Directed tagging of the Arabidopsis FATTY ACID ELONGATION1 (FAE1) gene with the maize transposon activator

The FATTY ACID ELONGATION1 (FAE1) gene of Arabidopsis is required for the synthesis of very long chain fatty acids in the seed. The product of the FAE1 gene is presumed to be a condensing enzyme that extends the chain length of fatty acids from C18 to C20 and C22. We report here the cloning of FAE1 by directed transposon tagging with the maize element Activator (Ac). An unstable fae1 mutant was isolated in a line carrying Ac linked to the FAE1 locus on chromosome 4. Cosegregation and reversion analyses established that the new mutant was tagged by Ac. A DNA fragment flanking Ac was cloned by inverse polymerase chain reaction and used to isolate FAE1 genomic clones and a cDNA clone from a library made from immature siliques. The predicted amino acid sequence of the FAE1 protein shares homology with those of other condensing enzymes (chalcone synthase, stilbene synthases, and beta-ketoacyl-acyl carrier protein synthase III), supporting the notion that FAE1 is the structural gene for a synthase or condensing enzyme. FAE1 is expressed in developing seed, but not in leaves, as expected from the effect of the fae1 mutation on the fatty acid compositions of those tissues.

Analysis of GLUT4 distribution in whole skeletal muscle fibers: identification of distinct storage compartments that are recruited by insulin and muscle contractions

The effects of insulin stimulation and muscle contractions on the subcellular distribution of GLUT4 in skeletal muscle have been studied on a preparation of single whole fibers from the rat soleus. The fibers were labeled for GLUT4 by a preembedding technique and observed as whole mounts by immunofluorescence microscopy, or after sectioning, by immunogold electron microscopy. The advantage of this preparation for cells of the size of muscle fibers is that it provides global views of the staining from one end of a fiber to the other and from one side to the other through the core of the fiber. In addition, the labeling efficiency is much higher than can be obtained with ultracryosections. In nonstimulated fibers, GLUT4 is excluded from the plasma membrane and T tubules. It is distributed throughout the muscle fibers with approximately 23% associated with large structures including multivesicular endosomes located in the TGN region, and 77% with small tubulovesicular structures. The two stimuli cause translocation of GLUT4 to both plasma membrane and T tubules. Quantitation of the immunogold electron microscopy shows that the effects of insulin and contraction are additive and that each stimulus recruits GLUT4 from both large and small depots. Immunofluorescence double labeling for GLUT4 and transferrin receptor (TfR) shows that the small depots can be further subdivided into TfR-positive and TfR-negative elements. Interestingly, we observe that colocalization of TfR and GLUT4 is increased by insulin and decreased by contractions. These results, supported by subcellular fractionation experiments, suggest that TfR-positive depots are only recruited by contractions. We do not find evidence for stimulation-induced unmasking of resident surface membrane GLUT4 transporters or for dilation of the T tubule system (Wang, W., P.A. Hansen, B.A. Marshall, J.O. Holloszy, and M. Mueckler. 1996. J. Cell Biol. 135:415-430).

Glycogen synthase localization and activity in rat skeletal muscle is strongly dependent on glycogen content

1. The influence of muscle glycogen content on glycogen synthase (GS) localization and GS activity was investigated in skeletal muscle from male Wistar rats. 2. Two groups of rats were obtained, preconditioned with a combination of exercise and diet to obtain either high (HG) or low (LG) muscle glycogen content. The cellular distribution of GS was studied using subcellular fractionation and confocal microscopy of immunostained single muscle fibres. Stimulation of GS activity in HG and LG muscle was obtained with insulin or contractions in the perfused rat hindlimb model. 3. We demonstrate that GS translocates from a glycogen-enriched membrane fraction to a cytoskeleton fraction when glycogen levels are decreased. Confocal microscopy supports the biochemical observations that the subcellular localization of GS is influenced by muscle glycogen content. GS was not found in the nucleus. 4. Investigation of the effect of glycogen content on GS activity in basal and insulin- and contraction-stimulated muscle shows that glycogen has a strong inhibitory effect on GS activity. Our data demonstrate that glycogen is a more potent regulator of glycogen synthase activity than insulin. Furthermore we show that the contraction-induced increase in GS activity is merely a result of a decrease in muscle glycogen content. 5. In conclusion, the present study shows that GS localization is influenced by muscle glycogen content and that not only basal but also insulin- and contraction-stimulated GS activity is strongly regulated by glycogen content in skeletal muscle.

