No abstract

The function of the Rhesus (Rh) complex in the human red cell membrane has been unknown for six decades. Based on the organismal, organ, and tissue distribution of Rh proteins, and on our evidence that their only known paralogues, the ammonium and methylammonium transport proteins (also called methylammonium permeases), are gas channels for NH 3, we recently speculated that Rh proteins are biological gas channels for CO 2. Like NH 3, CO2 differs from other gases in being readily hydrated. We have now tested our speculation by studying expression of the RH1 gene in the photosynthetic microbe Chlamydomonas reinhardtii. Expression of RH1 was high for cells grown in air supplemented with 3% CO 2 or shifted from air to high CO2 (3%) for 3 h. Conversely, RH1 expression was low for cells grown in air (0.035% CO 2) or shifted from high CO2 to air for 3 h. These results make viable the hypothesis that Rh1 and Rh proteins generally are gas channels for CO 2.T he Rhesus (Rh) blood group substance is the second most abundant protein in human red cell membranes (Ϸ10 5 copies per cell) (1). The related RhAG and Rh30 proteins, which constitute this complex (2, 3), have only one known paralogue, the ammonium and methylammonium transport (Amt) proteins [also called methylammonium permeases (MEP)] (4). Marini and colleagues (5) reported that both Amt͞MEP proteins and the human RhAG and RhCG proteins are active transporters for NH 4 ϩ . Their conclusion regarding Rh proteins was based on the properties of Saccharomyces cerevisiae strains lacking function of its three MEP proteins and carrying cloned human Rh genes. Contrary to the views of Marini et al., we have provided several lines of evidence that Amt and MEP proteins are gas channels for NH 3 and have speculated that Rh proteins are gas channels for CO 2 (6-9). To test the viability of our speculation regarding Rh, we have studied expression of the RH1 gene in the green alga Chlamydomonas reinhardtii, one of the few microbes to have RH genes. Materials and MethodsMedia and Growth Conditions. C. reinhardtii strains CC125 (137c; nit1 nit2 mtϩ) (10), 4Aϩ (nit1 nit2 mtϩ), and CC124 (nit1 nit2 mtϪ) were maintained at 24°C in TAP medium (11), under continuous illumination (40 mol photons m Ϫ2 s Ϫ1 ). Strain 4Aϩ was kindly provided by J.-D. Rochaix (Univ. of Geneva, Switzerland). For growth in high CO 2 , cells were cultured in 1 l bottles containing 700 ml of TP(-N) medium (TAP medium without acetate and nitrogen; ref. 11) under constant illumination (170 mol photons m Ϫ2 s Ϫ1 ) and were bubbled with air enriched with 3% (vol/vol) CO 2 . The nitrogen source was NH 4 Cl (10 mM), arginine (2.5 mM), or hypoxanthine (2.5 mM), as indicated. For growth in low CO 2 , cultures were bubbled with ordinary air [0.035% (vol/vol) CO 2 ]. Chlorophyll aϩb content was estimated after extracting cells with 96% (vol/vol) ethanol (12).

The clinical development of fedratinib, a Janus kinase (JAK2) inhibitor, was terminated after reports of Wernicke's encephalopathy in myelofibrosis patients. Since Wernicke's encephalopathy is induced by thiamine deficiency, investigations were conducted to probe possible mechanisms through which fedratinib may lead to a thiamine-deficient state. In vitro studies indicate that fedratinib potently inhibits the carrier-mediated uptake and transcellular flux of thiamine in Caco-2 cells, suggesting that oral absorption of dietary thiamine is significantly compromised by fedratinib dosing. Transport studies with recombinant human thiamine transporters identified the individual human thiamine transporter (hTHTR2) that is inhibited by fedratinib. Inhibition of thiamine uptake appears unique to fedratinib and is not shared by marketed JAK inhibitors, and this observation is consistent with the known structure-activity relationship for the binding of thiamine to its transporters. The results from these studies provide a molecular basis for the development of Wernicke's encephalopathy upon fedratinib treatment and highlight the need to evaluate interactions of investigational drugs with nutrient transporters in addition to classic xenobiotic transporters.

Structure-based design led to the discovery of novel (S)-isothiazolidinone ((S)-IZD) heterocyclic phosphotyrosine (pTyr) mimetics that when incorporated into dipeptides are exceptionally potent, competitive, and reversible inhibitors of protein tyrosine phosphatase 1B (PTP1B). The crystal structure of PTP1B in complex with our most potent inhibitor 12 revealed that the (S)-IZD heterocycle interacts extensively with the phosphate binding loop precisely as designed in silico. Our data provide strong evidence that the (S)-IZD is the most potent pTyr mimetic reported to date.