Malpighian tubules ofTrichoplusia ni: recycling ions via gap junctions and switching between secretion and reabsorption of Na+ and K+ in the distal ileac plexus

Abstract

The functional kidney in insects consists of the Malpighian tubules and hindgut. Malpighian tubules secrete ions and fluid aiding in hydromineral homeostasis, acid-base balance and metabolic waste excretion. In many insects, including lepidopterans, the Malpighian tubule epithelium consists of principal cells (PCs) and secondary cells (SCs). The SCs in the Malpighian tubules of larvae of the lepidopteran have been shown to reabsorb K, transporting it in a direction opposite to that in the neighbouring PCs that secrete K One of the mechanisms that could enable such an arrangement is a gap junction (GJ)-based coupling of the two cell types. In the current study, we have immunolocalized GJ protein Innexin-2 to the PC-PC and SC-PC cell-cell borders. We have demonstrated that GJs in the SC-containing region of the Malpighian tubules enable Na and K reabsorption by the SCs. We also demonstrated that in ion-loaded animals, PCs switch from Na/K secretion to reabsorption resulting in an ion-transporting phenotype similar to that of tubules with pharmacologically blocked GJs. Concomitantly, mRNA abundance encoding GJ proteins was downregulated. Finally, we observed that such PC-based reabsorption was only present in the distal ileac plexus connected to the rectal complex. We propose that this plasticity in the PC function in the distal ileac plexus is likely to be aimed at providing an ion supply for the SC function in this segment of the tubule.

Septate junction in the distal ileac plexus of larval lepidopteranTrichoplusia ni: alterations in paracellular permeability during ion transport reversal

“…Whole-mount immunohistochemistry (IHC) on the DIP of the larval T. ni was performed as previously described (Kolosov et al, 2018a). Briefly, DIPs from 5th instar larvae were dissected in lepidopteran saline and fixed in 4% paraformaldehyde (PFA) in phosphate-buffered saline (PBS, pH 7.4) overnight at 4°C.…”

“…Firstly, the SCs in this region reabsorb K + and Na + into the haemolymph, while PCs secrete them (O'Donnell and Ruiz-Sanchez, 2015). This ion transport mechanism is thought to be driven, at least in part, by the gap junction-based coupling between the SCs and PCs (Kolosov et al, 2018a). This arrangement is thought to play an important role in making ionoregulation and acid-base balance more efficient in growing larvae .…”

“…This paradoxical reabsorption in situ is thought to be aimed at haemolymph expansion and extraction of ions from the diet by way of the RC and MTs (Kolosov et al, 2018a, b;. Ion reabsorption by the DIP, however, is reversible; when excised and mounted for a Ramsay assay, an isolated DIP promptly reverts to ion secretion (Kolosov et al, 2018a). This probably takes place because of the removal of the DIP from the influence of endocrine factors circulating in the haemolymph and the innate ability of the DIP to autonomously sense changes in luminal ion supply from the rectal complex .…”

“…We have also demonstrated that ion transport reversal in the DIP is associated with a decrease in transcript abundance of many transcellular ion transport components (Kolosov et al, 2018a) and might be modulated by a lepidopteran kinin, helicokinin (HK) (Kolosov and O'Donnell, 2018). With this in mind, the goal of the current study was to identify the molecular components of the SJ, profile their expression and quantify changes in their abundance in the DIP of larvae fed ion-rich diets.…”

Water and ion transport across the eversible vesicles in the collophore of a springtailOrchesella cincta

“…The hardware, software and methodology for acquiring SIET data and calculating ion fluxes have been described previously (Donini and O'Donnell, 2005;O'Donnell and Ruiz-Sanchez, 2015;Kolosov et al, 2018a). Briefly, SIET measurements were made with hardware from Applicable Electronics (Forestdale, MA, USA) and Automated Scanning Electrode Technique (ASET) software (version 2.0; Science Wares, Falmouth, MA, USA).…”