Treatment approach and outcomes of patients with accelerated/blast-phase myeloproliferative neoplasms in the current era

Progression of myeloproliferative neoplasms (MPNs) to accelerated or blast-phase is associated with poor survival outcomes. Since 2017 there have been several therapies approved for use in acute myeloid leukemia (AML); these therapies have been incorporated into the management of accelerated/blast-phase MPNs (MPN-AP/BP). We performed a multi-center analysis to investigate outcomes of patients diagnosed with MPN-AP/BP in 2017 or later. Two-hundred two patients were identified; median overall survival (OS) was 0.86 years. We also analyzed patients based on first-line treatment; the three most common approaches were intensive chemotherapy (IC) (n=65), DNA methyltransferase inhibitor (DNMTi)-based regimens (n=65), and DNMTi + venetoclax (VEN)-based regimens (n=54). Median OS was not significantly different by treatment type. In addition, we evaluated response by 2017 European LeukemiaNet (ELN) AML criteria and 2012 MPN-BP criteria in an effort to understand the association of response with survival outcomes. We also analyzed outcomes in 65 patients that received allogeneic hematopoietic stem cell transplant (allo-HCT); median OS was 2.30 years from time of allo-HCT. Our study demonstrates that survival amongst patients with MPN-AP/BP is limited in the absence of allo-HCT even in the current era of therapeutics and underscores the urgent need for new agents and approaches.

Tolerability and Efficacy of the Anticluster of Differentiation 47 Antibody Magrolimab Combined With Azacitidine in Patients With Previously Untreated AML: Phase Ib Results

PURPOSE

Magrolimab is a first-in-class humanized monoclonal antibody against cluster of differentiation 47, an antiphagocytic signal used by cancer cells to evade phagocytosis. Azacitidine upregulates prophagocytic signals on AML cells, further increasing phagocytosis when combined with magrolimab. We report final phase Ib data for magrolimab with azacitidine in patients with untreated AML ineligible for intensive chemotherapy (ClinicalTrials.gov identifier: NCT03248479).

PATIENTS AND METHODS

Patients with previously untreated AML, including TP53-mutant AML, received magrolimab intravenously as an initial dose (1 mg/kg, days 1 and 4), followed by 15 mg/kg once on day 8 and 30 mg/kg once weekly or every 2 weeks as maintenance. Azacitidine 75 mg/m2 was administered intravenously/subcutaneously once daily on days 1-7 of each 28-day cycle. Primary end points were safety/tolerability and proportion with complete remission (CR).

RESULTS

Eighty-seven patients were enrolled and treated; 72 (82.8%) had TP53 mutations with a median variant allele frequency of 61% (range, 9.8-98.7). Fifty-seven (79.2%) of TP53-mutant patients had European LeukemiaNet 2017 adverse-risk cytogenetics. Patients received a median of 4 (range, 1-39) cycles of treatment. The most common treatment-emergent adverse events included constipation (49.4%), nausea (49.4%), and diarrhea (48.3%). Thirty (34.5%) experienced anemia, and the median hemoglobin change from baseline to first postdose assessment was -0.9 g/dL (range, -3.6 to 2.5 g/dL). Twenty-eight (32.2%) patients achieved CR, including 23 (31.9%) patients with TP53 mutations. The median overall survival in TP53-mutant and wild-type patients were 9.8 months and 18.9 months, respectively.

CONCLUSION

Magrolimab with azacitidine was relatively well tolerated with promising efficacy in patients with AML ineligible for intensive induction chemotherapy, including those with TP53 mutations, warranting further evaluation of magrolimab with azacitidine in AML. The phase III randomized ENHANCE-2 (ClinicalTrials.gov identifier: NCT04778397) and ENHANCE-3 (ClinicalTrials.gov identifier: NCT05079230) studies are recruiting frontline patients with AML.

Magrolimab in Combination With Azacitidine in Patients With Higher-Risk Myelodysplastic Syndromes: Final Results of a Phase Ib Study

PURPOSE

Magrolimab is a monoclonal antibody that blocks cluster of differentiation 47, a don't-eat-me signal overexpressed on cancer cells. Cluster of differentiation 47 blockade by magrolimab promotes macrophage-mediated phagocytosis of tumor cells and is synergistic with azacitidine, which increases expression of eat-me signals. We report final phase Ib data in patients with untreated higher-risk myelodysplastic syndromes (MDS) treated with magrolimab and azacitidine (ClinicalTrials.gov identifier: NCT03248479).

PATIENTS AND METHODS

Patients with previously untreated Revised International Prognostic Scoring System intermediate-/high-/very high-risk MDS received magrolimab intravenously as a priming dose (1 mg/kg) followed by ramp-up to a 30 mg/kg once-weekly or once-every-2-week maintenance dose. Azacitidine 75 mg/m2 was administered intravenously/subcutaneously once daily on days 1-7 of each 28-day cycle. Primary end points were safety/tolerability and complete remission (CR) rate.

RESULTS

Ninety-five patients were treated. Revised International Prognostic Scoring System risk was intermediate/high/very high in 27%, 52%, and 21%, respectively. Fifty-nine (62%) had poor-risk cytogenetics and 25 (26%) had TP53 mutation. The most common treatment-emergent adverse effects included constipation (68%), thrombocytopenia (55%), and anemia (52%). Median hemoglobin change from baseline to first postdose assessment was -0.7 g/dL (range, -3.1 to +2.4). CR rate and overall response rate were 33% and 75%, respectively. Median time to response, duration of CR, duration of overall response, and progression-free survival were 1.9, 11.1, 9.8, and 11.6 months, respectively. Median overall survival (OS) was not reached with 17.1-month follow-up. In TP53-mutant patients, 40% achieved CR with median OS of 16.3 months. Thirty-four patients (36%) had allogeneic stem-cell transplant with 77% 2-year OS.

CONCLUSION

Magrolimab + azacitidine was well tolerated with promising efficacy in patients with untreated higher-risk MDS, including those with TP53 mutations. A phase III trial of magrolimab/placebo + azacitidine is ongoing (ClinicalTrials.gov identifier: NCT04313881 [ENHANCE]).

Profile of Glasdegib for the Treatment of Newly Diagnosed Acute Myeloid Leukemia (AML): Evidence to Date

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy primarily affecting older adults. Historically, the highest rates of response have been achieved with intensive induction chemotherapy; however, a significant portion of older or unfit adults with AML are unable to tolerate intensive therapy or have chemotherapy-resistant disease, creating a large need for active and less intensive treatment strategies. Glasdegib, an oral inhibitor of the transmembrane protein Smoothened (SMO) involved in the Hedgehog (Hh) signaling pathway, was approved in 2018 for older or unfit adults with AML and attained a role in clinical practice after showing an overall survival (OS) advantage when combined with the established agent low-dose cytarabine (LDAC). Since that time, however, several other highly active lower intensity therapies such as venetoclax plus a hypomethylating agent (HMA) have garnered a dominant role in the treatment of this patient population. In this review, we summarize the role of glasdegib in the current treatment landscape of newly diagnosed AML and discuss ongoing investigations into its role in novel combination therapies.

