Kinetics of binding, endocytosis, and recycling of EGF receptor mutants

Dynamic Contrast-Enhanced Magnetic Resonance Imaging for the Prediction of Monoclonal Antibody Tumor Disposition

The prediction of monoclonal antibody (mAb) disposition within solid tumors for individual patients is difficult due to inter-patient variability in tumor physiology. Improved a priori prediction of mAb pharmacokinetics in tumors may facilitate the development of patient-specific dosing protocols and facilitate improved selection of patients for treatment with anti-cancer mAb. Here, we report the use of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), with tumor penetration of the contrast agent gadobutrol used as a surrogate, to improve physiologically based pharmacokinetic model (PBPK) predictions of cetuximab pharmacokinetics in epidermal growth factor receptor (EGFR) positive xenografts. In the initial investigations, mice bearing Panc-1, NCI-N87, and LS174T xenografts underwent DCE-MRI imaging with the contrast agent gadobutrol, followed by intravenous dosing of an 125Iodine-labeled, non-binding mAb (8C2). Tumor concentrations of 8C2 were determined following the euthanasia of mice (3 h-6 days after 8C2 dosing). Potential predictor relationships between DCE-MRI kinetic parameters and 8C2 PBPK parameters were evaluated through covariate modeling. The addition of the DCE-MRI parameter Ktrans alone or Ktrans in combination with the DCE-MRI parameter Vp on the PBPK parameters for tumor blood flow (QTU) and tumor vasculature permeability (σTUV) led to the most significant improvement in the characterization of 8C2 pharmacokinetics in individual tumors. To test the utility of the DCE-MRI covariates on a priori prediction of the disposition of mAb with high-affinity tumor binding, a second group of tumor-bearing mice underwent DCE-MRI imaging with gadobutrol, followed by the administration of 125Iodine-labeled cetuximab (a high-affinity anti-EGFR mAb). The MRI-PBPK covariate relationships, which were established with the untargeted antibody 8C2, were implemented into the PBPK model with considerations for EGFR expression and cetuximab-EGFR interaction to predict the disposition of cetuximab in individual tumors (a priori). The incorporation of the Ktrans MRI parameter as a covariate on the PBPK parameters QTU and σTUV decreased the PBPK model prediction error for cetuximab tumor pharmacokinetics from 223.71 to 65.02%. DCE-MRI may be a useful clinical tool in improving the prediction of antibody pharmacokinetics in solid tumors. Further studies are warranted to evaluate the utility of the DCE-MRI approach to additional mAbs and additional drug modalities.

Morphogen gradient scaling by recycling of intracellular Dpp

Morphogen gradients are fundamental to establish morphological patterns in developing tissues¹. During development, gradients scale to remain proportional to the size of growing organs^2,3. Scaling is a universal gear adjusting patterns to size in living organisms^3-8, yet its mechanisms remain unclear. Here, focusing on the Dpp gradient in the Drosophila wing disc, we unravel a cell biological basis behind scaling. From small to large discs, scaling of the Dpp gradient is achieved by increasing the contribution of the internalized Dpp molecules to Dpp transport: to expand the gradient, endocytosed molecules are re-exocytosed to spread extracellularly. To regulate the contribution of endocytosed Dpp to the spreading extracellular pool during tissue growth, it is the Dpp binding rates that are progressively modulated by the extracellular factor Pentagone, driving scaling. Thus, for some morphogens, evolution may act on endocytic trafficking to regulate the range of the gradient and its scaling, which could allow adaptation of shape and pattern to different sizes of organs in different species.

Transport of environmental natural organic matter coated silver nanoparticle across cell membrane based on membrane etching treatment and inhibitors

Environmental natural organic matters (NOMs) have great effects on the physicochemical properties of engineering nanoparticles, which may impact the transport of nanoparticles across plasma membrane and the cytotoxicity. Therefore, the kinetics, uptake pathway and mass of transporting into A549 cell membrane of silver nanoparticles (AgNPs) coated with citric acid (CA), tartaric acid (TA) and fulvic acid (FA) were investigated, respectively. CA, FA and TA enhanced the colloidal stability of AgNPs in culture medium and have greatly changed the surface plasmon resonance spectrum of AgNPs due to the absorption of CA, FA and TA on surface of AgNPs. Internalizing model showed that velocity of CA-, TA- and FA-nAg transporting into A549 cell were 5.82-, 1.69- and 0.29-fold higher than those of the control group, respectively. Intracellular mass of Ag was dependent on mass of AgNPs delivered to cell from suspension, which obeyed Logistic model and was affected by NOMs that CA- and TA-nAg showed a large promotion on intracellular mass of Ag. The lipid raft/caveolae-mediated endocytosis (LME) of A549 cell uptake of AgNPs were susceptible to CA, TA and FA that uptake of CA-, TA- and FA-nAg showed lower degree of dependent on LME than that of the control (uncoated AgNPs). Actin-involved uptake pathway and macropinocytosis would have less contribution to uptake of FA-nAg. Overall, transmembrane transport of NOMs-coated AgNPs differs greatly from that of the pristine AgNPs.

Correcting for targeted and control agent signal differences in paired-agent molecular imaging of cancer cell-surface receptors

Paired-agent kinetic modeling protocols provide one means of estimating cancer cell-surface receptors with in vivo molecular imaging. The protocols employ the coadministration of a control imaging agent with one or more targeted imaging agent to account for the nonspecific uptake and retention of the targeted agent. These methods require the targeted and control agent data be converted to equivalent units of concentration, typically requiring specialized equipment and calibration, and/or complex algorithms that raise the barrier to adoption. This work evaluates a kinetic model capable of correcting for targeted and control agent signal differences. This approach was compared with an existing simplified paired-agent model (SPAM), and modified SPAM that accounts for signal differences by early time point normalization of targeted and control signals (SPAMPN). The scaling factor model (SPAMSF) outperformed both SPAM and SPAMPN in terms of accuracy and precision when the scale differences between targeted and imaging agent signals (α) were not equal to 1, and it matched the performance of SPAM for α  =  1. This model could have wide-reaching implications for quantitative cancer receptor imaging using any imaging modalities, or combinations of imaging modalities, capable of concurrent detection of at least two distinct imaging agents (e.g., SPECT, optical, and PET/MR).

Using Percoll Gradient Fractionation to Study the Endocytic Trafficking of the EGFR

Scientists have separated subcellular compartments based on differences in their densities by gradient centrifugation for decades. With the proper equipment and thoughtful experimental design, density gradients are a reliable method for enriching various intracellular compartments to assess their protein composition, morphology, or biochemical activity. While variations of this basic technique have been used for separating whole cells, endosomes, synaptic vesicles, and viruses, we have found it especially useful for resolving the compartments of the endocytic pathway. In particular, this technique has been valuable for studying the regulation and signaling of the Epidermal Growth Factor Receptor (EGFR) while undergoing endocytic trafficking. In this article we detail the technical aspects of utilizing Percoll gradients to study the endocytic trafficking of the EGFR and associated proteins.

