Ichiro Takada

Pivotal role of cerebral interleukin-17|[ndash]|producing |[gamma]||[delta]|T cells in the delayed phase of ischemic brain injury

Lymphocyte recruitment and activation have been implicated in the progression of cerebral ischemia-reperfusion (I/R) injury, but the roles of specific lymphocyte subpopulations and cytokines during stroke remain to be clarified. Here we demonstrate that the infiltration of T cells into the brain, as well as the cytokines interleukin-23 (IL-23) and IL-17, have pivotal roles in the evolution of brain infarction and accompanying neurological deficits. Blockade of T cell infiltration into the brain by the immunosuppressant FTY720 reduced I/R-induced brain damage. The expression of IL-23, which was derived mostly from infiltrated macrophages, increased on day 1 after I/R, whereas IL-17 levels were elevated after day 3, and this induction of IL-17 was dependent on IL-23. These data, together with analysis of mice genetically disrupted for IL-17 and IL-23, suggest that IL-23 functions in the immediate stage of I/R brain injury, whereas IL-17 has an important role in the delayed phase of I/R injury during which apoptotic neuronal death occurs in the penumbra. Intracellular cytokine staining revealed that γδT lymphocytes, but not CD4+ helper T cells, were a major source of IL-17. Moreover, depletion of γδT lymphocytes ameliorated the I/R injury. We propose that T lymphocytes, including γδT lymphocytes, could be a therapeutic target for mitigating the inflammatory events that amplify the initial damage in cerebral ischemia.

A histone lysine methyltransferase activated by non-canonical Wnt signalling suppresses PPAR-gamma transactivation.

Histone modifications induced by activated signalling cascades are crucial to cell-lineage decisions. Osteoblast and adipocyte differentiation from common mesenchymal stem cells is under transcriptional control by numerous factors. Although PPAR-γ (peroxisome proliferator activated receptor-γ) has been established as a prime inducer of adipogenesis, cellular signalling factors that determine cell lineage in bone marrow remain generally unknown. Here, we show that the non-canonical Wnt pathway through CaMKII–TAK1–TAB2–NLK transcriptionally represses PPAR-γ transactivation and induces Runx2 expression, promoting osteoblastogenesis in preference to adipogenesis in bone marrow mesenchymal progenitors. Wnt-5a activates NLK (Nemo-like kinase), which in turn phosphorylates a histone methyltransferase, SETDB1 (SET domain bifurcated 1), leading to the formation of a co-repressor complex that inactivates PPAR-γ function through histone H3-K9 methylation. These findings suggest that the non-canonical Wnt signalling pathway suppresses PPAR-γ function through chromatin inactivation triggered by recruitment of a repressing histone methyltransferase, thus leading to an osteoblastic cell lineage from mesenchymal stem cells.

Dioxin receptor is a ligand-dependent E3 ubiquitin ligase

Fat-soluble ligands, including sex steroid hormones and environmental toxins, activate ligand-dependent DNA-sequence-specific transcriptional factors that transduce signals through target-gene-selective transcriptional regulation. However, the mechanisms of cellular perception of fat-soluble ligand signals through other target-selective systems remain unclear. The ubiquitin–proteasome system regulates selective protein degradation, in which the E3 ubiquitin ligases determine target specificity. Here we characterize a fat-soluble ligand-dependent ubiquitin ligase complex in human cell lines, in which dioxin receptor (AhR) is integrated as a component of a novel cullin 4B ubiquitin ligase complex, CUL4BAhR. Complex assembly and ubiquitin ligase activity of CUL4BAhR in vitro and in vivo are dependent on the AhR ligand. In the CUL4BAhR complex, ligand-activated AhR acts as a substrate-specific adaptor component that targets sex steroid receptors for degradation. Thus, our findings uncover a function for AhR as an atypical component of the ubiquitin ligase complex and demonstrate a non-genomic signalling pathway in which fat-soluble ligands regulate target-protein-selective degradation through a ubiquitin ligase complex.

Cytokines suppress adipogenesis and PPAR-gamma function through the TAK1/TAB1/NIK cascade.