Magrolimab in combination with azacitidine for untreated higher-risk myelodysplastic syndromes (HR-MDS): 5F9005 phase 1b study results

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Background: Magrolimab is a monoclonal antibody that blocks CD47, a “don’t eat me” signal overexpressed on cancer cells. CD47 blockade by magrolimab induces macrophage-mediated phagocytosis of tumor cells and is synergistic with azacitidine (AZA) via upregulation of “eat me” signals. Here we report final Phase 1b data in patients (pts) with untreated HR-MDS (NCT03248479). Methods: Pts with previously untreated intermediate-/high-/very high-risk MDS per IPSS-R received magrolimab IV as a priming dose (1 mg/kg) followed by ramp-up to a 30 mg/kg weekly or Q2W maintenance dose. AZA 75 mg/m2 was administered IV or SC on Days 1–7 of each 28-day cycle. Primary endpoints were safety/tolerability and complete remission (CR) rate. Results: 95 pts (median age 69 years [range 28, 91]) were treated. IPSS-R risk was intermediate, high, or very high in 27%, 52%, and 21%, respectively. MDS was therapy-related in 22%; 26% (n=25) had a TP53 mutation and 62% had poor-risk cytogenetics (27% complex). Median (range) number of cycles was 6 (1, 27). The most common TEAEs included constipation (68%), thrombocytopenia (55%), anemia (52%), neutropenia (47%), nausea (46%), and diarrhea (44%). The most common Grade 3/4 TEAEs included anemia (47%), neutropenia (46%), thrombocytopenia (46%), and WBC count decreased (30%). 6 pts discontinued treatment due to AEs. 60-day mortality was 2%. Median Hb change from baseline (BL) at first post-dose sample was –0.7 g/dL (range –3.1, +2.4). CR and objective response (OR) rates were 33% and 75% with 31% of evaluable OR pts with abnormal cytogenetics at BL having cytogenetic CR. Median time to first OR, duration of CR (DCR), duration of OR, and PFS were 1.9, 11.1, 9.8, and 11.6 mos. OS rates at 12 and 24 mos were 75% and 52%, respectively (median NR with 17.1 mos follow-up for OS). For patients evaluated with sequential WES with a VAF cutoff of 5%, 3 of 3 pts with TP53 mutation who achieved CR had TP53 VAF <5% by C5D1. Favorable outcomes were observed in both TP53 mutant (40% CR, median OS 16.3 months) and wildtype pts (31% CR, median OS NR; Table). Conclusions: Magrolimab+AZA was well tolerated with promising efficacy in pts with untreated HR-MDS including those with TP53-mut and TP53-wt disease. A Phase 3 trial of magrolimab/placebo+AZA (ENHANCE: NCT04313881) is ongoing. Clinical trial information: NCT03248479. [Table: see text]

Tolerability and efficacy of the first-in-class anti-CD47 antibody magrolimab combined with azacitidine in frontline TP53m AML patients: Phase 1b results

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Background: Magrolimab is a monoclonal antibody blocking CD47, a “don’t eat me” signal overexpressed on cancer cells such as acute myeloid leukemia (AML). This blockade induces phagocytosis of tumor cells and is synergistic with azacitidine (AZA) via upregulation of “eat me” signals. We report data from a Phase 1b trial of magrolimab+AZA in frontline TP53-mutant ( TP53m) AML. Methods: Patients (pts) with frontline AML not suitable for intensive chemotherapy received IV magrolimab starting with a priming dose (1 mg/kg) followed by ramp-up to 30 mg/kg QW or Q2W as maintenance dose. AZA 75 mg/m2 was given IV or SC on Days 1–7 of each 28-day cycle. Primary endpoints were safety/tolerability and complete remission (CR) rate by ELN 2017 criteria. Results: 72 TP53m AML pts were treated (Table). Common all-grade TEAEs were constipation (52.8%), diarrhea (47.2%), febrile neutropenia (45.8%), nausea (43.1%), fatigue (37.5%), decreased appetite (37.5%), thrombocytopenia (31.9%), peripheral edema (30.6%), and cough (30.6%). Most common Grade 3+ TEAEs were febrile neutropenia (37.5%), anemia (29.2%; Grade 3, 26.4%; Grade 4, 2.8%), thrombocytopenia (29.2%), pneumonia (26.4%), and neutropenia (20.8%). Objective response rate (ORR) by intent-to-treat was 48.6% (33.3% CR, 8.3% CR with incomplete hematologic recovery [CRi] / CR with partial hematologic recovery [CRh], 1.4% morphologic leukemia-free state [MLFS], 5.6% partial response). Stable disease was reported in 16.7%, progressive disease (PD) in 5.6%. 30- and 60-day mortalities were 8.3% and 18.1%, respectively. Response assessment was unavailable in 4.2% who discontinued due to AEs and 6.9% due to other, prior to the C3D1 assessment. Median time to CR/CRi was 2.2 months (mos; range 1.7–7.2) and to CR was 3.0 mos (range 1.8–9.6). 45.2% (14/31) of evaluable CR/CRi/CRh/MLFS pts achieved negative MRD by flow cytometry (investigator reported). Of 24 CR patients, 8 had a longitudinal TP53 VAF assessment, and 5/8 (63%) had VAF decreased to ≤5%. Treatment was stopped due to SCT in 9 pts (12.5%), PD 26 (36.1%), death 8 (11.1%), AE 13 (18.1%), and other 14 (19.4%). Median durations of CR and CR/CRi were 7.7 mos (95% CI: 4.7, 10.9) and 8.7 mos (95% CI: 5.3, 10.9), respectively. Median overall survival (OS) for the 72 pts was 10.8 mos (95% CI: 6.8, 12.8) with median follow up 8.3 mos. Conclusions: In high-risk frontline TP53m AML pts unsuitable for intensive chemotherapy, magrolimab+AZA showed durable responses and encouraging OS in a single-arm study. A Phase 3 trial in TP53m AML (ENHANCE-2; NCT04778397) of this combination vs standard of care is ongoing. Clinical trial information: NCT03248479. [Table: see text]

Molecular annotation of extramedullary acute myeloid leukemia to identify prevalence of targetable mutations

7024

Background: Extramedullary (EM) involvement, including myeloid sarcoma (MS) and leukemia cutis (LC), is uncommon in patients with acute myeloid leukemia (AML). Mutational landscape of EM-AML is not well characterized, including concordance of sequencing data from EM vs. non-EM site (blood or bone marrow) and the potential for personalized targeted therapy in this patient cohort. Methods: In a multicenter retrospective study, clinical and genomic data were collected on EM-AML patients treated at Moffitt Cancer Center, Memorial Healthcare System, and University of Miami, as well as sequenced cases at a central laboratory. Next generation sequencing (NGS) data come from panels that interrogated 24- 406 genes, with 15 genes covered by all panels, including notably, IDH1, IDH2, KIT, KRAS, NPM1, NRAS, and TP53. Survival estimates using Kaplan-Meier statistics and multivariate analysis with Cox-regression were performed in SPSS (v.26). Results: Our study included 58 patients with EM-AML. Median age at diagnosis was 62 years; 55% of patients were males. In our cohort, 34 (59%) patients had MS, and 19 (33%) had LC. EM-AML was noted during relapse in 60% of evaluable patients (n=45), and 31% had isolated EM disease. Patients with LC had a significantly worse median overall survival (OS) than those with MS (5.7 months vs. 21.9 months, p= 0.008); Pattern of EM involvement (MS vs. LC) remained an independent prognostic factor for OS (p= 0.04) in a multivariate analysis including disease setting (new diagnosis vs. relapse) and ELN risk category. Results of NGS performed during EM presentation were available in 48 patients, 19 of which had NGS data from EM site. Most commonly mutated genes were NRAS on EM site NGS (37%) and NPM1 on non-EM site NGS (28%). Based on EM NGS, 52% patients had a targetable genomic alteration, with 37% mutations in IDH, 21% NPM1, 5% FLT3, and 11% MLL-PTD. Five (two with concurrent M+EM disease) out of nine evaluable patients had significant discordance in targetable mutations between EM and non-EM NGS at EM-AML. Three of four patients who received treatment with IDH1/2 inhibitors based on EM NGS achieved complete response. Conclusions: EM-AML has a distinct molecular architecture with an inferior OS in LC vs. MS patients. We conclude that EM site NGS is critical in patients with EM-AML, as 52% have potentially targetable mutations and could benefit from specific targeted therapies.[Table: see text]

