Lecture 12- Neuronal migration in brain development III (cerebellum and RMS) Flashcards
What are the main functions of the cerebellum?
-coordinates motor patterns by monitoring and modifying motor programs
What percentage of all the neurons in the brain are in the cerebellum?
- 50%
- arranged in a highly ordered fashion
What are the cell types in the cerebellum? (4)
- Purkinje cells (PC)
- Granule cells (GC)
- Stellate cells (S)
- Golgi epithelial cells (GEC)
What are the three layers of the cerebellum?
- Molecular layer (ML)
- Purkinje cell layer (PCL)
- Granular layer (GL)
- cerebellum has laminated structure so needs migration during development
What are the Purkinje cells?
-projection neurons and the other layers contain various interneurons (stellate, granule, basket and golgi epithelial cell)
When does the cerebellum (cerebellar cortex) undergo morphogenetic changes?
-during development from E11 to postnatal day (P) 7
- tiny structure between midbrain and hindbrain at first (the cerebellum) then grows
- P- postnatal (birth)
- the adult picture= granular cell layer is darkly stained(in adult called inner granular layer)
- the migration occurs P0-P7 so after birth
What is the cerebellum derived from?
- from the rhombic lip situated in the dorsal metencephalon (hindbrain) adjacent to the fourth ventricle
- this layer of tissue that gives rise to the cerebellum is called external germinal layer
What is the external germinal layer (EGL)?
- this layer generates granule cells that migrate inwardly in a radial manner with the assistance of radial glia fibers of the Golgi epithelial cells
- the EGL is a transitory developmental structure not present in the adult
- this is involved in the development of the cerebellum
What are the domains of the developing cerebellum? (5)
- the developing cerebellum can be subdivided into domains:
1. External granule (germinal) layer
2. Molecular layer
3. Purkinje cell layer
4. Internal granule cell layer
5. Ventricular zone
What are the two zones in the developing cerebellum where cells are generated and what cells are born where?
- Ventricular zone= Purkinje cells are born here
- External granule (germinal zone)= Inhibitory granule neurons born here
What is the Purkinje cell migration in the cerebellum like?
-they are born in the ventricular zone and migrate outward
What is the migration of inhibitory granule neurons in the developing cerebellum like?
- born in the external granule layer (EGL)
- migrate through the molecular layer past the Purkinje cells to form the internal granule layer
- so during development have two granule layers, as and adult only he internal one remains
What do defects in the VLDLR gene lead to?
- pontocerebellar hypoplasia
- cerebellum shrinks and often doesn’t form at all
Does Lissencephaly have an effect on the cerebellum?
- yes
- when mutations in RELN (gene coding for Reelin) then big defects in the cerebellar lamination and formation
What are the mutants shown in the picture?
scm= scrambler is a natural mutant of the Dab1 protein
A= +/scm: heterozygote, control
B= scm/scm: Dab1 null mutant
C= rl/rl: Reelin null mutant
D and E compare the effects of B and C- very similar, thus Dab1 and Reelin are part of the same pathway
- Dab is an intracellular adaptor molecule for Reelin signalling
- these mutations lead to significant defects in the cerebellum
What are the two cerebellar abnormalities that Reelin mutations lead to?
- Purkinje cells form clumps and not a single layer
- Granule cells remain in the external layer and do not migrate past the Purkinje cells
What is the role of Reelin in cerebellar neuronal migration?
- Reelin released by external granule layer situated on the outer part.= Purkinje cells migrate towards these cells
- This is very similar to the cortex where Reelin is expressed by Cajal Retzius cells in the Marginal Zone (only similar to the Purkinje cell migration)
- in the cerebellum have cell moving towards reelin (Purkinje cells) and away from it (granule cells)
- when Reelin is absent the migration does not occur
- Reelin provides orientation cues
How is the RMS (rostral migratory stream) different to the othe rmigrations we have discussed?
- different migration, not setting up a structure in the brain, providing neurons for another part of the brain
- interneurons migrate from the lateral ventricle to the olfactory bulb via the RMS
- adult neurogenesis moment, this happens in adults too (based on rodent models)
- created in lateral ventricle (subventricular zone = diff to cortex) and migrate
- in humans there are not as many as in the rodents
- the neurons will enter the olfactory bulb only in a particular orientation, lot of guidance cues involved
What are the 2 neuogenic regions in the adult mammalian brain?
- Subgranular zone of the dentate gyrus in the hippocampus
- Subventricular zone (SVZ) of the lateral ventricle, post-mitotic cells migrate along the rostral migratory stream (RMS) to form interneurons in the olfactory bulb
What do olfactory bulb interneurons arise from?
- a pool of proliferating cells that reside in the SVZ of the walls of the lateral ventricle
How do the olfactory bulb interneurons migrate from the lateral ventricle wall?
- cells migrate tangentially and use a specialised form of neuronal migration called chain migration (this is different from the glial or guidance- guidance migration)
- migrating cells have an elongated morphology with a leading process and growth-cone like tip similar to those found on migrating interneurons
- cell adhesion molecules mediate the cell-cell interaction during migration, the cells stick together via polysialyted cell adhesion molecules (PSA-NCAM)
How is chain migration possible?
