The Identification and Heterologous Expression of the Biosynthetic Gene Cluster Encoding the Antibiotic and Anticancer Agent Marinomycin

With the rise in antimicrobial resistance, there is an urgent need for new classes of antibiotic with which to treat infectious disease. Marinomycin, a polyene antibiotic from a marine microbe, has been shown capable of killing methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecium (VREF), as well as having promising activity against melanoma. An attractive solution to the photoprotection of this antibiotic has been demonstrated. Here, we report the identification and analysis of the marinomycin biosynthetic gene cluster (BGC), and the biosynthetic assembly of the macrolide. The marinomycin BGC presents a challenge in heterologous expression due to its large size and high GC content, rendering the cluster prone to rearrangement. We demonstrate the transformation of Streptomyces lividans using a construct containing the cluster, and the heterologous expression of the encoded biosynthetic machinery and production of marinomycin B.

Expanding flavone and flavonol production capabilities in Escherichia coli

Flavones and flavonols are important classes of flavonoids with nutraceutical and pharmacological value, and their production by fermentation with recombinant microorganisms promises to be a scalable and economically favorable alternative to extraction from plant sources. Flavones and flavonols have been produced recombinantly in a number of microorganisms, with Saccharomyces cerevisiae typically being a preferred production host for these compounds due to higher yields and titers of precursor compounds, as well as generally improved ability to functionally express cytochrome P450 enzymes without requiring modification to improve their solubility. Recently, a rapid prototyping platform has been developed for high-value compounds in E. coli, and a number of gatekeeper (2S)-flavanones, from which flavones and flavonols can be derived, have been produced to high titers in E. coli using this platform. In this study, we extended these metabolic pathways using the previously reported platform to produce apigenin, chrysin, luteolin and kaempferol from the gatekeeper flavonoids naringenin, pinocembrin and eriodictyol by the expression of either type-I flavone synthases (FNS-I) or type-II flavone synthases (FNS-II) for flavone biosynthesis, and by the expression of flavanone 3-dioxygenases (F3H) and flavonol synthases (FLS) for the production of the flavonol kaempferol. In our best-performing strains, titers of apigenin and kaempferol reached 128 mg L-1 and 151 mg L-1 in 96-DeepWell plates in cultures supplemented with an additional 3 mM tyrosine, though titers for chrysin (6.8 mg L-1) from phenylalanine, and luteolin (5.0 mg L-1) from caffeic acid were considerably lower. In strains with upregulated tyrosine production, apigenin and kaempferol titers reached 80.2 mg L-1 and 42.4 mg L-1 respectively, without the further supplementation of tyrosine beyond the amount present in the rich medium. Notably, the highest apigenin, chrysin and luteolin titers were achieved with FNS-II enzymes, suggesting that cytochrome P450s can show competitive performance compared with non-cytochrome P450 enzymes in prokaryotes for the production of flavones.

Harnessing intercellular signals to engineer the soil microbiome

Covering: Focus on 2015 to 2020Plant and soil microbiomes consist of diverse communities of organisms from across kingdoms and can profoundly affect plant growth and health. Natural product-based intercellular signals govern important interactions between microbiome members that ultimately regulate their beneficial or harmful impacts on the plant. Exploiting these evolved signalling circuits to engineer microbiomes towards beneficial interactions with crops is an attractive goal. There are few reports thus far of engineering the intercellular signalling of microbiomes, but this article argues that it represents a tremendous opportunity for advancing the field of microbiome engineering. This could be achieved through the selection of synergistic consortia in combination with genetic engineering of signal pathways to realise an optimised microbiome.

