Sachin Mohan

NHE3 regulatory complexes

SUMMARY The epithelial brush border Na/H exchanger NHE3 is active under basal conditions and functions as part of neutral NaCl absorption in the intestine and renal proximal tubule, where it accounts for the majority of total Na absorbed. NHE3 is highly regulated. Both stimulation and inhibition occur post-prandially. This digestion related regulation of NHE3 is mimicked by multiple extracellular agonists and intracellular second messengers. The regulation of NHE3 depends on its C-terminal cytoplasmic domain, which acts as a scaffold to bind multiple regulatory proteins and links NHE3 to the cytoskeleton. The cytoskeletal association occurs by both direct binding to ezrin and by indirect binding via ezrin binding to the C-terminus of the multi-PDZ domain containing proteins NHERF1 and NHERF2. This is a review of the domain structure of NHE3 and of the scaffolding function and role in the regulation of NHE3 of the NHE3 C-terminal domain.

Casein Kinase 2 Binds to the C Terminus of Na+/H+ exchanger 3 (NHE3) and Stimulates NHE3 Basal Activity by Phosphorylating a Separate Site in NHE3

Na(+)/H(+) exchanger 3 (NHE3) is the epithelial-brush border isoform responsible for most intestinal and renal Na(+) absorption. Its activity is both up- and down-regulated under normal physiological conditions, and it is inhibited in most diarrheal diseases. NHE3 is phosphorylated under basal conditions and Ser/Thr phosphatase inhibitors stimulate basal exchange activity; however, the kinases involved are unknown. To identify kinases that regulate NHE3 under basal conditions, NHE3 was immunoprecipitated; LC-MS/MS of trypsinized NHE3 identified a novel phosphorylation site at S(719) of the C terminus, which was predicted to be a casein kinase 2 (CK2) phosphorylation site. This was confirmed by an in vitro kinase assay. The NHE3-S719A mutant but not NHE3-S719D had reduced NHE3 activity due to less plasma membrane NHE3. This was due to reduced exocytosis plus decreased plasma membrane delivery of newly synthesized NHE3. Also, NHE3 activity was inhibited by the CK2 inhibitor 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole DMAT when wild-type NHE3 was expressed in fibroblasts and Caco-2 cells, but the NHE3-S(719) mutant was fully resistant to DMAT. CK2 bound to the NHE3 C-terminal domain, between amino acids 590 and 667, a site different from the site it phosphorylates. CK2 binds to the NHE3 C terminus and stimulates basal NHE3 activity by phosphorylating a separate single site on the NHE3 C terminus (S(719)), which affects NHE3 trafficking.

Phospholipase C-γ Binds Directly to the Na+/H+ Exchanger 3 and Is Required for Calcium Regulation of Exchange Activity

Multiple studies suggest that phospholipase C-γ (PLC-γ) contributes to regulation of sodium/hydrogen exchanger 3 (NHE3) in the small intestine, although the mechanism(s) for this regulation remain unknown. We demonstrate here that PLC-γ binds directly to the C terminus of NHE3 and exists in similar sized multiprotein complexes as NHE3. This binding is dynamic and decreases with elevated [Ca2+]i. The PLC-γ-binding site in NHE3 was identified (amino acids 586–605) and shown to be a critical regulatory domain for protein complex formation, because when it is mutated, NHE3 binding to PLC-γ as well as NHERF2 is lost. An inhibitory peptide, which binds to the Src homology 2 domains contained in PLC-γ without interrupting binding of PLC-γ to NHE3, was used to probe a non-lipase-dependent role of PLC-γ. In the presence of this peptide, carbachol-stimulated calcium inhibition of NHE3 was lost. These results mirror previous studies with the transient receptor potential channel and suggest that PLC-γ may play a common role in regulating the cell-surface expression of ion transporters.

Calmodulin Kinase II Constitutively Binds, Phosphorylates, and Inhibits Brush Border Na/H Exchanger 3 (NHE3) by a NHERF2 Protein-dependent Process *

Background: NHE3 is regulated by a signaling complex on its C terminus, only some of the components of which are known. Results: CaMKIIγ binds to the NHE3 C terminus and phosphorylates and inhibits basal NHE3 activity by altering turnover number. Conclusion: CaMKIIγ is part of an NHE3 signaling complex. Significance: Signaling complexes that form on transport proteins take part in regulation of the transporter The epithelial brush border (BB) Na+/H+ exchanger 3 (NHE3) accounts for most renal and intestinal Na+ absorption. Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibits NHE3 activity under basal conditions in intact intestine, acting in the BB, but the mechanism is unclear. We now demonstrate that in both PS120 fibroblasts and polarized Caco-2BBe cells expressing NHE3, CaMKII inhibits basal NHE3 activity, because the CaMKII-specific inhibitors KN-93 and KN-62 stimulate NHE3 activity. This inhibition requires NHERF2. CaMKIIγ associates with NHE3 between aa 586 and 605 in the NHE3 C terminus in a Ca2+-dependent manner, with less association when Ca2+ is increased. CaMKII inhibits NHE3 by an effect on its turnover number, not changing surface expression. Back phosphorylation demonstrated that NHE3 is phosphorylated by CaMKII under basal conditions. This overall phosphorylation of NHE3 is not affected by the presence of NHERF2. Amino acids downstream of NHE3 aa 690 are required for CaMKII to inhibit basal NHE3 activity, and mutations of the three putative CaMKII phosphorylation sites downstream of aa 690 each prevented KN-93 stimulation of NHE3 activity. These studies demonstrate that CaMKIIγ is a novel NHE3-binding protein, and this association is reduced by elevated Ca2+. CaMKII inhibits basal NHE3 activity associated with phosphorylation of NHE3 by effects requiring aa downstream of NHE3 aa 690 and of the CaMKII-binding site on NHE3. CaMKII binding to and phosphorylation of the NHE3 C terminus are parts of the physiologic regulation of NHE3 that occurs in fibroblasts as well as in the BB of an intestinal Na+-absorptive cell.

