Is retrograde ureterography indicated in pelviureteric junction obstruction?

The role of retrograde ureterography in the management of pelviureteric junction (PUJ) obstruction remains controversial and it has recently been reiterated that visualisation of the entire ureter on the affected side is mandatory owing to the high incidence of other ureteric abnormalities. In a review of 119 consecutive adult pyeloplasties carried out over a 9-year period, only 2 ureteric abnormalities were found in association with PUJ obstruction (duplex ureter/3-cm proximal stricture). Other anatomical abnormalities included aberrant vessels, fascial bands and renal anomalies. Most secondary abnormalities were undiagnosed prior to surgery and all were easily dealt with per-operatively.

Management of yolk sac tumors in children

The current trend toward minimizing treatment morbidity in children with yolk sac carcinomas is laudable. As most children will present with stage I disease and be cured by radical orchiectomy alone, careful surveillance is adequate with the knowledge that effective chemotherapy is available should tumor recur. All new yolk sac carcinomas in children should be reported to the Prepubertal Testicular Tumor Registry and should ideally be treated at institutions that have experience with surveillance protocols and pediatric oncology. In this way, prospective studies on treatment options can be initiated, and management controversies may ultimately be ironed out. In particular, more information is needed on patients more than 2 years old, as the literature to date is contradictory regarding the prognosis for these patients and the need for adjuvant therapy when the older child presents with stage I disease.

Intensive care in an Irish district general hospital--a three year review

The International Missionary Training Hospital is a 340-bed acute general hospital with maternity and paediatric units. It serves a population of 120,000 people within the North Eastern Health Board Area and has approximately 14,000 admissions each year. This report retrospectively reviews the activity of a three bedded, Intensive Therapy Unit (ITU) over the three year period July 1987-June 1990. 805 patients (1.9% of hospital admissions) with an average age of 55 +/- 22 years (mean +/- SD, range 14-94 years) were admitted to the unit. There were 458 males (57%) and 347 females (43%). 68% of the patients were admitted from the general wards and the remainder from the accident unit. 59% of the admissions were immediate postoperative cases. 82% of patients had APACHE scores less than 20. There was a wide diversity of medical and surgical diagnoses requiring treatment. 219 cases required one or more systems to be supported and 586 (73%) were admitted as high dependency cases. For those requiring ventilation, the average ventilation time was 3.2 days (range 0.5-23 days). The average length of stay within in the unit was 2.3 days (range 1-23 days). 86% of the patients were discharged to the wards, 11% died and 3% were transferred to external specialist care facilities. ITU's in district general hospitals serve as both critical care areas and high dependency units. In our opinion they produce a positive contribution to progressive patient care for high risk medical and surgical patients.

A unique tubulin antiserum attenuates the rate of poleward chromosome movement in anaphase

An antiserum against tubulin, NS20, was previously shown to specifically attenuate both fast axonal transport in vivo (Johnston, K. M. et al., Brain Res. 385, 38-45 (1986)) and in vitro (Johnston, K. M. et al., Cell Motil. Cytoskel. 7, 110-115 (1987)) and flagellar motility (Goldsmith, M. et al., Cell Motil. Cytoskel. 20, 249-262 (1991)). We hypothesized that NS20 blocked motility by binding to a multifunctional motor binding domain on the microtubules (MTs), or axonemes. Here we have examined the effect of microinjecting NS20, at metaphase, into dividing PtK2 cells. Plotting chromosome separation (CS) as a function of time, we report here that CS rates for anaphase A (chromosome-to-pole movement) were reduced by approximately 50% relative to uninjected controls. CS rates for anaphase B (spindle pole elongation) were unaffected by the NS20 antiserum. The inhibition of CS rate during anaphase A by NS20 was significantly greater than the inhibition caused by a control antitubulin serum (PC5). Two possible mechanisms underlying NS20's inhibition of CS during anaphase A were considered. NS20 could block the binding of a kinetochore-associated motor to kinetochore MTs (kMTs) or, alternatively, NS20 could stabilize kMTs against depolymerization. Our results favor the first alternative. In a cold-induced depolymerization assay, NS20 had no selective stabilizing effect on MTs. Moreover, we show that NS20 can selectively block the binding of a well characterized MT-associated motor (kinesin) to MTs, in vitro. These results suggest that NS20 may be defining a unique tubulin binding domain common to the motors underlying vesicle transport, flagellar motility, and chromosome movements during anaphase A.

