Biomarkers and clinical outcomes after tezepelumab cessation: extended follow-up from the 2-year DESTINATION study

BACKGROUND

Long-term tezepelumab treatment in the DESTINATION study (NCT03706079) resulted in reduced asthma exacerbations, reduced biomarker levels and improved lung function and symptom control in patients with severe, uncontrolled asthma.

OBJECTIVE

To explore time course of changes in biomarkers and clinical manifestations following treatment cessation after 2 years of tezepelumab treatment.

METHODS

DESTINATION was a two-year, phase 3, multicenter, randomized, placebo-controlled, double-blind study of tezepelumab treatment in patients (12-80 years old) with severe asthma. Patients received their last treatment doses at week 100 and could enroll in an extended follow-up (EFU) period from week 104 to 140. Change over time in key biomarkers and clinical outcomes were assessed in tezepelumab versus placebo recipients for 40 weeks after stopping treatment.

RESULTS

Of 569 patients enrolled in the EFU period, 426 were included in the analysis (289 received tezepelumab and 137 placebo). Over the 40-week period after the last tezepelumab dose, blood eosinophil counts (BEC), fractional exhaled nitric oxide (FeNO) levels and Asthma Control Questionnaire-6 scores gradually increased from weeks 4-10, with a gradual reduction in pre-bronchodilator forced expiratory volume in 1 second such that BECs, FeNO levels and clinical outcomes returned to placebo levels; however, none of these outcomes returned to baseline levels. Total immunoglobulin E levels increased later from week 28 and remained well below placebo and baseline levels during the 40-week period after the last tezepelumab dose.

CONCLUSION

This analysis demonstrates benefits of continued tezepelumab treatment in the management of patients with severe, uncontrolled asthma, compared with stopping treatment after 2 years.

Effect of Tezepelumab on the Humoral Immune Response to Seasonal Quadrivalent Influenza Vaccination in Patients with Moderate to Severe Asthma: The Phase 3b VECTOR Study

INTRODUCTION

Annual influenza vaccinations are recommended for adolescents and adults with moderate to severe asthma. This study investigated the effect of tezepelumab, a human monoclonal antibody that blocks the activity of thymic stromal lymphopoietin, on the humoral immune response to the quadrivalent seasonal influenza vaccine in patients with moderate to severe asthma.

METHODS

VECTOR was a phase 3b, randomized, multicenter, double-blind, parallel-group, placebo-controlled study. Adolescents (aged 12-17 years) and young adults (aged 18-21 years) with moderate to severe asthma were enrolled across 15 centers in the USA. Patients received tezepelumab 210 mg or placebo subcutaneously at weeks 0, 4, 8, and 12, and a single dose of inactivated quadrivalent seasonal influenza vaccine at week 12 before receiving study treatment. Immediately before vaccination and at 4 weeks postvaccination (week 16), strain-specific antibody responses were assessed for four influenza antigens by hemagglutination inhibition (HAI) and microneutralization (MN) assays. Safety was assessed.

RESULTS

Seventy patients were randomized to tezepelumab (n = 35) or placebo (n = 35). There were no meaningful differences in HAI or MN antibody responses between treatment groups at week 16. HAI assay geometric mean fold rises (GMFRs) for influenza strains were 1.76-7.34 for tezepelumab and 1.46-4.75 for placebo. MN assay GMFRs were 4.00-14.56 for tezepelumab and 3.56-10.62 for placebo. In the HAI assay, a fourfold or larger rise in antibody titer from weeks 12 to 16 occurred in 15.2-78.8% and 15.2-51.5% of tezepelumab and placebo recipients, respectively, and 97.0-100% of patients in both treatment groups achieved an antibody titer of at least 40 at week 16. No unexpected safety findings occurred.

CONCLUSION

There was no observed suppression of the humoral immune response after influenza vaccination in adolescents and young adults with moderate to severe asthma treated with tezepelumab. Therefore, the influenza vaccine can be administered to this patient population during tezepelumab treatment.

CLINICALTRIALS

GOV IDENTIFIER

NCT05062759.

Erratum: Tezepelumab Efficacy in Patients with Severe, Uncontrolled Asthma with Comorbid Nasal Polyps in NAVIGATOR [Corrigendum]

[This corrects the article DOI: 10.2147/JAA.S413064.].

Tezepelumab Efficacy in Patients with Severe, Uncontrolled Asthma with Comorbid Nasal Polyps in NAVIGATOR

Purpose

Tezepelumab, a human monoclonal antibody, blocks thymic stromal lymphopoietin. In the phase 3 NAVIGATOR study (NCT03347279), tezepelumab reduced annualized asthma exacerbation rates (AAERs) versus placebo, irrespective of baseline disease characteristics, and improved lung function and symptom control versus placebo in adults and adolescents with severe, uncontrolled asthma. We assessed the efficacy of tezepelumab in patients with severe asthma with or without nasal polyps (NPs) in the 2 years before randomization in NAVIGATOR.

Methods

Patients with severe asthma (N=1059) were randomized (1:1) and received tezepelumab 210 mg or placebo every 4 weeks subcutaneously for 52 weeks. Prespecified exploratory analyses included: AAER over 52 weeks and changes from baseline to week 52 in pre-bronchodilator forced expiratory volume in 1 second, Sino-Nasal Outcome Test (SNOT)-22 scores, and asthma control and health-related quality life (HRQoL) outcomes in NP subgroups. Changes from baseline in fractional exhaled nitric oxide (FeNO), blood eosinophil counts, total immunoglobulin E (IgE), eosinophil-derived neurotoxin (EDN), matrix metalloproteinase-10 (MMP-10), and serum interleukin (IL)-5, IL-6, IL-8 and IL-13 were assessed (post hoc).

Results

Tezepelumab reduced the AAER over 52 weeks versus placebo by 85% (95% confidence interval [CI]: 72, 92; n=118) and 51% (95% CI: 40, 60; n=941) in patients with and without NPs, respectively. At week 52, tezepelumab improved lung function, asthma control and HRQoL versus placebo in patients with and without NPs. Tezepelumab reduced SNOT-22 total scores (least-squares mean difference versus placebo [95% CI]) in patients with NPs at 28 weeks (-12.57 points [-19.40, -5.73]) and 52 weeks (-10.58 points [-17.75, -3.41]). At week 52, tezepelumab reduced blood eosinophil counts and FeNO, IgE, IL-5, IL-13, EDN and MMP-10 levels versus placebo, irrespective of NP status.

Conclusion

Tezepelumab resulted in clinically meaningful improvements in sino-nasal symptoms and asthma outcomes in patients with severe asthma with comorbid NPs.

Long-term safety and efficacy of tezepelumab in people with severe, uncontrolled asthma (DESTINATION): a randomised, placebo-controlled extension study

BACKGROUND

Tezepelumab is a human monoclonal antibody that blocks thymic stromal lymphopoietin. The drug has been tested previously in the phase 3 NAVIGATOR (NCT03347279) and SOURCE (NCT03406078) studies, and was subsequently approved as a treatment for severe asthma. This extension study recruited from NAVIGATOR and SOURCE and aimed to evaluate the long-term safety and efficacy of tezepelumab in individuals with severe, uncontrolled asthma.

METHODS

DESTINATION was a phase 3, multicentre, randomised, double-blind, placebo-controlled, long-term extension study. The study was done across 182 sites (including hospitals, clinics, medical centres, clinical trial centres, and private practices) in 18 countries. Participants (aged 12-80 years) were required to have good treatment compliance in the parent study. Randomisation was stratified by the parent study and all participants were re-randomised. Those who were previously randomised to receive tezepelumab in either parent study continued treatment of subcutaneous tezepelumab (210 mg every 4 weeks); those who were previously randomised to receive placebo in either parent study were re-randomised 1:1 to receive either subcutaneous tezepelumab (210 mg every 4 weeks) or placebo (every 4 weeks) using a randomisation list prepared by a computerised system. Total treatment duration (including the parent studies) was 104 weeks for all groups. Participants, investigators, and site staff were masked to treatment assignment. The primary endpoints were exposure-adjusted incidence of adverse events and serious adverse events and the secondary endpoint was the annualised asthma exacerbation rate; these were assessed from week 0 of the parent studies to week 104 of DESTINATION in all participants who were randomised and who received at least one dose of tezepelumab or placebo in either of the parent studies. The trial is registered with ClinicalTrials.gov, NCT03706079, and is closed to new participants.

FINDINGS

Participants were recruited between Jan 7, 2019, and Oct 15, 2020. For individuals who initially received tezepelumab (n=528) in NAVIGATOR, incidence of adverse events over 104 weeks was 49·62 (95% CI 45·16 to 54·39) per 100 patient-years, compared with 62·66 (56·93 to 68·81) for those receiving placebo (n=531; difference -13·04, 95% CI -17·83 to -8·18). For serious adverse events, incidence was 7·85 (6·14 to 9·89) per 100 patient-years for individuals who initially received tezepelumab and 12·45 (9·97 to 15·35) for those who received placebo (difference -4·59, -7·69 to -1·65). In SOURCE, incidence of adverse events was 47·15 (36·06 to 60·56) per 100 patient-years for those who initially received tezepelumab (n=74) and 69·97 (54·54 to 88·40) for those who received placebo (n=76; difference -22·82, -34·77 to -10·01). For serious adverse events, incidence was 13·14 (7·65 to 21·04) per 100 patient-years for those who initially received tezepelumab and 17·99 (10·66 to 28·44) for those who received placebo (difference -4·85, -14·88 to 4·53). Tezepelumab reduced the annualised asthma exacerbation rate over 104 weeks compared with placebo. In participants initially from NAVIGATOR, the annualised asthma exacerbation rate ratio over 104 weeks was 0·42 (95% CI 0·35 to 0·51); in those initially from SOURCE, the ratio over 104 weeks was 0·61 (0·38 to 0·96).

INTERPRETATION

Tezepelumab treatment was well tolerated for up to 2 years and resulted in sustained, clinically meaningful reductions in asthma exacerbations in individuals with severe, uncontrolled asthma. These findings are consistent with previous randomised, placebo-controlled studies and show the long-term safety and sustained efficacy of tezepelumab in individuals with severe, uncontrolled asthma.

FUNDING

AstraZeneca and Amgen.

Effect of Age on Efficacy and Safety of Fluticasone Furoate/Vilanterol (FF/VI), Umeclidinium (UMEC), and UMEC + FF/VI in Patients with Chronic Obstructive Pulmonary Disease: Analyses of Five Randomized Clinical Trials

Introduction

Concerns have been raised about the practical use and clinical benefits of medications and inhalers in older patients with chronic obstructive pulmonary disease (COPD). Here, we report analyses according to age from five clinical trials evaluating medications administered using the ELLIPTA dry-powder inhaler (DPI).

Methods

Efficacy and safety according to age groups (<65 and ≥65 years) were assessed using data from five clinical trials in patients ≥40 years of age with symptomatic COPD. There was a mix of pre-specified and post hoc analyses of two 24-week trials with fluticasone furoate (FF)/vilanterol (VI) 100/25 µg; one 24-week trial with umeclidinium (UMEC) 62.5 µg; and two 12-week trials with UMEC 62.5 µg + FF/VI 100/25 µg. The primary endpoint was trough forced expiratory volume in 1 second (FEV₁) obtained 23 and 24 hours after dosing on the last day of the study.

Results

A total of 2876 patients <65 years of age and 2148 patients ≥65 years of age were enrolled across all studies of whom 1333 and 1111 patients, respectively, received treatment at the doses presented. Statistically significant and clinically meaningful treatment differences in improvement from baseline in mean trough FEV₁ were reported for active comparators versus placebo at study end for both <65 and ≥65 years subgroups (FF/VI vs placebo: 143 mL and 111 mL; UMEC vs placebo: 110 mL and 123 mL; UMEC + FF/VI vs placebo + FF/VI: 136 mL and 105 mL; p<0.001 for all comparisons). The incidence of adverse events reported for active treatments was similar between age groups.

Conclusion

These data provide evidence to support the use of FF/VI, UMEC, or UMEC + FF/VI, all delivered via the ELLIPTA DPI, to treat older (≥65 years) and younger (<65 years) patients with COPD.

A comparison of the tolerability of adapalene 0.1% cream and adapalene 0.1% lotion in healthy individuals

Two separate single-center, randomized, evaluator-blinded, bilateral (split-face) comparison studies compared the tolerability of adapalene 0.1% cream with adapalene 0.1% lotion in individuals with healthy skin treated once per day for 3 weeks. At each visit, the participants were graded on erythema, scaling, dryness, and stinging/burning (scale: 0 = none to 3 = severe). On the final study visit, the participants completed a Cosmetic Acceptability Questionnaire. Adverse events were recorded at each study visit. A total of 144 participants were enrolled and 130 completed the studies (study 1, n = 66; study 2, n = 64). The lotion formulation was non-inferior to the cream for the success rates and tolerability assessments in both studies. The frequency distributions of worst scores of either 0 (none) or 1 (mild) (study 1; study 2) for adapalene lotion were erythema (98.5%; 40.7%), scaling (100%; 73.5%), dryness (100%; 68.8%), and stinging/burning (98.5%; 100%). The most common treatment-related adverse event was dryness (study 1, cream 2.7% [2 of 75] and lotion 4.0% [3/75]); study 2, cream 2.9% [2 of 69] and lotion 4.3% [3 of 69]. Both the adapalene 0.1% cream and 0.1% lotion formulations were well tolerated and acceptable to the study participants. The adapalene 0.1% lotion provides clinicians with a retinoid for the treatment of acne in a lotion formulation.

Genetic analysis of the xenobiotic resistance-associated ABC gene subfamilies of the Lepidoptera

Some ATP-binding cassette (ABC) transporters of subfamilies B, C and G confer resistance to xenobiotics including insecticides. We identified genes of these subfamilies expressed by the lepidopterans Trichoplusia ni and Bombyx mori. The B. mori genome includes eight, six and 13 ABC-B, -C and -G genes, respectively, which encode P-glycoprotein, multidrug resistance protein, MRP, and breast cancer resistance protein, BCRP, homologues. Among the ABC-C and -G subfamilies, gene duplication contributes to protein diversity. We have identified three ABC-B and two ABC-C T. ni genes. Analyses of the T. ni MRP (TrnMRP) revealed unique features, including the potential for TrnMRP4 hyperglycosylation and the alternative splicing of TrnMRP1. Taken together, these attributes of moth multidrug resistance-associated ABCs may confer distinct functional capacities to xenobiotic efflux.

New observations on the secondary chemistry of world Ephedra (Ephedraceae)

For several millennia, stem extracts of Ephedra (Ephedraceae, Gnetales) have been used as folk medicines in both the Old and New World. Some species were used in treatments of questionable efficacy for venereal disease in North America during the last century. Many Eurasian species produce phenylethylamine alkaloids, mostly ephedrine and pseudoephedrine, that interact with adrenergic receptors in the mammalian sympathetic nervous system. Asian Ephedra have been used recently in the clandestine manufacture of a street drug, methamphetamine. Although ephedrine alkaloids are not detectable in New World species of Ephedra, together with Asian species they contain other nitrogen-containing secondary metabolites with known neuropharmacological activity. Many mesic and particularly xeric species worldwide accumulate substantial amounts of quinoline-2-carboxylic acids, or kynurenates, in their aerial parts. Many species of Ephedra accumulate cyclopropyl amino acid analogues of glutamate and proline in their stems and roots, and particularly in the seed endosperm. Mesic species synthesize substantial amounts of three L-2-(carboxycyclopropyl)glycine stereomers rarely seen in nature. A cyclopropyl analogue of proline with known antimicrobial activity, cis-3,4-methanoproline, is found in large amounts in the stems and seeds of many Ephedra species. The ability to synthesize cyclopropyl amino acids may be an ancestral feature in the taxon. The natural function in the taxon of these three groups of secondary compounds remains to be established.

Insecticidal activity of glufosinate through glutamine depletion in a caterpillar

The herbicide glufosinate-ammonium (GLA) is a competitive inhibitor of glutamine synthetase (GS), an enzyme converting glutamate to glutamine in both plants and animals. Because GS is essential for ammonia detoxification in plants, GLA treatment disrupts photorespiration by causing a build-up of ammonia and a loss of glutamine in plant tissues. This study reports that GLA applied to leaf surfaces is also toxic to 5th-instar caterpillars of the skipper butterfly Calpodes ethlius (LD50 = 400 mg kg-1). After ingesting GLA, caterpillars stopped feeding and became dehydrated through a loss of rectal function. Caterpillars showed symptoms of neurotoxicity, such as proleg tremors, body convulsions and complete paralysis before death. Incubation of several tissues isolated from normal feeding-stage caterpillars with the GS substrates glutamate and ammonium showed that GLA inhibited GS activity in vitro. Within 24 h of ingesting GLA, caterpillars had a greatly reduced glutamine content and the ammonium ion levels had more than doubled. Injection of ammonium chloride into non-GLA-treated caterpillars had no deleterious effect, suggesting that glutamine depletion, and not a rise in body ammonium, was the primary cause of GLA toxicity following GS inhibition. This was supported by the observation that the onset of the symptoms of GLA poisoning could be postponed by giving GLA-fed caterpillars several subsequent daily injections of glutamine. The effective GLA dose fed to 5th-instar caterpillars in this study was comparable to the amount that might realistically by acquired from feeding on GLA-treated crops.

L-Glutamate retrieved with the moulting fluid is processed by a glutamine synthetase in the pupal midgut of Calpodes ethlius

From apolysis until pupal ecdysis, the pharate pupa of the Brazilian Skipper (Calpodes ethlius) lies wrapped in a prepupal shell composed of the larval cuticle and an ecdysial space (ES) filled with enzyme-rich moulting fluid (MF). In the 4h before ecdysis the pharate pupa drinks the moulting fluid through its mouth and anus, and transfers the cuticular degradation products to its midgut (MG). At the same time, extra fluid passes across the body wall of the pharate pupa and flushes out the ES. The MF is recovered at an overall rate of 70µl/h and reabsorbed across the pharate pupal midgut at about 26µl/h. L-Glutamate was found to be the dominant amino acid in the moulting fluid. Total MF glutamate peaked at 850nmol about 8h before pupal ecdysis (P-8), but by ecdysis it had dropped to nearly zero as the MF became diluted with new fluid and was consumed. The drop in glutamate in the ES coincided with a rise in the glutamine content of the fluid in the midgut lumen. The highest rate of glutamine synthesis occurred in midguts isolated from pharate pupae actively drinking MF (P</=-4). The enzyme glutamine synthetase (GS) was found to be active in glutamate metabolism in the pharate pupal midgut. Glutamine synthesis in the midgut was L-glutamate-dependent and inhibited by two selective competitive inhibitors of GS activity, L-methionine sulfoximine (MSO) and glufosinate ammonium (GLA). Injection of GS inhibitors into the prepupal ES greatly reduced the glutamine content of the midgut epithelium by P+24. Although a corresponding increase in midgut glutamate levels was not seen, midgut serine levels in treated animals rose, suggesting that GS inhibitors shunted the MF-derived glutamate along an alternative metabolic pathway. GLA was much more toxic to pupae than MSO. Midgut GS appears to play a central role in the recycling of L-glutamate across the pupal MG epithelium at pupation.

Substrate-stereoselectivity of a high-affinity glutamate transporter cloned from the CNS of the cockroach Diploptera punctata

A cDNA encoding a Na(+)-dependent glutamate transporter has been cloned from the brain of the cockroach Diploptera punctata. The cDNA encodes a transporter protein of 481 amino acids, designated DipEAAT1, which when expressed in baculovirus infected insect cells, resulted in a 40-50 fold increase in [(3)H]L-glutamate uptake. DipEAAT1 mRNA is expressed in the brain, as is the RNA encoding TrnEAAT1, a related transporter recently isolated from the caterpillar Trichoplusia ni. The affinity of these transporters for L-glutamate and several structural analogues was compared. Both have a high affinity for L-glutamate, their presumed primary substrate, but quite different affinities for D-aspartate. TrnEAAT1 was found to be similar to other glutamate transporters in that its ability to transport [(3)H]L-glutamate into cells was inhibited strongly by D- and L- isomers of aspartate and its analogues. DipEAAT1, by contrast, was inhibited weakly by all D- isomers tested. The affinity of DipEAAT1 for [(3)H]D-aspartate was found to be an order of magnitude lower than that of TrnEAAT1, revealing an unusual stereoselectivity for aspartate substrates by the cockroach transporter. The activity of DipEAAT1 was also unaffected by the presence of Zn(++) in the bathing solution, despite the presence of a putative Zn(++)-binding motif conferring Zn(++)-sensitivity on some mammalian glutamate transporters.

Characterization of a chloride current in the larval epidermis of the beetle Tenebrio molitor

Voltage-clamp analysis of single cuticle-attached epidermal cells dissected from the newly-ecdysed mealworm revealed the presence of a large inwardly-rectifying anion (i.e. outwardly-going) current. In many cells this current formed spontaneously on breaking into the cell with the patch pipette when the bath solution was isoosmotic with the pipette solution (415 mosmol/l). The current was evoked rapidly by electrical stimulation or by bathing the cells in hyposmotic saline (335 mosmol/l). The reversal potential of the activated current shifted in agreement with the Nernst prediction for Cl(-) when the transmembrane chloride gradient was altered by partially substituting bath or patch pipette Cl(-) with gluconate(-). Substitution of Na(+) with choline(+) or K(+) with TEA and Ba(+) in the bath or pipette solutions did not alter the reversal potential. Addition of 200 &mgr;mol/l cyclic AMP or 1 mmol/l cyclic GMP to the pipette solution increased the initial current strength and reduced the time taken to reach half peak amplitude from 117 sec to 49 sec and 41 sec, respectively. Cyclic AMP also raised the threshold at which the current developed under hyperosmotic conditions by about 20 mosmol/l. Addition of the Cl(-) channel blockers diphenylamine-2-carboxylic acid (200 &mgr;mmol/l) and diisothiocyanostilbene-2,2'-disulphonic acid (250 &mgr;mol/l) to the bath solution reduced the inwardly-rectifying anion current by 50%. This current was barely detectable in cells prepared from the mid-instar integument. This non-constitutive pattern of expression suggests that cellular Cl(-) efflux (and that of other anions) may be required during moult-cycle specific processes such as moulting fluid formation and cell volume regulation. As the strength of the epidermal anion current could be raised by the exogenous application of cytosolic cyclic nucleotides, the activity of the anion channels responsible for this current may normally be regulated by yet-to-be-identified hormone(s) or neuropeptide(s) acting on this tissue.

Molecular cloning and functional characterization of a GABA transporter from the CNS of the cabbage looper, Trichoplusia ni

A cDNA encoding a GABA transporter in the caterpillar Trichoplusia ni has been cloned and expressed in baculovirus-infected insect cells. The cDNA contains an ORF encoding a 608-residue protein, designated TrnGAT. Hydropathy analysis of the deduced amino acid sequence suggests 12 transmembrane domains, a structure similar to that of all other cloned Na+/Cl(-)-dependent GABA transporters. The deduced amino acid sequence shows high identity with a GABA transporter (MasGAT) expressed in the embryo of Manduca sexta. Expression of TrnGAT mRNA was detected only in the brain. Sf21 cells infected with recombinant baculovirus exhibited a 20- to 30-fold increase in [3H]GABA uptake compared to control-infected cells. Several blockers of GABA uptake were used to determine the pharmacological profile of TrnGAT. Although most similar to mammalian neuronal GABA transporter GAT-1 in its kinetic properties, stoichiometry of ionic dependence and pharmacological properties, TrnGAT may be distinguished from mammalian GAT-1 by the inability of cyclic GABA analogues, such as nipecotic acid and its derivatives, to inhibit GABA uptake by the insect protein. The unique pharmacology of TrnGAT suggests that the GABA transport system in the lepidopteran CNS could be a useful target in the future development of rapidly-acting neuroactive agents used to control agriculturally-important insects.

Faecal firing in a skipper caterpillar is pressure-driven

Many leaf-rolling caterpillars have a rigid anal comb attached to the lower surface of the anal plate (or shield) situated above the anus. This comb is widely assumed to be a lever used to 'flick' away frass pellets. An alternative mechanism to explain pellet discharge is proposed on the basis of observations on the caterpillar of the skipper Calpodes ethlius. The model proposes that the underside of the anal plate serves as a blood-pressure-driven surface for the ejection of faecal pellets. Rather than acting as a lever, the anal comb serves as a latch to prevent the premature distortion of the lower wall of the anal plate until the anal haemocoel compartment is fully pressurized. The anal comb is swung into position during pellet extrusion by retractor muscles attached at its base and held in place by a catch formed by a blood-swollen torus of everted rectal wall. As the caterpillar raises the blood pressure in its anal compartment by contracting its anal prolegs, the comb eventually slips over the toral catch. This causes the underside of the anal plate to move rapidly backwards as the blood pressure is released, projecting the pellet resting against it through the air. Simulation suggests that a local blood pressure of at least 10 kPa (75 mmHg) would be required to accelerate the lower surface of the anal plate outwards at a rate fast enough to discharge a 10 mg pellet at an observed mean velocity of 1.3 m s-1.

Molecular cloning and functional expression of an insect high-affinity Na+-dependent glutamate transporter

Excitatory amino acid transporters in the central and peripheral nervous systems of insects are thought to assist in maintaining glutamate concentrations in the resting synapse below the activation threshold of glutamate receptors. We have isolated a cDNA from the caterpillar Trichoplusia ni which encodes a high-affinity Na+-dependent glutamate transporter, designated TrnEAAT1. The deduced amino acid sequence shows strong identity with known members of the vertebrate Na+- and K+-dependent amino acid transporter family. Expression of the insect transporter mRNA was predominantly localized in the caterpillar brain. The function of the TrnEAAT1 protein was analyzed in cultured insect cells using a baculovirus expression system. Cells infected with the recombinant virus were found to exhibit a 50-fold increase in ability to accumulate labeled L-glutamate compared to mock-infected cultures, and this activity was shown to be Na+-dependent. Transport activity was further demonstrated by chromatographic identification of various glutamate analogues accumulated by infected cells. Various glutamate uptake inhibitors were used to outline the pharmacological properties of the cloned transporter and to compare it with known mammalian transporters. Despite the significant differences between insect and vertebrate physiology, the characteristics of the respective transporters were found to be remarkably similar.