- the cells literally have to be in contact with other cells in order to migrate
- stick together using PSA-NCAM (polysialyted neural cell adhesion molecule)
Is the morpology of the migrating OB (olfactory bulb) interneuron similar to the migrating cortical interneuron?
-yes but the OB interneuron has less branching
How can the chain migration in RMS be monitored?
- using In vitro explant cultures, can thus monitor migration and examine factors regulating this process
- picture: neurons migrate away from the original RMS explant (dotted lines) and form cell aggregations resembling chains
What are the factors involved with migration in the RMS? (remember: only need one example)
- the migratio of neurons in the RMS is highly directed with no dispersion into the corpus callosum or the cortex
- this indicates a presence of chemotactic (attractive) cues within the RMS or that the surrounding environment prohibits invasion of migrating cells
Proteins involved in migration:
- PSA-NCAM (hallmark of RMS cells)
- NCAM
- Integrin
- Laminin
- Tenascin C
- GFRalpha1
- Slit 1/2
- EphA4, Ephrin B1-3
- Reelin
- ADAM2
What does the disruption of signalling pathways involved with RMS migration result in?
-thickening of the RMS or smaller olfactory bulbs
What happens to ADAM2 knockouts? (RMS migration)
- ADAM2 is part of the Reelin pathway
- similar effect to Reelin knockout
- the tangential migration of OB neurons in the RMS is disturbed
- in contrast to wildtype cells, fewer cells migrated out of the ADAM2 KO explant, and those that did migrated more slowly and tended to be present as single neuroblasts that were not in contact with other neuroblasts
What is the role of Reelin in the RMS migration?
- Reelin is involved with the initial detachment of migrating neurons from chains and facilitates migration into the OB (olfactory bulb)
- in Reelin knockouts the migration is disturbed and the interneurons don’t enter the olfactory bulb
- so Reelin is most crucial at the end of the RMS when it directs the cells to the OB
Where is Reelin expressed and where is the Reelin receptor on migrating neurons in the RMS?
- Reelin is expressed in the mitral cell layer
- receptor is expressed on migrating neurons
What is the human RMS like?
(interest only, non-examinable)
- the RMS in the adult human brain has been elusive and was only recently discovered. The reason being that it is not tangentially positioned as in the rodent, but takes a caudal (more posterior) path from the SVZ to the OB
- there is much debate on the existenc of proliferating cells withing the RMS in the adult brain
- recent studies suggest that infant human SVZ and RMS contains and extensive corridor of migrating immature neurons before 18 months of age but this subsides in older children and is nearly extinct by adulthood
- in addition a mohor migratory pathway has been identified that targets the prefrontal cortex
What are 3 common aspects during the migratory process in all the parts of the brain we discussed?
(revision)
- Neurons are frequently generated at a distance from a final destination where integration of specific circuits occur, born somewhere, reside somewhere else=need to migrate
- The process of migration requires neurons to synchronize multiple actions:
a) Initiation and cessation of movement
b) Interaction with guiding cells
c) Respond to multiple guidance cues - Neurons distingusih between multiple extracellular signals to determine direction of migration
What is the molecular network regulating migration like?
(revision, don’t have to know it, just to see how interconnected it all is)
-
What does the Reelin signalling pathway look like?
- Reelin is a large extracellular matrix protein
- we discussed the molecules in red circles: Reelin, LIS1,Dab1, DCX
- yellow circles: Reelin receptors
What do migratory defects result in?
- abnormal cortical layering
- the picture shows examples of these, they are mice with part of the Reelin signalling pathway missing
- Reelin is the ligand,
- ApoER2, VLDR are the receptors
- Dab1 (intracellular signalling molecule)
Fyn, Src (kinases= phosphorylate and activate Dab)
What are the hypotheses of Reelin function during brain development?
- Positive migration signal (pyramidal neurons in the cortex)
- Chemo-attractant (Purkinje cells)
- Stop signal, detachment signal (the RMS migration)
- Chemo-repellent (with external granular cell)
- probably all of these, has many functiona and many effects depending on where in the brain it is and what cells are involved
What determined the direction of migration?
- extrinsic factors
- the migration of interneurons from the MGE into the cortex demonstrated examples od chemoattractants (go to cortex), chemorepellent (avoid striatum) and motogens (promigratory) cues
What do intrinsic factor do with external signals?
- translate them into cell movement
- have effect on the cytoskeletal components of the cell
- myosin pushes the cel forward
picture: genes that if disrupted then the whole migration process won’t work
What are the three modes of neuronal migration? (revision)
- Glial gudied migration (cortex: pyramidal neurons)
- Guidance-directed migration, somal translocation (cortical interneurons and the hippocampal interneurons)
- Chain migration (RMS)
Summary of cortical development- pyramidal neurons:
- cortical plate formation occurs in an inside-out manner
- role of radial glia (glial guided migration)
- glial-dependent migration with transitory multipolar migration
- distinct populations of cells
- proliferative zones- difference between rodents and humans
- function of reelin
Summary: cortical development- cortical interneurons:
- tangential and radial migration
- guidance-directed migration
- guidance cues
- cellular and molecular mechanism of migration
Summary: Cerebellum development:
- granule cell migration
- glial-dependent migration
- function of Reelin
Summary: RMS:
- chain migration of interneurons
- guidance cues
- function of reelin
Summary: hippocampal development and adult neurogenesis:
-function of Reelin