A review of the nature and activity of a general surgical service in Ireland

The steady increase in hospital costs has led to demands for closer scrutiny of activity levels, workloads and outcomes. This study sought to examine these parameters in a typical district general surgical unit in this country. In the five year period 1985-89, 11,227 inpatient and 3,354 outpatient procedures were performed; 83% of the inpatient procedures were elective and 17% were emergency surgical operations. All inpatient procedures were categorized: minor (41%), intermediate (42%) and major (17%). A prospective one year review in 1990/1991 confirmed the accuracy of the retrospective data. During that year 2,335 inpatient operations and 765 outpatient operations were performed, of which 80.5% of the inpatient operations were elective and 19.5% were emergency procedures. Of these 16.6% were major, 34.4% intermediate and 49% were minor operations. 60% of the operations were performed by consultants. The in-hospital surgical mortality for the 1 year prospective review was 1.35% and the perioperative mortality was 0.64%. The overall operation morbidity rate was 9% and the procedure-related morbidity was 4.7%. The wound infection rate was 2%. Of the common operations performed throughout the six year study period appendicectomies and external hernia operations accounted for 20% of the caseload; 14% were urological, 7% were breast and 6% were biliary operations. The average waiting time for elective admissions was less than 4 weeks. The average length of hospital stay and the bed occupancy rates did not change.

Neuropil threads of Alzheimer's disease show a marked alteration of the normal cytoskeleton

Abnormal neurites, neuropil threads, are a widespread and characteristic lesion of Alzheimer's disease likely to play a major role in the cognitive impairment of this disease. Contrary to normal neurites, neuropil threads contain straight and paired helical filaments that contain the microtubule-associated protein tau and ubiquitin. It is not known whether these abnormal filaments are added to or replace the normal cytoskeleton. In this study, we examined the fine structure of neuropil threads and carried out a morphometric analysis of the neurofilaments and abnormal filaments contained in the neuropil threads by using an antiserum to tau and colloidal gold immuno-electron microscopy. Almost 70% of the neuropil threads contained straight or paired helical filaments with no neurofilaments. The total number of filaments in each neuropil thread remained essentially unchanged either when straight or paired helical filaments were present alone or when they coexisted either together or with neurofilaments. When the three types of filaments were expressed as a proportion of the total, a linear inverse correlation was found between neurofilaments and straight filaments as well as between straight and paired helical filaments. Approximately 10% of the neuropil threads were found to be myelinated axons. It is concluded that straight filaments are likely to replace neurofilaments, that they in turn might be replaced by paired helical filaments, and that this process occurs in axons as well as dendrites.

Conserved beta-tubulin binding domain for the microtubule-associated motors underlying sperm motility and fast axonal transport

An antiserum against tubulin, NS20, has been previously shown to inhibit anterograde and retrograde axonal transport by 50% in vivo and in vitro. We report here that Protein A purified NS20 antibodies also attenuate sperm motility by 50% in demembranated sea urchin sperm. This inhibition is absorbed out by preincubating the NS20 antibodies with a biochemically purified porcine microtubule preparation, with recombinant Trypanosoma beta- (but not alpha-) tubulin and most specifically, with a 37 amino acid (a.a.) synthetic peptide corresponding to a domain near (but not including) the porcine beta-tubulin C terminus. Furthermore, addition of this beta-tubulin peptide alone is sufficient to attenuate motility by 50% in demembranated sperm, indicating that this critical 37a.a. NS20 antigen is a motor binding domain. Together, the results suggest that at least two phenotypically distinct forms of microtubule-based motility, axonal transport and flagellar beating, are homologous at the fundamental level of the microtubule domains (the beta-tubulin peptide and we suggest a distinct but similarly located alpha-tubulin domain) mediating the attachment of tubulin-associated motors.

