The entry of enveloped viruses into cells by endocytosis

The penetration of therapeutics across the blood-brain barrier: Classic case studies and clinical implications

The blood-brain barrier (BBB) plays central roles in the maintenance and health of the brain. Its mechanisms to safeguard the brain against xenobiotics and endogenous toxins also make the BBB the primary obstacle to the development of drugs for the central nervous system (CNS). Here, we review classic examples of the intersection of clinical medicine, drug delivery, and the BBB. We highlight the role of lipid solubility (heroin), saturable brain-to-blood (efflux: opiates) and blood-to-brain (influx: nutrients, vitamins, and minerals) transport systems, and adsorptive transcytosis (viruses and incretin receptor agonists). We examine how the disruption of the BBB that occurs in certain diseases (tumors) can also be modulated (osmotic agents and microbubbles) and used to deliver treatments, and the role of extracellular pathways in gaining access to the CNS (albumin and antibodies). In summary, this review provides a historical perspective of the key role of the BBB in delivery of drugs to the brain in health and disease.

Blood-brain barrier penetration of non-replicating SARS-CoV-2 and S1 variants of concern induce neuroinflammation which is accentuated in a mouse model of Alzheimer's disease

COVID-19 and especially Long COVID are associated with severe CNS symptoms and may place persons at risk to develop long-term cognitive impairments. Here, we show that two non-infective models of SARS-CoV-2 can cross the blood-brain barrier (BBB) and induce neuroinflammation, a major mechanism underpinning CNS and cognitive impairments, even in the absence of productive infection. The viral models cross the BBB by the mechanism of adsorptive transcytosis with the sugar N-acetylglucosamine being key. The delta and omicron variants cross the BB B faster than the other variants of concern, with peripheral tissue uptake rates also differing for the variants. Neuroinflammation induced by icv injection of S1 protein was greatly enhanced in young and especially in aged SAMP8 mice, a model of Alzheimer's disease, whereas sex and obesity had little effect.

Transcellular routes of blood-brain barrier disruption

Disruption of the blood-brain barrier (BBB) can occur through different mechanisms and pathways. As these pathways result in increased permeability to different classes of substances, it is likely that the neurological insults that occur will also differ for these pathways. The major categories of BBB disruption are paracellular (between cells) and transcellular (across cells) with a subcategory of transcellular leakage involving vesicles (transcytotic). Older literature, as well as more recent studies, highlights the importance of the transcellular pathways in BBB disruption. Of the various transcytotic mechanisms that are thought to be active at the BBB, some are linked to receptor-mediated transcytosis, whereas others are likely involved in BBB disruption. For most capillary beds, transcytotic mechanisms are less clearly linked to permeability than are membrane spanning canaliculi and fenestrations. Disruption pathways share cellular mechanisms to some degree as exemplified by transcytotic caveolar and transcellular canaliculi formations. The discovery of some of the cellular components involved in transcellular mechanisms of BBB disruption and the ability to measure them are adding greatly to our classic knowledge, which is largely based on ultrastructural studies. Future work will likely address the conditions and diseases under which the various pathways of disruption are active, the different impacts that they have, and the cellular biology that underlies the different pathways to disruption.

The ins and outs of virus trafficking through acidic Ca2+ stores

Many viruses exploit host-cell Ca2+ signaling processes throughout their life cycle. This is especially relevant for viruses that translocate through the endolysosomal system, where cellular infection is keyed to the microenvironment of these acidic Ca2+ stores and Ca2+-dependent trafficking pathways. As regulators of the endolysosomal ionic milieu and trafficking dynamics, two families of endolysosomal Ca2+-permeable cation channels - two pore channels (TPCs) and transient receptor potential mucolipins (TRPMLs) - have emerged as important host-cell factors in viral entry. Here, we review: (i) current evidence implicating Ca2+ signaling in viral translocation through the endolysosomal system, (ii) the roles of these ion channels in supporting cellular infection by different viruses, and (iii) areas for future research that will help define the potential of TPC and TRPML ligands as progressible antiviral agents.

Developmental Aspects of SARS-CoV-2, Potential Role of Exosomes and Their Impact on the Human Transcriptome

With over 4.8 million deaths within 2 years, time is of the essence in combating COVID-19. The infection now shows devastating impacts on the younger population, who were not previously predicted to be vulnerable, such as in the older population. COVID-19-related complications have been reported in neonates whose mothers were infected with SARS-CoV-2 during pregnancy, and in children who get infected. Hence, a deeper understanding of the pathophysiology of COVID-19 during various developmental stages and placental transmission is essential. Although a connection has not yet been established between exosomal trafficking and the placental transmission of COVID-19, reports indicate that SARS-CoV-2 components may be trafficked between cells through exosomes. As the infection spreads, the transcriptome of cells is drastically perturbed, e.g., through the severe upregulation of several immune-related genes. Consequently, a major outcome of COVID-19 is an elevated immune response and the detection of viral RNA transcripts in host tissue. In this direction, this review focuses on SARS-CoV-2 virology, its in utero transmission from infected pregnant mothers to fetuses, SARS-CoV-2 and exosomal cellular trafficking, transcriptomic impacts, and RNA-mediated therapeutics against COVID-19. Future research will establish stronger connections between the above processes to develop diagnostic and therapeutic solutions towards COVID-19 and similar viral outbreaks.

Alphaviruses: Host pathogenesis, immune response, and vaccine & treatment updates

Human pathogens belonging to the Alphavirus genus, in the Togaviridae family, are transmitted primarily by mosquitoes. The signs and symptoms associated with these viruses include fever and polyarthralgia, defined as joint pain and inflammation, as well as encephalitis. In the last decade, our understanding of the interactions between members of the alphavirus genus and the human host has increased due to the re-appearance of the chikungunya virus (CHIKV) in Asia and Europe, as well as its emergence in the Americas. Alphaviruses affect host immunity through cytokines and the interferon response. Understanding alphavirus interactions with both the innate immune system as well as the various cells in the adaptive immune systems is critical to developing effective therapeutics. In this review, we summarize the latest research on alphavirus-host cell interactions, underlying infection mechanisms, and possible treatments.

