Selective externalization of the transferrin receptor by sheep reticulocytes in vitro. Response to ligands and inhibitors of endocytosis

Exosomal MicroRNAs: An Emerging Important Regulator in Acute Lung Injury

Acute lung injury (ALI) is a clinically life-threatening form of respiratory failure with a mortality of 30%-40%. Acute respiratory distress syndrome is the aggravated form of ALI. Exosomes are extracellular lipid vesicles ubiquitous in human biofluids with a diameter of 30-150 nm. They can serve as carriers to convey their internal cargo, particularly microRNA (miRNA), to the target cells involved in cellular communication. In disease states, the quantities of exosomes and the cargo generated by cells are altered. These exosomes subsequently function as autocrine or paracrine signals to nearby or distant cells, regulating various pathogenic processes. Moreover, exosomal miRNAs from multiple stem cells can provide therapeutic value for ALI by regulating different signaling pathways. In addition, changes in exosomal miRNAs of biofluids can serve as biomarkers for the early diagnosis of ALI. This study aimed to review the role of exosomal miRNAs produced by different sources participating in various pathological processes of ALI and explore their potential significance in the treatment and diagnosis.

Neural stem cell-derived exosomes and regeneration: cell-free therapeutic strategies for traumatic brain injury

Regenerative repair of the brain after traumatic brain injury (TBI) remains an extensive clinical challenge, inspiring intensified interest in therapeutic approaches to explore superior repair strategies. Exosome therapy is another research hotspot following stem cell alternative therapy. Prior research verified that exosomes produced by neural stem cells can participate in the physiological and pathological changes associated with TBI and have potential neuroregulatory and repair functions. In comparison with their parental stem cells, exosomes have superior stability and immune tolerance and lower tumorigenic risk. In addition, they can readily penetrate the blood‒brain barrier, which makes their treatment efficiency superior to that of transplanted stem cells. Exosomes secreted by neural stem cells present a promising strategy for the development of novel regenerative therapies. Their tissue regeneration and immunomodulatory potential have made them encouraging candidates for TBI repair. The present review addresses the challenges, applications and potential mechanisms of neural stem cell exosomes in regenerating damaged brains.

Erythrocyte tropism of malarial parasites: The reticulocyte appeal

Erythrocytes are formed from the enucleation of erythroblasts in the bone marrow, and as erythrocytes develop from immature reticulocytes into mature normocytes, they undergo extensive cellular changes through their passage in the blood. During the blood stage of the malarial parasite life cycle, the parasite sense and invade susceptible erythrocytes. However, different parasite species display varying erythrocyte tropisms (i.e., preference for either reticulocytes or normocytes). In this review, we explore the erythrocyte tropism of malarial parasites, especially their predilection to invade reticulocytes, as shown from recent studies. We also discuss possible mechanisms mediating erythrocyte tropism and the implications of specific tropisms to disease pathophysiology. Understanding these allows better insight into the role of reticulocytes in malaria and provides opportunities for targeted interventions.

The roles of small extracellular vesicles as prognostic biomarkers and treatment approaches in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a particularly aggressive and invasive breast cancer subtype and is associated with poor clinical outcomes. Treatment approaches for TNBC remain limited partly due to the lack of expression of well-known molecular targets. Small extracellular vesicles (sEVs) carrying a variety of bioactive contents play an important role in intercellular communications. The biomolecules including nucleic acids, proteins, and metabolites can be transferred locally or systematically to recipient cells and regulate their biological states and are involved in physiological and pathological processes. Recently, despite the extensive attraction to the physiological functions of sEVs, few studies focus on the roles of sEVs in TNBC. In this review, we will summarize the involvement of sEVs in the tumor microenvironment of TNBC. Moreover, we will discuss the potential roles of sEVs as diagnostic markers and treatment therapy in this heterogeneous breast cancer subtype. We finally summarize the clinical application of sEVs in TNBC.

Efficacy of Mesenchymal Stem Cells from Human Exfoliated DeciduousTeeth and their Derivatives in Inflammatory Diseases Therapy

Mesenchymal stem cells derived from postnatal orofacial tissues can be readily isolated and possess diverse origins, for example, from surgically removed teeth or gingiva. These cells exhibit stem cell properties, strong potential for self-renewal, and show multi-lineage differentiation, and they have therefore been widely employed in stem cell therapy, tissue regeneration, and inflammatory diseases. Among them, stem cells from human exfoliated deciduous teeth [SHED] and their derivatives have manifested wide application in the treatment of diseases because of their outstanding advantages- including convenient access, easy storage, and less immune rejection. Numerous studies have shown that most diseases are closely associated with inflammation and that inflammatory diseases are extremely destructive, can lead to necrosis of organ parenchymal cells, and can deposit excessive extracellular ma- trix in the tissues. Inflammatory diseases are thus the principal causes of disability and death from many diseases worldwide. SHED and their derivatives not only exhibit the basic characteristics of stem cells but also exhibit some special properties of their own, particularly with regard to their great potential in inhib- iting inflammation and tissue regeneration. SHED therapy may provide a new direction for the treatment of inflammation and corresponding tissue defects. In this review, we critically analyze and summarize the latest findings on the behaviors and functions of SHED, serum‑free conditioned medium from SHED [SHED-CM], and extracellular vesicles, especially exosomes, from SHED [SHED-Exos], and discuss their roles and underlying mechanisms in the control of inflammatory diseases, thus further highlighting additional functions for SHED and their derivatives in future therapies.

