Biochemistry of Nervous System

Report
Biochemistry of
the Nervous System
Transport through BBB
Metabolism of Neurotransmitters
Metabolism of CNS
Biochemical Aspects of CNS diseases
CSF chemical analysis
Biochemistry of the Nervous System
• Transport of substances through the blood-brain barrier (BBB)
• Metabolism of neurotransmitters
Synthesis (precursors – role of vitamins)
Release
Effect
Termination
• Metabolism of CNS :
Energy Metabolism of CNS
Lipid Metabolism in CNS
Myelin sheath
• Biochemical aspects of CNS diseases
Hypoglycemia
Cerebral ischemia
•
CSF chemical analysis
Transport through Blood-Brain Barrier (BBB)
Large number of compounds are transported through endothelial capillaries by
facilitated diffusion
1- Fuels
Glucose:
The principal fuel of the brain
Transported through endothelial membranes by facilitated diffusion via GLUT-1
At blood glucose of 60 m/dl, glucose is reduced to below Km of GLUT-1 leading to
appearance of symptoms of hypoglycemia
2- Others: as ketone bodies by another transport system
When blood levels of KB are elevated (during starvation)
KBs are important fuels for brain during prolonged starvation
Non-essential fatty acids (of diet or lipolysis) do not cross BBB
Essential fatty acids (linoleic & linolenic) can pass BBB
Transport through Blood-Brain Barrier (BBB)
2- Amino acids
Amino acids are transported by amino acid transporters
Amino acids are used in brain for synthesis of:
- Proteins of CNS
- Neurotransmitters (requires certain vitamins as B12, B6 & B1 …)
Types of transported amino acids:
1- Long neutral amino acids (by single amino acid transporters)
Essential:
Phenylalanine , Leucine, Isoleucine, Valine, Tryptophan, Methionine
Non-essential: Tyrosine
Semi-essential: Histidine
2- Small neutral amino acids (entry is markedly restricted as their influx markedly
change content of neurotransmitters)
Nonessential: Alanine, Glycine, Proline
3- Vitamins: transported by special transporters
Metabolism of Neurotransmitters
Neurotransmitters
Are chemicals released at synapses for transmission of nerve impulses
Generally, each neuron synthesizes only those neurotransmitters that it uses for transmission through
synapses So, the neuronal tracts are often identified by their neurotransmitters
Structurally divided into two categories:
- Small nitrogen-containing neurotransmitters
- Neuropeptides: Targeted in CNS as endorphins OR Released to circulation as GH & TSH
Major small nitrogen containing neurotransmitters:
•
•
•
•
•
•
•
•
Glutamate
GABA
Glycine
Acetylcholine
Dopamine
Norepinephrine
Serotonin
Histamine
In addition to: epinephrine, aspartate, nitric oxide
Metabolism of Neurotransmitters
General features of neurotransmitters synthesis, release & termination
1- Most are synthesized in presynaptic terminal from : Amino acids
Intermediates of glycolysis
Intermediates of TCA
2- Once synthesized, they are stored in vesicles (by active uptake)
3- Released in response to nerve impulse:
1- Nerve impulse causes Ca2+ influx (through Ca2+ channels) to presynaptic terminal
2- Exocytosis of neurotransmitters into synaptic cleft
3- Neurotransmitter binds to receptors on postsynaptic membrane-----EFFECT
4- Termination: by: Reuptake of the neurotransmitter into presynaptic terminal (or by glial cells)
Or/ Enzymatic inactivation (in presynaptic terminal, postsynaptic terminal or in astrocyte)
Metabolism of Neurotransmitters
Neurotransmitters: Catecholamines & Serotonin
Phenylalanine
Tyrosine
Tryptophan
Tyrosine Hydroxylase
Cu-dependent
BH4
DOPA
DOPA Decarboxylase
Tryptophan
Hydroxylase
BH4
Melanin
melanocytes
PLP
Dopamine
Dopamine Hydroxylase
Cu2+
Vit . C
Monoamine Oxidases
5 HIAA
MAO-A
Norepinephrine
MAO-A
Serotonin
VMA
in Adrenal
Medulla
Methyl transferase
(& few neurons)
using SAM
Vit. B12
Epinephrine
Folate
In URINE
Melatonin
Neurotransmitter: Histamine
• Histamine is an excitatory neurotransmitter in CNS
• Synthesized in CNS from the amino acid histidine by histidine decarboxylase
(requires PLP)
• Antihistaminic drugs (for treatment of allergy) cause drowsiness BUT new
generations of antihistaminics do not pass BBB & so do not cause CNS effects
Neurotransmitter: Acetylcholine
Synthesis in CNS (in presynapses)
Choline Acetyltransferase enzyme
Acetyl CoA + Choline
Acetyl Choline
Choline:
1- From diet
2- From phosphatidylcholine (PC) in membrane lipids
