In vivo imaging reveals PKA regulation of ERK activity during neutrophil recruitment to inflamed intestines

Prostaglandin E2 in the Tumor Microenvironment, a Convoluted Affair Mediated by EP Receptors 2 and 4

The involvement of the prostaglandin E2 (PGE2) system in cancer progression has long been recognized. PGE2 functions as an autocrine and paracrine signaling molecule with pleiotropic effects in the human body. High levels of intratumoral PGE2 and overexpression of the key metabolic enzymes of PGE2 have been observed and suggested to contribute to tumor progression. This has been claimed for different types of solid tumors, including, but not limited to, lung, breast, and colon cancer. PGE2 has direct effects on tumor cells and angiogenesis that are known to promote tumor development. However, one of the main mechanisms behind PGE2 driving cancerogenesis is currently thought to be anchored in suppressed antitumor immunity, thus providing possible therapeutic targets to be used in cancer immunotherapies. EP2 and EP4, two receptors for PGE2, are emerging as being the most relevant for this purpose. This review aims to summarize the known roles of PGE2 in the immune system and its functions within the tumor microenvironment. SIGNIFICANCE STATEMENT: Prostaglandin E2 (PGE2) has long been known to be a signaling molecule in cancer. Its presence in tumors has been repeatedly associated with disease progression. Elucidation of its effects on immunological components of the tumor microenvironment has highlighted the potential of PGE2 receptor antagonists in cancer treatment, particularly in combination with immune checkpoint inhibitor therapeutics. Adjuvant treatment could increase the response rates and the efficacy of immune-based therapies.

Cxcl1 monomer-dimer equilibrium controls neutrophil extravasation

The chemokine Cxcl1 plays a crucial role in recruiting neutrophils in response to infection. The early events in chemokine-mediated neutrophil extravasation involve a sequence of highly orchestrated steps including rolling, adhesion, arrest, and diapedesis. Cxcl1 function is determined by its properties of reversible monomer-dimer equilibrium and binding to Cxcr2 and glycosaminoglycans. Here, we characterized how these properties orchestrate extravasation using intravital microscopy of the cremaster. Compared to WT Cxcl1, which exists as both a monomer and a dimer, the trapped dimer caused faster rolling, less adhesion, and less extravasation. Whole-mount immunofluorescence of the cremaster and arrest assays confirmed these data. Moreover, the Cxcl1 dimer showed impaired LFA-1-mediated neutrophil arrest that could be attributed to impaired Cxcr2-mediated ERK signaling. We conclude that Cxcl1 monomer-dimer equilibrium and potent Cxcr2 activity of the monomer together coordinate the early events in neutrophil recruitment.

Estradiol Augments Tumor-Induced Neutrophil Production to Promote Tumor Cell Actions in Lymphangioleiomyomatosis Models

Lymphangioleiomyomatosis (LAM) is a rare cystic lung disease caused by smooth muscle cell-like tumors containing tuberous sclerosis (TSC) gene mutations and found almost exclusively in females. Patient studies suggest LAM progression is estrogen dependent, an observation supported by in vivo mouse models. However, in vitro data using TSC-null cell lines demonstrate modest estradiol (E2) responses, suggesting E2 effects in vivo may involve pathways independent of direct tumor stimulation. We previously reported tumor-dependent neutrophil expansion and promotion of TSC2-null tumor growth in an E2-sensitive LAM mouse model. We therefore hypothesized that E2 stimulates tumor growth in part by promoting neutrophil production. Here we report that E2-enhanced lung colonization of TSC2-null cells is indeed dependent on neutrophils. We demonstrate that E2 induces granulopoiesis via estrogen receptor α in male and female bone marrow cultures. With our novel TSC2-null mouse myometrial cell line, we show that factors released from these cells drive E2-sensitive neutrophil production. Last, we analyzed single-cell RNA sequencing data from LAM patients and demonstrate the presence of tumor-activated neutrophils. Our data suggest a powerful positive feedback loop whereby E2 and tumor factors induce neutrophil expansion, which in turn intensifies tumor growth and production of neutrophil-stimulating factors, resulting in continued TSC2-null tumor growth.

Low-dose sumatriptan improves the outcome of acute mesenteric ischemia in rats via downregulating kynurenine

BACKGROUND

Mesenteric ischemia has remained without effective pharmacological management for many years. Sumatriptan, an abortive medication for migraine and cluster headaches, has potent anti-inflammatory properties and ameliorated organ ischemia in previous animal studies. Similarly, inhibition of the kynurenine pathway ameliorated renal and myocardial ischemia/reperfusion (I/R) in many preclinical studies. Herein, we assessed the effect of sumatriptan on experimental mesenteric I/R and investigated whether kynurenine pathway inhibition is a mechanism underlying its action.

METHODS

Ischemia was induced by ligating the origin of the superior mesenteric artery (SMA) and its anastomosis with the inferior mesenteric artery (IMA) with bulldog clamps for 30 min. Ischemia was followed by 1 h of reperfusion. Sumatriptan (0.1, 0.3, and 1 mg/kg ip) was injected 5 min before the reperfusion phase, 1-methyltryptophan (1-MT) (100 mg/kg iv) was used to inhibit kynurenine production. At the end of the reperfusion phase, samples were collected from the jejunum of rats for H&E staining and molecular assessments.

RESULTS

Sumatriptan improved the integrity of intestinal mucosa after I/R, and 0.1 mg/kg was the most effective dose of sumatriptan in this study. Sumatriptan decreased the increased levels of TNF-α, kynurenine, and p-ERK but did not change the decreased levels of NO. Furthermore, sumatriptan significantly increased the decreased ratio of Bcl2/Bax. Similarly, 1-MT significantly decreased TNF-α and kynurenine and protected against mucosal damage.

CONCLUSIONS

This study demonstrated that sumatriptan has protective effects against mesenteric ischemia and the kynurenine inhibition is potentially involved in this process. Therefore, it can be assumed that sumatriptan has the potential to be repurposed as a treatment for acute mesenteric ischemia.

Salmonella Typhimurium induces genome-wide expression and phosphorylation changes that modulate immune response, intracellular survival and vesicle transport in infected neutrophils

Salmonella Typhimurium is a food-borne pathogen that causes salmonellosis. When in contact with the host, neutrophils are rapidly recruited to act as first line of defense. To better understand the pathogenesis of this infection, we used an in vitro model of neutrophil infection to perform dual RNA-sequencing (both host and pathogen). In addition, and given that many pathogens interfere with kinase-mediated phosphorylation in host signaling, we performed a phosphoproteomic analysis. The immune response was overall diminished in infected neutrophils, mainly JAK/STAT and toll-like receptor signaling pathways. We found decreased expression of proinflammatory cytokine receptor genes and predicted downregulation of the mitogen-activated protein (MAPK) signaling pathway. Also, Salmonella infection inhibited interferons I and II signaling pathways by upregulation of SOCS3 and subsequent downregulation of STAT1 and STAT2. Additionally, phosphorylation of PSMC2 and PSMC4, proteasome regulatory proteins, was decreased in infected neutrophils. Cell viability and survival was increased by p53 signaling, cell cycle arrest and NFkB-proteasome pathways activation. Combined analysis of RNA-seq and phosphoproteomics also revealed inhibited vesicle transport mechanisms mediated by dynein/dynactin and exocyst complexes, involved in ER-to-Golgi transport and centripetal movement of lysosomes and endosomes. Among the overexpressed virulence genes from Salmonella we found potential effectors responsible of these dysregulations, such as spiC, sopD2, sifA or pipB2, all of them involved in intracellular replication. Our results suggest that Salmonella induces (through overexpression of virulence factors) transcriptional and phosphorylation changes that increases neutrophil survival and shuts down immune response to minimize host response, and impairing intracellular vesicle transport likely to keep nutrients for replication and Salmonella-containing vacuole formation and maintenance.

Mead acid inhibits retinol-induced irritant contact dermatitis via peroxisome proliferator-activated receptor alpha

Retinol is widely used in topical skincare products to ameliorate skin aging and treat acne and wrinkles; however, retinol and its derivatives occasionally have adverse side effects, including the induction of irritant contact dermatitis. Previously, we reported that mead acid (5,8,11-eicosatrienoic acid), an oleic acid metabolite, ameliorated skin inflammation in dinitrofluorobenzene-induced allergic contact hypersensitivity by inhibiting neutrophil infiltration and leukotriene B₄ production by neutrophils. Here, we showed that mead acid also suppresses retinol-induced irritant contact dermatitis. In a murine model, we revealed that mead acid inhibited keratinocyte abnormalities such as keratinocyte hyperproliferation. Consistently, mead acid inhibited p38 MAPK (mitogen-activated protein kinase) phosphorylation, which is an essential signaling pathway in the keratinocyte hyperplasia induced by retinol. These inhibitory effects of mead acid were associated with the prevention of both keratinocyte hyperproliferation and the gene expression of neutrophil chemoattractants, including Cxcl1 and Cxcl2, and they were mediated by a PPAR (peroxisome proliferator-activated receptor)-α pathway. Our findings identified the anti-inflammatory effects of mead acid, the use of which can be expected to minimize the risk of adverse side effects associated with topical retinoid application.

Intravital imaging of immune responses in intestinal inflammation

To date, many kinds of immune cells have been identified, but their precise roles in intestinal immunity remain unclear. Understanding the in vivo behavior of these immune cells and their function in gastrointestinal inflammation, including colitis, inflammatory bowel disease, ischemia-reperfusion injury, and neutrophil extracellular traps, is critical for gastrointestinal research to proceed to the next step. Additionally, understanding the immune responses involved in gastrointestinal tumors and tissue repair is becoming increasingly important for the elucidation of disease mechanisms that have been unknown. In recent years, the application of intravital microscopy in gastrointestinal research has provided novel insights into the mechanisms of intestine-specific events including innate and adaptive immunities. In this review, we focus on the emerging role of intravital imaging in gastrointestinal research and describe how to observe the intestines and immune cells using intravital microscopy. Additionally, we outline novel findings obtained by this new technique.

Neutrophil Extracellular Traps Promote Metastases of Colorectal Cancers through Activation of ERK Signaling by Releasing Neutrophil Elastase

Neutrophil extracellular traps (NETs) play important roles in host immunity, as there is increasing evidence of their contribution to the progression of several types of cancers even though their role in colorectal cancers (CRCs) remains unclear. To investigate the clinical relevance of NETs in CRCs, we examined the expression of citrullinated histone H3 using immunohistochemistry and preoperative serum myeloperoxidase-DNA complexes in CRC patients using an enzyme-linked immunosorbent assay. High expression of intratumoral or systemic NETs was found to correlate with poor relapse-free survival (RFS), for which it is an independent prognostic factor. In vitro investigations of CRC cells (HCT116, HT29) revealed that NETs did not affect their proliferation but did promote the migration of CRC cells mediated by neutrophil elastase (NE) released during NETosis to increase extracellular signal-regulated kinase (ERK) activity. In vivo experiments using nude mice (KSN/slc) revealed that NE inhibition suppressed liver metastases in CRC cells, although it did not affect the growth of subcutaneously implanted tumors. Taken together, these results suggest that NET formation correlates with poor prognoses of patients with CRC and that the inhibition of NE could be a potential therapy for CRC metastases.

Protective effects of 4‐geranyloxy‐2,6‐dihydroxybenzophenonel on DSS ‐induced ulcerative colitis in mice via regulation of cAMP / PKA / CREB and NF‐κB signaling pathways

Hypericum sampsonii Hance has traditionally been used to treat enteritis and diarrhea. As one of the main benzophenones isolated from H. sampsonii, 4-geranyloxy-2,6-dihydroxybenzophenonel (4-GDB) has been shown to possess anti-inflammatory effects. However, the therapeutic effect and potential mechanisms of 4-GDB in ulcerative colitis (UC) remain unclear. This study aimed to evaluate the role of 4-GDB in UC using a dextran sulfate sodium-induced colitis mouse model. Intragastric administration of 4-GDB (20 mg/kg/day) for 8 days significantly attenuated colonic injury, reduced the expression of inflammatory mediators, and improved colonic barrier function in mice with colitis. Furthermore, in vivo and in vitro experiments indicated that 4-GDB could activate cAMP/PKA/CREB and inhibit the NF-κB pathway. Collectively, 4-GDB may be a potential agent for treating UC by regulating the cAMP/PKA/CREB and NF-κB pathways.

Atypical CXCL12 signaling enhances neutrophil migration by modulating nuclear deformability

To reach inflamed tissues from the circulation, neutrophils must overcome physical constraints imposed by the tissue architecture, such as the endothelial barrier or the three-dimensional (3D) interstitial space. In these microenvironments, neutrophils are forced to migrate through spaces smaller than their own diameter. One of the main challenges for cell passage through narrow gaps is the deformation of the nucleus, the largest and stiffest organelle in cells. Here, we showed that chemokines, the extracellular signals that guide cell migration in vivo, modulated nuclear plasticity to support neutrophil migration in restricted microenvironments. Exploiting microfabricated devices, we found that the CXC chemokine CXCL12 enhanced the nuclear pliability of mouse bone marrow–derived neutrophils to sustain their migration in 3D landscapes. This previously uncharacterized function of CXCL12 was mediated by the atypical chemokine receptor ACKR3 (also known as CXCR7), required protein kinase A (PKA) activity, and induced chromatin compaction, which resulted in enhanced cell migration in 3D. Thus, we propose that chemical cues regulate the nuclear plasticity of migrating leukocytes to optimize their motility in restricted microenvironments.

PGE2 -EP3 axis promotes brown adipose tissue formation through stabilization of WTAP RNA methyltransferase

Brown adipose tissue (BAT) functions as a thermogenic organ and is negatively associated with cardiometabolic diseases. N⁶ -methyladenosine (m⁶ A) modulation regulates the fate of stem cells. Here, we show that the prostaglandin E₂ (PGE₂ )-E-prostanoid receptor 3 (EP3) axis was activated during mouse interscapular BAT development. Disruption of EP3 impaired the browning process during adipocyte differentiation from pre-adipocytes. Brown adipocyte-specific depletion of EP3 compromised interscapular BAT formation and aggravated high-fat diet-induced obesity and insulin resistance in vivo. Mechanistically, activation of EP3 stabilized the Zfp410 mRNA via WTAP-mediated m⁶ A modification, while knockdown of Zfp410 abolished the EP3-induced enhancement of brown adipogenesis. EP3 prevented ubiquitin-mediated degradation of WTAP by eliminating PKA-mediated ERK1/2 inhibition during brown adipocyte differentiation. Ablation of WTAP in brown adipocytes abrogated the protective effect of EP3 overexpression in high-fat diet-fed mice. Inhibition of EP3 also retarded human embryonic stem cell differentiation into mature brown adipocytes by reducing the WTAP levels. Thus, a conserved PGE₂ -EP3 axis promotes BAT development by stabilizing WTAP/Zfp410 signaling in a PKA/ERK1/2-dependent manner.

