In maintenance of the body’s homeostasis the role of kidneys is significant. Histologically kidneys consist of two zones – the cortex and the medulla. The morphofunctional unit of the kidney is the nephron. Funcionally nephrons consists of glomeruli and tubules. Nephrons are responsible for filtering the blood plasma to eliminate waste products as well as conserving glucose and water. In birds, the homeostasis of fluid and ions needs the proper functioning of several organ systems and is a more complex phenomenon than in other vertebrates [
37]. Unlike mammals, the kidneys of chicken contain different types of glomeruli, all noticed in the cortical region: small reptilian type glomerulus near the surface of cortex; larger, mammalian type, glomerulus near medulla and intermediate type glomerulus in the cortex deeper regions. The kidney maintains the glucose homeostasis by reabsorption of glucose in the proximal tubules, release of glucose into the circulation via gluconeogenesis and uptake of glucose from the circulatory sytem to provide itself with energy [
6]. Glomeruli are the primary filtration units, because they filter glucose from plasma, which is then reabsorbed through glucose transporter proteins that are found in the cell membranes within the proximal tubules of the kidney. The proximal renal tubule, a part of the nephron, can be further divided into two sections, the proximal convoluted tubule and the proximal straight tubule. These two sections are subdivided into ultrastructural divisions, so-called segments, that consist either of higher or lower cell complexity. While the S1 segment corresponds to the convoluted proximal tubule, the straight proximal tubule has the correspondence of the S2 and S3 [
38]. The studies on mammalian renal tissue have shown that the kidney’s proximal tubules contain the necessary enzymes for gluconeogenesis (lactate, glutamine, glycerol, alanine) as well as the main transporters which are active in the process of glucose reabsorption - the Na (+) -dependent glucose co-transporters SGLT2 and SGLT1 on the apical membrane and the facilitated diffusion glucose transporter GLUT2 on the basolateral membrane. Glucose reabsorbed or produced in the proximal tubule (gluconeogenesis) is mainly absorbed in the peritubular capillaries and returned to the systemic circulation or used as an energy source in the distal tubule segments, which take up glucose via basolateral GLUT1. The SGLTs are considered active transporters and GLUTs belong to the family of passive transporters [
39]. In our previous study the immunolocalization of SGLT1 and SGLT2 has been revealed in the kidneys of chickens of different ages [
40]. For SGLT1 strong staining in the apical parts of the epithelial cells of the straight proximal tubules was noted and for SGLT2 strong staining of the renal proximal tubules and unstained distal tubules were observed in kidneys of laying chicken of different ages. The immunohistochemical localization of SGLT1 and SGLT2 in the kidneys of hen chickens of different ages was detected on the apical side of the epithelial cells of the proximal renal tubules. The findings on the localization of the sodium-dependent glucose co-transporters SGLT1 and SGLT2 in renal tissue of chickens of different age groups of the previous studies are in accordance with the findings of our current study on young laying chicken where the immunolocalization of SGLT1 and SGLT2 was detected on the apical membrane of the epithelial cells of the chicken kidney’s proximal tubules.
Mycotoxins are low-molecular-weight secondary metabolites of fungi [
41]. Mycotoxins can be divided according to the organ affected as nephro-, hepato-, immunotoxic etc. or into general groups such as allergens, teratogens or carcinogens etc. [
42]. The T-2 toxin studied in our current experiment, is a carcinogenic mycotoxin that belongs in the trichothecene family to the group of Fusarium mycotoxins that is widely encountered as natural contaminant known to elicit toxin responses in different organs and tissues [
43]. Mycotoxins, which are secondary metabolites produced by several fungal species, can exert severe toxic effects on humans and animals [
44]. In the trichothecene family, the T-2 mycotoxin is amongst the most toxic members as compared to other mycotoxins causing serous hemorrhagic inflammation, necrosis and ulceration in the digestive tract, dystrophy in kidney, liver, heart, brain and peripheral ganglia of the vegetative nervous system. Additionally, T-2 has been shown to reduce egg production and weight gain of chickens, as well as impeding the ability of egg hatching [
45]. In poultry the T-2 toxin also has been observed as the causative agent for the impairment of immune responses, destruction of the hematopoietic system, declining egg production, the serous-haemorrhagic necrotic-ulcerative inflammation of the digestive tract, internal hemorrhaging, mouth and skin lesions [
44]. Pathohistological studies usually reveal fatty changes and strong granular degeneration mainly in the kidneys and liver. In chronically T-2 intoxicated poultry, interstitial nephritis, glomerunephritis and kidney sclerosis are among the main pathologies that can be observed [
46]. As the kidneys are among the main affected organs by T-2 mycotoxin, having an essential role in the glucose homeostasis, and as the immunolocalization of sodium-dependent glucose transporters in birds’ kidney tissue has not yet been fully elucidated, our current study was carried out on domestic chickens’ kidney material, investigating the effect of T-2 on SGLT1 and SGLT2 - members of the SLC5A gene family contributing to the renal glucose reabsorption [
47]. Compared to the staining of SGLT1 in chicken kidney tissue our study revealed a strong staining of SGLT2 in the epithelial cells of the renal proximal tubules, presumambly because SGLT2 is considered as the main glucose transporter involved in glucose reabsorption in the kidneys carrying out 80–90% of glomerular filtered glucose reabsorption in renal tubular system [
12,
13]. In eye visual control it could be observed that compared to SGLT1, which localized mainly only in the apical region of the epithelial cells, SGLT2 was immunolocalized throughout the epithelial cells of the proximal tubules.
In the T-2 toxin group pale staining
of proximal renal tubules was noted- the weak expression of both studied antibodies and the morphological changes in the T-2 intoxicated bird group noted in our current study suggest the reduced glucose transport caused by the intoxication. Furthermore,
irregular and damaged brush border membranes of proximal tubule’s epithelial cells in the T-2 toxicated group chicken has been observed in our present study which is also in accordance with previous studies showing that T-2 induces oxidative stress with detrimental effects like nuclear and mitochondrial DNA damage, disturbances in cell-signaling as the toxins can affect the cell cycle and lead to apoptosis and ultimatively death of the cells [
48,
49,
50]. A few of the T-2 main toxic effects on the kidney specifically are apoptosis of proximal convoluted tubule’s epithelial cells, degeneration of the epithelial lining, swelling and vacuolar degeneration of the tubular epithelium and necrosis of renal tubules [
25]. These pathological alterations indicate the nephrotoxicity of T-2 and are in accordance with the findings of our study. The
pale staining of proximal renal tubules in the T-2 toxin group of both major glucose transporters in kidneys indicates to the reduced ability to adequately carry out glucose transport caused by T-2 mycotoxicosis supporting the hypothesis that there are morphological changes in the kidney tissue during intoxication.
To specify the glucose transporters’ localization and clarify the patomorphological changes in laying chicken’s kidney tissue during mycotoxicosis in greater detail, more studies using different research methods such as Immunofluorescence or Western blot should be carried out in the future.