Sialin (SLC17A5) Functions As a Nitrate Transporter in the Plasma Membrane

Sialin (SLC17A5) functions as a nitrate transporter in the plasma membrane

Lizheng Qina,1, Xibao Liub,1, Qifei Suna, Zhipeng Fana, Dengsheng Xiaa, Gang Dinga, Hwei Ling Ongb, David Adamsc, William A. Gahlc, Changyu Zhengb, Senrong Qia, Luyuan Jina, Chunmei Zhanga, Liankun Gud, Junqi Hee, Dajun Dengd,2, Indu S. Ambudkarb,2, and Songlin Wanga,e,2

aSalivary Gland Disease Center and Beijing Key Laboratory of Tooth Regeneration and Function Reconstruction, Capital Medical University School of Stomatology, Beijing 100050, China; bMolecular Physiology and Therapeutics Branch, National Institute of Dental and Craniofacial Research, Bethesda, MD 20892; cMedical Genetics Branch, National Human Genome Research Institute, Bethesda, MD 20892; dKey Laboratory of Carcinogenesis and Translational Research, Peking University Cancer Hospital and Institute, Beijing 100142, China; and eDepartment of Biochemistry and Molecular Biology, Capital Medical University School of Basic Medicine, Beijing 100069, China

Edited by Mark Gladwin, University of Pittsburg Medical Center, Pittsburgh, PA and accepted by the Editorial Board June 5, 2012 (received for review October 13, 2011)

