Neurogenesis

What happens when the Shh pathway is activated?

→ glial activators go into the nucleus and transcriptionally upregulate a gene

What cells can progenitor cells give rise to?

→ neurons, astrocytes and glia

Do all progenitor cells become neurons?

→ no, some stay as progenitors next to the lumen in a region called the ‘ventricular zone’

How many cells wide is the early neural tube?

→ 1 cell wide - called the neuroepithelium

→ progenitor cells that are being patterned by BMPs or Shh

Why does the neural tube look as though it is several cells wide?

→ progenitor cells are undergoing mitosis, because of the movement of the cells as they undergo mitosis, nucleus is migrating in the cell, neural tube looks several cells wide

What is interkinetic migration?

→ whilst progenitor cells are proliferating, in G1 and S phase, nucleus is away from the lumen

→ at M phase and cytokinesis, nucleus moves close to the lumen

→ at cytokinesis, lateral attachment is lost and then reforms

How do neuroepithelial cells divide?

→ symmetrically - generating two identical daughter cells

→ they start to change shape and become radial glial-like - cells with a longer process

How do radial glia cells divide?

→ asymmetrically - giving rise to one daughter which is identical to the mother and a 2nd daughter that will differentiate into a neuron

→ 2nd daughter uses the scaffold provided by its sister to migrate away from ventricular zone

What determines whether a cell will give rise to 2 radial glial cells or 1 radial glial and 1 neuron?

→ cell division plane

→ notch signalling

Why is the cell division plane the first determining factor?

→ division along the meridian (north to south) generates tow identical daughter cells

→ division along the equator (aka asymmetrical localisation) generates 2 different daughter cells

Why is notch signalling another determining factor?

→ in the vertebrate neural tube - asymmetric cell division leads to imbalance in Notch

→ a cell where notch pathway is activated gives rise to supporter cells whereas in the cell where delta signalling is increased, neuronal cell develops

→ notch signalling is inhibited in neuronal precursors

What is lateral inhibition?

→ start off with every cell being equal with the same chances of survival

→ only one winner cell emerges by putting every other cell down (inhibiting)

→ loser cells start spiralling downwards rapidly in their ability to become a winner - become cemented in staying a progenitor cell

How does lateral inhibition work?

→ two cells start equally capable of making and receiving an inhibitory signal

→ a change/bias is introduced so one cell makes more inhibitory signal

→ the second cells receives more inhibitory signal and becomes inhibited, to stabilise this change, the inhibited cell must be prevented from continuing to send the inhibitory signal

How is Notch signalling an example of lateral inhibition?

→ achaete-scute proteins activate delta signal to notch receptor

→ one cell begins to overexpress delta, activating notch to a higher level in neighbour cell

→ notch pathway is established and amplified by feedback pathway involving suppressor of hairless and enhancer of split proteins, inhibiting Achaete-scute proteins - cell becomes a support cell

→ notch signalling is now low in the cell with high delta expression, activating bHLH protein to progress the cell into neuronal precursor

How was the notch pathway discovered in drosophila?

→ clusters of cells in the neurogenic region that have activated ‘proneural’ genes - they all have potential to become neuroblasts but only a few do

→ in proneural - Achaete-scute - mutants, no cells become neurons

→ in neurogenic mutant - notch mutant - more neurons are formed - if the cell does not see notch more neurons form

What happens to mice with the Shh gene knocked out?

→ fetus develops but shows holoprosencephaly and cyclopia

→ they don’t have progenitor cells or neurons forming so develop cyclopia = one single brain/eye

→ the floor plate usually separates the eyes into two but with no Shh there is no floor plate to induce this

Why is our understanding of signals and TFs that build the nervous system important to types of clinical work?

→ for drug discovery, disease understanding and regenerative medicine

→ through studies in mice, chicks and zebrafish we learned neural plate is posteriorised by combination of Wnt and retinoids acid signalling and ventralised through Shh

→ eg fetal alcohol syndrome - alcohol has a large effect on Shh signalling pathway, leads to mental health and learning difficulties in kids

→ used mice embryonic stem cells to have a dish of motor neurons they could develop

How are different types of neurons born at the same level along the A-P axis?

→ due to the intersection of Shh with factors that govern A-P regionalisation

→ we can use this information to direct es cells to different neuronal fates

What are the advantages of iPS?

→ no need for an embryo

→ can get individual-specific pluripotent cells

→ ability to differentiate into multiple cell types

→ vastly renewable, easily accessible

What are gastruloids?

→ 3D aggregates of embryonic stem cells that recapitulate axial organisation of post-implantation embryos

→ human gastruloids can be generated from human iPS cells

→ Wnt inhibition can poromote some aspects of brain development in gastruloids but not in human ones

→ gastruloids from some species eg mice can develop beating hearts