Cellular localization of a putative Na+/H+ exchanger 3 during ontogeny in the pronephros and mesonephros of the Japanese black salamander (Hynobius nigrescens Stejneger)

Renal protein reabsorption impairment related to a myxosporean infection in the grass frog (Rana temporaria L.)

A morphophysiological study of tubular reabsorption and mechanisms of protein endocytosis in the kidney of frogs (Rana temporaria L.) during parasitic infection was carried out. Pseudoplasmodia and spores of myxosporidia, beforehand assigned to the genus Sphaerospora, were detected in Bowman's capsules and in the lumen of individual renal tubules by light and electron microscopy. Remarkable morphological alteration and any signs of pathology in kidney tissue related to this myxosporean infection have not been noted. At the same time, significant changes in protein reabsorption and distribution of molecular markers of endocytosis in the proximal tubule (PT) cells in infected animals were detected by immunofluorescence confocal microscopy. In lysozyme injection experiments, the endocytosed protein and megalin expression in the infected PTs were not revealed. Tubular expression of cubilin and clathrin decreased, but endosomal recycling marker Rab11 increased or remained unchanged. Thus, myxosporean infection resulted in the alterations in lysozyme uptake and expression of the main molecular determinants of endocytosis. The inhibition of receptor-mediated clathrin-dependent protein endocytosis in amphibian kidneys due to myxosporidiosis was shown for the first time. Established impairment of the endocytic process is a clear marker of tubular cell dysfunction that can be used to assess the functioning of amphibian kidneys during adaptation to adverse environmental factors.

Teleostean fishes may have developed an efficient Na+ uptake for adaptation to the freshwater system

Understanding Na⁺ uptake mechanisms in vertebrates has been a research priority since vertebrate ancestors were thought to originate from hyperosmotic marine habitats to the hypoosmotic freshwater system. Given the evolutionary success of osmoregulator teleosts, these freshwater conquerors from the marine habitats are reasonably considered to develop the traits of absorbing Na⁺ from the Na⁺-poor circumstances for ionic homeostasis. However, in teleosts, the loss of epithelial Na⁺ channel (ENaC) has long been a mystery and an issue under debate in the evolution of vertebrates. In this study, we evaluate the idea that energetic efficiency in teleosts may have been improved by selection for ENaC loss and an evolved energy-saving alternative, the Na⁺/H⁺ exchangers (NHE3)-mediated Na⁺ uptake/NH₄⁺ excretion machinery. The present study approaches this question from the lamprey, a pioneer invader of freshwater habitats, initially developed ENaC-mediated Na⁺ uptake driven by energy-consuming apical H⁺-ATPase (VHA) in the gills, similar to amphibian skin and external gills. Later, teleosts may have intensified ammonotelism to generate larger NH₄⁺ outward gradients that facilitate NHE3-mediated Na⁺ uptake against an unfavorable Na⁺ gradient in freshwater without consuming additional ATP. Therefore, this study provides a fresh starting point for expanding our understanding of vertebrate ion regulation and environmental adaptation within the framework of the energy constraint concept.

Receptor-mediated endocytosis of lysozyme in renal proximal tubules of the frog Rana temporaria

The mechanism of protein reabsorption in the kidney of lower vertebrates remains insufficiently investigated in spite of raising interest to the amphibian and fish kidneys as a useful model for physiological and pathophysiological examinations. In the present study, we examined the renal tubular uptake and the internalization rote of lysozyme after its intravenous injection in the wintering frog Rana temporaria using immunohisto- and immunocytochemistry and specific markers for some endocytic compartments. The distinct expression of megalin and cubilin in the proximal tubule cells of lysozyme-injected frogs was revealed whereas kidney tissue of control animals showed no positive immunoreactivity. Lysozyme was detected in the apical endocytic compartment of the tubular cells and colocalized with clathrin 10 min after injection. After 20 min, lysozyme was located in the subapical compartment negative to clathrin (endo-somes), and intracellular trafficking of lysozyme was coincided with the distribution of megalin and cubilin. However, internalized protein was retained in the endosomes and did not reach lysosomes within 30 min after treatment that may indicate the inhibition of intra-cellular trafficking in hibernating frogs. For the first time, we provided the evidence that lysozyme is filtered through the glomeruli and absorbed by receptor-mediated clathrin-dependent endocytosis in the frog proximal tubule cells. Thus, the protein uptake in the amphibian mesonephros is mediated by megalin and cubilin that confirms a critical role of endocytic receptors in the renal reabsorption of proteins in amphibians as in mammals.

Immunohistological classification of ionocytes in the external gills of larval Japanese black salamander, Hynobius nigrescens Stejneger

In this cytological and immunohistological study, we clarified the localization of the membrane transporters Na(+) , K(+) -ATPase (NKA), vacuolar-type H(+) -ATPase (VHA), and epithelial sodium channel (ENaC) and distinguished ionocyte subtypes in the gill of the Japanese salamander (Hynobius nigrescens). In larvae (IY stages 43-65), NKA immunoreactivity was observed on the basolateral plasma membrane in more than 60% cells and less than 20% cells in the primary filaments and secondary lamellae of the external gills, respectively. VHA immunoreactivity was observed on the apical membrane of some epithelial cells in the secondary lamellae of the external gills. High ENaCα immunoreactivity was widely observed on the apical cell membrane of a population of squamous cells, presumably pavement cells (PVCs), and mitochondria-rich cells (MRCs), in the primary filaments and secondary lamellae of the external gills. Using double immunofluorescence microscopy, epithelial cell types involved in ionic regulation were characterized and divided into three ionocyte types: NKA-, NKA- and ENaC-, and VHA-positive cells. VHA-immunoreactive cells as well as NKA-positive cells were observed during IY stages 43-65 of the salamander larvae. During late stages of metamorphosis, NKA, VHA, and ENaCα immunoreactivities in the external gills decreased and finally disappeared during the completion of metamorphosis (IY stage 68). PVCs and MRCs in the external gills are probably involved in acid-base balance regulation and osmoregulation in urodele amphibian larvae. The results are discussed in relation to the ionocytes previously reported in fish gills and the frog skin epithelium.

Functional characterization of the vertebrate primary ureter: structure and ion transport mechanisms of the pronephric duct in axolotl larvae (Amphibia)

BACKGROUND

Three kidney systems appear during vertebrate development: the pronephroi, mesonephroi and metanephroi. The pronephric duct is the first or primary ureter of these kidney systems. Its role as a key player in the induction of nephrogenic mesenchyme is well established. Here we investigate whether the duct is involved in urine modification using larvae of the freshwater amphibian Ambystoma mexicanum (axolotl) as model.

RESULTS

We investigated structural as well as physiological properties of the pronephric duct. The key elements of our methodology were: using histology, light and transmission electron microscopy as well as confocal laser scanning microscopy on fixed tissue and applying the microperfusion technique on isolated pronephric ducts in combination with single cell microelectrode impalements. Our data show that the fully differentiated pronephric duct is composed of a single layered epithelium consisting of one cell type comparable to the principal cell of the renal collecting duct system. The cells are characterized by a prominent basolateral labyrinth and a relatively smooth apical surface with one central cilium. Cellular impalements demonstrate the presence of apical Na+ and K+ conductances, as well as a large K+ conductance in the basolateral cell membrane. Immunolabeling experiments indicate heavy expression of Na+/K+-ATPase in the basolateral labyrinth.

CONCLUSIONS

We propose that the pronephric duct is important for the subsequent modification of urine produced by the pronephros. Our results indicate that it reabsorbs sodium and secretes potassium via channels present in the apical cell membrane with the driving force for ion movement provided by the Na+/K+ pump. This is to our knowledge the first characterization of the pronephric duct, the precursor of the collecting duct system, which provides a model of cell structure and basic mechanisms for ion transport. Such information may be important in understanding the evolution of vertebrate kidney systems and human diseases associated with congenital malformations.

African lungfish, Protopterus annectens, possess an arginine vasotocin receptor homologous to the tetrapod V2-type receptor

In tetrapods, arginine vasopressin and its counterpart, arginine vasotocin (AVT), are involved in renal water conservation through vascular V1a-type and tubular V2-type receptors, and only the former has thus far been cloned in fish. We successfully cloned the V1a-type and V2-type AVT receptor from the kidney of the African lungfish, Protopterus annectens, and the deduced amino acid sequences exhibited high homology with amphibian V1a- and V2-type receptors, respectively. Functional analysis showed that AVT addition to CHO cells transfected with lungfish V1a-type receptor increased [Ca2+]i in a concentration-dependent manner, whereas CHO cells transfected with lungfish V2-type receptor responded with cAMP accumulation after AVT stimulation. Lungfish V2-type receptor mRNA was strongly expressed in the heart and kidney, while V1a-type receptor mRNA was ubiquitously expressed in all the tissues examined. In the kidney, immunohistochemistry using a specific antibody to lungfish V2-type receptor showed localization in the basolateral area of the cells in the late part of the distal tubules. Artificial estivation (EST) for 90 days significantly increased plasma osmolality and sodium and urea concentrations. There was no significant difference in the V2-type receptor mRNA and protein expression levels in the kidney between the freshwater and EST lungfish, while the AVT precursor mRNA level in the hypothalamus was remarkably higher in the EST lungfish. Our results indicate that African lungfish possess a functional V2-type receptor similar to that in tetrapods, suggesting that elevated plasma AVT during estivation exerts a renal tubular antidiuretic effect through the V2-type receptor expressed in the distal segments of lungfish kidney.