Biochemistry Ch 48 903-926

Metabolism of Nervous System

Cell Types of Nervous System – consists of neurons and neuroglia (oligodendrocytes and astrocytes = glial cells, microglial cells, ependymal cells, Shwann cells)

-neuroglia support and sustain neurons by surrounding them and holding them in place, supplying nutrients and O2, and insulating them for quicker conduction

Neurons – have soma containing long axon and short dendrites; dendrites receive information from axons and axons transmit information to other neurons

-axon-dendrite connection is known as synapse

-most neurons have multiple dendrites, and most axons branch to multiple targets (divergence)

-signaling requires neurotransmitter binding to receptor to initiate electrical signal

-injured neurons have limited capacity for regeneration

Astrocytes – found in CNS, star-shaped, and provide physical/nutritional support for neurons

-astrocytes guide neuronal migration to final adult position during development and secrete matrix in order to hold neurons in place

-astrocytes phagocytize debris left by cells, provide lactate (from glucose metabolism) as a carbon source for neurons, and control brain extracellular ionic environment

Oligodendrocytes – myelinate CNS neurons; myelin is lipid-protein covering of axons

-oligodendrocytes form myelin sheaths around multiple neurons in CNS by sending out processes that bind to axons on target neurons

-speed of neuron conduction is directly proportional to degree of myelination

-oligodendrocytes support similar to astrocytes, and have limited regeneration capability

Schwann Cells – supporting cells of PNS and act to form myelin sheaths around axon; unlike oligodendrocytes, Schwann cells can only myelinate ONE axon

-Schwann cells clean up cellular debris in PNS and help peripheral axon regeneration

Microglial Cells – smallest glial cells; they are immunologically responsive similar to macrophages and they destroy microorganisms/phagocytize debris

Ependymal Cells – ciliated cells lining cavities (ventricles) of CNS and spinal cord; these cells secrete cerebrospinal fluid (CSF) into ventricular system, and beating of ependymal cilia allows for efficient circulation of CSF

-CSF acts as shock absorber and a system of removing metabolic wastes

-cells in ependymal layer can act as neural stem cells, which can regenerate into neurons

Blood Brain Barrier (BBB)

Capillary Structure – in most organs, rapid passage of molecules occurs from blood through endothelial wall of capillaries into interstitial fluid, and interstitial fluid resembles blood

-in brain, transcapillary movement of substrates from circulation is restricted due to BBB, which limits accessibility of toxins and harmful compounds to neurons in CNS

-BBB begins with endothelial cells forming inner lining of vessels supplying blood to CNS; these endothelial cells are joined by tight junctions that don’t permit movement of POLAR MOLECULES from blood into interstitial fluid that bathes neurons.

-Also, the endothelial cells in CNS lack mechanisms for transendothelial transport such as fenestrations (pores) or transpinocytosis (vesicular transport from one side to the other)

-endothelial cells contain drug-metabolizing enzymes found in liver, so that they can metabolize toxic chemicals and neurotransmitters as an enzyme-forming barrier

-they actively pump hydrophobic molecules that diffuse into endothelial cells back into blood with P-glycoproteins, which act as transmembranous, ATP-dependent pumps

-further BBB protection comes from continuous collagen-containing basement membrane that surrounds capillaries surrounded by foot of astrocytes

-compounds must pass through endothelial cell membranes, enzymatic barrier, basement membranes, and astrocyte cell barriers to get to CNS neurons

Transport Through Blood Brain Barrier – nonpolar compounds: drugs/gases can diffuse through. Non-essential fatty acids CANNOT cross, essential fatty acids CAN cross

1.  Fuels – glucose is principal fuel of brain, and is transported thorugh endothelial membranes by facilitated diffusion by the GLUT-1 transporter

  1. GLUT-3 on neurons then allow neurons to transport glucose from ECF
  2. GLUT-1 is also found on glial cells
  3. Rate of glucose transport into ECF normally exceeds rate of energy metabolism, but in hypoglycemic states, glucose is reduced to lower than Km for GLUT-1 in endothelial cells of barrier
  4. Monocarboxylic acids (lactate, acetate, pyruvate, acetoacetate, B-hydroxybutyrate) are transported by separate system slower than glucose
  5. Facilitated glucose transporter protein type I (GLUT-1) deficiency syndrome – glut-1 transporters are impaired resulting in low glucose concentration in CSF (known as hypoglycorrhachia); diagnostic is normal blood glucose and low CSF levels; clinical features are seizures, developmental delay, motor disorder

2.  Amino Acids and Vitamins – Large Neutral Amino Acids (LNAAs) such as F, L, Y, I, V, W, M, and H all enter CSF rapidly via single AA transporter; many are essential in diet and must be imported for protein synthesis or as neurotransmitter precursors

  1. All of these amino acids compete for transporter binding
  2. Entry of small amino acids (A, G, P, and GABA) Is restricutred because they could change content of neurotransmitters, and instead are synthesized in brain and transported out of CNS via the Alanine-preferring system carrier
  3. If one AA is in excess, it can compete for binding to the single transporter to lower transport of the other amino acids, providing a reason for the mental retardation seen in phenylketonuria caused by high phenylalanine

3.  Receptor Mediated Transcytosis – proteins such as insulin, transferrin, and IGFs cross BBB by receptor-mediated transcytosis; protein binds membrane receptor, then complex is endocytosed into a vesicle and released on other side of endothelial cell

  1. Absorptive-mediated endocytosis can occur where protein binds nonspecifically to membrane and not a distinct receptor

Synthesis of Small Nitrogen-Containing Neurotransmitters – neurotransmitters fall into 2 categories: small nitrogen-containing molecules and neuropeptides

-small nitrogen containing molecules are: GABA, glycine, acetylcholine, dopamine, norepinephrine, serotonin, histamine, epinephrine, aspartate, nitric oxide

-each neuron synthesizes only the neurotransmitter it uses for transmission

-neuropeptides are small peptides synthesized and processed in CNS; some have targets in CNS (endorphins binding to opioid receptors to block pain), whereas others are released into circulation and bind to receptors on other organs, such as GH and TSH

-many neuropeptides are synthesized as larger precursors later cut

-neurons can contain all kinds of varieties of small molecule/neuropeptide neurotransmitters

General Features of Neurotransmitter Synthesis – most are synthesized from amino acids, intermediates of glycolosys and TCA, and O2 in cytoplasm of presynaptic cell

-rate of synthesis regulated to relate to rate of neuron firing

-synthesis occurs in cytoplasm of presynaptic nerve terminal; enzymes synthesized in soma and transported to nerve terminal

-neurotransmitters transported into vesicles by ATP-pump linked with proton gradient

-release of storage vesicle triggered by impulse depolarizing membrane = influx of Ca to promote vesicle fusion with synaptic membrane and release of neurotransmitter

-action of neurotransmitter is terminated through reuptake into presynaptic terminal, uptake into glial cells, diffusion away from synapse, or enzymatic inactivation

-Drugs have been developed to block neurotransmitter uptake into storage vesicles; Reserpine blocks catecholamine uptake into vesicles, had been used as antihypertensive/antiepileptic

Dopamine, Norepinephrine, Epinephrine:

1.  Synthesis of the Catecholamine Neurotransmitters – dopamine, norepinephrine, and epinephrine are synthesized from a common pathway starting at TYROSINE, which is supplied by diet or converted to from phenylalanine by phenylalanine hydroxylase

  1. RATE LIMITING STEP in synthesis of neurotransmitters from tyrosine is hydroxylation of tyrosine by tyrosine hydroxylase (requires hydrobiopterin BH4)+), and the product formed is dihydroxyphenylalanine DOPA
  2. Second step is decarboxylation of DOPA à dopamine, requiring pyridoxal phosphate (vitamin B6); dopaminergic neurons STOP synthesis here
  3. Neurons synthesize epinephrine from dopamine in a hydroxylation reaction catalyzed by dopamine β-hydroxylase (DBG), present only in storage vesicles
  4. Enzyme requires electron donor from vitamin C; Cu2+ also required
  5. Epinephrine synthesized by adrenal medulla AND few neurons; the neurons synthesize epinephrine from norepinephrine with the enzyme phenylethanolamine N-Methyl transferase that uses S-adenosylmethionine (SAM) as a methyl donor to norepinephrine à epinephrine

2.  Storage and Release of Catecholamines – low concentrations of catecholamines are free in cytosol and high concentrations are inside vesicles; conversion of L-dopa à dopamine occurs in cytosol after which dopamine is stored in vesicles.