Interactions of SARS-CoV-2 with the Blood-Brain Barrier

Emerging data indicate that neurological complications occur as a consequence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The blood-brain barrier (BBB) is a critical interface that regulates entry of circulating molecules into the CNS, and is regulated by signals that arise from the brain and blood compartments. In this review, we discuss mechanisms by which SARS-CoV-2 interactions with the BBB may contribute to neurological dysfunction associated with coronavirus disease of 2019 (COVID-19), which is caused by SARS-CoV-2. We consider aspects of peripheral disease, such as hypoxia and systemic inflammatory response syndrome/cytokine storm, as well as CNS infection and mechanisms of viral entry into the brain. We also discuss the contribution of risk factors for developing severe COVID-19 to BBB dysfunction that could increase viral entry or otherwise damage the brain.

Enveloped artificial viral capsids self-assembled from anionic β-annulus peptide and cationic lipid bilayer

Anionic artificial viral capsids were self-assembled from β-annulus-EE peptide, then complexed with lipid-bilayer-containing cationic lipids via electrostatic interaction to form enveloped artificial viral capsids. The critical aggregation concentration of the enveloped artificial viral capsid was significantly lower than that of the uncomplexed artificial viral capsid, indicating that the lipid bilayer stabilised the capsid structure.

Membrane Fusion and Infection of the Influenza Hemagglutinin

The influenza virus is a major health concern associated with an estimated 5000 to 30,000 deaths every year (Reed et al. 2015) and a significant economic impact with the development of treatments, vaccinations and research (Molinari et al. 2007). The entirety of the influenza genome is comprised of only eleven coding genes. An enormous degree of variation in non-conserved regions leads to significant challenges in the development of inclusive inhibitors for treatment. The fusion peptide domain of the influenza A hemagglutinin (HA) is a promising candidate for treatment since it is one of the most highly conserved sequences in the influenza genome (Heiny et al. 2007), and it is vital to the viral life cycle. Hemagglutinin is a class I viral fusion protein that catalyzes the membrane fusion process during cellular entry and infection. Impediment of the hemagglutinin's function, either through incomplete post-translational processing (Klenk et al. 1975; Lazarowitz and Choppin 1975) or through mutations (Cross et al. 2001), leads to non-infective virus particles. This review will investigate current research on the role of hemagglutinin in the virus life cycle, its structural biology and mechanism as well as the central role of the hemagglutinin fusion peptide (HAfp) to influenza membrane fusion and infection.

Role of the immune system in HIV-associated neuroinflammation and neurocognitive implications

Individuals living with HIV who are optimally treated with combination antiretroviral therapy (cART) can now lead an extended life. In spite of this remarkable survival benefit from viral suppression achieved by cART in peripheral blood, the rate of mild to moderate cognitive impairment remains high. A cognitive decline that includes impairments in attention, learning and executive function is accompanied by increased rates of mood disorders that together adversely impact the daily life of those with chronic HIV infection. The evidence is clear that cells in the brain are infected with HIV that has crossed the blood-brain barrier both as cell-free virus and within infected monocytes and T cells. Viral proteins that circulate in blood can induce brain endothelial cells to release cytokines, invoking another source of neuroinflammation. The difficulty of efficient delivery of cART to the central nervous system (CNS) contributes to elevated viral load in the CNS, resulting in a persistent HIV-associated neurocognitive disorders (HAND). The pathogenesis of HAND is multifaceted, and mounting evidence indicates that immune cells play a major role. HIV-infected monocytes and T cells not only infect brain resident cells upon migration into the CNS but also produce proinflammatory cytokines such as TNF and IL-1ß, which in turn, further activate microglia and astrocytes. These activated brain resident cells, along with perivascular macrophages, are the main contributors to neuroinflammation in HIV infection and release neurotoxic factors such as excitatory amino acids and inflammatory mediators, resulting in neuronal dysfunction and death. Cytokines, which are elevated in the blood of patients with HIV infection, may also contribute to brain inflammation by entering the brain from the blood. Host factors such as aging and co-morbid conditions such as cytomegalovirus co-infection and vascular pathology are important factors that affect the HIV-host immune interactions in HAND pathogenesis. By these diverse mechanisms, HIV-1 induces a neuroinflammatory response that is likely to be a major contributor to the cognitive and behavior changes seen in HIV infection.

Kinetics of particle wrapping by a vesicle

We present theoretical results on kinetics for the passive wrapping of a single, rigid particle by a flexible membrane. Using a simple geometric ansatz for the shape of the membrane/particle complex we first compute free energy profiles as a function of the particle size, attraction strength between the particle and vesicle, and material properties of the vesicle--bending stiffness and stretching modulus. The free energy profiles thus computed are taken as input to a stochastic model of the wrapping process, described by a Fokker-Planck equation. We compute average uptake rates of the particle into the vesicle. We find that the rate of particle uptake falls to zero outside of a thermodynamically allowed range of particle sizes. Within the thermodynamically allowed range of particle size, the rate of uptake is variable and we compute the optimal particle size and maximal uptake rate as a function of the attraction strength, the vesicle size, and vesicle material properties.

GRB2 interaction with the ecotropic murine leukemia virus receptor, mCAT-1, controls virus entry and is stimulated by virus binding

For retroviruses such as HIV-1 and murine leukemia virus (MLV), active receptor recruitment and trafficking occur during viral entry. However, the underlying mechanisms and cellular factors involved in the process are largely uncharacterized. The viral receptor for ecotropic MLV (eMLV), a classical model for retrovirus infection mechanisms and pathogenesis, is mouse cationic amino acid transporter 1 (mCAT-1). Growth factor receptor-bound protein 2 (GRB2) is an adaptor protein that has been shown to couple cell surface receptors, such as epidermal growth factor receptor (EGFR) and hepatocyte growth factor receptor, to intracellular signaling events. Here we examined if GRB2 could also play a role in controlling infection by retroviruses by affecting receptor function. The GRB2 RNA interference (RNAi)-mediated suppression of endogenous GRB2 resulted in a consistent and significant reduction of virus binding and membrane fusion. The binding between eMLV and cells promoted increased GRB2-mCAT-1 interactions, as detected by immunoprecipitation. Consistently, the increased colocalization of GRB2 and mCAT-1 signals was detected by confocal microscopy. This association was time dependent and paralleled the kinetics of cell-virus membrane fusion. Interestingly, unlike the canonical binding pattern seen for GRB2 and growth factor receptors, GRB2-mCAT-1 binding does not depend on the GRB2-SH2 domain-mediated recognition of tyrosine phosphorylation on the receptor. The inhibition of endogenous GRB2 led to a reduction in surface levels of mCAT-1, which was detected by immunoprecipitation and by a direct binding assay using a recombinant MLV envelope protein receptor binding domain (RBD). Consistent with this observation, the expression of a dominant negative GRB2 mutant (R86K) resulted in the sequestration of mCAT-1 from the cell surface into intracellular vesicles. Taken together, these findings suggest a novel role for GRB2 in ecotropic MLV entry and infection by facilitating mCAT-1 trafficking.