Rinsing paired-agent model (RPAM) to quantify cell-surface receptor concentrations in topical staining applications of thick tissues

Conventional molecular assessment of tissue through histology, if adapted to fresh thicker samples, has the potential to enhance cancer detection in surgical margins and monitoring of 3D cell culture molecular environments. However, in thicker samples, substantial background staining is common despite repeated rinsing, which can significantly reduce image contrast. Recently, 'paired-agent' methods-which employ co-administration of a control (untargeted) imaging agent-have been applied to thick-sample staining applications to account for background staining. To date, these methods have included (1) a simple ratiometric method that is relatively insensitive to noise in the data but has accuracy that is dependent on the staining protocol and the characteristics of the sample; and (2) a complex paired-agent kinetic modeling method that is more accurate but is more noise-sensitive and requires a precise serial rinsing protocol. Here, a new simplified mathematical model-the rinsing paired-agent model (RPAM)-is derived and tested that offers a good balance between the previous models, is adaptable to arbitrary rinsing-imaging protocols, and does not require calibration of the imaging system. RPAM is evaluated against previous models and is validated by comparison to estimated concentrations of targeted biomarkers on the surface of 3D cell culture and tumor xenograft models. This work supports the use of RPAM as a preferable model to quantitatively analyze targeted biomarker concentrations in topically stained thick tissues, as it was found to match the accuracy of the complex paired-agent kinetic model while retaining the low noise-sensitivity characteristics of the ratiometric method.

Synthesis and biochemical characterization of EGF receptor in a water-soluble membrane model system

ErbB (Erythroblastic Leukemia Viral Oncogene Homolog) receptor tyrosine kinases are critical for tissue development and maintenance, and frequently become oncogenic when mutated or overexpressed. In vitro analysis of ErbB receptor kinases can be difficult because of their large size and poor water solubility. Here we report improved production and assembly of the correctly folded full-length EGF receptor (EGFR) into nanolipoprotein particles (NLPs). NLPs are ~10 nm in diameter discoidal cell membrane mimics composed of apolipoproteins surrounding a lipid bilayer. NLPs containing EGFR were synthesized via incubation of baculovirus-produced recombinant EGFR with apolipoprotein and phosphoplipids under conditions that favor self-assembly. The resulting EGFR-NLPs were the correct size, formed dimers and multimers, had intrinsic autophosphorylation activity, and retained the ability to interact with EGFR-targeted ligands and inhibitors consistent with previously-published in vitro binding affinities. We anticipate rapid adoption of EGFR-NLPs for structural studies of full-length receptors and drug screening, as well as for the in vitro characterization of ErbB heterodimers and disease-relevant mutants.

Surface coupling strength of gold nanoparticles affects cytotoxicity towards neurons

Weakly bound gold nanoparticles reveal awful toxicity towards neurons.

High affinity nanobodies against human epidermal growth factor receptor selected on cells by E. coli display

Most therapeutic antibodies (Abs) target cell surface proteins on tumor and immune cells. Cloning of Ab gene libraries in E. coli and their display on bacteriophages is commonly used to select novel therapeutic Abs binding target antigens, either purified or expressed on cells. However, the sticky nature of bacteriophages renders phage display selections on cells challenging. We previously reported an E. coli display system for expression of VHHs (i.e., nanobodies, Nbs) on the surface of bacteria and selection of high-affinity clones by magnetic cell sorting (MACS). Here, we demonstrate that E. coli display is also an attractive method for isolation of Nbs against cell surface antigens, such as the epidermal growth factor receptor (EGFR), upon direct selection and screening of Ab libraries on live cells. We employ a whole cell-based strategy using a VHH library obtained by immunization with human tumor cells over-expressing EGFR (i.e., A431), and selection of bacterial clones bound to murine fibroblast NIH-3T3 cells transfected with human EGFR, after depletion of non-specific clones on untransfected cells. This strategy resulted in the isolation of high-affinity Nbs binding distinct epitopes of EGFR, including Nbs competing with the ligand, EGF, as characterized by flow cytometry of bacteria displaying the Nbs and binding assays with purified Nbs using surface plasmon resonance. Hence, our study demonstrates that E. coli display of VHH libraries and selection on cells enables efficient isolation and characterization of high-affinity Nbs against cell surface antigens.

Scale-up of a physiologically-based pharmacokinetic model to predict the disposition of monoclonal antibodies in monkeys

Preclinical assessment of monoclonal antibody (mAb) disposition during drug development often includes investigations in non-human primate models. In many cases, mAb exhibit non-linear disposition that relates to mAb-target binding [i.e., target-mediated disposition (TMD)]. The goal of this work was to develop a physiologically-based pharmacokinetic (PBPK) model to predict non-linear mAb disposition in plasma and in tissues in monkeys. Physiological parameters for monkeys were collected from several sources, and plasma data for several mAbs associated with linear pharmacokinetics were digitized from prior literature reports. The digitized data displayed great variability; therefore, parameters describing inter-antibody variability in the rates of pinocytosis and convection were estimated. For prediction of the disposition of individual antibodies, we incorporated tissue concentrations of target proteins, where concentrations were estimated based on categorical immunohistochemistry scores, and with assumed localization of target within the interstitial space of each organ. Kinetics of target-mAb binding and target turnover, in the presence or absence of mAb, were implemented. The model was then employed to predict concentration versus time data, via Monte Carlo simulation, for two mAb that have been shown to exhibit TMD (2F8 and tocilizumab). Model predictions, performed a priori with no parameter fitting, were found to provide good prediction of dose-dependencies in plasma clearance, the areas under plasma concentration versu time curves, and the time-course of plasma concentration data. This PBPK model may find utility in predicting plasma and tissue concentration versus time data and, potentially, the time-course of receptor occupancy (i.e., mAb-target binding) to support the design and interpretation of preclinical pharmacokinetic-pharmacodynamic investigations in non-human primates.

Juxtacrine signaling is inherently noisy

Juxtacrine signaling is an important class of signaling systems that plays a crucial role in various developmental processes ranging from coordination of differentiation between neighboring cells to guiding axon growth during neurogenesis. Such signaling systems rely on the interaction between receptors on one cell and trans-membrane ligands on the membrane of a neighboring cell. Like other signaling systems, the ability of signal-receiving cells to accurately determine the concentration of ligands, is affected by stochastic diffusion processes. However, it is not clear how restriction of ligand movement to the two-dimensional (2D) cell membrane in juxtacrine signaling affects the accuracy of ligand sensing. In this study, we use a statistical mechanics approach, to show that long integration times, from around one second to several hours, are required to reach high-sensing accuracy (better than 10%). Surprisingly, the accuracy of sensing cannot be significantly improved, neither by increasing the number of receptors above three to five receptors per contact area, nor by increasing the contact area between cells. We show that these results impose stringent constraints on the dynamics of processes relying on juxtacrine signaling systems, such as axon guidance mediated by Ephrins and developmental patterns mediated by the Notch pathway.

