Int J Biol Sci 2014; 10(7):771-776. doi:10.7150/ijbs.9539 This issue

Short Research Communication

Direct Evidence for Calcineurin Binding to the Exon-7 Loop of the Sodium-Bicarbonate Cotransporter NBCn1

Harindarpal S. Gill1,2✉, Eric D. Roush3, Lauren Dutcher4, Samir Patel2

1. Department of Medicine, The George Washington University, Washington, DC 20052
2. Division of Renal Diseases and Hypertension, The George Washington Medical Faculty Associates, Washington, DC 20037
3. GE Healthcare, Piscataway, NJ 08554
4. Hospital of University of Pennsylvania, Philadelphia, PA 19104

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Gill HS, Roush ED, Dutcher L, Patel S. Direct Evidence for Calcineurin Binding to the Exon-7 Loop of the Sodium-Bicarbonate Cotransporter NBCn1. Int J Biol Sci 2014; 10(7):771-776. doi:10.7150/ijbs.9539. Available from

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The NaHCO3 cotransporter NBCn1 plays a role in neutralizing intracellular acid loads at the basolateral membrane in cells of the medullary thick ascending limb (mTAL). Calcineurin inhibitors (Cn-Is) are known to both downregulate NBCn1 expression in the distal nephron and cause renal tubular acidosis (RTA), a risk factor for nephrocalcinosis and nephrolithiasis. In this report, we provide a new perspective on concurrent studies of NBCn1 in various tissues by using cell-free binding assays to investigate the interaction of NBCn1 with the calcineurin (Cn) isoform PPP3CA. Surface plasmon resonance (SPR) analyses show that the protein domain Exon 7 (translated from cassette II of NBCn1) binds Cn with an equilibrium dissociation constant (KD) of 30 +/- 15 nm. Linked-reaction tests suggest that the binding involves a conformational change. Nested PCR reactions were used to show that NBCn1-Exon 7 splice variants with alternative N-termini regions are expressed in the kidney, as well as other tissues. Additionally, we discuss NBCn1-Exon 7 implication in acid-base balance and calcium crystallization in the kidney.

Keywords: bicarbonate, transporters, calcineurin, renal physiology, surface plasmon resonance