Clinical Responsiveness to All-trans Retinoic Acid Is Potentiated by LSD1 Inhibition and Associated with a Quiescent Transcriptome in Myeloid Malignancies

PURPOSE

In preclinical studies, the lysine-specific histone demethylase 1A (LSD1) inhibitor tranylcypromine (TCP) combined with all-trans retinoic acid (ATRA) induces differentiation and impairs survival of myeloid blasts in non-acute promyelocytic leukemia acute myeloid leukemia (AML). We conducted a phase I clinical trial (NCT02273102) to evaluate the safety and activity of ATRA plus TCP in patients with relapsed/refractory AML and myelodysplasia (MDS).

PATIENTS AND METHODS

Seventeen patients were treated with ATRA and TCP (three dose levels: 10 mg twice daily, 20 mg twice daily, and 30 mg twice daily).

RESULTS

ATRA-TCP had an acceptable safety profile. The MTD of TCP was 20 mg twice daily. Best responses included one morphologic leukemia-free state, one marrow complete remission with hematologic improvement, two stable disease with hematologic improvement, and two stable disease. By intention to treat, the overall response rate was 23.5% and clinical benefit rate was 35.3%. Gene expression profiling of patient blasts showed that responding patients had a more quiescent CD34⁺ cell phenotype at baseline, including decreased MYC and RARA expression, compared with nonresponders that exhibited a more proliferative CD34⁺ phenotype, with gene expression enrichment for cell growth signaling. Upon ATRA-TCP treatment, we observed significant induction of retinoic acid-target genes in responders but not nonresponders. We corroborated this in AML cell lines, showing that ATRA-TCP synergistically increased differentiation capacity and cell death by regulating the expression of key gene sets that segregate patients by their clinical response.

CONCLUSIONS

These data indicate that LSD1 inhibition sensitizes AML cells to ATRA and may restore ATRA responsiveness in subsets of patients with MDS and AML.

Leveraging Hypomethylating Agents for Better MDS Therapy

PURPOSE OF REVIEW

Myelodysplastic syndrome (MDS) is a clinically and molecularly heterogeneous disease, which primarily occurs in older adults. Although hypomethylating agents have survival benefit and are the current standard of care, many MDS patients will not garner a response from therapy. For those who do respond, most responses are not durable, and the only hope for a cure is allogeneic stem cell transplant. New therapies to improve outcomes are urgently needed.

RECENT FINDINGS

Clinical trials combining standard hypomethylating agents with novel experimental agents are underway in an effort to improve clinical outcomes in MDS patients. Several of these small molecules have demonstrated the ability to augment the response rates of hypomethylating agents alone, including complete remission rates, in both the front line and refractory settings. Combination approaches utilizing hypomethylating agents and novel-targeted therapies have demonstrated the ability to improve response rates in MDS patients in both the front line and salvage settings, and thus may change the standard of care.

Early determinants of atherosclerosis in paediatric systemic lupus erythematosus

OBJECTIVES

To assess traditional and non-traditional cardiovascular risk factors and to determine the prevalence and correlates of early vascular markers of atherosclerosis in paediatric systemic lupus erythematosus (pSLE).

METHODS

Fifty-four adolescents with pSLE had cardiovascular risk factor assessment, disease activity and vascular testing including carotid intima-media thickness (CIMT), flow-mediated dilatation (FMD), arterial stiffness measures, and myocardial perfusion studies.

RESULTS

The traditional risk factors of hypertension, elevated triglycerides, apolipoprotein B, haemoglobin A1c and insulin levels and non-traditional risk factors of elevated homocysteine and fibrinogen were present (all p<0.001). Some arterial stiffness measures, central pulse wave velocity and characteristic impedance were elevated (p<0.001), but CIMT, FMD and myocardial perfusion were normal. Cumulative prednisone dose correlated with total cholesterol (r=0.5790, p<0.001) and elevated LDL-C (r=0.4488, p=0.0012). Hydroxychloroquine treatment correlated negatively with total cholesterol (r=-0.4867, p=0.0002), LDL-C (r=-0.4805, p=0.0002) and apolipoprotein B (r=-0.4443, p=0.0011). In multivariate analysis LDL-C correlated with cumulative prednisone dose and negatively with hydroxychloroquine treatment (R2=0.40, p<0.001).

CONCLUSIONS

An increased burden of traditional and non-traditional risk factors and early evidence of insulin resistance and increased central arterial stiffness were present in paediatric SLE. Disease-specific and therapy-related factors are likely modifying these cardiovascular risk profiles warranting prospective longitudinal studies.

Interaction of aluminium with bacteria isolated from wounds

The effect of aluminium as metal foil and as a weak solution of potassium aluminium sulphate on the growth of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Proteus vulgaris, has been examined over a range of pH values. Aluminium foil, and an aluminium metal film deposited by evaporation on a bonded cellulose fabric, failed to inhibit the growth of the organisms on agar plates. In contact with saline metal foil yielded trace amounts of aluminium in solution but no detectable amounts of aluminium were found in human serum which had been in prolonged contact with aluminium metal. Staph, aureus and Ps. aeruginosa were adsorbed from aqueous suspension by aluminium surfaces. Below pH 5 the growth of these organisms was inhibited by low concentrations of aluminium in aqueous solution. Preliminary measurements of the uptake of aluminium by these sensitive organisms have been made.

Transitions in insect respiratory patterns are controlled by changes in metabolic rate

We examined the respiratory patterns of Rhodnius prolixus and Gromphadorhina portentosa as metabolic rates varied with temperature to determine whether insects transition from discontinuous (DGC), cyclical and continuous respiration as a response to increasing aerobic demand. Using flow through respirometry we: (1) determined the effects of temperature on metabolic rate; (2) objectively defined periods of spiracular closure; (3) observed whether there was a correlation between metabolic rate and length of spiracular closure. At low temperatures both species exhibit lengthy periods of spiracular closure reflecting a discontinuous respiratory pattern. As metabolic rate increased, periods of spiracular closure decreased and insects displayed a more cyclical pattern of respiration. As metabolic rates increased even further under the highest experimental temperatures, periods of spiracular closure decreased even more and a continuous respiratory pattern was employed by both species. Our results suggest that the three described respiratory patterns in insects are not distinct but are instead a continuum of respiratory responses driven by the metabolic demand experienced by the insect.

Electroless metallization onto pulsed plasma deposited poly(4-vinylpyridine) surfaces

Pulsed plasma-chemical deposition of poly(4-vinylpyridine) is found to be a highly effective way of functionalizing solid surfaces with pyridine ring centers. These surfaces can be metallized via complexation to Pd2+ ions from solution, followed by autocatalytic electroless deposition of either copper or nickel films.

