Critical roles of IκBα and RelA phosphorylation in transitional oscillation in NF-κB signaling module.

The transcription factor NF-κB performs various cell functions, such as regulating proliferation and differentiation and blocking apoptosis, by inducing the expression of multiple genes. The shuttling of NF-κB between the cytoplasm and nucleus is involved in its transcriptional activity in the canonical NF-κB pathway. The transcription of the NF-κB target genes is regulated by the phosphorylation of both IκBα and the RelA subunit of NF-κB, suggesting that these phosphorylation events are crucial for the oscillation. In this study, we constructed a new mathematical model of NF-κB activation to explore the modulation of the oscillation by the phosphorylation of IκBα and RelA. Based on a stability analysis around the equilibrium point, we confirmed that IκBα phosphorylation added a structure with a stable periodic solution to the phosphorylation model. The stable periodic solution appeared to transiently respond to the attenuation of the concentration of active IKKß. Because the NF-κB oscillation is caused by the periodic solution, the amplitude and period of the NF-κB oscillation in the phosphorylation model was constant regardless of the initial conditions; we defined this property as the reproducibility of the oscillation. On the other hand, the amplitude and period of the NF-κB oscillation depended on a parameter related to the RelA phosphorylation, suggesting that the oscillation period is regulated by RelA phosphorylation. In addition, the region of the periodic solution that is dependent on active IKKß also depends on a parameter related to RelA phosphorylation. Therefore, we conclude that the phosphorylation of both IκBα and RelA regulates the robustness of the NF-κB signaling module oscillation. That is, by appropriately controlling the phosphorylation process, it becomes possible to control the NF-κB oscillation and appropriately induce the NFkB-dependent expression gene. We anticipate that this study will contribute to the future elucidation of the mechanism underlying the nuclear cytoplasmic (N-C) oscillation of NF-κB.

[Aging-associated Cyst Formation and Fibrosis].

Cysts are abnormal fluid-filled sacs found in various human organs, including the liver. Liver cysts can be associated with known causes such as parasite infections and gene mutations, or simply aging. Among these causes, simple liver cysts are often found in elderly people. While they are generally benign, they may occasionally grow but rarely shrink with age, indicating their clear association with aging. However, the mechanism behind the formation of simple liver cysts has not been thoroughly investigated. Recently, we have generated transgenic mice that specifically overexpress fibroblast growth factor (FGF)18 in hepatocytes. These mice exhibit severe liver fibrosis without inflammation and spontaneously develop liver cysts that grow with age. Our findings suggest that simple liver cysts can be induced by fibrosis accompanied by sterile inflammation or injury, whereas fibrosis accompanied by severe inflammation or injury may lead to cirrhosis. We also discuss the detrimental effects of disease- and aging-associated fibrosis in various organs, such as the heart, lungs, and kidneys. Additionally, we provide a brief summary of the two currently approved anti-fibrotic drugs for idiopathic pulmonary fibrosis, nintedanib and pirfenidone, as well as their possibility of future expansion of application toward other fibrotic diseases.

Egr2 drives the differentiation of Ly6Chi monocytes into fibrosis-promoting macrophages in metabolic dysfunction-associated steatohepatitis in mice.

Metabolic dysfunction-associated steatohepatitis (MASH), previously called non-alcoholic steatohepatitis (NASH), is a growing concern worldwide, with liver fibrosis being a critical determinant of its prognosis. Monocyte-derived macrophages have been implicated in MASH-associated liver fibrosis, yet their precise roles and the underlying differentiation mechanisms remain elusive. In this study, we unveil a key orchestrator of this process: long chain saturated fatty acid-Egr2 pathway. Our findings identify the transcription factor Egr2 as the driving force behind monocyte differentiation into hepatic lipid-associated macrophages (hLAMs) within MASH liver. Notably, Egr2-deficiency reroutes monocyte differentiation towards a macrophage subset resembling resident Kupffer cells, hampering hLAM formation. This shift has a profound impact, suppressing the transition from benign steatosis to liver fibrosis, demonstrating the critical pro-fibrotic role played by hLAMs in MASH pathogenesis. Long-chain saturated fatty acids that accumulate in MASH liver emerge as potent inducers of Egr2 expression in macrophages, a process counteracted by unsaturated fatty acids. Furthermore, oral oleic acid administration effectively reduces hLAMs in MASH mice. In conclusion, our work not only elucidates the intricate interplay between saturated fatty acids, Egr2, and monocyte-derived macrophages but also highlights the therapeutic promise of targeting the saturated fatty acid-Egr2 axis in monocytes for MASH management.

Mitochondrial protein C15ORF48 is a stress-independent inducer of autophagy that regulates oxidative stress and autoimmunity.

