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Metal homeostasis in plants is regulated by diverse mechanisms that act together to maintain optimal metal ion concentrations inside the cell. P1B-ATPases are heavy metal transport ATPases that are likely to be related to these processes. The sequencing of the genome of Arabidopsis thaliana revealed the presence of eight putative P1B-ATPases, HMA1-8. The main goal in this work is to characterize of the role of P1B-ATPases in plant metal homeostasis. Toward this goal, the P1B-ATPases HMA1 and HMA6 from Arabidopsis thaliana were cloned from leaves and sequenced. Results from RT-PCR experiments show ubiquitous expression in planta of this two ATPases, except for HMA1 that does not express in roots. Upon Cu2+ exposure during growth, expression of HMA6 increases in seedlings. HMA1 expression increases when seedlings are grown in high Cu2+ and Co2+ media, and decreases when grown in high concentrations of Zn2+ and Ni2+. hma1-1 plants have smaller size and less chlorophyll content than WT plants. Growth is affected in hma1-1 seedlings when grown in Zn2+, Mn2+, Fe2+, Co2+ and Cu2+ deficient media, or when these metals are in excess. Moreover, hma1-1 plants show an increase in Zn2+, Mn2+ and Fe2+ content in whole plants compared to WT plants. Mutant plants also show increased levels of HMA3 and HMA4 transcripts (Zn2+/Cd2+/Pb2+ P1B-ATPases), upregulation of metallothioneins 1a and 2b, downregulation of metallothionein 1c, and a decrease in the phytochellatin synthases 1 and 2 transcripts, compared to WT plants. Homozygous for mutation in HMA6 seems to be lethal, given that none was recovered after screening. These results indicate HMA1 and HMA6 as essential components of plant metal homeostasis in Arabidopsis thaliana.

Thermogenic brown and beige adipocytes convert chemical energy to heat by metabolizing glucose and lipids via uncoupling protein 1 (Ucp1), a process known as non-shivering thermogenesis. Serotonin (5-HT) neurons in the ventral medulla are known to regulate sympathetic efferent neurons in the intermediolateral nucleus (IML) necessary to maintain brown adipose tissue (BAT) activity. Previous studies show that mice lacking central 5-HT neurons are incapable of maintaining body temperature in cold, ambient conditions. Due to this direct linkage between 5-HT and thermoregulation, we hypothesized that central 5-HT neurons may be vital to the regulation of brown and beige adipocyte activity. Given that BAT consumes large amounts of substrate when active, we also hypothesized that inactivation of BAT due to deletion of the regulatory neural circuitry (5-HT neurons) would cause metabolic dysregulation. To test this, we generated mice in which the human diphtheria toxin (DT) receptor was selectively expressed in central 5-HT neurons under control of a Pet-1 promoter. Pet-1 is a transcription factor selectively located in mature, central 5-HT neurons. Coincidentally, some cells within pancreatic islets also express Pet-1, and contain adequate machinery to produce, release, and uptake 5-HT. Systemic treatment with DT eliminated 5-HT neurons and caused loss of thermoregulation, BAT steatosis, and a >50% decrease in Ucp1 expression in BAT and beige fat, indicative of reduced thermal production. In parallel, blood glucose increased 3.5-fold, free fatty acids 13.4-fold and triglycerides 6.5-fold. Intracerebroventricular (ICV) treatment with 1/30th the systemic dose of DT induced an even greater thermoregulatory impairment. The metabolic deficits following systemic DT treatment indicate that central 5-HT neurons are essential for proper metabolic regulation. However, such high levels of glucose and lipids also indicate failure of the pancreatic endocrine program following systemic treatment, likely due to moderate destruction of β-cells expressing Pet-1 and the DT receptor. Because ICV treatment caused even greater thermoregulatory and metabolic deficits, where little, if any, of the toxin would spread systemically, central 5-HT neurons are clearly essential for normal central regulation of metabolism. Interestingly, similar BAT and beige fat defects occurred in Lmx1bf/f/p mice, in which 5-HT neurons fail to develop in utero. Assessment of systemically treated animals using a euglycemic/hyperinsulinemic clamp showed extensive fasting hyperglycemia and systemic insulin resistance, coinciding with reduced glucose uptake in skeletal muscle and BAT. The hyperinsulinemic clamp failed to suppress hepatic glucose and fatty acid production, leading to the conclusion that loss of central 5-HT neurons disrupts central hepatic regulation. In attempts to induce BAT thermogenesis and metabolism, we optogenetically stimulated 5-HT neurons in the rostral raphe pallidus and measured BAT and body temperature along with blood glucose. Unfortunately, these stimulations were incapable of increasing BAT temperature and lowering blood glucose, perhaps limiting therapeutic potential of these 5-HT neurons. We conclude that 5-HT neurons are major players in central regulation of metabolic homeostasis, in part through recruitment and activation of brown and beige adipocytes and hepatic substrate production. Data also suggest that 5-HT neurons regulate glucose homeostasis via undefined neural mechanisms independently of BAT activity and pancreatic insulin secretion. Cumulative data on central 5-HT neurons indicate they are master regulators of whole-body metabolism.

The biosynthesis of [NiFe]-hydrogenase and urease in Helicobacter pylori requires several accessory proteins for proper assembly of the nickel-containing active sites. Critical to the maturation of both enzymes in H. pylori is the GTPase HypB. In this work, the metal-binding properties of H. pylori HypB (HpHypB) were investigated and a link between metal binding and the other biochemical properties of HpHypB was established. HpHypB binds stoichiometric nickel or zinc with nanomolar affinities, in partially overlapping sites located between two major GTPase motifs. Upon metal binding, the GTP hydrolysis activity and oligomeric properties of the protein are modulated. Furthermore, the stoichiometry and affinity of the nickel is altered when HpHypB is bound to nucleotide, a change not observed for zinc. Mutagenesis of the metal ligands suggest that a conserved cysteine is responsible for transducing the metal-bound state to altered GTPase activity and a conserved histidine is a required nickel ligand only in the nucleotide-bound state. Together, these results suggest that the metal-binding and GTP hydrolysis properties of HpHypB are intimately linked and may comprise a mechanism through which the [NiFe]-hydrogenase and urease maturation pathways can discriminate between Ni(II) and Zn(II). Characterization of the Escherichia coli GTPase YjiA, a member of the same GTPase family as HpHypB, demonstrated that YjiA can bind Ni(II), Zn(II), or Co(II) at a site in a similar location as in HpHypB. Metal binding also regulates the GTPase activity and oligomerization of YjiA. This finding suggests that metal-responsive GTPase activity may be a trait of this family of GTPases. Together, this work describes a unique link between the metal-binding and biochemical properties of the G3E GTPases and provides insight into the role of HpHypB in [NiFe]-hydrogenase and urease maturation.

The biosynthesis of [NiFe]-hydrogenase and urease in Helicobacter pylori requires several accessory proteins for proper assembly of the nickel-containing active sites. Critical to the maturation of both enzymes in H. pylori is the GTPase HypB. In this work, the metal-binding properties of H. pylori HypB (HpHypB) were investigated and a link between metal binding and the other biochemical properties of HpHypB was established. HpHypB binds stoichiometric nickel or zinc with nanomolar affinities, in partially overlapping sites located between two major GTPase motifs. Upon metal binding, the GTP hydrolysis activity and oligomeric properties of the protein are modulated. Furthermore, the stoichiometry and affinity of the nickel is altered when HpHypB is bound to nucleotide, a change not observed for zinc. Mutagenesis of the metal ligands suggest that a conserved cysteine is responsible for transducing the metal-bound state to altered GTPase activity and a conserved histidine is a required nickel ligand only in the nucleotide-bound state. Together, these results suggest that the metal-binding and GTP hydrolysis properties of HpHypB are intimately linked and may comprise a mechanism through which the [NiFe]-hydrogenase and urease maturation pathways can discriminate between Ni(II) and Zn(II). Characterization of the Escherichia coli GTPase YjiA, a member of the same GTPase family as HpHypB, demonstrated that YjiA can bind Ni(II), Zn(II), or Co(II) at a site in a similar location as in HpHypB. Metal binding also regulates the GTPase activity and oligomerization of YjiA. This finding suggests that metal-responsive GTPase activity may be a trait of this family of GTPases. Together, this work describes a unique link between the metal-binding and biochemical properties of the G3E GTPases and provides insight into the role of HpHypB in [NiFe]-hydrogenase and urease maturation.

Hypothalamic lactate metabolism regulates hepatic glucose and lipid homeostasis, however it remains unclear whether hypothalamic lactate also controls energy homeostasis. Furthermore, the precise downstream molecular and signaling pathway(s) involved in hypothalamic lactate-sensing is yet to be fully elucidated. To specifically address these two questions, we tested the hypothesis that hypothalamic lactate metabolism regulates energy homeostasis (Study 1) and assessed whether the activation of N-methyl-D-aspartate (NMDA) receptors in the nucleus of the solitary tract (NTS) of the brainstem is required for hypothalamic lactate, and sufficient per se, to regulate glucose homeostasis (Study 2). In an in vivo rat model, we reported in Study 1 that central lactate lowers food intake and body weight through its metabolism into pyruvate. In Study 2, we identified that hypothalamic lactate metabolism requires the activation of NMDA receptors in the NTS to lower hepatic glucose production. Moreover, we showed that the activation of NTS NMDA receptors per se lowers hepatic glucose production. In summary, these findings advance the understanding of central nutrient-sensing in the regulation of energy and glucose homeostasis, which is critical in bridging the therapeutic gap of obesity and type 2 diabetes.

Hypothalamic lactate metabolism regulates hepatic glucose and lipid homeostasis, however it remains unclear whether hypothalamic lactate also controls energy homeostasis. Furthermore, the precise downstream molecular and signaling pathway(s) involved in hypothalamic lactate-sensing is yet to be fully elucidated. To specifically address these two questions, we tested the hypothesis that hypothalamic lactate metabolism regulates energy homeostasis (Study 1) and assessed whether the activation of N-methyl-D-aspartate (NMDA) receptors in the nucleus of the solitary tract (NTS) of the brainstem is required for hypothalamic lactate, and sufficient per se, to regulate glucose homeostasis (Study 2). In an in vivo rat model, we reported in Study 1 that central lactate lowers food intake and body weight through its metabolism into pyruvate. In Study 2, we identified that hypothalamic lactate metabolism requires the activation of NMDA receptors in the NTS to lower hepatic glucose production. Moreover, we showed that the activation of NTS NMDA receptors per se lowers hepatic glucose production. In summary, these findings advance the understanding of central nutrient-sensing in the regulation of energy and glucose homeostasis, which is critical in bridging the therapeutic gap of obesity and type 2 diabetes.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2006. / Härtill 4 uppsatser.

Diabetes is a group of metabolic diseases characterized by hyperglycaemia resulting from defects in insulin secretion, insulin action, or both. Several models have been developed for describing the glucose-insulin system. Silber and Jauslin developed a semi-mechanistic integrated glucose insulin (IGI) model which simultaneously describe glucose and insulin profiles in either healthy subjects or type 2 diabetis mellitus (T2DM) patients. The model was developed for describing the basal system, i.e. when no drugs are present in the body. In this thesis the IGI model was extended to also include the effects of anti-diabetic drugs on glucose homeostasis. The model was extended to describe postprandial glucose and insulin excursions in T2DM patients treated with either biphasic insulin aspart or the GLP-1 receptor agonist liraglutide. These extensions make the model a useful tool in drug development as it can be used for elucidating the effects of new products as well as for clinical trial simulation. In this thesis several modelling tasks were also performed to get a more mechanistic description of the glucose-insulin system. A model was developed which describes the release of the incretin hormones glucosedependent insulinotropic polypeptide and glucagon-like peptide-1 following the ingestion of various glucose doses. The effects of these hormones on the beta cell function were incorporated in a model describing both the C-peptide and insulin concentrations in healthy subjects and T2DM patients during either an oral glucose tolerance test or an isoglycaemic intravenous glucose infusion. By including measurements of both C-peptide and insulin concentrations in the model it could also be used to characterize the hepatic extraction of insulin.