Cellular & Molecular Mechanisms of Development II

Question 1

What are growth factors?

Answer 1

smalld iffusible proteins that cells release into the extracellular environment

Question 2

When growth factors are received by specific receptors on another cell, they can bring about a number of changes to that cell’s behavior, including:

Answer 2

• stimulation or inhibition of cell proliferation
• cell differentiation
• cell shape change
• stimulation or inhibition of cell migration

*all of these are important for regulating embryonic development

Question 2

f

Answer 2

f

Question 3

What is the general mode of action of growth factors?

Answer 3

cell (ex: notochord cell) secretes diffusible growth factor that can cross several membranes, the growth factor binds to corresponding receptor, which then changes intracellular and transcriptional properties of the cell

• final effect is change in protein synthesis in recipient cell. Leads to altered behavior
• only cells with the specific receptor can respond

Question 4

Wnt growth factor family

Answer 4

• named after a Drosophila mutation called wingless and a human oncogene called int
• there are about 20 different Wnt proteins in mammals; all are expressed in the embryo

ex: fly with Wnt mutation won’t have wings!

Question 5

The Transforming Growth Factor-Beta family

Answer 5

(involved in wound healing, Marfan’s syndrome)

• TGF-Beta1, Beta2, and Beta3
• Bone Morphogenetic proteins (BMPs) ??
• the receptors for these are threonine/serine kinases
  i. e. they modify intracellular proteins by adding a phosphate to a serine or a threonine

Question 6

Hedgehog growth factors

Answer 6

three hedgehog proteins called:

• sonic hedgehog
• indian hedgehog
• desert hedgehog

Question 8

What are the other growth factor families?

Answer 8

• fibroblast growth factors (FGFs) - about 10 proteins
• platelet derived growth factors (PDGFs)
• epithelial growth factors (EGFs)
• vascular endothelial growth factors (VEGFs; very important in vascular formation)
• others

*all of these proteins bind to specific receptors

Question 9

Even with this large number of growth factors, exactly the same proteins are used for regulation of numerous different developmental events.
for ex: BMP4 is active during blood formation, neural development, lung development, and bone growth

Answer 9

fix

Question 10

Given the potency of growth factors, their expression is very closely regulated. Regulation occurs at the level of:

Answer 10

• transcription
• mRNA processing and stability
• translation
• post-translation activity

*when start releasing growth factors, cells also preparing to halt the process (cancer cells lose this ‘break’)

Question 10

What are the inhibitors of BMP family proteins?

Answer 10

noggin and chordin

Question 11

What are some examples of post-translational regulation? (inhibition)
***important I would assume

Answer 11

• BMP family proteins are bound by inhibitors called noggin and chordin
• Wnt family proteins are bound by inhibitors called Dickkopf and crescent
• Hedgehog is bound by an inhibitor called Hip (Hedgehog interacting protein)

*all of these inhibitors prevent the growth factors from binding to their receptors, thus blocking the signal

Question 13

What are the inhibitors of Wnt

Answer 13

Dickkopf and crescent

Question 14

What are the inhibitors of Hedgehog?

Answer 14

Hedgehog interacting protein (Hip)

Question 15

Growth factor signaling is so fundamental to embryonic development, that it is very rare for embryos with growth factor defects to survive until birth. Approximately 50% of all pregnancies fail to reach term. What do you think many of these are possibly due to?

Answer 15

probable that defects in growth factor signaling account for many of these spontaneous terminations

Question 15

A failure in holoprosencephaly causes what?

Answer 15

• brain may fail to form two lobes
• at its most extreme, holoprosencephaly may present as cyclopia
• cyclopia is a telltale sign of loss of Hedgehog signaling

Question 16

What is holoprosencephaly?

Answer 16

• a specific growth factor
• results from reduction of sonic hedgehog signaling
• relates to a spectrum of defects primarily due to failure of formation of midline structures in neural tissues

Question 17

What is induction?

Answer 17

At the large scale level, induction can be interpreted as one tissue (or region of the embryo) instructing another region.

ex:

• notochord inducing epidermis to form neural tissue
• notochord inducing scleretome
• endoderm induction of heart tissue
• heart induction of liver tissue

*all due to cell-signaling events mediated by growth factors

Question 18

Induction is the result of what action?

Answer 18

result of action by signaling proteins (growth factors or inhibitors)

ex: Shh (Sonihc hedgehog) signaling from the notochord and the base of the neural tube induces nearby cells in somite to become sclerotome cells

Question 19

Explain the induction of the heart.

