For human parainfluenza virus type 3 and many other paramyxoviruses, membrane fusion mediated by the fusion protein (F) has a stringent requirement for the presence of the homotypic hemagglutinin-neuraminidase protein (HN). With the goal of gaining further insight into the role of HN in the fusion process, we developed a simple method for quantitative comparison of the ability of wild-type and variant HNs to activate F. In this method, HN/F-coexpressing cells with red blood cells (RBC) bound to them at 4°C are transferred to 22°C, and at different times after transfer 4-guanidino-neu5Ac2en (4-GU-DANA) is added; this inhibitor of the HNreceptor interaction then releases all reversibly bound RBC but not those in which F insertion in the target membrane or fusion has occurred. Thus, the amount of irreversibly bound (nonreleased) RBC provides a measure of F activation, and the use of fluorescently labeled RBC permits microscopic assessment of the extent to which F insertion has progressed to fusion. We studied two neuraminidase-deficient HN variants, C28a, which has two mutations, P111S and D216N, and C28, which possesses the D216N mutation only. C28a but not C28 exhibits a slow fusion phenotype, although determination of the HNs' receptor-binding avidity (with our sensitive method, employing RBC with different degrees of receptor depletion) showed that the receptorbinding avidity of C28a or C28 HN was not lower than that of the wild type. The F activation assay, however, revealed fusion-triggering defects in C28a HN. After 10 and also 20 min at 22°C, irreversible RBC binding was significantly less for cells coexpressing wild-type F with C28a HN than for cells coexpressing wild-type F with wild-type HN. In addition, F insertion progressed to fusion more slowly in the case of C28a HN-expressing cells than of wild-type HN-expressing cells. Identical defects were found for P111S HN, whereas for C28 HN, representing the 216 mutation of C28a, F activation and fusion were as rapid as for wild-type HN. The diminished fusion promotion capacity of C28a HN is therefore attributable to P111S, a mutation in the stalk region of the molecule that causes no decrease in receptor-binding avidity. C28a HN is the first parainfluenza virus variant found so far to be specifically defective in HNs F-triggering and fusion promotion functions and may contribute to our understanding of transmission of the activating signal from HN to F.

Three discrete activities of the paramyxovirus hemagglutinin-neuraminidase (HN) protein, receptor binding, receptor cleaving (neuraminidase), and triggering of the fusion protein, each affect the promotion of viral fusion and entry. For human parainfluenza virus type 3 (HPIV3), the effects of specific mutations that alter these functions of the receptor-binding protein have been well characterized using cultured monolayer cells, which have identified steps that are potentially relevant to pathogenesis. In the present study, proposed mechanisms that are relevant to pathogenesis were tested in natural host cell cultures, a model of the human airway epithelium (HAE) in which primary HAE cells are cultured at an air-liquid interface and retain functional properties. Infection of HAE cells with wild-type HPIV3 and variant viruses closely reflects that seen in an animal model, the cotton rat, suggesting that HAE cells provide an ideal system for assessing the interplay of host cell and viral factors in pathogenesis and for screening for inhibitory molecules that would be effective in vivo. Both HNs receptor avidity and the function and timing of F activation by HN require a critical balance for the establishment of ongoing infection in the HAE, and these HN functions independently modulate the production of active virions. Alterations in HNs F-triggering function lead to the release of noninfectious viral particles and a failure of the virus to spread. The finding that the dysregulation of F triggering prohibits successful infection in HAE cells suggests that antiviral strategies targeted to HNs F-triggering activity may have promise in vivo.Paramyxoviruses are enveloped viruses that enter cells by fusing directly with the cell membrane. During entry, the viral surface glycoproteins hemagglutinin-neuraminidase (HN) (the receptor-binding molecule) and F (the fusion protein) cooperate in a highly specific way to mediate fusion upon receptor binding. To understand these mechanisms, elucidate how paramyxoviruses enter cells, and develop strategies to prevent or treat infection, we study human parainfluenza virus (HPIV), an important cause of croup and bronchiolitis in children. Our results have uncovered fundamental roles of the receptor-binding protein in paramyxovirus fusion and principles of coordinated interaction between the glycoproteins during the viral life cycle.To understand how the diverse functions of the viral glycoproteins are regulated during the viral life cycle, we have used viruses bearing variant HN molecules with mutations at the binding/F-triggering site (and/or the primary receptor-binding site) to study how this molecule works to trigger F (2,3,7,10,15,18,20). The correct timing of F activation (triggering) by HN is essential for entry. For infection to occur, triggering must occur only when F is in proximity to the target cell membrane, and we propose that the regulation of F triggering is essential for the survival of the virus. The outcome of infection is determined by the target cell's propertie...

Quantitative stereological methods have been adapted for the measurement of the volume of liver attributable to parenchymal, hematopoietic, and Kupffer cells and for the measurement of the relative and absolute number (per unit volume) of these cell types and the mean volume of the parenchymal cell . These morphological parameters are the main ones for interpreting the biochemical differentiation of liver . Quantitative changes in these parameters, in rat liver between the 15th day of gestation and adult life, are presented . Despite the large number of hematopoietic cells, the parenchymal cells fill more than half of the liver volume between the 15th and 18th days of gestation and 0 .85 of the liver volume at term . The fraction of liver volume occupied by Kupffer cells is never more than 0 .02 ; the number of Kupffer cells per cubic centimeter increases less than twofold between fetal and adult life . The mean volume of individual parenchymal cells undergoes a threefold rise during late fetal life, declines in the neonatal period, and doubles between the 12th and 28th postnatal days . With the morphometric data obtained, it is impossible to convert enzyme concentrations (units per gram, determined in homogenates of whole liver) to enzyme amounts per unit volume of parenchymal or hematopoietic tissue or per individual cell of either type . In late fetal liver, only rises in enzyme concentration less than twofold may be attributed to the enrichment of parenchymal tissue at the expense of hematopoietic elements . The sudden upsurge, by more than twofold, of hepatic enzymes of the late fetal cluster (and also of the neonatal and late suckling cluster) reflects rises per parenchymal mass and per parenchymal cell . Thyroxine and glucagon, the administration of which to fetal rats promotes enzyme differentiation in liver, are without appreciable effect on the cytological parameters studied . Hydrocortisone accelerates the involution of hematopoietic tissue in fetal liver . Enzymes that are diminished by prenatal injection of hydrocortisone may be concentrated in hematopoietic cells .