Autophagy is primarily activated by cellular stress, such as starvation or mitochondrial damage. However, stress-independent autophagy is activated by unclear mechanisms in several cell types, such as thymic epithelial cells (TECs). Here we report that the mitochondrial protein, C15ORF48, is a critical inducer of stress-independent autophagy. Mechanistically, C15ORF48 reduces the mitochondrial membrane potential and lowers intracellular ATP levels, thereby activating AMP-activated protein kinase and its downstream Unc-51-like kinase 1. Interestingly, C15ORF48-dependent induction of autophagy upregulates intracellular glutathione levels, promoting cell survival by reducing oxidative stress. Mice deficient in C15orf48 show a reduction in stress-independent autophagy in TECs, but not in typical starvation-induced autophagy in skeletal muscles. Moreover, C15orf48-/- mice develop autoimmunity, which is consistent with the fact that the stress-independent autophagy in TECs is crucial for the thymic self-tolerance. These results suggest that C15ORF48 induces stress-independent autophagy, thereby regulating oxidative stress and self-tolerance.

Fibroblast growth factor 18 stimulates the proliferation of hepatic stellate cells, thereby inducing liver fibrosis.

Liver fibrosis results from chronic liver injury triggered by factors such as viral infection, excess alcohol intake, and lipid accumulation. However, the mechanisms underlying liver fibrosis are not fully understood. Here, we demonstrate that the expression of fibroblast growth factor 18 (Fgf18) is elevated in mouse livers following the induction of chronic liver fibrosis models. Deletion of Fgf18 in hepatocytes attenuates liver fibrosis; conversely, overexpression of Fgf18 promotes liver fibrosis. Single-cell RNA sequencing reveals that overexpression of Fgf18 in hepatocytes results in an increase in the number of Lrat+ hepatic stellate cells (HSCs), thereby inducing fibrosis. Mechanistically, FGF18 stimulates the proliferation of HSCs by inducing the expression of Ccnd1. Moreover, the expression of FGF18 is correlated with the expression of profibrotic genes, such as COL1A1 and ACTA2, in human liver biopsy samples. Thus, FGF18 promotes liver fibrosis and could serve as a therapeutic target to treat liver fibrosis.

The early neutrophil-committed progenitors aberrantly differentiate into immunoregulatory monocytes during emergency myelopoiesis.

Inflammatory stimuli cause a state of emergency myelopoiesis leading to neutrophil-like monocyte expansion. However, their function, the committed precursors, or growth factors remain elusive. In this study we find that Ym1+Ly6Chi monocytes, an immunoregulatory entity of neutrophil-like monocytes, arise from progenitors of neutrophil 1 (proNeu1). Granulocyte-colony stimulating factor (G-CSF) favors the production of neutrophil-like monocytes through previously unknown CD81+CX3CR1lo monocyte precursors. GFI1 promotes the differentiation of proNeu2 from proNeu1 at the cost of producing neutrophil-like monocytes. The human counterpart of neutrophil-like monocytes that also expands in response to G-CSF is found in CD14+CD16- monocyte fraction. The human neutrophil-like monocytes are discriminated from CD14+CD16- classical monocytes by CXCR1 expression and the capacity to suppress T cell proliferation. Collectively, our findings suggest that the aberrant expansion of neutrophil-like monocytes under inflammatory conditions is a process conserved between mouse and human, which may be beneficial for the resolution of inflammation.

Generation of transgenic mice expressing a FRET biosensor, SMART, that responds to necroptosis.

Necroptosis is a regulated form of cell death involved in various pathological conditions, including ischemic reperfusion injuries, virus infections, and drug-induced tissue injuries. However, it is not fully understood when and where necroptosis occurs in vivo. We previously generated a Forster resonance energy transfer (FRET) biosensor, termed SMART (the sensor for MLKL activation by RIPK3 based on FRET), which monitors conformational changes of MLKL along with progression of necroptosis in human and murine cell lines in vitro. Here, we generate transgenic (Tg) mice that express the SMART biosensor in various tissues. The FRET ratio is increased in necroptosis, but not apoptosis or pyroptosis, in primary cells. Moreover, the FRET signals are elevated in renal tubular cells of cisplatin-treated SMART Tg mice compared to untreated SMART Tg mice. Together, SMART Tg mice may provide a valuable tool for monitoring necroptosis in different types of cells in vitro and in vivo.

Integrative analysis of scRNA-seq and scATAC-seq revealed transit-amplifying thymic epithelial cells expressing autoimmune regulator.