Rewritable DNA Microarrays

Thiol-terminated single-stranded deoxyribonucleic acids (ssDNA) can be immobilized onto pulsed plasma deposited poly(allylmercaptan) surfaces via disulfide bridge chemistry and are found to readily undergo nucleic acid hybridization. Unlike other methods for oligonucleotide attachment to solid surfaces, this approach is shown to be independent of substrate material or geometry, and amenable to highly efficient rewriting.

Malarial infection in Aedes aegypti : effects on feeding, fecundity and metabolic rate

We have examined metabolic rate, lipid and carbohydrate of female Aedes aegypti during 10 days following a malaria-infected bloodmeal. In parallel, we determined bloodmeal size, portions retained and diuresed, and subsequent fecundity. We found that mosquitoes obtained identical masses of blood when feeding on an infected or control host. However, infected mosquitoes lost more mass during diuresis and retained a smaller mass. Infection led to a significant reduction in fecundity, the extent of which could not be explained by the difference in post-diuresis bloodmeal mass alone. We found no differences in lipid or carbohydrate content between infected and control mosquitoes during the 10 days post-infection, although infected mosquitoes had a lower body mass than controls. Metabolic rates were not different between groups, except during blood digestion, where the metabolic rate was lower in infected mosquitoes. These results suggest that infection by malaria does not lead to an increase in metabolic rate during the phases of midgut invasion and sporogony. However, infection does have a measurable effect on fecundity and subsequent body mass of the infected females.

Water acquisition and partitioning inDrosophila melanogaster: effects of selection for desiccation-resistance

We examined physiological features related to water balance in five replicate populations of Drosophila melanogaster that have undergone selection for enhanced resistance to desiccation (D populations) and in five replicate control (C) populations. Adult D flies contain 34 % more water than the control flies. We examined two hypotheses for increased water acquisition in the D flies: (i) that they accumulate more water early in development and (ii) that they have a reduced post-eclosion diuretic water loss. We found no evidence of differential water or dry mass acquisition between the C and D populations prior to adulthood. We also found no evidence of differential post-eclosion diuresis, i.e. both C and D groups showed insignificant changes in water volume in the 4 h period immediately after eclosion. In addition, we quantified water content in the intra- and extracellular compartments of the C and D populations and were able to identify the hemolymph as the primary storage site of the ‘extra’ water carried by the desiccation-resistant flies. We estimated that 68 % of the increased water volume observed in the D flies was contained in the hemolymph. Desiccation-resistance was strongly correlated with hemolymph volume and only weakly with intracellular water volume. Survival during desiccation was also strongly related to the carbohydrate content of the D flies. It has been presumed that the D flies accumulate carbohydrate primarily as intracellular glycogen, which would result in a significant increase in intracellular water volume. We found that carbohydrate content was weakly correlated with intracellular water volume and more strongly with hemolymph volume. The carbohydrate pool in the D flies may, therefore, be contained in the extracellular compartment as well as in cells. These results are suggestive of the importance of modifications in hemolymph volume and hemolymph solute concentrations in the evolution of enhanced desiccation-tolerance in populations of Drosophila melanogaster.

Sodium and chloride regulation in freshwater and osmoconforming larvae ofCulexmosquitoes

In this study, we examined aspects of Na+ and Cl– regulation in mosquito larvae of the genus Culex, a group that includes species that tolerate high salinity as well as other forms that are restricted to fresh water. When the euryhaline osmoconformer C. tarsalis was acutely transferred from 30 % to 50 % sea water, the patterns of hemolymph Na+ and Cl– regulation were similar. The underlying regulatory mechanisms for these two ions have very different characteristics. In C. tarsalis, Na+ efflux was significantly elevated compared with the rates measured in the freshwater-restricted C. quinquefasciatus, while Cl– influx was relatively lower. The modulation of Na+ efflux and Cl– influx allowed C. tarsalis to avoid a potential salt load and ionic disturbance in the hemolymph during an acute increase  in salinity. The observed adjustment of NaCl regulation departs from that determined for other euryhaline organisms and is integral to the osmoconforming response. At the other extreme of the salinity spectrum, we observed that C. tarsalis faces difficulties in ion regulation in habitats with low NaCl levels because of its inability to reduce ion efflux and adjust ion absorption rates to maintain hemolymph ion balance. In contrast, C. quinquefasciatus exhibited a reduced ion efflux and the ability to upregulate Na+ uptake, traits necessary to extend its lower salinity limit.

Postponed aging and desiccation resistance in Drosophila melanogaster

Studies with the fruit fly, Drosophila melanogaster, have repeatedly shown that selection for postponed reproduction leads to increases in mean life span and increased stress resistance; including increased resistance to desiccation, starvation and ethanol vapors. We show that desiccation resistance declines with age in both short- and long-lived flies suggesting that desiccation resistance may serve as a useful biomarker for aging-related declines in physiological performance. We examined the physical basis of desiccation resistance in five replicate populations selected for postponed reproduction and five replicate control populations. The variables examined were water content, rates of water loss during desiccation, and water content at time of death due to desiccation. In the absence of desiccation stress, both the flies exhibiting postponed senescence and their controls maintained constant water content throughout their lifetimes. In the presence of desiccation stress, the short-lived flies showed significantly higher rates of water loss at all ages than did the long-lived flies. Flies from the two treatments did not differ in water content at death. Our results indicate that water loss rates are the major determinant of desiccation resistance. Water loss rates are under genetic control and covary with age in populations with genetically-determined postponed senescence.

The physiology of salinity tolerance in larvae of two species of Culex mosquitoes: the role of compatible solutes

We investigated the physiological basis for differences in salinity tolerance ranges in mosquito larvae of the genus Culex. We examined the response of larvae of C. quinquefasciatus, a freshwater obligate, and C. tarsalis, a euryhaline osmoconformer, following transfer from fresh water to 34% sea water. Hemolymph Na(+) and Cl(−) levels increased similarly in both species, indicating that ion regulation does not differ under these conditions. C. quinquefasciatus responded to increased environmental salinity with increased hemolymph levels of serine, but suffered a significant reduction in levels of trehalose. C. tarsalis responded to increased environmental salinity with increased hemolymph levels of both proline and trehalose. When C. tarsalis larvae were held in 64% sea water, which C. quinquefasciatus larvae cannot tolerate, hemolymph proline and trehalose were accumulated approximately 50-fold and twofold, respectively, relative to freshwater values. We found that proline serves as both an intra- and extracellular compatible solute in C. tarsalis, the first such circumstance documented in an animal in response to increased environmental salinity. Analyses of the acute responses of the two species to an increase in salinity (from 30% to 50% sea water) indicate that larvae of C. tarsalis are able to volume-regulate via drinking and to attenuate increases in hemolymph NaCl concentration using unknown mechanisms during large, rapid increases in salinity.

Regulation of compatible solute accumulation in larvae of the mosquito Culex tarsalis: osmolarity versus salinity

In this study, we demonstrate that two of the osmolytes utilized in the osmoconforming strategy of larval Culex tarsalis are regulated by two fundamentally different signals. When the external osmolality was increased using salinity (sea salts), hemolymph NaCl, proline and trehalose concentrations increased significantly. When sorbitol was used to increase the external osmolality without an elevation in salt concentration, hemolymph NaCl and proline concentrations decreased, whereas hemolymph trehalose concentration increased. The results suggest that proline accumulation was cued by increases in salinity, whereas trehalose levels followed increases in osmolality. Interestingly, we found that C. tarsalis larvae accumulated the exogenous sorbitol in the hemolymph in an osmoconforming manner. We conducted further studies in which changes in hemolymph NaCl concentrations were manipulated using changes in environmental salinity. The results suggested that hemolymph proline accumulation was cued by the proximal signal of hemolymph NaCl levels. Regardless of which solute (sea salts, sorbitol or mixtures thereof) was used to raise the external osmolality, trehalose accumulation tracked the increase in total osmolality of the medium. These findings indicate that the synthesis and accumulation of these two osmolytes are regulated by two independent signals.

