Impact of hypoglycemia on brain metabolism during diabetes.

Abstract

Diabetes is a metabolic disease afflicting millions of people worldwide. A substantial fraction of world’s total healthcare expenditure is spent on treating diabetes. Hypoglycemia is a serious consequence of anti-diabetic drug therapy, because it induces metabolic alterations in brain. Metabolic alterations are one of the central mechanisms mediating hypoglycemia-related functional changes in brain. Acute, chronic, and/or recurrent hypoglycemia modulates multiple metabolic pathways, and exposure to hypoglycemia increases consumption of alternate respiratory substrates such as ketone bodies, glycogen and monocarboxylates in brain. The aim of this review is to discuss hypoglycemia-induced metabolic alterations in brain in glucose counterregulation, uptake, utilization and metabolism, cellular respiration, amino acid and lipid metabolism, and the significance of other sources of energy. The present review summarizes information on hypoglycemia-induced metabolic changes in brain of diabetic and non-diabetic subjects, and the manner in which they may affect brain function.

Introduction

Diabetes is a metabolic disease afflicting 425 million people worldwide. About $727 billion (12 percent of the world’s total healthcare expenditure) are spent on diabetes [1]. Multiple effective therapeutic approaches are available to treat diabetes and its related complications by attenuating hyperglycemia. However, these approaches also elicit hypoglycemia as a side effect [2–4]. Therefore, drug therapy-related hypoglycemia is an important problem in the management of both type 1 (T1D) and type 2 diabetes (T2D) [5]. The American Diabetes Association defines hypoglycemia as plasma glucose level of ≤70 mg/dL (≤3.9 mmol/L) [6]. Based on epidemiological and clinical studies, it appears that episodes of hypoglycemia are commonly observed in both T1D and T2D patients [3,7,8]. Episodes of hypoglycemia are frequently observed in either patients suffering from severe insulin-dependent T1D or longstanding T2D [3,9].

Severe hypoglycemia impairs cognition and is associated with dementia in diabetic subjects [10–13]. Long-term cerebral hypoperfusion in T1D patients is associated with repeated episodes of hypoglycemia [14] and is linked with increased risk of cognitive impairment and dementia [15]. Hypoglycemia-associated cognitive impairment is associated with vascular changes in brain [11]. Oxidative stress is concomitant with hypoglycemia-associated cognitive deficit [16]. Thus, the literature suggests that exposure to hypoglycemia causes brain-related dysfunction in diabetics. A detailed understanding of the effect of hypoglycemia on brain metabolites may help us design approaches to lower hypoglycemia-induced brain damage.

In this review article, we provide an overview of brain metabolism and how it is altered by exposure to acute, chronic and/or recurrent hypoglycemia (RH) (Fig. 1). We also summarized information about hypoglycemia-induced increase in consumption of sources of energy other than glucose.

Figure 1:

Schematic representation of the interplay of signaling pathways modulated by hypoglycemia. GLUT, Glucose transporter; GDH, Glutamate dehydrogenase; MCT2, Monocarboxylate transporter 2; RH, Recurrent hypoglycemia; RCR, Respiratory control ratio; SDH, Succinate dehydrogenase

