Transcript de Robertis EM, Evo-Devo: Variations on Ancestral themes. Cell 132

Molecular Regulation of Development.
Growth factor signaling,
Hox genes and the body plan
张咸宁
[email protected]
Tel：13105819271; 88208367
Office: A705, Research Building
2012/09
出生缺陷（Birth defect）：即先天性疾病
• Malformation is a primary morphologic defect of an organ or body part
resulting from an intrinsically abnormal developmental process (e.g., cleft
lip, polydactyly).
• Dysplasia is a primary defect involving abnormal organization of cells into
tissue (e.g., vascular malformation).
• Sequence is a primary defect with its secondary structural changes (e.g.,
Pierre Robin sequence, a disorder in which a primary defect in mandibular
development produces a small jaw, secondary glossoptosis, and a cleft
palate)
• Syndrome is a pattern of multiple primary malformations with a single
etiology (e.g., trisomy 13 syndrome).
• Deformation is alteration of the form, shape, or position of a normally
formed body part by mechanical forces. It usually occurs in the fetal period,
not in embryogenesis. It is a secondary alteration. It can be extrinsic, as in
oligohydramnios (reduced amniotic fluid), or intrinsic, as in congenital
myotonic dystrophy.
• Disruption is a morphological defect of an organ, part of an organ, or a
larger region of the body resulting from the extrinsic breakdown of, or
interference with, an originally normal developmental process. It is a
secondary malformation (e.g., secondary limb defect resulting from a
vascular event).
Three questions:
1. How is dorsal-ventral (D-V背腹 ) cell
differentiation regulated by morphogen（形态发
生素）gradients（梯度）. BMP（骨形态发生
蛋白） signal transduction.
2. Hox is the the antero-posterior (A-P前后 )
pattern regulated by Hox genes?
- Colinearity（共线性）
- Activation by retinoic acid（视黄酸）.
Retinoid receptors.
3. How have conserved（保守）gene networks
affected Evolution and Development? Evo-Devo.
1. During development groups of inducing cells called
organizing centers secrete graded growth factor signals.
The concentration gradient of a “morphogen” can induce
multiple cell differentiation choices.
Fig. 1
D-V patterning can
be studied in the
frog embryo.
Fig. 2
The best example of a morphogen is the gradient of BMP
signaling that controls D-V tissue differentiation.
Fig. 3a
BMP signaling
Mesoderm（中胚层） cell differentiation is
determined by a morphogen gradient of BMP.
ventral
Lateral plate
Somite
Notochord
dorsal
BMPs are members of the TGFβ superfamily.
Fig. 3b
Genes specifically expressed in the dorsal
blastopore lip (胚孔唇Spemann organizer) of
the gastrula（原肠胚）were cloned.
Organizer-specific
Genes
Fig. 4a
Chordin（脊索发生素）mRNA is expressed in Spemann’s
organizer.
Chordin protein is secreted and diffuses in the embryo.
Fig. 4b
Chordin is an antagonist（拮抗剂）that binds BMP growth factors
in the extracellular space, inhibiting binding to cell surface
receptors. Chordin establishes a BMP4 activity gradient at
gastrula. Another protein, Noggin（头发生素）, has similar activity.
Secreted antagonists diffuse and are used in development to
generate morphogen gradients.
Chordin
inhibits
Fig. 5
Signal transduction: membrane receptors transduce the signal so that
transcription factors are activated through phosphorylation. TGFβ
family members (30 different ligands in humans) activate cell surface
receptors (Serine-Threonine kinases).
Xnr
BMP4
Fig. 6a
Fig. 6b
Visualizing a morphogen gradient: phosphorylated
Smad1 forms a gradient, maximal in the ventral, in
the Xenopus（爪蟾） gastrula（原肠胚）
Dorsal
Blastopore
Side view
Ventral
Transverse section at the
level of white arrows
Fig. 7a
Epidermis
CNS
Dorsal
Ventral
Mesoderm
Endoderm
Spemann’s
Organizer
At gastrula a gradient of BMP4 is established by a ventral source of
BMP4 and a dorsal source of Chordin and Noggin, two BMP antagonists
secreted by the dorsal organizing center.
Fig. 7b
The BMP gradient induces different tissues in mesoderm中
胚层 and ectoderm外胚层 (because the DNA-binding partners are
Mesoderm
differentiation
Lateral plate Somite
Notochord
BMP signaling
BMP signaling
different)
Ectoderm
differentiation
Epidermis Neural
crest
CNS
Fig. 7c
Conclusion：a morphogen gradient can be generated by a
source of growth factor (such as BMP) or by a localized
source of inhibitor (such as Chordin). Both mechanisms
are used.
This is how organizing centers work in embryonic induction.