PC is synthesized from PE utilizing methyl groups of S-adenosyl methionine (SAM)
Requires vitamins B12 & B6
Acetyl group
From glucose oxidation (requires oxygen) is the major source (little FA oxidation in CNS)
Thiamine (vit. B1)
Glucose
Pyruvate
Acetyl CoA
ATP
Pyruvate Decarboxylase
N.B. In thiamine deficiency & hypoxia: no ATP & no acetylcholine neurotransmitter
Neurotransmitter: Glutamate & GABA
•
•
Glutamate is the main excitatory neurotransmitter in the CNS
Neurons that respond to glutamate are referred to as glutaminergic neurons
Sources of glutamate in nerve terminals:
1- Synthesized from glucose through glucose metabolism in neurons (main source)
Glucose --- a Ketoglutarate (a KG) ------ glutamate (Requires PLP)
2- From glutamine (of glial cells) by glutaminase
2- From blood (few as no cross BBB)
Mechanism of action of glutamate as a neurotransmitter:
1- Synthesis from glucose metabolism & concentration in vesicles (in presynapses)
2- Release by exocytosis to synaptic cleft
3- Uptake by postsynaptic
4- Binding to glutaminergic receptors in postsynapses
5- Functional effect
6- Termination: glutamate reuptake by astrocytes (glial cells) .. REQUIRES ATP (ENERGY)
In astrocytes, glutamate is converted to glutamine (REQUIRES ATP)
Glutamine is released from astrocytes & is taken up by neurons
In neurons, glutamine is converted to glutamate by glutaminase
Neurotransmitter: Glutamate & GABA
GABA Is an inhibitory neurotransmitters in CNS
In presynaptic neurons, GABA is synthesized from glutamate by glutamate
decarboxylase (GAD) by a reaction that requires PLP
Then, GABA is released to synaptic cleft. It is recognized by receptors on postsynaptic
neurons.
Termination: GABA in synaptic cleft is uptaken by glial cells (as astrocytes) & converted
to glutamate
Glutamate is converted to glutamine by glutamine synthetase (requires ATP)
Fate of glutamine of astrocytes:
1- A fraction of glutamine is released from astrocytes & is taken up by neurons.
In neurons, glutamine is deaminated to glutamate by glutaminase
2- Another fraction of glutamine is released to blood------to kidney --- ammonia
Tiagabine is used as an antiepileptic (anticonvulsant) as it inhibits the reuptake of GABA
`
Neurotransmitter: Glutamate & GABA
Glutamate metabolism in hyperammonemia:
• During hyperammonemia, ammonia can diffuse into the brain from
the blood to neurons.
• The ammonia is able to inhibit the glutaminase in neurons, thereby
decreasing formation of glutamate in presynaptic neurons (not
regenerated)
This effect of ammonia might contribute to the lethargy associated
with the hyperammonemia found in patients with hepatic disease.
(hepatic encephalopathy)
Neurotransmitter: Glutamate & GABA
Relation between glutamate synthesis & citric acid cycle:
•
•
In neurons, synthesis of glutamate removes a ketoglutarate from the citric acid
cycle ending in a decrease in regeneration of oxalacetate
Regeneration of oxalacetate is necessary for oxidation of acetyl CoA & this is
performed by two major anaplerotic pathways:
1- Degradation of isoleucine & valine amino acids to butyric succinyl CoA,
which yields oxalacetate.
This reaction requires vitamin B12 (coenzyme for methylmalonyl CoA mutase)
2- Pyruvate carboxylation to oxalacetate (by pyruvate carboxylase, requires
the vitamin biotin as a coenzyme).
Metabolism of CNS
Glucose & Energy Metabolism
Energy source of the brain
• The mass of the brain is only 2% of the total body mass, yet its energy
requirement is more than seven times than that of the other organs
• Thus for brain metabolism, there is a high requirement for glucose and
oxygen at steady rate.
• The main source of energy is the generation of ATP by the aerobic
metabolism of glucose
Aerobic Glycolysis
In Cytosol
Mitochondria & Oxygen
Glucose ---- Pyruvate ----- Acetyl CoA ---- With oxalacetate in CAC ------- ATP
Metabolism of CNS
Glucose & Energy Metabolism
Glucose metabolism & neurotransmitters
(in CNS)
There is a relationship between the oxidation of
glucose in glycolysis and the supply of precursors
for the synthesis of neurotransmitters in neurons
within CNS.
Accordingly, deficiencies of either glucose or oxygen
(hypoglycemia or hypoxia) affect brain
function because they influence:
1- ATP production for CNS neurons
2- Supply of precursors for neurotransmitter
synthesis.
Glucose Metabolism & Neurotransmitter Synthesis
Metabolism of CNS
Brain Lipids Synthesis & Oxidation
Sources of lipids to CNS:
• BBB significantly inhibits entry of certain fatty acids & lipids into CNS. So, all lipids found
in CNS must be synthesized within CNS
(e.g. non-essential fatty acids, cholesterol, sphingolipids, glycosphingolipids &
cerebrosides)
All these are needed for neurological functions & synthesis of myelin by glial cells (nonneuronal cells)
•
EXCEPTION is:
Essential fatty acids (linoleic & linolenic FAs) can enter the brain
Within CNS, these two FAs are elongated & desaturated to yield the very-long chain
fatty acids required for synthesis of myelin sheath.
Fatty acid oxidation as a source of energy:
• Intake of fatty acids (coming from diet and/or lipolysis of TG ) to CNS is insufficient to
meet the energy demands of CNS (by FA oxidation) & hence the requirement for aerobic
glucose metabolism
• Recall that ketone bodies are sources of energy to brain during prolonged starvation as
they can pass BBB easily….
Metabolism of CNS
Brain Lipids Synthesis & Oxidation
• Peroxisomal fatty acid oxidation is important in the brain as
the brain contains very-long-chain fatty acids & branchedchain fatty acids as phytanic acid (of diet)
Both are oxidized by a oxidation in the peroxisomes
• Refsumes disease: a disorder that affects the peroxisomes –
severely affects the brain due to inability to metabolize both
very long chain & branched chain fatty acids
Myelin Synthesis
Rapid rate of nerve conduction in PNS & CNS depends on the formation of myelin.
Myelin is a multilayered lipid (sphingolipids) & protein structure that is formed by the
plasma membrane of glial cells to wrap around the axon.
In PNS, myelin is synthesized by Schwan cells
In CNS, myelin is synthesized by oligodendrocytes
Multiple sclerosis (MS)
Progressive demyelination of CNS neurons
May be due to an event that triggers the formation of autoimmune antibodies
directed against components of the nervous system (as viral or bacterial infection)
Loss of myelin (insulator) in the white matter of the brain that interferes with nerve
conduction along demyelinated area
CNS compensates by stimulating the oligodendrocytes to remyelinate the damaged
axon(& hence remission is activated)
Remyelination is accompanied by slowing in conduction (speed is proportional to
myelin thickness)
Clinical Manifestation
Hypoglycemic Encephalopathy
Clinical manifestations of hypoglycemia:
Early clinical signs in hypoglycemia initiated by hypothalamic
sensory nuclei as sweating, palpitations, anxiety and hunger.
In late stages, these symptoms give way to serious manifestations
of CNS disorders as confusion, lethargy, seizures & coma
Biochemistry of
Hypoglycemic Encephalopathy
• As blood glucose falls below 45 mg/dL
the brain attempts to use internal substrates such as glutamate and TCA cycle intermediates
as fuels for ATP production.
Because the pool size of these substrates is quite small, they are quickly depleted.
• As the blood glucose drops from 45 to 36 mg/dL
NO EEG changes are observed
Symptoms appear to arise from decreased synthesis of neurotransmitters in particular
regions of the brain (hippocampal & cortical structures)
•
If blood glucose levels continue to fall below 18 mg/dL
EEG becomes isoelectric
Neuronal cell death ensues that may be caused by glutamate excitotoxicity ?? (as result of
ATP depletion)
SO, GLUTAMATE METABOLIDSM HAS TO BE UNDERSTOOD
Glutamate as a neurotransmitter
Role of glutamate as a transmitter in CNS:
Within the CNS, glutaminergic neurons are responsible for the mediation of many
vital processes such as the encoding of information, the formation and retrieval of
memories, spatial recognition and the maintenance of consciousness.
Postsynaptic glutaminergic neurons perform their roles through:
1- Ionotropic receptors that bind glutamate released from presynaptic neurons
referred to as kainate, 2-amino-3-hydroxy-5-methyl-4-isoxalone propionic
acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors.
2- Metabotropic glutamate receptors that are members of the G-protein coupled
receptor (GPCR) family.
Ionotropic Glutamate Receptors
Non- NMDA
KAINATE
NMDA
AMPA
AMPA and Kainate receptors
generally allow the passage of
Na+ and K+
NMDA receptors allows the
passage of both Na+ and Ca++
ions. (More permeable to Ca++ )
Glutamate Excitotoxicity
Exciotoxicity is the pathological process by which nerve cells are damaged
and killed by glutamate (and similar substances).
This occurs when receptors for glutamate such as the NMDA & AMPA
receptor are over activated (overstimulated).