Glucose counterregulatory mechanisms

Typically, during hypoglycemia, patients experience palpitations, anxiety, hunger, sweating, confusion, loss of consciousness and seizures, and if the hypoglycemia is sustained and severe it may result in coma and ultimately death [17]. An episode of hypoglycemia activates a number of adaptive mechanisms, which correct the fall in blood glucose levels. These mechanisms include drop in pancreatic insulin and epinephrine secretion, increase in glucagon secretion, and modulation of the autonomic nervous system [18–20]. Reduction in insulin secretion decreases metabolic dissipation of glucose from circulation and an increase in glucagon secretion stimulates hepatic glucose production by activating glycogenolysis and gluconeogenesis, ultimately leading to increased blood glucose levels [21,22]. Epinephrine activates adrenergic pathways that increase blood glucose levels by modulating the secretion of insulin and glucagon, and the rates of glucose uptake and utilization [21,22]. Sustained hypoglycemia causes the release of growth hormone and cortisol, which corrects hypoglycemia [23–25]. A prior episode of acute hypoglycemia causes decreased glucose counterregulation and autonomic responses to hypoglycemia leading to hypoglycemia unawareness, hypoglycemia-associated autonomic failure (HAAF) and problems with hypoglycemia management [26–28]. HAAF increases the risk of severe hypoglycemia many fold when the patient is under intensive anti-diabetic therapy [29,30]. Prior exposure to hypoglycemia decreases hypoglycemia-linked changes in glucagon and epinephrine levels [26,31]. Therefore, the literature cited above shows that intrinsic adaptive mechanisms of glucose counterregulation are blunted in T1D patients previously exposed to hypoglycemia, resulting in increased severity of pronounced hypoglycemia.

Alterations of metabolism in brain during hypoglycemia

Brain glucose levels during systemic hypoglycemia

Glucose is a critical energy substrate required for brain functioning [32]. During euglycemic conditions as well as during mild to moderate hypoglycemia, normal human brain depends primarily on glucose as the principal metabolic substrate obtained from the systemic circulation [33,34]. A study observed a linear relationship between plasma glucose levels and brain glucose concentration when plasma glucose values were within the physiological range [35]. It is predicted that during hypoglycemia when blood glucose concentration is 2.1 mM, the brain glucose level goes down to zero owing to glucose consumption [36]. Therefore, the glucose content in brain is regulated by blood glucose levels, and systemic hypoglycemia is expected to reduce glucose availability to the brain.

Glucose uptake

Glucose transporters (GLUTs) belong to the Major Facilitator Superfamily group of proteins [37] that mediates facilitative passive diffusion of glucose across membranes. GLUTs are approximately 500 amino acid-long proteins containing 12 transmembrane-spanning α helices and N-linked oligosaccharide moieties [38–41]. GLUTs are subdivided into three classes based on amino acid sequence homology. Class I is comprised of GLUT1 to GLUT4 (differentiated by varied tissue distributions and hormonal regulation). Class II contains GLUT5 (a fructose-specific transporter) and three related transporter proteins - GLUT7, GLUT9, and GLUT11 [42]. Class III of GLUTs are characterized by an absence of a glycosylation site on the first extracellular linker domain and the presence of glycosylation sites in the ninth transmembrane domain [43]. GLUT1, GLUT3, and GLUT5 are expressed in brain [44–52]. Glucose transporters expressed on microglia are GLUT5 [51]. Glucose is transported from the systemic circulation into the brain across the blood brain barrier via GLUT1 present on microvascular endothelial cells [53,54]. GLUT1 is also expressed on astrocytes [54]. Glucose transport occurs in neurons via GLUT3 (an isoform of GLUT abundantly expressed in the neurons) [55]. Hypoglycemia causes upregulation of GLUT1 mRNA as well as protein levels in the blood brain barrier and brain tissue [53,56–58]. Further, chronic hypoglycemia increases GLUT1 levels in brain capillaries [58]. Repeated insulin administration-induced chronic hypoglycemia for 4–8 days increases the levels of GLUT3 in rat brain [59–61]. Hypoglycemia causes upregulation of monocarboxylic acid transporters (MCT-2) and GLUT3 [62–64]. Continued hypoglycemia enhances glucose transport in brain by increasing GLUT levels, extent of glucose uptake, and related brain functions [58,65–67]. An in vitro study showed that hypoglycemia along with serum deprivation also enhances glucose uptake [68]. Chronic hypoglycemia causes increased transportation and utilization of glucose in brain [66,67]. A normal rate of brain glucose uptake is maintained during hypoglycemia [65,69]. After antecedent RH, brain glucose metabolism during euglycemic conditions increases in terms of higher neuronal glucose oxidation [70]. The cerebral metabolic rate of glucose correlates with hypoglycemia-induced changes in GLUT levels [71], whereas hypoglycemia in animals previously exposed to RH causes a decrease in total tricarboxylic acid (TCA) cycle flux [70]. Moreover, the extent of glucose metabolism in brain increases after RH along with a concomitant increase in lactate uptake [72].