A model for the modulation of muscle cell determination and differentiation by growth factors

In an adult organism three principal types of muscle tissue can be found: skeletal, smooth, and cardiac. While each display subtle differences, for the most part they express a common set of genes that are representative of differentiated muscle. Several in vitro muscle cell lines have provided clues as to how the developmental programs of muscle cell proliferation, determination, and differentiation are controlled. In this paper we will explore recent advances in our understanding of how growth factors, acting through specific signal transduction pathways, control muscle gene expression. The transcription of muscle genes is controlled by specific cis-acting regulatory sequences. We will discuss how growth factors may exert their effects on muscle genes by modulating the expression of nuclear DNA-binding proteins that directly regulate muscle gene expression.

Pattern formation in the mammalian forebrain: patch neurons from the rat striatum selectively reassociate in vitro

Mechanisms involved in the developmental organization of the rat striatum were investigated in vitro. The neurons of the patch and matrix compartments were preferentially labeled in vivo with a [3H]thymidine injection on embryonic day (E) 13 or 18, respectively. Two or 7 days later the striatum was removed, dissociated into a single cell suspension and plated on a collagen-coated substrate. After 5 days in culture the neurons had migrated into aggregates. Within an individual aggregate, neurons labeled on E13 tended to clump together, whereas neurons labeled on E18 were randomly dispersed. Comparing between aggregates, [3H]thymidine-labeled E13 cells were located in aggregates containing numerous other labeled E13 cells, whereas [3H]thymidine-labeled E18 cells were dispersed randomly between aggregates. These results suggest that early born striatal neurons (primarily patch cells) selectively associate with each other, and that this process may be crucial to the developmental compartmentalization of the rat striatum.

Interleukin 3-stimulated proliferation is sensitive to pertussis toxin: evidence for a guanyl nucleotide regulatory protein-mediated signal transduction mechanism

Interleukin 3 (IL-3) stimulates several biochemical and biological responses in IL-3-dependent tissue culture cells. We examined the possibility that guanyl nucleotide regulatory (G) proteins may transduce signals from IL-3 receptors. We report here that pertussis toxin (PT), which can covalently modify a subclass of G proteins, is capable of inhibiting IL-3-stimulated proliferation in a dose-dependent fashion. PT inhibition of IL-3-stimulated proliferation could be overcome by using the Ca++ ionophore A23187 in conjunction with TPA. PT could also inhibit IL-3-stimulated hexose transport. In the absence of IL-3, hexose transport could be stimulated by introducing GTP-gamma S into intact cells. From these data we propose that IL-3 receptors transduce signals via a PT-sensitive G protein(s).

FGF and EGF act synergistically to induce proliferation in BC3H1 myoblasts

BC3H1 muscle cells proliferate when grown in high concentrations of FBS (20%). Lowering the FBS concentration to 0.5% causes the cells to stop proliferating and is permissive for the morphological and biochemical differentiation of BC3H1 cells. Exposure of differentiated BC3H1 myocytes to high concentrations of serum or to the purified growth factors FGF or TGF-b induced a shutdown of this differentiation program but did not induce cell proliferation (Olson et al., J. Cell Biol., 103:1799-1805, 1986; Lathrop et al., J. Cell Biol., 100:1540-1547, 1985, and J. Cell Biol., 101:2194-2198, 1985). We explored the possibility that BC3H1 cells require factors to act synergistically to induce proliferation. We found that EGF and FGF function in a synergistic fashion to stimulate BC3H1 proliferation. Moreover, the temporal requirement for these growth factors suggest that they are functioning as competence and progression factors for BC3H1 cell proliferation.

