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Studies have shown that the renal tubular epithelium can adapt to alterations in the sulfur amino acid composition of the diet, particularly in terms of reabsorption of the slufur-containing β-amino acid taurine(1–4). Taurine is a compound whose body pool size is regulated by the kidney(2). The renal adaptive response is expressed by enhanced NaCl-dependent taurine transport by rat renal BBMV after a LTD as compared with a NTD. These changes in tubule and BBMV taurine accumulation are associated with a change in the rate of taurine uptake (Vmax) rather than in the affinity of the uptake system for taurine (Km) after dietary manipulation(1, 5, 6). This adaptive regulation of taurine transport has been demonstrated in LLC-PK1 cells (a cell line of porcine proximal tubular origin), and in MDCK cells (Madin-Darby canine kidney cell line, presumably of distal tubule origin). Incubation of cell monolayers with taurine-free medium rapidly induced an increase in Na+-dependent taurine uptake when compared with cells exposed to standard medium containing 50 μM taurine. When cells were incubated in medium containing a high concentration of taurine (500 μM), taurine uptake was decreased when compared with control cells. This adaptive response is the result of changes in the apparent transport maximum (Vmax) rather than the apparent Km of taurine uptake, as was found in rat renal BBMV(1–4, 7).

Taurine functions as an osmolyte in MDCK cells(8). When MDCK cells cultured in isotonic medium were switched to hypertonic medium, the intracellular taurine content doubled(8). Taurine transport in MDCK cells is dependent on Na+ and Cl- and is localized primarily in the basolateral plasma membrane. Medium hypertonicity increased the Vmax of the taurine transporter in the basolateral plasma membrane without a change in Km(8). This regulation of taurine transport activity by medium hypertonicity occurs at the level of mRNA accumulation(9). However, regulation of taurine transport by hypertonicity and regulation by medium taurine are independent of each other(8).

A possible method of identifying membrane transport mechanisms is by their functional expression in Xenopus laevis oocytes, a strategy that has been used successfully to clone the intestinal Na+/D-glucose transporter(10). Several amino acid transporters have been expressed in X. laevis oocytes(11–13), including the MDCK cell taurine transporter (pNCT)(14). To date, the cDNA of several taurine transporters has been cloned from different tissues and species, including human thyroid, human placenta, mouse brain, rat brain, and canine kidney cells(9, 15–18). These taurine transporters share a high percentage (>90%) of amino acid identity.

We used the X. laevis oocyte expression system to begin examining the molecular basis for adaptive regulation of the renal taurine transporter. The specific aims were to determine whether this adaptive regulation is based on changes in RNA transcription or novel protein synthesis.

METHODS

Animals. Four male Sprague-Dawley rats (Harlan Sprague-Dawley Inc., Indianapolis, IN) per group, aged 56-60 d and weighing 250-300 g each, were fed a low sulfur amino acid diet (LTD), a NTD, or HTD for 28 d before sacrifice. This time interval permits full expression of the prolonged adaptive response(4). HTD is supplemented with 3% taurine and 0.5% methionine (wt/wt), NTD is supplemented with 0.5% methionine, and LTD is not supplemented with either amino acid. The composition of each diet has previously been described in complete detail(19), as has its effects on tissue taurine values(1, 2, 19). Adult female frogs (Xenopus I Co., Ann Arbor, MI) were kept in 14°C water and fed with chicken liver twice a week. New Zealand White rabbits were from Myrtle's Rabbitry, Thompson Station, TN.

RNA preparation. Rats were decapitated and exsanguinated; the kidneys were removed and decapsulated, and both total RNA and poly(A)+ RNA were prepared as previously described(20). Briefly, guanidinium thiocyanate was used to disrupt rat kidney cortex tissues, and the resulting homogenate was layered on a cushion of a dense solution of CsCl (5.7 M, 0.01 M EDTA, pH 7.5). Total RNA was pelleted from the lysate after centrifugation at 32 000 × g for 18 h at 25 °C. Poly(A)+ RNA was isolated by oligo(dT)-cellulose chromatography, reprecipitated in ethanol, and then dissolved in diethyl pyrocarbonate-treated water to a final concentration of 1.0 μg/μL. The concentrations of total RNA and poly(A)+ RNA were measured by reading the OD at 260 nm using a Perkin-Elmer model 552 spectrophotometer (Perkin-Elmer, Norwalk, CT).

