Transcriptomic analysis of the Malpighian tubules of Trichoplusia ni: Clues to mechanisms for switching from ion secretion to ion reabsorption in the distal ileac plexus
Kolosov, D., Donly, C., MacMillan, H., O'Donnell, M.J. (2019). Transcriptomic analysis of the Malpighian tubules of Trichoplusia ni: Clues to mechanisms for switching from ion secretion to ion reabsorption in the distal ileac plexus. Journal of Insect Physiology, [online] 112 73-89. http://dx.doi.org/10.1016/j.jinsphys.2018.12.005
Plain language summary
In insects, metabolic wastes and toxins are excreted by specialized organs called Malpighian tubules (MTs) and this activity is coupled to the secretion of ions and water. The larvae of lepidopterans (moths and butterflies) have complex and regionalized MTs which include a specialized segment called the distal iliac plexus (DIP). In the caterpillar of the cabbage looper, Trichoplusia ni, the DIP region of the tubule can reverse the direction of ion transport in response to changing amounts of ions in the diet. In this paper, mass sequencing of all the mRNAs expressed in this tissue was used to identify the transport mechanisms, and the regulation systems controlling them, that enable this unique transport reversal process. The genes whose expression changed when dietary ion levels changed were found to be diverse, and affect many physiological processes including ion and nutrient transport, toxin elimination, hormone signaling, cell structure maintenance, and others. This information will be useful in identifying effective molecular targets for disruption in pest insects that can be used to make crop damaging larvae more susceptible to control products.
Excretion of metabolic wastes and toxins in insect Malpighian tubules (MTs) is coupled to secretion of ions and fluid. Larval lepidopterans demonstrate a complex and regionalized MT morphology, and recent studies of larvae of the cabbage looper, Trichoplusia ni, have revealed several unusual aspects of ion transport in the MTs. Firstly, cations are reabsorbed via secondary cells (SCs) in T. ni, whereas in most insects SCs secrete ions. Secondly, SCs are coupled to neighbouring principal cells (PCs) via gap junctions to enable such ion reabsorption. Thirdly, PCs in the SC-containing distal ileac plexus (DIP) region of the tubule reverse from cation secretion to reabsorption in response to dietary ion loading. Lastly, antidiuresis is observed in response to a kinin neuropeptide, which targets both PCs and SCs, whereas in most insects kinins are diuretics that act exclusively via SCs. Recent studies have generated a basic model of ion transport in the DIP of the larval T. ni. RNAseq was used to elucidate previously uncharacterised aspects of ion transport and endocrine regulation in the DIP, with the aim of painting a composite picture of ion transport and identifying putative regulatory mechanisms of ion transport reversal in this tissue. Results indicated an overall expression of 9103 transcripts in the DIP, 993 and 382 of which were differentially expressed in the DIP of larvae fed high-K+ and high-Na+ diets respectively. Differentially expressed transcripts include ion-motive ATPases, ion channels and co-transporters, aquaporins, nutrient and xenobiotic transporters, cell adhesion and junction components, and endocrine receptors. Notably, several transcripts for voltage-gated ion channels and cell volume regulation-associated products were detected in the DIP and differentially expressed in larvae fed ion-rich diet. The study provides insights into the transport of solutes (sugars, amino acids, xenobiotics, phosphate and inorganic ions) by the DIP of lepidopterans. Our data suggest that this region of the MT in lepidopterans (as previously reported) transports cations, fluid, and xenobiotics/toxic metals. Besides this, the DIP expresses genes coding for the machinery involved in Na+- and H+-dependent reabsorption of solutes, chloride transport, and base recovery. Additionally, many of the transcripts expressed by the DIP a capacity of this region to respond to, process, and sometimes produce, neuropeptides, steroid hormones and neurotransmitters. Lastly, the DIP appears to possess an arsenal of septate junction components, differential expression of which may indicate junctional restructuring in the DIP of ion-loaded larvae.