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Figure 1. Pronephric kidney development and the global expression of slc and cldn genes. (a) Hallmarks of pronephric kidney development in Xenopus laevis. Schematic representations of Xenopus embryos are shown with the embryonic stages and hours postfertilization (hpf), in accordance with the terminology established by Nieuwkoop and Faber [28]. Stage 12.5 and 20 embryos are dorsal views with anterior to the left. All other embryos are shown as lateral views. (b,c) Complexity of slc (panel b) and cldn (panel c) gene expression at defined stages of pronephric kidney development. The number of expressed genes at a given stage of pronephric kidney development was determined by whole-mount in situ hybridization.
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Figure 2. Models of the segmental organization of the Xenopus pronephric and mammalian metanephric nephrons. The color coding of analogous nephron segments is based on the comparison of marker gene expression as shown in Figure 7. (a) Schematic representation of the stage 35/36 Xenopus pronephric kidney. The glomerular filtration apparatus (G; also known as glomus) is derived from the splanchnic layer of the intermediate mesoderm and receives blood from vessels that branch from the dorsal aorta. All other parts of the pronephric nephron are derivatives of the somatic layer of the intermediate mesoderm. On the basis of molecular markers, four distinct tubular compartments can be recognized. Each tubule may be further subdivided into distinct segments: proximal tubule (PT, yellow; PT1, PT2, and PT3), intermediate tubule (IT, green; IT1 and IT2), distal tubule (DT, orange; DT1 and DT2), and connecting tubule (CT, gray). The nephrostomes (NS) are ciliated peritoneal funnels that connect the coelomic cavity (C) to the nephron. The scheme was adapted from Reggiani and coworkers [22]. (b) Scheme depicting a short-looped and a long-looped nephron of the adult mammalian metanephric kidney. The figure was taken and adapted from Kriz and Bankir [2]. Abbreviations used for the mammalian nephron segments are as follows: ATL, ascending thin limb; CD, collecting duct; CNT, connecting tubule; DCT, distal convoluted tubule; DTL, descending thin limb; S1, S2, and S3, segments of the proximal tubule; TAL, thick ascending limb.
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Figure 3. The expression domains of slc genes identify three distinct segments in the proximal tubule. Stage 35/36 Xenopus embryos were stained for marker gene expression by whole-mount in situ hybridization. For each distinct class of expression pattern obtained, lateral views of embryos stained for two representative slc genes are shown accompanied by enlargements of the pronephric region. A color-coded scheme of the nephron depicts the deduced segmental expression domains. (a) Examples of slc genes expressed in all segments of the proximal tubule. (b-d) Examples of slc genes with expression confined to proximal tubule (PT)1 (panel b), PT2 (panel c), or PT3 (panel d) alone. Arrowheads are shown to highlight specific proximal tubule segments stained. (e,f) Examples of slc genes with expression either in PT1 and PT2 (panel e) or in PT2 and PT3 (panel f). In panel e, arrowheads and arrows highlight the PT1 and PT2 segments, respectively. In panel f, arrowheads and arrows highlight the PT2 and PT3 segments, respectively. The localization of the slc7a13 expression domains has previously been reported [22]. They are shown here for comparative purposes.
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Figure 4. Slc gene expression defines segmentation of the intermediate, distal, and connecting tubules. Stage 35/36 Xenopus embryos were stained for marker gene expression by whole-mount in situ hybridization. Lateral views of stained embryos are shown accompanied by enlargements of the pronephric region and a color-coded scheme of the nephron depicting the deduced segmental expression domains. (a) slc19a2: intermediate tubule. (b) slc20a1: intermediate tubule (IT)1 (arrowheads). (c) slc5a8: proximal tubule (PT)2, PT3, IT2, and distal tubule (DT)1. In the upper panel, the embryo was stained to reveal slc5a8 expression in IT2 (arrow) and DT1 (arrowhead). The embryo shown in the lower panel was stained shorter to demonstrate expression in PT2 (arrowhead) and PT3 (arrows). (d) slc4a4: proximal tubules, DT1. Arrowheads illustrate expression in PT1. (e) slc12a1: intermediate tubule, DT1. (f) rhcg/slc42a3: DT2. (g) slc8a1: connecting tubule (CT). (h) slc12a3: DT2, CT. Note that there is also strong slc12a3 expression in the cloaca (arrowhead). The localization of the expression domains for slc12a1 and slc12a3 has previously been reported [22]. They are shown here for comparative purposes.
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Figure 5. Expression domains of selected molecular marker genes validates the pronephric segmentation model. Whole-mount in situ hybridizations of stage 35/36 Xenopus embryos were performed. Lateral views of whole embryos (left panels), enlargements of the pronephric region (middle panels), and color-coded schematic representations of the segment-restricted expression domains (right panels) are shown. (a) cldn8: intermediate tubule (IT)2. Note that the expression levels are low. Arrowheads indicate the proximal and distal boundaries of the cldn8 expression domain. (b,d) cldn14 and cldn16: intermediate tubule, distal tubule (DT)1. (d) cldn19: nephrostomes (arrowheads), intermediate tubule. (e) clcnk: intermediate tubule, distal tubule, connecting tubule. Note that the dotted staining pattern localizes to cells of the epidermis. The localization of the clcnk expression domains has previously been reported [22] and is shown here for comparative purposes. (f) kcnj1: IT1, distal tubule, connecting tubule. The arrowhead indicates the location of IT2, which fails to express kcnj1. (g) calb1: connecting tubule. The arrowhead indicates the proximal boundary of the expression domain. Expression is highest in the most distal parts of the connecting tubule.
