Cell migration in the cerebral cortex

How does it work?

What happens when it goes wrong?

Story of interplay between basic and clinical science

Inside-out programme of neurogenesis

Somal translocation Glial-guided locomotion

“Reeler” mutant mice

- Mice are ataxic, fall over a lot

- Lack cerebellum

- Layers in cerebral cortex are inverted

Inverted cortical layers in reeler mutants

“Reeler” mutation mapped to gene, named Reelin.

- Encodes large secreted protein

- Specifically expressed by Cajal-Retzius cells in marginal zone of developing cortex

- Mutations in the human Reelin gene result in Lissencephaly with Cerebellar Hypoplasia

Lissencephaly with Cerebellar Hypoplasia

(OMIM: 257320)

- Autosomal recessive

- Simplified folding of cortex (“smooth” cortex)

- Cerebellum very underdeveloped

- Severe ataxia, mental retardation and epilepsy

“Scrambler” mutant mice show same behaviouraland anatomical defects as Reeler mice

- Mapped to gene Dab1

- Encodes cytoplasmic “adaptor” protein

- Link cytoplasmic tails of transmembrane proteins to intracellular signaling proteins

=> Maybe in same biochemical pathway as Reelin?

What’s in between them?

?

Reelin (secreted from MZ cells)

Dab1

Outside

Inside

(inside migratingneuron)

Dab1 protein found to bind to cytoplasmic tails of two transmembrane proteins (related to each other):

- VLDLR and ApoER2(very low density lipoprotein receptor and ApoE receptor 2)

- These proteins found to bind Reelin through their extracellular domains

- Binding of Reelin leads to phosphorylation of Dab1 protein

VL

DL

R

Reelin (secreted from MZ cells)

Dab1

Outside

Inside

(inside migratingneuron)

Ap

oE

R2

P

What happens when these genes are mutated?

- Knock out VLDLR in mice: very little effect

- Knock out ApoER2 in mice: very little effect

- Make double KO: looks just like reeler mutants

=> These proteins are redundant in this process

Upon Reelin binding to VLDLR or ApoER2:

- Dab1 recruited

- Dab1 phosphorylated by Src or Fyn(Src;Fyn double mutant has same phenotype as Dab1)

- Signaling to cytoskeleton

- Phosphorylated Dab1 degraded (entire complex including receptors endocytosed and degraded)

VL

DL

R

Reelin (secreted from MZ cells)

Dab1

Outside

Inside

(inside migratingneuron)

Ap

oE

R2

P Src/Fyn

But what is the process at a cellular level?

- Ultimate defect is inverted layers but what causes that?

- What cellular effects does Reelin have? - Not a chemoattractant

- Not a “stop signal”

Reelin protein shown to stimulate detachment of migrating neurons in vitro

- in this case it was cells migrating to the olfactorybulb but concept same

- Dab signaling shown to:

(i) reduce expression of adhesion molecules (3-integrin)(ii) promote detachment from radial glia

GFP transfection can label radial units (glia and neurons)

Migrating neurons fail to detach from radial glia

wild-type dab1 mutants

But what is the process at a cellular level?

- Failure to detach from radial glia

- Creates “logjam”; newly generated neurons can’t get past first ones

Layers form in outside-in order, instead of inside-out order

Other types of cortical defects:

- Classical lissencephaly

- Double cortex syndrome

Classical Lissencephaly

(OMIM: 607432)

- Autosomal dominant

- Very little folding of cortex (“smooth” cortex)

- Four primitive layers instead of six

- Cerebellum normal

- Mental retardation, microcephaly and epilepsy

Caused by mutations in LIS1 gene

Double cortex syndrome

(OMIM: 300067)

- X-linked dominant

- Cortex completely smooth and highly thickened

- Four primitive layers instead of six

- Severe mental retardation and epilepsy in males

- Milder symptoms in heterozygous females

Caused by mutations in DCX gene

QuickTime™ and a decompressor

are needed to see this picture.

Staining for DCX protein highlights microtubulues

LIS1 and DCX proteins both bind to microtubules(part of cytoskeleton that controls migration)

- Required early for somal translocation (within cell’s own process) as well as later for migration along glia

- Both interact with microtubules in dynamic fashion

- LIS1 also interacts with VLDLR and binds phsophorylated Dab1 in response to Reelin signaling

link from Dab1 to cytoskeleton

Actin filament-interacting proteins

Microtubule-interacting proteins

DCX

Prenatal/neonatal risk factors

Early prodromal signs

Distributed neuropathology - local and long-range

Genes implicated - Nrg1, DISC1, DTNBP1- have roles in neurodevelopment

Schizophrenia as a neurodevelopmental disorder

Summary of observed pathological changes

(Frankle, 2003)

Cell migration defects in schizophrenia?

DISC1

- Broken by translocation in certain families with SZ

- Protein binds NUDEL (which binds LIS1)

- Truncated protein does not bind

- Truncated form reduces neurite outgrowth when transfected into PC12 cells

Mutations in DISC1 are associated with schizophrenia and other psychiatric disorders

Mice with mutations in DISC1 show behavioral defects thought to be “endophenotypes” of schizophrenia:

- hyperactivity- prepulse inhibition (suppression of startle response)- working memory

Disrupting DISC1 function ONLY during development still results in behavioural defects

=> Neurodevelopmental functions of DISC1 essential for normal brain structure and function

Tangential migration also very important

- Especially for GABAergic (inhibitory) interneurons

- Not generated in cortical ventricular zone

- Generated subcortically in ganglionic eminences

- Migrate long distance to cortex

- Controlled by positive and negative guidance cues

Interneurons generated subcortically and migrate to cortex

(Flames et al., 2004, Neuron 44, 251)

Neuregulin-1 in control of radial migration

Nrg1 protein attracts interneurons to cortex

Slice of developing mouse brain - interneurons (red) normally migrate up to cortex but will be attracted to cells expressing Nrg1 protein placed on slice (green)

Mutant mice lacking Nrg1 in cortex have fewer interneurons in cortex

ErbB4 is receptor for secreted Nrg1 protein

Nrg1 and ErbB4 both genetically associated with SZ in humans

Nrg1 and ErbB4 mutants lethal in mice

But heterozygotes live and show hyperactivity and other “endophenotypes” of schizophrenia - e.g., working memory deficits

Reversed by clozapine (no effect on wild-type activity)

Neuregulin-1 and ErbB4 in schizophrenia

Summary

- Cell migration is a complex process that is crucial for nervous system development in general and cortical

function in particular

- Severe clinical syndromes caused by mutations in a number of single genes

- More subtle genetic variation in cell migration genes may contribute to multigenic disorders such as epilepsy,

schizophrenia and autism.

(Caveat: genes like DISC1 and Nrg1 also have non-developmental roles in adult)

References:

Olson and Walsh (2002)Curr. Opin. Genetics and Development 12, 320

Marin and Rubenstein (2001)Nat. Rev. Neurosci. 2, 780

Websites with nice movies:

http://www.ipmc.cnrs.fr/~duprat/neurophysiology/video.htm

http://www.rockefeller.edu/labheads/hatten/movies.html

3-integrin not downregulated in scrambler mice

ntegrins bind extracellular matrix molecules and mediate cell adhesion

Blocking 3-integrin in scm cortical slice restores normal detachment

Several types of defects observed: