live-imaging-assay-for-assessing-roles-ca2-sphingomyelinase-repair

Live Imaging Assay for Assessing the Roles of Ca<sup>2+</sup> and Sphingomyelinase in the Repair of Pore-forming Toxin Wounds

Plasma membrane injury is a frequent event, and wounds have to be rapidly repaired to ensure cellular survival. Influx of Ca2+ is a key signaling event ...

University of Maryland 

The yeast V-ATPase belongs to a family of V-type ATPases present in all eucaryotic organisms. In Saccharomyces cerevisiae the V-ATPase is localized to the membrane of the vacuole as well as the Golgi complex and endosomes. The V-ATPase brings about the acidification of these organelles by the transport of protons coupled to the hydrolysis of ATP. In yeast, the V-ATPase is composed of 13 subunits consisting of a catalytic V1 domain of peripherally associated proteins and a proton-translocating V0 domain of integral membrane proteins. The regulatory subunit, Vma13p, was the first V-ATPase subunit to have its crystal structure determined. In addition to proteins forming the functional V-ATPase complex, three ER-localized proteins facilitate the assembly of the V0 subunits following their translation and insertion into the membrane of the ER. Homologues of the Vma21p assembly factor have been identified in many higher eukaryotes supporting a ubiquitous assembly pathway for this important enzyme complex.

Structure and assembly of the yeast V-ATPase.

The vacuolar ATPase (V-ATPase) is a multisubunit enzyme that acidifies intracellular organelles in eukaryotes. Similar to the F-type ATP synthase (FATPase), the V-ATPase is composed of two subcomplexes, V(1) and V(0). Hydrolysis of ATP in the V(1) subcomplex is tightly coupled to proton translocation accomplished by the V(0) subcomplex, which is composed of five unique subunits (a, d, c, c', and c"). Three of the subunits, subunit c (Vma3p), c' (Vma11p), and c" (Vma16p), are small highly hydrophobic integral membrane proteins called "proteolipids" that share sequence similarity to the F-ATPase subunit c. Whereas subunit c from the F-ATPase spans the membrane bilayer twice, the V-ATPase proteolipids have been modeled to have at least four transmembrane-spanning helices. Limited proteolysis experiments with epitope-tagged copies of the proteolipids have revealed that the N and the C termini of c (Vma3p) and c' (Vma11p) were in the lumen of the vacuole. Limited proteolysis of epitope-tagged c" (Vma16p) indicated that the N terminus is located on the cytoplasmic face of the vacuole, whereas the C terminus is located within the vacuole. Furthermore, a chimeric fusion between Vma16p and Vma3p, Vma16-Vma3p, was found to assemble into a fully functional V-ATPase complex, further supporting the conclusion that the C terminus of Vma16p resides within the lumen of the vacuole. These results indicate that subunits c and c' have four transmembrane segments with their N and C termini in the lumen and that c" has five transmembrane segments, with the N terminus exposed to the cytosol and the C terminus lumenal.

Topological characterization of the c, c', and c" subunits of the vacuolar ATPase from the yeast Saccharomyces cerevisiae.

Previous two-hybrid analysis of the 17 soluble class E Vps yeast proteins revealed that Vps46p/Did2p interacts with Vta1p and the AAA (ATPase associated with a variety of cellular activities) ATPase Vps4p. Here we report that the binding of Vps46p to Vps4p and Vta1p is direct and not mediated by additional proteins, and the binding of Vps46p to Vps4p is ATP independent. Vps46p regulates the membrane association of Vps4p and is required for the interaction of Vta1p with Vps32p/Snf7p of the ESCRT-III complex. Vta1p is a potent activator of Vps4p, stimulating the ATPase activity by 6- to 8-fold. These results reveal functional roles for the Vps46p and Vta1p proteins in regulating the ESCRT complex assembly/disassembly cycle in protein sorting at the yeast late endosome.

Vta1p and Vps46p regulate the membrane association and ATPase activity of Vps4p at the yeast multivesicular body.

