Eye Development Flashcards
How many times are eyes thought to have evolved?
At least 40-50 times.
What condition results in a “small eye” mutant mouse? Does it come from a heterozygous or homozygous mutation?
Microphthalmia. Comes from a heterozygous mutation.
What is anophthalmia? Does it come from a heterozygous or homozygous mutation?
A total loss of eye, nose, and brain structures (die at birth). Basically a homozygous version of microphthalmia.
What gene is responsible for producing “small eye” mutant mice?
Pax6 (encodes a homeodomain TF).
What steps outline early eye formation in mice?
- Evagination of optic vesicles
- Invagination of lens placode
- Forms optic cup with pinching off of optic vesicle
What is aniridia? What are the morphological characteristics?
The human version of microphthalmia in mice. Missing an iris, cloudy lens, under-developed retina.
Is Pax6 gene dosage important for eye development? What happens if you have too much? Too little?
Yes. Adding Pax6 to “small eye” mutant can rescue development.
Excess = “small eye” mutant
Lack = “small eye” mutant
How is Pax6 gene dosage controlled in the eye? How did Bridget’s experiments determine this?
Controlled by ~12 miRNAs. Determined using a PITA to predict potential miRNA and then narrowed the range from here.
What happens if you knock out one of the miRNA binding sites for Pax6? What if you knock out both?
No effect with just one knock-out. miRNA repression lost and Pax6 over-expressed if both knocked out.
What does miRNA -> Pax6 binding site knockout tell us about the miRNA function in regulating Pax6?
miRNA must function in a cooperative, redundant manner.
What kinds of eyes do drosophila have? What kind of photoreceptors do they have?
Compound eyes with rhabdomeric photoreceptors.
What kind of photoreceptors do protostomes have? What about deuterostomes?
Protostomes: rhabdomeric
Deuterostomes: ciliary
What differentiates rhabdomeric photoreceptors from ciliary photoreceptors?
Rhabdomeric express opsins in the microvilli while ciliary express opsins in a modified cilium.
How were researchers able to induce ectopic eye expression in drosophila?
Using the GAL4 UAS system to express “eyeless” in the legs and antennae, and wings.
What gene is critical for eye development in drosophila? What about mice? Are these functionally conserved?
Drosophila: eyeless
Mice: Pax6
Yes. Replacing one with the other has little effect.
Is Pax6’s function as a master control gene conserved in vertebrate development?
Yep, also induced ectopic eyes on Xenopus when tested by Bob :)
In the vertebrate eye, what does misexpression of Optx2 cause?
Formation of “big” eyes.
In the vertebrate eye, what does misexpression of Rx cause?
Expansion of eye structures.
Does Pax6 act alone as the “dictator” of eye expression?
No. Acts as a key member of a larger “junta” which controls eye development.
Once the regulatory eye-specific TF network begins, does it require continuous stimulation?
No, can upregulate itself to direct eye development independently.
How did eye structures evolve in protostomes and deuterostomes?
Protostomes and deuterostomes likely shared a single ancestor with both kinds of opsins and signalling pathways.
What are the implications of protostomes and deuterostomes having a common ancestor with both kinds of opsins and signalling pathways?
Some protostomes have ciliary opsins while some deuterostomes have rhabdomeric opsins.
What early optic cup tissue is the retina derived from?
The pseudostratified neuroepithelium.
How does the pseudostratified neuroepithelium grow? What is this process called?
Undergoes cell division but only at the apical side. Cell nuclei can move from one side to the other by “interkinetic nuclear migration”.
What gene is involved in retina formation that is only expressed in the retina?
Chx10.
Why do mutations in Chx10 affect the development of the whole eye if Chx10 is only expressed in the retina?
Because the retina can influence the development of the cornea and lens.
Where on Chx10 is the mutation causing mouse microphthalmia? What kind of mutation is it?
A nonsense mutation at the beginning of the homeodomain.