Fig. 8
Cell-cell communication is controlled by
surprisingly few signal transduction pathways:
1)
2)
3)
4)
5)
6)
7)
TGFβ/BMP Serine/Threonine kinase receptors
Receptor Tyrosine kinases such as FGF, EGF, IGF, Insulin
Wnts
Sonic Hedgehog
Notch
G protein-coupled receptors (7-transmembrane receptors)
Nuclear hormone receptors
Only a few signaling pathways pattern the embryo,
but there are hundreds of differentiated cell types in
the human body. The same signals can trigger
different types of cell differentiation responses in cells
of different developmental history (because of
different DNA-binding partners).
Fig. 9
A-P patterning outline:
A.
Hox genes: colinearity between
the gene order in genomic DNA
and the body plan
B.
Hox genes and Retinoic acid
C.
Hox genes in Evolution and
Development (Evo-Devo)
Fig. 10
2. Hox genes and the development of body plans
Homeotic transformations（同源异型转化） in humans. A cervical
vertebra（颈椎）transformed into a thoracic one with ribs（胸肋）.
Fig. 11
Homeotic Mutations – the Homeobox story
Fig. 12
Edward B. Lewis
Homeotic genes specify body segment identity in Drosophila.
Lewis predicted Hox genes would be duplicated.
Fig. 13
Homeobox（同源〔异型〕框）refers to nucleic acid.
Homeodomain（同源〔异型〕域）refers to protein.
The homeodomain is a 60 aa helixturn-helix DNA-binding domain
Define
Hox,conserved
homeobox
that
is very
during
evolution. It fits into the major
groove of the DNA.
The term homeobox is reserved for the nucleic acid sequences that
encode homeodomains. Since they are highly conserved, and can be
detected by low-stringency hybridization across species.
Fig. 14
Hox complexes are conserved between Drosophila and mammals
(from de Robertis et al., Scientific American, 1990)
Fig. 15
Vertebrates have 4 Hox complexes, with about 10 genes each.
They display colinearity（共线性）:
a) Spatial（空间）colinearity: the more anteriorly（前部）
expressed genes are in one end, the more posterior ones at the
other end of the gene complex.
b) Temporal（时间）colinearity: genes on one end of the
complex are expressed first, those on the other (posterior。后
部) end are turned on last.
c) Anterior Hox genes are activated sequentially by retinoic acid.
Hox genes can be aligned in 13 groups of paralogues (种内同源
基因。that were duplicated twice).
Fig. 16
Extensive conservations between Drosophila and the
four human Hox complexes
High RA response
Low RA response
From de Robertis E.M. Evo-Devo: Variations on Ancestral themes. Cell
132, 185-195 (2008)
Fig. 17
Hox-C6 protein is seen
in 8 thoracic segments
of the mouse embryo.
Translation is blocked
in the tail region,
probably through the
action of microRNAs.
The inset shows that
Hox-C6 mRNA is
expressed all the way
to the tip of the tail.
From De Robertis, Cell 132, 185-195 (2008)
Fig. 18
Spatial and temporal colinearity: order of Hox genes
in DNA follows the antero-posterior body axis.
Why have Hox genes stayed together in a complex?
Fig. 19
Hox knockouts in mice cause homeotic transformations, in
this case an extra rib in the lumbar region (HoxC-8 mutant).
Treatment with retinoic acid can also cause lumbar ribs（腰
肋）.
Your patient this week has 13 ribs.
Fig. 20
mRNA amount
B. Retinoic acid activates HOX genes sequentially in cultured
human teratocarcinoma（畸胎癌）cells
Fig. 21
How do Retinoic acid receptors work?
DBD
LBD
Dimerization
ligand
coactivator
Fig. 22
Retinoic acid receptor is a DNA-binding protein that works as a ligandactivated transcription factor. Many hydrophobic hormone receptors
work in this way. Nuclear receptors work very differently from cell
surface receptors.
(RA)
RA
Fig. 23
Hox complexes have a retinoic acid receptor response element (RARE) in the
DNA before paralogue 1. This DNA enhancer element controls expression of
many genes in the complex. In retinoic acid teratogenesis, Hox gene expression
borders move into more anterior regions.
RARE
Fig. 24
Pharyngeal arch 1 does not express any Hox gene. It gives rise to maxillary
（上颌骨）and mandibular（下颌骨）structures. Retinoic acid can cause
cleft palate（腭裂）and micrognathia（下颌）
Fig. 25
III. The common ancestor Urbilateria（原生两侧对称动物）
used Hox genes and Chordin/BMP to pattern the embryo.
30 of the 35 animal phyla（门）are bilaterans（两侧对称
动物）.
BMP
Hox
Chd
Fig. 26
Evo-Devo: Urbilateria had a Hox gene complex.
Developmental control genes placed evolutionary
constraints on the types of animal shapes that evolved
by Natural Selection（自然选择）.
High RA response
Low RA response
From: de Robertis EM, Evo-Devo: Variations on Ancestral themes. Cell 132, 185-195 (2008)
Fig. 27
Overview
Langman’s Medical Embryology
Fig. 28
Acknowledge（PPT特别鸣谢！）
• UCLA David Geffen School of Medicine
• www.medsch.ucla.edu/ANGEL/
• Prof. Robertis E (UCLA), et al.