LPS-treated primary microglia showing colocalization of NR4A1 with DCP1A. Scale bar, 50 and 5 μm. (G, H) In situ proximity ligation assay assessing the interactions between NR4A1 and DCP1A (red) in negative control, unstimulated and ATP+LPS-treated primary microglia. Scale bar, 50 and 10 μm (n = 50 cells counted in each group). (I) Immunoblot analysis of DCP1A in BV2 cell (expressing FLAG-NR4A1) lysates immunoprecipitated with an anti-FLAG antibody. Data are presented as mean ± SEM. In ( H ), one-way ANOVA with post hoc Dunnett’s test. ***P < 0.001. The underlying data for this figure can be found in S1 Data . The original blot for this figure can be found in S1 Raw Images . Co-IP, coimmunoprecipitation; WT, wild-type.
NR4A1 is expressed in microglia and exhibits anti-inflammatory effect under inflammatory stimuli [ 12 ]. However, the underlying mechanism is still unclear. Injured neurons release various damage-associated molecular patterns (DAMPs), which activates microglia through Toll-like receptors and purinergic receptors [ 15 , 16 ], we therefore used ATP and LPS to activate primary microglia and confirmed that NR4A1 was significantly up-regulated after ATP treatment and ATP+LPS co-stimulation ( Figs 1A and S1A ). Notably, we found that ATP+LPS stimulation induced an upper band of NR4A1 as compared to ATP alone ( S1A Fig ). Since NR4A1 is a canonical transcription factor with its nuclear localization, and previous studies also have demonstrated that NR4A1 is present in the cytoplasm of neurons and PC12 cells in its phosphorylated form [ 17 , 18 ], we further examined the subcellular localization of NR4A1 in microglia after ATP+LPS treatment. Western blot analysis of compartmentalized proteins showed that ATP+LPS stimulation resulted in an up-regulation of an upper band of NR4A1 in the cytoplasm but not the nucleus ( Fig 1B ).
Discussion
In the present study, we found that NR4A1 was significantly up-regulated in the cytoplasm of activated microglia, which was inconsistent with the role of NR4A1 as a transcription factor, indicating microglial NR4A1 might also function as a non-transcriptional regulator. We then discovered that microglial NR4A1 localized to P-bodies, where it destabilized Tnf mRNA in an m6A-dependent manner. Global and conditional microglial knockout of Nr4a1 up-regulated Tnf expression and worsened stroke outcomes. Therefore, we uncovered a previously unidentified role of NR4A1 as an RBP, highlighting that microglial NR4A1 is a posttranscriptional brake that suppresses Tnf expression.
NR4A1 is widely expressed in the CNS, including the cortex, hippocampus, striatum, and other brain regions [32]. Additionally, it was reported that NR4A1 expression in activated microglia is markedly lower in an MPTP-PD mouse model than that in WT mice, and NR4A1 suppresses deleterious inflammatory responses and protects dopaminergic cells from inflammation-induced death [13]. Rapid up-regulation of NR4A1 in microglia upon exposure to the neuronal-derived stress signal ATP is critical for the maintenance of a resting and noninflammatory phenotype of microglia, which was observed in the EAE model [12].
One outstanding question is the mechanisms underlying NR4A1-mediated anti-inflammatory effect in microglia. It was reported that NR4A1 inhibits the expression of Tnf at the transcriptional level after LPS stimulation by suppressing the activation of the NF-κB signaling pathway [33]. Here, we discovered that NR4A1 was significantly up-regulated in the cytoplasm and located in P-bodies in activated microglia, then directly binding Tnf mRNA and promoting its destabilization. P-bodies belong to a group of nonmembrane-bound organelles and are assembled by RNAs and RBPs [34]. Posttranscriptional modulation of mRNAs in P-bodies are common mechanisms of mRNA degradation and stabilization [34,35]. DNA-binding proteins (DBPs) and RBPs were considered as 2 distinct categories and studied separately. However, this standpoint has been outdated. Recent studies have found that some DBDs are evolutionarily conserved, which enables them to bind and regulate RNA metabolism [36]. Similar to NR4A1, the glucocorticoid receptor (GCR, also referred to as NR3C1), a classic ligand-activated transcription factor that also has 2 highly conserved Cys 4 zinc-fingers, was found to destabilize Ccl2 mRNA through direct RNA binding and the recruitment of UPF1 [37,38]. In addition, P-bodies have also been associated with microRNA (miRNA)-mediated gene silencing, and Tnf have been reported to be regulated by multiple miRNAs [39–41]. Therefore, the effect of NR4A1 on miRNA-mediated Tnf decay was not fully excluded and needs further investigation.
