The Emergence of the Conserved AAA+ ATPases Pontin and Reptin on the Signaling Landscape

Rosenbaum, Jean,Baek, Sung Hee,Dutta, Anindya,Houry, Walid A.,Huber, Otmar,Hupp, Ted R.,Matias, Pedro M. AAAS 2013 Science signaling Vol.6 No.266

<P>Pontin (also known as RUVBL1 and RVB1) and Reptin (also called RUVBL2 and RVB2) are related members of the large AAA+ (adenosine triphosphatase associated with diverse cellular activities) superfamily of conserved proteins. Various cellular functions depend on Pontin and Reptin, mostly because of their functions in the assembly of protein complexes that play a role in the regulation of cellular energetic metabolism, transcription, chromatin remodeling, and the DNA damage response. Little is known, though, about the interconnections between these multiple functions, how the relevant signaling pathways are regulated, whether the interconnections are affected in human disease, and whether components of these pathways are suitable targets for therapeutic intervention. The First International Workshop on Pontin (RUVBL1) and Reptin (RUVBL2), held between 16 and 19 October 2012, discussed the nature of the oligomeric organization of these proteins, their structures, their roles as partners in various protein complexes, and their involvement in cellular regulation, signaling, and pathophysiology, as well as their potential for therapeutic targeting. A major outcome of the meeting was a general consensus that most functions of Pontin and Reptin are related to their roles as chaperones or adaptor proteins that are important for the assembly and function of large signaling protein complexes.</P>

The Scaffold Protein Prohibitin Is Required for Antigen-Stimulated Signaling in Mast Cells

Kim, Do Kyun,Kim, Hyuk Soon,Kim, A-Ram,Jang, Geun Hyo,Kim, Hyun Woo,Park, Young Hwan,Kim, Bokyung,Park, Yeong Min,Beaven, Michael A.,Kim, Young Mi,Choi, Wahn Soo AAAS 2013 Science signaling Vol.6 No.292

<P><B>How to Prohibit Mast Cell Activation</B></P><P>Mast cells are the major effector cells of the allergic response. Binding of antigen to immunoglobulin E (IgE) molecules bound to the cell-surface, high-affinity IgE receptor FcεRI results in receptor clustering, mast cell activation, and degranulation, resulting in the release of factors that mediate the allergic response. Phosphorylation and activation of the Src family kinases Lyn and Syk are critical for mast cell activation. Kim <I>et al</I>. found that the scaffold protein prohibitin (PHB), which mediates functions such as mitochondrial biogenesis and transcriptional regulation (see commentary by Yurugi and Rajalingam), was abundant in intracellular granules in mouse mast cells. Stimulation of mast cells with antigen resulted in the translocation of PHB to plasma membrane lipid rafts, which was required for the association of FcεRI with Syk, the activation of Syk, and degranulation. Knockdown of PHB in mice inhibited mast cell activation and anaphylaxis, suggesting that targeting PHB therapeutically may reduce allergic responses.</P>

A Systems Approach for Decoding Mitochondrial Retrograde Signaling Pathways

Chae, Sehyun,Ahn, Byung Yong,Byun, Kyunghee,Cho, Young Min,Yu, Myeong-Hee,Lee, Bonghee,Hwang, Daehee,Park, Kyong Soo AAAS 2013 Science signaling Vol.6 No.264

<P><B>Signaling Mitochondrial Dysfunction</B></P><P>The mitochondrial and nuclear genomes contribute to mitochondrial function, and when mitochondrial function is compromised, mitochondrial retrograde signaling alters nuclear gene expression. Chae <I>et al</I>. performed gene expression profiling of engineered cells that had mitochondria containing a disease-associated mutation that causes mitochondrial dysfunction. By generating networks of transcription factors that targeted these genes, the authors revealed putative mitochondrial retrograde signaling pathways. One such pathway involved retinoic X receptor α (RXRA), the mRNA for which was reduced in the mutant cells. Network analysis and experiments in cells suggested that mitochondrial dysfunction caused by the mutation initiated a positive feedback loop that aggravated mitochondrial dysfunction: Reduced RXRA abundance further compromised expression of genes encoding products involved in mitochondrial function and translation. This gene transcription factor mapping network approach may reveal targets for therapeutic intervention of diseases associated with mitochondrial dysfunction.</P>

