The Brain Hypometabolism Hypothesis – Overview of Parts 1-120

glucose_2

This post summarises the previous posts on the Brain Hypometabolism Hypothesis. The post summarises the hypothesis and then looks at physiological principles relating to cerebral glucose metabolism and how this is impacted with age. Broadly speaking there are four main sections

  1. An examination of the relationship between Diabetes and neurodegeneration
  2. An examination of the GLUT receptors
  3. Linking Oxidative Phosphorylation with Hypoxic Ischaemic Brain Injury (HIBI)
  4. An examination of the consequences of Hypoxic Ischaemic Brain Injury
  5. An examination of the key metabolites in energy metabolism pathways

Brain Hypometabolism Hypothesis

The Brain Hypometabolism Hypothesis focuses on energy metabolism. More specifically the hypothesis states that

Energy hypometabolism in the brain leads to neuropathology

What is the value for Brain Glucose Metabolism?

One of the key concepts in understanding brain glucose metabolism is the cerebral metabolic rate of glucose. This was calculated in one paper by comparing the glucose content of cerebral arterial and cerebral venous blood. When this difference is calculated it can be combined with the cerebral blood flow values to estimate the rate of glucose metabolism by the brain.

The value given in the article is (see Appendix A for calculations)

6–7 mg/100 g/min

or approximately

31 μmol/100 g/min

Is there a Link Between Alzheimer’s Disease and Brain Glucose Metabolism?

In their 2016 paper, Cunnane and colleagues outline several supporting lines of evidence as the basis for their paper

(i) Lower glucose uptake in the frontal cortex of older adults

(ii) Regional deficits in brain glucose uptake in younger adults at risk of Alzheimer’s Type Dementia

(iii) Preservation of ketone uptake in the brain

(iv) Evidence from ketone based studies.

Is there Lower Glucose Uptake in the Frontal Cortex of Older Adults?

Cannane and colleagues published a paper on brain metabolism and aging based on a database they had compiled using radioactive tracers for glucose and ketone metabolism in the brain.

fnmol-09-00053-g002

Figure 2 from Cunnane and colleagues, CC BY

Is Insulin Resistance a Risk Factor?

The Rotterdam study looked at insulin resistance in a large prospective cohort. Researchers found a correlation between insulin resistance and incident Alzheimer’s Type Dementia but only within the first three years.

However we know from Kuhn’s work on ‘The Structure of Scientific Revolutions‘ that central paradigms are successful even if there are lines of evidence that do not support the paradigm (as models are approximations to reality).

https://youtu.be/wdG5z0Xj-Hg

A Review of ‘The Structure of Scientific Revolutions’

Is Diabetes a Risk Factor for Dementia?

Gudala and colleagues published a meta-analysis of prospective observational studies in 2013 looking at the relationship between Diabetes and the risk of Dementia. The researchers screened 67,083 papers and identified 28 papers for inclusion. The papers included are found in Table 1 and include the Rotterdam study.

Demographic Chart

The meta-analysis included 1,148,041 patients and of these 89,708 had a diagnosis of Diabetes (although I couldn’t see a distinction between NIDDM and IDDM).

In people with Diabetes the relative risk of Dementia was:-

(1) Alzheimer’s Type Dementia: Pooled RR 1.56 (95% CI 1.41–1.73)

(2) Vascular Dementia: Pooled RR 2.27 (95% CI 1.94–2.66)

The researchers suggested several mechanisms that may lead to the increased risk including advanced glycation end-products.

Is Insulin Dependent Diabetes a Risk Factor for Dementia?

The question of whether Insulin Dependent Diabetes Mellitus (IDDM) is associated with an increased risk of Dementia was asked in one study that was presented at the Alzheimer’s Association International Conference in 2015. There are two sources for the presentation here and here.

There were 490,344 persons aged >= 60 years and followed up over 12 years. During this time 16% of people with IDDM developed Dementia compared to 12% of those without IDDM. However after adjustment for other risk factors the hazards ratio for Dementia with IDDM compared to Dementia without IDDM was 1.83 (95% confidence interval [CI], 1.3 – 2.5).

Is There a Diabetes Type 3?

There is a concept of a Type 3 Diabetes where the pathology occurs in the brain and is described in this paper by Dr Suzanne de la Monte.

To the best of my knowledge this subject has been on the periphery of research into Alzheimer’s Disease. The concept of Diabetes Type 3 is not widely accepted and looking at the ICD-11 Browser Beta (not final) version I couldn’t find any reference to this diagnosis as a specific category.

At this stage, this looks to be an emerging discussion but I am not clear on whether it will become an established diagnosis. It is still useful to know about this concept because even if it does not become established it involves various models of glucose metabolism in the brain.

The concept appears to date back to 2005 and from the paper above, the 2 initial papers are here and here. I will refer to this from here on in as the Type 3 Diabetes model (T3DM).