Exosomes Immunity Strategy: A Novel Approach for Ameliorating Intervertebral Disc Degeneration

Low back pain (LBP), which is one of the most severe medical and social problems globally, has affected nearly 80% of the population worldwide, and intervertebral disc degeneration (IDD) is a common musculoskeletal disorder that happens to be the primary trigger of LBP. The pathology of IDD is based on the impaired homeostasis of catabolism and anabolism in the extracellular matrix (ECM), uncontrolled activation of immunologic cascades, dysfunction, and loss of nucleus pulposus (NP) cells in addition to dynamic cellular and biochemical alterations in the microenvironment of intervertebral disc (IVD). Currently, the main therapeutic approach regarding IDD is surgical intervention, but it could not considerably cure IDD. Exosomes, extracellular vesicles with a diameter of 30–150 nm, are secreted by various kinds of cell types like stem cells, tumor cells, immune cells, and endothelial cells; the lipid bilayer of the exosomes protects them from ribonuclease degradation and helps improve their biological efficiency in recipient cells. Increasing lines of evidence have reported the promising applications of exosomes in immunological diseases, and regarded exosomes as a potential therapeutic source for IDD. This review focuses on clarifying novel therapies based on exosomes derived from different cell sources and the essential roles of exosomes in regulating IDD, especially the immunologic strategy.

The Role of Exosomes in Lysosomal Storage Disorders

Exosomes, small membrane-bound organelles formed from endosomal membranes, represent a heterogenous source of biological and pathological biomarkers capturing the metabolic status of a cell. Exosomal cargo, including lipids, proteins, mRNAs, and miRNAs, can either act as inter-cellular messengers or are shuttled for autophagic/lysosomal degradation. Most cell types in the central nervous system (CNS) release exosomes, which serve as long and short distance communicators between neurons, astrocytes, oligodendrocytes, and microglia. Lysosomal storage disorders are diseases characterized by the accumulation of partially or undigested cellular waste. The exosomal content in these diseases is intrinsic to each individual disorder. Emerging research indicates that lysosomal dysfunction enhances exocytosis, and hence, in lysosomal disorders, exosomal secretion may play a role in disease pathogenesis. Furthermore, the unique properties of exosomes and their ability to carry cargo between adjacent cells and organs, and across the blood-brain barrier, make them attractive candidates for use as therapeutic delivery vehicles. Thus, understanding exosomal content and function may have utility in the treatment of specific lysosomal storage disorders. Since lysosomal dysfunction and the deficiency of at least one lysosomal enzyme, glucocerebrosidase, is associated with the development of parkinsonism, the study and use of exosomes may contribute to an improved understanding of Parkinson disease, potentially leading to new therapeutics.

Small extracellular vesicles (sEVs): discovery, functions, applications, detection methods and various engineered forms

INTRODUCTION

Extracellular vesicles (EVs) are cell-created delivery systems of proteins, lipids, or nucleic acids, and means of extracellular communication. Though sEVs were initially considered to be the waste disposal mechanism, today they are at the forefront of research with different biological and pathological functions. Such EVs play a key role in the immunoregulation, CNS development, nervous system physiology, mammary gland development, induction of immunosuppression in pregnancy, the developmental signaling pathways, regeneration of different tissues, inflammation, angiogenesis, coagulation, apoptosis, stem cell differentiation, and extracellular matrix turnover.

AREAS COVERED

SEVs contribute to the pathogenesis of different cancers and the progression of various neurodegenerative diseases, infections, as well as metabolic and cardiovascular diseases. Expert Opinion: There is no exact classification for EVs; however, according to size, density, morphological features, content, and biogenesis, they can be categorized into three major classes: microvesicles (ectosomes or microparticles), apoptotic bodies, and sEVs. SEVs, as an important class of EVs, have a crucial role in distinct biological functions. Moreover, shedding light on different structural and molecular aspects of sEV has led to their application in various therapeutic, diagnostic, and drug delivery fields. In this review, we have endeavored to elaborate on different aspects of EVs, especially sEVs.