NHE3 activity is dependent on direct phosphoinositide binding at the N terminus of its intracellular cytosolic region.

The small intestinal BB Na+/H+ antiporter NHE3 accounts for the majority of intestinal sodium and water absorption. It is highly regulated with both postprandial inhibition and stimulation sequentially occurring. Phosphatidylinositide 4,5-bisphosphate (PI(4,5)P2) and phosphatidylinositide 3,4,5-trisphosphate (PI(3,4,5)P3) binding is involved with regulation of multiple transporters. We tested the hypothesis that phosphoinositides bind NHE3 under basal conditions and are necessary for its acute regulation. His6 proteins were made from the NHE3 C-terminal region divided into four parts as follows: F1 (amino acids 475–589), F2 (amino acids 590–667), F3 (amino acids 668–747), and F4 (amino acids 748–832) and purified by a nickel column. Mutations were made in the F1 region of NHE3 and cloned in pet30a and pcDNA3.1 vectors. PI(4,5)P2 and PI(3,4,5)P3 bound only to the NHE3 F1 fusion protein (amino acids 475–589) on liposomal pulldown assays. Mutations were made in the putative lipid binding region of the F1 domain and studied for alterations in lipid binding and Na+/H+ exchange as follows: Y501A/R503A/K505A; F509A/R511A/R512A; R511L/R512L; R520/FR527F; and R551L/R552L. Our results indicate the following. 1) The F1 domain of the NHE3 C terminus has phosphoinositide binding regions. 2) Mutations of these regions alter PI(4,5)P2 and PI(3,4,5)P3 binding and basal NHE3 activity. 3) The magnitude of serum stimulation of NHE3 correlates with PI(4,5)P2 and PI(3,4,5)P3 binding of NHE3. 4) Wortmannin inhibition of PI3K did not correlate with PI(4,5)P2 or PI(3,4,5)P3 binding of NHE3. Two functionally distinct phosphoinositide binding regions (Tyr501–Arg512 and Arg520–Arg552) are present in the NHE3 F1 domain; both regions are important for serum stimulation, but they display differences in phosphoinositide binding, and the latter but not the former alters NHE3 surface expression.

A genetic database can be utilized to identify potential biomarkers for biphenotypic hepatocellular carcinoma-cholangiocarcinoma

BACKGROUND Biphenotypic hepatocellular carcinoma-cholangiocarcinoma (HCC-CC) is an uncommon primary liver neoplasm. Due to limitations in radiologic imaging for the diagnosis of this condition, biopsy is a common method for diagnosis, which is invasive and holds potential complications. To identify alternative means for obtaining the diagnosis and assessing the prognosis of this condition, we evaluated biomarkers for biphenotypic HCC-CC using a genetic database. METHODS To evaluate the genetic associations with each variable we utilized GeneCards(®), The Human Gene Compendium (http://www.genecards.org). The results of our search were entered into the Pathway Interaction Database from the National Cancer Institute (PID-NCI) (http://pid.nci.nih.gov), to generate a biomolecule interaction map. RESULTS The results of our query yielded 690 genes for HCC, 98 genes for CC and 50 genes for HCC-CC. Genes depicted in this analysis demonstrate the role of hormonal regulation, embryonic development, cell surface adhesion, cytokeratin stability, mucin production, metalloproteinase regulation, Ras signaling, metabolism and apoptosis. Examples of previously described markers included hepatocyte growth factor (HGF), mesenchymal epithelial transition (MET) and Kirsten rat sarcoma viral oncogene homolog (KRAS). Novel markers included phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha (PIK3CA), GPC3, choline kinase alpha (CHKA), prostaglandin-endoperoxide synthase 2 (PTGS2), telomerase reverse transcriptase (TERT), myeloid cell leukemia 1 (MCL1) and N-acetyltransferase 2 (NAT2). CONCLUSIONS GeneCards is a useful research tool in the genetic analysis of low frequency malignancies. Utilizing this tool we identified several biomarkers are methods for diagnosing HCC-CC. Finally, utilizing these methods, HCC-CC was found to be predominantly a subtype of CC.