Cdc42-dependent structural development of auditory supporting cells is required for wound healing at adulthood

Cdc42 regulates the initial establishment of cytoskeletal and junctional structures, but only little is known about its role at later stages of cellular differentiation. We studied Cdc42′s role in vivo in auditory supporting cells, epithelial cells with high structural complexity. Cdc42 inactivation was induced early postnatally using the Cdc42^loxP/loxP;Fgfr3-iCre-ER^T2 mice. Cdc42 depletion impaired elongation of adherens junctions and F-actin belts, leading to constriction of the sensory epithelial surface. Fragmented F-actin belts, junctions containing ectopic lumens and misexpression of a basolateral membrane protein in the apical domain were observed. These defects and changes in aPKCλ/ι expression suggested that apical polarization is impaired. Following a lesion at adulthood, supporting cells with Cdc42 loss-induced maturational defects collapsed and failed to remodel F-actin belts, a process that is critical to scar formation. Thus, Cdc42 is required for structural differentiation of auditory supporting cells and this proper maturation is necessary for wound healing in adults.

Abnormal cerebellar development and ataxia in CARP VIII morphant zebrafish

Congenital ataxia and mental retardation are mainly caused by variations in the genes that affect brain development. Recent reports have shown that mutations in the CA8 gene are associated with mental retardation and ataxia in humans and ataxia in mice. The gene product, carbonic anhydrase-related protein VIII (CARP VIII), is predominantly present in cerebellar Purkinje cells, where it interacts with the inositol 1,4,5-trisphosphate receptor type 1, a calcium channel. In this study, we investigated the effects of the loss of function of CARP VIII during embryonic development in zebrafish using antisense morpholino oligonucleotides against the CA8 gene. Knockdown of CA8 in zebrafish larvae resulted in a curved body axis, pericardial edema and abnormal movement patterns. Histologic examination revealed gross morphologic defects in the cerebellar region and in the muscle. Electron microscopy studies showed increased neuronal cell death in developing larvae injected with CA8 antisense morpholinos. These data suggest a pivotal role for CARP VIII during embryonic development. Furthermore, suppression of CA8 expression leads to defects in motor and coordination functions, mimicking the ataxic human phenotype. This work reveals an evolutionarily conserved function of CARP VIII in brain development and introduces a novel zebrafish model in which to investigate the mechanisms of CARP VIII-related ataxia and mental retardation in humans.

Progressive sheet-to-tubule transformation is a general mechanism for endoplasmic reticulum partitioning in dividing mammalian cells

During mitosis, ER network reorganization can lead to packing of the ER into tight concentric layers at the cell cortex and occurs in tandem with rounding of the cell. Morphometric and 3D EM analysis shows that in addition to reorganization, ER sheets undergo transformation toward more fenestrated and tubular forms before anaphase in mammalian cells.

The endoplasmic reticulum (ER) is both structurally and functionally complex, consisting of a dynamic network of interconnected sheets and tubules. To achieve a more comprehensive view of ER organization in interphase and mitotic cells and to address a discrepancy in the field (i.e., whether ER sheets persist, or are transformed to tubules, during mitosis), we analyzed the ER in four different mammalian cell lines using live-cell imaging, high-resolution electron microscopy, and three dimensional electron microscopy. In interphase cells, we found great variation in network organization and sheet structures among different cell lines. In mitotic cells, we show that the ER undergoes both spatial reorganization and structural transformation of sheets toward more fenestrated and tubular forms. However, the extent of spatial reorganization and sheet-to-tubule transformation varies among cell lines. Fenestration and tubulation of the ER correlates with a reduced number of membrane-bound ribosomes.

Callose biosynthesis regulates symplastic trafficking during root development

Plant cells are connected through plasmodesmata (PD), membrane-lined channels that allow symplastic movement of molecules between cells. However, little is known about the role of PD-mediated signaling during plant morphogenesis. Here, we describe an Arabidopsis gene, CALS3/GSL12. Gain-of-function mutations in CALS3 result in increased accumulation of callose (β-1,3-glucan) at the PD, a decrease in PD aperture, defects in root development, and reduced intercellular trafficking. Enhancement of CALS3 expression during phloem development suppressed loss-of-function mutations in the phloem abundant callose synthase, CALS7 indicating that CALS3 is a bona fide callose synthase. CALS3 alleles allowed us to spatially and temporally control the PD aperture between plant tissues. Using this tool, we are able to show that movement of the transcription factor SHORT-ROOT and microRNA165 between the stele and the endodermis is PD dependent. Taken together, we conclude that regulated callose biosynthesis at PD is essential for cell signaling.