Cerebral glucose utilization

Normal cerebral glucose utilization varies in different parts of brain, e.g., cerebral glucose consumption in gray matter is more heterogeneous and relatively high as compared to the white matter [73,74]. An acute episode of hypoglycemia decreases cerebral glucose consumption [67,74–77]. The extent of hypoglycemia-induced decrease in glucose utilization is more prominent in areas of brain where normal cerebral glucose utilization is higher [74,76]. Segal et al have reported that glucose uptake and consumption rate were not significantly different in human subjects after 24 h of hypoglycemia [78]. A single episode of hypoglycemia does not produce any significant effect on metabolic fluxes affecting mechanisms of glucose catabolism in the brains of healthy subjects [79]. Continued hypoglycemia for a period of one week decreases cerebral glucose utilization [71]. However, subjects suffering from uncontrolled T1D display an impaired stimulation of hyperinsulinemic hypoglycemia-induced glucose oxidation [80]. Cerebral glucose utilization in animals previously exposed to RH later increases when they are euglycemic, but decreases when they are hypoglycemic [70]. During consequent euglycemia, RH-exposed animals perform better on spatial memory tests but they did worse when exposed to hypoglycemia, likely owing to limited availability of glucose [81].

Glycolysis

Once glucose enters the cells, it serves as the principal substrate of glycolysis [82]. A study by Marín-Hernández et al on HeLa cells exposed to hypoglycemia for 24 hours showed that acute hypoglycemia causes increase in the protein levels of GLUT1, GLUT3 and hexokinase I, as well as glycolytic flux [83]. Prior exposure to recurrent hypoglycemia increases glucose phosphorylation in rat hypothalamus [84]. These observations implicate that hypoglycemia affects delivery of glucose as well as glycolysis. A 48 hour period of fasting decreases glucose metabolism and production of malonyl-coenzyme A in the hypothalamus and cortex [85]. This study implies that fasting, to compensate the limited availability of glucose, causes reprogramming of substrate utilization away from glycolysis and more toward lipid oxidation. A study in cerebellar neuron cultures showed that aglycemia is associated with decreased consumption of glucose, lactate production rate, and lactate to glucose ratio resulting in an increased extent of glucose oxidation by approximately 35% [86]. They also demonstrated that hypoglycemia increased the ratio of TCA cycle to glycolytic fluxes, indicative of enhanced oxidative metabolism when compared to glycolysis. An acute episode of hypoglycemia inhibits glycolysis and glycogenolysis [87]. Severe hypoglycemia leads to a decrease in the levels of glucose 1,6-bisphosphate, fructose 2,6-bisphosphate, and fructose 1,6-bisphosphate, allosteric activators of phosphofructokinase (the rate-limiting enzyme in glycolysis) [88]. Glucose consumption increases during euglycemia in brain previously exposed to RH

TCA cycle

After glycolysis, which takes place in the cytosol, mitochondria play a critical role in mediating ATP generation via the TCA cycle and electron transport chain [82]. TCA cycle flux in healthy subjects at glucose levels of approximately 3 mmol/l was not significantly different from that observed at euglycemic levels [79]. However, hypoglycemia in type 1 diabetic subjects caused increase in TCA cycle flux in comparison to heathy subjects, possibly due to cerebral adaptations to RH [89]. During the substantial decline in brain energy production associated with severe hypoglycemia, a modified form of TCA cycle continues to work which involves aspartate aminotransferase-mediated formation of α-ketoglutarate from oxaloacetate and supports neurons during hypoglycemia-associated energy deprivation [90]. Using a radio-labeled lactate microdialysis study followed by nuclear magnetic resonance analysis of microdialysate, Gallagher et al. showed that lactate may be directly used as a TCA cycle substrate [91]. In previously RH-exposed animals, euglycemia increases glucose utilization and TCA cycle flux, but hypoglycemia causes a decrease in the same [70]. An acute episode of hypoglycemia decreases activities of succinate dehydrogenase and glutamate dehydrogenase [87]. Overall, these studies indicate differential effects of hypoglycemia on TCA cycle during euglycemia and hypoglycemia.

