Ultrafast distant wound response is essential for whole-body regeneration.

Injury induces systemic responses, but their functions remain elusive. Mechanisms that can rapidly synchronize wound responses through long distances are also mostly unknown. Using planarian flatworms capable of whole-body regeneration, we report that injury induces extracellular signal-regulated kinase (Erk) activity waves to travel at a speed 10-100 times faster than those in other multicellular tissues. This ultrafast propagation requires longitudinal body-wall muscles, elongated cells forming dense parallel tracks running the length of the organism. The morphological properties of muscles allow them to act as superhighways for propagating and disseminating wound signals. Inhibiting Erk propagation prevents tissues distant to the wound from responding and blocks regeneration, which can be rescued by a second injury to distal tissues shortly after the first injury. Our findings provide a mechanism for long-range signal propagation in large, complex tissues to coordinate responses across cell types and highlight the function of feedback between spatially separated tissues during whole-body regeneration.

GPR146 Deficiency Protects against Hypercholesterolemia and Atherosclerosis.

Although human genetic studies have implicated many susceptible genes associated with plasma lipid levels, their physiological and molecular functions are not fully characterized. Here we demonstrate that orphan G protein-coupled receptor 146 (GPR146) promotes activity of hepatic sterol regulatory element binding protein 2 (SREBP2) through activation of the extracellular signal-regulated kinase (ERK) signaling pathway, thereby regulating hepatic very low-density lipoprotein (VLDL) secretion, and subsequently circulating low-density lipoprotein cholesterol (LDL-C) and triglycerides (TG) levels. Remarkably, GPR146 deficiency reduces plasma cholesterol levels substantially in both wild-type and LDL receptor (LDLR)-deficient mice. Finally, aortic atherosclerotic lesions are reduced by 90% and 70%, respectively, in male and female LDLR-deficient mice upon GPR146 depletion. Taken together, these findings outline a regulatory role for the GPR146/ERK axis in systemic cholesterol metabolism and suggest that GPR146 inhibition could be an effective strategy to reduce plasma cholesterol levels and atherosclerosis.

A transient transcriptional activation governs unpolarized-to-polarized morphogenesis during embryo implantation.

During implantation, embryos undergo an unpolarized-to-polarized transition to initiate postimplantation morphogenesis. However, the underlying molecular mechanism is unknown. Here, we identify a transient transcriptional activation governing embryonic morphogenesis and pluripotency transition during implantation. In naive pluripotent embryonic stem cells (ESCs), which represent preimplantation embryos, we find that the microprocessor component DGCR8 can recognize stem-loop structures within nascent mRNAs to sequester transcriptional coactivator FLII to suppress transcription directly. When mESCs exit from naive pluripotency, the ERK/RSK/P70S6K pathway rapidly activates, leading to FLII phosphorylation and disruption of DGCR8/FLII interaction. Phosphorylated FLII can bind to transcription factor JUN, activating cell migration-related genes to establish poised pluripotency akin to implanting embryos. Resequestration of FLII by DGCR8 drives poised ESCs into formative pluripotency. In summary, we identify a DGCR8/FLII/JUN-mediated transient transcriptional activation mechanism. Disruption of this mechanism inhibits naive-poised-formative pluripotency transition and the corresponding unpolarized-to-polarized transition during embryo implantation, which are conserved in mice and humans.

Costimulation molecules differentially regulate the ERK-Zfp831 axis to shape T follicular helper cell differentiation.