Normal development of preimplantation mouse embryos deficient in gap junctional coupling

The connexin multigene family (13 characterized members in rodents) encodes the subunits of gap junction channels. Gap junctional intercellular coupling, established during compaction of the preimplantation mouse embryo, is assumed to be necessary for development of the blastocyst. One member of the connexin family, connexin43, has been shown to contribute to the gap junctions that form during compaction, yet embryos homozygous for a connexin43 null mutation develop normally, at least until implantation. We show that this can be explained by contributions from one or more additional connexin genes that are normally expressed along with connexin43 in preimplantation development. Immunogold electron microscopy confirmed that roughly 30% of gap junctions in compacted morulae contain little or no connexin43 and therefore are likely to be composed of another connexin(s). Confocal immunofluorescence microscopy was then used to demonstrate that connexin45 is also assembled into membrane plaques, beginning at the time of compaction. Correspondingly, embryos homozygous for the connexin43 null mutation were found to retain the capacity for cell-to-cell transfer of fluorescent dye (dye coupling), but at a severely reduced level and with altered permeability characteristics. Whereas mutant morulae showed no evidence of dye coupling when tested with 6-carboxyfluorescein, dye coupling could be demonstrated using 2′,7′-dichlorofluorescein, revealing permeability characteristics previously established for connexin45 channels. We conclude that preimplantation development in the mouse can proceed normally even though both the extent and nature of gap junctional coupling have been perturbed. Despite the distinctive properties of connexin43 channels, their role in preimplantation development can be fulfilled by one or more other types of gap junction channels.

Double whole-cell patch-clamp characterization of gap junctional channels in isolated insect epidermal cell pairs

Double whole-cell patch-clamp methods were used to characterize junctional membrane conductances in epidermal cell pairs isolated from the prepupal integument of the flour beetle, Tenebrio molitor. The mean initial junctional conductance in 267 cell pairs was 9.5 +/- 1.0 nS (range 0-95 nS). Well-coupled cell pairs uncoupled spontaneously with a half-time of 7.6 min. Adding 5 mM ATP to the pipette solution stabilized coupling with less than a 50% drop occurring after 30 min. Nonjunctional membrane potential was the major determinant of junctional conductance with transjunctional potential playing a minor role. Junctional conductance approached 0 pA at nonjunctional membrane potentials greater than 0 mV and increased with hyperpolarization. The voltage at half-maximal conductance was -26 mV. The time course of the reversible changes in junctional conductance were slow (< or = 30 sec) with time-dependent decay occurring faster and recovery occurring slower with increasing depolarization. Single gap junctional channel activity was recorded in uncoupling cell pairs and in poorly coupled ATP-stabilized cell pairs. One main single channel conductance was observed in each cell pair. The mean single channel conductances from all cell pairs in this study ranged from 197-347 pS (mean 248 pS). Single channel conductance was linear over the +/- 60 mV transjunctional voltage range tested. A broad range of subconductance states of the main state representing 5% of the total open time of measurable main state events was observed. Single channel activity was strongly dependent on the nonjunctional membrane potential, increasing with hyperpolarization.

ISOLATION OF EPIDERMAL CELL PAIRS FROM AN INSECT, TENEBRIO MOLITOR, FOR DUAL WHOLE-CELL RECORDING OF LARGE-CONDUCTANCE GAP-JUNCTIONAL CHANNELS

The segmented insect integument, composed of an epidermal monolayer and its cuticular secretion, is a paradigm for the study of pattern formation during development (Lawrence, 1992). Epidermal activity during development may be coordinated by the transfer of cytoplasmic molecules through cell-cell gap-junctional channels (reviewed in Caveney, 1985). Gap junctions within intact epidermal segments exhibit dynamic changes in their permeability properties during the moult cycle in vivo (Caveney, 1978) and with exposure to the developmental hormone 20-hydroxyecdysone in vitro (Caveney and Blennerhassett, 1980). In addition, the row of epidermal cells at segment borders has distinct permeability properties creating developmental and communication compartments (Warner and Lawrence, 1982; Blennerhassett and Caveney, 1984). This paper documents a method for isolating epidermal cells that are suitable for dual whole-cell voltage-clamp studies of gap-junctional currents. We have identified a large-conductance gap-junctional channel in cell pairs with octanol- reduced junctional currents. This cell model may be useful for examining mechanisms of gap- junctional channel gating during development.

Rapid de novo formation of gap junctions between insect hemocytes in vitro: a freeze-fracture, dye- transfer and patch-clamp study

Gap junctions form between insect hemocytes (blood cells) when they encapsulate foreign objects in the hemocoel (body cavity). In this study we show that hemocytes from cockroach (Periplaneta americana) form gap-junctions rapidly in vitro. Freeze-fracture replicas of hemocyte aggregates fixed 5 minutes after bleeding contain gap-junctional plaques. Dye passage was detected between carboxyfluorescein diacetate- labelled and unlabelled hemocytes within 3 minutes of bleeding, when the cells made contact as they flattened rapidly onto coverslips. When double whole-cell voltage-clamp was used to measure gap-junction formation between cells which were pushed together, electrical coupling was detected within one second of cell-cell contact. To prevent extensive flattening, cells were plated onto lipophorin-coated coverslips. Junctional conductance increased in staircase fashion with steps corresponding to an average single channel conductance of 345 pS. Assuming all channels to have this conductance, the maximal accretion rate of channels to the growing junction was one channel per second. Junctional currents and dye-coupling were detected in the absence of Ca2+, indicating that involvement of Ca2+-dependent adhesion molecules is not a prerequisite for gap-junction formation in hemocytes. Hemocytes from distantly related insects (cockroach and moth) form functional gap junctions with each other, suggesting sequence homology among gap- junction proteins in insects. The function of rapid gap-junction formation between hemocytes during encapsulation and wound healing in vivo are discussed.