Pluripotent mesenchymal stem cells in bone marrow differentiate into adipocytes, osteoblasts and other cells. Balanced cytodifferentiation of stem cells is essential for the formation and maintenance of bone marrow; however, the mechanisms that control this balance remain largely unknown. Whereas cytokines such as interleukin-1 (IL-1) and tumour-necrosis factor-α (TNF-α) inhibit adipogenesis, the ligand-induced transcription factor peroxisome proliferator-activated receptor-γ (PPAR-γ), is a key inducer of adipogenesis. Therefore, regulatory coupling between cytokine- and PPAR-γ-mediated signals might occur during adipogenesis. Here we show that the ligand-induced transactivation function of PPAR-γ is suppressed by IL-1 and TNF-α, and that this suppression is mediated through NF-κB activated by the TAK1/TAB1/NF-κB-inducing kinase (NIK) cascade, a downstream cascade associated with IL-1 and TNF-α signalling. Unlike suppression of the PPAR-γ transactivation function by mitogen-activated protein kinase-induced growth factor signalling through phosphorylation of the A/B domain, NF-κB blocks PPAR-γ binding to DNA by forming a complex with PPAR-γ and its AF-1-specific co-activator PGC-2. Our results suggest that expression of IL-1 and TNF-α in bone marrow may alter the fate of pluripotent mesenchymal stem cells, directing cellular differentiation towards osteoblasts rather than adipocytes by suppressing PPAR-γ function through NF-κB activated by the TAK1/TAB1/NIK cascade.

Wnt and PPARγ signaling in osteoblastogenesis and adipogenesis

Osteoblasts and adipocytes differentiate from a common pluripotent precursor, the mesenchymal stem cell (MSC). Studies have identified numerous transcription factors, and multiple extracellular and intracellular signaling pathways that regulate the closely linked processes of adipogenesis and osteoblastogenesis. Interestingly, inducers of differentiation along one lineage often inhibit differentiation along the other; for example, the transcription factor peroxisome proliferator-activated receptor γ (PPARγ) is a prime inducer of adipogenesis that inhibits osteoblastogenesis. The latest research has shown that inducers of osteoblastogenesis (such as bone morphogenetic protein 2 and Wnt ligands) use different mechanisms to suppress the transactivation function of PPARγ during osteoblastogenesis from MSCs. Signaling via the canonical Wnt–β-catenin pathway inhibits PPARγ mRNA expression, whereas signaling via the noncanonical Wnt pathway results in activation of a histone methyltransferase SETDB1 that represses PPARγ transactivation through histone H3K9 methylation of target genes. This article summarizes Wnt and PPARγ signaling in MSCs and the crosstalk between these pathways, and speculates on future clinical application of this knowledge as the basis of novel approaches for regeneration therapy.

Wnt5a-Ror2 signaling between osteoblast-lineage cells and osteoclast precursors enhances osteoclastogenesis

The signaling molecule Wnt regulates bone homeostasis through β-catenin–dependent canonical and β-catenin–independent noncanonical pathways. Impairment of canonical Wnt signaling causes bone loss in arthritis and osteoporosis; however, it is unclear how noncanonical Wnt signaling regulates bone resorption. Wnt5a activates noncanonical Wnt signaling through receptor tyrosine kinase-like orphan receptor (Ror) proteins. We showed that Wnt5a-Ror2 signaling between osteoblast-lineage cells and osteoclast precursors enhanced osteoclastogenesis. Osteoblast-lineage cells expressed Wnt5a, whereas osteoclast precursors expressed Ror2. Mice deficient in either Wnt5a or Ror2, and those with either osteoclast precursor-specific Ror2 deficiency or osteoblast-lineage cell-specific Wnt5a deficiency showed impaired osteoclastogenesis. Wnt5a-Ror2 signals enhanced receptor activator of nuclear factor-κB (RANK) expression in osteoclast precursors by activating JNK and recruiting c-Jun on the promoter of the gene encoding RANK, thereby enhancing RANK ligand (RANKL)-induced osteoclastogenesis. A soluble form of Ror2 acted as a decoy receptor of Wnt5a and abrogated bone destruction in mouse arthritis models. Our results suggest that the Wnt5a-Ror2 pathway is crucial for osteoclastogenesis in physiological and pathological environments and represents a therapeutic target for bone diseases, including arthritis.

DNA demethylation in hormone-induced transcriptional derepression

Epigenetic modifications at the histone level affect gene regulation in response to extracellular signals. However, regulated epigenetic modifications at the DNA level, especially active DNA demethylation, in gene activation are not well understood. Here we report that DNA methylation/demethylation is hormonally switched to control transcription of the cytochrome p450 27B1 (CYP27B1) gene. Reflecting vitamin-D-mediated transrepression of the CYP27B1 gene by the negative vitamin D response element (nVDRE), methylation of CpG sites (5mCpG) is induced by vitamin D in this gene promoter. Conversely, treatment with parathyroid hormone, a hormone known to activate the CYP27B1 gene, induces active demethylation of the 5mCpG sites in this promoter. Biochemical purification of a complex associated with the nVDRE-binding protein (VDIR, also known as TCF3) identified two DNA methyltransferases, DNMT1 and DNMT3B, for methylation of CpG sites, as well as a DNA glycosylase, MBD4 (ref. 10). Protein-kinase-C-phosphorylated MBD4 by parathyroid hormone stimulation promotes incision of methylated DNA through glycosylase activity, and a base-excision repair process seems to complete DNA demethylation in the MBD4-bound promoter. Such parathyroid-hormone-induced DNA demethylation and subsequent transcriptional derepression are impaired in Mbd4-/- mice. Thus, the present findings suggest that methylation switching at the DNA level contributes to the hormonal control of transcription.

