Dystrophin is a microtubule-associated protein

Cytolinkers are giant proteins that can stabilize cells by linking actin filaments, intermediate filaments, and microtubules (MTs) to transmembrane complexes. Dystrophin is functionally similar to cytolinkers, as it links the multiple components of the cellular cytoskeleton to the transmembrane dystroglycan complex. Although no direct link between dystrophin and MTs has been documented, costamere-associated MTs are disrupted when dystrophin is absent. Using tissue-based cosedimentation assays on mice expressing endogenous dystrophin or truncated transgene products, we find that constructs harboring spectrinlike repeat 24 through the first third of the WW domain cosediment with MTs. Purified Dp260, a truncated isoform of dystrophin, bound MTs with a K_d of 0.66 µM, a stoichiometry of 1 Dp260/1.4 tubulin heterodimer at saturation, and stabilizes MTs from cold-induced depolymerization. Finally, α- and β-tubulin expression is increased ∼2.5-fold in mdx skeletal muscle without altering the tubulin–MT equilibrium. Collectively, these data suggest dystrophin directly organizes and/or stabilizes costameric MTs and classifies dystrophin as a cytolinker in skeletal muscle.

Microtubule plus-end binding protein EB1 is necessary for muscle cell differentiation, elongation and fusion

During muscle differentiation, microtubule stability, nucleation and orientation all undergo profound changes, which are simultaneous with and possibly necessary for the elongation and fusion of muscle cells. We do not yet understand these events, but they present similarities with the polarized migration of fibroblasts, in which EB1 is necessary for microtubule stabilization. However, it was recently reported that EB3, not EB1, is involved in muscle cell elongation and fusion, and that neither of these two proteins influences microtubule stabilization. To re-examine the role of EB1, we have generated C2 cell lines permanently expressing EB1-targeted shRNAs. In these lines, EB1 is specifically knocked down by more than 90% before any differentiation-related changes can take place. We find that differentiation (assessed by myogenin expression), elongation and fusion are prevented. In addition, two early events that normally precede differentiation - microtubule stabilization and the accumulation of cadherin and beta-catenin on the plasma membrane - are inhibited. Re-expression of EB1 as EB1-GFP restores all aspects of normal differentiation, whereas overexpression of EB3-GFP restores elongation but not fusion. We conclude that EB1 is necessary for the early stages of muscle differentiation.

When more is less: excess and deficiency of autophagy coexist in skeletal muscle in Pompe disease

The role of autophagy, a catabolic lysosome-dependent pathway, has recently been recognized in a variety of disorders, including Pompe disease, which results from a deficiency of the glycogen-degrading lysosomal hydrolase acid-alpha glucosidase (GAA). Skeletal and cardiac muscle are most severely affected by the progressive expansion of glycogen-filled lysosomes. In both humans and an animal model of the disease (GAA KO), skeletal muscle pathology also involves massive accumulation of autophagic vesicles and autophagic buildup in the core of myofibers, suggesting an induction of autophagy. Only when we suppressed autophagy in the skeletal muscle of the GAA KO mice did we realize that the excess of autophagy manifests as a functional deficiency. This failure of productive autophagy is responsible for the accumulation of potentially toxic aggregate-prone ubiquitinated proteins, which likely cause profound muscle damage in Pompe mice. Also, by generating muscle-specific autophagy-deficient wild-type mice, we were able to analyze the role of autophagy in healthy skeletal muscle.

Suppression of autophagy in skeletal muscle uncovers the accumulation of ubiquitinated proteins and their potential role in muscle damage in Pompe disease

The role of autophagy, a catabolic lysosome-dependent pathway, has recently been recognized in a variety of disorders, including Pompe disease, the genetic deficiency of the glycogen-degrading lysosomal enzyme acid-alpha glucosidase. Accumulation of lysosomal glycogen, presumably transported from the cytoplasm by the autophagic pathway, occurs in multiple tissues, but pathology is most severe in skeletal and cardiac muscle. Skeletal muscle pathology also involves massive autophagic buildup in the core of myofibers. To determine if glycogen reaches the lysosome via autophagy and to ascertain whether autophagic buildup in Pompe disease is a consequence of induction of autophagy and/or reduced turnover due to defective fusion with lysosomes, we generated muscle-specific autophagy-deficient Pompe mice. We have demonstrated that autophagy is not required for glycogen transport to lysosomes in skeletal muscle. We have also found that Pompe disease involves induction of autophagy but manifests as a functional deficiency of autophagy because of impaired autophagosomal-lysosomal fusion. As a result, autophagic substrates, including potentially toxic aggregate-prone ubiquitinated proteins, accumulate in Pompe myofibers and may cause profound muscle damage.