Alteration of blood-brain barrier function by methamphetamine and cocaine

The integrity of the blood-brain barrier (BBB) plays an important role in maintaining a safe neural microenvironment in the brain. Loss of BBB integrity has been recognized as a major cause of profound brain alterations. Psychoactive drugs such as methamphetamine (METH) or cocaine are well-known drugs of abuse that can alter the permeability of the BBB via various mechanisms. In addition, the neurotoxicity of METH is well documented, and alterations in BBB function can contribute to this toxicity. A great deal of effort has been devoted to understanding the cellular and molecular mechanisms of the action of these drugs in the central nervous system. However, only a few investigations have focused on the effects of METH and cocaine on BBB function. The aim of this short review is to summarize our present knowledge of this subject.

Chapter 9 Fusion of Viral Envelopes with Cellular Membranes

This chapter reviews some characteristic features of membrane fusion activity for each virus and discusses the mechanisms of membrane fusion, especially low pH-induced membrane fusion. It concentrates on the interaction of the hydrophobic segment with the target cell membrane lipid bilayer and suggests the entrance of the segment into the lipid bilayer hydrophobic core as a key step in fusion. The envelope is a lipid bilayer membrane with the virus specific glycoproteins spanning it. The bilayer originates from the host cell membrane and has a lipid composition and transbilayer distribution quite similar to the host's. The viral glycoproteins have the functions of binding to the target cell surface and fusion with the cell membranes. The two functions are carried by a single glycoprotein in influenza virus (HA), vesicular stomatitis virus (VSV) G glycoprotein, and Semliki Forest virus SFV E glycoprotein. In Sendai virus (HVJ), the functions are carried by separate glycoproteins, hemagglutinin-neuraminidase (HN) for binding and fusion glycoprotein (F) for fusion. When viruses encounter target cells, they first bind to the cell surface through an interaction of the viral glycoprotein with receptors.

Histidine protonation and the activation of viral fusion proteins

Many viral fusion proteins only become activated under mildly acidic condition (pH 4.5–6.5) close to the pKa of histidine side-chain protonation. Analysis of the sequences and structures of influenza HA (haemagglutinin) and flaviviral envelope glycoproteins has led to the identification of a number of histidine residues that are not only fully conserved themselves but have local environments that are also highly conserved [Kampmann, Mueller, Mark, Young and Kobe (2006) Structure 14, 1481–1487]. Here, we summarize studies aimed at determining the role, if any, that protonation of these potential switch histidine residues plays in the low-pH-dependent conformational changes associated with fusion activation of a flaviviral envelope protein. Specifically, we report on MD (Molecular Dynamics) simulations of the DEN2 (dengue virus type 2) envelope protein ectodomain sE (soluble E) performed under varied pH conditions designed to test the histidine switch hypothesis of Kampmann et al. (2006).

Mutagenicity of diesel exhaust particles mediated by cell-particle interaction in mammalian cells

Diesel exhaust particle (DEP) has been identified as a class 2A human carcinogen and closely related to the increased incidence of respiratory allergy, cardiopulmonary morbidity and mortality, and risk of lung cancer. However, the molecular mechanisms of DEP mutagenicity/carcinogenicity are still largely unknown. In the present study, we focused on the mutagenicity of DEPs in human-hamster hybrid (A(L)) cells and evaluated the role of cell-particle interaction in mediating mutagenic process. We found that DEPs formed micron-sized aggregates in the medium and located mainly in large cytoplasmic vacuoles of cells by 24h treatment. The cellular granularity was increased by DEP treatment in a dose-dependent manner. DEPs resulted in a dose-dependent increase of mutation yield at CD59 locus in A(L) cells, while inflicting minimal cytotoxicity. There was a more than two-fold increase of mutation yield at CD59 locus in A(L) cells exposed to DEPs at a dose of 50mug/ml. Such induction was significantly reduced by concurrent treatment with phagocytosis inhibitors, cytochalasin B and ammonium chloride (p<0.05). These results provided direct evidence that DEPs was mutagenic in mammalian cells and that cell-particle interaction played an essential role in the process.

Improved attachment of natural HCV isolate to Daudi cells upon elimination of immune complexes and close pH control

OBJECTIVES

To study the viral requirements for attachment, entry and infection using natural isolates of HCV (infectious plasma of a patient infected with HCV from genotype 1b) and a cell line which has previously been shown to support HCV replication (Daudi cells).

METHODS

We studied attachment of HCV to Daudi cells, a human B-cell line. Quantification was done by real-time RT-PCR.

RESULTS

The best attachment levels were obtained using plasma depleted of immunocomplexes. Results of kinetics of HCV attachment to Daudi cells show that attachment is maximum at pH 7.0, and that it decreased drastically at any other pH studied (5.5, 6.0, 6.5, 7.5 and 8.0).

CONCLUSIONS

Depletion of immunocomplexes and pH control during infection are important parameters to improve attachment of HCV to Daudi cells using plasma as a natural source of virus.

Ligand-directed gene targeting to mammalian cells by pseudotype baculoviruses

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) can infect a variety of mammalian cells, as well as insect cells, facilitating its use as a viral vector for gene delivery into mammalian cells. Glycoprotein gp64, a major component of the budded AcMNPV envelope, is involved in viral entry into cells by receptor-mediated endocytosis and subsequent membrane fusion. We examined the potential production of pseudotype baculovirus particles transiently carrying ligands of interest in place of gp64 as a method of ligand-directed gene delivery into target cells. During amplification of a gp64-null pseudotype baculovirus carrying a green fluorescent protein gene in gp64-expressing insect cells, however, we observed the high-frequency appearance of a replication-competent virus incorporating the gp64 gene into the viral genome. To avoid generation of replication-competent revertants, we prepared pseudotype baculoviruses by transfection with recombinant bacmids without further amplification in the gp64-expressing cells. We constructed gp64-null recombinant bacmids carrying cDNAs encoding either vesicular stomatitis virus G protein (VSVG) or measles virus receptors (CD46 or SLAM). The VSVG pseudotype baculovirus efficiently transduced a reporter gene into a variety of mammalian cell lines, while CD46 and SLAM pseudotype baculoviruses allowed ligand-receptor-directed reporter gene transduction into target cells expressing measles virus envelope glycoproteins. Gene transduction mediated by the pseudotype baculoviruses could be inhibited by pretreatment with specific antibodies. These results indicate the possible application of pseudotype baculoviruses in ligand-directed gene delivery into target cells.

In vitro methods in the study of viral and prion permeability across the blood-brain barrier

(1) Infectious agents capable of entering the central nervous system (CNS) produce some of the most dreaded diseases known to man. The infectious agent within the CNS is often protected by the blood-brain barrier (BBB), shielded from endogenous and exogenous anti-infectious agents. (2) The use of in vitro methods offers many advantages to the study of how infectious agents interact with the BBB. Two such agents which negotiate the BBB early in the course of disease before damage to the BBB are the autoimmune deficiency syndrome virus, or human immunodeficiency virus 1, and scrapie prion. Our laboratories have used in vitro methods to study these agents. (3) Here, we review some of the results form our laboratories and those of others.

Role of protein kinase C betaII in influenza virus entry via late endosomes

Many viruses take advantage of receptor-mediated endocytosis in order to enter target cells. We have utilized influenza virus and Semliki Forest virus (SFV) to define a role for protein kinase C betaII (PKCbetaII) in endocytic trafficking. We show that specific PKC inhibitors prevent influenza virus infection, suggesting a role for classical isoforms of PKC. We also examined virus entry in cells overexpressing dominant-negative forms of PKCalpha and -beta. Cells expressing a phosphorylation-deficient form of PKCbetaII (T500V), but not an equivalent mutant form of PKCalpha, inhibited successful influenza virus entry-with the virus accumulating in late endosomes. SFV, however, believed to enter cells from the early endosome, was unaffected by PKCbetaII T500V expression. We also examined the trafficking of two cellular ligands, transferrin and epidermal growth factor (EGF). PKCbetaII T500V expression specifically blocked EGF receptor trafficking and degradation, without affecting transferrin receptor recycling. As with influenza virus, in PKCbetaII kinase-dead cells, EGF receptor was trapped in a late endosome compartment. Our findings suggest that PKCbetaII is an important regulator of a late endosomal sorting event needed for influenza virus entry and infection.

Persistent Ross River virus infection of murine macrophages: an in vitro model for the study of viral relapse and immune modulation during long-term infection

A clinical feature of Ross River virus disease (RRVD) is the periodic relapse of symptoms months after the initial onset of disease. The underlying mechanisms responsible for this relapse have not been determined. In a long-term (148 days) in vitro study of persistently infected murine macrophages we established that RRV infection periodically fell to undetectable biological levels that required genetic detection. However, the virus concentration spontaneously relapsed to biologically detectable levels that corresponded with enhanced viral mRNA expression, cellular detachment, and cytopathic effect. By altering the cell culture conditions we found that relapse could also be induced. We propose that the periodic relapse of symptoms in RRVD may be associated with spontaneous or stress-induced increases in RRV within persistently infected macrophages. This study also established that RRV enhanced macrophage phagocytic activity and dysregulated the immunoregulatory molecules CD80, IFN-gamma, and TNF-alpha that may facilitate persistence of RRV and avoidance of immune responses.

Dissecting virus entry via endocytosis

Numerous virus families utilize endocytosis to infect host cells, mediating virus internalization as well as trafficking to the site of replication. Recent research has demonstrated that viruses employ the full endocytic capabilities of the cell. The endocytic pathways utilized include clathrin-mediated endocytosis, caveolae, macropinocytosis and novel non-clathrin, non-caveolae pathways. The tools to study endocytosis and, consequently, virus entry are becoming more effective and specific as the amount of information on endocytic component structure and function increases. The use of inhibitory drugs, although still quite common, often leads to non-specific disruptions in the cell. Molecular inhibitors in the form of dominant-negative proteins have surpassed the use of chemical inhibitors in terms of specificity to individual pathways. Dominant-negative molecules are derived from both structural proteins of endocytosis, such as dynamin and caveolin, and regulatory proteins, primarily small GTPases and kinases. This review focuses on the experimental approaches taken to examine virus entry and provides both classic examples and recent research on a variety of virus families.

RSV entry inhibitors block F-protein mediated fusion with model membranes

RSV fusion is mediated by F-protein, a major viral surface glycoprotein. CL-309623, a specific inhibitor of RSV, interacts tightly with F-protein, which results in a hydrophobic environment at the binding site. The binding is selective for F-protein and does not occur with G-protein, a surface glycoprotein that facilitates the binding of RSV to target cells, or with lipid membranes at concentrations in the sub-millimolar range. Using an assay based on the relief of self-quenching of octadecyl rhodamine (R18) incorporated in the RSV envelope, we show that the virus fuses efficiently with large unilamellar vesicles containing cholesterol, in the absence of specific receptor analogs. Fusion of cp-52, a mutant virus lacking the G and SH surface glycoproteins, with vesicles is inhibited by CL-309623 and RFI-641 due to specific interactions of the inhibitor(s) with the fusion protein. Both virus-vesicle and virus-cell fusion are inhibited with equal potency. The formation of the binary complex of CL-309623 with F-protein in its native state, resulting in the inhibition of fusion and entry of virus, is a prerequisite for the observed anti-RSV activity in cell cultures.