T follicular helper (Tfh) cells play essential roles in regulating humoral immunity, especially germinal center reactions. However, how CD4+ T cells integrate the antigenic and costimulatory signals in Tfh cell development is still poorly understood. Here, we found that phorbol 12-myristate 13-acetate (PMA) + ionomycin (P+I) stimulation, together with interleukin-6 (IL-6), potently induce Tfh cell-like transcriptomic programs in vitro. The ERK kinase pathway was attenuated under P+I stimulation; ERK2 inhibition enhanced Tfh cell development in vitro and in vivo. We observed that inducible T cell costimulator (ICOS), but not CD28, lacked the ability to activate ERK, which was important in sustaining Tfh cell development. The transcription factor Zfp831, whose expression was repressed by ERK, promoted Tfh cell differentiation by directly upregulating the expression of the transcription factors Bcl6 and Tcf7. We have hence identified an ERK-Zfp831 axis, regulated by costimulation signaling, in critical regulation of Tfh cell development.

The TGF-ß superfamily cytokine Activin-A is induced during autoimmune neuroinflammation and drives pathogenic Th17 cell differentiation.

Th17 cells are known to exert pathogenic and non-pathogenic functions. Although the cytokine transforming growth factor ß1 (TGF-ß1) is instrumental for Th17 cell differentiation, it is dispensable for generation of pathogenic Th17 cells. Here, we examined the T cell-intrinsic role of Activin-A, a TGF-ß superfamily member closely related to TGF-ß1, in pathogenic Th17 cell differentiation. Activin-A expression was increased in individuals with relapsing-remitting multiple sclerosis and in mice with experimental autoimmune encephalomyelitis. Stimulation with interleukin-6 and Activin-A induced a molecular program that mirrored that of pathogenic Th17 cells and was inhibited by blocking Activin-A signaling. Genetic disruption of Activin-A and its receptor ALK4 in T cells impaired pathogenic Th17 cell differentiation in vitro and in vivo. Mechanistically, extracellular-signal-regulated kinase (ERK) phosphorylation, which was essential for pathogenic Th17 cell differentiation, was suppressed by TGF-ß1-ALK5 but not Activin-A-ALK4 signaling. Thus, Activin-A drives pathogenic Th17 cell differentiation, implicating the Activin-A-ALK4-ERK axis as a therapeutic target for Th17 cell-related diseases.

ARAF protein kinase activates RAS by antagonizing its binding to RASGAP NF1.

RAF protein kinases are effectors of the GTP-bound form of small guanosine triphosphatase RAS and function by phosphorylating MEK. We showed here that the expression of ARAF activated RAS in a kinase-independent manner. Binding of ARAF to RAS displaced the GTPase-activating protein NF1 and antagonized NF1-mediated inhibition of RAS. This reduced ERK-dependent inhibition of RAS and increased RAS-GTP. By this mechanism, ARAF regulated the duration and consequences of RTK-induced RAS activation and supported the RAS output of RTK-dependent tumor cells. In human lung cancers with EGFR mutation, amplification of ARAF was associated with acquired resistance to EGFR inhibitors, which was overcome by combining EGFR inhibitors with an inhibitor of the protein tyrosine phosphatase SHP2 to enhance inhibition of nucleotide exchange and RAS activation.

Lyso-PAF, a biologically inactive phospholipid, contributes to RAF1 activation.

Phospholipase A2, group VII (PLA2G7) is widely recognized as a secreted, lipoprotein-associated PLA2 in plasma that converts phospholipid platelet-activating factor (PAF) to a biologically inactive product Lyso-PAF during inflammatory response. We report that intracellular PLA2G7 is selectively important for cell proliferation and tumor growth potential of melanoma cells expressing mutant NRAS, but not cells expressing BRAF V600E. Mechanistically, PLA2G7 signals through its product Lyso-PAF to contribute to RAF1 activation by mutant NRAS, which is bypassed by BRAF V600E. Intracellular Lyso-PAF promotes p21-activated kinase 2 (PAK2) activation by binding to its catalytic domain and altering ATP kinetics, while PAK2 significantly contributes to S338-phosphorylation of RAF1 in addition to PAK1. Furthermore, the PLA2G7-PAK2 axis is also required for full activation of RAF1 in cells stimulated by epidermal growth factor (EGF) or cancer cells expressing mutant KRAS. Thus, PLA2G7 and Lyso-PAF exhibit intracellular signaling functions as key elements of RAS-RAF1 signaling.