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.

Deconstructing Pompe disease by analyzing single muscle fibers: to see a world in a grain of sand

Autophagy is a major pathway for delivery of proteins and organelles to lysosomes where they are degraded and recycled. We have previously shown excessive autophagy in a mouse model of Pompe disease (glycogen storage disease type II), a devastating myopathy caused by a deficiency of the glycogen-degrading lysosomal enzyme acid alpha-glucosidase. The autophagic buildup constituted a major pathological component in skeletal muscle and interfered with delivery of the therapeutic enzyme. To assess the role of autophagy in the pathogenesis of the human disease, we have analyzed vesicles of the lysosomal-degradative pathway in isolated single muscle fibers from Pompe patients. Human myofibers showed abundant autophagosome formation and areas of autophagic buildup of a wide range of sizes. In patients, as in the mouse model, the enormous autophagic buildup causes greater skeletal muscle damage than the enlarged, glycogenfilled lysosomes outside the autophagic regions. Clearing or preventing autophagic buildup seems, therefore, a necessary target of Pompe disease therapy.

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.

Autophagy and lysosomes in Pompe disease

In Pompe disease, a deficiency of lysosomal acid alpha-glucosidase, intralysosomal glycogen accumulates in multiple tissues, with skeletal and cardiac muscle most severely affected.(1) Complete enzyme deficiency results in rapidly progressive infantile cardiomyopathy and skeletal muscle myopathy that is fatal within the first two years of life. Patients with partial enzyme deficiency suffer from skeletal muscle myopathy and experience shortened lifespan due to respiratory failure. The major advance has been the development of enzyme replacement therapy, which recently became available for Pompe patients. However, the effective clearance of skeletal muscle glycogen, as shown by both clinical and preclinical studies, has proven more difficult than anticipated.(2-4) Our recent work published in Annals of Neurology(5) was designed to cast light on the problem, and was an attempt to look beyond the lysosomes by analyzing the downstream events affected by the accumulation of undigested substrate in lysosomes. We have found that the cellular pathology in Pompe disease spreads to affect both endocytic (the route of the therapeutic enzyme) and autophagic (the route of glycogen) pathways, leading to excessive autophagic buildup in therapy-resistant skeletal muscle fibers of the knockout mice.

Autophagy and mistargeting of therapeutic enzyme in skeletal muscle in Pompe disease

Enzyme replacement therapy (ERT) became a reality for patients with Pompe disease, a fatal cardiomyopathy and skeletal muscle myopathy caused by a deficiency of glycogen-degrading lysosomal enzyme acid alpha-glucosidase (GAA). The therapy, which relies on receptor-mediated endocytosis of recombinant human GAA (rhGAA), appears to be effective in cardiac muscle, but less so in skeletal muscle. We have previously shown a profound disturbance of the lysosomal degradative pathway (autophagy) in therapy-resistant muscle of GAA knockout mice (KO). Our findings here demonstrate a progressive age-dependent autophagic buildup in addition to enlargement of glycogen-filled lysosomes in multiple muscle groups in the KO. Trafficking and processing of the therapeutic enzyme along the endocytic pathway appear to be affected by the autophagy. Confocal microscopy of live single muscle fibers exposed to fluorescently labeled rhGAA indicates that a significant portion of the endocytosed enzyme in the KO was trapped as a partially processed form in the autophagic areas instead of reaching its target--the lysosomes. A fluid-phase endocytic marker was similarly mistargeted and accumulated in vesicular structures within the autophagic areas. These findings may explain why ERT often falls short of reversing the disease process and point toward new avenues for the development of pharmacological intervention.

