Lecture 8 - Developmntal Neurobiology - neuronal migration in CNS development

What happens to newborn neurons?

• they don’t stay put

• they migrate either radially or tangentially out from the ventricular zone (VZ)

what is radial migration?

• the process by which newborn neurons travel outward from the ventricular zone to reach their final positions in the cortex

why is the early neuroepithelium pseudostratified?

• due to migration of mitotically dividing nuclei

What does it mean when the early neuroepithelium is described as pseudostratified during interkinetic nuclear migration?

• migration of nuclei up and down the cell gives the appearance of multiple layers = pseudostratified

what does interkinetic nuclear migration reflect?

• reflects that the nuclei are going through different phases of the cell cycle as their nuclei move up (G1) and down (G2)

Where is the neuroepithelial cell at each stage of the cell cycle?

• G1 - nuclei moves up

• G2 - nuclei moves down

• S phase (DNA replication) - occurs at basal surface

• M phase (division) - occurs at apical surface (ventricular)

What are the apical and basal surfaces flipped in neuroscience once the neural tube is formed?

Apical surface is initially the top of the neural plate, as neural tube forms, apical surface ends up being on the inside as it folds up

What are the early divisions in the neuroepithelium like?

• symmetrical divisions

• the singular neuroepithelia cell divides and gives rise to 2 identical daughters

• expanding the size of the neuroepithelium

• division is at a right angle to the apical surface

• SAME COMPONENTS INSIDE THE 2 DAUGHTER CELLS

What are the later divisions in the neuroepithelium like?

• asymmetrical division

• resulting in 1 daughter cell similar to parent and 1 daughter cell that will become a mature neuron

• generating radial glia and neuronal precursors

• radial glia are capable of symmetrical and asymmetrical division to amplify number - stem like cell

• DIFFERENT COMPONENTS INSIDE OF THE DAUGHTER CELLS

why are the components of the daughter cells important?

• they determine whether the daughter becomes/stays the same as the parent or becomes different and starts to differentiate

what does the plane of division determine?

• the localisation of asymmetric localised cytoplasmic components in the daughter cells

what is radial migration?

• mature neurons accumulate over time in the mantle zone by migrating radially from ventricular zone

• neurons migrate from ventricular zone (VZ) to the mantle zone along radial glial cells

How do we know where neurons move to?

• Birth-dating to follow neurogenesis and migration

• tritiated (3H) thymidine used - incorporated into newly synthesised DNA

• the thymidine was injected into pregnant females - so it was incorporates into cells in S phase

• only the cells in their final division retain the label over time

• cells that have left the cell cycle and are differentiated can be seen (called label-retaining cells)

what are the label retaining cells?

• only the cells which have gone through one round of replication and then exited the cell cycle will retain the label

• this is because they have a lot of thymidine incorporated into their DNA

what happens to the cells that continue to divide?

• they thymidine label gets diluted out - cannot be seen as thymidine is dispersed

What can we find from using thymidine?

• can work out birth-dates of these cells allowing us to trace migration and neurogenesis to final destination over time

• suggested that neurones born at different times are migrating to different areas of the cortex, there is sequential layering of the cortex

How is the cortex organised based on time of neuronal birth?

• first birth neurons = occupy deepest layers - apical

• latest born neurons = occupy superficial layers - basal

• as time passes neurons that are born later move into higher areas of the cortex

What do we know about the different layers of the cortex?

• each layer is characterised by the expression of a specific set of transcription factors

• indicating that neurons born at different times have different fates, they move into different layers with different functions

How do cortical neurons fates change over time?

• the fates that cortical neurons can adopt become more strict - they lose potency over time

• early precursors have the ability to adopt many fates but this ability is lost in older precursors

• this suggests that something in the environment influences their fate

How do we know how cortical neuronal fates change over time?

• classical heterochronic transplants test whether fates of neuronal precursors at different ages are fixed or plastic

  1. early precursors are transplanted into an older host - these early precursors migrate and adopt the fate of the cells to match the new environment → their fate is still plastic

  2. later precursors transplanted to younger hosts - they adopt the fate they would’ve had before transplanting

     → their fate has already been decided - fixed (potency is lost)

  → this strongly suggests that early precursors have the ability to adopt many fates, but in older precursors they lose this flexibility and become fixed

→ also suggest that for early precursors have something in the environment that influences their fate

How can mutations affecting migration lead to Lissencephaly?

