Attività delle sinapsi glutamatergiche e sindromi dello spettro autistico

Attività delle sinapsi glutamatergiche e sindromi
dello spettro autistico
17 JANUARY 2013 | VOL 493 | NATURE |
* Neuronal activity induces the post-translational modification of synaptic molecules,
promotes localized protein synthesis within dendrites and activates gene transcription,
thereby regulating synaptic function and allowing neuronal circuits to respond
dynamically to experience.
* Many of the genes that are mutated in autism spectrum disorder are crucial
components of the activity-dependent signalling networks that regulate synapse
development and plasticity. Dysregulation of activity-dependent signalling pathways in
neurons may, therefore, have a key role in the aetiology of autism spectrum disorder.
-1/100 children display signs and symptoms that lead to a diagnosis of autism
spectrum disorder (ASD).
-This debilitating developmental disorder is characterized by impairments in
social interaction and communication, and by restricted, repetitive and
stereotyped behaviour and interests. In addition, individuals with ASD often
have a seizure disorder and intellectual disability.
-Most of the features of ASD manifest in the first years of life, at the time of
brain development when sensory experience is modifying excitatory synapse
maturation and elimination, and promoting the development of inhibitory
synapses
ASD may be due to a disruption of the normal process of experiencedependent synaptic development, resulting in an imbalance between
excitation and inhibition in the developing brain
Recent evidence indicate that:
ASD is often due to newly arising gene copy number variants (CNVs) — such as a
deletion or duplication of a region of a chromosome — or a rare mutation that arises in
the germ cell, particularly in the sperm of older fathers. In cases with CNVs, ASD is
hypothesized to be a result of the increased or decreased expression of one or several
genes that lie within the region of the genome in which the CNV mutation resides.
This lead to hypothesize that a convergent molecular pathway dysregulated in ASD is
the signalling network that controls synapse development and function.
In fact, sensory, cognitive and emotional experiences shape synapse and neural-circuit
development. Neuronal activity triggers local changes at the synapse, altering the
composition, shape and strength of the synapse, inducing specific changes in messenger
RNA translation near synapses and sending signals to the nucleus to induce gene
transcription programs that control synaptic maturation and function. These neuronal
activity-dependent pathways are crucial for learning and memory and for adaptive
behavioural responses
Regulation of synaptic development and function by neuronal activity
- Adhesion molecules and components of the
postsynaptic density organize and regulate the
formation of excitatory synapses on dendritic
spines and can stimulate activity-dependent signalling
networks within the postsynaptic neuron.
- At the synapse, the excitatory neurotransmitter
glutamate can bind several glutamate receptors, the
NMDA (NMDAR), the AMPA (AMPAR) and the
metabotropic glutamate receptor (mGluR). Signalling
from stimulated mGluR regulates mRNA translation,
which is required for long-lasting forms of synaptic
plasticity.
- Modification and cell-surface expression of AMPA
receptors underlies many aspects of
synaptic plasticity. Stimulated NMDA receptors flux
calcium, inducing calcium-dependent signalling
networks, at the synapse, that regulate AMPA receptor
function and actin reorganization.
- Calcium influx through NMDA receptors and L-type
voltage-sensitive calcium channels (L-VSCCs) triggers
calcium-dependent signalling to the nucleus, leading to
the modification of
transcriptional regulators and resulting in the induction
of activity-dependent gene expression.
- Genes induced by neuronal activity (including Bdnf,
Arc and Ube3A) function to regulate synapse formation,
maturation, elimination and plasticity. P,
phosphorylation; TF, transcription factor.
Synapses are stabilized, matured and eliminated in response to neuronal activity during
postnatal development
Genes and proteins associated with ASD are both regulated by and control
activity-dependent pathways that modulate synaptic function. Many of the
mutations associated with ASD lead to alterations in excitatory or inhibitory
neurotransmission that disrupt activity-dependent signalling and activitydependent synapse development, maturation and refinement. In addition,
neuronal activity clearly regulates the function, localization and expression of
many of the proteins that are associated with ASD.cia
As an example
David Hubel and Torsten Wiesel discovered that occluding one eye during a crucial
period of development disrupts the formation of ocular dominance columns in the
visual cortex, demonstrating a central role for experience in the development of
neural circuits
Durante lo sviluppo del sistema visivo:
Le connessioni si rifiniscono e si adattano in
funzione dell’attività neurale che, partendo
dalla retina, oscilla costantemente nel sistema
visivo.