Dysfunction of endocytic and autophagic pathways in a lysosomal storage disease

OBJECTIVE

To understand the mechanisms of skeletal muscle destruction and resistance to enzyme replacement therapy in Pompe disease, a deficiency of lysosomal acid alpha-glucosidase (GAA), in which glycogen accumulates in lysosomes primarily in cardiac and skeletal muscles.

METHODS

We have analyzed compartments of the lysosomal degradative pathway in GAA-deficient myoblasts and single type I and type II muscle fibers isolated from wild-type, untreated, and enzyme replacement therapy-treated GAA knock-out mice.

RESULTS

Studies in myoblasts from GAA knock-out mice showed a dramatic expansion of vesicles of the endocytic/autophagic pathways, decreased vesicular movement in overcrowded cells, and an acidification defect in a subset of late endosomes/lysosomes. Analysis by confocal microscopy of isolated muscle fibers demonstrated that the consequences of the lysosomal glycogen accumulation are strikingly different in type I and II muscle fibers. Only type II fibers, which are the most resistant to therapy, contain large regions of autophagic buildup that span the entire length of the fibers.

INTERPRETATION

The vastly increased autophagic buildup may be responsible for skeletal muscle damage and prevent efficient trafficking of replacement enzyme to lysosomes.

Structural and functional roles of desmin in mouse skeletal muscle during passive deformation

Mechanical interactions between desmin and Z-disks, costameres, and nuclei were measured during passive deformation of single muscle cells. Image processing and continuum kinematics were used to quantify the structural connectivity among these structures. Analysis of both wild-type and desmin-null fibers revealed that the costamere protein talin colocalized with the Z-disk protein alpha-actinin, even at very high strains and stresses. These data indicate that desmin is not essential for mechanical coupling of the costamere complex and the sarcomere lattice. Within the sarcomere lattice, significant differences in myofibrillar connectivity were revealed between passively deformed wild-type and desmin-null fibers. Connectivity in wild-type fibers was significantly greater compared to desmin-null fibers, demonstrating a significant functional connection between myofibrils that requires desmin. Passive mechanical analysis revealed that desmin may be partially responsible for regulating fiber volume, and consequently, fiber mechanical properties. Kinematic analysis of alpha-actinin strain fields revealed that knockout fibers transmitted less shear strain compared to wild-type fibers and experienced a slight increase in fiber volume. Finally, linkage of desmin intermediate filaments to muscle nuclei was strongly suggested based on extensive loss of nuclei positioning in the absence of desmin during passive fiber loading.

Reorganization of microtubule nucleation during muscle differentiation

Skeletal muscle differentiation involves a complete reorganization of the microtubule network. Nearly 20 years ago, Tassin et al. [1985: J Cell Biol 100:35-46] suggested a mechanism for this reorganization by showing a redistribution of the microtubule organizing center from the centrosome to the nuclear membrane. Little progress has been made since. It is still not clear whether centrosomal proteins are redistributed together, whether microtubules are nucleated at the nuclear membrane or transported there post-nucleation, and whether gamma-tubulin (gammatub) remains necessary for nucleation in myotubes. To investigate these questions, we have examined the redistribution of the centrosomal proteins pericentrin (PC), gammatub, and ninein in the C2 muscle cell line. Immunofluorescence of differentiated myotubes shows PC along the nuclear membrane whereas gammatub is only detected there after pre-fixation detergent extraction. After expression of a GFP-tagged gammatub, we observe a weak fluorescence along the nuclear membrane, confirming the presence of gammatub at a low concentration relative to PC. Microinjection of anti-gammatub antibodies into myotubes blocks microtubule growth from both nuclear membranes and centrosomal sites. The centrosomal microtubule-anchoring protein, ninein, is found at the nuclear membrane as well and its distribution appears independent of microtubule integrity. We conclude that centrosomal proteins are redistributed independently during muscle differentiation, to sites that nucleate microtubules both along the nuclear membranes and through the cytoplasm.