HIV-1 capsid protein forms spherical (immature-like) and tubular (mature-like) particles in vitro: structure switching by pH-induced conformational changes

The viral genome and replicative enzymes of the human immunodeficiency virus are encased in a shell consisting of assembled mature capsid protein (CA). The core shell is a stable, effective protective barrier, but is also poised for dissolution on cue to allow transmission of the viral genome into its new host. In this study, static light scattering (SLS) and dynamic light scattering (DLS) were used to examine the entire range of the CA protein response to an environmental cue (pH). The CA protein assembled tubular structures as previously reported but also was capable of assembling spheres, depending on the pH of the protein solution. The switch from formation of one to the other occurred within a very narrow physiological pH range (i.e., pH 7.0 to pH 6.8). Below this range, only dimers were detected. Above this range, the previously described tubular structures were detected. The ability of the CA protein to form a spherical structure that is detectable by DLS but not by electron microscopy indicates that some assemblages are inherently sensitive to perturbation. The dimers in equilibrium with these assemblages exhibited distinct conformations: Dimers in equilibrium with the spherical form exhibited a compact conformation. Dimers in equilibrium with the rod-like form had an extended conformation. Thus, the CA protein possesses the inherent ability to form metastable structures, the morphology of which is regulated by an environmentally-sensitive molecular switch. Such metastable structures may exist as transient intermediates during the assembly and/or disassembly of the virus core.

RFI-641 inhibits entry of respiratory syncytial virus via interactions with fusion protein

BACKGROUND

RFI-641, a small dendrimer-like compound, is a potent and selective inhibitor of respiratory syncytial virus (RSV), which is currently a clinical candidate for the treatment of upper and lower respiratory tract infections caused by RSV. RFI-641 inhibits RSV growth with an IC(50) value of 50 nM and prevents syncytia formation in tissue culture. RSV contains of three surface glycoproteins, a small hydrophobic (SH) protein of unknown function, and attachment (G) and fusion (F) proteins that enable binding and fusion of virus, respectively, with target cells. Because of their role in attachment and fusion, the G and F surface proteins are prominent targets for therapeutic intervention. RFI-641 was previously shown to bind purified preparations of RSV fusion protein. Based on this observation, in conjunction with the biological results, it was speculated that the fusion event might be the target of these inhibitors.

RESULTS

A fusion assay based upon the relief of self-quenching of octadecyl rhodamine R18 was used to determine effects of the inhibitors on binding and fusion of RSV. The results show that RFI-641 inhibits both RSV-cell binding and fusion events. The inhibition of RSV is mediated via binding to the fusion protein on the viral surface. A closely related analog, WAY-158830, which is much less active in the virus-infectivity assay does not inhibit binding and fusion of RSV with Vero cells.

CONCLUSIONS

RFI-641, an in vivo active RSV inhibitor, is shown to inhibit both binding and fusion of RSV with cells, events that are early committed steps in RSV entry and pathogenicity. The results described here demonstrate that a non-peptidic, small molecule can inhibit binding and fusion of enveloped virus specifically via interaction with the viral fusion protein.

Transport of human immunodeficiency virus type 1 pseudoviruses across the blood-brain barrier: role of envelope proteins and adsorptive endocytosis

Blood-borne human immunodeficiency virus type 1 (HIV-1) crosses the blood-brain barrier (BBB) to induce brain dysfunction. How HIV-1 crosses the BBB is unclear. Most work has focused on the ability of infected immune cells to cross the BBB, with less attention devoted to the study of free virus. Since the HIV-1 coat glycoprotein gp120 can cross the BBB, we postulated that gp120 might be key in determining whether free virus can cross the BBB. We used radioactive virions which do (Env+) or do not (Env-) bear the envelope proteins to characterize the ability of HIV-1 to be taken up by the murine BBB. In vivo and in vitro studies showed that the envelope proteins are key to the uptake of free virus and that uptake was enhanced by wheat germ agglutinin, strongly suggesting that the envelope proteins induce viral adsorptive endocytosis and transcytosis in brain endothelia. Capillary depletion showed that Env+ virus completely crossed the vascular BBB to enter the parenchyma of the brain. Virus also entered the cerebrospinal fluid, suggesting passage across the choroid plexus as well. About 0.22% of the intravenously injected dose was taken up per g of brain. In vitro studies showed that postinternalization membrane cohesion (membrane binding not reversed with acid wash or cell lysis) was a regulated event. Intact virus was recovered from the brain endothelial cytosol and was effluxed from the endothelial cells. These results show that free HIV-1 can cross the BBB by an event related to adsorptive endocytosis and mediated by the envelope proteins.

Role of endosomes in gene transfection mediated by photochemical internalisation (PCI)

BACKGROUND

Most non-viral gene therapy vectors deliver transgenes into cells through the endocytic pathway. Lack of escape from endocytic vesicles in many cases constitutes a major barrier for delivery of the functional gene. We have developed a new technology named photochemical internalisation (PCI) to achieve light-inducible cytosolic delivery of the transgene. The technology is based on a photochemical treatment employing photosensitisers localised in endocytic vesicles. In this work mechanisms involved in PCI-mediated transfection (photochemical transfection) were studied.

METHODS

Human melanoma or colon carcinoma cells were pre-incubated with the photosensitiser aluminium phthalocyanine disulfonate (AlPcS2a) followed by treatment with plasmid encoding enhanced green fluorescent protein (EGFP) complexed with poly-L-lysine, N-(1-(2,3-dioleoxyloxy)propyl)-N,N,N,-trimethylammonium-methyl-sulfate (DOTAP) or polyethylenimine (PEI) and light exposure. The expression of the EGFP-gene was scored by fluorescence microscopy and flow cytometry.

RESULTS

The photochemical treatment using light doses corresponding to D50 substantially improves the efficiency of transfection mediated by poly-L-lysine and PEI, but not by DOTAP. The treatment does not enhance the delivery of the plasmid complex across the plasma membrane, since the amount of internalised plasmid is similar for irradiated and non-irradiated cells. Light-inducible transfection occurs only under temperature conditions allowing endocytic uptake and is not improved by chloroquine or ammonium chloride, but is inhibited by bafilomycin A1 (agents that increase vesicular pH and interfere with the endocytic transport).

