Lactate receptor HCAR1 regulates neurogenesis and microglia activation after neonatal hypoxia-ischemia

Neonatal cerebral hypoxia-ischemia (HI) is the leading cause of death and disability in newborns with the only current treatment being hypothermia. An increased understanding of the pathways that facilitate tissue repair after HI may aid the development of better treatments. Here, we study the role of lactate receptor HCAR1 in tissue repair after neonatal HI in mice. We show that HCAR1 knockout mice have reduced tissue regeneration compared with wildtype mice. Furthermore, proliferation of neural progenitor cells and glial cells, as well as microglial activation was impaired. Transcriptome analysis showed a strong transcriptional response to HI in the subventricular zone of wildtype mice involving about 7300 genes. In contrast, the HCAR1 knockout mice showed a modest response, involving about 750 genes. Notably, fundamental processes in tissue repair such as cell cycle and innate immunity were dysregulated in HCAR1 knockout. Our data suggest that HCAR1 is a key transcriptional regulator of pathways that promote tissue regeneration after HI.

Endonuclease V Regulates Atherosclerosis Through C-C Motif Chemokine Ligand 2-Mediated Monocyte Infiltration

Background

In cardiovascular diseases, atherosclerotic disorder are the most frequent and important with respect to morbidity and mortality. Inflammation mediated by immune cells is central in all parts of the atherosclerotic progress, and further understanding of the underlying mechanisms is needed. Growing evidence suggests that deamination of adenosine‐to‐inosine in RNA is crucial for a correct immune response; nevertheless, the role of adenosine‐to‐inosine RNA editing in atherogenesis has barely been studied. Several proteins have affinity for inosines in RNA, one being ENDOV (endonuclease V), which binds and cleaves RNA at inosines. Data on ENDOV in atherosclerosis are lacking.

Methods and Results

Quantitative polymerase chain reaction on ENDOV mRNA showed an increased level in human carotid atherosclerotic plaques compared with control veins. Inosine‐ribonuclease activity as measured by an enzyme activity assay is detected in immune cells relevant for the atherosclerotic process. Abolishing EndoV in atherogenic apolipoprotein E‐deficient (ApoE^−/−) mice reduces the atherosclerotic plaque burden, both in size and lipid content. In addition, in a brain stroke model, mice without ENDOV suffer less damage than control mice. Finally, lack of EndoV reduces the recruitment of monocytes to atherosclerotic lesions in atherogenic ApoE^−/− mice.

Conclusions

ENDOV is upregulated in human atherosclerotic lesions, and data from mice suggest that ENDOV promotes atherogenesis by enhancing the monocyte recruitment into the atherosclerotic lesion, potentially by increasing the effect of CCL2 activation on these cells.

OXR1A, a Coactivator of PRMT5 Regulating Histone Arginine Methylation

Oxidation resistance gene 1 (OXR1) protects cells against oxidative stress. We find that male mice with brain-specific isoform A knockout (Oxr1A^-/-) develop fatty liver. RNA sequencing of male Oxr1A^-/- liver indicates decreased growth hormone (GH) signaling, which is known to affect liver metabolism. Indeed, Gh expression is reduced in male mice Oxr1A^-/- pituitary gland and in rat Oxr1A^-/- pituitary adenoma cell-line GH3. Oxr1A^-/- male mice show reduced fasting-blood GH levels. Pull-down and proximity ligation assays reveal that OXR1A is associated with arginine methyl transferase PRMT5. OXR1A-depleted GH3 cells show reduced symmetrical dimethylation of histone H3 arginine 2 (H3R2me2s), a product of PRMT5 catalyzed methylation, and chromatin immunoprecipitation (ChIP) of H3R2me2s shows reduced Gh promoter enrichment. Finally, we demonstrate with purified proteins that OXR1A stimulates PRMT5/MEP50-catalyzed H3R2me2s. Our data suggest that OXR1A is a coactivator of PRMT5, regulating histone arginine methylation and thereby GH production within the pituitary gland.