Deletion of the yeast gene PKR1 (YMR123W) results in an inability to grow on iron-limited medium. Pkr1p is localized to the membrane of the endoplasmic reticulum. Cells lacking Pkr1p show reduced levels of the V-ATPase subunit Vph1p due to increased turnover of the protein in mutant cells. Reduced levels of the V-ATPase lead to defective copper loading of Fet3p, a component of the high affinity iron transport system. Levels of Vph1p in cells lacking Pkr1p are similar to cells unable to assemble a functional V-ATPase due to lack of a V0 subunit or an endoplasmic reticulum (ER) assembly factor. However, unlike yeast mutants lacking a V0 subunit or a V-ATPase assembly factor, low levels of Vph1p present in cells lacking Pkr1p are assembled into a V-ATPase complex, which exits the ER and is present on the vacuolar membrane. The V-ATPase assembled in the absence of Pkr1p is fully functional because the mutant cells are able to weakly acidify their vacuoles. Finally, overexpression of the V-ATPase assembly factor Vma21p suppresses the growth and acidification defects of pkr1Delta cells. Our data indicate that Pkr1p functions together with the other V-ATPase assembly factors in the ER to efficiently assemble the V-ATPase membrane sector.

PKR1 encodes an assembly factor for the yeast V-type ATPase.

The yeast Saccharomyces cerevisiae vacuolar H(+)-ATPase (V-ATPase) is a multisubunit complex responsible for acidifying intracellular organelles and is highly regulated. One of the regulatory subunits, subunit H, is encoded by the VMA13 gene in yeast and is composed of two domains, the N-terminal domain (amino acids (aa) 1-352) and the C-terminal domain (aa 353-478). The N-terminal domain is required for the activation of the complex, whereas the C-terminal domain is required for coupling ATP hydrolysis to proton translocation (Liu, M., Tarsio, M., Charsky, C. M., and Kane, P. M. (2005) J. Biol. Chem. 280, 36978-36985). Experiments with epitope-tagged copies of Vma13p revealed that there is only one copy of Vma13p/subunit H per V-ATPase complex. Analysis of the N-terminal domain shows that the first 179 amino acids are not required for the activation and full function of the V-ATPase complex and that the minimal region of Vma13p/subunit H capable of activating the V-ATPase is aa 180-353 of the N-terminal domain. Subunit H is expressed as two splice variants in mammals, and deletion of 18 amino acids in yeast Vma13p corresponding to the mammalian subunit H beta isoform results in reduced V-ATPase activity and significantly lower coupling of ATPase hydrolysis to proton translocation. Intriguingly, the yeast Vma13p mimicking the mammalian subunit H beta isoform is functionally equivalent to Vma13p lacking the entire C-terminal domain. These results suggest that the mammalian V-ATPase complexes with subunit H splice variant SFD-alpha or SFD-beta are likely to have different activities and may perform distinct cellular functions.

Functional characterization of the N-terminal domain of subunit H (Vma13p) of the yeast vacuolar ATPase.

Syt VII is a Ca(2+) sensor that regulates lysosome exocytosis and plasma membrane repair. Because it lacks motifs that mediate lysosomal targeting, it is unclear how Syt VII traffics to these organelles. In this paper, we show that mutations or inhibitors that abolish palmitoylation disrupt Syt VII targeting to lysosomes, causing its retention in the Golgi complex. In macrophages, Syt VII is translocated simultaneously with the lysosomal tetraspanin CD63 from tubular lysosomes to nascent phagosomes in a Ca(2+)-dependent process that facilitates particle uptake. Mutations in Syt VII palmitoylation sites block trafficking of Syt VII, but not CD63, to lysosomes and phagosomes, whereas tyrosine replacement in the lysosomal targeting motif of CD63 causes both proteins to accumulate on the plasma membrane. Complexes of CD63 and Syt VII are detected only when Syt VII palmitoylation sites are intact. These findings identify palmitoylation-dependent association with the tetraspanin CD63 as the mechanism by which Syt VII is targeted to lysosomes.

Palmitoylation-dependent association with CD63 targets the Ca2+ sensor synaptotagmin VII to lysosomes.