Stabilization of ERK-Phosphorylated METTL3 by USP5 Increases m6A Methylation.

N6-methyladenosine (m6A) is the most abundant mRNA modification and is installed by the METTL3-METTL14-WTAP methyltransferase complex. Although the importance of m6A methylation in mRNA metabolism has been well documented recently, regulation of the m6A machinery remains obscure. Through a genome-wide CRISPR screen, we identify the ERK pathway and USP5 as positive regulators of the m6A deposition. We find that ERK phosphorylates METTL3 at S43/S50/S525 and WTAP at S306/S341, followed by deubiquitination by USP5, resulting in stabilization of the m6A methyltransferase complex. Lack of METTL3/WTAP phosphorylation reduces decay of m6A-labeled pluripotent factor transcripts and traps mouse embryonic stem cells in the pluripotent state. The same phosphorylation can also be found in ERK-activated human cancer cells and contribute to tumorigenesis. Our study reveals an unrecognized function of ERK in regulating m6A methylation.

ERK2 Phosphorylates PFAS to Mediate Posttranslational Control of De Novo Purine Synthesis.

The RAS-ERK/MAPK (RAS-extracellular signal-regulated kinase/mitogen-activated protein kinase) pathway integrates growth-promoting signals to stimulate cell growth and proliferation, at least in part, through alterations in metabolic gene expression. However, examples of direct and rapid regulation of the metabolic pathways by the RAS-ERK pathway remain elusive. We find that physiological and oncogenic ERK signaling activation leads to acute metabolic flux stimulation through the de novo purine synthesis pathway, thereby increasing building block availability for RNA and DNA synthesis, which is required for cell growth and proliferation. We demonstrate that ERK2, but not ERK1, phosphorylates the purine synthesis enzyme PFAS (phosphoribosylformylglycinamidine synthase) at T619 in cells to stimulate de novo purine synthesis. The expression of nonphosphorylatable PFAS (T619A) decreases purine synthesis, RAS-dependent cancer cell-colony formation, and tumor growth. Thus, ERK2-mediated PFAS phosphorylation facilitates the increase in nucleic acid synthesis required for anabolic cell growth and proliferation.

FGF signaling regulates development by processes beyond canonical pathways.

FGFs are key developmental regulators that engage a signal transduction cascade through receptor tyrosine kinases, prominently engaging ERK1/2 but also other pathways. However, it remains unknown whether all FGF activities depend on this canonical signal transduction cascade. To address this question, we generated allelic series of knock-in Fgfr1 and Fgfr2 mouse strains, carrying point mutations that disrupt binding of signaling effectors, and a kinase dead allele of Fgfr2 that broadly phenocopies the null mutant. When interrogated in cranial neural crest cells, we identified discrete functions for signaling pathways in specific craniofacial contexts, but point mutations, even when combined, failed to recapitulate the single or double null mutant phenotypes. Furthermore, the signaling mutations abrogated established FGF-induced signal transduction pathways, yet FGF functions such as cell-matrix and cell-cell adhesion remained unaffected, though these activities did require FGFR kinase activity. Our studies establish combinatorial roles of Fgfr1 and Fgfr2 in development and uncouple novel FGFR kinase-dependent cell adhesion properties from canonical intracellular signaling.

The adrenergic-induced ERK3 pathway drives lipolysis and suppresses energy dissipation.