Notably, TNF-α is a well-characterized proinflammatory cytokine, and increased TNF-α expression is observed in the early phase of stroke in rodent models and postmortem human brain tissues [42–44]. Microglia have been reported to be the dominant source of TNF-α in the brain [45]. TNF-α secreted by microglia can directly act on TNFR1 on neurons and strongly trigger neuronal death by activating caspase-8 signaling after stroke [46,47]. In addition, we previously discovered that TNF-α mediates the interaction between microglia and double-negative T cells after ischemia, leading to aggravated brain injury [48]. Previous studies have identified several RBPs (e.g., TTP, HuR) that target Tnf mRNA 3′ UTR to regulate its stability, whose disruption would cause dysregulated TNF-α production and inflammatory diseases [49,50]. Nevertheless, our studies indicated that Tnf was significantly affected by NR4A1-deletion in rodent stroke model, while NR4A1 regulates multiple proinflammatory cytokines in in vitro-activated microglia. We speculated that in MCAO mice, NR4A1 was up-regulated in the cytoplasm but absent in the nucleus of microglia; however, in in vitro studies, NR4A1 was significantly induced after microglial activation in both cytoplasm and nucleus, suggesting that Il1b and Il6 were mainly transcriptionally regulated by nuclear NR4A1. We also overexpressed NR4A1-NES in BV2 cells to confirm that cytoplasmic NR4A1 specifically regulate Tnf expression, which probably revealed a de novo posttranscriptional role of cytoplasmic NR4A1, therefore expanding the mechanisms underlying its anti-inflammatory role, as well as replenishing the RBP pool that posttranscriptionally regulate the stability of Tnf mRNA.
We also found that TSA, a pan-HDAC inhibitor, is effective in increasing NR4A1 expression and down-regulating microglia-derived Tnf expression. We proposed that TSA-promoted Tnf mRNA degradation in an NR4A1-dependent manner, further supporting our finding that NR4A1 functions as an RBP to posttranscriptionally regulate Tnf mRNA in microglia. Several compounds have been found to potentially act as agonists of NR4A1, but the side effects of these compounds limit their application for neurological diseases. Cytosporone B (Csn-B), a natural compound, can suppress microglia-mediated inflammation in an NR4A1-dependent manner, but the specific mechanism underlying the anti-inflammatory effect of Csn-B remains elusive [12]. In addition, since Csn-B has been reported to translocate NR4A1 to mitochondria to induce apoptosis, it might initiate neuronal death, suggesting that its utility for CNS diseases is limited [51]. Celastrol also alleviates inflammation by inducing the interaction between NR4A1 and TRAF2 to promote mitophagy [52]. However, the immunosuppressive effect of celastrol can cause serious infection in patients [53]. Considering the detrimental role of neuroinflammation in multiple sclerosis, Alzheimer’s disease, and Parkinson’s disease, TSA might also be used for treatment of these CNS diseases.
N6-methyladenosine (m6A) methylation is the most prevalent modification of eukaryotic mRNAs and is interpreted by its readers to regulate mRNA fate [54,55]. Previous studies have demonstrated that m6A writers, erasers, and readers are involved in the immune response under various pathological context. As the most well-identified writer, METTL3 played diverse roles in different diseases. In endotoxemia, METTL3-mediated m6A-modification of Tlr4 showed increased translation and slowed degradation, which was critical for neutrophil activation [56]. METTL3 also enhanced the stabilization of Tab3 and contributed to the aggravation of renal injury and inflammation [57]. In the spinal cord injury, however, METTL3-methylated Yap1 mRNA and promoted its stability, leading to reactive astrogliosis and functional recovery [58]. Furthermore, another m6A writer, METTL14, was reported to increase the translation of Foxo1 and aggravated endothelial inflammation and atherosclerosis [24]. ALKBH5, a well-characterized m6A eraser, demethylated various target transcripts in neutrophils and promoted neutrophil migration, which is essential for antibacterial defense [59]. As to m6A readers, IGF2BP2 could stabilize m6A-modified target transcripts (e.g., Cebpd, Tsc1) to augment autoimmune inflammation [60,61]. However, inactivation of YTHDF2 increased m6A-modified inflammation-related transcripts and impaired hematopoietic stem cell function [62]. A previous study also identified m6A-hypermethylated mRNAs related to proinflammatory cytokines in the ischemic brain, including Tnf mRNA, indicating m6A-modification was involved in ischemic stroke [63]. Accordingly, our study identified microglial NR4A1 functions as an RBP to destabilize Tnf mRNA in an m6A-dependent manner and alleviates post-stroke neuroinflammation, highlighting the role of m6A in ischemic stroke.