Answer 19

• the heart forms from mesoderm where Wnt signaling is blocked
• this is achieved by a Wnt inhibitor called Crescent
• Crescent is expressed in the endoderm
  (all of mesoderm is experiencing Wnt signaling; in region where Wnt signaling is switched off, heart can be formed)

*note: splanchnic mesoderm forms heart

Question 20

Explain the induction of neural tissues

Answer 20

• in ectoderm (–> epidermis, skin), BMP (BMP-4) is present/expressed everywhere
• notochord secretes Noggin to inhibit BMP-4
• Neural tissue forms where BMP-4 signaling is absent

Question 21

What are homeobox genes?

Answer 21

• important for many aspects of embryonic development
• all of these genes contain a conserved DNA sequence called the homeobox
• the homeobox encodes a DNA binding protein domain called the HOMEODOMAIN
• box is DNA sequence
• all homeodomain proteins are transcriptional regulators - either positive or negative
• there are about 150 homeobox genes in the human genome
• homeodomain is the protein sequence

Question 22

What is the homeodomain?

Answer 22

• a DNA binding protein domain in the homeobox of homeobox genes
• all homeodomain proteins are transcriptional regulators - either positive or negative

about 150, all expressed in embryo

Question 23

What are Hox genes?

Answer 23

• these are a subset of the homeobox genes that provide positional information in the embryo
• the Hox genes are arranged in clusters with up to nine genes in a row on the chromosome (mammals have 4 Hox clusters, flies have just 1; the order of the genes is the same in human and fly)
• these genes must therefore have arisen in an evolutionary ancestor of both mammals and insects

Question 24

In relation to Hox gene expression, the higher the number of the gene, where is expression in relation to cranial and caudal ends?

Answer 24

the higher the number of the gene, the more caudal is the expression
(you are moving from head to tail, downwards - think of it like with each higher number, you’ve drank more out of the glass, so the level of water (the cranial most expression) continues to be lowered)

Question 25

Which of the following would have a more cranial expression, b1 or d11?

Answer 25

b1

Question 26

Hox genes - also, the more genes expressed (kind of related to high number of gene ex: d12), the more caudal you are. Give example.

Answer 26

legs take more genes to be formed

need to express all previous genes as you get higher in number if you’re trying to express b9, b1-8 must also all be expressed

Question 27

If Hoxc-8 is mutated, the first lumbar vertebra becomes rib-bearing. It has been transformed into a more cranial structure
Similarly, if Hoxb-4 is mutated, cervical vertebra C2 takes on structural features like C1 which means what? More cranial or caudal?

Answer 27

more cranial

review this!

Question 29

Human Hox mutation is common defect affecting cervical ribs

Answer 29

• estimated 1% of population
• normally has no clinical significance, but in some cases the extra rib and associated cartilage can compress nerves of the branchial plexus leading to pain

Question 30

Differentiation (or Determination ) genes

Answer 30

• these are genes that are responsible for directing differentiation of a specific cell or tissue type
• they are sometimes called master regulators because they CONTROL TRANSCRIPTION of many subordinate genes (regulate all genes downstream)
• these subordinate genes will include the differentiation products that comprise the tissue, e.g. muscle proteins or eye proteins

ex: MyoD

Question 31

MyoD

Answer 31

• determination gene
• myocyte determination (muscle determination)
• small family of related muscle determination genes have been identified, including myogenin, myf-5, and MRF-4
• MyoD can convert most cells into muscle cells, i.e. it can initiate the entire muscle development program

Question 31

What can allow a fat cell to grow into a muscle cell?

Answer 31

MyoD

Question 33

Pax6

Answer 33

• determination gene
• this is a homeodomain protein (but not a Hox protein; not involved in positional identity)
• essential for development of the eye
• exctopic expression (i.e. in the wrong place) can cause eyes to develop in ectopic locations. Therefore, Pax6 is sufficient to initiate the eye development pathway

Question 34

If you express a Pox6 gene in the wrong location, what would happen?

Answer 34

you would grow an eye there!

Question 35

In the total absence of Pax6 function, the eye fails to form completely, and there are other neural defects that result in embryonic lethality. Absence of one Px6 gene (i.e. heterozygosity) results in what human disease of the eyes?

Answer 35

aniridia, which can result in complete lack of the iris