A mechanistic tumor penetration model to guide antibody drug conjugate design

Antibody drug conjugates (ADCs) represent novel anti-cancer modalities engineered to specifically target and kill tumor cells expressing corresponding antigens. Due to their large size and their complex kinetics, these therapeutic agents often face heterogeneous distributions in tumors, leading to large untargeted regions that escape therapy. We present a modeling framework which includes the systemic distribution, vascular permeability, interstitial transport, as well as binding and payload release kinetics of ADC-therapeutic agents in mouse xenografts. We focused, in particular, on receptor dynamics such as endocytic trafficking mechanisms within cancer cells, to simulate their impact on tumor mass shrinkage upon ADC administration. Our model identified undesirable tumor properties that can impair ADC tissue homogeneity, further compromising ADC success, and explored ADC design optimization scenarios to counteract upon such unfavorable intrinsic tumor tissue attributes. We further demonstrated the profound impact of cytotoxic payload release mechanisms and the role of bystander killing effects on tumor shrinkage. This model platform affords a customizable simulation environment which can aid with experimental data interpretation and the design of ADC therapeutic treatments.

Rapid quantification of live cell receptors using bioluminescence in a flow-based microfluidic device

The number of receptors expressed by cells plays an important role in controlling cell signaling events, thus determining its behaviour, state and fate. Current methods of quantifying receptors on cells are either laborious or do not maintain the cells in their native form. Here, a method integrating highly sensitive bioluminescence, high precision microfluidics and small footprint of lensfree optics is developed to quantify cell surface receptors. This method is safe to use, less laborious, and faster than the conventional radiolabelling and near field scanning methods. It is also more sensitive than fluorescence based assays and is ideal for high throughput screening. In quantifying β(1) adrenergic receptors expressed on the surface of H9c2 cardiomyocytes, this method yields receptor numbers from 3.12 × 10(5) to 9.36 × 10(5) receptors/cell which are comparable with current methods. This can serve as a very good platform for rapid quantification of receptor numbers in ligand/drug binding and receptor characterization studies, which is an important part of pharmaceutical and biological research.

Toward a magic or imaginary bullet? Ligands for drug targeting to cancer cells: principles, hopes, and challenges

There are many problems directly correlated with the systemic administration of drugs and how they reach their target site. Targeting promises to be a hopeful strategy as an improved means of drug delivery, with reduced toxicity and minimal adverse side effects. Targeting exploits the high affinity of cell-surface-targeted ligands, either directly or as carriers for a drug, for specific retention and uptake by the targeted diseased cells. One of the most important parameters which should be taken into consideration in the selection of an appropriate ligand for targeting is the binding affinity (K D). In this review we focus on the importance of binding affinities of monoclonal antibodies, antibody derivatives, peptides, aptamers, DARPins, and small targeting molecules in the process of selection of the most suitable ligand for targeting of nanoparticles. In order to provide a critical comparison between these various options, we have also assessed each technology format across a range of parameters such as molecular size, immunogenicity, costs of production, clinical profiles, and examples of the level of selectivity and toxicity of each. Wherever possible, we have also assessed how incorporating such a targeted approach compares with, or is superior to, original treatments.

Regulation of EGFR signal transduction by analogue-to-digital conversion in endosomes

An outstanding question is how receptor tyrosine kinases (RTKs) determine different cell-fate decisions despite sharing the same signalling cascades. Here, we uncovered an unexpected mechanism of RTK trafficking in this process. By quantitative high-resolution FRET microscopy, we found that phosphorylated epidermal growth factor receptor (p-EGFR) is not randomly distributed but packaged at constant mean amounts in endosomes. Cells respond to higher EGF concentrations by increasing the number of endosomes but keeping the mean p-EGFR content per endosome almost constant. By mathematical modelling, we found that this mechanism confers both robustness and regulation to signalling output. Different growth factors caused specific changes in endosome number and size in various cell systems and changing the distribution of p-EGFR between endosomes was sufficient to reprogram cell-fate decision upon EGF stimulation. We propose that the packaging of p-RTKs in endosomes is a general mechanism to ensure the fidelity and specificity of the signalling response.

Coronary Microvascular Function and Beyond: The Crosstalk between Hormones, Cytokines, and Neurotransmitters

Beyond its hemodynamic function, the heart also acts as a neuroendocrine and immunoregulatory organ. A dynamic communication between the heart and other organs takes place constantly to maintain cardiovascular homeostasis. The current understanding highlights the importance of the endocrine, immune, and nervous factors to fine-tune the crosstalk of the cardiovascular system with the entire body. Once disrupted, this complex interorgan communication may promote the onset and the progression of cardiovascular diseases. Thus, expanding our knowledge on how these factors influence the cardiovascular system can lead to novel therapeutic strategies to improve patient care. In the present paper, we review novel concepts on the role of endocrine, immune, and nervous factors in the modulation of microvascular coronary function.

Spatial regulation of epidermal growth factor receptor signaling by endocytosis

Signaling by cell surface receptors appears to be relatively straight-forward: ligand binds to the extracellular domain of the receptor and biochemical changes are communicated into the cell. However, this process is more complex than it first seems due to the various mechanisms that regulate signaling. In order to effectively target these receptors for pharmacological purposes, a more complete understanding of how their signaling is regulated is needed. Here, how the endocytic pathway regulates receptor signaling is discussed, using the epidermal growth factor receptor (EGFR) as a model. In particular, the spatial regulation of signaling is examined. Areas of discussion include: how endocytic trafficking affects biology/pathology, varying approaches for studying the relationship between receptor endocytosis and signaling, and developments in how the endocytic pathway controls EGFR:effector communication and EGFR-mediated cell biology.

Neuropilin-1-dependent regulation of EGF-receptor signaling

Neuropilin-1 (NRP1) is a coreceptor for multiple extracellular ligands. NRP1 is widely expressed in cancer cells and in advanced human tumors; however, its functional relevance and signaling mechanisms are unclear. Here, we show that NRP1 expression controls viability and proliferation of different cancer cells, independent of its short intracellular tail. We found that the extracellular domain of NRP1 interacts with the EGF receptor (EGFR) and promotes its signaling cascade elicited upon EGF or TGF-α stimulation. Upon NRP1 silencing, the ability of ligand-bound EGFR to cluster on the cell surface, internalize, and activate the downstream AKT pathway is severely impaired. EGFR is frequently activated in human tumors due to overexpression, mutation, or sustained autocrine/paracrine stimulation. Here we show that NRP1-blocking antibodies and NRP1 silencing can counteract ligand-induced EGFR activation in cancer cells. Thus our findings unveil a novel molecular mechanism by which NRP1 can control EGFR signaling and tumor growth.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Novel outcomes and end points: biomarkers in chronic obstructive pulmonary disease clinical trials

Biomarker development in chronic obstructive pulmonary disease (COPD) is a nascent field, in part because of the complexity underlying COPD pathogenesis. The objective of this review is to provide examples of how biomarkers may be effectively applied in clinical trials of COPD by limiting their use to specific contexts and using them to answer well delineated questions. Types of novel outcomes or "biomarkers" that may be useful in clinical trials in COPD include analyses performed on bronchoscopically obtained samples, sputum, exhaled gases, blood, and urine and "ex vivo" assays performed using biological samples obtained from trial participants. These novel biological outcomes are rarely useful as primary end points in phase III clinical trials in COPD, because they are not typically recognized by the U.S. Food and Drug Administration or other regulatory agencies. More commonly, the applications of these outcomes include "proof-of-concept" decisions, demonstration that the intervention had the intended pharmacologic or biological effect, identification of patient subgroups that benefit most, and safety monitoring. Examples given in this review include outcomes used in a phase IIA study of an inhaled small molecule inhibitor of epidermal growth factor receptor. Large observational studies of COPD, including the ECLIPSE, COPDGene, and SPIROMICS studies will further inform our use of biomarkers in COPD clinical trials. To encourage the application of novel biomarkers in clinical trials, the Food and Drug Administration has developed a new process for biomarker "qualification." This process has been designed to be more efficient and to promote consensus building and sharing of preclinical data.

