Simultaneous occlusion of Na+ and phosphate by the intestinal brush border membrane Na+/phosphate cotransporter

Development of a Novel Backbone Cyclic Peptide Inhibitor of the Innate Immune TLR/IL1R Signaling Protein MyD88

MyD88 is a cytoplasmic adaptor protein that plays a central role in signaling downstream of the TLRs and the IL1R superfamily. We previously demonstrated that MyD88 plays a critical role in EAE, the murine model of multiple sclerosis, and showed that the MyD88 BB-loop decoy peptide RDVLPGT ameliorates EAE. We now designed and screened a library of backbone cyclized peptides based on the linear BB loop peptide, to identify a metabolically stable inhibitor of MyD88 that retains the binding properties of the linear peptide. We identified a novel cyclic peptide protein mimetic that inhibits inflammatory responses to TLR ligands, and NFκB activation in response to IL-1 activation. The inhibitor, c(MyD 4-4), is metabolically stable in comparison to the linear peptide, blocks MyD88 in a specific manner, and inhibits MyD88 function by preventing MyD88 dimerization. Finally, treatment of mice with c(MyD 4-4) reduced the severity of clinical disease in the murine EAE model of multiple sclerosis. Thus, modulation of MyD88-dependent signaling using c(MyD 4-4) is a potential therapeutic strategy to lower innate immune inflammation in autoimmune CNS disease.

NaPi-mediated transcellular permeation is the dominant route in intestinal inorganic phosphate absorption in rats

Inorganic phosphate in food is absorbed two ways, the transcellular route via the brush border membrane and the paracellular route via tight junctions. NaPi, a sodium-dependent inorganic phosphate transporter, is expressed in rat and human intestine. However, the relative contribution of NaPi to total carrier-mediated transport of physiological concentrations of inorganic phosphate in rat intestine is not clear. Here, we characterized inorganic phosphate transport across the rat small intestine using a voltage-clamp analysis which allowed the diffrentiation of inorganic phosphate permeation through these two (transcellular and paracellular) routes. Results showed that, under a physiologically normal transmucosal electrical potential difference (about 2 mV), permeation of inorganic phosphate by the transcellular route was greater than that by the paracellular route. Further, transport was significantly decreased by the addition to the incubation medium of phosphonoformic acid, a sodium-dependent phosphate transporter inhibitor, and severely inhibited under sodium-free conditions. Similar results were obtained without the voltage-clamp. Together, these results suggest that NaPi-mediated transcellular permeation is the dominant route in the absorption of inorganic phosphate across the small intestine.

A 40-kDa polypeptide from papain digestion of the rabbit intestinal Na+/phosphate cotransporter retains Na+ and phosphate cotransport

The rabbit intestinal brush border membrane Na+/phosphate cotransporter was digested with a variety of proteolytic enzymes. Limited papain digestion generated a 40-kDa polypeptide (P40) which retained putative substrate site markers, fluorescein isothiocyanatophenyl glyoxal and eosin n-acetyl imidazole. P40 retained Na+- and phosphate-selective tryptophan fluorescence quenching, pH sensitivity of ion-induced conformational changes, and tight Na+ and H(2)PO(4)(-) binding. Reconstituted into proteoliposomes, P40 catalyzed Na+-dependent phosphate uptake. The N-terminus of P40 was blocked. An internal sequence of P40 demonstrated that it was derived from NaPi II b. These results suggest that P40 may be a useful model system for studies of the molecular mechanism of Na+-dependent phosphate cotransport and a starting point for structural studies.

Interaction of substrates with the intestinal brush border membrane Na/phosphate cotransporter

The interaction of Na+ and phosphate with the intestinal brush border membrane Na+/phosphate cotransporter was examined using stopped-flow tryptophan fluorescence and ion-exchange Dowex columns coupled to a light-activated microsecond timer (LAM timer) which measures exchange kinetics between protein-bound ions and the external medium Na+ or Na+ + H2PO4- induced tryptophan fluorescence quenching with apparent rate constants of 35 s-1 and 13 s-1, respectively. Dilution of substrate-bound cotransporter resulted in tryptophan fluorescence recovery consistent with cotransporter return to the substrate-free conformation. Recovery of the substrate-free conformation was slow (1.6 s-1) in the absence of phosphate, was accelerated by H2PO4 (7 s-1) and was inhibited by HPO4(2) (1.1 s-1). The effects of substrates on tryptophan fluorescence were sensitive to substrate site blockers consistent with tryptophan fluorescence monitoring cotransporter conformations and substrate-induced changes in conformation. Equivalent experiments using the LAM timer and either (22Na+) or Na+ + (32P) phosphate verified the rate constants for the substrate-induced quenching of tryptophan fluorescence, suggested that 2 Na+ 's were occluded by the cotransporter as part of the Na(+)-induced conformational change and that H2PO4 accelerated deocclusion of Na+. The association of phosphate with the cotransporter was also examined. Although cotransporter-bound phosphate was medium anion-insensitive, a cotransporter conformational change preceding the release of phosphate from the cotransporter was not observed. However, three lines of evidence suggest that release of phosphate from the cotransporter involved a unique cotransporter conformation which may suggest that phosphate was also occluded by the intestinal brush border Na+/phosphate cotransporter.