Using laboratory selection for desiccation resistance to examine the relationship between respiratory pattern and water loss in insects

We conducted concurrent measurements of rates of CO2 and H2O release from individual fruit flies Drosophila melanogaster taken from populations subjected to three different selective regimes: (1) populations selected for resistance to desiccation (D flies); (2) populations maintained as their controls (C flies); and (3) the ancestral populations of the D and C populations (O flies). In the D flies, water loss rates were significantly reduced, the standard error of the regression (SER) of the CO2 release pattern measured over the survival period of the flies was increased, and the ratio of CO2 loss to H2O loss (VCO2/VH2O) was increased. Correlations across all 15 populations from the three selection treatments indicate that survival time was negatively correlated with water loss rate, positively correlated with the SER of CO2 release and positively correlated with the VCO2/VH2O ratio. We did not, however, find a significant correlation between the SER of CO2 release and rates of water loss or the VCO2/VH2O ratio.

The effect of respiratory pattern on water loss in desiccation-resistant Drosophila melanogaster

We measured CO2 and H2O release from individual fruit flies from five populations of Drosophila melanogaster selected for resistance to desiccation (D flies). Our previous work found that these flies survive for an extended period in dry air, have an increase in the peak height and frequency of CO2 release, as measured by the standard error of a linear regression (SER) of CO2 release for the entire survival period, and have reduced water loss rates (VH2O) compared with their control or ancestor populations. In the present study, we examined the following respiratory characteristics: VCO2, VH2O, the SER of CO2 release and the ratio of VCO2 to VH2O in the D flies. Correlations between these characters were calculated in order to determine the effect of respiratory pattern on water loss. We found that, within the D flies, neither periodic release of CO2 nor an increased SER for CO2 release was associated with reduced water loss. In addition, an increased SER was positively correlated with both an increased water loss rate and a decreased survival time. Therefore, although selection for desiccation resistance leads to both an increased SER and a decreased rate of water loss in the D flies, the increased SER does not significantly reduce respiratory water loss.

Metabolic reserves and evolved stress resistance in Drosophila melanogaster

We have examined starvation and desiccation resistance in 43 outbred populations of Drosophila melanogaster that have diverged from a common ancestral population as a result of a variety of defined selection protocols. The populations differ up to 8.5-fold in desiccation resistance and up to 10-fold in starvation resistance. We used these populations to search for evolved physiological changes that might explain the differences in stress resistance. We examined two hypotheses for increased stress resistance that had been proposed previously in the literature: (1) that increments in starvation resistance are principally the result of differential lipid accumulation, and (2) that changes in glycogen accumulation play a role in evolved increases in resistance to desiccation stress. By quantifying desiccation resistance, starvation resistance, lipid content, and carbohydrate content in each of our populations of flies, we were able to demonstrate strong correlations between the capacity of the flies to resist starvation and the quantity of lipid or carbohydrate that the flies had stored. The strongest correlation (R2 = 0.99) was observed when the total energy content of both the lipid and carbohydrate stores was regressed against starvation resistance. These results demonstrate that the flies responded to selection for starvation resistance through a genetically determined increase in both lipid and carbohydrate storage. Similar analyses of the correlation between lipid storage or total energy storage and desiccation resistance revealed no significant correlations. Carbohydrate storage was significantly correlated with desiccation resistance in female but not in male flies. These results suggest that different forms of stress are resisted with distinct physiological mechanisms and that the evolutionary response of the flies to stress selection is specific to the stress imposed.

CO2 release patterns in Drosophila melanogaster: the effect of selection for desiccation resistance

We used laboratory natural selection on insects as a means of investigating the role of patterns of gas exchange in desiccation resistance. We used 15 populations of Drosophila melanogaster: five selected for desiccation resistance, five control populations and five ancestral populations. Using flow-through respirometry, we found that D. melanogaster from all populations produced irregular peaks of CO2 release. To quantify the height and frequency of these peaks, we used the standard error of a linear regression (SER) through the recordings of CO2 release. The values for the SER were significantly larger in the populations selected for desiccation resistance than in the control and ancestral populations. Occasionally, highly periodic peaks of CO2 release were observed in the desiccation-resistant populations only. Maximum SER was found to be strongly correlated with survival time in dry air among selection treatments, but not among individuals within a population. Access to dietary water resulted in lower SER values. These data demonstrate that gas exchange is physiologically controlled in Drosophila melanogaster and that the pattern of gas exchange can change under selection. The relationship between these CO2 release patterns and classic discontinuous ventilation is discussed.

Lectin binding to extracellularly melanized microfilariae of Brugia malayi from the hemocoel of Anopheles quadrimaculatus

Binding patterns of fluorescein isothiocyanate (FITC)- and gold-conjugated lectins to extracellularly melanized sheathed and exsheathed microfilariae of subperiodic Brugia malayi, isolated from and in situ in the abdominal hemocoel of Anopheles quadrimaculatus 72-hr postinfection, were examined. Five FITC-conjugated lectins [Helix pomatia agglutinin (HPA), Arachis hypogaea (peanut agglutinin-PNA), Triticum vulgaris (wheat germ agglutinin-WGA), Lens culinaris (lentil-LCH), and Concanavalin A (Con A)] with specificities for different carbohydrate moieties were tested for binding to isolated melanized microfilariae and observed with transmitted light and fluorescence microscopy. All five FITC-lectins bound strongly to the acellular material accompanying the melanin deposits on the surface of isolated melanized microfilariae. Significant inhibition of FITC-lectin binding occurred when lectins were preincubated with their complementary carbohydrates before testing. H. pomatia agglutinin binding was totally inhibited by N-acetyl-D-glucosamine and N-acetyl-D-galactosamine. Other lectins were partially inhibited, such as PNA by galactose and lactose; WGA by N-acetylneuraminic acid; LCH by N-acetyl-D-glucosamine, mannose, glucose, and methyl alpha-D-mannopyranoside; and Con A by mannose and methyl alpha-D-mannopyranoside. Three gold-conjugated lectins (HPA, PNA, and Con A), examined by using transmission electron microscopy, bound to the outer surface of the acellular material associated with the melanin deposits on isolated melanized microfilarial sheaths and melanized microfilariae and to the remnants of lysed hemocytes found in the proximity of the melanized deposits. Con A in the presence of gold-labeled horseradish peroxidase, examined by using transmission electron microscopy, showed random binding within the melanized capsule formed around the microfilarial sheath in situ. These results indicate that the acellular material accompanying melanin deposits on melanized microfilarial sheaths and sheathed and exsheathed microfilariae contain several glycoconjugates with exposed carbohydrate moieties and are possibly glycoproteins. These glycoproteins could be the by-products of the activation of the prophenoloxidase by the microfilariae.