Obesity-induced diabetes affects >400 million people worldwide. Uncontrolled lipolysis (free fatty acid release from adipocytes) can contribute to diabetes and obesity. To identify future therapeutic avenues targeting this pathway, we performed a high-throughput screen and identified the extracellular-regulated kinase 3 (ERK3) as a hit. We demonstrated that ß-adrenergic stimulation stabilizes ERK3, leading to the formation of a complex with the cofactor MAP kinase-activated protein kinase 5 (MK5), thereby driving lipolysis. Mechanistically, we identified a downstream target of the ERK3/MK5 pathway, the transcription factor FOXO1, which promotes the expression of the major lipolytic enzyme ATGL. Finally, we provide evidence that targeted deletion of ERK3 in mouse adipocytes inhibits lipolysis, but elevates energy dissipation, promoting lean phenotype and ameliorating diabetes. Thus, ERK3/MK5 represents a previously unrecognized signaling axis in adipose tissue and an attractive target for future therapies aiming to combat obesity-induced diabetes.

ERK signaling expands mammalian cortical radial glial cells and extends the neurogenic period.

The molecular basis for cortical expansion during evolution remains largely unknown. Here, we report that fibroblast growth factor (FGF)-extracellular signal-regulated kinase (ERK) signaling promotes the self-renewal and expansion of cortical radial glial (RG) cells. Furthermore, FGF-ERK signaling induces bone morphogenic protein 7 (Bmp7) expression in cortical RG cells, which increases the length of the neurogenic period. We demonstrate that ERK signaling and Sonic Hedgehog (SHH) signaling mutually inhibit each other in cortical RG cells. We provide evidence that ERK signaling is elevated in cortical RG cells during development and evolution. We propose that the expansion of the mammalian cortex, notably in human, is driven by the ERK-BMP7-GLI3R signaling pathway in cortical RG cells, which participates in a positive feedback loop through antagonizing SHH signaling. We also propose that the relatively short cortical neurogenic period in mice is partly due to mouse cortical RG cells receiving higher SHH signaling that antagonizes ERK signaling.

The expression of congenital Shoc2 variants induces AKT-dependent crosstalk activation of the ERK1/2 pathway.

The Shoc2 scaffold protein is crucial in transmitting signals within the Epidermal Growth Factor Receptor (EGFR)-mediated Extracellular signal-Regulated Kinase (ERK1/2) pathway. While the significance of Shoc2 in this pathway is well-established, the precise mechanisms through which Shoc2 governs signal transmission remain to be fully elucidated. Hereditary variants in Shoc2 are responsible for Noonan Syndrome with Loose anagen Hair (NSLH). However, due to the absence of known enzymatic activity in Shoc2, directly assessing how these variants affect its function is challenging. ERK1/2 phosphorylation is used as a primary parameter of Shoc2 function, but the impact of Shoc2 mutants on the pathway activation is unclear. This study investigates how the NSLH-associated Shoc2 variants influence EGFR signals in the context of the ERK1/2 and AKT downstream signaling pathways. We show that when the ERK1/2 pathway is a primary signaling pathway activated downstream of EGFR, Shoc2 variants cannot upregulate ERK1/2 phosphorylation to the level of the WT Shoc2. Yet, when the AKT and ERK1/2 pathways were activated, in cells expressing Shoc2 variants, ERK1/2 phosphorylation was higher than in cells expressing WT Shoc2. In cells expressing the Shoc2 NSLH mutants, we found that the AKT signaling pathway triggers the PAK activation, followed by phosphorylation of Raf-1/MEK1/2 and activation of the ERK1/2 signaling axis. Hence, our studies reveal a previously unrecognized feedback regulation downstream of the EGFR and provide additional evidence for the role of Shoc2 as a "gatekeeper" in controlling the selection of downstream effectors within the EGFR signaling network.

USP33 deubiquitinates and stabilizes HIF-2alpha to promote hypoxia response in glioma stem cells.

