Dietary zinc deficiency increases uroguanylin accumulation in rat kidney

Receptor Guanylyl Cyclase C and Cyclic GMP in Health and Disease: Perspectives and Therapeutic Opportunities

Receptor Guanylyl Cyclase C (GC-C) was initially characterized as an important regulator of intestinal fluid and ion homeostasis. Recent findings demonstrate that GC-C is also causally linked to intestinal inflammation, dysbiosis, and tumorigenesis. These advances have been fueled in part by identifying mutations or changes in gene expression in GC-C or its ligands, that disrupt the delicate balance of intracellular cGMP levels and are associated with a wide range of clinical phenotypes. In this review, we highlight aspects of the current knowledge of the GC-C signaling pathway in homeostasis and disease, emphasizing recent advances in the field. The review summarizes extra gastrointestinal functions for GC-C signaling, such as appetite control, energy expenditure, visceral nociception, and behavioral processes. Recent research has expanded the homeostatic role of GC-C and implicated it in regulating the ion-microbiome-immune axis, which acts as a mechanistic driver in inflammatory bowel disease. The development of transgenic and knockout mouse models allowed for in-depth studies of GC-C and its relationship to whole-animal physiology. A deeper understanding of the various aspects of GC-C biology and their relationships with pathologies such as inflammatory bowel disease, colorectal cancer, and obesity can be leveraged to devise novel therapeutics.

Brain-Delivery of Zinc-Ions as Potential Treatment for Neurological Diseases: Mini Review

Homeostasis of metal ions such as Zn(2+) is essential for proper brain function. Moreover, the list of psychiatric and neurodegenerative disorders involving a dysregulation of brain Zn(2+)-levels is long and steadily growing, including Parkinson's and Alzheimer's disease as well as schizophrenia, attention deficit and hyperactivity disorder, depression, amyotrophic lateral sclerosis, Down's syndrome, multiple sclerosis, Wilson's disease and Pick's disease. Furthermore, alterations in Zn(2+)-levels are seen in transient forebrain ischemia, seizures, traumatic brain injury and alcoholism. Thus, the possibility of altering Zn(2+)-levels within the brain is emerging as a new target for the prevention and treatment of psychiatric and neurological diseases. Although the role of Zn(2+) in the brain has been extensively studied over the past decades, methods for controlled regulation and manipulation of Zn(2+) concentrations within the brain are still in their infancy. Since the use of dietary Zn(2+) supplementation and restriction has major limitations, new methods and alternative approaches are currently under investigation, such as the use of intracranial infusion of Zn(2+) chelators or nanoparticle technologies to elevate or decrease intracellular Zn(2+) levels. Therefore, this review briefly summarizes the role of Zn(2+) in psychiatric and neurodegenerative diseases and highlights key findings and impediments of brain Zn(2+)-level manipulation. Furthermore, some methods and compounds, such as metal ion chelation, redistribution and supplementation that are used to control brain Zn(2+)-levels in order to treat brain disorders are evaluated.

The rat kidney contains high levels of prouroguanylin (the uroguanylin precursor) but does not express GC-C (the enteric uroguanylin receptor)

The peptide uroguanylin (Ugn) regulates enteric and renal electrolyte transport. Previous studies have shown that Ugn and its receptor GC-C (a ligand-activated guanylate cyclase) are abundant in the intestine. Less is known about Ugn and GC-C expression in the kidney. Here, we identify a 9.4-kDa polypeptide in rat kidney extracts that appears, based on its biochemical and immunological properties, to be authentic prouroguanylin (proUgn). This propeptide is relatively plentiful in the kidney (~16% of intestinal levels), whereas its mRNA is marginally present (<1% of intestinal levels), and free Ugn peptide levels are below detection limits (<0.4% of renal proUgn levels). The paucity of preproUgn-encoding mRNA and free Ugn peptide raises the possibility that the kidney might absorb intact proUgn from plasma, where the concentration of propeptide greatly exceeds that of Ugn. However, immunocytochemical analysis reveals that renal proUgn is found exclusively in distal tubular segments, sites previously shown not to accumulate radiolabeled proUgn after intravascular infusions. Thus proUgn appears to be synthesized within the kidney, but the factors that determine its abundance (rates of transcription, translation, processing, and secretion) must be balanced quite differently than in the gut. Surprisingly, we also find negligible expression of GC-C in the rat kidney, a result confirmed both by RT-PCR and by functional assays that measure Ugn-activated cGMP synthesis. Taken together, these data provide evidence for an intrarenal Ugn system that differs from the well-described intestinal system in its regulatory mechanisms and in the receptor targeted by the peptide.

