Corneal avascularity—the absence of blood vessels in the cornea—is required for optical clarity and optimal vision, and has led to the cornea being widely used for validating pro- and anti-angiogenic therapeutic strategies for many disorders1-4. But the molecular underpinnings of the avascular phenotype have until now remained obscure5-10 and are all the more remarkable given the presence in the cornea of vascular endothelial growth factor (VEGF)-A, a potent stimulator of angiogenesis, and the proximity of the cornea to vascularized tissues. Here we show that the cornea expresses soluble VEGF receptor-1 (sVEGFR-1; also known as sflt-1) and that suppression of this endogenous VEGF-A trap11 by neutralizing antibodies, RNA interference or Cre-lox-mediated gene disruption abolishes corneal avascularity in mice. The spontaneously vascularized corneas of corn1 and Pax6+/− mice12,13 and Pax6+/− patients with aniridia14 are deficient in sflt-1, and recombinant sflt-1 administration restores corneal avascularity in corn1 and Pax6+/− mice. Manatees, the only known creatures uniformly to have vascularized corneas15, do not express sflt-1, whereas the avascular corneas of dugongs, also members of the order Sirenia, elephants, the closest extant terrestrial phylogenetic relatives of manatees, and other marine mammals (dolphins and whales) contain sflt-1, indicating that it has a crucial, evolutionarily conserved role. The recognition that sflt-1 is essential for preserving the avascular ambit of the cornea can rationally guide its use as a platform for angiogenic modulators, supports its use in treating neovascular diseases, and might provide insight into the immunological privilege of the cornea.

The tapetum lucidum represents a remarkable example of neural cell and tissue specialization as an adaptation to a dim light environment and, despite these differences, all tapetal variants act to increase retinal sensitivity by reflecting light back through the photoreceptor layer. These variations regarding both its location and structure, as well as the choice of reflective material, may represent selective visual adaptations associated with their feeding behavior, in response to the use of specific wavelengths and amount of reflectance required.

O ur understanding of how Zn is metabolized in animals and humans is advancing rapidly by examination of membrane proteins that facilitate Zn transport. Zn transporters fall into two gene families, SLC30A and SLC39A (1). These families are commonly referred to as ZnT and Zip transporters, respectively.Zn metabolism has been defined partially in studies with humans and animals by using radioactive and stable isotopes (reviewed in ref. 2). Kinetic analyses and metabolic modeling have established the major pathways by which this micronutrient is processed and how organ systems produce an effective homeostatic control over absorption and excretion. The current Dietary Reference Intake (Institute of Medicine of the National Academies,Washington, DC) recommendations for Zn intake by humans are based on a balance between intestinal absorption and endogenous losses by using these metabolic assessments (3).Dietary Zn restriction leads to an up-regulation of the mediated component of the Zn-absorptive pathway in rodents, presumably by means of changes programmed in intestinal cells (4). Loss of endogenous Zn from intestinal and pancreatic secretions are concomitantly reduced during Zn restriction (reviewed in refs. 5 and 6). Urinary losses of Zn are low, indicative of high reabsorptive capacity, and they are refractory to change over a wide Zn-intake range (6). Furthermore, metabolic studies have identified organ systems that influence Zn metabolism and its response to physiologic stimuli, including hormones, cytokines, and growth factors (reviewed in ref. 5). Induction of metallothionein (MT) gene expression, which is usually concurrent with enhanced cellular Zn acquisition, has been integrated into all of these aspects of cellular Zn trafficking.Zn transporters are essential components of systems that influence Zn trafficking in times of dietary depletion or excess, such as during acute and chronic physiologic stress (e.g., infection and inflammation) and during pregnancy and lactation. Therefore, experiments with mice were designed to show the differential expression of ZnT and Zip transporter genes associated with dietary Zn restriction and excess. The data presented here identify the transporters critical for regulation of Zn homeostasis in intestinal absorptive cells, as well as pancreatic acinar cells. Materials and MethodsMice and Treatments. Young adult male CD-1 strain mice (Charles River Breeding Laboratories) were housed and fed as described in detail in refs. 7 and 8. Mice were fed one of three dietary Zn levels [Zn Ϫ (Ͻ1 mg͞kg), Zn normal (ZnN; 30 mg͞kg), or Zn ϩ (150 mg͞kg)], which represent depleted, adequate, or excessive intakes of Zn, respectively. Mice were fed individually for up to 21 days. Blood was collected by cardiac puncture under halothane anesthesia for measurement of the serum Zn concentration (7), and preparation of peripheral blood mononuclear cells (PBMCs) was performed by using NycoPrep 1.077 (Life Technologies, Grand Island, NY) gradient centrifugation. MT knockout mice and appropriate 1...

Intestinal epithelial tight junction (TJ) barrier dysfunction may lead to inflammation and mucosal injury. Glutamine (GLN) plays a role in maintenance of intestinal barrier function in various animal models and critically ill humans. Recent evidence from intestinal cell monolayers indicates that GLN maintains transepithelial resistance and decreases permeability. The mechanisms of these effects remain undefined. We hypothesized that GLN affects proteins involved in the intercellular junctional complex. GLN availability was controlled in Caco-2 monolayers by addition to the medium and treatment with methionine sulfoximine (MSO) to inhibit glutamine synthetase (GS). Expression of TJ proteins, claudin-1, occludin, and zonula occluden (ZO)-1 was measured by immunoblotting. Localization of TJ proteins was evaluated by immunofluorescence light microscopy. Structure of TJ was determined by transmission electron microscopy (TEM). Deprivation of GLN decreased claudin-1, occludin, and ZO-1 protein expression and caused a disappearance of perijunctional claudin-1 and a reduction of occludin but had no effect on ZO-1. TEM revealed that MSO-treated cells in the absence of GLN formed irregular junctional complexes between the apical lateral margins of adjoining cells. These findings indicate that TJ protein expression and cellular localization in Caco-2 cell monolayers rely on GLN. This mechanism may similarly relate to GLN-mediated modulation of intestinal barrier function in stressed animals and humans.