Effect of endogenous beta-hydroxybutyrate on glucose metabolism in the diabetic rabbit brain: a (13)C-magnetic resonance spectroscopy study of [U-(13)C]glucose metabolites

Brain Energy Metabolism in Ischemic Stroke: Effects of Smoking and Diabetes

Proper regulation of energy metabolism in the brain is crucial for maintaining brain activity in physiological and different pathophysiological conditions. Ischemic stroke has a complex pathophysiology which includes perturbations in the brain energy metabolism processes which can contribute to worsening of brain injury and stroke outcome. Smoking and diabetes are common risk factors and comorbid conditions for ischemic stroke which have also been associated with disruptions in brain energy metabolism. Simultaneous presence of these conditions may further alter energy metabolism in the brain leading to a poor clinical prognosis after an ischemic stroke event. In this review, we discuss the possible effects of smoking and/or diabetes on brain glucose utilization and mitochondrial energy metabolism which, when present concurrently, may exacerbate energy metabolism in the ischemic brain. More research is needed to investigate brain glucose utilization and mitochondrial oxidative metabolism in ischemic stroke in the presence of smoking and/or diabetes, which would provide further insights on the pathophysiology of these comorbid conditions and facilitate the development of therapeutic interventions.

Human amylin proteotoxicity impairs protein biosynthesis, and alters major cellular signaling pathways in the heart, brain and liver of humanized diabetic rat model in vivo

INTRODUCTION

Chronic hypersecretion of the 37 amino acid amylin is common in type 2 diabetics (T2D). Recent studies implicate human amylin aggregates cause proteotoxicity (cell death induced by misfolded proteins) in both the brain and the heart.

OBJECTIVES

Identify systemic mechanisms/markers by which human amylin associated with cardiac and brain defects might be identified.

METHODS

We investigated the metabolic consequences of amyloidogenic and cytotoxic amylin oligomers in heart, brain, liver, and plasma using non-targeted metabolomics analysis in a rat model expressing pancreatic human amylin (HIP model).

RESULTS

Four metabolites were significantly different in 3 or more of the the four compartments (heart, brain, liver, and plasma) in HIP rats. When compared to a T2D rat model, HIP hearts uniquely had significant DECREASES in five amino acids (lysine, alanine, tyrosine, phenylalanine, serine), with phenylalanine decreased across all four tissues investigated, including plasma. In contrast, significantly INCREASED circulating phenylalanine is reported in diabetics in multiple recent studies.

CONCLUSION

DECREASED phenylalanine may serve as a unique marker of cardiac and brain dysfunction due to hyperamylinemia that can be differentiated from alterations in T2D in the plasma. While the deficiency in phenylalanine was seen across tissues including plasma and could be monitored, reduced tyrosine was seen only in the brain. The 50% reduction in phenylalanine and tyrosine in HIP brains is significant given their role in supporting brain chemistry as a precursor for catecholamines (dopamine, norepinephrine, epinephrine), which may contribute to the increased morbidity and mortality in diabetics at a multi-system level beyond the effects on glucose metabolism.

Effects of diabetes on brain metabolism--is brain glycogen a significant player?

Brain glycogen, being an intracellular glucose reservoir, contributes to maintain energy and neurotransmitter homeostasis under physiological as well as pathological conditions. Under conditions with a disturbance in systemic glucose metabolism such as in diabetes, the supply of glucose to the brain may be affected and have important impacts on brain metabolism and neurotransmission. This also implies that brain glycogen may serve an essential role in the diabetic state to sustain appropriate brain function. There are two main types of diabetes; type 1 and type 2 diabetes and both types may be associated with brain impairments e.g. cognitive decline and dementia. It is however, not clear how these impairments on brain function are linked to alterations in brain energy and neurotransmitter metabolism. In this review, we will illuminate how rodent diabetes models have contributed to a better understanding of how brain energy and neurotransmitter metabolism is affected in diabetes. There will be a particular focus on the role of brain glycogen to support glycolytic and TCA cycle activity as well as glutamate-glutamine cycle in type 1 and type 2 diabetes.