m6A demethylase ALKBH5 is required for antibacterial innate defense by intrinsic motivation of neutrophil migration

Neutrophil migration into the site of infection is necessary for antibacterial innate defense, whereas impaired neutrophil migration may result in excessive inflammation and even sepsis. The neutrophil migration directed by extracellular signals such as chemokines has been extensively studied, yet the intrinsic mechanism for determining neutrophil ability to migrate needs further investigation. N⁶-methyladenosine (m⁶A) RNA modification is important in immunity and inflammation, and our preliminary data indicate downregulation of RNA m⁶A demethylase alkB homolog 5 (ALKBH5) in neutrophils during bacterial infection. Whether m⁶A modification and ALKBH5 might intrinsically modulate neutrophil innate response remain unknown. Here we report that ALKBH5 is required for antibacterial innate defense by enhancing intrinsic ability of neutrophil migration. We found that deficiency of ALKBH5 increased mortality of mice with polymicrobial sepsis induced by cecal ligation and puncture (CLP), and Alkbh5-deficient CLP mice exhibited higher bacterial burden and massive proinflammatory cytokine production in the peritoneal cavity and blood because of less neutrophil migration. Alkbh5-deficient neutrophils had lower CXCR2 expression, thus exhibiting impaired migration toward chemokine CXCL2. Mechanistically, ALKBH5-mediated m⁶A demethylation empowered neutrophils with high migration capability through altering the RNA decay, consequently regulating protein expression of its targets, neutrophil migration-related molecules, including increased expression of neutrophil migration-promoting CXCR2 and NLRP12, but decreased expression of neutrophil migration-suppressive PTGER4, TNC, and WNK1. Our findings reveal a previously unknown role of ALKBH5 in imprinting migration-promoting transcriptome signatures in neutrophils and intrinsically promoting neutrophil migration for antibacterial defense, highlighting the potential application of targeting neutrophil m⁶A modification in controlling bacterial infections.

Neutrophils: Amoeboid Migration and Swarming Dynamics in Tissues

Neutrophils are key cells of our innate immune response with essential roles for eliminating bacteria and fungi from tissues. They are also the prototype of an amoeboid migrating leukocyte. As one of the first blood-recruited immune cell types during inflammation and infection, these cells can invade almost any tissue compartment. Once in the tissue, neutrophils undergo rapid shape changes and migrate at speeds higher than most other immune cells. They move in a substrate-independent manner in interstitial spaces and do not follow predetermined tissue paths. Instead, neutrophil navigation is largely shaped by the chemokine and chemoattractant milieu around them. This highlights the decisive role of attractant-sensing G-protein coupled receptors (GPCRs) and downstream molecular pathways for controlling amoeboid neutrophil movement in tissues. A diverse repertoire of cell-surface expressed GPCRs makes neutrophils the perfect sentinel cell type to sense and detect danger-associated signals released from wounds, inflamed interstitium, dying cells, complement factors or directly from tissue-invading microbes. Moreover, neutrophils release attractants themselves, which allows communication and coordination between individual cells of a neutrophil population. GPCR-mediated positive feedback mechanisms were shown to underlie neutrophil swarming, a population response that amplifies the recruitment of amoeboid migrating neutrophils to sites of tissue injury and infection. Here we discuss recent findings and current concepts that counteract excessive neutrophil accumulation and swarm formation. In particular, we will focus on negative feedback control mechanisms that terminate neutrophil swarming to maintain the delicate balance between tissue surveillance, host protection and tissue destruction.

Analysis of localized cAMP perturbations within a tissue reveal the effects of a local, dynamic gap junction state on ERK signaling

Beyond natural stimuli such as growth factors and stresses, the ability to experimentally modulate at will the levels or activity of specific intracellular signaling molecule(s) in specified cells within a tissue can be a powerful tool for uncovering new regulation and tissue behaviors. Here we perturb the levels of cAMP within specific cells of an epithelial monolayer to probe the time-dynamic behavior of cell-cell communication protocols implemented by the cAMP/PKA pathway and its coupling to the ERK pathway. The time-dependent ERK responses we observe in the perturbed cells for spatially uniform cAMP perturbations (all cells) can be very different from those due to spatially localized perturbations (a few cells). Through a combination of pharmacological and genetic perturbations, signal analysis, and computational modeling, we infer how intracellular regulation and regulated cell-cell coupling each impact the intracellular ERK response in single cells. Our approach reveals how a dynamic gap junction state helps sculpt the intracellular ERK response over time in locally perturbed cells.

Dual blockade of macropinocytosis and asparagine bioavailability shows synergistic anti-tumor effects on KRAS-mutant colorectal cancer

Mutations of KRAS gene are found in various types of cancer, including colorectal cancer (CRC). Despite intense efforts, no pharmacological approaches are expected to be effective against KRAS-mutant cancers. Macropinocytosis is an evolutionarily conserved actin-dependent endocytic process that internalizes extracellular fluids into large vesicles called macropinosomes. Recent studies have revealed macropinocytosis's important role in metabolic adaptation to nutrient stress in cancer cells harboring KRAS mutations. Here we showed that KRAS-mutant CRC cells enhanced macropinocytosis for tumor growth under nutrient-depleted conditions. We also demonstrated that activation of Rac1 and phosphoinositide 3-kinase were involved in macropinocytosis of KRAS-mutant CRC cells. Furthermore, we found that macropinocytosis was closely correlated with asparagine metabolism. In KRAS-mutant CRC cells engineered with knockdown of asparagine synthetase, macropinocytosis was accelerated under glutamine-depleted condition, and albumin addition could restore the glutamine depletion-induced growth suppression by recovering the intracellular asparagine level. Finally, we discovered that the combination of macropinocytosis inhibition and asparagine depletion dramatically suppressed the tumor growth of KRAS-mutant CRC cells in vivo. These results indicate that dual blockade of macropinocytosis and asparagine bioavailability could be a novel therapeutic strategy for KRAS-mutant cancers.

Protein Kinase A Detection in Human Urine Samples

Actively involved in tumor maintenance, cAMP-dependent protein kinase A (PKA) has been proposed as a putative biomarker in cancer. Recently, an active PKA form has been identified in human sera and PKA autoantibodies have been detected in cancer patients. However, their serum functions, as well as diagnostic significance, remain largely unknown. Although several PKA detection assays have been developed, none refer to a laboratory diagnostic procedure. Among these, ELISA and Western blotting (WB) assays have been employed in PKA detection. Since, to the best of our knowledge, there are no data showing its presence in human urine samples, herein, we explore the possibility of PKA's existence in this biological specimen. Interestingly, among the 30 screened urines by quantitative sandwich ELISA, we recognized detectable PKA levels in 5 different samples, and of those two exhibited a considerable high concentration. To corroborate these results, we also evaluated PKA's presence in both positive and negative ELISA urines by WB. Remarkably, immunoblotting analysis confirmed PKA's existence in certain, but not in all, human urine specimens. Despite being quite preliminary, these findings firstly identify PKA in urine samples and provide evidence for its potential clinic usage as a diagnostic analyte in laboratory medicine.

