Surface redistribution of 125I-insulin in cultured human lymphocytes

Removal of cellular protrusions

Cell-cell communications are central to a variety of physiological and pathological processes in multicellular organisms. Cells often rely on cellular protrusions to communicate with one another, which enable highly selective and efficient signaling within complex tissues. Owing to significant improvements in imaging techniques, identification of signaling protrusions has increased in recent years. These protrusions are structurally specialized for signaling and facilitate interactions between cells. Therefore, physical regulation of these structures must be key for the appropriate strength and pattern of signaling outcomes. However, the typical approaches for understanding signaling regulation tend to focus solely on changes in signaling molecules, such as gene expression, protein-protein interaction, and degradation. In this short review, we summarize the studies proposing the removal of different types of signaling protrusions-including cilia, neurites, MT (microtubule based)-nanotubes and microvilli-and discuss their mechanisms and significance in signaling regulation.

Surfing on Membrane Waves: Microvilli, Curved Membranes, and Immune Signaling

Microvilli are finger-like membrane protrusions, supported by the actin cytoskeleton, and found on almost all cell types. A growing body of evidence suggests that the dynamic lymphocyte microvilli, with their highly curved membranes, play an important role in signal transduction leading to immune responses. Nevertheless, challenges in modulating local membrane curvature and monitoring the high dynamicity of microvilli hampered the investigation of the curvature-generation mechanism and its functional consequences in signaling. These technical barriers have been partially overcome by recent advancements in adapted super-resolution microscopy. Here, we review the up-to-date progress in understanding the mechanisms and functional consequences of microvillus formation in T cell signaling. We discuss how the deformation of local membranes could potentially affect the organization of signaling proteins and their biochemical activities. We propose that curved membranes, together with the underlying cytoskeleton, shape microvilli into a unique compartment that sense and process signals leading to lymphocyte activation.

Membrane protrusion powers clathrin-independent endocytosis of interleukin-2 receptor

Endocytosis controls many functions including nutrient uptake, cell division, migration and signal transduction. A clathrin- and caveolin-independent endocytosis pathway is used by important physiological cargos, including interleukin-2 receptors (IL-2R). However, this process lacks morphological and dynamic data. Our electron microscopy (EM) and tomography studies reveal that IL-2R-pits and vesicles are initiated at the base of protrusions. We identify the WAVE complex as a specific endocytic actor. The WAVE complex interacts with IL-2R, via a WAVE-interacting receptor sequence (WIRS) present in the receptor polypeptide, and allows for receptor clustering close to membrane protrusions. In addition, using total internal reflection fluorescent microscopy (TIRF) and automated analysis we demonstrate that two timely distinct bursts of actin polymerization are required during IL-2R uptake, promoted first by the WAVE complex and then by N-WASP. Finally, our data reveal that dynamin acts as a transition controller for the recruitment of Arp2/3 activators required for IL-2R endocytosis. Altogether, our work identifies the spatio-temporal specific role of factors initiating clathrin-independent endocytosis by a unique mechanism that does not depend on the deformation of a flat membrane, but rather on that of membrane protrusions.

Algal photoreceptors: in vivo functions and potential applications

Many algae, particularly microalgae, possess a sophisticated light-sensing system including photoreceptors and light-modulated signaling pathways to sense environmental information and secure the survival in a rapidly changing environment. Over the last couple of years, the multifaceted world of algal photobiology has enriched our understanding of the light absorption mechanisms and in vivo function of photoreceptors. Moreover, specific light-sensitive modules have already paved the way for the development of optogenetic tools to generate light switches for precise and spatial control of signaling pathways in individual cells and even in complex biological systems.

Light scattering and morphology of the lymphocyte as applied to flow cytometry for distinguishing healthy and infected individuals

A simple optical model of single lymphocytes with smooth and nonsmooth surfaces has been developed for healthy and infected individuals. The model can be used for rapid (in the real-time scale) solution of the inverse light-scattering problem on the basis of optical data measured by label-free flow cytometry. Light scattering patterns have been calculated for the model developed. It has been shown that the smooth and nonsmooth cells can be resolved using the intensities of the sideward- and backward-scattered light. We have found by calculations and validated by the flow cytometer experiments that intensity distributions for the cells of lymphocyte populations can be used as a preliminary signatures of some virus infections. Potential biomedical applications of the findings for label-free flow cytometry detection of individuals infected with viruses of hepatitis B or C and some others viruses are presented.

Rapid insulin-dependent endocytosis of the insulin receptor by caveolae in primary adipocytes

Background

The insulin receptor is localized in caveolae and is dependent on caveolae or cholesterol for signaling in adipocytes. When stimulated with insulin, the receptor is internalized.

Methodology/Principal Findings

We examined primary rat adipocytes by subcellular fractionation to examine if the insulin receptor was internalized in a caveolae-mediated process. Insulin induced a rapid, t_1/2<3 min, endocytosis of the insulin receptor in parallel with receptor tyrosine autophosphorylation. Concomitantly, caveolin-1 was phosphorylated at tyrosine(14) and endocytosed. Vanadate increased the phosphorylation of caveolin-1 without affecting insulin receptor phosphorylation or endocytosis. Immunocapture of endosomal vesicles with antibodies against the insulin receptor co-captured caveolin-1 and immunocapture with antibodies against tyrosine(14)-phosphorylated caveolin-1 co-captured the insulin receptor, demonstrating that the insulin receptor was endocytosed together with tyrosine(14)-phosphorylated caveolin-1. By immunogold electron microscopy the insulin receptor and caveolin-1 were colocalized in endosome vesicles that resembled caveosomes. Clathrin was not endocytosed with the insulin receptor and the inhibitor of clathrin-coated pit-mediated endocytosis, chlorpromazine, did not inhibit internalization of the insulin receptor, while transferrin receptor internalization was inhibited.