Peroxiredoxin family proteins are key initiators of post-ischemic inflammation in the brain

Post-ischemic inflammation is an essential step in the progression of brain ischemia-reperfusion injury. However, the mechanism that activates infiltrating macrophages in the ischemic brain remains to be clarified. Here we demonstrate that peroxiredoxin (Prx) family proteins released extracellularly from necrotic brain cells induce expression of inflammatory cytokines including interleukin-23 in macrophages through activation of Toll-like receptor 2 (TLR2) and TLR4, thereby promoting neural cell death, even though intracellular Prxs have been shown to be neuroprotective. The extracellular release of Prxs in the ischemic core occurred 12 h after stroke onset, and neutralization of extracellular Prxs with antibodies suppressed inflammatory cytokine expression and infarct volume growth. In contrast, high mobility group box 1 (HMGB1), a well-known damage-associated molecular pattern molecule, was released before Prx and had a limited role in post-ischemic macrophage activation. We thus propose that extracellular Prxs are previously unknown danger signals in the ischemic brain and that its blocking agents are potent neuroprotective tools.

GlcNAcylation of a histone methyltransferase in retinoic-acid-induced granulopoiesis

The post-translational modifications of histone tails generate a ‘histone code’ that defines local and global chromatin states. The resultant regulation of gene function is thought to govern cell fate, proliferation and differentiation. Reversible histone modifications such as methylation are under mutual controls to organize chromosomal events. Among the histone modifications, methylation of specific lysine and arginine residues seems to be critical for chromatin configuration and control of gene expression. Methylation of histone H3 lysine 4 (H3K4) changes chromatin into a transcriptionally active state. Reversible modification of proteins by β-N-acetylglucosamine (O-GlcNAc) in response to serum glucose levels regulates diverse cellular processes. However, the epigenetic impact of protein GlcNAcylation is unknown. Here we report that nuclear GlcNAcylation of a histone lysine methyltransferase (HKMT), MLL5, by O-GlcNAc transferase facilitates retinoic-acid-induced granulopoiesis in human HL60 promyelocytes through methylation of H3K4. MLL5 is biochemically identified in a GlcNAcylation-dependent multi-subunit complex associating with nuclear retinoic acid receptor RARα (also known as RARA), serving as a mono- and di-methyl transferase to H3K4. GlcNAcylation at Thr 440 in the MLL5 SET domain evokes its H3K4 HKMT activity and co-activates RARα in target gene promoters. Increased nuclear GlcNAcylation by means of O-GlcNAc transferase potentiates retinoic-acid-induced HL60 granulopoiesis and restores the retinoic acid response in the retinoic-acid-resistant HL60-R2 cell line. Thus, nuclear MLL5 GlcNAcylation triggers cell lineage determination of HL60 through activation of its HKMT activity.

Suppression of PPAR transactivation switches cell fate of bone marrow stem cells from adipocytes into osteoblasts.

Abstract:  Osteoblasts and adipocytes differentiate from common pleiotropic mesenchymal stem cells under transcriptional controls by numerous factors and multiple intracellular signalings. However, cellular signaling factors that determine cell fates of mensenchymal stem cells in bone marrow remain to be largely uncovered, though peroxisome proliferator‐activated receptor‐γ (PPAR‐γ) is well established as a prime inducer of adipogenesis. Here, we describe two signaling pathways that induce the cell fate decision into osteoblasts from adipocytes. One signaling is a TAK1/TAB1/NIK cascade activated by TNF‐α and IL‐1, and the activated NF‐κB blocked the DNA binding of PPAR‐γ, attenuating the activated PPAR‐mediated adipogenesis. The second signaling is the noncanonical Wnt pathway through CaMKII‐TAK1/TAB2‐NLK. Activated NLK by a noncanonical Wnt ligand (Wnt‐5a) transrepresses PPAR transactivation through a histone methyltransferase, SETDB1. Wnt‐5a induces phosphorylation of NLK, leading to the formation of a corepressor complex that inactivates PPAR function through histone H3‐K9 methylation. Thus, two signaling pathways lead to an osteoblastic cell lineage decision from mesenchymal stem cells through two distinct modes of PPAR transrepression.