β2-Adrenoceptor Agonists Promote ERK1/2 Dephosphorylation in Human Airway Epithelial Cells by Canonical, cAMP-driven Signaling Independently of β-Arrestin 2

Chronic use of β₂-adrenoceptor agonists as a monotherapy in asthma is associated with a loss of disease control and an increased risk of mortality. Herein, we tested the hypothesis that β₂-adrenoceptor agonists, including formoterol, promote biased, β-arrestin 2 (βArr2)-dependent activation of the mitogen-activated protein (MAP) kinases, ERK1/2, in human airway epithelial cells and, thereby, effect changes in gene expression that could contribute to their adverse clinical outcomes. Three airway epithelial cell models were used: the BEAS-2B cell line, human primary bronchial epithelial cells (HBEC) grown in submersion culture and HBEC that were highly differentiated at an air-liquid interface. Unexpectedly, treatment of all epithelial cell models with formoterol decreased basal ERK1/2 phosphorylation. This was mediated by cAMP-dependent protein kinase and involved the inactivation of C-rapidly-activated fibrosarcoma, which attenuated down-stream ERK1/2 activity, and the induction of dual-specificity phosphatase-1. Formoterol also inhibited the basal expression of early growth response-1, an ERK1/2-regulated gene that controls cell growth and repair in the airways. Neither carvedilol, a β₂-adrenoceptor agonist biased towards βArr2, nor formoterol promoted ERK1/2 phosphorylation in BEAS-2B cells although β₂-adrenoceptor desensitization was compromised in ARRB2-deficient cells. Collectively, these results contest the hypothesis that formoterol activates ERK1/2 in airway epithelia by nucleating a βArr2 signaling complex; instead, they indicate that β₂-adrenoceptor agonists inhibit constitutive ERK1/2 activity in a cAMP-dependent manner. These findings are the antithesis of results obtained using acutely challenged native and engineered HEK293 cells, which have been used extensively to study mechanisms of ERK1/2 activation, and highlight the cell-type-dependence of β₂-adrenoceptor-mediated signaling. Significance Statement It has been proposed that the adverse-effects of β₂-adrenoceptor agonist monotherapy in asthma are mediated by genomic mechanisms that occur principally in airway epithelial cells and are the result of β-arrestin 2-dependent activation of ERK1/2. This study shows that β₂-adrenoceptor agonists, paradoxically, reduced ERK1/2 phosphorylation in airway epithelia by disrupting upstream Ras-C-Raf complex formation and inducing DUSP1. Moreover, these effects were PKA-dependent suggesting that β₂-adrenoceptor agonists were not biased toward β-arrestin 2 and acted via canonical, cAMP-dependent signaling.

Live-imaging of endothelial Erk activity reveals dynamic and sequential signalling events during regenerative angiogenesis

The formation of new blood vessel networks occurs via angiogenesis during development, tissue repair, and disease. Angiogenesis is regulated by intracellular endothelial signalling pathways, induced downstream of vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs). A major challenge in understanding angiogenesis is interpreting how signalling events occur dynamically within endothelial cell populations during sprouting, proliferation, and migration. Extracellular signal-regulated kinase (Erk) is a central downstream effector of Vegf-signalling and reports the signalling that drives angiogenesis. We generated a vascular Erk biosensor transgenic line in zebrafish using a kinase translocation reporter that allows live-imaging of Erk-signalling dynamics. We demonstrate the utility of this line to live-image Erk activity during physiologically relevant angiogenic events. Further, we reveal dynamic and sequential endothelial cell Erk-signalling events following blood vessel wounding. Initial signalling is dependent upon Ca²⁺ in the earliest responding endothelial cells, but is independent of Vegfr-signalling and local inflammation. The sustained regenerative response, however, involves a Vegfr-dependent mechanism that initiates concomitantly with the wound inflammatory response. This work reveals a highly dynamic sequence of signalling events in regenerative angiogenesis and validates a new resource for the study of vascular Erk-signalling in real-time.

Hierarchical modeling of mechano-chemical dynamics of epithelial sheets across cells and tissue

Collective cell migration is a fundamental process in embryonic development and tissue homeostasis. This is a macroscopic population-level phenomenon that emerges across hierarchy from microscopic cell-cell interactions; however, the underlying mechanism remains unclear. Here, we addressed this issue by focusing on epithelial collective cell migration, driven by the mechanical force regulated by chemical signals of traveling ERK activation waves, observed in wound healing. We propose a hierarchical mathematical framework for understanding how cells are orchestrated through mechanochemical cell-cell interaction. In this framework, we mathematically transformed a particle-based model at the cellular level into a continuum model at the tissue level. The continuum model described relationships between cell migration and mechanochemical variables, namely, ERK activity gradients, cell density, and velocity field, which could be compared with live-cell imaging data. Through numerical simulations, the continuum model recapitulated the ERK wave-induced collective cell migration in wound healing. We also numerically confirmed a consistency between these two models. Thus, our hierarchical approach offers a new theoretical platform to reveal a causality between macroscopic tissue-level and microscopic cellular-level phenomena. Furthermore, our model is also capable of deriving a theoretical insight on both of mechanical and chemical signals, in the causality of tissue and cellular dynamics.

Visualization of Spatially-Controlled Vasospasm by Sympathetic Nerve-Mediated ROCK Activation

Contraction of vascular smooth muscle is regulated primarily by calcium concentration and secondarily by ROCK activity within the cells. In contrast to the wealth of information regarding regulation of calcium concentration, little is known about the spatiotemporal regulation of ROCK activity in live blood vessels. Here, we report ROCK activation in subcutaneous arterioles in a transgenic mouse line that expresses a genetically encoded ROCK biosensor based on the principle of Fӧrster resonance energy transfer by two-photon excitation in vivo imaging. Rapid vasospasm was induced upon laser ablation of arterioles, concomitant with a transient increase in calcium concentration in arteriolar smooth muscles. Unlike the increase in calcium concentration, vasoconstriction and ROCK activation continued for several minutes after irradiation. Both the ROCK inhibitor, fasudil, and the ganglionic nicotinic acetylcholine receptor blocker, hexamethonium, inhibited laser-induced ROCK activation and reduced the duration of vasospasm at the segments distant from the irradiated point. These observations suggest that vasoconstriction is initially triggered by a rapid surge of cytoplasmic calcium and then maintained by sympathetic nerve-mediated ROCK activation.

Rhodopsin-mediated light-off-induced protein kinase A activation in mouse rod photoreceptor cells

Light-induced extrasynaptic dopamine release in the retina reduces adenosine 3',5'-cyclic monophosphate (cAMP) in rod photoreceptor cells, which is thought to mediate light-dependent desensitization. However, the fine time course of the cAMP dynamics in rods remains elusive due to technical difficulty. Here, we visualized the spatiotemporal regulation of cAMP-dependent protein kinase (PKA) in mouse rods by two-photon live imaging of retinal explants of PKAchu mice, which express a fluorescent biosensor for PKA. Unexpectedly, in addition to the light-on-induced suppression, we observed prominent light-off-induced PKA activation. This activation required photopic light intensity and was confined to the illuminated rods. The estimated maximum spectral sensitivity of 489 nm and loss of the light-off-induced PKA activation in rod-transducin-knockout retinas strongly suggest the involvement of rhodopsin. In support of this notion, rhodopsin-deficient retinal explants showed only the light-on-induced PKA suppression. Taken together, these results suggest that, upon photopic light stimulation, rhodopsin and dopamine signals are integrated to shape the light-off-induced cAMP production and following PKA activation. This may support the dark adaptation of rods.