Transcriptome and proteome analysis identifies the pathways that increase hepatic lipid accumulation in zinc-deficient rats

For identification of the underlying molecular changes in hepatic lipid metabolism in zinc deficiency, rats were force-fed a zinc-deficient diet. Subsequently DNA-microarray and proteome profiling was performed in combination with hepatic lipid analysis. Of 6200 target sequences analyzed, 268 transcripts showed altered expression levels in livers of zinc-deficient rats, with 43 genes thereof related to hepatic lipid metabolism. Northern blot analysis and quantitative real-time RT-PCR were employed to confirm changes in mRNA levels. Proteins involved in lipid metabolism were identified by proteome analysis. Functional gene clusters with uniform changes in transcript levels suggested that the pathways required for lipolysis and mitochondrial as well as peroxisomal fatty acid degradation were downregulated, whereas those needed for de novo fatty acid synthesis and triglyceride assembly were increased. Subsequent enzymatic analysis of liver tissues confirmed an almost 40% greater triacylglycerol concentration in zinc-depleted rats, as well as an altered fatty acid composition of the lipid fraction as determined by gas chromatography. Liver lipids of zinc-deficient rats had significantly greater proportions of cis-9-oleic acid, cis-11-vaccenic acid, caprylic acid, myristic acid, alpha-linolenic acid, and eicosapentaenoic acid, and significantly less stearic and arachidonic acids. These alterations in hepatic metabolism are discussed in the context of changes in mRNA and protein levels of enzymes and transporters responsible for fatty acid metabolism, sequestration, and their transcriptional control.

Regulation of zinc metabolism and genomic outcomes

Differential mRNA display and cDNA array analysis have identified zinc-regulated genes in small intestine, thymus and monocytes. The vast majority of the transcriptome is not influenced by dietary zinc intake, high or low. Of the genes that are zinc regulated, most are involved in signal transduction (particularly influencing the immune response), responses to stress and redox, growth and energy utilization. Among the genes identified are uroguanylin (UG), cholecystokinin, lymphocyte-specific protein tyrosine kinase (LCK), T-cell cytokine receptor, heat shock proteins and the DNA damage repair and recombination protein-23B. Zinc transporters (ZnT) help regulate the supply of this micronutrient to maintain cellular functions. Expression of ZnT-1 and -2 is regulated by dietary zinc in many organs including small intestine and kidney. ZnT-4 is ubiquitously expressed but is refractory to zinc intake. Expression of ZnT-1, -2 and -4 changes markedly during gestation and lactation from highly abundant to undetectable. Each ZnT has an endosomal-like appearance in the tissues examined. Upregulation of ZnT-1 and ZnT-2 by dietary zinc strongly implicates these transporters in zinc acquisition and/or storage for subsequent systemic needs. THP-1 cells were used as a model to examine the response of human cells to changes in zinc status. Based on mRNA quantities, Zip1 and ZnT-5 were the most highly expressed. Zinc depletion of these cells decreased expression of all transporters except Zip2, where expression increased markedly. Collectively, these findings provide a genomic footprint upon which to address the biological and clinical significance of zinc and new avenues for status assessment.

Dietary zinc modulates gene expression in murine thymus: results from a comprehensive differential display screening