CONCLUSIONS

Photochemical transfection occurs through endocytosis, followed by cytosolic release of the transfecting DNA from photochemically permeabilised endocytic vesicles. Release of plasmid from early endosomes seems to be of importance in photochemical transfection, although a role of later endocytic vesicles can, however, not be ruled out.

Sindbis virus entry into cells triggers apoptosis by activating sphingomyelinase, leading to the release of ceramide

Sindbis virus (SV) causes acute encephalomyelitis by infecting and inducing the death of neurons. Induction of apoptosis occurs during virus entry and involves acid-induced conformational changes in the viral surface glycoproteins and sphingomyelin (SM)-dependent fusion of the virus envelope with the endosomal membrane. We have studied neuroblastoma cells to determine how this entry process triggers cell death. Acidic sphingomyelinase was activated during entry followed by activation of neutral sphingomyelinase, SM degradation, and a sustained increase in ceramide. Ceramide-induced apoptosis and SV-induced apoptosis could be inhibited by treatment with Z-VAD-fmk, a caspase inhibitor, and by overexpression of Bcl-2, an antiapoptotic cellular protein. Acid ceramidase, expressed in a recombinant SV, decreased intracellular ceramide and protected cells from apoptosis. The data suggest that acid-induced SM-dependent virus fusion initiates the apoptotic cascade by inducing SM degradation and ceramide release.

Cytoplasmic trafficking of the canine parvovirus capsid and its role in infection and nuclear transport

To begin a successful infection, viruses must first cross the host cell plasma membrane, either by direct fusion with the membrane or by receptor-mediated endocytosis. After release into the cytoplasm those viruses that replicate in the nucleus must target their genome to that location. We examined the role of cytoplasmic transport of the canine parvovirus (CPV) capsid in productive infection by microinjecting two antibodies that recognize the intact CPV capsid into the cytoplasm of cells and also by using intracellular expression of variable domains of a neutralizing antibody fused to green fluorescence protein. The two antibodies tested and the expressed scFv all efficiently blocked virus infection, probably by binding to virus particles while they were in the cytoplasm and before entering the nucleus. The injected antibodies were able to block most infections even when injected 8 h after virus inoculation. In control studies, microinjected capsid antibodies did not interfere with CPV replication when they were coinjected with an infectious plasmid clone of CPV. Cytoplasmically injected full and empty capsids were able to move through the cytosol towards the nuclear membrane in a process that could be blocked by nocodazole treatment of the cells. Nuclear transport of the capsids was slow, with significant amounts being found in the nucleus only 3 to 6 h after injection.

Characterization of a hyaluronan receptor on rat sinusoidal liver endothelial cells and its functional relationship to scavenger receptors

Hyaluronan is a widely distributed extracellular component of connective tissue with several mechanical and cell biological functions. The serum level of hyaluronan is elevated in rheumatic and liver diseases and in certain malignancies. The major route of hyaluronan clearance from the blood is via the liver, taken up predominantly by sinusoidal liver endothelial cells. We have purified a novel hyaluronan binding protein from liver that also has an affinity for the N-terminal propeptide of type I procollagen, a physiological scavenger receptor ligand. A polyclonal antibody raised against the protein was found to inhibit the binding and degradation of hyaluronan as well as two scavenger receptor ligands by cultured sinusoidal liver endothelial cells. Immunostaining of nonpermeabilized liver cells and liver sections showed that the antibody specifically stains the surface of sinusoidal liver endothelial cells. After pretreatment with monensin to block the recirculation of endocytic receptors, the immunostaining was specifically associated with early endosomes of these cells. Thus, this rat sinusoidal liver endothelial cell hyaluronan receptor shares functional properties with the scavenger receptor family, a group of proteins shown to play a key role in the uptake of atherogenic lipids and other waste products from the tissues.

Replication-competent rhabdoviruses with human immunodeficiency virus type 1 coats and green fluorescent protein: entry by a pH-independent pathway

We describe a replication-competent, recombinant vesicular stomatitis virus (VSV) in which the gene encoding the single transmembrane glycoprotein (G) was deleted and replaced by an env-G hybrid gene encoding the extracellular and transmembrane domains of a human immunodeficiency virus type 1 (HIV-1) envelope protein fused to the cytoplasmic domain of VSV G. An additional gene encoding a green fluorescent protein was added to permit rapid detection of infection. This novel surrogate virus infected and propagated on cells expressing the HIV receptor CD4 and coreceptor CXCR4. Infection was blocked by SDF-1, the ligand for CXCR4, by antibody to CD4 and by HIV-neutralizing antibody. This virus, unlike VSV, entered cells by a pH-independent pathway and thus supports a pH-independent pathway of HIV entry. Additional recombinants carrying hybrid env-G genes derived from R5 or X4R5 HIV strains also showed the coreceptor specificities of the HIV strains from which they were derived. These surrogate viruses provide a simple and rapid assay for HIV-neutralizing antibodies as well as a rapid screen for molecules that would interfere with any stage of HIV binding or entry. The viruses might also be useful as HIV vaccines. Our results suggest wide applications of other surrogate viruses based on VSV.

Direct visualization of HIV-1 entry: mechanisms and role of cell surface receptors

Highly fluorescent virions of T- and M-tropic HIV-1 strains were obtained by incorporation of the viral accessory protein Vpr, fused to the green fluorescent protein, in trans. The fluorescent virions displayed normal morphology, were infectious, and could be used for direct visualization of HIV-1 attachment and trafficking in various cell lines. More than 90% of the viral particles were found to enter the cells by direct membrane fusion in T-cells, CD4+ HeLa cells, and macrophages. Visualizing HIV-1 attachment and entry in the absence or presence of CD4 and/or the appropriate coreceptors indicated that CD4 is the major receptor for virus attachment in the case of JR-CSF and NL-4-3 HIV-1 isolates; however, the coreceptors are required for membrane fusion. Internalization of the coreceptor CXCR4 inhibited entry, but did not prevent virus binding suggesting that transient downregulation of the coreceptor(s) may not be the most efficient way of blocking HIV infection in vivo.