Conclusion

It is concluded that in response to insulin stimulation the autophosphorylated insulin receptor in primary adipocytes is rapidly endocytosed in a caveolae-mediated process, involving tyrosine phosphorylation of caveolin-1.

The neck of caveolae is a distinct plasma membrane subdomain that concentrates insulin receptors in 3T3-L1 adipocytes

Insulin receptors (IRs) segregate on plasma membrane microvilli, but in cells devoid of microvilli, such as adipocytes, the localization of IRs is a matter of controversy. In the present study, we examined the distribution of IRs in the plasma membrane of 3T3-L1 adipocytes. Quantitative electron microscopy indicates that IRs are predominantly associated with the neck, but not the bulb, of caveolae. Caveola necks represent distinct microdomains of the plasma membrane. Indeed, as shown by freeze-fracture analysis, intramembrane particles are concentrated as necklaces around the craters of caveolae. In addition, subcellular fractionation suggests that the neck and the bulb of caveolae present a different resistance to detergent solubility. Finally, cytoskeletal components, including actin, are highly enriched in the membrane area underlying the neck part of caveolae. IRs coimmunoprecipitate with cytoskeletal components, and disruption of the actin cytoskeleton alters IRs expression, localization, and signaling, thus supporting the notion that caveola necks are involved in intracellular signaling by IRs. Together, these results suggest that cytoskeletal proteins anchor IRs to microdomains in the caveola necks of 3T3-L1 adipocytes. By homology with IR localization in other cell types, we suggest that the necks of caveolae may represent the counterpart of microvillar domains in cells poor in microvilli such as adipocytes and that they play an important role as signaling platforms.

Role of two dileucine-like motifs in insulin receptor anchoring to microvilli

In the absence of ligand, the insulin receptor is maintained on microvilli on the cell surface. A dileucine motif (LL(986-987)) is necessary but not sufficient for this anchoring, which also required the presence of additional sequence(s) downstream of position 1000. The aim of the present study was to identify this (these) additional sequence(s). First, exons 16 or 17 were fused to the extracellular and transmembrane domains of complement receptor 1 and stably expressed in Chinese hamster ovary cells. Results obtained indicate that exon 17 is sufficient for anchoring to microvilli. Second, analysis of insulin receptor mutants truncated within exon 17 demonstrated that whereas receptors truncated at position 1000 showed no preferential association with microvilli, receptors truncated at position 1012 displayed a level of association identical to that of the full-length insulin receptor. Third, mutation of a diisoleucine motif (II(1006-1007)) present within this 12-amino acid stretch abrogated the preferential association of the receptor with microvilli. These results indicate that the domain required for association of insulin receptor with microvilli is contained within the region encoded by exon 17 and that, within this sequence, two dileucine-like motifs (LL(986-987) and II(1006-1007)) play a crucial role.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Nef-mediated clathrin-coated pit formation

The sequence of events leading to clathrin-coated pit (CCP) nucleation on the cell surface and to the incorporation of receptors into these endocytic structures is still imperfectly understood. In particular, the question remains as to whether receptor tails initiate the assembly of the coat proteins or whether receptors migrate into preformed CCP. This question was approached through a dissection of the mechanisms implemented by Nef, an early protein of human and simian immunodeficiency virus (HIV and SIV, respectively), to accelerate the endocytosis of cluster of differentiation antigen type 4 (CD4), the major receptor for these viruses. Results collected showed that: (a) Nef promotes CD4 internalization via an increased association of CD4 with CCP; (b) the Nef-mediated increase of CD4 association with CCP is related to a doubling of the plasma membrane area occupied by clathrin-coated structures; (c) this increased CCP number at the plasma membrane has functional consequences preferentially on CD4 uptake and does not significantly affect transferrin receptor internalization or fluid-phase endocytosis; (d) the presence of a CD4 cytoplasmic tail including a critical dileucine motif is required to induce CCP formation via Nef; and (e) when directly anchored to the cytoplasmic side of the plasma membrane, Nef itself can promote CCP formation. Taken together, these observations lead us to propose that CD4 can promote CCP generation via the connector molecule Nef. In this model, Nef interacts on one side with CD4 through a dileucine-based motif present on CD4 cytoplasmic tail and on the other side with components of clathrin-coated surface domain (i.e., adaptins). These Nef-generated complexes would then initiate the nucleation of CCP.

Binding of 125I-insulin on capillary endothelial and myofiber cell membranes in normal and streptozotocin-induced diabetic perfused rat hearts