Subito dopo la nascita, è la stimolazione
ambientale che genera l’attività neurale nel
sistema visivo.
Rappresentazione schematica della retrazione degli assoni
del LGN che proiettano allo strato 4 della corteccia visiva
(nel gatto)
La deprivazione monoculare
• La formazione delle colonne di dominanza oculare
dipende dal bilanciamento dell’attività dei due occhi.
• La chiusura di un occhio in un animale in via di sviluppo
(deprivazione monoculare) riduce drasticamente le
capacità percettive dell’occhio.
• I neuroni dell’LGN diminuiscano del 40%
• Cellule della corteccia visiva striata non rispondono a
stimoli presentati all’occhio deprivato.
alla nascita la corteccia visiva dei mammiferi è
immatura sia anatomicamente che funzionalmente
nei primi mesi di vita l'influenza ambientale regola
una maturazione strutturale geneticamente
predeterminata
lo sviluppo dipende dall’esperienza visiva acquisita
in un breve periodo plastico: “periodo critico”
l’esperienza visiva modula il livello e la
conformazione dell’attività neuronale
nella vita fetale le connessioni tra CGL e corteccia visiva sono
sovrapposte
tutte le cellule corticali visive sono binoculari
dalla nascita, attraverso fenomeni competitivi legati alla
visione, inizia il processo della “segregazione”
le cellule si aggregano in colonne più responsive allo stimolo di
un occhio rispetto all’altro
Neuronal activity regulates mRNA translation and synaptic plasticity
- At the excitatory synapse, the cell-adhesion molecules neurexin and
neuroligin and structural proteins in the postsynaptic density, including
SHANK proteins, regulate, and are regulated by, neuronal activitydependent signalling networks
UBE3A degrades ARC, which can affect the trafficking of AMPA
receptors. During plasticity,
- FMRP inhibits mRNA translation, and mGluR signalling regulates FMRP
activity. Growth factors, including BDNF, whose expression is induced by
neuronal activity, bind receptor tyrosine kinases (including TrkB), which
activate multiple signalling pathways, including the PI(3)K–AKT pathway.
The PI(3)K–AKT pathway, when activated, leads to phosphorylation of the
TSC1–TSC2 complex to control mTOR activity.
- Mutations in neurexins, neuroligins, SHANKs, GKAP, UBE3A,
FMRP and TSC1–TSC2 are associated with ASD.
Mutazioni nei geni codificanti per proteine che svolgono ruoli funzionali diversi
alle sinapsi glutamatergiche: strutturale, sintesi e degradazione locale di
molecole, e regolazione dell’espressione genica sono associate a ASD.
Quali ASD syndromes?
Mutations in the L-type voltage-sensitive calcium channels that flux calcium to
initiate activity-dependent gene transcription are associated with Timothy
syndrome, which has ASD phenotypes.
- Mutations in RSK2, CBP, the methyl binding protein MECP2 and the ubiquitin
ligase UBE3A are causes of Coffin–Lowry syndrome, Rubinstein–Taybi
syndrome, Rett syndrome and Angelman syndrome, respectively.
Activity-dependent gene
expression and ASD
Synaptic proteins implicated in ASD
Mutations in multiple synaptic cell-adhesion molecules and
components of the PSD are associated with ASD.
-Neuroligins and neurexins
Neurexins (presynaptic) Neuroligins (postsynaptic) are localized specifically to
inhibitory or excitatory synapses, suggesting a role in synapses formation.
Mutazioni nei geni codificanti per neurexins and neuroligins osservate in ASD,
sono associate ad alterazioni nella neurotrasmissione eccitatoria ed inibitoria
E’ tuttavia interessante che……..