Where on Chx10 is the mutation causing human microphthalmia? What kind of mutation is it?
A nonsense mutation in the middle of the homeodomain.
When does the vertebrate lens stop growing?
It never does, but growth slows to a crawl later in life (think “tree trunk” analogy).
What 2 components make up the vertebrate lens? What happens if you invert the lens (in a young organism)?
Think “tree trunk” analogy!
- Lens epithelium (“bark”)
- Fibre cells(“core”)
Inversion causes the cell types to switch, eventually becomes a viable eye again.
How does Chx10 interact with Kip1 (+Cdk2/cyclin D2) to regulate retinal development?
In a double-negative gate.
Chx10 -| Kip1 -| Cdk2 -> cell cycle continues
If Chx10 were nonfunctional, what would likely occur regarding Kip1 and the cell cycle?
Kip1 (normally inhibited) would inhibit Cdk2/cyclin D1, causing cells to go to G0 prematurely and resulting in a small eye.
What occurs when retinal cells exit the cell cycle and go to G0?
They differentiate into the different retinal cell types.
How does Chx10 regulate Kip1 (i.e. at what level)?
At the protein translation level.
What happens to retinal development in mice if both Chx10 and Kip1 are missing?
Would expect huge eyes, but actually get a rescue and more-or-less normal eye development.
What occurs to a mouse which has a Chx10 mutation causing loss of bipolar cells?
That mouse is blind because no photoreceptor signals are reaching the ganglion cells.
What are the 2 functional roles of Chx10?
- Regulation of cell-cycle exit
2. Specification of bipolar cells
How are retinal cell fates determined? Unique progenitors? Multipotent progenitors?
Actually limited potency progenitors. Differentiation partially controlled by temporal ordering.
Which 4 retinal cell types differentiate earlier in development?
- Ganglion
- Amacrine
- Cone
- Horizontal
Which 3 retinal cell types differentiate later in development?
- Rod
- Bipolar
- Muller Glia
What transcription factor regulates retinal progenitor multipotency? What retinal cell type is this not required for?
Pax6. Not required to specify amacrine cells though.
What are the 2 ways to control retinal cell specification?
- Basic helix loop helix
2. Homeodomain TFs
What are the symptoms of retinal degeneration?
Photoreceptor loss, progressive vision loss leading to eventual blindness.
What are the 4 challenges associated with retinal cell transplantation therapies?
- Generation of target cells
- Integration with retina
- Synapse formation
- Visual signalling
What is cyclopia?
You just get one big eye in the middle of your face instead of two…
How does cyclopia occur? What TFs are involved in this?
A failure of the early eye-field to split into two separate eyes. Pax6 and Pax2 implicated.
What mechanism causes the early eye-field to split?
The growth of the prechordal plate mesoderm inhibits medial Pax6.
How does the prechordal plate mesoderm inhibit Pax6?
By expressing shh that stimulates Pax2 to transcriptionally repress medial Pax6.
What eye development defect arises from the loss of shh expression?
Holoprosencephaly. Failure of face/brain to divide into R and L halves.
What molecule was causing farm animals to give birth to offspring expressing cyclopia?
Certain plants have cyclopamine which actually inhibits smoothened activity in the shh pathway.
How did blind cavefish lose their eyes?
They have expanded shh expression and therefore broad inhibition of Pax6 (no eyes).
Why might repression of Pax6 and upregulation of Pax2 be advantageous for cave fish?
shh and Pax 2 linked to ear development. But cave fish actually have worse hearing that helps them block out ambient noises which interfere with important sounds.
Why does reversal of the lens actually have no effect in the long run?
Because there are factors in the cornea and surrounding tissues that determine proliferation and differentiation of lens cells. They eventually change cell type to become the other, restoring function.
What occurs to mouse eye development if Chx10 is present but Kip1 is non-functional?
The mouse develops big eyes (up to a point).
What 2 factors keep retinal progenitor cells in a proliferative state?
- Sonic hedgehog
2. Notch
