PIP2 hydrolysis stimulates the electrogenic Na+–bicarbonate cotransporter NBCe1-B and -C variants expressed in Xenopus laevis oocytes

Authors


M. O. Bevensee: Department of Cell, Developmental and Integrative Biology, University of Alabama at Birmingham, 1918 University Blvd, 812 MCLM, Birmingham, AL 35294-0005, USA. Email: bevensee@uab.edu

Key points

  • • The Na+–bicarbonate cotransporter NBCe1 regulates cell and tissue pH, as well as ion movement across cell layers in organs such as kidney, gut, and pancreas.
  • • We previously showed that the signalling molecule PIP2 stimulates the cloned A variant of NBCe1 in a patch of biological membrane.
  • • In the current study, we characterize the effect of injecting PIP2 into intact oocytes expressing an NBCe1 variant (A, B, or C).
  • • PIP2 stimulates the B and C variants, but not the A variant, through hydrolysis to IP3. Stimulation requires an intracellular Ca2+ store and kinase activity.
  • • The results will contribute to our understanding of multiple HCO3-dependent transporters with different modes of regulation, as well as how molecules that stimulate specific membrane receptors lead to changes in cell/tissue pH, and perhaps how pathologies such as stroke and ischaemia that lead to energy deficiency cause tissue acidosis.

Abstract  Electrogenic Na+–bicarbonate cotransporter NBCe1 variants contribute to pHi regulation, and promote ion reabsorption or secretion by many epithelia. Most Na+-coupled bicarbonate transporter (NCBT) families such as NBCe1 contain variants with differences primarily at the cytosolic N and/or C termini that are likely to impart on the transporters different modes of regulation. For example, N-terminal regions of NBCe1 autoregulate activity. Our group previously reported that cytosolic phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates heterologously expressed rat NBCe1-A in inside-out macropatches excised from Xenopus laevis oocytes. In the current study on whole oocytes, we used the two-electrode voltage-clamp technique, as well as pH- and voltage-sensitive microelectrodes, to characterize the effect of injecting PIP2 on the activity of heterologously expressed NBCe1-A, -B, or -C. Injecting PIP2 (10 μm estimated final) into voltage-clamped oocytes stimulated NBC-mediated, HCO3-induced outward currents by >100% for the B and C variants, but not for the A variant. The majority of this stimulation involved PIP2 hydrolysis and endoplasmic reticulum (ER) Ca2+ release. Stimulation by PIP2 injection was mimicked by injecting IP3, but inhibited by either applying the phospholipase C (PLC) inhibitor U73112 or depleting ER Ca2+ with prolonged thapsigargin/EGTA treatment. Stimulating the activity of store-operated Ca2+ channels (SOCCs) to trigger a Ca2+ influx mimicked the PIP2/IP3 stimulation of the B and C variants. Activating the endogenous Gq protein-coupled receptor in oocytes with lysophosphatidic acid (LPA) also stimulated the B and C variants in a Ca2+-dependent manner, although via an increase in surface expression for the B variant. In simultaneous voltage-clamp and pHi studies on NBCe1-C-expressing oocytes, LPA increased the NBC-mediated pHi-recovery rate from a CO2-induced acid load by ∼80%. Finally, the general kinase inhibitor staurosporine completely inhibited the IP3-induced stimulation of NBCe1-C. In summary, injecting PIP2 stimulates the activity of NBCe1-B and -C expressed in oocytes through an increase in IP3/Ca2+ that involves a staurosporine-sensitive kinase. In conjunction with our previous macropatch findings, PIP2 regulates NBCe1 through a dual pathway involving both a direct stimulatory effect of PIP2 on at least NBCe1-A, as well as an indirect stimulatory effect of IP3/Ca2+ on the B and C variants.

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