A heart-perfusion technique was employed to measure 125I-insulin binding on capillary endothelial and myocyte cell membranes in Sprague-Dawley rats. Animals were anesthetized, and the anterior chest wall excised to expose the mediastinal contents. The right and left superior and inferior venae cavae were dissected and tied, and another tie was passed around the aorta. A polyethylene catheter was introduced into the aortic lumen from cephalad to caudad to sit with its tip above the aortic valve. Another catheter was introduced into the cavity of the right atrium and both were anchored by sutures. Oxygenated Ringer-Lock buffer containing 20 mM/L K+ and 125I-insulin was perfused at a rate of 1 mL/min via the aortic catheter. Concomitantly, the distal ascending aorta and venae cavae were ligated. The effluent was collected from the right atrial catheter at the same infusion rate. Animals were divided into two groups, the normal group and streptozotocin-induced diabetic group. Heart perfusion was done on both groups either without or after treatment with detergent (CHAPS) to remove the capillary endothelial lining. A physical model for 125I-insulin sequestration as a ligand to its receptors on endothelial and/or myocyte plasma membranes was proposed. The model described a reversible binding of ligand on cellular surface receptor concentration to fit a conservation equation and a first order Bessel function. The binding constants (kn), reversal constants (k-n), dissociation constants kd = k-n/kn, and residency time constants tau = 1/k-n of 125I-insulin in normal untreated, normal CHAPS-treated, diabetic untreated, and diabetic CHAPS-treated hearts were estimated using a theoretically generated curve-fit to the data. Since insulin receptor binding on the capillary endothelial cell surfaces may serve to transport insulin from the intravascular to the subendothelial space, and since streptozotocin-induced diabetes was shown to diminish receptor autophosphorylation and kinase activity and hence internalization of insulin, then one can conclude the following from the data. In the normal heart, removal of the capillary endothelial lining with CHAPS did not alter kn, k-n, kd, and tau of insulin binding as compared to the normal untreated, whereas in the diabetic untreated heart these constants were altered, compared to the diabetic treated. Furthermore, the kn and k-n values in the diabetic CHAPS-treated hearts were the same as for the normals untreated and CHAPS-treated, respectively. In conclusion, the dissociation constants and residency time constants of all groups indicated the possible existence of two types of insulin receptors: the capillary endothelial cell surface insulin receptors with lower residency time (low affinity receptor or combination of insulin and IGF-1 receptors) and the myocyte plasma membrane insulin receptors with higher residency times (high affinity).

Second-messenger regulation of receptor association with clathrin-coated pits: a novel and selective mechanism in the control of CD4 endocytosis

CD4, a member of the immunoglobulin superfamily, is not only expressed in T4 helper lymphocytes but also in myeloid cells. Receptor-mediated endocytosis plays a crucial role in the regulation of surface expression of adhesion molecules such as CD4. In T lymphocytes p56lck, a CD4-associated tyrosine kinase, prevents CD4 internalization, but in myeloid cells p56lck is not expressed and CD4 is constitutively internalized. In this study, we have investigated the role of cyclic AMP (cAMP) in the regulation of CD4 endocytosis in the myeloid cell line HL-60. Elevations of cellular cAMP were elicited by 1) cholera toxin, 2) pertussis toxin, 3) forskolin and IBMX, 4) NaF, or 5) the physiological receptor agonist prostaglandin E1. All five interventions led to an inhibition of CD4 internalization. Increased cAMP levels did not inhibit endocytosis per se, because internalization of insulin receptors and transferrin receptors and fluid phase endocytosis were either unchanged or slightly enhanced. The mechanism of cAMP inhibition was further analyzed at the ultrastructural level. CD4 internalization, followed either by quantitative electron microscopy autoradiography or by immunogold labeling, showed a rapid and temperature-dependent association of CD4 with clathrin-coated pits in control cells. This association was markedly inhibited in cells with elevated cAMP levels. Thus these findings suggest a second-messenger regulation of CD4 internalization through an inhibition of CD4 association with clathrin-coated pits in p56lck-negative cells.

Annexin II is a novel player in insulin signal transduction. Possible association between annexin II phosphorylation and insulin receptor internalization

Annexin II is a Ca2+-, phospholipid-, and actin- binding protein that was implicated in the regulation of vesicular traffic and endosome fusion. It is a known substrate for protein kinases including the platelet-derived growth factor receptor, src protein-tyrosine kinase, and protein kinase C. In the present study we investigated the possible involvement of annexin II in insulin signal transduction. Phosphorylation of annexin II in response to insulin treatment of intact Chinese hamster ovary (CHO)-T cells was detected by 5 min and reached maximal levels after a 2-3-h incubation with the hormone. However, unlike other receptor substrates, annexin II failed to undergo insulin-induced Tyr phosphorylation under conditions where receptor internalization was inhibited. This was evident in CHO cells, overexpressing the insulin receptor, in which internalization was inhibited either by tyrosine kinase inhibitors or by lowering the temperature to 4 degrees C, and in CHO cells overexpressing various insulin receptor mutants in which normal internalization was impaired. Hence, Tyr phosphorylation of annexin II could be part of the internalization and sorting mechanism of the insulin receptor.

Insulin receptor kinase activation releases a constraint maintaining the receptor on microvilli

To examine whether the surface redistribution of the insulin receptor from microvilli, where it sits in its unoccupied form, to the nonvillous domain, where it is internalized through clathrin-coated pits, is an active movement or a passive redistribution linked to the release of a restraint maintaining it on microvilli, we have generated a mutated insulin receptor with a truncation of exons 17-22 and tracked it biochemically and morphologically. Biochemical analysis indicates that this mutated receptor is constitutively internalized and recycled even in the absence of ligand. Quantitative electron microscope autoradiography analysis reveals that it does not preferentially associate with microvilli in its unoccupied form but is normally segregated in clathrin-coated pits through the preserved signal sequence(s) of exon 16. We conclude that (a) insulin receptor internalization initiated through receptor kinase activation and autophosphorylation, which free the receptor from constraints maintaining it on microvilli; (b) the signal sequences contained in exon 16 are entirely sufficient to promote clathrin-coated pit-mediated internalization of insulin receptors; (c) these sequences are not uncovered by kinase activation; and (d) the "code" maintaining the unoccupied receptors on microvilli is contained within exons 17-21 of the receptor.

