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Plasma membrane proteins of the solute carrier family 5 (SLC5) are responsible for sodium-coupled uptake of ions, sugars and nutrients in the vertebrate body. Mutations in SLC5 genes are the cause of several inherited human disorders. We have recently reported the cloning and transport properties of SGLT-1L, a Xenopus homologue of the sodium-dependent glucose cotransporter 1 (SGLT-1) [Nagata et al. (1999) Am. J. Physiol. 276: G1251 -G 1259]. Here, we describe the phylogenetic relationship of SGLT-1L with other members of the SLC5 family and characterize its expression during Xenopus embryogenesis and in organ cultures. Sequence comparisons and phylogenetic analyses of all known vertebrate SLC5 sequences indicated that Xenopus SGLT-1L encodes a novel SLC5 member, which shares highest amino acid identity with mammalian ST-1 proteins. Temporal and spatial expression of SGLT-1L during Xenopus embryogenesis was examined by whole mount in situ hybridization. Initiation of SGLT-1L expression occurred in the late tailbudembryo. Remarkably, expression was restricted to the developing pronephric kidney. SGLT-1L was highly expressed in tubular epithelia, but completely absent from the epithelia of the duct. Analysis of growth factor-treated animal caps indicated that expression of SGLT-1L could also be induced in organ cultures. Taken together, our findings indicate that the expression of sodium-dependent solute cotransporter genes in early segments of the excretory system appears to be conserved between pronephric and metanephric kidneys. Furthermore, we establish SGLT-1L as a novel, highly specific molecular marker for pronephric tubule epithelia undergoing maturation and terminal differentiation in Xenopus.
FIG. 2. Expression of SGLT-1L is confined to
tubules of the pronephric kidney. SGLT-1L and
CLC-K transcripts were detected by whole-mount in
situ hybridization. Lateral views of Xenopus embryos
with anterior to the left are shown in (A-E, G,
H). Transverse sections (F, I) are oriented with dorsal
to the left. (A-D) Embryonic SGLT-1L expression
started at stage 30 (A) and remained exclusively
associated with the developing pronephric tubules
throughout tadpole stages (B-D). (E) Enlargement
of the pronephric region of the embryo shown in (D)
illustrating SGLT-1L expression in connecting tubules
(arrowheads) and the common tubule (arrow).
(F) Transverse section of stage 37/38 embryo hybridized
in whole mount for SGLT-1L expression.
Embryos were embedded in plastic and 3 μm sections
were cut at the level of the pronephros. SGLT-
1L transcripts were detected in tubular epithelia (pt),
but not in the duct (pd). (G) Double in situ hybridization
of a stage 36 embryo demonstrating SGLT-1L
(magenta) and CLC-K (blue) expression. (H) A closeup
view of the pronephric region of the embryo
shown in (G). SGLT-1L expression is present in pronephric tubules (pt), whereas CLC-K transcripts are found in the pronephric duct (pd). Arrowheads
indicate the junction between the common tubule and the duct. Selected epidermal cells express CLC-K (arrow). (I) Transverse section through the
pronephric kidney of a stage 36 embryo hybridized in whole mount for SGLT-1L and CLC-K expression. The plane of section is indicated by the dotted
line in (H). SGLT-1L and CLC-K transcripts were associated with distinct tubular (pt) and ductal (pd) epithelia, respectively.
FIG. 3. Induction of SGLT-1L expression in explant cultures.
(A-E) Induction of SGLT-1L expression assayed by whole mount
in situ hybridization. Animal caps removed from stage 9 embryos
were either cultured without any added factors (A),
retinoic acid (RA) alone (B), activin alone (C), or a combination
of activin and RA (D). Expression of SGLT-1L was analyzed by
in situ hybridization or RT-PCR once reference embryos reached
stage 33/34 (E). Tissues expressing SGLT-1L are indicated with
arrows. (F-H) Histological analysis of animal cap cultures.
Vibratome sections were cut through selected control (F),
activin-treated (G), and activin plus RA-treated animal caps (H).
Sections were cut at 30 μm. (I) Expression of SGLT-1L as
assayed by semi-quantitative reverse transcription (RT)-PCR.
(Upper panel) RT-PCR performed with SGLT1-L specific primers.
Sources of the RNA preparations used in the RT-PCRs are
as indicated. (Lower panel) Control RT-PCRs for equal RNA
amounts were carried out in parallel with elongation factor-1α
(EF-1α) specific primers. �RT, control in which reverse transcriptase
was omitted.
slc5a11 (solute carrier family 5 (sodium/inositol cotransporter), member 11) gene expression in a Xenopus laevis embryo as assayed by in situ hybridization, NF stage 36. Lateral view: anteriorleft, dorsal up
