Post-feeding induction of trypsin in the midgut of Aedes aegypti L. (Diptera: Culicidae) is separable into two cellular phases

Early trypsin, a female-specific midgut protease in Aedes aegypti: isolation, aminoterminal sequence determination, and cloning and sequencing of the gene

Early trypsin is a female-specific protease present in the Aedes aegypti midgut during the first hours after ingestion of a blood meal. It plays an essential role in the transcriptional activation of the late trypsin form, the major midgut endoprotease involved in the blood meal digestion. Early trypsin is the most abundant midgut polypeptide isolated by benzamidine-sepharose affinity chromatography 3 h after feeding. The amino-terminal sequence of the early trypsin protein matches that of the 3a1 cDNA for a putative trypsinogen described by Kalhok et al. (Insect. Molec. Biol., 2, 71-79, 1993). The early trypsin cDNA was over expressed in Escherichia coli. Polyclonal antibodies generated against this recombinant protein were used to show that the enzyme was present in the midgut during the first 4 h after feeding. A 2.5 kb genomic clone of the early trypsin was isolated, mapped and subcloned. A 1.56 kb subclone, corresponding to 1303 bp of the upstream regulatory region and 265 bp of the coding region, was sequenced. The gene contains a 64 nucleotide intron which interrupts the codon for Val at position 18 of the protein. This Val is located toward the end of the putative signal sequence of the protein.

Aedes aegypti midgut early trypsin is post-transcriptionally regulated by blood feeding

Early trypsin is a female-specific protease present in the Aedes aegypti midgut during the first hours after ingestion of a blood meal. Early trypsin gene expression was studied by Northern blot analysis. The early trypsin mRNA, absent in larvae, pupae and newly emerged females, reaches detectable levels at 24 h post-emergence and attains a maximum level at an adult age of 4-7 days. After the first week there is a decrease in the steady-state level of the transcript, but it remains readily detectable for up to a month after emergence. Despite the high levels of early trypsin mRNA present in the midgut of the unfed female, translation of the early trypsin mRNA occurs only after a blood or a protein meal. Early trypsin mRNA levels rapidly decrease during the first 24 h after feeding, but the steady-state level of the transcript rises again at the end of the blood digestion cycle (60 h), as the mosquito prepares for a second blood meal.

Early trypsin activity is part of the signal transduction system that activates transcription of the late trypsin gene in the midgut of the mosquito, Aedes aegypti

Trypsin activity during the first hours after feeding is essential to induce late trypsin gene expression. These results are consistent with the idea that free amino acids or other products released during digestion might be the initial signal for transcriptional activation of late trypsin. Besides early trypsin, some other factor(s) have to be translated for induction of late trypsin. This is the first case in which the proteolytic activity of a digestive enzyme is part of the signal transduction system which regulates expression of a second gene. The presence of two trypsins allows the mosquito to assess the quality of the meal and adjust the levels of late trypsin for a particular meal with remarkable flexibility.

Dietary control of late trypsin gene transcription in Aedes aegypti

In Aedes aegypti the levels of midgut trypsin activity after feeding are directly proportional to the protein concentration in the meal. The mechanisms of this up-regulatory event were investigated by analyzing the expression of the late trypsin gene under different dietary conditions. Transcription of the gene was dependent on both the quality and quantity of protein in the meal. As measured by Northern blot analysis, the levels of late trypsin gene expression increased up to 100-fold 24 h after feeding on gamma-globulin, hemoglobin or albumin (100 mg/ml). In contrast, gelatin, histone, amino acids, saline or agarose were very poor inducers of transcription. The rates of late trypsin transcription induced during the first 24 h were directly proportional to the concentration of protein in the meal. These data further support the suggestion that the primary mechanism that regulates the synthesis of trypsin in the mosquito midgut is transcriptional regulation of the gene. This regulatory mechanism enables the midgut to maintain the appropriate balance between protease synthesis and the protein content of the meal.

Approaches to vector control: new and trusted. 2. Molecular targets in the insect midgut

The midgut of blood-feeding insects is an important site for the activity of antibodies and drugs ingested with the blood meal. These agents can be directed at molecular targets in the midgut, and may affect the insect directly by reducing its fitness, or indirectly by blocking transmission of a disease organism to the vector. Both of these result in eventual disease control. Immunization with crude vector or parasite preparations can result in isolation of vaccine candidates which are very effective but often of unknown function. Conversely, by examining carefully the vector's biology and its interactions with the parasite, it is possible to identify various physiological or cellular systems and vector-parasite interactions that can be interfered with. Examples of both approaches are presented in this paper. Anti-vector vaccines offer tremendous potential for disease control, as they can affect the parasite reproductive rate in a number of ways--reducing vector longevity, fecundity or competence. This is most striking if life expectancy of the vector is reduced to a period less than the extrinsic life cycle of the parasite. The rationale often presented for examining molecular systems in vectors is their control potential; it is clearly appropriate to re-examine many of these systems and realistically assess their applicability.

Vector-parasite interactions for vaccine development

The ingestion of blood by arthropod vectors of disease can be exploited in order to either kill the vector or render it incapable of disease transmission. This paper examines some approaches to identifying target molecules of vector origin, against which immunisation could result in blocking parasite transmission. Manipulation of the blood meal of vectors through such techniques as membrane feeding can help identify true target sites for attack, but just as useful, can identify structures or molecules that play no significant role in parasite development. Examples, mostly derived from the interactions between the malaria parasite, Plasmodium, and the mosquito midgut, illustrate the real need to understand the multiple aspects of vector-parasite interactions before they can be exploited for control purposes. The approaches outlined are however applicable directly to any vector-borne disease. Careful examination of the parasite life cycle in the vector, and comparisons with other parasites, vectors, non-vector insects and analogous vertebrate systems (the latter being often relatively well advanced) can result in the identification of specific and definable interactions which can then be further developed for vaccine purposes.

A family of serine protease genes expressed in adult buffalo fly (Haematobia irritans exigua)

Gene fragments encoding serine proteases expressed in adult buffalo fly (Haematobia irritans exigua) were amplified from cDNA using generic oligonucleotide PCR primers, based on conserved residues surrounding the active-site His and Ser amino acids found in all serine proteases. The PCR product consisted of a broad band extending from about 450 bp to 520 bp, which suggested that the PCR product actually consisted of numerous DNA fragments of slightly variable sizes. Seventeen independent clones of these fragments, each with an insert of approximately 480 bp, were digested with HaeIII. Comparison of restriction fragment patterns indicated that 13 of these clones harboured different PCR products. This was confirmed by DNA sequence analysis of 9 clones. Each of the sequenced clones contained an open reading frame which included structurally conserved regions characteristic of the serine protease superfamily. This study reveals the expression of a large and highly variable repertoire of serine proteases in adult buffalo fly. Importantly, these data also demonstrate the utility of such an approach in obtaining DNA probes for use in further investigations of gene family organization and expression, as well as providing recombinant antigens in the form of fusion proteins which may be used as candidates for vaccine production.