Endosomal actin remodeling by coronin-1A controls lipoprotein uptake and degradation in macrophages

RATIONALE

The actin cytoskeleton has been implicated in the processing of atherogenic lipoproteins in macrophages. However, the functional role of actin and the regulatory proteins involved are unknown.

OBJECTIVE

Coronin-1A (Coro1A) was identified as a differentially expressed transcript in wild-type versus Niemann-Pick type C1 deficient macrophages exposed to acetylated low-density lipoproteins (AcLDL). We investigated whether Coro1A plays a role in the uptake or processing of modified lipoproteins in macrophages and if this is related to its actin regulatory functions.

METHODS AND RESULTS

In wild-type primary macrophages, filamentous actin transiently decorated AcLDL containing endosomes that also recruited Coro1A. This dynamic association of F-actin with endosomes was disturbed in Coro1A deficient macrophages. In Coro1A knockout macrophages the uptake of AcLDL was increased, rate of AcLDL delivery to lysosomes enhanced, and lipoprotein-derived cholesteryl ester hydrolysis accelerated. Overexpression of wild-type Coro1A normalized AcLDL uptake in Coro1A knockout macrophages while a Coro1A actin binding mutant did not. Furthermore, the effects of macrophage Coro1A silencing on endosomal actin association and AcLDL delivery to lysosomes resembled those of cofilin silencing.

CONCLUSIONS

Coro1A controls actin association with endocytic organelles, thereby negatively regulating endo-lysosomal delivery, degradation of modified lipoproteins and cholesterol deposition in macrophages.

Callose biosynthesis regulates symplastic trafficking during root development

Plant cells are connected through plasmodesmata (PD), membrane-lined channels that allow symplastic movement of molecules between cells. However, little is known about the role of PD-mediated signaling during plant morphogenesis. Here, we describe an Arabidopsis gene, CALS3/GSL12. Gain-of-function mutations in CALS3 result in increased accumulation of callose (β-1,3-glucan) at the PD, a decrease in PD aperture, defects in root development, and reduced intercellular trafficking. Enhancement of CALS3 expression during phloem development suppressed loss-of-function mutations in the phloem abundant callose synthase, CALS7 indicating that CALS3 is a bona fide callose synthase. CALS3 alleles allowed us to spatially and temporally control the PD aperture between plant tissues. Using this tool, we are able to show that movement of the transcription factor SHORT-ROOT and microRNA165 between the stele and the endodermis is PD dependent. Taken together, we conclude that regulated callose biosynthesis at PD is essential for cell signaling.

Pinkbar is an epithelial-specific BAR domain protein that generates planar membrane structures

Bin/amphipysin/Rvs (BAR)-domain proteins sculpt cellular membranes and have key roles in processes such as endocytosis, cell motility and morphogenesis. BAR domains are divided into three subfamilies: BAR- and F-BAR-domain proteins generate positive membrane curvature and stabilize cellular invaginations, whereas I-BAR-domain proteins induce negative curvature and stabilize protrusions. We show that a previously uncharacterized member of the I-BAR subfamily, Pinkbar, is specifically expressed in intestinal epithelial cells, where it localizes to Rab13-positive vesicles and to the plasma membrane at intercellular junctions. Notably, the BAR domain of Pinkbar does not induce membrane tubulation but promotes the formation of planar membrane sheets. Structural and mutagenesis analyses reveal that the BAR domain of Pinkbar has a relatively flat lipid-binding interface and that it assembles into sheet-like oligomers in crystals and in solution, which may explain its unique membrane-deforming activity.

