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Disordini del ciclo dell’urea:la neurotossicità dell'iperammoniemia

Napoli 26 - 28 Novembre 2013

Diego MartinelliUnità Operativa Complessa Patologia Metabolica

diego.martinelli@opbg.net

…in general the age of onset, duration and degree of hyperammonemia may predictprognosis….

……….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

Pathogenetic Mechanisms of Hyperammonemia

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

Genetic causes• urea cycle defects • organic acidurias -oxidation def.• mitochondrial• hyperinsulinism/hyperam

monemia

Non-genetic causes• THAN• liver bypass/insufficiency• asphyxia-shock-heart fail.• infections• intoxication

0 500 1000 1500

Urea Cycle Defects

Organic Acidurias

-OX - mito

Hi-Ha

0 500 1000 15000 500 1000 1500

Urea Cycle Defects

Organic Acidurias

-OX - mito

Hi-Ha

Patient’s age• fetus• newborn• infant• child• adolescent• adult

Hyperammonemia: possible scenarios

The Urea Cycle

0 2000 4000 6000 8000 10000

NH4

glutamate

alanine

glutamine

urea

6 enzymes 2 carriers

Clay Chest 2007

normal UCDs – liver failure

M

Msall et al. NEJM 1984;310:1500-5

PROGNOSIS DEPENDS ON COMA DURATION

22.210.1

48.811.2

4711*

7813

Picca et al. Pediatr Nephrol 2001;16:862-7

2210*

4911

Good 2y

Bad 2y

*p<0.02pre-treatment

• PERI-INSULAR• FRONTAL• PARIETAL• OCCIPITAL

• THALAMIC RESTRICTED DIFFUSION (unusual in UCDs)

…suggesting that brain MRI may assist in determining prognosis & helpingclinicians with subsequent treatment decisions

ADC map

ADC map

neuronal damage

Pathogenetic Mechanisms of Hyperammonemia

• Glutamine induced cerebral edema• Neurotransmitter perturbation• Oxidative stress• Neuroinflammation• Energy failure• Other……

PORTALblood

NH4+ 300 M

SYSTEMIC blood

NH4+ 50 M

glutamine

ureaPerivenous hepatocyte (astrocyte, muscle)

Periportal hepatocyte

Ammonia detoxification

Plasma gln during neonatal hyperammonemiaScriver 1995

ATP ADP

glutamine

Is glutamine synthesis the principal means of NH4 detoxification?

NH4+ p. Gln CSF Gln

900 420 1380

NH4+ mol/L

< 100 >100p. Gln 397 386

Propionic ac.

NH4+ Gln

470 640560 229970 369

Methylmalonic ac.

The concentration changes of the nitrogen scavengerglutamine have to be interpreted in the light of NH4 levels.

In contrast to other hyperammonemic syndromes, in PAplasma glutamine do not increase in hyperammonemia, whereas CSF glutamine concentrations are elevated.

2010

depletion of oxaloacetate (>methylcitrate production)reduced supply of succinyl-CoA

alfa-ketoglutarate

glutamate > glutamine

NH4

GlutamineGlutamate

NH4

GS GlutamateGlutamine

The osmotic action of glutamine

Glutamine synthesis the principal means of NH4 detoxification?

CEREBRAL EDEMA

NEURON

ASTROCYTESWELLING

ASTROCYTESWELLING

Alzheimer type II astrocytosis

Alzheimer type II astrocytosis

Glutamine induced cerebral edema

hyperintensity basal ganglia, talami subcortical white matter

cytotoxic edema

Citrullinemia 8 d NH3 2083 M

Majoie 2004

OTC 2 d NH3 1000 M

Multicystic lesions

Yamanouchi 2002

hyperammonemia

↑ glutamine

astrocyte swelling astrocyte dysfunction

brain edema

brain stem compression

encephalopathy

Albrecht & Norenberg Hepatol 2006

Glutamine synthesis the principal means of NH4 detoxification?

NH4

glutamine

NH4

regenerated in mitochondria

ROS MTP

astroglyal dysfunction

ENCEPHALOPATHYGlnGlu

NH4

GS

Gln

Gln Glu

NH4

Gase

ASTROCYTE

NEURON

ROS

Gln Glu

NH4

Gase

ROS

GlnGlu

NH4

GS

Gln

Gln Glu

NH4

Gase

ASTROCYTE

NEURON

ROS

Gln Glu

NH4

Gase

ROS

neuronal damage

Pathogenetic Mechanisms of Hyperammonemia

• Glutamine induced cerebral edema• Neurotransmitter perturbation• Oxidative stress• Neuroinflammation• Energy failure• Other……

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

glutamate

NMDA

POST-SYNAPTICNEURON

Ca++ Na+

Ca++ROS

ATP MTP

CalmodulinNOS

NO

Glnglutamate

NH4

GS

ASTROCYTE

Calcineurin

Na+ Na+

ATP

neuronal damage

Pathogenetic Mechanisms of Hyperammonemia

• Glutamine induced cerebral edema• Neurotransmitter perturbation• Oxidative stress• Neuroinflammation• Energy failure• Other……

The role of oxidative stress

NH4

ROSMPT

NEURONALastroglyal dysfunction

ENCEPHALOPATHY

GlnGlu

NH4

GS

Gln

Gln Glu

NH4

Gase

ASTROCYTE

NEURON

ROS

Gln Glu

NH4

Gase

ROS

GlnGlu

NH4

GS

Gln

Gln Glu

NH4

Gase

ASTROCYTE

NEURON

ROS

Gln Glu

NH4

Gase

ROS

glutamate

NMDA

POST-SYNAPTICNEURON

Ca++ Na+

Ca++ROS

ATP MTP

Ca++ROS

ATP MTP

CalmoduliniNOS

NO

Glnglutamate

NH4

GS

ASTROCYTE

CalmoduliniNOS

NO

Glnglutamate

NH4

GS

ASTROCYTE

CalmoduliniNOS

NO

Glnglutamate

NH4

GS

ASTROCYTE

Glnglutamate

NH4

GS

ASTROCYTE

Calcineurin

Na+ Na+

ATP

Calcineurin

Na+ Na+

ATP

Calcineurin

Na+ Na+

ATP

neuronal damage

Pathogenetic Mechanisms of Hyperammonemia

• Glutamine induced cerebral edema• Neurotransmitter perturbation• Oxidative stress• Neuroinflammation• Energy failure• Other……

Hepatic encephalopathy: role of inflammation

NEURON

blood capillary

BBB

INFECTION – INFLAMMATION NH4 – LACTATE

NEUROINFLAMMATION

MICROGLIALACTIVATION

TNF- IL-6IL-1

MICROGLIALACTIVATION

TNF- IL-6IL-1

HYPOTHERMIAN-ACETYL-CYSTEINE

IBUPROFEN

neuronal damage

Pathogenetic Mechanisms of Hyperammonemia

• Glutamine induced cerebral edema• Neurotransmitter perturbation• Oxidative stress• Neuroinflammation• Energy failure• Other……

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

Pathogenesis of brain damage in HA: others

Modified from Braissant; J Inherit Metab Dis (2013) 36:595–612*KMG : α-ketoglutaramate; AKGM are increased in UCDs

**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

«Trojan horse» hypothesis

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

ROS ↑↑ MPT open

Astrocyte swelling can cause a secondary release of Glu into the intercellular space

KGM neurotoxic ?

↑↑*

SNAT5↓↓ SNAT5. Trapping GLn Altered Neurotrasnsmitter system

**

• 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.

Pathogenesis underlying brain dysfunction. Acute and chronic hyperammonemia Conclusions

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