The key molecules in the T3DM are Insulin and Insulin-like Growth Factors. In the 2014 paper Dr de la Monte outlines the various important functions that Insulin has in the brain ranging from anti-apoptosis through to growth, plasticity and metabolism. Although the model is more nuanced I will summarise this as

Insulin and Insulin-like Growth Factor Dysfunction Leads to Neuropathology

The dysfunction is not clear but reframing this we can say that in this model the actions of Insulin and Insulin-like Growth Factor are not as expected.

The video above is from the NDSU Virtual Cell Animations collection and illustrates the intracellular mechanism of action of Insulin.

Dr de la Monte refers to the effects of glucose uptake and underutilisation in the brain as three-fold

(a) Oxidative stress

(b) Impaired homeostasis

(c) Cell death

The paper elaborates on the GLUT4 receptors:-

(1) Insulin regulates both the expression of the receptors and the transfer of the receptors to the cell membrane

(2) GLUT4 receptors are expressed in the medial temporal lobe

(3) The GLUT4 receptors are not reduced in Alzheimer’s Type Dementia

(4) There may possibly be a reduced transfer of GLUT4 to the cell membrane

Does atrophy in a brain region account for brain hypometabolism?

Cunnane and colleagues answer this question in two ways.

Firstly there is a specific research methodology that is able to answer this question – correction for atrophy. In their 2011 paper, Cunnane and colleagues review the literature on brain glucose metabolism studies in Alzheimer’s Type Dementia and summarise the results in Table 1 in the paper.

They note that in some studies, there is correction for atrophy and in those studies there is still a preservation of the relationship between Alzheimer’s Type Dementia and brain glucose hypometabolism.

Secondly Cunnane and colleagues also answer this question by referencing risk factors for Alzheimer’s Type Dementia where brain glucose hypometabolism occurs even in the absence of cognitive impairment

  • Pre-senilin-1 mutation,
  • Apolipoprotein E4 carrier status
  • Matrilinear AD
  • Cognitively healthy aging
  • Insulin resistance

There are five lines of evidence for further investigation of the question above. However the relationship cited above looks at cognitive impairment. Cognitive impairment is a proxy marker. The key question is whether there is brain atrophy which is causing the brain hypometabolism.

So the question can be asked for each of the five risk factors identified above.

What is the Mechanism for Glucose Uptake in the Brain?

In their 2011 paper, Cunnane and colleagues note that there are three separate isoforms of GLUT1 that facilitate glucose uptake in the brain

(1) One isoform facilitates glucose uptake across the blood-brain barrier

(2) Another isoform facilitates glucose uptake in astrocytes

(3) A third isoform facilitates glucose uptake in neurons

There are however other mechanisms for glucose uptake in the brain.

What are the GLUT’s?

GLUT’s are short for Glucose Transporters. They constitute a set of molecules which transport substrates across cell membranes. They play a central role in Glucose transport and hence the name. However their role is not limited to the transport of Glucose.

Professors Bernard Thorens and Mike Mueckler have written a review titled ‘Glucose Transporters in the 21st Century’. In terms of a Brain Hypometabolism Hypothesis, it is important to understand how Glucose is handled in the brain. Thorens and Mueckler reference 14 Glucose Transporters but not all of them are expressed in the brain.

In the article they note that the glucose transporters (members of the GLUT family) are part of the Major Facilitator Superfamily of Membrane Transporters (e.g. see this paper).

This superfamily of membrane transporters is responsible for transporting a large variety of compounds across the cell membrane and along an electrochemical gradient.

In their paper they also note that Glucose not just a source of energy but is also a signalling molecule. Thus they state that Glucose influences various processes including

(a) The activity of neurons that regulate Glucose (e.g. see this paper)

(b) Gene transcription

(c) Enzyme activity

In this paper it is noted that the GLUT family are divided into three classes.

Class I – GLUT1, GLUT2, GLUT3, GLUT4, GLUT14

Class II – GLUT5, GLUT7, GLUT9, GLUT11. Fructose is a subtrate for Class II GLUT’s.

Class III – GLUT6, GLUT8, GLUT10, GLUT12, GLUT13

This paper goes into some of the more subtle nuances of the GLUT’s.

GLUT 1

Addressing GLUT1, Thorens and Mueckler refer to this as the most intensively studied of this family.

GLUT1 is expressed in the endothelial cells that constitute the blood brain barrier. There is an emerging discussion about how GLUT1 specifically might be involved in neurodegenerative conditions such as Alzheimer’s Type Dementia (e.g. see this paper).

GLUT 2

In the paper they note that there is evidence that GLUT 2 plays a role in glucose ‘sensors’ in the central nervous system and the periphery.

There are a number of papers on GLUT2 and the brain including this one on a possible drug interaction with GLUT2 and this one on Ghrelin and the Hypothalamus.

GLUT 3

In their paper, Thorens and Mueckler note that GLUT 3 is the main glucose transporter in the brain but also plays an important role in embryogenesis.

GLUT 4

In their paper, Thorens and Mueckler note that GLUT 4 is one of the most well studied Glucose transporters and is linked to Glucose homeostasis throughout the body. There are various studies that have examined GLUT 4 in the brain including this one which looks at synaptic activity.