Safety and efficacy of an inhaled epidermal growth factor receptor inhibitor (BIBW 2948 BS) in chronic obstructive pulmonary disease

RATIONALE

Epidermal growth factor receptor (EGFR) activation is implicated in mucin hypersecretion in chronic obstructive pulmonary disease (COPD).

OBJECTIVES

To investigate the safety and efficacy of an inhaled EGFR antagonist (BIBW 2948) in COPD.

METHODS

Multicenter, double-blind, placebo-controlled trial of 4 weeks of treatment with two doses of BIBW 2948 (15 and 30 mg twice a day) on safety and mucin-related outcomes in 48 patients with COPD. The effect of BIBW 2948 on EGFR activation in airway epithelial cells was assessed using an ex vivo assay. Efficacy measures included the volume of mucin in the airway epithelium (Vs mu,bala) in bronchial biopsies and the expression of mucin genes in bronchial brushings.

MEASUREMENTS AND MAIN RESULTS

Inhaled BIBW 2948 induced a dose-related inhibition of EGFR internalization (reflecting decreased EGFR activation) in epithelial cells from treated subjects. However, BIBW 2948 was associated with a dose-related increase in adverse events, including reversible liver enzyme elevation (n = 2), and reduction in FEV(1). The changes in mucin stores and mucin gene expression were not significantly different in the pooled BIBW 2948 group versus placebo (volume of mucin per surface area of basal lamina = 0.22 +/- 7.11 vs. 0.47 +/- 8.06 microm(3)/microm(2); P = 0.93). However, in the 30 mg twice a day group, the reduction in epithelial mucin stores was greatest in subjects with the greatest degree of EGFR inhibition (Pearson r = 0.98; 95% confidence interval, 0.71-0.99).

CONCLUSIONS

Four-week treatment with BIBW 2948 did not significantly decrease epithelial mucin stores and was poorly tolerated in patients with COPD. Ex vivo analyses suggest that higher doses may be more effective at both EGFR inhibition and decreases in mucin stores but that adverse events should be expected. Clinical trial registered with www.clinicaltrials.gov (NCT00423137).

Expression of excess receptors and negative feedback control of signal pathways are required for rapid activation and prompt cessation of signal transduction

Background

Cellular signal transduction is initiated by the binding of extracellular ligands to membrane receptors. Receptors are often expressed in excess, and cells are activated when a small number of receptors bind ligands. Intracellular signal proteins are activated at a high level soon after ligand binding, and the activation level decreases in a negative feedback manner without ligand clearance. Why are excess receptors required? What is the physiological significance of the negative feedback regulation?

Results

To answer these questions, we developed a Monte Carlo simulation program to kinetically analyze signal pathways using the model in which ligands are bound to receptors and then membrane complexes with other membrane proteins are formed. Our simulation results showed that excess receptors are not required for cell activation when the dissociation constant (Kd) of the ligand-receptor complex is 10^-10 M or less. However, such low Kd values cause delayed signal shutdown after ligand clearance from the extracellular space. In contrast, when the Kd was 10^-8 M and the ligand level was less than 1 μM, excess receptors were required for prompt signal propagation and rapid signal cessation after ligand clearance. An initial increase in active cytosolic signal proteins to a high level is required for rapid activation of cellular signal pathways, and a low level of active signal proteins is essential for the rapid shutdown of signal pathways after ligand clearance.

Conclusion

The present kinetic analysis revealed that excess receptors and negative feedback regulation promote activation and cessation of signal transduction with a low amount of extracellular ligand.

Understanding the role of surface charges in cellular adsorption versus internalization by selectively removing gold nanoparticles on the cell surface with a I2/KI etchant

This Letter presents a new method for differentiating the Au nanospheres attached to the cell surface from those being internalized into the cells. We introduced an etching solution based on I2 and KI that can selectively dissolve the Au nanospheres on the cell surface within a short period of time. The advantage of this etchant is its low toxicity to the cells because it is capable of etching away a relatively large amount of Au nanospheres at a low molar concentration. By combining with quantitative elemental analysis, we found that the deposition of Au nanospheres on the surface of cancer cells was highly dependent on the sign of surface charges on the Au nanospheres. In addition, by fitting the uptake data with a kinetic model, we were able to derive the overall and internalization rate constants for Au nanospheres and both of them were found to be governed by the surface charges on Au nanospheres.

Epidermal growth factor receptor and cancer: control of oncogenic signalling by endocytosis

The epidermal growth factor receptor (EGFR) and other members of the EGFR/ErbB receptor family of receptor tyrosine kinases (RTKs) are important regulators of proliferation, angiogenesis, migration, tumorigenesis and metastasis. Overexpression, mutations, deletions and production of autocrine ligands contribute to aberrant activation of the ErbB proteins. The signalling output from EGFR is complicated given that other ErbB proteins are often additionally expressed and activated in the same cell, resulting in formation of homo-and/or heterodimers. In particular, association of EGFR with ErbB2 prevents its down-regulation, underscoring the importance of the cellular background for EGFR effects. Signalling from ErbB proteins can either be terminated by dissociation of ligand resulting in dephosphorylation, or blunted by degradation of the receptors. Although proteasomal targeting of ErbB proteins has been described, lysosomal degradation upon ligand-induced endocytosis seems to play the major role in EGFR down-regulation. Preclinical and clinical data have demonstrated that EGFR is a central player in cancer, especially in carcinomas, some brain tumours and in non-small cell lung cancer. Such studies have further validated EGFR as an important molecular target in cancer treatment. This review focuses on mechanisms involved in ligand-induced EGFR activation and endocytic down-regulation. A better understanding of EGFR biology should allow development of more tumour-selective therapeutic approaches targeting EGFR-induced signalling.

Identification of EGF receptor C-terminal sequences 1005–1017 and di-leucine motif 1010LL1011 as essential in EGF receptor endocytosis

Most studies regarding the role of epidermal growth factor (EGF) receptor (EGFR) C-terminal domain in EGFR internalization are done in the context of EGFR kinase activation. We recently showed that EGF-induced EGFR internalization is directly controlled by receptor dimerization, rather than kinase activation. Here we studied the role of EGFR C-terminus in EGF-induced EGFR internalization with or without EGFR kinase activation. We showed that graduate truncation of EGFR from C-terminus to 1044 did not affect EGF-induced EGFR endocytosis with or without kinase activation. However, truncation to 991 or further completely inhibited EGFR endocytosis. Graduate truncation within 991-1044 progressively lower EGF-induced EGFR endocytosis with most significant effects observed for residues 1005-1017. The endocytosis patterns of mutant EGFRs are independent of EGFR kinase activation. The residues 1005-1017 were also required for EGFR internalization triggered by non-ligand-induced receptor dimerization. This indicates that residues 1005-1017 function as an internalization motif, rather than a dimerization motif, to mediate EGFR internalization. Furthermore, we showed that the di-leucine motif 1010LL1011 within this region is essential in mediating EGF-induced rapid EGFR internalization independent of kinase activation. We conclude that EGFR C-terminal sequences 1005-1017 and the 1010LL1011 motif are essential for EGF-induced EGFR endoytosis independent of EGFR kinase activation and autophosphorylation.