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. II. Two signaling pathways distinguished by pertussis toxin and a potential role for the ras oncogene

In the preceding report (Kelvin, D.J., G. Simard, H.H. Tai, T.P. Yamaguchi, and J.A. Connolly. 1989. J. Cell Biol. 108:159-167) we demonstrated that pertussis toxin (PT) blocked proliferation and induced differentiation in BC3H1 muscle cells. In the present study, we have used PT to examine specific growth factor signaling pathways that may regulate these processes. Inhibition of [3H]thymidine by PT in 20% FBS was reversed in a dose-dependent fashion by purified fibroblast growth factor (FGF). In 0.5% FBS, the normally induced increase in creatine kinase (CK) activity was blocked by FGF in both the presence and absence of PT. Similar results were obtained with purified epidermal growth factor (EGF). We subsequently examined the effect of a family of growth factors linked to inositol lipid hydrolysis and found that thrombin, like FGF, would increase [3H]thymidine incorporation and block CK synthesis. However, PT blocked thymidine incorporation induced by thrombin, and blocked the inhibition of CK turn-on in 0.5% FBS by thrombin. The ras oncogene, a G protein homologue, has previously been shown to block muscle cell differentiation in C2 muscle cells (Olson, E.N., G. Spizz, and M.A. Tainsky. 1987. Mol. Cell. Biol. 7:2104-2111); we have characterized a BC3H1 cell line, BCT31, which we transfected with the val12 oncogenic Harvey ras gene. This cell line did not express CK in response to serum deprivation. Whereas [3H]thymidine incorporation was inhibited by 70-80% by increasing doses of PT in control cells, BCT31 cells were only inhibited by 15-20%. ADP ribosylation studies indicate this PT-insensitivity is not because of the lack of a PT substrate in this cell line. Furthermore, PT could not induce CK expression in BCT31 cells as it did in parental cells. We conclude that there are at least two distinct growth factor pathways that play a key role in regulating proliferation and differentiation in BC3H1 muscle cells, one of which is PT sensitive, and postulate that a G protein is involved in transducing signals from the thrombin receptor. We believe that ras functions in the transduction of growth factor signals in the nonPT-sensitive pathway or downstream from the PT substrate in the second pathway.

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. I. A pertussis toxin-sensitive pathway is involved

Cells of the nonfusing muscle cell line BC3H1 stop proliferating and express a family of muscle-specific proteins when the FBS concentration is reduced from 20 to 0.5% (Munson, R., K.L. Caldwell, and L. Glaser. 1982. J. Cell Biol. 92:350-356). Several growth factors have been shown to block differentiation in this cell line. To begin to investigate the potential role of G proteins in signal transducing pathways from these receptors, we have examined the effects of cholera toxin (CT) and pertussis toxin (PT) on proliferation and differentiation in BC3H1 cells. PT specifically ADP ribosylates a protein with an apparent molecular mass of 40 kD in BC3H1 cell membranes, whereas CT specifically ADP ribosylates three proteins of 35-43 kD. When added to exponentially growing cells in 20% FBS, CT and PT inhibited [3H]thymidine incorporation by up to 75% in a dose-dependent fashion. We found the synthesis of creatine kinase (CK) and skeletal muscle myosin light chain was reversibly induced in cells in 20% FBS treated with PT, but no increased synthesis was seen in cells treated with CT or in control cells; Northern analysis indicated this induction was at the level of mRNA. In cells shifted to 0.5% FBS, CT inhibited the normally induced synthesis of CK whereas PT potentiated it by approximately 50%. Forskolin also inhibited growth in 20% FBS and differentiation in 0.5% FBS medium in a dose-dependent fashion. both forskolin and CT elevated cAMP levels compared with control or PT-treated cells, suggesting that CT is blocking proliferation and differentiation by elevating cAMP levels. These results establish that a PT-sensitive pathway is involved in regulating proliferation and differentiation in BC3H1 cells, and we postulate that PT functions by ADP ribosylating a G protein that transduces signals from growth factor receptors in these cells.

The disappearance of a cyclin-like protein and the appearance of statin is correlated with the onset of differentiation during myogenesis in vitro

We have used monoclonal antibodies to statin (S-44) and a cyclin-like protein (S-132) to examine the distribution of these two antigens in proliferating and in nonproliferating populations of cells. We have found that this cyclin-like protein is present in proliferating fibroblasts, whereas statin is absent from these same cell populations; in contrast, in senescent populations of fibroblasts the cyclin-like antigen disappears and statin labeling of nuclei appears. During myogenesis in rat muscle cell cultures, S-132 labeling is present in proliferating myoblasts and disappears after cells fuse and differentiate as multinucleated myotubes. In contrast, statin is absent from proliferating myoblasts, but appears when these cells become postmitotic and begin to differentiate. Similar results were seen during chick myogenesis. We have also found similar results during serum-starvation-induced differentiation in neuroblastoma cells. These results indicate that the cyclin-like protein disappears and statin appears upon commitment to differentiation in vitro, and the presence or the absence of these proteins appears to provide cellular markers for the transition from the proliferative to the nonproliferative state during differentiation.

Membrane glycoproteins are involved in the differentiation of the BC3H1 muscle cell line

The nonfusing muscle cell line BC3H1 expresses a family of muscle-specific proteins when the fetal bovine serum (FBS) concentration is reduced from 20 to 1%. We have used a series of glycosylation inhibitors to assess the role played by glycoproteins in the initiation of differentiation in this cell line. Tunicamycin (TNM) and 2-deoxy-D-glucose, added to cells when the FBS concentration was reduced, blocked creatine phosphokinase (CPK) induction by 70-95%. These effects were dose dependent and reversible. TNM and 2-deoxy-D-glucose also reversed CPK induction in differentiated cells. Leupeptin and N-acetylglucosamine did not reverse these effects. 1-Deoxynojirimycin, 1-deoxymannojirimycin, and swainsonine have no effect on induced CPK expression, whereas castanospermine, a glucosidase I inhibitor, blocked its induction completely. As attempts to use conditioned medium from cells grown in 1 or 20% FBS have no effect on this differentiation process we conclude that high mannose structures, but not complex form glycoproteins, bound to the surface of BC3H1 cells play a role in transducing signals for differentiation and are probable mediators of cell/cell contact.

A unique tubulin antibody which disrupts particle movement in squid axoplasm

Microtubules have been demonstrated to be a substrate for organelle transport and particle translocation in vitro and in vivo. Subsequent to a previous report of inhibition of axonal transport of exogenous tracers in vivo using antiserum NS-20 against tubulin (Johnston et al: Brain Res. 1986), we now show disruption of particle movement in extruded squid axoplasm using this unique immunological probe. Using video-enhanced contract-differential interference contrast (AVEC-DIC) microscopy, we examined the properties of particle movement along microtubules and demonstrated that both the velocity of particle movement and the numbers of particles moving are decreased in the presence of NS-20 antiserum or NS-20 affinity-purified antibodies but not in the presence of another antiserum against tubulin. The amount of microtubule substrate does not change in the presence of any of the antisera. In conclusion, we suggest that NS-20 antibodies bind near or at a site on the tubulin molecule which is critical in the mechanism of particle transport, and provide a direct immunological probe to examine the mechanism of microtubule involvement in axonal transport.

