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Flashcards in lecture 26 Deck (19)
1

What are the aims of these lectures?

- focus on drosophila melanogaster as a model organism for the study of mechanisms that regulate organ and/or body size
- extrinsic hormone/peptide circuits within the whole animal that regulate organ size
- intrinsic systems (salvador/warts/Hpo signal transduction pathway) that regulate organ growth (imaginal discs)

2

What experiments revealed that there are both extrinsic and intrinsic factor that regulate organ size?

- 1930s that peoples started to think about the regulation of organ size
- two types of experiments done
1. if you starve an animal during development, it will be smaller than its nutritionally supplied/well-fed counterparts. Same for mammals: if you have mouse that have been nutritionally deprived, their progeny will be smaller. Nutrition has a profound influence on body size. This is an example of an extrinsic factor.

2. intrinsic factors: simple experiment by Twitty and Schwind, took the limb from a larger embryo and grafted it onto a smaller embryo (axolotl). As they developed, the grafted limbs maintained the body size of the original animal from that it came. Therefore there must be systems within organs/body tissues/organisms that regulate organ size.

3

What are examples of extrinsic factors that regulate organ growth?

- include nutrition and hormones
A. environmental regulation of body size: increasing temperature, smaller, increasing protein, bigger
B. physiological regulation of body size: hormones,
C. genetic regulation of body size: male or female (female much larger)
D. coordination of organ growth: organ systems crosstalk so that the brain doesn't outgrow/pace heart, limbs etc

4

What is the difference between extrinsic and intrinsic factors that control organ size?

Extrinsic
- hormones and nutrient status (glucose, fats, amino acids, ecdysone, insulin)
- cell-cell and organ-organ communication

Intrinsic
- genetic programmes within the cell that are unaffected by neighbouring cells e.g. salvador/warts/Hpo pathway

5

What is the drosophila life cycle?

- egg - 24 hours
- 1st instar larva
- 2nd instar larva
- 3rd instar larva
- pupa
- adult

- life cycle takes 10 days --> fantastic for studying in a laboratory

- cheap compared to mouse studies
- at end of each larval stage cuticle is malted
- all the energy and nutrients that are going to make the adult are present in the pupal stage - no more feeding, climbs up wall

6

What is nutrient dependent growth control in Drosophila?

- Minimum viable weight: threshold minimum weight to survive metamorphosis or eclose to adult when starved
- critical weight: minimal size at which starvation no longer delays metamorphosis

- larvae feed during larval stage 2, then they undergo a transition from larval stage 2 to larval stage 3, larvae senses that it is fed enough and that there is enough energy stored to allow the larvae to pupate, and undergo metamorphosis
- unique biological system
- these terms allude to the fact that in the drosophila there is this critical period during which the animal knows through sensing systems that it is time to undergo metamorphosis

- if you starve the animal you get smaller adults because you haven't reached the minimal viable weight

7

What is hormone dependent growth control in Drosophila?

- Critical weight - insulin like peptides are released from the Fat Body and Imaginal Discs
- Ecdysone released from the Ring Gland
- larvae feed on sugars
- this signals in the fat body (liver equivalent) and the brain (ring gland/neurosecretory cells)
- as larvae feeds, fat body has the ability to detect amount of amino acids, sugars, fat in the body
- fat body releases insulin like peptides that enter the circulatory system and are eventually detected by both the neurosecretory cells (also secreting insulin like peptides) and the ring gland
- when critical balance of secreted insulin like peptides is reached, the ring gland releases a hormone called ecdysone
- ecdysone travels throughout the animal via the circulatory system
- tells the animal to stop feeding and begin the process of morphogenesis
- crosstalk between organs can thereby regulate developmental timing etc, when it should stop feeding
- if imaginal discs are undeveloped (release insulin like proteins), then development will be delayed

8

What is the ring gland?

- sits between the lobes of the adult brain
- releases ecdysone
- prothoracic gland and corpus allutum

9

How is the ring gland important for organ size?

A) wildtype ring gland
B) small ring gland
C) pupae from larvae with reduced ring gland size
- starved larvae
- non-starved but reduced larval ring gland size = bigger pupae

- decreasing ring gland size, increase time to pupation, increase organ and body size

10

What do insulin like peptides in the wing disc regulate?

