Membrane-Associated, Not Cytoplasmic or Nuclear, FGFR1 Induces Neuronal Differentiation

Expression of fibroblast growth factor receptor 1 correlates inversely with the efficacy of single-agent fibroblast growth factor receptor-specific inhibitors in pancreatic cancer

BACKGROUND AND PURPOSE

Elevated fibroblast growth factor receptor (FGFR) activity correlates with pancreatic adenocarcinoma (PDAC) progression and poor prognosis. However, its potential as a therapeutic target remains largely unexplored.

EXPERIMENTAL APPROACH

The mechanisms of action and therapeutic effects of selective pan-FGFR inhibitors (pan-FGFRi) were explored using in vitro and in vivo PDAC models ranging from gemcitabine-sensitive to highly gemcitabine-resistant (GemR). Gain-/loss-of-function investigations were employed to define the role of individual FGFRs in cell proliferation, migration, and treatment response and resistance.

RESULTS

The pan-FGFRi NVP-BGJ398 significantly inhibited cell proliferation, migration, and invasion, and downregulated key cell survival- and invasiveness markers in multiple PDAC cell lines. Gemcitabine is a standard-of-care for PDAC, but development of resistance to gemcitabine (GemR) compromises its efficacy. Acquired GemR was modelled experimentally by developing highly GemR cells using escalating gemcitabine exposure in vitro and in vivo. FGFRi treatment inhibited GemR cell proliferation, migration, GemR marker expression, and tumour progression. FGFR2 or FGFR3 loss-of-function by shRNA knockdown failed to decrease cell growth, whereas FGFR1 knockdown was lethal. FGFR1 overexpression promoted cell migration more than proliferation, and reduced FGFRi-mediated inhibition of proliferation and migration. Single-agent FGFRi suppressed the viability and growth of multiple patient-derived xenografts inversely with respect to FGFR1 expression, underscoring the influence of FGFR1-dependent tumour responses to FGFRi. Importantly, secondary data analysis showed that PDAC tumours expressed FGFR1 at lower levels than in normal pancreas tissue.

CONCLUSIONS AND IMPLICATIONS

Single-agent FGFR inhibitors mediate selective, molecularly-targeted suppression of PDAC proliferation, and their effects are greatest in PDAC tumours expressing low-to-moderate levels of FGFR1.

The TNFR Wengen regulates the FGF pathway by an unconventional mechanism

Unveiling the molecular mechanisms of receptor activation has led to much understanding of development as well as the identification of important drug targets. We use the Drosophila tracheal system to study the activity of two families of widely used and conserved receptors, the TNFRs and the RTK-FGFRs. Breathless, an FGFR, controls the program of differentiation of the tracheal terminal cells in response to ligand activation. Here we identify a role for Wengen, a TNFR, in repressing the terminal cell program by regulating the MAPK pathway downstream of Breathless. We find that Wengen acts independently of both its canonical ligand and downstream pathway genes. Wengen does not stably localise at the membrane and is instead internalised-a trafficking that seems essential for activity. We show that Breathless and Wengen colocalise in intracellular vesicles and form a complex. Furthermore, Wengen regulates Breathless accumulation, possibly regulating Breathless trafficking and degradation. We propose that, in the tracheal context, Wengen interacts with Breathless to regulate its activity, and suggest that such unconventional mechanism, involving binding by TNFRs to unrelated proteins, may be a general strategy of TNFRs.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Light-activated receptor tyrosine kinases: Designs and applications

Receptor tyrosine kinases (RTKs) are a large and essential membrane receptor family. The molecular mechanisms and physiological consequences of RTK activation depend on, for example, ligand identity, subcellular localization, and developmental or disease stage. In the past few years, genetically-encoded light-activated RTKs (Opto-RTKs) have been developed to dissect these complexities by providing reversible and spatio-temporal control over cell signaling. These methods have very recently matured to include highly-sensitive multi-color actuators. The new ability to regulate RTK activity with high precision has been recently harnessed to gain mechanistic insights in subcellular, tissue, and animal models. Because of their sophisticated engineering, Opto-RTKs may only mirror some aspects of natural activation mechanisms but nevertheless offer unique opportunities to study RTK signaling and physiology.

