SUMMARY Reduced expression of the Indy (= I am Not Dead, Yet) gene in D. melanogaster and C. elegans prolongs life span, and in D. melanogaster augments mitochondrial biogenesis in a manner akin to caloric restriction. However, the cellular mechanism by which Indy does this is unknown. Here, we report on the knockout-mouse model of the mammalian Indy (mIndy) homologue, SLC13A5. Deletion of mIndy in mice (mINDY−/− mice) reduces hepatocellular ATP/ADP ratio, activates hepatic AMPK, induces PGC-1α, inhibits ACC-2, and reduces SREBP-1c levels. This signaling network promotes hepatic mitochondrial biogenesis, lipid oxidation, and energy expenditure and attenuates hepatic de novo lipogenesis. Together, these traits protect mINDY−/− mice from the adiposity and insulin resistance that evolve with high-fat feeding and aging. Our studies demonstrate a profound effect of mIndy on mammalian energy metabolism and suggest that mINDY might be a therapeutic target for the treatment of obesity and type 2 diabetes.

A key function of the proximal tubule is retrieval of most of the vast quantities of NaCl and water filtered by the kidney. Physiological studies using brush border vesicles and perfused tubules have indicated that a major fraction of Cl ؊ reabsorption across the apical membrane of proximal tubule cells occurs via Cl ؊ -formate exchange. The molecular identity of the transporter responsible for renal brush border Cl ؊ -formate exchange has yet to be elucidated. As a strategy to identify one or more anion exchangers responsible for mediating Cl ؊ reabsorption in the proximal tubule, we screened the expressed sequence tag database for homologs of pendrin, a transporter previously shown to mediate Cl ؊ -formate exchange. We now report the cDNA cloning of CFEX, a mouse pendrin homolog with expression in the kidney by Northern analysis. Sequence analysis indicated that CFEX very likely represents the mouse ortholog of human SLC26A6. Immunolocalization studies detected expression of CFEX, but not pendrin, on the brush border membrane of proximal tubule cells. Functional expression studies in Xenopus oocytes demonstrated that CFEX mediates Cl ؊ -formate exchange. Taken together, these observations identify CFEX as a prime candidate to mediate Cl ؊ -formate exchange in the proximal tubule and thereby to contribute importantly to renal NaCl reabsorption. Given its wide tissue distribution, CFEX also may contribute to transcellular Cl ؊ transport in additional epithelia such as the pancreas and contribute to transmembrane Cl ؊ transport in nonepithelial tissues such as the heart.

Oxalate nephropathy with renal failure is caused by multiple disorders causing hyperoxaluria due to either overproduction of oxalate (primary hyperoxaluria) or excessive absorption of dietary oxalate (enteric hyperoxaluria). To study the etiology of renal failure in crystal-induced kidney disease, we created a model of progressive oxalate nephropathy by feeding mice a diet high in soluble oxalate (high oxalate in the absence of dietary calcium). Renal histology was characterized by intratubular calcium-oxalate crystal deposition with an inflammatory response in the surrounding interstitium. Oxalate nephropathy was not found in mice fed a high oxalate diet that also contained calcium. NALP3, also known as cryopyrin, has been implicated in crystal-associated diseases such as gout and silicosis. Mice fed the diet high in soluble oxalate demonstrated increased NALP3 expression in the kidney. Nalp3-null mice were completely protected from the progressive renal failure and death that occurred in wild-type mice fed the diet high in soluble oxalate. NALP3-deficiency did not affect oxalate homeostasis, thereby excluding differences in intestinal oxalate handling to explain the observed phenotype. Thus, progressive renal failure in oxalate nephropathy results primarily from NALP3-mediated inflammation.

Caloric restriction extends life span in a variety of species, highlighting the importance of energy balance in aging. A new longevity gene, Indy (for I'm not dead yet), which doubles the average life span of flies without a loss of fertility or physical activity, was postulated to extend life by affecting intermediary metabolism. We report that functional studies in Xenopus oocytes show INDY is a metabolite transporter that mediates the high-affinity, disulfonic stilbene-sensitive flux of dicarboxylates and citrate across the plasma membrane by a mechanism that is not coupled to Na ؉ , K ؉ , or Cl ؊ . Immunocytochemical studies localize INDY to the plasma membrane with most prominent expression in adult fat body, oenocytes, and the basolateral region of midgut cells and show that life-extending mutations in Indy reduce INDY expression. We conclude that INDY functions as a novel sodium-independent mechanism for transporting Krebs and citric acid cycle intermediates through the epithelium of the gut and across the plasma membranes of organs involved in intermediary metabolism and storage. The life-extending effect of mutations in Indy is likely caused by an alteration in energy balance caused by a decrease in INDY transport function. C aloric restriction is the only known means of extending life span in mammals (1). Although the mechanism by which caloric restriction extends life span is not understood, it is likely to include alterations in energy utilization. Caloric restriction has been shown to increase life span in a variety of other species including fruit flies, supporting the idea that energy balance is a critical element in the aging of many species (1, 2). A new class of longevity gene, Indy (for I'm not dead yet), was identified in the fly and postulated to extend life span through its effect on intermediary metabolism (3). Mutations in the Indy gene result in a near doubling of the average life span of adult flies without a loss of fertility or physical activity (3). The function of the INDY protein was not known, but its closest homology (34% identity) to mammalian sodium-dicarboxylate cotransporters (4-12) and the tissue distribution of its transcriptional activity (fat body, oenocytes, and midgut; ref.3) suggested that mutations in this gene may be reducing transport of important nutrients in tissues critical for intermediary metabolism. Direct knowledge of the function of the INDY protein and its subcellular distribution is essential to our understanding of how life span can be so dramatically increased by a single gene mutation. Materials and MethodsCloning of Indy for Expression Studies. The coding sequence was amplified by PCR using primers designed to incorporate upstream and downstream BamHI and XbaI sites, respectively (5Ј primer: ATA AGA AGGATCCACCATGGA A AT TGA-AATTGGCGAACAACC-oh; 3Ј primer: CGGTCTAGAC-TAGTGCGTCTTGTTTCCCAGTG-oh). The BamHI and XbaI sites then were used to subclone the PCR product into the Xenopus expression plasmid pGH19 between the 5Ј and 3Ј UTRs of the Xenopus ␤-globin g...