Deoxyribonucleic acid homologies among some Pseudomonas species

Phylogenetic relationships among a number of strains belonging to the genus Pseudomonas were explored by the use of in vitro deoxyribonucleic acid (DNA) hybridization. The fluorescent nomenspecies (P. fluorescens, P. putida, P. aeruginosa, P. cichorii, P. syringae, and related species), as well as the nonfluorescent species P. stutzeri, P. mendocina, P. alcaligenes, and P. pseudoalcaligenes, were shown to belong to a single DNA homology complex which is isolated from other Pseudomonas species that have been studied [P. cepacia (= P. multivorans), P. caryophylli, P. marginata (= P. alliicola), P. pseudomallei, P. acidovorans, P. testosteroni, P. solanacearum, P. diminuta, P. facilis, P. delafieldii, P. saccharophila, P. palleronii]. A limited numerical analysis of the phenotypic properties of the examined strains supported, with some exceptions, their previous allocation to nomenspecies and biotypes. The internal structure and nomenclature of the "P. fluorescens homology complex" are discussed.

Topological mapping of acetylcholine receptor: evidence for a model with five transmembrane segments and a cytoplasmic COOH-terminal peptide

Antibodies were raised against two synthetic peptides whose sequences correspond respectively to the COOH-terminal end (residues 501-516) of the protein encoded by the gene for the delta chain and to a proposed cytoplasmic region (residues 350-358) of the beta chain of the acetylcholine receptor from Torpedo californica. Binding of the COOH-terminal antibody to the acetylcholine receptor in intact, receptor-rich vesicles was tested by radioimmunoassay and by precipitation with immobilized protein A. In both cases, binding was detected only after treatment of the vesicles with detergent, suggesting that the segment of the receptor that is recognized by this antibody is on the cytoplasmic side of the membrane. Electron microscopy of tissue from Torpedo electric organ labeled with colloidal gold-conjugated second antibodies established that both anti-receptor antibodies bind to the cytoplasmic surface of the postsynaptic membrane. These experiments give ultrastructural evidence that the COOH-terminal segment of the delta chain as well as residues 350-358 of the beta chain are on the cytoplasmic surface. They strongly support a model in which each of the receptor subunits crosses the membrane five times in which one transmembrane segment of each chain contributes to the formation of a central ion channel.

Golgi complex reorganization during muscle differentiation: visualization in living cells and mechanism

During skeletal muscle differentiation, the Golgi complex (GC) undergoes a dramatic reorganization. We have now visualized the differentiation and fusion of living myoblasts of the mouse muscle cell line C2, permanently expressing a mannosidase-green fluorescent protein (GFP) construct. These experiments reveal that the reorganization of the GC is progressive (1-2 h) and is completed before the cells start fusing. Fluorescence recovery after photobleaching (FRAP), immunofluorescence, and immunogold electron microscopy demonstrate that the GC is fragmented into elements localized near the endoplasmic reticulum (ER) exit sites. FRAP analysis and the ER relocation of endogenous GC proteins by phospholipase A2 inhibitors demonstrate that Golgi-ER cycling of resident GC proteins takes place in both myoblasts and myotubes. All results support a model in which the GC reorganization in muscle reflects changes in the Golgi-ER cycling. The mechanism is similar to that leading to the dispersal of the GC caused, in all mammalian cells, by microtubule-disrupting drugs. We propose that the trigger for the dispersal results, in muscle, from combined changes in microtubule nucleation and ER exit site localization, which place the ER exit sites near microtubule minus ends. Thus, changes in GC organization that initially appear specific to muscle cells, in fact use pathways common to all mammalian cells.