A marine viral halogenase that iodinates diverse substrates

Oceanic cyanobacteria are the most abundant oxygen-generating phototrophs on our planet, and therefore, important to life. These organisms are infected by viruses called cyanophages, recently shown to encode metabolic genes that modulate host photosynthesis, phosphorus cycling and nucleotide metabolism. Herein, we report the characterisation of a wild type flavin-dependent viral halogenase (VirX1) from a cyanophage. Notably, halogenases have been previously associated with secondary metabolism, tailoring natural products. Exploration of this viral halogenase reveals it capable of regioselective halogenation of a diverse range of substrates, with a preference for forming aryl iodide species; this has potential implications for the metabolism of the infected host. Until recently, a flavin-dependent halogenase (FDH) capable of iodination in vitro had not been reported. VirX1 is interesting from a biocatalytic perspective showing strikingly broad substrate flexibility, and a clear preference for iodination, as illustrated by kinetic analysis. These factors together render it an attractive tool for synthesis.

Defining the genes for the final steps in biosynthesis of the complex polyketide antibiotic mupirocin by Pseudomonas fluorescens NCIMB10586

The mupirocin trans-AT polyketide synthase pathway, provides a model system for manipulation of antibiotic biosynthesis. Its final phase involves removal of the tertiary hydroxyl group from pseudomonic acid B, PA-B, producing the fully active PA-A in a complex series of steps. To further clarify requirements for this conversion, we fed extracts containing PA-B to mutants of the producer strain singly deficient in each mup gene. This additionally identified mupM and mupN as required plus the sequence but not enzymic activity of mupL and ruled out need for other mup genes. A plasmid expressing mupLMNOPVCFU + macpE together with a derivative of the producer P. fluorescens strain NCIMB10586 lacking the mup cluster allowed conversion of PA-B to PA-A. MupN converts apo-mAcpE to holo-form while MupM is a mupirocin-resistant isoleucyl tRNA synthase, preventing self-poisoning. Surprisingly, the expression plasmid failed to allow the closely related P. fluorescens strain SBW25 to convert PA-B to PA-A.

Fine Tuning of Antibiotic Activity by a Tailoring Hydroxylase in a Trans-AT Polyketide Synthase Pathway

The addition or removal of hydroxy groups modulates the activity of many pharmacologically active biomolecules. It can be integral to the basic biosynthetic factory or result from associated tailoring steps. For the anti-MRSA antibiotic mupirocin, removal of a C8-hydroxy group late in the biosynthetic pathway gives the active pseudomonic acid A. An extra hydroxylation, at C4, occurs in the related but more potent antibiotic thiomarinol A. We report here in vivo and in vitro studies that show that the putative non-haem-iron(II)/α-ketoglutaratedependent dioxygenase TmuB, from the thiomarinol cluster, 4-hydroxylates various pseudomonic acids whereas C8-OH, and other substituents around the tetrahydropyran ring, block enzyme action but not substrate binding. Molecular modelling suggested a basis for selectivity, but mutation studies had a limited ability to rationally modify TmuB substrate specificity. 4-Hydroxylation had opposite effects on the potency of mupirocin and thiomarinol. Thus, TmuB can be added to the toolbox of polyketide tailoring technologies for the in vivo generation of new antibiotics in the future.

Outreach with street-involved youth: a quantitative and qualitative review of the literature

Outreach workers meet with street-involved youth in their environment, and attempt to engage them in services vital to their well-being. The goal of this study is to conduct a systematic review of both the quantitative and qualitative research that explores outreach with street-involved youth. Using 16 outreach programs with quantitative information, our meta-analysis found that 63% of youth who are contacted through outreach later participate in the offered service. Our meta-synthesis of 31 qualitative articles on outreach uncovered 13 themes across 4 conceptual domains of interest. Themes pertaining to the therapeutic relationship, flexibility, and youth-centric programming had a large presence among the majority of the articles. Outreach with street-involved youth, when characterized through a strong bond between worker and youth, is an effective strategy for involving youth in agency services.

Eating disturbances in girls with diabetes: the contribution of adolescent self-concept, maternal weight and shape concerns and mother-daughter relationships

BACKGROUND

This study examined the relative contribution of adolescent self-concept, maternal weight and shape concerns (WSC), and mother-daughter relationships to eating disturbances among girls with type 1 diabetes mellitus (DM).

METHOD

Eighty-eight adolescent girls (mean = 15.0 years, S.D. = 2.2) and their mothers completed self-report measures of disordered eating and weight control behaviours, with teens also reporting on disturbed eating and body attitudes. Based on reported symptoms, adolescents were classified as highly (N = 18), mildly (N = 30) and non-eating disturbed (N = 40). Self-concept was assessed by adolescent self-report. Mother-daughter relationships were assessed by adolescent self-report and by observed mother-daughter interactions that were rated using a macroanalytic coding system that assesses intimacy and autonomy in these relationships.

RESULTS

Hierarchical regressions illustrated that adolescent self-concept deficits, maternal WSC, and impaired mother-daughter relationships significantly predicted eating disturbances in girls with DM, accounting for 57% of the variance. Mothers who engaged in dieting and binge-eating were more impaired in their ability to support their daughters' emerging autonomy. The quality of mother-daughter relationships partly mediated the influence of maternal WSC on adolescent eating disturbances. Moreover, the impact of maternal WSC and mother-daughter relationships on eating disturbances was mediated by adolescent self-concept.

CONCLUSIONS

Findings illustrate two pathways through which mother-daughter relationships may impact upon risk of eating disturbances in girls with DM and highlight the need to evaluate family-based interventions specifically tailored for this high-risk population.

Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth's mantle

Volatiles, most notably CO₂, are recycled back into the Earth's interior at subduction zones^1,2. The amount of CO₂ emitted from arc volcanism appears to be less than that subducted, which implies that a significant amount of CO₂ either is released before reaching the depth at which arc magmas are generated or is subducted to deeper depths. Few high-pressure experimental studies^3,4,5 have addressed this problem and therefore metamorphic decarbonation in subduction zones remains largely unquantified, despite its importance to arc magmatism, palaeoatmospheric CO₂ concentrations and the global carbon cycle⁶. Here we present computed phase equilibria to quantify the evolution of CO₂ and H₂O through the subduction-zone metamorphism of carbonate-bearing marine sediments (which are considered to be a major source for CO₂ released by arc volcanoes⁶). Our analysis indicates that siliceous limestones undergo negligible devolatilization under subduction-zone conditions. Along high-temperature geotherms clay-rich marls completely devolatilize before reaching the depths at which arc magmatism is generated, but along low-temperature geotherms, they undergo virtually no devolatilization. And from 80 to 180 km depth, little devolatilization occurs for all carbonate-bearing marine sediments. Infiltration of H₂O-rich fluids therefore seems essential to promote subarc decarbonation of most marine sediments. In the absence of such infiltration, volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile fluxes and represent a mechanism for return of carbon to the Earth's mantle.

ESkine, a novel beta-chemokine, is differentially spliced to produce secretable and nuclear targeted isoforms

Using the murine embryonal stem cell system, we have identified a novel gene encoding a highly divergent member of the beta-chemokine family of proinflammatory mediators and have called this protein ESkine. Much of the coding sequence for ESkine overlaps with the 3'-end of a novel interleukin 11 receptor alpha-like sequence on murine chromosome 4. ESkine is produced as two splice variants. One of these variants encodes a classical chemokine with an associated signal peptide, while the other variant (PESKY) possesses the main body of the chemokine but has replaced the signal peptide with an alternative stretch of amino acids that allows for nuclear targeting of this isoform. This differential splicing arises as a result of alternative 5' exon usage. These differentially spliced forms are expressed at discrete tissue loci. Thus, while ESkine is highly expressed in the placenta, PESKY is mainly expressed in the Testes and brain and weakly in the developing embryo. Studies on the proinflammatory properties of ESkine reveal it to be active in inducing polarization of CD4(+) T cells but to be inactive on other hemopoietic cellular populations.