GLUT 5

In their paper, Thorens and Mueckler note that GLUT 5 specialised for Fructose transport and expressed predominantly in the intestine although also in other tissues including the brain.

Does the brain metabolise fructose? There is evidence presented in this paper.

GLUT 6

In their paper, Thorens and Mueckler note that there is little data on GLUT 6.

GLUT 7

In their paper, Thorens and Mueckler note that there is little data on GLUT 7 other than to say it is similar in structure to GLUT 5 and is not very effective for either Glucose or Fructose as a transporter. Therefore Thorens and Mueckler suggest it is effective for another substrate. The suggestion of another substrate is reiterated in this paper.

GLUT 8

In their paper, Thorens and Mueckler note that GLUT8 has been linked to neuronal proliferation in the Hippocampus. This paper implicates GLUT8 in both Hippocampal neuroproliferation and also an increase in the atrial p-wave duration.

GLUT 9

In their paper, Thorens and Mueckler discuss evidence to suggest that GLUT 9 is a urate transporter and is implicated in hyperuricaemia which can lead to Gout and is also seen in Lesch-Nyhan syndrome.

In terms of a hypothesis about metabolism this may not be relevant. Urate is an end-product of Purine metabolism and is usually excreted. Possibly the only significance here is that there is a conserved mechanism for molecular transport that is shared by metabolites such as Glucose. Nevertheless there is this paper which suggests that Urate may be neuroprotective (although high levels can also cause neuronal damage). There are also lines of evidence suggesting a possible interaction between Insulin and Urate.

As with other substrates the body has elegant mechanisms for homeostasis.

GLUT 10

Mutations in GLUT 10 are linked to Arterial Tortuosity Syndrome. A more recent literature search reveals a number of other papers in relation to ATS as well as a study looking at peripheral vascular disease in Diabetes. At the time of writing, the NIH Gene Database describes the gene as playing a role in glucose homeostasis.

GLUT 11

Thorens and Mueckler note in their 2009 paper that not much is known about GLUT 11 other than an affinity for both Glucose and Fructose. A 2017 medline search using the search term “GLUT11” retrieved only 14 results and for “SLC2A11” also. Few of these papers post-dated the 2009 review and there was little discussion about the brain. This does not discount the possibility of a significant role in brain metabolism but it looks as though this is not an area of active research.

GLUT 12

This paper suggests properties of the GLUT12 transporter that differ from other members of the GLUT family and hint at an as yet undiscovered substrate. The authors note that previously it has been proposed as an ancestor of GLUT4 with ancillary function. They identify complex aspects of its action and also transport to the plasma membrane. They suggest that GLUT12 is involved in Glucose homeostasis throughout the body.

GLUT 13

In their paper, Professors Bernard Thorens and Mike Mueckler note that GLUT 13 is

  1. A myoinositol transporter
  2. A symporter
  3. Expressed mainly in the brain

As of April 2017 there is no solved structure for GLUT 13 according to PhosphoSite Plus.

GLUT 13 is described as a symporter

What is a Symporter?

A symporter is a membrane protein that transports more than one substance across the membrane in the same direction. This contrasts with the antiporter (which transports substances in opposite directions) and a uniporter which transports a single substance.

Inositol_structure

Myo-Inositol by Edgar181

What is Myo-Inositol?

Myo-Inositol is a molecule derived from Glucose-6-phosphate. Inositol is a substrate of GLUT 13 (one of the GLUT’s). Inositol plays a role in energy metabolism and is relevant to a discussion of the Brain Hypometabolism Hypothesis.

GLUT 14

Professors Bernard Thorens and Mike Mueckler have written a review titled ‘Glucose Transporters in the 21st Century’. In terms of a Brain Hypometabolism Hypothesis, it is important to understand how Glucose is handled in the brain. Thorens and Mueckler reference 14 Glucose Transporters but not all of them are expressed in the brain.

In their paper, Professors Bernard Thorens and Mike Mueckler note that GLUT 14 is not primarily expressed in the brain and also that the role of GLUT 14 in glucose metabolism is not characterised.

The Context of Hypoxic Ischaemic Brain Injury

Sekhon, Ainslie and Griesdale have written an open access article on hypoxic ischaemic brain injury titled “Clinical Pathophysiology of Hypoxic Ischemic Brain Injury after Cardiac Arrest:A “two-hit” Model“. This paper can be used as a starting point for discussion of the events that lead to brain injury following hypoxia. This in turn is relevant to the question of energy usage in the Brain Hypometabolism Hypothesis.

Sekhon, Ainslie and Griesdale posit a simple two stage model of brain injury following cardiac arrest in which injury results from

  1. Primary cerebral hypoxia
  2. Secondary mechanisms after return of cerebral perfusion

In Sekhon, Ainslie and Griesdale’s model they discuss primary and secondary brain injury following a cardiac arrest.