Exosomes in intercellular communication and implications for osteoarthritis

Osteoarthritis (OA) is the most prevalent of the musculoskeletal conditions and represents a significant public health burden. While degeneration of articular cartilage is a key feature, it is now increasingly recognized as a complex condition affecting the whole joint, with synovial inflammation present in a significant proportion of patients. As a secretory tissue, the OA synovium is a rich source of both soluble inflammatory mediators and extracellular vesicles, including exosomes, which have been implicated in cell-cell communication. Exosome cargo has been found to include proteins, lipids and various RNA subtypes such as mRNA and miRNA, potentially capable of regulating gene expression in target cells and tissues. Profiling of exosome cargo and understanding effects on cartilage could elucidate novel regulatory mechanisms within the joint, providing insight for targeted treatment. The aim of this article is to review current literature on exosome biology, highlighting the relevance and application for OA pathogenesis.

Exosomes: cell-created drug delivery systems

Exosomes are 40- to 100- nm cell-originated vesicles derived from endocytic compartments that are released into almost all biological fluids. Exosomes are cell-created vesicles that inherit identical phospholipid membrane, explaining a wide application of electroporation as a technique for exosomes loading with exogenous cargoes. Another way of loading exosomes with therapeutic cargo is to overexpress a certain gene in exosome-donor cells or treat cell line with drug of interest that later will be gently enveloped into vesicles based on the process of EV biogenesis. Similarly, to visualize siRNA loading into exosomes as well as the exosomal product delivery to recipient cells, we have conducted an experiment where chemical-based exosome transfection was used. In this review, we discuss different ways of extracellular vesicle loading with exogenous cargoes and their advantages/limitations as well as novel alternative techniques of substance incorporation into nanoparticles.

Small RNAs detected in exosomes derived from the MH7A synovial fibroblast cell line with TNF-α stimulation

Rheumatoid arthritis (RA) is an autoimmune disease that causes the chronic inflammation of the joints. Intercellular communication containing synovial fibroblasts seems to play a major role in RA pathogenesis. In this study, to better understand intercellular communication related to RA pathogenesis, we identified exosomal microRNAs (miRNAs) derived from synovial fibroblasts. Exosomes were collected from an RA synovial fibroblast (RASF) cell line, namely, MH7A, with or without stimulation by tumor necrosis factor alpha (TNF-α). We used small RNA sequencing to analyze the profile of small RNAs, including miRNAs, in MH7A exosomes and cells. By using differential expression analysis, we identified four miRNAs (miR-155-5p, miR-146a-5p, miR-323a-5p, and miR-1307-3p) that are upregulated in exosomes with TNF-α stimulation. The identification of miR-155-5p and miR-146a-5p which have been reported in RA patients demonstrated the validity of our experimental model. Other two miRNAs were newly identified. miR-323a-5p was predicted to target the protein encoding gene CD6, which attenuates T-cell activation signals, and miR-1307-3p was predicted to target the protein encoding gene N-myc downstream-regulated gene 2 (NDRG2), which inhibits osteoclast-related gene expression. The results suggested that these miRNAs might be involved in RA pathogenesis. We hope our results will help us understand the role of RASF exosomes in RA pathogenesis.

Cancer-derived extracellular vesicles: friend and foe of tumour immunosurveillance

Extracellular vesicles (EVs) are important players of intercellular signalling mechanisms, including communication with and among immune cells. EVs can affect the surrounding tissue as well as peripheral cells. Recently, EVs have been identified to be involved in the aetiology of several diseases, including cancer. Tumour cell-released EVs or exosomes have been shown to promote a tumour-supporting environment in non-malignant tissue and, thus, benefit metastasis. The underlying mechanisms are numerous: loss of antigen expression, direct suppression of immune effector cells, exchange of nucleic acids, alteration of the recipient cells' transcription and direct suppression of immune cells. Consequently, tumour cells can subvert the host's immune detection as well as suppress the immune system. On the contrary, recent studies reported the existence of EVs able to activate immune cells, thus promoting the tumour-directed immune response. In this article, the immunosuppressive capabilities of EVs, on the one hand, and their potential use in immunoactivation and therapeutic potential, on the other hand, are discussed.

This article is part of the discussion meeting issue ‘Extracellular vesicles and the tumour microenvironment’.

Non-coding RNAs as a new dawn in tumor diagnosis

The current knowledge about non-coding RNAs (ncRNAs) as important regulators of gene expression in both physiological and pathological conditions, has been the main engine for the design of innovative platforms to finalize the pharmacological application of ncRNAs as either therapeutic tools or as molecular biomarkers in cancer. Biochemical alterations of cancer cells are, in fact, largely supported by ncRNA disregulation in the tumor site, which, in turn, reflects the cancer-associated specific modification of circulating ncRNA expression pattern. The aim of this review is to describe the state of the art of pre-clinical and clinical studies that analyze the involvement of miRNAs and lncRNAs in cancer-related processes, such as proliferation, invasion and metastases, giving emphasis to their functional role. A central node of our work has been also the examination of advantages and criticisms correlated with the clinical use of ncRNAs, taking into account the pressing need to refine the profiling methods aimed at identify novel diagnostic and prognostic markers and the request to optimize the delivery of such nucleic acids for a therapeutic use in an imminent future.

