Transcript Migration Medicine

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 behavioural
and 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
?
Outside
(secreted from
MZ cells)
Inside
Dab1
(inside
migrating
neuron)
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
Reelin
VLDLR
Inside
ApoER2
Outside
(secreted from
MZ cells)
P
Dab1
(inside
migrating
neuron)
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)
(secreted from
MZ cells)
Reelin
VLDLR
Inside
ApoER2
Outside
P
Src/Fyn
Dab1
(inside
migrating
neuron)
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 olfactory
bulb but concept same
- Dab signaling shown to:
(i) reduce expression of adhesion molecules (a3-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
Staining for
DCX protein
highlights
microtubulues
QuickTime™ and a
decompressor
are needed to see this picture.
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
Microtubuleinteracting proteins
DCX
Actin filamentinteracting proteins
Schizophrenia as a neurodevelopmental disorder
 Prenatal/neonatal risk factors
 Early prodromal signs
 Distributed neuropathology
- local and long-range
 Genes implicated - Nrg1, DISC1, DTNBP1
- have roles in neurodevelopment
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
Neuregulin-1 in control of radial migration
(Flames et al., 2004, Neuron 44, 251)
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
Neuregulin-1 and ErbB4 in schizophrenia
 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)
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 nondevelopmental 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
a3-integrin not downregulated in scrambler mice
Integrins bind extracellular matrix molecules and
mediate cell adhesion
Blocking a3-integrin in scm cortical slice restores
normal detachment
Several types of defects observed: