Lecture 6- Neurogenesis and determination of cell fate I (Drosophila) Flashcards
What is specification?
- imparting of positional information to cell populations, restriction
- The process whereby a cell becomes capable of differentiating autonomously into a [cell type] cell in an environment that is neutral with respect to the developmental pathway. Upon specification, the cell fate can be reversed.
What is determination?
- Imparting information that commits a cell to become (differentiate into) a certain cell type
- expresses a phenotype of a cell
- state where a cell has irreversibly acquired fate (potency = fate)
- The process whereby a cell becomes capable of differentiating autonomously into a [cell type] cell regardless of its environment; upon determination, the cell fate cannot be reversed.
What controls specification/differentiation?
- Cell extrinsic- communication with other cells (Fgf, Shh, Delta/Jagged)
- Cell intrinsic programmes- gene specific transcription factors (Suppressor of Hairless Su(H), achaete scute (asc)).
- cell intrinsic programmes= commit a cell, gene transcription in the cell
What is the effect of Notch in drosophila?
-mutant/Notch has notched wing
What does Notch do?
- inhibits neurogenesis
- Notch mutant embryos contain too many neurons, therefore Notch is required to restrict the number of neurons
(white dots= neurons)
What is the story with Drosophila bristles?
- each bristle represents a neuron as it is connected to one on 1-1 basis
- in Notch mutants there are more bristles and are less well ordered
What does neurogenesis require?
- positive and negative signals
- have pro-bristle genes that promote neurogenesis and bristle genes that are required to inhibit neurons
- the interaction of positive and negative signals is crucial in successful neurogenesis
What is a Notch mosaic animal?
-animal made up of two distinct genetic cell populations
What did the Notch mosaics tell us?
- Nocth acts cell autonomously (intrinsically), acts on its own, doesn’t affect surrounding tissue
- Delta acts non-cell autonomously (extrinisically), affects cells in the surrounding tissue
What is lateral inhibition?
- discovered thanks to the analysis of Notch mosaics
- the NB(=neuroblast) tells the cells around it to not become NB as well, that is how you get a distinct number of neuroblasts in the embryo
- the cell destined to become the NB inhibits other competent cells from adopting that fate
What is Notch?
- transmembrane receptor (protein)
- extracellular domain consists of multiple epidermal growth factor-like (EGF) repeats, and an intracellular domain consists of multiple, different domain types
What is a neuroblast?
- a dividing cell that will develop into a neuron often after a migration phase
- the main difference between a neuroblast and a neuron is the ability to divide; neuroblasts can still undergo mitosis, whereas neurons are postmitotic
What is delta?
-a ligand that binds to Notch initiating the Notch signalling pathway
What is the structure and cleavage sites of the Notch receptor?
- the extracellular domain (containing the EGF-like repeats) interacts with the ligand Delta
- when Delta binds, conformational change in the Notch receptor takes place
- this activates proteases that cleave the Notch receptor, first the ADAM cleaves it at site 2 (mimicked by NotchdeltaE)
- then gamma secretase cleaves it at site 3 (mimicked by Notch(intra))
- the intracellular domain of the Notch receptor is now free to go to the nucleus to alter gene transcription
What is the Delta/Notch pathway critical for?
- for signalling out of the neuroblast
- the Notch signal allows one cell in the proneural cluster to maintain achaete scute levels (so will become neuroblast and later neuron)
- low Notch receptor activity= cell will adopt neural fate and become neuroblast
- high Notch receptor activity= epidermal fate
What is the initial setup of two cells in the proneural cluster in the Drosophila?
-ectodermal cells are initially equivalent
- in the embryonic epiderm none are commited to become neurons
- two cells, both experessing delta and notch receptors
- delta expression is upregulated in one cell via ASC
What happens when Delta expression is increased in 1 cell?
- Delta binds ti Notch on adjacent cell
- once it has more delta then it binds to the notch receptor of the receiving/adjacent cell
What does Notch activation release?
- releases SuH which induces E(spl) expression
- notch intracellular domain is associated with SuH after cleavage events and then translocated to the nucleus
- enhancer of supressor hair gene interact with proteins of the ASC proteins
- ASC is downregulated in one cell and active in the other
- the ASC are the ones responsible for neuorgenesis
- the one with no ASC will not be neuron
- the one with more delta and active ASC will be neuron
What does E(spl) do after Notch activation?
-blocks ASC therefore Delta and Neural gene expression
What does decrease in Delta in the inhibited cell mean for the developing neuroblast?
-leads to more Delta and Neural gene expression in putative neuroblast
What happens at the end of the Notch interaction between the two cells?
-one cell now neuroblast (neuronal progenitor) and one epidermal
How are proneural genes initially expressed?
-in patches but become successively restricted to single cells as the feedback mechanism of Delta/Notch pathway takes place
What do drosophila embryos with:
a) normal neural cell determination/differentiation
b) overexpression of Notch
c) lack of Notch
look like?
a) normal, SuH and ASC balanced
b) all ectoderm/epiblast (Su(H)on)
c) all neuroblasts when lack of Notch (ASC on)