The BPV-4 co-carcinogen quercetin induces cell cycle arrest and up-regulates transcription from the LCR of BPV-4

Bracken fern is the environmental co-carcinogen of BPV-4 in the induction of neoplasias of the upper alimentary canal of cattle. The flavonoid quercetin is one of the most potent and best characterised mutagens present in the fern. We have shown that transfection with BPV-4 DNA and exposure to a single dose of quercetin leads to tumorigenic transformation of primary bovine cells. We now show that quercetin induces cell cycle arrest and up-regulates transcription from the BPV-4 long control region (LCR). This up-regulation is mediated by a 21 nucleotide-long cis-element in the LCR, designated QRE-1, which is located immediately downstream of the TATA box. Cellular proteins bind to QRE-1 and removal or substitution of QRE-1 lead to the abrogation of the response to quercetin. As expression of the viral oncogenes is controlled by the LCR, perturbation in this control and increased oncoprotein expression are likely to contribute to fully malignant cell transformation by overcoming the cell cycle arrest induced by quercetin, thus forcing damaged cells to proliferate.

Locally recurrent prostate tumors following either radiation therapy or radical prostatectomy have changes in Ki-67 labeling index, p53 and bcl-2 immunoreactivity

PURPOSE

We compare the biological phenotype of recurrent prostatic tumors after definitive local therapy (radiation or radical prostatectomy) with that of the same tumors before treatment.

MATERIALS AND METHODS

Cellular proliferation (Ki-67 labeling index), p53 nuclear reactivity and bcl-2 immunoreactivity were determined in pretreatment and posttreatment tumor specimens from 13 patients with local tumor recurrence following radiation, and in 18 patients with local tumor recurrence following radical prostatectomy.

RESULTS

Mean Ki-67 labeling index increased approximately 2-fold in locally recurrent tumors after radiation (10.5 versus 5.6%, p=0.0008) or surgery (6.0 versus 3.2%, p=0.0025) when compared with pretreatment tumors. We noted p53 nuclear reactivity in a significantly higher proportion of recurrences than in pretreatment tumors following radiation (54 versus 8%, p=0.032) and surgery (39 versus 5%, p=0.022). Although bcl-2 immunoreactivity was also seen in a higher proportion of recurrent tumors, this difference did not reach statistical significance for either radiation or surgery.

CONCLUSIONS

Recurrent tumors following either radiation or surgery differ significantly from the corresponding pretreatment tumors with respect to cellular proliferation and p53 nuclear reactivity.

Cryosurgery for locally advanced (T3) prostate cancer

Cryosurgery of prostate cancer results in epithelial destruction. An undetectable serum prostate-specific antigen and absence of cancer by biopsy is related to cancer stage. The best results are seen in patients with low stage disease. Recurrence is more common in those patients with seminal vesicle invasion or apical involvement by cancer. Morbidity is not insignificant. The risk of urinary retention can be minimized by the use of an effective urethral warming device and more prolonged urethral catheterization after the procedure. Impotence is common in patients who undergo complete cryoablation of the prostate.

Accelerated tumor proliferation rates in locally recurrent prostate cancer after radical prostatectomy

PURPOSE

We compared the growth rates of primary cancer and prostatic fossa recurrence after radical prostatectomy.

MATERIALS AND METHODS

Tumor proliferative rates were studied in 26 patients with biopsy proved prostatic fossa recurrences after radical prostatectomy. Proliferation was calculated in the prostatectomy specimens and in recurrent cancer using Ki-67 antibody to detect dividing cells.

RESULTS

Mean and median labeling indexes for radical prostatectomy specimens were 2.96 and 2.51, respectively. Labeling indexes in locally recurrent tumors were significantly higher (mean 6.47, median 5.59, p < 0.001). The increase in labeling index between parent and recurrent tumors was unrelated to pathological staging at prostatectomy or interval from radical prostatectomy.

CONCLUSIONS

Tumors that recur locally after radical prostatectomy appear to have a higher proliferative rate compared to parent tumors.

Prostate-specific antigen after cryosurgical ablation of the prostate. Defining the appropriate response

The ability of prostate-specific antigen (PSA) to predict long-term cure after cryotherapy for localized prostate cancer is not known because experience with this treatment modality is limited; however, it appears that a PSA value of 0.5 ng/mL or less at 6 months or longer after cryotherapy is associated with a high probability of a negative post-treatment biopsy (greater than 95%). An undetectable PSA (less than 0.1 ng/mL) 3 months after the procedure is associated with a likelihood of a negative biopsy at 6 months and low, stable PSA levels up to 2 years.