DNA glycosylase Neil3 regulates vascular smooth muscle cell biology during atherosclerosis development

BACKGROUND AND AIMS

Atherogenesis involves a complex interaction between immune cells and lipids, processes greatly influenced by the vascular smooth muscle cell (VSMC) phenotype. The DNA glycosylase NEIL3 has previously been shown to have a role in atherogenesis, though whether this is due to its ability to repair DNA damage or to other non-canonical functions is not yet clear. Hereby, we investigate the role of NEIL3 in atherogenesis, specifically in VSMC phenotypic modulation, which is critical in plaque formation and stability.

METHODS

Chow diet-fed atherosclerosis-prone Apoe^-/- mice deficient in Neil3, and NEIL3-abrogated human primary aortic VSMCs were characterized by qPCR, and immunohistochemical and enzymatic-based assays; moreover, single-cell RNA sequencing, mRNA sequencing, and proteomics were used to map the molecular effects of Neil3/NEIL3 deficiency in the aortic VSMC phenotype. Furthermore, BrdU-based proliferation assays and Western blot were performed to elucidate the involvement of the Akt signaling pathway in the transdifferentiation of aortic VSMCs lacking Neil3/NEIL3.

RESULTS

We show that Neil3 deficiency increases atherosclerotic plaque development without affecting systemic lipids. This observation was associated with a shift in VSMC phenotype towards a proliferating, lipid-accumulating and secretory macrophage-like cell phenotype, without changes in DNA damage. VSMC transdifferentiation in Neil3-deficient mice encompassed increased activity of the Akt signaling pathway, supported by cell experiments showing Akt-dependent proliferation in NEIL3-abrogated human primary aortic VSMCs.

CONCLUSIONS

Our findings show that Neil3 deficiency promotes atherosclerosis development through non-canonical mechanisms affecting VSMC phenotype involving activation of the Akt signaling pathway.

Deletion of Endonuclease V suppresses chemically induced hepatocellular carcinoma

Endonuclease V (EndoV) is a conserved inosine-specific ribonuclease with unknown biological function. Here, we present the first mouse model lacking EndoV, which is viable without visible abnormalities. We show that endogenous murine EndoV cleaves inosine-containing RNA in vitro, nevertheless a series of experiments fails to link an in vivo function to processing of such transcripts. As inosine levels and adenosine-to-inosine editing often are dysregulated in hepatocellular carcinoma (HCC), we chemically induced HCC in mice. All mice developed liver cancer, however, EndoV^−/− tumors were significantly fewer and smaller than wild type tumors. Opposed to human HCC, adenosine deaminase mRNA expression and site-specific editing were unaltered in our model. Loss of EndoV did not affect editing levels in liver tumors, however mRNA expression of a selection of cancer related genes were reduced. Inosines are also found in certain tRNAs and tRNAs are cleaved during stress to produce signaling entities. tRNA fragmentation was dysregulated in EndoV^−/− livers and apparently, inosine-independent. We speculate that the inosine-ribonuclease activity of EndoV is disabled in vivo, but RNA binding allowed to promote stabilization of transcripts or recruitment of proteins to fine-tune gene expression. The EndoV^−/− tumor suppressive phenotype calls for related studies in human HCC.

DNA glycosylase Neil2 contributes to genomic responses in the spleen during clinical prion disease