Growth retardation in glioma cells cocultured with cells overexpressing a gap junction protein

To examine the role of gap-junctional intercellular communication in controlling cell proliferation, we have transfected C6 glioma cells with connexin 43 cDNA. The growth of transfected clones was dramatically reduced compared with nontransfected glioma cells. To further characterize the role of gap junctions in controlling proliferation, we have examined the growth of C6 cells cocultured with transfected cells overexpressing connexin 43. Although C6 cells grew at their normal rate when cocultured with nontransfected C6 cells, when cocultured with connexin 43-overexpressing cells they displayed a dramatic reduction in growth rate. Furthermore, a significant, dose-dependent reduction in cell proliferation was noted when C6 cells were cultured in medium conditioned by transfected cells. This effect correlated with the level of connexin 43 expression. These results suggest that the decreased cell proliferation rate of transfected cells and C6 cells cultured with them is due to the secretion of a growth inhibitory factor(s) and that the secretion of this factor may be linked to the level of gap junctional intercellular communication.

Expression of gap junction genes in astrocytes and C6 glioma cells

The expression of the gap junction genes coding for the liver-type connexin32 and the heart-type connexin43 was examined in primary cultures of astrocytes and in cultures of C6 glioma cells. In both cell types, only connexin43 mRNA was detectable. However, the level of this mRNA was greatly reduced in C6 glioma cells compared to astrocytes. This was consistent with the further observation that astrocytes in primary culture were extensively dye-coupled, whereas such coupling was very restricted in cultures of C6 glioma cells. Connexin43 was immunocytochemically localized in astrocytes, but was not readily detected in C6 cells.

Transfection of C6 glioma cells with connexin 43 cDNA: analysis of expression, intercellular coupling, and cell proliferation

C6 glioma cells express low levels of the gap junction protein connexin 43 and its mRNA and display very weak dye coupling. When implanted into the rat cerebrum, these cells quickly give rise to a large glioma. To investigate the role of gap junctions in the tumor characteristics of these cells, we have used Lipofectin-mediated transfection to introduce a full-length cDNA encoding connexin 43. Several transfected clones were obtained that exhibited various amounts of connexin 43 mRNA transcribed from the inserted cDNA. Immunocytochemical analysis revealed an increase in the amount of connexin 43 immunoreactivity in the transfected cells, being localized at areas of intercellular contact as well as in the cytoplasm. The level of dye coupling was also assessed and found to correlate with the amount of connexin 43 mRNA. When cell proliferation was followed over several days, cells expressing the transfected cDNA grew more slowly than non-transfected cells. These transfected cells will be useful in examining the role of gap junctions in tumorigenesis.

Engrailed gene expression in the abdominal segment of Oncopeltus: gradients and cell states in the insect segment

A monoclonal antibody that recognizes the product of the segmental gene, engrailed (en), of Drosophila has been used to analyse expression of the homologous gene of Oncopeltus. engrailed expression in the abdominal segment of larval Oncopeltus is confined to a narrow band of epidermal cells localized immediately anterior to the segment border. Expression varies in intensity during postembryonic development: no gene product is detectable in newly moulted larvae, but reappears soon after initiation of intermoult activities. One possible function of en in this system is revealed by a series of operations confronting cells from different anteroposterior levels in the segment. New segment borders are generated only when en-expressing cells confront cells from the anteriormost region of the segment. All other combinations result in intercalation of intermediate intrasegmental levels. It is therefore suggested that the most important function of en is the establishment of new, and presumably the maintenance of existing, segment borders.

Relative roles of gap junction channels and cytoplasm in cell-to-cell diffusion of fluorescent tracers

Intercellular (tissue) diffusion of molecules requires cytoplasmic diffusion and diffusion through gap junctional (or cell-to-cell) channels. The rates of tissue and cytoplasmic diffusion of fluorescent tracers, expressed as an effective diffusion coefficient, De, and a cytoplasmic diffusion coefficient, Dcyt, have been measured among the developing epidermal cells of a larval beetle, Tenebrio molitor L., to determine the contribution of the junctional channels to intercellular diffusion. Tracer diffusion was measured by injecting fluorescent tracers into cells and quantitating the rate of subsequent spread into adjacent cells. Cytoplasmic diffusion was determined by fluorescence photobleaching. These experiments show that gap junctional channels constitute approximately 70-80% of the total cell-to-cell resistance to the diffusion of organic tracers at high concentrations in this tissue. At low concentrations, however, the binding of tracer to cytoplasm slows down the cytoplasmic diffusion, which may limit intercellular diffusion.

Rates of diffusion of fluorescent molecules via cell-to-cell membrane channels in a developing tissue

Diffusion coefficients for the intercellular movement of fluorescent tracers have been measured in the epidermis of a larval beetle. Fluorescent tracer was injected into a cell and the spread of tracer from cell to cell in this monolayer was recorded by a TV camera. Fluorescence intensities were digitized from the TV images at successive times after the start of injection at various distances from the source by a microcomputer interfaced with a video analyzer. From the relationship between concentration (measured as light intensity), time and distance, an effective diffusion coefficient (De) is calculated for the tracer in the tissue. In newly ecdysed epidermis, De for carboxyfluorescein (CF) is 2.7 X 10(-7) cm2/s, and De for lissamine rhodamine B (LRB) is 1.2 X 10(-7) cm2/s, whereas in intermolt epidermis the De's for CF and LRB are 3.7 X 10(-7) and 1.2 X 10(-7) cm2/s, respectively. These diffusion coefficients are only an order of magnitude lower than their values in water. The ratio of De for the two tracers at these two stages of development differs from the ratio predicted in cytoplasm alone, with the movement of the slightly larger molecule (LRB) being impeded relative to that of the smaller molecule (CF). This suggests that the properties of the membrane channels amplify differences in the rates of movement of molecules of similar size. This may be important during cell patterning in development. De for CF was also monitored as junctional resistance was increased in the epidermis. During 30 min of exposure to 0.25 mM chlorpromazine, De dropped to 20% of its initial value of 5 X 10(-7) cm2/s, implying that the junctional membrane, rather than cytoplasm, is the major barrier to molecular diffusion among the cells.

Active muscle migration during insect metamorphosis

The R1 abdominal retractor muscles of the insect Tenebrio molitor change position during the course of metamorphosis. These muscles detach from the epidermal tendon cells at their anterior ends, and migrate in a posterior direction, parallel to the body axis, to form completely new attachments shortly before adult emergence. Movement is preceded by the loss of sarcomere structure, and the muscles migrate in a partially dedifferentiated condition, closely accompanied by satellite cells and haemocytes. Movement appears to result from the extension of muscle processes towards the epidermis posterior to the larval attachment sites, which contact reciprocal processes extended from the epidermis. Contacts at the new posterior sites are then reinforced, and relinquished at the anterior. This cycle is subsequently repeated. It is envisaged that migration ceases when the muscles encounter a contour in the epidermal gradient known to specify the position of the adult muscle attachment sites. This positional information may be encoded in the epidermal basal lamina. The muscles then redifferentiate, with concurrent differentiation of new epidermal tendon cells. Development of adult muscle attachments appears to require reciprocal morphogenetic interactions between muscle and epidermis.