Jak-TGFβ cross-talk links transient adipose tissue inflammation to beige adipogenesis

Babaei, Rohollah,Schuster, Maximilian,Meln, Irina,Lerch, Sarah,Ghandour, Rayane A.,Pisani, Didier F.,Bayindir-Buchhalter, Irem,Marx, Julia,Wu, Shuang,Schoiswohl, Gabriele,Billeter, Adrian T.,Krunic, D AAAS 2018 Science signaling Vol.11 No.527

<P>The transient activation of inflammatory networks is required for adipose tissue remodeling including the 'browning' of white fat in response to stimuli such as. 3-adrenergic receptor activation. In this process, white adipose tissue acquires thermogenic characteristics through the recruitment of so-called beige adipocytes. We investigated the downstream signaling pathways impinging on adipocyte progenitors that promote de novo formation of adipocytes. We showed that the Jak family of kinases controlled TGF beta signaling in the adipose tissue microenvironment through Stat3 and thereby adipogenic commitment, a function that was required for beige adipocyte differentiation of murine and human progenitors. Jak/Stat3 inhibited TGF beta signaling to the transcription factors Srf and Smad3 by repressing local Tgfb3 and Tgfb1 expression before the core transcriptional adipogenic cascade was activated. This pathway cross-talk was triggered in stromal cells by ATGL-dependent adipocyte lipolysis and a transient wave of IL-6 family cytokines at the onset of adipose tissue remodeling induced by beta 3-adrenergic receptor stimulation. Our results provide insight into the activation of adipocyte progenitors and are relevant for the therapeutic targeting of adipose tissue inflammatory pathways.</P>

An Increase in Synaptic NMDA Receptors in the Insular Cortex Contributes to Neuropathic Pain

Qiu, Shuang,Chen, Tao,Koga, Kohei,Guo, Yan-yan,Xu, Hui,Song, Qian,Wang, Jie-jie,Descalzi, Giannina,Kaang, Bong-Kiun,Luo, Jian-hong,Zhuo, Min,Zhao, Ming-gao AAAS 2013 Science signaling Vol.6 No.275

<P><B>Stopping the Pain</B></P><P>Damage to the central or peripheral nervous system can trigger the development of neuropathic pain, which can manifest as painful sensations in response to stimuli that are not normally painful. Qiu <I>et al.</I> found that mice that had developed neuropathic pain after peripheral nerve injury showed changes in synaptic plasticity and increased abundance of synaptic NMDA receptors in the insular cortex, a region of the brain that is activated by acute and chronic pain. Using pharmacological inhibitors and transgenic mice, they mimicked these changes in vitro with insular cortical slices and thus identified the signaling pathway responsible. Mice injected with NMDA receptor inhibitors showed reduced behavioral signs of neuropathic pain after peripheral nerve injury. Thus, blocking NMDA receptor function in the insular cortex may prevent the development of neuropathic pain.</P>

Excitatory neuron–specific SHP2-ERK signaling network regulates synaptic plasticity and memory

Ryu, Hyun-Hee,Kim, TaeHyun,Kim, Jung-Woong,Kang, Minkyung,Park, Pojeong,Kim, Yong Gyu,Kim, Hyopil,Ha, Jiyeon,Choi, Ja Eun,Lee, Jisu,Lim, Chae-Seok,Kim, Chul-Hong,Kim, Sang Jeong,Silva, Alcino J.,Kaang AAAS 2019 Science signaling Vol.12 No.571