Inhibition of axonal transport 'in vivo' by a tubulin-specific antibody

We have used antibodies against the major proteins of the cytoskeleton-tubulin, the neurofilament triplet proteins and actin-as in vivo probes to determine the contribution of separate components of the cytoskeleton in axonal transport. The injection of either Fast Blue or wheat germ agglutinin conjugated horseradish peroxidase into the caudate nucleus of adult rats resulted in the retrograde transport of these tracers to the neuronal cell bodies in the substantia nigra pars compacta. In experimental animals these tracer injections were immediately preceded by injections of antiserum against tubulin, neurofilament triplet protein or actin, into multiple sites in the caudate. Preimmune serum injection preceded tracer injection as a control in the contralateral caudate of the same animal. One antiserum against electrophoretically purified pig brain tubulin (NS-20) produced a dramatic decrease in the normal retrograde and anterograde transport of both tracers to the SN. Other antisera against tubulin, as well as neurofilament and actin antisera, had no effect on the axonal transport of the tracers. Affinity purified antibodies prepared from the NS-20 antitubulin serum also blocked axonal transport of the tracers. These results provide further support for a critical role of microtubules in axonal transport in vivo. Moreover, an antigenic determinant on tubulin that is uniquely recognized by the NS-20 antibodies may provide us with a way to define the site of association of transfer vesicles with microtubules.

Calmodulin and acetylcholine receptor clustering in embryonic chick myotubes

We have used the calmodulin antagonists, trifluoperazine (TFP) and calmidazolium, to study the potential role of this protein in the movement of acetylcholine receptors (AChRs) to and from the myotube membrane, as well as in the formation of clusters of AChRs within the plasma membrane. Neither calmidazolium (up to 10(-6) M) nor TFP (10(-5) M) inhibited receptor degradation or the incorporation of new receptors (12 to 24 h). In addition, neither drug blocked the increased synthesis of receptors induced by chick brain extract, nor significantly affected AChR clusters already in the plane of the membrane at the time of drug addition. However, both drugs blocked new receptor clusters (induced by a basement membrane extract from Torpedo electric organ) from forming. These results indicate that receptors can move to and from the cell membrane in a calmodulin-independent fashion, but movement in the plane of the membrane to form a cluster requires the participation of calmodulin.

Interleukin 3 and cell cycle progression

Interleukin 3 (IL-3) is a regulatory glycoprotein required for the proliferation and differentiation of cells from many if not all hemopoietic lineages. With the emergence of the competence-progression model of cell proliferation, which predicts that growth factors function at specific stages of the cell cycle, we examined the possibility that IL-3 functions at a specific stage of the cell cycle. C-63 cells were developed as a cell line from normal murine bone marrow. They have a mast cell phenotype and require pokeweed-stimulated spleen cell-conditioned medium (CM), a rich source of IL-3, for their continued growth. Exponentially growing cells were transferred from growth medium, which contains CM, to medium lacking CM or IL-3. After 24 hours, cell viability had decreased 40-50%. The remaining viable cells did not incorporate 3H-thymidine, and displayed a single peak at G1 in a DNA histogram. Restimulation of these cells with CM or IL-3 resulted in a dramatic rise in 3H-thymidine uptake 20-24 hours after restimulation. DNA histograms of restimulated cultures indicated that the cells were progressing in a wave-like fashion throughout the remainder of the cell cycle. The length of time necessary for cells to be in contact with CM or IL-3 before they could progress into the remainder of the cell cycle was also examined. Cells incubated with CM or IL-3 for less than 16 hours could not progress into S phase, whereas cells incubated for 16 hours or longer could progress into S phase and through the remainder of the cell cycle. These data suggest that IL-3 exerts its function at a specific stage of the cell cycle.

Centrioles are lost as embryonic myoblasts fuse into myotubes in vitro

Embryonic chick myoblasts possess an extensive network of cytoplasmic microtubules which emanate from a single, perinuclear centrosome containing a microtubule-organizing center (MTOC) and the centrioles. However, after myoblasts fuse into myotubes the centrosome is no longer apparent, and instead long parallel arrays of microtubules are seen. From ultrastructural studies on developing muscle tissue, it has been proposed that centrioles are present in myoblasts but are absent from fused muscle fibers. We have examined this hypothesis in vitro in cultures of chick embryonic muscle cells using sera which specifically label centrioles. Almost all (90-97%) mononucleated cells in these cultures, including myoblasts aligned just prior to fusion, contain a pair of centrioles in close proximity to the nucleus. However, in newly fused multinucleated myotubes as well as in older myotubes that had developed myofibrils, centrioles were rarely found (1-10% positive cells). This study thus provides direct evidence for a loss of centrioles from muscle cells soon after they fuse to form myotubes.

Microtubules and the formation of acetylcholine receptor clusters in chick embryonic muscle cells

We have used the microtubule-stabilizing drug taxol to examine the relationship between microtubules and the appearance and cell surface distribution of acetylcholine receptors (AChRs) in primary cultures of chick embryonic muscle cells. Taxol at a 5-microM concentration induced the large scale polymerization of tubulin in muscle cells that was most obvious as intermittent bundles of microtubules along the myotube. Prominent bundles of microtubules were also clearly visible in the fibroblasts. This concentration of taxol had no significant effect on the incorporation rate, increased synthesis induced by brain extract or the total cell surface number of AChRs measured over a 24-h period. Thus, excess polymerization of microtubules does not affect the movement of receptors to the cell surface. However, when cell surface AChR distribution was examined using rhodamine-conjugated alpha-bungarotoxin, taxol treatment of myotubes was shown to induce the aggregation of receptors. If receptors were labeled before taxol addition, aggregation of these prelabeled receptors was also seen, a result indicating that taxol can induce the movement of receptors already in the membrane. We believe this evidence further implicates microtubules as being involved in the movement of these cell surface receptors in the plane of the myotube membrane.

Microtubules, microfilaments and the transport of acetylcholine receptors in embryonic myotubes

Both microtubules and microfilaments have been implicated in the exocytotic and endocytotic transport of coated and smooth surfaced membrane vesicles. We have reexamined this question by using specific pharmacological agents to disrupt these filaments and assess the effect on the movement of acetylcholine receptor (AChR) containing membrane vesicles in embryonic chick myotubes. Myotube cultures treated with nocodazole (0.6 microgram/ml) or colcemid (0.5 microgram/ml) (to disrupt microtubules) show only a 20-25% decrease in the number of cell surface AChRs after 48 h. Addition of chick brain extract (CBE) to cultured myotubes causes a significant increase in the total number of cell surface AChRs (measured by [125I]alpha-bungarotoxin (alpha-BGT) binding), thus providing us with a way to manipulate receptor and transport vesicle populations. Cultures treated with CBE plus nocodazole or colcemid show a 1.7-fold increase in AChR number over drug treatment alone, the same increase seen in cultures treated with CBE alone, although the total number remains about 20-25% less than that seen in control cultures. In cultures treated with cytochalasin D (0.2 microgram/ml) or dihydrocytochalasin B (5.0 micrograms/ml) (to disrupt microfilaments), 35 and 65% decreases in cell surface AChR number were seen after 48 h. However, in cultures treated with CBE and cytochalasin D, the same total number of AChRs was found as in cultures treated with CBE alone. No significant effects were seen with any of these drugs on the receptor incorporation rate (the appearance of new alpha-BGT-binding sites) after 6 h. The half-life for AChRs in control cultures was 23.0 h. In cytochalasin D and dihydrocytochalasin B it was 21.9 and 19.0 h, respectively; with colcemid and nocodazole, it increased to 37.1 and 28.1 h. These results suggest that non-myofibrillar microfilament bundles are not involved in the movement of AChR-containing membrane vesicles; further, the small effects seen with microtubule inhibitors tend to rule out a major role for microtubules in this transport.

