Immunolocalization of cuticular proteins in Johnston's organ and the corneal lens of Anopheles gambiae

A cuticular protein, BgCPLCP1, contributes to insecticide resistance by thickening the cockroach endocuticle

Overuse of insecticides has led to severe environmental problems. Insect cuticle, which consists mainly of chitin, proteins and a thin outer lipid layer, serves multiple functions. Its prominent role is as a physical barrier that impedes the penetration of xenobiotics, including insecticides. Blattella germanica (L.) is a major worldwide indoor pest that causes allergic disease and asthma. Extensive use of pyrethroid insecticides, including β-cypermethrin, has selected for the rapid and independent evolution of resistance in cockroach populations on a global scale. We demonstrated that BgCPLCP1, the first CPLCP (cuticular proteins of low complexity with a highly repetitive proline-rich region) family cuticular protein in order Blattodea, contributes to insecticide penetration resistance. Silencing BgCPLCP1 resulted in 85.0 %-85.7 % and 81.0 %-82.0 % thinner cuticle (and especially thinner endocuticle) in the insecticide-susceptible (S) and β-cypermethrin-resistant (R) strains, respectively. The thinner and more permeable cuticles resulted in 14.4 % and 20.0 % lower survival of β-cypermethrin-treated S- and R-strain cockroaches, respectively. This study advances our understanding of cuticular penetration resistance in insects and opens opportunities for the development of new efficiently and environmentally friendly insecticides targeting the CPLCP family of cuticular proteins.

Mosquitoes Possess Specialized Cuticular Proteins That Are Evolutionarily Related to the Elastic Protein Resilin

Resilin is an elastic protein that is vital to insects' vigorous movement. Canonical resilin proteins possess the R&R Consensus, a chitin-binding domain conserved in a family of cuticular proteins, and highly repetitive sequences conferring elastic properties. In the malaria vector mosquito, Anopheles gambiae, however, a cuticular protein has been found that has an R&R Consensus resembling that of resilin but lacks the repetitive sequences (here, we call it resilin-related or resilin-r). The relationship between resilin-r and resilin was unclear. It was also unknown whether resilin-r is conserved in mosquitoes. In this paper, phylogenetic and structural analyses were performed to reveal the relationship of resilin homologous proteins from holometabolous insects. Their chitin-binding abilities were also assessed. A resilin-r was found in each mosquito species, and these proteins constitute a clade with resilin from other insects based on the R&R Consensus sequences, indicating an evolutionary relationship between resilin-r and resilin. The resilin-r showed chitin-binding activity as same as resilin, but had distinct structural features from resilin, suggesting that it plays specialized roles in the mosquito cuticle. Another resilin-like protein was found to exist in each holometabolous insect that possesses resilin-like repetitive sequences but lacks the R&R Consensus. These results suggest that similar evolutionary events occurred to create resilin-r and resilin-like proteins.

Global Gene Expression Analysis Reveals Complex Cuticle Organization of the Tribolium Compound Eye

The red flour beetle Tribolium castaneum is a resource-rich model for genomic and developmental studies. To extend previous studies on Tribolium eye development, we produced transcriptomes for normal-eyed and eye-depleted heads of pupae and adults to identify differentially transcript-enriched (DE) genes in the visual system. Unexpectedly, cuticle-related genes were the largest functional class in the pupal compound eye DE gene population, indicating differential enrichment in three distinct cuticle components: clear lens facet cuticle, highly melanized cuticle of the ocular diaphragm, which surrounds the Tribolium compound eye for internal fortification, and newly identified facet margins of the tanned cuticle, possibly enhancing external fortification. Phylogenetic, linkage, and high-throughput gene knockdown data suggest that most cuticle proteins (CPs) expressed in the Tribolium compound eye stem from the deployment of ancient CP genes. Consistent with this, TcasCPR15, which we identified as the major lens CP gene in Tribolium, is a beetle-specific but pleiotropic paralog of the ancient CPR RR-2 CP gene family. The less abundant yet most likely even more lens-specific TcasCP63 is a member of a sprawling family of noncanonical CP genes, documenting a role of local gene family expansions in the emergence of the Tribolium compound eye CP repertoire. Comparisons with Drosophila and the mosquito Anopheles gambiae reveal a steady turnover of lens-enriched CP genes during insect evolution.

A knockout screen of genes expressed specifically in Ae. aegypti pupae reveals a critical role for stretchin in mosquito flight

Aedes aegypti is a critical vector for transmitting Zika, dengue, chikungunya, and yellow fever viruses to humans. Genetic strategies to limit mosquito survival based upon sex distortion or disruption of development may be valuable new tools to control Ae. aegypti populations. We identified six genes with expression limited to pupal development; osi8 and osi11 (Osiris protein family), CPRs and CPF (cuticle protein family), and stretchin (a muscle protein). Heritable CRISPR/Cas9-mediated gene knockout of these genes did not reveal any defects in pupal development. However, stretchin-null mutations (strn^Δ35/Δ41) resulted in flightless mosquitoes with an abnormal open wing posture. The inability of adult strn^Δ35/Δ41 mosquitoes to fly restricted their escape from aquatic rearing media following eclosion, and substantially reduced adult survival rates. Transgenic strains which contain the EGFP marker gene under the control of strn regulatory regions (0.8 kb, 1.4 kb, and 2.2 kb upstream, respectively), revealed the gene expression pattern of strn in muscle-like tissues in the thorax during late morphogenesis from L₄ larvae to young adults. We demonstrated that Ae. aegypti pupae-specific strn is critical for adult mosquito flight capability and a key late-acting lethal target for mosquito-borne disease control.

