Low Phosphate Diet Exacerbates Hypercalciuria in the Jimbee Mouse Model of SGLT2 Loss-of Function

Abstract

Selective sodium-dependent glucose co-transporter 2 inhibitors (SGLT2is) are a class of anti-hyperglycemic drugs that lower blood glucose in an insulin-independent manner by inhibiting renal glucose reabsorption and promoting glucosuria. In persons with chronic kidney disease, a potential therapeutic target group for such SGLT2i treatment, dietary phosphate restriction is a mainstay of treatment for metabolic bone disease. We investigated the impact of a low phosphate (LP) diet on the physiology of Jimbee mice which, via deletion in exon 10 of the sglt2 gene, provide a model of SGLT2 loss-of-function, albeit with otherwise normal renal function. Male (M) and female (F), 12-week (wk) old, C57BL/6J (genetic control) and Jimbee mice were randomized 1:1 to a kcal/g equivalent 0.1% phosphate (LP) or 0.4% phosphate (normal P = NP) diet and monitored for 12 wks (n=9–12 per group x 8 groups). At study end (~24 wks of age), male Jimbee vs. C57BL/6J mice had lower body mass (BM: p<0.0001), more-so on LP diet (C57BL/6J vs. Jimbee; (M) NP: 31.4 ± 2.1 vs. 28.6 ± 2.0. LP: 30.8 ± 2.0 vs. 26.0 ± 1.6 g). Female mice exhibited no differences in BM. By MRI, male mice demonstrated proportionate decrements in body composition of Jimbees, as the % fat vs. lean mass and % total body water were comparable between genotypes. HbA1c and random blood glucose were no different between groups, while glucosuria was increased in M and F Jimbee mice (p<0.0001) on either diet [C57BL/6J vs. Jimbee; (M) NP: 0.2 ± 0.2 vs. 10.2 ± 4.5. LP: 0.2 ± 0.2 vs. 7.8 ± 2.0 mg/g (body weight)/day. (F) NP: 0.5 ± 0.5 vs. 8.2 ± 2.7. LP: 0.4 ± 0.3 vs. 7.0 ± 2.9 mg/g/day]. Serum calcium and phosphorus were no different between any groups. However, Jimbee mice also exhibited hypercalciuria and hyperphosphaturia (p<0.001 for both). Hypercalciuria was amplified by LP diet in both strains, with a significant diet x strain interaction in males (p=0.01) [C57BL/6J vs. Jimbee; (M) NP: 4.7 ± 2.3 vs. 15.5 ± 8.2. LP: 27.8 ± 31.5 vs. 73.4 ± 25.8 µg/g/day of urine calcium (Ca2+). (F) NP: 4.9 ± 2.8 vs. 22.7 ± 16.9. LP: 45.8 ± 29.5 vs. 62.6 ± 39.8 µg/g/day]. In contrast, hyperphosphaturia was attenuated by LP diet [C57BL/6J vs. Jimbee; (M) NP: 8.7 ± 8.5 vs. 14.7 ± 10.4. LP: 0.9 ± 0.5 vs. 3.2 ± 2.9 µg/g/day of urine phosphate (PO4). (F) NP: 4.4 ± 6.1 vs. 16.3 ± 9.7. LP: 1.2 ± 0.8 vs. 2.9 ± 1.0 µg/g/day]. Plasma PTH levels were significantly lower (p<0.001) in male Jimbee mice on either diet (C57BL/6J vs. Jimbee; NP: 81.1 ± 31.0 vs. 41.3 ± 10.7. LP: 38.2 ± 1.9 vs. 24.1 ± 6.2 pg/mL) and negatively correlated with daily urine Ca2+ (r = -0.62; p=0.006). Consistent with PTH, renal 1-α hydroxylase gene expression was decreased by ~60% in Jimbee males, specifically on LP diet (p=0.02). Together, these data suggest that, in mice, dietary phosphate restriction might exacerbate SGLT2i-related hypercalciuria during prolonged treatment, independent of PTH, becoming potentially detrimental to bone mineralization and growth over time.