Microinjection of oocytes. Ovarian lobes were dissected from anesthetized frogs (Xenopus LTD), and oocytes were separated by incubation of ovarian fragments for 40 min with 2 mg/mL collagenase type II in calcium-free buffer at room temperature. Defolliculated stage V-VI oocytes were selected and incubated overnight at 18 °C in medium containing 50% Leibovitz L-15 medium (Life Technologies, Inc., Gaithersburg, MD), 1 mM L-glutamine, 15 mM HEPES, pH 7.6, and 100 μg/mL gentamicin sulfate before injection(21–23). Forty nanoliters of water containing 0-40 ng of poly(A)+ RNA were injected into each oocyte; injected oocytes were maintained at 18 °C in the above medium for 1-3 d.

Oocyte uptake studies. Oocytes were transferred to Na+-containing uptake solution (2 mM KCl, 1 mM MgCl2, 100 mM NaCl) or Na+-free uptake solution (100 mM choline chloride replaced NaCl) containing 10 μM unlabeled taurine and 0.5 μCi/mL[14C]taurine (DuPont NEN, Boston, MA). After incubation for an appropriate time at room temperature, oocytes were transferred to a 24-well cell cluture plate in which they were washed rapidly five times with 1 mL of ice-cold Na+-free uptake solution. Finally, individual oocytes were transferred to miniscintillation vials, solubilized in 100 μL of 10% SDS, and counted in 2 mL of scintillation cocktail (Aquasol, DuPont NEN).

RT-PCR. To determine whether the renal adaptive response occurs at the level of taurine transporter mRNA, two primers were designed such that a 902-bp fragment of rB16a could be amplified from cDNA and detected by Southern blot analysis. The RT reactions were performed individually from each sample of mRNA in 20 μL of 1 × RT buffer (10 mM Tris-HCl, pH 8.8, 50 mM KCl, 0.1% Triton X-100, 5 mM MgCl2) containing 1 mM dNTP, RNasin ribonuclease inhibitor at 1 U/μL, 1 μg of oligonucleotide(dT)15 primer, 1 μg of mRNA, and 30 U of AMV reverse transcriptase (Promega, Madison, WI) at 42 °C for 30 min. After cDNA synthesis, the reactions were stopped by heating at 95 °C for 5 min, then the tubes were quickly chilled on ice. PCR reactions were carried out in 50 μL of 1 × PCR buffer (10 mM Tris-HCl, pH 8.3, 50 mM KCl) containing 200 μM of each dNTP, 1.5 mM MgCl2, 0.001% gelatin, 0.4 μM of each primer, 2.5 U of Taq polymerase (Perkin-Elmer), and either cDNA or water using the following temperature profile: denaturation, 45 s at 94 °C; annealing, 45 s at 66°C (58 °C for β-actin); polymerization, 2 min (in the last cycle, 6 min) at 72 °C for 30 cycles. The sequences of the sense and antisense primers were derived from amino acids 40-47(5′-TCAGAGGGAGAAGTGGTCCAGCAAG-3′)(18), and 335-341 (5′-AGCCAGACACAAAACTGGTACCA-3′) and were designed based on information reported previously by Smith et al.(18). As a control, primers were also designed to amplify the cDNA encoding β-actin (sense, 5′-ATGGTAGTCTGTGACGATATCGCTG-3′, antisense, 5′-ATGGTAGTCTGTCAGGT-3′, which generate a product of 568 bp)(24). To detect amplified products, an oligonucleotide probe was synthesized (corresponding to amino acids 249-271) that is specific for rB16a(18), and an internal oligonucleotide was used(5′-AGCAAGAGAGGTATTCCT-3′) for the β-actin gene. The PCR products were separated on 1.2% agarose gels, and the PCR fragments were positively identified by Southern blotting with 32P-labeled internal oligonucleotide probes. Oligonucleotides were synthesized by the Molecular Resource Center at the University of Tennessee.