• lissencephaly - smooth brain, agyria meaning no grooves in the surface

• sulci and gyri (make the grooves) are diminished or absent

• majority of the neurons found in the deeper layers due to failure/abnormality of neuronal migration

• failed migration = lissencephaly

• causative mutations in the genes of proteins associated with micro tubule function:

  1. TUBA1A (alpha tubulin)

  2. TUBB2B (beta tubulin)

  3. LIS1 and DCX (micro tubule associated proteins)

• MICROTUBULES ARE CRITICAL FOR MIGRATION

• mutations in these genes causes high risk of lissencephaly

what do first migrations do?

• they delineate the boundaries of cortical layers

• they help to set up where each cortical layer will go

what can be found at the preplate (PP)?

• a form of very specialised cell type called Cajal Retzius (CR) cell in the marginal zone - superficial

• subplate neurons - deeper (below)

How are boundaries of the cortical layers formed?

• earliest post-mitotic cells migrating from VZ form the preplate (PP)

• Cajal-Retzius cells in marginal zone - superficial

• Subplate neurons - deeper (below)

• later migrating neurons form the cortical plate - in which the major layers of cortex form

• cortical plate sits between marginal zone (CR cells) and subplate

• these subplate neurons eventually die but play an important part in guiding thalamic axons

How were CR (Cajal Retzius) cells discovered?

• first visualised through the use of Golgi method which relies on silver nitrate

• also can be visualised with GFP - green fluorescence protein

• first post-mitosis cells to appear - characteristic morphology

• these cells change shape and die in post-natal period

what do Cajal-Retzius cells do?

• they tell migrating cells when to stop

• Cajal-Retzius cells secrete a protein called Reelin

• as neurons migrate outward along radial glia - Reelin acts as a stop signal

• when neurons reach the top near the CR cell they respond to Reelin and stop migrating

• this ensures they settle in the correct cortical layer

How do we know CR cells tell migrating cells when to stop?

• analysis of Reeler mouse mutant

• mutations in the Reelin gene encoding large (3460aa) ECM protein expressed by CR cells

• loss of Reelin leads to failure of CR and subplate cells to separate - causing disruption to layering of cortex

• migrating neurons fail to stop

• in humans mutations in Reelin leads to lissencephaly

What happens to radial glia over time?

• they deplete over time

• intermediate progenitors (IPs) accumulate (increase) in subventricular zone (SVZ) - and continue to divide producing upper layer neurons

• IPs have increased capacity to develop bigger neuronal cells

• in most regions proliferation ceases in adulthood

what are the adult neuronal cells?

• a subset of radial glial cells become astrocyte-like

• they are put aside to become adult neural stem cells that can produce new neurons

what are the 2 major zones if adult stem cells?

1. Subventricular zone (SVZ) of 4th ventricle

   • give rise to neuronal precursors for olfactory neurons so we are able to constantly learn new smells

   • these neurons migrate via Rostral Migratory stream (RMS) to the olfactory bulb

2. Dentate gyrus of hippocampus

   • granule neurons - involved in spatial memory

   • important for us to learn more spatial situations

What will happen if radial migration was the only form of migrations?

• pattern of TF expression (cell fates) induced in the neuroepithelium would persist in neurons in the mantle zone

• largely true eg spinal cord

What happens during tangential migrations?

• inhibitory interneurons of the cortex migrate in from subpallium (ganglionic eminence)

• subpallium = the source of many interneurons for the cortex and other regions

• precursors of these interneurons migrate tangentially over large distances to distribute themselves in the cortex

What are examples of tangential migration from the subpallium?

1. GABA-ergic inhibitory interneurons - to cortex

2. Dopaminergic inhibitory interneurons - to olfactory bulb

3. Cholinergic inhibitory interneurons - to striatum

Radial migration

Cells are migrating out from the ventricular zone and out towards the surface

Tangential migration

• cells move parallel to brain surface (across/around region)

What did Altman & Bayer notice about the neurons born at different times?

• they tracked them by injections at different stages of pregnancy

• and found that the neurons migrated to different layers of the cortex