Control of gap junction formation in early mouse embryos

Intercellular communication via gap junctions begins in the eight-cell stage in early mouse embryos. We have studied the timing of this event in relation to compaction, and have begun to explore some of the possible control mechanisms underlying it. Gap junction formation was inferred by measuring ionic coupling as well as by observing the intercellular transfer of fluorescent dye. Embryos were obtained early on Day 3 of pregnancy by flushing the oviducts of HA/ICR mice that had been mated with CB6F1/J males. Gap junctions were detected only in those embryos which had achieved the fully compacted state. Inhibition of protein synthesis by cycloheximide treatment beginning as early as the late four-cell stage failed to block compaction or the acquisition of gap junctions, demonstrating that the necessary proteinaceous components are present in advance of these events. In order to test the possibility that gap junctions could be induced to form prematurely, fully compacted, communication-competent eight-cell embryos were aggregated with two- or four-cell embryos. Even after 10 hr of aggregation, no interembryonic gap junctions could be detected. Fully compacted eight-cell embryos when aggregated with each other, however, became ionically coupled within 3-5 hr. The number of interembryonic junctional channels was judged to be effectively small, since the aggregated embryos exhibited obvious ionic coupling but very weak dye coupling. In contrast to gap junction formation within embryos, junction formation between embryos was blocked by cycloheximide. These results demonstrate that gap junction formation in early mouse embryos is under precise temporal control, involving the assembly or mobilisation of preexisting components. This stockpile of components is either unavailable or insufficient to allow the formation of additional gap junctions between aggregated communication-competent embryos without new protein synthesis.

Drugs that block calmoduLin activity inhibit cell-to-cell coupling in the epidermis of Tenebrio molitor

In many cell systems, the permeability of membrane junctions is modulated by the cytoplasmic level of free Ca++. To examine whether the calcium-dependent regulatory protein calmodulin is involved in this process, the ability of anticalmodulin drugs to influence the cell-to-cell passage of injected current and an organic tracer was tested using standard intracellular glass microelectrode techniques. Several antipsychotics and local anesthetics were found to block junctional communication in the epidermis of the beetle Tenebrio molitor. Treatment of the epidermis with chlorpromazine (0.25 mM) raised intercellular resistance two- to threefold within 20 to 25 min; cell-to-cell passage of electrical current was abolished within 41 +/- 5 min. Loss of electrotonic coupling was accompanied by a block in the cell-to-cell movement of the organic tracer carboxyfluorescein. The reaction is fully reversible, with normal electrotonic coupling being restored within 2 to 4 hr. Other antipsychotics and local anesthetics had similar effects on cell coupling. The order of potency found was: trifluoperazine greater than thioridazine greater than D-butaclamol greater than chlorprothixine = chlorpromazine greater than L-butaclamol greater than dibucaine greater than tetracaine. The relative uncoupling potencies of these drugs correlate well with their known ability to inhibit calmodulin-dependent phosphodiesterase activity. Other anesthetic compounds, procaine and pentobarbital, did not block cell-to-cell communication. Altering the extracellular Ca++ concentration did not affect the rate of uncoupling by antipsychotics, while chelation of extracellular Ca++ with EGTA raised electrotonic coupling. The effect of three metabolic inhibitors on coupling was also examined. Iodoacetate uncoupled the epidermal cells while DNP and cyanide did not. These results are discussed in terms of possible mechanisms by which calmodulin may control junctional communication in this tissue.

A gradient of morphogenetic information involved in muscle patterning

The results of grafts performed in the larva of the beetle Tenebrio molitor reveal that the position at which certain adult muscle attachments form is specified by a regulative, rather than a mosaic, mechanism. The results of grafts involving the rotation of squares of integument through 180 degrees or 90 degrees, or the antero-posterior transposition of two adjacent rectangles of integument, show that the site of muscle attachments in the antero-posterior axis of an abdominal sternite is specified by an epidermal segmental gradient of positional information. This gradient is presumed to be identical to the gradient which specifies cuticular patterns in this insect. There is a good correlation between the effect of grafts on adult muscle morphology and cuticular patterns.

A gradient of morphogenetic information involved in muscle patterning

The results of grafts performed in the larva of the beetle Tenebrio molitor reveal that the position at which certain adult muscle attachments form is specified by a regulative, rather than a mosaic, mechanism. The results of grafts involving the rotation of squares of integument through 180° or 90°, or the antero-posterior transposition of two adjacent rectangles of integument, show that the site of muscle attachments in the antero-posterior axis of an abdominal sternite is specified by an epidermal segmental gradient of positional information. This gradient is presumed to be identical to the gradient which specifies cuticular patterns in this insect. There is a good correlation between the effect of grafts on adult muscle morphology and cuticular patterns.

Changing muscle patterns in a segmental epidermal field

The spatial rearrangements that take place during metamorphosis in the abdominal sternites and associated retractor muscles of the beetle Tenebrio molitor are described. This paper provides the descriptive background needed to consider whether a morphogenetic gradient is involved in specifying the position at which adult muscle attachments develop. (Experimental work in support of this gradient hypothesis is published in a companion paper.)

The ventral abdominal retractor muscles are extensively remodelled at metamorphosis so that the adult muscles differ considerably in appearance from the larval muscles from which they are derived. In particular, there is a change in both the absolute and relative positions of the sites of muscle attachment. Rearrangement of muscles takes place during both the prepupal period and the pupal stage. It is achieved by means of two separate and temporally distinct mechanisms. Epidermal remodelling in the pre-pupal period results in the movement of attached retractor muscles (epidermokinetic muscle movement). In the pupal stage, however, the muscles move over the basal surface of the epidermis (myokinetic muscle movement). Myokinetic movements may be brought about by extension of myoblast processes from the metamorphosing muscles. These findings are considered in terms of Poyarkoff's theory that the pupa serves as an integumental mould, approximating the shape of the adult, within which certain adult muscles develop.

