Blood-Brain Glucose Transfer in Alzheimer's disease: Effect of GLP-1 Analog Treatment.

There are fewer than normal glucose transporters at the blood-brain barrier (BBB) in Alzheimer's disease (AD). When reduced expression of transporters aggravates the symptoms of AD, the transporters become a potential target of therapy. The incretin hormone GLP-1 prevents the decline of cerebral metabolic rate for glucose (CMRglc) in AD, and GLP-1 may serve to raise transporter numbers. We hypothesized that the GLP-1 analog liraglutide would prevent the decline of CMRglc in AD by raising blood-brain glucose transfer, depending on the duration of disease. We randomized 38 patients with AD to treatment with liraglutide (n = 18) or placebo (n = 20) for 6 months, and determined the blood-brain glucose transfer capacity (T max) in the two groups and a healthy age matched control group (n = 6). In both AD groups at baseline, T max estimates correlated inversely with the duration of AD, as did the estimates of CMRglc that in turn were positively correlated with cognition. The GLP-1 analog treatment, compared to placebo, highly significantly raised the T max estimates of cerebral cortex from 0.72 to 1.1 umol/g/min, equal to T max estimates in healthy volunteers. The result is consistent with the claim that GLP-1 analog treatment restores glucose transport at the BBB.

Capillary dysfunction is associated with symptom severity and neurodegeneration in Alzheimer's disease.

INTRODUCTION: We examined whether cortical microvascular blood volume and hemodynamics in Alzheimer's disease (AD) are consistent with tissue hypoxia and whether they correlate with cognitive performance and the degree of cortical thinning. METHODS: Thirty-two AD patients underwent cognitive testing, structural magnetic resonance imaging (MRI), and perfusion MRI at baseline and after 6 months. We measured cortical thickness, microvascular cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), and capillary transit time heterogeneity (CTH) and estimated tissue oxygen tension (PtO2). RESULTS: At baseline, poor cognitive performance and regional cortical thinning correlated with lower CBF and CBV, with higher MTT and CTH and with low PtO2 across the cortex. Cognitive decline over time was associated with increasing whole brain relative transit time heterogeneity (RTH = CTH/MTT). DISCUSSION: Our results confirm the importance of microvascular pathology in AD. Deteriorating microvascular hemodynamics may cause hypoxia, which is known to precipitate amyloid retention.

In Alzheimer's Disease, 6-Month Treatment with GLP-1 Analog Prevents Decline of Brain Glucose Metabolism: Randomized, Placebo-Controlled, Double-Blind Clinical Trial.

In animal models, the incretin hormone GLP-1 affects Alzheimer's disease (AD). We hypothesized that treatment with GLP-1 or an analog of GLP-1 would prevent accumulation of Aß and raise, or prevent decline of, glucose metabolism (CMRglc) in AD. In this 26-week trial, we randomized 38 patients with AD to treatment with the GLP-1 analog liraglutide (n = 18), or placebo (n = 20). We measured Aß load in brain with tracer [(11)C]PIB (PIB), CMRglc with [(18)F]FDG (FDG), and cognition with the WMS-IV scale (ClinicalTrials.gov NCT01469351). The PIB binding increased significantly in temporal lobe in placebo and treatment patients (both P = 0.04), and in occipital lobe in treatment patients (P = 0.04). Regional and global increases of PIB retention did not differ between the groups (P ≥ 0.38). In placebo treated patients CMRglc declined in all regions, significantly so by the following means in precuneus (P = 0.009, 3.2 µmol/hg/min, 95% CI: 5.45; 0.92), and in parietal (P = 0.04, 2.1 µmol/hg/min, 95% CI: 4.21; 0.081), temporal (P = 0.046, 1.54 µmol/hg/min, 95% CI: 3.05; 0.030), and occipital (P = 0.009, 2.10 µmol/hg/min, 95% CI: 3.61; 0.59) lobes, and in cerebellum (P = 0.04, 1.54 µmol/hg/min, 95% CI: 3.01; 0.064). In contrast, the GLP-1 analog treatment caused a numerical but insignificant increase of CMRglc after 6 months. Cognitive scores did not change. We conclude that the GLP-1 analog treatment prevented the decline of CMRglc that signifies cognitive impairment, synaptic dysfunction, and disease evolution. We draw no firm conclusions from the Aß load or cognition measures, for which the study was underpowered.

Glucagon-like peptide-1 (GLP-1) raises blood-brain glucose transfer capacity and hexokinase activity in human brain.