Comparative study of hemolymph phenoloxidase activity in Aedes aegypti and Anopheles quadrimaculatus and its role in encapsulation of Brugia malayi microfilariae

Hemolymph phenoloxidase activity of sugar-fed and blood-fed females of Anopheles quadrimaculatus and Aedes aegypti showed similar characteristics. Phenoloxidase was present as an inactive proenzyme in both mosquito species and was partially activated during collection of the hemolymph. In both mosquito species, phenoloxidase activity was modulated by different buffers and activated phenoloxidase did not need Ca2+. Enzymatic activity was higher in the hemocytes than in the plasma in both mosquito species. Trypsin, laminarin, and blood-feeding on uninfected and Brugia malayi-infected jirds enhanced hemolymph phenoloxidase activity in both mosquito species. The appearance of hemolymph phenoloxidase activity was inhibited by p-nitrophenyl p'-guanidinobenzoate HCl, soybean trypsin inhibitor, ethylenediaminetetraacetic acid, diethyldithiocarbamic acid, saturated 1-phenyl-2-thiourea and reduced glutathione, but not by benzamidine in A. quadrimaculatus. The appearance of hemolymph phenoloxidase activity was inhibited by benzamidine, diethyldithiocarbamic acid, saturated 1-phenyl-2-thiourea, reduced glutathione, p-nitrophenyl p'-guanidinobenzoate and soybean trypsin inhibitor, but not by ethylenediamine-tetraacetic acid in A. aegypti. It is suggested that in both mosquito species, blood-feeding and migration of sheathed microfilariae in the homocoel activated the prophenoloxidase in the hemolymph and caused the encapsulation and melanization of microfilarial sheaths and microfilariae of B. malayi.

Ultrastructural comparison of extracellular and intracellular encapsulation of Brugia malayi in Anopheles quadrimaculatus

Ultrastructural aspects of extracellular humoral encapsulation of microfilariae of Brugia malayi in the hemocoel of Anopheles quadrimaculatus were compared with those of intracellular encapsulation of first-stage larvae (L1) of the same parasite species, in the thoracic muscle cells of the same species of mosquito. The results showed that extracellular humoral encapsulation of microfilarial sheaths, and sheathed and exsheathed microfilariae, in the hemocoel of mosquitoes occurs around the parasite within the first 6 hr postingestion, apparently without initial participation of hemocytes. Hemocytes and their remnants were observed near the parasite during the first 6 hr postingestion. Within the next 24 hr, hemocytes attach to the initial humoral capsule. By contrast, intracellular encapsulation of L1S is initiated by the accumulation of a dense cytoplasmic layer derived from the infected thoracic muscle cell. Melanin deposits accumulate in this layer adjacent to the parasite cuticle, again without visible participation of hemocytes.

Intracellular development of subperiodic Brugia malayi influenced by mosquito thoracic muscle cells

The in vitro development of 1-day-old intracellularly lodged larvae of Brugia malayi cultured in infected excised thoraces of selected susceptible and refractory strains of Aedes aegypti and Anopheles quadrimaculatus was compared with larvae reared in vivo. In susceptible mosquitoes, both in vitro and in vivo, larvae developed normally and abnormally. In refractory mosquitoes this pattern of both normal and abnormal development was also observed, except that comparatively fewer larvae developed to the infective third-stage larvae (L3) in vitro than in vivo and that more first-stage larvae (L1) were intracellularly melanized in vivo than in vitro. These studies indicate that factors in the thoracic muscle cells of the mosquito greatly affect the development of B. malayi microfilariae to L3. Intracellular melanization of L1 in An. quadrimaculatus, previously demonstrated in vivo, rarely occurred in vitro. These studies therefore suggest that refractoriness and melanization of B. malayi larvae in the thoraces of An. quadrimaculatus are controlled by two different and separate mechanisms.

Characterization of the intracellular melanization response in Anopheles quadrimaculatus against subperiodic Brugia malayi larvae

Intracellular melanization, a defense or an immune response in the thoracic muscle cells, was investigated in a refractory strain of Anopheles quadrimaculatus infected with larvae of Brugia malayi. In mosquitoes fed on B. malayi-infected jirds, intracellular melanization against first-stage larvae (L1) was better expressed when fewer than 40 microfilariae reached the thoracic muscle cells than when more than 40 microfilariae reached the thoracic muscle cells. This result suggests that when large numbers of microfilariae invade the thoracic muscle cells, the immune response of the mosquito may become overloaded. Intracellular melanization response against L1 in the thoracic muscle cells also showed a significant decrease in older females (14-16-day-old) as compared to the younger ones (4-9-day-old). A comparison is made between intracellular and extracellular responses of mosquitoes to filarial larvae. It is significant that in both cases high rate of infection can reduce both the number and percentage of larvae melanized.

Fluid secretion and microvillar ultrastructure in mosquito malpighian tubules

The Malpighian tubules of fourth instar larvae, pupae, and female adults of the mosquito Aedes taeniorhynchus were examined with regard to in vitro fluid secretion rate and the ultrastructural features of the microvillar border of the primary cells. In vitro fluid secretion rates were determined after stimulation with 5-hydroxytryptamine. While larval tubules are capable of rapid fluid secretion, the tubules of pupae exhibit very low rates of secretion, indistinguishable from 0 nl/h. The capacity to secrete fluid returns after the pupal-adult molt and is further enhanced after blood feeding. Similar results were obtained in tubules stimulated in vitro with dibutyryl adenosine 3',5'-cyclic monophosphate. Ultrastructural examination of the microvillar border of the primary cells of the Malpighian tubules revealed that the period of reduced secretion capacity in the pupal tubules is correlated with a marked reduction in microvillar volume, microvillar surface area, and mitochondrial content in the microvillar border. The results suggest that microvilli of a certain size and containing extensions of mitochondria are required for rapid fluid transport. The absence of these conditions in pupal tubules cannot be overcome by in vitro stimulation with known secretagogues and therefore represents a physiological limit on transport performance in the pupal tubules of mosquitoes.

Membrane dynamics in insect malpighian tubules

Urine formation in insects occurs in the Malpighian tubules by means of active ion transport and osmotically coupled water flow. The rates of urine formation can vary with time and can be modulated by diuretic hormones, developmental events, and intracellular parasitism. This paper reviews a number of recent studies in which it has been demonstrated that variations in transport rate are associated with substantial changes in tubule ultrastructure in the form of membrane insertion into and deletion from the apical microvilli. The principal driving force for fluid movement in Malpighian tubules is thought to be a common cation pump located in the apical membranes. It is proposed that modulation of the apical microvillar membrane may reflect regulation by the cells of the number of common cation pump units involved in fluid secretion.

Further characterization of refractoriness in Aedes aegypti (L.) to infection by Dirofilaria immitis (Leidy)

Factors which control the expression of the refractory or susceptible condition to infection with Dirofilaria immitis in the mosquito. Aedes aegypti, were investigated using three protocols. (1) Microfilariae and prelarvae were injected into the hemocoel of susceptible A. aegypti. Some microfilariae and prelarvae developed to the L1 larval stage but they failed to complete development to the infective stage. (2) Enema of microfilariae and prelarvae from infected susceptible and refractory donor females were given into the midgut of uninfected susceptible and refractory recipient females. The results indicate that the conditions which inhibit the initiation of development are present in the Malpighian tubules and not in the midgut of the refractory mosquitoes. (3) Transplants of infected Malpighian tubules from susceptible and refractory donor females were made into the abdominal hemocoel of uninfected susceptible and refractory recipient females. The results showed that the refractory condition depends on the genetic makeup of the donor, not the recipient, mosquito. The above results taken as a whole indicate that the factors which control refractoriness are not present in the midgut but are present in the Malpighian tubule cells of refractory A. aegypti.