DNA subcloning. A 902-bp DNA fragment was amplified by RT-PCR as described above and subcloned into pGEM vector (Promega), transformed into Escherichia coli JM109, and screened by colony hybridization with an oligonucleotide probe (corresponding to amino acids 249-271 of rB16a) labeled with [32P]ATP using T4 polynucleotide kinase (Promega). Positive clones were isolated and sequenced using Sequenase Version 2.0 DNA sequencing kit (U. S. Biochemical Corp., Cleveland, OH).

Northern blot analysis. Ten micrograms of poly(A)+ RNA isolated from kidney cortex of rats on each diet were separated in a 1.2% agarose gel containing 6.5% formaldehyde and transferred to a nylon membrane(Gene-Screen Plus, DuPont NEN) by overnight capillary blotting in 10 × SSC (1.5 M sodium chloride, 1.6 M sodium citrate buffer). Before hybridization the membrane was incubated for 1 h at 42 °C in a prehybridization solution containing 50% formamide, 1 M NaCl, 5% dextran sulfate, and 1% SDS. The Northern blot was hybridized overnight at 42 °C with 32P-labeled rB16a subclone (described above) riboprobe in the hybridization solution. The blot was washed successively in 2 × SSC/0.1% SDS, 1 × SSC/0.1% SDS, and 0.1 × SSC/0.1% SDS at 65 °C for 30 min in each buffer, then exposed to Kodak film with one intensifying screen at -80 °C for 3 wk. Autoradiograms were quantified by a Phospho-Imager (Sony). Data were normalized to the β-actin densitometric value.

Polyclonal antibody. A 14-mer synthetic peptide (SPS4), corresponding to the fourth intracellular fragment of predicted rB16a taurine transporter (GSYNKYKYNSYRDC, 313-326 residues), was coupled to keyhole limpet hemocyanin using a kit (Imject SuperCarrier EDC System for Peptides, Pierce Chemical Co., Rockford, IL) and used to generate the antibody. The peptide was synthesized by the Molecular Resource Center at the University of Tennessee, Memphis, and its purity was >95%. Preimmune IgG was purified from serum collected from the rabbits before antigenic challenge to serve as a control in various experiments. Rabbits were injected with approximately 1000 μg of immunogen and boosted with an equivalent amount of antigen 2 wk later. The animals were bled through the ear vein on d 21 and 36 to measure the antibody response. Antisera were collected on d 49 and purified by affinity column chromatography. The specificity of the antibody was analyzed by Western blot in preliminary experiments using BBMV from kidney cortex of control NTD-fed rats.

Western blot analysis. Equal amounts of BBMV protein from rats fed different diets were separated by electrophoresis on a 12% SDS-polyacrylamide gel and transferred to a nitrocellulose membrane(Millipore, Bedford, MA) using a semidry electrophoretic transfer system(Bio-Rad, Richmond, CA). Membranes were incubated with a 1:1000 dilution of antibody against the conserved peptide sequence of the taurine transporter(rB16a). The blots were detected by chemiluminescence (ECL, Amersham Corp., Arlington Heights, IL) and quantified by a densitometer (Bio-Rad GS 700).

Materials. [14C]Taurine (92.1 mCi/mmol), [32P]UTP(3000 Ci/mmol), and [32P]dATP (3000 Ci/mmol) were purchased from DuPont NEN. AMV reverse transcriptase, T4 polynucleotide kinase, T7 RNA polymerase, pGEM vector, and E. coli JM109 were obtained from Promega. Taq DNA polymerase was purchased from Perkin-Elmer. Imject SuperCarrier EDC System for Peptides was purchased from Pierce Chemical Co. Nylon filters (Gene-Screen Plus) were from DuPont NEN, and nitrocellulose membranes were from Millipore. Other reagents were of molecular biologic grade and were purchased from Sigma Chemical Co. (St. Louis, MO), Promega (Madison, WI), and American Bioanalytical (Natick, MA).

Data analysis. All experiments were performed at least twice; most were done in triplicate. Two-way repeated measures ANOVA, one-way ANOVA, and a t test were applied to determine significant differences in the means.