In hyperglycemia, glucagon-like peptide-1 (GLP-1) lowers brain glucose concentration together with increased net blood-brain clearance and brain metabolism, but it is not known whether this effect depends on the prevailing plasma glucose (PG) concentration. In hypoglycemia, glucose depletion potentially impairs brain function. Here, we test the hypothesis that GLP-1 exacerbates the effect of hypoglycemia. To test the hypothesis, we determined glucose transport and consumption rates in seven healthy men in a randomized, double-blinded placebo-controlled cross-over experimental design. The acute effect of GLP-1 on glucose transfer in the brain was measured by positron emission tomography (PET) during a hypoglycemic clamp (3 mM plasma glucose) with (18)F-fluoro-2-deoxy-glucose (FDG) as tracer of glucose. In addition, we jointly analyzed cerebrometabolic effects of GLP-1 from the present hypoglycemia study and our previous hyperglycemia study to estimate the Michaelis-Menten constants of glucose transport and metabolism. The GLP-1 treatment lowered the vascular volume of brain tissue. Loading data from hypo- to hyperglycemia into the Michaelis-Menten equation, we found increased maximum phosphorylation velocity (V max) in the gray matter regions of cerebral cortex, thalamus, and cerebellum, as well as increased blood-brain glucose transport capacity (T max) in gray matter, white matter, cortex, thalamus, and cerebellum. In hypoglycemia, GLP-1 had no effects on net glucose metabolism, brain glucose concentration, or blood-brain glucose transport. Neither hexokinase nor transporter affinities varied significantly with treatment in any region. We conclude that GLP-1 changes blood-brain glucose transfer and brain glucose metabolic rates in a PG concentration-dependent manner. One consequence is that hypoglycemia eliminates these effects of GLP-1 on brain glucose homeostasis.

Effects of liraglutide on neurodegeneration, blood flow and cognition in Alzheimer´s disease - protocol for a controlled, randomized double-blinded trial.

INTRODUCTION: Type 2 diabetes (DM-2) increases the risk of developing Alzheimer´s disease (AD), and patients with AD are more likely to develop DM-2. DM-2 and AD share some pathophysiological features. In AD, amyloid-ß (Aß) is accumulated as extracellular plaques in the gray matter of the brain, while in DM-2 islet amyloid polypeptide (IAPP) is accumulated in the pancreas. Premature cellular degeneration is seen in both diseases. Glucagon-like peptide-1 (GLP-1) reduces the amount of Aß and improves cognition in animal studies. The present study tests the hypothesis that treatment with the long-acting GLP-1 receptor agonist liraglutide affects the accumulation of Aß in patients with AD. MATERIAL AND METHODS: This is a randomized, controlled, double-blinded intervention study with AD patients treated for six months with liraglutide (n = 20) or placebo (n = 20). The primary outcome is change in deposition of Aß in the central nervous system (CNS) by Pittsburgh compound B positron emission tomography (PET). The secondary outcome is evaluation of cognition using a neuro-psychological test battery, and examination of changes in glucose uptake in the CNS by 18F-fluoro-deoxy-glucose PET. Finally, a perfusion-weighted magnetic resonance imaging with contrast will be performed to evaluate blood flow. CONCLUSION: No registered drug affects the deposition of Aß in the brain of AD patients. Our goal is to find a new therapeutic agent that alters the pathophysiology in AD patients by decreasing the formation of Aß plaques and thereby presumably improves the cognitive function. FUNDING: The trial is investigator-initiated and investigator-driven and is supported by Novo Nordisk Scandinavia. TRIAL REGISTRATION: ClinicalTrials.gov: NCT01469351.

Glucagon-like peptide-1 decreases intracerebral glucose content by activating hexokinase and changing glucose clearance during hyperglycemia.

Type 2 diabetes and hyperglycemia with the resulting increase of glucose concentrations in the brain impair the outcome of ischemic stroke, and may increase the risk of developing Alzheimer's disease (AD). Reports indicate that glucagon-like peptide-1 (GLP-1) may be neuroprotective in models of AD and stroke: Although the mechanism is unclear, glucose homeostasis appears to be important. We conducted a randomized, double-blinded, placebo-controlled crossover study in nine healthy males. Positron emission tomography was used to determine the effect of GLP-1 on cerebral glucose transport and metabolism during a hyperglycemic clamp with (18)fluoro-deoxy-glucose as tracer. Glucagon-like peptide-1 lowered brain glucose (P=0.023) in all regions. The cerebral metabolic rate for glucose was increased everywhere (P=0.039) but not to the same extent in all regions (P=0.022). The unidirectional glucose transfer across the blood-brain barrier remained unchanged (P=0.099) in all regions, while the unidirectional clearance and the phosphorylation rate increased (P=0.013 and 0.017), leading to increased net clearance of the glucose tracer (P=0.006). We show that GLP-1 plays a role in a regulatory mechanism involved in the actions of GLUT1 and glucose metabolism: GLP-1 ensures less fluctuation of brain glucose levels in response to alterations in plasma glucose, which may prove to be neuroprotective during hyperglycemia.

Targeting amyloid-beta by glucagon-like peptide -1 (GLP-1) in Alzheimer's disease and diabetes.