  1. norepinephrine-containing neurons: final β-hydroxylation reaction is inside vesicle
  2. catecholamines are transported into vesicles by VMAT2, a protein homologous to bacterial drug-resistant transporters (P-glycoprotein)
  3. mechanism of transport in storage vesicles is an ATP-dependent process linked to proton pump; protons pumped into vesicles by vesicular ATPase (V-ATPase); protons then exchange for catecholamine by VMAT2
  4. in vesicles, catecholamines exist in complex with ATP and acidic proteins known as chromogranins
  5. elevated chromogranins can be found in patients with neuroendocrine tumors such as pheochromocytomas
  6. vesicles maintain supply of catecholamines at nerve terminal available for immediate release, AND they mediate process of release; when action potential reaches nerve, Ca channels open to allow influx of Ca to promote fusion of vesicles with neuronal membrane; vesicles discharge contents, including neurotransmitters, ATP, chromogranins, and DBH by exocytosis

3.  Inactivation and Degradation of Catecholamine Neurotransmitters – action is terminated through REUPTAKE into presynaptic terminal and diffusion away from synapse. Also, degradative enzymes are present in presynaptic terminal and glial + endothelial cells

-Two major reactions in process of inactivation/degradation are catalyzed by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)

-MAO – on outer mitochondrial membrane of many cells and oxidizes carbon containing amino group to an aldehyde to release NH4

-in presynaptic terminal, MAO inactivates catecholamines not protected in vesicles

-MAO-A preferentially deaminates norepinephrine and serotonin

-MAO-B acts on wide spectrum of phenylethylamines

-MAO in liver protects against ingestion of dietary amines such as tyramine from cheese

-tyarmine is degradation product of tyrosine, can cause headaches, palpitations, nausea, vomiting, and elevated blood pressure; leads to norepinephrine release

-tyramine is inactivated by MAO-A

COMT – found on many cells including RBC and works on a broad spectrum of extraneuronal catechols diffused away from synapse

-COMT transfers methyl from SAM to OH group on cateholamine or its degradation product. Because it uses SAM, it is dependent on vitamin B12 and folate

-cerebrospinal homovanillylmandelic acid (HVA) is indicator of dopamine degradation

-in albinism, either Cu-dependent tyrosine hydroxylase of melanocytes or other enzymes converting tyrosine to melanins is defective

4. Regulation of Tyrosine Hydroxylase – since this enzyme is the rate-limiting step in catecholamine synthesis, it is heavily regulated by FEEDBACK INHIBITION coordinated with depolarization of nerve terminal

-also inhibited by free cytosolic catecholamines

-depolarization of nerve terminal activates enzyme and activates several protein kinases to phosphorylate tyrosine hydroxylase to cause it to bind more tightly to BH4, making it less sensitive to product inhibition

-long-term process involves alterations in amounts of tyrosine hydroxylase and dopamine B-hydroxylase present in nerve terminals

-when sympathetic activity is increased, mRNA for tyrosine hydroxylase and dopamine B-hydroxylase are increased in soma, resulting from phosphorylation of CREB by kinases

-CREB binds CRE in promoter of gene to stimulate enzyme

Metabolism of Serotonin – serotonin is synthesized from TRYPTOPHAN in a similar method to the pathway of norepinephrine from tyrosine

-first step uses tryptophan hydroxylase and requires BH4 to hydroxylate the ring of tryptophan

-seconds step is decarboxylation catalyzed by same enzyme that decarboxylates DOPA