Two steps of insulin receptor internalization depend on different domains of the beta-subunit

The internalization of signaling receptors such as the insulin receptor is a complex, multi-step process. The aim of the present work was to determine the various steps in internalization of the insulin receptor and to establish which receptor domains are implicated in each of these by the use of receptors possessing in vitro mutations. We find that kinase activation and autophosphorylation of all three regulatory tyrosines 1146, 1150, and 1151, but not tyrosines 1316 and 1322 in the COOH-terminal domain, are required for the ligand-specific stage of the internalization process; i.e., the surface redistribution of the receptor from microvilli where initial binding occurs to the nonvillous domain of the cell. Early intracellular steps in insulin signal transduction involving the activation of phosphatidylinositol 3'-kinase are not required for this redistribution. The second step of internalization consists in the anchoring of the receptors in clathrin-coated pits. In contrast to the first ligand specific step, this step is common to many receptors including those for transport proteins and occurs in the absence of kinase activation and receptor autophosphorylation, but requires a juxta-membrane cytoplasmic segment of the beta-subunit of the receptor including a NPXY sequence. Thus, there are two independent mechanisms controlling insulin receptor internalization which depend on different domains of the beta-subunit.

Robert Feulgen Prize Lecture 1993. The journey of the insulin receptor into the cell: from cellular biology to pathophysiology

The data that we have reviewed indicate that insulin binds to a specific cell-surface receptor. The complex then becomes involved in a series of steps which lead the insulin-receptor complex to be internalized and rapidly delivered to endosomes. From this sorting station, the hormone is targeted to lysosomes to be degraded while the receptor is recycled back to the cell surface. This sequence of events presents two degrees of ligand specificity: (a) The first step is ligand-dependent and requires insulin-induced receptor phosphorylation of specific tyrosine residues. It consists in the surface redistribution of the receptor from microvilli where it preferentially localizes in its unoccupied form. (b) The second step is more general and consists in the association with clathrin-coated pits which represents the internalization gate common to many receptors. This sequence of events participates in the regulation of the biological action of the hormone and can thus be implicated in the pathophysiology of diabetes mellitus and various extreme insulin resistance syndromes, including type A extreme insulin resistance, leprechaunism, and Rabson-Mendehall syndrome. Alterations of the internalization process can result either from intrinsic abnormalities of the receptor or from more general alteration of the plasma membrane or of the cell metabolism. Type I diabetes is an example of the latter possibility, since general impairment of endocytosis could contribute to extracellular matrix accumulation and to an increase in blood cholesterol. Thus, better characterization of the molecular and cellular biology of the insulin receptor and of its journey inside the cell definitely leads to better understanding of disease states, including diabetes.

Insulin-induced surface redistribution regulates internalization of the insulin receptor and requires its autophosphorylation

The role of insulin-induced receptor autophosphorylation in its internalization was analyzed by comparing 125I-labeled insulin (125I-insulin) internalization in Chinese hamster ovary (CHO) cell lines transfected with normal (CHO.T) or mutated insulin receptors. In four cell lines with a defect of insulin-induced autophosphorylation, 125I-insulin internalization was impaired. By contrast, in CHO.T cells and in two other CHO cell lines with amino acid deletions or insertions that do not perturb autophosphorylation, 125I-insulin internalization was not affected. A morphological analysis showed that the inhibition is linked to the ligand-specific surface redistribution in which the insulin-receptor complexes leave microvilli and concentrate on nonvillous segments of the membrane where endocytosis occurs.

Receptor-mediated endocytosis of insulin by cultured endothelial cells

Label-fracture immunochemistry and pre-embedding indirect immunocytochemistry were applied to investigate insulin uptake by endothelial cells. Freeze fracture replicas showed that a small percentage of native insulin receptors are associated with non-coated pits (4%) and coated pits (2%). After warming, receptor bound insulin became increasingly associated with such endocytotic vesicles. After 2 min the percentage of detectable insulin associated with non-coated and coated pits increased to 16% and 8%, respectively. Pre-embedding immunocytochemical localization of insulin gave results consistent with those obtained from the label-fracture studies. Both non-coated and coated vesicles appeared labelled after 5 min of warming. Non-coated vesicles contained 25% of the cell associated insulin while 9% was associated with coated pits and vesicles. After 10 min of warming, 9% of label was located in non-coated vesicles and 7% in coated vesicles. A large proportion (29%) of the label was found in tubular-vesicular endosomes at this time. After 15 min of warming, 30% of the remaining cell-associated gold label was found in multivesicular bodies. These experiments demonstrate that insulin uptake by endothelium is mediated by both coated and non-coated vesicles and that, once internalized, insulin is routed through endosomal pathways that primarily result in transcytosis.

Prenatal exposure to ethanol alters plasma membrane glycoproteins of astrocytes during development in primary culture as revealed by concanavalin A binding and 5'-nucleotidase activity

We have investigated the effect of prenatal exposure to ethanol on the extent of binding and surface distribution of the lectin concanavalin A (con A) on rat cortical astrocytes during the periods of proliferation and differentiation in primary culture. The enzymatic activity of the plasma membrane glycoprotein 5'-nucleotidase was also assessed. The cells were obtained from control fetuses (no exposure to ethanol) and from fetuses prenatally exposed to ethanol. The main findings were: 1) both proliferating and differentiating control astrocytes showed two distinct types of surface con A receptors that could correspond to high- and low-affinity binding sites; 2) the extent of con A binding was greater in mature than in proliferating control cells; 3) the distribution of con A on cell surface components changed with differentiation; 4) the activity of 5'-nucleotidase showed a substantial increment during the period of differentiation; and 5) prenatal exposure to ethanol clearly decreased the ability of astrocytes to bind con A, altered the surface distribution of the receptors for this lectin, and decreased the activity of 5'-nucleotidase. These effects were more marked in proliferating cells. In conclusion, it is shown that the extent of con A labeling and the activity of 5'-nucleotidase in astrocytes are dependent on the stage of cell differentiation and that prenatal exposure to ethanol alters the plasma membrane structure of these cells during development.