Insight into cell-entry mechanisms of CPPs by electron microscopy

Despite the quickly widening application of cell-penetrating peptides (CPP) for the cellular delivery of various macromolecules, the cell entry mechanisms of these peptides have remained elusive so far. The basic features of the translocation of CPPs into cells have been mapped by fluorescence microscopy and activity-based assays revealing that endocytotic mechanisms are mainly responsible for the uptake at physiological temperature. However, the high concentration of CPP or the lowering of the incubation temperature below 10°C (re)activates a nonvesicular cell entry mode. The fluorescence microscopy can hardly provide detailed information about the interaction of CPP molecules with the extracellular structures, the induced changes in the morphology of the plasma membrane, etc. Therefore, application of electron microscopy could help to shed light on the nature of nonvesicular uptake mechanism. Transmission electron microscopy (TEM) has been a valuable tool for the morphological characterization of biological material at high resolution. It can provide useful information at the ultrastructural level about the interaction and arrangement of CPPs on the cell surface, the entrapment in cellular organelles and the translocation to the cytoplasm. In this chapter, we present a method for the tagging of CPPs covalently with a 1.4 nm gold cluster and provide a flat-embedding protocol for the mapping of Nanogold™-labeled CPPs in cultured cells by TEM. This method enables to retain the cell monolayers in their in situ orientation. The Nanogold™ tag is putatively not interfering with the uptake of CPPs and enables the production of specimens with excellent morphology and good contrast.

Protein delivery with transportans is mediated by caveolae rather than flotillin-dependent pathways

Delivery of large bioactive cargoes into cells with the help of cell-penetrating peptides (CPPs) is mostly based on endocytic processes. Here we map the cellular pathways used by transportan and transportan 10 (TP10) for protein transduction in HeLa cells. CPP-mediated cellular delivery is often suggested to be lipid-raft-dependent; therefore, we used flotillin-1, caveolin, Rab5, and PI3P as markers to elucidate the involvement of these particular endosomal pathways in the protein uptake process. Confocal laser scanning and electron microscopy reveal only a negligible overlap of avidin/neutravidin conveyed into cells by transportans with the raft marker flotillin-1 or early endosomal markers Rab5 and PI3P. However, about 20% of protein-CPP complexes colocalize with the caveolar/caveosomal marker caveolin, and down-regulation of caveolin-1 by siRNA treatment leads to the inhibition of the CPP-mediated protein uptake by 30-50%. On the contrary, the lack of flotillin-1 increases rather than decreases the CPP-mediated protein transport. The participation of the caveolin-1-dependent pathway in CPP-mediated protein delivery was also corroborated by using caveolin-1 knockout mouse embryonic fibroblasts.

3D tomography reveals connections between the phagophore and endoplasmic reticulum

Autophagosomes have been reported to form in the vicinity of the endoplasmic reticulum (ER). In many cases, the phagophore membrane is observed between two cisternae of rough ER, but it is not known whether these two membranes are directly connected. To investigate the relationship of the phagophore membrane and the ER, we used electron microscopic tomography of serum and amino acid starved normal rat kidney cells. The cells were fixed in glutaraldehyde and reduced osmium tetroxide and embedded in Epon. Dual axis tilt image series were acquired from two successive 250-nm sections. To analyze the three-dimensional (3D) morphology of phagophores and the associated rough ER, 3D tomograms were used to model the ER and phagophore membranes. The tomographic reconstructions revealed connections between the phagophore/autophagosome membrane and the closely located ER cisternae, especially with the ER located inside the autophagosome. The connections were typically formed by narrow extensions from the phagophore/autophagosome to the ER. This finding has potential implications on the origin of autophagosome membranes, and on the mechanism of phagophore membrane extension. In addition, we observed lipid droplets in very close contact with the phagophores/autophagosomes.

Monitoring autophagy by electron microscopy in Mammalian cells

Electron microscopy remains one of the most accurate methods for the detection of autophagy and quantification of autophagic accumulation. Compared to fluorescence microscopy, the resolution of transmission electron microscopy is superior. In this chapter we describe the fine structure of early and late autophagic compartments in mammalian cells. Instructions are given for the preparation of samples for conventional electron microscopy using three different protocols suitable for cultured cells and animal tissues. We also introduce tomography as a tool to study the three-dimensional morphology of autophagic organelles and show the morphology of a phagophore as an example. Finally, we describe a protocol for the quantification of autophagic compartments by electron microscopy and point counting.

