GLP-1-RA Corrects Mitochondrial Labile Iron Accumulation and Improves β-Cell Function in Type 2 Wolfram Syndrome

CONTEXT

Type 2 Wolfram syndrome (T2-WFS) is a neuronal and β-cell degenerative disorder caused by mutations in the CISD2 gene. The mechanisms underlying β-cell dysfunction in T2-WFS are not known, and treatments that effectively improve diabetes in this context are lacking.

OBJECTIVE

Unraveling the mechanisms of β-cell dysfunction in T2-WFS and the effects of treatment with GLP-1 receptor agonist (GLP-1-RA).

DESIGN AND SETTING

A case report and in vitro mechanistic studies.

PATIENT AND METHODS

We treated an insulin-dependent T2-WFS patient with the GLP-1-RA exenatide for 9 weeks. An iv glucose/glucagon/arginine stimulation test was performed off-drug before and after intervention. We generated a cellular model of T2-WFS by shRNA knockdown of CISD2 (nutrient-deprivation autophagy factor-1 [NAF-1]) in rat insulinoma cells and studied the mechanisms of β-cell dysfunction and the effects of GLP-1-RA.

RESULTS

Treatment with exenatide resulted in a 70% reduction in daily insulin dose with improved glycemic control, as well as an off-drug 7-fold increase in maximal insulin secretion. NAF-1 repression in INS-1 cells decreased insulin content and glucose-stimulated insulin secretion, while maintaining the response to cAMP, and enhanced the accumulation of labile iron and reactive oxygen species in mitochondria. Remarkably, treatment with GLP-1-RA and/or the iron chelator deferiprone reversed these defects.

CONCLUSION

NAF-1 deficiency leads to mitochondrial labile iron accumulation and oxidative stress, which may contribute to β-cell dysfunction in T2-WFS. Treatment with GLP-1-RA and/or iron chelation improves mitochondrial function and restores β-cell function. Treatment with GLP-1-RA, probably aided by iron chelation, should be considered in WFS and other forms of diabetes associated with iron dysregulation.

Fourier transforms infrared difference spectroscopy of secondary quinone acceptor photoreduction in proton transfer mutants of Rhodobacter sphaeroides

In order to investigate the changes of protonation or environment of carboxylic residues occurring upon photoreduction of the secondary quinone acceptor (QB) in the reaction center (RC) of the photosynthetic bacteria Rhodobacter sphaeroides 2.4.1., we have performed light-induced Fourier transform infrared (FTIR) spectroscopy on RCs from wild-type (Wt) and several site-directed mutants. The FTIR QB-/QB spectra have been obtained at pH 7 upon single-saturating flash excitation for native RCs and RC mutants containing either a single-site mutation, with Gln at L212 (EQ L212), Asn at L213 (DN L213), or Asn at L210 (DN L210), or a double-site mutation with both Gln at L212 and Asn at L213 (EQ L212 + DN L213). The assignment of an IR band to the protonation/deprotonation of a particular carboxylic side chain was analyzed by combining the effects of site-directed mutagenesis and 1H/2H isotope exchange. A positive band at 1728 cm-1 in the QB-/QB spectra was observed in Wt, DN L213, and DN L210 and was absent in the mutants EQ L212 and EQ L212 + DN L213. The intensity of the 1728 cm-1 band was significantly reduced in 2H2O, and a new feature appears at 1717 +/- 1 cm-1. Furthermore, the amplitude of the 1728 cm-1 band was similar in native and DN L210 RCs but was increased in DN L213. This band is attributed to partial proton uptake by Glu L212 estimated to be 0.3-0.4 H+/QB- in native and DN L210 RCs and O.5-0.6 H+/QB- in DN L213 RCs. In contrast, the FTIR QB-/QB spectra show no evidence for change of protonation or environment of Asp L213 upon QB- formation. The increased protonation of Glu L212 in DN L213 RCs is explained by a decreased Glu L212 pKa value due to the loss of a negatively charged Asp L213. Part of a small differential signal at 1732 (+)/1740 (-) cm-1 that is affected by 1H/2H exchange is tentatively assigned to an environmental shift of the protonated Asp L210. A negative signal at 1685 cm-1 is propose to arise from the absorption change of the amide I carbonyl mode of Glu L212.(ABSTRACT TRUNCATED AT 400 WORDS)

Protonation of Glu L212 following QB- formation in the photosynthetic reaction center of Rhodobacter sphaeroides: evidence from time-resolved infrared spectroscopy

The protonation events that occur upon QA-QB-->QAQB- electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides were investigated by time-resolved infrared spectroscopy using tunable diode lasers as previously described [Mäntele, W., Hienerwadel, R., Lenz, F., Riedel, E. J., Grisar, R., & Tacke, M. (1990) Spectrosc. Int. 2, 29-35; Hienerwadel, R., Thibodeau, D. L., Lenz, F., Nabedryk, E., Breton, J., Kreutz, W., & Mäntele, W. (1992) Biochemistry 31, 5799-5808]. In the mid-infrared region between 1695 and 1780 cm-1, transient signals associated with QA-QB-->QAQB- electron transfer were observed and characterized. The dominant transient absorbance changes are three positive signals at 1732, 1725, and 1706 cm-1 and two negative signals at 1716 and at 1698 cm-1. The 1725 cm-1-signal disappears upon 1H-->2H exchange as expected for an accessible COOH group and is absent in Glu L212 Gln mutant reaction centers. On this basis, we propose an assignment of this signal to the COOH group of Glu L212. The other signals could correspond to intensity changes and/or shifts of other carboxylic residues, although contributions from ester C = O groups of bacteriopheophytins cannot be ruled out. In native reaction centers at pH 7 and at 4 degrees C, biphasic kinetics of the transient components were observed at most frequencies. The major signal at 1725 cm-1 exhibits a fast kinetic component of t 1/2 = 0.18 ms (25% of the total amplitude) and a slow one of t1/2 = 1 ms (75% of the total amplitude). A global fit analysis of the signals between 1695 and 1780 cm-1 revealed that the spectral distributions of the fast and the slow components are different. Biphasic kinetics with comparable half-times were also observed for the Glu L212 to Gln mutant. The simplest model to explain these results is that the fast phase represents electron transfer and the slow phase represents proton transfer and/or conformational changes coupled to electron transfer. The difference spectra of the slow component from native reaction centers show that the 1725 cm-1 band corresponds to an absorbance increase and not to a shift of an existing band. The signal is therefore proposed to arise from the protonation of Glu L212. The amplitude of the 1725 cm-1 signal varies distinctly with pH as expected for protonation of a COO- group. With increasing pH, the amplitude of the slow component increases while that of the fast component decreases slightly.(ABSTRACT TRUNCATED AT 400 WORDS)