Acrp30͞adiponectin is reduced in the serum of obese and diabetic individuals, and the genetic locus of adiponectin is linked to the metabolic syndrome. Recombinant adiponectin, administered to diet-induced obese mice, induced weight loss and improved insulin sensitivity. In muscle and liver, adiponectin stimulates AMP-activated protein kinase activation and fatty acid oxidation. To expression-clone molecules capable of binding adiponectin, we transduced a C2C12 myoblast cDNA retroviral expression library into Ba͞F3 cells and panned infected cells on recombinant adiponectin linked to magnetic beads. We identified T-cadherin as a receptor for the hexameric and high-molecular-weight species of adiponectin but not for the trimeric or globular species. Only eukaryotically expressed adiponectin bound to T-cadherin, implying that posttranslational modifications of adiponectin are critical for binding. An adiponectin mutant lacking a conserved N-terminal cysteine residue required for formation of hexamer and high-molecularweight species did not bind T-cadherin in coimmunoprecipitation studies. Although lacking known cellular functions, T-cadherin is expressed in endothelial and smooth muscle cells, where it is positioned to interact with adiponectin. Because T-cadherin is a glycosylphosphatidylinositol-anchored extracellular protein, it may act as a coreceptor for an as-yet-unidentified signaling receptor through which adiponectin transmits metabolic signals.A dipose tissue is not only a storage depot for lipid but also a regulator of metabolism, through hormones known as adipokines. One adipokine is adiponectin (also denoted Acrp30, for adipocyte complement-related protein of 30 kDa), a molecule secreted exclusively by differentiated adipocytes (1). Adiponectin, which has homology to C1q, is found in the serum as three distinct oligomers, namely trimer, hexamer, and a highmolecular-weight (HMW) species (2). Adiponectin levels are decreased in the serum of obese and diabetic people (3) and animal models of obesity and diabetes. Replenishment by any of several methods induces weight loss and correction of insulin resistance (4-6). The mechanisms by which adiponectin influences metabolism are not fully understood but involve increasing fatty-acid oxidation in muscle through AMP-activated protein kinase (AMPK) activation, as well as synergizing with insulin in the liver to increase glycogen stores and to inhibit gluconeogenesis (6, 7). In tissue culture and isolated muscle, the trimeric isoform and a trimeric globular C-terminal subdomain activate AMPK (7,8), whereas the hexamer and HMW isoforms activate NF-B (9). Adiponectin also has been implicated in the inflammatory process of the metabolic syndrome, and reduced adiponectin levels have been correlated with impaired forearm blood flow, possibly linking endothelial dysfunction with adiponectin levels (10).Analysis of the transmembrane pathways linking adiponectin to downstream signaling events has yielded conflicting results. Two recently described receptors that bind ad...

Insulin regulates glucose uptake and storage in peripheral tissues, and has been shown to act within the hypothalamus to acutely regulate food intake and metabolism. The machinery for transduction of insulin signaling is also present in other brain areas, particularly in the hippocampus, but a physiological role for brain insulin outside the hypothalamus has not been established. Recent studies suggest that insulin may be able to modulate cognitive functions including memory. Here we report that local delivery of insulin to the rat hippocampus enhances spatial memory, in a PI-3-kinase dependent manner, and that intrahippocampal insulin also increases local glycolytic metabolism. Selective blockade of endogenous intrahippocampal insulin signaling impairs memory performance. Further, a rodent model of type 2 diabetes mellitus produced by a high-fat diet impairs basal cognitive function and attenuates both cognitive and metabolic responses to hippocampal insulin administration. Our data demonstrate that insulin is required for optimal hippocampal memory processing. Insulin resistance within the telencephalon may underlie the cognitive deficits commonly reported to accompany type 2 diabetes.

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.