Multiple acid‐base and electrolyte disturbances upregulate NBCn1, NBCn2, IRBIT and L‐IRBIT in the mTAL

Abstract

Key points The roles of the Na+/HCO3– cotransporters NBCn1 and NBCn2 as well as their activators IRBIT and L‐IRBIT in the regulation of the mTAL transport of NH4+, HCO3–, and NaCl are investigated. Dietary challenges of NH4Cl, NaHCO3 or NaCl all increase the abundance of NBCn1 and NBCn2 in the outer medulla. The three challenges generally produce parallel increases in the abundance of IRBIT and L‐IRBIT in the outer medulla. Both IRBIT and L‐IRBIT powerfully stimulate the activities of the mTAL isoforms of NBCn1 and NBCn2 as expressed in Xenopus oocytes. Our findings support the hypothesis that NBCn1, NBCn2, IRBIT and L‐IRBIT appropriately promote NH4+ shunting but oppose HCO3– and NaCl reabsorption in the mTAL, and thus are at the nexus of the regulation pathways for multiple renal transport processes. Abstract The medullary thick ascending limb (mTAL) plays a key role in urinary acid and NaCl excretion. NBCn1 and NBCn2 are present in the basolateral mTAL, where NBCn1 promotes NH4+ shunting. IRBIT and L‐IRBIT (the IRBITs) are two powerful activators of certain acid‐base transporters. Here we use western blotting and immunofluorescence to examine the effects of multiple acid‐base and electrolyte disturbances on expression of NBCn1, NBCn2 and the IRBITs in rat kidney. We also use electrophysiology to examine the functional effects of IRBITs on NBCn1 and NBCn2 in Xenopus oocytes. NH4Cl‐induced metabolic acidosis (MAc) substantially increases protein expression of NBCn1 and NBCn2 in the outer medulla (OM) of rat kidney. Surprisingly, NaHCO3‐induced metabolic alkalosis (MAlk) and high‐salt diet (HSD) also increase expression of NBCn1 and NBCn2 (effect of NaHCO3 > HSD). Moreover, all three challenges generally increase OM expression of the IRBITs. In Xenopus oocytes, the IRBITs substantially increase the activities of NBCn1 and NBCn2. We propose that upregulation of basolateral NBCn1 and NBCn2 plus the IRBITs in the mTAL: (1) promotes NH4+ shunting by increasing basolateral HCO3– uptake to neutralize apical NH4+ uptake during MAc; (2) inhibits HCO3– reabsorption during MAlk by opposing HCO3− efflux via the basolateral anion exchanger AE2; and (3) inhibits NaCl reabsorption by mediating (with AE2) net NaCl backflux into the mTAL cell during HSD. Thus, NBCn1, NBCn2 and the IRBITs are at the nexus of the regulatory pathways for multiple renal transport processes.

IRBIT activates NBCe1‐B by releasing the auto‐inhibition module from the transmembrane domain

“…IRBIT was originally identified as a binding partner that inhibits the activity of inositol 1,4,5-trisphosphate receptor (IP3R), a Ca 2+ channel expressed in the endoplasmic reticulum membrane (Ando et al 2003(Ando et al , 2006. IRBIT can stimulate a series of acid-base transporters, such as Na + /HCO 3 − cotransporter NBCe1 (Shirakabe et al 2006), NBCn1 (Hong et al 2013;Wang et al 2020a), NBCn2 (Wang et al 2020a), Na + /H + exchanger NHE3 (He et al 2008;He et al 2010), Cl − /HCO 3 − exchanger SLC26A6 (Park et al 2013), Na + /dicarboxylate cotransporter SLC13A3 (Khamaysi et al 2019). IRBIT can also regulate the calcium calmodulin-dependent kinase II alpha (Kawaai et al 2015), ribonucleotide reductase involved in regulation of DNA synthesis (Arnaoutov & Dasso, 2014), the cleavage and polyadenylation specificity factor involved in RNA splicing (Kiefer et al 2009), and apoptosis regulator Bcl-2 (Bonneau et al 2016).…”

Potential Novel Role of Membrane-Associated Carbonic Anhydrases in the Kidney

“…In contrast to PTs, where NBCe1-A exports Na + and CO 3 2− across the BLM, here in the TAL, AE2 exchanges cytosolic HCO 3 − for Cl − at the BLM. In addition to AE2, investigators have identified NBCn1 and (in rat) NBCn2 in the BLM of the TAL [ 131 , 151 , 152 , 153 ]. Because NBCn1 and NBCn2 normally operate in an inward direction [ 154 ], it seems unlikely that they contribute to HCO 3 − reabsorption per se.…”

Metabolic Reprogramming and Renal Fibrosis

“…The responses of different regions of the kidney to hypoxia also differ. The medullary thick ascending limbs (mTALs) play a central role in maintaining systemic acid-base and electrolyte homeostasis ( 150 ). The mTALs and straight segment of the proximal tubule have a strong demand for extramedullary oxygen transport from the renal tubules, which are very vulnerable to hypoxia ( 22 ).…”