Single-molecule analysis of epidermal growth factor binding on the surface of living cells

Global cellular responses induced by epidermal growth factor (EGF) receptor (EGFR) occur immediately with a less than 1% occupancy among tens of thousands of EGFR molecules on single cell surface. Activation of EGFR requires the formation of a signaling dimer of EGFR bound with a single ligand to each molecule. How sufficient numbers of signaling dimers are formed at such low occupancy rate is still not known. Here, we have analyzed the kinetics of EGF binding and the formation of the signaling dimer using single-molecule imaging and mathematical modeling. A small number of EGFR on the cell surface formed dimeric binding sites, which bound EGF two orders of magnitude faster than the monomeric binding sites. There was a positive cooperative binding of EGF to the dimeric binding sites through a newly discovered kinetic intermediate. These two mechanisms facilitate the formation of signaling dimers of EGF/EGFR complexes.

Control of epidermal growth factor receptor endocytosis by receptor dimerization, rather than receptor kinase activation

Given that ligand binding is essential for the rapid internalization of epidermal growth factor receptor (EGFR), the events induced by ligand binding probably contribute to the regulation of EGFR internalization. These events include receptor dimerization, activation of intrinsic tyrosine kinase activity and autophosphorylation. Whereas the initial results are controversial regarding the role of EGFR kinase activity in EGFR internalization, more recent data suggest that EGFR kinase activation is essential for EGFR internalization. However, we have shown here that inhibition of EGFR kinase activation by mutation or by chemical inhibitors did not block EGF-induced EGFR internalization. Instead, proper EGFR dimerization is necessary and sufficient to stimulate EGFR internalization. We conclude that EGFR internalization is controlled by EGFR dimerization, rather than EGFR kinase activation. Our results also define a new role for EGFR dimerization: by itself it can drive EGFR internalization, independent of its role in the activation of EGFR kinase.

Sorting motifs in receptor trafficking

A variety of receptors have been analyzed in sufficient detail to identify sorting motifs. Initial studies focused on the identification of sequences in the cytoplasmic tails of the LDL and transferrin receptors that mediated their internalization. These motifs have since been found in the cytoplasmic domains of a wide variety of receptors and provide for numerous sorting functions. This review will outline the early studies on LDL and transferrin receptors and will then focus on two classes of signaling receptors, receptor tyrosine kinases (EGF and the insulin receptors) and heterotrimeric G-protein coupled receptors (beta2-adrenergic receptors). The identification of sorting motifs and proteins that bind these motifs will be discussed. Importantly, the studies identify a variety of potential targets for modulating the sorting and hence activity of these medically important receptors.

Genetics of dark skin in mice

Chemical mutagenesis in the mouse is a powerful approach for phenotype-driven genetics, but questions remain about the efficiency with which new mutations ascertained by their phenotype can be localized and identified, and that knowledge applied to a specific biological problem. During a global screen for dominant phenotypes in about 30,000 animals, a novel class of pigmentation mutants were identified by dark skin (Dsk). We determined the genetic map location, homozygous phenotype, and histology of 10 new Dsk and 2 new dark coat (Dcc) mutations, and identified mutations in Agouti (Met1Leu, Dcc4), Sox18 (Leu220ter, Dcc1), Keratin 2e (Thr500Pro, Dsk2), and Egfr (Leu863Gln, Dsk5). Cutaneous effects of most Dsk mutations are limited to melanocytes, except for the Keratin 2e and Egfr mutations, in which hyperkeratosis and epidermal thickening precede epidermal melanocytosis by 3-6 wk. The Dsk2 mutation is likely to impair intermediate filament assembly, leading to cytolysis of suprabasal keratinocytes and secondary hyperkeratosis and melanocytosis. The Dsk5 mutation causes increased tyrosine kinase activity and a decrease in steady-state receptor levels in vivo. The Dsk mutations represent genes or map locations not implicated previously in pigmentation, and delineate a developmental pathway in which mutations can be classified on the basis of body region, microscopic site, and timing of pigment accumulation.

Submicromolar doses of alkyl-lysophospholipids induce rapid internalization, but not activation, of epidermal growth factor receptor and concomitant MAPK/ERK activation in A431 cells

Synthetic ALPs, e.g., Et-18-OCH(3) and HePC, are anticancer agents that accumulate in cell membranes, where they interfere with lipid-mediated signal transduction. We previously reported that ALPs, when added at micromolar concentrations (5-25 microM), inhibit growth factor-induced MAPK/ERK activation and enhance radiation-induced apoptosis. We now show that, at nanomolar doses (10-500 nM), ALPs activate the MAPK/ERK pathway in A431 cells without stimulating cell proliferation. Strikingly, ALPs (500 nM) also trigger rapid clustering and internalization of the EGFR in A431 cells. Tyrphostin AG1478, an EGFR tyrosine kinase inhibitor, blocks ALP-induced MAPK/ERK activation but not EGFR internalization. We found no evidence for ALPs acting via G protein-coupled receptors and/or transactivation of EGFRs, as determined by calcium mobilization, EGFR phosphorylation and Grb2 binding assays. Since ALPs readily intercalate into the plasma membrane, our data suggest that they induce subtle changes in the lipid microenvironment of the EGFR, resulting in clustering and internalization of the EGFR and concomitant MAPK/ERK activation.

Effect of tyrosine kinase inhibitors on clathrin-coated pit recruitment and internalization of epidermal growth factor receptor

Several inhibitors of epidermal growth factor receptor (EGFR) kinase and Src family kinases (SFK) were employed to study the role of these kinases in EGFR internalization through clathrin-coated pits. The EGFR kinase-specific compound PD158780 substantially diminished EGFR internalization. PP2, an inhibitor of SFK, had a moderate effect on EGFR internalization in several types of cells, including cells lacking SFK, indicating that the inhibition of endocytosis by PP2 is mediated by kinases other than SFK. In contrast, SU6656, a more specific inhibitor of SFK, did not affect EGFR internalization. To examine what stage of internalization requires receptor kinase activity, we established a quantitative assay based on three-dimensional fluorescence microscopy that measures co-localization of an EGF-rhodamine conjugate and a fluorescently tagged clathrin adaptor protein complex, AP-2. Interestingly, recruitment of EGFR into coated pits did not require physiological temperature because the maximal accumulation of EGFR in coated pits was observed at 4 degrees C. Pretreatment of the cells with PD158780 prevented EGFR recruitment into coated pits, whereas the inhibitor did not block the internalization of receptors that had first been allowed to enter the coated pits at 4 degrees C. These data demonstrate that the activation of receptor kinase is essential for the initial, coated pit recruitment step of endocytosis.