Mitochondrial respiration

Mitochondrial respiration is the principal source of energy in neurons. Electron donors like NADH and FADH₂ produced in the TCA cycle supply electrons to the electron transport chain in mitochondria [92]. This produces a proton gradient across mitochondrial membrane, which drives production of ATP via complex V [92,93]. Insulin-induced acute hypoglycemia decreases the respiratory control ratio (RCR: ratio of state 3 to state 4 respiration) when compared with control groups, indicating hypoglycemia-induced impairment of electron transport chain function in mitochondria [94]. Exposure to chronic moderate hypoglycemia in normal as well as streptozotocin-diabetic rats for a period of one week causes lower state 3 respiration and RCR [95]. Chronic hypoglycemia exacerbates mitochondrial respiratory chain impairments in the hippocampus of an animal model of T1D as assessed in terms of RCR and ADP/O index [96]. Our laboratory has previously shown that RH changes the ratio of mitochondrial respiratory chain complex I subunits, indicating that hippocampal mitochondria are very sensitive to variations in glucose levels in diabetics [97]. Hypoglycemia-induced mitochondrial substrate limitation causes increased mitochondrial free radical production [98]. Therefore, hypoglycemia causes impairment of mitochondrial respiration and results in an increase in free radical production, a pathway having a significant potential to cause brain damage.

Amino acid metabolism

Physiological hyperinsulinemia produces hypoglycemia by causing increased tissue glucose uptake and inhibiting release of newly synthesized glucose into the circulation [99,100]. Leucine oxidation data has implied that insulin inhibits release of glucose from protein stores [101,102]. Hypoglycemia causes an adaptive release of regulatory hormones such as glucagon, epinephrine and cortisol to counter hypoglycemia as described in a previous section of this article [18–20,23–25]. Besides increasing glucose production, glucagon also enhances proteolysis, uptake of glutamine, and oxidation of amino acids [103–108]. Epinephrine and cortisol increases amino acid uptake and proteolysis [109–114]. Hyperinsulinemic hypoglycemia induces proteolysis as assessed in terms of leucine kinetics and oxidation [115]. Acute hypoglycemia among human subjects increases uptake of glutamine, a gluconeogenic amino acid, without affecting protein and leucine kinetics [116]. Sustained moderate hypoglycemia causes a decrease in amino acid levels in blood [117]. During the 12 hr period of moderate progressive hypoglycemia, the level of plasma branched-chain amino acids decreases in the first 6 hours but increases in the later 6-hour period. In contrast, the levels of essential non-branched-chain amino acids continued to decrease at a slower rate [117]. Amino acid supplementation suppresses glucose oxidation in fasted human subjects [118]. An earlier study observed that an episode of acute hypoglycemia caused in streptozotocin (Stz)-diabetic rats increases plasma levels of aspartate and GABA when compared to Stz-diabetic rats that were not subjected to hypoglycemia [119]. The same study also observed increased synaptosomal levels of glutamate and GABA in Stz-diabetic rats exposed to hypoglycemia when compared to Stz-diabetic that were not exposed to hypoglycemia. Evaluation of extracellular (microdialysate) amino acid levels in striatum of hypoglycemia-exposed perinatal rats (P7) observed that the levels of glutamate, aspartate and taurine increased, while levels of glutamine decreased over time during hypoglycemia [120]. Exposure of synaptosomes to hypoglycemia led to lower levels of ATP and increased levels of ADP, and subsequent depletion in synaptosomal membrane potential with increased release of aspartate [121]. This depleted energy status may be responsible for increased cytosolic free Ca²⁺ levels, which in turn, may contribute to brain damage during severe hypoglycemia [122]. Thus, the existing literature shows a differential effect of hypoglycemia on amino acid metabolism in brain. However, future research is required to determine the potential role of amino acid metabolism in hypoglycemia-induced brain dysfunction.

Lipid metabolism

Hypoglycemia causes activation of counterregulatory mechanisms for the attainment of euglycemia [18–20,23–25]. Lipolysis causes generation of glycerol and free fatty acids as a source of energy and substrates for the gluconeogenic processes. Fasting enhances fatty acid and ketone levels in blood [123]. Moderate hypoglycemia for a short duration (23 min) does not stimulate lipolysis [124]. However, moderate hypoglycemia for 4 hours inhibits the effect of insulin-associated decrease in free fatty acid levels [125]. Attenuation of insulin-induced hypoglycemia is associated not only with alterations in glucose kinetics but also with a rebound increase in lipolysis-mediated production of glucose in liver [125,126]. Free fatty acids mediate glucose counter-regulation by modulating hepatic glucose production [126]. Lipolysis participates in catecholamine-mediated acute phase of glucose counter-regulation elicited by hypoglycemia [127]. Lipolysis also mediates delayed glucose counter-regulatory pathways stimulated by growth hormone and cortisol [24,25]. Free fatty acids in the blood mediates the development of post-hypoglycemic insulin resistance [128]. Non-esterified fatty acids production increases with hyperinsulinemic hypoglycemia indicating the potential role of the lipids as an alternate source of energy [129]. Hypoglycemia inhibits uptake of arachidonate into glycerophospholipids of brain membranes [130]. It is possible that this decreased fatty acid uptake may lead to altered membrane function as well as synaptic processes during and after hypoglycemia. However, the extent of the role of lipid metabolism in mediating hypoglycemia-induced metabolic adaptations in brain is unknown and requires critical assessment.

Effect of hypoglycemia on cellular metabolism

Severe hypoglycemia causes a significant decrease in the number of metabolites in brain in terms of the phosphocreatine, ATP, ADP, and AMP concentrations [131]. Starvation for a period of 3–4 days causes decrease in the levels of alanine, glutamate and glutamine as well as increases in the levels of glycine in brain [132]. Insulin-induced moderate to severe hypoglycemia in pregnant mothers increases glutamate and glutamine levels in the fetal brain, and decreases alanine and GABA levels in young adults. Moreover, an NMR study indicates that hypoglycemia increases glucose flux in young adults via the pyruvate carboxylase and pyruvate dehydrogenase pathways [133]. MRS-based metabolomic analysis shows that insulin-induced hypoglycemia causes a decrease in the levels of alanine, β-hydroxybutyrate, lactate, threonine, and valine, and increase in the phenylalanine and Ƭ-methyl histidine concentrations in brain [134]. Repeated and severe hypoglycemia in neonates increases the levels of creatine, glutamate, glutamine, γ-aminobutyric acid, aspartate, succinate, taurine, and myoinositol in the occipital cortex, and levels of N-acetyl aspartate and choline were increased in hippocampus of hypoglycemia-exposed neonates [135]. In addition to the metabolite increases in neonates, levels of lactate, N-acetyl aspartate, alanine, choline, glycine, acetate, and ascorbate are also observed to be higher in the occipital cortex of adolescent animals that were exposed to hypoglycemia during the neonatal period. This study observed brain area-specific effects as patterns of metabolic alteration were different in hippocampus of adolescent animals that were exposed to hypoglycemia during the neonatal period [135]. A recent study investigated the effect of RH on reduced glutathione levels (GSH) in parietal cortex, striatum, and hippocampus [136]. They observed that levels of GSH decreased when measured at 12 and 24 h after last hypoglycemia exposure in all three brain regions studied. Their results suggest overall oxidative alteration of proteins. It is possible that oxidized proteins may affect overall cellular metabolism owing to altered functioning capacities of these proteins. These studies demonstrate a profound effect of hypoglycemia on the metabolome. However, detailed studies identifying the effect of recurrent hypoglycemia on cellular metabolism are thus warranted.

