Cellular mechanisms of brain hypoglycemia

β-Hydroxybutyrate Alleviates Low Glucose-Induced Apoptosis via Modulation of ROS-Mediated p38 MAPK Signaling

Hypoglycemia has emerged as a prominent complication in anti-diabetic drug therapy or negative energy balance of animals, which causes brain damage, cognitive impairment, and even death. Brain injury induced by hypoglycemia is closely related to oxidative stress and the production of reactive oxygen species (ROS). The intracellular accumulation of ROS leads to neuronal damage, even death. Ketone body β-hydroxybutyrate (BHBA) not only serves as alternative energy source for glucose in extrahepatic tissues, but is also involved in cellular signaling transduction. Previous studies showed that BHBA reduces apoptosis by inhibiting the excessive production of ROS and activation of caspase-3. However, the effects of BHBA on apoptosis induced by glucose deprivation and its related molecular mechanisms have been seldom reported. In the present study, PC12 cells and primary cortical neurons were used to establish a low glucose injury model. The effects of BHBA on the survival and apoptosis in a glucose deficient condition and related molecular mechanisms were investigated by using flow cytometry, immunofluorescence, and western blotting. PC12 cells were incubated with 1 mM glucose for 24 h as a low glucose cell model, in which ROS accumulation and cell mortality were significantly increased. After 24 h and 48 h treatment with different concentrations of BHBA (0 mM, 0.05 mM, 0.5 mM, 1 mM, 2 mM), ROS production was significantly inhibited. Moreover, cell apoptosis rate was decreased and survival rate was significantly increased in 1 mM and 2 mM BHBA groups. In primary cortical neurons, at 24 h after treatment with 2 mM BHBA, the injured length and branch of neurites were significantly improved. Meanwhile, the intracellular ROS level, the proportion of c-Fos⁺ cells, apoptosis rate, and nuclear translocation of NF-κB protein after treatment with BHBA were significantly decreased when compared with that in low glucose cells. Importantly, the expression of p38, p-p38, NF-κB, and caspase-3 were significantly decreased, while the expression of p-ERK was significantly increased in both PC12 cells and primary cortical neurons. Our results demonstrate that BHBA decreased the accumulation of intracellular ROS, and further inhibited cell apoptosis by mediating the p38 MAPK signaling pathway and caspase-3 apoptosis cascade during glucose deprivation. In addition, BHBA inhibited apoptosis by activating ERK phosphorylation and alleviated the damage of low glucose to PC12 cells and primary cortical neurons. These results provide new insight into the anti-apoptotic effect of BHBA in a glucose deficient condition and the related signaling cascade.

Reappraisal of metabolic dysfunction in neurodegeneration: Focus on mitochondrial function and calcium signaling

The cellular and molecular mechanisms that drive neurodegeneration remain poorly defined. Recent clinical trial failures, difficult diagnosis, uncertain etiology, and lack of curative therapies prompted us to re-examine other hypotheses of neurodegenerative pathogenesis. Recent reports establish that mitochondrial and calcium dysregulation occur early in many neurodegenerative diseases (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, and others. However, causal molecular evidence of mitochondrial and metabolic contributions to pathogenesis remains insufficient. Here we summarize the data supporting the hypothesis that mitochondrial and metabolic dysfunction result from diverse etiologies of neuropathology. We provide a current and comprehensive review of the literature and interpret that defective mitochondrial metabolism is upstream and primary to protein aggregation and other dogmatic hypotheses of NDDs. Finally, we identify gaps in knowledge and propose therapeutic modulation of mCa2+ exchange and mitochondrial function to alleviate metabolic impairments and treat NDDs.

Glycemic Variability and CNS Inflammation: Reviewing the Connection

Glucose is the primary energy source for the brain, and exposure to both high and low levels of glucose has been associated with numerous adverse central nervous system (CNS) outcomes. While a large body of work has highlighted the impact of hyperglycemia on peripheral and central measures of oxidative stress, cognitive deficits, and vascular complications in Type 1 and Type 2 diabetes, there is growing evidence that glycemic variability significantly drives increased oxidative stress, leading to neuroinflammation and cognitive dysfunction. In this review, the latest data on the impact of glycemic variability on brain function and neuroinflammation will be presented. Because high levels of oxidative stress have been linked to dysfunction of the blood-brain barrier (BBB), special emphasis will be placed on studies investigating the impact of glycemic variability on endothelial and vascular inflammation. The latest clinical and preclinical/in vitro data will be reviewed, and clinical/therapeutic implications will be discussed.