Differential endocytosis of CD4 in lymphocytic and nonlymphocytic cells

The endocytosis of the T cell differentiation antigen CD4 has been investigated in CD4-transfected HeLa cells, the promyelocytic HL-60 cell line, and in a number of leukemia- or lymphoma-derived T cell lines. CD4 internalization was followed using radioiodinated antibodies in an acid-elution endocytosis assay, or by covalently modifying cell surface proteins with biotin and analyzing CD4 distributions by immunoprecipitation; both approaches gave equivalent results. The assays demonstrated that in transfected HeLa cells and in HL-60 cells CD4 was constitutively internalized and recycled in the absence of ligand. Immunogold labeling and electron microscopy demonstrated that CD4 enters cells through coated pits. In contrast to the nonlymphocytic cells, T cell lines showed very little endocytosis of CD4. Measurements of fluid phase endocytosis and morphometric analysis of the endosome compartment indicated that the endocytic capacities of HeLa and lymphoid cells are equivalent and suggested that the low level of CD4 uptake in lymphocytic cells is due to exclusion of CD4 from coated pits. This conclusion was supported by experiments using truncated CD4 molecules, lacking the bulk of the cytoplasmic domain, which were internalized equally efficiently in both transfected lymphocytes and HeLa cells. Together, these results indicate that the cytoplasmic domain of CD4 mediates the different interactions with the endocytic apparatus in lymphoid and nonlymphoid cells. We suggest that the CD4-associated lymphocyte-specific protein tyrosine kinase p56lck may be involved in preventing CD4 endocytosis in T cells.

Internalization pathway of C3b receptors in human neutrophils and its transmodulation by chemoattractant receptors stimulation

On the surface of phagocytes, C3b receptors (CR1) bind C3b-coated particles and promote their ingestion after activation by appropriate stimuli such as lymphokines or the chemoattractant formyl methionyl leucyl phenylalanine (fMLP) and fibronectin. The aims of the present study were 1) to define at the electron microscopic level the nature of the process responsible for CR1 internalization and 2) to dissect the mechanism by which a physiological activator (fMLP) stimulates this process. CR1 was visualized either by the immunogold technique or by quantitative electron microscopic autoradiography using a monoclonal anti-CR1 antibody. Both techniques revealed that after anti-CR1 binding, CR1 cluster on the neutrophil surface in a time-, temperature-, and antibody-dependent fashion, but do not concentrate in coated pits. CR1 internalization requires receptor cross-linking (does not occur in the presence of Fab fragments of anti-CR1) and intact microfilaments. It results in the association of the internalized material with large flattened vacuoles, organized in stacks. Together with the surface localization of CR1 close to cytoplasmic projections (ruffles), these observations suggest that uptake of CR1 occurs through a macropinocytotic process. Eventually, CR1 concentrate in lysosomal structures. fMLP markedly stimulates this pattern of CR1 internalization without affecting their clustering or their lack of association with coated pits. Stimulation by fMLP is inhibited by pertussis toxin, unaffected by preventing receptor-triggered cytosolic free calcium [Ca2+]i elevations, and mimicked by phorbol myristate acetate. Taken together our data demonstrate 1) that, in neutrophils, CR1 is internalized via a coated pit independent macropinocytotic process, dependent on intact microfilaments and receptor cross-linking; 2) that, in the same cells, fMLP is internalized via the classical coated pits pathway; and 3) that fMLP amplifies CR1 uptake possibly via protein kinase C stimulation.

The receptor for urokinase type plasminogen activator polarizes expression of the protease to the leading edge of migrating monocytes and promotes degradation of enzyme inhibitor complexes

Receptor-bound urokinase-type plasminogen activator (uPA) remains associated to the surface of human monocytes for many hours. Monocytes induced to migrate in a chemotactic gradient of f-Met-Leu-Phe rapidly polarize their uPA receptors to the leading front of the cells. Receptor-bound enzyme can be inhibited by plasminogen activator inhibitor 2 (PAI-2), with a kinetics comparable to that determined for the free enzyme, and uPA/PAI-2 complexes can bind to the uPA receptor. In contrast to the active enzyme, the uPA/PAI-2 complex is rapidly cleared from the monocyte cell surface; this involves an initial cleavage of the complex at the cell surface, followed by endocytosis and degradation. These results indicate that the uPA receptor can function both to focus plasmin-mediated extracellular matrix degradation in front of migrating cells, and to target uPA/PAI-2 enzyme/inhibitor complexes for degradation; they suggest that this receptor is a key determinant in the control of uPA-catalyzed extracellular proteolysis.

Down-regulation of insulin receptors is related to insulin internalization

In the present study, we have tested the influence of inhibition of endocytosis by hypertonic medium on the regulation of cell surface insulin receptors. We show that active internalization of 125I-insulin is markedly inhibited by hypertonic media and that, in parallel, cell surface invaginations are significantly diminished. These two events are accompanied by a marked inhibition of cell surface insulin receptor down-regulation. These data provide further strong evidence that receptor-mediated endocytosis is the major mechanism by which insulin receptors are regulated at the surface of target cells.

The cell biology of the insulin receptor

Following initial binding to specific cell surface receptors insulin is internalized in target cells. The fate of the internalized insulin-receptor complexes and how the processes involved are regulated is reviewed. The implications of these events in the effects of insulin on its target cells and in the physiopathology of diabetes and insulin resistance states are also considered.