Actin filaments and acetylcholine receptor clusters in embryonic chick myotubes

The tumor-promoting drug 4 beta-phorbol 12-myristate 13-acetate (TPA) causes the loss of myofibrils in primary cultures of chick embryonic myotubes [9]. When myofibrils in chick myotubes are dispersed by TPA treatment (5 X 10(-8) M), there remains a class of non-myofibrillar actin filaments that are sensitive to subsequent breakdown by cytochalasin D. Microfilament bundles in fibroblasts in the same cultures seem unaffected by this TPA treatment, but are also broken down by cytochalasin D (0.2 micrograms/ml); this dose has little effect on myofibrils. We have previously shown that treatment of chick myotubes with cytochalasins would destabilize clusters of acetylcholine receptors (AChRs) [6]. In order to further examine the relationship between actin filaments and cell surface AChRs, we have used the receptor-specific ligand alpha bungarotoxin (a-BGT) to study the fate of AChR clusters in drug-treated and control myotubes. Cells treated with TPA showed no loss in the number of receptor clusters present on their surface. However, if these cells were also treated with cytochalasin D, cluster number was reduced to approximately the same value as seen for cytochalasin treatment alone (50% of the control value). These results suggest that the cytoskeletal link to these cell surface receptors is not mediated by attachment to the alpha actin-containing myofibrils, but rather clustered AChRs are stabilized by a class of non-myofibrillar actin filaments.

Is there an endogenous bungarotoxin-like molecule in the vertebrate central nervous system?

It has been postulated that there is an endogenous bungarotoxin-like ligand in rat brain which can inhibit the binding of alpha-bungarotoxin (alpha-BGT) to its target nicotinic acetylcholine (ACh) receptor. We have examined this further by testing the ability of rat and chick brain and spinal cord extracts to inhibit the binding of alpha-BGT to ACh receptors in cultured chick and rat myotubes. We find no evidence for inhibition by any of these extracts, and thus cannot support the hypothesis of an endogenous alpha-BGT-like ligand.

Role of the cytoskeleton in the formation, stabilization, and removal of acetylcholine receptor clusters in cultured muscle cells

We have examined the effects of microtubule- and microfilament-disrupting drugs on the stability, formation, and removal of acetylcholine (ACh) receptors and ACh receptor clusters on the surface of aneurally cultured chick embryonic myotubes. (a) In muscle cell cultures, cytochalasin D (0.2 microgram/ml) or B (2.0 micrograms/ml) causes the dispersal of 50-60% of the existing clusters over a 24-h period (visualized with rhodamine-conjugated alpha-bungarotoxin); Colcemid (0.5 micrograms/ml) has no affect on these clusters. The total number of cell surface ACh receptors does not decline during this period (measured by [125I]alpha-bungarotoxin binding) in the presence of either drug. (b) When cells are treated with biotinylated alpha-bungarotoxin and fluorescent avidin, ACh receptors are cross-linked and rapidly internalized (Axelrod, D., 1980, Proc. Natl. Acad. Sci. USA., 77: 4823-4827). Within 6 h, I have found that 0-15% of the existing large clusters remain. Cytochalasin D or B had no effect on this removal of clusters; however, Colcemid completely prevented the removal of clusters from the cell surface. (c) Addition of chick brain extract to chick myotubes causes an increase in the synthesis and clustering of ACh receptors (Jessell et al., 1979, Proc. Natl. Acad. Sci. USA. 76: 5397-5401). Cytochalasin D caused a slight increase in the number of receptors synthesized in the presence of brain extract whereas Colcemid had no effect on the synthesis and insertion of new receptors into the plasma membrane induced by the brain extract. However, both drugs prevented the increase in the number of receptor clusters. These results are consistent with the hypothesis that receptor clusters are stabilized by actin-containing filaments, but that the movement of receptors in the plane of the membrane requires Colcemid-sensitive microtubules.