Preferential transposition of the maize element Activator to linked chromosomal locations in tobacco

The autonomous maize transposon Activator (Ac) has been used in maize for gene isolation by tagging and may prove similarly useful in other species. To test the feasibility of gene tagging with heterospecific transposons, we have examined three key genetic properties of a slightly modified Ac in tobacco. First, we show that frequencies of germinal excision of this Ac element from the antibiotic resistance gene streptomycin phosphotransferase can be comparable with or slightly lower than in maize. Second, we show that about half of the progeny carrying a germinal excision product also carry a transposed Ac. Last, we have mapped transposed Ac locations relative to the streptomycin transferase gene excision product and have shown that as in maize Ac in tobacco preferentially transposes to genetically linked sites.

Expression of the adrenoleukodystrophy protein in the human and mouse central nervous system

The gene mutated in X-linked adrenoleukodystrophy (ALD), a progressive demyelinating disease, codes for a protein (ALDP) involved in very-long-chain fatty acid (VLCFA) transport. The expression of ALDP and of two peroxisomal enzymes involved in beta-oxidation of VLCFA, acyl-CoA oxidase, and catalase was studied in human and mouse brain. The pattern of expression was similar in both species. While acyl-CoA oxidase and catalase are found in all types of CNS cells, including neurons and oligodendrocytes, ALDP expression is restricted mostly to the white matter and endothelial cells. ALDP is highly expressed in astrocytes and microglial cells in vivo and in regenerating oligodendrocytes in vitro. In contrast, in vivo, ALDP is detected in much fewer oligodendrocytes and quantitative Western blot analysis confirmed the lower abundance of ALDP in these cells than in astrocytes. Only oligodendrocytes localized in corpus callosum, internal capsules, and anterior commissure express ALDP at levels comparable to those seen in astrocytes. In ALD, demyelination is first detected in these white matter regions, suggesting that the ALD gene mutation selectively affects those oligodendrocytes strongly expressing ALDP. Because of their failure to express ALDP, microglia and astrocytes may also contribute to demyelination in ALD patients.

Changes in architecture of the Golgi complex and other subcellular organelles during myogenesis

Myogenesis involves changes in both gene expression and cellular architecture. Little is known of the organization, in muscle in vivo, of the subcellular organelles involved in protein synthesis despite the potential importance of targeted protein synthesis for formation and maintenance of functional domains such as the neuromuscular junction. A panel of antibodies to markers of the ER, the Golgi complex, and the centrosome were used to localize these organelles by immunofluorescence in myoblasts and myotubes of the mouse muscle cell line C2 in vitro, and in intact single muscle fibers from the rat flexor digitorum brevis. Antibodies to the ER stained structures throughout the cytoplasm of both C2 myoblasts and myotubes. In contrast, the spatial relationship between nucleus, centrosome, and Golgi complex was dramatically altered. These changes could also be observed in a low-calcium medium that allowed differentiation while preventing myoblast fusion. Muscle fibers in vivo resembled myotubes except that the ER occupied a smaller volume of cytoplasm and no staining was found for one of the Golgi complex markers, the enzyme alpha-mannosidase II. Electron microscopy, however, clearly showed the presence of stacks of Golgi cisternae in both junctional and extrajunctional regions of muscle fibers. The perinuclear distribution of the Golgi complex was also observed in live muscle fibers stained with a fluorescent lipid. Thus, the distribution of subcellular organelles of the secretory pathway was found to be similar in myotubes and muscle fibers, and all organelles were found in both junctional and extrajunctional areas of muscle.

Transfer of a protein encoded by a single nucleus to nearby nuclei in multinucleated myotubes

Specialized regions of muscle fibers may result from differential gene expression within a single fiber. In order to investigate the range of action of individual nuclei in multinucleated myotubes, C2 myoblasts were transfected to obtain stable cell lines that express a reporter protein that is targeted to the nucleus. Hybrid myotubes were then formed containing one or a few transfected nuclei as well as a large number of nuclei from the parental strain. In order to determine how far the products of a single nucleus extend, transfected nuclei were labeled with [3H]thymidine before fusion and the myotubes were stained to identify the reporter protein. In such myotubes the fusion protein was not confined to its nucleus of origin, but was restricted to nearby nuclei.