Hypoxia regulates tumor angiogenesis, metabolism, and therapeutic response in malignant cancers including glioblastoma, the most lethal primary brain tumor. The regulation of HIF transcriptional factors by the ubiquitin-proteasome system is critical in the hypoxia response, but hypoxia-inducible deubiquitinases that counteract the ubiquitination remain poorly defined. While the activation of ERK1/2 also plays an important role in hypoxia response, the relationship between ERK1/2 activation and HIF regulation remains elusive. Here, we identified USP33 as essential deubiquitinase that stabilizes HIF-2alpha protein in an ERK1/2-dependent manner to promote hypoxia response in cancer cells. USP33 is preferentially induced in glioma stem cells by hypoxia and interacts with HIF-2alpha, leading to its stabilization through deubiquitination. The activation of ERK1/2 upon hypoxia promoted HIF-2alpha phosphorylation, enhancing its interaction with USP33. Silencing of USP33 disrupted glioma stem cells maintenance, reduced tumor vascularization, and inhibited glioblastoma growth. Our findings highlight USP33 as an essential regulator of hypoxia response in cancer stem cells, indicating a novel potential therapeutic target for brain tumor treatment.

Sclerotome-derived PDGF signaling functions as a niche cue responsible for primitive erythropoiesis.

Primitive erythropoiesis serves a vital role in embryonic development, generating primitive red blood cells responsible for transportation of oxygen throughout the body. Although diverse niche factors are known to function in definitive hematopoiesis, the microenvironment contributing to primitive hematopoiesis remains largely elusive. Here, we report that platelet-derived growth factor (PDGF) signaling is required for erythroid progenitor differentiation in zebrafish. Ablating pdgfαa (also known as pdgfaa) and pdgfαb (also known as pdgfab) or blocking PDGF signaling with an inhibitor impairs erythroid progenitor differentiation, thus resulting in a significant decrease in the number of erythrocytes. We reveal that pdgfαb is expressed in sclerotomal cells, and that its receptor genes, pdgfra and pdgfrb, are expressed in the adjacent erythroid progenitor cells. Sclerotome-specific overexpression of pdgfαb effectively restores primitive erythropoiesis in pdgfαa-/-;pdgfαb-/- mutant embryos. In addition, we have defined ERK1/2 signaling as a downstream pathway of PDGF signaling during embryonic erythropoiesis. Taken together, our findings indicate that PDGF signaling derived from sclerotome functions as a niche cue for primitive erythropoiesis.

Dynamics of an incoherent feedforward loop drive ERK-dependent pattern formation in the early Drosophila embryo.

Positional information in development often manifests as stripes of gene expression, but how stripes form remains incompletely understood. Here, we use optogenetics and live-cell biosensors to investigate the posterior brachyenteron (byn) stripe in early Drosophila embryos. This stripe depends on interpretation of an upstream ERK activity gradient and the expression of two target genes, tailless (tll) and huckebein (hkb), that exert antagonistic control over byn. We find that high or low doses of ERK signaling produce transient or sustained byn expression, respectively. Although tll transcription is always rapidly induced, hkb converts graded ERK inputs into a variable time delay. Nuclei thus interpret ERK amplitude through the relative timing of tll and hkb transcription. Antagonistic regulatory paths acting on different timescales are hallmarks of an incoherent feedforward loop, which is sufficient to explain byn dynamics and adds temporal complexity to the steady-state model of byn stripe formation. We further show that 'blurring' of an all-or-none stimulus through intracellular diffusion non-locally produces a byn stripe. Overall, we provide a blueprint for using optogenetics to dissect developmental signal interpretation in space and time.

A Loss of Epigenetic Control Can Promote Cell Death through Reversing the Balance of Pathways in a Signaling Network.

Epigenetic control of regulatory networks is only partially understood. Expression of Insulin-like growth factor-II (IGF2) is controlled by genomic imprinting, mediated by silencing of the maternal allele. Loss of imprinting of IGF2 (LOI) is linked to intestinal and colorectal cancers, causally in murine models and epidemiologically in humans. However, the molecular underpinnings of the LOI phenotype are not clear. Surprisingly, in LOI cells, we find a reversal of the relative activities of two canonical signaling pathways triggered by IGF2, causing further rebalancing between pro- and anti-apoptotic signaling. A predictive mathematical model shows that this network rebalancing quantitatively accounts for the effect of receptor tyrosine kinase inhibition in both WT and LOI cells. This mechanism also quantitatively explains both the stable LOI phenotype and the therapeutic window for selective killing of LOI cells, and thus prevention of epigenetically controlled cancers. These findings suggest a framework for understanding epigenetically modified cell signaling.

