Cleavage of disulfide bonds in endocytosed macromolecules. A processing not associated with lysosomes or endosomes

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.

Disulfide bonds are essential for the stability of the Sindbis virus envelope

Sindbis virus is a membrane-containing virus which has two glycoproteins organized in an icosahedral lattice. Protein-protein associations have been identified which participate in the formation of the icosahedron and these associations are stabilized by intramolecular disulfide bridges (Anthony, R. P., and Brown, D. T., 1990, J. Virol. 65, 1187-1194). The present study further examines the role of disulfides in the structure and function of Sindbis virus by following the effect of dithiothreitol on the protease sensitivity of envelope proteins as well as the electron microscopic appearance and infectivity of Sindbis virus. Treatment of isolated virus with 5 mM dithiothreitol for 6 hr causes a marked increase in trypsin sensitivity of both E1 and E2, profound morphological alterations in the viral envelope, increased susceptibility of the nucleocapsid to RNase, and 95% loss of infectivity. These effects are greatly enhanced and accelerated when treatment with DTT is preceded by a brief exposure of the virus to pH 5.3, suggesting that acid-induced conformational changes render structurally critical disulfides more accessible to reductive cleavage by DTT. When compared to other manipulations known to change the conformation of the viral envelope, such as heating to 51 or 60 degrees or exposure to acid pH, only the exposure to DTT with or without prior acid treatment caused marked structural changes correlated with a loss of infectivity. These data provide electron microscopic and functional evidence that intact disulfide bonds are critical for the stability of the virus envelope and suggest that the cleavage of critical disulfide(s) may play a role in the process of virus infection.

Recent developments in peptide drug delivery to the brain

Peptide-based therapeutics are highly water-soluble compounds that do not readily enter brain from blood owing to poor transport through the brain capillary endothelial wall, i.e., the blood-brain barrier (BBB). Strategies available for peptide drug delivery to brain include: (a) neurosurgical-based (intraventricular drug infusion, hyperosmotic opening of the BBB); (b) pharmacological-based (peptide lipidization, liposomes); and (c) physiological-based (biochemical opening of the BBB, chimeric peptides). Chimeric peptides are formed by the covalent coupling of a pharmaceutical peptide (that is normally not transported through the BBB) to a brain transport vector that undergoes absorptive-mediated or receptor-mediated transcytosis through the BBB. The most efficient brain transport vector known to date is a monoclonal antibody to the transferrin receptor, and this vector achieves a brain volume of distribution approximately 18-fold greater than the plasma space by 5 hr after a single intravenous injection of antibody. The chimeric peptides are formed generally with chemical-based linkers. However, avidin/biotin-based linkers allow for high yield coupling of drug to vector, and for the release of biologically-active peptide following cleavage of the chimeric peptide linker. These strategies may also be used for the delivery of antisense oligonucleotide-based therapeutics to brain. In conclusion, the development of efficacious neuropharmaceuticals in the future will require the development of both drug delivery and drug discovery strategies that operate in parallel.

Polarity in the transcytotic processing of apical and basal membrane-bound peroxidase-polylysine conjugates in MDCK cells

A conjugate of horseradish peroxidase (HRP) to poly(L-lysine) (PLL) was used to characterize a non-lysosomal proteolytic compartment in the MDCK Strain I epithelial cell line. This compartment is expressed in a polar fashion, and is capable of degradation of the PLL moiety in the conjugate followed by release of HRP via a basal-to-apical, but not apical-to-basal, transcytotic pathway. This uptake, cleavage, and transport process appears to require approximately 2 hr, as there is a 2 hr lag-time between conjugate administration to the basal surface and HRP release to the apical medium. Monensin (10 microM) failed to inhibit this process, indicating that participation of the trans-Golgi network (TGN) in the trafficking of internalized conjugate is not the rate-determining step. Inhibition of HRP transport was found to be elicited by 50 micrograms/ml leupeptin, but only when applied to the basal surface. Brief trypsinization of either the basal or apical surfaces of cells preloaded with HRP conjugate showed no appreciable inhibitory effect on the apical release of HRP, indicating that an intracellular compartment rather than surface-bound enzymes is responsible for the degradation of the PLL moiety in the conjugate. Our results demonstrate the presence of an intracellular proteolytic compartment which is accessible in the basal-to-apical, but not apical-to-basal, transport pathway; and this compartment can be exploited for the transcytosis of membrane-bound molecules.

Isolation of variants of Chinese hamster ovary cells with abnormally low levels of GSH: decreased ability to cleave endocytosed disulfide bonds

Mutants of Chinese hamster ovary cells were selected for resistance to a 3 hour exposure to 4 microgram/ml N-methyl-N'-nitro-N-nitrosoguanidine and tested for glutathione (GSH) levels. Six of eight clones that survived the initial treatment had reduced GSH levels ranging from 26 to 61% of wild-type values. These eight cell lines were tested for their susceptibility to a drug conjugate in which methotrexate (MTX) is disulfide-linked to poly(D-lysine) (MTX-SS-PDL) to test their capacity to cleave the endocytosed disulfide bond and release free MTX from this otherwise undegradable drug conjugate. We had shown that wild-type cells were killed by approximately 1 x 10(-7) M MTX given as free drug, as MTX-poly(L-lysine) or as MTX-SS-PDL, but were not affected by MTX-poly(D-lysine). All six lines with abnormally low levels of GSH were resistant to MTX-SS-PDL. The variants with the lowest levels of GSH (MNR-5 and MNR-10) were tested further and showed near-normal sensitivity to MTX and MTX poly(L-lysine). As expected, both lines were hypersensitive to melphalan. They were, however, normally sensitive to diphtheria toxin and ricin, indicating that some cleavage of the interchain disulfides in these protein toxins occurs even when cellular GSH is abnormally low. The lesser GSH requirement for toxin activation may be due to their extraordinary potency.