Binding and internalization of somatostatin, insulin, and glucagon by cultured rat islet cells

The pathways by which islet B, A, and D cells bind and internalize homologous (self) and heterologous (other) islet hormones were compared. [125I-Tyr]Somatostatin-14 (S-14), 125I-insulin, and 125I-glucagon were incubated with monolayer cultures of neonatal rat islet cells. Tissues were processed for quantitative electron microscopic autoradiography by the probability circle method coupled to morphometry. For all three radioligands and all three cell types surface labeling was rapidly followed by internalization of the radioligands into endocytotic vesicles. The further intracellular movement of the ligand occurred in a time- and temperature-related manner and depended on whether it was homologous or heterologous for the cell in question. Thus [125I-Tyr]S-14 in B and A cells, 125I-insulin in A and D cells, and 125I-glucagon in B and D cells were rapidly transferred from endocytotic vesicles to lysosomal structures. By contrast, [125I-Tyr]S-14 in D cells, 125I-insulin in B cells, and 125I-glucagon in A cells showed poor progression from endocytotic vesicles to downstream vesicular structures. We conclude that (a) each of the three radioligands is internalized by islet cells in a time- and temperature-dependent manner; (b) after initial internalization the further intracellular progression of the endocytosed radioligand occurs freely in cells heterologous for the radioligand but poorly in cells homologous for the radioligand; and (c) binding and endocytosis can be uncoupled from lysosomal degradation of ligand.

A role for the cytoplasmic domain in transferrin receptor sorting and coated pit formation during endocytosis

The cytoplasmic domain of transferrin receptor (TR) is essential for endocytosis of this transmembrane protein. We have investigated by electron microscopy the association of wild-type and cytoplasmic deletion mutant human TR with coated pits at the surface of transfected L cell lines. Approximately 15% of wild-type TR was concentrated in coated pits, regardless of the level of TR expression. In contrast, only 2% of deletion mutant TR was present in these structures. We also correlated the frequency of coated pits with the level of TR expression in different transfected L cell lines. Expression of more than 3 x 10(6) wild-type TR per cell was accompanied by up to a 4-fold increase in coated pits compared with nontransfected Ltk- cells. No such increase was observed in a cell line expressing a similarly high level of cytoplasmic deletion mutant TR. These results indicate that the cytoplasmic domain plays an active role in sorting and endocytosis of TR by providing an assembly site for coated pit formation.

Uptake and intracellular fate of [14C]sucrose-insulin in perfused rat livers

Insulin was covalently linked to [14C]sucrose by means of cyanuric chloride to provide a label that would remain entrapped within the vacuolar system. The uptake of the conjugate by the perfused rat liver was rapid (half-life = 2.9 min), competitively inhibited by native insulin, and abolished by alkali denaturation. As assessed by its distribution on self-generating gradients of colloidal silica-povidone, label in lysosome-enriched samples of liver taken at different times after the addition of the conjugate moved progressively during 15 min from the plasma membrane into an intermediate peak and then to dense lysosomal fractions. After 30-60 min, the label had equilibrated throughout the lysosomal-vacuolar system. The initial movement from the plasma membrane to the intermediate peak occurred between 2 and 5 min. Because label in the peak could be physically separated from the lysosomal marker, beta-acetylglucosaminidase, by dispersing the sample through the gradient mixture before centrifugation rather than layering it, we concluded that the intermediate particles in question were not lysosomal in nature. On gel-filtration chromatography, label extracted from the intermediate peak did not move with insulin but rather as a broad band of lower molecular weight products, suggesting that insulin is subject to early proteolytic attack within a nonlysosomal compartment.

Regulation and function of an insulin-sensitive glycosyl-phosphatidylinositol during T lymphocyte activation

A combination of metabolic labeling and chemical or enzymatic modification was employed to isolate and biochemically characterize a set of glycosyl-phosphatidylinositol (gly-PI) molecules synthesized by T lymphocytes. Gly-PI displayed unique patterns of synthesis following mitogen activation relative to the phosphoinositides and major structural lipids. The increase with time in gly-PI was paralleled by the appearance of insulin receptors. Gly-PI molecules were sensitive to hydrolysis by a PI-specific phospholipase C and were rapidly (15 sec) degraded in response to insulin binding. The product of this hydrolysis is believed to be a novel inositol phosphate-glycan (IP-gly) that was shown to inhibit the activity of a cAMP-dependent protein kinase. These results demonstrate that T cells contain a structurally related set of gly-PI molecules, at least one of which is sensitive to insulin and may function as a second messenger of hormone action.

Insulin-induced myosin light-chain phosphorylation during receptor capping in IM-9 human B-lymphoblasts

We have examined further the interaction between insulin surface receptors and the cytoskeleton of IM-9 human lymphoblasts. Using immunocytochemical techniques, we determined that actin, myosin, calmodulin and myosin light-chain kinase (MLCK) are all accumulated directly underneath insulin-receptor caps. In addition, we have now established that the concentration of intracellular Ca2+ (as measured by fura-2 fluorescence) increases just before insulin-induced receptor capping. Most importantly, we found that the binding of insulin to its receptor induces phosphorylation of myosin light chain in vivo. Furthermore, a number of drugs known to abolish the activation properties of calmodulin, such as trifluoperazine (TFP) or W-7, strongly inhibit insulin-receptor capping and myosin light-chain phosphorylation. These data imply that an actomyosin cytoskeletal contraction, regulated by Ca2+/calmodulin and MLCK, is involved in insulin-receptor capping. Biochemical analysis in vitro has revealed that IM-9 insulin receptors are physically associated with actin and myosin; and most interestingly, the binding of insulin-receptor/cytoskeletal complex significantly enhances the phosphorylation of the 20 kDa myosin light chain. This insulin-induced phosphorylation is inhibited by calmodulin antagonists (e.g. TFP and W-7), suggesting that the phosphorylation is catalysed by MLCK. Together, these results strongly suggest that MLCK-mediated myosin light-chain phosphorylation plays an important role in regulating the membrane-associated actomyosin contraction required for the collection of insulin receptors into caps.