Importance of alternate sources of energy for brain during RH

Although the brain depends on glucose as its main source of energy, it possesses an ability to use other sources of energy such as lactate and ketones; viz., β-hydroxybutyrate and acetoacetate. After being converted into pyruvate or acetyl coenzyme A, these substrates can enter the TCA cycle and contribute toward energy production. Such alternate sources of energy facilitate brain physiology during hypoglycemia. For example, increased lactate is taken up into the brain for respiration but without a concomitant increase in lactate oxidation particularly during hypoglycemia [137,138]. Although glucose is the only metabolic substrate that can rescue brain from hypoglycemia [139], supplementation of alternate fuels like lactate and β-hydroxybutyrate during hypoglycemia can enhance oxidative metabolism, decrease autonomic and neuroglycopenic symptoms and enhance glucose counterregulatory mechanisms [72,140–143]. RH increases neuronal uptake of lactate, which serves as a metabolic regulator that preserves metabolism of glucose in brain during hypoglycemia [72]. Brain acetate concentrations, metabolism and monocarboxylic acid transport increases two-fold in T1D patients in comparison to non-diabetic control subjects [144]. Developing rats (age equivalent to full-term newborn human infants) were able to maintain phosphocreatine/creatine ratio in the physiological range for almost 2.5 hrs during experimentally-induced neuroglycopenia [145]. This study also observed that lactate levels decreased during initial phase of hypoglycemia suggesting that lactate may be utilized for energy production during this phase. Results also indicated that glutamate and glutamine were major energy substrates in the subsequent phase of hypoglycemia and once those energy substrates were depleted, aspartate was used as the final energy source. Their results indicate that brain relies on various sources of energy during different phases of hypoglycemia. The use of glutamate and glutamine as a source of energy substrates is further implicated by an earlier study [146]. Using immunogold staining in hippocampus and striatum, they observed that levels of both glutamate and glutamine decreased in most tissue compartments following hypoglycemia. In vitro study observed that isolated synaptosomes were able to maintain high ATP/ADP ratios when exposed to hypoglycemia (glucose-free media) further supporting the role of alternate fuels in maintaining energy status during hypoglycemia [147]. An earlier study observed reduced cerebral arteriovenous difference for glucose during the period of hypoglycemia, further supporting a view that brain relies on alternate energy substrates during hypoglycemia [148].

Ketone bodies

Ketone bodies like acetoacetate, 3-β-hydroxybutyrate and acetone are products of fatty acid metabolism, which serve as a source of energy during conditions of nutrient deprivation as reviewed previously [149,150]. Prolonged fasting increases the level of ketone bodies in blood [151]. Ketone metabolism in brain is dependent on its blood levels and blood-brain barrier permeability [152–154]. Ketone bodies are metabolized into acetyl-CoA which feeds the TCA cycle to meet the increased metabolic demand in the brain [155,156]. β-hydroxybutyric acid contributes toward the neuronal synthesis of glutamate and glutamine [157]. Although β-hydroxybutyric acid facilitates ATP synthesis in brain, it cannot replace glucose as a respiratory substrate [142,158–160]. Lipid administration normalizes hypoglycemia-induced changes in cognitive function of brain and glucose counterregulation [142,161,162]. Ketone bodies decrease oxidative stress, energy deficit, excitotoxicity and neuronal death associated with glycolysis inhibition and hypoglycemia [163–167]. Moreover, a ketogenic diet ameliorates hypoglycemia-induced neuronal death in rats [168]. Therefore, current data implicates that ketone bodies may serve as an alternate metabolic fuel during hypoglycemia. In comparison to other metabolites, understanding the time course and extent of hypoglycemia-induced increase in ketone bodies and their metabolism can provide valuable insights.