Primary Brain Injury after Hypoxia

Looking more closely at the primary brain injury they state that with a reduction in cerebral oxygen ATP production decreases and there is a switch to anaerobic respiration. This in turn leads to a reduction in ATP dependent ion channel action. There are three main effects

  1. Accumulation of Na+ ions
  2. Accumulation of lactate with acidosis
  3. An influx of Calcium ions into the cells

Secondary Brain Injury after Hypoxia

Sekhon, Ainslie and Griesdale identify 7 factors associated with secondary brain injury after hypoxia in their two stage model. These 7 factors are

  1. Microvascular Dysfunction
  2. Cerebral Oedema
  3. Anaemia
  4. Impaired Cerebral Autoregulation
  5. Carbon Dioxide
  6. Hyperoxia
  7. Hyperthermia

Human_Metabolism_-_Pathways

Human Metabolism by Frozen Man (CC BY 4.0)

What is Metabolism?

Metabolism can be defined as the chemical processes that occur in living organisms. There are three types of metabolic processes

(a) Generation of energy

(b) Generation of basic chemicals including fatty acids, amino acids and sugars

(c) Elimination of Nitrogen waste products

Glycolysis

Glycolysis

Glycolysis by Dr Thomas Shafee (CC BY 4.0)

Glycolysis is one of the key pathways for energy metabolism in the human body. In this metabolic pathway the molecule Glucose is converted into Pyruvate. This pathway generates energy in the form of ATP. This pathway however does not use oxygen although the products generated are metabolised using oxygen. This is relevant to the bigger picture of energy metabolism in the brain.

1024px-Acetyl-CoA-3D-vdW

Acetyl CoA Space Filling Molecule by Benjah-bmm27 (Public Domain)

Acetyl Coenzyme A is an important molecule for many pathways involved in energy metabolism. Acetyl Coenzyme A is derived from

(a) Glucose via the Glycolysis pathway

(b) Amino acids via Acetoacetyl-CoA, Pyruvate and directly through multiple pathways

(c) Fatty acids via Beta-oxidation

Vitamin B5 is required for the synthesis of Acetyl CoA.

The Citric Acid Cycle

The Citric Acid Cycle (CC BY 3.0) by Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins, Wadester16

The Citric Acid Cycle is one of the main energy metabolism pathways in humans. Acetyl Co-A which is generated from other pathways is utilised in the Citric Acid Cycle. The Citric Acid Cycle has a number of properties

  1. Generation of energy in the form of ATP
  2. Generating NADH which is utilised in oxidative phosphorylation
  3. Citric Acid is regenerated
  4. Carbon Dioxide is produced

The Citric Acid Cycle takes place in the Mitochondria.

The Citric Acid Cycle is important for the discussion of the Brain Hypometabolism Hypothesis where we have already discussed the metabolism of Glucose.

What is Oxidative Phosphorylation?

Oxidative phosphorylation is a series of chemical reactions in which electrons are transferred, nutrients are metabolised and ATP is formed. Nutrients are oxidised and the donated electrons are processed in the electron transport chain. ATP formation via ATP Synthase utilises the electron/proton gradient across the mitochondrial membrane according to the Chemiosmotic Theory.

What is the Chemiosmotic Theory?

The Chemiosmotic Theory is central to the understanding of Oxidative Phosphorylation. Proposed by Dr Peter Mitchell in 1961, the theory states that the energy for ATP generation derives from electrical and chemical gradients resulting from the transfer of electrons and protons across the mitochondrial membrane in the electron transport chain.

1280px-Adenosine-diphosphate-3D-balls

Ball and Stick Model of ADP by Jynto (Public Domain)

What is ADP?

Adenosine Diphosphate (ADP) is a precursor of ATP. ATP is synthesised from ADP and inorganic Phosphate by the enzyme ATP Synthase. ADP contains Adenine and Ribose both of which are also found in RNA.

What is ATP Synthase?

Atp_exp.qutemol-ball

ATP Synthase by ALoopingIcon using QuteMol (CC BY 2.5)

ATP Synthase is an enzyme that combines inorganic phosphate and Adenosine Diphosphate to form Adenosine Triphosphate (ATP). This in turn is used as a source of energy.

What is Complex I?

Complex1

Complex I by Tim Vickers (Public Domain)

The first step in Oxidative Phosphorylation in humans is the transfer of electrons from NAD via Complex I. The structure of Complex I is shown above. Complex I is also known as NADH-coenzyme Q Oxidoreductase. NADH donates electrons to Complex I in a reaction requiring Coenzyme Q10. The electrons are further transferred via Flavin Mononucleotide and Iron-Sulfur Complexes before the transfer of proteins into the intermembrane space.

What is NAD+?

NAD+-from-xtal-2003-3D-balls

NAD+ by Ben Miller (Public Domain)

Nicotinamide Adenine Dinucleotide (NAD) has a number of properties

  1. NAD exists in a reduced (NADH) and oxidised (NAD+) form
  2. NAD is a key molecule in oxidative phosphorylation
  3. NAD is formed by two nucleotides

What is Complex II?

Complex_II

Complex II by FVasconcellos and TimVickers (Public Domain)

Complex II is involved in Oxidative Phosphorylation and is also known as Succinate Dehydrogenase. Succinate is oxidised (donating electrons) to form Fumarate. The donated electrons enter the electron transport chain.

Complex_III_reaction

Complex III by FVasconcellos and TimVickers (Public Domain)

What is Complex III?

Complex III is also known as Q-cytochrome C Oxidoreductase. Complex III contains Cytochromes. Ubiquinol (a reduced form of Coenzyme Q10) donates electrons to Cytochrome C. Electrons are transferred between molecules in a circuit which causes four protons to be transferred across the Mitochondrial membrane for every 2 electrons. This forms part of the electron transport chain.

ATP Synthase is an enzyme that combines inorganic phosphate and Adenosine Diphosphate to form Adenosine Triphosphate (ATP). This in turn is used as a source of energy.

800px-Complex_IV

Complex IV by FVasconcellos and TimVickers (Public Domain)

What is Complex IV?

Complex IV is also known as Cytochrome C Oxidase. Complex IV contains Heme groups, Copper, Magnesium and Zinc. Complex IV facilitates the transfer of electrons to Oxygen in a reaction which results in the formation of water.

Linking Oxidative Phosphorylation with Hypoxic Ischaemic Brain Injury

Linking Oxidative Phosphorylation to Hypoxic Ischaemic Brain Injury is an important strand in the Brain Hypometabolism Hypothesis.

Hypoxic Ischaemic Brain Injury occurs when there is an interruption of cerebral blood flow leading to a reduction or cessation of delivery of Oxygen to the cells in the Brain. The term denotes the combination of hypoxia and ischaemia both of which can contribute to pathology.

However one clear pathological mechanism is the cessation or reduction in Oxygen supply. In the energy metabolic pathways various nutrients are involved in the production of Acetyl CoA which is incorporated into the Citric Acid Cycle in which NADH is produced. This is utilised in Oxidative Phosphorylation where Oxygen acts as an electron acceptor.

Oxidative Phosphorylation is a key energy metabolism pathway. Since Oxidative Phosphorylation generates a relatively large amount of ATP, the reduction or cessation of the Oxygen supply interrupts this important supply of energy. This in turn interrupts the supply of energy to the ATP dependent ion channels resulting in membrane depolarisation and an influx of Calcium ions.

The Context of Hypoxic Ischaemic Brain Injury

Sekhon, Ainslie and Griesdale have written an open access article on hypoxic ischaemic brain injury titled “Clinical Pathophysiology of Hypoxic Ischemic Brain Injury after Cardiac Arrest:A “two-hit” Model“. This paper can be used as a starting point for discussion of the events that lead to brain injury following hypoxia. This in turn is relevant to the question of energy usage in the Brain Hypometabolism Hypothesis.

Sekhon, Ainslie and Griesdale posit a simple two stage model of brain injury following cardiac arrest in which injury results from

  1. Primary cerebral hypoxia
  2. Secondary mechanisms after return of cerebral perfusion

In Sekhon, Ainslie and Griesdale’s model they discuss primary and secondary brain injury following a cardiac arrest.

Primary Brain Injury after Hypoxia

Looking more closely at the primary brain injury they state that with a reduction in cerebral oxygen ATP production decreases and there is a switch to anaerobic respiration. This in turn leads to a reduction in ATP dependent ion channel action. There are three main effects

  1. Accumulation of Na+ ions
  2. Accumulation of lactate with acidosis
  3. An influx of Calcium ions into the cells

Secondary Brain Injury after Hypoxia

Sekhon, Ainslie and Griesdale identify 7 factors associated with secondary brain injury after hypoxia in their two stage model. These 7 factors are

  1. Microvascular Dysfunction
  2. Cerebral Oedema
  3. Anaemia
  4. Impaired Cerebral Autoregulation
  5. Carbon Dioxide
  6. Hyperoxia
  7. Hyperthermia

Activated_NMDAR

NMDA Receptor by RicHard-59 (CC BY SA 3.0)

The NMDA Receptor is an ion channel which is activated by Glutamate. The NMDA Receptor is involved in the response of neurons to ischaemia. The NMDA Receptor is also involved in memory formation in a process referred to as Long Term Potentiation.

There is an interesting paper by Lai, Zhang and Wang on excitotoxicity and stroke. The authors look at the targets for neuroprotection following a stroke. Excitotoxicity is related to the NMDA Receptor. This in turn is relevant to the Brain Hypometabolism Hypothesis.

Monosodium_glutamate_crystals

Crystalline Monosodium Glutamate by Ragesoss (CC BY 3.0)

Lai, Zhang and Wang outline the role of Monosodium Glutamate in their paper. Monosodium Glutamate is a food additive. In a paper published in 1957, Lucas and Newhhouse reported the results of their study looking at the effects of Monosodium Glutamate (MSG) when applied to the mouse retina. The researchers found evidence of neurotoxicity. Further findings confirmed that Glutamate (MSG dissociates into Sodium and Glutamate in solution) is excitatory (i.e. increases the chance of a neuron firing).