Expression of X-chromosome linked inhibitor of apoptosis protein in mature Purkinje cells and in retinal bipolar cells in transgenic mice induces neurodegeneration

Transgenic mice with overexpression of the caspase-inhibitor, X-chromosome-linked inhibitor of apoptosis protein (XIAP) in Purkinje cell (PC) and in retinal bipolar cells (RBCs) were produced to study the regulation of cell death. Unexpectedly, an increased neurodegeneration was observed in the PCs in these L7-XIAP mice after the third postnatal week with the mice exhibiting severe ataxia. The loss of PCs was independent of Bax as shown by crossing the L7-XIAP mice with Bax gene-deleted mice. Electron microscopy revealed intact organelles in PCs but with the stacking of ER cisterns indicative of cell stress. Immunostaining for cell death proteins showed an increased phosphorylation of c-Jun in the PCs, suggesting an involvement in cell degeneration. Apart from PCs, the number of RBCs was decreased in adult retina in line with the expression pattern for the L7 promoter. The data show that overexpression of the anti-apoptotic protein XIAP in vulnerable neurons leads to enhanced cell death. The mechanisms underlying this neurodegeneration can be related to the effects of XIAP on cell stress and altered cell signaling.

Reactive oxygen species induce signals that lead to apoptotic DNA degradation in primary CD4+ T cells

Reactive oxygen species are toxic to cells but they may also have active roles in transducing apoptotic events. To study the role of reactive oxygen species in growth factor depletion induced apoptosis of human primary CD4+ T cells, we used a synthetic manganese porphyrin superoxide dismutase mimetic to detoxify superoxide anions formed during apoptosis. Apoptosis of primary CD4+ T cells was characterized by generation of superoxide anions, plasma membrane phosphatidyl-serine translocation, loss of mitochondrial membrane potential, activation of caspase 3, condensation of chromatin, as well as DNA degradation. The detoxification of superoxide anions did not influence plasma membrane phosphatidyl-serine translocation, or chromatin condensation, and only marginally inhibited the loss of mitochondrial membrane potential and the formation of DNA strand breaks. In contrast, the detoxification of superoxide anions significantly reduced caspase 3 activity and almost completely inhibited the apoptotic decrease in total cellular DNA content as measured by propidium iodide staining. Our results indicate that reactive oxygen anions induce signals leading to efficient DNA degradation after the initial formation of DNA strand breaks. Thus, reactive oxygen anions have active roles in signaling that lead to the apoptotic events.

Cholesterol substitution increases the structural heterogeneity of caveolae

Caveolin-1 binds cholesterol and caveola formation involves caveolin-1 oligomerization and cholesterol association. The role of cholesterol in caveolae has so far been addressed by methods that compromise membrane integrity and abolish caveolar invaginations. To study the importance of sterol specificity for the structure and function of caveolae, we replaced cholesterol in mammalian cells with its immediate precursor desmosterol by inhibiting 24-dehydrocholesterol reductase. Desmosterol could substitute for cholesterol in maintaining cell growth, membrane integrity, and preserving caveolar invaginations. However, in desmosterol cells the affinity of caveolin-1 for sterol and the stability of caveolin oligomers were decreased. Moreover, caveolar invaginations became more heterogeneous in dimensions and in the number of caveolin-1 molecules per caveola. Despite the altered caveolar structure, caveolar ligand uptake was only moderately inhibited. We found that in desmosterol cells, Src kinase phosphorylated Cav1 at Tyr(14) more avidly than in cholesterol cells. Taken the role of Cav1 Tyr(14) phosphorylation in caveolar endocytosis, this may help to preserve caveolar uptake in desmosterol cells. We conclude that a sterol C24 double bond interferes with caveolin-sterol interaction and perturbs caveolar morphology but facilitates Cav1 Src phosphorylation and allows caveolar endocytosis. More generally, substitution of cholesterol by a structurally closely related sterol provides a method to selectively modify membrane protein-sterol affinity, structure and function of cholesterol-dependent domains without compromising membrane integrity.

Endoplasmic reticulum remains continuous and undergoes sheet-to-tubule transformation during cell division in mammalian cells

The endoplasmic reticulum (ER) is a multifaceted cellular organelle both structurally and functionally, and its cell cycle–dependent morphological changes are poorly understood. Our quantitative confocal and EM analyses show that the ER undergoes dramatic reorganization during cell division in cultured mammalian cells as mitotic ER profiles become shorter and more branched. 3D modeling by electron tomography reveals that the abundant interphase structures, sheets, are lost and subsequently transform into a branched tubular network that remains continuous. This is confirmed by observing the most prominent ER subdomain, the nuclear envelope (NE). A NE marker protein spreads to the mitotic ER tubules, although it does not show a homogenous distribution within the network. We mimicked the mitotic ER reorganization using puromycin to strip the membrane-bound ribosomes from the interphase ER corresponding to the observed loss of ribosomes normally occurring during mitosis. We propose that the structural changes in mitotic ER are linked to ribosomal action on the ER membranes.