-knock-in mice that harbour the neuroligin-3 ASD missense mutation Nlgn3(R451C)
display impaired social interactions, recapitulating a key feature of ASD. These knockin mice also have enhanced inhibitory neurotransmission with no alterations in
excitatory neurotransmission, resulting in a defect in excitatory–inhibitory balance in
the brain.
-Knock-in mice with another ASD-associated missense mutation, Nlgn3(R704C)
displayed a decrease in AMPA-receptor-mediated synaptic transmission in the
hippocampus, but no alteration in NMDA-receptor or GABA (!-aminobutyric acid)receptor-mediated neurotransmission.
Mutations in NRXN1 are also associated with ASD in humans. Nrxn1 knockout mice
have defects in excitatory postsynaptic current (EPSC) frequency and evoked
postsynaptic potentials, but show no change in inhibitory neurotransmission in the
hippocampus.
Effetto mutazione-specifico
These findings indicate that particular ASD-associated mutations in neurexins or
neuroligins disrupt excitatory OR inhibitory neurotransmission in the brain in specific
ways and suggest that modelling ASD-associated mutations in mice will be important
for elucidating how each mutation affects synaptic function and gives rise to ASD.
SHANKs
Shanks are scaffolds proteins in the PSD of excitatory synapses that regulate the
organization of postsynaptic signalling complexes, as well as the morphology and function
of synapses.
Rare mutations associated with ASD have been found in SHANK2 and SHANK3, and
recently in SHANK1. Knockout mice with deletions in members of the Shank family of
genes exhibit behaviours similar to those observed in ASD. For example, Shank3
knockout mice have deficits in social interactions and engage in repetitive behaviours,
such as excessive grooming, that lead to self injury.
- Shank3 knockout mice have reduced cortico-striatal synaptic transmission;
- Shank2 knockout mouse models show defects in NMDA-receptor-dependent excitatory
neurotransmission and synaptic plasticity in the hippocampus. Partial agonist of NMDA receptor
(d-cycloserine) or positive allosteric modulator of mGluR5 normalize NMDA receptor function and
decrease autistic behaviours, suggesting that impairment of NMDA receptor functioning may be
a key mechanism through which mutations in genes that encode the SHANK family of proteins
lead to ASD.
Activity-dependent regulation of mRNA translation
Synaptic plasticity elicited by glutamate binding to NMDA receptors or group 1 mGluRs, require protein
synthesis mediated by mRNAs and ribosomes that are localized near synapses.
Several genes that are mutated in ASD — FMR1, TSC1, TSC2 and PTEN — have key roles
in protein-synthesis-dependent plasticity of synapses
Fragile X syndrome
- Fmr1 knockout show
levels of protein synthesis and
suggesting that under normal conditions FMRP
LTD in response to mGluR stimulation,
mGluR-dependent protein synthesis and LTD.
- FMRP negatively regulates the translation of specific mRNAs at the synapse: Arc is one of the beststudied mRNA targets of FMRP. This protein promotes the internalization of AMPA receptors at excitatory
synapses. Translation of Arc mRNA is enhanced during, and crucial for, mGluR-dependent LTD
…….
During early postnatal brain development, neuronal activity promotes changes in
glutamate receptor subtype that are required for proper maturation of
excitatory thalamocortical synapses in the somatosensory cortex.
In Fmr1 knockout mice, this activity-dependent maturation of the excitatory
thalamocortical synapses is dysregulated, resulting in a persistence of silent
NMDA-receptor-only synapses at times during brain development when these
synapses would normally express both NMDA and AMPA receptors. The absence
of AMPA receptors at these synapses may be due, in part, to the dysregulation
of Arc mRNA translation that occurs in Fmr1-knockout mice.
FMRP and synapse elimination:
- FMRP has also been suggested to be crucial for activity-dependent synapse elimination, a key
process during postnatal brain development that may be defective in fragile X syndrome
Rett syndrome
Mutations in MECP2 lead to Rett syndrome, which is a form of ASD characterized by impaired
language development, loss of social engagement, stereotyped hand movements, seizures and
motor-system disabilities. MECP2 binds methylated cytosines within DNA and seems to function
mainly as a transcriptional repressor.
