lecture 16 Flashcards
what are the dominos that fall
reactions of cells & proteins, interactions they make, signal transduction pathways they control
do all those dominoes fall at the same speed
no; not at the same speed/time, some processes are slowed down/sped up as necessary
what does speed of a response depend on
turnover of signaling molecules (like fast proteins are already made, slower proteins take time to be made etc.)
describe fast signals
use proteins already made just waiting for activation signaling to turn them on so they can bind & activate downstream signaling protein [seconds to minutes]
give an example of fast signals
rac1 triggering switch of GDP to GTP, activates Arp2/3, triggers formation of branched actin networks important for cell-cell junction formation & protrusion of leading edge of migrating cell
why is rac 1 fast
everything (rac1, actin, arp 2/3) is already expressed, just waiting for signal
what are slow signals
when the protein needs to be made / expression needs to be changed before the next domino can fall
give an example of slow signals
if you need to produce a new protein, signal needs to go into nucleus, need to have an activation of trans. regulator that increase/decreases expression of certain genes, produces messenger RNA processed in nucleus and then exported
basically what are slow signals
bunch of different steps, each one takes time
what does mesoderm form
somites
what are somites
paired blocks of mesoderm that form early in development, lie on either side
what do somites give rise to
vertebrae, muscles, connective tissue; depending on where they are in the embryo
how do somites occur
always in pairs, extend the length of the embryo
what do somites represent
differentiation (mesoderm differentiates into somites)
what does the fate of each somite pair depend on
where they are on embryo
what happens as the embryo elongates
mesoderm differentiates into somites
visually describe differentiation of mesoderm into somites
goes from uniform field of cells into morphologically distinct little squares
what part of embryo are somites formed
at head region
what happens as somites go from right to left
embryo itself is getting longer
basically describe what happens to somites as embryo elongates
head is stationary, tail is moving away from the head while somites are forming in same direction left to right
fundamental question
what controls the clocklike timing of somite creation, that leads to regularly spaced somites
what does mutation of notch lead to
delays formation of somites
what else does a mutation of notch lead to
morphology is a little screwed up (doesn’t have sharp boundaries)
what is the key component of this timing mechanism
notch
what happens if notch is mutated
somites still form, just their size and position is disrupted
what specifically is Notch required for
timing of somite formation rather than formation itself [for coordinate segmentation of somites]
describe mechanism of Notch
lateral inhibition; protein X is delta, notch is receptor for delta
what is lateral inhibition for
way for a uniform field of cells to differentiate into 2 diff cell types
who is delta expressed by
expressed by one cell that acts on its neighbor trying to turn off delta
who expresses Notch
both cells express notch, it’s in plasma membrane
where in cell is delta expressed
also plasma memmbrane
what kind of signaling is notch and delta binding
cell-cell contact dependent signaling
describe lateral inhibition w/r to delta & notch
botch cells express delta; when delta binds notch, it tries to block its expression of delta
does it lead to a stalemate forever or does someone win
one cell wins this competition –> one cell doesn’t express delta while another cell expresses a lot of it
what does one cell expressing none while another cell expresses a lot of it mean
differentiation
describe the receptor notch
latent transcription regulatory protein
describe the binding of notch and delta
cell-cell dependent signaling; notch in one cell, delta in another
what happens after notch and delta bind
triggers proteolytic event, tail of Notch enters nucleus where it acts like a transcriptional regulator
is lateral inhibition in somite formation
no; different processes, but they both use Notch
what is Hes
one of the genes controlled by notch fragment in nucleus (when it acts as a transcriptional regulator)
what is integral component of developmental clock that controls the timing of somite formation
expression of Hes
what activates notch
morphogens FGF and Wnt
where is notch activated
FGF and Wnt, when expressed, activate Notch in mesoderm
what does notch do and where
in mesoderm, it controls the production of Hes
what does a notch gene-expression oscillator act as
acts as a clock to control vertebrate segmentation
what happens in somite formation
tail is moving back, farther away from the head
what is occurring as the tail moves back
proliferation AND growth (embryo gets bigger)
what happens as tail elongates
uniform mesoderm is condensing and forming paired blocks of somites
what do the wavy blue lines represent
oscillations of Hes protein within that tissue
what does the line signify
level of Hes protein is oscillating, going up and down within individiual cells
how long is a cycle of Hes protein
90 mins
what happens as you get closer to presomitic mesoderm differentiating in to somiets
oscillations begin to slow down
which is produced first mRNA or Hes
mRNA first, because its mRNA and then protein
what needs to happen if we increase Hes expression
Hes mRNA has to increase first
what is secreted by the cells in the tail
Wnt and FGF proteins
what do Wnt and FGF maintain
maintain the oscillation of a transcriptional regulator within the presomitic mesoderm
what is the oscillation of this regulator required for
to prevent presomitic mesoderm from forming somites
what happens as Wnt and FGF move farther away
oscillations begin to slow down and eventually arrest
where/when is oscillating transcriptional regulator arrested at
either high expression or low expression
what is Hes protein that’s oscillating a product of
notch signaling
what activates notch signaling
Wnt and FGF
where do Wnt and FGF come from
mesodermic cells at very tip of the tail
describe [ ] of Wnt and FGF
highest next to cells producing it, decreases the farther you go away
what dictates the oscillations of Hes
[ ] of Wnt and FGF
is this [ ] gradient static?
