Developmental Flashcards
What is epistasis?
Ordering genes into pathways
What is the hedgehog signalling pathway in vertebrates?
Ptc inhibits Smo
Shh inhibits Ptc
Frees up Smo and allows it to activate Gli transcription factor
Gli enters nucleus and activates transcription of target genes
What inhibits Ptc?
Shh
When Ptc is inhibited, what does this do to Smo?
Activates
What occurs to signalling pathways in Basal cell carcinoma?
Mutations that predispose:
- Ptc - loss of function mutation (amorphic) - cannot inhibit Smo so activation of Gli, cell division
- Smo: gain of function mutation (hypermorphic) - overactivity of pathway - cell divisions
What causes APC and colon cancer in terms of signalling pathways?
- Inappropriate activation of Wnt pathway
- Polyps (small tumours) form in colon
What characteristics do tumour cells share with embryonic cells?
- Rapidly dividing
- Undifferentiated
- Undergo epithelial to mesenchymal transition (metastasis)
How do tumour cells differ to embryonic cells?
- Tumour cells escape normal controls
- Proliferation
- Differentiation
- Growth
Where do most cancers occur and what does this contain?
In epithelial - contain stem cells which continue to proliferate
What is cancer caused by overactivation of Hh signalling called?
How can this be treated?
Medulloblastoma: caused by mutation in Ptch 1 (tumour supressor)
- Smo active even in absence of Hh
- GDC-449: blocks Smo activity (like cyclopamine)
- Second mutation in one tumour cell causes drug resistance so cyclopamine can’t bind
- Smo can continue to activate Gli (treatment doesnt last long)
Is Ptc a tumour supressor or oncogene?
Tumour supressor
Is Smo a tumour supressor or oncogene?
Oncogene
How can we make new pancreatic beta cells from stem cells?
- Making a stem cell: best to take cells from patients to turn into stem cells before beta cells
- Make into endoderm: differentiation factors FoxA2, Pdx1 - put through protocol with growth factors
- Differentiate back to beta cells
- Check cell function: check cells can release insulin in response to lgucose
- Introduce cells back into patient
What is totipotent?
Cells with potential to form everything
What is pluripotent?
Cell that can form any cell type within embryo
Can gene expression in nuclei from differentiated cells be changed?
Yes under special circumstances
Can gene expression be controlled by cytoplasmic factors?
Yes
How was nuclear transfer from adult skin cells used to test nuclear potency?
Conclusions?
Skin cells taken from webbing of Xenopus foot
Looked at keratin expression
Trafer nuclei to irradiated, unfertilised egg
Initially doesn’t develop
Carry out serial nuclear transfer
Clones of tadpoles formed
Conclusion: Nuclei of differentiated cells can be reprogrammed to become pluripotent
What are the exceptions to rule that the genome remains constant during development?
- Chromosome loss in Parascaris (a complete chromosome set is retained only in the germ cells)
- Immunoglobulin gene rearrangements in differentiated B lymphocytes
(generating diversity at the DNA level in genes coding for antibody molecules)
Are genes lost or retained during cell differentiation?
Retained
What is the experimental evidence to show that genes are not lost in differentiation, and that nuclei retain potency?
- Lens regeneration in the newt
- Activation of gene expression in heterokaryons
- Activation of gene expression following nuclear transfer in oocytes
- Genomic sequencing
What is the experimental evidence to show that a somatic nucleus contains all genetic information to direct cell differentiation?
- Nuclear transfer exp. in frogs
2. Nuclear transfer in mammals (Dolly)
What are proneural clusters?
Groups of cells in neurectoderm
- Beginning to differentiate apart from neural cells
- One cell becomes neuroblast and others become epidermis
What is the position of proneural clusters determined by?
AP and DV genes
Normally bicoid and toll
How does neurectoderm become neuroblast?
Lateral inhibition:
- One cell becomes neuroblast so inhibits cell fate of others
- Notch and Delta phenotypes give evidence
What is the Notch phenotype?
Wing phenotype - notches in wing
What is the Delta phenotype?
Wing vein phenotype
Notch and Delta: what acts as the ligand and which is the receptor?
Notch = Receptor Delta = Ligand
How does lateral inhibition involving notch and delta work?
- Ligand (delta) tethered to membrane - only cells immediately touching can signal
- When ligand bind, In. domain of Notch cleaved away, becomes TF effector protein
- Both cells express notch and delta
- One has more delta
- Sends stronger signal to notch on surrounding so this forms negative feedback of delta in other cell
- Less signal sent to notch in first cell
- Difference between cells
- Cell with more delta becomes neuroblast
What is delamination?
A separation into layers
How do neuroblasts become neurons?
Neuroblast –> ganglion mother cell –> Neuron
- Neuroblast delaminates from neurectoderm
- Segregation of cytoplasmic factors into one daughter cell (Numb, Prospero)
- Reorientation of spindle
- Undergo asymmetric division
- Neuroblast remains as stem cell
- Progeny of GMC differentiate as neurons
What are the cell types in mechnosensory bristles?
N: sensory neuron
H: hair cell/bristle (triogen)
So: Socket cell (Tormogen)
Sh: Sheath cell (Theocogen)
How are sensory organ precurosors singled out by lateral inhibition?
Lateral inhibition
- Delta-notch singalling singles out sensory organ precursors (SOPs) from proneural clusters
- Each SOP undergoes further division to form cells of sensory bristle
What are mechnosensory bristles?
Adult fly covered in them
How does cell lineage in the sensory bristle work?
