Insulin secretion by pancreatic β-cells is primarily regulated by glucose; however, hormones and additional nutrients, such as long-chain fatty acids, also play an important role in adjusting insulin output to physiologic needs. To examine the role of short-chain fatty acids (SCFAs) in β-cell function, we analyzed mouse models of gain and loss of function of GPR41 (FFAR3), a receptor for SCFAs, vs. wild-type control mice. GPR41 gain of function [GPR41-overexpressing transgenic (41 Tg) model] and GPR41 loss of function [GPR41-knockout (KO 41) model] resulted in complementary changes in glucose tolerance, without significant effects on insulin sensitivity. KO 41 mice showed fasting hypoglycemia, which was consistent with increased basal and glucose-induced insulin secretion by islets in vitro Mirroring this, 41 Tg islets showed impaired glucose responsiveness in vitro Microarray analysis of islets from 41 Tg mice indicated significant alterations in gene expression patterns; several of the altered genes were chosen for further analysis and were also observed to change upon incubation of islets‏‏ and cultured β-cells with SCFAs in a GPR41-dependent manner. Taken together, our results indicate that GPR41 and its ligands, SCFAs, may play an important role in the fine-tuning of insulin secretion in fed and fasting states.-Veprik, A., Laufer, D., Weiss, S., Rubins, N., Walker, M. D. GPR41 modulates insulin secretion and gene expression in pancreatic β-cells and modifies metabolic homeostasis in fed and fasting states.

The divalent cation specificity of ATP-linked reactions catalyzed by the H+-translocating F0F! ATP synthase-ATPase complex has been followed in the Rhodospirillum rubrum chromatophore bound complex. In the presence of Mg2+ and Mn2+ the complex catalyzes ATP synthesis and hydrolysis as well as ATP-driven H+ translocation, but in the presence of Ca2+ it catalyzes only ATP hydrolysis, which is not coupled to H+ tanslocation. The inability of Ca2+ to maintain the coupling process is not due to opening of a proton leak in its presence nor to any release of p! from the membrane, because (a) an identical light-induced H+ translocation is observed in the absence or presence of Ca-ATP and (b) the Ca-ATPase, as well as the Mg-and Mn-ATPase activities, is blocked by specific F0 inhibitors. These results indicate that the divalent cations play an important role in the regulation of H+-coupled ATP synthesis and hydrolysis by the F0Fj complex. Further tests suggest that their site of action is located on the Fr/3 subunit. The isolated ß subunit of the R. rubrum FqF, has been reported to contain two nucleotide binding sites, a Mg-independent and a Mg-dependent site [Gromet-Elhanan, Z., & Khananshvili, D. (1984) Biochemistry 23, 1022-1028]. Addition of Mn2+ also enables the binding of 2 mol of ATP/mol of this isolated ß subunit. But under identical conditions, Ca2+ does not enable ATP binding to this cation-dependent site and inhibits its binding in the presence of Mg2+ or Mn2+. In light of these results we propose that the binding of Mg-ATP or -ATP, but not of Ca-ATP, to the R. rubrum ¥3-ß subunit forms the trigger that opens the pathway for H+ translocation through the FqF] complex during catalysis.Electron-transport coupled ATP synthesis in respiratory and photosynthetic organisms is catalyzed by a reversible prot Supported in part by grants from the Minerva Foundation, Munich, West Germany, and from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel. ton-translocating ATP synthase-ATPase complex that is composed of two distinct structures (Senior & Wise, 1983; Merchant & Selman, 1985): a readily solubilized catalytic ATPase sector, Fb and an intrinsic membrane sector that is involved in proton fluxes, F0. The F,-ATPase from many different sources has a 3ß3• ( subunit structure and contains 0006-2960/89/0428-3645S01.50/0 © 1989 American Chemical Society

Solubilized Rhodospirillum rubrum RrF1-ATPase, depleted of loosely bound nucleotides, retains 2.6 mol of tightly bound ATP and ADP/mol of enzyme. Incubation of the depleted RrF1 with Mg(2+)-ATP or Mg(2+)-AMP-PNP, followed by passage through two successive Sephadex centrifuge columns, results in retention of a maximal number of 4 mol of tightly bound nucleotides/mol of RrF1. They include 1.5 mol of nonexchangeable ATP, whereas all tightly bound ADP is fully exchangeable. A similar retention of only four out of the six nucleotide binding sites present on CF1 has been observed after its passage through one or two centrifuge columns. These results indicate that the photosynthetic, unlike the respiratory, F1-ATPases have faster koff constants for two of the Mg-dependent nucleotide binding sites. This could be the reason for the tenfold lower Mg2+ than Ca(2+)-ATPase activity observed with native RrF1, as with epsilon-depleted, activated CF1. An almost complete conversion of both RrF1 and CF1 from Ca(2+)- to Mg(2+)-dependent ATPases is obtained upon addition of octylglucoside, at concentrations below its CMC, to the ATPase assay medium. Thus, octylglucoside seems to affect directly the RrF1 and CF1 divalent cation binding site(s), in addition to its proposed role in relieving their inhibition by free Mg2+ ions. The RrF1-ATPase activity is 30-fold more sensitive than CF1 to efrapeptin, and completely resistant to either inhibition or stimulation by the CF1 effector, tentoxin. Octylglucoside decreases the inhibition by efrapeptin and tentoxin, but exposes on CF1 a low-affinity, stimulatory site for tentoxin.

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