The DNA glycosylase Neil2 is a member of the base excision repair (BER) family of enzymes, which are important for repair of oxidative DNA damage. Specifically, Neil2 participates in repair of oxidized bases in single-stranded DNA of transcriptionally active genes. Mice with genetic ablation of Neil2 (Neil2^-/-) display no overt phenotypes, but an age-dependent accumulation of oxidative DNA damage and increased inflammatory responsiveness. In young mice intra-cerebrally inoculated with prions, vigorous prion propagation starts rapidly in the germinal follicles of the spleen due to inoculum spillover. Here, we compare experimental prion disease in Neil2^-/- mice with that in wild-type mice at disease onset and end-stage. Specifically, we investigated disease progression, accumulation of DNA damage, and mitochondrial respiratory complex activity in brain and spleen. We used genome-wide RNA sequencing of the spleen to compare the immune responses to prion propagation between the two groups of mice, at both onset and end-stage prion disease. The Neil2^-/- mice deteriorated more rapidly than wild-type mice after onset of clinical signs. Levels of DNA damage in brain increased in both mouse groups, slightly more in the Neil2^-/- mice. Transcriptome data from spleen at disease onset were similar between the mouse groups with moderate genomic responses. However, at end-stage a substantial response was evident in the wild-type mice but not in Neil2^-/- mice. Our data show that Neil2 counteracts toxic signaling in clinical prion disease, and this is separate from gross pathological manifestations and PrP^Sc accumulation.

Neuromodulatory Effect of NLRP3 and ASC in Neonatal Hypoxic Ischemic Encephalopathy

BACKGROUND

Following birth asphyxia there is a robust inflammatory response. NLRP3 is a receptor of the innate immune system. Upon activation, NLRP3 forms an inflammasome together with ASC and procaspase-1 to mediate release of IL-1β and IL-18. NLRP3 has previously been shown to be upregulated following neonatal hypoxic-ischemic (HI) brain injury in mice, but with no early effect on brain injury.

OBJECTIVE

We aimed to evaluate if deficiency of NLRP3 or ASC protects against neonatal HI brain damage 7 days after hypoxia-ischemia.

METHODS

C57BL/6J, NLRP3-/-, and ASC-/- mice were subjected to unilateral common carotid artery ligation followed by hypoxia at P9. Brain infarction, apoptosis, and microglial response were evaluated, as well as total RNA sequencing and examination of plasma levels of systemic proinflammatory cytokines.

RESULTS

NLRP3-/- mice showed significantly increased brain infarction volumes compared to wild-type (Wt) mice, while ASC-/- mice showed reduced brain infarction volumes after neonatal hypoxia-ischemia. The amount of activated microglia was increased in NLRP3-/- mice, while decreased in ASC-/- mice compared to Wt mice. Total RNA sequencing showed an impaired inflammatory transcriptional response in the hippocampus of NLRP3-/- mice. Plasma levels of IL-1β and IL-18 were not affected, but TNF was lower in NLRP3-/- and ASC-/- mice compared to Wt mice.

CONCLUSION

ASC deficiency is neuroprotective in neonatal HI brain damage in mice, while NLRP3 deficiency increases brain damage.

Neonatal Ogg1/Mutyh knockout mice have altered inflammatory gene response compared to wildtype mice in the brain and lung after hypoxia-reoxygenation

Background 8-Oxoguanine DNA-glycosylase 1 (OGG1) and mutY DNA glycosylase (MUTYH) are crucial in the repair of the oxidative DNA lesion 7,8-dihydro-8-oxoguanine caused by hypoxia-reoxygenation injury. Our objective was to compare the gene expression changes after hypoxia-reoxygenation in neonatal Ogg1-Mutyh double knockout mice (OM) and wildtype mice (WT), and study the gene response in OM after hyperoxic reoxygenation compared to normoxic. Methods Postnatal day 7 mice were subjected to 2 h of hypoxia (8% O2) followed by reoxygenation in either 60% O2 or air, and sacrificed right after completed reoxygenation (T0h) or after 72 h (T72h). The gene expression of 44 a priori selected genes was examined in the hippocampus/striatum and lung. Results We found that OM had an altered gene response compared to WT in 21 genes in the brain and 24 genes in the lung. OM had a lower expression than WT of inflammatory genes in the brain at T0h, and higher expression at T72h in both the brain and lung. In the lung of OM, five genes were differentially expressed after hyperoxic reoxygenation compared to normoxic. Conclusion For the first time, we report that Ogg1 and Mutyh in combination protect against late inflammatory gene activation in the hippocampus/striatum and lung after neonatal hypoxia-reoxygenation.

