Local actin polymerization and dynamin recruitment in SV40-induced internalization of caveolae

Contrasting roles of endosomal pH and the cytoskeleton in infection of human glial cells by JC virus and simian virus 40

Infection of eukaryotic cells by pathogens requires the efficient use of host cell endocytic and cytoplasmic transport mechanisms. Understanding how these cellular functions are exploited by microorganisms allows us to better define the basic biology of pathogenesis while providing better insight into normal cellular functions. In this report we compare and contrast intracellular transport and trafficking of the human polyomavirus JC virus (JCV) with that of simian virus 40 (SV40). We have previously shown that infection of human glial cells by JCV requires clathrin-dependent endocytosis. In contrast, infection of cells by SV40 proceeds by caveola-dependent endocytosis. We now examine the roles of endosomal pH and the cellular cytoskeleton during infection of glial cells by both viruses. Our results demonstrate that JCV infection is sensitive to disruption of endosomal pH, whereas SV40 infection is pH independent. Infection by JCV is inhibited by treatment of glial cells with cytochalasin D, nocodazole, and acrylamide, whereas SV40 infection is affected only by nocodazole. These data point to critical differences between JCV and SV40 in terms of endocytosis and intracellular trafficking of their DNA genomes to the nucleus. These data also suggest a unique sequential involvement of cytoskeletal elements during infection of glial cells by JCV.

Escherichia coli K1 internalization via caveolae requires caveolin-1 and protein kinase Calpha interaction in human brain microvascular endothelial cells

The morbidity and mortality associated with Escherichia coli K1 meningitis during the neonatal period have remained significant over the last decade and are once again on the rise. Transcytosis of brain microvascular endothelial cells (BMEC) by E. coli within an endosome to avoid lysosomal fusion is crucial for dissemination into the central nervous system. Central to E. coli internalization of BMEC is the expression of OmpA (outer membrane protein A), which interacts with its receptor for the actin reorganization that leads to invasion. However, nothing is known about the nature of the signaling events for the formation of endosomes containing E. coli K1. We show here that E. coli K1 infection of human BMEC (HBMEC) results in activation of caveolin-1 for bacterial uptake via caveolae. The interaction of caveolin-1 with phosphorylated protein kinase Calpha (PKCalpha) at the E. coli attachment site is critical for the invasion of HBMEC. Optical sectioning of confocal images of infected HBMEC indicates continuing association of caveolin-1 with E. coli during transcytosis. Overexpression of a dominant-negative form of caveolin-1 containing mutations in the scaffolding domain blocked the interaction of phospho-PKCalpha with caveolin-1 and the E. coli invasion of HBMEC, but not actin cytoskeleton rearrangement or the phosphorylation of PKCalpha. The interaction of caveolin-1 with phospho-PKCalpha was completely abrogated in HBMEC overexpressing dominant-negative forms of either focal adhesion kinase or PKCalpha. Treatment of HBMEC with a cell-permeable peptide that represents the scaffolding domain, which was coupled to an antennapedia motif of a Drosophila transcription factor significantly blocked the interaction of caveolin-1 with phospho-PKCalpha and E. coli invasion. These results show that E. coli K1 internalizes HBMEC via caveolae and that the scaffolding domain of caveolin-1 plays a significant role in the formation of endosomes.

ARF6-GTP recruits Nm23-H1 to facilitate dynamin-mediated endocytosis during adherens junctions disassembly

ARF6-regulated endocytosis of E-cadherin is essential during the disassembly of adherens junctions in epithelial cells. Here, we show that activation of ARF6 promotes clathrin-dependent internalization of E-cadherin and caveolae at the basolateral cell surface. Furthermore, we demonstrate that ARF6-GTP, a constitutively activate form of ARF6, interacts with and recruits Nm23-H1, a nucleoside diphosphate (NDP) kinase that provides a source of GTP for dynamin-dependent fission of coated vesicles during endocytosis. Finally, we show that ARF6-mediated recruitment of Nm-23-H1 to cell junctions is accompanied by a decrease in the cellular levels of Rac1-GTP, consistent with previous findings that Nm23-H1 down-regulates activation of Rac1. These studies provide a molecular basis for ARF6 function in polarized epithelia during adherens junction disassembly.