Using confocal microscopy to study virus binding and entry into cells

The chapter presents a discussion on the study of virus binding and entry into cells by using confocal microscopy. For the study new approaches to study vaccinia virus (VV), binding and entry based on confocal microscopy are developed. These techniques do not require virus purification or labeling and generate data that reveal the absolute numbers of virus particles that have bound to or have entered into individual cells. The chapter describes these techniques and then illustrates with some of the results obtained. These methods should be applicable to any virus larger than 50 nm. The chapter discusses the way these techniques have generated data that cannot be obtained with classical binding or entry assays. Vaccina virus is the prototype of the poxvirus family. These are DNA viruses that replicate in the cell cytoplasm and have genomes between 150 and 300 kbp. These techniques are presented with a study of the binding and entry of VV. The methods have been particularly useful for studying VV because this virus produces two different forms of infectious virion that are antigenically and biologically distinct and are produced in widely differing amounts. Moreover, the extracellular enveloped virus (EEV) form of VV cannot be purified from contaminating IMV without disrupting the integrity of the outer envelope.

Characterization of lectin-mediated brain uptake of HIV-1 GP120

The viral coat of the HIV-1 virus, gp120, has been shown to cross the blood-brain barrier (BBB) in lectin-like fashion by inducing adsorptive endocytosis (AE), a vesicular mechanism that could provide pathways into and across brain endothelial cells for virus and infected immune cells. Here, we extended those findings to show that gp120 slowly crossed the BBB with about 0.15% of an intravenously injected dose entering the brain after about 2 hr. The plant lectin glycoprotein wheat germ agglutinin (WGA) greatly enhanced gp120 crossing without disrupting the BBB. WGA enhanced the uptake of gp120 into all peripheral tissues studied, but the greatest percent increase occurred for brain, whereas another barrier tissue, the testis, had the least increase. Five other plant lectins tested had little or no effect on gp120 uptake by brain, suggesting a key role for sialic acid and N-acetyl-beta-D-glucosaminyl acid, the sugars to which WGA binds, in the uptake of gp120 by brain endothelial cells. WGA did not enhance the uptake of nonglycosylated gp120 and the uptake of gp120 was not self-inhibitable or altered by pretreatment of mice with aluminum. In conclusion, these studies show that gp120 crosses the BBB by a lectin-like mechanism resembling AE that is likely mediated by binding to specific sugar moieties and is rather selective for brain.

Adsorptive endocytosis mediates the passage of HIV-1 across the blood-brain barrier: evidence for a post-internalization coreceptor

HIV-1 induces the AIDS dementia complex and infects brain endothelial and glial cells. Because the endothelial cells comprising the blood-brain barrier (BBB) do not possess CD4 receptors or galactosylceramide binding sites, it is unclear how HIV-1 negotiates the BBB. Previous work has suggested that gp120, the glycoprotein viral coat of HIV-1, is capable of inducing adsorptive endocytosis. Glycoprotein lectins like wheatgerm agglutinin induce adsorptive endocytosis and greatly potentiate the uptake by and passage across mouse endothelial cells in vivo and in vitro. We show here that the wheatgerm agglutinin-induced binding of gp120 is dose-dependent and involves components of the cytoskeleton. The uptake is partially dependent on temperature and energy and is modestly enhanced by potassium depletion. Glycosylation of gp120 is critical for its uptake by adsorptive endocytosis since the non-glycosylated form of gp120 is unaffected by wheatgerm agglutinin. Evidence is presented for the existence of a coreceptor sensitive to protamine sulfate that is primarily involved in membrane fusion after 125I-gp120 has bound to the cell membrane and is probably activated after internalization. This coreceptor probably contains a negatively charged heparin sulfate group and could be a member of the chemokine receptor family.

Receptor-mediated targeted drug or toxin delivery

The new approach to the treatment of cancer or to immunomodulation is drug targeting. Cellular uptake of drugs bound to a targeting carrier or to a targetable polymeric carrier is mostly restricted to receptor-mediated endocytosis. Factors that influence the efficiency of receptor-mediated uptake of targeted drug conjugate are the affinity of the targeting moieties, the affinity and nature of the target antigen, density of the target antigen, the epitope of the target antigen, the type of cell target, the rate of endocytosis, the route of internalization of the ligand-receptor complex, the ability of the drug or toxin to release from its targeted carrier, the ability of the drug or toxin to escape from a vesicular compartment into the cytosol, the affinity of the carrier to the drug and the concentration of the carrier. Targeted chemotherapy is also significantly influenced by the antigenic modulation and/or immunoselection of tumor cells. The binding of drug (toxin) to targetable polymeric carrier considerably decreases unwanted side toxicity.

Intracellular route of canine parvovirus entry

The present study was designed to investigate the endocytic pathway involved in canine parvovirus (CPV) infection. Reduced temperature (18 degrees C) or the microtubule-depolymerizing drug nocodazole was found to inhibit productive infection of canine A72 cells by CPV and caused CPV to be retained in cytoplasmic vesicles as indicated by immunofluorescence microscopy. Consistent with previously published results, these data indicate that CPV enters a host cell via an endocytic route and further suggest that microtubule-dependent delivery of CPV to late endosomes is required for productive infection. Cytoplasmic microinjection of CPV particles was used to circumvent the endocytosis and membrane fusion steps in the entry process. Microinjection experiments showed that CPV particles which were injected directly into the cytoplasm, thus avoiding the endocytic pathway, were unable to initiate progeny virus production. CPV treated at pH 5.0 prior to microinjection was unable to initiate virus production, showing that factors of the endocytic route other than low pH are necessary for the initiation of infection by CPV.

