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Developmental Biology II(Molecular Mechanisms of Development)
Anke van Eekelen, PhDTelethon Institute for Child Health Research
Basic Principles of Development
1- Cell Division • multicellular organism
2- Pattern Formation • Defining the Axes: Body Plan
• initiating Germ Layer Formation
3- Morphogenesis • Formation of 3 Germ layers
4- Cell Differentiation • Blood, Muscle, Nerves …
5- Growth • maturity
Cleavage
Gastrulation
Organogenesis
Event Principle Outcome
Developmental events
are controlled by differential gene expression,
……..which drives cascades of gene-regulatory events,
…………………..which define differential cell behaviours,
………………………………which underlie major developmental processes
crucial to study gene expression & protein function
DNA
mRNA
Protein(e.g.transcription factors)
Differential gene expression:- type of enhancer
- processing of mRNA intodifferent splice product
- protein modification to createfunctional proteins
TF = transcription factordevelopmental specificcascades of gene regulatory events
Transcriptional protein complex:
= downstream gene(e.g. new TF!!)
During cleavage, gastrulation and cell differentiation:
‘Autonomous’ signals Cell Conditioning signals
Maternal & Zygotic determinants
Genes encoding Genes encodingtranscription factors secreted proteins(or factors affecting translation)
direct effect on gene indirect effect on gene expressionexpression in same cell in another (target ) cell
Wolpert: “The early development of Drosophila is probably the best understood developmental system of all animals, at the molecular level. This is due largely to a saturation mutagenesis screen for embryonic pattern defects carried out in the 1970s by C. Nusslein-Volhard and E. Wieschaus (Nobel Prize in 1995). Approximately 100 genes were identified which can account for most of the pattern formation and morphogenetic events of the early embryo. This set of genes complement a different set of genes studied by E. Lewis, whose activities were involved in specifying the identity of segments observable in the adult fly. Together, these genes became the Rosetta stones that allowed fly development to be deciphered. At the time, no one suspected that they would provide the keys to understanding development in all animals, yet that is what has occurred; even humans use the genetic networks first discovered in flies, albeit in sometimes very different ways. For this reason, Drosophila development deserves special attention in studies in developmental biology.”
Wolpert, Beddington, Jessell, Lawrence, Meyerowitz and Smith (2002)“Principles of Development” 2nd edition (Oxford University Press)
P (p
oste
rior)
bicoid (oocyte mRNA encoding a TF)
Nanos (translation repressor)
Patterning of A-P axis during cleavage event (Fruit fly) A
(ant
erio
r)
Putting A and P determinants together…..
hunchbackmRNA
Hunchbackprotein
Zygotic hunchback
The expression of each transcription factor in developmentat the right time in the right location
is regulated by a complex interactive network of gene regulatory events
Over time the type of expressed developmental specific TF and other determinants change!
Nature of protein : TF (different families of TFs based on structure)secretion proteinmembrane coupled receptor
Cell Signalling regulation of development
Interactive network ofdeterminants regulating
gene expression in neighbouring cells
e.g. Segment polarity genes
Cell Autonomous regulation of development
Interactive network ofdeterminants regulating gene expression within
cellse.g. Gap genes
Similarity between invertebrates and (higher)vertebrates:- many common genes- maternal determinants in oocyte- overlap in cascades of gene-regulatory events driving development
Why study flies……if we are mostly interested in development of mammals like
mice and humans?
Homeotic selector genes• Hom (flies) vs Hox in (mammals)• Homeobox family of TFs: highly conserved homeodomain DNA binding region• Specify order of type of structures along a line
(“memory” for the A-P order already established by earlier developmental specific genes)
Fruit fly: 1 cluster of Hox genesmammals: 4 Hox clusters, each preserving the homologies and order along the chromosome of the fly
cluster.
• Secreted factors noggin and chordin both bind to the TGF-beta type growth factor BMP-4.• The secreted factor Frizbee is related to Frizzled, the cell membrane receptor for Wnt-family
growth factors. It acts as a competitive inhibitor of Wnt/Frizzled signalling.
Example of the molecular mechanism underlying initiation of gastrulation in the frog:
The somite (Pax3 +ve) is target of numerous known signals from all its adjacent tissues at
the end of gastrulation and neurulation,
that sub-divide it into regions expressing distinct gene signatures. These regions are
specified for distinct fates.
Example of the molecular mechanism underlying mesodermal tissue differentiation (somite)in the chicken:
Various commonly used Signalling Pathways in developmental events of
tissue organization and organogenesisG
row
th fa
ctor
sign
alin
gH
edge
hog
sign
alin
g
Wnt
sign
alin
g
0
Organizer AER:• controls proximal to distal (ie shoulder to finger tip) growth & development
• morphogen = diffusible growth factors FGF4 and FGF 8
Tbx-5
Tbx-4
Proximal distal
mesenchym
ectoderm
AER
Example of the molecular mechanism underlying mesodermal limb formationin the chicken:
Example of cascade of gene regulatory events in heart development(organogenesis)
Example of crucial regulatory genes in muscle development
HLH TFs
How to study molecular mechanisms underlying development ?
previously- Mutagenesis in fruit fliescurrently- Genetic manipulation - gene knockout loss of function
Tutorial in week 6
Future research will be inclusive of Alternative ways to influence gene expression ?-Epigenetics
Lecture in week 5
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