Optogenetic activation of intracellular signaling based on light-inducible protein-protein homo-interactions

Dynamic protein-protein interactions are essential for proper cell functioning. Homo-interaction events—physical interactions between the same type of proteins—represent a pivotal subset of protein-protein interactions that are widely exploited in activating intracellular signaling pathways. Capacities of modulating protein-protein interactions with spatial and temporal resolution are greatly desired to decipher the dynamic nature of signal transduction mechanisms. The emerging optogenetic technology, based on genetically encoded light-sensitive proteins, provides promising opportunities to dissect the highly complex signaling networks with unmatched specificity and spatiotemporal precision. Here we review recent achievements in the development of optogenetic tools enabling light-inducible protein-protein homo-interactions and their applications in optical activation of signaling pathways.

A Cardiac Mitochondrial FGFR1 Mediates the Antithetical Effects of FGF2 Isoforms on Permeability Transition

Mitochondria, abundant organelles in high energy demand cells such as cardiomyocytes, can determine cell death or survival by regulating the opening of mitochondrial permeability transition pore, mPTP. We addressed the hypothesis that the growth factor FGF2, known to reside in intracellular locations, can directly influence mitochondrial susceptibility to mPTP opening. Rat cardiac subsarcolemmal (SSM) or interfibrillar (IFM) mitochondrial suspensions exposed directly to rat 18 kDa low molecular weight (Lo-) FGF2 isoform displayed increased resistance to calcium overload-induced mPTP, measured spectrophotometrically as "swelling", or as cytochrome c release from mitochondria. Inhibition of mitochondrial protein kinase C epsilon abrogated direct Lo-FGF2 mito-protection. Exposure to the rat 23 kDa high molecular weight (Hi) FGF2 isoform promoted cytochrome c release from SSM and IFM under nonstressed conditions. The effect of Hi-FGF2 was prevented by mPTP inhibitors, pre-exposure to Lo-FGF2, and okadaic acid, a serine/threonine phosphatase inhibitor. Western blotting and immunoelectron microscopy pointed to the presence of immunoreactive FGFR1 in cardiac mitochondria in situ. The direct mito-protective effect of Lo-FGF2, as well as the deleterious effect of Hi-FGF2, were prevented by FGFR1 inhibitors and FGFR1 neutralizing antibodies. We propose that intracellular FGF2 isoforms can modulate mPTP opening by interacting with mito-FGFR1 and relaying isoform-specific intramitochondrial signal transduction.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

Optogenetic Control of the Canonical Wnt Signaling Pathway During Xenopus laevis Embryonic Development

Optogenetics uses light-inducible protein-protein interactions to precisely control the timing, localization, and intensity of signaling activity. The precise spatial and temporal resolution of this emerging technology has proven extremely attractive to the study of embryonic development, a program faithfully replicated to form the same organism from a single cell. We have previously performed a comparative study for optogenetic activation of receptor tyrosine kinases, where we found that the cytoplasm-to-membrane translocation-based optogenetic systems outperform the membrane-anchored dimerization systems in activating the receptor tyrosine kinase signaling in live Xenopus embryos. Here, we determine if this engineering strategy can be generalized to other signaling pathways involving membrane-bound receptors. As a proof of concept, we demonstrate that the cytoplasm-to-membrane translocation of the low-density lipoprotein receptor-related protein-6 (LRP6), a membrane-bound coreceptor for the canonical Wnt pathway, triggers Wnt activity. Optogenetic activation of LRP6 leads to axis duplication in developing Xenopus embryos, indicating that the cytoplasm-to-membrane translocation of the membrane-bound receptor could be a generalizable strategy for the construction of optogenetic systems.