<P><B>Cell type–specific RASopathy</B></P><P>The neurodevelopmental disorder Noonan syndrome is often caused by activating mutations in the phosphatase SHP2 that enhance RAS signaling. However, SHP2 is present in multiple neuron types as well as glia; thus, where the mutant protein has its pathological effects is unclear. Ryu <I>et al.</I> examined one NS-associated SHP2 mutation in isolated cell types from mice and determined that its presence in only excitatory neurons resulted in electrophysiological and cognitive effects. This was because certain adaptor proteins that interact with SHP2 to mediate RAS signaling are abundant in excitatory but not inhibitory neurons. These findings reveal that cell type–specific variations within the RAS signaling network underlie the phenotypes of NS and possibly other “RASopathies”.</P><P>Mutations in RAS signaling pathway components cause diverse neurodevelopmental disorders, collectively called RASopathies. Previous studies have suggested that dysregulation in RAS–extracellular signal–regulated kinase (ERK) activation is restricted to distinct cell types in different RASopathies. Some cases of Noonan syndrome (NS) are associated with gain-of-function mutations in the phosphatase SHP2 (encoded by <I>PTPN11</I>); however, SHP2 is abundant in multiple cell types, so it is unclear which cell type(s) contribute to NS phenotypes. Here, we found that expressing the NS-associated mutant SHP2<SUP>D61G</SUP> in excitatory, but not inhibitory, hippocampal neurons increased ERK signaling and impaired both long-term potentiation (LTP) and spatial memory in mice, although endogenous SHP2 was expressed in both neuronal types. Transcriptomic analyses revealed that the genes encoding SHP2-interacting proteins that are critical for ERK activation, such as GAB1 and GRB2, were enriched in excitatory neurons. Accordingly, expressing a dominant-negative mutant of GAB1, which reduced its interaction with SHP2<SUP>D61G</SUP>, selectively in excitatory neurons, reversed SHP2<SUP>D61G</SUP>-mediated deficits. Moreover, ectopic expression of GAB1 and GRB2 together with SHP2<SUP>D61G</SUP> in inhibitory neurons resulted in ERK activation. These results demonstrate that RAS-ERK signaling networks are notably different between excitatory and inhibitory neurons, accounting for the cell type–specific pathophysiology of NS and perhaps other RASopathies.</P>

Interleukin-10–producing CD5⁺ B cells inhibit mast cells during immunoglobulin E–mediated allergic responses

Kim, Hyuk Soon,Kim, A-Ram,Kim, Do Kyun,Kim, Hyun Woo,Park, Young Hwan,Jang, Geun Hyo,Kim, Bokyung,Park, Yeong Min,You, Jueng Soo,Kim, Hyung Sik,Beaven, Michael A.,Kim, Young Mi,Choi, Wahn Soo AAAS 2015 Science signaling Vol.8 No.368

<P>Subsets of B cells inhibit various immune responses through their production of the cytokine interleukin-10 (IL-10). We found that IL-10-producing CD5(+) B cells suppressed the immunoglobulin E (IgE)- and antigen-mediated activation of mast cells in vitro as well as allergic responses in mice in an IL-10-dependent manner. Furthermore, the suppressive effect of these B cells on mast cells in vitro and in vivo depended on direct cell-to-cell contact through the costimulatory receptor CD40 on CD5(+) B cells and the CD40 ligand on mast cells. This contact enhanced the production of IL-10 by the CD5(+) B cells. Through activation of the Janus-activated kinase-signal transducer and activator of transcription 3 pathway, IL-10 decreased the abundance of the kinases Fyn and Fgr and inhibited the activation of the downstream kinase Syk in mast cells. Together, these findings suggest that an important function of IL-10-producing CD5(+) B cells is inhibiting mast cells and IgE-mediated allergic responses.</P>

A Network of Substrates of the E3 Ubiquitin Ligases MDM2 and HUWE1 Control Apoptosis Independently of p53

Kurokawa, Manabu,Kim, Jiyeon,Geradts, Joseph,Matsuura, Kenkyo,Liu, Liu,Ran, Xu,Xia, Wenle,Ribar, Thomas J.,Henao, Ricardo,Dewhirst, Mark W.,Kim, Wun-Jae,Lucas, Joseph E.,Wang, Shaomeng,Spector, Neil L AAAS 2013 Science signaling Vol.6 No.274

<P><B>Breaking Down to Build Resistance</B></P><P>Chemotherapeutic resistance often arises because of the rewiring of signaling pathways in cancer cells. Kurokawa <I>et al.</I> found that the ubiquitin E3 ligase MDM2 triggered the breakdown of another ubiquitin E3 ligase, HUWE1. In breast cancer cells that died when exposed to the HER2 (human epidermal growth factor receptor 2) EGFR (epidermal growth factor receptor) tyrosine kinase inhibitor lapatinib, MDM2 was degraded, which enabled HUWE1 to trigger the degradation of a prosurvival protein and promote assembly and activation of a protein complex required for the execution of cell death. However, MDM2 degradation did not occur in lapatinib-resistant breast cancer cells, and thus, the abundance of HUWE1 was decreased, promoting cell survival. In a mouse xenograft model, an inhibitor of MDM2 reduced the growth of tumors generated from lapatinib-resistant breast cancer cells. Thus, MDM2 could be targeted to circumvent resistance to lapatinib in breast cancers.</P>