Mitochondria, OxPhos, and neurodegeneration: cells are not just running out of gas

Mitochondrial respiratory deficiencies have been observed in numerous neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. For decades, these reductions in oxidative phosphorylation (OxPhos) have been presumed to trigger an overall bioenergetic crisis in the neuron, resulting in cell death. While the connection between respiratory defects and neuronal death has never been proven, this hypothesis has been supported by the detection of nonspecific mitochondrial DNA mutations in these disorders. These findings led to the notion that mitochondrial respiratory defects could be initiators of these common neurodegenerative disorders, instead of being consequences of a prior insult, a theory we believe to be misconstrued. Herein, we review the roots of this mitochondrial hypothesis and offer a new perspective wherein mitochondria are analyzed not only from the OxPhos point of view, but also as a complex organelle residing at the epicenter of many metabolic pathways.

Biochemical phenotyping unravels novel metabolic abnormalities and potential biomarkers associated with treatment of GLUT1 deficiency with ketogenic diet

Global metabolomic profiling offers novel opportunities for the discovery of biomarkers and for the elucidation of pathogenic mechanisms that might lead to the development of novel therapies. GLUT1 deficiency syndrome (GLUT1-DS) is an inborn error of metabolism due to reduced function of glucose transporter type 1. Clinical presentation of GLUT1-DS is heterogeneous and the disorder mirrors patients with epilepsy, movement disorders, or any paroxysmal events or unexplained neurological manifestation triggered by exercise or fasting. The diagnostic biochemical hallmark of the disease is a reduced cerebrospinal fluid (CSF)/blood glucose ratio and the only available treatment is ketogenic diet. This study aimed at advancing our understanding of the biochemical perturbations in GLUT1-DS pathogenesis through biochemical phenotyping and the treatment of GLUT1-DS with a ketogenic diet. Metabolomic analysis of three CSF samples from GLUT1-DS patients not on ketogenic diet was feasible inasmuch as CSF sampling was used for diagnosis before to start with ketogenic diet. The analysis of plasma and urine samples obtained from GLUT1-DS patients treated with a ketogenic diet showed alterations in lipid and amino acid profiles. While subtle, these were consistent findings across the patients with GLUT1-DS on ketogenic diet, suggesting impacts on mitochondrial physiology. Moreover, low levels of free carnitine were present suggesting its consumption in GLUT1-DS on ketogenic diet. 3-hydroxybutyrate, 3-hydroxybutyrylcarnitine, 3-methyladipate, and N-acetylglycine were identified as potential biomarkers of GLUT1-DS on ketogenic diet. This is the first study to identify CSF, plasma, and urine metabolites associated with GLUT1-DS, as well as biochemical changes impacted by a ketogenic diet. Potential biomarkers and metabolic insights deserve further investigation.

ROS Production and ERK Activity Are Involved in the Effects of d-β-Hydroxybutyrate and Metformin in a Glucose Deficient Condition

Hypoglycemia, a complication of insulin or sulfonylurea therapy in diabetic patients, leads to brain damage. Furthermore, glucose replenishment following hypoglycemic coma induces neuronal cell death. In this study, we investigated the molecular mechanism underlying glucose deficiency-induced cytotoxicity and the protective effect of d-β-hydroxybutyrate (D-BHB) using SH-SY5Y cells. The cytotoxic mechanism of metformin under glucose deficiency was also examined. Cell viability under 1 mM glucose (glucose deficiency) was significantly decreased which was accompanied by increased production of reactive oxygen species (ROS) and decreased phosphorylation of extracellular signal-regulated kinase (ERK) and glycogen synthase 3 (GSK3β). ROS inhibitor reversed the glucose deficiency-induced cytotoxicity and restored the reduced phosphorylation of ERK and GSK3β. While metformin did not alter cell viability in normal glucose media, it further increased cell death and ROS production under glucose deficiency. However, D-BHB reversed cytotoxicity, ROS production, and the decrease in phosphorylation of ERK and GSK3β induced by the glucose deficiency. ERK inhibitor reversed the D-BHB-induced increase in cell viability under glucose deficiency, whereas GSK3β inhibitor did not restore glucose deficiency-induced cytotoxicity. Finally, the protective effect of D-BHB against glucose deficiency was confirmed in primary neuronal cells. We demonstrate that glucose deficiency-induced cytotoxicity is mediated by ERK inhibition through ROS production, which is attenuated by D-BHB and intensified by metformin.