Prostatic Acid Phosphatase Is Not a Prostate Specific Target

Prostatic acid phosphatase (PAP) is currently evaluated as a target for vaccine immunotherapy of prostate cancer. This is based on the previous knowledge about secretory PAP and its high prostatic expression. We describe a novel PAP spliced variant mRNA encoding a type I transmembrane (TM) protein with the extracellular NH(2)-terminal phosphatase activity and the COOH-terminal lysosomal targeting signal (YxxPhi). TM-PAP is widely expressed in nonprostatic tissues like brain, kidney, liver, lung, muscle, placenta, salivary gland, spleen, thyroid, and thymus. TM-PAP is also expressed in fibroblast, Schwann, and LNCaP cells, but not in PC-3 cells. In well-differentiated human prostate cancer tissue specimens, the expression of secretory PAP, but not TM-PAP, is significantly decreased. TM-PAP is localized in the plasma membrane-endosomal-lysosomal pathway and is colocalized with the lipid raft marker flotillin-1. No cytosolic PAP is detected. We conclude that the wide expression of TM-PAP in, for instance, neuronal and muscle tissues must be taken into account in the design of PAP-based immunotherapy approaches.

Characterization and subcellular localization of human neutral class IIα-mannosidase cytosolic enzymes/free oligosaccharides/glycosidehydrolase family 38/M2C1/N-glycosylation

A glycosyl hydrolase family 38 enzyme, neutral alpha-mannosidase, has been proposed to be involved in hydrolysis of cytosolic free oligosaccharides originating either from ER-misfolded glycoproteins or the N-glycosylation process. Although this enzyme has been isolated from the cytosol, it has also been linked to the ER by subcellular fractionations. We have studied the subcellular localization of neutral alpha-mannosidase by immunofluorescence microscopy and characterized the human recombinant enzyme with natural substrates to elucidate the biological function of this enzyme. Immunofluorescence microscopy showed neutral alpha-mannosidase to be absent from the ER, lysosomes, and autophagosomes, and being granularly distributed in the cytosol. In experiments with fluorescent recovery after photo bleaching, neutral alpha-mannosidase had slower than expected two-phased diffusion in the cytosol. This result together with the granular appearance in immunostaining suggests that portion of the neutral alpha-mannosidase pool is somehow complexed. The purified recombinant enzyme is a tetramer and has a neutral pH optimum for activity. It hydrolyzed Man(9)GlcNAc to Man(5)GlcNAc in the presence of Fe(2+), Co(2+), and Mn(2+), and uniquely to neutral alpha-mannosidases from other organisms, the human enzyme was more activated by Fe(2+) than Co(2+). Without activating cations the main reaction product was Man(8)GlcNAc, and Cu(2+) completely inhibited neutral alpha-mannosidase. Our findings from enzyme-substrate characterizations and subcellular localization studies support the suggested role for neutral alpha-mannosidase in hydrolysis of soluble cytosolic oligomannosides.

Intracellular localization and effects of individually expressed human parechovirus 1 non-structural proteins

Human parechovirus 1 (HPEV-1) has many unique features compared with other picornaviruses and it has been shown that the replication complex formed during HPEV-1 infection is different from that of other picornaviruses. Here, the intracellular localization and functional effects of individually expressed HPEV-1 non-structural proteins were studied. The 2A and 3D proteins were found diffusely in the cytoplasm and nucleus of the cell. The 3A and 3AB proteins were observed to co-localize with the markers for the Golgi apparatus, whereas 2B co-localized with markers for the endoplasmic reticulum and the 2C and 2BC proteins were observed mainly on the surface of lipid droplets. The 2C protein, which has been implicated in replication-complex formation in enterovirus-infected cells, was not able to induce vesicles similar to those seen in HPEV-1-infected cells when expressed individually. However, in superinfected cells, the fusion protein was able to relocate to the virus replication complexes. Similar to other picornaviruses, HPEV-1 was found to interfere with cellular secretion, but this function could not be ascribed to any of the individually expressed non-structural proteins.

