Specific expression and regulation of glucose transporters in zebrafish ionocytes

Glucose, a carbohydrate metabolite, plays a major role in the energy supply for fish iono- and osmoregulation, and the way that glucose is transported in ionocytes is a critical process related to the functional operations of ionocytes. Eighteen members of glucose transporters (GLUTs, SLC2A) were cloned and identified from zebrafish. Previously, Na⁺,K⁺-ATPase- rich (NaR), Na⁺-Cl^- cotransporter-expressing (NCC), H ⁺-ATPase-rich (HR), and glycogen-rich (GR) cells have been identified to be responsible for Ca²⁺ uptake, Cl^- uptake, Na ⁺ uptake, and the energy deposition, respectively, in zebrafish skin/gills. The purpose of the present study was to test the hypothesis of whether GLUT isoforms are specifically expressed and function in ionocytes to supply energy for ion regulatory mechanisms. On the basis of translational knockdown of foxi3a/3b (2 transcriptional factors related to the ionocytes' differentiation) and triple in situ hybridization/ immunocytochemistry, 3 GLUT isoforms, zglut1a, -6, and -13.1, were specifically localized in NaR/NCC cells, GR cells, and HR cells, respectively. mRNA expression of zglut1a in embryos and adult gills were stimulated by the low Ca²⁺ or low Cl^- freshwater, which has been previously reported to upregulate the functions (monitored by epithelial Ca²⁺ channel, NCC mRNA) of NaR/NCC cells, respectively while that of zglut13.1 was stimulated only by low Na⁺, a situation to upregulate the function (monitored by carbonic anhydrase 15a mRNA) of HR cells. On the other hand, ambient ion compositions did not affect the zglut6 mRNA expression. Taken together, zGLUT1a, -6, and 13.1, the specific transporters in NaR/NCC cells, GR cells, and HR cells, may absorb glucose into the respective cells to fulfill different physiological demands.