Should cryosurgery be considered a therapeutic option in localized prostate cancer?

Cryosurgical ablation of the prostate currently is under investigation as a potential treatment for localized prostate cancer. Results to date indicate that the majority of patients have negative biopsies and a marked reduction in prostate-specific antigen levels following cryotherapy. This treatment, however, is associated with significant side effects, notably bladder outflow obstruction, impotence, and incontinence, and its long-term durability is still unknown.

Ultrasound evaluation of the penis for assessment of impotence

Image directed Doppler ultrasonography of the cavernous arteries provides functional, quantifiable assessment of penile arterial flow during a pharmacological erection. In this respect, this modality is superior to arteriography as a means of evaluating arteriogenic impotence. Peak flow velocity, arterial dilatation, and vessel pulsation are the most reliable ultrasonic indicators of arterial health. Aberrant arterial anatomy should be noted as this may contribute significantly to total penile blood flow. A thorough understanding of erectile physiology and anatomy is necessary to properly perform and interpret Doppler ultrasound results.

Cryosurgical treatment of localized prostate cancer (stages T1 to T4): preliminary results

PURPOSE

We determined the posttreatment biopsy results, prostate specific antigen (PSA) levels and complications associated with cryosurgical ablation of the prostate performed for localized prostate cancer.

MATERIALS AND METHODS

Within 18 months 102 patients underwent cryosurgery as definitive therapy for localized prostate cancer. Mean patient age was 68 years and 57% had advanced local disease (stage T3 or T4). Mean preoperative PSA was 21.8 ng./ml.

RESULTS

PSA was undetectable at 6 months in 48% of patients who received no androgen deprivation therapy following cryosurgery. Of 91 patients with postoperative biopsies 77% had no evidence of cancer but 71% had benign epithelial a elements. The complication rate (excluding impotence) was 51%. Biopsy and PSA results improved with experience and changes in technique, that is double freezing, more lateral placement of cryoprobes and more aggressive freezing beyond the prostatic capsule. The most recent cohort of 77 patients had a detectable PSA rate of 23% and a positive post-cryosurgical biopsy rate of 11%. The most common serious complication encountered was bladder outflow obstruction requiring transurethral resection in 23% of the patients. Impotence occurred in 84% of patients potent preoperatively.

CONCLUSIONS

Cryosurgical ablation of the prostate can result in negative posttreatment biopsies and undetectable serum PSA levels. However, it is associated with significant side effects and the long-term durability of the procedure is unknown.

Local recurrence after radical prostatectomy: characteristics in size, location, and relationship to prostate-specific antigen and surgical margins

OBJECTIVES

To define the sonographic characteristics of local cancer recurrence after radical prostatectomy.

METHODS

in 114 patients with an elevated prostate-specific antigen (PSA) and negative bone scan, 156 ultrasound-guided prostate fossa biopsies were carried out.

RESULTS

in 53.5%, biopsy proved local recurrence. More than one ultrasound-guided biopsy session was required to make the diagnosis in 33% of patients. Local recurrence was seen on ultrasound at the anastomotic site (66%), the bladder neck (16%), and posterior to the trigone (13%). in 5% of patients there was a normal-appearing anastomotic site. Transrectal ultrasound was greater than 90% sensitive in detecting local recurrence, but lacked specificity. Examination of the radical prostatectomy specimens in patients with local recurrence showed positive surgical margins in 66% and organ-confined disease in 20%.

CONCLUSIONS

Transrectal ultrasonography is a useful adjunct to PSA and digital rectal examination in the detection of local recurrences following radical prostatectomy.

Voiding dysfunction in human immunodeficiency virus infections

PURPOSE

We prospectively evaluated the current spectrum of urodynamic pathology in patients infected with human immunodeficiency virus (HIV) who presented with voiding dysfunction.

MATERIALS AND METHODS

We obtained a directed genitourinary and neurological history, and performed a physical examination and urodynamic testing in 18 patients. A 4-channel membrane urethral catheter was used to record intravesical and intraurethral pressures simultaneously.