Insulin and alpha 2-macroglobulin-methylamine undergo endocytosis by different mechanisms in rat adipocytes: I. Comparison of cell surface events

This ultrastructural study compared the endocytosis of a peptide hormone, ferritin-labeled insulin (Fm-I) or gold-labeled insulin (Au-I), and a non-hormonal ligand, gold-labeled alpha-2-macroglobulin-methylamine (Au-alpha 2MGMA), by rat adipocytes. Quantitative analysis of the cell surface showed that coated pits occupied 0.4% of the adipocyte surface. This was one fifth to one tenth of that which has been reported on fibroblasts and hepatocytes, cell types in which receptor-mediated endocytosis has been extensively studied. In contrast, uncoated micropinocytotic invaginations were quite numerous and occupied 13.1% of the adipocyte cell surface. The frequency of micropinocytotic invaginations, 13.8 per micron 2 of plasma membrane, was 7-12 times greater than has been reported on fibroblasts. Therefore, the ultrastructure of the endocytic apparatus on rat adipocytes was different from more commonly studied cell types. At 4 degrees C, Au-alpha 2MGMA concentrated within coated pits to a density that was 52 times greater than that on the uncoated plasma membrane. Au-alpha 2MGMA was excluded from micropinocytotic invaginations by more than 93%; this exclusion was unrelated to the size of the Au-alpha 2MGMA particle. In contrast, at 4 degrees C, Fm-I did not concentrate within coated pits and occupied micropinocytotic invaginations in a random manner. At 37 degrees C, coated pits accounted for all of the endocytosis of Au-alpha 2MGMA, proving that these structures were functional despite their atypically low density. In contrast, greater than 99% of the endocytosis of Fm-I or Au-I occurred through micropinocytotic invaginations. These results demonstrated for the first time by a comparative, quantitative, ultrastructural method that insulin and Au-alpha 2MGMA undergo endocytosis by dissimilar mechanisms on rat adipocytes. Dissimilarities in the endocytosis of insulin and Au-alpha 2MGMA may be related to the different biological roles of these two molecules.

Expression of human interleukin-2 receptor cDNA in E. coli

cDNAs for human interleukin-2 receptor were recently cloned and sequenced (Leonard et al., 1984, Nature 311, 626-631; Nikaido et al., 1984, Nature 311, 631-635; Cosman et al., Nature 312, 768-771). In the studies reported here, we describe the expression of a cDNA clone for the human interleukin-2 receptor in E. coli using an "open reading frame" expression vector pMR100. The inserted cDNA was expressed in E. coli transformants as a tripartite fusion polypeptide fused to the lambda cI protein at its amino terminus and to beta-galactosidase at its carboxy terminus. We demonstrate that the bacterially produced IL-2 receptor protein can bind to IL-2.

Role of intracellular calcium and protein kinase C in the endocytosis of transferrin and insulin by HL60 cells

The role of the cytosolic free calcium concentration ([Ca2+]i) and of protein kinase C on the internalization of transferrin and insulin in the human promyelocytic cell line HL60 was investigated. [Ca2+]i was selectively monitored and manipulated by the use of the fluorescent Ca2+ indicator and buffer quin2, while receptor-ligand internalization was studied directly by quantitative electron microscope autoradiography. Decreasing the [Ca2+]i up to 10-fold below resting level had no effect on the internalization of transferrin or insulin. Similarly, a 10-fold elevation of the [Ca2+]i using the calcium ionophore ionomycin caused little or no change in the endocytosis of the two ligands. In contrast, activation of protein kinase C by phorbol myristate acetate markedly stimulated the internalization of both occupied and unoccupied transferrin receptors, even in cells with very low [Ca2+]i. The insulin receptor was found to behave differently in response to phorbol myristate acetate, however, in that only the occupied receptors were stimulated to internalize. We conclude that the [Ca2+]i plays only a minor role in regulating receptor-mediated endocytosis, whereas protein kinase C can selectively modulate receptor internalization depending on receptor type and occupancy.

Expression of a functional human insulin receptor from a cloned cDNA in Chinese hamster ovary cells

We have placed human insulin receptor cDNA into a vector under the control of the simian virus 40 (SV40) early promoter and tested its function by transient expression in microinjected Xenopus oocytes and by expression in stably transformed CHO cells. The precursor and the alpha and beta subunits of the receptor were detected by immunoprecipitation from extracts of these cells. The human insulin receptor expressed in CHO cells specifically binds 125I-labeled insulin but not insulin-like growth factor I, displays insulin-stimulated autophosphorylation of the beta subunit, and mediates insulin-stimulated 2-deoxyglucose uptake. We conclude that the human insulin receptor is synthesized, processed normally, and functional in this heterologous cell system.

Insulin receptor internalization and recycling: mechanism and significance

Using a 125I-photoreactive insulin analogue that can be covalently coupled to its receptor we have shown that in rat hepatocytes the insulin receptor is concomitantly internalized with the labeled hormone and afterwards is progressively recycled back to the cell surface. In the course of the internalization process the insulin-receptor complex associates with clear vesicles and later on with lysosomes from which it is recycled through clear vesicles. On the basis of these observations it is suggested that modulation of the rates of internalization and of recycling of the insulin receptor can regulate the number of available surface insulin receptors. This hypothesis is supported by the results of experiments showing that monensin, an inhibitor of receptor recycling enhances insulin induced loss of its own surface receptors (down regulation) in U-937 monocytes.