Optogenetic Approaches for the Spatiotemporal Control of Signal Transduction Pathways

Biological signals are sensed by their respective receptors and are transduced and processed by a sophisticated intracellular signaling network leading to a signal-specific cellular response. Thereby, the response to the signal depends on the strength, the frequency, and the duration of the stimulus as well as on the subcellular signal progression. Optogenetic tools are based on genetically encoded light-sensing proteins facilitating the precise spatiotemporal control of signal transduction pathways and cell fate decisions in the absence of natural ligands. In this review, we provide an overview of optogenetic approaches connecting light-regulated protein-protein interaction or caging/uncaging events with steering the function of signaling proteins. We briefly discuss the most common optogenetic switches and their mode of action. The main part deals with the engineering and application of optogenetic tools for the control of transmembrane receptors including receptor tyrosine kinases, the T cell receptor and integrins, and their effector proteins. We also address the hallmarks of optogenetics, the spatial and temporal control of signaling events.

Fibroblast Growth Factor Signalling in the Diseased Nervous System

Fibroblast growth factors (FGFs) act as key signalling molecules in brain development, maintenance, and repair. They influence the intricate relationship between myelinating cells and axons as well as the association of astrocytic and microglial processes with neuronal perikarya and synapses. Advances in molecular genetics and imaging techniques have allowed novel insights into FGF signalling in recent years. Conditional mouse mutants have revealed the functional significance of neuronal and glial FGF receptors, not only in tissue protection, axon regeneration, and glial proliferation but also in instant behavioural changes. This review provides a summary of recent findings regarding the role of FGFs and their receptors in the nervous system and in the pathogenesis of major neurological and psychiatric disorders.

Photoreaction Mechanisms of Flavoprotein Photoreceptors and Their Applications

Three classes of flavoprotein photoreceptors, cryptochromes (CRYs), light-oxygen-voltage (LOV)-domain proteins, and blue light using FAD (BLUF)-domain proteins, have been identified that control various physiological processes in multiple organisms. Accordingly, signaling activities of photoreceptors have been intensively studied and the related mechanisms have been exploited in numerous optogenetic tools. Herein, we summarize the current understanding of photoactivation mechanisms of the flavoprotein photoreceptors and review their applications.

On-section correlative light and electron microscopy of large cellular volumes using STEM tomography

The application of both fluorescence and electron microscopy results in a powerful combination of imaging modalities called "correlative light and electron microscopy" (CLEM). Whereas conventional transmission electron microscopy (TEM) tomography is only able to image sections up to a thickness of ~300nm, scanning transmission electron microscopy (STEM) tomography at 200kV allows the analysis of sections up to a thickness of 900nm in three dimensions. In the current study we have successfully integrated STEM tomography into CLEM as demonstrated for human retinal pigment epithelial 1 (RPE1) cells expressing various fluorescent fusion proteins which were high-pressure frozen and then embedded in Lowicryl HM20. Fluorescently labeled gold nanoparticles were applied onto resin sections and imaged by fluorescence and electron microscopy. STEM tomograms were recorded at regions of interest, and overlays were generated using the eC-CLEM software package. Through the nuclear staining of living cells, the use of fluorescently labeled gold fiducials for the generation of overlays, and the integration of STEM tomography we have markedly extended the application of the Kukulski protocol (Kukulski et al., 2011, 2012). Various fluorescently tagged proteins localizing to different cellular organelles could be assigned to their ultrastructural compartments. By combining STEM tomography with on-section CLEM, fluorescently tagged proteins can be localized in three-dimensional ultrastructural environments with a volume of at least 2.7×2.7×0.5μm.