Excessive excitation of glutamate receptors has been associated with
hypoglycemia & stroke
(cases in which there is lack of glucose and/or oxygen ending in lack of energy
production in CNS)
Glutamate Excitotoxicity
occurs when the cellular energy reserves are depleted
(as in hypoglycemia or stroke, etc )
Failure of the energy-dependent reuptake pumps of glutamate
Accumulation of glutamate in the synaptic cleft
Overstimulation of the postsynaptic glutamate receptors
Prolonged glutamate receptor activation leads to prolonged opening of the
receptor ion channel and the influx of lethal amounts of Ca2 ions, which can
activate cytotoxic intracellular pathways in the postsynaptic neurons
Biochemistry of Cerebral Ischemia
Cerebral ischemia
•
It is the potentially reversible altered state of brain physiology and biochemistry
that occurs when substrate delivery is cut off or substantially reduced by
vascular stenosis or occlusion
Stroke
•
is defined as “an acute neurologic dysfunction of vascular origin with sudden
(within seconds) or at least rapid (within hours) occurence of symptoms and
signs corresponding to the involvement of focal areas in the brain”
(Goldstein et al, 1989)
Pathophysiology of Cerebral Ischemia
↓↓ ATP
Induction
Amplification
Expression
Lack of oxygen supply to ischemic neurones
The cell switches to anaerobic metabolism, producing lactic acid.
ATP depletion
malfunctioning of membrane ion system
Depolarisation of neurones
Influx of calcium
Induction
Phase
Release of neurotransmitters as glutamate
(causing glutamate excitotoxicity)
Amplification
Phase
Accumulation of more intracellular
levels of Ca2+ which causes
additional release of glutamate
(viscious cycle)
Expression Phase
Ca2+
1-overexcites cells and causes the generation of harmful chemicals like
free radicals ( causing oxidative stress)
2- Activation of calcium-dependent enzymes such as:
calpain ( causing apoptosis)
phospholipases (causing membrane breakdown)
3- Calcium can also cause the release of more glutamate (glutamate excitotoxicity)
The cell's membrane is broken down by phospholipases
Cell membrane becomes more permeable, and more ions and harmful
chemicals flow into the cell.
+
Mitochondria membrane break down, releasing toxins and apoptotic factors
into the cell
lactic acid is produced in excess in ischemia
In cerebral ischemia, lack of oxygen switches metabolism of glucose to the
anaerobic pathway & lactic acid production
Lactic acid contribute to the pathophysiology of ischemia as:
1- It decreases pH that may injure and inactivate mitochondria.
2- Lactic acid degradation of NADH (which is needed for ATP synthesis)
may also interfere with adequate post-ischemic recovery of ATP levels.
3- Lactic acid increase the amount of free-radical mediated injury.
 Lactic acid in neurons  acidosis  promotes the pro-oxidant effect
 ↑ the rate of conversion of O2- to H2O2 or to hydroxyperoxyl radical
Oxidative stress is caused by ischemia
What is meant by ROS?
Reactive oxygen species (ROS) are formed from partial reduction of molecular O2 i.e.
adding electrons to oxygen leading to the formation of superoxide, hydrogen
peroxide & hydroxyl radical.
Generally, ROS cause damage to DNA, protein and unsaturated lipids of the cells.
What is meant by oxidative stress
A condition in which cells are subjected to excessive levels of ROS (free radicals) &
they are unable to counterbalance their deleterious effects with antioxidants
Oxidative stress is caused by ischemia
Cellular Effects of Reactive Oxygen Species (ROS) in CNS
• Nitric oxide is over produced and turns to be a neurotoxic
mediator as it reacts with superoxide anions to generate toxic
peroxynitrite which leads to production of more potent
neurotoxin such as hydroxyl radicals
• Lipid peroxidation
• Inactivation of enzymes
• Nucleic acid (DNA & RNA) damage
• Release of calcium ions from intracellular stores with more
damage to neurons
• Damage to cytoskeleton
Apoptosis & necrosis are caused by ischemia
• Necrosis:
is commonly observed early after severe ischemic insults
• Apoptosis:
occurs with more mild insults and with longer survival periods
• The mechanism of cell death involves calcium-induced calpainmediated proteolysis of brain tissue
• Substrates for calpain include:
– Cytoskeletal proteins
– Membrane proteins
– Regulatory and signaling proteins
Apoptosis
Mitochondria break down, releasing toxins and apoptotic factors into the cell.
The caspase-dependent apoptosis cascade is initiated, causing cells to "commit suicide."
Broughton et al., 2009; Stroke
Caplains are cytosolic proteinases
Whose irreversible proteolytic activity
is against cytoskeleton
and regulatory proteins
Broughton et al., 2009; Stroke
Broughton et al., 2009; Stroke

similar documents