SPAK dependent regulation of peptide transporters PEPT1 and PEPT2

Stimulatory Role of SPAK Signaling in the Regulation of Large Conductance Ca2+-Activated Potassium (BK) Channel Protein Expression in Kidney

SPS1-related proline/alanine-rich kinase (SPAK) plays important roles in regulating the function of numerous ion channels and transporters. With-no-lysine (WNK) kinase phosphorylates SPAK kinase to active the SPAK signaling pathway. Our previous studies indicated that WNK kinases regulate the activity of the large-conductance Ca²⁺-activated K⁺ (BK) channel and its protein expression via the ERK1/2 signaling pathway. It remains largely unknown whether SPAK kinase directly modulates the BK protein expression in kidney. In this study, we investigated the effect of SPAK on renal BK protein expression in both HEK293 cells and mouse kidney. In HEK293 cells, siRNA-mediated knockdown of SPAK expression significantly reduced BK protein expression and increased ERK1/2 phosphorylation, whereas overexpression of SPAK significantly enhanced BK expression and decreased ERK1/2 phosphorylation in a dose-dependent manner. Knockdown of ERK1/2 prevented SPAK siRNA-mediated inhibition of BK expression. Similarly, pretreatment of HEK293 cells with either the lysosomal inhibitor bafilomycin A1 or the proteasomal inhibitor MG132 reversed the inhibitory effects of SPAK knockdown on BK expression. We also found that there is no BK channel activity in PCs of CCD in SPAK KO mice using the isolated split-open tubule single-cell patching. In addition, we found that BK protein abundance in the kidney of SPAK knockout mice was significantly decreased and ERK1/2 phosphorylation was significantly enhanced. A high-potassium diet significantly increased BK protein abundance and SPAK phosphorylation levels, while reducing ERK1/2 phosphorylation levels. These findings suggest that SPAK enhances BK protein expression by reducing ERK1/2 signaling-mediated lysosomal and proteasomal degradations of the BK channel.

Meat proteins had different effects on oligopeptide transporter PEPT1 in the small intestine of young rats

The peptide transporter 1 (PEPT1) in the apical membrane of enterocytes is the central mechanism for regulating the absorption of di- and tripeptides. Dietary proteins may affect PEPT1 abundance and peptide absorption. The present study aimed to characterize changes in PEPT1 mRNA and PEPT1 protein levels in the duodenum and jejunum of young rats after 7-day diet intervention with casein (reference), soy, beef, pork, chicken and fish proteins and further evaluate the impact on the epithelial absorption capacity. RT-PCR and western blot analyses showed that: (1) PEPT1 protein level in duodenum was higher (p < 0.05) for soy protein group than that for casein group. However, no difference was observed in jejunal PEPT1 protein level between any two diet groups (p > 0.05). The soy protein group had lower crypt depth and higher V/C ratio in the jejunum (p < 0.05). (2) PEPT1 mRNA levels were lower (p < 0.05) in rat duodenum and jejunum in pork, chicken and fish protein groups, whose trend was contrary to the results of jejunual histological observation with lower crypt depth, greater villus height and higher V/C ratio. In conclusion, different meat proteins alter distinct PEPT1 expression level and absorption capacity as reflected by gut morphology in small intestine.

Caveolin-1 Sensitivity of Excitatory Amino Acid Transporters EAAT1, EAAT2, EAAT3, and EAAT4

Excitatory amino acid transporters EAAT1 (SLC1A3), EAAT2 (SLC1A2), EAAT3 (SLC1A1), and EAAT4 (SLC1A6) serve to clear L-glutamate from the synaptic cleft and are thus important for the limitation of neuronal excitation. EAAT3 has previously been shown to form complexes with caveolin-1, a major component of caveolae, which participate in the regulation of transport proteins. The present study explored the impact of caveolin-1 on electrogenic transport by excitatory amino acid transporter isoforms EAAT1-4. To this end cRNA encoding EAAT1, EAAT2, EAAT3, or EAAT4 was injected into Xenopus oocytes without or with additional injection of cRNA encoding caveolin-1. The L-glutamate (2 mM)-induced inward current (I Glu) was taken as a measure of glutamate transport. As a result, I Glu was observed in EAAT1-, EAAT2-, EAAT3-, or EAAT4-expressing oocytes but not in water-injected oocytes, and was significantly decreased by coexpression of caveolin-1. Caveolin-1 decreased significantly the maximal transport rate. Treatment of EAATs-expressing oocytes with brefeldin A (5 µM) was followed by a decrease in conductance, which was similar in oocytes expressing EAAT together with caveolin-1 as in oocytes expressing EAAT1-4 alone. Thus, caveolin-1 apparently does not accelerate transporter protein retrieval from the cell membrane. In conclusion, caveolin-1 is a powerful negative regulator of the excitatory glutamate transporters EAAT1, EAAT2, EAAT3, and EAAT4.