Changing muscle patterns in a segmental epidermal field

The spatial rearrangements that take place during metamorphosis in the abdominal sternites and associated retractor muscles of the beetle Tenebrio molitor are described. This paper provides the descriptive background needed to consider whether a morphogenetic gradient is involved in specifying the position at which adult muscle attachments develop. (Experimental work in support of this gradient hypothesis is published in a companion paper.) The ventral abdominal retractor muscles are extensively remodelled at metamorphosis so that the adult muscles differ considerably in appearance from the larval muscles from which they are derived. In particular, there is a change in both the absolute and relative positions of the sites of muscle attachment. Rearrangement of muscles takes place during both the prepupal period and the pupal stage. It is achieved by means of two separate and temporally distinct mechanisms. Epidermal remodelling in the pre-pupal period results in the movement of attached retractor muscles (epidermokinetic muscle movement). In the pupal stage, however, the muscles move over the basal surface of the epidermis (myokinetic muscle movement). Myokinetic movements may be brought about by extension of myoblast processes from the metamorphosing muscles. These findings are considered in terms of Poyarkoff's theory that the pupa serves as an integumental mould, approximating the shape of the adult, within which certain adult muscles develop.

Intercellular communication in insect development is hormonally controlled

Cellular coupling in the insect epidermis changes in a characteristic way during metamorphosis. In vitro, beta-ecdysone mimics the initial phase of these changes by increasing electrical coupling. Both adenosine 3',5'-monophosphate (cyclic AMP) and Ca2+ reverse natural and beta-ecdysone-stimulated changes, which suggests that ecdysone could work on communication through changes in cyclic AMP and Ca2+ levels. The transient changes in intercellular communication before metamorphosis may reflect the timing of the signals that trigger proliferation and the generation of new spatial patterns in the epidermis.

Intercellular communication in a positional field. Ultrastructural correlates and tracer analysis of communication between insect epidermal cells

The junctional membrane in the epidermal cells of the larval beetle (Tenebrio molitor L.) is comprised of macular gap junctions embedded in septate junctions. Ultrastructural and morphometric analysis of the distribution of gap junctions within the segmental epidermis suggests that this junction alone could account for the high electrotonic coupling recorded for the epidermal sheet. Analysis of the lanthanum-impregnated septate junction makes it doubtful that this junction serves as a communicating channel between beetle cells. A new model for the septate junction is presented in which pleated septa, less than 30 A thick, connect adjacent plasma membranes; the septa themselves are interconnected by two interseptal platforms that are coplanar with the plasma membranes. Iontophoretic injection of organic tracers into single epidermal cells suggests that only molecules of less than MW 1000 can transfer between cells through low-resistance junctions.

Cuticle reflectivity and optical activity in scarab beetles: the rôle of uric acid

The iridescent cuticle of certain Rutelino scarab beetles, which is a form optically active and selectively reflects circularly polarized light, incorporates an NH 4 OH -extractable component The ultraviolet absorption spectrum of this component, together with its chromatographic and refractive properties, identify it as uric acid (2,6,8-trihydroxypurine). All species of Plusiotis examined have uric acid in their reflecting layers, as do several species of Anoplognathus. Plusiotis resplendens has a reflecting layer with a uric acid volume fraction of 0.7, P . optima a volume fraction of 0.6. The reflecting layer of P . resplenden s has an anticlockwise helicoidal architecture, the optical thickness of the helicoidal p itch being such that it constructively interferes with visible light wavelengths. An anticlockwise helicoid constructively interferes with only the left circularly polarized component of incident light, right circularly polarized light being transmitted without attenuation. P. resplendens has a 1.8 /xm thick unidirectional layer embedded within the helicoid which functions as a perfect halfwave retardation plate for wavelength 590 nm . This halfwave plate enables the helicoidal reflector in this species to reflect both left and right circularly polarized components of incident light. After passing through the halfwave plate, transmitted right circularly polarized light becomes left circularly polarized ; this light is now reflected and emerges from the cuticle right circularly polarized, after passing back through the halfwave plate. Consequently the total reflectivity of circularly polarized incident light is greater in P. resplendens than in any other species examined; the plate also reduces multiple internal reflexions. Interferometric analysis of the refractive properties of the helicoidal reflectors in species of Plusiotis showed that the ordered incorporation of uric acid increases the birefringence of the system by a factor of five times, e.g. the in tact birefringence of the unidirectional layer of P . resplendens is 0.166 at wavelength 560 nm ; after uric acid extraction the birefringence is reduced to 0.034. As the coefficient of reflexion of a helicoidal reflector is directly proportional to the birefringence of the individual planes comprising the helicoid, beetles incorporating uric acid into their reflecting surfaces reflect circularly polarized light far more efficiently than beetles lacking uric acid. Refractive index values for a single multicomponent plane of the helicoid have been summarized as a biaxial indicatrix, with the Z axis tilte dat 45° to the plane of the epicuticle. Beetle reflecting layers which incorporate uric acid have twenty times greater optical rotatory power compared with reflecting layers lacking this component. Mathematical treatments dealing with helicoidal reflectors predict the form optical rotatory power to be a function of the square of the birefringence, which is in agreement with the experimental observations. To enable uric acid to have the optical effects mentioned above, an epitaxial incorporation into the helicoidal framework is necessary. Although uric acid is a common cytoplasmic reflecting material in arthropods, this is the first record of its presence in an extracellular (cuticular) reflector.

Muscle Attachment Related to Cuticle Architecture in Apterygota

In Apterygota muscles are attached to the cuticle by a series of discrete structures. The junction of the muscle and epidermal cells is demarcated by regular interdigitations of the two tissues, with desmosomes lining these processes. Within the epidermal cells, microtubules link up the desmosomes of the interdigitated region with dense material associated with cone-like depressions in the apical plasma membranes of the epidermal cells. Each of these ‘conical hemidesmosomes’ is situated opposite a pore canal. From within each cone, an electron-dense ‘muscle attachment fibre’ extends up the corresponding pore canal through the procuticle and is inserted on the epicuticle. There is no direct link between the microtubules and the muscle attachment fibres. The muscle attachment fibres are slightly elliptical in cross-section, and are twisted, this twist being in phase with the orientation of the chitin-protein microfibrils forming the lamellae of the procuticle. The attachment fibres are straight, and not helically arranged; patterns obtained in oblique sections of procuticle including these structures support the twisted ribbon model of pore canal shape. The cuticle, particularly in Thysanuran and Dipluran intersegmental membrane, displays the parabolic patterning typical of softer insect procuticle and procuticle deposition zones. The epicuticular insertion of the muscle attachment fibre is characterized by a pit in the cuticulin layer, the fibre passing through the middle of this pit.

The microtubule-associated conical hemidesmosomes appear to be cytoskeletal in function. The muscle attachment fibres are rigid structures which are not digested by the moulting fluid enzymes. New muscle attachment fibres may only become attached to the epicuticle during its formation. The structures described in regions of muscle attachment in Apterygota are similar to those recorded for other arthropods.