In their paper Lai, Zhang and Wang reference a classic study by Olney and colleagues (Olney et al, 1974). This study involved Kainic acid which is an analogue of Glutamate. This paper generated the hypothesis that

‘excitotoxicity is “in essence, an exaggeration of the excitatory effect”’

Professor John Olney and colleagues conducted research in 1971 into the excitotoxic effects of Glutamate analogues (Olney et al, 1971). They found that analogues which were excitatory were excitotoxic and those which were not excitatory were not excitotoxic. Contextualising this – the effects of Glutamate are mediated via the NMDA Receptor and excitotoxicity is a pathological process which can result from reduced energy metabolism. This in turn is relevant to the Brain Hypometabolism Hypothesis.

In their paper Lai, Zhang and Wang write that although excitotoxicity was initially investigated in relation to the properties of Monosodium Glutamate it plays an important role in brain trauma and a number of neurodegenerative conditions including

  • Huntington’s Disease
  • Alzheimer’s Disease
  • Amyotrophic Lateral Sclerosis

In their paper Lai, Zhang and Wang cite research by Berdichevsky and colleagues (Berdichevsky et al, 1983) identifying the N-Methyl-DL-Aspartate receptor as the most potent inducer of excitoxocity and facilitator of Calcium influx. This leads to the following hypothesis.

The N-Methyl-DL-Aspartate receptor (NMDA Receptor) is a potent inducer of excitotoxicity and Calcium influx. The NMDA Receptor mediates Glutamate induced excitotoxicity.

Lai, Zhang and Wang discuss the regulation of Calcium. They refer to the Sodium-Calcium Exchanger as an important regulatory of Calcium influx into the neuron. This is turn is relevant to Excitotoxicity. NCX removes Calcium from the cell in exchange for Sodium entering the cell. The authors state cite evidence that shows NMDA receptor mediated dysfunction in NCX and that Glutamate induced Calcium influx is associated with a reversal of the action of NCX.

Lai, Zhang and Wang discuss the regulation of Calcium by the Sodium Calcium Exchanger (NCX). They reference a study (Bano et al, 2005) in which it was demonstrated that substituting an NCX isoform that was not influenced by the NMDA receptor avoided subsequent excitotoxicity. This reinforced the importance of the NMDA receptor for excitoxicity.

Lai, Zhang and Wang provide evidence that the Mitochondria play a key role in Calcium homeostasis.

Calcium plays a role in NMDA Receptor mediated excitotoxicity. This in turn is relevant to the Brain Hypometabolism Hypothesis as hypometabolism can lead to excitotoxicity.

Lai, Zhang and Wang provide evidence that the Mitochondria produce reactive Oxygen species during Glutamate induced excitotoxicity (Castilho et al, 1999). This can be summarised thus

Glutamate induces Calcium influx in excitoxicity.

The Mitochondria incorporate Calcium ions resulting from the Calcium influx.

The Mitochondria produce reactive Oxygen species

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. They cite research evidence that the Calcium influx leads to the permeability transition pore opening and a subsequent depolarisation of the Mitochondrial membrane.

Restating

Glutamate induced excitoxicity leads to Calcium influx into the neuron.

This leads to Calcium influx into the Mitochondria.

This leads to opening of the permeability transition pore.

This leads to depolarisation of the Mitochondrial membrane’

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. They cite research by (Stout et al, 1998) providing evidence that the Mitochondria mediates neuronal cell death.

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. They cite research which identifies the Mitochondrial Calcium Uniporter as necessary for excitotoxic cell death. The sequence of events can be summarised as

Mitochondrial Calcium Uniporter imports Calcium into the Mitochondria in response to Glutamate.

This leads to Mitochondrial depolarisation.

This leads to Excitotoxic cell death

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. They cite research (Qiu et al, 2013) showing that Mitochondrial Calcium Uniporter overexpression leads to an increase in Calcium influx into Mitochondria and an increase neuronal injury.

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. They note that there are separate Calcium pathways. Some pathways are neurotoxic and others play a role in the protection of neurons.

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. The authors reference a set-point hypothesis which states that there is an optimum intracellular Calcium range. Outside of this range there is an increased risk of neuronal toxicity. It is useful to note that extracellular Calcium is maintained within a narrow range as a result of Calcium homeostasis. The authors also suggest that there are subpopulations of NMDA receptors which determine the effects of the Calcium influx into the cell.

Lai, Zhang and Wang examine the effects of Glutamate induced excitotoxicity on the Mitochondria. As per the previous posts, the authors have noted that there is a set-point hypothesis which effectively states that NMDA receptor mediated Calcium influx into the cell is associated either with excitotoxicity or else the promotion of neuronal survival. The authors cite studies on Cerebellar Granule cells which show that low levels of NMDA stimulation of NMDA receptors promotes neuronal survival in contrast with high levels of NMDA stimulation which leads to excitotoxicity.

In the section on neuronal survival and synaptic NMDA receptors, the authors cite research evidence of environmental stimulation reducing excitotoxic injury. The key reference they cite is (Young et al, 1999).