Neuronal activity induces the phosphorylation of MECP2 at Ser 421, raising the possibility that
activity-dependent phosphorylation of MECP2 mediates a genome-wide chromatin response to
neuronal activity
A Ser 421Ala knock-in mice display increased dendritic complexity and increased inhibitory
synaptic strength in the cortex. Behaviourally, the Ser 421Ala knock-in mice had deficits in their
response to new objects or mice. These findings indicate that activity-dependent
phosphorylation of MECP2 regulates synapse development and function and behavioural
responses to environmental stimuli.
NEUROGENESI ADULTA
Today’s question:
what key diseases—AD, schizophrenia,
seizure disorders, and psychiatric disorders like depression
and addiction and animal models of these disorders—can
reveal about the relationship between interneurons and
neurogenesis.
The Interesting Interplay Between Interneurons and Adult
Hippocampal Neurogenesis
Adult neurogenesis is a unique form of plasticity found in the hippocampus, a
brain region key to learning and memory formation.
While many external stimuli are known to modulate the generation of new
neurons in the hippocampus, little is known about the local circuitry mechanisms
that regulate the process of adult neurogenesis.
The neurogenic niche in the hippocampus is highly complex and consists of a
heterogeneous population of cells including interneurons.
Because interneurons are already highly integrated into the hippocampal
circuitry, they are in a prime position to influence the proliferation, survival,
and maturation of adult-generated cells in the dentate gyrus.
The dentate gyrus (DG) of the
hippocampus contains a neurogenic
niche, the subgranular
zone (SGZ), which is inhabited by a
heterogeneous population of cells
And cellular elements
Note the large cell bodies of the
MOPP interneurons
(mol layer perforant path cell, #1, blue);
the HICAP interneurons (hilar comsassociatl pathway related cell,
#2, orange); the HIPP interneurons
(hilar perforant pathassociated
cell, #3, purple); the L–M interneurons
(s. lacunosum/s.moleculare cells, #4, green);
and a typical
DG basket cell (also called
pyramidal basket cell, #5, pink)
Hippocampus dentate gyrus: SGZ, subgranular zone; GCL, granule cell layer
Neurogenesi e gliogenesi durante la formazione della corteccia cerebrale. Il tubo neurale è
formato da cellule neuroepiteliali che si estendono dalla superficie ventricolare a quella piale ed il
cui numero aumenta rapidamente (fase di espansione) mediante divisioni simmetriche. Nella fase
neurogenica alcune cellule neuroepiteliali diventano cellule della glia radiale che mediante divisioni
simmetriche aumentano di numero, mentre mediante divisioni asimmetriche generano cellule postmitotiche che di fatto sono i precursori delle cellule neuronali. Queste ultime si allontanano dalla
superficie ventricolare disponendosi in prossimità della superficie piale. La fase neurogenica si
accompagna pertanto ad una espansione radiale della superficie del tubo neurale. Con l’inizio della
gliogenesi (periodo perinatale) le cellule della glia radiale smettono di generare neuroni e danno
origine ad astrociti ed oligodendrociti.
Initially the newly generated neurons are “silent”, meaning that they
have no spontaneous or evoked postsynaptic currents to any
of the commonly applied agents (e.g., GABA, NMDA,
AMPA, glycine).
However, upon the formation of GABAergic synapses
they become sensitive to depolarization by GABA, followed by the
development of glutamatergic inputs, and lastly a switch to
hyperpolarization by GABA, a sign of neuronal maturity.
Adult generated neurons in the DG SGZ go through an almost
identical progression of steps initiating synaptic connectivity
with the surrounding and preexisting hippocampal
circuitry
As adult-generated cells differentiate into mature DG
GCLs, they not only respond to tonic GABA but also
receive phasic GABAergic inputs.
This phasic input is important because it depolarizes the
maturing cells and elevates their intracellular Ca2+ ([Ca2+]i)
levels via activation of voltage-gated calcium channels.
This increase in [Ca2+]i has been shown to stimulate
the expression of NeuroD, a transcription factor necessary
for the survival and differentiation of adult-generated cells in
The SGZ.
In this activity-dependent manner, GABAergic
interneurons in the existing hippocampal circuitry have
the power to regulate the differentiation of adult-generated
DG cells (Fig. 2).