Neutrophil: A New Player in Metastatic Cancers

The interaction between cancer cells and immune cells is important for the cancer development. However, much attention has been given to T cells and macrophages. Being the most abundant leukocytes in the blood, the functions of neutrophils in cancer have been underdetermined. They have long been considered an “audience” in the development of cancer. However, emerging evidence indicate that neutrophils are a heterogeneous population with plasticity, and subpopulation of neutrophils (such as low density neutrophils, polymorphonuclear-myeloid-derived suppressor cells) are actively involved in cancer growth and metastasis. Here, we review the current understanding of the role of neutrophils in cancer development, with a specific focus on their pro-metastatic functions. We also discuss the potential and challenges of neutrophils as therapeutic targets. A better understanding the role of neutrophils in cancer will discover new mechanisms of metastasis and develop new immunotherapies by targeting neutrophils.

Experimental pathology by intravital microscopy and genetically encoded fluorescent biosensors

The invention of two‐photon excitation microscopes widens the potential application of intravital microscopy (IVM) to the broad field of experimental pathology. Moreover, the recent development of fluorescent protein‐based, genetically encoded biosensors provides an ideal tool to visualize the cell function in live animals. We start from a brief review of IVM with two‐photon excitation microscopes and genetically encoded biosensors based on the principle of Förster resonance energy transfer (FRET). Then, we describe how IVM using biosensors has revealed the pathogenesis of several disease models.

Imaging of the immune system - towards a subcellular and molecular understanding

Immune responses involve many types of leukocytes that traffic to the site of injury, recognize the insult and respond appropriately. Imaging of the immune system involves a set of methods and analytical tools that are used to visualize immune responses at the cellular and molecular level as they occur in real time. We will review recent and emerging technological advances in optical imaging, and their application to understanding the molecular and cellular responses of neutrophils, macrophages and lymphocytes. Optical live-cell imaging provides deep mechanistic insights at the molecular, cellular, tissue and organism levels. Live-cell imaging can capture quantitative information in real time at subcellular resolution with minimal phototoxicity and repeatedly in the same living cells or in accessible tissues of the living organism. Advanced FRET probes allow tracking signaling events in live cells. Light-sheet microscopy allows for deeper tissue penetration in optically clear samples, enriching our understanding of the higher-level organization of the immune response. Super-resolution microscopy offers insights into compartmentalized signaling at a resolution beyond the diffraction limit, approaching single-molecule resolution. This Review provides a current perspective on live-cell imaging in vitro and in vivo with a focus on the assessment of the immune system.

Multiphoton intravital microscopy in small animals: motion artefact challenges and technical solutions

Since its invention 29 years ago, two‐photon laser‐scanning microscopy has evolved from a promising imaging technique, to an established widely available imaging modality used throughout the biomedical research community. The establishment of two‐photon microscopy as the preferred method for imaging fluorescently labelled cells and structures in living animals can be attributed to the biophysical mechanism by which the generation of fluorescence is accomplished. The use of powerful lasers capable of delivering infrared light pulses within femtosecond intervals, facilitates the nonlinear excitation of fluorescent molecules only at the focal plane and determines by objective lens position. This offers numerous benefits for studies of biological samples at high spatial and temporal resolutions with limited photo‐damage and superior tissue penetration. Indeed, these attributes have established two‐photon microscopy as the ideal method for live‐animal imaging in several areas of biology and have led to a whole new field of study dedicated to imaging biological phenomena in intact tissues and living organisms. However, despite its appealing features, two‐photon intravital microscopy is inherently limited by tissue motion from heartbeat, respiratory cycles, peristalsis, muscle/vascular tone and physiological functions that change tissue geometry. Because these movements impede temporal and spatial resolution, they must be properly addressed to harness the full potential of two‐photon intravital microscopy and enable accurate data analysis and interpretation. In addition, the sources and features of these motion artefacts are varied, sometimes unpredictable and unique to specific organs and multiple complex strategies have previously been devised to address them. This review will discuss these motion artefacts requirement and technical solutions for their correction and after intravital two‐photon microscopy.

Prostaglandin E2/EP Signaling in the Tumor Microenvironment of Colorectal Cancer

The number of colorectal cancer (CRC) patients is increasing worldwide. Accumulating evidence has shown that the tumor microenvironment (TME), including macrophages, neutrophils, and fibroblasts, plays an important role in the development and progression of CRC. Although targeting the TME could be a promising therapeutic approach, the mechanisms by which inflammatory cells promote CRC tumorigenesis are not well understood. When inflammation occurs in tissues, prostaglandin E2 (PGE2) is generated from arachidonic acid by the enzyme cyclooxygenase-2 (COX-2). PGE2 regulates multiple functions in various immune cells by binding to the downstream receptors EP1, EP2, EP3, and EP4, and plays an important role in the development of CRC. The current therapies targeting PGE2 using non-steroidal anti-inflammatory drugs (NSAIDs) or COX-2 inhibitors have failed due to the global prostanoid suppression resulting in the severe adverse effects despite the fact they could prevent tumorigenesis. Therefore, therapies targeting the specific downstream molecules of PGE2 signaling could be a promising approach. This review highlights the role of each EP receptor in the TME of CRC tumorigenesis and their therapeutic potential.

Intravital multiphoton microscopy as a novel tool in the field of immunopharmacology

Intravital microscopy with multiphoton excitation is a recently developed optical imaging technique for deep tissue imaging without fixation or sectioning, which permits examination of fundamental concepts regarding the dynamic nature of cells under physiological and pathological conditions in living animals. This novel technique also offers exciting opportunities for pharmacological research by providing new platforms for the study of cellular dynamics in response to drugs in vivo. Moreover, fluorescent chemical probes for functional or molecular analysis in single cells in vivo play important roles in pharmacology. For example, we have recently revealed the pharmacodynamic actions of different biological agents for the treatment of rheumatoid arthritis (RA) in vivo by directly visualizing drug-induced cellular behaviors and functions of osteoclasts on bone surfaces. This review focuses on the principles and advantages of intravital imaging for the dissection of pharmacological mechanisms, and discusses how such imaging can contribute to the drug development process, introducing recent trials that evaluated the in vivo pharmacological effects of various agents.

Divergent Dynamics and Functions of ERK MAP Kinase Signaling in Development, Homeostasis and Cancer: Lessons from Fluorescent Bioimaging

The extracellular signal-regulated kinase (ERK) signaling pathway regulates a variety of biological processes including cell proliferation, survival, and differentiation. Since ERK activation promotes proliferation of many types of cells, its deregulated/constitutive activation is among general mechanisms for cancer. Recent advances in bioimaging techniques have enabled to visualize ERK activity in real-time at the single-cell level. Emerging evidence from such approaches suggests unexpectedly complex spatiotemporal dynamics of ERK activity in living cells and animals and their crucial roles in determining cellular responses. In this review, we discuss how ERK activity dynamics are regulated and how they affect biological processes including cell fate decisions, cell migration, embryonic development, tissue homeostasis, and tumorigenesis.

ERK Activity Imaging During Migration of Living Cells In Vitro and In Vivo

Extracellular signal-regulated kinase (ERK) is a major downstream factor of the EGFR-RAS-RAF signalling pathway, and thus the role of ERK in cell growth has been widely examined. The development of biosensors based on fluorescent proteins has enabled us to measure ERK activities in living cells, both after growth factor stimulation and in its absence. Long-term imaging unexpectedly revealed the oscillative activation of ERK in an epithelial sheet or a cyst in vitro. Studies using transgenic mice expressing the ERK biosensor have revealed inhomogeneous ERK activities among various cell species. In vivo Förster (or fluorescence) resonance energy transfer (FRET) imaging shed light on a novel role of ERK in cell migration. Neutrophils and epithelial cells in various organs such as intestine, skin, lung and bladder showed spatio-temporally different cell dynamics and ERK activities. Experiments using inhibitors confirmed that ERK activities are required for various pathological responses, including epithelial repair after injuries, inflammation, and niche formation of cancer metastasis. In conclusion, biosensors for ERK will be powerful and valuable tools to investigate the roles of ERK in situ.

