Mass spectrometric analysis of TRPM6 and TRPM7 from small intestine of omeprazole-induced hypomagnesemic rats

Abstract

Disruption of small intestinal Mg2+ absorption has been reported as the underlying mechanism of proton pump inhibitor-induced hypomagnesemia (PPIH); hence, this study evaluated the expression, localization, phosphorylation, and oxidation of transient receptor potential melastatin 6 (TRPM6) and TRPM7 in the small intestine of rats subjected to PPIH. The expression and localization of cyclin M4 (CNNM4) was also analyzed. We show that, compared to control rats, membrane expression of the TRPM6/7 heterodimer and TRPM7 was markedly lower in the duodenum and the jejunum of PPIH rats; in contrast, expression of membrane TRPM6 and CNNM4 was higher in these organs. Mass spectrometric analysis of TRPM6 demonstrated hyper-phosphorylation, especially T1851, and hyper-oxidation at M1755, both of which can suppress its channel permeability. Further, hypo-phosphorylation of S141 and the dimerization motif domain of TRPM6 in PPIH rats might be involved in lower TRPM6/7 heterodimer expression. Hypo-phosphorylation, especially at S138 and S1360 in TRPM7 from PPIH rats disrupted stability of TRPM7 at the cell membrane; hyper-oxidation of TRPM7 was also observed. These results help explain the mechanism underlying the disruption of small intestinal Mg2+ absorption in PPIH.

Current opinion on the regulation of small intestinal magnesium absorption

“…Transcellular Mg 2+ absorption occurs through mucosal Mg 2+ uptake by TRPM6 and TRPM7 homodimer channel, both of which were markedly detected in the small intestinal epithelium of human and murine cells[ 10 - 14 ]. In addition, recent mass spectrometric peptide sequence analysis confirmed the expression of TRPM6 and TRPM7 in the duodenum and jejunum[ 15 ]. The channel activities of both homodimers of TRPM6 and of TRPM7 are negatively regulated by physiological Mg·ATP and Mg 2+ levels[ 10 , 16 - 19 ].…”

“…The channel activities of both homodimers of TRPM6 and of TRPM7 are negatively regulated by physiological Mg·ATP and Mg 2+ levels[ 10 , 16 - 19 ]. A recent study reported the expression of a heterodimer TRPM6/7 channel in the plasma membrane of duodenal and jejunal epithelium[ 15 ]; therefore, Mg 2+ enters the small intestinal epithelial cells through TRPM6/7, TRMP6, and TRPM7. However, the heterodimer TRPM6/7 channels do not respond to physiological intracellular Mg 2+ and Mg·ATP[ 17 , 19 ]; thus, continuous epithelial Mg 2+ absorption can occur through the TRPM6/7 channel, regardless of intracellular Mg 2+ and concentrations.…”

“…These findings suggest that PPIs could suppress intestinal Mg 2+ absorption. Our group has extensively studied the underlying mechanisms of PPI-suppressed intestinal Mg 2+ absorption for a decade[ 11 , 12 , 15 , 30 , 35 , 53 , 58 , 59 ]. Our results suggest that PPIs mainly suppressed small intestinal Mg 2+ absorption.…”

“…The proposed mechanism of PPI-suppression of small intestinal Mg 2+ absorption is shown in Figure 6 . PPIs markedly suppress membranous TRPM7 and TRPM6/7[ 15 ]. Membranous TRPM6-channel activity is suppressed by hyperphosphorylation at the T1851 residue and hyperoxidation at the M1755 residue[ 15 ].…”

“…PPIs markedly suppress membranous TRPM7 and TRPM6/7[ 15 ]. Membranous TRPM6-channel activity is suppressed by hyperphosphorylation at the T1851 residue and hyperoxidation at the M1755 residue[ 15 ]. Phosphorylation of the T1851 residue of the TRPM6 protein induces TRPM6-channel suppression by intracellular free Mg 2+ and activated 5 C-kinase 1[ 62 ].…”