In vivo recycling of nitrate (NO −) and nitrite (NO −) is an important Nitrate uptake into salivary glands represents the key initial step 3 2 − alternative pathway for the generation of nitric oxide (NO) and in NO clearance from the serum; however, the mechanism me- 3 − maintenance of systemic nitrate–nitrite–NO balance. More than diating transport of NO3 in salivary gland epithelial cells has not − − fl 25% of the circulating NO3 is actively removed and secreted by yet been established. In this study, we examined NO3 in ux in − + salivary glands. Oral commensal bacteria convert salivary NO3 to salivary gland cells. Here we report that the sialic acid (SA)/H − SLC17A5 NO2 , which enters circulation and leads to NO generation. The cotransporter, sialin ( ), mutations in which result in Salla − transporters for NO3 in salivary glands have not yet been identi- disease and ISSD, is involved in nitrate uptake into salivary glands. fied. Here we report that sialin (SLC17A5), mutations in which cause Our data suggest a similar function for sialin in several other cell fi Salla disease and infantile sialic acid storage disorder (ISSD), func- types as well, including broblasts. We show that sialin is endoge- − + nously localized in the lysosomes as well as in the plasma membrane tions as an electrogenic 2NO3 /H cotransporter in the plasma − fi of salivary gland cells, where it functions as an electrogenic 2NO / membrane of salivary gland acinar cells. We have identi ed an ex- + 3 − H cotransporter mediating influx of nitrate into the cell. We also tracellular pH-dependent anion current that is carried by NO3 or − provide evidence that plasma membrane sialin is a multifunctional sialic acid (SA), but not by Br , and is accompanied by intracellular − + acidification. Both responses were reduced by knockdown of sialin anion transporter that can mediate electrogenic A /H cotransport expression and increased by the plasma membrane-targeted sialin of anions such as SA, glutamate, and aspartate. Importantly, we mutant (L22A-L23A). Fibroblasts from patients with ISSD displayed have assessed the function of sialin in vivo in pig salivary glands and − provide evidence for the physiological relevance of sialin in medi- reduced SA- and NO -induced currents compared with healthy − 3 ating nitrate uptake NO influx into pig salivary glands. In ag- controls. Furthermore, expression of disease-associated sialin mu- 3 + gregate, our findings suggest that sialin is a versatile anion tants in fibroblasts and salivary gland cells suppressed the H -de- − transporter, and that functional defects in the protein may have pendent NO conductance. Importantly, adenovirus-dependent 3 a deleterious impact on several critical physiological functions. expression of the sialinH183R mutant in vivo in pig salivary glands − − decreased NO3 secretion in saliva after intake of a NO3 -rich diet. Results Taken together, these data demonstrate that sialin mediates nitrate − Coupled NO Currents and Intracellular Acidification in Human influx into salivary gland and other cell types. We suggest that the 3 − + Submandibular Gland Cells. Human submandibular gland cell line 2NO /H transport function of sialin in salivary glands can contrib- 3 (HSG) cells did not display constitutive currents in standard ex- ute significantly to clearance of serum nitrate, as well as nitrate − − tracellular solution. Replacement of Cl with 150 mM NO3 recycling and physiological nitrite-NO homeostasis. produced a relatively slow, but significant, spontaneous increase in the outward current that was dramatically enhanced by decreasing pH | proton theexternalpHfrom7.4to4.0(Fig.1A); the current density in the − ± n = − − 150 mM NO3 solution was 42.4 5.4 pA/pF ( 21). Both the he anions NO3 and NO2 were once thought to be inert end constitutive and low pH-induced currents had similar outwardly Tproducts of NO metabolism. However, it is now evident that rectifying characteristics with a reversal potential of −15 ± 2mV nitrate and nitrite can be recycled in vivo to form NO, and thus (n = 19) (Fig. 1B). Low pH also increased the outward current in − these anions complement the nitric oxide synthase (NOS)-de- normal standard extracellular solution (Cl -containing), although − pendent activity (1). The nitrate-nitrite-NO pathway is emerging the amplitude of the current was smaller than that seen with NO3 as a potential therapeutic target in such diseases as myocardial (23.1 ± 4.3 pA/pF; n = 18) (Fig. 1C), and the current reversed at infarction, stroke, gastric ulcers, and pulmonary hypertension (1, 2). There are two major sources of nitrate and nitrite: the L-arginine–NO synthase pathway and diet. Dietary intake of ni- − Author contributions: L.Q., X.L., D.D., I.S.A., and S.W. designed research; L.Q., X.L., Q.S., Z. trate leads to a relatively rapid increase in NO3 concentration F., D.X., G.D., H.L.O., D.A., W.A.G., C. Zheng, S.Q., L.J., L.G., and S.W. performed research; in serum. Although a large part of the anion is excreted via the L.Q., X.L., C. Zheng, and L.J. contributed new reagents/analytic tools; L.Q., X.L., C. Zheng, kidneys, up to 25% of the circulating nitrate is actively taken up L.J., C. Zhang, J.H., D.D., I.S.A., and S.W. analyzed data; and L.Q., X.L., D.D., I.S.A., and S.W. by the salivary glands and concentrated ∼10-fold in the saliva wrote the paper. secreted from the glands (3–5). Conditions that compromise The authors declare no conflict of interest. − salivary gland function have been linked to decreased NO se- This article is a PNAS Direct Submission. M.G. is a guest editor invited by the Editorial − 3 Board. cretion from the salivary glands and increased NO3 levels in the serum and urine (5, 6). Although nitrate can be reduced to nitrite See Commentary on page 13144. by the commensal bacteria in the oral cavity, most of the salivary 1L.Q. and X.L. contributed equally to this work. nitrite escapes gastric conversion to NO and enters the systemic 2To whom correspondence may be addressed. E-mail: [email protected], iambudkar@dir. circulation, where it generates NO. Thus, salivary nitrate is nidcr.nih.gov, or [email protected]. − recycled back to NO and is critical for the maintenance of 2 − This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. physiological levels of NO and NO2 in the serum (1, 2, 7). 1073/pnas.1116633109/-/DCSupplemental.

13434–13439 | PNAS | August 14, 2012 | vol. 109 | no. 33 www.pnas.org/cgi/doi/10.1073/pnas.1116633109 Downloaded by guest on September 28, 2021 SEE COMMENTARY

− − Fig. 1. Coupled NO3 currents and intracellular acidification in HSG cells. NO3 currents in HSG cells (A–F) and primary huSMG cells (G) measured by the whole-cell patch-clamp technique. NaCl was replaced with NaNO3, NaBr, or Na-gluconate as indicated. Changes in extracellular pH are shown in the traces − (bar). I-V curves are shown in B and D.(H) External pH and NO3 (5 mM)-dependent acidification of HSG cells measured using BCECF fluorescence. (I)pH − dependence of NO3 currents in HSG cells. (J) Intracellular pH changes under the same experimental conditions as shown in I.(K) Data from I and J were used − to determined the relationship of NO3 currents and intracellular acidification (Hill coefficient: 2.0 ± 0.1).

− − 0 mV (Fig. 1D). Decreasing the external pH in Br - or gluconate- conditions demonstrated pH- and NO3 -dependent intracellular containing medium did not generate any detectable current (Fig. 1 acidification (Fig. 1J).Thedatawerefitted (R2 = 0.999) using the E and F). Similar findings were observed in primary human sub- Hill equation, yielding a Hill coefficient of 2.0 ± 0.1 (Fig. 1K), mandibular gland (huSMG) cells (Fig. 1G), human parotid gland consistent with the electrophysiological data. These data strongly − + ductal cells, and freshly dispersed acini cells prepared from mouse suggest that an electrogenic 2NO /H cotransporter is involved in − − 3 salivary glands. Cl channels in salivary gland cells are quite per- mediating NO uptake in salivary gland epithelial cells. − − 3 meable to NO and Br , unlike the activity described here (8–11). 3 − Nonetheless, NO conductance was inhibited by several anion Involvement of Sialin in 2NO −/H+ Cotransport. To examine nitrate 3 − 3 − channel blockers that block various Cl channels (Fig. S1). Fur- transport via salivary gland, [NO ] was measured in the serum − − 3 thermore, unlike known Cl channels in salivary gland cells, NO and saliva of miniature pigs fed with regular or nitrate-rich 3 −

conductance was not regulated by cAMP, intracellular or extra- fodder. In both cases, the [NO ] level in saliva exceeded that in PHYSIOLOGY + 3 cellular Ca2 , or muscarinic or purinergic receptor agonists (Fig. serum within 1 h after feeding (Fig. 2A), suggesting that salivary + + S2). Replacing Na in the medium with Cs also had no affect on glands take up and secrete nitrate. The nitrate transporters the current (Fig. S3 A and B). Relatively lower (and likely more identified to date belong to the highly conserved major facilitator − physiological) NO concentrations (0.05–0.5 mM) also induced superfamily (MFS) (11–15). To determine the molecular com- 3 − + currents in HSG cells, with channel densities of 2.3 ± 0.3 pA/pF ponent mediating 2NO3 /H cotransport in salivary glands, we (n = 5) and 3.4 ± 0.5 pA/pF (n = 6), respectively (Fig. S3C). Taken analyzed the expression of 127 MFS genes (11, 16, 17) in human together, these findings demonstrate the unique properties of the parotid and submandibular glands (n = 3) using Affymetrix − NO conductance detected in salivary gland cells. U133Plus 2.0 microarrays. SLC17A5 (which encodes sialin) was 3 − To examine the modulation of NO conductance by pH, we expressed at high levels in both types of salivary glands (Table S1). 3 + monitored intracellular pH using 2′-7′-bis(carboxyethyl)-5(6)-car- Sialin functions as a lysosomal SA/H transporter involved in SA boxyfluorescein (BCECF). Decreasing the external pH in gluco- efflux, although it is detected in the plasma membrane of neurons − nate-containing medium (ambient [Cl ] in gluconate-containing (18, 19) where it mediates aspartate and glutamate transport. The medium was <10 mM) did not change intracellular pH, whereas protein is widely expressed in such tissues as brain, heart, lung, − including 5 mM NO induced rapid acidification that was reversed liver, kidney, and mouse submandibular glands (20–22). Impor- − 3 by replacing NO with gluconate in the medium (Fig. 1H). When tantly, mutations in sialin are causative factors in the neurode- 3 − external pH was varied, keeping [NO3 ] constant at 5 mM, the generative disorders Salla disease and ISSD (18, 23). We validated outward current was changed as a function of external pH (Fig. 1I). the expression of sialin in various human tissues by qPCR (Fig. Importantly, measurement of intracellular pH under the same 2B) and by Western blot analysis using samples of various tissues

Qin et al. PNAS | August 14, 2012 | vol. 109 | no. 33 | 13435 Downloaded by guest on September 28, 2021 − + − Fig. 2. Involvement of sialin in 2NO3 /H cotransport. (A) Saliva and serum [NO3 ] in miniature pigs fed on regular fodder or supplemented with 100 mg/kg of nitrate. (B) Validation of gene expression of SLC17A5 based on the MFS gene expression pattern (Table S1). (C) Detection of sialin (i and iv) in human salivary gland. Na/K-ATPase (ii) or LAMP-1 (v) are markers for basolateral membrane and lysosomes, respectively. Colocalization of the two proteins is shown (iii and vi, yellow, indicated by arrows). (D) Nitrate uptake in HSG cells transfected with sh-sialin or scram-sh. The values indicated by * or ** are signiﬁcantly different from the unmarked values. (P < 0.05 or P < 0.01; n ≥ 3). (E and F) Knockdown of sialin by sh-sialin and overexpression of WT-sialin. (G and H) Effect of − sialin knockdown on constitutive and low-pH–induced NO3 current. Levels of current and intracellular pH in control HSG cells transfected with scram-sh (G)or sh-sialin (H) were as described in Fig. 1. (I) Average data obtained from the experiments shown in G and H. The number of cells tested is indicated. Statistically signiﬁcant differences are indicated by ** (P < 0.01).

from miniature pigs (Fig. S4). Sialin was highly expressed in sali- biopsies and in lysosomes as detected by its colocalization with the + + vary glands, and liver, with lower levels of expression in brain, respective markers Na /K ATPase and LAMP-1 (Fig. 2C). spleen and kidney, and even lower in muscle and pancreas. Sialin Based on the expression and function of known solute carrier was localized in the basolateral regions of human parotid gland (SLC) transporters (Table S1) and the channel activity described in

− + + Fig. 3. Sialin mediates NO3 /H as well as SA/H cotransporter in HSG cells. (A) Current measurement in cells perfused with medium at pH 4.0 containing − − 5 mM SA, NO3 , or both. (B) I-V curves of the current (Erev= 0mVfor5mMSAorNO3 ) obtained under the different conditions shown in A.(C) Intracellular − − pH in medium of pH 4.0 containing either SA or NO3 .(D) Currents generated by substituting NO3 with glutamate (Glu) or aspartate (Asp), with other anions − transported by sialin. (E and F) SA and NO3 currents induced at pH 4.0 in control HSG cells (E) and cells treated with sh-sialin (F). The y-axis scale is the same in E and F.(G) Average of data from the experiments shown in E and F.(H) Effect of expression of plasma membrane-targeted mutant of sialin L22A-L23A on − NO3 and SA currents. (I) Average data and statistical evaluation for H.(J) Surface expression of sialin in WT cells and in cells transfected with scram-sh, sh- − sialin, or L22A-L23A mutant (AA). (K)NO3 and SA uptake measured at 10 min after loading in HSG cells transfected with sh-sialin or scram-sh. *Values signiﬁcantly different from the unmarked values (P < 0.05; n = 6).

13436 | www.pnas.org/cgi/doi/10.1073/pnas.1116633109 Qin et al. Downloaded by guest on September 28, 2021 − + Fig. 1, we assessed the involvement of sialin in 2NO /H co- the action of commensal bacteria in the oral cavity. This is not a − 3 transport. HSG cells accumulated NO , and this function was completely unexpected finding, given that the physiological SEE COMMENTARY 3 − significantly decreased in cells expressing sh-sialin compared to function of salivary gland cells is to mediate transepithelial NO3 cells expressing scrambled shRNA (scram-sh) (Fig. 2D; protein transport and concentrate the anion in saliva. A similar conclu- expression and mRNA levels are shown in Fig. 2 E and F). In- sion was reached in an earlier study, which showed that salivary + + − − tracellular levels of K ,Na ,andCl were not changed by sh-sialin glands deliver NO from the serum into the oral cavity with − 3 expression. Importantly, NO3 current in HSG cells maintained at minimal metabolism (2). normal or low pH (Fig. 2 G–I) was significantly reduced in sh- − + + sialin–treated cells. Sialin Mediates NO3 /H and SA/H Cotransport in HSG Cells. In- In addition, incubation of HSG and other cell types, including clusion of SA, a major substrate for sialin, in the external me- human colon carcinoma (RKO), human gastric mucosal epi- dium also induced currents when ambient pH was decreased. − thelial cells (GES-1), and human umbilical vein endothelial cells Although NO3 and SA individually (5 mM each) generated − (HE-CV-304), in medium containing NO resulted in increased outward currents at low external pH (Fig. 3A), the amplitude 3 − levels of intracellular NO as detected by diaminofluorescein with NO3 (11.2 ± 2.2 pA/pF; n = 15) was greater than that with (DAF) (Fig. S5 A and B); nitrite was not detected in the nitrate SA (5.4 ± 0.7 pA/pF; n = 7). When both anions were present loading solution. Athough HSG cells displayed nitrate conduc- together (10.6 ± 0.7 pA/pF; n = 9), the current amplitude was tance when exposed to physiological levels of nitrate (50–1,000 slightly more than that seen with SA alone, although it was not μM), indicating that these cells have an influx pathway that can a sum of the individual currents. The characteristics of currents − transport nitrate at relatively low [NO ], generation of NO and were similar in all of the conditions (Fig. 3B). As seen with 3 − cGMP was detected only at relatively high, nonphysiological levels NO3 , decreasing extracellular pH in the presence of 5 mM SA − fi C of the anions (≥15 mM nitrate and 3 mM NO2 , respectively) also led to intracellular acidi cation (Fig. 3 ). In addition, (Fig. S5 C and D). Note that other tissues, such as liver, can proton-dependent aspartate and glutamate currents, similar to − effectively metabolize both anions at lower concentrations (Fig. NO3 current, were detected in these cells (Fig. 3D), consistent S5E). This indicates that direct nitrate uptake via sialin is not a with a previous study indicating that sialin transports both of physiological pathway for NO generation in these cell types; these anions (20). Sialin knockdown decreased low-pH stimu- rather, once secreted, salivary nitrate is metabolized to nitrite by lated currents in HSG cells perfused with media containing SA + PHYSIOLOGY

− Fig. 4. Assessment of sialin-mediated NO3 transport in fibroblasts from patients with ISSD, showing the effect of the Salla disease and ISSD sialin mutations − on anion transport. (A–D)NO3 and SA (5 mM each) currents in HSG cells transfected with sialinR39C (B) or sialinH183R (C). (C, Insert) Average data and − − statistical evaluation. (D) I-V curve of the NO3 current shown in A and B.(E–H) Sialin-mediated current (150 mM SA or NO3 )infibroblasts from healthy volunteers (HV) (E and F) and patients with ISSD (G and H). (I and J) Effect of sialinH183R expression on anion transport in fibroblasts from patients with ISSD. (K) Average data from the experiments with cells from patients.

Qin et al. PNAS | August 14, 2012 | vol. 109 | no. 33 | 13437 Downloaded by guest on September 28, 2021 − − − NO3 , SA, or NO3 (Fig. 3 E–G). Importantly, expression of the levels of NO3 in the saliva (152.5 ± 17.3 μM vs. 221.6 ± 8.5 μMat sialin mutant L22A-L23A, which reportedly is targeted to the 30 min and 204.1 ± 31.5 vs. 305.3 ± 27.8 μM after 60 min of feeding; plasma membrane (24, 25), increased the currents by approxi- P < 0.05; data obtained from fours pigs and eight parotid glands in − − mately twofold with either NO3 or SA in the external solution each group) (Fig. 5A). [NO3 ] in the saliva from animals over- (Fig. 3 H and I). This increase was also detected when the anions expressing WT-sialin was not significantly different than that from − were used at low concentrations (5 mM). Surface biotinylation control animals, and although serum and urine [NO3 ] levels were confirmed the presence of endogenous sialin in the plasma higher after feeding, there was no difference between the two membrane, which was decreased by the expression of sh-sialin groups. Western blot analysis (Fig. 5B) revealed greater sialin ex- J − and increased by expression of sialin L22A-L23A (Fig. 3 ). NO3 pression in parotid glands in pigs receiving the sialinH183R plasmid − > and SA were transported into HSG cells (NO3 uptake SA compared with controls (receiving AdCMV-EGFP-PEI alone). uptake), and uptake of both anions was reduced when sialin Furthermore, immunohistochemistry demonstrated a higher sialin expression was suppressed by sh-sialin (Fig. 3K). − signal in glands receiving sialinH183R compared with those in the Sialin also transports NO , as detected by activation of low- C D 2 − − control group (Fig. 5 and ). Taken together with the data pH currents when either NO3 or NO2 was included in the bath shown in Fig. 4, these data strongly suggest that nitrate transport D − ± n = solution (Fig. S3 ). NO2 current (7.5 1.0 pA/pF; 8) was in salivary glands could be potentially altered in patients with − ± n = smaller than NO3 current (11.2 2.2 pA/pF; 15), but was ISSD or Salla disease. not additive in solutions containing both anions (12.8 ± 2.4 pA/ pF; n = 5). Nitrite uptake was confirmed in cells for which in- Discussion + cluding nitrite or nitrate in the medium led to intracellular Herein we report a novel function for the lysosomal SA/H increases in the accumulation of the respective anion, and in cells SLC17A5 fi cotransporter, sialin ( ), which has been linked to Salla for which including SA with the anions did not signi cantly alter disease and ISSD. We show that sialin can also function as an the uptake of either anion (Fig. S3E). − + electrogenic 2NO3 /H cotransporter in the plasma membrane. − Other findings include that (i) sialin mutants associated with Assessment of Sialin-Mediated NO Transport in Fibroblasts from − 3 Salla disease and ISSD induce dominant suppression of 2NO / + 3 Patients with ISSD and Effect of the Salla Disease and ISSD Sialin H cotransporter activity, (ii) cotransporter activity is relatively Mutations on Anion Transport. To establish the link between sia- low in fibroblasts obtained from patients with ISSD compared lin and nitrate transport, we examined the effect of two non- with healthy volunteers, and (iii) expression of the ISSD-asso- functional sialin mutants that have been associated with Salla ciated sialin mutant (H183R) suppresses cotransporter function disease (R39C) and ISSD (H183R) (23–25). Expression of each in cells from healthy volunteers. Taken together, these data of these mutants in HSG cells induced dominant suppression of + − − provide strong evidence that sialin mediates H -dependent both NO and SA currents without altering the current-voltage − 3 NO uptake into cells. Although it is well established that the (I-V) characteristics (Fig. 4 A–D). A major finding of the present 3 + − study was that fibroblasts from patients with ISSD displayed lysosomal H /SA transport function mediated by sialin con- − a substantial reduction in pH-dependent NO or SA currents tributes to lysosomal recycling of SA, further studies are needed 3 to determine the exact physiological function of nitrate transport compared with the current amplitudes in cells from healthy fi volunteers (Fig. 4 E and G) with no change in the I-V re- mediated by sialin in broblasts and other cell types. However, it − lationship (Fig. 4 F and H). Note that a relatively higher [NO ] has been well established that salivary glands serve as a major − 3 level was required to detect NO3 conductance in fibroblasts. Although we could not induce recovery of function in cells obtained from patients with ISSD by expressing the WT-sialin (likely due to a dominant negative effect of the endogenously expressed mutant), expression of sialinH183R in cells from healthy volunteers strongly suppressed transporter function to levels seen in the cells from patients with ISSD (Fig. 4 I–K). Although we appreciate that including data demonstrating salivary deficiencies in patients would have strengthened our findings, several major issues preclude conducting such a study at the present time. There are very few patients with Salla disease or ISSD worldwide. In Salla disease, found primarily in Finland, newborns develop intellectual impairment gradually. ISSD, although not geographically restricted, is more severe, and patients generally die early in childhood or even in utero. Both disorders cause developmental delays. The majority of patients are chil- dren with severe developmental defects and poor overall health. Thus, although assessment of saliva in patients might yield significant data, this is beyond the scope of the present study. De- spite lack of such data, the findings presented herein establish a strong link between nitrate transport and sialin. We further demonstrate that disease-causing mutants of sialin also decreases − NO3 transport.

Effect of Adenovirus-Dependent Expression of Sialinh183r Mutant in Vivo in Pig Salivary Glands on Salivary Nitrate Secretion. To provide Fig. 5. Effect of in vivo suppression of sialin function in salivary glands on further evidence that sialin mediates salivary gland nitrate trans- salivary nitrate secretion. (A) Salivary nitrate concentrations at 30 min and 60 port, we used adenoviral vector containing cytomegalovirus pro- min after feeding a nitrate-rich diet to miniature pigs after in vivo delivery moter (AdCMV)-EGFP-polyethylenimine (PEI) complex as of plasmids encoding WT-sialin or sialinH183R (Methods). *Values signiﬁ- a carrier to deliver sialinH183R or WT-sialin vector into pig salivary < − cantly different from the control values (P 0.05). (B) Sialin expression in glands. At 3 d after infection, the animals were fed a NO3 -rich diet control parotid glands and glands receiving the plasmid-PEI-Ad complex as for 30 min and then stimulated with pilocarpine to induce saliva determined by Western blot analysis. (C and D) Detection of sialin in salivary secretion. Compared with control animals or those that received glands at 3 d after transduction with AdCMV-EGFP-PEI plus sialinH183R WT-sialin, the pigs receiving sialinH183R displayed relatively lower vector (C) or in control parotid gland (D). (Scale bar: 50 μM.)