Prostaglandin-cytokine crosstalk in chronic inflammation

Chronic inflammation underlies various debilitating disorders including autoimmune, neurodegenerative, vascular and metabolic diseases as well as cancer, where aberrant activation of the innate and acquired immune systems is frequently seen. Since non-steroidal anti-inflammatory drugs exert their effects by inhibiting COX and suppressing PG biosynthesis, PGs have been traditionally thought to function mostly as mediators of acute inflammation. However, an inducible COX isoform, COX-2, is often highly expressed in tissues of the chronic disorders, suggesting an as yet unidentified role of PGs in chronic inflammation. Recent studies have shown that in addition to their short-lived actions in acute inflammation, PGs crosstalk with cytokines and amplify the cytokine actions on various types of inflammatory cells and drive pathogenic conversion of these cells by critically regulating their gene expression. One mode of such PG-mediated amplification is to induce the expression of relevant cytokine receptors, which is typically observed in Th1 cell differentiation and Th17 cell expansion, events leading to chronic immune inflammation. Another mode of amplification is cooperation of PGs with cytokines at the transcription level. Typically, PGs and cytokines synergistically activate NF-κB to induce the expression of inflammation-related genes, one being COX-2 itself, which makes PG-mediated positive feedback loops. This signalling consequently enhances the expression of various NF-κB-induced genes including chemokines to macrophages and neutrophils, which enables sustained infiltration of these cells and further amplifies chronic inflammation. In addition, PGs are also involved in tissue remodelling such as fibrosis and angiogenesis. In this article, we review these findings and discuss their relevance to human diseases.

Single-Cell Live Imaging

Recent fluorescence microscopy allows for high-throughput acquisition of 5D (X, Y, Z, T, and Color) images in various targets such as cultured cells, 3D spheroid/organoid, and even living tissue with single-cell resolution. The technology is considered promising to augment insights on heterogeneous features of both physiological and pathological cell phenotypes, for instance, distinct responses of cancer cells to anticancer drug treatment. Here we overview microscopic applications to capture live cell events for different types of targets, together with a couple of proof of concepts. The 2D live imaging will be exemplified by a FRET-based time-lapse cultured cell imaging, and 3D tissue imaging protocol will be complemented with a method for mouse skin live imaging.

Active K-RAS induces the coherent rotation of epithelial cells: A model for collective cell invasion in vitro

At the invasive front of adenocarcinomas, single cells and multicellular structures exist; the latter include glands and cell clusters, such as tumor buddings and poorly differentiated clusters. Recent reports suggest the importance of collective cell migration in metastasis; however, it is technically difficult to observe the movement of multicellular structures in vivo. We utilized MDCK cells as a model for epithelial cells and established a method to quantify their motility in 3D structures in vitro. A single MDCK cell grows as a cell cluster in the gel and later proliferates and forms a cyst. Active K-RAS expression induced rotation of both the cell clusters and the cysts. The rotation speed of cell clusters was 4 times higher than that of cysts. The screening of inhibitors for their effects on cell clusters and cysts revealed that cyclin B1 and β-catenin were the key molecules for their rotation, respectively. Regulators for cyst rotation, such as vorinostat and β-catenin, were not effective for inducing cell cluster rotation. These results indicate that the signaling pathways of cell dynamics are different between cell clusters and cysts. As cell clusters are related to lymph node involvement and the prognosis of various carcinomas, our in vitro quantitative system may be useful for the screening of drugs to prevent lymphatic invasion.

In vivo imaging reveals unique neutrophil transendothelial migration patterns in inflamed intestines

Neutrophil (PMN) infiltration of the intestinal mucosa is a hallmark of gastrointestinal inflammation, with significant implications for host defense, injury and repair. However, phenotypic and mechanistic aspects of PMN recruitment in inflamed intestines have not been explored in vivo. Using novel epithelial/PMN fluorescence reporter mice, advanced intravital imaging and 3D reconstruction analysis, we mapped the microvasculature architecture across the intestinal layers and determined that in response to Salmonella/endotoxin-induced inflammation, PMN transendothelial migration (TEM) was restricted to submucosal vessels. PMN TEM was not observed in villus or crypt vessels, proximal to the epithelium that underlies the intestinal lumen, and was partially dependent on (C-X-C motif) ligands 1 (CXCL1) and 2 (CXCL2) expression, which was found to be elevated in the submucosa layer. Restricted PMN extravasation at the submucosa and subsequent PMN interstitial migration may serve as a novel regulatory step of PMN effector function and recruitment to the luminal space in inflamed intestines.

Circadian gene period1b regulates proinflammatory cytokine expression through NF-κB signalling in zebrafish

The circadian clock plays a critical role in regulating the immune system. Our previous publication revealed that a mutation in the circadian gene period1b (per1b) in zebrafish significantly decreased proinflammatory gene expression, particularly under constant darkness (DD) conditions; however, the underlying mechanisms remain unclear. In this study, using per1b-null mutant zebrafish and a larval tail fin injury model, we observed that the loss of per1b resulted in the downregulation expression of proinflammatory cytokines, such as IL-6 and TNF-α, at protein level. Furthermore, the loss of per1b downregulated ERK phosphorylation and inhibited p65 phosphorylation, leading to reduced NF-κB activation, which could downregulate the expression of proinflammatory cytokines, such as IL-6 and TNF-α, in zebrafish. These results provided insight into the communication between the circadian clock and immune functions.

Gliding Basal Cell Migration of the Urothelium during Wound Healing

Collective cell migration during wound healing has been extensively studied in the epidermis. However, it remains unknown whether the urothelium repairs wounds in a manner similar to the epidermis. By in vivo two-photon excitation microscopy of transgenic mice that express fluorescent biosensors, we studied the collective cell migration of the urothelium in comparison with that of the epidermis. In vivo time-lapse imaging revealed that, even in the absence of a wound, urothelial cells continuously moved and sometimes glided as a sheet over the underlying lamina propria. On abrasion of the epithelium, the migration speed of each epidermal cell was inversely correlated with the distance to the wound edge. Repetitive activation waves of extracellular signal-regulated kinase (ERK) were generated at and propagated away from the wound edge. In contrast, urothelial cells glided as a sheet over the lamina propria without any ERK activation waves. Accordingly, the mitogen-activated protein kinase/ERK kinase inhibitor PD0325901 decreased the migration velocity of the epidermis but not the urothelium. Interestingly, the tyrosine kinase inhibitor dasatinib inhibited migration of the urothelium as well as the epidermis, suggesting that the gliding migration of the urothelium is an active, not a passive, migration. In conclusion, the urothelium glides over the lamina propria to fill wounds in an ERK-independent manner, whereas the epidermis crawls to cover wounds in an ERK-dependent manner.