An ultrastructural study of Dirofilaria immitis infection in the Malpighian tubules of Anopheles quadrimaculatus

An ultrastructural study was conducted of the Malpighian tubules of Anopheles quadrimaculatus, both uninfected and following infection with Dirofilaria immitis. The Malpighian tubules in Anopheles are composed of primary and stellate cells. The primary cells are the predominant cell type and are characterized by the presence of membrane-bound, intracellular, mineralized concretions and large apical microvilli containing mitochondria. Following the infective blood meal, the microfilariae enter the primary cells of the Malpighian tubules and reside in the cytoplasm in a clear zone without a delimiting membrane. Cells in infected tubules differ from those in uninfected tubules in that the membranes of the vacuoles surrounding the concretions are disrupted in many specimens. The apical and basal cell membranes and the mitochondria associated with these are not disrupted during the first 6-8 days of infection. These observations differ sharply from those previously described in Aedes taeniorhynchus infected with D. immitis. The observations are consistent with the hypothesis that the extended transport capacity observed in previous physiological studies of An. quadrimaculatus infected with D. immitis are dependent on the prolonged normal ultrastructure of the apical microvilli, mitochondria, and basal membranes.

Extracellular accumulation of proline, serine and trehalose in the haemolymph of osmoconforming brackish-water mosquitoes

Larvae of Culex tarsalis, a mosquito, are capable of surviving and developing in dilutions of sea water ranging from 0 mosmol l-1 to 700 mosmol l-1. In waters more dilute than 400 mosmol l-1, the larvae osmoregulate, whereas in those more concentrated than 400 mosmol l-1, the osmotic strength of the haemolymph parallels that of the medium, i.e. the larvae osmoconform. Over the full range of external concentrations tested, the larvae regulate the levels of Na+, K+, Mg2+, Ca2+ and Cl- in the haemolymph. Analyses of haemolymph samples from larvae adapted to media of 50 mosmol l-1 or 600 mosmol l-1 indicate that the increase in haemolymph osmotic concentration observed in media above 400 mosmol l-1 is due to the accumulation of organic compounds, particularly proline, serine and trehalose.

Dissociation of insect Malpighian tubules into single, viable cells

The Malpighian tubules of insects are generally composed of more than one cell type. In the hemipteran Rhodnius prolixus, the tubules are divided into two regions, termed the upper and lower tubules, each of which is composed of a distinct cell type. In the dipteran Aedes taeniorhynchus, primary and stellate cells are interspersed throughout the length of the tubules. We report here techniques for the dissociation of the Malpighian tubules of both of these species into single cells. Tubules are removed from the insect and placed for 1 h in insect Ringer containing elastase (Sigma, type III) at 4 mg/ml. This treatment fully removes the basal lamella. Mild agitation by hand produces a suspension of single cells, which remain viable as determined by Trypan Blue exclusion. Isolated cells have been maintained in cell culture for one week. Using light and scanning electron microscopy, upper and lower tubule cells of Rhodnius and primary and stellate cells of Aedes can be distinguished on the basis of size, shape, microvillar length, and the presence or absence of intracellular crystals.

Ultrastructure of osmoregulatory organs in larvae of the brackish-water mosquito, Culiseta inornata (Williston)

The ultrastructure of the Malpighian tubules, ileum, rectum, anal canal, and anal papillae of larvae of the mosquito Culiseta inornata was examined. The Malpighian tubules, rectum, and anal papillae have many of the ultrastructural features characteristic of ion transport tissues, i.e., elaboration of the basal and apical membranes and a close association of these membranes with mitochondria. The Malpighian tubules possess two cell types, primary and stellate. The larval rectum of C. inornata is composed of a single segment containing a homogenous population of cells. In this respect, the larval rectum of C. inornata is distinct from that of saline-water species of Aedes. The cells in the larval rectum of C. inornata, however, closely resemble those of one cell type, the anterior rectal cells, of the saline-water mosquito Aedes campestris with regard to cell and nuclear size, the percentage of the cell occupied by apical folds, and mitochondrial density and distribution. No similarities can be found between the rectum of C. inornata and the posterior segment of the saline-water Aedes, which functions as a salt gland. On this basis, we have postulated that the rectum of C. inornata does not function as a site of hyperosmotic fluid secretion. The ultrastructure of the anal papillae of C. inornata is consistent with a role in ion transport. The significance of these findings to comparative aspects of osmoregulatory strategies in mosquito larvae is discussed.

The pattern of osmotic regulation in larvae of the mosquito Culiseta inornata

Larvae of Culiseta inornata (Williston) can survive and complete development in dilutions of sea water ranging from 50–700 mosmol kg-1. The larvae hyperregulate with regard to haemolymph osmotic concentration in dilute media (50–400 mosmol kg-1) and osmoconform when external salinities exceed 400 mosmol kg-1. This pattern of osmoregulation is distinct from that observed in freshwater and saline-water mosquito species. We propose that mosquitoes exhibiting this osmoregulatory pattern should be described as ‘brackish-water’ species. Larvae of Culiseta inornata are able closely to regulate both sodium and chloride ion concentrations in the haemolymph over the full range of salinities tested (50–750 mosmol kg-1). The Malpighian tubules produce an isosmotic, potassium-rich fluid. In vitro and in vivo sampling of rectal fluids demonstrates that rectal secretions are isosmotic or only slightly hyperosmotic to the haemolymph and the surrounding saline media, and that they are isotonic with regard to sodium.

Early cellular responses in the Malpighian tubules of the mosquito Aedes taeniorhynchus to infection with Dirofilaria immitis (Nematoda)

Early ultrastructural changes in the Malpighian tubules of the mosquito, Aedes taeniorhynchus, were examined following infection with the nematode, Dirofilaria immitis. After ingestion by the mosquito, the microfilariae enter the cells of the Malpighian tubules, becoming intracellular. During early development, the filarial prelarvae reside in the cell cytoplasm surrounded by a clear zone without a delimiting membrane. Cells infected with prelarvae differed from uninfected cells and from cells in uninfected mosquitoes in that the volume of the apical microvilli was reduced and mitochondria were retracted from these microvilli. Morphometric analysis was used to quantify the ultrastructural consequences of infection. In infected cells, microvillar volume, the percent of microvillar volume occupied by mitochondria, and volume of mitochondria within the microvilli were significantly reduced.

Removal of insect basal laminae using elastase

We have used the enzyme elastase to remove the basal lamina of epithelia from two insects: the upper Malpighian tubules of Rhodnius prolixus and imaginal discs of Drosophila melanogaster. Removal of the basal lamina was confirmed using scanning and transmission electron microscopy. Use of the technique on the Malphighian tubules of Rhodnius reveals for the first time the three-dimensional organization of the circumferential folds of the basal plasma membrane. Elastase is much more effective in removing the basal lamina than are the enzymes hyaluronidase, collagenase, and chymotrypsin, either alone or in combination. Following elastase treatment, cells of the Malpighian tubules dissociate with only mild mechanical agitation into single, viable cells. Treatment with elastase removes the basal laminae of imaginal discs of Drosophila and accelerates evagination as has been previously described for trypsin. To obtain single cell preparations from elastase-treated imaginal discs, mechanical stirring in Ringer low in Ca2+ was required. In addition to its usefulness in cell isolation, elastase treatment allows examination of the effect of removal of basal laminae on the physiology and development of insect epithelia.