Differential mRNA display was used to comprehensively screen the murine thymic transcriptome for genes modulated in vivo by dietary zinc. A moderate feeding protocol rendered young adult, outbred mice zinc-deficient and zinc-supplemented without alterations in feeding behavior or growth. However, these levels of deficiency and supplementation altered specific mRNA abundances in a manner detectable by differential display. In total, 240 primer-pair combinations were used to generate >48,000 interpretable cDNA bands derived from thymic total RNA, of which only 265 or 0.55% were identified as zinc-modulated under these moderate dietary conditions. The most strongly zinc-modulated cDNAs identified by display were reamplified and sequenced. No cDNAs encoding zinc-metalloenzymes or zinc-finger transcription factors were identified as zinc-modulated in this global screening. Those zinc-regulated genes independently confirmed by quantitative PCR included: heat shock proteins 40 and 60; heat shock cognate 70; histocompatibility 2, class II antigen A, alpha; and the T cell cytokine receptor. In addition, a variety of transcription- and translation-related factors (such as ribosomal proteins L3, L5, and L28; nuclear matrix protein 84; matrin cyclophilin; the H3 histone family 3A protein; beta(2) microglobulin; and a cleavage and polyadenylation factor) were identified as zinc-modulated. These profiling data show that differential expression of genes in the thymus in response to the dietary zinc supply precedes many of the phenotypic effects on thymic function associated with severe zinc restriction or supplementation. Several genes involved in T cell development were identified as regulated by zinc and will be targets to evaluate the effects of zinc on immune function.

Changes in rat hepatic gene expression in response to zinc deficiency as assessed by DNA arrays

Zinc deficiency affects hepatic functions and due to the central role of the liver in metabolism, this may contribute to metabolic alterations in other tissues in zinc deficiency. In addition to clinical manifestations of zinc deficiency, we used cDNA- and oligonucleotide-arrays to compare the expression of > 2500 different genes in liver of rats force-fed a zinc-adequate or a zinc-deficient diet for 11 d. Radio- or fluorescence-labeled cDNAs from liver of control and zinc-deficient rats were hybridized to arrays. Approximately 1550 mRNAs were detected above background levels; by comparing expression profiles of the two groups, the mRNA levels of 66 genes were found to be altered by zinc deficiency. Steady-state expression levels of 35 genes were reduced, whereas the mRNA-levels of 31 genes were elevated. Array data were verified by Northern blot analysis for 24 selected genes and 19 were confirmed to be up- or down-regulated. Among those, predominantly gene products that participate in growth (i.e., insulin-like growth factor binding proteins), lipid metabolism (long-chain acyl-CoA synthetase), xenobiotic metabolism (cytochrome P(450) isoenzymes), the stress response (glutathione transferase), nitrogen metabolism (cytosolic aspartate aminotransferase), intracellular trafficking (syntaxin isoforms) and signal transduction (G-protein-coupled receptors) were identified. Additionally, regulation of mRNA levels of genes important for porphyrin synthesis and collagen metabolism was observed. In conclusion, we have identified in vivo a number of mammalian genes from different cellular pathways whose expression changes in response to zinc depletion. The characterization of the identified genes and their products will allow a more comprehensive analysis of the role of zinc in metabolism; moreover, the mRNAs identified could be useful in establishing biomarkers for the determination of zinc status in mammals.

Modulation of intestinal gene expression by dietary zinc status: effectiveness of cDNA arrays for expression profiling of a single nutrient deficiency

Mammalian nutritional status affects the homeostatic balance of multiple physiological processes and their associated gene expression. Although DNA array analysis can monitor large numbers of genes, there are no reports of expression profiling of a micronutrient deficiency in an intact animal system. In this report, we have tested the feasibility of using cDNA arrays to compare the global changes in expression of genes of known function that occur in the early stages of rodent zinc deficiency. The gene-modulating effects of this deficiency were demonstrated by real-time quantitative PCR measurements of altered mRNA levels for metallothionein 1, zinc transporter 2, and uroguanylin, all of which have been previously documented as zinc-regulated genes. As a result of the low level of inherent noise within this model system and application of a recently reported statistical tool for statistical analysis of microarrays [Tusher, V.G., Tibshirani, R. & Chu, G. (2001) Proc. Natl. Acad. Sci. USA 98, 5116-5121], we demonstrate the ability to reproducibly identify the modest changes in mRNA abundance produced by this single micronutrient deficiency. Among the genes identified by this array profile are intestinal genes that influence signaling pathways, growth, transcription, redox, and energy utilization. Additionally, the influence of dietary zinc supply on the expression of some of these genes was confirmed by real-time quantitative PCR. Overall, these data support the effectiveness of cDNA array expression profiling to investigate the pleiotropic effects of specific nutrients and may provide an approach to establishing markers for assessment of nutritional status.