Disordini del ciclo dell'urea: la neurotossicità dell'iperammoniemia. Autore: Diego Martinelli, Unità Operativa Complessa Patologia Metabolica.

1. 1
Disordini del ciclo dell’urea:
la neurotossicità dell'iperammoniemia
Diego Martinelli
Unità Operativa Complessa Patologia Metabolica
diego.martinelli@opbg.net
Napoli 26 - 28 Novembre 2013

2. Pathogenetic Mechanisms of Hyperammonemia
…in general the age of onset, duration and degree of hyperammonemia may predict
prognosis….
……….the underlying mechanisms of hyperammonemic encephalopathy are not
completely understood………
Gropman A, Summar M, Leonard JV
JIMD 2007;30:865-79
...although several theories exist, it is
not well understood how
hyperammonemia
disrupts brain function…..
Gropman A
MGM 2010;100:S20-S30

3. Clinical Manifestations
NEONATAL
• severe encephalopathy
INFANCY – CHILDHOOD  ADULTHOOD
• abnormal feeding with protein aversion
• abnormal behaviour
• recurrent vomiting
• intermittent encephalopatic episodes
• developmental delay
• hepatitis like attacks
• epilepsy
• migraine
• psychiatric symptoms
• ataxia
• stroke-like episodes

4. Hyperammonemia: possible scenarios
•
•

•
•
Genetic causes
urea cycle defects
organic acidurias
-oxidation def.
mitochondrial
hyperinsulinism/hyperam
monemia
•
•
•
•
•
Urea Cycle Defects
Organic Acidurias
-OX - mito
Hi-Ha
0
500
1000
1500
Non-genetic causes
THAN
liver bypass/insufficiency
asphyxia-shock-heart fail.
infections
intoxication
Patient’s age
• fetus
• newborn
• infant
• child
• adolescent
• adult

5. The Urea Cycle
urea
glutamine
alanine
glutamate
6 enzymes 2 carriers
NH4
0
normal
Clay Chest 2007
2000
4000
6000
UCDs – liver failure
8000
10000 M

6. PROGNOSIS DEPENDS ON COMA DURATION
Msall et al. NEJM 1984;310:1500-5
Picca et al. Pediatr Nephrol 2001;16:862-7
Good 2y
22.210.1
2210*
pre-treatment
Bad 2y
4711*
*p<0.02
48.811.2
4911
7813

7. • PERI-INSULAR
• FRONTAL
• PARIETAL
• OCCIPITAL
ADC map
ADC map
• THALAMIC RESTRICTED DIFFUSION (unusual in UCDs)
…suggesting that brain MRI may assist in determining prognosis & helping
clinicians with subsequent treatment decisions

8. Pathogenetic Mechanisms of Hyperammonemia
neuronal damage
•
•
•
•
•
•
Glutamine induced cerebral edema
Neurotransmitter perturbation
Oxidative stress
Neuroinflammation
Energy failure
Other……

9. Ammonia detoxification
Periportal hepatocyte
PORTAL
blood
NH4+ 300 M
SYSTEMIC
blood
NH4+ 50 M
Perivenous hepatocyte (astrocyte, muscle)
ATP
urea
ADP
glutamine
Plasma gln during
neonatal hyperammonemia
Scriver 1995

10. Is glutamine synthesis the principal means of NH4
detoxification?
glutamine

11. NH4+
900
Propionic ac.
Methylmalonic ac.
p. Gln
420
Gln
1380
CSF
NH4+ mol/L
< 100
>100
p. Gln 397
386
NH4+
470
560
970
Gln
640
229
369

12. The concentration changes of the nitrogen scavenger
glutamine have to be interpreted in the light of NH4 levels.
In contrast to other hyperammonemic syndromes, in PA
plasma glutamine do not increase in hyperammonemia,
whereas CSF glutamine concentrations are elevated.
2010

13. depletion of oxaloacetate (>methylcitrate production)
reduced supply of succinyl-CoA
 alfa-ketoglutarate
 glutamate >  glutamine

14. Glutamine synthesis the principal means of NH4 detoxification?
The osmotic action of glutamine
CEREBRAL EDEMA
NEURON
ASTROCYTE
SWELLING
ASTROCYTE
SWELLING
NH4
NH4
Glutamate
Glutamine
GS
Alzheimer type II
astrocytosis
Glutamine
Alzheimer type II
astrocytosis
Glutamate

15. Glutamine induced cerebral edema
hyperammonemia
↑ glutamine
astrocyte swelling
brain edema
astrocyte dysfunction
encephalopathy
brain stem compression
Citrullinemia 8 d NH3 2083 M
hyperintensity basal ganglia, talami
subcortical white matter
cytotoxic edema
Majoie 2004
OTC 2 d NH3 1000 M
Multicystic lesions
Yamanouchi 2002

16. Glutamine synthesis the principal means of NH4 detoxification?
NEURON
NH4
Gln
Gase
glutamine
ROS
NH4
Glu
NH4
Gln
regenerated in
mitochondria
ASTROCYTE
Gln
Gase
NH4
Gln
Glu
GS
NH4
Glu
ROS
MTP
ROS
astroglyal
dysfunction
ENCEPHALOPATHY
Albrecht & Norenberg Hepatol 2006