What is interesting to note is that the NMDA receptor is involved in both long term potentiation (memory formation) and excitotoxic injury. The authors hypothesise that these pathways are mediated by different receptor subpopulations. More specifically they suggest the hypotheses that

  • Synaptic NMDA receptors mediate neuronal survival
  • Extrasynaptic NMDA receptors mediate excitotoxic injury

In their paper, they suggest that extrasynpatic NMDA receptors are stimulated when there is an excess of Glutamate resulting from causes such as ischaemia.

Lei et al cite a number of studies which demonstrate a difference between synaptic and extra-synaptic NMDA receptors dating back to 2001 (Lu et al, 2001).

With reference to the synaptic NMDA receptor they note that this mediates

  1. The activity of the Extracellular Signal Regulated Kinase (ERK)
  2. An increase in Calcium in the nucleus which leads to the activation of CREB (a transcription factor) and BDNF production

They note that extrasynaptic NMDA receptor activation

  • Reduces Extracellular Signal-Related Kinase (ERK) activity
  • Decreases CREB production
  • Decreases BDNF production

The authors distinguish between synaptic and extra-synaptic NMDA receptors. The authors note that a global activation of NMDA receptors can lead to excitoxicity and that this can be inhibited by inhibiting the activation of NMDA receptors containing the N2B subunit (which occur extrasynaptically) (Hardingham et al, 2002).

Lai et al have outlined a broad division between synaptic and extra-synaptic NMDA receptors. They describe

  • Synaptic NMDA receptors are pro-survival (of neurons)
  • Extra-Synaptic NMDA receptors are excitotoxic

They also provide evidence that this division is not so straightforward. There are several lines of evidence that suggest that there is overlap in the NMDA receptor functions across locations.

Thus for example they note that synaptic NMDA-receptor induced Glutamate release is associated with primary hippocampal neuronal death secondary to hypoxic ischaemia and cite (Rothman, 1983 and 1984).

They identify several subunits of the NMDA Receptor

  • GluN1: Found in multiple locations in the Brain
  • GluN2A: Located in the Forebrain and Cerebellum
  • GluN2B: Located in the Forebrain
  • GluN2C: Located in the Cerebellum

They note that two NMDA Receptor subunits – GLU2NA and GLU2NB interact with proteins involved in synaptic plasticity – in Long Term Depression (LTD) and Long Term Potentiation (LTP).

They discuss the NMDA Receptor Glu2NBR and Glu2NAR subunits

They cite evidence to suggest that NMDA Receptor GLU2NBR antagonists are neuroprotective and the opposite may be the case for GLU2NAR antagonists.

They note that the C-Terminus of the NMDA receptor subunits Glu2NA and Glu2NB mediates function and swapping the C-Terminus between the subunits changes the functional properties.

The authors cite an important study by Harreveld which identifies a role for Glutamate and led to the suggestion that Glutamate mediates stroke related brain injury (Harreveled, 1959). This study provided evidence that an extract from the Pallium induced Cortical depression and muscle contraction.

The authors cite evidence that excitotoxicity is mediated via the synaptic Glutamate receptor

  1. Cultured Hippocampal neurons without synapses can remain unaffected for up to 24 hours in comparison with Hippocampal neurons with synaptic connections.
  2. Increased intracerebral Glutamate and Aspartate levels have been identified following ischaemia.
  3. The findings in 2. are reduced by a reduction in afferent glutaminergic neurons.
  4. There is experimental evidence from AP7 studies

The authors cite a number of studies dating back to (Benviste et al, 1984) which provide evidence for an elevation of Glutamate following Stroke. The authors suggest that this rapid increase in Glutamate is the initial step that leads to excitotoxicity.

The authors note that the NMDA receptor activates the protein kinase Akt. This is done via two mechanisms

(1) Phosphorylation of Insulin Receptor Substrate-1

(2) Activation of Akt via the protein kinase CaM-KK

The authors note that there is activation of Akt via Protein Kinase CaM-KK as one of two mechanisms for the activation of Akt.

The authors note that the activation of calcium–calmodulin dependent protein kinase kinase (CaM-KK) is independent of PI3K which is involved in the other pathway.

The authors note that there is activation of Akt via PI3K is one of two mechanisms for the activation of Akt.

The authors note that NMDA receptor activation leads to calcium dependent Tyrosine phosphorylation of Insulin Receptor Substrate-1 which in turn binds and activates PI3K (phosphatidylinositol 3-kinase) which in turn leads to the activation of Akt.

The authors note the actions of Phosphatase and Tensin Homolog (PTEN) as involved in NMDA receptor mediated death signalling and can be mutated in many types of cancer cells.

The authors outline a number of properties of Phosphatase and Tensin Homolog (PTEN).