The powerfull GABAergic interneuron
Relina e plasticità sinaptica adulta
Relina, recettori per le lipoproteine e plasticità sinaptica
L’Apolipoproteina E (ApoE) è una proteina coinvolta
nel trasporto del colesterolo ed alcune delle sue isoforme (ApoE!4)
rappresentano un fattore di rischio per la neurodegenerazione
della Malattia di Alzheimer.
I recettori per le lipoproteine legano non solo ApoE ma nche la
relina, contribuendo in tal modo alla trasduzione di segnali cruciali
non solo durante il neurosviluppo, ma anche nel cervello adulto, con
particolare riferimento alla plasticità sinaptica.
In tal modo, ApoE, il colesterolo, la relina ed i recettori per ApoE
rivestono ruoli essenziali alle funzioni cognitive quali, l’apprendimento,
la memoria e la stessa sopravvivenza neuronale
Metabolismo del colesterolo
Il colesterolo è insolubile in acqua e viaggia nel circolo sanguigno
legato a proteine sotto forma di particelle dette lipoproteine a bassa
lensità (LDL, low density particles). Le LDL si legano a recettori
situati sulla superficie cellulare, i complessi recettore-LDL vengono
ingeriti per endocitosi mediata dai recettori e recapitati agli endosomi
Ogni LDL
contiene
1500 mol
colesterolo
esterificato
ad acidi
grassi
Endocitosi di LDL mediata da recettori
I recettori per le LDL
La trasduzione del segnale mediata dalla relina
La trasduzione del segnale mediata dalla relina
La relina si lega con elevata affinità ai recettori per le LDL, VLDLR
and APOER2. Ciò attiva DAB1 mediante fosforilazione, ed a cascata
le proteine SRC (appartenenti alla famiglia delle tirosin kinasi), che a
loro volta aumentano il grado di fosforilazione di DAB1con
conseguente attivazione della PI3K e di PKB.
In sintesi, l’attivazione di DAB1:
i)
Favorisce la polimerizzazione dei microtubuli;
migrazione neur.
ii) Modifica la permeabilità agli ioni Ca2++ dei recettori glutammater
gici, NMDA, con conseguente attivazione di CREB;
plasticità sinapt
ApoE e demenza di Alzheimer
Il ruolo esercitato dai recettori di ApoE nella trasduzione del
segnale può contribuire alla comprensine della neuropatologia di AD.
ApoE, infatti legando i recettori APOER2 e VLDLR può esercitare
la sua influenza sul grado di fosforilazione della proteina Tau, con un
effetto diretto sulla polimerizzazione dei microtubuli.
Aggregati neurofibrillari
ApoE, funzione sinaptica e neurotrasmissione
Un ulteriore meccanismo attraverso il quale i recettori per ApoE
possono influenzare la funzione sinaptica, riguarda il loro contributo
al metabolismo del colesterolo.
Un adeguato contenuto in colesterolo delle membrane plasmatiche
assicura una normale funzione dei recettori NMDA.
In accordo con ciò, in ApoE-deficient mice sono stati riscontrati
deficit nello LTP.
Tale effetto è dipendente dalla isoforma di ApoE, essendo nello
specifico associato all’isoforma epsilon4.
Tale isoforma, è meno efficiente nelle normali funzioni richieste
per LTP e ciò spiegherebbe la ridotta frequenza di tale allele,
rispetto all’epsilon 3 e 2
Relina……neurosviluppo e non solo!?
Durante il neurosviluppo la relina è prodotta dai neuroni
Cajal-Retzius localizzati nella corteccia in via di sviluppo. Più
tardi, la relina è invece sintetizzata e rilasciata nello spazio
extracellulare da interneuroni GABA-ergici.
Questo shift sembra marcare un analogo shift della funzione
della relina:
Migrazione….vs…..plasticità sinaptica
Evidenze genetiche e non solo, hanno dimostrato che attraverso
il legame ai recettori APOER2 e VLDLR, la relina regola la
plasticità sinaptica
d i N
i
Come la relina regola
il recettore NMDA