Effects of acute and chronic hyperglycemia on the neurochemical profiles in the rat brain with streptozotocin-induced diabetes detected using in vivo ¹H MR spectroscopy at 9.4 T

Chronic hyperglycemia could lead to cerebral metabolic alterations and CNS injury. However, findings of metabolic alterations in poorly managed diabetes in humans and animal models are rather inconsistent. We have characterized the cerebral metabolic consequences of untreated hyperglycemia from the onset to the chronic stage in a streptozotocin-induced rat model of diabetes. In vivo ¹H magnetic resonance spectroscopy was used to measure over 20 neurochemicals longitudinally. Upon the onset of hyperglycemia (acute state), increases in brain glucose levels were accompanied by increases in osmolytes and ketone bodies, all of which remained consistently high through the chronic state of over 10 weeks of hyperglycemia. Only after over 4 weeks of hyperglycemia, the levels of other neurochemicals including N-acetylaspartate and glutathione were significantly reduced and these alterations persisted into the chronic stage. However, glucose transport was not altered in chronic hyperglycemia of over 10 weeks. When glucose levels were acutely restored to euglycemia, some neurochemical changes were irreversible, indicating the impact of prolonged uncontrolled hyperglycemia on the CNS. Furthermore, progressive changes in neurochemical levels from control to acute and chronic conditions demonstrated the utility of ¹H magnetic resonance spectroscopy as a non-invasive tool in monitoring the disease progression in diabetes.

Impaired formalin-evoked changes of spinal amino acid levels in diabetic rats

To investigate mechanisms by which diabetes alters sensory processing, we measured levels of amino acid neurotransmitters in spinal dialysates from awake, unrestrained control and diabetic rats under resting conditions and following hind paw formalin injection. Under resting conditions, glutamate concentrations in spinal dialysates were significantly (P<0.05) decreased in diabetic rats compared to those of control rats whereas aspartate, taurine, glycine and citrulline remained unchanged and GABA was significantly (P<0.05) increased. Noxious stimulation of the hind paw by subcutaneous injection of 0.5% formalin into the dorsum caused a defined flinching behavior in the afflicted paw, and the amount of flinching was significantly (P<0.05) greater in diabetic rats than in controls. Paw formalin injection significantly (P<0.05) increased dialysate levels of glutamate, aspartate, taurine, glycine and citrulline by 3- to 4-fold above basal in both control and diabetic rats. The concentration of glutamate in dialysate samples collected immediately after paw formalin injection remained significantly (P<0.05) lower in diabetic rats compared to those in controls. Formalin injection did not alter dialysate GABA concentrations in control rats, whereas in diabetic rats there was an increase of 151+/-15% above basal levels. These findings indicate that the selective depression of basal and stimulus-evoked glutamate levels in the spinal cord of diabetic rats occurs in parallel with elevated spinal GABA levels. Because increased pain-associated behavior is accompanied by an attenuated spinal glutamate spike following paw formalin injection, hyperalgesia in diabetic rats does not appear to be secondary to enhanced glutamatergic input to the spinal cord.

Effect of endogenous beta-hydroxybutyrate on brain glucose metabolism in fetuses of diabetic rabbits, studied by (13)C magnetic resonance spectroscopy

The aim of our research was to uncover perturbations in in-utero fetal cerebral metabolism resulting from hyperglycemia and hyperketonemia, which occur during maternal diabetes. Therefore, we examined the effects of glucose overload and hyperketonemia on glucose metabolism in the diabetic fetal brain; more specifically, the effect of diabetes on the glucose flux via pyruvate dehydrogenase (PDH) and pyruvate carboxylase (PC) and subsequent metabolism in the fetal cerebral tricarboxylic acid (TCA) cycle were examined, as well as the effect of diabetes on energy fuel utilization in the neurons and glia. Diabetes was induced in pregnant rabbits, and towards term, [U-(13)C(6)]glucose was infused into maternal circulation, and [(13)C]glucose metabolites were subsequently studied in fetal brain extracts by (13)C MRS isotopomer analysis. Significantly elevated maternal and fetal plasma glucose levels (three- and up to fivefold, respectively) and fetal brain glucose levels (up to eightfold) accompanied by an increase of beta-hydroxybutyrate (beta-HBA) levels (approximately 20-fold) were found in the hyperketonemic diabetic animals, whereas fetal cerebral lactate levels were decreased. Alterations in the (13)C labeling patterns, mainly of glutamine, led us to suggest that the entry of beta-HBA-derived acetyl-CoA inhibits formation and entry of labeled glucose-derived acetyl-CoA into the TCA cycle, mainly in glia. Accumulation of glucose and the decrease in lactate levels in the fetal brain are most likely the result of an inhibitory effect of beta-HBA on glycolysis. In addition, loss of (13)C enrichment of TCA cycle intermediates and products, glutamate and glutamine, in the hyperketonemic diabetic fetal brain may be attributed to the effect of beta-HBA fuel utilization by the fetal brain.