Lecture 34- Developmental disorders II

Question 1

How does the dendritic arbor develop?

Answer 1

-Basic plan is genetically specified– programs of gene transcription -Growth and branching influenced by environmental factors: • local signals (contact dependent or diffusible cues) • active synapses

Question 2

How do dendritic spines acquire their mature shape and density?

Answer 2

-from active neuronal inputs

• Spine size and shape reflects strength of synapse
• Changing inputs changes in size/shape: dynamic system (morphological and functional “plasticity”)
• Large spines with prominent heads (“mushroom”) correlate with more glutamate receptors and stronger “mature” synapses filopodium is the early stage, as mature get to the mushroom morphology this is when they are mature

Question 3

What is the definition of mental retardation and what are the symptoms?

Answer 3

• MR defined by an intelligence quotient (IQ) below 70 (two standard deviations below the average IQ of 100), corresponding to ~2% of the general population • Mental retardation often associated with other clinical symptoms – called syndromic MR • In syndromic MR, the cognitive disability is associated with a defined array of body and brain malformations, neurological or psychiatric symptoms or metabolic defects • In non syndromic MR, no abnormalities are observed apart from sub normal intelligence

Question 4

What are the two syndromes involving mental retardation that are genetically caused?

Answer 4

-Down and Patau’s syndromes

Question 5

What is Down syndrome?

Answer 5

• Trisomy (or partial triplication) of Chr 21 -Most common chromosomal abnormality in humans (~1/1000 births) - Neurological: intellectual delay, progressive neurodegeneration similar to Alzheimer’s disease, also associated with epilepsy –Other: distinct facial features, problems with the heart and digestive tract -doesn’t have to be the whole chromosome 21 that is extra, can be partial -accelerated neurodegeneration

Question 6

What is Patau’s syndrome?

Answer 6

-Trisomy (or partial triplication) of Chr 13 - Complex dysmorphic syndrome: -Neurological: small brain, cognitive and motor problems, spina bifida (myelomenigocoele), eye problems – Other: musculoskeletal, urogenital, polydactyly -Most patients die within the first year of life

Question 7

What are the autism spectrum disorders (ASD’s)?

Answer 7

-family of related disorders with varied clinical manifestations

Question 8

What are the core symptoms of autism spectrum?

Answer 8

• impairment in social interaction, verbal and non verbal communication -repetitive, stereotyped behaviours, restricted interests -definition: - have to have the core symptom -adjunct symptoms= sometimes, not every case

Question 9

What are adjunct symptoms of autism?

Answer 9

-intellectual disability -seizure disorders • ~25% of cases have a known genetic cause, some chromosomal loci (e.g.16p11.2 containing 25 genes incl. Sez6L2) and risk genes known

Question 10

What are some autism spectrum disorders that can be caused by single gene mutations?

Answer 10

• Fragile X syndrome CGG repeat expansion causes “fragile” site on the end of the X chromosome that silences the Fragile X Mental Retardation 1 gene (FMR1) coding for FXMR1 protein) • Rett syndrome Mutation of MeCP2 gene (on X chr, affects females)– transcription repressor -Between 2 10 years of age, language and motor skills regress, seizures, and stereotypic movements and MR develop

Question 11

What are abnormal dendrite and synapse development in syndromic MR?

Answer 11

• Patau’s syndrome (trisomy 13): • Protrusions have immature, filopodial like morphologies • Down syndrome (trisomy 21): • Very small, poorly developed spines

Question 12

What are the dendrite and dendritic spine abnormalities in ASDs, Rett syndrome, Fragile X MR and non-specific mental retardation?

Answer 12

• Post-mortem analysis of brains from patients with an autism spectrum disorder or Rett syndrome showed:

– Smaller neuron cell bodies,reduced size of dendritic trees

• Neurons from patients with MR/ASD’s bear fewer mature spines:

– Non-syndromic MR–long, thin with small heads, reduced in number

– Fragile X syndrome: long tortuous protrusions with immature filopodial morphology, increased in number

Question 13

What are the spinal abnormalities commonly associated with cognitive disorders?