RESULTS

Functional expression of rat renal taurine transporter in oocytes. The renal taurine transporter was functionally expressed in oocytes after injection of poly(A)+ RNA (40 ng/40 nl) from NTD-fed rat kidney cortex, as shown in Fig. 1. Taurine uptake increased continuously during the first 90 min of the incubation assay. The difference between the values in poly(A)+ RNA-injected and water-injected oocytes eliminates the endogenous components of taurine uptake and represents expressed uptake. This expressed taurine uptake was significantly higher than that of the water-injected oocytes during the first 60 min (p < 0.0001). Thereafter, the background increased in water-injected oocytes. In subsequent experiments taurine uptake was routinely analyzed during a 60-min incubation period in both water-injected and poly(A)+ RNA-injected oocytes, and the expressed uptake was calculated.

Figure 1
figure 1

Time course of taurine uptake by oocytes. Oocytes were injected with 40 ng of poly(A)+ RNA and assayed for taurine uptake 3 d postinjection as described in “Methods.” Results are expressed as the mean ± SEM of six oocytes. The data are representative of three experiments.

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Expression of the renal taurine transporter by oocytes was time-dependent, as shown in Fig. 2. Marked expression of taurine uptake was observed after 2 d and increased further up to 3 d. Taurine uptake in water-injected oocytes was indistinguishable from the background. As demonstrated in Fig. 3, expression of taurine uptake showed a clear dose-response relationship between increased transport and the amount of poly(A)+ RNA injected. These results indicated that functional renal taurine transport can be expressed and transport kinetics studied by simply injecting oocytes with poly(A)+ RNA from rat kidney cortex.

Figure 2
figure 2

Time-dependent expression of taurine uptake by oocytes injected with 40 ng of mRNA from rat kidney cortex. Water- or mRNA-injected oocytes were incubated in medium for 1-3 d, then transferred to Na+-containing uptake solution and incubated for 1 h at room temperature in the presence of [14C]taurine. Results are from six oocytes from multiple donors and are expressed as mean ± SEM. Taurine uptake by water-injected oocytes was negligible. The data are representative of three experiments.

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Figure 3
figure 3

Dose-dependent expression of taurine uptake by oocytes. Oocytes were injected with 10-40 ng of poly(A)+ RNA from rat kidney cortex and assayed for taurine uptake 3 d postinjection. Results are from six oocytes from multiple donors and are expressed as mean ± SEM.

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The transporter expressed in oocytes after injection of poly(A)+ RNA from LTD-fed rat kidney cortex was characterized to confirm that it was the same as that found in BBMV. Expressed taurine uptake obtained by subtracting the uptake in poly(A)+ RNA-injected oocytes from that in water-injected oocytes showed Michaelis-Menten kinetics (Fig. 4). Eadie-Hafstee transformation yielded an apparent Km and Vmax of 22.5 μM and 8.35 pmol/h/oocyte, respectively.

Figure 4
figure 4

Saturation curves and Eadie-Hofstee transformations for mRNA-injected oocytes. Oocytes were injected with 40 ng of poly(A)+ RNA from LTD-fed rat kidney cortex and taurine uptake was measured 3 d postinjection. Taurine accumulation was measured for taurine concentrations ranging from 1.25 to 60 μM. (Inset) Eadie-Hofstee plots were calculated from the saturation curves. V, taurine uptake(pmol/h/oocyte); S, free substrate concentration. Results are from six oocytes from multiple donors and are expressed as mean ± SEM.

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Oocytes were injected with 40 ng of mRNA from LTD-fed rat kidney cortex. Experiments were performed in which Na+ was replaced with choline in the uptake media, and Cl- was replaced with various anions. Oocytes were preincubated for 30 min in Na+- and taurine-free uptake solution, then switched into salt-containing taurine uptake solution. Uptake of taurine in the presence of external NaCl (100 mM) was up to six times greater than in the presence of external choline. The substitution of Cl- with I- or SCN- completely abolished taurine uptake by poly(A)+ RNA-injected oocytes. Br-, however, partially restored taurine transport by oocytes. Hence, taurine uptake by oocytes injected with poly(A)+ RNA from LTD-fed rat kidney cortex is NaCl-dependent(Table 1), consistent with the ionic requirements for taurine uptake by rat renal brush border membranes(25).

Table 1 Taurine uptake by oocytes in the presence of various anions
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Expression of taurine transport activity by poly(A)+ RNA-injected oocytes was adaptively regulated by dietary manipulation, as shown in Fig. 5. Taurine uptake by oocytes injected with poly(A)+ RNA from the kidney cortex of rats fed LTD was significantly higher (p < 0.01) than that from NTD-fed rats, and taurine uptake was significantly decreased when oocytes were injected with poly(A)+ RNA from the kidney cortex of rats fed HTD (p < 0.01). Thus there exists a diet-induced alteration in taurine uptake by oocytes after injection of a fixed amount (40 ng) of poly(A)+ RNA.

Figure 5
figure 5

Dietary regulation of taurine transport activity expressed in oocytes. Oocytes were injected with 40 ng of poly(A)+ RNA from rats fed LTD, NTD, or HTD, and uptakes were performed as described in“Methods.” Results are from 18 oocytes from multiple donors and are expressed as mean ± SEM. *p < 0.0001vs control (NTD).

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Analysis of expression of rB16a mRNA by RT-PCR. Studies have shown that rB16a, a rat brain taurine transporter, is expressed in various tissues, including the kidney(18). RT-PCR was used to amplified an expected 902-bp fragment of rB16a as described in“Methods.” Identical ≈900-bp PCR products were amplified from rat brain and rat kidney. To determine whether this product was homologous to the same portion of rB16a, PCR products were detected by Southern blot using an rB16a-specific probe (Fig. 6A). The ≈900-bp product was also subcloned into a pGEM-T vector (Promega) and sequenced by the dideoxy chain termination method(24). The nucleotide sequence was identical to that of rB16a.

Figure 6
figure 6

Analysis of expression of rB16a mRNA by RT-PCR, RT-PCR was used to amplify an expected 902-bp fragment of rB16a as described in the text and “Methods.” (A) A ≈900-bp PCR product was amplified from both rat brain and rat kidney, and a Southern blot of RT-PCR products was probed by an rB16a-specific probe, as described in the text.(B) Effect of diet on expression of rB16a as analyzed by RT-PCR and Southern blot. The templates for PCR were used at dilutions of 1:1, 1:10, and 1:100. β-Actin was used as an internal control.

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The effect of dietary manipulation on expression of rB16a in rat kidney was also analyzed by RT-PCR. The PCR templates were diluted as indicated in Fig. 6B. Southern blot analysis of PCR cDNA fragments showed that rB16a transcripts were ≈10 times more abundant in kidney from rats fed LTD and ≈10 times less abundant in kidney from rats fed HTD, as compared with kidney from rats fed NTD. Expression of β-actin was not affected by diet.

Northern blot analysis. Northern blot analysis showed that two transcripts, 6.2 and 2.4 kb, corresponding to the rB16a subclone probe were detected in mRNA isolated from the kidney cortex of each group of rats.β-Actin was used as an internal control for gel loading. As shown in Fig. 7,A and B, the abundance of 6.2- and 2.4-kb mRNA was increased in renal cortex from rats fed LTD and reduced in tissue from rats fed HTD, as compared with rats fed NTD. This finding is consistent with the result shown in Fig. 6B. Again, diet had no influence on the abundance of β-actin.

Figure 7
figure 7

Expression and regulation of taurine transporter mRNA by diet. (A) Ten micrograms of poly(A)+ RNA from kidney cortex of rats fed each diet were loaded per well. The Northern blot was hybridized using a riboprobe derived from an rB16a subclone and subjected to autoradiography. Size standards are indicated to the right of the blot in kilobase units. The transcripts detected are 6.2 and 2.4 kb. (B) Densitometric analysis of the data illustrated in A. Values are expressed as mean ± SEM of three independent experiments. Data were normalized to the β-actin densitometric value. *p < 0.05 vs control.

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Western blot analysis. The specificity of the antibody was determined by Western blots prepared from BBMV derived from NTD-fed rat kidney (Fig. 8A). Preimmune IgG purified from rabbits before antigenic challenge was used to probe Western blots: no band was detected in the lysate prepared from rat BBMV. When the polyclonal antibody to the synthetic peptide SPS4 was used as a probe, a ≈70-kD protein corresponding to the predicted molecular mass of rB16a was detected(18). A mixture of the antibody and the peptide failed to detect this protein product. These results indicated that the ≈70-kD protein detected by the antibody is the taurine transporter protein expressed in rat kidney and that the antibody specifically recognized the S4 segment of the transporter. The antibody was then used to probe Western blots prepared from BBMV of rats fed the different diets. The ≈70-kD protein corresponding to the predicted molecular mass of rB16a was detected, and the abundance of the protein was up- or down-regulated in correspondence with the restriction or excess of taurine in the diet (Fig. 8,B and C).