Up-Regulation of Intestinal Phosphate Transporter NaPi-IIb (SLC34A2) by the Kinases SPAK and OSR1

BACKGROUND/AIMS

SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1), kinases controlled by WNK (with-no-K[Lys] kinase), are powerful regulators of cellular ion transport and blood pressure. Observations in gene-targeted mice disclosed an impact of SPAK/OSR1 on phosphate metabolism. The present study thus tested whether SPAK and/or OSR1 contributes to the regulation of the intestinal Na(+)-coupled phosphate co-transporter NaPi-IIb (SLC34A2).

METHODS

cRNA encoding NaPi-IIb was injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding wild-type SPAK, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 or catalytically inactive (D164A)OSR1. The phosphate (1 mM)-induced inward current (I(Pi)) was taken as measure of phosphate transport.

RESULTS

I(Pi) was observed in NaPi-IIb expressing oocytes but not in water injected oocytes, and was significantly increased by co-expression of SPAK, (T233E)SPAK, OSR1, (T185E)OSR1 or SPAK+OSR1, but not by co-expression of (T233A)SPAK, (D212A)SPAK, (T185A)OSR1, or (D164A)OSR1. SPAK and OSR1 both increased the maximal transport rate of the carrier.

CONCLUSIONS

SPAK and OSR1 are powerful stimulators of the intestinal Na+-coupled phosphate co-transporter NaPi-IIb.

Down-Regulation of Excitatory Amino Acid Transporters EAAT1 and EAAT2 by the Kinases SPAK and OSR1

SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1) are cell volume-sensitive kinases regulated by WNK (with-no-K[Lys]) kinases. SPAK/OSR1 regulate several channels and carriers. SPAK/OSR1 sensitive functions include neuronal excitability. Orchestration of neuronal excitation involves the excitatory glutamate transporters EAAT1 and EAAT2. Sensitivity of those carriers to SPAK/OSR1 has never been shown. The present study thus explored whether SPAK and/or OSR1 contribute to the regulation of EAAT1 and/or EAAT2. To this end, cRNA encoding EAAT1 or EAAT2 was injected into Xenopus oocytes without or with additional injection of cRNA encoding wild-type SPAK or wild-type OSR1, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 or catalytically inactive (D164A)OSR1. The glutamate (2 mM)-induced inward current (I Glu) was taken as a measure of glutamate transport. As a result, I Glu was observed in EAAT1- and in EAAT2-expressing oocytes but not in water-injected oocytes, and was significantly decreased by coexpression of SPAK and OSR1. As shown for EAAT2, SPAK, and OSR1 decreased significantly the maximal transport rate but significantly enhanced the affinity of the carrier. The effect of wild-type SPAK/OSR1 on EAAT1 and EAAT2 was mimicked by (T233E)SPAK and (T185E)OSR1, but not by (T233A)SPAK, (D212A)SPAK, (T185A)OSR1, or (D164A)OSR1. Coexpression of either SPAK or OSR1 decreased the EAAT2 protein abundance in the cell membrane of EAAT2-expressing oocytes. In conclusion, SPAK and OSR1 are powerful negative regulators of the excitatory glutamate transporters EAAT1 and EAAT2.

SPAK and OSR1 Sensitivity of Excitatory Amino Acid Transporter EAAT3

BACKGROUND/AIMS

Kinases involved in the regulation of epithelial transport include SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1). SPAK and OSR1 are both regulated by WNK (with-no-K(Lys)) kinases. The present study explored whether SPAK and/or OSR1 influence the excitatory amino acid transporter EAAT3, which accomplishes glutamate and aspartate transport in kidney, intestine and brain.

METHODS

cRNA encoding EAAT3 was injected into Xenopus laevis oocytes with or without additional injection of cRNA encoding wild-type SPAK, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 and catalytically inactive (D164A)OSR1. Glutamate-induced current was taken as measure of electrogenic glutamate transport and was quantified utilizing dual electrode voltage clamp. Furthermore, Ussing chamber was employed to determine glutamate transport in the intestine from gene-targeted mice carrying WNK insensitive SPAK (spak(tg/tg)) and from corresponding wild-type mice (spak(+/+)).

RESULTS

EAAT3 activity was significantly decreased by wild-type SPAK and (T233E)SPAK, but not by (T233A)SPAK and (D212A)SPAK. SPAK decreased maximal transport rate without affecting significantly affinity of the carrier. Similarly, EAAT3 activity was significantly downregulated by wild-type OSR1 and (T185E)OSR1, but not by (T185A)OSR1 and (D164A)OSR1. Again OSR1 decreased maximal transport rate without affecting significantly affinity of the carrier. Intestinal electrogenic glutamate transport was significantly lower in spak(+/+) than in spak(tg/tg) mice.

CONCLUSION

Both, SPAK and OSR1 are negative regulators of EAAT3 activity.