Answer 13

-Changes in spine density and shape caused by both GENETIC and ENVIRONMENTAL factors

Question 14

What types of genes/proteins cause mental retardation and/or ASD’s when mutated?

Answer 14

• >450 different genes or genetic loci now known • Can be classified according to the function of their encoded proteins • Functional groups strongly represented include synapse formation, reorganization of the cytoskeleton, synapse signalling

Question 15

What are the proteins involved in mental retardation and autism spectrum disorder?

Answer 15

• neurexin, neuregulin
• CASK
• Munc18
• AMPA and NMDA receptors
• GKAP
• SHANK

Question 16

What are the dendrite and spine cytoskeleton regulated by?

Answer 16

• dendrite and spine cytoskeleton regulated by small GTPases of the Rho family
• Dendrite & spine growth via actin
• Dendrite formation & stabilization via effects on microtubules
• Dendrite & spine growth via actin
• Gene transcription for dendrite formation
• Small GTPases of Rho family (e.g. Rho, Rac, cdc42) signal to alter actin cytoskeleton
• ultimately all 3 end up in affecting the cytoskeletal components of the cell
• small GTPases of Rho family: Rho, Rac, cdc42= these act like signalling swithces that switch on signals and affect cytoskleteon

Question 17

What are the Rho small GTPases?

Answer 17

-molecular switches regulating the actin cytoskeleton

• Most studied Rho GTPases: RhoA, Rac1 and Cdc42

• Act as molecular switches:
– active, GTP bound
– inactive, GDP bound state

• Activated by guanine nucleotide exchange factors – GEFs
• Inactivated: Bound GTP converted to GDP by the GTPase activity of the protein, stimulated by GTPase activating proteins GAPs
• have activators= GEFs
• inactivators= GAPs

-cycle between active and inactive (GDP- to GTP)

Question 18

What do Rac and cdc42 do?

Answer 18

• stimulate neurite outgrowth and spine growth by actin filament assembly

Question 19

What does Rho do?

Answer 19

-– inhibits outgrowth, causes actin filament break down and retraction

Question 20

What are the RhoGTPase effectors?

Answer 20

-Include kinases (signal via adding phosphate groups) and regulators of the actin / myosin cytoskeleton

(know only the overall effects of cdc42 + Rac and Rho)

Question 21

What happens in many single-gene causes of mental retardation?

Answer 21

-in many single-gene causes of Mental Retardation, mutations occur in genesencoding:
• Rho GTPase activators (GEFs)

• inactivators (GAPs)
• effector

-Abnormal cytoskeletal responses (growth cones, dendritic filopodia and spines)

aberrant circuit development reduction in spine growth & plasticity intellectual disability

Question 22

What is actin cytoskeleton required for?

Answer 22

-for spine morphological and functional plastictiy

• Long term potentiation (LTP) inducing stimuli stabilize actin polymerization and increase size of spine head
• Actin cytoskeleton also required for synapse strengthening– increased trafficking of AMPA receptors into PSD

Question 23

What is the likely pathology in fragule X mental retardation?

Answer 23

• most likely it is exeggerated long term depression (LTD)
• Loss of FXMR1 interferes with local translation

of mRNA’s in dendritic spines

-FXMR1 normally acts to repress translation

so get excess protein synthesis when FXMR1

is silenced (in FraX syndrome)

• LTD induction through “metabotropic” glutamate

receptors (mGluRs) needs synthesis of particular

proteins at synapse

-No FXMR1 = more LTD protein synthesis more LTD

(synapse weakening through AMPA receptor

internalization)

-Loss of AMPA receptors at synapse leads to

shrinking of spine head and transformation back

into filopodia (trying to re-establish connection?)