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Figure 6.
Expression of selected renal marker genes in the adult mouse kidney. In situ hybridizations were performed on paraffin sections of adult kidneys taken from 12-week old mice. Whole transverse sections (upper panels) and magnifications (lower panels) are shown to illustrate marker gene expression in detail. (a) Slc5a2: proximal tubules (S1, S2). (b) Slc7a13: proximal tubules (S2, S3). (c) Slc8a1: connecting tubule. (d) Slc12a1: thick ascending limb. (e) Slc12a3: distal convoluted tubule. (f) Slc16a7: thick ascending limb, connecting tubule. (g) Cldn8: descending thin limb, connecting tubule, collecting duct. (h) Calb1: distal convoluted tubule, connecting tubule.
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slc3a1 ( solute carrier family 3 (amino acid transporter heavy chain), member 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc7a7 (solute carrier family 7 (amino acid transporter light chain, y+L system), member 7 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc25a10 (solute carrier family 25 (mitochondrial carrier; dicarboxylate transporter), member 10 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc1a1 ( solute carrier family 1 (neuronal/epithelial high affinity glutamate transporter, system Xag), member 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc6a19 (solute carrier family 6 (neutral amino acid transporter), member 19 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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hypothetical LOC495492 (slc2a11) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc26a6 (solute carrier family 26 (anion exchanger), member 6 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc7a8 ( solute carrier family 7 (amino acid transporter light chain, L system), member 8) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc26a11 (solute carrier family 26 (anion exchanger), member 11) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc7a13 (solute carrier family 7 (anionic amino acid transporter), member 13) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc15a2 (solute carrier family 15 (oligopeptide transporter), member 2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc5a1 (solute carrier family 5 (sodium/glucose cotransporter), member 1, gene 2 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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UNNMED (Putative ortholog of thiamine transporter 1 (Solute carrier family 19 (Thiamine transporter), member 2) (High affinity thiamine transporter THTR-1) (High-affinity thiamin transporter), 1 of 1 (slc19a2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc20a1 (solute carrier family 20 (phosphate transporter), member 1 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc5a8 (solute carrier family 5 (sodium/monocarboxylate cotransporter), member 8 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc5a8 (solute carrier family 5 (sodium/monocarboxylate cotransporter), member 8 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization ( light staining), NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc4a4 (solute carrier family 4 (sodium bicarbonate cotransporter), member 4 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc12a1 (solute carrier family 12 (sodium/potassium/chloride transporter), member 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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rhcg (Rh family, C glycoprotein) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc8a1 (solute carrier family 8 (sodium/calcium exchanger), member 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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slc12a3 ( solute carrier family 12 (sodium/chloride transporters), member 3 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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cldn8.4 (claudin 8, gene 4 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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cldn14 (claudin 14) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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cldn16 ( ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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cldn19 (claudin 19 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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clcnk (chloride channel, voltage-sensitive Kb) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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kcnj1 (potassium inwardly rectifying channel subfamily J member 1 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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calb1 (calbindin 1, 28kDa ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 35 & 36, lateral view, anterior left, dorsal up.
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Figure 7. Expression domains of selected marker genes in the Xenopus pronephros and the rodent metanephros. The expression domains of selected marker genes in the nephrons of (a) Xenopus stage 35/36 pronephric kidneys and (b) adult rodent metanephric kidneys are depicted schematically. The nephrostomes, which may correspond to the neck segment found in some mammalian species such as rabbits [67,68], are not shown. Dark and pale blue colors indicate strong and low expression levels, respectively. Note that only a single clcnk gene is known in Xenopus, whereas there are two mouse Clcnk genes. The expression of Xenopus clcnk in the intermediate, distal, and connecting tubules has therefore to be compared with the combined renal expression domains of mouse Clcnka and Clcnkb. The abbreviations for segments of the pronephric and metanephric nephrons are given in the legend to Figure 2.
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Figure 8. Comparison of vertebrate nephron segmentation models. Schematic representations of (a) the original model, (b) the improved model of Zhou and Vize [19], and (c) our novel model of pronephric nephron segmentation in the stage 35/36 Xenopus embryo. For comparison, (d) a simplified scheme of the model of the segmental organization of the mammalian metanephric nephron, according to Kriz and Bankir [2], is shown. The glomerulus and nephrostomes (neck segments) are not shown. Abbreviations: DE, distal early; DL, distal late; PE, proximal early; PL, proximal late. For other abbreviations, see the legends to Figure 2.
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