Medullary thymic epithelial cells (mTECs) are critical for self-tolerance induction in T cells via promiscuous expression of tissue-specific antigens (TSAs), which are controlled by the transcriptional regulator, AIRE. Whereas AIRE-expressing (Aire+) mTECs undergo constant turnover in the adult thymus, mechanisms underlying differentiation of postnatal mTECs remain to be discovered. Integrative analysis of single-cell assays for transposase-accessible chromatin (scATAC-seq) and single-cell RNA sequencing (scRNA-seq) suggested the presence of proliferating mTECs with a specific chromatin structure, which express high levels of Aire and co-stimulatory molecules, CD80 (Aire+CD80hi). Proliferating Aire+CD80hi mTECs detected using Fucci technology express a minimal number of Aire-dependent TSAs and are converted into quiescent Aire+CD80hi mTECs expressing high levels of TSAs after a transit amplification. These data provide evidence for the existence of transit-amplifying Aire+mTEC precursors during the Aire+mTEC differentiation process of the postnatal thymus.

Impact of spaceflight on the murine thymus and mitigation by exposure to artificial gravity during spaceflight.

The environment experienced during spaceflight may impact the immune system and the thymus appears to undergo atrophy during spaceflight. However, molecular aspects of this thymic atrophy remain to be elucidated. In this study, we analysed the thymi of mice on board the international space station (ISS) for approximately 1 month. Thymic size was significantly reduced after spaceflight. Notably, exposure of mice to 1 × g using centrifugation cages in the ISS significantly mitigated the reduction in thymic size. Although spaceflight caused thymic atrophy, the global thymic structure was not largely changed. However, RNA sequencing analysis of the thymus showed significantly reduced expression of cell cycle-regulating genes in two independent spaceflight samples. These reductions were partially countered by 1 × g exposure during the space flights. Thus, our data suggest that spaceflight leads to reduced proliferation of thymic cells, thereby reducing the size of the thymus, and exposure to 1 × g might alleviate the impairment of thymus homeostasis induced by spaceflight.

Down-regulation of GATA1-dependent erythrocyte-related genes in the spleens of mice exposed to a space travel.

Secondary lymphoid organs are critical for regulating acquired immune responses. The aim of this study was to characterize the impact of spaceflight on secondary lymphoid organs at the molecular level. We analysed the spleens and lymph nodes from mice flown aboard the International Space Station (ISS) in orbit for 35 days, as part of a Japan Aerospace Exploration Agency mission. During flight, half of the mice were exposed to 1 g by centrifuging in the ISS, to provide information regarding the effect of microgravity and 1 g exposure during spaceflight. Whole-transcript cDNA sequencing (RNA-Seq) analysis of the spleen suggested that erythrocyte-related genes regulated by the transcription factor GATA1 were significantly down-regulated in ISS-flown vs. ground control mice. GATA1 and Tal1 (regulators of erythropoiesis) mRNA expression was consistently reduced by approximately half. These reductions were not completely alleviated by 1 g exposure in the ISS, suggesting that the combined effect of space environments aside from microgravity could down-regulate gene expression in the spleen. Additionally, plasma immunoglobulin concentrations were slightly altered in ISS-flown mice. Overall, our data suggest that spaceflight might disturb the homeostatic gene expression of the spleen through a combination of microgravity and other environmental changes.

Visualization of RelB expression and activation at the single-cell level during dendritic cell maturation in Relb-Venus knock-in mice.

RelB is activated by the non-canonical NF-κB pathway, which is crucial for immunity by establishing lymphoid organogenesis and B-cell and dendritic cell (DC) maturation. To elucidate the mechanism of the RelB-mediated immune cell maturation, a precise understanding of the relationship between cell maturation and RelB expression and activation at the single-cell level is required. Therefore, we generated knock-in mice expressing a fusion protein between RelB and fluorescent protein (RelB-Venus) from the Relb locus. The Relb(Venus/Venus) mice developed without any abnormalities observed in the Relb(-/-) mice, allowing us to monitor RelB-Venus expression and nuclear localization as RelB expression and activation. Relb(Venus/Venus) DC analyses revealed that DCs consist of RelB(-), RelB(low) and RelB(high) populations. The RelB(high) population, which included mature DCs with projections, displayed RelB nuclear localization, whereas RelB in the RelB(low) population was in the cytoplasm. Although both the RelB(low) and RelB(-) populations barely showed projections, MHC II and co-stimulatory molecule expression were higher in the RelB(low) than in the RelB(-) splenic conventional DCs. Taken together, our results identify the RelB(low) population as a possible novel intermediate maturation stage of cDCs and the Relb(Venus/Venus) mice as a useful tool to analyse the dynamic regulation of the non-canonical NF-κB pathway.