Figure 8
figure 8

Expression of taurine transporter in rat kidney.(A) Western blot demonstrating the specificity of the antibody.(B) Equal amounts of BBMV from rats fed LTD, NTD, or HTD were loaded on an SDS-polyacrylamide gel (12% acrylamide), and the taurine transporter protein was detected by chemiluminescence. Sizes of the relevant molecular mass standards are shown in kilodaltons. (C) Densitometric analysis of the data illustrated in B. Values are expressed as mean ± SEM of three independent experiments. *p < 0.05vs control, **p < 0.01 vs control.

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DISCUSSION

The renal adaptive response to altered sulfur amino acid intake has been described in man, cat, mouse, rat, dog, and pig(26). The observed phenomenon involves increased or decreased initial rate activity of the NaCl-dependent taurine transporter at the brush border membrane surface of the proximal tubule(1–4). The enhanced accumulation is accompanied by a 10-fold increase in the renal reabsorption of taurine after restriction of methionine, cystine and/or taurine in the diet. After consumption of a diet containing a high concentration of taurine there is reduced NaCl-taurine symport across the brush border surface coupled with an 18-fold increase in urinary taurine excretion. This adaptive response does not extend to methionine or cystine uptake or excretion(2, 26). At least two types of adaptive response are evident: a rapid (4-8-h) response, best examined in LLC-PK1 and MDCK cells in culture(7), and a prolonged (3-6-d) response, best observed in vivo and in slices, isolated tubules, or BBMV(25).

This study was conducted to begin examining the molecular basis for adaptive regulation of the renal taurine transporter. The transporter was functionally expressed and characterized in the X. laevis oocyte expression system. Oocytes injected with poly(A)+ RNA from rat kidney showed a time- and dose-dependent increase in NaCl-taurine cotransport, and the Km of the expressed taurine transporter was 22.5 μM, similar to the Km of 17 μM found in rat BBMV. Adaptive regulation of taurine transport activity was observed in oocytes after injection of poly(A)+ RNA from rats fed each diet, consistent with results obtained from rat renal brush border membranes(1–4). These results suggest that the renal taurine transporter expressed in oocytes is the same as that located on rat BBMV. The slight difference in the observed Km of the transporter may be an artifact of the system itself, because the oocyte is a living cell and its membrane permeability different from that of a membrane vesicle. Factors such as ion gradients may also affect the Km of the transporter.

It has been shown that the transcription rate of genes can be regulated by the presence of or concentration of a particular substrate. Recent work by Murer's group demonstrates the adaptive regulation of a NaPi contransporter in rabbit renal cortex to the concentration of phosphate in the diet(27). Transporter mRNA and protein were increased in kidneys of rabbits fed a diet low in Pi as compared with rabbits fed a high Pi diet, and the effect was correlated with increased transport by BBM prepared from rabbits fed a low Pi diet.

Studies have shown that rB16a, a rat brain taurine transporter, is expressed in various tissues, including the kidney(18). To investigate if this up-regulation by LTD and down-regulation by HTD are based on the accumulation of mRNA, and if rB16a is involved in renal adaptive regulation, expression of rB16a mRNA in renal cortex was analyzed by RT-PCR. Southern blots showed that expression of rB16a in rat kidney was up-regulated about 10-fold by LTD and down-regulated about 10-fold by HTD, as compared with NTD. These results clearly indicate that the renal adaptive response to dietary manipulation is based on the transcription of taurine transporter mRNA, and that rB16a is directly involved in this adaptive regulation. The results also suggest that increased uptake by oocytes injected with poly(A)+ RNA from LTD-fed rats results from a higher percentage of accumulated taurine transporter mRNA in kidney tissue. The apparent affinity of the transporter expressed in poly(A)+ RNA-injected oocytes was 22.5μM, whereas the affinity of rB16a expressed in COS cells was 40 μM(18). We speculate that the difference in the observed Km may be caused by differences between the oocyte and COS cell expression systems. It is logical that, because oocytes have no endogenous Na+-dependent or independent taurine transport system, as demonstrated in this study, the Km of the transporter derived from the oocyte expression system after injection of poly(A)+ RNA from rat kidney should be similar to the Km of rat BBMV.