Endocytosis and sorting of glycosphingolipids in sphingolipid storage disease

Recent studies on the endocytic itinerary of glycosphingolipids (GSLs) in sphingolipid storage disease (SLSD) fibroblasts have yielded new insights into the mechanisms underlying the endocytosis and intracellular sorting of lipids in normal and disease cells. Here we highlight new data on clathrin-independent endocytosis of GSLs, the involvement of sphingolipid-cholesterol interactions in perturbation of endocytic trafficking, and potential roles for rab proteins in regulation of GSL transport in SLSDs.

Microbial entry through caveolae: variations on a theme

Caveolae and lipid rafts are increasingly being recognized as a significant portal of entry into host cells for a wide variety of pathogenic microorganisms. Entry through this mechanism appears to afford the microbes protection from degradation in lysosomes, though the level to which each microbe actively participates in avoiding lysosomal fusion may vary. Other possible variations in microbial entry through caveolae or lipid rafts may include (i) the destination of trafficking after entry and (ii) how actively the microbe contributes to the caveolae lipid/raft mediated entry. It seems that, though a wide variety of microorganisms are capable of utilizing caveolae/lipid rafts in various stages of their intracellular lifestyle, there can be distinct differences in how each microbe interacts with these structures. By studying these variations, we may learn more about the normal functioning of these cellular microdomains, and perhaps of more immediate importance, how to incorporate the use of these structures into the treatment of both infectious and non-infectious disease.

Role of cholesterol in human immunodeficiency virus type 1 envelope protein-mediated fusion with host cells

In this study we examined the effects of target membrane cholesterol depletion and cytoskeletal changes on human immunodeficiency virus type 1 (HIV-1) Env-mediated membrane fusion by dye redistribution assays. We found that treatment of peripheral blood lymphocytes (PBL) with methyl-beta-cyclodextrin (MbetaCD) or cytochalasin reduced their susceptibility to membrane fusion with cells expressing HIV-1 Env that utilize CXCR4 or CCR5. However, treatment of human osteosarcoma (HOS) cells expressing high levels of CD4 and coreceptors with these agents did not affect their susceptibility to HIV-1 Env-mediated membrane fusion. Removal of cholesterol inhibited stromal cell-derived factor-1alpha- and macrophage inflammatory protein 1beta-induced chemotaxis of both PBL and HOS cells expressing CD4 and coreceptors. The fusion activity as well as the chemotactic activity of PBL was recovered by adding back cholesterol to these cells. Confocal laser scanning microscopy analysis indicated that treatment of lymphocytes with MbetaCD reduced the colocalization of CD4 or of CXCR4 with actin presumably in microvilli. These findings indicate that, although cholesterol is not required for HIV-1 Env-mediated membrane fusion per se, its depletion from cells with relatively low coreceptor densities reduces the capacity of HIV-1 Env to engage coreceptor clusters required to trigger fusion. Furthermore, our results suggest that coreceptor clustering may occur in microvilli that are supported by actin polymerization.

Using small molecules to study big questions in cellular microbiology

High-throughput screening of small molecules is used extensively in pharmaceutical settings for the purpose of drug discovery. In the case of antimicrobials, this involves the identification of small molecules that are significantly more toxic to the microbe than to the host. Only a small percentage of the small molecules identified in these screens have been studied in sufficient detail to explain the molecular basis of their antimicrobial effect. Rarer still are small molecule screens undertaken with the explicit goal of learning more about the biology of a particular microbe or the mechanism of its interaction with its host. Recent technological advances in small molecule synthesis and high-throughput screening have made such mechanism-directed small molecule approaches a powerful and accessible experimental option. In this article, we provide an overview of the methods and technical requirements and we discuss the potential of small molecule approaches to address important and often otherwise experimentally intractable problems in cellular microbiology.

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.