17. Pathogenetic Mechanisms of Hyperammonemia
neuronal damage
•
•
•
•
•
•
Glutamine induced cerebral edema
Neurotransmitter perturbation
Oxidative stress
Neuroinflammation
Energy failure
Other……

18. Transmitters and Receptors
• Increased aromatic amino acids uptake precursors of
neurotransmitters
• Glutamatergic excitotoxicity
• Impairement of cholinergic transmission
• Increased serotoninergic transmission
• False transmitters compete with normal transmitters

Tyramine

Octopamine

Phenylethanolamine

19. glutamate
NMDA
Ca++
ROS
ATP 
NOS
ASTROCYTE
Ca
MTP
Na+
ATP 
++
Na+
Calcineurin
Calmodulin
NO
POST-SYNAPTIC
NEURON
glutamate
NH4
GS
Gln
Na+

20. Pathogenetic Mechanisms of Hyperammonemia
neuronal damage
•
•
•
•
•
•
Glutamine induced cerebral edema
Neurotransmitter perturbation
Oxidative stress
Neuroinflammation
Energy failure
Other……

21. The role of oxidative stress
glutamate
NEURON
NMDA
Ca++
Gln
Gase
Na+
ROS
NH4
Glu
ROS
Gln
ATP 
Ca++
Na+
ASTROCYTE
ATP 
Gln
Gase
NH4
Glu
iNOS
ROS
ASTROCYTE
MTP
Calcineurin
Calmodulin
NO
POST-SYNAPTIC
NEURON
NH4
Glu
Gln
glutamate
GS
NH4
MPT
NH4
ROS
NEURONAL
astroglyal
dysfunction
ENCEPHALOPATHY
GS
Gln
Na+

22. Pathogenetic Mechanisms of Hyperammonemia
neuronal damage
•
•
•
•
•
•
Glutamine induced cerebral edema
Neurotransmitter perturbation
Oxidative stress
Neuroinflammation
Energy failure
Other……

23. Hepatic encephalopathy: role of inflammation
HYPOTHERMIA
N-ACETYL-CYSTEINE
IBUPROFEN
NEURON
NEUROINFLAMMATION
TNF-
IL-6
IL-1
TNF-
IL-6
IL-1
MICROGLIAL
ACTIVATION
MICROGLIAL
ACTIVATION
BBB
INFECTION – INFLAMMATION
blood capillary
NH4 – LACTATE

24. Pathogenetic Mechanisms of Hyperammonemia
neuronal damage
•
•
•
•
•
•
Glutamine induced cerebral edema
Neurotransmitter perturbation
Oxidative stress
Neuroinflammation
Energy failure
Other……

25. The role of energy failure
NH4+ exposure
generates
secondary Cr
deficiency in
brain cell
cultures
Braissant 2010
• altered oxidative
phosphorylation
• cessation of ATP
synthesis
• production of
ROS and cell
death

26. Pathogenesis of brain damage in HA: others
Astrocyte swelling can cause a secondary release of Glu into the intercellular space
KGM neurotoxic ?
**
*
↑↑
«Trojan horse» hypothesis
SNAT5
Modified from Braissant; J Inherit Metab Dis (2013) 36:595–612
*KMG : α-ketoglutaramate; AKGM are increased in UCDs
ROS ↑↑
MPT open
↓↓ SNAT5. Trapping GLn
Altered Neurotrasnsmitter
system
**Imp:, brain NO metabolism is affected in a number of ways by NH4 + exposure. Effects vary
depending on whether the exposure is acute or chronic, on brain cell type, and whether Arg
supply is normal or decreased
Braissant; J Inherit Metab Dis (2013) 36:595–612;Albrecht; Hepatolgoy . 2006 Oct;44(4):788-94; Halámková; Talanta. 2012 Oct 15;100:7-11;
Vergara F et al; Science 1974;183:81-83; P. Desjardins et al. / Neurochemistry International 60 (2012) 690–696

27. Pathogenesis underlying brain dysfunction. Acute and chronic
hyperammonemia
Conclusions
•
How HA can lead to severe consequences in the central nervous system (CNS) remains unclear.
•
The rise in ammonia levels, the elevations of glutamine, and the effect of glutamine on the brain are
proposed to account for the different effects of acute (vs chronic) hyperammonemia on the brain.
•
In acute hyperammonemia the excessive NMDA receptors activation could be inducing neuronal death
•
In chronic hyperammonemia the impaired function of the glutamate-nitric oxide-cGMP pathway,
associated to NMDA receptors could be inducing cognitive impairment.
•
N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA) receptors are fundamental for
learning because they are the major modulators of the long-term potentiation, the electrophysiologic
mechanism for learning.
Braissant; J Inherit Metab Dis (2013) 36:595–612;Albrecht; Hepatolgoy . 2006 Oct;44(4):788-94; Halámková; Talanta. 2012 Oct 15;100:7-11;
Vergara F et al; Science 1974;183:81-83; P. Desjardins et al. / Neurochemistry International 60 (2012) 690–696;
Cauli O et al Metab Brain Disease ; 2009 Mar;24(1):69-80; Alison S. Et al ; Chest Chest 2007;132;1368-1378