Upon host cell contact, the protozoan parasite Trypanosoma cruzi triggers cytosolic Ca(2+) transients that induce exocytosis of lysosomes, a process required for cell invasion. However, the exact mechanism by which lysosomal exocytosis mediates T. cruzi internalization remains unclear. We show that host cell entry by T. cruzi mimics a process of plasma membrane injury and repair that involves Ca(2+)-dependent exocytosis of lysosomes, delivery of acid sphingomyelinase (ASM) to the outer leaflet of the plasma membrane, and a rapid form of endocytosis that internalizes membrane lesions. Host cells incubated with T. cruzi trypomastigotes are transiently wounded, show increased levels of endocytosis, and become more susceptible to infection when injured with pore-forming toxins. Inhibition or depletion of lysosomal ASM, which blocks plasma membrane repair, markedly reduces the susceptibility of host cells to T. cruzi invasion. Notably, extracellular addition of sphingomyelinase stimulates host cell endocytosis, enhances T. cruzi invasion, and restores normal invasion levels in ASM-depleted cells. Ceramide, the product of sphingomyelin hydrolysis, is detected in newly formed parasitophorous vacuoles containing trypomastigotes but not in the few parasite-containing vacuoles formed in ASM-depleted cells. Thus, T. cruzi subverts the ASM-dependent ceramide-enriched endosomes that function in plasma membrane repair to infect host cells.

Trypanosoma cruzi subverts the sphingomyelinase-mediated plasma membrane repair pathway for cell invasion.

The protozoan parasite Leishmania is the causative agent of serious human infections worldwide. The parasites alternate between insect and vertebrate hosts and cause disease by invading macrophages, where they replicate. Parasites lacking the ferrous iron transporter LIT1 cannot grow intracellularly, indicating that a plasma membrane-associated mechanism for iron uptake is essential for the establishment of infections. Here, we identify and functionally characterize a second member of the Leishmania iron acquisition pathway, the ferric iron reductase LFR1. The LFR1 gene is up-regulated under iron deprivation and accounts for all the detectable ferric reductase activity exposed on the surface of Leishmania amazonensis. LFR1 null mutants grow normally as promastigote insect stages but are defective in differentiation into the vertebrate infective forms, metacyclic promastigotes and amastigotes. LFR1 overexpression partially restores the abnormal morphology of infective stages but markedly reduces parasite viability, precluding its ability to rescue LFR1 null replication in macrophages. However, LFR1 overexpression is not toxic for amastigotes lacking the ferrous iron transporter LIT1 and rescues their growth defect. In addition, the intracellular growth of both LFR1 and LIT1 null parasites is rescued in macrophages loaded with exogenous iron. This indicates that the Fe(3+) reductase LFR1 functions upstream of LIT1 and suggests that LFR1 overexpression results in excessive Fe(2+) production, which impairs parasite viability after intracellular transport by LIT1.

LFR1 ferric iron reductase of Leishmania amazonensis is essential for the generation of infective parasite forms.

The protozoan parasite Trypanosoma cruzi, the aetiological agent of Chagas' disease, has two infective life cycle stages, trypomastigotes and amastigotes. While trypomastigotes actively enter mammalian cells, highly infective extracellular amastigotes (type Iâ€…T.â€‰cruzi) rely on actin-mediated uptake, which is generally inefficient in non-professional phagocytes. We found that extracellular amastigotes (EAs) of T.â€‰cruziâ€…G strain (type I), but not Y strain (type II), were taken up 100-fold more efficiently than inert particles. Mammalian cell lines showed levels of parasite uptake comparable to macrophages, and extensive actin recruitment and polymerization was observed at the site of entry. EA uptake was not dependent on parasite-secreted molecules and required the same molecular machinery utilized by professional phagocytes during large particle phagocytosis. Transcriptional silencing of synaptotagmin VII and CD63 significantly inhibited EA internalization, demonstrating that delivery of supplemental lysosomal membrane to form the phagosome is involved in parasite uptake. Importantly, time-lapse live imaging using fluorescent reporters revealed phagosome-associated modulation of phosphoinositide metabolism during EA uptake that closely resembles what occurs during phagocytosis by macrophages. Collectively, our results demonstrate that T.â€‰cruziâ€…EAs are potent inducers of phagocytosis in non-professional phagocytes, a process that may facilitate parasite persistence in infected hosts.

Extracellular amastigotes of Trypanosoma cruzi are potent inducers of phagocytosis in mammalian cells.