-if don’t have enough signalling then receptor reduction, LTD excessive exaggarated LTD

Question 24

What is the therapeutic approach for Fragile X mental retardation?

Answer 24

• If excess protein synthesis after mGluR signalling contributes to FXMR pathology,

then drugs blocking mGluR (antagonists, negative modulators) should help:

• Does it?

-Yes mGluR5 blocker improves behavioural outcomes in Fragile X

patients

Question 25

Do other mental retardation/autism spectrum disorder mutations negatively affect mGluR signalling?

Answer 25

-Shank3 part of protein scaffold in PSD of excitatory synapses; mutated in an ASD called Phelan McDermid syndrome - Tsc2 (tuberous sclerosis) relays mGluR signal - Loss of Shank3 or Tsc2 reduces mGluR5 downstream signalling, protein translation and synaptic plasticity (opposite to FXMR; interbreeding Tsc2 mutant mouse line with Fmr1 rescued the deficits of the parent lines!) -Therapeutic approach for ASD patients with mutant Shank 3 or tuberous sclerosis patients – mGluR5 potentiator should restore normal function -if parents one has top much and one too little = recues the offspring

Question 26

What is the state of GABA-ergic synapse in Rett syndrome?

Answer 26

- Inhibitory GABA ergic synapse deficits in Rett syndrome - Rett syndrome caused by Mecp2 mutation - Inhibitory synapse defects contribute to Rett syndrome - Mecp2 deficiency reduces pre-synaptic Gad1/Gad2 (enzymes for GABA synthesis), GABA, inhibitory synaptic currents (mIPSCs) - Therapeutic approach: GABA receptor potentiators could rescue deficient presynaptic GABA release in Rett syndrome -Mecp2 mutation= don’t have enough GABA synthesis and release -way to solve it= GABA receptor potentiators can recuse parts of the deficiency = the ret syndrome

Question 27

When should treatment be started?

Answer 27

* “The earlier, the better” is common view * For ASD’s, early detection is important as early behavioural therapy can improve social and language deficits -Atypical antipsychotics (e.g serotonin receptor blocker used for schizophrenia) are able to improve ASD adjunct symptoms (aggressive behaviour and tantrums) but may not be suitable for early/chronic treatment of children -Surprisingly, even adult treatment may be successful for patients with certain “neurodevelopmental” disorders: -Mouse Rett syndrome models: • Mecp2 required for adult neuron function (same symptoms as total knockout mouse when Mecp2 expression lost in adult) •Restoration of Mecp2 expression in adult Mecp2 deficient mice can rescue nearly all deficits, including LTP

Question 28

Why study brain development?

Answer 28

-High throughput DNA sequencing and availability of the human genome – huge influx of new information about the genetics of disease -What is the function of these disease associated genes in the normal brain?

Question 29

Lecture summary I:

Answer 29

- Structure of outgrowing dendritic processes, particularly spines, is dynamic - Changes in dendritic spine shape (via actin filament extension/retraction) are closely correlated to neuronal activity Abnormalities in spine shape & number (e.g. small/sparse, long/distorted/thin) correlate with mental retardation indicating problems with information processing • Spine formation and morphological plasticity (growth or shrinkage depending on synapse potentiation or depression) compromised, even in MR that results from single gene mutations – abnormal synapses are key to development of these diseases

Question 30

Lecture summary II?

Answer 30

-Many synaptic proteins found to be mutated in MR/ASDs, including pre synaptic vesicle release proteins and post synaptic organizers -One major class of actin cytoskeletal regulatory proteins Rho GTPases and their activators (GEF’s), inactivators (GAP’s) and effectors (kinases and actomyosin regulatory proteins) is strongly implicated in MR • Post-natal developmental disorder (Rett syndrome) is (genetically) reversible and may be amenable to treatment in adult -Possible therapeutic approaches to rectify synaptic pathologies: