Net Intestinal Transport of Oxalate Reflects Passive Absorption and SLC26A6-mediated Secretion

Abstract

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calciumoxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [14 C]oxalate simultaneously with the flux of [ 3 H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion. 22: 224722: -225522: , 201122: . doi: 10.1681 Nephrolithiasis is the second most common chronic kidney condition after hypertension and demonstrates a rising prevalence and association with chronic kidney disease. 1 Calcium oxalate (CaOx) is the predominant component of most stones, and the level of urinary oxalate is an important risk factor for CaOx nephrolithiasis. 2,3 The amount of urinary oxalate depends on the net effect of metabolic production, intestinal absorption, and renal excretion. 4 The integral role of the intestine in oxalate homeostasis becomes most evident in diseases of intestine. Inflammatory bowel disease and small-bowel resections are conditions that are strongly associated with hyperoxaluria and CaOx nephrolithiasis. 5 New insight into the importance of the intestine in the pathogenesis of CaOx nephrolithiasis has come from studies of anion transporter SLC26A6. Slc26a6-null mice have a defect in in- J Am Soc Nephrol

Transcellular oxalate and Cl−absorption in mouse intestine is mediated by the DRA anion exchanger Slc26a3, and DRA deletion decreases urinary oxalate

“…Of the three segments we examined in this study, the cecum exhibited the largest steady-state, DRA-dependent (J Role of paracellular oxalate transport. Our conclusion regarding the role of DRA in transcellular oxalate transport in the mouse intestine is strikingly different from the recent proposal of Knauf et al (27) that transepithelial oxalate absorption (J ms Ox ) is predominantly passive and paracellular in all major intestinal segments. The key observation in their report was that, in isolated, short-circuited segments of WT mouse duodenum, oxalate and mannitol permeabilities (P Ox and P Man , respectively) were numerically similar when measured in the MS direction, but P Ox Ͼ P Man when measured in the SM direction.…”

“…In their initial report (Table 2), WT duodenum exhib-ited a net secretion of oxalate (Ϫ73 pmol·cm Ϫ2 ·h Ϫ1 ), but a net oxalate absorption (ϩ28 pmol·cm Ϫ2 ·h Ϫ1 ) was revealed in the PAT1 KO mouse duodenum. As indicated below, this net absorptive flux reported in their PAT1 KO model was obtained using a sampling protocol that commenced 2 h earlier than that employed in the later report of Knauf et al (27). In WT mouse ileum, we previously observed (9) a small net secretion of oxalate under short-circuit conditions (Ϫ10 pmol·cm Ϫ2 ·h Ϫ1 ), but in PAT1 KO mouse ileum there was a very large net absorption (ϩ75 pmol·cm Ϫ2 ·h…”

SLC26A6 and NaDC-1 Transporters Interact to Regulate Oxalate and Citrate Homeostasis

“…8 Dietary oxalate is an important source of exogenous oxalate and is absorbed by the intestinal epithelium via the paracellular pathway. 9 The liver is the main source of endogenous oxalate; however, under physiologic conditions, a small fraction of the bodily oxalate is derived from hepatic production. 10 Studies using slc26a6 2/2 mice showed that increased serum oxalate is the result of impaired intestinal excretion that, in turn, leads to increased filtered renal oxalate load and formation of Ca 2+ -oxalate stones.…”

“…Slc26a6 mediates oxalate clearance via the intestine, 9 and urinary citrate chelates the Ca 2+ to reduce the free Ca 2+ available for binding to oxalate. 11 Citrate binds Ca 2+ at a higher affinity than does oxalate; 12 thus, in the presence of a high citrate concentration, Ca…”

Sat1 is dispensable for active oxalate secretion in mouse duodenum

“…SLC26A6 expression in mouse intestinal tissue is highest in duodenum (18), and active oxalate secretion in this tissue is completely ablated in Slc26a6-null mice (7,9). In contrast to the evidence for active oxalate secretion in mouse duodenum, oxalate absorption in this tissue is predominantly, if not exclusively, passive and paracellular (9). However, no studies to date have addressed the nature of the basolateral membrane oxalate transport process that operates in series with SLC26A6 to mediate active oxalate secretion across mouse duodenum.…”