Transcriptomic, proteomic and ultrastructural studies on salinity-tolerant Aedes aegypti in the context of rising sea levels and arboviral disease epidemiology

BACKGROUND

Aedes aegypti mosquito, the principal global vector of arboviral diseases, lays eggs and undergoes larval and pupal development to become adult mosquitoes in fresh water (FW). It has recently been observed to develop in coastal brackish water (BW) habitats of up to 50% sea water, and such salinity tolerance shown to be an inheritable trait. Genomics of salinity tolerance in Ae. aegypti has not been previously studied, but it is of fundamental biological interest and important for controlling arboviral diseases in the context of rising sea levels increasing coastal ground water salinity.

RESULTS

BW- and FW-Ae. aegypti were compared by RNA-seq analysis on the gut, anal papillae and rest of the carcass in fourth instar larvae (L4), proteomics of cuticles shed when L4 metamorphose into pupae, and transmission electron microscopy of cuticles in L4 and adults. Genes for specific cuticle proteins, signalling proteins, moulting hormone-related proteins, membrane transporters, enzymes involved in cuticle metabolism, and cytochrome P450 showed different mRNA levels in BW and FW L4 tissues. The salinity-tolerant Ae. aegypti were also characterized by altered L4 cuticle proteomics and changes in cuticle ultrastructure of L4 and adults.

CONCLUSIONS

The findings provide new information on molecular and ultrastructural changes associated with salinity adaptation in FW mosquitoes. Changes in cuticles of larvae and adults of salinity-tolerant Ae. aegypti are expected to reduce the efficacy of insecticides used for controlling arboviral diseases. Expansion of coastal BW habitats and their neglect for control measures facilitates the spread of salinity-tolerant Ae. aegypti and genes for salinity tolerance. The transmission of arboviral diseases can therefore be amplified in multiple ways by salinity-tolerant Ae. aegypti and requires appropriate mitigating measures. The findings in Ae. aegypti have attendant implications for the development of salinity tolerance in other fresh water mosquito vectors and the diseases they transmit.

Proteomics reveals localization of cuticular proteins in Anopheles gambiae

Anopheles gambiae devotes over 2% of its protein coding genes to its 298 structural cuticular proteins (CPs). This paper provides new LC-MS/MS data on two adult structures, proboscises and palps, as well as three larval samples - 4th instar larvae, just their terminal segment, and a preparation enriched in their tracheae. These data were combined with our previously published results of proteins from five other adult structures, whole adults, and two preparations chosen for their relatively clean cuticle, the larval head capsules left behind after ecdysis and the pupal cuticles left behind after adult eclosion. Peptides from 28 CPs were recovered in all adult structures; 24 CPs were identified for the first time, 6 of these were members of the TWDL family. Most newly identified proteins came from the larval sources. Based solely on peptide recovery, from our data and from other investigators, most available on VectorBase, there were only 4 CPs that were restricted to a single adult structure. More were restricted to a single metamorphic stage, 14 in larvae, 0 in pupae and 32 in adults. Expression data from our earlier RT-qPCR studies reduces these numbers. Charting restriction of CPs to stage or structure is a step forward in establishing their specific roles.

Culex pipiens pallens cuticular protein CPLCG5 participates in pyrethroid resistance by forming a rigid matrix

Background

Chemical insecticides have hugely reduced the prevalence of vector-borne diseases around the world, but resistance threatens their continued effectiveness. Despite its importance, cuticle resistance is an under-studied area, and exploring the detailed molecular basis of resistance is critical for implementing suitable resistance management strategies.

Methods

We performed western blotting of cuticular protein CPLCG5 in deltamethrin-susceptible (DS) and laboratory-produced deltamethrin-resistant (DR) strains of Culex pipiens pallens. Immunofluorescence assays using a polyclonal antibody to locate cuticular CPLCG5 in mosquitoes. EM immunohistochemical analysis of the femur segment was used to compare the cuticle in control and CPLCG5-deficient siRNA experimental groups.

Results

The gene CPLCG5 encodes a cuticle protein that plays an important role in pyrethroid resistance. Based on a prior study, we found that expression of CPLCG5 was higher in the resistant (DR) strain than the susceptible (DS) strain. CPLCG5 transcripts were abundant in white pupae and 1-day-old adults, but expression was dramatically decreased in 3-day-old adults, then remained stable thereafter. Western blotting revealed that the CPLCG5 protein was ~2.2-fold higher in the legs of the DR strain than the DS strain. Immunofluorescence assays revealed CPLCG5 expression in the head, thorax, abdomen, wing, and leg, and expression most abundant in the leg and wing. EM immunohistochemical analysis suggested that the exocuticle thickness of the femur was significantly thinner in the CPLCG5-deficient siCPLCG5 strain (0.717 ± 0.110 μm) than the siNC strain (0.946 ± 0.126 μm). Depletion of CPLCG5 by RNA interference resulted in unorganised laminae and a thinner cuticle.

Conclusions

The results suggest CPLCG5 participates in pyrethroid resistance by forming a rigid matrix and increasing the thickness of the cuticle.

Electronic supplementary material

The online version of this article (doi: 10.1186/s13071-017-2567-9) contains supplementary material, which is available to authorized users.

Transcriptome Analysis of the Triatoma infestans (Hemiptera: Reduviidae) Integument

The insect integument, formed by the cuticle and the underlying epidermis, is essential for insect fitness, regulation of lipid biosynthesis and storage, insect growth and feeding, together with development progress. Its participation in insecticide resistance has also been outlined. Triatoma infestans Klug (Hemiptera: Reduviidae) is one of the major vectors of Chagas disease in South America; however, genomic data are scarce. In this study, we performed a transcriptome analysis of the nymph integument in order to identify which genes are expressed and their putative role. Using the 454 GS-FLX sequencing platform, we obtained approximately 144,620 reads from the integument tissue. These reads were assembled into 6,495 isotigs and 8,504 singletons. Based on BLAST similarity searches, about 8,000 transcripts were annotated with known genes, conserved domains, and/or Gene Ontology terms.The most abundant transcripts corresponded to transcription factors and nucleic acid metabolism, membrane receptors, cell signaling, and proteins related to cytoskeleton, transport, and cell energy processes, among others. More than 10% of the transcripts-encoded proteins putatively involved in the metabolism of fatty acids and related components (fatty acid synthases, elongases, desaturases, fatty alcohol reductases), structural integument proteins, and the insecticide detoxification system (among them, cytochrome P450s, esterases, and glutathione transferases). Real-time qPCR assays were used to investigate their putative participation in the resistance mechanism. This preliminary study is the first transcriptome analysis of a triatomine integument, and together with prior biochemical information, will help further understandthe role of the integument in a wide array of mechanisms.

Contributions of cuticle permeability and enzyme detoxification to pyrethroid resistance in the major malaria vector Anopheles gambiae

To tackle the problem of insecticide resistance, all resistance mechanisms need to be studied. This study investigated the involvement of the cuticle in pyrethroid resistance in a strain of Anopheles gambiae, MRS, free of kdr mutations. Bioassays revealed MRS to be resistant to pyrethroids and DDT, indicated by increasing knockdown times and resistance ratios. Moreover, biochemical analysis indicated that metabolic resistance based on enhanced CYP450 activity may also play a role. Insecticide penetration assays showed that there were significantly lower amounts of insecticide in the MRS strain than in the susceptible control. Analysis of the levels of the selected transcripts by qPCR showed that CYP6M2, a major pyrethroid metaboliser, CYP4G16, a gene implicated in resistance via its contribution to the biosynthesis of elevated epicuticular hydrocarbons that delay insecticide uptake, and the cuticle genes CPAP3-E and CPLCX1 were upregulated after insecticide exposure. Other metabolic (CYP6P3, GSTe2) and cuticle (CPLCG3, CPRs) genes were also constitutively upregulated. Microscopic analysis showed that the cuticle layers of the MRS strain were significantly thicker than those of the susceptible strain. This study allowed us to assess the contribution made by the cuticle and metabolic mechanisms to pyrethroid resistance in Anopheles gambiae without target-site mutations.

Properties of the cuticular proteins of Anopheles gambiae as revealed by serial extraction of adults

How cuticular proteins (CPs) interact with chitin and with each other in the cuticle remains unresolved. We employed LC-MS/MS to identify CPs from 5–6 day-old adults of Anopheles gambiae released after serial extraction with PBS, EDTA, 2-8M urea, and SDS as well as those that remained unextracted. Results were compared to published data on time of transcript abundance, localization of proteins within structures and within the cuticle, as well as properties of individual proteins, length, pI, percent histidine, tyrosine, glutamine, and number of AAP[A/V/L] repeats. Thirteen proteins were solubilized completely, all were CPRs, most belonging to the RR-1 group. Eleven CPs were identified in both soluble fractions and the final pellet, including 5 from other CP families. Forty-three were only detected from the final pellet. These included CPRs and members of the CPAP1, CPF, CPFL, CPLCA, CPLCG, CPLCP, and TWDL families, as well as several low complexity CPs, not assigned to families and named CPLX. For a given protein, many histidines or tyrosines or glutamines appear to be potential participants in cross-linking since we could not identify any peptide bearing these residues that was consistently absent. We failed to recover peptides from the amino-terminus of any CP. Whether this implicates that location in sclerotization or some modification that prevents detection is not known. Soluble CPRs had lower isoelectric points than those that remained in the final pellet; most members of other CP families had isoelectric points of 8 or higher. Obviously, techniques beyond analysis of differential solubility will be needed to learn how CPs interact with each other and with chitin.