Structure-activity relationships in the fusion of small unilamellar phosphatidylcholine vesicles induced by a model peptide

Limited proteolysis of fatty acid-free bovine serum albumin by pepsin yields several well characterized peptides, one of which (P9, M(r) 9,000), induces fusion of small unilamellar vesicles (SUV) of phosphatidylcholine at pH 3.6. Circular dichroism (CD) of P9 solutions confirmed that the peptide undergoes a reversible transition between pH 7 and pH 3.6. The spectral changes observed with CD suggest that in the low pH conformation there is a decrease in the alpha-helical contents and an exposure of hydrophobic residues. CD and differential ultraviolet spectroscopy demonstrated that P9 binds to micelles of hexadecylphosphorylcholine and the binding produces changes in the tertiary structure of the peptide. Reduction and carboxymethylation of the two disulfide bridges of P9 produced loss of the ability to induce fusion of SUV, although the reduced peptide binds to vesicles, induces loss of entrapped marker and produces vesicle disruption. In the active form P9 exposes hydrophobic groups, one amphiphilic alpha-helix and requires the integrity of the disulfide bridge-stabilized tertiary structure.

pH-independent uptake of hepatitis B virus in primary human hepatocytes

The replication cycle of the hepatitis B virus (HBV) is still incompletely understood. In particular, the early steps of the viral life cycle, such as absorption, penetration, uncoating, and nuclear translocation require further clarification. In this study we performed infection experiments with HBV in primary human hepatocyte cultures. To further elucidate the possible mechanism of virus uptake, infection experiments were performed at different pH levels, after pretreatment of viral particles with acidic buffers and in the presence of lysosomotropic agents (chloroquine and ammonium chloride, respectively). Using a selective PCR technique which discriminates between input virus DNA and the earliest replicative form, we could demonstrate viral replication 36 hr after inoculation. HBV was taken up most efficiently at a pH of 7.4. Infection was still successful after pretreatment of viral particles at low pH and was unaffected by the presence of lysosomotropic agents. In conclusion, this suggests HBV to be a pH-independent virus.

Migration of vesicular stomatitis virus glycoprotein to the nucleus of infected cells

A new means of direct visualization of the early events of viral infection by selective fluorescence labeling of viral proteins coupled with digital imaging microscopy is reported. The early phases of viral infection have great importance for understanding viral replication and pathogenesis. Vesicular stomatitis virus, the best-studied rhabdovirus, is composed of an RNA genome of negative sense, five viral proteins, and membrane lipids derived from the host cell. The glycoprotein of vesicular stomatitis virus was labeled with fluorescein isothiocyanate, and the labeled virus was incubated with baby hamster kidney cells. After initiation of infection, the fluorescence of the labeled glycoprotein was first seen inside the cells in endocytic vesicles. The fluorescence progressively migrated to the nucleus of infected cells. After 1 h of infection, the virus glycoprotein was concentrated in the nucleus and could be recovered intact in a preparation of purified nuclei. These results suggest that uncoating of the viral RNA occurs close to the nuclear membrane, which would precede transcription of the leader RNA that enters the nucleus to shut off cellular RNA synthesis and DNA replication.

Involvement of oxidants and oxidant-generating enzyme(s) in tumour-necrosis-factor-alpha-mediated apoptosis: role for lipoxygenase pathway but not mitochondrial respiratory chain

Cellular signalling by the inflammatory cytokine tumour necrosis factor alpha (TNF alpha) has been suggested to involve generation of low levels of reactive oxygen species (ROS). Certain antioxidants and metal chelators can inhibit cytotoxicity and gene expression in response to TNF alpha in numerous cell types. However, neither the source nor function of TNF alpha-induced oxidant generation is known. Using specific inhibitors, we ruled out involvement of several oxidant-generating enzymes [cyclo-oxygenase (indomethacin), cytochrome P-450 (metyrapone), nitric oxide synthase (NG-methyl-L-arginine), NADPH oxidase (iodonium diphenyl), xanthine oxidase (allopurinol), ribonucleotide reductase (hydroxyurea)] in TNF alpha-mediated apoptosis of the murine fibrosarcoma line, L929. We also demonstrated no role for mitochondrial-derived radicals/respiratory chain in the lytic pathway using specific inhibitors/uncouplers (rotenone, KCN, carboxin, fluoroacetate, antimycin, malonate, carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and chloramphenicol-derived respiration-deficient cells. Significant ROS (H2O2, O2-.) generation was not observed in response to TNF alpha in L929 cells using four separate assays. Also, prevention of intracellular H2O2 removal by inhibition of catalase did not potentiate TNF alpha-mediated cell death. These data suggest that neither H2O2 nor O2-. plays a direct role in TNF alpha cytotoxicity. Finally, we suggest a central role for lipoxygenase in TNF alpha-mediated lysis. Three inhibitors of this radical-generating signalling pathway, including an arachidonate analogue (5,8,11,14-eicosatetraynoic acid), could protect cells against TNF alpha. The inhibitor nordihydroguaiaretic acid is also a radical scavenger, but it could not protect cells from ROS toxicity at concentrations that effectively prevented TNF alpha killing. Therefore protection by nordihydroguaiaretic acid cannot be due to scavenging of cytotoxic H2O or O2-.. The lipoxygenase product, (12S)-hydroxyeicosatetraenoic acid, was also significantly protective. As this analogue can act as a substrate for certain lipoxygenases, this effect may be due to prevention of generation of physiological products.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Analysis of receptor clustering on cell surfaces by imaging fluorescent particles

Fluorescently labeled low density lipoproteins (LDL) and influenza virus particles were bound to the surface of human fibroblasts and imaged with a cooled slow-scan CCD camera attached to a fluorescence microscope. Particles were also imaged after attachment to polylysine-coated microscope slides. The digital images were analyzed by fitting data points in the region of fluorescent spots by a two-dimensional Gaussian function, thus obtaining a measure of spot intensity with correction for local background. The intensity distributions for particles bound to polylysine slides were mainly accounted for by particle size distributions as determined by electron microscopy. In the case of LDL, the intensity distributions for particles bound to fibroblasts were considerably broadened, indicative of clustering. The on-cell intensity distributions were deconvolved into 1-particle, 2-particle, 3-particle, etc. components using the data obtained with LDL bound to polylysine-coated slides as an empirical measure of the single particle intensity distribution. This procedure yielded a reasonably accurate measure of the proportion of single particles, but large errors were encountered in the proportions of larger cluster sizes. The possibility of studying the dynamics of clustering was investigated by binding LDL to cells at 4 degrees C and observing changes in the intensity distribution with time after warming to 20 degrees C.