Functions of Cancer-Derived Extracellular Vesicles in Immunosuppression

Extracellular vesicles, including exosomes, constitute an important element of intercellular communication by carrying a variety of molecules from producer to target cells. The transport of mRNA and miRNA can directly modulate gene expression in the target cells. The miRNA content in exosomes is characteristic for the cell from which the vesicles were derived enabling the usage of exosomes as biomarkers for the diagnosis various diseases, including cancer. Cancer-derived exosomes support the survival and progression of tumors in many ways and also contribute to the neutralization of the anti-cancer immune response. Exosomes participate in all known mechanisms by which cancer evades the immune system. They influence the differentiation and activation of immune suppressor cells, they modulate antigen presentation, and are able to induce T-cell apoptosis. Although cancer-derived exosomes mainly suppress the immune system and facilitate tumor progression, they are also important sources of tumor antigens with potential clinical application in stimulating immune responses. This review summarizes how exosomes assist cancer to escape immune recognition and to acquire control over the immune system.

Beyond soluble transferrin receptor: old challenges and new horizons

Disturbances of iron metabolism are a frequent challenge in outpatient and inpatient care. Although several established biomarkers are commonly used by clinicians for differential diagnosis, the discrimination between latent or classic iron deficiency, anaemia of chronic disease or a combination of functional iron deficiency (iron-restricted erythropoiesis) with anaemia of chronic disease in patients affected by inflammatory disease can be demanding. Soluble transferrin receptor (sTfR) is a cleaved monomer of transferrin receptor 1 and correlates positively with tissue iron deficiency as well as with stimulated erythropoiesis. The ratio between sTfR and ferritin in combination with reticulocyte haemoglobin content further helps to identify different states of iron deficiency. In this review, we will focus on biological aspects of iron metabolism and sTfR, established clinical applications and limitations of sTfR and derived indices, and prospects of future research and applications.

Exosome and exosomal microRNA: trafficking, sorting, and function

Exosomes are 40-100 nm nano-sized vesicles that are released from many cell types into the extracellular space. Such vesicles are widely distributed in various body fluids. Recently, mRNAs and microRNAs (miRNAs) have been identified in exosomes, which can be taken up by neighboring or distant cells and subsequently modulate recipient cells. This suggests an active sorting mechanism of exosomal miRNAs, since the miRNA profiles of exosomes may differ from those of the parent cells. Exosomal miRNAs play an important role in disease progression, and can stimulate angiogenesis and facilitate metastasis in cancers. In this review, we will introduce the origin and the trafficking of exosomes between cells, display current research on the sorting mechanism of exosomal miRNAs, and briefly describe how exosomes and their miRNAs function in recipient cells. Finally, we will discuss the potential applications of these miRNA-containing vesicles in clinical settings.

First demonstration of transmissible spongiform encephalopathy-associated prion protein (PrPTSE) in extracellular vesicles from plasma of mice infected with mouse-adapted variant Creutzfeldt-Jakob disease by in vitro amplification

The development of variant Creutzfeldt-Jakob disease (vCJD) in three recipients of non-leukoreduced red blood cells from asymptomatic donors who subsequently developed the disease has confirmed existing concerns about the possible spread of transmissible spongiform encephalopathies (TSEs) via blood products. In addition, the presence of disease-associated misfolded prion protein (PrP(TSE)), generally associated with infectivity, has been demonstrated in the blood of vCJD patients. However, its origin and distribution in this biological fluid are still unknown. Various studies have identified cellular prion protein (PrP(C)) among the protein cargo in human blood-circulating extracellular vesicles released from endothelial cells and platelets, and exosomes isolated from the conditioned media of TSE-infected cells have caused the disease when injected into experimental mice. In this study, we demonstrate the detection of PrP(TSE) in extracellular vesicles isolated from plasma samples collected during the preclinical and clinical phases of the disease from mice infected with mouse-adapted vCJD and confirm the presence of the exosomal marker Hsp70 in these preparations.