Smith et al.(18) detected a 6.2-kb transcript corresponding to rB16a mRNA in rat kidney. In the present study we found that, in addition to the 6.2-kb transcript, a 2.4-kb transcript was also detected in rat kidney by the rB16a riboprobe. The abundance of both transcripts was regulated by dietary manipulation. The smaller transcript may represent an isoform of rB16a, because a PCR product of a single size was generated from rat kidney using rB16a-specific primers. Alternatively, the 2.4-kb transcript may be caused by RNA splicing. Expression of multiple transcripts for the taurine transporter has been reported in studies carried out in MDCK cells(9), as well as in studies of human tissues(16).

To identify the carrier protein(s) of taurine, a conserved 14-mer peptide sequence (SPS4) of cloned taurine transporters was used as an antigen to develop a polyclonal antibody intended for use as a probe to search for novel proteins that might function as taurine transporters. The specificity of the antibody was determined by Western blots prepared from rat BBMV. A≈70-kD protein corresponding to the predicted molecular mass of rB16a was detected by the antibody. A mixture of the antibody and the peptide, as well as preimmune IgG, were used as controls: both failed to detect the protein. These results indicated that the ≈70-kD protein detected by the antibody is taurine transporter protein and that the antibody specifically recognized rB16a.

With this evidence that the antibody was able to recognize the taurine transporter protein in rat BBMV, the antibody was used to probe Western blots prepared from BBMV of rats fed each diet. The results showed that the≈70-kD protein was adaptively regulated by dietary manipulation,e.g. the abundance of the transporter protein was increased in BBMV from rats fed LTD and decreased in BBMV of rats fed HTD, as compared with BBMV from control rats fed NTD.

Our results suggest that a transcriptional mechanism may be involved in the dietary regulation of expression of the renal taurine transporter gene. The signal for the long-term adaptive regulation of taurine transport appears to be the extracellular taurine concentration or, more specifically, the tissue taurine content(7). The transcription rate of the taurine transporter gene in MDCK cells was regulated by the concentration of medium taurine over 24 h(28). The abundance of specific transporter mRNA is related to medium taurine content, being highest in cells exposed to taurine-free medium and lowest in cells exposed to 500 μM taurine medium. Similarly, the abundance of mRNA in rat kidney varied in response to taurine in the diet, being highest in LTD-fed rats and lowest in HTD-fed rats. It has been shown by in situ hybridization that mRNA for the taurine transporter is overexpressed in rat kidney S3 segments after 2 wk of LTD and underexpressed after 2 wk of HTD(29).

A short-term adaptive response occurring in the first few hours after diet or medium taurine content alteration probably occurs at the level of posttranscriptional modification, because classical inhibitors of protein trafficking or the translocation of taurine transporter between membrane and cytoplasm, including colchicine, prevent adaptive regulation(30, 31). Inhibitors of protein synthesis, such as cycloheximide, both influence gene transcription rate and inhibit expression of the adaptive response in MDCK cells(28). Most importantly, neither taurine concentration nor the presence of cycloheximide has any influence on taurine transporter mRNA stability, suggesting that ongoing protein synthesis is required for adaptive regulation of taurine transporter gene expression. Finally, the amount of membrane-bound transporter protein, as measured by Western blot, is related to dietary taurine intake. Hence, a model in which taurine content influences, transporter gene transcription rate, the concentration and localization of taurine transporter mRNA, the abundance of taurine transporter protein in the membrane, as well as taurine transporter initial rate activity in renal cortex slices, brush border membrane vesicles, or into microinjected oocytes, is supported by our data. A mechanism involving diet- or medium taurine content-induced changes in mRNA stability is not likely. The precise molecular mechanism by which expression of the renal taurine transporter is adaptively regulated by dietary manipulation remains to be determined.

In conclusion, expression of the taurine transporter is adaptively regulated by dietary taurine manipulation at the level of mRNA. The rat brain taurine transporter rB16a is expressed in rat kidney and is involved in the adaptive regulation of taurine transport.