Ythdf2-mediated m6A mRNA clearance modulates neural development in mice

BACKGROUND

N 6 -methyladenosine (m6A) modification in mRNAs was recently shown to be dynamically regulated, indicating a pivotal role in multiple developmental processes. Most recently, it was shown that the Mettl3-Mettl14 writer complex of this mark is required for the temporal control of cortical neurogenesis. The m6A reader protein Ythdf2 promotes mRNA degradation by recognizing m6A and recruiting the mRNA decay machinery.

RESULTS

We show that the conditional depletion of the m6A reader protein Ythdf2 in mice causes lethality at late embryonic developmental stages, with embryos characterized by compromised neural development. We demonstrate that neural stem/progenitor cell (NSPC) self-renewal and spatiotemporal generation of neurons and other cell types are severely impacted by the loss of Ythdf2 in embryonic neocortex. Combining in vivo and in vitro assays, we show that the proliferation and differentiation capabilities of NSPCs decrease significantly in Ythdf2 -/- embryos. The Ythdf2 -/- neurons are unable to produce normally functioning neurites, leading to failure in recovery upon reactive oxygen species stimulation. Consistently, expression of genes enriched in neural development pathways is significantly disturbed. Detailed analysis of the m6A-methylomes of Ythdf2 -/- NSPCs identifies that the JAK-STAT cascade inhibitory genes contribute to neuroprotection and neurite outgrowths show increased expression and m6A enrichment. In agreement with the function of Ythdf2, delayed degradation of neuron differentiation-related m6A-containing mRNAs is seen in Ythdf2 -/- NSPCs.

CONCLUSIONS

We show that the m6A reader protein Ythdf2 modulates neural development by promoting m6A-dependent degradation of neural development-related mRNA targets.

Neil3 induced neurogenesis protects against prion disease during the clinical phase

Base excision repair (BER) is the major pathway for repair of oxidative DNA damage. Mice with genetic knockout of the BER enzyme Neil3 display compromised neurogenesis in the sub-ventricular zone of the lateral ventricle and sub-granular layer of the dentate gyrus of the hippocampus. To elucidate the impact of oxidative DNA damage-induced neurogenesis on prion disease we applied the experimental prion disease model on Neil3-deficient mice. The incubation period for the disease was similar in both wild type and Neil3^−/− mice and the overall neuropathology appeared unaffected by Neil3 function. However, disease in the Neil3^−/− mice was of shorter clinical duration. We observed a mildly reduced astrogliosis in the hippocampus and striatum in the Neil3-deficient mice. Brain expression levels of neuronal progenitor markers, nestin (Nestin), sex determining region Box 2 (Sox2), Class III beta-tubulin (Tuj1) decreased towards end-stage prion disease whereas doublecortin (Dcx) levels were less affected. Neuronal nuclei (NeuN), a marker for mature neurons declined during prion disease and more pronounced in the Neil3^−/− group. Microglial activation was prominent and appeared unaffected by loss of Neil3. Our data suggest that neurogenesis induced by Neil3 repair of oxidative DNA damage protects against prion disease during the clinical phase.

Neil3-dependent base excision repair regulates lipid metabolism and prevents atherosclerosis in Apoe-deficient mice

Increasing evidence suggests that oxidative DNA damage accumulates in atherosclerosis. Recently, we showed that a genetic variant in the human DNA repair enzyme NEIL3 was associated with increased risk of myocardial infarction. Here, we explored the role of Neil3/NEIL3 in atherogenesis by both clinical and experimental approaches. Human carotid plaques revealed increased NEIL3 mRNA expression which significantly correlated with mRNA levels of the macrophage marker CD68. Apoe^−/−Neil3^−/− mice on high-fat diet showed accelerated plaque formation as compared to Apoe^−/− mice, reflecting an atherogenic lipid profile, increased hepatic triglyceride levels and attenuated macrophage cholesterol efflux capacity. Apoe^−/−Neil3^−/− mice showed marked alterations in several pathways affecting hepatic lipid metabolism, but no genotypic alterations in genome integrity or genome-wide accumulation of oxidative DNA damage. These results suggest a novel role for the DNA glycosylase Neil3 in atherogenesis in balancing lipid metabolism and macrophage function, potentially independently of genome-wide canonical base excision repair of oxidative DNA damage.