Glycogen

Glycogen is a reserve respiratory substrate for brain during metabolic stress conditions like hypoglycemia. Although the basal level of glycogen in brain is modest, its exceptionally slow respiratory consumption [169,170] causes its increased metabolism during brain activation [171–174]. Glycogen is stored in cytosol of astrocytes in the form of electron-dense isodiametric (10–30 nm) β-particles adjacent to the enzymes needed for its synthesis and degradation, namely glycogen synthase and glycogen phosphorylase, respectively [175–177]. During hypoglycemia, stored glycogen is mobilized and acted upon by glycogen phosphorylase in astrocytes and is converted into glucose-1-phosphate and glucose-6-phosphate to eventually feed glycolysis [178,179]. Metabolism of astrocytic glycogen into lactate fuels brain cells during glucose deprivation and increased metabolic demand [171,180–182]. There is a correlation between glucose deprivation-induced decrease in glycogen levels and delay in the loss of action potential of neurons [181,182]. Inhibition of glycogen phosphorylase-induced increase in astrocytic glycogen levels preserves normal neuronal activity during hypoglycemia and decreases associated neuronal death [183]. Acute moderate hypoglycemia enhances glycogen utilization to produce glucose [178]. During restoration of euglycemia after hypoglycemia, glycogen levels in brain go well above the pre-hypoglycemia levels within few hours indicating the presence of ‘glycogen supercompensation’ [178]. Duarte and colleagues evaluated the effect of post-hypoglycemia glucose levels on hypoglycemia-induced glycogen supercompensation [184]. They observed that brain glycogen concentrations remained elevated in studied brain areas (cortex, hippocampus, and striatum) when measured at 24 h post-hypoglycemia. They also observed that post-hypoglycemia, glucose (normoglycemia vs hyperglycemia) levels did not affect the extent of post-hypoglycemia glycogen supercompensation. During RH, glycogen supercompensation is blunted and does not mediate hypoglycemia unawareness and HAAF [185,186]. Further studies are required to understand the role of glycogen as an alternative fuel for brain during hypoglycemia.

Other alternate sources of energy

During hypoglycemia, pyruvate treatment produces neuroprotection in brain [187,188]. Pyruvate treatment of hippocampal slices before hypoglycemia improves neuronal function [189]. Pyruvate administration attenuates RH-induced brain damage in diabetic rats by circumventing sustained impairment of glycolysis induced via activation of PARP-1 [190]. Hypoglycemia increases monocarboxylate transport and acetate uptake in diabetic brains [144]. Lactate may be directly used as a TCA cycle substrate [91]. While RH decreases glucose metabolism during an eventual episode of hypoglycemia, acetate metabolism remains unchanged [70]. During hypoglycemia, an increase in aspartate, and decrease in the glutamine and glutamate levels is associated with a low energy status of brain [90,191]. Glutamine [86] serves as an alternate source of energy for neurons during recovery from hypoglycemia. The pyruvate recycling pathway is involved in maintaining respiration during hypoglycemia [86]. Typically, pyruvate recycling involves glutamate or glutamine oxidation in the TCA cycle by facilitating the outward movement of a four-carbon unit from the malate or oxaloacetate steps and reentry at the pyruvate step of the cycle [192,193]. Glutamate/glutamine oxidation via pyruvate recycling maintains energy supply for homeostasis during sustained hypoglycemia in developing rat hippocampus [145], while a 12-hour exposure of cultured cerebellar neurons to aglycemia decreases glucose uptake and glycolysis and increases glutamine uptake [86]. Hypoglycemia enhances fluxes involved in the pyruvate recycling pathway; viz., pyruvate-Acetyl-CoA flux, α-ketoglutarate-succinyl-CoA flux, glutamine-glutamate flux and malate pyruvate flux. However, the sites of these biochemical changes are debated between neurons and astrocytes and are yet to be determined [192–198]. Glutamate oxaloacetate transaminase mediated metabolism of experimentally increased extracellular glutamate levels takes place through the truncated TCA cycle under hypoglycemic conditions [199].