13438 | www.pnas.org/cgi/doi/10.1073/pnas.1116633109 Qin et al. Downloaded by guest on September 28, 2021 − route for NO clearance from the serum. Approximately 25% large amount of ingested nitrite survives hydrolysis in the stomach 3 − of the circulating NO3 is taken up by the salivary glands, where and enters the systemic circulation, where it is reduced to NO and SEE COMMENTARY it is concentrated and secreted (at millimolar levels) in the saliva. other bioactive nitrogen oxides (1, 2). Thus, salivary nitrate fl − Thus, it is likely that transepithelial ux of NO3 occurs across transport provides an alternative, noncanonical pathway for the the salivary gland cells as the anion is taken up from the serum generation of nitrite and NO. This pathway appears to be par- and secreted into saliva. fi − ticularly signi cant under conditions of hypoxia and acidosis. We propose that sialin can function as the NO uptake system − 3 Taken together, these findings suggest that NO3 secretion via the in salivary gland cells. Consistent with this proposal, we found that gland can significantly impact the nitrate–nitrite–NO balance in the protein is localized in the basal and lateral regions. Further − fl the serum, which is of major importance in such conditions as studies are needed to identify the NO3 ef ux pathway likely to be high blood pressure, platelet aggregation, and vascular damage localized in the apical membrane of the cells. The physiological − (26–30). We suggest that disruption of sialin function, as seen in relevance of sialin in NO3 transport via the salivary gland is fi fi fi patients with Salla disease or ISSD, can have signi cant effects con rmed by our nding that in vivo expression of the dominant – – negative sialin mutant (sialinH183R) in pig parotid glands re- on the nitrate nitrite NO balance, in addition to the previously − − recognized impairment in SA storage and aspartergic neuro- duced NO3 secretion in the saliva in response to a NO3 -rich diet compared with the function in control animals and those transmission. Furthermore, loss of salivary gland secretory ac- expressing WT-sialin in the glands. Together with our findings tivity as a result of radiation treatment for head and neck cancers − or autoimmune disease (e.g., Sjogren’s syndrome) can have po- demonstrating that sialin mediates NO3 uptake into salivary gland cells, these data provide evidence for a physiological role tential systemic consequences owing to an imbalance of nitrite– of sialin in nitrate uptake into these glands. In addition, sialin NO homeostasis. Together, the findings presented herein dem- appears to be a versatile anion transporter that also has the ability onstrate that sialin, as a nitrate cotransporter in the salivary + − to mediate H -dependent transport of SA, NO2 , aspartate, or glands, can play an important role in the physiological regulation glutamate. Given the protein’s relatively wide distribution in dif- of systemic nitrate–nitrite–NO balance. ferent tissues, its function in the plasma membrane as well as lysosomes can have a significant impact on cell function, including Methods SA recycling as well as nitrate–NO balance. In neuronal tissues, All reagents and detailed methods are described in SI Methods. These include there might be additional consequences related to sialin’s ability cell culture, electrophysiology, confocal imaging, and biochemical techni- to transport glutamate and aspartate. ques such as surface biotinylation and western blotting. Descriptions of In summary, our data provide insight into the function of sialin experiments with miniature pigs are also provided in SI Methods. and demonstrate its unique function as a nitrate transporter. In − mammals, the diet is a major source of NO ; absorption of di- − 3 − ACKNOWLEDGMENTS. We thank Dr. Jim Turner, Dr. Shmuel Muallem, Dr. etary NO3 results in an increase in serum NO3 . Approximately Jing Jiang, and Professor Liangbiao Chen for their invaluable help during − the course of this work. This study was supported by the National Nature 25% of the circulating NO3 is taken up into salivary gland and secreted via saliva, where it is reduced to nitrite by the action of Science Foundation of China (Grants 30430690, 30125042, and 81170975), the National Basic Research Program of China (Grants 2007CB947304 and commensal bacteria in the oral cavity and then converted to NO 2010CB944801), and the Divisions of Intramural Research of the National in the stomach. NO is suggested to have an important role in the Institute of Dental and Craniofacial Research and the National Human protection of gastric tissues from stress-induced injury; however, a Genome Research Institute.

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