Missing-in-metastasis and IRSp53 deform PI(4,5)P2-rich membranes by an inverse BAR domain-like mechanism

The actin cytoskeleton plays a fundamental role in various motile and morphogenetic processes involving membrane dynamics. We show that actin-binding proteins MIM (missing-in-metastasis) and IRSp53 directly bind PI(4,5)P(2)-rich membranes and deform them into tubular structures. This activity resides in the N-terminal IRSp53/MIM domain (IMD) of these proteins, which is structurally related to membrane-tubulating BAR (Bin/amphiphysin/Rvs) domains. We found that because of a difference in the geometry of the PI(4,5)P(2)-binding site, IMDs induce a membrane curvature opposite that of BAR domains and deform membranes by binding to the interior of the tubule. This explains why IMD proteins induce plasma membrane protrusions rather than invaginations. We also provide evidence that the membrane-deforming activity of IMDs, instead of the previously proposed F-actin-bundling or GTPase-binding activities, is critical for the induction of the filopodia/microspikes in cultured mammalian cells. Together, these data reveal that interplay between actin dynamics and a novel membrane-deformation activity promotes cell motility and morphogenesis.

p37 is a p97 adaptor required for Golgi and ER biogenesis in interphase and at the end of mitosis

We previously reported that p97/p47-assisted membrane fusion is important for the reassembly of organelles at the end of mitosis, but not for their maintenance during interphase. We have now identified a p97 adaptor protein, p37, which forms a complex with p97 in the cytosol and localizes to the Golgi and ER. siRNA experiments revealed that p37 is required for Golgi and ER biogenesis. Injection of anti-p37 antibodies into cells at different cell cycle stages showed that p37 plays an important role in both Golgi and ER maintenance during interphase as well as in their reassembly at the end of mitosis. In an in vitro Golgi reassembly assay, the p97/p37 complex has membrane fusion activity. In contrast to the p97/p47 pathway, this pathway requires p115-GM130 tethering and SNARE GS15, but not syntaxin5. Interestingly, although VCIP135 is also required, its deubiquitinating activity is unnecessary for p97/p37-mediated activities.

Complement activation associates with saccular cerebral artery aneurysm wall degeneration and rupture

OBJECTIVE

Saccular cerebral artery aneurysm (SCAA) wall degeneration and inflammatory cell infiltrations associate with aneurysm rupture and subarachnoid hemorrhage, resulting in a devastating form of stroke. The complement system is the key mediator of inflammation and household processing of injured tissue. We studied how complement activation associates with SCAA wall degeneration and rupture to better understand the pathobiology of SCAA wall rupture.

METHODS

Unruptured (n = 26) and ruptured (n = 32) SCAA fundi resected after microsurgical clipping were studied by immunostaining for complement activation (membrane attack complex [MAC]) and by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling reaction for related cell death. Complement activation was correlated with clinical and other histological parameters. Electromicroscopy and immunoelectron microscopy were used for locating MAC depositions at the ultrastructural level.

RESULTS

MAC localized consistently in a decellularized layer in the outer SCAA wall, and was found in all SCAA samples. The percentage of MAC-positive area relative to the total SCAA wall surface area (range, 5-77%) was greater in ruptured (n = 25; median, 39%) than in unruptured SCAAs (n = 18; median, 20%; P = 0.005). It also associated significantly with SCAA wall degeneration (P < 0.001), de-endothelialization(P < 0.001), and CD163+ macrophage (P = 0.023) and T-lymphocyte (P = 0.030) infiltrations. Apoptotic terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling-positive nuclei and MAC were located at the same wall areas in four out of 14 double-stained samples, but no double-positive cells were found. Electromicroscopy and immunoelectron microscopy of an unruptured SCAA showed cell death in the MAC-positive layers in the outer SCAA wall.

CONCLUSION

These data suggests that complement activation and MAC formation are involved in SCAA wall degeneration and rupture.