Arginine Methylation of CRTC2 Is Critical in the Transcriptional Control of Hepatic Glucose Metabolism

Han, Hye-Sook,Jung, Chang-Yun,Yoon, Young-Sil,Choi, Seri,Choi, Dahee,Kang, Geon,Park, Keun-Gyu,Kim, Seong-Tae,Koo, Seung-Hoi AAAS 2014 Science signaling Vol.7 No.314

<P><B>Getting a Sugar Rush from Arginine Methylation</B></P><P>Fasting causes the liver to produce and release glucose into the bloodstream, a process called gluconeogenesis. Fasting triggers the transcriptional activation of genes encoding gluconeogenic enzymes by CREB (cAMP response element–binding protein) and its coactivator CRTC2 (CREB-regulated transcription coactivator 2). Han <I>et al.</I> found that CRTC2 was targeted by the arginine methyltransferase PRMT6. In cells, arginine methylation of CRTC2 enhanced the association of CRTC2 with CREB and increased CREB activity. In mice, overexpression of PRMT6 increased the expression of genes encoding gluconeogenic enzymes and blood glucose concentrations, effects that required CRTC2. Obesity or insulin resistance leads to higher blood glucose concentrations, and acute deletion of PRMT6 restored blood glucose concentrations to normal in mice that were obese or insulin-resistant either through genetics or diet. Thus, hyperglycemia triggered by obesity or diet could be controlled by inhibiting the arginine methylation of CRTC2 by PRMT6.</P>

Mer signaling increases the abundance of the transcription factor LXR to promote the resolution of acute sterile inflammation

Choi, Ji-Yeon,Seo, Jeong Yeon,Yoon, Young-So,Lee, Ye-Ji,Kim, Hee-Sun,Kang, Jihee Lee AAAS 2015 Science signaling Vol.8 No.365

<P><B>Resolving inflammation with Mer</B></P><P>Unchecked inflammatory responses in the body can do more harm than good; thus, the body has mechanisms for resolving inflammation and restoring normal tissue function. Choi <I>et al</I>. found that mice with impaired signaling by the receptor tyrosine kinase Mer had exacerbated inflammation in response to acute sterile tissue damage, as well as decreased abundance of liver X receptor (LXR) transcription factors (which are implicated in inhibiting macrophage responses) in their macrophages, spleens, and lungs. Conversely, treating macrophages in culture with an agonist of Mer increased LXR abundance and activity. Furthermore, an LXR agonist reduced inflammation in mice even in the context of reduced Mer signaling. The finding that Mer-dependent increases in LXR abundance lead to the resolution of inflammation might be exploited therapeutically.</P><P>The receptor tyrosine kinase Mer plays a central role in inhibiting the inflammatory response of immune cells to pathogens. We aimed to understand the function of Mer signaling in the resolution of sterile inflammation in experiments with a Mer-neutralizing antibody or with Mer-deficient (<I>Mer</I><SUP>−/−</SUP>) mice in a model of sterile, zymosan-induced acute inflammation. We found that inhibition or deficiency of Mer enhanced local and systemic inflammatory responses. The exacerbated inflammatory responses induced by the lack of Mer signaling were associated with reduced abundance of the transcription factors liver X receptor α (LXRα) and LXRβ and decreased expression of their target genes in peritoneal macrophages, spleens, and lungs. Similarly, treatment of mice with a Mer/Fc fusion protein, which prevents the Mer ligand Gas6 (growth arrest–specific protein 6) from binding to Mer, exacerbated the inflammatory response and decreased the abundance of LXR. Coadministration of the LXR agonist T0901317 with the Mer-neutralizing antibody inhibited the aggravating effects of the antibody on inflammation in mice. In vitro exposure of RAW264.7 cells or primary peritoneal macrophages to Gas6 increased LXR abundance in an Akt-dependent manner. Thus, we have elucidated a previously uncharacterized pathway involved in the resolution of acute sterile inflammation: Enhanced Mer signaling during the recovery phase increases the abundance and activity of LXR to inactivate the inflammatory response in macrophages.</P>