Characterization of the transport activity of SGLT2/MAP17, the renal low-affinity Na⁺-glucose cotransporter

Abstract

The cotransporter SGLT2 is responsible for 90% of renal glucose reabsorption, and we recently showed that MAP17 appears to work as a required β-subunit. We report in the present study a detailed functional characterization of human SGLT2 in coexpression with human MAP17 in oocytes. Addition of external glucose generates a large inward current in the presence of Na, confirming an electrogenic transport mechanism. At a membrane potential of -50 mV, SGLT2 affinity constants for glucose and Na are 3.4 ± 0.4 and 18 ± 6 mM, respectively. The change in the reversal potential of the cotransport current as a function of external glucose concentration clearly confirms a 1:1 Na-to-glucose transport stoichiometry. SGLT2 is selective for glucose and α-methylglucose but also transports, to a lesser extent, galactose and 3--methylglucose. SGLT2 can be inhibited in a competitive manner by phlorizin ( = 31 ± 4 nM) and by dapagliflozin ( = 0.75 ± 0.3 nM). Similarly to SGLT1, SGLT2 can be activated by Na, Li, and protons. Pre-steady-state currents for SGLT2 do exist but are small in amplitude and relatively fast (a time constant of ~2 ms). The leak current defined as the phlorizin-sensitive current in the absence of substrate was extremely small in the case of SGLT2. In summary, in comparison with SGLT1, SGLT2 has a lower affinity for glucose, a transport stoichiometry of 1:1, very small pre-steady-state and leak currents, a 10-fold higher affinity for phlorizin, and an affinity for dapagliflozin in the subnanomolar range.

Sodium–Glucose Cotransporter 2 Inhibition and Diabetic Kidney Disease

“…1). SGLT1 is expressed on the luminal surface of the epithelial cells of the late proximal tubule and reabsorbs most of the remaining ;10% of filtered glucose (21)(22)(23).…”

Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2

“…Other differences include the higher affinity of phlorizin to SGLT2 vs SGLT1 (K i 11 vs 140 nmol/l), the higher affinity of gliflozins, such as dapagliflozin to SGLT2 (K i 4 vs 400 nmol/l), and a narrower sugar selectivity of SGLT2 (galactose is a poor substrate for SGLT2). Michael Coady and his colleagues recently elucidated the reason for low SGLT2 expression in oocytes with the discovery that co-expression of MAP17 was required [ 41 , 42 ]. Apart from small differences in kinetic variables (likely owing to the difference in temperature of culture conditions), the kinetic properties of SGLT2 expressed in oocytes (cultured at 22°C) were identical to those in HEK 293 cells (cultured at 37°C).…”

PDZK1-interacting protein 1 (PDZK1IP1) traps Smad4 protein and suppresses transforming growth factor-β (TGF-β) signaling

“…Furthermore, PDZK1IP1 can clearly contribute to the internalization of sodium-dependent phosphate transport protein 2b (NaPiIIa; alternatively termed SLC34A1) in the trans-Golgi network (10). In addition, PDZK1IP1 showed stimulation of Na-dependent transport of mannose and glucose in Xenopus oocytes and mammalian cell lines because PDZK1IP1 acted as a required ␤-subunit for sodium-dependent glucose cotransporter 2 (SGLT2) (11)(12)(13). Therefore, PDZK1IP1 is thought to participate in enhancement of the endogenous uphill transport system in the kidney as well.…”