Role of the amphipathic peptide of Semliki forest virus replicase protein nsP1 in membrane association and virus replication

Semliki Forest virus RNA replication takes place in association with specific cytoplasmic vacuoles, derived from the endosomal apparatus. Of the four virus-encoded replicase proteins, nsP1 serves as the membrane anchor of the replication complex. An amphipathic peptide segment, G245STLYTESRKLLRSWHLPSV264, has been implicated in the membrane binding of nsP1. nsP1 variants with changes within the peptide were studied after protein expression and in the context of virus infection. Proteins with mutations R253E and W259A accumulated in the cytoplasm and were very poorly palmitoylated. The same mutations also drastically affected the localization of the precursor polyprotein P123, and they were lethal when introduced into the virus genome. Mutations R253A and L255A+L256A partially changed the localization of nsP1, and the respective viruses acquired compensatory changes. L255A+L256A only yielded virus encoding L255A+L256V, indicating the importance of a hydrophobic residue in the central 256 position. When fused to green fluorescent protein, the peptide was required in at least two tandem copies to effect a change in localization, but even then the fusion protein was associated with membranes in a nonspecific manner. Thus, the amphipathic peptide is a crucial element for the membrane association of nsP1 and the replication complex. It provides essential affinity for membranes, and other regions of nsP1 also appear to contribute to the localization of the protein.

Defective insulin receptor activation and altered lipid rafts in Niemann-Pick type C disease hepatocytes

Niemann-Pick type C (NPC) disease is a neuro-visceral cholesterol storage disorder caused by mutations in the NPC-1 or NPC-2 gene. In the present paper, we studied IR (insulin receptor) activation and the plasma-membrane lipid assembly in primary hepatocytes from control and NPC1-/- mice. We have previously reported that, in hepatocytes, IR activation is dependent on cholesterol-sphingolipid rafts [Vainio, Heino, Mansson, Fredman, Kuismanen, Vaarala and Ikonen (2002) EMBO Rep. 3, 95-100]. We found that, in NPC hepatocytes, IR levels were up-regulated and the receptor activation was compromised. Defective IR activation was reproduced in isolated NPC plasma-membrane preparations, which displayed an increased cholesterol content and saturation of major phospholipids. The NPC plasma membranes were less fluid than control membranes as indicated by increased DPH (1,6-diphenyl-1,3,5-hexatriene) fluorescence anisotropy values. Both in NPC hepatocytes and plasma-membrane fractions, the association of IR with low-density DRMs (detergent-resistant membranes) was increased. Moreover, the detergent resistance of both cholesterol and phosphatidylcholine were increased in NPC membranes. Finally, cholesterol removal inhibited IR activation in control membranes but restored IR activation in NPC membranes. Taken together, the results reveal a lipid imbalance in the NPC hepatocyte, which increases lipid ordering in the plasma membrane, alters the properties of lipid rafts and interferes with the function of a raft-associated plasma-membrane receptor. Such a mechanism may participate in the pathogenesis of NPC disease and contribute to insulin resistance in other disorders of lipid metabolism.

Effect of chronic nicotine treatment on localization of neuronal nicotinic acetylcholine receptors at cellular level

Chronic nicotine treatment increases the number of neuronal nicotinic acetylcholine receptors (nAChRs). Localization of nAChRs at a cellular level determines their functional role. However, changes in the localization of nAChRs caused by chronic nicotine treatment are not well known. In this study, we have examined the effects of chronic nicotine treatment on alpha7 and beta2 nAChR subunits in vitro in cell lines and in vivo in mouse striatum. In vitro, two different cell lines were used, SH-SY5Y cells endogenously expressing several nAChR subtypes and SH-EP1-halpha7 cells, transfected with the human alpha7 nAChR subunit gene. Effects of chronic nicotine treatment (10 microM, 3 days) were studied in vitro by using confocal and electron microscopy and calcium fluorometry. In vitro in SH-SY5Y cells, alpha7 and beta2 subunits formed groups, unlike alpha7 subunits in SH-EP1-halpha7 cells, which were partially localized on endoplastic reticulum. Chronic nicotine treatment did not change the localization of nAChRs in endosomes, but caused clustering of alpha7 subunits in SH-EP1-halpha7 cells. In vivo, nicotine was given to mice in their drinking water for 7 weeks. Results showed that alpha7 and beta2 subunits formed groups, and that chronic nicotine treatment increased the size of the clusters. As a conclusion, our data show that there are large intracellular pools of nAChR subunits, which are partially localized on endoplastic reticulum. Chronic nicotine treatment does not change endocytotic trafficking of nAChRs. Chronic nicotine treatment increased clustering of nAChRs, which could have a role in the release of dopamine (DA) evoked by nicotine.