Cearoin Induces Autophagy, ERK Activation and Apoptosis via ROS Generation in SH-SY5Y Neuroblastoma Cells

Neuroblastomas are the most common solid extracranial tumors in childhood. We investigated the anticancer effect of cearoin isolated from Dalbergia odorifera in human neuroblastoma SH-SY5Y cells. SH-SY5Y cells were treated with various doses of cearoin. The viability was measured by MTT assay. DCFDA fluorescence assay and Griess assay were used for the measurement of intracellular reactive oxygen species (ROS) and nitric oxide (NO), respectively. Western blot analysis was performed to clarify the molecular pathway involved. Cearoin induced cell death in a dose-dependent manner. Cearoin increased the phosporylation of ERK, the conversion of LC3B-I to LC3B-II, decrease in Bcl2 expression, the activation of caspase-3, and the cleavage of PARP, indicating the induction of autophagy and apoptosis. Furthermore, cearoin treatment increased the production of ROS and NO. Co-treatment with the antioxidant N-acetylcysteine completely abolished cearoin-mediated autophagy, ERK activation and apoptosis, suggesting the critical role of ROS in cearoin-induced anticancer effects. Moreover, co-treatment with ERK inhibitor PD98059 partially reversed cearoin-induced cell death, indicating the involvement of ERK in cearoin anticancer effects. These data reveal that cearoin induces autophagy, ERK activation and apoptosis in neuroblastoma SH-SY5Y cells, which is mediated primarily by ROS generation, suggesting its therapeutic application for the treatment of neuroblastomas.

Death by a thousand cuts in Alzheimer's disease: hypoxia--the prodrome

A wide range of clinical consequences may be associated with obstructive sleep apnea (OSA) including systemic hypertension, cardiovascular disease, pulmonary hypertension, congestive heart failure, cerebrovascular disease, glucose intolerance, impotence, gastroesophageal reflux, and obesity, to name a few. Despite this, 82 % of men and 93 % of women with OSA remain undiagnosed. OSA affects many body systems, and induces major alterations in metabolic, autonomic, and cerebral functions. Typically, OSA is characterized by recurrent chronic intermittent hypoxia (CIH), hypercapnia, hypoventilation, sleep fragmentation, peripheral and central inflammation, cerebral hypoperfusion, and cerebral glucose hypometabolism. Upregulation of oxidative stress in OSA plays an important pathogenic role in the milieu of hypoxia-induced cerebral and cardiovascular dysfunctions. Strong evidence underscores that cerebral amyloidogenesis and tau phosphorylation--two cardinal features of Alzheimer's disease (AD), are triggered by hypoxia. Mice subjected to hypoxic conditions unambiguously demonstrated upregulation in cerebral amyloid plaque formation and tau phosphorylation, as well as memory deficit. Hypoxia triggers neuronal degeneration and axonal dysfunction in both cortex and brainstem. Consequently, neurocognitive impairment in apneic/hypoxic patients is attributable to a complex interplay between CIH and stimulation of several pathological trajectories. The framework presented here helps delineate the emergence and progression of cognitive decline, and may yield insight into AD neuropathogenesis. The global impact of CIH should provide a strong rationale for treating OSA and snoring clinically, in order to ameliorate neurocognitive impairment in aged/AD patients.

Glucose starvation stimulates Zn2+ toxicity in cultures of cerebellar granule neurons

Zinc chloride (0.02 mM, 3h) did not have any influence on the survival of cerebellar granule neurons (CGNs) incubated in balanced salt solution (BSS). However, in the absence of glucose ZnCl(2) caused severe neuronal damage, decreasing cell survival to 12±2%. Either the blockade of ionotropic glutamate NMDA-receptors with MK-801 or APV or supplementation the medium with ruthenium red (mitochondrial Ca(2+) uniporter blocker) almost entirely protected CGNs from the toxic effect of ZnCl(2) during glucose deprivation (GD). However, NBQX (AMPA/kainate glutamate receptor blocker) did not show protective effect. Measurements of intracellular calcium ions concentration using fluorescent probe (Fluo-4 AM) and zinc ions (FluoZin-3AM) demonstrated that 1.5h-exposure to GD induced intensive increase of Fluo-4 fluorescence and small increase of FluoZin-3 fluorescence in neurons. The supplementation of medium with ZnCl(2) caused equal increase of FluoZin-3 fluorescence at both GD and normoglycemia, whereas the potentiation of Fluo-4 fluorescence by zinc was observed only under GD and could be prevented by MK-801. However, neither MK-801 nor NBQX could influence [Zn(2+)](i) increase caused by zinc addition under GD, while ruthenium red did cause significant increase of [Zn(2+)](i). This data implies that zinc ions during GD induce an additional overload of CGNs with calcium ions that get transported through activated NMDA-channel. Zinc and calcium ions accumulate in mitochondria and amplify individual destructive action on these organelles leading to neuronal death.