Functional design of microvilli in the Malpighian tubules of the insect Rhodnius prolixus

The Malpighian tubules of Rhodnius prolixus are divided into two regions; the upper tubule, which is the site of isosmotic secretion and haemolymph filtration, and the lower tubule where water and KCl are resorbed. In the upper tubule the microvilli are arranged in clumps consisting of several hundred microvilli lying closely parallel. The microvillar plasma membranes do not touch but are held approximately equal to 16 nm apart along the full length of the microvilli. As a consequence, the extracellular space between the microvilli consists of long narrow channels. A morphometric analysis of extracellular, cytoplasmic, endoplasmic reticular and mitochondrial volume within the clumps was conducted. Using the secretion rate of the epithelium and the channel dimensions, it was calculated that the mean residence time for secreted fluid in the intermicrovillar spaces was approximately equal to 0.4s. In view of our current knowledge of the physiology and morphology of the upper tubule, it is argued: (1) that osmotically driven water passes principally through the cells, not the junctional spaces; and (2) that the microvillar clumps are a morphological specialization, which serves to maximize solute-water coupling in the upper tubule. The microvilli in the lower tubule are free-standing, with no pattern of clumping as in the upper tubule. The axopods are about twice as long as the microvilli (10-14 micron) and are found in all regions of the lower tubule. This is in agreement with the proposal that the motile axopods serve to propel uric acid crystals through the lower tubule. No morphological difference was found between the upper and lower halves of the lower tubule, although the two portions are known to be physiologically distinct.

The ultrastructure of the larval malpighian tubules of a saline-water mosquito

The larval Malpighian tubules of the saline-water mosquito Aedes taeniorhynchus were examined using light and electron microscopy. The tubules contain two cell types; primary cells and stellate cells. Primary cells are characterized by their size (70 microns x 70 microns x 10 microns) and an abundance of intracellular membrane-bound crystals. Two types of microvilli are found on the luminal surface of the primary cells: (1) small microvilli containing core microfilaments and extensions of endoplasmic reticulum, and (2) larger microvilli (approximately equal to 3 microns in length) which in addition to the above components contain a mitochondrion along their entire length. Both microvillar types have abundant knobs lining the cytoplasmic surface of the microvillar membrane. These knobs, which are often found in insect ion transporting tissues, have been termed 'portasomes' by Harvey (1980). The possible role of these structures in ion transport and mitochondrial positioning is discussed. The stellate cells are much smaller than the primary cells, and lack intracellular crystals. Their microvilli are smaller as well (approximately equal to 0 x 6 microns in length) and contain no endoplasmic reticulum, mitochondria or knobs. The cells types found in the saline-water mosquito larva, Aedes taeniorhynchus, are identical to those found in Aedes aegypti, indicating that the unique capacity of saline-water mosquito larvae to transport Mg2+ and SO42 is not associated with the presence of an additional cell type.

5-hydroxytryptamine-stimulated mitochondrial movement and microvillar growth in the lower malpighian tubule of the insect, Rhodnius prolixus

Rapid initiation of ion transport occurs in the lower Malpighian tubule of the insect Rhodnius prolixus following feeding in vivo or stimulation with 5-hydroxytryptamine (5-HT) in vitro. Using the electron microscope, we have conducted a morphometric analysis of cells in the lowest one-third of the lower tubule, demonstrating that 5-HT also induces mitochondrial movement and microvillar growth simultaneously with, but independent of, the onset of ion transport. Mitochondria move from a position below the cell cortex to one inside the microvilli within 10 min of stimulation with 5-HT, resulting in an 8- to 10-fold increase in the volume of mitochondria within the microvilli. Previous findings indicated that mitochondrial movement is dependent on actin-containing microfilaments, but not microtubules. As the mitochondria enter the microvillus, the core microfilaments are reorganized into a sheath of microfilaments, which extends closely parallel to the outer mitochondrial membrane down into the cell interior. This sheath of microfilaments is also observed around mitochondria in the axopods. We suggest that the core microfilaments are responsible for mitochondrial movement into the microvilli and axopods. Stimulation with 5-HT induces a shift in mitochondrial configuration from orthodox to condensed, indicating a possible increase in oxidative phosphorylation. Following stimulation, the microvilli grow about 3 X in volume and 2.5 X in surface area. These increases are more than can be accounted for by mitochondrial invasion and must involve the addition of new membrane and microfilament polymerization. The observed changes - microvillar growth, insertion of additional membrane, activation and movement of mitochondria adjacent to the ion transport membrane - are described in the light of their significance in ion transport. A simple model is proposed which unifies the observed ultrastructural changes and known ion movements in the lower tubule.

Insect axopods

Using the elctron microscope we have found axopods, a cell organelle previously undescribed in multicellular animals, in the lower Malpighian tubule of the insect. Rhodnius prolixus. The axopods, which are 0.2 to 0.8 micrometer in diameter and 10 or more micrometer in length, derive from the luminal surface of the tubule and contain an array of 1 to about 46 microtubules each. These microtubules arise within the cell near the cell junctions or near clumps of mitochondria. Uric acid crystals which occur naturally in the lower tubule have been observed to move down the tubule under experimental conditions where peristalsis and fluid secretion can be ruled out. We suggest that the axopods are motile and serve to transport the crystals along the narrow tubule lumen. Since cilia are not found on somatic cells of arthropods, we suggest that axopods have evolved in the lower tubule to perform a function analogous to a ciliated epithelium in other animals.

Evidence of microfilament-associated mitochondrial movement

The mitochondria in the lower Malpighian tubule of the insect Rhodnius prolixus can be stimulated by feeding in vivo and by 5-hydroxytryptamine in vitro, to move from a position below the cell cortex to one inside the apical microvilli. During and following their movement into the microvilli, the mitochondria are intimately associated with the microfilaments of the cell cortex and microvillar core bundle. Bridges approximately 14 nm in length and 4 nm in diameter are observed connecting the microvillar microfilaments to the outer mitochondrial membrane and microvillar plasma membrane. Depolymerization of all visible microtubules with colchicine does not inhibit 5-HT-stimulated mitochondrial movement. On the other hand, treatment with cytochalasin B does block mitochondrial movement, suggesting that microfilaments play a role in the mitochondrial motility. We have labeled the microvillar microfilaments, which are 6 nm in diameter, with heavy meromyosin, which supports the contention that they contain actin. A model of the mechanism of mitochondrial movement is presented in which mitochondria slide into position in the microvilli along actin-containing microfilaments in a manner analogous to the sliding actin-myosin model of skeletal muscle.

The effect of external salinity on drinking rate and rectal secretion in the larvae of the saline-water mosquito Aedes taeniorhynchus

1. The drinking rate of the saline-water mosquito larva Aedes taeniorhyncus (100 nl.mg-1.h-1) is unaffected by the salinity of the external medium, but is directly proportional to the surface area of the animal. 2. Haemolymph Na+, Mg2+, K+, Cl-, SO42- and osmotic concentrations were measured in larvae adapted to 10%, 100% and 200% seawater and were found to be regulated within a narrow range. 3. With the exception of potassium, ionic concentrations in rectal secretion were found to increase with increasing concentrations of the sea water in which larvae were reared. 4. The osmotic concentration of rectal secretion was unaffected by changes in haemolymph osmotic concentration but did rise when sodium or chloride concentrations of the haemolymph were increased. High levels of these ions also stimulated the rate of fluid secretion. 5. Transport of chloride and sodium by the rectum exhibits the kinetics of allosteric rather than classical enzymes.