UV induces tyrosine kinase-independent internalisation and endosome arrest of the EGF receptor

We have compared the activation and trafficking of epidermal growth factor receptor (EGFR) induced by UV light and EGF. Tyrosine phosphorylation of EGFR was not detected in UV-exposed cells by immunoblotting of whole cell lysates or EGFR immunoprecipitates with antibodies specific for each of the five activated autophosphorylation sites of EGFR. In addition, EGFR of UV-irradiated cells did not demonstrate increased 32P-incorporation. However, UV-exposed cells demonstrated a gel mobility shift of EGFR, which was not abolished by alkaline phosphatase treatment. UV-exposure did not induce dimerisation of EGFR. Furthermore, UV induced internalisation of EGFR without polyubiquitination or degradation. UV-exposed EGFR was transferred to early endosomes and arrested in transferrin-accessible endosomes close to the cell surface. Whereas inhibition of the EGFR tyrosine kinase effectively inhibited tyrosine phosphorylation and internalisation of EGF-activated EGFR, internalisation of UV-exposed EGFR was unaffected. UV induced neither relocalisation of Shc and Grb2 nor activation of Raf, but activation of MEK and MAPK was observed.

Our work indicates that UV induces internalisation of EGFR independent of its phosphorylation or receptor tyrosine kinase activation, and altered EGFR trafficking compared with ligand-activated receptor. In addition, MAPK activation by UV does not appear to be mediated by EGFR activation.

c-Cbl ubiquitinates the EGF receptor at the plasma membrane and remains receptor associated throughout the endocytic route

Cbl family members have an evolutionarily conserved role in attenuating receptor tyrosine kinase function. Their negative regulatory capacity depends on a Ring finger domain that interacts with ubiquitin conjugating enzymes. Cbl molecules constitute a novel type of E3 or ubiquitin ligase family that is recruited to phosphotyrosine motifs. Ubiquitination of the receptor system is coupled to its downregulation, but it is unclear at which point in the endocytic pathway Cbl molecules come into play. Using low temperature and a dynamin mutant, we find that c-Cbl associates with and ubiquitinates the activated epidermal growth factor (EGF) receptor at the plasma membrane in the absence of endocytosis. With the aid of confocal microscopy and immunogold electron microscopy, we could demonstrate that c-Cbl associates with the EGF receptor at the plasma membrane prior to receptor recruitment into clathrin-coated pits and remains associated throughout the clathrin-mediated endocytic pathway. c-Cbl and the EGF receptor also colocalize in internal vesicles of multivesicular endosomes. Our data are consistent with a role for c-Cbl in clathrin-mediated endocytosis of tyrosine kinase receptors, as well as their intracellular sorting.

Growth factor receptor signalling: location, location, location

Ligand binding to plasma membrane receptors initiates a series of events culminating in a variety of changes in cellular phenotypes. Although numerous publications have documented the activation/inactivation of signalling molecules following receptor binding, relatively few investigations have focused on the cellular compartment responsible for either initiating or selecting the particular pathway that mediates the response. Specifically, does receptor signalling occur only at the plasma membrane; is signalling dependent upon the location of defined endosome populations; or are components of both plasma membrane and endosomal activity operative depending upon the particular signalling pathway or cell type? This review addresses aspects of these questions by discussing the evidence supporting or contrasting the interplay between the endocytic and signalling systems for a subset of tyrosine kinase, serine/threonine kinase and G-protein-coupled receptors.

Down-regulation of cell surface receptors is modulated by polar residues within the transmembrane domain

How recycling receptors are segregated from down-regulated receptors in the endosome is unknown. In previous studies, we demonstrated that substitutions in the transferrin receptor (TR) transmembrane domain (TM) convert the protein from an efficiently recycling receptor to one that is rapidly down regulated. In this study, we demonstrate that the "signal" within the TM necessary and sufficient for down-regulation is Thr(11)Gln(17)Thr(19) (numbering in TM). Transplantation of these polar residues into the wild-type TR promotes receptor down-regulation that can be demonstrated by changes in protein half-life and in receptor recycling. Surprisingly, this modification dramatically increases the TR internalization rate as well ( approximately 79% increase). Sucrose gradient centrifugation and cross-linking studies reveal that propensity of the receptors to self-associate correlates with down-regulation. Interestingly, a number of cell surface proteins that contain TM polar residues are known to be efficiently down-regulated, whereas recycling receptors for low-density lipoprotein and transferrin conspicuously lack these residues. Our data, therefore, suggest a simple model in which specific residues within the TM sequences dramatically influence the fate of membrane proteins after endocytosis, providing an alternative signal for down-regulation of receptor complexes to the well-characterized cytoplasmic tail targeting signals.

Missense mutations interfere with VEGFR-3 signalling in primary lymphoedema

Primary lymphoedema is a rare, autosomal dominant disorder that leads to a disabling and disfiguring swelling of the extremities and, when untreated, tends to worsen with time. Here we link primary human lymphoedema to the FLT4 locus, encoding vascular endothelial growth factor receptor-3 (VEGFR-3), in several families. All disease-associated alleles analysed had missense mutations and encoded proteins with an inactive tyrosine kinase, preventing downstream gene activation. Our study establishes that VEGFR-3 is important for normal lymphatic vascular function and that mutations interfering with VEGFR-3 signal transduction are a cause of primary lymphoedema.

Epidermal growth factor receptor dimerization monitored in live cells

We present a method for monitoring receptor dimerization at the membrane of live cells. Chimeric proteins containing the epidermal growth factor (EGF) receptor extracellular and transmembrane domains fused to weakly complementing beta-galactosidase (beta-gal) deletion mutants were expressed in cells in culture. Treatment of the cells with EGF-like compounds for as little as 15 s resulted in chimeric receptor dimerization detectable as beta-gal enzymatic activity. The dose response of chimeric receptors was ligand specific. beta-galactosidase complementation was reversible upon removal of ligand and could be reinduced. Antibodies that block ligand binding inhibited receptor dimerization and beta-gal complementation. These results demonstrate that beta-gal complementation provides a rapid, simple, and sensitive assay for protein interactions and for detecting and monitoring the kinetics of receptor dimerization.

The evolutionarily conserved N-terminal region of Cbl is sufficient to enhance down-regulation of the epidermal growth factor receptor

The mammalian proto-oncoprotein Cbl and its homologues in Caenorhabditis elegans and Drosophila are evolutionarily conserved negative regulators of the epidermal growth factor receptor (EGF-R). Overexpression of wild-type Cbl enhances down-regulation of activated EGF-R from the cell surface. We report that the Cbl tyrosine kinase-binding (TKB) domain is essential for this activity. Whereas wild-type Cbl enhanced ligand-dependent EGF-R ubiquitination, down-regulation from the cell surface, accumulation in intracellular vesicles, and degradation, a Cbl TKB domain-inactivated mutant (G306E) did not. Furthermore, the transforming truncation mutant Cbl-N (residues 1-357), comprising only the Cbl TKB domain, functioned as a dominant negative protein. It colocalized with EGF-R in intracellular vesicular structures, yet it suppressed down-regulation of EGF-R from the surface of cells expressing endogenous wild-type Cbl. Therefore, Cbl-mediated down-regulation of EGF-R requires the integrity of both the N-terminal TKB domain and additional C-terminal sequences. A Cbl truncation mutant comprising amino acids 1-440 functioned like wild-type Cbl in down-regulation assays. This mutant includes the evolutionarily conserved TKB and RING finger domains but lacks the less conserved C-terminal sequences. We conclude that the evolutionarily conserved N terminus of Cbl is sufficient to effect enhancement of EGF-R ubiquitination and down-regulation from the cell surface.