no; tail is growing, so the gradient is continuously shifting to the right
how is the highest concentration of Wnt and FGF moving
moving to the right
describe the rate of concentration
the concentration decreases at a rate equal to the growth of its tail
what is directly linked to how fast the tail is growing
when [ ] of Wnt and FGF slows down and arrest, it correlates to how fast tail is growing
if tail grows faster
soites form faster
what happens as source of wnt and fgf move farther away
oscillations slow down and eventually arrest
where the somites are formed is determined by what
relative expression level of Hes within tissue
are oscillations rapid?
yeah, Hes goes up and then down every 90 mins
describe signaling stuff closer to somites
Wnt and FGF much less concentrated, so notch signaling is not as active
what happens when these oscillations stop
cells are locked in place at level of Hes protein that they were expressed
does it mean cells have uniform level of Hes when they are stopped
naur
what dictates boundaries of somites
peaks and troughs; one edge is determined by peak, other edge is determined by trough
how can the length of somites be controlled
by changing the frequency of oscillations
what is the Hes gene essentially
essential component of this developmental clock… it’s essentially the developmental clock
what is Hes doing
controls its own expression thru delayed negative feedback
what does delayed negative feedback lead to
oscillation
what happens to Hes once produced in cytoplasm
Hes protein moves into nucleus to turn off its own transcription (oscillation peak)
what happens to existing Hes protein
will be degraded over time due to normal protein homeostasis mechanisms
what happens after that to Hes
Hes gene transcription begins again (oscillation trough)
describe the end of the tail
Wnt and FGF in abundance, notch signaling is going full guns
what is Hes
inhibitory protein
is there Hes expressed at the starting point
no
what happens after this
notch is active (due to Wnt and FGF in high concentration), proteolytically cleaved and enters nucleus
what does notch turn on
expression of Hes
what happens as Hes and mRNA rises
mRNA exported to cytoplasm to create Hes, Hes enters nucleus and tries to turn itself off
how does hes try to turn itself off
thru delayed negative feedback
mRNA and Hes levels drop until what
drop until there’s not enough Hes around to turn itself off
what happens after levels drop
mRNA and Hes are produced again, exported back to nucleus, turns itself off, begins to decrease
what are oscillations
delayed negative feedback of Hes BY Hes
what happens as tail moves away
oscillations slow until they become arrested
what are regions w/ high expression remaining
defined as one end of somite
what are regions w/ low Hes expression
other side of somite
what kinda protein is Hes
transcriptional regulator
what happens at the start
no inhibitory protein (no Hes)
what happens after notch pathway is activated
fragment of Notch that acts as its own transcriptional regulator enters nucleus, binds to DNA, recruits RNA Pol to produce Hes mRNA
what happens after mRNA is translated into protein at cytoplasm
Hes protein shows up
what is job of Hes
to turn itself off
what happens once it’s produced in cytoplasm
Hes will be imported into nucleus thru nuclear pore complexes
what happens after Hes is imported into nucleus
binds to its own regulatory sequence (upstream of Hes)
how does Hes turn itself off
by preventing RNA pol from acting wit the gene
what do Wnt and FGF trigger
Hes segmental clock to form regularly spaced somites
what does this whole system, oscillations depend on
delayed negative feedback