Insulin receptor capping and its correlation with calmodulin-dependent myosin light chain kinase

Both fluorescence microscopy and fluorometric analysis techniques have been used to characterize insulin receptor capping in IM-9 human lymphoblastoid cells. Morphologically, insulin caps appear similar to lectin or antiimmunoglobulin-induced caps displaying a preferential accumulation of actin, myosin, and actin-binding protein directly underneath the cap structure. Using the fluorescent calcium indicator quin2 we have detected no change in the calcium activity following insulin stimulation. However, in the presence of a number of calmodulin inhibitors, such as W-5, W-7, W-12, and trifluoperazine (TFP), insulin capping is significantly inhibited, which implies that a calmodulin-regulated process is involved. Using double immunofluorescence microscopy, we have found that the calmodulin-dependent myosin light chain kinase (MLCK) is concentrated directly beneath insulin caps. Upon treatment with trifluoperazine (TFP), the redistribution of both MLCK and insulin receptors are inhibited concomitantly. Our data indicate that the calmodulin-dependent myosin light chain kinase may be directly responsible for the activation of actomyosin-mediated contractility during insulin receptor capping.

The endosomal compartment of rat hepatocytes. Its characterization in the course of [125I]insulin internalization

When freshly isolated hepatocytes are incubated with [125I]insulin in the presence of the microtubule-disrupting agent colchicine, internalization of the labelled hormone is not significantly altered. However, the drug limits the endocytosis of the labelled material to a peripheral band of cytoplasm extending 1 micron beyond the plasma membrane. Both in the presence and absence of colchicine, internalized [125I]insulin preferentially associates with clear vesicles (endosomes) and lysosome-like structures, but the relative amount of labelled material associated with clear vesicles is higher in the presence of the drug than in its absence. An inverse pattern is observed for the lysosome-like structures. As demonstrated by cytochemical methods, clear vesicles do not contain the lysosomal enzyme aryl sulfatase. Moreover, colchicine induces an increase of the clear vesicle diameter without affecting their frequency, while it perturbs multivesicular bodies and dense bodies in an opposite way by increasing their frequency without affecting their size. By reducing and/or delaying the fusion between internalized endocytotic vesicles and lysosomes, colchicine allows better characterization of the endosomal compartment of isolated rat hepatocytes and allows it to be distinguished from other compartments, such as multivesicular bodies and the Golgi apparatus.

Ultrastructural localization of voltage-sensitive sodium channels using [125I]alpha scorpion toxin

The distribution of alpha scorpion toxin (alpha-ScTx) receptors was examined in differentiated mouse neuroblastoma cell cultures (N IE 115 clone) by electron microscope autoradiography using [125I]alpha-ScTx. This neurotoxin binds specifically to voltage-sensitive sodium channels, slowing down the inactivation of the sodium permeability. Quantitative analysis demonstrated that only plasma membranes were labelled. The alpha-ScTx receptors seemed to be randomly dispersed on both cell bodies and cell processes. Microvilli protruding from the cell bodies carried more sodium channels than other parts of the membrane. The specific binding site density for alpha-ScTx varied from 4 (cell body membrane) to 13 (cell process membrane) per square micrometer.

Ultrastructural analysis of the organization and distribution of insulin receptors on the surface of 3T3-L1 adipocytes: rapid microaggregation and migration of occupied receptors

Monomeric ferritin-insulin and high-resolution electron microscopic analysis were used to study the organization, distribution, and movement of insulin receptors on differentiated 3T3-L1 adipocytes. Analysis of the binding to prefixed cells showed that insulin initially occupied single and paired receptors preferentially located on microvilli. The majority of receptors (60%) were found as single molecules and 30% were pairs. In 1 min at 37% C, 50% of the receptors on nonfixed cells were found on the intervillous plasma membrane and more than 70% of the total receptors had microaggregated. By 30 min only 7% of the receptors were single or paired molecules on microvilli. The majority were on the intervillous membrane, with 95% of those receptors in groups. The receptor groups on the intervillous plasma membrane could be found in both noncoated invaginations and coated pits. The concentration of occupied receptors in the noncoated invaginations and the coated pits was similar; however, ten times more noncoated invaginations than coated pits contained occupied insulin receptors. The observations in this study contrast with those reported on rat adipocytes using identical techniques (Jarett and Smith, 1977). Insulin receptors on adipocytes were initially grouped and randomly distributed over the entire cell surface and did not microaggregate into larger groups. Insulin receptors on rat adipocytes were found in noncoated invaginations but were excluded from the coated pits. The differences in the organization and behavior of the insulin receptor between rat and 3T3-L1 adipocytes suggest that the mechanisms regulating the initial organization of insulin receptors and the aggregation of occupied receptors may be controlled by tissue-specific processes. Since both of these cell types are equally insulin sensitive, the differences in the initial organization and distribution of the insulin receptors on the cell surface may not be related to the sensitivity or biological responsiveness of these cells to insulin but may affect other processes such as receptor regulation and internalization. On the other hand, the microaggregates of occupied receptors on both cell types may relate to biological responsiveness.

The human insulin receptor cDNA: the structural basis for hormone-activated transmembrane signalling

A cloned approximately 5 kb cDNA (human placenta) contains the coding sequences for the insulin receptor. The nucleotide sequence predicts a 1382 amino acid precursor. The alpha subunit comprises the N-terminal portion of the precursor and contains a striking cysteine-rich "cross-linking" domain. The beta-subunit (the C-terminal portion of the precursor) contains a transmembrane domain and, in the intracellular region, the elements of a tyrosine phosphokinase: an ATP-binding site and a possible tyrosine autophosphorylation site or sites. The overall structure is reminiscent of the EGF receptor; the cross-linking domain of the alpha subunit and several regions of the beta subunit exhibit sequence homology with the EGF receptor. The phosphokinase domain also exhibits homology with some oncogenic proteins that have tyrosine phosphokinase activity, in particular, a striking homology with v-ros. Southern blotting experiments suggest that the coding region spans more than 45 kb. The insulin receptor gene is located on chromosome 19.