The two subunits of the interleukin-4 receptor mediate independent and distinct patterns of ligand endocytosis

Interleukin-4 (IL-4) triggers cellular responses by interaction with the bipartite interleukin-4 receptor (IL-4R). IL-4-responsive cells specifically endocytose IL-4. We studied the ligand internalization properties of the human IL-4R and analyzed the specific functions of its two subunits IL-4Ralpha and gammac in this process. IL-4 mutant RY, which binds to IL-4Ralpha but does not recruit gammac into the receptor complex was used as a tool to show that IL-4Ralpha can promote independent ligand uptake in human T cells. Internalization was limited, however, by rapid IL-4 dissociation, suggesting that one important function of gammac in IL-4 endocytosis is to retain the ligand sufficiently long within the ternary receptor complex. We then measured IL-4 internalization by murine Ba/F3 cells that were stably transfected with various human IL-4R constructs. Efficient IL-4 uptake required the cytoplasmic section of the receptor. The intracellular domains of IL-4Ralpha and gammac were responsible for independent endocytosis processes with distinct kinetics. IL-4Ralpha-mediated internalization resulted in long-term intracellular maintainance of IL-4, whereas gammac directed the associated radioligand to intracellular breakdown and rapid release in the form of degraded protein. Mutants of either IL-4R subunit deficient in Janus kinase activation were not impaired in internalization, indicating that IL-4 endocytosis is not functionally connected to signal transduction.

Endocytosis deficiency of epidermal growth factor (EGF) receptor-ErbB2 heterodimers in response to EGF stimulation

Epidermal growth factor (EGF) stimulates the homodimerization of EGF receptor (EGFR) and the heterodimerization of EGFR and ErbB2. The EGFR homodimers are quickly endocytosed after EGF stimulation as a means of down-regulation. However, the results from experiments on the ability of ErbB2 to undergo ligand-induced endocytosis are very controversial. It is unclear how the EGFR-ErbB2 heterodimers might behave. In this research, we showed by subcellular fractionation, immunoprecipitation, Western blotting, indirect immunofluorescence, and microinjection that, in the four breast cancer cell lines MDA453, SKBR3, BT474, and BT20, the EGFR-ErbB2 heterodimerization levels were positively correlated with the ratio of ErbB2/EGFR expression levels. ErbB2 was not endocytosed in response to EGF stimulation. Moreover, in MDA453, SKBR3, and BT474 cells, which have very high levels of EGFR-ErbB2 heterodimerization, EGF-induced EGFR endocytosis was greatly inhibited compared with that in BT20 cells, which have a very low level of EGFR-ErbB2 heterodimerization. Microinjection of an ErbB2 expression plasmid into BT20 cells significantly inhibited EGF-stimulated EGFR endocytosis. Coexpression of ErbB2 with EGFR in 293T cells also significantly inhibited EGF-stimulated EGFR endocytosis. EGF did not stimulate the endocytosis of ectopically expressed ErbB2 in BT20 and 293T cells. These results indicate that ErbB2 and the EGFR-ErbB2 heterodimers are impaired in EGF-induced endocytosis. Moreover, when expressed in BT20 cells by microinjection, a chimeric receptor composed of the ErbB2 extracellular domain and the EGFR intracellular domain underwent normal endocytosis in response to EGF, and this chimera did not block EGF-induced EGFR endocytosis. Thus, the endocytosis deficiency of ErbB2 is due to the sequence of its intracellular domain.

An endocytosed TGN38 chimeric protein is delivered to the TGN after trafficking through the endocytic recycling compartment in CHO cells

To examine TGN38 trafficking from the cell surface to the TGN, CHO cells were stably transfected with a chimeric transmembrane protein, TacTGN38. We used fluorescent and 125I-labeled anti-Tac IgG and Fab fragments to follow TacTGN38's postendocytic trafficking. At steady-state, anti-Tac was mainly in the TGN, but shortly after endocytosis it was predominantly in early endosomes. 11% of cellular TacTGN38 is on the plasma membrane. Kinetic analysis of trafficking of antibodies bound to TacTGN38 showed that after short endocytic pulses, 80% of internalized anti-Tac returned to the cell surface (t1/2 = 9 min), and the remainder trafficked to the TGN. When longer filling pulses and chases were used to load anti-Tac into the TGN, it returned to the cell surface with a t1/2 of 46 min. Quantitative confocal microscopy analysis also showed that fluorescent anti-Tac fills the TGN with a 46-min t1/2. Using the measured rate constants in a simple kinetic model, we predict that 82% of TacTGN38 is in the TGN, and 7% is in endosomes. TacTGN38 leaves the TGN slowly, which accounts for its steady-state distribution despite the inefficient targeting from the cell surface to the TGN.

Temporal Synthesis of Band 3 Oligomers During Terminal Maturation of Mouse Erythroblasts. Dimers and Tetramers Exist in the Membrane as Preformed Stable Species

Band 3, the anion transport protein of the erythrocyte membrane, exists in the membrane as a mixture of dimers (B3D) and tetramers (B3T). The dimers are not linked to the skeleton and constitute the free mobile band 3 fraction. The tetramers are linked to the skeleton by their interaction with ankyrin. In this report we have examined the temporal synthesis and assembly of band 3 oligomers into the plasma membrane during red cell maturation. The oligomeric state of newly synthesized band 3 in early and late erythroblasts was analyzed by size-exclusion high-pressure liquid chromatography of band 3 extracts derived by mild extraction of plasma membranes with the nonionic detergent C12E8 (octaethylene glycol n-dodecyl monoether). This analysis revealed that at the early erythroblast stage, the newly synthesized band 3 is present predominantly as tetramers, whereas at the late stages of erythroid maturation, it is present exclusively as dimers. To examine whether the dimers and tetramers exist in the membrane as preformed stable species or whether they are interconvertible, the fate of band 3 species synthesized during erythroblast maturation was examined by pulse-chase analysis. We showed that the newly synthesized band 3 dimers and tetramers are stable and that there is no interconversion between these species in erythroblast membranes. Pulse-chase analysis followed by cellular fractionation showed that, in early erythroblasts, the newly synthesized band 3 tetramers are initially present in the microsomal fraction and later incorporated stably into the plasma membrane fraction. In contrast, in late erythroblasts the newly synthesized band 3 dimers move rapidly to the plasma membrane fraction but then recycle between the plasma membrane and microsomal fractions. Fluorescence photobleaching recovery studies showed that significant fractions of B3T and B3D are laterally mobile in early and late erythroblast plasma membranes, respectively, suggesting that many B3T-ankyrin complexes are unattached to the membrane skeleton in early erythroblasts and that the membrane skeleton has yet to become tightly organized in late erythroblasts. We postulate that in early erythroblasts, band 3 tetramers are transported through microsomes and stably incorporated into the plasma membrane. However, when ankyrin synthesis is downregulated in late erythroblasts, it appears that B3D are rapidly transported to the plasma membrane but then recycled between the plasma membrane and microsomal compartments. These observations may suggest novel roles for membrane skeletal proteins in stabilizing integral membrane protein oligomers at the plasma membrane and in regulating the endocytosis of such proteins.