RESULTS

Detrusor hyperreflexia was present in 28% of our patients and detrusor-sphincter dyssynergia in 28%. Detrusor areflexia, previously described as the most frequent abnormality, was uncommon in our series (6% of patients).

CONCLUSIONS

This changing proportion of urodynamic diagnoses may reflect a changing pattern of neurological manifestations of HIV infection due to more aggressive management. Urodynamic evaluation remains critical for precise diagnosis and treatment in patients with HIV who present with urinary symptoms.

Prevalence and repair of inguinal hernias in children with bladder exstrophy

PURPOSE

We delineated the prevalence, recurrence rates and optimal treatment of inguinal hernia in the exstrophy population.

MATERIALS AND METHODS

Of 181 children with exstrophy followed at our hospital inguinal hernias developed in 121 (66.8%).

RESULTS

In a 12-year period inguinal hernias developed in 81.8% of the boys and 10.5% of the girls. In 18.2% of the cases the hernia was repaired via a preperitoneal approach at the same time as exstrophy closure. The remaining patients underwent an inguinal operation. Most patients had a wide defect at the internal ring in addition to a patent processus vaginalis. The overall recurrence rate was 8.3%. The incidence of synchronous or asynchronous bilaterality was 81.8%.

CONCLUSIONS

Children with bladder exstrophy should be carefully examined for inguinal hernias before bladder closure. If a unilateral hernia is present, the contralateral side should be explored. Careful preperitoneal repair should emphasize repair of the internal ring.

Renal transplantation in patients with posterior urethral valves: favorable long-term outcome

PURPOSE

We assessed the long-term efficacy of renal transplantation in patients with posterior urethral valves.

MATERIALS AND METHODS

We reviewed the outcome in 23 patients with posterior urethral valves who underwent renal transplantation since 1979.

RESULTS

Graft survival was 69% at 5 years and 63% at 10 years. Seven patients with followup of 7 years or longer had a mean serum creatinine level of 1.5 mg/dl. Three patients (13%) demonstrated significant bladder dysfunction postoperatively.

CONCLUSIONS

Renal transplantation into a valve bladder is not associated with an abnormally high rate of failure. Deterioration of graft function secondary to lower urinary tract dysfunction is uncommon, with the majority of patients able to use the unmodified native bladder as a receptacle for the transplanted kidney.

Anterior bladder neck tube reconstruction at radical prostatectomy preserves functional urethral length--a comparative urodynamic study

OBJECTIVE

To characterize the continence mechanism of anterior bladder neck tube reconstruction at the time of radical prostatectomy using urodynamic evaluation.

PATIENTS AND METHODS

Prospective pre- and post-operative urodynamic profiles were performed in patients receiving conventional posterior 'racket handle' closures (control group--eight patients) and those receiving anterior bladder neck tube reconstruction at radical prostatectomy (study group--nine patients). Bladder neck tube reconstruction was performed by constructing 2 cm anterior bladder neck tubes over 22 F catheters.

RESULTS

A significantly longer post-operative functional urethral length was demonstrated using urodynamic assessment in patients receiving an anterior bladder neck tube closure (4.6 vs 3.4 cm; P < 0.01).

CONCLUSION

The anterior bladder neck tube reconstruction may promote urinary continence in patients undergoing radical prostatectomy by preservation of functional urethral length.

Penile intracavernous hematoma: diagnosis and surgical considerations

Soft tissue lesions of the penile corporeal bodies are rare and usually represent benign or malignant tumors, or Peyronie's disease. We report a unique case of a deep seated penile mass that at exploration proved to be an encapsulated hematoma of the inter-corporeal septum. Preoperative diagnosis of lesions of the corporeal bodies is facilitated by high resolution Doppler ultrasonography. Surgical exploration to exclude malignancy must be done unless the clinical and/or radiographic findings are unequivocally benign (Peyronie's plaque, simple cyst and so forth). Surgical exploration must be done according to the penile neurovascular anatomy to preserve potency and glanular sensation.

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.