Regulation of Voltage-Gated K+ Channel Kv1.5 by the Janus Kinase JAK3

The tyrosine kinase Janus kinase 3 (JAK3) participates in the regulation of cell proliferation and apoptosis. The kinase further influences ion channels and transport proteins. The present study explored whether JAK3 contributes to the regulation of the voltage-gated K(+) channel Kv1.5, which participates in the regulation of diverse functions including atrial cardiac action potential and tumor cell proliferation. To this end, cRNA encoding Kv1.5 was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild-type JAK3, constitutively active (A568V)JAK3, or inactive (K851A)JAK3. Voltage-gated K(+) channel activity was measured utilizing dual electrode voltage clamp, and Kv1.5 channel protein abundance in the cell membrane was quantified utilizing chemiluminescence of Kv1.5 containing an extracellular hemagglutinin epitope (Kv1.5-HA). As a result, Kv1.5 activity and Kv1.5-HA protein abundance were significantly decreased by wild-type JAK3 and (A568V)JAK3, but not by (K851A)JAK3. Inhibition of Kv1.5 protein insertion into the cell membrane by brefeldin A (5 μM) resulted in a decline of the voltage-gated current, which was similar in the absence and presence of (A568V)JAK3, suggesting that (A568V)JAK3 did not accelerate Kv1.5 protein retrieval from the cell membrane. A 24 h treatment with ouabain (100 µM) significantly decreased the voltage-gated current in oocytes expressing Kv1.5 without or with (A568V)JAK3 and dissipated the difference between oocytes expressing Kv1.5 alone and oocytes expressing Kv1.5 with (A568V)JAK3. In conclusion, JAK3 contributes to the regulation of membrane Kv1.5 protein abundance and activity, an effect sensitive to ouabain and thus possibly involving Na(+)/K(+) ATPase activity.

USP18 Sensitivity of Peptide Transporters PEPT1 and PEPT2

USP18 (Ubiquitin-like specific protease 18) is an enzyme cleaving ubiquitin from target proteins. USP18 plays a pivotal role in antiviral and antibacterial immune responses. On the other hand, ubiquitination participates in the regulation of several ion channels and transporters. USP18 sensitivity of transporters has, however, never been reported. The present study thus explored, whether USP18 modifies the activity of the peptide transporters PEPT1 and PEPT2, and whether the peptide transporters are sensitive to the ubiquitin ligase Nedd4-2. To this end, cRNA encoding PEPT1 or PEPT2 was injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding USP18. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp. As a result, in Xenopus laevis oocytes injected with cRNA encoding PEPT1 or PEPT2, but not in oocytes injected with water or with USP18 alone, application of the dipeptide gly-gly (2 mM) was followed by the appearance of an inward current (I_gly-gly). Coexpression of USP18 significantly increased I_gly-gly in both PEPT1 and PEPT2 expressing oocytes. Kinetic analysis revealed that coexpression of USP18 increased maximal I_gly-gly. Conversely, overexpression of the ubiquitin ligase Nedd4-2 decreased I_gly-gly. Coexpression of USP30 similarly increased I_gly-gly in PEPT1 expressing oocytes. In conclusion, USP18 sensitive cellular functions include activity of the peptide transporters PEPT1 and PEPT2.

Regulation of the voltage gated K channel Kv1.3 by recombinant human klotho protein

BACKGROUND/AIMS

Klotho, a protein mainly produced in the kidney and released into circulating blood, contributes to the negative regulation of 1,25(OH)2D3 formation and is thus a powerful regulator of mineral metabolism. As β-glucuronidase, alpha Klotho protein further regulates the stability of several carriers and channels in the plasma membrane and thus regulates channel and transporter activity. Accordingly, alpha Klotho protein participates in the regulation of diverse functions seemingly unrelated to mineral metabolism including lymphocyte function. The present study explored the impact of alpha Klotho protein on the voltage gated K+ channel Kv1.3.

METHODS

cRNA encoding Kv1.3 (KCNA3) was injected into Xenopus oocytes and depolarization induced outward current in Kv1.3 expressing Xenopus oocytes determined utilizing dual electrode voltage clamp. Experiments were performed without or with prior treatment with recombinant human Klotho protein (50 ng/ml, 24 hours) in the absence or presence of a β-glucuronidase inhibitor D-saccharic acid-1,4-lactone (DSAL, 10 µM). Moreover, the voltage gated K+ current was determined in Jcam lymphoma cells by whole cell patch clamp following 24 hours incubation without or with recombinant human Klotho protein (50 ng/ml, 24 hours). Kv1.3 protein abundance in Jcam cells was determined utilising fluorescent antibodies in flow cytometry.

RESULTS

In Kv1.3 expressing Xenopus oocytes the Kv1.3 currents and the protein abundance of Kv1.3 were both significantly enhanced after treatment with recombinant human Klotho protein (50 ng/ml, 24 hours), an effect reversed by presence of DSAL. Moreover, treatment with recombinant human Klotho protein increased Kv currents and Kv1.3 protein abundance in Jcam cells.

CONCLUSION

Alpha Klotho protein enhances Kv1.3 channel abundance and Kv1.3 currents in the plasma membrane, an effect depending on its β-glucuronidase activity.