Composite regulation of ERK activity dynamics underlying tumour-specific traits in the intestine

Acting downstream of many growth factors, extracellular signal-regulated kinase (ERK) plays a pivotal role in regulating cell proliferation and tumorigenesis, where its spatiotemporal dynamics, as well as its strength, determine cellular responses. Here, we uncover the ERK activity dynamics in intestinal epithelial cells (IECs) and their association with tumour characteristics. Intravital imaging identifies two distinct modes of ERK activity, sustained and pulse-like activity, in IECs. The sustained and pulse-like activities depend on ErbB2 and EGFR, respectively. Notably, activation of Wnt signalling, the earliest event in intestinal tumorigenesis, augments EGFR signalling and increases the frequency of ERK activity pulses through controlling the expression of EGFR and its regulators, rendering IECs sensitive to EGFR inhibition. Furthermore, the increased pulse frequency is correlated with increased cell proliferation. Thus, ERK activity dynamics are defined by composite inputs from EGFR and ErbB2 signalling in IECs and their alterations might underlie tumour-specific sensitivity to pharmacological EGFR inhibition.

The ERK signalling pathway regulates homeostasis of the intestinal epithelium. Here the authors identify two modes of ERK activity generated independently from EGFR and ErbB2 receptor and whose balance in cancer is shifted by Wnt pathway activation, resulting in enhanced sensitivity to EGFR inhibitors.

Molecular mobility and activity in an intravital imaging setting - implications for cancer progression and targeting

Molecular mobility, localisation and spatiotemporal activity are at the core of cell biological processes and deregulation of these dynamic events can underpin disease development and progression. Recent advances in intravital imaging techniques in mice are providing new avenues to study real-time molecular behaviour in intact tissues within a live organism and to gain exciting insights into the intricate regulation of live cell biology at the microscale level. The monitoring of fluorescently labelled proteins and agents can be combined with autofluorescent properties of the microenvironment to provide a comprehensive snapshot of in vivo cell biology. In this Review, we summarise recent intravital microscopy approaches in mice, in processes ranging from normal development and homeostasis to disease progression and treatment in cancer, where we emphasise the utility of intravital imaging to observe dynamic and transient events in vivo We also highlight the recent integration of advanced subcellular imaging techniques into the intravital imaging pipeline, which can provide in-depth biological information beyond the single-cell level. We conclude with an outlook of ongoing developments in intravital microscopy towards imaging in humans, as well as provide an overview of the challenges the intravital imaging community currently faces and outline potential ways for overcoming these hurdles.

Kinome and phosphoproteome of high-grade meningiomas reveal AKAP12 as a central regulator of aggressiveness and its possible role in progression

There is a need to better understand meningioma oncogenesis for biomarker discovery and development of targeted therapies. Histological or genetic criteria do not accurately predict aggressiveness. Post-translational studies in meningioma progression are lacking. In the present work, we introduce a combination of mass spectrometry-based phosphoproteomics and peptide array kinomics to profile atypical and anaplastic (high-grade) meningiomas. In the discovery set of fresh-frozen tissue specimens (14), the A-kinase anchor protein 12 (AKAP12) protein was found downregulated across the grades. AKAP12 knockdown in benign meningioma cells SF4433 increases proliferation, cell cycle, migration, invasion, and confers an anaplastic profile. Differentially regulated pathways were characteristic of high-grade meningiomas. Low AKAP12 expression in a larger cohort of patients (75) characterized tumor invasiveness, recurrence, and progression, indicating its potential as a prognostic biomarker. These results demonstrate AKAP12 as a central regulator of meningioma aggressiveness with a possible role in progression.

Live Imaging of Fetal Intra-abdominal Organs Using Two-Photon Laser-Scanning Microscopy

The processes by which the intra-abdominal organ circulatory system develops in the embryo and during organogenesis are unclear. Previous studies have used fixed tissues to study the development of abdominal organ vasculature in the embryo; however, the intravital circulation of intra-abdominal organs in rodent fetal development has not been studied. This protocol describes a system that uses two-photon laser-scanning microscopy (TPLSM) for real-time observation and quantification of normal and pathologic live fetal intra-abdominal dynamics while the fetus is still connected to the mother via the umbilical cord.

Intravital imaging of neutrophil recruitment in intestinal ischemia-reperfusion injury

BACKGROUND

Neutrophils are known to be key players in innate immunity. Activated neutrophils induce local inflammation, which results in pathophysiologic changes during intestinal ischemia-reperfusion injury (IRI). However, most studies have been based on static assessments, and few have examined real-time intravital neutrophil recruitment. We herein report a method for imaging and evaluating dynamic changes in the neutrophil recruitment in intestinal IRI using two-photon laser scanning microscopy (TPLSM).

METHODS

LysM-eGFP mice were subjected to 45 min of warm intestinal ischemia followed by reperfusion. Mice received an intravenous injection of tetramethylrhodamine isothiocyanate-labeled albumin to visualize the microvasculature. Using a time-lapse TPLSM technique, we directly observed the behavior of neutrophils in intestinal IRI.

RESULTS

We were able to image all layers of the intestine without invasive surgical stress. At low-magnification, the number of neutrophils per field of view continued to increase for 4 h after reperfusion. High-magnification images revealed the presence or absence of blood circulation. At 0-2 h after reperfusion, rolling and adhesive neutrophils increased along the vasculature. At 2-4 h after reperfusion, the irregularity of crypt architecture and transmigration of neutrophils were observed in the lamina propria. Furthermore, TPLSM imaging revealed the villus height, the diameters of the crypt, and the number of infiltrating neutrophils in the crypt. In the IRI group, the villus height 4 h after reperfusion was significantly shorter than in the control group.

CONCLUSIONS

TPLSM imaging revealed the real-time neutrophil recruitment in intestinal IRI. Z-stack imaging was useful for evaluating pathophysiological changes in the intestinal wall.

Propagating Wave of ERK Activation Orients Collective Cell Migration

The biophysical framework of collective cell migration has been extensively investigated in recent years; however, it remains elusive how chemical inputs from neighboring cells are integrated to coordinate the collective movement. Here, we provide evidence that propagation waves of extracellular signal-related kinase (ERK) mitogen-activated protein kinase activation determine the direction of the collective cell migration. A wound-healing assay of Mardin-Darby canine kidney (MDCK) epithelial cells revealed two distinct types of ERK activation wave, a "tidal wave" from the wound, and a self-organized "spontaneous wave" in regions distant from the wound. In both cases, MDCK cells collectively migrated against the direction of the ERK activation wave. The inhibition of ERK activation propagation suppressed collective cell migration. An ERK activation wave spatiotemporally controlled actomyosin contraction and cell density. Furthermore, an optogenetic ERK activation wave reproduced the collective cell migration. These data provide new mechanistic insight into how cells sense the direction of collective cell migration.

Activation of Erk in ileal epithelial cells engaged in ischemic-injury repair

Intestinal epithelial cells function as a barrier to protect our body from various agents; therefore, any damage to these cells must be immediately repaired. Several in vivo and vitro studies have shown the involvement of Erk (extracellular signal-regulated kinase) in the regeneration process; however, the spatial regulation of Erk related to tissue morphology has not been well documented. Using two-photon microscopy and mice carrying a Förster resonance energy transfer-based biosensor, we here monitored the Erk activity in the ileal epithelial cells of living mice. Forty-eight h after ischemia-induced injury, epithelial cells were observed as a monolayer covering the injured area. The Erk activity in these cells was higher than that in the epithelial cells at the surrounding crypts, and treatment with an epidermal growth factor receptor inhibitor cancelled the higher Erk activity. The resealing epithelial cells were not in the G₂/M phase of the cell cycle, and Yap (Yes-associated protein) was localized to the nucleus. Immunostaining of intestinal ulcers from patients revealed ERK phosphorylation and nucleus localization of YAP without Ki-67 staining in the resealing epithelial cells. These findings led us to propose that the YAP-EGFR-ERK axis is involved in migration, but not in proliferation, of the resealing epithelial cells.