INTRODUCTION: Epidemiological evidence suggests an association between type 2 diabetes (T2DM) and Alzheimer's disease (AD), in that one disease increases the risk of the other. T2DM and AD share several molecular processes which underlie the tissue degeneration in either disease. Disturbances in insulin signaling may be the link between the two conditions. Drugs originally developed for T2DM are currently being considered as possible novel agents in the treatment of AD. AREAS COVERED: This review discusses the potential role of glucagon-like peptide -1 (GLP-1) treatment in AD. GLP-1 receptors are expressed in areas of the brain important to memory and learning, and GLP-1 has growth-factor-like properties similar to insulin. A key neuropathological feature of AD is the accumulation of amyloid-beta (Aß). In preclinical studies, GLP-1 and longer lasting analogues have been shown to have both neuroprotective and neurotrophic effects, and to protect synaptic activity in the brain from Aß toxicity. EXPERT OPINION: A convincing amount of evidence has shown a beneficial effect of GLP-1 agonist treatment on cognitive function, memory and learning in experimental models of AD. GLP-1 analogues may therefore be the new therapeutic agent of choice for intervention in AD.

siRNA-mediated knock-down of ZnT3 and ZnT8 affects production and secretion of insulin and apoptosis in INS-1E cells.

Zinc is essential for the crystallization of insulin in pancreatic ß-cells and is thought to induce apoptosis in a dose-dependent manner, thereby regulating ß-cell mass. Therefore, a tight intracellular regulation of Zn²(+) is required. The zinc-transporter family SLC30A is an important factor in the regulation of zinc homeostasis. The aim of this study was to examine the effect of the zinc transporters ZnT3 and ZnT8 on insulin metabolism and apoptosis. Both these proteins are present in pancreatic ß-cells and have been linked to diabetes. The objective of our study was to perform a considerable siRNA-mediated knock-down of ZnT3 and ZnT8 in INS-1E cells, a pancreatic ß-cell model, and afterwards examine the impact on cell viability and insulin metabolism. Increased levels of apoptosis were observed after knock-down of both ZnT3 and ZnT8. Insulin secretion was significantly reduced by ZnT3 knock-down, whereas knock-down of ZnT8 resulted in increased intracellular content of insulin accompanied by a relatively lowered secretion. Both zinc transporters in this way seem to play a role in ß-cell survival and the ability of these cells to react appropriately to surrounding glucose concentrations.

Zinc, alpha cells and glucagon secretion.

Zinc concentrates in islet cells and is related to insulin secretion. Islet cells act as a unit within islets and hormone secretion in the islets is profoundly influenced by paracrine and autocrine regulation. Zinc has been recognised as a candidate paracrine inhibitor of glucagon secretion in alpha-cells. Further zinc fluxes may contribute to regulation of cell mass, Zn2+ may be cytotoxic and Zn2+ depletion by itself can cause cell death induced by oxidative stress. Recently, both free zinc ions and a number of zinc transporters have been localized in alpha-cells. These include zinc importers, ZIP1, ZIP10, and ZIP14 of the SLC39A family and zinc exporters, ZnT1, and ZnT4-8 of the SLC30A family. Furthermore, the redox state of thiol groups and Voltage Gated Ca2+ Channels (VGCC) add to the maintenance of a tight cytoplasmatic zinc homeostasis in the alpha-cells. The ZnT8 protein has emerged as particularly interesting since this zinc transporter has been identified as a genetic risk factor for the development of both type 1 and type 2 diabetes in which both alpha- and beta-cell functions are affected. Recent data discussed here suggest specific effects of Zn2+ on glucagon secretion and other alpha-cell functions.

Zinc transporter gene expression is regulated by pro-inflammatory cytokines: a potential role for zinc transporters in beta-cell apoptosis?

BACKGROUND: Beta-cells are extremely rich in zinc and zinc homeostasis is regulated by zinc transporter proteins. beta-cells are sensitive to cytokines, interleukin-1beta (IL-1beta) has been associated with beta-cell dysfunction and -death in both type 1 and type 2 diabetes. This study explores the regulation of zinc transporters following cytokine exposure. METHODS: The effects of cytokines IL-1beta, interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha) on zinc transporter gene expression were measured in INS-1-cells and rat pancreatic islets. Being the more sensitive transporter, we further explored ZnT8 (Slc30A8): the effect of ZnT8 over expression on cytokine induced apoptosis was investigated as well as expression of the insulin gene and two apoptosis associated genes, BAX and BCL2. RESULTS: Our results showed a dynamic response of genes responsible for beta-cell zinc homeostasis to cytokines: IL-1beta down regulated a number of zinc-transporters, most strikingly ZnT8 in both islets and INS-1 cells. The effect was even more pronounced when mixing the cytokines. TNF-alpha had little effect on zinc transporter expression. IFN-gamma down regulated a number of zinc transporters. Insulin expression was down regulated by all cytokines. ZnT8 over expressing cells were more sensitive to IL-1beta induced apoptosis whereas no differences were observed with IFN-gamma, TNF-alpha, or a mixture of cytokines. CONCLUSION: The zinc transporting system in beta-cells is influenced by the exposure to cytokines. Particularly ZnT8, which has been associated with the development of diabetes, seems to be cytokine sensitive.