Blood vessels and desmin control the positioning of nuclei in skeletal muscle fibers

Skeletal muscle fibers contain hundreds to thousands of nuclei which lie immediately under the plasmalemma and are spaced out along the fiber, except for a small cluster of specialized nuclei at the neuromuscular junction. How the nuclei attain their positions along the fiber is not understood. Here we show that the nuclei are preferentially localized near blood vessels (BV), particularly in slow-twitch, oxidative fibers. Thus, in rat soleus muscle fibers, 81% of the nuclei appear next to BV. Lack of desmin markedly perturbs the distribution of nuclei along the fibers but does not prevent their close association with BV. Consistent with a role for desmin in the spacing of nuclei, we show that denervation affects the organization of desmin filaments as well as the distribution of nuclei. During chronic stimulation of denervated muscles, new BV form, along which muscle nuclei align themselves. We conclude that the positioning of nuclei along muscle fibers is plastic and that BV and desmin intermediate filaments each play a distinct role in the control of this positioning.

Restricted distribution of mRNA produced from a single nucleus in hybrid myotubes

Although the proteins encoded by a single nucleus in multinucleated myotubes have a wide range of distributions within the myofiber, little is known about the distributions of their mRNAs. We have used hybrid myotubes in which one or a few nuclei are derived from myoblasts that express nonmuscle proteins to investigate this question. We find that three different mRNAs, encoding proteins that are, respectively, nuclear, cytoplasmic, and targeted to the ER, have similar distributions within myotubes. Each is confined to an area within approximately 100 microns of the nucleus that expresses it.

Chromosome-breaking structure in maize involving a fractured Ac element

Chromosome breakage in maize can result from an interaction between certain transposable elements. When an Ac (Activator) element and a state I Ds (Dissociation) element are present together in the genome, either linked or unlinked, breaks occur regularly at the locus of the Ds element. We show here that breaks occur with high frequency at or near the locus of a structure consisting of a 2.5-kilobase (kb) terminally deleted or fractured Ac element very tightly linked to a second, intact 4.6-kb Ac element. This structure has the features of a macrotransposon and may behave like one. Loss of the tight linkage abolishes chromosome breakage. A model based on transposition of the macrotransposon is proposed to explain the chromosome-breaking properties of Ac and Ds.

Acetylcholine receptor in a C2 muscle cell variant is retained in the endoplasmic reticulum

We have examined the properties and intracellular localization of acetylcholine receptors in the C2 muscle cell line and in a variant (T-) that accumulates AChR intracellularly. On immunoblots, the subunit structures of the AChR from wild-type and T- cells were similar except that the gamma and delta subunits of the variant AChR had altered mobilities. Digestion with endoglycosidases H and F demonstrated that this difference results from a failure of high-mannose N-linked oligosaccharides on AChR subunits to be processed to complex forms in the variant. N-linked glycosylation of other proteins in the variant was normal. When examined by immunocytochemistry, the distribution of internal AChR in wild-type cells was consistent with a location both in the endoplasmic reticulum and in the Golgi. Variant cells, however, showed no evidence of Golgi staining. Subcellular fractionation experiments also demonstrated AChR in the Golgi fractions of wild-type cells, but not in those derived from T- cells. We conclude that in T- myotubes most of the AChR fails to be transported out of the endoplasmic reticulum.

Caveolin-3 is associated with the T-tubules of mature skeletal muscle fibers

Caveolae are abundant in skeletal muscle and their coat contains a specific isoform of caveolin, caveolin-3. It has been suggested that during muscle development, caveolin-3 is associated with the T-tubules, but that in adult muscle it is found on the plasma membrane only. We have studied the distribution of caveolin-3 in single skeletal muscle fibers from adult rat soleus by confocal immunofluorescence and by immunogold electron microscopy. We found that caveolin-3 occurs at the highest density on the plasma membrane but is also present in the core of the fibers, at the I-band/A-band interface where it is associated with the T-tubules. In neither domain of the muscle surface does caveolin-3 colocalize with the glucose transporter GLUT4 and there is no evidence for internalization of the caveolae in muscle.

Detection and imaging of non-contractile inclusions and sarcomeric anomalies in skeletal muscle by second harmonic generation combined with two-photon excited fluorescence

The large size of the multinucleated muscle fibers of skeletal muscle makes their examination for structural and pathological defects a challenge. Sections and single fibers are accessible to antibodies and other markers but imaging of such samples does not provide a three-dimensional view of the muscle. Regrettably, bundles of fibers cannot be stained or imaged easily. Two-photon microscopy techniques overcome these obstacles. Second harmonic generation (SHG) by myosin filaments and two-photon excited fluorescence (2PEF) of mitochondrial and lysosomal components provides detailed structural information on unstained tissue. Furthermore, the infrared exciting light can penetrate several layers of muscle fibers and the minimal processing is particularly valuable for fragile biopsies. Here we demonstrate the usefulness of SHG, combined with 2PEF, to reveal enlarged lysosomes and accumulations of non-contractile material in muscles from the mouse model for the lysosomal storage disorder Pompe disease (PD), and in biopsies from adult and infant PD patients. SHG and 2PEF also detect sarcomeric defects that may presage the loss of myofibrils in atrophying muscle and signify loss of elasticity. The combination of SHG and 2PEF should be useful in the analysis and diagnosis of a wide range of skeletal muscle pathologies.

GLUT4 in cultured skeletal myotubes is segregated from the transferrin receptor and stored in vesicles associated with TGN

There is little consensus on the nature of the storage compartment of the glucose transporter GLUT4, in non-stimulated cells of muscle and fat. More specifically, it is not known whether GLUT4 is localized to unique, specialized intracellular storage vesicles, or to vesicles that are part of the constitutive endosomal-lysosomal pathway. To address this question, we have investigated the localization of the endogenous GLUT4 in non-stimulated skeletal myotubes from the cell line C2, by immunofluorescence and immunoelectron microscopy. We have used a panel of antibodies to markers of the Golgi complex (alpha mannosidase II and giantin), of the trans-Golgi network (TGN38), of lysosomes (lgp110), and of early and late endosomes (transferrin receptor and mannose-6-phosphate receptor, respectively), to define the position of their subcellular compartments. By immunofluorescence, GLUT4 appears concentrated in the core of the myotubes. It is primarily found around the nuclei, in a pattern suggesting an association with the Golgi complex, which is further supported by colocalization with giantin and by immunogold electron microscopy. GLUT4 appears to be in the trans-most cisternae of the Golgi complex and in vesicles just beyond, i.e. in the structures that constitute the trans-Golgi network (TGN). In myotubes treated with brefeldin A, the immunofluorescence pattern of GLUT4 is modified, but it differs from both Golgi complex markers and TGN38. Instead, it resembles the pattern of the transferrin receptor, which forms long tubules. In untreated cells, double staining for GLUT4 and transferrin receptor by immunofluorescence shows similar but distinct patterns. Immunoelectron microscopy localizes transferrin receptor, detected by immunoperoxidase, to large vesicles, presumably endosomes, very close to the GLUT4-containing tubulo-vesicular elements. In brefeldin A-treated cells, a network of tubules of approximately 70 nm diameter, studded with varicosities, stains for both GLUT4 and transferrin receptor, suggesting that brefeldin A has caused fusion of the transferrin receptor and GLUT4-containing compartments. The results suggest that GLUT4 storage vesicles constitute a specialized compartment that is either a subset of the TGN, or is very closely linked to it. The link between GLUT4 vesicles and transferrin receptor containing endosomes, as revealed by brefeldin A, may be important for GLUT4 translocation in response to muscle stimulation.