Mitochondrial DNA damage and repair during ischemia-reperfusion injury of the heart

Ischemia-reperfusion (IR) injury of the heart generates reactive oxygen species that oxidize macromolecules including mitochondrial DNA (mtDNA). The 8-oxoguanine DNA glycosylase (OGG1) works synergistically with MutY DNA glycosylase (MYH) to maintain mtDNA integrity. Our objective was to study the functional outcome of lacking the repair enzymes OGG1 and MYH after myocardial IR and we hypothesized that OGG1 and MYH are important enzymes to preserve mtDNA and heart function after IR. Ex vivo global ischemia for 30min followed by 10min of reperfusion induced mtDNA damage that was removed within 60min of reperfusion in wild-type mice. After 60min of reperfusion the ogg1(-/-) mice demonstrated increased mtDNA copy number and decreased mtDNA damage removal suggesting that OGG1 is responsible for removal of IR-induced mtDNA damage and copy number regulation. mtDNA damage was not detected in the ogg1(-/-)/myh(-/-), inferring that adenine opposite 8-oxoguanine is an abundant mtDNA lesion upon IR. The level and integrity of mtDNA were restored in all genotypes after 35min of regional ischemia and six week reperfusion with no change in cardiac function. No consistent upregulation of other mitochondrial base excision repair enzymes in any of our knockout models was found. Thus repair of mtDNA oxidative base lesions may not be important for maintenance of cardiac function during IR injury in vivo. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease."

Accelerated clinical course of prion disease in mice compromised in repair of oxidative DNA damage

The detailed mechanisms of prion-induced neurotoxicity are largely unknown. Here, we have studied the role of DNA damage caused by reactive oxygen species in a mouse scrapie model by characterizing prion disease in the ogg1(-/-)mutyh(-/-) double knockout, which is compromised in oxidative DNA base excision repair. Ogg1 initiates removal of the major oxidation product 8-oxoguanine (8-oxoG) in DNA, and Mutyh initiates removal of adenine that has been misincorporated opposite 8-oxoG. Our data show that the onset of clinical signs appeared unaffected by Mutyh and Ogg1 expression. However, the ogg1(-/-)mutyh(-/-) mice displayed a significantly shorter clinical phase of the disease. Thus, accumulation of oxidative DNA damage might be of particular importance in the terminal clinical phase of prion disease. The prion-induced pathology and lesion profile were similar between knockout mice and controls. The fragmentation pattern of protease-resistant PrP as revealed in Western blots was also identical between the groups. Our data show that the fundamentals of prion propagation and pathological manifestation are not influenced by the oxidative DNA damage repair mechanisms studied here, but that progressive accumulation of oxidative lesions may accelerate the final toxic phase of prion disease.

Hippocampal adult neurogenesis is maintained by Neil3-dependent repair of oxidative DNA lesions in neural progenitor cells

Accumulation of oxidative DNA damage has been proposed as a potential cause of age-related cognitive decline. The major pathway for removal of oxidative DNA base lesions is base excision repair, which is initiated by DNA glycosylases. In mice, Neil3 is the main DNA glycosylase for repair of hydantoin lesions in single-stranded DNA of neural stem/progenitor cells, promoting neurogenesis. Adult neurogenesis is crucial for maintenance of hippocampus-dependent functions involved in behavior. Herein, behavioral studies reveal learning and memory deficits and reduced anxiety-like behavior in Neil3(-/-) mice. Neural stem/progenitor cells from aged Neil3(-/-) mice show impaired proliferative capacity and reduced DNA repair activity. Furthermore, hippocampal neurons in Neil3(-/-) mice display synaptic irregularities. It appears that Neil3-dependent repair of oxidative DNA damage in neural stem/progenitor cells is required for maintenance of adult neurogenesis to counteract the age-associated deterioration of cognitive performance.