Notably, a previous study found that m6A-loss in Tnf mRNA did not directly affect its degradation in macrophages using various approaches [64]. However, there are many m6A readers with opposite functions in regulating mRNA stability. For example, IGF2BP proteins could stabilize mRNA, whereas FMR1 was reported to promote the degradation of its target mRNAs [65]. Since the stability of Tnf mRNA probably be regulated by both types of m6A readers, m6A-loss in Tnf mRNA might disable all the m6A readers to regulate its stability, leading to the net effect of m6A-loss on Tnf stability to be insignificant. In addition, according to this study, simultaneously knocking down YTHDF proteins significantly down-regulated Tnf expression, but the authors did not investigate the effect of YTHDF-knockout on Tnf stability. Here, our study used Nr4a1-deficient cells rather than Mettl3/14-deficient cells to demonstrate that genetic manipulation of specific m6A readers could directly regulate Tnf stability, which might explain the inconsistency. However, the domain of NR4A1 that binds to m6A sites remains elusive. Future structural studies exploring the specific domain that binds to m6A-modified mRNAs are warranted. In addition, our study also used BV2 and HEK293T cells instead of primary microglia to demonstrate that NR4A1 regulates Tnf by directly binding to the m6A sites because these experiments require a large number of cells, and BV2 cells share common features in immune response with primary microglia. Still, the function of NR4A1 as an RBP needs to be further validated in ex vivo microglia (e.g., primary microglia, iPSC-derived microglia).
In addition, NR4A1 was reported to be expressed in various immune cells, including macrophages and T cells [11,66,67]. Since a large number of peripheral immune cells could infiltrate the brain after ischemic stroke, it would be difficult to differ the function of microglial NR4A1 and NR4A1 in other immune cells in Nr4a1-knockout mice. Previous studies have confirmed that Nr4a1-deficiency caused a significant decrease in Ly6C- monocyte, which might also affect the outcome of ischemic brain injury in mice [68]. In addition, neuronal NR4A1 also protected dopaminergic neurons from MPTP, suggesting the pro-survival role of NR4A1 in neurons [69]. However, in Tmem119-CreERT2; Nr4a1fl/fl mice, microglial NR4A1 was specifically knockout, enabling us to confirm the neuroprotective role of microglial NR4A1 in the context of ischemic stroke.
Accordingly, our study found that global and conditional microglial knockout of Nr4a1-enhanced Tnf expression and remarkably exacerbated ischemic brain injury. We also used Etanercept to block the effect of TNF-α in MCAO model and found that Etanercept could rescue the detrimental effect of Nr4a1-knockout in microglia. However, the effect of TNF-α in ischemic stroke is controversial. Previous studies using Tnf-knockout mice showed that TNF-α was neuroprotective in ischemic stroke, while pharmacologically blocking TNF-α attenuated brain injury [45,70,71]. Moreover, neurons pretreated with TNF-α were more resistant to death [72,73]. It has also been suggested TNF-α was essential for maintaining neuronal function [74]. However, ablation of soluble TNF-α is neuroprotective in ischemic stroke [75]. Thus, we speculated that loss of TNF-α under steady state probably make neurons susceptible to ischemia, while acute blocking increased soluble TNF-α in the injured brain could be neuroprotective. Furthermore, although Etanercept was reported to be neuroprotective in MCAO rats, we found 2 studies reported that Etanercept did not show therapeutic effect in MCAO mice [76,77]. Several differences between our study and theirs might explain the inconsistencies. Sumbria and colleagues [77] used Etanercept at a dose of 1 mg/kg, which is lower than ours (10 mg/kg), indicating that higher dose of Etanercept might exhibit neuroprotective effect. In another study, Etanercept (10 mg/kg) improve neurological function but did not reduce the infarct size of MCAO mice [76]. The reason might be that they used permanent MCAO model, which there is no salvageable penumbra with reperfusion. However, in our study, we used transient MCAO model, and the penumbra exist for at least 24 to 48 h post-stroke in this model [78].
In summary, we found that NR4A1 is critical for accelerating Tnf mRNA decay in an m6A-dependent manner in P-bodies of activated microglia, which is distinct from its canonical function as a transcription factor. Furthermore, NR4A1 is significantly induced in the cytoplasm of microglia after ischemia and contributes to the suppression of post-stroke Tnf up-regulation, leading to improved outcomes. Thus, compounds with the ability to induce NR4A1 expression may improve stroke progression and be potential candidates for stroke therapy.
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