- Each division involves segregation of Numb pro. and regulation by Notch
- Each sensory bristle organ is a clone of four cells that derive from SOPs
- SOPs undergo series of asymmetrical divisions to form different cell types
- Numb protein inhibits notch function
What happens when Notch is genetically inactivated in SOPs?
Both cells become IIb and have low numb expression:
Four neurons form
What happens when Notch is over expressed in SOPs?
Both cells become IIa with high notch activity:
Four socket cells produced
How do neurons differentiate?
- Axon outgrowth and path finding
2. Synapse formation and pruning - each neuron sends out axons, refines 1:1 mapping
How does axon guidance work?
Growth cone at the tip sends out filopodia which explore env.
2. Growth cone responds to short-range and long-rangec cues
3. Synapses when axons reaches target
4
How are neural connections refined?
Pruning and cell death
- Only the neurons that make appropriate connections survive - Neurons produced in excess
What are the sites for limb outgrowth called?
Limb bud
What determines the sites of limb bud formation?
Hox genes
Is Fibroblast growth factor (FGF) required for limb bud formation?
Yes
Is Fibroblast growth factor (FGF) sufficient for limb bud formation?
Yes - evidence from chick, ectopic limb buds when FGF4 protein applied locally
What is the apical ectodermal ridge?
Ridge of thickened epithelium around the tip of the limb bud
- Maintains progress zone
What happens if AER is removed?
Truncation of limb - not complete pattern
Can FGFs substitute AER?
Yes
What is the progress zone?
Distal region of limb bud mesenchyme (mesodermal)
- Undifferentiated, rapidly dividing cells
- Length of time in PZ determines cell fate
What happens when an old limb bud is replaced with a young tip?
Full limb forms
What happens when a young limb bud is replaced with an old tip?
Limb truncated
What is the zone of polarising activity?
Region of posterior limb bud mesenchyme
- When transplanted underneath anterior AER, induces symmetrical duplication of normal limb elements about AP axis
What Hox genes are expressed in the limb bud?
Hoxa and Hoxd genes expressed in nested domains - may specify ZPA
What happens when nested domains of Hox are deleted?
Results in absence of Shh expression and truncation of limb
Where is Shh expressed in limb bud??
ZPA
What is involved in the formation of digits?
Programmed cell death in mammals and birds to remove the cells in between digits
- Differential growth in amphibians
What determines the pattern of cell death?
Mesoderm
Regulated by Hox genes
What do somites give rise to?
Muscle, bone and epidermis
What are rhomberes?
Segmented regions in the hindbrain which gives rise to the brainstem
In Drosophila, what segments make up the body?
T1-T3: Thoracic segments (where limb appendages)
A1-A8: abdominal segments
How are Hox genes involved in segmentation?
Specify the identity of segments
All Hox genes code for TFs
How are Hox genes arranged in Drosophila?
In two gene clusters
- Bithorax Complex (BX-C)
- Antennapedia Complex (ANT-C)
Make up the HOM-C complex
What is Ultrabithorax gene?
A selector gene to specify haltere identity
- It regulates the developmental program followed by all cells in T3
What does a recessive mutation of Ultrabithorax gene cause?
Transformation of third thoracic segment to second thoracic segment
What is a homeotic transformation?
One structure is replaced by another
What is autonomous function?
Mutation affects only the cells in which it is present
What is non-autonomous function?
Mutation affects other cells
What do selector genes do?
Causes cells in different segments to interpret positional information differently
What makes up the bithorax complex (BX-C)?
Ubx and AbdB genes (+AbdA)
What is a homeobox?
Highly conserved 180-nucleotide motif
- Codes for the homeodomain
What is a homeodomain?
A sequence-specific DNA binding motif
What does the homeodomain do in Ubx and AbdB?
Codes for 3 alpha helices, one of which contacts the main group within the DNA
What happens when there is a variation in combination of Hox gene?
Different combinations of expressed genes results in different development
What happens if the whole of BX-C is mutated?
All segments in body plan look like T2
What happens if AbdA and AbdB are mutated?
Ubx is functional and all segments develop into A1
If all genes in BX-C are expressed, what identity is shown?
A8
Is BX-C required for segmentation to occur?
No
What does the antennapedia complex contain?
Homeobox genes controlling head and thoracic segments
What happens when there are mutations in ANT-C?
Antennae develop as legs - homeotic transformation
Where is Antp gene expressed?
In T2 segment and controls identity
Is ANT-C dominant or recessive mutant?
Dominant mutant
What is a Hox cluster?
Highly conserved
- Hox genes expressed in spatially restricted patterns along AP axis of vertebrae embryos
What can mutations in Hox cluster cause?
Homeotic transformations in vertebrates
What is Synpolydactlyl?
Extra digits in humans
- Mutations in HOXD13 gene
What is Bicoid?
Transcription factor that starts turning genes on in zygotic genome
In maternal genes
What are examples of axial patterning in model organisms?
Frog - sperm entry and organiser graft (DV)
Drosophila - Bicoid
Chick limb AP axis
Drosophila DV axis
How is the DV axis established in the cellular blastoderm?
By the cell fates of the 3 germ layers
How is the DV axis of the embryos established?
In the oocyte by the presence of the nucleus in the anterodorsal position
What are the DV cell fates in the Drosophila cellular blastoderm?
Dorsal ectoderm = Future epidermis
Neurectoderm = Future NS
Mesoderm = Future muscle
What are the dorsalised class of maternal effect mutants affecting DV pattern? What are they required for?
Dorsal
Spatzle
Toll
Development of ventral cell fates in embyro
What are the ventralised class of maternal effect mutants affecting DV pattern? What are they required for?
Cactus
Development of dorsal cell fates in embryos