  1. PTEN interacts with the GluN1 subunit of GLU2NBR
  2. PTEN doesn’t interact with the GluN1 subunit of GLU2NAR (which is implicated in neuronal survival pathways).
  3. The lipid phosphatase activity inhibits the Akt activation pathway
  4. The protein phosphatase activity potentiates the NMDA receptor mediated current extrasynaptically

The authors note that the NMDA receptor can cause nuclear translocation of PTEN via the GluN2B subunit. In nuclear translocation, Cytoplasmic proteins are transported into the nucleus where they can modify cell function. The authors also note that GluN2A impairs nuclear translocation.

They note that another PIK3 product – PtdIns(3,4)P2(Phosphatidylinositol 3,4-bisphosphate) potentiates excitotoxicity via the NMDA receptor. PtdIns(3,4)P2 is a phospholipid and secondary messenger in the cell.

References

 

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Barros LF, San Martín A, Ruminot I, Sandoval PY, Fernández-Moncada I Baeza-Lehnert F, Arce-Molina R, Contreras-Baeza Y, Cortés-Molina F, Galaz A, Alegría K.J Neurosci Res. 2017 Feb 2. doi: 10.1002/jnr.23998. [Epub ahead of print]Near-critical GLUT1 and Neurodegeneration.

Bell S, Kolobova I, Crapper L, Ernst C, Lesch-Nyhan Syndrome: Models, Theories, and Therapies. Mol Syndromol 2016;7:302-311

H. Benveniste, J. Drejer, A. Schousboe, N.H. Diemer. Elevation of the extracellular concentrations of glutamate and aspartate in rat hippocampus during transient cerebral ischemia monitored by intracerebral microdialysis
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Ebert K, Ludwig M, Geillinger KE, Schoberth GC, Essenwanger J, Stolz J, Daniel H, Witt H.Reassessment of GLUT7 and GLUT9 as Putative Fructose and Glucose Transporters.
J Membr Biol. 2017 Jan 12. doi: 10.1007/s00232-016-9945-7. [Epub ahead of print]

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D.R. Lucas, J.P. Newhouse. The toxic effect of sodium l-glutamate on the inner layers of the retina. AMA Arch. Ophthalmol., 58 (1957), pp. 193–201

Meaney E, Alva F, Moguel R, Meaney A, Alva J, Webel R. Formula and nomogram for the sphygmomanometric calculation of the mean arterial pressure. Heart. 2000 Jul;84(1):64.

Membrez M, Hummler E, Beermann F, et al. GLUT8 Is Dispensable for Embryonic Development but Influences Hippocampal Neurogenesis and Heart Function. Molecular and Cellular Biology. 2006;26(11):4268-4276. doi:10.1128/MCB.00081-06.

Mypinder S. Sekhon, Philip N. Ainslie and Donald E. Griesdale
Clinical pathophysiology of hypoxic ischemic brain injury after cardiac arrest: a “two-hit” model. Critical Care. 2017. 21:90. DOI: 10.1186/s13054-017-1670-9

Oppelt SA, Zhang W, Tolan DR.Specific regions of the brain are capable of fructose metabolism. Brain Res. 2017 Feb 15;1657:312-322. doi: 10.1016/j.brainres.2016.12.022. Epub 2016 Dec 27.

Pao SS, Paulsen IT, Saier MH. Major Facilitator Superfamily. Microbiology and Molecular Biology Reviews. 1998;62(1):1-34.

Rivera EJ, Goldin A, Fulmer N, Tavares R, Wands JR, de la Monte SM.
Insulin and insulin-like growth factor expression and function deteriorate with progression of Alzheimer’s disease: link to brain reductions in acetylcholine. J Alzheimers Dis. 2005 Dec;8(3):247-68.

S. Rothman. Synaptic release of excitatory amino acid neurotransmitter mediates anoxic neuronal death. J. Neurosci., 4 (1984), pp. 1884-1891

S.M. Rothman. Synaptic activity mediates death of hypoxic neurons
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Spindelboeck W, Gemes G, Strasser C, Toescher K, Kores B, Metnitz P, Haas J, Prause G. Arterial blood gases during and their dynamic changes after cardiopulmonary resuscitation: A prospective clinical study.Resuscitation. 2016 Sep;106:24-9. doi: 10.1016/j.resuscitation.2016.06.013. Epub 2016 Jun 18.

Steen E, Terry BM, Rivera EJ, Cannon JL, Neely TR, Tavares R, Xu XJ, Wands JR, de la Monte SM. Impaired insulin and insulin-like growth factor expression and signaling mechanisms in Alzheimer’s disease–is this type 3 diabetes? J Alzheimers Dis. 2005 Feb;7(1):63-80.

A.K. Stout, H.M. Raphael, B.I. Kanterewicz, E. Klann, I.J. Reynolds. Glutamate-induced neuron death requires mitochondrial calcium uptake. Nat. Neurosci., 1 (1998), pp. 366–373

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Appendix A – Calculations for Unit Conversion

A mole is 6 x 10²³ molecules.

The molecular weight for glucose is 180.1559 g per mole.

Therefore 31 μmol of glucose is equivalent to 180.1559 g x 31/1000000

= 0.00558 g

= 5.58 mg

which is in the range described above.

There is also a conversion calculator here.

 

Citations

 

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