lec 9- patterning the dorsal ventral axis Flashcards
what maternal effect gene plays a role in the patterning of the dorsal-ventral axis?
Dorsal and the activation of its receptors
how is dorsal distributed in the embryo?
it is distributed throughout the cytoplasm that surrounds the nuclei on the periphery of the syncytium
what is Toll?
a receptor
where is the receptor Toll located in the syncytium blastoderm?
on the membrane that surrounds the blastoderm, all around it
what protein is the fragmented ligand for Toll made of?
Spatzle
where is the Spatzle located in the blastoderm?
it is located outside the embryo in the perivitelline space and it is evenly distributed while also being inactive
where is the proteinase that cleaves Spatzle to make ligands for Toll located?
on the ventral side
once Spatzle has been cleaved and the ligands binds to Toll on the ventral side, what happens?
-the activated receptors send a signal to Dorsal allowing it to enter the nuclei, resulting in Dorsal protein to create a gradient and has a high concentration in ventral region while low in the dorsal.
-this means Dorsal protein activates the genes needed to specify ventral region of embryo
what are the 3 steps for Dorsal to move into nuclei?
- Dorsal is held in the cytoplasm by a protein called cactus
- when Toll is activated by binding to the Spatzle fragment, its cytoplasmic domain binds to a protein called Tube which activates kinases
- The kinases phosphorylate Cactus and it is then degraded, Dorsal is now able to enter the nucleus and activate genes needed to specify the ventral region of the embryo
does Dorsal act as a morphogen?
yes
what does Dorsal do once in the nuclei?
-acting as a morphogen, it specifies the initial pattern along the DV axis
- It turns transcription of different zygotic genes either on or off
-after the gradient is established, the embryo cellularizes and transcription factors can no longer diffuse between nuclei. This means signalling molecules and their receptors must be used to transmit signals between cells
what is expressed where dorsal is low and what is it?
Decapentaplegic (Dpp), a secreted signalling molecule
what is expressed where dorsal is high?
Twist
why does it take a lot of Dorsal to activate Twist?
Twist has low affinity for Dorsal
in what cells are the amnioserosa and dorsal ectoderm specified in?
Dpp cells
where are the mesoderm and neurectoderm specified in?
Twist cells, in the ventral region where Dorsal is present
what else is expressed with Dpp where Dorsal is low?
a protease called Tolloid (Tld)
what is expressed on the side with high Dorsal where the region is specified as the neurectoderm?
Short gastrulation protein (Sog)
what happens to Sog and Dpp?
-Sog protein is released to the outside of cells and it diffuses from the cells where it is expressed, forming a gradient
-where Sog meets Dpp, it binds to it which prevents Dpp signalling from stopping it from binding with its receptors
-As Sog diffuses towards the dorsal region, it continues to bind to Dpp, but due to Sog not being produced there, the number of unbound Dpp increases, with the most at the peak of the dorsal region
-Tld helps create more active Dpp at the peak by cleaving Sog resulting in more released Dpp which can bind to its receptors
what are the first zygotic genes expressed that all encode transcription factors
gap genes
how were the gap genes first identified?
by their mutant phenotypes
what controls gap gene expression?
anterior-posterior gradients of maternally-expressed proteins together with interactions between the gap genes
when the embryo is a multinucleate syncytium, what does the bicoid do?
initiates gap gene expression
what gap gene does bicoid activate?
hunchback, which is important to turn on other gap genes
how do we know there is a relationship between bicoid and hunchback?
when bicoid levels increase, so does hunchback expression
what does the hunchback transcription factor restrict?
it restricts gap gene expression to stripes along the AP axis
when is the kruppel gene activated?
when there are low levels of hunchback, high represses it
if there is a very low threshold of hunchback, is kruppel gene still expressed?
no
how was the role of hunchback tested?
when there are high levels of hunchback, the kruppel gene is pushed to the posterior, but in an embryo that lacks bicoid proteins, the hunchback gene is less and kruppel is more towards the anterior
can gap genes also repress one another?
yes
where do gap genes repress one another?
in regions of overlapping
how is the anterior and the posterior side of knirps expressed?
the anterior is expressed below a certain hunchback threshold whilel the posterior is expressed by a similar interaction with tailless
do gap genes divide the embryo up into broad domains?
yes
is the dividing of different regions of the embryo only possible because the syncytial blastoderm allows free diffusion of transcription factors?
yes