- growth and developmental timing
- dILP8 secreted by wing discs and regulates organ size
- eight insulin like peptides in drosophila
- imaginal discs express ILP - unexpected, also wing disc, fat body (salivary gland doesn't express ILP)
- eyeful - eye disc keeps growing, never mutate
- expression of ILP8 is dramatically higher in eyeful mutants
- this suggests that secretion of ILPs from the imaginal discs is also feeding back onto the ring gland and affecting the secretion of ecdysone
- what happens then is that in the developing larvae, there are a number of organs, fat body, neurosecretory cells in the brain, imaginal discs and fat body that are all secreting ILPs, (and the ring gland)
- all those ILPs are ultimately sensed by the ring gland
- searching for equivalent system in mammalian embryo
- coordinated growth of insulin like peptides

11

What is the insulin signalling pathway?

- small secreted proteins (30kDa)
- enter the circulatory system
- bind the insulin receptor
- through a series of kinases and phosphatases they ultimately regulate a transcription factor called FOXO
- FOXO goes into the nucleus and binds a number of gene promoters where it will regulate the transcription of target genes
- genes involved in the regulation of protein synthesis, degradation and cell death
- FOXOs part of sensing amino acids and glucose
- regulate organ size

12

What two hormone systems within Drosophila larvae signal between organs to regulate growth?

- insulin-like-peptides (ILPs) in wing discs and fat body
- ILPs signal to Ring Gland to secrete Ecdysone to initiate morphogenesis

13

What are the molecular mechanisms that control organ size?

Extrinsic
- energy production/nutrient status
-- insulin/PI3 kinase pathway (Akt, Dp110)
- hormones (Ecdysone)

Intrinsic
- cell proliferation
-- cycle regulators e.g. cyclin E (cyc E), Retinoblastoma (Rb)
- Apoptosis/Cell death (reaper (rpr), sickle (skl), Drosophila inhibitor of apoptosis (diap1)
- Cell size
-- myc
- protein synthesis
-- minutes (M)
-- myc

14

What are drosophila imaginal discs?

- precursors of adult organs
- imaginal discs are comprised of epithelial cells
- imaginal discs are the cornerstone of genetic analysis of organ size

15

How do drosophila wing discs develop?

- drosophila wing disc has an excellent cell fate map
- in the ~30,000 cell wing disc, fates of almost all are known
- cells 'in here' will form the notum = sort of shoulder of drosophila
- pouch cells will form adult wing blade
- certain cells will go on to form vein tissue

- drosophila wing disc has known rates of cell proliferation
- some cells are actively dividing and some are dividing less e.g. wing pouch vs notum
- cells in hinge region don't divide a great deal
- cells in pouch region undergo a great deal of cell division

- developing imaginal discs align within the larvae, within pupal case
- to form adult structures have to undergo a huge amount of cell migration

16

How do drosophila eye-antennal discs develop?

- eye made up of ommatidia and bristle cells --> precise organ organisation
- stems back to organisation in the larvae
- drosophila eye disc has an excellent cell fate map
- posterior to the morphogenetic furrow all cells are fully differentiated --> these are the cells that will contribute to the adult eye and the ommatidia
- photoreceptors etc are present here
- drosophila eye disc cell fate is coupled to cell proliferation
-- very very coordinated process
- when undifferentiated cells divide at random
- apical constriction --> arising the morphogenetic furrow
- when cells enter the morphogenetic furrow they cease to divide - G1 morphogenetic arrest
- begin to differentiate
- proliferation tightly coupled to division
- after differentiated will only undergo one more division
- drosophila has a precise pattern of cell division

17

What is the patterning of the eye/ommatidia?

- cone cells in the centre
- underneath which sit photoreceptors
- surrounded by pigment cells
- hexagaonally shaped
- surrounded by insulating cells that allow for discrete neural circuits
- bristle cells in corners
- precise pattern

18

How is drosophila eye disc cell fate coupled to cell death?

- drosophila pupal eye disc has a precise pattern of cell death
- e.g. hpo mutant
- these cells don't undergo apoptosis in the development of the eye
- precise pattern for cell death
- doesn't happen in the wing disc
- normally eye disc cells die so there aren't any extra cells to disrupt the pattern

19

What are key learning points?

- development is an integrated process that incorporates pattern formation, morphogens and organ size control
- body/organ size control is regulated by extrinsic (nutrients and hormones) mechanisms and intrinsic mechanisms (SWH pathway)