Live imaging of extracellular signal-regulated kinase and protein kinase A activities during thrombus formation in mice expressing biosensors based on Förster resonance energy transfer

Essentials Spatiotemporal regulation of protein kinases during thrombus formation remains elusive in vivo. Activities of protein kinases were live imaged in mouse platelets at laser-ablated arterioles. Protein kinase A was activated in the dislodging platelets at the downstream side of the thrombus. Extracellular signal-regulated kinase was activated at the core of contracting platelet aggregates.

SUMMARY

Background The dynamic features of thrombus formation have been visualized by conventional video widefield microscopy or confocal microscopy in live mice. However, owing to technical limitations, the precise spatiotemporal regulation of intracellular signaling molecule activities, which have been extensively studied in vitro, remains elusive in vivo. Objectives To visualize, by the use of two-photon excitation microscopy of transgenic mice expressing Förster resonance energy transfer (FRET) biosensors for extracellular signal-regulated kinase (ERK) and protein kinase A (PKA), ERK and PKA activities during thrombus formation in laser-injured subcutaneous arterioles. Results When a core of densely packed platelets had developed, ERK activity was increased from the basal region close to the injured arterioles. PKA was activated at the downstream side of an unstable shell overlaying the core of platelets. Intravenous administration of a MEK inhibitor, PD0325901, suppressed platelet tethering and dislodged platelet aggregates, indicating that ERK activity is indispensable for both initiation and maintenance of the thrombus. A cAMP analog, dbcAMP, inhibited platelet tethering but failed to dislodge the preformed platelet aggregates, suggesting that PKA can antagonize thrombus formation only in the early phase. Conclusion In vivo imaging of transgenic mice expressing FRET biosensors will open a new opportunity to visualize the spatiotemporal changes in signaling molecule activities not only during thrombus formation but also in other hematologic disorders.

Intravital microscopy in historic and contemporary immunology

In this review, we discuss intravital microscopy of immune cells, starting from its historic origins to current applications in diverse organs. It is clear from a quantitative review of the literature that intravital microscopy is a key tool in both historic and contemporary immunological research, providing unique advances in our understanding of immune responses. We have chosen to focus this review on how intravital microscopy methodologies are used to image specific organs or systems and we present recent descriptions of fundamental immunological processes that could not have been achieved by other methods. The following target organs/systems are discussed in more detail: cremaster muscle, skin (ear and dorsal skin fold chamber), lymph node, liver, lung, mesenteric vessels, carotid artery, bone marrow, brain, spleen, foetus and lastly vessels of the knee joint.

NR4A orphan nuclear receptor family members, NR4A2 and NR4A3, regulate neutrophil number and survival

The lifespan of neutrophils is plastic and highly responsive to factors that regulate cellular survival. Defects in neutrophil number and survival are common to both hematologic disorders and chronic inflammatory diseases. At sites of inflammation, neutrophils respond to multiple signals that activate protein kinase A (PKA) signaling, which positively regulates neutrophil survival. The aim of this study was to define transcriptional responses to PKA activation and to delineate the roles of these factors in neutrophil function and survival. In human neutrophil gene array studies, we show that PKA activation upregulates a significant number of apoptosis-related genes, the most highly regulated of these being NR4A2 and NR4A3 Direct PKA activation by the site-selective PKA agonist pair N6/8-AHA (8-AHA-cAMP and N6-MB-cAMP) and treatment with endogenous activators of PKA, including adenosine and prostaglandin E2, results in a profound delay of neutrophil apoptosis and concomitant upregulation of NR4A2/3 in a PKA-dependent manner. NR4A3 expression is also increased at sites of neutrophilic inflammation in a human model of intradermal inflammation. PKA activation also promotes survival of murine neutrophil progenitor cells, and small interfering RNA to NR4A2 decreases neutrophil production in this model. Antisense knockdown of NR4A2 and NR4A3 homologs in zebrafish larvae significantly reduces the absolute neutrophil number without affecting cellular migration. In summary, we show that NR4A2 and NR4A3 are components of a downstream transcriptional response to PKA activation in the neutrophil, and that they positively regulate neutrophil survival and homeostasis.

Context-dependent intravital imaging of therapeutic response using intramolecular FRET biosensors

Intravital microscopy represents a more physiologically relevant method for assessing therapeutic response. However, the movement into an in vivo setting brings with it several additional considerations, the primary being the context in which drug activity is assessed. Microenvironmental factors, such as hypoxia, pH, fibrosis, immune infiltration and stromal interactions have all been shown to have pronounced effects on drug activity in a more complex setting, which is often lost in simpler two- or three-dimensional assays. Here we present a practical guide for the application of intravital microscopy, looking at the available fluorescent reporters and their respective expression systems and analysis considerations. Moving in vivo, we also discuss the microscopy set up and methods available for overlaying microenvironmental context to the experimental readouts. This enables a smooth transition into applying higher fidelity intravital imaging to improve the drug discovery process.

Intravital Förster resonance energy transfer imaging reveals osteopontin-mediated polymorphonuclear leukocyte activation by tumor cell emboli

Myeloid‐derived suppressor cells (MDSCs) cause paraneoplastic leukemoid reactions and facilitate tumor cell metastasis. However, the interaction of MDSCs with tumor cells in live tissue has not been adequately visualized. To accomplish this task, we developed an intravital imaging protocol to observe metastasized tumor cells in mouse lungs. For visualization of the activation of MDSCs, bone marrow cells derived from transgenic mice expressing a Förster resonance energy transfer biosensor for ERK were implanted into host mice. Under a two‐photon excitation microscope, numerous polymorphonuclear cells (PMNs) were found to infiltrate the lungs of tumor‐bearing mice in which 4T1 mammary tumor cells were implanted into the footpads. By Förster resonance energy transfer imaging, we found ERK activation in PMNs around the 4T1 tumor emboli in the lungs. Because antibody array analysis implied the involvement of osteopontin (OPN) in the metastasis of 4T1 cells, we further analyzed the effect of OPN knockdown. The OPN knockdown in 4T1 cells did not affect the cell growth, but markedly suppressed lung metastasis of 4T1 cells and ERK activation in PMNs in the lung. Intravenous injection of recombinant OPN restored the lung metastasis of OPN‐deficient 4T1 cells, suggesting that OPN functioned in a paracrine manner. It has been reported that ERK activation of neutrophils causes NETosis and that PMNs promote metastasis of tumor cells by NETosis. In agreement with previous reports, the NETosis inhibitor DNase I inhibited lung metastasis of 4T1 cells. These observations suggest that OPN promotes metastasis of 4T1 cells by activating PMNs and inducing NETosis.

Measuring ERK Activity Dynamics in Single Living Cells Using FRET Biosensors

Fluorescence resonance energy transfer (FRET)-based biosensors are powerful tools for measuring spatio-temporal signaling dynamics in single living cells with subcellular resolution. There are quite a number of already existing sensors and this technology is increasingly used to obtain quantitative dynamic datasets. In this chapter, we describe the analysis of endogenous extracellular signal-regulated kinase (ERK) activity in living cells using the EKAR2G (ERK activity reporter second generation) probe. We focus on the generation of stable cell lines expressing the EKAR2G sensor as well as data acquisition and analysis.