Internalization of the interleukin 6 signal transducer gp130 does not require activation of the Jak/STAT pathway

Signalling receptors often undergo receptor-mediated endocytosis. In many cases this internalization is stimulated by ligand binding and activation of intrinsic receptor tyrosine kinases, resulting in a receptor down-regulation. We have analysed whether internalization of the interleukin 6 signal transducer gp130 is dependent on the activation of receptor-associated Jak kinases. By using a chimaeric receptor system we found that receptor mutants that lack box1 and therefore are not capable of activating Jak and signal transducer and activator of transcription (STAT) proteins are still endocytosed efficiently. A chimaeric receptor with the recently identified dileucine internalization motif being replaced by two alanine residues was not efficiently internalized but still capable of recruiting STATs. Furthermore an antagonistic antibody that inhibits the signalling of all interleukin-6-type cytokines via gp130 was internalized as efficiently as an agonistic one that activates the Jak/STAT pathway. Our findings suggest that the endocytosis of gp130 is signal-independent.

Rabphilin-3A: a multifunctional regulator of synaptic vesicle traffic

We have investigated the function of the synaptic vesicle protein Rabphilin-3A in neurotransmitter release at the squid giant synapse. Presynaptic microinjection of recombinant Rabphilin-3A reversibly inhibited the exocytotic release of neurotransmitter. Injection of fragments of Rabphilin-3A indicate that at least two distinct regions of the protein inhibit neurotransmitter release: the NH2-terminal region that binds Rab3A and is phosphorylated by protein kinases and the two C2 domains that interact with calcium, phospholipid, and beta-adducin. Each of the inhibitory fragments and the full-length protein had separate effects on presynaptic morphology, suggesting that individual domains were inhibiting a subset of the reactions in which the full-length protein participates. In addition to inhibiting exocytosis, constructs containing the NH2 terminus of Rabphilin-3A also perturbed the endocytotic pathway, as indicated by changes in the membrane areas of endosomes, coated vesicles, and the plasma membrane. These results indicate that Rabphilin-3A regulates synaptic vesicle traffic and appears to do so at distinct stages of both the exocytotic and endocytotic pathways.

Biochemical characterization of mutant EGF receptors expressed in the hemopoietic cell line BaF/3

The Epidermal Growth Factor (EGF) receptor appears to require a fully active tyrosine kinase domain to transmit mitogenic signals. However, waved-2 mice carrying a mutation in the alpha-helix C of their EGF-R, which abolishes tyrosine kinase activity, only display a mild phenotype and are fully viable. This suggests that the mutant EGF-R signals through heterodimerization with endogenous, kinase active members of the EGF-R family such as ErbB-2 or ErbB-4. We have examined the biochemistry of EGF-Rs carrying mutations in the alpha-helix C of the human EGF-R (V741G and Y740F), in the ATP binding site (K721R) and at the C-terminus (CT957), by expression in BaF/3 cells which are devoid of EGF-R family members. The in vitro kinase activity of the alpha-helix C EGF-R mutants was severely impaired as a result of reduced phosphotransfer activity without appreciable changes in the affinity for either ATP or peptide substrate. Surprisingly, EGF stimulation of cells carrying the different mutant or wild type EGF-Rs resulted in tyrosine phosphorylation of EGF-R proteins; this phosphorylation was abolished in crude plasma membrane preparations, and appears to be due to activation of a membrane-associated or a cytosolic kinase. Receptor-mediated internalization of EGF was profoundly suppressed in the V741G, K721R and CT957 receptor mutant, and high affinity EGF binding was undetectable in the V741G and K721R receptors. We conclude that specific residues in the C-helix of the EGF-R kinase are essential for full kinase activity; mutations in this region do not affect ATP binding, but impair the receptors' phosphotransfer ability. High affinity binding of EGF is not dependent on tyrosine kinase activity or sequences in the C-terminus.

Cholesterol derivative of poly(ethylene glycol) inhibits clathrin-independent, but not clathrin-dependent endocytosis

The effect of poly(ethylene glycol) cholesteryl ethers (PEG(n)-Chols) with two different numbers of units (n = 50 and 200) in the hydrophilic PEG moiety on cellular endocytic activity was studied on HT-1080 cells. The amphipathic molecules were soluble in aqueous solution. When fluorescein derivatives of PEG-Chols (one fluorescein at the distal end of PEG) were incubated with the cells in culture, the cellular fluorescence was localized at the plasma membrane level and in intracellular vesicles. Fluorescence quantification indicated that for the same external concentration, twice more FPEG(50)-Chol than FPEG(200)-Chol was associated with the cells under the same conditions. Regardless of the length of PEG moiety, PEG-Chols' interaction with cells reduced the endocytic internalization of a fluid phase marker, horseradish peroxidase (HRP) depending on the cell-associated amount. In contrast, internalization of 125I-labeled epidermal growth factor (EGF) through receptor-mediated endocytosis did not change upon incubation with PEG(50)-Chol. The effect of PEG(200)-Chol was also small, since EGF internalization showed a reduction of 10-20%, while at the same concentration as much as 80% of HRP uptake was inhibited. PEG(50)-Chol did not influence the internalization of a larger ligand, 125I-transferrin (Tfn). However, in the presence of PEG(200)-Chol, the uptake of 125I-Tfn decreased remarkably, and yet, PEG(200)-Chol has no influence on the binding and internalization of a monoclonal antibody directed toward the ectodomain of the Tfn-receptor. These results suggested that incorporation of PEG-Chols in the outer monolayer of the plasma membrane specifically inhibited clathrin-independent, but not clathrin-dependent endocytosis.

Nonphosphorylated peptide ligands for the Grb2 Src homology 2 domain

Critical intracellular signals in normal and malignant cells are transmitted by the adaptor protein Grb2 by means of its Src homology 2 (SH2) domain, which binds to phosphotyrosyl (pTyr) residues generated by the activation of tyrosine kinases. To understand this important control point and to design inhibitors, previous investigations have focused on the molecular mechanisms by which the Grb2 SH2 domain selectively binds pTyr containing peptides. In the current study, we demonstrate that the Grb2 SH2 domain can also bind in a pTyr independent manner. Using phage display, an 11-amino acid cyclic peptide, G1, has been identified that binds to the Grb2 SH2 domain but not the src SH2 domain. Synthetic G1 peptide blocks Grb2 SH2 domain association (IC50 10-25 microM) with a 9-amino acid pTyr-containing peptide derived from the SHC protein (pTyr317). These data and amino acid substitution analysis indicate that G1 interacts in the phosphopeptide binding site. G1 peptide requires a YXN sequence similar to that found in natural pTyr-containing ligands, and phosphorylation of the tyrosine increases G1 inhibitory activity. G1 also requires an internal disulfide bond to maintain the active binding conformation. Since the G1 peptide does not contain pTyr, it defines a new type of SH2 domain binding motif that may advance the design of Grb2 antagonists.