Bacterial Phytochrome as a Scaffold for Engineering of Receptor Tyrosine Kinases Controlled with Near-Infrared Light

Optically controlled receptor tyrosine kinases (opto-RTKs) allow regulation of RTK signaling using light. Until recently, the majority of opto-RTKs were activated with blue-green light. Fusing a photosensory core module of Deinococcus radiodurans bacterial phytochrome (DrBphP-PCM) to the kinase domains of neurotrophin receptors resulted in opto-RTKs controlled with light above 650 nm. To expand this engineering approach to RTKs of other families, here we combined the DrBpP-PCM with the cytoplasmic domains of EGFR and FGFR1. The resultant Dr-EGFR and Dr-FGFR1 opto-RTKs are rapidly activated with near-infrared and inactivated with far-red light. The opto-RTKs efficiently trigger ERK1/2, PI3K/Akt, and PLCγ signaling. Absence of spectral crosstalk between the opto-RTKs and green fluorescent protein-based biosensors enables simultaneous Dr-FGFR1 activation and detection of calcium transients. Action mechanism of the DrBphP-PCM-based opto-RTKs is considered using the available RTK structures. DrBphP-PCM represents a versatile scaffold for engineering of opto-RTKs that are reversibly regulated with far-red and near-infrared light.

A Generalizable Optogenetic Strategy to Regulate Receptor Tyrosine Kinases during Vertebrate Embryonic Development

Ligand-independent activation of receptor tyrosine kinases (RTKs) allows for dissecting out the receptor-specific signaling outcomes from the pleiotropic effects of the ligands. In this regard, RTK intracellular domains (ICD) are of interest due to their ability to recapitulate signaling activity in a ligand-independent manner when fused to chemical or optical dimerizing domains. A common strategy for synthetic activation of RTKs involves membrane tethering of dimerizer-RTK ICD fusions. Depending on the intrinsic signaling capacity, however, this approach could entail undesirable baseline signaling activity in the absence of stimulus, thereby diminishing the system's sensitivity. Here, we observed toxicity in early Xenopus laevis embryos when using such a conventional optogenetic design for the fibroblast growth factor receptor (FGFR). To surpass this challenge, we developed a cytoplasm-to-membrane translocation approach, where FGFR ICD is recruited from the cytoplasm to the plasma membrane by light, followed by its subsequent activation via homo-association. This strategy results in the optical activation of FGFR with low background activity and high sensitivity, which allows for the light-mediated formation of ectopic tail-like structures in developing X. laevis embryos. We further generalized this strategy by developing optogenetic platforms to control three neurotrophic tropomyosin receptor kinases, TrkA, TrkB, and TrkC. We envision that these ligand-independent optogenetic RTKs will provide useful toolsets for the delineation of signaling sub-circuits in developing vertebrate embryos.

RhoA-GTPase Modulates Neurite Outgrowth by Regulating the Expression of Spastin and p60-Katanin

RhoA-GTPase (RhoA) is widely regarded as a key molecular switch to inhibit neurite outgrowth by rigidifying the actin cytoskeleton. However, during neurite outgrowth, whether and how microtubule dynamics are regulated by RhoA remains to be elucidated. Herein, CT04 and Y27632 were used to inactivate RhoA and its downstream effector Rho-associated coiled coil-forming kinase (ROCK), while the RhoAQ63L lentiviral vector was utilized to overexpress the constitutively activated RhoA in dorsal root ganglion (DRG) neurons or neuronal differentiated PC12 cells. The current data illustrate that the RhoA signaling pathway negatively modulates neurite outgrowth and elevates the expression of Glu-tubulin (a marker for a stabilized microtubule). Meanwhile, the microtubule-severing proteins spastin and p60-katanin were downregulated by the RhoA signaling pathway. When spastin and p60-katanin were knocked down, the effects of RhoA inhibition on neurite outgrowth were significantly reversed. Taken together, this study demonstrates that the RhoA pathway-mediated inhibition of neurite outgrowth is not only related to the modulation of microfilament dynamics but is also attributable to the regulation of the expression of spastin and p60-katanin and thus influences microtubule dynamics.