Nuclear PGK1 Alleviates ADP-Dependent Inhibition of CDC7 to Promote DNA Replication.

DNA replication is initiated by assembly of the kinase cell division cycle 7 (CDC7) with its regulatory activation subunit, activator of S-phase kinase (ASK), to activate DNA helicase. However, the mechanism underlying regulation of CDC7-ASK complex is unclear. Here, we show that ADP generated from CDC7-mediated MCM phosphorylation binds to an allosteric region of CDC7, disrupts CDC7-ASK interaction, and inhibits CDC7-ASK activity in a feedback way. EGFR- and ERK-activated casein kinase 2α (CK2α) phosphorylates nuclear phosphoglycerate kinase (PGK) 1 at S256, resulting in interaction of PGK1 with CDC7. CDC7-bound PGK1 converts ADP to ATP, thereby abrogating the inhibitory effect of ADP on CDC7-ASK activity, promoting the recruitment of DNA helicase to replication origins, DNA replication, cell proliferation, and brain tumorigenesis. These findings reveal an instrumental self-regulatory mechanism of CDC7-ASK activity by its kinase reaction product ADP and a nonglycolytic role for PGK1 in abrogating this negative feedback in promoting tumor development.

Visualizing Dynamics of Cell Signaling In Vivo with a Phase Separation-Based Kinase Reporter.

Visualizing dynamics of kinase activity in living animals is essential for mechanistic understanding of cell and developmental biology. We describe GFP-based kinase reporters that phase-separate upon kinase activation via multivalent protein-protein interactions, forming intensively fluorescent droplets. Called SPARK (separation of phases-based activity reporter of kinase), these reporters have large dynamic range (fluorescence change), high brightness, fast kinetics, and are reversible. The SPARK-based protein kinase A (PKA) reporter reveals oscillatory dynamics of PKA activities upon G protein-coupled receptor activation. The SPARK-based extracellular signal-regulated kinase (ERK) reporter unveils transient dynamics of ERK activity during tracheal metamorphosis in live Drosophila. Because of intensive brightness and simple signal pattern, SPARKs allow easy examination of kinase signaling in living animals in a qualitative way. The modular design of SPARK will facilitate development of reporters of other kinases.

ERK and c-Myc signaling in host-derived tumor endothelial cells is essential for solid tumor growth.

The limited efficacy of the current antitumor microenvironment strategies is due in part to the poor understanding of the roles and relative contributions of the various tumor stromal cells to tumor development. Here, we describe a versatile in vivo anthrax toxin protein delivery system allowing for the unambiguous genetic evaluation of individual tumor stromal elements in cancer. Our reengineered tumor-selective anthrax toxin exhibits potent antiproliferative activity by disrupting ERK signaling in sensitive cells. Since this activity requires the surface expression of the capillary morphogenesis protein-2 (CMG2) toxin receptor, genetic manipulation of CMG2 expression using our cell-type-specific CMG2 transgenic mice allows us to specifically define the role of individual tumor stromal cell types in tumor development. Here, we established mice with CMG2 only expressed in tumor endothelial cells (ECs) and determined the specific contribution of tumor stromal ECs to the toxin's antitumor activity. Our results demonstrate that disruption of ERK signaling only within tumor ECs is sufficient to halt tumor growth. We discovered that c-Myc is a downstream effector of ERK signaling and that the MEK-ERK-c-Myc central metabolic axis in tumor ECs is essential for tumor progression. As such, disruption of ERK-c-Myc signaling in host-derived tumor ECs by our tumor-selective anthrax toxins explains their high efficacy in solid tumor therapy.