Molecular mechanisms involved in GLUT4 translocation in muscle during insulin and contraction stimulation

Studies in mammalian cells have established the existence of numerous intracellular signaling cascades that are critical intermediates in the regulation of various biological functions. Over the past few years considerable research has shown that many of these signaling proteins are expressed in skeletal muscle. However, the detailed mechanisms involved in the regulation of glucose transporter (GLUT4) translocation from intracellular compartments to the cell surface membrane in response to insulin and contractions in skeletal muscle are not well understood. In the present essay we report three different approaches to unravel the GLUT4 translocation mechanism: 1. specific pertubation of the insulin and/or contraction signaling pathways; 2. characterization of the protein composition of GLUT4-containing vesicles with the expectation that knowledge of the constituent proteins of the vesicles may help in understanding their trafficking; 3. degree of co-immunolocalization of the GLUT4 glucose transporters with other membrane marker proteins assessed by immunofluorescense and electron microscopy.

Improved lipid-mediated gene transfer into primary cultures of hippocampal neurons

We have examined lipids as transfection agents to introduce recombinant plasmids into primary cultures of rat hippocampal neurons. By modifying the protocol for transfection mediated by the commercial reagent DOTAP, we were able to achieve a transfection efficiency of about 3%. Expression of various transfected gene products was sustained for several weeks in culture, the neurons developed normally and the transfected gene products were targeted to the appropriate subcellular compartment.

A single genetic unit specifies two transposition functions in the maize element activator

The self-mobile maize transposable element Ac (Activator) displays two trans-acting genetic functions: it induces transposition of the element Ds (Dissociation) but, as its dosage is increased, it also inhibits transposition. Previous work has shown that the 4563 base pair (bp)-long Ac element contains three open reading frames (ORF's) and that a deletion in ORF 1 in wx-m9(Ds), a Ds derivative from Ac isolated at the wx (waxy) locus, results in loss of transposition. The Ds element in the bronze allele bz-m2(DI) is shown to have arisen from Ac by a 1312-bp deletion that is located almost entirely within ORF 2 and does not affect ORF 1. The Ds elements in wx-m9(Ds) and bzm2(DI), defective in ORF 1 and ORF 2, respectively, do not complement genetically to restore the transposition function of Ac; therefore, this function must be specified jointly by ORF's 1 and 2. Furthermore, since bz-m2(DI) does not contribute to Ac's inhibitory dosage effect, both Ac properties result from the expression of the same genetic functional unit.

Ribosome association contributes to restricting mRNAs to the cell body of hippocampal neurons

In neurons, mRNAs are differentially sorted to axons, dendrites, and the cell body. Recently, regions of certain mRNAs have been identified that target those mRNAs for translocation to the processes. However, the mechanism by which many, if not most mRNAs are retained in the cell body is not understood. Total inhibition of translation, by puromycin or cycloheximide, results in the mislocalization of cell body mRNAs to dendrites. We have examined the effect of translational inhibitors on the localization of ferritin mRNA, the translation of which can also be inhibited specifically by reducing iron levels. Using nonisotopic in situ hybridization, ferritin mRNA is found restricted to the cell body of cultured rat hippocampal neurons. Following treatment with either puromycin or cycloheximide, it migrates into dendrites. Control experiments reveal that the drugs affect neither the viability of the neuronal cultures, nor the steady-state level of ferritin mRNA. When transcription and protein synthesis are inhibited simultaneously, ferritin mRNA is found in the dendrites of puromycin, but not of cycloheximide-treated neurons. However, the localization of ferritin mRNA is unaffected by changes in iron concentration that regulate its translation rate specifically. We propose a model whereby cell body-restricted mRNAs are maintained in that location by association with ribosomes and with another cell component, which traps mRNAs when they are freed of ribosome association. The release of all mRNA species, as happens after total protein synthesis inhibition, floods the system and allows cell body mRNAs to diffuse into dendrites. In contrast, the partial release of the single ferritin mRNA species does not saturate the trapping system and the mRNA is retained in the cell body.