Preliminary characterization of the murine membrane reticulocyte proteome

The maturation from reticulocyte (immature red blood cell) to erythrocyte (mature red blood cell) includes the loss or decreased expression of cell surface molecules through exosome formation and secretion. Identifying the molecules lost and the molecular events involved is important to our understanding of this final stage of erythropoiesis and of diseases where it is deranged. Also, the presence of certain cell surface molecules is likely responsible for the invasion of certain malaria parasites into reticulocytes. Using a global proteomics approach, we identified proteins potentially lost during and/or involved in the reticulocyte maturation process. The reticulocyte proteome has not yet been published, as previous such studies have focused on the mature erythrocyte. Membrane-rich fractions were fractionated by electrophoresis followed by analysis with tandem mass spectrometry. Seven hundred forty four proteins were identified in the reticulocyte-rich membrane fraction, 192 proteins in the erythrocyte-rich membrane fraction, with 157 common to both fractions. Many of the proteins found uniquely in the reticulocyte were associated with structures known to be in reticulocytes (mitochondria, Golgi). Additional proteins detected are or may be specifically involved in vesicle trafficking, a process important in the maturation process. A number of unique plasma membrane proteins were also identified. These results provide the groundwork for future targeted studies to improve our understanding of the mechanism of reticulocyte maturation and the role of reticulocytes in disease.

Selective transfer of exosomes from oligodendrocytes to microglia by macropinocytosis

The transfer of antigens from oligodendrocytes to immune cells has been implicated in the pathogenesis of autoimmune diseases. Here, we show that oligodendrocytes secrete small membrane vesicles called exosomes, which are specifically and efficiently taken up by microglia both in vitro and in vivo. Internalisation of exosomes occurs by a macropinocytotic mechanism without inducing a concomitant inflammatory response. After stimulation of microglia with interferon-γ, we observe an upregulation of MHC class II in a subpopulation of microglia. However, exosomes are preferentially internalised in microglia that do not seem to have antigen-presenting capacity. We propose that the constitutive macropinocytotic clearance of exosomes by a subset of microglia represents an important mechanism through which microglia participate in the degradation of oligodendroglial membrane in an immunologically 'silent' manner. By designating the capacity for macropinocytosis and antigen presentation to distinct cells, degradation and immune function might be assigned to different subtypes of microglia.

Mycobacterium tuberculosis synergizes with ATP to induce release of microvesicles and exosomes containing major histocompatibility complex class II molecules capable of antigen presentation

Major histocompatibility complex class II (MHC-II) molecules are released by murine macrophages upon lipopolysaccharide (LPS) stimulation and ATP signaling through the P2X7 receptor. These studies show that infection of macrophages with Mycobacterium tuberculosis or M. bovis strain BCG enhances MHC-II release in synergy with ATP. Shed MHC-II was contained in two distinct organelles, exosomes and plasma membrane-derived microvesicles, which were both able to present exogenous antigenic peptide to T hybridoma cells. Furthermore, microvesicles from mycobacterium-infected macrophages were able to directly present M. tuberculosis antigen (Ag) 85B(241-256)-I-A(b) complexes that were generated by the processing of M. tuberculosis Ag 85B in infected cells to both M. tuberculosis-specific T hybridoma cells and naïve P25 M. tuberculosis T-cell receptor (TCR)-transgenic T cells. In the presence of prefixed macrophages, exosomes from mycobacterium-infected macrophages provided weak stimulation to M. tuberculosis-specific T hybridoma cells but not naïve P25 T cells. Thus, infection with M. tuberculosis primes macrophages for the increased release of exosomes and microvesicles bearing M. tuberculosis peptide-MHC-II complexes that may generate antimicrobial T-cell responses.

Exosomes in erythropoiesis

Multivesicular endosomes contain membrane vesicles which can be released into the extracellular environment as exosomes. This review describes the role of exosome secretion in the remodeling of the red cell plasma membrane during the last stage of erythropoietic differentiation. Herein, we propose that the exosome biogenesis involves several mechanisms of protein sorting and leads to partial or complete loss of membrane activities, in some cases in a regulated way.

Proteomic analysis of secreted exosomes

This chapter focuses on the contribution of proteomic analysis to the understanding of the process of exosome secretion and the mechanism and function of exosomes. It also describes the potential of exosome proteomic analysis to aid in the development of exosomes for therapeutic use.

Revisiting the road to the discovery of exosomes

A look-back in time is presented to review the 'Alice in Blunderland' approach which led to the discovery of exosomes in and their extrusion from maturing reticulocytes. The key experiments which established that these structures are naturally occurring in both nucleated and non-nucleated reticulocytes are reviewed. The protein content of reticulocyte exosomes is largely that of plasma membrane proteins which are known to diminish in the course of reticulocyte maturation. Thus their protein content may vary depending on the species origin of the reticulocyte. To date, all exosomes also contain HSP 70, a major cellular chaperone which is not transmembrane bound. Significantly, the proteins externalized are intact and retain catalytic activity as well as native ligand binding activity. Their fate remains unknown.