The protozoan parasite Leishmania amazonensis is a heme auxotroph, and must acquire this essential factor from the environment. Previous studies showed that L. amazonensis incorporates heme through the transmembrane protein LHR1 (Leishmania Heme Response-1). LHR1 null promastigotes were not viable, suggesting that this transporter is essential for survival. Here we compared the growth, differentiation, and infectivity for macrophages and mice of wild type, LHR1-single knockout (LHR1/Î”lhr1) and LHR1-complemented (LHR1/Î”lhr1 + LHR1) L. amazonensis strains. LHR1/Î”lhr1 promastigotes replicated poorly in heme-deficient media and had lower intracellular heme content when compared to wild type parasites. LHR1/Î”lhr1 promastigotes were also less effective in reducing ferric to ferrous iron, a reaction mediated by the heme-containing parasite enzyme LFR1 (Leishmania Ferric Reductase 1). LHR1/Î”lhr1 parasites differentiated normally into aflagellated forms expressing amastigote-specific markers, but were not able to replicate intracellularly after infecting macrophages. Importantly, the intracellular growth of LHR1/Î”lhr1 amastigotes was fully restored when macrophages were allowed to phagocytose red blood cells prior to infection. LHR1/Î”lhr1 parasites were also severely defective in the development of cutaneous lesions in mice. All phenotypes observed in LHR1/Î”lhr1 L. amazonensis were rescued by expression of episomal LHR1. Our results reveal the importance of efficient heme uptake for L. amazonensis replication and vertebrate host infectivity, reinforcing the potential usefulness of LHR1 as a target for new anti-leishmanial drugs.

Heme uptake mediated by LHR1 is essential for Leishmania amazonensis virulence.

Iron is essential for many metabolic pathways, but is toxic in excess. Recent identification of the ferric iron reductase LFR1, the ferrous iron transporter LIT1, and the heme transporter LHR1 greatly advanced our understanding of how Leishmania parasites acquire iron and regulate its uptake. LFR1 and LIT1 have close orthologs in plants, and are required for Leishmania virulence. Consistent with the lack of heme biosynthesis in trypanosomatids, LHR1 and LABCG5, a protein involved in heme salvage from hemoglobin, seem essential for Leishmania survival. LFR1, LIT1 and LHR1 are upregulated under low iron availability, in agreement with the need to prevent excessive iron uptake. Future studies should clarify how Leishmania interacts with the iron homeostasis machinery of its host cell, the macrophage.

Article	Year
Structure and assembly of the yeast V-ATPase. Journal of bioenergetics and biomembranes\| PubMed ID: 14635776	2003
Topological characterization of the c, c', and c" subunits of the vacuolar ATPase from the yeast Saccharomyces cerevisiae. The Journal of biological chemistry\| PubMed ID: 15252052	2004
Vta1p and Vps46p regulate the membrane association and ATPase activity of Vps4p at the yeast multivesicular body. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 16601096	2006
PKR1 encodes an assembly factor for the yeast V-type ATPase. The Journal of biological chemistry\| PubMed ID: 16926153	2006
Functional characterization of the N-terminal domain of subunit H (Vma13p) of the yeast vacuolar ATPase. The Journal of biological chemistry\| PubMed ID: 18708638	2008
Palmitoylation-dependent association with CD63 targets the Ca2+ sensor synaptotagmin VII to lysosomes. The Journal of cell biology\| PubMed ID: 21041449	2010
Trypanosoma cruzi subverts the sphingomyelinase-mediated plasma membrane repair pathway for cell invasion. The Journal of experimental medicine\| PubMed ID: 21536739	2011
LFR1 ferric iron reductase of Leishmania amazonensis is essential for the generation of infective parasite forms. The Journal of biological chemistry\| PubMed ID: 21558274	2011
Extracellular amastigotes of Trypanosoma cruzi are potent inducers of phagocytosis in mammalian cells. Cellular microbiology\| PubMed ID: 23241026	2013
Heme uptake mediated by LHR1 is essential for Leishmania amazonensis virulence. Infection and immunity\| PubMed ID: 23876801	2013
Pathways of iron acquisition and utilization in Leishmania. Current opinion in microbiology\| PubMed ID: 23962817	2013

Andrew R. Flannery

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