Disruption of microtubules in rat skeletal muscle does not inhibit insulin- or contraction-stimulated glucose transport

Insulin and muscle contractions stimulate glucose transport in skeletal muscle through a translocation of intracellular GLUT4 glucose transporters to the cell surface. Judged by immunofluorescence microscopy, part of the GLUT4 storage sites is associated with the extensive microtubule cytoskeleton found in all muscle fibers. Here, we test whether microtubules are required mediators of the effect of insulin and contractions. In three different incubated rat muscles with distinct fiber type composition, depolymerization of microtubules with colchicine for < or =8 h did not inhibit insulin- or contraction-stimulated 2-deoxyglucose transport or force production. On the contrary, colchicine at least partially prevented the approximately 30% decrease in insulin-stimulated transport that specifically developed during 8 h of incubation in soleus muscle but not in flexor digitorum brevis or epitrochlearis muscles. In contrast, nocodazole, another microtubule-disrupting drug, rapidly and dose dependently blocked insulin- and contraction-stimulated glucose transport. A similar discrepancy between colchicine and nocodazole was also found in their ability to block glucose transport in muscle giant "ghost" vesicles. This suggests that the ability of insulin and contractions to stimulate glucose transport in muscle does not require an intact microtubule network and that nocodazole inhibits glucose transport independently of its microtubule-disrupting effect.

Gene gun bombardment-mediated expression and translocation of EGFP-tagged GLUT4 in skeletal muscle fibres in vivo

Cellular protein trafficking has been studied to date only in vitro or with techniques that are invasive and have a low time resolution. To establish a gentle method for analysis of glucose transporter-4 (GLUT4) trafficking in vivo in fully differentiated rat skeletal muscle fibres we combined the enhanced green fluorescent protein (EGFP) labelling technique with physical transfection methods in vivo: intramuscular plasmid injection or gene gun bombardment. During optimisation experiments with plasmid coding for the EGFP reporter alone EGFP-positive muscle fibres were counted after collagenase treatment of in vivo transfected flexor digitorum brevis (FDB) muscles. In contrast to gene gun bombardment, intramuscular injection produced EGFP expression in only a few fibres. Regardless of the transfection technique, EGFP expression was higher in muscles from 2-week-old rats than in those from 6-week-old rats and peaked around 1 week after transfection. The gene gun was used subsequently with a plasmid coding for EGFP linked to the C-terminus of GLUT4 (GLUT4-EGFP). Rats were anaesthetised 5 days after transfection and insulin given i.v. with or without accompanying electrical hindleg muscle stimulation. After stimulation, the hindlegs were fixed by perfusion. GLUT4-EGFP-positive FDB fibres were isolated and analysed by confocal microscopy. The intracellular distribution of GLUT4-EGFP under basal conditions as well as after translocation to the plasma membrane in response to insulin, contractions, or both, was in accordance with previous studies of endogenous GLUT4. Finally, GLUT4-EGFP trafficking in quadriceps muscle in vivo was studied using time-lapse microscopy analysis in anaesthetised mice and the first detailed time-lapse recordings of GLUT4-EGFP translocation in fully differentiated skeletal muscle in vivo were obtained.

Exploring the whereabouts of GLUT4 in skeletal muscle (review)

The glucose transporter GLUT4 is expressed in muscle, fat cells, brain and kidney. In contrast to other glucose transporters, GLUT4 in unstimulated cells is mostly intracellular. Stimuli such as insulin and muscle contractions then cause the translocation of GLUT4 to the cell surface. Questions related to GLUT4 storage compartments, trafficking to the surface membrane, and nature of the intracellular pools, have kept many groups busy for the past 20 years. Yet, one of the main questions in the field remains the universality of GLUT4 features. Can one extrapolate work done on fat cells to muscle or brain? Or vice-versa? Can one use cultures to predict GLUT4 behaviour in fully differentiated tissues? This review summarizes the authors' knowledge of GLUT4 biology in skeletal muscle, which is the predominant tissue for glucose homeostasis. The results are compared to those obtained with the fat cell system, and an attempt is made to assess the universality principle.

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.

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.

Golgi complex, endoplasmic reticulum exit sites, and microtubules in skeletal muscle fibers are organized by patterned activity

The Golgi complex of skeletal muscle fibers is made of thousands of dispersed elements. The distributions of these elements and of the microtubules they associate with differ in fast compared with slow and in innervated compared with denervated fibers. To investigate the role of muscle impulse activity, we denervated fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of adult rats and stimulated them directly with patterns that resemble the impulse patterns of normal fast EDL (25 pulses at 150 Hz every 15 min) and slow SOL (200 pulses at 20 Hz every 30 sec) motor units. After 2 weeks of denervation plus stimulation, peripheral and central regions of muscle fibers were examined by immunofluorescence microscopy with regard to density and distribution of Golgi complex, microtubules, glucose transporter GLUT4, centrosomes, and endoplasmic reticulum exit sites. In extrajunctional regions, fast pattern stimulation preserved normal fast characteristics of all markers in EDL type IIB/IIX fibers, although inducing changes toward the fast phenotype in originally slow type I SOL fibers, such as a 1.5-fold decrease of the density of Golgi elements at the fiber surface. Slow pattern stimulation had converse effects such as a 2.2-fold increase of the density of Golgi elements at the EDL fiber surface. In junctional regions, where fast and slow fibers are similar, both stimulation patterns prevented a denervation-induced accumulation of GLUT4. The results indicate that patterns of muscle impulse activity, as normally imposed by motor neurons, play a major role in regulating the organization of Golgi complex and related proteins in the extrajunctional region of muscle fibers.

The organization of the Golgi complex and microtubules in skeletal muscle is fiber type-dependent

Skeletal muscle has a nonconventional Golgi complex (GC), the organization of which has been a subject of controversy in the past. We have now examined the distribution of the GC by immunofluorescence and immunogold electron microscopy in whole fibers from different rat muscles, both innervated and experimentally denervated. The total number of GC elements, small polarized stacks of cisternae, is quite similar in all fibers, but their intracellular distribution is fiber type-dependent. Thus, in slow-twitch, type I fibers, approximately 75% of all GC elements are located within 1 micrometer from the plasma membrane, and each nucleus is surrounded by a belt of GC elements. In contrast, in the fast-twitch type IIB fibers, most GC elements are in the fiber core, and most nuclei only have GC elements at their poles. Intermediate, type IIA fibers also have an intermediate distribution of GC elements. Interestingly, the distribution of microtubules, with which GC elements colocalize, is fiber type-dependent as well. At the neuromuscular junction, the distribution of GC elements and microtubules is independent of fiber type, and junctional nuclei are surrounded by GC elements in all fibers. After denervation of the hindlimb muscles, GC elements as well as microtubules converge toward a common pattern, that of the slow-twitch fibers, in all fibers. Our data suggest that innervation regulates the distribution of microtubules, which in turn organize the Golgi complex according to muscle fiber type.

A simple method to enrich an Agrobacterium-transformed population for plants containing only T-DNA sequences

A simple modification to standard binary vector design has been utilized to enrich an Agrobacterium-transformed population for plants containing only T-DNA sequences. A lethal gene was incorporated into the non-T-DNA portion of a binary vector, along with a screenable marker. The resulting class of vectors is designated as NTL T-DNA vectors (non-T-DNA lethal gene-containing T-DNA vectors). The lethal gene used here is a CaMV 35S-barnase gene with an intron in the coding sequence (barnase-INT); the screenable marker is a pMAS-luciferase gene with an intron in the coding sequence (LUC-int). To evaluate the utility of this vector design, tobacco plants were transformed with either the NTL T-DNA vector or a control vector from which most of the barnase-INT gene was deleted. Populations of 50 transgenic plants were scored for LUC expression. The results indicated a dramatic reduction in the presence of non-T-DNA sequences in the transgenic population using the NTL T-DNA vector. Only one transgenic plant was found to be LUC+ using the NTL vector, compared with 42 of 50 plants using the control vector. Importantly, the efficiency with which transformed tobacco plants was obtained was reduced by no more than 30%. The reduction in LUC+ transgenics was partially reversed when a barstar-expressing tobacco line was transformed, indicating that barnase expression was responsible for the reduced frequency of incorporating non-T-DNA sequences. Similar transformation results were obtained with tomato and grape. The incorporation of a barnase-INT gene outside the left border appears to provide a generally applicable tool for enriching an Agrobacterium-transformed population for plants containing only T-DNA sequences.

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.

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.

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.