Exosome release by reticulocytes—An integral part of the red blood cell differentiation system

Reticulocyte maturation into erythrocytes is the final step of erythropoiesis that occurs in the blood circulation. This terminal differentiation period corresponds to a cellular remodeling phase following expulsion of the nucleus into the bone marrow. Among other events, this remodeling leads to the disappearance of intracellular organelles and acquisition of the typical cellular biconcave form. Here, we propose that exosome biogenesis and secretion, which contributes to net loss of the cell surface membrane via selective vesicular membrane secretion, is also closely interconnected with upstream (nucleus expulsion), accompanying (mitoptosis) and downstream (vesicle clearance) events.

Haemoglobin synthesis in erythropoietin-stimulated J2E cells does not require increased numbers of transferrin receptors

Changes in transferrin-receptor numbers and iron utilisation were monitored during erythropoietin-induced maturation of J2E erythroid cells. Uptake of transferrin and iron doubled 24 h after exposure to erythropoietin, due to a twofold rise in surface transferrin receptors. In addition, a tenfold increase in iron incorporation into haem was observed after erythropoietin stimulation, as iron taken up from transferrin was directed towards haem biosynthesis and away from storage in ferritin. The rise in iron chelation into haem correlated extremely well with haemoglobin synthesis. However, the increase in numbers of transferrin receptors was not essential for haemoglobin synthesis; rather, it was linked with a burst in proliferation stimulated by erythropoietin. We have shown previously that amiloride blocks erythropoietin-enhanced proliferation of J2E cells, but potentiates maturation [Callus, B. A., Tilbrook, P. A., Busfield, S. J. & Klinken, S. P. (1995) Exp. Cell Res. 219, 39-46]. Here we demonstrate that amiloride suppressed the hormone-induced increase in transferrin receptors, whereas the enhanced incorporation of iron into haem was not inhibited. Similarly, when sodium butyrate was used to induce differentiation of J2E cells, proliferation ceased and surface transferrin receptors remained unaltered, while haemoglobin production was accelerated. It was concluded from these experiments that the erythropoietin-stimulated rise in transferrin receptors during the final stages of J2E cell maturation is linked to cell division, and is not essential for haemoglobin synthesis.

Iron metabolism: a comprehensive review

Despite its abundance in the earth's crust, iron deficiency is a serious health issue in many parts of the world. Although fundamental observations about iron metabolism and the significance of iron nutriture were first noted some time ago, the molecular mechanisms involved in iron metabolism are just now being defined.

Cleavage of the transferrin receptor by human granulocytes: differential proteolysis of the exosome-bound TFR

In contrast with other mammalian granulocytes, human granulocytes rapidly cleave the transferrin receptor (TFR) from sheep exosomes. Proteolysis of TFR from exosomes is more rapid and more extensive than that from the sheep reticulocyte cell surface itself, although little difference in cleavage is seen when immunoprecipitates or when immobilized, solubilized receptors from the two sources are compared. Circulating exosomes but not the plasma membrane fraction from seven species of immature red cells or erythropoietic cells show the presence of a peptide of approximately 18 kD recognized by an antibody to the cytoplasmic domain of the TFR. This 18 kD peptide is virtually absent from the corresponding cellular plasma membranes including human reticulocyte membranes. Taken together, the data are consistent with the conclusion that the exosomes released to the circulation from maturing red cells are the principal source of the soluble, circulating, truncated TFR. The granulocyte protease appears to be present on the cell surface and not released into the medium after short (30 min) periods of incubation at 37 degrees C. The protease is inhibited by PMSF but only at high (1 mM) concentrations. Using sheep exosome bound-TFR as substrate, human granulocytes exceed other granulocytes in their capacity to cleave TFR, suggesting that this may be a key factor for the prominent amount of circulating, soluble receptor found in human sera during periods of elevated reticulocyte levels. Human exosomes, unlike those from other species, contain little native size TFR. Truncated receptor containing the cytoplasmic domain being predominant in human exosomes.

Assessment of iron status

OBJECTIVE

To review the clinical assessment of iron deficiency and excess.

CONCLUSIONS

Two key iron-related proteins in the human body are ferritin which is the iron storage protein, and the transferrin receptor, which controls the entry of iron-bearing transferrin to cells. Intact ferritin and truncated transferrin-receptor molecules are present in serum in direct quantitative proportion to their total tissue content. Ferritin and transferrin-receptor production are precisely and reciprocally regulated at a posttranscriptional level. This is achieved by an iron-responsive element-binding protein that interacts with iron-responsive elements in the mRNA of each, but with contrary effects. Increases in serum ferritin reflect increased storage iron and increases in serum transferrin receptor reflect cellular iron deficits. The combined use of these two measurements allows accurate definition of the entire range of body iron status. This is valid even in situations where assessment of iron status has been notoriously problematic, including periods of rapid growth, in pregnancy, in conditions associated with inflammation, and in trained athletes.

Characterization of phospholipase A2 activity in reticulocyte endocytic vesicles

Electron spin resonance spectroscopy was used to investigate the presence of phospholipase A2 activity in endocytic vesicles prepared from reticulocytes and to define some of its characteristics. Using spin-labeled phospholipid analogues, we measured the hydrolysis rate of the ester bond at position 2 during incubation with reticulocyte endocytic vesicles. We have shown that this phospholipase A2 activity was membrane-associated, enriched in endocytic vesicles as compared to cytosol and plasma membrane. Enzymic activity was also observed in exosomes, vesicles coming from the endocytic compartment and released by reticulocytes during their maturation in erythrocytes. Neither the hydrolytic activity nor the membrane association was found to be Ca(2+)-dependent. Spin-labeled phospholipids with choline and serine polar heads were better substrates than glycerophosphoethanolamine analogues. Optimal pH was found to be close to neutral. 5,5'-Dithiobis(2-nitrobenzoic acid) and diisopropyl fluorophosphate very efficiently inhibited spin-labeled phospholipid hydrolysis. This phospholipase A2 activity was confirmed using a radioactive assay, although with much lower sensitivity. (E)-6-(Bromomethylene)-tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one, a specific mechanism-based inhibitor of calcium-independent phospholipases A2, was found to abolish the enzymic activity present in endocytic vesicles.

Effect of cell age and phenylhydrazine on the cation transport properties of rabbit erythrocytes

We studied the effect of cell age on the cation transport systems of rabbit erythrocytes by increasing the proportion of circulating young erythrocytes with either repeated bleeding or with phenylhydrazine (PHZ) treatment. We found that when the reticulocyte content of rabbit blood is increased by bleeding (from 1 to 40-50% of the circulating red cells), the response of the various transport pathways differs. The largest increase (fivefold) was found in the activity of K-Cl cotransport, which peaked 3 days after the last bleeding. The Na-K pump activity peaked at a similar time, but the % increase was twofold less than the K-Cl cotransport. There was a very small increase in the activity of the Na-Li exchange, whereas the Na-H exchange reached peak values 10 days after the last bleeding (twofold increase), when activities of K-Cl cotransport and Na-K pump had returned to almost normal levels. In vivo PHZ treatment resulted in anemia and marked reticulocytosis (80-90% of circulating cells). Transport rates were markedly increased (Na-K pump 9.6-fold, Na-H exchange 6.8-fold, Na-Li exchange 2.75-fold; K-Cl cotransport: 10-20-fold). When blood from PHZ-treated rabbits was incubated in vitro for 24-48 hours, red cell volume and K content decreased. This process was associated with a 70% reduction in the activity of the K-Cl cotransport after 24 hours and a 90% reduction after 48 hours. The activity of the other systems also declined and approached baseline values after 48 hours. Loss of transport activity was not affected by 10 microM E-64, whereas 10 mM methylamine reduced the inactivation of the Na-H exchange and of the Na-Li exchange. PHZ treatment of rabbit red cells in vitro resulted in marked increase of the K-Cl cotransport and inhibition of Na-K pump, Na-H exchange, and Na-Li exchange. These effects were abolished by DTT, with the exception of the Na-K pump inhibition, which was DTT insensitive. Thus both cell age and oxidative damage are important determinants of cation transport in rabbit red cells.

The transferrin receptor: its diagnostic value and its potential as therapeutic target

Transferrin receptors are present on almost all mammalian cells. The receptor participates in the cellular acquisition of iron from transferrin by receptor-mediated endocytosis. Receptor abundancy is generally regulated by two factors: i) cellular iron status and ii) cell growth. These two factors form the basis for the utilization of transferrin receptor determination as a diagnostic tool. In the assessment of body iron status and erythropoietic activity the measurement of circulating transferrin receptor has proved to be of value as a measure of mild tissue iron deficiency, to distinguish iron deficiency anemia from the anemias of chronic disease, and as a sensitive index of iron deficiency during pregnancy. Histochemical analysis of the presence and abundancy of the transferrin receptor will continue to serve as an additional tool in special cases to distinguish between malignant and normal cell growth, and to provide additional information about the biological behaviour of tumor cells. Finally, the transferrin receptor holds a potential as a target for direct and indirect drug delivery in the therapy of malignant cell growth.

Maturation-associated loss and incomplete de novo synthesis of the transferrin receptor in peripheral sheep reticulocytes: response to heme and iron

Hemin, but not iron, in the culture medium stimulates the maturation-associated loss of the transferrin receptor from sheep reticulocytes (t1/2 for loss approximately 6 hr) and its appearance in a population of externalized vesicles. A similar pattern is seen with nucleoside binding (a measure of the nucleoside transporter), where hemin increases the loss of binding activity from the cells during culture, concomitant with an increase in nucleoside binding in the externalized vesicles. Sheep reticulocytes retain the ability to synthesize the transferrin receptor, but the 35S-labeled receptors are not detected in released vesicles. Whereas hemin stimulates the loss of 35S-labeled transferrin receptors from the cell (t1/2 for loss approximately 20 hr), nonheme iron is more effective than heme. This difference in response of native and 35S-labeled receptor to hemin and iron supplements appears to be related to the differences in the two classes of receptors. Although the 35S-labeled receptor binds transferrin and both native and 35S-labeled peptides comigrate after chemical deglycosylation, the 35S-receptor is approximately 2 kD smaller than the native receptor and fails to acquire its complete size even when chased for up to 24 hr. Moreover, the 35S-labeled receptor is not expressed at the cell surface, but is retained in a nonrecycling compartment, where it is insensitive to digestion by trypsin at both 0 degrees C and 37 degrees C.

Blood-brain barrier transcytosis of insulin in developing rabbits

Previous studies with isolated brain microvessels have suggested that blood insulin is selectively transported through the brain capillary, i.e. the blood-brain barrier (BBB), by receptor-mediated transcytosis. The purpose of the present study is to demonstrate in vivo the uptake of circulating 125I-insulin by brain using thaw-mount autoradiography. However, metabolism of systemic 125I-insulin to 125I-tyrosine would allow for brain uptake of 125I-tyrosine and this would preclude interpretation of the autoradiogram. Therefore, the present studies were performed in developing rabbits, since plasma protein degradation of peptides is greatly reduced in developing animals. 125I-insulin was infused via the carotid artery at a rate of 0.25 ml/min for 1, 5, or 10 min, and the mean brain uptake, relative to a [3H]albumin reference, was 99.3 +/- 5.5%, 110.1 +/- 4.3%, and 143.6 +/- 7.9%, respectively. This uptake was saturable by simultaneously infusing unlabeled insulin. Thaw-mount autoradiography of rabbit brain after a 10-min infusion of 125I-insulin revealed silver grains in the pericapillary space and well within the brain parenchyma. HPLC analysis of acid-ethanol extracts of rabbit blood after a 10-min infusion showed virtually all of the 125I-radioactivity co-migrated with a known insulin standard on a reverse-phase column, indicating minimal degradation of infused 125I-insulin. HPLC analysis of brain radioactivity showed the major peak co-migrated with 125I-insulin and this peak was precipitated by an anti-insulin antiserum. The correlation of the transport data, the autoradiography, and the HPLC analysis support the model that brain insulin originates from blood via receptor-mediated transport of the peptide at the BBB.

Human blood-brain barrier transferrin receptor

The kinetics of binding and endocytosis of 125I-human holotransferrin by isolated human brain capillaries was examined using this system as a model of the human blood-brain barrier (BBB). Both binding and endocytosis of the peptide by human brain capillaries was temperature-dependent and the binding was saturated by holotransferrin, but not by insulin, somatostatin, or vasopressin. Scatchard analysis of the binding reaction revealed a dissociation constant of 448 +/- 110 ng/mL (5.6 +/- 1.4 nmol/L) and a maximal binding constant (Ro) of 8.0 +/- 1.5 ng/mg protein. Thus, the affinity and capacity of the BBB transferrin receptor is within the same order of magnitude as the affinity and capacity of the BBB receptors for insulin, insulinlike growth factor-I, or insulinlike growth factor-II. The human brain capillary transferrin receptor was also detected with a mouse monoclonal antibody to the receptor using the avidin/biotin/peroxidase technique. In conclusion, these studies characterize the human BBB transferrin receptor and support the hypothesis that this receptor acts as a transport system which mediates the transcytosis of transferrin-bound iron through the brain capillary endothelial cell in man.

Evidence for a pool of non-recycling transferrin receptors in peripheral sheep reticulocytes

Sheep reticulocytes from phlebotomized animals have a total transferrin binding potential that may exceed by an order of magnitude the surface binding capacity. Steady state uptake of transferrin at 37 degrees C is generally less than 50% of the total transferrin binding capacity. During long-term incubation of the reticulocytes, all transferrin binding ability is lost, the ability to internalize being lost most rapidly. The loss in ability to bind transferrin during long-term incubation is independent of the number of surface transferrin binding sites, since removal of surface receptors with pronase does not affect the rate of loss of the internal pool of receptors during long-term incubation. Moreover, after removing surface receptors with pronase, only a fraction of the original number of receptors is restored to the surface, despite the presence of a large pool of internal receptors. These data suggest that only a fraction of the internal pool of receptors is capable of recycling to the cell surface in sheep reticulocytes.