-Serotonin can be inactivated by the enzyme MAO

-melatonin is also synthesized from tryptophan; produced in pineal gland in response to light-dark cycle (levels rise in blood in the dark), involved in circadian rhythms

-antipsychotic drugs were based on inhibiting MAO: Deprenyl (MAO-B specific), Clorgyline (MAO-A specific), Moclobemide (MAO-A specific, reversible)

Metabolism of Histamine – histamine is produced by both mast cells and certain neuronal fibers. Mast cells produced in the bone marrow and are present in thalamus, hypothalamus, dura mater, leptomeninges, and choroid plexus

-histaminergic neuron bodies are found in tuberomammillary nucleus of posterior basal hypothalamus and project fibers into all areas of CNS

-Histamine is synthesized from histidine in a single enzymatic step using histidine decarboxylase requiring pyridoxal phosphate (vitamin B6), similar to dopa decarboxylase

-histamine is stored in nerve terminal vesicles; depolarization activates release of histamine through Ca dependent mechanism

-histamine activates both postsynaptic AND presynaptic receptors, and it is NOT recycled into presynaptic terminal

-astrocytes have high-affinity uptake system for histamine

-first step in histamine inactivation is methylation by histamine methyltransferase which transfers methyl group from SAM to nitrogen group of histamine à methylhistamine; second step is oxidation by MAO-B

-histamine is major mediator of allergic response à vasodilation and increased permeability of vascular endothelium; bronchoconstrictor to remove allergic material… histamine in brain is an excitatory neurotransmitter

Acetylcholine

1.  Synthesis – acetylcholine is synthesized from acetyl CoA and choline, catalyzed by choline acetyltransferase (ChAT) occurring in presynaptic terminal and ACh is stored in vesicles released by Ca-mediated exocytosis

  1. Choline is taken up by presynaptic terminal from blood via low-affinity (high Km) system and from synaptic cleft via high-affinity (low Km) system; it is also derived from hydrolysis of phosphatidylcholine in membrane lipids
  2. Supply of choline in brain can become rate limiting for acetylcholine synthesis, and so supplementing diet with lecithin increases brain acetylcholine in patients suffering from tardive dyskinesia (involuntary movements of facial muscles/tongue)
  3. Choline is component of diet but can also be synthesized for phospholipids by route of triple methylation (taken from SAM) to ethanolamine portion of phosphatidylethanolamine to form phosphatidylcholine; in brain and liver
  4. Vitamin B12 is requirement for choline synthesis
  5. Acetyl group used for acetylcholine synthesis is derived from glucose oxidation to pyruvate and decarboxylation of pyruvate to form acetyl-Coa via pyruvate dehydrogenase

2.  Inactivation of Acetylcholine – inactivated by acetylcholinesterase, an enzyme that is inhibited by wide variety of compounds and toxins

  1. Neurotoxins such as Sarin
  2. Acetylcholine is major neurotransmitter at neuromuscular junctions, inability to inactivate it leads to constant activation of nerve-muscle synapses which can lead to paralysis

Glutamate and γ-Aminobutyric Acid (GABA) –

1.  Synthesis of Glutamate – glutamate is an excitatory neurotransmitter in CNS leading to nerve depolarization and is synthesized de novo from GLUCOSE rather than picked up from blood because it doesn’t cross BBB

  1. Glutamate is synthesized from TCA cycle intermediate α-ketoglutarate and occurs through 2 routes:
  2. glutamate dehydrogenase reduces alpha-ketoglutarate to glutamate and uses free ammonia (provided by AA degradation or diffusion across BBB)
  3. transamination reactions where amino group is transferred from other amino acids to α-ketoglutarate to form glutamate
  4. glutamate can be synthesized from glutamine using glutaminase
  5. glutamate is stored in vesicles and its release is Ca dependent
  6. glutamate is removed from synapse through high-affinity uptake systems in nerve terminals and glial cells

2.  GABA – major INHIBITORY neurotransmitter in CNS – it is synthesized by decarboxylation of glutamate in a single step catalyzed by glutamic acid decarboxylase (GAD)