Research implicating the therapeutic potential of modulating metabolic changes in brain during RH

Considering the literature cited above, it is evident that RH induces multiple changes in brain metabolism. Modulation of some of these changes may have a potential beneficial effect on the hypoglycemic brain. An intraperitoneal dose of sodium L-lactate prevents hypoglycemia-induced neuronal death by serving as an alternate source of energy in the brain during hypoglycemia. This effect is due to lactate-induced correction of PARP-1 activation-linked decrease in NAD(+) levels, causing direct activation of the TCA cycle and maintenance of cellular ATP levels [200]. Pyruvate treatment along with glucose administration post-hypoglycemia protects neurons from oxidative injury, microglial activation and loss of endogenous antioxidant system in the cerebral cortex after RH [190]. Administration of both a ketogenic diet and β-hydroxybutyrate produces a beneficial effect on neuronal death associated with hypoglycemia [164,165,168]. Moreover, medium chain fatty acid supplementation improves cognitive deficit in treated T1D patients undergoing hypoglycemia [142]. Therefore, it may be deduced that modulation of metabolic changes in hypoglycemic brain possesses therapeutic potential, provided it is augmented by future in-depth mechanistic studies.

Summary and Future Directions

Therefore, as discussed above, RH exposure induces adaptive changes in glucose counter-regulatory mechanisms, uptake, utilization, cellular respiration, amino acid metabolism, and lipid metabolism. Further, we summarized literature on RH-induced adaptive changes in brain involving utilization of alternate sources of energy. Additional information about changes in activity of key respiratory enzymes and determination of the extent of dependence of hypoglycemic brain on alternate sources of energy during diabetes can contribute towards attainment of the clinical potential of modulating such changes in brain. Therefore, studies on the effect produced by mild, moderate and severe hypoglycemia on the individual metabolic pathways, their influence on overall brain energetics and functioning will help us understand the metabolomic changes in brain. This may help in tailoring new clinical approaches to treat RH-induced dysfunction of brain seen during diabetes. Further, studying the crosstalk between different metabolic pathways and their overall influence on temporal changes in brain function may help to identify key pathways which are involved in the pathophysiology of diabetes-induced brain dysfunction. Studying the time course of changes in brain metabolism with the progression of diabetes may also help us identify the therapeutic windows of potential treatment strategies. Diabetic patients are suffering from other comorbid conditions like aging, hyperlipidemia and hypertension [201–204]. These comorbidities produce their independent effects on brain metabolism as well [205–207]. Therefore, it is worthwhile to study the extent and time course of the cumulative metabolic effect of the concomitant presence of hypoglycemia during diabetes with these comorbid conditions. Moreover, prior to studying the beneficial effect of modulating the metabolic changes in brain induced by RH, it is also important to test the effect of such interventions on brain functioning per se, hypoglycemia unawareness, and the pathogenesis of RH. Overall, such analysis may help us continue to study the clinical potential of modulating the metabolic changes induced by hypoglycemia in brain.

Conclusions

Considering the profound effect of hypoglycemia on brain in diabetes and that not much information is available on its mechanism, studying hypoglycemia-induced adaptive changes in brain metabolism promises to have important pathological ramifications. Even in the presence of literature produced by exhaustive research work carried out by several research groups world-wide, some basic information on the effect of hypoglycemia on metabolic pathways is still missing. Given the epidemiological significance of hypoglycemia in diabetics, continued studies on metabolomic changes in brain elicited by RH are expected to identify novel therapeutic principles for hypoglycemia-induced detrimental effects on brains of diabetic subjects. Additionally, research integrating the qualitative and quantitative differences between metabolic effects produced by hypoglycemia on brain might help us further appreciate the overall influence of diabetes on brain function.